Photocurrent spectroscopy of InAs/GaAs self-assembled quantum dots, studied as a function of applied electric field, is used to probe the nature of the confined electronic states. A field asymmetry of the quantum confined Stark effect is observed, consistent with the dots possessing a permanent dipole moment. The sign of this dipole indicates that for zero field the hole wavefunction lies above that of the electron, in disagreement with the predictions of all recent calculations. Comparison with a theoretical model demonstrates that the experimentally determined alignment of the electron and hole can only be explained if the dots contain a nonzero and non-uniform Ga content. The role of two different carrier escape mechanisms, tunneling and thermal excitation, is studied.

We present an InAs QDs structure designed to separate and store photo-generated electron-hole pairs. Charge separation in the structure is demonstrated using power dependent photoluminescence and biased photoluminescence. Preliminary data from time resolved photoluminescence suggest storage times in the device in the μsec range.

Self-assembled InAs/InP and InAs/InGaAsP quantum dots (QDs) grown by metal-organic chemical vapour epitaxy (MOVPE) exhibit emission at 1.5–1.6 μm at room temperature. P-I-N diodes incorporating a single InAs/InGaAsP QD layer exhibit strong electroluminescence over a wide range of input currents and emit significantly more light per layer than a InGaAs/InGaAsP multi-quantum well device.

We report an in-situ molecular beam epitaxy – scanning tunneling microscopy study of three dimensional (3D) self organized InAs islands on (AI,Ga)As surfaces. The influence of the presence of Al atoms on the roughness of the starting surface and on the island density is shown by investigating several Al compositions. We emphasize the case of InAs/AlAs and point out the major differences between this system and the widely studied InAs/GaAs system.

A stress-driven formation of self-assembled InGaAs islands has been studied by the growth on GaAs (100) substrates with sub-micron platinum stripe pattern. Islands or quantum dots preferentially nucleate at the boundary of metal patterns. In addition, a quantum dot-free region near the boundary of the metal pattern is found. Those results are attributed to the stress between metal stripe and GaAs surface, which produces a laterally stressed region around the metal stripe. Adatoms on this region preferentially migrate toward the edge of metal stripes with maximum stress. This result may show a possible way for the interconnection between randomly distributed self-assembled quantum dots and metal stripes.

We study coupled semiconductor quantum dots theoretically using a generalized Hubbard approach, where intra- and inter-dot Coulomb correlation, as well as tunneling effects are described on the basis of realistic electron wavefunctions. We find that the ground-state configuration of vertically coupled double dots undergoes non-trivial quantum transitions as the inter-dot distance d changes; at intermediate values of d we predict a new phase that should be observable in the addition spectra and in the magnetization changes.

We study injection and transport in thin disordered films of CdSe nanocrystals between metal electrodes, We investigate the current-voltage characteristics of these devices as a function of electrode material, nanocrystal size, and temperature. We also measure the photocurrent response of these devices, and find that the photocurrent action spectra follow the quantum-confined absorption spectra of the nanocrystals. For dissimilar top and bottom electrodes, we find that the devices are highly rectifying. By studying space charge limited currents in these devices, we are able to place a lower bound on the effective carrier mobility in such films, and we find that the effective mobility is strongly field dependent. We find that the conductivity is strongly temperature dependent, and is qualitatively consistent with an activated hopping process at temperatures above 180 K.

We demonstrate photoconductivity and conductivity in three-dimensional close-packed solids of colloidal CdSe quantum dots. We observe quantum dot size and surface passivation dependent photoconductivity that can be qualitatively understood by considering the energy required in order to overcome the Coulomb energy of the initial electron-hole pair. Our results suggest that surface ligands that promote initial separation of the electron and hole reduce the electric field required for the onset of the photocurrent. The dark conductance is much smaller than the photoconductance. Hysteretic behaviour and extremely long-lived current transients are observed in the dark current that are suggestive of Coulomb-glass behaviour.

The dynamics of photoinduced electron transfer between 3.0 nm diameter MoS2 nanoclusters and adsorbed electron acceptors have been studied using static and time resolved optical spectroscopy. Two types of electron acceptors are adsorbed on these nanoclusters: 2,2'-bipyridine and 4,4',5,5'-tetramethyl-2,2'-bipyridine. The results reported here focus on the interpretation of the strongly non-exponential electron transfer kinetics. Electron/hole recombination kinetics have previously been analyzed in terms of a simple distributed kinetics model, and this model is extended to include the case of interfacial electron transfer.

A time evolution method is used to compute the electron escape time and dark current of a quantum dot under electric field. We found that for quantum dots with just one bound state, the ground state wave function can be well described by the product of a 2D confined wave function in the x, y plane and a 1D confined wave function in the z direction with an effective quantum well potential. A comparison of phase-shift analysis and the full time-dependent calculation is presented. Good agreement between the two in the large time scale is found, but discrepancy exists in the small time scale. Our study shows that the electron escape rate (which determines dark current) of quantum dots is much lower than that of quantum wells with the same bound-to-continuum transition energy.

We have fabricated and characterized the lateral electron transport through InAs quantum dots with double barrier system. Aluminum metal electrodes with the inter-electrode spacing of 30 nm have been deposited on an InAs self-assembled quantum dot wafer to form the planar type quantum dot devices. Current peak structure and negative differential resistance effects are observed above 77 K in current-voltage characteristics. These results are interpreted as due to 3D-0D resonant tunneling through the single quantum dot positioned in between the electrodes.

