Transition from two-dimensional to three-dimensional quantum confinement in semiconductor quantum wires/quantum dots

We report the photoluminescence (PL) and polarization-resolved PL characteristics of a novel GaAs/AlGaAs quantum wire/dot semiconductor system, realized by metalorganic vapor-phase epitaxy of site-controlled, self-assembled nanostructures in inverted tetrahedral pyramids. By systematically changing the length of the quantum wires, we implement a continuous transition between the regimes of two-dimensional and three-dimensional quantum confinement. The two main evidences for this transition are observed experimentally and confirmed theoretically: (i) strongly blue-shifted ground-state emission, accompanied by increase separation of ground and excited transition energies; and (ii) change in the orientation of the main axis of linear polarization of the photoluminescence, from parallel to perpendicular with respect to the wire axis. This latter effect, whose origin is shown to be purely due to quantum confinement and valence band mixing, sets in at wire lengths of only approximately 30 nm.     

Calculating potential and electron density for strained semiconductor quantum dots

Distributions of the electrostatic potential, electric field strength, and electron concentration for a strained semiconductor quantum dot (QD) have been calculated within the nonlinear Poisson model with allowance for the deformation potential that arises due to a lattice misfit between the QD and matrix.     

Unraveling the physical properties of Mn-doped CdS diluted magnetic semiconductor quantum dots for potential application in quantum spintronics

Journal of Materials Science: Materials in Electronics - The tunability of structural, optical, electronic, and magnetic properties in semiconductor quantum dots (QDs) makes them promising...     

Polarization rotation in asymmetric semiconductor quantum dots

Rotation of input polarization arising due to the recombination of electrons from the 1s-excitonic state to the hybrid valence band states have been theoretically examined in asymmetric semiconductor quantum dots. The Jones matrix calculations suggest that the polarization rotation directly depends on the asymmetry of the quantum dot and strength of hh–lh coupling. The results advocate the suitability of quantum dots as polarizing devices.     

Many electron effects in semiconductor quantum dots

Semiconductor quantum dots (QDs) exhibit shell structures, very similar to atoms. Termed as ’artificial atoms’ by some, they are much larger (1 100 nm) than real atoms. One can study a variety of manyelectron effects in them, which are otherwise difficult to observe in a real atom. We have treated these effects within the local density approximation (LDA) and the Harbola-Sahni (HS) scheme. HS is free of the self-interaction error of the LDA. Our calculations have been performed in a three-dimensional quantum dot. We have carried out a study of the size and shape dependence of the level spacing. Scaling laws for the Hubbard ‘U’ are established.     

Transport and photodetection in self-assembled semiconductor quantum dots

A great step forward in science and technology was made when it was discovered that lattice mismatch can be used to grow highly ordered, artificial atom-like structures called self-assembled quantum dots. Several groups have in the meantime successfully demonstrated useful infrared photodetection devices which are based on this technology. The new physics is fascinating, and there is no doubt that many new applications will be found when we have developed a better understanding of the underlying physical processes, and in particular when we have learned how to integrate the exciting new developments made in nanoscopic addressing and molecular self-assembly methods with semiconducting dots. In this paper we examine the scientific and technical questions encountered in current state of the art infrared detector technology and suggest ways of overcoming these difficulties. Promoting simple physical pictures, we focus in particular on the problem of high temperature detector operation and discuss the origin of dark current, noise, and photoresponse.     

Ferromagnetic self-assembled quantum dots on semiconductor nanowires

Ferromagnetic self-assembled alpha-MnAs quantum dots (QD) were grown epitaxially on metal catalyst-grown InAs nanowires (NW) by chemical vapor deposition. Magnetic force microscopy measurements demonstrated that the QDs are stable, single-domain ferromagnets with T(c) values of approximately 310 K. Single QD switching was demonstrated at fields as low as 60 Oe. The hybrid ferromagnetic/semiconductor QD/NW properties provide a promising basis for the development of nanowire spin-valves and magnetic memory devices.     

Transport properties of coupled semiconductor quantum dots

We have measured the electronic transport properties of the coupled quantum dot devices at low temperatures. The interplay between the strong many body spin interaction and the molecular states are probed in linear and non-linear transport regime. We observe the formation of strong coherent molecular states clearly visible in the double dot conductance phase diagram. In our study, the spin configuration in multiply coupled quantum dots could be identified using Kondo phenomenon. In addition, the characteristics of the spin dependent molecular states and phase dependant tunneling have been also observed using non-linear conductance measurement of the double dots. The results suggest the importance of the diverse spin related physical issues in artificial quantum dot devices.     

Electron properties of open semiconductor quantum dots

The energy spectrum and lifetimes of electron states in an open semiconductor quantum dot (QD) have been studied using the scattering S-matrix method. It is established that the lifetime of electron states in the QD is highly sensitive to changes in the QD radius and the thickness of an external coating layer. As the coating layer thickness increases from one to five monolayers, the electron lifetime grows by a factor of 20–60.     

Electrical transport phenomena in systems of semiconductor quantum dots

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

Binding of semiconductor quantum dots to cellular integrins

There is currently a major international effort aimed at integrating semiconductor nanostructures with biological structures. This paper reports the functionalization of cadmium sulfide quantum dots with peptides that facilitate the selective binding of these quantum-dot-peptide complexes to integrins in the membranes of cancer cells of the MDA-MB-435 cell line. In addition, this paper focuses on the roles that biological environments play in altering and determining the optical and vibrational properties of these nanostructures.     

