All sample spectra exhibited a trough-like feature around 2200 nm. It is likely linked to the samples rather than external absorbing sources since instrumental effects were eliminated by data normalization. Given that the feature appears in the NIR range, it is likely associated with vibrational modes rather than electronic transitions. A possible explanation is the presence of hydroxyl (OH) group on the sample surface, as OH absorption bands have been observed around 2200 nm in previous studies. To determine the optical band gap of Ag 3 Au(Se,Te) 2 , the Tauc equation and Kubelka-Munk theory were combined as: