Raman Shifts in Electron-Irradiated Monolayer MoS₂

Abstract

We report how the presence of electron-beam-induced sulfur vacancies affects first-order Raman modes and correlate the effects with the evolution of the in situ transmission-electron microscopy two-terminal conductivity of monolayer MoS2 under electron irradiation. We observe a red-shift in the E′ Raman peak and a less pronounced blue-shift in the A′1 peak with increasing electron dose. Using energy-dispersive X-ray spectroscopy and selected-area electron diffraction, we show that irradiation causes partial removal of sulfur and correlate the dependence of the Raman peak shifts with S vacancy density (a few %). This allows us to quantitatively correlate the frequency shifts with vacancy concentration, as rationalized by first-principles density functional theory calculations. In situ device current measurements show an exponential decrease in channel current upon irradiation. Our analysis demonstrates that the observed frequency shifts are intrinsic properties of the defective systems and that Raman spectroscopy can be used as a quantitative diagnostic tool to characterize MoS2-based transport channels.

E’’ Raman Mode in Thermal Strain-Fractured CVD-MoS2

“…Theoretically, with the increasing of sulfur vacancies concentration (Vs) in 1H-MoS 2 , the wavenumber difference between E' and A' 1 modes increase obviously. When Vs increases from 0% to 50%, the wavenumber of E 1 2g mode is decreased by 100.6 cm −1 while A 1g mode is increased by 42.3 cm −1 [38]. Experimentally, a similar evolution is observed in single layered MoS 2 either bombarded by manganese ion (Mn+) [33] or treated by oxygen plasma [31].…”

Uncovering edge states and electrical inhomogeneity in MoS 2 field-effect transistors

“…The MIM maps thus provide both quantitative measurements and direct visualization of the mesoscopic potential landscape in the sample (32), which is the combined effect from defects within the MoS 2 layer, charges inside the substrate and the capping layer, and impurities across the interface. The percolation network is vividly demonstrated in the subthreshold regime, which was only indirectly inferred from atomic scale or macroscale studies (18,19). Note that a similar technique, the alternating current STM (40), can be applied to study the atomicscale defects in MoS 2 , which would provide complementary information to our MIM work.…”

“…TMD films in actual devices, however, are far from electronically uniform. Due to the relatively large amount of intrinsic defects and the inevitable charged states in the substrates, mesoscopic electrical inhomogeneity is not uncommon in TMDs, leading to hopping transport and percolation transition in the devices (6,(16)(17)(18)(19). Little is known, however, about the magnitude and characteristic length scale of such conductance fluctuations.…”

Spectroscopic investigation of defects in two-dimensional materials

“…After introducing vacancy defects into MoS 2 by electron-beam irradiation, the redshift of E′ peak and blueshift of the A′ 1 peak, accompanied by the broadening of E′ and A′ 1 peaks, are observed, as shown in Figure 7A [47,108]. With the increase of vacancy concentration, there are fewer Mo-S bonds involved in the in-plane vibrations, and thus the restoring force constant of the E′ peak is continuously weakened, resulting in the redshift of E′ peak.…”