Direct Transformation of Crystalline MoO3 into Few-Layers MoS2

We fabricated large-area atomically thin MoS2 layers through the direct transformation of crystalline molybdenum trioxide (MoO3) by sulfurization at relatively low temperatures. The obtained MoS2 sheets are polycrystalline (~10–20 nm single-crystal domain size) with areas of up to 300 × 300 µm2, 2–4 layers in thickness and show a marked p-type behavior. The synthesized films are characterized by a combination of complementary techniques: Raman spectroscopy, X-ray diffraction, transmission electron microscopy and electronic transport measurements.


Introduction
Two-dimensional (2D) transition metal dichalcogenides (TMDCs) have recently gained interest among the scientific community to solve the weakness of the lack of a bandgap in graphene, which limits its applications in field-effect transistors and digital integrated circuits [1]. The TMDC molybdenum disulphide (MoS2) was the first 2D material with an intrinsic bandgap that was isolated [2] and it consists of S-Mo-S layers that are held by weak van der Waal forces in a trigonal prismatic structure [3][4][5][6]. In its bulk form, this material displays an indirect bandgap of about 1.2 eV; nevertheless, it becomes a direct bandgap semiconductor (1.8 eV) when it is thinned down to a monolayer [7]. In addition, when a single-layer MoS2 is used as the channel in a field-effect transistor, it exhibits high in-plane mobility and large current ON/OFF ratio [8]. These are the reasons why molybdenum disulphide has attracted interest for electronic and optoelectronics applications [8][9][10]. Furthermore, it is an attractive candidate for energy conversion [11,12] and storage [13,14], hydrogen evolution reactions [15][16][17] or oxygen reduction reactions [18].
First methods that were reported for the synthesis of 2D MoS2 consisted of mechanical and chemical exfoliation from bulk crystals [2,19,20] and, in fact, a lot of studies still use these methods since they provide high-quality single layers. However, these techniques present some problems like randomly deposited flakes, relatively small coverage area of material and a poor control over thickness. A solution for these issues is critical to achieve real-life electronic devices based on MoS2 This is the authors' version (post peer-review) of the manuscript: F Carrascoso et al. Materials, 13(10), 2293, 2020. https://doi.org/10.3390/ma13102293 That has been published in its final form: https://www.mdpi.com/1996-1944/13/10/2293 and, therefore, synthesis of large-area MoS2 films is a very active research area. The most explored methods to synthesise large-area MoS2 thin films are the chemical vapour deposition (CVD) [21,22] and the sulfuration of sputtered molybdenum thin films [23][24][25].
Here, we explore an alternative route to obtain atomically thin MoS2 layers: the direct transformation of crystalline molybdenum trioxide (MoO3) layers into MoS2 nanosheets by sulfurization at moderate temperatures. Up to now the sulfurization of crystalline MoO3 have only demonstrated to produce MoS2 fullerenes and nanotubes but here we demonstrate that it can be also employed to fabricate large area MoS2 layers [26,27]. We characterized the resulting layers by Raman spectroscopy, X-ray diffraction and transmission electron microscopy, finding that the resulting layers show all the characteristics of polycrystalline MoS2. We transferred the as-synthesized films to pre-patterned electrodes to fabricate electronic devices and we found that they are strongly p-doped, which can be an interesting feature to complement the marked n-doping of mechanically exfoliated or CVD grown MoS2. Our synthesis method does not require a tube furnace with flow gas control as the sulfurization is carried out in a sealed ampoule, simplifying considerably its implementation, and reducing the cost.

