Quantum Confined Silicon Clathrate Quantum Dots
NASA Astrophysics Data System (ADS)
Lusk, Mark; Brawand, Nicholas
2013-03-01
Silicon (Si) allotropes can be synthesized in such a way that tetrahedrally bonded atoms form cage-like structures with bulk mechanical and opto-electronic properties distinct from those of diamond silicon (dSi). We use DFT, supplemented with many-body Green function analysis, to explore the structural stability of clathrate Si quantum dots (QDs) and to characterize their confinement as a function of crystal symmetry and size. Our results show that that there is a simple relationship between the confinement character of the QDs and the effective mass of the associated bulk crystals. Clathrate QDs and dSiQDs of the same size can exhibit differences of gap energies by as much as 2 eV. This offers the potential of synthesizing Si dots on the order of 1 nm that have optical gaps in the visible range but that do not rely on high-pressure routes such as those explored for the metastable BC8 and R8 phases. These results prompt the question as to how minimal quantum confinement can be in dots composed of Si. More broadly, clathrate QDs can in principle be synthesized for a wide range of semiconductors, and the design space can be further enriched via doping. Silicon (Si) allotropes can be synthesized in such a way that tetrahedrally bonded atoms form cage-like structures with bulk mechanical and opto-electronic properties distinct from those of diamond silicon (dSi). We use DFT, supplemented with many-body Green function analysis, to explore the structural stability of clathrate Si quantum dots (QDs) and to characterize their confinement as a function of crystal symmetry and size. Our results show that that there is a simple relationship between the confinement character of the QDs and the effective mass of the associated bulk crystals. Clathrate QDs and dSiQDs of the same size can exhibit differences of gap energies by as much as 2 eV. This offers the potential of synthesizing Si dots on the order of 1 nm that have optical gaps in the visible range but that do not rely on high-pressure routes such as those explored for the metastable BC8 and R8 phases. These results prompt the question as to how minimal quantum confinement can be in dots composed of Si. More broadly, clathrate QDs can in principle be synthesized for a wide range of semiconductors, and the design space can be further enriched via doping. NSF Renewable Energy Materials Research Science and Engineering Center (REMRSEC) and the Golden Energy Computing Organization (GECO)
Quantum Dot Photovoltaics in the Extreme Quantum Confinement Regime
of possible har- vesting of multiple excitons,7 9 and also as the small-bandgap junction in a tandem or triple-junctionQuantum Dot Photovoltaics in the Extreme Quantum Confinement Regime: The Surface-Chemical Origins reaching the earth. One oppor- tunity for further improvement in solution- cast solar cells' efficiency
Electron confinement in single layer graphene quantum dots: Semiclassical approach
G. Giavaras; P. A. Maksym; M. Roy
2010-01-01
A semiclassical approach is used to investigate the physics of electrons confined in a graphene quantum dot in a magnetic field. The states near the gap in the spectrum are localised between the turning points for classical massless particles. This fact is used to obtain approximate energies and states in the case of non-zero, positive angular momentum. The agreement with
Confinement of Dirac electrons in graphene quantum dots
NASA Astrophysics Data System (ADS)
Jolie, Wouter; Craes, Fabian; Petrovi?, Marin; Atodiresei, Nicolae; Caciuc, Vasile; Blügel, Stefan; Kralj, Marko; Michely, Thomas; Busse, Carsten
2014-04-01
We observe spatial confinement of Dirac states on epitaxial graphene quantum dots with low-temperature scanning tunneling microscopy after using oxygen as an intercalant to suppress the surface state of Ir(111) and to effectively decouple graphene from its metal substrate. We analyze the confined electronic states with a relativistic particle-in-a-box model and find a linear dispersion relation. The oxygen-intercalated graphene is p doped [ED=0.64±0.07 eV] and has a Fermi velocity close to the one of free-standing graphene [vF=0.96±0.07×106 m/s].
Iyengar, Srinivasan S.
Atomic and Molecular Quantum Theory Course Number: C561 10 Quantum Confinement in "Quantum dots S. Iyengar (instructor) #12;Atomic and Molecular Quantum Theory Course Number: C561 5 and Molecular Quantum Theory Course Number: C561 happens to optical transitions in quantum dots. As a result
Exciton States in a Quantum Dot with Parabolic Confinement
NASA Astrophysics Data System (ADS)
Do?an, Ü.; Sakiro?lu, S.; Yildiz, A.; Akgüngör, K.; Epik, H.; Sökmen, I.; Sari, H.; Ergün, Y.
In this study the electronic eigenstructure of an exciton in a parabolic quantum dot (QD) has been calculated with a high accuracy by using Finite element method (FEM). We have converted the coordinates of electron-light-hole system to relative and center of mass coordinate, then placed the Spherical Harmonics into Schrödinger equation analytically and obtained the Schrödinger equation which depends only on the radial variable. Finally we used FEM with only radial variable in order to get the accurate numerical results. We also showed first 21 energy level spectra of exciton depending on confinement and Coulomb interaction parameters.
G. Giavaras; P. A. Maksym; M. Roy
2009-01-01
Massless Dirac particles cannot be confined by an electrostatic potential. This is a problem for making graphene quantum dots but confinement can be achieved with a magnetic field and here general conditions for confined and deconfined states are derived. There is a class of potentials for which the character of the state can be controlled at will. Then a confinement-deconfinement
Quantum confinement effects across two-dimensional planes in MoS2 quantum dots
NASA Astrophysics Data System (ADS)
Gan, Z. X.; Liu, L. Z.; Wu, H. Y.; Hao, Y. L.; Shan, Y.; Wu, X. L.; Chu, Paul K.
2015-06-01
The low quantum yield (˜10-5) has restricted practical use of photoluminescence (PL) from MoS2 composed of a few layers, but the quantum confinement effects across two-dimensional planes are believed to be able to boost the PL intensity. In this work, PL from 2 to 9 nm MoS2 quantum dots (QDs) is excluded from the solvent and the absorption and PL spectra are shown to be consistent with the size distribution. PL from MoS2 QDs is also found to be sensitive to aggregation due to the size effect.
Engineering the hole confinement for CdTe-based quantum dot molecules
NASA Astrophysics Data System (ADS)
K?opotowski, ?.; Wojnar, P.; Kret, S.; Parli?ska-Wojtan, M.; Fronc, K.; Wojtowicz, T.; Karczewski, G.
2015-06-01
We demonstrate an efficient method to engineer the quantum confinement in a system of two quantum dots grown in a vertical stack. We achieve this by using materials with a different lattice constant for the growth of the outer and inner barriers. We monitor the resulting dot morphology with transmission electron microscopy studies and correlate the results with ensemble quantum dot photoluminescence. Furthermore, we embed the double quantum dots into diode structures and study photoluminescence as a function of bias voltage. We show that in properly engineered structures, it is possible to achieve a resonance of the hole states by tuning the energy levels with electric field. At the resonance, we observe signatures of a formation of a molecular state, hybridized over the two dots.
Reinhold Egger; Alessandro De Martino; Heinz Siedentop; Edgardo Stockmeyer
2010-01-01
We study the energy of quasi-particles in graphene within the Hartree-Fock approximation. The quasi-particles are confined via an inhomogeneous magnetic field and interact via the Coulomb potential. We show that the associated functional has a minimizer and determines the stability conditions for the N-particle problem in such a graphene quantum dot.
NASA Astrophysics Data System (ADS)
Schaibley, J. R.; Burgers, A. P.; McCracken, G. A.; Duan, L.-M.; Berman, P. R.; Steel, D. G.; Bracker, A. S.; Gammon, D.; Sham, L. J.
2013-04-01
The electron spin state of a singly charged semiconductor quantum dot has been shown to form a suitable single qubit for quantum computing architectures with fast gate times. A key challenge in realizing a useful quantum dot quantum computing architecture lies in demonstrating the ability to scale the system to many qubits. In this Letter, we report an all optical experimental demonstration of quantum entanglement between a single electron spin confined to a single charged semiconductor quantum dot and the polarization state of a photon spontaneously emitted from the quantum dot’s excited state. We obtain a lower bound on the fidelity of entanglement of 0.59±0.04, which is 84% of the maximum achievable given the timing resolution of available single photon detectors. In future applications, such as measurement-based spin-spin entanglement which does not require sub-nanosecond timing resolution, we estimate that this system would enable near ideal performance. The inferred (usable) entanglement generation rate is 3×103s-1. This spin-photon entanglement is the first step to a scalable quantum dot quantum computing architecture relying on photon (flying) qubits to mediate entanglement between distant nodes of a quantum dot network.
NASA Astrophysics Data System (ADS)
Ganguly, Jayanta; Pal, Suvajit; Ghosh, Manas
2013-11-01
We investigate the excitation kinetics of a repulsive impurity doped quantum dot initiated by the application of additive Gaussian white noise. The noise and the dot confinement sources of electric and magnetic origin have been found to fabricate the said kinetics in a delicate way. In addition to this the dopant location also plays some prominent role. The present study sheds light on how the individual or combined variation of different confinement sources could design the excitation kinetics in presence of noise. The investigation reveals emergence of maximization and saturation in the excitation kinetics as a result of complex interplay between various parameters that affect the kinetics. The phase space plots are often invoked and they lend credence to the findings. The present investigation is believed to provide some useful perceptions of the functioning of mesoscopic systems where noise plays some profound role.
Electrostatic confinement of electrons in an integrable graphene quantum dot.
Bardarson, J H; Titov, M; Brouwer, P W
2009-06-01
We compare the conductance of an undoped graphene sheet with a small region subject to an electrostatic gate potential for the cases that the dynamics in the gated region is regular (disc-shaped region) and classically chaotic (stadium). For the disc, we find sharp resonances that narrow upon reducing the area fraction of the gated region. We relate this observation to the existence of confined electronic states. For the stadium, the conductance loses its dependence on the gate voltage upon reducing the area fraction of the gated region, which signals the lack of confinement of Dirac quasiparticles in a gated region with chaotic classical electron dynamics. PMID:19658887
Strongly confining bare core CdTe quantum dots in polymeric microdisk resonators
Flatae, Assegid, E-mail: assegid.flatae@kit.edu; Grossmann, Tobias; Beck, Torsten; Wiegele, Sarah; Kalt, Heinz [Institute of Applied Physics and DFG-Center for Functional Nanostructures (CFN), Karlsruhe Institute of Technology (KIT), Wolfgang-Gaede-Str.1, 76131 Karlsruhe (Germany)
2014-01-01
We report on a simple route to the efficient coupling of optical emission from strongly confining bare core CdTe quantum dots (QDs) to the eigenmodes of a micro-resonator. The quantum emitters are embedded into QD/polymer sandwich microdisk cavities. This prevents photo-oxidation and yields the high dot concentration necessary to overcome Auger enhanced surface trapping of carriers. In combination with the very high cavity Q-factors, interaction of the QDs with the cavity modes in the weak coupling regime is readily observed. Under nanosecond pulsed excitation the CdTe QDs in the microdisks show lasing with a threshold energy as low as 0.33 ?J.
Dynamics of a Mn spin coupled to a single hole confined in a quantum dot
NASA Astrophysics Data System (ADS)
Varghese, B.; Boukari, H.; Besombes, L.
2014-09-01
Using the emission of the positively charged exciton as a probe, we analyze the dynamics of the optical pumping and the dynamics of the relaxation of a Mn spin exchange coupled with a confined hole spin in a II-VI semiconductor quantum dot. The hole-Mn spin can be efficiently initialized in a few tens of ns under optical injection of spin-polarized carriers. We show that this optical pumping process and its dynamics are controlled by electron-Mn flip-flops within the positively charged exciton-Mn complex. The pumping mechanism and its magnetic field dependence are theoretically described by a model including the dynamics of the electron-Mn complex in the excited state and the dynamics of the hole-Mn complex in the ground state of the positively charged quantum dot. We measure at zero magnetic field a spin-relaxation time of the hole-Mn spin in the ?s range or shorter. This hole-Mn spin relaxation is induced by the presence of valence-band mixing in self-assembled quantum dots.
Wave-Function Mapping of Graphene Quantum Dots with Soft Confinement
NASA Astrophysics Data System (ADS)
Subramaniam, D.; Libisch, F.; Li, Y.; Pauly, C.; Geringer, V.; Reiter, R.; Mashoff, T.; Liebmann, M.; Burgdörfer, J.; Busse, C.; Michely, T.; Mazzarello, R.; Pratzer, M.; Morgenstern, M.
2012-01-01
Using low-temperature scanning tunneling spectroscopy, we map the local density of states of graphene quantum dots supported on Ir(111). Because of a band gap in the projected Ir band structure around the graphene K point, the electronic properties of the QDs are dominantly graphenelike. Indeed, we compare the results favorably with tight binding calculations on the honeycomb lattice based on parameters derived from density functional theory. We find that the interaction with the substrate near the edge of the island gradually opens a gap in the Dirac cone, which implies soft-wall confinement. Interestingly, this confinement results in highly symmetric wave functions. Further influences of the substrate are given by the known moiré potential and a 10% penetration of an Ir surface resonance into the graphene layer.
Zhang, Zhenkui; Dai, Ying; Yu, Lin; Guo, Meng; Huang, Baibiao; Whangbo, Myung-Hwan
2012-03-01
In light of the established differences between the quantum confinement effect and the electron affinities between hydrogen-passivated C and Si quantum dots, we carried out theoretical investigations on SiC quantum dots, with surfaces uniformly terminated by C-H or Si-H bonds, to explore the role of surface terminations on these two aspects. Surprisingly, it was found that the quantum confinement effect is present (or absent) in the highest occupied (or lowest unoccupied) molecular orbital of the SiC quantum dots regardless of their surface terminations. Thus, the quantum confinement effect related to the energy gap observed experimentally (Phys. Rev. Lett., 2005, 94, 026102) is contributed to by the size-dependence of the highest occupied states; the absence of quantum confinement in the lowest unoccupied states is in contrary to the usual belief based on hydrogen-passivated C quantum dots. However, the cause of the absence of the quantum confinement in C nanodots is not transferable to SiC. We propose a model that provides a clear explanation for all findings on the basis of the nearest-neighbor and next-nearest-neighbor interactions between the valence atomic p-orbital in the frontier occupied/unoccupied states. We also found that the electron affinities of the SiC quantum dots, which closely depend on the surface environments, are negative for the C-H termination and positive for the Si-H termination. The prediction of negative electron affinities in SiC quantum dots by simple C-H termination indicates a promising application for these materials in electron-emitter devices. Our model predicts that GeC quantum dots with hydrogen passivation exhibit similar features to SiC quantum dots and our study confirms the crucial role that the surface environment plays in these nanoscale systems. PMID:22294210
NASA Astrophysics Data System (ADS)
Ganguly, Jayanta; Pal, Suvajit; Ghosh, Manas
2013-11-01
We investigate the excitation kinetics of a repulsive impurity doped quantum dot initiated by the application of multiplicative Gaussian white noise. The noise strength and the dot confinement sources of electric and magnetic origin have been found to produce the said kinetics in a subtle way. In addition to this the dopant location also plays some crucial role. The present study sheds light on how the individual or combined variation of different confinement sources could design the excitation kinetics in presence of noise. The investigation reveals maximization and saturation in the excitation kinetics as a result of complex interplay between the confinement potentials of the dot, the dopant location, and the noise strength. The present investigation is believed to provide some useful perceptions of the functioning of mesoscopic systems where noise plays some profound role.
In situ tunable g factor for a single electron confined inside an InAs quantum dot
NASA Astrophysics Data System (ADS)
Liu, W.; Sanwlani, S.; Hazbun, R.; Kolodzey, J.; Bracker, A. S.; Gammon, D.; Doty, M. F.
2011-09-01
Tailoring the properties of single spins confined in self-assembled quantum dots (QDs) is critical to the development of new optoelectronic logic devices. However, the range of heterostructure engineering techniques that can be used to control spin properties is severely limited by the requirements of QD self-assembly. We demonstrate a new strategy for rationally engineering the spin properties of single confined electrons or holes by adjusting the composition of the barrier between a stacked pair of InAs QDs coupled by coherent tunneling to form a quantum dot molecule (QDM). We demonstrate this strategy by designing, fabricating, and characterizing a QDM in which the g-factor for a single confined electron can be tuned in situ by over 50% with a minimal change in applied voltage.
Anas, M. M.; Othman, A. P.; Gopir, G. [School of Applied Physics, Faculty of Science and Technology, Universiti Kebangsaan Malaysia, 43600, Bangi, Selangor (Malaysia)
2014-09-03
Density functional theory (DFT), as a first-principle approach has successfully been implemented to study nanoscale material. Here, DFT by numerical basis-set was used to study the quantum confinement effect as well as electronic properties of silicon quantum dots (Si-QDs) in ground state condition. Selection of quantum dot models were studied intensively before choosing the right structure for simulation. Next, the computational result were used to examine and deduce the electronic properties and its density of state (DOS) for 14 spherical Si-QDs ranging in size up to ? 2 nm in diameter. The energy gap was also deduced from the HOMO-LUMO results. The atomistic model of each silicon QDs was constructed by repeating its crystal unit cell of face-centered cubic (FCC) structure, and reconstructed until the spherical shape obtained. The core structure shows tetrahedral (T{sub d}) symmetry structure. It was found that the model need to be passivated, and hence it was noticed that the confinement effect was more pronounced. The model was optimized using Quasi-Newton method for each size of Si-QDs to get relaxed structure before it was simulated. In this model the exchange-correlation potential (V{sub xc}) of the electrons was treated by Local Density Approximation (LDA) functional and Perdew-Zunger (PZ) functional.
Cosentino, S; Mio, A M; Barbagiovanni, E G; Raciti, R; Bahariqushchi, R; Miritello, M; Nicotra, G; Aydinli, A; Spinella, C; Terrasi, A; Mirabella, S
2015-06-25
Quantum confinement (QC) typically assumes a sharp interface between a nanostructure and its environment, leading to an abrupt change in the potential for confined electrons and holes. When the interface is not ideally sharp and clean, significant deviations from the QC rule appear and other parameters beyond the nanostructure size play a considerable role. In this work we elucidate the role of the interface on QC in Ge quantum dots (QDs) synthesized by rf-magnetron sputtering or plasma enhanced chemical vapor deposition (PECVD). Through a detailed electron energy loss spectroscopy (EELS) analysis we investigated the structural and chemical properties of QD interfaces. PECVD QDs exhibit a sharper interface compared to sputter ones, which also evidences a larger contribution of mixed Ge-oxide states. Such a difference strongly modifies the QC strength, as experimentally verified by light absorption spectroscopy. A large size-tuning of the optical bandgap and an increase in the oscillator strength occur when the interface is sharp. A spatially dependent effective mass (SPDEM) model is employed to account for the interface difference between Ge QDs, pointing out a larger reduction in the exciton effective mass in the sharper interface case. These results add new insights into the role of interfaces on confined systems, and open the route for reliable exploitation of QC effects. PMID:26077313
Electrochromic Nanocrystal Quantum Dots
Congjun Wang; Moonsub Shim; Philippe Guyot-Sionnest
2001-01-01
The optical properties of colloidal semiconductor nanocrystal quantum dots can be tuned by an electrochemical potential. The injection of electrons into the Lowest Unoccupied Quantum Confined Orbital (LUQCO) leads to an extraordinary electrochromic response with novel characteristics. These include a strong size-tunable mid-infrared absorption corresponding to an intraband transition, a bleach of the visible interband exciton transitions and a quench
Photoinduced band filling in strongly confined colloidal PbS quantum dots
Ullrich, B., E-mail: bruno@fis.unam.mx [Instituto de Ciencias Físicas, Universidad Nacional Autónoma de México, Cuernavaca, Morelos 62210 (Mexico); Ullrich Photonics LLC, Wayne, Ohio 43466 (United States); Xi, H. [Department of Physics and Astronomy, Bowling Green State University, Bowling Green, Ohio 43403-0209 (United States); Wang, J. S. [Air Force Research Laboratory, Materials and Manufacturing Directorate, Wright Patterson AFB, Ohio 45433-7707 (United States)
2014-06-21
Increase in continuous wave laser excitation (6?W/cm{sup 2} to 120?W/cm{sup 2}) of colloidal PbS quantum dots in the strongly quantized regime (diameters 2.0?nm and 4.7?nm) deposited on semi-insulating GaAs and glass causes a clear blue shift (0.019?eV and 0.080?eV) of the emission spectra. Proof of the applicability of a dynamic three-dimensional band filling model is the significance of the presented results and demonstrates the effective electronic coupling in quantum dot arrays similar to superlattices. The work also reveals the influence of quantum dot sizes on photo-doping effects.
NASA Astrophysics Data System (ADS)
Mardilovich, Pavel; Yang, Lihmei; Huang, Huan; Krol, Denise M.; Risbud, Subhash H.
2013-04-01
Optically tunable mesoscale structures offer unparalleled potential for photonic device applications. Here, we report the creation of composite photonic structures consisting of CdSxSe1-x quantum dots (QDs) customized within lines, first written in a glass by femtosecond laser pulses. CdSxSe1-x-doped borosilicate glasses were pulsed with a fs-laser using a 473 kHz repetition rate to create chemically distinct microscopic regions. Upon further heat treatment, these regions served as "micro-crucibles" within which quantum dots were precipitated exclusively. These results open prospects of developing other semiconductor doped glasses for versatile photonic structures useful over broader optical wavelengths.
NASA Astrophysics Data System (ADS)
Musia?, A.; Gold, P.; Andrzejewski, J.; Löffler, A.; Misiewicz, J.; Höfling, S.; Forchel, A.; Kamp, M.; Sek, G.; Reitzenstein, S.
2014-07-01
In this paper we present a comprehensive and detailed analysis of carrier/exciton wave function extension in large low-strain In0.3Ga0.7As quantum dots (QDs). They exhibit rather shallow confinement potential with electron/hole localization energy below 30 meV and confinement strength substantially weakened in comparison to typical epitaxial quasi-zero-dimensional semiconductor nanostructures. The aim of this study is to investigate the influence of different factors on the wave function (probability density distribution) for carriers or excitons in this regime, i.e., object shape anisotropy as well as strain, piezoelectricity, and Coulomb interactions, and to identify the physical mechanisms determining the properties of optical emission. To probe the wave function symmetry, polarization-resolved photoluminescence has been performed, and the spatial extensions of the corresponding probability densities have been verified in magneto-optical measurements. The observed diamagnetic coefficients in the range of (15-31) ?eV/T2 reflect large in-plane QD size. These studies also enable us to investigate the importance of light hole states admixture to the valence band ground state in such nanostructures, which can be addressed via the degree of linear polarization of emission as well as the exciton gX factor. The linear-polarization-resolved measurements revealed an exceptionally low exciton fine structure splitting of 5?eV on average as well as a low emission polarization degree of -0.05, with the polarization perpendicular to the QD elongation direction dominating. The increased light hole contribution to the lowest energy hole level is reflected in the decreased exciton gX factor (in the range of 0-1) and is consistent with the results of the eight-band k.p modelling. Based on the temperature dependence of the diamagnetic coefficient, the problem of individual QD uniformity has additionally been discussed. To evaluate the impact of the confinment potential and the structure geometry on the optical properties of the QDs, a comparison between the investigated dots and InAs/InGaAlAs/InP quantum dashes exhibiting a much deeper confining potential is presented.
Mejía-Salazar, J R; Porras-Montenegro, N; Oliveira, L E
2009-11-11
We have performed a theoretical study of the quantum confinement (geometrical and barrier potential confinements) and axis-parallel applied magnetic-field effects on the conduction-electron effective Landé g factor in GaAs-(Ga, Al)As cylindrical quantum dots. Numerical calculations of the g factor are performed by using the Ogg-McCombe effective Hamiltonian-which includes non-parabolicity and anisotropy effects-for the conduction-band electrons. The quantum dot is assumed to consist of a finite-length cylinder of GaAs surrounded by a Ga(1-x)Al(x)As barrier. Theoretical results are given as functions of the Al concentration in the Ga(1-x)Al(x)As barrier, radius, lengths and applied magnetic fields. We have studied the competition between the quantum confinement and applied magnetic field, finding that in this type of heterostructure the geometrical confinement and Al concentration determine the behavior of the electron effective Landé [Formula: see text] factor, as compared to the effect of the applied magnetic field. Present theoretical results are in good agreement with experimental reports in the limiting geometry of a quantum well, and with previous theoretical findings in the limiting case of a quantum well wire. PMID:21694007
Yong Zhang
1994-01-01
This thesis describes an experimental and theoretical investigation of semiconductor quantum wires and dots. I have studied the formation and decay of excitons and related problems: luminescent efficiency, carrier relaxation, energy transfer, effects of localization, etc. in these quasi one-dimensional (1-D) and quasi zero-dimensional (0-D) semiconductor nanostructures. The quantum wires and dots are made by strain patterning GaAs\\/rm Al _{x}Ga_{1-x}As
Yannouleas, Constantine; Landman, Uzi
2006-01-01
We discuss the formation of crystalline electron clusters in semiconductor quantum dots and of crystalline patterns of neutral bosons in harmonic traps. In a first example, we use calculations for two electrons in an elliptic quantum dot to show that the electrons can localize and form a molecular dimer. The calculated singlet–triplet splitting (J) as a function of the magnetic field (B) agrees with cotunneling measurements with its behavior reflecting the effective dissociation of the dimer for large B. Knowledge of the dot shape and of J(B) allows determination of the degree of entanglement. In a second example, we study strongly repelling neutral bosons in two-dimensional harmonic traps. Going beyond the Gross–Pitaevskii (GP) mean-field approximation, we show that bosons can localize and form polygonal-ring-like crystalline patterns. The total energy of the crystalline phase saturates in contrast to the GP solution, and its spatial extent becomes smaller than that of the GP condensate. PMID:16740665
NASA Astrophysics Data System (ADS)
P J, Monisha; Mukhopadhyay, Soma
2015-05-01
The effect of electron-phonon interaction on a few low-lying energy levels in a polar semiconductor quantum dot with Gaussian confinement is studied by using an improved Wigner-Brillouin perturbation theory (IWBPT). In the absence of the electron-phonon interaction, the electronic ground state plus one phonon state is degenerate with the first excited electronic state plus the zero-phonon state at some value of the confinement length. Similarly, the electronic ground state plus one phonon state is also degenerate with the second excited electronic state plus the zero-phonon state at a larger value of the confinement length. It is shown that the electron-phonon interaction lifts these degeneracies and as a result, the excited state energy levels bend downward and get pinned to the ground state plus one phonon state as the confinement frequency is increased. Our calculations are finally applied to GaAs and InSb quantum dots.
Quantum optics with single nanowire quantum dots
NASA Astrophysics Data System (ADS)
Akopian, N.; van Weert, M.; van Kouwen, M.; Algra, R.; Liu, L.; Patriarche, G.; Harmand, J.-C.; Bakkers, E.; Kouwenhoven, L.; Zwiller, V.
2010-01-01
In this paper we present our recent developments in control and manipulation of individual spins and photons in a single nanowire quantum dot. Specific examples include demonstration of optical excitation of single spin states, charge tunable quantum devices and single photon sources. We will also discuss our recent discovery of a new type of charge confinement - crystal phase quantum dots. They are formed from the same material with different crystal structure, and today can only be realized in nanowires.
Presto, Jorge Michael M; Prieto, Elizabeth Ann P; Omambac, Karim M; Afalla, Jessica Pauline C; Lumantas, Deborah Anne O; Salvador, Arnel A; Somintac, Armando S; Estacio, Elmer S; Yamamoto, Kohji; Tani, Masahiko
2015-06-01
We present experimental demonstration of photocarrier dynamics in InAs quantum dots (QDs) via terahertz (THz) time-domain spectroscopy (TDS) using two excitation wavelengths and observing the magnetic field polarity characteristics of the THz signal. The InAs QDs was grown using standard Stranski-Krastanow technique on semi-insulating GaAs substrate. Excitation pump at 800 nm- and 910 nm-wavelength were used to distinguish THz emission from the InAs/GaAs matrix and InAs respectively. THz-TDS at 800 nm pump revealed intense THz emission comparable to a bulk p-InAs. For 910 nm pump, the THz emission generally weakened and upon applying external magnetic field of opposite polarities, the THz time-domain plot exhibited anomalous phase-shifting. This was attributed to the possible current-surge associated with the permanent dipole in the QD. PMID:26072813
Zhu, Nan; Zheng, Kaibo; Karki, Khadga J; Abdellah, Mohamed; Zhu, Qiushi; Carlson, Stefan; Haase, Dörthe; Žídek, Karel; Ulstrup, Jens; Canton, Sophie E; Pullerits, Tõnu; Chi, Qijin
2015-01-01
Quantum dots (QDs) and graphene are both promising materials for the development of new-generation optoelectronic devices. Towards this end, synergic assembly of these two building blocks is a key step but remains a challenge. Here, we show a one-step strategy for organizing QDs in a graphene matrix via interfacial self-assembly, leading to the formation of sandwiched hybrid QD-graphene nanofilms. We have explored structural features, electron transfer kinetics and photocurrent generation capacity of such hybrid nanofilms using a wide variety of advanced techniques. Graphene nanosheets interlink QDs and significantly improve electronic coupling, resulting in fast electron transfer from photoexcited QDs to graphene with a rate constant of 1.3?×?10(9) s(-1). Efficient electron transfer dramatically enhances photocurrent generation in a liquid-junction QD-sensitized solar cell where the hybrid nanofilm acts as a photoanode. We thereby demonstrate a cost-effective method to construct large-area QD-graphene hybrid nanofilms with straightforward scale-up potential for optoelectronic applications. PMID:25996307
2011-01-01
A method to determine the effects of the geometry and lateral ordering on the electronic properties of an array of one-dimensional self-assembled quantum dots is discussed. A model that takes into account the valence-band anisotropic effective masses and strain effects must be used to describe the behavior of the photoluminescence emission, proposed as a clean tool for the characterization of dot anisotropy and/or inter-dot coupling. Under special growth conditions, such as substrate temperature and Arsenic background, 1D chains of In0.4Ga0.6 As quantum dots were grown by molecular beam epitaxy. Grazing-incidence X-ray diffraction measurements directly evidence the strong strain anisotropy due to the formation of quantum dot chains, probed by polarization-resolved low-temperature photoluminescence. The results are in fair good agreement with the proposed model.
Troncale, V; Karlsson, K F; Kapon, E
2010-07-16
The engineering of the three-dimensional (3D) heterostructure potential in GaAs/AlGaAs pyramidal quantum dot-in-dots (DiDs) provides control over the valence band symmetry and hence on the polarization of the emitted photons. We propose a technique for dynamic switching of hole character and photon polarization in DiDs by means of an applied electric field. The structural parameters required for producing this effect are discussed. Asymmetric DiDs are found to be particularly suitable for obtaining switching with fields smaller than 1 kV cm( - 1). The proposed device enables generation of single photons with dynamic control on the photon polarization, with potential applications in quantum information technology. PMID:20562488
NASA Astrophysics Data System (ADS)
Rezaei, G.; Shojaeian Kish, S.
2012-08-01
Based on the effective-mass approximation, simultaneous effects of external electric and magnetic fields, hydrostatic pressure and temperature on the binding energy of a hydrogenic donor impurity confined in a two-dimensional parabolic quantum dot have been studied, using the direct matrix diagonalization method. The binding energy dependencies upon the external electric field strength and direction, magnetic field, hydrostatic pressure and temperature are reported. The results reveal that the external electric and magnetic fields, hydrostatic pressure and temperature have a great influence on the impurity binding energy. Moreover, an increase in the tilt angle reduces the binding energy, especially at weak confinement regimes.
Electron localization in graphene quantum dots
Prabath Hewageegana; Vadym Apalkov
2008-01-01
We study theoretically a localized state of an electron in a graphene quantum dot with a sharp boundary. Due to Klein's tunneling, the ``relativistic'' electron in graphene cannot be localized by any confinement potential. In this case the electronic states in a graphene quantum dot become resonances with finite trapping time. We consider these resonances as the states with complex
Electron localization in graphene quantum dots
Vadym Apalkov; Prabath Hewageegana
2009-01-01
We study theoretically a localized state of an electron in a graphene quantum dot with a sharp boundary. Due to Klein's tunneling, the relativistic electron in graphene cannot be localized by a confinement potential. In this case electron states in a graphene quantum dot become resonances with finite trapping time. We consider these resonances as the states with complex energy.
Tunneling current through a quantum dot array
David M.-T. Kuo; G. Y. Guo; Yia-Chung Chang
2001-01-01
The tunneling current through a quantum dot array (QDA) is studied theoretically. Strong electron correlation effect is taken into account in the QDA in which the quantum dots provide a strong three-dimensional confinement effect. A mixed Hubbard and Anderson model is used to simulate the system. It is found that Coulomb charging splits the band resulting from interdot coupling into
NASA Astrophysics Data System (ADS)
Nakamura, Yoshiaki; Masada, Akiko; Ichikawa, Masakazu
2007-07-01
The authors observed a quantum-confinement effect in individual Ge1-xSnx quantum dots (QDs) on Si (111) substrates covered with ultrathin SiO2 films using scanning tunneling spectroscopy at room temperature. The quantum-confinement effect was featured by an increase in the energy band gap of ˜1.5eV with a decrease in QD diameter from 35to4nm. The peaks for quantum levels of QDs became broader with a decrease in the height-diameter aspect ratio of QDs, demonstrating the gradual emergence of two dimensionality in density of states of quasi zero-dimensional QDs with the QD flattening.
Kosemura, Daisuke, E-mail: d-kose@isc.meiji.ac.jp; Mizukami, Yuki; Takei, Munehisa; Numasawa, Yohichiroh; Ogura, Atsushi [School of Science and Technology, Meiji University, Kawasaki 214-8571 (Japan)] [School of Science and Technology, Meiji University, Kawasaki 214-8571 (Japan); Ohshita, Yoshio [Toyota Technological Institute, Nagoya 468-8511 (Japan)] [Toyota Technological Institute, Nagoya 468-8511 (Japan)
2014-01-15
100-nm-thick nanocrystalline silicon (nano-Si)-dot multi-layers on a Si substrate were fabricated by the sequential repetition of H-plasma surface treatment, chemical vapor deposition, and surface oxidation, for over 120 times. The diameter of the nano-Si dots was 5–6 nm, as confirmed by both the transmission electron microscopy and X-ray diffraction analysis. The annealing process was important to improve the crystallinity of the nano-Si dot. We investigated quantum confinement effects by Raman spectroscopy and photoluminescence (PL) measurements. Based on the experimental results, we simulated the Raman spectrum using a phenomenological model. Consequently, the strain induced in the nano-Si dots was estimated by comparing the experimental and simulated results. Taking the estimated strain value into consideration, the band gap modulation was measured, and the diameter of the nano-Si dots was calculated to be 5.6 nm by using PL. The relaxation of the q ? 0 selection rule model for the nano-Si dots is believed to be important to explain both the phenomena of peak broadening on the low-wavenumber side observed in Raman spectra and the blue shift observed in PL measurements.
Ultrasmall silicon quantum dots
NASA Astrophysics Data System (ADS)
Zwanenburg, F. A.; van Loon, A. A.; Steele, G. A.; van Rijmenam, C. E. W. M.; Balder, T.; Fang, Y.; Lieber, C. M.; Kouwenhoven, L. P.
2009-06-01
We report the realization of extremely small single quantum dots in p-type silicon nanowires, defined by Schottky tunnel barriers with Ni and NiSi contacts. Despite their ultrasmall size the NiSi-Si-NiSi nanowire quantum dots readily allow spectroscopy of at least ten consecutive holes, and additionally they display a pronounced excited-state spectrum. The Si channel lengths are visible in scanning electron microscopy images and match the dimensions predicted by a model based on the Poisson equation. The smallest dots (<12 nm) allow identification of the last charge and thus the creation of a single-charge quantum dot.
Vukmirovic, Nenad; Wang, Lin-Wang
2009-11-10
This review covers the description of the methodologies typically used for the calculation of the electronic structure of self-assembled and colloidal quantum dots. These are illustrated by the results of their application to a selected set of physical effects in quantum dots.
Substrate Modulated Graphene Quantum Dot
Qiong Ma; Zhi-Rong Lin; Tao Tu; Guang-Can Guo; Guo-Ping Guo
2009-01-01
We propose a new method to use gapped graphene as barrier to confine electrons in gapless graphene and form a good quantum dot, which can be realized on an oxygen-terminated $SiO_{2}$ substrate partly H-passivated. In particular, we use ferromagnetic insulators deposited on top of barrier which give rise to a spin related energy spectrum and transport properties. Compared to the
NASA Technical Reports Server (NTRS)
Raffaelle, Ryne P.; Castro, Stephanie L.; Hepp, Aloysius; Bailey, Sheila G.
2002-01-01
We have been investigating the synthesis of quantum dots of CdSe, CuInS2, and CuInSe2 for use in an intermediate bandgap solar cell. We have prepared a variety of quantum dots using the typical organometallic synthesis routes pioneered by Bawendi, et. al., in the early 1990's. However, unlike previous work in this area we have also utilized single-source precursor molecules in the synthesis process. We will present XRD, TEM, SEM and EDS characterization of our initial attempts at fabricating these quantum dots. Investigation of the size distributions of these nanoparticles via laser light scattering and scanning electron microscopy will be presented. Theoretical estimates on appropriate quantum dot composition, size, and inter-dot spacing along with potential scenarios for solar cell fabrication will be discussed.
Quantum-dot supercrystals for future nanophotonics
Baimuratov, Anvar S.; Rukhlenko, Ivan D.; Turkov, Vadim K.; Baranov, Alexander V.; Fedorov, Anatoly V.
2013-01-01
The study of supercrystals made of periodically arranged semiconductor quantum dots is essential for the advancement of emerging nanophotonics technologies. By combining the strong spatial confinement of elementary excitations inside quantum dots and exceptional design flexibility, quantum-dot supercrystals provide broad opportunities for engineering desired optical responses and developing superior light manipulation techniques on the nanoscale. Here we suggest tailoring the energy spectrum and wave functions of the supercrystals' collective excitations through the variation of different structural and material parameters. In particular, by calculating the excitonic spectra of quantum dots assembled in two-dimensional Bravais lattices we demonstrate a wide variety of spectrum transformation scenarios upon alterations in the quantum dot arrangement. This feature offers unprecedented control over the supercrystal's electromagnetic properties and enables the development of new nanophotonics materials and devices.
Optical Properties of Semiconductor Quantum Dots
U. Perinetti
2011-01-01
This thesis presents different optical experiments performed on semiconductor quantum dots. These structures allow to confine a small number of electrons and holes to a tiny region of space, some nm across. The aim of this work was to study the basic properties of different types of quantum dots made of various materials and with different techniques.\\u000a\\u000aFirst we studied
Tunneling current through a quantum dot array
NASA Astrophysics Data System (ADS)
Kuo, David M.-T.; Guo, G. Y.; Chang, Yia-Chung
2001-12-01
The tunneling current through a quantum dot array (QDA) is studied theoretically. Strong electron correlation effect is taken into account in the QDA in which the quantum dots provide a strong three-dimensional confinement effect. A mixed Hubbard and Anderson model is used to simulate the system. It is found that Coulomb charging splits the band resulting from interdot coupling into two subbands. The tunneling current is thus influenced significantly by both Coulomb charging and interdot coupling.
Magneto-transport of graphene quantum dots
Kuei-Lin Chiu; Charles Smith; Malcolm Connolly; Simon Chorley; Jonathan Griffiths
2011-01-01
Graphene nanostructures continue to attract attention due to their customizable electronic properties and compatability with existing semiconductor device processing. The promise of long spin relaxation times makes graphene quantum dots - small islands of confined charge - particularly suited to quantum computing architectures that manipulate the spin degree of freedom. In order to probe the spin and charge dynamics of
Graphene quantum dots embedded in hexagonal boron nitride sheets
NASA Astrophysics Data System (ADS)
Li, Junwen; Shenoy, Vivek B.
2011-01-01
We have carried out first-principles calculations on electronic properties of graphene quantum dots embedded in hexagonal boron nitride monolayer sheets. The calculations with density functional theory show that the band gaps of quantum dots are determined by the quantum confinement effects and the hybridization of ? orbitals from B, N, and C atoms. The energy states near the Fermi level are found to be strongly localized within and in the vicinity of the quantum dots.
Graphene quantum dots embedded in hexagonal boron nitride sheets
Junwen Li; Vivek B. Shenoy
2011-01-01
We have carried out first-principles calculations on electronic properties of graphene quantum dots embedded in hexagonal boron nitride monolayer sheets. The calculations with density functional theory show that the band gaps of quantum dots are determined by the quantum confinement effects and the hybridization of pi orbitals from B, N, and C atoms. The energy states near the Fermi level
Numerical simulation of a laterally confined double dot with tunable interaction potential
Finck, Aaron David Kiyoshi
2005-01-01
Recent technological advances have allowed for the construction of small (on the order of 100-1000 nm) systems of confined electrons called quantum dots. Often kept within semiconductor heterostructures, these systems are ...
Das Arulsamy, A. [School of Physics, University of Sydney, Sydney, New South Wales 2006 (Australia); Rider, A. E. [School of Physics, University of Sydney, Sydney, New South Wales 2006 (Australia); CSIRO Materials Science and Engineering, P.O. Box 218, Lindfield, New South Wales 2070 (Australia); Cheng, Q. J.; Ostrikov, K. [CSIRO Materials Science and Engineering, P.O. Box 218, Lindfield, New South Wales 2070 (Australia); School of Physics, University of Sydney, Sydney, New South Wales 2006 (Australia); Xu, S. [Plasma Sources and Applications Center, NIE, Nanyang Technological University, 1 Nanyang Walk, Singapore 637616 (Singapore)
2009-05-01
A high level of control over quantum dot (QD) properties such as size and composition during fabrication is required to precisely tune the eventual electronic properties of the QD. Nanoscale synthesis efforts and theoretical studies of electronic properties are traditionally treated quite separately. In this paper, a combinatorial approach has been taken to relate the process synthesis parameters and the electron confinement properties of the QDs. First, hybrid numerical calculations with different influx parameters for Si{sub 1-x}C{sub x} QDs were carried out to simulate the changes in carbon content x and size. Second, the ionization energy theory was applied to understand the electronic properties of Si{sub 1-x}C{sub x} QDs. Third, stoichiometric (x=0.5) silicon carbide QDs were grown by means of inductively coupled plasma-assisted rf magnetron sputtering. Finally, the effect of QD size and elemental composition were then incorporated in the ionization energy theory to explain the evolution of the Si{sub 1-x}C{sub x} photoluminescence spectra. These results are important for the development of deterministic synthesis approaches of self-assembled nanoscale quantum confinement structures.
Pejova, Biljana, E-mail: biljana@pmf.ukim.mk
2014-05-01
Raman scattering in combination with optical spectroscopy and structural studies by X-ray diffraction was employed to investigate the phonon confinement and strain-induced effects in 3D assemblies of variable-size zincblende ZnSe quantum dots close packed in thin film form. Nanostructured thin films were synthesized by colloidal chemical approach, while tuning of the nanocrystal size was enabled by post-deposition thermal annealing treatment. In-depth insights into the factors governing the observed trends of the position and half-width of the 1LO band as a function of the average QD size were gained. The overall shifts in the position of 1LO band were found to result from an intricate compromise between the influence of phonon confinement and lattice strain-induced effects. Both contributions were quantitatively and exactly modeled. Accurate assignments of the bands due to surface optical (SO) modes as well as of the theoretically forbidden transverse optical (TO) modes were provided, on the basis of reliable physical models (such as the dielectric continuum model of Ruppin and Englman). The size-dependence of the ratio of intensities of the TO and LO modes was studied and discussed as well. Relaxation time characterizing the phonon decay processes in as-deposited samples was found to be approximately 0.38 ps, while upon post-deposition annealing already at 200 °C it increases to about 0.50 ps. Both of these values are, however, significantly smaller than those characteristic for a macrocrystalline ZnSe sample. - Graphical abstract: Optical phonons in nanostructured thin films composed by zincblende zinc selenide quantum dots in strong size-quantization regime: competition between phonon confinement and strain-related effects. - Highlights: • Phonon confinement vs. strain-induced effects in ZnSe 3D QD assemblies were studied. • Shifts of the 1LO band result from an intricate compromise between the two effects. • SO and theoretically forbidden TO modes were accurately assigned. • Phonon relaxation time in as-deposited and annealed samples is 0.38 ps and 0.50 ps. • Both values are significantly smaller than in macrocrystalline ZnSe sample.
Pai, Yi-Hao; Lin, Gong-Ru
2011-01-17
By depositing Si-rich SiOx nano-rod in nano-porous anodic aluminum oxide (AAO) membrane using PECVD, the spatially confined synthesis of Si quantum-dots (Si-QDs) with ultra-bright photoluminescence spectra are demonstrated after low-temperature annealing. Spatially confined SiOx nano-rod in nano-porous AAO membrane greatly increases the density of nucleated positions for Si-QD precursors, which essentially impedes the route of thermally diffused Si atoms and confines the degree of atomic self-aggregation. The diffusion controlled growth mechanism is employed to determine the activation energy of 6.284 kJ mole(-1) and diffusion length of 2.84 nm for SiO1.5 nano-rod in nano-porous AAO membrane. HRTEM results verify that the reduced geometric dimension of the SiOx host matrix effectively constrain the buried Si-QD size at even lower annealing temperature. The spatially confined synthesis of Si-QD essentially contributes the intense PL with its spectral linewidth shrinking from 210 to 140 nm and its peak intensity enhancing by two orders of magnitude, corresponding to the reduction on both the average Si-QD size and its standard deviation from 2.6 to 2.0 nm and from 25% to 12.5%, respectively. The red-shifted PL wavelength of the Si-QD reveals an inverse exponential trend with increasing temperature of annealing, which is in good agree with the Si-QD size simulation via the atomic diffusion theory. PMID:21263629
NASA Astrophysics Data System (ADS)
Prabhakar, Sanjay; Raynolds, James
2009-03-01
Among recent proposals for next-generation, non-charge-based logic is the notion that a single electron can be trapped and its spin can be manipulated through the application of gate voltages (Rev. Mod. Phys.79, 1217 (2007)). In this talk we present numerical simulations of Berry Phase of electron spins in single electron devices for realistic asymmetric confining potentials in support of experimental work at the University at Albany, State University of New York aimed at the practical development of post-CMOS concepts and devices. We solve the Schr"odinger equation including spin-orbit effects using a numerical finite-element based technique. We will discuss the calculation of Berry Phase for electrons (Phys. Rev. B 73, 125330 (2006)) in electrostatically defined quantum dots including the Rashba and Dresselhaus spin-orbit interactions computed numerically from realistic asymmetric confining potentials. The new simulation results open the possibility of spin manipulation through the gate induced Berry phase. This work is supported through funding from the DARPA/NRI INDEX center.
Ahmed, Ghada H; Aly, Shawkat M; Usman, Anwar; Eita, Mohamed S; Melnikov, Vasily A; Mohammed, Omar F
2015-05-11
Here, we report a ground-state interaction between the positively charged cationic porphyrin and the negatively charged carboxylate groups of the thiol ligands on the surface of CdTe quantum dots (QDs), leading to the formation of a stable nanoassembly between the two components. Our time-resolved data clearly demonstrate that we can dramatically tune the intersystem crossing (ISC) and the triplet state lifetime of porphyrin by changing the size of the QDs in the nanoassembly. PMID:25846495
Phonon bottleneck effects in rectangular graphene quantum dots
Jun Qian; A. S. Michael; Mitra Dutta
2010-01-01
This paper considers dimensionally-confined graphene quantum dots, provides formulations for the electronic states and the optical phonons in these quantum dots. These results are used to calculate scattering rates for the optical deformation potential and phonon bottleneck effects are identified.
A. J Nozik
2002-01-01
Quantum dot (QD) solar cells have the potential to increase the maximum attainable thermodynamic conversion efficiency of solar photon conversion up to about 66% by utilizing hot photogenerated carriers to produce higher photovoltages or higher photocurrents. The former effect is based on miniband transport and collection of hot carriers in QD array photoelectrodes before they relax to the band edges
Strathclyde, University of
Nanoparticles & Quantum Dots in the SEM Paul Edwards & Robert Martin #12;Outline & Acknowledgments, David Flint & Nial Wheate (SIPBS); David Stirling (UWS) · Gold nanorods Yinan Zhang & Yu Chen (Physics Society: S.Brown, P.Nativo, J.-A.Smith, D.Stirling, P.R.Edwards, D.J.Flint, D.Graham & N
Infrared Quantum Dot Intersubband
Krishna, Sanjay
, targeting and tracking, medical diagnosis, law enforcement, environmental monitoring, and space science [1Infrared Quantum Dot Intersubband Photodetectors Are a Promising Technology for Multiwavelength IR, or QDIP. Recent advances in the epitaxial growth of strained heterostructures, such as Ga(In)As on Ga
Colloidal quantum dot photovoltaics
Susanna M. Thon; Edward H. Sargent
2011-01-01
Colloidal quantum dot solar cells offer the possibility of combining low-cost, low-temperature solution-processing with efficient photon harvesting over the entire solar spectrum. Their quantum size effect tunability offers a path to tandem and triple-junction cells. The first solution-processed infrared solar cells were reported in 2005; the latest devices offer greater than 5% AM1.5 PCE and many paths remain for further
NASA Astrophysics Data System (ADS)
Zhuo, Ning; Liu, Feng Qi; Zhang, Jin Chuan; Wang, Li Jun; Liu, Jun Qi; Zhai, Shen Qiang; Wang, Zhan Guo
2014-03-01
We demonstrated an unambiguous quantum dot cascade laser based on InGaAs/GaAs/InAs/InAlAs heterostructure by making use of self-assembled quantum dots in the Stranski-Krastanow growth mode and two-step strain compensation active region design. The prototype generates stimulated emission at ? ~ 6.15 ?m and a broad electroluminescence band with full width at half maximum over 3 ?m. The characteristic temperature for the threshold current density within the temperature range of 82 to 162 K is up to 400 K. Moreover, our materials show the strong perpendicular mid-infrared response at about 1,900 cm-1. These results are very promising for extending the present laser concept to terahertz quantum cascade laser, which would lead to room temperature operation.
2014-01-01
We demonstrated an unambiguous quantum dot cascade laser based on InGaAs/GaAs/InAs/InAlAs heterostructure by making use of self-assembled quantum dots in the Stranski-Krastanow growth mode and two-step strain compensation active region design. The prototype generates stimulated emission at ??~?6.15 ?m and a broad electroluminescence band with full width at half maximum over 3 ?m. The characteristic temperature for the threshold current density within the temperature range of 82 to 162 K is up to 400 K. Moreover, our materials show the strong perpendicular mid-infrared response at about 1,900 cm-1. These results are very promising for extending the present laser concept to terahertz quantum cascade laser, which would lead to room temperature operation. PACS 42.55.Px; 78.55.Cr; 78.67.Hc PMID:24666965
Chaotic Dirac billiard in graphene quantum dots.
