  In this study, we systematically study on crystal and electronic structures
and optical absorption properties of perfect monolayer MoS2 (M), M with S
vacancy (M@SV), M with N doping at S site (M@ND) and M with both S vacancy and
N doping at S site (M@V-D) using first-principles method. It is showed that the
N atom is tend to located between Mo and S layers, leaving one vacancy at
original site, to form interstitial N atom. Thus, the interstitial N atom and
the S vacancy make up the NI-VS dimer. We study M@V-D with five atomic
configurations and find the most stable structure having the NI-VS-VS trimer.
It is showed that the absorbance for the stable M@V-D in the most infrared
region is obviously higher than that for the other systems. It is revealed that
large enhancement of infrared absorption for the stable M@V-D is mainly
attributable to the special electronic structure determined by the crystal
structure with the trimer. It is considered that M@V-D could be the promising
candidate for infrared materials.