Materials and Methods
The crystalline MoO3 source is obtained by heating up a molybdenum foil (99.99% purity) to 540 °C in air using a laboratory hot plate. At this temperature, the MoO3 starts to sublime. A mica substrate is placed above the hot molybdenum foil. The MoO3 gas sublimed from the hot molybdenum foil crystalizes on the slightly cooler mica substrate placed on top, as we show in Figure  1(a). As reported by Molina-Mendoza et al. [26], this method produces continuous crystalline thin films through a van der Waals epitaxy process thanks to the van der Waals interaction with the mica surface. Note that in the van der Waals epitaxy process there is no need for lattice matching between the substrate and the grown MoO3 overlayer.
Prior to the sulfuration of the MoO3 crystals, they are reduced by heating them at 300 °C for 24 hours in a tube furnace in forming gas atmosphere, Figure 1(b). This process yields MoO3-x crystals. We found that this step is crucial to avoid the evaporation of MoO3 during the sulfuration process as MoO3 is a highly volatile material. On the contrary, MoO2 is a more stable oxide [28], in fact, by partially reducing the molybdenum trioxide we observe an improved stability of the material upon temperature increase. Then the MoO3-x layers are converted to MoS2 by a sulfuration process in a closed glass ampoule. The sample containing the MoO3-x layers is sealed with sulphur powder at 10 -5 mbar pressure. The ampoule is placed in a furnace at 500 °C for 5 hours and then the temperature is increased at 600 °C for another 5 hours. Once the sulfuration process is concluded, the temperature is slowly lowered to room temperature, Fig 1(c). The number of MoS2 layers that we obtain depends MoO3-x is formed after placing the MoO3 in a tube furnace at 300 °C in a forming gas atmosphere for 24 hours. (c) Sulfuration process is performed in a closed glass ampoule at 500 °C -600 °C. This is the authors' version (post peer-review) of the manuscript: F Carrascoso et al. Materials, 13(10), 2293, 2020. https://doi.org/10.3390/ma13102293 That has been published in its final form: https://www.mdpi.com/1996-1944/13/10/2293 on the starting MoO3 thickness. Therefore, with this method we are able to obtain MoS2 continuous layers covering most of the mica substrate with regions of up to 300×300 µm 2 with <5 layers in thickness, an example is shown in Figure 2(a). As discussed below, also single layer MoS2 could be observed (see the discussion related to the scanning transmission electron microscopy results). It is important to note that, when we tried to sulfurize the as-grown MoO3 layers, without the reduction step, we obtained thick MoS2 crystallites randomly deposited on both the ampoule surface and on the substrate. Scanning transmission electron microscopy (STEM) data was acquired in an aberrationcorrected JEOL JEM-ARM200cF electron microscope operated at 80 kV.

Raman characterization
In Figure 2(a) we show an optical image of a thin and large-area MoS2 film on mica. We employ Raman spectroscopy to characterize the MoS2 film as this technique has been demonstrated to be a very powerful tool to characterize 2D materials [29,30]. Figure 2(b) presents the Raman spectra acquired on two locations (indicated in the figure) of the MoS2 film shown in Figure 2(a). The characteristic E 1 2g and A1g phonon modes of MoS2 (around 380 cm -1 and 415 cm -1 ) are clearly visible in the spectra [23,31]. One can determine the number of layers from the frequency difference between these two Raman modes. In the inset in Figure 2(b) we show the relation between this frequency difference and the number of layers of MoS2, obtained from the literature [32,33], and we compare these values with those obtained in two spots in our sample to determine the number of layers finding that the MoS2 specimen is composed of a bilayer and a four-layer region. We address the reader to the Supporting Information for a Raman map of another thin MoS2 region.

XRD characterization
The crystal structure of the films has been characterized with X-ray diffraction (XRD). XRD was performed at room temperature on the initial sample (MoO3 grown on mica 18 mm x 2 mm substrate) and on the final sample (MoS2 obtained after the sulfuration process). Figure 3 illustrates the X-ray diffractograms that were taken for the initial sample and for the final sample in green and blue, respectively. In red, we also show the X-ray diffractogram for a bare mica substrate in order to be That has been published in its final form: https://www.mdpi.com/1996-1944/13/10/2293 able to differentiate the peaks that belong to the substrate from the peaks that correspond to the growth film.
Notice that the green spectrum exhibits peaks that correspond to (0 2 0), (0 4 0) and (0 6 0) reflections, which belong to the diffraction peaks of MoO3. The appearance of the (0 k 0) peaks, parallel to the plane (0 1 0), is a product of a preferred orientation of the MoO3 crystal with respect to the mica (0 0 1) surface due to the van der Waals epitaxy type of growth [34]. The blue spectrum obtained for the same sample after the sulfuration process shows a peak that corresponds to the (0 0 2) reflection of MoS2 [6]. Thus, we further confirm that we are able to obtain MoS2 from MoO3 deposited onto a mica substrate. In some works it is proposed that the average thickness of a thin sample can be obtained from the analysis of the XRD peaks using the Scherrer equation (D=kλ/βcosθ, where k is the shape factor, λ is the X-ray wavelength, β is the full width at half maximum of the peak and 2θ is the scattering angle) [35,36]. By analysing the (002) peak of the MoS2 XRD pattern we estimated a cstacking height for the analyzed sample of 10 nm, which corresponds to 15 layers of MoS2. Note that this value corresponds to the average thickness of the whole sample; however thinner regions (as those shown in Figure 2) can be found on it. It is also worth mentioning that the single-crystal domain size observed in our samples is also of the order of ~10 nm (see STEM discussion below) and thus it is not completely clear if the Scherrer equation provides accurate values of the average thickness of the sample or simply the single-crystal domain size.

