Microscopic Kinetics of Heat-induced Oxidative Etching of Thick MoS2

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C: Surfaces, Interfaces, Porous Materials, and Catalysis 2

Microscopic Kinetics of Heat-induced Oxidative Etching of Thick MoS Crystals Ugonna Ukegbu, and Robert Szoszkiewicz J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b02739 • Publication Date (Web): 26 Aug 2019 Downloaded from pubs.acs.org on August 31, 2019

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The Journal of Physical Chemistry

Microscopic Kinetics of Heat-induced Oxidative Etching of Thick MoS2 Crystals Ugonna Ukegbu and Robert Szoszkiewicz* Faculty of Chemistry, Biological and Chemical Research Centre, University of Warsaw, Żwirki i Wigury 101, 02-089 Warsaw, Poland.

(*) Corresponding author, [email protected]

Abstract We have studied the kinetics of microscopic heat-induced oxidative etching in the case of thick, mechanically exfoliated, geological MoS2 crystals in air. We have measured spatial dimensions of microscopically obtained triangular etch pits during a series of sample heating increments at a given temperature. The data has been collected for the samples heated at 320, 350, 370 and 390 °C. Using our data we have extracted an Arrhenius apparent activation energy, Ea = 1.15 ± 0.25 eV, as well as an Arrhenius kinetic constant, A= 10x s-1 with x = 9,09 ± 2,03. The obtained value of Ea compares extremely well with another study of oxidative etching, but done via in situ Raman spectroscopy on an collection of thin MoS2 flakes. We notice that apparent activation energy relates to a weighted average of microscopic Arrheniuslike processes. It might need a correction due to yet unknown fractions of removed MoOx species at the investigated temperatures. Based on the existing literature the most expected reaction is a series of etching events proceeding along zig-zag Mo edges and with each event being comprised of three stages. First, an oxygen molecule reacts with unsaturated Mo atoms - accessed via abundant single sulfur vacancies - to produce MoO3 as well as Mo vacancies with exposed S atoms. The unsaturated S-terminated layer reacts subsequently with two O2 molecules to produce two SO2 molecules and to expose yet new unsaturated Mo atoms along the Mo zig-zig edge. Finally, the obtained here value of Ea suggests that oxidative etching competes with two other surface reactions with very similar apparent activation energies. These are dissociative O2 adsorption on defected MoS2 surfaces and oxygen induced single sulfur vacancy creation.

