Strain Effects on the Band Gap and Diameter of CdSe Core and CdSe/ZnS Core/Shell Quantum Dots at Any Temperature

Md Rezaul Karim,Hilmi Ünlü,Mesut Balaban

doi:10.1155/2019/3764395

Abstract

We present the results of an experimental study about strain effects on the core band gap and diameter of spherical bare CdSe core and CdSe/ZnS core/shell quantum dots (QDs) synthesized by using a colloidal technique at varying temperatures. Structural characterizations were made by using X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM) techniques. Optical characterizations were made by using UV-Vis absorption and fluorescence emission spectroscopies. The XRD analysis suggests that the synthesized bare CdSe core and CdSe/ZnS core/shell QDs have zinc blende crystal structure. HRTEM results indicate that the CdSe core and CdSe/ZnS QDs have average particle sizes about 3.50 nm and 4.84 nm, respectively. Furthermore, compressive strain causes an increase (decrease) in the core band gap (diameter) of spherical CdSe/ZnS core/shell QDs at any temperature. An elastic strain-modified effective mass approximation (EMA) predicts that there is a parabolic decrease (increase) in the core band gap (diameter) of QDs with temperature. The diameter of spherical bare CdSe core and CdSe/ZnS core/shell QDs calculated by using the strain-modified EMA, with core band gap extracted from absorption spectra, are in excellent agreement with the HRTEM data.

Highlights

Group II-VI compound semiconductor-based spherical bare CdSe core and CdSe/ZnS core/shell quantum dots (QDs) have generated much interest among device scientists and engineers because of their potential use in fabrication of new generation solar cells, light-emitting diodes (LEDs), fluorescent biosensors etc. [1,2,3,4,5,6]
high-resolution transmission electron microscopy (HRTEM) images shown in Figure 2 indicate that both the bare CdSe core and CdSe/ZnS core/Shell NCs are uniform in size and shape. e nanocrystal size becomes larger as reaction time is increased
E peak intensity values of UV-Vis absorption and PL emission spectra, Stokes shift, full width at half maximum (FWHM), and quantum yield (QY) of CdSe/ZnS core/ shell QDs synthesized at 160°C and 170°C are listed in Table 2 for reaction times between 1 and 20 min., respectively. e fluorescence quantum yield (QY) of CdSe/ ZnS core/shell QDs synthesized at 160°C and 170°C increases monotonically from 27% to 45% with reaction time increase and decreases as reaction time is increased further. is result suggests that one can optimize the reaction time to get maximum quantum yield

Summary

Introduction

Group II-VI compound semiconductor-based spherical bare CdSe core and CdSe/ZnS core/shell quantum dots (QDs) have generated much interest among device scientists and engineers because of their potential use in fabrication of new generation solar cells, light-emitting diodes (LEDs), fluorescent biosensors etc. [1,2,3,4,5,6]. Interface strain effects on structural and electronic properties of core/shell QDs become important in determining their tunable properties such as the core band gap and diameter, which are essential for predicting their potential as electronic and optical devices. E aim of this work is to prove that elastic strain has parabolic effects on the band gap and diameter of bare CdSe core and CdSe/ZnS core/shell QDs as a function of temperature, which were synthesized by using a colloidal technique and characterized by using X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), UV-Vis absorption, and fluorescence emission techniques, respectively. E core diameter of QDs calculated by using strain–modified two-band effective mass approximation, with the core band gap extracted from UV-Vis absorption spectra, will be compared with the results of HRTEM analysis. After the fine isolation of growth CdSe/ZnS core/shell QDs, the precipitation was dissolved with different volume of hexane. After the fine isolation of growth CdSe/ZnS core/shell QDs, the precipitation was dissolved with different volume of hexane. e reaction was monitored with Shimadzu UV-Vis NIR absorption spectrometer with aliquots taken at different time and temperature

Results and Discussion

Unstrained

Conclusion