Review of strategies toward the development of alloy two-dimensional (2D) transition metal dichalcogenides

Summary Atomically thin two-dimensional (2D) transition metal dichalcogenides (TMDCs) have attracted significant attention owing to their prosperity in material research. The inimitable features of TMDCs triggered the emerging applications in diverse areas. In this review, we focus on the tailored and engineering of the crystal lattice of TMDCs that finally enhance the efficiency of the material properties. We highlight several preparation techniques and recent advancements in compositional engineering of TMDCs structure. We summarize different approaches for TMDCs such as doping and alloying with different materials, alloying with other 2D metals, and scrutinize the technological potential of these methods. Beyond that, we also highlight the recent significant advancement in preparing 2D quasicrystals and alloying the 2D TMDCs with MAX phases. Finally, we highlight the future perspectives for crystal engineering in TMDC materials for structure stability, machine learning concept marge with materials, and their emerging applications.


INTRODUCTION
After discovering graphene, an impressive improvement in fundamental science and technology implementation has been explored using two-dimensional (2D) materials. Over the past few years, a broad range of atomically thin 2D materials, for example, graphene-based 2D materials, transition metal dichalcogenides (TMDCs), transition metal carbides and nitrides (MXenes), layered oxides, 2D metal-organic frameworks, and their layered derivative structures, has been prepared owing to their novel structural properties, extensive applications in electronics, optoelectronics, energy generation/conversion, biomedical technology, and catalysis (Anasori et al., 2017;George et al., 2019;Kochat et al., 2017Kochat et al., , 2018Kumbhakar et al., 2021). Most significantly, 2D layered materials show several physical properties ranging from insulator to narrow-gap semiconductor to semimetal or metal. Multiple structural phases have been observed at the molecular level owing to the variations of crystal configurations. Different synthesis method has been employed to prepare these 2D materials as summarized in several reviews (Mannix et al., 2017). Synthesis strategies for 2D materials can be classified into two broad groups: top-down and bottom-up methods. Among them, mechanical exfoliation, liquid exfoliation, ion-intercalation, chemical vapor deposition (CVD), wet-chemical synthesis, and so on are suitable for producing 2D materials on a large scale. However, for the synthesis of 2D materials, the synthesis parameters are crucial such as in mechanical exfoliation, the peeling methods, mechanical properties of the materials are crucial. In the liquid-phase exfoliation method, the most critical parameters are the choice of solvent because of the surface energy, which can control the exfoliation and aggregation of sheets. In the CVD process, controlling the temperature, precursor ratio, substrate, and gas flow rate also affects the morphology of the synthesized material. Therefore, these traditional synthesis methods have some shortcomings.
Despite the significant potential of 2D materials, controlling the structural, physical, chemical properties, optimization of materials compositions, tuning of defects, etc., are the most daunting challenges for the materials field. Different approaches such as alloying, doping, mixing of multiple atoms, functionalization, etc., have been employed in this context. Owing to the weak van der Waals (vdW) interaction between layers, layered TMDCs with different structural modifications have attracted extensive attention. Recently, creating defects by removing specific atoms on the TMDC structures show many fascinating optoelectronic applications (Yin et al., 2021). The insertion of foreign atoms (dopant) has altered the properties of layered TMDCs. Apart from doping, the engineering of compositions has been demonstrated to be an active method for resolving the challenges.
