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1

The confinement energy of quantum dots

One of the most significant research interests in the field of electronics is that on quantum dot, because such materials have electronic properties intermediate between those of bulk semiconductors and those of discrete molecules. Confinement energy is a very important property of quantum dot. In this study, quantum confinement energy of a quantum dot is concluded to be h2/8md2 (d being the diameter of the confinement) and not h2/8ma2 (a being the radius of the confinement), as reported in the available literature. This is in the light of a recent study [1]. This finding should have a significant impact in the understanding of the physics of quantum dot and its technological application.

Dey, Samrat; Chakraborty, kishan; Dasgupta, Debasmita; Bordoloi, Darsana; Saikia, Rituja; Neog, Darsana; Shimray, Shishila; Paul, Supriyanka; Brahma, Kabita; Dey, Joydeep; Choudhury, Saurav

2012-01-01

2

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

3

Electronic confinement in modulation doped quantum dots

Modulation doping, an effective way to dope quantum dots (QDs), modifies the confinement energy levels in the QDs. We present a self-consistent full multi-grid solver to analyze the effect of modulation doping on the confinement energy levels in large-area structures containing Si QDs in SiO{sub 2} and Si{sub 3}N{sub 4} dielectrics. The confinement energy was found to be significantly lower when QDs were in close proximity to dopant ions in the dielectric. This effect was found to be smaller in Si{sub 3}N{sub 4}, while smaller QDs in SiO{sub 2} were highly susceptible to energy reduction. The energy reduction was found to follow a power law relationship with the QD size.

Puthen Veettil, B., E-mail: b.puthen-veettil@unsw.edu.au; König, D.; Patterson, R.; Smyth, S.; Conibeer, G. [Australian Centre for Advanced Photovoltaics, UNSW, Sydney 2052 (Australia)

2014-04-14

4

Femtosecond optical gain in strongly confined quantum dots

NASA Astrophysics Data System (ADS)

Optical gain was found in strongly confined CdSe quantum dots. As a result of a multitude of one- and two-electron-hole pair transitions, the gain region is broad and quasi-continuous and stretches below the absorption edge. We present a model for gain in a quasi-zero-dimensional quantum confined semiconductor system that agrees well with the femtosecond experiments.

Giessen, H.; Woggon, U.; Fluegel, B.; Mohs, G.; Hu, Y. Z.; Koch, S. W.; Peyghambarian, N.

1996-07-01

5

Computational study of confined states in quantum dots by an efficient finite difference method.

??Semiconductor quantum dot systems have gained more attention in quantum computation and optoelectronic applications due to the ease of bandstructure tailoring and three-dimensional quantum confinement.… (more)

Butt, Salman

2010-01-01

6

Properties of a polaron confined in a spherical quantum dot

NASA Astrophysics Data System (ADS)

A Frohlich Hamiltonian describing the electron-phonon interaction in a spherical quantum dot embedded in another polar material is derived, taking into account interactions with both bulk longitudinal optical and surface optical phonons. The Hamiltonian is appropriate to the general case of a finite confining potential originating from a bandgap mismatch between the materials of the dot and the surrounding matrix. This Hamiltonian is then applied to treat the electron-phonon interaction in the adiabatic approximation for various quantum dot systems. It was found that, as the radius of the dot decreases, the magnitude of the electron-phonon interaction energy first increases, passes through a maximum, and then gradually decreases to the value appropriate to the situation where the electron is weakly localized inside the dot. For most dot radii the polaron properties are described well by a model assuming perfect electron confinement. Based on this result, the problem of the bound polaron confined perfectly in the quantum dot was investigated within the adiabatic and all-coupling variational approaches. The polaron properties have been studied performing both analytical and numerical calculations for various radii of the quantum dot and for different impurity positions inside the dot. Within the adiabatic approximation, it was found that the magnitude of the electron-phonon interaction increases as the radius decreases for any impurity position. It was also shown that the input from the electron-surface-phonon interaction to the total polaron energy is much larger than was found earlier for the free polaron confined in the dot. As a function of the impurity position, the electron-surface-phonon interaction energy increases as the impurity is shifted towards the surface, reaches its maximum when the impurity is positioned inside the dot and then decreases as the impurity moved close to surface. The all-coupling approach gave rise to the following results: for any value of coupling and for small binding strength the total electron-phonon interaction energy depends weakly on the impurity position and has a maximum when the impurity is at the center of the dot. As the binding strength increases, the maximum of the electron-phonon interaction energy shifts inside the dot. For small values of the coupling the magnitude of the electron-surface-phonon interaction has a maximum on the surface of the dot, unlike the results of the adiabatic limit.

Melnikov, Dmitriy V.

7

Sensitivity of exciton spin relaxation in quantum dots to confining potential

We observe a strong dependence of the exciton spin relaxation in CdTe quantum dots on the average dot size and the depth of the confining potential. After rapid thermal annealing, which increases the average dot size and leads to weaker confinement, we measure the spin relaxation time of the quantum dot excitons to be 1.5 ns, as compared to 4.8

S. Mackowski; T. Gurung; H. E. Jackson; L. M. Smith; W. Heiss; J. Kossut; G. Karczewski

2005-01-01

8

Impurity binding energies in quantum dots with parabolic confinement

NASA Astrophysics Data System (ADS)

We present an effective numerical procedure to calculate the binding energies and wave functions of the hydrogen-like impurity states in a quantum dot (QD) with parabolic confinement. The unknown wave function was expressed as an expansion over one-dimensional harmonic oscillator states, which describes the electron's movement along the defined z-axis. Green's function technique used to obtain the solution of Schredinger equation for electronic states in a transverse plane. Binding energy of impurity states is defined as poles of the wave function. The dependences of the binding energy on the position of an impurity, the size of the QD and the magnetic field strength are presented and discussed.

Abramov, Arnold

2015-03-01

9

The three-dimensional confinement created by the ultrasmall semiconductor structures known as quantum dots greatly modifies the optical properties of the spatially localized carriers. These systems are presently of great interest to the research community, both for the improved understanding of the physics of confined structures, and for the potential applications of these systems to areas such as optoelectronics and quantum

Anthony Stephen Lenihan

2002-01-01

10

NASA Astrophysics Data System (ADS)

Semiconducting quantum dots - more fancifully dubbed artificial atoms - are quasi-zero dimensional, tiny, man-made systems with charge carriers completely confined in all three dimensions. The scientific quest behind the synthesis of quantum dots is to create and control future electronic and optical nanostructures engineered through tailoring size, shape, and composition. The complete confinement - or the lack of any degree of freedom for the electrons (and/or holes) - in quantum dots limits the exploration of spatially localized elementary excitations such as plasmons to direct rather than reciprocal space. Here we embark on a thorough investigation of the magneto-optical absorption in semiconducting spherical quantum dots characterized by a confining harmonic potential and an applied magnetic field in the symmetric gauge. This is done within the framework of Bohm-Pines' random-phase approximation that enables us to derive and discuss the full Dyson equation that takes proper account of the Coulomb interactions. As an application of our theoretical strategy, we compute various single-particle and many-particle phenomena such as the Fock-Darwin spectrum; Fermi energy; magneto-optical transitions; probability distribution; and the magneto-optical absorption in the quantum dots. It is observed that the role of an applied magnetic field on the absorption spectrum is comparable to that of a confining potential. Increasing (decreasing) the strength of the magnetic field or the confining potential is found to be analogous to shrinking (expanding) the size of the quantum dots: resulting into a blue (red) shift in the absorption spectrum. The Fermi energy diminishes with both increasing magnetic-field and dot-size; and exhibits saw-tooth-like oscillations at large values of field or dot-size. Unlike laterally confined quantum dots, both (upper and lower) magneto-optical transitions survive even in the extreme instances. However, the intra-Landau level transitions are seen to be forbidden. The spherical quantum dots have an edge over the strictly two-dimensional quantum dots in that the additional (magnetic) quantum number makes the physics richer (but complex). A deeper grasp of the Coulomb blockade, quantum coherence, and entanglement can lead to a better insight into promising applications involving lasers, detectors, storage devices, and quantum computing.

Kushwaha, Manvir S.

2014-12-01

11

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

12

Efficient exciton transport between strongly quantum-confined silicon quantum dots.

Many-body Green function analysis and first-order perturbation theory 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 for the same center-to-center separation. This surprising result offers the prospect of designing assemblies of quantum dots through which excitons can travel for long distances, a game-changing paradigm shift for next-generation solar energy harvesting. We also find that surface reconstruction significantly influences the absorption cross section and leads to a large reduction in both transport rate and efficiency. Further, exciton transport efficiency is higher for hydrogen-passivated dots as compared with those terminated with more electronegative ligands, a result not predicted by Förster theory. PMID:22468899

Lin, Zhibin; Li, Huashan; Franceschetti, Alberto; Lusk, Mark T

2012-05-22

13

Imaging quantum-dot-confined electron density in transition to fractional quantum Hall regime

NASA Astrophysics Data System (ADS)

We consider mapping the charge density confined in quantum dots by the Coulomb blockade microscopy (CBM) at the transition to the fractional quantum Hall regime. We apply an exact diagonalization method to determine the exact charge density and its reaction to the scanning probe as well as to calculate the energy maps as functions of the position of the probe. From the energy maps—which are the only experimentally accessible quantity in CBM—we evaluate an apparent charge density solving an inverse integral problem given by the perturbation theory. We discuss the exact and apparent charge densities derived from the energy maps. We find that for magnetic fields corresponding to the integer fillings of the lowest Landau level, when the electron system exhibits a liquid-like reaction to the potential of the probe, the confined charge density can be quite accurately mapped by the CBM. For fractional fillings of the lowest Landau level the probe induces nucleation of single-electron islands which in circular quantum dots evade imaging by CBM. We demonstrate that mapping the molecular charge densities is possible for confinement potentials of lower symmetry that is consistent with the geometry of the single-electron islands distribution.

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

2015-01-01

14

A general scheme is established within the effective-mass approximation to calculate systematically the excitonic energy spectra in a semiconductor quantum dot including the dielectric confinement effect. This effect is found to appear most pronounced in the quantum-dot structure in comparison with the quantum-well and quantum-wire structures. A formula of the lowest exciton energy in the strong confinement regime is derived

T. Takagahara

1993-01-01

15

XANES: observation of quantum confinement in the conduction band of colloidal PbS quantum dots

NASA Astrophysics Data System (ADS)

The presented investigations aimed at development of inexpensive method for synthesized materials suitable for utilization of solar energy. This important issue was addressed by focusing, mainly, on electronic local structure studies with supporting x-ray diffraction (XRD) and transmission electron microscopy (TEM) analysis of colloidal galena nano-particles (NPs) and quantum dots (QDs) synthesized using wet chemistry under microwave irradiation. Performed x-ray absorption near edge structure (XANES) analysis revealed an evidence of quantum confinement for the sample with QDs, where the bottom of the conduction band was shifted to higher energy. The QDs were found to be passivated with oxides at the surface. Existence of sulfate/sulfite and thiosulfate species in pure PbS and QDs, respectively, was identified.

Demchenko, I. N.; Chernyshova, M.; He, X.; Minikayev, R.; Syryanyy, Y.; Derkachova, A.; Derkachov, G.; Stolte, W. C.; Piskorska-Hommel, E.; Reszka, A.; Liang, H.

2013-04-01

16

Temperature-Dependent Electron Transport in Quantum Dot Photovoltaics

Covalency in semiconductor quantum dots. Chemical SocietySemiconductor Properties Before examining quantum confinement effects in quantum dots,gap of the quantum dots. Besides semiconductor-semiconductor

Padilla, Derek

2013-01-01

17

Strong exciton confinement in site-controlled GaN quantum dots embedded in nanowires

NASA Astrophysics Data System (ADS)

The optical properties of site-controlled single GaN quantum dots (QDs) embedded in nanowires are presented. These properties are largely determined by a strong carrier confinement due to the small size of the dots (0.5 ˜ 1 nm in height). The QDs emit in the deep-ultraviolet region of the electromagnetic spectrum and exhibit the biexciton binding energy of 52 meV, which is the largest value ever reported in III-V semiconductor QDs. Furthermore, the dots luminesce at room-temperature and have short exciton decay times of ˜300 ps due to an increased oscillator strength. These findings indicate that these site-controlled QDs are promising for various applications in quantum information devices.

Choi, Kihyun; Kako, Satoshi; Holmes, Mark J.; Arita, Munetaka; Arakawa, Yasuhiko

2013-10-01

18

Effect of confinement potential geometry on entanglement in quantum dot-based nanostructures

We calculate the spatial entanglement between two electrons trapped in a nanostructure for a broad class of confinement potentials, including single and double quantum dots, and core-shell quantum dot structures. By using a parametrized confinement potential, we are able to switch from one structure to the others with continuity and to analyze how the entanglement is influenced by the changes in the confinement geometry. We calculate the many-body wave function by `exact' diagonalization of the time independent Schr\\"odinger equation. We discuss the relationship between the entanglement and specific cuts of the wave function, and show that the wave function at a single highly symmetric point could be a good indicator for the entanglement content of the system. We analyze the counterintuitive relationship between spatial entanglement and Coulomb interaction, which connects maxima (minima) of the first to minima (maxima) of the latter. We introduce a potential quantum phase transition which relates quantum states characterized by different spatial topology. Finally we show that by varying shape, range and strength of the confinement potential, it is possible to induce strong and rapid variations of the entanglement between the two electrons. This property may be used to tailor nanostructures according to the level of entanglement required by a specific application.

S. Abdullah; J. P. Coe; I. D'Amico

2009-08-19

19

We used Raman scattering for study the phonon modes of self-organized Ge/Si quantum dots, grown by a molecular-beam epitaxy method. It is revealed, that Ge-Ge and Si-Ge vibrational modes considerably intensify at excitation of exciton between the {Lambda}3 valence and {Lanbda}1 conduction bands (transitions E1 and E1+{Delta}1), that allows to observe Raman scattering spectrum from extremely small volumes of Ge, even from one layer of quantum dots with the layer thickness of ~ 10 A. It is shown that Si diffuses into the Ge quantum dots from the Si spacer layers forming Ge_xSi_{1-x} solid solution, and Si concentration was estimated. It is revealed, that the frequency of Ge-Ge mode decreases in 10 1/cm at decreasing of the Ge layer thickness from 10 up to 6 A as a result of phonon size confinement effect.

Kucherenko, I V; Melnik, N N; Arapkina, L V; Chapnin, V A; Chizh, K V; Yuryev, V A

2011-01-01

20

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

21

Dynamics of a Mn spin coupled to a single hole confined in a quantum dot

NASA Astrophysics Data System (ADS)

Using the emission of the positively charged exciton as a probe, we analyze the dynamics of the optical pumping and the dynamics of the relaxation of a Mn spin exchange coupled with a confined hole spin in a II-VI semiconductor quantum dot. The hole-Mn spin can be efficiently initialized in a few tens of ns under optical injection of spin-polarized carriers. We show that this optical pumping process and its dynamics are controlled by electron-Mn flip-flops within the positively charged exciton-Mn complex. The pumping mechanism and its magnetic field dependence are theoretically described by a model including the dynamics of the electron-Mn complex in the excited state and the dynamics of the hole-Mn complex in the ground state of the positively charged quantum dot. We measure at zero magnetic field a spin-relaxation time of the hole-Mn spin in the ?s range or shorter. This hole-Mn spin relaxation is induced by the presence of valence-band mixing in self-assembled quantum dots.

Varghese, B.; Boukari, H.; Besombes, L.

2014-09-01

22

Spin blockade and exchange in Coulomb-confined silicon double quantum dots

NASA Astrophysics Data System (ADS)

Electron spins confined to phosphorus donors in silicon are promising candidates as qubits because of their long coherence times, exceeding seconds in isotopically purified bulk silicon. With the recent demonstrations of initialization, readout and coherent manipulation of individual donor electron spins, the next challenge towards the realization of a Si:P donor-based quantum computer is the demonstration of exchange coupling in two tunnel-coupled phosphorus donors. Spin-to-charge conversion via Pauli spin blockade, an essential ingredient for reading out individual spin states, is challenging in donor-based systems due to the inherently large donor charging energies (~45 meV), requiring large electric fields (>1 MV m-1) to transfer both electron spins onto the same donor. Here, in a carefully characterized double donor-dot device, we directly observe spin blockade of the first few electrons and measure the effective exchange interaction between electron spins in coupled Coulomb-confined systems.

Weber, Bent; Tan, Y. H. Matthias; Mahapatra, Suddhasatta; Watson, Thomas F.; Ryu, Hoon; Rahman, Rajib; Hollenberg, Lloyd C. L.; Klimeck, Gerhard; Simmons, Michelle Y.

2014-06-01

23

Spin blockade and exchange in Coulomb-confined silicon double quantum dots.

Electron spins confined to phosphorus donors in silicon are promising candidates as qubits because of their long coherence times, exceeding seconds in isotopically purified bulk silicon. With the recent demonstrations of initialization, readout and coherent manipulation of individual donor electron spins, the next challenge towards the realization of a Si:P donor-based quantum computer is the demonstration of exchange coupling in two tunnel-coupled phosphorus donors. Spin-to-charge conversion via Pauli spin blockade, an essential ingredient for reading out individual spin states, is challenging in donor-based systems due to the inherently large donor charging energies (?45 meV), requiring large electric fields (>1 MV m(-1)) to transfer both electron spins onto the same donor. Here, in a carefully characterized double donor-dot device, we directly observe spin blockade of the first few electrons and measure the effective exchange interaction between electron spins in coupled Coulomb-confined systems. PMID:24727686

Weber, Bent; Tan, Y H Matthias; Mahapatra, Suddhasatta; Watson, Thomas F; Ryu, Hoon; Rahman, Rajib; Hollenberg, Lloyd C L; Klimeck, Gerhard; Simmons, Michelle Y

2014-06-01

24

NASA Astrophysics Data System (ADS)

Monolayer transition metal dichalcogenides (TMDs) offer new opportunities for realizing quantum dots (QDs) in the ultimate two-dimensional (2D) limit. Given the rich control possibilities of electron valley pseudospin discovered in the monolayers, this quantum degree of freedom can be a promising carrier of information for potential quantum spintronics exploiting single electrons in TMD QDs. An outstanding issue is to identify the degree of valley hybridization, due to the QD confinement, which may significantly change the valley physics in QDs from its form in the 2D bulk. Here we perform a systematic study of the intervalley coupling by QD confinement potentials on extended TMD monolayers. We find that the intervalley coupling in such geometry is generically weak due to the vanishing amplitude of the electron wavefunction at the QD boundary, and hence valley hybridization will be well quenched by the much stronger spin–valley coupling in monolayer TMDs and the QDs can well inherit the valley physics of the 2D bulk. We also discover sensitive dependence of intervalley coupling strength on the central position and the lateral length scales of the confinement potentials, which may possibly allow tuning of intervalley coupling by external controls.

Liu, Gui-Bin; Pang, Hongliang; Yao, Yugui; Yao, Wang

2014-10-01

25

Effect of phonon confinement on one- and two-polar optical phonon capture processes in quantum dots

NASA Astrophysics Data System (ADS)

The electron capture in spherical quantum dot-quantum well structure is studied theoretically. The capture rate in one- and two-polar-optical-phonon-mediated capture processes has been studied by taking into account the phonon confinement. We have derived the explicit analytic expressions for carrier capture rates which can be conveniently applied to practical calculations for the spherical quantum dot systems. The numerical results of the capture rate as function of dot radius, lattice temperature and electron density in GaAs/AlAs QD systems are obtained and discussed. The dependence of the carrier capture rate for a fixed dot radius shows a maximum as a function of carrier density. It is shown that the capture rate of an electron from the barrier region to the quantum dot ground-state, via emission of one and two phonons, exhibits the existence of the bands of dot radii where capture is energetically allowed. We found that the height of the capture rate peaks obtained for one-phonon assisted processes is lowered as QD radius decreases when the phonon confinement is taken into account. The capture rates due to emission single and two optical phonon modes are compared. Carrier capture is shown to proceed with rates as high as 1010 s-1 at temperature T > 100 K. A short capture time is also achieved for a low carrier density.

Vardanyan, K. A.; Vartanian, A. L.; Mughnetsyan, V. N.; Dvurechenskii, A. V.; Kirakosyan, A. A.

2015-02-01

26

Quantum Confinement in Amorphous Silicon Quantum Dots Embedded in Silicon Nitride

Amorphous silicon quantum dots ( a-Si QDs) were grown in a silicon nitride film by plasma enhanced chemical vapor deposition. Transmission electron micrographs clearly demonstrated that a-Si QDs were formed in the silicon nitride. Photoluminescence and optical absorption energy measurement of a-Si QDs with various sizes revealed that tuning of the photoluminescence emission from 2.0 to 2.76 eV is possible

Nae-Man Park; Chel-Jong Choi; Tae-Yeon Seong; Seong-Ju Park

2001-01-01

27

NASA Astrophysics Data System (ADS)

Density functional theory (DFT), as a first-principle approach has successfully been implemented to study nanoscale material. Here, DFT by numerical basis-set was used to study the quantum confinement effect as well as electronic properties of silicon quantum dots (Si-QDs) in ground state condition. Selection of quantum dot models were studied intensively before choosing the right structure for simulation. Next, the computational result were used to examine and deduce the electronic properties and its density of state (DOS) for 14 spherical Si-QDs ranging in size up to ˜ 2 nm in diameter. The energy gap was also deduced from the HOMO-LUMO results. The atomistic model of each silicon QDs was constructed by repeating its crystal unit cell of face-centered cubic (FCC) structure, and reconstructed until the spherical shape obtained. The core structure shows tetrahedral (Td) symmetry structure. It was found that the model need to be passivated, and hence it was noticed that the confinement effect was more pronounced. The model was optimized using Quasi-Newton method for each size of Si-QDs to get relaxed structure before it was simulated. In this model the exchange-correlation potential (Vxc) of the electrons was treated by Local Density Approximation (LDA) functional and Perdew-Zunger (PZ) functional.

Anas, M. M.; Othman, A. P.; Gopir, G.

2014-09-01

28

NASA Astrophysics Data System (ADS)

An exactly solvable problem of impurity states is considered in core/shell/shell spherical quantum dot. Kratzer molecular potential is taken for confinement potential. The analytical expressions are obtained for the energy spectrum and wave functions of the impurity electron. The dependencies of the total energy and the binding energy of the impurity on the parameters of the confining potential are investigated. The possibility of the impurity electron leakage is shown in the external environment, due to the specific form of the Kratzer potential. The character of the electrostatic field created by the impurity and the electron is observed on the basis of obtained results. The multipole corrections caused by the dipole and quadrupole moments of the electron are calculated. It is shown that the dipole moment is absent, and the problem reduces to the calculation of only z component for the average values of the diagonal elements of the quadrupole moment tensor. The dependencies of the average values of the quadrupole moment on the Kratzer potential parameters are studied.

Hayrapetyan, D. B.; Kazaryan, E. M.; Petrosyan, L. S.; Sarkisyan, H. A.

2015-02-01

29

Spin-orbit splitting in semiconductor quantum dots with a parabolic confinement potential

We present a theoretical study of the effect of spin-orbit interaction on the electron energy spectrum of cylindrical semiconductor quantum dots. The study is based on a simple effective one-band approximation. The dependence of energy levels on parameters of the dots and the applied external magnetic field is studied. Contributions of the bulk inversion asymmetry (the Dresselhaus term) and the

O. Voskoboynikov; C. P. Lee; O. Tretyak

2001-01-01

30

Exciton states and interband absorption of cylindrical quantum dot with Morse confining potential

NASA Astrophysics Data System (ADS)

In this paper the exciton and electron sates in cylindrical quantum dot with Morse potential made of GaAs are studied. For the regime of strong size quantization, energy spectrum with the parabolic approximation case are compared. For strong and weak size quantization regimes analytic expressions for the particle energy spectrum, absorption coefficient and dependencies of effective threshold frequencies of absorption on the geometrical parameters quantum dot are obtained. For the intermediate size quantization regime the problem solved in the framework of variation method. The selection rules corresponding to different transitions between quantum levels are found. The size dispersion distribution of growing quantum dots by the radius and height by two experimentally realizing distribution functions have been taken into account. Distribution functions of Gauss, Lifshits-Slezov have been considered.

Hayrapetyan, D. B.; Kazaryan, E. M.; Kotanjyan, T. V.; Tevosyan, H. Kh.

2015-02-01

31

Photoinduced band filling in strongly confined colloidal PbS quantum dots

Increase in continuous wave laser excitation (6?W/cm{sup 2} to 120?W/cm{sup 2}) of colloidal PbS quantum dots in the strongly quantized regime (diameters 2.0?nm and 4.7?nm) deposited on semi-insulating GaAs and glass causes a clear blue shift (0.019?eV and 0.080?eV) of the emission spectra. Proof of the applicability of a dynamic three-dimensional band filling model is the significance of the presented results and demonstrates the effective electronic coupling in quantum dot arrays similar to superlattices. The work also reveals the influence of quantum dot sizes on photo-doping effects.

Ullrich, B., E-mail: bruno@fis.unam.mx [Instituto de Ciencias Físicas, Universidad Nacional Autónoma de México, Cuernavaca, Morelos 62210 (Mexico); Ullrich Photonics LLC, Wayne, Ohio 43466 (United States); Xi, H. [Department of Physics and Astronomy, Bowling Green State University, Bowling Green, Ohio 43403-0209 (United States); Wang, J. S. [Air Force Research Laboratory, Materials and Manufacturing Directorate, Wright Patterson AFB, Ohio 45433-7707 (United States)

2014-06-21

32

NASA Astrophysics Data System (ADS)

Optically tunable mesoscale structures offer unparalleled potential for photonic device applications. Here, we report the creation of composite photonic structures consisting of CdSxSe1-x quantum dots (QDs) customized within lines, first written in a glass by femtosecond laser pulses. CdSxSe1-x-doped borosilicate glasses were pulsed with a fs-laser using a 473 kHz repetition rate to create chemically distinct microscopic regions. Upon further heat treatment, these regions served as "micro-crucibles" within which quantum dots were precipitated exclusively. These results open prospects of developing other semiconductor doped glasses for versatile photonic structures useful over broader optical wavelengths.

Mardilovich, Pavel; Yang, Lihmei; Huang, Huan; Krol, Denise M.; Risbud, Subhash H.

2013-04-01

33

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

2013-11-20

34

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

35

Q-size metal or semiconductor nanoclusters, more commonly known as quantum-confined nanoclusters or quantum dots, are currently the focus of much intensive investigation as these interesting new materials hold the potential for revolutionary impact on the electronics industry. The photoinduced decomposition of dimethylformamide suspensions of 10 different surface-substituted thiolate-capped 30 {angstrom} cadmium sulfide quantum-confined nanoclusters was studied. HPLC analysis demonstrates that decomposition proceeds at a constant rate and produces only precipitated cluster aggregates and the symmetric disulfide photogeneration kinetics on the Hammett {sigma}{sub p} parameter for the remote substituent indicates that both electron donors and acceptors enhance the rate and is mimicked by nanocluster fluorescence quenching efficiencies of the substituents, suggesting a photodecomposition mechanism involving facile electronic communication between the quantum dot core and the remote substituent. Further evidence for the mechanism was obtained from the product distribution ratios of the photodecomposition of a 4-nitrothiolate/4-methylthiolate-mixed surface nanocluster.

Veinot, J.G.C.; Galloro, J.; Pugliese, L.; Pestrin, R.; Pietro, W.J. [York Univ., Toronto, Ontario (Canada). Dept. of Chemistry] [York Univ., Toronto, Ontario (Canada). Dept. of Chemistry

1999-03-01

36

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. PMID:23826867

2013-01-01

37

Quantum confinement effect in Bi anti-dot thin films with tailored pore wall widths and thicknesses

We investigated quantum confinement effects in Bi anti-dot thin films grown on anodized aluminium oxide templates. The pore wall widths (w{sub Bi}) and thickness (t) of the films were tailored to have values longer or shorter than Fermi wavelength of Bi (?{sub F}?=??40?nm). Magnetoresistance measurements revealed a well-defined weak antilocalization effect below 10?K. Coherence lengths (L{sub ?}) as functions of temperature were derived from the magnetoresistance vs field curves by assuming the Hikami-Larkin-Nagaoka model. The anti-dot thin film with w{sub Bi} and t smaller than ?{sub F} showed low dimensional electronic behavior at low temperatures where L{sub ?}(T) exceed w{sub Bi} or t.

Park, Y., E-mail: youngok@chem.s.u-tokyo.ac.jp [Department of Chemistry, The University of Tokyo, Bunkyo, Tokyo 113-0033 (Japan); Hirose, Y.; Fukumura, T.; Hasegawa, T. [Department of Chemistry, The University of Tokyo, Bunkyo, Tokyo 113-0033 (Japan); Kanagawa Academy of Science and Technology (KAST), Kawasaki 213-0012 (Japan); CREST, JST, Bunkyo, Tokyo 113-0033 (Japan); Nakao, S. [Kanagawa Academy of Science and Technology (KAST), Kawasaki 213-0012 (Japan); CREST, JST, Bunkyo, Tokyo 113-0033 (Japan); Xu, J. [School of Engineering, Brown University, Providence, Rhode Island 02912 (United States)

2014-01-13

38

NASA Astrophysics Data System (ADS)

Semiconductor quantum dots (QDs) have potential applications in quantum information processing due to the fact that they are potential on-demand sources of single and entangled photons. Generation of polarization-entangled photon pairs was demonstrated using the biexciton-exciton radiative cascade. One obvious way to increase the number of quantum correlated photons that the QDs emit is to use higher-order multiexcitons, in particular, the triexciton. Towards achieving this goal, we first demonstrate deterministic generation of the QD-confined triexciton in a well-defined coherent state and then spectrally identify and directly measure a three-photon radiative cascade resulting from the sequential triexciton-biexciton-exciton radiative recombination.

Schmidgall, E. R.; Schwartz, I.; Gantz, L.; Cogan, D.; Raindel, S.; Gershoni, D.

2014-12-01

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. PMID:16740665

Yannouleas, Constantine; Landman, Uzi

2006-01-01

40

NASA Astrophysics Data System (ADS)

Quantum dots (QDs) with the highest possible photoluminescence quantum yields are necessary for modern nanotechnology applications to biosensing and optoelectronics. To date, core-shell QDs are the best. We suggest and demonstrate a novel approach to enhancement of charge-carrier confinement in the core of CdSe QDs by creating a ZnS/CdS/ZnS shell with staggered potential barrier. The CdS interlayer breaks the ZnS-shell structure continuity, which allows combining the benefits of a single ZnS-monolayer inner shell, creating the highest possible confinement potential, with a sufficient overall shell thickness and suitability for common surface modification techniques. This approach allows the preparation of CdSe-ZnS/CdS/ZnS QDs with photoluminescence quantum yields approaching 100% and small photoluminescence peak width.

Samokhvalov, Pavel; Linkov, Pavel; Michel, Jean; Molinari, Michael; Nabiev, Igor

2014-03-01

41

Electron Transport in Side Coupled Quantum Dots Fano Fano

the detection of the spin polarization in a semiconductor using the side coupled quantum dot as a spin probeElectron Transport in Side Coupled Quantum Dots 21 12 #12;#12;Fano Fano Fermi Fermi Fermi Fermi dots. A quantum dot is an artificial quantum system to confine electrons in a small region. Different

Katsumoto, Shingo

42

Transport through graphene quantum dots.

We review transport experiments on graphene quantum dots and narrow graphene constrictions. In a quantum dot, electrons are confined in all lateral dimensions, offering the possibility for detailed investigation and controlled manipulation of individual quantum systems. The recently isolated two-dimensional carbon allotrope graphene is an interesting host to study quantum phenomena, due to its novel electronic properties and the expected weak interaction of the electron spin with the material. Graphene quantum dots are fabricated by etching mono-layer flakes into small islands (diameter 60-350 nm) with narrow connections to contacts (width 20-75 nm), serving as tunneling barriers for transport spectroscopy. Electron confinement in graphene quantum dots is observed by measuring Coulomb blockade and transport through excited states, a manifestation of quantum confinement. Measurements in a magnetic field perpendicular to the sample plane allowed to identify the regime with only a few charge carriers in the dot (electron-hole transition), and the crossover to the formation of the graphene specific zero-energy Landau level at high fields. After rotation of the sample into parallel magnetic field orientation, Zeeman spin splitting with a g-factor of g ? 2 is measured. The filling sequence of subsequent spin states is similar to what was found in GaAs and related to the non-negligible influence of exchange interactions among the electrons. PMID:23144122

Güttinger, J; Molitor, F; Stampfer, C; Schnez, S; Jacobsen, A; Dröscher, S; Ihn, T; Ensslin, K

2012-12-01

43

The engineering of the three-dimensional (3D) heterostructure potential in GaAs/AlGaAs pyramidal quantum dot-in-dots (DiDs) provides control over the valence band symmetry and hence on the polarization of the emitted photons. We propose a technique for dynamic switching of hole character and photon polarization in DiDs by means of an applied electric field. The structural parameters required for producing this effect are discussed. Asymmetric DiDs are found to be particularly suitable for obtaining switching with fields smaller than 1 kV cm( - 1). The proposed device enables generation of single photons with dynamic control on the photon polarization, with potential applications in quantum information technology. PMID:20562488

Troncale, V; Karlsson, K F; Kapon, E

2010-07-16

44

Magnetooptic properties of semiconductor quantum dots in glass composites

Low dimensional systems of semiconductor quantum dots in glass composites exhibit interesting physical properties arising from spatial confinement effects; an example is the discretization of the energy spectrum. In semiconductor quantum dots, electronic wave functions experience effects of quantum confinement arising from the dot-glass interface acting as an infinite potential barrier, effectively creating an infinite potential well. This leads to

Joseph H. Kratzer; John Schroeder

2004-01-01

45

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

46

Modeling of the quantum dot filling and the dark current of quantum dot infrared photodetectors

NASA Astrophysics Data System (ADS)

A generalized drift-diffusion model for the calculation of both the quantum dot filling profile and the dark current of quantum dot infrared photodetectors is proposed. The confined electrons inside the quantum dots produce a space-charge potential barrier between the two contacts, which controls the quantum dot filling and limits the dark current in the device. The results of the model reasonably agree with a published experimental work. It is found that increasing either the doping level or the temperature results in an exponential increase of the dark current. The quantum dot filling turns out to be nonuniform, with a dot near the contacts containing more electrons than one in the middle of the device where the dot occupation approximately equals the number of doping atoms per dot, which means that quantum dots away from contacts will be nearly unoccupied if the active region is undoped.

Ameen, Tarek A.; El-Batawy, Yasser M.; Abouelsaood, A. A.

2014-02-01

47

Modeling of the quantum dot filling and the dark current of quantum dot infrared photodetectors

A generalized drift-diffusion model for the calculation of both the quantum dot filling profile and the dark current of quantum dot infrared photodetectors is proposed. The confined electrons inside the quantum dots produce a space-charge potential barrier between the two contacts, which controls the quantum dot filling and limits the dark current in the device. The results of the model reasonably agree with a published experimental work. It is found that increasing either the doping level or the temperature results in an exponential increase of the dark current. The quantum dot filling turns out to be nonuniform, with a dot near the contacts containing more electrons than one in the middle of the device where the dot occupation approximately equals the number of doping atoms per dot, which means that quantum dots away from contacts will be nearly unoccupied if the active region is undoped.

Ameen, Tarek A.; El-Batawy, Yasser M.; Abouelsaood, A. A. [Department of Engineering Mathematics and Physics, Faculty of Engineering, Cairo University, Giza (Egypt)

2014-02-14

48

Electronic structure and photon absorption in semiconductor quantum dots

Quantum dot structures have been the subject of intense investigation in recent years with the advances of molecular beam epitaxy growth technology. Quantum dot infrared photodetector (QDIP) which rely on intersubband transitions in quantum wells or quantum dots for infrared detection, as the three-dimensional confinement of electrons leads to a distribution of energy levels which impedes electron-phonon scattering. This suggests

Gregory von Winckel

2006-01-01

49

100-nm-thick nanocrystalline silicon (nano-Si)-dot multi-layers on a Si substrate were fabricated by the sequential repetition of H-plasma surface treatment, chemical vapor deposition, and surface oxidation, for over 120 times. The diameter of the nano-Si dots was 5–6 nm, as confirmed by both the transmission electron microscopy and X-ray diffraction analysis. The annealing process was important to improve the crystallinity of the nano-Si dot. We investigated quantum confinement effects by Raman spectroscopy and photoluminescence (PL) measurements. Based on the experimental results, we simulated the Raman spectrum using a phenomenological model. Consequently, the strain induced in the nano-Si dots was estimated by comparing the experimental and simulated results. Taking the estimated strain value into consideration, the band gap modulation was measured, and the diameter of the nano-Si dots was calculated to be 5.6 nm by using PL. The relaxation of the q ? 0 selection rule model for the nano-Si dots is believed to be important to explain both the phenomena of peak broadening on the low-wavenumber side observed in Raman spectra and the blue shift observed in PL measurements.

Kosemura, Daisuke, E-mail: d-kose@isc.meiji.ac.jp; Mizukami, Yuki; Takei, Munehisa; Numasawa, Yohichiroh; Ogura, Atsushi [School of Science and Technology, Meiji University, Kawasaki 214-8571 (Japan)] [School of Science and Technology, Meiji University, Kawasaki 214-8571 (Japan); Ohshita, Yoshio [Toyota Technological Institute, Nagoya 468-8511 (Japan)] [Toyota Technological Institute, Nagoya 468-8511 (Japan)

2014-01-15

50

Far infrared spectroscopy of quantum-dots and antidot arrays

Advances in submicron technology make it possible to realize man-made low-dimensional electronic systems with quantum confined energy states, i.e., quantum wires, quantum dots and antidots. With typical confinement energies in the meV regime far-infrared spectroscopy gives the most direct access to the quantum confined energy levels in these systems. One is now approaching the limit to prepare quantum dots with

Detlef Heitmann

1995-01-01

51

Multipolar interband absorption in a semiconductor quantum dot.

Multipolar interband absorption in a semiconductor quantum dot. I. Electric quadrupole enhancement a theoretical investigation of a semiconductor quantum dot interacting with a strongly localized optical field rates and selection rules. For a semiconductor quantum dot in the strong confinement limit we calcu

Novotny, Lukas

52

NASA Astrophysics Data System (ADS)

We have been investigating the synthesis of quantum dots of CdSe, CuInS2, and CuInSe2 for use in an intermediate bandgap solar cell. We have prepared a variety of quantum dots using the typical organometallic synthesis routes pioneered by Bawendi, et. al., in the early 1990's. However, unlike previous work in this area we have also utilized single-source precursor molecules in the synthesis process. We will present XRD, TEM, SEM and EDS characterization of our initial attempts at fabricating these quantum dots. Investigation of the size distributions of these nanoparticles via laser light scattering and scanning electron microscopy will be presented. Theoretical estimates on appropriate quantum dot composition, size, and inter-dot spacing along with potential scenarios for solar cell fabrication will be discussed.

Raffaelle, Ryne P.; Castro, Stephanie L.; Hepp, Aloysius; Bailey, Sheila G.

2002-10-01

53

NASA Technical Reports Server (NTRS)

We have been investigating the synthesis of quantum dots of CdSe, CuInS2, and CuInSe2 for use in an intermediate bandgap solar cell. We have prepared a variety of quantum dots using the typical organometallic synthesis routes pioneered by Bawendi, et. al., in the early 1990's. However, unlike previous work in this area we have also utilized single-source precursor molecules in the synthesis process. We will present XRD, TEM, SEM and EDS characterization of our initial attempts at fabricating these quantum dots. Investigation of the size distributions of these nanoparticles via laser light scattering and scanning electron microscopy will be presented. Theoretical estimates on appropriate quantum dot composition, size, and inter-dot spacing along with potential scenarios for solar cell fabrication will be discussed.

Raffaelle, Ryne P.; Castro, Stephanie L.; Hepp, Aloysius; Bailey, Sheila G.

2002-01-01

54

Electronic structure of quantum dots Stephanie M. Reimann

III. Addition Energy Spectra 1293 A. Many-body effects in quantum dots 1294 B. Density with flattened bottom 1298 G. Three-dimensionality of the confinement 1299 H. Triangular quantum dots 1300 I spectroscopy of vertical dots 1313 2. Gated transport spectroscopy in magnetic fields 1314 3. B-N phase diagram

Wu, Zhigang

55

Colloidal quantum dots intraband photodetectors.

Photoconductivity is demonstrated with monodispersed HgSe colloidal quantum dots that are illuminated with radiation resonant with 1S(e)-1P(e) intraband electronic absorption, between 3 and 5 ?m. A doping of two electrons per dot gives the lowest dark current, and a detectivity of 8.5 × 10(8) Jones is obtained at 80 K. Photoluminescence of the intraband transition is also observed. The detector properties are discussed in terms of the measured photoluminescence quantum yield, the electron mobility in the 1P(e) state, and the responsivity. The intraband photoresponse allows to fully harness the quantum confined states in colloidal nanostructures, extending the prior limited use of interband transition. PMID:25343383

Deng, Zhiyou; Jeong, Kwang Seob; Guyot-Sionnest, Philippe

2014-11-25

56

Quantum properties of spherical semiconductor quantum dots

Quantum properties of spherical semiconductor quantum dots B. Billaud and T.-T. Truong Laboratoire of having a QD-LASER emitting in the range of visible light. Keywords spherical Quantum Dot, semiconductor.22.Dj. 1 Introduction Semiconductor quantum dots (QDs), as well as quantum wires or quantum wells, show

Paris-Sud XI, UniversitÃ© de

57

Electron Spin Dynamics in Semiconductor Quantum Dots

An electron spin confined to a semiconductor quantum dot is not subject to the classical spin relaxation mechanisms known for free carriers but it strongly interacts with the nuclear spin system via the hyperfine interaction. We show in time resolved photoluminescence spectroscopy experiments on ensembles of self assembled InAs quantum dots in GaAs that this interaction leads to strong electron spin dephasing.

