Single Quantum Dots Level Research Articles

Quantitative photocurrent scanning probe microscopy on PbS quantum dot monolayers.

Photoconductive atomic force microscopy can probe monolayers of PbS/perovskite quantum dots (QDs) with a contact area of 1-3 QDs in stable and reproducible acquisition conditions for I/V curves and photocurrent maps. From the measurements, quantitative values for the barrier height, built-in voltage, diffusion constant and ideality factor are deduced with high precision. The data analysis is based on modelling a superposition of the drift current of the photo-excited charges and a diffusion current across the interface barriers, providing physical insight into the underlying processes. Besides looking into PbS/perovskite on an indium tin oxide substrate, it is shown how the photocurrent is modified by changing either the QD ligand (to thiocyanate) or the substrate (to micro- and nanostructured gold). The dependence of the photocurrent on the light irradiance is found to follow a power law with an exponent of 0.64. Generally, quantitative measurements with high spatial resolution (on the single QD level) can provide significant insight into the processes in nanostructured hybrid optoelectronic components.

Excitation wavelength-dependent photoluminescence decay of single quantum dots near plasmonic gold nanoparticles.

Changing the excitation wavelength is a simple but effective strategy to modulate the photophysical cha racteristics of colloidal quantum dots (QDs) near plasmonic nanostructures. It has been observed that the photoluminescence (PL) decay of QDs near plasmonic nanostructures differs when the excitation wavelength is varied, but the exact mechanism is still unclear today. Here, we studied the excitation wavelength dependence of the PL decay of CdSe/CdS core/shell QDs near plasmonic gold nanoparticles at the single QD level. With the aid of statistical science, we demonstrated that the PL decay of a single QD near gold nanoparticles is generally faster when the QD is excited spectrally close to the localized surface plasmon resonance of gold nanoparticles. This excitation wavelength dependence is mainly caused by the varied proportion of photons coming from biexciton emission, which is the result of different local electric field enhancement by gold nanoparticles upon excitation.

Biexciton Dynamics in Single Colloidal CdSe Quantum Dots.

The investigation of biexciton dynamics in single colloidal quantum dots (QDs) is critical to biexciton-based applications. Generally, a biexciton exhibits an extremely low photoluminescence (PL) quantum yield as well as very fast PL decay due to strong nonradiative Auger recombination, making it difficult to investigate the biexciton dynamics. Here, we develop a quantitative method based on intensity- and time-resolved photon statistics to investigate the biexciton dynamics in single colloidal QDs. This robust method can be used under high-excitation conditions to determine the absolute radiative and Auger recombination rates of both neutral and charged biexciton states in a single QD level, and the corresponding ratios between the two states agree with the theoretical predictions of the asymmetric band structures of CdSe-based QDs. Furthermore, the surface traps are found to provide additional nonradiative recombination pathways for the biexcitons, and their contributions are quantified by the method.

Self-Assembly of Semiconductor Quantum Dots using Organic Templates.

Colloidal semiconductor nanocrystals, known as quantum dots (QDs), are regarded as brightly photoluminescent nanomaterials possessing outstanding photophysical properties, such as high photodurability and tunable absorption and emission wavelengths. Therefore, QDs have great potential for a wide range of applications, such as in photoluminescent materials, biosensors and photovoltaic devices. Since the development of synthetic methods for accessing high-quality QDs with uniform morphology and size, various types of QDs have been designed and synthesized, and their photophysical properties dispersed in solutions and at the single QD level have been reported in detail. In contrast to dispersed QDs, the photophysical properties of assembled QDs have not been revealed, although the structures of the self-assemblies are closely related to the device performance of the solid-state QDs. Therefore, creating and controlling the self-assembly of QDs into well-defined nanostructures is crucial but remains challenging. In this Minireview, we discuss the notable examples of assembled QDs such as dimers, trimers and extended QD assemblies achieved using organic templates. This Minireview should facilitate future advancements in materials science related to the assembled QDs.

Plasmonic Effect on Exciton and Multiexciton Emission of Single Quantum Dots.

Quantum dots are nanoscale quantum emitters with high quantum yield and size-dependent emission wavelength, holding promises in many optical and electronic applications. When quantum dots are situated close to noble metal nanoparticles, their emitting behavior can be conveniently tuned because of the interaction between the excitons of the quantum dots and the plasmons of the metal nanoparticles. This interaction at the single quantum dot level gives rise to reduced or suppressed photoluminescence blinking and enhanced multiexciton emission, which is difficult to achieve in isolated quantum dots. However, the mechanism of how plasmonic structures cause the changes in the quantum dot emission remains unclear. Because of the complexity of the system, the interfaces between metal, semiconductor, and ligands must be considered, in addition to factors such as geometry, interparticle distance, and spectral overlap. The challenges in the design and fabrication of the hybrid nanostructures as well as in understanding the exciton-plasmon coupling mechanism can be overcome by a cooperative effort in synthesis, optical spectroscopy, and theoretical modeling.

Resonant and Inelastic Andreev Tunneling Observed on a Carbon Nanotube Quantum Dot.

