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Sample records for acceptor bound excitons

  1. Nanosecond spin coherence of excitons bound to acceptors in a CdTe quantum well

    NASA Astrophysics Data System (ADS)

    Grinberg, P.; Bernardot, F.; Eble, B.; Karczewski, G.; Testelin, C.; Chamarro, M.

    2016-03-01

    We have studied the coherent spin dynamics of excitons bound to acceptors, A0X, immersed in a CdTe quantum well by using time resolved photo-induced Faraday rotation. We have also measured the time-resolved differential transmission in order to determine a A0X lifetime of 220 ps, which is independent of the applied magnetic field. We show that at low magnetic field, the spin of A0X is completely frozen during a time, ≅ 4.5 ns, at least twenty times longer than its lifetime. We compare the spin properties of A0X with the spin properties of other charged excitons systems, and we conclude that the hyperfine interaction of the photo-created electron spin with nuclear spins is very likely to be at the origin of the observed spin dephasing times.

  2. Fourier-transform photoluminescence spectroscopy of excitons bound to group-III acceptors in silicon: Uniaxial stress

    NASA Astrophysics Data System (ADS)

    Karasyuk, V. A.; Thewalt, M. L. W.; An, S.; Lightowlers, E. C.

    1997-12-01

    Photoluminescence of excitons bound to Al, Ga, In, and Tl acceptors in Si crystals subjected to <001>, <111>, or <110> uniaxial stress was studied at liquid-He temperatures with 0.0025-meV spectral resolution. The deformation-potential constants of the group-III acceptors in the ground state are (in eV) b=-1.01+/-0.02, d=-3.31+/-0.06 for Al, b=-1.03+/-0.02, d=-3.10+/-0.06 for Ga, b=-0.43+/-0.01, d=-2.41+/-0.05 for In, and b=-0.30+/-0.03, d=-1.95+/-0.2 for Tl. The shear deformation-potential constant for electrons in acceptor bound excitons Ξu=8.6 eV for all group-III acceptors within an experimental error of +/-0.15 eV for Al, Ga, and In, and +/-0.8 eV for Tl. The order of the valley-orbit states in Tl bound excitons is Γ1, Γ3, Γ5 with the Γ5 energy 1.21 meV above Γ1, and 0.10 meV above Γ3. All details of the spectra including positions, relative amplitudes, and polarizations of the components have been explained on the basis of a simple model of acceptor bound excitons with holes in the J=0 state taking into account the valley-orbit splitting and the spin-orbit coupling of the electron. Significant deviations from the theoretical predictions were observed only for very small strains producing acceptor splittings comparable with the intrinsic zero-stress splitting.

  3. The impact of time-varying phosphorus doping on ZnMgO thin films and achievement of dominant acceptor-bound-exciton peak

    NASA Astrophysics Data System (ADS)

    Saha, S.; Nagar, S.; Gupta, S. K.; Chakrabarti, S.

    2014-03-01

    ZnO is a highly efficient and promising semiconductor material because of its large bandgap (3.37 eV) and exciton binding energy (60 meV). MgO also has a very high bandgap (7.8 eV), and the incorporation of Mg into ZnO can result in an alloy with a bandgap of more than 4 eV . We used plasma immersion ion implantation to dope phosphorus into Zn0.85Mg0.15O for achieving p-type ZnMgO. RF sputtering was used to deposit ZnMgO on a Si substrate. Phosphorus doping was conducted from 10 s to 70 s. Rapid thermal annealing of the samples was performed to remove any implantation defects. A highly dominant acceptor-bound-exciton peak was observed at 3.36 eV by photoluminescence measurements, which continued to dominate from low temperature to room temperature. Donor-bound acceptor and free-electron acceptor peaks were also observed at 3.24 eV and 3.28 eV, respectively.

  4. Aluminum acceptor four particle bound exciton complex in 4H, 6H, and 3C SiC

    NASA Technical Reports Server (NTRS)

    Clemen, L. L.; Devaty, R. P.; Macmillan, M. F.; Yoganathan, M.; Choyke, W. J.; Larkin, D. J.; Powell, J. A.; Edmond, J. A.; Kong, H. S.

    1993-01-01

    Evidence is presented for a four particle acceptor complex in 3C, 6H, and 4H SiC, obtained in low-temperature photoluminescence and cathodoluminescence experiments. The new lines were observed in p-type films lightly doped with aluminum, of 6H, 4H, and 3C SiC grown on the silicon (0001) face of 6H SiC under special conditions. The lines increase in intensity as more aluminum is added during growth. The multiplicity of observed lines is consistent with symmetry-based models similar to those which have been proposed to describe 4A centers in p-type zincblende semiconductors.

  5. Electronic and vibronic states of the acceptor-bound-exciton complex (A0,X) in CdS. II. Determination of the fine structure of the (A0,XB) electronic states by high-resolution excitation spectroscopy

    NASA Astrophysics Data System (ADS)

    Gutowski, J.

    1985-03-01

    In a previous paper [R. Baumert, I. Broser, J. Gutowski, and A. Hoffman, Phys. Rev. B 27, 6263 (1983)] it has been shown that high-density, high-resolution excitation spectroscopy gives new information on the electronic and vibronic excited states of the acceptor-bound-exciton complex (A0,XA) with two holes from the A valence band in CdS. We now report on corresponding results for the (A0,XB) configuration which includes one hole from the second B valence band. This complex is unstable for a very fast B-->A hole conversion, and therefore gives rise to a set of excitation resonances of the I1 luminescence arising from the (A0,XA) recombination. A detailed theoretical analysis of the energetic structure of the (A0,XB) complex including the dependence on the excitation intensity and on an applied magnetic field allows the correct assignment of the excitation resonances to the (A0,XB) fine-structure levels originating from the interparticle-exchange interactions. It is shown that the magnetic field is a suitable means of distinguishing the different (A0,XB) ground-state levels. The magnetic field also creates allowed transitions which are dipole forbidden in the zero-field case. A self-contained model of the (A0,XB) complex thus can be developed, including all symmetry states and yielding adequate values for the exchange energies within the complex.

  6. Localized and bound excitons in type-II ZnMnSe/ZnSSe quantum wells.

    PubMed

    Chernenko, A V; Brichkin, A S

    2014-10-22

    Photoluminescence of ZnMnSe/ZnSSe multiple quantum wells under a bandgap continuous wave and fs-pulsed excitations is measured in magnetic fields up to 10 T in Faraday geometry at temperatures within the range of 1.6-20 K. The measurements reveal two dominant lines in the spectra and LO-phonon replicas of the lower-energy line. The photoluminescence and time-resolved studies show dramatically different behaviour of the lines. Analysis of their properties reveals that they correspond to recombination of indirect localized excitons and indirect acceptor-bound excitons (A0X). Crossing of exciton and A0X lines because of the difference in magnitudes of their Zeeman shifts is observed. Analysis of LO-phonon replicas of photoluminescence lines provides additional evidence for strong carrier localization bound to A0X. A model of phonon-assisted recombination of indirect acceptor-bound excitons is proposed. The fitting of photoluminescence lines with this model gives the Huang-Rhys factor S≃0.25 for A0X and the hole localization size ah≃30 Å. Contrary to expectations the exciton magnetic polaron effect is hardly observed in these structures. PMID:25273841

  7. Photocurrent generation through electron-exciton interaction at the organic semiconductor donor/acceptor interface.

    PubMed

    Chen, Lijia; Zhang, Qiaoming; Lei, Yanlian; Zhu, Furong; Wu, Bo; Zhang, Ting; Niu, Guoxi; Xiong, Zuhong; Song, Qunliang

    2013-10-21

    In this work, we report our effort to understand the photocurrent generation that is contributed via electron-exciton interaction at the donor/acceptor interface in organic solar cells (OSCs). Donor/acceptor bi-layer heterojunction OSCs, of the indium tin oxide/copper phthalocyanine (CuPc)/fullerene (C60)/molybdenum oxide/Al type, were employed to study the mechanism of photocurrent generation due to the electron-exciton interaction, where CuPc and C60 are the donor and the acceptor, respectively. It is shown that the electron-exciton interaction and the exciton dissociation processes co-exist at the CuPc/C60 interface in OSCs. Compared to conventional donor/acceptor bi-layer OSCs, the cells with the above configuration enable holes to be extracted at the C60 side while electrons can be collected at the CuPc side, resulting in a photocurrent in the reverse direction. The photocurrent thus observed is contributed to primarily by the charge carriers that are generated by the electron-exciton interaction at the CuPc/C60 interface, while charges derived from the exciton dissociation process also exist at the same interface. The mechanism of photocurrent generation due to electron-exciton interaction in the OSCs is further investigated, and it is manifested by the transient photovoltage characteristics and the external quantum efficiency measurements. PMID:24002235

  8. Atomistic model for excitons: Capturing Strongly Bound Excitons in Monolayer Transition-Metal Dichalcogenides

    NASA Astrophysics Data System (ADS)

    Tseng, Frank; Simsek, Ergun; Gunlycke, Daniel

    2015-03-01

    Monolayer transition-metal dichalcogenides form a direct bandgap predicted in the visible regime making them attractive host materials for various electronic and optoelectronic applications. Due to a weak dielectric screening in these materials, strongly bound electron-hole pairs or excitons have binding energies up to at least several hundred meV's. While the conventional wisdom is to think of excitons as hydrogen-like quasi-particles, we show that the hydrogen model breaks down for these experimentally observed strongly bound, room-temperature excitons. To capture these non-hydrogen-like photo-excitations, we introduce an atomistic model for excitons that predicts both bright excitons and dark excitons, and their broken degeneracy in these two-dimensional materials. For strongly bound exciton states, the lattice potential significantly distorts the envelope wave functions, which affects predicted exciton peak energies. The combination of large binding energies and non-degeneracy of exciton states in monolayer transition metal dichalogendies may furthermore be exploited in room temperature applications where prolonged exciton lifetimes are necessary. This work has been funded by the Office of Naval Research (ONR), directly and through the Naval Research Laboratory (NRL). F.T and E.S acknowledge support from NRL through the NRC Research Associateship Program and ONR Summer Faculty Program, respectively.

  9. Exciton dissociation at organic small molecule donor-acceptor interfaces (Presentation Recording)

    NASA Astrophysics Data System (ADS)

    Robey, Steven W.

    2015-08-01

    Exciton dissociation at organic semiconductor donor-acceptor (D-A) heterojunctions is critical for the performance of organic photovoltaic (OPV) structures. Interfacial charge separation and recombination processes control device efficiency. We have investigated these fundamental interfacial issues using time-resolved two-photon photoemission (TR-2PPE), coupled with the formation of well-controlled D-A structures by organic molecular beam epitaxy. The interfacial electronic and molecular structure of these model interfaces was well-characterized using scanning tunneling microscopy and ultraviolet photoemission. Exciton dissociation dynamics were investigated by using a sub-picosecond pump pulse to create Pc π-->π* transitions, producing a population of singlet (S1) Pc excitons. The subsequent decay dynamics of this population was monitored via photoemission with a time-delayed UV pulse. For CuPcC60 interfaces, S1 exciton population decay in the interfacial CuPc layer was much faster than decay in the bulk due to interfacial charge separation. The rate constant for exciton dissociation was found to be ≍ 7 x 10 12 sec-1 (≍ 140 fs). Excitons that lose energy via intersystem crossing (ISC) to triplet levels dissociate approximately 500 to 1000 times slower. The dependence of exciton dissociation on separation was also studied. Exciton dissociation falls of rapidly with distance from the interface. Dissociation from the 2nd, and subsequent, layers of H2Pc is reduced by at least a factor of 10 from that in the interfacial layer. Finally, investigations of the relative efficiency for interfacial exciton dissociation by alternative acceptors based on perylene cores, (perylene tetracarboxylic dianhydride, or PTCDA) compared to fullerene-based acceptors such as C60 will also be discussed.

  10. Non-fullerene acceptors: exciton dissociation with PTCDA versus C60.

    PubMed

    Dutton, Gregory J; Robey, Steven W

    2015-06-28

    Extensive development of new polymer and small molecule donors has helped produce a steady increase in the efficiency of organic photovoltaic (OPV) devices. However, OPV technology would also benefit from the introduction of non-fullerene acceptors. Unfortunately, efforts to replace fullerenes have typically led to significantly reduced efficiencies. A number of possible explanations for reduced efficiencies with non-fullerene acceptors compared to fullerene acceptors have been suggested, including the formation of unfavorable morphologies in non-fullerene systems and/or favorable excitation/carrier delocalization in fullerenes. In addition, enhanced exciton dissociation associated with fundamental characteristics of the fullerene molecular electronic states has also been suggested. We used time-resolved two-photon photoemission (TR-2PPE) to directly compare exciton dissociation at interfaces between zinc phthalocyanine (ZnPc) interfaces and the non-fullerene acceptor, perylene tetracarboxylic dianhydride (PTCDA) versus dissociation measured at the analogous interface with C60, and thus help discriminate between these potential explanations. Exciton dissociation rates are comparable for phthalocyanine interfaces with both acceptors, allowing us to suggest a hierarchy for the importance of various effects producing higher efficiencies with fullerene acceptors. PMID:26027544

  11. Dynamics of exciton dissociation in donor-acceptor polymer heterojunctions.

    PubMed

    Sun, Zhen; Stafström, Sven

    2013-04-28

    Exciton dissociation in a donor-accepter polymer heterojunction has been simulated using a nonadiabatic molecular dynamics approach, which allows for the coupled evolution of the nuclear degrees of freedom and the electronic degrees of freedom described by multiconfigurational electronic wavefunctions. The simulations reveal important details of the charge separation process: the exciton in the donor polymer first dissociates into a "hot" charge transfer state, which is best described as a polaron pair. The polaron pair can be separated into free polaron charge carriers if a sufficiently strong external electric field is applied. We have also studied the effects of inter-chain interaction, temperature, and the external electric field strength. Increasing inter-chain interactions makes it easier for the exciton to dissociate into a polaron pair state, but more difficult for the polaron pair to dissociate into free charge carriers. Higher temperature and higher electric field strength both favor exciton dissociation as well as the formation of free charge carriers. PMID:23635169

  12. Charge transfer fluorescence and 34 nm exciton diffusion length in polymers with electron acceptor end traps

    DOE PAGESBeta

    Zaikowski, Lori; Mauro, Gina; Bird, Matthew; Karten, Brianne; Asaoka, Sadayuki; Wu, Qin; Cook, Andrew R.; Miller, John R.

    2014-12-22

    Photoexcitation of conjugated poly-2,7-(9,9-dihexylfluorene) polyfluorenes with naphthylimide (NI) and anthraquinone (AQ) electron-acceptor end traps produces excitons that form charge transfer states at the end traps. Intramolecular singlet exciton transport to end traps was examined by steady state fluorescence for polyfluorenes of 17 to 127 repeat units in chloroform, dimethylformamide (DMF), tetrahydrofuran (THF), and p-xylene. End traps capture excitons and form charge transfer (CT) states at all polymer lengths and in all solvents. The CT nature of the end-trapped states is confirmed by their fluorescence spectra, solvent and trap group dependence and DFT descriptions. Quantum yields of CT fluorescence are asmore » large as 46%. This strong CT emission is understood in terms of intensity borrowing. Energies of the CT states from onsets of the fluorescence spectra give the depths of the traps which vary with solvent polarity. For NI end traps the trap depths are 0.06 (p-xylene), 0.13 (THF) and 0.19 eV (CHCl3). For AQ, CT fluorescence could be observed only in p-xylene where the trap depth is 0.27 eV. Quantum yields, emission energies, charge transfer energies, solvent reorganization and vibrational energies were calculated. Fluorescence measurements on chains >100 repeat units indicate that end traps capture ~50% of the excitons, and that the exciton diffusion length LD =34 nm, which is much larger than diffusion lengths reported in polymer films or than previously known for diffusion along isolated chains. As a result, the efficiency of exciton capture depends on chain length, but not on trap depth, solvent polarity or which trap group is present.« less

  13. Charge transfer fluorescence and 34 nm exciton diffusion length in polymers with electron acceptor end traps

    SciTech Connect

    Zaikowski, Lori; Mauro, Gina; Bird, Matthew; Karten, Brianne; Asaoka, Sadayuki; Wu, Qin; Cook, Andrew R.; Miller, John R.

    2014-12-22

    Photoexcitation of conjugated poly-2,7-(9,9-dihexylfluorene) polyfluorenes with naphthylimide (NI) and anthraquinone (AQ) electron-acceptor end traps produces excitons that form charge transfer states at the end traps. Intramolecular singlet exciton transport to end traps was examined by steady state fluorescence for polyfluorenes of 17 to 127 repeat units in chloroform, dimethylformamide (DMF), tetrahydrofuran (THF), and p-xylene. End traps capture excitons and form charge transfer (CT) states at all polymer lengths and in all solvents. The CT nature of the end-trapped states is confirmed by their fluorescence spectra, solvent and trap group dependence and DFT descriptions. Quantum yields of CT fluorescence are as large as 46%. This strong CT emission is understood in terms of intensity borrowing. Energies of the CT states from onsets of the fluorescence spectra give the depths of the traps which vary with solvent polarity. For NI end traps the trap depths are 0.06 (p-xylene), 0.13 (THF) and 0.19 eV (CHCl3). For AQ, CT fluorescence could be observed only in p-xylene where the trap depth is 0.27 eV. Quantum yields, emission energies, charge transfer energies, solvent reorganization and vibrational energies were calculated. Fluorescence measurements on chains >100 repeat units indicate that end traps capture ~50% of the excitons, and that the exciton diffusion length LD =34 nm, which is much larger than diffusion lengths reported in polymer films or than previously known for diffusion along isolated chains. As a result, the efficiency of exciton capture depends on chain length, but not on trap depth, solvent polarity or which trap group is present.

  14. Observation of selective plasmon-exciton coupling in nonradiative energy transfer: donor-selective versus acceptor-selective plexcitons.

    PubMed

    Ozel, Tuncay; Hernandez-Martinez, Pedro Ludwig; Mutlugun, Evren; Akin, Onur; Nizamoglu, Sedat; Ozel, Ilkem Ozge; Zhang, Qing; Xiong, Qihua; Demir, Hilmi Volkan

    2013-07-10

    We report selectively plasmon-mediated nonradiative energy transfer between quantum dot (QD) emitters interacting with each other via Förster-type resonance energy transfer (FRET) under controlled plasmon coupling either to only the donor QDs (i.e., donor-selective) or to only the acceptor QDs (i.e., acceptor-selective). Using layer-by-layer assembled colloidal QD nanocrystal solids with metal nanoparticles integrated at carefully designed spacing, we demonstrate the ability to enable/disable the coupled plasmon-exciton (plexciton) formation distinctly at the donor (exciton departing) site or at the acceptor (exciton feeding) site of our choice, while not hindering the donor exciton-acceptor exciton interaction but refraining from simultaneous coupling to both sites of the donor and the acceptor in the FRET process. In the case of donor-selective plexciton, we observed a substantial shortening in the donor QD lifetime from 1.33 to 0.29 ns as a result of plasmon-coupling to the donors and the FRET-assisted exciton transfer from the donors to the acceptors, both of which shorten the donor lifetime. This consequently enhanced the acceptor emission by a factor of 1.93. On the other hand, in the complementary case of acceptor-selective plexciton we observed a 2.70-fold emission enhancement in the acceptor QDs, larger than the acceptor emission enhancement of the donor-selective plexciton, as a result of the combined effects of the acceptor plasmon coupling and the FRET-assisted exciton feeding. Here we present the comparative results of theoretical modeling of the donor- and acceptor-selective plexcitons of nonradiative energy transfer developed here for the first time, which are in excellent agreement with the systematic experimental characterization. Such an ability to modify and control energy transfer through mastering plexcitons is of fundamental importance, opening up new applications for quantum dot embedded plexciton devices along with the development of new

  15. Bound excitons at nitrogen and bismuth isoelectronic impurities

    NASA Astrophysics Data System (ADS)

    Christian, Theresa; Alberi, Kirstin; Beaton, Daniel; Fluegel, Brian; Mascarenhas, Angelo

    When nitrogen and bismuth dopants are simultaneously incorporated into a host lattice such as gallium arsenide (GaAs) or gallium phosphide (GaP), each dopant species contributes to the evolution of the electronic structure. Bound excitons in these systems luminescence from localized states whose distinctive radiative signatures provide invaluable clues into the nature of impurity clustering and inter-impurity interactions within the host lattice. Spectroscopic studies of these states will be presented for a series of samples grown by molecular beam epitaxy. Research was supported by the U. S. Department of Energy, Basic Energy Sciences, Materials Sciences and Engineering Division under contract DE-AC36-08GO28308 and by the Department of Energy Office of Science Graduate Fellowship Program (DOE SCGF), made possible in part by the American Recovery and Reinvestment Act of 2009, administered by ORISE-ORAU under Contract No. DE-AC05-06OR23100.

  16. Three holes bound to a double acceptor - Be(+) in germanium

    NASA Technical Reports Server (NTRS)

    Haller, E. E.; Mcmurray, R. E., Jr.; Falicov, L. M.; Haegel, N. M.; Hansen, W. L.

    1983-01-01

    A double acceptor binding three holes has been observed for the first time with photoconductive far-infrared spectroscopy in beryllium-doped germanium single crystals. This new center, Be(+), has a hole binding energy of about 5 meV and is only present when free holes are generated by ionization of either neutral shallow acceptors or neutral Be double acceptors. The Be(+) center thermally ionizes above 4 K. It disappears at a uniaxial stress higher than about a billion dyn/sq cm parallel to (111) as a result of the lifting of the valence-band degeneracy.

  17. Cathodoluminescence of stacking fault bound excitons for local probing of the exciton diffusion length in single GaN nanowires

    SciTech Connect

    Nogues, Gilles Den Hertog, Martien; Auzelle, Thomas; Gayral, Bruno; Daudin, Bruno

    2014-03-10

    We perform correlated studies of individual GaN nanowires in scanning electron microscopy combined to low temperature cathodoluminescence, microphotoluminescence, and scanning transmission electron microscopy. We show that some nanowires exhibit well localized regions emitting light at the energy of a stacking fault bound exciton (3.42 eV) and are able to observe the presence of a single stacking fault in these regions. Precise measurements of the cathodoluminescence signal in the vicinity of the stacking fault give access to the exciton diffusion length near this location.

  18. Giant permanent dipole moment of two-dimensional excitons bound to a single stacking fault

    NASA Astrophysics Data System (ADS)

    Karin, Todd; Linpeng, Xiayu; Glazov, M. M.; Durnev, M. V.; Ivchenko, E. L.; Harvey, Sarah; Rai, Ashish K.; Ludwig, Arne; Wieck, Andreas D.; Fu, Kai-Mei C.

    2016-07-01

    We investigate the magneto-optical properties of excitons bound to single stacking faults in high-purity GaAs. We find that the two-dimensional stacking fault potential binds an exciton composed of an electron and a heavy hole, and we confirm a vanishing in-plane hole g -factor, consistent with the atomic-scale symmetry of the system. The unprecedented homogeneity of the stacking-fault potential leads to ultranarrow photoluminescence emission lines (with a full width at half-maximum ≲80 μ eV ) and reveals a large magnetic nonreciprocity effect that originates from the magneto-Stark effect for mobile excitons. These measurements unambiguously determine the direction and magnitude of the giant electric dipole moment (≳e ×10 nm ) of the stacking-fault exciton, making stacking faults a promising new platform to study interacting excitonic gases.

  19. Combined impact of entropy and carrier delocalization on charge transfer exciton dissociation at the donor-acceptor interface

    NASA Astrophysics Data System (ADS)

    Ono, Shota; Ohno, Kaoru

    2016-08-01

    Several models of the charge transfer exciton (CTE) have been proposed to explain its dissociation at the donor-acceptor (DA) interface. However, the underlying physics is still under debate. Here, we derive a temperature (T ) dependent tight-binding model for an electron-hole pair at the DA interface. The main finding is the existence of the localization-delocalization transition at a critical T , which can explain the CTE dissociation. The present study highlights the combined effect of entropy (finite T ) and carrier delocalization in the CTE dissociation.

  20. Phonon coupling in optical transitions for singlet-triplet pairs of bound excitons in semiconductors

    NASA Astrophysics Data System (ADS)

    Pistol, M. E.; Monemar, B.

    1986-05-01

    A model is presented for the observed strong difference in selection rules for coupling of phonons in the one-phonon sideband of optical spectra related to bound excitons in semiconductors. The present treatment is specialized to the case of a closely spaced pair of singlet-triplet character as the lowest electronic states, as is common for bound excitons associated with neutral complexes in materials like GaP and Si. The optical transition for the singlet bound-exciton state is found to couple strongly only to symmetric A1 modes. The triplet state has a similar coupling strength to A1 modes, but in addition strong contributions are found for replicas corresponding to high-density-of-states phonons TAX, LAX, and TOX. This can be explained by a treatment of particle-phonon coupling beyond the ordinary adiabatic approximation. A weak mixing between the singlet and triplet states is mediated by the phonon coupling, as described in first-order perturbation theory. The model derived in this work, for such phonon-induced mixing of closely spaced electronic states, is shown to explain the observed phonon coupling for several bound-exciton systems of singlet-triplet character in GaP. In addition, the observed oscillator strength of the forbidden triplet state may be explained as partly derived from phonon-induced mixing with the singlet state, which has a much larger oscillator strength.

  1. Complete quantum control of exciton qubits bound to isoelectronic centres.

    PubMed

    Éthier-Majcher, G; St-Jean, P; Boso, G; Tosi, A; Klem, J F; Francoeur, S

    2014-01-01

    In recent years, impressive demonstrations related to quantum information processing have been realized. The scalability of quantum interactions between arbitrary qubits within an array remains however a significant hurdle to the practical realization of a quantum computer. Among the proposed ideas to achieve fully scalable quantum processing, the use of photons is appealing because they can mediate long-range quantum interactions and could serve as buses to build quantum networks. Quantum dots or nitrogen-vacancy centres in diamond can be coupled to light, but the former system lacks optical homogeneity while the latter suffers from a low dipole moment, rendering their large-scale interconnection challenging. Here, through the complete quantum control of exciton qubits, we demonstrate that nitrogen isoelectronic centres in GaAs combine both the uniformity and predictability of atomic defects and the dipole moment of semiconductor quantum dots. This establishes isoelectronic centres as a promising platform for quantum information processing. PMID:24875932

  2. Transition from bound to free excitons observed in deep- ultraviolet photoluminescence of AlN grown by MOCVD

    NASA Astrophysics Data System (ADS)

    Wang, Weiying; Jin, Peng; Tang, Ning; Liu, Yali; Fu, Lei; Xu, Fujun; Qin, Zhixin; Ge, Weikun; Shen, Bo

    2016-07-01

    The transition from bound exciton to free exciton and exciton–phonon interaction in an AlN epilayer have been investigated by time resolved deep ultraviolet photoluminescence spectroscopy. Based on the analysis of the energy position (S-shaped dependence with temperature), integrated intensity as well as decay time, the main X peak located at 6.06 eV at 7.7 K is assigned to originate from radiative recombination of excitons bound to some unintentionally doped Si or O impurities. While the other two peaks on the lower energy side should be from the bound exciton’s phonon replicas. The corresponding small Huang–Rhys factor indicates weak interaction between phonon and bound excitons, in comparison to the case of free exciton, for which our experimental results are in good agreement with the theoretical calculation of the Huang–Rhys factors.

  3. Observation of Excitonic N-Body Bound States: Polyexcitons in Diamond

    NASA Astrophysics Data System (ADS)

    Omachi, J.; Suzuki, T.; Kato, K.; Naka, N.; Yoshioka, K.; Kuwata-Gonokami, M.

    2013-07-01

    We have found a series of resonances associated with the bound state (polyexcitons, PENs) of N excitons up to N=6 in the emission spectra of diamond under two-photon excitation at around 10 K. Time-resolved spectra show a stepwise formation of PENs with smaller to larger N, as well as a successive decay from larger to smaller N. At higher excitation levels, the transformation of PENs into a condensed phase of electron-hole droplets occurs. The binding energies of the PENs, normalized to the exciton Rydberg energy, agree well with those of silicon, suggesting the universality of the phenomena.

  4. Carbon related donor bound exciton transitions in ZnO nanowires

    SciTech Connect

    Mohammadbeigi, F.; Kumar, E. Senthil; Alagha, S.; Anderson, I.; Watkins, S. P.

    2014-08-07

    Several shallow donor bound exciton photoluminescence (PL) transitions are reported in ZnO nanowires doped with carbon. The emission energies are in the range of 3360.8–3361.9 meV, close to previously reported emission lines due to excitons bound to donor point defects, such as Ga, Al, In, and H. The addition of small amounts of hydrogen during growth results in a strong enhancement of the PL of these carbon related emission lines, yet PL and annealing measurements indicate no appreciable bulk hydrogen. The observation of two electron satellites for these emission lines enables the determination of the donor binding energies. The dependence of exciton localization energy on donor binding energy departs somewhat from the usual linear relationship observed for group III donors, indicating a qualitatively different central cell potential, as one would expect for a complex. Emission lines due to excitons bound to ionized donors associated with these defects are also observed. The dependence of the PL emission intensities on temperature and growth conditions demonstrates that the lines are due to distinct complexes and not merely excited states of each other.

  5. Dynamics of excitons bound to nitrogen isoelectronic centers in GaAs

    NASA Astrophysics Data System (ADS)

    St-Jean, P.; Éthier-Majcher, G.; Francoeur, S.

    2015-03-01

    A detailed analysis of the dynamics of excitons bound to two nitrogen atoms forming an isoelectronic center of C2 v symmetry in GaAs is presented. The temperature dependence of photoluminescence (PL) intensities under both continuous and pulsed excitations reveals (1) overall decay rates significantly slower than that of spontaneous emission, (2) a decay rate anisotropy between states of orthogonal symmetry representations, and (3) a complementary behavior of relative intensities measured from states of identical symmetry representations. A comprehensive model of the exciton fine structure and the exciton dynamics allows the determination of the strength of the exchange and crystal-field interactions, the light- and heavy-hole splitting and composition of exciton states, the exciton capture time, the rates of spontaneous and nonradiative emission, and the rates of interlevel transfers induced by interactions with nuclear spins and by longitudinal acoustic phonons. It is found that the rate of electron spin flips is comparable to that measured in quantum dots but that the near degeneracy of light and heavy holes results in an efficient transfer channel between light- and heavy-hole states of identical symmetry representation.

  6. Repulsively bound exciton-biexciton states in high-spin fermions in optical lattices

    SciTech Connect

    Argueelles, A.; Santos, L.

    2011-03-15

    We show that the interplay between spin-changing collisions and quadratic Zeeman coupling provides a mechanism for the formation of repulsively bound composites in high-spin fermions, which we illustrate by considering spin flips in an initially polarized hard-core one-dimensional Mott insulator of spin-3/2 fermions. We show that after the flips the dynamics is characterized by the creation of two types of exciton-biexciton composites. We analyze the conditions for the existence of these bound states and discuss their intriguing properties. In particular we show that the effective mass and stability of the composites depends nontrivially on spin-changing collisions, on the quadratic Zeeman effect, and on the initial exciton localization. Finally, we show that the composites may remain stable against inelastic collisions, opening the possibility of interesting quantum composite phases.

  7. The identification and nature of bound exciton I-line PL systems in ZnO

    SciTech Connect

    Johnston, K.; Cullen, J.; Henry, M. O.; McGlynn, Enda; Khawaga, Rehab I.

    2013-12-04

    The chemical identification of donor bound excitons in ZnO has been studied using radioactive ions. Implantations of {sup 117}Ag - which decays to radioactive Cd and In - have enabled the identification of the I{sub 2} optical feature as being the ionized donor counterpart of I{sub 9}, one of the most prominent optical features in the photoluminescence spectrum of ZnO. Both of these lines are consistent with In occupying a Zn site.

  8. The identification and nature of bound exciton I-line PL systems in ZnO

    NASA Astrophysics Data System (ADS)

    Johnston, K.; Cullen, J.; Henry, M. O.; McGlynn, Enda; Khawaga, Rehab I.

    2013-12-01

    The chemical identification of donor bound excitons in ZnO has been studied using radioactive ions. Implantations of 117Ag - which decays to radioactive Cd and In - have enabled the identification of the I2 optical feature as being the ionized donor counterpart of I9, one of the most prominent optical features in the photoluminescence spectrum of ZnO. Both of these lines are consistent with In occupying a Zn site.

  9. Efficient photoinduced orthogonal energy and electron transfer reactions via phospholipid membrane-bound donors and acceptors

    SciTech Connect

    Clapp, P.J.; Armitage, B.; Roosa, P.; O'Brien, D.F. )

    1994-10-05

    A three component, liposome-bound photochemical molecular device (PMD) consisting of energy and electron transfer reactions is described. Bilayer membrane surface-associated dyes, 5,10,15,20-tetrakis[4-(trimethylammonio)-phenyl]-21H,2 3H-porphine tetra-p-tosylate salt and N,N[prime]-bis[(3-trimethylammonio)propyl]thiadicarbocya nine tribromide, are the energy donor and acceptor, respectively, in a blue light stimulated energy transfer reaction along the vesicle surface. The electronically excited cyanine is quenched by electron transfer from the phospholipid membrane bound triphenylbenzyl borate anion, which is located in the lipid bilayer interior. The PMD exhibits sequential reactions following electronic excitation with the novel feature that the steps proceed with orthogonal orientation: energy transfer occurs parallel to the membrane surface, and electron transfer occurs perpendicular to the surface. Photobleaching and fluorescence quenching experiments verify the transfer reactions, and Stern-Volmer analysis was used to estimate the reaction rate constants. At the highest concentrations examined of energy and electron acceptor ca. 60% of the photoexcited porphyrins were quenched by energy transfer to the cyanine. 56 refs., 6 figs., 3 tabs.

  10. Magneto-optical properties and recombination dynamics of isoelectronic bound excitons in ZnO

    SciTech Connect

    Chen, S. L.; Chen, W. M.; Buyanova, I. A.

    2014-02-21

    Magneto-optical and time-resolved photoluminescence (PL) spectroscopies are employed to evaluate electronic structure of a bound exciton (BX) responsible for the 3.364 eV line (labeled as I{sub 1}{sup *}) in bulk ZnO. From time-resolved PL spectroscopy, I{sub 1}{sup *} is concluded to originate from the exciton ground state. Based on performed magneto-PL studies, the g-factors of the involved electron and hole are determined as being g{sub e} = 1.98 and g{sub h}{sup ∥}(g{sub h}{sup ⊥}) = 1.2(1.62), respectively. These values are nearly identical to the reported g-factors for the I{sup *} line in ZnO (Phys. Rev. B 86, 235205 (2012)), which proves that I{sub 1}{sup *} should have a similar origin as I{sup *} and should arise from an exciton bound to an isoelectronic center with a hole-attractive potential.

  11. Observation of excitonic N-body bound states: polyexcitons in diamond.

    PubMed

    Omachi, J; Suzuki, T; Kato, K; Naka, N; Yoshioka, K; Kuwata-Gonokami, M

    2013-07-12

    We have found a series of resonances associated with the bound state (polyexcitons, PE(N)s) of N excitons up to N=6 in the emission spectra of diamond under two-photon excitation at around 10 K. Time-resolved spectra show a stepwise formation of PE(N)s with smaller to larger N, as well as a successive decay from larger to smaller N. At higher excitation levels, the transformation of PE(N)s into a condensed phase of electron-hole droplets occurs. The binding energies of the PE(N)s, normalized to the exciton Rydberg energy, agree well with those of silicon, suggesting the universality of the phenomena. PMID:23889423

  12. Hybrid optical-electrical detection of donor electron spins with bound excitons in silicon.

    PubMed

    Lo, C C; Urdampilleta, M; Ross, P; Gonzalez-Zalba, M F; Mansir, J; Lyon, S A; Thewalt, M L W; Morton, J J L

    2015-05-01

    Electrical detection of spins is an essential tool for understanding the dynamics of spins, with applications ranging from optoelectronics and spintronics, to quantum information processing. For electron spins bound to donors in silicon, bulk electrically detected magnetic resonance has relied on coupling to spin readout partners such as paramagnetic defects or conduction electrons, which fundamentally limits spin coherence times. Here we demonstrate electrical detection of donor electron spin resonance in an ensemble by transport through a silicon device, using optically driven donor-bound exciton transitions. We measure electron spin Rabi oscillations, and obtain long electron spin coherence times, limited only by the donor concentration. We also experimentally address critical issues such as non-resonant excitation, strain, and electric fields, laying the foundations for realizing a single-spin readout method with relaxed magnetic field and temperature requirements compared with spin-dependent tunnelling, enabling donor-based technologies such as quantum sensing. PMID:25799326

  13. Hybrid optical-electrical detection of donor electron spins with bound excitons in silicon

    NASA Astrophysics Data System (ADS)

    Lo, C. C.; Urdampilleta, M.; Ross, P.; Gonzalez-Zalba, M. F.; Mansir, J.; Lyon, S. A.; Thewalt, M. L. W.; Morton, J. J. L.

    2015-05-01

    Electrical detection of spins is an essential tool for understanding the dynamics of spins, with applications ranging from optoelectronics and spintronics, to quantum information processing. For electron spins bound to donors in silicon, bulk electrically detected magnetic resonance has relied on coupling to spin readout partners such as paramagnetic defects or conduction electrons, which fundamentally limits spin coherence times. Here we demonstrate electrical detection of donor electron spin resonance in an ensemble by transport through a silicon device, using optically driven donor-bound exciton transitions. We measure electron spin Rabi oscillations, and obtain long electron spin coherence times, limited only by the donor concentration. We also experimentally address critical issues such as non-resonant excitation, strain, and electric fields, laying the foundations for realizing a single-spin readout method with relaxed magnetic field and temperature requirements compared with spin-dependent tunnelling, enabling donor-based technologies such as quantum sensing.

  14. Towards building artificial light harvesting complexes: enhanced singlet-singlet energy transfer between donor and acceptor pairs bound to albumins.

    PubMed

    Kumar, Challa V; Duff, Michael R

    2008-12-01

    Specific donor and acceptor pairs have been assembled in bovine serum albumin (BSA), at neutral pH and room temperature, and these dye-protein complexes indicated efficient donor to acceptor singlet-singlet energy transfer. For example, pyrene-1-butyric acid served as the donor and Coumarin 540A served as the acceptor. Both the donor and the acceptor bind to BSA with affinity constants in excess of 2x10(5) M(-1), as measured in absorption and circular dichroism (CD) spectral titrations. Simultaneous binding of both the donor and the acceptor chromophores was supported by CD spectra and one chromophore did not displace the other from the protein host, even when limited concentrations of the host were used. For example, a 1:1:1 complex between the donor, acceptor and the host can be readily formed, and spectral data clearly show that the binding sites are mutually exclusive. The ternary complexes (two different ligands bound to the same protein molecule) provided opportunities to examine singlet-singlet energy transfer between the protein-bound chromophores. Donor emission was quenched by the addition of the acceptor, in the presence of limited amounts of BSA, while no energy transfer was observed in the absence of the protein host, under the same conditions. The excitation spectra of the donor-acceptor-host complexes clearly show the sensitization of acceptor emission by the donor. Protein denaturation, as induced by the addition of urea or increasing the temperature to 360 K, inhibited energy transfer, which indicate that protein structure plays an important role. Sensitization also proceeded at low temperature (77 K) and diffusion of the donor or the acceptor is not required for energy transfer. Stern-Volmer quenching plots show that the quenching constant is (3.1+/-0.2)x10(4) M(-1), at low acceptor concentrations (<35 microM). Other albumins such as human and porcine proteins also served as good hosts for the above experiments. For the first time, non

  15. Donor bound excitons in ZnSe nanoresonators - Applications in quantum information science

    SciTech Connect

    Pawlis, A.; Lischka, K.; Sanaka, K.; Yamamoto, Y.; Sleiter, D.

    2014-05-15

    Here we summarize the advantages of excitons bound to isolated fluorine donor in ZnSe/ZnMgSe quantum well nano-structures. Devices based on these semiconductors, are particularly suited to implement concepts of the optical manipulation of quantum states in solid-state material. The fluorine donor in ZnSe provides a physical qubit with potential advantages over previously researched qubits. In this context we show several initial demonstrations of devices, such as a low-threshold microdisk laser and an indistinguishable single photon source. Additionally we demonstrate the realization of a controllable three-level-system qubit consisting of a single Fluorine donor in a ZnSe nano-pillar, which provides an optical accessible single electon spin qubit.

  16. Improved Exciton Dissociation at Semiconducting Polymer:ZnO Donor:Acceptor Interfaces via Nitrogen Doping of ZnO

    PubMed Central

    Musselman, Kevin P; Albert-Seifried, Sebastian; Hoye, Robert L Z; Sadhanala, Aditya; Muñoz-Rojas, David; MacManus-Driscoll, Judith L; Friend, Richard H

    2014-01-01

    Exciton dissociation at the zinc oxide/poly(3-hexylthiophene) (ZnO/P3HT) interface as a function of nitrogen doping of the zinc oxide, which decreases the electron concentration from approximately 1019 cm−3 to 1017 cm−3, is reported. Exciton dissociation and device photocurrent are strongly improved with nitrogen doping. This improved dissociation of excitons in the conjugated polymer is found to result from enhanced light-induced de-trapping of electrons from the surface of the nitrogen-doped ZnO. The ability to improve the surface properties of ZnO by introducing a simple nitrogen dopant has general applicability. PMID:25520604

  17. Thermal dissociation of free and acceptor-bound positive trions from magnetophotoluminescence studies of high quality GaAs/Al xGa1 -xAs quantum wells

    NASA Astrophysics Data System (ADS)

    Bryja, L.; Jadczak, J.; Ryczko, K.; Kubisa, M.; Misiewicz, J.; Wójs, A.; Liu, F.; Yakovlev, D. R.; Bayer, M.; Nicoll, C. A.; Farrer, I.; Ritchie, D. A.

    2016-04-01

    Thermal dissociation of free and acceptor-bound quasi-two-dimensional positive trions is investigated by measuring the temperature dependence of the integrated emission intensity in magnetic fields up to 17 T in high quality GaAs/Al xGa1 -xAs quantum wells. Three distinct dissociation processes are observed for the well-resolved hole cyclotron replicas ("shake-up") of positive trions bound to neutral acceptors in the spin-doublet state (SU -A0Xd+ ). To demonstrate that the hole involved in the shake-up process is not bound by the Coulomb interaction to the charged A0X+ complex, we calculate the valence Landau levels using the Luttinger model beyond the axial approximation. The calculated value of the hole cyclotron energy agrees well with the experimental data for the energy separation of the A0X+ and SU -A0X+ lines, determined from the emission spectra. At low temperatures, below 6 K, the dominant dissociation results in a free hole and an exciton bound to the neutral acceptor in the spin-singlet or -triplet state, (A0Xd+→A0Xs+h or A0Xt+h ). At higher temperatures, above 9 K, the dissociation into the free positive trion and the neutral acceptor (A0Xd+→A0+X+ ) predominates. From the temperature evolution of the integrated emission of the free trion lines (X+) we evaluate the transition energy between the two triplet trion states, the dark one (Xtd +) and the bright one (Xtb +). The ionization energies of all detected dissociation processes are compared with the spectral positions of the relevant radiative recombination lines from which excellent quantitative agreement is achieved.

  18. Donor bound or negatively charged excitons in thin CdTe/Cd1-xMnxTe quantum wells

    NASA Astrophysics Data System (ADS)

    Paganotto, N.; Siviniant, J.; Coquillat, D.; Scalbert, D.; Lascaray, J.-P.; Kavokin, A. V.

    1998-08-01

    Magnetophotoluminescence spectroscopy of unintentionally doped thin CdTe/(Cd,Mn)Te single and double quantum wells (QW's) revealed a pronounced excitonic transition that can be associated with either an exciton bound to a neutral donor (D0X) or a negatively charged exciton (X-). Comparative experimental study and theoretical analysis of this transition in quantum wells of different thicknesses allowed us to attribute it to the D0X complex in a single QW and to the X- state in the double QW. A record X- binding energy of 3.7 meV has been detected. The double QW structure was shown to be favorable for the formation of X- in the wide well due to the efficient interwell electron tunneling.

  19. Effect of microwave irradiation on the photoluminescence of bound excitons in CdTe:Cl single crystals

    SciTech Connect

    Korbutyak, D. V.; Lotsko, A. P.; Vakhnyak, N. D.; Demchuna, L. A.; Konakova, R. V. Milenin, V. V.; Red'ko, R. A.

    2011-09-15

    The effect of microwave radiation on the transformation of impurity-based structural complexes in Cd{sub Te}:Cl single crystals is studied using low-temperature photoluminescence measurements. It is shown that microwave radiation activates Cl{sub Te} centers, resulting in an increase in the intensity of photoluminescence line of excitons bound at the corresponding Cl{sub Te} donor centers. A nonmonotonic dependence of the integrated photoluminescence intensity on the duration of microwave irradiation is observed. At the initial stage of microwave irradiation (t = 30 s), an increase in the integrated excitonic photoluminescence intensity is observed; as the duration of microwave irradiation is increased, the photoluminescence intensity decreases. The experimentally observed variations in the photoluminescence intensity are athermal in nature. The hypothetical mechanism of transformation of impurity-based structural complexes is described.

  20. Bi donor hyperfine state populations studied by optical transitions of donor bound excitons in enriched {sup 28}Si

    SciTech Connect

    Ilkhchy, K. Saeedi; Steger, M.; Thewalt, M. L. W.; Abrosimov, N.; Riemann, H.; Becker, P.; Pohl, H.-J.

    2013-12-04

    We report on the first optical studies of Bi donor bound excitons in {sup 28}Si, using absorption rather than emission spectroscopy, and a new noncontact photoconductivity method which has much higher sensitivity and spectral resolution than photoluminescence spectroscopy. Individual hyperfine components of this potential semiconductor qubit can be resolved under an applied magnetic field, and we find that strong nonresonant optical hyperpolarization towards both the I{sub z} = +9/2 and −9/2 hyperfine states can be observed, depending on the intensity of the above-gap excitation.

  1. Electric field distribution and exciton recombination line shape in GaAs

    NASA Astrophysics Data System (ADS)

    Schuster, J.; Kim, T. Y.; Batke, E.; Reuter, D.; Wieck, A. D.

    2016-05-01

    We studied the photoluminescence line shapes of free and bound excitons in a n-modulation doped {{Al}}1-x{{Ga}}x{As}–GaAs heterostructure with linearly increasing electric field in the p-doped buffer. At small laser excitation power the line shapes of the neutral donor bound and free excitons deviate strongly from a simple Lorentzian, whereas the neutral acceptor bound exciton is not obviously affected. Asymmetric lines of sawtooth-type form are observed for the donor bound and the free exciton. The line asymmetry could be traced back to the field dependent exciton binding energy and the field distribution in our heterostructure. A simple analytical model can account for the field dependent line shapes and a fit to the experimental lines gives a satisfactory agreement.

  2. Charge Transfer Excitons at van der Waals Interfaces.

    PubMed

    Zhu, Xiaoyang; Monahan, Nicholas R; Gong, Zizhou; Zhu, Haiming; Williams, Kristopher W; Nelson, Cory A

    2015-07-01

    The van der Waals interfaces of molecular donor/acceptor or graphene-like two-dimensional (2D) semiconductors are central to concepts and emerging technologies of light-electricity interconversion. Examples include, among others, solar cells, photodetectors, and light emitting diodes. A salient feature in both types of van der Waals interfaces is the poorly screened Coulomb potential that can give rise to bound electron-hole pairs across the interface, i.e., charge transfer (CT) or interlayer excitons. Here we address common features of CT excitons at both types of interfaces. We emphasize the competition between localization and delocalization in ensuring efficient charge separation. At the molecular donor/acceptor interface, electronic delocalization in real space can dictate charge carrier separation. In contrast, at the 2D semiconductor heterojunction, delocalization in momentum space due to strong exciton binding may assist in parallel momentum conservation in CT exciton formation. PMID:26001297

  3. The FX iron-sulfur cluster serves as the terminal bound electron acceptor in heliobacterial reaction centers.

    PubMed

    Romberger, Steven P; Golbeck, John H

    2012-03-01

    Phototrophs of the family Heliobacteriaceae contain the simplest known Type I reaction center (RC), consisting of a homodimeric (PshA)(2) core devoid of bound cytochromes and antenna proteins. Unlike plant and cyanobacterial Photosystem I in which the F(A)/F(B) protein, PsaC, is tightly bound to P(700)-F(X) cores, the RCs of Heliobacterium modesticaldum contain two F(A)/F(B) proteins, PshBI and PshBII, which are loosely bound to P(800)-F(X) cores. These two 2[4Fe-4S] ferredoxins have been proposed to function as mobile redox proteins, reducing downstream metabolic partners much in the same manner as does [2Fe-2S] ferredoxin or flavodoxin (Fld) in PS I. Using P(800)-F(X) cores devoid of PshBI and PshBII, we show that iron-sulfur cluster F(X) directly reduces Fld without the involvement of F(A) or F(B) (Fld is used as a proxy for soluble redox proteins even though a gene encoding Fld is not identified in the H. modesticaldum genome). The reduction of Fld is suppressed by the addition of PshBI or PshBII, an effect explained by competition for the electron on F(X). In contrast, P(700)-F(X) cores require the presence of the PsaC, and hence, the F(A)/F(B) clusters for Fld (or ferredoxin) reduction. Thus, in H. modesticaldum, the interpolypeptide F(X) cluster serves as the terminal bound electron acceptor. This finding implies that the homodimeric (PshA)(2) cores should be capable of donating electrons to a wide variety of yet-to-be characterized soluble redox partners. PMID:22297911

  4. Generation of Nitrogen Acceptors in ZnO using Pulse Thermal Processing

    SciTech Connect

    Xu, Jun; Ott, Ronald D; Sabau, Adrian S; Pan, Zhengwei; Xiu, Faxian; Liu, Jilin; Erie, Jean-Marie; Norton, David P

    2008-01-01

    Bipolar doping in wide bandgap semiconductors is difficult to achieve under equilibrium conditions because of the spontaneous formation of compensating defects and unfavorable energetics for dopant substitution. In this work, we explored the use of rapid pulse thermal processing for activating nitrogen dopants into acceptor states in ZnO. Low-temperature photoluminescence spectra revealed both acceptor-bound exciton (A{sup 0}X) and donor-acceptor pair emissions, which present direct evidence for acceptors generated after pulse thermal processing of nitrogen-doped ZnO. This work suggests that pulse thermal processing is potentially an effective method for p-type doping of ZnO.

  5. Charge-transfer excitons at organic semiconductor surfaces and interfaces.

    PubMed

    Zhu, X-Y; Yang, Q; Muntwiler, M

    2009-11-17

    When a material of low dielectric constant is excited electronically from the absorption of a photon, the Coulomb attraction between the excited electron and the hole gives rise to an atomic H-like quasi-particle called an exciton. The bound electron-hole pair also forms across a material interface, such as the donor/acceptor interface in an organic heterojunction solar cell; the result is a charge-transfer (CT) exciton. On the basis of typical dielectric constants of organic semiconductors and the sizes of conjugated molecules, one can estimate that the binding energy of a CT exciton across a donor/acceptor interface is 1 order of magnitude greater than k(B)T at room temperature (k(B) is the Boltzmann constant and T is the temperature). How can the electron-hole pair escape this Coulomb trap in a successful photovoltaic device? To answer this question, we use a crystalline pentacene thin film as a model system and the ubiquitous image band on the surface as the electron acceptor. We observe, in time-resolved two-photon photoemission, a series of CT excitons with binding energies < or = 0.5 eV below the image band minimum. These CT excitons are essential solutions to the atomic H-like Schrodinger equation with cylindrical symmetry. They are characterized by principal and angular momentum quantum numbers. The binding energy of the lowest lying CT exciton with 1s character is more than 1 order of magnitude higher than k(B)T at room temperature. The CT(1s) exciton is essentially the so-called exciplex and has a very low probability of dissociation. We conclude that hot CT exciton states must be involved in charge separation in organic heterojunction solar cells because (1) in comparison to CT(1s), hot CT excitons are more weakly bound by the Coulomb potential and more easily dissociated, (2) density-of-states of these hot excitons increase with energy in the Coulomb potential, and (3) electronic coupling from a donor exciton to a hot CT exciton across the D

  6. Using bound exciton transitions to optically resolve neutral donor hyperfine states of various donor species in Silicon-28

    NASA Astrophysics Data System (ADS)

    Salvail, Jeff; Dluhy, Phillip; Saeedi, Kamyar; Szech, Michael; Riemann, Helge; Abromisov, Nikolai; Becker, Peter; Pohl, Hans-Joachim; Thewalt, Michael

    2014-03-01

    Phosphorus in silicon is established as a promising resource for use in quantum information processing tasks. The neutral donor hyperfine states have been shown to have record long coherence times, high fidelity gates via RF pulses, and projective readout via optical bound exciton transitions. As Shannon's theory of information tells us, we can process more information in an alphabet of more symbols, so there is motivation to look at donors with higher nuclear spin than the I = 1 / 2 of 31P, which provide access to Hilbert spaces of dimension greater than two. In this talk I will describe optical studies of the donors 75As (I = 3 / 2), 121Sb (I = 5 / 2), and 209Bi (I = 9 / 2) in 28Si.

  7. Analysis of rocking curve width and bound exciton linewidth of MOCVD grown CdTe layers in relation with substrate type and crystalline orientation

    NASA Astrophysics Data System (ADS)

    Tromson-Carli, A.; Svob, L.; Marfaing, Y.; Druilhe, R.; Desjonqueres, F.; Triboulet, R.

    1991-12-01

    X-ray double diffraction and photoluminescence experiments were performed on a series of CdTe layers grown by MOVPE on CdTe, CdZnTe and GaAs substrates. Some correlation appears between the measured rocking curve widths and impurity-bound exciton linewidth. To analyze these results, a model relating the exciton linewidth to the average strain induced by an array of random dislocations has been developed. It appears that X-ray diffraction is also sensitive to non-random dislocation configurations which do not affect luminescence linewidth.

  8. Optical activation behavior of ion implanted acceptor species in GaN

    SciTech Connect

    Skromme, B.J.; Martinez, G.L.

    2000-07-01

    Ion implantation is used to investigate the spectroscopic properties of Mg, Be, and C acceptors in GaN. Activation of these species is studied using low temperature photoluminescence (PL). Low dose implants into high quality undoped hydride vapor phase epitaxial (HVPE) material are used in conjunction with high temperature (1300 C) rapid thermal anneals to obtain high quality spectra. Dramatic, dose-dependent evidence of Mg acceptor activation is observed without any co-implants, including a strong, sharp neutral Mg acceptor-bound exciton and strong donor-acceptor pair peaks. Variable temperature measurements reveal a band-to-acceptor transition, whose energy yields an optical binding energy of 224 meV. Be and C implants yield only slight evidence of shallow acceptor-related features and produce dose-correlated 2.2 eV PL, attributed to residual implantation damage. Their poor optical activation is tentatively attributed to insufficient vacancy production by these lighter ions. Clear evidence is obtained for donor-Zn acceptor pair and acceptor-bound exciton peaks in Zn-doped HVPE material.

  9. Bound states and quantization of screening in the Wannier-Mott excitons

    SciTech Connect

    Makowski, Adam J.

    2011-02-15

    The problem of validity of the simple relation {lambda}=(n+2l)(n+2l+1) between the screening length {lambda} and the number of bound states in the Stern-Howard potential is discussed and the results are compared with those obtained for some potential with the same limiting behavior as the former one. The main conclusion of the present study is that the above relation does not hold for the Stern-Howard potential but most likely it is very close to an unknown relation to be discovered for the potential.

  10. Covalently Bound Clusters of Alpha-Substituted PDI-Rival Electron Acceptors to Fullerene for Organic Solar Cells.

    PubMed

    Wu, Qinghe; Zhao, Donglin; Schneider, Alexander M; Chen, Wei; Yu, Luping

    2016-06-15

    A cluster type of electron acceptor, TPB, bearing four α-perylenediimides (PDIs), was developed, in which the four PDIs form a cross-like molecular conformation while still partially conjugated with the BDT-Th core. The blend TPB:PTB7-Th films show favorable morphology and efficient charge dissociation. The inverted solar cells exhibited the highest PCE of 8.47% with the extraordinarily high Jsc values (>18 mA/cm(2)), comparable with those of the corresponding PC71BM/PTB7-Th-based solar cells. PMID:27219665

  11. Excitonic Heterodimer Formation in an HIV-1 Oligonucleotide Labeled with a Donor-Acceptor Pair Used for Fluorescence Resonance Energy Transfer

    PubMed Central

    Bernacchi, Serena; Piémont, Etienne; Potier, Noelle; Dorsselaer, Alain van; Mély, Yves

    2003-01-01

    In this study, we investigated the absorbance and fluorescence properties of cTAR, the complementary DNA sequence of the transactivation response element of the HIV-1 genome, doubly end-labeled by different dyes, 5(and 6)-carboxyfluorescein (Fl) and 5(and 6)-carboxytetramethylrhodamine (TMR), frequently used in fluorescence resonance energy transfer (FRET) studies. This oligonucleotide forms a stable stem-loop structure. The absorption spectrum of this species clearly differed from that of a doubly labeled cTAR derivative in which the terminal part of the stem is melted and from an equimolecular mixture of singly labeled species. Moreover, no significant TMR fluorescence change accompanies the dramatic Fl intensity increase when the doubly labeled native cTAR was melted by temperature or annealed with its complementary sequence. Both elements suggest the formation of an H-type ground-state heterodimer between Fl and TMR that may be described by the molecular exciton model. Moreover, time-resolved fluorescence further suggests that the nonfluorescent heterodimer is in equilibrium with a small population of partially melted species showing FRET. Based on the spectral shifts associated with heterodimer formation, an interchromophore distance of 7.7 Å was calculated. Both the excitonic signal and the Fl fluorescence were used as sensitive tools to monitor the temperature-mediated and HIV nucleocapsid protein-mediated annealing of cTAR with its complementary sequence. PMID:12524317

  12. Excitonic heterodimer formation in an HIV-1 oligonucleotide labeled with a donor-acceptor pair used for fluorescence resonance energy transfer.

    PubMed

    Bernacchi, Serena; Piémont, Etienne; Potier, Noelle; Dorsselaer, Alain van; Mély, Yves

    2003-01-01

    In this study, we investigated the absorbance and fluorescence properties of cTAR, the complementary DNA sequence of the transactivation response element of the HIV-1 genome, doubly end-labeled by different dyes, 5(and 6)-carboxyfluorescein (Fl) and 5(and 6)-carboxytetramethylrhodamine (TMR), frequently used in fluorescence resonance energy transfer (FRET) studies. This oligonucleotide forms a stable stem-loop structure. The absorption spectrum of this species clearly differed from that of a doubly labeled cTAR derivative in which the terminal part of the stem is melted and from an equimolecular mixture of singly labeled species. Moreover, no significant TMR fluorescence change accompanies the dramatic Fl intensity increase when the doubly labeled native cTAR was melted by temperature or annealed with its complementary sequence. Both elements suggest the formation of an H-type ground-state heterodimer between Fl and TMR that may be described by the molecular exciton model. Moreover, time-resolved fluorescence further suggests that the nonfluorescent heterodimer is in equilibrium with a small population of partially melted species showing FRET. Based on the spectral shifts associated with heterodimer formation, an interchromophore distance of 7.7 A was calculated. Both the excitonic signal and the Fl fluorescence were used as sensitive tools to monitor the temperature-mediated and HIV nucleocapsid protein-mediated annealing of cTAR with its complementary sequence. PMID:12524317

  13. Efficient Organic Light-Emitting Diode through Triplet Exciton Reharvesting by Employing Blended Electron Donor and Acceptor as the Emissive Layer.

    PubMed

    Zhang, Lu; Cai, Chao; Li, King Fai; Tam, Hoi Lam; Chan, Kin Long; Cheah, Kok Wai

    2015-11-18

    A blended bimolecular exciplex formation was demonstrated between two individual donor and acceptor molecules, which are tris(4-carbazoyl-9-ylphenyl)amine (TCTA) and 2,4,6-tris(3'-(pyridin-3-yl)biphenyl-3-yl)-1,3,5-triazine (Tm3PyBPZ). The photoluminescence spectrum of the exciplex in the solid state showed an emission with a peak around 514 nm (∼2.49 eV). By applying this exciplex as an emitting layer, a highly efficient all-fluorescent organic lighting emitting diode with maximum efficiencies of 13.1% and 53.4 lm/W can be realized under an extremely low turn-on voltage of only 2.4 V. The thermally activated delayed fluorescence (TADF) process is believed to be responsible for the excellent device performance. PMID:26529382

  14. Optical Characterization of Strong UV Luminescence Emitted from the Excitonic Edge of Nickel Oxide Nanotowers

    PubMed Central

    Ho, Ching-Hwa; Kuo, Yi-Ming; Chan, Ching-Hsiang; Ma, Yuan-Ron

    2015-01-01

    NiO had been claimed to have the potential for application in transparent conducting oxide, electrochromic device for light control, and nonvolatile memory device. However, the detailed study of excitonic transition and light-emission property of NiO has rarely been explored to date. In this work, we demonstrate strong exciton-complex emission of high-quality NiO nanotowers grown by hot-filament metal-oxide vapor deposition with photoluminescence as an evaluation tool. Fine and clear emission features coming from the excitonic edge of the NiO are obviously observed in the photoluminescence spectra. A main excitonic emission of ~3.25 eV at 300 K can be decomposed into free exciton, bound excitons, and donor-acceptor-pair irradiations at lowered temperatures down to 10 K. The band-edge excitonic structure for the NiO nanocrystals has been evaluated and analyzed by transmission and thermoreflectacne measurements herein. All the experimental results demonstrate the cubic NiO thin-film nanotower is an applicable direct-band-gap material appropriate for UV luminescence and transparent-conducting-oxide applications. PMID:26506907

  15. Optical Characterization of Strong UV Luminescence Emitted from the Excitonic Edge of Nickel Oxide Nanotowers

    NASA Astrophysics Data System (ADS)

    Ho, Ching-Hwa; Kuo, Yi-Ming; Chan, Ching-Hsiang; Ma, Yuan-Ron

    2015-10-01

    NiO had been claimed to have the potential for application in transparent conducting oxide, electrochromic device for light control, and nonvolatile memory device. However, the detailed study of excitonic transition and light-emission property of NiO has rarely been explored to date. In this work, we demonstrate strong exciton-complex emission of high-quality NiO nanotowers grown by hot-filament metal-oxide vapor deposition with photoluminescence as an evaluation tool. Fine and clear emission features coming from the excitonic edge of the NiO are obviously observed in the photoluminescence spectra. A main excitonic emission of ~3.25 eV at 300 K can be decomposed into free exciton, bound excitons, and donor-acceptor-pair irradiations at lowered temperatures down to 10 K. The band-edge excitonic structure for the NiO nanocrystals has been evaluated and analyzed by transmission and thermoreflectacne measurements herein. All the experimental results demonstrate the cubic NiO thin-film nanotower is an applicable direct-band-gap material appropriate for UV luminescence and transparent-conducting-oxide applications.

  16. Enhancement Of Free Exciton Peak Intensity In Reactively Sputtered ZnO Thin Films On (0001) Al2O3

    SciTech Connect

    Tuezemen, S.; Guer, Emre; Yildirim, T.; Xiong, G.; Williams, R. T.

    2007-04-23

    Wide bandgap materials such as GaN with its direct bandgap structure have been developed rapidly for applications in short wavelength light emission. ZnO, II-VI oxide semiconductor, is also promising for various technological applications, especially for optoelectronic light emitting devices in the visible and ultraviolet (UV) range of the electromagnetic spectrum. Above-band-edge absorption spectra of reactively sputtered Zn- and O-rich samples exhibit free exciton (FX) and neutral acceptor bound exciton (A deg. X) features. It is shown that the residual acceptors which bind excitons with an energy of 75 meV reside about 312 meV above the valence band, according to effective mass theory. An intra-bandgap absorption feature peaking at 2.5 eV shows correlation with the characteristically narrow A-free exciton peak intensity. Relevant annealing processes are presented as a function of time and temperature dependently for both Zn- and O- rich thin films. Enhancement of the free exciton peak intensity is observed without disturbing the residual shallow acceptor profile which is necessary for at least background p-type conductivity.

  17. Singlet exciton fission photovoltaics.

    PubMed

    Lee, Jiye; Jadhav, Priya; Reusswig, Philip D; Yost, Shane R; Thompson, Nicholas J; Congreve, Daniel N; Hontz, Eric; Van Voorhis, Troy; Baldo, Marc A

    2013-06-18

    Singlet exciton fission, a process that generates two excitons from a single photon, is perhaps the most efficient of the various multiexciton-generation processes studied to date, offering the potential to increase the efficiency of solar devices. But its unique characteristic, splitting a photogenerated singlet exciton into two dark triplet states, means that the empty absorption region between the singlet and triplet excitons must be filled by adding another material that captures low-energy photons. This has required the development of specialized device architectures. In this Account, we review work to develop devices that harness the theoretical benefits of singlet exciton fission. First, we discuss singlet fission in the archetypal material, pentacene. Pentacene-based photovoltaic devices typically show high external and internal quantum efficiencies. They have enabled researchers to characterize fission, including yield and the impact of competing loss processes, within functional devices. We review in situ probes of singlet fission that modulate the photocurrent using a magnetic field. We also summarize studies of the dissociation of triplet excitons into charge at the pentacene-buckyball (C60) donor-acceptor interface. Multiple independent measurements confirm that pentacene triplet excitons can dissociate at the C60 interface despite their relatively low energy. Because triplet excitons produced by singlet fission each have no more than half the energy of the original photoexcitation, they limit the potential open circuit voltage within a solar cell. Thus, if singlet fission is to increase the overall efficiency of a solar cell and not just double the photocurrent at the cost of halving the voltage, it is necessary to also harvest photons in the absorption gap between the singlet and triplet energies of the singlet fission material. We review two device architectures that attempt this using long-wavelength materials: a three-layer structure that uses

  18. The quadratic Zeeman effect used for state-radius determination in neutral donors and donor bound excitons in Si:P

    NASA Astrophysics Data System (ADS)

    Litvinenko, K. L.; Li, Juerong; Stavrias, N.; Meaney, A. J.; Christianen, P. C. M.; Engelkamp, H.; Homewood, K. P.; Pidgeon, C. R.; Murdin, B. N.

    2016-04-01

    We have measured the near-infrared photoluminescence spectrum of phosphorus doped silicon (Si:P) and extracted the donor-bound exciton (D0X) energy at magnetic fields up to 28 T. At high field the Zeeman effect is strongly nonlinear because of the diamagnetic shift, also known as the quadratic Zeeman effect (QZE). The magnitude of the QZE is determined by the spatial extent of the wave-function. High field data allows us to extract values for the radius of the neutral donor (D0) ground state, and the light and heavy hole D0X states, all with more than an order of magnitude better precision than previous work. Good agreement was found between the experimental state radius and an effective mass model for D0. The D0X results are much more surprising, and the radius of the m J = ±3/2 heavy hole is found to be larger than that of the m J = ±1/2 light hole.

  19. Size dependence of the polarizability and Haynes rule for an exciton bound to an ionized donor in a single spherical quantum dot

    SciTech Connect

    Feddi, E. Zouitine, A.; Oukerroum, A.; Zazoui, M.; Dujardin, F.; Assaid, E.

    2015-02-14

    We study the effect of an external electric field on an exciton bound to an ionized donor (D{sup +}, X) confined in a spherical quantum dot using a perturbative-variational method where the wave function and energy are developed in series of powers of the electric field strength. After testing this new approach in the determination of the band gap for some semiconductor materials, we generalize it to the case of (D{sup +}, X) in the presence of the electric field and for several materials ZnO, PbSe, and InAs, with significant values of the mass ratio. Three interesting results can be deduced: First, we show that the present method allows to determine the ground state energy in the presence of a weak electric field in a simple way (E = E{sub 0} − αf{sup 2}) using the energy without electric field E{sub 0} and the polarizability α. The second point is that our theoretical predictions show that the polarizability of (D{sup +}, X) varies proportionally to R{sup 3.5} and follows an ordering α{sub D{sup 0}}<α{sub X}<α{sub (D{sup +},X)}. The last point to highlight is that the Haynes rule remains valid even in the presence of a weak electric field.

  20. Free and bound excitonic effects in Al0.5Ga0.5N/Al0.35Ga0.65N MQWs with different Si-doping levels in the well layers

    PubMed Central

    He, Chenguang; Qin, Zhixin; Xu, Fujun; Hou, Mengjun; Zhang, Shan; Zhang, Lisheng; Wang, Xinqiang; Ge, Weikun; Shen, Bo

    2015-01-01

    Free exciton (FX) and bound exciton (BX) in Al0.5Ga0.5N/Al0.35Ga0.65N multiple quantum wells (MQWs) with different Si-doping levels in the well layers are investigated by photoluminescence (PL) spectra. Low temperature (10 K) PL spectra identify a large binding energy of 87.4 meV for the BX in undoped sample, and 63.6 meV for the BX in Si-doped (2 × 1018 cm−3) sample. They are attributed to O-bound and Si-bound excitons, respectively. The large binding energies of BX are assumed to originate from the strong quantum confinement in the quantum wells, which also leads to a stronger FX PL peak intensity in comparison with BX at 10 K. Si-doping is found to suppress the FX quenching by reducing threading dislocation density (TDD) in the well layers, leading to a significant improvement of IQE from 33.7% to 45%. PMID:26267249

  1. Theoretical study on the cooperative exciton dissociation process based on dimensional and hot charge-transfer state effects in an organic photocell

    NASA Astrophysics Data System (ADS)

    Shimazaki, Tomomi; Nakajima, Takahito

    2016-06-01

    This paper discusses the exciton dissociation process at the donor-acceptor interface in organic photocells. In our previous study, we introduced a local temperature to handle the hot charge-transfer (CT) state and calculated the exciton dissociation probability based on the 1D organic semiconductor model [T. Shimazaki and T. Nakajima, Phys. Chem. Chem. Phys. 17, 12538 (2015)]. Although the hot CT state plays an essential role in exciton dissociations, the probabilities calculated are not high enough to efficiently separate bound electron-hole pairs. This paper focuses on the dimensional (entropy) effect together with the hot CT state effect and shows that cooperative behavior between both effects can improve the exciton dissociation process. In addition, we discuss cooperative effects with site-disorders and external-electric-fields.

