Composition Dependence of the Band Gaps of Semiconducting GeS (x) Se1-x van der Waals Alloys

Alloyingof two-dimensional (2D)/layered chalcogenide semiconductorsby forming ternaries with properties that span the range between thebinary constituents allows tuning of the electronic and optical propertiesand achieving the full potential of these materials. While the focusso far has been on transition-metal dichalcogenides, alloying in layeredgroup IV chalcogenides Rpromising for optoelectronics, photovoltaics,ferroelectrics, etc.Rremains less understood. Here, we investigatealloying in the GeSe-GeS system and its effect on the fundamentalband gap. We synthesize single-crystalline layered GeS (x) Se1-x alloy micro-and nanowires whose compositions are tunable over the entire rangeof S content, x, via the GeS and GeSe precursor temperatures.Cathodoluminescence in scanning transmission electron microscopy isused to investigate the composition dependence of the band gaps ofGeS (x) Se1-x alloy micro- and nanowires. The band gaps of bulk-like microwiresincrease systematically with the sulfur content of the alloys, therebycovering the entire range between GeSe (1.27 eV) and GeS (1.6 eV).The composition dependence of the fundamental band gap is close tolinear with a bowing coefficient b = 0.173 eV. Densityfunctional theory calculations support the isomorphous behavior ofGeSe-GeS solid solutions and demonstrate that the band gapsare indirect and have similar small bowing as determined experimentally.Finally, we establish pronounced size effects in GeS (x) Se1-x alloy nanowiresthat provide access to higher-energy optoelectronic transitions thancan be realized in bulk alloys of the same composition. Our resultssupport applications of germanium monochalcogenide alloys in areassuch as optoelectronics and photovoltaics.