Ponomarenko, L A; Schedin, F; Katsnelson, M I; Yang, R; Hill, E W; Novoselov, K S; Geim, A K
2008-04-18
The exceptional electronic properties of graphene, with its charge carriers mimicking relativistic quantum particles and its formidable potential in various applications, have ensured a rapid growth of interest in this new material. We report on electron transport in quantum dot devices carved entirely from graphene. At large sizes (>100 nanometers), they behave as conventional single-electron transistors, exhibiting periodic Coulomb blockade peaks. For quantum dots smaller than 100 nanometers, the peaks become strongly nonperiodic, indicating a major contribution of quantum confinement. Random peak spacing and its statistics are well described by the theory of chaotic neutrino billiards. Short constrictions of only a few nanometers in width remain conductive and reveal a confinement gap of up to 0.5 electron volt, demonstrating the possibility of molecular-scale electronics based on graphene. PMID:18420930
Probing relaxation times in graphene quantum dots
Volk, Christian; Neumann, Christoph; Kazarski, Sebastian; Fringes, Stefan; Engels, Stephan; Haupt, Federica; Müller, André; Stampfer, Christoph
2013-01-01
Graphene quantum dots are attractive candidates for solid-state quantum bits. In fact, the predicted weak spin-orbit and hyperfine interaction promise spin qubits with long coherence times. Graphene quantum dots have been extensively investigated with respect to their excitation spectrum, spin-filling sequence and electron-hole crossover. However, their relaxation dynamics remain largely unexplored. This is mainly due to challenges in device fabrication, in particular concerning the control of carrier confinement and the tunability of the tunnelling barriers, both crucial to experimentally investigate decoherence times. Here we report pulsed-gate transient current spectroscopy and relaxation time measurements of excited states in graphene quantum dots. This is achieved by an advanced device design that allows to individually tune the tunnelling barriers down to the low megahertz regime, while monitoring their asymmetry. Measuring transient currents through electronic excited states, we estimate a lower bound for charge relaxation times on the order of 60–100?ns. PMID:23612294
A voltage tunable quantum dot photodetector for terahertz detection
Wei Wu; Dibyendu Dey; Hooman Mohseni
2010-01-01
A voltage tunable quantum dot (QD) photodetector for terahertz detection based on intersublevel transitions is proposed. The intersublevels are formed by the lateral electrical confinement applied on quantum wells and the transitions between them can be strongly tuned by the confinement. Under normal incidence, the peak detection wavelengths can be tuned from ~50 to ~90 µm (6.0 to ~3.3 THz)
Graphene quantum dots: Beyond a Dirac billiard
Florian Libisch; Christoph Stampfer; Joachim Burgdörfer
2009-01-01
We present realistic simulations of quantum confinement effects in phase-coherent graphene quantum dots with linear dimensions of 10-40 nm. We determine wave functions and energy-level statistics in the presence of disorder resulting from edge roughness, charge impurities, or short-ranged scatterers. Marked deviations from a simple Dirac billiard for massless fermions are found. We find a remarkably stable dependence of the
NASA Astrophysics Data System (ADS)
Taylor, Robert A.
2010-09-01
These conference proceedings contain the written papers of the contributions presented at Quantum Dot 2010 (QD2010). The conference was held in Nottingham, UK, on 26-30 April 2010. The conference addressed topics in research on: 1. Epitaxial quantum dots (including self-assembled and interface structures, dots defined by electrostatic gates etc): optical properties and electron transport quantum coherence effects spin phenomena optics of dots in cavities interaction with surface plasmons in metal/semiconductor structures opto-electronics applications 2. Novel QD structures: fabrication and physics of graphene dots, dots in nano-wires etc 3. Colloidal quantum dots: growth (shape control and hybrid nanocrystals such as metal/semiconductor, magnetic/semiconductor) assembly and surface functionalisation optical properties and spin dynamics electrical and magnetic properties applications (light emitting devices and solar cells, biological and medical applications, data storage, assemblers) The Editors Acknowledgements Conference Organising Committee: Maurice Skolnick (Chair) Alexander Tartakovskii (Programme Chair) Pavlos Lagoudakis (Programme Chair) Max Migliorato (Conference Secretary) Paola Borri (Publicity) Robert Taylor (Proceedings) Manus Hayne (Treasurer) Ray Murray (Sponsorship) Mohamed Henini (Local Organiser) International Advisory Committee: Yasuhiko Arakawa (Tokyo University, Japan) Manfred Bayer (Dortmund University, Germany) Sergey Gaponenko (Stepanov Institute of Physics, Minsk, Belarus) Pawel Hawrylak (NRC, Ottawa, Canada) Fritz Henneberger (Institute for Physics, Berlin, Germany) Atac Imamoglu (ETH, Zurich, Switzerland) Paul Koenraad (TU Eindhoven, Nethehrlands) Guglielmo Lanzani (Politecnico di Milano, Italy) Jungil Lee (Korea Institute of Science and Technology, Korea) Henri Mariette (CNRS-CEA, Grenoble, France) Lu Jeu Sham (San Diego, USA) Andrew Shields (Toshiba Research Europe, Cambridge, UK) Yoshihisa Yamamoto (Stanford University, USA) Artur Zrenner (Paderborn University, Germany) International Programme Committee: Alexander Eychmüller (TU Dresden, Germany) Jonathan Finley (TU Munich, Germany) Dan Gammon (NRL, Washington, USA) Alexander Govorov (Ohio University, USA) Neil Greenham (Cavendish Laboratory, UK) Vladimir Korenev (Ioffe Institute, Russia) Leo Kouwenhoven (TU Delft, Netherlands) Wolfgang Langbein (Cardiff University, UK) Xavier Marie (CNRS Toulouse, France) David Ritchie (Cambridge, UK) Andrew Sachrajda (IMS, Ottawa, Canada) Katerina Soulantica (University of Toulouse, France) Seigo Tarucha (University of Tokyo, Japan) Carlos Tejedor (UAM, Madrid, Spain) Euijoon Yoon (Seoul National University, Korea) Ulrike Woggon (Tu Berlin, Germany) Proceedings edited and compiled by Profesor Robert A Taylor, University of Oxford
NSDL National Science Digital Library
2014-09-18
Students are introduced to the physical concept of the colors of rainbows as light energy in the form of waves with distinct wavelengths, but in a different manner than traditional kaleidoscopes. Looking at different quantum dot solutions, they make observations and measurements, and graph their data. They come to understand how nanoparticles interact with absorbing photons to produce colors. They learn the dependence of particle size and color wavelength and learn about real-world applications for using these colorful liquids.
Vertical transport and tunnelling through squeezable quantum dots
NASA Astrophysics Data System (ADS)
Blanc, N.; Guéret, P.; Germann, R.; Rothuizen, H.
1993-06-01
Resonant tunnelling through zero-dimensional states in a laterally confined double-barrier quantum well heterostructure has been investigated. The lateral confinement is provided by a Schottky gate which allows continuous squeezing of the current channel to diameters smaller than 100 nm. The conductance data on submicron dots exhibit well-resolved peaks, the position of which depends on both channel diameter and applied magneticfield. The measured displacement of the conductance peaks with decreasing channel diameter provides direct evidence of quantum confinement in the dot. In addition magnetotunneling measurements lend experimental support to the observation of zero-dimensional states with positive and negative azimuthal quantum number ?.
NASA Astrophysics Data System (ADS)
Bhargava, Rameshwar
2001-03-01
When the size of Tb3+ doped Y2O3 nanoparticles is decreased from about 10 nm to 3 nm, the luminescent efficiency of Tb3+ green-emission (f-f transition) is increased by an order of magnitude1. This enhancement of oscillator strength is interpreted due to modulation of excited-states of the Tb3+ ion, brought about by the confinement-boundary of the Y2O3 nanocrystal2. This high efficiency of 'caged' single Tb3+ or Eu3+ ion in Y2O3 has led to the first observation of an atomic blinking3. The Quantum Confined Atoms (QCA) provide a novel way to modulate the luminescent properties of a single activator atom via quantum confinement of higher excited states. This QCA-effect occurs when the size of the host is about 5 to 10 times the size of the radius of the excited-state atom. This discovery is expected to impact many applications in the area of nanotechnology. 1. R.N Bhargava, V. Chhabra, B. Kulkarni and J.V. Veliadis Phys. stat. sol. (b) 210, 621 (1998) 2. R.N. Bhargava Jour. of Crystal Growth 214, 926 (2000) 3. M. Barnes, A. Mehta, T. Thundant, R.N. Bhargava, V. Chhabra, B .Kulkarni Jour. Chem. Phys. B 104, 6099 (2000)
Clinical Potential of Quantum Dots
Iga, Arthur M.; Robertson, John H. P.; Winslet, Marc C.; Seifalian, Alexander M.
2007-01-01
Advances in nanotechnology have led to the development of novel fluorescent probes called quantum dots. Quantum dots have revolutionalized the processes of tagging molecules within research settings and are improving sentinel lymph node mapping and identification in vivo studies. As the unique physical and chemical properties of these fluorescent probes are being unraveled, new potential methods of early cancer detection, rapid spread and therapeutic management, that is, photodynamic therapy are being explored. Encouraging results of optical and real time identification of sentinel lymph nodes and lymph flow using quantum dots in vivo models are emerging. Quantum dots have also superseded many of the limitations of organic fluorophores and are a promising alternative as a research tool. In this review, we examine the promising clinical potential of quantum dots, their hindrances for clinical use and the current progress in abrogating their inherent toxicity. PMID:18317518
Quantum dot micropatterning on si.
Lambert, K; Moreels, I; Thourhout, D Van; Hens, Z
2008-06-01
Using InP and PbSe quantum dots, we demonstrate that the Langmuir-Blodgett technique is well-suited to coat nonflat surfaces with quantum dot monolayers. This allows deposition on silicon substrates covered by a developed patterned resist, which results in monolayer patterns with micrometer resolution. Atomic force microscopy and scanning electron microscopy reveal the formation of a densely packed monolayer that replicates predefined structures with high selectivity after photoresist removal. A large variety of shapes can be reproduced and, due to the excellent adhesion of the quantum dots to the substrate, the hybrid approach can be repeated on the same substrate. This final possibility leads to complex, large-area quantum dot monolayer structures with micrometer spatial resolution that may combine different types of quantum dots. PMID:18442279
Blanton, Christopher J; Chakraborty, Arindam
2012-01-01
The effect of external electric field on electron-hole correlation in GaAs quantum dots is investigated. The electron-hole Schrodinger equation in the presence of external electric field is solved using explicitly correlated full configuration interaction (XCFCI) method and accurate exciton binding energy and electron-hole recombination probability are obtained. The effect of the electric field was included in the 1-particle single component basis functions by performing variational polaron transformation. The quality of the wavefunction at small inter-particle distances was improved by using Gaussian-type geminal function that depended explicitly on the electron-hole separation distance. The parameters of the explicitly correlated function were determined variationally at each field strength. The scaling of total exciton energy, exciton binding energy, and electron-hole recombination probability with respect to the strength of the electric field was investigated. It was found that a 500 kV/cm change in elect...
Quantum confined nanocrystalline silicon
NASA Astrophysics Data System (ADS)
Guan, Tianyuan; Kendrick, Chito; Theingi, San; Bagolini, Luigi; Riskey, Kory; Vitti, Lauren; Klafehn, Grant; Taylor, Craig; Lusk, Mark; Gorman, Brain; Collins, Reuben; Fields, Jeremy; Stradins, Pauls
2014-03-01
Quantum confined (QC) semiconductors have drawn much attention in photovoltaics due to their tunable optoelectronic properties and potential for efficiency improvements. Here, we report a study of nanocrystalline silicon (nc-Si:H), consisting of silicon nano-particles (SiNPs) embedded in hydrogenated amorphous silicon (a-Si:H) matrix. Films were grown by depositing the SiNPs and a-Si:H sequentially from separate plasma reactors in a common deposition chamber. Several characterizations were used to ensure the material had low defect density and that the SiNPs were highly crystalline and well within the QC regime. Optical properties of hybrid SiNP/a-Si:H films were explored using visible to near infrared photoluminescence (PL). At low temperature, PL revealed two primary emission features, one from conventional a-Si:H ~ 1.3 eV and a second peak which can be attributed to recombination in SiNPs. The energy of this peak is higher than the bulk c-Si bandgap (~ 1.2 eV), and with decreasing SiNP size, it increases to ~ 1.7 eV. This quantum confinement effect agrees with Density Functional Theory predictions. In addition, we also see that the PL peak for SiNPs surrounded by a-Si:H shifts to lower energy relative to the isolated SiNPs. This shift is also consistent with the modeling results which show that surrounding SiNPs with a-Si:H leads to a softening of the confinement barrier and a redshift in the optical gap.
Graphene quantum dots formed by a spatial modulation of the Dirac gap
G. Giavaras; Franco Nori
2010-01-01
An electrostatic quantum dot cannot be formed in monolayer graphene because of the Klein tunneling. However, a dot can be formed with the help of a uniform magnetic field. As shown here, a spatial modulation of the Dirac gap leads to confined states with discrete energy levels, thus defining a dot, without applying external electric and magnetic fields. Gap-induced dot
Quantum computation with two-dimensional graphene quantum dots
Li Jie-Sen; Li Zhi-Bing; Yao Dao-Xin
2012-01-01
We study an array of graphene nano sheets that form a two-dimensional S = 1\\/2 Kagome spin lattice used for quantum computation. The edge states of the graphene nano sheets are used to form quantum dots to confine electrons and perform the computation. We propose two schemes of bang-bang control to combat decoherence and realize gate operations on this array
The pinning effect in quantum dots
Monisha, P. J., E-mail: pjmonisha@gmail.com [School of Physics, University of Hyderabad, Hyderabad-500046 (India); Mukhopadhyay, Soma [Department of Physics, D V R College of Engineering and Technology, Hyderabad-502285 (India)
2014-04-24
The pinning effect is studied in a Gaussian quantum dot using the improved Wigner-Brillouin perturbation theory (IWBPT) in the presence of electron-phonon interaction. The electron ground state plus one phonon state is degenerate with the electron in the first excited state. The electron-phonon interaction lifts the degeneracy and the first excited states get pinned to the ground state plus one phonon state as we increase the confinement frequency.
Carrier Dynamics in Colloidal Graphene Quantum Dots
Cheng Sun; Xin Yan; Liang-Shi Li; John A. McGuire
2011-01-01
We describe carrier dynamics for single and multiple excitons in colloidal graphene quantum dots (GQDs). Strong confinement and corresponding size-tunable electronic structure make GQDs potentially useful sensitizers in photovoltaic devices. We have studied the optical response of GQDs consisting of 132 and 168 sp^2 hybridized carbon atoms dissolved in toluene with HOMO-LUMO transitions of 1.4-1.6 eV. From measurements of ultrafast
Zeeman Effect in Parabolic Quantum Dots
R. Rinaldi; P. V. Giugno; R. Cingolani; H. Lipsanen; M. Sopanen; J. Tulkki; J. Ahopelto
1996-01-01
An unprecedentedly well resolved Zeeman effect has been observed when confined carriers moving along a closed mesoscopic path experience an external magnetic field orthogonal to the orbit plane. Large Zeeman splitting of excited higher angular momentum states is observed in the magnetoluminescence spectrum of quantum dots induced by self-organized InP islands on InGaAs\\/GaAs. The measured effect is quantitatively reproduced by
Efficient Quantum Dot-Quantum Dot and Quantum Dot-Dye Energy Transfer in Biotemplated Assemblies
Achermann, Marc; Jeong, Sohee; Balet, Laurent; Montano, Gabriel A.; Hollingsworth, Jennifer A.
2011-01-01
CdSe semiconductor nanocrystal quantum dots are assembled into nanowire-like arrays employing microtubule fibers as nanoscale molecular “scaffolds.” Spectrally and time-resolved energy-transfer analysis is used to assess the assembly of the nanoparticles into the hybrid inorganic-biomolecular structure. Specifically, we demonstrate that a comprehensive study of energy transfer between quantum-dot pairs on the biotemplate, and, alternatively, between quantum dots and molecular dyes embedded in the microtubule scaffold, comprises a powerful spectroscopic tool for evaluating the assembly process. In addition to revealing the extent to which assembly has occurred, the approach allows determination of particle-to-particle (and particle-to-dye) distances within the bio-mediated array. Significantly, the characterization is realized in situ, without need for further sample workup or risk of disturbing the solution-phase constructs. Furthermore, we find that the assemblies prepared in this way exhibit efficient quantum dot-quantum dot and quantum dot-dye energy transfer that affords faster energy-transfer rates compared to densely packed quantum dot arrays on planar substrates and small-molecule-mediated quantum dot/dye couples, respectively. PMID:21314178
Single quantum dot nanowire photodetectors
NASA Astrophysics Data System (ADS)
van Kouwen, M. P.; van Weert, M. H. M.; Reimer, M. E.; Akopian, N.; Perinetti, U.; Algra, R. E.; Bakkers, E. P. A. M.; Kouwenhoven, L. P.; Zwiller, V.
2010-09-01
We report InP nanowire photodetectors with a single InAsP quantum dot as light absorbing element. With excitation above the InP band gap, the nanowire photodetectors are efficient (quantum efficiency of 4%). Under resonant excitation of the quantum dot, the photocurrent amplitude depends on the linear polarization direction of the incident light. The photocurrent is enhanced (suppressed) for a polarization parallel (perpendicular) to the axis of the nanowire (contrast 0.83). The active detection volume under resonant excitation is 7×103 nm3. These results show the promising features of quantum dots embedded in nanowire devices for electrical light detection at high spatial resolution.
Zero-energy states in graphene quantum dots and rings
C. A. Downing; D. A. Stone; M. E. Portnoi
2011-01-01
We present exact analytical zero-energy solutions for a class of smooth-decaying potentials, showing that the full confinement of charge carriers in electrostatic potentials in graphene quantum dots and rings is indeed possible without recourse to magnetic fields. These exact solutions allow us to draw conclusions on the general requirements for the potential to support fully confined states, including a critical
Electron counting in quantum dots
Fominov, Yakov
V] G2[mV] -7.5 -7 -6.5 -6 34 34.5 35 35.5 1 10 100 Counts/s #12;Microwave emission of a QPC · VoltageElectron counting in quantum dots Klaus Ensslin Solid State Physics ·Charge detection in quantum;Time-resolved detection of single electron transport source drain quantum dot kBT Schleser et al., APL
Atypical quantum confinement effect in silicon nanowires.
Sorokin, Pavel B; Avramov, Pavel V; Chernozatonskii, Leonid A; Fedorov, Dmitri G; Ovchinnikov, Sergey G
2008-10-01
The quantum confinement effect (QCE) of linear junctions of silicon icosahedral quantum dots (IQD) and pentagonal nanowires (PNW) was studied using DFT and semiempirical AM1 methods. The formation of complex IQD/PNW structures leads to the localization of the HOMO and LUMO on different parts of the system and to a pronounced blue shift of the band gap; the typical QCE with a monotonic decrease of the band gap upon the system size breaks down. A simple one-electron one-dimensional Schrodinger equation model is proposed for the description and explanation of the unconventional quantum confinement behavior of silicon IQD/PNW systems. On the basis of the theoretical models, the experimentally discovered deviations from the typical QCE for nanocrystalline silicon are explained. PMID:18785695
An optically driven quantum dot quantum computer
G. D. Sanders; K. W. Kim; W. C. Holton
1999-09-22
We propose a quantum computer structure based on coupled asymmetric single-electron quantum dots. Adjacent dots are strongly coupled by means of electric dipole-dipole interactions enabling rapid computation rates. Further, the asymmetric structures can be tailored for a long coherence time. The result maximizes the number of computation cycles prior to loss of coherence.
Multi-million atom electronic structure calculations for quantum dots
Muhammad Usman
2010-01-01
Quantum dots grown by self-assembly process are typically constructed by 50,000 to 5,000,000 structural atoms which confine a small, countable number of extra electrons or holes in a space that is comparable in size to the electron wavelength. Under such conditions quantum dots can be interpreted as artificial atoms with the potential to be custom tailored to new functionality. In
Factorization of the Dirac equation and a graphene quantum dot
NASA Astrophysics Data System (ADS)
Zahidi, Youness; Jellal, Ahmed; Bahlouli, Hocine; El Bouziani, Mohammed
2014-10-01
We consider a quantum dot described using a cylindrically symmetrical 2D Dirac equation. The potentials representing the quantum dot are taken to be of different types of potential configuration, scalar, vector, and pseudo-scalar to enable us to enrich our study. Using various potential configurations, we found that in the presence of a mass term, an electrostatically confined quantum dot can accommodate true bound states, which is in agreement with our previous work. The differential cross section associated with one specific potential configuration has been computed and discussed as a function of the various potential parameters.
Factorization of Dirac Equation and Graphene Quantum Dot
Youness Zahidi; Ahmed Jellal; Hocine Bahlouli; Mohammed El Bouziani
2014-05-14
We consider a quantum dot described by a cylindrically symmetric 2D Dirac equation. The potentials representing the quantum dot are taken to be of different types of potential configuration, scalar, vector and pseudo-scalar to enable us to enrich our study. Using various potential configurations, we found that in the presence of a mass term an electrostatically confined quantum dot can accommodate true bound states, which is in agreement with previous work. The differential cross section associated with one specific potential configuration has been computed and discussed as function of the various potential parameters.
NASA Astrophysics Data System (ADS)
Christina lily Jasmine, P.; John Peter, A.; Lee, Chang Woo
2015-05-01
Electronic and optical properties of a hydrogenic donor impurity in a CdTe/Zn0.2Cd0.8Te/ZnTe core/shell/shell quantum dot are discussed taking into consideration of geometrical confinement effect. The confining potentials on both the sides of the barrier are different and a two parametrical potential of Smorodinsky-Winternitz is considered in this problem. The dielectric mismatch is included in the Hamiltonian. The position dependent effective mass is applied. The electronic properties are studied using variational method and the optical properties are investigated using the density matrix approach. The intersubband optical absorption, the oscillator strength and the radiative life time between ground and the excited states are studied based on the wave functions and the confined energies with and without the impurity with various dot radii. The results show that the absorption wavelength in type-II core and shell semiconducting nanomaterials can be tuned over a wider range of wavelengths by altering their size and the composition.
Nori, Franco
2011-01-01
PHYSICAL REVIEW B 83, 165427 (2011) Dirac gap-induced graphene quantum dot in an electrostatic modulated Dirac gap in a graphene sheet leads to charge confinement, thus enabling a graphene quantum dot the electrostatic potential rises slowly compared to the magnetic vector potential.8,9 Thus a graphene quantum dot
All inorganic colloidal quantum dot LEDs
Wood, Vanessa Claire
2007-01-01
This thesis presents the first colloidal quantum dot light emitting devices (QD-LEDs) with metal oxide charge transport layers. Colloidally synthesized quantum dots (QDs) have shown promise as the active material in ...
NASA Astrophysics Data System (ADS)
Vagov, A.; Croitoru, M. D.; Glässl, M.; Axt, V. M.; Kuhn, T.
2011-03-01
Based on a representation of the functional integral as the time evolution of the augmented density matrix we have worked out an implementation of the real-time path integral approach that is applicable to the dynamics of quantum dissipative systems with superohmic coupling to the environment. As a prototype for such a system we consider a laser-driven strongly confined semiconductor quantum dot coupled to acoustic phonons. First applications of this approach to quantum dot systems have already been published. Here, we provide a detailed description of the implementation, including a discussion of numerical issues and extend the formalism from two-level quantum dot models with a pure-dephasing type carrier-phonon coupling to the case of multiple electronic levels. The method allows for numerically exact calculations of the dot dynamics at strong dot-phonon and dot-laser coupling and at long times, usually inaccessible by other approaches.
Two-electron quantum dots as scalable qubits
J. H. Jefferson; M. Fearn; D. L. Tipton; T. P. Spiller
2002-01-01
We show that two electrons confined in a square semiconductor quantum dot have two isolated low-lying energy eigenstates, which have the potential to form the basis of scalable computing elements (qubits). Initialization, one-qubit and two-qubit universal gates, and readout are performed using electrostatic gates and magnetic fields. Two-qubit transformations are performed via the Coulomb interaction between electrons on adjacent dots.
Two-electron quantum dots as scalable qubits
J. H. Jefferson; M. Fearn; D. L. J. Tipton; T. P. Spiller
2002-01-01
We show that two electrons confined in a square semiconductor quantum dot\\u000ahave two isolated low-lying energy eigenstates, which have the potential to\\u000aform the basis of scalable computing elements (qubits). Initialisation,\\u000aone-qubit and two-qubit universal gates, and readout are performed using\\u000aelectrostatic gates and magnetic fields. Two-qubit transformations are\\u000aperformed via the Coulomb interaction between electrons on adjacent dots.
Density functional calculation of the structural and electronic properties of germanium quantum dots
NASA Astrophysics Data System (ADS)
Anas, M. M.; Gopir, G.
2015-04-01
We apply first principles density functional computational methods to study the structures, densities of states (DOS), and higher occupied molecular orbital (HOMO) - lowest unoccupied molecular orbital (LUMO) gaps of selected free-standing Ge semiconductor quantum dots up to 1.8nm. Our calculations are performed using numerical atomic orbital approach where linear combination of atomic orbital was applied. The surfaces of the quantum dots was passivized by hydrogen atoms. We find that surface passivation does affect the electronic properties associated with the changes of surface state, electron localization, and the energy gaps of germanium nanocrystals as well as the confinement of electrons inside the quantum dots (QDs). Our study shows that the energy gaps of germanium quantum dots decreases with the increasing dot diameter. The size-dependent variations of the computed HOMO-LUMO gaps in our quantum dots model were found to be consistent with the effects of quantum confinement reported in others theoretical and experimental calculation.
Quantum Confined Semiconductors for High Efficiency Photovoltaics
NASA Astrophysics Data System (ADS)
Beard, Matthew
2014-03-01
Semiconductor nanostructures, where at least one dimension is small enough to produce quantum confinement effects, provide new pathways for controlling energy flow and therefore have the potential to increase the efficiency of the primary photon-to-free energy conversion step. In this discussion, I will present the current status of research efforts towards utilizing the unique properties of colloidal quantum dots (NCs confined in three dimensions) in prototype solar cells and demonstrate that these unique systems have the potential to bypass the Shockley-Queisser single-junction limit for solar photon conversion. The solar cells are constructed using a low temperature solution based deposition of PbS or PbSe QDs as the absorber layer. Different chemical treatments of the QD layer are employed in order to obtain good electrical communication while maintaining the quantum-confined properties of the QDs. We have characterized the transport and carrier dynamics using a transient absorption, time-resolved THz, and temperature-dependent photoluminescence. I will discuss the interplay between carrier generation, recombination, and mobility within the QD layers. A unique aspect of our devices is that the QDs exhibit multiple exciton generation with an efficiency that is ~ 2 to 3 times greater than the parental bulk semiconductor.
A colloidal quantum dot spectrometer.
Bao, Jie; Bawendi, Moungi G
2015-07-01
Spectroscopy is carried out in almost every field of science, whenever light interacts with matter. Although sophisticated instruments with impressive performance characteristics are available, much effort continues to be invested in the development of miniaturized, cheap and easy-to-use systems. Current microspectrometer designs mostly use interference filters and interferometric optics that limit their photon efficiency, resolution and spectral range. Here we show that many of these limitations can be overcome by replacing interferometric optics with a two-dimensional absorptive filter array composed of colloidal quantum dots. Instead of measuring different bands of a spectrum individually after introducing temporal or spatial separations with gratings or interference-based narrowband filters, a colloidal quantum dot spectrometer measures a light spectrum based on the wavelength multiplexing principle: multiple spectral bands are encoded and detected simultaneously with one filter and one detector, respectively, with the array format allowing the process to be efficiently repeated many times using different filters with different encoding so that sufficient information is obtained to enable computational reconstruction of the target spectrum. We illustrate the performance of such a quantum dot microspectrometer, made from 195 different types of quantum dots with absorption features that cover a spectral range of 300 nanometres, by measuring shifts in spectral peak positions as small as one nanometre. Given this performance, demonstrable avenues for further improvement, the ease with which quantum dots can be processed and integrated, and their numerous finely tuneable bandgaps that cover a broad spectral range, we expect that quantum dot microspectrometers will be useful in applications where minimizing size, weight, cost and complexity of the spectrometer are critical. PMID:26135449
Tunable quantum dot parametric source.
Andronico, A; Favero, I; Ducci, S; Gérard, J M; Leo, G
2013-09-23
We report on the modeling of an electrically pumped nonlinear source for spontaneous parametric down-conversion in an AlGaAs single-sided Bragg waveguide. Laser emission from InAs quantum dots embedded in the waveguide core is designed to excite a Bragg pump mode at 950 nm. This mode is phase matched with two cross-polarized total-internal-reflection fundamental signal and idler modes around 1900 nm. Besides numerically evaluating the source efficiency, we discuss the crucial role played by the quantum dots in the practical implementation of the phase-matching condition along with the tuning capabilities of this promising active device. PMID:24104126
Electronic levels in self-assembled quantum dots
NASA Astrophysics Data System (ADS)
Cocoletzi, Gregorio H.; López-Bolaños, R.; Ulloa, S. E.
2000-03-01
Self-assembled quantum dots may be produced as a result of deposition of a semiconductor on a substrate with different lattice parameter. Optical and electronic properties of these interesting systems have been extensively studied in recent years. In this work, we explore the confinement effects on the energy levels of electrons in isolated and coupled pairs of InAs quantum dots grown on GaAs, modelled as cylinders. To solve the Schroedinger equation in the effective mass approximation for the coupled dots, we use the superposition of the wave functions of the isolated dots to obtain the energy splitting as a function of the sample structural parameters and the separation between dots.
Graphene quantum dots embedded in hexagonal boron nitride sheets
Junwen Li; Vivek B. Shenoy
2010-01-01
We have carried out first-principles calculations on electronic properties of\\u000agraphene quantum dots embedded in hexagonal boron nitride monolayer sheets. The\\u000acalculations with density functional theory show that the band gaps of quantum\\u000adots are determined by the quantum confinement effects and the hybridization of\\u000a{\\\\pi} orbitals from B, N and C atoms. The energy states near the Fermi level
Charge transfer magnetoexciton formation at vertically coupled quantum dots
2012-01-01
A theoretical investigation is presented on the properties of charge transfer excitons at vertically coupled semiconductor quantum dots in the presence of electric and magnetic fields directed along the growth axis. Such excitons should have two interesting characteristics: an extremely long lifetime and a permanent dipole moment. We show that wave functions and the low-lying energies of charge transfer exciton can be found exactly for a special morphology of quantum dots that provides a parabolic confinement inside the layers. To take into account a difference between confinement potentials of an actual structure and of our exactly solvable model, we use the Galerkin method. The density of energy states is calculated for different InAs/GaAs quantum dots’ dimensions, the separation between layers, and the strength of the electric and magnetic fields. A possibility of a formation of a giant dipolar momentum under external electric field is predicted. PMID:23092373
Deformation potential dominated phonons in ZnS quantum dots
S. Dhara; A. K. Arora; Jay Ghatak; K. H. Chen; C. P. Liu; L. C. Chen; Y. Tzeng; Baldev Raj
2008-07-06
Strong deformation potential (DP) dominated Raman spectra are reported for quantum confined cubic ZnS nanoclusters under off-resonance conditions allowed only in quantum dots. A flurry of zone boundary phonons is demonstrated in the scattering process. Transverse optic (TO) mode in the multi-phonon process shows only even order overtones suggesting the dominance of a two-phonon process (having large DP value in ZnS) and its integral multiples. Two-phonon TO modes corresponding to A1 and B2 symmetries are also demonstrated under off-resonance conditions which are allowed only in quantum dots.
Quasiparticles for a quantum dot array in graphene and the associated magnetoplasmons
NASA Astrophysics Data System (ADS)
Berman, Oleg L.; Gumbs, Godfrey; Echenique, P. M.
2009-02-01
We calculate the low-frequency magnetoplasmon excitation spectrum for a square array of quantum dots on a two-dimensional (2D) graphene layer. The confining potential is linear in the distance from the center of the quantum dot. The electron eigenstates in a magnetic field and confining potential are mapped onto a 2D plane of electron-hole pairs in an effective magnetic field without any confinement. The tight-binding model for the array of quantum dots leads to a wave function with interdot mixing of the quantum numbers associated with an isolated quantum dot. For chosen confinement, magnetic field, wave vector, and frequency, we plot the dispersion equation as a function of the period d of the lattice. We obtain those values of d which yield collective plasma excitations. For the allowed transitions between the valence and conduction bands in our calculations, we obtain plasmons when d?100Å .
Quantum dot quantum cascade infrared photodetector
Wang, Xue-Jiao; Zhai, Shen-Qiang; Zhuo, Ning; Liu, Jun-Qi, E-mail: jqliu@semi.ac.cn, E-mail: fqliu@semi.ac.cn; Liu, Feng-Qi, E-mail: jqliu@semi.ac.cn, E-mail: fqliu@semi.ac.cn; Liu, Shu-Man; Wang, Zhan-Guo [Key Laboratory of Semiconductor Materials Science, Institute of Semiconductors, Chinese Academy of Sciences and Beijing Key Laboratory of Low Dimensional Semiconductor Materials and Devices, P.O. Box 912, Beijing 100083 (China)
2014-04-28
We demonstrate an InAs quantum dot quantum cascade infrared photodetector operating at room temperature with a peak detection wavelength of 4.3??m. The detector shows sensitive photoresponse for normal-incidence light, which is attributed to an intraband transition of the quantum dots and the following transfer of excited electrons on a cascade of quantum levels. The InAs quantum dots for the infrared absorption were formed by making use of self-assembled quantum dots in the Stranski–Krastanov growth mode and two-step strain-compensation design based on InAs/GaAs/InGaAs/InAlAs heterostructure, while the following extraction quantum stairs formed by LO-phonon energy are based on a strain-compensated InGaAs/InAlAs chirped superlattice. Johnson noise limited detectivities of 3.64?×?10{sup 11} and 4.83?×?10{sup 6} Jones at zero bias were obtained at 80?K and room temperature, respectively. Due to the low dark current and distinct photoresponse up to room temperature, this device can form high temperature imaging.
Zero-energy states in graphene quantum dots and rings
Downing, C A; Portnoi, M E
2011-01-01
We present exact analytical zero-energy solutions for a class of smooth decaying potentials, showing that the full confinement of charge carriers in electrostatic potentials in graphene quantum dots and rings is indeed possible without recourse to magnetic fields. These exact solutions allow us to draw conclusions on the general requirements for the potential to support fully confined states, including a critical value of the potential strength and spatial extent.
Zero-energy states in graphene quantum dots and rings
C. A. Downing; D. A. Stone; M. E. Portnoi
2011-10-29
We present exact analytical zero-energy solutions for a class of smooth decaying potentials, showing that the full confinement of charge carriers in electrostatic potentials in graphene quantum dots and rings is indeed possible without recourse to magnetic fields. These exact solutions allow us to draw conclusions on the general requirements for the potential to support fully confined states, including a critical value of the potential strength and spatial extent.
Quantum Dots Based Rad-Hard Computing and Sensors
NASA Technical Reports Server (NTRS)
Fijany, A.; Klimeck, G.; Leon, R.; Qiu, Y.; Toomarian, N.
2001-01-01
Quantum Dots (QDs) are solid-state structures made of semiconductors or metals that confine a small number of electrons into a small space. The confinement of electrons is achieved by the placement of some insulating material(s) around a central, well-conducting region. Thus, they can be viewed as artificial atoms. They therefore represent the ultimate limit of the semiconductor device scaling. Additional information is contained in the original extended abstract.
Single-photon superradiance from a quantum dot
Tighineanu, P; Lehmann, T B; Beere, H E; Ritchie, D A; Lodahl, P; Stobbe, S
2015-01-01
We report on the observation of single-photon superradiance from an exciton in a semiconductor quantum dot. The confinement by the quantum dot is strong enough for it to mimic a two-level atom, yet sufficiently weak to ensure superradiance. The electrostatic interaction between the electron and the hole comprising the exciton gives rise to an anharmonic spectrum, which we exploit to prepare the superradiant quantum state deterministically with a laser pulse. We observe a five-fold enhancement of the oscillator strength compared to conventional quantum dots. The enhancement is limited by the base temperature of our cryostat and may lead to oscillator strengths above 1000 from a single quantum emitter at optical frequencies.
Fabrication Technology and Measurement of Coupled Quantum Dot Devices
NASA Astrophysics Data System (ADS)
Burkhardt, Martin
1995-01-01
This thesis describes the fabrication and measurement of planar tunneling devices. X-ray lithography was used to define gate patterns in order to achieve lateral electrostatic confinement in a two-dimensional electron gas. Technologies were developed for the printing of features with linewidths of 50 nm and below, a lithographic resolution which is necessary for the fabrication of narrow tunneling barriers. Development of technologies such as this can also be used for large -scale fabrication of silicon and GaAs devices and circuits with critical dimensions of 100 nm and below. Quantum dots, in which the capacitances to the dot were minimized, were fabricated using high resolution lithography. Decreased capacitances to the dot increase the charging energy of a quantum dot, making it possible to observe single electron effects at elevated temperatures. The conductance of a device, featuring eight electrodes to control size and shape of a quantum dot, was measured in a Heliox insertion probe at a temperature of 300 mK. Measurements of several quantum dot sizes were performed and the results were discussed. The same device was biased to produce two unequal quantum dots in series. The results are discussed and compared with theoretical predictions. (Copies available exclusively from MIT Libraries, Rm. 14-0551, Cambridge, MA 02139-4307. Ph. 617-253-5668; Fax 617-253-1690.).
Energy spectrum and density of states for a graphene quantum dot in a magnetic field
Norman J. Morgenstern Horing; S. Y. Liu
2010-01-01
In this paper, we determine the spectrum and density of states of a graphene quantum dot in a normal quantizing magnetic field. To accomplish this, we employ the retarded Green function for a magnetized, infinite-sheet graphene layer to describe the dynamics of a tightly confined graphene quantum dot subject to Landau quantization. Considering a delta(2)(r) potential well that supports just
Signatures of classical chaos in gate-defined graphene quantum dots
Jens H. Bardarson; M. Titov; P. W. Brouwer
2009-01-01
A generic, non-integrable, gate-potential can not confine electrons in graphene. Integrable gate-defined quantum dots, in contrast, do have well defined bound states. This difference between integrable and non-integrable graphene quantum dots is revealed in e.g. the two terminal conductance, whose dependence on the gate potential strength is starkly different for the two cases.
Energy spectrum and density of states for a graphene quantum dot in a magnetic field
Norman J Morgenstern Horing; S Y Liu
2010-01-01
In this paper, we determine the spectrum and density of states of a graphene quantum dot in a normal quantizing magnetic field. To accomplish this, we employ the retarded Green function for a magnetized, infinite-sheet graphene layer to describe the dynamics of a tightly confined graphene quantum dot subject to Landau quantization. Considering a ?(2)(r) potential well that supports just
Coupling of quantum-dot light emission with a three-dimensional photonic-crystal nanocavity
Kanna Aoki; Denis Guimard; Masao Nishioka; Masahiro Nomura; Satoshi Iwamoto; Yasuhiko Arakawa
2008-01-01
We report on the first demonstration of the coupling of fully confined electrons and photons using a combination of three-dimensional photonic crystal nanocavities and quantum dots. The three dimensional photonic crystals were assembled by stacking planar components using a sophisticated micromanipulation technique. Point defects, containing embedded quantum dots, were introduced into the photonic crystals as active sites. By measuring the
Negative-Band-Gap Quantum Dots
NASA Astrophysics Data System (ADS)
Malkova, Natalia; Bryant, Garnett W.
2011-12-01
The spectrum of quantum dots (QDs) made from semiconductors like HgTe and HgS changes from negative gap to positive gap with decreasing size. Furthermore, intrinsic surface states, which are not related to dangling bonds, appear in the negative gap regime. We investigate theoretically the evolution of the spectrum of HgS QDs with decreasing size and show how states evolve from a negative gap to a positive gap as confinement is increased. The lowest confined electron level evolves into an intrinsic surface state with increasing size. This surface state is not derived from a bulk HgS bands. We demonstrate that surface states found do not have characteristic topological properties.
Experiments And Simulations On A Few-Electron Quantum Dot Circuit With Integrated Charge Read-Out
Experiments And Simulations On A Few-Electron Quantum Dot Circuit With Integrated Charge Read-Out R of the circuit. INTRODUCTION The spin of single electron confined in a semiconductor quantum dot [1 diagram for the first two electrons in the double dot circuit is calculated [7]. Then, we use
Gate-defined quantum confinement in suspended bilayer graphene.
Allen, M T; Martin, J; Yacoby, A
2012-01-01
Quantum-confined devices that manipulate single electrons in graphene are emerging as attractive candidates for nanoelectronics applications. Previous experiments have employed etched graphene nanostructures, but edge and substrate disorder severely limit device functionality. Here we present a technique that builds quantum-confined structures in suspended bilayer graphene with tunnel barriers defined by external electric fields that open a bandgap, thereby eliminating both edge and substrate disorder. We report clean quantum dot formation in two regimes: at zero magnetic field B using the energy gap induced by a perpendicular electric field and at B>0 using the quantum Hall ?=0 gap for confinement. Coulomb blockade oscillations exhibit periodicity consistent with electrostatic simulations based on local top-gate geometry, a direct demonstration of local control over the band structure of graphene. This technology integrates single electron transport with high device quality and access to vibrational modes, enabling broad applications from electromechanical sensors to quantum bits. PMID:22760633
Spin states in graphene quantum dots
Klaus Ensslin
2011-01-01
Graphene quantum dots [1,2], douple dots [3], rings [4] and nanoribbons [5] have been fabricated by electron beam lithography and dry etching. The orbital [1] properties of graphene quantum dots have been investigated in perpendicular magnetic fields and the details of the electron-hole crossover in graphene leads to a situation where electron (hole) states move down (up) in magnetic field
Electronic states and tunneling times in coupled self-assembled quantum dots
NASA Astrophysics Data System (ADS)
López-Bolaños, R.; Cocoletzi, G. H.; Ulloa, S. E.
2001-11-01
Electron energy levels in single dots, and energy splitting and tunneling times in stacked quantum dots are calculated as functions of structure parameters. An effective mass approach is used to solve the Schrödinger equation for cylindrical dots with finite confinement potentials. Strong confinement due to small sizes produces quantized energy levels in single dots and strong interactions of the wavefunctions with adjacent dots. This electronic coupling induces significant energy splittings and short tunneling times for characteristic structures used in experiments. This coupling may even yield coherent artificial molecular states with different optical properties.
Graphene quantum dots formed by a spatial modulation of the Dirac gap
G. Giavaras; Franco Nori
2011-01-01
An electrostatic quantum dot cannot be formed in monolayer graphene, because\\u000aof the Klein tunnelling. However, a dot can be formed with the help of a\\u000auniform magnetic field. As shown here, a spatial modulation of the Dirac gap\\u000aleads to confined states with discrete energy levels, thus defining a dot,\\u000awithout applying external electric and magnetic fields. Gap-induced dot
Tunable few-electron double quantum dots and Klein tunnelling in ultraclean carbon nanotubes
NASA Astrophysics Data System (ADS)
Steele, G. A.; Gotz, G.; Kouwenhoven, L. P.
2009-06-01
Quantum dots defined in carbon nanotubes are a platform for both basic scientific studies and research into new device applications. In particular, they have unique properties that make them attractive for studying the coherent properties of single-electron spins. To perform such experiments it is necessary to confine a single electron in a quantum dot with highly tunable barriers, but disorder has prevented tunable nanotube-based quantum-dot devices from reaching the single-electron regime. Here, we use local gate voltages applied to an ultraclean suspended nanotube to confine a single electron in both a single quantum dot and, for the first time, in a tunable double quantum dot. This tunability is limited by a novel type of tunnelling that is analogous to the tunnelling in the Klein paradox of relativistic quantum mechanics.
Tunable few-electron double quantum dots and Klein tunnelling in ultraclean carbon nanotubes.
Steele, G A; Gotz, G; Kouwenhoven, L P
2009-06-01
Quantum dots defined in carbon nanotubes are a platform for both basic scientific studies and research into new device applications. In particular, they have unique properties that make them attractive for studying the coherent properties of single-electron spins. To perform such experiments it is necessary to confine a single electron in a quantum dot with highly tunable barriers, but disorder has prevented tunable nanotube-based quantum-dot devices from reaching the single-electron regime. Here, we use local gate voltages applied to an ultraclean suspended nanotube to confine a single electron in both a single quantum dot and, for the first time, in a tunable double quantum dot. This tunability is limited by a novel type of tunnelling that is analogous to the tunnelling in the Klein paradox of relativistic quantum mechanics. PMID:19498397
Multi-million atom electronic structure calculations for quantum dots
NASA Astrophysics Data System (ADS)
Usman, Muhammad
Quantum dots grown by self-assembly process are typically constructed by 50,000 to 5,000,000 structural atoms which confine a small, countable number of extra electrons or holes in a space that is comparable in size to the electron wavelength. Under such conditions quantum dots can be interpreted as artificial atoms with the potential to be custom tailored to new functionality. In the past decade or so, these nanostructures have attracted significant experimental and theoretical attention in the field of nanoscience. The new and tunable optical and electrical properties of these artificial atoms have been proposed in a variety of different fields, for example in communication and computing systems, medical and quantum computing applications. Predictive and quantitative modeling and simulation of these structures can help to narrow down the vast design space to a range that is experimentally affordable and move this part of nanoscience to nano-Technology. Modeling of such quantum dots pose a formidable challenge to theoretical physicists because: (1) Strain originating from the lattice mismatch of the materials penetrates deep inside the buffer surrounding the quantum dots and require large scale (multi-million atom) simulations to correctly capture its effect on the electronic structure, (2) The interface roughness, the alloy randomness, and the atomistic granularity require the calculation of electronic structure at the atomistic scale. Most of the current or past theoretical calculations are based on continuum approach such as effective mass approximation or k.p modeling capturing either no or one of the above mentioned effects, thus missing some of the essential physics. The Objectives of this thesis are: (1) to model and simulate the experimental quantum dot topologies at the atomistic scale; (2) to theoretically explore the essential physics i.e. long range strain, linear and quadratic piezoelectricity, interband optical transition strengths, quantum confined stark shift, coherent coupling of electronic states in a quantum dot molecule etc.; (3) to assess the potential use of the quantum dots in real device implementation and to provide physical insight to the experimentalists. Full three dimensional strain and electronic structure simulations of quantum dot structures containing multi-million atoms are done using NEMO 3-D. Both single and vertically stacked quantum dot structures are analyzed in detail. The results show that the strain and the piezoelectricity significantly impact the electronic structure of these devices. This work shows that the InAs quantum dots when placed in the InGaAs quantum well red shifts the emission wavelength. Such InAs/GaAs-based optical devices can be used for optical-fiber based communication systems at longer wavelengths (1.3um -- 1.5um). Our atomistic simulations of InAs/InGaAs/GaAs quantum dots quantitatively match with the experiment and give the critical insight of the physics involved in these structures. A single quantum dot molecule is studied for coherent quantum coupling of electronic states under the influence of static electric field applied in the growth direction. Such nanostructures can be used in the implementation of quantum information technologies. A close quantitative match with the experimental optical measurements allowed us to get a physical insight into the complex physics of quantum tunnel couplings of electronic states as the device operation switches between atomic and molecular regimes. Another important aspect is to design the quantum dots for a desired isotropic polarization of the optical emissions. Both single and coupled quantum dots are studied for TE/TM ratio engineering. The atomistic study provides a detailed physical analysis of these computationally expensive large nanostructures and serves as a guide for the experimentalists for the design of the polarization independent devices for the optical communication systems.
Semiconductor double quantum dot micromaser
NASA Astrophysics Data System (ADS)
Liu, Y.-Y.; Stehlik, J.; Eichler, C.; Gullans, M. J.; Taylor, J. M.; Petta, J. R.
2015-01-01
The coherent generation of light, from masers to lasers, relies upon the specific structure of the individual emitters that lead to gain. Devices operating as lasers in the few-emitter limit provide opportunities for understanding quantum coherent phenomena, from terahertz sources to quantum communication. Here we demonstrate a maser that is driven by single-electron tunneling events. Semiconductor double quantum dots (DQDs) serve as a gain medium and are placed inside a high-quality factor microwave cavity. We verify maser action by comparing the statistics of the emitted microwave field above and below the maser threshold.
Electron states in semiconductor quantum dots
NASA Astrophysics Data System (ADS)
Dhayal, Suman S.; Ramaniah, Lavanya M.; Ruda, Harry E.; Nair, Selvakumar V.
2014-11-01
In this work, the electronic structures of quantum dots (QDs) of nine direct band gap semiconductor materials belonging to the group II-VI and III-V families are investigated, within the empirical tight-binding framework, in the effective bond orbital model. This methodology is shown to accurately describe these systems, yielding, at the same time, qualitative insights into their electronic properties. Various features of the bulk band structure such as band-gaps, band curvature, and band widths around symmetry points affect the quantum confinement of electrons and holes. These effects are identified and quantified. A comparison with experimental data yields good agreement with the calculations. These theoretical results would help quantify the optical response of QDs of these materials and provide useful input for applications.
Semiconductor quantum dot-sensitized solar cells
Tian, Jianjun; Cao, Guozhong
2013-01-01
Semiconductor quantum dots (QDs) have been drawing great attention recently as a material for solar energy conversion due to their versatile optical and electrical properties. The QD-sensitized solar cell (QDSC) is one of the burgeoning semiconductor QD solar cells that shows promising developments for the next generation of solar cells. This article focuses on recent developments in QDSCs, including 1) the effect of quantum confinement on QDSCs, 2) the multiple exciton generation (MEG) of QDs, 3) fabrication methods of QDs, and 4) nanocrystalline photoelectrodes for solar cells. We also make suggestions for future research on QDSCs. Although the efficiency of QDSCs is still low, we think there will be major breakthroughs in developing QDSCs in the future. PMID:24191178
InP quantum dots: Electronic structure, surface effects, and the redshifted emission
Huaxiang Fu; Alex Zunger
1997-01-01
We present pseudopotential plane-wave electronic-structure calculations on InP quantum dots in an effort to understand quantum confinement and surface effects and to identify the origin of the long-lived and redshifted luminescence. We find that (i) unlike the case in small GaAs dots, the lowest unoccupied state of InP dots is the Gamma1c-derived direct state rather than the X1c-derived indirect state
Quantum Dot Light Emitting Diode
Keith Kahen
2008-07-31
The project objective is to create low cost coatable inorganic light emitting diodes, composed of quantum dot emitters and inorganic nanoparticles, which have the potential for efficiencies equivalent to that of LEDs and OLEDs and lifetime, brightness, and environmental stability between that of LEDs and OLEDs. At the end of the project the Recipient shall gain an understanding of the device physics and properties of Quantum-Dot LEDs (QD-LEDs), have reliable and accurate nanocrystal synthesis routines, and have formed green-yellow emitting QD-LEDs with a device efficiency greater than 3 lumens/W, a brightness greater than 400 cd/m2, and a device operational lifetime of more than 1000 hours. Thus the aim of the project is to break the current cost-efficiency paradigm by creating novel low cost inorganic LEDs composed of inorganic nanoparticles.
Quantum Dot Light Emitting Diode
Kahen, Keith
2008-07-31
The project objective is to create low cost coatable inorganic light emitting diodes, composed of quantum dot emitters and inorganic nanoparticles, which have the potential for efficiencies equivalent to that of LEDs and OLEDs and lifetime, brightness, and environmental stability between that of LEDs and OLEDs. At the end of the project the Recipient shall gain an understanding of the device physics and properties of Quantum-Dot LEDs (QD-LEDs), have reliable and accurate nanocrystal synthesis routines, and have formed green-yellow emitting QD-LEDs with a device efficiency greater than 3 lumens/W, a brightness greater than 400 cd/m{sup 2}, and a device operational lifetime of more than 1000 hours. Thus the aim of the project is to break the current cost-efficiency paradigm by creating novel low cost inorganic LEDs composed of inorganic nanoparticles.
Spin fluctuations in quantum dots
NASA Astrophysics Data System (ADS)
Sharafutdinov, A. U.; Lyubshin, D. S.; Burmistrov, I. S.
