  Among two-dimensional (2D) transition metal dichalcogenides (TMDs), platinum
diselenide (PtSe2) stands at a unique place in the sense that it undergoes a
phase transition from type-II Dirac semimetal to indirect-gap semiconductor as
thickness decreases. Defects in 2D TMDs are ubiquitous and play crucial roles
in understanding and tuning electronic, optical, and magnetic properties. Here
we investigate intrinsic point defects in ultrathin 1T-PtSe2 layers grown on
mica through the chemical vapor transport (CVT) method, using scanning
tunneling microscopy and spectroscopy (STM/STS) and first-principles
calculations. We observed five types of distinct defects from STM topography
images and obtained the local density of states of the defects. By combining
the STM results with the first-principles calculations, we identified the types
and characteristics of these defects, which are Pt vacancies at the topmost and
next monolayers, Se vacancies in the topmost monolayer, and Se antisites at Pt
sites within the topmost monolayer. Our study shows that the Se antisite
defects are the most abundant with the lowest formation energy in a Se-rich
growth condition, in contrast to cases of 2D molybdenum disulfide (MoS2)
family. Our findings would provide critical insight into tuning of carrier
mobility, charge carrier relaxation, and electron-hole recombination rates by
defect engineering or varying growth condition in few-layer 1T-PtSe2 and other
related 2D materials.
