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1

Quantum Confined Silicon Clathrate Quantum Dots

NASA Astrophysics Data System (ADS)

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.

Lusk, Mark; Brawand, Nicholas

2013-03-01

2

Comparison of quantum confinement effects between quantum wires and dots

Dimensionality is an important factor to govern the electronic structures of semiconductor nanocrystals. The quantum confinement energies in one-dimensional quantum wires and zero-dimensional quantum dots are quite different. Using large-scale first-principles calculations, we systematically study the electronic structures of semiconductor (including group IV, III-V, and II-VI) surface-passivated quantum wires and dots. The band-gap energies of quantum wires and dots have the same scaling with diameter for a given material. The ratio of band-gap-increases between quantum wires and dots is material-dependent, and slightly deviates from 0.586 predicted by effective-mass approximation. Highly linear polarization of photoluminescence in quantum wires is found. The degree of polarization decreases with the increasing temperature and size.

Li, Jingbo; Wang, Lin-Wang

2004-03-30

3

Investigation of confinement effects in ZnO quantum dots

We report a simple method for the synthesis of Na+ doped and stable zinc oxide quantum dots, using the quantum confinement atom method. An intense broad green photoluminescence (PL) was observed with a maximum located at ~535 nm when excited by UV radiation of 332 nm. The PL peak intensity is found to be highly dependent on the size of

D. Haranath; Sonal Sahai; Amish G. Joshi; Bipin K. Gupta; V. Shanker

2009-01-01

4

Quantum-Confined Impurities as Single-Electron Quantum Dots: Application in Tereherz Emitters

This work will demonstrate that impurities within semiconductor crystals can be thought of as the ultimate in quantum dots, with the three-dimensional confining potential being provided by individual atoms which are each capable of localising single electrons or holes. A heterostruc- ture confining potential provides a means to tune the energy levels of the impurities. Just as im- portantly, quantum

P. Harrison; M. P. Halsall; W.-M. Zheng

2002-01-01

5

Two- versus three-dimensional quantum confinement in indium phosphide wires and dots

NASA Astrophysics Data System (ADS)

The size dependence of the bandgap is the most identifiable aspect of quantum confinement in semiconductors; the bandgap increases as the nanostructure size decreases. The bandgaps in one-dimensional (1D)-confined wells, 2D-confined wires, and 3D-confined dots should evolve differently with size as a result of the differing dimensionality of confinement. However, no systematic experimental comparisons of analogous 1D, 2D or 3D confinement systems have been made. Here we report growth of indium phosphide (InP) quantum wires having diameters in the strong-confinement regime, and a comparison of their bandgaps with those previously reported for InP quantum dots. We provide theoretical evidence to establish that the quantum confinement observed in the InP wires is weakened to the expected extent, relative to that in InP dots, by the loss of one confinement dimension. Quantum wires sometimes behave as strings of quantum dots, and we propose an analysis to generally distinguish quantum-wire from quantum-dot behaviour.

Yu, Heng; Li, Jingbo; Loomis, Richard A.; Wang, Lin-Wang; Buhro, William E.

2003-08-01

6

Two versus three-dimensional quantum confinement in indium phosphide wires and dots

The size dependence of the bandgap is the most identifiable aspect of quantum confinement in semiconductors; the bandgap increases as the nanostructure size decreases. The bandgaps in one-dimensional (1D)-confined wells, 2D-confined wires, and 3D-confined dots should evolve differently with size as a result of the differing dimensionality of confinement. However, no systematic experimental comparisons of analogous 1D, 2D or 3D

Heng Yu; Jingbo Li; Richard A. Loomis; Lin-Wang Wang; William E. Buhro

2003-01-01

7

NASA Astrophysics Data System (ADS)

Electronic energies of an exciton confined in a strained Zn1- x Cd x Se/ZnSe quantum dot have been computed as a function of dot radius with various Cd content. Calculations have been performed using Bessel function as an orthonormal basis for different confinement potentials of barrier height considering the internal electric field induced by the spontaneous and piezoelectric polarizations. The optical absorption coefficients and the refractive index changes between the ground state ( L = 0) and the first excited state ( L = 1) are investigated. It is found that the optical properties in the strained ZnCdSe/ZnSe quantum dot are strongly affected by the confinement potentials and the dot radii. The intensity of the total absorption spectra increases for the transition between higher levels. The obtained optical nonlinearity brings out the fact that it should be considered in calculating the optical properties in low dimensional semiconductors especially in quantum dots.

Mathan Kumar, K.; John Peter, A.; Lee, C. W.

2011-12-01

8

Efficient Exciton Transport Between Strongly Quantum-Confined Silicon Quantum Dots

NASA Astrophysics Data System (ADS)

First-order perturbation theory and many-body Green function analysis are used to quantify the influence of size, surface reconstruction and surface treatment on exciton transport between small silicon quantum dots. Competing radiative processes are also considered in order to determine how exciton transport efficiency is influenced. The analysis shows that quantum confinement causes small (˜ 1 nm) Si quantum dots to exhibit exciton transport efficiencies far exceeding that of their larger counterparts. We also find that surface reconstruction significantly influences the absorption cross-section and leads to a large reduction in both transport rate and efficiency. Exciton transport efficiency is higher for hydrogen-passivated dots as compared with those terminated with more electronegative ligands. This is because such ligands delocalize electron wave functions towards the surface and result in a lower dipole moment. This work [1] is a first step in the development of a framework for the design of quantum dot assemblies with improved exciton transfer efficiency. 2mm [1] Z. Lin, A. Franceschetti and M. T. Lusk, arXiv:1110.6456v1 [cond-mat.mes-hall

Lusk, Mark; Lin, Zhibin; Franceschetti, Alberto

2012-02-01

9

NASA Astrophysics Data System (ADS)

The binding energy of a shallow hydrogenic impurity of a spherical quantum dot confined by harmonic oscillator-like and by rectangular well-like potentials, using a variational procedure within the effective mass approximation, has been determined. The calculations of the binding energy of the donor impurity as a function of the system geometry, and the donor impurity position have been investigated. The binding energy of shallow donor impurity depends not only on the quantum confinements but also on the impurity position. Our results reveal that (i) the donor binding energy decreases as the dot size increases irrespective of the impurity position, and (ii) the binding energy values of rectangular confinement are larger than the values of parabolic confinement and (iii) the rectangular confinement is better than the parabolic confinement in a spherical quantum dot.

Peter, A. John

10

Patterning of confined-state energies in site-controlled semiconductor quantum dots

NASA Astrophysics Data System (ADS)

We demonstrate control of the confined-state energies of semiconductor quantum dots (QDs) grown on prepatterned substrates. The InGaAs/AlGaAs QDs self-order at the apex of self-limiting, inverted pyramids whose locations are fixed by lithography. The confinement energy in the dots is systematically varied across the substrate by changing the pattern of the pyramid array in their vicinity. The resulting energy- and site-controlled QDs show systematic and reproducible shifts of their emission wavelengths as well as antibunched photon emissions from confined single excitons. Such QDs should be useful for applications in quantum information processing and quantum communication devices, e.g., multiple-wavelength single-photon emitters.

Watanabe, S.; Pelucchi, E.; Leifer, K.; Malko, A.; Dwir, B.; Kapon, E.

2005-06-01

11

Two-electron quantum dot in tilted magnetic fields: Sensitivity to the confinement model

NASA Astrophysics Data System (ADS)

Semiconductor quantum dots are conventionally treated within the effective-mass approximation and a harmonic model potential in the two-dimensional plane for the electron confinement. The validity of this approach depends on the type of the quantum-dot device as well as on the number of electrons confined in the system. Accurate modeling is particularly demanding in the few-particle regime, where screening effects are diminished and thus the system boundaries may have a considerable effect on the confining potential. Here we solve the numerically exact two-electron states in both harmonic and hard-wall model quantum dots subjected to tilted magnetic fields. Our numerical results enable direct comparison against experimental singlet-triplet energy splittings. Our analysis shows that hard and soft wall models produce qualitatively different results for quantum dots exposed to tilted magnetic fields. Hence, we are able to address the sensitivity of the two-body phenomena to the modeling, which is of high importance in realistic spin-qubit design.

Frostad, T.; Hansen, J. P.; Wesslén, C. J.; Lindroth, E.; Räsänen, E.

2013-10-01

12

Excitons in artificial quantum dots in the weak spatial confinement regime

The exciton states in individual quantum dots prepared by the selective interdiffusion method in CdTe/CdMgTe quantum wells are studied by the methods of steady-state optical spectroscopy. The annealing-induced diffusion of Mg atoms inward to the bulk of the quantum well, which is significantly enhanced under the SiO{sub 2} mask, leads to a modulation of the bandgap width in the plane of the well, with the minima of the potential being located in the mask aperture areas. A lateral potential that arises, whose height is in the range 30-270 meV and characteristic scale is about 100 nm, efficiently localizes carriers, which form quasi-zero-dimensional excitons in the weak spatial confinement regime. Detailed magnetooptical studies show that Coulomb correlations play a significant role in the formation of exciton states under such a regime, which, in particular, manifests itself in the localization of the wavefunction of carriers on scales that are considerably smaller than the scale of the lateral potential. The particular features of the interlevel splitting, of the biexciton binding energy, and of the diamagnetic shift are discussed. A strong dependence of the interlevel relaxation on the interlevel splitting (the phonon neck) indicates that alternative relaxation mechanisms in the quantum dots studied are weak. The excited states are populated according to the Pauli principle, which indicates that it is possible to apply the shell model of many-exciton states to quantum dots under the weak spatial confinement conditions.

Zaitsev, S. V. [Russian Academy of Sciences, Institute of Solid State Physics (Russian Federation)], E-mail: zaitsev@issp.ac.ru; Welsch, M. K.; Forchel, A. [Universitaet Wuerzburg, Technische Physik (Germany); Bacher, G. [Universitaet Duisburg-Essen, Lehrstuhl fuer Werkstoffe der Elektrotechnik (Germany)

2007-12-15

13

NASA Astrophysics Data System (ADS)

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.

Ganguly, Jayanta; Pal, Suvajit; Ghosh, Manas

2013-11-01

14

The spectrum, the electron density distribution and the character of three two-electron quantum systems, namely the two-electron quantum dot, the negative hydrogen ion and the helium atom, confined by an anisotropic harmonic oscillator potential, have been studied for different confinement strengths, omega, by using the quantum chemical configuration interaction (CI) method employing a Cartesian anisotropic Gaussian basis set supplemented by

T. Sako; G. H. F. Diercksen

2003-01-01

15

Confinement effects on the vibrational properties of colloidal quantum dots

NASA Astrophysics Data System (ADS)

We present a first-principles study of the confinement effects on the vibrational properties of colloidal III-V and II-VI nanoclusters with thousand atoms and radii up to 16.2 å. We describe the connection between the vibrational properties including surface-optical and -acoustic modes, coherent acoustic modes and the structural changes induced by the surface. We highlight the qualitative difference between III-Vs and II-VIs. We can clearly ascribe most of the observations to the large relaxation of the clusters dominated by an inward relaxation of the surface penetrating deep inside the cluster in case of the III-Vs and a large distribution of bond length at the surface of II-VIs. These strong confinement effects tend to disappear for clusters with more than 1000 atoms, where a small red shift of the Raman peaks remains, due to a softening in response to undercoordination. The coherent acoustic phonons are identified and found to be in good agreement with results from the Lamb model and experiment. We explain why the simple model by Lamb gives an accurate description in case of the breathing modes while the vibrational properties of small NCs are poorly described by continuum models in general.

Han, Peng; Bester, Gabriel

2012-02-01

16

NASA Astrophysics Data System (ADS)

A recently developed computational technique, the complex absorbing potential (CAP) method for locating complex poles of resonance states, is applied to calculate the field-induced energy shifts and widths of the ground stale of a hydrogenic donor in a quantum dot (QD) subjected to parallel electric and magnetic fields. The present model demonstrates that resonances in a confined hydrogen atom lead to an anomalous behaviour due to the quantum confinement of the atom. We have studied the influence of magnetic field on the quantum-confined Stark effect. With increasing magnetic field, resonances shift to higher level and the width gets narrower with less impact on the oscillation amplitude. We will discuss our results in terms of a QD system realized in the GaAs/Ga1-xAlxAs sample.

Sahoo, Satyabrate; Lin, Y. C.; Ho, Y. K.

2008-09-01

17

Self-Induced Oscillation for Electron-Hole Pair Confined in Quantum Dot

We study the time-dependent (TD) phenomena of the electron-hole or electron-electron pair confined in the square quantum dot (SQD) system by computationally solving TD Schroedinger equation under the unrestricted Hartree-Fock (UHF) approach. A typical vacillation is found both in the electron and hole when the charged pair is strongly confined in the SQD while the charged particles have initially the same orbital symmetry. The FFT analysis elucidates that the transition matrix element due to the coulomb interaction involves the eigen frequency {omega} being equal to the excitation energy when the resonative vacillation appears. Thus, Coulomb potential has a potential to cause the self-induced ''Rabi'' oscillation when the charged-particle pair is confined only in the QD.

Tagawa, Tomoki; Tsubaki, Atsushi; Ishizuki, Masamu; Takeda, Kyozaburo [Department of Electrical Engineering and Bioscience, Waseda University, Tokyo 169-8555 (Japan)

2011-12-23

18

Confined states of a positronium (Ps) in the spherical and circular quantum dots (QDs) are theoretically investigated in two size quantization regimes: strong and weak. Two-band approximation of Kane’s dispersion law and parabolic dispersion law of charge carriers are considered. It is shown that electron-positron pair instability is a consequence of dimensionality reduction, not of the size quantization. The binding energies for the Ps in circular and spherical QDs are calculated. The Ps formation dependence on the QD radius is studied.

2013-01-01

19

Coherent confinement of plasmonic field in quantum dot-metallic nanoparticle molecules.

Interaction of a hybrid system consisting of a semiconductor quantum dot and a metallic nanoparticle (MNP) with a laser beam can replace the intrinsic plasmonic field of the MNP with a coherently normalized field (coherent-plasmonic or CP field). In this paper we show how quantum coherence effects in such a hybrid system can form a coherent barrier (quantum cage) that spatially confines the CP field. This allows us to coherently control the modal volume of this field, making it significantly smaller or larger than that of the intrinsic plasmonic field of the MNP. We investigate the spatial profiles of the CP field and discuss how the field barrier depends on the collective states of the hybrid system. PMID:23609222

Sadeghi, S M; Hatef, A; Fortin-Deschenes, Simon; Meunier, Michel

2013-04-23

20

NASA Astrophysics Data System (ADS)

Part I. Nanostructure Design and Structural Properties of Epitaxially Grown Quantum Dots and Nanowires: 1. Growth of III/V semiconductor quantum dots C. Schneider, S. Hofling and A. Forchel; 2. Single semiconductor quantum dots in nanowires: growth, optics, and devices M. E. Reimer, N. Akopian, M. Barkelid, G. Bulgarini, R. Heeres, M. Hocevar, B. J. Witek, E. Bakkers and V. Zwiller; 3. Atomic scale analysis of self-assembled quantum dots by cross-sectional scanning tunneling microscopy and atom probe tomography J. G. Keizer and P. M. Koenraad; Part II. Manipulation of Individual Quantum States in Quantum Dots Using Optical Techniques: 4. Studies of the hole spin in self-assembled quantum dots using optical techniques B. D. Gerardot and R. J. Warburton; 5. Resonance fluorescence from a single quantum dot A. N. Vamivakas, C. Matthiesen, Y. Zhao, C.-Y. Lu and M. Atature; 6. Coherent control of quantum dot excitons using ultra-fast optical techniques A. J. Ramsay and A. M. Fox; 7. Optical probing of holes in quantum dot molecules: structure, symmetry, and spin M. F. Doty and J. I. Climente; Part III. Optical Properties of Quantum Dots in Photonic Cavities and Plasmon-Coupled Dots: 8. Deterministic light-matter coupling using single quantum dots P. Senellart; 9. Quantum dots in photonic crystal cavities A. Faraon, D. Englund, I. Fushman, A. Majumdar and J. Vukovic; 10. Photon statistics in quantum dot micropillar emission M. Asmann and M. Bayer; 11. Nanoplasmonics with colloidal quantum dots V. Temnov and U. Woggon; Part IV. Quantum Dot Nano-Laboratory: Magnetic Ions and Nuclear Spins in a Dot: 12. Dynamics and optical control of an individual Mn spin in a quantum dot L. Besombes, C. Le Gall, H. Boukari and H. Mariette; 13. Optical spectroscopy of InAs/GaAs quantum dots doped with a single Mn atom O. Krebs and A. Lemaitre; 14. Nuclear spin effects in quantum dot optics B. Urbaszek, B. Eble, T. Amand and X. Marie; Part V. Electron Transport in Quantum Dots Fabricated by Lithographic Techniques: III-V Semiconductors and Carbon: 15. Electrically controlling single spin coherence in semiconductor nanostructures Y. Dovzhenko, K. Wang, M. D. Schroer and J. R. Petta; 16. Theory of electron and nuclear spins in III-V semiconductor and carbon-based dots H. Ribeiro and G. Burkard; 17. Graphene quantum dots: transport experiments and local imaging S. Schnez, J. Guettinger, F. Molitor, C. Stampfer, M. Huefner, T. Ihn and K. Ensslin; Part VI. Single Dots for Future Telecommunications Applications: 18. Electrically operated entangled light sources based on quantum dots R. M. Stevenson, A. J. Bennett and A. J. Shields; 19. Deterministic single quantum dot cavities at telecommunication wavelengths D. Dalacu, K. Mnaymneh, J. Lapointe, G. C. Aers, P. J. Poole, R. L. Williams and S. Hughes; Index.

Tartakovskii, Alexander

2012-07-01

21

Evaluation of quantum confinement effect in nanocrystal Si dot layer by Raman spectroscopy.

Quantum confinement effect in the nanocrystal-Si (nc-Si) was evaluated by Raman spectroscopy. The nc-Si dot layers were fabricated by the H2 plasma treatment for the nucleation site formation followed by the SiH4 irradiation for the nc-Si growth. Post-oxidation annealing was also performed to improve the crystalline quality. After post-oxidation annealing for 5 or 10 min, the asymmetric broadening on the lower frequency sides in Raman spectra were obtained, which can be attributed to the phonon confinement effect in nc-Si. Furthermore we confirmed that hydrostatic stress of approximately 500 MPa was induced in nc-Si after post-oxidation annealing. PMID:23421269

Mizukami, Y; Kosemura, D; Numasawa, Y; Ohshita, Y; Ogura, A

2012-11-01

22

Wave-function mapping of graphene quantum dots with soft confinement.

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. PMID:22400872

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

2012-01-23

23

Relation between growth procedure and confinement properties of CdSe/ZnSe quantum dots

NASA Astrophysics Data System (ADS)

The influence of the growth conditions of CdSe/ZnSe quantum structures on the confinement properties are systematically investigated by time integrated and time resolved photoluminescence. Three samples grown on GaAs (001) by molecular beam epitaxy consisting of three CdSe monolayers embedded between two 40nm ZnSe barriers are studied. We focus on the influence of the treatment performed after the growth of the strained CdSe layer before capping. The results clearly indicate the formation of quantum dots with excellent optical properties when a specific treatment is performed on the strained CdSe layer before capping, whereas when the CdSe layer is capped directly after growth a rough QW is obtained.

Robin, Ivan-Christophe; André, Régis; Gérard, Jean-Michel

2006-10-01

24

Confinement-dependent ferromagnetism in Mn-doped InAs quantum dots embedded in InP nanowires

NASA Astrophysics Data System (ADS)

Mn-doped InAs quantum dots (QDs) embedded in InP nanowires have been studied by first-principles calculations. The Mn-Mn ferromagnetic (FM) coupling is stronger in quantum dots than in nanowires and even greater than that of GaMnAs. The stabilization of the ferromagnetic phase is a consequence of the strong p-d exchange between the Mn 3d5 states and the hole bound to the magnetic impurity. We demonstrate that this interaction is dependent on the quantum-confinement effects, and this local antiferromagnetic p-d exchange will lead to a stable total FM coupling for InAs QDs embedded in InP nanowires. The magnetic active system is governed mostly by short-range interactions in such a way that the coupling for Mn ions at the same quantum dot is quite robust while that for Mn ions at neighboring dots is weaker.

Lima, Erika N.; Schmidt, Tome M.

2012-09-01

25

Anomalous quantum confinement of the longitudinal optical phonon mode in PbSe quantum dots

NASA Astrophysics Data System (ADS)

We have investigated the diameter dependence of the Raman spectra of lead selenide nanocrystals. The first-order Raman peak at about 136 cm-1 and its second-order overtone at twice this wavenumber move up in energy with decreasing nanocrystal diameter. This anomalous behavior is interpreted in terms of quantum confinement of the longitudinal optical (LO) phonon whose frequency displays a minimum at ? in the dispersion of bulk PbSe. We perform ab initio calculations of the phonons of PbSe slabs with up to 15 layers. The LO mode perpendicular to the slab shifts indeed upwards with decreasing layer thickness, thus validating the interpretation of the anomalous radius dependence of the Raman spectra in terms of quantum confinement.

Habinshuti, J.; Kilian, O.; Cristini-Robbe, O.; Sashchiuk, A.; Addad, A.; Turrell, S.; Lifshitz, E.; Grandidier, B.; Wirtz, L.

2013-09-01

26

NASA Astrophysics Data System (ADS)

We study hole intraband relaxation in strongly confined CdSe quantum dots. We observe a dramatic reduction in the hole energy-loss rate in the final stage of hole relaxation at the bottom of the valence band. This reduction occurs because of a significantly increased inter-level spacing near the band edge, and, in particular, because of a large energy gap separating the lowest (``emitting'') hole states from a dense quasi-continuum of higher lying states. A slowed population buildup of the lowest hole state indicates that the ``phonon bottleneck,'' which is bypassed in the conduction band due to Auger-type electron-hole interactions, still plays a significant role in hole relaxation.

Xu, S.; Mikhailovsky, A. A.; Hollingsworth, J. A.; Klimov, V. I.

2002-01-01

27

NASA Astrophysics Data System (ADS)

To further the objective of controlled manipulation of the electronic states in epitaxial island quantum dots (QDs), we introduce the notion of a lateral potential confinement layer (LPCL) whose judicious placement during island capping allows selective impact on ground and excited electron and hole states. The energy states of InAs/In0.15Ga0.85As QDs are manipulated using 10-monolayer-thick In0.15Al0.25Ga0.60As LPCLs positioned at the bottom, upper, and top region of the QDs. The changes in the photoluminescence (PL) and PL excitation spectra reveal the nature of the electronic transitions impacted selectively through the spatial charge distributions of the states involved.

Kim, Eui-Tae; Chen, Zhonghui; Madhukar, Anupam

2002-10-01

28

Heat capacity and entropy of a GaAs quantum dot with Gaussian confinement

NASA Astrophysics Data System (ADS)

The heat capacity and entropy effects in a GaAs quantum dot with Gaussian confinement are calculated in the presence of a magnetic field and its interaction with the electron spin using the canonical ensemble approach. It is shown that the heat capacity shows a Schottky-like anomaly at a low temperature, while it approaches a saturation value 2kB as the temperature increases. As a function of the magnetic field, the heat capacity shows a maximum and then reduces to zero. Also the width of the maximum becomes wider with temperature. It is also shown that the heat capacity remains constant up to a certain value of the confinement length beyond which it displays a monotonic increase. However as a function of the confinement strength, though the heat capacity initially shows a significant drop, it remains constant thereafter. At low temperatures like T = 10 and 20 K, the entropy is found to decrease with increasing magnetic field, but at higher temperatures, it remains almost independent of the magnetic field. At high temperatures, entropy shows a monotonic increase with temperature, but at a sufficiently low temperature as the magnetic field decreases, the entropy is found to develop a shoulder which becomes more and more pronounced with decreasing magnetic field.

Boyacioglu, B.; Chatterjee, A.

2012-10-01

29

Manipulating quantum-confined Stark shift in electroluminescence from quantum dots with side gates

NASA Astrophysics Data System (ADS)

Single quantum dot (QD) light-emitting diodes were fabricated with side gates in a lateral p-i-n structure. The electroluminescence (EL) energy from the QDs can be controlled independently by the side gates and by forward bias. Stark shifts in EL have been observed up to 0.4 meV as a function of forward injection current, and around 0.7 meV by applying an electric field of 36 kV cm-1 across the QDs. The independent control of the QD emission energy is an important step towards electrically tuning the coupling between QDs and cavities, and generating entangled-photon sources.

Xu, Xiulai; Andreev, Aleksey; Williams, David A.

2008-05-01

30

NASA Astrophysics Data System (ADS)

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.

Ganguly, Jayanta; Pal, Suvajit; Ghosh, Manas

2013-11-01

31

NASA Astrophysics Data System (ADS)

Periodic disposed quantum dot arrays are very useful for the large scale integration of single electron devices. Gold quantum dot arrays were self-assembled inside pore channels of ordered amino-functionalized mesoporous silica thin films, employing the neutralization reaction between chloroauric acid and amino groups. The diameters of quantum dots are controlled via changing the aperture of pore channels from 2.3 to 8.3 nm, which are characterized by HRTEM, SEM and FT-IR. UV-vis absorption spectra of gold nanoparticle/mesoporous silica composite thin films exhibit a blue shift and intensity drop of the absorption peak as the aperture of mesopores decreases, which represents the energy level change of quantum dot arrays due to the quantum size effect.

Yaqing, Chi; Haiqin, Zhong; Xueao, Zhang; Liang, Fang; Shengli, Chang

2009-12-01

32

Binding Energy of Donor States in a Quantum Dot with Parabolic Confinement

The donor binding energies associated with the ground state and a few excited states, are computed as a function of the dot size and the impurity position within two and three dimensional GaAs quantum dots. The calculation has been done using the Potential Morphing Method - a recently developed numerical method for the solution of time independent Schroedinger equation. The

S. Baskoutas; A. F. Terzis

2002-01-01

33

Quantum dot intersublevel light emitters

Self-organized quantum dots have been used with great success in quantum dot infrared photodetectors (QDIPs), wherein intersublevel transitions between confined electron states are utilized. Many attributes make self-organized quantum dots attractive for the realization of intersublevel infrared light sources for the mid- to far-infrared wavelength range. Intersublevel electroluminescence has been demonstrated from both bipolar and quantum cascade unipolar structures, with

Pallab Bhattacharya; Carl Fischer; Sanjay Krishna; Zetian Mi; Xiaohua Su

2004-01-01

34

Millimeter wave absorption by confined acoustic modes in CdSe\\/CdTe core-shell quantum dots

Taking advantage of the specific core-shell charge separation structure in the CdSe\\/CdTe core-shell Type-II quantum dots (QDs), we experimentally observed the resonant-enhanced dipolar interaction between millimeter-wave (MMW) photons and their corresponding (l = 1) confined acoustic phonons. With proper choice of size, the absorption band can be tuned to desired frequency of MMW imaging. Exploiting this characteristic absorption, in a

T.-M. Liu; J.-Y. Lu; C.-C. Kuo; Y.-C. Wen; C.-W. Lai; M.-J. Yang; P.-T. Chou; D. B. Murray; L. Saviot; C.-Kuang Sun

2007-01-01

35

NASA Astrophysics Data System (ADS)

The spectrum, the electron density distribution and the character of three two-electron quantum systems, namely the two-electron quantum dot, the negative hydrogen ion and the helium atom, confined by an anisotropic harmonic oscillator potential, have been studied for different confinement strengths, omega, by using the quantum chemical configuration interaction (CI) method employing a Cartesian anisotropic Gaussian basis set supplemented by a quantum chemical standard Cartesian Gaussian basis set, respectively, and a full CI wavefunction. Energy level diagrams and electron density distributions are displayed for low lying electronic singlet states and selected confinement parameters. The results for the three confined quantum systems are compared with each other: in general, the absolute energies of the states and the interval between states increase with increasing confinement, omega. The ordering of states may vary for different values of omega. The shape of the electron density distribution differs among the three systems. Its size increases in the order He < H- < two-electron quantum dot. Electron distributions in prolate type confinement are stretched along the z axis, while those in oblate type are compressed with respect to this axis. The CI wavefunction of most of the low lying states for the helium atom and the negative hydrogen ion is dominated by one configuration, while some of the CI wavefunctions corresponding to states of the two-electron quantum dot have more than one leading configuration. The ground state CI wavefunctions of the negative hydrogen ion and the helium atom confined in a spherical potential are dominated by harmonic oscillator functions up to 82.4 and 47.1%, respectively, for the smallest confinement of omega = 0.1. These values increase to 98.8 and 89.0%, respectively, for omega = 0.5.

Sako, T.; Diercksen, G. H. F.

2003-05-01

36

NASA Astrophysics Data System (ADS)

I present a systematic study of self-assembled InAs/InP and InAs/GaAs quantum dot single-particle and many-body properties as a function of the quantum dot–surrounding matrix valence band offset. I use an atomistic, empirical tight-binding approach and perform numerically demanding calculations for half-million-atom nanosystems. I demonstrate that the overall confinement in quantum dots is a non-trivial interplay of two key factors: strain effects and the valence band offset. I show that strain effects determine both the peculiar structure of confined hole states of lens type InAs/GaAs quantum dots and the characteristic ‘shell-like’ structure of confined hole states in the commonly considered ‘low-strain’ lens type InAs/InP quantum dot. I also demonstrate that strain leads to single-band-like behavior of hole states of disk type (‘indium flushed’) InAs/GaAs and InAs/InP quantum dots. I show how strain and valence band offset affect quantum dot many-body properties: the excitonic fine structure, an important factor for efficient entangled photon pair generation, and the biexciton and charged exciton binding energies.

>M Zieli?ski,

2013-11-01

37

I present a systematic study of self-assembled InAs/InP and InAs/GaAs quantum dot single-particle and many-body properties as a function of the quantum dot-surrounding matrix valence band offset. I use an atomistic, empirical tight-binding approach and perform numerically demanding calculations for half-million-atom nanosystems. I demonstrate that the overall confinement in quantum dots is a non-trivial interplay of two key factors: strain effects and the valence band offset. I show that strain effects determine both the peculiar structure of confined hole states of lens type InAs/GaAs quantum dots and the characteristic 'shell-like' structure of confined hole states in the commonly considered 'low-strain' lens type InAs/InP quantum dot. I also demonstrate that strain leads to single-band-like behavior of hole states of disk type ('indium flushed') InAs/GaAs and InAs/InP quantum dots. I show how strain and valence band offset affect quantum dot many-body properties: the excitonic fine structure, an important factor for efficient entangled photon pair generation, and the biexciton and charged exciton binding energies. PMID:24129261

Zieli?ski, M

2013-10-15

38

Multilayer GaSb\\/GaAs quantum dot (QD) structures grown by atmospheric pressure metalorganic chemical deposition (MOCVD) on semi-insulating (SI) GaAs (100) substrates with varying growth temperature of the confinement layers are studied by the cathodoluminescence (CL). Two main features assigned to wetting layer (WL) and quantum dots (QD) are observed in the CL spectra. Their relative positions strongly depend on the growth

Krystyna Drozdowicz-Tomsia; Ewa M. Goldys; M. Motlan

2004-01-01

39

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.

Yannouleas, Constantine; Landman, Uzi

2006-01-01

40

Relation between growth procedure and confinement properties of CdSe\\/ZnSe quantum dots

The influence of the growth conditions of CdSe\\/ZnSe quantum structures on the confinement properties are systematically investigated by time integrated and time resolved photoluminescence. Three samples grown on GaAs (001) by molecular beam epitaxy consisting of three CdSe monolayers embedded between two 40nm ZnSe barriers are studied. We focus on the influence of the treatment performed after the growth of

Ivan-Christophe Robin; Régis André; Jean-Michel Gérard

2006-01-01

41

NASA Astrophysics Data System (ADS)

We study the effects of exciton confinement on the nonlinear optical susceptibility of one-dimensional quantum dots. We use a direct numerical diagonalization to obtain the eigenenergies and eigenstates of the discretized Hamiltonian representing an electron-hole pair confined by a semiparabolic potential and interacting with each other via a Coulomb potential. Density matrix perturbation theory is used to compute the nonlinear optical susceptibilities due to third-harmonic generation and the corresponding nonlinear corrections to the refractive index and absorption coefficient. These quantities are analyzed as a function of ratio between the confinement length L and the exciton Bohr radius a0. The Coulomb potential degrades the uniformity of the level separation. We show that this effect promotes the emergence of multiple resonance peaks in the third-harmonic generation spectrum. In the weak confinement regime ? = L/a0 ? 1, the third-order susceptibility is shown to decay as 1/?8 due to the prevalence of the hydrogenoid character of the exciton eigenstates.

Bautista, Jessica E. Q.; Lyra, Marcelo L.; Lima, R. P. A.

2013-02-01

42

NASA Astrophysics Data System (ADS)

Electric field induced exciton binding energy as a function of dot radius in a ZnO/Zn1-xMgxO quantum dot is investigated. The interband emission as a function of dot radius is obtained in the presence of electric field strength. The Stark effect on the exciton as a function of the dot radius is discussed. The effects of strain, including the hydrostatic and the biaxial strain and the internal electric field, induced by spontaneous and piezoelectric polarization are taken into consideration in all the calculations. Numerical calculations are performed using variational procedure within the single band effective mass approximation. Some nonlinear optical properties are investigated for various electric field strengths in a ZnO/Zn1-xMgxO quantum dot taking into account the strain-induced piezoelectric effects. Our results show that the nonlinear optical properties strongly depend on the effects of electric field strength and the geometrical confinement.

Minimala, N. S.; Peter, A. John; Lee, Chang Woo

2013-02-01

43

Lateral Quantum Dots for Quantum Information Processing

NASA Astrophysics Data System (ADS)

The possibility of building a computer that takes advantage of the most subtle nature of quantum physics has been driving a lot of research in atomic and solid state physics for some time. It is still not clear what physical system or systems can be used for this purpose. One possibility that has been attracting significant attention from researchers is to use the spin state of an electron confined in a semiconductor quantum dot. The electron spin is magnetic in nature, so it naturally is well isolated from electrical fluctuations that can a loss of quantum coherence. It can also be manipulated electrically, by taking advantage of the exchange interaction. In this work we describe several experiments we have done to study the electron spin properties of lateral quantum dots. We have developed lateral quantum dot devices based on the silicon metal-oxide-semiconductor transistor, and studied the physics of electrons confined in these quantum dots. We measured the electron spin excited state lifetime, which was found to be as long as 30 ms at the lowest magnetic fields that we could measure. We fabricated and characterized a silicon double quantum dot. Using this double quantum dot design, we fabricated devices which combined a silicon double quantum dot with a superconducting microwave resonator. The microwave resonator was found to be sensitive to two-dimensional electrons in the transistor channel, which we measured and characterized. We developed a new method for extracting information from random telegraph signals, which are produced when we observe thermal fluctuations of electrons in quantum dots. The new statistical method, based on the hidden Markov model, allows us to detect spin-dependent effects in such fluctuations even though we are not able to directly observe the electron spin. We use this analysis technique on data from two experiments involving gallium arsenide quantum dots and use it to measure spin-dependent tunneling rates. Our results advance the understanding of electron spin physics in lateral quantum dots, in silicon and in gallium arsenide.

House, Matthew Gregory

44

Mid-Infrared Quantum Dot Emitters Utilizing Planar Photonic Crystal Technology.

National Technical Information Service (NTIS)

The three-dimensional confinement inherent in InAs self-assembled quantum dots (SAQDs) yields vastly different optical properties compared to one-dimensionally confined quantum well systems. Intersubband transitions in quantum dots can emit light normal t...

B. Passmore E. A. Shaner G. Subramania J. Cederberg S. K. Lyo

2008-01-01

45

Near-field magnetoabsorption of quantum dots

We investigate the effect of an external magnetic field of variable orientation and magnitude (up to 20T ) on the linear near-field optical absorption spectra of single and coupled III-V semiconductor quantum dots. We focus on the spatial as well as on the magnetic confinement, varying the dimensions of the quantum dots and the magnetic field. We show that the

Constantinos Simserides; Anna Zora; Georgios Triberis

2006-01-01

46

Resistive switching memory devices based on three-dimensionally confined Ag quantum dots (QDs) embedded in polyimide (PI) layers were fabricated by using spin-coating and thermal evaporation. The Ag QDs embedded in PI layer were distributed uniformly with sizes of approximately 4-6 nm and with surface density of approximately 1.25 x 10(11) cm(-2). The electrical properties of the Ag/PI (10 nm)/Ag QDs/PI (10 nm)/Ag devices were investigated at room temperature. Current-voltage (I-V) measurements on the devices showed a counterclockwise electrical hysteresis behavior with reliable and reproducible resistive switching to the existence of the Ag QDs. The memory device transformed from its original high-resistance state to low-resistance state under positive bias, and regained its original high-resistance state under negative bias. The maximum ON/OFF ratio of the current bistability was 1 x 10(4). The device also revealed excellent endurance ability at ambient conditions. The possible operating mechanisms concerning the interaction between Ag QDs and PI matrix for the resistance-transform phenomenon were analyzed on the basis of the I-V results. PMID:23646596

Wu, Chaoxing; Li, Fushan; Guo, Tailiang

2013-02-01

47

Optical properties of exciton confinement in spherical ZnO quantum dots embedded in SiO matrix

NASA Astrophysics Data System (ADS)

The optical characteristics of ZnO nanoparticles in SiO2 matrix have been determined by UV-visible and photoluminescence (PL) studies. The PL spectrum of the ZnO quantum dots shows two different bands: the first one is a broad green emission band related to deep level emission in the visible range and the second one is situated at 3.48 eV in the ultraviolet region, and is attributed to the recombination of electrons in the conduction band and holes in the valence band. The experimental result that we found was simulated numerically using different methods. Our calculations revealed a good agreement between the matrix element calculation method and the experimental result. We have also calculated the ground state binding energy, the oscillator strength and the radiative lifetime of the exciton in a ZnO spherical quantum dot. It was clearly shown from our calculation that these physical parameters are very sensitive to the quantum dot size. As a consequence, the optical and electronic properties of the dot can be controlled and also tuned through the nanoparticle size variation. These results could be particularly helpful, since they are closely related to experiments performed on such nanoparticles; this may allow us to improve the stability and efficiency of semiconductor quantum dot luminescence, which is considered critical.

Dallali, Lobna; Jaziri, Sihem; El Haskouri, Jamal; Amorós, Pedro

2009-12-01

48

Quantum computation with quantum dots

We propose an implementation of a universal set of one- and two-quantum-bit gates for quantum computation using the spin states of coupled single-electron quantum dots. Desired operations are effected by the gating of the tunneling barrier between neighboring dots. Several measures of the gate quality are computed within a recently derived spin master equation incorporating decoherence caused by a prototypical

Daniel Loss; David P. Divincenzo

1998-01-01

49

Quantum Computing with Quantum Dots

Our time at the Computing Beyond Silicon Summer School, hosted by Caltech, gave us a unique opportunity to explore new concepts and learn about advances in modern information processing that interest us. We chose to study quantum computation using quantum dot systems because of their potential for bringing forth a solid-sate quantum information processing system. Specifically we chose to investigate

Joon Ho Baek; Happy Hsin; Joshua LaForge; Daniel Nedelcu

50

The authors report the observation of strong self-pulsations in molecular-beam epitaxy-grown oxide-confined vertical-cavity surface-emitting lasers based on submonolayer InGaAs quantum dots. At continuous-wave operation, self-pulsations with pulse durations of 100-300 ps and repetition rates of 0.2-0.6 GHz were measured. The average optical power of the pulsations was 0.5-1.0 mW at the laser continuous-wave current values of 1.5-2.5 mA.

Kuzmenkov, A. G.; Ustinov, V. M.; Sokolovskii, G. S.; Maleev, N. A.; Blokhin, S. A.; Deryagin, A. G.; Chumak, S. V.; Shulenkov, A. S.; Mikhrin, S. S.; Kovsh, A. R.; McRobbie, A. D.; Sibbett, W.; Cataluna, M. A.; Rafailov, E. U. [Saint-Petersburg Physics and Technology Centre for Research and Education of the Russian Academy of Sciences, St. Petersburg 199034 (Russian Federation); Ioffe Physico-Technical Institute, St. Petersburg 194021 (Russian Federation); Minsk R and D Institute of Radiomaterials, Minsk 220024 (Belarus); Innolume GmbH, 44263 Dortmund (Germany); School of Physics and Astronomy, University of St Andrews, St. Andrews KY16 9SS (United Kingdom); Division of Electronic Engineering and Physics, University of Dundee, Dundee, DD1 4HN (United Kingdom)

2007-09-17

51

We report colloidal quantum dot (CQDs) photovoltaics having a approximately 930 nm bandgap. The devices exhibit AM1.5G power conversion efficiencies in excess of 2%. Remarkably, the devices are stable in air under many tens of hours of solar illumination without the need for encapsulation. We explore herein the origins of this orders-of-magnitude improvement in air stability compared to larger PbS dots. We find that small and large dots form dramatically different oxidation products, with small dots forming lead sulfite primarily and large dots, lead sulfate. The lead sulfite produced on small dots results in shallow electron traps that are compatible with excellent device performance; whereas the sulfates formed on large dots lead to deep traps, midgap recombination, and consequent catastrophic loss of performance. We propose and offer evidence in support of an explanation based on the high rate of oxidation of sulfur-rich surfaces preponderant in highly faceted large-diameter PbS colloidal quantum dots. PMID:20104859

Tang, Jiang; Brzozowski, Lukasz; Barkhouse, D Aaron R; Wang, Xihua; Debnath, Ratan; Wolowiec, Remigiusz; Palmiano, Elenita; Levina, Larissa; Pattantyus-Abraham, Andras G; Jamakosmanovic, Damir; Sargent, Edward H

2010-02-23

52

Collective modes in quantum-dot arrays in magnetic fields

We study collective excitations in a model array of parabolically confined quantum dots in an applied magnetic field. Treating the interactions between electrons on the same dot exactly and those between electrons on different dots in a simple approximation, we find that the Hamiltonian separates into a term involving center-of-mass coordinates of the electrons on each dot and a term

Jed Dempsey; N. F. Johnson; L. Brey; B. I. Halperin

1990-01-01

53

Quantum Dots: An Experiment for Physical or Materials Chemistry

ERIC Educational Resources Information Center

|An experiment is conducted for obtaining quantum dots for physical or materials chemistry. This experiment serves to both reinforce the basic concept of quantum confinement and providing a useful bridge between the molecular and solid-state world.|

Winkler, L. D.; Arceo, J. F.; Hughes, W. C.; DeGraff, B. A.; Augustine, B. H.

