Microscale Insight into Oxidation of Single MoS2 Crystals in Air - The

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Microscale Insight Into Oxidation of Single MoS Crystals in Air Wojciech L. Spychalski, Marcin Pisarek, and Robert Szoszkiewicz J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b05405 • Publication Date (Web): 07 Nov 2017 Downloaded from http://pubs.acs.org on November 9, 2017

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The Journal of Physical Chemistry

Microscale Insight into Oxidation of Single MoS2 Crystals in Air Wojciech Leon Spychalski1, Marcin Pisarek2 and Robert Szoszkiewicz*,1,3,4

(1) Faculty of Materials Science and Engineering, Warsaw University of Technology, Wołoska 141, 02-507 Warsaw, Poland. (2) Institute of Physical Chemistry, Polish Academy of Sciences, Kasprzaka 44/52, 01-224 Warsaw, Poland. (3) Faculty of Chemistry, University of Warsaw, Pasteura 1, 02-093 Warsaw, Poland. (4) Biological and Chemical Research Centre, University of Warsaw, Żwirki Wigury 101, 02-089 Warsaw, Poland.

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ABSTRACT Due to profound applications of MoS2 crystals in electronics their microscale oxidation is the subject of substantial interest. We report on oxidation of single MoS2 crystals, which were oxidized within a precision muffle furnace at a series of increasing temperatures up to 500 °C. Using Electron Dispersion X-ray Spectroscopy (EDS) at ambient conditions we observed an increase of oxide content with increasing heating temperature and obtained an apparent activation energy for the oxidation process of the order of 1 kCal/mol. This value is at least eight times smaller than an activation energy for surface formation of MoO3 and according to literature points rather to physisorbed oxygen species. Our Auger Electron Spectroscopy (AES) results also pointed out towards the physisorbed oxygen, similarly as our further heating studies within elevated relative humidity conditions. The Mo oxide leftovers on the sample were investigated using Atomic Force Microscopy (AFM) and showed dendritic structures. Surface appearance of those dendrites, their fractal dimension between 1.61 and 1.66, as well as their surface distribution were reminiscent of the diffusion limited aggregation (DLA) growth. Based on analysis of AFM topographs we hypothesized that the DLA process was controlled by a surface diffusion of the initially physisorbed oxygen, which had to diffuse to reaction centers in order to facilitate the subsequent chemical conversion of MoS2 layers to volatile Mo oxides.


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INTRODUCTION Recent progress in nanotechnology and nanoscale electronics enabled controlled preparation and characterization of several kinds of 2D materials including graphene, 1,2 graphene analogues such as boron nitride, 3,4 transition metal oxides including titania- and perovskite-based oxides 5,6 as well as transition metal dichalcogenides (TMDCs), such as MoS2 crystals. 7,8,9 Chemical stability and prowess for surface oxidation of these materials are the main features impacting long time performance of any devices based on 2D crystals. Thus, substantial interest has been directed into understanding local scale oxidation of 2D crystals. 10,11,12 Single MoS2 flakes have been given particular attention, since they are easy to prepare substrates for subsequent nano- and microscale electrical devices. One of the seminal studies reported that below 340 °C thin exfoliated MoS2 samples exposed to etching flow of O2 in Ar for times as substantial as several hours showed no formation of MoO3 oxides. Instead, triangular pits usually one monolayer deep were found on their surface 10. Those pits have been attributed to oxygen induced physical etching of MoS2 crystals. The pits were suggested to initiate at sulfur defects on MoS2 basal planes and to proceed along zigzag edges of the Mo-edge (1 0 ̅1̅ 0) termination. Heating of MoS2 crystals in oxygen rich atmosphere is expected to oxidize the top MoS2 layer to its most stable oxide according to a following reaction: 2MoS2 + 9O2 2MoO3 + 4SO3. The large energy barrier of 1.6 eV for this reaction confirms good stability of MoS2 against oxidation at room temperature. 13

However, thermodynamical calculations have shown that depending on conditions other Mo oxides

may form as well. 14 Macroscopic and isothermal oxidation of bulk MoS2 crystals has been found to happen at about 600 °C in air. 15 However, non-isothermal studies have found that oxidation of bulk MoS2 starts already at about 350 °C. 15 Similarly, oxidation of thin, e.g., less than 10 monolayers (ML) thick exfoliated MoS2 crystals in O2 flow has been reported also to start at 350 °C.

