The NIR absorption of Cu 3-x P was measured in six different solvents, namely tetrahydrofuran (THF), chloroform, carbon tetrachloride, toluene, tetrachloroethylene (PCE), and carbon disulfide. (Figure ) . With an increase in the refractive index, there was a redshift of the spectra. However, this variation wasn't linear as predicted by either eq 1 or COMSOL simulation based on the bulk dielectric fuction of Cu 3-x P thin film(Figure ). But in solvent suited for IR spectroscopy ( carbon tetrachloride, tetrachloroethylene, and carbon disulfide), there was a linear dependence of absorption peak on the refractive index as per eq 1 (Figure ) . The discrepancy arises likely due to interference from the vibrational features of solvents such as chloroform, THF, and toluene, and the coordinating nature of the solvents like THF (Figure ). Despite this, the influence of solvent refractive index on absorption peak position suggests that the NIR peak has a dominant plasmonic nature and the interband transition is either non-existent or may be hidden under the plasmon resonance.