2005-01-01

54

Optical gain and stimulated emission in nanocrystal quantum dots.

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 the wavelength of the emitting transition for close-packed solids of these dots. Narrowband stimulated emission with a pronounced gain threshold at wavelengths tunable with the size of the nanocrystal was observed, as expected from quantum confinement effects. These results unambiguously demonstrate the feasibility of nanocrystal quantum dot lasers. PMID:11030645

Klimov, V I; Mikhailovsky, A A; Xu, S; Malko, A; Hollingsworth, J A; Leatherdale, C A; Eisler, H; Bawendi, M G

2000-10-13

55

Nonlinear thermovoltage and thermocurrent in quantum dots

NASA Astrophysics Data System (ADS)

Quantum dots are model systems for quantum thermoelectric behavior because of their ability to control and measure the effects of electron-energy filtering and quantum confinement on thermoelectric properties. Interestingly, nonlinear thermoelectric properties of such small systems can modify the efficiency of thermoelectric power conversion. Using quantum dots embedded in semiconductor nanowires, we measure thermovoltage and thermocurrent that are strongly nonlinear in the applied thermal bias. We show that most of the observed nonlinear effects can be understood in terms of a renormalization of the quantum-dot energy levels as a function of applied thermal bias and provide a theoretical model of the nonlinear thermovoltage taking renormalization into account. Furthermore, we propose a theory that explains a possible source of the observed, pronounced renormalization effect by the melting of Kondo correlations in the mixed-valence regime. The ability to control nonlinear thermoelectric behavior expands the range in which quantum thermoelectric effects may be used for efficient energy conversion.

Fahlvik Svensson, S.; Hoffmann, E. A.; Nakpathomkun, N.; Wu, P. M.; Xu, H. Q.; Nilsson, H. A.; Sánchez, D.; Kashcheyevs, V.; Linke, H.

2013-10-01

56

Quantum optics with single quantum dot devices

A single radiative transition in a single-quantum emitter results in the emission of a single photon. Single quantum dots are single-quantum emitters with all the requirements to generate single photons at visible and near-infrared wavelengths. It is also possible to generate more than single photons with single quantum dots. In this paper we show that single quantum dots can be

Valéry Zwiller; Thomas Aichele; Oliver Benson

2004-01-01

57

Plasmonic fluorescent quantum dots

Combining multiple discrete components into a single multifunctional nanoparticle could be useful in a variety of applications. Retaining the unique optical and electrical properties of each component after nanoscale integration is, however, a long-standing problem. It is particularly difficult when trying to combine fluorophores such as semiconductor quantum dots with plasmonic materials such as gold, because gold and other metals

Yongdong Jin; Xiaohu Gao

2009-01-01

58

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.

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

59

Bound states in continuum: Quantum dots in a quantum well

NASA Astrophysics Data System (ADS)

We report on the existence of a bound state in the continuum (BIC) of quantum rods (QR). QRs are novel elongated InGaAs quantum dot nanostructures embedded in the shallower InGaAs quantum well. BIC appears as an excited confined dot state and energetically above the bottom of a well subband continuum. We prove that high height-to-diameter QR aspect ratio and the presence of a quantum well are indispensable conditions for accommodating the BIC. QRs are unique semiconductor nanostructures, exhibiting this mathematical curiosity predicted 83 years ago by Wigner and von Neumann.

Prodanovi?, Nikola; Milanovi?, Vitomir; Ikoni?, Zoran; Indjin, Dragan; Harrison, Paul

2013-11-01

60

Far-field patterns of Quantum Well, Quantum Dash, and Quantum Dot Laser Diodes

We perform measurements of far field pattern of various quantum-confined heterostructures namely quantum well (QW), quantum dash (Qdash), and quantum dot (Qdot) lasers to study the effect of different active gain mediums of semiconductor lasers on the optical beam quality and coupling efficiency. The beam pattern profile of the Qdash laser is similar to that of the QW laser (FWHM

Y. H. Ding; C. L. Tan; V. Hongpinyo; C. E. Dimas; Y. Wang; C. Chen; H. S. Djie; B. S. Ooi

2008-01-01

61

Optical processes in quantum dots and wires

Quantum wires and dots are unique condensed matter systems where electron and hole localization can be achieved by lateral confinement down to quasi 0-dimensions. Here we review how these nanostructures are realized and their optical properties, mainly from an experimental point of view. We discuss changes in the energy and momentum mechanisms as the lateral dimensions are reduced, such as

Clivia M. Sotomayor Torres; Pei D. Wang; N. N. Ledentsov; Yin-Sheng Tang

1994-01-01

62

Quantum-dot cellular automata: computing with coupled quantum dots

We discuss novel nanoelectronic architecture paradigms based on cells composed of coupled quantum-dots. Boolean logic functions may be implemented in speci® c arrays of cells representing binary information, the so-called quantum-dot cellular automata (QCA). Cells may also be viewed as carrying analogue information and we outline a network-theoretic description of such quantum-dot nonlinear net- works (Q-CNN). In addition, we discuss

WOLFGANG POROD; CRAIG S. LENT; GARY H. BERNSTEIN; ALEXEI O. ORLOV; ISLAMSHAH AMLANI; GREGORY L. SNIDER; JAMES L. MERZ

1999-01-01

63

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

Pai, Yi-Hao; Lin, Gong-Ru

2011-01-17

64

Quantum dots and spin qubits in graphene.

This is a review on graphene quantum dots and their use as a host for spin qubits. We discuss the advantages but also the challenges to use graphene quantum dots for spin qubits as compared to the more standard materials like GaAs. We start with an overview of this young and fascinating field and then discuss gate-tunable quantum dots in detail. We calculate the bound states for three different quantum dot architectures where a bulk gap allows for confinement via electrostatic fields: (i) graphene nanoribbons with armchair boundaries, (ii) a disc in single-layer graphene, and (iii) a disc in bilayer graphene. In order for graphene quantum dots to be useful in the context of spin qubits, one needs to find reliable ways to break the valley degeneracy. This is achieved here, either by a specific termination of graphene in (i) or in (ii) and (iii) by a magnetic field, without the need of a specific boundary. We further discuss how to manipulate spin in these quantum dots and explain the mechanism of spin decoherence and relaxation caused by spin-orbit interaction in combination with electron-phonon coupling, and by hyperfine interaction with the nuclear-spin system. PMID:20603538

Recher, Patrik; Trauzettel, Björn

2010-07-06

65

Spin readout and initialization in semiconductor quantum dots

US Patent & Trademark Office Database

A semiconductor quantum dot device converts spin information to charge information utilizing an elongated quantum dot having an asymmetric confining potential along its length so that charge movement occurs during orbital excitation. A single electron sensitive electrometer is utilized to detect the charge movement. Initialization and readout can be carried out rapidly utilizing RF fields at appropriate frequencies.

2006-11-14

66

Quantum dots in nanobiotechnology

Research on semiconductor nanocristals (also known as quantum dots of QD) in the field of nanobiotechnology is rapidly evolving\\u000a thanks to progresses in their synthesis and their surface chemistry. Two types of materials, water soluble and biocompatible\\u000a single QD and beads containing QDs, are becoming available and exciting applications based on these new materials are developed.\\u000a We will present the

N. Lequeux; B. Dubertret

2005-01-01

67

NASA Astrophysics Data System (ADS)

Effective exciton-g factor as functions of dot radius and Ga alloy content in a GaxIn1-xAs/GaAs quantum dot is computed. The computations are included with the effect of mass anisotropy and the non-parabolicity of the conduction band. The dependence of excitonic binding energy with respect to the dot radius is investigated for various composition of Ga alloy content.

Senthilkumar, N. R.; Peter, A. John

2013-02-01

68

Probing relaxation times in graphene quantum dots.

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

Volk, Christian; Neumann, Christoph; Kazarski, Sebastian; Fringes, Stefan; Engels, Stephan; Haupt, Federica; Müller, André; Stampfer, Christoph

2013-01-01

69

Adiabatic description of nonspherical quantum dot models

Within the effective mass approximation an adiabatic description of spheroidal and dumbbell quantum dot models in the regime of strong dimensional quantization is presented using the expansion of the wave function in appropriate sets of single-parameter basis functions. The comparison is given and the peculiarities are considered for spectral and optical characteristics of the models with axially symmetric confining potentials depending on their geometric size, making use of the complete sets of exact and adiabatic quantum numbers in appropriate analytic approximations.

Gusev, A. A., E-mail: gooseff@jinr.ru; Chuluunbaatar, O.; Vinitsky, S. I. [Joint Institute for Nuclear Research (Russian Federation); Dvoyan, K. G.; Kazaryan, E. M.; Sarkisyan, H. A. [Russian-Armenian (Slavonic) University (Armenia); Derbov, V. L.; Klombotskaya, A. S.; Serov, V. V. [Saratov State University (Russian Federation)

2012-10-15

70

Quantum Dots as Cellular Probes

Robust and bright light emitters, semiconductor nanocrystals[quantum dots (QDs)] have been adopted as a new class of fluorescent labels. Six years after the first experiments of their uses in biological applications, there have been dramatic improvements in understanding surface chemistry, biocompatibility, and targeting specificity. Many studies have shown the great potential of using quantum dots as new probes in vitro and in vivo. This review summarizes the recent advances of quantum dot usage at the cellular level, including immunolabeling, cell tracking, in situ hybridization, FRET, in vivo imaging, and other related technologies. Limitations and potential future uses of quantum dot probes are also discussed.

Alivisatos, A. Paul; Gu, Weiwei; Larabell, Carolyn

2004-09-16

71

Highly tunable hybrid quantum dots with charge detection

NASA Astrophysics Data System (ADS)

In order to employ solid state quantum dots as qubits, both a high degree of control over the confinement potential as well as sensitive charge detection are essential. We demonstrate that by combining local anodic oxidation with local Schottky-gates, these criteria are nicely fulfilled in the resulting hybrid device. To this end, a quantum dot with adjacent charge detector is defined. After tuning the quantum dot to contain only a single electron, we are able to observe the charge detector signal of the quantum dot state for a wide range of tunnel couplings.

Rössler, C.; Küng, B.; Dröscher, S.; Choi, T.; Ihn, T.; Ensslin, K.; Beck, M.

2010-10-01

72

Circular polarization memory in single Quantum Dots

Under quasi-resonant circularly polarized optical excitation, charged quantum dots may emit polarized light. We measured various transitions with either positive, negative or no circular-polarization memory. We explain these observations and quantitatively calculate the polarization spectrum. Our model use the full configuration-interaction method, including the electron-hole exchange interaction, for calculating the quantum dot's confined many-carrier states, along with one assumption regarding the spin relaxation of photoexcited carriers: Electrons maintain their initial spin polarization, while holes do not.

Khatsevich, S.; Poem, E.; Benny, Y.; Marderfeld, I.; Gershoni, D. [Physics Department and Solid State Institute, Technion, Haifa 32000 (Israel); Badolato, A.; Petroff, P. M. [Materials Department, University of California Santa Barbara, Santa Barbara, California 93106 (United States)

2010-01-04

73

Coherent Optical Spectroscopy of a Strongly Driven Quantum Dot

Quantum dots are typically formed from large groupings of atoms and thus may be expected to have appreciable many-body behavior under intense optical excitation. Nonetheless, they are known to exhibit discrete energy levels due to quantum confinement effects. We show that, like single-atom or single-molecule two- and three-level quantum systems, single semiconductor quantum dots can also exhibit interference phenomena when

Xiaodong Xu; Bo Sun; Paul R. Berman; Duncan G. Steel; Allan S. Bracker; Dan Gammon; L. J. Sham

2007-01-01

74

Evaluating dot-lead coupling in a non-parabolic quantum dot connected to two conducting leads

NASA Astrophysics Data System (ADS)

A manifestly novel approach is developed by considering excitons in a semiconductor non-parabolic quantum dot as hydrogen-atom-like quasi-particles confined in a spherical quantum box, the dot being connected to two conducting leads. In fact, the maximal dot-lead coupling energy is calculated in terms of maximum Fermi velocity within a new perspective.

Grado-Caffaro, M. A.; Grado-Caffaro, M.

2013-05-01

75

Electron charging in epitaxial Ge quantum dots on Si(100)

NASA Astrophysics Data System (ADS)

Electron confinement to heteroepitaxial Ge/Si(100) quantum dots encapsulated in a Si matrix was investigated using capacitance-voltage measurements. Optimized growth conditions produced dot ensembles comprised of either huts and pyramids or dome clusters allowing investigation of electron confinement to these distinct dot morphologies. At room temperature, 20-40 nm diameter hut and pyramid clusters confine ~0.7 electrons, while 60-80 nm diameter dome clusters confine ~6 electrons. The greater capacity of dome clusters may be attributed to the four distinct conduction band minima that are deeper than the single minimum found for pyramid clusters using a simple band structure model.

Ketharanathan, Sutharsan; Sinha, Sourabh; Shumway, John; Drucker, Jeff

2009-02-01

76

Quantum Dot Based Chemosensors: Selective Estimation of Cu2+ in Semi-aqueous Medium

NASA Astrophysics Data System (ADS)

Quantum dots are the semiconducting nanocrystals whose exicitons are restricted in all the three dimensions within a range of 2-10 nm. Due to the phenomenon called quantum confinement they have unique optical and photo physical properties which makes them useful as chemosensors. Since quantum dots provide a useful framework for the surface confinement of the receptors thereby resulting in changes in the receptor binding affinity. Therefore capturing this feature of the quantum dots different ligands are synthesized and then attached to the quantum dots. Finally studying the surface modification of the quantum dots helps us to make chemosensorsors.

Sheoran, Vandana; Saluja, Preeti; Singh, Narinder; Kaur, Navneet

2011-12-01

77

Quantum Dots for Infrared Devices

Tremendous progresses have been made in both fabrication and characterization of quantum dots, especially self-assembled dots grown by epitaxy. New physics has been discovered, e.g., unique luminescence section rules in the few-particle regime. Much efforts have been devoted to the research and development of quantum dot lasers relying on interband (conduction-to-valence) transitions and emitting in the near infrared to visible

H. C. Liu

2001-01-01

78

Induced quantum dots and wires: electron storage and delivery.

We show that quantum dots and quantum wires are formed underneath metal electrodes deposited on a planar semiconductor heterostructure containing a quantum well. The confinement is due to the self-focusing mechanism of an electron wave packet interacting with the charge induced on the metal surface. Induced quantum wires guide the transfer of electrons along metal paths and induced quantum dots store the electrons in specific locations of the nanostructure. Induced dots and wires can be useful for devices operating on the electron spin. An application for a spin readout device is proposed. PMID:18517899

Bednarek, S; Szafran, B; Dudek, R J; Lis, K

2008-03-28

79

Nonvolatile Si quantum memory with self-aligned doubly-stacked dots

We propose a novel Si dot memory whose floating gate consists of self-aligned doubly stacked Si dots. A lower Si dot exists immediately below an upper dot and lies between thin tunnel oxides. It is experimentally shown that charge retention is improved compared to the usual single-layer Si dot memory. A theoretical model considering quantum confinement and Coulomb blockade in

Ryuji Ohba; Naoharu Sugiyama; Ken Uchida; Junji Koga; Akira Toriumi

2002-01-01

80

Clinical Potential of Quantum Dots

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.

Iga, Arthur M.; Robertson, John H. P.; Winslet, Marc C.; Seifalian, Alexander M.

2007-01-01

81

Quantum dot composite light sources

NASA Astrophysics Data System (ADS)

In this effort, we present the most recent progress towards fabrication and testing of quantum dot composite materials. These materials are important steps towards easy integration of a light source on silicon. Quantum dot composites are formed by incorporating quantum dots in a host material that acts to protect and maintain the characteristics of the quantum dots, as well as to act as a patternable matrix for lithography and etching. In this effort, commercially available IR emitting PbS quantum dots (Evident Technologies) were incorporated with PMMA and poly (hexyl methacrylate) and their photoluminescence spectra was examined over time. A large shift in the photoluminescence emission peak over the course of several days was observed in all composites. This could be due to oxidation despite being in a host.

Grund, David W., Jr.; Olbricht, Benjamin C.; Prather, Dennis W.

2011-02-01

82

Quantum dot micropatterning on si.

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

Lambert, K; Moreels, I; Thourhout, D Van; Hens, Z

2008-04-29

83

QUANTUM CONFINEMENT IN NANOCRYSTALLINE SILICON

Quantum confinement effects in different kinds of nanocrystalline silicon systems are experimentally and theoretically investigated. Porous silicon structured as a nanowire network and silicon nanodots embedded in amorphous silicon dioxide are studied. The main quantum confinement effect in both cases is represented by the appearance of new energy levels in the silicon band gap. The corresponding energies can be experimentally

M. L. Ciurea

84

NASA Astrophysics Data System (ADS)

We study quantum dots defined by external potentials within finite flakes of bilayer graphene using the tight-binding approach. We find that in the limit of large flakes containing zigzag edges the dot-localized energy levels appear within the energy continuum formed by extended states. As a consequence no ionization threshold for the carriers contained within the dot exists. For smaller flakes with zigzag boundaries the dot-localized energy levels appear interlaced with the energy levels outside the flake, so in a charging experiment the electrons will be added alternately to the dot area and to its neighborhood. We demonstrate that for flakes with armchair boundaries only, an energy window accessible uniquely to the dot-localized states is opened. Then a number of electrons can be added to the dot before the external states start to be occupied. We also discuss coupling of the dot-localized states to the edge states in the context of the valley degeneracy lifting. Moreover, we extract smooth envelope wave functions from the tight-binding solution and discuss their spatial symmetries. The coupling of the dot localized energy levels with reconstructed zigzag edges and atomic vacancies present within the layers is also considered.

?ebrowski, D. P.; Wach, E.; Szafran, B.

2013-10-01

85

Efficient Quantum Dot-Quantum Dot and Quantum Dot-Dye Energy Transfer in Biotemplated Assemblies

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.

Achermann, Marc; Jeong, Sohee; Balet, Laurent; Montano, Gabriel A.; Hollingsworth, Jennifer A.

2011-01-01

86

Spin-resolved quantum-dot resonance fluorescence

Confined spins in self-assembled semiconductor quantum dots promise to serve both as probes for studying mesoscopic physics in the solid state and as stationary qubits for quantum-information science. Moreover, the excitations of self-assembled quantum dots can interact with near-infrared photons, providing an interface between stationary and `flying' qubits. Here, we report the observation of spin-selective photon emission from a resonantly

A. Nick Vamivakas; Yong Zhao; Chao-Yang Lu; Mete Atatüre

2009-01-01

87

Stability of quantum dots in live cells

Quantum dots are highly fluorescent and photostable, making them excellent tools for imaging. When using these quantum dots in cells and animals, however, intracellular biothiols (such as glutathione and cysteine) can degrade the quantum dot monolayer, compromising function. Here, we describe a label-free method to quantify the intracellular stability of monolayers on quantum dot surfaces that couples laser desorption\\/ionization mass

Zheng-Jiang Zhu; Yi-Cheun Yeh; Rui Tang; Bo Yan; Joshua Tamayo; Richard W. Vachet; Vincent M. Rotello

2011-01-01

88

Quantum confinement in Si and Ge nanostructures

NASA Astrophysics Data System (ADS)

We apply perturbative effective mass theory as a broadly applicable theoretical model for quantum confinement (QC) in all Si and Ge nanostructures including quantum wells (QWs), wires (Q-wires), and dots (QDs). Within the limits of strong, medium, and weak QC, valence and conduction band edge energy levels (VBM and CBM) were calculated as a function of QD diameters, QW thicknesses, and Q-wire diameters. Crystalline and amorphous quantum systems were considered separately. Calculated band edge levels with strong, medium, and weak QC models were compared with experimental VBM and CBM reported from X-ray photoemission spectroscopy (XPS), X-ray absorption spectroscopy (XAS), or photoluminescence (PL). Experimentally, the dimensions of the nanostructures were determined directly, by transmission electron microscopy (TEM), or indirectly, by x-ray diffraction (XRD) or by XPS. We found that crystalline materials are best described by a medium confinement model, while amorphous materials exhibit strong confinement regardless of the dimensionality of the system. Our results indicate that spatial delocalization of the hole in amorphous versus crystalline nanostructures is the important parameter determining the magnitude of the band gap expansion, or the strength of the quantum confinement. In addition, the effective masses of the electron and hole are discussed as a function of crystallinity and spatial confinement.

Barbagiovanni, E. G.; Lockwood, D. J.; Simpson, P. J.; Goncharova, L. V.

2012-02-01

89

Resonant tunnelling features in quantum dots

NASA Astrophysics Data System (ADS)

We present a systematic review of features due to resonant electron tunnelling, observable in transport spectroscopy experiments on quantum dots and single donors. The review covers features attributable to intrinsic properties of the dot (orbital, spin and valley states) as well as extrinsic effects (phonon/photon emission/absorption, features in the charge reservoirs, coupling to nearby charge centres). We focus on the most common operating conditions, neglecting effects due to strong coupling to the leads. By discussing the experimental signatures of each type of feature, we aim at providing practical methods to distinguish between their different physical origins. The correct classification of the resonant tunnelling features is an essential requirement to understand the details of the confining potential or to predict the performance of the dot for quantum information processing.

Escott, C. C.; Zwanenburg, F. A.; Morello, A.

2010-07-01

90

Dynamical localization of double quantum dots with two levels

We study the dynamics of a two-level double quantum dot system under the action of ac electric field, in which two electrons are confined. The results show that, although these electrons always transit between different energy levels, they can be localized in one dot with appropriate parameters. The localization can be significantly influenced by the external parameters, which provides an

Zhe Jiang; Duan Suqing; Xian-Geng Zhao

2005-01-01

91

Carrier Dynamics in Colloidal Graphene Quantum Dots

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

Cheng Sun; Xin Yan; Liang-Shi Li; John A. McGuire

2011-01-01

92

Pinned Magnetic Impurity Levels in Doped Quantum Dots

Spectroscopic data demonstrate that impurity D\\/A levels in doped semiconductor nanostructures are energetically pinned, resulting in variations in D\\/A binding energies with increasing quantum confinement. Using magnetic circular dichroism spectroscopy, the donor binding energies of Co2+ ions in colloidal ZnSe quantum dots have been measured as a function of quantum confinement and analyzed in conjunction with ab initio density functional

Nick S. Norberg; Gustavo M. Dalpian; James R. Chelikowsky; Daniel R. Gamelin

2006-01-01

93

Electron charging in epitaxial germanium quantum dots on silicon (100)

NASA Astrophysics Data System (ADS)

The electron charging behavior of self assembled epitaxial Ge quantum dots on Si(100) grown using molecular beam epitaxy has been studied. Ge quantum dots encapsulated in n-type Si matrix were incorporated into Schottky diodes to investigate their charging behavior using capacitance-voltage measurements. These experimental results were interpreted in the context of theoretical models to assess the degree of charge localization to the dot. Experiments involving Ge quantum dot growth, growth of Sb-doped Si and morphological evolution during encapsulation of the Ge dots during Si overgrowth were performed in order to optimize the conditions for obtaining distinct Ge quantum dot morphologies. This investigation included finding a suitable method to minimize Sb segregation while maintaining good dot epitaxy and overall crystal quality. Holes are confined to the Ge dots for which the valence band offsets are large (˜650 meV). Electrons are confined to the strained Si regions adjacent to the Ge quantum dots which have relatively smaller confinement potentials (˜100--150 meV). Experimentally, it was found that but and pyramid clusters in the range from 20--40 nm in diameter confine ˜1electron per dot while dome clusters in the range from 60--80 nm diameter confine ˜6--8 electrons per dot. Theoretical simulations predict that similar pyramid structures confine ˜0.4 electrons per dot and dome structures confine ˜2.2--3 electrons per dot. Even though the theory and the experimental results disagree due to various uncertainties and approximations, the ratio between theory and experiment agree remarkably well for both island types. We also investigated constructive three-dimensional nanolithography. Nanoscale Au rich dots and pure Ge dots were deposited on SiO2 and Si3N4 substrates by decomposing adsorbed precursors using a focused electron beam in an environmental transmission electron microscope. Dimethyl acetylacetonate gold was used for Au and digermane was used to deposit Ge. Deposited feature diameters were larger than that of the electron beam used for deposition by an amount comparable to the secondary electron escape depth. Real-time composition analysis during growth using electron energy-loss spectroscopy indicates that mature Au deposits entrain significant carbon while Ge deposits are pure with C contents below the carbon detection limit of less than 18%.

Ketharanathan, Sutharsan

94

Quantum dots are semiconductor nanocrystals that have broad excitation spectra, narrow emission spectra, tunable emission peaks, long fluorescence lifetimes, negligible photobleaching, and ability to be conjugated to proteins, making them excellent probes for bioimaging applications. Here the author reviews the advantages and disadvantages of using quantum dots in bioimaging applications, such as single-particle tracking and fluorescence resonance energy transfer, to study receptor-mediated transport.

Barroso, Margarida M.

2011-01-01

95

Long-wavelength infrared quantum-dot based interband photodetectors

We report on the design and fabrication of (Al)GaAs(Sb)\\/InAs tensile strained quantum-dot (QD) based detector material for thermal infrared imaging applications in the long-wavelength infrared (LWIR) regime. The detection is based on transitions between confined dot states and continuum states in a type-II band lineup, and we therefore refer to it as a dot-to-bulk (D2B) infrared photodetector with expected benefits

O. Gustafsson; J. Berggren; U. Ekenberg; A. Hallén; M. Hammar; L. Höglund; A. Karim; B. Noharet; Q. Wang; A. Gromov; S. Almqvist; A. Zhang; S. Junique; J. Y. Andersson; C. Asplund; R. Marcks von Würtemberg; H. Malm; H. Martijn

2011-01-01

96

Planar Dirac electrons in magnetic quantum dots.

In this paper, we explore the size- and mass-dependent energy spectra and the electronic correlation of two- and three-electron graphene magnetic quantum dots. It is found that only the magnetic dots with large size can well confine the electrons. For large graphene magnetic dots with massless (ultra-relativity) electrons, the energy level structures of two Dirac electrons and even the ground state spin and angular momentum of three electrons are quite different from those of the usual semiconductor quantum dots. Also we reveal that such differences are not due to the magnetic confinement but originate from the character of the Coulomb interaction of two-component electronic wavefunctions in graphene. We reveal that the increase of the mass leads to both the crossover of the energy spectrum structures from the ultra-relativity to non-relativity ones and the increasing of the crystallization. The results are helpful for the understanding of the mass and size effects and may be useful in controlling the few-electron states in graphene-based nanodevices. PMID:22543306

Yang, Ning; Zhu, Jia-Lin

2012-04-27

97

Emission in Mn-Doped Quantum Dot

NASA Astrophysics Data System (ADS)

We theoretically investigate the magneto-PL of Mn2+doped semiconductor core-shell colloidal quantum dot to explain the experiment result from a recent magnetophotoluminescence study of strongly confined diluted magnetic semiconductor (DMS) in Mn2+-doped ZnSe/CdSe core-shell colloidal nanocrystals. The yellow emission characterized for in Mn2+-which is associated with the d-d internal transition 4T1-6A1, was reported not suppressed in an applied B //z magnetic field and unpolarized as usual and instead, a Mn PL circular polarization has been observed. The in Mn2+- photoluminescence has been found to have a large splitting between ? + and ? {- }components which depends on the applied field. We show that this behavior, which has not been found in characteristics of the Mn2+ PL in bulks and other conventional DMS materials, is the result of the strong confinement of the nanocrystal and its properties. Our theory and calculation show that the reasons the yellow Mn2+ PL band in quantum dots is not suppressed under applied magnetic field originate due to the existence of the internal piezoelectric dipole moment and the Coulomb exchange interaction of the impurity ions with the confined electrons inside the dot.

Huong Nguyen, Que; Birman, Joseph L.

2013-03-01

98

Energy levels in self-assembled quantum arbitrarily shaped dots.

A model to determine the electronic structure of self-assembled quantum arbitrarily shaped dots is applied. This model is based principally on constant effective mass and constant potentials of the barrier and quantum dot material. An analysis of the different parameters of this model is done and compared with those which take into account the variation of confining potentials, bands, and effective masses due to strain. The results are compared with several spectra reported in literature. By considering the symmetry, the computational cost is reduced with respect to other methods in literature. In addition, this model is not limited by the geometry of the quantum dot. PMID:15740390

Tablero, C

2005-02-01

99

Electron-phonon interaction in a cylindrical quantum dot

The confined logitudinal-optical (LO) phonon and surface-optical (SO) phonon modes of a free-standing cylindrical quantum dot are discussed within the dielectric continuu approximation. It is found that there exist two types of SO phonon modes: top SO (TSO) mode and side SO (SSO) in cylindrical quantum dot. The operators describing the free phonon modes and their interactions with electrons in

Wai-Sang Li; Chuan-Yu Chen

1997-01-01

100

Electron-phonon interaction in a cylindrical quantum dot

NASA Astrophysics Data System (ADS)

The confined logitudinal-optical (LO) phonon and surface-optical (SO) phonon modes of a free-standing cylindrical quantum dot are discussed within the dielectric continuum approximation. It is found that there exist two types of SO phonon modes: top SO (TSO) mode and side SO (SSO) in a cylindrical quantum dot. The operators describing the free phonon modes and their interactions with electrons in the system are also derived.

Li, Wai-Sang; Chen, Chuan-Yu

1997-03-01

101

Stability of quantum dots in live cells

NASA Astrophysics Data System (ADS)

Quantum dots are highly fluorescent and photostable, making them excellent tools for imaging. When using these quantum dots in cells and animals, however, intracellular biothiols (such as glutathione and cysteine) can degrade the quantum dot monolayer, compromising function. Here, we describe a label-free method to quantify the intracellular stability of monolayers on quantum dot surfaces that couples laser desorption/ionization mass spectrometry with inductively coupled plasma mass spectrometry. Using this new approach we have demonstrated that quantum dot monolayer stability is correlated with both quantum dot particle size and monolayer structure, with appropriate choice of both particle size and ligand structure required for intracellular stability.

Zhu, Zheng-Jiang; Yeh, Yi-Cheun; Tang, Rui; Yan, Bo; Tamayo, Joshua; Vachet, Richard W.; Rotello, Vincent M.

2011-12-01

102

Diamagnetic Exciton Properties in Quantum Dot Molecules

NASA Astrophysics Data System (ADS)

The magnetic properties of nanostructures like quantum dots and rings are the subject of intense research. In particular, magnetic control of coupled quantum dots (artificial molecules) has become subject of interest. The diamagnetic shift of confined excitons complexes has been used as a measured of the wave function spatial extent in semiconductor nanostructures. In weak magnetic field, the diamagnetic shift is expected to exhibit quadratic dependence. However, for exciton complexes the diamagnetic behavior is expected to exhibit more complicated features related to electron-hole asymmetry effects on Coulomb interactions. In this work we study the magnetic response of neutral and charged excitons in InAs/GaAs asymmetric artificial molecules By using a first order perturbation approach, and within the effective mass approximation, we calculate magnetic field dependent electronic structures of confined excitons and trions in vertically coupled quantum dots. These predicted regions, which show coexistence of crossing and anticrossing exciton states, because of allowing control of charge localization and polarization of emitted photons. .

Ricardo, Fino Puerto Nelson; Hanz Yecid, Ramirez; S, Camacho Angela

2012-02-01

103

Silicon quantum dot nanostructures for tandem photovoltaic cells

Tandem PV cells – with their increased efficiency due to a multi-band gap approach – usually involve expensive materials and fabrication. Thin film approaches, with an engineered variation in band gap through the use of quantum confinement in Si quantum dots, offer a cheaper alternative. Presented are characterisation and modelling data on fabrication of such Si and Sn QD nanostructures

Gavin Conibeer; Martin Green; Eun-Chel Cho; Dirk König; Young-Hyun Cho; Thipwan Fangsuwannarak; Giuseppe Scardera; Edwin Pink; Yidan Huang; Tom Puzzer; Shujuan Huang; Dengyuan Song; Chris Flynn; Sangwook Park; Xiaojing Hao; Daniel Mansfield

2008-01-01

104

Synthesis and characterization of aqueous quantum dots for biomedical applications

Quantum Dots (QDs) are semiconductor nanocrystals (1˜20 nm) exhibiting distinctive photoluminescence (PL) properties due to the quantum confinement effect. Having many advantages over organic dyes, such as broad excitation and resistance to photobleaching, QDs are widely used in bioapplications as one of most exciting nanobiotechnologies. To date, most commercial QDs are synthesized through the traditional organometallic method and contain toxic

Hui Li

2008-01-01

105

Quantum entanglement and teleportation in quantum dot

We study the thermal entanglement and quantum teleportation using quantum dot as a resource. We first consider entanglement of the resource, and then focus on the effects of different parameters on the teleportation fidelity under different conditions. The critical temperature of disentanglement is obtained. Based on Bell measurements in two subspaces, we find the anisotropy measurements is optimal to the

Li-Guo Qin; Li-Jun Tian; Guo-Hong Yang

2011-01-01

106

Growth and Characterization of Quantum Dots and Quantum Dots Devices

Quantum dot nanostructures were investigated experimentally and theoretically for potential applications for optoelectronic devices. We have developed the foundation to produce state-of-the-art compound semiconductor nanostructures in a variety of materials: In(AsSb) on GaAs, GaSb on GaAs, and In(AsSb) on GaSb. These materials cover a range of energies from 1.2 to 0.7 eV. We have observed a surfactant effect in InAsSb nanostructure growth. Our theoretical efforts have developed techniques to look at the optical effects induced by many-body Coulombic interactions of carriers in active regions composed of quantum dot nanostructures. Significant deviations of the optical properties from those predicted by the ''atom-like'' quantum dot picture were discovered. Some of these deviations, in particular, those relating to the real part of the optical susceptibility, have since been observed in experiments.

CEDERBERG, JEFFREY G.; BIEFELD, ROBERT M.; SCHNEIDER, H.C.; CHOW, WENG W.

2003-04-01

107

Supercurrent reversal in quantum dots

NASA Astrophysics Data System (ADS)

When two superconductors are electrically connected by a weak link-such as a tunnel barrier-a zero-resistance supercurrent can flow. This supercurrent is carried by Cooper pairs of electrons with a combined charge of twice the elementary charge, e. The 2e charge quantum is clearly visible in the height of voltage steps in Josephson junctions under microwave irradiation, and in the magnetic flux periodicity of h/2e (where h is Planck's constant) in superconducting quantum interference devices. Here we study supercurrents through a quantum dot created in a semiconductor nanowire by local electrostatic gating. Owing to strong Coulomb interaction, electrons only tunnel one-by-one through the discrete energy levels of the quantum dot. This nevertheless can yield a supercurrent when subsequent tunnel events are coherent. These quantum coherent tunnelling processes can result in either a positive or a negative supercurrent, that is, in a normal or a ?-junction, respectively. We demonstrate that the supercurrent reverses sign by adding a single electron spin to the quantum dot. When excited states of the quantum dot are involved in transport, the supercurrent sign also depends on the character of the orbital wavefunctions.

van Dam, Jorden A.; Nazarov, Yuli V.; Bakkers, Erik P. A. M.; de Franceschi, Silvano; Kouwenhoven, Leo P.

2006-08-01

108

A two-electron system confined in two coupled semiconductor quantum dots is investigated as a candidate for performing quantum logic operations with spin qubits. We study different processes of swapping the electron spins by a controlled switching on and off of the exchange interaction. The resulting spin swap corresponds to an elementary operation in quantum-information processing. We perform direct simulations of

S. Moskal; S. Bednarek; J. Adamowski

2007-01-01

109

NASA Astrophysics Data System (ADS)

Quantum-dot Cellular Automata (QCA) [1] is a promising architecture which employs quantum dots for digital computation. It is a revolutionary approach that holds the promise of high device density and low power dissipation. A basic QCA cell consists of four quantum dots coupled capacitively and by tunnel barriers. The cell is biased to contain two excess electrons within the four dots, which are forced to opposite "corners" of the four-dot cell by mutual Coulomb repulsion. These two possible polarization states of the cell will represent logic "0" and "1". Properly arranged, arrays of these basic cells can implement Boolean logic functions. Experimental results from functional QCA devices built of nanoscale metal dots defined by tunnel barriers will be presented. The experimental devices to be presented consist of Al islands, which we will call quantum dots, interconnected by tunnel junctions and lithographically defined capacitors. Aluminum/ aluminum-oxide/aluminum tunnel junctions were fabricated using a standard e-beam lithography and shadow evaporation technique. The experiments were performed in a dilution refrigerator at a temperature of 70 mK. The operation of a cell is evaluated by direct measurements of the charge state of dots within a cell as the input voltage is changed. The experimental demonstration of a functioning cell will be presented. A line of three cells demonstrates that there are no metastable switching states in a line of cells. A QCA majority gate will also be presented, which is a programmable AND/OR gate and represents the basic building block of QCA systems. The results of recent experiments will be presented. 1. C.S. Lent, P.D. Tougaw, W. Porod, and G.H. Bernstein, Nanotechnology, 4, 49 (1993).

Snider, Gregory

2000-03-01

110

Mid-infrared quantum dot emitters utilizing planar photonic crystal technology

The three-dimensional confinement inherent in InAs self-assembled quantum dots (SAQDs) yields vastly different optical properties compared to one-dimensionally confined quantum well systems. Intersubband transitions in quantum dots can emit light normal to the growth surface, whereas transitions in quantum wells emit only parallel to the surface. This is a key difference that can be exploited to create a variety of

Ganapathi Subramanian Subramania; Sungkwun Kenneth Lyo; Jeffrey George Cederberg; Brandon Scott Passmore; Ihab Fathy El-Kady; Eric Arthur Shaner

2008-01-01

111

Researchers at the National Renewable Energy Laboratory (NREL) have certified the first all-quantum-dot photovoltaic cell, which was based on lead sulfide and demonstrated reasonable quantum dot solar cell performance for an initial efficiency measurement along with good stability. The certified open-circuit voltage of the quantum dot cell is greater than that possible from bulk lead sulfide because of quantum confinement.

Not Available

2011-02-01

112

Applications of quantum dots in cell biology

Quantum dots promise to revolutionize the way fluorescence imaging is used in the Cell Biology field. The unique fluorescent spectral characteristics, high photostability, low photobleaching and tight emission spectra of quantum dots, position them above traditional dyes. Here we will address the ability of EviTags, which are water stabilized quantum dot products from Evident Technologies, to behave as effective FRET

Margarida Barroso; Roshanak Mehdibeigi; Louise Brogan

2006-01-01

113

Synthesis and applications of quantum dots and magnetic quantum dots

We have developed a new synthetic method for producing high-quality quantum dots (QDs) in aqueous solution for biological imaging applications. The glutathione-capped CdTe, ZnSe and Zn1-xCdxSe alloyed QDs derived are tunable in fluorescence emissions between 360 nm and 700 nm. They show high quantum yields (QYs) of up to 50%, with narrow bandwidths of 19-55 nm. The synthesis of glutathione-capped

Jackie Y. Ying; Yuangang Zheng; S. Tamil Selvan

2008-01-01

114

Mesoscopic cavity quantum electrodynamics with quantum dots

We describe an electrodynamic mechanism for coherent, quantum-mechanical coupling between spatially separated quantum dots on a microchip. The technique is based on capacitive interactions between the electron charge and a superconducting transmission line resonator, and is closely related to atomic cavity quantum electrodynamics. We investigate several potential applications of this technique which have varying degrees of complexity. In particular, we demonstrate that this mechanism allows design and investigation of an on-chip double-dot microscopic maser. Moreover, the interaction may be extended to couple spatially separated electron-spin states while only virtually populating fast-decaying superpositions of charge states. This represents an effective, controllable long-range interaction, which may facilitate implementation of quantum information processing with electron-spin qubits and potentially allow coupling to other quantum systems such as atomic or superconducting qubits.

Childress, L.; Soerensen, A.S.; Lukin, M.D. [Department of Physics, Harvard University, Cambridge, Massachusetts 02138 (United States); Department of Physics, Harvard University, Cambridge, Massachusetts 02138, USA and ITAMP, Harvard-Smithsonian Center for Astrophysics, Cambridge, Massachusetts 02138 (United States)

2004-04-01

115

Binding energy of hydrogenic impurities in polar cylindrical quantum dot

NASA Astrophysics Data System (ADS)

We have studied the polaron effect on the binding energy of shallow hydrogenic impurities in a cylindrical quantum dot using a variational approach for the finite confinement potential. The interactions of charge carriers (electron and ion) with both the confined LO phonon and surface phonons (SSO and TSO) are taken into account. The effect of these three phonon modes on the binding energy of a shallow donor is examined. The emphasis is placed on the dependence of the polaronic corrections on the quantum dot size. It is found that the correction due to the LO phonons is more important than that due to the surface optical phonon modes.

Charrour, R.; Bouhassoune, M.; Fliyou, M.; Bria, D.; Nougaoui, A.

2000-06-01

116

Control of valence band states in pyramidal quantum dot-in-dot semiconductor heterostructures

NASA Astrophysics Data System (ADS)

The character of the hole states in a pyramidal GaAs/AlGaAs quantum dot-in-dot (DiD) heterostructure is shown to be controllable by tailoring the confinement potential shape. The change in ground valence band state from heavy hole like to light hole like is demonstrated by side-view polarization resolved photoluminescence measurements. The experimental findings are supported by three-dimensional numerical model calculations. The results are applicable for polarization control in quantum dot photonic devices.

Troncale, V.; Karlsson, K. F.; Pelucchi, E.; Rudra, A.; Kapon, E.

