A. MoS₂ film growth by two-step method

MoS₂ growth in our lab using the two-step method [Fig. 1(a)] involves the room temperature deposition of parent Mo films on Si/SiO₂ substrates as the first step. Mo films with different thicknesses were prepared by controlling the sputter deposition time. A high-purity (99.99%) Mo metal target of 50 mm in diameter was used for sputtering Mo thin films. The substrates were initially cleaned thoroughly with acetone in an ultrasonic bath, followed by cleaning in ethanol, methanol, and deionized (DI) water. The substrates were fixed on the heater, and the chamber was evacuated to a vacuum level of 10⁻⁷ Torr. Before every sputtering run, the target was presputtered for 5 min to ascertain the same state of Mo target for each sample. In the second step, the deposited Mo films were subsequently placed in a low-pressure CVD system (graphene square CVD) equipped with a 4 inch diameter quartz tube furnace. A ceramic boat containing pure sulfur (∼1 gm, Sigma Aldrich, St. Louis, Missouri) was placed in the upstream of the quartz tube. Argon was used as a carrier gas to convey sulfur vapor species to the Mo films. The tube was pumped down to a pressure of 10⁻³ Torr and flushed with Ar gas repeatedly to guarantee a favorable growth atmosphere. In the flow atmosphere of 200 sccm Ar with a chamber pressure of 5 Torr, the furnace was heated to 600 °C at the center zone in 30 min. After 60 min, the furnace was cooled down naturally to room temperature. Figures 1(b) and 1(c) show the optical images of large area MoS₂ film grown on Si/SiO₂ and quartz substrates, respectively. These films can be further transferred to any arbitrary substrates using KOH solution and DI water. The sample names S₁, S₂, S₃, S₄, and S₅ are used for Mo films deposited for 4, 10, 30, 60, and 180 s, respectively.

FIG. 1.

(a) Schematic showing the two-step growth method; Step 1: Deposition of Mo film using magnetron sputtering, Step 2: Sulfurization of Mo films in a CVD furnace. Large area growth of MoS₂ on (b) Si/SiO₂ and (c) transparent (quartz) substrate.

B. Characterization of grown MoS₂ film

MoS₂ films synthesized by using the two-step method were characterized to confirm their thickness, crystalline quality, and uniformity over the wafer scale. Figures 2(a) and 2(b) show the atomic force microscopy (AFM) image and corresponding height profile of MoS₂ film for sample S₁. A step height of 0.728 nm infers the presence of SL MoS₂. To further confirm the thickness, cross-sectional high resolution transmission electron microscopy (HRTEM) image was recorded as shown in Fig. 2(c). However, the HRTEM image represents a SL MoS₂ thickness, but the film was not uniform over the entire region of the cross-section. The formation of voids or breakage as seen in the image could be attributed to very low sputter deposition time and polycrystalline nature of the MoS₂ film. Raman spectroscopy is an excellent tool to address the evolution of structural properties in layered materials changing their thickness from the 3D bulk to 2D van der Waal layered structures. ^{Reference Bromley16} The Raman spectrum of bulk MoS₂ has two vibration modes: an in-plane (E_2g) mode located around 383 cm⁻¹ and an out-of-plane (A_1g) mode, which is located at 407 cm⁻¹. The difference of frequencies between these two modes could be used to identify the number of layers in MoS₂ films. Figure 2(d) shows the Raman spectra of MoS₂ films with different thicknesses, measured at 532 nm excitation laser line. A frequency difference (Δk) of ∼19.7 cm⁻¹ observed in sample S₁ confirms a SL MoS₂, ^{Reference Mak, Lee, Hone, Shan and Heinz17} whereas this frequency difference rises with increasing number of layers, indicating the modulation of MoS₂ film thickness. The thickness of the MoS₂ film corresponding to sample S₂, S₃, S₄, and S₅ as estimated by AFM height profile measurements was found to be 3.38, 5.48, 7.11, and 12.20 nm, respectively [Fig. 2(e)]. We also developed a simple and nondestructive method to identify the number of MoS₂ layers by using the optical contrast of thickness modulated MoS₂ films. The detail study of this method is published elsewhere. ^{Reference Park, Choudhary, Smith, Lee, Kim and Choi18} Figure 2(f) shows the optical images of MoS₂ films grown on Si/SiO₂ substrate taken by Nikon Eclipse ME600 microscope (Melville, New York). It is to be noted that the color of MoS₂ film changes from purple to green with increasing number of MoS₂ layers. The change in MoS₂ color could be due to the change in optical band gap energy and reflectance with changing thickness. ^{Reference Yim, O'Brien, McEvoy, Winters, Mirza, Lunney and Duesberg19} The facile optical contrast and color methods of thickness identification could be applied to a variety of substrate systems. The detail method for identifying MoS₂ thickness on different substrate systems is under investigation.