  2. Theoretical study on the cooperative exciton dissociation process based on dimensional and hot charge-transfer state effects in an organic photocell.

    PubMed

    Shimazaki, Tomomi; Nakajima, Takahito

    2016-06-21

    This paper discusses the exciton dissociation process at the donor-acceptor interface in organic photocells. In our previous study, we introduced a local temperature to handle the hot charge-transfer (CT) state and calculated the exciton dissociation probability based on the 1D organic semiconductor model [T. Shimazaki and T. Nakajima, Phys. Chem. Chem. Phys. 17, 12538 (2015)]. Although the hot CT state plays an essential role in exciton dissociations, the probabilities calculated are not high enough to efficiently separate bound electron-hole pairs. This paper focuses on the dimensional (entropy) effect together with the hot CT state effect and shows that cooperative behavior between both effects can improve the exciton dissociation process. In addition, we discuss cooperative effects with site-disorders and external-electric-fields. PMID:27334193

  3. DNA-mediated excitonic upconversion FRET switching

    NASA Astrophysics Data System (ADS)

    Kellis, Donald L.; Rehn, Sarah M.; Cannon, Brittany L.; Davis, Paul H.; Graugnard, Elton; Lee, Jeunghoon; Yurke, Bernard; Knowlton, William B.

    2015-11-01

    Excitonics is a rapidly expanding field of nanophotonics in which the harvesting of photons, ensuing creation and transport of excitons via Förster resonant energy transfer (FRET), and subsequent charge separation or photon emission has led to the demonstration of excitonic wires, switches, Boolean logic and light harvesting antennas for many applications. FRET funnels excitons down an energy gradient resulting in energy loss with each step along the pathway. Conversely, excitonic energy upconversion via upconversion nanoparticles (UCNPs), although currently inefficient, serves as an energy ratchet to boost the exciton energy. Although FRET-based upconversion has been demonstrated, it suffers from low FRET efficiency and lacks the ability to modulate the FRET. We have engineered an upconversion FRET-based switch by combining lanthanide-doped UCNPs and fluorophores that demonstrates excitonic energy upconversion by nearly a factor of 2, an excited state donor to acceptor FRET efficiency of nearly 25%, and an acceptor fluorophore quantum efficiency that is close to unity. These findings offer a promising path for energy upconversion in nanophotonic applications including artificial light harvesting, excitonic circuits, photovoltaics, nanomedicine, and optoelectronics.

  4. DNA-mediated excitonic upconversion FRET switching

    SciTech Connect

    Kellis, Donald L.; Rehn, Sarah M.; Cannon, Brittany L.; Davis, Paul H.; Graugnard, Elton; Lee, Jeunghoon; Yurke, Bernard; Knowlton, William B.

    2015-11-17

    Excitonics is a rapidly expanding field of nanophotonics in which the harvesting of photons, ensuing creation and transport of excitons via Förster resonant energy transfer (FRET), and subsequent charge separation or photon emission has led to the demonstration of excitonic wires, switches, Boolean logic and light harvesting antennas for many applications. FRET funnels excitons down an energy gradient resulting in energy loss with each step along the pathway. Conversely, excitonic energy up conversion via up conversion nanoparticles (UCNPs), although currently inefficient, serves as an energy ratchet to boost the exciton energy. Although FRET-based up conversion has been demonstrated, it suffers from low FRET efficiency and lacks the ability to modulate the FRET. We have engineered an up conversion FRET-based switch by combining lanthanide-doped UCNPs and fluorophores that demonstrates excitonic energy up conversion by nearly a factor of 2, an excited state donor to acceptor FRET efficiency of nearly 25%, and an acceptor fluorophore quantum efficiency that is close to unity. These findings offer a promising path for energy up conversion in nanophotonic applications including artificial light harvesting, excitonic circuits, photovoltaics, nanomedicine, and optoelectronics.

  5. DNA-mediated excitonic upconversion FRET switching

    DOE PAGESBeta

    Kellis, Donald L.; Rehn, Sarah M.; Cannon, Brittany L.; Davis, Paul H.; Graugnard, Elton; Lee, Jeunghoon; Yurke, Bernard; Knowlton, William B.

    2015-11-17

    Excitonics is a rapidly expanding field of nanophotonics in which the harvesting of photons, ensuing creation and transport of excitons via Förster resonant energy transfer (FRET), and subsequent charge separation or photon emission has led to the demonstration of excitonic wires, switches, Boolean logic and light harvesting antennas for many applications. FRET funnels excitons down an energy gradient resulting in energy loss with each step along the pathway. Conversely, excitonic energy up conversion via up conversion nanoparticles (UCNPs), although currently inefficient, serves as an energy ratchet to boost the exciton energy. Although FRET-based up conversion has been demonstrated, it suffersmore » from low FRET efficiency and lacks the ability to modulate the FRET. We have engineered an up conversion FRET-based switch by combining lanthanide-doped UCNPs and fluorophores that demonstrates excitonic energy up conversion by nearly a factor of 2, an excited state donor to acceptor FRET efficiency of nearly 25%, and an acceptor fluorophore quantum efficiency that is close to unity. These findings offer a promising path for energy up conversion in nanophotonic applications including artificial light harvesting, excitonic circuits, photovoltaics, nanomedicine, and optoelectronics.« less

  6. Exciton Transport in Organic Semiconductors

    NASA Astrophysics Data System (ADS)

    Menke, Stephen Matthew

    Photovoltaic cells based on organic semiconductors are attractive for their use as a renewable energy source owing to their abundant feedstock and compatibility with low-cost coating techniques on flexible substrates. In contrast to photovoltaic cells based traditional inorganic semiconductors, photon absorption in an organic semiconductor results in the formation of a coulombically bound electron-hole pair, or exciton. The transport of excitons, consequently, is of critical importance as excitons mediate the interaction between charge and light in organic photovoltaic cells (OPVs). In this dissertation, a strong connection between the fundamental photophysical parameters that control nanoscopic exciton energy transfer and the mesoscopic exciton transport is established. With this connection in place, strategies for enhancing the typically short length scale for exciton diffusion (L D) can be developed. Dilution of the organic semiconductor boron subphthalocyanine chloride (SubPc) is found to increase the LD for SubPc by 50%. In turn, OPVs based on dilute layers of SubPc exhibit a 30% enhancement in power conversion efficiency. The enhancement in power conversion efficiency is realized via enhancements in LD, optimized optical spacing, and directed exciton transport at an exciton permeable interface. The role of spin, energetic disorder, and thermal activation on L D are also addressed. Organic semiconductors that exhibit thermally activated delayed fluorescence and efficient intersystem and reverse intersystem crossing highlight the balance between singlet and triplet exciton energy transfer and diffusion. Temperature dependent measurements for LD provide insight into the inhomogeneously broadened exciton density of states and the thermal nature of exciton energy transfer. Additional topics include energy-cascade OPV architectures and broadband, spectrally tunable photodetectors based on organic semiconductors.

  7. Exciton Emission from Bare and Alq3/Gold Coated GaN Nanorods

    NASA Astrophysics Data System (ADS)

    Mohammadi, Fatemesadat; Kuhnert, Gerd; Hommel, Detlef; Schmitzer, Heidrun; Wagner, Hans-Peter

    We study the excitonic and impurity related emission in bare and aluminum quinoline (Alq3)/gold coated wurtzite GaN nanorods by temperature-dependent time-integrated (TI) and time-resolved (TR) photoluminescence (PL). The GaN nanorods were grown by molecular beam epitaxy. Alq3 as well as Alq3/gold covered nanorods were synthesized by organic molecular beam deposition. In the near-band edge region a donor-bound-exciton (D0X) emission is observed at 3.473 eV. Another emission band at 3.275 eV reveals LO-phonon replica and is attributed to a donor-acceptor-pair (DAP) luminescence. TR PL traces at 20 K show a nearly biexponential decay for the D0X with lifetimes of approximately 180 and 800 ps for both bare and Alq3 coated nanorods. In GaN nanorods which were coated with an Alq3 film and subsequently with a 10 nm thick gold layer we observe a PL quenching of D0X and DAP band and the lifetimes of the D0X transition shorten. The quenching behaviour is partially attributed to the energy-transfer from free excitons and donor-bound-excitons to plasmon oscillations in the gold layer.

  8. Ultrafast exciton dynamics at molecular surfaces

    NASA Astrophysics Data System (ADS)

    Monahan, Nicholas R.

    Further improvements to device performance are necessary to make solar energy conversion a compelling alternative to fossil fuels. Singlet exciton fission and charge separation are two processes that can heavily influence the power conversion efficiency of a solar cell. During exciton fission one singlet excitation converts into two triplet excitons, potentially doubling the photocurrent generated by higher energy photons. There is significant discord over the singlet fission mechanism and of particular interest is whether the process involves a multiexciton intermediate state. I used time-resolved two-photon photoemission to investigate singlet fission in hexacene thin films, a model system with strong electronic coupling. My results indicate that a multiexciton state forms within 40 fs of photoexcitation and loses singlet character on a 280 fs timescale, creating two triplet excitons. This is concordant with the transient absorption spectra of hexacene single crystals and definitively proves that exciton fission in hexacene proceeds through a multiexciton state. This state is likely common to all strongly-coupled systems and my results suggest that a reassessment of the generally-accepted singlet fission mechanism is required. Charge separation is the process of splitting neutral excitons into carriers that occurs at donor-acceptor heterojunctions in organic solar cells. Although this process is essential for device functionality, there are few compelling explanations for why it is highly efficient in certain organic photovoltaic systems. To investigate the charge separation process, I used the model system of charge transfer excitons at hexacene surfaces and time-resolved two-photon photoemission. Charge transfer excitons with sufficient energy spontaneously delocalize, growing from about 14 nm to over 50 nm within 200 fs. Entropy drives this delocalization, as the density of states within the Coulomb potential increases significantly with energy. This charge

  9. Coherent Exciton Dynamics in Atomically Thin Semiconductors

    NASA Astrophysics Data System (ADS)

    Li, Xiaoqin (Elaine)

    The near band-edge optical response of an emerging class of semiconductors, known as the transitional metal dichalcogenides (TMDs), is dominated by tightly-bound excitons and charged excitons (i.e. trions). A fundamental property of these quasiparticles (excitons and trions) is quantum decoherence time, which reflects irreversible quantum dissipation arising from system (excitons and trions) and bath (vacuum and other quasiparticles) interactions and determines the timescale during which excitons can be coherently manipulated. Dephasing time is also equivalent to the intrinsic homogeneous linewidth of exciton resonance. In addition, excitons in TMDs are localized at the corners of the Brillouin zone and provide a convenient way to optical manipulate the valley degree of freedom, which may act as a useful information carrier analogous to electronic charge or spin. Direct measurement of valley coherence time is challenging because it corresponds to a non-radiative coherence between two degenerate states. Using ultrafast multi-dimensional optical spectroscopy, we investigate the intrinsic homogeneous linewidth of excitons, exciton valley coherence as well as coupling between excitons and trions. Our studies reveal coherent electronic dynamics on the order of ~100 fs in monolayer TMDs. We gratefully acknowledge financial support from NSF, AFOSR, and the Welch Foundation.

  10. Thermodynamic efficiency limit of excitonic solar cells

    SciTech Connect

    Giebink, Noel C.; Wiederrecht, Gary P.; Wasielewski, Michael R.; Forrest, Stephen R.

    2011-01-01

    Excitonic solar cells, comprised of materials such as organic semiconductors, inorganic colloidal quantum dots, and carbon nanotubes, are fundamentally different than crystalline, inorganic solar cells in that photogeneration of free charge occurs through intermediate, bound exciton states. Here, we show that the Second Law of Thermodynamics limits the maximum efficiency of excitonic solar cells below the maximum of 31% established by Shockley and Queisser [J. Appl. Phys. 32, 510 (1961)] for inorganic solar cells (whose exciton-binding energy is small). In the case of ideal heterojunction excitonic cells, the free energy for charge transfer at the interface, ΔG, places an additional constraint on the limiting efficiency due to a fundamental increase in the recombination rate, with typical -ΔG in the range 0.3 to 0.5 eV decreasing the maximum efficiency to 27% and 22%, respectively.

  11. Observations of exciton and carrier spin relaxation in Be doped p-type GaAs

    SciTech Connect

    Asaka, Naohiro; Harasawa, Ryo; Tackeuchi, Atsushi; Lu, Shulong; Dai, Pan

    2014-03-17

    We have investigated the exciton and carrier spin relaxation in Be-doped p-type GaAs. Time-resolved spin-dependent photoluminescence (PL) measurements revealed spin relaxation behaviors between 10 and 100 K. Two PL peaks were observed at 1.511 eV (peak 1) and 1.497 eV (peak 2) at 10 K, and are attributed to the recombination of excitons bound to neutral Be acceptors (peak 1) and the band-to-acceptor transition (peak 2). The spin relaxation times of both PL peaks were measured to be 1.3–3.1 ns at 10–100 K, and found to originate from common electron spin relaxation. The observed existence of a carrier density dependence of the spin relaxation time at 10–77 K indicates that the Bir-Aronov-Pikus process is the dominant spin relaxation mechanism.

  12. Influence of Effects of Self-Polarization and Exciton-Phonon Interactions on the Exciton Energy in Lead Iodide Nanofilms

    NASA Astrophysics Data System (ADS)

    Kramar, V. M.; Pugantseva, O. V.

    2014-08-01

    In the approximation of effective masses for electronic and phononic - dielectric continuum - systems, the influence of spatial bounding, self-polarization, and exciton-phonon interactions on the exciton state in a flat double nanoheterostructure (a nanofilm) - lead iodide in a polymer matrix -is theoretically investigated for the model of a single infinitely deep quantum well. It is demonstrated that the dominating factor determining the energy of the bottom of the ground exciton band and its binding energy is spatial bounding. The relationship between two other effects depends on the nanofilm thickness, namely, the influence of the self-polarization effect in ultrathin films significantly exceeds that of exciton-phonon interaction.

  13. Theory of core excitons

    SciTech Connect

    Dow, J. D.; Hjalmarson, H. P.; Sankey, O. F.; Allen, R. E.; Buettner, H.

    1980-01-01

    The observation of core excitons with binding energies much larger than those of the valence excitons in the same material has posed a long-standing theoretical problem. A proposed solution to this problem is presented, and Frenkel excitons and Wannier excitons are shown to coexist naturally in a single material. (GHT)

  14. Theory of Primary Photoexcitations in Donor-Acceptor Copolymers

    NASA Astrophysics Data System (ADS)

    Aryanpour, Karan; Dutta, Tirthankar; Huynh, Uyen N. V.; Vardeny, Zeev Valy; Mazumdar, Sumit

    2015-12-01

    We present a generic theory of primary photoexcitations in low band gap donor-acceptor conjugated copolymers. Because of the combined effects of strong electron correlations and broken symmetry, there is considerable mixing between a charge-transfer exciton and an energetically proximate triplet-triplet state with an overall spin singlet. The triplet-triplet state, optically forbidden in homopolymers, is allowed in donor-acceptor copolymers. For an intermediate difference in electron affinities of the donor and the acceptor, the triplet-triplet state can have a stronger oscillator strength than the charge-transfer exciton. We discuss the possibility of intramolecular singlet fission from the triplet-triplet state, and how such fission can be detected experimentally.

  15. Theory of Primary Photoexcitations in Donor-Acceptor Copolymers.

    PubMed

    Aryanpour, Karan; Dutta, Tirthankar; Huynh, Uyen N V; Vardeny, Zeev Valy; Mazumdar, Sumit

    2015-12-31

    We present a generic theory of primary photoexcitations in low band gap donor-acceptor conjugated copolymers. Because of the combined effects of strong electron correlations and broken symmetry, there is considerable mixing between a charge-transfer exciton and an energetically proximate triplet-triplet state with an overall spin singlet. The triplet-triplet state, optically forbidden in homopolymers, is allowed in donor-acceptor copolymers. For an intermediate difference in electron affinities of the donor and the acceptor, the triplet-triplet state can have a stronger oscillator strength than the charge-transfer exciton. We discuss the possibility of intramolecular singlet fission from the triplet-triplet state, and how such fission can be detected experimentally. PMID:26765027

  16. Exciton-dominant electroluminescence from a diode of monolayer MoS{sub 2}

    SciTech Connect

    Ye, Yu; Ye, Ziliang; Gharghi, Majid; Zhu, Hanyu; Wang, Yuan; Zhao, Mervin; Yin, Xiaobo; Zhang, Xiang

    2014-05-12

    In two-dimensional monolayer MoS{sub 2}, excitons dominate the absorption and emission properties. However, the low electroluminescent efficiency and signal-to-noise ratio limit our understanding of the excitonic behavior of electroluminescence. Here, we study the microscopic origin of the electroluminescence from a diode of monolayer MoS{sub 2} fabricated on a heavily p-type doped silicon substrate. Direct and bound-exciton related recombination processes are identified from the electroluminescence. At a high electron-hole pair injection rate, Auger recombination of the exciton-exciton annihilation of the bound exciton emission is observed at room temperature. Moreover, the efficient electrical injection demonstrated here allows for the observation of a higher energy exciton peak of 2.255 eV in the monolayer MoS{sub 2} diode, attributed to the excited exciton state of a direct-exciton transition.

  17. Microscopic theories of excitons and their dynamics

    NASA Astrophysics Data System (ADS)

    Berkelbach, Timothy C.

    This thesis describes the development and application of microscopically-defined theories of excitons in a wide range of semiconducting materials. In Part I, I consider the topic of singlet exciton fission, an organic photophysical process which generates two spin-triplet excitons from one photoexcited spin-singlet exciton. I construct a theoretical framework that couples a realistic treatment of the static electronic structure with finite-temperature quantum relaxation techniques. This framework is applied separately, but consistently, to the problems of singlet fission in pentacene dimers, crystalline pentacene, and crystalline hexacene. Through this program, I am able to rationalize observed behaviors and make non-trivial predictions, some of which have been confirmed by experiment. In Part II, I present theoretical developments on the properties of neutral excitons and charged excitons (trions) in atomically thin transition metal dichalcogenides. This work includes an examination of material trends in exciton binding energies via an effective mass approach. I also present an experimental and theoretical collaboration, which links the unconventional disposition of excitons in the Rydberg series to the peculiar screening properties of atomically thin materials. The light-matter coupling in these materials is examined within low-energy models and is shown to give rise to bright and dark exciton states, which can be qualitatively labeled in analogy with the hydrogen series. In Part III, I explore theories of relaxation dynamics in condensed phase environments, with a focus on methodology development. This work is aimed towards biological processes, including resonant energy transfer in chromophore complexes and electron transfer in donor-bridge-acceptor systems. Specifically, I present a collaborative development of a numerically efficient but highly accurate hybrid approach to reduced dynamics, which exploits a partitioning of environmental degrees of freedom into

  18. Fundamental processes of exciton scattering at organic solar-cell interfaces: One-dimensional model calculation

    NASA Astrophysics Data System (ADS)

    Masugata, Yoshimitsu; Iizuka, Hideyuki; Sato, Kosuke; Nakayama, Takashi

    2016-08-01

    Fundamental processes of exciton scattering at organic solar-cell interfaces were studied using a one-dimensional tight-binding model and by performing a time-evolution simulation of electron–hole pair wave packets. We found the fundamental features of exciton scattering: the scattering promotes not only the dissociation of excitons and the generation of interface-bound (charge-transferred) excitons but also the transmission and reflection of excitons depending on the electron and hole interface offsets. In particular, the dissociation increases in a certain region of an interface offset, while the transmission shows resonances with higher-energy bound-exciton and interface bound-exciton states. We also studied the effects of carrier-transfer and potential modulations at the interface and the scattering of charged excitons, and we found trap dissociations where one of the carriers is trapped around the interface after the dissociation.

  19. Temperature dependence of free excitons in GaN

    NASA Astrophysics Data System (ADS)

    Reynolds, D. C.; Hoelscher, J.; Litton, C. W.; Collins, T. C.

    2002-11-01

    The excitons involved in this study are the longitudinal and the Gamma5 and Gamma6 free excitons, as well as the donor bound exciton (D0),X. The temperature dependence of the energy positions of the Gamma5, Gamma6, and (D0),X excitons are well accounted for by the Varshni equation (Y. P. Varshni, Physica (Amsterdam) 34, 149 (1967)). In the same temperature range, the energy positions of the longitudinal excitons depart from the predictions of the Varshni equation. The separation between the longitudinal- and transverse-mode free excitons has been previously reported. One component of this separation is the polarizability, which has a temperature dependence. The longitudinal exciton therefore has a band-gap temperature dependence, predicted by the Varshni equation, as well as an additional dependence due to polarizability. This temperature dependence has been accounted for by the Varshni equation, plus an additional linear and a quadratic temperature dependent term.

  20. High efficiency organic multilayer photodetectors based on singlet exciton fission

    NASA Astrophysics Data System (ADS)

    Lee, J.; Jadhav, P.; Baldo, M. A.

    2009-07-01

    We employ an exciton fission process that converts one singlet exciton into two triplet excitons to increase the quantum efficiency of an organic multilayer photodetector beyond 100%. The photodetector incorporates ultrathin alternating donor-acceptor layers of pentacene and C60, respectively. By comparing the quantum efficiency after separate pentacene and C60 photoexcitation we find that singlet exciton fission in pentacene enhances the quantum efficiency by (45±7)%. In quantitative agreement with this result, we also observe that the photocurrent generated from pentacene excitons is decreased by (2.7±0.2)% under an applied magnetic field of H =0.4 T, while the C60 photocurrent is relatively unchanged.

  1. Associative Memory Acceptors.

    ERIC Educational Resources Information Center

    Card, Roger

    The properties of an associative memory are examined in this paper from the viewpoint of automata theory. A device called an associative memory acceptor is studied under real-time operation. The family "L" of languages accepted by real-time associative memory acceptors is shown to properly contain the family of languages accepted by one-tape,…

  2. Charge carrier dynamics in organic semiconductors and their donor-acceptor composites: Numerical modeling of time-resolved photocurrent

    NASA Astrophysics Data System (ADS)

    Johnson, Brian; Kendrick, Mark J.; Ostroverkhova, Oksana

    2013-09-01

    We present a model that describes nanosecond (ns) time-scale photocurrent dynamics in functionalized anthradithiophene (ADT) films and ADT-based donor-acceptor (D/A) composites. By fitting numerically simulated photocurrents to experimental data, we quantify contributions of multiple pathways of charge carrier photogeneration to the photocurrent, as well as extract parameters that characterize charge transport (CT) in organic films including charge carrier mobilities, trap densities, hole trap depth, and trapping and recombination rates. In pristine ADT films, simulations revealed two competing charge photogeneration pathways: fast, occurring on picosecond (ps) or sub-ps time scales with efficiencies below 10%, and slow, which proceeds at the time scale of tens of nanoseconds, with efficiencies of about 11%-12%, at the applied electric fields of 40-80 kV/cm. The relative contribution of these pathways to the photocurrent was electric field dependent, with the contribution of the fast process increasing with applied electric field. However, the total charge photogeneration efficiency was weakly electric field dependent exhibiting values of 14%-20% of the absorbed photons. The remaining 80%-86% of the photoexcitation did not contribute to charge carrier generation at these time scales. In ADT-based D/A composites with 2 wt.% acceptor concentration, an additional pathway of charge photogeneration that proceeds via CT exciton dissociation contributed to the total charge photogeneration. In the composite with the functionalized pentacene (Pn) acceptor, which exhibits strong exciplex emission from a tightly bound D/A CT exciton, the contribution of the CT state to charge generation was small, ˜8%-12% of the total number of photogenerated charge carriers, dependent on the electric field. In contrast, in the composite with PCBM acceptor, the CT state contributed about a half of all photogenerated charge carriers. In both D/A composites, the charge carrier mobilities were

  3. Acceptor and Excitation Density Dependence of the Ultrafast Polaron Absorption Signal in Donor-Acceptor Organic Solar Cell Blends.

    PubMed

    Zarrabi, Nasim; Burn, Paul L; Meredith, Paul; Shaw, Paul E

    2016-07-21

    Transient absorption spectroscopy on organic semiconductor blends for solar cells typically shows efficient charge generation within ∼100 fs, accounting for the majority of the charge carriers. In this Letter, we show using transient absorption spectroscopy on blends containing a broad range of acceptor content (0.01-50% by weight) that the rise of the polaron signal is dependent on the acceptor concentration. For low acceptor content (<10% by weight), the polaron signal rises gradually over ∼1 ps with most polarons generated after 200 fs, while for higher acceptor concentrations (>10%) most polarons are generated within 200 fs. The rise time in blends with low acceptor content was also found to be sensitive to the pump fluence, decreasing with increasing excitation density. These results indicate that the sub-100 fs rise of the polaron signal is a natural consequence of both the high acceptor concentrations in many donor-acceptor blends and the high excitation densities needed for transient absorption spectroscopy, which results in a short average distance between the exciton and the donor-acceptor interface. PMID:27355877

  4. Triplet diffusion in singlet exciton fission sensitized pentacene solar cells

    NASA Astrophysics Data System (ADS)

    Tabachnyk, Maxim; Ehrler, Bruno; Bayliss, Sam; Friend, Richard H.; Greenham, Neil C.

    2013-10-01

    Singlet fission sensitized photovoltaics have the potential to surpass the Shockley-Queisser limit for a single-junction structure. We investigate the dynamics of triplet excitons resulting from singlet fission in pentacene and their ionization at a C60 heterojunction. We model the generation and diffusion of excitons to predict the spectral response. We find the triplet diffusion length in polycrystalline pentacene to be 40 nm. Poly(3-hexylthiophene) between the electrode and pentacene works both to confine triplet excitons and also to transfer photogenerated singlet excitons into pentacene with 30% efficiency. The lower bound for the singlet fission quantum efficiency in pentacene is 180 ± 15%.

  5. Organic photosensitive cells having a reciprocal-carrier exciton blocking layer

    DOEpatents

    Rand, Barry P.; Forrest, Stephen R.; Thompson, Mark E.

    2007-06-12

    A photosensitive cell includes an anode and a cathode; a donor-type organic material and an acceptor-type organic material forming a donor-acceptor junction connected between the anode and the cathode; and an exciton blocking layer connected between the acceptor-type organic material of the donor-acceptor junction and the cathode, the blocking layer consisting essentially of a material that has a hole mobility of at least 10.sup.-7 cm.sup.2/V-sec or higher, where a HOMO of the blocking layer is higher than or equal to a HOMO of the acceptor-type material.

  6. Bias activated dielectric response of excitons and excitonic Mott transition in quantum confined lasers structures.

    NASA Astrophysics Data System (ADS)

    Bhunia, Amit; Bansal, Kanika; Datta, Shouvik; Alshammari, Marzook S.; Henini, Mohamed

    In contrast to the widely reported optical techniques, there are hardly any investigations on corresponding electrical signatures of condensed matter physics of excitonic phenomena. We studied small signal steady state capacitance response in III-V materials based multi quantum well (AlGaInP) and MBE grown quantum dot (InGaAs) laser diodes to identify signatures of excitonic presence. Conductance activation by forward bias was probed using frequency dependent differential capacitance response (fdC/df), which changes characteristically with the onset of light emission indicating the occurrence of negative activation energy. Our analysis shows that it is connected with a steady state population of exciton like bound states. Calculated average energy of this bound state matches well with the binding energy of weakly confined excitons in this type of structures. Further increase in charge injection decreases the differential capacitive response in AlGaInP based diodes, indicating a gradual Mott transition of excitonic states into electron hole plasma. This electrical description of excitonic Mott transition is fully supplemented by standard optical spectroscopic signatures of band gap renormalization and phase space filling effects.

  7. Valley-Polarized Interlayer Excitons in 2D Semiconductor Heterostructures

    NASA Astrophysics Data System (ADS)

    Rivera, Pasqual; Seyler, Kyle; Yu, Hongyi; Schaibley, John; Yan, Jiaqiang; Mandrus, David; Xu, Xiaodong

    Vertically stacked monolayers of MoSe2 and WSe2 feature a type-II band alignment causing the formation of interlayer excitons, where the Coulomb bound hole and electron reside in different layers. This species of exciton has lifetime many orders of magnitude longer than intralayer valley excitons, providing a unique and advantageous system for investigating valley exciton physics. Here, we optically pump the MoSe2-WSe2 heterostructure with circularly polarized light, creating interlayer valley excitons with gate-tunable spin-valley polarization lifetime up to 40 ns. This long valley lifetime enables the diffusion of the interlayer valley exciton gas to be visualized. Under increasing excitation power we observe the formation of a ring in the spatial distribution of the valley polarization, a manifestation of significant valley-selective exchange interactions at high exciton densities. The combination of long valley polarization and spatial diffusion makes the interlayer exciton in semiconductor heterostructures an exciting platform for studies of valley exciton physics.

  8. Alternansucrase acceptor products

    Technology Transfer Automated Retrieval System (TEKTRAN)

    The regioselectivity of alternansucrase (EC 2.4.1.140) differs from dextransucrase (EC 2.4.1.5) in ways that can be useful for the synthesis of novel oligosaccharide structures. For example, it has been recently shown that the major oligosaccharides produced when maltose is the acceptor include one...

  9. Optically programmable excitonic traps

    PubMed Central

    Alloing, Mathieu; Lemaître, Aristide; Galopin, Elisabeth; Dubin, François

    2013-01-01

    With atomic systems, optically programmed trapping potentials have led to remarkable progress in quantum optics and quantum information science. Programmable trapping potentials could have a similar impact on studies of semiconductor quasi-particles, particularly excitons. However, engineering such potentials inside a semiconductor heterostructure remains an outstanding challenge and optical techniques have not yet achieved a high degree of control. Here, we synthesize optically programmable trapping potentials for indirect excitons of bilayer heterostructures. Our approach relies on the injection and spatial patterning of charges trapped in a field-effect device. We thereby imprint in-situ and on-demand electrostatic traps into which we optically inject cold and dense ensembles of excitons. This technique creates new opportunities to improve state-of-the-art technologies for the study of collective quantum behavior of excitons and also for the functionalisation of emerging exciton-based opto-electronic circuits. PMID:23546532

  10. Exciton-polariton laser

    NASA Astrophysics Data System (ADS)

    Moskalenko, S. A.; Tiginyanu, I. M.

    2016-05-01

    We present a review of the investigations realized in the last decades of the phenomenon of the Bose-Einstein condensation (BEC) in the system of two-dimensional cavity polaritons in semiconductor nanostructures. The conditions at which the excitons interacting with cavity photons form new type of quasiparticles named as polaritons are described. Since polaritons can form in a microcavity a weakly interacting Bose gas, similarly to the exciton gas in semiconductors, the microcavity exciton-polariton BEC emerged in the last decades as a new direction of the exciton BEC in solids, promising for practical applications. The high interest in BEC of exciton-polaritons in semiconductor microcavities is related to the ultra-low threshold lasing which has been demonstrated, in particular, for an electrically injected polariton laser based on bulk GaN microcavity diode working at room temperature.

  11. Probing excitonic dark states in single-layer tungsten disulphide

    NASA Astrophysics Data System (ADS)

    Ye, Ziliang; Cao, Ting; O'Brien, Kevin; Zhu, Hanyu; Yin, Xiaobo; Wang, Yuan; Louie, Steven G.; Zhang, Xiang

    2014-09-01

    Transition metal dichalcogenide (TMDC) monolayers have recently emerged as an important class of two-dimensional semiconductors with potential for electronic and optoelectronic devices. Unlike semi-metallic graphene, layered TMDCs have a sizeable bandgap. More interestingly, when thinned down to a monolayer, TMDCs transform from indirect-bandgap to direct-bandgap semiconductors, exhibiting a number of intriguing optical phenomena such as valley-selective circular dichroism, doping-dependent charged excitons and strong photocurrent responses. However, the fundamental mechanism underlying such a strong light-matter interaction is still under intensive investigation. First-principles calculations have predicted a quasiparticle bandgap much larger than the measured optical gap, and an optical response dominated by excitonic effects. In particular, a recent study based on a GW plus Bethe-Salpeter equation (GW-BSE) approach, which employed many-body Green's-function methodology to address electron-electron and electron-hole interactions, theoretically predicted a diversity of strongly bound excitons. Here we report experimental evidence of a series of excitonic dark states in single-layer WS2 using two-photon excitation spectroscopy. In combination with GW-BSE theory, we prove that the excitons are of Wannier type, meaning that each exciton wavefunction extends over multiple unit cells, but with extraordinarily large binding energy (~0.7 electronvolts), leading to a quasiparticle bandgap of 2.7 electronvolts. These strongly bound exciton states are observed to be stable even at room temperature. We reveal an exciton series that deviates substantially from hydrogen models, with a novel energy dependence on the orbital angular momentum. These excitonic energy levels are experimentally found to be robust against environmental perturbations. The discovery of excitonic dark states and exceptionally large binding energy not only sheds light on the importance of many

  12. Theory of exciton transfer and diffusion in conjugated polymers

    SciTech Connect

    Barford, William; Tozer, Oliver Robert

    2014-10-28

    We describe a theory of Förster-type exciton transfer between conjugated polymers. The theory is built on three assumptions. First, we assume that the low-lying excited states of conjugated polymers are Frenkel excitons coupled to local normal modes, and described by the Frenkel-Holstein model. Second, we assume that the relevant parameter regime is ℏω < J, i.e., the adiabatic regime, and thus the Born-Oppenheimer factorization of the electronic and nuclear degrees of freedom is generally applicable. Finally, we assume that the Condon approximation is valid, i.e., the exciton-polaron wavefunction is essentially independent of the normal modes. The resulting expression for the exciton transfer rate has a familiar form, being a function of the exciton transfer integral and the effective Franck-Condon factors. The effective Franck-Condon factors are functions of the effective Huang-Rhys parameters, which are inversely proportional to the chromophore size. The Born-Oppenheimer expressions were checked against DMRG calculations, and are found to be within 10% of the exact value for a tiny fraction of the computational cost. This theory of exciton transfer is then applied to model exciton migration in conformationally disordered poly(p-phenylene vinylene). Key to this modeling is the assumption that the donor and acceptor chromophores are defined by local exciton ground states (LEGSs). Since LEGSs are readily determined by the exciton center-of-mass wavefunction, this theory provides a quantitative link between polymer conformation and exciton migration. Our Monte Carlo simulations indicate that the exciton diffusion length depends weakly on the conformation of the polymer, with the diffusion length increasing slightly as the chromophores became straighter and longer. This is largely a geometrical effect: longer and straighter chromophores extend over larger distances. The calculated diffusion lengths of ∼10 nm are in good agreement with experiment. The spectral

  13. Control of Radiative Exciton Recombination by Charge Transfer Induced Surface Dipoles in MoS2 and WS2 Monolayers.

    PubMed

    Hu, Peng; Ye, Jun; He, Xuexia; Du, Kezhao; Zhang, Keke K; Wang, Xingzhi; Xiong, Qihua; Liu, Zheng; Jiang, Hui; Kloc, Christian

    2016-01-01

    Due to the two dimensional confinement of electrons in a monolayer of 2D materials, the properties of monolayer can be controlled by electrical field formed on the monolayer surface. F4TCNQ was evaporated on MoS2 and WS2 monolayer forming dipoles between strong acceptor, F4TCNQ, and monolayers of MoS2 or WS2. The strong acceptor attracts electrons (charge transfer) and decreases the number of the ionized excitons. Free excitons undergo radiative recombination in both MoS2 and WS2. Moreover, the photoluminescence enhancement is stronger in WS2 where the exciton-phonon coupling is weaker. The theoretical model indicates that the surface dipole controls the radiative exciton recombination and enhances photoluminescence radiation. Deposition of F4TCNQ on the 2D monolayers enables a convenient control of the radiative exciton recombination and leads to the applications of these materials in lasers or LEDs. PMID:27053440

  14. Control of Radiative Exciton Recombination by Charge Transfer Induced Surface Dipoles in MoS2 and WS2 Monolayers

    NASA Astrophysics Data System (ADS)

    Hu, Peng; Ye, Jun; He, Xuexia; Du, Kezhao; Zhang, Keke K.; Wang, Xingzhi; Xiong, Qihua; Liu, Zheng; Jiang, Hui; Kloc, Christian

    2016-04-01

    Due to the two dimensional confinement of electrons in a monolayer of 2D materials, the properties of monolayer can be controlled by electrical field formed on the monolayer surface. F4TCNQ was evaporated on MoS2 and WS2 monolayer forming dipoles between strong acceptor, F4TCNQ, and monolayers of MoS2 or WS2. The strong acceptor attracts electrons (charge transfer) and decreases the number of the ionized excitons. Free excitons undergo radiative recombination in both MoS2 and WS2. Moreover, the photoluminescence enhancement is stronger in WS2 where the exciton-phonon coupling is weaker. The theoretical model indicates that the surface dipole controls the radiative exciton recombination and enhances photoluminescence radiation. Deposition of F4TCNQ on the 2D monolayers enables a convenient control of the radiative exciton recombination and leads to the applications of these materials in lasers or LEDs.

  15. Control of Radiative Exciton Recombination by Charge Transfer Induced Surface Dipoles in MoS2 and WS2 Monolayers

    PubMed Central

    Hu, Peng; Ye, Jun; He, Xuexia; Du, Kezhao; Zhang, Keke K.; Wang, Xingzhi; Xiong, Qihua; Liu, Zheng; Jiang, Hui; Kloc, Christian

    2016-01-01

    Due to the two dimensional confinement of electrons in a monolayer of 2D materials, the properties of monolayer can be controlled by electrical field formed on the monolayer surface. F4TCNQ was evaporated on MoS2 and WS2 monolayer forming dipoles between strong acceptor, F4TCNQ, and monolayers of MoS2 or WS2. The strong acceptor attracts electrons (charge transfer) and decreases the number of the ionized excitons. Free excitons undergo radiative recombination in both MoS2 and WS2. Moreover, the photoluminescence enhancement is stronger in WS2 where the exciton-phonon coupling is weaker. The theoretical model indicates that the surface dipole controls the radiative exciton recombination and enhances photoluminescence radiation. Deposition of F4TCNQ on the 2D monolayers enables a convenient control of the radiative exciton recombination and leads to the applications of these materials in lasers or LEDs. PMID:27053440

  16. Spin injection effects on exciton distributions in conjugated organic semiconductors

    NASA Astrophysics Data System (ADS)

    Yunus, Mohammad; Ruden, P. Paul; Smith, Darryl

    2008-03-01

    Conjugated organic semiconductors are under rapid development as the active material in organic light emitting diodes (OLEDs). Electrons and holes injected into the organic semiconductor form bound singlet or triplet excitons. Singlet excitons may recombine radiatively giving rise to light emission whereas triplet excitons do not recombine radiatively. Thus the quantum efficiency of OLEDs is limited by the fraction of singlet excitons, χS. In this work, we explore theoretically an approach to control χS through spin-polarized electron and hole injection from ferromagnetic contacts. Conventional ferromagnetic transition metals and half-metallic materials, such as LSMO, are considered as candidate electrode materials. Electron and hole transport in the organic semiconductor is treated through the conventional device equations with the formation of excitons described by a Langevin process. Once formed, the excitons may recombine or diffuse. Triplet excitons have a lower recombination probability and hence a longer diffusion length. The model calculations yield steady state spatial profiles for singlet and triplet exciton densities in the organic semiconductor.

  17. Storing excitons in transition-metal dichalcogenides using dark states

    NASA Astrophysics Data System (ADS)

    Gunlycke, Daniel; Tseng, Frank; Simsek, Ergun

    Monolayer transition-metal dichalcogenides exhibit strongly bound excitons confined to two dimensions. One challenge in exploiting these excitons is that they have a finite life time and collapse through electron-hole recombination. We propose that the exciton life time could be extended by transitioning the exciton population into dark states. The symmetry of these dark states require the electron and hole to be spatially separated, which not only causes these states to be optically inactive but also inhibits electron-hole recombination. Based on an atomistic model we call the Triangular Lattice Exciton (3ALE) model, we derive transition matrix elements and approximate selection rules showing that excitons could be transitioned into and out of dark states using a pulsed infrared laser. For illustration, we also present exciton population scenarios based on different recombination decay constants. Longer exciton lifetimes could make these materials candidates for applications in energy management and quantum information processing. This work was supported by the Office of Naval Research, directly and through the Naval Research Laboratory.

  18. Temperature Dependence of the Free Excitons in GaN

    NASA Astrophysics Data System (ADS)

    Reynolds, D. C.; Collins, T. C.

    2002-03-01

    We are presenting the temperature dependence of the photoluminescence (PL) of the free excitons and donor bound exciton ( D^circ ,X) in GaN. The excitons involved are the longitudinal, the Γ5 and the Γ6 excitons as well as D^circ ,X. The temperature dependence of the energy positions of Γ_5, Γ_6, and D^circ ,X are well mapped out using the Varshni equation in a temperature range of 0 -- 60 K. In this temperature range, the energy positions of the longitudinal excitons depart from the predictions of the Varshni equation used for the other excitons. If more than one columnar structural direction is present, then in PL, more than one longitudinal mode might be observed. The sample being investigated shows two longitudinal modes. The energy separation between the longitudinal and transverse modes has been reported by Hopfield and Thomas. One component in the separation is the polarizability, which has some temperature dependence. The longitudinal exciton then has a band gap temperature dependence, which is predicted by the Varshni equation and an additional temperature dependence due to the polarizability. We have used the Varshni equation plus a linear and quadratic temperature dependence term to map the energy positions of the longitudinal excitons.

  19. Nonradiative exciton energy transfer in hybrid organic-inorganic heterostructures

    NASA Astrophysics Data System (ADS)

    Chanyawadee, S.; Lagoudakis, P. G.; Harley, R. T.; Lidzey, D. G.; Henini, M.

    2008-05-01

    Nonradiative energy transfer from a GaAs quantum well to a thin overlayer of an infrared organic semiconductor dye is unambiguously demonstrated. The dynamics of exciton transfer are studied in the time domain by using pump-probe spectroscopy at the donor site and fluorescence spectroscopy at the acceptor site. The effect is observed as simultaneous increase in the population decay rate at the donor and of the rise time of optical emission at the acceptor sites. The hybrid configuration under investigation provides an alternative nonradiative, noncontact pumping route to electrical carrier injection that overcomes the losses imposed by the associated low carrier mobility of organic emitters.

  20. Trion and exciton dephasing measurements in modulation-doped quantum wells: A probe for trion and carrier localization

    NASA Astrophysics Data System (ADS)

    Brinkmann, D.; Kudrna, J.; Gilliot, P.; Hönerlage, B.; Arnoult, A.; Cibert, J.; Tatarenko, S.

    1999-08-01

    To investigate the properties of a two-dimensional carrier gas at intermediate densities, we perform picosecond transient four-wave mixing experiments on trions (charged excitons) and neutral excitons in modulation p-doped CdTe/Cd1-x-yMgxZnyTe quantum wells. The determination of trion and exciton dephasing rates reveals a localization of both trions and holes in potential fluctuations induced by the ionized remote acceptors. We demonstrate that trions can be efficiently used as a charged optical probe sensitive to electrostatic potential fluctuations which are imperceptible for neutral excitons.

  1. Organic photovoltaic cell incorporating electron conducting exciton blocking layers

    SciTech Connect

    Forrest, Stephen R.; Lassiter, Brian E.

    2014-08-26

    The present disclosure relates to photosensitive optoelectronic devices including a compound blocking layer located between an acceptor material and a cathode, the compound blocking layer including: at least one electron conducting material, and at least one wide-gap electron conducting exciton blocking layer. For example, 3,4,9,10 perylenetetracarboxylic bisbenzimidazole (PTCBI) and 1,4,5,8-napthalene-tetracarboxylic-dianhydride (NTCDA) function as electron conducting and exciton blocking layers when interposed between the acceptor layer and cathode. Both materials serve as efficient electron conductors, leading to a fill factor as high as 0.70. By using an NTCDA/PTCBI compound blocking layer structure increased power conversion efficiency is achieved, compared to an analogous device using a conventional blocking layers shown to conduct electrons via damage-induced midgap states.

  2. Photoreflectance investigation of exciton-acoustic phonon scattering in GaN grown by MOVPE

    NASA Astrophysics Data System (ADS)

    Bouzidi, M.; Soltani, S.; Halidou, I.; Chine, Z.; El Jani, B.

    2016-04-01

    In this paper, we report a systematic investigation of the near band edge (NBE) excitonic states in GaN using low temperature photoluminescence (PL) and photoreflectance (PR) measurements. For this purpose, GaN films of different thicknesses have been grown on silicon nitride (SiN) treated c-plane sapphire substrates by atmospheric pressure metalorganic vapor phase epitaxy (MOVPE). Low temperature PR spectra exhibit well-defined spectral features related to the A, B and C free excitons denoted by FXA FXB and FXC, respectively. In contrast, PL spectra are essentially dominated by the A free and donor bound excitons. By combining PR spectra and Hall measurements a strong correlation between residual electron concentration and exciton linewidths is observed. From the temperature dependence of the excitonic linewidths, the exciton-acoustic phonon coupling constant is determined for FXA, FXB and FXC. We show that this coupling constant is strongly related to the exciton kinetic energy and to the strain level.

  3. Exciton Diffusion in Conjugated Polymers: From Fundamental Understanding to Improvement in Photovoltaic Conversion Efficiency.

    PubMed

    Tamai, Yasunari; Ohkita, Hideo; Benten, Hiroaki; Ito, Shinzaburo

    2015-09-01

    Singlet exciton diffusion plays a central role in the photovoltaic conversion in organic photovoltaics (OPVs). Upon light absorption, singlet excitons are promptly generated in organic materials instead of charge carriers because the dielectric constant (εr) is small (∼3-4), which is in sharp contrast to inorganic and perovskite solar cells. In order to convert to charge carriers, excitons need to diffuse into an interface between electron donor and acceptor materials before deactivating to the ground state. Therefore, fundamental understanding of exciton diffusion dynamics is one of the most important issues to further improve OPVs. We highlight recent leading studies in this field and describe several approaches for efficient exciton harvesting at the interface in OPVs. PMID:26269208

  4. The excitonic insulator route through a dynamical phase transition induced by an optical pulse

    NASA Astrophysics Data System (ADS)

    Brazovskii, S.; Kirova, N.

    2016-03-01

    We consider a dynamical phase transition induced by a short optical pulse in a system prone to thermodynamical instability. We address the case of pumping to excitons whose density contributes directly to the order parameter. To describe both thermodynamic and dynamic effects on equal footing, we adopt a view of the excitonic insulator for the phase transition and suggest a formation of the Bose condensate for the pumped excitons. The work is motivated by experiments in donor-acceptor organic compounds with a neutral- ionic phase transition coupled to the spontaneous lattice dimerization and to charge transfer excitons. The double nature of the ensemble of excitons leads to an intricate time evolution, in particular, to macroscopic quantum oscillations from the interference between the Bose condensate of excitons and the ground state of the excitonic insulator. The coupling of excitons and the order parameter also leads to self-trapping of their wave function, akin to self-focusing in optics. The locally enhanced density of excitons can surpass a critical value to trigger the phase transformation, even if the mean density is below the required threshold. The system is stratified in domains that evolve through dynamical phase transitions and sequences of merging. The new circumstances in experiments and theory bring to life, once again, some remarkable inventions made by L.V. Keldysh.

  5. Effect of the Pauli exclusion principle on the singlet exciton yield in conjugated polymers

    NASA Astrophysics Data System (ADS)

    Thilagam, A.

    2016-03-01

    Optical devices fabricated using conjugated polymer systems give rise to singlet exciton yields which are high compared to the statistically predicted estimate of 25 % obtained using simple recombination schemes. In this study, we evaluate the singlet exciton yield in conjugated polymers systems by fitting to a model that incorporates the Pauli exclusion principle. The rate equations which describe the exciton dynamics include quantum dynamical components (both density and spin-dependent) which arise during the spin-allowed conversion of composite intra-molecular excitons into loosely bound charge-transfer (CT) electron-hole pairs. Accordingly, a crucial mechanism by which singlet excitons are increased at the expense of triplet excitons is incorporated in this work. Non-ideal triplet excitons which form at high densities, are rerouted via the Pauli exclusion mechanism to form loosely bound CT states which subsequently convert to singlet excitons. Our derived expression for the yield in singlet exciton incorporates the purity measure and provides a realistic description of the carrier dynamics at high exciton densities.

  6. Oxygen as Acceptor.

    PubMed

    Borisov, Vitaliy B; Verkhovsky, Michael I

    2015-01-01

    Like most bacteria, Escherichia coli has a flexible and branched respiratory chain that enables the prokaryote to live under a variety of environmental conditions, from highly aerobic to completely anaerobic. In general, the bacterial respiratory chain is composed of dehydrogenases, a quinone pool, and reductases. Substrate-specific dehydrogenases transfer reducing equivalents from various donor substrates (NADH, succinate, glycerophosphate, formate, hydrogen, pyruvate, and lactate) to a quinone pool (menaquinone, ubiquinone, and dimethylmenoquinone). Then electrons from reduced quinones (quinols) are transferred by terminal reductases to different electron acceptors. Under aerobic growth conditions, the terminal electron acceptor is molecular oxygen. A transfer of electrons from quinol to O₂ is served by two major oxidoreductases (oxidases), cytochrome bo₃ encoded by cyoABCDE and cytochrome bd encoded by cydABX. Terminal oxidases of aerobic respiratory chains of bacteria, which use O₂ as the final electron acceptor, can oxidize one of two alternative electron donors, either cytochrome c or quinol. This review compares the effects of different inhibitors on the respiratory activities of cytochrome bo₃ and cytochrome bd in E. coli. It also presents a discussion on the genetics and the prosthetic groups of cytochrome bo₃ and cytochrome bd. The E. coli membrane contains three types of quinones that all have an octaprenyl side chain (C₄₀). It has been proposed that the bo₃ oxidase can have two ubiquinone-binding sites with different affinities. "WHAT'S NEW" IN THE REVISED ARTICLE: The revised article comprises additional information about subunit composition of cytochrome bd and its role in bacterial resistance to nitrosative and oxidative stresses. Also, we present the novel data on the electrogenic function of appBCX-encoded cytochrome bd-II, a second bd-type oxidase that had been thought not to contribute to generation of a proton motive force in E

  7. Singlet exciton fission in solution

    NASA Astrophysics Data System (ADS)

    Walker, Brian J.; Musser, Andrew J.; Beljonne, David; Friend, Richard H.

    2013-12-01

    Singlet exciton fission, the spin-conserving process that produces two triplet excited states from one photoexcited singlet state, is a means to circumvent the Shockley-Queisser limit in single-junction solar cells. Although the process through which singlet fission occurs is not well characterized, some local order is thought to be necessary for intermolecular coupling. Here, we report a triplet yield of 200% and triplet formation rates approaching the diffusion limit in solutions of bis(triisopropylsilylethynyl (TIPS)) pentacene. We observe a transient bound excimer intermediate, formed by the collision of one photoexcited and one ground-state TIPS-pentacene molecule. The intermediate breaks up when the two triplets separate to each TIPS-pentacene molecule. This efficient system is a model for future singlet-fission materials and for disordered device components that produce cascades of excited states from sunlight.

  8. Singlet exciton fission in solution.

    PubMed

    Walker, Brian J; Musser, Andrew J; Beljonne, David; Friend, Richard H

    2013-12-01

    Singlet exciton fission, the spin-conserving process that produces two triplet excited states from one photoexcited singlet state, is a means to circumvent the Shockley-Queisser limit in single-junction solar cells. Although the process through which singlet fission occurs is not well characterized, some local order is thought to be necessary for intermolecular coupling. Here, we report a triplet yield of 200% and triplet formation rates approaching the diffusion limit in solutions of bis(triisopropylsilylethynyl (TIPS)) pentacene. We observe a transient bound excimer intermediate, formed by the collision of one photoexcited and one ground-state TIPS-pentacene molecule. The intermediate breaks up when the two triplets separate to each TIPS-pentacene molecule. This efficient system is a model for future singlet-fission materials and for disordered device components that produce cascades of excited states from sunlight. PMID:24256865

  9. High magnetic field studies of charged exciton localization in GaAs/AlxGa1-xAs quantum wells

    NASA Astrophysics Data System (ADS)

    Jadczak, J.; Bryja, L.; Ryczko, K.; Kubisa, M.; Wójs, A.; Potemski, M.; Liu, F.; Yakovlev, D. R.; Bayer, M.; Nicoll, C. A.; Farrer, I.; Ritchie, D. A.

    2014-09-01

    We report on low temperature, polarization resolved, high magnetic field (up to 23 T) photoluminescence experiments on high mobility asymmetric GaAs quantum wells. At high magnetic fields, we detect two strong emission lines of the neutral and positively charged excitons (X and X+) and a series of weaker lines of the excitons bound to ionized acceptors (AX-). From polarization energy splittings of these lines, we determine the hole Landé factors (gh) of different complexes. For X and X+, gh initially grows with magnetic field and then saturates at gh = 0.88 and 1.55, respectively; for AX-'s, gh begins from a high value (from 6 to 11 at zero field) and decreases with the field growth. This contrasting behavior is traced to the structure of valence band Landau levels, calculated numerically in the Luttinger model, beyond axial approximation. This points to the coexistence (in the same well) of mobile X and X+ with localized and interface-pressed AX- states.

  10. Donor-acceptor electron transport mediated by solitons.

    PubMed

    Brizhik, L S; Piette, B M A G; Zakrzewski, W J

    2014-11-01

    We study the long-range electron and energy transfer mediated by solitons in a quasi-one-dimensional molecular chain (conjugated polymer, alpha-helical macromolecule, etc.) weakly bound to a donor and an acceptor. We show that for certain sets of parameter values in such systems an electron, initially located at the donor molecule, can tunnel to the molecular chain, where it becomes self-trapped in a soliton state, and propagates to the opposite end of the chain practically without energy dissipation. Upon reaching the end, the electron can either bounce back and move in the opposite direction or, for suitable parameter values of the system, tunnel to the acceptor. We estimate the energy efficiency of the donor-acceptor electron transport depending on the parameter values. Our calculations show that the soliton mechanism works for the parameter values of polypeptide macromolecules and conjugated polymers. We also investigate the donor-acceptor electron transport in thermalized molecular chains. PMID:25493866

  11. Donor-acceptor electron transport mediated by solitons

    NASA Astrophysics Data System (ADS)

    Brizhik, L. S.; Piette, B. M. A. G.; Zakrzewski, W. J.

    2014-11-01

    We study the long-range electron and energy transfer mediated by solitons in a quasi-one-dimensional molecular chain (conjugated polymer, alpha-helical macromolecule, etc.) weakly bound to a donor and an acceptor. We show that for certain sets of parameter values in such systems an electron, initially located at the donor molecule, can tunnel to the molecular chain, where it becomes self-trapped in a soliton state, and propagates to the opposite end of the chain practically without energy dissipation. Upon reaching the end, the electron can either bounce back and move in the opposite direction or, for suitable parameter values of the system, tunnel to the acceptor. We estimate the energy efficiency of the donor-acceptor electron transport depending on the parameter values. Our calculations show that the soliton mechanism works for the parameter values of polypeptide macromolecules and conjugated polymers. We also investigate the donor-acceptor electron transport in thermalized molecular chains.

  12. Time-resolved photoluminescence study of excitons in hexagonal GaN layers grown on sapphire

    NASA Astrophysics Data System (ADS)

    Pau, S.; Liu, Z. X.; Kuhl, J.; Ringling, J.; Grahn, H. T.; Khan, M. A.; Sun, C. J.; Ambacher, O.; Stutzmann, M.

    1998-03-01

    We performed time-resolved and continuous wave photoluminescence on two samples of hexagonal GaN, one with free exciton emission and the other without. For the sample with free exciton emission, very different decay dynamics are observed between the front and backside emission. We find that the strain caused by the lattice mismatch between the sapphire substrate and the GaN film has a large influence on the population decay of the sample with free exciton emission and a minor influence on the decay properties of the sample dominated by bound exciton emission. A polariton picture is used to describe the observed behavior.

  13. Excitonic surface lattice resonances

    NASA Astrophysics Data System (ADS)

    Humphrey, A. D.; Gentile, M. J.; Barnes, W. L.

    2016-08-01

    Electromagnetic resonances are important in controlling light at the nanoscale. The most studied such resonance is the surface plasmon resonance that is associated with metallic nanostructures. Here we explore an alternative resonance, the surface exciton-polariton resonance, one based on excitonic molecular materials. Our study is based on analytical and numerical modelling. We show that periodic arrays of suitable molecular nanoparticles may support surface lattice resonances that arise as a result of coherent interactions between the particles. Our results demonstrate that excitonic molecular materials are an interesting alternative to metals for nanophotonics; they offer the prospect of both fabrication based on supramolecular chemistry and optical functionality arising from the way the properties of such materials may be controlled with light.

  14. Photoluminescence study of Be acceptors in GaInNAs epilayers

    NASA Astrophysics Data System (ADS)

    Tsai, Y.; Barman, B.; Scrace, T.; Petrou, A.; Fukuda, M.; Sellers, I. R.; Leroux, M.; Khalfioui, M. A.

    2014-03-01

    We have studied the photoluminescence (PL) spectra from MBE grown GaInNAs epilayers doped p-type with Beryllium acceptors. The measurements were carried out in the 5 K - 70 K temperature range and in magnetic fields (B) up to 7 tesla. The PL spectra contain two features at T = 5 K: The exciton at 1093 meV and a second broader feature at 1058 meV. The intensity of this feature decreases with increasing temperature and disappears completely by 70K while the excitonic feature persists. The emission at 1058meV is identified as the conduction band to Beryllium acceptor transition. If we take into account the binding energy of the exciton [3] we get a value of 23 meV for the Beryllium acceptor binding energy. The acceptor related transition was studied as a function of magnetic field; the energy of this transition has a linear dependence on B with a slope of 055 meV/T. Research supported by Amethyst Research In. through the State of Oklahoma, ONAP program.

  15. Interlayer excitons with tunable dispersion relation

    NASA Astrophysics Data System (ADS)

    Skinner, Brian

    2016-06-01

    Interlayer excitons, comprising an electron in one material bound by Coulomb attraction to a hole in an adjacent material, are composite bosons that can assume a variety of many-body phases. The phase diagram of the bosonic system is largely determined by the dispersion relation of the bosons, which itself arises as a combination of the dispersion relations of the electron and hole separately. Here I show that in situations where either the electron or the hole has a nonmonotonic, "Mexican hat-shaped" dispersion relation, the exciton dispersion relation can have a range of qualitatively different forms, each corresponding to a different many-body phase at low temperature. This diversity suggests a platform for continuously tuning between different quantum phases using an external field.

  16. Semiconducting monolayer materials as a tunable platform for excitonic solar cells.

    PubMed

    Bernardi, Marco; Palummo, Maurizia; Grossman, Jeffrey C

    2012-11-27

    The recent advent of two-dimensional monolayer materials with tunable optical properties and high carrier mobility offers renewed opportunities for efficient, ultrathin excitonic solar cells alternative to those based on conjugated polymer and small molecule donors. Using first-principles density functional theory and many-body calculations, we demonstrate that monolayers of hexagonal BN and graphene (CBN) combined with commonly used acceptors such as PCBM fullerene or semiconducting carbon nanotubes can provide excitonic solar cells with tunable absorber gap, donor-acceptor interface band alignment, and power conversion efficiency, as well as novel device architectures. For the case of CBN-PCBM devices, we predict power conversion efficiency limits in the 10-20% range depending on the CBN monolayer structure. Our results demonstrate the possibility of using monolayer materials in tunable, efficient, ultrathin solar cells in which unexplored exciton and carrier transport regimes are at play. PMID:23062107

  17. Dielectric screening of excitons and trions in single-layer MoS2.

    PubMed

    Lin, Yuxuan; Ling, Xi; Yu, Lili; Huang, Shengxi; Hsu, Allen L; Lee, Yi-Hsien; Kong, Jing; Dresselhaus, Mildred S; Palacios, Tomás

    2014-10-01

    Photoluminescence (PL) properties of single-layer MoS2 are indicated to have strong correlations with the surrounding dielectric environment. Blue shifts of up to 40 meV of exciton or trion PL peaks were observed as a function of the dielectric constant of the environment. These results can be explained by the dielectric screening effect of the Coulomb potential; based on this, a scaling relationship was developed with the extracted electronic band gap and exciton and trion binding energies in good agreement with theoretical estimations. It was also observed that the trion/exciton intensity ratio can be tuned by at least 1 order of magnitude with different dielectric environments. Our findings are helpful to better understand the tightly bound exciton properties in strongly quantum-confined systems and provide a simple approach to the selective and separate generation of excitons or trions with potential applications in excitonic interconnects and valleytronics. PMID:25216267

  18. Control of Exciton Photon Coupling in Nano-structures

    NASA Astrophysics Data System (ADS)

    Liu, Xiaoze

    In this thesis, we study the interaction of excitons with photons and plasmons and methods to control and enhance this interaction. This study is categorized in three parts: light-matter interaction in microcavity structures, direct dipole-dipole interactions, and plasmon-exciton interaction in metal-semiconductor systems. In the microcavity structures, the light-matter interactions become significant when the excitonic energy is in resonance with microcavity photons. New hybrid quantum states named polariton states will be formed if the strong coupling regime is achieved, where the interaction rate is faster than the average decay rate of the excitons and photons. Polaritons have been investigated in zinc oxide (ZnO) nanoparticles based microcavity at room temperature and stimulated emission of the polaritons has also been observed with a low optical pump threshold. Exictons in organic semiconductors (modeled as Frenkel excitons) are tightly bound to molecular sites, and differ considerably from loosely bound hydrogen atom-like inorganic excitons (modeled as Wannier-Mott excitons). This fundamental difference results in distinct optoelectronic properties. Not only strongly coupled to Wannier-Mott excitons in ZnO, the microcavity photons have also been observed to be simultaneously coupled to Frenkel excitons in 3,4,7,8-naphthalene tetracarboxylic dianhydride (NTCDA). The photons here act like a glue combining Wannier-Mott and Frenkel excitons into new hybrid polaritons taking the best from both constituents. Two-dimensional (2D) excitons in monolayer transition metal dichalcogenides (TMDs) have emerged as a new and fascinating type of Wannier-Mott-like excitons due to direct bandgap transition, huge oscillator strength and large binding energy. Monolayer molybdenum disulfide (MoS2) has been incorporated into the microcavity structure and 2D exciton-polaritons have been observed for the first time with directional emission in the strong coupling regime. Valley

  19. Free Carrier Generation in Organic Photovoltaic Bulk Heterojunctions of Conjugated Polymers with Molecular Acceptors: Planar versus Spherical Acceptors

    SciTech Connect

    Nardes, Alexandre M.; Ferguson, Andrew J.; Wolfer, Pascal; Gui, Kurt; Burn, Paul L.; Meredith, Paul; Kopidakis, Nikos

    2014-03-05

    We present a comparative study of the photophysical performance of the prototypical fullerene derivative PC61BM with a planar small-molecule acceptor in an organic photovoltaic device. The small-molecule planar acceptor is 2-[{7-(9,9-di-n-propyl-9H-fluoren-2-yl)benzo[c][1,2,5]thiadiazol-4-yl}methylene]malononitrile, termed K12. We discuss photoinduced free charge-carrier generation and transport in blends of PC61BM or K12 with poly(3-n-hexylthiophene) (P3HT), surveying literature results for P3HT:PC61BM and presenting new results on P3HT:K12. For both systems we also review previous work on film structure and correlate the structural and photophysical results. In both cases, a disordered mixed phase is formed between P3HT and the acceptor, although the photophysical properties of this mixed phase differ markedly for PC61BM and K12. In the case of PC61BM the mixed phase acts as a free carrier generation region that can efficiently shuttle carriers to the pure polymer and fullerene domains. As a result, the vast majority of excitons quenched in P3HT:PC61BM blends yield free carriers detected by the contactless time-resolved microwave conductivity (TRMC) method. In contrast, approximately 85 % of the excitons quenched in P3HT:K12 do not result in free carriers over the nanosecond timescale of the TRMC experiment. We attribute this to poor electron-transport properties in the mixed P3HT:K12 phase. Here, we propose that the observed differences can be traced to the respective shapes of PC61BM and K12: the three-dimensional nature of the fullerene cage facilitates coupling between PC61BM molecules irrespective of their relative orientation, whereas for K12 strong electronic coupling is only expected for molecules oriented with their π systems parallel to each other. Comparison between the eutectic compositions of the P3HT:PC61BM and P3HT:K12 shows that the former contains enough fullerene to form a percolation pathway for electrons, whereas the latter contains a sub

  20. Probing the origin of excitonic states in monolayer WSe2

    NASA Astrophysics Data System (ADS)

    Huang, Jiani; Hoang, Thang B.; Mikkelsen, Maiken H.

    2016-03-01

    Two-dimensional transition metal dichalcogenides (TMDCs) have spurred excitement for potential applications in optoelectronic and valleytronic devices; however, the origin of the dynamics of excitons, trions, and other localized states in these low dimensional materials is not well-understood. Here, we experimentally probed the dynamics of excitonic states in monolayer WSe2 by investigating the temperature and polarization dependent photoluminescence (PL) spectra. Four pronounced PL peaks were identified below a temperature of 60 K at near-resonant excitation and assigned to exciton, trion and localized states from excitation power dependence measurements. We find that the localized states vanish above 65 K, while exciton and trion emission peaks remain up to room temperature. This can be explained by a multi-level model developed for conventional semiconductors and applied to monolayer TMDCs for the first time here. From this model, we estimated a lower bound of the exciton binding energy of 198 meV for monolayer WSe2 and explained the vanishing of the localized states. Additionally, we observed a rapid decrease in the degree of circular polarization of the PL at increasing temperatures indicating a relatively strong electron-phonon coupling and impurity-related scattering. Our results reveal further insight into the excitonic states in monolayer WSe2 which is critical for future practical applications.

  1. Probing the origin of excitonic states in monolayer WSe2

    PubMed Central

    Huang, Jiani; Hoang, Thang B.; Mikkelsen, Maiken H.

    2016-01-01

    Two-dimensional transition metal dichalcogenides (TMDCs) have spurred excitement for potential applications in optoelectronic and valleytronic devices; however, the origin of the dynamics of excitons, trions, and other localized states in these low dimensional materials is not well-understood. Here, we experimentally probed the dynamics of excitonic states in monolayer WSe2 by investigating the temperature and polarization dependent photoluminescence (PL) spectra. Four pronounced PL peaks were identified below a temperature of 60 K at near-resonant excitation and assigned to exciton, trion and localized states from excitation power dependence measurements. We find that the localized states vanish above 65 K, while exciton and trion emission peaks remain up to room temperature. This can be explained by a multi-level model developed for conventional semiconductors and applied to monolayer TMDCs for the first time here. From this model, we estimated a lower bound of the exciton binding energy of 198 meV for monolayer WSe2 and explained the vanishing of the localized states. Additionally, we observed a rapid decrease in the degree of circular polarization of the PL at increasing temperatures indicating a relatively strong electron-phonon coupling and impurity-related scattering. Our results reveal further insight into the excitonic states in monolayer WSe2 which is critical for future practical applications. PMID:26940069

  2. Probing the origin of excitonic states in monolayer WSe2.

    PubMed

    Huang, Jiani; Hoang, Thang B; Mikkelsen, Maiken H

    2016-01-01

    Two-dimensional transition metal dichalcogenides (TMDCs) have spurred excitement for potential applications in optoelectronic and valleytronic devices; however, the origin of the dynamics of excitons, trions, and other localized states in these low dimensional materials is not well-understood. Here, we experimentally probed the dynamics of excitonic states in monolayer WSe2 by investigating the temperature and polarization dependent photoluminescence (PL) spectra. Four pronounced PL peaks were identified below a temperature of 60 K at near-resonant excitation and assigned to exciton, trion and localized states from excitation power dependence measurements. We find that the localized states vanish above 65 K, while exciton and trion emission peaks remain up to room temperature. This can be explained by a multi-level model developed for conventional semiconductors and applied to monolayer TMDCs for the first time here. From this model, we estimated a lower bound of the exciton binding energy of 198 meV for monolayer WSe2 and explained the vanishing of the localized states. Additionally, we observed a rapid decrease in the degree of circular polarization of the PL at increasing temperatures indicating a relatively strong electron-phonon coupling and impurity-related scattering. Our results reveal further insight into the excitonic states in monolayer WSe2 which is critical for future practical applications. PMID:26940069

  3. PHOTOVOLTAIC PROPERTIES OF AU-MEROCYANINE-TiO2 SANDWICH CELLS. II. PROPERTIES OF ILLUMINATED CELLS AND EFFECTS OF DOPING WITH ELECTRON ACCEPTORS

    SciTech Connect

    Skotheim, T.; Yang, J.-M.; Otvos, J.; Klein, M.P.