10

This would result in patches of one



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monolayer thick MoO3 crystals, which were confirmed via AFM, Raman and XPS 16. Wu et al. observed both thermal thinning and oxidation of exfoliated thin MoS2 crystals at temperatures around 330 °C and above, in air. 11 Finally, well-formed microscopic crystals of Mo oxides have been found on bulk MoS2 samples after heating them above 400 °C. 10,17 However, confirmation of small amounts of Mo6+, pointing towards MoO3 presence on the surface already at room temperature has been provided by XPS studies by Sim et al. 18 Same authors showed a growing presence of Mo6+ after heating the MoS2 crystal above 250 °C at their experimental conditions. Furthermore, Ross and Sussman 19 have reported that MoS2 oxidation in the presence of water vapor may occur at temperatures below 100 °C and according to a following reaction: 2MoS2 + 9O2 + 4H2O 2MoO3 + 4 H2SO4. Since H2SO4 is known to be highly hygroscopic, this reaction is expected to self-propel itself in water vapors due local water condensation 20. Indeed, a recently published MoS2 oxidation study in water at room temperature has reported the presence of MoO3·H2O crystals on the MoS2 surface and molybdate ions in aqueous solution. 21 In addition, local surface oxidation studies of MoS2 flakes in humid air by means of oxidation scanning probe lithography reported oxidation induced buildups on MoS2 surface, which were suggested to contain MoO3 oxide. 22 Overall, thermal thinning, formation of Mo oxides and finally oxygen adsorption are the landmarks of MoS2 oxidation and heat-induced MoS2 thinning in oxygen and/or air.

23

Despite some

puzzling evidence, below 330 – 350 °C both oxygen adsorption on the surface and oxygen initiated thermal thinning of the MoS2 surface are considered to prevail. Above 330 – 350 °C formation of Mo oxides starts more abruptly, particularly in humid air, and the most likely product is the MoO3 oxide. One hypothesis for the oxidation mechanism would be that Mo oxide layer forms quickly and then the sample thins via evaporation of the oxide layer. A competing mechanism might include oxygen induced physical thinning, i.e., with formation of triangular pits, and exposure of underlying layers of the MoS2 crystals. Finally, another mechanism might involve oxidation process controlled by a surface diffusion of the


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physisorbed oxygen. Thus, a detailed picture related to local oxygen reactivity on the surface of the MoS2 crystals is quite complicated and its molecular basis are still not well understood. Here, we studied macroscopic oxygen adsorption via global heating of single and thick MoS2 flakes at a series of temperatures ranging from room temperature to 500 °C. Most of the reported studies use thin MoS2 flakes on SiO2 substrates and the oxygen from those SiO2 substrates might have interfered with the reported oxygen content on the MoS2 flakes. Thus, we used hydrogenated Si substrates. The resulting oxide content was monitored using EDS at ambient conditions after each oxidation step. We observed an increased oxide content with increasing heating temperature. Using the EDS data we calculated an activation energy of the order of 1 kCal/mol, which is at least eight times smaller than activation energy of any Mo oxide formation. According to literature this value points out towards physisorbed oxygen species. We obtained same activation energy after applying time-temperature superposition principle to a sample left for oxidation for two months at room temperature. Next, our AES studies confirmed a localized surface-only oxygen buildups related mostly physisorbed oxygen, similarly as our further heating studies within elevated relative humidity conditions and with subsequent annealing of the samples. Through an AFM study we obtained that Mo oxides form dendrites on the MoS2 sample. The morphology and corresponding fractal dimension of those dendrites suggested their growth through the mechanism of diffusion limited aggregation (DLA). In the light of the existing literature Mo oxide growth would initiate on single sulphur vacancies, which constitute a vast majority of existing MoS2 surface defects. Based on our AFM data, we have calculated a related density of defects to obtain a value of 3*1010 cm-2, which is substantially smaller then densities of 1013 cm-2 obtained from a published high resolution TEM study. This served as another indirect proof that majority of the Mo oxides must have evaporated shortly after their formation. Overall, we suggest the Mo oxides form on MoS2 flakes mostly due to initially physisorbed oxygen. Next, such oxygen diffuses to reaction centers in order to facilitate the subsequent chemical conversion of MoS2 to a volatile Mo oxides, most of which then leave the reaction environment.