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Introduction Semiconducting and several monolayer (ML) thick 2H MoS2 flakes with bandgaps depending on their thickness are expected to complement and/or enhance electronic circuits made of graphene in many electronic applications, which require transparent semiconductors.1,2 Field Effect Transistors based on MoS2 have been used as label-free biosensors3,4 and memory cells.5 Their further applications are looming, in particular in optoelectronics and energy harvesting. Even more interesting are the van der Waals heterostructures of 2H MoS2 with its 3R, 1T and 1T’ polytypes2,6 as well as with other transition metal dichalcogenides (TMDCs),2 layered chalcogenides7 and different 2D materials.2 These heterostructures allow for manipulations of other than electronic degrees of freedom. For example, groundbreaking room temperature manipulations of long living excitons in MoS2/WSe2 heterostructures have been accomplished recently.8 Furthermore, substantial research efforts have been devoted to manipulations of the valley degree of freedom in MoS2, which is associated with the minima (maxima) of the conduction (valence) band that a given current carrier is occupying.2,9 Any progress in MoS2 electronics, however, will benefit from gaining understanding about microscopic surface reactivity of the MoS2 crystals. One of the most widely applicable issues pertains to their microscopic chemistry change due to oxidation in ambient conditions. Macroscopically, MoS2 crystals are expected to react with oxygen and etch according to a following reaction: 2MoS2 + 7O2 → 2MoO3 + 4SO2. Here, SO2 can convert into SO3 at elevated temperatures to yield an alternative overall stoichiometry of 2MoS2 + 9O2 → 2MoO3 + 4SO3. Different stoichiometries leading to other Mo oxides than MoO3 are also possible.10 Thus, the sample thins via evaporation of SO2 molecules as well as through the production of MoO3 oxides and/or other Mo oxide species. Microscopically, O2 molecules have been showed uneager to react with S- or Mo- atoms on the MoS2 surface as well as to penetrate through the MoS2 layers at ambient conditions.11 The corresponding activation energies for these processes have been calculated to be several eV,11-13 although values below 1 eV were suggested for sufficiently defected samples. Sample heating above 330 – 350 °C have produced either triangular microscopic etch pits and/or surface buildups containing Mo oxides.14-16 These results have spurred many questions about the possible microscopic oxidation mechanisms. In addition, several other processes competing with oxidative MoS2 etching have been recently pointed out. These include: oxygen induced single sulfur vacancy creation at room temperatures,17 surface diffusion of initially adsorbed oxygen18 and purely thermal etching without oxygen.19 Overall, microscopic kinetics of oxidative etching is still not well understood both mechanistically as well as experimentally. In order to help to elucidate the actual mechanisms of MoS2 oxidative etching in air we report on a simple method for obtaining the microscopic kinetics of this process. We provide measurements of thermal growth of the microscopically obtained triangular etch pits resulting from heating MoS2 crystals in air. From such data we extract an Arrhenius apparent activation energy, Ea, as well as an Arrhenius kinetic constant, A. Our obtained value of Ea of 1.15 ± 0.25 eV compares extremely well with another kinetic study of oxidative etching, but done in situ on very thin MoS2 flakes via Raman spectroscopy.20 We note that an apparent activation energy relates to a weighted average of microscopic Arrhenius-like processes, but it might need a correction due to yet unknown fractions of removed MoOx species at the investigated temperatures.21 Based on the existing literature we infer that the an expected reaction is a series of etching events proceeding along zig-zag Mo edges and with each event being comprised of three stages. First, an oxygen molecule reacts with unsaturated Mo atoms - accessed most likely via single sulfur vacancies - to produce MoO3 as well as Mo vacancies with exposed S atoms. Next, the unsaturated and S-terminated layer produces two SO2 molecules via two subsequent sulfur eliminations by O2 molecules. Finally, the obtained here value of Ea suggests that oxidative etching competes with two other surface reactions with very similar apparent activation energies. These are dissociative O2 adsorption on defected MoS2 surfaces and oxygen induced single sulfur vacancy creation.


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Materials and Methods Preparation and heating of the MoS2 flakes. 2H MoS2 geological crystals have been bought from SPI Supplies, USA, cat. no. #429MM-AB. The MoS2 crystals have been mechanically exfoliated using a standard double-sided scotch tape and then transferred on flat p-doped Si crystals, cat. no. 647675, bought from Sigma-Aldrich. Prior to a sample transfer, the substrates were ultrasonically cleaned with acetone and isopropanol, and then dried with pure N2. Due to subsequent heating studies at temperatures more than 300°C no annealing nor any other cleaning methods have been used to remove any remaining traces of the scotch tape from the substrate. For sample heating we used a standard hot plate covered with a custom made quartz Petri dish to insure a controlled atmosphere and proper distribution of temperatures within a heating zone. After calibrating the hotplate with a standard thermocouple and a Pt thermometer we established a temperature error of ± 2K on the sample surface. We heated each sample at either 320, 350, 370 or 390 °C. After selecting one given temperature, each sample was heated in a series of five minute increments. After each heating increment we performed room temperature atomic force microscopy (AFM) studies to investigate any resulting topographical and frictional changes on the samples. Characterization of the MoS2 flakes. ZEISS AXIO Scope A1 light microscope with a CCD camera AxioCam ICc 5 and AxioCam software have been used for optical observations of the freshly prepared samples. AFM contact mode imaging of the samples has been conducted at room temperature using a Dimension Icon AFM from Bruker, USA. The MLCT cantilevers from Bruker have been used for that purpose and with setpoint forces of several nN. We have collected raw topography and lateral force microscopy (LFM) images of the investigated MoS2 flakes. The images were usually 5 µm x 5µm in size and have been collected with at least 512 points per line. We have treated these images using Gwyddion software22 and using a standard line-by-line first or second order flattening method. No other image treatment and/or conditioning has been performed. Lateral dimensions of the triangular etch pits have been calculated from the LFM images. We have also collected micro-Raman spectra of the MoS2 flakes at various stages of their heating. Raman spectra were collected in the backscattering configuration with a Labram HR800 (Horiba Jobin–Yvon) confocal microscope system, equipped with a Peltier-cooled CCD detector (1024 x 256 pixel). Diode pumped, frequency doubled Nd:YAG laser, emitting 532 nm wavelength was used as an excitation source. The confocal pinhole size was set to 200 mm and the holographic grating with 1800 lines / mm-1 was used. Spectra were acquired with a 100x magnification Olympus objective.

