Dimensional Quantum Dots Research Articles

Nonlinear optical rectification of hydrogenic impurity in a disk-like parabolic quantum dot: The role of applied magnetic field

We report a detailed theoretical study of the effect of combined electric and magnetic field on the nonlinear optical rectification of a hydrogenic impurity, confined in a two dimensional disk-like quantum dot, with parabolic confinement potential. We use the compact density matrix formalism and iterative method to obtain nonlinear optical rectification and absorption coefficients. To find energy levels and wave functions, we employ exact diagonalization method in the effective mass approximation. As main result, we found that the transition energy from ground to first excited state redshifts with increasing the magnetic field while blueshifts for transition from ground to second excited state, moreover, for former transition, nonlinear optical rectification coefficient decreases with increasing magnetic field in contrast to that occurs for latter one.

Two-Electron Resonances in Quasi-One Dimensional Quantum Dots with Gaussian Confinement

We consider a quasi one-dimensional quantum dot composed of two Coulombically interacting electrons confined in a Gaussian trap. Apart from bound states, the system exhibits resonances that are related to the autoionization process. Employing the complex-coordinate rotation method, we determine the resonance widths and energies and discuss their dependence on the longitudinal confinement potential and the lateral radius of the quantum dot. The stability properties of the system are discussed.

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

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.

The impact of disorder on charge transport in three dimensional quantum dot resonant tunneling structures

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.

Subwavelength focusing of light with orbital angular momentum.

The spatial structure of light with Orbital Angular Momentum, or "twisted light", closely resembles the shape of atomic wave functions. It could therefore make symmetry-forbidden transitions possible in quantum dots, or "artificial atoms". However, the vanishing intensity in the center of an OAM beam usually makes this effect weak. Here we show a plasmonic approach to focus OAM light to subwavelength dimensions using metallic nanoscale resonant optical antennas. This allows to increase the field intensity of OAM light at the typical dimensions of quantum dots to an intensity larger than a regular Gaussian beam, which corresponds to increasing the interaction strength by 3 orders of magnitude.

Non-polar InGaN quantum dots grown on the m-plane of n-GaN nanowires by a self-catalyst method using metal organic chemical vapor deposition

We propose a novel nanostructure semiconductor which integrated the nanowire (NW) template and the zero dimensional quantum dot (QD) active region. We successfully fabricated InGaN quantum dots on the nonpolar facet of n-GaN NWs via a new self-catalyst method using In droplets. The resultant InGaN QDs were of a quadrangle shape. Their average width and height were 30 and 7 nm, respectively. HR-TEM analysis verified that the InGaN QDs were epitaxially grown on the m-plane facet of n-GaN NWs and had high crystalline quality without any defects. The low-temperature PL spectra of InGaN QDs indicated a prominent emission in the range of 387–425 nm which consisted of several overlapping sharp peaks owing to the different sizes of InGaN QDs. As a result, we expect that this unique InGaN QD growth method can be widely applied to other nanostructure semiconductor systems and that this newly integrated nanostructure semiconductor will be a very promising material for high-quality optoelectronic device applications.

