Ultrathin indium selenide has recently become a promising 2D semiconductor for photodetector devices thanks to its outstanding optical and electronic properties [1, 2]. Nonetheless, InSe free surface is highly sensitive to environment and light exposure, affecting the performance of devices and reducing their lifetimes [3]. Specifically, intrinsic defects, such as selenium vacancies (V_Se) and indium interstitial atoms, do have a major impact on 2D InSe optoelectronic properties either doping the material or creating in-gap trapping states [4]. Therefore, understanding the physical processes affecting surfaces and interfaces can envision valuable strategies for stable 2D InSe optoelectronic device design.

In this work, we investigate the operation of 2D InSe nanosheets (20-40 nm) deposited on Pt electrodes. Current-voltage measurements confirmed the formation of a double Schottky junction. Photoconductivity ON/OFF cycles at low light excitation power conducted on pristine samples revealed a trap sensitized response that allows for a high responsivity at the expense of slow rise/decay times. However, after sufficiently high illumination, an effective photopassivation was observed: response times decreased remarkably along with a reduction of dark currents and an enhancement of photocurrent linearity with light power. To gain an understanding of these phenomena, light-assisted Kelvin probe force microscopy (KPFM) maps of the InSe nanosheets have been recorded for different light illumination intensities. We observe a gradual surface photovoltage (SPV) change on pristine samples: initially the SPV is negative, whereas for longer and more powerful exposures SPV becomes positive. This effect is permanent and causes a surface potential increase, around 100 mV. On the other hand, hBN-encapsulated InSe nanosheets show no evolution with illumination power and they are characterized by a fast response, steadier responsivity and negative SPV for all illuminations. Our SPV results are consistent with an increase of n-doping that shifts InSe Fermi energy level, and a transient behavior during irradiation related to band flattening in the air/InSe interface after trap states are deactivated. We concluded that photopassivation is taking place due to V_Se filling with reactive species, mainly oxygen. These processes are inhibited in encapsulated samples, as hBN suppresses oxygen leaking into InSe surface.