To date, several reviews have discussed the modern synthesis techniques of 2D materials and their potential applications. However, compositional engineering in 2D TMDCs, such as doping with different metals, alloying with other TMDCs and MXenes, has not been explored so much. This review mainly focuses on the recent progress related to developing novel 2D TMDCs materials and their potential applications. Here, we have discussed the doping of several metal ions in the TMDCs crystal. The used doping techniques for the characterization of the materials are systemically summarized in Figure 1. Then, we discuss alloying of 2D TMDCs with other materials; it can be classified into four subsections: metal replacement, dichalcogenide replacement, metal and dichalcogenide replacement, and multicomponent replacements. The MAX phases are polycrystalline nanolaminates of ternary nitrides or carbides, and their alloying with 2D TMDCs is discussed extensively. After that, the working mechanism of doping/alloying materials has been discussed in detail. After such a detailed analysis of the progress of 2D TMDCs, some challenges and perspectives on the further development of high-quality 2D TMDCs explore. The discussions provide a theoretical basis and technical support for implementing other functional strategies and could be extended to other 2D TMDCs-based optoelectronic devices.

RESULTS AND DISCUSSION
Characteristics of doped 2D transition metal dichalcogenide 2D TMDCs can be expressed as MX 2 , M is a transition metal element such as Mo, Ta, W, and Nb, and X is S, Se, and Te. Doping in 2D TMDC is achieved by either replacing the M site, X site, or interstitial site with a foreign atom. Approach reportedly helps to tune the material to achieve desired characteristics such as better thermal, electrical conductivity, photosensitivity, controlling phonon dispersion, etc. In Table 1, we have summarized the doping of various atoms in 2D TMDCs.
Experimental justification and theoretical studies suggested that substitutional doping of Group I to II in MX 2 will result in an n-type semiconductor with $7%-18% lattice expansion, mostly in the c-direction of the lattice which may indicate the phase transformations. Bulk MX 2 was treated with n-butyl lithium iScience Review (n-Bu Li) to obtain doped MX 2 with alkali metals. Purified Li x MX 2 was exfoliated in deionized water by ultrasonication. Lithium intercalation yields 90% 1T 0 MX 2 after exfoliation and gets transformed to 1T and 2H structure under electron beam microscopy imaging owing to its instability. Bond formation energy predicts the movement of atoms in lattice and final stable structure of new alloy. Bond formation energy of Li with substitutional MX 2 on M site or X site was greater than the bond formation energy in interstitial site. Doping of lithium and sodium in MX 2 resulted in an n-type semiconductor with lattice expansion, for which theoretical studies suggests that intercalation of alkali metals in MX 2 octahedral site is more favorable than tetrahedral site because of higher coordination number and approximately three times larger space. Intercalation in WX 2 lattice also showed increment in interlayer spacing, caused by the change in phase from 2H to 1T phase and Fermi level shift toward up, i.e., valence band maximum to the conduction band Onofrio et al., 2017). Similar phenomena were observed for MX 2 lattice where phase transformation of MX 2 from 2H to 1T is also supported by experimental justification by synthesis of Li intercalation using powder metallurgy route ( Figure 2C) (Padmajan Sasikala et al., 2020). For NbX 2 and TaX 2 , no such phase transfer occurs, but their Fermi level showed moving into a gap that is not suitable for electron transport . In-situ surface doping of rubidium (Rb) atoms in 2H TMDCs (WS 2 , MoS 2 , WSe 2 , and MoSe 2 ) resulted in n-type semiconductor of 0.8-2 eV bandgap for concentration of $ 10 12 cm À2 . Valence and conduction band get progressively closer owing to ionized Rb atoms on the surface of 2H-TMDCs. Rbdoped TMDCs reported causing structural changes because of potential difference between the layers that break inversion symmetry and leads to spin splitting of valence bands .
Furthermore, for Group III elements, the theoretical investigation suggested doping of Ni, Sc in MX 2 lattice may result in the distorted lattice as much as $12% lattice size reduction with MX 2 lattice and with tellurides exhibiting even more significant distortion than other dichalcogenides due to 1T 0 phase transformation. Scdoped MX 2 lattice was semiconducting material with a suggested bandgap of 0.14 eV, whereas Ni was the least stable dopant. It is being estimated that the Sc's atomic radius plays a critical role in its stability than its oxidation state. Recently, Williamson et al. and his group reported that W or Mo doping in MX 2 lattice has minimal impact on the density of state as the difference in atomic radii and oxidation state was near equal, resulting in p-type doping (Williamson et al., 2019).