2014-11-01
We explore the static longitudinal and dynamic transverse spin susceptibilities in quantum dots and nanoparticles within the framework of the Hamiltonian that extends the universal Hamiltonian to the case of uniaxial anisotropic exchange. For the limiting cases of Ising and Heisenberg exchange interactions, we ascertain how fluctuations of single-particle levels affect the Stoner instability in quantum dots. We reduce the problem to the statistics of extrema of a certain Gaussian process. We prove that, despite possible strong randomness of the single-particle levels, the spin susceptibility and all its moments diverge simultaneously at the point which is determined by the standard criterion of the Stoner instability involving the mean level spacing only.
Quantitative multiplexed quantum dot immunohistochemistry
Sweeney, E.; Ward, T.H. [Clinical and Experimental Pharmacology, Paterson Institute for Cancer Research, Wilmslow Road, Manchester, 420 4BX (United Kingdom); Gray, N.; Womack, C. [AstraZeneca, Alderley Park, Macclesfield, Cheshire, SK10 4TG (United Kingdom); Jayson, G. [Translational Angiogenesis Group, Paterson Institute for Cancer Research, Wilmslow Road, Manchester, M20 4BX (United Kingdom); Hughes, A. [AstraZeneca, Alderley Park, Macclesfield, Cheshire, SK10 4TG (United Kingdom); Dive, C. [Clinical and Experimental Pharmacology, Paterson Institute for Cancer Research, Wilmslow Road, Manchester, 420 4BX (United Kingdom); Byers, R. [School of Cancer and Imaging Studies, University of Manchester, Stopford Building, Oxford Road, Manchester M13 9PT (United Kingdom); Department of Histopathology, Manchester Royal Infirmary, Oxford Road, Manchester, M13 9WL (United Kingdom)], E-mail: richard.byers@cmmc.nhs.uk
2008-09-19
Quantum dots are photostable fluorescent semiconductor nanocrystals possessing wide excitation and bright narrow, symmetrical, emission spectra. These characteristics have engendered considerable interest in their application in multiplex immunohistochemistry for biomarker quantification and co-localisation in clinical samples. Robust quantitation allows biomarker validation, and there is growing need for multiplex staining due to limited quantity of clinical samples. Most reported multiplexed quantum dot staining used sequential methods that are laborious and impractical in a high-throughput setting. Problems associated with sequential multiplex staining have been investigated and a method developed using QDs conjugated to biotinylated primary antibodies, enabling simultaneous multiplex staining with three antibodies. CD34, Cytokeratin 18 and cleaved Caspase 3 were triplexed in tonsillar tissue using an 8 h protocol, each localised to separate cellular compartments. This demonstrates utility of the method for biomarker measurement enabling rapid measurement of multiple co-localised biomarkers on single paraffin tissue sections, of importance for clinical trial studies.
Charges, spins (and phonons) in graphene quantum dots
Charges, spins (and phonons) in graphene quantum dots Klaus Ensslin Solid State Physics ·graphene in a Graphene Quantum Dot #12;Graphene dot with charge detector C. Stampfer, S. Hellmüller, J. Güttinger, F #12;Graphene quantum dots: orbital and spin effects QD area: 50 nm x 80 nm #12;Quantum dot states
Breathing modes of confined skyrmions in ultrathin magnetic dots
NASA Astrophysics Data System (ADS)
Kim, Joo-Von; Garcia-Sanchez, Felipe; Sampaio, João; Moreau-Luchaire, Constance; Cros, Vincent; Fert, Albert
2014-08-01
The dynamics of individual magnetic skyrmions confined in ultrathin film dots is studied theoretically. The systems considered are transition-metal ferromagnets possessing perpendicular magnetic anisotropy and particular attention is given to the dynamic response of the skyrmions to perpendicular ac fields. By using micromagnetics simulations, it is shown that breathing modes can hybridize with geometrically quantized spin wave eigenmodes of the circular dots, leading to distinct features in the power spectrum that differ from the behavior expected for uniformly magnetized systems. The static field dependence of the breathing mode frequency offers a direct means of detecting and characterizing such skyrmion states in experiment.
Electronic properties of Graphene quantum dots
Pawel Hawrylak; Pawel Potasz; A. Devrim Güçlü
2009-01-01
We study electronic properties of Graphene quantum dots in magnetic fields. Graphene quantum dots are atomically thick nanometer-scale islands constructed by connecting benzene molecules. Quantum dots with triangular and hexagonal shape have shown to have different edge properties [1,2], and triangular zig-zag structures have recently attracted attention due to their half-filled zero-energy edge states. In this work, we investigate electronic
Functional microspheres of graphene quantum dots
Yi Ding; Huhu Cheng; Ce Zhou; Yueqiong Fan; Jia Zhu; Huibo Shao; Liangti Qu
2012-01-01
Graphene-quantum-dot microspheres (GQDSs) have been prepared by assembly of graphene quantum dots (GQDs) via a water-in-oil (W\\/O) emulsion technique without the addition of any surfactants. Although made of quantum-sized graphene dots, the as-formed GQDSs are solid and remain intact after slight ultrasonication. The versatile W\\/O emulsion method allows the in situ intercalation of functional nanocomponents into the GQDSs for specific
Surround-gated vertical nanowire quantum dots
NASA Astrophysics Data System (ADS)
van Weert, M. H. M.; den Heijer, M.; van Kouwen, M. P.; Algra, R. E.; Bakkers, E. P. A. M.; Kouwenhoven, L. P.; Zwiller, V.
2010-06-01
We report voltage dependent photoluminescence experiments on single indium arsenide phosphide (InAsP) quantum dots embedded in vertical surround-gated indium phosphide (InP) nanowires. We show that by tuning the gate voltage, we can access different quantum dot charge states. We study the anisotropic exchange splitting by polarization analysis, and identify the neutral and singly charged exciton. These results are important for spin addressability in a charge tunable nanowire quantum dot.
Optically active quantum dots in monolayer WSe2
NASA Astrophysics Data System (ADS)
Srivastava, Ajit; Sidler, Meinrad; Allain, Adrien V.; Lembke, Dominik S.; Kis, Andras; Imamo?lu, A.
2015-06-01
Semiconductor quantum dots have emerged as promising candidates for the implementation of quantum information processing, because they allow for a quantum interface between stationary spin qubits and propagating single photons. In the meantime, transition-metal dichalcogenide monolayers have moved to the forefront of solid-state research due to their unique band structure featuring a large bandgap with degenerate valleys and non-zero Berry curvature. Here, we report the observation of zero-dimensional anharmonic quantum emitters, which we refer to as quantum dots, in monolayer tungsten diselenide, with an energy that is 20–100?meV lower than that of two-dimensional excitons. Photon antibunching in second-order photon correlations unequivocally demonstrates the zero-dimensional anharmonic nature of these quantum emitters. The strong anisotropic magnetic response of the spatially localized emission peaks strongly indicates that radiative recombination stems from localized excitons that inherit their electronic properties from the host transition-metal dichalcogenide. The large ?1?meV zero-field splitting shows that the quantum dots have singlet ground states and an anisotropic confinement that is most probably induced by impurities or defects. The possibility of achieving electrical control in van der Waals heterostructures and to exploit the spin–valley degree of freedom renders transition-metal-dichalcogenide quantum dots interesting for quantum information processing.
Optically active quantum dots in monolayer WSe2.
Srivastava, Ajit; Sidler, Meinrad; Allain, Adrien V; Lembke, Dominik S; Kis, Andras; Imamo?lu, A
2015-06-01
Semiconductor quantum dots have emerged as promising candidates for the implementation of quantum information processing, because they allow for a quantum interface between stationary spin qubits and propagating single photons. In the meantime, transition-metal dichalcogenide monolayers have moved to the forefront of solid-state research due to their unique band structure featuring a large bandgap with degenerate valleys and non-zero Berry curvature. Here, we report the observation of zero-dimensional anharmonic quantum emitters, which we refer to as quantum dots, in monolayer tungsten diselenide, with an energy that is 20-100?meV lower than that of two-dimensional excitons. Photon antibunching in second-order photon correlations unequivocally demonstrates the zero-dimensional anharmonic nature of these quantum emitters. The strong anisotropic magnetic response of the spatially localized emission peaks strongly indicates that radiative recombination stems from localized excitons that inherit their electronic properties from the host transition-metal dichalcogenide. The large ?1?meV zero-field splitting shows that the quantum dots have singlet ground states and an anisotropic confinement that is most probably induced by impurities or defects. The possibility of achieving electrical control in van der Waals heterostructures and to exploit the spin-valley degree of freedom renders transition-metal-dichalcogenide quantum dots interesting for quantum information processing. PMID:25938570
Quantum dot enabled high color gamut LCDs
NASA Astrophysics Data System (ADS)
Chen, Jian; Kan, Shihai; Lee, Ernie; Gensler, Steve; Hartlove, Jason
2015-03-01
Quantum dots are a new generation of phosphor material that have high photon conversion efficiency, narrow spectral line-widths and can be continuously tuned in their emission wavelengths. Since 2013, quantum dots have been adopted by the consumer electronics industry into LCDs to significantly increase their color performance. Compared to the OLED solution, quantum dot LCDs have higher energy efficiency, larger color gamut, longer lifetime, and are offered at a fraction of the cost of OLED panels. In this paper, we demonstrate that quantum-dot based LCDs can achieve more than 90% coverage of the ultra-wide color gamut, Rec. 2020, which is the new color standard for UHDTV.
A light-hole exciton in a quantum dot
NASA Astrophysics Data System (ADS)
Huo, Y. H.; Witek, B. J.; Kumar, S.; Cardenas, J. R.; Zhang, J. X.; Akopian, N.; Singh, R.; Zallo, E.; Grifone, R.; Kriegner, D.; Trotta, R.; Ding, F.; Stangl, J.; Zwiller, V.; Bester, G.; Rastelli, A.; Schmidt, O. G.
2014-01-01
A light-hole exciton is a quasiparticle formed from a single electron bound to a single light hole. This type of fundamental excitation, if confined inside a semiconductor quantum dot, could be advantageous in quantum information science and technology. However, it has been difficult to access it so far, because confinement and strain in conventional quantum dots favour a ground-state single-particle hole with a predominantly heavy-hole character. Here we demonstrate the creation of a light-hole exciton ground state by applying elastic stress to an initially unstrained quantum dot. Its signature is clearly distinct from that of the well-known heavy-hole exciton and consists of three orthogonally polarized bright optical transitions and a fine-structure splitting of hundreds of microelectronvolts between in-plane and out-of-plane components. This work paves the way for the exploration of the fundamental properties and of the potential relevance of three-dimensionally confined light-hole states in quantum technologies.
Decoherence dynamics of two charge qubits in vertically coupled quantum dots
Ben Chouikha, W.; Bennaceur, R. [Laboratoire de Physique de la Matiere Condensee, Departement de Physique, Faculte des Sciences de Tunis, 1060 Tunis (Tunisia); Jaziri, S. [Departement de Physique, Faculte des Sciences de Bizerte, Jarzouna 7021 Bizerte (Tunisia)
2007-12-15
The decoherence dynamics of two charge qubits in a double quantum dot is investigated theoretically. We consider the quantum dynamics of two interacting electrons in a vertically coupled quantum dot driven by an external electric field. We derive the equations of motion for the density matrix, in which the presence of an electron confined in the double dot represents one qubit. A Markovian approach to the dynamical evolution of the reduced density matrix is adopted. We evaluate the concurrence of two qubits in order to study the effect of acoustic phonons on the entanglement. We also show that the disentanglement effect depends on the double dot parameters and increases with the temperature.
Valley-orbit hybrid states in Si quantum dots
NASA Astrophysics Data System (ADS)
Gamble, John; Friesen, Mark; Coppersmith, S. N.
2013-03-01
The conduction band for electrons in layered Si nanostructures oriented along (001) has two low-lying valleys. Most theoretical treatments assume that these valleys are decoupled from the long-wavelength physics of electron confinement. In this work, we show that even a minimal amount of disorder (a single atomic step at the quantum well interface) is sufficient to mix valley states and electron orbitals, causing a significant distortion of the long-wavelength electron envelope. For physically realistic electric fields and dot sizes, this valley-orbit coupling impacts all electronic states in Si quantum dots, implying that one must always consider valley-orbit hybrid states, rather than distinct valley and orbital degrees of freedom. We discuss the ramifications of our results on silicon quantum dot qubits. The conduction band for electrons in layered Si nanostructures oriented along (001) has two low-lying valleys. Most theoretical treatments assume that these valleys are decoupled from the long-wavelength physics of electron confinement. In this work, we show that even a minimal amount of disorder (a single atomic step at the quantum well interface) is sufficient to mix valley states and electron orbitals, causing a significant distortion of the long-wavelength electron envelope. For physically realistic electric fields and dot sizes, this valley-orbit coupling impacts all electronic states in Si quantum dots, implying that one must always consider valley-orbit hybrid states, rather than distinct valley and orbital degrees of freedom. We discuss the ramifications of our results on silicon quantum dot qubits. This work was supported in part by ARO (W911NF-08-1-0482) and NSF (DMR-0805045).
Chiral quantum dot based materials
NASA Astrophysics Data System (ADS)
Govan, Joseph; Loudon, Alexander; Baranov, Alexander V.; Fedorov, Anatoly V.; Gun'ko, Yurii
2014-05-01
Recently, the use of stereospecific chiral stabilising molecules has also opened another avenue of interest in the area of quantum dot (QD) research. The main goal of our research is to develop new types of technologically important quantum dot materials containing chiral defects, study their properties and explore their applications. The utilisation of chiral penicillamine stabilisers allowed the preparation of new water soluble white emitting CdS quantum nanostructures which demonstrated circular dichroism in the band-edge region of the spectrum. It was also demonstrated that all three types of QDs (D-, L-, and Rac penicillamine stabilised) show very broad emission bands between 400 and 700 nm due to defects or trap states on the surfaces of the nanocrystals. In this work the chiral CdS based quantum nanostructures have also been doped by copper metal ions and new chiral penicilamine stabilized CuS nanoparticles have been prepared and investigated. It was found that copper doping had a strong effect at low levels in the synthesis of chiral CdS nanostructures. We expect that this research will open new horizons in the chemistry of chiral nanomaterials and their application in biotechnology, sensing and asymmetric synthesis.
Realistic model of a vertical pillar quantum dot: Analysis of individual dot data
NASA Astrophysics Data System (ADS)
Maksym, P. A.; Nishi, Y.; Austing, D. G.; Hatano, T.; Kouwenhoven, L. P.; Aoki, H.; Tarucha, S.
2009-03-01
An accurate model of a vertical pillar quantum dot is described. The full three-dimensional structure of the device containing the dot is taken into account and this leads to an effective two-dimensional model in which electrons move in the two lateral dimensions, the confinement is parabolic, and the interaction potential is very different from the bare Coulomb potential. The potentials are found from the device structure and a few adjustable parameters. Numerically stable calculation procedures for the interaction potential are detailed and procedures for deriving parameter values from experimental addition energy and chemical potential data are described. The model is able to explain magnetic-field-dependent addition energy and chemical potential data for an individual dot to an accuracy of about 5%, the accuracy level needed to determine ground-state quantum numbers from experimental transport data. Applications to excited state transport data are also described.
Correlation energy of anisotropic quantum dots
Zhao Yan; Loos, Pierre-Francois; Gill, Peter M. W. [Research School of Chemistry, Australian National University, Canberra, ACT 0200 (Australia)
2011-09-15
We study the D-dimensional high-density correlation energy E{sub c} of the singlet ground state of two electrons confined by a harmonic potential with Coulombic repulsion. We allow the harmonic potential to be anisotropic and examine the behavior of E{sub c} as a function of the anisotropy {alpha}{sup -1}. In particular, we are interested in the limit where the anisotropy goes to infinity ({alpha}{yields}0) and the electrons are restricted to a lower-dimensional space. We show that tuning the value of {alpha} from 0 to 1 allows a smooth dimensional interpolation and we demonstrate that the usual model, in which a quantum dot is treated as a two-dimensional system, is inappropriate. Finally, we provide a simple function which reproduces the behavior of E{sub c} over the entire range of {alpha}.
Exchange in a silicon-based quantum dot quantum computer architecture
S. N. Coppersmith; Seungwon Lee; Paul von Allmen
2004-08-25
In bulk silicon, intervalley electronic interference has been shown to lead to strong oscillations in the exchange coupling between impurity electronic wavefunctions, posing a serious manufacturability problem for proposed quantum computers. Here we show that this problem does not arise in proposed architectures using Si/SiGe quantum dots because of the large in-plane strain in Si quantum wells together with the strong confinement potential typical of heterostructures.
Charge detection in graphene quantum dots
J. Güttinger; C. Stampfer; S. Hellmüller; F. Molitor; T. Ihn; K. Ensslin
2008-01-01
We report measurements on a graphene quantum dot with an integrated graphene charge detector. The quantum dot device consists of a graphene island (diameter of ~200 nm) connected to source and drain contacts via two narrow graphene constrictions. From Coulomb diamond measurements a charging energy of 4.3 meV is extracted. The charge detector is based on a 45 nm wide
STED nanoscopy with fluorescent quantum dots
Hanne, Janina; Falk, Henning J.; Görlitz, Frederik; Hoyer, Patrick; Engelhardt, Johann; Sahl, Steffen J.; Hell, Stefan W.
2015-01-01
The widely popular class of quantum-dot molecular labels could so far not be utilized as standard fluorescent probes in STED (stimulated emission depletion) nanoscopy. This is because broad quantum-dot excitation spectra extend deeply into the spectral bands used for STED, thus compromising the transient fluorescence silencing required for attaining super-resolution. Here we report the discovery that STED nanoscopy of several red-emitting commercially available quantum dots is in fact successfully realized by the increasingly popular 775?nm STED laser light. A resolution of presently ?50?nm is demonstrated for single quantum dots, and sub-diffraction resolution is further shown for imaging of quantum-dot-labelled vimentin filaments in fibroblasts. The high quantum-dot photostability enables repeated STED recordings with >1,000 frames. In addition, we have evidence that the tendency of quantum-dot labels to blink is largely suppressed by combined action of excitation and STED beams. Quantum-dot STED significantly expands the realm of application of STED nanoscopy, and, given the high stability of these probes, holds promise for extended time-lapse imaging. PMID:25980788
Low density InAs/(In)GaAs quantum dots emitting at long wavelengths.
Trevisi, G; Seravalli, L; Frigeri, P; Franchi, S
2009-10-14
We present research carried out on molecular beam epitaxy grown InAs/(In)GaAs quantum dot structures for single-photon operation at long wavelengths. The optical and morphological properties of the structures are studied as functions of quantum dot growth parameters and of the InGaAs upper confining layer thickness and composition. We show that low growth rate, high growth temperature and reduced quantum dot coverage are very effective in reducing the quantum dot density but, owing to In desorption effects and quantum dot size reduction, this result is not always concomitant with the achievement of long wavelength emission. To this aim, we show that the use of InGaAs upper confining layers allows the redshift of quantum dot emission energy without affecting their density. Both the thickness and composition of the InGaAs layer have to be carefully chosen to provide a complete coverage of quantum dots and not to exceed the critical thickness for plastic relaxation. Our results led to the preparation of quantum dot structures with densities in the low 10(9) cm(-2) range, 1.33 microm emission at 10 K and full widths at half maximum of 22 meV. PMID:19762951
Biocompatible Quantum Dots for Biological Applications
Rosenthal, Sandra J.; Chang, Jerry C.; Kovtun, Oleg; McBride, James R.; Tomlinson, Ian D.
2011-01-01
Semiconductor quantum dots are quickly becoming a critical diagnostic tool for discerning cellular function at the molecular level. Their high brightness, long-lasting, sizetunable, and narrow luminescence set them apart from conventional fluorescence dyes. Quantum dots are being developed for a variety of biologically oriented applications, including fluorescent assays for drug discovery, disease detection, single protein tracking, and intracellular reporting. This review introduces the science behind quantum dots and describes how they are made biologically compatible. Several applications are also included, illustrating strategies toward target specificity, and are followed by a discussion on the limitations of quantum dot approaches. The article is concluded with a look at the future direction of quantum dots. PMID:21276935
NASA Astrophysics Data System (ADS)
Tomczak, Nikodem; Liu, Rongrong; Vancso, Julius G.
2013-11-01
Quantum Dots (QDs) are semiconductor nanocrystals with distinct photophysical properties finding applications in biology, biosensing, and optoelectronics. Polymeric coatings of QDs are used primarily to provide long-term colloidal stability to QDs dispersed in solutions and also as a source of additional functional groups used in further chemical derivatization of the nanoparticles. We review the coating methods, including multidentate and amphiphilic polymeric coatings, and grafting-to and grafting-from approaches. We highlight the most commonly used polymers and discuss how their chemical structure influences the coating properties.
Scattering of two-dimensional Dirac fermions on gate-defined oscillating quantum dots
NASA Astrophysics Data System (ADS)
Schulz, C.; Heinisch, R. L.; Fehske, H.
2015-01-01
Within an effective Dirac-Weyl theory we solve the scattering problem for massless chiral fermions impinging on a cylindrical time-dependent potential barrier. The setup we consider can be used to model the electron propagation in a monolayer of graphene with harmonically driven quantum dots. For static small-sized quantum dots scattering resonances enable particle confinement and interference effects may switch forward scattering on and off. An oscillating dot may cause inelastic scattering by excitation of states with energies shifted by integer multiples of the oscillation frequency, which significantly modifies the scattering characteristics of static dots. Exemplarily the scattering efficiency of a potential barrier with zero bias remains finite in the limit of low particle energies and small potential amplitudes. For an oscillating quantum dot with finite bias, the partial wave resonances at higher energies are smeared out for small frequencies or large oscillation amplitudes, thereby dissolving the quasibound states at the quantum dot.
Energy levels of few-electron quantum dots imaged and characterized by atomic force microscopy
Cockins, Lynda; Miyahara, Yoichi; Bennett, Steven D.; Clerk, Aashish A.; Studenikin, Sergei; Poole, Philip; Sachrajda, Andrew; Grutter, Peter
2010-01-01
Strong confinement of charges in few-electron systems such as in atoms, molecules, and quantum dots leads to a spectrum of discrete energy levels often shared by several degenerate states. Because the electronic structure is key to understanding their chemical properties, methods that probe these energy levels in situ are important. We show how electrostatic force detection using atomic force microscopy reveals the electronic structure of individual and coupled self-assembled quantum dots. An electron addition spectrum results from a change in cantilever resonance frequency and dissipation when an electron tunnels on/off a dot. The spectra show clear level degeneracies in isolated quantum dots, supported by the quantitative measurement of predicted temperature-dependent shifts of Coulomb blockade peaks. Scanning the surface shows that several quantum dots may reside on what topographically appears to be just one. Relative coupling strengths can be estimated from these images of grouped coupled dots. PMID:20457938
Matthew C Beard; Joseph M Luther; Aaron G Midgett; Octavi E. Semonin; Justin C Johnson; Arthur J. Nozik
2010-01-01
Nanostructures of semiconductor materials exhibit quantization effects when the electronic particles of these materials are confined by potential barriers to small regions of space. The confinement can be in one dimension producing quantum films, also termed quantum wells in the early 1980s as the first examples of quantization in nanoscale materials, in two dimensions (producing quantum wires or rods), or
Tailoring 10 nm Scale Suspended Graphene Junctions and Quantum Dots
NASA Astrophysics Data System (ADS)
Tayari, Vahid; McRae, Andrew C.; Yi?en, Serap; Island, Joshua O.; Porter, James M.; Champagne, Alexandre R.
2015-01-01
The possibility to make 10 nm scale, and low-disorder, suspended graphene devices would open up many possibilities to study and make use of strongly coupled quantum electronics, quantum mechanics, and optics. We present a versatile method, based on the electromigration of gold-on-graphene bow-tie bridges, to fabricate low-disorder suspended graphene junctions and quantum dots with lengths ranging from 6 nm up to 55 nm. We control the length of the junctions, and shape of their gold contacts by adjusting the power at which the electromigration process is allowed to avalanche. Using carefully engineered gold contacts and a nonuniform downward electrostatic force, we can controllably tear the width of suspended graphene channels from over 100 nm down to 27 nm. We demonstrate that this lateral confinement creates high-quality suspended quantum dots. This fabrication method could be extended to other two-dimensional materials.
Tailoring 10 nm scale suspended graphene junctions and quantum dots.
Tayari, Vahid; McRae, Andrew C; Yi?en, Serap; Island, Joshua O; Porter, James M; Champagne, Alexandre R
2015-01-14
The possibility to make 10 nm scale, and low-disorder, suspended graphene devices would open up many possibilities to study and make use of strongly coupled quantum electronics, quantum mechanics, and optics. We present a versatile method, based on the electromigration of gold-on-graphene bow-tie bridges, to fabricate low-disorder suspended graphene junctions and quantum dots with lengths ranging from 6 nm up to 55 nm. We control the length of the junctions, and shape of their gold contacts by adjusting the power at which the electromigration process is allowed to avalanche. Using carefully engineered gold contacts and a nonuniform downward electrostatic force, we can controllably tear the width of suspended graphene channels from over 100 nm down to 27 nm. We demonstrate that this lateral confinement creates high-quality suspended quantum dots. This fabrication method could be extended to other two-dimensional materials. PMID:25490053
Density of states in randomly shaped graphene quantum dots
T. Espinosa-Ortega; Igor A. Luk'Yanchuk; Yuri G. Rubo
2011-01-01
By numerical diagonalization of honeycomb-lattice tight-binding Hamiltonian we calculate the density of state (DOS) of irregularly shaped graphene quantum dots fabricated in the form of graphene nano-flakes. The finite-size electron confinement and the edge states result in the central peak of DOS that is located at the zero-energy Dirac point. The amplitude and width of the peak are provided by
Electric transport through circular graphene quantum dots: Presence of disorder
G. Pal; W. Apel; L. Schweitzer
2011-01-01
The electronic states of an electrostatically confined cylindrical graphene quantum dot and the electric transport through this device are studied theoretically within the continuum Dirac-equation approximation and compared with numerical results obtained from a tight-binding lattice description. A spectral gap, which may originate from strain effects, additional adsorbed atoms, or substrate-induced sublattice-symmetry breaking, allows for bound and scattering states. As
Single to quadruple quantum dots with tunable tunnel couplings
Takakura, T.; Noiri, A.; Obata, T.; Yoneda, J.; Yoshida, K. [Department of Applied Physics, University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656 (Japan); Otsuka, T.; Tarucha, S. [Department of Applied Physics, University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656 (Japan); RIKEN, Center for Emergent Matter Science, 3-1 Wako-shi, Saitama 351-0198 (Japan)
2014-03-17
We prepare a gate-defined quadruple quantum dot to study the gate-tunability of single to quadruple quantum dots with finite inter-dot tunnel couplings. The measured charging energies of various double dots suggest that the dot size is governed by the gate geometry. For the triple and quadruple dots, we study the gate-tunable inter-dot tunnel couplings. For the triple dot, we find that the effective tunnel coupling between side dots significantly depends on the alignment of the center dot potential. These results imply that the present quadruple dot has a gate performance relevant for implementing spin-based four-qubits with controllable exchange couplings.
Hybrid superconductor-quantum dot devices.
De Franceschi, Silvano; Kouwenhoven, Leo; Schönenberger, Christian; Wernsdorfer, Wolfgang
2010-10-01
Advances in nanofabrication techniques have made it possible to make devices in which superconducting electrodes are connected to non-superconducting nanostructures such as quantum dots. The properties of these hybrid devices result from a combination of a macroscopic quantum phenomenon involving large numbers of electrons (superconductivity) and the ability to control single electrons, offered by quantum dots. Here we review research into electron transport and other fundamental processes that have been studied in these devices. We also describe potential applications, such as a transistor in which the direction of a supercurrent can be reversed by adding just one electron to a quantum dot. PMID:20852639
Hybrid superconductor-quantum dot devices
NASA Astrophysics Data System (ADS)
de Franceschi, Silvano; Kouwenhoven, Leo; Schönenberger, Christian; Wernsdorfer, Wolfgang
2010-10-01
Advances in nanofabrication techniques have made it possible to make devices in which superconducting electrodes are connected to non-superconducting nanostructures such as quantum dots. The properties of these hybrid devices result from a combination of a macroscopic quantum phenomenon involving large numbers of electrons (superconductivity) and the ability to control single electrons, offered by quantum dots. Here we review research into electron transport and other fundamental processes that have been studied in these devices. We also describe potential applications, such as a transistor in which the direction of a supercurrent can be reversed by adding just one electron to a quantum dot.
Charge detection in graphene quantum dots J. Gttinger,a
Ihn, Thomas
Charge detection in graphene quantum dots J. Güttinger,a C. Stampfer, S. Hellmüller, F. Molitor, T on a graphene quantum dot with an integrated graphene charge detector. The quantum dot device consists9 interference devices,1012 and graphene quantum dots QDs .1315 In this paper we present an integrated graphene
Optophononics with coupled quantum dots.
Kerfoot, Mark L; Govorov, Alexander O; Czarnocki, Cyprian; Lu, Davis; Gad, Youstina N; Bracker, Allan S; Gammon, Daniel; Scheibner, Michael
2014-01-01
Modern technology is founded on the intimate understanding of how to utilize and control electrons. Next to electrons, nature uses phonons, quantized vibrations of an elastic structure, to carry energy, momentum and even information through solids. Phonons permeate the crystalline components of modern technology, yet in terms of technological utilization phonons are far from being on par with electrons. Here we demonstrate how phonons can be employed to render a single quantum dot pair optically transparent. This phonon-induced transparency is realized via the formation of a molecular polaron, the result of a Fano-type quantum interference, which proves that we have accomplished making typically incoherent and dissipative phonons behave in a coherent and non-dissipative manner. We find the transparency to be widely tunable by electronic and optical means. Thereby we show amplification of weakest coupling channels. We further outline the molecular polaron's potential as a control element in phononic circuitry architecture. PMID:24534815
Unraveling the Mesoscopic Character of Quantum Dots in Nanophotonics
NASA Astrophysics Data System (ADS)
Tighineanu, P.; Sørensen, A. S.; Stobbe, S.; Lodahl, P.
2015-06-01
We provide a microscopic theory for semiconductor quantum dots that explains the pronounced deviations from the prevalent point-dipole description that were recently observed in spectroscopic experiments on quantum dots in photonic nanostructures. The deviations originate from structural inhomogeneities generating a large circular quantum current density that flows inside the quantum dot over mesoscopic length scales. The model is supported by the experimental data, where a strong variation of the multipolar moments across the emission spectrum of quantum dots is observed. Our work enriches the physical understanding of quantum dots and is of significance for the fields of nanophotonics, quantum photonics, and quantum-information science, where quantum dots are actively employed.
Quantum dot loaded immunomicelles for tumor imaging
Aristarchos Papagiannaros; Jaydev Upponi; William Hartner; Dmitriy Mongayt; Tatyana Levchenko; Vladimir Torchilin
2010-01-01
BACKGROUND: Optical imaging is a promising method for the detection of tumors in animals, with speed and minimal invasiveness. We have previously developed a lipid coated quantum dot system that doubles the fluorescence of PEG-grafted quantum dots at half the dose. Here, we describe a tumor-targeted near infrared imaging agent composed of cancer-specific monoclonal anti-nucleosome antibody 2C5, coupled to quantum
George, Steven C.
to Third-Generation Photovoltaic Solar Cells A. J. Nozik,*,, M. C. Beard, J. M. Luther, M. Law,§ R. J. Applications: Quantum Dot Solar Cells 6884 6.1. Quantum Dot Solar Cell Configurations 6885 6.1.1. Photoelectrodes Composed of Quantum Dot Arrays 6885 6.1.2. Quantum Dot-Sensitized Nanocrystalline TiO2 Solar Cells
Electron spin coherence near room temperature in magnetic quantum dots.
Moro, Fabrizio; Turyanska, Lyudmila; Wilman, James; Fielding, Alistair J; Fay, Michael W; Granwehr, Josef; Patanè, Amalia
2015-01-01
We report on an example of confined magnetic ions with long spin coherence near room temperature. This was achieved by confining single Mn(2+) spins in colloidal semiconductor quantum dots (QDs) and by dispersing the QDs in a proton-spin free matrix. The controlled suppression of Mn-Mn interactions and minimization of Mn-nuclear spin dipolar interactions result in unprecedentedly long phase memory (TM?~?8??s) and spin-lattice relaxation (T1?~?10?ms) time constants for Mn(2+) ions at T?=?4.5?K, and in electron spin coherence observable near room temperature (TM?~?1??s). PMID:26040432
Electron spin coherence near room temperature in magnetic quantum dots
Moro, Fabrizio; Turyanska, Lyudmila; Wilman, James; Fielding, Alistair J.; Fay, Michael W.; Granwehr, Josef; Patanè, Amalia
2015-01-01
We report on an example of confined magnetic ions with long spin coherence near room temperature. This was achieved by confining single Mn2+ spins in colloidal semiconductor quantum dots (QDs) and by dispersing the QDs in a proton-spin free matrix. The controlled suppression of Mn–Mn interactions and minimization of Mn–nuclear spin dipolar interactions result in unprecedentedly long phase memory (TM?~?8??s) and spin–lattice relaxation (T1?~?10?ms) time constants for Mn2+ ions at T?=?4.5?K, and in electron spin coherence observable near room temperature (TM?~?1??s). PMID:26040432
What Quantum Dots Can Do for You
NASA Astrophysics Data System (ADS)
Salamo, Gregory
2008-03-01
Recent clever techniques for fabricating nanosize materials, one-atomic-layer-at-a-time, have simultaneously opened a door to a fantastic adventure at the frontier of physics, chemistry, biology, and engineering. Nanosize materials simply do not behave as the bulk. Indeed, the rules that govern the growth and behavior of these tiny structures are unexplored. In this talk we will discuss our recent efforts to be the architect of their shape, size, density, and position of nanostructures and along the way, the interactions between them that lead to their optical and electrical behavior. While self-assembly is providing exciting quantum dot (QD) structures to explore, like the QD molecules shown here, it is equally exciting to try to use the rules we uncover to encourage QD formation to take a desired path. Can we understand the formation of faceted nanostructures? Can we encourage or seed dot structures to form specific arrays? Is it possible to engineer greater homogeneity of dot shape and size? Can we design both the optical and electrical behavior of either individual or arrays of nanostructures to mimic those we find in nature? In this talk we will review our progress to answer these questions and discuss the possibilities and challenges ahead. For example, we will discuss the formation of individual faceted nanostructures as well as the fabrication of a vertically and laterally ordered QD stacks forming three-dimensional QD arrays. As another example, we will discuss the importance of surfaces with high Miller indices, as a template to the formation of nanostructures as well as their potential role in determining the shape and increased size uniformity of the confined structures. Importantly, these observations lead to an even more basic question of when and why high index surfaces are stable. Indeed, we have found that in order to understand the origin of high index surfaces that bound nanostructures we have to study them directly.
Floquet Majorana Modes in Graphene Quantum Dots
NASA Astrophysics Data System (ADS)
Li, Yantao; Kundu, Arijit; Seradjeh, Babak
2015-03-01
We propose a possible way to realize Floquet Majorana fermions in graphene quantum dots connected by a superconducting island. The effective crossed Andreev reflection and hopping amplitudes between the dots are calculated as a function of system parameters. It is shown that the spin degeneracy is broken when the dots are driven out of phase. This all-electric, highly tunable device could be a realistic platform for uncovering dynamically generated Majorana fermions in graphene system.
Probing the size and environment induced phase transformation in CdSe quantum dots
Karakoti, Ajay S.; Sanghavi, Shail P.; Nachimuthu, Ponnusamy; Yang, Ping; Thevuthasan, Suntharampillai
2011-11-17
The structural and electronic properties of CdSe quantum dots in toluene and drop-casted on Si wafer were investigated by in-situ micro X-ray diffraction, X-ray photoelectron spectroscopy and UV-Vis absorption and emission spectroscopy. The in-situ micro diffraction data show that the CdSe quantum dots capped with TOPO or hexadecylamine (HDA) in toluene exhibit predominantly wurtzite crystal structure, which undergoes a phase transformation to zinc blende crystal structure following drop casting on Si and this phase transition increases with decreasing the size of the CdSe quantum dots. Decreasing the size of quantum dots also increases the Se vacancies that facilitate the phase transformation. The X-ray photoelectron spectra show a systematic increase in the core level binding energies of Cd 3d and Se 3d, the band gap and the Cd/Se ratio as the size of the quantum dots decreases from 6.6nm to 2.1nm. This is attributed to the quantum confinement of CdSe crystallites by the capping ligands in toluene which increases with decreasing the size of the quantum dots. However, drop-casting quantum dots on Si alter the density and arrangement of capping ligands and solvent molecules on the quantum dots which causes significant phase transformation.
Nanostructure assembly of indium sulphide quantum dots and their characterization.
Vigneashwari, B; Ravichandran, V; Parameswaran, P; Dash, S; Tyagi, A K
2008-02-01
Nanocrystals (approximately 5 nm) of the semiconducting wide band gap material beta-In2S3 obtained by chemical synthesis through a hydrothermal route were characterized for phase and compositional purity. These nanoparticles exhibited quantum confinement characteristics as revealed by a blue-shifted optical absorption. These quantum dots of beta-In2S3 were electrically driven from a monodisperse colloidal suspension on to conducting glass substrates by Electophoretic Deposition (EPD) technique and nanostructural thin films were obtained. The crystalline and morphological structures of these deposits were investigated by X-ray diffraction and nanoscopic techniques. We report here that certain interesting nanostructural morphologies were observed in the two-dimensional quantum dot assemblies of beta-In2S3. The effect of the controlling parameters on the cluster growth and deposit integrity was also systematically studied through a series of experiments and the results are reported here. PMID:18464393
Control of the cavity reflectivity using a single quantum dot spin
NASA Astrophysics Data System (ADS)
Sun, Shuo; Kim, Hyochul; Solomon, Glenn; Waks, Edo
2015-03-01
The implementation of quantum network and distributive quantum information processing relies on interaction between stationary matter qubits and flying photons. The spin of a single electron or hole confined in a quantum dot is considered as promising matter qubit as it possesses microsecond coherence time and allows picosecond timescale control using optical pulses. The quantum dot spin can also interact with a photon by controlling the optical response of a strongly coupled cavity. Yet all the experimental demonstrations of the cavity spectrum control have used neutral dots. The spin-dependent cavity spectrum for a strongly coupled charged quantum dot and cavity system has not been reported. Here, we report an experimental realization of a spin-photon interface using a strongly coupled quantum dot and cavity system. We show large modulation of the cavity reflection spectrum by manipulating the spin states of the quantum dot. The spin-photon interface is crucial for realizing a quantum logic gate or generating hybrid entanglement between a quantum dot spin and a photon. Our results represent an important step towards semiconductor based quantum logic devices and on-chip quantum networks.
Carey, Graham H; Levina, Larissa; Comin, Riccardo; Voznyy, Oleksandr; Sargent, Edward H
2015-06-01
Through a combination of chemical and mutual dot-to-dot surface passivation, high-quality colloidal quantum dot solids are fabricated. The joint passivation techniques lead to a record diffusion length for colloidal quantum dots of 230 ± 20 nm. The technique is applied to create thick photovoltaic devices that exhibit high current density without losing fill factor. PMID:25899173
Elias Towe; Dong Pan
2000-01-01
Semiconductor quantum-dot nanostructures are interesting objects for fundamental as well as practical reasons. Fundamentally, they can form the basis of systems in which to study the quantum mechanics of electrons confined in zero-dimensional (0-D) space. In practice, the dots can be embedded in the active regions of a new class of electronic and optoelectronic devices with novel functionalities. This paper
Visible InGaN\\/GaN Quantum-Dot Materials and Devices
Nicolas Grandjean; Marc Ilegems
2007-01-01
General properties of III-V nitride-based quantum dots (QDs) are presented, with a special emphasis on InGaN\\/GaN QDs for visible optoelectronic devices. Stranski-Krastanov GaN\\/AlN dots are first discussed as a prototypical system. It is shown that the optical transition energies are governed by a giant quantum-confined Stark effect, which is the consequence of the presence of a large built-in internal electric
Two-electron localization in a quantum dot molecule driven by a cosine squared field
NASA Astrophysics Data System (ADS)
Zhong, Guang-Hui; Wang, Li-Min
2015-04-01
We investigate the dynamics of two interacting electrons confined in a quantum dot molecule under the influence of cosine squared electric fields. The conditions for two-electron localization in the same quantum dot are analytically derived within the frame of the Floquet formalism. The analytical results are compared to numerical results obtained from the solution of the time-dependent Schödinger equation. Project supported by the Natural Science Foundation of Hebei Province, China (Grant No. A201405104).
Graphene quantum dots in perpendicular magnetic fields
J. Güttinger; C. Stampfer; T. Frey; T. Ihn; K. Ensslin
2009-01-01
We report transport experiments on graphene quantum dots. We focus on excited state spectra in the near vicinity of the charge neutrality point and signatures of the electron-hole crossover as a function of a perpendicular magnetic field. Coulomb blockade resonances of a 50 nm wide and 80 nm long dot are visible at all gate voltages across the transport gap
Coulomb blockade in graphene quantum dots
Qiong Ma; Tao Tu; Zhi-Rong Lin; Guang-Can Guo; Guo-Ping Guo
2009-01-01
We study the conductance spectrum of graphene quantum dots, both single and multiple cases. The single electron tunneling phenomenon is investigated and the periodicity, amplitude and line shape of the Coulomb blockade oscillations at low temperatures are obtained. Further, we discuss the transport behavior when multiple dots are assembled in array and find a phase transition of conductance spectra from
Spin-orbit induced two-electron spin relaxation in double quantum dots
NASA Astrophysics Data System (ADS)
Borhani, Massoud; Hu, Xuedong
2011-03-01
We study the spin decay of two electrons confined in a double quantum dots via the spin-orbit interaction and acoustic phonons. We have obtained a generic form for the spin Hamiltonian for two electrons confined in (elliptic) harmonic potentials in doubles dots and in the presence of an arbitrary applied magnetic field. Our focus is on the interdot bias regime where singlet-triplet splitting is small, in contrast to the spin-blockade regime. Our results clarify the spin-orbit mediated two-spin relaxation in lateral/nanowire quantum dots, particularly when the confining potentials are different in each dot. We thank support by NSA/LPS thorugh ARO.
NASA Astrophysics Data System (ADS)
Ehsanfard, Najmeh; Kazerani Vahdani, Mohammad Reza
2015-07-01
The effects of smooth confining potential on the linear and nonlinear optical properties are investigated with Gaussian confining potential in Quantum Dots (GQDs). To this end the effective mass approximation, the compact density matrix approach, and iterative method are used to calculate linear and nonlinear Absorption Coefficient (AC) and Refractive Index (RI) change of an electron confined in GQD. The effects of confining potential and radius of GQD and also incident optical intensity have been investigated. The results show that increasing confining potential decreases the refractive index magnitude and shifts the peak values to the higher energy regions. It is also shown that the peak values of absorption coefficient increase by increasing confining potential. Both absorption coefficient and refractive index experience red shift by increasing the size of quantum dot. It is also shown that transition from step potential to Gaussian potential makes AC and RI to experience a blue shift.
Quantum dots and prion proteins
Sobrova, Pavlina; Blazkova, Iva; Chomoucka, Jana; Drbohlavova, Jana; Vaculovicova, Marketa; Kopel, Pavel; Hubalek, Jaromir; Kizek, Rene; Adam, Vojtech
2013-01-01
A diagnostics of infectious diseases can be done by the immunologic methods or by the amplification of nucleic acid specific to contagious agent using polymerase chain reaction. However, in transmissible spongiform encephalopathies, the infectious agent, prion protein (PrPSc), has the same sequence of nucleic acids as a naturally occurring protein. The other issue with the diagnosing based on the PrPSc detection is that the pathological form of prion protein is abundant only at late stages of the disease in a brain. Therefore, the diagnostics of prion protein caused diseases represent a sort of challenges as that hosts can incubate infectious prion proteins for many months or even years. Therefore, new in vivo assays for detection of prion proteins and for diagnosis of their relation to neurodegenerative diseases are summarized. Their applicability and future prospects in this field are discussed with particular aim at using quantum dots as fluorescent labels. PMID:24055838
Quantum dot-based theranostics.
Ho, Yi-Ping; Leong, Kam W
2010-01-01
Luminescent semiconductor nanocrystals, also known as quantum dots (QDs), have advanced the fields of molecular diagnostics and nanotherapeutics. Much of the initial progress for QDs in biology and medicine has focused on developing new biosensing formats to push the limit of detection sensitivity. Nevertheless, QDs can be more than passive bio-probes or labels for biological imaging and cellular studies. The high surface-to-volume ratio of QDs enables the construction of a "smart" multifunctional nanoplatform, where the QDs serve not only as an imaging agent but also a nanoscaffold catering for therapeutic and diagnostic (theranostic) modalities. This mini review highlights the emerging applications of functionalized QDs as fluorescence contrast agents for imaging or as nanoscale vehicles for delivery of therapeutics, with special attention paid to the promise and challenges towards QD-based theranostics. PMID:20648364
Quantum dot-based theranostics
Ho, Yi-Ping; Leong, Kam W.
2010-01-01
Luminescent semiconductor nanocrystals, also known as quantum dots (QDs), have advanced the fields of molecular diagnostics and nanotherapeutics. Much of the initial progress for QDs in biology and medicine has focused on developing new biosensing formats to push the limit of detection sensitivity. Nevertheless, QDs can be more than passive bio-probes or labels for biological imaging and cellular studies. The high surface-to-volume ratio of QDs enables the construction of a “smart” multifunctional nanoplatform, where the QDs serve not only as an imaging agent but also a nanoscaffold catering for therapeutic and diagnostic (theranostic) modalities. This mini review highlights the emerging applications of functionalized QDs as fluorescence contrast agents for imaging or as nanoscale vehicles for delivery of therapeutics, with special attention paid to the promise and challenges towards QD-based theranostics. PMID:20648364
Quantum dot-based theranostics
NASA Astrophysics Data System (ADS)
Ho, Yi-Ping; Leong, Kam W.
2010-01-01
Luminescent semiconductor nanocrystals, also known as quantum dots (QDs), have advanced the fields of molecular diagnostics and nanotherapeutics. Much of the initial progress for QDs in biology and medicine has focused on developing new biosensing formats to push the limit of detection sensitivity. Nevertheless, QDs can be more than passive bio-probes or labels for biological imaging and cellular studies. The high surface-to-volume ratio of QDs enables the construction of a ``smart'' multifunctional nanoplatform, where the QDs serve not only as an imaging agent but also a nanoscaffold catering for therapeutic and diagnostic (theranostic) modalities. This mini review highlights the emerging applications of functionalized QDs as fluorescence contrast agents for imaging or as nanoscale vehicles for delivery of therapeutics, with special attention paid to the promise and challenges towards QD-based theranostics.
Optical Studies of Semiconductor Quantum Dots
NASA Astrophysics Data System (ADS)
Yükselici, H.; Allahverdi, Ç.; A??ko?lu, A.; Ünlü, H.; Baysal, A.; Çulha, M.; ?nce, R.; ?nce, A.; Feeney, M.; Athalin, H.
Optical absorption (ABS), steady-state photoluminescence (PL), resonant Raman, and photoabsorption (PA) spectroscopies are employed to study quantum-size effects in II-VI semiconductor quantum dots (QDs) grown in glass samples. We observe a size-dependent shift in the energetic position of the first exciton peak and have examined the photoinduced evolution of the differential absorption spectra. The Raman shifts of the phonon modes are employed to monitor stoichiometric changes in the composition of the QDs during growth. Two sets of glass samples were prepared from color filters doped with CdS x Se1 - x and Zn x Cd1 - x Te. We analyze the optical properties of QDs through the ABS, PL, resonant Raman, and PA spectroscopies. The glass samples were prepared from commercially available semiconductor doped filters by a two-step thermal treatment. The average size of QDs is estimated from the energetic position of the first exciton peak in the ABS spectrum. A calculation based on a quantized-state effective mass model in the strong confinement regime predicts that the average radius of QDs in the glass samples ranges from 2.9 to 4.9 nm for CdTe and from 2.2 to 9.3 nm for CdS0. 08Se0. 92. We have also studied the nonlinear optical properties of QDs by reviewing the results of size-dependent photoinduced modulations in the first exciton band of CdTe QDs studied by PA spectroscopy.
Synthesis and characterization of infrared quantum dots
Harris, Daniel Kelly
2014-01-01
This thesis focuses on the development of synthetic methods to create application ready quantum dots (QDs) in the infrared for biological imaging and optoelectronic devices. I concentrated primarily on controlling the size ...
Photodetectors based on colloidal quantum dots
Oertel, David C. (David Charles)
2007-01-01
Inspired by recent work demonstrating photocurrent enhancement in quantum-dot (QD) solids via post-deposition chemical annealing and by recent successes incorporating single monolayers of QDs in light-emitting devices ...
Luminescence blinking of a reacting quantum dot.
Routzahn, Aaron L; Jain, Prashant K
2015-04-01
Luminescence blinking is an inherent feature of optical emission from individual fluorescent molecules and quantum dots. There have been intense efforts, although not with complete resolution, toward the understanding of the mechanistic origin of blinking and also its mitigation in quantum dots. As an advance in our microscopic view of blinking, we show that the luminescence blinking of a quantum dot becomes unusually heavy in the temporal vicinity of a reactive transformation. This stage of heavy blinking is a result of defects/dopants formed within the quantum dot on its path to conversion. The evolution of blinking behavior along the reaction path allows us to measure the lifetime of the critical dopant-related intermediate in the reaction. This work establishes luminescence blinking as a single-nanocrystal level probe of catalytic, photocatalytic, and electrochemical events occurring in the solid-state or on semiconductor surfaces. PMID:25730168
Scanning Tunneling Spectroscopy of Semiconductor Quantum Dots and Nanocrystals
Giuseppe Maruccio; Roland Wiesendanger
2010-01-01
\\u000a Quantum dots (QDs) and nanocrystals (NCs) have attracted great attention for applications in nano- and opto-electronics, quantum\\u000a computation, biosensing, and nanomedicine. Three-dimensional electronic confinement can be achieved based on lateral or vertical\\u000a QDs in a two-dimensional electron gas, by strain-induced QDs, or by colloidal NCs.\\u000a \\u000a \\u000a In this chapter, we will focus on tunneling spectroscopy on semiconductor QDs and NCs. First,
Artificial atoms in interacting graphene quantum dots
Wolfgang Häusler; Reinhold Egger
2009-01-01
We describe the theory of few Coulomb-correlated electrons in a magnetic quantum dot formed in graphene. While the corresponding nonrelativistic (Schrödinger) problem is well understood, a naive generalization to graphene's ``relativistic'' (Dirac-Weyl) spectrum encounters divergencies and is ill defined. We employ Sucher's projection formalism to overcome these problems. Exact diagonalization results for the two-electron quantum dot, i.e., the artificial helium
Transmission and scarring in graphene quantum dots
Liang Huang; Ying-Cheng Lai; David K. Ferry; Richard Akis; Stephen M. Goodnick
2009-01-01
We study electronic transport in quantum-dot structures made of graphene. Focusing on the rectangular dot geometry and utilizing the non-equilibrium Green's function to calculate the transmission in the tight-binding framework, we find significant fluctuations in the transmission as a function of the electron energy. The fluctuations are correlated with the formation of quantum scarring states, or pointer states in the
Spin relaxation in graphene quantum dots
Guido Burkard; Philipp Struck
2010-01-01
With its low concentration of nuclear spins and relatively weak spin-orbit coupling, graphene is a promising host material for electron spin qubits. We have calculated the spin relaxation time T1 of a single spin in graphene quantum dots [1,2] as a function of the externally applied magnetic field B. We find that in quantum dots without coupling between the valleys
Spin States in Graphene Quantum Dots
J. Güttinger; T. Frey; C. Stampfer; T. Ihn; K. Ensslin
2010-01-01
We investigate ground and excited state transport through small (d≈70nm) graphene quantum dots. The successive spin filling of orbital states is detected by measuring the difference between ground-state energies as a function of a magnetic field. For a magnetic field in-plane of the quantum dot the Zeeman splitting of spin states is measured. The results are compatible with a g
Tunneling through Quantum Dots with Discrete Symmetries
Yshai Avishai; Konstantin Kikoin
2008-01-01
We describe in this short review the influence of discrete symmetries in complex quantum dots on the Kondo co-tunneling through these nano-objects. These discrete symmetries stem from the geometrical structure of the tunneling devices (e.g spatial symmetry of multivalley quantum dot in a tunneling contact with leads). They affect the dynamical symmetry of spin multiplets characterizing the ground state and
Tunneling Through Quantum Dots with Discrete Symmetries
Yshai Avishai; Konstantin Kikoin
2009-01-01
We describe in this short review the influence of discrete symmetries in\\u000acomplex quantum dots on the Kondo co-tunneling through these nano-objects.\\u000aThese discrete symmetries stem from the geometrical structure of the tunneling\\u000adevices (e.g spatial symmetry of multivalley quantum dot in a tunneling contact\\u000awith leads). They affect the dynamical symmetry of spin multiplets\\u000acharacterizing the ground state and
Spatially separated excitons in quantum-dot quantum well structures
Kai Chang; Jian-Bai Xia
1998-01-01
In the framework of the effective-mass envelope-function theory, the electronic and optical properties of a spherical core-shell quantum-dot quantum well (QDQW) structure with one and two wells have been investigated. The results show that the energies of electron and hole states depend sensitively on the well thickness and core radius of quantum-dot quantum well structure. An interesting spatially separated characteristic
Quantum-Well to Quantum-Dot Tunneling
Shun Lien Chuang Holonyak Jr.
