MoS2 oxidative etching caught in the act: formation of single (MoO3)n molecules

We report the presence of sub-nm MoOx clusters formed on basal planes of the 2H MoS2 crystals during thermal oxidative etching in air at a temperature of 370 °C. Using high resolution non-contact atomic force microscopy (AFM) we provide a histogram of their preferred heights. The AFM results combined with density functional theory (DFT) simulations show remarkably well that the MoOx clusters are predominantly single MoO3 molecules and their dimers at the sulfur vacancies. Additional Raman spectroscopy, and energy and wavelength dispersive X-ray spectroscopies as well as Kelvin probe AFM investigations confirmed the presence of the MoO3/MoOx species covering the MoS2 surface only sparsely. The X-ray absorption near edge spectroscopy data confirm the MoO3 stoichiometry. Taken together, our results show that oxidative etching and removal of Mo atoms at the atomic level follow predominantly via formation of single MoO3 molecules. Such findings confirm the previously only proposed oxidative etching stoichiometry.

. High resolution 14 x 14 µm 2 NC-AFM topography of the same region of the Fig. 1(a) in the main paper after rastering the whole area in a contact-mode AFM. Image Z-scale is 5.6 nm. Brighter colors represent higher Z-scale.    S4. (a) Height histogram of more than 1100 movable oxide clusters which were randomly chosen from sample 1 in Fig. 1 (a). The bin size is 0.05 nm. Single Gaussian fit: 0.391 ± 0.019 nm / double Gaussian fit: 0.236 ± 0.007 nm, 0.475 ± 0.021 nm.
(b) Height histogram of more than 350 movable oxide clusters which were randomly chosen from sample 2 in Fig. S2. The bin size is 0.05 nm. Single Gaussian fit: 0.362 ± 0.026 nm / double Gaussian fit: 0.236 ± 0.007 nm, 0.438 ± 0.031 nm. The second peak position is different in parts (a) and (b). However, they are the same when considering errors. To understand this one must appreciate that an error for each bin scales with the square root of the counts. Therefore, summing up the overall data from these two histograms is expected to decrease an overall error, see the main manuscript.

Gaussian fitting of the histograms
Two Gaussian fits (Eqs. S2 & S3) were used to find the peaks of cluster height histograms in Fig. 2 (main paper). Their optimized parameters obtained by Igor Pro 6.37 program (WaveMetrics, Inc., Lake Oswego, Oregon, USA) are shown in Table S1.

Single Gaussian fit:
Eq. S2   As seen in Table S2,

Discussion of the Raman data in Figs. S6-S9:
An area including a rastered MoS 2 crystalline surface and an unrastered MoS 2 zone with tentative Mo oxides was selected for a detailed Raman analysis. This area is depicted by a green box in Fig. S6(a) and its Raman map is in Fig. S6(b).
By a quick look on Fig. S6(b), one can only see slightly higher intensity on the unrastered vs. rastered zones, which comes from background difference arising after Si normalization. Individual Raman spectra were too noisy to observe any differences between rastered and unrastered zones. This is expected since an amount of tentative Mo oxides even at the border region is very small in comparison to strong presence of thick MoS 2 flake and Si in the Raman data. Therefore, we averaged several Raman spectra from the rastered and border regions in the map. This procedure helped us to increase the signal-to-noise (S/N) ratio and enhance visibility of any possible weak Raman peaks arising from the oxides. Figs. S7-9 show the average Raman spectra taken for 2x2 pixels in several places on the sample starting from rastered to unrastered regions. As can be noticed, there are some peaks -marked by yellow windows -from the border or near border regions, which are absent in the Raman spectra taken on the rastered area far away from the border. Furthermore, intensity of such new peaks is the highest at the border. Table S3 lists such peaks and their attributions. Raman shifts, which were absent in the data from 2x2 boxes, but can also be attributed to MoO 3 and MoO 3-x . They are marked in yellow in the Table S3 and were not included in the main paper. Generally all the peaks from 4x4 boxes appear smaller in comparison to the data from the 2x2 boxes. This is because increasing number of averaged pixels in each box indeed enhances the S/N ratio, but at the same time increases a collection area. The latter might diminish a contribution from single clusters.
Overall, quite many Raman peaks are listed in the Table S3 and discussed above, since they can be attributed to MoO 3 and MoO 3-x based on the literature data.   We have also attempted a semi-quantitative SEM-EDS analysis of the oxygen, sulfur and molybdenum content and indeed observed an elevated oxygen content on the large agglomerated clusters accumulated near the border between rastered and unrastered regions, but quantifications and deconvolutions of the oxygen content with respect to Mo and S have showed to be unreliable. Some of the reasons could be that the bands of Mo and S coincide at ca. 2.3 keV and cannot be deconvoluted at low beam energies, which in turn are necessary to detect properly light elements such as oxygen. Nevertheless, we considered this issue further and used another SEM setup with WDS (wavelength dispersive X-ray spectroscopy) detectors. Therein, due to a beam size diameter of ca. 1 micron, we could not observe the signal from isolated clusters, but observed quite clearly the difference in oxygen content between rastered and unrastered zones, i.e., respectively, the portions of the samples with and without single MoO x clusters. We present these SEM WDS results in the Table S4 below under rows: "New sample, rastered zone" and "New sample, unrastered zone". The new MoS 2 sample was freshly thermally etched at exactly the same conditions as described in the Experimental section of the manuscript.

Additional SEM-EDS and SEM-WDS investigations
Furthermore, we have also investigated the MoS 2 flake from Fig. 1 in the main paper (as well as Fig. S10 above), which is now ca. one year old. A comparisons between the borderline region and rastered zones shows substantially higher oxygen content at the border, which follows the results in Fig. S10(f) and shows additional oxidation, which took place in the sample. See the data in Table S4, rows "Flake from Figs. 1 and S10, border region" and "Flake from Figs. 1 and S10, rastered region".
Finally, the silicon substrate with native silica film near the aforementioned flake was also measured to yield an amount of the oxygen present therein. This data is presented in a row "SiOx/Si substrate in the vicinity of the flake from Figs. 1 and S10" in Table S4. Due to the SEM WDS signal collected from substantial penetration depths and substantial Si percentage present in all the results in the To conclude, the SEM EDS and SEM WDS analyses did not provide unambiguous information about the stoichiometry of the MoO x clusters discussed in the main paper, but showed the following results. First, the presence of isolated and sparsely present sub-nm MoOx clusters produced detectable increase in the O/Mo ratio (Table S4, rows 1-2). Second, agglomerated MoO x clusters present at the border between rastered and unrastered zones displayed oxygen content pointing towards MoO 3 , but other stoichiometries could not be excluded ( Fig. S10 and Table S4, rows 3-5).