Substitutional doping of transition elements in 2D TMDCs from group IV to group XII results in better dopants. There is a negligible distortion because of similar atomic radii of element and nearby oxidation state. The substitutional doping of Vanadium (V) in WS 2 and WSe 2 results in p-type with negligible lattice distortion, approximately the same lattice parameters, and low contact resistance. Doping of V in MoS 2 was reported as poor p-type conduction behavior than WS 2 and WSe 2 as the doped sample was determined to have a degenerate state ( Figure 2B) Williamson et al., 2019). Doping of Fe in WS 2 lattice resulted in n-type with defect coupling and lattice distortion similar to Mo-doped WS 2 lattice (Padmajan Sasikala et al., 2020). Doping of Fe in MoTe 2 lattice-induced spins on nearby Te and Mo atoms is parallel to dopants. Similar phenomena were observed for Cr, Mn, Co, and V doped in MoTe 2 exhibiting magnetic properties, and Ni doping exhibited non-magnetic properties (Kanoun, 2018;Tiwari et al., 2021). The atomic doping changes the band structure of MoS 2 layers, and new defect states are introduced near conduction band minima ( Figure 2A). The approach was attempted by the CVD method -VLS growth of layers using molten salts, which was also found to be applicable in liquid precursor-assisted technique of in situ substitutional doping Zhang et al., 2020a). Cu intercalation into 2D TMDC was 1.2 atomic % in NbS 2 . Substitutional doping of Cu in a tetrahedral site is coordinated by S atoms, where doping leads to an upshift Fermi level in-band structure with a 0-0.65 eV bandgap. Similar Cu doping in Group IV and V TMDC 2H led to no Cu species are intercalated into group VI TMDC, such as MoS 2 or WS 2 . Most significantly, substitutional doping of Group XIII to XVI mostly resulted in p-type semiconductors, whereas doping of Group XVII elements resulted in n-type doping, as shown in Figure 2A (Onofrio et al., 2017). Surface doping of oxygen resulted in p-type semiconductor in TMDCs (MoTe 2 , WSe 2 , MoSe 2 , PtSe 2 , and PdSe 2 ) for concentration of $ 10 13 -10 14 cm À2 ( Figure 2D). Isoelectronic substitution of chalcogen atoms, Se to O atoms, induces trapping potential due to electronegativity difference resulting in modulation of free carrier density and conduction type (Liang et al., 2020). Doping of Re, in MoS 2 , resulted in n-type with lattice scattering and distorted 1T phase structure. Monolayer CVD growth was performed at 750 C. Salt precursors mixing were in designated ratios in a tube furnace. As the doping concentration increased, more distortion has been reported contributing to the ReS 2 structure that increased degenerate electron transport and improved conductivity at 2.1 atomic % Re-doping. Leonhardt et al. highlighted the importance of band alignment by demonstrating low-temperature atomic layer deposition of Al x O y in MoS 2 ( Figure 2F) (Leonhardt et al., 2019). Atomic layer deposition of Al x O y on using tetra methyl aluminum and H 2 O precursors results in high doping on synthetic MoS 2 -exfoliated flakes. This approach resulted in electrons donation as the conduction bands' edge of MoS 2 lies at similar or lower energy than Al x O y donor states; it is energetically favorable for electrons from donor states to move into acceptor states of MoS 2 , ReS 2 , and converting them into n-type semiconducting material. This result was not applicable for WS 2 as the WS 2 conduction band lies at higher energy, and electrons cannot be donated.

