Two-dimensional (2D)
GeSe is an important IVA–VIA semiconductor
for future applications in electronics and optoelectronics because
of its high absorption coefficient, mobility, and photoresponsivity.
However, the controllable synthesis of 2D GeSe flakes is still a huge
problem. Here, high-quality single-crystalline ultrathin 2D GeSe flakes
are synthesized by a salt-assisted chemical vapor deposition method.
The flakes tend to grow along the [010] crystal orientation presenting
a rectangular shape with a thickness down to 5 nm. Then, the electrical
and optoelectronic properties have been systematically investigated.
A thickness-dependent Schottky barrier is shown in GeSe field-effect
transistors. The p-type conductivity of GeSe is mainly caused by the
Ge deficiency, which is proven by a variable-temperature experiment
and theoretical calculations. In addition, the phototransistors based
on as-grown GeSe flakes present an ultrahigh responsivity of 1.8 ×
104 A/W and an excellent external quantum efficiency of
4.2 × 106%.