2002-01-01
We have developed a tunneling formula for injection of carriers from a quantum well (QW) into a quantum dot (QD). Our theory is based on the transition-probability approach using the tunneling Hamiltonian of Bardeen [1]. An analytical expression is found for the tunneling rate. We show that the quantum-well to quantum-dot tunneling can be extremely fast when the well, barrier,
Modeling of an electrically tunable quantum dot photodetector for terahertz detection
Shahriar, Selim
confinement from the quantum barrier forms a quantum dot structure. Using the energy states and intersublevel absorption of water vapor from high altitude to ground. To realize these applications in terahertz region , and silicon bolometers12 etc. However, these detectors either need sophisticated instrumentation
Role of Symmetry Breaking on the Optical Transitions in Lead-Salt Quantum Dots
Van Stryland, Eric
Role of Symmetry Breaking on the Optical Transitions in Lead-Salt Quantum Dots Gero Nootz of Optics and Photonics, University of Central Florida, Orlando, Florida 32816, Physics Department, M5S3G4 ABSTRACT The influence of quantum confinement on the one- and two-photon absorption spectra
NASA Astrophysics Data System (ADS)
Malkova, Natalia; Bryant, Garnett W.
2010-10-01
The spectrum of quantum dots made from semiconductors such as HgTe and HgS changes from negative gap to positive gap with decreasing size. Furthermore, intrinsic surface states, which are not related to dangling bonds, appear in the negative-gap regime. We investigate theoretically the evolution of the spectrum of HgS quantum dots with decreasing size and show how states evolve from a negative gap to a positive gap as confinement is increased. The lowest confined electron level evolves into an intrinsic surface state with increasing size and, thus, is not derived directly from a bulk HgS band. Due to strong band mixing in narrow-gap semiconductors, spacing between confined levels decreases more slowly with increasing size than for quantum dots made from wide-gap semiconductors. Moreover, dielectric screening becomes nearly metallic as the gap closes. As a consequence, confinement energies dominate exciton binding energies for all dot sizes up to the gap closure. Excitons remain in the strong confinement limit as size increases until the gap closes. Nonetheless, the exciton binding exceeds the single-particle gap for sizes near gap closure, opening up the possibility of an excitonic insulator phase in quantum dots not possible in positive-gap quantum dots. Signatures in the quantum-dot optical response for gap collapse and surface states are identified.
Quantum confinement-induced tunable exciton states in graphene oxide
Lee, Dongwook; Seo, Jiwon; Zhu, Xi; Lee, Jiyoul; Shin, Hyeon-Jin; Cole, Jacqueline M.; Shin, Taeho; Lee, Jaichan; Lee, Hangil; Su, Haibin
2013-01-01
Graphene oxide has recently been considered to be a potential replacement for cadmium-based quantum dots due to its expected high fluorescence. Although previously reported, the origin of the luminescence in graphene oxide is still controversial. Here, we report the presence of core/valence excitons in graphene-based materials, a basic ingredient for optical devices, induced by quantum confinement. Electron confinement in the unreacted graphitic regions of graphene oxide was probed by high resolution X-ray absorption near edge structure spectroscopy and first-principles calculations. Using experiments and simulations, we were able to tune the core/valence exciton energy by manipulating the size of graphitic regions through the degree of oxidation. The binding energy of an exciton in highly oxidized graphene oxide is similar to that in organic electroluminescent materials. These results open the possibility of graphene oxide-based optoelectronic device technology. PMID:23872608
Optical properties of PbSe nanocrystal quantum dots under pressure
Kirill K. Zhuravlev; Jeffrey M. Pietryga; Robert K. Sander; Richard D. Schaller
2007-01-01
The optical properties of PbSe nanocrystal quantum dots (NQDs) were studied as a function of applied hydrostatic pressure over the range from ambient to 5.4 GPa. PbSe NQDs exhibit an energy gap that is dominated by quantum confinement. Despite such strong confinement, the authors find that the energy gaps of 3, 5, and 7 nm diameter PbSe NQDs change monotonically
Optical Properties of PbSe Nanocrystal Quantum Dots Under Pressure
Kirill K. Zhuravlev; Jeffrey M. Pietryga; Robert K. Sander; Richard D. Schaller
2007-01-01
The optical properties of PbSe nanocrystal quantum dots (NQDs) were studied as a function of applied hydrostatic pressure over the range from ambient to 4 GPa. PbSe NQDs exhibit an energy gap that is dominated by quantum confinement energy. Despite such strong confinement, we find that the energy gaps of 3, 5, and 7 nm PbSe NQDs change monotonically with
Computational intelligence applied to the growth of quantum dots
NASA Astrophysics Data System (ADS)
Singulani, Anderson P.; Vilela Neto, Omar P.; Aurélio Pacheco, Marco C.; Vellasco, Marley B. R.; Pires, Maurício P.; Souza, Patrícia L.
2008-11-01
We apply two computational intelligence techniques, namely, artificial neural network and genetic algorithm to the growth of self-assembled quantum dots. The method relies on an existing database of growth parameters with a resulting quantum dot characteristic to be able to later obtain the growth parameters needed to reach a specific value for such a quantum dot characteristic. The computational techniques were used to associate the growth input parameters with the mean height of the deposited quantum dots. Trends of the quantum dot mean height behavior as a function of growth parameters were correctly predicted and the growth parameters required to minimize the quantum dot mean height were provided.
Evaporation-Induced Assembly of Quantum Dots into Nanorings
Chen, Jixin; Liao, Wei-Ssu; Chen, Xin; Yang, Tinglu; Wark, Stacey E.; Son, Dong Hee; Batteas, James D.; Cremer, Paul S.
2011-01-01
Herein, we demonstrate the controlled formation of two-dimensional periodic arrays of ring-shaped nanostructures assembled from CdSe semiconductor quantum dots (QDs). The patterns were fabricated by using an evaporative templating method. This involves the introduction of an aqueous solution containing both quantum dots and polystyrene microspheres onto the surface of a planar hydrophilic glass substrate. The quantum dots became confined to the meniscus of the microspheres during evaporation, which drove ring assembly via capillary forces at the polystyrene sphere/glass substrate interface. The geometric parameters for nanoring formation could be controlled by tuning the size of the microspheres and the concentration of the QDs employed. This allowed hexagonal arrays of nanorings to be formed with thicknesses ranging from single dot necklaces to thick multilayer structures over surface areas of many square millimeters. Moreover, the diameter of the ring structures could be simultaneously controlled. A simple model was employed to explain the forces involved in the formation of nanoparticle nanorings. PMID:19206264
Thermoelectric transport through strongly correlated quantum dots
T. A. Costi; V. Zlatic
2010-07-08
The thermoelectric properties of strongly correlated quantum dots, described by a single level Anderson model coupled to conduction electron leads, is investigated using Wilson's numerical renormalization group method. We calculate the electronic contribution, $K_{\\rm e}$, to the thermal conductance, the thermopower, $S$, and the electrical conductance, $G$, of a quantum dot as a function of both temperature, $T$, and gate voltage, ${\\rm v}_g$, for strong, intermediate and weak Coulomb correlations, $U$, on the dot. For strong correlations and in the Kondo regime, we find that the thermopower exhibits two sign changes, at temperatures $T_{1}({\\rm v}_g)$ and $T_{2}({\\rm v}_g)$ with $T_{1}< T_{2}$. Such sign changes in $S(T)$ are particularly sensitive signatures of strong correlations and Kondo physics. The relevance of this to recent thermopower measurements of Kondo correlated quantum dots is discussed. We discuss the figure of merit, power factor and the degree of violation of the Wiedemann-Franz law in quantum dots. The extent of temperature scaling in the thermopower and thermal conductance of quantum dots in the Kondo regime is also assessed.
Quantum-dot-in-perovskite solids.
Ning, Zhijun; Gong, Xiwen; Comin, Riccardo; Walters, Grant; Fan, Fengjia; Voznyy, Oleksandr; Yassitepe, Emre; Buin, Andrei; Hoogland, Sjoerd; Sargent, Edward H
2015-07-15
Heteroepitaxy-atomically aligned growth of a crystalline film atop a different crystalline substrate-is the basis of electrically driven lasers, multijunction solar cells, and blue-light-emitting diodes. Crystalline coherence is preserved even when atomic identity is modulated, a fact that is the critical enabler of quantum wells, wires, and dots. The interfacial quality achieved as a result of heteroepitaxial growth allows new combinations of materials with complementary properties, which enables the design and realization of functionalities that are not available in the single-phase constituents. Here we show that organohalide perovskites and preformed colloidal quantum dots, combined in the solution phase, produce epitaxially aligned 'dots-in-a-matrix' crystals. Using transmission electron microscopy and electron diffraction, we reveal heterocrystals as large as about 60 nanometres and containing at least 20 mutually aligned dots that inherit the crystalline orientation of the perovskite matrix. The heterocrystals exhibit remarkable optoelectronic properties that are traceable to their atom-scale crystalline coherence: photoelectrons and holes generated in the larger-bandgap perovskites are transferred with 80% efficiency to become excitons in the quantum dot nanocrystals, which exploit the excellent photocarrier diffusion of perovskites to produce bright-light emission from infrared-bandgap quantum-tuned materials. By combining the electrical transport properties of the perovskite matrix with the high radiative efficiency of the quantum dots, we engineer a new platform to advance solution-processed infrared optoelectronics. PMID:26178963
Charge transport through a semiconductor quantum dot-ring nanostructure.
Kurpas, Marcin; K?dzierska, Barbara; Janus-Zygmunt, Iwona; Gorczyca-Goraj, Anna; Wach, El?bieta; Zipper, El?bieta; Ma?ka, Maciej M
2015-07-01
Transport properties of a gated nanostructure depend crucially on the coupling of its states to the states of electrodes. In the case of a single quantum dot the coupling, for a given quantum state, is constant or can be slightly modified by additional gating. In this paper we consider a concentric dot-ring nanostructure (DRN) and show that its transport properties can be drastically modified due to the unique geometry. We calculate the dc current through a DRN in the Coulomb blockade regime and show that it can efficiently work as a single-electron transistor (SET) or a current rectifier. In both cases the transport characteristics strongly depend on the details of the confinement potential. The calculations are carried out for low and high bias regime, the latter being especially interesting in the context of current rectification due to fast relaxation processes. PMID:26052631
Charge transport through a semiconductor quantum dot-ring nanostructure
NASA Astrophysics Data System (ADS)
Kurpas, Marcin; K?dzierska, Barbara; Janus-Zygmunt, Iwona; Gorczyca-Goraj, Anna; Wach, El?bieta; Zipper, El?bieta; Ma?ka, Maciej M.
2015-07-01
Transport properties of a gated nanostructure depend crucially on the coupling of its states to the states of electrodes. In the case of a single quantum dot the coupling, for a given quantum state, is constant or can be slightly modified by additional gating. In this paper we consider a concentric dot–ring nanostructure (DRN) and show that its transport properties can be drastically modified due to the unique geometry. We calculate the dc current through a DRN in the Coulomb blockade regime and show that it can efficiently work as a single-electron transistor (SET) or a current rectifier. In both cases the transport characteristics strongly depend on the details of the confinement potential. The calculations are carried out for low and high bias regime, the latter being especially interesting in the context of current rectification due to fast relaxation processes.
Full-colour quantum dot displays fabricated by transfer printing
Tae-Ho Kim; Kyung-Sang Cho; Eun Kyung Lee; Sang Jin Lee; Jungseok Chae; Jung Woo Kim; Do Hwan Kim; Jang-Yeon Kwon; Gehan Amaratunga; Sang Yoon Lee; Byoung Lyong Choi; Young Kuk; Jong Min Kim; Kinam Kim
2011-01-01
Light-emitting diodes with quantum dot luminophores show promise in the development of next-generation displays, because quantum dot luminophores demonstrate high quantum yields, extremely narrow emission, spectral tunability and high stability, among other beneficial characteristics. However, the inability to achieve size-selective quantum dot patterning by conventional methods hinders the realization of full-colour quantum dot displays. Here, we report the first demonstration
Excitonic optical properties of wurtzite ZnS quantum dots under pressure.
Zeng, Zaiping; Garoufalis, Christos S; Baskoutas, Sotirios; Bester, Gabriel
2015-03-21
By means of atomistic empirical pseudopotentials combined with a configuration interaction approach, we have studied the optical properties of wurtzite ZnS quantum dots in the presence of strong quantum confinement effects as a function of pressure. We find the pressure coefficients of quantum dots to be highly size-dependent and reduced by as much as 23% in comparison to the bulk value of 63 meV/GPa obtained from density functional theory calculations. The many-body excitonic effects on the quantum dot pressure coefficients are found to be marginal. The absolute gap deformation potential of quantum dots originates mainly from the energy change of the lowest unoccupied molecular orbital state. Finally, we find that the exciton spin-splitting increases nearly linearly as a function of applied pressure. PMID:25796247
Quantum Dots for quantitative flow cytometry
Buranda, Tione; Wu, Yang; Sklar, Larry A.
2014-01-01
Summary In flow cytometry, the quantitation of fluorophore-tagged ligands and receptors on cells or at particulate surfaces is achieved by the use of standard beads of known calibration. To the best of our knowledge, only those calibration beads based on fluorescein, EGFP, phycoerythyrin and allophycocyanine are readily available from commercial sources. Because fluorophore based standards are specific to the selected fluorophore tag, their applicability is limited to the spectral region of resonance. Since quantum dots can be photo-excited over a continuous and broad spectral range governed by their size, it is possible to match the spectral range and width (absorbance and emission) of a wide range of fluorophores with appropriate quantum dots. Accordingly, quantitation of site coverage of the target fluorophores can be readily achieved using quantum dots whose emission spectra overlaps with the target fluorophore. This chapter will focus on the relevant spectroscopic concepts and molecular assembly of quantum dot fluorescence calibration beads. We will first examine the measurement and applicability of spectroscopic parameters, ?, ?, and %T to fluorescence calibration standards. Where, ? is the absorption coefficient of the fluorophore, ? is the quantum yield of the fluorophore and %T is the percent fraction of emitted light that is transmitted by the bandpass filter at the detector PMT. The modular construction of beads decorated with discrete quantities of quantum dots with defined spectroscopic parameters is presented in the context of a generalizable approach to calibrated measurements of fluorescence in flow cytometry. PMID:21116979
Quantum dots for quantitative flow cytometry.
Buranda, Tione; Wu, Yang; Sklar, Larry A
2011-01-01
In flow cytometry, the quantitation of fluorophore-tagged ligands and receptors on cells or at particulate surfaces is achieved by the use of standard beads of known calibration. To the best of our knowledge, only those calibration beads based on fluorescein, EGFP, phycoerythyrin and allophycocyanine are readily available from commercial sources. Because fluorophore-based standards are specific to the selected fluorophore tag, their applicability is limited to the spectral region of resonance. Since quantum dots can be photo-excited over a continuous and broad spectral range governed by their size, it is possible to match the spectral range and width (absorbance and emission) of a wide range of fluorophores with appropriate quantum dots. Accordingly, quantitation of site coverage of the target fluorophores can be readily achieved using quantum dots whose emission spectra overlaps with the target fluorophore.This chapter focuses on the relevant spectroscopic concepts and molecular assembly of quantum dot fluorescence calibration beads. We first examine the measurement and applicability of spectroscopic parameters, ?, ?, and %T to fluorescence calibration standards, where ? is the absorption coefficient of the fluorophore, ? is the quantum yield of the fluorophore, and %T is the percent fraction of emitted light that is transmitted by the bandpass filter at the detector PMT. The modular construction of beads decorated with discrete quantities of quantum dots with defined spectroscopic parameters is presented in the context of a generalizable approach to calibrated measurements of fluorescence in flow cytometry. PMID:21116979
Single Electron Charging Effects in Quantum Dot Nanostructures.
NASA Astrophysics Data System (ADS)
Kumar, Arvind
This thesis focuses on the study of GaAs quantum dot devices, in which an electron gas is electrostatically confined to a small conducting island. The device dimensions are sufficiently small that striking effects due to the capacitive charging of the island by a single electron can be observed in the transistor characteristics, leading to a periodic dependence of the current on gate voltage. In particular, we study a quantum dot structure in which a novel gate geometry allows the island to be contacted by three electron reservoirs. When the dot charge is well -confined, periodic conductance oscillations due to Coulomb charging are observed in-phase with each other at two of the leads in response to a small excitation voltage at the third. As the tunnel barriers are made softer by changing the gate voltage, a strikingly different phenomenon is observed: conductance peaks at the two output leads evolve from perfect correlation to perfect anti-correlation with each other. Two simple models of transport in the weakly blockaded regime are presented as possible explanations. (Cpies available exclusively from MIT Libraries, Rm. 14-0551, Cambridge, MA 02139-4307. Ph. 617-253-5668; Fax 617-253 -1690.).
Quantum dots for light emitting diodes.
Qasim, Khan; Lei, Wei; Li, Qing
2013-05-01
In this article we discuss the development and key advantages of quantum dot based light emitting diode (QD-LED) and other applications based on their color purity, stability, and solution processibility. Analysis of quantum dot based LEDs and the main challenges faced in this field, such as the QD luminescence quenching, QD charging in thin films, and external quantum efficiency are discussed in detail. The description about how different optical down-conversion and structures enabled researchers to overcome these challenges and to commercialize the products. The recent developments about how to overcome these difficulties have also been discussed in this article. PMID:23858829
Pulse-gated quantum dot hybrid qubit
Teck Seng Koh; John King Gamble; Mark Friesen; M. A. Eriksson; S. N. Coppersmith
2012-07-24
A quantum dot hybrid qubit formed from three electrons in a double quantum dot has the potential for great speed, due to presence of level crossings where the qubit becomes charge-like. Here, we show how to take full advantage of the level crossings in a pulsed gating scheme, which decomposes the spin qubit into a series of charge transitions. We develop one and two-qubit dc quantum gates that are simpler than the previously proposed ac gates. We obtain closed form solutions for the control sequences and show that these sub-nanosecond gates can achieve high fidelities.
Chaotic Dirac Billiard in Graphene Quantum Dots
L. A. Ponomarenko; F. Schedin; M. I. Katsnelson; R. Yang; E. W. Hill; K. S. Novoselov; A. K. Geim
2008-01-01
The exceptional electronic properties of graphene, with its charge carriers mimicking relativistic quantum particles and its formidable potential in various applications, have ensured a rapid growth of interest in this new material. We report on electron transport in quantum dot devices carved entirely from graphene. At large sizes (>100 nanometers), they behave as conventional single-electron transistors, exhibiting periodic Coulomb blockade
Optical properties of quantum wires and dots
T. L. Reinecke; P. A. Knipp
1996-01-01
Recent work on the optical properties of quantum wire and quantum dot systems is discussed, including carrier, phonon and photon states, electronphonon scattering and excitonic effects. In realistic systems the geometry often results in the equations for the elementary excitations being non-separable. Numerical methods for calculating these excitations are discussed with emphasis on \\
Preparation of chiral quantum dots.
Moloney, Mícheál P; Govan, Joseph; Loudon, Alexander; Mukhina, Maria; Gun'ko, Yurii K
2015-04-01
Chiral quantum dots (QDs) are expected to have a range of potential applications in photocatalysis, as specific antibacterial and cytotoxic drug-delivery agents, in assays, as sensors in asymmetric synthesis and enantioseparation, and as fluorescent chiral nanoprobes in biomedical and analytical technologies. In this protocol, we present procedures for the synthesis of chiral optically active QD nanostructures and their quality control using spectroscopic studies and transmission electron microscopy imaging. We closely examine various synthetic routes for the preparation of chiral CdS, CdSe, CdTe and doped ZnS QDs, as well as of chiral CdS nanotetrapods. Most of these nanomaterials can be produced by a very fast (70 s) microwave-induced heating of the corresponding precursors in the presence of D- or L-chiral stabilizing coating ligands (stabilizers), which are crucial to generating optically active chiral QDs. Alternatively, chiral QDs can also be produced via the conventional hot injection technique, followed by a phase transfer in the presence of an appropriate chiral stabilizer. We demonstrate that the properties, structure and behavior of chiral QD nanostructures, as determined by various spectroscopic techniques, strongly depend on chiral stabilizers and that the chiral effects induced by them can be controlled via synthetic procedures. PMID:25741991
Bichromatic dressing of a quantum dot detected by a remote second quantum dot
NASA Astrophysics Data System (ADS)
Maragkou, M.; Sánchez-Muñoz, C.; Lazi?, S.; Chernysheva, E.; van der Meulen, H. P.; González-Tudela, A.; Tejedor, C.; Martínez, L. J.; Prieto, I.; Postigo, P. A.; Calleja, J. M.
2013-08-01
We demonstrate an information transfer mechanism between two dissimilar remote InAs/GaAs quantum dots weakly coupled to a common photonic crystal microcavity. Bichromatic excitation in the s state of one of the dots leads to the formation of dressed states due to the coherent coupling to the laser field, in resonance with the quantum dot. Information on the resulting dressed structure is read out through the photoluminescence spectrum of the other quantum dot, as well as the cavity mode. The effect is also observed upon exchange of the excitation and detection quantum dots. This quantum dot intertalk is interpreted in terms of a cavity-mediated coupling involving acoustic phonons. A master equation for a three-level system coherently pumped by the two lasers quantitatively describes the behavior of our system. Our result presents an important step towards scalable solid-state quantum networking based on coupled multi-quantum-dot-cavity systems, without the need to use identical quantum emitters.
Nanostructured architectures for colloidal quantum dot solar cells
Jean, Joel, S.M. Massachusetts Institute of Technology
2013-01-01
This thesis introduces a novel ordered bulk heterojunction architecture for colloidal quantum dot (QD) solar cells. Quantum dots are solution-processed nanocrystals whose tunable bandgap energies make them a promising ...
Electron tunneling and spin relaxation in a lateral quantum dot
Amasha, Sami
2008-01-01
We report measurements that use real-time charge sensing to probe a single-electron lateral quantum dot. The charge sensor is a quantum point contact (QPC) adjacent to the dot and the sensitivity is comparable to other ...
Biexciton induced refractive index changes in a semiconductor quantum dot
NASA Astrophysics Data System (ADS)
Shojaei, S.
2015-06-01
We present a detailed theoretical study of linear and third order nonlinear refractive index changes in a optically driven disk-like GaN quantum dot. In our numerical calculations, we consider the three level system containing biexciton, exciton, and ground states and use the compact density matrix formalism and iterative method to obtain refractive index changes. Variational method through effective mass approximation are employed to calculate the ground state energy of biexciton and exciton states. The evolution of refractive index changes around one, two and three photon resonance is investigated and discussed for different quantum dot sizes and light intensities. Size-dependent three-photon nonlinear refractive index change versus incident photon energy compared to that of two-photon is obtained and analyzed. As main result, we found that around resonance frequency at exciton-biexciton transition the quantum confinement has great influence on the linear change in refractive index so that for very large quantum dots, it decreases. Moreover, it was found that third order refractive index changes for three photon process is strongly dependent on QD size and light intensity. Our study reveals that considering our simple model leads to results which are in good agreement with other rare numerical results. Comparison with experimental results has been done.
Polaron coupling in quantum dot molecules
Verzelen, O.; Ferreira, R.; Bastard, G.
2001-08-15
We report on the calculation of polaron energies in InAs quantum dot molecules. Polaron effects are larger in vertical than in lateral molecules. The far infrared absorption associated with molecular polaron transitions is calculated. It may show prominent lines associated with inter dot polaron transitions. We have also calculated the polaron relaxation time to thermodynamical equilibrium when its lifetime is limited by the decay of its phonon component due to crystal anharmonicity.
Multi-Junction Solar Cell Spectral Tuning with Quantum Dots
Ryne P. Raffaelle; Samar Sinharoy; John Andersen; David M. Wilt; Sheila G. Bailey
2006-01-01
We have theoretically analyzed the potential efficiency improvement to multi-junction solar cell efficiencies which are available through the incorporation of quantum dot using detailed balance calculations. We have also experimentally investigated the Stranski-Krastanov growth of self-organized InAs quantum dots and quantum dot arrays on lattice-matched GaAs by metallorganic vapor phase epitaxy (MOVPE). The morphology of the quantum dots were investigated
Quantum computer aided design simulation and optimization of semiconductor quantum dots
NASA Astrophysics Data System (ADS)
Gao, X.; Nielsen, E.; Muller, R. P.; Young, R. W.; Salinger, A. G.; Bishop, N. C.; Lilly, M. P.; Carroll, M. S.
2013-10-01
We present the Quantum Computer Aided Design (QCAD) simulator that targets modeling multi-dimensional quantum devices, particularly silicon multi-quantum dots (QDs) developed for quantum bits (qubits). This finite-element simulator has three differentiating features: (i) its core contains nonlinear Poisson, effective mass Schrodinger, and Configuration Interaction solvers that have massively parallel capability for high simulation throughput and can be run individually or combined self-consistently for 1D/2D/3D quantum devices; (ii) the core solvers show superior convergence even at near-zero-Kelvin temperatures, which is critical for modeling quantum computing devices; and (iii) it interfaces directly with the full-featured optimization engine Dakota. In this work, we describe the capabilities and implementation of the QCAD simulation tool and show how it can be used to both analyze existing experimental QD devices through capacitance calculations and aid in the design of few-electron multi-QDs. In particular, we observe that computed capacitances are in rough agreement with experiment, and that quantum confinement increases capacitance when the number of electrons is fixed in a quantum dot. Coupling of QCAD with the optimizer Dakota allows for rapid identification and improvement of device layouts that are likely to exhibit few-electron quantum dot characteristics.
Dirac electrons in graphene-based quantum wires and quantum dots
NASA Astrophysics Data System (ADS)
Peres, N. M. R.; Rodrigues, J. N. B.; Stauber, T.; Lopes dos Santos, J. M. B.
2009-08-01
In this paper we analyse the electronic properties of Dirac electrons in finite-size ribbons and in circular and hexagonal quantum dots. We show that due to the formation of sub-bands in the ribbons it is possible to spatially localize some of the electronic modes using a p-n-p junction. We also show that scattering of confined Dirac electrons in a narrow channel by an infinitely massive wall induces mode mixing, giving a qualitative reason for the fact that an analytical solution to the spectrum of Dirac electrons confined in a square box has not yet been found. A first attempt to solve this problem is presented. We find that only the trivial case k = 0 has a solution that does not require the existence of evanescent modes. We also study the spectrum of quantum dots of graphene in a perpendicular magnetic field. This problem is studied in the Dirac approximation, and its solution requires a numerical method whose details are given. The formation of Landau levels in the dot is discussed. The inclusion of the Coulomb interaction among the electrons is considered at the self-consistent Hartree level, taking into account the interaction with an image charge density necessary to keep the back-gate electrode at zero potential. The effect of a radial confining potential is discussed. The density of states of circular and hexagonal quantum dots, described by the full tight-binding model, is studied using the Lanczos algorithm. This is necessary to access the detailed shape of the density of states close to the Dirac point when one studies large systems. Our study reveals that zero-energy edge states are also present in graphene quantum dots. Our results are relevant for experimental research in graphene nanostructures. The style of writing is pedagogical, in the hope that newcomers to the subject will find this paper a good starting point for their research.
The different effect of electron-electron interaction on the spectrum of atoms and quantum dots
NASA Astrophysics Data System (ADS)
Keren, K.; Stern, A.; Sivan, U.
2000-11-01
The electron-electron scattering rate of single particle excitations in atoms is estimated and compared with the corresponding rate in quantum dots. It is found that in alkali atoms single particle excitations do not acquire a width due to electron-electron interaction, while in complex atoms they may. This width is typically smaller than the single particle level spacing, and hence does not affect the number of discrete single particle excitations resolved below the ionization threshold. This situation is contrasted with that of quantum dots where electron-electron interaction severely limits the number of resolved excitations. Unlike the case of quantum dots, the scattering rate in atoms is found to decrease with increasing excitation energy. The different effect of electron-electron interaction on the spectrum of quantum dots and atoms is traced to the different confining potentials in the two systems.
Excitonic absorption in gate-controlled graphene quantum dots
A. D. Güçlü; P. Potasz; P. Hawrylak
2010-01-01
We present a theory of excitonic processes in gate controlled graphene quantum dots. The dependence of the energy gap on shape, size, and edge for graphene quantum dots with up to a million atoms is predicted. Using a combination of tight-binding, Hartree-Fock and configuration interaction methods, we show that triangular graphene quantum dots with zigzag edges exhibit optical transitions simultaneously
Quantum Dot Focal Plane Array with Plasmonic Resonator Sanjay Krishna
Krishna, Sanjay
. Various optoelectronic devices such as lasers, detectors, filters and solar cells are expected to benefitQuantum Dot Focal Plane Array with Plasmonic Resonator Sanjay Krishna Electrical and Computer structures with quantum dot focal plane arrays. Keywords: infrared detectors, quantum dots-in-a-well (DWELL
Optical Gain and Stimulated Emission in Nanocrystal Quantum Dots
V. I. Klimov; A. A. Mikhailovsky; Su Xu; A. Malko; J. A. Hollingsworth; C. A. Leatherdale; H.-J. Eisler; M. G. Bawendi
2000-01-01
The development of optical gain in chemically synthesized semiconductor nanoparticles (nanocrystal quantum dots) has been intensely studied as the first step toward nanocrystal quantum dot lasers. We examined the competing dynamical processes involved in optical amplification and lasing in nanocrystal quantum dots and found that, despite a highly efficient intrinsic nonradiative Auger recombination, large optical gain can be developed at
Imaging vasculature and lymphatic flow in mice using quantum dots.
Ballou, Byron; Ernst, Lauren A; Andreko, Susan; Fitzpatrick, James A J; Lagerholm, B Christoffer; Waggoner, Alan S; Bruchez, Marcel P
2009-01-01
Quantum dots are ideal probes for fluorescent imaging of vascular and lymphatic tissues. On injection into appropriate sites, red- and near-infrared-emitting quantum dots provide excellent definition of vasculature, lymphoid organs, and lymph nodes draining both normal tissues and tumors. We detail methods for use with commercially available quantum dots and discuss common difficulties. PMID:19685300
Theory of the Optical Response of Singleand Coupled Semiconductor Quantum Dots
C. Weber; M. Richter; S. Ritter; A. Knorr
Due to their quasi-zero-dimensional structure, quantum dots show optical properties which are different from those of nanostructures\\u000a with spatial confinement in less than three dimensions. In this chapter, the theory of both the linear optical properties\\u000a and nonlinear dynamics of semiconductor quan- tum dots is discussed. The main focus is on the experimentally accessible quantities\\u000a such as absorption\\/luminescence and pump-probe
Functional microspheres of graphene quantum dots.
Ding, Yi; Cheng, Huhu; Zhou, Ce; Fan, Yueqiong; Zhu, Jia; Shao, Huibo; Qu, Liangti
2012-06-29
Graphene-quantum-dot microspheres (GQDSs) have been prepared by assembly of graphene quantum dots (GQDs) via a water-in-oil (W/O) emulsion technique without the addition of any surfactants. Although made of quantum-sized graphene dots, the as-formed GQDSs are solid and remain intact after slight ultrasonication. The versatile W/O emulsion method allows the in situ intercalation of functional nanocomponents into the GQDSs for specific applications. As exemplified by the Fe(3)O(4)-containing GQDSs, Fe(3)O(4)-GQDSs exhibit a large magnetic response. Furthermore, the embedded Fe(3)O(4) nanoparticles in GQDSs can act as the catalysts for the growth of carbon nanotubes (CNTs), which opens the opportunities for fabricating new complex structures of CNTs surrounding GQDSs by simple chemical vapor deposition. PMID:22653222
Functional microspheres of graphene quantum dots
NASA Astrophysics Data System (ADS)
Ding, Yi; Cheng, Huhu; Zhou, Ce; Fan, Yueqiong; Zhu, Jia; Shao, Huibo; Qu, Liangti
2012-06-01
Graphene-quantum-dot microspheres (GQDSs) have been prepared by assembly of graphene quantum dots (GQDs) via a water-in-oil (W/O) emulsion technique without the addition of any surfactants. Although made of quantum-sized graphene dots, the as-formed GQDSs are solid and remain intact after slight ultrasonication. The versatile W/O emulsion method allows the in situ intercalation of functional nanocomponents into the GQDSs for specific applications. As exemplified by the Fe3O4-containing GQDSs, Fe3O4-GQDSs exhibit a large magnetic response. Furthermore, the embedded Fe3O4 nanoparticles in GQDSs can act as the catalysts for the growth of carbon nanotubes (CNTs), which opens the opportunities for fabricating new complex structures of CNTs surrounding GQDSs by simple chemical vapor deposition.
Three-terminal quantum-dot refrigerators
NASA Astrophysics Data System (ADS)
Zhang, Yanchao; Lin, Guoxing; Chen, Jincan
2015-05-01
Based on two capacitively coupled quantum dots in the Coulomb-blockade regime, a model of three-terminal quantum-dot refrigerators is proposed. With the help of the master equation, the transport properties of steady-state charge current and energy flow between two quantum dots and thermal reservoirs are revealed. It is expounded that such a structure can be used to construct a refrigerator by controlling the voltage bias and temperature ratio. The thermodynamic performance characteristics of the refrigerator are analyzed, including the cooling power, coefficient of performance (COP), maximum cooling power, and maximum COP. Moreover, the optimal regions of main performance parameters are determined. The influence of dissipative tunnel processes on the optimal performance is discussed in detail. Finally, the performance characteristics of the refrigerators operated in two different cases are compared.
Collective Excitations in Cylindrical Quantum Dots Chains
NASA Astrophysics Data System (ADS)
Vergara, Jimena; Camacho, Angela
2009-03-01
We are interested in the study of collective excitations in quantum dot chains because these can be used to effectively transmit information at nano scale and to control spontaneous and stimulate electromagnetic emission in the quantum dots. [1] This work is centered in the study of semiconductor one-dimensional quantum dot arrays. Based on a tight-binding bandstructure calculation combined with a self consistent field approximation we obtain the dispersion relations and we analyze how the geometry of the dot affects the collective oscillation of charge and its propagation. We focus our study first on Coulomb interaction between charges as the main cause of the 1D plasmons neglecting tunneling to finally compare with the case where tunneling is allowed. We find out that Coulomb interaction plays an important role in these systems and that tunneling opens the energy spectrum permitting new excitations, which are good candidates to be used in nanometric devices. [1] A.V.Akimov, A.Mukherjee, C.L. Yu, D.E Chang, A.S.Zybrov, P.R. Hemmer, H Park and M.D Lukin, Generation of Single optical plasmons in metallic nanowires coupled to quantum dots, Nature 450, 402 (2007).
Pulse-gated quantum dot hybrid qubit
NASA Astrophysics Data System (ADS)
Coppersmith, S. N.; Koh, Teck Seng; King Gamble, John; Eriksson, M. A.; Friesen, Mark
2013-03-01
A quantum dot hybrid qubit formed from three electrons in a double quantum dot has the potential for great speed, due to presence of level crossings where the qubit becomes charge-like. Here, we show how to exploit the level crossings to implement fast pulsed gating. We develop one- and two-qubit dc quantum gates that are simpler than the previously proposed ac gates. We obtain closed-form solutions for the control sequences and show that the gates are fast (sub-nanosecond) and can achieve high fidelities. A quantum dot hybrid qubit formed from three electrons in a double quantum dot has the potential for great speed, due to presence of level crossings where the qubit becomes charge-like. Here, we show how to exploit the level crossings to implement fast pulsed gating. We develop one- and two-qubit dc quantum gates that are simpler than the previously proposed ac gates. We obtain closed-form solutions for the control sequences and show that the gates are fast (sub-nanosecond) and can achieve high fidelities. Work supported by ARO (W911NF-08-1-0482) and NSF (DMR-0805045, PHY-1104660), and the National Science Foundation Graduate Research Fellowship (DGE-0718123).
Sized controlled synthesis, purification, and cell studies with silicon quantum dots
NASA Astrophysics Data System (ADS)
Shiohara, Amane; Prabakar, Sujay; Faramus, Angelique; Hsu, Chia-Yen; Lai, Ping-Shan; Northcote, Peter T.; Tilley, Richard D.
2011-08-01
This article describes the size control synthesis of silicon quantum dots with simple microemulsion techniques. The silicon nanocrystals are small enough to be in the strong confinement regime and photoluminesce in the blue region of the visible spectrum and the emission can be tuned by changing the nanocrystal size. The silicon quantum dots were capped with allylamine either a platinum catalyst or UV-radiation. An extensive purification protocol is reported and assessed using 1H NMR to produce ultra pure silicon quantum dots suitable for biological studies. The highly pure quantum dots were used in cellular uptake experiments and monitored using confocal microscopy. The results showed that the amine terminated silicon nanocrystals accumulated in lysosome but not in nuclei and could be used as bio-markers to monitor cancer cells over long timescales.This article describes the size control synthesis of silicon quantum dots with simple microemulsion techniques. The silicon nanocrystals are small enough to be in the strong confinement regime and photoluminesce in the blue region of the visible spectrum and the emission can be tuned by changing the nanocrystal size. The silicon quantum dots were capped with allylamine either a platinum catalyst or UV-radiation. An extensive purification protocol is reported and assessed using 1H NMR to produce ultra pure silicon quantum dots suitable for biological studies. The highly pure quantum dots were used in cellular uptake experiments and monitored using confocal microscopy. The results showed that the amine terminated silicon nanocrystals accumulated in lysosome but not in nuclei and could be used as bio-markers to monitor cancer cells over long timescales. Electronic supplementary information (ESI) available. See DOI: 10.1039/c1nr10458f
Adrienne D. Stiff-Roberts; Kevin R. Lantz; Ryan Pate
2009-01-01
A unique and distinct approach to unipolar, intraband transitions appropriate for room-temperature, mid- and long-wave-infrared (IR) photodetection is to use active regions comprising colloidal quantum dots (CQDs) synthesized by inorganic chemistry embedded in conjugated polymers. The polymer not only enhances quantum confinement of and electron localization in CQDs, but it also assists in the conduction of electrons photogenerated by the
Single-dot optical emission from ultralow density well-isolated InP quantum dots
Ugur, A.; Hatami, F.; Masselink, W. T. [Department of Physics, Humboldt-Universitaet zu Berlin, Newtonstrasse 15, D-12489 Berlin (Germany); Vamivakas, A. N.; Lombez, L.; Atatuere, M. [Cavendish Laboratory, University of Cambridge, JJ Thomson Avenue, Cambridge CB3 0HE (United Kingdom)
2008-10-06
We demonstrate a straightforward way to obtain single well-isolated quantum dots emitting in the visible part of the spectrum and characterize the optical emission from single quantum dots using this method. Self-assembled InP quantum dots are grown using gas-source molecular-beam epitaxy over a wide range of InP deposition rates, using an ultralow growth rate of about 0.01 atomic monolayers/s, a quantum-dot density of 1 dot/{mu}m{sup 2} is realized. The resulting isolated InP quantum dots embedded in an InGaP matrix are individually characterized without the need for lithographical patterning and masks on the substrate. Such low-density quantum dots show excitonic emission at around 670 nm with a linewidth limited by instrument resolution. This system is applicable as a single-photon source for applications such as quantum cryptography.
NASA Astrophysics Data System (ADS)
Kannan, V.; Rhee, J. K.
2011-10-01
We report a Ti-TiOx/quantum dot based bipolar nonvolatile resistive memory device. The device has ON/OFF ratio 100 and is reproducible. The memory device showed good retention characteristics under stress and excellent stability even after 100 000 cycles of switching operation. The memory devices are solution processed at room temperature in ambient atmosphere. The operating mechanism is discussed based on charge trapping in quantum dots resulting in Coulomb blockade effect with the metal-oxide layer acting as the barrier to confine the trapped charges. The mechanism is supported by negative differential resistance (NDR) observed exclusively in the ON state.
Low-energy trions in graphene quantum dots
NASA Astrophysics Data System (ADS)
Cheng, H.-C.; Lue, N.-Y.; Chen, Y.-C.; Wu, G. Y.
2014-06-01
We investigate, within the envelope function approximation, the low-energy states of trions in graphene quantum dots (QDs). The presence of valley pseudospin in graphene as an electron degree of freedom apart from spin adds convolution to the interplay between exchange symmetry and the electron-electron interaction in the trion, leading to new states of trions as well as a low-energy trion spectrum different from those in semiconductors. Due to the involvement of valley pseudospin, it is found that the low-energy spectrum is nearly degenerate and consists of states all characterized by having an antisymmetric (pseudospin) ? (spin) component in the wave function, with the spin (pseudospin) part being either singlet (triplet) or triplet (singlet), as opposed to the spectrum in a semiconductor whose ground state is known to be nondegenerate and always a spin singlet in the case of X- trions. We investigate trions in the various regimes determined by the competition between quantum confinement and electron-electron interaction, both analytically and numerically. The numerical work is performed within a variational method accounting for electron mass discontinuity across the QD edge. The result for electron-hole correlation in the trion is presented. Effects of varying quantum dot size and confinement potential strength on the trion binding energy are discussed. The "relativistic effect" on the trion due to the unique relativistic type electron energy dispersion in graphene is also examined.
Bilayer graphene quantum dot defined by topgates
Müller, André; Kaestner, Bernd; Hohls, Frank; Weimann, Thomas; Pierz, Klaus; Schumacher, Hans W., E-mail: hans.w.schumacher@ptb.de [Physikalisch-Technische Bundesanstalt, Bundesallee 100, 38116 Braunschweig (Germany)
2014-06-21
We investigate the application of nanoscale topgates on exfoliated bilayer graphene to define quantum dot devices. At temperatures below 500 mK, the conductance underneath the grounded gates is suppressed, which we attribute to nearest neighbour hopping and strain-induced piezoelectric fields. The gate-layout can thus be used to define resistive regions by tuning into the corresponding temperature range. We use this method to define a quantum dot structure in bilayer graphene showing Coulomb blockade oscillations consistent with the gate layout.
Bilayer graphene quantum dot defined by topgates
NASA Astrophysics Data System (ADS)
Müller, André; Kaestner, Bernd; Hohls, Frank; Weimann, Thomas; Pierz, Klaus; Schumacher, Hans W.
2014-06-01
We investigate the application of nanoscale topgates on exfoliated bilayer graphene to define quantum dot devices. At temperatures below 500 mK, the conductance underneath the grounded gates is suppressed, which we attribute to nearest neighbour hopping and strain-induced piezoelectric fields. The gate-layout can thus be used to define resistive regions by tuning into the corresponding temperature range. We use this method to define a quantum dot structure in bilayer graphene showing Coulomb blockade oscillations consistent with the gate layout.
Potential clinical applications of quantum dots
Medintz, Igor L; Mattoussi, Hedi; Clapp, Aaron R
2008-01-01
The use of luminescent colloidal quantum dots in biological investigations has increased dramatically over the past several years due to their unique size-dependent optical properties and recent advances in biofunctionalization. In this review, we describe the methods for generating high-quality nanocrystals and report on current and potential uses of these versatile materials. Numerous examples are provided in several key areas including cell labeling, biosensing, in vivo imaging, bimodal magnetic-luminescent imaging, and diagnostics. We also explore toxicity issues surrounding these materials and speculate about the future uses of quantum dots in a clinical setting. PMID:18686776
NASA Astrophysics Data System (ADS)
Misra, Nirmal; Roy, Mohendra; Mohanta, Dambarudhar; Baruah, Kishor Kumar; Choudhury, Amarjyoti
2008-03-01
ZnO:Mn semiconductor quantum dots were prepared by solution casting led microemulsion route. Quantum dots of average size ˜2 nm were noticed in transmission electron micrographs. The present work highlights colour change phenomena (photochromic effect) of quantum dots while subjected to photon illumination. The magneto-optic measurements e.g. magnetic field ( H) vs angle of rotation ( ?) show step like behavior and is ascribed to the quantum confinement effect of diluted magnetic ZnO:Mn nanostructures. Further, underlying mechanism responsible for exhibiting photochromism and magneto-optic effects are also discussed.
NASA Astrophysics Data System (ADS)
Misra, Nirmal; Roy, Mohendra; Mohanta, Dambarudhar; Baruah, Kishor; Choudhury, Amarjyoti
2008-03-01
ZnO:Mn semiconductor quantum dots were prepared by solution casting led microemulsion route. Quantum dots of average size ˜2 nm were noticed in transmission electron micrographs. The present work highlights colour change phenomena (photochromic effect) of quantum dots while subjected to photon illumination. The magneto-optic measurements e.g. magnetic field (H) vs angle of rotation (?) show step like behavior and is ascribed to the quantum confinement effect of diluted magnetic ZnO:Mn nanostructures. Further, underlying mechanism responsible for exhibiting photochromism and magneto-optic effects are also discussed.
Theory of a double-quantum-dot spaser
NASA Astrophysics Data System (ADS)
Andrianov, E. S.; Pukhov, A. A.; Dorofeenko, A. V.; Vinogradov, A. P.; Lisyansky, A. A.
2015-03-01
We consider the influence of the number of quantum dots on spaser operation. It is shown that even in the presence of only two quantum dots, the spaser behaviour is qualitatively different from that of the previously studied spaser consisting of a nanoparticle and a single quantum dot. In particular, for nonzero detuning of resonant frequencies of a nanoparticle and quantum dots, an increase in the interaction constant between quantum dots first leads to a decrease in the spasing threshold and then to its growth and even the spasing breakdown.
Fast Electrical Control of a Quantum Dot Strongly Coupled to a Nano-resonator
Andrei Faraon; Arka Majumdar; Hyochul Kim; Pierre Petroff; Jelena Vuckovic
2009-06-03
The resonance frequency of an InAs quantum dot strongly coupled to a GaAs photonic crystal cavity was electrically controlled via quantum confined Stark effect. Stark shifts up to 0.3meV were achieved using a lateral Schottky electrode that created a local depletion region at the location of the quantum dot. We report switching of a probe laser coherently coupled to the cavity up to speeds as high as 150MHz, limited by the RC constant of the transmission line. The coupling rate and the magnitude of the Stark shift with electric field were investigated while coherently probing the system.
Composite-fermion description of correlated electrons in quantum dots: Low-Zeeman-energy limit
R. K. Kamilla; J. K. Jain
1995-01-01
We study the applicability of composite-fermion theory to electrons in two-dimensional parabolically confined quantum dots in a strong perpendicular magnetic field in the limit of low Zeeman energy. The noninteracting composite fermion spectrum correctly specifies the primary features of this system. Additional features are relatively small, indicating that the residual interaction between the composite fermions is weak.
Direct interband light absorption in a cylindrical quantum dot in quantizing magnetic field
M. S. Atoyan; E. M. Kazaryan; H. A. Sarkisyan
2004-01-01
In this paper the direct interband transitions in cylindrical quantum dot (QD) made of GaAs are studied in the presence of a magnetic field. Two models of QD confinement potential are discussed. For both models the expressions for absorption coefficients and dependencies of effective threshold frequencies of absorption on the value of applied magnetic field and on geometrical sizes of
Jürgen Wurm; Adam Rycerz; Inanç Adagideli; Michael Wimmer; Klaus Richter; Harold U. Baranger
2009-01-01
We study the symmetry classes of graphene quantum dots, both open and closed, through the conductance and energy level statistics. For abrupt termination of the lattice, these properties are well described by the standard orthogonal and unitary ensembles. However, for smooth mass confinement, special time-reversal symmetries associated with the sublattice and valley degrees of freedom are critical: they lead to
Dirac gap-induced graphene quantum dot in an electrostatic potential
G. Giavaras; Franco Nori
2011-01-01
A spatially modulated Dirac gap in a graphene sheet leads to charge confinement, thus enabling a graphene quantum dot to be formed without the application of external electric and magnetic fields [G. Giavaras and F. Nori, Appl. Phys. Lett. 97, 243106 (2010)]. This can be achieved provided the Dirac gap has a local minimum in which the states become localized.
Tuning the optical properties of dilute nitride site controlled quantum dots
NASA Astrophysics Data System (ADS)
Juska, G.; Dimastrodonato, V.; Mereni, L. O.; Gocalinska, A.; Pelucchi, E.
2013-12-01
We show that deterministic control of the properties of pyramidal site-controlled quantum dots (QD) could be achieved by exposing the QD layer to nitrogen precursor unsymmetrical dimethylhydrazine (UDMHy). The properties that could be tuned include an expected emission reduction in dilute nitride materials, excitonic pattern (biexciton binding energy) and improved carrier confinement potential symmetry (reduced fine-structure splitting).
Damping of Rabi oscillations in quantum dots due to lattice dynamics
Pawel Machnikowski; Lucjan Jacak
2004-01-01
We show that the interaction between carriers confined in a quantum dot and the surrounding lattice under external driving of carrier dynamics has a dynamical, resonant character. The quality of Rabi oscillations in such a system depends on the relation between nonlinear spectral characteristics of the driven dynamics and the spectral density of effectively coupled lattice modes (phonon frequencies and
A Surface Plasmon Enhanced Infrared Photodetector Based on InAs Quantum Dots
Krishna, Sanjay
A Surface Plasmon Enhanced Infrared Photodetector Based on InAs Quantum Dots Chun-Chieh Chang to a level suitable for emerging surveillance and medical diagnostic applications. KEYWORDS Surface plasmon, there has been a great deal of interest in confining light strongly at the metal surface by surface plasmons
Tuning the optical properties of dilute nitride site controlled quantum dots
Juska, G.; Dimastrodonato, V.; Mereni, L. O.; Gocalinska, A.; Pelucchi, E. [Tyndall National Institute, University College Cork, Lee Maltings, Cork (Ireland)
2013-12-04
We show that deterministic control of the properties of pyramidal site-controlled quantum dots (QD) could be achieved by exposing the QD layer to nitrogen precursor unsymmetrical dimethylhydrazine (UDMHy). The properties that could be tuned include an expected emission reduction in dilute nitride materials, excitonic pattern (biexciton binding energy) and improved carrier confinement potential symmetry (reduced fine-structure splitting)
Absorption Spectra and Refractive Index Changes of an Exciton in a Core/Shell Quantum Dot
NASA Astrophysics Data System (ADS)
Xie, Wen-Fang
2015-05-01
The absorption spectra and the refractive index changes are calculated theoretically for an exciton in a core/shell quantum dot. The advantage of our methodology is that one can investigate the influence of the repulsive core by varying two parameters in the confinement potential. The dimensionality effect of exciton quantum dots on the optical absorptions has been studied. It has been found that in the same regime, the optical absorption intensities of excitons are much smaller for the core/shell quantum dots than for the two-dimensional quantum rings. The linear and the nonlinear optical absorption coefficients and refractive index changes have been examined with the change of the confinement potential. The results show that the optical absorptions and the refractive index changes are strongly affected by the repulsive core of core/shell quantum dots. Moreover, the calculated results also reveal that as the inner radius increases, the peak values of the absorption coefficients and the refractive index changes of an exciton will show the optical Aharonov-Bohm oscillation in core/shell quantum dots.