Characteristics of 2D transition metal dichalcogenide alloys
Alloying is a powerful technique to tailor the structures in order to tune the desired property. Alloying of 2D TMDCs has gained vast popularity over the past few years. Even so, they have several drawbacks, including the synthesis of these structures at a large scale. The alloys can be classified into four subsections: metal replacement, dichalcogenide replacement, metal and dichalcogenide replacement, and multicomponent replacements. We will be discussing briefly on each of these sections. The structure control during material synthesis, which involves thermodynamic stability, is crucial during alloying 2D chalcogenides. 2D materials also face the challenge of being unstable and highly reactive compared to their bulk counterpart. 2D TMDCs reportedly have bipolar behavior, and significant band engineering can be approached with substituting alloying components. Various reported alloying attempts had been summarized in Table 2. Most of the alloying is carried about by CVD method, process starts with mixing powders in stoichiometric ratio; growth of bulk crystals inside a quartz tube is placed in furnace and followed by standard mechanical exfoliation to obtain monolayer flakes. Alloying in the metal component reported increasing p-type behavior of 2D TMDCs with few distortions in the lattice. Alloying W in Mo site, Mo (1-x) W x X 2 , where X is S, Se reported to have 0.268 eV and 0.167eV owing to bond-stretching effect as the contribution is attributed to the Coulomb electrostatic force as the anion undergoes an offset when the cation is doped. The study may imply the growing band shift with an increase in the dopant composition (Dong et al., 2019). While alloying Mo in W site, Mo x W 1-x Se 2 showed p-type characteristics with a bandgap of 1.44-1.53 eV. As the Mo concentration is increased, valence band dispersion and spin-splitting size are monotonically reduced. Hence, it highlighted the importance of controlling stoichiometric ratio in-band tuning (Xie et al., 2020). Another approach (Wang et al., 2020a) explained the phase-dependent characteristics of Nb alloyed in Mo site, Mo (1-x) Nb x Se 2 , where 2H phase exhibiting metallic behavior and 1T phase with distortion cause semiconductor behavior with a bandgap of $0.4 eV. Figure 3A shows the phase mixture state of Mo (1-x) Nb x Se 2 .
Alloying in chalcogen component reported having n-type, p-type, or bipolar behavior characteristics. When alloying in Te in chalcogen site, WS 2(1-x) Te 2x reportedly showed bipolar behavior with structural change from 2H phase to 1T 0 , when Te alloyed with chalcogen site. Less than 0.5 atomic % of doping, material reported as p-type semiconductor while more significant than 0.5 atomic % resulted in n-type (Figure 3C). The root cause of the phase transition behavior is mainly attributed to the nature of Te for stronger metallicity and higher melting point (Wang et al., 2020d iScience Review reportedly has a stable 2H structure, but alloying of telluride, Te, increased the electron effective mass inband structure mainly because Te showed metallicity characteristics ( Figure 3C) Kanoun and Goumri-Said, 2020). Ko et al. demonstrated that alloying sulfur, S, in chalcogen site in WSe 2 results in enhancing NO 2 gas sensing properties by 2.5 times from WSe 2 (Ko et al., 2018). Alloying Se in chalcogen site, Mo S 2(1-x) Se 2x resulted in p-type semiconductor for 0 to 1 atomic % with a bandgap of 1.82 to 1.53 eV using low-pressure CVD method ( Figure 3D). Bandgap values decrease with Se doping in MoS 2 lattice as the Se occupies the highest occupied molecular orbital and lowest unoccupied orbital ( Figure 3E) (Yao et al., 2020). It is also reported to have less distortion energy of 0.06 eV, resulting in the valence band orbital and conduction band orbital having an increase or decrease monotonically with the dopant composition (Dong et al., 2019).
Alloying in both metal and chalcogenide sites may show p-type semiconducting behavior. Susarla et al. demonstrated material optical properties with alloying, Mo x W (1-x) S 2y Se 2(1-y) , with CVD synthesis (Avdizhiyan et al., 2021;Susarla et al., 2017). CVD Synthesis of the quaternary alloy at 750 C provided a better distribution of composition throughout the sample with a bandgap range of 1.61-1.85 eV. Interestingly, if the quaternary alloy has a miscibility gap, then annealing may result in the heterostructure, as demonstrated for Mo (1-x) W x S 2(1-y) Se 2y that resulted in MoS 2(1-x) Se 2x /WS 2(1-y) Se 2y .
