Photochemical Reaction in Monolayer MoS₂ via Correlated Photoluminescence, Raman Spectroscopy, and Atomic Force Microscopy

Abstract

Photoluminescence (PL) from monolayer MoS2 has been modulated using plasma treatment or thermal annealing. However, a systematic way of understanding the underlying PL modulation mechanism has not yet been achieved. By introducing PL and Raman spectroscopy, we analyze that the PL modulation by laser irradiation is associated with structural damage and associated oxygen adsorption on the sample in ambient conditions. Three distinct behaviors were observed according to the laser irradiation time: (i) slow photo-oxidation at the initial stage, where the physisorption of ambient gases gradually increases the PL intensity; (ii) fast photo-oxidation at a later stage, where chemisorption increases the PL intensity abruptly; and (iii) photoquenching, with complete reduction of PL intensity. The correlated confocal Raman spectroscopy confirms that no structural deformation is involved in slow photo-oxidation stage; however, the structural disorder is invoked during the fast photo-oxidation stage, and severe structural degradation is generated during the photoquenching stage. The effect of oxidation is further verified by repeating experiments in vacuum, where the PL intensity is simply degraded with laser irradiation in a vacuum due to a simple structural degradation without involving oxygen functional groups. The charge scattering by oxidation is further explained by the emergence/disappearance of neutral excitons and multiexcitons during each stage.

Manipulation of local optical properties and structures in molybdenum-disulfide monolayers using electric field-assisted near-field techniques

“…This result, no etching using a positive bias voltage under near-field illumination, suggests that the observed etching mechanisms were neither thermal heating due to near-field illumination nor mechanical scratches due to cantilever contact. We assume that unique photochemical reactions, such as photooxidation25, could be one of the possible origins for the observed etching of MoS 2 crystals by near-field illumination in combination with the applied negative bias voltages.…”

“…However, its very low light intensity, typically in the order of 10 nW μm −2 , presents a major technical challenge, and accordingly hardly any one has expected near-field light can be applied to etching of TMDCs. Laser thinning and patterning of MoS 2 have been reported252829. However, a laser power of several 10 μW μm −2 is required for them, which is by an order of 3 higher than the maximum available intensity of near-field light, several 10 nW μm −2 .…”

Rapid visualization of grain boundaries in monolayer MoS2 by multiphoton microscopy

“…25). Further, there have been several recent reports about the effects of chemical doping on the optical and electronic properties of MoS 2 (refs 26, 27, 28, 29). These studies show that both chemical doping by various solvents and laser treatment can significantly modify the PL of monolayer MoS 2 .…”

Facile Doping in Two-Dimensional Transition-Metal Dichalcogenides by UV Light

“…3 On the other hand, currently the detailed structures of defects [9][10][11][12][13][14][15][16] and oxygen passivation effects on the defects 17,18 in 2D TMdCs remain under hot debate. Several previous works [19][20][21][22][23] have shown that the enhancement of photoluminescence (PL) intensity achieved by thermal annealing or weak plasma irradiation is closely related to strong oxygen bonding at the defect sites of 1L-MoS 2 19,22,23 and the quenching of PL intensity by strong plasma irradiation is associated with the lattice distortion or creation of MoO 3 defects. [19][20][21] The gradual oxidation of TMdCs are prevalent for these atomic-thick layer when exposed to air, even under ambient conditions.…”

“…Several previous works [19][20][21][22][23] have shown that the enhancement of photoluminescence (PL) intensity achieved by thermal annealing or weak plasma irradiation is closely related to strong oxygen bonding at the defect sites of 1L-MoS 2 19,22,23 and the quenching of PL intensity by strong plasma irradiation is associated with the lattice distortion or creation of MoO 3 defects. [19][20][21] The gradual oxidation of TMdCs are prevalent for these atomic-thick layer when exposed to air, even under ambient conditions. [24][25][26][27] The environmental effects play key roles on the optical and electronic properties of monolayer TMdCs.…”