    1980-07-01

    Photocurrent generation in thin films of a merocyanine photosensitizing dye sandwiched between a TiO{sub 2} single crystal doped n type and an Au overlayer has been studied using photovoltaic techniques. A theoretical model was developed to explain the observed photovoltaic properties. The model assumes that the principal route for the formation of charge carriers is via singlet excitons diffusing to the merocyanine - TiO{sub 2} interface followed by dissociation of the excitons into electron-hole pairs, the electrons being injected into the TiO{sub 2} conduction band and the holes into the merocyanine. The model also incorporates field dependence of the quantum efficiency for charge generation. An exciton diffusion length of 79 {angstrom} was determined by analyzing the short circuit action spectra using the theoretical model developed. The low fill factor of 0.35 for these cells was attributed to the field dependence of the quantum efficiency and the high series resistance of the undoped merocyanine films. Doping the merocyanine films with iodine was found to increase both the dark conductivity and the steady state photoconductivity, the latter by as much as a factor of 5. This resulted in a quantum yield of 12% for a 500 {angstrom} thick film and an increase in the fill factor to 0.44 giving a monochromatic power conversion efficiency of 0.4% at 520 nm. The carrier generation in iodine doped films is shown to result from a bulk process, possibly involving collisions between singlet excitons and acceptor-hole complexes resulting in activation out of the bound states formed by the charge-transfer complex. The quenching of excitons in the immediate vicinity of the metal surface was studied by monitoring the photoconductive response of a 200 {angstrom} merocyanine film with varying thickness of perylene sandwiched between the metal and the merocyanine. Perylene was shown to be able to transport the photoexcited holes from the merocyanine to the Au electrode. The

  4. Photovoltaic properties of Au-merocyanine-TiO2 sandwich cells. II. Properties of illuminated cells and effects of doping with electron acceptors

    NASA Astrophysics Data System (ADS)

    Skotheim, T.; Yang, J.-M.; Otvos, J.; Klein, M. P.

    1982-12-01

    Photocurrent generation in thin films of a merocyanine photosensitizing dye sandwiched between a TiO2 single crystal doped n type and an Au overlayer has been studied using photovoltaic techniques. A theoretical model was developed to explain the observed photovoltaic properties. The model assumes that the principal route for the formation of charge carriers is via singlet excitons diffusing to the merocyanine-TiO2 interface followed by dissociation of the excitons into electron-hole pairs, the electrons being injected into the TiO2 conduction band and the holes into the merocyanine. The model also incorporates field dependence of the quantum efficiency for charge generation. An exciton diffusion length of 79 Å was determined by analyzing the short circuit action spectra using the theoretical model developed. The low fill factor of 0.35 for these cells was attributed to the field dependence of the quantum efficiency and the high series resistance of the undoped merocyanine films. Doping the merocyanine films with iodine was found to increase both the dark conductivity and the steady state photoconductivity, the latter by as much as a factor of 5. This resulted in a quantum yield of 12% for a 500 Å thick film and an increase in the fill factor to 0.44 giving a monochromatic power conversion efficiency of 0.4% at 520 nm. The carrier generation in iodine doped films is shown to result from a bulk process, possibly involving collisions between singlet excitons and acceptor-hole complexes resulting in activation out of the bound states formed by the charge-transfer complex. The quenching of excitons in the immediate vicinity of the metal surface was studied by monitoring the photoconductive response of a 200 Å merocyanine film with varying thickness of perylene sandwiched between the metal and the merocyanine. Perylene was shown to be able to transport the photoexcited holes from the merocyanine to the Au electrode. The quantum efficiency for photocarrier production

  5. Optical Study of Exciton Localization Phenomena in Semimagnetic Semiconductors and Their Multiple Quantum Wells.

    NASA Astrophysics Data System (ADS)

    Zhang, Xi-Cheng

    1986-12-01

    field accelerates the localized exciton recombination processes. The dynamics of the excitons in CdMnTe MQW samples suggests that quasi-2D bound magnetic polarons (BMP) exist. A discussion of this quasi-2D BMP and the influence of an external magnetic field on exciton lifetime and exciton dynamics in Cd(,1 -x)Mn(,x)Te MQW is included.

  6. Valley excitons in two-dimensional semiconductors

    DOE PAGESBeta

    Yu, Hongyi; Cui, Xiaodong; Xu, Xiaodong; Yao, Wang

    2014-12-30

    Monolayer group-VIB transition metal dichalcogenides have recently emerged as a new class of semiconductors in the two-dimensional limit. The attractive properties include: the visible range direct band gap ideal for exploring optoelectronic applications; the intriguing physics associated with spin and valley pseudospin of carriers which implies potentials for novel electronics based on these internal degrees of freedom; the exceptionally strong Coulomb interaction due to the two-dimensional geometry and the large effective masses. The physics of excitons, the bound states of electrons and holes, has been one of the most actively studied topics on these two-dimensional semiconductors, where the excitons exhibitmore » remarkably new features due to the strong Coulomb binding, the valley degeneracy of the band edges, and the valley dependent optical selection rules for interband transitions. Here we give a brief overview of the experimental and theoretical findings on excitons in two-dimensional transition metal dichalcogenides, with focus on the novel properties associated with their valley degrees of freedom.« less

  7. Valley excitons in two-dimensional semiconductors

    SciTech Connect

    Yu, Hongyi; Cui, Xiaodong; Xu, Xiaodong; Yao, Wang

    2014-12-30

    Monolayer group-VIB transition metal dichalcogenides have recently emerged as a new class of semiconductors in the two-dimensional limit. The attractive properties include: the visible range direct band gap ideal for exploring optoelectronic applications; the intriguing physics associated with spin and valley pseudospin of carriers which implies potentials for novel electronics based on these internal degrees of freedom; the exceptionally strong Coulomb interaction due to the two-dimensional geometry and the large effective masses. The physics of excitons, the bound states of electrons and holes, has been one of the most actively studied topics on these two-dimensional semiconductors, where the excitons exhibit remarkably new features due to the strong Coulomb binding, the valley degeneracy of the band edges, and the valley dependent optical selection rules for interband transitions. Here we give a brief overview of the experimental and theoretical findings on excitons in two-dimensional transition metal dichalcogenides, with focus on the novel properties associated with their valley degrees of freedom.

  8. Monolayer excitonic laser

    NASA Astrophysics Data System (ADS)

    Ye, Yu; Wong, Zi Jing; Lu, Xiufang; Ni, Xingjie; Zhu, Hanyu; Chen, Xianhui; Wang, Yuan; Zhang, Xiang

    2015-11-01

    Two-dimensional van der Waals materials have opened a new paradigm for fundamental physics exploration and device applications because of their emerging physical properties. Unlike gapless graphene, monolayer transition-metal dichalcogenides (TMDCs) are two-dimensional semiconductors that undergo an indirect-to-direct bandgap transition, creating new optical functionalities for next-generation ultra-compact photonics and optoelectronics. Although the enhancement of spontaneous emission has been reported on TMDC monolayers integrated with photonic crystals and distributed Bragg reflector microcavities, coherent light emission from a TMDC monolayer has not been demonstrated. Here, we report the realization of a two-dimensional excitonic laser by embedding monolayer WS2 in a microdisk resonator. Using a whispering gallery mode with a high quality factor and optical confinement, we observe bright excitonic lasing at visible wavelengths. This demonstration of a two-dimensional excitonic laser marks a major step towards two-dimensional on-chip optoelectronics for high-performance optical communication and computing applications.

  9. Evidence of Hybrid Excitons in Weakly Interacting Nanopeapods

    PubMed Central

    2013-01-01

    Nanopeapods, consisting of optically active π-conjugated molecules encapsulated inside of the cavity of carbon nanotubes, exhibit efficient photon emission in the visible spectral range. Combining optical experiments with ab initio theory, we show that the puzzling features observed in photoluminescence spectra are of excitonic nature. The subunits though being van der Waals bound are demonstrated to interact in the excited state, giving rise to the formation of hybrid excitons. We rationalize why this many-body effect makes such nanohybrids useful for optoelectronic devices. PMID:23991266

  10. Selectively Modulating Triplet Exciton Formation in Host Materials for Highly Efficient Blue Electrophosphorescence.

    PubMed

    Li, Huanhuan; Bi, Ran; Chen, Ting; Yuan, Kai; Chen, Runfeng; Tao, Ye; Zhang, Hongmei; Zheng, Chao; Huang, Wei

    2016-03-23

    The concept of limiting the triplet exciton formation to fundamentally alleviate triplet-involved quenching effects is introduced to construct host materials for highly efficient and stable blue phosphorescent organic light-emitting diodes (PhOLEDs). The low triplet exciton formation is realized by small triplet exciton formation fraction and rate with high binding energy and high reorganization energy of triplet exciton. Demonstrated in two analogue molecules in conventional donor-acceptor molecule structure for bipolar charge injection and transport with nearly the same frontier orbital energy levels and triplet excited energies, the new concept host material shows significantly suppressed triplet exciton formation in the host to avoid quenching effects, leading to much improved device efficiencies and stabilities. The low-voltage-driving blue PhOLED devices exhibit maximum efficiencies of 43.7 cd A(-1) for current efficiency, 32.7 lm W(-1) for power efficiency, and 20.7% for external quantum efficiency with low roll-off and remarkable relative quenching effect reduction ratio up to 41%. Our fundamental solution for preventing quenching effects of long-lived triplet excitons provides exciting opportunities for fabricating high-performance devices using the advanced host materials with intrinsically small triplet exciton formation cross section. PMID:26937742

  11. Intrinsic homogeneous linewidth and broadening mechanisms of excitons in monolayer transition metal dichalcogenides.

    PubMed

    Moody, Galan; Kavir Dass, Chandriker; Hao, Kai; Chen, Chang-Hsiao; Li, Lain-Jong; Singh, Akshay; Tran, Kha; Clark, Genevieve; Xu, Xiaodong; Berghäuser, Gunnar; Malic, Ermin; Knorr, Andreas; Li, Xiaoqin

    2015-01-01

    The band-edge optical response of transition metal dichalcogenides, an emerging class of atomically thin semiconductors, is dominated by tightly bound excitons localized at the corners of the Brillouin zone (valley excitons). A fundamental yet unknown property of valley excitons in these materials is the intrinsic homogeneous linewidth, which reflects irreversible quantum dissipation arising from system (exciton) and bath (vacuum and other quasiparticles) interactions and determines the timescale during which excitons can be coherently manipulated. Here we use optical two-dimensional Fourier transform spectroscopy to measure the exciton homogeneous linewidth in monolayer tungsten diselenide (WSe2). The homogeneous linewidth is found to be nearly two orders of magnitude narrower than the inhomogeneous width at low temperatures. We evaluate quantitatively the role of exciton-exciton and exciton-phonon interactions and population relaxation as linewidth broadening mechanisms. The key insights reported here—strong many-body effects and intrinsically rapid radiative recombination—are expected to be ubiquitous in atomically thin semiconductors. PMID:26382305

  12. Excitons in asymmetric quantum wells

    NASA Astrophysics Data System (ADS)

    Grigoryev, P. S.; Kurdyubov, A. S.; Kuznetsova, M. S.; Ignatiev, I. V.; Efimov, Yu. P.; Eliseev, S. A.; Petrov, V. V.; Lovtcius, V. A.; Shapochkin, P. Yu.

    2016-09-01

    Resonance dielectric response of excitons is studied for the high-quality InGaAs/GaAs heterostructures with wide asymmetric quantum wells (QWs). To highlight effects of the QW asymmetry, we have grown and studied several heterostructures with nominally square QWs as well as with triangle-like QWs. Several quantum confined exciton states are experimentally observed as narrow exciton resonances. A standard approach for the phenomenological analysis of the profiles is generalized by introducing different phase shifts for the light waves reflected from the QWs at different exciton resonances. Good agreement of the phenomenological fit to the experimentally observed exciton spectra for high-quality structures allowed us to reliably obtain parameters of the exciton resonances: the exciton transition energies, the radiative broadenings, and the phase shifts. A direct numerical solution of the Schrödinger equation for the heavy-hole excitons in asymmetric QWs is used for microscopic modeling of the exciton resonances. Remarkable agreement with the experiment is achieved when the effect of indium segregation is taken into account. The segregation results in a modification of the potential profile, in particular, in an asymmetry of the nominally square QWs.

  13. High magnetic field studies of charged exciton localization in GaAs/Al{sub x}Ga{sub 1−x}As quantum wells

    SciTech Connect

    Jadczak, J.; Bryja, L. Ryczko, K.; Kubisa, M.; Wójs, A.; Potemski, M.; Liu, F.; Yakovlev, D. R.; Bayer, M.; Nicoll, C. A.; Farrer, I.; Ritchie, D. A.

    2014-09-15

    We report on low temperature, polarization resolved, high magnetic field (up to 23 T) photoluminescence experiments on high mobility asymmetric GaAs quantum wells. At high magnetic fields, we detect two strong emission lines of the neutral and positively charged excitons (X and X{sup +}) and a series of weaker lines of the excitons bound to ionized acceptors (AX{sup −}). From polarization energy splittings of these lines, we determine the hole Landé factors (g{sub h}) of different complexes. For X and X{sup +}, g{sub h} initially grows with magnetic field and then saturates at g{sub h} = 0.88 and 1.55, respectively; for AX{sup −}'s, g{sub h} begins from a high value (from 6 to 11 at zero field) and decreases with the field growth. This contrasting behavior is traced to the structure of valence band Landau levels, calculated numerically in the Luttinger model, beyond axial approximation. This points to the coexistence (in the same well) of mobile X and X{sup +} with localized and interface-pressed AX{sup −} states.

  14. Efficient Exciton Diffusion and Resonance-Energy Transfer in Multilayered Organic Epitaxial Nanofibers

    PubMed Central

    2015-01-01

    Multilayered epitaxial nanofibers are exemplary model systems for the study of exciton dynamics and lasing in organic materials because of their well-defined morphology, high luminescence efficiencies, and color tunability. We use temperature-dependent continuous wave and picosecond photoluminescence (PL) spectroscopy to quantify exciton diffusion and resonance-energy transfer (RET) processes in multilayered nanofibers consisting of alternating layers of para-hexaphenyl (p6P) and α-sexithiophene (6T) serving as exciton donor and acceptor material, respectively. The high probability for RET processes is confirmed by quantum chemical calculations. The activation energy for exciton diffusion in p6P is determined to be as low as 19 meV, proving p6P epitaxial layers also as a very suitable donor material system. The small activation energy for exciton diffusion of the p6P donor material, the inferred high p6P-to-6T resonance-energy-transfer efficiency, and the observed weak PL temperature dependence of the 6T acceptor material together result in an exceptionally high optical emission performance of this all-organic material system, thus making it well suited, for example, for organic light-emitting devices. PMID:26191119

  15. Exciton absorption of entangled photons in semiconductor quantum wells

    NASA Astrophysics Data System (ADS)

    Rodriguez, Ferney; Guzman, David; Salazar, Luis; Quiroga, Luis; Condensed Matter Physics Group Team

    2013-03-01

    The dependence of the excitonic two-photon absorption on the quantum correlations (entanglement) of exciting biphotons by a semiconductor quantum well is studied. We show that entangled photon absorption can display very unusual features depending on space-time-polarization biphoton parameters and absorber density of states for both bound exciton states as well as for unbound electron-hole pairs. We report on the connection between biphoton entanglement, as quantified by the Schmidt number, and absorption by a semiconductor quantum well. Comparison between frequency-anti-correlated, unentangled and frequency-correlated biphoton absorption is addressed. We found that exciton oscillator strengths are highly increased when photons arrive almost simultaneously in an entangled state. Two-photon-absorption becomes a highly sensitive probe of photon quantum correlations when narrow semiconductor quantum wells are used as two-photon absorbers. Research funds from Facultad de Ciencias, Universidad de los Andes

  16. Singlet exciton fission in polycrystalline pentacene: from photophysics toward devices.

    PubMed

    Wilson, Mark W B; Rao, Akshay; Ehrler, Bruno; Friend, Richard H

    2013-06-18

    generated via singlet fission in pentacene can be dissociated at an interface with a suitable electron acceptor, such as fullerenes and infrared-absorbing inorganic semiconducting quantum dots. We highlight our recent reports of a pentacene/PbSe hybrid solar cell with a power conversion efficiency of 4.7% and of a pentacene/PbSe/amorphous silicon photovoltaic device. Although substantive challenges remain, both to better our understanding of the mechanism of singlet exciton fission and to optimize device performance, this realization of a solar cell where photocurrent is simultaneously contributed from a blue-absorbing fission-capable material and an infrared-absorbing conventional cell is an important step towards a dual-bandgap, single-junction, fission-enhanced photovoltaic device, which could one day surpass the Shockley-Queisser limit. PMID:23656886

  17. A photoluminescence study of excitonic grade CuInSe{sub 2} single crystals irradiated with 6 MeV electrons

    SciTech Connect

    Yakushev, M. V.; Mudryi, A. V.; Borodavchenko, O. M.; Volkov, V. A.; Martin, R. W.

    2015-10-21

    High-quality single crystals of CuInSe{sub 2} with near-stoichiometric elemental compositions were irradiated with 6 MeV electrons, at doses from 10{sup 15} to 3 × 10{sup 18 }cm{sup −2}, and studied using photoluminescence (PL) at temperatures from 4.2 to 300 K. Before irradiation, the photoluminescence spectra reveal a number of sharp and well resolved lines associated with free- and bound-excitons. The spectra also show broader bands relating to free-to-bound transitions and their phonon replicas in the lower energy region below 1.0 eV. The irradiation with 6 MeV electrons reduces the intensity of the free- and the majority of the bound-exciton peaks. Such a reduction can be seen for doses above 10{sup 16 }cm{sup −2}. The irradiation induces new PL lines at 1.0215 eV and 0.9909 eV and also enhances the intensity of the lines at 1.0325 and 1.0102 eV present in the photoluminescence spectra before the irradiation. Two broad bands at 0.902 and 0.972 eV, respectively, are tentatively associated with two acceptor-type defects: namely, interstitial selenium (Se{sub i}) and copper on indium site (Cu{sub In}). After irradiation, these become more intense suggesting an increase in the concentration of these defects due to irradiation.

  18. Revealing the dark side of a bright exciton-polariton condensate.

    PubMed

    Ménard, J-M; Poellmann, C; Porer, M; Leierseder, U; Galopin, E; Lemaître, A; Amo, A; Bloch, J; Huber, R

    2014-01-01

    Condensation of bosons causes spectacular phenomena such as superfluidity or superconductivity. Understanding the nature of the condensed particles is crucial for active control of such quantum phases. Fascinating possibilities emerge from condensates of light-matter-coupled excitations, such as exciton-polaritons, photons hybridized with hydrogen-like bound electron-hole pairs. So far, only the photon component has been resolved, while even the mere existence of excitons in the condensed regime has been challenged. Here we trace the matter component of polariton condensates by monitoring intra-excitonic terahertz transitions. We study how a reservoir of optically dark excitons forms and feeds the degenerate state. Unlike atomic gases, the atom-like transition in excitons is dramatically renormalized on macroscopic ground state population. Our results establish fundamental differences between polariton condensation and photon lasing and open possibilities for coherent control of condensates. PMID:25115964

  19. A simple model for exploring the role of quantum coherence and the environment in excitonic energy transfer.

    PubMed

    Manikandan, Sreenath K; Shaji, Anil

    2015-07-28

    We investigate the role of quantum coherence in modulating the energy transfer rate between two independent energy donors and a single acceptor participating in an excitonic energy transfer process. The energy transfer rate depends explicitly on the nature of the initial coherent superposition state of the two donors and we connect it to the observed absorption profile of the acceptor and the stimulated emission profile of the energy donors. We consider simple models with mesoscopic environments interacting with the donors and the acceptor and compare the expression we obtained for the energy transfer rate with the results of numerical integration. PMID:26123739

  20. Excitonic condensation in bilayer systems

    NASA Astrophysics Data System (ADS)

    Su, Jung-Jung

    Among the many examples of Bose condensation considered in physics, electron-hole-pair (exciton) condensation has maintained special interest because it has been difficult to realize experimentally, and because of controversy about condensate properties. In this thesis, we studied the various aspects of spontaneous symmetry broken state of exciton in bilayer using mean field theory. We calculated the photoluminescence of excitonic condensation created by laser. We developed a one-dimensional toy model of excitonic supercurrent using mean field theory plus non-equilibrium Green's function (NEGF) which give qualitatively consistent results with experiments. We proposed graphene bilayer as a novel system for excitonic condensation to occur and estimate it to exist even at temperature as high as room temperature.

  1. Energy Level Tuning of Non-Fullerene Acceptors in Organic Solar Cells.

    PubMed

    Cnops, Kjell; Zango, German; Genoe, Jan; Heremans, Paul; Martinez-Diaz, M Victoria; Torres, Tomas; Cheyns, David

    2015-07-22

    The use of non-fullerene acceptors in organic photovoltaic (OPV) devices could lead to enhanced efficiencies due to increased open-circuit voltage (VOC) and improved absorption of solar light. Here we systematically investigate planar heterojunction devices comprising peripherally substituted subphthalocyanines as acceptors and correlate the device performance with the heterojunction energetics. As a result of a balance between VOC and the photocurrent, tuning of the interface energy gap is necessary to optimize the power conversion efficiency in these devices. In addition, we explore the role of the charge transport layers in the device architecture. It is found that non-fullerene acceptors require adjusted buffer layers with aligned electron transport levels to enable efficient charge extraction, while the insertion of an exciton-blocking layer at the anode interface further boosts photocurrent generation. These adjustments result in a planar-heterojunction OPV device with an efficiency of 6.9% and a VOC above 1 V. PMID:26104833

  2. External quantum efficiency exceeding 100% in a singlet-exciton-fission-based solar cell

    NASA Astrophysics Data System (ADS)

    Baldo, Marc

    2013-03-01

    Singlet exciton fission can be used to split a molecular excited state in two. In solar cells, it promises to double the photocurrent from high energy photons, thereby breaking the single junction efficiency limit. We demonstrate organic solar cells that exploit singlet exciton fission in pentacene to generate more than one electron per incident photon in the visible spectrum. Using a fullerene acceptor, a poly(3-hexylthiophene) exciton confinement layer, and a conventional optical trapping scheme, the peak external quantum efficiency is (109 +/-1)% at λ = 670 nm for a 15-nm-thick pentacene film. The corresponding internal quantum efficiency is (160 +/-10)%. Independent confirmation of the high internal efficiency is obtained by analysis of the magnetic field effect on photocurrent, which determines that the triplet yield approaches 200% for pentacene films thicker than 5 nm. To our knowledge, this is the first solar cell to generate quantum efficiencies above 100% in the visible spectrum. Alternative multiple exciton generation approaches have been demonstrated previously in the ultraviolet, where there is relatively little sunlight. Singlet exciton fission differs from these other mechanisms because spin conservation disallows the usual dominant loss process: a thermal relaxation of the high-energy exciton into a single low-energy exciton. Consequently, pentacene is efficient in the visible spectrum at λ = 670 nm because only the collapse of the singlet exciton into twotriplets is spin-allowed. Supported as part of the Center for Excitonics, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Award Number DE-SC0001088.

  3. Molecular helices as electron acceptors in high-performance bulk heterojunction solar cells

    PubMed Central

    Zhong, Yu; Trinh, M. Tuan; Chen, Rongsheng; Purdum, Geoffrey E.; Khlyabich, Petr P.; Sezen, Melda; Oh, Seokjoon; Zhu, Haiming; Fowler, Brandon; Zhang, Boyuan; Wang, Wei; Nam, Chang-Yong; Sfeir, Matthew Y.; Black, Charles T.; Steigerwald, Michael L.; Loo, Yueh-Lin; Ng, Fay; Zhu, X.-Y.; Nuckolls, Colin

    2015-01-01

    Despite numerous organic semiconducting materials synthesized for organic photovoltaics in the past decade, fullerenes are widely used as electron acceptors in highly efficient bulk-heterojunction solar cells. None of the non-fullerene bulk heterojunction solar cells have achieved efficiencies as high as fullerene-based solar cells. Design principles for fullerene-free acceptors remain unclear in the field. Here we report examples of helical molecular semiconductors as electron acceptors that are on par with fullerene derivatives in efficient solar cells. We achieved an 8.3% power conversion efficiency in a solar cell, which is a record high for non-fullerene bulk heterojunctions. Femtosecond transient absorption spectroscopy revealed both electron and hole transfer processes at the donor−acceptor interfaces. Atomic force microscopy reveals a mesh-like network of acceptors with pores that are tens of nanometres in diameter for efficient exciton separation and charge transport. This study describes a new motif for designing highly efficient acceptors for organic solar cells. PMID:26382113

  4. Molecular helices as electron acceptors in high-performance bulk heterojunction solar cells

    DOE PAGESBeta

    Yu M. Zhong; Nam, Chang -Yong; Trinh, M. Tuan; Chen, Rongsheng; Purdum, Geoffrey E.; Khlyabich, Petr P.; Sezen, Melda; Oh, Seokjoon; Zhu, Haiming; Fowler, Brandon; et al

    2015-09-18

    Despite numerous organic semiconducting materials synthesized for organic photovoltaics in the past decade, fullerenes are widely used as electron acceptors in highly efficient bulk-heterojunction solar cells. None of the non-fullerene bulk heterojunction solar cells have achieved efficiencies as high as fullerene-based solar cells. Design principles for fullerene-free acceptors remain unclear in the field. Here we report examples of helical molecular semiconductors as electron acceptors that are on par with fullerene derivatives in efficient solar cells. We achieved an 8.3% power conversion efficiency in a solar cell, which is a record high for non-fullerene bulk heterojunctions. Femtosecond transient absorption spectroscopy revealedmore » both electron and hole transfer processes at the donor–acceptor interfaces. Atomic force microscopy reveals a mesh-like network of acceptors with pores that are tens of nanometres in diameter for efficient exciton separation and charge transport. As a result, this study describes a new motif for designing highly efficient acceptors for organic solar cells.« less

  5. Molecular helices as electron acceptors in high-performance bulk heterojunction solar cells.

    PubMed

    Zhong, Yu; Trinh, M Tuan; Chen, Rongsheng; Purdum, Geoffrey E; Khlyabich, Petr P; Sezen, Melda; Oh, Seokjoon; Zhu, Haiming; Fowler, Brandon; Zhang, Boyuan; Wang, Wei; Nam, Chang-Yong; Sfeir, Matthew Y; Black, Charles T; Steigerwald, Michael L; Loo, Yueh-Lin; Ng, Fay; Zhu, X-Y; Nuckolls, Colin

    2015-01-01

    Despite numerous organic semiconducting materials synthesized for organic photovoltaics in the past decade, fullerenes are widely used as electron acceptors in highly efficient bulk-heterojunction solar cells. None of the non-fullerene bulk heterojunction solar cells have achieved efficiencies as high as fullerene-based solar cells. Design principles for fullerene-free acceptors remain unclear in the field. Here we report examples of helical molecular semiconductors as electron acceptors that are on par with fullerene derivatives in efficient solar cells. We achieved an 8.3% power conversion efficiency in a solar cell, which is a record high for non-fullerene bulk heterojunctions. Femtosecond transient absorption spectroscopy revealed both electron and hole transfer processes at the donor-acceptor interfaces. Atomic force microscopy reveals a mesh-like network of acceptors with pores that are tens of nanometres in diameter for efficient exciton separation and charge transport. This study describes a new motif for designing highly efficient acceptors for organic solar cells. PMID:26382113

  6. Molecular helices as electron acceptors in high-performance bulk heterojunction solar cells

    SciTech Connect

    Yu M. Zhong; Nam, Chang -Yong; Trinh, M. Tuan; Chen, Rongsheng; Purdum, Geoffrey E.; Khlyabich, Petr P.; Sezen, Melda; Oh, Seokjoon; Zhu, Haiming; Fowler, Brandon; Zhang, Boyuan; Wang, Wei; Sfeir, Matthew Y.; Black, Charles T.; Steigerwald, Michael L.; Loo, Yueh -Lin; Ng, Fay; Zhu, X. -Y.; Nuckolls, Colin

    2015-09-18

    Despite numerous organic semiconducting materials synthesized for organic photovoltaics in the past decade, fullerenes are widely used as electron acceptors in highly efficient bulk-heterojunction solar cells. None of the non-fullerene bulk heterojunction solar cells have achieved efficiencies as high as fullerene-based solar cells. Design principles for fullerene-free acceptors remain unclear in the field. Here we report examples of helical molecular semiconductors as electron acceptors that are on par with fullerene derivatives in efficient solar cells. We achieved an 8.3% power conversion efficiency in a solar cell, which is a record high for non-fullerene bulk heterojunctions. Femtosecond transient absorption spectroscopy revealed both electron and hole transfer processes at the donor–acceptor interfaces. Atomic force microscopy reveals a mesh-like network of acceptors with pores that are tens of nanometres in diameter for efficient exciton separation and charge transport. As a result, this study describes a new motif for designing highly efficient acceptors for organic solar cells.

  7. Exciton transport, charge extraction, and loss mechanisms in organic photovoltaics

    NASA Astrophysics Data System (ADS)

    Scully, Shawn Ryan

    the primary losses that puts stringent requirements on the charge carrier mobilities in these cells is the recombination losses due to space charge build up at the heterojunction. Because electrons are confined to the acceptor and holes to the donor, net charge density always exists even when mobilities are matched, in contrast to bulk heterojunctions wherein matched mobilities lead to zero net charge. This net charge creates an electric field which opposes the built-in field and limits the current that can be carried away from this heterojunction. Using simulations we show that for relevant current densities charge carrier mobilities must be higher than 10-4 cm2/V.s to avoid significant losses due to space charge formation. In the last part of this work, we will focus on the second class of architectures in which exciton harvesting is efficient. We will present a systematic analysis of one of the leading polymer:fullerene bulk heterojunction cells to show that losses in this architecture are due to charge recombination. Using optical measurements and simulations, exciton harvesting measurements, and device characteristics we will show that the dominant loss is likely due to field-dependent geminate recombination of the electron and hole pair created immediately following exciton dissociation. No losses in this system are seen due to bimolecular recombination or space charge which provides information on charge-carrier mobility targets necessary for the future design of high efficiency organic photovoltaics.

  8. Impact of Molecular Organization on Exciton Diffusion in Photosensitive Single-Crystal Halogenated Perylenediimides Charge Transfer Interfaces.

    PubMed

    Pinto, Rui M; Gouveia, Wilson; Maçôas, Ermelinda M S; Santos, Isabel C; Raja, Sebastian; Baleizão, Carlos; Alves, Helena

    2015-12-23

    The efficiency of organic photodetectors and optoelectronic devices is strongly limited by exciton diffusion, in particular for acceptor materials. Although mechanisms for exciton diffusion are well established, their correlation to molecular organization in real systems has received far less attention. In this report, organic single-crystals interfaces were probed with wavelength-dependent photocurrent spectroscopy and their crystal structure resolved using X-ray diffraction. All systems present a dynamic photoresponse, faster than 500 ms, up to 650 nm. A relationship between molecular organization and favorable exciton diffusion in substituted butyl-perylenediimides (PDIB) is established. This is demonstrated by a set of PDIBs with different intra- and interstack distances and short contacts and their impact on photoresponse. Given the short packing distances between PDIs cores along the same stacking direction (3.4-3.7 Å), and across parallel stacks (2.5 Å), singlet exciton in these PDIBs can follow both Förster and Dexter exciton diffusion, with the Dexter-type mechanism assuming special relevance for interstack exciton diffusion. Yet, the response is maximized in substituted PDIBs, where a 2D percolation network is formed through strong interstack contacts, allowing for PDIBs primary excitons to reach with great efficiency the splitting interface with crystalline rubrene. The importance of short contacts and molecular distances, which is often overlooked as a parameter to consider and optimize when choosing materials for excitonic devices, is emphasized. PMID:26599347

  9. Quantum confined acceptors and donors in InSe nanosheets

    SciTech Connect

    Mudd, G. W.; Patanè, A. Makarovsky, O.; Eaves, L.; Kudrynskyi, Z. R.; Kovalyuk, Z. D.; Fay, M. W.; Zólyomi, V.; Falko, V.

    2014-12-01

    We report on the radiative recombination of photo-excited carriers bound at native donors and acceptors in exfoliated nanoflakes of nominally undoped rhombohedral γ-polytype InSe. The binding energies of these states are found to increase with the decrease in flake thickness, L. We model their dependence on L using a two-dimensional hydrogenic model for impurities and show that they are strongly sensitive to the position of the impurities within the nanolayer.

  10. A study of polaritonic transparency in couplers made from excitonic materials

    SciTech Connect

    Singh, Mahi R.; Racknor, Chris

    2015-03-14

    We have studied light matter interaction in quantum dot and exciton-polaritonic coupler hybrid systems. The coupler is made by embedding two slabs of an excitonic material (CdS) into a host excitonic material (ZnO). An ensemble of non-interacting quantum dots is doped in the coupler. The bound exciton polariton states are calculated in the coupler using the transfer matrix method in the presence of the coupling between the external light (photons) and excitons. These bound exciton-polaritons interact with the excitons present in the quantum dots and the coupler is acting as a reservoir. The Schrödinger equation method has been used to calculate the absorption coefficient in quantum dots. It is found that when the distance between two slabs (CdS) is greater than decay length of evanescent waves the absorption spectrum has two peaks and one minimum. The minimum corresponds to a transparent state in the system. However, when the distance between the slabs is smaller than the decay length of evanescent waves, the absorption spectra has three peaks and two transparent states. In other words, one transparent state can be switched to two transparent states when the distance between the two layers is modified. This could be achieved by applying stress and strain fields. It is also found that transparent states can be switched on and off by applying an external control laser field.

  11. Stimulated Terahertz Emission from Intra-Excitonic Transitions inCu2O

    SciTech Connect

    Huber, Rupert; Schmid, Ben A.; Shen, Y. Ron; Chemla, Daniel S.; Kaindl, Robert A.

    2005-06-16

    We report the first observation of stimulated emission of terahertz radiation from internal transitions of excitons. The far-infrared electromagnetic response of Cu{sub 2}O is monitored via broadband terahertz pulses after ultrafast resonant excitation of three-dimensional 3p excitons. Stimulated emission from the 3p to the energetically lower 2s bound level occurs at a photon energy of 6.6 meV, with a cross section of {approx} 10{sup 14} cm{sup 2}. Simultaneous excitation of both exciton levels, in turn, drives quantum beats which lead to efficient terahertz emission sharply peaked at the difference frequency.

  12. Slow light enhanced singlet exciton fission solar cells with a 126% yield of electrons per photon

    NASA Astrophysics Data System (ADS)

    Thompson, Nicholas J.; Congreve, Daniel N.; Goldberg, David; Menon, Vinod M.; Baldo, Marc A.

    2013-12-01

    Singlet exciton fission generates two triplet excitons per absorbed photon. It promises to increase the power extracted from sunlight without increasing the number of photovoltaic junctions in a solar cell. We demonstrate solar cells with an external quantum efficiency of 126% by enhancing absorption in thin films of the singlet exciton fission material pentacene. The device structure exploits the long photon dwell time at the band edge of a distributed Bragg reflector to achieve enhancement over a broad range of angles. Measuring the reflected light from the solar cell establishes a lower bound of 137% for the internal quantum efficiency.

  13. Slow light enhanced singlet exciton fission solar cells with a 126% yield of electrons per photon

    SciTech Connect

    Thompson, Nicholas J.; Congreve, Daniel N.; Baldo, Marc A. E-mail: baldo@mit.edu; Goldberg, David; Menon, Vinod M. E-mail: baldo@mit.edu

    2013-12-23

    Singlet exciton fission generates two triplet excitons per absorbed photon. It promises to increase the power extracted from sunlight without increasing the number of photovoltaic junctions in a solar cell. We demonstrate solar cells with an external quantum efficiency of 126% by enhancing absorption in thin films of the singlet exciton fission material pentacene. The device structure exploits the long photon dwell time at the band edge of a distributed Bragg reflector to achieve enhancement over a broad range of angles. Measuring the reflected light from the solar cell establishes a lower bound of 137% for the internal quantum efficiency.

  14. Acceptors in ZnO

    SciTech Connect

    McCluskey, Matthew D. Corolewski, Caleb D.; Lv, Jinpeng; Tarun, Marianne C.; Teklemichael, Samuel T.; Walter, Eric D.; Norton, M. Grant; Harrison, Kale W.; Ha, Su

    2015-03-21

    Zinc oxide (ZnO) has potential for a range of applications in the area of optoelectronics. The quest for p-type ZnO has focused much attention on acceptors. In this paper, Cu, N, and Li acceptor impurities are discussed. Experimental evidence indicates these point defects have acceptor levels 3.2, 1.4, and 0.8 eV above the valence-band maximum, respectively. The levels are deep because the ZnO valence band is quite low compared to conventional, non-oxide semiconductors. Using MoO{sub 2} contacts, the electrical resistivity of ZnO:Li was measured and showed behavior consistent with bulk hole conduction for temperatures above 400 K. A photoluminescence peak in ZnO nanocrystals is attributed to an acceptor, which may involve a Zn vacancy. High field (W-band) electron paramagnetic resonance measurements on the nanocrystals revealed an axial center with g{sub ⊥} = 2.0015 and g{sub //} = 2.0056, along with an isotropic center at g = 2.0035.

  15. Acceptors in ZnO

    SciTech Connect

    Mccluskey, Matthew D.; Corolewski, Caleb; Lv, Jinpeng; Tarun, Marianne C.; Teklemichael, Samuel T.; Walter, Eric D.; Norton, M. G.; Harrison, Kale W.; Ha, Su Y.

    2015-03-21

    Zinc oxide (ZnO) has potential for a range of applications in the area of optoelectronics. The quest for p-type ZnO has focused much attention on acceptors. In this paper, Cu, N, and Li acceptor impurities are discussed. Experimental evidence shows that these point defects have acceptor levels 3.2, 1.5, and 0.8 eV above the valence-band maximum, respectively. The levels are deep because the ZnO valence band is quite low compared to conventional, non-oxide semiconductors. Using MoO2 contacts, the electrical resistivity of ZnO:Li was measured and showed behavior consistent with bulk hole conduction for temperatures above 400 K. A photoluminescence peak in ZnO nanocrystals has been attributed to an acceptor, which may involve a zinc vacancy. High field (W-band) electron paramagnetic resonance measurements on the nanocrystals revealed an axial center with g = 2.0033 and g = 2.0075, along with an isotropic center at g = 2.0053.

  16. Exciton Transfer and Emergent Excitonic States in Oppositely-Charged Conjugated Polyelectrolyte Complexes.

    PubMed

    Hollingsworth, William R; Segura, Carmen; Balderrama, Jonathan; Lopez, Nathaniel; Schleissner, Pamela; Ayzner, Alexander L

    2016-08-11

    Photosynthetic organisms have mastered the use of "soft" macromolecular assemblies for light absorption and concentration of electronic excitation energy. Nature's design centers on an optically inactive protein-based backbone that acts as a host matrix for an array of light-harvesting pigment molecules. The pigments are organized in space such that excited states can migrate between molecules, ultimately delivering the energy to the reaction center. Here we report our investigation of an artificial light-harvesting energy transfer antenna based on complexes of oppositely charged conjugated polyelectrolytes (CPEs). The conjugated backbone and the charged side chains of the CPE lead to an architecture that simultaneously functions as a structural scaffold and an electronic energy "highway". We find that the process of ionic complex formation leads to a remarkable change in the excitonic wavefunction of the energy acceptor, which manifests in a dramatic increase in the fluorescence quantum yield. We argue that the extended backbone of the donor CPE effectively templates a planarized acceptor polymer, leading to excited states that are highly delocalized along the polymer backbone. PMID:27428604

  17. Momentum dependence of the excitons in pentacene

    SciTech Connect

    Roth, Friedrich; Schuster, Roman; Koenig, Andreas; Knupfer, Martin; Berger, Helmuth

    2012-05-28

    We have carried out electron energy-loss investigations of the lowest singlet excitons in pentacene at 20 K. Our studies allow to determine the full exciton band structure in the a*, b* reciprocal lattice plane. The lowest singlet exciton can move coherently within this plane, and the resulting exciton dispersion is highly anisotropic. The analysis of the energetically following (satellite) features indicates a strong admixture of charge transfer excitations to the exciton wave function.

  18. External quantum efficiency above 100% in a singlet-exciton-fission-based organic photovoltaic cell.

    PubMed

    Congreve, Daniel N; Lee, Jiye; Thompson, Nicholas J; Hontz, Eric; Yost, Shane R; Reusswig, Philip D; Bahlke, Matthias E; Reineke, Sebastian; Van Voorhis, Troy; Baldo, Marc A

    2013-04-19

    Singlet exciton fission transforms a molecular singlet excited state into two triplet states, each with half the energy of the original singlet. In solar cells, it could potentially double the photocurrent from high-energy photons. We demonstrate organic solar cells that exploit singlet exciton fission in pentacene to generate more than one electron per incident photon in a portion of the visible spectrum. Using a fullerene acceptor, a poly(3-hexylthiophene) exciton confinement layer, and a conventional optical trapping scheme, we show a peak external quantum efficiency of (109 ± 1)% at wavelength λ = 670 nanometers for a 15-nanometer-thick pentacene film. The corresponding internal quantum efficiency is (160 ± 10)%. Analysis of the magnetic field effect on photocurrent suggests that the triplet yield approaches 200% for pentacene films thicker than 5 nanometers. PMID:23599489

  19. Photophysics and charge transfer in donor-acceptor triblock copolymer photovoltaic materials

    NASA Astrophysics Data System (ADS)

    Schwarz, Kyra N.; Jones, David J.; Smith, Trevor A.; Ghiggino, Kenneth P.

    2014-10-01

    Efficient conversion of solar energy to electricity in low-cost organic photovoltaic (OPV) devices requires the complex interplay between multiple processes and components over various length and time scales. Optimizing device morphology to ensure efficient exciton diffusion and charge transport as well as ensuring efficient charge photogeneration is necessary to achieve optimum performance in new materials. The conjugated polymer electron donor PFM (poly(9,9-diocetyluorene-co-bis-N,N-(4-methylphenyl)-bis-N,N-phenyl-1,4-phenylenediamine)) and electron acceptor F8BT (poly[(9,9-di-n-octyluorenyl-2,7-diyl)-alt-(benzo[2,1,3]thiadiazol-4,8-diyl)), comprise the novel triblock copolymer PFM-F8BT-PFM. This copolymer is designed to phase separate on the 20-30 nm scale, a domain size ideal for maximizing exciton collection at the donor-acceptor interface. Using steady-state and ultrafast spectroscopic characterization including high repetition rate transient absorption spectroscopy, the dynamics of charge and energy transfer of the component polymers and the triblock co-polymer have been investigated. The results demonstrate that for the homopolymers solvent dependent exciton transport processes dominate, while in the triblock copolymer solutions transient spectroscopy provides evidence for interfacial charge separation.

  20. Fine structure of the Mn acceptor in GaAs

    NASA Astrophysics Data System (ADS)

    Krainov, I. V.; Debus, J.; Averkiev, N. S.; Dimitriev, G. S.; Sapega, V. F.; Lähderanta, E.

    2016-06-01

    We reveal the electronic level structure of the Mn acceptor, which consists of a valence-band hole bound to an Mn2 + ion, in presence of applied uniaxial stress and an external magnetic field in bulk GaAs. Resonant spin-flip Raman scattering is used to measure the g factor of the AMn0 center in the ground and excited states with the total angular momenta F =1 and F =2 and characterize the optical selection rules of the spin-flip transitions between these Mn-acceptor states. We determine the random stress fields near the Mn acceptor, the constant of the antiferromagnetic exchange interaction between the valence-band holes and the electrons of the inner Mn2 + shell as well as the deformation potential for the exchange energy. The p -d exchange energy, in particular, decreases significantly with increasing compressive stress. By combining the experimental Raman study with the developed theoretical model on the scattering efficiency, in which also the random local and external uniaxial stresses and magnetic field are considered, the fine structure of the Mn acceptor is determined in full detail.

  1. Loss mechanisms in organic solar cells based on perylene diimide acceptors studied by time-resolved photoluminescence

    NASA Astrophysics Data System (ADS)

    Gerhard, Marina; Gehrig, Dominik; Howard, Ian A.; Arndt, Andreas P.; Bilal, Mühenad; Rahimi-Iman, Arash; Lemmer, Uli; Laquai, Frédéric; Koch, Martin

    2016-04-01

    In organic photovoltaics (OPV), perylene diimide (PDI) acceptor materials are promising candidates to replace the commonly used, but more expensive fullerene derivatives. The use of alternative acceptor materials however implies new design guidelines for OPV devices. It is therefore important to understand the underlying photophysical processes, which either lead to charge generation or geminate recombination. In this contribution, we investigate radiative losses in a series of OPV materials based on two polymers, P3HT and PTB7, respectively, which were blended with different PDI derivatives. Our time-resolved photoluminescence measurements (TRPL) allow us to identify different loss mechanisms by the decay characteristics of several excitonic species. In particular, we find evidence for unfavorable morphologies in terms of large-scale pure domains, inhibited exciton transport and incomplete charge transfer. Furthermore, in one of the P3HT-blends, an interfacial emissive charge transfer (CT) state with strong trapping character is identified.

  2. Electronic structure and absorption spectrum of biexciton obtained by using exciton basis

    SciTech Connect

    Shiau, Shiue-Yuan; Combescot, Monique; Chang, Yia-Chung

    2013-09-15

    We approach the biexciton Schrödinger equation not through the free-carrier basis as usually done, but through the free-exciton basis, exciton–exciton interactions being treated according to the recently developed composite boson many-body formalism which allows an exact handling of carrier exchange between excitons, as induced by the Pauli exclusion principle. We numerically solve the resulting biexciton Schrödinger equation with the exciton levels restricted to the ground state and we derive the biexciton ground state as well as the bound and unbound excited states as a function of hole-to-electron mass ratio. The biexciton ground-state energy we find, agrees reasonably well with variational results. Next, we use the obtained biexciton wave functions to calculate optical absorption in the presence of a dilute exciton gas in quantum well. We find an asymmetric peak with a characteristic low-energy tail, identified with the biexciton ground state, and a set of Lorentzian-like peaks associated with biexciton unbound states, i.e., exciton–exciton scattering states. Last, we propose a pump–probe experiment to probe the momentum distribution of the exciton condensate. -- Highlights: •New composite boson many-body theory is used to derive exactly the biexciton Schrödinger equation using the exciton basis. •We solved the 2D and 3D biexciton ground- and excited-state binding energies for various electron-to-hole mass ratios. •The absorption spectrum shows an asymmetric low-energy peak identified with the biexciton ground state. •High-energy Lorentzian-like peaks in the absorption spectrum are associated with the exciton–exciton scattering states. •The exciton gas momentum distribution can be determined by the absorption spectrum via the biexciton wave functions.

  3. Exciton Transport and Perfect Coulomb Drag

    NASA Astrophysics Data System (ADS)

    Nandi, Debaleena

    2013-03-01

    Exciton condensation is realized in closely-spaced bilayer quantum Hall systems at νT = 1 when the total density in the two 2D electron layers matches the Landau level degeneracy. In this state, electrons in one layer become tightly bound to holes in the other layer, forming a condensate similar to the Cooper pairs in a superconductor. Being charge neutral, these excitons ought to be free to move throughout the bulk of the quantum Hall fluid. One therefore expects that electron current driven in one layer would spontaneously generate a ``hole'' current in the other layer, even in the otherwise insulating bulk of the 2D system. We demonstrate precisely this effect, using a Corbino geometry to defeat edge state transport. Our sample contains two essentially identical two-dimensional electron systems (2DES) in GaAs quantum wells separated by a thin AlGaAs barrier. It is patterned into an annulus with arms protruding from each rim that provide contact to each 2DES separately. A current drag geometry is realized by applying a drive voltage between the outer and inner rim on one 2DES layer while the two rims on the opposite layer are connected together in a closed loop. There is no direct electrical connection between the two layers. At νT = 1 the bulk of the Corbino annulus becomes insulating owing to the quantum Hall gap and net charge transport across the bulk is suppressed. Nevertheless, we find that in the drag geometry appreciable currents do flow in each layer. These currents are almost exactly equal magnitude but, crucially, flow in opposite directions. This phenomenon reflects exciton transport within the νT = 1 condensate, rather than its quasiparticle excitations. We find that quasiparticle transport competes with exciton transport at elevated temperatures, drive levels, and layer separations. This work represents a collaboration with A.D.K. Finck, J.P. Eisenstein, L.N. Pfeiffer and K.W. West. This work is supported by the NSF under grant DMR-1003080.

  4. Photosynthetic light harvesting: excitons and coherence

    PubMed Central

    Fassioli, Francesca; Dinshaw, Rayomond; Arpin, Paul C.; Scholes, Gregory D.

    2014-01-01

    Photosynthesis begins with light harvesting, where specialized pigment–protein complexes transform sunlight into electronic excitations delivered to reaction centres to initiate charge separation. There is evidence that quantum coherence between electronic excited states plays a role in energy transfer. In this review, we discuss how quantum coherence manifests in photosynthetic light harvesting and its implications. We begin by examining the concept of an exciton, an excited electronic state delocalized over several spatially separated molecules, which is the most widely available signature of quantum coherence in light harvesting. We then discuss recent results concerning the possibility that quantum coherence between electronically excited states of donors and acceptors may give rise to a quantum coherent evolution of excitations, modifying the traditional incoherent picture of energy transfer. Key to this (partially) coherent energy transfer appears to be the structure of the environment, in particular the participation of non-equilibrium vibrational modes. We discuss the open questions and controversies regarding quantum coherent energy transfer and how these can be addressed using new experimental techniques. PMID:24352671

  5. An Infinite Order Discrete Variable Representation of an Effective Mass Hamiltonian: Application to Exciton Wave Functions in Quantum Confined Nanostructures.

    PubMed

    Kaledin, Alexey L; Lian, Tianquan; Hill, Craig L; Musaev, Djamaladdin G

    2014-08-12

    We describe an extension of the conventional Fourier grid discrete variable representation (DVR) to the bound state problem of a particle with a position-dependent mass. An infinite order DVR, derived for a variable mass kinetic energy operator, coupled with an efficient grid contraction scheme yields essentially exact eigenvalues for a chosen grid spacing. Implementation of the method is shown to be very practical due to the fact that in a DVR no integral evaluation is necessary and that the resultant kinetic energy matrix is sparse. Numerical calculations are presented for exciton states of spherical, cylindrical, and toric Type I (CdSe/ZnS) core-shell quantum dots. In these examples, electron-hole interaction is treated explicitly by solving a self-consistent Schrödinger-Poisson equation on a contracted DVR grid. Prospective applications of the developed approach to calculating electron transfer rates between adsorbed molecular acceptors and quantum confined nanocrystals of generic shape, dimensionality, and composition are also discussed. PMID:26588309

  6. Coherent spin dynamics of donor bound electrons in GaAs

    NASA Astrophysics Data System (ADS)

    Phelps, Carey; O'Leary, Shannon; Prineas, John; Wang, Hailin

    2011-08-01

    We report experimental studies of coherent spin dynamics of donor-bound electrons in high-purity GaAs by using transient differential transmission. The donor-bound exciton transitions, which are not visible in the linear absorption spectrum, are spectrally resolved in the nonlinear differential transmission spectra. The spin beats in the transient differential transmission response, arising from electron spin precession in an external magnetic field, are investigated with the pump and probe coupling to various donor-bound exciton transitions. The spectral dependence of the spin beats provides important information on the polarization selection rule for the underlying donor-bound exciton transitions. The polarization selection rules deduced from these experiments indicate that contributions from higher-energy donor-bound exciton transitions can severely limit the effectiveness of optical spin control using mechanisms such as polarization-dependent optical Stark shifts.

  7. Plasmonic, excitonic and exciton-plasmonic photoinduced nanocomposites

    NASA Astrophysics Data System (ADS)

    Bityurin, N.; Ermolaev, N.; Smirnov, A. A.; Afanasiev, A.; Agareva, N.; Koryukina, T.; Bredikhin, V.; Kamensky, V.; Pikulin, A.; Sapogova, N.

    2016-03-01

    UV irradiation of materials consisting of a polymer matrix that possesses precursors of different kinds can result in creation of nanoparticles within the irradiated domains. Such photoinduced nanocomposites are promising for photonic applications due to the strong alteration of their optical properties compared to initial non-irradiated materials. We report our results on the synthesis and investigation of plasmonic, excitonic and exciton-plasmonic photoinduced nanocomposites. Plasmonic nanocomposites contain metal nanoparticles of noble metals with a pronounced plasmon resonance. Excitonic nanocomposites possess semiconductor nanoclusters (quantum dots). We consider the CdS-Au pair because the luminescent band of CdS nanoparticles enters the plasmon resonance band of gold nanoparticles. The obtaining of such particles within the same composite materials is promising for the creation of media with exciton-plasmon resonance. We demonstrate that it is possible to choose appropriate precursor species to obtain the initially transparent poly(methyl methacrylate) (PMMA) films containing both types of these molecules either separately or together. Proper irradiation of these materials by a light-emitting diode operating at the wavelength of 365 nm provides material alteration demonstrating light-induced optical absorption and photoluminescent properties typical for the corresponding nanoparticles. Thus, an exciton-plasmonic photoinduced nanocomposite is obtained. It is important that here we use the precursors that are different from those usually employed.

  8. Highly anisotropic and robust excitons in monolayer black phosphorus.

    PubMed

    Wang, Xiaomu; Jones, Aaron M; Seyler, Kyle L; Tran, Vy; Jia, Yichen; Zhao, Huan; Wang, Han; Yang, Li; Xu, Xiaodong; Xia, Fengnian

    2015-06-01

    Semi-metallic graphene and semiconducting monolayer transition-metal dichalcogenides are the most intensively studied two-dimensional materials of recent years. Lately, black phosphorus has emerged as a promising new two-dimensional material due to its widely tunable and direct bandgap, high carrier mobility and remarkable in-plane anisotropic electrical, optical and phonon properties. However, current progress is primarily limited to its thin-film form. Here, we reveal highly anisotropic and strongly bound excitons in monolayer black phosphorus using polarization-resolved photoluminescence measurements at room temperature. We show that, regardless of the excitation laser polarization, the emitted light from the monolayer is linearly polarized along the light effective mass direction and centres around 1.3 eV, a clear signature of emission from highly anisotropic bright excitons. Moreover, photoluminescence excitation spectroscopy suggests a quasiparticle bandgap of 2.2 eV, from which we estimate an exciton binding energy of ∼0.9 eV, consistent with theoretical results based on first principles. The experimental observation of highly anisotropic, bright excitons with large binding energy not only opens avenues for the future explorations of many-electron physics in this unusual two-dimensional material, but also suggests its promising future in optoelectronic devices. PMID:25915195

  9. Highly anisotropic and robust excitons in monolayer black phosphorus

    NASA Astrophysics Data System (ADS)

    Wang, Xiaomu; Jones, Aaron M.; Seyler, Kyle L.; Tran, Vy; Jia, Yichen; Zhao, Huan; Wang, Han; Yang, Li; Xu, Xiaodong; Xia, Fengnian

    2015-06-01

    Semi-metallic graphene and semiconducting monolayer transition-metal dichalcogenides are the most intensively studied two-dimensional materials of recent years. Lately, black phosphorus has emerged as a promising new two-dimensional material due to its widely tunable and direct bandgap, high carrier mobility and remarkable in-plane anisotropic electrical, optical and phonon properties. However, current progress is primarily limited to its thin-film form. Here, we reveal highly anisotropic and strongly bound excitons in monolayer black phosphorus using polarization-resolved photoluminescence measurements at room temperature. We show that, regardless of the excitation laser polarization, the emitted light from the monolayer is linearly polarized along the light effective mass direction and centres around 1.3 eV, a clear signature of emission from highly anisotropic bright excitons. Moreover, photoluminescence excitation spectroscopy suggests a quasiparticle bandgap of 2.2 eV, from which we estimate an exciton binding energy of ∼0.9 eV, consistent with theoretical results based on first principles. The experimental observation of highly anisotropic, bright excitons with large binding energy not only opens avenues for the future explorations of many-electron physics in this unusual two-dimensional material, but also suggests its promising future in optoelectronic devices.

  10. Intrinsic homogeneous linewidth and broadening mechanisms of excitons in monolayer transition metal dichalcogenides

    PubMed Central

    Moody, Galan; Kavir Dass, Chandriker; Hao, Kai; Chen, Chang-Hsiao; Li, Lain-Jong; Singh, Akshay; Tran, Kha; Clark, Genevieve; Xu, Xiaodong; Berghäuser, Gunnar; Malic, Ermin; Knorr, Andreas; Li, Xiaoqin

    2015-01-01

    The band-edge optical response of transition metal dichalcogenides, an emerging class of atomically thin semiconductors, is dominated by tightly bound excitons localized at the corners of the Brillouin zone (valley excitons). A fundamental yet unknown property of valley excitons in these materials is the intrinsic homogeneous linewidth, which reflects irreversible quantum dissipation arising from system (exciton) and bath (vacuum and other quasiparticles) interactions and determines the timescale during which excitons can be coherently manipulated. Here we use optical two-dimensional Fourier transform spectroscopy to measure the exciton homogeneous linewidth in monolayer tungsten diselenide (WSe2). The homogeneous linewidth is found to be nearly two orders of magnitude narrower than the inhomogeneous width at low temperatures. We evaluate quantitatively the role of exciton–exciton and exciton–phonon interactions and population relaxation as linewidth broadening mechanisms. The key insights reported here—strong many-body effects and intrinsically rapid radiative recombination—are expected to be ubiquitous in atomically thin semiconductors. PMID:26382305

  11. Tuning excitons in monolayer and few-layer MoS2

    NASA Astrophysics Data System (ADS)

    Qiu, Diana Y.; da Jornada, Felipe H.; Louie, Steven G.

    2014-03-01

    Our recent ab initio GW-BSE calculations showed that monolayer MoS2 is a computationally challenging system, requiring a large number of empty bands and very fine k-point sampling to converge its quasiparticle band structure and optical properties. Careful convergence of a GW-BSE calculation reveals that MoS2 has a large number of bound excitons with varying k-space characteristics. Specifically, there are two series of excitons: a low-energy series with k-space wavefunctions localized at the K/K' valleys in the Brillouin zone and a higher energy series localized in a ring around the Γ point. There is very little hybridization between these two exciton series in monolayer MoS2, but changes in electronic structure and screening due to additional layers, strain, or doping can lead to changes in exciton binding energies, character, and hybridization. Thus, we have carried out ab initio GW-BSE calculations to study the excitonic properties of few-layer MoS2. We find that layering and straining MoS2 systematically changes the exciton binding energies, the peak positions and amount of absorbance in the optical spectrum, and the character and hybridization of the excitons near Γ. This work was supported by NSF grant No. DMR10-1006184 and U.S. DOE under Contract No. DE-AC02-05CH11231.

  12. Dynamical reconstruction of the exciton in LiF with inelastic x-ray scattering

    PubMed Central

    Abbamonte, Peter; Graber, Tim; Reed, James P.; Smadici, Serban; Yeh, Chen-Lin; Shukla, Abhay; Rueff, Jean-Pascal; Ku, Wei

    2008-01-01

    The absorption of light by materials proceeds through the formation of excitons, which are states in which an excited electron is bound to the valence hole it vacated. Understanding the structure and dynamics of excitons is important, for example, for developing technologies for light-emitting diodes or solar energy conversion. However, there has never been an experimental means to study the time-dependent structure of excitons directly. Here, we use causality-inverted inelastic x-ray scattering (IXS) to image the charge-transfer exciton in the prototype insulator LiF, with resolutions Δt = 20.67 as (2.067 × 10−17 s) in time and Δx = 0.533 Å (5.33 × 10−11 m) in space. Our results show that the exciton has a modulated internal structure and is coherently delocalized over two unit cells of the LiF crystal (≈8 Å). This structure changes only modestly during the course of its life, which establishes it unambiguously as a Frenkel exciton and thus amenable to a simplified theoretical description. Our results resolve an old controversy about excitons in the alkali halides and demonstrate the utility of IXS for imaging attosecond electron dynamics in condensed matter. PMID:18711146

  13. Probing the spin states of a single acceptor atom.

    PubMed

    van der Heijden, Joost; Salfi, Joe; Mol, Jan A; Verduijn, Jan; Tettamanzi, Giuseppe C; Hamilton, Alex R; Collaert, Nadine; Rogge, Sven

    2014-03-12

    We demonstrate a single-hole transistor using an individual acceptor dopant embedded in a silicon channel. Magneto-transport spectroscopy reveals that the ground state splits as a function of magnetic field into four states, which is unique for a single hole bound to an acceptor in a bulk semiconductor. The two lowest spin states are heavy (|m(j)| = 3/2) and light (|m(j)| = 1/2) hole-like, a two-level system that can be electrically driven and is characterized by a magnetic field dependent and long relaxation time, which are properties of interest for qubits. Although the bulklike spin splitting of a boron atom is preserved in our nanotransistor, the measured Landé g-factors, |g(hh)| = 0.81 ± 0.06 and |g(lh)| = 0.85 ± 0.21 for heavy and light holes respectively, are lower than the bulk value. PMID:24571637

  14. Interface effects on acceptor qubits in silicon and germanium.

    PubMed

    Abadillo-Uriel, J C; Calderón, M J

    2016-01-15

    Dopant-based quantum computing implementations often require the dopants to be situated close to an interface to facilitate qubit manipulation with local gates. Interfaces not only modify the energies of the bound states but also affect their symmetry. Making use of the successful effective mass theory we study the energy spectra of acceptors in Si or Ge taking into account the quantum confinement, the dielectric mismatch and the central cell effects. The presence of an interface puts constraints to the allowed symmetries and leads to the splitting of the ground state in two Kramers doublets (Mol et al 2015 Appl. Phys. Lett. 106 203110). Inversion symmetry breaking also implies parity mixing which affects the allowed optical transitions. Consequences for acceptor qubits are discussed. PMID:26618443

  15. Excitons and charged excitons in semiconductor quantum wells

    SciTech Connect

    Riva, C.; Peeters, F. M.; Varga, K.

    2000-05-15

    A variational calculation of the ground-state energy of neutral excitons and of positively and negatively charged excitons (trions) confined in a single-quantum well is presented. We study the dependence of the correlation energy and of the binding energy on the well width and on the hole mass. The conditional probability distribution for positively and negatively charged excitons is obtained, providing information on the correlation and the charge distribution in the system. A comparison is made with available experimental data on trion binding energies in GaAs-, ZnSe-, and CdTe-based quantum well structures, which indicates that trions become localized with decreasing quantum well width. (c) 2000 The American Physical Society.

  16. Novel Exciton States in Monolayer MoS2: Unconventional Effective Hamiltonian

    NASA Astrophysics Data System (ADS)

    da Jornada, Felipe; Qiu, Diana; Louie, Steven

    2014-03-01

    Recent well-converged ab inito GW-BSE calculations show that monolayer MoS2 has a large number of strongly bound excitons with varying characters. We show that these excitonic states cannot be even qualitatively described by an effective mass hydrogenic model without a detailed understanding of the 2D screening. Additionally, we identify and analyze one exciton series having an unusually high binding energy, which originates around the Γ point of the Brillouin zone. We show that this excitonic series arises from a fundamentally different effective Hamiltonian with a kinetic energy term resembling a Mexican hat in momentum space, which is responsible for the unusual ordering of the energy levels and distribution of oscillator strength. This work was supported by NSF grant No. DMR10-1006184 and the U.S. DOE under Contract No. DE-AC02-05CH11231.

  17. Ab-initio calculation of excitons in conventional and anorganic semiconductors

    NASA Astrophysics Data System (ADS)

    Ambrosch-Draxl, Claudia; Laskowsky, Robert

    2005-03-01

    The excitonic effects on the optical absorption properties of organic as well as inorganic semiconductors are studied from first-principles. The Coulomb interaction between the electron and the hole is accounted for by solving the two-particle Bethe-Salpeter equation. In the organic semiconductors the exciton binding energies strongly depend on the molecular size, the crystalline packing, as well as the polarization direction of the incoming light. We show that the electron-hole interaction can lead to strongly bound excitons with binding energies of the order of 1eV or to a mere redistribution of oscillator strength. In several cases, the screening is efficient enough such that free charge carriers govern the optical absorption process. In the inorganic counterparts the sensitivity of the exciton binding energy is tested against the structural parameters and the screening of the electron-hole Coulomb interaction.

  18. Tuning the driving force for exciton dissociation in single-walled carbon nanotube heterojunctions

    NASA Astrophysics Data System (ADS)

    Ihly, Rachelle; Mistry, Kevin S.; Ferguson, Andrew J.; Clikeman, Tyler T.; Larson, Bryon W.; Reid, Obadiah; Boltalina, Olga V.; Strauss, Steven H.; Rumbles, Garry; Blackburn, Jeffrey L.

    2016-06-01

    Understanding the kinetics and energetics of interfacial electron transfer in molecular systems is crucial for the development of a broad array of technologies, including photovoltaics, solar fuel systems and energy storage. The Marcus formulation for electron transfer relates the thermodynamic driving force and reorganization energy for charge transfer between a given donor/acceptor pair to the kinetics and yield of electron transfer. Here we investigated the influence of the thermodynamic driving force for photoinduced electron transfer (PET) between single-walled carbon nanotubes (SWCNTs) and fullerene derivatives by employing time-resolved microwave conductivity as a sensitive probe of interfacial exciton dissociation. For the first time, we observed the Marcus inverted region (in which driving force exceeds reorganization energy) and quantified the reorganization energy for PET for a model SWCNT/acceptor system. The small reorganization energies (about 130 meV, most of which probably arises from the fullerene acceptors) are beneficial in minimizing energy loss in photoconversion schemes.

  19. Excitons in a mirror: Formation of “optical bilayers” using MoS{sub 2} monolayers on gold substrates

    SciTech Connect

    Mertens, Jan; Baumberg, Jeremy J.; Shi, Yumeng; Yang, Hui Ying; Molina-Sánchez, Alejandro; Wirtz, Ludger

    2014-05-12

    We report coupling of excitons in monolayers of molybdenum disulphide to their mirror image in an underlying gold substrate. Excitons at the direct band gap are little affected by the substrate whereas strongly bound C-excitons associated with a van-Hove singularity change drastically. On quartz substrates only one C-exciton is visible (in the blue) but on gold substrates a strong red-shifted extra resonance in the green is seen. Exciton coupling to its image leads to formation of a “mirror biexciton” with enhanced binding energy. Estimates of this energy shift in an emitter-gold system match experiments well. The absorption spectrum of MoS{sub 2} on gold thus resembles a bilayer of MoS{sub 2} which has been created by optical coupling. Additional top-mirrors produce an “optical bulk.”.

  20. Talbot Effect for Exciton Polaritons.

    PubMed

    Gao, T; Estrecho, E; Li, G; Egorov, O A; Ma, X; Winkler, K; Kamp, M; Schneider, C; Höfling, S; Truscott, A G; Ostrovskaya, E A

    2016-08-26

    We demonstrate, experimentally and theoretically, a Talbot effect for hybrid light-matter waves-an exciton-polariton condensate formed in a semiconductor microcavity with embedded quantum wells. The characteristic "Talbot carpet" is produced by loading the exciton-polariton condensate into a microstructured one-dimensional periodic array of mesa traps, which creates an array of phase-locked sources for coherent polariton flow in the plane of the quantum wells. The spatial distribution of the Talbot fringes outside the mesas mimics the near-field diffraction of a monochromatic wave on a periodic amplitude and phase grating with the grating period comparable to the wavelength. Despite the lossy nature of the polariton system, the Talbot pattern persists for distances exceeding the size of the mesas by an order of magnitude. Thus, our experiment demonstrates efficient shaping of the two-dimensional flow of coherent exciton polaritons by a one-dimensional "flat lens." PMID:27610883

  1. Exciton coupling in molecular crystals

    NASA Technical Reports Server (NTRS)

    Ake, R. L.

    1976-01-01

    The implications of perfect exciton coupling and molecular vibrations were investigated, as well as the effect they have on the lifetime of singlet and triplet excitons coupled in a limiting geometry. Crystalline bibenzyl, Cl4Hl4, provided a situation in which these mechanisms involving exciton coupling can be studied in the limit of perfect coupling between units due to the crystal's geometry. This geometry leads to a coupling between the two halves of the molecule resulting in a splitting of the molecular excited states. The study reported involves an experimental spectroscopic approach and begins with the purification of the bibenzyl. The principal experimental apparatus was an emission spectrometer. A closed cycle cryogenic system was used to vary the temperature of the sample between 20 K and 300 K. The desired results are the temperature-dependent emission spectra of the bibenzyl; in addition, the lifetimes and quantum yields measured at each temperature reveal the effect of competing radiationless processes.

  2. Negative Polaron and Triplet Exciton Diffusion inOrganometallic “Molecular Wires”

    SciTech Connect

    Schanze, K.S.; Miller, J.; Keller, J.M.; Sean McIlroy, S.; Sreearuothai, P.; Danilov, E.O.; Jiang, H.; Glusac, K.D.; Miller, J.R.