The theoretical investigation suggested that MoS 2 /Mo (1-x) W x Se 2 heterostructure was reported as p-type while Mo (1-x) W x S 2 /MoSe 2 heterostructure reported being bipolar behavior transitioning from indirect to direct band structure. W (1-x) Mo x S 2 /MoSe 2 heterostructure resulting in n-type when doping concentration is less than 0.57 atomic %, with increasing doping concentration resulted in a p-type semiconductor. Doping mechanism is similar to the other alloy formation. The change in stoichiometric ratio during the compound formation has a significant impact on the electron transfer and availability of states during bonding. As the doping concentration increased, it induces gap in the state which indicates p-type behavior. Similarly, as the doping concentration decreased, the density of state increased at fermi level indicating stronger hybridization and n-type behavior. The underlying cause of bipolar behavior is attributed to the doping lowering of Mo ''d'' orbital energy in the WS 2 layer (Zi et al., 2019). This study can be scaled for the band engineering tuning of TMDC heterostructures as a function of composition and compounds interlayer coupling.
Molybdenum systems have been widely explored for their stability to form alloys. In a recent study, Apte et al. alloyed 2D monolayer of molybdenum by CBD method with a selenide-telluride combination. HAADF image of MoSe 2(1-x) Te 2x system is shown in Figure 4A, where the semiconducting 2H phase was observed. The 2H phase is isotropic, and the images confirmed the atomistic nature of the alloy formed. The increased tellurium concentration of Se: Te in 1:5 ratios (Alloy B) resulted in a redshift of emission spectra to 1.42 eV. The induced structural change in the 2D monolayer provides an opportunity for optical properties like absorption and emission spectral ranges to be efficiently engineered in 2D TMD alloys . In a similar study, large area of MoTe x Se 2-x alloy was formed by the molten salt-assisted CVD method. The photoluminescence spectra analysis showed a 1.55 to 1.38 eV shift in bandgap with increasing tellurium concentrations; thus, finding applications in near-infrared device applications (Zhao et al., 2019b). Studies have shown that Te alloying in excess leads to phase transition from 2H to 1T 0 in WSe 2(1-x) Te 2x system, which is not favorable . There are instances where tuning in between sulfide and selenide bandgap range for molybdenum compositions during the alloying of the MoSSe system (Mann et al., 2014). The tunable bandgap of such systems ranges from 1.5-2.0 eV (Fu et al., 2015;Lin et al., 2017;Wu et al., 2017).
In the subsequent study, the Se-Te combination was replaced with sulfide-tellurium. The thickness of the layers was $1.52 nm (three atomic layers), which lies between the monolayer thickness of MoS 2 and MoTe 2 . Figure 4B shows the high-resolution STEM images of the MoS 0.81 Te 1.19 . Inset shows the rotation between two basal planes by 11.87 , resulting in overlapping these honeycomb structures. Figure 4C shows the dependence of valence band structure on the composition of the material. The composition changes of MoS 2x Te 2(1Àx) from x R 0.93 to x % 0.8 resulted in a shift in band structure from n-type to p-type . We also observe the usage of such alloys as a light-emitting diode. Significant area synthesis of Mo 1-x W x S 2 as a hole transport layer (HTL) was realized for organic light-emitting diodes (OLED). Figure 4D shows the schematic representation of the fabricated OLED device, which was comparable with the already existing PEDOT: PSS units (Kwon et al., 2021). Figure 4F demonstrates the current density and luminance-voltage curve for the OLED device. The highest luminance of 22,000 cd/m 2 was achieved

OPEN ACCESS
iScience 24, 103532, December 17, 2021 7 iScience Review using Mo 0.2 W 0.8 S 2 as HTL, comparable with PEDOT: PSS. As the alloys are favorable in flexible electronic devices, they were also studied for their mechanical performance. The material behavior on extreme stress, strain, and fracture conditions plays a crucial role. Susarla et al. studied the strain-induced structural deformation in 2D Mo x W (1-x) S 2 alloy. Figure 4E shows the snapshots taken during MD simulation depicting crack propagation in the TMDCs alloy. The increase in weight content to 40% reduces the failure strain. The study also shows that the weakness is at the interface of the two transition metals and not the MoS 2 matrix. Figure 4F shows that the weight concentration has a major impact on the fracture limit, as seen in these biaxial iScience Review stresses vs. strain curves (Susarla et al., 2019). Therefore, exploring alloys in between different chalcogens with different d-electrons can help in tuning the bandgap of the 2D TMDCs and also understand their structural limitations. Forming alloys of metals and chalcogens at the 2D level is challenging rather than forming them in a 3D structure as they form non-directional atomic bonding. They are also energetically favored in bulk alloys.