Results and discussion Fig. 1 shows several AFM topographs of our single 2H MoS2 crystals on silicon substrates. For further research we have selected ten thick MoS2 flakes with thickness of at least 20 nm, i.e., more than 30 MoS2 MLs. This is due to known dependencies of several physico-chemical properties of the MoS2 flakes on their thickness, particularly for the flakes thinner than 10 ML.23 (a)

(b)

(c)

Fig. 1. AFM topographs of several investigated here thick 2H MoS2 crystals on silicon substrates. Colors represent height (Z-scale) and brighter color means “higher”. Image (a) presents a flake with a mean height of a central zone of 83.3 ± 1.3 nm and a Z-scale of an image of 110 nm. Image (b) shows a



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flake with a mean height of a central zone (excluding several lone MoS2 pieces) of 47.7 ± 0.8 nm and a total Z-scale of 96 nm. Image (c) shows a flake with a mean height of a central zone of 30.2 ± 2.1 nm and a total Z-scale of 56 nm. Next, we heated the MoS2 flakes at 370 °C for 10 minutes and then reimaged them at room temperature with AFM. We repeated the same process several times, each time with an additional five minutes heating at 370 °C. After the last AFM imaging we have selected 14 separate triangular etch pits on three different MoS2 flakes and investigated their growth. Figure 2 presents an example of the obtained data. The investigated triangular etch pits have been usually one monolayer deep. The triangles were equilateral and with the same orientation along each given basal layer. Their orientation did not change during each round of heating. Eventually, the neighboring triangles merged with each other until the top layer was etched off entirely. There has been almost no size reduction of the MoS2 crystal between the heating steps. Similar observations have been made so-far by several other published studies, while heating in the presence of oxygen as well as nitrogen.14,15,19,24 However, according to our knowledge, none of these studies used such observations to infer about the insitu kinetics of the oxidative etching processes, which we develop in this manuscript based on the following considerations. (a)

(b)

(c)

(d)

(e)

Fig. 2. Growth of local triangular etch pits on MoS2 crystals due to their heating investigated by AFM. All AFM images are 5 µm by 5 µm. For better visualization we show only one way LFM (~friction) images. Images from (a) to (e) were obtained after heating 10, 15, 20, 25 and 30 minutes at 370 °C, respectively. The 2H MoS2 structure is the most popular naturally occurring MoS2 crystal structure. It is composed of two weakly interacting S-Mo-S layers portrayed in Figure 3. Structural insight from Fig. 3 yields three main features of each MoS2 layer: i) its hexagonal symmetry, ii) presence of only one kind of atoms within each MoS2 plane, and iii) a structural motif of each MoS2 layer being a trigonal bipyramid with four mirror symmetry axes. Due to those features one expects exactly three equally probable spatial etching directions within each S-Mo-S layer. From Fig. 3 it is also clear that in order to etch a secondary layer within the 2H MoS2 its first layer needs to be removed first.