    2011-07-27

    The dynamics of negative polaron and triplet exciton transport within a series of monodisperse platinum (Pt) acetylide oligomers is reported. The oligomers consist of Pt-acetylide repeats, [PtL{sub 2}-C {triple_bond} C-Ph-C {triple_bond} C-]{sub n} (where L = PBu{sub 3} and Ph = 1,4-phenylene, n = 2, 3, 6, and 10), capped with naphthalene diimide (NDI) end groups. The Pt-acetylide segments are electro- and photoactive, and they serve as conduits for transport of electrons (negative polaron) and triplet excitons. The NDI end groups are relatively strong acceptors, serving as traps for the carriers. Negative polaron transport is studied by using pulse radiolysis/transient absorption at the Brookhaven National Laboratory Laser-Electron Accelerator Facility (LEAF). Electrons are rapidly attached to the oligomers, with some fraction initially residing upon the Pt-acetylide chains. The dynamics of transport are resolved by monitoring the spectral changes associated with transfer of electrons from the chain to the NDI end group. Triplet exciton transport is studied by femtosecond-picosecond transient absorption spectroscopy. Near-UV excitation leads to rapid production of triplet excitons localized on the Pt-acetylide chains. The excitons transport to the chain ends, where they are annihilated by charge separation with the NDI end group. The dynamics of triplet transport are resolved by transient absorption spectroscopy, taking advantage of the changes in spectra associated with decay of the triplet exciton and rise of the charge-separated state. The results indicate that negative polarons and excitons are transported rapidly, on average moving distances of 3 nm in less than 200 ps. Analysis of the dynamics suggests diffusive transport by a site-to-site hopping mechanism with hopping times of 27 ps for triplets and <10 ps for electrons.

  3. Exciton-photon correlations in bosonic condensates of exciton-polaritons

    PubMed Central

    Kavokin, Alexey V.; Sheremet, Alexandra S.; Shelykh, Ivan A.; Lagoudakis, Pavlos G.; Rubo, Yuri G.

    2015-01-01

    Exciton-polaritons are mixed light-matter quasiparticles. We have developed a statistical model describing stochastic exciton-photon transitions within a condensate of exciton polaritons. We show that the exciton-photon correlator depends on the rate of incoherent exciton-photon transformations in the condensate. We discuss implications of this effect for the quantum statistics of photons emitted by polariton lasers. PMID:26153979

  4. Exciton-photon correlations in bosonic condensates of exciton-polaritons.

    PubMed

    Kavokin, Alexey V; Sheremet, Alexandra S; Shelykh, Ivan A; Lagoudakis, Pavlos G; Rubo, Yuri G

    2015-01-01

    Exciton-polaritons are mixed light-matter quasiparticles. We have developed a statistical model describing stochastic exciton-photon transitions within a condensate of exciton polaritons. We show that the exciton-photon correlator depends on the rate of incoherent exciton-photon transformations in the condensate. We discuss implications of this effect for the quantum statistics of photons emitted by polariton lasers. PMID:26153979

  5. Direct measurement of exciton valley coherence in monolayer WSe2

    NASA Astrophysics Data System (ADS)

    Hao, Kai; Moody, Galan; Wu, Fengcheng; Dass, Chandriker Kavir; Xu, Lixiang; Chen, Chang-Hsiao; Sun, Liuyang; Li, Ming-Yang; Li, Lain-Jong; MacDonald, Allan H.; Li, Xiaoqin

    2016-07-01

    In crystals, energy band extrema in momentum space can be identified by a valley index. The internal quantum degree of freedom associated with valley pseudospin indices can act as a useful information carrier, analogous to electronic charge or spin. Interest in valleytronics has been revived in recent years following the discovery of atomically thin materials such as graphene and transition metal dichalcogenides. However, the valley coherence time--a crucial quantity for valley pseudospin manipulation--is difficult to directly probe. In this work, we use two-dimensional coherent spectroscopy to resonantly generate and detect valley coherence of excitons (Coulomb-bound electron-hole pairs) in monolayer WSe2 (refs ,). The imposed valley coherence persists for approximately one hundred femtoseconds. We propose that the electron-hole exchange interaction provides an important decoherence mechanism in addition to exciton population recombination. This work provides critical insight into the requirements and strategies for optical manipulation of the valley pseudospin for future valleytronics applications.

  6. Excitonic effects in two-dimensional semiconductors: Path integral Monte Carlo approach

    NASA Astrophysics Data System (ADS)

    Velizhanin, Kirill A.; Saxena, Avadh

    2015-11-01

    One of the most striking features of novel two-dimensional semiconductors (e.g., transition metal dichalcogenide monolayers or phosphorene) is a strong Coulomb interaction between charge carriers resulting in large excitonic effects. In particular, this leads to the formation of multicarrier bound states upon photoexcitation (e.g., excitons, trions, and biexcitons), which could remain stable at near-room temperatures and contribute significantly to the optical properties of such materials. In the present work we have used the path integral Monte Carlo methodology to numerically study properties of multicarrier bound states in two-dimensional semiconductors. Specifically, we have accurately investigated and tabulated the dependence of single-exciton, trion, and biexciton binding energies on the strength of dielectric screening, including the limiting cases of very strong and very weak screening. The results of this work are potentially useful in the analysis of experimental data and benchmarking of theoretical and computational models.

  7. Excitonic effects in two-dimensional semiconductors: Path integral Monte Carlo approach

    SciTech Connect

    Velizhanin, Kirill A.; Saxena, Avadh

    2015-11-11

    The most striking features of novel two-dimensional semiconductors (e.g., transition metal dichalcogenide monolayers or phosphorene) is a strong Coulomb interaction between charge carriers resulting in large excitonic effects. In particular, this leads to the formation of multicarrier bound states upon photoexcitation (e.g., excitons, trions, and biexcitons), which could remain stable at near-room temperatures and contribute significantly to the optical properties of such materials. In our work we have used the path integral Monte Carlo methodology to numerically study properties of multicarrier bound states in two-dimensional semiconductors. Specifically, we have accurately investigated and tabulated the dependence of single-exciton, trion, and biexciton binding energies on the strength of dielectric screening, including the limiting cases of very strong and very weak screening. Our results of this work are potentially useful in the analysis of experimental data and benchmarking of theoretical and computational models.

  8. Excitonic effects in two-dimensional semiconductors: Path integral Monte Carlo approach

    DOE PAGESBeta

    Velizhanin, Kirill A.; Saxena, Avadh

    2015-11-11

    The most striking features of novel two-dimensional semiconductors (e.g., transition metal dichalcogenide monolayers or phosphorene) is a strong Coulomb interaction between charge carriers resulting in large excitonic effects. In particular, this leads to the formation of multicarrier bound states upon photoexcitation (e.g., excitons, trions, and biexcitons), which could remain stable at near-room temperatures and contribute significantly to the optical properties of such materials. In our work we have used the path integral Monte Carlo methodology to numerically study properties of multicarrier bound states in two-dimensional semiconductors. Specifically, we have accurately investigated and tabulated the dependence of single-exciton, trion, and biexcitonmore » binding energies on the strength of dielectric screening, including the limiting cases of very strong and very weak screening. Our results of this work are potentially useful in the analysis of experimental data and benchmarking of theoretical and computational models.« less

  9. Effect of heteroboundary spreading on the properties of exciton states in Zn(Cd)Se/ZnMgSSe quantum wells

    SciTech Connect

    Adiyatullin, A. F. Belykh, V. V.; Kozlovsky, V. I.; Krivobok, V. S. Martovitsky, V. P.; Nikolaev, S. N.

    2012-11-15

    Exciton states in Zn(Cd)Se/ZnMgSSe quantum wells with different diffusion spreading of interfaces are studied by optical spectroscopy methods. It is shown that the emission spectrum of quantum wells at low temperatures is determined by free excitons and bound excitons on neutral donors. The nonlinear dependence of the stationary photoluminescence intensity on the excitation power density and the biexponential luminescence decay are explained by the neutralization of charged defects upon photoexcitation of heterostructures. With the stationary illumination on, durable (about 40 min) reversible changes in the reflection coefficient near the exciton resonances of quantum wells are observed. It is shown that, along with the shift of exciton levels, the spreading of heteroboundaries leads to three effects: an increase in the excitonphonon interaction, an increase in the energy shift between the emission lines of free and bound excitons, and a decrease in the decay time of exciton luminescence in a broad temperature range. The main reasons for these effects are discussed.

  10. Observation of excitonic super-radiance in quantum well structures and its application for laser cooling of solids

    NASA Astrophysics Data System (ADS)

    Hassani Nia, Iman; Weinberg, David; Wheaton, Skylar; Weiss, Emily A.; Mohseni, Hooman

    2016-03-01

    Excitons, bound electron-hole pairs, possess distinct physical properties from free electrons and holes that can be employed to improve the performance of optoelectronic devices. In particular, the signatures of excitons are enhanced optical absorption and radiative emission. These characteristics could be of major benefit for the laser cooling of semiconductors, a process which has stringent requirements on the parasitic absorption of incident radiation and the internal quantum efficiency. Here we experimentally demonstrate the dominant ultrafast excitonic super-radiance of our quantum well structure from 78 K up to room temperature. The experimental results are followed by our detailed discussions about the advantages and limitations of this method.

  11. Exciton size and quantum transport in nanoplatelets

    SciTech Connect

    Pelzer, Kenley M. Gray, Stephen K.; Darling, Seth B.; Schaller, Richard D.

    2015-12-14

    Two-dimensional nanoplatelets (NPLs) are an exciting class of materials with promising optical and energy transport properties. The possibility of efficient energy transport between nanoplatelets raises questions regarding the nature of energy transfer in these thin, laterally extended systems. A challenge in understanding exciton transport is the uncertainty regarding the size of the exciton. Depending on the material and defects in the nanoplatelet, an exciton could plausibly extend over an entire plate or localize to a small region. The variation in possible exciton sizes raises the question how exciton size impacts the efficiency of transport between nanoplatelet structures. Here, we explore this issue using a quantum master equation approach. This method goes beyond the assumptions of Förster theory to allow for quantum mechanical effects that could increase energy transfer efficiency. The model is extremely flexible in describing different systems, allowing us to test the effect of varying the spatial extent of the exciton. We first discuss qualitative aspects of the relationship between exciton size and transport and then conduct simulations of exciton transport between NPLs for a range of exciton sizes and environmental conditions. Our results reveal that exciton size has a strong effect on energy transfer efficiency and suggest that manipulation of exciton size may be useful in designing NPLs for energy transport.

  12. Exciton size and quantum transport in nanoplatelets.

    PubMed

    Pelzer, Kenley M; Darling, Seth B; Gray, Stephen K; Schaller, Richard D

    2015-12-14

    Two-dimensional nanoplatelets (NPLs) are an exciting class of materials with promising optical and energy transport properties. The possibility of efficient energy transport between nanoplatelets raises questions regarding the nature of energy transfer in these thin, laterally extended systems. A challenge in understanding exciton transport is the uncertainty regarding the size of the exciton. Depending on the material and defects in the nanoplatelet, an exciton could plausibly extend over an entire plate or localize to a small region. The variation in possible exciton sizes raises the question how exciton size impacts the efficiency of transport between nanoplatelet structures. Here, we explore this issue using a quantum master equation approach. This method goes beyond the assumptions of Förster theory to allow for quantum mechanical effects that could increase energy transfer efficiency. The model is extremely flexible in describing different systems, allowing us to test the effect of varying the spatial extent of the exciton. We first discuss qualitative aspects of the relationship between exciton size and transport and then conduct simulations of exciton transport between NPLs for a range of exciton sizes and environmental conditions. Our results reveal that exciton size has a strong effect on energy transfer efficiency and suggest that manipulation of exciton size may be useful in designing NPLs for energy transport. PMID:26671357

  13. Exciton size and quantum transport in nanoplatelets

    NASA Astrophysics Data System (ADS)

    Pelzer, Kenley M.; Darling, Seth B.; Gray, Stephen K.; Schaller, Richard D.

    2015-12-01

    Two-dimensional nanoplatelets (NPLs) are an exciting class of materials with promising optical and energy transport properties. The possibility of efficient energy transport between nanoplatelets raises questions regarding the nature of energy transfer in these thin, laterally extended systems. A challenge in understanding exciton transport is the uncertainty regarding the size of the exciton. Depending on the material and defects in the nanoplatelet, an exciton could plausibly extend over an entire plate or localize to a small region. The variation in possible exciton sizes raises the question how exciton size impacts the efficiency of transport between nanoplatelet structures. Here, we explore this issue using a quantum master equation approach. This method goes beyond the assumptions of Förster theory to allow for quantum mechanical effects that could increase energy transfer efficiency. The model is extremely flexible in describing different systems, allowing us to test the effect of varying the spatial extent of the exciton. We first discuss qualitative aspects of the relationship between exciton size and transport and then conduct simulations of exciton transport between NPLs for a range of exciton sizes and environmental conditions. Our results reveal that exciton size has a strong effect on energy transfer efficiency and suggest that manipulation of exciton size may be useful in designing NPLs for energy transport.

  14. Highly Efficient Nondoped Green Organic Light-Emitting Diodes with Combination of High Photoluminescence and High Exciton Utilization.

    PubMed

    Wang, Chu; Li, Xianglong; Pan, Yuyu; Zhang, Shitong; Yao, Liang; Bai, Qing; Li, Weijun; Lu, Ping; Yang, Bing; Su, Shijian; Ma, Yuguang

    2016-02-10

    Photoluminescence (PL) efficiency and exciton utilization efficiency are two key parameters to harvest high-efficiency electroluminescence (EL) in organic light-emitting diodes (OLEDs). But it is not easy to simultaneously combine these two characteristics (high PL efficiency and high exciton utilization) into a fluorescent material. In this work, an efficient combination was achieved through two concepts of hybridized local and charge-transfer (CT) state (HLCT) and "hot exciton", in which the former is responsible for high PL efficiency while the latter contributes to high exciton utilization. On the basis of a tiny chemical modification in TPA-BZP, a green-light donor-acceptor molecule, we designed and synthesized CzP-BZP with this efficeient combination of high PL efficiency of η(PL) = 75% in the solid state and maximal exciton utilization efficiency up to 48% (especially, the internal quantum efficiency of η(IQE) = 35% substantially exceed 25% of spin statistics limit) in OLED. The nondoped OLED of CzP-BZP exhibited an excellent performance: a green emission with a CIE coordinate of (0.34, 0.60), a maximum current efficiency of 23.99 cd A(-1), and a maximum external quantum efficiency (EQE, η(EQE)) of 6.95%. This combined HLCT state and "hot exciton" strategy should be a practical way to design next-generation, low-cost, high-efficiency fluorescent OLED materials. PMID:26785427

  15. Electronic structure disorder, vibronic coupling, and charge transfer excitons in poly(fluorene-alt-bithiophene):fullerene films

    SciTech Connect

    Riisness, I.; Gordon, M. J.

    2013-03-18

    Charge transfer processes in conjugated polymer:fullerene blends play an important role in the operation of organic solar cells and organic light emitting diodes. Herein, near-infrared emission from poly-(9,9-dioctylfluorene-alt-bithiophene) (F8T2) and [6,6]-phenyl-C{sub 61}-butyric acid methyl ester blends was studied and attributed to charge transfer exciton (CTX) recombination. Polymer and CTX emission were monitored via low-temperature/transient photoluminescence and absorbance to elucidate the effects of annealing and composition on donor-acceptor morphology. CTX emission decreased and F8T2 vibronic structure was partially restored due to lower fullerene dispersion and polymer realignment upon annealing. Differences in the temperature-dependent emissions of the polymer singlet vs. CTX were attributed to exciton diffusion in the polymer phase vs. enhanced quenching at the donor-acceptor interface, respectively.

  16. DONOR-ACCEPTOR INTERACTIONS OF NITROGEN*

    PubMed Central

    Kimura, J. E.; Szent-Györgyi, A.

    1969-01-01

    The nitrogen atoms of organic molecules readily enter into donor-acceptor interactions, giving off an electron from their lone pair. Under favorable conditions the acceptor can form free radicals. S and O atoms behave likewise but less intensely. PMID:4306047

  17. Anaerobic electron acceptor chemotaxis in Shewanella putrefaciens

    NASA Technical Reports Server (NTRS)

    Nealson, K. H.; Moser, D. P.; Saffarini, D. A.

    1995-01-01

    Shewanella putrefaciens MR-1 can grow either aerobically or anaerobically at the expense of many different electron acceptors and is often found in abundance at redox interfaces in nature. Such redox interfaces are often characterized by very strong gradients of electron acceptors resulting from rapid microbial metabolism. The coincidence of S. putrefaciens abundance with environmental gradients prompted an examination of the ability of MR-1 to sense and respond to electron acceptor gradients in the laboratory. In these experiments, taxis to the majority of the electron acceptors that S. putrefaciens utilizes for anaerobic growth was seen. All anaerobic electron acceptor taxis was eliminated by the presence of oxygen, nitrate, nitrite, elemental sulfur, or dimethyl sulfoxide, even though taxis to the latter was very weak and nitrate and nitrite respiration was normal in the presence of dimethyl sulfoxide. Studies with respiratory mutants of MR-1 revealed that several electron acceptors that could not be used for anaerobic growth nevertheless elicited normal anaerobic taxis. Mutant M56, which was unable to respire nitrite, showed normal taxis to nitrite, as well as the inhibition of taxis to other electron acceptors by nitrite. These results indicate that electron acceptor taxis in S. putrefaciens does not conform to the paradigm established for Escherichia coli and several other bacteria. Carbon chemo-taxis was also unusual in this organism: of all carbon compounds tested, the only positive response observed was to formate under anaerobic conditions.

  18. Glucansucrase acceptor reactions with D-mannose

    Technology Transfer Automated Retrieval System (TEKTRAN)

    The main acceptor product of glucansucrases with D-mannose has not previously been identified. We used glucansucrases that form water-insoluble a-D-glucans to produce increased yields of acceptor products from D-mannose, and identified the major product as 6-O-a-D-glucopyranosyl-D-mannose. Glucansuc...

  19. Probing excitonic states in suspended two-dimensional semiconductors by photocurrent spectroscopy

    SciTech Connect

    Klots, A. R.; Newaz, A. K. M.; Wang, Bin; Prasai, D.; Krzyzanowska, H.; Lin, Junhao; Caudel, D.; Ghimire, N. J.; Yan, J.; Ivanov, B. L.; Velizhanin, K. A.; Burger, A.; Mandrus, D. G.; Tolk, N. H.; Pantelides, S. T.; Bolotin, K. I.

    2014-10-16

    The optical response of semiconducting monolayer transition-metal dichalcogenides (TMDCs) is dominated by strongly bound excitons that are stable even at room temperature. However, substrate-related effects such as screening and disorder in currently available specimens mask many anticipated physical phenomena and limit device applications of TMDCs. Here, we demonstrate that that these undesirable effects are strongly suppressed in suspended devices. Extremely robust (photogain > 1,000) and fast (response time < 1 ms) photoresponse allow us to study, for the first time, the formation, binding energies, and dissociation mechanisms of excitons in TMDCs through photocurrent spectroscopy. By analyzing the spectral positions of peaks in the photocurrent and by comparing them with first-principles calculations, we obtain binding energies, band gaps and spin-orbit splitting in monolayer TMDCs. For monolayer MoS2, in particular, we obtain an extremely large binding energy for band-edge excitons, Ebind ≥ 570 meV. Along with band-edge excitons, we observe excitons associated with a van Hove singularity of rather unique nature. In conclusion, the analysis of the source-drain voltage dependence of photocurrent spectra reveals exciton dissociation and photoconversion mechanisms in TMDCs.

  20. Probing excitonic states in suspended two-dimensional semiconductors by photocurrent spectroscopy

    DOE PAGESBeta

    Klots, A. R.; Newaz, A. K. M.; Wang, Bin; Prasai, D.; Krzyzanowska, H.; Lin, Junhao; Caudel, D.; Ghimire, N. J.; Yan, J.; Ivanov, B. L.; et al

    2014-10-16

    The optical response of semiconducting monolayer transition-metal dichalcogenides (TMDCs) is dominated by strongly bound excitons that are stable even at room temperature. However, substrate-related effects such as screening and disorder in currently available specimens mask many anticipated physical phenomena and limit device applications of TMDCs. Here, we demonstrate that that these undesirable effects are strongly suppressed in suspended devices. Extremely robust (photogain > 1,000) and fast (response time < 1 ms) photoresponse allow us to study, for the first time, the formation, binding energies, and dissociation mechanisms of excitons in TMDCs through photocurrent spectroscopy. By analyzing the spectral positions ofmore » peaks in the photocurrent and by comparing them with first-principles calculations, we obtain binding energies, band gaps and spin-orbit splitting in monolayer TMDCs. For monolayer MoS2, in particular, we obtain an extremely large binding energy for band-edge excitons, Ebind ≥ 570 meV. Along with band-edge excitons, we observe excitons associated with a van Hove singularity of rather unique nature. In conclusion, the analysis of the source-drain voltage dependence of photocurrent spectra reveals exciton dissociation and photoconversion mechanisms in TMDCs.« less

  1. Probing excitonic states in suspended two-dimensional semiconductors by photocurrent spectroscopy

    PubMed Central

    Klots, A. R.; Newaz, A. K. M.; Wang, Bin; Prasai, D.; Krzyzanowska, H.; Lin, Junhao; Caudel, D.; Ghimire, N. J.; Yan, J.; Ivanov, B. L.; Velizhanin, K. A.; Burger, A.; Mandrus, D. G.; Tolk, N. H.; Pantelides, S. T.; Bolotin, K. I.

    2014-01-01

    The optical response of semiconducting monolayer transition-metal dichalcogenides (TMDCs) is dominated by strongly bound excitons that are stable even at room temperature. However, substrate-related effects such as screening and disorder in currently available specimens mask many anticipated physical phenomena and limit device applications of TMDCs. Here, we demonstrate that that these undesirable effects are strongly suppressed in suspended devices. Extremely robust (photogain > 1,000) and fast (response time < 1 ms) photoresponse allow us to study, for the first time, the formation, binding energies, and dissociation mechanisms of excitons in TMDCs through photocurrent spectroscopy. By analyzing the spectral positions of peaks in the photocurrent and by comparing them with first-principles calculations, we obtain binding energies, band gaps and spin-orbit splitting in monolayer TMDCs. For monolayer MoS2, in particular, we obtain an extremely large binding energy for band-edge excitons, Ebind ≥ 570 meV. Along with band-edge excitons, we observe excitons associated with a van Hove singularity of rather unique nature. The analysis of the source-drain voltage dependence of photocurrent spectra reveals exciton dissociation and photoconversion mechanisms in TMDCs. PMID:25318849

  2. Ultrafast formation of interlayer hot excitons in atomically thin MoS2/WS2 heterostructures.

    PubMed

    Chen, Hailong; Wen, Xiewen; Zhang, Jing; Wu, Tianmin; Gong, Yongji; Zhang, Xiang; Yuan, Jiangtan; Yi, Chongyue; Lou, Jun; Ajayan, Pulickel M; Zhuang, Wei; Zhang, Guangyu; Zheng, Junrong

    2016-01-01

    Van der Waals heterostructures composed of two-dimensional transition-metal dichalcogenides layers have recently emerged as a new family of materials, with great potential for atomically thin opto-electronic and photovoltaic applications. It is puzzling, however, that the photocurrent is yielded so efficiently in these structures, despite the apparent momentum mismatch between the intralayer/interlayer excitons during the charge transfer, as well as the tightly bound nature of the excitons in 2D geometry. Using the energy-state-resolved ultrafast visible/infrared microspectroscopy, we herein obtain unambiguous experimental evidence of the charge transfer intermediate state with excess energy, during the transition from an intralayer exciton to an interlayer exciton at the interface of a WS2/MoS2 heterostructure, and free carriers moving across the interface much faster than recombining into the intralayer excitons. The observations therefore explain how the remarkable charge transfer rate and photocurrent generation are achieved even with the aforementioned momentum mismatch and excitonic localization in 2D heterostructures and devices. PMID:27539942

  3. Ultrafast formation of interlayer hot excitons in atomically thin MoS2/WS2 heterostructures

    PubMed Central

    Chen, Hailong; Wen, Xiewen; Zhang, Jing; Wu, Tianmin; Gong, Yongji; Zhang, Xiang; Yuan, Jiangtan; Yi, Chongyue; Lou, Jun; Ajayan, Pulickel M.; Zhuang, Wei; Zhang, Guangyu; Zheng, Junrong

    2016-01-01

    Van der Waals heterostructures composed of two-dimensional transition-metal dichalcogenides layers have recently emerged as a new family of materials, with great potential for atomically thin opto-electronic and photovoltaic applications. It is puzzling, however, that the photocurrent is yielded so efficiently in these structures, despite the apparent momentum mismatch between the intralayer/interlayer excitons during the charge transfer, as well as the tightly bound nature of the excitons in 2D geometry. Using the energy-state-resolved ultrafast visible/infrared microspectroscopy, we herein obtain unambiguous experimental evidence of the charge transfer intermediate state with excess energy, during the transition from an intralayer exciton to an interlayer exciton at the interface of a WS2/MoS2 heterostructure, and free carriers moving across the interface much faster than recombining into the intralayer excitons. The observations therefore explain how the remarkable charge transfer rate and photocurrent generation are achieved even with the aforementioned momentum mismatch and excitonic localization in 2D heterostructures and devices. PMID:27539942

  4. A Comparison Between Magnetic Field Effects in Excitonic and Exciplex Organic Light-Emitting Diodes

    NASA Astrophysics Data System (ADS)

    Sahin Tiras, Kevser; Wang, Yifei; Harmon, Nicholas J.; Wohlgenannt, Markus; Flatte, Michael E.

    In flat-panel displays and lighting applications, organic light emitting diodes (OLEDs) have been widely used because of their efficient light emission, low-cost manufacturing and flexibility. The electrons and holes injected from the anode and cathode, respectively, form a tightly bound exciton as they meet at a molecule in organic layer. Excitons occur as spin singlets or triplets and the ratio between singlet and triplet excitons formed is 1:3 based on spin degeneracy. The internal quantum efficiency (IQE) of fluorescent-based OLEDs is limited 25% because only singlet excitons contribute the light emission. To overcome this limitation, thermally activated delayed fluorescent (TADF) materials have been introduced in the field of OLEDs. The exchange splitting between the singlet and triplet states of two-component exciplex systems is comparable to the thermal energy in TADF materials, whereas it is usually much larger in excitons. Reverse intersystem crossing occurs from triplet to singlet exciplex state, and this improves the IQE. An applied small magnetic field can change the spin dynamics of recombination in TADF blends. In this study, magnetic field effects on both excitonic and exciplex OLEDs will be presented and comparison similarities and differences will be made.

  5. Optical spectroscopy of free excitons in a CuInS{sub 2} chalcopyrite semiconductor compound

    SciTech Connect

    Mudryi, A. V. Ivanyukovich, A. V.; Yakushev, M. V.; Martin, R.; Saad, A.

    2008-01-15

    The spectra of reflectance and luminescence of high-quality CuInS{sub 2} single crystals grown by oriented crystallization are studied at the temperature 4.2 K. In the region of the fundamental absorption edge, the two excitonic resonance reflectance peaks, nondegenerate peak A at the energy {approx}1.5356 eV and doubly degenerate peak BC at the energy {approx}1.5567 eV, and the luminescence signal produced by free and bound excitons are observed. The luminescence lines, A{sub UPB} at {approx}1.5361 eV and A{sub LPB} at {approx}1.5347 eV, with a half-width {approx}1 meV, are attributed to exciton-polariton recombination. From the experimentally observed energy position of the exciton ground state and excited states, the binding energy of free excitons is determined to be {approx}18.5 meV. In studying the photoluminescence in magnetic fields up to 10 T, a diamagnetic shift of the ground state of free excitons A is observed.

  6. Probing excitonic states in suspended two-dimensional semiconductors by photocurrent spectroscopy

    NASA Astrophysics Data System (ADS)

    Klots, A. R.; Newaz, A. K. M.; Wang, Bin; Prasai, D.; Krzyzanowska, H.; Lin, Junhao; Caudel, D.; Ghimire, N. J.; Yan, J.; Ivanov, B. L.; Velizhanin, K. A.; Burger, A.; Mandrus, D. G.; Tolk, N. H.; Pantelides, S. T.; Bolotin, K. I.

    2014-10-01

    The optical response of semiconducting monolayer transition-metal dichalcogenides (TMDCs) is dominated by strongly bound excitons that are stable even at room temperature. However, substrate-related effects such as screening and disorder in currently available specimens mask many anticipated physical phenomena and limit device applications of TMDCs. Here, we demonstrate that that these undesirable effects are strongly suppressed in suspended devices. Extremely robust (photogain > 1,000) and fast (response time < 1 ms) photoresponse allow us to study, for the first time, the formation, binding energies, and dissociation mechanisms of excitons in TMDCs through photocurrent spectroscopy. By analyzing the spectral positions of peaks in the photocurrent and by comparing them with first-principles calculations, we obtain binding energies, band gaps and spin-orbit splitting in monolayer TMDCs. For monolayer MoS2, in particular, we obtain an extremely large binding energy for band-edge excitons, Ebind >= 570 meV. Along with band-edge excitons, we observe excitons associated with a van Hove singularity of rather unique nature. The analysis of the source-drain voltage dependence of photocurrent spectra reveals exciton dissociation and photoconversion mechanisms in TMDCs.

  7. Spatially indirect excitons in coupled quantum wells

    SciTech Connect

    Lai, Chih-Wei Eddy

    2004-03-01

    Microscopic quantum phenomena such as interference or phase coherence between different quantum states are rarely manifest in macroscopic systems due to a lack of significant correlation between different states. An exciton system is one candidate for observation of possible quantum collective effects. In the dilute limit, excitons in semiconductors behave as bosons and are expected to undergo Bose-Einstein condensation (BEC) at a temperature several orders of magnitude higher than for atomic BEC because of their light mass. Furthermore, well-developed modern semiconductor technologies offer flexible manipulations of an exciton system. Realization of BEC in solid-state systems can thus provide new opportunities for macroscopic quantum coherence research. In semiconductor coupled quantum wells (CQW) under across-well static electric field, excitons exist as separately confined electron-hole pairs. These spatially indirect excitons exhibit a radiative recombination time much longer than their thermal relaxation time a unique feature in direct band gap semiconductor based structures. Their mutual repulsive dipole interaction further stabilizes the exciton system at low temperature and screens in-plane disorder more effectively. All these features make indirect excitons in CQW a promising system to search for quantum collective effects. Properties of indirect excitons in CQW have been analyzed and investigated extensively. The experimental results based on time-integrated or time-resolved spatially-resolved photoluminescence (PL) spectroscopy and imaging are reported in two categories. (i) Generic indirect exciton systems: general properties of indirect excitons such as the dependence of exciton energy and lifetime on electric fields and densities were examined. (ii) Quasi-two-dimensional confined exciton systems: highly statistically degenerate exciton systems containing more than tens of thousands of excitons within areas as small as (10 micrometer){sup 2} were

  8. Exciton formation in monolayer transition metal dichalcogenides.

    PubMed

    Ceballos, Frank; Cui, Qiannan; Bellus, Matthew Z; Zhao, Hui

    2016-06-01

    Two-dimensional transition metal dichalcogenides provide a unique platform to study excitons in confined structures. Recently, several important aspects of excitons in these materials have been investigated in detail. However, the formation process of excitons from free carriers has yet to be understood. Here we report time-resolved measurements on the exciton formation process in monolayer samples of MoS2, MoSe2, WS2, and WSe2. The free electron-hole pairs, injected by an ultrashort laser pulse, immediately induce a transient absorption signal of a probe pulse tuned to the exciton resonance. The signal quickly drops by about a factor of two within 1 ps and is followed by a slower decay process. In contrast, when excitons are resonantly injected, the fast decay component is absent. Based both on its excitation excess energy and intensity dependence, this fast decay process is attributed to the formation of excitons from the electron-hole pairs. This interpretation is also consistent with a model that shows how free electron-hole pairs can be about twice more effective than excitons in altering the exciton absorption strength. From our measurements and analysis of our results, we determined that the exciton formation times in these monolayers to be shorter than 1 ps. PMID:27219022

  9. Contrastive analysis of multiple exciton generation theories

    NASA Astrophysics Data System (ADS)

    Tan, Hengyu; Chang, Qing

    2015-10-01

    Multiple exciton generation (MEG) is an effect that semiconductor nanocrystals (NCs) quantum dots (QDs) generate multiple excitons (electron-hole pairs) through absorbing a single high energy photon. It can translate the excess photon energy of bandgap (Eg) into new excitons instead of heat loss and improve the photovoltaic performance of solar cells. However, the theories of MEG are not uniform. The main MEG theories can be divided into three types. The first is impact ionization. It explains MEG through a conventional way that a photogenerated exciton becomes multiple excitons by Coulomb interactions between carriers. The Second is coherent superposition of excitonic states. Multiple excitons are generated by the coherent superposition of single photogenerated exciton state with enough excess momentum and the two-exciton state with the same momentum. The third is excitation via virtual excitonic states. The nanocrystals vacuum generates a virtual biexciton by coulomb coupling between two valence band electrons. The virtual biexciton absorbing a photon with an intraband optical transition is converted into a real biexciton. This paper describes the MEG influence on solar photoelectric conversion efficiency, concludes and analyzes the fundamentals of different MEG theories, the MEG experimental measure, their merits and demerits, calculation methods of generation efficiency.

  10. Exciton formation in monolayer transition metal dichalcogenides

    NASA Astrophysics Data System (ADS)

    Ceballos, Frank; Cui, Qiannan; Bellus, Matthew Z.; Zhao, Hui

    2016-06-01

    Two-dimensional transition metal dichalcogenides provide a unique platform to study excitons in confined structures. Recently, several important aspects of excitons in these materials have been investigated in detail. However, the formation process of excitons from free carriers has yet to be understood. Here we report time-resolved measurements on the exciton formation process in monolayer samples of MoS2, MoSe2, WS2, and WSe2. The free electron-hole pairs, injected by an ultrashort laser pulse, immediately induce a transient absorption signal of a probe pulse tuned to the exciton resonance. The signal quickly drops by about a factor of two within 1 ps and is followed by a slower decay process. In contrast, when excitons are resonantly injected, the fast decay component is absent. Based both on its excitation excess energy and intensity dependence, this fast decay process is attributed to the formation of excitons from the electron-hole pairs. This interpretation is also consistent with a model that shows how free electron-hole pairs can be about twice more effective than excitons in altering the exciton absorption strength. From our measurements and analysis of our results, we determined that the exciton formation times in these monolayers to be shorter than 1 ps.

  11. Two-exciton excited states of J-aggregates in the presence of exciton-exciton annihilation

    NASA Astrophysics Data System (ADS)

    Levinsky, B.; Fainberg, B. D.; Nesterov, L. A.; Rosanov, N. N.

    2016-07-01

    We study decay of two-exciton states of a J-aggregate that is collective in nature. We use mathematical formalism based on effective non-Hermitian Hamiltonian suggested in nuclear theory. We show that decay of two-exciton states is strongly affected by the interference processes in the exciton-exciton annihilation. Our evaluations of the imaginary part of the effective Hamiltonian show that it exceeds the spacing between real energies of the two-exciton states that gives rise to the transition to the regime of overlapping resonances supplying the system by the new collectivity - the possibility of coherent decay in the annihilation channel. The decay of two-exciton states varies from twice bimolecular decay rate to the much smaller values that is associated with population trapping. We have also considered the corresponding experiment in the framework of our approach, the picture of which appears to be more complex and richer than it was reasoned before.

  12. Bright Solid-State Emission of Disilane-Bridged Donor-Acceptor-Donor and Acceptor-Donor-Acceptor Chromophores.

    PubMed

    Shimada, Masaki; Tsuchiya, Mizuho; Sakamoto, Ryota; Yamanoi, Yoshinori; Nishibori, Eiji; Sugimoto, Kunihisa; Nishihara, Hiroshi

    2016-02-01

    The development of disilane-bridged donor-acceptor-donor (D-Si-Si-A-Si-Si-D) and acceptor-donor-acceptor (A-Si-Si-D-Si-Si-A) compounds is described. Both types of compound showed strong emission (λem =ca. 500 and ca. 400 nm, respectively) in the solid state with high quantum yields (Φ: up to 0.85). Compound 4 exhibited aggregation-induced emission enhancement in solution. X-ray diffraction revealed that the crystal structures of 2, 4, and 12 had no intermolecular π-π interactions to suppress the nonradiative transition in the solid state. PMID:26822564

  13. Analytical approach to excitonic properties of MoS2

    NASA Astrophysics Data System (ADS)

    Berghäuser, Gunnar; Malic, Ermin

    2014-03-01

    We present an analytical investigation of the optical absorption spectrum of monolayer molybdenum-disulfide. Based on the density matrix formalism, our approach gives insights into the microscopic origin of excitonic transitions, their relative oscillator strength, and binding energy. We show analytical expressions for the carrier-light coupling element, which contains the optical selection rules and describes well the valley-selective polarization in MoS2. In agreement with experimental results, we find the formation of strongly bound electron-hole pairs due to the efficient Coulomb interaction. The absorption spectrum of MoS2 features two pronounced peaks corresponding to the A and B exciton. For MoS2 on a SiO2 substrate, these are characterized by binding energies of 455 meV and 465 meV, respectively. Our calculations reveal their relative oscillator strength and predict the appearance of further low-intensity excitonic transitions at higher energies. The presented approach is applicable to other transition metal dichalcogenides and can be extended to investigations of trion and biexcitonic effects.

  14. The dissociation of excitons at indium tin oxide-copper phthalocyanine interface in organic solar cells

    NASA Astrophysics Data System (ADS)

    Sun, X. Y.; Song, Q. L.; Wang, M. L.; Ding, X. M.; Hou, X. Y.; Zhou, Z. G.; Li, F. Y.

    2008-11-01

    Exciton dissociation process at indium tin oxide (ITO)/copper phthalocyanine (CuPc) interface of ITO/CuPc(370 nm)/Al is studied by transient photovoltage method. A negative-to-positive change in the polarity of photovoltage upon pulsed laser irradiation is observed in CuPc thin film. The polarity change is regarded as a summation of the effect of exciton dissociation at ITO/CuPc interface (fast process) and that of free carrier separation by built-in field (slow process). Further experiments confirm the existence of exciton dissociation at ITO/CuPc interface, and the direction of which is electron injected into ITO, with holes left in CuPc film. This is opposite to that of the interfacial dissociation at donor/acceptor (D/A) interface in single heterojunction cells (ITO/D/A/buffer/Al). 3-nm-thick LiF insulating layer is inserted between ITO and CuPc to inhibit the exciton dissociation at ITO/CuPc interface. Thereby, the open-circuit voltage and power conversion efficiency of the single layer cell have been increased by several times.

  15. A design strategy for intramolecular singlet fission mediated by charge-transfer states in donor-acceptor organic materials

    NASA Astrophysics Data System (ADS)

    Busby, Erik; Xia, Jianlong; Wu, Qin; Low, Jonathan Z.; Song, Rui; Miller, John R.; Zhu, X.-Y.; Campos, Luis M.; Sfeir, Matthew Y.

    2015-04-01

    The ability to advance our understanding of multiple exciton generation (MEG) in organic materials has been restricted by the limited number of materials capable of singlet fission. A particular challenge is the development of materials that undergo efficient intramolecular fission, such that local order and strong nearest-neighbour coupling is no longer a design constraint. Here we address these challenges by demonstrating that strong intrachain donor-acceptor interactions are a key design feature for organic materials capable of intramolecular singlet fission. By conjugating strong-acceptor and strong-donor building blocks, small molecules and polymers with charge-transfer states that mediate population transfer between singlet excitons and triplet excitons are synthesized. Using transient optical techniques, we show that triplet populations can be generated with yields up to 170%. These guidelines are widely applicable to similar families of polymers and small molecules, and can lead to the development of new fission-capable materials with tunable electronic structure, as well as a deeper fundamental understanding of MEG.

  16. A design strategy for intramolecular singlet fission mediated by charge-transfer states in donor-acceptor organic materials.

    PubMed

    Busby, Erik; Xia, Jianlong; Wu, Qin; Low, Jonathan Z; Song, Rui; Miller, John R; Zhu, X-Y; Campos, Luis M; Sfeir, Matthew Y

    2015-04-01

    The ability to advance our understanding of multiple exciton generation (MEG) in organic materials has been restricted by the limited number of materials capable of singlet fission. A particular challenge is the development of materials that undergo efficient intramolecular fission, such that local order and strong nearest-neighbour coupling is no longer a design constraint. Here we address these challenges by demonstrating that strong intrachain donor-acceptor interactions are a key design feature for organic materials capable of intramolecular singlet fission. By conjugating strong-acceptor and strong-donor building blocks, small molecules and polymers with charge-transfer states that mediate population transfer between singlet excitons and triplet excitons are synthesized. Using transient optical techniques, we show that triplet populations can be generated with yields up to 170%. These guidelines are widely applicable to similar families of polymers and small molecules, and can lead to the development of new fission-capable materials with tunable electronic structure, as well as a deeper fundamental understanding of MEG. PMID:25581625

  17. Excitonic transport in ZnO

    NASA Astrophysics Data System (ADS)

    Noltemeyer, Martin; Bertram, Frank; Hempel, Thomas; Bastek, Barbara; Christen, Juergen; Brandt, Matthias; Lorenz, Michael; Grundmann, Marius

    2012-02-01

    The temperature dependence of diffusion length and lifetime or diffusivity of the free exciton is measured in a commercial ZnO-substrate and in an epitaxial ZnO quantum well using nm-spatially and ps-time resolved cathodoluminescence (CL) spectroscopy. The characteristic temperature dependence of the exciton mobility is a fingerprint of the underlying excitonic scattering processes. Since excitons are neutral particles scattering at ionized impurities should be not effective. With decreasing temperature diffusion lengths and lifetimes give rise to a monotonous increase of the excitonic mobility. Two different methods for determining the excitonic transport parameters will be presented. On the one hand we are able to perform completely pulsed excitation experiments and on the other hand a combination of cw- and pulsed excitation in two independent measurements are needed.

  18. Exciton Seebeck effect in molecular systems

    SciTech Connect

    Yan, Yun-An; Cai, Shaohong

    2014-08-07

    We investigate the exciton dynamics under temperature difference with the hierarchical equations of motion. Through a nonperturbative simulation of the transient absorption of a heterogeneous trimer model, we show that the temperature difference causes exciton population redistribution and affects the exciton transfer time. It is found that one can reproduce not only the exciton population redistribution but also the change of the exciton transfer time induced by the temperature difference with a proper tuning of the site energies of the aggregate. In this sense, there exists a site energy shift equivalence for any temperature difference in a broad range. This phenomenon is similar to the Seebeck effect as well as spin Seebeck effect and can be named as exciton Seebeck effect.

  19. Interface-split Kramers doublets for acceptor-based qubits in silicon

    NASA Astrophysics Data System (ADS)

    Mol, Jan; Salfi, Joseph; Rahman, Rajib; Rogge, Sven

    2013-03-01

    Single dopants in silicon form a particular attractive platform for hosting spin quantum bits (qubits). The effective spin-3/2 states of acceptor-bound holes in silicon can be used to store bits of quantum information for several μs. Strong coupling of spin and momentum in the silicon valence band allows for rapid electrical manipulation of the hole spin. Acceptors in silicon have a four-fold degenerate ground-state, reflecting character of the top of the valence band. Symmetry breaking, by an electric field, strain or confinement, lifts this degeneracy, resulting in two Kramers doublets. The states within these isolated Kramers doublets are protected against decoherence by time reversal symmetry and form the working levels of a hole spin qubit. Here we investigate the effect of the presence of an interface on the ground-state energy splitting of individual sub-surface acceptors, as a function of dopant depth, by means of low temperature scanning tunneling spectroscopy. The depth of individual acceptors is determined by probing the Coulomb potential of the ionized acceptor nuclei. Resonant tunneling through the localized acceptor states provides a direct measure of the excited state spectrum of single dopants.

  20. Localized Excitons in Carbon Nanotubes.

    NASA Astrophysics Data System (ADS)

    Adamska, Lyudmyla; Doorn, Stephen K.; Tretiak, Sergei

    2015-03-01

    It has been historically known that unintentional defects in carbon nanotubes (CNTs) may fully quench the fluorescence. However, some dopants may enhance the fluorescence by one order of magnitude thus turning the CNTs, which are excellent light absorbers, in good emitters. We have correlated the experimentally observed photoluminescence spectra to the electronic structure simulations. Our experiment reveals multiple sharp asymmetric emission peaks at energies 50-300 meV red-shifted from that of the lowest bright exciton peak. Our simulations suggest an association of these peaks with deep trap states tied to different specific chemical adducts. While the wave functions of excitons in undoped CNTs are delocalized, those of the deep-trap states are strongly localized and pinned to the dopants. These findings are consistent with the experimental observation of asymmetric broadening of the deep trap emission peaks, which can result from scattering of acoustic phonons on localized excitons. Our work lays the foundation to utilize doping as a generalized route for wave function engineering and direct control of carrier dynamics in SWCNTs toward enhanced light emission properties for photonic applications.

  1. Excitons in Organics Using Time-Dependent Density Functional Theory: PPV, Pentacene, and Picene.

    PubMed

    Sharma, S; Dewhurst, J K; Shallcross, S; Madjarova, G K; Gross, E K U

    2015-04-14

    We apply the bootstrap kernel within time-dependent density functional theory to study the one-dimensional chain of polymer polyphenylenevinylene and molecular crystals of picene and pentacene. The absorption spectra of poly(p-phenylenevinylene) has a bound excitonic peak that is well-reproduced. Pentacene and picene, electronically similar materials, have remarkably different excitonic physics, and this difference is also well captured. We show that the inclusion of local-field effects dramatically changes the spectra of both picene and pentacene but not for poly(p-phenylenevinylene). PMID:26574381

  2. Exciton Binding Energy of Monolayer WS2

    PubMed Central

    Zhu, Bairen; Chen, Xi; Cui, Xiaodong

    2015-01-01

    The optical properties of monolayer transition metal dichalcogenides (TMDC) feature prominent excitonic natures. Here we report an experimental approach to measuring the exciton binding energy of monolayer WS2 with linear differential transmission spectroscopy and two-photon photoluminescence excitation spectroscopy (TP-PLE). TP-PLE measurements show the exciton binding energy of 0.71 ± 0.01 eV around K valley in the Brillouin zone. PMID:25783023

  3. Charge-transfer excitons in DNA.

    PubMed

    Conwell, E M; McLaughlin, P M; Bloch, S M

    2008-02-21

    There have been a number of theoretical treatments of excitons in DNA, most neglecting both the intrachain and interchain wavefunction overlaps of the electron and hole, treating them as Frenkel excitons. Recently, the importance of the intrachain and interchain coupling has been highlighted. Experiments have shown that in (dA)n oligomers and in duplex (dA)n.(dT)n, to be abbreviated (A/T), where A is adenine and T is thymine, the exciton wavefunction is delocalized over several bases. In duplexes it is possible to have charge-transfer (CT) excitons. Theoretical calculations have suggested that CT excitons in DNA may have lower energy than single chain excitons. In all the calculations of excitons in DNA, the polarization of the surrounding water has been neglected. Calculations have shown, however, that polarization of the water by an excess electron or a hole in DNA lowers its energy by approximately 1/2 eV, causing it to become a polaron. It is therefore to be expected that polarization charge induced in the surrounding water has a significant effect on the properties of the exciton. In what follows, we present calculations of some properties CT excitons would have in an A/T duplex taking into account the wavefunction overlaps, the effect of the surrounding water, which results in the electron and hole becoming polarons, and the ions in the water. As expected, the CT exciton has lowest energy when the electron and hole polarons are directly opposite each other. By appropriate choice of the dielectric constant, we can obtain a CT exciton delocalized over the number of sites found in photoinduced absorption experiments. The absorption threshold that we then calculate for CT exciton creation in A/T is in reasonable agreement with the lowest singlet absorption deduced from available data. PMID:18232682

  4. Macrocrystals of Colloidal Quantum Dots in Anthracene: Exciton Transfer and Polarized Emission

    NASA Astrophysics Data System (ADS)

    Soran-Erdem, Zeliha; Erdem, Talha; Hernandez-Martinez, Pedro Ludwig; Akgul, Mehmet Zafer; Gaponik, Nikolai; Demir, Hilmi Volkan

    We systematically investigate the exciton energy transfer from anthracene host (donor) to quantum dots (acceptor) in a centimeter-scale macrocrystal of nonpolar colloidal quantum dots incorporated into anthracene. The decrease in photoluminescence lifetime of the donor anthracene indicate a strong energy transfer with increasing quantum dot concentration in the macrocrystals. In addition, anisotropic emission from the isotropic quantum dots in anthracene macrocrystals was observed. The quantum dots inside the anthracene host acquired a polarization ratio of ~1.5 at 0 degree collection angle, and this increases to ~2.5 at the collection angle of 60 degree. Finally, a proof-of-concept application of these excitonic macrocrystals as tunable color converters was employed in light-emitting diodes. Bilkent University.

  5. Distinct exciton dissociation behavior of organolead trihalide perovskite and excitonic semiconductors studied in a same device

    DOE PAGESBeta

    Hu, Miao; Bi, Cheng; Yuan, Yongbo; Xiao, Zhengguo; Dong, Qingfeng; Shao, Yuchuan; Huang, Jinsong

    2015-01-15

    The nonexcitonic character for organometal trihalide perovskites is demonstrated by examining the field-dependent exciton dissociation behavior. Moreover, it is found that photogenerated excitons can be effectively dissociated into free charges inside perovskite without the assistance of charge extraction layer or external field, which is a stark contrast to the charge-separation behavior in excitonic materials in the same photovoltaic operation system.

  6. Cross-polarized excitons in carbon nanotubes.

    PubMed

    Kilina, Svetlana; Tretiak, Sergei; Doorn, Stephen K; Luo, Zhengtang; Papadimitrakopoulos, Fotios; Piryatinski, Andrei; Saxena, Avadh; Bishop, Alan R

    2008-05-13

    Polarization of low-lying excitonic bands in finite-size semiconducting single-walled carbon nanotubes (SWNTs) is studied by using quantum-chemical methodologies. Our calculations elucidate properties of cross-polarized excitons, which lead to the transverse optical absorption of nanotubes and presumably couple to intermediate-frequency modes recently observed in resonance Raman excitation spectroscopy. We identify up to 12 distinct excitonic transitions below the second fundamental band associated with the E(22) van Hove singularity. Calculations for several chiral SWNTs distinguish the optically active "bright" excitonic band polarized parallel to the tube axis and several optically "weak" cross-polarized excitons. The rest are optically (near) forbidden "dark" transitions. An analysis of the transition density matrices related to excitonic bands provides detailed information about delocalization of excitonic wavefunction along the tube. Utilization of the natural helical coordinate system accounting for the tube chirality allows one to disentangle longitudinal and circumferential components. The distribution of the transition density matrix along a tube axis is similar for all excitons. However, four parallel-polarized excitons associated with the E(11) transition are more localized along the circumference of a tube, compared with others related to the E(12) and E(21) cross-polarized transitions. Calculated splitting between optically active parallel- and cross-polarized transitions increases with tube diameter, which compares well with experimental spectroscopic data. PMID:18463293

  7. Exciton spin dynamics in GaSe

    SciTech Connect

    Tang, Yanhao; Xie, Wei; McGuire, John A. Lai, Chih Wei; Mandal, Krishna C.

    2015-09-21

    We analyze exciton spin dynamics in GaSe under nonresonant circularly polarized optical pumping with an exciton spin-flip rate-equation model. The model reproduces polarized time-dependent photoluminescence measurements in which the initial circular polarization approaches unity even when pumping with 0.15 eV excess energy. At T = 10 K, the exciton spin relaxation exhibits a biexponential decay with sub-20 ps and >500 ps time constants, which are also reproduced by the rate-equation model assuming distinct spin-relaxation rates for hot (nonequilibrium) and cold band-edge excitons.

  8. Plasmon and Exciton Coupling and Purcell Enhancement

    NASA Astrophysics Data System (ADS)

    Rice, Quinton; Rigo, Maria Veronica; Fudala, Rafal; Cho, Hyoyeong; Kim, Wan-Joong; Rich, Ryan; Tabibi, Bagher; Gryczynski, Zygmunt; Gryczynski, Ignacy; Yu, William; Seo, Jaetae

    2014-05-01

    The photoluminescence from plasmon-coupled exciton is of great interest for optoelectronic applications, because of the large quantum yield with localized field enhancement and reduced nonradiative transition. The Coulomb interaction through plasmon-exciton coupling results in the Purcell enhancement of quantum dots (QDs) in the vicinity of metal nanoparticles (MNPs). With plasmon-exciton coupling, the radiative and non-radiative decay rates and the coupling rates compete with each other. The coupling rate is closely related to the coupling distance between QDs and MNPs. The optimized coupling distance scales the re-excitation density of localized fields and the plasmon-exciton coupling rates. If the plasmon-exciton coupling rate is much faster than the radiative and non-radiative transitions of excitons, the re-excitations of excitons by the localized plasmonic field and the reduction of non-radiative transitions may occur. This presentation includes plasmon-exciton coupling dynamics, large enhancement and temporal properties of PL, and dipole-PL polarization fidelity of hybrid optical materials of plasmonic nanometals and excitonic semiconductor QDs. The work at Hampton University was supported by the National Science Foundation (NSF HRD-1137747), and Army Research Office (ARO W911NF-11-1-0177). The work at University of North Texas was supported by National Institutes of Health (NIH R01EB12003, and 5R21CA14897 (Z.G.)).

  9. Cross-polarized excitons in carbon nanotubes

    PubMed Central

    Kilina, Svetlana; Tretiak, Sergei; Doorn, Stephen K.; Luo, Zhengtang; Papadimitrakopoulos, Fotios; Piryatinski, Andrei; Saxena, Avadh; Bishop, Alan R.

    2008-01-01

    Polarization of low-lying excitonic bands in finite-size semiconducting single-walled carbon nanotubes (SWNTs) is studied by using quantum-chemical methodologies. Our calculations elucidate properties of cross-polarized excitons, which lead to the transverse optical absorption of nanotubes and presumably couple to intermediate-frequency modes recently observed in resonance Raman excitation spectroscopy. We identify up to 12 distinct excitonic transitions below the second fundamental band associated with the E22 van Hove singularity. Calculations for several chiral SWNTs distinguish the optically active “bright” excitonic band polarized parallel to the tube axis and several optically “weak” cross-polarized excitons. The rest are optically (near) forbidden “dark” transitions. An analysis of the transition density matrices related to excitonic bands provides detailed information about delocalization of excitonic wavefunction along the tube. Utilization of the natural helical coordinate system accounting for the tube chirality allows one to disentangle longitudinal and circumferential components. The distribution of the transition density matrix along a tube axis is similar for all excitons. However, four parallel-polarized excitons associated with the E11 transition are more localized along the circumference of a tube, compared with others related to the E12 and E21 cross-polarized transitions. Calculated splitting between optically active parallel- and cross-polarized transitions increases with tube diameter, which compares well with experimental spectroscopic data. PMID:18463293

  10. Ground state energy of N Frenkel excitons

    NASA Astrophysics Data System (ADS)

    Pogosov, W.; Combescot, M.

    2009-03-01

    By using the composite many-body theory for Frenkel excitons we have recently developed, we here derive the ground state energy of N Frenkel excitons in the Born approximation through the Hamiltonian mean value in a state made of N identical Q = 0 excitons. While this quantity reads as a density expansion in the case of Wannier excitons, due to many-body effects induced by fermion exchanges between N composite particles, we show that the Hamiltonian mean value for N Frenkel excitons only contains a first order term in density, just as for elementary bosons. Such a simple result comes from a subtle balance, difficult to guess a priori, between fermion exchanges for two or more Frenkel excitons appearing in Coulomb term and the ones appearing in the N exciton normalization factor - the cancellation being exact within terms in 1/Ns where Ns is the number of atomic sites in the sample. This result could make us naively believe that, due to the tight binding approximation on which Frenkel excitons are based, these excitons are just bare elementary bosons while their composite nature definitely appears at various stages in the precise calculation of the Hamiltonian mean value.

  11. Acceptor Products of Alternansucrase with Gentiobiose

    Technology Transfer Automated Retrieval System (TEKTRAN)

    In the presence of suitable acceptor molecules, dextransucrase makes a homologous series of oligosaccharides in which the isomers differ by a single glucosyl unit, whereas alternansucrase synthesizes one trisaccharide, two tetrasaccharides, etc. Previously, we showed that alternansucrase only forms...

  12. Synthetic CO.sub.2 acceptor

    DOEpatents

    Lancet, Michael S.; Curran, George P.

    1981-08-18

    A synthetic CO.sub.2 acceptor consisting essentially of at least one compound selected from the group consisting of calcium oxide and calcium carbonate supported in a refractory carrier matrix, the carrier having the general formula Ca.sub.5 (SiO.sub.4).sub.2 CO.sub.3. A method for producing the synthetic CO.sub.2 acceptor is also disclosed.

  13. Features of exciton dynamics in molecular nanoclusters (J-aggregates): Exciton self-trapping (Review Article)

    NASA Astrophysics Data System (ADS)

    Malyukin, Yu. V.; Sorokin, A. V.; Semynozhenko, V. P.

    2016-06-01

    We present thoroughly analyzed experimental results that demonstrate the anomalous manifestation of the exciton self-trapping effect, which is already well-known in bulk crystals, in ordered molecular nanoclusters called J-aggregates. Weakly-coupled one-dimensional (1D) molecular chains are the main structural feature of J-aggregates, wherein the electron excitations are manifested as 1D Frenkel excitons. According to the continuum theory of Rashba-Toyozawa, J-aggregates can have only self-trapped excitons, because 1D excitons must adhere to barrier-free self-trapping at any exciton-phonon coupling constant g = ɛLR/2β, wherein ɛLR is the lattice relaxation energy, and 2β is the half-width of the exciton band. In contrast, very often only the luminescence of free, mobile excitons would manifest in experiments involving J-aggregates. Using the Urbach rule in order to analyze the low-frequency region of the low-temperature exciton absorption spectra has shown that J-aggregates can have both a weak (g < 1) and a strong (g > 1) exciton-phonon coupling. Moreover, it is experimentally demonstrated that under certain conditions, the J-aggregate excited state can have both free and self-trapped excitons, i.e., we establish the existence of a self-trapping barrier for 1D Frenkel excitons. We demonstrate and analyze the reasons behind the anomalous existence of both free and self-trapped excitons in J-aggregates, and demonstrate how exciton-self trapping efficiency can be managed in J-aggregates by varying the values of g, which is fundamentally impossible in bulk crystals. We discuss how the exciton-self trapping phenomenon can be used as an alternate interpretation of the wide band emission of some J-aggregates, which has thus far been explained by the strongly localized exciton model.

  14. Correlation between charge transfer exciton recombination and photocurrent in polymer/fullerene solar cells

    SciTech Connect

    Hallermann, Markus; Da Como, Enrico; Feldmann, Jochen; Izquierdo, Marta; Filippone, Salvatore; Martin, Nazario; Juechter, Sabrina; Hauff, Elizabeth von

    2010-07-12

    We correlate carrier recombination via charge transfer excitons (CTEs) with the short circuit current, J{sub sc}, in polymer/fullerene solar cells. Near infrared photoluminescence spectroscopy of CTE in three blends differing for the fullerene acceptor, gives unique insights into solar cell characteristics. The energetic position of the CTE is directly correlated with the open-circuit voltage, V{sub oc}, and more important J{sub sc} decreases with increasing CTE emission intensity. CTE emission intensity is discussed from the perspective of blend morphology. The work points out the fundamental role of CTE recombination and how optical spectroscopy can be used to derive information on solar cell performances.

  15. Achieving effective terminal exciton delivery in quantum dot antenna-sensitized multistep DNA photonic wires.

    PubMed

    Spillmann, Christopher M; Ancona, Mario G; Buckhout-White, Susan; Algar, W Russ; Stewart, Michael H; Susumu, Kimihiro; Huston, Alan L; Goldman, Ellen R; Medintz, Igor L

    2013-08-27

    Assembling DNA-based photonic wires around semiconductor quantum dots (QDs) creates optically active hybrid architectures that exploit the unique properties of both components. DNA hybridization allows positioning of multiple, carefully arranged fluorophores that can engage in sequential energy transfer steps while the QDs provide a superior energy harvesting antenna capacity that drives a Förster resonance energy transfer (FRET) cascade through the structures. Although the first generation of these composites demonstrated four-sequential energy transfer steps across a distance >150 Å, the exciton transfer efficiency reaching the final, terminal dye was estimated to be only ~0.7% with no concomitant sensitized emission observed. Had the terminal Cy7 dye utilized in that construct provided a sensitized emission, we estimate that this would have equated to an overall end-to-end ET efficiency of ≤ 0.1%. In this report, we demonstrate that overall energy flow through a second generation hybrid architecture can be significantly improved by reengineering four key aspects of the composite structure: (1) making the initial DNA modification chemistry smaller and more facile to implement, (2) optimizing donor-acceptor dye pairings, (3) varying donor-acceptor dye spacing as a function of the Förster distance R0, and (4) increasing the number of DNA wires displayed around each central QD donor. These cumulative changes lead to a 2 orders of magnitude improvement in the exciton transfer efficiency to the final terminal dye in comparison to the first-generation construct. The overall end-to-end efficiency through the optimized, five-fluorophore/four-step cascaded energy transfer system now approaches 10%. The results are analyzed using Förster theory with various sources of randomness accounted for by averaging over ensembles of modeled constructs. Fits to the spectra suggest near-ideal behavior when the photonic wires have two sequential acceptor dyes (Cy3 and Cy3.5) and

  16. Josephson effects in condensates of excitons and exciton polaritons

    SciTech Connect

    Shelykh, I. A.; Solnyshkov, D. D.; Pavlovic, G.; Malpuech, G.

    2008-07-15

    We analyze theoretically the phenomena related to the Josephson effect for exciton and polariton condensates, taking into account their specific spin degrees of freedom. We distinguish between two types of Josephson effects: the extrinsic effect, related to the coherent tunneling of particles with the same spin between two spatially separated potential traps, and the intrinsic effect, related to the 'tunneling' between different spinor components of the condensate within the same trap. We show that the Josephson effect in the nonlinear regime can lead to nontrivial polarization dynamics and produce spontaneous separation of the condensates with opposite polarization in real space.

  17. Exciton dynamics at a single dislocation in GaN probed by picosecond time-resolved cathodoluminescence

    NASA Astrophysics Data System (ADS)

    Liu, W.; Carlin, J.-F.; Grandjean, N.; Deveaud, B.; Jacopin, G.

    2016-07-01

    We investigate the dynamics of donor bound excitons (D°XA) at T = 10 K around an isolated single edge dislocation in homoepitaxial GaN, using a picosecond time-resolved cathodoluminescence (TR-CL) setup with high temporal and spatial resolutions. An ˜ 1.3 meV dipole-like energy shift of D°XA is observed around the dislocation, induced by the local strain fields. By simultaneously recording the variations of both the exciton lifetime and the CL intensity across the dislocation, we directly assess the dynamics of excitons around the defect. Our observations are well reproduced by a diffusion model. It allows us to deduce an exciton diffusion length of ˜24 nm as well as an effective area of the dislocation with a radius of ˜95 nm, where the recombination can be regarded as entirely non-radiative.

  18. Exciton photoluminescence and energy in a percolation cluster of ZnSe quantum dots as a fractal object

    SciTech Connect

    Bondar, N. V. Brodyn, M. S.

    2012-05-15

    The results of studies of samples containing ZnSe quantum dots with a density corresponding to or considerably higher than the exciton percolation threshold, at which quantum dots form conglomerates, are reported. Excitonic emission from a percolation cluster of bound quantum dots as a fractal object is observed for the first time. Analysis of the structure of the photoluminescence spectra shows that the spectra are determined by the contribution of exciton states that belong to different structural elements of the percolation cluster, specifically, to the skeleton (backbone), dangling (dead) ends, and internal hollow spaces. A qualitative model is proposed to interpret the dependence of the exciton energy in these structural elements on the concentration of quantum dots in the material.

  19. Cavity-less on-chip optomechanics using excitonic transitions in semiconductor heterostructures

    NASA Astrophysics Data System (ADS)

    Okamoto, Hajime; Watanabe, Takayuki; Ohta, Ryuichi; Onomitsu, Koji; Gotoh, Hideki; Sogawa, Tetsuomi; Yamaguchi, Hiroshi

    2015-10-01

    The hybridization of semiconductor optoelectronic devices and nanomechanical resonators provides a new class of optomechanical systems in which mechanical motion can be coupled to light without any optical cavities. Such cavity-less optomechanical systems interconnect photons, phonons and electrons (holes) in a highly integrable platform, opening up the development of functional integrated nanomechanical devices. Here we report on a semiconductor modulation-doped heterostructure-cantilever hybrid system, which realizes efficient cavity-less optomechanical transduction through excitons. The opto-piezoelectric backaction from the bound electron-hole pairs enables us to probe excitonic transition simply with a sub-nanowatt power of light, realizing high-sensitivity optomechanical spectroscopy. Detuning the photon energy from the exciton resonance results in self-feedback cooling and amplification of the thermomechanical motion. This cavity-less on-chip coupling enables highly tunable and addressable control of nanomechanical resonators, allowing high-speed programmable manipulation of nanomechanical devices and sensor arrays.

  20. Cavity-less on-chip optomechanics using excitonic transitions in semiconductor heterostructures.

    PubMed

    Okamoto, Hajime; Watanabe, Takayuki; Ohta, Ryuichi; Onomitsu, Koji; Gotoh, Hideki; Sogawa, Tetsuomi; Yamaguchi, Hiroshi

    2015-01-01

    The hybridization of semiconductor optoelectronic devices and nanomechanical resonators provides a new class of optomechanical systems in which mechanical motion can be coupled to light without any optical cavities. Such cavity-less optomechanical systems interconnect photons, phonons and electrons (holes) in a highly integrable platform, opening up the development of functional integrated nanomechanical devices. Here we report on a semiconductor modulation-doped heterostructure-cantilever hybrid system, which realizes efficient cavity-less optomechanical transduction through excitons. The opto-piezoelectric backaction from the bound electron-hole pairs enables us to probe excitonic transition simply with a sub-nanowatt power of light, realizing high-sensitivity optomechanical spectroscopy. Detuning the photon energy from the exciton resonance results in self-feedback cooling and amplification of the thermomechanical motion. This cavity-less on-chip coupling enables highly tunable and addressable control of nanomechanical resonators, allowing high-speed programmable manipulation of nanomechanical devices and sensor arrays. PMID:26477487

  1. Cavity-less on-chip optomechanics using excitonic transitions in semiconductor heterostructures

    PubMed Central

    Okamoto, Hajime; Watanabe, Takayuki; Ohta, Ryuichi; Onomitsu, Koji; Gotoh, Hideki; Sogawa, Tetsuomi; Yamaguchi, Hiroshi

    2015-01-01

    The hybridization of semiconductor optoelectronic devices and nanomechanical resonators provides a new class of optomechanical systems in which mechanical motion can be coupled to light without any optical cavities. Such cavity-less optomechanical systems interconnect photons, phonons and electrons (holes) in a highly integrable platform, opening up the development of functional integrated nanomechanical devices. Here we report on a semiconductor modulation-doped heterostructure–cantilever hybrid system, which realizes efficient cavity-less optomechanical transduction through excitons. The opto-piezoelectric backaction from the bound electron–hole pairs enables us to probe excitonic transition simply with a sub-nanowatt power of light, realizing high-sensitivity optomechanical spectroscopy. Detuning the photon energy from the exciton resonance results in self-feedback cooling and amplification of the thermomechanical motion. This cavity-less on-chip coupling enables highly tunable and addressable control of nanomechanical resonators, allowing high-speed programmable manipulation of nanomechanical devices and sensor arrays. PMID:26477487

  2. Valley dynamics probed through charged and neutral exciton emission in monolayer WSe2

    NASA Astrophysics Data System (ADS)

    Wang, G.; Bouet, L.; Lagarde, D.; Vidal, M.; Balocchi, A.; Amand, T.; Marie, X.; Urbaszek, B.

    2014-08-01

    Optical interband transitions in monolayer transition metal dichalcogenides such as WSe2 and MoS2 are governed by chiral selection rules. This allows efficient optical initialization of an electron in a specific K valley in momentum space. Here we probe the valley dynamics in monolayer WSe2 by monitoring the emission and polarization dynamics of the well-separated neutral excitons (bound electron-hole pairs) and charged excitons (trions) in photoluminescence. The neutral exciton photoluminescence intensity decay time is about 4 ps, whereas the trion emission occurs over several tens of ps. The trion polarization dynamics shows a partial, fast initial decay within tens of ps before reaching a stable polarization of ≈20%, for which a typical valley polarization decay time of the order of 1 ns can be inferred.

  3. Excitonic luminescence of SiGe/Si quantum wells δ-doped with boron

    SciTech Connect

    Bagaev, V. S.; Nikolaev, S. N.; Onishchenko, E. E.; Pruchkina, A. A.; Krivobok, V. S.; Novikov, A. V.

    2015-05-14

    Low-temperature photoluminescence of undoped and moderately δ-doped Si{sub 1−x}Ge{sub x}/Si (x < 0.1) quantum wells has been studied. The influence of boron δ-layer on the excitonic luminescence and the luminescence caused by a dense electron plasma was demonstrated. The conditions under which the luminescence spectra of quantum wells are dominated by impurity-bound excitons (BE) have been established. Some unusual properties of these BE are explained in terms of type II band-offset in Si{sub 1−x}Ge{sub x}/Si (x < 0.1) quantum wells, which favors a spatial separation of electrons and holes. It is shown that the temperature dependence of an excitonic emission in the quantum wells allows to calculate the BE-related density of states and, thus, can be used for contactless estimation of the impurity concentration in quantum wells.