2D metals alloy
Tuning of 2D crystals is dependent on the synthesis route taken to achieve the maximum attainable potential-desired properties. Various attempts have been made for 2D crystal synthesizing from their precursors using electron beam irradiation, etching, mechanically straining, or thermal decomposition. Synthesizing with electron beam irradiation using STEM highlights the relation between controlling beam intensity and changing the material's morphology. If the electron beam is controlled optimistically, irradiation can help synthesize 2D crystals from their precursor. This method induces the loss of atoms that can facilitate structural transformation, as it provides enough energy to overcome the barrier for migration leading to vacancy creation (Elibol et al., 2018;Zhu et al., 2017). In the case of amorphous material, with in situ heating, electron beam irradiation can induce mobility of atoms resulted in inversion domain grain boundaries that can shift to allowing the variation of inversion domain size. These domains are crystalline, restructured from amorphous material, and are limited to the formation of voids near inversion domains. As the void size grows under prolonged exposure to electron irradiation, inversion domains shrink (Bayer et al., 2018;Chen et al., 2019).
Similarly, bulk materials having a layered structure can also lead to 2D crystal formation using selective electron beam ionization. This approach may help remaining atoms retain parent crystal structure as bonding between atoms is easy to dissociate (Zhao et al., 2018). However, not every 2D crystal can retain its parent identity , demonstrated the scotch-tape method to synthesize 2D crystal structure originating from bulk PdSe 2 layered structure by introducing Se vacancies in PdSe 2 via electron irradiation. Interestingly, the formation of 2D structure resulted in Pd 2 Se 3 and did not resemble the PdSe 2 crystal structure. This deviation from most of the reported literature trends (Zhu et al., 2021) may be due to Pd-Se's strong interlayer interaction. Electron beam irradiation by STEM, Ad-atoms diffusion, and re-arrangement can also lead to 2D crystal formation. Adatoms preferentially occupy sites with more surrounding atom columns and vacancy sites with the movement of atoms for repair governed by controlling beam intensity (Sang et al., 2018;Shen et al., 2018). Another approach for 2D crystal formation has been approached by thermal decomposition of salts. Providing in situ heating with STEM helped form short-range order from amorphous salt, as the thermolysis temperature helps develop larger grain size with a layered structure (Sang et al., 2019;. Yadav et al. highlighted that the 2D metallic alloys' exfoliation from the bulk materials could be difficult due to isotropic forces using a bottom-up approach (Yadav et al., 2018). The study suggested a novel approach synthesizing 2D Quasicrystal (QC) using N-dimethyl-formamide (DMF) solvent, and ultrasonification-assisted exfoliation method ( Figure 5A). The novelty of the approach was demonstrated by exfoliation of Al-Pd-Mn poly-grained icosahedral QC and explored its application for electro-catalytic performance where the overpotential was lowered to $100mV. The increment in hydrogen evolution reaction performance is mainly because of the morphology change resulting in the presence of active catalytic atoms. In the latter study, similar phenomena were demonstrated for Al-Co-Cu. Firstly, 2D QC was exfoliated and exploring its further application; at the second step, 2D QC was synthesized with WS 2 to make heterostructure ( Figure 5B). The 2D QC/WS 2 showed that overpotential was lowered to $60mV (Yadav et al., 2020).