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MATERIALS AND METHODS Preparation of MoS2 flakes: MoS2 crystals for our experiments have been prepared using a standard mechanical exfoliation technique with a scotch tape. Single MoS2 crystals from SPI Supplies, USA, have been used. Furnace sample heating: MoS2 flakes were heated within a standard muffle precision furnace model MRT3 from SEL, Rybnik, Poland. The furnace was equipped with a precision driving electronics with PID regulators allowing a temperature error of ±2 degrees Celsius. We heated the samples for either 5 minutes or 10 minutes (depending which sample) and at a series of temperatures ranging from the room temperature through 270 °C, 360 °C, 410 °C, and finally to 500 °C. Light microscopy: ZEISS AXIO Scope A1 light microscope with a CCD camera AxioCam ICc 5 was used for optical observations of the samples. AxioCam software was used to process the data. The image contrast in green channel was used to evaluate thickness of the MoS2 flakes. SEM-EDS: SEM imaging and microanalysis were performed utilizing a Hitachi S-3500N (LaB6 Filaments) microscope with EDS detector Thermo NORAN Pioneer NORVAR. We used 5kV acceleration voltage. Atomic Force Microscopy: AFM imaging was conducted in a contact mode with several nN setpoint forces. We used a commercial MFP3D-BIO AFM from Asylum Research/Oxford Instruments with its extended 90 µm - 90 µm - 40 µm scanner (X-Y-Z). We used RC800PSA-no.3 cantilevers with lengths of ca.100 µm and widths of ca. 40 µm. The cantilevers, provided by Olympus, were calibrated in-situ using an equipartition method with standard corrections 24 implemented into Asylum Research Software. Their calibrated elastic spring constants were between 600 pN/nm to 700 pN/nm. The AFM imaging was performed at 25 ± 2 °C. Auger Electron Spectroscopy: A high-resolution scanning Auger microprobe, a Microlab 350 (Thermo Electron), was employed in order to monitor the chemical composition of the MoS2 flakes utilizing the 6 ACS Paragon Plus Environment

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AES technique (Auger electron spectroscopy—high sensitivity surface analysis) with a lateral resolution of about 30 nm and a depth information of a few MoS2 monolayers. Differential AES spectra recorded at 10 keV (Avantage software, version 4.88) and pph signals for detected elements: Si, O, C, Mo, S were used for a qualitative and quantitative analysis of the materials. Sensitivity factors for calculation of the chemical composition were taken from the Handbook of Auger Electron Spectroscopy. 25 An Ar+ ion gun EX05 with ion energy 3 keV, sputtering time of 60 s, and at estimated etching speed of about 0.08 nm/s was applied to thin the surface of analyzed materials.

RESULTS AND DISCUSSION In order to study oxidation of MoS2 crystals single MoS2 flakes were transferred on silicon substrates and visualized using optical microscopy to differentiate between thin and thick MoS2 samples, see Fig. S1 in Supporting Information. We concentrated on thick samples, see Fig. 1, with thickness of more than 20 nm to avoid their disappearance during extensive heating at elevated temperatures. A)

B)

C)

D)

Fig. 1. Examples of investigated here single MoS2 flakes. (A), (B) SEM images, (C) 40 µm x 40 µm AFM topography image with a total height scale of 29.5 nm, (D) 30 µm x 30 µm AFM topography image with a total height scale of 25.0 nm. Images B and D were obtained after sample heating at 270 °C. The MoS2 samples like the ones presented in Fig. 1 have been heated in air within a muffle furnace and at a series of temperatures starting from room temperature and increasing through 270 °C, 360 °C, 410 °C to 500 °C. Most of those temperatures were above 350 °C. Thus, we expected distinct formation of Mo 7 ACS Paragon Plus Environment


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oxides with only minimal (if any) oxygen induced surface etching. Oxidation was monitored at room temperature via EDS measurements of oxygen content within MoS2 flakes after each heating round, see Fig. 2. A)

B)

C)