Optical properties of quantum-dot-doped liquid scintillators
Aberle, C.; Li, J.J.; Weiss, S.; Winslow, L.
2014-01-01
Semiconductor nanoparticles (quantum dots) were studied in the context of liquid scintillator development for upcoming neutrino experiments. The unique optical and chemical properties of quantum dots are particularly promising for the use in neutrinoless double-beta decay experiments. Liquid scintillators for large scale neutrino detectors have to meet specific requirements which are reviewed, highlighting the peculiarities of quantum-dot-doping. In this paper, we report results on laboratory-scale measurements of the attenuation length and the fluorescence properties of three commercial quantum dot samples. The results include absorbance and emission stability measurements, improvement in transparency due to filtering of the quantum dot samples, precipitation tests to isolate the quantum dots from solution and energy transfer studies with quantum dots and the fluorophore PPO. PMID:25392711
Optical properties of quantum-dot-doped liquid scintillators.
Aberle, C; Li, J J; Weiss, S; Winslow, L
2013-10-14
Semiconductor nanoparticles (quantum dots) were studied in the context of liquid scintillator development for upcoming neutrino experiments. The unique optical and chemical properties of quantum dots are particularly promising for the use in neutrinoless double-beta decay experiments. Liquid scintillators for large scale neutrino detectors have to meet specific requirements which are reviewed, highlighting the peculiarities of quantum-dot-doping. In this paper, we report results on laboratory-scale measurements of the attenuation length and the fluorescence properties of three commercial quantum dot samples. The results include absorbance and emission stability measurements, improvement in transparency due to filtering of the quantum dot samples, precipitation tests to isolate the quantum dots from solution and energy transfer studies with quantum dots and the fluorophore PPO. PMID:25392711
Second-order nonlinear susceptibility in quantum dot structure under applied electric field
NASA Astrophysics Data System (ADS)
Abdullah, M.; Noori, Farah T. Mohammed; Al-Khursan, Amin H.
2015-06-01
A model for quantum dot (QD) subbands, when the dots are in the form of quantum disks, under applied electric field was stated. Then, subbands of dots with different disk radii and heights were calculated under applied field. The competition between the shift due to confinement by field and the size was shown for subbands. Second-order nonlinear susceptibility in quantum dots (QDs) was derived using density matrix theory which is, then, simulated using the calculated subbands. Both interband (IB) and intersubband (ISB) transitions were discussed. High second-order susceptibility in QDs was predicted. The results show a reduction in the susceptibility with the applied field while the peak wavelength was mainly relates to energy difference between subbands. A good match between theory and laboratory experiments was observed. Laboratory experiments at terahertz region might be possible using valence intersubband which is important in many device applications.
Mohseni, Hooman
A voltage tunable quantum dot photodetector for terahertz detection This article has been (6pp) doi:10.1088/0022-3727/43/15/155101 A voltage tunable quantum dot photodetector for terahertz, the photodetector can be used to detect the terahertz region. Changing the confinement voltage, the intersublevel
Wang, Deli
, the single-electron charging energy can become comparable to the quantum confinement energies in the dot. Consider a spherical semiconductor quantum dot of radius a in which the semiconductor material has band gap Eg = 1.0 eV, dielectric constant = 100, electron effective mass 0 * 1.0 mmn = , and (heavy) hole
Wang, Deli
, the single-electron charging energy can become comparable to the quantum confinement energies in the dot. Consider a spherical semiconductor quantum dot of radius a in which the semiconductor material has band gap Eg = 1.0 eV, dielectric constant = 100, electron effective mass 0 * 1.0 mmn , and (heavy) hole
Transport properties of graphene quantum dots
J. W. González; M. Pacheco; L. Rosales; P. A. Orellana
2011-01-01
In this work we present a theoretical study of transport properties of a double crossbar junction composed of segments of graphene ribbons with different widths forming a graphene quantum dot structure. The systems are described by a single-band tight binding Hamiltonian and the Green's function formalism using real space renormalization techniques. We show calculations of the local density of states,
Spin qubits in graphene quantum dots
Bjoern Trauzettel; Denis V. Bulaev; Daniel Loss; Guido Burkard
2007-01-01
The main characteristics of good qubits are long coherence times in combination with fast operating times. It is well known that carbon-based materials could increase the coherence times of spin qubits, which are among the most developed solid-state qubits. Here, we propose how to form spin qubits in graphene quantum dots. A crucial requirement to achieve this goal is to
REVIEW ARTICLE Semiconductor quantum dot-sensitized
Cao, Guozhong
of dye-sensitized solar cells (DSCs), which were first reported by O'Regan and Gra¨tzel in 1991 (8REVIEW ARTICLE Semiconductor quantum dot-sensitized solar cells Jianjun Tian1 * and Guozhong Cao2-sensitized solar cell (QDSC) is one of the burgeon- ing semiconductor QD solar cells that shows promising
Quantum Dot Based Infrared Focal Plane Arrays
Sanjay Krishna; Sarath D. Gunapala; Sumith V. Bandara; Cory Hill; David Z. Ting
2007-01-01
In the past decade, there has been active research on infrared detectors based on intersubband transitions in self-assembled quantum dots (QDs). In the past two years, at least four research groups have independently demonstrated focal plane arrays based on this technology. In this paper, the progress from the first raster scanned image obtained with a QD detector to the demonstration
New small quantum dots for neuroscience
NASA Astrophysics Data System (ADS)
Selvin, Paul
2014-03-01
In "New Small Quantum Dots for Neuroscience," Paul Selvin (University of Illinois, Urbana-Champaign) notes how the details of synapsis activity in the brain involves chemical receptors that facilitate the creation of the electrical connection between two nerves. In order to understand the details of this neuroscience phenomenon you need to be able to "see" what is happening at the scale of these receptors, which is around 10 nanometers. This is smaller than the diffraction limit of normal microscopy and it takes place on a 3 dimensional structure. Selvin describes the development of small quantum dots (on the order of 6-9 microns) that are surface-sensitized to interact with the receptors. This allows the application of photo-activated localized microscopy (PALM), a superresolution microscopy that can be scanned through focus to develop a 3D map on a scale that is the same size as the emitter, which in this case are the small quantum dots. The quantum dots are stable in time and provide access to the receptors which allows the imaging of the interactions taking place at the synoptic level.
Quantum dots in molecular detection of disease
V. J. Bailey; C. M. Puleo; Y. P. Ho; H. C. Yeh; T. H. Wang
2009-01-01
The unique photophysical properties of semiconductor quantum dots (QDs) have made them ideal for use as spectral labels and luminescent probes. In this review, applications are presented in which QDs function as active participants in nanoscale biosensor assemblies, where replacing traditional molecular fluorophores results in improved assay performance. Specific focus is on disease detection with applications including multiplexed target detection,
Buljan, M; Radi?, N; Sancho-Paramon, J; Janicki, V; Grenzer, J; Bogdanovi?-Radovi?, I; Siketi?, Z; Ivanda, M; Utrobi?i?, A; Hübner, R; Weidauer, R; Valeš, V; Endres, J; Car, T; Jer?inovi?, M; Roško, J; Bernstorff, S; Holy, V
2015-02-13
We report on the formation of Ge/Si quantum dots with core/shell structure that are arranged in a three-dimensional body centered tetragonal quantum dot lattice in an amorphous alumina matrix. The material is prepared by magnetron sputtering deposition of Al2O3/Ge/Si multilayer. The inversion of Ge and Si in the deposition sequence results in the formation of thin Si/Ge layers instead of the dots. Both materials show an atomically sharp interface between the Ge and Si parts of the dots and layers. They have an amorphous internal structure that can be crystallized by an annealing treatment. The light absorption properties of these complex materials are significantly different compared to films that form quantum dot lattices of the pure Ge, Si or a solid solution of GeSi. They show a strong narrow absorption peak that characterizes a type II confinement in accordance with theoretical predictions. The prepared materials are promising for application in quantum dot solar cells. PMID:25605224
Silicon based quantum dot hybrid qubits
NASA Astrophysics Data System (ADS)
Kim, Dohun
2015-03-01
The charge and spin degrees of freedom of an electron constitute natural bases for constructing quantum two level systems, or qubits, in semiconductor quantum dots. The quantum dot charge qubit offers a simple architecture and high-speed operation, but generally suffers from fast dephasing due to strong coupling of the environment to the electron's charge. On the other hand, quantum dot spin qubits have demonstrated long coherence times, but their manipulation is often slower than desired for important future applications. This talk will present experimental progress of a `hybrid' qubit, formed by three electrons in a Si/SiGe double quantum dot, which combines desirable characteristics (speed and coherence) in the past found separately in qubits based on either charge or spin degrees of freedom. Using resonant microwaves, we first discuss qubit operations near the `sweet spot' for charge qubit operation. Along with fast (>GHz) manipulation rates for any rotation axis on the Bloch sphere, we implement two independent tomographic characterization schemes in the charge qubit regime: traditional quantum process tomography (QPT) and gate set tomography (GST). We also present resonant qubit operations of the hybrid qubit performed on the same device, DC pulsed gate operations of which were recently demonstrated. We demonstrate three-axis control and the implementation of dynamic decoupling pulse sequences. Performing QPT on the hybrid qubit, we show that AC gating yields ? rotation process fidelities higher than 93% for X-axis and 96% for Z-axis rotations, which demonstrates efficient quantum control of semiconductor qubits using resonant microwaves. We discuss a path forward for achieving fidelities better than the threshold for quantum error correction using surface codes. This work was supported in part by ARO (W911NF-12-0607), NSF (PHY-1104660), DOE (DE-FG02-03ER46028), and by the Laboratory Directed Research and Development program at Sandia National Laboratories under contract DE-AC04-94AL85000.
Solution-Processed Quantum Dot Photodetectors
Gerasimos Konstantatos; Edward H. Sargent
2009-01-01
Digital imaging has traditionally been enabled by single-crystalline photodetectors. This approach typically either mandates the use of silicon as photon-to-electron converter or requires a hybrid-integrated solution. In contrast, solution-processed optoelectronic materials offer convenient integration of light-sensing materials atop an electronic readout circuit. Colloidal quantum dots offer particular advantages, combining solution-processing with the spectral tunability afforded by the quantum size effect.
Optical properties of quantum wires and dots
T. L. ReineckeI; P. Knipp
Recent work on the optical properties of quantum wire and quantum dot systems is discussed, including carrier, phonon and\\u000a photon states, electronphonon scattering and excitonic effects. In realistic systems the geometry often results in the equations\\u000a for the elementary excitations being non-separable. Numerical methods for calculating these excitations are discussed with\\u000a emphasis on “boundary element methods”, which we have recently
Electronic transient processes and optical spectra in quantum dots for quantum computing
Karel Král; P. Zdenek; Z. Khas
2004-01-01
Quantum dot systems are studied theoretically from the point of view of realization of quantum bit using the orbital state of electronic motion in a quantum dot. Attention is paid to several effects which can influence significantly the application of quantum dot electronic orbital states in quantum computing, for example, the effect of upconversion of the population and the incomplete
Magneto-optic properties of II-VI semiconductor quantum dots
NASA Astrophysics Data System (ADS)
Kratzer, Joseph H.
Low dimensional systems of semiconductor quantum dots in glass composites exhibit interesting physical properties arising from spatial confinement effects; an example is the discretization of the energy spectrum. In semiconductor quantum dots, electronic wave functions experience effects of quantum confinement arising from the dot-glass interface acting as an infinite potential barrier. This leads to electronic transitions having higher energies with decreasing dot size. The shift of the electronic energies is generally associated with an increase in the Faraday rotation due to exciton confinement. Magnetooptic measurements of II-VI semiconductor quantum dots in a boro-silicate glass matrix have been studied. The Faraday rotation of the quantum dot glass composite shows an increase over both the bulk semiconductor crystal for the same volume fraction, and the pure boro-silicate glass. The Verdet constant, initially constant, demonstrates an increase to another, higher constant value, at higher magnetic fields. This kink in the slope is achieved at lower fields with smaller quantum dots, and this finding is noted to be invariant, throughout the various samples studied. Kinks in Faraday rotation curves have been found by others in rare earth doped glasses as well. Kinks seen in those glasses occur at lower magnetic fields than for the quantum dots. Several possible explanations are examined to explain this phenomenon. Greater homogeneity of nanocrystal size and higher degree of sphericity is suggested as an explanation for the higher Verdet Constants seen in the samples more homogeneous in size and sphericity. Given the presence of kinks for both semiconductor quantum dots and rare earth doped glasses; one commonality is suggested to be regions of electron localization. Here, regions of intermediate-range order of 2--4 nm. in rare earth doped glasses are suggested to be associated with electron localization in a manner analogous to quantum dots. Ultimately, it is the localization of a small number of electrons elevated to the conduction band which is suggested to be affected by the incident magnetic field. Magnetically-induced orientation of the spins of those electrons is hypothesized to be the cause of the jumps or kinks in the Faraday rotation seen.
Non-Markovian full counting statistics in quantum dot molecules
Xue, Hai-Bin; Jiao, Hu-Jun; Liang, Jiu-Qing; Liu, Wu-Ming
2015-01-01
Full counting statistics of electron transport is a powerful diagnostic tool for probing the nature of quantum transport beyond what is obtainable from the average current or conductance measurement alone. In particular, the non-Markovian dynamics of quantum dot molecule plays an important role in the nonequilibrium electron tunneling processes. It is thus necessary to understand the non-Markovian full counting statistics in a quantum dot molecule. Here we study the non-Markovian full counting statistics in two typical quantum dot molecules, namely, serially coupled and side-coupled double quantum dots with high quantum coherence in a certain parameter regime. We demonstrate that the non-Markovian effect manifests itself through the quantum coherence of the quantum dot molecule system, and has a significant impact on the full counting statistics in the high quantum-coherent quantum dot molecule system, which depends on the coupling of the quantum dot molecule system with the source and drain electrodes. The results indicated that the influence of the non-Markovian effect on the full counting statistics of electron transport, which should be considered in a high quantum-coherent quantum dot molecule system, can provide a better understanding of electron transport through quantum dot molecules. PMID:25752245
Optical, magnetic and electronic properties of graphene quantum dots
A. Devrim Guclu
2011-01-01
We present a theory of optical, magnetic and electronic properties of graphene quantum dots. We demonstrate that there exists a class of triangular graphene quantum dots with zigzag edges [1-8] which combines magnetic, optical and transport properties in a single-material structure. These dots exhibit robust magnetic moment and optical transitions simultaneously in the THz, visible and UV spectral ranges due
Single-photon detection mechanism in a quantum dot transistor
N. S. Beattie; B. E. Kardynal; A. J. Shields; I. Farrer; D. A. Ritchie; M. Pepper
2005-01-01
We study the transport mechanisms in a quantum dot MODFET by tuning the localization induced by charge stored on the quantum dots with light. The temperature dependence of the resistivity of a macroscopic sample reveals a hopping transport when the dots contain an excess of electrons. The resistance of a mesoscopic sample however, which is capable of detecting single photons,
Application of Synthesized Quantum Dots for Cell Imaging
Hengyi Xu; Feng Xu; Yonghua Xiong; Cuixiang Wan; Jingfei Zhang; Hua Wei; Jiang Zhu
2009-01-01
Quantum dots (QDs) have received considerable attention due to their advantages and are widely used in biological studies, especially for multiplexed staining assays and immunological assays. Here we report an easy method for quantum dot synthesis and encapsulation, and use for efficient bioconjugation with secondary antibody. For the application of QDs-antibody conjugates, we approached a fast dot blotting immunological assay,
Optical levitation of microdroplet containing a single quantum dot
Minowa, Yosuke; Ashida, Masaaki
2014-01-01
Semiconductor nanocrystals, also known as quantum dots (QDs), are key ingredients in current quantum optics experiments. They serve as quantum emitters and memories and have tunable energy levels that depend not only on the material but also, through the quantum confinement effect, on the size. The resulting strongly confined electron and hole wave functions lead to large transition dipole moments, which opens a path to ultra strong coupling and even deep strong coupling between light and matter. Such efficient coupling requires the precise positioning of the QD in an optical cavity with a high quality factor and small mode volume, such as micro-Fabry--Perot cavity, whispering-gallery-mode microcavity, or photonic-crystal cavity. However, the absence of a technique for free-space positioning has limited the further research on QD-based cavity quantum electrodynamics. In this paper, we present a technique to overcome this challenge by demonstrating the optical levitation or trapping in helium gas of a single Q...
Coherent control of multipartite excitonic entanglement in quantum dot arrays
NASA Astrophysics Data System (ADS)
Rolon, Juan E.; Drut, Joaquin E.
2015-03-01
We propose a coherent control scheme for multipartite entanglement of exciton states in optically driven quantum dot arrays (QDAs) coupled by charge tunneling and resonant energy transfer (RET) processes. An adiabatic manipulation of the entanglement dynamics is devised by pulse shaping and time-dependent electric field sweeps. By varying the inter-dot distance and number of quantum dots (QDs) comprising the QDA, the excitonic qubit manifolds are obtained by a Feshbach projection over the resulting multilevel exciton configurations. We identify regimes in which the dynamics is confined to decoherence-free excitonic qubit manifolds taking into account spontaneous recombination and non-Markovian effects introduced by a phonon bath. We present results for entanglement monotones and measures such as the entanglement of formation and entanglement entropy for different QDA geometries and carrier injection conditions. Our results indicate that in spite of the effects of phonon-assisted relaxation, entanglement can be optimized and transferred between QDs by the controlled interplay of system geometry, pulse shaping, RET and carrier tunneling.
Unraveling the mystery of quantum-dot April 3, 2012
. At these tiny dimensions, the rules of quantum physics allow scientists to produce particles with finely tunable- 1 - Unraveling the mystery of quantum-dot blinking April 3, 2012 Unraveling the mystery of quantum-dot blinking Significant progress is being made in understanding the phenomenon of quantum
Longitudinal wave function control in single quantum dots with an applied magnetic field
Cao, Shuo; Tang, Jing; Gao, Yunan; Sun, Yue; Qiu, Kangsheng; Zhao, Yanhui; He, Min; Shi, Jin-An; Gu, Lin; Williams, David A.; Sheng, Weidong; Jin, Kuijuan; Xu, Xiulai
2015-01-01
Controlling single-particle wave functions in single semiconductor quantum dots is in demand to implement solid-state quantum information processing and spintronics. Normally, particle wave functions can be tuned transversely by an perpendicular magnetic field. We report a longitudinal wave function control in single quantum dots with a magnetic field. For a pure InAs quantum dot with a shape of pyramid or truncated pyramid, the hole wave function always occupies the base because of the less confinement at base, which induces a permanent dipole oriented from base to apex. With applying magnetic field along the base-apex direction, the hole wave function shrinks in the base plane. Because of the linear changing of the confinement for hole wave function from base to apex, the center of effective mass moves up during shrinking process. Due to the uniform confine potential for electrons, the center of effective mass of electrons does not move much, which results in a permanent dipole moment change and an inverted electron-hole alignment along the magnetic field direction. Manipulating the wave function longitudinally not only provides an alternative way to control the charge distribution with magnetic field but also a new method to tune electron-hole interaction in single quantum dots. PMID:25624018
Longitudinal wave function control in single quantum dots with an applied magnetic field
Shuo Cao; Jing Tang; Yunan Gao; Yue Sun; Kangsheng Qiu; Yanhui Zhao; Min He; Jin-An Shi; Lin Gu; David A. Williams; Weidong Sheng; Kuijuan Jin; Xiulai Xu
2015-01-29
Controlling single-particle wave functions in single semiconductor quantum dots is in demand to implement solid-state quantum information processing and spintronics. Normally, particle wave functions can be tuned transversely by an perpendicular magnetic field. We report a longitudinal wave function control in single quantum dots with a magnetic field. For a pure InAs quantum dot with a shape of pyramid or truncated pyramid, the hole wave function always occupies the base because of the less confinement at base, which induces a permanent dipole oriented from base to apex. With applying magnetic field along the base-apex direction, the hole wave function shrinks in the base plane. Because of the linear changing of the confinement for hole wave function from base to apex, the center of effective mass moves up during shrinking process. Due to the uniform confine potential for electrons, the center of effective mass of electrons does not move much, which results in a permanent dipole moment change and an inverted electron-hole alignment along the magnetic field direction. Manipulating the wave function longitudinally not only provides an alternative way to control the charge distribution with magnetic field but also a new method to tune electron-hole interaction in single quantum dots.
The couple electronic state of the stack quantum dots by axial symmetrical finite element analysis
NASA Astrophysics Data System (ADS)
Liu, Yumin; Yu, Zhongyuan; Ren, Xiaomin
2007-11-01
Semiconductor quantum dots have been of major interest in recent years. This has largely been simulated by progress in quantum dot growth technology, whereby self-organized quantum dots array can be fabricated by MBE and MOCVD facilities using Stranski Krastanow growth mode. Quantum does material has achieved broad applications in optoelectronic devices and quantum information fields because of the unique 3D electron confinement. However, a good understanding about the electronic, excitonic and optoelectronics properties of the quantum materials are very important in fabrication nanostructure devices based on quantum dots. Based on the 1-band effective-mass theory, a finite element numerical technique is developed to calculate the electronic structure of truncated conical shaped InAs GaAs vertical aligned quantum dot molecular, including the wetting layer. Using the axis-symmetry model, the 3D effective-mass Schrödinger equation with step potential barrier can be reduced to a 2D problem by separating variable technique, which greatly reduced the calculation cost. Form the calculated results, we found that the coupling effects is obviously when the separation distance is in the range of the less than 10nm. The wave functions will exhibits large probability in the region between the quantum dots. In order to consider the effect of the distance between the two layers of quantum dots on the electronic state coupling, we calculated the results when the distance is 6nm, 11nm, 14nm and 17nm. The ground state, the second excited and the highest excited state will lower its energy with decreasing the distance between the quantum dots, but the second excited state will increase its energy. With increasing the distance between the two quantum dots, the coupling effect will become weaker, and for the ground state, the wave function distribution will tend to localized only in one of the quantum dot, the energy become something degenerate. The calculated results show that the ground state and the first excited state are degenerate. With decreasing of the distance, the degenerate states are broken, and the energy levels are separated. In our simulations, the strain effects are ignored. In the future woks, strain should be taken in to account as an easy way. The calculated results can help us to examine optoelectronic properties of the semiconductor nanostructure based on multi sheet of quantum dots with wetting layers.
NASA Astrophysics Data System (ADS)
Bochorishvili, Beka
2011-02-01
A theoretical study of electron and hole electronic states and oscillator strength of interband transitions in a graded spherical HgS/CdS/HgS/CdS quantum dot quantum well (QDQW) nanostructure is presented; also the Coulomb interaction energies of excitons are calculated. The Finite Element Method (FEM) is used for solving the problem with the position dependent effective mass approximation. The results of calculations show that gradation of the potential decreases energies of confined particles and considerably reduces the oscillator strength for certain transitions. The gradation of the potential can be used as an additional parameter for experimental purposes to manipulate needed properties of the QDQW structure.
Mitigation of Quantum Dot Cytotoxicity by Microencapsulation
Romoser, Amelia; Ritter, Dustin; Majitha, Ravish; Meissner, Kenith E.; McShane, Michael; Sayes, Christie M.
2011-01-01
When CdSe/ZnS-polyethyleneimine (PEI) quantum dots (QDs) are microencapsulated in polymeric microcapsules, human fibroblasts are protected from acute cytotoxic effects. Differences in cellular morphology, uptake, and viability were assessed after treatment with either microencapsulated or unencapsulated dots. Specifically, QDs contained in microcapsules terminated with polyethylene glycol (PEG) mitigate contact with and uptake by cells, thus providing a tool to retain particle luminescence for applications such as extracellular sensing and imaging. The microcapsule serves as the “first line of defense” for containing the QDs. This enables the individual QD coating to be designed primarily to enhance the function of the biosensor. PMID:21814567
Resonant tunneling through quantum-dot arrays
Chen, G.; Klimeck, G.; Datta, S. (School of Electrical Engineering, Purdue University, West Lafayette, Indiana 47907 (United States)); Chen, G.; Goddard, W.A. III (Material and Molecular Simulation Center, Beckman Institute, California Institute of Technology, Pasadena, California 91125 (United States))
1994-09-15
We apply the Hubbard Hamiltonian to describe quantum-dot arrays weakly coupled to two contacts. Exact diagonalization is used to calculate the eigenstates of the arrays containing up to six dots and the linear-response conductance is then calculated as a function of the Fermi energy. In the atomic limit the conductance peaks form two distinct groups separated by the intradot Coulomb repulsion, while in the band limit the peaks occur in pairs. The crossover is studied. A finite interdot repulsion is found to cause interesting rearrangements in the conductance spectrum.
Effect of shells on photoluminescence of aqueous CdTe quantum dots
Yuan, Zhimin; Yang, Ping, E-mail: mse_yangp@ujn.edu.cn
2013-07-15
Graphical abstract: Size-tunable CdTe coated with several shells using an aqueous solution synthesis. CdTe/CdS/ZnS quantum dots exhibited high PL efficiency up to 80% which implies the promising applications for biomedical labeling. - Highlights: • CdTe quantum dots were fabricated using an aqueous synthesis. • CdS, ZnS, and CdS/ZnS shells were subsequently deposited on CdTe cores. • Outer ZnS shells provide an efficient confinement of electron and hole inside the QDs. • Inside CdS shells can reduce the strain on the QDs. • Aqueous CdTe/CdS/ZnS QDs exhibited high stability and photoluminescence efficiency of 80%. - Abstract: CdTe cores with various sizes were fabricated in aqueous solutions. Inorganic shells including CdS, ZnS, and CdS/ZnS were subsequently deposited on the cores through a similar aqueous procedure to investigate the effect of shells on the photoluminescence properties of the cores. In the case of CdTe/CdS/ZnS quantum dots, the outer ZnS shell provides an efficient confinement of electron and hole wavefunctions inside the quantum dots, while the middle CdS shell sandwiched between the CdTe core and ZnS shell can be introduced to obviously reduce the strain on the quantum dots because the lattice parameters of CdS is situated at the intermediate-level between those of CdTe and ZnS. In comparison with CdTe/ZnS core–shell quantum dots, the as-prepared water-soluble CdTe/CdS/ZnS quantum dots in our case can exhibit high photochemical stability and photoluminescence efficiency up to 80% in an aqueous solution, which implies the promising applications in the field of biomedical labeling.
Quantum dots in Si/SiGe 2DEGs with Schottky top-gated leads
NASA Astrophysics Data System (ADS)
Slinker, K. A.; Lewis, K. L. M.; Haselby, C. C.; Goswami, S.; Klein, L. J.; Chu, J. O.; Coppersmith, S. N.; Joynt, Robert; Blick, R. H.; Friesen, Mark; Eriksson, M. A.
2005-11-01
We report on the fabrication and characterization of quantum-dot devices in a Schottky-gated silicon/silicon germanium modulation-doped two-dimensional electron gas (2DEG). The dots are confined laterally inside an etch-defined channel, while their potential is modulated by an etch-defined 2DEG gate in the plane of the dot. For the first time in this material, Schottky top gates are used to define and tune the tunnel barriers of the dot. The leakage current from the gates is reduced by minimizing their active area. Further suppression of the leakage is achieved by increasing the etch depth of the channel. The top gates are used to put the dot into the Coulomb-blockade regime, and conductance oscillations are observed as the voltage on the side gate is varied.
Ultranarrow ionization resonances in a quantum dot under broadband excitation
Rudner, M. S.
Semiconductor quantum dots driven by the broadband radiation fields of nearby quantum point contacts provide an interesting setting for probing dynamics in driven quantum systems at the nanoscale. We report on real-time ...
NASA Astrophysics Data System (ADS)
Elyasi, P.; SalmanOgli, A.
2014-05-01
This paper investigates GaAs/AlGaAs modified quantum dot nanocrystal and GaAs/AlGaAs/GaAs/AlGaAs quantum dot-quantum well heteronanocrystal. These quantum dots have been analyzed by the finite element numerical methods. Simulations carried out for state n=1, l=0, and m=0 which are original, orbital, and magnetic state of quantum numbers. The effects of variation in radius layers such as total radius, GaAs core, shell and AlGaAs barriers radius on the wavelength and emission coefficient are studied. For the best time, it has also investigated the effect of mole fraction on emission coefficient. Meanwhile, one of the problems in biological applications is alteration of the emission wavelength of a quantum dot by changing in its dimension. This problem will be resolved by changing in potential profile.
n-Type silicon quantum dots and p-type crystalline silicon heteroface solar cells
Sangwook Park; Eunchel Cho; Dengyuan Song; Gavin Conibeer; Martin A. Green
2009-01-01
Heteroface devices have been realized by depositing phosphorus-doped silicon (Si) quantum dots (QDs) (n-type) on a p-type crystalline silicon substrate. To compare the quantum confinement effect, different sizes (3, 4, 5, and 8±1nm) of Si QD were fabricated, whose optical energy bandgaps are in the ranges of 1.3–1.65eV. The electrical and photovoltaic properties of heterojunction devices were characterized by illuminated
DC Electrical Transport Properties Of PbSe Nanocrystal Quantum Dot Solids
Hugo Romero
2005-01-01
We have studied temperature-dependent electronic charge transport in three-dimensional, closed-packed arrays of PbSe colloidal nanocrystals in the form of thin disordered films. PbSe nanocrystal quantum dots offer unique access to the regime of extreme quantum confinement because of the large Bohr radii of electrons and holes. These materials are expected to have significantly different physical properties from those of the
NASA Astrophysics Data System (ADS)
Saravanamoorthy, S. N.; Peter, A. John
2015-06-01
Binding energies of the exciton and the interband optical transition energies are studied in a CdSe/Pb1-xCdxSe/CdSe spherical quantum dot-quantum well nanostructure taking into account the geometrical confinement effect. The core and shell are taken as the same material. The initial and final states of energy and the overlap integrals of electron and hole wave functions are determined by the oscillator strength. The oscillator strength and the radiative transition life time with the dot radius are investigated for various Cd alloy content in the core and shell materials.
Theory of the Quantum Dot Hybrid Qubit
NASA Astrophysics Data System (ADS)
Friesen, Mark
2015-03-01
The quantum dot hybrid qubit, formed from three electrons in two quantum dots, combines the desirable features of charge qubits (fast manipulation) and spin qubits (long coherence times). The hybridized spin and charge states yield a unique energy spectrum with several useful properties, including two different operating regimes that are relatively immune to charge noise due to the presence of optimal working points or ``sweet spots.'' In this talk, I will describe dc and ac-driven gate operations of the quantum dot hybrid qubit. I will analyze improvements in the dephasing that are enabled by the sweet spots, and I will discuss the outlook for quantum hybrid qubits in terms of scalability. This work was supported in part by ARO (W911NF-12-0607), NSF (PHY-1104660), the USDOD, and the Intelligence Community Postdoctoral Research Fellowship Program. The views and conclusions contained in this presentation are those of the authors and should not be interpreted as representing the official policies or endorsements, either expressed or implied, of the US government.
Inverted Singlet-Triplet Qubit Coded on a Two-Electron Double Quantum Dot
Sebastian Mehl; David P. DiVincenzo
2014-11-26
The $s_z=0$ spin configuration of two electrons confined at a double quantum dot (DQD) encodes the singlet-triplet qubit (STQ). We introduce the inverted STQ (ISTQ) that emerges from the setup of two quantum dots (QDs) differing significantly in size and out-of-plane magnetic fields. The strongly confined QD has a two-electron singlet ground state, but the weakly confined QD has a two-electron triplet ground state in the $s_z=0$ subspace. Spin-orbit interactions act nontrivially on the $s_z=0$ subspace and provide universal control of the ISTQ together with electrostatic manipulations of the charge configuration. GaAs and InAs DQDs can be operated as ISTQs under realistic noise conditions.
Melnik, Roderick
2013-01-01
in semiconductor quantum dots: Rashba versus Dresselhaus spin-orbit coupling Sanjay Prabhakar,1 Roderick Melnik,1 dominates over the Rashba spin-orbit coupling in GaAs and GaSb QDs, it is important to find the exact electron spin with the application of gate controlled electric fields in confined semiconductor quantum
Ha, S.-K.; Song, J. D.; Lim, J. Y.; Choi, W. J.; Han, I. K.; Lee, J. I. [Nano Convergence Devices Center, KIST, Seoul 136-791 (Korea, Republic of); Bounouar, S.; Donatini, F.; Dang, L. S.; Poizat, J. P. [CEA/CNRS/UJF team 'Nanophysics and semiconductors', Institute Neel/CNRS-UJF, 38042 Grenoble (France); Kim, J. S. [Department of Physics, Yeungnam University, Gyeongsan 712-749 (Korea, Republic of)
2011-12-23
The GaAs quantum dots in AlGaAs barriers were grown by droplet epitaxy, emitting around 700 nm in wavelength which is compatible with low cost Si based detectors. The excitation power dependent and time resolved micro-photoluminescence measurements identified optical characteristics of exciton and biexciton states which are attributed to good quantum confinements in GaAs QDs.
NASA Astrophysics Data System (ADS)
Juska, G.; Dimastrodonato, V.; Mereni, L. O.; Chung, T. H.; Gocalinska, A.; Pelucchi, E.; Van Hattem, B.; Ediger, M.; Corfdir, P.
2014-05-01
A study of previously overlooked structural and optical properties of InGaAs heterostructures grown on (111)B oriented GaAs substrates patterned with inverted 7.5-?m pitch pyramidal recesses is presented. First, the composition of the confinement barrier material (GaAs in this work) and its growth temperature are shown as some of the key parameters that determine the main quantum dot properties, including nontrivial emission energy dependence, excitonic pattern, and unusual photoluminescence energetic ordering of the InGaAs ensemble nanostructures. Second, the formation of a formerly unidentified type of InGaAs nanostructures—three corner quantum dots—is demonstrated in our structures next to the well-known ones (a quantum dot and three lateral quantum wires and quantum wells). The findings show the complexity of the pyramidal quantum dot system which strongly depends on the sample design and which should be considered when selecting highly symmetric (central) quantum dots in newly designed experimental projects.
Light-emitting quantum dot transistors: emission at high charge carrier densities.
Schornbaum, Julia; Zakharko, Yuriy; Held, Martin; Thiemann, Stefan; Gannott, Florentina; Zaumseil, Jana
2015-03-11
For the application of colloidal semiconductor quantum dots in optoelectronic devices, for example, solar cells and light-emitting diodes, it is crucial to understand and control their charge transport and recombination dynamics at high carrier densities. Both can be studied in ambipolar, light-emitting field-effect transistors (LEFETs). Here, we report the first quantum dot light-emitting transistor. Electrolyte-gated PbS quantum dot LEFETs exhibit near-infrared electroluminescence from a confined region within the channel, which proves true ambipolar transport in ligand-exchanged quantum dot solids. Unexpectedly, the external quantum efficiencies improve significantly with current density. This effect correlates with the unusual increase of photoluminescence quantum yield and longer average lifetimes at higher electron and hole concentrations in PbS quantum dot thin films. We attribute the initially low emission efficiencies to nonradiative losses through trap states. At higher carrier densities, these trap states are deactivated and emission is dominated by trions. PMID:25652433
Light-Emitting Quantum Dot Transistors: Emission at High Charge Carrier Densities
2015-01-01
For the application of colloidal semiconductor quantum dots in optoelectronic devices, for example, solar cells and light-emitting diodes, it is crucial to understand and control their charge transport and recombination dynamics at high carrier densities. Both can be studied in ambipolar, light-emitting field-effect transistors (LEFETs). Here, we report the first quantum dot light-emitting transistor. Electrolyte-gated PbS quantum dot LEFETs exhibit near-infrared electroluminescence from a confined region within the channel, which proves true ambipolar transport in ligand-exchanged quantum dot solids. Unexpectedly, the external quantum efficiencies improve significantly with current density. This effect correlates with the unusual increase of photoluminescence quantum yield and longer average lifetimes at higher electron and hole concentrations in PbS quantum dot thin films. We attribute the initially low emission efficiencies to nonradiative losses through trap states. At higher carrier densities, these trap states are deactivated and emission is dominated by trions. PMID:25652433
Mechanisms for Electric Field Control of Single Spin Relaxation in Double Quantum Dots
NASA Astrophysics Data System (ADS)
Srinivasa, V.; Nowack, K. C.; Shafiei, M.; Vandersypen, L. M. K.; Taylor, J. M.
2013-03-01
We theoretically investigate electrically-tunable spin-flip transitions for a single electron confined within a double quantum dot. In the presence of spin-orbit and hyperfine interactions, the rate at which phonon-induced spin relaxation occurs depends non-monotonically on the detuning between the dots. We analyze this detuning dependence for both direct decay to the ground state and indirect decay via an intermediate excited state of the double dot. A description in terms of a simple toy model captures characteristic features of the relaxation rate recently measured for GaAs double quantum dots. Our results suggest that spin-orbit mediated relaxation via phonons serves as the dominant mechanism through which the electron spin-flip rate in these systems varies with detuning. We theoretically investigate electrically-tunable spin-flip transitions for a single electron confined within a double quantum dot. In the presence of spin-orbit and hyperfine interactions, the rate at which phonon-induced spin relaxation occurs depends non-monotonically on the detuning between the dots. We analyze this detuning dependence for both direct decay to the ground state and indirect decay via an intermediate excited state of the double dot. A description in terms of a simple toy model captures characteristic features of the relaxation rate recently measured for GaAs double quantum dots. Our results suggest that spin-orbit mediated relaxation via phonons serves as the dominant mechanism through which the electron spin-flip rate in these systems varies with detuning. Support from DARPA MTO and IARPA is gratefully acknowledged.
Miniband formation in a quantum dot crystal Olga L. Lazarenkovaa)
structure in a three-dimensional regimented array of semiconductor quantum dots using an envelope function regimented array of semiconductor quantum dots using an envelope function approximation. The regi- mentation are different from those of bulk and quantum well superlattices. It has also been established
Facile labeling of lipoglycans with quantum dots
Morales Betanzos, Carlos; Gonzalez-Moa, Maria [Center for Innovations in Medicine, Biodesign Institute, Arizona State University, 1001 S. McAllister Av, Tempe, AZ 85287 (United States); Johnston, Stephen Albert [Center for Innovations in Medicine, Biodesign Institute, Arizona State University, 1001 S. McAllister Av, Tempe, AZ 85287 (United States); School of Life Sciences, Arizona State University 1711 S. Rural Road, Tempe, AZ 85287 (United States); Svarovsky, Sergei A. [Center for Innovations in Medicine, Biodesign Institute, Arizona State University, 1001 S. McAllister Av, Tempe, AZ 85287 (United States)], E-mail: Sergei.Svarovsky@asu.edu
2009-02-27
Bacterial endotoxins or lipopolysaccharides (LPS) are among the most potent activators of the innate immune system, yet mechanisms of their action and in particular the role of glycans remain elusive. Efficient non-invasive labeling strategies are necessary for studying interactions of LPS glycans with biological systems. Here we report a new method for labeling LPS and other lipoglycans with luminescent quantum dots. The labeling is achieved by partitioning of hydrophobic quantum dots into the core of various LPS aggregates without disturbing the native LPS structure. The biofunctionality of the LPS-Qdot conjugates is demonstrated by the labeling of mouse monocytes. This simple method should find broad applicability in studies concerned with visualization of LPS biodistribution and identification of LPS binding agents.
Nanobeam photonic crystal cavity quantum dot laser
Gong, Yiyang; Shambat, Gary; Sarmiento, Tomas; Harris, James S; Vuckovic, Jelena
2010-01-01
The lasing behavior of one dimensional GaAs nanobeam cavities with embedded InAs quantum dots is studied at room temperature. Lasing is observed throughout the quantum dot PL spectrum, and the wavelength dependence of the threshold is calculated. We study the cavity lasers under both 780 nm and 980 nm pump, finding thresholds as low as 0.3 uW and 19 uW for the two pump wavelengths, respectively. Finally, the nanobeam cavity laser wavelengths are tuned by up to 7 nm by employing a fiber taper in near proximity to the cavities. The fiber taper is used both to efficiently pump the cavity and collect the cavity emission.
Nanobeam photonic crystal cavity quantum dot laser.
Gong, Yiyang; Ellis, Bryan; Shambat, Gary; Sarmiento, Tomas; Harris, James S; Vuckovic, Jelena
2010-04-26
The lasing behavior of one dimensional GaAs nanobeam cavities with embedded InAs quantum dots is studied at room temperature. Lasing is observed throughout the quantum dot PL spectrum, and the wavelength dependence of the threshold is calculated. We study the cavity lasers under both 780 nm and 980 nm pump, finding thresholds as low as 0.3 microW and 19 microW for the two pump wavelengths, respectively. Finally, the nanobeam cavity laser wavelengths are tuned by up to 7 nm by employing a fiber taper in near proximity to the cavities. The fiber taper is used both to efficiently pump the cavity and collect the cavity emission. PMID:20588722
IR quantum dot detectors with miniband tunnel extraction
F. F. Schrey; D. P. Nguyen; T. Mueller; L. Rebohle; N. Regnault; R. Ferreira; G. Bastard; G. Strasser; K. Unterrainer
2004-01-01
By combining band gap engineering with the self-organised growth of quantum dots we present a scheme of adjusting the mid-infrared absorption properties to desired energy transitions in quantum dot based photodetectors (QDIPs). Embedding of the self organised InAs quantum dots into an AlAs\\/GaAs superlattice enables us to tune the optical transition energy by changing the superlattice period as well as
Molecular spintronics: Coherent spin transfer in coupled quantum dots
Florian Meier; Veronica Cerletti; Oliver Gywat; Daniel Loss; D. D. Awschalom
2004-01-01
Time-resolved Faraday rotation has recently demonstrated coherent transfer of electron spin between quantum dots coupled by conjugated molecules. Using a transfer Hamiltonian ansatz for the coupled quantum dots, we calculate the Faraday rotation signal as a function of the probe frequency in a pump-probe setup using neutral quantum dots. Additionally, we study the signal of one spin-polarized excess electron in
Ultrafast nonlinear spectroscopy characterization of CdSe quantum dots
Qiguang Yang; Seongmin Ma; Bagher Tabibi; Jaetae Seo; William Yu
2008-01-01
Frequency degenerate and nondegenerate two-photon absorption spectra of direct band gap semiconductor quantum dots, such as CdSe and CdTe, have attracted great attention recently because of their potential applications in nonlinear photonic devices. In this work, we used the femtosecond time-resolved photon echo technique to characterize the third-order nonlinear optical properties of CdSe quantum dots in toluene. The quantum dots
Photoluminescence of Silicon-Germanium Quantum Dots
NSDL National Science Digital Library
Kolodzey, James
This presentation, given at the Arizona Nanotechnology Cluster Symposium, introduces the topic of the photoluminescence of silicon-germainium quantum dots. Dr. James Kolodzey, of University of Delaware, presents the topic in powerpoint format. The presentation is loaded with helpful diagrams and images that capture the essence of Kolodzey's research. Overall, while the topic is advanced, the presentations allows users to better understand due to the helpful resources it contains.
Optical Spectroscopy Of Single Quantum Dots
Jan Valenta; Jan Linnros; Robert Juhasz; Frank Cichos; JÖrg Martin
\\u000a The technique of single quantum dot spectroscopy (SQDS) measurements of individual nanocrystals (NCs) is nowadays widely applied\\u000a to study NCs of III-V and II- VI semiconductors [1]. It enables to observe effects that are hidden by significant inhomogeneous broadening inevitable in measurements on ensemble\\u000a of NCs. The SQDS measurements revealed several intriguing phenomena such as photoluminescence (PL) intermittence, spectral\\u000a diffusion
Dark pulse quantum dot diode laser.
Feng, Mingming; Silverman, Kevin L; Mirin, Richard P; Cundiff, Steven T
2010-06-21
We describe an operating regime for passively mode-locked quantum dot diode laser where the output consists of a train of dark pulses, i.e., intensity dips on a continuous background. We show that a dark pulse train is a solution to the master equation for mode-locked lasers. Using simulations, we study stability of the dark pulses and show they are consistent with the experimental results. PMID:20588468
Nitride-based quantum dot visible lasers
NASA Astrophysics Data System (ADS)
Banerjee, A.; Frost, T.; Bhattacharya, P.
2013-07-01
Blue- and green-emitting laser heterostructures are grown by molecular beam epitaxy, incorporating InGaN/GaN quantum dots as the active medium. The quantum dot growth parameters are optimized to obtain the highest photoluminescence (PL) intensity and radiative efficiency. Injected carrier lifetimes in the quantum dots are measured by temperature-dependent and time-resolved PL measurements. The blue lasers (? = 479 nm) are characterized by threshold current densities of 1.8 kA cm-2 and 2.3 kA cm-2 under quasi-continuous wave bias for devices with Al0.08Ga0.92N and GaN waveguide cladding layers, respectively. The differential gain of these devices is 1.03 × 10-16 cm-2. A threshold current density of ˜1.65 kA cm-2 is measured for a green laser (? = 545 nm) with Al0.08Ga0.92N cladding under quasi-continuous wave bias.
TOPICAL REVIEW: Quantum dots: lasers and amplifiers
NASA Astrophysics Data System (ADS)
Bimberg, Dieter; Ledentsov, Nikolai
2003-06-01
Continuous wave room-temperature output power of ~ 3 W for edge emitters and of 1.2 mW for vertical-cavity surface-emitting lasers is realized for GaAs-based devices using InAs quantum dots (QDs) operating at 1.3 µm. Characteristic temperatures up to 170 K below 330 K are realized. Simultaneously, differential efficiency exceeds 80% for these devices. Lasers emitting up to 12 W at 1140 1160 nm are useful as pump sources for Tm3+-doped fibres for frequency up-conversion to 470 nm. Both types of lasers show transparency current densities of 6 A cm-2 per dot layer, ?int = 98% and ?i around 1.5 cm-1. Long operation lifetimes (above 3000 h at 50 °C heatsink temperature at 1.5 W CW) and improved radiation hardness as compared to quantum well (QW) devices are manifested. Cut-off frequencies of about 10 GHz at 1100 nm and 6 GHz at 1300 nm and low ? factors resulting in reduced filamentation and improved M2 values in single-mode operation are realized. Quantum dot semiconductor optical amplifiers (QD SOAs) demonstrate gain recovery times of 120 140 fs, 4 7 times faster than bulk/QW SOAs. The breakthrough became possible due to the development of self-organized growth in QD technology.
Imaging ligand-gated ion channels with quantum dots
NASA Astrophysics Data System (ADS)
Tomlinson, I. D.; Orndorff, Rebecca L.; Gussin, Hélène; Mason, John N.; Blakely, Randy D.; Pepperberg, David R.; Rosenthal, Sandra J.
2007-02-01
In this paper we report two different methodologies for labeling ligand-gated receptors. The first of these builds upon our earlier work with serotonin conjugated quantum dots and our studies with pegilated quantum dots to reduce non specific binding. In this approach a pegilated derivative of muscimol was synthesized and attached via an amide linkage to quantum dots coated in an amphiphillic polymer derivative of poly acrylamide. These conjugates were used to image the GABA C receptor in oocytes. An alternative approach was used to image tissue sections to study nicotinic acetylcholine receptors in the neuro muscular junction with biotinylated Bungerotoxin and streptavidin coated quantum dots.
Ultra-bright alkylated graphene quantum dots
NASA Astrophysics Data System (ADS)
Feng, Lan; Tang, Xing-Yan; Zhong, Yun-Xin; Liu, Yue-Wen; Song, Xue-Huan; Deng, Shun-Liu; Xie, Su-Yuan; Yan, Jia-Wei; Zheng, Lan-Sun
2014-10-01
Highly efficient and stable photoluminescence (PL) are urgently desired for graphene quantum dots (GQDs) to facilitate their prospective applications as optical materials. Here, we report the facile and straightforward synthesis of alkylated graphene quantum dots (AGQDs) via the solvothermal reaction of propagatively alkylated graphene sheets (PAGenes). In contrast to most GQDs reported so far, the synthesized AGQDs process pH-independent and ultra-bright PL with a relative quantum yield of up to 65%. Structural and chemical composition characterization demonstrated that the synthesized AGQDs are nearly oxygen-defect-free with alkyl groups decorated on edges and basal plane, which may contribute to their greatly improved pH tolerance and high quantum efficiency. The photocatalytic performance of AGQDs-P25 nanocomposites was evaluated by the degradation of Rhodamine B under visible light. The photocatalytic rate is ca. 5.9 times higher than that of pure P25, indicating that AGQDs could harness the visible spectrum of sunlight for energy conversion or environmental therapy.Highly efficient and stable photoluminescence (PL) are urgently desired for graphene quantum dots (GQDs) to facilitate their prospective applications as optical materials. Here, we report the facile and straightforward synthesis of alkylated graphene quantum dots (AGQDs) via the solvothermal reaction of propagatively alkylated graphene sheets (PAGenes). In contrast to most GQDs reported so far, the synthesized AGQDs process pH-independent and ultra-bright PL with a relative quantum yield of up to 65%. Structural and chemical composition characterization demonstrated that the synthesized AGQDs are nearly oxygen-defect-free with alkyl groups decorated on edges and basal plane, which may contribute to their greatly improved pH tolerance and high quantum efficiency. The photocatalytic performance of AGQDs-P25 nanocomposites was evaluated by the degradation of Rhodamine B under visible light. The photocatalytic rate is ca. 5.9 times higher than that of pure P25, indicating that AGQDs could harness the visible spectrum of sunlight for energy conversion or environmental therapy. Electronic supplementary information (ESI) available: Additional figures (Fig. S1-S12). See DOI: 10.1039/c4nr03506b
Engineered quantum dot single-photon sources
NASA Astrophysics Data System (ADS)
Buckley, Sonia; Rivoire, Kelley; Vu?kovi?, Jelena
2012-12-01
Fast, high efficiency and low error single-photon sources are required for the implementation of a number of quantum information processing applications. The fastest triggered single-photon sources to date have been demonstrated using epitaxially grown semiconductor quantum dots (QDs), which can be conveniently integrated with optical microcavities. Recent advances in QD technology, including demonstrations of high temperature and telecommunications wavelength single-photon emission, have made QD single-photon sources more practical. Here we discuss the applications of single-photon sources and their various requirements, before reviewing the progress made on a QD platform in meeting these requirements.