  4. Splitting between bright and dark excitons in transition metal dichalcogenide monolayers

    NASA Astrophysics Data System (ADS)

    Echeverry, J. P.; Urbaszek, B.; Amand, T.; Marie, X.; Gerber, I. C.

    2016-03-01

    The optical properties of transition metal dichalcogenide monolayers such as the two-dimensional semiconductors MoS2 and WSe2 are dominated by excitons, Coulomb bound electron-hole pairs. The light emission yield depends on whether the electron-hole transitions are optically allowed (bright) or forbidden (dark). By solving the Bethe-Salpeter equation on top of G W wave functions in density functional theory calculations, we determine the sign and amplitude of the splitting between bright and dark exciton states. We evaluate the influence of the spin-orbit coupling on the optical spectra and clearly demonstrate the strong impact of the intra-valley Coulomb exchange term on the dark-bright exciton fine structure splitting.

  5. Outward Bound.

    ERIC Educational Resources Information Center

    Outward Bound, Inc., Andover, MA.

    The Outward Bound concept was developed in Germany and Great Britain with the saving of human life as the ultimate goal. Courses are designed to help students discover their true physical and mental limits through development of skills including emergency medical aid, firefighting, search and rescue, mountaineering, and sailing. Five Outward Bound…

  6. Excitonic superconductivity in copper oxides

    SciTech Connect

    Tesanovic, Z.; Bishop, A.R.; Martin, R.L.; Harris, C.

    1988-01-01

    We discuss the possibility of excitonic superconductivity in high T/sub c/ copper oxides. The Hamiltonians describing CuO/sub 2/ planes supports both antiferromagnetism and low-lying Cu /longleftrightarrow/ O intra- and interband charge fluctuations. One crosses from one regime to another as the number of holes per unit cell increases. The high T/sub c/ superconductivity takes place at hole concentrations most favorable for intraband charge transfer excitations. The dynamic polarizability of the environment surrounding CuO/sub 2/ planes plays an important role in enhancing T/sub c/. 15 refs., 4 figs.

  7. Pyridine as proton acceptor in the concerted proton electron transfer oxidation of phenol.

    PubMed

    Bonin, Julien; Costentin, Cyrille; Robert, Marc; Savéant, Jean-Michel

    2011-06-01

    Taking pyridine as a prototypal example of biologically important nitrogen bases involved in proton-coupled electron transfers, it is shown with the example of the photochemically triggered oxidation of phenol by Ru(III)(bpy)(3) that this proton acceptor partakes in a concerted pathway whose kinetic characteristics can be extracted from the overall kinetic response. The treatment of these data, implemented by the results of a parallel study carried out in heavy water, allowed the determination of the intrinsic kinetic characteristics of this proton acceptor. Comparison of the reorganization energies and of the pre-exponential factors previously derived for hydrogen phosphate and water (in water) as proton acceptors suggests that, in the case of pyridine, the proton charge is delocalized over a primary shell of water molecules firmly bound to the pyridinium cation. PMID:21499600

  8. Crystalline Nanoporous Frameworks: a Nanolaboratory for Probing Excitonic Device Concepts.

    SciTech Connect

    Allendorf, Mark D.; Azoulay, Jason; Ford, Alexandra Caroline; Foster, Michael E.; El Gabaly Marquez, Farid; Leonard, Francois Leonard; Leong-Hau, Kirsty; Stavila, Vitalie; Talin, Albert Alec; Wong, Brian M.; Brumbach, Michael T.; Van Gough, D.; Lambert, Timothy N.; Rodriguez, Mark A.; Spoerke, Erik David; Wheeler, David Roger; Deaton, Joseph C.; Centrone, Andrea; Haney, Paul; Kinney, R.; Szalai, Veronika; Yoon, Heayoung P.

    2014-09-01

    in disordered systems. Implementing this concept also creates entirely new dimensions for device fabrication that could both improve performance, increase durability, and reduce costs with unprecedented control of over properties. This report summarizes the key results of this project and is divided into sections based on publications that resulted from the work. We begin in Section 2 with an investigation of light harvesting and energy transfer in a MOF infiltrated with donor and acceptor molecules of the type typically used in OPV devices (thiophenes and fullerenes, respectively). The results show that MOFs can provide multiple functions: as a light harvester, as a stabilizer and organizer or the infiltrated molecules, and as a facilitator of energy transfer. Section 3 describes computational design of MOF linker groups to accomplish light harvesting in the visible and facilitate charge separation and transport. The predictions were validated by UV-visible absorption spectroscopy, demonstrating that rational design of MOFs for light-harvesting purposes is feasible. Section 4 extends the infiltration concept discussed in Section to, which we now designate as "Molecule@MOF" to create an electrically conducting framework. The tailorability and high conductivity of this material are unprecedented, meriting publication in the journal Science and spawning several Technical Advances. Section 5 discusses processes we developed for depositing MOFs as thin films on substrates, a critical enabling technology for fabricating MOF-based electronic devices. Finally, in Section 6 we summarize results showing that a MOF thin film can be used as a sensitizer in a DSSC, demonstrating that MOFs can serve as active layers in excitonic devices. Overall, this project provides several crucial proofs-of- concept that the potential of MOFs for use in optoelectronic devices that we predicted several years ago [ 3 ] can be realized in practice.

  9. Assembly and Comparison of Plasma Membrane SNARE Acceptor Complexes.

    PubMed

    Kreutzberger, Alex J B; Liang, Binyong; Kiessling, Volker; Tamm, Lukas K

    2016-05-24

    Neuronal exocytotic membrane fusion occurs on a fast timescale and is dependent on interactions between the vesicle SNARE synaptobrevin-2 and the plasma membrane SNAREs syntaxin-1a and SNAP-25 with a 1:1:1 stoichiometry. Reproducing fast fusion rates as observed in cells by reconstitution in vitro has been hindered by the spontaneous assembly of a 2:1 syntaxin-1a:SNAP-25 complex on target membranes that kinetically alters the binding of synaptobrevin-2. Previously, an artificial SNARE acceptor complex consisting of 1:1:1 syntaxin-1a(residues 183-288):SNAP-25:syb(residues 49-96) was found to greatly accelerate the rates of lipid mixing of reconstituted target and vesicle SNARE proteoliposomes. Here we present two (to our knowledge) new procedures to assemble membrane-bound 1:1 SNARE acceptor complexes that produce fast and efficient fusion without the need of the syb(49-96) peptide. In the first procedure, syntaxin-1a is purified in a strictly monomeric form and subsequently assembled with SNAP-25 in detergent with the correct 1:1 stoichiometry. In the second procedure, monomeric syntaxin-1a and dodecylated (d-)SNAP-25 are separately reconstituted into proteoliposomes and subsequently assembled in the plane of merged target lipid bilayers. Examining single particle fusion between synaptobrevin-2 proteoliposomes and planar-supported bilayers containing the two different SNARE acceptor complexes revealed similar fast rates of fusion. Changing the stoichiometry of syntaxin-1a and d-SNAP-25 in the target bilayer had significant effects on docking, but little effect on the rates of synaptobrevin-2 proteoliposome fusion. PMID:27178662

  10. Resonant internal quantum transitions and femtosecond radiative decay of excitons in monolayer WSe2

    NASA Astrophysics Data System (ADS)

    Poellmann, C.; Steinleitner, P.; Leierseder, U.; Nagler, P.; Plechinger, G.; Porer, M.; Bratschitsch, R.; Schüller, C.; Korn, T.; Huber, R.

    2015-09-01

    Atomically thin two-dimensional crystals have revolutionized materials science. In particular, monolayer transition metal dichalcogenides promise novel optoelectronic applications, owing to their direct energy gaps in the optical range. Their electronic and optical properties are dominated by Coulomb-bound electron-hole pairs called excitons, whose unusual internal structure, symmetry, many-body effects and dynamics have been vividly discussed. Here we report the first direct experimental access to all 1s A excitons, regardless of momentum--inside and outside the radiative cone--in single-layer WSe2. Phase-locked mid-infrared pulses reveal the internal orbital 1s-2p resonance, which is highly sensitive to the shape of the excitonic envelope functions and provides accurate transition energies, oscillator strengths, densities and linewidths. Remarkably, the observed decay dynamics indicates an ultrafast radiative annihilation of small-momentum excitons within 150 fs, whereas Auger recombination prevails for optically dark states. The results provide a comprehensive view of excitons and introduce a new degree of freedom for quantum control, optoelectronics and valleytronics of dichalcogenide monolayers.

  11. Photocurrent spectroscopy of exciton and free particle optical transitions in suspended carbon nanotube pn-junctions

    SciTech Connect

    Chang, Shun-Wen; Theiss, Jesse; Hazra, Jubin; Aykol, Mehmet; Kapadia, Rehan; Cronin, Stephen B.

    2015-08-03

    We study photocurrent generation in individual, suspended carbon nanotube pn-junction diodes formed by electrostatic doping using two gate electrodes. Photocurrent spectra collected under various electrostatic doping concentrations reveal distinctive behaviors for free particle optical transitions and excitonic transitions. In particular, the photocurrent generated by excitonic transitions exhibits a strong gate doping dependence, while that of the free particle transitions is gate independent. Here, the built-in potential of the pn-junction is required to separate the strongly bound electron-hole pairs of the excitons, while free particle excitations do not require this field-assisted charge separation. We observe a sharp, well defined E{sub 11} free particle interband transition in contrast with previous photocurrent studies. Several steps are taken to ensure that the active charge separating region of these pn-junctions is suspended off the substrate in a suspended region that is substantially longer than the exciton diffusion length and, therefore, the photocurrent does not originate from a Schottky junction. We present a detailed model of the built-in fields in these pn-junctions, which, together with phonon-assistant exciton dissociation, predicts photocurrents on the same order of those observed experimentally.

  12. The Structure and Dynamics of Molecular Excitons

    NASA Astrophysics Data System (ADS)

    Bardeen, Christopher J.

    2014-04-01

    The photophysical behavior of organic semiconductors is governed by their excitonic states. In this review, I classify the three different exciton types (Frenkel singlet, Frenkel triplet, and charge transfer) typically encountered in organic semiconductors. Experimental challenges that arise in the study of solid-state organic systems are discussed. The steady-state spectroscopy of intermolecular delocalized Frenkel excitons is described, using crystalline tetracene as an example. I consider the problem of a localized exciton diffusing in a disordered matrix in detail, and experimental results on conjugated polymers and model systems suggest that energetic disorder leads to subdiffusive motion. Multiexciton processes such as singlet fission and triplet fusion are described, emphasizing the role of spin state coherence and magnetic fields in studying singlet -- triplet pair interconversion. Singlet fission provides an example of how all three types of excitons (triplet, singlet, and charge transfer) may interact to produce useful phenomena for applications such as solar energy conversion.

  13. Excitons and optical spectra of phosphorene nanoribbons

    NASA Astrophysics Data System (ADS)

    Nourbakhsh, Zahra; Asgari, Reza

    2016-07-01

    On the basis of many-body ab initio calculations, using the single-shot G0W0 method and Bethe-Salpeter equation, we study phosphorene nanoribbons (PNRs) in the two typical zigzag and armchair directions. The electronic structure, optical absorption, electron-hole (exciton) binding energy, exciton exchange splitting, and exciton wave functions are calculated for different sizes of PNRs. The typically strong splitting between singlet and triplet excitonic states make PNRs favorable systems for optoelectronic applications. Quantum confinement occurs in both kinds of PNRs, and it is stronger in the zPNRs, which behave like quasi-zero-dimensional systems. Scaling laws are investigated for the size-dependent behaviors of PNRs. The first bright excitonic state in PNRs is explored in detail.

  14. Fixed-Node Monte Carlo Studies of Excitons in Confined Geometries

    NASA Astrophysics Data System (ADS)

    He, Song; Zacharia, I.; Zacharia, I.

    1997-11-01

    We study the properties of excitons in CEO quantum wires by the fixed-node Monte Carlo method in connection with recent experiments. The results of our calculations indicate that 1. as a function of the well width in a stem-arm symmetric structure at Al concentration of x = 0.3, the energy shift between the well and wire excitons reaches a peak value of about 25 meV at a well width of 25 Å; 2. the Coulomb interaction plays an important if not the dominant role. Depending on the well width, the correlated electron-hole Coulomb energy can be as large as 20 meV. The anisotropy in the projected density of the hole is significantly suppressed by electron-hole correlation; 3. the binding energy enhancement can be up to 70% at the optimal well widths; 4. there is more than one bound state in the center of mass motion of the exciton in the direction perpendicular to the quantum wire. This is markedly different from the single particle case where only one bound state exists; 5. comparison between our numerical results and experimental data show that the effective mass approximation with one hole band is sufficient for the computation of various energies of the exciton in typical experimental structures around well widths 70 Å.

  15. Controlled energy transfer between isolated donor-acceptor molecules intercalated in thermally self-ensemble two-dimensional hydrogen bonding cages

    NASA Astrophysics Data System (ADS)

    Al Attar, Hameed A.; Monkman, Andrew P.

    2012-12-01

    Thermally assembled hydrogen bonding cages which are neither size nor guest specific have been developed using a poly (vinyl alcohol) (PVA) host. A water-soluble conjugated polymer poly(2,5-bis(3-sulfonatopropoxy)-1,4-phenylene, disodium salt-alt-1,4-phenylene) (PPP-OPSO3) as a donor and tris(2,2-bipyridyl)- ruthenium(II) [Ru(bpy)32+] as an acceptor have been isolated and trapped in such a PVA matrix network. This is a unique system that shows negligible exciton diffusion and the donor and acceptor predominantly interact by a direct single step excitation transfer process (DSSET). Singlet and triplet exciton quenching have been studied. Time-resolved fluorescence lifetime measurement at different acceptor concentrations has enabled us to determine the dimensionality of the energy-transfer process within the PVA scaffold. Our results reveal that the PVA hydrogen bonding network effectively isolates the donor-acceptor molecules in a two-dimensional layer structure (lamella) leading to the condition where a precise control of the energy and charge transfer is possible.

  16. Conduction electrons in acceptor-doped GaAs/GaAlAs heterostructures: a review

    NASA Astrophysics Data System (ADS)

    Zawadzki, Wlodek; Raymond, Andre; Kubisa, Maciej

    2016-05-01

    We review magneto-optical and magneto-transport effects in GaAs/GaAlAs heterostructures doped in GaAlAs barriers with donors, providing two-dimensional (2D) electron gas (2DEG) in GaAs quantum wells (QWS), and additionally doped with smaller amounts of acceptors (mostly Be atoms) in the vicinity of 2DEG. One may also deal with residual acceptors (mostly C atoms). The behavior of such systems in the presence of a magnetic field differs appreciably from those doped in the vicinity of 2DEG with donors. Three subjects related to the acceptor-doped heterostructures are considered. First is the problem of bound states of conduction electrons confined to the vicinity of negatively charged acceptors by the joint effect of a QW and an external magnetic field parallel to the growth direction. A variational theory of such states is presented, demonstrating that an electron turning around a repulsive center has discrete energies above the corresponding Landau levels. Experimental evidence for the discrete electron energies comes from the work on interband photo-magneto-luminescence, intraband cyclotron resonance and quantum magneto-transport (the Quantum Hall and Shubnikov-de Haas effects). An electron rain-down effect at weak electric fields and a boil-off effect at strong electric fields are introduced. It is demonstrated, both theoretically and experimentally, that a negatively charged acceptor can localize more than one electron. The second subject describes experiment and theory of asymmetric quantized Hall and Shubnikov-de Haas plateaus in acceptor-doped GaAs/GaAlAs heterostructures. It is shown that the main features of the plateau asymmetry can be attributed to asymmetric density of Landau states in the presence of acceptors. However, at high magnetic fields, the rain-down effect is also at work. The third subject deals with the so-called disorder modes (DMs) in the cyclotron resonance of conduction electrons. The DMs originate from random distributions of negatively

  17. Orbital diamagnetic susceptibility in excitonic condensation phase

    NASA Astrophysics Data System (ADS)

    Sugimoto, Koudai; Ohta, Yukinori

    2016-08-01

    We study the orbital diamagnetic susceptibility in excitonic condensation phase using the mean-field approximation for a two-band model defined on a square lattice. We find that, in semiconductors, the excitonic condensation acquires a finite diamagnetic susceptibility due to spontaneous hybridization between the valence and the conduction bands, whereas in semimetals, the diamagnetic susceptibility in the normal phase is suppressed by the excitonic condensation. We also study the orbital diamagnetic and Pauli paramagnetic susceptibilities of Ta2NiSe5 using a two-dimensional three-band model and find that the calculated temperature dependence of the magnetic susceptibility is in qualitative agreement with experiment.

  18. Excitons in the Fractional Quantum Hall Effect

    DOE R&D Accomplishments Database

    Laughlin, R. B.

    1984-09-01

    Quasiparticles of charge 1/m in the Fractional Quantum Hall Effect form excitons, which are collective excitations physically similar to the transverse magnetoplasma oscillations of a Wigner crystal. A variational exciton wavefunction which shows explicitly that the magnetic length is effectively longer for quasiparticles than for electrons is proposed. This wavefunction is used to estimate the dispersion relation of these excitons and the matrix elements to generate them optically out of the ground state. These quantities are then used to describe a type of nonlinear conductivity which may occur in these systems when they are relatively clean.

  19. Exciton effects in strained armchair graphene nanoribbons

    NASA Astrophysics Data System (ADS)

    Jia, Yonglei; Liu, Junlin

    2016-01-01

    The exciton effects in 1-nm-wide armchair graphene nanoribbons (AGNRs) under the uniaxial strain were studied within the nonorthogonal tight-binding (TB) model, supplemented by the long-range Coulomb interactions. The obtained results show that both the excitation energy and exciton binding energy are modulated by the uniaxial strain. The variation of these energies depends on the ribbon family. In addition, the results show that the variation of the exciton binding energy is much weaker than the variation of excitation energy. Our results provide new guidance for the design of optomechanical systems based on graphene nanoribbons.

  20. Exciton Regeneration at Polymeric Semiconductor Heterojunctions

    NASA Astrophysics Data System (ADS)

    Morteani, Arne C.; Sreearunothai, Paiboon; Herz, Laura M.; Friend, Richard H.; Silva, Carlos

    2004-06-01

    Control of the band-edge offsets at heterojunctions between organic semiconductors allows efficient operation of either photovoltaic or light-emitting diodes. We investigate systems where the exciton is marginally stable against charge separation and show via E-field-dependent time-resolved photoluminescence spectroscopy that excitons that have undergone charge separation at a heterojunction can be efficiently regenerated. This is because the charge transfer produces a geminate electron-hole pair (separation 2.2 3.1nm) which may collapse into an exciplex and then endothermically (EA=100 200 meV) back transfer towards the exciton.

  1. Phonon-assisted exciton transfer into silicon using nanoemitters: the role of phonons and temperature effects in Förster resonance energy transfer.

    PubMed

    Yeltik, Aydan; Guzelturk, Burak; Hernandez-Martinez, Pedro Ludwig; Govorov, Alexander O; Demir, Hilmi Volkan

    2013-12-23

    We study phonon-assisted Förster resonance energy transfer (FRET) into an indirect band-gap semiconductor using nanoemitters. The unusual temperature dependence of this energy transfer, which is measured using the donor nanoemitters of quantum dot (QD) layers integrated on the acceptor monocrystalline bulk silicon as a model system, is predicted by a phonon-assisted exciton transfer model proposed here. The model includes the phonon-mediated optical properties of silicon, while considering the contribution from the multimonolayer-equivalent QD film to the nonradiative energy transfer, which is derived with a d(-3) distance dependence. The FRET efficiencies are experimentally observed to decrease at cryogenic temperatures, which are well explained by the model considering the phonon depopulation in the indirect band-gap acceptor together with the changes in the quantum yield of the donor. These understandings will be crucial for designing FRET-enabled sensitization of silicon based high-efficiency excitonic systems using nanoemitters. PMID:24274734

  2. Multiple Exciton Generation Solar Cells

    SciTech Connect

    Luther, J. M.; Semonin, O. E.; Beard, M. C.; Gao, J.; Nozik, A. J.

    2012-01-01

    Heat loss is the major factor limiting traditional single junction solar cells to a theoretical efficiency of 32%. Multiple Exciton Generation (MEG) enables efficient use of the solar spectrum yielding a theoretical power conversion efficiency of 44% in solar cells under 1-sun conditions. Quantum-confined semiconductors have demonstrated the ability to generate multiple carriers but present-day materials deliver efficiencies far below the SQ limit of 32%. Semiconductor quantum dots of PbSe and PbS provide an active testbed for developing high-efficiency, inexpensive solar cells benefitting from quantum confinement effects. Here, we will present recent work of solar cells employing MEG to yield external quantum efficiencies exceeding 100%.

  3. Temperature-dependent excitonic effects in the optical properties of single-layer MoS2

    NASA Astrophysics Data System (ADS)

    Molina-Sánchez, Alejandro; Palummo, Maurizia; Marini, Andrea; Wirtz, Ludger

    2016-04-01

    Temperature influences the performance of two-dimensional (2D) materials in optoelectronic devices. Indeed, the optical characterization of these materials is usually realized at room temperature. Nevertheless, most ab initio studies are still performed without including any temperature effect. As a consequence, important features are thus overlooked, such as the relative height of the excitonic peaks and their broadening, directly related to the temperature and to the nonradiative exciton relaxation time. We present ab initio calculations of the optical response of single-layer MoS2, a prototype 2D material, as a function of temperature using density functional theory and many-body perturbation theory. We compute the electron-phonon interaction using the full spinorial wave functions, i.e., fully taking into account the effects of spin-orbit interaction. We find that bound excitons (A and B peaks) and resonant excitons (C peak) exhibit different behavior with temperature, displaying different nonradiative linewidths. We conclude that the inhomogeneous broadening of the absorption spectra is mainly due to electron-phonon scattering mechanisms. Our calculations explain the shortcomings of previous (zero-temperature) theoretical spectra and match well with the experimental spectra acquired at room temperature. Moreover, we disentangle the contributions of acoustic and optical phonon modes to the quasiparticles and exciton linewidths. Our model also allows us to identify which phonon modes couple to each exciton state, which is useful for the interpretation of resonant Raman-scattering experiments.

  4. Quantum and classical solutions for statically screened two-dimensional Wannier-Mott excitons

    SciTech Connect

    Makowski, Adam J.

    2011-08-15

    Quantum solutions and classical orbits are discussed for statically screened Wannier-Mott excitons for two closely related potentials: the Stern-Howard potential and a suggested simple focusing one. Bound states and exact ''quantized'' values of screening are obtained as well. For the suggested potential, the scattering matrix, the Regge poles, and the transmission coefficient are calculated exactly. We argue that the simple potential can be utilized in applications instead of the Stern-Howard potential, which is difficult to handle.

  5. Probing the effect of electron acceptor structure and morphology on charge separation in ZnO/P3HT hybrid photovoltaics using steady-state transient photoinduced absorption.

    SciTech Connect

    Davis, Robert Jackson; Lloyd, Matthew T.; Ferreira, Summer Rhodes; Lee, Yun-Ju; Hsu, Julia W. P.

    2010-04-01

    Hybrid cells based on ZnO/P3HT heterojunctions have the advantage of better device stability, but suffer poor photovoltaic performance compared to all-organic cells which use PCBM as the electron acceptor. The photovoltaic effect in these hybrid systems is accomplished via photoinduced charge separation at the interface between the absorbing polymer (P3HT) and the electron acceptor (ZnO). Efforts to improve device performance in these hybrid systems have centered on reducing the required diffusion length for P3HT excitons by creating bulk heterojunctions from either ZnO nanoparticles and P3HT or using ZnO precursors which convert in situ to form ZnO networks inside a polymer matrix. In this study, we use transient photoinduced absorption to access the lifetimes of P3HT polarons and excitons in bulk heterojunctions constructed using P3HT and ZnO nanoparticles or ZnO precursors and compare to those in planar ZnO/P3HT devices. Steady-state photoinduced absorption spectra of ZnO/P3HT show characteristic of sub-bandgap transitions associated with the formation of long-lived (msec lifetimes) radical cations (polarons) in P3HT. Similar short-lived polarons (psec lifetimes) are observed by picosecond transient photoinduced absorption in addition to infrared absorption due to excitons. Here we examine the lifetimes of both the excitons and polarons in ZnO:P3HT bulk heterojunctions using both picosecond and millisecond techniques in an effort to understand the effect of the structure and morphology of the electron acceptor on charge separation. We will also compare the relative photoexitation lifetimes, hence charge separation efficiency, for the planar and bulk heterojunction hybrid system to an all-organic P3HT:PCBM system.

  6. Electron Donor Acceptor Interactions. Final Progress Report

    SciTech Connect

    2002-08-16

    The Gordon Research Conference (GRC) on Electron Donor Acceptor Interactions was held at Salve Regina University, Newport, Rhode Island, 8/11-16/02. Emphasis was placed on current unpublished research and discussion of the future target areas in this field.

  7. Observation of long-lived interlayer excitons in monolayer MoSe2–WSe2 heterostructures

    DOE PAGESBeta

    Rivera, Pasqual; Schaibley, John R.; Jones, Aaron M.; Ross, Jason S.; Wu, Sanfeng; Aivazian, Grant; Klement, Philip; Seyler, Kyle; Clark, Genevieve; Ghimire, Nirmal J.; et al

    2015-02-24

    Van der Waals bound heterostructures constructed with two-dimensional materials, such as graphene, boron nitride and transition metal dichalcogenides, have sparked wide interest in both device physics and technologies at the two-dimensional limit. One highly coveted heterostructure is that of differing monolayer transition metal dichalcogenides with type-II band alignment, with bound electrons and holes localized in individual monolayers, that is, interlayer excitons. Here, we report the observation of interlayer excitons in monolayer MoSe2–WSe2 heterostructures by photoluminescence and photoluminescence excitation spectroscopy. The energy and luminescence intensity are highly tunable by an applied vertical gate voltage. Moreover, we measure an interlayer exciton lifetimemore » of ~1.8 ns, an order of magnitude longer than intralayer excitons in monolayers. Ultimately, our work demonstrates optical pumping of interlayer electric polarization, which may provoke further exploration of interlayer exciton condensation, as well as new applications in two-dimensional lasers, light-emitting diodes and photovoltaic devices.« less

  8. Electron transport mechanism of bathocuproine exciton blocking layer in organic photovoltaics.

    PubMed

    Lee, Jeihyun; Park, Soohyung; Lee, Younjoo; Kim, Hyein; Shin, Dongguen; Jeong, Junkyeong; Jeong, Kwangho; Cho, Sang Wan; Lee, Hyunbok; Yi, Yeonjin

    2016-02-21

    Efficient exciton management is a key issue to improve the power conversion efficiency of organic photovoltaics (OPVs). It is well known that the insertion of an exciton blocking layer (ExBL) having a large band gap promotes the efficient dissociation of photogenerated excitons at the donor-acceptor interface. However, the large band gap induces an energy barrier which disrupts the charge transport. Therefore, building an adequate strategy based on the knowledge of the true charge transport mechanism is necessary. In this study, the true electron transport mechanism of a bathocuproine (BCP) ExBL in OPVs is comprehensively investigated by in situ ultraviolet photoemission spectroscopy, inverse photoemission spectroscopy, density functional theory calculation, and impedance spectroscopy. The chemical interaction between deposited Al and BCP induces new states within the band gap of BCP, so that electrons can transport through these new energy levels. Localized trap states are also formed upon the Al-BCP interaction. The activation energy of these traps is estimated with temperature-dependent conductance measurements to be 0.20 eV. The Al-BCP interaction induces both transport and trap levels in the energy gap of BCP and their interplay results in the electron transport observed. PMID:26821701

  9. Coherent Dynamics of Mixed Frenkel and Charge-Transfer Excitons in Dinaphtho[2,3-b:2'3'-f]thieno[3,2-b]-thiophene Thin Films: The Importance of Hole Delocalization.

    PubMed

    Fujita, Takatoshi; Atahan-Evrenk, Sule; Sawaya, Nicolas P D; Aspuru-Guzik, Alán

    2016-04-01

    Charge-transfer states in organic semiconductors play crucial roles in processes such as singlet fission and exciton dissociation at donor/acceptor interfaces. Recently, a time-resolved spectroscopy study of dinaphtho[2,3-b:2'3'-f]thieno[3,2-b]-thiophene (DNTT) thin films provided evidence for the formation of mixed Frenkel and charge-transfer excitons after the photoexcitation. Here, we investigate optical properties and excitation dynamics of the DNTT thin films by combining ab initio calculations and a stochastic Schrödinger equation. Our theory predicts that the low-energy Frenkel exciton band consists of 8-47% CT character. The quantum dynamics simulations show coherent dynamics of Frenkel and CT states in 50 fs after the optical excitation. We demonstrate the role of charge delocalization and localization in the mixing of CT states with Frenkel excitons as well as the role of their decoherence. PMID:27011327

  10. Hybrid interlayer excitons with tunable dispersion relation

    NASA Astrophysics Data System (ADS)

    Skinner, Brian

    When two semiconducting materials are layered on top of each other, interlayer excitons can be formed by the Coulomb attraction of an electron in one layer to a hole in the opposite layer. The resulting exciton is a composite boson with a dispersion relation that is a hybrid between the dispersion relations of the electron and the hole separately. In this talk I show how such hybridization is particularly interesting when one layer has a ``Mexican hat''-shaped dispersion relation and the other has a conventional parabolic dispersion. In this case the interlayer exciton can have a range of qualitatively different dispersion relations, which can be continuously altered by an external field. This tunability in principle allows one to continuously tune a collection of interlayer excitons between different quantum many-body phases, including Bose-Einstein condensate, Wigner crystal, and fermion-like ``moat band'' phases.

  11. Topologically protected excitons in porphyrin thin films

    NASA Astrophysics Data System (ADS)

    Yuen-Zhou, Joel; Saikin, Semion K.; Yao, Norman Y.; Aspuru-Guzik, Alán

    2014-11-01

    The control of exciton transport in organic materials is of fundamental importance for the development of efficient light-harvesting systems. This transport is easily deteriorated by traps in the disordered energy landscape. Here, we propose and analyse a system that supports topological Frenkel exciton edge states. Backscattering of these chiral Frenkel excitons is prohibited by symmetry, ensuring that the transport properties of such a system are robust against disorder. To implement our idea, we propose a two-dimensional periodic array of tilted porphyrins interacting with a homogeneous magnetic field. This field serves to break time-reversal symmetry and results in lattice fluxes that mimic the Aharonov-Bohm phase acquired by electrons. Our proposal is the first blueprint for realizing topological phases of matter in molecular aggregates and suggests a paradigm for engineering novel excitonic materials.

  12. Topologically protected excitons in porphyrin thin films.

    PubMed

    Yuen-Zhou, Joel; Saikin, Semion K; Yao, Norman Y; Aspuru-Guzik, Alán

    2014-11-01

    The control of exciton transport in organic materials is of fundamental importance for the development of efficient light-harvesting systems. This transport is easily deteriorated by traps in the disordered energy landscape. Here, we propose and analyse a system that supports topological Frenkel exciton edge states. Backscattering of these chiral Frenkel excitons is prohibited by symmetry, ensuring that the transport properties of such a system are robust against disorder. To implement our idea, we propose a two-dimensional periodic array of tilted porphyrins interacting with a homogeneous magnetic field. This field serves to break time-reversal symmetry and results in lattice fluxes that mimic the Aharonov-Bohm phase acquired by electrons. Our proposal is the first blueprint for realizing topological phases of matter in molecular aggregates and suggests a paradigm for engineering novel excitonic materials. PMID:25242533

  13. Triplet excitons: Bringing dark states to light

    NASA Astrophysics Data System (ADS)

    Bardeen, Christopher J.

    2014-11-01

    Semiconducting quantum dots have been used to harvest triplet excitons produced through singlet fission in organic semiconductors. These hybrid organic-inorganic materials may boost the efficiency of solar cells.

  14. Theory of Orbital Susceptibility on Excitonic Insulator

    NASA Astrophysics Data System (ADS)

    Matsuura, Hiroyasu; Ogata, Masao

    2016-09-01

    We study the temperature dependence of the orbital susceptibility of an excitonic insulator on the basis of a two-band model. It is shown that a drastic change (an anomalous enhancement) in susceptibility as a function of temperature occurs owing to the occurrence of additional orbital susceptibility due to the excitonic gap. We calculate explicitly the temperature dependence of orbital susceptibility for a model of Ta2NiSe5, and show that the result is consistent with experimental results.

  15. Exciton properties of selected aromatic hydrocarbon systems

    NASA Astrophysics Data System (ADS)

    Roth, Friedrich; Mahns, Benjamin; Hampel, Silke; Nohr, Markus; Berger, Helmuth; Büchner, Bernd; Knupfer, Martin

    2013-02-01

    We have examined the singlet excitons in two representatives of acene-type (tetracene and pentacene) and phenacene-type (chrysene and picene) molecular crystals, respectively, using electron energy-loss spectroscopy at low temperatures. We show that the excitation spectra of the two hydrocarbon families significantly differ. Moreover, close inspection of the data indicates that there is an increasing importance of charge-transfer excitons at lowest excitation energy with increasing length of the molecules.

  16. Exciton transport by surface acoustic waves

    NASA Astrophysics Data System (ADS)

    Rudolph, J.; Hey, R.; Santos, P. V.

    2007-05-01

    Long-range acoustic transport of excitons in GaAs quantum wells (QWs) is demonstrated. The mobile strain field of a surface acoustic wave creates a dynamic lateral type I modulation of the conduction and valence bands in a double-quantum-well (DQW) structure. This mobile potential modulation transports long-living indirect excitons in the DQW over several hundreds of μm.

  17. Förster energy transfer of dark excitons enhanced by a magnetic field in an ensemble of CdTe colloidal nanocrystals

    NASA Astrophysics Data System (ADS)

    Liu, Feng; Rodina, A. V.; Yakovlev, D. R.; Golovatenko, A. A.; Greilich, A.; Vakhtin, E. D.; Susha, A.; Rogach, A. L.; Kusrayev, Yu. G.; Bayer, M.

    2015-09-01

    We present a systematic experimental study along with theoretical modeling of the energy transfer in an ensemble of closely packed CdTe colloidal nanocrystals identified as the Förster resonant energy transfer (FRET). We prove that at low temperature of 4.2 K, mainly the ground dark exciton states in the initially excited small-size (donor) nanocrystals participate in the dipole-dipole FRET leading to additional excitation of the large-size (acceptor) nanocrystals. The FRET becomes possible due to the weak admixture of the bright exciton states to the dark states. The admixture takes place even in zero magnetic field and allows the radiative recombination of the dark excitons. An external magnetic field considerably enhances this admixture, thus increasing the energy transfer rate by a factor of 2-3 in a field of 15 T, as well as the radiative rates of the dark excitons in the donor and acceptor nanocrystals. The theoretical modeling allows us to determine the spectral dependence of the probability for the NC to serve as a donor for larger nanocrystals, to evaluate the energy transfer rates as well as to predict their dependencies on the magnetic field, to describe the spectral shift of the photoluminescence maximum due to the energy transfer, and to reproduce the experimentally observed spectral dependencies of the photoluminescence recombination dynamics in the magnetic field.

  18. Orientation of luminescent excitons in layered nanomaterials

    NASA Astrophysics Data System (ADS)

    Schuller, Jon A.; Karaveli, Sinan; Schiros, Theanne; He, Keliang; Yang, Shyuan; Kymissis, Ioannis; Shan, Jie; Zia, Rashid

    2013-04-01

    In nanomaterials, optical anisotropies reveal a fundamental relationship between structural and optical properties. Directional optical properties can be exploited to enhance the performance of optoelectronic devices, optomechanical actuators and metamaterials. In layered materials, optical anisotropies may result from in-plane and out-of-plane dipoles associated with intra- and interlayer excitations, respectively. Here, we resolve the orientation of luminescent excitons and isolate photoluminescence signatures arising from distinct intra- and interlayer optical transitions. Combining analytical calculations with energy- and momentum-resolved spectroscopy, we distinguish between in-plane and out-of-plane oriented excitons in materials with weak or strong interlayer coupling--MoS2 and 3,4,9,10-perylene tetracarboxylic dianhydride (PTCDA), respectively. We demonstrate that photoluminescence from MoS2 mono-, bi- and trilayers originates solely from in-plane excitons, whereas PTCDA supports distinct in-plane and out-of-plane exciton species with different spectra, dipole strengths and temporal dynamics. The insights provided by this work are important for understanding fundamental excitonic properties in nanomaterials and designing optical systems that efficiently excite and collect light from exciton species with different orientations.

  19. Orientation of luminescent excitons in layered nanomaterials.

    PubMed

    Schuller, Jon A; Karaveli, Sinan; Schiros, Theanne; He, Keliang; Yang, Shyuan; Kymissis, Ioannis; Shan, Jie; Zia, Rashid

    2013-04-01

    In nanomaterials, optical anisotropies reveal a fundamental relationship between structural and optical properties. Directional optical properties can be exploited to enhance the performance of optoelectronic devices, optomechanical actuators and metamaterials. In layered materials, optical anisotropies may result from in-plane and out-of-plane dipoles associated with intra- and interlayer excitations, respectively. Here, we resolve the orientation of luminescent excitons and isolate photoluminescence signatures arising from distinct intra- and interlayer optical transitions. Combining analytical calculations with energy- and momentum-resolved spectroscopy, we distinguish between in-plane and out-of-plane oriented excitons in materials with weak or strong interlayer coupling-MoS₂ and 3,4,9,10-perylene tetracarboxylic dianhydride (PTCDA), respectively. We demonstrate that photoluminescence from MoS₂ mono-, bi- and trilayers originates solely from in-plane excitons, whereas PTCDA supports distinct in-plane and out-of-plane exciton species with different spectra, dipole strengths and temporal dynamics. The insights provided by this work are important for understanding fundamental excitonic properties in nanomaterials and designing optical systems that efficiently excite and collect light from exciton species with different orientations. PMID:23455984

  20. Exciton Resonances in Novel Silicon Carbide Polymers

    NASA Astrophysics Data System (ADS)

    Burggraf, Larry; Duan, Xiaofeng

    2015-05-01

    A revolutionary technology transformation from electronics to excitionics for faster signal processing and computing will be advantaged by coherent exciton transfer at room temperature. The key feature required of exciton components for this technology is efficient and coherent transfer of long-lived excitons. We report theoretical investigations of optical properties of SiC materials having potential for high-temperature excitonics. Using Car-Parinello simulated annealing and DFT we identified low-energy SiC molecular structures. The closo-Si12C12 isomer, the most stable 12-12 isomer below 1100 C, has potential to make self-assembled chains and 2-D nanostructures to construct exciton components. Using TDDFT, we calculated the optical properties of the isomer as well as oligomers and 2-D crystal formed from the isomer as the monomer unit. This molecule has large optical oscillator strength in the visible. Its high-energy and low-energy transitions (1.15 eV and 2.56 eV) are nearly pure one-electron silicon-to-carbon transitions, while an intermediate energy transition (1.28 eV) is a nearly pure carbon-to-silicon one-electron charge transfer. These results are useful to describe resonant, coherent transfer of dark excitons in the nanostructures. Research supported by the Air Force Office of Scientific Research.

  1. Longitudinal Excitons in GaN

    NASA Astrophysics Data System (ADS)

    Reynolds, D. C.; Jogai, B.; Collins, T. C.

    2002-05-01

    Many of the previous investigations of longitudinal excitons have involved reflection and absorption measurements rather than emission. In these measurements it is more difficult to resolve the longitudinal exciton from the Γ5 and Γ6 free excitons in wurtzite material. The longitudinal excitons have energies and oscillator strengths that depend on the direction of propagation and they are not observable along the principal axis of the crystal. In the wurtzite structure, such as GaN, the Γ5 exciton is the pure transverse mode, whereas the longitudinal is a mixed mode going from pure longitudinal, for the propogation direction K perpendicular to C, to pure transverse for K parallel to C. If more than one orientation is present in the sample, it is clear that more than one longitudinal exciton may be seen since it is a mixed mode. In the current experiment we observe more than one mode, which we associate with more than one crystal orientation. This may result from the columnar growth often observed in GaN.

  2. Exciton formation and diffusion in OLEDs (Presentation Recording)

    NASA Astrophysics Data System (ADS)

    Ingram, Grayson L.; Lu, Zheng-Hong

    2015-10-01

    This talk will discuss recent experiments designed to study the formation of excitons and their subsequent diffusions in OLEDs. These experimental results suggest that contrary to conventional wisdom, host singlet exciton diffusion can occur over long distances, while host triplet excitons are confined close to the exciton formation region for the archetype host and hole transport layer CBP. The exciton formation mechanism is studied and we show that the ratio of excitons formed on the host to excitons formed on the dopant varies strongly with the applied voltage. Refinements to models of efficiency roll off are discussed in light of the improved understanding of exciton formation and we suggest design guidelines to improve efficiency by engineering exciton formation.

  3. Decoherence processes during optical manipulation of excitonic qubits in semiconductor quantum dots

    NASA Astrophysics Data System (ADS)

    Wang, Q. Q.; Muller, A.; Bianucci, P.; Rossi, E.; Xue, Q. K.; Takagahara, T.; Piermarocchi, C.; MacDonald, A. H.; Shih, C. K.

    2005-07-01

    Using photoluminescence spectroscopy, we have investigated the nature of Rabi oscillation damping during optical manipulation of excitonic qubits in self-assembled quantum dots. Rabi oscillations were recorded by varying the pulse amplitude for fixed pulse durations between 4ps and 10ps . Up to five periods are visible, making it possible to quantify the excitation dependent damping. We find that this damping is more pronounced for shorter pulse widths and show that its origin is the nonresonant excitation of carriers in the wetting layer, most likely involving bound-to-continuum and continuum-to-bound transitions.

  4. Nonlinear Exciton Dynamics in IndiumGalliumArsenic Quantum Wells

    NASA Astrophysics Data System (ADS)

    Zaks, Benjamin Rene

    Near infrared and optical light applied near the band edge of a semiconductor can lead to the formation of bound electron-hole pairs known as excitons. Using growth methods such as molecular beam epitaxy, semiconductor heterostructures can be engineered to have properties beneficial to particular experiments. The formation of thin layers of semiconductor can lead electrons and holes to be confined along a particular direction, and one structure that can be grown is called a quantum well. Confinement of charge in the quantum well increases the Coulomb interaction between the electron and hole, increasing exciton formation. When excitons are driven with an intense THz field, changes to the optical properties of the semiconductor are observed. These changes to the optical properties can not only provide interesting information about the exciton system, but also may provide insight on modulating optical beams at THz frequencies, information that may be necessary to further improve the speed of our cable and internet connections. By growing InGaAs quantum wells with AlGaAs barriers on a GaAs substrate, we have observed strong changes to the optical spectrum due to intense THz fields. We find that when the strong THz field is applied to an intersubband transition in the quantum well, the applied field can significantly shift the energy of that intersubband transition. This shift is unexpected within the approximations often used to describe this system, and we find that full numerical simulations of the system are necessary to interpret our results. When the strong THz field is polarized in the plane of the quantum well, we are able to observe optical light at up to 11 frequencies that were not present before application of the THz. The new frequencies are separated from the optical frequency by multiples of the THz frequency and are often referred to as sidebands. To understand the origin of the high-order sidebands observed, which are present up to 8 th order, a

  5. Interaction of Dirac Fermion excitons and biexciton-exciton cascade in graphene quantum dots

    NASA Astrophysics Data System (ADS)

    Ozfidan, Isil; Korkusinski, Marek; Hawrylak, Pawel

    2015-03-01

    We present a microscopic theory of interacting Dirac quasi-electrons and quasi-holes confined in graphene quantum dots. The single particle states of quantum dots are described using a tight binding model and screened direct, exchange, and scattering Coulomb matrix elements are computed using Slater pz orbitals. The many-body ground and excited states are expanded in a finite number of electron-hole pair excitations from the Hartree-Fock ground state and computed using exact diagonalization techniques. The resulting exciton and bi-exciton spectrum reflects the degeneracy of the top of the valence and bottom of the conduction band characteristic of graphene quantum dots with C3 symmetry. We study the interaction of multi-electron and hole complexes as a function of quantum dot size, shape and strength of Coulomb interactions. We identify two degenerate bright exciton (X) states and a corresponding biexciton (XX) state as XX-X cascade candidates, a source of entangled photon pairs. We next calculate the exciton to bi-exciton transitions detected in transient absorption experiments to extract the strength of exciton-exciton interactions and biexciton binding energies. We further explore the possibility of excitonic instability.

  6. G-factors of hole bound states in spherically symmetric potentials in cubic semiconductors

    NASA Astrophysics Data System (ADS)

    Miserev, Dmitry; Sushkov, Oleg

    2016-03-01

    Holes in cubic semiconductors have effective spin 3/2 and very strong spin orbit interaction. Due to these factors properties of hole bound states are highly unusual. We consider a single hole bound by a spherically symmetric potential, this can be an acceptor or a spherically symmetric quantum dot. Linear response to an external magnetic field is characterized by the bound state Lande g-factor. We calculate analytically g-factors of all bound states.

  7. A charge carrier transport model for donor-acceptor blend layers

    NASA Astrophysics Data System (ADS)

    Fischer, Janine; Widmer, Johannes; Kleemann, Hans; Tress, Wolfgang; Koerner, Christian; Riede, Moritz; Vandewal, Koen; Leo, Karl

    2015-01-01

    Highly efficient organic solar cells typically comprise donor-acceptor blend layers facilitating effective splitting of excitons. However, the charge carrier mobility in the blends can be substantially smaller than in neat materials, hampering the device performance. Currently, available mobility models do not describe the transport in blend layers entirely. Here, we investigate hole transport in a model blend system consisting of the small molecule donor zinc phthalocyanine (ZnPc) and the acceptor fullerene C60 in different mixing ratios. The blend layer is sandwiched between p-doped organic injection layers, which prevent minority charge carrier injection and enable exploiting diffusion currents for the characterization of exponential tail states from a thickness variation of the blend layer using numerical drift-diffusion simulations. Trap-assisted recombination must be considered to correctly model the conductivity behavior of the devices, which are influenced by local electron currents in the active layer, even though the active layer is sandwiched in between p-doped contacts. We find that the density of deep tail states is largest in the devices with 1:1 mixing ratio (Et = 0.14 eV, Nt = 1.2 × 1018 cm-3) directing towards lattice disorder as the transport limiting process. A combined field and charge carrier density dependent mobility model are developed for this blend layer.

  8. A charge carrier transport model for donor-acceptor blend layers

    SciTech Connect

    Fischer, Janine Widmer, Johannes; Koerner, Christian; Vandewal, Koen; Leo, Karl; Kleemann, Hans; Tress, Wolfgang; Riede, Moritz

    2015-01-28

    Highly efficient organic solar cells typically comprise donor-acceptor blend layers facilitating effective splitting of excitons. However, the charge carrier mobility in the blends can be substantially smaller than in neat materials, hampering the device performance. Currently, available mobility models do not describe the transport in blend layers entirely. Here, we investigate hole transport in a model blend system consisting of the small molecule donor zinc phthalocyanine (ZnPc) and the acceptor fullerene C{sub 60} in different mixing ratios. The blend layer is sandwiched between p-doped organic injection layers, which prevent minority charge carrier injection and enable exploiting diffusion currents for the characterization of exponential tail states from a thickness variation of the blend layer using numerical drift-diffusion simulations. Trap-assisted recombination must be considered to correctly model the conductivity behavior of the devices, which are influenced by local electron currents in the active layer, even though the active layer is sandwiched in between p-doped contacts. We find that the density of deep tail states is largest in the devices with 1:1 mixing ratio (E{sub t} = 0.14 eV, N{sub t} = 1.2 × 10{sup 18 }cm{sup −3}) directing towards lattice disorder as the transport limiting process. A combined field and charge carrier density dependent mobility model are developed for this blend layer.

  9. Optical spacing effect in organic photovoltaic cells incorporating a dilute acceptor layer

    SciTech Connect

    Menke, S. Matthew; Lindsay, Christopher D.; Holmes, Russell J.

    2014-06-16

    The addition of spacing layers in organic photovoltaic cells (OPVs) can enhance light absorption by optimizing the spatial distribution of the incident optical field in the multilayer structure. We explore the optical spacing effect in OPVs achieved using a diluted electron acceptor layer of C{sub 60}. While optical spacing is often realized by optimizing buffer layer thickness, we find that optical spacing via dilution leads to cells with similar or enhanced photocurrent. This is observed despite a smaller quantity of absorbing molecules, suggesting a more efficient use of absorbed photons. In fact, dilution is found to concentrate optical absorption near the electron donor-acceptor interface, resulting in a marked increase in the exciton diffusion efficiency. Contrasting the use of changes in thickness to engineer optical absorption, the use of dilution does not significantly alter the overall thickness of the OPV. Optical spacing via dilution is shown to be a viable alternative to more traditional optical spacing techniques and may be especially useful in the continued optimization of next-generation, tandem OPVs where it is important to minimize competition for optical absorption between individual sub-cells.

  10. Optically decomposed near-band-edge structure and excitonic transitions in Ga2S3

    PubMed Central

    Ho, Ching-Hwa; Chen, Hsin-Hung

    2014-01-01

    The band-edge structure and band gap are key parameters for a functional chalcogenide semiconductor to its applications in optoelectronics, nanoelectronics, and photonics devices. Here, we firstly demonstrate the complete study of experimental band-edge structure and excitonic transitions of monoclinic digallium trisulfide (Ga2S3) using photoluminescence (PL), thermoreflectance (TR), and optical absorption measurements at low and room temperatures. According to the experimental results of optical measurements, three band-edge transitions of EA = 3.052 eV, EB = 3.240 eV, and EC = 3.328 eV are respectively determined and they are proven to construct the main band-edge structure of Ga2S3. Distinctly optical-anisotropic behaviors by orientation- and polarization-dependent TR measurements are, respectively, relevant to distinguish the origins of the EA, EB, and EC transitions. The results indicated that the three band-edge transitions are coming from different origins. Low-temperature PL results show defect emissions, bound-exciton and free-exciton luminescences in the radiation spectra of Ga2S3. The below-band-edge transitions are respectively characterized. On the basis of experimental analyses, the optical property of near-band-edge structure and excitonic transitions in the monoclinic Ga2S3 crystal is revealed. PMID:25142550

  11. Temporal evolution of Trion-Exciton coupling in transition metal dichalcogenide

    NASA Astrophysics Data System (ADS)

    Tran, Kha; Singh, Akshay; Moody, Galan; Wu, Sanfeng; Ross, Jason; Xu, Xiaodong; Li, Xiaoqin

    2015-03-01

    Transition Metal Dichalcogenides (TMD's), especially in the two-dimensional limit, show remarkable physical phenomena including large light absorption by single layers (up to 10%) and coherent many-body effects. Increased interactions in these two-dimensional materials have been attributed to reduced screening and these are reasoned to cause large binding energies of quasiparticles including excitons (coulomb bound electron-hole pairs) and trions (charged excitons). The coupling amongst these quasiparticles is an interesting fundamental problem relating to strength of electronic interactions as well as having applications in photo-detectors working beyond diffusive transport. We use two-color pump-probe spectroscopy to measure these quasiparticle interactions as a function of time. We concentrate on the special condition of pumping at the trion transition, which is lower in energy than the exciton, and probing the exciton transition. We observe an optical response with distinct temporal response compared to other excitation conditions. The observations are well explained by a phenomenological model incorporating Optical Bloch Equations which suggests many-body effects, including excitation induced shifts, are involved. We acknowledge NSF and AFOSR.

  12. Tuning the driving force for exciton dissociation in single-walled carbon nanotube heterojunctions.

    PubMed

    Ihly, Rachelle; Mistry, Kevin S; Ferguson, Andrew J; Clikeman, Tyler T; Larson, Bryon W; Reid, Obadiah; Boltalina, Olga V; Strauss, Steven H; Rumbles, Garry; Blackburn, Jeffrey L

    2016-06-01

    Understanding the kinetics and energetics of interfacial electron transfer in molecular systems is crucial for the development of a broad array of technologies, including photovoltaics, solar fuel systems and energy storage. The Marcus formulation for electron transfer relates the thermodynamic driving force and reorganization energy for charge transfer between a given donor/acceptor pair to the kinetics and yield of electron transfer. Here we investigated the influence of the thermodynamic driving force for photoinduced electron transfer (PET) between single-walled carbon nanotubes (SWCNTs) and fullerene derivatives by employing time-resolved microwave conductivity as a sensitive probe of interfacial exciton dissociation. For the first time, we observed the Marcus inverted region (in which driving force exceeds reorganization energy) and quantified the reorganization energy for PET for a model SWCNT/acceptor system. The small reorganization energies (about 130 meV, most of which probably arises from the fullerene acceptors) are beneficial in minimizing energy loss in photoconversion schemes. PMID:27219706

  13. Dynamics of exciton recombination in strong magnetic fields in ultrathin GaAs/AlAs quantum wells with indirect band gap and type-II band alignment

    NASA Astrophysics Data System (ADS)

    Shamirzaev, T. S.; Debus, J.; Yakovlev, D. R.; Glazov, M. M.; Ivchenko, E. L.; Bayer, M.

    2016-07-01

    The exciton recombination dynamics is studied experimentally and theoretically in two-monolayer-thick GaAs/AlAs quantum wells characterized by an indirect band gap and a type-II band alignment. At cryogenic temperatures, the lifetimes of the excitons that are indirect both in real and k space are in the millisecond range. The exciton recombination time and the photoluminescence (PL) intensity are strongly dependent on strength and orientation of an applied magnetic field. In contrast to the very weak influence of an in-plane field, at 2 K temperature a field applied parallel to the growth axis drastically slows down the recombination and reduces the PL intensity. With increasing temperature the magnetic field effects on PL intensity and decay time are vanishing. The experimental data are well described by a model for the exciton dynamics that takes into account the magnetic-field-induced redistribution of the indirect excitons between their bright and dark states. It allows us to evaluate the lower bound of the heavy-hole longitudinal g factor of 2.5, the radiative recombination time for the bright excitons of 0.34 ms, and the nonradiative recombination time of the bright and dark excitons of 8.5 ms.

  14. Optical properties of one- and two-dimensional excitons in m-plane ZnO/MgZnO multiple quantum wells

    NASA Astrophysics Data System (ADS)

    Chen, H. R.; Tsai, C. Y.; Huang, Y. C.; Kuo, C. C.; Hsu, H. C.; Hsieh, W. F.

    2016-03-01

    Five pairs of ZnO/Zn0.9Mg0.1O multiple quantum wells (MQWs) with fixed barrier thickness of 55 nm and three well widths of 4, 8 and 16 nm have been grown on m-plane sapphires by pulsed laser deposition. Due to 2D quantum confinement with decreasing well width, the emission of excitons bound to the basal-plane stacking faults, which are one-dimensionally confined in MQWs, encounters larger blue shift than that of the near-band edge excitons. Furthermore, remarkably reducing coupling of free excitons with A 1 longitudinal optical phonons is closely correlated with increasing exciton binding energy but enhanced coupling of E 2-low phonons is a result of increasing interaction with the interface phonons with decreasing well width.

  15. Photoinduced gap closure in an excitonic insulator

    NASA Astrophysics Data System (ADS)

    Golež, Denis; Werner, Philipp; Eckstein, Martin

    2016-07-01

    We study the dynamical phase transition out of an excitonic insulator phase after photoexcitation using a time-dependent extension of the self-consistent GW method. We connect the evolution of the photoemission spectra to the dynamics of the excitonic order parameter and identify two dynamical phase transition points marked by a slowdown in the relaxation: one critical point is connected with the trapping in a nonthermal state with reduced exciton density and the second corresponds to the thermal phase transition. The transfer of kinetic energy from the photoexcited carriers to the exciton condensate is shown to be the main mechanism for the gap melting. We analyze the low energy dynamics of screening, which strongly depends on the presence of the excitonic gap, and argue that it is difficult to interpret the static component of the screened interaction as the effective interaction of some low energy model. Instead we propose a phenomenological measure for the effective interaction which indicates that screening has minor effects on the low energy dynamics.

  16. Plasmon-Exciton Coupling Using DNA Templates.

    PubMed

    Roller, Eva-Maria; Argyropoulos, Christos; Högele, Alexander; Liedl, Tim; Pilo-Pais, Mauricio

    2016-09-14

    Coherent energy exchange between plasmons and excitons is a phenomenon that arises in the strong coupling regime resulting in distinct hybrid states. The DNA-origami technique provides an ideal framework to custom-tune plasmon-exciton nanostructures. By employing this well controlled self-assembly process, we realized hybrid states by precisely positioning metallic nanoparticles in a defined spatial arrangement with fixed nanometer-sized interparticle spacing. Varying the nanoparticle diameter between 30 nm and 60 nm while keeping their separation distance constant allowed us to precisely adjust the plasmon resonance of the structure to accurately match the energy frequency of a J-aggregate exciton. With this system we obtained strong plasmon-exciton coupling and studied far-field scattering at the single-structure level. The individual structures displayed normal mode splitting up to 170 meV. The plasmon tunability and the strong field confinement attained with nanodimers on DNA-origami renders an ideal tool to bottom-up assembly plasmon-exciton systems operating at room temperature. PMID:27531635

  17. Pairing interaction effects in exciton level densities

    SciTech Connect

    Fu, C.Y.

    1989-01-01

    Recent progress in pairing corrections for exciton state-density formulas used in pre-compound nuclear reaction theories is reviewed. These correction factors are, strictly speaking, dependent on the nuclear excitation energy U and the exciton number n. A simple formula for (U,n)-dependent pairing corrections has been derived, based on the BCS pairing equations for constant single-particle spacing, for the exciton state-density formula for one kind of Fermion. It has been shown that the constant-pairing-energy correction used in standard state-density formulas, such U{sub 0} in Gilbert and Cameron, is a limiting case of the general (U,n)-dependent results. Spin cutoff factors with pairing effects were also obtained using the same theory and parameterized into an explicit (U,n)-dependent function, thereby defining a simple exciton level-density formula for applications in quantum mechanical precompound theories. Preliminary results from extending such simple pairing-interaction representations to level-density formulas for two kinds of Fermions are summarized. The results show that the ratios in the exciton level densities in the one-Fermion and two-Fermion approaches vary with both U and n, thus likely leading to differences in calculated compound to precompound ratios. However, the differences in the spin cutoff factors in the two cases are found to be rather small. 12 refs., 3 figs.

  18. Attractive Coulomb interaction of two-dimensional Rydberg excitons

    NASA Astrophysics Data System (ADS)

    Shahnazaryan, V.; Shelykh, I. A.; Kyriienko, O.

    2016-06-01

    We analyze theoretically the Coulomb scattering processes of highly excited excitons in the direct-band-gap semiconductor quantum wells. We find that contrary to the interaction of ground-state excitons, the electron and hole exchange interaction between excited excitons has an attractive character both for s - and p -type two-dimensional (2D) excitons. Moreover, we show that similar to the three-dimensional highly excited excitons, the direct interaction of 2D Rydberg excitons exhibits van der Waals-type long-range interaction. The results predict the linear growth of the absolute value of exchange interaction strength with an exciton principal quantum number and point the way towards enhancement of optical nonlinearity in 2D excitonic systems.

  19. Exciton-Exciton Annihilation in Copper-Phthalocyanine Single-Crystal Nanowires

    SciTech Connect

    Ma, Yingzhong; Xiao, Kai; Shaw, Robert W

    2012-01-01

    Femtosecond one-color pump-probe spectroscopy was applied to study exciton dynamics in single-crystal copper-phthalocyanine (CuPc) nanowires grown on an opaque silicon substrate. The transient reflectance kinetics measured at different pump fluences exhibit a remarkable intensity-dependent decay behavior which accelerates significantly with increasing pump pulse intensity. All the kinetic decays can be satisfactorily described using a bi-exponential decay function with lifetimes of 22 and 204 ps, and corresponding relative amplitudes depending on the pump intensity. The accelerated decay behavior observed at high pump intensities arises from a nonlinear exciton-exciton annihilation process. While this phenomenon has been found previously in crystalline metallophthalocyanine (MPc) polymorphs such as colloidal particles and thin films, the results obtained using the CuPc nanowires are markedly distinct, namely, much longer decay times and a linear intensity dependence of the initial peak amplitudes. Despite these differences, detailed data analysis further shows that, as found for other metal-phthalocyanine polymorphs, exciton-exciton annihilation in the CuPc nanowires is one-dimensional (1D) diffusion-limited, which possibly involves intra-chain exciton diffusion along 1D molecular stacks. The significantly long-lived excitons of CuPc nanowires in comparison to those of other crystalline polymorphs make them particularly suitable for photovoltaic applications.

  20. Identification of effective exciton-exciton annihilation in squaraine-squaraine copolymers.

    PubMed

    Hader, Kilian; May, Volkhard; Lambert, Christoph; Engel, Volker

    2016-05-11

    Ultrafast time-resolved transient absorption spectroscopy is able to monitor the fate of the excited state population in molecular aggregates or polymers. Due to many competing decay processes, the identification of exciton-exciton annihilation (EEA) is difficult. Here, we use a microscopic model to describe exciton annihilation processes in squaraine-squaraine copolymers. Transient absorption time traces measured at different laser powers exhibit an unusual time-dependence. The analysis points towards dynamics taking place on three time-scales. Immediately after laser-excitation a localization of excitons takes place within the femtosecond time-regime. This is followed by exciton-exciton annihilation which is responsible for a fast decay of the exciton population. At later times, excitations being localized on units which are not directly connected remain so that diffusion dominates the dynamics and leads to a slower decay. We thus provide evidence for EEA tracked by time-resolved spectroscopy which has not been reported that clearly before. PMID:27120976

  1. Microscopic theory of two-dimensional spatially-indirect-exciton condensates and exciton-polariton condensates

    NASA Astrophysics Data System (ADS)

    Xue, Fei; Wu, Feng Cheng; MacDonald, Allan

    BEC of excitons and polaritons have drawn attention in recent years because of the demonstration of their ability to host macroscopic quantum phenomena and because of their promise for applications. We study the case of a system containing two TMD monolayers that are separated and surrounded by h-BN. Under appropriate conditions this system is expected to support a spatially indirect thermal equilibrium exciton condensate. We combine a microscopic mean-field calculation and a weakly interacting boson model to explore the bilayer exciton condensates phase diagram. By varying the layer separation and exciton density, we find a phase transition occurs between states containing one and two condensate flavors. We also use a microscopic time-dependent mean-field theory to address condensate collective mode spectra and quantum fluctuations. Next we study the case of exciton-polariton formed by strong coupling between quantum well excitons and confined photon modes when the system is placed in a vertical microcavity. We build a microscopic mean-field theory starting from electrons and holes, and account for their coupling to coherent light field. We compare our model with the normal weakly interacting boson model that starts from weakly interacting excitons that are coupled to photons. This work was supported by the SRC and NIST under the Nanoelectronic Research Initiative (NRI) and SWAN, by the Welch Foundation under Grant No. F1473, and by the ARO Grant No. 26-3508-81.

  2. Excitonic effects in 2D semiconductors: Path Integral Monte Carlo approach

    NASA Astrophysics Data System (ADS)

    Velizhanin, Kirill; Saxena, Avadh

    One of the most striking features of novel 2D semiconductors (e.g., transition metal dichalcogenide monolayers or phosphorene) is a strong Coulomb interaction between charge carriers resulting in large excitonic effects. In particular, this leads to the formation of multi-carrier bound states (e.g., excitons, trions and biexcitons), which could remain stable at near-room temperatures and contribute significantly to optical properties of such materials. In my talk, I will report on our recent progress in using the Path Integral Monte Carlo methodology to numerically study properties of multi-carrier bound states in 2D semiconductors. Incorporating the effect of the dielectric confinement (via Keldysh potential), we have investigated and tabulated the dependence of single exciton, trion and biexciton binding energies on the strength of dielectric screening, including the limiting cases of very strong and very weak screening. The implications of the obtained results and the possible limitations of the used model will be discussed. The results of this work are potentially useful in the analysis of experimental data and benchmarking of theoretical and computational models.

  3. Hyperspectral imaging of exciton photoluminescence in individual carbon nanotubes controlled by high magnetic fields.

    PubMed

    Alexander-Webber, Jack A; Faugeras, Clement; Kossacki, Piotr; Potemski, Marek; Wang, Xu; Kim, Hee Dae; Stranks, Samuel D; Taylor, Robert A; Nicholas, Robin J

    2014-09-10

    Semiconducting carbon nanotubes (CNTs) provide an exceptional platform for studying one-dimensional excitons (bound electron-hole pairs), but the role of defects and quenching centers in controlling emission remains controversial. Here we show that, by wrapping the CNT in a polymer sheath and cooling to 4.2 K, ultranarrow photoluminescence (PL) emission line widths below 80 μeV can be seen from individual solution processed CNTs. Hyperspectral imaging of the tubes identifies local emission sites and shows that some previously dark quenching segments can be brightened by the application of high magnetic fields, and their effect on exciton transport and dynamics can be studied. Using focused high intensity laser irradiation, we introduce a single defect into an individual nanotube which reduces its quantum efficiency by the creation of a shallow bound exciton state with enhanced electron-hole exchange interaction. The emission intensity of the nanotube is then reactivated by the application of the high magnetic field. PMID:25158099

  4. Long-range coupling of electron-hole pairs in spatially separated organic donor-acceptor layers

    PubMed Central

    Nakanotani, Hajime; Furukawa, Taro; Morimoto, Kei; Adachi, Chihaya

    2016-01-01

    Understanding exciton behavior in organic semiconductor molecules is crucial for the development of organic semiconductor-based excitonic devices such as organic light-emitting diodes and organic solar cells, and the tightly bound electron-hole pair forming an exciton is normally assumed to be localized on an organic semiconducting molecule. We report the observation of long-range coupling of electron-hole pairs in spatially separated electron-donating and electron-accepting molecules across a 10-nanometers-thick spacer layer. We found that the exciton energy can be tuned over 100 megaelectron volts and the fraction of delayed fluorescence can be increased by adjusting the spacer-layer thickness. Furthermore, increasing the spacer-layer thickness produced an organic light-emitting diode with an electroluminescence efficiency nearly eight times higher than that of a device without a spacer layer. Our results demonstrate the first example of a long-range coupled charge-transfer state between electron-donating and electron-accepting molecules in a working device. PMID:26933691

  5. Theory of exciton linewidth in II VI semiconductor mixed crystals

    NASA Astrophysics Data System (ADS)

    Zimmermann, R.

    1990-04-01

    The disorder-induced broadening of excitons in mixed crystals is discussed, using a novel expression for the relevant exciton volume. Earlier experimental data on CdS 1-xSe x are successfully explained. The exciton broadening in quantum wells due to well-width fluctuations is obtained along similar lines.

  6. Exciton dynamics in perturbed vibronic molecular aggregates

    PubMed Central

    Brüning, C.; Wehner, J.; Hausner, J.; Wenzel, M.; Engel, V.

    2015-01-01

    A site specific perturbation of a photo-excited molecular aggregate can lead to a localization of excitonic energy. We investigate this localization dynamics for laser-prepared excited states. Changing the parameters of the electric field significantly influences the exciton localization which offers the possibility for a selective control of this process. This is demonstrated for aggregates possessing a single vibrational degree of freedom per monomer unit. It is shown that the effects identified for the molecular dimer can be generalized to larger aggregates with a high density of vibronic states. PMID:26798840

  7. Control of exciton transport using quantum interference

    NASA Astrophysics Data System (ADS)

    Lusk, Mark T.; Stafford, Charles A.; Zimmerman, Jeramy D.; Carr, Lincoln D.

    2015-12-01

    It is shown that quantum interference can be employed to create an exciton transistor. An applied potential gates the quasiparticle motion and also discriminates between quasiparticles of differing binding energy. When implemented within nanoscale assemblies, such control elements could mediate the flow of energy and information. Quantum interference can also be used to dissociate excitons as an alternative to using heterojunctions. A finite molecular setting is employed to exhibit the underlying discrete, two-particle, mesoscopic analog to Fano antiresonance. Selected entanglement measures are shown to distinguish regimes of behavior which cannot be resolved from population dynamics alone.

  8. Multiple Exciton Generation in Silicon QD arrays

    NASA Astrophysics Data System (ADS)

    Kryjevski, Andrei; Kilin, Dmitri

    2014-03-01

    We use Density Functional Theory (DFT) combined with the many body perturbation theory to calculate multiple exciton generation (MEG) in several semiconductor nanosystems. Hydrogen-passivated Si29H36 quantum dots (QDs) with crystalline and amorphous core structures, the quasi one dimensional (1-D) arrays constructed from these QDs, as well as crystalline and amorphous Si nanowires have been studied. Quantum efficiency, the average number of excitons created by a single photon, has been calculated in these nanoparticles to the leading order in the screened Coulomb interaction. Amorphous nanostructures are predicted to have more effective carrier multiplication.