2007-12-01

117

A Review of Quantum Confinement

NASA Astrophysics Data System (ADS)

A succinct history of the Confined Atom problem is presented. The hydrogen atom confined to the centre of an impenetrable sphere counts amongst the exactly soluble problems of physics, alongside much more noted exact solutions such as Black Body Radiation and the free Hydrogen atom in absence of any radiation field. It shares with them the disadvantage of being an idealisation, while at the same time encapsulating in a simple way particular aspects of physical reality. The problem was first formulated by Sommerfeld and Welker [1]-henceforth cited as SW-in connection with the behaviour of atoms at very high pressures, and the solution was published on the occasion of Pauli's 60th birthday celebration. At the time, it seemed that there was not much other connection with physical reality beyond a few simple aspects connected to the properties of atoms in solids, for which more appropriate models were soon developed. Thus, confined atoms attracted little attention until the advent of the metallofullerene, which provided the first example of a confined atom with properties quite closely related to those originally considered by SW. Since then, the problem has received much more attention, and many more new features of quantum confinement, quantum compression, the quantum Faraday cage, electronic reorganisation, cavity resonances, etc have been described, which are relevant to real systems. Also, a number of other situations have been uncovered experimentally to which quantum confinement is relevant. Thus, studies of the confined atom are now more numerous, and have been extended both in terms of the models used and the systems to which they can be applied. Connections to thermodynamics are explored through the properties of a confined two-level atom adapted from Einstein's celebrated model, and issues of dynamical screening of electromagnetic radiation by the confining shell are discussed in connection with the Faraday cage produced by a confining conducting shell. The conclusions are shown to be relevant to a proposed `quantum computer'. The description of the actual geometry of C60, as opposed to a purely spherical approximation, leads to some qualification of the computed results.

Connerade, Jean-Patrick

2009-12-01

118

Zeno-logic applications of semiconductor quantum dots

Microscopic calculations show that CdSe-based semiconductor quantum dots with confined exciton and biexciton states are suitable candidates for Zeno-logic applications. The frequencies of the control and signal fields are chosen to guarantee very high transmission of the individual beams. If both fields are present simultaneously, they are strongly absorbed due to efficient ground-state-to-biexciton transitions. The optical Bloch equations for a three-level quantum-dot model with self-consistent light-matter coupling are solved numerically. The influence of dephasing and/or inhomogeneous dot distributions is analyzed and the conditions for satisfactory device operation are identified.

Schneebeli, L.; Peyghambarian, N. [Optical Sciences Center, University of Arizona, Tucson, Arizona 85721 (United States); Feldtmann, T.; Kira, M. [Department of Physics and Material Sciences Center, Philipps-University, D-35032 Marburg (Germany); Koch, S. W. [Optical Sciences Center, University of Arizona, Tucson, Arizona 85721 (United States); Department of Physics and Material Sciences Center, Philipps-University, D-35032 Marburg (Germany)

2010-05-15

119

NASA Astrophysics Data System (ADS)

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.

Vagov, A.; Croitoru, M. D.; Glässl, M.; Axt, V. M.; Kuhn, T.

2011-03-01

120

Dependence of quantum-dot formation on substrate orientation studied by magnetophotoluminescence

NASA Astrophysics Data System (ADS)

We have investigated the substrate orientation-dependence of InAs/GaAs quantum dot growth by photoluminescence spectroscopy in very high magnetic fields. An abrupt change from one-dimensional to three-dimensional charge confinement is observed for InAs deposited on (100) GaAs. On the tilted (311)B substrates, the quantum dot morphology is different, resulting in a weaker charge confinement that gradually increases with the amount of deposited InAs. At 1.9 monolayers, the quantum-dot confinement on this substrate orientation is as effective as for the (100) oriented substrates. By studying the confinement of the charges in samples with quantum dots at different stages of development, we are able to give insight into the quantum-dot formation process.

Maes, J.; Hayne, M.; Moshchalkov, V. V.; Patane, A.; Henini, M.; Eaves, L.; Main, P. C.

2002-08-01

121

Probing of Unembedded Metallic Quantum Dots with Positrons

We employed the two detector coincident Doppler Broadening Technique (coPAS) to investigate Ag, Au and Ag/Au alloy quantum dots of varying sizes which were deposited in thin layers on glass slides. The Ag quantum dots range from 2 to 3 nm in diameter, while the Ag/Au alloy quantum dots exhibit Ag cores of 2 nm and 3 nm and Au shells of varying thickness. We investigate the possibility of positron confinement in the Ag core due to positron affinity differences between Ag and Au. We describe the results and their significance to resolving the issue of whether positrons annihilate within the quantum dot itself or whether surface and positron escape effects play an important role.

Fischer, C G; Denison, A B; Weber, M H; Wilcoxon, J P; Woessner, S; Lynn, K G

2003-08-01

122

CORRELATIONS IN CONFINED QUANTUM PLASMAS

This is the final report for the project 'Correlations in Confined Quantum Plasmas', NSF-DOE Partnership Grant DE FG02 07ER54946, 8/1/2007 - 7/30/2010. The research was performed in collaboration with a group at Christian Albrechts University (CAU), Kiel, Germany. That collaboration, almost 15 years old, was formalized during the past four years under this NSF-DOE Partnership Grant to support graduate students at the two institutions and to facilitate frequent exchange visits. The research was focused on exploring the frontiers of charged particle physics evolving from new experimental access to unusual states associated with confinement. Particular attention was paid to combined effects of quantum mechanics and confinement. A suite of analytical and numerical tools tailored to the specific inquiry has been developed and employed

DUFTY J W

2012-01-11

123

Quantum-dot heterostructure lasers

Quantum-dot (QD) heterostructures are nanoscale coherent insertions of narrow-gap material in a single-crystalline matrix. These tiny structures provide unique opportunities to modify and extend all basic principles of heterostructure lasers and advance their applications. Despite early predictions, fabrication of QD heterostructure (QDHS) lasers appeared to be a much more challenging task, as compared to quantum well (QW) devices. The breakthrough

Nikolai N. Ledentsov; M. Grundmann; F. Heinrichsdorff; Dieter Bimberg; V. M. Ustinov; A. E. Zhukov; M. V. Maximov; Zh. I. Alferov; J. A. Lott

2000-01-01

124

Pinned Magnetic Impurity Levels in Doped Quantum Dots

Spectroscopic data demonstrate that impurity D\\/A levels in doped\\u000asemiconductor nanostructures are energetically pinned, resulting in variations\\u000ain D\\/A binding energies with increasing quantum confinement. Using magnetic\\u000acircular dichroism spectroscopy, the donor binding energies of Co2+ ions in\\u000acolloidal ZnSe quantum dots have been measured as a function of quantum\\u000aconfinement and analyzed in conjunction with ab initio density functional

Nick S. Norberg; Gustavo M. Dalpian; James R. Chelikowsky; Daniel R. Gamelin

2006-01-01

125

Quantum Dots for Molecular Pathology

Assessing malignant tumors for expression of multiple biomarkers provides data that are critical for patient management. Quantum dot-conjugated probes to specific biomarkers are powerful tools that can be applied in a multiplex manner to single tissue sections of biopsies to measure expression levels of multiple biomarkers.

True, Lawrence D.; Gao, Xiaohu

2007-01-01

126

Colloidal quantum dot solar cells

NASA Astrophysics Data System (ADS)

Solar cells based on solution-processed semiconductor nanoparticles -- colloidal quantum dots -- have seen rapid advances in recent years. By offering full-spectrum solar harvesting, these cells are poised to address the urgent need for low-cost, high-efficiency photovoltaics.

Sargent, Edward H.

2012-03-01

127

Quantization of multiparticle auger rates in semiconductor quantum dots

We have resolved single-exponential relaxation dynamics of the 2-, 3-, and 4-electron-hole pair states in nearly monodisperse cadmium selenide quantum dots with radii ranging from 1 to 4 nanometers. Comparison of the discrete relaxation constants measured for different multiple-pair states indicates that the carrier decay rate is cubic in carrier concentration, which is characteristic of an Auger process. We observe that in the quantum-confined regime, the Auger constant is strongly size-dependent and decreases with decreasing the quantum dot size as the radius cubed. PMID:10669406

Klimov; Mikhailovsky; McBranch; Leatherdale; Bawendi

2000-02-11

128

Modeling and fabrication of electrically tunable quantum dot intersubband devices

NASA Astrophysics Data System (ADS)

We propose an idea of forming quantum dot intersubband transition devices based on lateral electrical confinement on quantum wells. Numerical simulations show that the energy level separation in the structure can be as large as about 50 meV, and with different electric field, the energy levels can be tuned. We also demonstrate the fabrication of a large number of field-induced quantum dots by our super lens lithography technique. We achieved uniform arrays of contacts that are about 200 nm using a conventional UV source of ?~400 nm.

Wu, Wei; Dey, Dibyendu; Memis, Omer G.; Mohseni, Hooman

2009-05-01

129

Charge transfer magnetoexciton formation at vertically coupled quantum dots.

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

Gutiérrez, Willian; Marin, Jairo H; Mikhailov, Ilia D

2012-10-23

130

Artificial Arom in Carbon Nanotube Quantum Dots and its Quantum Teraherz Response

Quantum dots have been fabricated in an individual single-wall carbon nanotube (SWCNT), and single electron transport measurements have been carried out at low temperatures. It is demonstrated that the SWCNT quantum-dot can be considered as an artificial atom where electrons are confined in a one-dimensional hard-wall potential. A shell structure and shell filling in magnetic fields are shown, and the

Koji Ishibashi; Satoshi Moriyama; Tomoko Fuse; Yukio Kawano; Seiko Toyokawa; Tomohiro Yamaguchi

2008-01-01

131

Hole storage in GaSb\\/GaAs quantum dots for memory devices

The hole confinement of self-organized GaSb\\/GaAs quantum dots embedded in n+p-diodes is investigated experimentally by admittance spectroscopy. The highest thermal activation energy obtained, 400 meV, refers to only weakly charged quantum dots. Detailed bias-dependent investigations allow to study state- filling and Coulomb charging effects. State filling lowers the activation energy down to 150 meV in quantum dots charged with the

M. Geller; C. Kapteyn; L. Müller-Kirsch; R. Heitz; D. Bimberg

2003-01-01

132

Size tunable visible and near-infrared photoluminescence from vertically etched silicon quantum dots

NASA Astrophysics Data System (ADS)

Corrugated etching techniques were used to fabricate size-tunable silicon quantum dots that luminesce under photoexcitation, tunable over the visible and near infrared. By using the fidelity of lithographic patterning and strain limited, self-terminating oxidation, uniform arrays of pillar containing stacked quantum dots as small as 2 nm were patterned. Furthermore, an array of pillars, with multiple similar sized quantum dots on each pillar, was fabricated and tested. The photoluminescence displayed a multiple, closely peaked emission spectra corresponding to quantum dots with a narrow size distribution. Similar structures can provide quantum confinement effects for future nanophotonic and nanoelectronic devices.

Walavalkar, Sameer S.; Homyk, Andrew P.; Hofmann, Carrie E.; Henry, M. David; Shin, Claudia; Atwater, Harry A.; Scherer, Axel

2011-04-01

133

Dot-Height Dependence of Photoluminescence from ZnO Quantum Dots

NASA Astrophysics Data System (ADS)

We report on the optical properties of ZnO nanodots and how they are influenced by the height of the dot configuration. The nanodots were grown on SiO2/Si substrates by remote-plasma enhanced metalorganic chemical vapor deposition (RPE-MOCVD). The dot configuration was regulated by controlling the growth time. The photoluminescence (PL) spectra of nanodots having average dot height of 2.5, 3.5, and 4.5 nm grown on SiO2/Si substrates were measured at 16 K. A blue shift in the excitonic emission was observed for shorter dots due to a larger quantum confinement effect. The fractional-dimensional space model was used to estimate a dimensionality of 2.35 for ZnO nano dots with an average dot height of 2.5 nm.

Nakamura, Atsushi; Okamatsu, Kota; Tawara, Takehiko; Gotoh, Hideki; Temmyo, Jiro; Matsui, Yoshio

2008-04-01

134

Low Disorder Si MOSFET Dots for Quantum Computing

NASA Astrophysics Data System (ADS)

Silicon quantum dot based qubits have emerged as an appealing approach to extending the success of GaAs spin based double quantum dot qubits. Research in this field is motivated by the promise of long spin coherence times, and within a MOS system the potential for variable carrier density, very small dot sizes, and CMOS compatibility. In this work, we will present results on the fabrication and transport properties of quantum dots in novel double gated Si MOS structures. Coulomb blockade is observed from single quantum dots with extracted charging energies up to an including 5meV. Observed dots were formed both from disorder within a quantum point contact, and through disorder free electrostatic confinement. Extracted capacitances, verified with 3D finite element simulations confirm the location of the disorder free dot to be within the designed lithographic structure. Distinctions will be made regarding the effects of feature sizes and sample processing. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000.

Nordberg, E. P.; Tracy, L. A.; Ten Eyck, G. A.; Eng, K.; Stalford, H. L.; Childs, K. D.; Stevens, J.; Grubbs, R. K.; Lilly, M. P.; Eriksson, M. A.; Carroll, M. S.

2009-03-01

135

Zero-energy states in graphene quantum dots and rings

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.

Downing, C. A.; Stone, D. A. [School of Physics, University of Exeter, Stocker Road, Exeter EX4 4QL (United Kingdom); Portnoi, M. E. [School of Physics, University of Exeter, Stocker Road, Exeter EX4 4QL (United Kingdom); International Institute of Physics, Av. Odilon Gomes de Lima, 1722, Capim Macio, CEP: 59078-400, Natal - RN (Brazil)

2011-10-15

136

Single Dot Spectroscopy of Two-Color Quantum Dot\\/Quantum Shell Nanostructures

Single dot spectroscopy is performed on two-color CdSe\\/ZnS\\/CdSe core\\/barrier\\/shell nanostructures. Unlike quantum dots cores, these systems have two phases with which to emit and ultimately examine for blinking analysis. These particles are brighter than conventional quantum dots and also show the photoluminescence (PL) intensity and energy fluctuations characteristic of quantum dots. Single dot spectral diffusion analysis yields no measureable energy

Eva A. Dias; Amy F. Grimes; Douglas S. English; Patanjali Kambhampati

2008-01-01

137

Luminescence upconversion in colloidal double quantum dots

NASA Astrophysics Data System (ADS)

Luminescence upconversion nanocrystals capable of converting two low-energy photons into a single photon at a higher energy are sought-after for a variety of applications, including bioimaging and photovoltaic light harvesting. Currently available systems, based on rare-earth-doped dielectrics, are limited in both tunability and absorption cross-section. Here we present colloidal double quantum dots as an alternative nanocrystalline upconversion system, combining the stability of an inorganic crystalline structure with the spectral tunability afforded by quantum confinement. By tailoring its composition and morphology, we form a semiconducting nanostructure in which excited electrons are delocalized over the entire structure, but a double potential well is formed for holes. Upconversion occurs by excitation of an electron in the lower energy transition, followed by intraband absorption of the hole, allowing it to cross the barrier to a higher energy state. An overall conversion efficiency of 0.1% per double excitation event is achieved.

Deutsch, Zvicka; Neeman, Lior; Oron, Dan

2013-09-01

138

Luminescence upconversion in colloidal double quantum dots.

Luminescence upconversion nanocrystals capable of converting two low-energy photons into a single photon at a higher energy are sought-after for a variety of applications, including bioimaging and photovoltaic light harvesting. Currently available systems, based on rare-earth-doped dielectrics, are limited in both tunability and absorption cross-section. Here we present colloidal double quantum dots as an alternative nanocrystalline upconversion system, combining the stability of an inorganic crystalline structure with the spectral tunability afforded by quantum confinement. By tailoring its composition and morphology, we form a semiconducting nanostructure in which excited electrons are delocalized over the entire structure, but a double potential well is formed for holes. Upconversion occurs by excitation of an electron in the lower energy transition, followed by intraband absorption of the hole, allowing it to cross the barrier to a higher energy state. An overall conversion efficiency of 0.1% per double excitation event is achieved. PMID:23912060

Deutsch, Zvicka; Neeman, Lior; Oron, Dan

2013-08-04

139

TOPICAL REVIEW: Quantum dots and spin qubits in graphene

NASA Astrophysics Data System (ADS)

This is a review on graphene quantum dots and their use as a host for spin qubits. We discuss the advantages but also the challenges to use graphene quantum dots for spin qubits as compared to the more standard materials like GaAs. We start with an overview of this young and fascinating field and then discuss gate-tunable quantum dots in detail. We calculate the bound states for three different quantum dot architectures where a bulk gap allows for confinement via electrostatic fields: (i) graphene nanoribbons with armchair boundaries, (ii) a disc in single-layer graphene, and (iii) a disc in bilayer graphene. In order for graphene quantum dots to be useful in the context of spin qubits, one needs to find reliable ways to break the valley degeneracy. This is achieved here, either by a specific termination of graphene in (i) or in (ii) and (iii) by a magnetic field, without the need of a specific boundary. We further discuss how to manipulate spin in these quantum dots and explain the mechanism of spin decoherence and relaxation caused by spin-orbit interaction in combination with electron-phonon coupling, and by hyperfine interaction with the nuclear-spin system.

Recher, Patrik; Trauzettel, Björn

2010-07-01

140

Gate-defined Quantum Confinement in Suspended Bilayer Graphene

NASA Astrophysics Data System (ADS)

Quantum confined devices in carbon-based materials offer unique possibilities for applications ranging from quantum computation to sensing. In particular, nanostructured carbon is a promising candidate for spin-based quantum computation due to the ability to suppress hyperfine coupling to nuclear spins, a dominant source of spin decoherence. Yet graphene lacks an intrinsic bandgap, which poses a serious challenge for the creation of such devices. We present a novel approach to quantum confinement utilizing tunnel barriers defined by local electric fields that break sublattice symmetry in suspended bilayer graphene. This technique electrostatically confines charges via band structure control, thereby eliminating the edge and substrate disorder that hinders on-chip etched nanostructures to date. We report clean single electron tunneling through gate-defined quantum dots in two regimes: at zero magnetic field using the energy gap induced by a perpendicular electric field and at finite magnetic fields using Landau level confinement. The observed Coulomb blockade periodicity agrees with electrostatic simulations based on local top-gate geometry, a direct demonstration of local control over the band structure of graphene. This technology integrates quantum confinement with pristine device quality and access to vibrational modes, enabling wide applications from electromechanical sensors to quantum bits. More broadly, the ability to externally tailor the graphene bandgap over nanometer scales opens a new unexplored avenue for creating quantum devices.

Allen, Monica

2013-03-01

141

Spintronics and Quantum Dots for Quantum Computing and Quantum Communication

Control over electron-spin states, such as coherent manipulation, filtering and measurement promises access to new technologies in conventional as well as in quantum computation and quantum communication. We review our proposal of using electron spins in quantum confined structures as qubits and discuss the requirements for implementing a quantum computer. We describe several realizations of one- and two-qubit gates and

Guido Burkard; Hans-Andreas Engel; Daniel Loss

2000-01-01

142

NASA Astrophysics Data System (ADS)

We consider a system of two quantum dots, coupled to each other and to separate leads by tunnel junctions with controllable barriers, in the regime where the Coulomb charging energy is large compared to the single-particle level spacing but small compared to the Fermi energy. When the dots are isolated from the leads, the interdot tunneling produces a shift in the groundstate energy which is a singular function of the system parameters. The shift is different for cases where the total number of electrons on the dots is even or odd, and depends also on any asymmetry between the dots. The tunneling strength may be independently determined from the conductivity measured when the dots are strongly coupled to the leads. Theoretical resultsfootnote J. M. Golden and B. I. Halperin, Phys. Rev. B (in press) and unpublished work^,footnote K. A. Matveev, L. I. Glazman, and H. U. Baranger, preprint may be compared with recent experiments by Waugh et alfootnote F. R. Waugh, M. J. Berry, D. J. Mar, R. M. Westervelt, K. C. Campman, and A. C. Gossard, Phys. Rev. Lett. 75, 705 (1995). This work has been supported by the NSF through the Harvard MRSEC, grant DMR94-00396.

Halperin, Bertrand I.

1996-03-01

143

Electronic structure of quantum dots

The properties of quasi-two-dimensional semiconductor quantum dots are reviewed. Experimental techniques for measuring the electronic shell structure and the effect of magnetic fields are briefly described. The electronic structure is analyzed in terms of simple single-particle models, density-functional theory, and ``exact'' diagonalization methods. The spontaneous magnetization due to Hund's rule, spin-density wave states, and electron localization are addressed. As a

Stephanie M. Reimann; Matti Manninen

2002-01-01

144

Surface Functionalized Carbogenic Quantum Dots

Surface functionalized carbon-based quantum dots (C-QDs) are formed in-situ in a single-step process via thermal carbonization of suitable molecular precursors based on ammonium citrate salts. The as-synthesized nanoparticles have near spherical morphology and size around 7nm. Using different surface modifiers, we can form hydrophobic or hydrophilic capped C-QDs, which can be dispersed in organic or aqueous solvents, respectively. These C-QDs

A. B. Bourlinos; Andreas Stassinopoulos; A. Anglos; S. H. Anastasiadis; R. Zboril; M. Karakassides; E. P. Giannelis

2008-01-01

145

Applications of colloidal quantum dots

This paper addresses a number of major trends underlying the continuing effort to realize practical optoelectronic, electronic, and information-processing devices based on ensembles of quantum dots assembled in a variety of matrix materials. The great diversity of such structures makes it possible to fabricate numerous ensemble-based devices for applications underlying photoluminescent devices, light-emitting diodes, displays, photodetectors, photovoltaic devices, and solar

Ke Sun; Milana Vasudev; Hye-Son Jung; Jianyong Yang; Ayan Kar; Yang Li; Kitt Reinhardt; Preston Snee; Michael A. Stroscio; Mitra Dutta

2009-01-01

146

Electronic states and curved surface effect of silicon quantum dots

NASA Astrophysics Data System (ADS)

The calculation results show that the bonding energy and electronic states of silicon quantum dots (Si QDs) are different on various curved surfaces (CS), for example, a Si-O-Si bridge bond on curved surface provides the localized levels in band gap and its bonding energy is shallower than that on facet. Curved surface breaks symmetrical shape of silicon quantum dots on which some bonds can produce localized electronic states in band gap. The red-shifting of photoluminescence spectra on smaller silicon quantum dots can be explained by CS effect. In CS effect, surface curvature is determined by the shape of Si QDs or silicon nanostructures, which is independent of their sizes. The CS effect has the interesting fundamental physical properties in nanophysics as that of quantum confinement effect.

Huang, Wei-Qi; Huang, Zhong-Mei; Cheng, Han-Qiong; Miao, Xin-Jian; Shu, Qin; Liu, Shi-Rong; Qin, Chao-Jian

2012-10-01

147

We report on the theoretical investigation of the elementary electronic excitations in a quantum wire made up of vertically stacked self-assembled InAs\\/GaAs quantum dots. The length scales (of a few nanometers) involved in the experimental setups prompt us to consider an infinitely periodic system of two-dimensionally confined (InAs) quantum dot layers separated by GaAs spacers. The resultant quantum wire is

Manvir S. Kushwaha

2011-01-01

148

Nano-laser on silicon quantum dots

NASA Astrophysics Data System (ADS)

A new conception of nano-laser is proposed in which depending on the size of nano-clusters (silicon quantum dots (QD)), the pumping level of laser can be tuned by the quantum confinement (QC) effect, and the population inversion can be formed between the valence band and the localized states in gap produced from the surface bonds of nano-clusters. Here we report the experimental demonstration of nano-laser on silicon quantum dots fabricated by nanosecond pulse laser. The peaks of stimulated emission are observed at 605 nm and 693 nm. Through the micro-cavity of nano-laser, a full width at half maximum of the peak at 693 nm can reach to 0.5 nm. The theoretical model and the experimental results indicate that it is a necessary condition for setting up nano-laser that the smaller size of QD (d < 3 nm) can make the localized states into band gap. The emission energy of nano-laser will be limited in the range of 1.7-2.3 eV generally due to the position of the localized states in gap, which is in good agreement between the experiments and the theory.

Huang, Wei-Qi; Liu, Shi-Rong; Qin, Chao-Jian; Lü, Quan; Xu, Li

2011-04-01

149

Ultrafast intraband relaxation in colloidal quantum dots

We independently determine the subpicosecond cooling rates for holes and electrons in CdSe quantum dots using time-resolved luminescence and time-resolved TeraHertz spectroscopy. The rate of hole cooling, following photoexcitation of the quantum dots, depends critically on the electron excess energy. This constitutes a direct proof of electron-to-hole energy transfer, the hypothesis behind the Auger cooling mechanism proposed in quantum dots,

J. J. H. Pijpers; E. Hendry; M. Bonn

2008-01-01

150

Impurity effects on coupled quantum dot spin qubits in semiconductors

NASA Astrophysics Data System (ADS)

Localized electron spins confined in semiconductor quantum dots are being studied by many groups as possible elementary qubits for solid-state quantum computation. We theoretically consider the effects of having unintentional charged impurities in laterally coupled two-dimensional double (GaAs) quantum dot systems, where each dot contains one or two electrons and a single charged impurity in the presence of an external magnetic field. We calculate the effect of the impurity on the 2-electron energy spectrum of each individual dot as well as on the spectrum of the coupled-double-dot 2-electron system. We find that the singlet-triplet exchange splitting between the two lowest energy states, both for the individual dots and the coupled dot system, depends sensitively on the location of the impurity and its coupling strength (i.e. the effective charge). We comment on the impurity effect in spin qubit operations in the double dot system based on our numerical results. This work is supported by LPS-CMTC and CNAM.

Nguyen, Nga; Das Sarma, Sankar

2011-03-01

151

Quantum Logic Using Excitonic Quantum Dots in External Optical Microcavities.

National Technical Information Service (NTIS)

An experimental project was undertaken to develop means to achieve quantum optical strong coupling between a single GaAs quantum dot and the optical mode of a microcavity for the purpose of quantum control of dot and photon states for quantum information ...

M. G. Raymer

2003-01-01

152

Successful growth of two different quantum dots on one substrate

We report the successful growth of ZnSe and ZnTe quantum dots (QDs) embedded in ZnS on GaAs substrate. These QDs have good optical properties and show quantum confinement effect. High-resolution electron scanning microscope studies show that these QDs are grown in Volmer–Weber mode. It is found that the size of the QDs is controlled by the growth duration. When the

C. F Tsai; Y. H Chang; J. H Cheng; S. C Yang; C. C Hsu; Y. F Chen; L. C Chen

2004-01-01

153

Successful growth of two different quantum dots on one substrate

We report the successful growth of ZnSe and ZnTe quantum dots (QDs) embedded in ZnS on GaAs substrate. These QDs have good optical properties and show quantum confinement effect. High-resolution electron scanning microscope studies show that these QDs are grown in Volmer-Weber mode. It is found that the size of the QDs is controlled by the growth duration. When the

C. F. Tsai; Y. H. Chang; J. H. Cheng; S. C. Yang; C. C. Hsu; Y. F. Chen; L. C. Chen

2004-01-01

154

Carrier dynamics in quantum dot and gallium arsenide-based quantum dot cascade laser

Self-organized quantum dots provide unique atomic-like density of states and have important applications in semiconductor lasers. Energy relaxation of charge carriers in quantum dots is important for understanding the physics of devices fabricated from these artificially structured materials. Because the charge carriers relax through discrete energy levels, quantum dots provide a means to study the charge carrier interactions in the

Chuanshun Cao

2004-01-01

155

Externally mode-matched cavity quantum electrodynamics with charge-tunable quantum dots.

We present coherent reflection spectroscopy on a charge and dc Stark tunable quantum dot embedded in a high-quality and externally mode-matched microcavity. The addition of an exciton to a single-electron-charged quantum dot forms a trion that interacts with the microcavity just below the strong-coupling regime of cavity quantum electrodynamics. Such an integrated, monolithic system is a crucial step towards the implementation of scalable hybrid quantum-information schemes that are based on an efficient interaction between a single photon and a confined electron spin. PMID:19392565

Rakher, M T; Stoltz, N G; Coldren, L A; Petroff, P M; Bouwmeester, D

2009-03-05

156

The last few years have seen rapid advances in nanoscience and nanotechnology, allowing unprecedented manipulation of nanostructures controlling solar energy capture, conversion, and storage. Quantum confined nanostructures, such as quantum wells (QWs) and quantum dots (QDs) have been projected as potential candidates for the implementation of some high efficiency photovoltaic device concepts, including the intermediate band solar cell (IBSC). In

Anup Pancholi

2008-01-01

157

Magneto-exciton transitions in laterally coupled quantum dots

NASA Astrophysics Data System (ADS)

We present a study of the electronic and optical properties of laterally coupled quantum dots. The excitonic spectra of this system under the effects of an external magnetic field applied perpendicular to the plane of the dots is obtained, with the potential of every individual dot taken as the superposition of a quantum well potential along the axial direction with a lateral parabolic confinement potential, and the coupled two- dot system then modeled by a superposition of the potentials of each dot, with their minima at different positions and truncated at the intersection plane. The wave functions and eigenvalues are obtained in the effective-mass approximation by using an extended variational approach in which the magneto- exciton states are simultaneously obtained [1]. The allowed magneto-exciton transitions are investigated by using circularly polarized radiation in the plane perpendicular to the magnetic field. We present results on the excitonic absorption coefficient as a function of the photon energy for different geometric quantum-dot confinement and magnetic-field values. Reference: [1] Z. Barticevic, M. Pacheco, C. A. Duque and L. E. Oliveira, Phys. Rev. B 68, 073312 (2003).

Barticevic, Zdenka; Pacheco, Monica; Duque, Carlos A.; Oliveira, Luiz E.

2008-03-01

158

Synthesis and applications of quantum dots and magnetic quantum dots

NASA Astrophysics Data System (ADS)

We have developed a new synthetic method for producing high-quality quantum dots (QDs) in aqueous solution for biological imaging applications. The glutathione-capped CdTe, ZnSe and Zn1-xCdxSe alloyed QDs derived are tunable in fluorescence emissions between 360 nm and 700 nm. They show high quantum yields (QYs) of up to 50%, with narrow bandwidths of 19-55 nm. The synthesis of glutathione-capped QDs is simple and cost-effective compared to the conventional organometallic approaches. It can be easily scaled up for the commercial production of alloyed nanocrystals of various compositions. We have also demonstrated the fabrication of magnetic quantum dots (MQDs) through a seed-mediated approach. The formation and assembly of these bifunctional nanocomposites have been elucidated by high-resolution transmission electron microscopy (HRTEM). The MQDs exhibit superparamagnetism and tunable emissions characteristic of the components in this hybrid system. We have created biocompatible silica-coated MQDs that effectively target the cell membranes.

Ying, Jackie Y.; Zheng, Yuangang; Selvan, S. Tamil

2008-03-01

159

Raman-Controlled Quantum Dots for Quantum Computing.

National Technical Information Service (NTIS)

Optical control is fundamental to our project objective of demonstration of key quantum operations for quantum computation with spin qubits of electrons in semiconductor quantum dots. Sophia Economou, the graduate student supported by this fellowship, wor...

L. J. Sham

2005-01-01

160

Growth and Characterization of Quantum Dots and Quantum Dots Devices

Quantum dot nanostructures were investigated experimentally and theoretically for potential applications for optoelectronic devices. We have developed the foundation to produce state-of-the-art compound semiconductor nanostructures in a variety of materials: In(AsSb) on GaAs, GaSb on GaAs, and In(AsSb) on GaSb. These materials cover a range of energies from 1.2 to 0.7 eV. We have observed a surfactant effect in InAsSb

JEFFREY G. CEDERBERG; ROBERT M. BIEFELD; H. C. SCHNEIDER; WENG W. CHOW

2003-01-01

161

Quasiparticles for a quantum dot array in graphene and the associated magnetoplasmons

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

Oleg L. Berman; Godfrey Gumbs; P. M. Echenique

2009-01-01

162

Stress Relaxation Phenomena in Buried Quantum Dots

We report on the results of experimental and theoretical investigation of mechanical stress relaxation in heterostructures with buried quantum dots. Quan- tum dot is viewed as a dilatational inclusion with eigenstrain (transformation strain) caused by crystal lattice mismatch between the dot and matrix materials. Stresses and energies for spheroid inclusions in an infinite medium, in a half-space, and in a

N. A. Bert; V. V. Chaldyshev; A. L. Kolesnikova; A. E. Romanov

163

Single spins in semiconductor quantum dot microcavities

NASA Astrophysics Data System (ADS)

Semiconductor quantum dots can be utilized to capture single electron or hole spins and they have therewith promise for various applications in fields like spintronics, spin based quantum information processing and chiral photonics. We integrate quantum dots into semiconductor microcavities to enhance light-matter interaction for ultrafast optical manipulation and read-out. Single electron and single hole spins can be statistically or deterministically loaded into the quantum dots and coherently controlled. Within the about ?s-coherence times of the spins about 105 complete single qubit rotations can be performed with ultrafast optical pulses. By utilizing a ?-type energy level system of a single quantum-dot electron spin in a magnetic field and ultrafast non-linear frequency conversion, quantum-dot spin-photon entanglement is observed.

Höfling, Sven; De Greve, Kristiaan; McMahon, Peter L.; Press, David; Yu, Leo; Pelc, Jason S.; Natarajan, Chandra M.; Kim, Na Young; Ladd, Thaddeus; Abe, Eisuke; Maier, Sebastian; Bisping, Dirk; Langer, Fabian; Schneider, Christian; Kamp, Martin; Hadfield, Robert H.; Forchel, Alfred; Fejer, M. M.; Yamamoto, Yoshihisa

2013-09-01

164

InP quantum dots: Electronic structure, surface effects, and the redshifted emission

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

Huaxiang Fu; Alex Zunger

1997-01-01

165

Nanotwinning in CdS quantum dots

NASA Astrophysics Data System (ADS)

High resolution transmission electron microscopy, X-ray diffraction and photoluminescence measurements are carried out in order to study the defects in CdS quantum dots (QDs), synthesized in cubic phase by chemical co-precipitation method. The nanotwinning structures in CdS quantum dots (˜2.7 nm) are reported for the first time. Mostly CdS QDs are characterized by existence of nanotwin structures. The twinning structures are present together with stacking faults in some QDs while others exist with grain boundaries. Raman spectroscopy analysis shows intense and broad peaks corresponding to fundamental optical phonon mode (LO) and the first over tone mode (2LO) of CdS at 302 cm-1 and 605 cm-1 respectively. A noticeable shift is observed in Raman lines indicating the effect of phonon confinement. Fourier transform infrared spectroscopy analysis confirms the presence of Cd-S stretching bands at 661 cm-1 and 706 cm-1. The photoluminescence spectrum shows emission in yellow and red regions of visible spectrum. The presence of stacking faults and other defects are explained on the basis of X-rays diffraction patterns and are correlated with photoluminescence spectrum. These nanotwinning and microstructural defects are responsible for different emissions from CdS QDs.

Kumar, Pragati; Saxena, Nupur; Singh, F.; Agarwal, Avinash

2012-09-01

166

Nonlinear thermoelectric response of quantum dots

NASA Astrophysics Data System (ADS)

The thermoelectric transport properties of nanostructured devices continue to attract attention from theorists and experimentalist alike as the spatial confinement allows for a controlled approach to transport properties of correlated matter. Most of the existing work, however, focuses on thermoelectric transport in the linear regime despite the fact that the nonlinear conductance of correlated quantum dots has been studied in some detail throughout the last decade. To go beyond the linear response regime, we use a recently developed scheme [1], to address the low-energy behavior near the strong-coupling fixed point at finite bias voltage and finite temperature drop at the quantum dot. We test the reliability of the method against the numerical renormalization group [2] and determine the charge, energy, and heat current through the nanostructure. This allows us to determine the nonlinear transport coefficients, the entropy production, and the fate of the Wiedemann-Franz law in the non-thermal steady-state [3].[4pt] [1] E. Munoz et al, arXiv:1111.4076.[0pt] [2] L. Merker et al, in preparation.[0pt] [3] S. Kirchner, F. Zamani, and E. Munoz, in ``New Materials for Thermoelectric Applications: Theory and Experiment,'' Springer (2012).

Kirchner, Stefan; Zamani, Farzaneh; Munoz, Enrique; Merker, Lukas; Costi, Theo

2013-03-01

167

Electric-dipole-induced spin resonance in quantum dots

NASA Astrophysics Data System (ADS)

An alternating electric field, applied to a quantum dot, couples to the electron spin via the spin-orbit interaction. We analyze different types of spin-orbit coupling known in the literature and find two efficient mechanisms of spin control in quantum dots. The linear in momentum Dresselhaus and Rashba spin-orbit couplings give rise to a fully transverse effective magnetic field in the presence of a Zeeman splitting at lowest order in the spin-orbit interaction. The cubic in momentum Dresselhaus terms are efficient in a quantum dot with anharmonic confining potential and give rise to a spin-electric coupling proportional to the orbital magnetic field. We derive an effective spin Hamiltonian, which can be used to implement spin manipulation on a time scale of 10ns with the current experimental setups.

Golovach, Vitaly N.; Borhani, Massoud; Loss, Daniel

2006-10-01

168

Coherent photonic coupling of semiconductor quantum dots.

We report a new type of coupling between quantum dot excitons mediated by the strong single-photon field in a high-finesse micropillar cavity. Coherent exciton coupling is observed for two dots with energy differences of the order of the exciton-photon coupling. The coherent coupling mode is characterized by an anticrossing with a particularly large line splitting of 250 microeV. Because of the different dispersion relations with temperature, the simultaneous photonic coupling of quantum dot excitons can be easily distinguished from cases of sequential strong coupling of two quantum dots. PMID:16688279

Reitzenstein, S; Löffler, A; Hofmann, C; Kubanek, A; Kamp, M; Reithmaier, J P; Forchel, A; Kulakovskii, V D; Keldysh, L V; Ponomarev, I V; Reinecke, T L

2006-06-01

169

Nonradiative resonance energy transfer between quantum dots

NASA Astrophysics Data System (ADS)

In the work we examined the mechanism of nonradiactive resonant energy transfer between quantum dots (QD), the probability of this process was calculated. The valence band has difficult structure due to the additional matrix element connected with another polarization of heavy holes. Dependences of transfer probability on distance between quantum dots and barrier heights for electrons were studied.

Stepashkina, A. S.; Samosvat, D. M.; Chikalova-Luzina, O. P.; Zegrya, G. G.

2013-08-01

170

Phonon runaway in carbon nanotube quantum dots

We explore electronic transport in a nanotube quantum dot strongly coupled with vibrations and weakly with leads and the thermal environment. We show that the recent observation of anomalous conductance signatures in single-walled carbon nanotube quantum dots [B. J. LeRoy , Nature (London) 395, 371 (2004) and B. J. LeRoy , Phys. Rev. B 72, 075413 (2005)] can be understood

L. Siddiqui; A. W. Ghosh; S. Datta

2007-01-01

171

Quantum dots as handles for optical manipulation

NASA Astrophysics Data System (ADS)

Individual colloidal quantum dots can be optically trapped and manipulated by a single infrared laser beam operated at low laser powers.1, 2 If the absorption spectrum and the emission wavelength of the trapping laser are appropriately chosen, the trapping laser light can act as a source for two-photon excitation of the trapped quantum dot. This eliminates the need for an additional excitation laser in experiments where individual quantum dots are used both as force transducers and for visualization of the system. To use quantum dots as handles for quantitative optical force transduction, it is crucial to perform a precise force calibration. Here, we present an Allan variance analysis3 of individual optically trapped quantum dots and show that the optimal measurement time for experiments involving individual quantum dots is on the order of 0.3 seconds. Due to their small size and strong illumination, quantum dots are optimal for single molecule assays where, optimally, the presence of the tracer particle should not dominate the dynamics of the system. As an example, we investigated the thermal fluctuations of a DNA tether using an individual colloidal quantum dot as marker, this being the smallest tracer for tethered particle method reported.

Jauffred, Liselotte; Sletmoen, Marit; Czerwinski, Fabian; Oddershede, Lene

2010-08-01

172

Spin noise spectroscopy of quantum dot molecules

NASA Astrophysics Data System (ADS)

We discuss advantages and limitations of the spin noise spectroscopy for characterization of interacting quantum dot systems on specific examples of individual singly and doubly charged quantum dot molecules (QDMs). It is shown that all the relevant parameters of the QDMs, including tunneling amplitudes with spin-conserving and spin-nonconserving interactions, decoherence rates, Coulomb repulsions, anisotropic g factors and the distance between the dots, can be determined by measuring properties of the spin noise power spectrum.

Roy, Dibyendu; Li, Yan; Greilich, Alex; Pershin, Yuriy V.; Saxena, Avadh; Sinitsyn, Nikolai A.

2013-07-01

173

Ferritin-Templated Quantum-Dots for Quantum Logic Gates.

National Technical Information Service (NTIS)

Quantum logic gates (QLGs) or other logic systems are based on quantum-dots (QD) with a stringent requirement of size uniformity. The QD are widely known building units for QLGs. The size control of QD is a critical issue in quantum-dot fabrication. The w...

S. H. Choi J. W. Kim S. H. Chu Y. Park G. C. King

2005-01-01

174

Antimony-based quantum dot memories

NASA Astrophysics Data System (ADS)

As a type-II heterostructure with exclusive hole confinement GaSb/(Al,Ga)As QDs are an ideal candidate for a QD based memory device operating at room temperature. We investigated different Antimony-based QDs in respect of localization energies and storage times with 8-band-k•p calculations as well as time-resolved capacitance spectroscopy. In addition, we present a memory concept based on self-organized quantum dots (QDs) which could fuse the advantages of today's main semiconductor memories DRAM and Flash. First results on the performance of such a memory cell are shown and a closer look at Sb-based QDs as a storage unit is taken.

Bimberg, Dieter; Marent, Andreas; Nowozin, Tobias; Schliwa, Andrei

2011-02-01

175

Carrier Dynamics in Colloidal Graphene Quantum Dots

NASA Astrophysics Data System (ADS)

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 (˜100 fs) transient absorption over nanosecond timescales, we extract the single-photon absorption cross-section and observe carrier-induced Stark shifts of the order of 0.1 eV indicating strong carrier-carrier interactions, as expected for the relatively weak screening of a two-dimensional nanostructure. Multiexcitons are observed to decay nonradiatively on ˜1 to 20 ps timescales, while single excitons display dynamics on multiple timescales due to carrier cooling, singlet-to-triplet intersystem crossing, and, on nanosecond to microsecond timescales, radiative recombination.

Sun, Cheng; Yan, Xin; Li, Liang-Shi; McGuire, John A.

2011-03-01

176

Biocompatible Quantum Dots for Biological Applications

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.

Rosenthal, Sandra J.; Chang, Jerry C.; Kovtun, Oleg; McBride, James R.; Tomlinson, Ian D.

2011-01-01

177

Biocompatible Quantum Dots for Biological Applications

Semiconductor quantum dots are quickly becoming a critical diagnostic tool for discerning cellular function at the molecular level. Their high brightness, long-lasting, size-tunable, 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.