Marie, X.; Belhadj, T.; Urbaszek, B.; Amand, T. [Universite de Toulouse, LPCNO, INSA-CNRS-UPS, 135 avenue de Rangueil, 31077 Toulouse (France); Krebs, O.; Lemaitre, A.; Voisin, P. [Laboratoire de Photonique et Nanostructures, route de Nozay, 91460 Marcoussis (France)

2011-07-15

58

Far-infrared spectroscopy of quantum dots

Quantum dots on semiconductors are few-electron systems with discrete energy spectra. They are fabricated from semiconductor structures with two-dimensional electron gases like metal-oxide-semiconductor (MOS) capacitors or GaAs\\/GaAlAs heterostructures by laterally confining the electrons. Here some important fabrication schemes employing nanostructure technologies to create isolated quantum dots are outlined and the present status of far-infrared spectroscopy on these atomic-like systems is

Ulrich Merkt

1993-01-01

59

Quantum dots with single-atom precision.

Quantum dots are often called artificial atoms because, like real atoms, they confine electrons to quantized states with discrete energies. However, although real atoms are identical, most quantum dots comprise hundreds or thousands of atoms, with inevitable variations in size and shape and, consequently, unavoidable variability in their wavefunctions and energies. Electrostatic gates can be used to mitigate these variations by adjusting the electron energy levels, but the more ambitious goal of creating quantum dots with intrinsically digital fidelity by eliminating statistical variations in their size, shape and arrangement remains elusive. We used a scanning tunnelling microscope to create quantum dots with identical, deterministic sizes. By using the lattice of a reconstructed semiconductor surface to fix the position of each atom, we controlled the shape and location of the dots with effectively zero error. This allowed us to construct quantum dot molecules whose coupling has no intrinsic variation but could nonetheless be tuned with arbitrary precision over a wide range. Digital fidelity opens the door to quantum dot architectures free of intrinsic broadening-an important goal for technologies from nanophotonics to quantum information processing as well as for fundamental studies of confined electrons. PMID:24974937

Fölsch, Stefan; Martínez-Blanco, Jesús; Yang, Jianshu; Kanisawa, Kiyoshi; Erwin, Steven C

2014-07-01

60

Quantum dots with single-atom precision

NASA Astrophysics Data System (ADS)

Quantum dots are often called artificial atoms because, like real atoms, they confine electrons to quantized states with discrete energies. However, although real atoms are identical, most quantum dots comprise hundreds or thousands of atoms, with inevitable variations in size and shape and, consequently, unavoidable variability in their wavefunctions and energies. Electrostatic gates can be used to mitigate these variations by adjusting the electron energy levels, but the more ambitious goal of creating quantum dots with intrinsically digital fidelity by eliminating statistical variations in their size, shape and arrangement remains elusive. We used a scanning tunnelling microscope to create quantum dots with identical, deterministic sizes. By using the lattice of a reconstructed semiconductor surface to fix the position of each atom, we controlled the shape and location of the dots with effectively zero error. This allowed us to construct quantum dot molecules whose coupling has no intrinsic variation but could nonetheless be tuned with arbitrary precision over a wide range. Digital fidelity opens the door to quantum dot architectures free of intrinsic broadening--an important goal for technologies from nanophotonics to quantum information processing as well as for fundamental studies of confined electrons.

Fölsch, Stefan; Martínez-Blanco, Jesús; Yang, Jianshu; Kanisawa, Kiyoshi; Erwin, Steven C.

2014-07-01

61

Carbon Nanotube Quantum Dot with Superconducting Electrodes Bachelor of Science Thesis Faculty of march 2010. The study of the electrical properties of carbon nanotubes falls under meso- scopic physics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 3 Theory 8 3.1 Carbon Nanotubes . . . . . . . . . . . . . . . . . . . . . . . . 8 3

NygÃ¥rd, Jesper

62

Designing quantum dots for solotronics

NASA Astrophysics Data System (ADS)

Solotronics, optoelectronics based on solitary dopants, is an emerging field of research and technology reaching the ultimate limit of miniaturization. It aims at exploiting quantum properties of individual ions or defects embedded in a semiconductor matrix. It has already been shown that optical control of a magnetic ion spin is feasible using the carriers confined in a quantum dot. However, a serious obstacle was the quenching of the exciton luminescence by magnetic impurities. Here we show, by photoluminescence studies on thus-far-unexplored individual CdTe dots with a single cobalt ion and CdSe dots with a single manganese ion, that even if energetically allowed, nonradiative exciton recombination through single-magnetic-ion intra-ionic transitions is negligible in such zero-dimensional structures. This opens solotronics for a wide range of as yet unconsidered systems. On the basis of results of our single-spin relaxation experiments and on the material trends, we identify optimal magnetic-ion quantum dot systems for implementation of a single-ion-based spin memory.

Kobak, J.; Smole?ski, T.; Goryca, M.; Papaj, M.; Gietka, K.; Bogucki, A.; Koperski, M.; Rousset, J.-G.; Suffczy?ski, J.; Janik, E.; Nawrocki, M.; Golnik, A.; Kossacki, P.; Pacuski, W.

2014-01-01

63

Designing quantum dots for solotronics

Solotronics, optoelectronics based on solitary dopants, is an emerging field of research and technology reaching the ultimate limit of miniaturization. It aims at exploiting quantum properties of individual ions or defects embedded in a semiconductor matrix. It has already been shown that optical control of a magnetic ion spin is feasible using the carriers confined in a quantum dot. However, a serious obstacle was the quenching of the exciton luminescence by magnetic impurities. Here we show, by photoluminescence studies on thus-far-unexplored individual CdTe dots with a single cobalt ion and CdSe dots with a single manganese ion, that even if energetically allowed, nonradiative exciton recombination through single-magnetic-ion intra-ionic transitions is negligible in such zero-dimensional structures. This opens solotronics for a wide range of as yet unconsidered systems. On the basis of results of our single-spin relaxation experiments and on the material trends, we identify optimal magnetic-ion quantum dot systems for implementation of a single-ion-based spin memory. PMID:24463946

Kobak, J.; Smole?ski, T.; Goryca, M.; Papaj, M.; Gietka, K.; Bogucki, A.; Koperski, M.; Rousset, J.-G.; Suffczy?ski, J.; Janik, E.; Nawrocki, M.; Golnik, A.; Kossacki, P.; Pacuski, W.

2014-01-01

64

NSDL National Science Digital Library

This topic-in-depth addresses the characteristics and numerous applications of the semiconductor nanocrystals, quantum dots. Find out how quantum dot are being used in computer science, chemistry, and medicine.

2010-09-20

65

Coherent control and decoherence of single semiconductor quantum dots in a microcavity

Semiconductor quantum dots tightly confine excited electron-hole pairs, called excitons, resulting in discrete energy levels similar to those of single atoms. Transition energies in the visible or near-infrared make quantum dots suitable for many applications in quantum optics and quantum information science, but to take advantage of all the properties of quantum dot emission, it is necessary to excite them

Edward B. Flagg II

2008-01-01

66

Electron cotunneling in a semiconductor quantum dot

We report transport measurements on a semiconductor quantum dot with a small\\u000anumber of confined electrons. In the Coulomb blockade regime, conduction is\\u000adominated by cotunneling processes. These can be either elastic or inelastic,\\u000adepending on whether they leave the dot in its ground state or drive it into an\\u000aexcited state, respectively. We are able to discriminate between these

S. De Franceschi; S. Sasaki; J. M. Elzerman; W. G. van der Wiel; S. Tarucha; L. P. Kouwenhoven

2001-01-01

67

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

68

Optical study of Ge quantum dots and infrared photodetectors

Stacked Ge quantum dots were grown on Si(100) by ultra-high vacuum chemical vapor deposition (UHV\\/CVD). Obvious blueshift (87 meV) observed from PL spectrum under 10 K demonstrates strong quantum confinement in Ge dots. Based on the material, PIiN structure quantum dot infrared photodetectors (QDIPs) were fabricated. At room temperature, I-V measurement showed a low dark current density of 1.7 ×

Rongshan Wei; Ning Deng; Minsheng Wang; Shuang Zhang; Peiyi Chen

2005-01-01

69

Biexciton and Triexciton States in Quantum Dots

NASA Astrophysics Data System (ADS)

The formation, the binding energy and the structure of biexcitons and triexcitons in a strong magnetic field are studied within the method of hyperspherical harmonics. The magnetic field confines the excitons in quantum dots and results in formation of Wigner-crystal-like states of 4- and 6-particles.

Kezerashvili, R. Ya.; Tsiklauri, Sh. M.

2013-08-01

70

From a recent study of the growth and optical properties of quantum dots (QD's), we demonstrated that artificial atoms with sharp electronic shells can be fabricated with good control, using self-assembled QD's grown by molecular beam epitaxy. Size and shape engineering of the QD's during growth permits the tailoring of their intersublevel energy spacings. We demonstrate a much improved uniformity

Simon Fafard; Hui C. Liu; Z. R. Wasilewski; John P. McCaffrey; M. Spanner; Sylvain Raymond; C. N. Allen; K. Hinzer; Jean M. Lapointe; C. Struby; M. Gao; Pawel Hawrylak; C. Gould; A. Sachrajda; P. Zawadzki

2000-01-01

71

Biexcitons in semiconductor quantum dots

Theoretical and experimental results are reported which provide the first evidence for biexciton states in semiconductor quantum dots. The theory predicts an increasing biexciton binding energy with decreasing dot size. Unlike bulk semiconductors, quantum dots have excited biexciton states which are stable. These biexciton states are observed as pronounced induced absorption features on the high-energy side of the bleached exciton

Y. Z. Hu; S. W. Koch; M. Lindberg; N. Peyghambarian; E. L. Pollock; Farid F. Abraham

1990-01-01

72

Stark Effect of Interactive Electron-hole pairs in Spherical Semiconductor Quantum Dots

Stark Effect of Interactive Electron-hole pairs in Spherical Semiconductor Quantum Dots B. Billaud1, on the quantum-confinement Stark effects for spherical semiconducting quantum dots in the regime of strong are found to be in good agreement with experimental data over a significant domain of quantum dot sizes

73

Semicond.Sci.Technol.7 (1992). 812-814. Printed in the UK Semiconductor quantum dot

Semicond.Sci.Technol.7 (1992). 812-814. Printed in the UK Semiconductor quantum dot resonant tunne1. Recently,three-dimensionallylaterally confined semiconductor quantum wells ('quantum dots') have been)-'dependence on the energy separation.The electronic transport through quantum dots is presented and analysed. The spectra

Reed, Mark

74

Design and simulation of an electrically tunable quantum dot cascade laser

Design and simulation of an electrically tunable quantum dot cascade laser Dibyendu Dey, Wei Wu quantum dot with electrical confinement and apply it to a Quantum Cascade Laser structure to realize a Quantum Dot Cascade Laser. A two-dimensional finite element method has been used to numerically simulate

Mohseni, Hooman

75

NASA Astrophysics Data System (ADS)

We demonstrated an unambiguous quantum dot cascade laser based on InGaAs/GaAs/InAs/InAlAs heterostructure by making use of self-assembled quantum dots in the Stranski-Krastanow growth mode and two-step strain compensation active region design. The prototype generates stimulated emission at ? ~ 6.15 ?m and a broad electroluminescence band with full width at half maximum over 3 ?m. The characteristic temperature for the threshold current density within the temperature range of 82 to 162 K is up to 400 K. Moreover, our materials show the strong perpendicular mid-infrared response at about 1,900 cm-1. These results are very promising for extending the present laser concept to terahertz quantum cascade laser, which would lead to room temperature operation.

Zhuo, Ning; Liu, Feng Qi; Zhang, Jin Chuan; Wang, Li Jun; Liu, Jun Qi; Zhai, Shen Qiang; Wang, Zhan Guo

2014-03-01

76

Electronic structure and photon absorption in semiconductor quantum dots

NASA Astrophysics Data System (ADS)

Quantum dot structures have been the subject of intense investigation in recent years with the advances of molecular beam epitaxy growth technology. Quantum dot infrared photodetector (QDIP) which rely on intersubband transitions in quantum wells or quantum dots for infrared detection, as the three-dimensional confinement of electrons leads to a distribution of energy levels which impedes electron-phonon scattering. This suggests that such devices have the potential for higher operating temperatures in comparison with the traditional quantum well devices. However, the complexity of the QDIP geometry prevents closed-form solutions of the corresponding Schrodinger eigenvalue problems. We propose and demonstrate a numerically robust and efficient discretization scheme for one-dimensional and three-dimensional axisymmetric quantum structures using spectral elements. The variations in energy levels of isolated conical and lenticular InAs quantum dots are investigated with respect to changes in the dot dimension, as is the coupling between vertically stacked dots. Given measured distributions of quantum dot sizes, we give an uncertainty analysis for the range of energy levels. Finally we consider quantum dot-in-a-well (DWELL) structures and investigate dependence of eigenvalues and eigenfunctions on well width, dot size, and dot location. The effective quantum dot density-of-states and photon absorption oscillator strength are computed as functions of photon wavelength and polarization for a variety of configurations and applied electric fields.

von Winckel, Gregory

77

NASA Astrophysics Data System (ADS)

These conference proceedings contain the written papers of the contributions presented at Quantum Dot 2010 (QD2010). The conference was held in Nottingham, UK, on 26-30 April 2010. The conference addressed topics in research on: 1. Epitaxial quantum dots (including self-assembled and interface structures, dots defined by electrostatic gates etc): optical properties and electron transport quantum coherence effects spin phenomena optics of dots in cavities interaction with surface plasmons in metal/semiconductor structures opto-electronics applications 2. Novel QD structures: fabrication and physics of graphene dots, dots in nano-wires etc 3. Colloidal quantum dots: growth (shape control and hybrid nanocrystals such as metal/semiconductor, magnetic/semiconductor) assembly and surface functionalisation optical properties and spin dynamics electrical and magnetic properties applications (light emitting devices and solar cells, biological and medical applications, data storage, assemblers) The Editors Acknowledgements Conference Organising Committee: Maurice Skolnick (Chair) Alexander Tartakovskii (Programme Chair) Pavlos Lagoudakis (Programme Chair) Max Migliorato (Conference Secretary) Paola Borri (Publicity) Robert Taylor (Proceedings) Manus Hayne (Treasurer) Ray Murray (Sponsorship) Mohamed Henini (Local Organiser) International Advisory Committee: Yasuhiko Arakawa (Tokyo University, Japan) Manfred Bayer (Dortmund University, Germany) Sergey Gaponenko (Stepanov Institute of Physics, Minsk, Belarus) Pawel Hawrylak (NRC, Ottawa, Canada) Fritz Henneberger (Institute for Physics, Berlin, Germany) Atac Imamoglu (ETH, Zurich, Switzerland) Paul Koenraad (TU Eindhoven, Nethehrlands) Guglielmo Lanzani (Politecnico di Milano, Italy) Jungil Lee (Korea Institute of Science and Technology, Korea) Henri Mariette (CNRS-CEA, Grenoble, France) Lu Jeu Sham (San Diego, USA) Andrew Shields (Toshiba Research Europe, Cambridge, UK) Yoshihisa Yamamoto (Stanford University, USA) Artur Zrenner (Paderborn University, Germany) International Programme Committee: Alexander Eychmüller (TU Dresden, Germany) Jonathan Finley (TU Munich, Germany) Dan Gammon (NRL, Washington, USA) Alexander Govorov (Ohio University, USA) Neil Greenham (Cavendish Laboratory, UK) Vladimir Korenev (Ioffe Institute, Russia) Leo Kouwenhoven (TU Delft, Netherlands) Wolfgang Langbein (Cardiff University, UK) Xavier Marie (CNRS Toulouse, France) David Ritchie (Cambridge, UK) Andrew Sachrajda (IMS, Ottawa, Canada) Katerina Soulantica (University of Toulouse, France) Seigo Tarucha (University of Tokyo, Japan) Carlos Tejedor (UAM, Madrid, Spain) Euijoon Yoon (Seoul National University, Korea) Ulrike Woggon (Tu Berlin, Germany) Proceedings edited and compiled by Profesor Robert A Taylor, University of Oxford

Taylor, Robert A.

2010-09-01

78

Confined-but-connected quantum solids via controlled ligand displacement.

Confined-but-connected quantum dot solids (QDS) combine the advantages of tunable, quantum-confined energy levels with efficient charge transport through enhanced electronic interdot coupling. We report the fabrication of QDS by treating self-assembled films of colloidal PbSe quantum dots with polar nonsolvents. Treatment with dimethylformamide balances the rates of self-assembly and ligand displacement to yield confined-but-connected QDS structures with cubic ordering and quasi-epitaxial interdot connections through facets of neighboring dots. The QDS structure was analyzed by a combination of transmission electron microscopy and wide-angle and small-angle X-ray scattering. Excitonic absorption signatures in optical spectroscopy confirm that quantum confinement is preserved. Transport measurements show significantly enhanced conductivity in treated films. PMID:23777454

Baumgardner, William J; Whitham, Kevin; Hanrath, Tobias

2013-07-10

79

We study transfer of a single-electron through a quantum ring capacitively coupled to the charged quantum dot placed in its center. For this purpose we solve the time-dependent Schrödinger equation for the pair of particles: the electron traveling through the ring and the other carrier confined within the quantum dot. The correlation effects due to the interaction between the charge

T. Chwiej; K. Kutorasinski

2010-01-01

80

NSDL National Science Digital Library

Students are introduced to the physical concept of the colors of rainbows as light energy in the form of waves with distinct wavelengths, but in a different manner than traditional kaleidoscopes. Looking at different quantum dot solutions, they make observations and measurements, and graph their data. They come to understand how nanoparticles interact with absorbing photons to produce colors. They learn the dependence of particle size and color wavelength and learn about real-world applications for using these colorful liquids.

2014-09-18

81

Quantum dot solar concentrators

The luminescent properties of core-shell quantum dots are being exploited in an unconventional solar concentrator, which promises\\u000a to reduce the cost of photovoltaic electricity. Luminescent solar collectors have advantages over geometric concentrators\\u000a in that tracking is unnecessary and both direct and diffuse radiation can be collected. However, development has been limited\\u000a by the performance of luminescent dyes. We present experimental

A. J. Chatten; K. W. J. Barnham; B. F. Buxton; N. J. Ekins-Daukes; M. A. Malik

2004-01-01

82

Mach-zender devices are an ideal modulation source for communication networks at 1.3 mum and 1.55 mum. Superlinear electro-optical effects are a desirable feature in mach-zender modulators since their large second order electro-optical coefficient would give complete signal extinction at a small voltage. Quantum dot devices show promise for such applications in the 1.3 mum band. In this project we performed

Brendan Turner; Manish Mehta; Ramesh Laghumavarapu; Diana Huffaker

2006-01-01

83

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

84

Zeno Quantum Gates in Semiconductor Quantum Dots

We propose a scheme for a two-qubit conditional phase gate by quantum Zeno effect with semiconductor quantum dots. The system consists of two charged dots and one ancillary dot that can perform Rabi oscillations under a resonant laser pulse. The quantum Zeno effect is induced by phonon-assisted exciton relaxation between the ancillary dot and the charged dots, which is equivalent to a continuous measurement. We solve analytically the master equation and simulate the dynamics of the system using a realistic set of parameters. In contrast to standard schemes, larger phonon relaxation rates increase the fidelity of the operations.

K. J. Xu; Y. P. Huang; M. G. Moore; C. Piermarocchi

2008-10-24

85

Increased normal incidence photocurrent in quantum dot infrared photodetectors

NASA Astrophysics Data System (ADS)

We have increased the ratio of s-polarization (normal incidence) to p-polarization photocurrent to 50% in a quantum dot-in-a-well based infrared photodetector form the typical s-p polarization ratio about 20%. This improvement was achieved by engineering the dot geometry and the quantum confinement via post growth capping materials of the Stranski Krastanov growth mode quantum dots (QDs). The TEM images show that the height to base ratio of shape engineered QDs was increased to 8 nm/12 nm from the control sample's ratio 4 nm/17 nm. The dot geometry correlates with the polarized photocurrent measurements of the detector.

Shao, Jiayi; Vandervelde, Thomas E.; Barve, Ajit; Stintz, Andreas; Krishna, Sanjay

2012-12-01

86

Magnetic anisotropies of quantum dots

NASA Astrophysics Data System (ADS)

Magnetic anisotropies in quantum dots (QDs) doped by magnetic ions are discussed in terms of two frameworks: anisotropic g-factors and magnetocrystalline anisotropy energy [1]. Two examples, related to zinc-blende p-doped materials, are given of how these frameworks are utilized: four-level Hamiltonian of a flat QD and a cuboid infinite-well QD containing a single hole. The latter model, despite being an idealization of a real QD, displays a rich phenomenology of anisotropies. We quantify the anisotropy constants for ZnSe and CdTe QDs, confirming that the Ising-like effective Hamiltonians apply to magnetic QDs [2]. Compared to bulk systems, confinement tuning offers a new way to control easy axes in magnetic QDs. [1] K. Vyborny et al., preprint (2011). [2] C. Le Gall et al., Phys. Rev. Lett. 107, 057401 (2011).

Vyborny, Karel; Han, J. E.; Oszwaldowski, Rafal; Zutic, Igor; Petukhov, A. G.

2012-02-01

87

The main theme of this thesis is the hyperfine interaction between the many lattice nuclear spins and electron spins localized in GaAs quantum dots. This interaction is an intrinsic property of the material. Despite the fact that this interaction is rather weak, it can, as shown in this thesis, strongly influence the dynamics of electron spins in quantum dots. In

S. I. Erlingsson

2003-01-01

88

Frequency cavity pulling induced by a single semiconductor quantum dot

We investigate the emission properties of a single semiconductor quantum dot deterministically coupled to a confined optical mode in the weak coupling regime. A strong pulling, broadening and narrowing of the cavity mode emission is evidenced when changing the spectral detuning between the emitter and the cavity. These features are theoretically accounted for by considering the phonon assisted emission of the quantum dot transition. These observations highlight a new situation for cavity quantum electrodynamics involving spectrally broad emitters.

Daniel Valente; Jan Suffczy?ski; Tomasz Jakubczyk; Adrien Dousse; Aristide Lemaître; Isabelle Sagnes; Loïc Lanco; Paul Voisin; Alexia Auffeves; Pascale Senellart

2013-07-23

89

1 Quantum Dot Applications forQuantum Dot Applications for Flash Memory, SemiconductorFlash Memory cell #12;3 5 Quantum dots and highQuantum dots and high--K tunnelingK tunneling oxide reduce size and quantum dots 6 HighHigh--K tunneling layerK tunneling layer Replace current materials with a high

Lightsey, Glenn

90

Resonant tunnelling features in quantum dots.

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. PMID:20571205

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

2010-07-01

91

Optical properties of two-electron quantum dots in low lying para- and ortho-states

NASA Astrophysics Data System (ADS)

Two low lying energy levels of 3D two-electron quantum dot with rigid confinement (the wave functions vanish at the surface of the quantum dot) are obtained by the variational and perturbation methods. There are two kind of quantum dots: para- and ortho-dots with antiparallel and parallel electron spins, respectively. An ensemble of the two-electron quantum dots contains para-dots in the ground state and ortho-dots in the lowest metastable state at low enough temperatures. The optical parameters of GaAs two-electron quantum dot are calculated with the help of obtained energy levels and compared with the optical parameters known for the one electron GaAs quantum dot. The Coulomb interaction between electrons is responsible for the blue shift of maxima of the absorption coefficient and refractive index of two-electron quantum dots.

Mengesha, Menberu; Mal'nev, Vadim

2012-07-01

92

Red-Emitting Semiconductor Quantum Dot Lasers

Visible-stimulated emission in a semiconductor quantum dot (QD) laser structure has been demonstrated. Red-emitting, self-assembled QDs of highly strained InAlAs have been grown by molecular beam epitaxy on a GaAs substrate. Carriers injected electrically from the doped regions of a separate confinement heterostructure thermalized efficiently into the zero-dimensional QD states, and stimulated emission at ~707 nanometers was observed at 77

S. Fafard; K. Hinzer; S. Raymond; M. Dion; J. McCaffrey; Y. Feng; S. Charbonneau

1996-01-01

93

The pinning effect in quantum dots

The pinning effect is studied in a Gaussian quantum dot using the improved Wigner-Brillouin perturbation theory (IWBPT) in the presence of electron-phonon interaction. The electron ground state plus one phonon state is degenerate with the electron in the first excited state. The electron-phonon interaction lifts the degeneracy and the first excited states get pinned to the ground state plus one phonon state as we increase the confinement frequency.

Monisha, P. J., E-mail: pjmonisha@gmail.com [School of Physics, University of Hyderabad, Hyderabad-500046 (India); Mukhopadhyay, Soma [Department of Physics, D V R College of Engineering and Technology, Hyderabad-502285 (India)

2014-04-24

94

Zeeman Effect in Parabolic Quantum Dots

An unprecedentedly well resolved Zeeman effect has been observed when confined carriers moving along a closed mesoscopic path experience an external magnetic field orthogonal to the orbit plane. Large Zeeman splitting of excited higher angular momentum states is observed in the magnetoluminescence spectrum of quantum dots induced by self-organized InP islands on InGaAs\\/GaAs. The measured effect is quantitatively reproduced by

R. Rinaldi; P. V. Giugno; R. Cingolani; H. Lipsanen; M. Sopanen; J. Tulkki; J. Ahopelto

1996-01-01

95

Quantum transport in quantum dot cascade structures

In last years, a significant experimental and theoretical interest has been put into the possibility of development of intraband lasers based on quantum dots (1)-(5). Due to truly discrete electronic spectrum of quantum dots, most of the undesired scattering and relaxation processes are suppressed, and such devices are expected to have two orders of magnitude lower threshold currents than the

N. Vukmirovic; Z. Ikoni ´; D. Indjin; P. Harrison

96

Numerical simulation of optical feedback on a quantum dot lasers

We use multi-population rate equations model to study feedback oscillations in the quantum dot laser. This model takes into account all peculiar characteristics in the quantum dots such as inhomogeneous broadening of the gain spectrum, the presence of the excited states on the quantum dot and the non-confined states due to the presence of wetting layer and the barrier. The contribution of quantum dot groups, which cannot follow by other models, is simulated. The results obtained from this model show the feedback oscillations, the periodic oscillations which evolves to chaos at higher injection current of higher feedback levels. The frequency fluctuation is attributed mainly to wetting layer with a considerable contribution from excited states. The simulation shows that is must be not using simple rate equation models to express quantum dots working at excited state transition.

Al-Khursan, Amin H., E-mail: ameen_2all@yahoo.com [Thi-Qar University, Nassiriya Nanotechnology Research Laboratory (NNRL), Science College (Iraq); Ghalib, Basim Abdullattif [Babylon University, Laser Physics Department, Science College for Women (Iraq); Al-Obaidi, Sabri J. [Al-Mustansiriyah University, Physics Department, Science College (Iraq)

2012-02-15

97

Quantum dot (QD) solar cells have emerged as promising low-cost alternatives to existing photovoltaic technologies. Here, we investigate charge transfer and separation at PbS QDs and phenyl-C61-butyric acid methyl ester (PCBM) interfaces using a combination of femtosecond broadband transient absorption (TA) spectroscopy and steady-state photoluminescence quenching measurements. We analyzed ultrafast electron injection and charge separation at PbS QD/PCBM interfaces for four different QD sizes and as a function of PCBM concentration. The results reveal that the energy band alignment, tuned by the quantum size effect, is the key element for efficient electron injection and charge separation processes. More specifically, the steady-state and time-resolved data demonstrate that only small-sized PbS QDs with a bandgap larger than 1 eV can transfer electrons to PCBM upon light absorption. We show that these trends result from the formation of a type-II interface band alignment, as a consequence of the size distribution of the QDs. Transient absorption data indicate that electron injection from photoexcited PbS QDs to PCBM occurs within our temporal resolution of 120 fs for QDs with bandgaps that achieve type-II alignment, while virtually all signals observed in smaller bandgap QD samples result from large bandgap outliers in the size distribution. Taken together, our results clearly demonstrate that charge transfer rates at QD interfaces can be tuned by several orders of magnitude by engineering the QD size distribution. The work presented here will advance both the design and the understanding of QD interfaces for solar energy conversion. PMID:24521255

El-Ballouli, Ala'a O; Alarousu, Erkki; Bernardi, Marco; Aly, Shawkat M; Lagrow, Alec P; Bakr, Osman M; Mohammed, Omar F

2014-05-14

98

The effect of external electric field on electron-hole (eh) correlation in gallium arsenide quantum dots is investigated. The electron-hole Schrodinger equation in the presence of an external electric field is solved using explicitly correlated full configuration interaction method and accurate exciton binding energy and electron-hole recombination probability are obtained. The effect of the electric field was included in the 1-particle single component basis functions by performing variational polaron transformation. The quality of the wavefunction at small inter-particle distances was improved by using Gaussian-type geminal function that depended explicitly on the electron-hole separation distance. The parameters of the explicitly correlated function were determined variationally at each field strength. The scaling of total exciton energy, exciton binding energy, and electron-hole recombination probability with respect to the strength of the electric field was investigated. It was found that a 500 kV/cm change in field strength reduces the binding energy and recombination probability by a factor of 2.6 and 166, respectively. The results show that the eh-recombination probability is affected much more strongly by the electric field than the exciton binding energy. Analysis using the polaron-transformed basis indicates that the exciton binding should asymptotically vanish in the limit of large field strength. PMID:23406105

Blanton, Christopher J; Brenon, Christopher; Chakraborty, Arindam

2013-02-01

99

Two-photon Absorption Process in Semiconductor Quantum Dots

Two-photon Absorption Process in Semiconductor Quantum Dots J. LÂ´opez Gondar1 , R. Cipolatti1 and G. Introduction As a result of the spatial confinement of charged carriers in small volume, semiconductor quantum very difficult from the experimental point of view. Nevertheless, when dealing with semiconductor QD

Cipolatti, Rolci

100

Quantum optical phenomena in semiconductor quantum dots

Quantum optical phenomena are explored in artificial atoms well known as semiconductor quantum dots, in the presence of excitons and biexcitons. The analytical results are obtained using the conventional time-dependent perturbation technique. Numerical estimations are made for arealistic sample of CdS quantum dots in a high-Q cavity. Quantum optical phenomena such as quantum Rabi oscillations, photon statistics and collapse and revival of population inversion in exciton and biexciton states are observed. In the presence of biexcitons the collapse and revival phenomenon becomes faster due to the strong coupling of biexciton with cavity field.

J. Thomas Andrews

2002-11-22

101

Unity quantum yield of photogenerated charges and band-like transport in quantum-dot solids.

Solid films of colloidal quantum dots show promise in the manufacture of photodetectors and solar cells. These devices require high yields of photogenerated charges and high carrier mobilities, which are difficult to achieve in quantum-dot films owing to a strong electron-hole interaction and quantum confinement. Here, we show that the quantum yield of photogenerated charges in strongly coupled PbSe quantum-dot films is unity over a large temperature range. At high photoexcitation density, a transition takes place from hopping between localized states to band-like transport. These strongly coupled quantum-dot films have electrical properties that approach those of crystalline bulk semiconductors, while retaining the size tunability and cheap processing properties of colloidal quantum dots. PMID:21946709

Talgorn, Elise; Gao, Yunan; Aerts, Michiel; Kunneman, Lucas T; Schins, Juleon M; Savenije, T J; van Huis, Marijn A; van der Zant, Herre S J; Houtepen, Arjan J; Siebbeles, Laurens D A

2011-11-01

102

electrons confined in GaAs quantum dots (QDs) are probed by resonant inelastic light scattering. We in quantum dots 39 L ow-lying collective excitations above highly correlated ground states of few interacting. West #12;Probing collective modes of correlated states of few electrons in quantum dots NEST

Abbondandolo, Alberto

103

Density and temperature dependence of carrier dynamics in self-organized InGaAs quantum dots

We have used two- and three-pulse femtosecond differential transmission spectroscopy to study the dependence of quantum dot carrier dynamics on temperature. At low temperatures and densities, the rates for relaxation between the quantum dot confined states and for capture from the barrier region into the various dot levels could be directly determined. For electron-hole pairs generated directly in the quantum

T. B. Norris; K. Kim; J. Urayama; Z. K. Wu; J. Singh; P. K. Bhattacharya

2005-01-01

104

All inorganic colloidal quantum dot LEDs

This thesis presents the first colloidal quantum dot light emitting devices (QD-LEDs) with metal oxide charge transport layers. Colloidally synthesized quantum dots (QDs) have shown promise as the active material in ...

Wood, Vanessa Claire

2007-01-01

105

Holonomic quantum computation with electron spins in quantum dots

With the help of the spin-orbit interaction, we propose a scheme to perform holonomic single-qubit gates on the electron spin confined to a quantum dot. The manipulation is done in the absence (or presence) of an applied magnetic field. By adiabatic changing the position of the confinement potential, one can rotate the spin state of the electron around the Bloch sphere in semiconductor heterostructures. The dynamics of the system is equivalent to employing an effective non-Abelian gauge potential whose structure depends on the type of the spin-orbit interaction. As an example, we find an analytic expression for the electron spin dynamics when the dot is moved around a circular path (with radius R) on the two dimensional electron gas (2DEG) and show that all single-qubit gates can be realized by tuning the radius and orientation of the circular paths. Moreover, using the Heisenberg exchange interaction, we demonstrate how one can generate two-qubit gates by bringing two quantum dots near each other, yielding a scalable scheme to perform quantum computing on arbitrary N qubits. This proposal shows a way of realizing holonomic quantum computers in solid-state systems.

Golovach, Vitaly N. [Arnold Sommerfeld Center for Theoretical Physics and Center for Nanoscience Department of Physics, Ludwig-Maximilians-Universitaet, Theresienstrasse 37, D-80333 Munich (Germany); Borhani, Massoud [Department of Physics, University of Basel, Klingelbergstrasse 82, 4056 Basel (Switzerland); Department of Physics, University at Buffalo, SUNY, Buffalo, New York 14260-1500 (United States); Loss, Daniel [Department of Physics, University of Basel, Klingelbergstrasse 82, 4056 Basel (Switzerland)

2010-02-15

106

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

107

Gate-defined coupled quantum dots in topological insulators

NASA Astrophysics Data System (ADS)

We consider electrostatically coupled quantum dots in topological insulators, otherwise confined and gapped by a magnetic texture. By numerically solving the (2 + 1) Dirac equation for the wave packet dynamics, we extract the energy spectrum of the coupled dots as a function of bias-controlled coupling and an external perpendicular magnetic field. We show that the tunneling energy can be controlled to a large extent by the electrostatic barrier potential. Particularly interesting is the coupling via Klein tunneling through a resonant valence state of the barrier. The effective three-level system nicely maps to a model Hamiltonian, from which we extract the Klein coupling between the confined conduction and valence dots levels. For large enough magnetic fields Klein tunneling can be completely blocked due to the enhanced localization of the degenerate Landau levels formed in the quantum dots.

Ertler, Christian; Raith, Martin; Fabian, Jaroslav

2014-02-01

108

NASA Astrophysics Data System (ADS)

We study the neutral exciton (X) and charged exciton (CX) transitions from (Al,Ga)As shell quantum dots located in core-shell nanowires, in the presence of a magnetic field. The g-factors and the diamagnetic coefficients of both the X and the CX depend on the orientation of the field with respect to the nanowire axis. The aspect ratio of the X wavefunction is quantified based on the anisotropy of the diamagnetic coefficient. For specific orientations of the magnetic field, it is possible to cancel the g-factor of the bright states of the X and the CX by means of an inversion of the sign of the hole's g-factor, which is promising for quantum information processing applications.

Corfdir, P.; Fontana, Y.; Van Hattem, B.; Russo-Averchi, E.; Heiss, M.; Fontcuberta i Morral, A.; Phillips, R. T.

2014-12-01

109

Vacancy clusters in graphane as quantum dots.

Complementary electronic properties and a tendency to form sharp graphene-graphane interfaces open tantalizing possibilities for two-dimensional nanoelectronics. First-principles density functional and tight-binding calculations show that graphane can serve as natural host for graphene quantum dots, clusters of vacancies in the hydrogen sublattice. Their size n, shape, and stability are governed by the aromaticity and interfaces, resulting in formation energies approximately 1/ radicaln eV/atom and preference to hexagonal clusters congruent with lattice hexagons (i.e., with armchair edge). Clusters exhibit large gaps approximately 15/ radicaln eV with size dependence typical for confined Dirac fermions. PMID:20465240

Singh, Abhishek K; Penev, Evgeni S; Yakobson, Boris I

2010-06-22

110

Optical control of single excitons in semiconductor quantum dots

NASA Astrophysics Data System (ADS)

The fundamental building block of quantum information processing technologies is the quantum-bit a ‘qubit.’ These technologies require the ability to prepare, control, and read out a qubit state. Spins confined in self-assembled quantum dots are promising candidates for a quantum bit, because semiconductors are compatible with mature modern opto- and micro-electronics. These quantum dot systems offer two more advantages: they are excellent interfaces between the spin state—an anchored qubit and a photon—a ‘flying qubit’ and they provide means to coherently control the spin qubit by ultrashort optical pulses. In this review, we thoroughly discuss the qubit provided by an optically-excited electron in a quantum dot-the exciton qubit. We demonstrate its spin state initialization, coherent control and read-out using ultrashort optical pulses.

Kodriano, Y.; Schmidgall, E. R.; Benny, Y.; Gershoni, D.

2014-05-01

111

Quantum Monte Carlo study of quantum dots in magnetic fields

NASA Astrophysics Data System (ADS)

We have studied the ground state energies and quantum numbers of confined two-dimensional (2D) electrons in weak and intermediate magnetic field strengths using quantum Monte Carlo methods. These 2D quantum dots are of theoretical interest, because it is possible to go from a weakly to a strongly correlated system by tuning the relative strength of the external potential to the electron-electron interaction. The accuracy of current spin density functional theory, and of the variational and diffusion Monte Carlo methods using single and multi configuration wave functions, is studied by comparison with results obtained by exact diagonalization. Using optimized trial wave functions, we calculate pair correlation functions as a function of the magnetic field and confinement strength. (Supported by the DOE and NSF)

Geist, Wolfgang; Zeng, Lang; Chou, Mei-Yin

2004-03-01

112

Quantum Confinement in Hydrogen Bond

In this work, the quantum confinement effect is proposed as the cause of the displacement of the vibrational spectrum of molecular groups that involve hydrogen bonds. In this approach the hydrogen bond imposes a space barrier to hydrogen and constrains its oscillatory motion. We studied the vibrational transitions through the Morse potential, for the NH and OH molecular groups inside macromolecules in situation of confinement (when hydrogen bonding is formed) and non-confinement (when there is no hydrogen bonding). The energies were obtained through the variational method with the trial wave functions obtained from Supersymmetric Quantum Mechanics (SQM) formalism. The results indicate that it is possible to distinguish the emission peaks related to the existence of the hydrogen bonds. These analytical results were satisfactorily compared with experimental results obtained from infrared spectroscopy.

Santos, Carlos da Silva dos; Ricotta, Regina Maria

2015-01-01

113

Quantum dot quantum cascade infrared photodetector

We demonstrate an InAs quantum dot quantum cascade infrared photodetector operating at room temperature with a peak detection wavelength of 4.3??m. The detector shows sensitive photoresponse for normal-incidence light, which is attributed to an intraband transition of the quantum dots and the following transfer of excited electrons on a cascade of quantum levels. The InAs quantum dots for the infrared absorption were formed by making use of self-assembled quantum dots in the Stranski–Krastanov growth mode and two-step strain-compensation design based on InAs/GaAs/InGaAs/InAlAs heterostructure, while the following extraction quantum stairs formed by LO-phonon energy are based on a strain-compensated InGaAs/InAlAs chirped superlattice. Johnson noise limited detectivities of 3.64?×?10{sup 11} and 4.83?×?10{sup 6} Jones at zero bias were obtained at 80?K and room temperature, respectively. Due to the low dark current and distinct photoresponse up to room temperature, this device can form high temperature imaging.

