We report on the temperature dependence of the photoluminescence (PL) spectrum and of the PL relaxation dynamics for colloidal $\mathrm{Cd}\mathrm{Se}∕\mathrm{Zn}\mathrm{S}$ core/shell quantum dots (QDs) embedded in an inert polystyrene matrix. We demonstrate that the confinement energy in the QDs is independent of the temperature. The coupling with both acoustic and optical phonons is also studied. Quantum confinement results in a strong increase of the exciton\char21{}acoustic-phonon coupling constant, up to $71\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{eV}∕\mathrm{K}$, and in a reduced exciton\char21{}longitudinal-optical (LO)-phonon coupling constant, down to $21\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$, with respect to bulk CdSe. In addition, we demonstrate that the main nonradiative process that limits the quantum efficiency of the QD at room temperature is the thermal escape from the dot assisted by scattering with four LO phonons. Thermally activated trapping in surface states is also observed at low temperature, with an activation energy of about $15\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$.

Temperature dependence of the photoluminescence properties of colloidal Cd Se ∕ Zn S core/shell quantum dots embedded in a polystyrene matrix

We collect and resolve spectrally and temporally the photoluminescence of single CdSe nanoplatelets. The emission intensity of single nanoplatelets at room temperature shows ON and OFF periods with a usual blinking statistics, while at 20 K, their emission intensity can be extremely stable in time. At room temperature, the emission spectra of single nanoplatelets are similar to ensemble measurements with a full width at half-maximum of 40 meV. At 20 K, we obtain a resolution-limited spectral line width (<0.4 meV). The fluorescence lifetime of single nanoplatelets decreases when the temperature decreases to reach 200 ps at 20 K. This lifetime shortening is concomitant with an increase of the nanoplatelets’ emission intensity.

Spectroscopy of single CdSe nanoplatelets.

Exciton-LO-phonon couplings in CdS x Sc 1-x semiconductor microcrystallites (x=0.12±0.05) are investigated by measuring the temperature dependence of the width and energy of excitons by electroabsorption. The LO phonons are shown semiquantitatively to contribute to the experimentally obtained temperaure dependencies of the width and energy of excitons. The dependence of the coupling constant (the Huang-Rhys parameter) on the radius of microcrystallites is calculated for CdSe and GaAs microcrystallites. The phonon confinement effects are considered with "free-standing" and "rigid"-boundary conditions. As for the exciton state, nonparabolicity of the conduction band and the valence-band mixing are considered in order to obtain a precise exciton wave function, which is crucially important in calculating the Huang-Rhys parameter in a microcrystallite. The exciton-confined-optical-phonon interaction Harniltonian is constructed for a microcrystallite. It is found that the Huang-Rhys parameters have a minimum at a radius of 70 A for CdSe and 270 A for GaAs microcrystallites. The size dependence of the Huang-Rhys parameter is also calculated for a microcrystallite with an extra charge at the spherical-particle center. The lowest (s,S 3/2 ) state in the trapped state is found to have small transition probability and g values of 1 in CdSe (R =30 A) and 0.01 in GaAs? = 100 A). The higher states are found to have larger transition probability and g values of 0.7 in CdSe (R =30 A) and 0.01 in GaAs (R = 100 A). These results suggest that large g values observed experimentally in CdS and CdSe microcrystallites originate from extrinsic effects such as the presence of charged point defects inside the microcrystallite.

Exciton-LO-phonon couplings in spherical semiconductor microcrystallites

Recent synthetic developments have generated intense interest in the use of cesium lead halide perovskite nanocrystals for light-emitting applications. This work presents the photoluminescence (PL) of cesium lead halide perovskite nanocrystals with tunable halide composition recorded as function of temperature from 80 to 550 K. CsPbBr3 nanocrystals show the highest resilience to temperature while chloride-containing samples show relatively poorer preservation of photoluminescence at elevated temperatures. Thermal cycling experiments show that PL loss of CsPbBr3 is largely reversible at temperatures below 450 K, but shows irreversible degradation at higher temperatures. Time-resolved measurements of CsPbX3 samples show an increase in the PL lifetime with temperature elevation, consistent with exciton fission to form free carriers, followed by a decrease in the apparent PL lifetime due to trapping. PL persistence measurements and time-resolved spectroscopies implicate thermally assisted trapping, most likely to halogen vacancy traps, as the mechanism of reversible PL loss.

