室温下CdSe胶体量子点超快自旋动力学

利用时间分辨法拉第旋转光谱技术研究了室温下CdSe胶体量子点的自旋相干特性.获得了不同磁场下的自旋退相干时间,并分析了自旋退相干的物理机理.零磁场时量子点激子自旋退相干时间为102 ps,主要受电子与核自旋之间的超精细相互作用所影响.当外加横向磁场强度为250 mT时,激子自旋退相干时间为294 ps;增大磁场强度,自旋退相干时间逐渐减小.在较强磁场环境中(≥250mT),量子点激子自旋动力学由非均匀退相干机制所主导.     

Spin-flip limited exciton dephasing in CdSe/ZnS colloidal quantum dots

The dephasing time of the lowest bright exciton in CdSe/ZnS wurtzite quantum dots is measured from 5 to 170 K and compared with density dynamics within the exciton fine structure using a sensitive three-beam four-wave-mixing technique unaffected by spectral diffusion. Pure dephasing via acoustic phonons dominates the initial dynamics, followed by an exponential zero-phonon line dephasing of 109 ps at 5 K, much faster than the ~10 ns exciton radiative lifetime. The zero-phonon line dephasing is explained by phonon-assisted spin flip from the lowest bright state to dark-exciton states. This is confirmed by the temperature dependence of the exciton lifetime and by direct measurements of the bright-dark-exciton relaxation. Our results give an unambiguous evidence of the physical origin of the exciton dephasing in these nanocrystals.     

Optical performance evolutions of reductive glutathione coated CdSe quantum dots in different environments

Optical performances of reductive glutathione coated CdSe quantum dots were studied under different ageing conditions. The enhancements of luminescence were obviously occurred for the samples ageing under illumination. The quantum yield of CdSe was enhanced continuously over 44 days at room temperature, and reached as high as 36.6%. O₂ was proved to make a certain contribute to the enhancement. The evolutions of the systems during the ageing time were deduced according to the variations of pH values with ageing time and the XRD results of the samples ageing in air with illumination. We conferred that the reduction of surface defects resulted from the photo-induced decomposition of CdSe quantum dots was the main reason for the enhancement of fluorescence. The production of CdO as a result of the surface reaction with O₂ made contributions to the enhancement for a certain extent. The curves of quantum yield versus ageing time were fitted with a stretched exponential function. It was found that the course of fluorescence enhancement accorded with the dynamics of system with strongly coupled hierarchical degrees of freedom.     

Spectroscopic study of oscillator strength and radiative decay time of colloidal CdSe quantum dots

Characterization of samples of cadmium selenide quantum dots (CdSe) QDs dissolved in toluene colloidal solutions at a concentration of 1.4 mg/ml was carried out through UV–Vis absorption and photoluminescence (PL) spectroscopy. The size-dependent absorption and red-shifted PL emission peak wavelengths could be tuned between 510–576 and 545–606 nm respectively. Optical absorption spectral measurements yielded CdSe QDs having diameters about ~ 2.44–3.69 nm with energy gaps 2.32–2.08 eV which are higher than the bulk CdSe (1.74 eV) reminiscent of quantum confinement. This is found to be in good agreement with the semi-empirical pseudopotential model. In addition, the first excitonic absorption transition 1S_(e)1S_3/2(h) oscillator strength and the corresponding fluorescence radiative decay time of CdSe QDs are assessed using relevant Einstein relations for absorption and emission in a two-level system. The elaborated calculations would anticipate that the transition oscillator scale with the CdSe QD radius as ~ R^2.54. Correspondingly, the calculated radiative decay times decrease from 56.4 to 23.2 ns which scale with CdSe QDs radius as ~ R^?2.155 in fairly good agreement with experimental values reported in the literature.     

Light quenching and dark states in colloidal solutions of semiconductor CdSe/ZnS quantum dots

The photodynamics of optical limiting in colloidal solutions of different-size CdSe/ZnS quantum dots is studied. The behavior of the dependences points to a multistage process that includes the bleaching and optical limiting stages. The limiting photodynamics is compared with luminescence dynamics of quantum dots under conditions of high-power excitation. It is shown that the optical limiting efficiency in such media is determined by the position of exciting radiation with respect to the main exciton absorption band of the quantum dot. The roles played by so-called dark states and light quenching in limiting photodynamics are discussed.     

