Phonons and excitons in ZrSe2 – ZrS2 alloys

ZrS2 has received increased attention due to its indirect band gap that matches with the visible spectrum and which is predicted to undergo an indirect-to-direct transition with strain. Alloying with ZrSe2 to produce ZrSxSe2-x (x=0...2) would provide continuous control over key optical and electronic parameters for photonics. Here, we combine Raman spectroscopy, spectroscopic ellipsometry (SE), and density functional theory to conduct the first comprehensive exploration of phonons and excitons in this system. We find that long-range Coulomb interactions activate optically-silent infrared phonons that completely dominate Raman spectra in some alloys. SE measurements of excitons reveal strong light-matter interactions with low optical losses for near-infrared wavelengths. This study suggests ZrSxSe2-x alloys will find applications in atomically-thin, tunable photodetectors and photovoltaics.