(Invited) Novel Luminescent Materials Based on Semiconductor Nanowires

We report novel luminescent materials based on group III-V compound semiconductor nanowires. Semiconductor nanowires are essentially one-dimensional rods with length of several microns and diameter below 100 nm. Hence, nanowires exhibit interesting quantum confinement and carrier transport properties. Nanowires are grown using metal seed particles by the vapor-liquid-solid (VLS) process in a molecular beam epitaxy or metalorganic chemical vapor deposition system. By varying the material deposition during growth, axial or radial nanowire heterostructures and p-n junctions may be formed for various device applications including light emitting diodes, lasers, and photodetectors. Due to the large surface area to volume ratio of a nanowire, lattice mismatch strain may be accommodated by elastic distortion of the nanowire without detrimental misfit dislocations, which gives a much greater ability to perform bandgap engineering in nanowires as compared to thin films. Hence, unique heterostructures are possible in nanowires that would be impossible in thin films, opening up new device applications and possibilities in condensed matter physics. We will report our recent work on the photoluminescence properties of InAsP/InP nanowires. InP nanowires were grown on <111> Si substrates by the Au-assisted vapor-liquid-solid process in a gas source molecular beam epitaxy system. InAsyP1-y segments were grown in the middle of the InP nanowires, creating a multiple quantum dot structure or superlattice. The quantum dot dimensions and composition were determined by scanning electron microscopy (SEM), transmission electron microscopy (TEM), and energy dispersive x-ray spectroscopy (EDX). Photoluminescence (PL) from the quantum dot structure could be tuned by the InAsyP1-y composition (y), or by the size of the quantum dot via the quantum confinement effect. Cathodoluminescence (CL) measurements confirmed localized emission from the quantum dots. To reduce detrimental surface states, the nanowires were passivated with an AlInP shell, which resulted in strong PL emission.The growth mechanism of the quantum dots were inferred from the InAsP and InP segment lengths as a function of nanowire diameter. Short InAsP segment lengths were found to grow by depletion of In from the Au particle as well as by direct impingement, while longer segments of InAsP and InP grew by diffusive transport of adatoms from the nanowire sidewalls. The present study offers a manner to engineer the lengths of InAsP quantum dots embedded in InP barriers to better control the PL or CL emission. A novel group III and V gas switching sequence is presented to improve compositional control of the QD.

(Invited) Novel Luminescent Materials Based on Semiconductor Nanowires

Recent work on the photoluminescence (PL) properties of InAsP/InP nanowires are reported. InP nanowires were grown on <111> Si substrates by the Au-assisted vapor-liquid-solid process in a gas source molecular beam epitaxy system. InAsyP1-y segments were grown within the InP nanowires, creating single or multiple quantum dots structures. The quantum dot dimensions and composition were determined by scanning electron microscopy (SEM), transmission electron microscopy (TEM), and energy dispersive x-ray spectroscopy (EDX). PL from the quantum dot structures could be tuned by the InAsyP1-y composition (y), or by the size of the quantum dot via the quantum confinement effect. Cathodoluminescence (CL) measurements confirmed localized emission from the quantum dots. To reduce detrimental surface states, the nanowires were passivated with an AlInP shell, which resulted in strong PL emission.

Strain-induced vertical self-organization of semiconductor quantum dots: A computational study

Atomistic strain simulations based on the valence force field method are employed to study the vertical arrangements of semiconductor quantum dot (QD) multilayers. The effects of the QD shape, dimensions, and materials parameters are systematically investigated, varying independently the following parameters: spacer width H, QD lateral spacing D, base b, and height h, slope of the side facets, elastic properties of the dot and the substrate materials, and lattice mismatch between the dot and the substrate. The transition between vertically aligned and anti-aligned structures is found to be determined mainly by the ratios H/D and b/D, as well as by the strain anisotropy of the substrate and to a lesser extent of the QD. The dependence on the QD height h is significant only for steep side facets and large aspect ratios h/b, and the effects of the lattice mismatch strain and the bulk elastic moduli are found to be negligible. The comparison with experimental data shows an excellent agreement with the results from the simulations, demonstrating that the presented analysis results in precise theoretical predictions for the vertical self-organization regime in a wide range of QD materials systems.

Finite difference method for the arbitrary potential in two dimensions: Application to double/triple quantum dots

A finite difference method (FDM) applicable to a two dimensional (2D) quantum dot was developed as a non-conventional approach to the theoretical understandings of quantum devices. This method can be applied to a realistic potential with an arbitrary shape. Using this method, the Hamiltonian in a tri-diagonal matrix could be obtained from any 2D potential, and the Hamiltonian could be diagonalized numerically for the eigenvalues. The legitimacy of this method was first checked by comparing the results with a finite round well with the analytic solutions. Two truncated harmonic wells were examined as a realistic model potential for lateral double quantum dots (DQDs) and for triple quantum dots (TQDs). The successful applications of the 2D FDM were observed with the entanglements in the DQDs. The level-splitting and anticrossing behaviors of the DQDs could be obtained by varying the distance between the dots and by introducing asymmetry in the well-depths. The 2D FDM results for linear/triangular TQDs were compared with the tight binding approximations.