Rosenthal, Sandra [ORNL; Chang, Jerry [Vanderbilt University; Kovtun, Oleg [Department of Chemistry, Vanderbilt University, 7300 Stevenson Ctr Ln, Nashville, TN 37235, USA.; McBride, James [Vanderbilt University; Tomlinson, Ian [Oak Ridge National Laboratory (ORNL)

2011-01-01

178

The Coulomb Blockade in Coupled Quantum Dots

NASA Astrophysics Data System (ADS)

Individual quantum dots are often referred to as ``artificial atoms''. Two tunnel-coupled quantum dots may be considered an ``artificial molecule''. One advantage of studying quantum dot molecules is that the amount of coupling between the dots may be experimentally varied. We present measurements of conductance through a double quantum dot with adjustable interdot tunneling rate that show the evolution from two separate ``atoms'' to one composite ``molecule''.(C. Livermore, C.H. Crouch, R.M. Westervelt, K.L. Campman, and A.C. Gossard, Science) 274, 1332 (1996). The quantum dots are defined in a two-dimensional electron gas in a GaAs/AlGaAs heterostructure by ten independent electrostatic surface gates. The gates are used to separately control the total induced charge on the double dot, the induced charge difference between the dots, and the interdot tunnel conductance. We measure the double dot conductance as a function of the total induced charge and the induced charge difference to measure the charging diagram and probe the minimum energy surface of the double dot; we repeat this measurement for a series of conductances ranging from near zero to 2e^2/h. For weak interdot tunneling each dot is individually governed by the Coulomb blockade. As interdot tunnel conductance increases, the charging diagram departs from simple Coulomb blockade theory and evolves to that of a single large dot. The evolution is controlled by quantum mechanical charge sharing between dots. The results are in excellent quantitative agreement with recent theory.(J.M. Golden and B.I. Halperin, Phys. Rev. B), in press (1996); K.A. Matveev, L.I. Glazman and H.U. Baranger, Phys. Rev. B 54, 5637 (1996).

Livermore, C.

1997-03-01

179

Nanometer Distance Measurements Between Multicolor Quantum Dots

Quantum dot dimers made of short double-stranded DNA molecules labeled with different color quantum dots at each end were imaged using multicolor stage-scanning confocal microscopy. This approach eliminates chromatic aberration and color registration issues usually encountered in other multicolor imaging techniques. We demonstrate nanometer accuracy in individual distance measurement by suppression of quantum dot blinking, and thoroughly characterize the contribution of different effects to the variability observed between measurements. Our analysis opens the way to accurate structural studies of biomolecules and biomolecular complexes using multicolor quantum labeling.

Antelman, Josh; Wilking-Chang, Connie; Weiss, Shimon; Michalet, Xavier

2009-01-01

180

Decoherence dynamics of two charge qubits in vertically coupled quantum dots

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.

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

181

Thermoelectromechanical effects in quantum dots

NASA Astrophysics Data System (ADS)

Electromechanical effects are important in semiconductor nanostructures as most of the semiconductors are piezoelectric in nature. These nanostructures find applications in electronic and optoelectronic devices where they may face challenges for thermal management. Low dimensional semiconductor nanostructures, such as quantum dots (QD) and nanowires, are the nanostructures where such challenges must be particularly carefully addressed. In this contribution we report a study on thermoelectromechanical effects in QDs. For the first time a coupled model of thermoelectroelasticity has been applied to the analysis of quantum dots and the influence of thermoelectromechanical effects on bandstructures of low dimensional nanostructures has been quantified. Finite element solutions are obtained for different thermal loadings and their effects on the electromechanical properties and bandstructure of QDs are presented. Our model accounts for a practically important range of internal and external thermoelectromechanical loadings. Results are obtained for typical QD systems based on GaN/AlN and CdSe/CdS (as representatives of III-V and II-VI group semiconductors, respectively), with cylindrical and truncated conical geometries. The wetting layer effect on electromechanical quantities is also accounted for. The energy bandstructure calculations for various thermal loadings are performed. Electromechanical fields are observed to be more sensitive to thermal loadings in GaN/AlN QDs as compared to CdSe/CdS QDs. The results are discussed in the context of the effect of thermal loadings on the performance of QD-based nanosystems.

Patil, Sunil R.; Melnik, Roderick V. N.

2009-03-01

182

Dicke states in multiple quantum dots

NASA Astrophysics Data System (ADS)

We present a theoretical study of the collective optical effects which can occur in groups of three and four quantum dots. We define conditions for stable subradiant (dark) states, rapidly decaying super-radiant states, and spontaneous trapping of excitation. Each quantum dot is treated like a two-level system. The quantum dots are, however, realistic, meaning that they may have different transition energies and dipole moments. The dots interact via a short-range coupling which allows excitation transfer across the dots, but conserves the total population of the system. We calculate the time evolution of single-exciton and biexciton states using the Lindblad equation. In the steady state the individual populations of each dot may have permanent oscillations with frequencies given by the energy separation between the subradiant eigenstates.

Sitek, Anna; Manolescu, Andrei

2013-10-01

183

Series-Coupled Triple Quantum Dot Molecules

NASA Astrophysics Data System (ADS)

We present the electronic properties of a triple quantum dot molecule embedded inside a sub-micron mesa, made from a quadruple-barrier triple-quantum-well structure, and surrounded by a single gate electrode. We outline the design principles of the quadruple-barrier triple-quantum-well structure and calculate the energy of the three lowest states as a function of center well thickness. We observe regular and irregular shaped Coulomb diamond regions similar to those for double quantum dot devices. Variation in the Coulomb blockade region shape is introduced by fluctuation in the offset energies between the quantum dots likely associated with device processing and random impurity potential in the material. We also present Coulomb blockade patterns calculated with a constant interaction model for sequential tunneling through the three series-coupled quantum dots.

Amaha, Shinichi; Hatano, Tsuyoshi; Izumida, Wataru; Teraoka, Soichiro; Ono, Keiji; Kono, Kimitoshi; Tarucha, Seigo; Aers, Geof; Gupta, James; Austing, Guy

2012-02-01

184

Optical Nonlinearities and Ultrafast Carrier Dynamics in Semiconductor Quantum Dots

Low-dimensional semiconductors have attracted great interest due to the potential for tailoring their linear and nonlinear optical properties over a wide-range. Semiconductor nanocrystals (NC's) represent a class of quasi-zero-dimensional objects or quantum dots. Due to quantum cordhement and a large surface-to-volume ratio, the linear and nonlinear optical properties, and the carrier dynamics in NC's are significantly different horn those in bulk materials. napping at surface states can lead to a fast depopulation of quantized states, accompanied by charge separation and generation of local fields which significantly modifies the nonlinear optical response in NC's. 3D carrier confinement also has a drastic effect on the energy relaxation dynamics. In strongly confined NC's, the energy-level spacing can greatly exceed typical phonon energies. This has been expected to significantly inhibit phonon-related mechanisms for energy losses, an effect referred to as a phonon bottleneck. It has been suggested recently that the phonon bottleneck in 3D-confined systems can be removed due to enhanced role of Auger-type interactions. In this paper we report femtosecond (fs) studies of ultrafast optical nonlinearities, and energy relaxation and trap ping dynamics in three types of quantum-dot systems: semiconductor NC/glass composites made by high temperature precipitation, ion-implanted NC's, and colloidal NC'S. Comparison of ultrafast data for different samples allows us to separate effects being intrinsic to quantum dots from those related to lattice imperfections and interface properties.

Klimov, V.; McBranch, D.; Schwarz, C.

1998-08-10

185

Two-photon absorption in single site-controlled InGaN/GaN quantum dots

NASA Astrophysics Data System (ADS)

We present micro-photoluminescence measurements on single site-controlled InGaN/GaN quantum dots using two-photon excitation Furthermore, measurements of photoluminescence excitation and time-resolved photoluminescence are also presented. We show that two-photon excitation results in total suppression of the emission from the underlying quantum well, to which the quantum dots are couple, and yet strong quantum dot emission. We attribute this effect to the enhancement of the two-photon absorption in the quantum dots as a result of the zero-dimensional confinement compared to that of the quantum wells.

Jarjour, A. F.; Parker, T. J.; Taylor, R. A.; Martin, R. W.; Watson, I. M.

2005-11-01

186

Quantum dot loaded immunomicelles for tumor imaging

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

Aristarchos Papagiannaros; Jaydev Upponi; William Hartner; Dmitriy Mongayt; Tatyana Levchenko; Vladimir Torchilin

2010-01-01

187

Optical pumping of a single hole spin in a quantum dot

The spin of an electron is a natural two-level system for realizing a quantum bit in the solid state. For an electron trapped in a semiconductor quantum dot, strong quantum confinement highly suppresses the detrimental effect of phonon-related spin relaxation. However, this advantage is offset by the hyperfine interaction between the electron spin and the 104 to 106 spins of

Brian D. Gerardot; Daniel Brunner; Paul A. Dalgarno; Patrik Öhberg; Stefan Seidl; Martin Kroner; Khaled Karrai; Nick G. Stoltz; Pierre M. Petroff; Richard J. Warburton

2008-01-01

188

Optical Vibration Modes in Spherical Core-Shell Quantum Dots

NASA Astrophysics Data System (ADS)

Using a dielectric continuum approach, the optical vibration modes in a spherical core-shell quantum dots (QDs) imbedded in a host nonpolar material are studied. The dispersion relation and the corresponding electron-phonon interaction Hamiltonian are derived. The numerical calculations for the CdSe/ZnS system are performed. The results reveal that there are three branches frequencies of interface/surface optical phonon in the system. A detailed discussion of the combined effects of the spatial confinement and dielectric mismatch between the dot and the host medium is given.

Xing, Y.; Liang, X. X.; Wang, Z. P.

2013-07-01

189

Optical Detection of Single-Electron Spin Resonance in a Quantum Dot

We demonstrate optically detected spin resonance of a single electron confined to a self-assembled quantum dot. The dot is rendered dark by resonant optical pumping of the spin with a laser. Contrast is restored by applying a radio frequency (rf) magnetic field at the spin resonance. The scheme is sensitive even to rf fields of just a few muT. In

Martin Kroner; Kathrina M. Weiss; Benjamin Biedermann; Stefan Seidl; Stephan Manus; Alexander W. Holleitner; Antonio Badolato; Pierre M. Petroff; Brian D. Gerardot; Richard J. Warburton; Khaled Karrai

2008-01-01

190

GaN quantum dots as optical transducers for chemical sensors

GaN/AlN quantum dots were investigated as optical transducers for field effect chemical sensors. The structures were synthesized by molecular-beam epitaxy and covered by a semitransparent catalytic Pt top contact. Due to the thin (3 nm) AlN barriers, the variation of the quantum dot photoluminescence with an external electric field along the [0001] axis is dominated by the tunneling current rather than by the quantum confined Stark effect. An increasing field results in a blueshift of the luminescence and a decreasing intensity. This effect is used to measure the optical response of quantum dot superlattices upon exposure to molecular hydrogen.

Weidemann, O.; Jegert, G.; Stutzmann, M. [Walter Schottky Institut, Technische Universitaet Muenchen, Am Coulombwall 3, 85748 Garching (Germany); Kandaswamy, P. K.; Monroy, E. [CEA-CNRS group 'Nanophysique et semiconducteurs', INAC/SP2M/NPSC, CEA-Grenoble, 17 rue des Martyrs, 38054 Grenoble cedex 9 (France); Eickhoff, M. [Walter Schottky Institut, Technische Universitaet Muenchen, Am Coulombwall 3, 85748 Garching (Germany); I. Physikalisches Institut, Justus-Liebig-Universitaet Giessen, 35392 Giessen (Germany)

2009-03-16

191

Ultralong Dephasing Time in InGaAs Quantum Dots

We measure a dephasing time of several hundred picoseconds at low temperature in the ground-state transition of strongly confined InGaAs quantum dots, using a highly sensitive four-wave mixing technique. Between 7 and 100 K the polarization decay has two distinct components resulting in a non-Lorentzian line shape with a lifetime-limited zero-phonon line and a broadband from elastic exciton-acoustic phonon interactions.

P. Borri; W. Langbein; S. Schneider; U. Woggon; R. L. Sellin; D. Ouyang; D. Bimberg

2001-01-01

192

CNT Quantum dots as Terahertz detectors

NASA Astrophysics Data System (ADS)

We study Carbon Nanotube (CNT) quantum dots as detectors of THz radiation via photon assisted single electron tunneling. Although successful detection was recently demonstrated [1], the coupling between the CNT and THz radiations was very weak. Here, we implement a novel device design where the radiation is effectively coupled to the CNT quantum dot through broad band on-chip antennas. We show that the enhanced coupling yields a highly sensitive broad band Terahertz sensor. [4pt] [1] Y. Kawano, S. Toyokawa, T. Uchida and K. Ishibashi, THz photon assisted tunneling in carbon-nanotube quantum dots, Journal of Applied Physics 103, 034307 (2008).

Rinzan, Mohamed; Jenkins, Greg; Drew, Dennis; Shafranjuk, Serhii; Barbara, Paola

2011-03-01

193

Magneto-Absorption in Ellipsoidal Quantum Dot

NASA Astrophysics Data System (ADS)

The absorption of light in ellipsoidal quantum dot in the presence of a magnetic field is discussed using perturbation theory. Also the same problem is discussed using the normal modes. Quantum dot absorption coefficient is calculated - as well as threshold frequency of absorption - as a function of applied magnetic field. Theoretical results are compared with experimental data obtained by magneto-luminescence method in In0.53Ga0.47As quantum dot (M. Bayer et al, PRB vol.53, pp. 15810-15814, 1996).

Dvoyan, K. G.; Evoyan, V. V.; Kazaryan, E. M.; Nazmitdinov, R. G.; Sarkisyan, H. A.

194

Solid-state quantum optics with quantum dots in photonic nanostructures

NASA Astrophysics Data System (ADS)

Quantum nanophotonics has become a new research frontier where quantum optics is combined with nanophotonics in order to enhance and control the interaction between strongly confined light and quantum emitters. Such progress provides a promising pathway towards quantum-information processing on an all-solid-state platform. Here we review recent progress on experiments with quantum dots in nanophotonic structures with special emphasis on the dynamics of single-photon emission. Embedding the quantum dots in photonic band-gap structures offers a way of controlling spontaneous emission of single photons to a degree that is determined by the local light-matter coupling strength. Introducing defects in photonic crystals implies new functionalities. For instance, efficient and strongly confined cavities can be constructed enabling cavity-quantum-electrodynamics experiments. Furthermore, the speed of light can be tailored in a photonic-crystal waveguide forming the basis for highly efficient single-photon sources where the photons are channeled into the slowly propagating mode of the waveguide. Finally, we will discuss some of the surprises that arise in solid-state implementations of quantum-optics experiments in comparison to their atomic counterparts. In particular, it will be shown that the celebrated point-dipole description of light-matter interaction can break down when quantum dots are coupled to plasmon nanostructures.

Lodahl, Peter; Stobbe, Søren

2013-02-01

195

Quantum dot-based theranostics

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.

Ho, Yi-Ping; Leong, Kam W.

2010-01-01

196

Quantum dot-based theranostics

NASA Astrophysics Data System (ADS)

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.

Ho, Yi-Ping; Leong, Kam W.

2010-01-01

197

Quantum dot lasers: from promise to reality

Nanoscale coherent insertions of narrow gap material in a single-crystalline matrix, or Quantum Dot (QD) provide a possibility to extend the basic principles of heterostructure lasers. The idea to use heterostructures with dimensionality lower than two in semiconductor lasers appeared a quarter of a century ago, simultaneously with the proposal of a quantum well laser. However, fabrication of quantum wire-

Dieter Bimberg; Nikolai N. Ledentsov

2000-01-01

198

Triple quantum dots as charge rectifiers.

We theoretically analyze electronic spin transport through a triple quantum dot in series, attached to electrical contacts, where the drain contact is coupled to the central dot. We show that current rectification is observed in the device due to current blockade. The current blocking mechanism is originated by a destructive interference of the electronic wavefunction at the drain dot. There, the electrons are coherently trapped in a singlet two-electron dark state, which is a coherent superposition of the electronic wavefunction in the source dot and in the dot isolated from the contacts. Its formation gives rise to zero current and current rectification as the voltage is swept. We analyze this behavior analytically and numerically for both zero and finite magnetic dc fields. On top of that, we include phenomenologically a finite spin relaxation rate and calculate the current numerically. Our results show that triple dots in series can be designed to behave as quantum charge rectifiers. PMID:22442135

Busl, M; Platero, G

2012-03-23

199

Energy levels of few-electron quantum dots imaged and characterized by atomic force microscopy

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.

Cockins, Lynda; Miyahara, Yoichi; Bennett, Steven D.; Clerk, Aashish A.; Studenikin, Sergei; Poole, Philip; Sachrajda, Andrew; Grutter, Peter

2010-01-01

200

Open quantum dots: II. Probing the classical to quantum transition

NASA Astrophysics Data System (ADS)

Open quantum dots provide a natural system in which to study both classical and quantum features of transport. From the classical point of view these dots possess a mixed phase space which yields families of closed, regular orbits as well as an expansive sea of chaos. An important question concerns the manner in which these classical states evolve into the set of quantum states that populate the dot in the quantum limit. In the reverse direction, the manner in which the quantum states evolve to the classical world is governed strongly by Zurek’s decoherence theory. This was discussed from the quantum perspective in an earlier review (Ferry et al 2011 Semicond. Sci. Technol. 26 043001). Here, we discuss the nature of the various classical states, how they are formed, how they progress to the quantum world, and the signatures that they create in magnetotransport and general conductance studies of these dots.

Brunner, R.; Ferry, D. K.; Akis, R.; Meisels, R.; Kuchar, F.; Burke, A. M.; Bird, J. P.

2012-08-01

201

Theory Of Alkyl Terminated Silicon Quantum Dots

We have carried out a series of ab-initio calculations to investigate changes in the optical properties of Si quantum dots as a function of surface passivation. In particular, we have compared hydrogen passivated dots with those having alkyl groups at the surface. We find that, while on clusters with reconstructed surfaces a complete alkyl passivation is possible, steric repulsion prevents full passivation of Si dots with unreconstructed surfaces. In addition, our calculations show that steric repulsion may have a dominant effect in determining the surface structure, and eventually the stability of alkyl passivated clusters, with results dependent on the length of the carbon chain. Alkyl passivation weakly affects optical gaps of silicon quantum dots, while it substantially decreases ionization potentials and electron affinities and affect their excited state properties. On the basis of our results we propose that alkyl terminated quantum dots may be size selected taking advantage of the change in ionization potential as a function of the cluster size.

Reboredo, F; Galli, G

2004-08-19

202

Time-resolved optical investigation of spatially indirect excitons in GaAs\\/GaSb quantum dots

Summary form only given. Self-organized quantum dots formed under Stranski-Krastanav growth has been an active area of quantum structure research. Traditional quantum dots grown with III-V materials usually have a normal band lineup, where the electron and hole are confined within the dot, and spatially direct excitons are observed in these structures. Highly strained material systems such as GaSb\\/GaAs exhibit

G. Wang; C.-K. Sun; H. R. Blank; B. Brar; J. E. Bowers; H. Kroemer; M. H. Pilkuhn

1996-01-01

203

Anticrossings in Förster coupled quantum dots

We consider two coupled generic quantum dots, each modelled by a simple\\u000apotential which allows the derivation of an analytical expression for the\\u000ainter-dot Foerster coupling, in the dipole-dipole approximation. We investigate\\u000athe energy level behaviour of this coupled two-dot system under the influence\\u000aof an external applied electric field and predict the presence of anticrossings\\u000ain the optical spectra

Ahsan Nazir; Brendon W. Lovett; Sean D. Barrett; John H. Reina; G. Andrew

2005-01-01

204

Gate defined quantum confinement in suspended bilayer graphene

NASA Astrophysics Data System (ADS)

Devices that confine electrons in graphene have sparked substantial interest due to applications ranging from spin-based quantum computation to valley filters. However, the absence of an intrinsic bandgap in graphene has limited such devices to on-chip nanopatterned structures to date. Here we present high quality quantum dots in suspended bilayer graphene with tunnel barriers defined by external electric fields that break layer inversion symmetry, thereby eliminating both edge and substrate disorder. We demonstrate clean electron confinement in two regimes: at zero magnetic field B using the single particle energy gap induced by a perpendicular electric field and at B>0 using the quantum Hall ferromagnet v=0 gap. Our devices exhibit clean quantum transport behavior at magnetic fields ranging from zero to seven Tesla, including a highly resistive v=0 quantum Hall state and over forty consecutive Coulomb blockade oscillations with symmetric source-drain coupling. The data indicate that the dots are defined by local top gating and are not disorder formed. Geometric control over oscillation periodicity is confirmed by electrostatic simulations based on lithographic gate geometry.

Allen, Monica; Martin, Jens; Yacoby, Amir

2012-02-01

205

Nanomaterials: Earthworms lit with quantum dots

NASA Astrophysics Data System (ADS)

Yeast, bacteria and fungi have been used to synthesize a variety of nanocrystals. Now, the metal detoxification process in the gut of an earthworm is exploited to produce biocompatible cadmium telluride quantum dots.

Tilley, Richard D.; Cheong, Soshan

2013-01-01

206

Quantum Dot Array Formation through Biomolecular Nanopatterning.

National Technical Information Service (NTIS)

Report developed under SBIR contract for Topic A98-028: The objective is to demonstrate the feasibility of forming precisely ordered and precisely located arrays of semiconductor quantum dots by using biomolecular templates and Low Energy Electron Enhance...

H. P. Gillis

1999-01-01

207

Measurement of Charge Fluctuations on a Quantum Dot

NASA Astrophysics Data System (ADS)

We report direct measurements of the charge on a single quantum dot with variable tunnel coupling to a neighboring electron reservoir. The dot is 1 ?m x 1 ?m in size and is defined in a high-mobility near surface (570 A) two-dimensional electron gas by independent split gate point contacts and confining gates on the surface of a GaAs/AlGaAs heterostructure. The dot charge is sensed by an SET electrometer defined in the 2DEG adjacent to the dot. When one point contact is fully closed and the second has very weak tunneling ((G << 2e^2/h)), such that no electron transport is observed through the quantum dot, the dot charge increases stepwise in units of e as gate charge is induced. As one point contact is opened to increase the tunnel coupling to the reservoir, charge quantization weakens and the steps broaden in agreement with theory.(K.A. Matveev, Phys. Rev. B 51, 1743 (1995).)

Livermore, C.; Duncan, D. S.; Westervelt, R. M.; Maranowski, K. D.; Gossard, A. C.

1998-03-01

208

Electronic states in zigzag carbon nanotube quantum dots

NASA Astrophysics Data System (ADS)

Local electronic properties of quantum dot nanotubes modeled by connecting pure semiconducting and metallic nanotubes via appropriate junctions are studied following a single ?-band tight-binding Hamiltonian. The junctions are formed by introducing pair defects composed of heptagons and pentagons along the axial direction of pure nanotubes. We investigate the dependence of the confined electronic states with the characteristic sizes of the quantum dots taking into account different nanotube-based heterostructures. Quantum-well-like and interface states are characterized by investigating the spatial dependence of the local density of states of the discrete levels. We follow the Green's function formalism and adopt real-space renormalization techniques to calculate local density of states. The conductance of metal/semiconductor(metal)/metal carbon heterostructures is also investigated and we found exponentially the decay and oscillatory behaviors that may be associated with the electronic structure of the tube constituents and the details of the junctions.

Rocha, C. G.; Dargam, T. G.; Latgé, A.

2002-04-01

209

Biomolecule-quantum dot systems for bioconjugation applications.

In the present work, it is reported for the first time the bioconjugation of CdS quantum dots (QDs) directly with bovine serum albumin (BSA) using a one-step procedure via aqueous route at room temperature by methods of colloidal chemistry. Essentially, the bioconjugates were developed based on BSA as capping ligand for the nucleation and stabilization of CdS nanoparticles using cadmium perchlorate and thioacetamide as precursors. UV-visible spectroscopy was used to characterize the kinetics and the relative stability of CdS nanoparticles. The CdS nanocrystals were produced with the calculated average particle size below 4.0 nm, indicating they were in the so-called "quantum-size confinement range". The results have clearly indicated that BSA was effective on nucleating and stabilizing the colloidal CdS quantum dots. PMID:21353498

Mansur, Herman S; González, J C; Mansur, Alexandra A P

2011-02-25

210

Synthesis, Characterization and Application Of PbS Quantum Dots

Lead Chalcogenides (PbS, PbSe, PbTe) quantum dots (QDs) are ideal for fundamental studies of strongly quantum confined systems with possible technological applications. Tunable electronic transitions at near--infrared wavelengths can be obtained with these QDs. Applications of lead chalcogenides encompass quite a good number of important field viz. the fields of telecommunications, medical electronics, optoelectronics etc. Very recently, it has been proposed that 'memristor'(Memory resistor) can be realized in nanoscale systems with coupled ionic and electronic transports. The hystersis characteristics of 'memristor' are observed in many nanoscale electronic devices including semiconductor quantum dot devices. This paper reports synthesis of PbS QDs by chemical route. The fabricated samples are characterized by UV-Vis, XRD, SEM, TEM, EDS, etc. Observed characteristics confirm nano formation. I-V characteristics of the sample are studied for investigating their applications as 'memristor'.

Sarma, Sweety; Datta, Pranayee [Department of Electronics Science, Gauhati University, Gnwahait-781014, Assam (India); Barua, Kishore Kr. [Department of Physics, Central University, Tezpur-784028, Assam (India); Karmakar, Sanjib [Department of Instrumentation and USIC, Gauhati University, Guwahati-781014, Assam (India)

2009-06-29

211

Femtosecond laser deposition of semiconductor quantum dot films

NASA Astrophysics Data System (ADS)

We report new results on the deposition of high-density films of semiconductor nanostructures by ultrafast pulsed laser deposition (UFPLD). Such materials are of interest for advanced optoelectronic applications such as quantum dot lasers and energy harvesting devices. The deposition method utilizes the interaction of a focused chirped pulse amplified (CPA) Ti-sapphire laser beam with a solid target (a rotating semiconductor wafer) to produce a hot-dense plasma at the target surface with a power density in excess of 1014 W/cm2. The plasma then undergoes rapid expansion and the resulting condensation process produces a high density of nanoscale particles (average size of a few nm) on a substrate placed a few cm from the target. We have investigated several semiconductor quantum dot systems including silicon and germanium. We observed a significant blue-shift of the optical absorption edge indicating quantum confinement effects which may be of interest for photovoltaic applications.

Oraiqat, Ibrahim; Kennedy, Jack; Mathis, James; Clarke, Roy

2012-07-01

212

Optical Studies of Semiconductor Quantum Dots

NASA Astrophysics Data System (ADS)

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.

Yükselici, H.; Allahverdi, Ç.; A??ko?lu, A.; Ünlü, H.; Baysal, A.; Çulha, M.; ?nce, R.; ?nce, A.; Feeney, M.; Athalin, H.

213

Quantum dots: Paradigm changes in semiconductor physics

Deposition of one or a few monolayers of a semiconductor having a lattice constant largely different from the underlying substrate\\u000a leads to formation of coherent “quantum dot arrays” of densities beyond 1011 cm?2 in a matter of seconds. Self-organization effects govern their massively parallel formation. Fundamental paradigms of semiconductor\\u000a physics must be changed in describing such quantum dots or their

D. Bimberg

1999-01-01

214

Quantum dots for fiber laser sources

In this invited paper, we will discuss the use of quantum dots as nonlinear optical elements in fiber laser sources. Furthemore, a review of the fabrication of the first low-loss (< 0.5 dB\\/cm) ion-exchanged waveguides in a quantum-dot-doped glass will be presented. We will discuss the coupling, propagation, absorption, and scattering losses in these waveguides. The near-field mode profile along

Jason M. Auxier; Axel Schülzgen; Michael M. Morrell; Brian R. West; Seppo Honkanen; Sabyasachi Sen; Nicholas F. Borrelli; Nasser N. Peyghambarian

2005-01-01

215

Magnetically-doped semiconductor quantum dots (nanocrystals)

Previously, we have shown how to use self-assembly and colloidal semiconductor quantum dots (or nanocrystals) to form semiconductor photonic crystals - structures that are three-dimensionally periodic on an optical-length scale. One route to further enhance the behavior of such structures is to modify the properties of the individual quantum dots prior to assembly. For example, properties may be altered by

David Norris

2001-01-01

216

Effects of excitons in nonlinear optical rectification in semiparabolic quantum dots

We study the effects of excitons in nonlinear optical rectification in one-dimensional semiparabolic quantum dots. We consider the cases that the electron and the hole are confined in semiparabolic potentials (i) with the same oscillator frequency and (ii) with the same width. In the first case we present approximate analytical results in the strong confinement regime and find a closed-form

Sotirios Baskoutas; Emmanuel Paspalakis; Andreas F. Terzis

2006-01-01

217

Quantum well and quantum dot energy harvesting devices

Quantum structured solar cells seek to harness a wide spectrum of photons at high voltages by embedding low energy-gap wells or dots within a high energy-gap matrix. Quantum well and quantum dot solar cells have the potential to deliver ultra-high power conversion efficiencies in single junction devices, efficiencies that in theory can approach 45% in un-concentrated sunlight over a wide

Roger E. Welser; Ashok K. Sood; Oleg A. Laboutin; Louis J. Guido; Nibir K. Dhar; Priyalal S. Wijewarnasuriya

2011-01-01

218

The electronic structure of a spherical quantum dot with parabolic confinement that contains a hydrogenic impurity and is subjected to a DC electric field is studied. In our calculations we vary the position of the impurity and the electric field strength. The calculated electronic structure is further used for determining the nonlinear optical rectification coefficient of the quantum dot structure.

S. Baskoutas; E. Paspalakis; A. F. Terzis

2007-01-01

219

Excitons in quantum dots manifest a lower-energy spin-forbidden "dark" state below a spin-allowed "bright" state; this splitting originates from electron-hole (e-h) exchange interactions, which are strongly enhanced by quantum confinement. The e-h exchange interaction may have both a short-range and a long-range component. Calculating numerically the e-h exchange energies from atomistic pseudopotential wave functions, we show here that in direct-gap quantum dots (such as InAs) the e-h exchange interaction is dominated by the long-range component, whereas in indirect-gap quantum dots (such as Si) only the short-range component survives. As a result, the exciton dark/bright splitting scales as 1/R(2) in InAs dots and 1/R(3) in Si dots, where R is the quantum-dot radius. PMID:19583283

Luo, Jun-Wei; Franceschetti, Alberto; Zunger, Alex

2009-07-01

220

Interplay of Aharonov-Bohm and Berry phases in gate-defined graphene quantum dots

NASA Astrophysics Data System (ADS)

We study the influence of a magnetic flux tube on the possibility to electrostatically confine electrons in a graphene quantum dot. Without a magnetic flux tube, the graphene pseudospin is responsible for a quantization of the total angular momentum to half-integer values. On the other hand, with a flux tube containing half a flux quantum, the Aharonov-Bohm phase and Berry phase precisely cancel, and we find a state at zero angular momentum that cannot be confined electrostatically. In this case, true bound states only exist in regular geometries for which states without zero-angular-momentum component exist, while nonintegrable geometries lack confinement. We support these arguments with a calculation of the two-terminal conductance of a gate-defined graphene quantum dot, which shows resonances for a disk-shaped geometry and for a stadium-shaped geometry without flux tube, but no resonances for a stadium-shaped quantum dot with a ?-flux tube.

Heinl, Julia; Schneider, Martin; Brouwer, Piet W.

2013-06-01

221

Metamorphic quantum dots: Quite different nanostructures

In this work, we present a study of InAs quantum dots deposited on InGaAs metamorphic buffers by molecular beam epitaxy. By comparing morphological, structural, and optical properties of such nanostructures with those of InAs/GaAs quantum dot ones, we were able to evidence characteristics that are typical of metamorphic InAs/InGaAs structures. The more relevant are: the cross-hatched InGaAs surface overgrown by dots, the change in critical coverages for island nucleation and ripening, the nucleation of new defects in the capping layers, and the redshift in the emission energy. The discussion on experimental results allowed us to conclude that metamorphic InAs/InGaAs quantum dots are rather different nanostructures, where attention must be put to some issues not present in InAs/GaAs structures, namely, buffer-related defects, surface morphology, different dislocation mobility, and stacking fault energies. On the other hand, we show that metamorphic quantum dot nanostructures can provide new possibilities of tailoring various properties, such as dot positioning and emission energy, that could be very useful for innovative dot-based devices.

Seravalli, L.; Frigeri, P.; Nasi, L.; Trevisi, G.; Bocchi, C. [CNR-IMEM, Parco delle Scienze 37a, I-43100 Parma (Italy)

2010-09-15

222

Optimal spin-state transition in singly occupied quantum dots network

NASA Astrophysics Data System (ADS)

We present a general model to study the spin-state transition in a network of singly occupied lateral quantum dots. The perturbative expansion of the extended Hubbard model is used to describe the dynamics of confined electrons as an effective spin Hamiltonian in the strong correlation regime. To optimize the transition, we apply the gradient ascent pulse engineering algorithm to control the exchange couplings constrained to the manifolds evaluated by Heitler-London approximation. The method is applicable to an arbitrary number of quantum dots in any arrangement. Results of the method applied to the case of triple quantum dot are presented for linear and triangular topologies.

Yaghouti, M.; Sharifi, M. J.; Kazemi, A.

2013-01-01

223

Evaluation of InAs quantum dots on Si as optical modulator

NASA Astrophysics Data System (ADS)

The potential of using InAs quantum dots, epitaxially grown on a Si substrate, as an optical modulator have been investigated. By exploiting the quantum-confined Stark effect across the quantum dot layers we were able to increase the absorption in the dot layers at a chosen wavelength. This resulted in the first demonstration of an extinction ratio of 5.1 dB at 1310 nm with a reverse bias of 20 V. Higher extinction ratios of 8.6 dB at 7 V and 21.6 dB at 20 V bias were observed at a wavelength of 1355 nm.

Sandall, I. C.; Ng, J. S.; David, J. P. R.; Liu, H.; Tan, C. H.

2013-09-01

224

Visible InGaN\\/GaN Quantum-Dot Materials and Devices

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

Nicolas Grandjean; Marc Ilegems

2007-01-01

225

Quasiparticles for quantum dot array in graphene and the associated Magnetoplasmons

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 displacement from the center of the quantum dot. Consequently, the corresponding Klein-Gordon equation may be solved analytically for the single-particle eigenstates since they are given by a simple harmonic oscillator operator. The electron

Godfrey Gumbs; Oleg Berman; Pedro Echenique

2009-01-01

226

A Model for second harmonic generation in a TWO-Dimensional array of quantum dots

NASA Astrophysics Data System (ADS)

We present a model for second harmonic generation in a two-dimensional array of quantum dots. We show that the combined effect of the electromagnetic local field of the array and the intrinsic electronic resonances due to the spatial confinement in the quantum dot produce giant enhancements of the second order non-linear susceptibilities, hence giving a very large efficiency in the second harmonic generation.

Mendoza, Bernardo S.

1994-01-01

227

Resonant states of N-electron quantum dots in eletric and magnetic fields

In this work, we calculated the resonant energies and level widths of interacting electrons in a quantum dot subjected to electric and magnetic fields using the complex coordinate rotation method. The quantum dot model we consider is disk-like. There are two barriers along the growth direction (z), sandwiching a potential well at z=0, the in-plane(x-y) confinement is represented by a

W. Y. Ruan; Yia-Chung Chang

2002-01-01

228

Multiple Exciton Generation in PbSe Quantum Dots and Quantum Dot Solar Cells

Multiple exciton generation in quantum dots (QDs) has been intensively studied as a way to enhance solar energy conversion by channeling the excess photon energy (energy greater than the bandgap) to produce multiple electron-hole pairs. Among other useful properties, quantum confinement can both increase Coulomb interactions that drive the MEG process and decrease the electron-phonon coupling that cools hot-excitons in bulk semiconductors. We have demonstrated that MEG in PbSe QDs is about two times as efficient at producing multiple electron-hole pairs than bulk PbSe. I will discuss our recent results investigating MEG in PbSe, PbS and PbSxSe1-x, which exhibits an interesting size-dependence of the MEG efficiency. Thin films of electronically coupled PbSe QDs have shown promise in simple photon-to-electron conversion architectures with power conversion efficiencies above 5%. We recently reported an enhancement in the photocurrent resulting from MEG in PbSe QD-based solar cells. We find that the external quantum efficiency (spectrally resolved ratio of collected charge carriers to incident photons) peaked at 114% in the best devices measured, with an internal quantum efficiency of 130%. These results demonstrate that MEG charge carriers can be collected in suitably designed QD solar cells. We compare our results to transient absorption measurements and find reasonable agreement.

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

2012-01-01

229

TOPICAL REVIEW: Labelling of cells with quantum dots

Colloidal quantum dots are semiconductor nanocrystals well dispersed in a solvent. The optical properties of quantum dots, in particular the wavelength of their fluorescence, depend strongly on their size. Because of their reduced tendency to photobleach, colloidal quantum dots are interesting fluorescence probes for all types of labelling studies. In this review we will give an overview on how quantum

Wolfgang J. Parak; Teresa Pellegrino; Christian Plank

2005-01-01

230

Detecting quantum and classical correlations using quantum dot system

NASA Astrophysics Data System (ADS)

We investigate the thermal classical and quantum correlations in an isolated quantum dot system (QDS) including the effects of different parameters. The thermal density operator is generated by simplifying the Hamiltonian of the quantum dot to the nature Hamiltonian by integrating and finding the unitary matrix. We find that the quantum discord (QD) is more resistant against temperature effect and might be finite even for higher temperatures in the asymptotic limit. Furthermore, we show that there is an optimal value of temperature such that the different kinds of correlations are maximal. Our results show that QDS is a useful resource and may open new perspectives in different quantum information tasks.

Berrada, K.

2013-12-01

231

Quantum phase transition in a single-molecule quantum dot.

Quantum criticality is the intriguing possibility offered by the laws of quantum mechanics when the wave function of a many-particle physical system is forced to evolve continuously between two distinct, competing ground states. This phenomenon, often related to a zero-temperature magnetic phase transition, is believed to govern many of the fascinating properties of strongly correlated systems such as heavy-fermion compounds or high-temperature superconductors. In contrast to bulk materials with very complex electronic structures, artificial nanoscale devices could offer a new and simpler means of understanding quantum phase transitions. Here we demonstrate this possibility in a single-molecule quantum dot, where a gate voltage induces a crossing of two different types of electron spin state (singlet and triplet) at zero magnetic field. The quantum dot is operated in the Kondo regime, where the electron spin on the quantum dot is partially screened by metallic electrodes. This strong electronic coupling between the quantum dot and the metallic contacts provides the strong electron correlations necessary to observe quantum critical behaviour. The quantum magnetic phase transition between two different Kondo regimes is achieved by tuning gate voltages and is fundamentally different from previously observed Kondo transitions in semiconductor and nanotube quantum dots. Our work may offer new directions in terms of control and tunability for molecular spintronics. PMID:18509439

Roch, Nicolas; Florens, Serge; Bouchiat, Vincent; Wernsdorfer, Wolfgang; Balestro, Franck

2008-05-29

232

Near-Infrared Localized Surface Plasmon Resonances Arising from Free Carriers in Doped Quantum Dots

Quantum confinement of electronic wavefunctions in semiconductor quantum dots (QDs) yields discrete atom-like and tunable electronic levels, thereby allowing the engineering of excitation and emission spectra. Metal nanoparticles, on the other hand, display strong resonant interactions with light from localized surface plasmon resonance (LSPR) oscillations of free carriers, resulting in enhanced and geometrically tunable absorption and scattering resonances. The complementary

Prashant K. Jain; Joey Luther; Trevor Ewers; A. Paul Alivisatos

2010-01-01

233

Transparent conducting films of CdSe(ZnS) core(shell) quantum dot xerogels

A method of fabricating sol-gel quantum dot (QD) films is demonstrated, and their optical, structural and electrical properties are evaluated. The CdSe(ZnS) xerogel films remain quantum confined, yet are highly conductive (10?3 S· cm?1). This approach provides a pathway for the exploitation of QD gels in optoelectronic applications.

Korala, Lasantha; Li, Li

2012-01-01

234

Optical studies of quantum confined nanostructures

Recent advances in material growth techniques have led to the laboratory realization of quantum confined nanostructures. By engineering the geometry of these systems it is possible to tailor their optical, electrical and vibrational properties. We now envision integrated electronic and optical devices potentially harnessing quantum mechanical properties of photons, electrons or even phonons. The realization of these next generation devices

Anthony Nickolas Vamivakas

2008-01-01

235

Transient Currents Through Quantum Dots

NASA Astrophysics Data System (ADS)

Kubo formula is used to get the time dependent current that flows through a quantum dot after switching on a small voltage bias. Technically, the calculation involves the evaluation of the linear response function for all frequencies and is, therefore, sensibly more expensive from the computational point of view than the evaluation of the d.c. conductance. Previous estimations of the transient current were done by Prigodin et al. in Phys. Rev. Lett. 72, 546 (1994) for chaotic mesoscopic systems. Our numerical results are completely different from the purely inductive results given in the mentioned paper. Both the regular and the chaotic system show initially a linear increase of the conductance that grows well beyond its static value. Afterwards, it decreases in an oscillating fashion towards its stationary value. While oscillations quickly attenuate in the chaotic model, a power law decay is obtained for the ideal system. Apart for the rapid oscillations, the result can be modelled by a classic circuit having resistive, inductive and also capacitive elements. In principle, our result opens a straightforward experimental way allowing a clear distinction between chaotic and regular systems.

Vergés, J. A.; Louis, E.