Quantum Dot Intermediate Band Solar Cells: Design Criteria and Optimal Materials
NASA Astrophysics Data System (ADS)
Jenks, Steven Evans
The main limitation of the conventional solar conversion device is that low energy photons cannot. excite charge carriers to the conduction band, therefore do not contribute to the devices's current, and high energy photons are not efficiently used due to a poor match of the solar spectrum to the energy gap. However, if intermediate levels are introduced into the energy gap of a conventional device, then low energy photons can be used to promote charge carriers in a stepwise mariner to the conduction band thereby enhancing the current while maintaining a large open-circuit voltage. This concept is called the intermediate band solar cell and increases the efficiency beyond the thermodynamic limits of the conventional device. A device based on the confined electron levels of quantum dots called the quantum dot intermediate band solar cell is a physical realization of the intermediate band solar cell. In this work, we propose design criteria and optimal material systems that are considered candidates for the quantum dot intermediate band solar cell. To search for optimal materials. the finite element method is developed and MATLAB code is designed in the context of quantum and continuum mechanics with the sophistication necessary to allow for three dimensional numerical simulations that incorporate realistic assumptions about the quantum dot. The materials considered in this work are the technologically important III-V compound semiconductors and their alloys. Numerical simulations are carried out on quantum dot geometries that have been experimentally observed during self-assembled growth, the technology proposed to achieve the quantum dot intermediate band solar cell, and those material systems that have properties that match those of the intermediate band solar cell with efficiency greater than 46% for unconcentrated light and greater than 62% for fully concentrated light are identified as optimal materials for the quantum dot intermediate band solar cell.
Transport through evanescent waves in ballistic graphene quantum dots
M. I. Katsnelson; F. Guinea
2008-01-01
We study the transport through evanescent waves in graphene quantum dots of different geometries. The transmission is suppressed when the leads are attached to edges of the same majority sublattice. Otherwise, the transmission depends exponentially on the distance between leads in rectangular dots and as a power law in circular dots. The transmission through junctions, where the transmitted and reflected
Morphology effects of self-assembled quantum dots on the energy spectrum of magneto-excitons
NASA Astrophysics Data System (ADS)
Villamil, Pablo; Sierra Ortega, José
2014-02-01
In this paper we analyze the changes experienced by the energy spectra of a confined exciton in type II semiconductor quantum dots, considering the quantum dot as a possible functional part that, in the future devices, can be applied in spintronics, optoelectronics, and quantum information technologies. We studied the lowest energy states of an exciton (X) confined in type II InP/GaInP self-assembled quantum dot (SAQDs), with axial symmetry in the presence of a uniformly applied magnetic field in the growth direction. In our model, it is considered that the electron is located within the point of InP and the hole is in the GaInP barrier. The solution of the Schrödinger equation for this system is obtained by a variational separation process of variables in the adiabatic approximation limit and within the effective mass approximation. We study the energy levels associated with the electron and the hole, and the energy of the exciton. Due to the axial symmetry of the problem the z component of the total orbital angular momentum, Lz=le+lh, is preserved and the exciton states are classified according to the values of this component. Quantum dots have a finite and variable thickness, with the purpose of analyzing the effects related to the variation of the morphology and the presence of a wet layer.
Enhanced performance of quantum dot solar cells based on type II quantum dots
NASA Astrophysics Data System (ADS)
Xu, Feng; Yang, Xiao-Guang; Luo, Shuai; Lv, Zun-Ren; Yang, Tao
2014-10-01
The characteristics of quantum dot solar cells (QDSCs) based on type II QDs are investigated theoretically. Based on a drift-diffusion model, we obtained a much higher open circuit voltage (Voc) as well as conversion efficiency in a type II QDSC, compared to type I QDSCs. The improved Voc and efficiency are mainly attributed to the much longer Auger recombination lifetime in type II QDs. Moreover, the influence of the carrier lifetime on devices' performance is discussed and clarified. In addition, an explicit criterion to determine the role of quantum dots in solar cells is put forward.
Wei, Hai-Rui; Deng, Fu-Guo
2013-07-29
We investigate the possibility of achieving scalable photonic quantum computing by the giant optical circular birefringence induced by a quantum-dot spin in a double-sided optical microcavity as a result of cavity quantum electrodynamics. We construct a deterministic controlled-not gate on two photonic qubits by two single-photon input-output processes and the readout on an electron-medium spin confined in an optical resonant microcavity. This idea could be applied to multi-qubit gates on photonic qubits and we give the quantum circuit for a three-photon Toffoli gate. High fidelities and high efficiencies could be achieved when the side leakage to the cavity loss rate is low. It is worth pointing out that our devices work in both the strong and the weak coupling regimes. PMID:23938640
Photoluminescence imaging of focused ion beam induced individual quantum dots.
Lee, Jieun; Saucer, Timothy W; Martin, Andrew J; Tien, Deborah; Millunchick, Joanna M; Sih, Vanessa
2011-03-01
We report on scanning microphotoluminescence measurements that spectrally and spatially resolve emission from individual InAs quantum dots that were induced by focused ion beam patterning. Multilayers of quantum dots were spaced 2 ?m apart, with a minimum single dot emission line width of 160 ?eV, indicating good optical quality for dots patterned using this technique. Mapping 16 array sites, at least 65% were occupied by optically active dots and the spectral inhomogeneity was within 30 meV. PMID:21302932
Quantum-CORMs: quantum dot sensitized CO releasing molecules.
Ruggi, A; Zobi, F
2015-06-28
The synthesis and photodecomposition behaviour of a family of CO releasing molecules (CORMs) based on [Mn(CO)3bpy] derivatives connected to a semiconductor Quantum Dot (QD) sensitizer is described here. Compared to the non-sensitized complexes, such systems show a 2 to 6-fold increase of the photodecomposition rate upon irradiation with visible light. PMID:25997860
POSSIBLE DEFINTION OF QUANTUM BITS IN COUPLED QUANTUM DOTS
Ludwig-Maximilians-Universität, München
reduction is the single-electron-transistor (SET). It consists of a small electron island also called interaction of single electrons in this quantum dot and in the leads is becoming dominant. The repulsive then enables single electrons to tunnel sequentially through the SET. At first these SETs only could
NASA Astrophysics Data System (ADS)
Safarpour, Gh.; Izadi, M. A.; Niknam, E.; Moradi, M.; Golshan, M. M.
2014-03-01
Based on the effective-mass approximation, the effects of external electric field and laser radiation on the binding energy and Stark shifts of electronic energy levels of a system consist of an on-center hydrogenic donor impurity in a spherical quantum dot which is located at the center of a cylindrical nano-wire have been studied. The energy eigenvalues and corresponding wave functions are calculated using the finite difference approximation. The binding energy and Stark shifts dependencies are reported upon the electric field strength, aluminum concentration and laser radiation. The results reveal that Stark shifts of ground and first excited states are strongly affected by presence of impurity, laser radiation and Al concentration. Additionally, the binding energies decreases as the electric field increases and become negligible for large values of electric field; decreases as the laser radiation increases and increases as the Al concentration increases.
Klotz, F; Huebl, H; Heiss, D; Klein, K; Finley, J J; Brandt, M S
2011-07-01
We report on the development and testing of a coplanar stripline antenna that is designed for integration in a magneto-photoluminescence experiment to allow coherent control of individual electron spins confined in single self-assembled semiconductor quantum dots. We discuss the design criteria for such a structure which is multi-functional in the sense that it serves not only as microwave delivery but also as electrical top gate and shadow mask for the single quantum dot spectroscopy. We present test measurements on hydrogenated amorphous silicon, demonstrating electrically detected magnetic resonance using the in-plane component of the oscillating magnetic field created by the coplanar stripline antenna necessary due to the particular geometry of the quantum dot spectroscopy. From reference measurements using a commercial electron spin resonance setup in combination with finite element calculations simulating the field distribution in the structure, we obtain a magnetic field of 0.12 mT at the position where the quantum dots would be integrated into the device. The corresponding ?-pulse time of ?0.5??s meets the requirements set by the high sensitivity optical spin read-out scheme developed for the quantum dot. PMID:21806214
Hybrid passivated colloidal quantum dot solids
NASA Astrophysics Data System (ADS)
Ip, Alexander H.; Thon, Susanna M.; Hoogland, Sjoerd; Voznyy, Oleksandr; Zhitomirsky, David; Debnath, Ratan; Levina, Larissa; Rollny, Lisa R.; Carey, Graham H.; Fischer, Armin; Kemp, Kyle W.; Kramer, Illan J.; Ning, Zhijun; Labelle, André J.; Chou, Kang Wei; Amassian, Aram; Sargent, Edward H.
2012-09-01
Colloidal quantum dot (CQD) films allow large-area solution processing and bandgap tuning through the quantum size effect. However, the high ratio of surface area to volume makes CQD films prone to high trap state densities if surfaces are imperfectly passivated, promoting recombination of charge carriers that is detrimental to device performance. Recent advances have replaced the long insulating ligands that enable colloidal stability following synthesis with shorter organic linkers or halide anions, leading to improved passivation and higher packing densities. Although this substitution has been performed using solid-state ligand exchange, a solution-based approach is preferable because it enables increased control over the balance of charges on the surface of the quantum dot, which is essential for eliminating midgap trap states. Furthermore, the solution-based approach leverages recent progress in metal:chalcogen chemistry in the liquid phase. Here, we quantify the density of midgap trap states in CQD solids and show that the performance of CQD-based photovoltaics is now limited by electron-hole recombination due to these states. Next, using density functional theory and optoelectronic device modelling, we show that to improve this performance it is essential to bind a suitable ligand to each potential trap site on the surface of the quantum dot. We then develop a robust hybrid passivation scheme that involves introducing halide anions during the end stages of the synthesis process, which can passivate trap sites that are inaccessible to much larger organic ligands. An organic crosslinking strategy is then used to form the film. Finally, we use our hybrid passivated CQD solid to fabricate a solar cell with a certified efficiency of 7.0%, which is a record for a CQD photovoltaic device.
Lifetime blinking in nonblinking nanocrystal quantum dots
Galland, Christophe; Ghosh, Yagnaseni; Steinbrück, Andrea; Hollingsworth, Jennifer A.; Htoon, Han; Klimov, Victor I.
2012-01-01
Nanocrystal quantum dots are attractive materials for applications as nanoscale light sources. One impediment to these applications is fluctuations of single-dot emission intensity, known as blinking. Recent progress in colloidal synthesis has produced nonblinking nanocrystals; however, the physics underlying blinking suppression remains unclear. Here we find that ultra-thick-shell CdSe/CdS nanocrystals can exhibit pronounced fluctuations in the emission lifetimes (lifetime blinking), despite stable nonblinking emission intensity. We demonstrate that lifetime variations are due to switching between the neutral and negatively charged state of the nanocrystal. Negative charging results in faster radiative decay but does not appreciably change the overall emission intensity because of suppressed nonradiative Auger recombination for negative trions. The Auger process involving excitation of a hole (positive trion pathway) remains efficient and is responsible for charging with excess electrons, which occurs via Auger-assisted ionization of biexcitons accompanied by ejection of holes. PMID:22713750
Can inorganic salts tune electronic properties of graphene quantum dots?
Colherinhas, Guilherme; Fileti, Eudes Eterno; Chaban, Vitaly V
2015-06-24
Electronic properties of graphene quantum dots (GQDs) constitute a subject of intense scientific interest. Being smaller than 20 nm, GQDs contain confined excitons in all dimensions simultaneously. GQDs feature a non-zero band gap and luminescence on excitation. Tuning their electronic structure is an attractive goal with technological promise. In this work, we apply density functional theory to study the effect of neutral ionic clusters adsorbed on the GQD surface. We conclude that both the HOMO and the LUMO of GQDs are very sensitive to the presence of ions and to their distance from the GQD surface. However, the alteration of the band gap itself is modest, as opposed to the case of free ions (recent reports). Our work fosters progress in modulating electronic properties of nanoscale carbonaceous materials. PMID:26076677
Terahertz hot electron bolometric detectors based on graphene quantum dots
NASA Astrophysics Data System (ADS)
El Fatimy, A.; Myers-Ward, R. L.; Boyd, A. K.; Daniels, K. M.; Gaskill, D. K.; Barbara, P.
2015-03-01
We study graphene quantum dots patterned from epitaxial graphene on SiC with a resistance strongly dependent on temperature. The combination of weak electron-phonon coupling and small electronic heat capacity in graphene makes these quantum dots ideal hot-electron bolometers. We measure and characterize the THz optical response of devices with different dot sizes, at operating temperatures from 2.5K to 80K. The high responsivity, the potential for operation above 80 K and the process scalability show great promise towards practical applications of graphene quantum dot THz detectors. This work was sponsored by the U.S. Office of Naval Research (Award Number N000141310865).
Quantum confinement in Si and Ge nanostructures: effect of crystallinity
NASA Astrophysics Data System (ADS)
Barbagiovanni, Eric G.; Lockwood, David J.; Costa Filho, Raimundo N.; Goncharova, Lyudmila V.; Simpson, Peter J.
2013-10-01
We look at the relationship between the preparation method of Si and Ge nanostructures (NSs) and the structural, electronic, and optical properties in terms of quantum confinement (QC). QC in NSs causes a blue shift of the gap energy with decreasing NS dimension. Directly measuring the effect of QC is complicated by additional parameters, such as stress, interface and defect states. In addition, differences in NS preparation lead to differences in the relevant parameter set. A relatively simple model of QC, using a `particle-in-a-box'-type perturbation to the effective mass theory, was applied to Si and Ge quantum wells, wires and dots across a variety of preparation methods. The choice of the model was made in order to distinguish contributions that are solely due to the effects of QC, where the only varied experimental parameter was the crystallinity. It was found that the hole becomes de-localized in the case of amorphous materials, which leads to stronger confinement effects. The origin of this result was partly attributed to differences in the effective mass between the amorphous and crystalline NS as well as between the electron and hole. Corrections to our QC model take into account a position dependent effective mass. This term includes an inverse length scale dependent on the displacement from the origin. Thus, when the deBroglie wavelength or the Bohr radius of the carriers is on the order of the dimension of the NS the carriers `feel' the confinement potential altering their effective mass. Furthermore, it was found that certain interface states (Si-O-Si) act to pin the hole state, thus reducing the oscillator strength.
Design and fabrication of quantum-dot lasers
Nabanja, Sheila
2008-01-01
Semiconductor lasers using quantum-dots in their active regions have been reported to exhibit significant performance advantages over their bulk semiconductor and quantum-well counterparts namely: low threshold current, ...
Complete All-Optical Quantum Control of Electron Spins in InAs/GaAs Quantum Dot Molecule
Guy Z. Cohen
2015-01-08
The spin states of electrons and holes confined in InAs quantum dot molecules have recently come to fore as a promising system for the storage or manipulation of quantum information. We describe here a feasible scheme for complete quantum optical control of two electron spin qubits in two vertically-stacked singly-charged InAs quantum dots coupled by coherent electron tunneling. With an applied magnetic field transverse to the growth direction, we construct a universal set of gates that corresponds to the possible Raman transitions between the spin states. We detail the procedure to decompose a given two-qubit unitary operation, so as to realize it with a successive application of up to 8 of these gates. We give the pulse shapes for the laser pulses used to implement this universal set of gates and demonstrate the realization of the two-qubit quantum Fourier transform with fidelity of 0.881 and duration of 414 ps. Our proposal therefore offers an accessible path to universal computation in quantum dot molecules and points to the advantages of using pulse shaping incoherent manipulation of optically active quantum dots to mitigate the negative effects of unintended dynamics and spontaneous emission.
Aperiodic arrays of quantum dots: Influence of external magnetic and electric fields
NASA Astrophysics Data System (ADS)
Kaputkina, N. E.; Lozovik, Yu E.; Muntyanu, R. F.; Vekilov, Yu Kh
2010-04-01
Electronic and excitonic excitations in aperiodic sequences of quantum dots (Thue-Morse, Cantor, Fibonacci, Double-period) were studied in external electrical and magnetic field. Single-particle and two-particle tunneling probability was taken into consideration. Transmission coefficient was determined using quasi-classical approximation and range of values of confining potential and interdot distances when tunneling is essential was estimated. An external electrical and magnetic field effect on electron localization was taken into consideration, an effective steepness of confining potential in magnetic field was appeared as control parameter of the problem. Energy spectrum of aperiodic quantum dot sequences in external magnetic field was obtained. Possibility to tune the state of the system by magnetic field was studied. The increase of the external electrical field shifts the energy states of the particle in a quantum dot and contributes to particle localization. The localization of the excitations is possible at the finite values of the perturbation in the case of aperiodic sequences of quantum dots (contrary to the case of periodical sequences).
Robustness of edge states in graphene quantum dots
M. Wimmer; A. R. Akhmerov; F. Guinea
2010-01-01
We analyze the single-particle states at the edges of disordered graphene quantum dots. We show that generic graphene quantum dots support a number of edge states proportional to circumference of the dot over the lattice constant. Our analytical theory agrees well with numerical simulations. Perturbations breaking electron-hole symmetry such as next-nearest-neighbor hopping or edge impurities shift the edge states away
Adiabatic charge and spin pumping through interacting quantum dots
Fernanda Deus; Alexis R Hernández; Mucio A Continentino
2012-01-01
In this paper we investigate adiabatic charge and spin pumping through interacting quantum dots using non-equilibrium Green’s function techniques and the equation-of-motion method. We treat the electronic correlations inside the dot using a Hartree–Fock approximation and succeed in obtaining closed analytic expressions for the Keldysh Green’s functions. These allow us to compute charge and spin currents through the quantum dot.
Magneto-optics from type-II single quantum dots
NASA Astrophysics Data System (ADS)
Godoy, M. P. F.; Nakaema, M. K. K.; Iikawa, F.; Brasil, M. J. S. P.; Bortoleto, J. R. R.; Cotta, M. A.; Ribeiro, E.; Medeiros-Ribeiro, G.
2004-02-01
We investigated single InP quantum dots embedded in GaAs using micro-photoluminescence as a function of the excitation intensity. InP/GaAs dots exhibit a type-II band alignment, which leads to a spatial separation of the carriers. The effect of a magnetic field on these type-II quantum dots were also investigated through micro-photoluminescence measurements.
Charge detection in a bilayer graphene quantum dot
Stefan Fringes; Christian Volk; Caroline Norda; Bernat Terrés; Jan Dauber; Stephan Engels; Stefan Trellenkamp; Christoph Stampfer
2011-01-01
We show measurements on a bilayer graphene quantum dot with an integrated charge detector. The focus lies on enabling charge detection with a 30 nm wide bilayer graphene nanoribbon located approximately 35 nm next to a bilayer graphene quantum dot with an island diameter of about 100 nm. Local resonances in the nanoribbon can be successfully used to detect individual
Geometry-dependent conductance oscillations in graphene quantum dots
Liang Huang; Rui Yang; Ying-Cheng Lai
2011-01-01
Utilizing rectangular graphene quantum dots with zigzag horizontal boundaries as a paradigm, we find that the conductance of the dots can exhibit significant oscillations with the position of the leads. The oscillation patterns are a result of quantum interference determined by the band structure of the underlying graphene nanoribbon. In particular, the power spectrum of the conductance variation concentrates on
Temperature dependent responsivity of quantum dot infrared photodetectors
Perera, A. G. Unil
Temperature dependent responsivity of quantum dot infrared photodetectors S.Y. Wang a,*, M.C. Lo b of the responsivity of InAs/GaAs quantum dot infrared photodetectors was investigated with detailed measurement constant at different device temperatures and changes linearly with the bias voltage for BC type device
Nonequilibrium electron transport through quantum dots in the Kondo regime
Buse, Karsten
Nonequilibrium electron transport through quantum dots in the Kondo regime P. Wölfle , J. Paaske voltage through quantum dots in the Kondo regime is described within the perturbative renormalization and local spectral function are calculated. We show how the Kondo effect is suppressed by nonequilibrium
Manipulating Quantum Dots to Nanometer Precision by Control of Flow
Waks, Edo
Manipulating Quantum Dots to Nanometer Precision by Control of Flow Chad Ropp, Roland Probst of Standards and Technology, Gaithersburg, Maryland 20899 ABSTRACT We present a method for manipulating on the dynamically manipulated QD. KEYWORDS Quantum dots, control, electroosmotic flow, subpixel averaging, photon
Localisation microscopy with quantum dots using non-negative matrix
Williams, Chris
Localisation microscopy with quantum dots using non-negative matrix factorisation Ondrej Mandula,1, Randall Division, London, UK 5Institute for Adaptive and Neural Computation, University of Edinburgh and blinking behaviour. This allows us to use quantum dots as bright and stable fluorophores for localisation
Colloidal quantum dot photovoltaics: a path forward.
Kramer, Illan J; Sargent, Edward H
2011-11-22
Colloidal quantum dots (CQDs) offer a path toward high-efficiency photovoltaics based on low-cost materials and processes. Spectral tunability via the quantum size effect facilitates absorption of specific wavelengths from across the sun's broad spectrum. CQD materials' ease of processing derives from their synthesis, storage, and processing in solution. Rapid advances have brought colloidal quantum dot photovoltaic solar power conversion efficiencies of 6% in the latest reports. These achievements represent important first steps toward commercially compelling performance. Here we review advances in device architecture and materials science. We diagnose the principal phenomenon-electronic states within the CQD film band gap that limit both current and voltage in devices-that must be cured for CQD PV devices to fulfill their promise. We close with a prescription, expressed as bounds on the density and energy of electronic states within the CQD film band gap, that should allow device efficiencies to rise to those required for the future of the solar energy field. PMID:21967723
Quantum Dots in Diagnostics and Detection: Principles and Paradigms
Pisanic, T. R.; Zhang, Y.; Wang, T. H.
2014-01-01
Quantum dots are semiconductor nanocrystals that exhibit exceptional optical and electrical behaviors not found in their bulk counterparts. Following seminal work in the development of water-soluble quantum dots in the late 1990's, researchers have sought to develop interesting and novel ways of exploiting the extraordinary properties of quantum dots for biomedical applications. Since that time, over 10,000 articles have been published related to the use of quantum dots in biomedicine, many of which regard their use in detection and diagnostic bioassays. This review presents a didactic overview of fundamental physical phenomena associated with quantum dots and paradigm examples of how these phenomena can and have been readily exploited for manifold uses in nanobiotechnology with a specific focus on their implementation in in vitro diagnostic assays and biodetection. PMID:24770716
Non-blinking quantum dot with a plasmonic nanoshell resonator
NASA Astrophysics Data System (ADS)
Ji, Botao; Giovanelli, Emerson; Habert, Benjamin; Spinicelli, Piernicola; Nasilowski, Michel; Xu, Xiangzhen; Lequeux, Nicolas; Hugonin, Jean-Paul; Marquier, Francois; Greffet, Jean-Jacques; Dubertret, Benoit
2015-02-01
Colloidal semiconductor quantum dots are fluorescent nanocrystals exhibiting exceptional optical properties, but their emission intensity strongly depends on their charging state and local environment. This leads to blinking at the single-particle level or even complete fluorescence quenching, and limits the applications of quantum dots as fluorescent particles. Here, we show that a single quantum dot encapsulated in a silica shell coated with a continuous gold nanoshell provides a system with a stable and Poissonian emission at room temperature that is preserved regardless of drastic changes in the local environment. This novel hybrid quantum dot/silica/gold structure behaves as a plasmonic resonator with a strong Purcell factor, in very good agreement with simulations. The gold nanoshell also acts as a shield that protects the quantum dot fluorescence and enhances its resistance to high-power photoexcitation or high-energy electron beams. This plasmonic fluorescent resonator opens the way to a new family of plasmonic nanoemitters with robust optical properties.
Fast Hybrid Silicon Double-Quantum-Dot Qubit
NASA Astrophysics Data System (ADS)
Shi, Zhan; Simmons, C. B.; Prance, J. R.; Gamble, John King; Koh, Teck Seng; Shim, Yun-Pil; Hu, Xuedong; Savage, D. E.; Lagally, M. G.; Eriksson, M. A.; Friesen, Mark; Coppersmith, S. N.
2012-04-01
We propose a quantum dot qubit architecture that has an attractive combination of speed and fabrication simplicity. It consists of a double quantum dot with one electron in one dot and two electrons in the other. The qubit itself is a set of two states with total spin quantum numbers S2=3/4 (S=1/2) and Sz=-1/2, with the two different states being singlet and triplet in the doubly occupied dot. Gate operations can be implemented electrically and the qubit is highly tunable, enabling fast implementation of one- and two-qubit gates in a simpler geometry and with fewer operations than in other proposed quantum dot qubit architectures with fast operations. Moreover, the system has potentially long decoherence times. These are all extremely attractive properties for use in quantum information processing devices.
Fast hybrid silicon double-quantum-dot qubit.
Shi, Zhan; Simmons, C B; Prance, J R; Gamble, John King; Koh, Teck Seng; Shim, Yun-Pil; Hu, Xuedong; Savage, D E; Lagally, M G; Eriksson, M A; Friesen, Mark; Coppersmith, S N
2012-04-01
We propose a quantum dot qubit architecture that has an attractive combination of speed and fabrication simplicity. It consists of a double quantum dot with one electron in one dot and two electrons in the other. The qubit itself is a set of two states with total spin quantum numbers S(2)=3/4 (S=1/2) and S(z)=-1/2, with the two different states being singlet and triplet in the doubly occupied dot. Gate operations can be implemented electrically and the qubit is highly tunable, enabling fast implementation of one- and two-qubit gates in a simpler geometry and with fewer operations than in other proposed quantum dot qubit architectures with fast operations. Moreover, the system has potentially long decoherence times. These are all extremely attractive properties for use in quantum information processing devices. PMID:22540779
Quantum beats due to excitonic ground-state splitting in colloidal quantum dots
NASA Astrophysics Data System (ADS)
Bylsma, J.; Dey, P.; Paul, J.; Hoogland, S.; Sargent, E. H.; Luther, J. M.; Beard, M. C.; Karaiskaj, D.
2012-09-01
The dephasing of PbS quantum dots has been carefully measured using three pulse four-wave mixing and two-dimensional nonlinear optical spectroscopy. The temperature dependence of the homogeneous linewidth obtained from the two-dimensional spectra indicates significant scattering by acoustic phonons, whereas the excitation density dependence shows negligible excitation induced broadening in agreement with previous results. The rapid dephasing is attributed to elastic scattering by acoustic phonons. However, two dephasing components emerge, the short component that dominates the decay and a weaker longer decay, likely due to ``zero-phonon'' dephasing. Quantum beats originating from two separate states can be observed, possibly revealing an ˜23.6 meV splitting of the excitonic ground state. Finally, the emergence of biexcitonic effects enhanced by the high quantum confinement is discussed.
Competitive Performance of Carbon “Quantum” Dots in Optical Bioimaging
Cao, Li; Yang, Sheng-Tao; Wang, Xin; Luo, Pengju G.; Liu, Jia-Hui; Sahu, Sushant; Liu, Yamin; Sun, Ya-Ping
2012-01-01
Carbon-based “quantum” dots or carbon dots are surface-functionalized small carbon nanoparticles. For bright fluorescence emissions, the carbon nanoparticles may be surface-doped with an inorganic salt and then the same organic functionalization. In this study, carbon dots without and with the ZnS doping were prepared, followed by gel-column fractionation to harvest dots of 40% and 60% in fluorescence quantum yields, respectively. These highly fluorescent carbon dots were evaluated for optical imaging in mice, from which bright fluorescence images were obtained. Of particular interest was the observed competitive performance of the carbon dots in vivo to that of the well-established CdSe/ZnS QDs. The results suggest that carbon dots may be further developed into a new class of high-performance yet nontoxic contrast agents for optical bioimaging. PMID:22448196
Energy spectrum and density of states for a graphene quantum dot in a magnetic field.
Horing, Norman J Morgenstern; Liu, S Y
2010-01-20
In this paper, we determine the spectrum and density of states of a graphene quantum dot in a normal quantizing magnetic field. To accomplish this, we employ the retarded Green function for a magnetized, infinite-sheet graphene layer to describe the dynamics of a tightly confined graphene quantum dot subject to Landau quantization. Considering a ?((2))(r) potential well that supports just one subband state in the well in the absence of a magnetic field, the effect of Landau quantization is to 'splinter' this single energy level into a proliferation of many Landau-quantized states within the well. Treating the graphene sheet and dot as a closed system subject to a fully Hermitian Hamiltonian (including boundary conditions), there is no indication of decay of the Landau-quantized graphene dot states into the quantized states of the host graphene sheet for 'tight' confinement by the ?((2))(r) potential well, notwithstanding extension of the dot Green function (and eigenfunctions) outside the ?((2))(r) potential well. PMID:21386256
Electrically driven reverse overhauser pumping of nuclear spins in quantum dots.
Rudner, M S; Levitov, L S
2007-12-14
We propose a new mechanism for polarizing nuclear spins in quantum dots, based on periodic modulation of the hyperfine coupling by electric driving at the electron spin resonance frequency. Dynamical nuclear polarization results from resonant excitation rather than hyperfine relaxation mediated by a thermal bath, and thus is not subject to Overhauser-like detailed balance constraints. This allows polarization in the direction opposite to that expected from the Overhauser effect. Competition of the electrically driven and bath-assisted mechanisms can give rise to spatial modulation and sign reversal of polarization on a scale smaller than the electron confinement radius in the dot. PMID:18233468
Spin relaxation in a nanowire quantum dot due to electrical noises
NASA Astrophysics Data System (ADS)
Hung, Jo-Tzu; Hu, Xuedong
2015-03-01
Semiconductor nanowire with strong spin-orbit couplings makes fast electrical coherent control feasible for spin qubits. One example is the spin-orbit qubit, confined by nanowire based quantum dots made from InAs or InSb. Because of the strong spin-orbit coupling, such a qubit is naturally sensitive to electrical noises. We theoretically investigate the influence of electrical noise on spin-orbit qubit by considering fluctuations from the gates and/or defects. We start from a three-dimensional Hamiltonian, and consider spin-orbit couplings for nanowires with zincblende structure grown along [111] and those with wurtzite structure grown along [001], respectively. We then analyze spin relaxation as we vary the parameters for the system, such as the magnitude and direction of the applied magnetic field, nanowire thickness, and the quantum dot confinement. We thank financial support by US ARO.
Fabrication of a graphene quantum dot device
NASA Astrophysics Data System (ADS)
Lee, Jeong Il; Kim, Eunseong
2014-03-01
Graphene, which exhibits a massless Dirac-like spectrum for its electrons, has shown impressive properties for nano-electronics applications including a high mobility and a width dependent bandgap. We will report the preliminary report on the transport property of the suspended graphene nano-ribbon(GNR) quantum dot device down to dilution refrigerator temperature. This GNR QD device was fabricated to realize an ideal probe to investigate Kondo physics--a characteristic phenomenon in the physics of strongly correlated electrons. We gratefully acknowledge the financial support by the National Research Foundation of Korea through the Creative Research Initiatives.
Quantum Dots for Molecular Diagnostics of Tumors
Zdobnova, T.A.; Lebedenko, E.N.; Deyev, S.?.
2011-01-01
Semiconductor quantum dots (QDs) are a new class of fluorophores with unique physical and chemical properties, which allow to appreciably expand the possibilities for the current methods of fluorescent imaging and optical diagnostics. Here we discuss the prospects of QD application for molecular diagnostics of tumors ranging from cancer-specific marker detection on microplates to non-invasive tumor imagingin vivo. We also point out the essential problems that require resolution in order to clinically promote QD, and we indicate innovative approaches to oncology which are implementable using QD. PMID:22649672
Liu, Fei; Jang, Min-Ho; Ha, Hyun Dong; Kim, Je-Hyung; Cho, Yong-Hoon; Seo, Tae Seok
2013-07-19
Pristine graphene quantum dots and graphene oxide quantum dots are synthesized by chemical exfoliation from the graphite nanoparticles with high uniformity in terms of shape (circle), size (less than 4 nm), and thickness (monolayer). The origin of the blue and green photoluminescence of GQDs and GOQDs is attributed to intrinsic and extrinsic energy states, respectively. PMID:23712762
Trion X+ in vertically coupled type II quantum dots in threading magnetic field
2012-01-01
We analyze the energy spectrum of a positively charged exciton confined in a semiconductor heterostructure formed by two vertically coupled, axially symmetrical type II quantum dots located close to each other. The electron in the structure is mainly located inside the dots, while the holes generally move in the exterior region close to the symmetry axis. The solutions of the Schrödinger equation are obtained by a variational separation of variables in the adiabatic limit. Numerical results are shown for bonding and anti-bonding lowest-lying of the trion states corresponding to the different quantum dots morphologies, dimensions, separation between them, thicknesses of the wetting layers, and the magnetic field strength. PMID:23013605
Luca Sapienza; Marcelo Davanco; Antonio Badolato; Kartik Srinivasan
2015-03-24
Self-assembled, epitaxially-grown InAs/GaAs quantum dots are promising semiconductor quantum emitters that can be integrated on a chip for a variety of photonic quantum information science applications. However, self-assembled growth results in an essentially random in-plane spatial distribution of quantum dots, presenting a challenge in creating devices that exploit the strong interaction of single quantum dots with highly confined optical modes. Here, we present a photoluminescence imaging approach for locating single quantum dots with respect to alignment features with an average (minimum) position uncertainty < 30 nm (< 10 nm), which represents an enabling technology for the creation of optimized single quantum dot devices. To that end, we create quantum dot single-photon sources, based on a circular Bragg grating geometry, that simultaneously exhibit high collection efficiency (48 % +/- 5 % into a 0.4 numerical aperture lens, close to the theoretically predicted value of 50 %), low multiphoton probability (g(2)(0) <1 %), and a significant Purcell enhancement factor (~ 3).
Nielsen, Steven O.
FIG. 1: Size-dependent color emission of quantum dots. This is a purely quantum mechanical effect. FIG. 2: Size-dependent color emission of quantum dots. This is a purely quantum mechanical effect. 1 #12;FIG. 3: Size-dependent color emission of quantum dots. This is a purely quantum mechanical effect
Kondo and mixed-valence regimes in multilevel quantum dots
Chudnovskiy, A. L.; Ulloa, S. E.
2001-04-15
We investigate the dependence of the ground state of a multilevel quantum dot on the coupling to an external fermionic system and on the interactions in the dot. As the coupling to the external system increases, the rearrangement of the effective energy levels in the dot signals the transition from the Kondo regime to a mixed-valence (MV) regime. The MV regime in a two-level dot is characterized by an intrinsic mixing of the levels in the dot, resulting in nonperturbative subtunneling and supertunneling phenomena that strongly influence the Kondo effect.
Wavefunction dynamics in a quantum-dot electron pump under a high magnetic field
NASA Astrophysics Data System (ADS)
Ryu, Sungguen; Kataoka, Masaya; Sim, Heung-Sun
2015-03-01
A quantum-dot electron pump, formed and operated by applying time-dependent potential barriers to a two dimensional electron gas system, provides a promising redefinition of ampere. The pump operation consists of capturing an electron from a reservoir into a quantum dot and ejecting it to another reservoir. The capturing process has been theoretically understood by a semi-classical treatment of the tunneling between the dot and reservoir. But the dynamics of the wavefunction of the captured electron in the ejection process has not been theoretically addressed, although it is useful for enhancing pump accuracy and for utilizing the pump as a single-electron source for mesoscopic quantum electron devices. We study the dynamics under a strong magnetic field that leads to magnetic confinement of the captured electron, which dominates over the electrostatic confinement of the dot. We find that the wave packet of the captured electron has the Gaussian form with the width determined by the strength of the magnetic field, and that the time evolution of the packet follows the classical drift motion, with maintaining the Gaussian form. We discuss the possible signatures of the wave packet dynamics in experiments.
High responsivity, LWIR dots-in-a-well quantum dot infrared photodetectors
D. T. Le; C. P. Morath; H. E. Norton; D. A. Cardimona; S. Raghavan; P. Rotella; S. A. Stintz; B. Fuchs; S. Krishna
2003-01-01
In this paper we report studies on normal incidence, InAs\\/In0.15Ga0.85As quantum dot infrared photodetectors (QDIPs) in the dots-in-a-well (DWELL) configuration. Three QDIP structures with similar dot and well dimensions were grown and devices were fabricated from each wafer. Of the three devices studied, the first served as the control, the second was grown with an additional 400 Å AlGaAs blocking
NASA Astrophysics Data System (ADS)
Abbarchi, M.; Troiani, F.; Mastrandrea, C.; Goldoni, G.; Kuroda, T.; Mano, T.; Sakoda, K.; Koguchi, N.; Sanguinetti, S.; Vinattieri, A.; Gurioli, M.
2008-10-01
We experimentally and theoretically investigate the photoluminescence broadening of different excitonic complexes in single self-assembled GaAs /AlGaAs quantum dots. We demonstrate that the excitonic fine-structure splitting leads to a sizable line broadening whenever the detection is not resolved in polarization. The residual broadening in polarized measurements is systematically larger for the exciton with respect to both the trion and the biexciton recombination. The experimental data agree with calculations of the quantum confined Stark effect induced by charge defects in the quantum dot (QD) environment, denoting the role of the QD spectator carrier rearrangement in reducing the perturbation of the fluctuating environment.
Biosensing with Luminescent Semiconductor Quantum Dots
Sapsford, Kim E.; Pons, Thomas; Medintz, Igor L.; Mattoussi, Hedi
2006-01-01
Luminescent semiconductor nanocrystals or quantum dots (QDs) are a recently developed class of nanomaterial whose unique photophysical properties are helping to create a new generation of robust fluorescent biosensors. QD properties of interest for biosensing include high quantum yields, broad absorption spectra coupled to narrow size-tunable photoluminescent emissions and exceptional resistance to both photobleaching and chemical degradation. In this review, we examine the progress in adapting QDs for several predominantly in vitro biosensing applications including use in immunoassays, as generalized probes, in nucleic acid detection and fluorescence resonance energy transfer (FRET) - based sensing. We also describe several important considerations when working with QDs mainly centered on the choice of material(s) and appropriate strategies for attaching biomolecules to the QDs.
Semiconductor Quantum Dots for Biomedicial Applications
Shao, Lijia; Gao, Yanfang; Yan, Feng
2011-01-01
Semiconductor quantum dots (QDs) are nanometre-scale crystals, which have unique photophysical properties, such as size-dependent optical properties, high fluorescence quantum yields, and excellent stability against photobleaching. These properties enable QDs as the promising optical labels for the biological applications, such as multiplexed analysis of immunocomplexes or DNA hybridization processes, cell sorting and tracing, in vivo imaging and diagnostics in biomedicine. Meanwhile, QDs can be used as labels for the electrochemical detection of DNA or proteins. This article reviews the synthesis and toxicity of QDs and their optical and electrochemical bioanalytical applications. Especially the application of QDs in biomedicine such as delivering, cell targeting and imaging for cancer research, and in vivo photodynamic therapy (PDT) of cancer are briefly discussed. PMID:22247690
Enhancement of photoluminescence in ZnS/ZnO quantum dots interfacial heterostructures
Rajalakshmi, M., E-mail: Rajingmmsd@gmail.com [Condensed Matter Physics Division, Indira Gandhi Centre for Atomic Research, Kalpakkam 603102 (India); Sohila, S.; Ramesh, R. [Centre for Materials Science and Nano Devices, Department of Physics, SRM University, Kattankulathur 603 203, Chennai (India)] [Centre for Materials Science and Nano Devices, Department of Physics, SRM University, Kattankulathur 603 203, Chennai (India); Bhalerao, G.M. [UGC-DAE Consortium for Scientific Research (Kalpakkam node), Kalpakkam 603102 (India)] [UGC-DAE Consortium for Scientific Research (Kalpakkam node), Kalpakkam 603102 (India)
2012-09-15
Highlights: ? ZnS/ZnO quantum dots (QDs) were synthesized by controlled oxidation of ZnS nanoparticles. ? Interfacial heterostructure formation of ZnS/ZnO QDs is seen in HRTEM. ? Enormous enhancement of UV emission (?10 times) in ZnS/ZnO QDs heterostructure is observed. ? Phonon confinement effect is seen in the Raman spectrum. -- Abstract: ZnS/ZnO quantum dots (QDs) were synthesized by controlled oxidation of ZnS nanoparticles. HRTEM image showed small nanocrystals of size 4 nm and the magnified image of single quantum dot shows interfacial heterostructure formation. The optical absorption spectrum shows a blue shift of 0.19 and 0.23 eV for ZnO and ZnS QDs, respectively. This is due to the confinement of charge carries within the nanostructures. Enormous enhancement in UV emission (10 times) is reported which is attributed to interfacial heterostructure formation. Raman spectrum shows phonons of wurtzite ZnS and ZnO. Phonon confinement effect is seen in the Raman spectrum wherein LO phonon peaks of ZnS and ZnO are shifted towards lower wavenumber side and are broadened.
Filling of hole arrays with InAs quantum dots.
Lee, Jennifer Y; Noordhoek, Mark J; Smereka, Peter; McKay, Hugh; Millunchick, Joanna M
2009-07-15
Focused ion beams are used to pattern GaAs(001) surfaces with an array of nanometer-deep holes upon which deposition of InAs results in quantum dot formation at the hole location. Experiments show that the size and quantity of quantum dots formed depend on growth parameters, and ion dose, which affects the size and shape of the resulting holes. Quantum dots fabricated in this fashion have a photoluminescence peak at 1.28 eV at 77 K, indicating that the ion irradiation due to patterning does not destroy their optical activity. Kinetic Monte Carlo simulations that include elastic relaxation qualitatively model the growth of dots in nanometer-deep holes, and demonstrate that growth temperature, depth of the holes, and the angle of the hole sidewalls strongly influence the number of quantum dots that form at their perimeter. PMID:19546494
Filling of hole arrays with InAs quantum dots
NASA Astrophysics Data System (ADS)
Lee, Jennifer Y.; Noordhoek, Mark J.; Smereka, Peter; McKay, Hugh; Millunchick, Joanna M.
2009-07-01
Focused ion beams are used to pattern GaAs(001) surfaces with an array of nanometer-deep holes upon which deposition of InAs results in quantum dot formation at the hole location. Experiments show that the size and quantity of quantum dots formed depend on growth parameters, and ion dose, which affects the size and shape of the resulting holes. Quantum dots fabricated in this fashion have a photoluminescence peak at 1.28 eV at 77 K, indicating that the ion irradiation due to patterning does not destroy their optical activity. Kinetic Monte Carlo simulations that include elastic relaxation qualitatively model the growth of dots in nanometer-deep holes, and demonstrate that growth temperature, depth of the holes, and the angle of the hole sidewalls strongly influence the number of quantum dots that form at their perimeter.
RKKY interaction in a chirally coupled double quantum dot system
Heine, A. W.; Tutuc, D.; Haug, R. J. [Institut für Festkörperphysik, Leibniz Universität Hannover, Appelstr. 2, 30167 Hannover (Germany); Zwicknagl, G. [Institut für Mathematische Physik, TU Braunschweig, Mendelssohnstr. 3, 38106 Braunschweig (Germany); Schuh, D. [Institut für Experimentelle und Angewandte Physik, Universität Regensburg, Universitätstr. 31, 93053 Regensburg (Germany); Wegscheider, W. [Laboratorium für Festkörperphysik, ETH Zürich, Schafmattstr. 16, 8093 Zürich, Switzerland and Institut für Experimentelle und Angewandte Physik, Universität Regensburg, Universitätstr. 31, 93053 Regens (Germany)
2013-12-04
The competition between the Kondo effect and the Ruderman-Kittel-Kasuya-Yoshida (RKKY) interaction is investigated in a double quantum dots system, coupled via a central open conducting region. A perpendicular magnetic field induces the formation of Landau Levels which in turn give rise to the so-called Kondo chessboard pattern in the transport through the quantum dots. The two quantum dots become therefore chirally coupled via the edge channels formed in the open conducting area. In regions where both quantum dots exhibit Kondo transport the presence of the RKKY exchange interaction is probed by an analysis of the temperature dependence. The thus obtained Kondo temperature of one dot shows an abrupt increase at the onset of Kondo transport in the other, independent of the magnetic field polarity, i.e. edge state chirality in the central region.
Electron Spin Qubits in Si/SiGe Quantum Dots
NASA Astrophysics Data System (ADS)
Eriksson, Mark
2010-10-01
It is intriguing that silicon, the central material of modern classical electronics, also has properties well suited to quantum electronics. Recent advances in Si/SiGe quantum devices have enabled the creation of high-quality silicon quantum dots, also known as artificial atoms. Motivated in part by the potential for very long spin coherence times in this material, we are pursuing the development of individual electron spin qubits in silicon quantum dots. I will discuss recent demonstrations of single-shot spin measurement in a Si/SiGe quantum dot spin qubit, and the demonstration of spin-relaxation times longer than one second in such a system. These and similar measurements depend on a knowledge of tunnel rates between quantum dots and nearby reservoirs or between pairs of quantum dots. Measurements of such rates provide an opportunity to revisit classic experiments in quantum mechanics. At the same time, the unique features of the silicon conduction band lead to novel and unexpected effects, demonstrating that Si/SiGe quantum dots provide a highly controlled experimental system in which to study ideas at the heart of quantum physics.
Monolithic quantum dot sensitized solar cells
NASA Astrophysics Data System (ADS)
Samadpour, M.; Ghane, Z.; Ghazyani, N.; Tajabadi, F.; Taghavinia, N.
2013-12-01
We report a new design of solar cells based on semiconductor quantum dots (QDs), monolithic quantum dot sensitized solar cells (MQDSCs). MQDSCs offer the prospect of having lower cost and a simpler manufacturing process in comparison to conventional double substrate QDSCs. Our proposed monolithic QDSCs have a triple-layer structure, composed of a CdS sensitized mesoporous TiO2 photoanode, a scattering layer made by a core-shell structure of TiO2/SiO2, and a carbon active/graphite counter electrode layer, which are all deposited on a single fluorine doped tin oxide (FTO) glass substrate. Mesoporous TiO2 was sensitized with CdS QDs by successive ionic layer adsorption and reaction. Here, non-conventional solvents were utilized, which made it possible to deposit the CdS QDs in our monolithic structure. The measured photovoltaic properties and simple preparation method show that MQDSCs can be introduced as promising structures to make low-cost QDSCs in the near future.
Quantum Dot Enabled Molecular Sensing and Diagnostics
Zhang, Yi; Wang, Tza-Huei
2012-01-01
Since its emergence, semiconductor nanoparticles known as quantum dots (QDs) have drawn considerable attention and have quickly extended their applicability to numerous fields within the life sciences. This is largely due to their unique optical properties such as high brightness and narrow emission band as well as other advantages over traditional organic fluorophores. New molecular sensing strategies based on QDs have been developed in pursuit of high sensitivity, high throughput, and multiplexing capabilities. For traditional biological applications, QDs have already begun to replace traditional organic fluorophores to serve as simple fluorescent reporters in immunoassays, microarrays, fluorescent imaging applications, and other assay platforms. In addition, smarter, more advanced QD probes such as quantum dot fluorescence resonance energy transfer (QD-FRET) sensors, quenching sensors, and barcoding systems are paving the way for highly-sensitive genetic and epigenetic detection of diseases, multiplexed identification of infectious pathogens, and tracking of intracellular drug and gene delivery. When combined with microfluidics and confocal fluorescence spectroscopy, the detection limit is further enhanced to single molecule level. Recently, investigations have revealed that QDs participate in series of new phenomena and exhibit interesting non-photoluminescent properties. Some of these new findings are now being incorporated into novel assays for gene copy number variation (CNV) studies and DNA methylation analysis with improved quantification resolution. Herein, we provide a comprehensive review on the latest developments of QD based molecular diagnostic platforms in which QD plays a versatile and essential role. PMID:22916072
Competing interactions in semiconductor quantum dots
van den Berg, R.; Brandino, G. P.; El Araby, O.; Konik, R. M.; Gritsev, V.; Caux, J. -S.
2014-10-01
We introduce an integrability-based method enabling the study of semiconductor quantum dot models incorporating both the full hyperfine interaction as well as a mean-field treatment of dipole-dipole interactions in the nuclear spin bath. By performing free induction decay and spin echo simulations we characterize the combined effect of both types of interactions on the decoherence of the electron spin, for external fields ranging from low to high values. We show that for spin echo simulations the hyperfine interaction is the dominant source of decoherence at short times for low fields, and competes with the dipole-dipole interactions at longer times. Onmore »the contrary, at high fields the main source of decay is due to the dipole-dipole interactions. In the latter regime an asymmetry in the echo is observed. Furthermore, the non-decaying fraction previously observed for zero field free induction decay simulations in quantum dots with only hyperfine interactions, is destroyed for longer times by the mean-field treatment of the dipolar interactions.« less
Colloidal quantum dot light-emitting devices.
Wood, Vanessa; Bulovi?, Vladimir
2010-01-01
Colloidal quantum dot light-emitting devices (QD-LEDs) have generated considerable interest for applications such as thin film displays with improved color saturation and white lighting with a high color rendering index (CRI). We review the key advantages of using quantum dots (QDs) in display and lighting applications, including their color purity, solution processability, and stability. After highlighting the main developments in QD-LED technology in the past 15 years, we describe the three mechanisms for exciting QDs - optical excitation, Förster energy transfer, and direct charge injection - that have been leveraged to create QD-LEDs. We outline the challenges facing QD-LED development, such as QD charging and QD luminescence quenching in QD thin films. We describe how optical downconversion schemes have enabled researchers to overcome these challenges and develop commercial lighting products that incorporate QDs to achieve desirable color temperature and a high CRI while maintaining efficiencies comparable to inorganic white LEDs (>65 lumens per Watt). We conclude by discussing some current directions in QD research that focus on achieving higher efficiency and air-stable QD-LEDs using electrical excitation of the luminescent QDs. PMID:22110863
Competing interactions in semiconductor quantum dots
van den Berg, R. [Univ. of Amsterdam (Netherlands). Inst. for Theoretical Physics; Brandino, G. P. [Univ. of Amsterdam (Netherlands). Inst. for Theoretical Physics; El Araby, O. [Univ. of Amsterdam (Netherlands). Inst. for Theoretical Physics; Konik, R. M. [Brookhaven National Lab. (BNL), Upton, NY (United States); Gritsev, V. [Univ. of Amsterdam (Netherlands). Inst. for Theoretical Physics; Caux, J. -S. [Univ. of Amsterdam (Netherlands). Inst. for Theoretical Physics
2014-10-01
We introduce an integrability-based method enabling the study of semiconductor quantum dot models incorporating both the full hyperfine interaction as well as a mean-field treatment of dipole-dipole interactions in the nuclear spin bath. By performing free induction decay and spin echo simulations we characterize the combined effect of both types of interactions on the decoherence of the electron spin, for external fields ranging from low to high values. We show that for spin echo simulations the hyperfine interaction is the dominant source of decoherence at short times for low fields, and competes with the dipole-dipole interactions at longer times. On the contrary, at high fields the main source of decay is due to the dipole-dipole interactions. In the latter regime an asymmetry in the echo is observed. Furthermore, the non-decaying fraction previously observed for zero field free induction decay simulations in quantum dots with only hyperfine interactions, is destroyed for longer times by the mean-field treatment of the dipolar interactions.