Photoluminescence of single quantum wires and quantum dots

The results of a study into the photoluminescence spectra of a set of quantum dots based on GaAs enclosed in AlGaAs nanowires are presented. The steady state and time resolved spectra of photoluminescence under optical excitation both from an array of quantum wires/dots and a single quantum wire/dot have been measured. In the photoluminescence spectra of single quantum dots, emission lines of excitons, biexcitons and tritons have been found. The binding energy of the biexciton in the studied structures was deduced to be 8 meV.     

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.     

Photoluminescence dynamics of coupled quantum dots

We have investigated effects of the strain-compensating spacer layer on photoluminescence (PL) properties of excitons in the multilayered self-assembled quantum dots (SAQDs). The PL decay time of excitons in the sample with a thin spacer layer is longer than that with a thick spacer layer. This is attributed to the lowering of the oscillator strengths owing to reduction of the overlap integral between the electrons and holes envelope functions because of spatially extended electron envelope functions in the growth direction. In addition, the PL intensity of the transverse-magnetic mode in the sample with the thinner spacer layer has been found to be enhanced in the polarized PL measurement from the cleaved edge surface of the samples. These results indicate the possibility of QDs connecting along the growth direction via penetration of the electron envelope function.     

IR-active vibrational modes of CdTe and CdSe colloidal quantum dots and CdTe/CdSe core/shell nanoparticles and coupling effects

The frequencies of the vibrational modes of CdTe and CdSe quantum dots and CdTe/CdSe core/shell nanoparticles prepared by the colloid chemistry method are determined using IR transmission and IR reflection spectroscopy. The experimental IR transmission spectrum of CdTe and CdSe nanocrystals exhibits a broad minimum located between the frequencies of the transverse optical (TO) and longitudinal optical (LO) phonons of bulk CdTe and CdSe crystals. The frequencies of the modes for ensembles of CdTe and CdSe quantum dots are considerably shifted toward lower frequencies as compared to those calculated for single quantum dots. This is explained by the dipole-dipole interaction between quantum dots. The frequencies of modes for the structures with core/shell nanoparticles differ little from the calculated frequencies. This suggests a weakening of the interaction in these structures due to the enhancement of dielectric screening.     

Electroluminescence of colloidal ZnSe quantum dots

The article reports a green chemical synthesis of colloidal ZnSe quantum dots at a moderate temperature. The prepared colloid sample is characterised by UV-vis absorption spectroscopy and transmission electron microscopy. UV-vis spectroscopy reveals as-expected blue-shift with strong absorption edge at 400 nm and micrographs show a non-uniform size distribution of ZnSe quantum dots in the range 1-4 nm. Further, photoluminescence and electroluminescence spectroscopies are carried out to study optical emission. Each of the spectroscopies reveals two emission peaks, indicating band-to-band transition and defect related transition. From the luminescence studies, it can be inferred that the recombination of electrons and holes resulting from interband transition causes violet emission and the recombination of a photon generated hole with a charged state of Zn-vacancy gives blue emission. Meanwhile electroluminescence study suggests the application of ZnSe quantum dots as an efficient light emitting device with the advantage of colour tuning (violet-blue-violet).     

Exciton tunnelling in ZnCdSe quantum well/CdSe quantum dots

Exciton tunnelling through a ZnSe barrier layer of various thicknesses is investigated in a Zn_0.72Cd_0.28Se/CdSe coupled quantum well/quantum dots (QW/QDs) structure using photoluminescence (PL) spectra and near resonant pump-probe technique. Fast exciton tunnelling from quantum well to quantum dots is observed by transient differential transmission. The tunnelling time is 1.8, 4.4 and 39 ps for barrier thickness of 10, 15 and 20 nm, respectively.     

Size-dependent electron-hole exchange interaction in CdSe quantum dots

Il Nuovo Cimento D - We have investigated the fine structure of luminecence of CdSe nanocrystals observed with size-selective excitation. We show that the luminescence line closest to the laser...     

Size-dependent electron-hole exchange interaction in CdSe quantum dots

Il Nuovo Cimento D - We have investigated the fine structure of luminecence of CdSe nanocrystals observed with size-selective excitation. We show that the luminescence line closest to the laser...     