Fig. 2. Microscopically measured oxidation of single MoS2 flakes via SEM EDS. (A) Two locations on a single MoS2 flake, where EDS spectra were collected. In each spot a spectrum before and after oxidation was collected. (B) an EDS spectrum before oxidation. (C) an EDS spectrum after a final round of heating at 500 °C and collected at the same spot. A clear a clear increase in oxygen content is noticeable. We have calculated that at our experimental conditions EDS spectra contain information from the sample depth of about 200 nm, see Fig. S2 in Supporting Information. Thus, any change of the detected oxygen content in the EDS study is low, particularly if oxygen locates mostly on the sample surface, which we discuss below. Similarly, a local Raman study conducted after a final round of heating (not presented here) barely detected faint markings of the MoO3 presence on the MoS2 surface via peaks between 600 to 800 cm-1. 26 Next, we conducted an AES analysis combined with ion beam sputtering. First, we obtained the AES spectra showing the presence of oxide on the surface before Ar ion sputtering process, see Fig. 3(b). The signals in Fig. 3(b) from Si (LMM), O (KLL), C (KLL), S (LMM), Mo (MNN) are clearly detectable, in accordance with the chemical composition of the fabricated samples deposited on a Si substrate. Then, we did controlled etching of ca. 5 nm of the MoS2 flake. Finally, we have obtained another set of AES spectra showing the lack of oxygen, see Fig. 3(c). Observed signals from Mo and S in


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Fig. 3(c) show local concentration of these elements at 20.5 at. % (Mo) and 49.1 at.% (S), respectively, yielding a S/Mo ratio of 2.40, which is close to the stoichiometry of MoS2. Those results pointed out that MoS2 oxidation was limited only to the sample surface. Furthermore, an analysis of the AES spectra like the one in Fig. 3(b) suggested that the detected amount of oxygen is too low to point towards any presence of the Mo oxides. A)

B)

C)

Fig. 3. Auger spectroscopy assisted with sample sputtering. (A) A single MoS2 flake has been investigated under AES setup. (B) Initially delivered flake just after oxidation at 500 degrees Celsius for 10 minutes in the furnace showed a clear presence of oxygen. (C) After 60 s ion beam (Ar+) sputtering process on the top layer of the flake the oxygen is removed. Carbon content (present due to prior SEM investigations) is also removed. This confirms that thermal oxidation has been limited only to the MoS2 sample surface. Summing it up, the EDS data have showed unambiguously an increased oxygen content in the sample after each round of heating. In addition, the AES studies have showed that oxidation in not a volumetric process, but affected only the very surface of the MoS2 crystals. Finally, while the presence of Mo oxides could not be ruled out, we observed only a faint presence of MoO3 oxides in Raman studies (not presented here) and we were not able to detect any clear presence of MoO3 in AES. Thus, only a small amount of the MoO3 oxide appeared to be present on the MoS2 surface after its oxidation. Next, we have embarked on a more quantitative study of the oxidation process based on the EDS data. We monitored and equilibrium oxide content after each heating cycle. Supposing oxidation is a 9 ACS Paragon Plus Environment


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thermally activated process we calculated its apparent activation energy using a standard Arrhenius equation. In particular, we linked the per cents of oxidation, % Oxid, with an apparent activation energy, Eact, via a following equation: % Oxid = A*exp (-Eact/(kBT))

Eq. 1

where: % Oxid refers to the EDS measured surface oxide content, A is a pre-factor, Eact is an apparent activation energy, i.e., the activation energy at our experimental conditions, kB is the Bolztmann’s constant and T is absolute temperature. We note that using Eq, 1, we catch only a surface bound oxygen, i.e., without accounting on any volatile oxidation products. Figure 4 shows the results of Eq. 1 applied to the data from the two spots in the MoS2 flake presented in Fig. 2(a). We obtained apparent activation energy Eact = 1.13 ± 0.08 kCal/mole and 0.88 ± 0.28 kCal/mole in the presented two cases, respectively.

Fig. 4. Arrhenius plots of surface oxygen content vs temperature. EDS measured oxygen content (% Oxidation) plotted versus an inverse of absolute temperature of the oxidation steps (22 °C, 270 °C, 360 °C, 410 °C and 500 °C) for the two data points in the MoS2 flake from Fig. 2(a). An obtained value of Eact being of the order of 1 kCal/mole or 50 meV is quite small comparing with the calculated oxidation energy barrier on MoS2, which is 1.6 eV and was obtained for a monolayer MoS2 using DFT based calculations based on the nudged elastic band method.