Wang, Xue-Jiao; Zhai, Shen-Qiang; Zhuo, Ning; Liu, Jun-Qi, E-mail: jqliu@semi.ac.cn, E-mail: fqliu@semi.ac.cn; Liu, Feng-Qi, E-mail: jqliu@semi.ac.cn, E-mail: fqliu@semi.ac.cn; Liu, Shu-Man; Wang, Zhan-Guo [Key Laboratory of Semiconductor Materials Science, Institute of Semiconductors, Chinese Academy of Sciences and Beijing Key Laboratory of Low Dimensional Semiconductor Materials and Devices, P.O. Box 912, Beijing 100083 (China)

2014-04-28

114

Ge\\/Si Self-Assembled Quantum Dots and Their Optoelectronic Device Applications

In recent years, quantum dots have been successfully grown by self-assembling processes. For optoelectronic device applications, the quantum-dot structures have advantages such as reduced phonon scattering, longer carrier lifetime, and lower detector noise due to low-dimensional confinement effect. Comparing to traditional optoelectronic III-V and other materials, self-assembled Ge quantum dots grown on Si substrates have a potential to be monolithically

Kang L. Wang; Dongho Cha; Jianlin Liu; Christopher Chen

2007-01-01

115

We present an electrically-controllable multi-spectral quantum dot infrared photodetector (QDIP). The QDIP consists of vertically-stacked InAs quantum dots layers with two different capping layers for MWIR and LWIR absorption, respectively. The multi-spectral QDOP is capable of simultaneously detecting multi-spectral normal incidence through inter-subband transitions in the three-dimensional (3-D) confined quantum dot nanostructures. The QDIP showed multi-color IR detection bands centered

Xuejun Lu; Jarrod Vaillancourt; Mark J. Meisner

2007-01-01

116

Electronic structure and optical properties of quantum dots

NASA Astrophysics Data System (ADS)

Because of complete (three dimensional) confinement, quantum dots have a most dramatic quantum size effect. Due to the finite size of the dot, the conduction and valence bands of semiconductor dots are quantized and quantum dot spectra exhibit a series of discrete electronic transitions and depend strongly on the size of the nanocrystal. In this thesis we study the electronic structure and the optical properties of semiconductor quantum dots and semiconductor quantum dot systems. Different properties and different dots and dot systems are described. The first topic is the electric polarization in a semiconductor dot (II-VI compound). A simple theoretical model for the origin of spontaneous polarization in single nanocrystals is developed, based on the proposal that the origin of the spontaneous polarization is in the strained layer between "cap" and the nanocrystal. The internal electric field in the dot is due to the piezoelectric effect caused by the strain existing in the interface region of material with different lattice constants. Based on spherical rotation symmetry without inversion SO(3), the model employs a distribution of polarization with symmetry which is a subgroup of SO(3), consistent with the hexagonal structure of wurtzite structure. The second topic we study is a distribution of many quantum dots, which are arranged together in an array. We present a new model to implement organic exciton-inorganic (semiconductor) exciton hybridization. We consider embedding a quantum dot array into an organic medium. A Wannier-Mott transfer exciton is formed when the exciton in each semiconductor dot interacts via multipole-multipole coupling with other excitons in the different semiconductor dots of the array. A new hybrid exciton appears in the system owing to strong dipole-dipole interaction of the Frenkel exciton of organic molecules with the Wannier Mott transfer exciton of the quantum dot array. This hybrid exciton has both a large oscillator strength (Frenkel-like) and a large exciton Bohr radius (Wannier-like). At resonance between these two types of excitons, the optical non-linearity is very high and can be controlled by changing parameters of the system such as dot radius and dot-dot spacing. As the third topic, which differs from the pure nanocrystal in the above study, we also present our study on Mn-doped semiconductor nanocrystals such as the ZnS:Mn quantum dot. The effect of an extra electron "injected" into the doped quantum dot with a substitutional Mn 2+ at the center is considered. The electron confined in the dot will be strongly coupled by exchange interaction with the Mn ion, and will split and mix Mn crystal-field energy levels. As a result, this will strongly break the previous selection rules. The optical transition of interest is the 4T1 - 6 A1 transition. Using this model we evaluate the energy structure, wavefunctions, luminescent efficiency and transition life time of a Mn doped quantum dot and compare our results with experimental data.

Nguyen, Thique Huong

117

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

2012-01-01

118

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

2012-01-01

119

Group-velocity slowdown in quantum-dots and quantum-dot molecules

NASA Astrophysics Data System (ADS)

We investigate theoretically the slowdown of optical pulses due to quantum-coherence effects in InGaAs-based quantum dots and quantum dot molecules. Simple models for the electronic structure of quantum dots and, in particular, quantum-dot molecules are described and calibrated using numerical simulations. It is shown how these models can be used to design optimized quantum-dot molecules for quantum coherence applications. The wave functions and energies obtained from the optimizations are used as input for a microscopic calculation of the quantum-dot material dynamics including carrier scattering and polarization dephasing. The achievable group velocity slowdown in quantum-coherence V schemes consisting of quantum-dot molecule states is shown to be substantially higher than what is achievable from similar transitions in typical InGaAs-based single quantum dots.

Michael, Stephan; Chow, Weng W.; Schneider, Hans Christian

2014-03-01

120

Biexciton versus Exciton Lifetime in a Single Semiconductor Quantum Dot

The decay characteristics of excitons and biexcitons in one single semiconductor quantum dot (QD) are directly monitored using time- and spatially resolved photoluminescence spectroscopy. The experiments are performed on a CdSe\\/ZnSe QD, occupied by either one or two excitons at a time, allowing a direct comparison between the radiative lifetime of a biexciton and an exciton confined in the same

G. Bacher; R. Weigand; J. Seufert; V. D. Kulakovskii; N. A. Gippius; A. Forchel; K. Leonardi; D. Hommel

1999-01-01

121

Quantum Dots Based Rad-Hard Computing and Sensors

NASA Technical Reports Server (NTRS)

Quantum Dots (QDs) are solid-state structures made of semiconductors or metals that confine a small number of electrons into a small space. The confinement of electrons is achieved by the placement of some insulating material(s) around a central, well-conducting region. Thus, they can be viewed as artificial atoms. They therefore represent the ultimate limit of the semiconductor device scaling. Additional information is contained in the original extended abstract.

Fijany, A.; Klimeck, G.; Leon, R.; Qiu, Y.; Toomarian, N.

2001-01-01

122

We propose an all-optical implementation of quantum-information processing in semiconductor quantum dots, where electron-hole excitations (excitons) serve as the computational degrees of freedom (qubits). We show that the strong dot confinement leads to an overall enhancement of Coulomb correlations and to a strong renormalization of the excitonic states, which can be exploited for performing conditional and unconditional qubit operations.

Filippo Troiani; Ulrich Hohenester; Elisa Molinari

2000-01-01

123

We propose an all-optical implementation of quantum-information processing in semiconductor quantum dots, where electron-hole excitations (excitons) serve as the computational degrees of freedom (qubits). We show that the strong dot confinement leads to an overall enhancement of Coulomb correlations and to a strong renormalization of the excitonic states, which can be exploited for performing conditional and unconditional qubit operations.

Filippo Troiani; Ulrich Hohenester; Elisa Molinari

2000-05-17

124

Optically Induced Magnetization of CdMnTe Self-Assembled Quantum Dots

We demonstrate that resonant excitation of CdMnTe self-assembled quantum dots creates an ensemble of spin-polarized magnetic polarons at B=0 T. The strong spatial confinement characteristic of quantum dots significantly increases the stability of magnetic polarons so that the optically induced spin alignment is observed for temperatures > 120 K.

S. Mackowski; T. Gurung; T. A. Nguyen; H. E. Jackson; L. M. Smith; G. Karczewski; J. Kossut

2004-03-11

125

Semiconductor double quantum dot micromaser.

The coherent generation of light, from masers to lasers, relies upon the specific structure of the individual emitters that lead to gain. Devices operating as lasers in the few-emitter limit provide opportunities for understanding quantum coherent phenomena, from terahertz sources to quantum communication. Here we demonstrate a maser that is driven by single-electron tunneling events. Semiconductor double quantum dots (DQDs) serve as a gain medium and are placed inside a high-quality factor microwave cavity. We verify maser action by comparing the statistics of the emitted microwave field above and below the maser threshold. PMID:25593187

Liu, Y-Y; Stehlik, J; Eichler, C; Gullans, M J; Taylor, J M; Petta, J R

2015-01-16

126

A tunable few electron triple quantum dot

NASA Astrophysics Data System (ADS)

We report on a new design to realize a fully tunable lateral triple quantum dot. The electrostatically defined quantum dots are arranged in series. The number of electrons in the quantum dots can be controlled and fundamental electronic configurations such as the (0,0,0) and (1,1,1) are obtained. Control of the number of electrons is important to perform quantum information processes using electron spins as qubits. Individual control of the tunnel barriers between the dots as well as resonant conditions called quadruple points where the chemical potential of the three quantum dots are aligned are achieved. A neighboring quantum point contact is used as a charge sensor to map out the charge stability diagram. We explore the back-action effects of the charge sensor on the triple quantum dot system and observe a series of additional resonances directly linked to the bias applied across the charge sensor.

Gaudreau, Louis; Kam, Alicia; Granger, Ghislain; Zawadzki, Piotr; Studenikin, Sergei; Clerk, Aashish; Sachrajda, Andrew

2010-03-01

127

Quantum computation with quantum dot excitons

Potential application of elementary excitation in semiconductor quantum dot to quantum computation is discussed. We propose a scalable hardware and all optical implementation of a logic gate that exploits the discrete nature of electron-hole states and their well-concentrated oscillator strength for ultrafast gate operation. A multiple-bit gate function is based on the nearest neighbour dipole-dipole coupling. Rabi population oscillation and

H. Kamada; H. Gotoh

2004-01-01

128

Decay dynamics and exciton localization in large GaAs quantum dots grown by droplet epitaxy

We investigate the optical emission and decay dynamics of excitons confined in large strain-free GaAs quantum dots grown by droplet epitaxy. From time-resolved measurements combined with a theoretical model we show that droplet-epitaxy quantum dots have a quantum efficiency of about 75% and an oscillator strength between 8 and 10. The quantum dots are found to be fully described by a model for strongly-confined excitons, in contrast to the theoretical prediction that excitons in large quantum dots exhibit the so-called giant oscillator strength. We attribute these findings to localized ground-state excitons in potential minima created by material intermixing during growth. We provide further evidence for the strong-confinement regime of excitons by extracting the size of electron and hole wavefunctions from the phonon-broadened photoluminescence spectra. Furthermore, we explore the temperature dependence of the decay dynamics and, for some quantum dots, observe a pronounced reduction in the effective transition strength with temperature. We quantify and explain these effects as being an intrinsic property of large quantum dots owing to thermal excitation of the ground-state exciton. Our results provide a detailed understanding of the optical properties of large quantum dots in general, and of quantum dots grown by droplet epitaxy in particular.

Petru Tighineanu; Raphaël Daveau; Eun Hye Lee; Jin Dong Song; Søren Stobbe; Peter Lodahl

2014-04-04

129

Electron states in semiconductor quantum dots

NASA Astrophysics Data System (ADS)

In this work, the electronic structures of quantum dots (QDs) of nine direct band gap semiconductor materials belonging to the group II-VI and III-V families are investigated, within the empirical tight-binding framework, in the effective bond orbital model. This methodology is shown to accurately describe these systems, yielding, at the same time, qualitative insights into their electronic properties. Various features of the bulk band structure such as band-gaps, band curvature, and band widths around symmetry points affect the quantum confinement of electrons and holes. These effects are identified and quantified. A comparison with experimental data yields good agreement with the calculations. These theoretical results would help quantify the optical response of QDs of these materials and provide useful input for applications.

Dhayal, Suman S.; Ramaniah, Lavanya M.; Ruda, Harry E.; Nair, Selvakumar V.

2014-11-01

130

Semiconductor quantum dot-sensitized solar cells

Semiconductor quantum dots (QDs) have been drawing great attention recently as a material for solar energy conversion due to their versatile optical and electrical properties. The QD-sensitized solar cell (QDSC) is one of the burgeoning semiconductor QD solar cells that shows promising developments for the next generation of solar cells. This article focuses on recent developments in QDSCs, including 1) the effect of quantum confinement on QDSCs, 2) the multiple exciton generation (MEG) of QDs, 3) fabrication methods of QDs, and 4) nanocrystalline photoelectrodes for solar cells. We also make suggestions for future research on QDSCs. Although the efficiency of QDSCs is still low, we think there will be major breakthroughs in developing QDSCs in the future. PMID:24191178

Tian, Jianjun; Cao, Guozhong

2013-01-01

131

Quantification of graphene based core/shell quantum dots from first principles

NASA Astrophysics Data System (ADS)

Density functional calculations are performed to study the electronic structure of recently proposed graphene/graphane based core/shell quantum dots, which have a type I band alignment and exhibit quantized carrier energy levels. Strong confinement is robust with shell thickness. The bandgap, band offset, and the number of confined carrier orbitals with different size and geometry are determined. Our findings indicate that these core/shell dots are potentially well suited for the design of advanced diode lasers and room-temperature single electron devices. The proposed method to determine the number of confined orbitals is applicable for other quantum dot systems.

Cui, X. Y.; Zheng, R. K.; Liu, Z. W.; Li, L.; Stampfl, C.; Ringer, S. P.

2011-10-01

132

Thermoelectric energy harvesting with quantum dots

NASA Astrophysics Data System (ADS)

We review recent theoretical work on thermoelectric energy harvesting in multi-terminal quantum-dot setups. We first discuss several examples of nanoscale heat engines based on Coulomb-coupled conductors. In particular, we focus on quantum dots in the Coulomb-blockade regime, chaotic cavities and resonant tunneling through quantum dots and wells. We then turn toward quantum-dot heat engines that are driven by bosonic degrees of freedom such as phonons, magnons and microwave photons. These systems provide interesting connections to spin caloritronics and circuit quantum electrodynamics.

Sothmann, Björn; Sánchez, Rafael; Jordan, Andrew N.

2015-01-01

133

Thermoelectric energy harvesting with quantum dots.

We review recent theoretical work on thermoelectric energy harvesting in multi-terminal quantum-dot setups. We first discuss several examples of nanoscale heat engines based on Coulomb-coupled conductors. In particular, we focus on quantum dots in the Coulomb-blockade regime, chaotic cavities and resonant tunneling through quantum dots and wells. We then turn toward quantum-dot heat engines that are driven by bosonic degrees of freedom such as phonons, magnons and microwave photons. These systems provide interesting connections to spin caloritronics and circuit quantum electrodynamics. PMID:25549281

Sothmann, Björn; Sánchez, Rafael; Jordan, Andrew N

2015-01-21

134

Vertically stacked quantum dot pairs fabricated by nanohole filling

NASA Astrophysics Data System (ADS)

Strain-free, vertically coupled GaAs quantum dots (QDs) with an ultra-low density below 1\\times {{10}^{7}} \\text{c}{{\\text{m}}^{-2}} are fabricated by filling of self-assembled nanoholes with a GaAs/AlGaAs/GaAs layer sequence. The sizes of the two QDs, forming a QD pair (QDP), as well as the AlGaAs tunnel-barrier between the dots are tuned independently. We present atomic force microscopy studies of the QDP formation steps. We have performed photoluminescence studies of single QDPs with varied dot size and tunnel-barrier thickness. The data indicate non-resonant tunnelling between the dots. Furthermore, we apply the quantum confined Stark effect to tune the photoluminescence energy by up to 25 meV.

Sonnenberg, D.; Küster, A.; Graf, A.; Heyn, Ch; Hansen, W.

2014-05-01

135

Energy calculations of quantum dot

NASA Astrophysics Data System (ADS)

We calculated the total energy of a semiconductor quantum dot formed in gate and etching defined devices. A 3D Poisson equation is solved self-consistently to obtain the electron density and potential profile. The total energies of electrons in the quantum dots with two different sizes are calculated with three different approximations by using the density and potential obtained from self-consistent procedure. In our calculation we used a recently developed energy functional called “orbital-free energy functional”, Thomas-Fermi approximation and standard local-density approximation within density functional theory. The comparison of these methods reveals the efficacy of the used newly developed orbital-free energy functional which facilitates the calculation of Hartree integral for treatment of electron-electron interaction.

Bilgeç Akyüz, G.; Akgüngör, K.; ?akiro?lu, S.; Siddiki, A.; Sökmen, ?.

2011-06-01

136

Quantum Dot Light Emitting Diode

The project objective is to create low cost coatable inorganic light emitting diodes, composed of quantum dot emitters and inorganic nanoparticles, which have the potential for efficiencies equivalent to that of LEDs and OLEDs and lifetime, brightness, and environmental stability between that of LEDs and OLEDs. At the end of the project the Recipient shall gain an understanding of the device physics and properties of Quantum-Dot LEDs (QD-LEDs), have reliable and accurate nanocrystal synthesis routines, and have formed green-yellow emitting QD-LEDs with a device efficiency greater than 3 lumens/W, a brightness greater than 400 cd/m2, and a device operational lifetime of more than 1000 hours. Thus the aim of the project is to break the current cost-efficiency paradigm by creating novel low cost inorganic LEDs composed of inorganic nanoparticles.

Keith Kahen

2008-07-31

137

Quantum Dot Light Emitting Diode

The project objective is to create low cost coatable inorganic light emitting diodes, composed of quantum dot emitters and inorganic nanoparticles, which have the potential for efficiencies equivalent to that of LEDs and OLEDs and lifetime, brightness, and environmental stability between that of LEDs and OLEDs. At the end of the project the Recipient shall gain an understanding of the device physics and properties of Quantum-Dot LEDs (QD-LEDs), have reliable and accurate nanocrystal synthesis routines, and have formed green-yellow emitting QD-LEDs with a device efficiency greater than 3 lumens/W, a brightness greater than 400 cd/m{sup 2}, and a device operational lifetime of more than 1000 hours. Thus the aim of the project is to break the current cost-efficiency paradigm by creating novel low cost inorganic LEDs composed of inorganic nanoparticles.

Kahen, Keith

2008-07-31

138

Generation of superradiation in quantum dot nanoheterostructures

Conditions for the generation of Dicke supperradiation are studied for two types of quantum dot nanoheterostructures: a planar waveguide and a microdisk. The domain model for the generation of superradiation suggested earlier for quantum wells is generalized to inhomogeneous quantum dot arrays. Different scenarios of the generation of superradiation in a semiconductor microdisk are classified.

Savelyev, A. V., E-mail: savelev@mail.ioffe.ru; Karachinsky, L. Ya.; Novikov, I. I.; Gordeev, N. Yu.; Seisyan, R. P.; Zegrya, G. G. [Russian Academy of Sciences, Ioffe Physicotechnical Institute (Russian Federation)

2008-06-15

139

Electron counting in quantum dots

We use time-resolved charge detection techniques to investigate single-electron tunneling in semiconductor quantum dots. The ability to detect individual charges in real-time makes it possible to count electrons one-by-one as they pass through the structure. The setup can thus be used as a high-precision current meter for measuring ultra-low currents, with resolution several orders of magnitude better than that of

S. Gustavsson; R. Leturcq; M. Studer; I. Shorubalko; T. Ihn; K. Ensslin; D. C. Driscoll; A. C. Gossard

2009-01-01

140

Electron microscopy of quantum dots.

This brief review describes the different types of semiconductor quantum dot systems, their main applications and which types of microscopy methods are used to characterize them. Emphasis is put on the need for a comprehensive investigation of their size distribution, microstructure, chemical composition, strain state and electronic properties, all of which influence the optical properties and can be measured by different types of imaging, diffraction and spectroscopy methods in an electron microscope. PMID:25406030

Walther, T

2015-03-01

141

Probing the Excitonic States of Site-Controlled GaN Nanowire Quantum Dots.

We report the detection of fully confined excited states and the zero-absorption region of individual site-controlled GaN/AlGaN nanowire quantum dots using photoluminescence excitation spectroscopy, which provides evidence of the true zero-dimensional discrete density of states of such quantum dots. Because of the strong quantum confinement in these dots, the p-shell, d-shell, and even higher energy (including some f-shell) states of a single quantum dot are observed, which provides unprecedented insight into the electronic structure. Several emitters are measured and used to build up an average picture of the electronic structure of a single quantum dot via comparison to theoretical simulations. PMID:25574691

Holmes, Mark J; Kako, Satoshi; Choi, Kihyun; Podemski, Pawel; Arita, Munetaka; Arakawa, Yasuhiko

2015-02-11

142

Electron counting in quantum dots

NASA Astrophysics Data System (ADS)

We use time-resolved charge detection techniques to investigate single-electron tunneling in semiconductor quantum dots. The ability to detect individual charges in real-time makes it possible to count electrons one-by-one as they pass through the structure. The setup can thus be used as a high-precision current meter for measuring ultra-low currents, with resolution several orders of magnitude better than that of conventional current meters. In addition to measuring the average current, the counting procedure also makes it possible to investigate correlations between charge carriers. Electron correlations are conventionally probed in noise measurements, which are technically challenging due to the difficulty to exclude the influence of external noise sources in the experimental setup. Using real-time charge detection techniques, we circumvent the problem by studying the electron correlation directly from the counting statistics of the tunneling electrons. In quantum dots, we find that the strong Coulomb interaction makes electrons try to avoid each other. This leads to electron anti-bunching, giving stronger correlations and reduced noise compared to a current carried by statistically independent electrons. The charge detector is implemented by monitoring changes in conductance in a nearby capacitively coupled quantum point contact. We find that the quantum point contact not only serves as a detector but also causes a back-action onto the measured device. Electron scattering in the quantum point contact leads to emission of microwave radiation. The radiation is found to induce an electronic transition between two quantum dots, similar to the absorption of light in real atoms and molecules. Using a charge detector to probe the electron transitions, we can relate a single-electron tunneling event to the absorption of a single photon. Moreover, since the energy levels of the double quantum dot can be tuned by external gate voltages, we use the device as a frequency-selective single-photon detector operating at microwave energies. The ability to put an on-chip microwave detector close to a quantum conductor opens up the possibility to investigate radiation emitted from mesoscopic structures and gives a deeper understanding of the role of electron-photon interactions in quantum conductors. A central concept of quantum mechanics is the wave-particle duality; matter exhibits both wave- and particle-like properties and cannot be described by either formalism alone. To investigate the wave properties of the electrons, we perform experiments on a structure containing a double quantum dot embedded in the Aharonov-Bohm ring interferometer. Aharonov-Bohm rings are traditionally used to study interference of electron waves traversing different arms of the ring, in a similar way to the double-slit setup used for investigating interference of light waves. In our case, we use the time-resolved charge detection techniques to detect electrons one-by-one as they pass through the interferometer. We find that the individual particles indeed self-interfere and give rise to a strong interference pattern as a function of external magnetic field. The high level of control in the system together with the ability to detect single electrons enables us to make direct observations of non-intuitive fundamental quantum phenomena like single-particle interference or time-energy uncertainty relations.

Gustavsson, S.; Leturcq, R.; Studer, M.; Shorubalko, I.; Ihn, T.; Ensslin, K.; Driscoll, D. C.; Gossard, A. C.

2009-06-01

143

in semiconductor quantum dots: Rashba versus Dresselhaus spin-orbit coupling Sanjay Prabhakar,1 Roderick Melnik,1; revised manuscript received 15 May 2013; published 7 June 2013) In symmetric quantum dots (QDs electron spin with the application of gate controlled electric fields in confined semiconductor quantum

Melnik, Roderick

144

Quantum dot quantum computation in III-V type semiconductor

NASA Astrophysics Data System (ADS)

Among recent proposals for next-generation, non-charge-based logic is the notion that a single electron can be trapped and spin of the electron can be manipulated through the application of gate potentials. In the thesis, there are two major contributions of the manipulation of electron spin. In regard to the first contribution, we present numerical simulations of such a spin in single electron devices for realistic asymmetric potentials in electrostatically confined quantum dot. Using analytical and numerical techniques we show that breaking in-plane rotational symmetry of the confining potential by applied gate voltage leads to a significant effect on the tuning of the electron g-factor. In particular, we find that anisotropy extends the tunability to larger quantum dots in the GaAs case. Although the same extension of tunability exists in the InAs quantum dot case, we find a new effect in the InAs case. The new discovery is that broken in-plane rotational symmetry due to the Rashba spin-orbit coupling in an asymmetric potential results in a significant reverse effect in the tuning of the electron g-factor. This effect can not be observed in symmetric case. The derivative of the g-factor with respect to the electric field has the opposite sign in the above two potentials. The manipulation of Berry phases of spin in nano-scale devices is a topic that has received recent attention as a promising candidate for solid state quantum computation and non-charge-based logic devices. A single electron in an electrostatically defined quantum dot located in a 2 dimensional electron gas (2DEG), for example, can be trapped and the spin can be manipulated by simply moving the center of mass of the quantum dot adiabatically along a closed loop in the 2D plane via the application of gate potentials. In relation to the second contribution, we present numerical simulations and analytical expressions for the spin-dependent electron propagator (a matrix-valued function of position) for an electron trapped in a quantum dot, while the center of mass of the quantum dot is adiabatically moved in the 2D plane in the presence of the Rashba and Dresselhaus spin-orbit interactions. We apply the Feynman disentangling technique to determine the non-abelian matrix Berry phase, we find exact analytical expression for the propagator in three cases: (a) pure Rashba coupling; (b) pure Dresselhaus coupling; and (c) a combination of equally strong Rashba and Dresselhaus couplings. For other cases of interest where the solution of the propagator can not be found analytically, we present results obtained by numerically solving the Riccati equation resulting from the disentangling procedure. We also find that the presence of both spin-orbit couplings leads to a larger spin-flip probability than what would result from either mechanism considered separately.

Prabhakar, Sanjay Kumar

145

Chiral quantum dot based materials

NASA Astrophysics Data System (ADS)

Recently, the use of stereospecific chiral stabilising molecules has also opened another avenue of interest in the area of quantum dot (QD) research. The main goal of our research is to develop new types of technologically important quantum dot materials containing chiral defects, study their properties and explore their applications. The utilisation of chiral penicillamine stabilisers allowed the preparation of new water soluble white emitting CdS quantum nanostructures which demonstrated circular dichroism in the band-edge region of the spectrum. It was also demonstrated that all three types of QDs (D-, L-, and Rac penicillamine stabilised) show very broad emission bands between 400 and 700 nm due to defects or trap states on the surfaces of the nanocrystals. In this work the chiral CdS based quantum nanostructures have also been doped by copper metal ions and new chiral penicilamine stabilized CuS nanoparticles have been prepared and investigated. It was found that copper doping had a strong effect at low levels in the synthesis of chiral CdS nanostructures. We expect that this research will open new horizons in the chemistry of chiral nanomaterials and their application in biotechnology, sensing and asymmetric synthesis.

Govan, Joseph; Loudon, Alexander; Baranov, Alexander V.; Fedorov, Anatoly V.; Gun'ko, Yurii

2014-05-01

146

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 Zn 1-xCd xSe 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

2008-02-01

147

The quantum Hall effect in quantum dot systems

NASA Astrophysics Data System (ADS)

It is proposed to use quantum dots in order to increase the temperatures suitable for observation of the integer quantum Hall effect. A simple estimation using Fock-Darwin spectrum of a quantum dot shows that good part of carriers localized in quantum dots generate the intervals of plateaus robust against elevated temperatures. Numerical calculations employing local trigonometric basis and highly efficient kernel polynomial method adopted for computing the Hall conductivity reveal that quantum dots may enhance peak temperature for the effect by an order of magnitude, possibly above 77 K. Requirements to potentials, quality and arrangement of the quantum dots essential for practical realization of such enhancement are indicated. Comparison of our theoretical results with the quantum Hall measurements in InAs quantum dot systems from two experimental groups is also given.

Beltukov, Y. M.; Greshnov, A. A.

2014-12-01

148

Photoluminescence of a quantum-dot molecule

NASA Astrophysics Data System (ADS)

The coherent coupling of quantum dots is a sensitive indicator of the energy and phase relaxation processes taking place in the nanostructure components. We formulate a theory of low-temperature, stationary photoluminescence from a quantum-dot molecule composed of two spherical quantum dots whose electronic subsystems are resonantly coupled via the Coulomb interaction. We show that the coupling leads to the hybridization of the first excited states of the quantum dots, manifesting itself as a pair of photoluminescence peaks with intensities and spectral positions strongly dependent on the geometric, material, and relaxation parameters of the quantum-dot molecule. These parameters are explicitly contained in the analytical expression for the photoluminescence differential cross section derived in the paper. The developed theory and expression obtained are essential in interpreting and analyzing spectroscopic data on the secondary emission of coherently coupled quantum systems.

Kruchinin, Stanislav Yu.; Rukhlenko, Ivan D.; Baimuratov, Anvar S.; Leonov, Mikhail Yu.; Turkov, Vadim K.; Gun'ko, Yurii K.; Baranov, Alexander V.; Fedorov, Anatoly V.

2015-01-01

149

Spin-based optical quantum computation via Pauli blocking in semiconductor quantum dots

We present a solid-state implementation of an all-optical spin-based quantum computer. Our proposal for a quantum-computing device is based on the spin degrees of freedom of electrons confined in semiconductor quantum dots, thus benefitting from relatively long coherence times. Combining Pauli blocking effects with properly tailored ultrafast laser pulses, we obtain sub-picosecond spin-dependent switching of the Coulomb interaction, which is

E. Pazy; E. Biolatti; T. Calarco; I. D'Amico; P. Zanardi; F. Rossi; P. Zoller

2003-01-01

150

Correlation energy of anisotropic quantum dots

We study the D-dimensional high-density correlation energy E{sub c} of the singlet ground state of two electrons confined by a harmonic potential with Coulombic repulsion. We allow the harmonic potential to be anisotropic and examine the behavior of E{sub c} as a function of the anisotropy {alpha}{sup -1}. In particular, we are interested in the limit where the anisotropy goes to infinity ({alpha}{yields}0) and the electrons are restricted to a lower-dimensional space. We show that tuning the value of {alpha} from 0 to 1 allows a smooth dimensional interpolation and we demonstrate that the usual model, in which a quantum dot is treated as a two-dimensional system, is inappropriate. Finally, we provide a simple function which reproduces the behavior of E{sub c} over the entire range of {alpha}.

Zhao Yan; Loos, Pierre-Francois; Gill, Peter M. W. [Research School of Chemistry, Australian National University, Canberra, ACT 0200 (Australia)

2011-09-15

151

Charge state hysteresis in semiconductor quantum dots

NASA Astrophysics Data System (ADS)

Semiconductor quantum dots provide a two-dimensional analogy for real atoms and show promise for the implementation of scalable quantum computers. Here, we investigate the charge configurations in a silicon metal-oxide-semiconductor double quantum dot tunnel coupled to a single reservoir of electrons. By operating the system in the few-electron regime, the stability diagram shows hysteretic tunnelling events that depend on the history of the dots charge occupancy. We present a model which accounts for the observed hysteretic behaviour by extending the established description for transport in double dots coupled to two reservoirs. We demonstrate that this type of device operates like a single-electron memory latch.

Yang, C. H.; Rossi, A.; Lai, N. S.; Leon, R.; Lim, W. H.; Dzurak, A. S.

2014-11-01

152

Creating atomic order in semiconductors quantum dots

1 Creating atomic order in semiconductors quantum dots Peter Moeck Department of Physics (MC 273. Nanoscience and technology 3. Self-assembled semiconductor quantum dots: What are they? How are they made-8 ~ 0.01 diameter of a human hair #12;4 "for developing semiconductor heterostructures used in high

Moeck, Peter

153

Slow light using semiconductor quantum dots

A variable semiconductor optical buffer based on the electromagnetically induced transparency in a quantum dot waveguide is theoretically investigated with feasible parameters for applications to a 40 Gbps optical network. We show the refractive index and absorption spectra of the quantum dot waveguide at various pump levels, which exhibit an optimal pump power for maximum slow-down factor, in agreement with

J Kim; S L Chuang; P C Ku; C J Chang-Hasnain

2004-01-01

154

Thick-shell nanocrystal quantum dots

Colloidal nanocrystal quantum dots comprising an inner core having an average diameter of at least 1.5 nm and an outer shell, where said outer shell comprises multiple monolayers, wherein at least 30% of the quantum dots have an on-time fraction of 0.80 or greater under continuous excitation conditions for a period of time of at least 10 minutes.

Hollingsworth, Jennifer A. (Los Alamos, NM); Chen, Yongfen (Eugene, OR); Klimov, Victor I. (Los Alamos, NM); Htoon, Han (Los Alamos, NM); Vela, Javier (Los Alamos, NM)

2011-05-03

155

QUANTUM DOTS: USING NANOTECHNOLOGY TO RESTORE VISION

Quantum dots are a newly developed nanotechnology that has the potential to restore impaired vision. We will explain what quantum dots are, how they operate, and how they are made, including the material makeup and fabrication processes. We will then proceed by giving brief background information about the composition of the eye and how it functions. Furthermore, we will explain

Catherine Nalesnik

156

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

157

NASA Technical Reports Server (NTRS)

The photoluminescence emission from InGaAs/GaAs quantum-well and quantum-dot (QD) structures are compared after controlled irradiation with 1.5 MeV proton fluxes. Results presented here show a significant enhancement in radiation tolerance with three-dimensional quantum confinement.

Leon, R.; Swift, G. M.; Magness, B.; Taylor, W. A.; Tang, Y. S.; Wang, K. L.; Dowd, P.; Zhang, Y. H.

2000-01-01

158

Wave function engineering in quantum dot-ring nanostructures

NASA Astrophysics Data System (ADS)

Modern nanotechnology allows the production of, depending on the application, various quantum nanostructures with selected properties. These properties are strongly influenced by the confinement potential which can be modified e.g. by electrical gating. In this paper, we analyze a nanostructure composed of a quantum dot surrounded by a quantum ring. We show that, depending on the details of the confining potential, the electron wave functions can be located in different parts of the structure. Since many properties of such a nanostructure strongly depend on the distribution of the wave functions, by varying the applied gate voltage one can easily control them. In particular, we illustrate the high controllability of the nanostructure by demonstrating how its coherent, optical and conducting properties can be drastically changed by a small modification of the confining potential.

Zipper, El?bieta; Kurpas, Marcin; Ma?ka, Maciej M.

2012-09-01

159

Strain-induced fundamental optical transition in (In,Ga)As/GaP quantum dots

The nature of the ground optical transition in an (In,Ga)As/GaP quantum dot is thoroughly investigated through a million atoms supercell tight-binding simulation. Precise quantum dot morphology is deduced from previously reported scanning-tunneling-microscopy images. The strain field is calculated with the valence force field method and has a strong influence on the confinement potentials, principally, for the conduction band states. Indeed, the wavefunction of the ground electron state is spatially confined in the GaP matrix, close to the dot apex, in a large tensile strain region, having mainly Xz character. Photoluminescence experiments under hydrostatic pressure strongly support the theoretical conclusions.

Robert, C., E-mail: cedric.robert@insa-rennes.fr, E-mail: cedric.robert@tyndall.ie; Pedesseau, L.; Cornet, C.; Jancu, J.-M.; Even, J.; Durand, O. [Université Européenne de Bretagne, INSA Rennes, France and CNRS, UMR 6082 Foton, 20 Avenue des Buttes de Coësmes, 35708 Rennes (France)] [Université Européenne de Bretagne, INSA Rennes, France and CNRS, UMR 6082 Foton, 20 Avenue des Buttes de Coësmes, 35708 Rennes (France); Nestoklon, M. O. [Ioffe Physico-Technical Institute, Russian Academy of Sciences, 194021 St. Petersburg (Russian Federation)] [Ioffe Physico-Technical Institute, Russian Academy of Sciences, 194021 St. Petersburg (Russian Federation); Pereira da Silva, K. [ICMAB-CSIC, Campus UAB, 08193 Bellaterra (Spain) [ICMAB-CSIC, Campus UAB, 08193 Bellaterra (Spain); Departamento de Física, Universidade Federal do Ceará, P.O. Box 6030, Fortaleza–CE, 60455-970 (Brazil); Alonso, M. I. [ICMAB-CSIC, Campus UAB, 08193 Bellaterra (Spain)] [ICMAB-CSIC, Campus UAB, 08193 Bellaterra (Spain); Goñi, A. R. [ICMAB-CSIC, Campus UAB, 08193 Bellaterra (Spain) [ICMAB-CSIC, Campus UAB, 08193 Bellaterra (Spain); ICREA, Passeig Lluís Companys 23, 08010 Barcelona (Spain); Turban, P. [Equipe de Physique des Surfaces et Interfaces, Institut de Physique de Rennes UMR UR1-CNRS 6251, Université de Rennes 1, F-35042 Rennes Cedex (France)] [Equipe de Physique des Surfaces et Interfaces, Institut de Physique de Rennes UMR UR1-CNRS 6251, Université de Rennes 1, F-35042 Rennes Cedex (France)

2014-01-06

160

Strain-induced fundamental optical transition in (In,Ga)As/GaP quantum dots

NASA Astrophysics Data System (ADS)

The nature of the ground optical transition in an (In,Ga)As/GaP quantum dot is thoroughly investigated through a million atoms supercell tight-binding simulation. Precise quantum dot morphology is deduced from previously reported scanning-tunneling-microscopy images. The strain field is calculated with the valence force field method and has a strong influence on the confinement potentials, principally, for the conduction band states. Indeed, the wavefunction of the ground electron state is spatially confined in the GaP matrix, close to the dot apex, in a large tensile strain region, having mainly Xz character. Photoluminescence experiments under hydrostatic pressure strongly support the theoretical conclusions.

Robert, C.; Nestoklon, M. O.; Pereira da Silva, K.; Pedesseau, L.; Cornet, C.; Alonso, M. I.; Goñi, A. R.; Turban, P.; Jancu, J.-M.; Even, J.; Durand, O.

2014-01-01

161

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

162

Tailoring 10 nm Scale Suspended Graphene Junctions and Quantum Dots

The possibility to make 10 nm scale, and low-disorder, suspended graphene devices would open up many possibilities to study and make use of strongly coupled quantum electronics, quantum mechanics, and optics. We present a versatile method, based on the electromigration of gold-on-graphene bow-tie bridges, to fabricate low-disorder suspended graphene junctions and quantum dots with lengths ranging from 6 nm up to 55 nm. We control the length of the junctions, and shape of their gold contacts by adjusting the power at which the electromigration process is allowed to avalanche. Using carefully engineered gold contacts and a nonuniform downward electrostatic force, we can controllably tear the width of suspended graphene channels from over 100 nm down to 27 nm. We demonstrate that this lateral confinement creates high-quality suspended quantum dots. This fabrication method could be extended to other two-dimensional materials.

V. Tayari; A. C. McRae; S. Yigen; J. O. Island; J. M. Porter; A. R. Champagne

2015-02-05

163

Tailoring 10 nm Scale Suspended Graphene Junctions and Quantum Dots.

The possibility to make 10 nm scale, and low-disorder, suspended graphene devices would open up many possibilities to study and make use of strongly coupled quantum electronics, quantum mechanics, and optics. We present a versatile method, based on the electromigration of gold-on-graphene bow-tie bridges, to fabricate low-disorder suspended graphene junctions and quantum dots with lengths ranging from 6 nm up to 55 nm. We control the length of the junctions, and shape of their gold contacts by adjusting the power at which the electromigration process is allowed to avalanche. Using carefully engineered gold contacts and a nonuniform downward electrostatic force, we can controllably tear the width of suspended graphene channels from over 100 nm down to 27 nm. We demonstrate that this lateral confinement creates high-quality suspended quantum dots. This fabrication method could be extended to other two-dimensional materials. PMID:25490053

Tayari, Vahid; McRae, Andrew C; Yi?en, Serap; Island, Joshua O; Porter, James M; Champagne, Alexandre R

2015-01-14

164

Coherent optoelectronics with single quantum dots

NASA Astrophysics Data System (ADS)

The optical properties of semiconductor quantum dots are in many respects similar to those of atoms. Since quantum dots can be defined by state-of-the-art semiconductor technologies, they exhibit long-term stability and allow for well-controlled and efficient interactions with both optical and electrical fields. Resonant ps excitation of single quantum dot photodiodes leads to new classes of coherent optoelectronic functions and devices, which exhibit precise state preparation, phase-sensitive optical manipulations and the control of quantum states by electrical fields.

Zrenner, A.; Ester, P.; Michaelis de Vasconcellos, S.; Hübner, M. C.; Lackmann, L.; Stufler, S.; Bichler, M.

2008-11-01

165

Hybrid superconductor-quantum dot devices.

Advances in nanofabrication techniques have made it possible to make devices in which superconducting electrodes are connected to non-superconducting nanostructures such as quantum dots. The properties of these hybrid devices result from a combination of a macroscopic quantum phenomenon involving large numbers of electrons (superconductivity) and the ability to control single electrons, offered by quantum dots. Here we review research into electron transport and other fundamental processes that have been studied in these devices. We also describe potential applications, such as a transistor in which the direction of a supercurrent can be reversed by adding just one electron to a quantum dot. PMID:20852639

De Franceschi, Silvano; Kouwenhoven, Leo; Schönenberger, Christian; Wernsdorfer, Wolfgang

2010-10-01

166

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

167

Two-photon transitions in systems with semiconductor quantum dots

Optical interband and intraband transitions in semiconductor quantum dots (QD's) are analyzed theoretically. It is found that three-dimensional confinement essentially modifies intraband matrix elements of the electron-photon interaction in the QD's as compared to the bulk materials. This effect is quite important for the multiphoton processes. It is shown that two competitive types of two-photon transitions with different selection rules

A. V. Fedorov; A. V. Baranov; K. Inoue

1996-01-01

168

Exciton spin relaxation in resonantly excited CdTe/ZnTe self-assembled quantum dots

We study the exciton spin relaxation in CdTe self-assembled quantum dots by using polarized photoluminescence spectroscopy in magnetic field. The experiments on single CdTe quantum dots and on large quantum dot ensembles show that by combining phonon-assisted absorption with circularly polarized resonant excitation the spin-polarized excitons are photo-excited directly into the ground states of quantum dots. We find that for single symmetric quantum dots at B=0 T, where the exciton levels are degenerate, the spins randomize very rapidly, so that no net spin polarization is observed. In contrast, when this degeneracy is lifted by applying external magnetic field, optically created spin-polarized excitons maintain their polarization on a time scale much longer than the exciton recombination time. We also observe that the exciton spin polarization is conserved when the splitting between exciton states is caused by quantum dot shape asymmetry. Similar behavior is found in a large ensemble of CdTe quantum dots. These results show that while exciton spins scatter rapidly between degenerate states, the spin relaxation time increases by orders of magnitude as the exciton spin states in a quantum dot become non-degenerate. Finally, due to strong electronic confinement in CdTe quantum dots, the large spin polarization of excitons shows no dependence on the number of phonons emitted between the virtual state and the exciton ground state during the excitation.

