Structural analysis of the chemical vapour deposition grown molybdenum disulphide nanofilms for multifaceted applications

Background/Objectives: In recent years, the research on molybdenum disulphide (MoS2) has gained significance because of its unique properties and ease of incorporation in hybrid structures, which makes it one of the most suitable materials for devices and multifaceted Applications. The objective of the study is to synthesize MoS2nanofilms and then to characterize them through X-ray diffraction (XRD) technique. Methods: In this study, MoS2 nanofilms are synthesized on silicon dioxide substrates by the thermal Chemical Vapour Deposition (CVD) technique, where molybdenum trioxide (MoO3) (VI) powder and sulphur (S) flakes are used as precursors. Findings: X-ray diffraction (XRD) measurements have been carried out for the thermal CVD grown MoS2 nanofilm samples. Further, the observed XRD data has been analyzed and the structural analysis of synthesized MoS2 nanofilms is presented in this report. Furthermore, the experimentally observed findings are compared with the standard findings and shown that they are resembling closely. Novelty/Applications: In order to highlight the scope of our work, the important applications, of the molybdenum disulphide nanostructures are also discussed, that make MoS2nanostructures attractive candidates in fields as diverse as energy, environmental, biomedical and semiconductors.

The need for smaller, more efficient electronic devices has made two dimensional (2-D) materials a focal point of research in materials science. Graphene, the most prominent member in the family of 2-D materials, has been a good candidate for nanoelectronics owing to its exceptional electronic behaviour -a characteristic quantum Hall effect, high thermal conductivity, high electron mobility, superior elastic strength and optical performance, and exploitable thermal stability. However, it is still far from being an ideal material for small scale transistors, owing to the absence of an electronic band gap (12)(13)(14)(15)(16)(17)(18)(19)(20)(21)(22) . Efforts focused on finding an alternate material has led to the enhanced prominence of a whole new variety of 2-D materials, among which transition metal dichalcogenides (TMDCs) such as molybdenum disulphide have been quite popular.
Molybdenum disulphide belongs to the family of transition metal dichalcogenides, materials having the generalized formula MX 2 (M = transition metal, X = chalcogen (S, Se, Te)), with electronic properties resembling those of semiconducting, metallic and superconducting materials. MoS 2 nanostructures from monolayers to bulk have been fabricated on a various number of rigid as well as flexible substrates by utilizing a variety of methods such as mechanical exfoliation, electrochemical exfoliation, thermal vapour sulphurisation of molybdenum or CVD of molybdenum oxide (MoO 3 ) and decomposition of thiomolybdates (23)(24)(25)(26)(27)(28) .
In the presented work, we have synthesized two MoS 2 thin film samples using Thermal Chemical Vapour Deposition. All the samples were then characterized using X-Ray Diffraction (XRD) method. Hence, the structural analysis of thermal CVD synthesized MoS 2 nanofilms is presented in this report. The final aim is to judge how closely our results match to the standard values of structural parameters of MoS 2 , and thus comment on its usability in commercial applications.

Materials and Methods
In order to synthesize MoS 2 thin films, commercially procured high purity molybdenum trioxide (MoO 3 ) (VI) powder and sulphur (S) flakes were used as precursors in a horizontal quartz tube furnace CVD system. A schematic diagram of the thermal Chemical Vapour Deposition apparatus for the growth of molybdenum disulphide is given in Figure 1 .
All the vessels, beakers and crucible used during the sample preparation underwent a standard cleaning procedure: prior to its insertion in the reaction chamber, the quartz tube was thoroughly cleansed with sulphuric acid and the substrate holder was washed with an organic solvent (acetone) and put under ultrasonic cleaning. The ultrasonic cleaning was performed in acetone for about 15-20 minutes to remove all the residual dirt from the material. MoO 3 powder and S flakes were kept in separate ceramic crucibles separated by a distance of 15 cm. Sulphur was placed upstream in the furnace and MoO 3 downstream, in the central zone. Substrate was placed facing down towards the reactants. Reaction by-products were removed by the carrier gas, argon (Ar). After purging the quartz tube with Ar gas for 15 minutes, the temperature of the quartz tube was raised to 700 o C while keeping the flow rate through the tube at 200 sccm.
The tube was maintained at the mentioned temperature for 30 minutes to allow complete sulphurisation of MoO 3 by thermal evaporation. Later, the CVD system was turned off keeping the flow rate constant, to allow the temperature of the quartz tube to come down to room temperature. This process took around 10 hours. After the completion the carrier gas flow was turned off and the crucibles were taken out. The crucible containing the substrate was found to have grown a thin film of MoS 2 along with powder form of MoS 2 during the reaction.
The MoS 2 samples prepared by CVD have been characterized by X-ray Diffraction to determine information about their crystal structures. For this purpose, a commercial X-Ray diffractometer (model number D8 Advance, DAVINCI design) from Bruker has been employed. Equipped with a 60-90 position sample changer, it consumes 1600 W power for XRD measurements and is used in Bragg-Brentano parafocusing geometry. This results in both high resolution and high intensity of the diffracted beam. https://www.indjst.org/