Quantum box energies as a route to the ground state levels of self-assembled InAs pyramidal dots
M. Califano; P. Harrison
2000-01-01
A theoretical investigation of the ground state electronic structure of InAs\\/GaAs quantum confined structures is presented. Energy levels of cuboids and pyramidal shaped dots are calculated using a single-band, constant-confining-potential model that in former applications has proved to reproduce well both the predictions of very sophisticated treatments and several features of many experimental photoluminescence spectra. A connection rule between their
Yiming Li; O. Voskoboynikov; C. P. Lee; S. M. Sze
2001-01-01
A computational technique for the energy levels calculation of an electron confined by a 3D InAs quantum dot (QD) embedded in GaAs semiconductor matrix is presented. Based on the effective one electronic band Hamiltonian, the energy and position dependent electron effective mass approximation, a finite height hard-wall 3D confinement potential, and the Ben Daniel–Duke boundary conditions, the problem is formulated
Effects of multiple organic ligands on size uniformity and optical properties of ZnSe quantum dots
Archana, J., E-mail: archana.jayaram@yahoo.com [Research Institute of Electronics, Shizuoka University, 3-5-1 Johoku, Naka-ku, Hamamatsu, Shizuoka 432-8011 (Japan); Navaneethan, M.; Hayakawa, Y. [Research Institute of Electronics, Shizuoka University, 3-5-1 Johoku, Naka-ku, Hamamatsu, Shizuoka 432-8011 (Japan)] [Research Institute of Electronics, Shizuoka University, 3-5-1 Johoku, Naka-ku, Hamamatsu, Shizuoka 432-8011 (Japan); Ponnusamy, S.; Muthamizhchelvan, C. [Department of Physics, SRM University, Kattankulathur 603203, Tamil Nadu (India)] [Department of Physics, SRM University, Kattankulathur 603203, Tamil Nadu (India)
2012-08-15
Highlights: ? Highly monodispersed ZnSe quantum dots have been synthesized by wet chemical route. ? Strong quantum confinement effect have been observed in ? 4 nm ZnSe quantum dots. ? Enhanced ultraviolet near band emission have been obtained using long chain polymer. -- Abstract: The effects of multi-ligands on the formation and optical transitions of ZnSe quantum dots have been investigated. The dots are synthesized using 3-mercapto-1,2-propanediol and polyvinylpyrrolidone ligands, and have been characterized by X-ray diffraction, transmission electron microscopy (TEM), UV–visible absorption spectroscopy, photoluminescence spectroscopy, and Fourier transform infrared spectroscopy. TEM reveals high monodispersion with an average size of 4 nm. Polymer-stabilized, organic ligand-passivated ZnSe quantum dots exhibit strong UV emission at 326 nm and strong quantum confinement in the UV–visible absorption spectrum. Uniform size and suppressed surface trap emission are observed when the polymer ligand is used. The possible growth mechanism is discussed.
Quantum dot mode locked lasers for coherent frequency comb generation
NASA Astrophysics Data System (ADS)
Martinez, A.; Calò, C.; Rosales, R.; Watts, R. T.; Merghem, K.; Accard, A.; Lelarge, F.; Barry, L. P.; Ramdane, A.
2013-12-01
Monolithic semiconductor passively mode locked lasers (MLL) are very attractive components for many applications including high bit rate telecommunications, microwave photonics and instrumentation. Owing to the three dimensional confinement of the charge carriers, quantum dot based mode-locked lasers have been the subject of intense investigations because of their improved performance compared to conventional material systems. Indeed, the inhomogeneous gain broadening and the ultrafast absorption recovery dynamics are an asset for short pulse generation. Moreover, the weak coupling of amplified spontaneous emission with the guided modes plus low loss waveguide leads to low timing jitter. Our work concentrates on InAs quantum dash nanostructures grown on InP substrate, intended for applications in the 1.55 ?m telecom window. InAs/InP quantum dash based lasers, in particular, have demonstrated efficient mode locking in single section Fabry-Perot configurations. The flat optical spectrum of about 12 nm, combined with the narrow RF beat note linewidth of about 10 kHz make them a promising technology for optical frequency comb generation. Coherence between spectral modes was assessed by means of spectral phase measurements. The parabolic spectral phase profile indicates that short pulses can be obtained provided the intracavity dispersion can be compensated by inserting a single mode fiber.
Hole States and Magnetic Anisotropy of a Quantum Dot
NASA Astrophysics Data System (ADS)
Rederth, Dan; Chapagain, Hari; Oszwaldowski, Rafal; Petukhov, A. G.
2015-03-01
In the era of spin-based advanced semiconductor materials, spin can be used for the control of quantum devices based on quantum dots (QDs). To facilitate the control of the electronic and magnetic properties, magnetic ions can be incorporated in the QDs. We study the properties of such a magnetic II-VI QD charged with one hole. To account for the complex structure of valence band, we propose a method based on the Luttinger-Kohn Hamiltonian. With a robust numerical algorithm suitable for any QD geometry, we study the interplay of quantum confinement and magnetic anisotropy of a flat QD. We go beyond the virtual crystal approximation; our model also allows for position-dependent direction of magnetization. We discuss the differences between our and previous results, as well as the effects of temperature (mean-field approximation), and of the spin-orbit split-off band. We also discuss possible fluctuations of magnetization in QDs. Supported by DOE DE-SC00004890. DoE
Tailoring Magnetism in Bulk Semiconductors and Quantum Dots
NASA Astrophysics Data System (ADS)
Zutic, Igor
2008-03-01
Carrier-mediated magnetism in semiconductors shows important and potentially useful differences from their metallic counterparts [1]. For example, in magnetically doped semiconductors the change in carrier density induced by light or bias could be sufficient to turn the ferromagnetism on and off. However, there remain many important challenges to fully understand these materials. Our density functional theory study of Mn- doped II-IV-V2 chalcopyrites [2] reveals that variation of magnetic properties across 64 different materials cannot be explained by the dominant models of ferromagnetism in semiconductors. We observe no qualitative similarity with the suggested Curie temperature scaling with the inverse cube of the lattice constant [3]. In contrast to most of the theoretical studies, we explicitly include the temperature dependence of the carrier density and propose a model which permits analysis of the thermodynamic stability of the competing magnetic states [4]. As an example we analyze the stability of a possible reentrant ferromagnetic semiconductor and discuss the experimental support for this prediction. An increasing temperature leads to an increased carrier density such that the enhanced coupling between magnetic impurities results in the onset of ferromagnetism as temperature is raised. We also use the real space finite-temperature local spin density approximation to examine magnetically doped quantum dots in which the interplay of quantum confinement and strong Coulomb interactions can lead to novel possibilities to tailor magnetism. We reveal that, even at a fixed number of carriers, the gate induced changes in the screening [5] or deviations from isotropic quantum confinement [6] could allow for a reversible control of magnetism and switching between zero and finite magnetization. Such magnetic quantum dots could also provide versatile voltage-control of spin currents and spin filtering. The work done in collaboration with S. C. Erwin (Naval Research Lab), A. G. Petukhov (South Dakota School of Mines and Technology), R. M. Abolfath (SUNY Buffalo) and P. Hawrylak (NRC, Canada). [1] T. Jungwirth et al., Rev. Mod. Phys 78, 1311 (2006); I. Zutic, J. Fabian, and S. Das Sarma, Rev. Mod. Phys. 76, 323 (2004). [2] S. C. Erwin and I. Zutic, Nature Mater. 3, 410 (2004). [3] T. Dietl et al., Science 287, 1019 (2000). [4] A. G. Petukhov, I. Zutic, and S. Erwin, Phys. Rev. Lett. 99, 257202 (2007) [5] R. M. Abolfath, P. Hawrylak, and I. Zutic, Phys. Rev. Lett. 98, 207203 (2007); New J. Phys. 9, 353 (2007). [6] R. M. Abolfath, A. G. Petukhov, and I. Zutic, arXiv:0707.2805.
NASA Astrophysics Data System (ADS)
Sarkar, Kanchan; Kumar Datta, Nirmal; Ghosh, Manas
2010-03-01
We explore the pattern of frequency-dependent linear and non-linear optical (NLO) response of electron impurity doped quantum dots harmonically confined in two dimensions. For some fixed values of transverse magnetic field strength ( ?c), and harmonic confinement potential ( ?0), the influence of impurity location ( x0,y0) on the diagonal components of frequency dependent linear ( ?xx and ?yy), and the first ( ?xxx and ?yyy) NLO responses of the dot is computed through linear variational route. The non-linear polarizabilities undergo maximization at some typical oscillation frequency of the external field depending upon the impurity location.
Orbital pathways for Mn2+-carrier sp-d exchange in diluted magnetic semiconductor quantum dots
NASA Astrophysics Data System (ADS)
Beaulac, Rémi; Feng, Yong; May, Joseph W.; Badaeva, Ekaterina; Gamelin, Daniel R.; Li, Xiaosong
2011-11-01
Manganese-carrier magnetic exchange interactions in strongly quantum-confined Mn2+-doped CdSe quantum dots (QDs) having dQD = 1.52, 2.08, and 2.54 nm have been investigated using a combination of density functional theory (DFT) and perturbation theory calculations. Established perturbation expressions have been tested by comparing the exchange energies predicted from these expressions (using DFT results as input parameters) with those calculated directly by DFT. These comparisons allow the dominant orbital pathways responsible for Mn2+-carrier exchange to be identified and analyzed. The Mn2+-valence-band(VB)-hole exchange interaction is described well using the long-accepted antiferromagnetic (AFM) p-d kinetic exchange pathway. The Mn2+-conduction-band(CB)-electron interaction is described well using the recently proposed ferromagnetic (FM) kinetic s-s exchange pathway. AFM kinetic s-d exchange interactions previously proposed to become dominant in quantum-confined diluted magnetic semiconductors (DMSs) have been evaluated quantitatively by both DFT and perturbation theory and are found to be weak compared to the FM s-s interaction, even in these strongly confined QDs. The magnitudes of the mean-field exchange parameters are found to be nearly independent of quantum confinement over this range of QD diameters, and the dominant orbital pathways are not fundamentally altered by quantum confinement.
Interaction effects on the tunneling of electron-hole pairs in coupled quantum dots
NASA Astrophysics Data System (ADS)
Guerrero, Hector M.; Cocoletzi, Gregorio H.; Ulloa, Sergio E.
2001-03-01
The transit time of carriers is beginning to be an important parameter in the physical operation of semiconductor quantum dot `devices'. In the present work, we study the coherent propagation of electron-hole pairs in coupled self-assembled quantum dots in close proximity. These systems, achieved experimentally in a number of different geometries, have been recently implemented as a novel storage of optical information that may give rise to smart pixel technology in the near future [1]. Here, we apply an effective mass hamiltonian approach and solve numerically the time dependent Schroedinger equation of a system of photo-created electron-hole pairs in the dots. Our approach takes into account both Coulomb interactions and confinement effects. The time evolution is investigated in terms of the structural parameters for typical InAs-GaAs dots. Different initial conditions are considered, reflecting the basic processes that would take place in these experiments. We study the probabilities of finding the electron and hole in either the same or adjacent quantum dot, and study carefully the role of interactions in this behavior. [1] T. Lundstrom, W. Schoenfeld, H. Lee, and P. M. Petroff, Science 286, 2312 (1999).
Numerical simulation of electronic properties of coupled quantum dots on wetting layers.
Betcke, M M; Voss, H
2008-04-23
Self-assembled quantum dots are grown on wetting layers and frequently in an array-like assembly of many similar but not exactly equal dots. Nevertheless, most simulations disregard these structural conditions and restrict themselves to simulating a pure single quantum dot. For reasons of numerical efficiency we advocate the effective one-band Hamiltonian with energy- and position-dependent effective mass approximation and a finite height hard-wall 3D confinement potential for computation of the energy levels of the electrons in the conduction band. Within this model we investigate the geometrical effects mentioned above on the electronic structure of a pyramidal InAs quantum dot embedded in a GaAs matrix. We find that the presence of a wetting layer may affect the electronic structure noticeably. Furthermore, we establish that, in spite of the large bandgap of the InAs/GaAs heterostructure, if the dots in a vertically aligned array are sufficiently close stacked there is considerable interaction between their eigenfunctions. Moreover, the eigenfunctions of such an array are quite sensitive to certain structural perturbations. PMID:21825638
Magneto-optical studies of quantum dots
NASA Astrophysics Data System (ADS)
Russ, Andreas Hans
Significant effort in condensed matter physics has recently been devoted to the field of "spintronics" which seeks to utilize the spin degree of freedom of electrons. Unlike conventional electronics that rely on the electron charge, devices exploiting their spin have the potential to yield new and novel technological applications, including spin transistors, spin filters, and spin-based memory devices. Any such application has the following essential requirements: 1) Efficient electrical injection of spin-polarized carriers; 2) Long spin lifetimes; 3) Ability to control and manipulate electron spins; 4) Effective detection of spin-polarized carriers. Recent work has demonstrated efficient electrical injection from ferromagnetic contacts such as Fe and MnAs, utilizing a spin-Light Emitting Diode (spin-LED) as a method of detection. Semiconductor quantum dots (QDs) are attractive candidates for satisfying requirements 2 and 3 as their zero dimensionality significantly suppresses many spin-flip mechanisms leading to long spin coherence times, as well as enabling the localization and manipulation of a controlled number of electrons and holes. This thesis is composed of three projects that are all based on the optical properties of QD structures including: I) Intershell exchange between spin-polarized electrons occupying adjacent shells in InAs QDs; II) Spin-polarized multiexitons in InAs QDs in the presence of spin-orbit interactions; III) The optical Aharonov-Bohm effect in AlxGa1-xAs/AlyGa1-yAs quantum wells (QWs). In the following we introduce some of the basic optical properties of quantum dots, describe the main tool (spin-LED) employed in this thesis to inject and detect spins in these QDs, and conclude with the optical Aharonov-Bohm effect (OAB) in type-II QDs.
Generation of singlet oxygen and other radical species by quantum dot and carbon dot nanosensitizers
NASA Astrophysics Data System (ADS)
Generalov, Roman; Christensen, Ingeborg L.; Chen, Wei; Sun, Ya-Ping; Kristensen, Solveig; Juzenas, Petras
2009-06-01
Medicinal applications of luminescent semiconductor quantum dots are of growing interest. In spite of the fact that their fabrication and imaging applications have been extensively investigated for the last decade, very little is documented on photodynamic action of quantum dots. In this study we demonstrate generation of singlet oxygen and other radical species upon exposure of quantum dots to blue light and therapeutic red light. Extent of radical production can be readily modified by antioxidants. Lay and scientific communities are two sites concerning potential hazards and enthusiastic applications of nanotechnology. Synthesis of quantum dots composed of less toxic materials is of great interest. A new candidate is a ubiquitous element carbon, which on nanoscale exhibits strong photoluminescence.
Imaging GABAc Receptors with Ligand-Conjugated Quantum Dots
Tomlinson, Ian D.; Gussin, Hélène A.; Little, Deborah M.; Warnement, Michael R.; Qian, Haohua; Pepperberg, David R.; Rosenthal, Sandra J.
2007-01-01
We report a methodology for labeling the GABAc receptor on the surface membrane of intact cells. This work builds upon our earlier work with serotonin-conjugated quantum dots and our studies with PEGylated quantum dots to reduce nonspecific binding. In the current approach, a PEGylated derivative of muscimol was synthesized and attached via an amide linkage to quantum dots coated in an amphiphilic polymer derivative of a modified polyacrylamide. These conjugates were used to image GABAC receptors heterologously expressed in Xenopus laevis oocytes. PMID:18437227
Single electron charging in optically active nanowire quantum dots.
van Kouwen, Maarten P; Reimer, Michael E; Hidma, Anne W; van Weert, Maarten H M; Algra, Rienk E; Bakkers, Erik P A M; Kouwenhoven, Leo P; Zwiller, Val
2010-05-12
We report optical experiments of a charge tunable, single nanowire quantum dot subject to an electric field tuned by two independent voltages. First, we control tunneling events through an applied electric field along the nanowire growth direction. Second, we modify the chemical potential in the nanowire with a back-gate. We combine these two field-effects to isolate a single electron and independently tune the tunnel coupling of the quantum dot with the contacts. Such charge control is a first requirement for opto-electrical single electron spin experiments on a nanowire quantum dot. PMID:20387798
Dot-bound and dispersive states in graphene quantum dot superlattices
NASA Astrophysics Data System (ADS)
Pieper, A.; Heinisch, R. L.; Wellein, G.; Fehske, H.
2014-04-01
We consider a square lattice configuration of circular gate-defined quantum dots in an unbiased graphene sheet and calculate the electronic, particularly spectral properties of finite albeit actual sample sized systems by means of a numerically exact kernel polynomial expansion technique. Analyzing the local density of states and the momentum resolved photoemission spectrum we find clear evidence for a series of quasibound states at the dots, which can be probed by optical measurements. We further analyze the interplay of the superlattice structure with dot-localized modes on the electron energy dispersion. Effects of disordered dot lattices are discussed too.
Interband optical absorption in a circular graphene quantum dot
M Gruji?; M Zarenia; M Tadi?; F M Peeters
2012-01-01
We investigate the energy levels and optical properties of a circular graphene quantum dot in the presence of an external magnetic field perpendicular to the dot. Based on the Dirac–Weyl equation and assuming zero outward current at the edge of the dot we present the results for two different types of boundary conditions, i.e. infinite-mass (IMBC) and zigzag boundary conditions.
Quantum confinement effects in strained silicon-germanium alloy quantum wells
Quantum confinement effects in strained silicon-germanium alloy quantum wells X. Xiao, C. W. Liu/strained Si, _ ,Ge,/Si single quantum wells. A quantum confinement energy of up to 45 meV has been observed for quantum wells as small as 33 A in width. The experimental results are in good agreement with a calculation
Linearly polarized emission from an embedded quantum dot using nanowire morphology control.
Foster, Andrew P; Bradley, John P; Gardner, Kirsty; Krysa, Andrey B; Royall, Ben; Skolnick, Maurice S; Wilson, Luke R
2015-03-11
GaAs nanowires with elongated cross sections are formed using a catalyst-free growth technique. This is achieved by patterning elongated nanoscale openings within a silicon dioxide growth mask on a (111)B GaAs substrate. It is observed that MOVPE-grown vertical nanowires with cross section elongated in the [21?1?] and [1?12] directions remain faithful to the geometry of the openings. An InGaAs quantum dot with weak radial confinement is realized within each nanowire by briefly introducing indium into the reactor during nanowire growth. Photoluminescence emission from an embedded nanowire quantum dot is strongly linearly polarized (typically >90%) with the polarization direction coincident with the axis of elongation. Linearly polarized PL emission is a result of embedding the quantum dot in an anisotropic nanowire structure that supports a single strongly confined, linearly polarized optical mode. This research provides a route to the bottom-up growth of linearly polarized single photon sources of interest for quantum information applications. PMID:25674919
M. Nikl; K. Nitsch; K. Polak; G. P. Pazzi; P. Fabeni; D. S. Citrin; M. Gurioli
1995-01-01
The quantum-confinement effect is demonstrated in the luminescence of the CsPbCl3-like aggregated phase in Pb2+-doped CsCl single crystals. Namely, microscopic excitonic superradiance is considered to explain the observed picosecond decay kinetics of the aggregated-phase emission band at 420 mn. The low- and high-temperature limits of the temperature dependence of the radiative decay time in the quantum dot are derived and
Few Electron Quantum Dots in Si/SiGe
NASA Astrophysics Data System (ADS)
Shaji, Nakul; Simmons, Christie; Klein, Levente; Savage, Don; Coppersmith, Susan; Friesen, Mark; Qin, Hua; Blick, Robert; Eriksson, Mark
2007-03-01
Quantum information processing in silicon-based materials offers potential advantages like low spin orbit coupling and long spin coherence times. We report the fabrication and measurement of few electron quantum dots in strained Si/SiGe heterostructures. The quantum dots are formed by depleting the underlying two-dimensional electron gas using Schottky top gates. The design incorporates a capacitively coupled quantum point contact charge sensor to enable the read out of the number of electrons in the quantum dot. Low-noise measurement through the quantum dot reveals stable coulomb diamonds in the few electron regime. Interesting effects such as Kondo coupling of electron spins with the leads and Fano lineshapes for the coulomb peaks are observed in our measurements. We have investigated in detail the ground state and excited state transport spectroscopy through the quantum dots in the few electron limit at a base temperature of 20mK. In the presence of an external magnetic field (up to 4 Tesla) applied normal to the plane of electron transport we observe shifts in peak height and position enabling a discussion of the nature of these transport channels in our quantum dot.
Quantum Phase Transitions in Cavity Coupled Dot systems
NASA Astrophysics Data System (ADS)
Kasisomayajula, Vijay; Russo, Onofrio
2011-03-01
We investigate a Quantum Dot System, in which the transconductance, in part, is due to spin coupling, with each dot subjected to a biasing voltage. When this system is housed in a QED cavity, the cavity dot coupling alters the spin coupling of the coupled dots significantly via the Purcell Effect. In this paper we show the extent to which one can control the various coupling parameters: the inter dot coupling, the individual dots coupling with the cavity and the coupled dots coupling with the cavity as a single entity. We show that the dots coupled to each other and to the cavity, the spin transport can be controlled selectively. We derive the conditions for such control explicitly. Further, we discuss the Quantum phase transition effects due to the charge and spin transport through the dots. The electron transport through the dots, electron-electron spin interaction and the electron-photon interaction are treated using the Non-equilibrium Green's Function Formalism. http://publish.aps.org/search/field/author/Trif_Mircea (Trif Mircea), http://publish.aps.org/search/field/author/Golovach_Vitaly_N (Vitaly N. Golovach), and http://publish.aps.org/search/field/author/Loss_Daniel (Daniel Loss), Phys. Rev. B 75, 085307 (2007)
InP quantum dots: Electronic structure, surface effects, and the redshifted emission
Fu, H.; Zunger, A. [National Renewable Energy Laboratory, Golden, Colorado 80401 (United States)] [National Renewable Energy Laboratory, Golden, Colorado 80401 (United States)
1997-07-01
We present pseudopotential plane-wave electronic-structure calculations on InP quantum dots in an effort to understand quantum confinement and surface effects and to identify the origin of the long-lived and redshifted luminescence. We find that (i) unlike the case in small GaAs dots, the lowest unoccupied state of InP dots is the {Gamma}{sub 1c}-derived direct state rather than the X{sub 1c}-derived indirect state and (ii) unlike the prediction of {bold k}{center_dot}{bold p} models, the highest occupied state in InP dots has a 1sd-type envelope function rather than a (dipole-forbidden) 1pf envelope function. Thus explanations (i) and (ii) to the long-lived redshifted emission in terms of an orbitally forbidden character can be excluded. Furthermore, (iii) fully passivated InP dots have no surface states in the gap. However, (iv) removal of the anion-site passivation leads to a P dangling bond (DB) state just above the valence band, which will act as a trap for photogenerated holes. Similarly, (v) removal of the cation-site passivation leads to an In dangling-bond state below the conduction band. While the energy of the In DB state depends only weakly on quantum size, its radiative lifetime increases with quantum size. The calculated {approximately}300-meV redshift and the {approximately}18 times longer radiative lifetime relative to the dot-interior transition for the 26-{Angstrom} dot with an In DB are in good agreement with the observations of full-luminescence experiments for unetched InP dots. Yet, (vi) this type of redshift due to surface defect is inconsistent with that measured in {ital selective} excitation for HF-etched InP dots. (vii) The latter type of ({open_quotes}resonant{close_quotes}) redshift is compatible with the calculated {ital screened} singlet-triplet splitting in InP dots, suggesting that the slow emitting state seen in selective excitation could be a triplet state. {copyright} {ital 1997} {ital The American Physical Society}
Quantum-dot cellular automata G. L. Snider,a)
Orlov, Alexei
and tunnel junctions. An improved design of the cell is presented in which all four dots of the cell are coupled by tunnel junctions. A noninvasive electrometer is presented which improves the sensitivity . A basic QCA cell consists of four quantum dots in a square array coupled by tunnel barriers. Electrons
Digital Logic Gate Using Quantum-Dot Cellular
Orlov, Alexei
electrons. The logic gate consists of a cell, composed of four dots connected in a ring by tunnel junctions architecture can be imple- mented in many systems, we choose the metal tunnel junction implementation described is a nanostructure-compatible compu- tation paradigm that uses arrays of quantum-dot cells to implement digital logic
Density-dependent carrier dynamics in a quantum dots-in-a-well heterostructure
Krishna, Sanjay
The incorporation of semiconductor quantum dots into different heterostructures for applications in nanoscale lasingDensity-dependent carrier dynamics in a quantum dots-in-a-well heterostructure R. P. Prasankumar,1 dots-in-a-well DWELL heterostructure. We observe excitation-dependent shifts of the quantum dot energy
NASA Astrophysics Data System (ADS)
Glässl, M.; Croitoru, M. D.; Vagov, A.; Axt, V. M.; Kuhn, T.
2011-09-01
We study the dynamics of strongly confined semiconductor quantum dots coupled to acoustic phonons and driven by external laser pulses by a numerical path integral method. The field-dependent damping, caused by the non-Markovian processes of pure dephasing and manifesting itself in the peculiar decay and reappearance phenomenon of Rabi rotations is found to depend notably on the dot size and the shape of the applied laser pulses. In the limit of strong fields rectangular pulses yield a significant weaker damping than Gaussian or other bell-shaped profiles. As a consequence, the undamping of Rabi rotations at high pulse areas is most clearly visible for rectangular pulses.
Hyper-parallel photonic quantum computation with coupled quantum dots
Ren, Bao-Cang; Deng, Fu-Guo
2014-01-01
It is well known that a parallel quantum computer is more powerful than a classical one. So far, there are some important works about the construction of universal quantum logic gates, the key elements in quantum computation. However, they are focused on operating on one degree of freedom (DOF) of quantum systems. Here, we investigate the possibility of achieving scalable hyper-parallel quantum computation based on two DOFs of photon systems. We construct a deterministic hyper-controlled-not (hyper-CNOT) gate operating on both the spatial-mode and the polarization DOFs of a two-photon system simultaneously, by exploiting the giant optical circular birefringence induced by quantum-dot spins in double-sided optical microcavities as a result of cavity quantum electrodynamics (QED). This hyper-CNOT gate is implemented by manipulating the four qubits in the two DOFs of a two-photon system without auxiliary spatial modes or polarization modes. It reduces the operation time and the resources consumed in quantum information processing, and it is more robust against the photonic dissipation noise, compared with the integration of several cascaded CNOT gates in one DOF. PMID:24721781
Hyper-parallel photonic quantum computation with coupled quantum dots
NASA Astrophysics Data System (ADS)
Ren, Bao-Cang; Deng, Fu-Guo
2014-04-01
It is well known that a parallel quantum computer is more powerful than a classical one. So far, there are some important works about the construction of universal quantum logic gates, the key elements in quantum computation. However, they are focused on operating on one degree of freedom (DOF) of quantum systems. Here, we investigate the possibility of achieving scalable hyper-parallel quantum computation based on two DOFs of photon systems. We construct a deterministic hyper-controlled-not (hyper-CNOT) gate operating on both the spatial-mode and the polarization DOFs of a two-photon system simultaneously, by exploiting the giant optical circular birefringence induced by quantum-dot spins in double-sided optical microcavities as a result of cavity quantum electrodynamics (QED). This hyper-CNOT gate is implemented by manipulating the four qubits in the two DOFs of a two-photon system without auxiliary spatial modes or polarization modes. It reduces the operation time and the resources consumed in quantum information processing, and it is more robust against the photonic dissipation noise, compared with the integration of several cascaded CNOT gates in one DOF.
Quantum dots: synthesis, bioapplications, and toxicity
2012-01-01
This review introduces quantum dots (QDs) and explores their properties, synthesis, applications, delivery systems in biology, and their toxicity. QDs are one of the first nanotechnologies to be integrated with the biological sciences and are widely anticipated to eventually find application in a number of commercial consumer and clinical products. They exhibit unique luminescence characteristics and electronic properties such as wide and continuous absorption spectra, narrow emission spectra, and high light stability. The application of QDs, as a new technology for biosystems, has been typically studied on mammalian cells. Due to the small structures of QDs, some physical properties such as optical and electron transport characteristics are quite different from those of the bulk materials. PMID:22929008
Capillary electrophoresis of quantum dots: minireview.
Stanisavljevic, Maja; Vaculovicova, Marketa; Kizek, Rene; Adam, Vojtech
2014-07-01
It has been already three decades, since the fluorescent nanocrystals called quantum dots (QDs) appeared and attracted attention of a broad scientific community. Their excellent not only optical but also electronic properties predetermined QDs for utilization in a variety of areas. Besides lasers, solar cells, and/or computers, QDs have established themselves in the field of (bio)chemical labeling as well as medical imaging. However, due to the numerous application possibilities of QDs, there are high demands on their properties that need to be precisely controlled and characterized. CE with its versatile modes and possibilities of detection was found to be an effective tool not only for characterization of QDs size and/or surface properties but also for monitoring of their interactions with other molecules of interest. In this minireview, we are giving short insight in analysis of QDs by CE, and summarizing the advantages of this method for QDs characterization. PMID:24648211
Luminescence studies of individual quantum dot photocatalysts.
Amirav, Lilac; Alivisatos, A Paul
2013-09-01
Using far-field optical microscopy we report the first measurements of photoluminescence from single nanoparticle photocatalysts. Fluence-dependent luminescence is investigated from metal-semiconductor heterojunction quantum dot catalysts exposed to a variety of environments, ranging from gaseous argon to liquid water containing a selection of hole scavengers. The catalysts each exhibit characteristic nonlinear fluence dependence. From these structurally and environmentally sensitive trends, we disentangle the separate rate-determining steps in each particle across the very wide range of time scales, which follow the initial light absorption process. This information will significantly benefit the design of effective artificial photocatalytic systems for renewable direct solar-to-fuel energy conversion. PMID:23895591
Highly Fluorescent Noble Metal Quantum Dots
Zheng, Jie; Nicovich, Philip R.; Dickson, Robert M.
2009-01-01
Highly fluorescent, water-soluble, few-atom noble metal quantum dots have been created that behave as multi-electron artificial atoms with discrete, size-tunable electronic transitions throughout the visible and near IR. These “molecular metals” exhibit highly polarizable transitions and scale in size according to the simple relation, Efermi/N1/3, predicted by the free electron model of metallic behavior. This simple scaling indicates that fluorescence arises from intraband transitions of free electrons and that these conduction electron transitions are the low number limit of the plasmon – the collective dipole oscillations occurring when a continuous density of states is reached. Providing the “missing link” between atomic and nanoparticle behavior in noble metals, these emissive, water-soluble Au nanoclusters open new opportunities for biological labels, energy transfer pairs, and light emitting sources in nanoscale optoelectronics. PMID:17105412
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.
Voltage-tunable dual-band quantum dot infrared photodetectors for temperature sensing.
Ling, Hong-Shi; Wang, Shiang-Yu; Hsu, Wei-Cheng; Lee, Chien-Ping
2012-05-01
We report voltage-tunable 3-5 ?m & 8-12 ?m dual-band detection in the InAs/Al0.3Ga0.7As/In0.15Ga0.85As confinement-enhanced dots-in-a-well quantum dot infrared photodetectors. The capability in temperature sensing is also demonstrated. Distinct response peaks at 5.0 ?m and 8.6 ?m were observed in the photocurrent spectra with working temperature up to 140K. The two peaks correspond to the transition paths from the quantum dot ground state to the quantum well state and the quantum dot excited state, respectively. At 77K, the response ratio of the 8.6 ?m peak over the 5.0 ?m peak changes from 0.29 at -3V to 5.8 at + 4.8V. Excellent selectivity between the two peaks with bias voltage makes the device attractive for third-generation imaging systems with pixel-level multicolor functionality. PMID:22565673
Quantum Dots for In Vivo Small-Animal Imaging
Bentolila, Laurent A.; Ebenstein, Yuval; Weiss, Shimon
2011-01-01
Nanotechnology is poised to transform research, prevention, and treatment of cancer through the development of novel diagnostic imaging methods and targeted therapies. In particular, the use of nanoparticles for imaging has gained considerable momentum in recent years. This review focuses on the growing contribution of quantum dots (QDs) for in vivo imaging in small-animal models. Fluorescent QDs, which are small nanocrystals (1–10 nm) made of inorganic semiconductor materials, possess several unique optical properties best suited for in vivo imaging. Because of quantum confinement effects, the emission color of QDs can be precisely tuned by size from the ultraviolet to the near-infrared. QDs are extremely bright and photostable. They are also characterized by a wide absorption band and a narrow emission band, which makes them ideal for multiplexing. Finally, the large surface area of QDs permits the assembly of various contrast agents to design multimodality imaging probes. To date, biocompatible QD conjugates have been used successfully for sentinel lymph node mapping, tumor targeting, tumor angiogenesis imaging, and metastatic cell tracking. Here we consider these novel breakthroughs in light of their potential clinical applications and discuss how QDs might offer a suitable platform to unite disparate imaging modalities and provide information along a continuum of length scales. PMID:19289434
Chlorine doped graphene quantum dots: Preparation, properties, and photovoltaic detectors
NASA Astrophysics Data System (ADS)
Zhao, Jianhong; Tang, Libin; Xiang, Jinzhong; Ji, Rongbin; Yuan, Jun; Zhao, Jun; Yu, Ruiyun; Tai, Yunjian; Song, Liyuan
2014-09-01
Graphene quantum dots (GQDs) are becoming one of the hottest advanced functional materials because of the opening of the bandgap due to quantum confinement effect, which shows unique optical and electrical properties. The chlorine doped GQDs (Cl-GQDs) have been fabricated by chemical exfoliation of HCl treated carbon fibers (CFs), which were prepared from degreasing cotton through an annealing process at 1000 °C for 30 min. Raman study shows that both G and 2D peaks of GQDs may be redshifted (softened) by chlorine doping, leading to an n-type doping. The first vertical (Cl)-GQDs based photovoltaic detectors have been demonstrated, both the light absorbing and electron-accepting roles for (Cl)-GQDs in photodetection have been found, resulting in an exceptionally big ratio of photocurrent to dark current as high as ˜105 at room temperature using a 405 nm laser irradiation under the reverse bias voltage. The study expands the application of (Cl)-GQDs to the important optoelectronic detection devices.
Graphene-quantum-dot nonvolatile charge-trap flash memories.
Sin Joo, Soong; Kim, Jungkil; Kang, Soo Seok; Kim, Sung; Choi, Suk-Ho; Hwang, Sung Won
2014-06-27
Nonvolatile flash-memory capacitors containing graphene quantum dots (GQDs) of 6, 12, and 27 nm average sizes (d) between SiO2 layers for use as charge traps have been prepared by sequential processes: ion-beam sputtering deposition (IBSD) of 10 nm SiO2 on a p-type wafer, spin-coating of GQDs on the SiO2 layer, and IBSD of 20 nm SiO2 on the GQD layer. The presence of almost a single array of GQDs at a distance of ?13 nm from the SiO2/Si wafer interface is confirmed by transmission electron microscopy and photoluminescence. The memory window estimated by capacitance-voltage curves is proportional to d for sweep voltages wider than ± 3 V, and for d = 27 nm the GQD memories show a maximum memory window of 8 V at a sweep voltage of ± 10 V. The program and erase speeds are largest at d = 12 and 27 nm, respectively, and the endurance and data-retention properties are the best at d = 27 nm. These memory behaviors can be attributed to combined effects of edge state and quantum confinement. PMID:24896068
Copper-indium-selenide quantum dot-sensitized solar cells.
Yang, Jiwoong; Kim, Jae-Yup; Yu, Jung Ho; Ahn, Tae-Young; Lee, Hyunjae; Choi, Tae-Seok; Kim, Young-Woon; Joo, Jin; Ko, Min Jae; Hyeon, Taeghwan
2013-12-21
We present a new synthetic process of near infrared (NIR)-absorbing copper-indium-selenide (CISe) quantum dots (QDs) and their applications to efficient and completely heavy-metal-free QD-sensitized solar cells (QDSCs). Lewis acid-base reaction of metal iodides and selenocarbamate enabled us to produce chalcopyrite-structured CISe QDs with controlled sizes and compositions. Furthermore, gram-scale production of CISe QDs was achieved with a high reaction yield of ~73%, which is important for the commercialization of low-cost photovoltaic (PV) devices. By changing the size and composition, electronic band alignment of CISe QDs could be finely tuned to optimize the energetics of the effective light absorption and injection of electrons into the TiO2 conduction band (CB). These energy-band-engineered QDs were applied to QDSCs, and the quantum-confinement effect on the PV performances was clearly demonstrated. Our best cell yielded a conversion efficiency of 4.30% under AM1.5G one sun illumination, which is comparable to the performance of the best solar cells based on toxic lead chalcogenide or cadmium chalcogenide QDs. PMID:24177572
Raman phonon emission in a driven double quantum dot.
Colless, J I; Croot, X G; Stace, T M; Doherty, A C; Barrett, S D; Lu, H; Gossard, A C; Reilly, D J
2014-01-01
The compound semiconductor gallium-arsenide (GaAs) provides an ultra-clean platform for storing and manipulating quantum information, encoded in the charge or spin states of electrons confined in nanostructures. The absence of inversion symmetry in the zinc-blende crystal structure of GaAs however, results in a strong piezoelectric interaction between lattice acoustic phonons and qubit states with an electric dipole, a potential source of decoherence during charge-sensitive operations. Here we report phonon generation in a GaAs double quantum dot, configured as a single- or two-electron charge qubit, and driven by the application of microwaves via surface gates. In a process that is a microwave analogue of the Raman effect, phonon emission produces population inversion of the two-level system and leads to rapid decoherence of the qubit when the microwave energy exceeds the level splitting. Comparing data with a theoretical model suggests that phonon emission is a sensitive function of the device geometry. PMID:24759675
Polarons in semiconductor quantum dots and their role in the quantum kinetics of carrier relaxation
J. Seebeck; T. R. Nielsen; P. Gartner; F. Jahnke
2005-01-01
While time-dependent perturbation theory shows inefficient carrier-phonon scattering in semiconductor quantum dots, we demonstrate that a quantum kinetic description of carrier-phonon interaction predicts fast carrier capture and relaxation. The considered processes do not fulfill energy conservation in terms of free-carrier energies because polar coupling of localized quantum-dot states strongly modifies this picture.
Physica E 26 (2005) 6366 Photoluminescence of tetrahedral quantum-dot quantum wells
Fonoberov, Vladimir
2005-01-01
-adiabatic approach a quantitative interpretation of the photoluminescence (PL) spectrum of a single CdS/HgS/CdS QDQW-adiabatic approach 1. Introduction Preparation of the CdS quantum dots with HgS quantum wells (QDQWs) was described dots (QDs). Excitons, localized in the HgS quantum well, are separated from the localized surface
Ihn, Thomas
with time-resolved charge detection resulted in a frequency- selective detector for microwave radiation the quantum point contact. We identify the dominant absorption and emission mechanisms in a double quantum dot quantum dot and its environment was investigated in detail in previous works410 using microwave
Study of field driven electroluminescence in colloidal quantum dot solids
Bozyigit, Deniz
Semiconductor nanocrystals, or quantum dots(QDs), promise to drive advances in electronic light generation. It was recently shown that long range transport of charge, which is typically required for electric excitation and ...
Franck-Condon blockade in suspended carbon nanotube quantum dots
von Oppen, Felix
Stampfer1,3* , Kevin Inderbitzin1 , Lukas Durrer3 , Christofer Hierold3 , Eros Mariani4 , Maximilian G suspended CNT quantum dot device are shown in Figs. 1a, 1b and 1c. The CNT is electrically and mechanically
Engineering a Robust Photovoltaic Device with Quantum Dots and Bacteriorhodopsin
Renugopalakrishnan, Venkatesan
We present a route toward a radical improvement in solar cell efficiency using resonant energy transfer and sensitization of semiconductor metal oxides with a light-harvesting quantum dot (QD)/bacteriorhodopsin (bR) layer ...
Heterovalent cation substitutional doping for quantum dot homojunction solar cells
Stavrinadis, Alexandros; Rath, Arup K.; de Arquer, F. Pelayo García; Diedenhofen, Silke L.; Magén, César; Martinez, Luis; So, David; Konstantatos, Gerasimos
2013-01-01
Colloidal quantum dots have emerged as a material platform for low-cost high-performance optoelectronics. At the heart of optoelectronic devices lies the formation of a junction, which requires the intimate contact of n-type and p-type semiconductors. Doping in bulk semiconductors has been largely deployed for many decades, yet electronically active doping in quantum dots has remained a challenge and the demonstration of robust functional optoelectronic devices had thus far been elusive. Here we report an optoelectronic device, a quantum dot homojunction solar cell, based on heterovalent cation substitution. We used PbS quantum dots as a reference material, which is a p-type semiconductor, and we employed Bi-doping to transform it into an n-type semiconductor. We then combined the two layers into a homojunction device operating as a solar cell robustly under ambient air conditions with power conversion efficiency of 2.7%. PMID:24346430
A fast "hybrid" silicon double quantum dot qubit
Shi, Zhan; Prance, J R; Gamble, John King; Koh, Teck Seng; Shim, Yun-Pil; Hu, Xuedong; Savage, D E; Lagally, M G; Eriksson, M A; Friesen, Mark; Coppersmith, S N
2011-01-01
We propose a quantum dot qubit architecture that has an attractive combination of speed and fabrication simplicity. It consists of a double quantum dot with one electron in one dot and two electrons in the other. The qubit itself is a set of two states with total spin quantum numbers $S^2=3/4$ ($S=\\half$) and $S_z = -\\half$, with the two different states being singlet and triplet in the doubly occupied dot. The architecture is relatively simple to fabricate, a universal set of fast operations can be implemented electrically, and the system has potentially long decoherence times. These are all extremely attractive properties for use in quantum information processing devices.
A fast "hybrid" silicon double quantum dot qubit
Zhan Shi; C. B. Simmons; J. R. Prance; John King Gamble; Teck Seng Koh; Yun-Pil Shim; Xuedong Hu; D. E. Savage; M. G. Lagally; M. A. Eriksson; Mark Friesen; S. N. Coppersmith
2011-10-30
We propose a quantum dot qubit architecture that has an attractive combination of speed and fabrication simplicity. It consists of a double quantum dot with one electron in one dot and two electrons in the other. The qubit itself is a set of two states with total spin quantum numbers $S^2=3/4$ ($S=\\half$) and $S_z = -\\half$, with the two different states being singlet and triplet in the doubly occupied dot. The architecture is relatively simple to fabricate, a universal set of fast operations can be implemented electrically, and the system has potentially long decoherence times. These are all extremely attractive properties for use in quantum information processing devices.
A fast ``hybrid'' silicon double quantum dot qubit
NASA Astrophysics Data System (ADS)
Koh, Teck Seng; Shi, Zhan; Simmons, C. B.; Prance, J. R.; King Gamble, John; Shim, Yun-Pil; Hu, Xuedong; Savage, D. E.; Lagally, M. G.; Eriksson, M. A.; Friesen, Mark; Coppersmith, S. N.
2012-02-01
We propose a quantum dot qubit architecture that has an attractive combination of speed and fabrication simplicity. It consists of a double quantum dot with one electron in one dot and two electrons in the other. The qubit itself is a set of two states with total spin quantum numbers S^2 = 3/4 (S = 1/2) and Sz = -1/2, with the two different states being singlet and triplet in the doubly occupied dot. The architecture is relatively simple to fabricate, a universal set of fast operations can be implemented electrically, and the system has potentially long decoherence times. These are all extremely attractive properties for use in quantum information processing devices.
Quantum dot conjugates in a sub-micrometer fluidic channel
Stavis, Samuel M.; Edel, Joshua B.; Samiee, Kevan T.; Craighead, Harold G.
2010-04-13
A nanofluidic channel fabricated in fused silica with an approximately 500 nm square cross section was used to isolate, detect and identify individual quantum dot conjugates. The channel enables the rapid detection of every fluorescent entity in solution. A laser of selected wavelength was used to excite multiple species of quantum dots and organic molecules, and the emission spectra were resolved without significant signal rejection. Quantum dots were then conjugated with organic molecules and detected to demonstrate efficient multicolor detection. PCH was used to analyze coincident detection and to characterize the degree of binding. The use of a small fluidic channel to detect quantum dots as fluorescent labels was shown to be an efficient technique for multiplexed single molecule studies. Detection of single molecule binding events has a variety of applications including high throughput immunoassays.
Linear and nonlinear absorption coefficients of spherical two-electron quantum dot
NASA Astrophysics Data System (ADS)
Yakar, Yusuf; Çak?r, Bekir; Özmen, Ayhan
2015-03-01
In this study, optical properties of two-electron quantum dot confined by an infinite spherical potential surface have been investigated. Linear, nonlinear and total absorption coefficients of S ? P, P ? D and D ? F dipole-allowed transitions between singlet-singlet and triplet-triplet states have been calculated as a function of dot radius and photon energy. The results show that the change of dot radius and incident optical intensity effects the peak positions and amplitudes of linear and nonlinear absorption coefficients. Besides, it has been found that the absorption coefficients of transitions between triplet states are stronger than those of the singlet states, and also triplet absorption transitions occur at higher energies.
Magnetoluminescence from trion and biexciton in type-II quantum dot
2011-01-01
We theoretically investigate optical Aharonov-Bohm (AB) effects on trion and biexciton in the type-II semiconductor quantum dots, in which holes are localized near the center of the dot, and electrons are confined in a ring structure formed around the dot. Many-particle states are calculated numerically by the exact diagonalization method. Two electrons in trion and biexciton are strongly correlated to each other, forming a Wigner molecule. Since the relative motion of electrons are frozen, the Wigner molecule behaves as a composite particle whose mass and charges are twice those of an electron. As a result, the period of AB oscillation for trion and biexciton becomes h/2e as a function of magnetic flux penetrating the ring. We find that the magnetoluminescence spectra from trion and biexciton change discontinuously as the magnetic flux increases by h/2e. PACS: 71.35.Ji, 73.21.-b, 73.21.La, 78.67.Hc PMID:21711894
NASA Astrophysics Data System (ADS)
Kushavah, Dushyant; Mohapatra, P. K.; Rustagi, K. C.; Bahadur, D.; Vasa, P.; Singh, B. P.
2015-05-01
We illustrate effect of charge transfer (CT) in type-II quantum confined heterostructure by comparing CdSe quantum dots (QDs), CdSe/CdTe heterostructure quantum dots (HQDs) and CdSe/CdTe/CdSe quantum well-quantum dots (QWQDs) heterostructures. CdSe core QDs were synthesized using a kinetic growth method where QD size depends on reaction time. For shell coating we used modified version of successive ionic layer adsorption and reaction (SILAR). Size of different QDs ˜5 to 7 nm were measured by transmission electron microscopy (TEM). Strong red shift from ˜597 to ˜746 nm in photoluminescence (PL) spectra from QDs to QWQDs shows high tunability which is not possible with single constituent semiconductor QDs. PL spectra have been recorded at different temperatures (10K-300K). Room temperature time correlated single photon counting (TCSPC) measurements for QDs to QWQDs show three exponential radiative decay. The slowest component decay constant in QWQDs comes around eight fold to ˜51 ns as compared to ˜6.5 ns in HQD suggesting new opportunities to tailor the radiative carrier recombination rate of CT excitons.
NASA Astrophysics Data System (ADS)
Sumi, R.; Warrier, Anita R.; Vijayan, C.
2014-03-01
We report on the visible-light-driven photocatalytic activity of highly stable ?-indium sulfide (In2S3) quantum dots embedded in Nafion matrix. ?-indium sulfide (In2S3) quantum dots (6-10 nm) embedded in Nafion matrix with strong quantum confinement were synthesized by a simple chemical route. The UV-Vis absorption spectrum shows a large blue shift (˜1 eV) which can be controlled by the reaction temperature and time. Strong broadband photoluminescence is observed in the blue, green and red regions of the emission spectrum with variation in particle size and stoichiometry of the quantum dots. Photocatalytic activity measurements show that these hybrid membranes synthesized with equimolar precursors of In and S show paramount photocatalytic activity under visible-light irradiation, with the degradation of Rhodamine-6G dyes up to 95% within 90 min. The photocatalytic membranes are tested for reusable and stable operation.
NASA Astrophysics Data System (ADS)
Kannan, V.; Kim, M. R.; Chae, Y. S.; Ramana, Ch V. V.; Rhee, J. K.
2011-01-01
Multi-layer heterostructure negative differential resistance devices based on poly-[2-methoxy-5-(2'-ethyl-hexyloxy)-1,4-phenylenevinylene] (MEH-PPV) conducting polymer and CdSe quantum dots is reported. The conducting polymer MEH-PPV acts as a barrier while CdSe quantum dots form the well layer. The devices exhibit negative differential resistance (NDR) at low voltages. For these devices, strong negative differential resistance is observed at room temperature. A maximum value of 51 for the peak-to-valley ratio of current is reported. Tunneling of electrons through the discrete quantum confined states in the CdSe quantum dots is believed to be responsible for the multiple peaks observed in the I-V measurement. Depending on the observed NDR signature, operating mechanisms are explored based on resonant tunneling and Coulomb blockade effects.
Observation of the ``Dark exciton'' in CdSe quantum dots
NASA Astrophysics Data System (ADS)
Nirmal, M.; Norris, D. J.; Kuno, M.; Bawendi, M. G.; Efros, Al. L.; Rosen, M.
1995-11-01
We use external magnetic fields to identify the band edge emitting state in CdSe quantum dots. The field dependence of emission decays and LO phonon spectra show the importance of exciton spin dynamics in the recombination mechanism. To interpret our results we calculate the band edge exciton structure, including the effects of the electron-hole exchange interaction and a nonspherical shape. The exchange term, negligible in the bulk, is strongly enhanced by quantum confinement and allows the observation of an optically passive ``dark'' excitonic state.
Electronic and Vibrational Spectra of InP Quantum Dots Formed by Sequential Ion Implantation
NASA Technical Reports Server (NTRS)
Hall, C.; Mu, R.; Tung, Y. S.; Ueda, A.; Henderson, D. O.; White, C. W.
1997-01-01
We have performed sequential ion implantation of indium and phosphorus into silica combined with controlled thermal annealing to fabricate InP quantum dots in a dielectric host. Electronic and vibrational spectra were measured for the as-implanted and annealed samples. The annealed samples show a peak in the infrared spectra near 320/cm which is attributed to a surface phonon mode and is in good agreement with the value calculated from Frolich's theory of surface phonon polaritons. The electronic spectra show the development of a band near 390 nm that is attributed to quantum confined InP.
Quantum Electrodynamics of Quantum Dot-Metal Nanoparticles Molecules
Ridolfo, A; Fina, N; Saija, R; Savasta, S
2010-01-01
We study theoretically the quantum optical properties of hybrid molecules composed of an individual quantum dot and a metallic nanoparticle. We calculate the resonance fluorescence of this hybrid system. Its incoherent part, the one arising from nonlinear quantum processes, results to be enhanced by more than two orders of magnitude as compared to that in the absence of the metallic nanoparticle. Scattering spectra at different excitation powers and nonperturbative calculations of intensity-field correlation functions show that this system can act as a nonlinear ultra-compact two-photon switch for incident photons, where the presence (or absence) of a single incident photon field is sufficient to allow (or prevent) the scattering of subsequent photons. We also find that a small frequency shift of the incident light field may cause changes in the intensity field correlation function of orders of magnitude.
Quantum Electrodynamics of Quantum Dot-Metal Nanoparticles Molecules
A. Ridolfo; O. Di Stefano; N. Fina; R. Saija; S. Savasta
2010-04-09
We study theoretically the quantum optical properties of hybrid molecules composed of an individual quantum dot and a metallic nanoparticle. We calculate the resonance fluorescence of this hybrid system. Its incoherent part, the one arising from nonlinear quantum processes, results to be enhanced by more than two orders of magnitude as compared to that in the absence of the metallic nanoparticle. Scattering spectra at different excitation powers and nonperturbative calculations of intensity-field correlation functions show that this system can act as a nonlinear ultra-compact two-photon switch for incident photons, where the presence (or absence) of a single incident photon field is sufficient to allow (or prevent) the scattering of subsequent photons. We also find that a small frequency shift of the incident light field may cause changes in the intensity field correlation function of orders of magnitude.
