Sanju Gupta1
Penn State University1
Modulating physical and chemical properties of two-dimensional (2D) transition metal dichalcogenides (TMDC) by defect-engineering induced by oxygen plasma is actively pursued. In this work, exfoliated 2D MoS<sub>2</sub> layers treated by medium power oxygen plasma for different times (0, 10, 20, 40, and 60 s) are investigated using Kelvin Probe Force microscopy and tip-enhanced Raman spectroscopy (TERS) besides micro-Raman and photoluminescence (PL) spectroscopy. Under oxygen plasma, defects (mono- and di-sulfur vacancies) and chemical oxidation is predominant from 0s (native defects) up to 40s, while etching becomes dominant beyond 40 s, for mono- (1L), bi- (2L), and tri- (3L) layer MoS<sub>2</sub> with optimal defect density for four- (4L) and more layers. While Raman spectra exhibited lattice distortion (broadening of phonon bands) and surface oxidation by the presence of sub-stoichiometric molytrioxide MoO<sub>3</sub> (<i>i.e.,</i> MoO<sub>3-x</sub> or MoS<sub>x</sub>O<sub>2-x</sub>) the increased spectral weight of trions and quenching in PL spectra are observed with treatment time. The localized nanodomains (~20-40 nm) and aggregated vacancies as nanovoids and intermixed MoS<sub>2</sub>/MoO<sub>3-x</sub> alloy are identified in near-field Raman spectra. The atomic force microscopy also showed defects aggregation and Kelvin probe force microscopy revealed the work function (WF) increase from 4.98 eV to 5.56 eV, corroborating the existence of MoO<sub>3-x</sub> phase which enables doping and shift Fermi level. We also highlight the unique interaction between the gold substrate and the formed MoO<sub>3-x</sub> facilitating Mo<sup>6+</sup> cation reduction to lower oxidation (<i>i.e.</i>, Mo<sup>4+</sup>) thereby yielding intermediate oxidation states responsible for lower WF (<i>ca.</i> theoretical 6.3 eV for stoichiometric MoO<sub>3</sub>). Strong correlations among the work function, vibrational and optical responses are established while exploring the oxygen plasma-induced defects and changing the landscape on oxygen doping at the nanoscale with varying MoS<sub>2</sub> layers, which are useful for heterogeneous electrocatalysis and applicable to other 2D TMDCs.