Current matching using CdSe quantum dots to enhance the power conversion efficiency of InGaP/GaAs/Ge tandem solar cells

A III-V multi-junction tandem solar cell is the most efficient photovoltaic structure that offers an extremely high power conversion efficiency. Current mismatching between each subcell of the device, however, is a significant challenge that causes the experimental value of the power conversion efficiency to deviate from the theoretical value. In this work, we explore a promising strategy using CdSe quantum dots (QDs) to enhance the photocurrent of the limited subcell to match with those of the other subcells and to enhance the power conversion efficiency of InGaP/GaAs/Ge tandem solar cells. The underlying mechanism of the enhancement can be attributed to the QD's unique capacity for photon conversion that tailors the incident spectrum of solar light; the enhanced efficiency of the device is therefore strongly dependent on the QD's dimensions. As a result, by appropriately selecting and spreading 7 mg/mL of CdSe QDs with diameters of 4.2 nm upon the InGaP/GaAs/Ge solar cell, the power conversion efficiency shows an enhancement of 10.39% compared to the cell's counterpart without integrating CdSe QDs.

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

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

Ionization induced by strong electromagnetic field in low dimensional systems bound by short range forces

Ionization processes for a two dimensional quantum dot subjected to combined electrostatic and alternating electric fields of the same direction are studied using quantum mechanical methods. We derive analytical equations for the ionization probability in dependence on characteristic parameters of the system for both extreme cases of a constant electric field and of a linearly polarized electromagnetic wave. The ionization probabilities for a superposition of dc and low frequency ac electric fields of the same direction are calculated. The impulse distribution of ionization probability for a system bound by short range forces is found for a superposition of constant and alternating fields. The total probability for this process per unit of time is derived within exponential accuracy. For the first time the influence of alternating electric field on electron tunneling probability induced by an electrostatic field is studied taking into account the pre-exponential term.

Evolution of visible photoluminescence of Si quantum dots embedded in silicon oxide matrix

Abstract Photoluminescence properties of nc-Si/SiOx nanocomposites produced by pressing the mixture of the silica aerogel powder with the porous silicon have been studied. The transformation of emission spectra of Si quantum dots, its quantum yield and emission kinetics during the long time (few months) aging in ambient conditions and formation conditions (external pressure) were observed. The different behavior of short and long wave range emission of Si quantum dots embedded in silicon oxide matrix has been obtained. The observed blue-shifting (from 2.02 to 2.05 eV) of emission peak associated with the free exciton recombination in Si quantum dots at forming pressure increasing is explained by the reduction of average Si quantum dot dimension due to the breaking away the surface irregularities and smoothing of the Si nanoparticles surface. Effect of a strong increase in intensity (more than 1 order) and blue shifting of emission of Si quantum dots were discovered in nc-Si/SiOx nanocomposites at few months’ storage in ambient conditions. Such effects were explained in terms of both surface passivation and the carrier confinement. It was also confirmed by the fact that for aged composites the photoluminescence kinetics transforms by adding the stretched exponential term to the single exponential low.

Optical properties of spherical quantum dot with position-dependent effective mass

In this paper, the effects of position-dependent effective mass on the optical properties of a three dimensional quantum dot are numerically investigated. For this purpose, by using the point canonical transformation method and numerical solution of the Schrödinger equation, the energy levels and wave functions of electrons in the confinement potential are obtained. Then the optical properties are studied by considering the compact density matrix approach based on the numerical calculation of the wave function. Our calculations were compared with the case of constant mass. Results show that the position-dependent effective mass plays an important role in the intersubband optical absorption coefficient and refractive index changes in a quantum dot.

Nano-bio interfaces probed by advanced optical spectroscopy: From model system studies to optical biosensors