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Fig. 3. 2H MoS2 structure. The 2H MoS2 structure is composed of the repeating pattern comprising two stacked and weakly interacting S-Mo-S layers.2 Each layer contains three basal planes of only sulfur atoms in a top plane, only molybdenium atoms in the middle plane and then only sulfur atoms again in a bottom plane. S atoms are in yellow, Mo in gray. In each of these planes sulfur and molybdenium atoms are arranged on a hexagonal honeycomb pattern and in such a way that along the principal symmetry axis any two sulfur atoms from the top and bottom planes within each layer lie perfectly above each other. This produces a unitary motif of a trigonal bipyramid with four mirror symmetry axes within each layer. However, along the principal symmetry axis any two corresponding sulfur atoms between the first and second MoS2 layers are shifted and twisted with respect to each other by /2 radians. Etching process starts from interactions of etching agents with a cluster containing several atoms. This leads to eventual substitution/elimination reactions and subsequent or concurrent reconstruction of a defected surface. However, there could be several etching mechanisms involved. According to our knowledge exact single-cluster reaction mechanisms are not known yet and possible pathways involve at least several substitution and/or elimination reactions.17,24 Etching speed may depend on the chemistry of etching agents, their energy and concentration and finally on the presence of any initial defects within the basal MoS2 planes. Nevertheless, to limit a number of possibilities one can make several observations applicable to our case, i.e., thermally induced oxidative etching in air. Thermal energy of oxygen or nitrogen molecules in air is several tens of meV, which is much less than several eV needed to break covalent bonds. Thus, each etching instance is expected to involve only a small cluster of atoms such as single S or Mo atoms with their nearest neighbors. Consequently, we suppose that etching will progress through the paths containing only the nearest neighboring atoms and predominantly only exclusively along S atoms or exclusively along Mo atoms. Any mixed S and Mo etching schemes, would result in several simultaneously occurring etching mechanisms, which is unlikely at low available thermal energies of air molecules. Due to involvement of only the same kinds of atoms and three available symmetry axes within a hexagonal lattice, each etching direction is expected to proceed either along the arm-chair (AC) orientation of any four proceeding same kinds of atoms (S or Mo), or along their zig-zag (ZZ) orientations. The AC etching orientations are expected to produce circular and rounded pits, similarly like in graphene.25 Only etching along the ZZ directions would produce triangular pits observed by us and by others. This mechanism is further confirmed by the fact that observed triangular etch pits change orientation when passing from n to (n-1) MoS2 layer.14,26 It remains to establish whether etching proceeds predominantly along zigzags formed only by sulfur (ZZ-S) or only by molybdenum (ZZ-Mo) atoms. Both kinds of etching schemes produce the same final effect, since ZZ-S and ZZ-Mo directions come in three parallel pairs in a hexagonal MoS2 surface lattice, i.e., the ZZ-S surface direction of (-1, 0, 1) is parallel to the ZZ-Mo direction of (1, 0, -1). Recent numerical simulations point out that etching along the ZZ-Mo directions produces more stable intermediate products.24 Recent high resolution scanning transmission electron microscopy studies point out towards much higher abundance of the ZZ-Mo edges within the etched structures.26 Thus, etching along the ZZ-Mo directions is likely a dominant reaction pathway in our experiments. Finally, existing literature data points out that for mechanically exfoliated geological 2H MoS2 samples, the number of single sulfur vacancies, #SSV, surpasses greatly the number of single Mo atom vacancies, #SMoV, as well as the number of any other kinds of defects in a MoS2 layer. For our geological mechanically exfoliated MoS2 sample STEM studies27 estimated that #SSV is of the order of 1013 cm-2 and a ratio of #SSV over #SMoV is of the order of 100. Thus, etching in mechanically exfoliated 2H MoS2 samples is most likely to proceed along the ZZ-Mo directions and unsaturated Mo atoms become available due to a large number of single sulfur vacancies. Having established theoretical grounds for our experiments, we embarked on calculations of the activation energy for oxidative etching of thick 2H MoS2 crystals in air. A good reaction coordinate for that process is a length increase of the etched triangular pits along their sides. Noteworthy, one could also



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compare an area change of a given etched triangle between each round of heating. However, in the light of the aforementioned discussion the lengths of their sides appeared for us more proper to be taken care of. First, we have followed a series of data gathered at 370 °C, such as the data presented in Figure 2. For each i-th triangle we calculated its averaged side length, aave,i, from the measured lengths of its three sides, aj, j=1,2,3. We calculated the corresponding errors of aave,i as the maximum absolute difference between aave,i and aj. Figure 4 presents the results of aave,i vs. the cumulative heating time, t, for several investigated triangular etch pits. We observe that the values of aave,i increase linearly with t. Thus, we calculate the growth speed, vi, for each i-th triangle from the slope of its aave,i vs. t. We average the values of vi for all of the investigated triangles to produce an average growth speed at 370 °C, vave(370 °C). We also calculate its standard deviation. The value of vave(370 °C) can be treated as the oxidative etching reaction rate at 370 °C, i.e., r(370 °C). A constant value of r(370 °C) is, in turn, reminiscent of the zero order chemical kinetics, where: vave(T) = r(T) = k(T) = C k0(T)