236

Optical Properties of ZnSe, ZnSTe Quantum Dots and Wires

Nanometer-scale dots and wires, cut into molecular-beam-epitaxy grown ZnSTe quantum well and ZnSe\\/ZnSTe superlattice by electron beam lithography and dry etching processing, were studied. They have been characterized by using photoluminescence at temperatures between 10K and 300K. The enhancement and the peak shift of the photoluminescence show that further quantum confinement effect in the (1-D) quantum wires and (0-D) quantum

Allen Ng; I. K. Sou; J. Wang; Y. Wang; Weikun Ge; Z. L. Yuan; Y. S. Tang; C. M. Solomayor Torres

1996-01-01

237

Hyperfine interactions in a charged quantum dot

NASA Astrophysics Data System (ADS)

The spin state of a single electron in a quantum dot (QD) is considered as one of the candidates for the basic building block of quantum computing -the qubit. One of the most important issues in this respect is the fact that an unpaired electron interacts with the underlying nuclear spins and its spin state is affected through the hyperfine interaction. An elegant and powerful probe of a single electron spin in a QD is provided through optical excitation of the trion state, which is composed of a confined electron-hole (e-h) pair (exciton) in the presence of an electron[1]. An applied magnetic field leads to Zeeman splitting, and under some conditions, an additional Overhauser shift due to polarization of the nuclei through the hyperfine interaction. Polarized nuclei exert an effective magnetic field on conduction electrons that adds or subtracts from the Zeeman splitting, depending on the relative directions of both fields. A neutral exciton contains an unpaired electron, and its Zeeman sublevels are therefore modified by the Overhauser effect[2]. A singlet trion has paired electron spins and therefore it is not subjected to the Overhauser effect. We present high-spectral resolution magneto-photoluminescence (PL) spectroscopy of individual quantum dots charged with single electrons using a Schottky diode structure. Our device allows direct comparison between the PL of the same individual QD, with and without the presence of an electron. We observed 0.13 meV saturated Overhauser shift for trion PL at externally applied magnetic field of 6 Tesla. This shift corresponds to an effective hyperfine magnetic field of more than 5 Tesla, which directly affect the ground electron spin state. We conjecture that fluctuations in this hyperfine field dominate spin dephasing and give rise to the recently measured spin relaxation times of 10 ns for this system[2,3]. [1]J.G. Tischler, et al, Phys. Rev. B 66, 081310(R) (2002).[2]D. Gammon et al, Phys. Rev. Lett., 86, 5176 (2001).[3]R. I. Dzhioev et al,Phys. Rev. B 66, 153409 (2002)[4]I A. Merkulov et al, Phys. Rev. B 65, 205309 (2002).

Bracker, Allen S.; Gammon, Dan; Tischler, Joseph; Ware, Morgan E.; Efros, Alexander L.; Gershoni, David

2003-03-01

238

Evaporation-Induced Assembly of Quantum Dots into Nanorings

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.

Chen, Jixin; Liao, Wei-Ssu; Chen, Xin; Yang, Tinglu; Wark, Stacey E.; Son, Dong Hee; Batteas, James D.; Cremer, Paul S.

2011-01-01

239

Wide-Bandgap Quantum Dot Based Microcavity VCSEL Structures

NASA Astrophysics Data System (ADS)

In this contribution we report on the optical properties of planar and pillar structured GaN- and ZnSe-based monolithic microcavities. These structures reveal three-dimensional confined optical modes with high quality factors and potentially small mode volumes especially for the ZnSe-based samples. The measurements are completed with theoretical calculations. Furthermore, the optical emission properties of CdSe quantum dots embedded into microcavities have been studied. The Purcell effect demonstrated by means of the pronounced enhancement of the spontaneous emission rate of quantum dots coupled to the discrete optical modes of the cavities. This enhancement depends systematically on the pillar diameter and thus on the Purcell factor of the individual pillars.

Sebald, K.; Lohmeyer, H.; Gutowski, J.; Kruse, C.; Yamaguchi, T.; Gust, A.; Hommel, D.; Wiersig, J.; Baer, N.; Jahnke, F.

240

Small mode-size waveguides in quantum-dot-doped glasses by Ag-film ion exchange

We present a silver-sodium ion-exchange process for fabricating small mode-size waveguides in glass doped with PbS semiconductor quantum dots. We show that the process does not alter the optical properties of the quantum dots by comparing the waveguide and bulk luminescence spectra. We also show that the optical mode is highly confined. This field confinement produces high optical intensities, which

Jason M. Auxier; Seppo Honkanen; Michael M. Morrell; Matthew A. Leigh; Sabyasachi Sen; Nicholas F. Borrelli; Axel Schülzgen

2006-01-01

241

Unconventional Nodal Wavefunctions in Quantum Dots

NASA Astrophysics Data System (ADS)

In a single band model such as one electron in a box, it is well known that the ground state wavefunction has no node maximizing its spatial symmetry. However, the ordering of eigenstates in a multiband system e.g., p-doped semiconductor quantum dots (QDs) can be very different due to spin-orbit interaction, symmetry of the underlying lattice and geometry of the confinement. Such unconventional ordering of states has appeared in the literature [1, 2] but it is often ignored or merely considered a shortcoming of k.p model [3]. We investigate spatial structure of hole envelope-wavefunctions in QDs with a focus on its symmetry. Our calculation shows a counter-intuitive ordering of eigenstates where a single hole ``ground-state'' has a node at the center. For simplicity, we start with a 2D QD tight-binding model and extend the discussion to 3D QD tight-binding and k.p models. We also discuss experimental implications of the wavefunction ordering described above. [1] K. V'yborn'y et al., PRB 85, 155312 (2012) [2] A. Bagga et al., PRB 71, 115327 (2005); P. Horodyská et al., PRB 81, 045301 (2010); J. Xia and J. Li, PRB 60, 11540 (1999) [3] L. W. Wang et al., APL 76, 339 (2000)

Lee, Jeongsu; Výborný, Karel; žuti?, Igor; Han, Jong

2013-03-01

242

Thermometry and Refrigeration using Quantum Dots

NASA Astrophysics Data System (ADS)

The 2D electron gas in GaAs/AlGaAs heterostructures has diverse applications at cryogenic temperatures, but is heated by unintended noise in the measurement set up. Our work involves the fabrication of a quantum dot refrigerator (QDR) which can cool the gas to below the ambient lattice temperature [1]. Lithographically defined gates define three quantum dots tunnel-coupled to an enclosed, macroscopic reservoir of electrons 100 ?m^2 in area. Energy selective transport of electrons via the discrete energy levels of two quantum dots through the electron reservoir modifies its Fermi-Dirac distribution, thus cooling it. The third quantum dot (the `thermometer') probes the temperature of the reservoir being cooled by monitoring the current flowing through an adjacent quantum point contact. We have demonstrated measuring electronic temperatures in the range 100 mK to 300 mK, with an estimated error of about 10%. We have also investigated the variation in electron temperature as a function of the energies of the entrance and exit dots. Our results are consistent with cooling an area of 64?m^2 by 30 mK, starting from 150mK, and agree qualitatively with theory [2].[1] Prance e. a. Phys. Rev. Lett. 102 146602 [2] Edwards e. a. Phys. Rev. B 52 5714

Mavalankar, Aquila; Smith, Charles; Chorley, Simon; Griffiths, Jonathan; Jones, Geb; Farrer, Ian; Ritchie, David

2013-03-01

243

In-situ encapsulation of quantum dots into polymer microspheres.

We have incorporated fluorescent quantum dots (QDs) into polystyrene microspheres using functionalized oligomeric phosphine (OP) ligands. We find that a uniform distribution of quantum dots is loaded inside each polymer bead. Some local close-packing of quantum dots in the beads is attributed to the self-polymerization of the functionalized ligands. The presence of quantum dots disturbs the nucleation and growth processes during the formation of polymer microspheres and results in a wider size distribution of the quantum dot-embedded polystyrene beads than for the control without dots. The change in quantum efficiency of the quantum dots before (approximately 20%) and after (12%) loading into the beads substantiates the protection of oligomeric phosphine ligands yet indicates that the properties of these quantum dots are still affected during processing. PMID:16584256

Sheng, Wenchao; Kim, Sungjee; Lee, Jinwook; Kim, Sang-Wook; Jensen, Klavs; Bawendi, Moungi G

2006-04-11

244

A two-electron system confined in two coupled semiconductor quantum dots is investigated as a candidate for performing quantum logic operations with spin qubits. We study different processes of swapping the electron spins by a controlled switching on and off of the exchange interaction. The resulting spin swap corresponds to an elementary operation in quantum-information processing. We perform direct simulations of the time evolution of the two-electron system. Our results show that, in order to obtain the full interchange of spins, the exchange interaction should change smoothly in time. The presence of jumps and spikes in the time characteristics of the confinement potential leads to a considerable increase of the spin-swap time. We propose several mechanisms to modify the exchange interaction by changing the confinement potential profile and discuss their advantages and disadvantages.

Moskal, S.; Bednarek, S.; Adamowski, J. [Faculty of Physics and Applied Computer Science, AGH University of Science and Technology, Cracow (Poland)

2007-09-15

245

Quantum Zeno effect rationalizes the phonon bottleneck in semiconductor quantum dots.

Quantum confinement can dramatically slow down electron-phonon relaxation in nanoclusters. Known as the phonon bottleneck, the effect remains elusive. Using a state-of-the-art time-domain ab initio approach, we model the observed bottleneck in CdSe quantum dots and show that it occurs under quantum Zeno conditions. Decoherence in the electronic subsystem, induced by elastic electron-phonon scattering, should be significantly faster than inelastic scattering. Achieved with multiphonon relaxation, the phonon bottleneck is broken by Auger processes and structural defects, rationalizing experimental difficulties. PMID:23683182

Kilina, Svetlana V; Neukirch, Amanda J; Habenicht, Bradley F; Kilin, Dmitri S; Prezhdo, Oleg V

2013-05-02

246

Spatial variation of available electronic excitations within individual quantum dots.

Quantum dots (QDs) allow for manipulation of the position and energy levels of electrons at sub-10 nm length scales through control of material chemistry, size, and shape. It is known from optical studies that the bandgap of semiconductor QDs increases as their size decreases due to the narrowing of the quantum confinement potential. The mechanism of quantum confinement also indicates that the localized properties within individual QDs should depend on their shape in addition to their size, but direct observations of this effect have proven challenging due to the limited spatial resolution of measurement techniques at this scale and the ability to remove contributions from the surroundings. Here we present experimental evidence of spatial variations in the lowest available electron transition energy within a series of single electrically isolated QDs due to a dome-shaped geometry, measured using electron energy-loss spectroscopy in a (scanning) transmission electron microscope [(S)TEM-EELS]. We observe a consistent increase in the energy onset of electronic excitations from the lateral center of the dot toward the edges, which we attribute purely to shape. This trend is in qualitative agreement with a simple quantum simulation of the local density of states in a dome-shaped QD. PMID:23276278

Jung, Hee Joon; Dasgupta, Neil P; Van Stockum, Philip B; Koh, Ai Leen; Sinclair, Robert; Prinz, Fritz B

2013-01-04

247

Red, green and blue lasing enabled by single-exciton gain in colloidal quantum dot films

NASA Astrophysics Data System (ADS)

Colloidal quantum dots exhibit efficient photoluminescence with widely tunable bandgaps as a result of quantum confinement effects. Such quantum dots are emerging as an appealing complement to epitaxial semiconductor laser materials, which are ubiquitous and technologically mature, but unable to cover the full visible spectrum (red, green and blue; RGB). However, the requirement for high colloidal-quantum-dot packing density, and losses due to non-radiative multiexcitonic Auger recombination, have hindered the development of lasers based on colloidal quantum dots. Here, we engineer CdSe/ZnCdS core/shell colloidal quantum dots with aromatic ligands, which form densely packed films exhibiting optical gain across the visible spectrum with less than one exciton per colloidal quantum dot on average. This single-exciton gain allows the films to reach the threshold of amplified spontaneous emission at very low optical pump energy densities of 90 µJ cm-2, more than one order of magnitude better than previously reported values. We leverage the low-threshold gain of these nanocomposite films to produce the first colloidal-quantum-dot vertical-cavity surface-emitting lasers (CQD-VCSEL). Our results represent a significant step towards full-colour single-material lasers.

Dang, Cuong; Lee, Joonhee; Breen, Craig; Steckel, Jonathan S.; Coe-Sullivan, Seth; Nurmikko, Arto

2012-05-01

248

Quantum dots: a new approach to low V? optical modulators

NASA Astrophysics Data System (ADS)

Preliminary analysis has shown that quantum dots enable tens of millivolt-range operation of phase-shifters in a semiconductor Mach-Zehnder interferometer modulator. Our methodology based upon the quantum dot experimental work of Hse et al, makes use of his measured exciton line shapes to estimate refractive index changes in a PIN structure in which the intrinsic laser is loaded with self-organizing quantum dots and their associated wetting layers. We consider both forward and reversed bias cases; in the former, the interferometer phase shift sections become DFB lasers, and in the latter, the phase shift is caused by the quantum-confined Stark effect (QCSE). With the latter, we found a trade-off between low operating voltage and modulating bandwidth. For a phase shifter insertion loss of 5 dB, a 250-micron long phase section will yield a pi/2 control voltage of 50 mV at a bandwidth of around 18 GHz. Ifi 90 mV control voltage swing can be tolerated, the modulator bandwidth increases to 30 GHz. If a resonant tunneling diode (RTD) is made part of the assembly, the local E-field is enhanced by a factor of 5 to 10, thereby reducing the drive requirements even further. Similar, though narrower bandwidth results were noted for the DFB laser phase modulator concept.

Twyford Kunkee, Elizabeth; Livingston, Peter M.; Holm, Steve

2003-06-01

249

Quantum Dot-Based Cell Motility Assay

Because of their favorable physical and photochemical properties, colloidal CdSe/ZnS-semiconductor nanocrystals (commonly known as quantum dots) have enormous potential for use in biological imaging. In this report, we present an assay that uses quantum dots as markers to quantify cell motility. Cells that are seeded onto a homogeneous layer of quantum dots engulf and absorb the nanocrystals and, as a consequence, leave behind a fluorescence-free trail. By subsequently determining the ratio of cell area to fluorescence-free track area, we show that it is possible to differentiate between invasive and noninvasive cancer cells. Because this assay uses simple fluorescence detection, requires no significant data processing, and can be used in live-cell studies, it has the potential to be a powerful new tool for discriminating between invasive and noninvasive cancer cell lines or for studying cell signaling events involved in migration.

Gu, Weiwei; Pellegrino, Teresa; Parak Wolfgang J; Boudreau,Rosanne; Le Gros, Mark A.; Gerion, Daniele; Alivisatos, A. Paul; Larabell, Carolyn A.

2005-06-06

250

Dynamics of Avalanche Quantum Dot Infrared Photodetectors

NASA Astrophysics Data System (ADS)

Time and frequency response of an avalanche quantum dot infrared photodetector (A-QDIP) operating at long infrared (IR) wavelengths is calculated and the effect of its structure on the dynamic behavior is studied. For this purpose, the rate equations of different regions are numerically solved considering the boundary conditions. Results show that detector with long multiplication region has a slower time response. Also frequency analysis predicts a 3-dB bandwidth above 100 GHz for a device with multiplication length of 200 nm. Gain bandwidth product (GBP) is calculated and a value of about 1000 GHz is obtained. Effect of charge layer doping on dynamic response of detector is also studied and results show that increase in doping improves the GBP while the bandwidth is reduced. We also study the effect of quantum dots of absorption region on frequency response of device and results show that longer electron relaxation time into quantum dot decreases the bandwidth of detector.

Zavvari, Mahdi; Ahmadi, Vahid

2012-12-01

251

Exciton binding energy in semiconductor quantum dots

In the adiabatic approximation in the context of the modified effective mass approach, in which the reduced exciton effective mass {mu} = {mu}(a) is a function of the radius a of the semiconductor quantum dot, an expression for the exciton binding energy E{sub ex}(a) in the quantum dot is derived. It is found that, in the CdSe and CdS quantum dots with the radii a comparable to the Bohr exciton radii a{sub ex}, the exciton binding energy E{sub ex}(a) is substantially (respectively, 7.4 and 4.5 times) higher than the exciton binding energy in the CdSe and CdS single crystals.

Pokutnii, S. I., E-mail: Pokutnyi_Sergey@inbox.ru [National Academy of Sciences of Ukraine, G.V. Kurdjumov Institute for Metal Physics (Ukraine)

2010-04-15

252

Generation of singlet oxygen and other radical species by quantum dot and carbon dot nanosensitizers

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

Roman Generalov; Ingeborg L. Christensen; Wei Chen; Ya-Ping Sun; Solveig Kristensen; Petras Juzenas

2009-01-01

253

Mapping image potential states on graphene quantum dots.

Free-electron-like image potential states are observed in scanning tunneling spectroscopy on graphene quantum dots on Ir(111) acting as potential wells. The spectrum strongly depends on the size of the nanostructure as well as on the spatial position on top, indicating lateral confinement. Analysis of the substructure of the first state by the spatial mapping of the constant energy local density of states reveals characteristic patterns of confined states. The most pronounced state is not the ground state, but an excited state with a favorable combination of the local density of states and parallel momentum transfer in the tunneling process. Chemical gating tunes the confining potential by changing the local work function. Our experimental determination of this work function allows us to deduce the associated shift of the Dirac point. PMID:23952430

Craes, Fabian; Runte, Sven; Klinkhammer, Jürgen; Kralj, Marko; Michely, Thomas; Busse, Carsten

2013-07-31

254

Quantum and semiclassical study of magnetic quantum dots

NASA Astrophysics Data System (ADS)

We study the energy level structure of two-dimensional charged particles in a circular quantum dot in inhomogeneous magnetic fields. In this system, the magnetic field is zero inside the dot and constant outside. Such a device can be fabricated with present-day technology. We present detailed semiclassical studies of such magnetic quantum dot systems and provide a comparison with exact quantum calculations. In the semiclassical approach we apply the Berry-Tabor formula for the density of states and the Borh-Sommerfeld quantization rules. In both cases we found good agreement with the exact spectrum in the weak magnetic field limit. The energy spectrum for a given missing flux quantum is classified in six possible classes of orbits and summarized in a so-called phase diagram. We also investigate the current flow patterns of different quantum states and show a clear correspondence with classical trajectories.

Kocsis, Bence; Palla, Gergely; Cserti, József

2005-02-01

255

Dirac electrons in graphene-based quantum wires and quantum dots.

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. PMID:21715777

Peres, N M R; Rodrigues, J N B; Stauber, T; Lopes Dos Santos, J M B

2009-07-27

256

Synergistic Tunability of Fluorescence Using Coupled Quantum Dots

Size and shape controlled semiconductor quantum dots have been widely adopted in tailoring nanomaterials properties, which basically utilize band edge engineering extending the band gap to larger values than the bulk. However, the long range electronic transition tunability is difficult to achieve using size or shape controlled quantum dots. Alternatively, chemically coupled quantum dots offer a novel route for tuning

Sucheta Sengupta; Somobrata Acharya

2011-01-01

257

A study of confined quantum systems using the Woods-Saxon potential

NASA Astrophysics Data System (ADS)

We propose the Woods-Saxon (WS) potential to simulate spatial confinement. The great advantage of our methodology is that it enables the study of a wide range of systems and confinement regimes by varying two parameters in the model potential. To test the methodology we have studied the confined harmonic oscillator in two different regimes: when the confinement potential exhibits a sudden jump; and when the confinement is described by a smooth function. We have also applied the present procedure to a realistic problem, a confined quantum dot-atom. The numerical calculation is performed with the equally spaced discrete variable representation (DVR). Our results are in close agreement with those available in the literature, and we believe our method to be a good alternative for studying confined quantum systems.

Costa, L. S.; Prudente, F. V.; Acioli, P. H.; Soares Neto, J. J.; Vianna, J. D. M.

1999-05-01

258

Collective Excitations in Cylindrical Quantum Dots Chains

NASA Astrophysics Data System (ADS)

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).

Vergara, Jimena; Camacho, Angela

2009-03-01

259

Spin states in graphene quantum dots

NASA Astrophysics Data System (ADS)

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 opposite to what has been observed in standard semiconductor based quantum dots. Graphene quantum dots are thought to be good candidates for spin-based quantum information processing since spin-orbit interactions and hyperfine coupling are both expected to be weak. We investigated graphene quantum dots in the single-level transport regime in in-plane magnetic fields where orbital effects are expected to have a minor effect [6]. The g-factor is found to be g 2 and the spin filling sequence of orbital levels can be understood in view of the strength of the exchange interaction which is independent of carrier density in graphene. [4pt] [1] J. Guttinger, C. Stampfer, F. Libisch, T. Frey, J. Burgdoerfer, T. Ihn, K. Ensslin, Phys. Rev. Lett. 103, 046810 (2009) [0pt] [2] T. Ihn, J. Guttinger, F. Molitor, S. Schnez, E. Schurtenberger, A. Jacobsen, S. Hellmuller, T. Frey, S. Droscher, C. Stampfer, and K. Ensslin, Materials Today 13, 44 (2010) [0pt] [3] F. Molitor, H. Knowles, S. Droscher, U. Gasser, T. Choi, P. Roulleau, J. Guttinger, A. Jacobsen, C. Stampfer, K. Ensslin and T. Ihn, Europhys. Lett. 89, 67005 (2010) [0pt] [4] M. Huefner, F. Molitor, A. Jacobsen, A. Pioda, C.Stampfer, K. Ensslin and T. Ihn, N. J. of Phys. 12, 043054 (2010) [0pt] [5] C. Stampfer, J. Guttinger, S. Hellmuller, F. Molitor, K. Ensslin, and T. Ihn, Phys. Rev. Lett. 102, 056403 (2009) [0pt] [6] J. Guttinger, T. Frey, C. Stampfer, T. Ihn, and K. Ensslin, Phys. Rev. Lett. 105, 116801 (2010)

Ensslin, Klaus

2011-03-01

260

Electron transport through a quantum dot assisted by cavity photons.

We investigate transient transport of electrons through a single quantum dot controlled by a plunger gate. The dot is embedded in a finite wire with length Lx assumed to lie along the x-direction with a parabolic confinement in the y-direction. The quantum wire, originally with hard-wall confinement at its ends, ±Lx/2, is weakly coupled at t = 0 to left and right leads acting as external electron reservoirs. The central system, the dot and the finite wire, is strongly coupled to a single cavity photon mode. A non-Markovian density-matrix formalism is employed to take into account the full electron-photon interaction in the transient regime. In the absence of a photon cavity, a resonant current peak can be found by tuning the plunger-gate voltage to lift a many-body state of the system into the source-drain bias window. In the presence of an x-polarized photon field, additional side peaks can be found due to photon-assisted transport. By appropriately tuning the plunger-gate voltage, the electrons in the left lead are allowed to undergo coherent inelastic scattering to a two-photon state above the bias window if initially one photon was present in the cavity. However, this photon-assisted feature is suppressed in the case of a y-polarized photon field due to the anisotropy of our system caused by its geometry. PMID:24132041

Abdullah, Nzar Rauf; Tang, Chi-Shung; Manolescu, Andrei; Gudmundsson, Vidar

2013-10-17

261

Electron transport through a quantum dot assisted by cavity photons

NASA Astrophysics Data System (ADS)

We investigate transient transport of electrons through a single quantum dot controlled by a plunger gate. The dot is embedded in a finite wire with length Lx assumed to lie along the x-direction with a parabolic confinement in the y-direction. The quantum wire, originally with hard-wall confinement at its ends, ±Lx/2, is weakly coupled at t = 0 to left and right leads acting as external electron reservoirs. The central system, the dot and the finite wire, is strongly coupled to a single cavity photon mode. A non-Markovian density-matrix formalism is employed to take into account the full electron–photon interaction in the transient regime. In the absence of a photon cavity, a resonant current peak can be found by tuning the plunger-gate voltage to lift a many-body state of the system into the source–drain bias window. In the presence of an x-polarized photon field, additional side peaks can be found due to photon-assisted transport. By appropriately tuning the plunger-gate voltage, the electrons in the left lead are allowed to undergo coherent inelastic scattering to a two-photon state above the bias window if initially one photon was present in the cavity. However, this photon-assisted feature is suppressed in the case of a y-polarized photon field due to the anisotropy of our system caused by its geometry.

Abdullah, Nzar Rauf; Tang, Chi-Shung; Manolescu, Andrei; Gudmundsson, Vidar

2013-11-01

262

Undoped Si/SiGe Depletion-Mode Few-Electron Double Quantum Dots

NASA Astrophysics Data System (ADS)

We have successfully formed a double quantum dot in the sSi/SiGe material system without need for intentional dopants. In our design, a two-dimensional electron gas is formed in a strained silicon well by forward biasing a global gate. Lateral definition of quantum dots is established with reverse-biased gates with ˜40 nm critical dimensions. Low-temperature capacitance and Hall measurements confirm electrons are confined in the Si-well with mobilities >10^4 cm^2/V-s. Further characterization identifies practical gate bias limits for this design and will be compared to simulation. Several double dot devices have been brought into the few-electron Coulomb blockade regime as measured by through-dot transport. Honeycomb diagrams and nonlinear through-dot transport measurements are used to quantify dot capacitances and addition energies of several meV. Sponsored by United States Department of Defense. Approved for Public Release, Distribution Unlimited.

Borselli, Matthew; Huang, Biqin; Ross, Richard; Croke, Edward; Holabird, Kevin; Hazard, Thomas; Watson, Christopher; Kiselev, Andrey; Deelman, Peter; Alvarado-Rodriguez, Ivan; Schmitz, Adele; Sokolich, Marko; Gyure, Mark; Hunter, Andrew

2011-03-01

263

Resonant tunneling in graphene pseudomagnetic quantum dots.

Realistic relaxed configurations of triaxially strained graphene quantum dots are obtained from unbiased atomistic mechanical simulations. The local electronic structure and quantum transport characteristics of y-junctions based on such dots are studied, revealing that the quasi-uniform pseudomagnetic field induced by strain restricts transport to Landau level- and edge state-assisted resonant tunneling. Valley degeneracy is broken in the presence of an external field, allowing the selective filtering of the valley and chirality of the states assisting in the resonant tunneling. Asymmetric strain conditions can be explored to select the exit channel of the y-junction. PMID:23659203

Qi, Zenan; Bahamon, D A; Pereira, Vitor M; Park, Harold S; Campbell, D K; Neto, A H Castro

2013-05-15

264

Resonant Tunneling in Graphene Pseudomagnetic Quantum Dots

NASA Astrophysics Data System (ADS)

Realistic relaxed configurations of triaxially strained graphene quantum dots are obtained from unbiased atomistic mechanical simulations. The local electronic structure and quantum transport characteristics of y-junctions based on such dots are studied, revealing that the quasi-uniform pseudomagnetic field induced by strain restricts transport to Landau level- and edge state-assisted resonant tunneling. Valley degeneracy is broken in the presence of an external field, allowing the selective filtering of the valley and chirality of the states assisting in the resonant tunneling. Asymmetric strain conditions can be explored to select the exit channel of the y-junction.

Qi, Zenan; Bahamon, D. A.; Pereira, Vitor M.; Park, Harold S.; Campbell, D. K.; Neto, A. H. Castro

2013-06-01

265

Trion X+ in vertically coupled type II quantum dots in threading magnetic field

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.

2012-01-01

266

Mid-infrared quantum dot emitters utilizing planar photonic crystal technology.

The three-dimensional confinement inherent in InAs self-assembled quantum dots (SAQDs) yields vastly different optical properties compared to one-dimensionally confined quantum well systems. Intersubband transitions in quantum dots can emit light normal to the growth surface, whereas transitions in quantum wells emit only parallel to the surface. This is a key difference that can be exploited to create a variety of quantum dot devices that have no quantum well analog. Two significant problems limit the utilization of the beneficial features of SAQDs as mid-infrared emitters. One is the lack of understanding concerning how to electrically inject carriers into electronic states that allow optical transitions to occur efficiently. Engineering of an injector stage leading into the dot can provide current injection into an upper dot state; however, to increase the likelihood of an optical transition, the lower dot states must be emptied faster than upper states are occupied. The second issue is that SAQDs have significant inhomogeneous broadening due to the random size distribution. While this may not be a problem in the long term, this issue can be circumvented by using planar photonic crystal or plasmonic approaches to provide wavelength selectivity or other useful functionality.

Subramania,Ganapathi Subramanian; Lyo, Sungkwun Kenneth; Cederberg, Jeffrey George; Passmore, Brandon Scott; El-Kady, Ihab Fathy; Shaner, Eric Arthur

2008-09-01

267

Coherence and control of single electron spins in quantum dots

NASA Astrophysics Data System (ADS)

Following our earlier work on single-shot read-out and relaxation of a single spin in a quantum dot, we now demonstrate coherent control of a single spin (detection is done using a second spin in a neighbouring dot). First, we manipulate the spin using conventional magnetic resonance. Next, we show that we can also rotate the spin using electric fields instead of magnetic fields. In both cases, 90 rotations can be realized in about 50 ns or less. We use these control techniques to probe decoherence of an isolated electron spin. The spin dephases in about 30 ns, due to the hyperfine interaction with the uncontrolled nuclear spin bath in the host material of the dot. However, since the nuclear spin dynamics is very slow, this dephasing can be largely reversed using a spin-echo pulse. Echo decay times of about 0.5 us are obtained at 70 mT. In parallel, we have started work on quantum dots in graphene, which is expected to offer superior coherence times. As a first step, we have succeeded in opening a bandgap in bilayer graphene, necessary for electrostatic confinement of carriers. F.H.L. Koppens et al., Nature 446, 56 (2006). K.C. Nowack et al., Science Express, 1 Nov 2007. F.H.L. Koppens et al., arXiv:0711.0479. J.B. Oostinga, Nature Mat., in press.

Vandersypen, Lieven

2008-03-01

268

NASA Astrophysics Data System (ADS)

We apply discoveries in nanoscience towards applications relevant to health, environment, security, and connectedness. A materials fundamental to our research is the quantum dot. Each quantum dot is a particle of semiconductor only a few nanometers in diameter. These semiconductor nanoparticles confine electrons to within their characteristic wavelength. Thus, just as changing the length of a guitar string changes the frequency of sound produced, so too does changing the size of a quantum dot alter the frequency - hence energy - the electron can adopt. As a result, quantum dots are tunable matter (Fig. 2). We work with colloidal quantum dots, nanoparticles produced in, and processed from, solution. They can be coated onto nearly anything - a semiconductor substrate, a window, a wall, fabric. Compared to epitaxially-grown semiconductors used to make optical detectors, lasers, and modulators, they are cheap, safe to work with, and easy to produce. Much of our work with quantum dots involves infrared light - its measurement, production, modulation, and harnessing. While there exists an abundance of work in colloidal quantum dots active in the visible, there are fewer results in the infrared. The wavelengths between 1000 and 2000 nm are nonetheless of great practical importance: half of the sun's power reaching the earth lies in this wavelength range; 'biological windows' in which tissue is relatively transparent and does not emit background light (autofluorescence) exist in the infrared; fiber-optic networks operate at 1.3 and 1.5 um.

Sargent, Edward H.

2006-03-01

269

Size dependence in tunneling spectra of PbSe quantum-dot arrays.

Interdot Coulomb interactions and collective Coulomb blockade were theoretically argued to be a newly important topic, and experimentally identified in semiconductor quantum dots, formed in the gate confined two-dimensional electron gas system. Developments of cluster science and colloidal synthesis accelerated the studies of electron transport in colloidal nanocrystal or quantum-dot solids. To study the interdot coupling, various sizes of two-dimensional arrays of colloidal PbSe quantum dots are self-assembled on flat gold surfaces for scanning tunneling microscopy and scanning tunneling spectroscopy measurements at both room and liquid-nitrogen temperatures. The tip-to-array, array-to-substrate, and interdot capacitances are evaluated and the tunneling spectra of quantum-dot arrays are analyzed by the theory of collective Coulomb blockade. The current-voltage of PbSe quantum-dot arrays conforms properly to a scaling power law function. In this study, the dependence of tunneling spectra on the sizes (numbers of quantum dots) of arrays is reported and the capacitive coupling between quantum dots in the arrays is explored. PMID:19546498

Ou, Y C; Cheng, S F; Jian, W B

2009-06-23

270

Nonlocal theory of collective excitations in quantum-dot arrays

We present a nonlocal theory of collective excitations in quantum-dot arrays. Selection rules, oscillator strengths, and Coulomb interactions inside a dot and between dots are discussed. The collective excitation energy is found to ``saturate'' for n0 (the number of electrons per dot) greater than 3. The depolarization energy shift in a quantum-dot array is found to be predominantly due to

Weiming Que; George Kirczenow; Eleuterio Castao

1991-01-01

271

Multiexciton Spectroscopy of a Single Self-Assembled Quantum Dot

NASA Astrophysics Data System (ADS)

We apply low temperature confocal optical microscopy to spatially resolve, and spectroscopically study, a single self-assembled quantum dot. By comparing the emission spectra obtained at various excitation levels to a theoretical many body model, we show that (a) single exciton radiative recombination is very weak, and (b) sharp spectral lines are due to optical transitions between confined multiexcitonic states among which excitons thermalize within their lifetimes. Once these few states are fully occupied, broadbands appear due to transitions between states which contain electrons in the continuum.

Dekel, E.; Gershoni, D.; Ehrenfreund, E.; Spektor, D.; Garcia, J. M.; Petroff, P. M.

1998-06-01

272

NASA Astrophysics Data System (ADS)

We study the potential for coherent transfer of a single electron between the ground states of two anharmonic coupled quantum dots using an adiabatic passage method. We consider the interaction of a quantum dot structure characterized by an asymmetric confining potential by two external electromagnetic fields. We use the method of stimulated Raman adiabatic passage (STIRAP) for control of the dynamics of the electron, and ultimately for succeeding the transfer of the electron from the initially occupied quantum dot to the other quantum dot. Results for several values of the parameters of the applied electromagnetic fields are presented, and we show that STIRAP can lead to efficient coherent single electron transfer for a wide range of these parameters.

Fountoulakis, Antonios; Paspalakis, Emmanuel

2013-05-01

273

Surface ligands increase photoexcitation relaxation rates in CdSe quantum dots.

Understanding the pathways of hot exciton relaxation in photoexcited semiconductor nanocrystals, also called quantum dots (QDs), is of paramount importance in multiple energy, electronics and biological applications. An important nonradiative relaxation channel originates from the nonadiabatic (NA) coupling of electronic degrees of freedom to nuclear vibrations, which in QDs depend on the confinement effects and complicated surface chemistry. To elucidate the role of surface ligands in relaxation processes of nanocrystals, we study the dynamics of the NA exciton relaxation in Cd(33)Se(33) semiconductor quantum dots passivated by either trimethylphosphine oxide or methylamine ligands using explicit time-dependent modeling. The large extent of hybridization between electronic states of quantum dot and ligand molecules is found to strongly facilitate exciton relaxation. Our computational results for the ligand contributions to the exciton relaxation and electronic energy-loss in small clusters are further extrapolated to larger quantum dots. PMID:22742432

Kilina, Svetlana; Velizhanin, Kirill A; Ivanov, Sergei; Prezhdo, Oleg V; Tretiak, Sergei

2012-07-11

274

Reconfigurable visible quantum dot microlasers integrated on a silicon chip

NASA Astrophysics Data System (ADS)

Developing on-chip, dynamically reconfigurable visible lasers that can be integrated with additional optical and electronic components will enable adaptive optical components. In the present work, we demonstrate a reconfigurable quantum dot laser based on an integrated silica ultra high-Q microcavity. By attaching the quantum dot using a reversible, non-destructive bioconjugation process, the ability to remove and replace it with an alternative quantum dot without damaging the underlying microcavity device has been demonstrated. As a result of the absorption/emission characteristics of quantum dots, the same laser source can be used to excite quantum dots with distinct emission wavelengths.

Mehrabani, Simin; Hunt, Heather K.; Armani, Andrea M.

2012-02-01

275

Synthesis and size dependent optical studies in CdSe quantum dots via inverse micelle technique

Cadmium selenide quantum dots (CdSe QDs) were successfully synthesized without using trioctylphosphine (TOP). The XRD pattern showed zinc-blend phase of the CdSe QDs. The absorption and PL spectra exhibit a strong blue shift as the QDs size decreases due to the quantum confinement effect. In addition, the quantum efficiency of CdSe QDs with TOP capping is higher than CdSe QDs

Nor Aliya Hamizi; Mohd Rafie Johan

2010-01-01

276

Magnetic properties of graphene quantum dots

NASA Astrophysics Data System (ADS)

Using the tight-binding approximation we calculated the diamagnetic susceptibility of graphene quantum dots (GQDs) of different geometrical shapes and characteristic sizes of 2-10 nm, when the magnetic properties are governed by the electron edge states. Two types of edge states can be discerned: the zero-energy states (ZESs), located exactly at the zero-energy Dirac point, and the dispersed edge states (DESs), with the energy close but not exactly equal to zero. DESs are responsible for a temperature-independent diamagnetic response, while ZESs provide a temperature-dependent spin paramagnetism. Hexagonal, circular, and randomly shaped GQDs contain mainly DESs, and, as a result, they are diamagnetic. The edge states of the triangular GQDs are of ZES type. These dots reveal the crossover between spin paramagnetism, dominating for small dots and at low temperatures, and orbital diamagnetism, dominating for large dots and at high temperatures.

Espinosa-Ortega, T.; Luk'yanchuk, I. A.; Rubo, Y. G.

2013-05-01

277

In this thesis, we study solution-processed lead sulfide quantum dots for near-infrared quantum information and communication applications. Quantum dots processed through synthetic routes and colloidally suspended in solution offer far-reaching device application possibilities that are unparalelled in traditional self-assembled quantum dots. Lead sulfide quantum dots are especially promising for near-infrared quantum optics due to their optical emission at the wavelengths

Ranojoy Bose

2009-01-01

278

Interaction effects in coupled quantum dots.

NASA Astrophysics Data System (ADS)

We study a linear array of coupled semiconductor quantum dots (``artificial molecules'') using an extended Hubbard Hamiltonian approach to account for the effects of intradot and interdot Coulomb interactions, as well as interdot tunneling. We obtain the electron addition spectrum from direct diagonalizations (C. A. Stafford and S. Das Sarma, Phys. Rev. Lett. 72), 3590 (1994) for identical double and triple quantum dots explaining qualitatively the splitting of the conductance peaks observed in recent experiments on these systems. We note that, depending on the number of particles in the system, the interdot Coulomb interaction gives rise to an interesting asymmetry in the conduction spectrum and is also responsible for splitting of the peaks even for weak coupling. We also consider dots of different sizes (``heteronuclear molecules'') and calculate the relevant many-body overlaps that enter in the calculation of the current in the nonlinear regime, to make contact with transport spectroscopy experiments.

Ramirez, F.; Cota, E.; Ulloa, S. E.

1996-03-01

279

Quantum-dot infrared photodetectors: a review

NASA Astrophysics Data System (ADS)

Quantum-dot infrared photodetectors (QDIPs) are positioned to become an important technology in the field of infrared (IR) detection, particularly for high-temperature, low-cost, high-yield detector arrays required for military applications. High-operating temperature (>=150 K) photodetectors reduce the cost of IR imaging systems by enabling cryogenic dewars and Stirling cooling systems to be replaced by thermo-electric coolers. QDIPs are well-suited for detecting mid-IR light at elevated temperatures, an application that could prove to be the next commercial market for quantum dots. While quantum dot epitaxial growth and intraband absorption of IR radiation are well established, quantum dot non-uniformity remains as a significant challenge. Nonetheless, state-of-the-art mid-IR detection at 150 K has been demonstrated using 70-layer InAs/GaAs QDIPs, and QDIP focal plane arrays are approaching performance comparable to HgCdTe at 77 K. By addressing critical challenges inherent to epitaxial QD material systems (e.g., controlling dopant incorporation), exploring alternative QD systems (e.g., colloidal QDs), and using bandgap engineering to reduce dark current and enhance multi-spectral detection (e.g. resonant tunneling QDIPs), the performance and applicability of QDIPs will continue to improve.

Stiff-Roberts, Adrienne D.

2009-04-01

280

Integrated photonics using colloidal quantum dots

NASA Astrophysics Data System (ADS)

Integrated photonic devices were realized using colloidal quantum dot composites such as flexible microcavity laser, microdisk emitters and integrated active-passive waveguides. The microcavity laser structure was realized using spin coating and consisted of an all-polymer distributed Bragg reflector with a poly-vinyl carbazole cavity layer embedded with InGaP/ZnS colloidal quantum dots. These microcavities can be peeled off the substrate yielding a flexible structure that can conform to any shape and whose emission spectra can be mechanically tuned. Planar photonic devices consisting of vertically coupled microring resonators, microdisk emitters, active-passive integrated waveguide structures and coupled active microdisk resonators were realized using soft lithography, photo-lithography, and electron beam lithography, respectively. The gain medium in all these devices was a composite consisting of quantum dots embedded in SU8 matrix. Finally, the effect of the host matrix on the optical properties of the quantum dots using results of steady-state and time-resolved luminescence measurements was determined. In addition to their specific functionalities, these novel device demonstrations and their development present a low-cost alternative to the traditional photonic device fabrication techniques.

Menon, Vinod M.; Husaini, Saima; Okoye, Nicky; Valappil, Nikesh V.

2009-11-01

281

PLAs in Quantum-dot Cellular Automata

Various implementations of the Quantum-dot Cel- lular Automata (QCA) device architecture may help many performance scaling trends continue as we approach the nano- scale. Experimental success has led to the evolution of a research track that looks at QCA-based design. The work presented in this paper follows that track and looks at implementation friendly, programmable QCA circuits. Specifically, we present

Xiaobo Sharon Hu; Michael Crocker; Michael T. Niemier; Minjun Yan; Gary H. Bernstein

2006-01-01

282

Charge pumping in carbon nanotube quantum dots

We investigate charge pumping in carbon nanotube quantum dots driven by the electric field of a surface acoustic wave. We find that at small driving amplitudes, the pumped current reverses polarity as the conductance is tuned through a Coulomb blockade peak using a gate electrode. We study the behavior as a function of wave amplitude, frequency and direction and develop

M. R. Buitelaar; V. Kashcheyevs; P. J. Leek; V. I. Talyanskii; C. G. Smith; D. Anderson; G. A. C. Jones; J. Wei; D. H. Cobden

283

Quantum dot lasers: Theory and experiment

Using of structures with size quantization in all three directions, or quantum dots (QD's) allows exciting possibilities in device engineering. Semiconductor heterostructures with self-organized QDs have experimentally exhibited properties expected for zero-dimensional systems. When used as active layer in the injection lasers, these advantages help to strongly increase material gain and differential gain, to improve temperature stability of the threshold

D. Bimberg; M. Grundmann; F. Heinrichsdorff; N. N. Ledentsov; Ch. Ribbat; R. Sellin; Zh. I. Alferov; P. S. Kop'ev; M. V. Maximov; V. M. Ustinov; A. E. Zhukov; J. A. Lott

2001-01-01

284

Quantum dot lasers: breakthrough in optoelectronics

Semiconductor heterostructures with self-organized quantum dots (QDs) have experimentally exhibited properties expected for zero-dimensional systems. When used as active layer in the injection lasers, these advantages help to strongly increase material gain and differential gain, to improve temperature stability of the threshold current, and to provide improved dynamic properties. Molecular beam epitaxy (MBE) represents a developed technology well suited for

D. Bimberg; M. Grundmann; F. Heinrichsdorff; N. N. Ledentsov; V. M. Ustinov; A. E. Zhukov; A. R. Kovsh; M. V. Maximov; Y. M. Shernyakov; B. V. Volovik; A. F. Tsatsul’nikov; P. S. Kop’ev; Zh. I. Alferov

2000-01-01

285

Peptide Coated Quantum Dots for Biological Applications

Quantum dots (QDOTs) have been widely recognized by the scientific community and the biotechnology industry, as witnessed by the exponential growth of this field in the past several years. We describe the synthesis and characterization of visible and near infrared QDots—a critical step for engineering organic molecules like proteins and peptides for building nanocomposite materials with multifunctional properties suitable for biological applications.

