  First-principles calculations are performed to investigate the effects of the
adsorption of gas molecules (CO, NO, NO2, H2S, N2, H2O, O2, NH3 and H2) on the
electronic properties of atomically thin indium selenium (InSe). Our study
shows that the lone-pair states of Se are located at the top of the valence
band of InSe and close to the Fermi energy level, implying its high sensitivity
to external adsorbates. Among these gas molecules, H2 and H2S are strong
donors, NO, NO2, H2O and NH3 are effective acceptors, while CO and N2 exhibit
negligible charge transfer. The O2 molecule has very limited oxidizing ability
and a relatively weak interaction with InSe which is comparable to the N2
adsorption. A clear band gap narrowing is found for the H2S, NO2, and NH3
adsorbed systems whereas a Fermi level shifting to the conduction band is
observed upon a moderate uptake of H2 molecules. Our analysis suggests several
interesting applications of InSe: 1) Due to the different interaction behaviors
with these external molecules, InSe can be used for gas sensing applications;
2) By monitoring the adsorption/desorption behavior of these gas molecules, the
population of hole states in InSe due to photon stimulation or defect
production can be quantitatively estimated; and 3) It is promising for novel
electronic and optoelectronic applications since the adsorption-induced in-gap
states and strong charge transfer are able to change the content and polarity
of charged carriers and lead to different optical properties.