S. Mackowski; T. A. Nguyen; T. Gurung; K. Hewaparkarama; H. E. Jackson; L. M. Smith; J. Wrobel; K. Fronc; J. Kossut; G. Karczewski

2003-07-28

169

Scattering of two-dimensional Dirac fermions on gate-defined oscillating quantum dots

NASA Astrophysics Data System (ADS)

Within an effective Dirac-Weyl theory we solve the scattering problem for massless chiral fermions impinging on a cylindrical time-dependent potential barrier. The setup we consider can be used to model the electron propagation in a monolayer of graphene with harmonically driven quantum dots. For static small-sized quantum dots scattering resonances enable particle confinement and interference effects may switch forward scattering on and off. An oscillating dot may cause inelastic scattering by excitation of states with energies shifted by integer multiples of the oscillation frequency, which significantly modifies the scattering characteristics of static dots. Exemplarily the scattering efficiency of a potential barrier with zero bias remains finite in the limit of low particle energies and small potential amplitudes. For an oscillating quantum dot with finite bias, the partial wave resonances at higher energies are smeared out for small frequencies or large oscillation amplitudes, thereby dissolving the quasibound states at the quantum dot.

Schulz, C.; Heinisch, R. L.; Fehske, H.

2015-01-01

170

Photon assisted tunneling spectroscopy on a double quantum dot

in a double-quantum-dot system. 1. Introduction Semiconductor quantum dots, in which charge states and singlePhoton assisted tunneling spectroscopy on a double quantum dot T Fujisawayz1 , T H Oosterkampy, W G quantum dots are often referred to as artiÂ¯cial atoms since they contain well-deÂ¯ned discrete levels. When

171

Kondo Effect in Coupled Quantum Dots A. M. Chang+

, with emphasis on the semiconductor quantum dot system. The rich variety of behaviors, such as distinct quantum and shapes, e.g. in semiconductor quantum dots [2Â19], metallic quan- tum dots [20Â23], tunnel junctionsKondo Effect in Coupled Quantum Dots A. M. Chang+ , J. C. Chen+ Department of Physics, Duke

Chang, Albert

172

Miniband formation in a quantum dot crystal Olga L. Lazarenkovaa)

structure in a three-dimensional regimented array of semiconductor quantum dots using an envelope function of semiconductor quantum dots or multiple arrays, e.g., stacks of quantum dots, which are also referred regimented array of semiconductor quantum dots using an envelope function approximation. The regi- mentation

173

Optophononics with coupled quantum dots.

Modern technology is founded on the intimate understanding of how to utilize and control electrons. Next to electrons, nature uses phonons, quantized vibrations of an elastic structure, to carry energy, momentum and even information through solids. Phonons permeate the crystalline components of modern technology, yet in terms of technological utilization phonons are far from being on par with electrons. Here we demonstrate how phonons can be employed to render a single quantum dot pair optically transparent. This phonon-induced transparency is realized via the formation of a molecular polaron, the result of a Fano-type quantum interference, which proves that we have accomplished making typically incoherent and dissipative phonons behave in a coherent and non-dissipative manner. We find the transparency to be widely tunable by electronic and optical means. Thereby we show amplification of weakest coupling channels. We further outline the molecular polaron's potential as a control element in phononic circuitry architecture. PMID:24534815

Kerfoot, Mark L; Govorov, Alexander O; Czarnocki, Cyprian; Lu, Davis; Gad, Youstina N; Bracker, Allan S; Gammon, Daniel; Scheibner, Michael

2014-01-01

174

Realization of quantum-dot cellular automata using semiconductor quantum dots

We demonstrate that a quantum-dot cellular automata device can be fabricated using electron beam lithographically defined gates on GaAs\\/AlGaAs heterostructure materials, and that by tuning the four quantum dot (J. Phys. C: Solid State Phys. 21 (1988) L893) system polarization of one double dot can lead to polarization in the neighboring double dot (Phys. Rev. B 67 (2003) 033302). The

C. G Smith; S. Gardelis; A. W Rushforth; R. Crook; J. Cooper; D. A Ritchie; E. H Linfield; Y. Jin; M. Pepper

2003-01-01

175

Surface Induced Magnetism in Quantum Dots

The study of nanometer sized semiconductor crystallites, also known as quantum dots (QDs), has seen rapid advancements in recent years in scientific disciplines ranging from chemistry, physics, biology, materials science, and engineering. QD materials of CdSe, ZnSe, InP, as well as many others, can be prepared in the size range of 1-10 nm producing uniform, nearly monodisperse materials that are typically coated with organic molecules [1-3]. The strength of charge carrier confinement, which dictates the size-dependent properties, in these QDs depends on the nature of the material and can be correlated to the Bohr radius for the system of interest. For instance, the Bohr radius for CdSe is {approx} 5 nm, while in the more covalent structure of InP, the Bohr radius approaches {approx} 10 nm. The study of CdSe QDs has been particularly extensive during the last decade because they exhibit unique and tunable optical properties and are readily synthesized with high-crystallinity and narrow size dispersions. Although the core electronic properties of CdSe are explained in terms of the quantum confinement model, experimental efforts to elucidate the surface structure of these materials have been limited. Typically, colloidal CdSe QDs are coated with an organic surfactant, which typically consists of an organo-phosphine, -thiol, or -amine, that has the function of energetically relaxing defect states via coordination to partially coordinated surface atoms. The organic surfactant also acts to enhance carrier confinement and prevent agglomeration of the particles. Chemically, it has been shown that the bonding of the surfactant to the CdSe QD occurs through Cd atoms resulting cleavage of the Se atoms and formation of a Cd-rich (i.e. non-stoichiometric) particle [5].

Meulenberg, R W; Lee, J I

2009-08-20

176

Single to quadruple quantum dots with tunable tunnel couplings

We prepare a gate-defined quadruple quantum dot to study the gate-tunability of single to quadruple quantum dots with finite inter-dot tunnel couplings. The measured charging energies of various double dots suggest that the dot size is governed by the gate geometry. For the triple and quadruple dots, we study the gate-tunable inter-dot tunnel couplings. For the triple dot, we find that the effective tunnel coupling between side dots significantly depends on the alignment of the center dot potential. These results imply that the present quadruple dot has a gate performance relevant for implementing spin-based four-qubits with controllable exchange couplings.

Takakura, T.; Noiri, A.; Obata, T.; Yoneda, J.; Yoshida, K. [Department of Applied Physics, University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656 (Japan); Otsuka, T.; Tarucha, S. [Department of Applied Physics, University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656 (Japan); RIKEN, Center for Emergent Matter Science, 3-1 Wako-shi, Saitama 351-0198 (Japan)

2014-03-17

177

Quantum dot light-emitting diode with quantum dots inside the hole transporting layers.

We report a hybrid, quantum dot (QD)-based, organic light-emitting diode architecture using a noninverted structure with the QDs sandwiched between hole transporting layers (HTLs) outperforming the reference device structure implemented in conventional noninverted architecture by over five folds and suppressing the blue emission that is otherwise observed in the conventional structure because of the excess electrons leaking towards the HTL. It is predicted in the new device structure that 97.44% of the exciton formation takes place in the QD layer, while 2.56% of the excitons form in the HTL. It is found that the enhancement in the external quantum efficiency is mainly due to the stronger confinement of exciton formation to the QDs. PMID:23731202

Leck, Kheng Swee; Divayana, Yoga; Zhao, Dewei; Yang, Xuyong; Abiyasa, Agus Putu; Mutlugun, Evren; Gao, Yuan; Liu, Shuwei; Tan, Swee Tiam; Sun, Xiao Wei; Demir, Hilmi Volkan

2013-07-24

178

Strain-Induced Localized States Within the Matrix Continuum of Self-Assembled Quantum Dots

Quantum dot-based infrared detectors often involve transitions from confined states of the dot to states above the minimum of the conduction band continuum of the matrix. We discuss the existence of two types of resonant states within this continuum in self-assembled dots: (i) virtual bound states, which characterize square wells even without strain and (ii) strain-induced localized states. The latter emerge due to the appearance of 'potential wings' near the dot, related to the curvature of the dots. While states (i) do couple to the continuum, states (ii) are sheltered by the wings, giving rise to sharp absorption peaks.

Popescu, V.; Bester, G.; Zunger, A.

2009-07-01

179

Spin-based quantum computation in multielectron quantum dots

In a quantum computer the hardware and software are intrinsically connected because the quantum Hamiltonian (or more precisely its time development) is the code that runs the computer. We demonstrate this subtle and crucial relationship by considering the example of electron-spin-based solid-state quantum computer in semiconductor quantum dots. We show that multielectron quantum dots with one valence electron in the

Xuedong Hu; S. Das Sarma

2001-01-01

180

Fluorescent Quantum Dots for Biological Labeling

NASA Technical Reports Server (NTRS)

Fluorescent semiconductor quantum dots that can serve as "on/off" labels for bacteria and other living cells are undergoing development. The "on/off" characterization of these quantum dots refers to the fact that, when properly designed and manufactured, they do not fluoresce until and unless they come into contact with viable cells of biological species that one seeks to detect. In comparison with prior fluorescence-based means of detecting biological species, fluorescent quantum dots show promise for greater speed, less complexity, greater sensitivity, and greater selectivity for species of interest. There are numerous potential applications in medicine, environmental monitoring, and detection of bioterrorism.

McDonald, Gene; Nadeau, Jay; Nealson, Kenneth; Storrie-Lomardi, Michael; Bhartia, Rohit

2003-01-01

181

Amplification Without Inversion in Semiconductor Quantum Dot

NASA Astrophysics Data System (ADS)

In this paper, we have realized amplification without inversion (AWI) in quantum dot (QD). A Y-type four-level system of InxGa1-xN quantum dot has been obtained and investigated for AWI. It has been shown that, with proper setting of control fields' amplitude, we can obtain reasonable gain. With proper setting of phase difference of control fields and probe field, we can obtain considerable gain in resonant wavelength. We have designed this system by solving the Schrödinger-Poisson equations for InxGa1-xN quantum dot in GaN substrate, self-consistently.

Hajibadali, A.; Abbasian, K.; Rostami, A.

182

An All-Optical Quantum Gate in a Semiconductor Quantum Dot

We report coherent optical control of a biexciton (two electron-hole pairs), confined in a single quantum dot, that shows coherent oscillations similar to the excited-state Rabi flopping in an isolated atom. The pulse control of the biexciton dynamics, combined with previously demonstrated control of the single-exciton Rabi rotation, serves as the physical basis for a two-bit conditional quantum logic gate.

Xiaoqin Li; Yanwen Wu; Duncan Steel; D. Gammon; T. H. Stievater; D. S. Katzer; C. Piermarocchi; L. J. Sham

2003-01-01

183

Quenching of phonon-induced processes in quantum dots due to electron-hole asymmetries

Differences in the confinement of electrons and holes in quantum dots are shown to profoundly impact the magnitude of scattering with acoustic phonons in materials where crystal deformation shifts the conduction and valence band in the same direction. Using an extensive model that includes the non-Markovian nature of the phonon reservoir, we show how the effect may be addressed by photoluminescence excitation spectroscopy of a single quantum dot. We also investigate the implications for cavity QED, i.e. a coupled quantum dot-cavity system, and demonstrate that the phonon scattering may be strongly quenched. The quenching is explained by a balancing between the deformation potential interaction strengths and the carrier confinement and depends on the quantum dot shape. Numerical examples suggest a route towards engineering the phonon scattering.

A. Nysteen; P. Kaer; J. Mork

2012-07-30

184

Quantum confinement in Si and Ge nanostructures: Theory and experiment

The role of quantum confinement (QC) in Si and Ge nanostructures (NSs) including quantum dots, quantum wires, and quantum wells is assessed under a wide variety of fabrication methods in terms of both their structural and optical properties. Structural properties include interface states, defect states in a matrix material, and stress, all of which alter the electronic states and hence the measured optical properties. We demonstrate how variations in the fabrication method lead to differences in the NS properties, where the most relevant parameters for each type of fabrication method are highlighted. Si embedded in, or layered between, SiO{sub 2}, and the role of the sub-oxide interface states embodies much of the discussion. Other matrix materials include Si{sub 3}N{sub 4} and Al{sub 2}O{sub 3}. Si NSs exhibit a complicated optical spectrum, because the coupling between the interface states and the confined carriers manifests with varying magnitude depending on the dimension of confinement. Ge NSs do not produce well-defined luminescence due to confined carriers, because of the strong influence from oxygen vacancy defect states. Variations in Si and Ge NS properties are considered in terms of different theoretical models of QC (effective mass approximation, tight binding method, and pseudopotential method). For each theoretical model, we discuss the treatment of the relevant experimental parameters.

Barbagiovanni, Eric G., E-mail: santino.gasparo@gmail.com [Laboratory for Simulation of Physical Systems, Beijing Computational Science Research Centre, Beijing 100084 (China); Lockwood, David J. [National Research Council, Ottawa, Ontario K1A 0R6 (Canada); Simpson, Peter J.; Goncharova, Lyudmila V. [Department of Physics and Astronomy, University of Western Ontario, London, Ontario N6A 3K7 (Canada)

2014-03-15

185

Phonon affected transport through molecular quantum dots.

To describe the interaction of molecular vibrations with electrons at a quantum dot contacted to metallic leads, we extend an analytical approach that we previously developed for the many-polaron problem. Our scheme is based on an incomplete variational Lang-Firsov transformation, combined with a perturbative calculation of the electron-phonon self-energy in the framework of generalized Matsubara functions. This allows us to describe the system at weak-to-strong coupling and intermediate-to-large phonon frequencies. We present results for the quantum dot spectral function and for the kinetic coefficient that characterizes the electron transport through the dot. With these results we critically examine the strengths and limitations of our approach, and discuss the properties of the molecular quantum dot in the context of polaron physics. We place particular emphasis on the importance of corrections to the concept of an anti-adiabatic dot polaron suggested by the complete Lang-Firsov transformation. PMID:21832393

Loos, J; Koch, T; Alvermann, A; Bishop, A R; Fehske, H

2009-09-30

186

Quantum dots and prion proteins

A diagnostics of infectious diseases can be done by the immunologic methods or by the amplification of nucleic acid specific to contagious agent using polymerase chain reaction. However, in transmissible spongiform encephalopathies, the infectious agent, prion protein (PrPSc), has the same sequence of nucleic acids as a naturally occurring protein. The other issue with the diagnosing based on the PrPSc detection is that the pathological form of prion protein is abundant only at late stages of the disease in a brain. Therefore, the diagnostics of prion protein caused diseases represent a sort of challenges as that hosts can incubate infectious prion proteins for many months or even years. Therefore, new in vivo assays for detection of prion proteins and for diagnosis of their relation to neurodegenerative diseases are summarized. Their applicability and future prospects in this field are discussed with particular aim at using quantum dots as fluorescent labels. PMID:24055838

Sobrova, Pavlina; Blazkova, Iva; Chomoucka, Jana; Drbohlavova, Jana; Vaculovicova, Marketa; Kopel, Pavel; Hubalek, Jaromir; Kizek, Rene; Adam, Vojtech

2013-01-01

187

Multipolar interband absorption in a semiconductor quantum dot.

Multipolar interband absorption in a semiconductor quantum dot. II. Magnetic dipole enhancement quantum dot. We find that electric dipole and magnetic dipole transitions are exclusive and therefore can- more, for various kinds of nanostructures (quantum dots, quantum wells, quantum wires) it has been

Novotny, Lukas

188

Quantum confinement and host/guest chemistry: probing a new dimension.

Nanoparticulate metals and semiconductors that have atomic arrangements at the interface of molecular clusters and "infinite" solid-state arrays of atoms have distinctive properties determined by the extent of confinement of highly delocalized valence electrons. At this interface, the total number of atoms and the geometrical disposition of each atom can be used to significantly modify the electronic and photonic response of the medium. In addition to teh novel inherent physical properties of the quantum-confined moieties, their "packaging" into nanocomposite bulk materials can be used to define the confinement surface states and environment, intercluster interactions, the quantum-confinement geometry, and the effective charge-carrier density of the bulk. Current approaches for generating nanostructures of conducting materials are briefly reviewed, especially the use of three-dimensional crystalline superlattices as hosts for quantum-confined semiconductor atom arrays (such as quantum wires and dots) with controlled inter-quantum-structure tunneling. PMID:17771883

Stucky, G D; Mac Dougall, J E

1990-02-01

189

Semiconductor Few-Electron Quantum Dots as Spin Qubits

Semiconductor Few-Electron Quantum Dots as Spin Qubits J.M. Elzerman1,2 , R. Hanson1 , L.H.W. van the experimental steps we have taken towards using a single electron spin, trapped in a semiconductor quantum dot dot. This is done by J.M. Elzerman et al.: Semiconductor Few-Electron Quantum Dots as Spin Qubits

190

Semiconductor quantum-dot nanostructures are interesting objects for fundamental as well as practical reasons. Fundamentally, they can form the basis of systems in which to study the quantum mechanics of electrons confined in zero-dimensional (0-D) space. In practice, the dots can be embedded in the active regions of a new class of electronic and optoelectronic devices with novel functionalities. This paper

Elias Towe; Dong Pan

2000-01-01

191

Universal Dephasing Mechanism in Semiconductor Quantum Dots Embedded in a Matrix

It has been clarified that the two excitations, the small energy excitation in the two-level system, and the confined acoustic phonons, determine the low-temperature dependence of the homogeneous linewidth in CuBr and CdSe quantum dots. The former dominates the temperature dependence below 15 K for CuBr quantum dots, causing a difference in temperature dependence between NaBr and glass matrix samples.

Kazuya Takemoto; Byung-Ryool Hyun; Masashi Furuya; Michio Ikezawa; Jialong Zhao; Yasuaki Masumoto

2003-01-01

192

Quantum Dots Investigated for Solar Cells

NASA Technical Reports Server (NTRS)

The NASA Glenn Research Center has been investigating the synthesis of quantum dots of CdSe and CuInS2 for use in intermediate-bandgap solar cells. Using quantum dots in a solar cell to create an intermediate band will allow the harvesting of a much larger portion of the available solar spectrum. Theoretical studies predict a potential efficiency of 63.2 percent, which is approximately a factor of 2 better than any state-of-the-art devices available today. This technology is also applicable to thin-film devices--where it offers a potential four-fold increase in power-to-weight ratio over the state of the art. Intermediate-bandgap solar cells require that quantum dots be sandwiched in an intrinsic region between the photovoltaic solar cell's ordinary p- and n-type regions (see the preceding figure). The quantum dots form the intermediate band of discrete states that allow sub-bandgap energies to be absorbed. However, when the current is extracted, it is limited by the bandgap, not the individual photon energies. The energy states of the quantum dot can be controlled by controlling the size of the dot. Ironically, the ground-state energy levels are inversely proportional to the size of the quantum dots. We have prepared a variety of quantum dots using the typical organometallic synthesis routes pioneered by Ba Wendi et al., in the early 1990's. The most studied quantum dots prepared by this method have been of CdSe. To produce these dots, researchers inject a syringe of the desired organometallic precursors into heated triocytlphosphine oxide (TOPO) that has been vigorously stirred under an inert atmosphere (see the following figure). The solution immediately begins to change from colorless to yellow, then orange and red/brown, as the quantum dots increase in size. When the desired size is reached, the heat is removed from the flask. Quantum dots of different sizes can be identified by placing them under a "black light" and observing the various color differences in their fluorescence (see the photograph).

Bailey, Sheila G.; Castro, Stephanie L.; Raffaelle, Ryne P.; Hepp, Aloysius F.

2001-01-01

193

Quantum confinement-induced tunable exciton states in graphene oxide.

Graphene oxide has recently been considered to be a potential replacement for cadmium-based quantum dots due to its expected high fluorescence. Although previously reported, the origin of the luminescence in graphene oxide is still controversial. Here, we report the presence of core/valence excitons in graphene-based materials, a basic ingredient for optical devices, induced by quantum confinement. Electron confinement in the unreacted graphitic regions of graphene oxide was probed by high resolution X-ray absorption near edge structure spectroscopy and first-principles calculations. Using experiments and simulations, we were able to tune the core/valence exciton energy by manipulating the size of graphitic regions through the degree of oxidation. The binding energy of an exciton in highly oxidized graphene oxide is similar to that in organic electroluminescent materials. These results open the possibility of graphene oxide-based optoelectronic device technology. PMID:23872608

Lee, Dongwook; Seo, Jiwon; Zhu, Xi; Lee, Jiyoul; Shin, Hyeon-Jin; Cole, Jacqueline M; Shin, Taeho; Lee, Jaichan; Lee, Hangil; Su, Haibin

2013-01-01

194

Quantum confinement-induced tunable exciton states in graphene oxide

NASA Astrophysics Data System (ADS)

Graphene oxide has recently been considered to be a potential replacement for cadmium-based quantum dots due to its expected high fluorescence. Although previously reported, the origin of the luminescence in graphene oxide is still controversial. Here, we report the presence of core/valence excitons in graphene-based materials, a basic ingredient for optical devices, induced by quantum confinement. Electron confinement in the unreacted graphitic regions of graphene oxide was probed by high resolution X-ray absorption near edge structure spectroscopy and first-principles calculations. Using experiments and simulations, we were able to tune the core/valence exciton energy by manipulating the size of graphitic regions through the degree of oxidation. The binding energy of an exciton in highly oxidized graphene oxide is similar to that in organic electroluminescent materials. These results open the possibility of graphene oxide-based optoelectronic device technology.

Lee, Dongwook; Seo, Jiwon; Zhu, Xi; Lee, Jiyoul; Shin, Hyeon-Jin; Cole, Jacqueline M.; Shin, Taeho; Lee, Jaichan; Lee, Hangil; Su, Haibin

2013-07-01

195

Giant Stark effect in the emission of single semiconductor quantum dots

We study the quantum-confined Stark effect in single InAs/GaAs quantum dots embedded within a AlGaAs/GaAs/AlGaAs quantum well. By significantly increasing the barrier height we can observe emission from a dot at electric fields of -500 kV/cm, leading to Stark shifts of up to 25 meV. Our results suggest this technique may enable future applications that require self-assembled dots with transitions at the same energy.

Bennett, Anthony J; Skiba-Szymanska, Joanna; Nicoll, Christine A; Farrer, Ian; Ritchie, David A; Shields, Andrew J

2012-01-01

196

Resonant Raman scattering on self-assembled GaN quantum dots

Self-assembled GaN quantum dots grown on Al0.15Ga0.85N using Si as antisurfactant have been investigated by resonant Raman scattering. Phonons of GaN quantum dots of different sizes and the Al0.15Ga0.85N barrier layer were probed selectively by varying the laser excitation energy from 3.53 to 5.08 eV. Phonon confinement effects were studied on GaN quantum dots of 2-3 nm height. We show

M. Kuball; J. Gleize; Satoru Tanaka; Yoshinobu Aoyagi

2001-01-01

197

Excited-state spectroscopy on a quantum dot side coupled to a quantum wire

.1063/1.2987424 Semiconductor quantum dots are promising candidates for quantum bits.1 In a conventional dot with two leadsExcited-state spectroscopy on a quantum dot side coupled to a quantum wire T. Otsuka,a E. Abe, Y report excited-state spectroscopy on a quantum dot side coupled to a quantum wire with accurate energy

Iye, Yasuhiro

198

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

199

Photodetectors based on colloidal quantum dots

Inspired by recent work demonstrating photocurrent enhancement in quantum-dot (QD) solids via post-deposition chemical annealing and by recent successes incorporating single monolayers of QDs in light-emitting devices ...

Oertel, David C. (David Charles)

2007-01-01

200

Synthesis and characterization of infrared quantum dots

This thesis focuses on the development of synthetic methods to create application ready quantum dots (QDs) in the infrared for biological imaging and optoelectronic devices. I concentrated primarily on controlling the size ...

Harris, Daniel Kelly

2014-01-01

201

Surface enhanced Raman scattering by CdS quantum dots

Surface enhanced Raman scattering is studied in nanostructures with CdS quantum dots formed using the Langmuir-Blodgett technology.\\u000a Features due to quantum dot longitudinal optical phonons are observed in the Raman spectra of both free CdS quantum dots and\\u000a such dots distributed in an organic matrix. The surface enhanced Raman scattering by nanostructures with CdS quantum dots\\u000a covered by an Ag

A. G. Milekhin; L. L. Sveshnikova; T. A. Duda; N. V. Surovtsev; S. V. Adichtchev; D. R. T. Zahn

2008-01-01

202

Single electron-spin memory with a semiconductor quantum dot

We show storage of the circular polarisation of an optical field, transferring it to the spin-state of an individual electron confined in a single semiconductor quantum dot. The state is subsequently readout through the electronically-triggered emission of a single photon. The emitted photon shares the same polarisation as the initial pulse but has a different energy, making the transfer of quantum information between different physical systems possible. With an applied magnetic field of 2 Tesla, spin memory is preserved for at least 1000 times more than the exciton's radiative lifetime.

Robert J. Young; Samuel J. Dewhurst; R. Mark Stevenson; Paola Atkinson; Anthony J. Bennett; Martin B. Ward; Ken Cooper; David A. Ritchie; Andrew J. Shields

2007-06-14

203

Spectroscopic characterization of streptavidin functionalized quantum dots

The spectroscopic properties of quantum dots can be strongly influenced by the conditions of their synthesis. In this work, we have characterized several spectroscopic properties of commercial, streptavidin functionalized quantum dots (QD525, lot 1005-0045, and QD585, lot 0905-0031, from Invitrogen). This is the first step in the development of calibration beads to be used in a generalizable quantification scheme of

Yang Wu; Gabriel P. Lopez; Larry A. Sklar; Tione Buranda

2007-01-01

204

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. PMID:22801641

Korala, Lasantha; Li, Li; Brock, Stephanie L

2012-09-01

205

Resonance fluorescence of semiconductor quantum dots: Signatures of the electron-phonon interaction

Using a fully quantized description of strongly confined electrons interacting with acoustic phonons and the photon field, the nonstationary resonance-fluorescence spectra of a semiconductor quantum dot are investigated. For excitation pulses with durations approaching typical electron-phonon scattering times, the virtual quantum processes yield an observable electron-phonon sideband broadening.

K. J. Ahn; J. Förstner; A. Knorr

2005-01-01

206

Magneto-biexciton in a group III-V diluted magnetic quantum dot

NASA Astrophysics Data System (ADS)

Binding energies of biexciton are computed in a GaMnAs/GaAlAs quantum dot with the effects of geometrical confinement, exchange interaction between the charge carrier and the magnetic impurities and the magnetic field. The size dependence of the binding energy of the biexciton in the presence of magnetic field is brought out. The optical transition energy, in the presence of magnetic field strength, is discussed for various magnetic impurities in a GaMnxAs quantum dot. Numerical calculations are performed using variational technique. The spin polaronic energy of the biexciton with the effect of spatial confinement is carried out taking into account the mean field approximation in the presence of magnetic field strength. The magnetization of magnetic ion impurities as a function of dot radius is computed in a GaMn0.02As quantum dot. The effective g-factor as a function of spatial confinement is found in the GaMnAs quantum dot. The results show that the shift in spin polaron has more influence for the larger dot radius and the nonlinearity to linear behavior of g-factor for a particular dot size is achieved due to the sign reversal of Zeeman splitting.

Lalitha, D.; Peter, A. John; Lee, Chang Woo

2014-07-01

207

We present evidence of cavity quantum electrodynamics from a sparse density of strongly quantum-confined Pb-chalcogenide nanocrystals (between 1 and 10) approaching single-dot levels on moderately high-Q mesoscopic silicon optical cavities. Operating at important near-infrared (1500-nm) wavelengths, large enhancements are observed from devices and strong modifications of the QD emission are achieved. Saturation spectroscopy of coupled QDs is observed at 77K, highlighting the modified nanocrystal dynamics for quantum information processing. PMID:20052171

Bose, Ranojoy; Gao, Jie; McMillan, James F; Williams, Alex D; Wong, Chee Wei

2009-12-01

208

Thermoelectric transport through strongly correlated quantum dots

The thermoelectric properties of strongly correlated quantum dots, described by a single level Anderson model coupled to conduction electron leads, is investigated using Wilson's numerical renormalization group method. We calculate the electronic contribution, $K_{\\rm e}$, to the thermal conductance, the thermopower, $S$, and the electrical conductance, $G$, of a quantum dot as a function of both temperature, $T$, and gate voltage, ${\\rm v}_g$, for strong, intermediate and weak Coulomb correlations, $U$, on the dot. For strong correlations and in the Kondo regime, we find that the thermopower exhibits two sign changes, at temperatures $T_{1}({\\rm v}_g)$ and $T_{2}({\\rm v}_g)$ with $T_{1}< T_{2}$. Such sign changes in $S(T)$ are particularly sensitive signatures of strong correlations and Kondo physics. The relevance of this to recent thermopower measurements of Kondo correlated quantum dots is discussed. We discuss the figure of merit, power factor and the degree of violation of the Wiedemann-Franz law in quantum dots. The extent of temperature scaling in the thermopower and thermal conductance of quantum dots in the Kondo regime is also assessed.

T. A. Costi; V. Zlatic

2010-04-09

209

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

210

Electron properties in directed self-assembly Ge/SiC/Si quantum dots

NASA Astrophysics Data System (ADS)

Artificially ordered semiconductor quantum dot (QD) patterns may be used to implement functionalities such as spintronic bandgap systems, quantum simulation and quantum computing, by manipulating the interaction between confined carriers via direct exchange coupling. In this dissertation, magnetotransport measurements have been conducted to investigate the electronic orbital and spin states of directed self-assembly single- and few-Ge/SiC/Si QD devices, fabricated by a directed self-assembly QD growth technique developed by our group. Diamagnetic and Zeeman energy shifts of electrons confined around the QD have been observed from the magnetotransport experiments. A triple-barrier resonant tunneling model has been proposed to describe the electron and spin transport. The strength of the Coulomb interaction between electrons confined at neighboring QDs has been observed dependent on the dot separation, and represents an important parameter for fabricating QD-based molecules and artificial arrays, which may be implemented as building blocks for future quantum simulation and quantum computing architectures.

Yang, Dongyue

211

Multi-band silicon quantum dots embedded in an amorphous matrix of silicon carbide.

Silicon quantum dots embedded in an amorphous matrix of silicon carbide were realized by a magnetron co-sputtering process and post-annealing. X-ray photoelectron spectroscopy, glancing x-ray diffraction, Raman spectroscopy and high-resolution transmission electron microscopy were used to characterize the chemical composition and the microstructural properties. The results show that the sizes and size distribution of silicon quantum dots can be tuned by changing the annealing atmosphere and the atom ratio of silicon and carbon in the matrix. A physicochemical mechanism is proposed to demonstrate this formation process. Photoluminescence measurements indicate a multi-band configuration due to the quantum confinement effect of silicon quantum dots with different sizes. The PL spectra are further widened as a result of the existence of amorphous silicon quantum dots. This multi-band configuration would be extremely advantageous in improving the photoelectric conversion efficiency of photovoltaic solar cells. PMID:20975214

Chang, Geng-rong; Ma, Fei; Ma, Da-yan; Xu, Ke-wei

2010-11-19

212

Two Exciton States in Semiconductor Quantum Dots

NASA Astrophysics Data System (ADS)

In a semiconductor quantum dot (QD), the conventional biexciton is formed by the coulomb attraction between two excitons. The interaction of two excitons can also create two-exciton states in a QD that in higher dimensional systems are unbound and part of the continuum. However, the confinement potential of the QD binds these excitons into an interaction that creates a biexciton whose energy is greater than the non-interacting two-exciton energy. We report here a nonlinear spectral signature consistent with the creation of an interdot ``anti-bound'' biexciton in GaAs interface fluctuation QDs obtained by high resolution nonlinear spectroscopy. These measurements show evidence of a coupled excitonic system distinct from both the conventional and the intradot ``anti-bound'' biexcitons [1,2] but similar to two interacting separately localized excitons. This work was supported by the ARO, AFOSR and ONR. [1] Y.Z. Hu, et al. Phys. Rev. Lett. 64, 1805 (1990). [2] H. Kamada, et al. Phys. Rev. B 58, 16243 (1998).

Batteh, E. T.; Chen, Gang; Cheng, Jun; Steel, D. G.; Gammon, D.; Katzer, D. S.; Park, D.; Chen, Pochung; Piermarocchi, C.; Sham, L. J.

2002-03-01

213

Quantum dots for light emitting diodes.

In this article we discuss the development and key advantages of quantum dot based light emitting diode (QD-LED) and other applications based on their color purity, stability, and solution processibility. Analysis of quantum dot based LEDs and the main challenges faced in this field, such as the QD luminescence quenching, QD charging in thin films, and external quantum efficiency are discussed in detail. The description about how different optical down-conversion and structures enabled researchers to overcome these challenges and to commercialize the products. The recent developments about how to overcome these difficulties have also been discussed in this article. PMID:23858829

Qasim, Khan; Lei, Wei; Li, Qing

2013-05-01

214

in Self-Assembled Semiconductor Quantum Dots R. J. Warburton,1 C. S. DÃ¼rr,1 K. Karrai,1 J. P. Kotthaus,1.40.Rw, 78.66.Fd A self-assembled semiconductor quantum dot confines electrons and holes on length scales photoluminescence excitation (PLE), have quite clearly yielded significant data on semiconductor quantum dots

Ludwig-Maximilians-UniversitÃ¤t, MÃ¼nchen

215

Intersublevel spectroscopy on single InAs-quantum dots by terahertz near-field microscopy.

Using scattering-type near-field infrared microscopy in combination with a free-electron laser, intersublevel transitions in buried single InAs quantum dots are investigated. The experiments are performed at room temperature on doped self-assembled quantum dots capped with a 70 nm GaAs layer. Clear near-field contrast of single dots is observed when the photon energy of the incident beam matches intersublevel transition energies, namely the p-d and s-d transition of conduction band electrons confined in the dots. The observed room-temperature line width of 5-8 meV of these resonances in the mid-infrared range is significantly below the inhomogeneously broadened spectral lines of quantum dot ensembles. The experiment highlights the strength of near-field microspectroscopy by demonstrating signals from bound-to-bound transitions of single electrons in a probe volume of the order of (100 nm)(3). PMID:22775149

Jacob, Rainer; Winnerl, Stephan; Fehrenbacher, Markus; Bhattacharyya, Jayeeta; Schneider, Harald; Wenzel, Marc Tobias; Ribbeck, Hans-Georg von; Eng, Lukas M; Atkinson, Paola; Schmidt, Oliver G; Helm, Manfred

2012-08-01

216

Qubit Protection in Nuclear-Spin Quantum Dot Memories

We present a mechanism to protect quantum information stored in an ensemble of nuclear spins in a semiconductor quantum dot. When the dot is charged the nuclei interact with the spin of the excess electron through the ...

Taylor, J. M.

217

Electron tunneling and spin relaxation in a lateral quantum dot

We report measurements that use real-time charge sensing to probe a single-electron lateral quantum dot. The charge sensor is a quantum point contact (QPC) adjacent to the dot and the sensitivity is comparable to other ...

Amasha, Sami

2008-01-01

218

Using resonantly excited photoluminescence along with photoluminescence excitation spectroscopies, we study the carrier excitation processes in CdTe/ZnTe and CdSe/ZnSe self-assembled quantum dots. Photoluminescence excitation spectra of single CdTe quantum dots reflect two major mechanisms for carrier excitation: The first, associated with the presence of sharp and intense lines in the spectrum, is a direct excited state ? ground state transition. The second, associated with the appearance of up to four much broader excitation lines, is a LO phonon-assisted absorption directly into the quantum dot ground states. LO phonons with energies of both quantum dots and ZnTe barrier material are identified in the photoluminescence excitation spectra. Resonantly excited PL measurements for the dot ensemble as a function of excitation energy makes it possible to separate the contributions of these two mechanisms. We find that for CdTe quantum dots the distribution of excited states coupled to the ground states reflects the energy distribution of the quantum dot emission, but shifted up in energy by 100 meV. This large splitting between excited and ground states in CdTe quantum dots suggests strong spatial confinement. In contrast, the LO phonon-assisted absorption shows significant size selectivity. In the case of CdTe dots the exciton-LO phonon coupling is strongly enhanced for smaller-sized dots which have higher emission energies. In contrast, for CdSe quantum dots the exciton-LO phonon coupling is uniform over the ensemble ? that is, the energy distribution determines the intensities of LO phonon replicas. We show that for CdTe quantum dots after annealing, that is after an increase in the average dot size, the exciton-LO phonon interaction reflects the dot energy distribution, as observed for CdSe quantum dots.

T. A. Nguyen; S. Mackowski; H. E. Jackson; L. M. Smith; J. Wrobel; K. Fronc; G. Karczewski; J. Kossut; M. Dobrowolska J. K. Furdyna; W. Heiss

2003-09-17

219

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

220

Energy Transfer in CdSe Quantum Dot Solids

Dipole-dipole interdot interactions in close packed quantum dot solids lead to observed electronic energy transfer between close packed quantum dots. We use photoluminescence spectroscopies to study electronic energy transfer in optically thin and clear, close packed quantum dot solids prepared from samples of CdSe quantum dots tunable in size from 17-150 Åin diameter with standard deviations of <4.5%. High resolution

C. R. Kagan; C. B. Murray; M. Nirmal; M. G. Bawendi

1996-01-01

221

Mesoscopic spatiotemporal theory for quantum-dot lasers

We present a mesoscopic theory for the spatiotemporal carrier and light-field dynamics in quantum-dot lasers. Quantum-dot Maxwell-Bloch equations have been set up that mesoscopically describe the spatiotemporal light-field and interlevel\\/intralevel carrier dynamics in each quantum dot (QD) of a typical QD ensemble in quantum-dot lasers. In particular, this includes spontaneous luminescence, counterpropagation of amplified spontaneous emission, and induced recombination as

Edeltraud Gehrig; Ortwin Hess

2002-01-01

222

Sizes and fluorescence of cadmium sulfide quantum dots

NASA Astrophysics Data System (ADS)

Cadmium sulfide quantum dots have been synthesized by wet chemical deposition from an aqueous solution. The sizes of the quantum dots determined by dynamic light scattering directly in the colloidal solution and by intermittent-contact atomic force microscopy in the dry sediment agree with each other. It has been found that splitting of the fluorescence peaks of the quantum dots can be affected by the disorder of the atomic structure of cadmium sulfide quantum dots.

Rempel', S. V.; Razvodov, A. A.; Nebogatikov, M. S.; Shishkina, E. V.; Shur, V. Ya.; Rempel', A. A.

2013-03-01

223

Quantum confinement of nanocrystals within amorphous matrices

NASA Astrophysics Data System (ADS)

Nanocrystals encapsulated within an amorphous matrix are computationally analyzed to quantify the degree to which the matrix modifies the nature of their quantum-confinement power—i.e., the relationship between nanocrystal size and the gap between valence- and conduction-band edges. A special geometry allows exactly the same amorphous matrix to be applied to nanocrystals of increasing size to precisely quantify changes in confinement without the noise typically associated with encapsulating structures that are different for each nanocrystal. The results both explain and quantify the degree to which amorphous matrices redshift the character of quantum confinement. The character of this confinement depends on both the type of encapsulating material and the separation distance between the nanocrystals within it. Surprisingly, the analysis also identifies a critical nanocrystal threshold below which quantum confinement is not possible—a feature unique to amorphous encapsulation. Although applied to silicon nanocrystals within an amorphous silicon matrix, the methodology can be used to accurately analyze the confinement softening of other amorphous systems as well.

Lusk, Mark T.; Collins, Reuben T.; Nourbakhsh, Zahra; Akbarzadeh, Hadi

2014-02-01

224

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.

2006-02-01

225

Quantum dot-based quantum buses for quantum computer hardware architecture

We propose a quantum bus based on semiconductor self-assembled quantum dots. This allows for transmission of qubits between the different quantum registers, and could be integrated in most of the present proposal for semiconductor quantum dot-based quantum computation.

Irene D’Amico

2006-01-01

226

Voltage-controlled motional narrowing in a semiconductor quantum dot

Voltage-controlled motional narrowing in a semiconductor quantum dot A. Berthelot,1,2 G. Cassabois.1088/1367-2630/11/9/093032 #12;Voltage-controlled motional narrowing in a semiconductor quantum dot 2 Tunneling is one of the environment- induced decoherence in a semiconductor quantum dot (QD) embedded in a gated field- effect device

Paris-Sud XI, UniversitÃ© de

227

A molecular state of correlated electrons in a quantum dot

by using inelastic light scattering16Â18 in quantum dots containing four electrons19 . Spectra of lowLETTERS A molecular state of correlated electrons in a quantum dot SOKRATIS KALLIAKOS1 , MASSIMO in a semiconductor quantum dot1Â3 , where at vanishing electron density the Coulomb interaction between electrons

Loss, Daniel

228

Controlling cavity reflectivity with a single quantum dot

on direct probing of the cavityÂquantum dot coupling, by means of resonant light scattering from stronglyLETTERS Controlling cavity reflectivity with a single quantum dot Dirk Englund1 *, Andrei Faraon1 nanocavities and semiconductor quantum dots have seen rapid progress. Recent experiments have allowed

Vuckovic, Jelena

229

Charge detection in graphene quantum dots J. Gttinger,a

oxide top layer. Electron beam lithography EBL is used for patterning the isolated graphene flakesCharge detection in graphene quantum dots J. GÃ¼ttinger,a C. Stampfer, S. HellmÃ¼ller, F. Molitor, T on a graphene quantum dot with an integrated graphene charge detector. The quantum dot device consists

Ihn, Thomas

230

NASA Astrophysics Data System (ADS)

InGaAs/GaAs self-assembled quantum dots are semiconductor nanostructures with 3-dimensional confinement. Their atomic-like density of states offers potential advantages over other semiconductor structures in optoelectronic devices and makes for an ideal semiconductor platform to study the physics of two-level systems. In this thesis, I analyze the unique properties of quantum dot lasers and discuss their potential application as optical amplifiers. I also directly measure the absorption coefficient of different quantum dot ensembles and determine an average dipole moment of 30 Debye. The carrier dynamics in the resonantly excited ground state transition are then measured using temperature dependent differential transmission spectroscopy. Ground state excitons escape from the quantum dot by multiple phonon emission with a time constant that increases from 32 ps to 130 ps as the sample temperature is reduced from 295 K to 230 K.

