Characterization of Engineering Defects in 2D-MoS₂ Layers by Oxygen Plasma Treatment Using Nanoscale TERS and KPFM Imaging

Transition metal dichalcogenides (TMDCs), such as molybdenum disulfide (MoS₂), exhibit interesting and technologically relevant optoelectronic characteristics owing to the strong dependence of the bandgap on dimensionality. Previous scientific investigations have tended to focus on electronic transport properties and device demonstrations that tend to employ electron-beam lithography for patterning [1, 2]. However, modulating the physical properties of two-dimensional 2D-MoS₂ induced by oxygen plasma and defect engineering at the nanoscale is actively pursued [1]. In this work, exfoliated atomically thin layers of two-dimensional 2D-MoS₂ treated with oxygen plasma for 0, 10, 20, 40, and 60 s are investigated using Kelvin Probe Force Microscopy (KPFM) and Tip-Enhanced Optical Spectroscopy (TEOS) imaging in addition to micro-Raman and photoluminescence spectroscopy. Under oxygen plasma, defects (mono- and di-sulfur vacancies) and chemical oxidation are predominant from 0s (native defects) up to 40s, while etching takes over beyond 40 s, for mono- (1L), bi- (2L), and tri- (3L) layer MoS₂ with optimal defect density for four- (4L) or more layers. While Raman spectra exhibited lattice distortion (broadening of phonon bands) and surface oxidation by the presence of sub-stoichiometric molytrioxide MoO₃ (i.e., MoO_3-x or MoS_xO_2-x), the quenching of PL and increased spectral weight of trions are observed with treatment time. The localized nanodomains (~20 nm) and aggregated vacancies nanovoids, intermixed MoS₂/MoO_3-x alloy, and trions are identified in near-field Raman spectra. The Kelvin probe force microscopy revealed the work function (WF) increase from 4.98 eV to 5.56 eV, corroborating the existence of MoO_3-x phase which enables doping and Fermi level shift. We also emphasize the unique interaction between the gold and MoO_3-x facilitating Mo⁶⁺ cation reduction to lower oxidation state (i.e., Mo⁴⁺) yielding intermediate oxidation states responsible for lowering WF (ca. 6.3 eV for stoichiometric MoO₃). Strong correlations among the work function, vibrational, and optical responses help in establishing a phase diagram and changing the landscape of nanoscale defects. The defects that confine charged defects constitute well-defined atomic-scale quantum systems explored as single photon emitters and spin qubits and heterogeneous electrocatalysis, applicable to other 2D material systems. Supported by NSF-MRI, USA. ¹ S. Gupta, A. Johnston, S. Khondaker, J. Appl. Phys. 131, 164303 (2022); ² S. Gupta, A. Johnston, S. Khondaker, J. Electron. Mater. 52, 1331-1346 (2023).