Optical vibrations confined in CdS nanocrystals (embedded in a glass film or forming a close-packed film with some organic-molecule links between them) have been studied experimentally, by means of FIR and Raman spectroscopies, and theoretically. The asymmetric Raman lineshape can be modelled considering resonant and non-resonant scattering on spherical modes with 1=0 (those with n>l are responsible for the low-frequency wing). A phonon-related dielectric response of the nanocrystal composites has been studied by measuring FIR reflectivity and transmission spectra. The theoretical consideration consists of the (i) calculation of the single-particle polarisability due to the confined 1=1 modes and (ii) including the dipole-dipole interaction between the particles. We show that, for the smallest particles, the FIR response has a bimodal structure, which disappears as the nanocrystal size increases. However, the experimentally observed bulk-CdS-like reflection band at 230–300 cm−1 is too broad and can not be explained by the model. Only making further assumptions, namely, of a fractal structure of the nanocrystal composites, enables us to fit well the experimental FIR spectra.

We present the results of our investigation of producing nanoclusters of gold, silver, copper and tin in 6H-SiC. This is accomplished by implanting 1.0 MeV Au, 2.0 MeV Ag, 2.0 MeV Cu, and 160 keV Sn into the Si face of SiC at room or elevated temperature followed by annealing at various temperatures. Using optical absorption spectrophotometry, we determined the location of the absorption band for each metal nanocluster in SiC. Elevated temperature implantation reduces optical absorption due to ion implantation induced defects. Using the Mie theory, we determined the index of refraction in the implanted volume.

We have synthesised CuO quantum dots by using a method of one-step solid state reaction under ambient conditions, and investigated them by X-ray diffraction (XRD), transmission electron microscopy (TEM), and Raman scattering technique. The XRD shows that the sample is composed of single phase CuO with a monoclinic structure. The particle size estimated from the x-ray diffraction peaks is about 12 nm, consistent with the TEM result. The Raman spectra show that there are three Raman peaks at 282, 332 and 618 cm−1, which are much broader and shifted several cm−l to lower frequencies in comparison with those of bulk CuO crystal. The temperature dependence of the Raman spectra in the range 77–873 K is also presented.

We have carried out a resonance Raman study of line-shape in silicon quantum dots synthesized on a quartz substrate by co-sputtering bulk Si and Si02. Optical transmission measurements are used to evaluate dot size distribution. The size distribution shows peaks around 1.0 and 1.4 nm. The Si dots exhibit photoluminescence in the visible region, which shifts to higher energy with decreasing size. The size dependence of Raman scattering shows phonon softening and increasing asymmetrical broadening for small dots (< 2nm). The observed spectra are compared with calculations considering electron-hole interactions at a quasi-direct gap of a spherical quantum dot.

We present a density-matrix approach for the description of nonequilibrium carrier dynamics in optically excited semiconductor quantum dots, that explicitly accounts for exciton-exciton as well as exciton-carrier interactions. Within this framework, we analyze few-particle effects in the optical spectra and provide a consistent description of additional peaks appearing at high photoexcitation density. We discuss possible applications of such optical nonlinearities in future coherent-control experiments.

In the last several years, great advances have been made in the ability to synthesize semiconductor quantum dots with very narrow size distributions. Here, we report the synthesis of a series of thiolate-capped CdS quantum dots having reasonably narrow size distributions and make optical property comparisons to the crystallographically defined CdS molecular clusters having essentially “zero” size distribution. These clusters contain a “Cd10S4” core and thiolate/halide capping ligands. The luminescence of the molecular clusters, like the nanoparticles, is greatly influenced by the nature of the capping ligands. Additionally, the luminescence of the molecular clusters can be quite similar to that observed for their larger quantum dot counterparts.

Second Harmonic Generation in CdSe nanocrystal quantum dots is observed by Hyper-Rayleigh scattering. We use a Ti-Sapphire femtosecond laser at 820 nm to induce the nonlinear optical response of the nanocrystals in solution. The unit cell normalized second harmonic coefficient βn, shows a substantial systematic enhancement in small sizes. The observed size dependence of the second harmonic generation process, is explained assuming two contributions. The first is a bulk-like contribution, from the non-centrosymmetric nanocrystal core, and the second, a contribution from the particle surface. The latter contribution is most significant in small nanocrystals with a substantial proportion of surface atoms.

We observed the quantum confined bandedge emission from ZnSe quantum dots and the size dependence of the energy states, spin-orbit interaction, and Stokes shift. The bandedge emission occurs in the UV-blue. The energy gap = Eg + C/dn where d is the diameter and n is 1. 19 ± 0.13 and 1.21 ± 0.13 for the first and second electron-hole transitions, respectively. The separation between these transitions approaches the bulk spin-orbit splitting, while the Stokes shift decreases with particle size. Effective mass theories can not explain these results. Trap emission is observed in some samples in the green and red, resulting from Se-related traps.

We have determined the growth conditions which result in a narrow linewidth and room temperature emission at 1.3pm from InAs/GaAs Quantum dots (QDs). The QDs formed under these conditions are extremely uniform in size and exhibit an emission linewidth of only 25meV. Single QD layers have been incorporated into p-i-n diodes which exhibit strong electroluminescence. We have compared the efficiency of these devices with a nominally identical quantum well device. The QD based device exhibits a higher electroluminescence efficiency, especially at low current densities. At higher current densities there is a loss of efficiency due to recombination from excited states.

Operated under reverse bias, the diodes act as photo-detectors and the measured photocurrent spectrum exhibits peaks due to absorption in the ground and excited states of the QDs as well as the 2D confining layer.