Mid-infrared luminescence from coupled quantum dots and wells

Coupled nanostructures have been developed in the InAs/InSb/GaSb materials system in order to extend the emission wavelength further into the infrared, beyond 2 μm. The samples studied consist of a single narrow InAs quantum well grown below a layer of InSb quantum dots in a GaSb matrix, in which the coupling has been altered by changing the thickness of a GaSb spacer layer. The overall transition energy of the combined dot–well system is generally reduced with respect to the dots and well only but the dependence on spacer thickness is more complex than that expected from a simple envelope function model.     

Coherent control of thermalized luminescence in semiconductor quantum dots

A theoretical study of the time-integrated signal of thermalized luminescence excited by a pair of phase-locked light pulses shows that the idea of coherent control should be efficient for measuring the total dephasing rates of high-energy excitonic transitions. This approach can be used for studying both the homogeneously and inhomogeneously broadened systems, which is highly important in studies of the semiconductor quantum dots. It is found that the method of coherent control makes it possible not only to obtain information about the relaxation constants but also to make conclusions about the statistical properties of frequency distributions of the excitonic transitions responsible for broadening the optical spectra.     

Synthesis and luminescence of CdS quantum dots capped with a silica precursor

A novel synthesis method for efficiently luminescing CdS nanocrystals is reported using (3-mercaptopropyl)-trimethoxysilane (MPS) as a capping agent in THF or MeOH. The synthesis yields particles capped with a precursor for coating the particle with a silica shell and offers control over a wide range of particle sizes (2-) by varying the MPS concentration. The temperature dependence of the luminescence and the luminescence life time was studied and explained in terms of a donor-acceptor recombination process in which both electron and hole are trapped at low temperature. In addition to varying the emission color by controlling the particle size, nanocrystals were doped with Zn or Cu resulting in a blue shift or a red shift of the emission.     

Room temperature luminescence in CuI/AgI quantum dots

Room temperature luminescence in a CuI/AgI glass system is investigated by irradiating the system at 410 nm (3.02 eV). The spectrum peaks at 635 nm (1.95 eV) and 700 nm (1.77 eV), while the intensity is significantly enhanced (centered at 635 nm) by increasing the amount of AgI. We propose a model based on an increase in the AgI:Cu⁺ species at higher AgI concentration at which the red emission is attributed to the radiative recombination from carriers trapped at the donor sites (e.g., interstitial silver ions) and the acceptor sites (e.g., a vacancy-compensated divalent cation). The PL efficiency is also estimated by comparison with a standard rhodamine B solution.     

Chemical role of amines in the colloidal synthesis of CdSe quantum dots and their luminescence properties

The role of organic amines in the colloidal synthesis of CdSe quantum dots (QDs) has been studied. CdSe QDs were synthesized from the source solutions containing 5 vol% of amines having various alkyl chain lengths, stereochemical sizes and electron donation abilities. The role of the additional amines was evaluated on the basis of the photoluminescence (PL) properties such as PL wavelength and intensity of the obtained CdSe QDs. The observed PL spectra were explained by the fact that the amines behaved as capping ligands on the surface of the QDs in the product colloidal solution and complex ligands for cadmium in the source solutions. It was shown that the particle size was controlled by the diffusion process depending on the mass and stereochemical shape of the amines, and the luminescence intensity increased with the increasing electron donation ability and capping density of the amines.     

Aqueous synthesis of Cu-doped ZnSe quantum dots

In this paper, we described a simple growth-doping approach for aqueous synthesis of Cu-doped ZnSe quantum dots (Cu:ZnSe d-dots) with mercaptopropionic acid as stabilizer. The influences of the ratios of precursors and the concentration of Cu dopant ions on Cu:ZnSe d-dots synthesis were studied in detail in this study. The Cu dopant ions had significant influence on the optical properties of ZnSe d-dots. The bandgap emission of ZnSe was effectively restrained through Cu doping. The prolonged reflux facilitates the doping of Cu, which led to the red-shift of the emission of Cu:ZnSe d-dots from 465 to 495 nm. The stable Cu:ZnSe d-dots with high quality can be obtained under optimal conditions. As compared with cadmium-based nanocrystals synthesized in aqueous solution, Cu:ZnSe d-dots have much lower toxicity, indicating that they can be applied as outstanding fluorescent labels for biological assays, imaging of cells and tissues, even in vivo investigations.     

Long-wavelength luminescence from GaSb quantum dots grown on GaAs substrates

Self-assembled GaSb quantum dots (QDs) with a photoluminescence wavelength longer than 1.3 μm were successfully grown by suppressing the replacement of As and Sb on the surface of the GaSb QDs. This result means that GaSb can thus join InAs or GaInAs as a suitable material for QD lasers for optical communications.     

