Composition Dependence of the Band Gaps of Semiconducting GeSₓSe_1–x van der Waals Alloys

Alloying of two-dimensional (2D)/layered chalcogenide semiconductors by forming ternaries with properties that span the range between the binary constituents allows tuning of the electronic and optical properties and achieving the full potential of these materials. While the focus so far has been on transition-metal dichalcogenides, alloying in layered group IV chalcogenides─promising for optoelectronics, photovoltaics, ferroelectrics, etc.─remains less understood. Here, we investigate alloying in the GeSe–GeS system and its effect on the fundamental band gap. We synthesize single-crystalline layered GeSₓSe_1–x alloy micro- and nanowires whose compositions are tunable over the entire range of S content, x, via the GeS and GeSe precursor temperatures. Cathodoluminescence in scanning transmission electron microscopy is used to investigate the composition dependence of the band gaps of GeSₓSe_1–x alloy micro- and nanowires. The band gaps of bulk-like microwires increase systematically with the sulfur content of the alloys, thereby covering the entire range between GeSe (1.27 eV) and GeS (1.6 eV). The composition dependence of the fundamental band gap is close to linear with a bowing coefficient b = 0.173 eV. Density functional theory calculations support the isomorphous behavior of GeSe–GeS solid solutions and demonstrate that the band gaps are indirect and have similar small bowing as determined experimentally. Finally, we establish pronounced size effects in GeSₓSe_1–x alloy nanowires that provide access to higher-energy optoelectronic transitions than can be realized in bulk alloys of the same composition. Our results support applications of germanium monochalcogenide alloys in areas such as optoelectronics and photovoltaics.