Control of spin state in CdSe quantum dots by coupling with magnetic semiconductor quantum dots

We studied spin states of CdSe quantum dots (QDs) coupled with CdMnSe QDs by probing circular polarization of photoluminescence spectrum under external magnetic fields. The bandgap energies of CdSe and CdMnSe QDs are close to each other and photoluminescence mainly originates from CdSe QDs due to relatively low radiation efficiency of CdMnSe QDs. The photoluminescence lifetime as well as its intensity was decreased with increasing magnetic field, which was ascribed to the increase in the ground state wavefunctions in CdMnSe QDs. The decrease was more pronounced for spin down electrons, which was explained by the difference in spin up and down wave functions under magnetic fields. Our results show that the spin state of CdSe QDs can be manipulated by coupling with CdMnSe QDs.     

CdSe/PMMA: Plastic fiber material containing CdSe quantum dots

An optical gain plastic, CdSe/PMMA, poly-methyl methacrylate doped with CdSe quantum dot (QD), is reported in this study. With a narrow size distribution of the QD prepared by organic synthesis, the measured results show that the photoluminescence (PL) peak intensity is enhanced by ～2.5 times, FWHM broadened by ～10 nm, and the PL peak wavelength blue shifted by ～25 nm in the CdSe/PMMA compared with plain CdSe QD. Under continuous irradiation of 473-nm laser, the PL peak intensity is positively correlated with time within the first hour after the samples are fabricated, but gradually stabilizes afterward. The obtained results suggest that the CdSe/PMMA material reported here is a desirable optical gain plastic. Potential applications include high gain wideband fiber amplifiers or other photonic devices in the future due to its high flexibility, low weight and low cost.     

Ultrafast spectroscopy of CdS/CdSe quantum dots

Results from the nonresonance spectroscopy of CdS/CdSe quantum dots (composites of CdSe–CdS nanoparticles (core–shell)) are presented. The nonlinear optical properties of CdS/CdSe QDs in PMMA are studied with fs pulses at 1053 nm using the transient lens technique. QDs generate rapidly oscillating signals with amplitude rise times of around 200 fs and decay times of around 500 fs, while pure PMMA polymer only generates an oscillating signal whose envelope coincides with its autocorrelation function.     

Pseudopotential theory of Auger processes in CdSe quantum dots

Auger rates are calculated for CdSe colloidal quantum dots using atomistic empirical pseudopotential wave functions. We predict the dependence of Auger electron cooling on size, on correlation effects (included via configuration interaction), and on the presence of a spectator exciton. Auger multiexciton recombination rates are predicted for biexcitons as well as for triexcitons. The results agree quantitatively with recent measurements and offer new predictions.     

Photoluminescence of charged magneto-excitons in InAs single quantum dots

We calculated the photoluminescence spectra of charged magneto-excitons in single two-dimensional parabolic quantum dots, using an unrestricted Hartree–Fock method. The calculated luminescence spectra explain well the observed red shifts of transition energies of InAs/GaAs single quantum dot by additional electron capture in a dot. The magnetic-field-induced transition of the ground state configuration of trapped electrons causes drastic change in the photoluminescence spectra. The dependence of photoluminescence intensities of charged excitons on the excess energies of photogenerated carriers above the bulk GaAs energy gap is studied phenomenologically, by calculating the steady state electron population probability in a dot.     

Multiexciton absorption and multiple exciton generation in CdSe quantum dots

Efficient multiple-exciton generation (MEG) in semiconductor quantum dots has been recently reported. The MEG efficiency has so far been evaluated assuming that the change (bleaching) of the absorption spectrum due to MEG is linearly proportional to the number of excitons N(X). Here, we critically examine this assumption using atomistic pseudopotential calculations for colloidal CdSe nanocrystals. We find that the bleaching of the first absorption peak depends nonlinearly on N(X), due to carrier-carrier interactions. This nonlinearity mandates an upper bound of 1.5 to the value of the normalized bleaching that can be attributed to MEG, significantly smaller than the limit of 2.0 predicted by the linear scaling assumption. Thus, measured values of the normalized bleaching in excess of 1.5 cannot be due entirely to MEG, but must originate in part from other mechanisms.     

Carrier capture and recombination in CdSe/ZnSe quantum dots

Journal of Experimental and Theoretical Physics - Time-resolved photoluminescence (PL) spectra of self-assembled CdSe/ZnSe quantum dots (QDs) are measured with a view to identifying the QD-size...