Iyer, Gopal; Pinaud, Fabien; Tsay, James; Li, Jack J.; Bentolila, Laurent A.; Michalet, Xavier; Weiss, Shimon

2011-01-01

286

Double Quantum Dots in Carbon Nanotubes.

National Technical Information Service (NTIS)

We study the few-electron eigenspectrum of a carbon-nanotube double quantum dot with spin-orbit coupling. Exact calculation are combined with a simple model to provide an intuitive and accurate description of single- particle and interaction effects. For ...

A. M. Rey B. Wunsch E. Demler J. Von Stecher M. Lukin

2010-01-01

287

Dark Current in Quantum Dot Infrared Photodetectors

We present the results of a new analytical model for the analysis of the dark current in realistic quantum dot infrared photodetectors (QDIPs). This model includes the effect of the space charge formed by electrons captured in QDs and donors, the self-consistent electric potential in the QDIP active region, the activation character of the electron capture and its limitation by

Victor Ryzhii; Victor Pipa; Irina Khmyrova; Vladimir Mitin; Magnus Willander

2000-01-01

288

Applications of quantum dots in cell biology

NASA Astrophysics Data System (ADS)

Quantum dots promise to revolutionize the way fluorescence imaging is used in the Cell Biology field. The unique fluorescent spectral characteristics, high photostability, low photobleaching and tight emission spectra of quantum dots, position them above traditional dyes. Here we will address the ability of EviTags, which are water stabilized quantum dot products from Evident Technologies, to behave as effective FRET donors in cells. EviTag-Hops Yellow (HY; Emission 566nm; Donor) conjugated to biotin were bound to stretapvidin-Alexa568 (Acceptor) conjugates. These HYbiotin-streptavidin-Alexa568 FRET EviTag conjugates were then internalized by fluid-phase into non-polarized MDCK cells. Confocal microscopy detects these FRET EviTag conjugates in endocytic compartments, suggesting that EviTags can be used to track fluid-phase internalization and trafficking. EviTags are shown here to be effective FRET donors when internalized into cells. Upon pairing with the appropriate acceptor dyes, quantum dots will reduce the laborious data processing that is required to compensate for bleed through contamination between organic dye donor and acceptor pair signals. The EviTag technology will simplify and expand the use of FRET in the analysis of cellular processes that may involve protein-protein interactions and other complex cellular processes.

Barroso, Margarida; Mehdibeigi, Roshanak; Brogan, Louise

2006-03-01

289

Nanocomposites of POC and quantum dots

NASA Astrophysics Data System (ADS)

New luminescent polymer nanocomposites were synthesized combining carbazole/oxadiazole copolymer (POC) and CdSe/ZnS quantum dots (QDs) surface passivated by ionic liquids. Ionic liquid ligands improve the photostability of QDs and their compatibility with polymer allowing the deposition of homogeneous nanocomposites films. The nanocomposites were characterized by UV and photoluminescence spectroscopy.

Borriello, C.; Concilio, S.; Minarini, C.; Iannelli, P.; Di Luccio, T.

2012-07-01

290

Optical properties of quantum-dot-doped liquid scintillators

NASA Astrophysics Data System (ADS)

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.

Aberle, C.; Li, J. J.; Weiss, S.; Winslow, L.

2013-10-01

291

Laterally-biased quantum dot infrared photodetector

NASA Astrophysics Data System (ADS)

At the Air Force Research Laboratory, Space Vehicles Directorate, we are interested in improving the performance of or modifying the capabilities of infrared detectors in order to locate and identify dim and/or distant objects in space. One characteristic we are very interested in is multicolor detection. To this end, we have turned to a novel detector design that we have come to call a Lateral Quantum Dot Infrared Photodetector (LQDIP). In this design, InAs quantum dots are buried in a GaAs quantum well, which in turn is tunnel-coupled to another GaAs quantum well. Photoexcited electrons from the quantum dots tunnel over to the second well and are then swept out via a lateral (perpendicular to the growth direction) bias voltage. This architecture should exhibit the ability to tune to select infrared frequencies with reduced dark current and unity gain. The lateral photocurrent is directed by a vertical (parallel to the growth direction) gate voltage. We will discuss this detector architecture and the LQDIP operating principles and conditions, and we will present some preliminary results of current-voltage, photocurrent, differential conductance, and spectral measurements.

Cardimona, D. A.; Morath, C. P.; Guidry, D. H.; Cowan, V. M.

2013-07-01

292

Radiation Damage Resistance of Quantum Wells and Self-Assembled Quantum Dots

NASA Astrophysics Data System (ADS)

The growth of self-assembled InAs quantum dots (QDs) has recently allowed for new devices which could exploit their zero dimensional quantum confinement and delta function density of states( G.Medeiros-Ribeiro,F.G.Pikus,P.M.Petroff,A.L.Efros, Phys. Rev. B55, 3, 1568 (1997).). The luminescence efficiency of a pseudomorphic In_xGa_1-xAs (x=0.2) QW and a QDs layer after exposure to an Ar^+ (E = 400 eV) plasma was studied for doses up to 10^15 cm-2. Photoluminescence spectroscopy was preformed using both an Ar^+ and a Ti-sapphire laser, allowing for both resonant and non-resonant pumping of the QW or QDs. The data shows that QDs have a greater radiation resistance relative to QWs by a factor of 8. This is attributed to zero dimensional quantum confinement and exciton localization in QDs.

Schoenfeld, Winston V.; Chen, Ching-Hui; Petroff, Pierre M.; Hu, Evelyn L.

1998-03-01

293

Quantum dot circuits: Single-electron switch and few-electron quantum dots

NASA Astrophysics Data System (ADS)

A strongly capacitively-coupled parallel double quantum dot was studied as a single-electron switch. The double dot was fabricated in a two-dimensional electron gas (2DEG) in a GaAs/AlGaAs heterostructure. An electrically-floating coupling gate increased capacitive-coupling between the dots, while an etched trench prevented tunnel-coupling between them. Split Coulomb blockade peaks were observed in each dot, and the Coulomb blockade conductance of the double dot formed a hexagonal pattern characteristic of coupled dots. A fractional peak splitting f = 0.34 was measured, which corresponds to a fractional capacitive-coupling alpha ? CINT/CSigma = 0.20. This is an order of magnitude larger than reported for similar lateral quantum dots, and shows that the coupling gate works. The strong capacitive-coupling in our device allowed the charge state of one dot to strongly influence the conductance of the other dot and enabled it to work as a single-electron switch. By moving in a combination of gate voltages, electrons are induced in one dot (the "trigger" dot) only. In response to the change in the charge state, the conductance of the other dot (the "switched" dot) is turned on and off. The abruptness of the conductance switching in gate voltage (the switching lineshape) is determined by how well charge is quantized on the trigger dot, and was found to follow tanh and arctan forms for (respectively) good and poor charge quantization in the trigger dot. A few-electron tunnel-coupled series double dot was studied for possible application to quantum computing. The device was fabricated in a square-well 2DEG in a GaAs/AlGaAs heterostructure. The dots were emptied of electrons in order to define the absolute number of electrons in the dot. Finite bias Coulomb blockade measurements on each dot showed that the last Coulomb blockade diamonds did not close and thus that both dots could be emptied. A three-dimensional conductance measurement of one dot in the one sidegate and the two quantum point contact voltages also showed that Coulomb blockade peaks ended, and corroborated that the dot could be emptied of electrons. The Zeeman energy of electrons in a few-electron dot, deduced from the Coulomb blockade peak spacings, was measured with an in-plane magnetic field of up to 7 T. The g-factor was found to be no different from that of bulk GaAs | g| = 0.44. Tunnel-coupling between the few-electron double dot was demonstrated, and a tunnel-coupling strength of 1.2e 2/h was estimated from the fractional peak splitting f = 0.3.

Chan, Ian Hin-Yun

294

Application of Synthesized Quantum Dots for Cell Imaging

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,

Hengyi Xu; Feng Xu; Yonghua Xiong; Cuixiang Wan; Jingfei Zhang; Hua Wei; Jiang Zhu

2009-01-01

295

Quantum-dot cellular automata at a molecular scale

ABSTRACT: Quantum - dot cellular automata (QCA) is a scheme for molecular electronics in which information is transmitted and processed through electro - static interactions between charges in an array of quantum dots QCA wires, majority gates, clocked cell operation, and (recently) true power gain between QCA cells has been demonstrated in a metal - dot prototype system at cryogenic

M. Lieberman

2002-01-01

296

Tuning of Long range Visible Emissions Using Coupled Quantum Dots

Size and shape controlled semiconductor quantum dots have been widely adopted in tailoring nanomaterials properties. Alternatively, chemically coupled quantum dots offer a novel route for tuning long range electronic transitions of semiconductors via band offset engineering at the material interface. We report on a simple route of tailoring visible emissions over long range by chemically designing coupled dots comprising of

Sucheta Sengupta; Nirmal Ganguli; I. Dasgupta; D. D. Sarma; Somobrata Acharya

2011-01-01

297

Mitigation of Quantum Dot Cytotoxicity by Microencapsulation

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.

Romoser, Amelia; Ritter, Dustin; Majitha, Ravish; Meissner, Kenith E.; McShane, Michael; Sayes, Christie M.

2011-01-01

298

Mitigation of quantum dot cytotoxicity by microencapsulation.

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

Romoser, Amelia; Ritter, Dustin; Majitha, Ravish; Meissner, Kenith E; McShane, Michael; Sayes, Christie M

2011-07-21

299

Structural analysis of strained quantum dots using nuclear magnetic resonance.

Strained semiconductor nanostructures can be used to make single-photon sources, detectors and photovoltaic devices, and could potentially be used to create quantum logic devices. The development of such applications requires techniques capable of nanoscale structural analysis, but the microscopy methods typically used to analyse these materials are destructive. NMR techniques can provide non-invasive structural analysis, but have been restricted to strain-free semiconductor nanostructures because of the significant strain-induced quadrupole broadening of the NMR spectra. Here, we show that optically detected NMR spectroscopy can be used to analyse individual strained quantum dots. Our approach uses continuous-wave broadband radiofrequency excitation with a specially designed spectral pattern and can probe individual strained nanostructures containing only 1 × 10(5) quadrupole nuclear spins. With this technique, we are able to measure the strain distribution and chemical composition of quantum dots in the volume occupied by the single confined electron. The approach could also be used to address problems in quantum information processing such as the precise control of nuclear spins in the presence of strong quadrupole effects. PMID:22922539

Chekhovich, E A; Kavokin, K V; Puebla, J; Krysa, A B; Hopkinson, M; Andreev, A D; Sanchez, A M; Beanland, R; Skolnick, M S; Tartakovskii, A I

2012-08-26

300

Entrapment in phospholipid vesicles quenches photoactivity of quantum dots

Quantum dots have emerged with great promise for biological applications as fluorescent markers for immunostaining, labels for intracellular trafficking, and photosensitizers for photodynamic therapy. However, upon entry into a cell, quantum dots are trapped and their fluorescence is quenched in endocytic vesicles such as endosomes and lysosomes. In this study, the photophysical properties of quantum dots were investigated in liposomes as an in vitro vesicle model. Entrapment of quantum dots in liposomes decreases their fluorescence lifetime and intensity. Generation of free radicals by liposomal quantum dots is inhibited compared to that of free quantum dots. Nevertheless, quantum dot fluorescence lifetime and intensity increases due to photolysis of liposomes during irradiation. In addition, protein adsorption on the quantum dot surface and the acidic environment of vesicles also lead to quenching of quantum dot fluorescence, which reappears during irradiation. In conclusion, the in vitro model of phospholipid vesicles has demonstrated that those quantum dots that are fated to be entrapped in endocytic vesicles lose their fluorescence and ability to act as photosensitizers.

Generalov, Roman; Kavaliauskiene, Simona; Westr?m, Sara; Chen, Wei; Kristensen, Solveig; Juzenas, Petras

2011-01-01

301

Semiconductor quantum dot scintillation under gamma-ray irradiation

We recently demonstrated the ability of semiconductor quantum dots to convert alpha radiation into visible photons. In this letter, we report on the scintillation of quantum dots under gamma-ray irradiation, and compare the energy resolution of the 59 keV line of Americium 241 obtained with our quantum dot-glass nanocomposite material to that of a standard sodium iodide scintillator. A factor 2 improvement is demonstrated experimentally and interpreted theoretically using a combination of energy-loss and photon transport models. These results demonstrate the potential of quantum dots for room-temperature gamma-ray detection, which has applications in medical imaging, environmental monitoring, as well as security and defense. Present technology in gamma radiation detection suffers from flexibility and scalability issues. For example, bulk Germanium provides fine energy resolution (0.2% energy resolution at 1.33 MeV) but requires operation at liquid nitrogen temperature. On the other hand, Cadmium-Zinc-Telluride is a good room temperature detector ( 1% at 662 keV) but the size of the crystals that can be grown is limited to a few centimeters in each direction. Finally, the most commonly used scintillator, Sodium Iodide (NaI), can be grown as large crystals but suffers from a lack of energy resolution (7% energy resolution at 662 keV). Recent advancements in nanotechnology6-10 have provided the possibility of controlling materials synthesis at the molecular level. Both morphology and chemical composition can now be manipulated, leading to radically new material properties due to a combination of quantum confinement and surface to volume ratio effects. One of the main consequences of reducing the size of semiconductors down to nanometer dimensions is to increase the energy band gap, leading to visible luminescence, which suggests that these materials could be used as scintillators. The visible band gap of quantum dots would also ensure both efficient photon counting (better coupling with photomultipliers optimized for the visible region), and high photon output (smaller individual photon energy results in more photons produced) at room temperature, which is essential for effective Poisson counting (the energy resolution {Delta}E/E is inversely proportional to the square root of the number of photons collected).

Letant, S E; Wang, T

2006-08-23

302

Spin-orbit coupling in many-electron quantum dots

NASA Astrophysics Data System (ADS)

The spin-orbit (SO) coupling in semiconductor quantum dots (QD's) is an important area of research, as it determines the spin relaxation rates in these systems. Full understanding of this phenomenon is essential for control and manipulation of coherent interactions in various QD computer architectures. In this work, we consider how the inclusion of SO coupling can affect the energy spectrum of spherical and cylindrical dots. Our approach includes electron-electron interactions in a Hartree-Fock approximation, and considers the full symmetry of the single-particle states to evaluate needed matrix elements. We focus on Dresselhaus and Rashba SO mechanisms, which account for coupling via cubic and linear terms of momentum. The relative strength of the different effects is shown to depend on the geometry of the confinement and to affect the resulting multi-electron states. Inclusion of external magnetic fields allow evaluation of effective g-factors in the dot, allowing us comparisons with peak spacings and spin states obtained from Coulomb blockade experiments. Spin relaxation rates related to the different SO mechanisms are compared and related to different dot geometries.

Destefani, Carlos F.; Marques, Gilmar E.; Ulloa, Sergio E.

2003-03-01

303

Dynamic Localization of a One-Dimensional Quantum Dot Array in an ac Electric Field

We investigate the dynamics of two interaction electrons confined to one-dimensional quantum dot array in an ac electric field. We find that initially localized electrons will remain localized in the absence of Coulomb interaction if the ratio of the ac field magnitude to the frequency is a root of the ordinary zero-order Bessel function. In contrast to the case without

Ying Luo; Su-Qing Duan; Wen-Bin Fan; Xian-Geng Zao; Li-Min Wang; Ben-Kun Ma

2002-01-01

304

Principles of conjugating quantum dots to proteins via carbodiimide chemistry

NASA Astrophysics Data System (ADS)

The covalent coupling of nanomaterials to bio-recognition molecules is a critical intermediate step in using nanomaterials for biology and medicine. Here we investigate the carbodiimide-mediated conjugation of fluorescent quantum dots to different proteins (e.g., immunoglobulin G, bovine serum albumin, and horseradish peroxidase). To enable these studies, we developed a simple method to isolate quantum dot bioconjugates from unconjugated quantum dots. The results show that the reactant concentrations and protein type will impact the overall number of proteins conjugated onto the surfaces of the quantum dots, homogeneity of the protein-quantum dot conjugate population, quantum efficiency, binding avidity, and enzymatic kinetics. We propose general principles that should be followed for the successful coupling of proteins to quantum dots.

Song, Fayi; Chan, Warren C. W.

2011-12-01

305

Nonequilibrium transport through coupled quantum dots with electron phonon interaction

NASA Astrophysics Data System (ADS)

We theoretically study transport properties of coupled quantum dots in parallel in the presence of electron phonon (e ph) interaction. Nonequilibrium transport under finite bias is calculated using the Keldysh Green function method. Firstly, we examine a double-dot interferometer with a penetrating magnetic flux (Aharonov Bohm phase phiv) between the two quantum dots. The differential conductance shows a sharp dip between double resonant peaks, as a function of energy levels in the quantum dots, when the two dots are equivalently coupled to external leads and 0 < phiv < ?. The e ph interaction significantly decreases the dip, reflecting an emission of phonons from one of the quantum dots. This dephasing effect is more prominent under larger bias voltage. Secondly, we study a T-shaped double-dot system in which one of the dots is connected to the external leads (dot 1) and the other is disconnected (dot 2). The differential conductance shows a dip between two resonant peaks, as in the double-dot interferometer. The dip is weakly reduced by an emission of phonons from dot 2. Phonon emission from dot 1 does not result in dephasing and hence does not influence the dip. Therefore the dip of the conductance is more robust against the e ph interaction in the T-shaped double-dot system than in the double-dot interferometer.

Ueda, Akiko; Eto, Mikio

2007-05-01

306

Magnetoexcitons in type-II semiconductor quantum dots

NASA Astrophysics Data System (ADS)

We present a theoretical investigation of excitons in type-II semiconductor quantum dots (QD). In these systems the confinement of electrons inside the QD and the hole outside the QD produces a ring-like structure [1-2]. Recently, Ribeiro et al [3], in a magnetophotoluminescence study of type-II InP/GaAs self-assembled quantum dots, observed Aharonov-Bohm-type oscillations characteristic of the ring topology for neutral excitons. Using a simple model they have derived the groundstate hole energy as a function of the magnetic field, and obtained values for the ring parameters which are in good agreement with the measured values. However, some of the features observed experimentally, in the photoluminescence intensity, can not be well explained under that approach. In this work we present a more realistic model which considers the finite width of the ring and the electron-hole interaction included via a perturbative approach. The calculations are performed within the oneparticle formalism using the effective mass approximation. The confinement potential for electrons is modelled as the superposition of a quantum well potential along the axial direction, and a parabolic lateral confinement potential. The energies for the hole in the ring plane are calculated using the method of reference [4]. Theoretical calculations are in good agreement with the experimental results of reference [3] provided that excitonic effects are properly taken into account. References 1. A.O. Govorov et al., Physica E 13 , 297 (2002). 2. K. L. Janssens et al. Phys. Rev B64, 155324 (2001), and Phys. Rev. B66, 075314 (2002). 3. E. Ribeiro, G. Medeiros-Ribeiro, and W.Carvalho Jr., and A.O. Govorov, condmat/0304092 (2003). 4. Z. Barticevic, G. Fuster, and M. Pacheco,Phys. Rev. B 65, 193307 (2002).

Fuster, Gonzalo; Barticevic, Zdenka; Pacheco, Monica; Oliveira, Luiz E.

2004-03-01

307

Magneto-optical single dot spectroscopy of GaSb\\/GaAs type II quantum dots

We have investigated magneto-optical properties of GaSb\\/GaAs self-assemble type II quantum dots by single dot spectroscopy in magnetic field. We have observed clear Zeeman splitting and diamagnetic shift of GaSb\\/GaAs quantum dots. The diamagnetic coefficient ranges from 5 to 30?eV\\/T2. The large coefficient and their large distribution are attributed to the size inhomogeneity and electron localization outside the dot. The

Tomohiko Sato; Toshihiko Nakaoka; Makoto Kudo; Yasuhiko Arakawa

2006-01-01

308

NASA Astrophysics Data System (ADS)

We present the model of a quantum dot (QD) consisting of a spherical core-bulk heterostructure made of three-dimensional (3D) topological insulator (TI) materials, such as PbTe/Pb0.31Sn0.69Te, with bound massless and helical Weyl states existing at the interface and being confined in all three dimensions. The number of bound states can be controlled by tuning the size of the QD and the magnitude of the core and bulk energy gaps, which determine the confining potential. We demonstrate that such bound Weyl states can be realized for QD sizes of few nanometers. We identify the spin locking and the Kramers pairs, both hallmarks of 3D TIs. In contrast to topologically trivial semiconductor QDs, the confined massless Weyl states in 3D TI QDs are localized at the interface of the QD and exhibit a mirror symmetry in the energy spectrum. We find strict optical selection rules satisfied by both interband and intraband transitions that depend on the polarization of electron-hole pairs and therefore give rise to the Faraday effect due to the Pauli exclusion principle. We show that the semiclassical Faraday effect can be used to read out spin quantum memory. When a 3D TI QD is embedded inside a cavity, the single-photon Faraday rotation provides the possibility to implement optically mediated quantum teleportation and quantum information processing with 3D TI QDs, where the qubit is defined by either an electron-hole pair, a single electron spin, or a single hole spin in a 3D TI QD. Remarkably, the combination of interband and intraband transition gives rise to a large dipole moment of up to 450 Debye. Therefore, the strong-coupling regime can be reached for a cavity quality factor of Q?104 in the infrared wavelength regime of around 10?m.

Paudel, Hari P.; Leuenberger, Michael N.

2013-08-01

309

We investigate the tunneling escape of confined excitons in CdSe core-shell quantum dots by means of the time-dependent Schrodinger equation. Our results indicate that obtaining efficient charge extraction strongly depends on the interplay between parameters such as nanocrystal sizes, shell thicknesses, external electric fields, and confinement barrier heights.

J. S. de Sousa; J. A. K. Freire; G. A. Farias

2007-01-01

310

NASA Astrophysics Data System (ADS)

Semiconductor quantum dots have attracted much interest in implementing solid-state quantum information processing. Using InAs based quantum dots, we demonstrate quantum coupling between two stacked quantum dot molecules in electroluminescence, controlling Stark shifts of single quantum dots in electroluminescence, and 'plug and play' single photon emission at telecommunication wavelengths.

Xu, Xiulai; Andreev, Aleksey; Brossard, Frederic; Hammura, Kiyotaka; Williams, David

2009-06-01

311

Light absorption enhancement in closely packed Ge quantum dots

NASA Astrophysics Data System (ADS)

Multilayers of Ge quantum dots (QDs, 3 nm in diameter) embedded in SiO2, separated by SiO2 barrier layer (3, 9, or 20 nm thick), have been synthesized by sputter deposition and characterized by transmission electron microscopy and light absorption spectroscopy. Quantum confinement affects the optical bandgap energy (1.9 eV for QDs, 0.8 eV for bulk Ge); moreover, the absorption probability greatly depends on the QD-QD distance. A strong electronic coupling among Ge QDs is evidenced, with a significant increase of the light absorption efficiency when the QD-QD distance is reduced. These data unveil promising aspects for light harvesting with nanostructures.

Mirabella, S.; Cosentino, S.; Failla, M.; Miritello, M.; Nicotra, G.; Simone, F.; Spinella, C.; Franzò, G.; Terrasi, A.

2013-05-01

312

Freestanding silicon quantum dots: origin of red and blue luminescence

NASA Astrophysics Data System (ADS)

In this paper, we studied the behavior of silicon quantum dots (Si-QDs) after etching and surface oxidation by means of photoluminescence (PL) measurements, Fourier transform infrared spectroscopy (FTIR) and electron paramagnetic resonance spectroscopy (EPR). We observed that etching of red luminescing Si-QDs with HF acid drastically reduces the concentration of defects and significantly enhances their PL intensity together with a small shift in the emission spectrum. Additionally, we observed the emergence of blue luminescence from Si-QDs during the re-oxidation of freshly etched particles. Our results indicate that the red emission is related to the quantum confinement effect, while the blue emission from Si-QDs is related to defect states at the newly formed silicon oxide surface.

Gupta, Anoop; Wiggers, Hartmut

2011-02-01

313

On-demand single-electron transfer between distant quantum dots.

Single-electron circuits of the future, consisting of a network of quantum dots, will require a mechanism to transport electrons from one functional part of the circuit to another. For example, in a quantum computer decoherence and circuit complexity can be reduced by separating quantum bit (qubit) manipulation from measurement and by providing a means of transporting electrons between the corresponding parts of the circuit. Highly controlled tunnelling between neighbouring dots has been demonstrated, and our ability to manipulate electrons in single- and double-dot systems is improving rapidly. For distances greater than a few hundred nanometres, neither free propagation nor tunnelling is viable while maintaining confinement of single electrons. Here we show how a single electron may be captured in a surface acoustic wave minimum and transferred from one quantum dot to a second, unoccupied, dot along a long, empty channel. The transfer direction may be reversed and the same electron moved back and forth more than sixty times-a cumulative distance of 0.25 mm-without error. Such on-chip transfer extends communication between quantum dots to a range that may allow the integration of discrete quantum information processing components and devices. PMID:21938065

McNeil, R P G; Kataoka, M; Ford, C J B; Barnes, C H W; Anderson, D; Jones, G A C; Farrer, I; Ritchie, D A

2011-09-21

314

Ultra-small carbon-dot patterns for directed Ge quantum dot growth

The controlled growth of patterns of small quantum dots in the diameter range of 10 nm is critical for the application in quantum computer architectures. It is well known that Ge quantum dots with a diameter smaller than 10 nm nucleate on the Si(100) surface after carbon pre-adsorption in a random pattern 1. We follow this approach to generate ultimately

Olivier Guise; Hubertus Marbach; Sergey Mezhenny; Jeremy Levy; Joachim Ahner; J. R. Yates

2003-01-01

315

Quantum entanglement of excitons in coupled quantum dots

Optically controlled exciton dynamics in coupled quantum dots is studied. We show that the maximally entangled Bell states and Greenberger-Horne-Zeilinger (GHZ) states can be robustly generated by manipulating the system parameters to be at the avoided crossings in the eigenenergy spectrum. The analysis of population transfer is systematically carried out by using a dressed-state picture. In addition to the quantum

Ping Zhang; C. K. Chan; Qi-Kun Xue; Xian-Geng Zhao

2003-01-01

316

Transport properties of graphene quantum dots

NASA Astrophysics Data System (ADS)

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, linear conductance, and I-V characteristics. Our results depict a resonant behavior of the conductance in the quantum dot structures, which can be controlled by changing geometrical parameters such as the nanoribbon segment widths and the distance between them. By application of a gate voltage on determined regions of the structure, it is possible to modulate the transport response of the systems. We show that negative differential resistance can be obtained for low values of applied gate and bias voltages.

González, J. W.; Pacheco, M.; Rosales, L.; Orellana, P. A.

2011-04-01

317

Hyperspectral fluorescence tomography of quantum dots

NASA Astrophysics Data System (ADS)

Hyperspectral excitation-resolved fluorescence tomography (HEFT) exploits the spectrally-dependent absorption properties of biological tissue for recovering the unknown three-dimensional (3D) fluorescent reporter distribution inside tissue. Only a single light source with macro-illumination and wavelength-discrimination is required for the purpose of light emission stimulation and 3D image reconstruction. HEFT is built on fluorescent sources with a relatively broad spectral absorption profile (quantum dots) and a light propagation model for strongly absorbing tissue between wavelengths 560 nm and 660 nm (simplified spherical harmonics - SPN, - equations). The measured partial current of fluorescence light is cast into an algebraic system of equations, which is solved for the unknown quantum dot distribution with an expectation-maximization (EM) method. HEFT requires no source-detector multiplexing for 3D image reconstruction and, hence, offers a technologically simple design.

Klose, Alexander D.

2011-02-01

318

Conductance Peaks in Open Quantum Dots

NASA Astrophysics Data System (ADS)

We present a simple measure of the conductance fluctuations in open ballistic chaotic quantum dots, extending the number of maxima method originally proposed for the statistical analysis of compound nuclear reactions. The average number of extreme points (maxima and minima) in the dimensionless conductance T as a function of an arbitrary external parameter Z is directly related to the autocorrelation function of T(Z). The parameter Z can be associated with an applied gate voltage causing shape deformation in quantum dot, an external magnetic field, the Fermi energy, etc. The average density of maxima is found to be ??Z?=?Z/Zc, where ?Z is a universal constant and Zc is the conductance autocorrelation length, which is system specific. The analysis of ??Z? does not require large statistic samples, providing a quite amenable way to access information about parametric correlations, such as Zc.

Ramos, J. G. G. S.; Bazeia, D.; Hussein, M. S.; Lewenkopf, C. H.

2011-10-01

319

Nonlinear optical properties of a Woods-Saxon quantum dot under an electric field

NASA Astrophysics Data System (ADS)

A theoretical study of the effect of the confining potential on the nonlinear optical properties of two dimensional quantum dots is performed. A three-parameter Woods-Saxon potential is used within the density matrix formalism. The control of confinement by three parameters and an applied electric field gives one quite an advantage in studying their effects on the nonlinear properties. The coefficients investigated include the optical rectification, second and third-harmonic generation and the change in the refractive index. Their dependence on the electric field values, dot size and the energy of the incoming photons is studied extensively.

Aytekin, O.; Turgut, S.; Üsto?lu Ünal, V.; Ak?ahin, E.; Tomak, M.

2013-12-01

320

Solution-processable graphene quantum dots.

This minireview describes recent progress in solution-processable graphene quantum dots (SGQDs). Advances in the preparation, modification, properties, and applications of SGQDs are highlighted in detail. As one of emerging nanostructured materials, possible ongoing research related to the precise control of the lateral size, edge structure and surface functionality; the manipulation and characterization; the relationship between the properties and structure; and interfaces with biological systems of SGQDs have been speculated upon. PMID:23733526

Zhou, Xuejiao; Guo, Shouwu; Zhang, Jingyan

2013-06-03

321

Quantum Dots with Perfectly Resonant Emission Energies

Semiconductor quantum dots (QDs) with excellent structural, optical and electronic properties can be easily fabricated by the self-assembled Stranski-Krastanow growth mode. However, in spite of numerous efforts and encouraging results, it has become clear that the bottom-up approach alone can not yield QDs with deterministically controllable electronic properties. Post-growth processing seems at present the only viable path to achieve this

A. Rastelli; Lijuan Wang; S. Kiravittaya; O. Schmidt

2007-01-01

322

Toxicity of carbon group quantum dots

Carbon group quantum dots (QDs) such as carbon, silicon and germanium, have potential for biomedical applications such as bio-imaging markers and drug delivery systems and are expected to demonstrate several advantages over conventional fluorescent QDs such as CdSe, especially in biocompatibility. We assessed biocompatibility of newly manufactured silicon QDs (Si-QDs), by means of both MTT assay and LDH assay for

Sanshiro Hanada; Kouki Fujioka; Akiyoshi Hoshino; Noriyoshi Manabe; Kenji Hirakuri; Kenji Yamamoto

2009-01-01

323

Luminescence properties of semiconductor quantum dots

Semiconductor quantum dot (QD) heterostructures created using self-ordering phenomena on crystal surfaces exhibit luminescence properties predicted for zero-dimensional systems, e.g. ultrasharp luminescence lines up to high temperatures, giantly increased material gain and practically complete temperature insensitivity of the laser threshold current. Faster than expected exciton capture and energy relaxation processes manifest minor role of the so-called phonon bottleneck effect. Formation

D. Bimberg; N. N. Ledentsov; M. Grundmann; R. Heitz; J. Böhrer; V. M. Ustinov; P. S. Kop'ev; Zh. I. Alferov

1997-01-01

324

Photoluminescence of Silicon-Germanium Quantum Dots

NSDL National Science Digital Library

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.

Kolodzey, James

2008-10-27

325

Quantum-dot semiconductor disk lasers

We demonstrate quantum-dot (QD)-based, optically pumped semiconductor disk lasers (SDLs) for wavelengths ranging from 950 to 1210nm. QDs grown either in the submonolayer (SML) or in the Stranski–Krastanow (SK) regime are employed as active layers of the SDLs which are based on two different design concepts. Output power of up to 1.4W continuous wave (CW) is achieved with an InAs\\/GaAs-SML

T. D. Germann; A. Strittmatter; U. W. Pohl; D. Bimberg; J. Rautiainen; M. Guina; O. G. Okhotnikov

2008-01-01

326

Quantum Dot Nanotechnology for Prostate Cancer Research

Quantum dots (QDs), tiny light-emitting particles on the nanometer scale, are emerging as a new class of fluorescent probes\\u000a for cancer cell imaging and molecular profiling. In comparison with organic dyes and fluorescent proteins, QDs have unique\\u000a optical and electronic properties, such as size-tunable light emission, improved signal brightness, resistance against photobleaching,\\u000a and simultaneous excitation of multiple fluorescence colors. These

Xiaohu Gao; Yun Xing; Leland W. K. Chung; Shuming Nie

327

Quantum dot encapsulation in viral capsids.

Incorporation of CdSe/ZnS semiconductor quantum dots (QDs) into viral particles provides a new paradigm for the design of intracellular microscopic probes and vectors. Several strategies for the incorporation of QDs into viral capsids were explored; those functionalized with poly(ethylene glycol) (PEG) can be self-assembled into viral particles with minimal release of photoreaction products and enhanced stability against prolonged irradiation. PMID:16968014

Dixit, Suraj K; Goicochea, Nancy L; Daniel, Marie-Christine; Murali, Ayaluru; Bronstein, Lyudmila; De, Mrinmoy; Stein, Barry; Rotello, Vincent M; Kao, C Cheng; Dragnea, Bogdan

2006-09-01

328

Electron properties of open semiconductor quantum dots

The energy spectrum and lifetimes of electron states in an open semiconductor quantum dot (QD) have been studied using the\\u000a scattering S-matrix method. It is established that the lifetime of electron states in the QD is highly sensitive to changes in the QD\\u000a radius and the thickness of an external coating layer. As the coating layer thickness increases from one

N. V. Tkach; Yu. A. Seti; G. G. Zegrya

2007-01-01

329

Quantum-Dot Semiconductor Optical Amplifiers

This paper reviews the recent progress of quantum-dot semiconductor optical amplifiers developed as ultrawideband polarization-insensitive high-power amplifiers, high-speed signal regenerators, and wideband wavelength converters. A semiconductor optical amplifier having a gain of > 25 dB, noise figure of 20 dBm, over the record widest bandwidth of 90 nm among all kinds of optical amplifiers, and also having a penalty-free output

T. Akiyama; M. Sugawara; Y. Arakawa

2007-01-01

330

Engineered quantum dot single-photon sources

NASA Astrophysics Data System (ADS)

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.

Buckley, Sonia; Rivoire, Kelley; Vu?kovi?, Jelena

2012-12-01

331

Plasma Synthesis of Luminescent Silicon Quantum Dots

NASA Astrophysics Data System (ADS)

Semiconductor nanoparticles with photoluminescent properties are of interest for a wide range of applications from solid state lighting, to solid state lasers, to imaging biological systems. A wide range of compound semiconductor quantum dots has been produced with excellent photoluminescent properties such as, for instance, cadmium-selenide. However, many of these compound quantum dot materials are of environmental concerns due to their content of toxic heavy metals. Hence there is continued interest in silicon quantum dots as a luminescent material, since silicon is widely considered as environmentally benign. In this presentation we discuss a plasma process that is capable of producing crystalline, nonagglomerated silicon nanoparticles of less than 10 nm in size. The reactor is designed to be a flow through reactor which enables online analysis of the particle size distribution with a fast scanning nano-Differential Mobility Analyzer (DMA). The particles structural properties are studied with high resolution transmission electron microscopy. The luminescent properties of the particles are investigated by studying photoluminescence of the particles on illumination with UV radiation.

Thimsen, Elijah; Mangolini, Lorenzo; Kortshagen, Uwe

2004-09-01

332

The Effects of Electric Field on Oscillation Period of Triangular Bound Potential Quantum Dot Qubit

NASA Astrophysics Data System (ADS)

This paper calculates the oscillation period of an electron by using variational method of Pekar type on the condition of electric-LO-phonon strong coupling in a triangular bound potential quantum dot. It obtains the eigenenergies of the ground state and the first excited state, the eigenfunctions of the ground state and the first excited state. This system in a quantum dot may be employed as a two-level quantum system qubit. The superposition state electron density oscillates in the quantum dot with a period when the electron is in the superposition state of the ground and the first-excited state. It studies the influence of the electric field on the period of oscillation at different electron-LO-phonon coupling strength, different polar angle and different confinement length.

Yu, Yi-Fu; Li, Hong-juan; Yin, Ji-wen

2013-10-01

333

Electron cotunneling through doubly occupied quantum dots: effect of spin configuration

A microscopic theory is presented for electron cotunneling through doubly occupied quantum dots in the Coulomb blockade regime. Beyond the semiclassic framework of phenomenological models, a fully quantum mechanical solution for cotunneling of electrons through a one-dimensional quantum dot is obtained using a quantum transmitting boundary method without any fitting parameters. It is revealed that the cotunneling conductance exhibits strong dependence on the spin configuration of the electrons confined inside the dot. Especially for the triplet configuration, the conductance shows an obvious deviation from the well-known quadratic dependence on the applied bias voltage. Furthermore, it is found that the cotunneling conductance reveals more sensitive dependence on the barrier width than the height.

2011-01-01

334

Few-electron physics in Double quantum dots in carbon nanotubes

NASA Astrophysics Data System (ADS)

Recent experimental progress on few-electron quantum dots (also known as artificial atoms) has allowed the controllable manipulation of the spin degrees of freedom of the confined electrons. Such control is at the heart of semiconductor-based spintronics and quantum-information proposals. Double-well quantum dot in semiconducting carbon nanotubes exhibit rich physics due to the additional valley degree of freedom. Here, we study the few-electron spectrum of a carbon-nanotube double quantum dot with spin-orbit coupling. We find that Coulomb interactions can cause strong correlation effects which lead to different ground state transitions. In particular, we show that such strong correlations can produce the disappearance of the Pauli blockade in transport experiments and an interaction-induced ferromagnetic ground state.

von Stecher, Javier; Wunsch, Bernhard; Lukin, Mikahil; Demler, Eugene; Rey, Ana Maria

2010-03-01

335

Fine structure of light-hole excitons in nanowire quantum dots

NASA Astrophysics Data System (ADS)

Quantum dots with light-hole ground states could find numerous applications including faster quantum bit operations or coherent conversion of photons into electron spins. Typically, however, holes confined in epitaxial quantum dots are of heavy-hole character. I show, by use of atomistic tight-binding theory, that the hole ground state undergoes a transition from heavy holelike to light holelike with increasing height of a nanowire InAs/InP quantum dot. The fine structure of the light-hole exciton consists of a dark ground state and three bright states. Two of the bright states are quasidegenerate and are in-plane polarized, whereas, the third energetically higher bright state is polarized in the perpendicular out-of-plane direction. The light-hole exciton fine structure is robust against alloying.

Zieli?ski, M.

2013-09-01

336

Ultrafast Coherent Spectroscopy of Single Semiconductor Quantum Dots

This chapter summarizes our recent work—performed within the project B6 of the Sonderforschungsbereich 296—on combining ultrafast\\u000a spectroscopy and near-field microscopy to probe the nonlinear optical response of a single quantum dot and of a pair of dipole-coupled\\u000a quantum dots on a femtosecond time scale. We demonstrate coherent control of both amplitude and phase of the coherent quantum\\u000a dot polarization by

Christoph Lienau; Thomas Elsaesser

337

Molecular spintronics: Coherent spin transfer in coupled quantum dots

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

Florian Meier; Veronica Cerletti; Oliver Gywat; Daniel Loss; D. D. Awschalom

2004-01-01

338

Growth and characterization of HgSe:Fe quantum dots

Applying three different sophisticated growth methods using MBE we succeeded in growing unburied and buried quantum-dot structures of HgSe:Fe on a ZnSe\\/GaAs buffer\\/substrate system in (001) orientation. For the buried quantum dots the buffer\\/substrate system was specially prepared by a thermal desorption process resulting in the generation of surface dips acting as a mould for the quantum dots. The advantage

T Tran-Anh; S Hansel; A Kirste; H. U Müller; M von Ortenberg; J Barner; J. P Rabe

2004-01-01

339

The development of a Quantum Dot Solar Concentrator

The aim of this study was to investigate the feasibility of Quantum Dot Solar Concentrators (QDSCs). Quantum Dots offer the advantages of having broad absorption specr4a, tunable emission and improved stability. A range of Cadmium Selenide\\/|Zince Sulphide Quantum Dots were characterized in solution and composite form using time-resolved and steady state spectroscopie techniques and the stability of the composite samples

Brenda Rowan

2007-01-01

340

Optical investigation of CdSe\\/Zn(Be)Se quantum dot structures: size and Cd composition

The optical properties of CdSe\\/Zn0.97Be0.03Se and CdSe\\/ZnSe quantum dots (QDs) are investigated using photoluminescence (PL) and PL excitation spectroscopies. We show that the addition of Be into the bar- rier enhances the Cd composition and the overall quantum confinement of optically active QDs. The temperature behavior of PL supports such a conclusion. We also show that the room temperature QD

Y. Gu; Igor L. Kuskovsky; J. Fung; G. F. Neumark; X. Zhou; S. P. Guo; M. C. Tamargo

2004-01-01

341

InAsSb\\/InP quantum dots for midwave infrared emitters: A theoretical study

A theoretical study of the electronic properties of InAsSb quantum dots (QDs) deposited on InP substrate is presented. Unstrained bulk materials present a direct gap between 0.1 and 0.35 eV suitable for mid-infrared emitters (2-5 mum). However, strain and quantum-confinement effects may limit the extension of the emission spectrum of these nanostructures towards the higher wavelengths. Various associations of materials

C. Cornet; F. Doré; A. Ballestar; J. Even; N. Bertru; A. Le Corre; S. Loualiche

2005-01-01

342

Transparent non-volatile memory device using silicon quantum dots

NASA Astrophysics Data System (ADS)

A transparent non-volatile memory device was fabricated using silicon quantum dots in silicon nitride film as a gate insulator. A silicon quantum dots were grown in-situ in the film by plasma-enhanced chemical vapor deposition. The silicon quantum dot film had a high optical transmittance of over 95% at 550 nm with a thickness of 50 nm. A large hysteresis curve was observed in a current-voltage measurement. When we increased the voltage sweep range, electrons were charged into the silicon quantum dots because of the electrical n-type channel in an oxide thin film transistor.

