Applied Science and Convergence Technology 2020; 29(6): 133-142
Published online November 30, 2020
Copyright © The Korean Vacuum Society.
Seoung-Woong Parka,b, Yong Jun Joa, Sukang Baea, Byung Hee Hongb, and Seoung-Ki Leea,*
aFunctional Composite Materials Research Center, Institute of Advanced Composite Materials, Korea Institute of Science and Technology (KIST), Jeonbuk 55324, Republic of Korea
bDepartment of Chemistry, Seoul National University, Seoul 08826, Republic of Korea
Correspondence to:E-mail: firstname.lastname@example.org
Transition metal dichalcogenides (TMDC) have been identified as excellent platforms for developing the next-generation commercial flexible logic devices and sensors, owing to their outstanding mechanical, optical, and electrical properties. The TMDCs can be used to produce novel form-factors for wearable electronic devices. Typically, synthesis of large-scale TMDC thin film have been achieved by complexity vacuum-based approach. Therefore, it is essential to develop a simple and effective method to boost-up mass production of TMDC thin films on a large scale upon arbitrary substrates. In this regard, the solution-based TMDC synthesis method is advantageous because it proposes a simplification of the fabrication processes and an easy scaling-up of the material with a non-vacuum system. In this review, we summarize the evolution of the solution-based thin-film preparation and synthesis of the TMDCs; subsequently, we discuss the merits and drawbacks of the recently developed methods to form TMDC thin films directly from the deposited precursor. Finally, we discuss the practical applications of the TMDC thin films, which demonstrate the feasibility of their commercialized applications in electronic devices and sensors.
Keywords: Transition metal dichalcogenide, Large-scale synthesis, Solution-based process, Transistor, Sensor, Hydrogen evolution reaction
Two-dimensional (2D) materials provide a versatile platform for investigating various electronic and optoelectronic phenomena. Since the exfoliated graphene was demonstrated in 2004, 2D materials have been one of the central research topics in the last two decades in the field of material science , physics , and electronics, [3,4] providing. Various merits and applications of 2D materials have been verified through numerous analytical methods. In the next step, the synthesis of large-area 2D materials for their commercialization has become the focus of subsequent investigations. Bae et al.  developed the method of scaling-up of graphene, grown up to 30 inches by the chemical vapor deposition (CVD) method. Since then, various progressive strategies have been introduced for the production of highquality graphene, even by companies, such as Sony  and Samsung . The successful scalability of graphene boosted the mass production of other 2D materials, which belong to the family of transition metal dichalcogenide (TMDC) . The TMDCs, which have semiconducting characteristics, are preferred as the target materials for developing the next-generation electronic devices because of their unique properties, such as direct or indirect band gap modulation , quantum-confinement , transparency , and flexibility . Therefore, the uniform synthesis of TMDCs on a large-scale is important to accelerate their mass production [13,14]. Large-area TMDC thin films can be successfully fabricated via CVD, which can be used to grow vertically or horizontally stacked heterostructures [15–17]. However, the CVDbased synthesis is limited by several factors, such as the requirement of high temperatures (≈ 1000 ℃), difficulties in modulating the deposited film thickness to a desired value, and long processing time, which cause delay in the material preparation . Recently, solutionphase deposition methods have been reported, which have several technical advantages, such as relatively low processing temperature , compatibility with various substrates, including polymer film (e.g. polyimide (PI)) , easily controlled layer thickness , rapid synthesis, and scalability with the help of existing coating techniques . Recently, the lithography-free approach has been developed to form patterns directly on a TMDC film [22–24]. In this paper, we review an industrially applicable solution-based TMDC synthesis method, involving trial and error of the solution-phase deposition and its underlying mechanisms. Further, we discuss the corresponding applications, such as in transistors, sensors, and diodes, in detail. The achievements of the advanced synthesis method include large-scale fabrication of novel TMDC thin films for industrial applications.
Solution-phase synthesis of TMDC thin film is generally performed in several steps: the preparation of a precursor solution, deposition of the precursor, and conversion of the precursor’s chemical structure. The thickness or uniformity of the synthesized TMDC film can be adjusted by controlling the precursor concentration, solution composition, and coating environment. In this section, we review the development of various deposition and synthesis methods for fabricating TMDC thin films.
