Critical Crystallographic Transition in Violation of Kasha’s Rule of Size-Specific ZnO Quantum Dots

Abstract

Investigation of the intrinsic relationship between crystallography and electronic properties may provide an avenue to unravel the photophysical aspects of semiconductor nanostructures. Since the 1980s, semiconducting materials at the nanoscale dimensions have been of immense scientific interest because of the unique quantum characteristics beyond a particular size limit. Prudent advances in the synthetic strategies of naked ZnO quantum dots have offered the opportunity to imbue the structure−property relationship of nanostructured systems. Seven size-specific ZnO nanospheres have been synthesized through alkaline hydrolysis of the precursor salt in the presence of alcohol in the reaction medium. The photoluminescence behavior of colloidal dispersion of ZnO nanoparticles has been studied as a function of excitation wavelength with a periodic interval of 10 nm in the range of 290−430 nm. It has been observed that changes in the excitation wavelength shift the emission spectrum in a regular manner independent of the size of ZnO particles under investigation. The empirical proposition in the framework of Kasha's rule has been established as a milestone of spectroscopy, being validated in tens of thousands of fluorophore systems; meanwhile, although obscure, anomalous cases have also been observed both in molecular and nanoscale systems. Crystallographic analysis has shown that there occurs a structural transition that determines the localization of electronic energy levels in the experimental size range of the ZnO particles. Therefore, the salient feature of physical significance is that crystallographic transition appears to exhibit the excitation wavelength-dependent shift of the photoluminescence maximum and is therefore the violation of Kasha's rule for the ZnO quantum dots.

Growth of ZnO Polytypes: Multiple Facets of Diverse Applications

“…We have investigated the excitation wavelength dependent photophysical characteristics of the colloidal dispersion of size-specific ZnO quantum dots. It has been revealed that a critical crystallographic transition correlates the excitation wavelength dependent photoluminescence shift and the violation of Kasha’s rule by these ultrasmall particles . A detailed crystallographic analysis of the size-selective ZnO particles of 2.0 ± 0.3, 3.2 ± 0.4, and 9.0 ± 1.0 nm has been shown in Figure .…”

“…(a–c) X-ray diffraction patterns, (d–f) corresponding crystalline phases, and (g–i) unit cell patterns of 2.0, 3.2, and 9.0 nm ZnO quantum dots, respectively. Reprinted with permission from ref . Copyright 2023 American Chemical Society.…”

“…It has been revealed that a critical crystallographic transition correlates the excitation wavelength dependent photoluminescence shift and the violation of Kasha's rule by these ultrasmall particles. 52 A detailed crystallographic analysis of the size-selective ZnO particles of 2.0 ± 0.3, 3.2 ± 0.4, and 9.0 ± 1.0 nm has been shown in Figure 10. The transition in the crystallographic structure varies from cubic crystal (space group Fm3m) to hexagonal wurtzite (space group P6 3 ) to hexagonal wurtzite (space group P6 3 mc) that differs on the number of symmetry operators associated with relative orientations of the lattice sites and dimensions with an increase in the size of the particles.…”