Park, Nae-Man; Shin, Jaeheon; Kim, Bosul; Kim, Kyung Hyun; Cheong, Woo-Seok

2013-07-01

343

Imaging ligand-gated ion channels with quantum dots

NASA Astrophysics Data System (ADS)

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 GABAC 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.

Tomlinson, I. D.; Orndorff, Rebecca L.; Gussin, Hélène; Mason, John N.; Blakely, Randy D.; Pepperberg, David R.; Rosenthal, Sandra J.

2007-02-01

344

Quantum transport through an array of quantum dots

NASA Astrophysics Data System (ADS)

The transient current through an array of as many as 1000 quantum dots is simulated with two newly developed quantum mechanical methods. To our surprise, upon switching on the bias voltage, the current increases linearly with time before reaching its steady state value. And the time required for the current to reach its steady state value is proportional to the length of the array, and more interestingly, is exactly the time for a conducting electron to travel through the array at the Fermi velocity. These quantum phenomena can be understood by a simple analysis on the energetics of an equivalent classical circuit. An experimental design is proposed to confirm the numerical findings.

Chen, Shuguang; Xie, Hang; Zhang, Yu; Cui, Xiaodong; Chen, Guanhua

2012-12-01

345

Experimental studies on quantum-dot cellular automata devices

NASA Astrophysics Data System (ADS)

Quantum-dot Cellular Automata is an exciting novel device architecture for implementation of digital logic using bistable elements. This architecture offers a number of advantages such as logical completeness, low power dissipation and possibility of miniaturization of devices into the nanometer scale. In this dissertation, we demonstrate the operation of metal-based QCA devices such as double-dot, cell, latch and shift register, and investigate properties such as memory, power gain and errors in these devices. The devices are fabricated using the aluminum tunnel junction technology. Charge is confined on islands of aluminum connected to each other by tunnel junctions formed by a thin layer of aluminum oxide. These islands or 'dots' are arranged in the form of cells so that each cell has two degenerate ground states depending on the position of electrons in the dots. Various digital logic gates can be formed using arrangements of these cells with respect to one another. We start with the demonstration of a leadless QCA double-dot and cell. Switching is accomplished in the QCA cell by application of input voltage signals through gate capacitors. Electron transfer between the dots in a QCA cell is detected by measuring the dot potentials using SET electrometers. Control of switching in a QCA cell by an external signal can be accomplished by using an extra dot between the top and bottom dots of a half-cell and modulating its potential using a clock voltage signal. We demonstrate clocking in QCA devices using a half cell containing three dots (triple-dot), with inputs applied to the top and bottom dots and clock applied to the middle dot. Memory is demonstrated in a clocked QCA half-cell by suppressing co-tunneling between the top and bottom dots by fabricating multiple tunnel junctions between them. This device is called the QCA latch. A QCA shift register can be made by placing multiple latches next to each other and applying phase-shifted clock signals. A two stage QCA shift register is demonstrated using two latches capacitively coupled to each other. Power gain is demonstrated experimentally in a latch and a shift register by calculating the work done by each latch on the next, in a row of latches. Further, the types and properties of errors in the operation of the QCA latch and shift register are investigated by statistically measuring error rates under various conditions of input magnitude and bias. Finally a circuit for microwave frequency measurements of QCA devices using an RFSET is discussed. The experiments presented in this dissertation demonstrate leadless operation, clocking, memory and power gain in QCA devices. Error analysis performed on the latch shows that as the charging energy of these devices is increased, the error rates would fall exponentially.

Kummamuru, Ravi Kiran

346

Nondestructive optical measurements of a single electron spin in a quantum dot.

Kerr rotation measurements on a single electron spin confined in a charge-tunable semiconductor quantum dot demonstrate a means to directly probe the spin off-resonance, thus minimally disturbing the system. Energy-resolved magneto-optical spectra reveal information about the optically oriented spin polarization and the transverse spin lifetime of the electron as a function of the charging of the dot. These results represent progress toward the manipulation and coupling of single spins and photons for quantum information processing. PMID:17095655

Berezovsky, J; Mikkelsen, M H; Gywat, O; Stoltz, N G; Coldren, L A; Awschalom, D D

2006-11-09

347

Singly ionized double-donor complex in vertically coupled quantum dots

The electronic states of a singly ionized on-axis double-donor complex (D2+) confined in two identical vertically coupled, axially symmetrical quantum dots in a threading magnetic field are calculated. 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 antibonding lowest-lying artificial molecule states corresponding to different quantum dot morphologies, dimensions, separation between them, thicknesses of the wetting layers, and magnetic field strength.

2012-01-01

348

Finite-Temperature Effects on Correlation of Electrons in Quantum Dots

NASA Astrophysics Data System (ADS)

The path-integral Monte Carlo method is used to examine the two-electron state of a model quantum dot. Electrons in the two-dimensional quantum dot are confined by a harmonic oscillator potential of strength hbar?=1eV. Mixed state densities, energies and pair correlation functions are evaluated at various temperatures, and their temperature dependencies are analyzed. Also, the two-electron pure state energetics is resolved and the correlation induced shifts of the first and second excited states are evaluated.

Leino, Markku; Rantala, Tapio T.

2004-01-01

349

Quantum dot lasers: from promise to reality

NASA Astrophysics Data System (ADS)

Nanoscale coherent insertions of narrow gap material in a single-crystalline matrix, or Quantum Dot (QD) provide a possibility to extend the basic principles of heterostructure lasers. The idea to use heterostructures with dimensionality lower than two in semiconductor lasers appeared a quarter of a century ago, simultaneously with the proposal of a quantum well laser. However, fabrication of quantum wire- and, particularly, QD heterostructure (QDHS) lasers appeared to be much more difficult. The breakthrough occurred when techniques for self-organized growth of QDs allowed fabrication of dense arrays of uniform in shape and size coherent islands free from undesirable defects. Recently, some key parameters of QD lasers were significantly improved as compared to those for QW devices. High-power operation, record low threshold current densities, strongly reduced chirp and extension of the wavelength range on GaAs substrates up to 1.3 micrometer range were demonstrated. It also became clear that unique properties of QDs may give rise to a new generation of semiconductor lasers, such as far and middle infrared light emitters based on interlevel electron transitions in QDs or single quantum dot vertical-cavity surface-emitting lasers.

Bimberg, Dieter; Ledentsov, Nikolai N.

2000-07-01

350

Coulomb Oscillations in Partially Open Quantum Dots

NASA Astrophysics Data System (ADS)

Coulomb blockade conductance oscillations in quantum dots are ordinarily observed with weak dot-to-lead tunneling. Recent theoryfootnote A. Furusaki and K.A. Matveev, Phys. Rev. Lett. 75 (1995), 709. examines the effect of strong tunneling on Coulomb oscillations. We have measured the zero magnetic field conductance through a 500 × 800 nm^2 quantum dot connected to leads through tunable tunnel barriers, defined in a high mobility near-surface two-dimensional electron gas by four independently tunable split gates on a GaAs/AlGaAs heterostructure. We set the conductance of one barrier much less than e^2/h and vary the conductance of the other (G_open) between e^2/h and 6e^2/h. We observe well-defined Coulomb oscillations at 50 mK for the entire range of G_open, except when G_open ? 2e^2/h, where the oscillations are suppressed. The temperature at which the oscillations disappear decreases with increasing G_open; for G_ open>2e^2/h, the oscillations are suppressed at temperatures above the single-particle level spacing.

Crouch, C. H.; Livermore, C.; Westervelt, R. M.; Campman, K. L.; Gossard, A. C.

1996-03-01

351

Multiple stack quantum dot infrared photodetectors

NASA Astrophysics Data System (ADS)

Quantum dot infrared photodetectors (QDIP) have established themselves as promising devices for detecting infrared (IR) radiation for wavelengths <20?m due to their sensitivity to normal incidence radiation and long excited carrier lifetimes. A limiting factor of QDIPs at present is their relatively small absorption volume, leading to a lower quantum efficiency and detectivity than in quantum well infrared photodetectors and mercury cadmium telluride based detectors. One means of increasing the absorption volume is to incorporate a greater number of quantum dot (QD) stacks, thereby increasing the probability of photon capture. Growth of InAs/InGaAs dot-in-a-well (DWELL) QDIPs with greater than 10 stacks is challenging due to the increased strain between layers, leading to high dark current. It is known that strain can be reduced in QDIPs by reducing the width of the InGaAs well and incorporating a second well consisting of GaAs and barriers consisting of AlGaAs. A number of InAs/InGaAs/GaAs DWELL QDIPs with 30-80 stacks have been grown, fabricated and characterised. Dark current in these layers appears to be constant at given electric field, suggesting strain does not increase significantly if the number of QD stacks is increased. IR spectral measurements show well defined peaks at 5.5?m, 6.5?m and 8.4?m. In this work a comparison between dark current, noise, gain, responsivity and detectivity in these layers is presented and compared to existing data from conventional DWELL QDIPs.

Vines, P.; Tan, C. H.; David, J. P. R.; Attaluri, R. S.; Vandervelde, T. E.; Krishna, S.

2008-10-01

352

Polarization Resolved Single Dot Spectroscopy of (211)B InAs Quantum Dots

We report on single dot spectroscopy of (211)B InAs quantum dots, grown by molecular beam epitaxy. The dots exhibit sharp emission lines, the origin of which has been identified. Polarization dependent microphotoluminescence spectra confirm fine structure splittings from 20{mu}eV down to the determination limit of our setup (10 {mu}eV).

Germanis, S.; Dialynas, G. E.; Deligeorgis, G.; Hatzopoulos, Z. [Physics Department, University of Crete, P.O. Box 2208, 71003 Heraklion, Crete (Greece); Savvidis, P. G.; Pelekanos, N. T. [Materials Science and Technology Department, University of Crete, P.O. Box 2208, 71003 Heraklion, Crete (Greece)

2011-12-23

353

Confinement of Fractional Quantum Hall States

NASA Astrophysics Data System (ADS)

Confinement of small-gapped fractional quantum Hall states facilitates quasiparticle manipulation and is an important step towards quasiparticle interference measurements. Demonstrated here is conduction through top gate defined, narrow channels in high density, ultra-high mobility heterostructures. Transport evidence for the persistence of a correlated state at filling fraction 5/3 is shown in channels of 2?m length but gated to near 0.3?m in width. The methods employed to achieve this confinement hold promise for interference devices proposed for studying potential non-Abelian statistics at filling fraction 5/2. R.L. Willett, M.J. Manfra, L.N. Pfeiffer, K.W. West, Appl. Phys. Lett. 91, 052105 (2007).

Willett, Robert; Manfra, Michael; West, Ken; Pfeiffer, Loren

2008-03-01

354

In-plane polarization anisotropy of ground state optical intensity in InAs/GaAs quantum dots

NASA Astrophysics Data System (ADS)

The design of optical devices such as lasers and semiconductor optical amplifiers for telecommunication applications requires polarization insensitive optical emissions in the region of 1500 nm. Recent experimental measurements of the optical properties of stacked quantum dots have demonstrated that this can be achieved via exploitation of inter-dot strain interactions. In particular, the relatively large aspect ratio (AR) of quantum dots in the optically active layers of such stacks provide a two-fold advantage, both by inducing a red shift of the gap wavelength above 1300 nm, and increasing the TM001-mode, thereby decreasing the anisotropy of the polarization response. However, in large aspect ratio quantum dots (AR > 0.25), the hole confinement is significantly modified compared with that in lower AR dots--this modified confinement is manifest in the interfacial confinement of holes in the system. Since the contributions to the ground state optical intensity (GSOI) are dominated by lower-lying valence states, we therefore propose that the room temperature GSOI be a cumulative sum of optical transitions from multiple valence states. This then extends previous theoretical studies of flat (low AR) quantum dots, in which contributions arising only from the highest valence state or optical transitions between individual valence states were considered. The interfacial hole distributions also increases in-plane anisotropy in tall (high AR) quantum dots (TE110 ? TE-110), an effect that has not been previously observed in flat quantum dots. Thus, a directional degree of polarization (DOP) should be measured (or calculated) to fully characterize the polarization response of quantum dot stacks. Previous theoretical and experimental studies have considered only a single value of DOP: either [110] or [-110].

Usman, Muhammad

2011-11-01

355

Toward quantitatively fluorescent carbon-based ``quantum'' dots

Carbon-based ``quantum'' dots (or ``carbon dots'') are generally defined as surface-passivated small carbon nanoparticles that are brightly fluorescent. Apparently, the carbon particle surface passivation in carbon dots is critical to their fluorescence performance. An effective way to improve the surface passivation is to dope the surface of the precursor carbon nanoparticles with an inorganic salt, followed by the typical functionalization

Parambath Anilkumar; Xin Wang; Li Cao; Sushant Sahu; Jia-Hui Liu; Ping Wang; Katerina Korch; Kenneth N. Tackett II; Alexander Parenzan; Ya-Ping Sun

2011-01-01

356

Bipolar spin filter in a quantum dot molecule

We show that the tunable hybridization between two lateral quantum dots connected to a nonmagnetic current leads in a ``hanging-dot'' configuration that can be used to implement a bipolar spin filter. The competition between Zeeman, exchange interaction, and interdot tunneling (molecular hybridization) yields a singlet-triplet transition of the double dot ground state that allows spin filtering in Coulomb blockade experiments.

F. Mireles; S. E. Ulloa; F. Rojas; E. Cota

2006-01-01

357

NASA Astrophysics Data System (ADS)

Quantum cascade lasers based on planar quantum wells have emerged as a leading candidate for infrared laser sources. However, these lasers are ultimately limited by phonon emission, and exhibit useful optical gain only for the tranverse magnetic polarization. Quantum dot (QD) gain material to replace the planar gain regions is very attractive because the unipolar approach can then lead to both a phonon bottleneck, and surface emission. However, tunneling phenomenon is quite different for unipolar QD injection, and designs that follow the now standard approaches based on planar quantum wells are known to have unfavorable tunneling characteristics. In this paper we present a new device design based on QDs that can lead to important advantages for realizing high performance unipolar injection infrared lasers. The new quantum dot cascade laser design is based on controlling electron tunneling in the different quantum dimensional systems, from zero-dimensional to two-dimensional, to both block as well as enhance tunneling in a gain stage so as to obtain the population inversion necessary for infrared gain. This new device, the quantum dot cascade laser (QDCL), can operate with a phonon bottleneck, and therefore can exhibit improved high temperature performance in contrast to planar heterostructure unipolar devices. In addition, the zero-dimensional confinement can also provide transverse electric polarization in the radiation field, and therefore surface emission. Epitaxial growth experiments based on self-organized quantum dots to realize the new QDCL approach are presented and discussed.

Gobet, Mathilde; Deppe, Dennis G.

2005-06-01

358

Coherent Nonlinear Optical Response and Control of Single Quantum Dots.

National Technical Information Service (NTIS)

Work on this program is aimed at developing and understanding nano- optical structures with emphasis on developing quantum optical-based devices. Specific work focused on semiconductor quantum dots. The major achievements include the first demonstration o...

D. G. Steel

2005-01-01

359

A top-down strategy towards monodisperse colloidal lead sulphide quantum dots.

Monodisperse colloidal quantum dots with size dispersions <10% are of great importance in realizing functionality manipulation, as well as building advanced devices, and have been normally synthesized via 'bottom-up' colloidal chemistry. Here we report a facile and environmentally friendly 'top-down' strategy towards highly crystalline monodisperse colloidal PbS quantum dots with controllable sizes and narrow dispersions 5.5%quantum dots demonstrate size-tunable near-infrared photoluminescence, and self-assemble into well-ordered two-dimensional or three-dimensional superlattices due to the small degree of polydispersity and surface capping of 1-dodecanethiol, not only serving as a surfactant but also a sulphur source. The acquisition of monodisperse colloidal PbS quantum dots is ascribed to both the quantum-confinement effect of quantum dots and the size-selective-vaporization effect of the millisecond pulse laser with monochromaticity and low intensity. PMID:23591862

Yang, Jing; Ling, Tao; Wu, Wen-Tian; Liu, Hui; Gao, Min-Rui; Ling, Chen; Li, Lan; Du, Xi-Wen

2013-01-01

360

Controlling the Aspect Ratio of Quantum Dots: From Columnar Dots to Quantum Rods

We demonstrate the feasibility and flexibility of artificial shape engineering of epitaxial semiconductor nanostructures. Novel nanostructures including InGaAs quantum rods (QRs), nanocandles, and quantum dots (QDs)-in-rods were realized on a GaAs substrate. They were formed by depositing a short-period GaAs\\/InAs superlattice (SL) on a seed QD layer by molecular beam epitaxy growth. The InAs layer thickness in the SL plays

Lianhe Li; Gilles Patriarche; Nicolas Chauvin; Philipp Ridha; Marco Rossetti; Janusz Andrzejewski; Jan Misiewicz; Andrea Fiore

2008-01-01

361

Lifetime blinking in nonblinking nanocrystal quantum dots.

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

Galland, Christophe; Ghosh, Yagnaseni; Steinbrück, Andrea; Hollingsworth, Jennifer A; Htoon, Han; Klimov, Victor I

2012-06-19

362

Toward Quantum-dot Cellular Automata units: thiolated-carbazole linked bisferrocenes.

Quantum-dot Cellular Automata (QCA) exploit quantum confinement, tunneling and electrostatic interaction for transistorless digital computing. Implementation at the molecular scale requires carefully tailored units which must obey several structural and functional constraints, ranging from the capability to confine charge efficiently on different 'quantum-dot centers'-in order to sharply encode the Boolean states-up to the possibility of having their state blanked out upon application of an external signal. In addition, the molecular units must preserve their geometry in the solid state, to interact electrostatically in a controlled way. Here, we present a novel class of organometallic molecules, 6-3,6-bis(1-ethylferrocen)-9H-carbazol-9-yl-6-hexan-1-thiols, which are engineered to satisfy all such crucial requirements at once, as confirmed by electrochemistry and scanning tunneling microscopy measurements, and first principles density functional calculations. PMID:22159165

Arima, Valentina; Iurlo, Matteo; Zoli, Luca; Kumar, Susmit; Piacenza, Manuel; Della Sala, Fabio; Matino, Francesca; Maruccio, Giuseppe; Rinaldi, Ross; Paolucci, Francesco; Marcaccio, Massimo; Cozzi, Pier Giorgio; Bramanti, Alessandro Paolo

2011-12-12

363

Surface Relaxations in Quantum Confined Pb

The relationship between electronic and atomic structure of quantum confined objects is a key issue in nanoscience. We performed low-energy electron diffraction to determine the surface relaxations of ultrathin Pb films on Si(111)7 x 7 with thicknesses ranging from four to nine atomic layers. The results indicate a contraction of the first interlayer spacing d{sub 12} for all films. The d{sub 12} contraction exhibits a small bilayer modulation as a function of thickness, indicative of a quantum size effect. The oscillatory relaxations furthermore suggest an interesting correlation with the work function oscillations predicted from density functional theory [Wei and Chou, Phys. Rev. B 66, 233408 (2002)] and can be understood qualitatively on the basis of the jellium model.

Mans, A. [Delft University of Technology, Delft, Netherlands; Dil, J. H. [Delft University of Technology, Delft, Netherlands; Ettema, A. R. [Delft University of Technology, Delft, Netherlands; Weitering, Harm H [ORNL

2005-01-01

364

Quantum beats due to excitonic ground-state splitting in colloidal quantum dots

NASA Astrophysics Data System (ADS)

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.

Bylsma, J.; Dey, P.; Paul, J.; Hoogland, S.; Sargent, E. H.; Luther, J. M.; Beard, M. C.; Karaiskaj, D.

2012-09-01

365

Colloidal quantum dot photovoltaics: a path forward.

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

Kramer, Illan J; Sargent, Edward H

2011-10-12

366

Non-parabolic model for InAs/GaAs quantum dot capacitance spectroscopy

NASA Astrophysics Data System (ADS)

InAs/GaAs quantum dot electron spectra obtained from the capacitance voltage measurements by B.T. Miller et al. [B.T. Miller, W. Hansen, S. Manus, R.J. Luyken, A. Lorke, J.P. Kotthaus, S. Huant, G. Medeiros-Ribeiro, P.M. Petroff, Phys. Rev. B 56 (1997) 6764] are quantitatively interpreted by applying a three-dimensional model of a semiconductor quantum dot with energy-dependent electron effective mass and finite confinement potential. The Coulomb interaction between tunnelled electrons is taken into account by perturbation theory. The observed significant increase in the electron effective mass of the quantum dot in respect to its bulk value is explained by the non-parabolic effect.

Filikhin, I.; Deyneka, E.; Vlahovic, B.

2006-12-01

367

We report observation of electric-field-modulated infrared absorption at room temperature in electrochemically self-assembled CdS quantum dots produced by electrodepositing the semiconductor in 50-nm pores of an anodic alumina film. The absorption is associated with photoassisted real space transfer of electrons from the CdS dots to surrounding trap sites in the alumina. Similar absorption was observed in the past [Appl. Phys. Lett. 79, 4423 (2001)] and was the basis of a room temperature near infrared photodetector. An electric field modulates this absorption by altering the overlap between the wavefunctions of electronic states in the quantum dots and the trap states in the surrounding alumina, thereby affecting the matrix element for radiative transitions, similar to the quantum confined Stark or Franz-Keldysh effect. The ability to electrically modulate absorption in these structures can result in inexpensive infrared signal processing devices operating at room temperature. PMID:17025128

Wang, Y; Ramanathan, S; Fan, Q; Yun, F; Morkoc, H; Bandyopadhyay, S

2006-07-01

368

Optical absorption in semiconductor quantum dots coupling to dispersive phonons of infinite modes

NASA Astrophysics Data System (ADS)

Optical absorption spectrum of semiconductor quantum dot is investigated by means of an analytical approach based on the Green's function for different forms of coupling strength in an unified method by using the standard model with valence and conduction band levels coupled to dispersive quantum phonons of infinite modes. The analytical expression of the optical absorption coefficient in semiconductor quantum dots is obtained and by this expression the line shape and the peak position of the absorption spectrum are procured. The relation between the properties of absorption spectrum and the forms of coupling strength is clarified, which can be referenced for choosing the proper form of the coupling strength or spectral density to control the features of absorption spectrum of quantum dot. The coupling and confinement induced energy shift and intensity decrease in the absorption spectrum are determined precisely for a wide range of parameters. The results show that the activation energy of the optical absorption is reduced by the effect of exciton-phonon coupling and photons with lower frequencies could also be absorbed in absorption process. With increase of the coupling constant, the line shape of optical absorption spectrum broadens and the peak position moves to lower photon energy with a rapid decrease in intensity at the same time. Both the coupling induced red shift and the confinement induced blue shift conduce to decrease in the intensity of absorption spectrum. Furthermore, this method may have application potential to other confined quantum systems.

Ding, Zhiwen; Wang, Qin; Zheng, Hang

2012-10-01

369

Energy level engineering in InAs quantum dot nanostructures

NASA Astrophysics Data System (ADS)

We present an advanced method to tailor the optical and electrical properties of semiconductor quantum dot structures. By embedding vertically stacked quantum dots in a two-dimensional superlattice, the advantages of self-organized growth and of band structure engineering can be combined. The transition energies between the dot levels and the extended states of the superlattice can be adjusted by the period of the superlattice. We apply this scheme for photodetectors made of InAs quantum dots embedded in an AlAs/GaAs superlattice. The dark current of these devices is reduced by more than one order of magnitude compared to the devices without a superlattice.

Rebohle, L.; Schrey, F. F.; Hofer, S.; Strasser, G.; Unterrainer, K.

2002-09-01

370

Mesoscopic thermodynamics and chaos in quantum dots

NASA Astrophysics Data System (ADS)

In this thesis we present our work on the thermodynamic and transport properties of isolated or nearly isolated mesoscopic systems. These mesoscopic systems are phase coherent and their thermodynamic and transport properties do not self-average and exhibit sample-specific, reproducible fluctuations. Significant research in mesoscopic physics revealed the applicability of Random Matrix Theory (RMT), which is a statistical theory of electron eigenenergies and eigenfunctions in complex quantum systems. For the mesoscopic systems considered we assume the applicability of the RMT and calculate universal properties. The thermodynamics of isolated mesoscopic systems require the use of Canonical Ensemble because the number of electrons in each isolated sample must be constant in order to keep the samples charge neutral. We implement exact Canonical Ensemble techniques numerically and use RMT to calculate the mesoscopic averages and fluctuations for thermodynamic and transport properties. For an ensemble of metallic grains we calculate the average and fluctuations of spin magnetism, which is a fundamental thermodynamic quantity like the specific heat. We show the applicability of calculating mesoscopic corrections using an equivalent grand canonical ensemble, as proposed by Imry. We study the transport properties of chaotic quantum dots at temperatures that are comparable to the mean level spacing. Our results indicate the non-triviality of the well-known Landauer-Buttiker formula for such systems. In particular, the finite temperature conductance peak height distributions in quantum dots in Coulomb blockade regime clearly show the signatures of the charging energy. In addition, we calculate the temperature dependent peak-to-peak correlator. Our collaboration with experimentalists on the temperature dependence of peak height correlations demonstrates the scrambling of electronic levels as electrons are added to the dot.

Gokcedag, Mehmet

371

Quantum dots for multimodal molecular imaging of angiogenesis

Quantum dots exhibit unique optical properties for bioimaging purposes. We have previously developed quantum dots with a paramagnetic and functionalized coating and have shown their potential for molecular imaging purposes. In the current mini-review we summarize the synthesis procedure, the in vitro testing and, importantly, the in vivo application for multimodal molecular imaging of tumor angiogenesis.

Strijkers, Gustav J.; Nicolay, Klaas; Griffioen, Arjan W.

2010-01-01

372

Solid-state tunneling spectroscopy of individual quantum dots

Quantum dots are gaining importance for their potential applications in the fields of energy harvesting, bio-labeling and treatment, in opto-electronic devices and in photonic devices. For all these applications, it is imperative to know their electronic structure. Although currently several spectroscopic techniques exist to study the electronic structure of these quantum dots, they are accompanied by other effects and do

Ramkumar Subramanian

2010-01-01

373

Visualization and Transport of Quantum Dot Nanomaterials in Porous Media

This paper presents our research on the visualization and transport phenomena of quantum dot nanomaterials in porous media. It includes the development of a non-intrusive, high spatial and temporal resolution method to visualize transport and measure quantum dot nanomaterials concentration in porous media, allowing to characterize the mechanisms that control the transport, or lack of mobility, of engineered nanomaterials ---

C. J. G. Darnault; S. M. C. Bonina; B. Uyusur; P. T. Snee

2009-01-01

374

Metallic carbon nanotube quantum dots under magnetic fields

Quantum dots made of individual metallic carbon nanotubes are theoretically studied under the influence of a magnetic field applied in the axial direction. After assessing the mechanical stability of the heterostructure by Monte Carlo simulations, the dependence of the electronic properties on the size of the nanotube quantum dot and applied magnetic field has been investigated within the Peierls approximation

C. G. Rocha; A. Latgé; L. Chico

2005-01-01

375

Integrated Quantum Dot Schottky Diodes for RECTENNA (Rectifying Antenna)

In this paper we will present our latest research results of integrated quantum dot Schottky diodes and integrated silicon antenna for RF applications. Both, the quantum dot Schottky diodes and the antenna are integrated on Si substrates forming a simple mm-wave detection system, the rectifying antenna (RECTENNA). Within this work a specific antenna design, 1-dimensional array (single line antenna), will

H. Xu; A. Karmous; M. Morschbach; O. Kirfel; S. Spiessberger; E. Kasper

2009-01-01

376

Spin and charge polarization in quantum dot arrays

The role of spin and charge distribution in a planar semiconductor quantum dot array (square geometry) with two electrons is studied in the presence of a driver cell and magnetic nanoparticles located near two quantum dots providing a local magnetic field. We use an extended Hubbard model to describe the electrons in the cell, taking into account intra and intercell

F. Rojas; E. Cota; F. Mireles; S. E. Ulloa

2005-01-01

377

Quantum Dots for Molecular Diagnostics of Tumors

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.

Zdobnova, T.A.; Lebedenko, E.N.; Deyev, S.?.

2011-01-01

378

Mollow quintuplets from coherently excited quantum dots.

Charge-neutral excitons in semiconductor quantum dots (QDs) have a small finite energy separation caused by the anisotropic exchange splitting. Coherent excitation of neutral excitons will generally excite both exciton components, unless the excitation is parallel to one of the dipole axes. We present a polaron master equation model to describe two-exciton pumping using a coherent continuous wave pump field in the presence of a realistic anisotropic exchange splitting. We predict a five-peak incoherent spectrum, namely a Mollow quintuplet under general excitation conditions. We experimentally confirm such spectral quintuplets for In(Ga)As QDs and obtain very good agreement with theory. PMID:23938913

Ge, Rong-Chun; Weiler, S; Ulhaq, A; Ulrich, S M; Jetter, M; Michler, P; Hughes, S

2013-05-15

379

Stability of Quantum Dots in Live Cells

Quantum dots (QDs) are highly fluorescent and photostable, making them excellent tools for imaging. When using these QDs in cells and animals, however, intracellular biothiols (e.g., glutathione and cysteine) can degrade the QD monolayer compromising function. Here, we describe a label-free method to quantify the intracellular stability of monolayers on QD surfaces that couples laser desorption/ionization mass spectrometry (LDI-MS) with inductively coupled plasma mass spectrometry (ICP-MS). Using this new approach we have demonstrated that QD monolayer stability is correlated with both QD particle size and monolayer structure, with proper choice of both particle size and ligand structure required for intracellular stability.

Zhu, Zheng-Jiang; Yeh, Yi-Cheun; Tang, Rui; Yan, Bo; Tamayo, Joshua; Vachet, Richard W.; Rotello, Vincent M.

2011-01-01

380

Quantum dot loaded immunomicelles for tumor imaging

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 dot (QD)-containing polymeric micelles, prepared from a polyethylene glycol/phosphatidylethanolamine (PEG-PE) conjugate. Its production is simple and involves no special equipment. Its imaging potential is great since the fluorescence intensity in the tumor is twofold that of non-targeted QD-loaded PEG-PE micelles at one hour after injection. Methods Para-nitrophenol-containing (5%) PEG-PE quantum dot micelles were produced by the thin layer method. Following hydration, 2C5 antibody was attached to the PEG-PE micelles and the QD-micelles were purified using dialysis. 4T1 breast tumors were inoculated subcutaneously in the flank of the animals. A lung pseudometastatic B16F10 melanoma model was developed using tail vein injection. The contrast agents were injected via the tail vein and mice were depilated, anesthetized and imaged on a Kodak Image Station. Images were taken at one, two, and four hours and analyzed using a methodology that produces normalized signal-to-noise data. This allowed for the comparison between different subjects and time points. For the pseudometastatic model, lungs were removed and imaged ex vivo at one and twenty four hours. Results The contrast agent signal intensity at the tumor was double that of the passively targeted QD-micelles with equally fast and sharply contrasted images. With the side views of the animals only tumor is visible, while in the dorsal view internal organs including liver and kidney are visible. Ex vivo results demonstrated that the agent detects melanoma nodes in a lung pseudometastatic model after a 24 hours wash-out period, while at one hour, only a uniform signal is detected. Conclusions The targeted agent produces ultrabright tumor images and double the fluorescence intensity, as rapidly and at the same low dose as the passively targeted agents. It represents a development that may potentially serve to enhance early detection for metastases.

2010-01-01

381

Single photon sources with single semiconductor quantum dots

NASA Astrophysics Data System (ADS)

In this contribution, we briefly recall the basic concepts of quantum optics and properties of semiconductor quantum dot (QD) which are necessary to the understanding of the physics of single-photon generation with single QDs. Firstly, we address the theory of quantum emitter-cavity system, the fluorescence and optical properties of semiconductor QDs, and the photon statistics as well as optical properties of the QDs. We then review the localization of single semiconductor QDs in quantum confined optical microcavity systems to achieve their overall optical properties and performances in terms of strong coupling regime, efficiency, directionality, and polarization control. Furthermore, we will discuss the recent progress on the fabrication of single photon sources, and various approaches for embedding single QDs into microcavities or photonic crystal nanocavities and show how to extend the wavelength range. We focus in particular on new generations of electrically driven QD single photon source leading to high repetition rates, strong coupling regime, and high collection efficiencies at elevated temperature operation. Besides, new developments of room temperature single photon emission in the strong coupling regime are reviewed. The generation of indistinguishable photons and remaining challenges for practical single-photon sources are also discussed.

Shan, Guang-Cun; Yin, Zhang-Qi; Shek, Chan Hung; Huang, Wei

2013-09-01

382

Minimal Self-Contained Quantum Refrigeration Machine Based on Four Quantum Dots

NASA Astrophysics Data System (ADS)

We present a theoretical study of an electronic quantum refrigerator based on four quantum dots arranged in a square configuration, in contact with as many thermal reservoirs. We show that the system implements the minimal mechanism for acting as a self-contained quantum refrigerator, by demonstrating heat extraction from the coldest reservoir and the cooling of the nearby quantum dot.

Venturelli, Davide; Fazio, Rosario; Giovannetti, Vittorio

2013-06-01

383

Interaction of porphyrins with CdTe quantum dots

NASA Astrophysics Data System (ADS)

Porphyrins may be used as photosensitizers for photodynamic therapy, photocatalysts for organic pollutant dissociation, agents for medical imaging and diagnostics, applications in luminescence and electronics. The detection of porphyrins is significantly important and here the interaction of protoporphyrin-IX (PPIX) with CdTe quantum dots was studied. It was observed that the luminescence of CdTe quantum dots was quenched dramatically in the presence of PPIX. When CdTe quantum dots were embedded into silica layers, almost no quenching by PPIX was observed. This indicates that PPIX may interact and alter CdTe quantum dots and thus quench their luminescence. The oxidation of the stabilizers such as thioglycolic acid (TGA) as well as the nanoparticles by the singlet oxygen generated from PPIX is most likely responsible for the luminescence quenching. The quenching of quantum dot luminescence by porphyrins may provide a new method for photosensitizer detection.

Zhang, Xing; Liu, Zhongxin; Ma, Lun; Hossu, Marius; Chen, Wei

2011-05-01

384

Growth and photoluminescence study of ZnSe quantum dots

The authors report detailed photoluminescence (PL) studies of ZnSe quantum dots grown by controlling the flow duration of the precursors in a metal-organic chemical vapor deposition system. The growth time of the quantum dots determines the amount of blue shift observed in the PL measurements. Blue shift as large as 320 meV was observed, and the emission was found to persist up to room temperature. It is found that changing the flow rate and the total number of quantum dot layers also affect the peak PL energy. The temperature dependence of the peak PL energy follows the Varshni relation. From analyzing the temperature-dependent integrated intensity of the photoluminescence spectra, it is found that the activation energy for the quenching of photoluminescence increases with decreasing quantum dot size, and is identified as the binding energy of the exciton in ZnSe quantum dot.

Chang, Y.H.; Chieng, M.H.; Tsai, C.C.; Harris Liao, M.C.; Chen, Y.F.

2000-01-01

385

Enhanced intratumoral uptake of quantum dots concealed within hydrogel nanoparticles

NASA Astrophysics Data System (ADS)

Effective nanomedical devices for tumor imaging and drug delivery are not yet available. In an attempt to construct a more functional device for tumor imaging, we have embedded quantum dots (which have poor circulatory behavior) within hydrogel nanoparticles made of poly-N-isopropylacrylamide. We found that the hydrogel encapsulated quantum dots are more readily taken up by cultured tumor cells. Furthermore, in a melanoma model, hydrogel encapsulated quantum dots also preferentially accumulate in the tumor tissue compared with normal tissue and have ~16-fold greater intratumoral uptake compared to non-derivatized quantum dots. Our results suggest that these derivatized quantum dots, which have greatly improved tumor localization, may enhance cancer monitoring and chemotherapy.

Nair, Ashwin; Shen, Jinhui; Thevenot, Paul; Zou, Ling; Cai, Tong; Hu, Zhibing; Tang, Liping

2008-12-01

386

Prospects for Spin-Based Quantum Computing in Quantum Dots

NASA Astrophysics Data System (ADS)

Experimental and theoretical progress toward quantum computation with spins in quantum dots (QDs) is reviewed, with particular focus on QDs formed in GaAs heterostructures, on nanowire-based QDs, and on self-assembled QDs. We report on a remarkable evolution of the field, where decoherence—one of the main challenges for realizing quantum computers—no longer seems to be the stumbling block it had originally been considered. General concepts, relevant quantities, and basic requirements for spin-based quantum computing are explained; opportunities and challenges of spin-orbit interaction and nuclear spins are reviewed. We discuss recent achievements, present current theoretical proposals, and make several suggestions for further experiments.

Kloeffel, Christoph; Loss, Daniel

2013-04-01

387

NASA Astrophysics Data System (ADS)

Recent rapid advances in atomic-scale crystal growth and nanofabrication techniques have enabled researchers to realise various kinds of three-dimensional (3D) electron confinement in semiconductors. This paper presents a numerical simulation for quantum-dot-based electronic devices. The electronic structures calculations of a CdS/SiO2 quantum dot have been have been performed based on the resolution of the three-dimensional time-independent Schrödinger equation in the effective mass theory (EMT) and the band parabolicity approximation. The influence of the strain and dot separation on the formation of coupled quantum dot levels has been discussed. The oscillator strengths for intersubband electronic transitions have been also calculated. Besides, it is found that the subband energies and intersubband optical absorption are rather sensitive to the applied electric field. This gives a further parameter in various device applications based on the intersubband transitions. We hope that the present results may make a contribution to experimental studies of CdS/SiO2 quantum dot based structures which can be produced by inexpensive means. Our calculations provide good guidance for the selection of dot parameters to obtain absorption in quantum dot infrared photodetectors and a better understanding of the intersubband transition phenomena in quantum dot.

Abdi-Ben Nasrallah, S.; Bouazra, A.; Poncet, A.; Said, M.

2010-11-01

388

We developed a technique for forming epitaxial GaSb quantum dots on Si substrates using ultrathin SiO{sub 2} films that contain epitaxial Ge nuclei. Unlike Volmer-Weber-type GaSb quantum dots on Si, the dot density was higher (10{sup 9}-10{sup 12} cm{sup -2}) and the dot size was controlled in the range of approximately 10-100 nm. The nucleation of quantum dots was initiated by trapping Ga atoms on the Ge nuclei. Photoluminescence spectroscopy measurement at 5 K revealed the quantum-confinement effect in GaSb dots causing the photoluminescence peak to be continuously blueshifted from 0.76 eV by {approx}30 meV when the base length of the dots decreases from 100 to 17 nm.

Nakamura, Yoshiaki; Sugimoto, Tomohiro; Ichikawa, Masakazu [Department of Applied Physics, Quantum-Phase Electronics Center, School of Engineering, University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656 (Japan)

2009-01-01

389

Controlling cavity reflectivity with a single quantum dot

Solid-state cavity quantum electrodynamics (QED) systems offer a robust and scalable platform for quantum optics experiments and the development of quantum information processing devices. In particular, systems based on photonic crystal nanocavities and semiconductor quantum dots have seen rapid progress. Recent experiments have allowed the observation of weak and strong coupling regimes of interaction between the photonic crystal cavity and

Dirk Englund; Andrei Faraon; Ilya Fushman; Nick Stoltz; Pierre Petroff; Jelena Vuckovic

2007-01-01

390

Backaction dephasing by a quantum dot detector

NASA Astrophysics Data System (ADS)

We derive an analytical expression for the backaction dephasing rate, which characterizes the disturbance induced by coupling with an environment containing a quantum dot detector (QDD). In this paper, we show in an explicit form that the charge noise induces backaction dephasing. In equilibrium, this backaction dephasing induced by the charge noise can be explained as a relaxation by an inelastic electron-electron scattering in Fermi-liquid theory. Unlike the quantum point contact, the backaction dephasing rate increases or decreases with the bias at low bias and finite-temperature condition. This behavior depends on the QDD energy level with respect to the Fermi energy and the asymmetry of the QDD coupling to the reservoirs. In the high bias voltage regime, the dephasing rate becomes insensitive to the bias because of the saturation of the charge noise.

Kubo, Toshihiro; Tokura, Yasuhiro

2013-10-01

391

FAST TRACK COMMUNICATION: Graphene based quantum dots

NASA Astrophysics Data System (ADS)

Laterally localized electronic states are identified on a single layer of graphene on ruthenium by low temperature scanning tunneling spectroscopy (STS). The individual states are separated by 3 nm and comprise regions of about 90 carbon atoms. This constitutes a highly regular quantum dot-array with molecular precision. It is evidenced by quantum well resonances (QWRs) with energies that relate to the corrugation of the graphene layer. The dI/dV conductance spectra are modeled by a layer height dependent potential-well with a delta-function potential that describes the barrier for electron penetration into graphene. The resulting QWRs are strongest and lowest in energy on the isolated 'hill' regions with a diameter of 2 nm, where the graphene is decoupled from the surface.

Zhang, H. G.; Hu, H.; Pan, Y.; Mao, J. H.; Gao, M.; Guo, H. M.; Du, S. X.; Greber, T.; Gao, H.-J.

