Synthesis, Optical and Structural Characterization, and Exciton Dynamics of Doped ZnSe Nanocrystals, and, Simultaneous X-ray Emission Spectroscopy of Two Elements Using Energy Dispersive Spectrometer

Abstract

Doped semiconductor quantum dots (QDs) comprise an important subclass of nanomaterials in which a small quantity of impurity is added intentionally, adding another degree of freedom to alter their size-dependent physical and electronic properties. Intense, tunable, long lived and stable photoluminescence make them quintessential candidates for many opto-electronic applications including solid-state lighting, display devices and biomedical imaging. ZnSe QDs, which are blue-emitting fluorophores, were doped with Cu⁺¹ to redshift their photoluminescence (PL) to green region of the visible spectrum. These Cu-doped ZnSe QDs were then codoped with Al³⁺, Ga ³⁺ and In³⁺ to improve the PL quantum yield (QY) by eliminating the defect states originating from charge imbalance created by aliovalent doping. Codoping also resulted in further redshifting of the PL, covering most of the visible spectrum, making them potential candidates for use in solid-state lighting and as optical down converters in next generation light emitting diodes (LEDs). To better understand the optical properties of these materials, local structure around the luminescent centers was investigated by extended X-ray absorption fine structure (EXAFS). Cu was found to occupy a distorted tetrahedral site with the codopant residing in a substitutional Zn site. Based on the structural information obtained by EXAFS, density functional theory calculations (DFT) were performed to get a clear picture of the energy levels associated with the electronic transitions. Furthermore, the dynamics studies of the exciton and charge carriers were carried out to get deeper insight of the various photophysical processes involved. The fluorescence lifetime was increased approximately 10 times after doping.

The multielectron catalytic reactions often involve multimetallic clusters, where the reaction is controlled by the electronic and spin coupling between metals and ligands to facilitate charge transfer, bond formation/breaking, substrate binding, and release of products. A method was developed to detect X-ray emission signal from multiple elements simultaneously to probe the electronic structure and sequential chemistry that occurs between the elements. A wavelength dispersive spectrometer based on the von-Hamos geometry was used, that disperses Kβ emission signals of multiple elements onto an area detector, and enables an XES spectrum to be measured in a single-shot mode. This overcomes the scanning needs of the Rowland circle spectrometers, and the data is free from temporal and normalization errors, and therefore ideal to follow sequential chemistry at multiple sites. This method was applied to MnO_x based electrocatalysts, and the effect of Ni addition was investigated. Electro-deposited Mn oxide catalyses oxygen-evolution reaction (OER) and oxygen-reduction reaction (ORR) at different electrochemical potentials under alkaline condition. Incorporation of Ni reduced the low valent Mn component resulting in higher average oxidation state of Mn in MnNiOx under ORR and OER conditions, when compared to MnO _x under similar conditions. The reversibility of the electrocatalyst was also found to improve by the inclusion of Ni.