  Selenium has attracted intensive attention as a promising material candidate
for future optoelectronic applications. However, selenium has a strong tendency
to grow into nanowire forms due to its anisotropic atomic structure, which has
largely hindered the exploration of its potential applications. In this work,
using a physical vapor deposition method, we have demonstrated the synthesis of
large-size, high-quality 2D selenium nanosheets, the minimum thickness of which
could be as thin as 5 nm. The Se nanosheet exhibits a strong in-plane
anisotropic property, which is determined by angle-resolved Raman spectroscopy.
Back-gating field-effect transistors based on a Se nanosheet exhibit p-type
transport behaviors with on-state current density around 20 mA/mm at Vds = 3 V.
Four-terminal field effect devices are also fabricated to evaluate the
intrinsic hole mobility of the selenium nanosheet, and the value is determined
to be 0.26 cm2 Vs at 300 K. The selenium nanosheet phototransistors show an
excellent photoresponsivity of up to 263 A/W, with a rise time of 0.1 s and
fall time of 0.12 s. These results suggest that crystal selenium as a 2D form
of a 1D van der Waals solid opens up the possibility to explore device
applications.
