Line-defect mediated formation of hole and Mo clusters in monolayer molybdenum disulfide

Abstract

The production of hole and Mo cluster by electron beam irradiation in molybdenum disulfide (MoS 2 ), which consists of S-Mo-S layers, is monitored over time using atomic resolution transmission electron microscopy. S vacancies are firstly formed due to knocking off of S atoms and then line defects are induced due to accumulation of S vacancies in MoS 2 sheet instead of forming a hole. The line defects tend to be merged at a point and a hole is formed subsequently at the point. Mo atoms tend to be clustered discretely as a nano sheet along the edge of the hole due to difference in displacement threshold energy between Mo and S atoms under electron irradiation. After Mo clusters are nearly separated from MoS 2 sheet, the clusters are transformed into body-centered cubic nanocrystal of Mo during prolonged electron beam irradiation. The line defect mediated formation of hole and Mo cluster only occurs within a single grain of monolayer MoS 2 sheet.

Evolution of Metastable Defects and Its Effect on the Electronic Properties of MoS2 Films

“…Once formed, these vacancies have been observed to diffuse throughout the lattice and coalesce, forming extended line defects 26 , 28 and mobile defect complexes, that could eventually get absorbed into crystal grain boundaries or layer edges 27 . Indeed, we observe that the domains of non-equilibrium work function always terminate at either a grain boundary or layer edge.…”

“…Recent experiments carried out using transmission electron microscopy show that sulfur vacancies exhibit non-trivial dynamics, in which a single point defect can migrate through the crystal lattice. Collective mobility of many point defects leads to formation of extended line defects 26 and other mobile complex defect structures 27 , which have tendency to migrate through the crystal and annihilate at grain boundaries or crystal edges 28 . Prolonged exposure to electron beam irradiation in ultra-high vacuum conditions has been also shown to cause complete dissociation of MoS 2 , with sulfur atoms evaporating from the material, decoupling the molybdenum sheet and allowing it to recrystallize into its native bcc crystal structure 28 .…”

“…Collective mobility of many point defects leads to formation of extended line defects 26 and other mobile complex defect structures 27 , which have tendency to migrate through the crystal and annihilate at grain boundaries or crystal edges 28 . Prolonged exposure to electron beam irradiation in ultra-high vacuum conditions has been also shown to cause complete dissociation of MoS 2 , with sulfur atoms evaporating from the material, decoupling the molybdenum sheet and allowing it to recrystallize into its native bcc crystal structure 28 . Understanding of these processes in synthetically grown MoS 2 is important for design of thin film MoS 2 electronic devices, particularly for device stability and the choice of material for contacts and gates 16 , where homogeneity and predictable behavior of the work function and doping is crucial.…”

Analyzing the microstructure and related properties of 2D materials by transmission electron microscopy

“…The S vacancies induced by electron beam irradiation during atomic resolution TEM accumulated and resulted in line defects. These line defects clustered and formatted the hold in the MoS 2 (Ryu et al 2016). In MoSe 2 , Se vacancies induced by radiation agglomerated into line defects and sequential atomic movement, filling the vacancies and resulting in triangular inversion symmetry.…”

<italic>In-situ</italic> study of electron irradiation on two-dimensional layered materials

“…此外, 如前文所述, MoS 2 中S原子比Mo原子更容 易发生离位, 因此Mo原子容易在纳米孔边缘富集及 重构 [5] . Koh等人 [47] 认为重构形成的边缘结构同辐照 形成的纳米线结构相同; Ryu等人 [48] 则认为重构后的 边缘为Mo. 显然, 不同课题组对边缘结构的认识仍 存在着较大的差异, 有待进一步深化.…”