Silverman, Kevin Lawrence

231

Structural Origin of Enhanced Luminescence Efficiency of Antimony Irradiated InAs Quantum Dots

We report that Sb irradiation combined with the presence of a GaAs intermediate layer previous to the deposition of a GaSb layer over InAs quantum dots grown by molecular beam epitaxy improves the crystalline quality of these nanostructures. Moreover, this approach to develop III-V-Sb nanostructures causes the formation of quantum dots buried by a confining GaSb layer and, in this way, achieving a type II band alignment. Both phenomena, studied by Conventional transmission electron microscopy (CTEM) and scanning-transmission electron microscope (STEM) techniques are keys to achieve the best room temperature photoluminescence results from InAs/GaAs (001) quantum dots. The Sb flux contributes to the preservation of the quantum dots size and at the same time reduces In diffusion from the wetting layer.

Beltran, AM [Universidad de Cadiz, Spain; Ben, Teresa [Universidad de Cadiz, Spain; Sales, David [Universidad de Cadiz, Spain; Sanchez, AM [University of Warwick, UK; Ripalda, JM [Instituto de Microelectronica de Madrid (CNM, CSIC); Taboada, Alfonso G [Instituto de Microelectronica de Madrid (CNM, CSIC); Varela del Arco, Maria [ORNL; Pennycook, Stephen J [ORNL; Molina, S. I. [Universidad de Cadiz, Spain

2011-01-01

232

Sized controlled synthesis, purification, and cell studies with silicon quantum dots.

This article describes the size control synthesis of silicon quantum dots with simple microemulsion techniques. The silicon nanocrystals are small enough to be in the strong confinement regime and photoluminesce in the blue region of the visible spectrum and the emission can be tuned by changing the nanocrystal size. The silicon quantum dots were capped with allylamine either a platinum catalyst or UV-radiation. An extensive purification protocol is reported and assessed using (1)H NMR to produce ultra pure silicon quantum dots suitable for biological studies. The highly pure quantum dots were used in cellular uptake experiments and monitored using confocal microscopy. The results showed that the amine terminated silicon nanocrystals accumulated in lysosome but not in nuclei and could be used as bio-markers to monitor cancer cells over long timescales. PMID:21727983

Shiohara, Amane; Prabakar, Sujay; Faramus, Angelique; Hsu, Chia-Yen; Lai, Ping-Shan; Northcote, Peter T; Tilley, Richard D

2011-08-01

233

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

234

NASA Astrophysics Data System (ADS)

This thesis explores the understanding of the chemistry and physics of colloidal quantum dots for practical solar energy photoconversion. Solar cell devices that make use of PbS quantum dots generally rely on constant and unchanged optical properties such that band gap energies remain tuned within the device. The design and development of unique experiments to ascertain mechanisms of optical band gap shifts occurring in PbS quantum dot thin-films exposed to air are discussed. The systematic study of the absorption properties of PbS quantum dot films exposed to air, heat, and UV illumination as a function of quantum dot size has been described. A method to improve the air-stability of films with atomic layer deposition of alumina is demonstrated. Encapsulation of quantum dot films using a protective layer of alumina results in quantum dot solids that maintain tuned absorption for 1000 hours. This thesis focuses on the use of atomic force microscopy and electrical variants thereof to study the physical and electrical characteristics of quantum dot arrays. These types of studies have broad implications in understanding charge transport mechanisms and solar cell device operation, with a particular emphasis on quantum dot transistors and solar cells. Imaging the channel potential of a PbSe quantum dot thin-film in a transistor showed a uniform distribution of charge coinciding with the transistor current voltage characteristics. In a second study, solar cell device operation of ZnO/PbS heterojunction solar cells was investigated by scanning active cross-sections with Kelvin probe microscopy as a function of applied bias, illumination and device architecture. This technique directly provides operating potential and electric field profiles to characterize drift and diffusion currents occurring in the device. SKPM established a field-free region occurring in the quantum dot layer, indicative of diffusion-limited transport. These results provide the path to optimization of future architectures that may employ drift-based transport in the quantum dot layer for enhanced charge extraction and power conversion efficiency.

Ihly, Rachelle

235

Scalable quantum computer architecture with coupled donor-quantum dot qubits

A quantum bit computing architecture includes a plurality of single spin memory donor atoms embedded in a semiconductor layer, a plurality of quantum dots arranged with the semiconductor layer and aligned with the donor atoms, wherein a first voltage applied across at least one pair of the aligned quantum dot and donor atom controls a donor-quantum dot coupling. A method of performing quantum computing in a scalable architecture quantum computing apparatus includes arranging a pattern of single spin memory donor atoms in a semiconductor layer, forming a plurality of quantum dots arranged with the semiconductor layer and aligned with the donor atoms, applying a first voltage across at least one aligned pair of a quantum dot and donor atom to control a donor-quantum dot coupling, and applying a second voltage between one or more quantum dots to control a Heisenberg exchange J coupling between quantum dots and to cause transport of a single spin polarized electron between quantum dots.

Schenkel, Thomas; Lo, Cheuk Chi; Weis, Christoph; Lyon, Stephen; Tyryshkin, Alexei; Bokor, Jeffrey

2014-08-26

236

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

237

Cavity quantum electrodynamics with quantum dot - photonic crystal nanocavities

High quality factor, small mode volume photonic crystal cavities and single emitter quantum dots are the topic of this dissertation. They are studied as both a combined system with InAs quantum dots grown in the center of a 2D GaAs photonic crystal slab nanocavity as well as individually. The individual studies are concerned with passive 1D silicon photonic crystal nanobeam

Joshua R. Hendrickson

2010-01-01

238

Non-destructive Kerr rotation measurements of a single spin in a quantum dot

NASA Astrophysics Data System (ADS)

A single electron spin in a quantum dot forms a natural two state system for use in quantum information processing. The ability to measure this spin without destroying the system is an important step towards observing various quantum measurement-related phenomena. In contrast to previous experiments, we have performed non-destructive Kerr rotation measurements on a single electron spin confined in a charge-tunable semiconductor quantum dot. This measurement technique provides a means to directly probe the spin off-resonance, thus minimally disturbing the system. Energy-resolved Kerr rotation spectra demonstrate that we are probing a single electron, and also yield information about the optically-pumped spin polarization as a function of quantum dot charging. These results point the way towards quantum non-demolition measurements and optically-mediated entanglement of spins in the solid state. J. Berezovsky et al., Science Express, 9 November 2006, (10.1126/science.1133862).

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

2007-03-01

239

NASA Astrophysics Data System (ADS)

For the past fifteen years, single semiconductor quantum dots, often referred to as solid-state artificial atoms, have been at the forefront of various research direction lines for experimental quantum information science, in particular in the development of practical sources of quantum states of light. Here we review the research to date, on the tailoring of the emission properties from single quantum dots producing single photons, indistinguishable single photons and entangled photon pairs. Finally, the progress and future prospects for applications of single dots in quantum information processing is considered.

Beveratos, Alexios; Abram, Izo; Gérard, Jean-Michel; Robert-Philip, Isabelle

2014-12-01

240

Luminescence of a semiconductor quantum dot system

. \\u000a A microscopic theory is used to study photoluminescence of\\u000a semiconductor quantum dots under the influence of Coulomb and carrier-photon correlation effects\\u000a beyond the Hartree-Fock level. We investigate the emission spectrum and the decay properties of the time-resolved luminescence\\u000a from initially excited quantum dots. The influence of\\u000a the correlations is included within a cluster expansion scheme up to the singlet-doublet

N. Baer; C. Gies; J. Wiersig; F. Jahnke

2006-01-01

241

Bilayer graphene quantum dot defined by topgates

We investigate the application of nanoscale topgates on exfoliated bilayer graphene to define quantum dot devices. At temperatures below 500 mK, the conductance underneath the grounded gates is suppressed, which we attribute to nearest neighbour hopping and strain-induced piezoelectric fields. The gate-layout can thus be used to define resistive regions by tuning into the corresponding temperature range. We use this method to define a quantum dot structure in bilayer graphene showing Coulomb blockade oscillations consistent with the gate layout.

Müller, André; Kaestner, Bernd; Hohls, Frank; Weimann, Thomas; Pierz, Klaus; Schumacher, Hans W., E-mail: hans.w.schumacher@ptb.de [Physikalisch-Technische Bundesanstalt, Bundesallee 100, 38116 Braunschweig (Germany)

2014-06-21

242

Potential clinical applications of quantum dots

The use of luminescent colloidal quantum dots in biological investigations has increased dramatically over the past several years due to their unique size-dependent optical properties and recent advances in biofunctionalization. In this review, we describe the methods for generating high-quality nanocrystals and report on current and potential uses of these versatile materials. Numerous examples are provided in several key areas including cell labeling, biosensing, in vivo imaging, bimodal magnetic-luminescent imaging, and diagnostics. We also explore toxicity issues surrounding these materials and speculate about the future uses of quantum dots in a clinical setting. PMID:18686776

Medintz, Igor L; Mattoussi, Hedi; Clapp, Aaron R

2008-01-01

243

Unconventional superconductivity in double quantum dots

NASA Astrophysics Data System (ADS)

The formation of electron pairs is a prerequisite of superconductivity. The fermionic nature of electrons yields four classes of superconducting correlations with definite symmetry in spin, space, and time. Here, we suggest double quantum dots coupled to conventional s -wave superconductors in the presence of inhomogeneous magnetic fields as a model system exhibiting unconventional pairing. Due to their small number of degrees of freedom, tunable by gate voltages, quantum-dot systems are ideal to gain fundamental insight into unconventional pairing. We propose two detection schemes for unconventional superconductivity, based on either Josephson or Andreev spectroscopy.

Sothmann, Björn; Weiss, Stephan; Governale, Michele; König, Jürgen

2014-12-01

244

Low-energy trions in graphene quantum dots

NASA Astrophysics Data System (ADS)

We investigate, within the envelope function approximation, the low-energy states of trions in graphene quantum dots (QDs). The presence of valley pseudospin in graphene as an electron degree of freedom apart from spin adds convolution to the interplay between exchange symmetry and the electron-electron interaction in the trion, leading to new states of trions as well as a low-energy trion spectrum different from those in semiconductors. Due to the involvement of valley pseudospin, it is found that the low-energy spectrum is nearly degenerate and consists of states all characterized by having an antisymmetric (pseudospin) ? (spin) component in the wave function, with the spin (pseudospin) part being either singlet (triplet) or triplet (singlet), as opposed to the spectrum in a semiconductor whose ground state is known to be nondegenerate and always a spin singlet in the case of X- trions. We investigate trions in the various regimes determined by the competition between quantum confinement and electron-electron interaction, both analytically and numerically. The numerical work is performed within a variational method accounting for electron mass discontinuity across the QD edge. The result for electron-hole correlation in the trion is presented. Effects of varying quantum dot size and confinement potential strength on the trion binding energy are discussed. The "relativistic effect" on the trion due to the unique relativistic type electron energy dispersion in graphene is also examined.

Cheng, H.-C.; Lue, N.-Y.; Chen, Y.-C.; Wu, G. Y.

2014-06-01

245

Tunnel-injection GaN quantum dot ultraviolet light-emitting diodes

NASA Astrophysics Data System (ADS)

We demonstrate a GaN quantum dot ultraviolet light-emitting diode that uses tunnel injection of carriers through AlN barriers into the active region. The quantum dot heterostructure is grown by molecular beam epitaxy on AlN templates. The large lattice mismatch between GaN and AlN favors the formation of GaN quantum dots in the Stranski-Krastanov growth mode. Carrier injection by tunneling can mitigate losses incurred in hot-carrier injection in light emitting heterostructures. To achieve tunnel injection, relatively low composition AlGaN is used for n- and p-type layers to simultaneously take advantage of effective band alignment and efficient doping. The small height of the quantum dots results in short-wavelength emission and are simultaneously an effective tool to fight the reduction of oscillator strength from quantum-confined Stark effect due to polarization fields. The strong quantum confinement results in room-temperature electroluminescence peaks at 261 and 340 nm, well above the 365 nm bandgap of bulk GaN. The demonstration opens the doorway to exploit many varied features of quantum dot physics to realize high-efficiency short-wavelength light sources.

Verma, Jai; Kandaswamy, Prem Kumar; Protasenko, Vladimir; Verma, Amit; Grace Xing, Huili; Jena, Debdeep

2013-01-01

246

Tunnel-injection GaN quantum dot ultraviolet light-emitting diodes

We demonstrate a GaN quantum dot ultraviolet light-emitting diode that uses tunnel injection of carriers through AlN barriers into the active region. The quantum dot heterostructure is grown by molecular beam epitaxy on AlN templates. The large lattice mismatch between GaN and AlN favors the formation of GaN quantum dots in the Stranski-Krastanov growth mode. Carrier injection by tunneling can mitigate losses incurred in hot-carrier injection in light emitting heterostructures. To achieve tunnel injection, relatively low composition AlGaN is used for n- and p-type layers to simultaneously take advantage of effective band alignment and efficient doping. The small height of the quantum dots results in short-wavelength emission and are simultaneously an effective tool to fight the reduction of oscillator strength from quantum-confined Stark effect due to polarization fields. The strong quantum confinement results in room-temperature electroluminescence peaks at 261 and 340 nm, well above the 365 nm bandgap of bulk GaN. The demonstration opens the doorway to exploit many varied features of quantum dot physics to realize high-efficiency short-wavelength light sources.

Verma, Jai; Kandaswamy, Prem Kumar; Protasenko, Vladimir; Verma, Amit; Grace Xing, Huili; Jena, Debdeep [Department of Electrical Engineering, University of Notre Dame, Indiana 46556 (United States)] [Department of Electrical Engineering, University of Notre Dame, Indiana 46556 (United States)

2013-01-28

247

Single-dot optical emission from ultralow density well-isolated InP quantum dots

We demonstrate a straightforward way to obtain single well-isolated quantum dots emitting in the visible part of the spectrum and characterize the optical emission from single quantum dots using this method. Self-assembled InP quantum dots are grown using gas-source molecular-beam epitaxy over a wide range of InP deposition rates, using an ultralow growth rate of about 0.01 atomic monolayers/s, a quantum-dot density of 1 dot/{mu}m{sup 2} is realized. The resulting isolated InP quantum dots embedded in an InGaP matrix are individually characterized without the need for lithographical patterning and masks on the substrate. Such low-density quantum dots show excitonic emission at around 670 nm with a linewidth limited by instrument resolution. This system is applicable as a single-photon source for applications such as quantum cryptography.

Ugur, A.; Hatami, F.; Masselink, W. T. [Department of Physics, Humboldt-Universitaet zu Berlin, Newtonstrasse 15, D-12489 Berlin (Germany); Vamivakas, A. N.; Lombez, L.; Atatuere, M. [Cavendish Laboratory, University of Cambridge, JJ Thomson Avenue, Cambridge CB3 0HE (United Kingdom)

2008-10-06

248

Perspective Materials for Thermoelectric ApplicationsThermoelectric Applications O.L. Lazarenkova, A.A. Balandin Y. Bao device it makes QDC very attractive as perspective materials for thermoelectric applications. [1]. OQuantum DotQuantum Dot SuperlatticesSuperlattices asas Perspective Materials for

249

Near-field resonant energy transfer between spherical quantum dots

NASA Astrophysics Data System (ADS)

Non-radiative (Forster-type) energy transfer of an exciton between two quantum dots (QDs) plays an important role in artificial structures where semiconductor nanocrystal QDs play the role of building blocks. We theoretically study the effect of surrounding medium (e.g. dielectric substrate) on the transfer rate. Applying a simple model to describe the QDs, we demonstrate that the transfer rate can be strongly enhanced in the vicinity of a metal surface if the donor QD is excited in resonance with surface plasmons characteristic of this surface. Then the scaling law with the interdot distance becomes more complex than R-6 and the characteristic Forster radius can increase by an order of magnitude. We also show that transfer rate between two QDs is not exactly / R-6 even within the dipole-dipole approximation, in free space if the electron and hole in the dot are in the weak confinement regime

Peres, Filipa C. R.; Vasilevskiy, Mikhail I.

2014-08-01

250

NASA Astrophysics Data System (ADS)

We report on the nonlinear optical properties of a quantum dot including the Rashba spin-orbit interaction (RSOI) with external electric and magnetic fields. The effect of dot size is considered. We do not make any assumptions about the strength of the confinement. We use the numerical diagonalization of the Hamiltonian to determine the electronic structure. The confining potential is taken to be of the Woods-Saxon type. We find the effect of RSOI on nonlinear optical coefficients.

Aytekin, O.; Turgut, S.; Tomak, M.

2014-11-01

251

Quantum dots-in-a-well infrared photodetectors

Novel InAs\\/InGaAs quantum dots-in-a-well (DWELL) infrared photodetectors are reported. These detectors, in which the active region consists of InAs quantum dots embedded in an InGaAs well quantum well, represent a hybrid between a conventional quantum well infrared photodetector (QWIP) and a quantum dot infrared photodetector (QDIP). Like QDIPs, the DWELL detectors display normal incidence operation without gratings or optocouplers while

Sanjay Krishna

2005-01-01

252

Fast Electrical Control of a Quantum Dot Strongly Coupled to a Nano-resonator

The resonance frequency of an InAs quantum dot strongly coupled to a GaAs photonic crystal cavity was electrically controlled via quantum confined Stark effect. Stark shifts up to 0.3meV were achieved using a lateral Schottky electrode that created a local depletion region at the location of the quantum dot. We report switching of a probe laser coherently coupled to the cavity up to speeds as high as 150MHz, limited by the RC constant of the transmission line. The coupling rate and the magnitude of the Stark shift with electric field were investigated while coherently probing the system.

Andrei Faraon; Arka Majumdar; Hyochul Kim; Pierre Petroff; Jelena Vuckovic

2009-06-03

253

Energy levels and optical properties of GaN spherical quantum dots

NASA Astrophysics Data System (ADS)

Using the pseudopotential approach, we have undertaken a detailed study of the energy levels and optical properties of zinc-blende GaN spherical quantum dots as a function of their radius taken in the range 1-10 nm. Features such as energy levels, energy band gaps, electron and heavy hole effective masses, refractive index, dielectric constants and transverse effective charge have been investigated. The quantum confinement is found to be of major effect on all the studied quantities for quantum dot radius below 5 nm. Beyond 5 nm, all properties of interest tend towards retrieving the bulk properties.

Gueddim, A.; Eloud, T.; Messikine, N.; Bouarissa, N.

2015-01-01

254

A theory is developed for the interaction of an electromagnetic field with one-particle quantum-confined states of charge carriers in semiconductor quantum dots. It is shown that the oscillator strengths and dipole moments for the transitions involving one-particle states in quantum dots are rather large, exceeding the corresponding typical parameters of bulk semiconductor materials. In the context of dipole approximation it is established that the large optical absorption cross sections and attenuation coefficients in the quasi-zero-dimensional systems make it possible to use the systems as new efficient absorbing materials.

Pokutnii, S. I. [Mechnikov National University, Center of Science and Education (Ilyichevsk Branch) (Ukraine)], E-mail: univer@ivt.ilyichevsk.odessa.ua

2006-02-15

255

Synthesis of CdSe quantum dots for quantum dot sensitized solar cell

CdSe Quantum Dots (QDs) of size 0.85 nm were synthesized using chemical route. ZnO based Quantum Dot Sensitized Solar Cell (QDSSC) was fabricated using CdSe QDs as sensitizer. The Pre-synthesized QDs were found to be successfully adsorbed on front ZnO electrode and had potential to replace organic dyes in Dye Sensitized Solar Cells (DSSCs). The efficiency of QDSSC was obtained to be 2.06 % at AM 1.5.

Singh, Neetu, E-mail: singh.neetu1985@gmail.com; Kapoor, Avinashi [Department of Electronic Science, University of Delhi South Campus, New Delhi-110 021 (India); Kumar, Vinod [Department of Physics, University of the Free State, Bloemfontein, ZA9300 (South Africa); Mehra, R. M. [School of Engineering and Technology, Sharda University, Greater Noida-201 306, U.P. (India)

2014-04-24

256

Optical properties of quantum-dot-doped liquid scintillators

Semiconductor nanoparticles (quantum dots) were studied in the context of liquid scintillator development for upcoming neutrino experiments. The unique optical and chemical properties of quantum dots are particularly promising for the use in neutrinoless double-beta decay experiments. Liquid scintillators for large scale neutrino detectors have to meet specific requirements which are reviewed, highlighting the peculiarities of quantum-dot-doping. In this paper, we report results on laboratory-scale measurements of the attenuation length and the fluorescence properties of three commercial quantum dot samples. The results include absorbance and emission stability measurements, improvement in transparency due to filtering of the quantum dot samples, precipitation tests to isolate the quantum dots from solution and energy transfer studies with quantum dots and the fluorophore PPO. PMID:25392711

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

2014-01-01

257

Ab initio study of ZnSe and CdTe semiconductor quantum dots

NASA Astrophysics Data System (ADS)

We apply first principles density functional and time dependent density functional computational methods to study the structures, densities of states, absorption spectra, and optical gaps of spherical ZnSe and CdTe semiconductor quantum dots in the size range 0.3-2.2 nm. Our calculations are performed in real space without an explicit basis. The surfaces of the quantum dots are passivated with partially charged hydrogen atoms. We find that this passivation technique effectively removes the electronic states associated with the surface atoms from the gaps of ZnSe and CdTe nanocrystals, but does not affect the energies of the quantum-confined electronic states. Our study shows that the absorption gaps of ZnSe and CdTe quantum dots decrease with increasing dot diameter. The size-dependent variations of the computed optical gaps in surface-passivated ZnSe and CdTe quantum dots are found to be consistent with the effects of quantum confinement observed in group IV and group III-V semiconductor nanocrystals.

Alnemrat, Sufian; Ho Park, Young; Vasiliev, Igor

2014-03-01

258

The exciton binding energy in a cylindrical quantum dot with Pöschl-Teller (PT) confining potential is studied with the use of the effective mass approximation and a variational calculation procedure. A GaAs prototype structure is particularly considered and different geometric configurations are taken into account by modifying the pair of parameters associated with the PT potential. We show that there is

M. E. Mora-Ramos; M. G. Barseghyan; C. A. Duque

2010-01-01

259

The exciton binding energy in a cylindrical quantum dot with Pöschl–Teller (PT) confining potential is studied with the use of the effective mass approximation and a variational calculation procedure. A GaAs prototype structure is particularly considered and different geometric configurations are taken into account by modifying the pair of parameters associated with the PT potential. We show that there is

M. E. Mora-Ramos; M. G. Barseghyan; C. A. Duque

2010-01-01

260

Tuning the optical properties of dilute nitride site controlled quantum dots

NASA Astrophysics Data System (ADS)

We show that deterministic control of the properties of pyramidal site-controlled quantum dots (QD) could be achieved by exposing the QD layer to nitrogen precursor unsymmetrical dimethylhydrazine (UDMHy). The properties that could be tuned include an expected emission reduction in dilute nitride materials, excitonic pattern (biexciton binding energy) and improved carrier confinement potential symmetry (reduced fine-structure splitting).

Juska, G.; Dimastrodonato, V.; Mereni, L. O.; Gocalinska, A.; Pelucchi, E.

2013-12-01

261

Reordering orbitals of semiconductor multi-shell quantum dot-quantum well heteronanocrystals

Reordering orbitals of semiconductor multi-shell quantum dot-quantum well heteronanocrystals Mehmet://jap.aip.org/about/rights_and_permissions #12;Reordering orbitals of semiconductor multi-shell quantum dot-quantum well heteronanocrystals in semiconductor quantum dots (QDs), which enables strongly size-dependent control of opti- cal properties

Demir, Hilmi Volkan

262

Electronic transient processes and optical spectra in quantum dots for quantum computing

Quantum dot systems are studied theoretically from the point of view of realization of quantum bit using the orbital state of electronic motion in a quantum dot. Attention is paid to several effects which can influence significantly the application of quantum dot electronic orbital states in quantum computing, for example, the effect of upconversion of the population and the incomplete

Karel Král; P. Zdenek; Z. Khas

2004-01-01

263

All quantum dot based femtosecond VECSEL

Using quantum well gain materials, ultrafast VECSELs have achieved higher output powers (2.1 W) and shorter pulses (60 fs) than any other semiconductor laser. Quantum dot (QD) gain materials offer a larger inhomogeneously broadened bandwidth, potentially supporting shorter pulse durations. We demonstrate the first femtosecond QD-based VECSEL using a QD-SESAM for modelocking, obtaining 63 mW at 3.2 GHz in 780-fs

M. Hoffmann; O. D. Sieber; W. P. Pallmann; V. J. Wittwer; I. L. Krestnikov; S. S. Mikhrin; D. A. Livshits; G. Malcolm; Y. Barbarin; T. Südmeyer; U. Keller

2011-01-01

264

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

265

Spin Relaxation Quenching in Semiconductor Quantum Dots

We have studied the spin dynamics in self-organized InAs\\/GaAs quantum dots by time-resolved photoluminescence performed under strictly resonant excitation. At low temperature, we observe strictly no decay of both the linear and the circular luminescence polarization. This demonstrates that the carrier spins are totally frozen on the exciton lifetime scale.

M. Paillard; X. Marie; P. Renucci; T. Amand; A. Jbeli; J. M. Gérard

2001-01-01

266

Excitonic Polarons in Semiconductor Quantum Dots

The discretization of the electronic spectrum in semiconductor quantum dots implies a strong coupling behavior between the optical phonons and the electron-hole pairs, despite the fact that a pair is electrically neutral. The excitonic polarons strongly modify the optical spectra. In particular, the ground excitonic polaron contains one or two phonon components, which leads to the existence of phonon replicas

O. Verzelen; R. Ferreira; G. Bastard

2002-01-01

267

New small quantum dots for neuroscience

NASA Astrophysics Data System (ADS)

In "New Small Quantum Dots for Neuroscience," Paul Selvin (University of Illinois, Urbana-Champaign) notes how the details of synapsis activity in the brain involves chemical receptors that facilitate the creation of the electrical connection between two nerves. In order to understand the details of this neuroscience phenomenon you need to be able to "see" what is happening at the scale of these receptors, which is around 10 nanometers. This is smaller than the diffraction limit of normal microscopy and it takes place on a 3 dimensional structure. Selvin describes the development of small quantum dots (on the order of 6-9 microns) that are surface-sensitized to interact with the receptors. This allows the application of photo-activated localized microscopy (PALM), a superresolution microscopy that can be scanned through focus to develop a 3D map on a scale that is the same size as the emitter, which in this case are the small quantum dots. The quantum dots are stable in time and provide access to the receptors which allows the imaging of the interactions taking place at the synoptic level.

Selvin, Paul

2014-03-01

268

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

269

Producing Quantum Dots by Spray Pyrolysis

NASA Technical Reports Server (NTRS)

An improved process for making nanocrystallites, commonly denoted quantum dots (QDs), is based on spray pyrolysis. Unlike the process used heretofore, the improved process is amenable to mass production of either passivated or non-passivated QDs, with computer control to ensure near uniformity of size.

Banger, Kulbinder; Jin, Michael H.; Hepp, Aloysius

2006-01-01

270

NASA Astrophysics Data System (ADS)

We report on the formation of Ge/Si quantum dots with core/shell structure that are arranged in a three-dimensional body centered tetragonal quantum dot lattice in an amorphous alumina matrix. The material is prepared by magnetron sputtering deposition of Al2O3/Ge/Si multilayer. The inversion of Ge and Si in the deposition sequence results in the formation of thin Si/Ge layers instead of the dots. Both materials show an atomically sharp interface between the Ge and Si parts of the dots and layers. They have an amorphous internal structure that can be crystallized by an annealing treatment. The light absorption properties of these complex materials are significantly different compared to films that form quantum dot lattices of the pure Ge, Si or a solid solution of GeSi. They show a strong narrow absorption peak that characterizes a type II confinement in accordance with theoretical predictions. The prepared materials are promising for application in quantum dot solar cells.

Buljan, M.; Radi?, N.; Sancho-Paramon, J.; Janicki, V.; Grenzer, J.; Bogdanovi?-Radovi?, I.; Siketi?, Z.; Ivanda, M.; Utrobi?i?, A.; Hübner, R.; Weidauer, R.; Valeš, V.; Endres, J.; Car, T.; Jer?inovi?, M.; Roško, J.; Bernstorff, S.; Holy, V.

2015-02-01

271

We report on the formation of Ge/Si quantum dots with core/shell structure that are arranged in a three-dimensional body centered tetragonal quantum dot lattice in an amorphous alumina matrix. The material is prepared by magnetron sputtering deposition of Al2O3/Ge/Si multilayer. The inversion of Ge and Si in the deposition sequence results in the formation of thin Si/Ge layers instead of the dots. Both materials show an atomically sharp interface between the Ge and Si parts of the dots and layers. They have an amorphous internal structure that can be crystallized by an annealing treatment. The light absorption properties of these complex materials are significantly different compared to films that form quantum dot lattices of the pure Ge, Si or a solid solution of GeSi. They show a strong narrow absorption peak that characterizes a type II confinement in accordance with theoretical predictions. The prepared materials are promising for application in quantum dot solar cells. PMID:25605224

Buljan, M; Radi?, N; Sancho-Paramon, J; Janicki, V; Grenzer, J; Bogdanovi?-Radovi?, I; Siketi?, Z; Ivanda, M; Utrobi?i?, A; Hübner, R; Weidauer, R; Valeš, V; Endres, J; Car, T; Jer?inovi?, M; Roško, J; Bernstorff, S; Holy, V

2015-02-13

272

Progress towards single spin optoelectronics using quantum dot nanostructures

We summarise recent progress in our understanding of the physics of fundamental charge and spin excitations in quantum dot semiconductor nanostructures. Many novel potential applications of these nanostructures have arisen from the strong optical non-linearities that exist in the few-particle quantum dot absorption spectrum. By comparison, the interaction of the electron spin with other localised charges in the dot and

Domink Heiss; Miro Kroutvar; Jonathan J. Finley; Gerhard Abstreiter

2005-01-01

273

Quantum Cascade of Photons in Semiconductor Quantum Dots

We have obtained pairs of correlated single photons from the emission cascade of an isolated InAs quantum dot. The cross-correlation function of the two photons in a pair exhibits the coexistence of asymmetric bunching and antibunching features, which is the signature for their sequential emission with a definite order. This observation opens the way to the use of semiconductor quantum

E. Moreau; I. Robert; L. Manin; V. Thierry-Mieg; J. M. Gérard; I. Abram

2001-01-01

274

Quantum criticality in a double-quantum-dot system.

We discuss the realization of the quantum-critical non-Fermi-liquid state, originally discovered within the two-impurity Kondo model, in double-quantum-dot systems. Contrary to common belief, the corresponding fixed point is robust against particle-hole and various other asymmetries and is unstable only to charge transfer between the two dots. We propose an experimental setup where such charge transfer processes are suppressed, allowing a controlled approach to the quantum-critical state. We also discuss transport and scaling properties in the vicinity of the critical point. PMID:17155422

Zaránd, Gergely; Chung, Chung-Hou; Simon, Pascal; Vojta, Matthias

2006-10-20

275

Elastic light scattering by semiconductor quantum dots of arbitrary shape

Elastic light scattering by low-dimensional quantum objects without a change in the frequency is theoretically investigated in terms of the quantum perturbation theory. The differential cross section of resonance light scattering from any excitons in any quantum dots is calculated. It is demonstrated that, when the light wavelengths considerably exceed the quantum-dot size, the polarization and angular distribution of the

I. G. Lang; L. I. Korovin; S. T. Pavlov

2007-01-01

276

Control of electron spin-orbit anisotropy in pyramidal InAs quantum dots

NASA Astrophysics Data System (ADS)

We investigate the electron spin-orbit interaction anisotropy of pyramidal InAs quantum dots using a fully three-dimensional Hamiltonian. The dependence of the spin-orbit interaction strength on the orientation of externally applied in-plane magnetic fields is consistent with recent experiments, and it can be explained from the interplay between Rashba and Dresselhaus spin-orbit terms in dots with asymmetric confinement. Based on this, we propose manipulating the dot composition and height as efficient means for controlling the spin-orbit anisotropy.

Segarra, C.; Planelles, J.; Climente, J. I.

2015-02-01

277

Structural analysis of strained quantum dots using nuclear magnetic resonance

NASA Astrophysics Data System (ADS)

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 × 105 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.

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.

2012-10-01

278

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

2012-10-01

279

Longitudinal wave function control in single quantum dots with an applied magnetic field

Controlling single-particle wave functions in single semiconductor quantum dots is in demand to implement solid-state quantum information processing and spintronics. Normally, particle wave functions can be tuned transversely by an perpendicular magnetic field. We report a longitudinal wave function control in single quantum dots with a magnetic field. For a pure InAs quantum dot with a shape of pyramid or truncated pyramid, the hole wave function always occupies the base because of the less confinement at base, which induces a permanent dipole oriented from base to apex. With applying magnetic field along the base-apex direction, the hole wave function shrinks in the base plane. Because of the linear changing of the confinement for hole wave function from base to apex, the center of effective mass moves up during shrinking process. Due to the uniform confine potential for electrons, the center of effective mass of electrons does not move much, which results in a permanent dipole moment change and an inverted electron-hole alignment along the magnetic field direction. Manipulating the wave function longitudinally not only provides an alternative way to control the charge distribution with magnetic field but also a new method to tune electron-hole interaction in single quantum dots. PMID:25624018

Cao, Shuo; Tang, Jing; Gao, Yunan; Sun, Yue; Qiu, Kangsheng; Zhao, Yanhui; He, Min; Shi, Jin-An; Gu, Lin; Williams, David A.; Sheng, Weidong; Jin, Kuijuan; Xu, Xiulai

2015-01-01

280

Longitudinal wave function control in single quantum dots with an applied magnetic field.

Controlling single-particle wave functions in single semiconductor quantum dots is in demand to implement solid-state quantum information processing and spintronics. Normally, particle wave functions can be tuned transversely by an perpendicular magnetic field. We report a longitudinal wave function control in single quantum dots with a magnetic field. For a pure InAs quantum dot with a shape of pyramid or truncated pyramid, the hole wave function always occupies the base because of the less confinement at base, which induces a permanent dipole oriented from base to apex. With applying magnetic field along the base-apex direction, the hole wave function shrinks in the base plane. Because of the linear changing of the confinement for hole wave function from base to apex, the center of effective mass moves up during shrinking process. Due to the uniform confine potential for electrons, the center of effective mass of electrons does not move much, which results in a permanent dipole moment change and an inverted electron-hole alignment along the magnetic field direction. Manipulating the wave function longitudinally not only provides an alternative way to control the charge distribution with magnetic field but also a new method to tune electron-hole interaction in single quantum dots. PMID:25624018

Cao, Shuo; Tang, Jing; Gao, Yunan; Sun, Yue; Qiu, Kangsheng; Zhao, Yanhui; He, Min; Shi, Jin-An; Gu, Lin; Williams, David A; Sheng, Weidong; Jin, Kuijuan; Xu, Xiulai

2015-01-01

281

Longitudinal wave function control in single quantum dots with an applied magnetic field

Controlling single-particle wave functions in single semiconductor quantum dots is in demand to implement solid-state quantum information processing and spintronics. Normally, particle wave functions can be tuned transversely by an perpendicular magnetic field. We report a longitudinal wave function control in single quantum dots with a magnetic field. For a pure InAs quantum dot with a shape of pyramid or truncated pyramid, the hole wave function always occupies the base because of the less confinement at base, which induces a permanent dipole oriented from base to apex. With applying magnetic field along the base-apex direction, the hole wave function shrinks in the base plane. Because of the linear changing of the confinement for hole wave function from base to apex, the center of effective mass moves up during shrinking process. Due to the uniform confine potential for electrons, the center of effective mass of electrons does not move much, which results in a permanent dipole moment change and an inverted electron-hole alignment along the magnetic field direction. Manipulating the wave function longitudinally not only provides an alternative way to control the charge distribution with magnetic field but also a new method to tune electron-hole interaction in single quantum dots.

Shuo Cao; Jing Tang; Yunan Gao; Yue Sun; Kangsheng Qiu; Yanhui Zhao; Min He; Jin-An Shi; Lin Gu; David A. Williams; Weidong Sheng; Kuijuan Jin; Xiulai Xu

2015-01-29

282

Thermoelectric transport through strongly correlated quantum dots

The thermoelectric properties of strongly correlated quantum dots, described by a single level Anderson model coupled to conduction electron leads, is investigated using Wilson's numerical renormalization group method. We calculate the electronic contribution, $K_{\\rm e}$, to the thermal conductance, the thermopower, $S$, and the electrical conductance, $G$, of a quantum dot as a function of both temperature, $T$, and gate voltage, ${\\rm v}_g$, for strong, intermediate and weak Coulomb correlations, $U$, on the dot. For strong correlations and in the Kondo regime, we find that the thermopower exhibits two sign changes, at temperatures $T_{1}({\\rm v}_g)$ and $T_{2}({\\rm v}_g)$ with $T_{1}< T_{2}$. Such sign changes in $S(T)$ are particularly sensitive signatures of strong correlations and Kondo physics. The relevance of this to recent thermopower measurements of Kondo correlated quantum dots is discussed. We discuss the figure of merit, power factor and the degree of violation of the Wiedemann-Franz law in ...

Costi, T A; 10.1103/PhysRevB.81.235127

2010-01-01

283

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.

2011-01-01

284

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

2011-09-22

285

Effect of shells on photoluminescence of aqueous CdTe quantum dots

Graphical abstract: Size-tunable CdTe coated with several shells using an aqueous solution synthesis. CdTe/CdS/ZnS quantum dots exhibited high PL efficiency up to 80% which implies the promising applications for biomedical labeling. - Highlights: • CdTe quantum dots were fabricated using an aqueous synthesis. • CdS, ZnS, and CdS/ZnS shells were subsequently deposited on CdTe cores. • Outer ZnS shells provide an efficient confinement of electron and hole inside the QDs. • Inside CdS shells can reduce the strain on the QDs. • Aqueous CdTe/CdS/ZnS QDs exhibited high stability and photoluminescence efficiency of 80%. - Abstract: CdTe cores with various sizes were fabricated in aqueous solutions. Inorganic shells including CdS, ZnS, and CdS/ZnS were subsequently deposited on the cores through a similar aqueous procedure to investigate the effect of shells on the photoluminescence properties of the cores. In the case of CdTe/CdS/ZnS quantum dots, the outer ZnS shell provides an efficient confinement of electron and hole wavefunctions inside the quantum dots, while the middle CdS shell sandwiched between the CdTe core and ZnS shell can be introduced to obviously reduce the strain on the quantum dots because the lattice parameters of CdS is situated at the intermediate-level between those of CdTe and ZnS. In comparison with CdTe/ZnS core–shell quantum dots, the as-prepared water-soluble CdTe/CdS/ZnS quantum dots in our case can exhibit high photochemical stability and photoluminescence efficiency up to 80% in an aqueous solution, which implies the promising applications in the field of biomedical labeling.

Yuan, Zhimin; Yang, Ping, E-mail: mse_yangp@ujn.edu.cn

2013-07-15

286

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. PMID:21931483

Generalov, Roman; Kavaliauskiene, Simona; Westrøm, Sara; Chen, Wei; Kristensen, Solveig; Juzenas, Petras

2011-01-01

287

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 10(4) to 10(6) spins of the host nuclei in the quantum dot. Random fluctuations in the nuclear spin ensemble lead to fast spin decoherence in about ten nanoseconds. Spin-echo techniques have been used to mitigate the hyperfine interaction, but completely cancelling the effect is more attractive. In principle, polarizing all the nuclear spins can achieve this but is very difficult to realize in practice. Exploring materials with zero-spin nuclei is another option, and carbon nanotubes, graphene quantum dots and silicon have been proposed. An alternative is to use a semiconductor hole. Unlike an electron, a valence hole in a quantum dot has an atomic p orbital which conveniently goes to zero at the location of all the nuclei, massively suppressing the interaction with the nuclear spins. Furthermore, in a quantum dot with strong strain and strong quantization, the heavy hole with spin-3/2 behaves as a spin-1/2 system and spin decoherence mechanisms are weak. We demonstrate here high fidelity (about 99 per cent) initialization of a single hole spin confined to a self-assembled quantum dot by optical pumping. Our scheme works even at zero magnetic field, demonstrating a negligible hole spin hyperfine interaction. We determine a hole spin relaxation time at low field of about one millisecond. These results suggest a route to the realization of solid-state quantum networks that can intra-convert the spin state with the polarization of a photon. PMID:18216849

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

2008-01-24

288

NASA Astrophysics Data System (ADS)

In this study, we have investigated the inter-sublevel optical properties of a core/shell/well/shell spherical quantum dot (QD) with the form of quantum dot-quantum well heterostructure. In order to determine the energy eigenvalues and corresponding wave functions, the Schrödinger equation has been solved full numerically by using shooting method in the effective mass approximation for a finite confining potential. The inter-sublevel optical absorption and the oscillator strength between ground (1 s) and excited (1 p) states have been examined based on the computed energies and wave functions. Also, the effect of a hydrogenic donor impurity, located at the center of the multi-shell spherical quantum dot (MSQD), has been researched for different core radii (R1), shell thicknesses (Ts), and well widths (Tw) in certain potential. It is observed that the oscillator strengths and the absorption coefficients are strongly depend on the core radii and layer thicknesses of the MSQD.