  9. Exciton dynamics in perturbed vibronic molecular aggregates.

    PubMed

    Brüning, C; Wehner, J; Hausner, J; Wenzel, M; Engel, V

    2016-07-01

    A site specific perturbation of a photo-excited molecular aggregate can lead to a localization of excitonic energy. We investigate this localization dynamics for laser-prepared excited states. Changing the parameters of the electric field significantly influences the exciton localization which offers the possibility for a selective control of this process. This is demonstrated for aggregates possessing a single vibrational degree of freedom per monomer unit. It is shown that the effects identified for the molecular dimer can be generalized to larger aggregates with a high density of vibronic states. PMID:26798840

  10. Evidence of Delocalization in Charge-Transfer State Manifold for Donor:Acceptor Organic Photovoltaics.

    PubMed

    Guan, Zhiqiang; Li, Ho-Wa; Zhang, Jinfeng; Cheng, Yuanhang; Yang, Qingdan; Lo, Ming-Fai; Ng, Tsz-Wai; Tsang, Sai-Wing; Lee, Chun-Sing

    2016-08-24

    How charge-transfer states (CTSs) assist charge separation of a Coulombically bound exciton in organic photovoltaics has been a hot topic. It is believed that the delocalization feature of a CTS plays a crucial role in the charge separation process. However, the delocalization of the "hot" and the "relaxed" CTSs is still under debate. Here, with a novel frequency dependent charge-modulated electroabsorption spectroscopy (CMEAS) technique, we elucidate clearly that both "hot" and "relaxed" CTSs are loosely bound and delocalized states. This is confirmed by comparing the CMEAS results of CTSs with those of localized polaron states. Our results reveal the role of CTS delocalization on charge separation and indicate that no substantial delocalization gradient exists in CTSs. PMID:27482867

  11. Excitons in van der Waals heterostructures: The important role of dielectric screening

    NASA Astrophysics Data System (ADS)

    Latini, S.; Olsen, T.; Thygesen, K. S.

    2015-12-01

    The existence of strongly bound excitons is one of the hallmarks of the newly discovered atomically thin semiconductors. While it is understood that the large binding energy is mainly due to the weak dielectric screening in two dimensions, a systematic investigation of the role of screening on two-dimensional (2D) excitons is still lacking. Here we provide a critical assessment of a widely used 2D hydrogenic exciton model, which assumes a dielectric function of the form ɛ (q )=1 +2 π α q , and we develop a quasi-2D model with a much broader applicability. Within the quasi-2D picture, electrons and holes are described as in-plane point charges with a finite extension in the perpendicular direction, and their interaction is screened by a dielectric function with a nonlinear q dependence which is computed ab initio. The screened interaction is used in a generalized Mott-Wannier model to calculate exciton binding energies in both isolated and supported 2D materials. For isolated 2D materials, the quasi-2D treatment yields results almost identical to those of the strict 2D model, and both are in good agreement with ab initio many-body calculations. On the other hand, for more complex structures such as supported layers or layers embedded in a van der Waals heterostructure, the size of the exciton in reciprocal space extends well beyond the linear regime of the dielectric function, and a quasi-2D description has to replace the 2D one. Our methodology has the merit of providing a seamless connection between the strict 2D limit of isolated monolayer materials and the more bulk-like screening characteristics of supported 2D materials or van der Waals heterostructures.

  12. Alkyl Chlorides as Hydrogen Bond Acceptors

    SciTech Connect

    Nadas, Janos I; Vukovic, Sinisa; Hay, Benjamin

    2012-01-01

    To gain an understanding of the role of an alkyl chloride as a hydrogen bond acceptor, geometries and interaction energies were calculated at the MP2/aug-cc-pVDZ level of theory for complexes between ethyl chloride and representative hydrogen donor groups. The results establish that these donors, which include hydrogen cyanide, methanol, nitrobenzene, pyrrole, acetamide, and N-methylurea, form X-H {hor_ellipsis} Cl hydrogen bonds (X = C, N, O) of weak to moderate strength, with {Delta}E values ranging from -2.8 to -5.3 kcal/mol.

  13. Temperature Evolution of Excitonic Absorptions in Cd(1-x)Zn(x)Te Materials

    NASA Technical Reports Server (NTRS)

    Quijada, Manuel A.; Henry, Ross

    2007-01-01

    The studies consist of measuring the frequency dependent transmittance (T) and reflectance (R) above and below the optical band-gap in the UV/Visible and infrared frequency ranges for Cd(l-x),Zn(x),Te materials for x=0 and x=0.04. Measurements were also done in the temperature range from 5 to 300 K. The results show that the optical gap near 1.49 eV at 300 K increases to 1.62 eV at 5 K. Finally, we observe sharp absorption peaks near this gap energy at low temperatures. The close proximity of these peaks to the optical transition threshold suggests that they originate from the creation of bound electron-hole pairs or excitons. The decay of these excitonic absorptions may contribute to a photoluminescence and transient background response of these back-illuminated HgCdTe CCD detectors.

  14. Exciton effects in the index of refraction of multiple quantum wells and superlattices

    NASA Technical Reports Server (NTRS)

    Kahen, K. B.; Leburton, J. P.

    1986-01-01

    Theoretical calculations of the index of refraction of multiple quantum wells and superlattices are presented. The model incorporates both the bound and continuum exciton contributions for the gamma region transitions. In addition, the electronic band structure model has both superlattice and bulk alloy properties. The results indicate that large light-hole masses, i.e., of about 0.23, produced by band mixing effects, are required to account for the experimental data. Furthermore, it is shown that superlattice effects rapidly decrease for energies greater than the confining potential barriers. Overall, the theoretical results are in very good agreement with the experimental data and show the importance of including exciton effects in the index of refraction.

  15. Bright and dark excitons in semiconductor carbon nanotubes

    SciTech Connect

    Tretiak, Sergei

    2008-01-01

    We report electronic structure calculations of finite-length semiconducting carbon nanotubes using the time dependent density functional theory (TD-DFT) and the time dependent Hartree Fock (TD-HF) approach coupled with semiempirical AM1 and ZINDO Hamiltonians. We specifically focus on the energy splitting, relative ordering, and localization properties of the optically active (bright) and optically forbidden (dark) states from the lowest excitonic band of the nanotubes. These excitonic states are very important in competing radiative and non-radiative processes in these systems. Our analysis of excitonic transition density matrices demonstrates that pure DFT functionals overdelocalize excitons making an electron-hole pair unbound; consequently, excitonic features are not presented in this method. In contrast, the pure HF and A111 calculations overbind excitons inaccurately predicting the lowest energy state as a bright exciton. Changing AM1 with ZINDO Hamiltonian in TD-HF calculations, predicts the bright exciton as the second state after the dark one. However, in contrast to AM1 calculations, the diameter dependence of the excitation energies obtained by ZINDO does not follow the experimental trends. Finally, the TD-DFT approach incorporating hybrid functions with a moderate portion of the long-range HF exchange, such as B3LYP, has the most generality and predictive capacity providing a sufficiently accurate description of excitonic structure in finite-size nanotubes. These methods characterize four important lower exciton bands. The lowest state is dark, the upper band is bright, and the two other dark and nearly degenerate excitons lie in-between. Although the calculated energy splittings between the lowest dark and the bright excitons are relatively large ({approx}0.1 eV), the dense excitonic manifold below the bright exciton allows for fast non-radiative relaxation leasing to the fast population of the lowest dark exciton. This rationalizes the low

  16. Engineering excitonic dynamics and environmental stability of post-transition metal chalcogenides by pyridine functionalization technique

    NASA Astrophysics Data System (ADS)

    Meng, Xiuqing; Pant, Anupum; Cai, Hui; Kang, Jun; Sahin, Hasan; Chen, Bin; Wu, Kedi; Yang, Sijie; Suslu, Aslihan; Peeters, F. M.; Tongay, Sefaattin

    2015-10-01

    Owing to their strong photon emission, low excitonic binding energies, and nearly-ideal band offset values for water splitting reactions, direct gap quasi-2D gallium chalcogenides are potential candidates for applications in energy harvesting, optoelectronics, and photonics. Unlike other 2D materials systems, chemical functionalization of gallium chalcogenides is still at its seminal stages. Here, we propose vapor phase pyridine intercalation technique to manipulate optical properties of gallium chalcogenides. After functionalization, the excitonic dynamics of quasi-2D GaSe change significantly as evidenced by an increase in integrated PL intensity and emergence of a new emission feature that is below the band edge. Based on our DFT calculations, we attribute these to formation of bound exciton complexes at the trap sites introduced by chemical reaction between pyridine and GaSe. On the contrary, pyridine functionalization does not impact the optical properties of GaTe, instead treats GaTe surface to prevent oxidization of tellurium atoms. Overall, results suggest novel ways to control properties of gallium chalcogenides on demand and unleash their full potential for a range of applications in photonics and optoelectronics.Owing to their strong photon emission, low excitonic binding energies, and nearly-ideal band offset values for water splitting reactions, direct gap quasi-2D gallium chalcogenides are potential candidates for applications in energy harvesting, optoelectronics, and photonics. Unlike other 2D materials systems, chemical functionalization of gallium chalcogenides is still at its seminal stages. Here, we propose vapor phase pyridine intercalation technique to manipulate optical properties of gallium chalcogenides. After functionalization, the excitonic dynamics of quasi-2D GaSe change significantly as evidenced by an increase in integrated PL intensity and emergence of a new emission feature that is below the band edge. Based on our DFT calculations

  17. Exciton fission and charge generation via triplet excitons in pentacene/C60 bilayers.

    PubMed

    Rao, Akshay; Wilson, Mark W B; Hodgkiss, Justin M; Albert-Seifried, Sebastian; Bässler, Heinz; Friend, Richard H

    2010-09-15

    Organic photovoltaic devices are currently studied due to their potential suitability for flexible and large-area applications, though efficiencies are presently low. Here we study pentacene/C(60) bilayers using transient optical absorption spectroscopy; such structures exhibit anomalously high quantum efficiencies. We show that charge generation primarily occurs 2-10 ns after photoexcitation. This supports a model where charge is generated following the slow diffusion of triplet excitons to the heterojunction. These triplets are shown to be present from early times (<200 fs) and result from the fission of a spin-singlet exciton to form two spin-triplet excitons. These results elucidate exciton and charge generation dynamics in the pentacene/C(60) system and demonstrate that the tuning of the energetic levels of organic molecules to take advantages of singlet fission could lead to greatly enhanced photocurrent in future OPVs. PMID:20735067

  18. Exciton-exciton annihilation and relaxation pathways in semiconducting carbon nanotubes

    NASA Astrophysics Data System (ADS)

    Chmeliov, Jevgenij; Narkeliunas, Jonas; Graham, Matt W.; Fleming, Graham R.; Valkunas, Leonas

    2016-01-01

    We present a thorough analysis of one- and two-color transient absorption measurements performed on single- and double-walled semiconducting carbon nanotubes. By combining the currently existing models describing exciton-exciton annihilation--the coherent and the diffusion-limited ones--we are able to simultaneously reproduce excitation kinetics following both E11 and E22 pump conditions. Our simulations revealed the fundamental photophysical behavior of one-dimensional coherent excitons and non-trivial excitation relaxation pathways. In particular, we found that after non-linear annihilation a doubly-excited exciton relaxes directly to its E11 state bypassing the intermediate E22 manifold, so that after excitation resonant with the E11 transition, the E22 state remains unpopulated. A quantitative explanation for the observed much faster excitation kinetics probed at E22 manifold, comparing to those probed at the E11 band, is also provided.

  19. Topological aspects of nonlinear excitonic processes in noncentrosymmetric crystals

    NASA Astrophysics Data System (ADS)

    Morimoto, Takahiro; Nagaosa, Naoto

    2016-07-01

    We study excitonic processes second order in the electric fields in noncentrosymmetric crystals. We derive formulas for shift current and second harmonic generation produced by exciton creation, by using the Floquet formalism combined with the Keldysh Green's function method. It is shown that (i) the steady dc shift current flows by exciton creation without dissociation into free carriers and (ii) second harmonic generation is enhanced at the exciton resonance. The obtained formulas clarify topological aspects of these second order excitonic processes which are described by Berry connections of the relevant valence and conduction bands.

  20. Photoluminescence properties and exciton dynamics in monolayer WSe2

    NASA Astrophysics Data System (ADS)

    Yan, Tengfei; Qiao, Xiaofen; Liu, Xiaona; Tan, Pingheng; Zhang, Xinhui

    2014-09-01

    In this work, comprehensive temperature and excitation power dependent photoluminescence and time-resolved photoluminescence studies are carried out on monolayer WSe2 to reveal its properties of exciton emissions and related excitonic dynamics. Competitions between the localized and delocalized exciton emissions, as well as the exciton and trion emissions are observed, respectively. These competitions are suggested to be responsible for the abnormal temperature and excitation intensity dependent photoluminescence properties. The radiative lifetimes of both excitons and trions exhibit linear dependence on temperature within the temperature regime below 260 K, providing further evidence for two-dimensional nature of monolayer material.

  1. Exciton localization and drift in tailored-potential quantum nanowires

    SciTech Connect

    Szeszko, J. Rudra, A.; Kapon, E.; Belykh, V. V.; Sibeldin, N. N.

    2014-06-30

    Exciton recombination dynamics in tailored-potential, site-controlled AlGaAs quantum wires (QWRs) are studied. Time-resolved photoluminescence spectra evidence exciton localization in weakly disordered “uniform” QWRs, whereas deterministic bandgap grading is shown to suppress localization and promote exciton drift along the potential gradient. Measured exciton transit times between two quantum dot probes placed at opposite ends of the potential gradient yield the effective 1D exciton mobility as >1300 cm{sup 2}/(eVs).

  2. electric dipole superconductor in bilayer exciton system

    NASA Astrophysics Data System (ADS)

    Sun, Qing-Feng; Jiang, Qing-Dong; Bao, Zhi-Qiang; Xie, X. C.

    Recently, it was reported that the bilayer exciton systems could exhibit many new phenomena, including the large bilayer counterflow conductivity, the Coulomb drag, etc. These phenomena imply the formation of exciton condensate superfluid state. On the other hand, it is now well known that the superconductor is the condensate superfluid state of the Cooper pairs, which can be viewed as electric monopoles. In other words, the superconductor state is the electric monopole condensate superfluid state. Thus, one may wonder whether there exists electric dipole superfluid state. In this talk, we point out that the exciton in a bilayer system can be considered as a charge neutral electric dipole. And we derive the London-type and Ginzburg-Landau-type equations of electric dipole superconductivity. From these equations, we discover the Meissner-type effect (against spatial variation of magnetic fields), and the dipole current Josephson effect. The frequency in the AC Josephson effect of the dipole current is equal to that in the normal (monopole) superconductor. These results can provide direct evidence for the formation of exciton superfluid state in the bilayer systems and pave new ways to obtain the electric dipole current. We gratefully acknowledge the financial support by NBRP of China (2012CB921303 and 2015CB921102) and NSF-China under Grants Nos. 11274364 and 11574007.

  3. Multiple Exciton Generation in Colloidal Silicon Nanocrystals

    SciTech Connect

    Beard, M. C.; Knutsen, K. P.; Yu, P.; Luther, J. M.; Song, Q.; Metzger, W. K.; Ellingson, R. J.; Nozik, A. M.

    2007-01-01

    Multiple exciton generation (MEG) is a process whereby multiple electron-hole pairs, or excitons, are produced upon absorption of a single photon in semiconductor nanocrystals (NCs) and represents a promising route to increased solar conversion efficiencies in single-junction photovoltaic cells. We report for the first time MEG yields in colloidal Si NCs using ultrafast transient absorption spectroscopy. We find the threshold photon energy for MEG in 9.5 nm diameter Si NCs (effective band gap {identical_to} Eg = 1.20 eV) to be 2.4 {+-} 0.1E{sub g} and find an exciton-production quantum yield of 2.6 {+-} 0.2 excitons per absorbed photon at 3.4E{sub g}. While MEG has been previously reported in direct-gap semiconductor NCs of PbSe, PbS, PbTe, CdSe, and InAs, this represents the first report of MEG within indirect-gap semiconductor NCs. Furthermore, MEG is found in relatively large Si NCs (diameter equal to about twice the Bohr radius) such that the confinement energy is not large enough to produce a large blue-shift of the band gap (only 80 meV), but the Coulomb interaction is sufficiently enhanced to produce efficient MEG. Our findings are of particular importance because Si dominates the photovoltaic solar cell industry, presents no problems regarding abundance and accessibility within the Earth's crust, and poses no significant environmental problems regarding toxicity.

  4. Quantum-confined emission and fluorescence blinking of individual exciton complexes in CdSe nanowires.

    PubMed

    Franz, Dennis; Reich, Aina; Strelow, Christian; Wang, Zhe; Kornowski, Andreas; Kipp, Tobias; Mews, Alf

    2014-11-12

    One-dimensional semiconductor nanostructures combine electron mobility in length direction with the possibility of tailoring the physical properties by confinement effects in radial direction. Here we show that thin CdSe quantum nanowires exhibit low-temperature fluorescence spectra with a specific universal structure of several sharp lines. The structure strongly resembles the pattern of bulk spectra but show a diameter-dependent shift due to confinement effects. Also the fluorescence shows a pronounced complex blinking behavior with very different blinking dynamics of different emission lines in one and the same spectrum. Time- and space-resolved optical spectroscopy are combined with high-resolution transmission electron microscopy of the very same quantum nanowires to establish a detailed structure-property relationship. Extensive numerical simulations strongly suggest that excitonic complexes involving donor and acceptor sites are the origin of the feature-rich spectra. PMID:25343231

  5. Indirect excitons in a potential energy landscape created by a perforated electrode

    NASA Astrophysics Data System (ADS)

    Dorow, C. J.; Kuznetsova, Y. Y.; Leonard, J. R.; Chu, M. K.; Butov, L. V.; Wilkes, J.; Hanson, M.; Gossard, A. C.

    2016-02-01

    We report on the principle and realization of an excitonic device: a ramp that directs the transport of indirect excitons down a potential energy gradient created by a perforated electrode at a constant voltage. The device provides an experimental proof of principle for controlling exciton transport with electrode density gradients. We observed that the exciton transport distance along the ramp increases with increasing exciton density. This effect is explained in terms of disorder screening by repulsive exciton-exciton interactions.

  6. Dynamics of the excitonic coupling in organic crystals.

    PubMed

    Aragó, Juan; Troisi, Alessandro

    2015-01-16

    We show that the excitonic coupling in molecular crystals undergoes a very large fluctuation at room temperature as a result of the combined thermal motions of the nuclei. This observation dramatically affects the description of exciton transport in organic crystals and any other phenomenon (like singlet fission or exciton dissociation) that originates from an exciton in a molecular crystal or thin film. This unexpected result is due to the predominance of the short-range excitonic coupling mechanisms (exchange, overlap, and charge-transfer mediated) over the Coulombic excitonic coupling for molecules in van der Waals contact. To quantify this effect we develop a procedure to evaluate accurately the short-range excitonic coupling (via a diabatization scheme) along a molecular dynamics trajectory of the representative molecular crystals of anthracene and tetracene. PMID:25635554

  7. Dynamics of the Excitonic Coupling in Organic Crystals

    NASA Astrophysics Data System (ADS)

    Aragó, Juan; Troisi, Alessandro

    2015-01-01

    We show that the excitonic coupling in molecular crystals undergoes a very large fluctuation at room temperature as a result of the combined thermal motions of the nuclei. This observation dramatically affects the description of exciton transport in organic crystals and any other phenomenon (like singlet fission or exciton dissociation) that originates from an exciton in a molecular crystal or thin film. This unexpected result is due to the predominance of the short-range excitonic coupling mechanisms (exchange, overlap, and charge-transfer mediated) over the Coulombic excitonic coupling for molecules in van der Waals contact. To quantify this effect we develop a procedure to evaluate accurately the short-range excitonic coupling (via a diabatization scheme) along a molecular dynamics trajectory of the representative molecular crystals of anthracene and tetracene.

  8. Ultrafast dynamics of exciton fission in polycrystalline pentacene.

    PubMed

    Wilson, Mark W B; Rao, Akshay; Clark, Jenny; Kumar, R Sai Santosh; Brida, Daniele; Cerullo, Giulio; Friend, Richard H

    2011-08-10

    We use ultrafast transient absorption spectroscopy with sub-20 fs time resolution and broad spectral coverage to directly probe the process of exciton fission in polycrystalline thin films of pentacene. We observe that the overwhelming majority of initially photogenerated singlet excitons evolve into triplet excitons on an ∼80 fs time scale independent of the excitation wavelength. This implies that exciton fission occurs at a rate comparable to phonon-mediated exciton localization processes and may proceed directly from the initial, delocalized, state. The singlet population is identified due to the brief presence of stimulated emission, which is emitted at wavelengths which vary with the photon energy of the excitation pulse, a violation of Kasha's Rule that confirms that the lowest-lying singlet state is extremely short-lived. This direct demonstration that triplet generation is both rapid and efficient establishes multiple exciton generation by exciton fission as an attractive route to increased efficiency in organic solar cells. PMID:21755937

  9. Efficient dipole-dipole coupling of Mott-Wannier and Frenkel excitons in (Ga,In)N quantum well/polyfluorene semiconductor heterostructures

    NASA Astrophysics Data System (ADS)

    Itskos, G.; Heliotis, G.; Lagoudakis, P. G.; Lupton, J.; Barradas, N. P.; Alves, E.; Pereira, S.; Watson, I. M.; Dawson, M. D.; Feldmann, J.; Murray, R.; Bradley, D. D. C.

    2007-07-01

    We investigate interactions between Mott-Wannier (MW) and Frenkel excitons in a family of hybrid structures consisting of thin organic (polyfluorene) films placed in close proximity (systematically adjusted by GaN cap layer thickness) to single inorganic [(Ga,In)N/GaN] quantum wells (QWs). Characterization of the QW structures using Rutherford backscattering spectrometry and atomic force microscopy allows direct measurement of the thickness and the morphology of the GaN cap layers. Time-resolved photoluminescence experiments in the 8-75K temperature range confirm our earlier demonstration that nonradiative energy transfer can occur between inorganic and organic semiconductors. We assign the transfer mechanism to resonant Förster (dipole-dipole) coupling between MW exciton energy donors and Frenkel exciton energy acceptors and at 15K we find transfer efficiencies of up to 43%. The dependence of the energy transfer rate on the distance R between the inorganic QW donor dipole and organic film acceptor dipole indicates that a plane-plane interaction, characterized by a 1/R2 variation, best describes the situation found in our structures.

  10. Analysis of Triplet Exciton Loss Pathways in PTB7:PC71BM Bulk Heterojunction Solar Cells

    NASA Astrophysics Data System (ADS)

    Kraus, Hannes; Heiber, Michael C.; Väth, Stefan; Kern, Julia; Deibel, Carsten; Sperlich, Andreas; Dyakonov, Vladimir

    2016-07-01

    A strategy for increasing the conversion efficiency of organic photovoltaics has been to increase the VOC by tuning the energy levels of donor and acceptor components. However, this opens up a new loss pathway from an interfacial charge transfer state to a triplet exciton (TE) state called electron back transfer (EBT), which is detrimental to device performance. To test this hypothesis, we study triplet formation in the high performing PTB7:PC71BM blend system and determine the impact of the morphology-optimizing additive 1,8-diiodoctane (DIO). Using photoluminescence and spin-sensitive optically detected magnetic resonance (ODMR) measurements at low temperature, we find that TEs form on PC71BM via intersystem crossing from singlet excitons and on PTB7 via EBT mechanism. For DIO blends with smaller fullerene domains, an increased density of PTB7 TEs is observed. The EBT process is found to be significant only at very low temperature. At 300 K, no triplets are detected via ODMR, and electrically detected magnetic resonance on optimized solar cells indicates that TEs are only present on the fullerenes. We conclude that in PTB7:PC71BM devices, TE formation via EBT is impacted by fullerene domain size at low temperature, but at room temperature, EBT does not represent a dominant loss pathway.

  11. Analysis of Triplet Exciton Loss Pathways in PTB7:PC71BM Bulk Heterojunction Solar Cells

    PubMed Central

    Kraus, Hannes; Heiber, Michael C.; Väth, Stefan; Kern, Julia; Deibel, Carsten; Sperlich, Andreas; Dyakonov, Vladimir

    2016-01-01

    A strategy for increasing the conversion efficiency of organic photovoltaics has been to increase the VOC by tuning the energy levels of donor and acceptor components. However, this opens up a new loss pathway from an interfacial charge transfer state to a triplet exciton (TE) state called electron back transfer (EBT), which is detrimental to device performance. To test this hypothesis, we study triplet formation in the high performing PTB7:PC71BM blend system and determine the impact of the morphology-optimizing additive 1,8-diiodoctane (DIO). Using photoluminescence and spin-sensitive optically detected magnetic resonance (ODMR) measurements at low temperature, we find that TEs form on PC71BM via intersystem crossing from singlet excitons and on PTB7 via EBT mechanism. For DIO blends with smaller fullerene domains, an increased density of PTB7 TEs is observed. The EBT process is found to be significant only at very low temperature. At 300 K, no triplets are detected via ODMR, and electrically detected magnetic resonance on optimized solar cells indicates that TEs are only present on the fullerenes. We conclude that in PTB7:PC71BM devices, TE formation via EBT is impacted by fullerene domain size at low temperature, but at room temperature, EBT does not represent a dominant loss pathway. PMID:27380928

  12. Analysis of Triplet Exciton Loss Pathways in PTB7:PC71BM Bulk Heterojunction Solar Cells.

    PubMed

    Kraus, Hannes; Heiber, Michael C; Väth, Stefan; Kern, Julia; Deibel, Carsten; Sperlich, Andreas; Dyakonov, Vladimir

    2016-01-01

    A strategy for increasing the conversion efficiency of organic photovoltaics has been to increase the VOC by tuning the energy levels of donor and acceptor components. However, this opens up a new loss pathway from an interfacial charge transfer state to a triplet exciton (TE) state called electron back transfer (EBT), which is detrimental to device performance. To test this hypothesis, we study triplet formation in the high performing PTB7:PC71BM blend system and determine the impact of the morphology-optimizing additive 1,8-diiodoctane (DIO). Using photoluminescence and spin-sensitive optically detected magnetic resonance (ODMR) measurements at low temperature, we find that TEs form on PC71BM via intersystem crossing from singlet excitons and on PTB7 via EBT mechanism. For DIO blends with smaller fullerene domains, an increased density of PTB7 TEs is observed. The EBT process is found to be significant only at very low temperature. At 300 K, no triplets are detected via ODMR, and electrically detected magnetic resonance on optimized solar cells indicates that TEs are only present on the fullerenes. We conclude that in PTB7:PC71BM devices, TE formation via EBT is impacted by fullerene domain size at low temperature, but at room temperature, EBT does not represent a dominant loss pathway. PMID:27380928

  13. Quantum computing with acceptor spins in silicon

    NASA Astrophysics Data System (ADS)

    Salfi, Joe; Tong, Mengyang; Rogge, Sven; Culcer, Dimitrie

    2016-06-01

    The states of a boron acceptor near a Si/SiO2 interface, which bind two low-energy Kramers pairs, have exceptional properties for encoding quantum information and, with the aid of strain, both heavy hole and light hole-based spin qubits can be designed. Whereas a light-hole spin qubit was introduced recently (arXiv:1508.04259), here we present analytical and numerical results proving that a heavy-hole spin qubit can be reliably initialised, rotated and entangled by electrical means alone. This is due to strong Rashba-like spin–orbit interaction terms enabled by the interface inversion asymmetry. Single qubit rotations rely on electric-dipole spin resonance (EDSR), which is strongly enhanced by interface-induced spin–orbit terms. Entanglement can be accomplished by Coulomb exchange, coupling to a resonator, or spin–orbit induced dipole–dipole interactions. By analysing the qubit sensitivity to charge noise, we demonstrate that interface-induced spin–orbit terms are responsible for sweet spots in the dephasing time {T}2* as a function of the top gate electric field, which are close to maxima in the EDSR strength, where the EDSR gate has high fidelity. We show that both qubits can be described using the same starting Hamiltonian, and by comparing their properties we show that the complex interplay of bulk and interface-induced spin–orbit terms allows a high degree of electrical control and makes acceptors potential candidates for scalable quantum computation in Si.

  14. Observation of long-lived interlayer excitons in monolayer MoSe2–WSe2 heterostructures

    SciTech Connect

    Rivera, Pasqual; Schaibley, John R.; Jones, Aaron M.; Ross, Jason S.; Wu, Sanfeng; Aivazian, Grant; Klement, Philip; Seyler, Kyle; Clark, Genevieve; Ghimire, Nirmal J.; Yan, Jiaqiang; Mandrus, D. G.; Yao, Wang; Xu, Xiaodong

    2015-02-24

    Van der Waals bound heterostructures constructed with two-dimensional materials, such as graphene, boron nitride and transition metal dichalcogenides, have sparked wide interest in both device physics and technologies at the two-dimensional limit. One highly coveted heterostructure is that of differing monolayer transition metal dichalcogenides with type-II band alignment, with bound electrons and holes localized in individual monolayers, that is, interlayer excitons. Here, we report the observation of interlayer excitons in monolayer MoSe2–WSe2 heterostructures by photoluminescence and photoluminescence excitation spectroscopy. The energy and luminescence intensity are highly tunable by an applied vertical gate voltage. Moreover, we measure an interlayer exciton lifetime of ~1.8 ns, an order of magnitude longer than intralayer excitons in monolayers. Ultimately, our work demonstrates optical pumping of interlayer electric polarization, which may provoke further exploration of interlayer exciton condensation, as well as new applications in two-dimensional lasers, light-emitting diodes and photovoltaic devices.

  15. Balance the Carrier Mobility To Achieve High Performance Exciplex OLED Using a Triazine-Based Acceptor.

    PubMed

    Hung, Wen-Yi; Chiang, Pin-Yi; Lin, Shih-Wei; Tang, Wei-Chieh; Chen, Yi-Ting; Liu, Shih-Hung; Chou, Pi-Tai; Hung, Yi-Tzu; Wong, Ken-Tsung

    2016-02-01

    A star-shaped 1,3,5-triazine/cyano hybrid molecule CN-T2T was designed and synthesized as a new electron acceptor for efficient exciplex-based OLED emitter by mixing with a suitable electron donor (Tris-PCz). The CN-T2T/Tris-PCz exciplex emission shows a high ΦPL of 0.53 and a small ΔET-S = -0.59 kcal/mol, affording intrinsically efficient fluorescence and highly efficient exciton up-conversion. The large energy level offsets between Tris-PCz and CN-T2T and the balanced hole and electron mobility of Tris-PCz and CN-T2T, respectively, ensuring sufficient carrier density accumulated in the interface for efficient generation of exciplex excitons. Employing a facile device structure composed as ITO/4% ReO3:Tris-PCz (60 nm)/Tris-PCz (15 nm)/Tris-PCz:CN-T2T(1:1) (25 nm)/CN-T2T (50 nm)/Liq (0.5 nm)/Al (100 nm), in which the electron-hole capture is efficient without additional carrier injection barrier from donor (or acceptor) molecule and carriers mobilities are balanced in the emitting layer, leads to a highly efficient green exciplex OLED with external quantum efficiency (EQE) of 11.9%. The obtained EQE is 18% higher than that of a comparison device using an exciplex exhibiting a comparable ΦPL (0.50), in which TCTA shows similar energy levels but higher hole mobility as compared with Tris-PCz. Our results clearly indicate the significance of mobility balance in governing the efficiency of exciplex-based OLED. Exploiting the Tris-PCz:CN-T2T exciplex as the host, we further demonstrated highly efficient yellow and red fluorescent OLEDs by doping 1 wt % Rubrene and DCJTB as emitter, achieving high EQE of 6.9 and 9.7%, respectively. PMID:26820247

  16. Exciton radiative lifetime in transition metal dichalcogenide monolayers

    NASA Astrophysics Data System (ADS)

    Robert, C.; Lagarde, D.; Cadiz, F.; Wang, G.; Lassagne, B.; Amand, T.; Balocchi, A.; Renucci, P.; Tongay, S.; Urbaszek, B.; Marie, X.

    2016-05-01

    We have investigated the exciton dynamics in transition metal dichalcogenide monolayers using time-resolved photoluminescence experiments performed with optimized time resolution. For MoS e2 monolayer, we measure τrad0=1.8 ±0.2 ps at T =7 K that we interpret as the intrinsic radiative recombination time. Similar values are found for WS e2 monolayers. Our detailed analysis suggests the following scenario: at low temperature (T ≲50 K ), the exciton oscillator strength is so large that the entire light can be emitted before the time required for the establishment of a thermalized exciton distribution. For higher lattice temperatures, the photoluminescence dynamics is characterized by two regimes with very different characteristic times. First the photoluminescence intensity drops drastically with a decay time in the range of the picosecond driven by the escape of excitons from the radiative window due to exciton-phonon interactions. Following this first nonthermal regime, a thermalized exciton population is established gradually yielding longer photoluminescence decay times in the nanosecond range. Both the exciton effective radiative recombination and nonradiative recombination channels including exciton-exciton annihilation control the latter. Finally the temperature dependence of the measured exciton and trion dynamics indicates that the two populations are not in thermodynamical equilibrium.

  17. The Orientation of Luminescent Excitons in Layered Organic Nanomaterials

    NASA Astrophysics Data System (ADS)

    Schuller, Jon; Karavelli, Sinan; He, Keliang; Yang, Shyuan; Shan, Jie; Kymissis, John; Zia, Rashid

    2012-02-01

    A fundamental understanding of optoelectronics in organic semiconductors is complicated by the diversity of excitons which can exist within a single material system. Measurements that distinguish between different exciton types are crucial for a complete understanding of organic materials. By fitting experimental curves of angle-, polarization-, and energy-dependent PL to analytical Purcell calculations we quantify the relative dipole moments for in-plane and out-of-plane oriented excitons in organic and inorganic layered nanomaterials. In mono- and bi-layers of Molybdenum Disulfide (MoS2) and Graphene Oxide the luminescence arises only from in-plane oriented excitons. In the perylene derivative PTCDA, however, we show that PL arises from both in-plane and out-of-plane excitons. We observe a difference in emission frequency between the dipole orientations which indicates the existence of two distinct exciton species: an in-plane oriented Frenkel exciton and an out-of-plane oriented Charge Transfer exciton. Based on these results we devise and implement a method for isolating luminescence from either exciton species. We observe different temporal dynamics for the two distinct excitons, highlighting the power of this technique for fundamental studies of organic materials.

  18. Effects of Be acceptors on the spin polarization of carriers in p-i-n resonant tunneling diodes

    NASA Astrophysics Data System (ADS)

    Awan, I. T.; Galeti, H. V. A.; Galvão Gobato, Y.; Brasil, M. J. S. P.; Taylor, D.; Henini, M.

    2014-08-01

    In this paper, we have investigated the effect of Be acceptors on the electroluminescence and the spin polarization in GaAs/AlAs p-i-n resonant tunneling diodes. The quantum well emission comprise two main lines separated by ˜20 meV attributed to excitonic and Be-related transitions, which intensities show remarkably abrupt variations at critical voltages, particularly at the electron resonant peak where it shows a high-frequency bistability. The circular-polarization degree of the quantum-well electroluminescence also shows strong and abrupt variations at the critical bias voltages and it attains relatively large values (of ˜-75% at 15 T). These effects may be explored to design novel devices for spintronic applications such as a high-frequency spin-oscillators.

  19. Effects of Be acceptors on the spin polarization of carriers in p-i-n resonant tunneling diodes

    SciTech Connect

    Awan, I. T.; Galvão Gobato, Y.; Galeti, H. V. A.; Brasil, M. J. S. P.; Taylor, D.; Henini, M.

    2014-08-07

    In this paper, we have investigated the effect of Be acceptors on the electroluminescence and the spin polarization in GaAs/AlAs p-i-n resonant tunneling diodes. The quantum well emission comprise two main lines separated by ∼20 meV attributed to excitonic and Be-related transitions, which intensities show remarkably abrupt variations at critical voltages, particularly at the electron resonant peak where it shows a high-frequency bistability. The circular-polarization degree of the quantum-well electroluminescence also shows strong and abrupt variations at the critical bias voltages and it attains relatively large values (of ∼−75% at 15 T). These effects may be explored to design novel devices for spintronic applications such as a high-frequency spin-oscillators.

  20. Aromatic donor-acceptor interactions in non-polar environments.

    PubMed

    Prentice, Giles M; Pascu, Sofia I; Filip, Sorin V; West, Kevin R; Pantoş, G Dan

    2015-05-14

    We have evaluated the strength of aromatic donor-acceptor interactions between dialkyl naphthalenediimide and dialkoxynaphthalene in non-polar environments. (1)H NMR, UV-vis spectroscopy and isothermal titration calorimetry were used to characterise this interaction. We concluded that the strength of donor-acceptor interactions in heptane is sufficient to drive supramolecular assemblies in this and other aliphatic solvents. PMID:25875729

  1. Engineering excitonic dynamics and environmental stability of post-transition metal chalcogenides by pyridine functionalization technique.

    PubMed

    Meng, Xiuqing; Pant, Anupum; Cai, Hui; Kang, Jun; Sahin, Hasan; Chen, Bin; Wu, Kedi; Yang, Sijie; Suslu, Aslihan; Peeters, F M; Tongay, Sefaattin

    2015-10-28

    Owing to their strong photon emission, low excitonic binding energies, and nearly-ideal band offset values for water splitting reactions, direct gap quasi-2D gallium chalcogenides are potential candidates for applications in energy harvesting, optoelectronics, and photonics. Unlike other 2D materials systems, chemical functionalization of gallium chalcogenides is still at its seminal stages. Here, we propose vapor phase pyridine intercalation technique to manipulate optical properties of gallium chalcogenides. After functionalization, the excitonic dynamics of quasi-2D GaSe change significantly as evidenced by an increase in integrated PL intensity and emergence of a new emission feature that is below the band edge. Based on our DFT calculations, we attribute these to formation of bound exciton complexes at the trap sites introduced by chemical reaction between pyridine and GaSe. On the contrary, pyridine functionalization does not impact the optical properties of GaTe, instead treats GaTe surface to prevent oxidization of tellurium atoms. Overall, results suggest novel ways to control properties of gallium chalcogenides on demand and unleash their full potential for a range of applications in photonics and optoelectronics. PMID:26419224

  2. Langevin Bimolecular Recombination Kinetics of a Layered Exciton-Trion Gas

    NASA Astrophysics Data System (ADS)

    Crowne, Frank; Birdwell, Anthony

    The use of rate equations to describe various many-body kinetic processes in highly photoexcited layered semiconductors is discussed. In these systems, pairs of electrons and holes generated by photons from an external laser combine to form a multicomponent plasma whose time evolution is governed by gas dynamics and various recombination processes. At high levels of illumination this leads to a variety of secondary components in addition to neutral excitons, notably the so-called trions, which consist of exciton-electron and exciton-hole bound states. Although the recombination is modeled as bimolecular for all pairs of carrier species, the structure of the rate terms is sensitive to the dimensionality of the system due to the Langevin nature of encounters between carriers. It is demonstrated that charge neutrality does not apply to individual carrier species, e.g., electron and hole densities need not be equal in the presence of trions. In order to track the full time evolution from laser initiation to steady state, the system of rate equations is simulated numerically.

  3. Optical properties of MgZnO alloys: Excitons and exciton-phonon complexes

    SciTech Connect

    Neumann, M. D.; Cobet, C.; Esser, N.; Laumer, B.; Wassner, T. A.; Eickhoff, M.; Feneberg, M.; Goldhahn, R.

    2011-07-01

    The characteristics of the excitonic absorption and emission around the fundamental bandgap of wurtzite Mg{sub x}Zn{sub 1-x}O grown on c-plane sapphire substrates by plasma assisted molecular beam epitaxy with Mg contents between x = 0 and x = 0.23 are studied using spectroscopic ellipsometry and photoluminescence (PL) measurements. The ellipsometric data were analyzed using a multilayer model yielding the dielectric function (DF). The imaginary part of the DF for the alloys exhibits a pronounced feature which is attributed to exciton-phonon coupling (EPC) similar to the previously reported results for ZnO. Thus, in order to determine reliable transition energies, the spectral dependence is analyzed by a model which includes free excitonic lines, the exciton continuum, and the enhanced absorption due to EPC. A line shape analysis of the temperature-dependent PL spectra yielded in particular the emission-related free excitonic transition energies, which are compared to the results from the DF line-shape analysis. The PL linewidth is discussed within the framework of an alloy disorder model.

  4. Acceptor impurity activation in III-nitride light emitting diodes

    SciTech Connect

    Römer, Friedhard Witzigmann, Bernd

    2015-01-12

    In this work, the role of the acceptor doping and the acceptor activation and its impact on the internal quantum efficiency (IQE) of a Gallium Nitride (GaN) based multi-quantum well light emitting diode is studied by microscopic simulation. Acceptor impurities in GaN are subject to a high activation energy which depends on the presence of proximate dopant atoms and the electric field. A combined model for the dopant ionization and activation barrier reduction has been developed and implemented in a semiconductor carrier transport simulator. By model calculations, we demonstrate the impact of the acceptor activation mechanisms on the decay of the IQE at high current densities, which is known as the efficiency droop. A major contributor to the droop is the electron leakage which is largely affected by the acceptor doping.

  5. Do Hydrogen Bonds Influence Excitonic Splittings?

    PubMed

    Balmer, Franziska A; Ottiger, Philipp; Leutwyler, Samuel

    2016-01-01

    The excitonic splitting and vibronic quenching of the inversion-symmetric homodimers of benzonitrile, (BN)2, and meta-cyanophenol, (mCP)2, are investigated by two-color resonant two-photon ionization spectroscopy. These systems have very different hydrogen bond strengths: the OH···N≡C bonds in (mCP)2 are ∼10 times stronger than the CH···N≡C hydrogen bonds in (BN)2. In (BN)2 the S0((1)Ag) → S1((1)Ag) transition is electric-dipole forbidden, while the S0((1)Ag) → S2((1)Bu) transition is allowed. The opposite holds for (mCP)2 due to the different transition dipole moment vector alignment. The S0 → S1S2 spectra of the dimers are compared and their excitonic splittings and vibronic quenchings are investigated by measuring the (13)C-substituted heterodimer isotopomers, for which the centrosymmetry is broken and both transitions are allowed. The excitonic splittings are determined as Δexc = 2.1 cm(-1) for (BN)2 and Δexc = 7.3 cm(-1) for (mCP)2. The latter exhibits a much stronger vibronic quenching, as the purely electronic splitting resulting from ab initio calculations is determined to be Δcalc = 179 cm-1, while in (BN)2 the calculated splitting is Δcalc = 10 cm(-1). The monomer site-shifts upon dimerization and comparing certain vibrations that deform the hydrogen bonds confirm that the OH···N≡C hydrogen bond is much stronger than the CH···N≡C bond. We show that the H-bonds have large effects on the spectral shifts, but little or no influence on the excitonic splitting. PMID:27131115

  6. Nanotransformation and current fluctuations in exciton condensate junctions.

    PubMed

    Soller, H; Dolcini, F; Komnik, A

    2012-04-13

    We analyze the nonlinear transport properties of a bilayer exciton condensate that is contacted by four metallic leads by calculating the full counting statistics of electron transport for arbitrary system parameters. Despite its formal similarity to a superconductor the transport properties of the exciton condensate turn out to be completely different. We recover the generic features of exciton condensates such as counterpropagating currents driven by excitonic Andreev reflections and make predictions for nonlinear transconductance between the layers as well as for the current (cross)correlations and generalized Johnson-Nyquist relationships. Finally, we explore the possibility of connecting another mesoscopic system (in our case a quantum point contact) to the bottom layer of the exciton condensate and show how the excitonic Andreev reflections can be used for transforming voltage at the nanoscale. PMID:22587267

  7. Ultrafast dynamics of excitons in tetracene single crystals

    NASA Astrophysics Data System (ADS)

    Birech, Zephania; Schwoerer, Markus; Schmeiler, Teresa; Pflaum, Jens; Schwoerer, Heinrich

    2014-03-01

    Ultrafast exciton dynamics in free standing 200 nm thin tetracene single crystals were studied at room temperature by femtosecond transient absorption spectroscopy in the visible spectral range. The complex spectrally overlapping transient absorption traces of single crystals were systematically deconvoluted. From this, the ultrafast dynamics of the ground, excited, and transition states were identified including singlet exciton fission into two triplet excitons. Fission is generated through both, direct fission of higher singlet states Sn on a sub-picosecond timescale, and thermally activated fission of the singlet exciton S1 on a 40 ps timescale. The high energy Davydov component of the S1 exciton is proposed to undergo fission on a sub-picoseconds timescale. At high density of triplet excitons their mutual annihilation (triplet-triplet annihilation) occurs on a <10 ps timescale.

  8. Ultrafast dynamics of excitons in tetracene single crystals.

    PubMed

    Birech, Zephania; Schwoerer, Markus; Schmeiler, Teresa; Pflaum, Jens; Schwoerer, Heinrich

    2014-03-21

    Ultrafast exciton dynamics in free standing 200 nm thin tetracene single crystals were studied at room temperature by femtosecond transient absorption spectroscopy in the visible spectral range. The complex spectrally overlapping transient absorption traces of single crystals were systematically deconvoluted. From this, the ultrafast dynamics of the ground, excited, and transition states were identified including singlet exciton fission into two triplet excitons. Fission is generated through both, direct fission of higher singlet states S(n) on a sub-picosecond timescale, and thermally activated fission of the singlet exciton S1 on a 40 ps timescale. The high energy Davydov component of the S1 exciton is proposed to undergo fission on a sub-picoseconds timescale. At high density of triplet excitons their mutual annihilation (triplet-triplet annihilation) occurs on a <10 ps timescale. PMID:24655187

  9. Exciton states and optical properties of carbon nanotubes.

    PubMed

    Ajiki, Hiroshi

    2012-12-01

    Exciton states and related optical properties of a single-walled carbon nanotube are reviewed, primarily from a theoretical viewpoint. The energies and wavefunctions of excitons are discussed using a screened Hartree-Fock approximation with an effective-mass or k·p approximation. The close relationship between a long-range electron-hole exchange interaction and a depolarization effect is clarified. I discuss optical properties including the radiative lifetime of excitons, absorption spectra and radiation force. To describe these properties in a unified scheme, a self-consistent method is introduced for calculating the scattering light and induced current density due to excitons. I also briefly review experimental results on the Aharonov-Bohm effect in excitons and quasi-dark excitons excited by light polarized perpendicular to the tube axis. PMID:23139202

  10. Excitonic gap formation and condensation in the bilayer graphene structure

    NASA Astrophysics Data System (ADS)

    Apinyan, V.; Kopeć, T. K.

    2016-09-01

    We have studied the excitonic gap formation in the Bernal Stacked, bilayer graphene (BLG) structures at half-filling. Considering the local Coulomb interaction between the layers, we calculate the excitonic gap parameter and we discuss the role of the interlayer and intralayer Coulomb interactions and the interlayer hopping on the excitonic pair formation in the BLG. Particularly, we predict the origin of excitonic gap formation and condensation, in relation to the furthermost interband optical transition spectrum. The general diagram of excitonic phase transition is given, explaining different interlayer correlation regimes. The temperature dependence of the excitonic gap parameter is shown and the role of the chemical potential, in the BLG, is discussed in details.

  11. Singlet Exciton Fission in Nanostructured Organic Solar Cells

    SciTech Connect

    Jadhav, P. J.; Mohanty, A.; Sussman, J.; Baldo, Marc

    2011-04-13

    Singlet exciton fission is an efficient multiexciton generation process in organic molecules. But two concerns must be satisfied before it can be exploited in low-cost solution-processed organic solar cells. Fission must be combined with longer wavelength absorption in a structure that can potentially surpass the single junction limit, and its efficiency must be demonstrated in nanoscale domains within blended devices. Here, we report organic solar cells comprised of tetracene, copper phthalocyanine, and the buckyball C{sub 6}0. Short wavelength light generates singlet excitons in tetracene. These are subsequently split into two triplet excitons and transported through the phthalocyanine. In addition, the phthalocyanine absorbs photons below the singlet exciton energy of tetracene. To test tetracene in nanostructured blends, we fabricate coevaporated bulk heterojunctions and multilayer heterojunctions of tetracene and C{sub 60}. We measure a singlet fission efficiency of (71 ± 18)%, demonstrating that exciton fission can efficiently compete with exciton dissociation on the nanoscale.

  12. Wannier-Mott excitons in semiconductors with a superlattice

    SciTech Connect

    Suris, R. A.

    2015-06-15

    The effect of the motion of a Wannier-Mott exciton in semiconductors with a superlattice formed by heterojunctions on the exciton binding energy and wave function is analyzed. This effect arises as a result of the fact that the dispersion laws of the electron and hole that form an exciton in a superlattice differ from the quadratic law. The investigated one-dimensional superlattice consists of alternating semiconductor layers with different energy positions of the conduction and valence bands, i.e., with one-dimensional wells and barriers. The exciton state in a superlattice consisting of quantum dots is analyzed. It is demonstrated that the closer the electron and hole effective masses, the greater the dependence of the binding energy on the exciton quasi-momentum. The possibility of replacing the tunneling excitation transfer between superlattice cells with the dipole-dipole one at certain exciton quasi-wave vector values is investigated.

  13. Fluorescence spectroscopy, exciton dynamics, and photochemistry of single allophycocyanin trimers

    SciTech Connect

    Ying, L.; Sie, X.S.

    1998-12-10

    The authors report a study of the allophycocyanin trimer (APC), a light-harvesting protein complex from cyanobacteria, by room-temperature single-molecule measurements of fluorescence spectra, lifetimes, intensity trajectories, and polarization modulation. Emission spectra of individual APC trimers are found to be homogeneous on the time scale of seconds. In contrast, their emission lifetimes are found to be widely distributed because of generation of long-lived exciton traps during the course of measurements. The intensity trajectories and polarization modulation experiments indicate reversible exciton trap formation within the three quasi-independent pairs of strong interacting {alpha}84 and {beta}84 chromophores in APC, as well as photobleaching of individual chromophores. Comparison experiments under continuous-wave and pulsed excitation reveal a two-photon mechanism for generating exciton traps and/or photobleaching, which involves exciton-exciton annihilation. These single-molecule experiments provide new insights into the spectroscopy, exciton dynamics, and photochemistry of light-harvesting complexes.

  14. Exciton mapping at subwavelength scales in two-dimensional materials.

    PubMed

    Tizei, Luiz H G; Lin, Yung-Chang; Mukai, Masaki; Sawada, Hidetaka; Lu, Ang-Yu; Li, Lain-Jong; Kimoto, Koji; Suenaga, Kazu

    2015-03-13

    Spatially resolved electron-energy-loss spectroscopy (EELS) is performed at diffuse interfaces between MoS2 and MoSe2 single layers. With a monochromated electron source (20 meV) we successfully probe excitons near the interface by obtaining the low loss spectra at the nanometer scale. The exciton maps clearly show variations even with a 10 nm separation between measurements; consequently, the optical band gap can be measured with nanometer-scale resolution, which is 50 times smaller than the wavelength of the emitted photons. By performing core-loss EELS at the same regions, we observe that variations in the excitonic signature follow the chemical composition. The exciton peaks are observed to be broader at interfaces and heterogeneous regions, possibly due to interface roughness and alloying effects. Moreover, we do not observe shifts of the exciton peak across the interface, possibly because the interface width is not much larger than the exciton Bohr radius. PMID:25815966

  15. Ultrafast dynamics of excitons in tetracene single crystals

    SciTech Connect

    Birech, Zephania; Schwoerer, Heinrich; Schwoerer, Markus; Schmeiler, Teresa; Pflaum, Jens

    2014-03-21

    Ultrafast exciton dynamics in free standing 200 nm thin tetracene single crystals were studied at room temperature by femtosecond transient absorption spectroscopy in the visible spectral range. The complex spectrally overlapping transient absorption traces of single crystals were systematically deconvoluted. From this, the ultrafast dynamics of the ground, excited, and transition states were identified including singlet exciton fission into two triplet excitons. Fission is generated through both, direct fission of higher singlet states S{sub n} on a sub-picosecond timescale, and thermally activated fission of the singlet exciton S{sub 1} on a 40 ps timescale. The high energy Davydov component of the S{sub 1} exciton is proposed to undergo fission on a sub-picoseconds timescale. At high density of triplet excitons their mutual annihilation (triplet-triplet annihilation) occurs on a <10 ps timescale.

  16. Size-dependent decoherence of excitonic states in semiconductor microcrystallites

    SciTech Connect

    Liu Yuxi; Miranowicz, Adam; Oezdemir, Sahin K.; Koashi, Masato; Imoto, Nobuyuki

    2003-03-01

    The size-dependent decoherence of the exciton states, resulting from the spontaneous emission, is investigated in a semiconductor spherical microcrystallite under the condition a{sub B}<excitonic coherent states, the coherence time depends on both the overlap of two excitonic coherent states and the size of the microcrystallite. When the system with fixed size is initially in the even or odd coherent state, the larger average number of the excitons corresponds to the faster decoherence. When the average number of the excitons is given, the bigger size of the microcrystallite corresponds to the faster decoherence. The decoherence of the exciton states for the materials GaAs and CdS is numerically studied by our theoretical analysis.

  17. Fractional Solitons in Excitonic Josephson Junctions

    PubMed Central

    Hsu, Ya-Fen; Su, Jung-Jung

    2015-01-01

    The Josephson effect is especially appealing to physicists because it reveals macroscopically the quantum order and phase. In excitonic bilayers the effect is even subtler due to the counterflow of supercurrent as well as the tunneling between layers (interlayer tunneling). Here we study, in a quantum Hall bilayer, the excitonic Josephson junction: a conjunct of two exciton condensates with a relative phase ϕ0 applied. The system is mapped into a pseudospin ferromagnet then described numerically by the Landau-Lifshitz-Gilbert equation. In the presence of interlayer tunneling, we identify a family of fractional sine-Gordon solitons which resemble the static fractional Josephson vortices in the extended superconducting Josephson junctions. Each fractional soliton carries a topological charge Q that is not necessarily a half/full integer but can vary continuously. The calculated current-phase relation (CPR) shows that solitons with Q = ϕ0/2π is the lowest energy state starting from zero ϕ0 – until ϕ0 > π – then the alternative group of solitons with Q = ϕ0/2π − 1 takes place and switches the polarity of CPR. PMID:26511770

  18. Fractional Solitons in Excitonic Josephson Junctions.

    PubMed

    Hsu, Ya-Fen; Su, Jung-Jung

    2015-01-01

    The Josephson effect is especially appealing to physicists because it reveals macroscopically the quantum order and phase. In excitonic bilayers the effect is even subtler due to the counterflow of supercurrent as well as the tunneling between layers (interlayer tunneling). Here we study, in a quantum Hall bilayer, the excitonic Josephson junction: a conjunct of two exciton condensates with a relative phase ϕ0 applied. The system is mapped into a pseudospin ferromagnet then described numerically by the Landau-Lifshitz-Gilbert equation. In the presence of interlayer tunneling, we identify a family of fractional sine-Gordon solitons which resemble the static fractional Josephson vortices in the extended superconducting Josephson junctions. Each fractional soliton carries a topological charge Q that is not necessarily a half/full integer but can vary continuously. The calculated current-phase relation (CPR) shows that solitons with Q = ϕ0/2π is the lowest energy state starting from zero ϕ0 - until ϕ0 > π - then the alternative group of solitons with Q = ϕ0/2π - 1 takes place and switches the polarity of CPR. PMID:26511770

  19. Fractional Solitons in Excitonic Josephson Junctions

    NASA Astrophysics Data System (ADS)

    Hsu, Ya-Fen; Su, Jung-Jung

    2015-10-01

    The Josephson effect is especially appealing to physicists because it reveals macroscopically the quantum order and phase. In excitonic bilayers the effect is even subtler due to the counterflow of supercurrent as well as the tunneling between layers (interlayer tunneling). Here we study, in a quantum Hall bilayer, the excitonic Josephson junction: a conjunct of two exciton condensates with a relative phase ϕ0 applied. The system is mapped into a pseudospin ferromagnet then described numerically by the Landau-Lifshitz-Gilbert equation. In the presence of interlayer tunneling, we identify a family of fractional sine-Gordon solitons which resemble the static fractional Josephson vortices in the extended superconducting Josephson junctions. Each fractional soliton carries a topological charge Q that is not necessarily a half/full integer but can vary continuously. The calculated current-phase relation (CPR) shows that solitons with Q = ϕ0/2π is the lowest energy state starting from zero ϕ0 - until ϕ0 > π - then the alternative group of solitons with Q = ϕ0/2π - 1 takes place and switches the polarity of CPR.

  20. Excitonic effects in oxyhalide scintillating host compounds

    SciTech Connect

    Shwetha, G.; Kanchana, V.; Valsakumar, M. C.

    2014-10-07

    Ab-initio calculations based on density functional theory have been performed to study the electronic, optical, mechanical, and vibrational properties of scintillator host compounds YOX (X = F, Cl, Br, and I). Semiempirical dispersion correction schemes are used to find the effect of van der Waals forces on these layered compounds and we found this effect to be negligible except for YOBr. Calculations of phonons and elastic constants showed that all the compounds studied here are both dynamically and mechanically stable. YOF and YOI are found to be indirect band gap insulators while YOCl and YOBr are direct band gap insulators. The band gap is found to decrease as we move from fluorine to iodine, while the calculated refractive index shows the opposite trend. As the band gap decreases on going down the periodic table from YOF to YOI, the luminescence increases. The excitonic binding energy calculated, within the effective mass approximation, is found to be more for YOF than the remaining compounds, suggesting that the excitonic effect to be more in YOF than the other compounds. The optical properties are calculated within the Time-Dependent Density Functional Theory (TDDFT) and compared with results obtained within the random phase approximation. The TDDFT calculations, using the newly developed bootstrap exchange-correlation kernel, showed significant excitonic effects in all the compounds studied here.

  1. Roles of Energy/Charge Cascades and Intermixed Layers at Donor/Acceptor Interfaces in Organic Solar Cells

    PubMed Central

    Nakano, Kyohei; Suzuki, Kaori; Chen, Yujiao; Tajima, Keisuke

    2016-01-01

    The secret to the success of mixed bulk heterojunctions (BHJs) in yielding highly efficient organic solar cells (OSCs) could reside in the molecular structures at their donor/acceptor (D/A) interfaces. In this study, we aimed to determine the effects of energy and charge cascade structures at the interfaces by using well-defined planar heterojunctions (PHJs) as a model system. The results showed that (1) the charge cascade structure enhanced VOC because it shuts down the recombination pathway through charge transfer (CT) state with a low energy, (2) the charge cascade layer having a wider energy gap than the bulk material decreased JSC because the diffusion of the excitons from the bulk to D/A interface was blocked; the energy of the cascade layers must be appropriately arranged for both the charges and the excitons, and (3) molecular intermixing in the cascade layer opened the recombination path through the low-energy CT state and decreased VOC. Based on these findings, we propose improved structures for D/A interfaces in BHJs. PMID:27404948

  2. Roles of Energy/Charge Cascades and Intermixed Layers at Donor/Acceptor Interfaces in Organic Solar Cells

    NASA Astrophysics Data System (ADS)

    Nakano, Kyohei; Suzuki, Kaori; Chen, Yujiao; Tajima, Keisuke

    2016-07-01

    The secret to the success of mixed bulk heterojunctions (BHJs) in yielding highly efficient organic solar cells (OSCs) could reside in the molecular structures at their donor/acceptor (D/A) interfaces. In this study, we aimed to determine the effects of energy and charge cascade structures at the interfaces by using well-defined planar heterojunctions (PHJs) as a model system. The results showed that (1) the charge cascade structure enhanced VOC because it shuts down the recombination pathway through charge transfer (CT) state with a low energy, (2) the charge cascade layer having a wider energy gap than the bulk material decreased JSC because the diffusion of the excitons from the bulk to D/A interface was blocked; the energy of the cascade layers must be appropriately arranged for both the charges and the excitons, and (3) molecular intermixing in the cascade layer opened the recombination path through the low-energy CT state and decreased VOC. Based on these findings, we propose improved structures for D/A interfaces in BHJs.

  3. Magnetic field enhanced electroluminescence in organic light emitting diodes based on electron donor-acceptor exciplex blends

    NASA Astrophysics Data System (ADS)

    Baniya, Sangita; Basel, Tek; Sun, Dali; McLaughlin, Ryan; Vardeny, Zeev Valy

    2016-03-01

    A useful process for light harvesting from injected electron-hole pairs in organic light emitting diodes (OLED) is the transfer from triplet excitons (T) to singlet excitons (S) via reverse intersystem crossing (RISC). This process adds a delayed electro-luminescence (EL) emission component that is known as thermally activated delayed fluorescence (TADF). We have studied electron donor (D)/acceptor(A) blends that form an exciplex manifold in which the energy difference, ΔEST between the lowest singlet (S1) and triplet (T1) levels is relatively small (<100 meV), and thus allows RISC at ambient temperature. We found that the EL emission in OLED based on the exciplex blend is enhanced up to 40% by applying a relatively weak magnetic field of 50 mT at ambient. Moreover the MEL response is activated with activation energy similar that of the EL emission. This suggests that the large magneto-EL originates from an additional spin-mixing channel between singlet and triplet states of the generated exciplexes, which is due to TADF. We will report on the MEL dependencies on the temperature, bias voltage, and D-A materials for optimum OLED performance. Supported by SAMSUNG Global Research Outreach (GRO) program, and also by the NSF-Material Science & Engineering Center (MRSEC) program at the University of Utah (DMR-1121252).

  4. Roles of Energy/Charge Cascades and Intermixed Layers at Donor/Acceptor Interfaces in Organic Solar Cells.

    PubMed

    Nakano, Kyohei; Suzuki, Kaori; Chen, Yujiao; Tajima, Keisuke

    2016-01-01

    The secret to the success of mixed bulk heterojunctions (BHJs) in yielding highly efficient organic solar cells (OSCs) could reside in the molecular structures at their donor/acceptor (D/A) interfaces. In this study, we aimed to determine the effects of energy and charge cascade structures at the interfaces by using well-defined planar heterojunctions (PHJs) as a model system. The results showed that (1) the charge cascade structure enhanced VOC because it shuts down the recombination pathway through charge transfer (CT) state with a low energy, (2) the charge cascade layer having a wider energy gap than the bulk material decreased JSC because the diffusion of the excitons from the bulk to D/A interface was blocked; the energy of the cascade layers must be appropriately arranged for both the charges and the excitons, and (3) molecular intermixing in the cascade layer opened the recombination path through the low-energy CT state and decreased VOC. Based on these findings, we propose improved structures for D/A interfaces in BHJs. PMID:27404948

  5. Electrochromism of a fused acceptor-donor-acceptor triad covering entire UV-vis and near-infrared regions.

    PubMed

    Yao, Bin; Ye, Xichong; Zhang, Jie; Wan, Xinhua

    2014-10-17

    A novel fused acceptor-donor-acceptor (A-D-A) type panchromatically electrochromic compound was synthesized. It exhibited intensive absorption bands covering entire UV-vis and near-infrared regions upon reduction to the radical anionic state, owing to the simultaneous presence of π*-π* transitions and intervalence charge transfer. PMID:25268224

  6. Transition metal-free generation of the acceptor/acceptor-carbene viaα-elimination: synthesis of fluoroacetyl cyclopropanes.

    PubMed

    Wang, Yongdong; Han, Jing; Chen, Jie; Cao, Weiguo

    2016-05-21

    An efficient transition metal-free approach for the generation of acceptor/acceptor-carbene followed by trapping with alkenes to provide fluoroacetyl cyclopropanes has been described. The resulting cyclopropanes could be further converted into the fluoromethyl dihydrofurans or fluorodihydropyrroles through ring-expansion processes. PMID:27125517

  7. Exciton-exciton annihilation and biexciton stimulated emission in graphene nanoribbons.

    PubMed

    Soavi, Giancarlo; Dal Conte, Stefano; Manzoni, Cristian; Viola, Daniele; Narita, Akimitsu; Hu, Yunbin; Feng, Xinliang; Hohenester, Ulrich; Molinari, Elisa; Prezzi, Deborah; Müllen, Klaus; Cerullo, Giulio

    2016-01-01

    Graphene nanoribbons display extraordinary optical properties due to one-dimensional quantum-confinement, such as width-dependent bandgap and strong electron-hole interactions, responsible for the formation of excitons with extremely high binding energies. Here we use femtosecond transient absorption spectroscopy to explore the ultrafast optical properties of ultranarrow, structurally well-defined graphene nanoribbons as a function of the excitation fluence, and the impact of enhanced Coulomb interaction on their excited states dynamics. We show that in the high-excitation regime biexcitons are formed by nonlinear exciton-exciton annihilation, and that they radiatively recombine via stimulated emission. We obtain a biexciton binding energy of ≈ 250 meV, in very good agreement with theoretical results from quantum Monte Carlo simulations. These observations pave the way for the application of graphene nanoribbons in photonics and optoelectronics. PMID:26984281

  8. Exciton-exciton annihilation and biexciton stimulated emission in graphene nanoribbons

    NASA Astrophysics Data System (ADS)

    Soavi, Giancarlo; Dal Conte, Stefano; Manzoni, Cristian; Viola, Daniele; Narita, Akimitsu; Hu, Yunbin; Feng, Xinliang; Hohenester, Ulrich; Molinari, Elisa; Prezzi, Deborah; Müllen, Klaus; Cerullo, Giulio

    2016-03-01

    Graphene nanoribbons display extraordinary optical properties due to one-dimensional quantum-confinement, such as width-dependent bandgap and strong electron-hole interactions, responsible for the formation of excitons with extremely high binding energies. Here we use femtosecond transient absorption spectroscopy to explore the ultrafast optical properties of ultranarrow, structurally well-defined graphene nanoribbons as a function of the excitation fluence, and the impact of enhanced Coulomb interaction on their excited states dynamics. We show that in the high-excitation regime biexcitons are formed by nonlinear exciton-exciton annihilation, and that they radiatively recombine via stimulated emission. We obtain a biexciton binding energy of ~250 meV, in very good agreement with theoretical results from quantum Monte Carlo simulations. These observations pave the way for the application of graphene nanoribbons in photonics and optoelectronics.

  9. Structural basis for acceptor-substrate recognition of UDP-glucose: anthocyanidin 3-O-glucosyltransferase from Clitoria ternatea

    PubMed Central

    Hiromoto, Takeshi; Honjo, Eijiro; Noda, Naonobu; Tamada, Taro; Kazuma, Kohei; Suzuki, Masahiko; Blaber, Michael; Kuroki, Ryota

    2015-01-01

    UDP-glucose: anthocyanidin 3-O-glucosyltransferase (UGT78K6) from Clitoria ternatea catalyzes the transfer of glucose from UDP-glucose to anthocyanidins such as delphinidin. After the acylation of the 3-O-glucosyl residue, the 3′- and 5′-hydroxyl groups of the product are further glucosylated by a glucosyltransferase in the biosynthesis of ternatins, which are anthocyanin pigments. To understand the acceptor-recognition scheme of UGT78K6, the crystal structure of UGT78K6 and its complex forms with anthocyanidin delphinidin and petunidin, and flavonol kaempferol were determined to resolutions of 1.85 Å, 2.55 Å, 2.70 Å, and 1.75 Å, respectively. The enzyme recognition of unstable anthocyanidin aglycones was initially observed in this structural determination. The anthocyanidin- and flavonol-acceptor binding details are almost identical in each complex structure, although the glucosylation activities against each acceptor were significantly different. The 3-hydroxyl groups of the acceptor substrates were located at hydrogen-bonding distances to the Nε2 atom of the His17 catalytic residue, supporting a role for glucosyl transfer to the 3-hydroxyl groups of anthocyanidins and flavonols. However, the molecular orientations of these three acceptors are different from those of the known flavonoid glycosyltransferases, VvGT1 and UGT78G1. The acceptor substrates in UGT78K6 are reversely bound to its binding site by a 180° rotation about the O1–O3 axis of the flavonoid backbones observed in VvGT1 and UGT78G1; consequently, the 5- and 7-hydroxyl groups are protected from glucosylation. These substrate recognition schemes are useful to understand the unique reaction mechanism of UGT78K6 for the ternatin biosynthesis, and suggest the potential for controlled synthesis of natural pigments. PMID:25556637

  10. Structural basis for acceptor-substrate recognition of UDP-glucose: anthocyanidin 3-O-glucosyltransferase from Clitoria ternatea.

    PubMed

    Hiromoto, Takeshi; Honjo, Eijiro; Noda, Naonobu; Tamada, Taro; Kazuma, Kohei; Suzuki, Masahiko; Blaber, Michael; Kuroki, Ryota

    2015-03-01

    UDP-glucose: anthocyanidin 3-O-glucosyltransferase (UGT78K6) from Clitoria ternatea catalyzes the transfer of glucose from UDP-glucose to anthocyanidins such as delphinidin. After the acylation of the 3-O-glucosyl residue, the 3'- and 5'-hydroxyl groups of the product are further glucosylated by a glucosyltransferase in the biosynthesis of ternatins, which are anthocyanin pigments. To understand the acceptor-recognition scheme of UGT78K6, the crystal structure of UGT78K6 and its complex forms with anthocyanidin delphinidin and petunidin, and flavonol kaempferol were determined to resolutions of 1.85 Å, 2.55 Å, 2.70 Å, and 1.75 Å, respectively. The enzyme recognition of unstable anthocyanidin aglycones was initially observed in this structural determination. The anthocyanidin- and flavonol-acceptor binding details are almost identical in each complex structure, although the glucosylation activities against each acceptor were significantly different. The 3-hydroxyl groups of the acceptor substrates were located at hydrogen-bonding distances to the Nε2 atom of the His17 catalytic residue, supporting a role for glucosyl transfer to the 3-hydroxyl groups of anthocyanidins and flavonols. However, the molecular orientations of these three acceptors are different from those of the known flavonoid glycosyltransferases, VvGT1 and UGT78G1. The acceptor substrates in UGT78K6 are reversely bound to its binding site by a 180° rotation about the O1-O3 axis of the flavonoid backbones observed in VvGT1 and UGT78G1; consequently, the 5- and 7-hydroxyl groups are protected from glucosylation. These substrate recognition schemes are useful to understand the unique reaction mechanism of UGT78K6 for the ternatin biosynthesis, and suggest the potential for controlled synthesis of natural pigments. PMID:25556637

  11. Upconverted photoluminescence induced by radiative coupling between excitons

    NASA Astrophysics Data System (ADS)

    Matsuda, Takuya; Yokoshi, Nobuhiko; Ishihara, Hajime

    2016-04-01

    We propose an unconventional scheme of photoluminescence in a semiconductor thin film, where the nonlocal correlation between an excitonic wave and light wave prominently enhances the interaction between different excitonic states via radiation beyond the long-wavelength approximation (the so-called excitonic superradiance regime). On the basis of the developed method extending input-output theory, we elucidate atypical photoluminescence effects due to the strong wave-wave correlation. In particular, the upconverted photoluminescence based on the coherent quantum superposition of excitons is found to be highly efficient, i.e., it can be realized by weak pumping without auxiliary systems such as cavities or photonic antennas.

