Tailoring Ultrafast Near-Band Gap Photoconductive Response in GeS by Zero-Valent Cu Intercalation

Zero-valent intercalation of atomic metals into the van der Waals gap of layered materials can be used to tune their electronic, optical, thermal, and mechanical properties. Here, we report the impact of intercalating ∼3 atm percent of zero-valent copper into germanium sulfide (GeS). Advanced many-body calculations predict that copper introduces quasi-localized intermediate band states, and time-resolved THz spectroscopy studies demonstrate that those states have prominent effects on the photoconductivity of GeS. Cu-intercalated GeS exhibits a faster rise of transient photoconductivity and a shorter lifetime of optically injected carriers following near-gap excitation with 800 nm pulses. At the same time, Cu intercalation improves free carrier mobility from 1100 to 1300 cm² V^-1 s^-1, which we attribute to the damping of acoustic phonons observed in Brillouin scattering and consequent reduction of phonon scattering.