Surface Characterization of MoS2 Atomic Layers Mechanically Exfoliated on a Si Substrate

Abstract

Mo disulfide overlayers with the thickness exceeding 1.77 nm were obtained on Si substrates through mechanical exfoliation. The resulting Mo disulfide flakes were then analyzed ex situ using combination of Auger electron spectroscopy (AES), elastic-peak electron spectroscopy (EPES) and scanning electron microscopy (SEM) in order to characterize their surface chemical composition, electron transport phenomena and surface morphology. Prior to EPES measurements, the Mo disulfide surface was sputter-cleaned and amorphized by 3 kV argon ions, and the resulting S/Mo atomic ratio varied in the range 1.80–1.88, as found from AES measurements. The SEM images revealed single crystalline small-area (up to 15 μm in lateral size) Mo disulfide flakes having polygonal or near-triangular shapes. Such irregular-edged flakes exhibited high crystal quality and thickness uniformity. The inelastic mean free path (IMFP), characterizing electron transport, was evaluated from the relative EPES using Au reference material for electron energies E = 0.5–2 keV. Experimental IMFPs, λ, determined for the AES-measured surface compositions were approximated by the simple function λ = kEp, where k = 0.0289 and p = 0.946 were fitted parameters. Additionally, these IMFPs were compared with IMFPs resulting from the two methods: (i) present calculations based on the formalism of the Oswald et al. model; (ii) the predictive equation of Tanuma et al. (TPP-2M) for the measured Mo0.293S0.551C0.156 surface composition (S/Mo = 1.88), and also for stoichiometric MoS2 composition. The fitted function was found to be reasonably consistent with the measured, calculated and predicted IMFPs. We concluded that the measured IMFP value at 0.5 keV was only slightly affected by residual carbon contamination at the Mo disulfide surface.

Direct Identification of Surface Bound MoO3 on Single MoS2 Flakes Heated in Dry and Humid Air

“…A high‐resolution scanning Auger microprobe Microlab 350 (Thermo Electron) was employed in order to monitor the chemical composition of the MoS 2 flakes with a lateral resolution of about 50 nm and a depth information from only a few MoS 2 monolayers. [ 56 ] Differential AES spectra were recorded at 10 keV and pph signals for detected elements Si, O, C, Mo, and S were used for a qualitative and quantitative analysis of the materials (Avantage software, version 4.88). Sensitivity factors for calculation of the chemical composition were taken from the Handbook of Auger electron spectroscopy.…”

Local Interactions of Atmospheric Oxygen with MoS2 Crystals

“…Early experimental studies of polycrystalline MoS 2 powders have shown that Mo oxide layers present on the surface form slowly and act as passivating layers till at least 100 • C [31,35]. However, freshly exfoliated single MoS 2 flakes did not show any direct manifestations of the protective oxide layers [44]. They displayed electronic density shifts within the MoS 2 above 200 • C [35].…”