Eq. 1

Fig. 4. Comparison of the growth speeds of the triangular etch pits during heating. Here, we plot several averaged side lengths of 10 selected triangles from several distinct locations onto five MoS2 flakes. In Eq. 1, T is the absolute temperature, k(T) is the reaction rate constant in nm/s, k0(T) is the reaction rate constant in s-1 and C is a normalizing constant, which recalculates the value of k(T) from nm/s to s-1. Supposing that etching propagates along the ZZ-Mo directions, we arbitrarily choose a value of C = cos(30 degr) * 0.3122 nm = 0.274 nm, where 0.3122 nm corresponds to the distance between first and second Mo atoms along the ZZ-Mo edge,28 and cos(30 degr) yields its projection on a macroscopically observed triangular side length. From the transition state theory the value of k0(T) relates to an apparent activation energy of the overall etching process, Eact, through a well-known Arrhenius equation: k0(T) = A exp{- Eact /(kBT)}

Eq. 2

where: A is the Arrhenius constant and kB are the Bolzmann’s constant.


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To obtain the values of A and Eact from Eq. 2, we repeated our heating studies for several new substrates with newly selected thick MoS2 flakes and at three other heating temperatures: 320 °C (5 thick flakes with a total of 50 triangular etch pits), 350 °C (8 thick flakes with 42 triangular etch pits) and 390 °C (3 thick flakes with 23 triangular etch pits). At each temperature, T, we obtained a corresponding value of the etching reaction rate constants k0(T) using Eq. 1. Figure 5 shows the results of natural logarithm of k0(T) vs. 1/T fitted to our experimental data. We obtained the respective values of A= 10x s-1 with x = 9,09 ± 2,03 and Eact = 1.15 ± 0.25 eV.

Fig. 5. Arrhenius equation fitted to our experimental results. We plot a natural logarithm of a reaction rate constant (in s-1) vs. inverse of the absolute temperature. The value of A= 10x s-1 with x = 9,09 ± 2,03 is in agreement with the transition state theory in which A is proportional to the ratio of kBT/h, where h is the Planck constant.29 This frequency represents the number of trials for converting the reactants to the product, which typically varies between 108 s-1 to 1012 s-1.29 Our mean value of A is somewhat on the lower end of the typically reported values. An explanation might relate to the fact that along a given growing triangular edge, there might be several initialization spots for this reaction, which merge, i.e., cannot develop fully on their own. Such a situation would lead to reducing an observed number of reaction events. For example, using #SSV of 1013 cm-2, one finds roughly that within a typically investigated slab with length of 500 nm and width of 20 nm (corresponding to a mean distance between any two data points), the number of potential reaction centers is on average: 1013 cm-2 * 1000 nm2 = 100. Clearly not all of those SSV might participate within etching. However, currently only high resolution in situ STEM studies are able to shed more light upon those considerations. In addition to A we also obtain an apparent activation energy barrier of Eact = 1.15 ± 0.25 eV. Our value of Eact falls precisely within the values obtained by reported oxidation studies of various bulk MoS2 forms, which yielded activation energies between 0.9 and 1.5 eV.30-32 However, to the best of our knowledge, there exist only one relevant direct study of this parameter for well characterized MoS2 flakes. Rao and al. have recently reported on the thermal oxidation kinetics of thin 2H MoS2 crystals.20 Their crystals were CVD grown. However, the authors reported the same quality, and consequently similar number of expected SSV, between their and our, geological, MoS2 samples. Rao et al. observed, via Raman spectroscopy, a temporal decay of the peak area for the out-of-plane A1g vibrational mode of a basal MoS2 layer. This mode, together with a related in-plane E12g vibrational mode yields a well-visible and wellestablished Raman signature of the MoS2 basal planes.2 The A1g mode is expected to vanish once the sample is entirely etched. From the plot of the natural logarithm of the A1g peak area decay rate vs. inverse of the absolute temperature Rao et al. inferred an activation energy for their oxidation/etching to be 0.54 eV ±