Ta?, Hatice; ?ahin, Mehmet

2012-09-01

289

Mode Competition in Dual-Mode Quantum Dots Semiconductor Microlaser

This paper describes the modeling of quantum dots lasers with the aim of assessing the conditions for stable cw dual-mode operation when the mode separation lies in the THz range. Several possible models suited for InAs quantum dots in InP barriers are analytically evaluated, in particular quantum dots electrically coupled through a direct exchange of excitation by the wetting layer or quantum dots optically coupled through the homogeneous broadening of their optical gain. A stable dual-mode regime is shown possible in all cases when quantum dots are used as active layer whereas a gain medium of quantum well or bulk type inevitably leads to bistable behavior. The choice of a quantum dots gain medium perfectly matched the production of dual-mode lasers devoted to THz generation by photomixing.

Chusseau, Laurent; Viktorovitch, P; Letartre, Xavier

2013-01-01

290

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

291

NASA Astrophysics Data System (ADS)

This paper investigates GaAs/AlGaAs modified quantum dot nanocrystal and GaAs/AlGaAs/GaAs/AlGaAs quantum dot-quantum well heteronanocrystal. These quantum dots have been analyzed by the finite element numerical methods. Simulations carried out for state n=1, l=0, and m=0 which are original, orbital, and magnetic state of quantum numbers. The effects of variation in radius layers such as total radius, GaAs core, shell and AlGaAs barriers radius on the wavelength and emission coefficient are studied. For the best time, it has also investigated the effect of mole fraction on emission coefficient. Meanwhile, one of the problems in biological applications is alteration of the emission wavelength of a quantum dot by changing in its dimension. This problem will be resolved by changing in potential profile.

Elyasi, P.; SalmanOgli, A.

2014-05-01

292

Unveiling structural properties of self-assembled quantum dots

NASA Astrophysics Data System (ADS)

A better understanding of structural properties and growth kinetics of Stranski-Krastanov (SK) quantum dots is necessary for applying dot potential in optical and electronic applications. In fact, We have recently demonstrated theoretically and experimentally, the ability of reflection high energy electron diffraction (RHEED) tool in quantitative analysis such as extracting average dot size, facet orientation and average dot density and real time monitoring of dot size during growth .As an extended study, in this work for the first time we present the experimental evidence on onset epitaxial quantum dot average shape evolution and theoretical predictions on QD facet orientations.

Gunasekera, M.; Rajapaksha, C.; Freundlich, A.

2013-03-01

293

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

294

First-principles study of the electronic structure of CdS/ZnSe coupled quantum dots

NASA Astrophysics Data System (ADS)

We have studied the electronic structure of CdS/ZnSe coupled quantum dots, a novel heterostructure at the nanoscale. Our calculations reveal CdS/ZnSe coupled quantum dots are type II in nature where the anion p states play an important role in deciding the band offset for the highest occupied molecular orbitals (HOMO). We show that the offsets of HOMO as well as the lowest unoccupied molecular orbitals (LUMO) can be tuned by changing the sizes of the components of the coupled quantum dots, thereby providing an additional control parameter to tune the band gap and the optical properties. Our investigations also suggest that the formation of an alloy near the interface has very little influence on the band offsets, although it affects the spatial localization of the quantum states from the individual components. Comparing the influence of strain on coupled quantum dots and core-shell nanowires, we find strain practically has no role in the electronic structure of coupled quantum dots as the small effective area of the interface in a coupled quantum dot helps a large part of the structure remain free from any substantial strain. We argue that in contrast to core-shell nanowires, quantum confinement is the key parameter that controls the electronic properties of coupled quantum dots and should therefore be an ideal candidate for the design of a quantum device.

Ganguli, Nirmal; Acharya, S.; Dasgupta, I.

2014-06-01

295

In this paper the electronic structure of nanoscale ellipsoid-torus-shaped semiconductor quantum dot and quantum ring is investigated of utilizing a unified model. This three-dimensional model considers the effective one-band Hamiltonian, the position- and energy-dependent effective mass approximation and Landé factor, the finite hard wall confinement potential, and the Ben Daniel-Duke boundary conditions. It is solved numerically without any fitting parameters

Yiming Li

2003-01-01

296

This dissertation describes the study of various nonlinear optical effects in both bulk and quantum-confined semiconductors. Transverse effects in increasing absorption optical bistability are considered in bulk CdS for both single beam and wave mixing geometries. Measurement of the temporal response of BiI_3 quantum dots is described using degenerate four-wave mixing and explained theoretically. Finally, the experimental techniques developed to

Brian Patrick McGinnis

1989-01-01

297

Homogeneous Linewidths in the Optical Spectrum of a Single Gallium Arsenide Quantum Dot

The homogeneous linewidths in the photoluminescence excitation spectrum of a single, naturally formed gallium arsenide (GaAs) quantum dot have been measured with high spatial and spectral resolution. The energies and linewidths of the homogeneous spectrum provide a new perspective on the dephasing dynamics of the exciton in a quantum-confined, solid-state system. The origins of the linewidths are discussed in terms

D. Gammon; E. S. Snow; B. V. Shanabrook; D. S. Katzer

1996-01-01

298

Electric Field effects on quantum correlations in semiconductor quantum dots

We study the effect of external electric bias on the quantum correlations in the array of optically excited coupled semiconductor quantum dots. The correlations are characterized by the quantum discord and concurrence and are observed using excitonic qubits. We employ the lower bound of concurrence for thermal density matrix at different temperatures. The effect of the F\\"orster interaction on correlations will be studied. Our theoretical model detects nonvanishing quantum discord when the electric field is on while concurrence dies, ensuring the existence of nonclassical correlations as measured by the quantum discord.

S. Shojaei; M. Mahdian; R. Yousefjani

2012-05-01

299

Electric Field effects on quantum correlations in semiconductor quantum dots

We study the effect of external electric bias on the quantum correlations in the array of optically excited coupled semiconductor quantum dots. The correlations are characterized by the quantum discord and concurrence and are observed using excitonic qubits. We employ the lower bound of concurrence for thermal density matrix at different temperatures. The effect of the F\\"orster interaction on correlations will be studied. Our theoretical model detects nonvanishing quantum discord when the electric field is on while concurrence dies, ensuring the existence of nonclassical correlations as measured by the quantum discord.

Shojaei, S; Yosefjani, R

2012-01-01

300

The GaAs quantum dots in AlGaAs barriers were grown by droplet epitaxy, emitting around 700 nm in wavelength which is compatible with low cost Si based detectors. The excitation power dependent and time resolved micro-photoluminescence measurements identified optical characteristics of exciton and biexciton states which are attributed to good quantum confinements in GaAs QDs.

Ha, S.-K.; Song, J. D.; Lim, J. Y.; Choi, W. J.; Han, I. K.; Lee, J. I. [Nano Convergence Devices Center, KIST, Seoul 136-791 (Korea, Republic of); Bounouar, S.; Donatini, F.; Dang, L. S.; Poizat, J. P. [CEA/CNRS/UJF team 'Nanophysics and semiconductors', Institute Neel/CNRS-UJF, 38042 Grenoble (France); Kim, J. S. [Department of Physics, Yeungnam University, Gyeongsan 712-749 (Korea, Republic of)

2011-12-23

301

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. PMID:22937880

2012-01-01

302

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

2006-12-22

303

Small bright charged colloidal quantum dots.

Using electrochemical charge injection, the fluorescence lifetimes of negatively charged core/shell CdTe/CdSe QDs are measured as a function of core size and shell thickness. It is found that the ensemble negative trion lifetimes reach a maximum (?4.5 ns) for an intermediate shell thickness. This leads to the smallest particles (?4.5 nm) with the brightest trion to date. Single dot measurements show that the negative charge suppresses blinking and that the trion can be as bright as the exciton at room temperature. In contrast, the biexciton lifetimes remain short and exhibit only a monotonous increase with shell thickness, showing no correlation with the negative trion decays. The suppression of the Auger process in small negatively charged CdTe/CdSe quantum dots is unprecedented and a significant departure from prior results with ultrathick CdSe/CdS core/shell or dot-in-rod structures. The proposed reason for the optimum shell thickness is that the electron-hole overlap is restricted to the CdTe core while the electron is tuned to have zero kinetic energy in the core for that optimum shell thickness. The different trend of the biexciton lifetime is not explained but tentatively attributed to shorter-lived positive trions at smaller sizes. These results improve our understanding of multiexciton recombination in colloidal quantum dots and may lead to the design of bright charged QDs for more efficient light-emitting devices. PMID:24350673

Qin, Wei; Liu, Heng; Guyot-Sionnest, Philippe

2014-01-28

304

Photoluminescence of silicon quantum dots in nanospheres

NASA Astrophysics Data System (ADS)

Si quantum dots (SiQDs) based nanospheres (SiNSs) were prepared via a novel synthetic strategy. These SiNSs were demonstrated to possess unique dot spacing dependent photoluminescence (PL) up-conversion and surface dependent (N modified surface) down-converted PL. It was demonstrated that a small distance between SiQDs (<5 nm) is the necessary condition for the PL up-conversion of SiNSs, while the surface state of SiQDs will affect the maximum emission wavelength and the PL intensity. The as-prepared SiNSs feature excellent aqueous dispersibility, and their optical properties were found to be stable enough in a specified temperature and pH range.Si quantum dots (SiQDs) based nanospheres (SiNSs) were prepared via a novel synthetic strategy. These SiNSs were demonstrated to possess unique dot spacing dependent photoluminescence (PL) up-conversion and surface dependent (N modified surface) down-converted PL. It was demonstrated that a small distance between SiQDs (<5 nm) is the necessary condition for the PL up-conversion of SiNSs, while the surface state of SiQDs will affect the maximum emission wavelength and the PL intensity. The as-prepared SiNSs feature excellent aqueous dispersibility, and their optical properties were found to be stable enough in a specified temperature and pH range. Electronic supplementary information (ESI) available. See DOI: 10.1039/c2nr32375c

Zhang, Yuxiao; Han, Xiao; Zhang, Jianming; Liu, Yang; Huang, Hui; Ming, Hai; Lee, Shuit-Tong; Kang, Zhenhui

2012-11-01

305

Theory of dynamic nuclear polarization and feedback in quantum dots

An electron confined in a quantum dot interacts with its local nuclear spin environment through the hyperfine contact interaction. This interaction combined with external control and relaxation or measurement of the electron spin allows for the generation of dynamic nuclear polarization. The quantum nature of the nuclear bath, along with the interplay of coherent external fields and incoherent dynamics in these systems renders a wealth of intriguing phenomena seen in recent experiments such as electron Zeeman frequency focusing, hysteresis, and line dragging. We develop in detail a fully quantum, self-consistent theory that can be applied to such experiments and that moreover has predictive power. Our theory uses the operator sum representation formalism in order to incorporate the incoherent dynamics caused by the additional, Markovian bath, which in self-assembled dots is the vacuum field responsible for electron-hole optical recombination. The beauty of this formalism is that it reduces the complexity of the problem by encoding the joint dynamics of the external coherent and incoherent driving in an effective dynamical map that only acts on the electron spin subspace. This together with the separation of timescales in the problem allows for a tractable and analytically solvable formalism. The key role of entanglement between the electron spin and the nuclear spins in the formation of dynamic nuclear polarization naturally follows from our solution. We demonstrate the theory in detail for an optical pulsed experiment and present an in-depth discussion and physical explanation of our results.

Sophia E. Economou; Edwin Barnes

2014-04-06

306

Theory of dynamic nuclear polarization and feedback in quantum dots

NASA Astrophysics Data System (ADS)

An electron confined in a quantum dot interacts with its local nuclear spin environment through the hyperfine contact interaction. This interaction combined with external control and relaxation or measurement of the electron spin allows for the generation of dynamic nuclear polarization. The quantum nature of the nuclear bath, along with the interplay of coherent external fields and incoherent dynamics in these systems renders a wealth of intriguing phenomena seen in recent experiments such as electron Zeeman frequency focusing, hysteresis, and line dragging. We develop in detail a fully quantum, self-consistent theory that can be applied to such experiments and that moreover has predictive power. Our theory uses the operator sum representation formalism in order to incorporate the incoherent dynamics caused by the additional, Markovian bath, which in self-assembled dots is the vacuum field responsible for electron-hole optical recombination. The beauty of this formalism is that it reduces the complexity of the problem by encoding the joint dynamics of the external coherent and incoherent driving in an effective dynamical map that only acts on the electron spin subspace. This, together with the separation of time scales in the problem, allows for a tractable and analytically solvable formalism. The key role of entanglement between the electron spin and the nuclear spins in the formation of dynamic nuclear polarization naturally follows from our solution. We demonstrate the theory in detail for an optical pulsed experiment and present an in-depth discussion and physical explanation of our results.

Economou, Sophia E.; Barnes, Edwin

2014-04-01

307

Photon-mediated interaction between distant quantum dot circuits.

Engineering the interaction between light and matter is an important goal in the emerging field of quantum opto-electronics. Thanks to the use of cavity quantum electrodynamics architectures, one can envision a fully hybrid multiplexing of quantum conductors. Here we use such an architecture to couple two quantum dot circuits. Our quantum dots are separated by 200 times their own size, with no direct tunnel and electrostatic couplings between them. We demonstrate their interaction, mediated by the cavity photons. This could be used to scale up quantum bit architectures based on quantum dot circuits or simulate on-chip phonon-mediated interactions between strongly correlated electrons. PMID:23360991

Delbecq, M R; Bruhat, L E; Viennot, J J; Datta, S; Cottet, A; Kontos, T

2013-01-01

308

NASA Astrophysics Data System (ADS)

A study of previously overlooked structural and optical properties of InGaAs heterostructures grown on (111)B oriented GaAs substrates patterned with inverted 7.5-?m pitch pyramidal recesses is presented. First, the composition of the confinement barrier material (GaAs in this work) and its growth temperature are shown as some of the key parameters that determine the main quantum dot properties, including nontrivial emission energy dependence, excitonic pattern, and unusual photoluminescence energetic ordering of the InGaAs ensemble nanostructures. Second, the formation of a formerly unidentified type of InGaAs nanostructures—three corner quantum dots—is demonstrated in our structures next to the well-known ones (a quantum dot and three lateral quantum wires and quantum wells). The findings show the complexity of the pyramidal quantum dot system which strongly depends on the sample design and which should be considered when selecting highly symmetric (central) quantum dots in newly designed experimental projects.

Juska, G.; Dimastrodonato, V.; Mereni, L. O.; Chung, T. H.; Gocalinska, A.; Pelucchi, E.; Van Hattem, B.; Ediger, M.; Corfdir, P.

2014-05-01

309

Geometric spin manipulation in semiconductor quantum dots

NASA Astrophysics Data System (ADS)

We propose a method to flip the spin completely by an adiabatic transport of quantum dots. We show that it is possible to flip the spin by inducing a geometric phase on the spin state of a quantum dot. We estimate the geometric spin flip time (approximately 2 ps) which turned out to be much shorter than the experimentally reported decoherence time (approximately 100 ns) that would provide an alternative means of fliping the spin before reaching decoherence. It is important that both the Rashba coupling and the Dresselhaus coupling are present for inducing a phase necessary for spin flip. If one of them is absent, the induced phase is trivial and irrelevant for spin-flip.

Prabhakar, Sanjay; Melnik, Roderick; Inomata, Akira

2014-04-01

310

Separability and dynamical symmetry of Quantum Dots

The separability and Runge–Lenz-type dynamical symmetry of the internal dynamics of certain two-electron Quantum Dots, found by Simonovi? et al. (2003), are traced back to that of the perturbed Kepler problem. A large class of axially symmetric perturbing potentials which allow for separation in parabolic coordinates can easily be found. Apart from the 2:1 anisotropic harmonic trapping potential considered in Simonovi? and Nazmitdinov (2013), they include a constant electric field parallel to the magnetic field (Stark effect), the ring-shaped Hartmann potential, etc. The harmonic case is studied in detail. -- Highlights: • The separability of Quantum Dots is derived from that of the perturbed Kepler problem. • Harmonic perturbation with 2:1 anisotropy is separable in parabolic coordinates. • The system has a conserved Runge–Lenz type quantity.

Zhang, P.-M., E-mail: zhpm@impcas.ac.cn [Institute of Modern Physics, Chinese Academy of Sciences, Lanzhou (China); Zou, L.-P., E-mail: zoulp@impcas.ac.cn [Institute of Modern Physics, Chinese Academy of Sciences, Lanzhou (China); Horvathy, P.A., E-mail: horvathy@lmpt.univ-tours.fr [Institute of Modern Physics, Chinese Academy of Sciences, Lanzhou (China); Laboratoire de Mathématiques et de Physique Théorique, Tours University (France); Gibbons, G.W., E-mail: G.W.Gibbons@damtp.cam.ac.uk [Department of Applied Mathematics and Theoretical Physics, Cambridge University, Cambridge (United Kingdom)

2014-02-15

311

Quantum dot behavior in bilayer graphene nanoribbons.

Bilayer graphene has recently earned great attention for its unique electronic properties and commendable use in electronic applications. Here, we report the observation of quantum dot (QD) behaviors in bilayer graphene nanoribbons (BL-GNRs). The periodic Coulomb oscillations indicate the formation of a single quantum dot within the BL-GNR because of the broad distribution function of the carrier concentration fluctuation at the charge neutrality point. The size of the QD changes as we modulate the relative position between the Fermi level and surface potential. Furthermore, the potential barriers forming the QD remain stable at elevated temperatures and external bias. In combination with the observation of transport gaps, our results suggest that the disordered surface potential creates QDs along the ribbon and governs the electronic transport properties in BL-GNRs. PMID:22017308

Wang, Minsheng; Song, Emil B; Lee, Sejoon; Tang, Jianshi; Lang, Murong; Zeng, Caifu; Xu, Guangyu; Zhou, Yi; Wang, Kang L

2011-11-22

312

Geometric spin manipulation in semiconductor quantum dots

We propose a method to flip the spin completely by an adiabatic transport of quantum dots. We show that it is possible to flip the spin by inducing a geometric phase on the spin state of a quantum dot. We estimate the geometric spin flip time (approximately 2 ps) which turned out to be much shorter than the experimentally reported decoherence time (approximately 100 ns) that would provide an alternative means of fliping the spin before reaching decoherence. It is important that both the Rashba coupling and the Dresselhaus coupling are present for inducing a phase necessary for spin flip. If one of them is absent, the induced phase is trivial and irrelevant for spin-flip.

Prabhakar, Sanjay, E-mail: sprabhakar@wlu.ca; Melnik, Roderick [M2NeT Laboratory, Wilfrid Laurier University, 75 University Avenue West, Waterloo, Ontario N2L 3C5 (Canada); Inomata, Akira [Department of Physics, State University of New York at Albany, Albany, New York 12222 (United States)

2014-04-07

313

Excitonic optical nonlinearity and exciton dynamics in semiconductor quantum dots

Two salient features of the excitonic state in semiconductor quantum dots are theoretically clarified. One is the enhanced excitonic optical nonlinearity arising from the state filling of discrete levels due to the quantum size effect. The calculated third-order nonlinear susceptibility explains successfully the recent experimental results. The other feature is the exciton dynamics in semiconductor quantum dots. A comprehensive interpretation

T. Takagahara

1987-01-01

314

Measurement back-action: Listening with quantum dots

NASA Astrophysics Data System (ADS)

Single electrons in quantum dots can be disturbed by the apparatus used to measure them. The disturbance can be mediated by incoherent phonons -- literally, noise. Engineering acoustic interference could negate these deleterious effects and bring quantum dots closer to becoming a robust quantum technology.

Ladd, Thaddeus D.

2012-07-01

315

Deposition of colloidal quantum dots by microcontact printing for LED display technology

This thesis demonstrates a new deposition method of colloidal quantum dots within a quantum dot organic light-emitting diode (QD-LED). A monolayer of quantum dots is microcontact printed as small as 20 ,Lm lines as well ...

Kim, LeeAnn

2006-01-01

316

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

317

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

318

Topological insulator quantum dot with tunable barriers.

Thin (6-7 quintuple layer) topological insulator Bi(2)Se(3) quantum dot devices are demonstrated using ultrathin (2-4 quintuple layer) Bi(2)Se(3) regions to realize semiconducting barriers which may be tuned from ohmic to tunneling conduction via gate voltage. Transport spectroscopy shows Coulomb blockade with large charging energy >5 meV and additional features implying excited states. PMID:22181853

Cho, Sungjae; Kim, Dohun; Syers, Paul; Butch, Nicholas P; Paglione, Johnpierre; Fuhrer, Michael S

2012-01-11

319

Multiple exciton generation in semiconductor quantum dots

A review is presented of recent work on (1) the origin of the concept of enhanced multiple electron–hole pair (i.e. exciton) production in semiconductor quantum dots (QDs), (2) various experiments based on time-resolved fs to ns spectroscopy (transient IR absorption, transient visible to near-IR bleaching due to state filling, terahertz spectroscopy, and time-resolved photoluminescence) that support the occurrence of highly

Arthur J. Nozik

2008-01-01

320

Coulomb effects in semiconductor quantum dots

Coulomb correlations in the optical spectra of semiconductor quantum dots are investigated using a full-diagonalization approach. The resulting multi-exciton spectra are discussed in terms of the symmetry of the involved states. Characteristic features of the spectra like the nearly equidistantly spaced s-shell emission lines and the approximately constant p-shell transition energies are explained using simplified Hamiltonians that are derived taking

Norman Baer; Paul Gartner; Frank Jahnke

2004-01-01

321

Dark pulse quantum dot diode laser.

We describe an operating regime for passively mode-locked quantum dot diode laser where the output consists of a train of dark pulses, i.e., intensity dips on a continuous background. We show that a dark pulse train is a solution to the master equation for mode-locked lasers. Using simulations, we study stability of the dark pulses and show they are consistent with the experimental results. PMID:20588468

Feng, Mingming; Silverman, Kevin L; Mirin, Richard P; Cundiff, Steven T

2010-06-21

322

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

323

Quantum Dot Charging By Means Of Temperature And Magnetic Field

A micro-photoluminescence study of individual InAs/GaAs quantum dots is presented. It is demonstrated that by varying the strength of an applied magnetic field and/or the temperature, the charge state of a quantum dot can be tuned. The charge tuning mechanism is shown to be due to the modification of the electron and hole transport in the wetting layer plane prior to their capture into the quantum dot.

Larsson, L. A.; Holtz, P. O. [IFM, Semiconductor Materials, Linkoeping University, SE-58183 Linkoeping (Sweden); Moskalenko, E. S. [A. F. Ioffe Physical-Technical Institute, 194021, Polytechnicheskaya 26, St. Petersburg (Russian Federation)

2011-12-23

324

Carrier and spin dynamics in charged quantum dots

Carrier and spin dynamics are measured in meutral, positively and negatively charged quantum dots using polarization-sensitive time-resolved photoluminescence. Carrier capture rates are observed to be strongly enhanced in charged quantum dots, suggesting that electron-hole scattering dominates this process. For positive quantum dots, the enhanced spin-polarized electron capture rate eliminates loss of electron spin information in the GaAs barriers prior to

Kimberley C. Hall; Kenan Gundogdu; Thomas F. Boggess; Oleg B. Shchekin; Dennis G. Deppe

2004-01-01

325

Quantum dot dipole orientation and excitation efficiency of micropillar modes.

The relative intensity of photonic modes in microcavity pillars with embedded self-assembled quantum dots is shown to be a sensitive function of quantum dot dipole orientation and position. This is deduced from a comparison of experiment and calculated intensities of light emission for many nominally identical pillars. We are able to obtain the overall degree of in-plane polarization of the quantum dot ensemble and also to obtain information on the degree of polarization along the growth axis. PMID:19582012

Silva, A G; Parra-Murillo, C A; Valentim, P T; Morais, J S V; Plentz, F; Guimarães, P S S; Vinck-Posada, H; Rodriguez, B A; Skolnick, M S; Tahraoui, A; Hopkinson, M

2008-11-10

326

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

327

Optical field distribution in ZnO/MgZnO quantum dot nanostructure at 375-nm wavelength

NASA Astrophysics Data System (ADS)

Computer analysis of near and far-field intensities in zinc oxide based quantum dot nanostructure has been carried out for the optimization of smaller active region volume of quantum dot to achieve desired modal gain. Near and far-field intensity distribution along junction plane has been studied as a function of mole fractions of Mg and for varying quantum dot thickness. Solutions to the wave equations have been obtained for the transverse electric mode. Effective index method has been used to deduce the propagation constants in various regions of optical wave-guide. Excellent confinement of near field intensity for greater values of Mg composition has been attributed to the increased values of refractive index step between dot and barrier regions. For the thicker dots, greater fraction of the optical field has been confined to the dot region. Near field intensity spread has been deduced and articulated in terms of full width of half maximum (FWHM) as a function of Mg mole fraction and a dot thickness. It clearly shows nonlinear decrease with increase of both the Mg mole fraction and a dot thickness. The surface image clearly reveals a bright spot in the center corresponding to far-field emitted from the dot region at 375-nm wavelength. Far-field intensity reveals divergence of 16°.

Zope, Ujwala; Samuel, E. P.; Bhole, M. P.; Patil, D. S.

2009-11-01

328

Quantum Computation Using Optically Coupled Quantum Dot Arrays

NASA Technical Reports Server (NTRS)

A solid state model for quantum computation has potential advantages in terms of the ease of fabrication, characterization, and integration. The fundamental requirements for a quantum computer involve the realization of basic processing units (qubits), and a scheme for controlled switching and coupling among the qubits, which enables one to perform controlled operations on qubits. We propose a model for quantum computation based on optically coupled quantum dot arrays, which is computationally similar to the atomic model proposed by Cirac and Zoller. In this model, individual qubits are comprised of two coupled quantum dots, and an array of these basic units is placed in an optical cavity. Switching among the states of the individual units is done by controlled laser pulses via near field interaction using the NSOM technology. Controlled rotations involving two or more qubits are performed via common cavity mode photon. We have calculated critical times, including the spontaneous emission and switching times, and show that they are comparable to the best times projected for other proposed models of quantum computation. We have also shown the feasibility of accessing individual quantum dots using the NSOM technology by calculating the photon density at the tip, and estimating the power necessary to perform the basic controlled operations. We are currently in the process of estimating the decoherence times for this system; however, we have formulated initial arguments which seem to indicate that the decoherence times will be comparable, if not longer, than many other proposed models.

Pradhan, Prabhakar; Anantram, M. P.; Wang, K. L.; Roychowhury, V. P.; Saini, Subhash (Technical Monitor)

1998-01-01

329

Quantum dot solar cells: The surface plays a core role

NASA Astrophysics Data System (ADS)

Mastering the impact of surface chemistry on the electronic properties and stability of colloidal quantum dots enables the realization of architectures with enhanced photovoltaic performance and air stability.

Milliron, Delia J.

2014-08-01

330

We investigate the possibility of achieving scalable photonic quantum computing by the giant optical circular birefringence induced by a quantum-dot spin in a double-sided optical microcavity as a result of cavity quantum electrodynamics. We construct a deterministic controlled-not gate on two photonic qubits by two single-photon input-output processes and the readout on an electron-medium spin confined in an optical resonant microcavity. This idea could be applied to multi-qubit gates on photonic qubits and we give the quantum circuit for a three-photon Toffoli gate. High fidelities and high efficiencies could be achieved when the side leakage to the cavity loss rate is low. It is worth pointing out that our devices work in both the strong and the weak coupling regimes. PMID:23938640

Wei, Hai-Rui; Deng, Fu-Guo

2013-07-29

331

Characterization of InGaAs quantum dot lasers with a single quantum dot layer as an active region

Quantum dot lasers with an active region consisting of just a single quantum dot layer have been grown using molecular beam epitaxy and characterized from 80 to 300K. The quantum dot lasers lase from excited states over the entire temperature range. The characteristic temperature is 185±10K over the temperature range 80–141K and decreases to 111±2K from 141–304K. The effects of

Richard P Mirin; Arthur C Gossard; John E Bowers

1998-01-01

332

Characterization of InGaAs quantum dot lasers with a single quantum dot layer as an active region

Quantum dot lasers with an active region consisting of just a single quantum dot layer have been grown using molecular beam epitaxy and characterized from 80 to 300 K. The quantum dot lasers lase from excited states over the entire temperature range. The characteristic temperature is 185+\\/-10 K over the temperature range 80-141 K and decreases to 111+\\/-2 K from

Richard P. Mirin; Arthur C. Gossard; John E. Bowers

1998-01-01

333

Enhanced performance of quantum dot solar cells based on type II quantum dots

The characteristics of quantum dot solar cells (QDSCs) based on type II QDs are investigated theoretically. Based on a drift-diffusion model, we obtained a much higher open circuit voltage (V{sub oc}) as well as conversion efficiency in a type II QDSC, compared to type I QDSCs. The improved V{sub oc} and efficiency are mainly attributed to the much longer Auger recombination lifetime in type II QDs. Moreover, the influence of the carrier lifetime on devices' performance is discussed and clarified. In addition, an explicit criterion to determine the role of quantum dots in solar cells is put forward.

Xu, Feng; Yang, Xiao-Guang; Luo, Shuai; Lv, Zun-Ren; Yang, Tao, E-mail: tyang@semi.ac.cn [Key Laboratory of Semiconductor Materials Science, Institute of Semiconductors, Chinese Academy of Sciences, P.O. Box 912, Beijing 100083 (China)

2014-10-07

334

Enhanced performance of quantum dot solar cells based on type II quantum dots

NASA Astrophysics Data System (ADS)

The characteristics of quantum dot solar cells (QDSCs) based on type II QDs are investigated theoretically. Based on a drift-diffusion model, we obtained a much higher open circuit voltage (Voc) as well as conversion efficiency in a type II QDSC, compared to type I QDSCs. The improved Voc and efficiency are mainly attributed to the much longer Auger recombination lifetime in type II QDs. Moreover, the influence of the carrier lifetime on devices' performance is discussed and clarified. In addition, an explicit criterion to determine the role of quantum dots in solar cells is put forward.

Xu, Feng; Yang, Xiao-Guang; Luo, Shuai; Lv, Zun-Ren; Yang, Tao

2014-10-01

335

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

336

Quantum confinement in Si and Ge nanostructures: effect of crystallinity

NASA Astrophysics Data System (ADS)

We look at the relationship between the preparation method of Si and Ge nanostructures (NSs) and the structural, electronic, and optical properties in terms of quantum confinement (QC). QC in NSs causes a blue shift of the gap energy with decreasing NS dimension. Directly measuring the effect of QC is complicated by additional parameters, such as stress, interface and defect states. In addition, differences in NS preparation lead to differences in the relevant parameter set. A relatively simple model of QC, using a `particle-in-a-box'-type perturbation to the effective mass theory, was applied to Si and Ge quantum wells, wires and dots across a variety of preparation methods. The choice of the model was made in order to distinguish contributions that are solely due to the effects of QC, where the only varied experimental parameter was the crystallinity. It was found that the hole becomes de-localized in the case of amorphous materials, which leads to stronger confinement effects. The origin of this result was partly attributed to differences in the effective mass between the amorphous and crystalline NS as well as between the electron and hole. Corrections to our QC model take into account a position dependent effective mass. This term includes an inverse length scale dependent on the displacement from the origin. Thus, when the deBroglie wavelength or the Bohr radius of the carriers is on the order of the dimension of the NS the carriers `feel' the confinement potential altering their effective mass. Furthermore, it was found that certain interface states (Si-O-Si) act to pin the hole state, thus reducing the oscillator strength.

Barbagiovanni, Eric G.; Lockwood, David J.; Costa Filho, Raimundo N.; Goncharova, Lyudmila V.; Simpson, Peter J.

2013-10-01

337

Coherent population transfer in coupled semiconductor quantum dots

We propose a solid-state implementation of stimulated Raman adiabatic passage in two coupled semiconductor quantum dots. Proper combination of two pulsed laser fields allows the coherent carrier transfer between the two nanostructures without suffering significant losses due to environment coupling. By use of a general solution scheme for the carrier states in the double-dot structure, we identify the pertinent dot

Ulrich Hohenester; Filippo Troiani; Elisa Molinari; Giovanna Panzarini; Chiara Macchiavello

2000-01-01

338

Hyperfine-induced decay in triple quantum dots

NASA Astrophysics Data System (ADS)

We analyze the effects of hyperfine interactions on coherent control experiments in triple quantum dots. By exploiting Hamiltonian symmetries and the SU(3) structure of the triple-dot system under pseudoexchange and longitudinal hyperfine couplings, we provide analytic formulas for the hyperfine decay of triple-dot Rabi and dephasing experiments.

Ladd, Thaddeus D.

2012-09-01

339

We report on the development and testing of a coplanar stripline antenna that is designed for integration in a magneto-photoluminescence experiment to allow coherent control of individual electron spins confined in single self-assembled semiconductor quantum dots. We discuss the design criteria for such a structure which is multi-functional in the sense that it serves not only as microwave delivery but also as electrical top gate and shadow mask for the single quantum dot spectroscopy. We present test measurements on hydrogenated amorphous silicon, demonstrating electrically detected magnetic resonance using the in-plane component of the oscillating magnetic field created by the coplanar stripline antenna necessary due to the particular geometry of the quantum dot spectroscopy. From reference measurements using a commercial electron spin resonance setup in combination with finite element calculations simulating the field distribution in the structure, we obtain an average magnetic field of ~0.2mT at the posit...

Klotz, F; Heiss, D; Klein, K; Finley, J J; Brandt, M S

2011-01-01

340

NASA Astrophysics Data System (ADS)

We present an electrically-controllable multi-spectral quantum dot infrared photodetector (QDIP). The QDIP consists of vertically-stacked InAs quantum dots layers with two different capping layers for MWIR and LWIR absorption, respectively. The multi-spectral QDOP is capable of simultaneously detecting multi-spectral normal incidence through inter-subband transitions in the three-dimensional (3-D) confined quantum dot nanostructures. The QDIP showed multi-color IR detection bands centered at 5.6?m, 7.7 ?m and 10.0?m, respectively. By tuning the bias voltage, the detection band can be individual turned on. High photodetectivity of > 2.3×10 10cmHz 1/2/W were obtained for these IR detection bands. The voltage-controllable detection band selection enables real-time system reconfiguration to focus on the band of interest. The vertically-stacked device structure allows easy construction of focal plane arrays (FPA).

Lu, Xuejun; Vaillancourt, Jarrod; Meisner, Mark J.

2007-04-01

341

We show that through the resonant optical excitation of spin-polarized excitons into CdMnTe magnetic quantum dots, we can induce a macroscopic magnetization of the Mn impurities. We observe very broad (4 meV linewidth) emission lines of single dots, which are consistent with the formation of strongly confined exciton magnetic polarons. Therefore we attribute the optically induced magnetization of the magnetic dots results to the formation of spin-polarized exciton magnetic polarons. We find that the photo-induced magnetization of magnetic polarons is weaker for larger dots which emit at lower energies within the QD distribution. We also show that the photo-induced magnetization is stronger for quantum dots with lower Mn concentration, which we ascribe to weaker Mn-Mn interaction between the nearest neighbors within the dots. Due to particular stability of the exciton magnetic polarons in QDs, where the localization of the electrons and holes is comparable to the magnetic exchange interaction, this optically induced spin alignment persists to temperatures as high as 160 K.

T. Gurung; S. Mackowski; H. E. Jackson; L. M. Smith; W. Heiss; J. Kossut; G. Karczewski

2004-08-30

342

Strain induced optical properties of exciton in a CdTe/ZnTe quantum dot

NASA Astrophysics Data System (ADS)

The influence of strain on the binding energies of heavy and light hole is obtained in a ZnxCd1-xTe/ZnTe quantum dot taking into account the phonon confinement effect. The band offsets are calculated using model-solid approach. The dielectric mismatch effect is introduced between the dielectric constants of dot and the barrier. The strain induced energies of excitonic transitions in a CdZnTe quantum dot with ZnTe barrier are brought out considering the internal electric field induced by the spontaneous and piezoelectric polarizations. Calculations have been obtained using Bessel function as an orthonormal basis for different confinement potentials of barrier height, strain induced linear and third-order nonlinear optical absorption coefficients and the changes of refractive index with the incident photon energy are observed. Our results show that the exciton binding energy is enhanced with the inclusion of potential taking into account the effects of dielectric mismatch and the geometry of quantum dot with various Zinc alloy content has a great influence on the optical properties of the dot.

Sangeetha, R.; John Peter, A.; Lee, Chang Woo

2014-02-01

343

Si, Ge, and SiGe quantum wires and quantum dots

NASA Astrophysics Data System (ADS)

This document is part of subvolume C3 'Optical Properties' of volume 34 'Semiconductor quantum structures' of Landolt-Börnstein, Group III, Condensed Matter, on the optical properties of quantum structures based on group IV semiconductors. It discusses Si, Ge, and SiGe quantum wire and quantum dot structures, the synthesis of quantum wires and quantum dots, and applications of SiGe quantum-dot structures as photodetectors, light-emitting diodes, for optical amplification and as Si quantum-dot memories.

Pearsall, T. P.

344

Production and Targeting of Monovalent Quantum Dots

The multivalent nature of commercial quantum dots (QDs) and the difficulties associated with producing monovalent dots have limited their applications in biology, where clustering and the spatial organization of biomolecules is often the object of study. We describe here a protocol to produce monovalent quantum dots (mQDs) that can be accomplished in most biological research laboratories via a simple mixing of CdSe/ZnS core/shell QDs with phosphorothioate DNA (ptDNA) of defined length. After a single ptDNA strand has wrapped the QD, additional strands are excluded from the surface. Production of mQDs in this manner can be accomplished at small and large scale, with commercial reagents, and in minimal steps. These mQDs can be specifically directed to biological targets by hybridization to a complementary single stranded targeting DNA. We demonstrate the use of these mQDs as imaging probes by labeling SNAP-tagged Notch receptors on live mammalian cells, targeted by mQDs bearing a benzylguanine moiety. PMID:25407345

Southard, Kade; Jun, Young-wook; Gartner, Zev J.

2014-01-01

345

Predicted Ultrafast Single Qubit Operations in Semiconductor Quantum Dots

Several recently proposed implementations of scalable quantum computation rely on the ability to manipulate the spin polarization of individual electrons in semiconductors. The most rapid single-spin-manipulation technique to date relies on the generation of an effective magnetic field via a spin-sensitive optical Stark effect. This approach has been used to split spin states in colloidal CdSe quantum dots and to manipulate ensembles of spins in ZnMnSe quantum wells with femtosecond optical pulses. Here we report that the process will produce a coherent rotation of spin in quantum dots containing a single electron. The calculated magnitude of the effective magnetic field depends on the dot bandgap and the strain. We predict that in InAs/InP dots, for reasonable experimental parameters, the magnitude of the rotation is sufficient and the intrinsic error is low enough for them to serve as elements of a quantum dot based quantum computer.

C. E. Pryor; M. E. Flatté

2002-11-25

346

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.

2012-01-01

347

Design and fabrication of quantum-dot lasers

Semiconductor lasers using quantum-dots in their active regions have been reported to exhibit significant performance advantages over their bulk semiconductor and quantum-well counterparts namely: low threshold current, ...

Nabanja, Sheila

2008-01-01

348

Multi-electron double quantum dot spin qubits

NASA Astrophysics Data System (ADS)

Double quantum dot (DQD) spin quits in a solid state environment typically consist of two electron spins confined to a DQD potential. We analyze the viability and potential advantages of DQD qubits which use greater then two electrons, and present results for six-electron qubits using full configuration interaction methods. The principal results of this work are that such six electron DQDs can retain an isolated low-energy qubit space that is more robust to charge noise due to screening. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000.

Nielsen, Erik; Kestner, Jason; Barnes, Edwin; Das Sarma, Sankar

2013-03-01

349

Two-electron volcano-shaped quantum dot

NASA Astrophysics Data System (ADS)

We propose a simple model of non-uniform volcano-shaped quantum dot that reflects the confinement details of the morphology of really fabricated GaAs/InAs nanorings and whose profile geometry, on the one hand, is described by means of simple analytical functions and, on the other hand, allows us to find exact one-particle wave functions. By using them as a basis function we calculate two-electron lower energies as functions of the external magnetic field applied along the growth axis. We show that the ring morphology and electron-electron interaction have great influence on the energy spectrum structure of nanoring and the Aharonov-Bohm oscillations.

Garcia, L. F.; Gutiérrez, W.; Mikhailov, I. D.

2014-12-01

350

Auger recombination in In(Ga)Sb/InAs quantum dots

NASA Astrophysics Data System (ADS)

We report on the epitaxial formation of type II In0.5Ga0.5Sb/InAs and InSb/InAs quantum dot ensembles using metal organic vapor phase epitaxy. Employing scanning tunneling spectroscopy, we determine spatial quantum dot dimensions smaller than the de Broglie wavelength of InGaSb, which strongly indicates a three dimensional hole confinement. Photoluminescence spectroscopy at low temperatures yields an enhanced radiative recombination in the mid-infrared regime at energies of 170-200 meV. This luminescence displays a strong excitation power dependence with a blueshift indicating a filling of excited quantum dot hole states. Furthermore, a rate equation model is used to extract the Auger recombination coefficient from the power dependent intensity at 77 K yielding values of 1.35 × 10-28 cm6/s for In0.5Ga0.5Sb/InAs quantum dots and 1.47 × 10-27 cm6/s for InSb/InAs quantum dots, which is about one order of magnitude lower as previously obtained values for InGaSb superlattices.

Zabel, T.; Reuterskiöld Hedlund, C.; Gustafsson, O.; Karim, A.; Berggren, J.; Wang, Q.; Ernerheim-Jokumsen, C.; Soldemo, M.; Weissenrieder, J.; Götelid, M.; Hammar, M.