Research in biology and medicine is a rapidly expanding field incorporating some of the most fundamental questions concerning structure, function, and purpose. The forefront of new research demands access to advanced techniques and instrumentation capable of probing these unanswered questions. Over the past several decades, nano-scale materials and devices ranging from quasi-one dimensional quantum dots to two dimensional graphene sheets have been engineered and have found applications in nano-bio imaging and spectroscopy. In this review, the incorporation of nanomaterials into three influential spectroscopic and microscopic techniques including fluorescence microscopy, surface plasmon resonance, and sum frequency generation will be introduced. Fluorescence imaging has visualized nanomaterials as compliments or replacements to comparable organic fluorphores, act as a quencher for FRET-based sensing, and serve as a nanoscaffold for molecular beacons. Their versatility in coating materials makes nanomaterials an excellent targeting molecule for any cellular macromolecule or structure. In addition to the targeting capabilities of nanomaterials in fluorescence imaging, surface plasmon resonance has incorporated nanomaterials for applications in signal enhancement, selectivity of target molecules, and the development of more refined and accurate detection. Functionalized nanoparticles enhance the capabilities of sum frequency generation vibrational spectroscopy by providing unique surface chemistry which alters target molecule interactions and orientations. In summary, the incorporation of nanomaterials has greatly enhanced the field of biology and medicine and has allowed for the continual advancement of not only research but instrument development.

Photoluminescence sensitivity to methanol vapours of surface InP quantum dot: Effect of dot size and coverage

We investigate the effect of morphology and coverage of self-assembled epitaxial InP quantum dot on the photoluminescence sensitivity to methanol vapour. The photoluminescence of quantum dots is in the near infrared (750–900nm) for quantum dot height and lateral size of 2–3nm and 20–30nm, respectively. Photoluminescence intensity undergoes an enhancement when the samples were exposed to methanol vapours. The dimension, density and effective surface of quantum dots have been estimated by statistical analysis of the dot morphology based on atomic force micrographs. We observed that it is possible to increase the magnitude of the intensity change due to methanol vapour by tailoring quantum dot size and density to increase the sample coverage. We observe a linear dependence on the coverage of the luminescence intensity changes induced by exposition to methanol vapours.

Optical anisotropy of InGaAs quantum dots

Polarization studies of InGaAs/GaAs quantum dots (QDs) synthesized in the submonolayer deposition mode (SMLQDs) on a singular GaAs (100) surface are carried out using photoluminescence spectroscopy. The influence of the effective In content in InGaAs SMLQDs and the effect of a wide-gap AlGaAs matrix on the optical anisotropy of the QDs are investigated. The highest degree (>15%) of optical anisotropy between the [011] and [0 $$ \bar 1 $$ 1] directions in the emission corresponding to the ground state of InGaAs/GaAs SMLQDs is observed for an effective In content of ∼40%. The use of a wide-gap AlGaAs matrix resulted in an increase in the optical anisotropy of InGaAs SMLQDs by a factor of 1.5. It is found that vertical stacking of In(Ga)As/AlGaAs SMLQDs in the vertical-coupling mode (with spacer-layer thicknesses of 5–10 nm) leads to a further increase in the degree of optical anisotropy, which becomes as high as 25% on average. According to the data of transmission electron microscopy, the optical anisotropy of the ground-state photo-luminescence is predominantly caused by the anisotropy of the lateral dimensions of QDs in the [011] and [0 $$ \bar 1 $$ 1] directions.

Nonlinear Optical Spectra of Intersubband Transitions in a CdSe/ZnS/CdSe/SiO2 Spherical Quantum Dot

In this paper the electronic spectra and optical properties of one-electron spherical quantum dot (QD) are presented. QD is composed of CdSe core surrounded by ZnS shell (barrier) CdSe shell and finally by medium of very high potential barrier (SiO2) compared to CdSe. The energy eigenvalues of s, p and d states and corresponding wave functions were calculated using effective mass approximation. We have calculated oscillator strengths and the linear and third-order nonlinear intersubband optical absorption coefficients as function of the QD dimension, incident photon energy and incident optical intensity for the 1s-1p, 1p-1d and 1p-2s transitions.

Time-dependent investigation of charge injection in a quantum dot containing one electron

The interaction of an injected electron towards a quantum dot (QD) containing a single confined electron is investigated using a flexible time-dependent quantum mechanics formalism, which allows both electrons to move and undergo quantum transitions. Different scenarios combining quantum dot dimensions, dielectric constant, injected wave packet energy, and width were explored, and our main results are: (i) due to the large characteristic transitions times between the confined state in the quantum dot and the delocalized state in the continuum, it is relatively difficult to ionize the occupied QD by Coulomb interaction solely and (ii) the charging state of the quantum dot can be sensed by direct injection of charges.