  12. Superradiance of High Density Frenkel Excitons at Room Temperature

    NASA Astrophysics Data System (ADS)

    Wang, H. Z.; Zheng, X. G.; Zhao, F. L.; Gao, Z. L.; Yu, Z. X.

    1995-05-01

    Superradiance of high density Frenkel excitons in an R-phycoerythrin single crystal is observed at room temperature for the first time. No fluorescence is observed except the emission at the sharp exciton band when the superradiance of excitons occurs, and the higher the pump density, the sharper the emission bandwidth. A redshift and a blueshift are observed at the rise time and the fall time of the emission pulse, respectively. The experimental results also imply deformed-boson properties of high density Frenkel excitons.

  13. Polarization-dependent exciton dynamics in tetracene single crystals.

    PubMed

    Zhang, Bo; Zhang, Chunfeng; Xu, Yanqing; Wang, Rui; He, Bin; Liu, Yunlong; Zhang, Shimeng; Wang, Xiaoyong; Xiao, Min

    2014-12-28

    We conduct polarization-dependent ultrafast spectroscopy to study the dynamics of singlet fission (SF) in tetracene single crystals. The spectrotemporal species for singlet and triplet excitons in transient absorption spectra are found to be strongly dependent on probe polarization. By carefully analyzing the polarization dependence, the signals contributed by different transitions related to singlet excitons have been disentangled, which is further applied to construct the correlation between dynamics of singlet and triplet excitons. The anisotropy of exciton dynamics provides an alternative approach to tackle the long-standing challenge in understanding the mechanism of singlet fission in organic semiconductors. PMID:25554147

  14. Exciton dynamics in organic molecular crystals and nanostructures

    NASA Astrophysics Data System (ADS)

    Bardeen, Chris

    2014-03-01

    The photophysical behavior of organic semiconductors is governed by their excitonic states. In this talk, we classify the three different exciton types (Frenkel singlet, Frenkel triplet, and charge-transfer) typically encountered in organic semiconductors. The availability of several different exciton bands provides the possibility of interband processes. One such process is singlet fission, where an initially excited singlet exciton can spontaneously split into a pair of spin-entangled triplet excitons. We analyze this phenomenon in detail, emphasizing the role of spin state coherence and magnetic fields in studying singlet <-- --> triplet pair interconversion. Singlet fission provides an example of how all three types of excitons (triplet, singlet, and charge-transfer) interact to generate unique nonlinear excitonic processes in molecular systems. These processes may be useful for applications like solar energy conversion, where the generation of two excitons per absorbed photon could lead to significant enhancements in the efficiency of single junction photovoltaic cells. Finally, we will briefly describe how excitons can also be used to initiate photochemical reactions in molecular crystal nanostructures, resulting in large shape changes and deformations.

  15. Polarization-dependent exciton dynamics in tetracene single crystals

    NASA Astrophysics Data System (ADS)

    Zhang, Bo; Zhang, Chunfeng; Xu, Yanqing; Wang, Rui; He, Bin; Liu, Yunlong; Zhang, Shimeng; Wang, Xiaoyong; Xiao, Min

    2014-12-01

    We conduct polarization-dependent ultrafast spectroscopy to study the dynamics of singlet fission (SF) in tetracene single crystals. The spectrotemporal species for singlet and triplet excitons in transient absorption spectra are found to be strongly dependent on probe polarization. By carefully analyzing the polarization dependence, the signals contributed by different transitions related to singlet excitons have been disentangled, which is further applied to construct the correlation between dynamics of singlet and triplet excitons. The anisotropy of exciton dynamics provides an alternative approach to tackle the long-standing challenge in understanding the mechanism of singlet fission in organic semiconductors.

  16. Localization length scales of triplet excitons in singlet fission materials

    NASA Astrophysics Data System (ADS)

    Bayliss, Sam L.; Thorley, Karl J.; Anthony, John E.; Bouchiat, Hélène; Greenham, Neil C.; Chepelianskii, Alexei D.

    2015-09-01

    We measure the dielectric confinement length scales of triplet excitons in organic semiconductors by jointly measuring their microwave-domain electric and magnetic susceptibilities. We apply this technique to characterize triplet excitons in two singlet fission materials with distinct solid-state packing and correlate the extracted localization length scales with the role of the excitonic environment. By using the magnetic susceptibility simultaneously determined through our experiments, we compare the independently extracted dielectric and spin-spin localization length scales, highlighting the role of local anisotropy on the properties of excitonic triplet states.

  17. Revealing and Characterizing Dark Excitons through Coherent Multidimensional Spectroscopy.

    PubMed

    Tollerud, Jonathan O; Cundiff, Steven T; Davis, Jeffrey A

    2016-08-26

    Dark excitons are of fundamental importance in a broad range of contexts but are difficult to study using conventional optical spectroscopy due to their weak interaction with light. We show how coherent multidimensional spectroscopy can reveal and characterize dark states. Using this approach, we identify parity-forbidden and spatially indirect excitons in InGaAs/GaAs quantum wells and determine details regarding lifetimes, homogeneous and inhomogeneous linewidths, broadening mechanisms, and coupling strengths. The observations of coherent coupling between these states and bright excitons hint at a role for a multistep process by which excitons in the barrier can relax into the quantum wells. PMID:27610881

  18. Disorder-enhanced exciton delocalization in an extended dendrimer.

    PubMed

    Pouthier, Vincent

    2014-08-01

    The exciton dynamics in a disordered extended dendrimer is investigated numerically. Because a homogeneous dendrimer exhibits few highly degenerate energy levels, a dynamical localization arises when the exciton is initially located on the periphery. However, it is shown that the disorder lifts the degeneracy and favors a delocalization-relocalization transition. Weak disorder enhances the delocalized nature of the exciton and improves any quantum communication, whereas strong disorder prevents the exciton from propagating in accordance with the well-known Anderson theory. PMID:25215792

  19. Binding energies of indirect excitons in double quantum well systems

    NASA Astrophysics Data System (ADS)

    Rossokhaty, Alex; Schmult, Stefan; Dietsche, Werner; von Klitzing, Klaus; Kukushkin, Igor

    2011-03-01

    A prerequisite towards Bose-Einstein condensation is a cold and dense system of bosons. Indirect excitons in double GaAs/AlGaAs quantum wells (DQWs) are believed to be suitable candidates. Indirect excitons are formed in asymmetric DQW structures by mass filtering, a method which does not require external electric fields. The exciton density and the electron-hole balance can be tuned optically. Binding energies are measured by a resonant microwave absorption technique. Our results show that screening of the indirect excitons becomes already relevant at densities as low as ~ 5 × 109 cm-2 and results in their destruction.

  20. Simulations of singlet exciton diffusion in organic semiconductors: a review

    SciTech Connect

    Bjorgaard, Josiah A.; Kose, Muhammet Erkan

    2014-12-22

    Our review describes the various aspects of simulation strategies for exciton diffusion in condensed phase thin films of organic semiconductors. Several methods for calculating energy transfer rate constants are discussed along with procedures for how to account for energetic disorder. Exciton diffusion can be modelled by using kinetic Monte-Carlo methods or master equations. Recent literature on simulation efforts for estimating exciton diffusion lengths of various conjugated polymers and small molecules are introduced. Moreover, these studies are discussed in the context of the effects of morphology on exciton diffusion and the necessity of accurate treatment of disorder for comparison of simulation results with those of experiment.

  1. Surface photovoltage in exciton absorption range in CdS

    NASA Technical Reports Server (NTRS)

    Morawski, A.; Banisch, R.; Lagowski, J.

    1977-01-01

    The high resolution, intrinsic spectra of surface photovoltage are reported for semiconducting n-type CdS single crystals. At reduced temperatures (120-160 K) the spectra exhibit three sharp maxima due to A, B and C free exciton transitions. Energy positions of these lines and valence band parameters (spin-orbit and crystal field splittings) estimated from surface photovoltage are in good agreement with values obtained by other methods. The excitonic transitions are very sensitive to surface treatment, i.e. polishing, etching, background illumination and surface doping. The mechanism of direct interaction of free excitons with surface states is proposed to explain exciton lines in surface photovoltage.

  2. Exciton annihilation in dye-sensitized nanocrystalline semiconductor films

    NASA Astrophysics Data System (ADS)

    Namekawa, Akihiro; Katoh, Ryuzi

    2016-08-01

    Exciton annihilation in dye-sensitized nanocrystalline semiconductor (Al2O3) films has been studied through laser-induced fluorescence spectroscopy. The relative quantum yield of the fluorescence decreases with increasing excitation light intensity, the indication being that exciton annihilation occurred. The rate constants of the annihilation were estimated for three dyes, N719, D149, and MK2, that are known to be sensitizing dyes for efficient dye-sensitized solar cells. The hopping time between dye molecules and the diffusion length of excitons within their lifetime were also estimated to facilitate discussion of the relevance of exciton annihilation to primary processes in dye-sensitized solar cells.

  3. Measurement of Exciton Binding Energy of Monolayer WS2

    NASA Astrophysics Data System (ADS)

    Chen, Xi; Zhu, Bairen; Cui, Xiaodong

    Excitonic effects are prominent in monolayer crystal of transition metal dichalcogenides (TMDCs) because of spatial confinement and reduced Coulomb screening. Here we use linear differential transmission spectroscopy and two-photon photoluminescence excitation spectroscopy (TP-PLE) to measure the exciton binding energy of monolayer WS2. Peaks for excitonic absorptions of the direct gap located at K valley of the Brillouin zone and transitions from multiple points near Γ point of the Brillouin zone, as well as trion side band are shown in the linear absorption spectra of WS2. But there is no gap between distinct excitons and the continuum of the interband transitions. Strong electron-phonon scattering, overlap of excitons around Γ point and the transfer of the oscillator strength from interband continuum to exciton states make it difficult to resolve the electronic interband transition edge even down to 10K. The gap between excited states of the band-edge exciton and the single-particle band is probed by TP-PLE measurements. And the energy difference between 1s exciton and the single-particle gap gives the exciton binding energy of monolayer WS2 to be about 0.71eV. The work is supported by Area of excellency (AoE/P-04/08), CRF of Hong Kong Research Grant Council (HKU9/CRF/13G) and SRT on New Materials of The University of Hong Kong.

  4. Polarization-dependent exciton dynamics in tetracene single crystals

    SciTech Connect

    Zhang, Bo; Zhang, Chunfeng Xu, Yanqing; Wang, Rui; He, Bin; Liu, Yunlong; Zhang, Shimeng; Wang, Xiaoyong; Xiao, Min

    2014-12-28

    We conduct polarization-dependent ultrafast spectroscopy to study the dynamics of singlet fission (SF) in tetracene single crystals. The spectrotemporal species for singlet and triplet excitons in transient absorption spectra are found to be strongly dependent on probe polarization. By carefully analyzing the polarization dependence, the signals contributed by different transitions related to singlet excitons have been disentangled, which is further applied to construct the correlation between dynamics of singlet and triplet excitons. The anisotropy of exciton dynamics provides an alternative approach to tackle the long-standing challenge in understanding the mechanism of singlet fission in organic semiconductors.

  5. Charge Transfer States in Dilute Donor-Acceptor Blend Organic Heterojunctions.

    PubMed

    Liu, Xiao; Ding, Kan; Panda, Anurag; Forrest, Stephen R

    2016-08-23

    We study the charge transfer (CT) states in small-molecule blend heterojunctions comprising the nonpolar donor, tetraphenyldibenzoperiflanthene (DBP), and the acceptor, C70, using electroluminescence and steady-state and time-resolved photoluminescence spectroscopy along with density functional theory calculations. We find that the CT exciton energy blue shifts as the C70 concentration in the blend is either decreased or increased away from 50 vol %. At 20 K, the increase in CT state lifetime is correlated with the increasing diameter of C70 nanocrystallites in the blends. A quantum confinement model is used to quantitatively describe the dependence of both CT energy and lifetime on the C70 or DBP domain size. Two discrete CT emission peaks are observed for blends whose C70 concentration is >65%, at which point C70 nanocrystallites with diameters >4 nm appear in high-resolution transmission electron micrographs. The presence of two CT states is attributed to coexistence of crystalline C70 and amorphous phases in the blends. Furthermore, analysis of CT dissociation efficiency versus photon energy suggests that the >90% dissociation efficiency of delocalized CT2 states from the crystalline phase significantly contributes to surprisingly efficient photogeneration in highly dilute (>80% C70) DBP/C70 heterojunctions. PMID:27487403

  6. Trap-limited carrier recombination in single-walled carbon nanotube heterojunctions with fullerene acceptor layers

    NASA Astrophysics Data System (ADS)

    Ferguson, Andrew J.; Dowgiallo, Anne-Marie; Bindl, Dominick J.; Mistry, Kevin S.; Reid, Obadiah G.; Kopidakis, Nikos; Arnold, Michael S.; Blackburn, Jeffrey L.

    2015-06-01

    Single-walled carbon nanotube (SWCNT)-fullerene (C60) bilayers represent an attractive "donor-acceptor" binary system for solar photoconversion, where the kinetics of photoinduced processes depend critically on the properties of the interface between the two materials. Using photoconductivity measurements we identify the kinetic scheme that describes the free carrier kinetics in such bilayers where the dominant SWCNT species is the (7,5) semiconducting nanotube. Following charge separation, the carrier kinetics, covering up to four orders of magnitude in volumetric hole density, are described by a recombination process that is limited by capture and emission at traps or states at the SWCNT-C60 interface. The high-frequency mobility of holes in the (7,5) SWCNT phase is lower than in multichiral films, potentially due to differences in SWCNT defect density for nanotubes that have been purified more aggressively. The results obtained here provide fundamental insights into the transport and recombination of both charges and excitons within SWCNT thin films and bilayers, and point to several potential ways to improve SWCNT-C60 photovoltaic devices.

  7. Intramolecular charge transfer in donor-acceptor molecules

    SciTech Connect

    Slama-Schwok, A.; Blanchard-Desce, M.; Lehn, J.M. )

    1990-05-17

    The photophysical properties of donor-acceptor molecules, push-pull polyenes and carotenoids, have been studied by absorption and fluorescence spectroscopy. The compounds bear various acceptor and donor groups, linked together by chains of different length and structure. The position of the absorption and fluorescence maxima and their variation in solvents of increasing polarity are in agreement with long-distance intramolecular charge-transfer processes, the linker acting as a molecular wire. The effects of the linker length and structure and of the nature of acceptor and donor are presented.

  8. Bound states and the Bekenstein bound

    SciTech Connect

    Bousso, Raphael

    2003-10-16

    We explore the validity of the generalized Bekenstein bound, S<= pi M a. We define the entropy S as the logarithm of the number of states which have energy eigenvalue below M and are localized to a flat space region of width alpha. If boundary conditions that localize field modes are imposed by fiat, then the bound encounters well-known difficulties with negative Casimir energy and large species number, as well as novel problems arising only in the generalized form. In realistic systems, however, finite-size effects contribute additional energy. We study two different models for estimating such contributions. Our analysis suggests that the bound is both valid and nontrivial if interactions are properly included, so that the entropy S counts the bound states of interacting fields.

  9. Single and double acceptor-levels of a carbon-hydrogen defect in n-type silicon

    NASA Astrophysics Data System (ADS)

    Stübner, R.; Scheffler, L.; Kolkovsky, Vl.; Weber, J.

    2016-05-01

    In the present study, we discuss the origin of two dominant deep levels (E42 and E262) observed in n-type Si, which is subjected to hydrogenation by wet chemical etching or a dc H-plasma treatment. Their activation enthalpies determined from Laplace deep level transient spectroscopy measurements are EC-0.06 eV (E42) and EC-0.51 eV (E262). The similar annealing behavior and identical depth profiles of E42 and E262 correlate them with two different charge states of the same defect. E262 is attributed to a single acceptor state due to the absence of the Poole-Frenkel effect and the lack of a capture barrier for electrons. The emission rate of E42 shows a characteristic enhancement with the electric field, which is consistent with the assignment to a double acceptor state. In samples with different carbon and hydrogen content, the depth profiles of E262 can be explained by a defect with one H-atom and one C-atom. From a comparison with earlier calculations [Andersen et al., Phys. Rev. B 66, 235205 (2002)], we attribute E42 to the double acceptor and E262 to the single acceptor state of the CH1AB configuration, where one H atom is directly bound to carbon in the anti-bonding position.

  10. Exciton-vibrational coupling in the dynamics and spectroscopy of Frenkel excitons in molecular aggregates

    NASA Astrophysics Data System (ADS)

    Schröter, M.; Ivanov, S. D.; Schulze, J.; Polyutov, S. P.; Yan, Y.; Pullerits, T.; Kühn, O.

    2015-03-01

    The influence of exciton-vibrational coupling on the optical and transport properties of molecular aggregates is an old problem that gained renewed interest in recent years. On the experimental side, various nonlinear spectroscopic techniques gave insight into the dynamics of systems as complex as photosynthetic antennae. Striking evidence was gathered that in these protein-pigment complexes quantum coherence is operative even at room temperature conditions. Investigations were triggered to understand the role of vibrational degrees of freedom, beyond that of a heat bath characterized by thermal fluctuations. This development was paralleled by theory, where efficient methods emerged, which could provide the proper frame to perform non-Markovian and non-perturbative simulations of exciton-vibrational dynamics and spectroscopy. This review summarizes the state of affairs of the theory of exciton-vibrational interaction in molecular aggregates and photosynthetic antenna complexes. The focus is put on the discussion of basic effects of exciton-vibrational interaction from the stationary and dynamics points of view. Here, the molecular dimer plays a prominent role as it permits a systematic investigation of absorption and emission spectra by numerical diagonalization of the exciton-vibrational Hamiltonian in a truncated Hilbert space. An extension to larger aggregates, having many coupled nuclear degrees of freedom, becomes possible with the Multi-Layer Multi-Configuration Time-Dependent Hartree (ML-MCTDH) method for wave packet propagation. In fact it will be shown that this method allows one to approach the limit of almost continuous spectral densities, which is usually the realm of density matrix theory. Real system-bath situations are introduced for two models, which differ in the way strongly coupled nuclear coordinates are treated, as a part of the relevant system or the bath. A rather detailed exposition of the Hierarchy Equations Of Motion (HEOM) method will be

  11. Quantum-Chemical Studies on Excitation Energy Transfer Processes in BODIPY-Based Donor-Acceptor Systems.

    PubMed

    Spiegel, J Dominik; Kleinschmidt, Martin; Larbig, Alexander; Tatchen, Jörg; Marian, Christel M

    2015-09-01

    BODIPY-based excitation energy transfer (EET) cassettes are experimentally extensively studied and serve as excellent model systems for the investigation of photophysical processes, since they occur in any photosynthetic system and in organic photovoltaics. In the present work, the EET rates in five BODIPY-based EET cassettes in which anthracene serves as the donor have been determined, employing the monomer transition density approach (MTD) and the ideal dipole approximation (IDA). To this end, a new computer program has been devised that calculates the direct and exchange contributions to the excitonic coupling (EC) matrix element from transition density matrices generated by a combined density functional and multireference configuration interaction (DFT/MRCI) calculation for the monomers. EET rates have been calculated according to Fermi's Golden Rule from the EC and the spectral overlap, which was obtained from the calculated vibrationally resolved emission and absorption spectra of donor and acceptor, respectively. We find that the direct contribution to the EC matrix element is dominant in the studied EET cassettes. Furthermore, we show that the contribution of the molecular linker to the EET rate cannot be neglected. In our best fragment model, the molecular linker is attached to the donor moiety. For cassettes in which the transition dipole moments of donor and acceptor are oriented in parallel manner, our results confirm the experimental findings reported by Kim et al. [J. Phys. Chem. A 2006, 110, 20-27]. In cassettes with a perpendicular orientation of the donor and acceptor transition dipole moments, dynamic effects turn out to be important. PMID:26575926

  12. Nitrogen is a deep acceptor in ZnO

    DOE PAGESBeta

    Tarun, M. C.; Iqbal, M. Zafar; McCluskey, M. D.

    2011-04-14

    Zinc oxide is a promising material for blue and UV solid-state lighting devices, among other applications. Nitrogen has been regarded as a potential p-type dopant for ZnO. However, recent calculations indicate that nitrogen is a deep acceptor. This paper presents experimental evidence that nitrogen is, in fact, a deep acceptor and therefore cannot produce p-type ZnO. A broad photoluminescence (PL) emission band near 1.7 eV, with an excitation onset of ~2.2 eV, was observed, in agreement with the deep-acceptor model of the nitrogen defect. Thus the deep-acceptor behavior can be explained by the low energy of the ZnO valence bandmore » relative to the vacuum level.« less

  13. Nitrogen is a deep acceptor in ZnO

    SciTech Connect

    Tarun, M. C.; Iqbal, M. Zafar; McCluskey, M. D.

    2011-04-14

    Zinc oxide is a promising material for blue and UV solid-state lighting devices, among other applications. Nitrogen has been regarded as a potential p-type dopant for ZnO. However, recent calculations indicate that nitrogen is a deep acceptor. This paper presents experimental evidence that nitrogen is, in fact, a deep acceptor and therefore cannot produce p-type ZnO. A broad photoluminescence (PL) emission band near 1.7 eV, with an excitation onset of ~2.2 eV, was observed, in agreement with the deep-acceptor model of the nitrogen defect. Thus the deep-acceptor behavior can be explained by the low energy of the ZnO valence band relative to the vacuum level.

  14. Excitonic luminescence upconversion in a two-dimensional semiconductor

    NASA Astrophysics Data System (ADS)

    Jones, Aaron M.; Yu, Hongyi; Schaibley, John R.; Yan, Jiaqiang; Mandrus, David G.; Taniguchi, Takashi; Watanabe, Kenji; Dery, Hanan; Yao, Wang; Xu, Xiaodong

    2016-04-01

    Photon upconversion is an elementary light-matter interaction process in which an absorbed photon is re-emitted at higher frequency after extracting energy from the medium. This phenomenon lies at the heart of optical refrigeration in solids, where upconversion relies on anti-Stokes processes enabled either by rare-earth impurities or exciton-phonon coupling. Here, we demonstrate a luminescence upconversion process from a negatively charged exciton to a neutral exciton resonance in monolayer WSe2, producing spontaneous anti-Stokes emission with an energy gain of 30 meV. Polarization-resolved measurements find this process to be valley selective, unique to monolayer semiconductors. Since the charged exciton binding energy closely matches the 31 meV A1' optical phonon, we ascribe the spontaneous excitonic anti-Stokes to doubly resonant Raman scattering, where the incident and outgoing photons are in resonance with the charged and neutral excitons, respectively. In addition, we resolve a charged exciton doublet with a 7 meV splitting, probably induced by exchange interactions, and show that anti-Stokes scattering is efficient only when exciting the doublet peak resonant with the phonon, further confirming the excitonic doubly resonant picture.

  15. Ultrafast exciton relaxation in monolayer transition metal dichalcogenides

    NASA Astrophysics Data System (ADS)

    Thilagam, A.

    2016-04-01

    We examine a mechanism by which excitons undergo ultrafast relaxation in common monolayer transition metal dichalcogenides. It is shown that at densities ≈1 × 1011 cm-2 and temperatures ≤60 K, excitons in well known monolayers (MoS2, MoSe2, WS2, and WSe2) exist as point-like structureless electron-hole quasi-particles. We evaluate the average rate of exciton energy relaxation due to acoustic phonons via the deformation potential and the piezoelectric coupling mechanisms and examine the effect of spreading of the excitonic wavefunction into the region perpendicular to the monolayer plane. Our results show that the exciton relaxation rate is enhanced with increase in the exciton temperature, while it is decreased with increase in the lattice temperature. Good agreements with available experimental data are obtained when the calculations are extrapolated to room temperatures. A unified approach taking into account the deformation potential and piezoelectric coupling mechanisms shows that exciton relaxation induced by phonons is as significant as defect assisted scattering and trapping of excitons by surface states in monolayer transition metal dichalcogenides.

  16. Measuring and Modeling Exciton Dynamics in Multichromophore Macromolecules

    NASA Astrophysics Data System (ADS)

    Weingarten, Daniel; Hu, Nan; Lacount, Michael; Ferguson, Andrew; Dessau, Daniel; Walba, David; Vandelagemaat, Jao; Lusk, Mark; Rumbles, Garry; Shaheen, Sean

    2014-03-01

    Attaining specific control over the dynamics of exciton movement in organic photovoltaics (OPV) has, thus far, been a largely unachieved goal of OPV design. Such an understanding of exciton transfer dynamics would allow for the design of macromolecules whose energetics, bandgaps, and conformational properties allow for control of exciton flow toward specific reaction site chromophores, potentially enabling non-linear improvements in energy harvesting. To better understand exciton movement we synthesized and characterized a multi-chromophoric macromolecule and measured the dynamics of exciton transfer across coupled chromophores. Our model system is a hexabenzocoronene molecule attached to six oligothiophene. We developed a kinetic model and by fitting it to the decay rates of excited states measured via time-correlated single photon counting, we were able to extract rates for exciton transfer between chromophores. Since this macromolecule exhibits liquid crystalline aggregation behavior, observing the dependence of exciton transfer rates on solution concentration yields an improved understanding of exciton movement within a single molecule as well as the dependence of that transfer process on local material structure.

  17. Mapping the exciton diffusion in semiconductor nanocrystal solids.

    PubMed

    Kholmicheva, Natalia; Moroz, Pavel; Bastola, Ebin; Razgoniaeva, Natalia; Bocanegra, Jesus; Shaughnessy, Martin; Porach, Zack; Khon, Dmitriy; Zamkov, Mikhail

    2015-03-24

    Colloidal nanocrystal solids represent an emerging class of functional materials that hold strong promise for device applications. The macroscopic properties of these disordered assemblies are determined by complex trajectories of exciton diffusion processes, which are still poorly understood. Owing to the lack of theoretical insight, experimental strategies for probing the exciton dynamics in quantum dot solids are in great demand. Here, we develop an experimental technique for mapping the motion of excitons in semiconductor nanocrystal films with a subdiffraction spatial sensitivity and a picosecond temporal resolution. This was accomplished by doping PbS nanocrystal solids with metal nanoparticles that force the exciton dissociation at known distances from their birth. The optical signature of the exciton motion was then inferred from the changes in the emission lifetime, which was mapped to the location of exciton quenching sites. By correlating the metal-metal interparticle distance in the film with corresponding changes in the emission lifetime, we could obtain important transport characteristics, including the exciton diffusion length, the number of predissociation hops, the rate of interparticle energy transfer, and the exciton diffusivity. The benefits of this approach to device applications were demonstrated through the use of two representative film morphologies featuring weak and strong interparticle coupling. PMID:25682881

  18. Excitons in atomically thin black phosphorus

    NASA Astrophysics Data System (ADS)

    Surrente, A.; Mitioglu, A. A.; Galkowski, K.; Tabis, W.; Maude, D. K.; Plochocka, P.

    2016-03-01

    Raman scattering and photoluminescence spectroscopy are used to investigate the optical properties of single layer black phosphorus obtained by mechanical exfoliation of bulk crystals under an argon atmosphere. The Raman spectroscopy, performed in situ on the same flake as the photoluminescence measurements, demonstrates the single layer character of the investigated samples. The emission spectra, dominated by excitonic effects, display the expected in-plane anisotropy. The emission energy depends on the type of substrate on which the flake is placed due to the different dielectric screening. Finally, the blueshift of the emission with increasing temperature is well described using a two-oscillator model for the temperature dependence of the band gap.

  19. Electro-optical properties of Rydberg excitons

    NASA Astrophysics Data System (ADS)

    Zielińska-Raczyńska, Sylwia; Ziemkiewicz, David; Czajkowski, Gerard

    2016-07-01

    We show how to compute the electro-optical functions (absorption, reflection, and transmission) when Rydberg exciton-polaritons appear, including the effect of the coherence between the electron-hole pair and the electromagnetic field. With the use of the real density matrix approach, numerical calculations applied for the Cu2O crystal are performed. We also examine in detail and explain the dependence of the resonance displacement on the state number and applied electric field strength. We report a fairly good agreement with recently published experimental data.

  20. Feasibility study of a nuclear exciton laser

    NASA Astrophysics Data System (ADS)

    ten Brinke, Nicolai; Schützhold, Ralf; Habs, Dietrich

    2013-05-01

    Nuclear excitons known from Mössbauer spectroscopy describe coherent excitations of a large number of nuclei—analogous to Dicke states (or Dicke super-radiance) in quantum optics. In this paper, we study the possibility of constructing a laser based on these coherent excitations. In contrast to the free-electron laser (in its usual design), such a device would be based on stimulated emission and thus might offer certain advantages, e.g., regarding energy-momentum accuracy. Unfortunately, inserting realistic parameters, the window of operability is probably not open (yet) to present-day technology; but our design should be feasible in the UV regime, for example.

  1. Entangled exciton states in quantum dot molecules

    NASA Astrophysics Data System (ADS)

    Bayer, Manfred

    2002-03-01

    Currently there is strong interest in quantum information processing(See, for example, The Physics of Quantum Information, eds. D. Bouwmeester, A. Ekert and A. Zeilinger (Springer, Berlin, 2000).) in a solid state environment. Many approaches mimic atomic physics concepts in which semiconductor quantum dots are implemented as artificial atoms. An essential building block of a quantum processor is a gate which entangles the states of two quantum bits. Recently a pair of vertically aligned quantum dots has been suggested as optically driven quantum gate(P. Hawrylak, S. Fafard, and Z. R. Wasilewski, Cond. Matter News 7, 16 (1999).)(M. Bayer, P. Hawrylak, K. Hinzer, S. Fafard, M. Korkusinski, Z.R. Wasilewski, O. Stern, and A. Forchel, Science 291, 451 (2001).): The quantum bits are individual carriers either on dot zero or dot one. The different dot indices play the same role as a "spin", therefore we call them "isospin". Quantum mechanical tunneling between the dots rotates the isospin and leads to superposition of these states. The quantum gate is built when two different particles, an electron and a hole, are created optically. The two particles form entangled isospin states. Here we present spectrocsopic studies of single self-assembled InAs/GaAs quantum dot molecules that support the feasibility of this proposal. The evolution of the excitonic recombination spectrum with varying separation between the dots allows us to demonstrate coherent tunneling of carriers across the separating barrier and the formation of entangled exciton states: Due to the coupling between the dots the exciton states show a splitting that increases with decreasing barrier width. For barrier widths below 5 nm it exceeds the thermal energy at room temperature. For a given barrier width, we find only small variations of the tunneling induced splitting demonstrating a good homogeneity within a molecule ensemble. The entanglement may be controlled by application of electromagnetic field. For

  2. A matrix lower bound

    SciTech Connect

    Grcar, Joseph F.

    2002-02-04

    A matrix lower bound is defined that generalizes ideas apparently due to S. Banach and J. von Neumann. The matrix lower bound has a natural interpretation in functional analysis, and it satisfies many of the properties that von Neumann stated for it in a restricted case. Applications for the matrix lower bound are demonstrated in several areas. In linear algebra, the matrix lower bound of a full rank matrix equals the distance to the set of rank-deficient matrices. In numerical analysis, the ratio of the matrix norm to the matrix lower bound is a condition number for all consistent systems of linear equations. In optimization theory, the matrix lower bound suggests an identity for a class of min-max problems. In real analysis, a recursive construction that depends on the matrix lower bound shows that the level sets of continuously differential functions lie asymptotically near those of their tangents.

  3. Synthesis, Properties, and Design Principles of Donor–Acceptor Nanohoops

    PubMed Central

    2015-01-01

    We have synthesized a series of aza[8]cycloparaphenylenes containing one, two, and three nitrogens to probe the impact of nitrogen doping on optoelectronic properties and solid state packing. Alkylation of these azananohoops afforded the first donor–acceptor nanohoops where the phenylene backbone acts as the donor and the pyridinium units act as the acceptor. The impact on the optoelectronic properties was then studied experimentally and computationally to provide new insight into the effect of functionalization on nanohoops properties. PMID:27162989

  4. Exciton complexes in low dimensional transition metal dichalcogenides

    NASA Astrophysics Data System (ADS)

    Thilagam, A.

    2014-08-01

    We examine the excitonic properties of layered configurations of low dimensional transition metal dichalcogenides (LTMDCs) using the fractional dimensional space approach. The binding energies of the exciton, trion, and biexciton in LTMDCs of varying layers are analyzed, and linked to the dimensionality parameter α, which provides insight into critical electro-optical properties (relative oscillator strength, absorption spectrum, exciton-exciton interaction) of the material systems. The usefulness of α is highlighted by its independence of the physical mechanisms underlying the confinement effects of geometrical structures. Our estimates of the binding energies of exciton complexes for the monolayer configuration of transition metal dichalcogenides suggest a non-collinear structure for the trion and a positronium-molecule-like square structure for the biexciton.

  5. Exciton complexes in low dimensional transition metal dichalcogenides

    SciTech Connect

    Thilagam, A.

    2014-08-07

    We examine the excitonic properties of layered configurations of low dimensional transition metal dichalcogenides (LTMDCs) using the fractional dimensional space approach. The binding energies of the exciton, trion, and biexciton in LTMDCs of varying layers are analyzed, and linked to the dimensionality parameter α, which provides insight into critical electro-optical properties (relative oscillator strength, absorption spectrum, exciton-exciton interaction) of the material systems. The usefulness of α is highlighted by its independence of the physical mechanisms underlying the confinement effects of geometrical structures. Our estimates of the binding energies of exciton complexes for the monolayer configuration of transition metal dichalcogenides suggest a non-collinear structure for the trion and a positronium-molecule-like square structure for the biexciton.

  6. Temperature effects in excitonic condensation driven by the lattice distortion

    NASA Astrophysics Data System (ADS)

    Do, Thi-Hong-Hai; Nguyen, Huu-Nha; Nguyen, Thi-Giang; Phan, Van-Nham

    2016-06-01

    The stability of the excitonic condensation at low temperature driven by a coupling of electrons to vibrational degrees of freedom in semimetal two-dimensional electronic system is discussed. In the framework of the unrestricted Hartree-Fock approximation, we derive a set of equations to determine both the excitonic condensate order parameter and lattice displacement self-consistently. By lowering temperature we find out a semimetal-insulator transition in the system if the coupling is large enough. The insulating state typifies an excitonic condensation accompanied by a finite lattice distortion. Increasing temperature, both excitonic condensate order parameter and the lattice distortion decrease and then disappear in the same manner. Microscopic analysis in momentum space strongly specifies that the excitonic condensate driven by the lattice distortion favours the BCS type.

  7. Influence of Exciton Localization on the Emission and Ultraviolet Photoresponse of ZnO/ZnS Core-Shell Nanowires.

    PubMed

    Fang, Xuan; Wei, Zhipeng; Chen, Rui; Tang, Jilong; Zhao, Haifeng; Zhang, Ligong; Zhao, Dongxu; Fang, Dan; Li, Jinhua; Fang, Fang; Chu, Xueying; Wang, Xiaohua

    2015-05-20

    The structural and optical properties of ZnO and ZnO/ZnS core-shell nanowires grown by a wet chemical method are investigated. The near-bandgap ultraviolet (UV) emission of the ZnO nanowires was enhanced by four times after coating with ZnS. The enhanced emission was attributed to surface passivation of the ZnO nanowires and localized states introduced during ZnS growth. The emission of the ZnO and ZnO/ZnS core-shell nanowires was attributed to neutral donor-bound excitons and localized excitons, respectively. Localized states prevented excitons from diffusing to nonradiative recombination centers, so therefore contributed to the enhanced emission. Emission from the localized exciton was not sensitive to temperature, so emission from the ZnO/ZnS core-shell nanowires was more stable at higher temperature. UV photodetectors based on the ZnO and ZnO/ZnS core-shell nanowires were fabricated. Under UV excitation, the device based on the ZnO/ZnS core-shell nanowires exhibited a photocurrent approximately 40 times higher than that of the device based on the ZnO nanowires. The differing photoresponse of the detectors was consistent with the existence of surface passivation and localized states. This study provides a means for modifying the optical properties of ZnO materials, and demonstrates the potential of ZnO/ZnS core-shell nanowires in UV excitonic emission and detection. PMID:25918945

  8. Description of the Adsorption and Exciton Delocalizing Properties of p-Substituted Thiophenols on CdSe Quantum Dots.

    PubMed

    Aruda, Kenneth O; Amin, Victor A; Thompson, Christopher M; Lau, Bryan; Nepomnyashchii, Alexander B; Weiss, Emily A

    2016-04-12

    This work describes the quantitative characterization of the interfacial chemical and electronic structure of CdSe quantum dots (QDs) coated in one of five p-substituted thiophenolates (X-TP, X = NH2, CH3O, CH3, Cl, or NO2), and the dependence of this structure on the p-substituent X. (1)H NMR spectra of mixtures of CdSe QDs and X-TPs yield the number of X-TPs bound to the surface of each QD. The binding data, in combination with the shift in the energy of the first excitonic peak of the QDs as a function of the surface coverage of X-TP and Raman and NMR analysis of the mixtures, indicate that X-TP binds to CdSe QDs in at least three modes, two modes that are responsible for exciton delocalization and a third mode that does not affect the excitonic energy. The first two modes involve displacement of OPA from the QD core, whereas the third mode forms cadmium-thiophenolate complexes that are not electronically coupled to the QD core. Fits to the data using the dual-mode binding model also yield the values of Δr1, the average radius of exciton delocalization due to binding of the X-TP in modes 1 and 2. A 3D parametrized particle-in-a-sphere model enables the conversion of the measured value of Δr1 for each X-TP to the height of the potential barrier that the ligand presents for tunneling of excitonic hole into the interfacial region. The height of this barrier increases from 0.3 to 0.9 eV as the substituent, X, becomes more electron-withdrawing. PMID:27002248

  9. Approximation of excitonic absorption in disordered systems using a compositional-component-weighted coherent-potential approximation

    NASA Astrophysics Data System (ADS)

    Schwabe, N. F.; Elliott, R. J.

    1996-03-01

    Employing a recently developed technique of component-weighted two-particle Green's functions in the coherent-potential approximation (CPA) of a binary substitutional alloy AcB1-c we extend the existing theory of excitons in such media using a contact potential model for the interaction between electrons and holes to an approximation which interpolates correctly between the limits of weak and strong disorder. With our approach we are also able to treat the case where the contact interaction between carriers varies between sites of different types, thus introducing further disorder into the system. Based on this approach we study numerically how the formation of exciton bound states changes as the strengths of the contact potentials associated with either of the two site types are varied through a large range of parameter values.

  10. Exciton Correlations in Intramolecular Singlet Fission.

    PubMed

    Sanders, Samuel N; Kumarasamy, Elango; Pun, Andrew B; Appavoo, Kannatassen; Steigerwald, Michael L; Campos, Luis M; Sfeir, Matthew Y

    2016-06-15

    We have synthesized a series of asymmetric pentacene-tetracene heterodimers with a variable-length conjugated bridge that undergo fast and efficient intramolecular singlet fission (iSF). These compounds have distinct singlet and triplet energies, which allow us to study the spatial dynamics of excitons during the iSF process, including the significant role of exciton correlations in promoting triplet pair generation and recombination. We demonstrate that the primary photoexcitations in conjugated dimers are delocalized singlets that enable fast and efficient iSF. However, in these asymmetric dimers, the singlet becomes more localized on the lower energy unit as the length of the bridge is increased, slowing down iSF relative to analogous symmetric dimers. We resolve the recombination kinetics of the inequivalent triplets produced via iSF, and find that they primarily decay via concerted processes. By identifying different decay channels, including delayed fluorescence via triplet-triplet annihilation, we can separate transient species corresponding to both correlated triplet pairs and uncorrelated triplets. Recombination of the triplet pair proceeds rapidly despite our experimental and theoretical demonstration that individual triplets are highly localized and unable to be transported across the conjugated linker. In this class of compounds, the rate of formation and yield of uncorrelated triplets increases with bridge length. Overall, these constrained, asymmetric systems provide a unique platform to isolate and study transient species essential for singlet fission, which are otherwise difficult to observe in symmetric dimers or condensed phases. PMID:27183040

  11. Excitons and charges at organic semiconductor heterojunctions.

    PubMed

    Friend, Richard H; Phillips, Matthew; Rao, Akshay; Wilson, Mark W B; Li, Zhe; McNeill, Christopher R

    2012-01-01

    All-organic heterojunction solar cells now provide very high quantum efficiencies for charge generation and rapidly-improving power conversion efficiencies. Charge generation and separation however, must overcome the strong Coulomb interactions between electrons and holes in these materials that is manifest also through the large exchange energies usually observed. We show for a polymer-polymer system with low charge generation efficiency that this arises through intersystem crossing from the photogenerated charge-transfer state to a lower lying triplet state, mediated by the proton hyperfine interaction, and that the activation barrier for full separation of electrons and holes is of the order of 250 meV. We observe, using transient optical spectroscopy, the processes of charge separation, recombination and sweep-out in efficient polymer-fullerene devices. We report also on the process of singlet exciton fission to form a pair of triplet excitons in pentacene that can later be dissociated against a heterojunction formed with C60. PMID:22470984

  12. Robust excitons inhabit soft supramolecular nanotubes

    PubMed Central

    Eisele, Dörthe M.; Arias, Dylan H.; Fu, Xiaofeng; Bloemsma, Erik A.; Steiner, Colby P.; Jensen, Russell A.; Rebentrost, Patrick; Eisele, Holger; Tokmakoff, Andrei; Lloyd, Seth; Nelson, Keith A.; Nicastro, Daniela; Knoester, Jasper; Bawendi, Moungi G.

    2014-01-01

    Nature's highly efficient light-harvesting antennae, such as those found in green sulfur bacteria, consist of supramolecular building blocks that self-assemble into a hierarchy of close-packed structures. In an effort to mimic the fundamental processes that govern nature’s efficient systems, it is important to elucidate the role of each level of hierarchy: from molecule, to supramolecular building block, to close-packed building blocks. Here, we study the impact of hierarchical structure. We present a model system that mirrors nature’s complexity: cylinders self-assembled from cyanine-dye molecules. Our work reveals that even though close-packing may alter the cylinders’ soft mesoscopic structure, robust delocalized excitons are retained: Internal order and strong excitation-transfer interactions—prerequisites for efficient energy transport—are both maintained. Our results suggest that the cylindrical geometry strongly favors robust excitons; it presents a rational design that is potentially key to nature’s high efficiency, allowing construction of efficient light-harvesting devices even from soft, supramolecular materials. PMID:25092336

  13. Exciton Dynamics in Semiconducting Carbon Nanotubes

    SciTech Connect

    Graham, Matt; Chmeliov, Javgenij; Ma, Yingzhong; Shinohara, Nori; Green, Alexander A.; Hersam, Mark C.; Valkunas, Leonas; Fleming, Graham

    2010-01-01

    We report femtosecond transient absorption spectroscopic study on the (6, 5) single-walled carbon nanotubes and the (7, 5) inner tubes of a dominant double-walled carbon nanotube species. We found that the dynamics of exciton relaxation probed at the first transition-allowed state (E11) of a given tube type exhibits a markedly slower decay when the second transition-allowed state (E22) is excited than that measured by exciting its first transition-allowed state (E11). A linear intensity dependence of the maximal amplitude of the transient absorption signal is found for the E22 excitation, whereas the corresponding amplitude scales linearly with the square root of the E11 excitation intensity. Theoretical modeling of these experimental findings was performed by developing a continuum model and a stochastic model with explicit consideration of the annihilation of coherent excitons. Our detailed numerical simulations show that both models can reproduce reasonably well the initial portion of decay kinetics measured upon the E22 and E11 excitation of the chosen tube species, but the stochastic model gives qualitatively better agreement with the intensity dependence observed experimentally than those obtained with the continuum model.

  14. Robust excitons inhabit soft supramolecular nanotubes.

    PubMed

    Eisele, Dörthe M; Arias, Dylan H; Fu, Xiaofeng; Bloemsma, Erik A; Steiner, Colby P; Jensen, Russell A; Rebentrost, Patrick; Eisele, Holger; Tokmakoff, Andrei; Lloyd, Seth; Nelson, Keith A; Nicastro, Daniela; Knoester, Jasper; Bawendi, Moungi G

    2014-08-19

    Nature's highly efficient light-harvesting antennae, such as those found in green sulfur bacteria, consist of supramolecular building blocks that self-assemble into a hierarchy of close-packed structures. In an effort to mimic the fundamental processes that govern nature's efficient systems, it is important to elucidate the role of each level of hierarchy: from molecule, to supramolecular building block, to close-packed building blocks. Here, we study the impact of hierarchical structure. We present a model system that mirrors nature's complexity: cylinders self-assembled from cyanine-dye molecules. Our work reveals that even though close-packing may alter the cylinders' soft mesoscopic structure, robust delocalized excitons are retained: Internal order and strong excitation-transfer interactions--prerequisites for efficient energy transport--are both maintained. Our results suggest that the cylindrical geometry strongly favors robust excitons; it presents a rational design that is potentially key to nature's high efficiency, allowing construction of efficient light-harvesting devices even from soft, supramolecular materials. PMID:25092336

  15. Robust excitons inhabit soft supramolecular nanotubes

    NASA Astrophysics Data System (ADS)

    Eisele, Dörthe M.; Arias, Dylan H.; Fu, Xiaofeng; Bloemsma, Erik A.; Steiner, Colby P.; Jensen, Russell A.; Rebentrost, Patrick; Eisele, Holger; Tokmakoff, Andrei; Lloyd, Seth; Nelson, Keith A.; Nicastro, Daniela; Knoester, Jasper; Bawendi, Moungi G.

    2014-08-01

    Nature's highly efficient light-harvesting antennae, such as those found in green sulfur bacteria, consist of supramolecular building blocks that self-assemble into a hierarchy of close-packed structures. In an effort to mimic the fundamental processes that govern nature's efficient systems, it is important to elucidate the role of each level of hierarchy: from molecule, to supramolecular building block, to close-packed building blocks. Here, we study the impact of hierarchical structure. We present a model system that mirrors nature's complexity: cylinders self-assembled from cyanine-dye molecules. Our work reveals that even though close-packing may alter the cylinders' soft mesoscopic structure, robust delocalized excitons are retained: Internal order and strong excitation-transfer interactions-prerequisites for efficient energy transport-are both maintained. Our results suggest that the cylindrical geometry strongly favors robust excitons; it presents a rational design that is potentially key to nature's high efficiency, allowing construction of efficient light-harvesting devices even from soft, supramolecular materials.

  16. Excitation and deexcitation dynamics of excitons in a GaN film based on the analysis of radiation from high-order states

    NASA Astrophysics Data System (ADS)

    Ishitani, Yoshihiro; Takeuchi, Kazuma; Oizumi, Naoyuki; Sakamoto, Hironori; Ma, Bei; Morita, Ken; Miyake, Hideto; Hiramatsu, Kazumasa

    2016-06-01

    The physical mechanism of excitation and deexcitation transitions of nonthermal exciton states in a GaN film is investigated at a measurement temperature of 23 K by time-resolved photoluminescence (PL) analysis involving phonon replica lines of the principal quantum number n  =  2 in addition to n  =  1 and bound states of the A exciton. A time region of 280 ps after a pulse excitation is mainly analyzed. The emission intensities of the constituent lines are obtained by spectrum fitting. Although the effective exciton temperature of the n  =  1 state shows a relaxation time within approximately 150 ps as a previous report, the temperature of the n  =  2 state is found to have a longer relaxation time. This is because the n  =  2 state strongly couples with the continuum by excitation and deexcitation transfers, while the n  =  1 state couples with the donor bound state. These two systems exhibit different dynamic properties. Overall population transfer is the direction of energy relaxation, however, cooling of the upper states is delayed when compared to the lower states by the increase in the excitation transfer rate to the continuum. This dynamics of the exciton has a similarity to that of hydrogen atoms in plasma.

  17. Tuning the Electron Acceptor in Phthalocyanine-Based Electron Donor-Acceptor Conjugates.

    PubMed

    Sekita, Michael; Jiménez, Ángel J; Marcos, M Luisa; Caballero, Esmeralda; Rodríguez-Morgade, M Salomé; Guldi, Dirk M; Torres, Tomás

    2015-12-21

    Zinc phthalocyanines (ZnPc) have been attached to the peri-position of a perylenemonoimide (PMI) and a perylenemonoanhydride (PMA), affording electron donor-acceptor conjugates 1 and 2, respectively. In addition, a perylene-monoimide-monoanhydride (PMIMA) has been connected to a ZnPc through its imido position to yield the ZnPc-PMIMA conjugate 10. The three conjugates have been studied for photoinduced electron transfer. For ZnPc-PMIMA 10, electron transfer occurs upon both ZnPc and PMIMA excitation, giving rise to a long-lived (340 ps) charge-separated state. For ZnPc-PMI 1 and ZnPc-PMA 2, stabilization of the radical ion pair states by using polar media is necessary. In THF, photoexcitation of either ZnPc or PMI/PMA produces charge-separated states with lifetimes of 375 and 163 ps, respectively. PMID:26593778

  18. The Impact of Heterogeneity and Dark Acceptor States on FRET: Implications for Using Fluorescent Protein Donors and Acceptors

    PubMed Central

    Vogel, Steven S.; Nguyen, Tuan A.; van der Meer, B. Wieb; Blank, Paul S.

    2012-01-01

    Förster resonance energy transfer (FRET) microscopy is widely used to study protein interactions in living cells. Typically, spectral variants of the Green Fluorescent Protein (FPs) are incorporated into proteins expressed in cells, and FRET between donor and acceptor FPs is assayed. As appreciable FRET occurs only when donors and acceptors are within 10 nm of each other, the presence of FRET can be indicative of aggregation that may denote association of interacting species. By monitoring the excited-state (fluorescence) decay of the donor in the presence and absence of acceptors, dual-component decay analysis has been used to reveal the fraction of donors that are FRET positive (i.e., in aggregates)._However, control experiments using constructs containing both a donor and an acceptor FP on the same protein repeatedly indicate that a large fraction of these donors are FRET negative, thus rendering the interpretation of dual-component analysis for aggregates between separately donor-containing and acceptor-containing proteins problematic. Using Monte-Carlo simulations and analytical expressions, two possible sources for such anomalous behavior are explored: 1) conformational heterogeneity of the proteins, such that variations in the distance separating donor and acceptor FPs and/or their relative orientations persist on time-scales long in comparison with the excited-state lifetime, and 2) FP dark states. PMID:23152925

  19. The Role of Polyprenol-Bound Saccharides as Intermediates in Glycoprotein Synthesis in Liver

    PubMed Central

    Parodi, Armando J.; Behrens, Nicolas H.; Leloir, Luis F.; Carminatti, Hector

    1972-01-01

    It has been reported that liver microsomes catalyze the transfer of glucose from uridine diphosphate glucose to dolichol monophosphate so as to produce dolichol monophosphate glucose. Dolichol is a polyprenol containing about 20 isoprene units. The glucosyl residue of dolichol monophosphate glucose is transferred to an endogenous acceptor on further incubation with liver microsomes. The glucosylated endogenous acceptor appears to be an oligosaccharide of about 20 monosaccharide units bound to dolichol through a phosphate or pyrophosphate bridge. In this paper it is reported that liver microsomes catalyze the transfer of the oligosaccharide from the glucosylated endogenous acceptor to an endogenous protein. This transfer reaction requires the presence of bivalent cations, manganese being more effective than magnesium. The presence of deoxycholate is also required. Besides the glycoprotein, several water-soluble products are also formed. Preliminary evidence indicates that they are glucose, iligosaccharides of different size, and possibly oligosaccharides bound to amino acids. PMID:4508319

  20. Physical Uncertainty Bounds (PUB)

    SciTech Connect

    Vaughan, Diane Elizabeth; Preston, Dean L.

    2015-03-19

    This paper introduces and motivates the need for a new methodology for determining upper bounds on the uncertainties in simulations of engineered systems due to limited fidelity in the composite continuum-level physics models needed to simulate the systems. We show that traditional uncertainty quantification methods provide, at best, a lower bound on this uncertainty. We propose to obtain bounds on the simulation uncertainties by first determining bounds on the physical quantities or processes relevant to system performance. By bounding these physics processes, as opposed to carrying out statistical analyses of the parameter sets of specific physics models or simply switching out the available physics models, one can obtain upper bounds on the uncertainties in simulated quantities of interest.

  1. Asymptotic entropy bounds

    NASA Astrophysics Data System (ADS)

    Bousso, Raphael

    2016-07-01

    We show that known entropy bounds constrain the information carried off by radiation to null infinity. We consider distant, planar null hypersurfaces in asymptotically flat spacetime. Their focusing and area loss can be computed perturbatively on a Minkowski background, yielding entropy bounds in terms of the energy flux of the outgoing radiation. In the asymptotic limit, we obtain boundary versions of the quantum null energy condition, of the generalized Second Law, and of the quantum Bousso bound.

  2. Energy Transfer of Excitons Between Quantum Wells Separated by a Wide Barrier

    SciTech Connect

    LYO,SUNGKWUN K.

    1999-12-06

    We present a microscopic theory of the excitonic Stokes and anti-Stokes energy transfer mechanisms between two widely separated unequal quantum wells with a large energy mismatch ({Delta}) at low temperatures (T). Exciton transfer through dipolar coupling, photon-exchange coupling and over-barrier ionization of the excitons through exciton-exciton Auger processes are examined. The energy transfer rate is calculated as a function of T and the center-to-center distance d between the two wells. The rates depend sensitively on T for plane-wave excitons. For located excitons, the rates depend on T only through the T-dependence of the localization radius.

  3. Exciton Transfer in Carbon Nanotube Aggregates for Energy Harvesting Applications

    NASA Astrophysics Data System (ADS)

    Davoody, Amirhossein; Karimi, Farhad; Knezevic, Irena

    Carbon nanotubes (CNTs) are promising building blocks for organic photovoltaic devices, owing to their tunable band gap, mechanical and chemical stability. We study intertube excitonic energy transfer between pairs of CNTs with different orientations and band gaps. The optically bright and dark excitonic states in CNTs are calculated by solving the Bethe-Salpeter equation. We calculate the exciton transfer rates due to the direct and exchange Coulomb interactions, as well as the second-order phonon-assisted processes. We show the importance of phonons in calculating the transfer rates that match the measurements. In addition, we discuss the contribution of optically inactive excited states in the exciton transfer process, which is difficult to determine experimentally. Furthermore, we study the effects of sample inhomogeneity, impurities, and temperature on the exciton transfer rate. The inhomogeneity in the CNT sample dielectric function can increase the transfer rate by about a factor of two. We show that the exciton confinement by impurities has a detrimental effect on the transfer rate between pairs of similar CNTs. The exciton transfer rate increases monotonically with increasing temperature. Support by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering under Award DE-SC0008712.

  4. Dynamical fermion mass generation and exciton spectra in graphene

    SciTech Connect

    Zhang Chunxu; Liu Guozhu; Huang Mingqiu

    2011-03-15

    The Coulomb interaction between massless Dirac fermions may induce dynamical chiral symmetry breaking by forming excitonic pairs in clean graphene, leading to semimetal-insulator transition. If the Dirac fermions have zero bare mass, an exact continuous chiral symmetry is dynamically broken and thus there are massless Goldstone excitons. If the Dirac fermions have a small bare mass, an approximate continuous chiral symmetry is dynamically broken and the resultant Goldstone-type excitons become massive, which is analogous to what happens in QCD. In this paper, after solving the Dyson-Schwinger gap equation in the presence of a small bare fermion mass, we found a remarkable reduction of the critical Coulomb interaction strength for excitonic pair formation and a strong enhancement of dynamical fermion mass. We then calculate the masses of Goldstone-type excitons using the Shifman-Vainshtein-Zakharov sum-rule method and operator product expansion technique developed in QCD and find that the exciton masses are much larger than bare fermion mass but smaller than the width of dynamical fermion mass gap. We also study the spin susceptibilities and estimate the masses of non-Goldstone-type excitons using the same tools.

  5. Novel exciton systems in 2D TMD monolayers and heterobilayers

    NASA Astrophysics Data System (ADS)

    Yu, Hongyi

    In this talk, two exciton systems in transition metal dichalcogenides (TMDs) monolayer and heterobilayer will be discussed. In TMD monolayers, the strong e-h Coulomb exchange interaction splits the exciton and trion dispersions into two branches with zero and finite gap, respectively. Each branch is a center-of-mass wave vector dependent coherent superposition of the two valleys, which leads to a valley-orbit coupling and possibly a trion valley Hall effect. The exchange interaction also eliminates the linear polarization of the negative trion PL emission. In TMD heterobilayers with a type-II band alignment, the low energy exciton has an interlayer configuration with the e and h localized in opposite layers. Because of the inevitable twist or/and lattice mismatch between the two layers, the bright interlayer excitons are located at finite center-of-mass velocities with a six-fold degeneracy. The corresponding photon emission is elliptically polarized, with the major axis locked to the direction of exciton velocity, and helicity determined by the valley indices of the e and h. Some experimental results on the interlayer excitons in the WSe2-MoSe2 heterobilayers will also be presented. The interlayer exciton exhibits a long lifetime as well as a long depolarization time, which facilitate the observation of a PL polarization ring pattern due to the valley dependent exciton-exciton interaction induced expansion. The works were supported by the Research Grant Council of Hong Kong (HKU17305914P, HKU705513P), the Croucher Foundation, and the HKU OYRA and ROP.

  6. Primary acceptor in bacterial photosynthesis: obligatory role of ubiquinone in photoactive reaction centers of Rhodopseudomonas spheroides.

    PubMed Central

    Okamura, M Y; Isaacson, R A; Feher, G

    1975-01-01

    Reaction centers were found to bind two ubiquinones, both of which could be removed by o-phenanthroline and the detergent lauryldimethylamine oxide. One ubiquinone was more easily removed than the other. The low-temperature light-induced optical and electron paramagnetic resonance (EPR) changes were eliminated and restored upon removal and readdition of ubiquinone and were quantitatively correlated with the amount of tightly bound ubiquinone. We, therefore, conclude that this ubiquinone plays an obligatory role in the primary photochemistry. The easily removed ubiquinone is thought to be the secondary electron acceptor. The low-temperature charge recombination kinetics, as well as the optical and EPR spectra, were the same for untreated reaction centers and for those reconstituted with ubiquinone. This indicates that extraction and reconstitution were accomplished without altering the conformation of the active site. Reaction centers reconstituted with other quinones also showed restored photochemical activity, although they exhibited changes in their low-temperature recombination kinetics and light-induced (g = 1.8) EPR signal is interpreted in terms of a magnetically coupled ubiquinone--Fe2+ acceptor complex. A possible role of iron is to facilitate electron transfer between the primary and secondary ubiquinones. PMID:1081231

  7. Electron acceptor-dependent respiratory and physiological stratifications in biofilms.

    PubMed

    Yang, Yonggang; Xiang, Yinbo; Sun, Guoping; Wu, Wei-Min; Xu, Meiying

    2015-01-01

    Bacterial respiration is an essential driving force in biogeochemical cycling and bioremediation processes. Electron acceptors respired by bacteria often have solid and soluble forms that typically coexist in the environment. It is important to understand how sessile bacteria attached to solid electron acceptors respond to ambient soluble alternative electron acceptors. Microbial fuel cells (MFCs) provide a useful tool to investigate this interaction. In MFCs with Shewanella decolorationis, azo dye was used as an alternative electron acceptor in the anode chamber. Different respiration patterns were observed for biofilm and planktonic cells, with planktonic cells preferred to respire with azo dye while biofilm cells respired with both the anode and azo dye. The additional azo respiration dissipated the proton accumulation within the anode biofilm. There was a large redox potential gap between the biofilms and anode surface. Changing cathodic conditions caused immediate effects on the anode potential but not on the biofilm potential. Biofilm viability showed an inverse and respiration-dependent profile when respiring with only the anode or azo dye and was enhanced when respiring with both simultaneously. These results provide new insights into the bacterial respiration strategies in environments containing multiple electron acceptors and support an electron-hopping mechanism within Shewanella electrode-respiring biofilms. PMID:25495895

  8. Coexistence of bound and virtual-bound states in shallow-core to valence x-ray spectroscopies

    NASA Astrophysics Data System (ADS)

    Sen Gupta, Subhra; Bradley, J. A.; Haverkort, M. W.; Seidler, G. T.; Tanaka, A.; Sawatzky, G. A.

    2011-08-01

    With the example of the non-resonant inelastic x-ray scattering (NIXS) at the O45 edges (5d→5f) of the actinides, we develop the theory for shallow-core to valence excitations, where the multiplet spread is larger than the core-hole attraction, such as if the core and valence orbitals have the same principal quantum number. This involves very strong final state configuration interaction (CI), which manifests itself as huge reductions in the Slater-Condon integrals, needed to explain the spectral shapes within a simple renormalized atomic multiplet theory. But more importantly, this results in a cross-over from bound (excitonic) to virtual-bound excited states with increasing energy, within the same core-valance multiplet structure, and in large differences between the dipole and high-order multipole transitions, as observed in NIXS. While the bound states (often higher multipole allowed) can still be modeled using local cluster-like models, the virtual-bound resonances (often dipole-allowed) cannot be interpreted within such local approaches. This is in stark contrast to the more familiar core-valence transitions between different principal quantum number shells, where all the final excited states almost invariably form bound core-hole excitons and can be modeled using local approaches. The possibility of observing giant multipole resonances for systems with high angular momentum ground states is also predicted. The theory is important to obtain ground state information from core-level x-ray spectroscopies of strongly correlated transition metal, rare-earth, and actinide systems.

  9. Ubiquity of Exciton Localization in Cryogenic Carbon Nanotubes.

    PubMed

    Hofmann, Matthias S; Noé, Jonathan; Kneer, Alexander; Crochet, Jared J; Högele, Alexander

    2016-05-11

    We present photoluminescence studies of individual semiconducting single-wall carbon nanotubes at room and cryogenic temperatures. From the analysis of spatial and spectral features of nanotube photoluminescence, we identify characteristic signatures of unintentional exciton localization. Moreover, we quantify the energy scale of exciton localization potentials as ranging from a few to a few tens of millielectronvolts and stemming from both environmental disorder and shallow covalent side-wall defects. Our results establish disorder-induced crossover from the diffusive to the localized regime of nanotube excitons at cryogenic temperatures as a ubiquitous phenomenon in micelle-encapsulated and as-grown carbon nanotubes. PMID:27105355

  10. Josephson oscillations between exciton condensates in electrostatic traps

    SciTech Connect

    Rontani, Massimo

    2009-08-15

    Technological advances allow for tunable lateral confinement of cold dipolar excitons in coupled quantum wells. We consider theoretically the Josephson effect between exciton condensates in two traps separated by a weak link. The flow of the exciton supercurrent is driven by the dipole-energy difference between the traps. The Josephson oscillations may be observed after ensemble average of the time correlation of photons separately emitted from the two traps. The fringe visibility is controlled by the trap coupling and is robust against quantum and thermal fluctuations.

  11. Controlling the dark exciton spin eigenstates by external magnetic field

    NASA Astrophysics Data System (ADS)

    Gantz, L.; Schmidgall, E. R.; Schwartz, I.; Don, Y.; Waks, E.; Bahir, G.; Gershoni, D.

    2016-07-01

    We study the dark exciton's behavior as a coherent physical two-level spin system (qubit) using an external magnetic field in the Faraday configuration. Our studies are based on polarization-sensitive intensity autocorrelation measurements of the optical transition resulting from the recombination of a spin-blockaded biexciton state, which heralds the dark exciton and its spin state. We demonstrate control over the dark exciton eigenstates without degrading its decoherence time. Our observations agree well with computational predictions based on a master equation model.

  12. Excitation of exciton states on a curved surface

    NASA Astrophysics Data System (ADS)

    Silotia, Poonam; Prasad, Vinod

    2016-05-01

    Excitonic transitions on the surface of a sphere have been studied in he presence of external static electric and laser fields. The spectrum and the various coupling matrix elements, <ψl,m | cosn ϑ |ψl‧,m‧ > (for n = 1 , 2 , 3), between few states of exciton have been evaluated in the absence and presence of excitonic Coulombic interaction with different values of dielectric constant. Variation of various physical quantities: energy eigenvalues, transition probability, orientational and alignment parameter, has been shown to have strong dependence on the laser field and static electric field.

  13. Exciton coupling of surface complexes on a nanocrystal surface.

    PubMed

    Xu, Xiangxing; Ji, Jianwei; Wang, Guan; You, Xiaozeng

    2014-08-25

    Exciton coupling may arise when chromophores are brought into close spatial proximity. Herein the intra-nanocrystal exciton coupling of the surface complexes formed by coordination of 8-hydroxyquinoline to ZnS nanocrystals (NCs) is reported. It is studied by absorption, photoluminescence (PL), PL excitation (PLE), and PL lifetime measurements. The exciton coupling of the surface complexes tunes the PL color and broadens the absorption and PLE windows of the NCs, and thus is a potential strategy for improving the light-harvesting efficiency of NC solar cells and photocatalysts. PMID:24863364

  14. Topological polaritons and excitons in garden-variety systems

    NASA Astrophysics Data System (ADS)

    Bardyn, Charles-Edouard; Karzig, Torsten; Refael, Gil; Liew, Timothy C. H.

    2015-04-01

    We present a practical scheme for creating topological polaritons in garden-variety systems based, for example, on zinc-blende semiconductor quantum wells. Our proposal requires a moderate magnetic field and a potential landscape which can be implemented, e.g., via surface acoustic waves or patterning. We identify indirect excitons in double quantum wells as an appealing alternative for topological states in exciton-based systems. Topological polaritons and indirect excitons open a new frontier for topological states in solid-state systems, which can be directly probed and manipulated while offering a system with nonlinear interactions.

  15. Confocal shift interferometry of coherent emission from trapped dipolar excitons

    SciTech Connect

    Repp, J.; Schinner, G. J.; Schubert, E.; Rai, A. K.; Wieck, A. D.; Reuter, D.; Wurstbauer, U.; Holleitner, A. W.; and others

    2014-12-15

    We introduce a confocal shift-interferometer based on optical fibers. The presented spectroscopy allows measuring coherence maps of luminescent samples with a high spatial resolution even at cryogenic temperatures. We apply the spectroscopy onto electrostatically trapped, dipolar excitons in a semiconductor double quantum well. We find that the measured spatial coherence length of the excitonic emission coincides with the point spread function of the confocal setup. The results are consistent with a temporal coherence of the excitonic emission down to temperatures of 250 mK.

  16. Excitonic AND Logic Gates on DNA Brick Nanobreadboards

    PubMed Central

    2015-01-01

    A promising application of DNA self-assembly is the fabrication of chromophore-based excitonic devices. DNA brick assembly is a compelling method for creating programmable nanobreadboards on which chromophores may be rapidly and easily repositioned to prototype new excitonic devices, optimize device operation, and induce reversible switching. Using DNA nanobreadboards, we have demonstrated each of these functions through the construction and operation of two different excitonic AND logic gates. The modularity and high chromophore density achievable via this brick-based approach provide a viable path toward developing information processing and storage systems. PMID:25839049

  17. Photocurrent, photoluminescence and exciton dynamics in rubrene molecular single crystals

    NASA Astrophysics Data System (ADS)

    Lyu, ByungGook

    This work discusses the photocurrent and photoluminescence that can be induced by short-pulse illumination in rubrene single crystals. The pulsed illumination excites a rubrene molecule from the ground state to its first optically accessible excited state, resulting in a singlet exciton state. In rubrene, a singlet exciton can transform into two triplet excitons - which together have a spin of zero - by an efficient spin-conserved fission process. On the other hand, two triplet excitons can interact to again form a singlet exciton by a fusion process. Quantitative modeling of the transformation of singlet excitons into triplet excitons and vice-versa shows that both photoconductivity dynamics and photocurrent dynamics after pulsed excitations can be understood within the same framework. The photoluminescence observed after pulsed excitation is only emitted upon radiative recombination of singlet excitons. A simple model of fission and fusion based on rate equations leads to a qualitatively different photoluminescence dynamics depending on the time scale. In particular, it predicts a fast exponential decay corresponding to the initial fission process, later a power-law (quadratic) decay corresponding to a regime when triplet-triplet interaction is dominant, and a final exponential decay with a time-constant which is half the triplet exciton lifetime. This last exponential decay corresponds to the case when only a lower density of triplet excitons is left. The same model can be used to predict the photocurrent dynamics after pulsed excitation. Experimental observations after pulsed illumination show that, for low excitation pulse energies, a large photocurrent grows exponentially with a time constant of the order of 100 microseconds. This photocurrent build-up time then becomes shorter at higher excitation energies, with the peak photocurrent also saturating. One finds that the observed photocurrent dynamics can be reproduced with the same model based on exciton

  18. Excitons and the lifetime of organic semiconductor devices

    PubMed Central

    Forrest, Stephen R.