2015-01-01

351

We report on the development and testing of a coplanar stripline antenna that is designed for integration in a magneto-photoluminescence experiment to allow coherent control of individual electron spins confined in single self-assembled semiconductor quantum dots. We discuss the design criteria for such a structure which is multi-functional in the sense that it serves not only as microwave delivery but also as electrical top gate and shadow mask for the single quantum dot spectroscopy. We present test measurements on hydrogenated amorphous silicon, demonstrating electrically detected magnetic resonance using the in-plane component of the oscillating magnetic field created by the coplanar stripline antenna necessary due to the particular geometry of the quantum dot spectroscopy. From reference measurements using a commercial electron spin resonance setup in combination with finite element calculations simulating the field distribution in the structure, we obtain a magnetic field of 0.12 mT at the position where the quantum dots would be integrated into the device. The corresponding ?-pulse time of ?0.5??s meets the requirements set by the high sensitivity optical spin read-out scheme developed for the quantum dot. PMID:21806214

Klotz, F; Huebl, H; Heiss, D; Klein, K; Finley, J J; Brandt, M S

2011-07-01

352

Power-law photoluminescence decay in quantum dots

Some quantum dot samples show a long-time (power-law) behavior of their luminescence intensity decay. This effect has been recently explained as being due to a cooperation of many tunneling channels transferring electrons from small quantum dots with triplet exciton to quantum dots at which the electrons can recombine with the holes in the valence band states. In this work we show that the long-time character of the sample luminescence decay can also be caused by an intrinsic property of a single dot, namely, by a non-adiabatic effect of the electron occupation up-conversion caused by the electron-phonon multiple scattering mechanism.

Král, Karel [Institute of Physics, Academy of Sciences of Czech Republic, v.v.i., Na Slovance 2, 18221 Prague 8 (Czech Republic); Menšík, Miroslav [Institute of Macromolecular Chemistry, Academy of Sciences of Czech Republic, v.v.i., Heyrovského nám. 2, 162 06 Prague 6 (Czech Republic)

2014-05-15

353

PREFACE: Quantum dots as probes in biology

NASA Astrophysics Data System (ADS)

The recent availability of nanostructured materials has resulted in an explosion of research focused on their unique optical, thermal, mechanical and magnetic properties. Optical imagining, magnetic enhancement of contrast and drug delivery capabilities make the nanoparticles of special interest in biomedical applications. These materials have been involved in the development of theranostics—a new field of medicine that is focused on personalized tests and treatment. It is likely that multimodal nanomaterials will be responsible for future diagnostic advances in medicine. Quantum dots (QD) are nanoparticles which exhibit luminescence either through the formation of three-dimensional excitons or excitations of the impurities. The excitonic luminescence can be tuned by changing the size (the smaller the size, the higher the frequency). QDs are usually made of semiconducting materials. Unlike fluorescent proteins and organic dyes, QDs resist photobleaching, allow for multi-wavelength excitations and have narrow emission spectra. The techniques to make QDs are cheap and surface modifications and functionalizations can be implemented. Importantly, QDs could be synthesized to exhibit useful optomagnetic properties and, upon functionalization with an appropriate biomolecule, directed towards a pre-selected target for diagnostic imaging and photodynamic therapy. This special issue on Quantum dots in Biology is focused on recent research in this area. It starts with a topical review by Sreenivasan et al on various physical mechanisms that lead to the QD luminescence and on using wavelength shifts for an improvement in imaging. The next paper by Szczepaniak et al discusses nanohybrids involving QDs made of CdSe coated by ZnS and combined covalently with a photosynthetic enzyme. These nanohybrids are shown to maintain the enzymatic activity, however the enzyme properties depend on the size of a QD. They are proposed as tools to study photosynthesis in isolated photosynthetic systems. The next paper, by Olejnik et al, discussed metallic QDs which enhance photosynthetic function in light-harvesting biomolecular complexes. Such hybrid structures with gold QDs are shown to exhibit a strong increase in the fluorescence quantum yield. The next two papers, by Sikora et al and Kaminska et al deal with the ZnO nanoparticles passivated by MgO. In the first of these two papers, the authors describe the behavior of ZnO/MgO when introduced to human cancer cells. In the second, the authors describe the QDs with an extra outer layer of Fe2O3 which makes the nanoparticles superparamagnetic and also capable of generation of reactive oxygen species which could be applied to form localized centers of toxicity for cancer treatment. Finally, in the last paper by Yatsunenko et al, the authors discuss several semiconducting QDs like ZnO with various rare-earth dopands. They propose a microwave-driven hydrothermal technology to make them, characterize their luminescence and demonstrate their usefulness in the early recognition of cancer tissues. Quantum dots as probes in biology contents Quantum dots as probes in biologyMarek Cieplak Luminescent nanoparticles and their applications in the life sciencesVarun K A Sreenivasan, Andrei V Zvyagin and Ewa M Goldys Ferredoxin:NADP+ oxidoreductase in junction with CdSe/ZnS quantum dots: characteristics of an enzymatically active nanohybrid Krzysztof Szczepaniak, Remigiusz Worch and Joanna Grzyb Spectroscopic studies of plasmon coupling between photosynthetic complexes and metallic quantum dotsMaria Olejnik, Bartosz Krajnik, Dorota Kowalska, Guanhua Lin and Sebastian Mackowski Luminescence of colloidal ZnO nanoparticles synthesized in alcohols and biological application of ZnO passivated by MgOBo?ena Sikora, Krzysztof Fronc, Izabela Kami?ska, Kamil Koper, Piotr St?pie? and Danek Elbaum Novel ZnO/MgO/Fe2O3 composite optomagnetic nanoparticles I Kami?ska, B Sikora, K Fronc, P Dziawa, K Sobczak, R Minikayev, W Paszkowicz and D Elbaum Impact of yttria stabilization on Tb3+ intra-shell luminescence efficiency in

Cieplak, Marek

2013-05-01

354

Visible single-photon generation from semiconductor quantum dots

In this paper we report recent results on single-photon generation with single InP and CdSe quantum dots. These dots produce single-photons on demand in the visible spectral range 510 690 nm. The emitted photoluminescence was characterized by measuring the autocorrelation function and by performing Fourier spectroscopy on several transitions in the quantum dot. We present the observation and interpretation of

Thomas Aichele; Valéry Zwiller; Oliver Benson

2004-01-01

355

Elastic light scattering by semiconductor quantum dots of arbitrary shape

Elastic light scattering by low-dimensional quantum objects without a change in the frequency is theoretically investigated\\u000a in terms of the quantum perturbation theory. The differential cross section of resonance light scattering from any excitons\\u000a in any quantum dots is calculated. It is demonstrated that, when the light wavelengths considerably exceed the quantum-dot\\u000a size, the polarization and angular distribution of the

I. G. Lang; L. I. Korovin; S. T. Pavlov

2007-01-01

356

Long-range radiative interaction between semiconductor quantum dots

We develop a Maxwell-Schrödinger formalism in order to describe the radiative interaction mechanism between semiconductor quantum dots. We solve the Maxwell equations for the electromagnetic field coupled to the polarization field of a quantum dot ensemble through a linear nonlocal susceptibility and compute the polariton resonances of the system. The radiative coupling, mediated by both radiative and surface photon modes,

Gaetano Parascandolo; Vincenzo Savona

2005-01-01

357

Coherent Acoustic Phonons in a Semiconductor Quantum Dot

The electronic and optical properties of semiconductor quantum dots (QD's) have attracted much attention re- cently due to their interesting physical properties and po- tential utility in applications. Exciton-phonon coupling is a crucial issue in nanocrystal physics, and this has moti- vated much of the previous work on the vibrational modes of quantum dots. The coupling to optical modes pro-

Todd D. Krauss; Frank W. Wise

1997-01-01

358

Spin Readout and Initialization in a Semiconductor Quantum Dot

Electron spin qubits in semiconductors are attractive from the viewpoint of long coherence times. However, single spin measurement is challenging. Several promising schemes incorporate ancillary tunnel couplings that may provide unwanted channels for decoherence. Here, we propose a novel spin-charge transduction scheme, converting spin information to orbital information within a single quantum dot by microwave excitation. The same quantum dot

Mark Friesen; Charles Tahan; Robert Joynt; M. A. Eriksson

2004-01-01

359

Mannosylated semiconductor quantum dots for the labeling of macrophages

Quantum dots show strong fluorescence emission and long stability compared with classical organic fluorescent dyes; therefore, quantum dots take the place of other dyes in Western blot, immunostaining and bioimaging. Since macrophage plays crucial roles in many pathophysiological processes, tracking macrophage migration, homing and fate is important for understanding the complex roles of macrophages in disease or developing disease diagnosis.

Yuriko Higuchi; Machiko Oka; Shigeru Kawakami; Mitsuru Hashida

2008-01-01

360

BCD computing structures in quantum- dot cellular automata

This paper proposes a detailed design analysis of BCD computing circuit for quantum-dot cellular automata (QCA). QCA is attracting a lot of attentions due to its very small sizes and low power consumption. The primary device, a quantum-dot cell, can be used to make gates, wires, and memories as such it is the basic building block of nanotechnology circuits. Because

Maryam Taghizadeh; Mehdi Askari; Khossro Fardad

2008-01-01

361

Modified Quantum Dots Could Lead to Improved Treatments for Cancer

NSDL National Science Digital Library

This online article from Carnegie Mellon Today, discusses how chemists are researching quantum dots to evaluate their effectiveness in treating diseases such as cancer. Readers can discover how the scientists were able to produce quantum dots that fluoresced for an unprecedented eight months, and how they might be useful in locating diseases like cancer.

Pavlak, Amy

362

Study of exciton transfer in dense quantum dot nanocomposites

Study of exciton transfer in dense quantum dot nanocomposites Burak Guzelturk,ab Pedro Ludwig. Govorov,c Xiao Wei Sun,b Qihua Xiongb and Hilmi Volkan Demir*ab Nanocomposites of colloidal quantum dots, contrary to the previous literature, efficient exciton transfer is demonstrated in the nanocomposites

Demir, Hilmi Volkan

363

Quantum-Dot Cellular Automata SPICE Macro Model Northeastern University

Quantum-Dot Cellular Automata SPICE Macro Model Rui Tang Northeastern University 360 Huntington Ave describes a SPICE model development method- ology for Quantum-Dot Cellular Automata (QCA) cells and presents a SPICE model for QCA cells. The model is val- idated by simulating the basic logic gates such as inverter

Ayers, Joseph

364

Minimized deterioration of ultrashort pulses in quantum dot optical amplifiers

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

Edeltraud Gehrig; Ortwin G. Hess

2004-01-01

365

Monovalent, reduced-size quantum dots for imaging

Monovalent, reduced-size quantum dots for imaging receptors on living cells Mark Howarth1,3, Wenhao Bawendi1 & Alice Y Ting1 We describe a method to generate monovalent quantum dots (QDs) using agarose gel particles or latex beads allow stable single-particle tracking via their scattering, but are generally very

366

Molecular-Scale Quantum Dots from Carbon Nanotube Heterojunctions

Molecular-Scale Quantum Dots from Carbon Nanotube Heterojunctions Bhupesh Chandra,,#,Â¶ Joydeep chiralities. These measurements reveal asymmetric IV-characteristics and the presence of a quantum dot (QD deposition across open slits (100 Âµm wide) on Si/SiO2 wafers,7 then characterized using Rayleigh scattering

Heinz, Tony F.

367

Quantum dot infrared photodetector enhanced by surface plasma wave excitation

Quantum dot infrared photodetector enhanced by surface plasma wave excitation S. C. Lee, S. Krishna: Up to a thirty-fold detectivity enhancement is achieved for an InAs quantum dot infrared photodetector (QDIP) by the excitation of surface plasma waves (SPWs) using a metal photonic crystal (MPC

Krishna, Sanjay

368

Photosensitive quantum dot composites and their applications in optical structures

Photosensitive quantum dot composites and their applications in optical structures Lin Pang,a Kevin poly methyl methacrylate PMMA -quantum-dot QD positive composite via a prepolymerization processing and an electron beam and ultraviolet UV light sensitive SU-8-QD negative composite via a direct dispersion

Fainman, Yeshaiahu

369

We present evidence for quantum dot cellular automata action in a cell consisting of four dots defined by submicron metal gates on the top surface of a molecular-beam-epitaxy-grown GaAs\\/AlGaAs heterostructure in which a two-dimensional electron gas layer was formed approximately 70 nm below the surface. The four-dot cell is separated by a strong barrier in two double-dot sets. We show

S. Gardelis; C. G. Smith; J. Cooper; D. A. Ritchie; E. H. Linfield; Y. Jin

2003-01-01

370

Longitudinal wave function control in single quantum dots with an applied magnetic field

Controlling single-particle wave functions in single semiconductor quantum dots is in demand to implement solid-state quantum information processing and spintronics. Normally, particle wave functions can be tuned transversely by an perpendicular magnetic field. We report a longitudinal wave function control in single quantum dots with a magnetic field. For a pure InAs quantum dot with a shape of pyramid or truncated pyramid, the hole wave function always occupies the base because of the less confinement at base, which induces a permanent dipole oriented from base to apex. With applying magnetic field along the base-apex direction, the hole wave function shrinks in the base plane. Because of the linear changing of the confinement for hole wave function from base to apex, the center of effective mass moves up during shrinking process. Due to the uniform confine potential for electrons, the center of effective mass of electrons does not move much, which results in a permanent dipole moment change and an inverted...

Cao, Shuo; Gao, Yunan; Sun, Yue; Qiu, Kangsheng; Zhao, Yanhui; He, Min; Shi, Jin-An; Gu, Lin; Williams, David A; Sheng, Weidong; Jin, Kuijuan; Xu, Xiulai

2015-01-01

371

Impurity position effect on optical properties of various quantum dots

NASA Astrophysics Data System (ADS)

In this work, we have investigated the effect of impurity position on optical properties of a pyramid and a cone like quantum dot. For this goal, we first obtain the energy levels and wave functions using finite element method (FEM) in the presence of impurity. Then, we have studied the influence of impurity location on refractive index changes and absorption coefficients of the two quantum dots. We found that there is a maximum value for total refractive index changes and absorption coefficients at a special impurity position. Also, we have found that the refractive index changes and absorption coefficients of a cone like quantum dot are greater than a pyramid quantum dot in same volume and height. According to the results, it is deduced that the impurity location plays an important and considerable role in the electronic and optical properties of a pyramid and a cone like quantum dot.

Khordad, R.; Bahramiyan, H.

2015-02-01

372

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

373

Hyperfine interactions in silicon quantum dots

We present an all-electron calculation of the hyperfine parameters for conduction electrons in Si, showing that: (i) all parameters scale linearly with the spin density at a $^{29}$Si site; (ii) the isotropic term is over 30 times larger than the anisotropic part; (iii) conduction electron charge density at a Si nucleus is consistent with experimental estimates; (iv) Overhauser fields in natural Si quantum dots (QDs) are two orders of magnitude smaller than in GaAs QDs. This reinforces the outstanding performance of Si in keeping spin coherence and opens access to reliable quantitative information aiming at spintronic applications.

Lucy V. C. Assali; Helena M. Petrilli; Rodrigo B. Capaz; Belita Koiller; Xuedong Hu; S. Das Sarma

2010-07-06

374

Hyperfine interactions in silicon quantum dots

We present an all-electron calculation of the hyperfine parameters for conduction electrons in Si, showing that: (i) all parameters scale linearly with the spin density at a $^{29}$Si site; (ii) the isotropic term is over 30 times larger than the anisotropic part; (iii) conduction electron charge density at a Si nucleus is consistent with experimental estimates; (iv) Overhauser fields in natural Si quantum dots (QDs) are two orders of magnitude smaller than in GaAs QDs. This reinforces the outstanding performance of Si in keeping spin coherence and opens access to reliable quantitative information aiming at spintronic applications.

Assali, Lucy V C; Capaz, Rodrigo B; Koiller, Belita; Hu, Xuedong; Sarma, S Das

2010-01-01

375

Mesoscopic admittance of a double quantum dot

We calculate the mesoscopic admittance G({omega}) of a double quantum dot (DQD), which can be measured directly using microwave techniques. This quantity reveals spectroscopic information on the DQD and is also directly sensitive to a Pauli spin blockade effect. We then discuss the problem of a DQD coupled to a high quality photonic resonator. When the photon correlation functions can be developed along a random-phase-approximation-like scheme, the response of the resonator gives an access to G({omega}).

Cottet, Audrey; Mora, Christophe; Kontos, Takis [Laboratoire Pierre Aigrain, Ecole Normale Superieure, CNRS (UMR 8551), Universite P. et M. Curie, Universite D. Diderot, 24 rue Lhomond, F-75231 Paris Cedex 05 (France)

2011-03-15

376

We show how to compute the optical spectra resulting from the scattering of a plane electromagnetic wave on a quantum dot (QD) in the region of the excitonic resonances. The method uses the microscopic calculation of quantum dot eigenfunctions and the macroscopic Stahl's density matrix approach to compute the electromagnetic field within and outside the QD, taking into account the

L. Silvestri; F. Bassani; G. Czajkowski

2000-01-01

377

We will describe, from both the theoretical and experimental points of view, a procedure leading to the realization of two-dimensional quantum dot superlattices (2D QDSLs). Two kinds of laterally coupled quantum dot arrays in which the exciton motions are coherent and incoherent are studied by photoluminescence (PL) measurements. The formation of minibands with coherent exciton motion is demonstrated in the

Sheng Lan; Kouichi Akahane; Kee-Youn Jang; Takahiro Kawamura; Yoshitaka Okada; Mitsuo Kawabe

1999-01-01

378

Subtle leakage of a Majorana mode into a quantum dot

NASA Astrophysics Data System (ADS)

We investigate quantum transport through a quantum dot connected to source and drain leads and side coupled to a topological superconducting nanowire (Kitaev chain) sustaining Majorana end modes. Using a recursive Green's-function approach, we determine the local density of states of the system and find that the end Majorana mode of the wire leaks into the dot, thus, emerging as a unique dot level pinned to the Fermi energy ?F of the leads. Surprisingly, this resonance pinning, resembling, in this sense, a "Kondo resonance," occurs even when the gate-controlled dot level ?dot(Vg) is far above or far below ?F. The calculated conductance G of the dot exhibits an unambiguous signature for the Majorana end mode of the wire: In essence, an off-resonance dot [?dot(Vg)??F], which should have G =0, shows, instead, a conductance e2/2h over a wide range of Vg due to this pinned dot mode. Interestingly, this pinning effect only occurs when the dot level is coupled to a Majorana mode; ordinary fermionic modes (e.g., disorder) in the wire simply split and broaden (if a continuum) the dot level. We discuss experimental scenarios to probe Majorana modes in wires via these leaked/pinned dot modes.

Vernek, E.; Penteado, P. H.; Seridonio, A. C.; Egues, J. C.

2014-04-01

379

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. PMID:23013605

2012-01-01

380

Uniform InGaAs quantum dot arrays fabricated using nanosphere lithography

We demonstrate the fabrication of optically active uniform InGaAs quantum dot arrays by combining nanosphere lithography and bromine ion-beam-assisted etching on a single InGaAs/GaAs quantum well. A wide range of lateral dot sizes was achieved from an oxygen plasma nanosphere resizing process. The increased lateral confinement of carriers in the dots results in low temperature photoluminescence blueshifts from 0.5 to 11 meV. Additional quantization was achieved using a selective wet-etch process. Our model suggests the presence of a 70 nm dead layer in the outer InGaAs radial edge, which we believe to be a result of defects and dislocations introduced during the dry-etch process.

Qian, X.; Li, J.; Wasserman, D.; Goodhue, W. D. [Department of Physics and Applied Physics, Photonics Center, University of Massachusetts Lowell, Massachusetts 01854 (United States)

2008-12-08

381

Quantum Dot Spin Cellular Automata for Realizing a Quantum Processor

We show how "single" quantum dots, each hosting a singlet-triplet qubit, can be placed in arrays to build a spin quantum cellular automaton. A fast ($\\sim 10$ ns) deterministic coherent singlet-triplet filtering, as opposed to current incoherent tunneling/slow-adiabatic based quantum gates (operation time $\\sim 300$ ns), can be employed to produce a two-qubit gate through capacitive (electrostatic) coupling that can operate over significant distances. This is the coherent version of the widely discussed charge and nano-magnet cellular automata and would offer speed, reduce dissipation, perform quantum computation, while interfacing smoothly with its classical counterpart. This combines the best of two worlds -- the coherence of spin pairs known from quantum technologies, and the strength and range of electrostatic couplings from the charge based classical cellular automata.

Abolfazl Bayat; Charles E. Creffield; John H. Jefferson; Michael Pepper; Sougato Bose

2013-10-16

382

NASA Astrophysics Data System (ADS)

Efficient iso-entropic energy filtering of electronic waves can be realized through nanostructures with three dimensional confinement, such as quantum dot resonant tunneling structures. Large-area deployment of such structures is useful for energy selective contacts but such configuration is susceptible to structural disorders. In this work, the transport properties of quantum-dot-based wide-area resonant tunneling structures, subject to realistic disorder mechanisms, are studied. Positional variations of the quantum dots are shown to reduce the resonant transmission peaks while size variations in the device are shown to reduce as well as broaden the peaks. Increased quantum dot size distribution also results in a peak shift to lower energy which is attributed to large dots dominating transmission. A decrease in barrier thickness reduces the relative peak height while the overall transmission increases dramatically due to lower "series resistance." While any shift away from ideality can be intuitively expected to reduce the resonance peak, quantification allows better understanding of the tolerances required for fabricating structures based on resonant tunneling phenomena.

Puthen-Veettil, B.; Patterson, R.; König, D.; Conibeer, G.; Green, M. A.

2014-10-01

383

Manipulating Single Spins in Quantum Dots Coupled to Ferromagnetic Leads

NASA Astrophysics Data System (ADS)

We discuss the possibility to generate, manipulate, and probe single spins in single-level quantum dots coupled to ferromagnetic leads. The spin-polarized currents flowing between dot and leads lead to a non-equilibrium spin accumulation, i.e., a finite polarization of the dot spin. Both the magnitude and the direction of the dot's spin polarization depends on the magnetic properties of leads and their coupling to the dot. They can be, furthermore, manipulated by either an externally applied magnetic field or an intrinsically present exchange field that arises due to the tunnel coupling of the strongly-interacting quantum-dot states to spin-polarized leads. The exchange field can be tuned by both the gate and bias voltage, which, therefore, provide convenient handles to manipulate the quantum-dot spin. Since the transmission through the quantum-dot spin valve sensitively depends on the state of the quantum-dot spin, all the dynamics of the latter is reflected in the transport properties of the device.

König, Jürgen; Braun, Matthias; Martinek, Jan

384

Structural Transformations in self-assembled Semiconductor Quantum Dots as inferred by Transmission of self-assembled semiconductor quantum dots are reported. III-V and II-VI quantum dots as grown and controlling structural transformations in self-assembled semiconductor quantum dots may also offer

Moeck, Peter

385

Discrete quantum Fourier transform in coupled semiconductor double quantum dot molecules

In this Letter, we present a physical scheme for implementing the discrete quantum Fourier transform in a coupled semiconductor double quantum dot system. The main controlled-R gate operation can be decomposed into many simple and feasible unitary transformations. The current scheme would be a useful step towards the realization of complex quantum algorithms in the quantum dot system.

Ping Dong; Ming Yang; Zhuo-Liang Cao

2008-11-28

386

Photovoltaic quantum dot quantum cascade infrared photodetector A. V. Barve and S. Krishna

Photovoltaic quantum dot quantum cascade infrared photodetector A. V. Barve and S. Krishna Citation subject to AIP license or copyright; see http://apl.aip.org/about/rights_and_permissions #12;Photovoltaic) Design and characterization of a quantum dot quantum cascade detector for photovoltaic midwave infrared

Krishna, Sanjay

387

Universal Braess paradox in open quantum dots

NASA Astrophysics Data System (ADS)

We present analytical and numerical results that demonstrate the presence of the Braess paradox in chaotic quantum dots. The paradox that we identify, originally perceived in classical networks, shows that the addition of more capacity to the network can suppress the current flow in the universal regime. We investigate the weak localization term, showing that it presents the paradox encoded in a saturation minimum of the conductance, under the presence of hyperflow in the external leads. In addition, we demonstrate that the weak localization suffers a transition signal depending on the overcapacity lead and presents an echo on the magnetic crossover before going to zero due to the full time-reversal symmetry breaking. We also show that the quantum interference contribution can dominate the Ohm term in the presence of constrictions and that the corresponding Fano factor engenders an anomalous behavior.

Barbosa, A. L. R.; Bazeia, D.; Ramos, J. G. G. S.

2014-10-01

388

Universal Braess paradox in open quantum dots.

We present analytical and numerical results that demonstrate the presence of the Braess paradox in chaotic quantum dots. The paradox that we identify, originally perceived in classical networks, shows that the addition of more capacity to the network can suppress the current flow in the universal regime. We investigate the weak localization term, showing that it presents the paradox encoded in a saturation minimum of the conductance, under the presence of hyperflow in the external leads. In addition, we demonstrate that the weak localization suffers a transition signal depending on the overcapacity lead and presents an echo on the magnetic crossover before going to zero due to the full time-reversal symmetry breaking. We also show that the quantum interference contribution can dominate the Ohm term in the presence of constrictions and that the corresponding Fano factor engenders an anomalous behavior. PMID:25375575

Barbosa, A L R; Bazeia, D; Ramos, J G G S

2014-10-01

389

Biosensing with Luminescent Semiconductor Quantum Dots

Luminescent semiconductor nanocrystals or quantum dots (QDs) are a recently developed class of nanomaterial whose unique photophysical properties are helping to create a new generation of robust fluorescent biosensors. QD properties of interest for biosensing include high quantum yields, broad absorption spectra coupled to narrow size-tunable photoluminescent emissions and exceptional resistance to both photobleaching and chemical degradation. In this review, we examine the progress in adapting QDs for several predominantly in vitro biosensing applications including use in immunoassays, as generalized probes, in nucleic acid detection and fluorescence resonance energy transfer (FRET) - based sensing. We also describe several important considerations when working with QDs mainly centered on the choice of material(s) and appropriate strategies for attaching biomolecules to the QDs.

Sapsford, Kim E.; Pons, Thomas; Medintz, Igor L.; Mattoussi, Hedi

2006-01-01

390

Quantum-Dot Photodetectors: High Sensitivity due to Controllable Kinetics

NASA Astrophysics Data System (ADS)

Comparing to the quantum wells, the quantum-dot structures provide more opportunities to control electron kinetics and to optimize operating regimes of quantum-dot photodetectors. At room temperatures, the photoelectron capture in quantum-dot structures is determined by the electron diffusion in the potential of intentionally or unintentionally charged quantum dots1. Therefore, the capture time can be drastically increased by a proper choice of geometry of the quantum-dot structure and modulation doping. Suppression of capture processes provides longer lifetimes of photoelectrons, thus increasing the photoconductive gain and responsivity. Here we exploit a model of the QD detectors operating at room temperatures and study electron diffusion in the self-consistent field of potential barriers surrounding quantum dots. Using the Monte-Carlo method and analytical evaluations, we investigate photoelectron capture and transit processes as functions of the quantum dot positions, sensor geometry, and external electric field applied. Finally, we calculate the photoconductive gain and discuss the optimal structures and regimes. [1] A. Sergeev, V. Mitin, and M. Stroscio, Physica B 316-317, 369 (2002).

Sergeev, Andrei; Chien, Li-Hsin; Vagidov, Nizami; Mitin, Vladimir

2008-03-01

391

Fractional Josephson effect in a quadruple quantum dot

NASA Astrophysics Data System (ADS)

A double quantum dot coupled to an s-wave superconductor and subject to an inhomogeneous magnetic field can host a pair of zero-energy Majorana fermions when the dot properties are tuned appropriately. Here, we demonstrate the possibility of generating a fractional 4? Josephson effect in two such double dots tunnel-coupled to each other. We discuss the robustness of this effect with respect to perturbations away from the special point in parameter space where the uncoupled double dots host Majorana fermions. We demonstrate the possibility of generating Josephson effects with a period of 8? and 12? in strongly coupled double dots.

Sothmann, Björn; Li, Jian; Büttiker, Markus

2013-08-01

392

Quantum and classical thermoelectric transport in quantum dot nanocomposites

Quantum dot nanocomposites are potentially high-efficiency thermoelectric materials, which could outperform superlattices and random nanocomposites in terms of manufacturing cost-effectiveness and material properties because of the reduction of thermal conductivity due to the phonon-interface scattering, the enhancement of Seebeck coefficient due to the formation of minibands, and the enhancement of electrical conductivity due to the phonon-bottleneck effect in electron-phonon scattering

Jun Zhou; Ronggui Yang

2011-01-01

393

Optically programmable electron spin memory using semiconductor quantum dots

The spin of a single electron subject to a static magnetic field provides a natural two-level system that is suitable for use as a quantum bit, the fundamental logical unit in a quantum computer. Semiconductor quantum dots fabricated by strain driven self-assembly are particularly attractive for the realization of spin quantum bits, as they can be controllably positioned, electronically coupled

Miro Kroutvar; Yann Ducommun; Dominik Heiss; Max Bichler; Dieter Schuh; Gerhard Abstreiter; Jonathan J. Finley

2004-01-01

394

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

395

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

396

Colloidal quantum dot light-emitting devices

Colloidal quantum dot light-emitting devices (QD-LEDs) have generated considerable interest for applications such as thin film displays with improved color saturation and white lighting with a high color rendering index (CRI). We review the key advantages of using quantum dots (QDs) in display and lighting applications, including their color purity, solution processability, and stability. After highlighting the main developments in QD-LED technology in the past 15 years, we describe the three mechanisms for exciting QDs - optical excitation, Förster energy transfer, and direct charge injection - that have been leveraged to create QD-LEDs. We outline the challenges facing QD-LED development, such as QD charging and QD luminescence quenching in QD thin films. We describe how optical downconversion schemes have enabled researchers to overcome these challenges and develop commercial lighting products that incorporate QDs to achieve desirable color temperature and a high CRI while maintaining efficiencies comparable to inorganic white LEDs (>65 lumens per Watt). We conclude by discussing some current directions in QD research that focus on achieving higher efficiency and air-stable QD-LEDs using electrical excitation of the luminescent QDs. PMID:22110863

Wood, Vanessa; Bulovi?, Vladimir

2010-01-01

397

Competing interactions in semiconductor quantum dots

NASA Astrophysics Data System (ADS)

We introduce an integrability-based method enabling the study of semiconductor quantum dot models incorporating both the full hyperfine interaction as well as a mean-field treatment of dipole-dipole interactions in the nuclear spin bath. By performing free-induction decay and spin-echo simulations we characterize the combined effect of both types of interactions on the decoherence of the electron spin, for external fields ranging from low to high values. We show that for spin-echo simulations the hyperfine interaction is the dominant source of decoherence at short times for low fields, and competes with the dipole-dipole interactions at longer times. On the contrary, at high fields the main source of decay is due to the dipole-dipole interactions. In the latter regime an asymmetry in the echo is observed. Furthermore, the nondecaying fraction previously observed for zero-field free-induction decay simulations in quantum dots with only hyperfine interactions, is destroyed for longer times by the mean-field treatment of the dipolar interactions.

van den Berg, R.; Brandino, G. P.; El Araby, O.; Konik, R. M.; Gritsev, V.; Caux, J.-S.

2014-10-01

398

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. PMID:22916072

Zhang, Yi; Wang, Tza-Huei

2012-01-01

399

Behavior of optical bistability in multifold quantum dot molecules

NASA Astrophysics Data System (ADS)

We analyze the optical bistability (OB) behavior in a multifold quantum dot (QD) molecule composed of five quantum dots controlled by the tunneling coupling. It is shown that the optical bistability can strongly be affected by the tunneling inter-dot coupling coefficients as well as detuning parameters. In addition, we find that the rate of an incoherent pump field has a leading role in modification of the OB threshold. We then generalize our analysis to the case of multifold quantum dot molecules where the number of the quantum dots is N (with a center dot and N-1 satellite dots). We compare the OB features that could occur in a multifold QD system consist of three (N= ), four (N=\\text{4} ), and five (N = 5) quantum dots. We realize that the OB threshold increases as the number of satellite QDs increases. Such controllable optical bistability in multiple QD molecules may provide some new possibilities for technological applications in optoelectronics and solid-state quantum information science.

Hamedi, H. R.; Mehmannavaz, M. R.

2015-02-01

400

Electronic transport properties of coupled quantum dots on carbon nanotubes.

We investigate the electronic transport properties of coupled quantum dots, controlled by local gates on carbon nanotubes. The inter-dot coupling can be tuned from weak to strong by changing gate voltages, and oscillates in short and long period with the distance between two gates. We introduce a one-dimensional scattering model to describe the mechanism of the electron transport through the carbon nanotube quantum dots. We show that pi and PI* channels contribute differently to the inter-dot coupling and the transport phase plays a key role in the oscillations of the coupling. PMID:21125895

Qian, Haiyun; Lu, Jun-Qiang

2010-08-01

401

Resonant hyper-Raman and second-harmonic scattering in a CdS quantum-dot system

NASA Astrophysics Data System (ADS)

Excitation profiles of resonant second-harmonic and hyper-Raman scattering in a CdS quantum-dot system are reported. Two maxima have been found in the related two-photon excitation spectra near the fundamental absorption region. Relative to the one-photon absorption spectrum, the maxima are more pronounced and shifted in energy. The result has revealed quantum-confined electron-hole pair states with a total angular momentum of odd number. Effective masses in the quantum dots have been estimated.

Baranov, A. V.; Inoue, K.; Toba, K.; Yamanaka, A.; Petrov, V. I.; Fedorov, A. V.

1996-01-01

402

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

403

Enhancement of photoluminescence in ZnS/ZnO quantum dots interfacial heterostructures

Highlights: ? ZnS/ZnO quantum dots (QDs) were synthesized by controlled oxidation of ZnS nanoparticles. ? Interfacial heterostructure formation of ZnS/ZnO QDs is seen in HRTEM. ? Enormous enhancement of UV emission (?10 times) in ZnS/ZnO QDs heterostructure is observed. ? Phonon confinement effect is seen in the Raman spectrum. -- Abstract: ZnS/ZnO quantum dots (QDs) were synthesized by controlled oxidation of ZnS nanoparticles. HRTEM image showed small nanocrystals of size 4 nm and the magnified image of single quantum dot shows interfacial heterostructure formation. The optical absorption spectrum shows a blue shift of 0.19 and 0.23 eV for ZnO and ZnS QDs, respectively. This is due to the confinement of charge carries within the nanostructures. Enormous enhancement in UV emission (10 times) is reported which is attributed to interfacial heterostructure formation. Raman spectrum shows phonons of wurtzite ZnS and ZnO. Phonon confinement effect is seen in the Raman spectrum wherein LO phonon peaks of ZnS and ZnO are shifted towards lower wavenumber side and are broadened.

Rajalakshmi, M., E-mail: Rajingmmsd@gmail.com [Condensed Matter Physics Division, Indira Gandhi Centre for Atomic Research, Kalpakkam 603102 (India); Sohila, S.; Ramesh, R. [Centre for Materials Science and Nano Devices, Department of Physics, SRM University, Kattankulathur 603 203, Chennai (India)] [Centre for Materials Science and Nano Devices, Department of Physics, SRM University, Kattankulathur 603 203, Chennai (India); Bhalerao, G.M. [UGC-DAE Consortium for Scientific Research (Kalpakkam node), Kalpakkam 603102 (India)] [UGC-DAE Consortium for Scientific Research (Kalpakkam node), Kalpakkam 603102 (India)

2012-09-15

404

Cavity Enhanced Faraday Rotation in Semiconductor Quantum Dots

NASA Astrophysics Data System (ADS)

The promise of quantum computation is helping fuel the development of single spin manipulation and measurement techniques. Photonic cavities provide an intriguing platform to increase the sensitivity of optical measurements, as well as the possibility to explore emergent light-matter interactions. The flexibility of a dielectric vertical cavity is exploited to study the spin dynamics within molecularly self-assembled CdSe quantum dots (QDs). Through the integration of QDs in microcavities, a twenty-fold enhancement of Faraday rotation is observed, which scales with the quality factor of the cavity. In this weak coupling regime, the amplified rotation is attributed to optically generated excited spins and multiple passes of the probe photons in the cavity. By applying this general planar cavity motif to Faraday rotation, dynamical measurements are accessible at extremely low powers on relatively small numbers of quantum confined states. In CdSe QDs, low power measurements reveal that contributions from both exciton and electron spin precession are largely dependent upon the power of excitation. This scheme is amenable to both soft and hard systems as a means to increase detection sensitivity.

Steuerman, D. W.; Li, Y. Q.; Berezovsky, J.; Seferos, D. S.; Bazan, G. C.; Awschalom, D. D.

2006-03-01

405

Quantum dot mode locked lasers for coherent frequency comb generation

NASA Astrophysics Data System (ADS)

Monolithic semiconductor passively mode locked lasers (MLL) are very attractive components for many applications including high bit rate telecommunications, microwave photonics and instrumentation. Owing to the three dimensional confinement of the charge carriers, quantum dot based mode-locked lasers have been the subject of intense investigations because of their improved performance compared to conventional material systems. Indeed, the inhomogeneous gain broadening and the ultrafast absorption recovery dynamics are an asset for short pulse generation. Moreover, the weak coupling of amplified spontaneous emission with the guided modes plus low loss waveguide leads to low timing jitter. Our work concentrates on InAs quantum dash nanostructures grown on InP substrate, intended for applications in the 1.55 ?m telecom window. InAs/InP quantum dash based lasers, in particular, have demonstrated efficient mode locking in single section Fabry-Perot configurations. The flat optical spectrum of about 12 nm, combined with the narrow RF beat note linewidth of about 10 kHz make them a promising technology for optical frequency comb generation. Coherence between spectral modes was assessed by means of spectral phase measurements. The parabolic spectral phase profile indicates that short pulses can be obtained provided the intracavity dispersion can be compensated by inserting a single mode fiber.

Martinez, A.; Calò, C.; Rosales, R.; Watts, R. T.; Merghem, K.; Accard, A.; Lelarge, F.; Barry, L. P.; Ramdane, A.

2013-12-01

406

Mechanisms of Auger recombination in semiconducting quantum dots

Microscopic calculation of the probability of Auger recombination of charge carriers localized in a semiconducting quantum dot (QD) is carried out. It is shown that two mechanism of Auger recombination (nonthreshold and quasi-threshold) operate in the QD. The nonthreshold Auger recombination mechanism is associated with scattering of a quasimomentum from a heterobarrier, while the quasi-threshold mechanism is connected with spatial confinement of the wave functions of charge carriers to the QD region; scattering of carriers occurs at the short-range Coulomb potential. Both mechanisms lead to a substantial enhancement of Auger recombination at the QD as compared to a homogeneous semiconductor. A detailed analysis of the dependence of Auger recombination coefficient on the temperature and QD parameters is carried out. It is shown that the nonthreshold Auger recombination process dominates at low temperatures, while the quasi-threshold mechanism prevails at high temperatures. The dependence of the Auger recombination coefficient on the QD radius experiences noticeable changes as compared to quantum wells and quantum filaments.

Zegrya, G. G., E-mail: zegrya@theory.ioffe.ru; Samosvat, D. M. [Russian Academy of Sciences, Ioffe Physicotechnical Institute (Russian Federation)

2007-06-15

407

Nonequilibrium electron transport in quantum dot and quantum point contact systems

Much experimental research has been performed in the equilibrium regime on individual quantum dots and quantum point contacts (QPCs). The focus of the research presented here is electron transport in the nonequilibrium regime in coupled quantum dot and QPC systems fabricated on AlGaAs\\/GaAs material using the split gate technique. Near equilibrium magnetoconductance measurements were performed on a quantum dot and

Anasuya Erin Krishnaswamy

1999-01-01

408

We design a photopolymerization, in which Mn-doped ZnS quantum dots (ZnS:Mn2+) initiate the polymerization of acrylic acid, to convert the non-cytotoxic quantum dots to water-soluble ones for biological chromophores The prepared quantum dots are nearly monodispersed in water and the resulting solution shows long-term stability for months. The water-soluble ZnS:Mn2+ quantum dots exhibit high quantum efficiency of fluorescence. The polymerization

Xufeng Liu; Xiuyuan Ni; Jiao Wang; Xinghai Yu

2008-01-01

409

Resonant Raman scattering by strained and relaxed germanium quantum dots

This paper reports on the results of resonant Raman scattering investigations of the fundamental vibrations in Ge\\/Si structures\\u000a with strained and relaxed germanium quantum dots. Self-assembled strained Ge\\/Si quantum dots are grown by molecular-beam epitaxy\\u000a on Si(001) substrates. An ultrathin SiO2 layer is grown prior to the deposition of a germanium layer with the aim of forming relaxed germanium quantum

A. G. Milekhin; A. I. Nikiforov; M. Yu. Ladanov; O. P. Pchelyakov; S. Schulze; D. R. T. Zahn

2004-01-01

410

Effects of multiple organic ligands on size uniformity and optical properties of ZnSe quantum dots

Highlights: ? Highly monodispersed ZnSe quantum dots have been synthesized by wet chemical route. ? Strong quantum confinement effect have been observed in ? 4 nm ZnSe quantum dots. ? Enhanced ultraviolet near band emission have been obtained using long chain polymer. -- Abstract: The effects of multi-ligands on the formation and optical transitions of ZnSe quantum dots have been investigated. The dots are synthesized using 3-mercapto-1,2-propanediol and polyvinylpyrrolidone ligands, and have been characterized by X-ray diffraction, transmission electron microscopy (TEM), UV–visible absorption spectroscopy, photoluminescence spectroscopy, and Fourier transform infrared spectroscopy. TEM reveals high monodispersion with an average size of 4 nm. Polymer-stabilized, organic ligand-passivated ZnSe quantum dots exhibit strong UV emission at 326 nm and strong quantum confinement in the UV–visible absorption spectrum. Uniform size and suppressed surface trap emission are observed when the polymer ligand is used. The possible growth mechanism is discussed.

