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.
A novel transparent conductive support structure for scalable integration of 2D materials is demonstrated, providing an electronically passive 2D-3D interface while also enabling facile interfacial charge transport. This structure, which comprises an evaporated nanocrystalline carbon (nc-C) film beneath nanometer-thin atomic layer deposited AlO x , is thermally stable and allows direct chemical vapor deposition of 2D materials onto the surface. The combination of spatial uniformity, enhanced charge screening, and low interface defect concentrations yields a tenfold enhancement of MoS 2 photoluminescence intensity compared to flakes on conventional Si/SiO 2 , while also retaining the strong optical contrast for monolayer flakes. Tunneling across the ultrathin AlO x enables facile interfacial charge injection, which is utilized for high-resolution scanning electron microscopy and photoemission electron microscopy with no detectable charging. Thus, this combination of scalable fabrication and electronic conductivity across a weakly interacting 2D-3D interface opens up new opportunities for device integration and characterization of 2D materials.
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