Alloying with MAX
The MAX phases are defined as M n+1 AX n , where M is a transition metal, A is an A group (IIIA or IVA) element, and X is C or N, and n varies from 1 to 3 (Ghosh and Harimkar, 2012). They are polycrystalline with ternary carbides and nitrides. MAX alloying and 2D MAX phases, MXenes, have been extensively explored for different kinds of application, including but not limited to battery storage applications (Naguib et al., 2013;Zhao et al., 2019a), catalyst, coating for corrosion resistance or thermal stability, cladding, and magnetic application Lu et al., 2017;Novoselova et al., 2018). Selection of synthesizing route and process change can determine the morphology of MAX phases properties and application . Mechanically milled Cu with Ti 3 AlC 2 -vol. 40% Cu powder mixture up to 10h followed with hot pressing at 950 C and 25 MPa for 1h. Alloying of Cu is mainly observed at Al (A Site iScience Review because of decreased Ti 3 AlC 2 crystallite size and microstructure homogeneity improve mechanism (Nechiche et al., 2017). Furthermore, mechanically milled Ti 3 AlC 2 -40 vol.% Cu powder mixture for 20 min, and cold compacted at 300 MPa, followed by sintering at 760 C for 2h. As a result, Cu was alloyed with Al with $50% composition and Ti with $3%. Selective etching was performed to dissolve composite Cu (Al, Ti) to get the solid solution. It showed (Ti (1-x) Cu x )(Al, Cu) that C 2 resulted in a higher lattice parameter, which may indicate distortion in the lattice. Ti 3 SiC 2 MAX Phase is intrinsically self-lubricating properties and TiC second phase for imparting strength (Magnus et al., 2020), which means that the Ti 3 SiC 2 can be easily dry ball milled for further alloying. Most of the attempts are synthesizing through powder metallurgy, which mainly depends upon either material self-lubricating properties or the addition of lubricating material for mechanical milling to avoid any foreign inclusion due to wear and tear (Haji Amiri et al., 2020;Ma et al., 2020;Wang et al., 2020b). There are also novel approaches recommended for formation, such as the arc-melting method, PVD and laser treatment, and molten salt route (Bahiraei et al., 2020;Fattahi and Zarezadeh Mehrizi, 2020;Galvin et al., 2018;Tunca et al., 2020). Wang et al. explored applying the Cr 2 AlC MAX phase as a coating using the arc/sputtering deposition method (Wang et al., 2020c). The coating showed good oxidation resistance at temperatures as high as 1100 C due to the Al 2 O 3 interface layer and (Cr, Al) 2 O 3 at the outermost layer ( Figure 5C). Similar phenomena were reported for 211 and 413 V-Al-C MAX phase corrosion resistance (Ghasali et al., 2021, p. 211). Application of coating can also impart additional mechanical properties. The Ti-Al-C MAX phase can be rapidly formed from elemental powder, and the presence of the Ti 2 AlC, TiC x , and Ti x Al may provide surface hardness of 811 Vickers hardness (Richardson et al., 2020). Another application of MAX phase coating (Akter et al., 2020) recommends that Hf 2 SN and Hf 2 Sc are good candidates to decrease solar heating. A recent study shows the problem of MXenes, the rate at which MXenes oxidize and degrade as stored as an aqueous suspension (Mathis et al., 2021). The experiment suggested that adding excess aluminum, Al, during synthesis leads to improved crystallinity iScience Review of MXene that can exhibit an increased shelf life ten times. This study supports the usefulness of an extensive study done on MXenes. showed etching using HF leads to MXene