0.14 eV. However, while omitting two clear outliers obtained at temperatures of more than 1250 K, which in fact might relate to the non-Arrhenius corrections, see Ref.21, one gets a much better fit to their data yielding an activation energy of 1.09 eV, which is extremely close to our value of Eact. To start pondering within a microscopic nature of the observed processes, we notice that the obtained here value of activation energy is much larger than thermal energy of oxygen molecules, kBT, which at 400 K and 500 K is 56 meV and 66 meV, respectively. Similarly, our value of Eact is much larger than activation energy for physiosorbtion of oxygen, which has been estimated – from experimental data to be also of the order of at least 50 meV.18 It is also larger than activation energy barrier of 0.122 eV obtained for oxygen chemisorption experiments in vacuum with a low oxygen content.33 The O2 molecules in contrast to molecular oxygen, have been showed uneager to directly react with S- or Mo- atoms on MoS2 surface. An energy barrier of 1.59 eV was obtained via DFT for oxygen dissociative adsorption of O 2 molecule on 1 ML thick MoS2,12 which would lead to two quite stable oxygen terminated sulfurs. Same process occurring on the S-defective site leads to substitutional oxygen and –S-O bonding. Then, its calculated energy barrier was 0.8 eV. Another recent study points out that there exists yet another process, which is preferential at room temperature: an oxygen induced SSV formation (O2 -> SO2 + vacancy) followed by the vacancy substitution by oxygen species.17 The process has been showed to occur via three stages, each of which with an activation energy barrier of 1 to 1.1 eV.17 Here, computational results have been backed by high resolution STM experimental studies showing a large number of the reported events due to their thermodynamic stability. However, both processes, i.e., dissociative O2 adsorption on sulfur atoms12 as well as oxygen induced SSV creation17 do not produce sample etching, but rather a local surface chemistry change. Nevertheless, based on the provided evidence and namely their activation energies, those processes compete with each other and with etching. Based primarily on X-ray photoelectron spectroscopy (XPS) data oxidative etching mechanisms have been proposed to be associated with the formation of oxidized MoOx areas that can subsequently volatilize to produce ditches on the sample surface.10,18,19 However, in contrary to AFM, XPS measurements are not local and inform rather about the lateral dimensions of the order of several microns, which are comparable to lateral dimensions of each given MoS2 flake, but not to the lateral dimensions of each etch pit, which are one order of magnitude smaller. Nerveless, volatilization of the MoOx species might also relate to the zero order chemical kinetics of etching, which we observed in our experiments. Accumulates of solid MoOx islands would obstruct the progress of etching. However, etching kinetics would become independent on the MoOx concentration in the case of its quick volatilization, e.g., via sublimation. Indeed observations of sublimations of high quality MoO3 crystals grown on gold showed their tendency to do so above 670 K.34 Thus, similar is expected on our samples, particularly that crystallographic dimensions and orientations of the most stable orthorhombic MoO3 crystals and 2H MoS2 crystals do not match. More light on the mechanisms of the oxidative etching associated with production of the MoOx species have been showed by first-principles computations of Zhou et al.24 They have identified the most exothermic reaction path for etching along the ZZ-Mo edge to fulfill a generally accepted reaction of MoS2 + 3.5O2 → MoO3 + 2SO2. In their scenario the reaction involves three stages as well as significant surface reconstructions after each step. First, an oxygen molecule reacts with unsaturated Mo atoms (accessed most likely via SSV) to produce MoO3 as well as Mo vacancies with exposed S atoms. The unsaturated S terminated layer produces two SO2 molecules via their two subsequent reactions with O2 molecules. Similar conclusions and explanations has been provided by experimental works of Lv et al. from high resolution STEM studies.26 However, we could not find a detailed examination of atom-by-atom reaction paths. Furthermore, recent studies with etching in pure N2 atmosphere without any oxygen have also showed triangular etch pits.19 Those have been associated by authors to “thermal etching”, but without a detailed mechanism outlined. Thus, a three stage oxidative etching along ZZ-Mo proposed by Zhou et al. and Lv et al. is not the only scenario, and an obtained here activation energy is certainly an apparent value for a combined number of plausible microscopic processes.