Transport Through Andreev Bound States in a Graphene Quantum Dot
Travis Dirks; Taylor L. Hughes; Siddhartha Lal; Bruno Uchoa; Yung-Fu Chen; Cesar Chialvo; Paul M. Goldbart; Nadya Mason
2011-01-01
We have performed transport measurements on a graphene-insulator-superconductor junction, and report the direct observation of sharp, gate-tunable Andreev bound states (ABS) in a graphene quantum dot (QD)[1]. The quantum dot is formed underneath the superconducting lead by local gating due to a work-function mismatch. We show that the ABS form when the discrete QD levels are proximity coupled to the
Time-dependent single-electron transport through quantum dots
Toshimasa Fujisawa; Toshiaki Hayashi; Satoshi Sasaki
2006-01-01
We describe time-dependent single-electron transport through quantum dots in the Coulomb blockade regime. Coherent dynamics of a single charge qubit in a double quantum dot is discussed with full one-qubit manipulation. Strength of decoherence is controlled with the applied voltage, but uncontrolled decoherence arises from electron-phonon coupling and background fluctuations. Then energy-relaxation dynamics is discussed for orbital and spin degree
Single Photon Detection with a Quantum Dot Transistor
Andrew J. Shields; Martin P. O'Sullivan; Ian Farrer; David A. Ritchie; Mark L. Leadbeater; Nalin K. Patel; Richard A. Hogg; Carl E. Norman; Neil J. Curson; Michael Pepper
2001-01-01
We propose and demonstrate a type of GaAs\\/AlGaAs modulation-doped field effect transistor (FET) which is sensitive to single photons. The FET contains a layer of InAs quantum dots formed using an in-situ, self-organising method, adjacent to the channel and separated from it by a thin AlGaAs barrier. Capture of a single photo-excited carrier by a quantum dot leads to a
Visualization and Transport of Quantum Dot Nanomaterials in Porous Media
C. J. G. Darnault; S. M. C. Bonina; B. Uyusur; P. T. Snee
This paper presents our research on the visualization and transport phenomena of quantum dot nanomaterials in porous media.\\u000a It includes the development of a non-intrusive, high spatial and temporal resolution method to visualize transport and measure\\u000a quantum dot nanomaterials concentration in porous media, allowing to characterize the mechanisms that control the transport,\\u000a or lack of mobility, of engineered nanomaterials —
Theory of relaxation oscillations in semiconductor quantum dot lasers
Ermin Malic; Kwang J. Ahn; Moritz J. P. Bormann; Philipp Hövel; Eckehard Schöll; Andreas Knorr; Matthias Kuntz; Dieter Bimberg
2006-01-01
A microscopic approach combining rate equations for photon and electron\\/hole occupations with kinetic equations for Coulomb scattering rates involving quantum dot and wetting layer states in InAs\\/GaAs quantum dot lasers is presented. The authors find strong damping of relaxation oscillations on a picosecond to nanosecond time scale depending on the type of the initial perturbation, similar to the damping observed
Point contact readout for a quantum dot terahertz sensor
NASA Astrophysics Data System (ADS)
Pelling, S.; Davis, R.; Kulik, L.; Tzalenchuk, A.; Kubatkin, S.; Ueda, T.; Komiyama, S.; Antonov, V. N.
2008-08-01
We introduce a terahertz radiation sensor in which the photon-induced ionization state of a quantum dot is monitored by a point contact formed in the same semiconductor heterostructure. For comparison we used a readout based on a single electron transistor coupled to the same quantum dot. The experiments prove functionality of the point contact-based device with additional practical advantage of a higher operation temperature up to 1.5K and ease of nanofabrication.
Coulomb Damped Relaxation Oscillations in Semiconductor Quantum Dot Lasers
Ermin Malic; Moritz J. P. Bormann; P. Hovel; M. Kuntz; D. Bimberg; Andreas Knorr; Eckehard Scholl
2007-01-01
We present a theoretical simulation of the turn-on dynamics of InAs\\/GaAs quantum dot semiconductor lasers driven by electrical current pulses. Our approach goes beyond standard phenomenological rate equations. It contains microscopically calculated Coulomb scattering rates, which describe Auger transitions between quantum dots and the wetting layer. In agreement with the experimental results, we predict a strong damping of relaxation oscillations
Long-distance coherent coupling in a quantum dot array.
Braakman, F R; Barthelemy, P; Reichl, C; Wegscheider, W; Vandersypen, L M K
2013-06-01
Controlling long-distance quantum correlations is central to quantum computation and simulation. In quantum dot arrays, experiments so far rely on nearest-neighbour couplings only, and inducing long-distance correlations requires sequential local operations. Here, we show that two distant sites can be tunnel-coupled directly. The coupling is mediated by virtual occupation of an intermediate site, with a strength that is controlled via the energy detuning of this site. It permits a single charge to oscillate coherently between the outer sites of a triple dot array without passing through the middle, as demonstrated through the observation of Landau-Zener-Stückelberg interference. The long-distance coupling significantly improves the prospects of fault-tolerant quantum computation using quantum dot arrays, and opens up new avenues for performing quantum simulations in nanoscale devices. PMID:23624695
Quantum confinement in transition metal oxide quantum wells
NASA Astrophysics Data System (ADS)
Choi, Miri; Lin, Chungwei; Butcher, Matthew; Rodriguez, Cesar; He, Qian; Posadas, Agham B.; Borisevich, Albina Y.; Zollner, Stefan; Demkov, Alexander A.
2015-05-01
We report on the quantum confinement in SrTiO3 (STO) quantum wells (QWs) grown by molecular beam epitaxy. The QW structure consists of LaAlO3 (LAO) and STO layers grown on LAO substrate. Structures with different QW thicknesses ranging from two to ten unit cells were grown and characterized. Optical properties (complex dielectric function) were measured by spectroscopic ellipsometry in the range of 1.0 eV-6.0 eV at room temperature. We observed that the absorption edge was blue-shifted by approximately 0.39 eV as the STO quantum well thickness was reduced to two unit cells. This demonstrates that the energy level of the first sub-band can be controlled by the QW thickness in a complex oxide material.
NASA Astrophysics Data System (ADS)
Ho, Y. K.; Lin, Y. C.; Sahoo, S.
2004-03-01
We will present calculations for the energy levels and the resonance widths of the quasi-bound states of a confined hydrogenic impurity in an isolate quantum dot subjected to external electric and magnetic fields in parallel directions. A method of complex absorbing potential [1] is used in our present investigation. Resonance positions and widths are reported for a wide range of dot sizes to demonstrate that Stark resonances in a confined hydrogen atom leads to a new phenomenon as a consequence of the quantum confinement of the atom, contrary to the Stark effect on a free atom. * This work was supported by the National Science Council of ROC. [1] S. Sahoo and Y. K. Ho, Chin. J. Phys. 38, 127 (2000); J. Phys. B 33, 2195 (2000); J. Phys. B 33, 5151 (2000); Phys. Rev. A 65, 015403 (2001);
Electronic states in vertically ordered Ge/Si quantum dots detected by photocurrent spectroscopy
NASA Astrophysics Data System (ADS)
Yakimov, A. I.; Kirienko, V. V.; Armbrister, V. A.; Bloshkin, A. A.; Dvurechenskii, A. V.
2014-07-01
We report on intraband photocurrent spectroscopy of sixfold stacked Ge/Si quantum dots embedded in a Si matrix and aligned along the growth direction. The dots are formed in a shape of pyramids with the average lateral size of 18 nm. The n-type heterostructures show broad spectral response ranging from 5 to 20 ?m, depending on the polarization of the incoming infrared light. The normal incidence photocurrent peak centered around 12-15 ?m is attributed to the transitions from the electron states localized in the Si region adjacent to the dots to continuum states of the Si matrix. The electron confinement is caused by a modification of the conduction band alignment induced by inhomogeneous tensile strain in Si around the buried Ge/Si quantum dots. Using the Ge content and dot shape determined by Raman and scanning tunneling microscopy analysis as input parameters for three-dimensional band structure simulations, a good agreement between measured and calculated electron binding energy is obtained. Photoluminescence spectroscopy and measurements of temperature dependence of dark conductance are used to correlate photocurrent results.
Optical properties of type-I PbSe/CdSe core/shell quantum dot
NASA Astrophysics Data System (ADS)
Saravanamoorthy, S. N.; John Peter, A.; Lee, Chang Woo
2015-06-01
Electronic properties and optical properties of exciton in a PbSe/CdSe core/shell quantum dot are investigated taking into account the spatial confinement effect. The present model is based on the PbSe/CdSe quantum dot for type-I confinement regime. The dielectric mismatch effect and the self polarization potential are taken into consideration in the PbSe/CdSe quantum dot nanostructure. Polarization charges are incorporated at the interface of the core/shell materials. Numerical calculations on the electronic and optical properties are found with the ratio of radius of inner and outer shell materials for various shell radii. The exciton binding energy and the interband optical transition energies are computed using variational formulism within the single band effective mass approximation. The oscillator strength and the recombination life time are determined with the ratio of radius of core to shell materials taking into account the dielectric mismatch between the materials. The nonlinear absorption coefficients and the changes of refractive index are computed for the ground and first excited state using compact density matrix method. The obtained results are found to be in good agreement with those reported by other investigators.
Prasankumar, Rohit P [Los Alamos National Laboratory; Taylor, Antoinette J [Los Alamos National Laboratory; Chow, W W [SNL; Attaluri, R S [UNM; Shenoi, R [UNM
2009-01-01
Semiconductor heterostructures incorporating multiple degrees of spatial confinement have recently attracted substantial interest for photonic applications. One example is the quantum dots-in-a-well (DWELL) heterostructure, consisting of zero-dimensional quantum dots embedded in a two-dimensional quantum well and surrounded by three-dimensional bulk material. This structure offers several advantages over conventional photonic devices while providing a model system for the study of light-matter interactions across multiple spatial dimensions. Here, we use ultrafast differential transmission spectroscopy2 to temporally and spectrally resolve density-dependent carrier dynamics in a DWELL heterostructure. We observe excitation-dependent shifts of the quantum dot energy levels at low densities, while at high densities we observe an anomalous induced absorption at the quantum dot excited state that is correlated to quantum well population dynamics. These studies of density-dependent light-matter interactions across multiple coupled spatial dimensions provide clues to the underlying physics governing quantum dot properties, with important implications for DWELL-based photonic devices.
Quantum Monte Carlo finite temperature electronic structure of quantum dots
NASA Astrophysics Data System (ADS)
Leino, Markku; Rantala, Tapio T.
2002-08-01
Quantum Monte Carlo methods allow a straightforward procedure for evaluation of electronic structures with a proper treatment of electronic correlations. This can be done even at finite temperatures [1]. We test the Path Integral Monte Carlo (PIMC) simulation method [2] for one and two electrons in one and three dimensional harmonic oscillator potentials and apply it in evaluation of finite temperature effects of single and coupled quantum dots. Our simulations show the correct finite temperature excited state populations including degeneracy in cases of one and three dimensional harmonic oscillators. The simulated one and two electron distributions of a single and coupled quantum dots are compared to those from experiments and other theoretical (0 K) methods [3]. Distributions are shown to agree and the finite temperature effects are discussed. Computational capacity is found to become the limiting factor in simulations with increasing accuracy. Other essential aspects of PIMC and its capability in this type of calculations are also discussed. [1] R.P. Feynman: Statistical Mechanics, Addison Wesley, 1972. [2] D.M. Ceperley, Rev.Mod.Phys. 67, 279 (1995). [3] M. Pi, A. Emperador and M. Barranco, Phys.Rev.B 63, 115316 (2001).
Optically induced magnetic moments in symmetric graphene quantum dots
NASA Astrophysics Data System (ADS)
Kavousanaki, Eleftheria G.; Dani, Keshav M.
2015-01-01
Using the tight-binding model, we classify the eigenstates of symmetric graphene quantum dots based on their discrete rotational symmetry. In the presence of an external magnetic field, we identify a Zeeman-like behavior arising from the electronic orbital wave functions, with a constant g value for a variety of quantum dot shapes, sizes, and edge structures. The discrete rotational symmetry of the dots also leads to anomalous optical selection rules, thereby suggesting a way to optically induce a magnetic moment in these nanostructures.
Near Field Photon Emission and Revival in Quantum Dot Qubits
NASA Astrophysics Data System (ADS)
Tafur, S.; Leuenberger, M. N.
2011-03-01
Modeling the spontaneous emission of photons coupled to the electronic states of quantum dots is important for understanding quantum interactions and entanglement in condensed matter as applied to proposed solid-state quantum computers, quantum networks, single photon emitters, and single photon detectors. A quantum dot initially in an excited state can be experimentally observed to decay to its ground state and the observed homodyne tomography of the emitted photon can yield information about the qubit state of the emitter. Though the characteristic lifetime of photon emission is traditionally modeled via the Weisskopf-Wigner approximation, we seek to model the fully quantized spontaneous emission, including near field effects, of a photon from the excited state of a quantum dot beyond the Markovian limit. We further investigate subsequent interactions between the emitted photon and adjacent quantum dots in an effort to describe multipartite entanglement. We propose the use of discretized central-difference approximations of space and time partial derivatives, similar to finite-difference time domain models, to describe single photon states via single photon operators. Additionally, within the future scope of this model, we seek results in the Purcell and Rabi regimes for spontaneous emission events from quantum dots embedded in micro-cavities. NSF (Grant No. ECCS-0725514), DARPA/MTO (Grant No. HR0011-08-1-0059), NSF (Grant No. ECCS-0901784), and AFOSR (Grant No. FA9550-09-1-0450).
Lead selenide quantum dot polymer nanocomposites
NASA Astrophysics Data System (ADS)
Waldron, Dennis L.; Preske, Amanda; Zawodny, Joseph M.; Krauss, Todd D.; Gupta, Mool C.
2015-02-01
Optical absorption and fluorescence properties of PbSe quantum dots (QDs) in an Angstrom Bond AB9093 epoxy polymer matrix to form a nanocomposite were investigated. To the authors’ knowledge, this is the first reported use of AB9093 as a QD matrix material and it was shown to out-perform the more common poly(methyl methacrylate) matrix in terms of preserving the optical properties of the QD, resulting in the first reported quantum yield (QY) for PbSe QDs in a polymer matrix, 26%. The 1-s first excitonic absorption peak of the QDs in a polymer matrix red shifted 65 nm in wavelength compared to QDs in a hexane solution, while the emission peak in the polymer matrix red shifted by 38 nm. The fluorescence QY dropped from 55% in hexane to 26% in the polymer matrix. A time resolved fluorescence study of the QDs showed single exponential lifetimes of 2.34 and 1.34 ?s in toluene solution and the polymer matrix respectively.
Effect of carrier dynamics and temperature on two-state lasing in semiconductor quantum dot lasers
Korenev, V. V., E-mail: korenev@spbau.ru; Savelyev, A. V.; Zhukov, A. E.; Omelchenko, A. V.; Maximov, M. V. [Saint Petersburg Academic University-Nanotechnology Research and Education Center (Russian Federation)] [Saint Petersburg Academic University-Nanotechnology Research and Education Center (Russian Federation)
2013-10-15
It is analytically shown that the both the charge carrier dynamics in quantum dots and their capture into the quantum dots from the matrix material have a significant effect on two-state lasing phenomenon in quantum dot lasers. In particular, the consideration of desynchronization in electron and hole capture into quantum dots allows one to describe the quenching of ground-state lasing observed at high injection currents both qualitatevely and quantitatively. At the same time, an analysis of the charge carrier dynamics in a single quantum dot allowed us to describe the temperature dependences of the emission power via the ground- and excited-state optical transitions of quantum dots.
Role of quantum confinement in luminescence efficiency of group IV nanostructures
Barbagiovanni, E. G., E-mail: santino.gasparo@gmail.com [Laboratory for Simulation of Physical Systems, Beijing Computational Science Research Centre, Beijing 100084 (China); Lockwood, D. J.; Rowell, N. L. [Measurement Science and Standards, National Research Council, Ottawa, Ontario K1A 0R6 (Canada); Costa Filho, R. N. [Departamento de Física, Universidade Federal do Ceará, Caixa Postal 6030, Campus do Pici, 60455-760 Fortaleza, Ceará (Brazil); Berbezier, I.; Amiard, G.; Favre, L.; Ronda, A. [Institut Matériaux Microélectronique Nanosciences de Provence, UMR CNRS, 6137, Avenue Normandie Niemen, 13397 Marseille Cedex 20 (France); Faustini, M.; Grosso, D. [Laboratoire Chimie de la Matière Condensée de Paris, UMR-7574 UPMC-CNRS, Collège de France, 11, place Marcelin Berthelot, 75231 Paris (France)
2014-01-28
Experimental results obtained previously for the photoluminescence efficiency (PL{sub eff}) of Ge quantum dots (QDs) are theoretically studied. A log-log plot of PL{sub eff} versus QD diameter (D) resulted in an identical slope for each Ge QD sample only when E{sub G}?(D{sup 2}+D){sup ?1}. We identified that above D ? 6.2?nm: E{sub G}?D{sup ?1} due to a changing effective mass (EM), while below D ? 4.6?nm: E{sub G}?D{sup ?2} due to electron/hole confinement. We propose that as the QD size is initially reduced, the EM is reduced, which increases the Bohr radius and interface scattering until eventually pure quantum confinement effects dominate at small D.
Cooper pair splitting in parallel quantum dot Josephson junctions.
Deacon, R S; Oiwa, A; Sailer, J; Baba, S; Kanai, Y; Shibata, K; Hirakawa, K; Tarucha, S
2015-01-01
Devices to generate on-demand non-local spin entangled electron pairs have potential application as solid-state analogues of the entangled photon sources used in quantum optics. Recently, Andreev entanglers that use two quantum dots as filters to adiabatically split and separate the quasi-particles of Cooper pairs have shown efficient splitting through measurements of the transport charge but the spin entanglement has not been directly confirmed. Here we report measurements on parallel quantum dot Josephson junction devices allowing a Josephson current to flow due to the adiabatic splitting and recombination of the Cooper pair between the dots. The evidence for this non-local transport is confirmed through study of the non-dissipative supercurrent while tuning independently the dots with local electrical gates. As the Josephson current arises only from processes that maintain the coherence, we can confirm that a current flows from the spatially separated entangled pair. PMID:26130172
Adiabatic charge and spin pumping through interacting quantum dots.
Deus, Fernanda; Hernández, Alexis R; Continentino, Mucio A
2012-09-01
In this paper we investigate adiabatic charge and spin pumping through interacting quantum dots using non-equilibrium Green's function techniques and the equation-of-motion method. We treat the electronic correlations inside the dot using a Hartree-Fock approximation and succeed in obtaining closed analytic expressions for the Keldysh Green's functions. These allow us to compute charge and spin currents through the quantum dot. Depending on the parameters of the quantum dot and its coupling to the reservoirs, we show that it can be found in two different regimes: the magnetic regime and the non-magnetic regime. In the magnetic regime we find a non-vanishing spin current in addition to the charge current present in both cases. PMID:22885672
NASA Astrophysics Data System (ADS)
Xue, Changfeng; Deng, Shaozhong
2011-03-01
In this article, the computational procedure to calculate wave-functions and energies of spherical quantum dots with finite barriers and a shallow donor impurity located anywhere inside is extended to prolate spheroidal quantum dots. As in the spherical case, the extension of the procedure fully takes into account polarization effects originating from the dielectric mismatch at the dot surface. Particular attentions are paid to the calculations of self-polarization potential energy so as to overcome the mathematical divergence in the self energy when the sharp step-like dielectric interface is assumed between the dot and the surrounding matrix. In this regard, the so-called three-layer dielectric models, including the novel quasi-harmonic dielectric model, are employed. The procedure is implemented to calculate energy states of a prolate spheroidal quantum dot with a hydrogenic donor impurity located at the polar axis of the dot, held at a finite confining potential. The resulting code is then employed to carry out an illustrative study on the effects of the dielectric mismatch, the impurity location, and the choice of dielectric models to the electronic energy in the dot.
Efficiency vs. multi-photon contribution test for quantum dots
Ana Predojevic; Miroslav Jezek; Tobias Huber; Harishankar Jayakumar; Thomas Kauten; Glenn S. Solomon; Radim Filip; Gregor Weihs
2014-03-19
The development of linear quantum computing within integrated circuits demands high quality semiconductor single photon sources. In particular, for a reliable single photon source it is not sufficient to have a low multi-photon component, but also to possess high efficiency. We investigate the photon statistics of the emission from a single quantum dot with a method that is able to sensitively detect the trade-off between the efficiency and the multi-photon contribution. Our measurements show, that the light emitted from the quantum dot when it is resonantly excited possess a very low multi-photon content. Additionally, we demonstrated, for the first time, the non-Gaussian nature of the quantum state emitted from a single quantum dot.
Thermopower of few-electron quantum dots with Kondo correlations
NASA Astrophysics Data System (ADS)
Ye, Lvzhou
2015-03-01
The thermopower of few-electron quantum dots is crucially influenced by on-dot electron-electron interactions, particularly in the presence of Kondo correlations. We present a comprehensive picture which elucidates the underlying relations between the thermopower and the spectral density function of two-level quantum dots. The effects of various electronic states, including the Kondo states originating from both spin and orbital degrees of freedom, are clearly unraveled. With these insights, we have exemplified an effective and viable way to control the sign of thermopower of Kondo-correlated quantum dots. This is realized by tuning the temperature and by selecting the appropriate level spacing and Coulomb repulsion strength. Such a physical picture is affirmed by accurate numerical data obtained with a hierarchical equations of motion approach. Our understandings and findings provide useful insights into controlling the direction of electric (heat) current through a quantum dot by applying a temperature (voltage) gradient across the two coupling leads. This may have important implications for novel thermoelectric applications of quantum dots. The support from the Natural Science Foundation of China (Grants No. 21033008, No. 21233007, No. 21303175, and No. 21322305) and the Strategic Priority Research Program (B) of the CAS (XDB01020000) is gratefully appreciated.
Electronic and optical properties of semiconductor and graphene quantum dots
NASA Astrophysics Data System (ADS)
Sheng, Wei-dong; Korkusinski, Marek; Güçlü, Alev Devrim; Zielinski, Michal; Potasz, Pawel; Kadantsev, Eugene S.; Voznyy, Oleksandr; Hawrylak, Pawel
2012-06-01
Our recent work on the electronic and optical properties of semiconductor and graphene quantum dots is reviewed. For strained self-assembled InAs quantum dots on GaAs or InP substrate atomic positions and strain distribution are described using valence-force field approach and continuous elasticity theory. The strain is coupled with the effective mass, k · p, effective bond-orbital and atomistic tight-binding models for the description of the conduction and valence band states. The single-particle states are used as input to the calculation of optical properties, with electron-electron interactions included via configuration interaction (CI) method. This methodology is used to describe multiexciton complexes in quantum dot lasers, and in particular the hidden symmetry as the underlying principle of multiexciton energy levels, manipulating emission from biexcitons for entangled photon pairs, and optical control and detection of electron spins using gates. The self-assembled quantum dots are compared with graphene quantum dots, one carbon atom-thick nanostructures. It is shown that the control of size, shape and character of the edge of graphene dots allows to manipulate simultaneously the electronic, optical, and magnetic properties in a single material system.
Influence of phonons on exciton-photon interaction and photon statistics of a quantum dot
M. Bagheri Harouni; R. Roknizadeh; M. H. Naderi
2009-01-01
In this paper, we investigate phonon effects on the optical properties of a spherical quantum dot. For this purpose, we consider the interaction of a spherical quantum dot with classical and quantum fields while the exciton of quantum dot interacts with a solid-state reservoir. We show that phonons strongly affect the Rabi oscillations and optical coherence on first picoseconds of
Ground States and Excited States in a Tunable Graphene Quantum Dot
Lin-Jun Wang; Gang Cao; Tao Tu; Hai-Ou Li; Cheng Zhou; Xiao-Jie Hao; Guang-Can Guo; Guo-Ping Guo
2011-01-01
We prepare an etched gate tunable quantum dot in single-layer graphene and present transport measurement in this system. We extract the information of the ground states and excited states of the graphene quantum dot, as denoted by the presence of characteristic Coulomb blockade diamond diagrams. The results demonstrate that the quantum dot in single-layer graphene bodes well for future quantum
Optical properties of spherical quantum dot with position-dependent effective mass
NASA Astrophysics Data System (ADS)
Keshavarz, A.; Zamani, N.
2013-06-01
In this paper, the effects of position-dependent effective mass on the optical properties of a three dimensional quantum dot are numerically investigated. For this purpose, by using the point canonical transformation method and numerical solution of the Schrödinger equation, the energy levels and wave functions of electrons in the confinement potential are obtained. Then the optical properties are studied by considering the compact density matrix approach based on the numerical calculation of the wave function. Our calculations were compared with the case of constant mass. Results show that the position-dependent effective mass plays an important role in the intersubband optical absorption coefficient and refractive index changes in a quantum dot.
Charge Sensing and Controllable Tunnel Coupling in a Si/SiGe Double Quantum Dot
NASA Astrophysics Data System (ADS)
Simmons, C. B.; Thalakulam, Madhu; Rosemeyer, B. M.; van Bael, B. J.; Sackmann, E. K.; Savage, D. E.; Lagally, M. G.; Joynt, R.; Friesen, Mark; Coppersmith, S. N.; Eriksson, M. A.
2009-09-01
We report integrated charge sensing measurements on a Si/SiGe double quantum dot. The quantum dot is shown to be tunable from a single, large dot to a well-isolated double dot. Charge sensing measurements enable the extraction of the tunnel coupling, t, between the quantum dots as a function of the voltage on the top gates defining the device. Control of the voltage on a single such gate tunes the barrier separating the two dots. The measured tunnel coupling is an exponential function of the gate voltage. The ability to control t is an important step towards controlling spin qubits in silicon quantum dots.
Charge sensing and controllable tunnel coupling in a Si/SiGe double quantum dot.
Simmons, C B; Thalakulam, Madhu; Rosemeyer, B M; Van Bael, B J; Sackmann, E K; Savage, D E; Lagally, M G; Joynt, R; Friesen, Mark; Coppersmith, S N; Eriksson, M A
2009-09-01
We report integrated charge sensing measurements on a Si/SiGe double quantum dot. The quantum dot is shown to be tunable from a single, large dot to a well-isolated double dot. Charge sensing measurements enable the extraction of the tunnel coupling t between the quantum dots as a function of the voltage on the top gates defining the device. Control of the voltage on a single such gate tunes the barrier separating the two dots. The measured tunnel coupling is an exponential function of the gate voltage. The ability to control t is an important step toward controlling spin qubits in silicon quantum dots. PMID:19645459
George Andre Pereira The; Rubens Viana Ramos; Sergio Antenor de Carvalho
2007-05-22
Analysis of quantum dot structures is a current topic with important applications in solid-state digital logic design, quantum information technology and quantum optics. In this work, we show a variational formulation for the solution of the effective two-level approach of the analysis of electron localization in two coupled quantum dots. Numerical results are presented as well the use of electron localization in the realization of a configurable logic circuit using quantum-dot cellular automata.
Entanglement of Electron Spins in Two Coupled Quantum Dots
NASA Astrophysics Data System (ADS)
Chen, Yuanzhen; Webb, Richard
2004-03-01
We study the entanglement of electron spins in a coupled quantum dots system at 70 mK. Two quantum dots are fabricated in a GaAs/AlGaAs heterostructure containing a high mobility 2-D electron gas. The two dots can be tuned independently and the electron spins in the dots are coupled through an exchange interaction between them. An exchange gate is used to vary the height and width of a potential barrier between the two dots, thus controlling the strength of the exchange interaction. Electrons are injected to the coupled dots by two independent DC currents and the output of the dots is incident on a beam splitter, which introduces quantum interferences. Cross-correlations of the shot noise of currents from the two output channels are measured and compared with theory (1). *Work supported by LPS and ARDA under MDA90401C0903 and NSF under DMR 0103223. (1) Burkard, Loss, & Sukhorukov, Phys. Rev. B61, R16303 (2000).
Controlled Photon Switch Assisted by Coupled Quantum Dots.
Luo, Ming-Xing; Ma, Song-Ya; Chen, Xiu-Bo; Wang, Xiaojun
2015-01-01
Quantum switch is a primitive element in quantum network communication. In contrast to previous switch schemes on one degree of freedom (DOF) of quantum systems, we consider controlled switches of photon system with two DOFs. These controlled photon switches are constructed by exploring the optical selection rules derived from the quantum-dot spins in one-sided optical microcavities. Several double controlled-NOT gate on different joint systems are greatly simplified with an auxiliary DOF of the controlling photon. The photon switches show that two DOFs of photons can be independently transmitted in quantum networks. This result reduces the quantum resources for quantum network communication. PMID:26095049
Controlled Photon Switch Assisted by Coupled Quantum Dots
Luo, Ming-Xing; Ma, Song-Ya; Chen, Xiu-Bo; Wang, Xiaojun
2015-01-01
Quantum switch is a primitive element in quantum network communication. In contrast to previous switch schemes on one degree of freedom (DOF) of quantum systems, we consider controlled switches of photon system with two DOFs. These controlled photon switches are constructed by exploring the optical selection rules derived from the quantum-dot spins in one-sided optical microcavities. Several double controlled-NOT gate on different joint systems are greatly simplified with an auxiliary DOF of the controlling photon. The photon switches show that two DOFs of photons can be independently transmitted in quantum networks. This result reduces the quantum resources for quantum network communication. PMID:26095049
Quantum confinement in GaP nanoclusters
Laurich, B.K.; Smith, D.C.; Healy, M.D.
1994-06-01
We have prepared GaP and GaAs nanoclusters from organometallic condensation reactions of E[Si(ChH{sub 3})3]3 (E = P, As) and GaCl{sub 3}. The size of the as synthesized clusters is 10 {Angstrom} to 15 {Angstrom}. Larger clusters of 20 {Angstrom} to 30 {Angstrom} size were obtained by thermal annealing of the as grown material. X-ray diffraction and transmission electron microscopy confirm the high crystalline quality. A lattice contraction of 6.7% could be seen for 10 {Angstrom} sized GaAs clusters. The clusters are nearly spherical in shape. Optical absorption spectra show a distinct line which can be assigned to the fundamental transition of the quantum confined electronic state. The measured blue shift, with respect to the GaP bulk absorption edge is 0.53 eV. As the cluster is smaller than the exciton radius, we can calculate the cluster size from this blue shift and obtain 20.2 {Angstrom}, consistent with the results from X-ray diffraction of 19.5 {Angstrom} for the same sample.
Glowing graphene quantum dots and carbon dots: properties, syntheses, and biological applications.
Zheng, Xin Ting; Ananthanarayanan, Arundithi; Luo, Kathy Qian; Chen, Peng
2015-04-01
The emerging graphene quantum dots (GQDs) and carbon dots (C-dots) have gained tremendous attention for their enormous potentials for biomedical applications, owing to their unique and tunable photoluminescence properties, exceptional physicochemical properties, high photostability, biocompatibility, and small size. This article aims to update the latest results in this rapidly evolving field and to provide critical insights to inspire more exciting developments. We comparatively review the properties and synthesis methods of these carbon nanodots and place emphasis on their biological (both fundamental and theranostic) applications. PMID:25521301
Mesoscopic to universal crossover of transmission phase of multi-level quantum dots
C. Karrasch; T. Hecht; A. Weichselbaum; Y. Oreg; J. von Delft; V. Meden
Transmission phasemeasurements of many-electron quantum dots (small mean level spacing ?) revealed universal phase lapses bybetween consecutive resonances. In contrast, for dots with only a few electrons (large ?), the appearance or not of a phase lapse depends on the dot parameters. We show that a model of a multi-level quantum dot with local Coulomb interactions and arbitrary level-lead couplings
On-chip generation and guiding of quantum light from a site-controlled quantum dot
Jamil, Ayesha; Farrer, Ian; Griffiths, Jonathan P.; Jones, Geb A. C.; Ritchie, David A. [Cavendish Laboratory, University of Cambridge, J. J. Thomson Avenue, Cambridge CB3 0HE (United Kingdom)] [Cavendish Laboratory, University of Cambridge, J. J. Thomson Avenue, Cambridge CB3 0HE (United Kingdom); Skiba-Szymanska, Joanna; Kalliakos, Sokratis; Ward, Martin B.; Ellis, David J. P.; Shields, Andrew J., E-mail: andrew.shields@crl.toshiba.co.uk [Cambridge Research Laboratory, Toshiba Research Europe Limited, 208 Science Park, Milton Road, Cambridge, CB4 0GZ (United Kingdom); Schwagmann, Andre; Brody, Yarden [Cavendish Laboratory, University of Cambridge, J. J. Thomson Avenue, Cambridge CB3 0HE (United Kingdom) [Cavendish Laboratory, University of Cambridge, J. J. Thomson Avenue, Cambridge CB3 0HE (United Kingdom); Cambridge Research Laboratory, Toshiba Research Europe Limited, 208 Science Park, Milton Road, Cambridge, CB4 0GZ (United Kingdom)
2014-03-10
We demonstrate the emission and routing of single photons along a semiconductor chip originating from carrier recombination in an actively positioned InAs quantum dot. Device–scale arrays of quantum dots are formed by a two–step regrowth process. We precisely locate the propagating region of a unidirectional photonic crystal waveguide with respect to the quantum dot nucleation site. Under pulsed optical excitation, the multiphoton emission probability from the waveguide's exit is 12%?±?5% before any background correction. Our results are a major step towards the deterministic integration of a quantum emitter with the waveguiding components of photonic quantum circuits.
Quantum Confinement in Hydrogen Bond of DNA and RNA
da Silva dos Santos; Elso Drigo Filho; Regina Maria Ricotta
2015-02-09
The hydrogen bond is a fundamental ingredient to stabilize the DNA and RNA macromolecules. The main contribution of this work is to describe quantitatively this interaction as a consequence of the quantum confinement of the hydrogen. The results for the free and confined system are compared with experimental data. The formalism to compute the energy gap of the vibration motion used to identify the spectrum lines is the Variational Method allied to Supersymmetric Quantum Mechanics.
Quantum Confinement in Hydrogen Bond of DNA and RNA
Santos, da Silva dos; Ricotta, Regina Maria
2015-01-01
The hydrogen bond is a fundamental ingredient to stabilize the DNA and RNA macromolecules. The main contribution of this work is to describe quantitatively this interaction as a consequence of the quantum confinement of the hydrogen. The results for the free and confined system are compared with experimental data. The formalism to compute the energy gap of the vibration motion used to identify the spectrum lines is the Variational Method allied to Supersymmetric Quantum Mechanics.
Quantum dot spontaneous emission control in a ridge waveguide
NASA Astrophysics Data System (ADS)
Stepanov, Petr; Delga, Adrien; Zang, Xiaorun; Bleuse, Joël; Dupuy, Emmanuel; Peinke, Emanuel; Lalanne, Philippe; Gérard, Jean-Michel; Claudon, Julien
2015-01-01
We investigate the spontaneous emission (SE) of self-assembled InAs quantum dots (QDs) embedded in GaAs ridge waveguides that lay on a low index substrate. In thin enough waveguides, the coupling to the fundamental guided mode is vanishingly small. A pronounced anisotropy in the coupling to non-guided modes is then directly evidenced by normal-incidence photoluminescence polarization measurements. In this regime, a measurement of the QD decay rate reveals a SE inhibition by a factor up to 4. In larger wires, which ensure an optimal transverse confinement of the fundamental guided mode, the decay rate approaches the bulk value. Building on the good agreement with theoretical predictions, we infer from calculations the fraction ? of SE coupled to the fundamental guided mode for some important QD excitonic complexes. For a charged exciton (isotropic in plane optical dipole), ? reaches 0.61 at maximum for an on-axis QD. In the case of a purely transverse linear optical dipole, ? increases up to 0.91. This optimal configuration is achievable through the selective excitation of one of the bright neutral excitons.
Capture delay and modulation bandwidth in a quantum dot laser
NASA Astrophysics Data System (ADS)
Asryan, Levon V.; Wu, Yuchang; Suris, Robert A.
2011-02-01
We show that the carrier capture from the optical confinement layer into quantum dots (QDs) can strongly limit the modulation bandwidth ?-3 dB of a QD laser. Closed-form analytical expressions are obtained for ?-3 dB in the limiting cases of fast and slow capture. ?-3 dB is highest in the case of instantaneous capture into QDs, when the cross-section of carrier capture into a QD ?n = ?. With reducing ?n, ?-3 dB decreases and becomes zero at a certain non-vanishing value ?nmin. This ?nmin presents the minimum tolerable capture cross-section for the lasing to occur at a given dc component j0 of the injection current density. The higher is j0, the smaller is ?nmin and hence the direct modulation of the output power is possible at a slower capture. The use of multiple layers with QDs is shown to considerably improve the modulation response of the laser - the same ?-3 dB is obtained in a multi-layer structure at a much lower j0 than in a single-layer structure. At a plausible value of ?n = 10-11 cm2, ?-3 dB as high as 19 GHz is attainable in a 5-QD-layer structure.
NASA Astrophysics Data System (ADS)
Naruse, Nobuyasu; Mera, Yutaka; Nakamura, Yoshiaki; Ichikawa, Masakazu; Maeda, Koji
2009-03-01
Spatially resolved Fourier-transform photoabsorption spectra of individual Ge1-xSnx nanodots, obtained by a technique based on scanning tunneling microscopy, exhibited a distinct peak far below the absorption edge of the Si substrate, which showed a clear blue shift with decreasing dot size. The energy position of the peak measured in high accuracy was in good agreement with the optical transition energy between discrete levels theoretically predicted by the size dependence due to a quantum-confinement effect, which was previously observed in scanning tunneling spectroscopic measurements.
Single-electron quantum dot in Si /SiGe with integrated charge sensing
NASA Astrophysics Data System (ADS)
Simmons, C. B.; Thalakulam, Madhu; Shaji, Nakul; Klein, Levente J.; Qin, Hua; Blick, R. H.; Savage, D. E.; Lagally, M. G.; Coppersmith, S. N.; Eriksson, M. A.
2007-11-01
Single-electron occupation is an essential component to the measurement and manipulation of spin in quantum dots, capabilities that are important for quantum information processing. Si /SiGe is of interest for semiconductor spin qubits, but single-electron quantum dots have not yet been achieved in this system. We report the fabrication and measurement of a top-gated quantum dot occupied by a single electron in a Si /SiGe heterostructure. Transport through the quantum dot is directly correlated with charge sensing from an integrated quantum point contact, and this charge sensing is used to confirm single-electron occupancy in the quantum dot.
Pulse-Gated Quantum-Dot Hybrid Qubit
NASA Astrophysics Data System (ADS)
Koh, Teck Seng; Gamble, John King; Friesen, Mark; Eriksson, M. A.; Coppersmith, S. N.
2012-12-01
A quantum-dot hybrid qubit formed from three electrons in a double quantum dot has the potential for great speed, due to the presence of level crossings where the qubit becomes chargelike. Here, we show how to exploit the level crossings to implement fast pulsed gating. We develop one- and two-qubit dc quantum gates that are simpler than the previously proposed ac gates. We obtain closed-form solutions for the control sequences and show that the gates are fast (subnanosecond) and can achieve high fidelities.
Pulse-gated quantum-dot hybrid qubit.
Koh, Teck Seng; Gamble, John King; Friesen, Mark; Eriksson, M A; Coppersmith, S N
2012-12-21
A quantum-dot hybrid qubit formed from three electrons in a double quantum dot has the potential for great speed, due to the presence of level crossings where the qubit becomes chargelike. Here, we show how to exploit the level crossings to implement fast pulsed gating. We develop one- and two-qubit dc quantum gates that are simpler than the previously proposed ac gates. We obtain closed-form solutions for the control sequences and show that the gates are fast (subnanosecond) and can achieve high fidelities. PMID:23368440
Bound states in the continuum in graphene quantum dot structures
J. W. González; M. Pacheco; L. Rosales; P. A. Orellana
2010-01-01
The existence of bound states in the continuum was predicted at the dawn of quantum mechanics by von Neumann and Wigner. In this work we discuss the mechanism of formation of these exotic states and the feasibility to observe them experimentally in symmetrical heterostructures composed by segments of graphene ribbons with different widths forming a graphene quantum dot. We identify
Efficient Single Photon Detection by Quantum Dot Resonant Tunneling Diodes
J. C. Blakesley; P. See; A. J. Shields; B. E. Kardynal; P. Atkinson; I. Farrer; D. A. Ritchie
2005-01-01
We demonstrate that the resonant tunnel current through a double-barrier structure is sensitive to the capture of single photoexcited holes by an adjacent layer of quantum dots. This phenomenon could allow the detection of single photons with low dark count rates and high quantum efficiencies. The magnitude of the sensing current may be controlled via the thickness of the tunnel
Electrical control of single hole spins in nanowire quantum dots
NASA Astrophysics Data System (ADS)
Pribiag, V. S.; Nadj-Perge, S.; Frolov, S. M.; van den Berg, J. W. G.; van Weperen, I.; Plissard, S. R.; Bakkers, E. P. A. M.; Kouwenhoven, L. P.
2013-03-01
The development of viable quantum computation devices will require the ability to preserve the coherence of quantum bits (qubits). Single electron spins in semiconductor quantum dots are a versatile platform for quantum information processing, but controlling decoherence remains a considerable challenge. Hole spins in III-V semiconductors have unique properties, such as a strong spin-orbit interaction and weak coupling to nuclear spins, and therefore, have the potential for enhanced spin control and longer coherence times. A weaker hyperfine interaction has previously been reported in self-assembled quantum dots using quantum optics techniques, but the development of hole-spin-based electronic devices in conventional III-V heterostructures has been limited by fabrication challenges. Here, we show that gate-tunable hole quantum dots can be formed in InSb nanowires and used to demonstrate Pauli spin blockade and electrical control of single hole spins. The devices are fully tunable between hole and electron quantum dots, which allows the hyperfine interaction strengths, g-factors and spin blockade anisotropies to be compared directly in the two regimes.
Quantum dots: Time to get the nukes out
NASA Astrophysics Data System (ADS)
Schroer, Michael D.; Petta, Jason R.
2008-07-01
The ability to electrically control spin dynamics in quantum dots makes them one of the most promising platforms for solid-state quantum-information processing. Minimizing the influence of the nuclear spin environment is an important step towards realizing such promise.
Emission Properties of Quantum Dots in a Levitated Microdrop
J. Schaefer; J. P. Mondia; R. Sharma; Z. H. Lu; L. J. Wang; A. S. Susha; A. L. Rogach
2007-01-01
Spherical microcavities doped with semiconductor nanocrystal quantum dots (QDs) have been extensively studied for their fundamental optical properties and for their potential in application such as biolabeling and telecommunications. The spherical microcavities provide a strong 3-dimensional resonant feedback system with a high Q and small mode volume. QDs have tunable emission properties and high quantum yields. To date, most studies
Controlled Dephasing of a Quantum Dot in the Kondo Regime
Alessandro Silva; Shimon Levit
2001-01-01
In this work we analyze how coherent transport through a Quantum Dot (QD) in the Kondo regime is affected by the weak capacitive interaction with a nearby biased Quantum Point Contact (QPC). We find that when the QD-QPC interaction is weak the width of the Kondo resonance is hardly affected by it. However, the spectral weight of the Kondo peak
Folded-light-path colloidal quantum dot solar cells.
Koleilat, Ghada I; Kramer, Illan J; Wong, Chris T O; Thon, Susanna M; Labelle, André J; Hoogland, Sjoerd; Sargent, Edward H
2013-01-01
Colloidal quantum dot photovoltaics combine low-cost solution processing with quantum size-effect tuning to match absorption to the solar spectrum. Rapid advances have led to certified solar power conversion efficiencies of over 7%. Nevertheless, these devices remain held back by a compromise in the choice of quantum dot film thickness, balancing on the one hand the need to maximize photon absorption, mandating a thicker film, and, on the other, the need for efficient carrier extraction, a consideration that limits film thickness. Here we report an architecture that breaks this compromise by folding the path of light propagating in the colloidal quantum dot solid. Using this method, we achieve a substantial increase in short-circuit current, ultimately leading to improved power conversion efficiency. PMID:23835564
Carbon Quantum Dots for Zebrafish Fluorescence Imaging
Kang, Yan-Fei; Li, Yu-Hao; Fang, Yang-Wu; Xu, Yang; Wei, Xiao-Mi; Yin, Xue-Bo
2015-01-01
Carbon quantum dots (C-QDs) are becoming a desirable alternative to metal-based QDs and dye probes owing to their high biocompatibility, low toxicity, ease of preparation, and unique photophysical properties. Herein, we describe fluorescence bioimaging of zebrafish using C-QDs as probe in terms of the preparation of C-QDs, zebrafish husbandry, embryo harvesting, and introduction of C-QDs into embryos and larvae by soaking and microinjection. The multicolor of C-QDs was validated with their imaging for zebrafish embryo. The distribution of C-QDs in zebrafish embryos and larvae were successfully observed from their fluorescence emission. the bio-toxicity of C-QDs was tested with zebrafish as model and C-QDs do not interfere to the development of zebrafish embryo. All of the results confirmed the high biocompatibility and low toxicity of C-QDs as imaging probe. The absorption, distribution, metabolism and excretion route (ADME) of C-QDs in zebrafish was revealed by their distribution. Our work provides the useful information for the researchers interested in studying with zebrafish as a model and the applications of C-QDs. The operations related zebrafish are suitable for the study of the toxicity, adverse effects, transport, and biocompatibility of nanomaterials as well as for drug screening with zebrafish as model. PMID:26135470
Photodynamic antibacterial effect of graphene quantum dots.
Ristic, Biljana Z; Milenkovic, Marina M; Dakic, Ivana R; Todorovic-Markovic, Biljana M; Milosavljevic, Momir S; Budimir, Milica D; Paunovic, Verica G; Dramicanin, Miroslav D; Markovic, Zoran M; Trajkovic, Vladimir S
2014-05-01
Synthesis of new antibacterial agents is becoming increasingly important in light of the emerging antibiotic resistance. In the present study we report that electrochemically produced graphene quantum dots (GQD), a new class of carbon nanoparticles, generate reactive oxygen species when photoexcited (470 nm, 1 W), and kill two strains of pathogenic bacteria, methicillin-resistant Staphylococcus aureus and Escherichia coli. Bacterial killing was demonstrated by the reduction in number of bacterial colonies in a standard plate count method, the increase in propidium iodide uptake confirming the cell membrane damage, as well as by morphological defects visualized by atomic force microscopy. The induction of oxidative stress in bacteria exposed to photoexcited GQD was confirmed by staining with a redox-sensitive fluorochrome dihydrorhodamine 123. Neither GQD nor light exposure alone were able to cause oxidative stress and reduce the viability of bacteria. Importantly, mouse spleen cells were markedly less sensitive in the same experimental conditions, thus indicating a fairly selective antibacterial photodynamic action of GQD. PMID:24612819
Graphene quantum dots derived from carbon fibers.
Peng, Juan; Gao, Wei; Gupta, Bipin Kumar; Liu, Zheng; Romero-Aburto, Rebeca; Ge, Liehui; Song, Li; Alemany, Lawrence B; Zhan, Xiaobo; Gao, Guanhui; Vithayathil, Sajna Antony; Kaipparettu, Benny Abraham; Marti, Angel A; Hayashi, Takuya; Zhu, Jun-Jie; Ajayan, Pulickel M
2012-02-01
Graphene quantum dots (GQDs), which are edge-bound nanometer-size graphene pieces, have fascinating optical and electronic properties. These have been synthesized either by nanolithography or from starting materials such as graphene oxide (GO) by the chemical breakdown of their extended planar structure, both of which are multistep tedious processes. Here, we report that during the acid treatment and chemical exfoliation of traditional pitch-based carbon fibers, that are both cheap and commercially available, the stacked graphitic submicrometer domains of the fibers are easily broken down, leading to the creation of GQDs with different size distribution in scalable amounts. The as-produced GQDs, in the size range of 1-4 nm, show two-dimensional morphology, most of which present zigzag edge structure, and are 1-3 atomic layers thick. The photoluminescence of the GQDs can be tailored through varying the size of the GQDs by changing process parameters. Due to the luminescence stability, nanosecond lifetime, biocompatibility, low toxicity, and high water solubility, these GQDs are demonstrated to be excellent probes for high contrast bioimaging and biosensing applications. PMID:22216895
Quantum dot mediated imaging of atherosclerosis
NASA Astrophysics Data System (ADS)
Jayagopal, Ashwath; Su, Yan Ru; Blakemore, John L.; Linton, MacRae F.; Fazio, Sergio; Haselton, Frederick R.
2009-04-01
The progression of atherosclerosis is associated with leukocyte infiltration within lesions. We describe a technique for the ex vivo imaging of cellular recruitment in atherogenesis which utilizes quantum dots (QD) to color-code different cell types within lesion areas. Spectrally distinct QD were coated with the cell-penetrating peptide maurocalcine to fluorescently-label immunomagnetically isolated monocyte/macrophages and T lymphocytes. QD-maurocalcine bioconjugates labeled both cell types with a high efficiency, preserved cell viability, and did not perturb native leukocyte function in cytokine release and endothelial adhesion assays. QD-labeled monocyte/macrophages and T lymphocytes were reinfused in an ApoE-/- mouse model of atherosclerosis and age-matched controls and tracked for up to four weeks to investigate the incorporation of cells within aortic lesion areas, as determined by oil red O (ORO) and immunofluorescence ex vivo staining. QD-labeled cells were visible in atherosclerotic plaques within two days of injection, and the two cell types colocalized within areas of subsequent ORO staining. Our method for tracking leukocytes in lesions enables high signal-to-noise ratio imaging of multiple cell types and biomarkers simultaneously within the same specimen. It also has great utility in studies aimed at investigating the role of distinct circulating leukocyte subsets in plaque development and progression.
Kondo effect in coupled quantum dots under magnetic fields
Aono, Tomosuke; Eto, Mikio
2001-08-15
The Kondo effect in coupled quantum dots is investigated theoretically under magnetic fields. We show that the magnetoconductance (MC) illustrates the peak structures of Kondo resonant spectra. When the dot-dot tunneling coupling V{sub C} is smaller than the dot-lead coupling {Delta} (level broadening), Kondo resonant levels appear at the Fermi level (E{sub F}). The Zeeman splitting of the levels weakens the Kondo effect, which results in a negative MC. When V{sub C} is larger than {Delta}, the Kondo resonances form bonding and antibonding levels, located below and above E{sub F}, respectively. We observe a positive MC since the Zeeman splitting increases the overlap between the levels at E{sub F}. In the presence of antiferromagnetic spin coupling between the dots, the sign of the MC can change as a function of the gate voltage.
Infrared Quantum Dots** By Edward H. Sargent*
rigid or flexible, smooth or rough, flat or curved, inorganic or organic (including biological luminescent dots are available in organic and aqueous solvents. Electroluminescent devices based on solution
Band-edge diagrams for strained III-V semiconductor quantum wells, wires, and dots
NASA Astrophysics Data System (ADS)
Pryor, C. E.; Pistol, M.-E.
2005-11-01
We have calculated band-edge energies for most combinations of zinc blende AlN, GaN, InN, GaP, GaAs, InP, InAs, GaSb, and InSb in which one material is strained to the other. Calculations were done for three different geometries (quantum wells, wires, and dots) and mean effective masses were computed in order to estimate confinement energies. For quantum wells, we have also calculated band-edges for ternary alloys. Energy gaps, including confinement, may be easily and accurately estimated using band energies and a simple effective mass approximation, yielding excellent agreement with experimental results. By calculating all material combinations we have identified interesting material combinations, such as artificial donors, that have not been experimentally realized. The calculations were perfomed using strain-dependent k•p theory and provide a comprehensive overview of band structures for strained heterostructures.
Berry, Vikas
: Graphene, quantum dots, electron tunneling, humidity sensor, nano arrays, microfibers Graphene quantum dots the polymer's hygroscopic nature for applications as humidity and pressure sensors operating via elec, Phenomenological Understanding, and Humidity/Pressure Sensing Applications T. S. Sreeprasad, Alfredo Alexander