    2015-01-01

    While excitons are responsible for the many beneficial optical properties of organic semiconductors, their non-radiative recombination within the material can result in material degradation due to the dumping of energy onto localized molecular bonds. This presents a challenge in developing strategies to exploit the benefits of excitons without negatively impacting the device operational stability. Here, we will briefly review the fundamental mechanisms leading to excitonic energy-driven device ageing in two example devices: blue emitting electrophosphorescent organic light emitting devices (PHOLEDs) and organic photovoltaic (OPV) cells. We describe strategies used to minimize or even eliminate this fundamental device degradation pathway. PMID:25987572

  19. Ubiquity of Exciton Localization in Cryogenic Carbon Nanotubes

    PubMed Central

    2016-01-01

    We present photoluminescence studies of individual semiconducting single-wall carbon nanotubes at room and cryogenic temperatures. From the analysis of spatial and spectral features of nanotube photoluminescence, we identify characteristic signatures of unintentional exciton localization. Moreover, we quantify the energy scale of exciton localization potentials as ranging from a few to a few tens of millielectronvolts and stemming from both environmental disorder and shallow covalent side-wall defects. Our results establish disorder-induced crossover from the diffusive to the localized regime of nanotube excitons at cryogenic temperatures as a ubiquitous phenomenon in micelle-encapsulated and as-grown carbon nanotubes. PMID:27105355

  20. Role of phonons in Josephson oscillations of excitonic and polaritonic condensates

    SciTech Connect

    Magnusson, E. B.; Flayac, H.; Malpuech, G.; Shelykh, I. A.

    2010-11-15

    We analyze theoretically the role of the exciton-phonon interactions in phenomena related to the Josephson effect between two spatially separated exciton and exciton-polariton condensates. We consider the role of the dephasing introduced by phonons in such phenomena as Josephson tunneling, self-trapping and spontaneous polarization separation. In the regime of cw pumping we find a remarkable bistability effect arising from exciton-exciton interactions as well as regimes of self-sustained regular and chaotic oscillations.

  1. Luminescence dynamics of bound exciton of hydrogen doped ZnO nanowires

    DOE PAGESBeta

    Yoo, Jinkyoung; Yi, Gyu -Chul; Chon, Bonghwan; Joo, Taiha; Wang, Zhehui

    2016-04-11

    In this study, all-optical camera, converting X-rays into visible photons, is a promising strategy for high-performance X-ray imaging detector requiring high detection efficiency and ultrafast detector response time. Zinc oxide is a suitable material for all-optical camera due to its fast radiative recombination lifetime in sub-nanosecond regime and its radiation hardness. ZnO nanostructures have been considered as proper building blocks for ultrafast detectors with spatial resolution in sub-micrometer scale. To achieve remarkable enhancement of luminescence efficiency n-type doping in ZnO has been employed. However, luminescence dynamics of doped ZnO nanostructures have not been thoroughly investigated whereas undoped ZnO nanostructures havemore » been employed to study their luminescence dynamics. Here we report a study of luminescence dynamics of hydrogen doped ZnO nanowires obtained by hydrogen plasma treatment. Hydrogen doping in ZnO nanowires gives rise to significant increase in the near-band-edge emission of ZnO and decrease in averaged photoluminescence lifetime from 300 to 140 ps at 10 K. The effects of hydrogen doping on the luminescent characteristics of ZnO nanowires were changed by hydrogen doping process variables.« less

  2. Excitons in a surface quantum well

    NASA Astrophysics Data System (ADS)

    Arulmozhi, M.; Anitha, A.

    2014-11-01

    Binding energies of excitons in a Surface Quantum Well (SQW) composed of vacuum/GaAs/AlxGa1-xAs as a function of wellwidth are calculated. The effect of non-parabolicity is considered by using an energy dependent effective mass. The effect of mass anisotropy and the effect of image charges which arise due to the large dielectric discontinuity at the vacuum/GaAs interface are also considered. The average distances of the electron and the hole from the vacuum/GaAs interface, with and without image charges and the integrated probability of finding an electron and a hole inside the well are also calculated. The results agree well with the available experimental data.

  3. Optical properties of Rydberg excitons and polaritons

    NASA Astrophysics Data System (ADS)

    Zielińska-Raczyńska, Sylwia; Czajkowski, Gerard; Ziemkiewicz, David

    2016-02-01

    We show how to compute the optical functions when Rydberg excitons appear, including the effect of the coherence between the electron-hole pair and the electromagnetic field. We use the real density matrix approach (RDMA), which, combined with the Green's function method, enables one to derive analytical expressions for the optical functions. Choosing the susceptibility, we performed numerical calculations appropriate to a Cu20 crystal, being a semiconductor with an indirect gap. The effect of the coherence is displayed in the line shape. We also examine in detail and explain the dependence of the oscillator strength and the resonance placement on the state number. We report good agreement with recently published experimental data. We also show that the presented method can be applied to semiconductors with a direct gap.

  4. Bounding Species Distribution Models

    NASA Technical Reports Server (NTRS)

    Stohlgren, Thomas J.; Jarnevich, Cahterine S.; Morisette, Jeffrey T.; Esaias, Wayne E.

    2011-01-01

    Species distribution models are increasing in popularity for mapping suitable habitat for species of management concern. Many investigators now recognize that extrapolations of these models with geographic information systems (GIS) might be sensitive to the environmental bounds of the data used in their development, yet there is no recommended best practice for "clamping" model extrapolations. We relied on two commonly used modeling approaches: classification and regression tree (CART) and maximum entropy (Maxent) models, and we tested a simple alteration of the model extrapolations, bounding extrapolations to the maximum and minimum values of primary environmental predictors, to provide a more realistic map of suitable habitat of hybridized Africanized honey bees in the southwestern United States. Findings suggest that multiple models of bounding, and the most conservative bounding of species distribution models, like those presented here, should probably replace the unbounded or loosely bounded techniques currently used [Current Zoology 57 (5): 642-647, 2011].

  5. Bounding species distribution models

    USGS Publications Warehouse

    Stohlgren, T.J.; Jarnevich, C.S.; Esaias, W.E.; Morisette, J.T.

    2011-01-01

    Species distribution models are increasing in popularity for mapping suitable habitat for species of management concern. Many investigators now recognize that extrapolations of these models with geographic information systems (GIS) might be sensitive to the environmental bounds of the data used in their development, yet there is no recommended best practice for "clamping" model extrapolations. We relied on two commonly used modeling approaches: classification and regression tree (CART) and maximum entropy (Maxent) models, and we tested a simple alteration of the model extrapolations, bounding extrapolations to the maximum and minimum values of primary environmental predictors, to provide a more realistic map of suitable habitat of hybridized Africanized honey bees in the southwestern United States. Findings suggest that multiple models of bounding, and the most conservative bounding of species distribution models, like those presented here, should probably replace the unbounded or loosely bounded techniques currently used. ?? 2011 Current Zoology.

  6. Causality and Tsirelson's bounds

    SciTech Connect

    Buhrman, H.; Massar, S.

    2005-11-15

    We study the properties of no-signaling correlations that cannot be reproduced by local measurements on entangled quantum states. We say that such correlations violate Tsirelson bounds. We show that if these correlations are obtained by some reversible unitary quantum evolution U, then U cannot be written in the product form U{sub A}xU{sub B}. This implies that U can be used for signaling and for entanglement generation. This result is completely general and in fact can be viewed as a characterization of Tsirelson bounds. We then show how this result can be used as a tool to study Tsirelson bounds and we illustrate this by rederiving the Tsirelson bound of 2{radical}(2) for the Clauser-Horn-Shimony-Holt inequality, and by deriving a new Tsirelson bound for qutrits.

  7. All-Polymer Solar Cell Performance Optimized via Systematic Molecular Weight Tuning of Both Donor and Acceptor Polymers.

    PubMed

    Zhou, Nanjia; Dudnik, Alexander S; Li, Ting I N G; Manley, Eric F; Aldrich, Thomas J; Guo, Peijun; Liao, Hsueh-Chung; Chen, Zhihua; Chen, Lin X; Chang, Robert P H; Facchetti, Antonio; Olvera de la Cruz, Monica; Marks, Tobin J

    2016-02-01

    The influence of the number-average molecular weight (Mn) on the blend film morphology and photovoltaic performance of all-polymer solar cells (APSCs) fabricated with the donor polymer poly[5-(2-hexyldodecyl)-1,3-thieno[3,4-c]pyrrole-4,6-dione-alt-5,5-(2,5-bis(3-dodecylthiophen-2-yl)thiophene)] (PTPD3T) and acceptor polymer poly{[N,N'-bis(2-octyldodecyl)naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,5'-(2,2'-bithiophene)} (P(NDI2OD-T2); N2200) is systematically investigated. The Mn effect analysis of both PTPD3T and N2200 is enabled by implementing a polymerization strategy which produces conjugated polymers with tunable Mns. Experimental and coarse-grain modeling results reveal that systematic Mn variation greatly influences both intrachain and interchain interactions and ultimately the degree of phase separation and morphology evolution. Specifically, increasing Mn for both polymers shrinks blend film domain sizes and enhances donor-acceptor polymer-polymer interfacial areas, affording increased short-circuit current densities (Jsc). However, the greater disorder and intermixed feature proliferation accompanying increasing Mn promotes charge carrier recombination, reducing cell fill factors (FF). The optimized photoactive layers exhibit well-balanced exciton dissociation and charge transport characteristics, ultimately providing solar cells with a 2-fold PCE enhancement versus devices with nonoptimal Mns. Overall, it is shown that proper and precise tuning of both donor and acceptor polymer Mns is critical for optimizing APSC performance. In contrast to reports where maximum power conversion efficiencies (PCEs) are achieved for the highest Mns, the present two-dimensional Mn optimization matrix strategy locates a PCE "sweet spot" at intermediate Mns of both donor and acceptor polymers. This study provides synthetic methodologies to predictably access conjugated polymers with desired Mn and highlights the importance of optimizing Mn for both polymer

  8. Hole-Burning Spectroscopy on Excitonically Coupled Pigments in Proteins: Theory Meets Experiment.

    PubMed

    Adolphs, Julian; Berrer, Manuel; Renger, Thomas

    2016-03-01

    A theory for the calculation of resonant and nonresonant hole-burning (HB) spectra of pigment-protein complexes is presented and applied to the water-soluble chlorophyll-binding protein (WSCP) from cauliflower. The theory is based on a non-Markovian line shape theory ( Renger and Marcus J. Chem. Phys. 2002 , 116 , 9997 ) and includes exciton delocalization, vibrational sidebands, and lifetime broadening. An earlier approach by Reppert ( J. Phys. Chem. Lett. 2011 , 2 , 2716 ) is found to describe nonresonant HB spectra only. Here we present a theory that can be used for a quantitative description of HB data for both nonresonant and resonant burning conditions. We find that it is important to take into account the excess energy of the excitation in the HB process. Whereas excitation of the zero-phonon transition of the lowest exciton state, that is, resonant burning allows the protein to access only its conformational substates in the neighborhood of the preburn state, any higher excitation gives the protein full access to all conformations present in the original inhomogeneous ensemble. Application of the theory to recombinant WSCP from cauliflower, reconstituted with chlorophyll a or chlorophyll b, gives excellent agreement with experimental data by Pieper et al. ( J. Phys. Chem. B 2011 , 115 , 4053 ) and allows us to obtain an upper bound of the lifetime of the upper exciton state directly from the HB experiments in agreement with lifetimes measured recently in time domain 2D experiments by Alster et al. ( J. Phys. Chem. B 2014 , 118 , 3524 ). PMID:26811003

  9. Hole-Burning Spectroscopy on Excitonically Coupled Pigments in Proteins: Theory Meets Experiment

    PubMed Central

    2016-01-01

    A theory for the calculation of resonant and nonresonant hole-burning (HB) spectra of pigment–protein complexes is presented and applied to the water-soluble chlorophyll-binding protein (WSCP) from cauliflower. The theory is based on a non-Markovian line shape theory (Renger and MarcusJ. Chem. Phys.2002, 116, 9997) and includes exciton delocalization, vibrational sidebands, and lifetime broadening. An earlier approach by Reppert (J. Phys. Chem. Lett.2011, 2, 2716) is found to describe nonresonant HB spectra only. Here we present a theory that can be used for a quantitative description of HB data for both nonresonant and resonant burning conditions. We find that it is important to take into account the excess energy of the excitation in the HB process. Whereas excitation of the zero-phonon transition of the lowest exciton state, that is, resonant burning allows the protein to access only its conformational substates in the neighborhood of the preburn state, any higher excitation gives the protein full access to all conformations present in the original inhomogeneous ensemble. Application of the theory to recombinant WSCP from cauliflower, reconstituted with chlorophyll a or chlorophyll b, gives excellent agreement with experimental data by Pieper et al. (J. Phys. Chem. B2011, 115, 405321417356) and allows us to obtain an upper bound of the lifetime of the upper exciton state directly from the HB experiments in agreement with lifetimes measured recently in time domain 2D experiments by Alster et al. (J. Phys. Chem. B2014, 118, 352424627983). PMID:26811003

  10. First-principles simulation of light propagation and exciton dynamics in metal cluster nanostructures

    NASA Astrophysics Data System (ADS)

    Lisinetskaya, Polina G.; Röhr, Merle I. S.; Mitrić, Roland

    2016-06-01

    We present a theoretical approach for the simulation of the electric field and exciton propagation in ordered arrays constructed of molecular-sized noble metal clusters bound to organic polymer templates. In order to describe the electronic coupling between individual constituents of the nanostructure we use the ab initio parameterized transition charge method which is more accurate than the usual dipole-dipole coupling. The electronic population dynamics in the nanostructure under an external laser pulse excitation is simulated by numerical integration of the time-dependent Schrödinger equation employing the fully coupled Hamiltonian. The solution of the TDSE gives rise to time-dependent partial point charges for each subunit of the nanostructure, and the spatio-temporal electric field distribution is evaluated by means of classical electrodynamics methods. The time-dependent partial charges are determined based on the stationary partial and transition charges obtained in the framework of the TDDFT. In order to treat large plasmonic nanostructures constructed of many constituents, the approximate self-consistent iterative approach presented in (Lisinetskaya and Mitrić in Phys Rev B 89:035433, 2014) is modified to include the transition-charge-based interaction. The developed methods are used to study the optical response and exciton dynamics of {Ag}3+ and porphyrin-Ag4 dimers. Subsequently, the spatio-temporal electric field distribution in a ring constructed of ten porphyrin-Ag4 subunits under the action of circularly polarized laser pulse is simulated. The presented methodology provides a theoretical basis for the investigation of coupled light-exciton propagation in nanoarchitectures built from molecular size metal nanoclusters in which quantum confinement effects are important.

  11. First-principles simulation of light propagation and exciton dynamics in metal cluster nanostructures

    NASA Astrophysics Data System (ADS)

    Lisinetskaya, Polina G.; Röhr, Merle I. S.; Mitrić, Roland

    2016-06-01

    We present a theoretical approach for the simulation of the electric field and exciton propagation in ordered arrays constructed of molecular-sized noble metal clusters bound to organic polymer templates. In order to describe the electronic coupling between individual constituents of the nanostructure we use the ab initio parameterized transition charge method which is more accurate than the usual dipole-dipole coupling. The electronic population dynamics in the nanostructure under an external laser pulse excitation is simulated by numerical integration of the time-dependent Schrödinger equation employing the fully coupled Hamiltonian. The solution of the TDSE gives rise to time-dependent partial point charges for each subunit of the nanostructure, and the spatio-temporal electric field distribution is evaluated by means of classical electrodynamics methods. The time-dependent partial charges are determined based on the stationary partial and transition charges obtained in the framework of the TDDFT. In order to treat large plasmonic nanostructures constructed of many constituents, the approximate self-consistent iterative approach presented in (Lisinetskaya and Mitrić in Phys Rev B 89:035433, 2014) is modified to include the transition-charge-based interaction. The developed methods are used to study the optical response and exciton dynamics of Ag3+ and porphyrin-Ag4 dimers. Subsequently, the spatio-temporal electric field distribution in a ring constructed of ten porphyrin-Ag4 subunits under the action of circularly polarized laser pulse is simulated. The presented methodology provides a theoretical basis for the investigation of coupled light-exciton propagation in nanoarchitectures built from molecular size metal nanoclusters in which quantum confinement effects are important.

  12. An overview of molecular acceptors for organic solar cells

    NASA Astrophysics Data System (ADS)

    Hudhomme, Piétrick

    2013-07-01

    Organic solar cells (OSCs) have gained serious attention during the last decade and are now considered as one of the future photovoltaic technologies for low-cost power production. The first dream of attaining 10% of power coefficient efficiency has now become a reality thanks to the development of new materials and an impressive work achieved to understand, control and optimize structure and morphology of the device. But most of the effort devoted to the development of new materials concerned the optimization of the donor material, with less attention for acceptors which to date remain dominated by fullerenes and their derivatives. This short review presents the progress in the use of non-fullerene small molecules and fullerene-based acceptors with the aim of evaluating the challenge for the next generation of acceptors in organic photovoltaics.

  13. Identification of C3 acceptors responsible for complement activation in Crithidia fasciculata

    SciTech Connect

    Guether, M.L.T.; Travassos, L.R.; Schenkman, S.

    1988-11-01

    Crithidia fasciculata, an insect trypanosomatid is readily lysed by normal human serum at concentrations as low as 3%. Lysis occurs in the presence of Mg+2-EGTA and is antibody independent, indicating that the alternative pathway of complement activation is involved. Analysis of (131I)C3 deposition on C. fasciculata cells using C8-deficient serum, revealed that about 4 x 10(5) C3 molecules bound to each cell. Most of the C3 was bound to cells as C3b, part of it forming high molecular weight complexes, which could be dissociated by methylamine treatment at alkaline pH. To characterize the C3 acceptors on C. fasciculata, surface-iodinated cells were incubated with C8D or heat-inactivated serum, extracted and immunoprecipitated with anti-C3 or anti-arabinogalactan antisera. Analysis of the immunoprecipitated material on SDS gels showed high-molecular weight components, which disappeared after methylamine treatment, giving rise to a component of 200 kDa molecular size. This 200-kDa component corresponded to a purified arabinogalactan complex, which was immunoprecipitated from labeled cell extracts, without incubation with C8D, using anti-arabinogalactan antibodies. These results suggest that the arabinogalactan glycoconjugate is a C3 acceptor in C. fasciculata during complement activation. Purified arabinogalactan complexes were able to inactivate C3 in vitro. Solubilization in KOH to cleave the peptide moiety rendered it unable to inactivate C3. Apparently, the aggregated state of the purified arabinogalactan component at the cell surface is important for C3 deposition and activation.

  14. Exciton-exciton interactions in CdWO{sub 4} irradiated by intense femtosecond vacuum ultraviolet pulses

    SciTech Connect

    Kirm, M.; Nagirnyi, V.; Feldbach, E.; De Grazia, M.; Carre, B.; Merdji, H.; Guizard, S.; Geoffroy, G.; Gaudin, J.; Fedorov, N.; Martin, P.; Vasil'ev, A.; Belsky, A.

    2009-06-15

    Exciton-exciton interaction is experimentally revealed and quantitatively analyzed in a wide band-gap scintillator material CdWO{sub 4}. Under high-intensity femtosecond vacuum ultraviolet excitation, the CdWO{sub 4} luminescence is quenched, while its decay becomes essentially nonexponential. We propose an analytical model, which successfully reproduces the decay kinetics recorded in a wide range of excitation densities. The dipole-dipole interaction between excitons leading to their nonradiative decay is shown to be the main cause of a nonproportional response common for many scintillators.

  15. Optical studies of magnons, excitons and polarons in CuO{sub 2}-layer compounds

    SciTech Connect

    Kastner, M.A.; Birgeneau, R.J.

    1996-12-31

    The optical properties of undoped and lightly doped lamellar copper oxides are reviewed. In the undoped materials the absorption below the charge-transfer gap is dominated by magnetic and crystal field excitations of the CuO{sub 2} layers. The temperature dependence of the charge-transfer absorption provides evidence that free charged excitations form large polarons. However, the optical ionization energy of holes bound to acceptors is much larger than the thermal ionization energy, indicating that the bound polarons are small. The parameters extracted from optical measurements predict the Hall mobility of holes in lightly doped La{sub 2}CuO{sub 4}, with no adjustable parameters, confirming that the carriers are polarons.

  16. Donor-acceptor chemistry in the main group.

    PubMed

    Rivard, Eric

    2014-06-21

    This Perspective article summarizes recent progress from our laboratory in the isolation of reactive main group species using a general donor-acceptor protocol. A highlight of this program is the use of carbon-based donors in combination with suitable Lewis acidic acceptors to yield stable complexes of parent Group 14 element hydrides (e.g. GeH2 and H2SiGeH2). It is anticipated that this strategy could be extended to include new synthetic targets from throughout the Periodic Table with possible applications in bottom-up materials synthesis and main group element catalysis envisioned. PMID:24788390

  17. Acceptors in bulk and nanoscale ZnO

    NASA Astrophysics Data System (ADS)

    McCluskey, M. D.

    2012-02-01

    Zinc oxide (ZnO) is a semiconductor that emits bright UV light, with little wasted heat. This intrinsic feature makes it a promising material for energy-efficient white lighting, nano-lasers, and other optical applications. For devices to be competitive, however, it is necessary to develop reliable p-type doping. Although substitutional nitrogen has been considered as a potential p-type dopant for ZnO, theoretical and experimental work indicates that nitrogen is a deep acceptor and will not lead to p-type conductivity. This talk will highlight recent experiments on ZnO:N at low temperatures. A red/near-IR photoluminescence (PL) band is correlated with the presence of deep nitrogen acceptors. PL excitation (PLE) measurements show an absorption threshold of 2.26 eV, in good agreement with theory. Magnetic resonance experiments provide further evidence for this assignment. The results of these studies seem to rule out group-V elements as shallow acceptors in ZnO, contradicting numerous reports in the literature. If these acceptors do not work as advertised, is there a viable alternative? Optical studies on ZnO nanocrystals show some intriguing leads. At liquid-helium temperatures, a series of sharp IR absorption peaks arise from an unknown acceptor impurity. The data are consistent with a hydrogenic acceptor 0.46 eV above the valence band edge. While this binding energy is still too deep for many practical applications, it represents a significant improvement over the ˜ 1.3 eV binding energy for nitrogen acceptors. Nanocrystals present another twist. Due to their high surface-to-volume ratio, surface states are especially important. Specifically, electron-hole recombination at the surface give rises to a red luminescence band. From our PL and IR experiments, we have developed a ``unified'' model that attempts to explain acceptor and surface states in ZnO nanocrystals. This model could provide a useful framework for designing future nanoscale ZnO devices.

  18. Sodium acceptor doping of ZnO crystals

    NASA Astrophysics Data System (ADS)

    Parmar, Narendra S.; Joni, I. Made; Lynn, Kelvin G.

    2016-02-01

    ZnO bulk single crystals were doped with sodium by thermal diffusion using sodium dispensers. Secondary-ion mass spectrometry measurement shows the diffusion of sodium with concentration ˜1×1018 cm-3 in near surface region. Photoluminescence (PL) measurements show donor acceptor pair (DAP) emission at 408 nm at room temperature which exhibits a blue-shift to 404 nm at 9 K. DC Hall measurements show the mixed conduction due to low Hall voltage in these samples. PL measurements and variable temperature resistivity measurements suggest that the sodium acceptor activation energy is ˜0.300 eV.

  19. Exciton generation/dissociation/charge-transfer enhancement in inorganic/organic hybrid solar cells by robust single nanocrystalline LnPxOy (Ln = Eu, Y) doping.

    PubMed

    Jin, Xiao; Sun, Weifu; Chen, Zihan; Wei, Taihuei; Chen, Chuyang; He, Xingdao; Yuan, Yongbiao; Li, Yue; Li, Qinghua

    2014-06-11

    Low-temperature solution-processed photovoltaics suffer from low efficiencies because of poor exciton or electron-hole transfer. Inorganic/organic hybrid solar cell, although still in its infancy, has attracted great interest thus far. One of the promising ways to enhance exciton dissociation or electron-hole transport is the doping of lanthanide phosphate ions. However, the underlying photophysical mechanism remains poorly understood. Herein, by applying femtosecond transient absorption spectroscopy, we successfully distinguished hot electron, less energetic electron, hole transport from electron-hole recombination. Concrete evidence has been provided that lanthanide phosphate doping improves the efficiency of both hot electron and "less energetic" electron transfers from donor to acceptor, but the hole transport almost remains unchanged. In particular, the hot electron transfer lifetime was shortened from 30.2 to 12.7 ps, that is, more than 60% faster than pure TiO2 acceptor. Such improvement was ascribed to the facts that the conduction band (CB) edge energy level of TiO2 has been elevated by 0.2 eV, while the valence band level almost remains unchanged, thus not only narrowing the energy offset between CB levels of TiO2 and P3HT, but also meanwhile enlarging the band gap of TiO2 itself that permits one to inhibit electron-hole recombination within TiO2. Consequently, lanthanide phosphate doped TiO2/P3HT bulk-heterojunction solar cell has been demonstrated to be a promising hybrid solar cell, and a notable power conversion efficiency of 2.91% is therefore attained. This work indicates that lanthanide compound ions can efficiently facilitate exciton generation, dissociation, and charge transport, thus enhancing photovoltaic performance. PMID:24835845

  20. Excitonic condensation in spatially separated one-dimensional systems

    SciTech Connect

    Abergel, D. S. L.

    2015-05-25

    We show theoretically that excitons can form from spatially separated one-dimensional ground state populations of electrons and holes, and that the resulting excitons can form a quasicondensate. We describe a mean-field Bardeen-Cooper-Schrieffer theory in the low carrier density regime and then focus on the core-shell nanowire giving estimates of the size of the excitonic gap for InAs/GaSb wires and as a function of all the experimentally relevant parameters. We find that optimal conditions for pairing include small overlap of the electron and hole bands, large effective mass of the carriers, and low dielectric constant of the surrounding media. Therefore, one-dimensional systems provide an attractive platform for the experimental detection of excitonic quasicondensation in zero magnetic field.

  1. Neutral and positively charged excitons in narrow quantum ring

    SciTech Connect

    Porras Monroy, L. C.; Rodríguez-Prada, F. A.; Mikhailov, I. D.

    2014-05-15

    We study theoretically quantized states of a neutral and a positively charged exciton (trion X{sup +}) confined in a heterostructure with the ring-like geometry. In order to assess the experimentally relevant domain of parameters, we adopt a simple model of a narrow ring when 3D wave equations for the neutral and positively charged excitons can be separated. By using the Fourier series method, we have calculated the energy spectra of excitons complexes in a quantum ring as a function of the electron-to-hole mass ratio, the ring radius, and the magnetic field strength. The quantum-size effect and the size-dependent magnetic oscillations of energy levels of excitons' complexes spectra have been revealed.

  2. Superconductivity in CuCl/Si superlattices: excitonic pairing?

    NASA Astrophysics Data System (ADS)

    Rhim, S. H.; Saniz, Rolando; Weinert, Michael; Freeman, A. J.

    2013-03-01

    Two-dimensional (2D) hetero-bonded semiconductor interfaces have been suggested as candidate geometries where excitonic superconductivity [2] - and the greatly enhanced where TC compared to phonon mechanisms mediation - can be realized. Among experimental efforts, epitaxially grown CuCl on Si (111) has reportedly exhibited excitonic superconductivity at 60 ~150 K. Our first-principles calculations confirm 2D metallicity at the interfaces due to charge transfer by valence mismatch. [3] The excitonic mechanism is investigated by calculating the kernel function, K (ω) , for the average of the electronic contributions to the effective interaction.[4] The attractive interaction found in the CuCl/Si superlattice indicates the feasibility of excitonic pairing for a certain frequency range. US Department of Energy (DE-FG02-05ER45372)

  3. Strong Quantum Coherence between Fermi Liquid Mahan Excitons

    NASA Astrophysics Data System (ADS)

    Paul, J.; Stevens, C. E.; Liu, C.; Dey, P.; McIntyre, C.; Turkowski, V.; Reno, J. L.; Hilton, D. J.; Karaiskaj, D.

    2016-04-01

    In modulation doped quantum wells, the excitons are formed as a result of the interactions of the charged holes with the electrons at the Fermi edge in the conduction band, leading to the so-called "Mahan excitons." The binding energy of Mahan excitons is expected to be greatly reduced and any quantum coherence destroyed as a result of the screening and electron-electron interactions. Surprisingly, we observe strong quantum coherence between the heavy hole and light hole excitons. Such correlations are revealed by the dominating cross-diagonal peaks in both one-quantum and two-quantum two-dimensional Fourier transform spectra. Theoretical simulations based on the optical Bloch equations where many-body effects are included phenomenologically reproduce well the experimental spectra. Time-dependent density functional theory calculations provide insight into the underlying physics and attribute the observed strong quantum coherence to a significantly reduced screening length and collective excitations of the many-electron system.

  4. Neutral and positively charged excitons in narrow quantum ring

    NASA Astrophysics Data System (ADS)

    Porras Monroy, L. C.; Rodríguez-Prada, F. A.; Mikhailov, I. D.

    2014-05-01

    We study theoretically quantized states of a neutral and a positively charged exciton (trion X+) confined in a heterostructure with the ring-like geometry. In order to assess the experimentally relevant domain of parameters, we adopt a simple model of a narrow ring when 3D wave equations for the neutral and positively charged excitons can be separated. By using the Fourier series method, we have calculated the energy spectra of excitons complexes in a quantum ring as a function of the electron-to-hole mass ratio, the ring radius, and the magnetic field strength. The quantum-size effect and the size-dependent magnetic oscillations of energy levels of excitons' complexes spectra have been revealed.

  5. Exciton-polariton localized wave packets in a microcavity

    NASA Astrophysics Data System (ADS)

    Voronych, Oksana; Buraczewski, Adam; Matuszewski, MichałÂ; Stobińska, Magdalena

    2016-06-01

    We investigate the possibility of creating X waves, or localized wave packets, in resonantly excited exciton-polariton superfluids. We demonstrate the existence of X-wave traveling solutions in the coupled exciton-photon system past the inflection point, where the effective mass of lower polaritons is negative in the direction perpendicular to the wave vector of the pumping beam. Contrary to the case of bright solitons, X waves do not require nonlinearity for sustaining their shape. Nevertheless, we show that nonlinearity is important for their dynamics, as it allows for their spontaneous formation from an initial Gaussian wave packet. Unique properties of exciton-polaritons may lead to applications of their X waves in long-distance signal propagation inside novel integrated optoelectronic circuits based on excitons.

  6. Topological Polaritons and Excitons in Garden Variety Systems

    NASA Astrophysics Data System (ADS)

    Bardyn, Charles-Edouard; Karzig, Torsten; Refael, Gil; Liew, Tim

    2015-03-01

    Topological polaritons (aka topolaritons) present a new frontier for topological behavior in solid-state systems. They combine light and matter, which allows to probe and manipulate them in a variety of ways. They can also be made strongly interacting, due to their excitonic component. Here we present a scheme which allows to realize topolaritons in garden variety zinc-blende quantum wells. Our proposal requires a moderate magnetic field and a potential landscape which can be implemented, e.g., via surface acoustic waves or patterning. We identify indirect excitons in double quantum wells as a particularly appealing alternative for topological states in exciton-based systems. Indirect excitons are robust and long lived (with lifetimes up to milliseconds), and, therefore, provide a flexible platform for the realization, probing, and utilization of topological coupled light-matter states. Funded by: Institute for Quantum Information and Matter, Swiss National Science Foundation, Packard Foundation, NSF.

  7. Hopping approach towards exciton dissociation in conjugated polymers

    SciTech Connect

    Emelianova, E. V.; Auweraer, M. van der; Baessler, H.

    2008-06-14

    By employing random walk an analytic theory for the dissociation of singlet excitons in a random organic solid, for instance, a conjugated polymer, has been developed. At variance of conventional three-dimensional Onsager theory, it is assumed that an exciton with finite lifetime can first transfer endothermically an electron to an adjacent site, thereby generating a charge transfer state whose energy is above the energy of that of the initial exciton. In a second step the latter can fully dissociate in accordance with Onsager's concept Brownian motion. The results indicate that, depending of the energy required for the first jump, the first jump contributes significantly to the field dependence of the dissociation yield. Disorder weakens the temperature dependence of the yield dramatically and precludes extracting information on the exciton binding energy from it.

  8. Singlet exciton fission-sensitized infrared quantum dot solar cells.

    PubMed

    Ehrler, Bruno; Wilson, Mark W B; Rao, Akshay; Friend, Richard H; Greenham, Neil C

    2012-02-01

    We demonstrate an organic/inorganic hybrid photovoltaic device architecture that uses singlet exciton fission to permit the collection of two electrons per absorbed high-energy photon while simultaneously harvesting low-energy photons. In this solar cell, infrared photons are absorbed using lead sulfide (PbS) nanocrystals. Visible photons are absorbed in pentacene to create singlet excitons, which undergo rapid exciton fission to produce pairs of triplets. Crucially, we identify that these triplet excitons can be ionized at an organic/inorganic heterointerface. We report internal quantum efficiencies exceeding 50% and power conversion efficiencies approaching 1%. These findings suggest an alternative route to circumvent the Shockley-Queisser limit on the power conversion efficiency of single-junction solar cells. PMID:22257168

  9. Organic-Inorganic Composites of Semiconductor Nanocrystals for Efficient Excitonics.

    PubMed

    Guzelturk, Burak; Demir, Hilmi Volkan

    2015-06-18

    Nanocomposites of colloidal semiconductor nanocrystals integrated into conjugated polymers are the key to soft-material hybrid optoelectronics, combining advantages of both plastics and particles. Synergic combination of the favorable properties in the hybrids of colloidal nanocrystals and conjugated polymers offers enhanced performance and new functionalities in light-generation and light-harvesting applications, where controlling and mastering the excitonic interactions at the nanoscale are essential. In this Perspective, we highlight and critically consider the excitonic interactions in the organic-inorganic nanocomposites to achieve highly efficient exciton transfer through rational design of the nanocomposites. The use of strong excitonic interactions in optoelectronic devices can trigger efficiency breakthroughs in hybrid optoelectronics. PMID:26266593

  10. Excitons in ultrathin organic-inorganic perovskite crystals

    NASA Astrophysics Data System (ADS)

    Yaffe, Omer; Chernikov, Alexey; Norman, Zachariah M.; Zhong, Yu; Velauthapillai, Ajanthkrishna; van der Zande, Arend; Owen, Jonathan S.; Heinz, Tony F.

    2015-07-01

    We demonstrate the formation of large sheets of layered organic-inorganic perovskite (OIPC) crystals, as thin as a single unit cell, prepared by mechanical exfoliation. The resulting two-dimensional OIPC nanosheets of 2.4 nm thickness are direct semiconductors with an optical band gap of 2.4 eV. They exhibit unusually strong light-matter interaction with an optical absorption as high as 25% at the main excitonic resonance, as well as bright photoluminescence. We extract an exciton binding energy of 490 meV from measurement of the series of excited exciton states. The properties of the excitons are shown to be strongly influenced by the changes in the dielectric surroundings. The environmental sensitivity of these ultrathin OIPC sheets is further reflected in the strong suppression of a thermally driven phase transition present in the bulk crystals.

  11. Anomalous magnetization of a carbon nanotube as an excitonic insulator

    NASA Astrophysics Data System (ADS)

    Rontani, Massimo

    2014-11-01

    We show theoretically that an undoped carbon nanotube might be an excitonic insulator—the long-sought phase of matter proposed by Keldysh, Kohn, and others fifty years ago. We predict that the condensation of triplet excitons, driven by intervalley exchange interaction, spontaneously occurs at equilibrium if the tube radius is sufficiently small. The signatures of exciton condensation are its sizable contributions to both the energy gap and the magnetic moment per electron. The increase of the gap might have already been measured, albeit with a different explanation [V. V. Deshpande, B. Chandra, R. Caldwell, D. S. Novikov, J. Hone, and M. Bockrath, Science 323, 106 (2009), 10.1126/science.1165799]. The enhancement of the quasiparticle magnetic moment is a pair-breaking effect that counteracts the weak paramagnetism of the ground-state condensate of excitons. This property could rationalize the anomalous magnitude of magnetic moments recently observed in different devices close to charge neutrality.

  12. Simulations of singlet exciton diffusion in organic semiconductors: a review

    DOE PAGESBeta

    Bjorgaard, Josiah A.; Kose, Muhammet Erkan

    2014-12-22

    Our review describes the various aspects of simulation strategies for exciton diffusion in condensed phase thin films of organic semiconductors. Several methods for calculating energy transfer rate constants are discussed along with procedures for how to account for energetic disorder. Exciton diffusion can be modelled by using kinetic Monte-Carlo methods or master equations. Recent literature on simulation efforts for estimating exciton diffusion lengths of various conjugated polymers and small molecules are introduced. Moreover, these studies are discussed in the context of the effects of morphology on exciton diffusion and the necessity of accurate treatment of disorder for comparison of simulationmore » results with those of experiment.« less

  13. Excitonic condensation in systems of strongly correlated electrons.

    PubMed

    Kuneš, Jan

    2015-08-26

    The idea of exciton condensation in solids was introduced in the 1960s with the analogy of superconductivity in mind. While exciton supercurrents have been realised only in artificial quantum-well structures so far, the application of the concept of excitonic condensation to bulk solids leads to a rich spectrum of thermodynamic phases with diverse physical properties. In this review we discuss recent developments in the theory of exciton condensation in systems described by Hubbard-type models. In particular, we focus on the connections to their various strong-coupling limits that have been studied in other contexts, e.g. cold atoms physics. One of our goals is to provide a 'dictionary' that would allow the reader to efficiently combine results obtained in these different fields. PMID:26218828

  14. Excitonic condensation in systems of strongly correlated electrons

    NASA Astrophysics Data System (ADS)

    Kuneš, Jan

    2015-08-01

    The idea of exciton condensation in solids was introduced in the 1960s with the analogy of superconductivity in mind. While exciton supercurrents have been realised only in artificial quantum-well structures so far, the application of the concept of excitonic condensation to bulk solids leads to a rich spectrum of thermodynamic phases with diverse physical properties. In this review we discuss recent developments in the theory of exciton condensation in systems described by Hubbard-type models. In particular, we focus on the connections to their various strong-coupling limits that have been studied in other contexts, e.g. cold atoms physics. One of our goals is to provide a ‘dictionary’ that would allow the reader to efficiently combine results obtained in these different fields.

  15. Electrical Activation of Dark Excitonic States in Carbon Nanotubes

    NASA Astrophysics Data System (ADS)

    Uda, Takushi; Yoshida, Masahiro; Ishii, Akihiro; Kato, Yuichiro K.

    Electrical activation of optical transitions to parity-forbidden dark excitonic states in individual carbon nanotubes is reported. We examine electric field effects on various excitonic states by simultaneously measuring both photocurrent and photoluminescence. As the applied field increases, we observe an emergence of new absorption peaks in the excitation spectra. From the diameter dependence of the energy separation between the new peaks and the ground state of E11 excitons, we attribute the peaks to the dark excited states which became optically active due to the applied field. A simple field-induced exciton dissociation model is introduced to explain the photocurrent threshold fields, and the edge of the E11 continuum states have been identified using this model. Work supported by JSPS (KAKENHI 24340066, 26610080), MEXT (Photon Frontier Network Program, Nanotechnology Platform), Canon Foundation, and Asahi Glass Foundation.

  16. Bound infragravity waves

    NASA Astrophysics Data System (ADS)

    Okihiro, Michele; Guza, R. T.; Seymour, R. J.

    1992-07-01

    Model predictions of bound (i.e., nonlinearly forced by and coupled to wave groups) infragravity wave energy are compared with about 2 years of observations in 8- to 13-m depths at Imperial Beach, California, and Barbers Point, Hawaii. Frequency-directional spectra of free waves at sea and swell frequencies, estimated with a small array of four pressure sensors, are used to predict the bound wave spectra below 0.04 Hz. The predicted total bound wave energy is always less than the observed infragravity energy, and the underprediction increases with increasing water depth and especially with decreasing swell energy. At most half, and usually much less, of the observed infragravity energy is bound. Bound wave spectra are also predicted with data from a single wave gage in 183-m depth at Point Conception, California, and the assumption of unidirectional sea and swell. Even with energetic swell, less than 10% of the total observed infragravity energy in 183-m depth is bound. Free waves, either leaky or edge waves, are more energetic than bound waves at both the shallow and deep sites. The low level of infragravity energy observed in 183-m depth compared with 8- to 13-m depths, with similarly moderate sea and swell energy, suggests that leaky (and very high-mode edge) waves contribute less than 10% of the infragravity energy in 8-13 m. Most of the free infragravity energy in shallow water is refractively trapped and does not reach deep water.

  17. Excitonic emissions and above-band-gap luminescence in the single-crystal perovskite semiconductors CsPbB r3 and CsPbC l3

    NASA Astrophysics Data System (ADS)

    Sebastian, M.; Peters, J. A.; Stoumpos, C. C.; Im, J.; Kostina, S. S.; Liu, Z.; Kanatzidis, M. G.; Freeman, A. J.; Wessels, B. W.

    2015-12-01

    The ternary compounds CsPb X3 (X =Br or Cl) have perovskite structures that are being considered for optical and electronic applications such as lasing and gamma-ray detection. An above-band-gap excitonic photoluminescence (PL) band is seen in both CsPb X3 compounds. An excitonic emission peak centered at 2.98 eV, ˜ 0.1 eV above the room-temperature band gap, is observed for CsPbC l3 . The thermal quenching of the excitonic luminescence is well described by a two-step quenching model, yielding activation energies of 0.057 and 0.0076 eV for high- and low-temperature regimes, respectively. CsPbB r3 exhibits bound excitonic luminescence peaks located at 2.29 and 2.33 eV that are attributed to recombination involving Br vacancy centers. Activation energies for thermal quenching of the excitonic luminescence of 0.017 and 0.0007 eV were calculated for CsPbB r3 . Temperature-dependent PL experiments reveal unexpected blueshifts for all excitonic emission peaks in CsPb X3 compounds. A phonon-assisted step-up process leads to the blueshift in CsPbB r3 emission, while there is a contribution from band-gap widening in CsPbC l3 . The absence of significant deep level defect luminescence in these compounds makes them attractive candidates for high-resolution, room-temperature radiation detection.

  18. Quantum confinement of excitons in wurtzite InP nanowires

    SciTech Connect

    Pemasiri, K.; Jackson, H. E.; Smith, L. M.; Wong, B. M.; Paiman, S.; Gao, Q.; Tan, H. H.; Jagadish, C.

    2015-05-21

    Exciton resonances are observed in photocurrent spectra of 80 nm wurtzite InP nanowire devices at low temperatures, which correspond to transitions between the A, B, and C valence bands and the lower conduction band. Photocurrent spectra for 30 nm WZ nanowires exhibit shifts of the exciton resonances to higher energy, which are consistent with finite element calculations of wavefunctions of the confined electrons and holes for the various bands.

  19. Excitonic effects in GeC hybrid: Many-body Green's function calculations

    NASA Astrophysics Data System (ADS)

    Drissi, L. B.; Ramadan, F. Z.

    2015-11-01

    Many-body effects on the electronic and optical absorption properties of a GeC sheet are studied by means of first principle many-body Green's function and Bethe-Salpeter equation formalism. The absence of soft modes in the phonon-spectrum indicates the stability of the system. The inclusion of quasiparticle corrections increases significantly the band gap. The local field effects induce significant change in the absorption spectra for the out-plane polarization rendering the GeC monolayer transparent below 7 eV. The excitonic effects are significant on the optical absorption properties. A detailed analysis of the spectrum shows a strong binding energy of 1.82 eV assigned to the lowest-energy bound excitons that is characterized by an effective mass of 1.68m0 and a Bohr radius of 2 Å. The results of this study hold the promise for potential applications of the GeC hybrid in optoelectronics.

  20. Electronic and optical properties of single excitons and biexcitons in type-II quantum dot nanocrystals

    SciTech Connect

    Koc, Fatih; Sahin, Mehmet E-mail: mehsahin@gmail.com

    2014-05-21

    In this study, a detailed investigation of the electronic and optical properties (i.e., binding energies, absorption wavelength, overlap of the electron-hole wave functions, recombination oscillator strength, etc.) of an exciton and a biexciton in CdTe/CdSe core/shell type-II quantum dot heterostructures has been carried out in the frame of the single band effective mass approximation. In order to determine the electronic properties, we have self-consistently solved the Poisson-Schrödinger equations in the Hartree approximation. We have considered all probable Coulomb interaction effects on both energy levels and also on the corresponding wave functions for both single exciton and biexciton. In addition, we have taken into account the quantum mechanical exchange-correlation effects in the local density approximation between same kinds of particles for biexciton. Also, we have examined the effect of the ligands and dielectric mismatch on the electronic and optical properties. We have used a different approximation proposed by Sahin and Koc [Appl. Phys. Lett. 102, 183103 (2013)] for the recombination oscillator strength of the biexciton for bound and unbound cases. The results obtained have been presented comparatively as a function of the shell thicknesses and probable physical reasons in behind of the results have been discussed in a detail.

  1. Simulation of Singlet Exciton Diffusion in Bulk Organic Materials.

    PubMed

    Kranz, Julian J; Elstner, Marcus

    2016-09-13

    We present a scheme for nonadiabatic direct dynamics simulation of Frenkel exciton diffusion in bulk molecular systems. The fluctuations of exciton couplings caused by the molecular motion can crucially influence exciton transport in such materials. This effect can be conveniently taken into account by computing the exciton couplings along molecular dynamics trajectories, as shown recently. In this work, we combine Molecular Dynamics simulations with a Frenkel Hamiltonian into a combined quantum-mechanical/molecular mechanics approach in order to allow for a simultaneous propagation of nuclear and electronic degrees of freedom using nonadiabatic dynamics propagation schemes. To reach the necessary time and length scales, we use classical force-fields and the semiempirical time-dependent density functional tight-binding method in combination with a fragmentation of the electronic structure. Fewest-switches surface-hopping, with adaptions to handle trivial crossings, and the Boltzmann-corrected Ehrenfest method are used to follow the excitonic quantum dynamics according to the classical evolution of the nuclei. As an application, we present the simulation of singlet exciton diffusion in crystalline anthracene, which allows us to address strengths and shortcomings of the presented methodology in detail. PMID:27434173

  2. Two-dminensional exciton states in monolayer semiconducting phosphorus alotropes

    NASA Astrophysics Data System (ADS)

    Rocha, Alexandre R.; Villegas, Cesar E. P.

    During the last decade, novel two-dimensional (2D) semiconducting materials have been synthesized and characterised. As a result, there have been several theoretical and experimental proposals to incorporate 2D materials for designing next generation electronic and optoelectronics devices. In particular, it has been demonstrated that light absorption in phosphorus-based monolayers can span the whole visible spectrum, suggesting they could be used for optolectronic applications. A key ingredient for optolectronic applications is the presence of excitons and their subsequent diffusion along a donor material. This is influenced by the character of the different excitations taking place, as well as, the exciton binding energy. Therefore, In this work we use accurate many-body corrected density functional theory by means of GW-BSE methodology to elucidate the most important optical transitions, exciton energy spectrum as well as exciton extension in different types of phosphorene materials. In addition, we solve the Schrodinger equation for different 2D screened potentials and estimate the 2D exciton energy levels and radius extension. Finally, in order to assess further studies based on these systems, we provide a simple analityc expression for estimating 2D exciton energy levels. Research funded by FAPESP-Brazil.

  3. Molecular packing determines singlet exciton fission in organic semiconductors.

    PubMed

    Kolata, Kolja; Breuer, Tobias; Witte, Gregor; Chatterjee, Sangam

    2014-07-22

    Carrier multiplication by singlet exciton fission enhances photovoltaic conversion efficiencies in organic solids. This decay of one singlet exciton into two triplet states allows the extraction of up to two electrons per harvested photon and, hence, promises to overcome the Shockley–Queisser limit. However, the microscopic mechanism of singlet exciton fission, especially the relation between molecular packing and electronic response, remains unclear, which therefore hampers the systematic improvement of organic photovoltaic devices. For the model system perfluoropentacene, we experimentally show that singlet exciton fission is greatly enhanced for a slip-stacked molecular arrangement by addressing different crystal axes featuring different packing schemes. This reveals that the fission process strongly depends on the intermolecular coupling: slip-stacking favors delocalization of excitations and allows for efficient exciton fission, while face-to-edge molecular orientations commonly found in the prevailing herringbone molecular stacking patterns even suppress it. Furthermore, we clarify the controversially debated role of excimer states as intermediary rather than competitive or precursory. Our detailed findings serve as a guideline for the design of next-generation molecular materials for application in future organic light-harvesting devices exploiting singlet exciton fission. PMID:24957197

  4. Singlet fission in pentacene through multi-exciton quantum states.

    PubMed

    Zimmerman, Paul M; Zhang, Zhiyong; Musgrave, Charles B

    2010-08-01

    Multi-exciton generation-the creation of multiple charge carrier pairs from a single photon-has been reported for several materials and may dramatically increase solar cell efficiency. Singlet fission, its molecular analogue, may govern multi-exciton generation in a variety of materials, but a fundamental mechanism for singlet fission has yet to be described. Here, we use sophisticated ab initio calculations to show that singlet fission in pentacene proceeds through rapid internal conversion of the photoexcited state into a dark state of multi-exciton character that efficiently splits into two triplets. We show that singlet fission to produce a pair of triplet excitons must involve an intermediate state that (i) has a multi-exciton character, (ii) is energetically accessible from the optically allowed excited state, and (iii) efficiently dissociates into multiple electron-hole pairs. The rational design of photovoltaic materials that make use of singlet fission will require similar ab initio analysis of multi-exciton states such as the dark state studied here. PMID:20651727

  5. Magnetic brightening of dark excitons in transitional metal dichalcogenides

    NASA Astrophysics Data System (ADS)

    Zhang, Xiao-Xiao; Lu, Zhengguang; Cao, Ting; Zhang, Fan; Hone, James; Louie, Steven G.; Li, Zhiqiang; Smirnov, Dmitry; Heinz, Tony

    Transitional metal dichalcogenides (TMDC) in the MX2 (M = Mo, W, X = S, Se) family represent an excellent platform to study of excitonic effects. At monolayer thickness, these materials exhibit both direct band-gap character and enhanced excitonic interactions. Theoretical studies suggest that both the valence and conduction bands are split and exhibit spin polarized character at the K/K' valleys. The lowest energy band-edge excitons are predicted to have different spin configurations for different materials in this family. When the lowest lying exciton has parallel electron and hole spin, radiative decay is forbidden and the state is dark. Here we demonstrate that by applying an in-plane magnetic field we can perturb the exciton spin configuration and brighten this state, allowing it to undergo radiative decay. We identify such a brightened dark state by the emergence of a new emission peak lying below the absorption peak, with a strength growing with applied in-plane magnetic field. On the other hand, for monolayer MoSe2, where no low-lying dark state is expected, we do not see the growth of a new emission feature under application of an in-plane magnetic field. Our experimental findings are in agreement with the calculated properties of dark excitons based on GW plus Bethe-Salpeter equation approach

  6. Femtosecond THz Studies of Intra-Excitonic Transitions

    SciTech Connect

    Huber, Rupert; Schmid, Ben A.; Kaindl, Robert A.; Chemla, Daniel S.

    2007-10-02

    Few-cycle THz pulses are employed to resonantly access the internal fine structure of photogenerated excitons in semiconductors, on the femtosecond time scale. This technique allows us to gain novel insight into many-body effects of excitons and reveal key quantum optical processes. We discuss experiments that monitor the density-dependent re?normalization of the binding energy of a high-density exciton gas in GaAs/AlGaAs quantum wells close to the Mott transition. In a dilute ensemble of 3p excitons in Cu2O, stimulated THz emission from internal transitions to the energetically lower 2s state is observed at a photon energy of 6.6 meV, with a cross section of 10-14 cm2. Simultaneous interband excitation of both exciton levels drives quantum beats, which cause efficient THz emission at the difference frequency. By extending this principle to various other exciton resonances, we develop a novel way of mapping the fine structure by two-dimensional THz emission spectroscopy.

  7. Trapping and transport of indirect excitons in coupled quantum wells

    NASA Astrophysics Data System (ADS)

    Wuenschell, Jeffrey K.

    Spatially indirect excitons are optically generated composite bosons with a radiative lifetime sufficient to reach thermal equilibrium. This work explores the physics of indirect excitons in coupled quantum wells in the GaAs/AlGaAs system, specifically in the low-temperature, high-density regime. Particular attention is paid to a technique whereby a spatially inhomogeneous strain field is used as a trapping potential. In the process of modeling the trapping profile in wide quantum wells, dramatic effects due to intersubband coupling were observed at high strain. Experimentally, this regime coincides with the abrupt appearance of a dark population of indirect excitons at trap center, an effect originally suspected to be related to Bose-Einstein condensation. Here, the role of band mixing due to the strain-induced distortion of the crystal symmetry will be explored in detail in the context of this effect. Experimental studies presented here and in the literature suggest that Bose-Einstein condensation in indirect exciton systems may be difficult to detect with optical means (e.g., coherence measurements, momentum-space narrowing), possibly due to the strong dipole interaction between indirect excitons. Due to similarities between this system and liquid helium, it may be more fruitful to look for transport-related signatures of condensation, such as super fluidity. Here, a method for performing transport measurements on optically generated indirect excitons is also outlined and preliminary results are presented.

  8. Monte Carlo Simulation of Exciton Dynamics in Supramolecular Semiconductor Architectures

    NASA Astrophysics Data System (ADS)

    Silva, Carlos; Beljonne, David; Herz, Laura; Hoeben, Freek

    2005-03-01

    Supramolecular chemistry is useful to construct molecular architectures with functional semiconductor properties. To explore the consequences of this approach in molecular electronics, we have carried out ultrafast measurements of exciton dynamics in supramolecular assemblies of an oligo-p-phenyl-ene-vinyl-ene derivative functionalized to form chiral stacks in dodecane solution in a thermotropically reversible manner. We apply a model of incoherent exciton hopping within a Monte Carlo scheme to extract microscopic physical quantities. The simulation first builds the chiral stacks with a Gaussian disorder of site energies and then simulates exciton hopping on the structure and exciton-exciton annihilation to reproduce ensemble-averaged experimental data. The exciton transfer rates are calculated beyond the point-dipole approximation using the so-called line-dipole approach in combination with the Förster expression. The model of incoherent hopping successfully reproduces the data and we extract a high diffusion coefficient illustrating the polymeric properties of such supramolecular assemblies. The scope and limitations of the line-dipole approximation as well as the resonance energy transfer concept in this system are discussed.

  9. Optical Absorption Spectra and Excitons of Dye-Substrate Interfaces: Catechol on TiO2(110).

    PubMed

    Mowbray, Duncan John; Migani, Annapaola

    2016-06-14

    Optimizing the photovoltaic efficiency of dye-sensitized solar cells (DSSC) based on staggered gap heterojunctions requires a detailed understanding of sub-band gap transitions in the visible from the dye directly to the substrate's conduction band (CB) (type-II DSSCs). Here, we calculate the optical absorption spectra and spatial distribution of bright excitons in the visible region for a prototypical DSSC, catechol on rutile TiO2(110), as a function of coverage and deprotonation of the OH anchoring groups. This is accomplished by solving the Bethe-Salpeter equation (BSE) based on hybrid range-separated exchange and correlation functional (HSE06) density functional theory (DFT) calculations. Such a treatment is necessary to accurately describe the interfacial level alignment and the weakly bound charge transfer transitions that are the dominant absorption mechanism in type-II DSSCs. Our HSE06 BSE spectra agree semiquantitatively with spectra measured for catechol on anatase TiO2 nanoparticles. Our results suggest deprotonation of catechol's OH anchoring groups, while being nearly isoenergetic at high coverages, shifts the onset of the absorption spectra to lower energies, with a concomitant increase in photovoltaic efficiency. Further, the most relevant bright excitons in the visible region are rather intense charge transfer transitions with the electron and hole spatially separated in both the [110] and [001] directions. Such detailed information on the absorption spectra and excitons is only accessible via periodic models of the combined dye-substrate interface. PMID:27183273

  10. Influence of dielectric environment on exciton and bi-exciton properties in colloidal, type II quantum dots

    NASA Astrophysics Data System (ADS)

    Miloszewski, Jacek M.; Walsh, Thomas; Tomić, Stanko

    2015-05-01

    We present theoretical calculations of type II CdSe/CdTe quantum dots systems. We use an 8-band k.p Hamiltonian that includes spin-orbit interaction, strain, and first order piezoelectric effects. Exciton and bi-exciton states are found using the configuration interaction (CI) method that explicitly includes the effects of the Coulomb interaction, as well as exchange and correlation effects between many-electron configurations. We study convergence of the CI Hamiltonian with respect to the number of single particle states used in creation of the Hamiltonian. We show that there is a very strong correlation between the dielectric constant of the environment and exciton and bi-exciton energies.

  11. Three Redox States of a Diradical Acceptor-Donor-Acceptor Triad: Gating the Magnetic Coupling and the Electron Delocalization.

    PubMed

    Souto, Manuel; Lloveras, Vega; Vela, Sergi; Fumanal, Maria; Ratera, Imma; Veciana, Jaume

    2016-06-16

    The diradical acceptor-donor-acceptor triad 1(••), based on two polychlorotriphenylmethyl (PTM) radicals connected through a tetrathiafulvalene(TTF)-vinylene bridge, has been synthesized. The generation of the mixed-valence radical anion, 1(•-), and triradical cation species, 1(•••+), obtained upon electrochemical reduction and oxidation, respectively, was monitored by optical and ESR spectroscopy. Interestingly, the modification of electron delocalization and magnetic coupling was observed when the charged species were generated and the changes have been rationalized by theoretical calculations. PMID:27231856

  12. Cooperative Singlet and Triplet Exciton Transport in Tetracene Crystals Visualized by Ultrafast Microscopys

    SciTech Connect

    Wan, Yan; Guo, Zhi; Zhu, Tong; Yan, Suxia; Johnson, Justin; Huang, Libai

    2015-09-14

    Singlet fission presents an attractive solution to overcome the Shockley–Queisser limit by generating two triplet excitons from one singlet exciton. Although triplet excitons are long-lived, their transport occurs through a Dexter transfer, making them slower than singlet excitons, which travel by means of a Förster mechanism. A thorough understanding of the interplay between singlet fission and exciton transport is therefore necessary to assess the potential and challenges of singlet-fission utilization. We report a direct visualization of exciton transport in single tetracene crystals using transient absorption microscopy with 200 fs time resolution and 50 nm spatial precision. Moreover, these measurements reveal a new singlet-mediated transport mechanism for triplets, which leads to an enhancement in effective triplet exciton diffusion of more than one order of magnitude on picosecond to nanosecond timescales. These results establish that there are optimal energetics of singlet and triplet excitons that benefit both singlet fission and exciton diffusion.

  13. Exciton dynamics in atomically thin MoS2: Interexcitonic interaction and broadening kinetics

    NASA Astrophysics Data System (ADS)

    Sim, Sangwan; Park, Jusang; Song, Jeong-Gyu; In, Chihun; Lee, Yun-Shik; Kim, Hyungjun; Choi, Hyunyong

    2013-08-01

    We report ultrafast pump-probe spectroscopy examining exciton dynamics in atomically thin MoS2. Spectrally and temporally resolved measurements are performed to investigate the interaction dynamics of two important direct-gap excitons (A and B) and their associated broadening kinetics. The two excitons show strongly correlated interexcitonic dynamic, in which the transient blue-shifted excitonic absorption originates from the internal A-B excitonic interaction. The observed complex spectral response is determined by the exciton collision-induced linewidth broadening; the broadening of the B-exciton linewidth in turn lowers the peak spectral amplitude of the A exciton. Resonant excitation at the B-exciton energy reveals that interexcitonic scattering plays a more important role in determining the broadening kinetics than free-carrier scattering.

  14. Cooperative singlet and triplet exciton transport in tetracene crystals visualized by ultrafast microscopy.

    PubMed

    Wan, Yan; Guo, Zhi; Zhu, Tong; Yan, Suxia; Johnson, Justin; Huang, Libai

    2015-10-01

    Singlet fission presents an attractive solution to overcome the Shockley-Queisser limit by generating two triplet excitons from one singlet exciton. However, although triplet excitons are long-lived, their transport occurs through a Dexter transfer, making them slower than singlet excitons, which travel by means of a Förster mechanism. A thorough understanding of the interplay between singlet fission and exciton transport is therefore necessary to assess the potential and challenges of singlet-fission utilization. Here, we report a direct visualization of exciton transport in single tetracene crystals using transient absorption microscopy with 200 fs time resolution and 50 nm spatial precision. These measurements reveal a new singlet-mediated transport mechanism for triplets, which leads to an enhancement in effective triplet exciton diffusion of more than one order of magnitude on picosecond to nanosecond timescales. These results establish that there are optimal energetics of singlet and triplet excitons that benefit both singlet fission and exciton diffusion. PMID:26391077

  15. Room-Temperature Transport of Indirect Excitons in (Al ,Ga )N /GaN Quantum Wells

    NASA Astrophysics Data System (ADS)

    Fedichkin, F.; Guillet, T.; Valvin, P.; Jouault, B.; Brimont, C.; Bretagnon, T.; Lahourcade, L.; Grandjean, N.; Lefebvre, P.; Vladimirova, M.

    2016-07-01

    We report on the exciton propagation in polar (Al ,Ga )N /GaN quantum wells over several micrometers and up to room temperature. The key ingredient to achieve this result is the crystalline quality of GaN quantum wells grown on GaN substrate that limits nonradiative recombination. From the comparison of the spatial and temporal dynamics of photoluminescence, we conclude that the propagation of excitons under continuous-wave excitation is assisted by efficient screening of the in-plane disorder. Modeling within drift-diffusion formalism corroborates this conclusion and suggests that exciton propagation is still limited by the exciton scattering on defects rather than by exciton-exciton scattering so that improving interface quality can boost exciton transport further. Our results pave the way towards room-temperature excitonic devices based on gate-controlled exciton transport in wide-band-gap polar heterostructures.

  16. Covalent non-fused tetrathiafulvalene-acceptor systems.

    PubMed

    Pop, Flavia; Avarvari, Narcis

    2016-06-28

    Covalent donor-acceptor (D-A) systems have significantly contributed to the development of many organic materials and to molecular electronics. Tetrathiafulvalene (TTF) represents one of the most widely studied donor precursors and has been incorporated into the structure of many D-A derivatives with the objective of obtaining redox control and modulation of the intramolecular charge transfer (ICT), in order to address switchable emissive systems and to take advantage of its propensity to form regular stacks in the solid state. In this review, we focus on the main families of non-fused TTF-acceptors, which are classified according to the nature of the acceptor: nitrogen-containing heterocycles, BODIPY, perylenes and electron poor unsaturated hydrocarbons, as well as radical acceptors. We describe herein the most representative members of each family with a brief mention of their synthesis and a special focus on their D-A characteristics. Special attention is given to ICT and its modulation, fluorescence quenching and switching, photoconductivity, bistability and spin distribution by discussing and comparing spectroscopic and electrochemical features, photophysical properties, solid-state properties and theoretical calculations. PMID:27193500

  17. Development of imide- and imidazole-containing electron acceptors for use in donor-acceptor conjugated compounds and polymers

    NASA Astrophysics Data System (ADS)

    Li, Duo

    Conjugated organic compounds and polymers have attracted significant attention due to their potential application in electronic devices as semiconducting materials, such as organic solar cells (OSCs). In order to tune band gaps, donor-acceptor (D-A) structure is widely used, which has been proved to be one of the most effective strategies. This thesis consists of three parts: 1) design, syntheses and characterization of new weak acceptors based on imides and the systematic study of the structure-property relationship; (2) introduction of weak and strong acceptors in one polymer to achieve a broad coverage of light absorption and improve the power conversion efficiency (PCE); (3) modification of benzothiadiazole (BT) acceptor in order to increase the electron withdrawing ability. Imide-based electron acceptors, 4-(5-bromothiophen-2-y1)-2-(2-ethylhexyl)-9- phenyl- 1H-benzo[f]isoindole-1,3(2H)-dione (BIDO-1) and 4,9-bis(5-bromothiophen-2-yl)-2-(2-ethylhexyl)-benzo[f]isoindole-1,3-dione (BIDO-2), were designed and synthesized. In this design, naphthalene is selected as its main core to maintain a planar structure, and thienyl groups are able to facilitate the bromination reaction and lower the band gap. BIDO-1 and BIDO-2 were successfully coupled with different donors by both Suzuki cross-coupling and Stille cross-coupling reactions. Based on the energy levels and band gaps of the BIDO-containing compounds and polymers, BIDO-1 and BIDO-2 are proved to be weak electron acceptors. Pyromellitic diimide (PMDI) was also studied and found to be a stronger electron acceptor than BIDO . In order to obtain broad absorption coverage, both weak acceptor ( BIDO-2) and strong acceptor diketopyrrolopyrrole (DPP) were introduced in the same polymer. The resulting polymers show two absorption bands at 400 and 600 nm and two emission peaks at 500 and 680 nm. The band gaps of the polymers are around 1.6 eV, which is ideal for OSC application. The PCE of 1.17% was achieved. Finally

  18. Anaerobic methanotrophy in tidal wetland: Effects of electron acceptors

    NASA Astrophysics Data System (ADS)

    Lin, Li-Hung; Yu, Zih-Huei; Wang, Pei-Ling

    2016-04-01

    Wetlands have been considered to represent the largest natural source of methane emission, contributing substantially to intensify greenhouse effect. Despite in situ methanogenesis fueled by organic degradation, methanotrophy also plays a vital role in controlling the exact quantity of methane release across the air-sediment interface. As wetlands constantly experience various disturbances of anthropogenic activities, biological burrowing, tidal inundation, and plant development, rapid elemental turnover would enable various electron acceptors available for anaerobic methanotrophy. The effects of electron acceptors on stimulating anaerobic methanotrophy and the population compositions involved in carbon transformation in wetland sediments are poorly explored. In this study, sediments recovered from tidally influenced, mangrove covered wetland in northern Taiwan were incubated under the static conditions to investigate whether anaerobic methanotrophy could be stimulated by the presence of individual electron acceptors. Our results demonstrated that anaerobic methanotrophy was clearly stimulated in incubations amended with no electron acceptor, sulfate, or Fe-oxyhydroxide. No apparent methane consumption was observed in incubations with nitrate, citrate, fumarate or Mn-oxides. Anaerobic methanotrophy in incubations with no exogenous electron acceptor appears to proceed at the greatest rates, being sequentially followed by incubations with sulfate and Fe-oxyhydroxide. The presence of basal salt solution stimulated methane oxidation by a factor of 2 to 3. In addition to the direct impact of electron acceptor and basal salts, incubations with sediments retrieved from low tide period yielded a lower rate of methane oxidation than from high tide period. Overall, this study demonstrates that anaerobic methanotrophy in wetland sediments could proceed under various treatments of electron acceptors. Low sulfate content is not a critical factor in inhibiting methane

  19. Broadband optical transparency in plasmonic nanocomposite polymer films via exciton-plasmon energy transfer.

    PubMed

    Dhama, R; Rashed, A R; Caligiuri, V; El Kabbash, M; Strangi, G; De Luca, A

    2016-06-27

    Inherent absorptive losses affect the performance of all plasmonic devices, limiting their fascinating applications in the visible range. Here, we report on the enhanced optical transparency obtained as a result of the broadband mitigation of optical losses in nanocomposite polymeric films, embedding core-shell quantum dots (CdSe@ZnS QDs) and gold nanoparticles (Au-NPs). Exciton-plasmon coupling enables non-radiative energy transfer processes from QDs to metal NPs, resulting in gain induced transparency of the hybrid flexible systems. Experimental evidences, such as fluorescence quenching and modifications of fluorescence lifetimes confirm the presence of this strong coupling between plexcitonic elements. Measures performed by means of an ultra-fast broadband pump-probe setup demonstrate loss compensation of gold NPs dispersed in plastic network in presence of gain. Furthermore, we compare two films containing different concentrations of gold NPs and same amount of QDs, to investigate the role of acceptor concentration (Au-NPs) in order to promote an effective and efficient energy transfer mechanism. Gain induced transparency in bulk systems represents a promising path towards the realization of loss compensated plasmonic devices. PMID:27410615

  20. Geometry and B(1s) core excitons of ortho-carborane

    SciTech Connect

    Green, T.A. )

    1991-07-01

    The optimized C{sub 2v} geometry of ortho-carborane, 1,2-C{sub 2}B{sub 10}H{sub 12}, is determined from Hartree-Fock calculations. For this geometry, a carbon atom is substituted for a boron atom at one of the 4 inequivalent boron sites and the ground-state unrestricted Hartree-Fock eigenvalues and molecular orbitals are found. One thus obtains the valence structure of the B(1s) core-excited molecule according to the Z+1 approximation. The eigenvalue of the highest occupied molecular orbital is then subtracted from the experimental B(1s) ionization energy of the same site in ortho-carborane. This determines the excitation energy of the most tightly bound exciton for that site. Three of the sites yield nearly identical excitation energies of 191.9 eV; the fourth site yields an excitation energy of 190.9 eV.