Real-Time Investigation of Sulfur Vacancy Generation and Passivation in Monolayer Molybdenum Disulfide via in situ X-ray Photoelectron Spectromicroscopy

Abstract

Understanding the chemical and electronic properties of point defects in two-dimensional materials, as well as their generation and passivation, is essential for the development of functional systems, spanning from next-generation optoelectronic devices to advanced catalysis. Here, we use synchrotron-based X-ray photoelectron spectroscopy (XPS) with submicron spatial resolution to create sulfur vacancies (SVs) in monolayer MoS2 and monitor their chemical and electronic properties in situ during the defect creation process. X-ray irradiation leads to the emergence of a distinct Mo 3d spectral feature associated with undercoordinated Mo atoms. Real-time analysis of the evolution of this feature, along with the decrease of S content, reveals predominant monosulfur vacancy generation at low doses and preferential disulfur vacancy generation at high doses. Formation of these defects leads to a shift of the Fermi level toward the valence band (VB) edge, introduction of electronic states within the VB, and formation of lateral pn junctions. These findings are consistent with theoretical predictions that SVs serve as deep acceptors and are not responsible for the ubiquitous n-type conductivity of MoS2. In addition, we find that these defects are metastable upon short-term exposure to ambient air. By contrast, in situ oxygen exposure during XPS measurements enables passivation of SVs, resulting in partial elimination of undercoordinated Mo sites and reduction of SV-related states near the VB edge. Correlative Raman spectroscopy and photoluminescence measurements confirm our findings of localized SV generation and passivation, thereby demonstrating the connection between chemical, structural, and optoelectronic properties of SVs in MoS2.

2 A cm–2 Level Large-Scale Production of Hydrogen Enabled by Constructing Higher Capacity of Interface “Electron Pocket”

“…To further confirm the existence of S-vacancies, electron paramagnetic resonance (EPR) spectra (Figure S4) were obtained for all samples. It can be seen that the characteristic peak signal of S-vacancies located at g = 2.003 in NM-IHJ-V is very strong, much higher than those of NM-IHJ and B-MoS 2 , which can be attributed to the Mo/Ni–S dangling bond and peak strength being proportional to the concentration of S-vacancies. − However, there is almost no signal in B-Ni 3 S 2 , which also validates the TEM data results (Figure S7).…”

“…More information about the chemical state of the hybrid surface can be obtained by X-ray photoelectron spectroscopy (XPS) (Figure f–h). The Ni 2p region (Figure f) shows two predominant peaks of Ni 2+ (855.7 and 873.4 eV), two shakeup satellites at 862.0 and 880.1 eV, and two broadened peaks indicative of Ni 3+ at 875.5 eV (Ni 2p 1/2 ) and 857.8 eV (Ni 2p 3/2 ). ,− In addition, compared with B-Ni 3 S 2 and NM-IHJ, the characteristic peaks of NM-IHJ-V move toward higher binding energy as a whole, indicating the occurrence of an oxidation reaction. ,− The S 2p region of NM-IHJ-V (Figure g) possesses two strong signals at ∼168.5 and ∼169.8 eV, classified as S–O and S-vacancy (S-v) characteristic peaks, respectively, and the S-v content obtained by integrating the peak area is as high as 44.1%. However, no S-v peak was detected in B-Ni 3 S 2 , which further confirms that the generation of S-v is synchronized with the formation of an interface heterojunction. ,, Besides, the characteristic signals of S 2p 3/2 at ∼162.8 eV that were revealed in NM-IHJ-V (Figure g) are detected, and the S 2p 3/2 content decreases with the decrease of S content (NM-IHJ-V < NM-IHJ < B-Ni 3 S 2 < B-MoS 2 ), which means that S 2p 3/2 sacrifice will be replaced by more S-v formation (Table S1).…”

Tunable Encapsulation and Doping of Monolayer MoS2 by In Situ Probing of Excitonic Properties During Atomic Layer Deposition

“…As previously reported, such flakes are also characterized by an absence of MoO 3 . [25] Atomic force microscopy images before and after encapsulation of the same monolayer MoS 2 flake (Figure 3) show a continuous and smooth ALD coating with a root mean square (rms) roughness, R rms , of 2.2 Å, which agrees well with reported roughness values of AlO x -coated MoS 2 on Si substrates. [24,26] Both monolayer MoS 2 and the AlO x adlayer on the single layer sheet conform to the topography of the underlying Si/SiO 2 substrate (R rms = 2.3 Å).…”

Gate-Tunable Bound Exciton Manifolds in Monolayer MoSe2

“…Previous density functional theory (DFT) calculations showed that a monoselenium vacancy (V Se ) gives rise to two unoccupied degenerate states near the conduction band minimum (CBM) and one occupied state below the valence band maximum (VBM) in monolayer MoSe 2 . − It is worth noting that other defects such as a diselenium vacancy (V 2Se ) and a V Se pair (V Se –V Se ) also result in in-gap states, , but we focus on the effect of V Se due to its predominance (Figure S6). While chalcogen vacancies were previously thought to be responsible for the commonly observed n-type behavior in TMDs, there is increasing evidence that they are in fact deep acceptors (A). − Theory shows that V Se is stable in either neutral or negatively charged states: − that is, it can accept an electron to compensate the native n-doping. This expectation is consistent with the reduction in X – /X 0 intensity ratio after irradiation.…”