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Indeed, we observe only an apparent, i.e., the overall, mean progress of the reaction, which otherwise might include several reaction stages, each with their own values of A and Eact. Gonsalvez et al. point out that a concept of “apparent” activation energy is only partially explained by an expected weighted average of the microscopic activation energies.21 A correction term accounting for the temperature dependence on the fractions of the removed particles should be included. A non-Arrhenius behavior can be expected when the fractions of the removed particles, here: MoOx species, change significantly over the considered range of temperatures. The issue merits further research, particularly at temperatures higher than 400 °C, which were not considered here, but have been showed to produced messy surface etching results depending on the way the experiments were conducted.14-16 To shed more light on the of heat-induced oxidative etching of thick MoS2 crystals, and in particular the role of MoOx species we performed as well some micro-Raman studies, see the Supplementary Information. Similarly as in other published reports, however performed on thin MoS2 flakes,14, 15, 24 we could not observe any direct signature of the MoOx species on the MoS2 surface within our experimental conditions. We have observed a slight decrease of the electronic densities within the MoS2 flakes associated with progression of thermal etching. This is also similar to the results obtained in Refs. 14, 15, 24 However this issue merits further discussion beyond the scope of this manuscript. Other overlooked issues with the presented here experiments are as follows. First, not all triangular etch pits are perfectly equilateral and some of their edges are quite rugged. Lv et al.26 have showed that about 80 – 90 % of found etched terminations were ZZ-Mo edges. Thus, other etching directions, and namely along the ZZ S edges and combined ZZ S-Mo edge etching events take place as well. We also mention that mechanical issues like sample edges and local bulging out of the sample are also expected to contribute to local fraying of the triangular etch pits. Finally, oxidation of thick MoS2 flakes should not be affected by boron dopants within the silicon substrate. This is because any meaningful boron diffusion to the Si/SiO2 interface is expected to happen at much more elevated temperatures than used in our experiments.35-37

Conclusions We have reported on the kinetics of local oxidative etching of the single thick MoS2 flakes. Our method to extract an Arrhenius apparent activation energy, Ea, as well as an Arrhenius kinetic constant, A, is based on the measurements of the thermal growth of the microscopically obtained triangular etch pits associated with the process. Our obtained value of an apparent activation energy of 1.15 ± 0.25 eV compares extremely well with another study of oxidative etching, but done on very thin MoS2 flakes via Raman spectroscopy. To explain the oxidative etching processes, we pondered into an existing literature and inferred that a predominant expected here etching process is comprised of a series of etching events proceeding along zigzag Mo edges and with each event being comprised of three stages. First, an oxygen molecule reacts with unsaturated Mo atoms, accessed most likely via SSV, to produce MoO3 as well as Mo vacancies with exposed S atoms. The unsaturated S terminated layer produces two SO2 molecules via their subsequent reactions with O2 molecules. Due to Eact of 1.15 eV, oxidative etching competes with thermal etching and with two other processes with very similar apparent activation energies: dissociative O 2 adsorption on defected MoS2 surfaces and oxygen induced single sulfur vacancy creation. Finally, an obtained here value of an apparent activation energy relates to a weighted average of microscopic Arrhenius-like processes. It might need a correction, however, due to yet unknown fractions of removed MoOx species at the investigated temperatures.

Associated Content Supporting Information Raman studies with related discussion of the previous literature reports.



Acknowledgments We acknowledge discussions with Prof. M. Jałochowski, UMC Lublin, Poland, and with Prof. M. Szymoński, UJ, Cracow, Poland, as well as help with data analyses by Aneta Mierzwa. The work was supported by the National Science Center, Poland, grant no. 2017/27/B/ST4/00697 and by the UW Statuary Funds.

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TOC Graphic

a1 a3

Single MoS2 flake oxidative etching

a2 Eact = 1.15 ± 0.25 eV


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Microscopic Kinetics of Heat-induced Oxidative Etching of Thick MoS2

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