Quantum confinement in graphene quantum dots

By performing density functional theory calculations, we studied the quantum confinement in charged graphene quantum dots (GQDs), which is found to be clearly edge and shape dependent. It is found that the excess charges have a large distribution at the edges of the GQD. The resulting energy spectrum shift is very nonuniform and hence the Coulomb diamonds in the charge stability diagram vary irregularly, in good agreement with the observed nonperiodic Coulomb blockade oscillation. We also illustrate that the level statistics of the GQDs can be described by a Gaussian distribution, as predicted for chaotic Dirac billiards.

     

Resonant Raman scattering of optical phonons in self-assembled quantum dots

We have investigated the carrier relaxation mechanism in InGaAs/GaAs quantum dots by photoluminescence excitation (PLE) spectroscopy. Near-field scanning optical microscope successfully shows that a PLE resonance at a relaxation energy of 36 meV can be seen in all single-dot luminescence spectra, and thus can be attributed to resonant Raman scattering by a GaAs LO phonon to the excitonic ground state. In addition, a number of sharp resonances observed in single-dot PLE spectra can be identified as resonant Raman features due to localized phonons, which are observed in the conventional Raman spectrum. The results reveal the mechanism for the efficient relaxation of carriers observed in self-assembled quantum dots: the carriers can relax within the continuum states, and make transitions to the excitonic ground state by phonon emission.     

Light absorption involving longitudinal optical phonons in semiconductor quantum dots

A theory of single-photon interband transitions involving optical phonons in semiconductor quantum dots (QDs) has been developed. This theory assumes that the electron subsystem of QDs with infinite potential walls is in strong confinement, and its energy spectrum can be described according to the two-band semiconductor model. Longitudinal optical phonons are considered to be related to the QD electron subsystem via polar (Fröhlich) electronphonon interaction. It is shown that, in these approximations, only the off-diagonal part of electron-phonon interaction leads to the generation of electron-hole pairs with the participation of phonons; the selection rules for these transitions differ from those for zero-phonon transitions. Analytical expressions for the light-absorption coefficients of ensembles of identical and size-distributed QDs have been obtained.     

Pressure effects on nanoprobe photoluminescence of quasi-zero-dimensional confinement quantum dots

Nanoprobe near-field photoluminescence (PL) of InGaAs(P) dots with quasi-zero-dimensional (quasi-0D) confinement with various degrees of 0D/2D has been investigated by studying probe-induced pressure effects and probe-bias effects. Fine PL peaks of 0D confinement are superimposed on quantum well (QW) peaks for quasi-0D structures, which proves the coexistence of 0D and two-dimensional (2D) confinement in the same layer. Large blue shifts of approximately 100 meV were observed to occur with pressure increase for 0D fine PL peaks, but no shift was observed for the QW peak. The fine 0D peaks were eliminated by larger probe-induced pressures, which should be attributed to carrier diffusion rather than to Γ–X crossover in energy levels. The QW peak increased with the positive probe bias, while 0D fine PL peaks showed a smaller increase with red shifts up to 8–9 meV. The results obtained can be explained by the excitation of immobile excitons in 0D potentials to mobile carriers in the 2D (QW) layer.     

Quantum confinement in amorphous silicon quantum dots embedded in silicon nitride

Amorphous silicon quantum dots (a-Si QDs) were grown in a silicon nitride film by plasma enhanced chemical vapor deposition. Transmission electron micrographs clearly demonstrated that a-Si QDs were formed in the silicon nitride. Photoluminescence and optical absorption energy measurement of a-Si QDs with various sizes revealed that tuning of the photoluminescence emission from 2.0 to 2.76 eV is possible by controlling the size of the a-Si QD. Analysis also showed that the photoluminescence peak energy E was related to the size of the a-Si QD, a (nm) by E(eV) = 1.56+2.40/a(2), which is a clear evidence for the quantum confinement effect in a-Si QDs.     