Colloidal semiconductor quantum dots with tunable surface composition

Colloidal CdS quantum dots (QDs) were synthesized with tunable surface composition. Surface stoichiometry was controlled by applying reactive secondary phosphine sulfide precursors in a layer-by-layer approach. The surface composition was observed to greatly affect photoluminescence properties. Band edge emission was quenched in sulfur terminated CdS QDs and fully recovered when QDs were cadmium terminated. Calculations suggest that electronic states inside the band gap arising from surface sulfur atoms could trap charges, thus inhibiting radiative recombination and facilitating nonradiative relaxation.     

Spectral properties of multiply charged semiconductor quantum dots

Spectrally resolved fluorescence imaging of single CdSe/ZnS quantum dots (QDs), charged by electrospray deposition under negative bias has revealed a surprising net blue shift (～60 meV peak-to-peak) in the distribution of center frequencies in QD band-edge luminescence. Electrostatic force microscopy (EFM) on the electrospray QD samples showed a subpopulation of charged QDs with 4.7 ± 0.7 excess electrons, as well as a significant fraction of uncharged QDs as evidenced by the distinct cantilever response under bias. We show that the blue-shifted peak recombination energy can be understood as a first-order electronic perturbation that affects the band-edge electron- and hole-states differently. These studies provide new insight into the role of electronic perturbations of QD luminescence by excess charges.     

Ultrafast near-field spectroscopy of single semiconductor quantum dots

Excitonic and spin excitations of single semiconductor quantum dots (QDs) currently attract attention as possible candidates for solid-state-based implementations of quantum logic devices. Due to their rather short decoherence times in the picosecond to nanosecond range, such implementations rely on using ultrafast optical pulses to probe and control coherent polarizations. We combine ultrafast spectroscopy and near-field microscopy to probe the nonlinear optical response of a single QD on a femtosecond time-scale. Transient reflectivity spectra show pronounced oscillations around the QD exciton line. These oscillations reflect phase-disturbing Coulomb interactions between the excitonic QD polarization and continuum excitations. The results show that although semiconductor QDs resemble in many respects atomic systems, Coulomb many-body interactions can contribute significantly to their optical nonlinearities on ultrashort time-scales.     

Photoluminescent two-dimensional SiC quantum dots for cellular imaging and transport

Two-dimensional (2D) ultrathin SiC has received intense attention due to its broad band gap and resistance to large mechanical deformation and external chemical corrosion. However, the synthesis and application of ultrasmall 2D SiC quantum dots (QDs) has not been explored. Herein, we synthesize a type of monolayered 2D SiC QDs with advanced photoluminescence (PL) properties via a facile hydrothermal route. Their average size and thickness can be easily adjusted by altering the reaction time. The ultrasmall 2D SiC QDs exhibit a long fluorescence lifetime of 2.59 μs due to efficient quantum confinement. The applications of SiC QDs are demonstrated through labeling A549, HeLa, and NHDF cells and delivering agents for intracellular low-abundant microRNA (miRNA) detection. This advance in preparing photoluminescent SiC QDs is of great significance for broadening their potential in biomedical and optical applications.

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Level structure of InAs quantum dots in two-dimensional assemblies

The electronic level structure of colloidal InAs quantum dots (QDs) in two-dimensional arrays, forming a QD-solid system, was probed using scanning tunneling spectroscopy. The band gap is found to reduce compared to that of the corresponding isolated QDs. Typically, the electron (conduction-band) ground state red shifts more than the hole (valence-band) ground state. This is assigned to the much smaller effective mass of the electrons, resulting in stronger electron delocalization and larger coupling between electron states of neighboring QDs compared to the holes. This is corroborated by comparing these results with those for InAs and CdSe nanorod assemblies, manifesting the effects of the electron effective mass and arrangement of nearest neighbors on the band gap reduction. In addition, in InAs QD arrays, the levels are broadened, and in some cases their discrete level structure was nearly washed out completely and the tunneling spectra exhibited a signature of two-dimensional density of states.     

A new method for amino acid diagnostics using semiconductor quantum dots

Advantages offered by the use of quantum dots (QDs) for the investigation and diagnostics of biological systems are considered. A new method for the QD diagnostics of amino acid is proposed. The interaction of isolated QDs with charged amino acid residues is analyzed. It is shown that this interaction can shift the luminescence spectrum of QDs by several tens of millielectronvolts, which opens broad possibilities for the identification of biological objects with the aid of QDs.     

Nonradiative resonance energy transfer between two semiconductor quantum dots

We consider the nonradiative resonance energy transfer between two semiconductor quantum dots (donor and acceptor), taking into account the nonparabolicity of the electron dispersion law, and the energy transfer due to the Coulomb interaction between charge carriers of the donor and acceptor. We show that, when nonparabolicity of the dispersion law is taken into account, a new term enters the matrix element of the energy transfer, which enhances the probability of the resonance energy transfer.     

Synthetic studies on II/VI semiconductor quantum dots

The chemical synthesis of nanocrystals associated to the study of quantum size effects on II/VI semiconductor compounds has been investigated extensively. Considerable advances in the use of chemical procedures to produce II/VI semiconductor quantum dots have been made and recent strategies to produce such materials will be reviewed here. This review is structured using a tentative classification for the most used solution chemical methods applied to II/VI semiconducting dots. Current progress on the enhancement of physical properties of these materials using chemical approaches will be reviewed.