Sonoelectrochemical synthesis of water-soluble CdTe quantum dots

A facile and fast one-pot method has been developed for the synthesis of CdTe quantum dots (QDs) in aqueous phase by a sonoelectrochemical route without the protection of N₂. The morphology, structure and composition of the as-prepared products were investigated by high resolution transmission electron microscopy (HRTEM), X-ray diffraction (XRD) and energy dispersive X-ray spectrometer (EDS). The influences of current intensity, current pulse width, and reaction temperature on the photoluminescence (PL) and quantum yield (QY) of the products were studied. The experimental results showed that the water-soluble CdTe QDs with high PL qualities can be conveniently synthesized without precursor preparation and N₂ protection, and the PL emission wavelength and QY can be effectively controlled by adjusting some parameters. This method can be expected to prepare other QDs as promising building blocks in solar cell, photocatalysis and sensors.     

Polymer-assisted synthesis of water-soluble PbSe quantum dots

Stable PbSe quantum dots were synthesised in water-based media using poly(amidehydroxyurethane) water-soluble polymer. The polymer acts like a precursor carrier, blocks the particles aggregation and assures their solubility. Atomic force microscopy data show that the particle radius is smaller than the Bohr radius of PbSe. Interactions studies, performed by Fourier transform IR spectroscopy, show that the quantum dots are capped with poly(amidehydroxyurethane). The proposed synthesis was realised in the absence of any organic solvent. As a result, the produced particles have good water solubility, stability and good arguments to be biologically compatible.     

Time-resolved spectroscopy of soantaneous luminescence of CdSSe1−x quantum dots

Picosecond time-resolved spectroscopy of the edge luminescence band of CdS _x Se_1–x quantum dots with crystallite diameters as small as a few nanometers under band-to-band excitation reveals strong enhancement of the radiative recombination rate compared to bulk CdS owing to quantum confinement. The splitting of the luminescence band into two lines originates from near-band-gap absorption. Analysis of the temperature as well as the spectral dependence of the decay time (leading to a red shift of the luminescence with increasing time) and of the total-light-decay law result in a new model for the dominant radiative recombination channel: donor-acceptor pair recombination instead of an excitonic mechanism as claimed in previous publications.Dedicated to H.-J. Queisser on the occasion of his 60th birthday     

Interband absorption and luminescence in small quantum dots under strong magnetic fields

Interband absorption and luminescence of quasi-two-dimensional, circularly symmetric, N_e-electron quantum dots are studied at high magnetic fields, 8B60 T, and low temperatures, T2 K. In the N_e=0 and 1 dots, the initial and final states of such processes are fixed, and thus the dependence on B of peak intensities is monotonic. For larger systems, ground state rearrangements with varying magnetic field lead to substantial modifications of the absorption and luminescence spectra. Collective effects are seen in the N_e=2 and 3 dots at “filling fractions” and .     

Laser synthesis and size tailor of carbon quantum dots

Carbon quantum dots (C-dots) with average sizes of about 3, 8, and 13 nm were synthesized by laser irradiation of graphite flakes in polymer solution. The obtained C-dots display size and excitation wavelength dependent photoluminescence behavior. The size control of C-dots can be realized by tuning laser pulse width. The original reason could be the effects of laser pulse width on the conditions of nucleation and growth of C-dots. Compared with short-pulse-width laser, the long-pulse-width laser would be better fitted to the size and morphology control of nanostructures in the different material systems.     

Synthesis and characterization of binary and ternary III–V quantum dots

Quantum dots of InP, GaP, GaInP₂, and GaAs with diameters ranging from 20–80 Å can be synthesized as well-crystallized nanoparticles with bulk zinc blende structure. The synthesis is achieved by heating appropriate organometallic precursors with stabilizers in high boiling solvents for several days to produce QDs, which can then be dissolved in nonpolar organic solvents to form transparent colloidal QD dispersions. The high sample quality of the InP and Gap QDs results in excitonic features in the absorption spectra; excitonic features could not be observed for GaAs or GaInP₂ QDs. The GaP and GaInP₂ QD colloids exhibit very intense (quantum yields of 15–25%) visible photoluminescence at room temperature. The photoluminescence for InP QDs preparations show two emission bands: one band is in the visible at the band edge of the QD, and a second band appears above 800 nm. The near-IR PL is attributed to deep traps, presumably phosphorus vacancies on the QD surface. This band can be removed after controlled addition of etchant; subsequently, very intense band-edge emission (quantum yield 30%), which is tunable with particle size, is obtained. The QDs were characterized by TEM, SAXS, AFM, powder X-ray diffraction, steady-state optical absorption and photoluminescence spectroscopy, ps to ns transient photoluminescence spectroscopy, and fs to ps pump-probe absorption (i.e., hole-burning) spectroscopy.     

