  Natural oxidation is a common degradation mechanism of both mechanical and
electronic properties for most of the new two-dimensional materials. From
another perspective, controlled oxidation is an option to tune material
properties, expanding possibilities for real-world applications. Understanding
the electronic structure modifications induced by oxidation is highly desirable
for new materials like monolayer GeSe, which is a new candidate for
near-infrared photodetectors. By means of first-principles calculations, we
study the influence of oxygen defects on the structure and electronic
properties of the single layer GeSe. Our calculations show that the oxidation
is an exothermic process, and it is nucleated in the germanium sites. The
oxidation can cause severe local deformations on the monolayer GeSe structure
and introduces a deep state in the bandgap or a shallow state near the
conduction band edge. Furthermore, the oxidation increases the bandgap by up to
23 %, and may induce direct to indirect bandgap transitions. These results
suggest that the natural or intentionally induced monolayer GeSe oxidation can
be a source of new optoelectronic properties, adding another important building
block to the two-dimensional layered materials.