STEM characterization
The crystal structure of the films can be further characterized in real space by STEM. Figure 4 displays a high-angle annular dark field (HAADF) image of a MoS2 layer transferred over a holey Si3N4 membrane support by an all-dry deterministic transfer process [37]. In order to transfer the MoS2 films on mica we stick a polydimethylsiloxane (PDMS) sheet on its surface and we immerse it in distilled water. Due to the hydrophilic character of mica, the water wedges between de MoS2 and the mica surface separating the MoS2 layer, which keeps attached to the PDMS substrate, from the  That has been published in its final form: https://www.mdpi.com/1996-1944/13/10/2293 mica surface. The MoS2 is easily transferred to the membrane by gently pressing the PDMS containing the MoS2 film against the acceptor substrate and peeling it off slowly.
The STEM characterization indicates that the MoS2 film is polycrystalline with a single-crystal domain size of 10-20 nm. Thinner regions can be found at the edges of the sulfurized film, where one can find monolayer, bilayer and trilayer areas ( Figure 4 shows the edge of a MoS2 film where mono-, bi-and tri-layer areas can be resolved). The fast Fourier transform (FFT) obtained from the monolayer region shows clearly the hexagonal symmetry of MoS2. The electrical properties of the fabricated MoS2 films have been characterized fabricating a fieldeffect device, by transferring a MoS2 film onto a SiO2/Si with pre-patterned drain-source electrodes separated 10 µm. Figure 5(a) shows the measured source-drain current vs. gate voltage (Isd-Vg) characteristic for a fixed source-drain voltage of Vsd = 1 V. Surprisingly, we obtain a decrease in the source-drain current upon gate voltage increases without reaching the OFF state, which corresponds to a strong p-doped field effect behaviour. To confirm this fact, we performed a thermopower measurement. Figure 5(b) displays the IV characteristics acquired applying a temperature difference between the two electrodes. It can be seen how a positive voltage offset at zero current appears (thermoelectric voltage) when the temperature different increases. The inset shows the thermoelectric voltage versus the temperature difference. The Seebeck coefficient can be extracted from the slope of a linear fit to the data: S = + 33.9 μV/K. This positive value confirms the p-doped nature of the MoS2 film obtained by the direct sulfurization of crystalline MoO3. The low magnitude of the Seebeck coefficient also indicates a high doping level We have carried out preliminary Hall effect measurements backing up the p-type electrical behavior of the MoS2 films observed in the Seebeck and electric-field measurements. Unfortunately, the large resistance of our samples precludes us from quantifying the charge carrier concentration as the electronics of our Hall effect measuring system is optimized for low impedance samples. The highly linear shape of the IVs, together with the high doping inferred from the shallow transconductance and low Seebeck coefficient, points to an Ohmic contact in the Au-MoS2 junction. We have also estimated the resistivity of the device ~100 Ω·cm, This is the authors' version (post peer-review) of the manuscript: F Carrascoso et al. Materials, 13(10), 2293, 2020. https://doi.org/10.3390/ma13102293
In order to get a deeper insight into the microscopic origin of this p-doping in our MoS2 layers we have done an electron energy loss spectra (EELS) analysis of the STEM data (see Supporting Information). Apart from the presence of Mo and S, we found C (which could come from e-beam induced deposition of amorphous carbon during the STEM measurement), O and B. The presence of O could be due to an incomplete MoO3 to MoS2 transformation and the presence of B impurities could come from unintentional cross-contamination from the surface of the glass ampoules used during the growth. The presence of these foreign species could be a plausible source of the unexpected p-type doping. Figure 6(a) represents the measured Isd-Vsd characteristics in dark condition and under light excitation with different wavelengths. The gate voltage was set to Vg = 0 V during the measurement. Fiber coupled LED light sources were employed to illuminate the device. The inset of this figure zooms on the high voltage region of the traces to distinguish the differences induced upon illumination. The photocurrent as a function of the wavelength can be calculated from these data, as we show in Figure 6(b). This spectrum reveals that the maximum photocurrent value is located between 530 nm and 595 nm, whereas it decreases at longer wavelengths. We were not able of measuring a sizeable photocurrent beyond 740 nm as expected for multilayer MoS2.

Conclusions
In summary, we presented an alternative method to obtain atomically thin MoS2 layers through the direct transformation of crystalline molybdenum trioxide (MoO3) layers into MoS2 nanosheets by sulfurization. The process can be carried out at moderate temperatures and using simple instrumentation. We obtained large area polycrystalline MoS2 sheets 2 to 4 layers thick and we characterized them by Raman spectroscopy, X-ray diffraction and transmission electron microscopy. Regarding their electronic properties, they are strongly p-doped.  This is the authors' version (post peer-review) of the manuscript: F Carrascoso et al. Materials, 13(10)