  Two-dimensional (2D) van der Waals ferroelectrics provide an unprecedented
architectural freedom for the creation of artificial multiferroics and
non-volatile electronic devices based on vertical and co-planar heterojunctions
of 2D ferroic materials. Nevertheless, controlled microscopic manipulation of
ferroelectric domains is still rare in monolayer-thick 2D ferroelectrics with
in-plane polarization. Here we report the discovery of robust ferroelectricity
with a critical temperature close to 400 K in SnSe monolayer plates grown on
graphene, and the demonstration of controlled room temperature ferroelectric
domain manipulation by applying appropriate bias voltage pulses to the tip of a
scanning tunneling microscope (STM). This study shows that STM is a powerful
tool for detecting and manipulating the microscopic domain structures in 2D
ferroelectric monolayers, which is difficult for conventional approaches such
as piezoresponse force microscopy, thus facilitating the hunt for other 2D
ferroelectric monolayers with in-plane polarization with important
technological applications.