2010-08-01

392

In Vivo Imaging of Quantum Dots

NASA Astrophysics Data System (ADS)

Noninvasive whole-body near-infrared fluorescence imaging is now acknowledged as a powerful method for the molecular mapping of biological events in live small animals such as mouse models. With outstanding optical properties such as high fluorescence quantum yields and low photobleaching rates, quantum dots (QDs) are labels of choice in the near-infrared domain. The main applications described in the literature for in vivo imaging of mice after injection of QDs encompass imaging of lymph nodes and tumors and cell tracking. Standard methods for the preparation, the purification, and the in vivo fluorescence whole-body imaging of QDs in the live mouse are described. Nanoparticles coated by PEG chains of different sizes and terminal groups are prepared using 705-nm-emitting commercial QDs. Their biodistribution after intravenous or intradermal injections in tumor-bearing mice is reported here.

Texier, Isabelle; Josser, Véronique

393

Long-wavelength infrared quantum-dot based interband photodetectors

NASA Astrophysics Data System (ADS)

We report on the design and fabrication of (Al)GaAs(Sb)/InAs tensile strained quantum-dot (QD) based detector material for thermal infrared imaging applications in the long-wavelength infrared (LWIR) regime. The detection is based on transitions between confined dot states and continuum states in a type-II band lineup, and we therefore refer to it as a dot-to-bulk (D2B) infrared photodetector with expected benefits including long carrier lifetime due to the type-II band alignment, suppressed Shockley-Read-Hall generation-recombination due to the relatively large-bandgap matrix material, inhibited Auger recombination processes due to the tensile strain and epitaxial simplicity. Metal-organic vapor-phase epitaxy was used to grow multiple (Al)GaAs(Sb) QD layers on InAs substrates at different QD nominal thicknesses, compositions, doping conditions and multilayer periods, and the material was characterized using atomic force and transmission electron microscopy, and Fourier-transform infrared absorption spectroscopy. Dot densities up to 1 × 1011 cm-2, 1 × 1012 cm-2 and 3 × 1010 cm-2 were measured for GaAs, AlGaAs and GaAsSb QDs, respectively. Strong absorption in GaAs, AlGaAs and GaAsSb multilayer QD samples was observed in the wavelength range 6-12 ?m. From the wavelength shift in the spectral absorption for samples with varying QD thickness and composition it is believed that the absorption is due to an intra- valance band transition. From this it is possible to estimate the type-II interband transition wavelength, thereby suggesting that (Al)GaAs(Sb) QD/InAs heterostructures are suitable candidates for LWIR detection and imaging.

Gustafsson, O.; Berggren, J.; Ekenberg, U.; Hallén, A.; Hammar, M.; Höglund, L.; Karim, A.; Noharet, B.; Wang, Q.; Gromov, A.; Almqvist, S.; Zhang, A.; Junique, S.; Andersson, J. Y.; Asplund, C.; Marcks von Würtemberg, R.; Malm, H.; Martijn, H.

2011-05-01

394

Single-molecule spectroscopy of turn-on quantum dots induced by NADPH-dependent biocatalyzed transformations reveals that the fluorescence intensities of quantum dots functionalized with Nile Blue are stepwisely and reversibly changed in the presence of NADPH. PMID:22117202

Kim, Yea Seul; Kim, Min Young; Song, Jae Kyu; Kim, Tae Jung; Kim, Young Dong; Hah, Sang Soo

2011-11-25

395

Fast and efficient photodetection in nanoscale quantum-dot junctions.

We report on a photodetector in which colloidal quantum dots directly bridge nanometer-spaced electrodes. Unlike in conventional quantum-dot thin film photodetectors, charge mobility no longer plays a role in our quantum-dot junctions as charge extraction requires only two individual tunnel events. We find an efficient photoconductive gain mechanism with external quantum efficiencies of 38 electrons-per-photon in combination with response times faster than 300 ns. This compact device-architecture may open up new routes for improved photodetector performance in which efficiency and bandwidth do not go at the cost of one another. PMID:23094869

Prins, Ferry; Buscema, Michele; Seldenthuis, Johannes S; Etaki, Samir; Buchs, Gilles; Barkelid, Maria; Zwiller, Val; Gao, Yunan; Houtepen, Arjan J; Siebbeles, Laurens D A; van der Zant, Herre S J

2012-10-26

396

Fast and Efficient Photodetection in Nanoscale Quantum-Dot Junctions

NASA Astrophysics Data System (ADS)

We report on a photodetector in which colloidal quantum-dots directly bridge nanometer-spaced electrodes. Unlike in conventional quantum-dot thin film photodetectors, charge mobility no longer plays a role in our quantum-dot junctions as charge extraction requires only two individual tunnel events. We find an efficient photoconductive gain mechanism with external quantum-efficiencies of 38 electrons-per-photon in combination with response times faster than 300 ns. This compact device-architecture may open up new routes for improved photodetector performance in which efficiency and bandwidth do not go at the cost of one another.

Prins, Ferry; Buscema, Michele; Seldenthuis, Johannes S.; Etaki, Samir; Buchs, Gilles; Barkelid, Maria; Zwiller, Val; Gao, Yunan; Houtepen, Arjan J.; Siebbeles, Laurens D. A.; van der Zant, Herre S. J.

2012-11-01

397

Ferritin-templated quantum dots for quantum logic gates (Invited Paper)

Quantum logic gates (QLGs) or other logic systems are based on quantum-dots (QD) with a stringent requirement of size uniformity. The QD are widely known building units for QLGs. The size control of QD is a critical issue in quantum-dot fabrication. The work presented here offers a new method to develop quantum-dots using a bio-template, called ferritin, that ensures QD

Sang H. Choi; Jae-Woo Kim; Sang-Hyon Chu; Glen C. King; Peter T. Lillehei; Seon-Jeong Kim; James R. Elliott

2005-01-01

398

Chaos and interactions in quantum dots

NASA Astrophysics Data System (ADS)

Random-matrix theory has proved to be a successful tool in understanding the statistics of transport measurements in mesoscopic systems. Fluctuations of conductance peak-heights due to the transfer of single electrons through diffusive and chaotic ballistic quantum dots in the Coulomb-blockade regime can be modeled by fluctuations of random-matrix wavefunctions at low temperatures in the simple constant interaction model. On the other hand, statistics of the spacing between successive peaks do not show the behaviour predicted by the simple model, indicating that interactions beyond constant charging energy play an important role. The central theme of this thesis is the study of interaction effects on the mesoscopic fluctuations in a weakly disordered or chaotic ballistic quantum dot. Mean-field calculations suggest that upon addition of electrons into the dot, single-particle wavefunctions are not significantly altered while their eigenvalues "scramble" with the change in the self-consistent potential, thus affecting the peak-spacing statistics. We model this variation in the mean field by an extension of random-matrix theory: Gaussian processes, where the ensemble of random matrices depends on a discrete parameter, the number of electrons. Using a Gaussian process we can explain the saturation of the peak-to-peak correlator versus temperature. Another experimental signature of interactions is the enhanced peak-height correlation length versus an experimental magnetic field. We calculate the parametric peak-height correlator in the random interaction matrix model whose single-particle Hamiltonian is modeled by a Gaussian process. We find an increase in peak-height correlation length with the interaction fluctuation strength, that explains qualitatively the experimental observations. A microscopic understanding of the peak-spacing distribution requires the inclusion of the spin degrees of freedom. We introduce the Hartree-Fock-Koopmans approach for electrons with spin and calculate the Coulomb-blockade spacing statistics in quantum dots with a large number of electrons and at low temperatures. We account for the exchange interaction, as well as the fluctuations of interaction matrix elements to leading order in inverse Thouless conductance. We explain various features of the experimental peak-spacing distributions, including the absence of bimodality.

Khandelwal, Swati

399

Coulomb blockade spectroscopy of tunnel-coupled quantum dots

NASA Astrophysics Data System (ADS)

This thesis presents experimental studies of interactions in systems of coupled quantum dots. Quantum dots are often referred to as "artificial atoms" because the number of electrons on a dot is quantized, and those electrons occupy quantized energy levels. Two quantum dots coupled together by interdot electron tunneling may be considered an "artificial molecule". In this thesis we study artificial molecules composed of two quantum dots connected in series and use Coulomb blockade spectroscopy to measure the analog of a molecular binding energy. These measurements reveal how the effects of charge quantization on each individual dot are destroyed as interdot electron tunneling increases in both zero and strong magnetic fields. In one set of experiments, we use transport measurements to monitor the double dot charge configuration as interdot electron tunneling is increased from near zero, where the dots are almost completely separated, to strong tunneling, where they are entirely joined by quantum mechanical charge sharing. These measurements demonstrate that in zero magnetic field, charge quantization effects on the two individual dots are destroyed when the interdot tunnel conductance is exactly 2esp2/h. The charging diagram is shown to evolve in quantitative agreement with recent many body theories. In other experiments, we study double quantum dots in the quantum Hall regime by applying a strong perpendicular magnetic field to the sample. We find that charge quantization weakens as the quantum Hall edge states on the two dots join, with quantization effects completely destroyed at esp2/h of interdot tunnel conductance. Finally, we find that as the magnetic field is varied, the electron distribution readjusts to minimize the energy. The adjustments form a pattern that repeats with magnetic field and with the addition of electrons.

Livermore, Carol

400

Quantum Dot Enabled Molecular Sensing and Diagnostics

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.

Zhang, Yi; Wang, Tza-Huei

2012-01-01

401

NASA Astrophysics Data System (ADS)

We present some deterministic schemes to construct universal quantum gates, that is, controlled- not, three-qubit Toffoli, and Fredkin gates, between flying photon qubits and stationary electron-spin qubits assisted by quantum dots inside double-sided optical microcavities. The control qubit of our gates is encoded on the polarization of the moving single photon and the target qubits are encoded on the confined electron spins in quantum dots inside optical microcavities. Our schemes for these universal quantum gates on a hybrid system have some advantages. First, all the gates are accomplished with a success probability of 100% in principle. Second, our schemes require no additional qubits. Third, the control qubits of the gates are easily manipulated and the target qubits are perfect for storage and processing. Fourth, the gates do not require that the transmission for the uncoupled cavity is balanceable with the reflectance for the coupled cavity, in order to get a high fidelity. Fifth, the devices for the three universal gates work in both the weak coupling and the strong coupling regimes, and they are feasible in experiment.

Wei, Hai-Rui; Deng, Fu-Guo

2013-02-01

402

Energetic pinning of magnetic impurity levels in quantum-confined semiconductors.

Donor- and acceptor-type (D/A) impurities play central roles in controlling the physical properties of semiconductors. With continued miniaturization of information processing devices, the relationship between quantum confinement and D/A ionization energies becomes increasingly important. Here, we provide direct spectroscopic evidence that impurity D/A levels in doped semiconductor nanostructures are energetically pinned, resulting in variations in D/A binding energies with increasing quantum confinement. Using magnetic circular dichroism spectroscopy, the donor binding energies of Co2+ ions in colloidal ZnSe quantum dots have been measured as a function of quantum confinement and analyzed in conjunction with ab initio density functional theory calculations. The resulting experimental demonstration of pinned impurity levels in quantum dots has far-reaching implications for physical phenomena involving impurity-carrier interactions in doped semiconductor nanostructures, including in the emerging field of semiconductor spintronics where magnetic-dopant-carrier exchange interactions define the functionally relevant properties of diluted magnetic semiconductors. PMID:17163725

Norberg, Nick S; Dalpian, Gustavo M; Chelikowsky, James R; Gamelin, Daniel R

2006-12-01

403

Electrical transport phenomena in systems of semiconductor quantum dots.

While a fairly good understanding of optical and transport properties that are associated with single quantum dots has emerged in recent years the understanding of the relation between these properties and the observed macroscopic optical and electrical properties of solid ensembles of such dots is still at a very rudimentary level. This is in particular so in regard to the transport properties where the interplay between inter-dot conduction and the connectivity of the dots network determines the macroscopic observations. Reviewing the basic concepts and issues associated with these two essential ingredients, and considering some recent experimental observations on quantum dot ensembles of CdSe and Si, an effort is made here to derive a whole-but-simple physical basis for the understanding of the transport and the optoelectronic properties of solid state ensemble of semiconductor quantum dots. PMID:18464401

Balberg, Isaac

2008-02-01

404

Open quantum dots in graphene: Scaling relativistic pointer states

NASA Astrophysics Data System (ADS)

Open quantum dots provide a window into the connection between quantum and classical physics, particularly through the decoherence theory, in which an important set of quantum states are not "washed out" through interaction with the environment-the pointer states provide connection to trapped classical orbits which remain stable in the dots. Graphene is a recently discovered material with highly unusual properties. This single layer, one atom thick, sheet of carbon has a unique bandstructure, governed by the Dirac equation, in which charge carriers imitate relativistic particles with zero rest mass. Here, an atomic orbital-based recursive Green's function method is used for studying the quantum transport. We study quantum fluctuations in graphene and bilayer graphene quantum dots with this recursive Green's function method. Finally, we examine the scaling of the domiant fluctuation frequency with dot size.

Ferry, D. K.; Huang, L.; Yang, R.; Lai, Y.-C.; Akis, R.

2010-04-01

405

Analysis of electronic structures of quantum dots using meshless Fourier transform k.p method

NASA Astrophysics Data System (ADS)

We develop a complete Fourier transform k.p method and present its application for a theoretical investigation on electronic structures of quantum dots with consideration of the built-in strain effects. The Fourier transform technique is applied to the periodic position-dependent Hamiltonian, and a simple and neat expression of the Hamiltonian matrix in the Fourier domain is formulated due to the orthogonality of exponential functions. Spurious solutions can be avoided due to the truncation of high Fourier frequencies. A kinetic Hamiltonian matrix in momentum domain is formulated by entering the analytical Fourier transform of the quantum-dot shape function into the neat Hamiltonian matrix easily, which allows meshless numerical implementation. The formulation of strain Hamiltonian matrix is done by convolution of Fourier series of strain components and Fourier series of the quantum-dot shape functions. Therefore, an original Fourier transform-based k.p approach is developed by combining the kinetic Hamiltonian matrix and the strain Hamiltonian. This approach is adopted to study the dimension effect and strain effect on the ground states of electrons and holes of pyramidal quantum dots that are truncated to different heights. The ground-state energy variation shows that the electron state is the most sensitive to these effects and the strain effect on E1, LH1, and HH1 is more prominent for sharperquantum dots. This investigation shows that band mixing between the conduction band and valence band, and band mixing between heavy-hole and light-hole bands are reduced due to the strain effect, whereas this effect is more prominent for nontruncated pyramidal quantum dots due to the stress concentration. Among the three ground states, light-hole states are more weakly confined in the nonpyramidal quantum dot and shift to the tip of the pyramid due to the strain.

Zhao, Qiuji; Mei, Ting

2011-03-01

406

Optically induced entanglement of excitons in a single quantum Dot

Optically induced entanglement is identified by the spectrum of the phase-sensitive homodyne-detected coherent nonlinear optical response in a single gallium arsenide quantum dot. The electron-hole entanglement involves two magneto-excitonic states differing in transition energy and polarization. The strong coupling needed for entanglement is provided through the Coulomb interaction involving the electrons and holes. The result presents a first step toward the optical realization of quantum logic operations using two or more quantum dots. PMID:10988065

Chen; Bonadeo; Steel; Gammon; Katzer; Park; Sham

2000-09-15

407

Effects of Confining Geometry on Ballistic Transport in Quantum Wires

The effects of confining geometry are investigated on ballistic transport in quasi-one-dimensional quantum wires. The coupled channel type method is used in order to calculate the conductance of the quantum wires as a function of the Fermi energy. It is shown that oscillations existing in the conductance of the wire with a confining potential of `step wise' type geometry disappear

Hideaki Kasai; Kunihiro Mitsutake; Ayao Okiji

1991-01-01

408

Tuning of Long range Visible Emissions Using Coupled Quantum Dots

NASA Astrophysics Data System (ADS)

Size and shape controlled semiconductor quantum dots have been widely adopted in tailoring nanomaterials properties. Alternatively, chemically coupled quantum dots offer a novel route for tuning long range electronic transitions of semiconductors via band offset engineering at the material interface. We report on a simple route of tailoring visible emissions over long range by chemically designing coupled dots comprising of ZnSe and CdS quantum dots. Long range tunability of visible emissions originated from strong interdot dispersion, which can not be realized from individual quantum dots. The first principles electronic structure calculations based on density functional theory reveals the type-II nature of chemical bonding at CdS/ZnSe interface which plays a crucial role in tailoring the long range emissions.

Sengupta, Sucheta; Ganguli, Nirmal; Dasgupta, I.; Sarma, D. D.; Acharya, Somobrata

2011-07-01

409

Quantum Confinement and Optical Gaps in Si Nanocrystals

Quasiparticle gaps, self-energy corrections, exciton Coulomb energies, and optical gaps in Si quantum dots are calculated from first principles using a real-space pseudopotential method. The calculations are performed on hydrogen-passivated spherical Si clusters with diameters up to 27.2 A (â¼800 Si and H atoms). It is shown that (i) the self-energy correction in quantum dots is enhanced substantially compared to

Serdar Ogut; James R. Chelikowsky; Steven G. Louie

1997-01-01

410

NASA Astrophysics Data System (ADS)

Quantum dot superlattices offer prospects for new generations of semiconductor devices. One possible recently suggested application is in tandem solar cells based entirely on silicon, using confinement in the quantum dot to control the cell band gap. In this paper, we use the effective mass approach to calculate the conduction band structure of a three-dimensional silicon quantum dot superlattice with the dots embedded in a matrix of silicon dioxide, silicon nitride, or silicon carbide. The quantum dot superlattice is modeled as a regularly spaced array of equally sized cubic dots in the respective matrix. Incorporating the effect of silicon anisotropic effective mass is shown to reduce both the degeneracies of the isotropic solutions and the energy separation between states. Electron densities of state and mobilities are derived from the band structure data. Theoretical results for the effect of dot size, interdot distance, and matrix material have been obtained. These results clarify the required design features of silicon quantum dot superlattices for the proposed all-silicon tandem solar cells.

Jiang, Chu-Wei; Green, Martin A.

2006-06-01

411

Quantum-Dot Cellular Automata: Line and Majority Logic Gate

NASA Astrophysics Data System (ADS)

An introduction to the operation of quantum-dot cellular automata is presented, along with recent experimental results. Quantum-dot cellular automata (QCA) is a transistorless computation paradigm that addresses the issues of device density and interconnection. The basic building blocks of the QCA architecture, such as AND, OR, and NOT are presented. The experimental devices presented are QCA cells where the dots are metal islands, coupled by capacitors and tunnel junctions. A line of three two-dot cells is presented, which demonstrates that there are no metastable states in a QCA line. The final experiment presented is a QCA majority gate, a programmable AND/OR logic gate.

Snider, Gregory; Orlov, Alexei; Amlani, Islamshah; Bernstein, Gary; Lent, Craig; Merz, James; Porod, Wolfgang

1999-12-01

412

Determination of the size of quantum dots by fluorescence spectroscopy.

There has been a lack of quick, simple and reliable methods for determination of nanoparticle size. An investigation of the size of hydrophobic (CdSe) and hydrophilic (CdSe/ZnS) quantum dots was performed by using the maximum position of the corresponding fluorescence spectrum. It has been found that fluorescence spectroscopy is a simple and reliable methodology to estimate the size of both quantum dot types. For a given solution, the homogeneity of the size of quantum dots is correlated to the relationship between the fluorescence maximum position (FMP) and the quantum dot size. This methodology can be extended to the other fluorescent nanoparticles. The employment of evolving factor analysis and multivariate curve resolution-alternating least squares for decomposition of the series of quantum dots fluorescence spectra recorded by a specific measuring procedure reveals the number of quantum dot fractions having different diameters. The size of the quantum dots in a particular group is defined by the FMP of the corresponding component in the decomposed spectrum. These results show that a combination of the fluorescence and appropriate statistical method for decomposition of the emission spectra of nanoparticles may be a quick and trusted method for the screening of the inhomogeneity of their solution. PMID:21491050

Mutavdži?, Dragosav; Xu, Jianmin; Thakur, Garima; Triulzi, Robert; Kasas, Sandor; Jeremi?, Milorad; Leblanc, Roger; Radoti?, Ksenija

2011-04-14

413

Single spins in self-assembled quantum dots.

Self-assembled quantum dots have excellent photonic properties. For instance, a single quantum dot is a high-brightness, narrow-linewidth source of single photons. Furthermore, the environment of a single quantum dot can be tailored relatively easily using semiconductor heterostructure and post-growth processing techniques, enabling electrical control of the quantum dot charge and control over the photonic modes with which the quantum dot interacts. A single electron or hole trapped inside a quantum dot has spintronics applications. Although the spin dephasing is rather rapid, a single spin can be manipulated using optical techniques on subnanosecond timescales. Optical experiments are also providing new insights into old issues, such as the central spin problem. This Review provides a snapshot of this active field, with some indications for the future. It covers the basic materials and optical properties of single quantum dots, techniques for initializing, manipulating and reading out single spin qubits, and the mechanisms that limit the electron-spin and hole-spin coherence. PMID:23695745

Warburton, Richard J

2013-06-01

414

Single spins in self-assembled quantum dots

NASA Astrophysics Data System (ADS)

Self-assembled quantum dots have excellent photonic properties. For instance, a single quantum dot is a high-brightness, narrow-linewidth source of single photons. Furthermore, the environment of a single quantum dot can be tailored relatively easily using semiconductor heterostructure and post-growth processing techniques, enabling electrical control of the quantum dot charge and control over the photonic modes with which the quantum dot interacts. A single electron or hole trapped inside a quantum dot has spintronics applications. Although the spin dephasing is rather rapid, a single spin can be manipulated using optical techniques on subnanosecond timescales. Optical experiments are also providing new insights into old issues, such as the central spin problem. This Review provides a snapshot of this active field, with some indications for the future. It covers the basic materials and optical properties of single quantum dots, techniques for initializing, manipulating and reading out single spin qubits, and the mechanisms that limit the electron-spin and hole-spin coherence.

Warburton, Richard J.

2013-06-01

415

Quantum dot: magic nanoparticle for imaging, detection and targeting.

Quantum dots (QDs) are one of the nanoparticles that use in Imaging, Detection and Targeting. Quantum dots are nanometer-size luminescent semiconductor crystals and have unique chemical and physical properties due to their size and their highly compact structure. They emit different wavelengths over a broad range of the light spectrum from visible to infrared, depending on their size and chemical composi tion. Eventual use of quantum dots to dramatically improve clinical diagnostic tests for the early detection of cancer. The use of quantum dots heralds a revolution in biological imaging. The current and widely used organic fluorophores have two shortcomings associated with their fluorescence. Signals from the labeled molecules can be obscured by cell autofluorescence, occurring in the visible spectrum and by photobleaching which seriously limits observation time. Colloidal quantum dots are bright, photostable fluorophores of a few nanometers in diameter. Because their size approximates that of individual biomolecules, water-solubl quantum dot complex have been used to target and image tumor cells. Despite their advantages the best materials for quantum dots; cadmium sulfide, CdS and cadmium selenide, CdSe can be highly toxic. While enhancing the biocompatibility of this nanoparticle various encapsulation techniques have also aided in their water-dispersibility and functionalization. QDs were introduced to cell biology as alternative fluorescent probes in recent years. Traditional fluorophores, e.g. organic dyes and fluorescent proteins are limited by thei narrow absorption range, broad emission spectra and short fluorescent lifetime. PMID:19848055

Ghasemi, Younes; Peymani, Payam; Afifi, Saba

2009-08-01

416

Quantum chromodynamics near the confinement limit

These nine lectures deal at an elementary level with the strong interaction between quarks and its implications for the structure of hadrons. Quarkonium systems are studied as a means for measuring the interquark interaction. This is presumably (part of) the answer a solution to QCD must yield, if it is indeed the correct theory of the strong interactions. Some elements of QCD are reviewed, and metaphors for QCD as a confining theory are introduced. The 1/N expansion is summarized as a way of guessing the consequences of QCD for hadron physics. Lattice gauge theory is developed as a means for going beyond perturbation theory in the solution of QCD. The correspondence between statistical mechanics, quantum mechanics, and field theory is made, and simple spin systems are formulated on the lattice. The lattice analog of local gauge invariance is developed, and analytic methods for solving lattice gauge theory are considered. The strong-coupling expansion indicates the existence of a confining phase, and the renormalization group provides a means for recovering the consequences of continuum field theory. Finally, Monte Carlo simulations of lattice theories give evidence for the phase structure of gauge theories, yield an estimate for the string tension characterizing the interquark force, and provide an approximate description of the quarkonium potential in encouraging good agreement with what is known from experiment.

Quigg, C.

1985-09-01

417

Spectroscopy of Collective Modes in Few-electron Quantum Dots

NASA Astrophysics Data System (ADS)

Quantum correlations among electrons confined in semiconductor quantum dots (QDs) are expected to lead to exotic states of matter, such as an electron molecule. In the limit of vanishing electron density, the distances between the confined electrons are rigidly fixed like those of nuclei in conventional molecules. The electronic excitations of such a molecule are quantized normal modes of roto-vibration, whose quanta have either a rigid-rotor or relative-motion character. Recent progress on the emergence of molecular roto-vibrational modes at experimentally attainable densities will be discussed. Signatures of the roto-vibrational spectrum are observed even if the localization in space of the electron wave functions is not yet fully achieved. I will present a joint experimental and theoretical investigation of the neutral electronic excitations of nanofabricated AlGaAs/GaAs QDs that contain four electrons. We use inelastic light scattering to probe electronic charge and spin excitations in an array of identical nanofabricated QDs. Spectra of low-lying excitations associated to changes of the relative-motion wave function -the analogues of the vibrational modes of a conventional molecule- do not depend on the rotational state represented by the angular momentum, which can be controlled by the application of a magnetic field. A theoretical model, based on full configuration-interaction method, offers an excellent quantitative agreement with the experimental findings. I will also demonstrate optical control of the number of electrons and lateral confining potential in our GaAs/AlGaAs QDs. This is achieved by illumination with a weak laser beam that is absorbed in the AlGaAs barrier. Precise tuning of the energy-level structure and number of electrons is manifested in the evolution of low-lying spin and charge excitations probed by inelastic light scattering. Our findings open a new venue towards the all-optical manipulation of single electrons in QDs.

Kalliakos, Sokratis

2009-03-01

418

Electron-spin resonance in a quantum dot

NASA Astrophysics Data System (ADS)

We discuss electron-spin resonance in a single quantum dot at filling factor ?<~2. The exchange interaction leads to an enhanced effective g*eff factor, which in turn gives rise to spatially separated regions of compressible and incompressible states with different spin orientations in the dot. These can be treated as electrostatically coupled dot systems within the original dot. Transport measurements under microwave radiation provide an experimental tool to induce spin transitions between these dots. Measurements indicating a spin-flip transition are presented.

Blick, Robert H.; Gudmundsson, Vidar; Haug, Rolf J.; von Klitzing, Klaus; Eberl, Karl

1998-05-01

419

Tailoring Magnetism in Bulk Semiconductors and Quantum Dots

NASA Astrophysics Data System (ADS)

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.

Zutic, Igor

2008-03-01

420

Minimized deterioration of ultrashort pulses in quantum dot optical amplifiers

NASA Astrophysics Data System (ADS)

The dynamics of ultrashort pulses propagating in a quantum dot amplifier is determined by a complex nonlinear coupling and dynamic interplay of light fields and carriers in the spatially inhomogeneous quantum dot ensemble. Computational modeling shows that in spite of the large complexity the strong localization of the carrier inversion and the low amplitude phase coupling may allow the amplification and transmission of ultrahort light pulses with minimum deterioration of the pulse properties (e.g. pulse shape, duration). The theoretical description is based on spatially resolved Quantum Dot Maxwell-Bloch equations that describe the spatio-temporal light field and inter-/intra-level carrier dynamics in each quantum dot of a typical quantum dot ensemble. In particular, this includes spontaneous luminescence, counterpropagation of amplified spontaneous emission and induced recombination as well as carrier diffusion in the wetting layer of the laser. Intradot scattering via emission and absorption of phonons, as well as the scattering with the carriers and phonons of the surrounding wetting layer are dynamically included on a mesoscopic level. Spatial fluctuations in size and energy levels of the quantum dots and irregularities in the spatial distribution of the quantum dots in the active layer are simulated via statistical methods. Simulation results of the nonlinear pulse propagation in quantum dot optical amplifiers allow visualization and interpretation of fundamental nonlinear processes such as selective depletion and re-filling of quantum dot energy levels leading to a complex gain and index dynamics that affect the amplitude and phase of a propagating light pulse. Computational modelling thus may lay the foundation for an optimization and tayloring of pulse properties.

Gehrig, Edeltraud; Hess, Ortwin G.

2004-09-01

421

Purcell effect on CdSe/ZnSe quantum dots in pillar microcavities

NASA Astrophysics Data System (ADS)

The Purcell effect leads to an enhancement of the spontaneous emission in a given cavity mode for an emitter located in a small volume cavity. This effect is well explained as a consequence of Fermi's golden rule in a particular photonic environment. In this paper we show an enhancement by a factor of up to 3.4 of the spontaneous emission of CdSe/ZnSe quantum dots located in a pillar microcavity. The sample consists of a ? /2-ZnSe cavity sandwiched between SiO2/TiO2 Bragg mirrors and is etched to provide lateral optical confinement. The present study is based on a comparison between time-resolved photoluminescence measurements for dots inserted in a pillar cavity or in a bulk ZnSe layer. The photoluminescence decay shortening is quantitatively interpreted by calculating the theoretical maximum Purcell factor expected for different cavity modes in resonance with quantum dots.

André, R.; Robin, I. C.; Balocchi, A.; Carayon, S.; Moehl, S.; Gérard, J. M.

422

GaSb quantum dot growth using InAs quantum dot stressors

Metalorganic vapor phase epitaxy of GaSb quantum dots (QDs) grown on top of a layer of InAs seed QDs shows a vertically aligned correlation if thin GaAs spacer layers deposited at low temperature are used. Introduction of an annealing step after spacer deposition strongly improves the crystalline quality of the spacer layer. Vertically anticorrelated ordering is found on annealed spacers

L. Müller-Kirsch; N. N. Ledentsov; R. Sellin; U. W. Pohl; D. Bimberg; I. Häusler; H. Kirmse; W. Neumann

2003-01-01

423

A Novel Particle Detector: Quantum Dot Doped Liquid Scintillator

NASA Astrophysics Data System (ADS)

Quantum dots are semiconducting nanocrystals. When excited by light shorter then their characteristic wavelength, they re-emit in a narrow band around this wavelength. The size of the quantum is proportional to the characteristic wavelength so they can be tuned for many applications. CdS quantum dots are made in wavelengths from 360nm to 460nm, a perfect range for the sensitivity of photo-multiplier tubes. The synthesis of quantum dots automatically leaves them in toluene, a good organic scintillator and Cd is a particularly interesting material as it has one of the highest thermal neutron cross sections and has several neutrinoless double beta decay and double electron capture isotopes. The performance of quantum dot loaded scintillator compared to standard scintillators is measured and some unique properties presented. )

Winslow, Lindley; Conrad, Janet; Jerry, Ruel

2010-02-01

424

Rare-earth doped GaN and InGaN quantum dots grown by plasma assisted MBE

NASA Astrophysics Data System (ADS)

We report on the MBE growth of GaN and InGaN quantum dots (QDs) doped with rare earth ions, namely Eu, Tm and Tb exhibiting red, blue and green luminescence, respectively. Intense photoluminescence/cathodoluminescence is observed, resulting from the spatial localization of rare earth ions in dots combined with the confinement properties of the carriers. White light emission has been produced by combining the three rare earths in a multilayer sample of stacked GaN QD planes.

Hori, Y.; Andreev, T.; Biquard, X.; Monroy, E.; Jalabert, D.; Dang, Le Si; Tanaka, M.; Oda, O.; Daudin, B.

2005-05-01

425

Rare-earth doped GaN and InGaN quantum dots grown by plasma assisted MBE

We report on the MBE growth of GaN and InGaN quantum dots (QDs) doped with rare earth ions, namely Eu, Tm and Tb exhibiting red, blue and green luminescence, respectively. Intense photoluminescence\\/cathodoluminescence is observed, resulting from the spatial localization of rare earth ions in dots combined with the confinement properties of the carriers. White light emission has been produced by

Y. Hori; T. Andreev; X. Biquard; E. Monroy; D. Jalabert; Le Si Dang; M. Tanaka; O. Oda; B. Daudin

2005-01-01

426

NASA Astrophysics Data System (ADS)

In this thesis, we study solution-processed lead sulfide quantum dots for near-infrared quantum information and communication applications. Quantum dots processed through synthetic routes and colloidally suspended in solution offer far-reaching device application possibilities that are unparalelled in traditional self-assembled quantum dots. Lead sulfide quantum dots are especially promising for near-infrared quantum optics due to their optical emission at the wavelengths of fiber-optic communications (1.3--1.5 microm). The broad absorption spectrum of these quantum dots can be used for solar light-harvesting applications, to which end the results of Chapter 2---where we study Forster resonance energy transfer in quantum dot solids---provide remarkable insights into photon emission from quantum-dot based solar cells. In subsequent chapters, we explore quantum-dot photonic crystal applications, where exciton-photon interactions in the cavity environment remarkably allow for the emission of indistinguishable single photons that are important for distribution of high-security quantum keys---being highly sensitive to 'eavesdropping'. Particularly, the suggestion of the solution-processed QED system is novel compared to traditional self-assembled systems, and as we will discuss, offer integration and processing capabilities that are unprecedented, and perform well at wavelength ranges where standard QED systems scale poorly. The results of chapters 3--6 are therefore significant in the general field of cavity quantum electrodynamics.

Bose, Ranojoy

427

Magnetic field dependence of triplet-singlet relaxation in quantum dots with spin-orbit coupling

We estimate the triplet-singlet relaxation time due to spin-orbit coupling assisted by phonon emission in weakly confined quantum dots. Calculations are performed taking into account Coulomb and spin-orbit interactions exactly within the full configuration interaction method, and Fermi golden rule. Our results for two and four electrons show that different triplet-singlet relaxation trends observed in recent experiments under magnetic fields

Juan I. Climente; Andrea Bertoni; Guido Goldoni; Massimo Rontani; Elisa Molinari

2007-01-01

428

Voltage-Controlled Electron-Hole Interaction in a Single Quantum Dot

The ground state of neutral and negatively charged excitons confined to a single self-assembled InGaAs quantum dot is probed in a direct absorption experiment by high resolution laser spectroscopy. We show how the anisotropic electron-hole exchange interaction depends on the exciton charge and demonstrate how the interaction can be switched on and off with a small dc voltage. Furthermore, we

A. Högele; S. Seidl; M. Kroner; K. Karrai; R. J. Warburton; M. Atatüre; J. Dreiser; A. Imamoglu; Brian D. Gerardot; Pierre M. Petroff

2005-01-01

429

Energy-selective charging of type-II GaSb\\/GaAs quantum dots

The hole confinement in type-II self-organized GaSb\\/GaAs quantum dots (QDs) was investigated by combining optical excitation and time-resolved capacitance spectroscopy. The experimental results indicate energy-selective charging even for type-II QDs. With increasing excitation energy the apparent hole activation energy decreases, which is attributed to light absorption in sub-ensembles of QDs with decreasing hole localization. The large localization energy of about

M. Geller; C. Kapteyn; E. Stock; L. Müller-Kirsch; R. Heitz; D. Bimberg

2004-01-01

430

Symbolic-Numeric Algorithms for Computer Analysis of Spheroidal Quantum Dot Models

A computation scheme for solving elliptic boundary value problems with\\u000aaxially symmetric confining potentials using different sets of one-parameter\\u000abasis functions is presented. The efficiency of the proposed symbolic-numerical\\u000aalgorithms implemented in Maple is shown by examples of spheroidal quantum dot\\u000amodels, for which energy spectra and eigenfunctions versus the spheroid aspect\\u000aratio were calculated within the conventional effective mass

A. A. Gusev; Ochbadrakh Chuluunbaatar; Vladimir P. Gerdt; V. A. Rostovtsev; Sergey I. Vinitsky; V. L. Derbov; V. V. Serov

2010-01-01

431

Exciton-Mediated Raman Scattering in One-Dimensional Quantum Dots

NASA Astrophysics Data System (ADS)

The differential cross section (DCS) for exciton-mediated Raman scattering (EMRS) in one-dimensional semiconductor quantum dots is presented. The exciton states are considered as intermediate states in the Raman scattering process. The selection rules for the EMRS process are studied. The numerical results show that the contribution to DCS indicated by exciton is larger than that by electron. DCS of EMRS is larger when there is a bigger confinement potential frequency.

Sun, Hai-Chao; Liu, Cui-Hong

432

Tracking bio-molecules in live cells using quantum dots

Single particle tracking (SPT) techniques were developed to explore bio-molecules dynamics in live cells at single molecule sensitivity and nanometer spatial resolution. Recent developments in quantum dots (Qdots) surface coating and bio-conjugation schemes have made them most suitable probes for live cell applications. Here we review recent advancements in using quantum dots as SPT probes for live cell experiments. The trajectory of single quantum dot bound to avidin-GPI (in black) is overlaid with the mean intensity of caveolin-1-EGFP (in green) to allow colocalization studies of avidin-GPI with caveolae.

Chang, Yun-Pei; Pinaud, Fabien; Antelman, Joshua; Weiss, Shimon

2009-01-01

433

The size of silicon quantum dots (Si QDs) embedded in silicon nitride (SiN{sub x}) has been controlled by varying the total pressure in the plasma-enhanced chemical vapor deposition (PECVD) reactor. This is evidenced by transmission electron microscopy and results in a shift in the light emission peak of the quantum dots. We show that the luminescence in our structures is attributed to the quantum confinement effect. These findings give a strong indication that the quality (density and size distribution) of Si QDs can be improved by optimizing the deposition parameters which opens a route to the fabrication of an all-Si tandem solar cell.

Rezgui, B.; Sibai, A.; Nychyporuk, T.; Lemiti, M.; Bremond, G. [Universite de Lyon, Institut des Nanotechnologies de Lyon INL-UMR5270, CNRS, INSA de Lyon, Villeurbanne F-69621 (France); Maestre, D.; Palais, O. [IM2NP, CNRS UMR 6242, Universite Aix-Marseille, Avenue Escadrille Normandie Niemen, Case 142, 13397, Marseille Cedex 20 (France)

2010-05-03

434

Highly Fluorescent Noble-Metal Quantum Dots

NASA Astrophysics Data System (ADS)

Highly fluorescent, water-soluble, few-atom noble-metal quantum dots have been created that behave as multielectron artificial atoms with discrete, size-tunable electronic transitions throughout the visible and near infrared. 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 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.

Zheng, Jie; Nicovich, Philip R.; Dickson, Robert M.

2007-05-01

435

Self-Formation of Semiconductor Quantum Dots

NASA Astrophysics Data System (ADS)

We reviewed the self-formation control of InAs/GaAs quantum dots (QDs) by molecular beam epitaxy. Uniform InAs/GaAs QDs were demonstrated by self-size-limiting effect, the optimized capping growth, and the closely stacked growth using the nanoholes. High-density InAs QDs were achieved by Sb-mediated growth. In addition, an intermittent growth method was presented for ultralow density InAs QDs. Furthermore, the vertical and in-plane arrangements of InAs/GaAs QDs were attempted by using the strain-controlled underlying layers. One-dimensional QD chains were spontaneously formed along the [1-10] direction on the GaAs/InGaAs/GaAs(001) buffer layers. Two-dimensional arrangement of InAs QDs was demonstrated by using GaAsSb/GaAs(001) buffer layers.

Yamaguchi, Koichi

436

Self-Formation of Semiconductor Quantum Dots

NASA Astrophysics Data System (ADS)

We reviewed the self-formation control of InAs/GaAs quantum dots (QDs) by molecular beam epitaxy. Uniform InAs/GaAs QDs were demonstrated by self size-limiting effect, the optimized capping growth, and the closely-stacked growth using the nanoholes. High-density InAs QDs were achieved by Sb-mediated growth. In addition, an intermittent growth method was presented for ultra-low density InAs QDs. Furthermore, the vertical and in-plane arrangements of InAs/GaAs QDs were attempted by using the strain-controlled underlying layers. One-dimensional QD chains were spontaneously formed along the [1-10] direction on the GaAs/InGaAs/GaAs(001) buffer layers. Two-dimensional arrangement of InAs QDs was demonstrated by using GaAsSb/GaAs(001) buffer layers.

Yamaguchi, Koichi

437

Semiconductor quantum dot-inorganic nanotube hybrids.

A synthetic route for preparation of inorganic WS(2) nanotube (INT)-colloidal semiconductor quantum dot (QD) hybrid structures is developed, and transient carrier dynamics on these hybrids are studied via transient photoluminescence spectroscopy utilizing several different types of QDs. Measurements reveal efficient resonant energy transfer from the QDs to the INT upon photoexcitation, provided that the QD emission is at a higher energy than the INT direct gap. Charge transfer in the hybrid system, characterized using QDs with band gaps below the INT direct gap, is found to be absent. This is attributed to the presence of an organic barrier layer due to the relatively long-chain organic ligands of the QDs under study. This system, analogous to carbon nanotube-QD hybrids, holds potential for a variety of applications, including photovoltaics, luminescence tagging and optoelectronics. PMID:22354096

Kreizman, Ronen; Schwartz, Osip; Deutsch, Zvicka; Itzhakov, Stella; Zak, Alla; Cohen, Sidney R; Tenne, Reshef; Oron, Dan

2012-02-21

438

[Application of quantum dots in biomedical detection].

Semiconductor quantum dots (QDs) are a new kind of biological fluorescence material, which has many advantages, such as broad excitation spectra, tunable emission spectra and good photostability. In the field of biomedical detection, the problems encountered in the traditional organic dye-based biomedical detections, such as short fluorescence lifetime and failure to simultaneous excitation of multiple colors, can be solved by using QDs. Water-soluble QDs combined with specific bio-molecules can label targeting bio-compound, which is useful in bio-molecule detection, cell labeling, tissue imaging, and can be used in fluorescence resonance energy transfer (FRET) technology. Combining QDs and protein chip technology to develop a new technology to detect multiple kinds of tumor markers will be one of the promising clinical applications of QDs with greater sensitivity, specificity, rapidity and convenience. PMID:21774239

Zhang, Luyao; Niu, Wanting; Yang, Hao; Pan, Min

2011-0