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.

The determination of optical constants of noble metals that govern the characteristics of the system has been found to be extremely important to retrospect the observed optical properties from theoretical perspectives to excavate the light–matter interaction at the bottom. Numerous experimental and theoretical approaches, often, followed by fitting through a specified model have been adopted in the literature to evaluate the optical constants either at the bulk, thin film, or nanoscale dimensions. Bulk optical constants have, often, been used for simulation of the optical extinction of noble metal clusters of arbitrary sizes. In 1900, Paul Drude proposed his model of free-electron conduction in a metal that allows expressing the plasmonic characteristics as a function of the common observables. Noble metals, like copper, silver, and gold, at the nanoscale dimension exhibit a characteristic strong absorption band in the UV–vis–NIR spectral region that can be ascribed to the localized surface plasmon resonance (LSPR) that is specific to nanostructures because of geometrical confinement effects of the free electrons. The spectral position and magnitude of the LSPR are, explicitly, governed by the density of conduction electrons, the effective electron mass, and the shape and the size of the charge distribution that can, solely, be attributed to the dielectric properties of both the materials and the surrounding medium. Prudent advances in the synthetic strategies have opened up avenues to achieve desired nanostructures with similar morphologies and stabilizing ligand shells dispersed under analogous conditions. The exquisite sensitivity of the plasmonic response under varieties of microenvironmental conditions could be employed to determine the optical constants of the corresponding metallic nanostructures. A comparative account of the plasmonic sensitivity of materials that requires the determination of the dielectric constant at the nanoscale dimension has been elucidated.