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However, similar to

graphene and h-BN, any presence of atomic-level defects can significantly alter the mechanism of oxygen 10 ACS Paragon Plus Environment

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adsorption on the MoS2 surface. Again, DFT based calculation using the nudged elastic band method, have showed that sulfur vacancies are expected to reduce the oxidation energy barrier to 0.8 eV for a monolayer thick MoS2 sample. 13 Despite a slight decrease of such energy for multilayer MoS2 substrates, this energy is still far too large comparing with our value of Eact. Qi et al. performed long time oxygen adsorption experiments at high vacuum and with low oxygen content to promote oxygen chemisorption to MoS2 surface. They found a corresponding activation energy barrier for oxygen chemisorption of 0.122 eV.

27

Nevertheless, at our experimental conditions, oxygen chemisorption competes with and may be

masked out by more dominant physisorption at defect-free regions due to an oversupply of oxygen. In fact, thermal energy of oxygen molecules, kBT, at 400 K and 500 K is 56 meV and 66 meV, respectively. Thus, we hypothesize that despite traces of molybdenum oxide on the surface, we have spotted a physisorbed oxygen. To follow upon that hypothesis, we have tracked oxygen content on the same samples after keeping them for substantial time at room temperature. At room temperature any further formation of MoO3 is highly unlikely due to a large activation energy for such a process. We have found that an oxygen content has increased from roughly 4% to about 8% after keeping the sample at room temperature for two months. After applying a time-temperature superposition principle, see Supporting Information, we can explain an increase of the oxide content with time using precisely the same activation energy of about 1 kCal/mol. This is indeed quite intriguing. Since Mo oxides are formed on the MoS2 surface at elevated temperatures, these findings as well as previously discussed EDS and AES results strongly suggest substantial surface presence of the physisorbed oxygen and only a faint surface presence of the Mo oxides. In order to address the very important issue of physisorbed oxygen in more detail, we conducted several rounds of new heating experiments in wet and dry heating conditions, and with postheating annealing. Annealing has been conducted for half an hour within Argon atmosphere at 150 °C, since at those conditions no oxidation has been observed in prior studies 10 and thermal energy is high enough to increase desorption of the physisorbed oxygen. We describe those results in the Supporting



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Information. They show that i) oxygen content increased in wet heated and dry heated samples and ii) oxygen content has dropped after annealing in wet heated and dry heated samples. Those experiments, and in particular the drop of the oxygen content on wet heated samples after their annealing, helped us to provide additional proofs that oxygen on the surface is mostly present in the form of the physisorbed oxygen, which leaves the samples during annealing. Next, we pondered within a more microscopic view and started to address the question of how any remainders of Mo oxides are distributed on the MoS2 surface. In order to address this questions we have performed a detailed AFM study of several MoS2 flakes after their final round of heating. Fig. 5(a) shows a 15 µm by 15 µm AFM topography image taken around the central portion of the MoS2 flake from Fig. 1(d). One can observe several regions of different heights, some surface terraces and tiny craters created by an electron beam during EDS measurements. Upon a closer look, the AFM topographs presented in Figs. 5(b) and 5(c) and collected at 5 µm by 5 µm and 1 µm by 1 µm, respectively, show some dendritic structures present on the MoS2 surface. They are several hundreds of nm long and arrange either as branched platelets in the vicinity of surface terraces in Fig. 5(b) or take more oblique and less branched shapes further away from the terraces in Fig. 5(c). Corresponding height histograms of those dendrites, see Fig. 5(d), yield three distinct heights with a most prominent height of 1.7 nm and two less prominent peaks of 1.1 nm and of 0.6 nm. According to literature those heights correspond to MoO3+MoS2 islands, MoO3 islands alone, and MoS2 islands alone, respectively (11). Furthermore, a related frictional analysis shows between 5 to 10 % non-calibrated friction increase on plateaus of the dendrites comparing with an underlying surface. Higher friction is expected for Mo oxides comparing to MoS2 substrates. MoS2 surfaces are used in industry as solid lubricants with known ultralow friction. Mo oxides are expected to be good substrates for forming water capillary bridges in ambient conditions, which in turn are responsible for an increase of frictional forces. 28,20,29. Consequently, the AFM detected height distribution of the dendrites and the AFM measured frictional forces strongly suggest that the observed dendrites are predominantly composed out of the Mo oxides. Those oxides must have formed on