Archana, J., E-mail: archana.jayaram@yahoo.com [Research Institute of Electronics, Shizuoka University, 3-5-1 Johoku, Naka-ku, Hamamatsu, Shizuoka 432-8011 (Japan); Navaneethan, M.; Hayakawa, Y. [Research Institute of Electronics, Shizuoka University, 3-5-1 Johoku, Naka-ku, Hamamatsu, Shizuoka 432-8011 (Japan)] [Research Institute of Electronics, Shizuoka University, 3-5-1 Johoku, Naka-ku, Hamamatsu, Shizuoka 432-8011 (Japan); Ponnusamy, S.; Muthamizhchelvan, C. [Department of Physics, SRM University, Kattankulathur 603203, Tamil Nadu (India)] [Department of Physics, SRM University, Kattankulathur 603203, Tamil Nadu (India)

2012-08-15

411

Spin Dynamics of a Single Mn Ion in a CdTe/(Cd, Mg, Zn)Te Quantum Dot

The spin dynamics of a single Mn ion confined in a CdTe/(Cd, Mg, Zn)Te quantum dot is determined by measurements of photon correlation of photoluminescence. The characteristic time of spin flip is a few nanoseconds and strongly depends on the excitation power.

Goryca, Mateusz; Kossacki, Piotr; Golnik, Andrzej; Kazimierczuk, Tomasz; Nawrocki, Michal [Institute of Experimental Physics, University of Warsaw, Hoza 69, 00-681 Warszawa (Poland); Wojnar, Piotr [Institute of Physics, Polish Academy of Sciences, al. Lotnikow 32/46, 02-668 Warszawa (Poland)

2010-01-04

412

A computational technique for the energy levels calculation of an electron confined by a 3D InAs quantum dot (QD) embedded in GaAs semiconductor matrix is presented. Based on the effective one electronic band Hamiltonian, the energy and position dependent electron effective mass approximation, a finite height hard-wall 3D confinement potential, and the Ben Daniel–Duke boundary conditions, the problem is formulated

Yiming Li; O. Voskoboynikov; C. P. Lee; S. M. Sze

2001-01-01

413

NASA Astrophysics Data System (ADS)

In this paper simultaneous effects of pressure, temperature, external electric field and laser radiation on the optical rectification and third harmonic generation of a spherical quantum dot with parabolic confinement and dressed impurity are studied. By means of matrix diagonalization technique, energy eigenvalues and functions are evaluated and used to find the optical rectification coefficient and third harmonic generation of the system via density operator method. It is shown that these nonlinear optical quantities strongly depend on pressure, temperature, electric field, confinement frequency and dressing laser intensity. Obvious effects of these external factors propose new facilities with different effects to control nonlinear optical properties of such systems.

Vaseghi, B.; Sadri, M.; Rezaei, G.; Gharaati, A.

2015-01-01

414

NASA Astrophysics Data System (ADS)

The binding energy of hydrogenic impurity associated with the ground state and some low-lying states in a GaAs spherical parabolic quantum dot with taking into account hydrostatic pressure and electric field are theoretically studied by using the configuration-integration method. The binding energies of these low-lying states of the impurity depend sensitively on the hydrostatic pressure, electric field and the strength of the parabolic confinement. Based on the analysis of these impurity states, we propose a way for preparation of quantum bit (qubit) by using the strong quantum confinement to the impurity in the quantum dot. Also we calculate the wave functions of some low-lying states to discuss the oscillator strength which is related to the electronic dipole-allowed transitions from 0s state to 0p state. The results show that the electronic dipole-allowed transitions mostly happen between the 0s state and 0p state, especially for the quantum confinement large enough.

Yuan, Jian-Hui; Zhang, Yan; Li, Meng; Wu, Zhi-Hui; Mo, Hua

2014-10-01

415

Shape, strain, and ordering of lateral InAs quantum dot molecules

The results of an x-ray study on freestanding, self-assembled InAs\\/GaAs quantum dots grown by molecular beam epitaxy are presented. The studied samples cover the range from statistically distributed single quantum dots to quantum dot bimolecules, and finally to quantum dot quadmolecules. The x-ray diffraction data of the single quantum dots and the bimolecules, obtained in grazing incidence geometry, have been

B. Krause; T. H. Metzger; A. Rastelli; R. Songmuang; S. Kiravittaya; O. G. Schmidt

2005-01-01

416

Quantum dot nanoparticle conjugation, characterization, and applications in neuroscience

NASA Astrophysics Data System (ADS)

Quantum dot are semiconducting nanoparticles that have been used for decades in a variety of applications such as solar cells, LEDs and medical imaging. Their use in the last area, however, has been extremely limited despite their potential as revolutionary new biological labeling tools. Quantum dots are much brighter and more stable than conventional fluorophores, making them optimal for high resolution imaging and long term studies. Prior work in this area involves synthesizing and chemically conjugating quantum dots to molecules of interest in-house. However this method is both time consuming and prone to human error. Additionally, non-specific binding and nanoparticle aggregation currently prevent researchers from utilizing this system to its fullest capacity. Another critical issue that has not been addressed is determining the number of ligands bound to nanoparticles, which is crucial for proper interpretation of results. In this work, methods to label fixed cells using two types of chemically modified quantum dots are studied. Reproducible non-specific artifact labeling is consistently demonstrated if antibody-quantum dot conditions are less than optimal. In order to explain this, antibodies bound to quantum dots were characterized and quantified. While other groups have qualitatively characterized antibody functionalized quantum dots using TEM, AFM, UV spectroscopy and gel electrophoresis, and in some cases have reported calculated estimates of the putative number of total antibodies bound to quantum dots, no quantitative experimental results had been reported prior to this work. The chemical functionalization and characterization of quantum dot nanocrystals achieved in this work elucidates binding mechanisms of ligands to nanoparticles and allows researchers to not only translate our tools to studies in their own areas of interest but also derive quantitative results from these studies. This research brings ease of use and increased reliability to nanoparticles in medical imaging.

Pathak, Smita

417

Magneto-optical studies of quantum dots

NASA Astrophysics Data System (ADS)

Significant effort in condensed matter physics has recently been devoted to the field of "spintronics" which seeks to utilize the spin degree of freedom of electrons. Unlike conventional electronics that rely on the electron charge, devices exploiting their spin have the potential to yield new and novel technological applications, including spin transistors, spin filters, and spin-based memory devices. Any such application has the following essential requirements: 1) Efficient electrical injection of spin-polarized carriers; 2) Long spin lifetimes; 3) Ability to control and manipulate electron spins; 4) Effective detection of spin-polarized carriers. Recent work has demonstrated efficient electrical injection from ferromagnetic contacts such as Fe and MnAs, utilizing a spin-Light Emitting Diode (spin-LED) as a method of detection. Semiconductor quantum dots (QDs) are attractive candidates for satisfying requirements 2 and 3 as their zero dimensionality significantly suppresses many spin-flip mechanisms leading to long spin coherence times, as well as enabling the localization and manipulation of a controlled number of electrons and holes. This thesis is composed of three projects that are all based on the optical properties of QD structures including: I) Intershell exchange between spin-polarized electrons occupying adjacent shells in InAs QDs; II) Spin-polarized multiexitons in InAs QDs in the presence of spin-orbit interactions; III) The optical Aharonov-Bohm effect in AlxGa1-xAs/AlyGa1-yAs quantum wells (QWs). In the following we introduce some of the basic optical properties of quantum dots, describe the main tool (spin-LED) employed in this thesis to inject and detect spins in these QDs, and conclude with the optical Aharonov-Bohm effect (OAB) in type-II QDs.

Russ, Andreas Hans

418

Improved dot size uniformity and luminescense of InAs quantum dots on InP substrate

NASA Technical Reports Server (NTRS)

InAs self-organized quantum dots have been grown in InGaAs quantum well on InP substrates by metalorganic vapor phase epitaxy. Atomic Force Microscopy confirmed of quantum dot formation with dot density of 3X10(sup 10) cm(sup -2). Improved dot size uniformity and strong room temperature photoluminescence up to 2 micron were observed after modifying the InGaAs well.

Qiu, Y.; Uhl, D.

2002-01-01

419

Full counting statistics of quantum dot resonance fluorescence

The electronic energy levels and optical transitions of a semiconductor quantum dot are subject to dynamics within the solid-state environment. In particular, fluctuating electric fields due to nearby charge traps or other quantum dots shift the transition frequencies via the Stark effect. The environment dynamics are mapped directly onto the fluorescence under resonant excitation and diminish the prospects of quantum dots as sources of indistinguishable photons in optical quantum computing. Here, we present an analysis of resonance fluorescence fluctuations based on photon counting statistics which captures the underlying time-averaged electric field fluctuations of the local environment. The measurement protocol avoids dynamic feedback on the electric environment and the dynamics of the quantum dot's nuclear spin bath by virtue of its resonant nature and by keeping experimental control parameters such as excitation frequency and external fields constant throughout. The method introduced here is experimentally undemanding. PMID:24810097

Matthiesen, Clemens; Stanley, Megan J.; Hugues, Maxime; Clarke, Edmund; Atatüre, Mete

2014-01-01

420

Full counting statistics of quantum dot resonance fluorescence.

The electronic energy levels and optical transitions of a semiconductor quantum dot are subject to dynamics within the solid-state environment. In particular, fluctuating electric fields due to nearby charge traps or other quantum dots shift the transition frequencies via the Stark effect. The environment dynamics are mapped directly onto the fluorescence under resonant excitation and diminish the prospects of quantum dots as sources of indistinguishable photons in optical quantum computing. Here, we present an analysis of resonance fluorescence fluctuations based on photon counting statistics which captures the underlying time-averaged electric field fluctuations of the local environment. The measurement protocol avoids dynamic feedback on the electric environment and the dynamics of the quantum dot's nuclear spin bath by virtue of its resonant nature and by keeping experimental control parameters such as excitation frequency and external fields constant throughout. The method introduced here is experimentally undemanding. PMID:24810097

Matthiesen, Clemens; Stanley, Megan J; Hugues, Maxime; Clarke, Edmund; Atatüre, Mete

2014-01-01

421

Full counting statistics of quantum dot resonance fluorescence

NASA Astrophysics Data System (ADS)

The electronic energy levels and optical transitions of a semiconductor quantum dot are subject to dynamics within the solid-state environment. In particular, fluctuating electric fields due to nearby charge traps or other quantum dots shift the transition frequencies via the Stark effect. The environment dynamics are mapped directly onto the fluorescence under resonant excitation and diminish the prospects of quantum dots as sources of indistinguishable photons in optical quantum computing. Here, we present an analysis of resonance fluorescence fluctuations based on photon counting statistics which captures the underlying time-averaged electric field fluctuations of the local environment. The measurement protocol avoids dynamic feedback on the electric environment and the dynamics of the quantum dot's nuclear spin bath by virtue of its resonant nature and by keeping experimental control parameters such as excitation frequency and external fields constant throughout. The method introduced here is experimentally undemanding.

Matthiesen, Clemens; Stanley, Megan J.; Hugues, Maxime; Clarke, Edmund; Atatüre, Mete

2014-05-01

422

Entanglement creation in semiconductor quantum dot charge qubit

We study theoretically the appearance of quantum correlations in two- and three-electron scattering in single and double dots. The key role played by transport resonances into entanglement formation between the single-particle states is shown. Both reflected and transmitted components of the scattered particle wavefunction are used to evaluate the quantum correlations between the incident carrier and the bound particle(s) in the dots. Our investigation provides a guideline for the analysis of decoherence effects due to the Coulomb scattering in semiconductor quantum dots structures.

Fabrizio Buscemi; Paolo Bordone; Andrea Bertoni

2010-06-03

423

Entanglement creation in semiconductor quantum dot charge qubit

We study theoretically the appearance of quantum correlations in two- and three-electron scattering in single and double dots. The key role played by transport resonances into entanglement formation between the single-particle states is shown. Both reflected and transmitted components of the scattered particle wavefunction are used to evaluate the quantum correlations between the incident carrier and the bound particle(s) in the dots. Our investigation provides a guideline for the analysis of decoherence effects due to the Coulomb scattering in semiconductor quantum dots structures.

Buscemi, Fabrizio; Bertoni, Andrea

2010-01-01

424

Semiconductor Quantum Dots in Chemical Sensors and Biosensors

Quantum dots are nanometre-scale semiconductor crystals with unique optical properties that are advantageous for the development of novel chemical sensors and biosensors. The surface chemistry of luminescent quantum dots has encouraged the development of multiple probes based on linked recognition molecules such as peptides, nucleic acids or small-molecule ligands. This review overviews the design of sensitive and selective nanoprobes, ranging from the type of target molecules to the optical transduction scheme. Representative examples of quantum dot-based optical sensors from this fast-moving field have been selected and are discussed towards the most promising directions for future research. PMID:22423206

Frasco, Manuela F.; Chaniotakis, Nikos

2009-01-01

425

Numerical simulation of electronic properties of coupled quantum dots on wetting layers.

Self-assembled quantum dots are grown on wetting layers and frequently in an array-like assembly of many similar but not exactly equal dots. Nevertheless, most simulations disregard these structural conditions and restrict themselves to simulating a pure single quantum dot. For reasons of numerical efficiency we advocate the effective one-band Hamiltonian with energy- and position-dependent effective mass approximation and a finite height hard-wall 3D confinement potential for computation of the energy levels of the electrons in the conduction band. Within this model we investigate the geometrical effects mentioned above on the electronic structure of a pyramidal InAs quantum dot embedded in a GaAs matrix. We find that the presence of a wetting layer may affect the electronic structure noticeably. Furthermore, we establish that, in spite of the large bandgap of the InAs/GaAs heterostructure, if the dots in a vertically aligned array are sufficiently close stacked there is considerable interaction between their eigenfunctions. Moreover, the eigenfunctions of such an array are quite sensitive to certain structural perturbations. PMID:21825638

Betcke, M M; Voss, H

2008-04-23

426

Interaction effects on the tunneling of electron-hole pairs in coupled quantum dots

NASA Astrophysics Data System (ADS)

The transit time of carriers is beginning to be an important parameter in the physical operation of semiconductor quantum dot `devices'. In the present work, we study the coherent propagation of electron-hole pairs in coupled self-assembled quantum dots in close proximity. These systems, achieved experimentally in a number of different geometries, have been recently implemented as a novel storage of optical information that may give rise to smart pixel technology in the near future [1]. Here, we apply an effective mass hamiltonian approach and solve numerically the time dependent Schroedinger equation of a system of photo-created electron-hole pairs in the dots. Our approach takes into account both Coulomb interactions and confinement effects. The time evolution is investigated in terms of the structural parameters for typical InAs-GaAs dots. Different initial conditions are considered, reflecting the basic processes that would take place in these experiments. We study the probabilities of finding the electron and hole in either the same or adjacent quantum dot, and study carefully the role of interactions in this behavior. [1] T. Lundstrom, W. Schoenfeld, H. Lee, and P. M. Petroff, Science 286, 2312 (1999).

Guerrero, Hector M.; Cocoletzi, Gregorio H.; Ulloa, Sergio E.

2001-03-01

427

Hyper-parallel photonic quantum computation with coupled quantum dots

NASA Astrophysics Data System (ADS)

It is well known that a parallel quantum computer is more powerful than a classical one. So far, there are some important works about the construction of universal quantum logic gates, the key elements in quantum computation. However, they are focused on operating on one degree of freedom (DOF) of quantum systems. Here, we investigate the possibility of achieving scalable hyper-parallel quantum computation based on two DOFs of photon systems. We construct a deterministic hyper-controlled-not (hyper-CNOT) gate operating on both the spatial-mode and the polarization DOFs of a two-photon system simultaneously, by exploiting the giant optical circular birefringence induced by quantum-dot spins in double-sided optical microcavities as a result of cavity quantum electrodynamics (QED). This hyper-CNOT gate is implemented by manipulating the four qubits in the two DOFs of a two-photon system without auxiliary spatial modes or polarization modes. It reduces the operation time and the resources consumed in quantum information processing, and it is more robust against the photonic dissipation noise, compared with the integration of several cascaded CNOT gates in one DOF.

Ren, Bao-Cang; Deng, Fu-Guo

2014-04-01

428

Hyper-parallel photonic quantum computation with coupled quantum dots

It is well known that a parallel quantum computer is more powerful than a classical one. So far, there are some important works about the construction of universal quantum logic gates, the key elements in quantum computation. However, they are focused on operating on one degree of freedom (DOF) of quantum systems. Here, we investigate the possibility of achieving scalable hyper-parallel quantum computation based on two DOFs of photon systems. We construct a deterministic hyper-controlled-not (hyper-CNOT) gate operating on both the spatial-mode and the polarization DOFs of a two-photon system simultaneously, by exploiting the giant optical circular birefringence induced by quantum-dot spins in double-sided optical microcavities as a result of cavity quantum electrodynamics (QED). This hyper-CNOT gate is implemented by manipulating the four qubits in the two DOFs of a two-photon system without auxiliary spatial modes or polarization modes. It reduces the operation time and the resources consumed in quantum information processing, and it is more robust against the photonic dissipation noise, compared with the integration of several cascaded CNOT gates in one DOF. PMID:24721781

Ren, Bao-Cang; Deng, Fu-Guo

2014-01-01

429

Coherent population transfer in coupled semiconductor quantum dots

We propose a solid-state implementation of stimulated Raman adiabatic passage in two coupled semiconductor quantum dots. Proper combination of two pulsed laser fields allows the coherent carrier transfer between the two nanostructures without suffering significant losses due to environment coupling. By use of a general solution scheme for the carrier states in the double-dot structure, we identify the pertinent dot and laser parameters.

Ulrich Hohenester; Giovanna Panzarini; Filippo Troiani; Elisa Molinari; Chiara Macchiavello

2000-08-03

430

Swapping Kondo resonances in coupled double quantum dots

Strong electron and spin correlations are studied in parallel-coupled double quantum dots with interdot spin superexchange J . In the Kondo regime with degenerate dot energy levels, coherent transport occurs at zero temperature, where two entangled (bonding and antibonding) resonances are formed near the Fermi energy. When increasing J or the dot-lead parallel-coupling asymmetry ratio Gamma2\\/Gamma1 , a swap between

Guang-Ming Zhang; Rong Lü; Zhi-Rong Liu; Lu Yu

2005-01-01

431

Band gap engineering of amorphous silicon quantum dots for light-emitting diodes

NASA Astrophysics Data System (ADS)

Amorphous silicon quantum dots (a-Si QDs), which show a quantum confinement effect were grown in a silicon nitride film by plasma-enhanced chemical vapor deposition. Red, green, blue, and white photoluminescence were observed from the a-Si QD structures by controlling the dot size. An orange light-emitting diode (LED) was fabricated using a-Si QDs with a mean size of 2.0 nm. The turn-on voltage was less than 5 V. An external quantum efficiency of 2×10-3% was also demonstrated. These results show that a LED using a-Si QDs embedded in the silicon nitride film is superior in terms of electrical and optical properties to other Si-based LEDs.

Park, Nae-Man; Kim, Tae-Soo; Park, Seong-Ju

2001-04-01

432

Semiconductor Quantum Dots B. Billaud and M. Picco Laboratoire de Physique ThÂ´eorique et Hautes Energies (LPTHE semiconductor quantum dots is put to question. A sharper theoretical approach is suggested based on a new pseudo on their dimensionality, these structures are called quantum dots (0D), quantum wires (1D) or quantum wells (2D

Paris-Sud XI, UniversitÃ© de

433

Nonequilibrium transport through a Josephson quantum dot

NASA Astrophysics Data System (ADS)

We study the electronic current through a quantum dot coupled to two superconducting leads which is driven by either a voltage V or temperature ?T bias. Finite biases beyond the linear response regime are considered. The local two-particle interaction U on the dot is treated using an approximation scheme within the functional renormalization group approach set up in Keldysh-Nambu space with U being the small parameter. For V >0, we compare our renormalization group enhanced results for the dc component of the current to earlier weak coupling approaches such as the Hartree-Fock approximation and second-order perturbation theory in U. We show that in parameter regimes in which finite-bias-driven multiple Andreev reflections prevail, small-|U| approaches become unreliable for interactions of appreciable strength. In the complementary regime, the convergence of the current with respect to numerical parameters becomes an issue, but can eventually be achieved, and interaction effects turn out to be smaller than expected based on earlier results. For ?T >0, we find a surprising increase of the current as a function of the superconducting phase difference in the regime which at T =0 becomes the ? (doublet) phase.

Rentrop, J. F.; Jakobs, S. G.; Meden, V.

2014-06-01

434

Ferritin-Templated Quantum-Dots for Quantum Logic Gates

NASA Technical Reports Server (NTRS)

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 production in uniform size of nano-scale proportion. The bio-template for uniform yield of QD is based on a ferritin protein that allows reconstitution of core material through the reduction and chelation processes. One of the biggest challenges for developing QLG is the requirement of ordered and uniform size of QD for arrays on a substrate with nanometer precision. The QD development by bio-template includes the electrochemical/chemical reconsitution of ferritins with different core materials, such as iron, cobalt, manganese, platinum, and nickel. The other bio-template method used in our laboratory is dendrimers, precisely defined chemical structures. With ferritin-templated QD, we fabricated the heptagonshaped patterned array via direct nano manipulation of the ferritin molecules with a tip of atomic force microscope (AFM). We also designed various nanofabrication methods of QD arrays using a wide range manipulation techniques. The precise control of the ferritin-templated QD for a patterned arrangement are offered by various methods, such as a site-specific immobilization of thiolated ferritins through local oxidation using the AFM tip, ferritin arrays induced by gold nanoparticle manipulation, thiolated ferritin positioning by shaving method, etc. In the signal measurements, the current-voltage curve is obtained by measuring the current through the ferritin, between the tip and the substrate for potential sweeping or at constant potential. The measured resistance near zero bias was 1.8 teraohm for single holoferritin and 5.7 teraohm for single apoferritin, respectively.

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

2005-01-01

435

Spectroscopy and energy level statistics in a disordered quantum dot

The spectrum of heavily doped quantum dots is found to be discrete only in close vicinity to the Fermi energy. Levels further away are broadened beyond the average level spacing and merge to form a quasi-continuous spectrum. This breakdown of a single particle picture is consistent with electron-electron interaction in the dot. For the discrete part of the spectrum, level

U. Sivan; F. P. Milliken; K. Milkove; S. Rishton; Y. Lee; J. M. Hong; V. Boegli; D. Kern; M. de Franza

1993-01-01

436

Interferometric Rayleigh Scattering by Excitons in a Single Quantum Dot

Considerable progress in realizing various regimes of cou- pling between the electromagnetic field and semiconductor quantum dots (QDs) has been achieved recently. Observations of phenomena related to interaction of photons with discrete states in self-assembled dots such as ground state Rabi oscil- lations (1), weak (2, 3, 4) and strong (5, 6) coupling regimes in various microcavity structures have strengthened

Benito Alen; Alexander Hogele; Martin Kroner; Stefan Seidl; Khaled Karrai; Richard J. Warburton; Antonio Badolato; Gilberto Medeiros-Ribeiro; Pierre M. Petroff

2005-01-01

437

Fractal behavior in magnetoconductance in coupled quantum dot systems

Fractal behavior in magnetoconductance fluctuations in coupled quantum dots has been studied by means of exact and statistical self-similarity. The fractal dimensions from the different features are not coincident exactly but show the similar gate voltage dependences, where the values increase with increasing negative gate voltage. Moreover, results of statistical fractal dimensions obtained from two types of dot-array samples show

Nobuyuki Aoki; Li-Hung Lin; Takahiro Morimoto; Takahiko Sasaki; Jun-Feng Song; Koji Ishibashi; Jonathan P. Bird; Agung Budiyono; Katsuhiro Nakamura; Takahisa Harayama; Yuichi Ochiai

2004-01-01

438

Spectroscopy of electronic states in InSb quantum dots

Arrays of quantum dots on InSb have been realized, and intraband transitions between their discrete (zero-dimensional) electronic states have been observed with far-infrared magnetospectroscopy. In the devices, the number of electrons can be adjusted by a gate voltage, and less than five electrons per dot are detectable.

Ch. Sikorski; U. Merkt

1989-01-01

439

Quantum Circuit based on Electron Spins in Semiconductor Quantum Dots

NASA Astrophysics Data System (ADS)

In this thesis, I present a microscopic theory of quantum circuits based on interacting electron spins in quantum dot molecules. We use the Linear Combination of Harmonic Orbitals-Configuration Interaction (LCHO-CI) formalism for microscopic calculations. We then derive effective Hubbard, t-J, and Heisenberg models. These models are used to predict the electronic, spin and transport properties of a triple quantum dot molecule (TQDM) as a function of topology, gate configuration, bias and magnetic field. With these theoretical tools and fully characterized TQDMs, we propose the following applications: 1. Voltage tunable qubit encoded in the chiral states of a half-filled TQDM. We show how to perform single qubit operations by pulsing voltages. We propose the "chirality-to-charge" conversion as the measurement scheme and demonstrate the robustness of the chirality-encoded qubit due to charge fluctuations. We derive an effective qubit-qubit Hamiltonian and demonstrate the two-qubit gate. This provides all the necessary operations for a quantum computer built with chirality-encoded qubits. 2. Berry's phase. We explore the prospect of geometric quantum computing with chirality-encoded qubit. We construct a Herzberg circuit in the voltage space and show the accumulation of Berry's phase. 3. Macroscopic quantum states on a semiconductor chip. We consider a linear chain of TQDMs, each with 4 electrons, obtained by nanostructuring a metallic gate in a field effect transistor. We theoretically show that the low energy spectrum of the chain maps onto that of a spin-1 chain. Hence, we show that macroscopic quantum states, protected by a Haldane gap from the continuum, emerge. In order to minimize decoherence of electron spin qubits, we consider using electron spins in the p orbitals of the valence band (valence holes) as qubits. We develop a theory of valence hole qubit within the 4-band k.p model. We show that static magnetic fields can be used to perform single qubit operations. We also show that the qubit-qubit interactions are sensitive to the geometry of a quantum dot network. For vertical qubit arrays, we predict that there exists an optimal qubit separation suitable for the voltage control of qubit-qubit interactions.

Hsieh, Chang-Yu

440

Luminescence studies of individual quantum dot photocatalysts.

Using far-field optical microscopy we report the first measurements of photoluminescence from single nanoparticle photocatalysts. Fluence-dependent luminescence is investigated from metal-semiconductor heterojunction quantum dot catalysts exposed to a variety of environments, ranging from gaseous argon to liquid water containing a selection of hole scavengers. The catalysts each exhibit characteristic nonlinear fluence dependence. From these structurally and environmentally sensitive trends, we disentangle the separate rate-determining steps in each particle across the very wide range of time scales, which follow the initial light absorption process. This information will significantly benefit the design of effective artificial photocatalytic systems for renewable direct solar-to-fuel energy conversion. PMID:23895591

Amirav, Lilac; Alivisatos, A Paul

2013-09-01

441

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

2012-03-28

442

Highly Fluorescent Noble Metal Quantum Dots

Highly fluorescent, water-soluble, few-atom noble metal quantum dots have been created that behave as multi-electron artificial atoms with discrete, size-tunable electronic transitions throughout the visible and near IR. These “molecular metals” exhibit highly polarizable transitions and scale in size according to the simple relation, Efermi/N1/3, predicted by the free electron model of metallic behavior. This simple scaling indicates that fluorescence arises from intraband transitions of free electrons and that these conduction electron transitions are the low number limit of the plasmon – the collective dipole oscillations occurring when a continuous density of states is reached. Providing the “missing link” between atomic and nanoparticle behavior in noble metals, these emissive, water-soluble Au nanoclusters open new opportunities for biological labels, energy transfer pairs, and light emitting sources in nanoscale optoelectronics. PMID:17105412

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

2009-01-01

443

Subdiffusive exciton transport in quantum dot solids.

Colloidal quantum dots (QDs) are promising materials for use in solar cells, light-emitting diodes, lasers, and photodetectors, but the mechanism and length of exciton transport in QD materials is not well understood. We use time-resolved optical microscopy to spatially visualize exciton transport in CdSe/ZnCdS core/shell QD assemblies. We find that the exciton diffusion length, which exceeds 30 nm in some cases, can be tuned by adjusting the inorganic shell thickness and organic ligand length, offering a powerful strategy for controlling exciton movement. Moreover, we show experimentally and through kinetic Monte Carlo simulations that exciton diffusion in QD solids does not occur by a random-walk process; instead, energetic disorder within the inhomogeneously broadened ensemble causes the exciton diffusivity to decrease over time. These findings reveal new insights into exciton dynamics in disordered systems and demonstrate the flexibility of QD materials for photonic and optoelectronic applications. PMID:24807586

Akselrod, Gleb M; Prins, Ferry; Poulikakos, Lisa V; Lee, Elizabeth M Y; Weidman, Mark C; Mork, A Jolene; Willard, Adam P; Bulovi?, Vladimir; Tisdale, William A

2014-06-11

444

Elastic analysis of an inhomogeneous quantum dot in multilayered semiconductors using a boundary examine the elastostatic field due to a buried quantum dot QD in multilayered semiconductors using quasizero- dimensional dots or quantum dots QDs and quasione- dimensional wires on the nanoscale

Pan, Ernie

445

Polarons in semiconductor quantum dots and their role in the quantum kinetics of carrier relaxation

While time-dependent perturbation theory shows inefficient carrier-phonon scattering in semiconductor quantum dots, we demonstrate that a quantum kinetic description of carrier-phonon interaction predicts fast carrier capture and relaxation. The considered processes do not fulfill energy conservation in terms of free-carrier energies because polar coupling of localized quantum-dot states strongly modifies this picture.

J. Seebeck; T. R. Nielsen; P. Gartner; F. Jahnke

2005-01-01

446

Continuous flow purification of nanocrystal quantum dots

NASA Astrophysics Data System (ADS)

Colloidal quantum dot (QD) purification is typically conducted via repeating precipitation-redispersion involving massive amounts of organic solvents and has been the main obstacle in mass production of QDs with dependable surface properties. Our results show that the electric field apparently affects the streamlining of QDs and that we could continuously collect stably dispersed QDs by the electrophoretic purification process. The purification yield increases as the electric potential difference increases or the flow rate decreases, but reaches an asymptotic value. The yield can be further improved by raising the absolute magnitude of the mobility of QDs with the addition of solvents with high dielectric constants. The continuous purification process sheds light on industrial production of colloidal nanomaterials.Colloidal quantum dot (QD) purification is typically conducted via repeating precipitation-redispersion involving massive amounts of organic solvents and has been the main obstacle in mass production of QDs with dependable surface properties. Our results show that the electric field apparently affects the streamlining of QDs and that we could continuously collect stably dispersed QDs by the electrophoretic purification process. The purification yield increases as the electric potential difference increases or the flow rate decreases, but reaches an asymptotic value. The yield can be further improved by raising the absolute magnitude of the mobility of QDs with the addition of solvents with high dielectric constants. The continuous purification process sheds light on industrial production of colloidal nanomaterials. Electronic supplementary information (ESI) available: Additional figures on the microfluidic chip fabrication, QD size analysis, and the effect of the solution environment (dilution) on removal of ligands. See DOI: 10.1039/c4nr04351k

Kim, Duckjong; Park, Hye Kyung; Choi, Hyekyoung; Noh, Jaehong; Kim, Kyungnam; Jeong, Sohee

2014-11-01

447

Annealing-induced change in quantum dot chain formation mechanism

NASA Astrophysics Data System (ADS)

Self-assembled InGaAs quantum dot chains were grown using a modified Stranski-Krastanov method in which the InGaAs layer is deposited under a low growth temperature and high arsenic overpressure, which suppresses the formation of dots until a later annealing process. The dots are capped with a 100 nm GaAs layer. Three samples, having three different annealing temperatures of 460°C, 480°C, and 500°C, were studied by transmission electron microscopy. Results indicate two distinct types of dot formation processes: dots in the 460°C and 480°C samples form from platelet precursors in a one-to-one ratio whereas the dots in the sample annealed at 500°C form through the strain-driven self-assembly process, and then grow larger via an additional Ostwald ripening process whereby dots grow into larger dots at the expense of smaller seed islands. There are consequently significant morphological differences between the two types of dots, which explain many of the previously-reported differences in optical properties. Moreover, we also report evidence of indium segregation within the dots, with little or no indium intermixing between the dots and the surrounding GaAs barrier.

Park, Tyler D.; Colton, John S.; Farrer, Jeffrey K.; Yang, Haeyeon; Kim, Dong Jun

2014-12-01

448

Raman phonon emission in a driven double quantum dot.

The compound semiconductor gallium-arsenide (GaAs) provides an ultra-clean platform for storing and manipulating quantum information, encoded in the charge or spin states of electrons confined in nanostructures. The absence of inversion symmetry in the zinc-blende crystal structure of GaAs however, results in a strong piezoelectric interaction between lattice acoustic phonons and qubit states with an electric dipole, a potential source of decoherence during charge-sensitive operations. Here we report phonon generation in a GaAs double quantum dot, configured as a single- or two-electron charge qubit, and driven by the application of microwaves via surface gates. In a process that is a microwave analogue of the Raman effect, phonon emission produces population inversion of the two-level system and leads to rapid decoherence of the qubit when the microwave energy exceeds the level splitting. Comparing data with a theoretical model suggests that phonon emission is a sensitive function of the device geometry. PMID:24759675

Colless, J I; Croot, X G; Stace, T M; Doherty, A C; Barrett, S D; Lu, H; Gossard, A C; Reilly, D J

2014-01-01

449

Graphene-quantum-dot nonvolatile charge-trap flash memories.

Nonvolatile flash-memory capacitors containing graphene quantum dots (GQDs) of 6, 12, and 27 nm average sizes (d) between SiO2 layers for use as charge traps have been prepared by sequential processes: ion-beam sputtering deposition (IBSD) of 10 nm SiO2 on a p-type wafer, spin-coating of GQDs on the SiO2 layer, and IBSD of 20 nm SiO2 on the GQD layer. The presence of almost a single array of GQDs at a distance of ?13 nm from the SiO2/Si wafer interface is confirmed by transmission electron microscopy and photoluminescence. The memory window estimated by capacitance-voltage curves is proportional to d for sweep voltages wider than ± 3 V, and for d = 27 nm the GQD memories show a maximum memory window of 8 V at a sweep voltage of ± 10 V. The program and erase speeds are largest at d = 12 and 27 nm, respectively, and the endurance and data-retention properties are the best at d = 27 nm. These memory behaviors can be attributed to combined effects of edge state and quantum confinement. PMID:24896068

Sin Joo, Soong; Kim, Jungkil; Kang, Soo Seok; Kim, Sung; Choi, Suk-Ho; Hwang, Sung Won

2014-06-27

450

Auger-assisted electron transfer from photoexcited semiconductor quantum dots.

Although quantum confined nanomaterials, such as quantum dots (QDs) have emerged as a new class of light harvesting and charge separation materials for solar energy conversion, theoretical models for describing photoinduced charge transfer from these materials remain unclear. In this paper, we show that the rate of photoinduced electron transfer from QDs (CdS, CdSe, and CdTe) to molecular acceptors (anthraquinone, methylviologen, and methylene blue) increases at decreasing QD size (and increasing driving force), showing a lack of Marcus inverted regime behavior over an apparent driving force range of ?0-1.3 V. We account for this unusual driving force dependence by proposing an Auger-assisted electron transfer model in which the transfer of the electron can be coupled to the excitation of the hole, circumventing the unfavorable Franck-Condon overlap in the Marcus inverted regime. This model is supported by computational studies of electron transfer and trapping processes in model QD-acceptor complexes. PMID:24359156

Zhu, Haiming; Yang, Ye; Hyeon-Deuk, Kim; Califano, Marco; Song, Nianhui; Wang, Youwei; Zhang, Wenqing; Prezhdo, Oleg V; Lian, Tianquan

2014-03-12

451

Quantum dots as optical labels for ultrasensitive detection of polyphenols.

Considering the fact that polyphenols have versatile activity in-vivo, its detection and quantification is very much important for a healthy diet. Laccase enzyme can convert polyphenols to yield mono/polyquinones which can quench Quantum dots fluorescence. This phenomenon of charge transfer from quinones to QDs was exploited as optical labels to detect polyphenols. CdTe QD may undergo dipolar interaction with quinones as a result of broad spectral absorption due to multiple excitonic states resulting from quantum confinement effects. Thus, "turn-off" fluorescence method was applied for ultrasensitive detection of polyphenols by using laccase. We observed proportionate quenching of QDs fluorescence with respect to polyphenol concentration in the range of 100 µg to 1 ng/mL. Also, quenching of the photoluminescence was highly efficient and stable and could detect individual and total polyphenols with high sensitivity (LOD-1 ng/mL). Moreover, proposed method was highly efficient than any other reported methods in terms of sensitivity, specificity and selectivity. Therefore, a novel optical sensor was developed for the detection of polyphenols at a sensitive level based on the charge transfer mechanism. PMID:24607583

Akshath, Uchangi Satyaprasad; Shubha, Likitha R; Bhatt, Praveena; Thakur, Munna Singh

2014-07-15

452

Chlorine doped graphene quantum dots: Preparation, properties, and photovoltaic detectors

NASA Astrophysics Data System (ADS)

Graphene quantum dots (GQDs) are becoming one of the hottest advanced functional materials because of the opening of the bandgap due to quantum confinement effect, which shows unique optical and electrical properties. The chlorine doped GQDs (Cl-GQDs) have been fabricated by chemical exfoliation of HCl treated carbon fibers (CFs), which were prepared from degreasing cotton through an annealing process at 1000 °C for 30 min. Raman study shows that both G and 2D peaks of GQDs may be redshifted (softened) by chlorine doping, leading to an n-type doping. The first vertical (Cl)-GQDs based photovoltaic detectors have been demonstrated, both the light absorbing and electron-accepting roles for (Cl)-GQDs in photodetection have been found, resulting in an exceptionally big ratio of photocurrent to dark current as high as ˜105 at room temperature using a 405 nm laser irradiation under the reverse bias voltage. The study expands the application of (Cl)-GQDs to the important optoelectronic detection devices.

Zhao, Jianhong; Tang, Libin; Xiang, Jinzhong; Ji, Rongbin; Yuan, Jun; Zhao, Jun; Yu, Ruiyun; Tai, Yunjian; Song, Liyuan

2014-09-01

453

Chlorine doped graphene quantum dots: Preparation, properties, and photovoltaic detectors

Graphene quantum dots (GQDs) are becoming one of the hottest advanced functional materials because of the opening of the bandgap due to quantum confinement effect, which shows unique optical and electrical properties. The chlorine doped GQDs (Cl-GQDs) have been fabricated by chemical exfoliation of HCl treated carbon fibers (CFs), which were prepared from degreasing cotton through an annealing process at 1000?°C for 30?min. Raman study shows that both G and 2D peaks of GQDs may be redshifted (softened) by chlorine doping, leading to an n-type doping. The first vertical (Cl)-GQDs based photovoltaic detectors have been demonstrated, both the light absorbing and electron-accepting roles for (Cl)-GQDs in photodetection have been found, resulting in an exceptionally big ratio of photocurrent to dark current as high as ?10{sup 5} at room temperature using a 405?nm laser irradiation under the reverse bias voltage. The study expands the application of (Cl)-GQDs to the important optoelectronic detection devices.

Zhao, Jianhong; Xiang, Jinzhong, E-mail: jzhxiang@ynu.edu.cn [School of Physical Science and Technology, Yunnan University, Kunming 650091 (China); Tang, Libin, E-mail: scitang@163.com; Ji, Rongbin, E-mail: jirongbin@gmail.com; Yuan, Jun; Zhao, Jun; Yu, Ruiyun; Tai, Yunjian; Song, Liyuan [Kunming Institute of Physics, Kunming 650223 (China)

2014-09-15

454

Coherent radiation by quantum dots and magnetic nanoclusters

The assemblies of either quantum dots or magnetic nanoclusters are studied. It is shown that such assemblies can produce coherent radiation. A method is developed for solving the systems of nonlinear equations describing the dynamics of such assemblies. The method is shown to be general and applicable to systems of different physical nature. Despite mathematical similarities of dynamical equations, the physics of the processes for quantum dots and magnetic nanoclusters is rather different. In a quantum dot assembly, coherence develops due to the Dicke effect of dot interactions through the common radiation field. For a system of magnetic clusters, coherence in the spin motion appears due to the Purcell effect caused by the feedback action of a resonator. Self-organized coherent spin radiation cannot arise without a resonator. This principal difference is connected with the different physical nature of dipole forces between the objects. Effective dipole interactions between the radiating quantum dots, appearing due to photon exchange, collectivize the dot radiation. While the dipolar spin interactions exist from the beginning, yet before radiation, and on the contrary, they dephase spin motion, thus destroying the coherence of moving spins. In addition, quantum dot radiation exhibits turbulent photon filamentation that is absent for radiating spins.

Yukalov, V. I. [Bogolubov Laboratory of Theoretical Physics, Joint Institute for Nuclear Research, Dubna 141980 (Russian Federation); Yukalova, E. P. [Laboratory of Information Technologies, Joint Institute for Nuclear Research, Dubna 141980 (Russian Federation)

2014-03-31

455