Laser-Induced Particle Adsorption on Atomically Thin MoS₂

Abstract

Atomically thin molybdenum disulfide (MoS2) shows great potential for use in nanodevices because of its remarkable electronic, optoelectronic, and mechanical properties. These material properties are often dependent on the thickness or the number of layers, and hence Raman spectroscopy is widely used to characterize the thickness of atomically thin MoS2 due to the sensitivity of the vibrational spectrum to thickness. However, the lasers used in Raman spectroscopy can increase the local surface temperature and eventually damage the upper layers of the MoS2, thereby changing the aforementioned material properties. In this work, the effects of lasers on the topography and material properties of atomically thin MoS2 were systematically investigated using Raman spectroscopy and atomic force microscopy. In detail, friction force microscopy was used to study the friction characteristics of atomically thin MoS2 as a function of laser powers from 0.5 to 20 mW and number of layers from 1 to 3. It was found that particles formed on the top surface of the atomically thin MoS2 due to laser-induced thermal effects. The degree of particle formation increased as the laser power increased, prior to the thinning of the atomically thin MoS2. In addition, the degree of particle formation increased as the number of MoS2 layers increased, which suggests that the thermal behavior of the supported MoS2 may differ depending on the number of layers. The particles likely originated from the atmosphere due to laser-induced heating, but could be eliminated via appropriate laser powers and exposure times, which were determined experimentally. The outcomes of this work indicate that thermal management is crucial in the design of reliable nanoscale devices based on atomically thin MoS2.

Layer-by-layer thinning of MoS2 via laser irradiation

“…The systematic investigation of MoS 2 laser thinning presented herein enables a mechanistic understanding of the etching process. The results demonstrate the presence of nanoparticles on the MoS 2 at sufficient laser treatment conditions prior to layer-by-layer thinning; previous work submits that the nanoparticles are due to the oxidation, etching, and redeposition of the MoS 2 [39,40]. It is hypothesized here that the nanoparticles act as nucleation sites for the subsequent layer-by-layer thinning.…”

“…The Raman spectra were collected through a 100× objective (NA≈0.9) with a laser spot size of about 720 nm, and the spectra resolution was set to be about 1.4 cm −1 (1800 lines mm −1 grating). The laser power was kept below 0.5 mW with an acquisition time of 10 s to avoid laser-induced thermal effects and nanoparticle formation on the specimens [39].…”

Recent Progress on Irradiation-Induced Defect Engineering of Two-Dimensional 2H-MoS2 Few Layers

“…Tran Khac et al [145] irradiated MoS 2 layers under laser power of 1 mW, 5 mW, and 10 mW for an exposure time of 60 s. No significant changes were observed in topographic images of the irradiated regions under the 1 mW laser. However, significant amounts of particles or adsorbates were formed on the MoS 2 surface after irradiated under 5 mW and 10 mW lasers.…”

“…To minimize thermal effects on atomically thin MoS 2 during Raman spectral measurements, low laser powers (0.14 mW to 2 mW) are usually employed [145] to avoid potential laser-induced local surface temperatures.…”

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 .…”