FIG. 2.

MoS₂ characterization fabricated by two-step method (a) AFM image and (b) height profile of MoS₂ film on SiO₂ substrate (sample S₁), (c) cross-section HRTEM image of S₁ showing voids and discontinuous film, (d) Raman spectra of MoS₂ film with different thicknesses. Peaks shifts apart with increasing thickness, (e) AFM height profiles of MoS₂ films with different thicknesses (S₂–S₅), and (f) optical images showing the color variation in MoS₂ films with increasing thickness from sample S₁ to S₅.

C. Field effect transistors

One of the most important application for semiconducting MoS₂ is in FETs for next generation electronic devices. ^{Reference Yoon, Park, Bae, Kim, Jang, Hyun, Lim, Kahng, Hong and Lee20} There have been several efforts in fabricating MoS₂ FETs using mechanical exfoliation and CVD methods but the mobility on SiO₂ substrates is typically characterized at low value of 0.01–10 cm²/(V s). ^{Reference Li and Li21,Reference Zhan, Liu, Najmaei, Ajayan and Lou22} Radisavljevic et al. ^{Reference Radisavljevic, Radenovic, Brivio, Giacometti and Kis6} used HfO₂ gate dielectric for SL MoS₂ transistor and reported an outstanding mobility of 200 cm²/(V s) and current on/off ratio of 1 × 10⁸ at room temperature. However, the fabrication of MoS₂ FET devices by using above methods resulted in non-uniform small flakes without any control over the film thickness. Therefore, it is imperative to develop new fabrication methods for scalable and layered controlled synthesis of MoS₂ films for commercial electronics applications.

The wafer scale and thickness modulated MoS₂ films fabricated by using our two-step method were characterized for electrical performance. Figure 3(a) shows the variation of electrical resistance with increasing MoS₂ film thickness, measured at room temperature. It could be observed from the graph that sample S₁ shows very high resistances while the resistance decreases with increasing MoS₂ thickness (S₂ & S₃) and shows a metallic conductance for thicker MoS₂ films (S₄ & S₅). The very high resistance in sample S₁ could be attributed to the discontinuity or void formation and large spatial constraints imposed by the underlying substrate. The electrical measurement for this sample did not show any FET behavior. However, a good FET behavior with p-type conduction was observed in sample S₂ as shown in Fig. 3(b). The mobility and current I _on/off recorded for sample S₂ were ∼12.24 cm²/(V s) and ∼1.56 × 10⁶, respectively. ^{Reference Choudhary, Park, Hwang and Choi23} Our results are better than many previous reports on mechanically exfoliated and CVD grown MoS₂ atomic layers. Generally, most of the MoS₂-based FETs showed n-type behavior but the origin of the electron doping is still unclear and need more investigation. On the other hand, there are only few studies showing a p-type conduction in MoS₂. ^{Reference Zeng, Yin, Huang, Li, He, Lu, Boey and Zhang24} We believe that the p-type behavior might have originated from the localized trap states that are expected to be present at the interface of the Si/SiO₂ substrate and MoS₂ film, which tends to relocate the Fermi level of the SiO₂/MoS₂ system just below the valence band maxima, making the system a p-type semiconductor. For higher MoS₂ thickness (12.29 nm), a conducting electrical behavior with a high value of drain current was observed in the FET measurements. It could be attributed to the presence of unsulfurized Mo films at MoS₂-substrate interface. Hence, there is a critical thickness limit upto which Mo can be fully sulfurized by using the same set of experimental conditions. Figure 3(c) shows the cross-section image of MoS₂ film from sample S₂ demonstrating high uniformity and complete sulfurization of the Mo film.