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the MoS2 surface and did not leave the reaction scene. As expected, we do not see any triangular pits, which otherwise might have been observed during pure oxygen etching at low vacuum and temperatures of less than 350 °C. Dendritic structures usually originate from diffusion controlled chemical reactions. One notices two kinds of dendrites: branched out dendrites with tip splitting events, which are located closer to the terraces as well as elongated dendrites, which are located further apart from the terraces. We obtained, a fractal dimension of dendrites of 1.66 ± 0.01 near terraces and 1.61 ± 0.01 far away from terraces, see Fig. S3 in the Supporting Information. Strikingly, those values of fractal dimension are very close to a value of 1.71, which is universally obtained in the case of the surface growth via the diffusion-limited aggregation (DLA) model. An example of a such a process is an epitaxial growth of a crystal, where adatoms move by diffusion and stick to the edges of islands that already nucleated. 30 Here, those adatoms would be physisorbed oxygen species. One readily observes that the dendrites are branched the most and their fractal dimension is the closest to the DLA process in the proximity of the surface edges on the surface, which is exactly where one would expect an increased amount of initially adsorbed oxygen. A complementary explanation of the DLA growth evokes Mullins-Sekerka and Gibbs-Thompson effects. Mullins-Sekerka effects relate to dendritic growth of higher surface energy crystal phase and Gibbs-Thompson effects related to dissolving sharp protrusions.

31

Both effects can have thermal and

chemical origins. Here, thermal factors include temperature drops in the heating oven, and chemical factors might include either the growth of the Mo oxide crystalline phase towards directions of the highest concentration of the physisorbed oxygen, or – as mentioned before - oxygen diffusion towards a growing dendrite.



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A)

B)

C)

D)

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Fig. 5. AFM topography of MoS2 flakes. AFM images were obtained at room temperature after a series of subsequent 5 minute long heating cycles at 270 °C, then 360 °C, then 410 °C, and finally at 500 °C. (A) 15 µm x 15 µm AFM topography image with a total height scale of 163 nm of a central zone of the MoS2 flake from Fig. 1(c). (B) 5 µm x 5 µm AFM topography image with a total height scale of 33 nm. One can notice two kinds of dendritic structures present on the MoS2 surface: branched platelets in the vicinity of a surface terrace and oblique and less branched shapes further away from the terrace. A 14 ACS Paragon Plus Environment

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central depletion zone, i.e., a diagonal zone without any dendrites, is visible there as well. (C) 1 µm x 1

µm AFM topography image with a total height scale of 4 nm obtained far away from a terrace. Directions of dendritic structures are marked by lines. Distinct dendritic nucleation sites are marked by circles. (D) Height histogram of the dendrites from (C). Three main heights are visible and marked with an asterisk at 0.6 nm, 1.15 nm and 1.7 nm, respectively. To guide the eye a multigaussian curve was fitted. An interesting aspect of verifying whether the DLA mechanism applies in our case would be to check the density of nucleation sites. This is because, if single sulphur vacancies on MoS2 surface are readily reacting with oxygen atoms, they will serve as nucleation sites for the DLA growth.

13

AFM

images presented in Fig. 5 have not been collected at atomic resolution, however, any turns in dendritic Mo oxide structures might well be caused by the presence of the corresponding structural defects in the MoS2 surface. Thus, we have counted any direction changes on those dendrites, which we marked by dots in Fig. 5(c), and obtained a related density of 3*1010 cm-2. In the results of other experimental study based on AFM imaging much lower defect densities have been obtained, i.e., between 106 to 108 cm-2, and no obvious correlation between the defect density and the sample thickness has been found. 10 However, those authors observed an increase of the defect density with the number of MoS2 layers from 107 - 108 cm-2 for 1 MoS2 monolayer (1 ML) and up to 109 cm-2 for 8-9 MoS2 MLs. Our value of 3*1010 cm-2 , obtained for the 30 ML thick MoS2 sample in Fig. 5(c), readily follows that trend. Nevertheless, a published high resolution TEM study yielded equilibrium density of sulfur vacancies in thick exfoliated MoS2 crystals to be of the order of 1013 cm-2, which is 2-3 orders more than our estimates. 32 Thus, even taking into account that only a small percentage of sulphur vacancies would serve as DLA growth sites, we have found only a small portion of the corresponding surface defects, e.g., several per cents or so. A compelling explanation is that a vast majority of sulphur defects could not be observed, since most of the Mo oxides must have left the scene leaving only a tiny portion of oxides present on the MoS2 substrate.



