  Through stacking engineering of two-dimensional (2D) materials, a switchable
interface polarization can be generated through interlayer sliding, so called
sliding ferroelectricity, which is advantageous over the traditional
ferroelectricity due to ultra-thin thickness, high switching speed and low
fatigue. However, 2D materials with intrinsic sliding ferroelectricity are
still rare, with the exception of rhombohedral-stacked MoS2, which limits
sliding ferroelectricity for practical applications such as high-speed storage,
photovoltaic, and neuromorphic computing. Here, we reported the observation of
sliding ferroelectricity with multiple states in undoped rhombohedral-stacked
InSe ({\gamma}-InSe) via dual-frequency resonance tracking piezoresponse force
microscopy, scanning Kelvin probe microscopy and conductive atomic force
microscopy. The tunable bulk photovoltaic effect via the electric field is
achieved in the graphene/{\gamma}-InSe/graphene tunneling device with a
photovoltaic current density of ~15 mA/cm2, which is attributed to the multiple
sliding steps in {\gamma}-InSe according to our theoretical calculations. The
vdw tunneling device also features a high photo responsivity of ~255 A/W and a
fast response time for real-time imaging. Our work not only enriches
rhombohedral-stacked 2D materials for sliding ferroelectricity, but also sheds
light on their potential for tunable photovoltaics and imaging applications.