High-Temperature Photoluminescence of CsPbX3 (X = Cl, Br, I) Nanocrystals

Electrochemical study of the effect of nano-zinc oxide on microperoxidase and its application to more sensitive hydrogen peroxide biosensor preparation.

A temperature-dependent photoluminescence measurement is performed in CdSe/ZnSe quantum dots with a ZnCdSe quantum well. We deduce the temperature dependence of the exciton linewidth and peak energy of the zero-dimensional exciton in the quantum dots and two-dimensional exciton in the CdSe wetting layer. The experimental data reveal a reduction of homogeneous broadening of the exciton line in the quantum dots in comparison with that in the two-dimensional wetting layer, which indicates the decrease of exciton and optical phonon coupling in the CdSe quantum dots.

https://iopscience.iop.org/article/10.1088/0256-307X/19/4/340/pdf

Exciton-Phonon Scattering in CdSe/ZnSe Quantum Dots

By employing lithium and nitrogen codoping method, p-type zinc oxide (ZnO) films have been prepared, and the p-type conduction can maintain for 207 days. ZnO p-n junctions have been constructed based on the p-type ZnO. Under the drive of continuous current, obvious emission has been observed from the p-n junctions, and the ZnO p-n junction light-emitting devices (LEDs) can still work after placing in air ambient for 180 days. Room temperature electroluminescence spectra of the p-ZnO:(Li, N)/n-ZnO structured LEDs under continuous current recorded intermittently for 180 days. (C) 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Stable p-type ZnO films obtained by lithium-nitrogen codoping method

MgNiO基板上の金属-半導体-金属紫外線検出器

We present a simple and useful method for preparing high-quality nanocrystalline ZnO thin films, i.e. the thermal oxidation of high-quality ZnS films prepared by the low-pressure metal-organic chemical vapour deposition technique. The x-ray diffraction measurements reveal that the nanocrystalline ZnO has a hexagonal wurtzite structure. Raman spectra show that the longitudinal optical phonon with the E1-mode appears at 578 cm-1. The multiple phonon scattering process is also observed, indicating the formation of a high-quality nanocrystalline ZnO thin film. The photoluminescence spectrum has a single emission peak at 3.264 eV from the free-exciton mission, under the condition of low excitation power at room temperature. However, when excitation intensities exceed the threshold of 150 kW/cm2, a new and narrow peak emerges at lower energies, which are attributed to exciton-exciton collisions, and is called the P line. The intensity of this peak increases superlinearly with the pumping power over a threshold value. This supplies strong evidence of stimulated emission. The multiple longitudinal cavity modes observed in the stimulated emission spectrum indicate the successful realization of optically pumped lasing from nanocrystalline ZnO films at room temperature.

https://iopscience.iop.org/article/10.1088/0256-307X/19/1/340/pdf

Photoluminescence and Optically Pumped Ultraviolet Lasing from Nanocrystalline ZnO Thin Films Prepared by Thermal Oxidation of High-Quality ZnS Thin Films

Shen De-Zhen

Papers

Exciton-Phonon Scattering in CdSe/ZnSe Quantum Dots

Stable p-type ZnO films obtained by lithium-nitrogen codoping method

MgNiO基板上の金属-半導体-金属紫外線検出器

Photoluminescence and Optically Pumped Ultraviolet Lasing from Nanocrystalline ZnO Thin Films Prepared by Thermal Oxidation of High-Quality ZnS Thin Films

Exciton tunnelling in ZnCdSe quantum well/CdSe quantum dots