FIG. 3.

(a) Electrical resistance of MoS₂ film with varying thickness, (b) FET properties shown by sample S₂, and (c) HRTEM cross-section of 2–3 layers MoS₂, indicating a complete sulfurization and uniform MoS₂ growth.

A. MoS₂ film growth by direct deposition

2D MoS₂ also has been attracted as a promising supercapacitor electrode material due to its intriguing sheet-like structure that provides large surface area for double layer charge storage and the presence of multiple oxidation states in the Mo atom facilitating pseudocapacitance by the intercalation of electrolyte ions (H⁺; Li⁺) into S–Mo–S nanosheets. However, relatively thick films are needed for the sufficient charge storage and for the reliable performance of the material. ^{Reference Yang, Fei, Ruan, Xiang and Tour25} We found that the direct deposition of MoS₂ by sputtering is an efficient method to make a desired structure of MoS₂ for a large scale supercapacitor electrode on any substrate including flexible substrates. Hence, we fabricated a flexible MoS₂ thin film electrode and characterized its performance in supercapacitor. It is also noted that this method offers the possibility of MoS₂ deposition on complex surfaces with high uniformity on larger cross-section and better adhesion as compared to previously reported results on chemically exfoliated or CVD grown films. In addition, these methods are accompanied with residues and contaminants. Figure 4(a) shows the schematic of MoS₂ deposition directly on various substrates including Cu foil, Si/SiO₂, and polyimide. The MoS₂ films were deposited at low substrate temperature of 300 °C by direct sputtering of MoS₂ target (99.8% purity) at a radio frequency (RF) power of 60 W in an inert gas plasma (Argon ∼99.999% purity).

FIG. 4.

(a) Schematic diagram showing the direct sputter deposition of MoS₂ films, (b) HRTEM image clearly shows the formation of high quality MoS₂ film by sputtering method, (c) schematic diagram showing the potential of direct deposition on flexible substrates. Camera images demonstrating MoS₂ film grown on polyimide substrate in our lab, (d) XRD patterns revealing 2H and 3R MoS₂ structure in MoS₂ films deposited on Si/SiO₂ substrate and Cu foil, respectively (peaks shown by asterisk are from Cu substrate), and (e) cross-sectional HRTEM and morphology of MoS₂ film on Si/SiO₂ and Cu substrates.

B. Characterization of direct deposited MoS₂ films

Although, there are several studies in the past that utilized sputtering to deposit MoS₂ coatings for solid lubricant applications, ^{Reference Christy26} the reports on the fabrication of MoS₂ films for electronics and energy applications by direct sputtering are sparse. Voevodin and co-workers ^{Reference Muratore, Hu, Wang, Haque, Bultman, Jespersen, Shamberger, McConney, Naguy and Voevodin27} attempted to grow 3–6 molecular layers of MoS₂ on thermally grown silicon oxide and (002) graphite using magnetron sputtering at a growth temperature of only 350 °C. In another study, Alam et al. ^{Reference Alam, Wang, Pulavarthy, Haque, Muratore, Glavin, Roy and Voevodin28} studied the dependence of grain size on temperature by in situ heating of free standing MoS₂ in transmission electron microscopy. In this context, our research group succeeded in fabricating high quality, large scale MoS₂ films on different substrates by using direct sputtering technique. The HRTEM image of Fig. 4(b) demonstrates the formation of high quality MoS₂ films on Cu foil. The thickness of individual layer was found to be 0.685 nm, which is the typical thickness of SL MoS₂. ^{Reference Yu, Li, Liu, Su, Zhang and Cao29} The possibility of low temperature deposition (∼300 °C) enabled us to grow MoS₂ layers on other flexible substrates like polyimide [Fig. 4(c)]. Interestingly, we found that MoS₂ films deposited on different substrates exhibited different crystal structures. It is clear from the X-ray diffraction (XRD) spectra of Fig. 4(d) that MoS₂ films grown on Si/SiO₂ substrates exhibited a hexagonal (2H) phase while a rhomobohedral (3R) phase was indexed by the XRD results when MoS₂ was deposited on Cu foil. The formation of different crystalline structures in our MoS₂ films could be attributed to the fact that the crystalline structure of sputtered deposited MoS₂ films depends on the chemically active sites, surface smoothness, and roughness of the substrates. ^{Reference Bertrand30} In our case, SiO₂ surface is much smoother as compared to the Cu foil and therefore possess high chances of layered structure growth (2H) parallel to the substrates. On the other hand, the uneven morphology in Cu foil with large surface roughness may promote the growth of nanoclustered MoS₂ films in highly un-equilibrium conditions and leads to a less stable (3R) polytype. Figure 4(e) shows the HRTEM cross-section and morphology of MoS₂ film on Si/SiO₂ and Cu substrates, respectively. It is observed that MoS₂ film crystallized into a well layered (2H) structure on Si/SiO₂ while a random distribution of MoS₂ layers with a un-equilibrium structure is formed on Cu foil, which is consistent with the XRD results.