X-Ray Diffractograms
X-ray Diffractograms were plotted, as shown in Figure 2 , on the basis of XRD data to confirm the crystallinity, phase purity and structure of the synthesized samples of MoS 2 . In the plotted diffractograms, a range of angles 2θ with the most prominent peaks were selected. Gaussian line profiling was performed for each peak using Origin software and the full width at halfmaximum (FWHM) of the profile, area, and height values calculated for each Bragg line. This information was used to reveal the correlation between microstructural parameters of the prepared nanofilms and the standard values.  Table 1 .
For hexagonal structure, plane spacing d is related to lattice constants a, c and miller indices by the relation: Combining with Bragg's law with this equation and rearranging with n = 1 approximation gives: For hexagonal lattice cells, 'a' is calculated looking at planes of type hk0. Thus, (3) gives: Similarly, 'c' can be calculated by looking for 001 type planes and substituting h = k = 0 to obtain: The diffraction peak width contains microstructural information about the material as peak broadening is caused by nanocrystallite size and additionally, may also be a result of lattice strain and lattice defects. This broadening is quantified in terms of Full Width at Half Maximum (FWHM) and integral breadth depending upon the type of curve fitting. We have used FWHM values generated by our software for each peak and used them in the Scherrer formula to get the value of crystallite size, D: Where, K is the Scherrer constant, β is the diffraction peak width (in radians) and θ is the Bragg angle. Microstrain or lattice strain is defined as the local deviation of d-spacings from the average value, which translates into variation in diffraction angles and results in diffraction peak broadening. The origin of these point defects may be interstitial or missing atoms, or substitution by different atoms; dislocations or twin boundary defects. The Williamson Hall method that we have used to calculate microstrain in this paper considers strain induced and crystallite size induced peak broadening as independent of each other. The diffraction line broadening being a result of strain induced as a result of crystal distortion and imperfection is calculated using the formula: ε = β 4 tan θ (7) ε Is the microstrain and β is the broadening Therefore, the strain-related diffraction peak broadening (β ε ) is inversely proportional to tanθ , whereas peak broadening resulting from crystallite size (β D ) varies as the inverse of cosθ . The total peak broadening is calculated as a sum of β ε and β D after accounting for the instrumental correction: Rearranging this equation gives the Williamson-Hall Equation: β hkl cos θ = Kλ D + 4 ∈ sin θ This equation results from a uniform deformation model that assumes that the crystal is isotropic in nature, i.e. crystallographic direction of measurement does not affect the value of strain.
For calculating values of lattice constants, plane (002) of the type (00l) was used to find 'c' in both the samples. Then another plane (102) of the type (h0l) for the first sample, and (100) of the type (h00) for the second sample were used to find 'a' after substituting the obtained value of c into (3). These values are tabulated in Table 2. The calculated parameters have been compared with the typical standardized values in Tables 3 and 4 , for validation of the results. The major peaks obtained in our XRD pattern are reasonably consistent with the standardized peak values for MoS 2 nanosheets. The calculated values of cell parameters of the crystal structure of MoS 2 (a = b and c) also agree with the typical values. The existence of a high intensity peak at (0 0 2) in both the samples implies periodicity in c-axis, which is indicative of multi-layer MoS 2 nanosheets on SiO 2 substrates.

