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 chalcogenidespromising 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 GeS
x
Se1–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
GeS
x
Se1–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 GeS
x
Se1–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.