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12 pages, 910 KiB

Open AccessArticle

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

by Mirosław Krawczyk, Marcin Pisarek, Robert Szoszkiewicz and Aleksander Jablonski

Materials 2020, 13(16), 3595; https://doi.org/10.3390/ma13163595 - 14 Aug 2020

Cited by 6 | Viewed by 2383

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) [...] Read more.

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 = kE^p, 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 Mo_0.293S_0.551C_0.156 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. Full article

(This article belongs to the Special Issue Nano- and Micro-Scale Surface Reactivity in Selected 2D Materials and Its Influence on Electronic Properties of Those Materials)

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14 pages, 2252 KiB

Open AccessArticle

Surface-Bound and Volatile Mo Oxides Produced During Oxidation of Single MoS₂ Crystals in Air and High Relative Humidity

by Robert Szoszkiewicz, Maciej Rogala and Paweł Dąbrowski

Materials 2020, 13(14), 3067; https://doi.org/10.3390/ma13143067 - 9 Jul 2020

Cited by 19 | Viewed by 2586

Abstract

We report on the MoO₃ oxides and their derivatives on microscopic 2H MoS₂ flakes oxidized in air and high relative humidity at a moderate temperature range below 410 °C. We combine XPS and AFM measurements such as topography, friction, creation of [...] Read more.

We report on the MoO₃ oxides and their derivatives on microscopic 2H MoS₂ flakes oxidized in air and high relative humidity at a moderate temperature range below 410 °C. We combine XPS and AFM measurements such as topography, friction, creation of nanoscale ripples and scratches on the MoS₂ flakes deposited on Si substrates. We detect MoO₃ oxides mostly by measuring selected nanomechanical properties of the MoO₃ layer, such as its compressive mechanical stress at the plastic yield. We discuss basal surface coverage of the single MoS₂ flakes by the MoO₃ oxides. We discuss conditions for appearance of all possible MoO₃ oxide derivatives, such as molybdenum(VI) hydroxyoxides and MoO₃ hydrates. Our findings agree with an expected mechanistic switch in thermal oxidation in water vapors vs. air. Full article

(This article belongs to the Special Issue Nano- and Micro-Scale Surface Reactivity in Selected 2D Materials and Its Influence on Electronic Properties of Those Materials)

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Review

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31 pages, 7092 KiB

Open AccessReview

Local Interactions of Atmospheric Oxygen with MoS₂ Crystals

by Robert Szoszkiewicz

Materials 2021, 14(20), 5979; https://doi.org/10.3390/ma14205979 - 11 Oct 2021

Cited by 14 | Viewed by 2760

Abstract

Thin and single MoS₂ flakes are envisioned to contribute to the flexible nanoelectronics, particularly in sensing, optoelectronics and energy harvesting. Thus, it is important to study their stability and local surface reactivity. Their most straightforward surface reactions in this context pertain to [...] Read more.

Thin and single MoS₂ flakes are envisioned to contribute to the flexible nanoelectronics, particularly in sensing, optoelectronics and energy harvesting. Thus, it is important to study their stability and local surface reactivity. Their most straightforward surface reactions in this context pertain to thermally induced interactions with atmospheric oxygen. This review focuses on local and thermally induced interactions of MoS₂ crystals and single MoS₂ flakes. First, experimentally observed data for oxygen-mediated thermally induced morphological and chemical changes of the MoS₂ crystals and single MoS₂ flakes are presented. Second, state-of-the-art mechanistic insight from computer simulations and arising open questions are discussed. Finally, the properties and fate of the Mo oxides arising from thermal oxidation are reviewed, and future directions into the research of the local MoS₂/MoO_x interface are provided. Full article

(This article belongs to the Special Issue Nano- and Micro-Scale Surface Reactivity in Selected 2D Materials and Its Influence on Electronic Properties of Those Materials)

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