A potential origin of the NIR absorption is the existence of intraband transition such as the intervalence band transition occurring in p-type semiconductors. A quick test to rule out this possibility and for assigning the NIR absorption to plasmon resonance is to check whether this peak is sensitive to the refractive index of the medium (ϵ m ). In fact, within the framework of Drude model, the plasmon band peak position (λ) is related to the