Tuning optical properties of water-soluble CdTe quantum dots for biological applications

In this study, two different synthetic methods in aqueous solution are presented to tune the optical properties of CdTe and CdSe semiconductor nanoparticles. Additionally, the influence of different temperatures, pressures, precursor ratios, surface ligands, bases, and core components in the synthesis was investigated with regard to the particle sizes and optical properties. As a result, a red shift of the emission and absorption maxima with increasing reaction temperature (100 to 220°C), pressure (1 to 25 bar), and different ratios of core components of alloyed semiconductor nanoparticles could be observed without a change of the particle size. An increase in particle size from 2.5 to 5 nm was only achieved by variation of the mercaptocarboxylic acid ligands in combination with the reaction time and used base. To get a first hint on the cytotoxic effects and cell uptake of the synthesized quantum dots, in vitro tests mesenchymal stem cells (MSCs) were carried out.

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Three-dimensional photon confinement in a spherical microcavity with CdTe quantum dots: Raman spectroscopy

We have studied the Raman spectra of a system consisting of a polystyrene latex microsphere coated by CdTe colloidal quantum dots. The cavity-induced enhancement of the Raman scattering allows the observation of Raman spectra from only one CdTe monolayer. Periodic structure with very narrow peaks in the Raman spectra of a single microsphere was detected both in Stokes and anti-Stokes spectral regions, arising from the coupling between the emission from quantum dots and spherical cavity modes.     

The influence of ligands on the preparation and optical properties of water-soluble CdTe quantum dots

In a systematic investigation has been found that ligands play an important role in both the water-phase preparation and optical properties of CdTe quantum dots. Experiments were performed using three typical thioalkyl acids as ligands, namely mercaptoacetic acid (MAA), l-cysteine (Cys) and reduced glutathione (GSH). The growth rate and size-distribution of CdTe quantum dots (QDs) are shown to depend on the type of ligands. A proper choice of ligand enables to make lager nanocrystals with narrower size-distribution. The effects of pH (buffer solution), illumination, heating and cations on the spectroscopic properties of CdTe QDs for the three ligands are reported. In addition, three same-size CdTe QDs were individually characterized by micellar electrokinetic capillary electrophoresis with laser-induced fluorescent detection, which proved their monodisperse size-distribution and different electric charge distribution on the surface for each of the three different type of QDs.     

Raman scattering of light by optical phonons in Si-Ge-Si structures with quantum dots

Raman scattering of light by optical phonons in Si-Ge-Si structures with pseudomorphic germanium quantum dots has been investigated. Resonance amplification of the scattering intensity on E ₀ (Γ₇−Γ₈) transitions has been observed. It is shown that as a result of the formation of the layer of germanium quantum dots, the resonance energy is ∼0.3 eV higher than in the two-dimensional case. Pis’ma Zh. éksp. Teor. Fiz. 64, No. 3, 203–207 (10 August 1996)     

Quantum size effects of CO reactivity on metallic quantum dots

We study the reactivity of a metallic quantum dot when exposed to a gas phase CO molecule. First, we perform a Newns-Anderson model calculation in which the valence electrons of the quantum dot are confined by a finite potential well and the molecule is characterized by its lowest unoccupied molecular orbital in the gas phase. A pronounced quantum size effect regarding the charge transfer between the quantum dot and molecule is observed. We then perform a first-principles calculation for a selected size interval. The quantum dot is described within the jellium model and the molecule by pseudopotentials. Our results show that the charge transfer between the quantum dot and the molecule depends critically on the size of the quantum dot, and that this dependence is intimately connected with the electronic structure. The key factor for charge transfer is the presence of states with the symmetry of the chemically active molecular orbital at the Fermi level.     

水溶性CdTe量子点的三阶光学非线性极化特性

利用超短脉冲Z扫描技术和光学Kerr效应研究了以巯基丙酸为稳定剂的CdTe量子点水溶液的三阶光学非线性极化特性. 在532nm,30ps和800nm,130fs脉冲激光激发下, 发现分别具有正负相反取值的三阶光学非线性折射率,自由载流子吸收和双光子吸收分别是这两种脉冲激光激发下三阶光学非线性吸收的起因. 测量得到CdTe量子点的三阶光学非线性极化率约为CS₂的32倍, 在520—700nm光谱区的CdTe量子点的光学响应时间小于400fs. 关键词： CdTe量子点(QDs) Z扫描 三阶光学非线性极化特性 双光子吸收     

Recent progresses of quantum confinement in graphene quantum dots

Graphene quantum dots (GQDs) not only have potential applications on spin qubit, but also serve as essential platforms to study the fundamental properties of Dirac fermions, such as Klein tunneling and Berry phase. By now, the study of quantum confinement in GQDs still attract much attention in condensed matter physics. In this article, we review the experimental progresses on quantum confinement in GQDs mainly by using scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS). Here, the GQDs are divided into Klein GQDs, bound-state GQDs and edge-terminated GQDs according to their different confinement strength. Based on the realization of quasi-bound states in Klein GQDs, external perpendicular magnetic field is utilized as a manipulation approach to trigger and control the novel properties by tuning Berry phase and electron–electron (e–e) interaction. The tip-induced edge-free GQDs can serve as an intuitive mean to explore the broken symmetry states at nanoscale and single-electron accuracy, which are expected to be used in studying physical properties of different two-dimensional materials. Moreover, high-spin magnetic ground states are successfully introduced in edge-terminated GQDs by designing and synthesizing triangulene zigzag nanographenes.     

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.     

Quantum cryptography based on quantum dots

Network protocols are formulated for the phase-coding and correlated-photon-pairs quantum cryptosystems. These protocols are free of the restriction imposed on the distance between the two legitimate users by the transmission loss in the optical fiber. A single-photon source and a source of correlated photon pairs based on quantum dots are proposed. Pis’ma Zh. éksp. Teor. Fiz. 63, No. 8, 646–651 (25 April 1996) Published in English in the original Russian journal. Edited by Steve Torstveit.     

Manifestation of CdTe quantum dots and interdiffusion in IR reflection spectra of CdTe/ZnTe structures with quantum dots

The IR reflection spectra of structures that consist of alternating layers of CdTe quantum dots and ZnTe barriers and are grown on ZnTe and CdTe/ZnTe buffer layers on GaAs(001) substrates are studied. The spectra are processed using dispersion analysis, and the parameters of the oscillators are determined. In the spectra, quantum dots manifest themselves in the form of a broad band at a frequency close to the frequency of the Fröhlich mode. It is revealed that the spectra contain features associated with the interdiffusion of the CdTe and ZnTe compounds, i.e., bands attributed to the local mode of Zn atoms in the CdTe compound and two gap modes of Cd atoms in the ZnTe compound.     

Few-electron semiconductor quantum dots with Gaussian confinement

We have performed Hartree-Fock calculations of the electronic structure of N ≤ 10 electrons in a quantum dot modeled with a confining Gaussian potential well. We discuss the conditions for the stability of N bound electrons in the system. We show that the most relevant parameter determining the number of bound electrons is V ₀ R ². Such a feature arises from widely valid scaling properties of the confining potential. Gaussian Quantum dots having N = 2, 5, and 8 electrons are particularly stable in agreement with the Hund rule. The shell structure becomes less and less noticeable as the well radius increases.      

CdTe量子点标记雌二醇衍生物的研究

以巯基乙酸为修饰剂合成高量子产率的CdTe量子点,并成功标记生物小分子17β-氨基雌二醇.紫外光谱、荧光光谱、红外光谱以及倒置显微镜照片分析表明CdTe量子点在活化剂N-羟基琥珀酰亚胺的作用下,通过表面的羧基与17β-氨基雌二醇的氨基以共价键结合,这为建立基于量子点探针的新型的药物筛选模型提供了实验基础.     

Interface phonons in semiconductor nanostructures with quantum dots

The vibrational spectra of structures with InAs quantum dots in an AlGaAs matrix and AlAs quantum dots in an InAs matrix are investigated experimentally and theoretically. The Raman spectra exhibit features that correspond to transverse-optical (TO), longitudinal-optical (LO), and interface phonons. The frequencies of interface phonons in InAs and AlAs quantum dots and in an AlGaAs matrix with various concentrations of aluminum are calculated with the use of experimental values of transverse-and longitudinal-optical phonons in the approximation of a dielectric continuum. It is shown that the model of a dielectric continuum adequately describes the behavior of interface phonons in structures with quantum dots under the assumption that the quantum dots are spheroidal.