Temperature- and size-dependent photoluminescence in colloidal CdTe and Cd _x Zn_1−x Te quantum dots

Abstract

Semiconductor colloidal quantum dots (QDs) of CdTe and alloyed Cd x Zn1−x Te QDs with N-acetyl-L-cysteine capping ligands are synthesized by a reflux method in aqueous solution. Alloying provides a new degree of freedom to tune the optical and electronic properties of the nanocrystals. The photoluminescence (PL) of Cd x Zn1−x Te QDs is sharper and displays a highly enhanced quantum yield (QY) of 65% relative to the 16% of CdTe QDs. The fluorescence of Cd x Zn1−x Te QDs is observed to be highly stable for over 12 months without degradation, while that of CdTe QDs begins to mildly flocculate around 8 months of storage. To characterise the material structure and composition, UV-Vis absorption spectroscopy, x-ray powder diffraction, transmission electron microscopy, and inductively coupled plasma mass spectrometry measurements are carried out. To understand the fundamental processes that play in the luminescence behaviour, temperature- and size-dependent PL spectra are investigated in the range 80–300 K. The Varshni and O’Donnell equations fit well on the PL peak emission energies and the Huang–Rhys parameter indicates the strengthening of exciton–phonon coupling in the nanocrystals upon alloying and with decreasing nanocrystal sizes. PL linewidth analysis reveals that the inhomogeneous broadening is considerably reduced in Cd x Zn1−x Te QDs relative to CdTe. Moreover, the quantum confinement effect of the nanocrystals leads to an increase in exciton–acoustic phonon interactions with the coefficients ranging between 26.9 and 95.6 µeV K−1 compared to the bulk CdTe value of 0.72 µeV K−1. Exciton–longitudinal optical phonon interactions are made stronger by the ZnTe alloying with the coefficients lying in the range between 24.8 and 41.7 meV and also with the effect of increasing crystal size. An Arrhenius plot of PL integrated area is used to calculate the thermal activation energy value E a of the non-radiative recombination channel, which is 132 meV for CdTe QDs and a higher value of 185 meV for Cd x Zn1−x Te QDs. This is consistent with the observed QY enhancement in Cd x Zn1−x Te QDs as a higher E a value indicates reduced generation of non-radiative recombination centres and a decrease in defect densities upon alloying. Cd x Zn1−x Te QDs with enhanced fluorescence properties serve both as a medium for studying fundamental effects of alloying and its properties, and for practical applications such as biomedical labelling and optoelectronics.

Investigation on nonlinear optical and optical limiting properties of Cd0.7Zn0.3Te quantum dots

“…The position of diffraction peaks at wider angle indicates the alloy effect. Balakrishanan et al and Qian et al also observed broadened and less intense CdZnTe diffraction peaks in their reports[2,11]…”

“…To this mixture, required quantities of sodium tellurite and reducing agent NaBH 4 are added as per the molar ratios and maintained a high stirring rate. When the solution began to show slight color changes a reaction temperature of 100 0 C provided for a time period of 7 hours to form the Cd 0.7 Zn 0.3 Te alloy quantum dots [1,2,7,10].…”

“…Semiconductor nanoparticles or quantum dots with size depended parameters have been studied thoroughly by many expertise in the material science owing to their excellent optoelectronic and luminescence characteristics. Semiconductor quantum dot finds the advantages of size tuneable structural, optical, chemical and electronic properties [1,2]. The strong quantum confinement effect and high surface to volume ratio exhibited by the quantum dots extends their possibilities in the nonlinear optical filed.…”

Size-dependent Thermal Shifts to MOF Nanocrystal Optical Gaps Induced by Dynamic Bonding

“…For example, fitting Eopt data for CdTe quantum dots to the O'Donnell and Chen model showed that decreasing particle sizes from 3.6 nm to 2.6 nm caused an increase in S from 1.9 to 2.4 and an increase in áħwñ from 12.3 meV to 12.7 meV. 34 This phenomenon has been widely observed for quantum dots and has been attributed to enhanced exciton-phonon interactions in smaller crystallite sizes. 35 Furthermore, theoretical analysis predicts S to be proportional to a 3 /V, where a represents the Bohr radius and V the particle volume.…”

Preparation and characterization of pure phase CdMnTe nanopowders by a hydrothermal route

“…16 Balakrishnan et al synthesized CdZnTe QDs in aqueous solution by a reflux method. 17 Compared to CdTe QDs, CdZnTe QDs have less nonradiative recombination centers and lower defect density. Some studies have been reported the synthesis of Mn 2+ partially substituted Cd 2+ ions in CdTe lattices.…”