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

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.

Determination of surface composition by X-ray photoelectron spectroscopy taking into account elastic photoelectron collisions.

It is now well known that elastic photoelectron scattering in the surface region of solids cannot be ignored in the mathematical formalism of quantitative XPS. Elastic collisions may increase or decrease the photoelectron signal intensity, depending on the experimental configuration. Consequently, it is advisable to take into account these effects in calculations of the surface composition. In certain experimental geometries, the photoelectron intensity is practically unaffected by elastic scattering events (configurations defined by the so-called "Master Angle"), and in principle such geometries should be recommended for measurements. Unfortunately, they usually cannot be implemented in typical constructions of spectrometers. In the present paper, different procedures for estimating corrections for elastic scattering events are overviewed. The influence of these correction procedures has been illustrated on examples of AuAgCu and AuAgPdCu alloys. It turned out that elastic photoelectron collisions substantially decrease the signal intensities selected for analysis. However, they are diminished by roughly the same factor. As a consequence, the calculated surface composition is only slightly modified by the correction procedure. This effect may not be of general validity for all solids, and the algorithms for calculating the surface composition should have an option for including any elastic scattering effects. Further efforts are needed to improve the predictive formulas providing corrections for elastic scattering effects.

Inelastic mean free path data for Si corrected for surface excitation.

Surface-sensitive electron spectroscopies, like Auger electron spectroscopy, X-ray photoelectron spectroscopy and elastic peak electron spectroscopy (EPES) are suitable techniques to investigate surfaces and thin layers. A theoretical model for electron transport is needed to process the observed electron spectra. Electron transport descriptions are based on the differential elastic cross sections for the sample atoms and the inelastic mean free path (IMFP) of backscattered electrons. An electron impinging on the sample can lose energy either due to surface or volume excitations. In the present work a Monte Carlo (MC) simulation of the elastic peak of Si, Ag, Ni, Cu, and Au for surface analysis is presented. The IMFP of Si was determined applying the EPES method. The integrated elastic peak ratio of Si with the standard metal reference samples corrected for surface excitation provided IMFP values of Si in the energy range E = 0.2-2.0 keV. Experiments were made with the ESA 31 HSA (ATOMKI) and with the DESA-100 (Staib) spectrometers. Surface correction was based on the application of Chen's model and material parameters. The Monte Carlo simulations of elastically backscattered electron trajectories were made using new EPESWIN software of Jablonski. An improvement of IMFP experimental results was achieved applying the presented procedure.