C. MoS₂ for supercapacitors

The MoS₂ films deposited on Cu foil were used as supercapacitor electrode and its electrochemical performance was evaluated by using cyclic voltammetry (CV). Charge storage in MoS₂ thin films can be done by three ways: (a) double layer charge storage by individual interlayer of MoS₂, (b) intra S–Mo–S double layer charge storage due to diffusion of electroactive species into the basal plane, and (c) Faradic charge transfer due to several oxidation states of Mo atoms ranging from +2 to +6 exhibiting pseudocapacitance due to efficient intercalation of alkali ions (H⁺, Li⁺, Na⁺, and K⁺). ^{Reference Soon and Loh31,Reference Choudhary, Patel, Ho, Dahotre, Lee, Hwang and Choi32}

Figure 5(a) shows the CV results of our MoS₂ electrode at different scan rates ranging from 10 mV/s to 500 mV/s. The elliptical S-shape curve of the CV indicates the combination of double layer charge storage and pseudocapacitance mechanism in our electrode. ^{Reference Alam, Wang, Pulavarthy, Haque, Muratore, Glavin, Roy and Voevodin28} The observed deviation from the ideal rectangular characteristics is mainly attributed to large polarization of electrode and physical/chemical adsorption at the interface. Furthermore, galvanostatic charge/discharge tests were performed at a current density of 25.47 mA/cm². An excellent electrochemical performance with a capacitance of ∼30 mF/cm² and cyclic stability up to 4000 cycles could be observed in Fig. 5(b). The existence of linear and triangular charge/discharge profile at different current densities [inset of Fig. 5(b)] represents the dominance of double layer charge storage mechanism. A small equivalent series resistance of only ∼3 Ω was also calculated from the Nyquist plot of our electrodes (not shown here). The excellent electrochemical performance observed in our MoS₂ electrodes could be attributed to the open porous structure with a 3D MoS₂ island structure shown by the HRTEM and AFM image of the top surface of the MoS₂ film [Fig. 5(c)]. This unique structure facilitates ample open channels for the large ion transport and penetration of electrolytic ions into the pores leads to high capacitive performance. Figure 5(d) compares the volumetric power and energy densities of our MoS₂ electrode with other commonly used power sources. It is observed that the power density of our supercapacitor electrode (∼80 W/cm³) surpasses the previously reported values and the volumetric energy density (1.62–2.33 mWh/cm³) exceeds conventional activated carbon and electrolytic capacitors. The interesting electrochemical properties of MoS₂ film makes it attractive as a promising electrode for the next generation supercapacitor energy storage devices.

FIG. 5.

(a) CV curves at different scan rates in 0.5 M H₂SO₄ electrolyte, (b) capacitance variation with cycle number a current density of 25.47 mA/cm². The inset shows the charge–discharge profiles at different current densities, (c) AFM and HRTEM image showing the formation of unique 3D porous MoS₂ structure, and (d) Ragone plot showing comparing the energy and power density of our supercapacitor electrode with different supercapacitor electrodes.