We report the observation of two fundamental subgap transport processes through a quantum dot (QD) with a superconducting contact. The device consists of a carbon nanotube contacted by a Nb superconducting and a normal metal contact. First, we find a single resonance with position, shape, and amplitude consistent with the theoretically predicted resonant Andreev tunneling (AT) through a single QD level. Second, we observe a series of discrete replicas of resonant AT at a separation of ~145 μeV, with a gate, bias, and temperature dependence characteristic for boson-assisted, inelastic AT, in which energy is exchanged between a bosonic bath and the electrons. The magnetic field dependence of the replica's amplitudes and energies suggest that two different bosons couple to the tunnel process.

Time-resolved photoluminescence and photostability of single semiconductor quantum dots

Time-resolved photoluminescence (TRPL) and photostability were studied for several core/shell-type semiconductor quantum dots (QDs) of CdTe/CdS, In(Zn)P/ZnS and CdZnS/ZnS using a TRPL microscopy at a single QD level, of which results were compared to that of CdSe/ZnS QD. The CdTe/CdS and In(Zn)P/ZnS QDs show unstable PL at a single QD level on both bare and polymer-coated glass coverslips, so that they mostly lose emissions within a few seconds. The CdZnS/ZnS QD shows better emission stability than those of the former two QDs, but still less stable than the case of the CdSe/ZnS.

Self-Limiting Growth of Hexagonal and Triangular Quantum Dots on (111)A

We report the formation of triangular GaAs quantum dots (QDs) on (111)A substrates using droplet epitaxy. Shape transition from hexagonal to triangular QDs is observed with increasing crystallizing temperature, as a result of different growth rates of step edges on a (111)A substrate. Size statistics illustrate the self-limiting growth of GaAs QDs whose characteristic size is determined by that of Ga droplets. Detailed structural analysis further reveals the three-dimensional structure of QDs. By decreasing the amount of Ga, low-density GaAs QDs are obtained while retaining the triangular shape, which allows the optical properties to be studied at a single QD level.

Self-aligned epitaxial metal-semiconductor hybrid nanostructures for plasmonics

We demonstrate self-alignment of epitaxial Ag nanocrystals on top of low-density near-surface InAs quantum dots (QDs) grown by molecular beam epitaxy. The Ag nanocrystals support a surface plasmon resonance that can be tuned to the emission wavelength of the QDs. Photoluminescence measurements of such hybrid metal-semiconductor nanostructures reveal large enhancement of the emission intensity. Our concept of epitaxial self-alignment enables the integration of plasmonic functionality with electronic and photonic semiconductor devices operating down to the single QD level.

Bias-Induced Photoluminescence Quenching of Single Colloidal Quantum Dots Embedded in Organic Semiconductors

We demonstrate reversible quenching of the photoluminescence from single CdSe/ZnS colloidal quantum dots embedded in thin films of the molecular organic semiconductor N,N'-diphenyl-N,N'-bis(3-methylphenyl)-(1,1'-biphenyl)-4,4'-diamine (TPD) in a layered device structure. Our analysis, based on current and charge carrier density, points toward field ionization as the dominant photoluminescence quenching mechanism. Blinking traces from individual quantum dots reveal that the photoluminescence amplitude decreases continuously as a function of increasing forward bias even at the single quantum dot level. In addition, we show that quantum dot photoluminescence is quenched by aluminum tris(8-hydroxyquinoline) (Alq3) in chloroform solutions as well as in thin solid films of Alq3 whereas TPD has little effect. This highlights the importance of chemical compatibility between semiconductor nanocrystals and surrounding organic semiconductors. Our study helps elucidate elementary interactions between quantum dots and organic semiconductors, knowledge needed for designing efficient quantum dot organic optoelectronic devices.

Coherent control of the biexciton in a single quantum dot

AbstractWe report on coherent control of the two‐photon transition between the ground state and the biexciton state on a single quantum dot level. The population of the biexciton is monitored by the cascaded spontaneous emission of the biexciton complex via the exciton states. The signal shows interference pattern at the double laser frequency and its contrast decays non‐exponentially at the time scale of 10 ps. The contrast decrease is attributed to a dynamical inhomogeneous broadening of the energy levels due to long‐term fluctuations in the dot environment. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Overview of fundamentals and applications of electrons, excitons and photons in confined structures

The advent of electron- and photon-confined structures has led to a wealth of novel physical phenomena and device applications. While the 1980s were devoted to 2D electronic quantum wells (QWs), the past decade has seen a flourish of 1D quantum wire (QWR) and 0D quantum dot (QD) studies. While remarkably sharp features were observed at the single QD level, fabrication of fluctuation-free ensembles of QDs remains a stumbling block for applications. On the other hand, more recently emerged photon-confined structures in the form of planar cavities, micropillars, photonic crystals have seen many achievements such as the Purcell effect, modification of light extraction efficiency, strong light–matter coupling. Advanced effects such as thresholdless lasers, squeezed photon generation, quantum condensation remain to be demonstrated. Although it might appear as a side effect, the interplay of electronic and photonic dimensionalities yields a profound insight on light–matter interaction.