From an application perspective, Cu 3-x P nanocrystals have been used in photodetectors, photothermal therapy , and as an electrocatalyst in hydrogen and oxygen evolution reactions . Recently, Cu 3-x P nanocrystals have been used as a template for cation exchange reactions for synthesizing wurtzite indium and gallium phosphide quantum dots. Interestingly, most of these are related to the inherent Cu vacancies which in turn can be gauged by the extinction and frequency of the surface plasmon resonance. Fine-tuning the plasmon peak is relevant for monitoring and controlling the activity of these nanocrystals in aforementioned applications.