Influence of pH on luminescence from water-soluble colloidal Mn-doped ZnSe quantum dots capped with different mercaptoacids

Water-soluble ZnSe/ZnS core–shell quantum dots with ZnSe core doped by manganese ions show different luminescence response to pH changes in aqueous solutions depending on the type of solubilizing agents (thioglycolic acid, mercaptoundecanoic acid, sodium mercaptopropylsulfonate). In the case of long-chain mercaptoundecanoic acid only excitonic emission is affected by pH changes. Short-chain thioglycolic acid brings about equal excitonic/Mn emission variations with pH, while mercaptopropylsulfonate-stabilized quantum dots are insensitive to pH. The mechanism discussed here is based on the competition between different relaxation channels for excited excitons in ZnSe: excitonic radiative recombination, energy transfer to Mn ion and the photogenerated electron trapping due to the presence of protonated carboxyl group. ZnSe:Mn/ZnS quantum dots stabilized with long-chain mercaptoacids may be used as a new type of fluorescence ratiometric pH-sensor or indicator.     

Temperature dependent time-resolved exciton luminescence in GaAs/AlGaAs quantum wires and dots

Transient photoluminescence of GaAs/AlGaAs quantum wires and quantum dots formed by strain confinement is studied as a function of temperature. At low temperature, luminescent decay times of the wires and dots correspond to the radiative decay times of localized excitons. The radiative decay time can be either longer or shorter than that of the host quantum well, depending on the size of the wires and dots. For small wires and dots (∼ 100 nm stressor), the exciton radiative recombination rate increases due to lateral confinement. Exciton localization due to the fluctuation of quantum well thickness plays an important role in the temperature dependence of luminescent decay time and exciton transfer in quantum wire and dot structures up to at least ∼ 80 K. Lateral exciton transfer in quantum wire and dot structures formed by laterally patterning quantum wells strongly affects the dynamics of wire and dot luminescence. The relaxation time of hot excitons increases with the depth of strain confinement, but we find no convincing evidence that it is significantly slower in quasi 1-D or 0-D systems than in quantum wells.     

Thermoelectrics with Coulomb-coupled quantum dots

In this article we review the thermoelectric properties of three terminal devices with Coulomb-coupled quantum dots (QDs) as observed in recent experiments [1], [2]. The system we consider consists of two Coulomb-blockade QDs, one of which can exchange electrons with only a single reservoir (heat reservoir), while the other dot is tunnel coupled with two reservoirs at a lower temperature (conductor). The heat reservoir and the conductor interact only via the Coulomb coupling of the quantum dots. It has been found that two regimes have to be considered. In the first one, the heat flow between the two systems is small. In this regime, thermally driven occupation fluctuations of the hot QD modify the transport properties of the conductor system. This leads to an effect called thermal gating. Experiments have shown how this can be used to control charge flow in the conductor by means of temperature in a remote reservoir. We further substantiate the observations with model calculations, and implications for the realisation of an all-thermal transistor are discussed. In the second regime, the heat flow between the two systems is relevant. Here the system works as a nanoscale heat engine, as proposed recently (Sánchez and Büttiker [3]). We review the conceptual idea, its experimental realisation and the novel features arising in this new kind of thermoelectric device such as decoupling of heat and charge flow.     

Entangled-photons generation with quantum dots

Entanglement between particles is a crucial resource in quantum information processing, an important example of which is the exploitation of entangled photons in quantum communication protocols. Among the different available sources of entangled photons, semiconductor quantum dots(QDs) excel owing to their deterministic emission properties, potential for electrical injections, and direct compatibility with semiconductor manufacturing techniques. Despite the great promises,QD-based sources are far from being ideal. In particular, such sources present several critical issues, which require the overcoming of challenges pertaining to spectral tunability, entanglement fidelity, photon indistinguishability and brightness.In this article, we will discuss the potential solutions to these problems and review the recent progress in the field.     

Controlled stacking growth of uniform InAs quantum dots by molecular beam epitaxy

Double-stacked InAs quantum dots (QDs) were grown by molecular beam epitaxy via Stranski–Krastanov growth mode. Transition of the facet formation from {1 3 6} plane to {1 1 0} plane was observed during the stacking growth of InAs QDs by reflection high-energy electron-beam diffraction. The enhanced growth rate and the different facet formation in the stacking growth were caused by tensile strain of the GaAs underlying layer. Low arsenic pressure and low growth rate conditions played an important role for a perfect coupling and uniformity in the size of the stacked QDs. The narrow photoluminescence line width of 17.6 meV was successfully obtained from the stacked InAs QDs.     

Towards hybrid circuit quantum electrodynamics with quantum dots

Cavity quantum electrodynamics allows one to study the interaction between light and matter at the most elementary level. The methods developed in this field have taught us how to probe and manipulate individual quantum systems like atoms and superconducting quantum bits with an exquisite accuracy. There is now a strong effort to extend further these methods to other quantum systems, and in particular hybrid quantum dot circuits. This could turn out to be instrumental for a noninvasive study of quantum dot circuits and a realization of scalable spin quantum bit architectures. It could also provide an interesting platform for quantum simulation of simple fermion–boson condensed matter systems. In this short review, we discuss the experimental state of the art for hybrid circuit quantum electrodynamics with quantum dots, and we present a simple theoretical modeling of experiments.