Applications
MoS 2 finds application in a diverse range of fields due to its novel and desirable electronic properties and chemical structure. It has been shown that multilayer plasma-treated MoS 2 transistors can act as affordable, highly durable, non-volatile memory devices. Moreover, the combination of two-dimensional materials like MoS 2 and graphene with high-κ dielectric materials can lead to the fabrication of non-volatile memory devices that possess increased memory window as well as stable retention, while also allowing multibit information storage (29,30) . A film composed of the combination of graphene oxide (GO) nanosheets with 2-D MoS 2 has been used to synthesize memory devices. The MoS 2 enhances the film conductivity, facilitating migration of oxygen in GO. This device has displayed non-volatile, rewritable, bistable switching with low voltage and an optimally high value of ON/OFF current ratio. Another MoS 2 -graphene composite has shown superior electrochemical performance, which can serve as an application for anode material in lithium-ion batteries. This composite consisted of a MoS 2 -coated 3D network of graphene and also showed superior high-current-density performance.
This composite also has applications in other areas of clean energy (31,32) . Owing to its unique structure and superior mechanical, biological and physicochemical properties, MoS 2 nanosheets can be used for various environmental applications related to water, such as membrane-based separation, contaminant absorption, or as disinfectants for biomedical devices, utilizing their antibacterial properties in absence of light (33) . Combining MoS 2 with other semiconductor materials to alter its bulk-to-single layer structure can enhance the photocatalytic performance. Different types of MoS 2 nanoparticles based on their structural components have been studied for their properties and applications in photocatalyst systems (8,34) . https://www.indjst.org/ Ultrasmall nanodots, complex nanostructures and printed 3-D nanoarchitecture forms of MoS 2 have been shown to be effective for photothermal therapy due to their desirable properties such as low cytotoxicity and stability. Using fluorescent quantum dots and arginine-glycine-aspartic (RGD) containing peptides has been used to produce functionalized single-layered MoS 2 nanosheets that have shown remarkable properties of fluorescence, photothermal conversion and cancer-targeting. MoS 2 -PEG can be used for fast targeted drug delivery as it shows minimal toxicity to cells (35) . Nanoparticle forms of MoS 2 have also shown promising behaviour as potential tools for treatment of cancer and Alzheimer's, with varying amounts of toxicities in different Mo based nanoforms (36) .
Mechanical flexibility of MoS 2 also makes it desirable for use in flexible electronics. It has been shown that mono and multi-layer MoS 2 can be used in conventional and tunnel field effect transistors, owing to their direct and indirect band-gap respectively. The effect of varying sulfurization temperature on the annealing of MoO 3 film has been studied in detail to obtain 2D MoS 2 films spanning a large area for nanoelectronic applications such as ferroelectric FETs (FeFET) memory (37) . Utilizing the optical properties of MoS 2 appropriately, an MoS 2 based photodetector has the ability to detect photons in the visible range (38) . MoS 2 based optoelectronic devices, when irradiated with UV -visible light, display useful properties for application in room-temperature optoelectronic NO 2 sensors. MoS 2 enriched with sulphur-vacancies, upon functionalization with ZnO quantum dots, showed superior performance, stability, fast response rate, reliable selectivity, full reversibility as well as subppb detection to NO 2 and humidity resistance at room temperature (39) . Moreover, apart from finding use in NO 2 sensors, a nanocomposite film incorporating indium oxide (In 2 O 3 ) and MoS 2 , in a nanccube or flower-like layer-by-layer synthesized structure,has found specific applications as a formaldehyde vapour sensor at room temperature (40) .

Conclusions
Molybdenum disulphide (MoS 2 ) nanofilms are successfully synthesized on silicon dioxide substrates by the thermal Chemical Vapour Deposition (CVD) technique, using molybdenum trioxide (MoO 3 ) (VI) powder and sulphur (S) flakes as precursors. Then, the CVD grown MoS 2 samples are characterized using X-ray Diffraction method in order to study the structural analysis of the prepared samples. As the obtained XRD-peaks and lattice parameters of the MoS 2 nanosheets are in accordance with the standardized values, and the calculated values of cell parameters of the crystal structure of MoS 2 (a = b and c) also agree with the typical values, the sheets have been grown properly via CVD mechanism and characterization through XRD provides information about the grown sample. The existence of a high intensity peak at (0 0 2) in both the samples implies periodicity in c-axis, which is indicative of multi-layer MoS 2 nanosheets on SiO 2 substrates. This report also calculates the microstrain and dislocation density to provide an insight into the defects present in the grown sample.
Research interest towards incorporating MoS 2 owing to its structure and properties has been steadily mounting since the past few years due to its novel applications as hybrid structures in next-generation devices. A few of these applications have been discussed in brief, each of which can be greatly benefitted by the wide range of benefits offered by MoS 2 structures. While the future scope of applications of MoS 2 is extensively varied, the authors of this paper intend to specifically work on incorporating MoS 2 in memory devices as well as solar cells for obtaining novel benefits in the field of energy.