The solution-deposited TMDC compounds are usually synthesized by thermolysis of a precursor [A]
For example [Fig. 1(a)], synthesis of MoS2 requires an ammonium thiomolybdate ([NH4]2[Mo3S13•2H2O]) precursor and a two-step ther- molysis . The precursor dissolves in a solvent as an anion [Mo3S13]2- and a cation [NH4]2+. In the first step, at temperatures 120 ≤
As an alternative precursor, ammonium tetrathiomolybdate (ATM, [NH4]2MoS4) is used to facilitate thermal decomposition for the fabrication of the MoS2 film  [Fig. 1(b)]. In the ATM structure, [NH4]2 is a cation; Mo maintains the bonding as the center of the pre- cursor, and S4 maintains the precursor structure. Prior to the two-step thermolysis, ATM is deposited on a specific substrate and all residual solvents are removed by heating at 100 ℃. Next, amorphous MoS3 is synthesized at 120 ≤
The Raman spectral characteristics are generally used to analyze the intrinsic property of the TMDCs (MoS2, WS2, MoSe2, WSe2, and etc). Liu et al. [12,27] comparatively analyzed the Raman spectrum of the thermally decomposed MoS2 films on sapphire substrate, under Ar and Ar+S atmosphere. Figure 1(c) shows two representative Raman absorption peaks, which are ascribed to the E1 and A1g modes of the MoS2 vibration. The E1 2g mode reflects the in-plane vibration and A1g mode reflects the out-of-plane vibration of the MoS2 as the difference in the energy level of electron from Raman absorption. A higher intensity of the Raman peak implies a better quality of the MoS2 film. Therefore, the sulfur gas aids in the filling of the sulfur vacancy, thereby enhancing the quality of the MoS2 film during thermolysis. In Fig. 1(d), the frequency difference between the A1g and E1 2g modes (∆ω=ωA1g-ωE2g) can be used to identify the number of layers of the synthesized MoS2 [Fig. 1(d), bottom]. The Raman mode spacing is considerably narrow (∆ω ≈ 25 cm−1), which indicates that the MoS2 film is composed of five layers; a mono layer has ∆ω ≈ 16.5 cm−1 [28,29]. As the number of layers of the synthesized MoS2 approaches one, this synthesized monolayer develops a direct band gap and has optical properties; it absorbs light of a specific wavelength (672 nm). Figure 1(e) shows the photoluminescence (PL) of a tri-layer MoS2 film, thermally decomposed on a sapphire substrate. The PL peaks also exhibit a stronger intensity when Ar+S gas mixture is injected; the result indicates that a MoS2 film of better quality is formed. Next, X-ray photoelectron spectroscopy (XPS) analysis was conducted to identify the chemical composition of the thermally decomposed MoS2 film [Fig. 1(f)]. The Mo 3d shows two peaks at 229.3 and 232.5 eV, which are attributed to Mo 3d5/2 and S 2s peaks, indicating that the chemical composition of the MoS2 film exists during the 2H phase formation. The S 2p peaks, shown in the inset, indicate the intra- molecular bonding of divalent sulfide ions (S2-). Accordingly, the two distinguished peaks are observed approximately at 163.3 and 162 eV, corresponding to the S 2p1/2 and S 2p3/2 orbital splits. As a result, it is possible to verify the intrinsic chemical composition of MoS2 through the thermolysis of [NH4]2MoS4. Additionally, the thickness modulation of a synthesized MoS2 film can be demonstrated by adjusting the concentration of the ATM precursor . Figure 1(f) shows the atomic force microscopy (AFM) image of a MoS2 thin film, synthesized through a two-step thermolysis reaction with different concentrations of ATM precursor. Thus, a MoS2 thin film, with one (0.65 nm) to a few layers (3.2 nm), can be synthesized by a two-step thermolysis method.
To synthesize a TMDC film from a solution on an arbitrary substrate (
Figure 2(a) illustrates the spin-coating strategy to obtain wafer- scale MoS2 thin films through the dissolution of (NH4)2MoS4 in n- methylpyrolidone (NMP) . Spin coating is widely used to deposit photoresists during the semiconductor manufacturing; the coating thickness can be controlled easily by varying the rotation speed. Therefore, using the spin coating method, the precursor film thickness can be controlled by varying the concentration of the solution and rotating speed, thereby yielding precisely tuned mono-/bi-/tri-layers of the thermally decomposed MoS2 film. Spin coating is advantageous for thickness control; however, only one solvent is used; therefore, the wetting of a substrate is low. As a result, the viscosity or surface tension of a precursor cannot be controlled easily. Therefore, this method is useful only to synthesize TMDC films on a relatively small scale.
Although the solution-engineered precursor deposition method increases the film area, this method has several drawbacks, such as low production yield, differences in the crystal growth depending on the substrate, and high production cost. Owing to these limitations, Lim
To use a TMDC thin film in a practical electronic device, the fabricated film should be isolated to form a source–drain array. Therefore, a TMDC pattern is typically fabricated via the additional photolithography or oxygen plasma treatment-based mask process. To simplify this process, the precursor deposition and synthesis methods have been developed to form TMDC patterns directly, without the additional patterning step. Lee
TMDC thin films obtained from a solution (e.g., MoS2, WS2, WSe2) can be synthesized on a large scale using simple thermal decomposition processes and therefore have a variety of applications in electronic devices. In this section, we review the electronic devices and sensors that have been developed by using the solution-based synthesis methods.
One of the key characteristics of a TMDC thin film is its semiconducting property. A TMDC thin film has a tunable band gap depending on the number of its layers [35,36]. Liu
Shifting the perspective to the optoelectronic properties of the solution-based MoS thin films, Lim
Another solution-based TMDC application has attracted considerable attention in the field of catalysis. Deng
The previously reported methods of TMDC-based strain sensor fabrication used thin films grown using CVD, with several subsequent photolithography steps to pattern the strain gauge [41,42]. Park
Direct-pattern formation by photothermal decomposition is used to fabricate self-powering haptic sensors that use an MoS2 active layer. Park
This review covers a wide range of large-scale, inexpensive, and simple methods for synthesizing TMDC thin films from solutions with direct-pattern formation, in addition to elucidating their practical applications. Various precursor deposition methods have been developed to increase the film area and modulate the solution composition, which yields the appropriate coating conditions for large-scale fabrication. Thus, the thickness of the TMDC film is easily modulated by adjusting the precursor concentration. Notably, direct formation of the TMDC patterns is also achieved using the solution-phase pre- cipitation and photothermal decomposition. These processes yield vertically stacked TMDC-based heterostructures, without complex processing steps. The synthesis of TMDCs from solutions can be practically applied to readily fabricate devices, such as FETs, photo- detectors, HER catalysts, diodes, and mechanical sensors.
However, the TMDC thin film synthesis method covered in this review is still problematic to be applied to high performance electronic devices because of the small TMDC grain size. Therefore, the development of another methodology for the mass production of highly crystallinity of TMDC thin films is crucial for their applications in the semiconductor industry and researches.
This research was supported by the Korea Institute of Science and Technology (KIST) Institutional Program and supported by the Tech-nology Innovation Program (20011317) funded by the Ministry of Trade Industry & Energy.