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Formation of Mo oxides would occur either due to pre-adsorbed oxygen diffusing towards sulphur defects to form a Mo oxide, or through the Mo oxide structures growing towards patches of the pre-adsorbed oxygen. A central depletion zone in Fig. 5(b), i.e., a zone without any dendritic structures, points out towards oxygen diffusion during oxidation rather than the growth of oxides towards the zones of highest concentration of surface adsorbed oxygen.

CONCLUSIONS We have provided novel evidence for understanding the microscale basis of the oxidation of thick MoS2 flakes. The results of our EDS, AES, Raman studies and obtained activation energies of about 1 kCal/mole as well as application of time-temperature superposition principle strongly suggest that: i) oxygen on the surface is present mostly in the form of physisorbed oxygen, ii) no bulk oxygen chemisorption has occurred, and iii) the presence of any surface-bound Mo oxides is very minimal. Through a complementary AFM study we obtained that Mo oxides are present in a form of surface dendrites. The morphology and corresponding fractal dimension of those dendrites suggested their DLA growth process. In the light of the existing literature Mo oxide growth would initiate on single sulphur vacancies, which constitute a vast majority of the existing MoS2 surface defects. We have calculated a related density of defects to obtain a value of 3*1010 cm-2, which is substantially smaller then densities of 1013 cm-2 obtained from a published high resolution TEM study. This served as another indirect proof that most of the Mo oxides must have evaporated shortly after their formation. Through further analysis of our AFM data, i.e., observations of dendrite depletion zones, we suggest that during oxidation of single thick MoS2 flakes at our experimental conditions the physisorbed oxygen diffuses to reaction centers. Diffused oxygen facilitates the subsequent chemical conversion of MoS2 to a volatile Mo oxide, which then mostly leaves the reaction environment.


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Overall, our findings point out that surface diffusion of initially physisorbed oxygen is an important step in the MoS2 oxidation mechanism, and that mechanism – in our experimental conditions follows most readily through the DLA process. A competing hypothesis could be that due to a very large number of single sulphur defects, which could act as prospective nucleation sites, a thin Mo oxide layer appears quickly during high temperature MoS2 oxidation, which then vaporizes leaving the dendritic leftovers. Nevertheless, even in such a case a quick formation of the continuous oxide layer would only be possible due to an oversupply of the pre-adsorbed surface oxygen. In order to address which portion of the originating Mo oxides leaves the scene, or whether detected dendrites are only the leftovers after vaporization of the thin Mo oxide layer, one would need to conduct a more thorough study. An example of such study would be in-situ heating combined with observation of the originating oxidation process. Since it is difficult to heat macroscopic samples inside an SEM/AFM, a localized heating setup is expected to fit that need. Appropriate DFT-based calculations suited to model the occurring process would also help in explaining the experimental findings. Finally, future studies should also concentrate not only of thick MoS2 flakes, but to include thin MoS2 flakes, which according to several reports are more stable against oxidation than multi-layer MoS2 flakes, and might show different oxidation mechanisms. Such a hypothesis is strengthened by the fact that prior experimental observations found that the density of observed surface defects have decreased in the case of thin MoS2 flakes.

ASSOCIATED CONTENT Supporting Information EDS signal simulations; applying time-temperature superposition to surface oxidation of MoS2 flakes; calculations of fractal dimensions of the dendrite Mo oxide; additional dry and wet oxidation studies.



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AUTHOR INFORMATION Corresponding Author *E-mail: [email protected].

ACKNOWLEDGEMENTS We acknowledge Prof. Wojciech Święszkowski for providing us access to an AFM microscope in his laboratories at Warsaw University of Technology. We are also grateful to Piotr Kaźmierczak and Prof. Andrzej Wysmołek for collecting Raman spectra (not presented here) of the investigated samples.

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Microscale Insight into Oxidation of Single MoS2 Crystals in Air - The

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