formation from MAX phase because of weak interlayer connection between M and A atoms ( Figure 5D). Study that synthesized (Nb 2/3 Sc 1/3 ) 2 AlC quaternary system and selective etching removed Sc and Al atoms, leading to the formation of Nb 1/3 C. Mo 2 ScAlC 2 suggested the existence of new quaternary MAX phase alloys with out-ofplane chemical order; Mo 2 ScAlC 2 with an Sc atoms layer sandwiched between two Mo-C layers. MAX phase selective etching of Al using HF leads to formation of MXene Mo 2 ScC 2 . (Aghamohammadi et al., 2018) studied the Ti 3 SiC 2 MAX phase immersion in Hydrofluoric acid (HF). The result showed that the Si layer disappeared and broke down to TiC. In addition, it changed the morphology of TiC, before TiC had low crystallinity, but after HF Treatment, TiC was shaped to hexagonal and then to truncated octahedron and cubic. showed the formation of MXenes from the i-MAX phases.
(Mo 2/3 Y 1/3 ) 2 AlC i-MAX phase was selective etched using HF, which led to the removal of Al atoms and formation of (Mo 2/3 Y 1/3 ) 2 C and M 1/3 C MXenes. MXenes application was further explored in the study.
(Mo 2/3 Y 1/3 ) 2 C showed better supercapacitor performance when used as an electrode with KOH electrolyte, whereas M 1/3 C showed higher capacitance with H 2 SO 4 solution (Chirica et al., 2021;Zhao et al., 2021). Selective etching of hybrid A layers (Al, Cu) resulted into Ti 3 (Al (1-x) Cu x )C 2 MXene, where its application was explored as a catalyst to efficiently electro-reduce CO 2 into methanol. Another application of selective etching in the MAX phase was attempted to find its application in batteries. Ti x Ta (4-x) AlC 3 was selective etched using HF and resulted into Ti x Ta (4-x) C 3 MXene. Transformation promoted its structural delamination with an expanded interlayer d-spacing, allowing effective reversible Li-ion storage (Syamsai et al., 2021). (Nä slund et al., 2020) show delocalized electron redistribution from Ti 3 C 2 to Al layers in Ti 3 AlC 2 electrostatic interaction. Selective etching may lead to MXene compound formation. The previous study shows 211 phase formation in (Zr,Nb) 2 (Al,Sn)C MAX phase quaternary system. They hypothesize that comparable atomic radii of M and A atoms obtain a stable 211 MAX phase (Lapauw et al., 2018). As a result, adding Nb, Sn reduces distortion of lattice parameter compared to pure ZrAlC and found two quaternary systems, (Zr Nb) 2 AlC and Zr 2 (Al Sn)C, stable over their entire compositional range. Similar phenomena were observed for (Zr Ti) 2 (Al,Sn)C MAX phase quaternary system, where Sn atoms alloying with A atoms help to achieve phase purity, and Ti atoms alloying with M atoms lead to a decrease in lattice parameters. (Tunca et al., 2019) explored the semi-conducting application of h-BN via fluorinating using Nafion, a perfluorinated polymer ( Figure 5E) (Radhakrishnan et al., 2017). The fluorination process showed fluorine (F) radicals interacting with B-N bonds and significantly change the charge density of nitrogen atoms, resulting in ferromagnetic ordering and widening its bandgap. This simple approach can be fruitfully used for the fluorination of other 2D materials.

Utilization of phase diagrams
Phase diagram plays a very crucial role in the development of materials. Utilizing the phase diagram, we can know the temperature, phase boundaries, and solubility of the target phase. In addition, it can help decide phase composition in the alloy system as we tend to explore multicomponent (binary, ternary, and more); it is crucial to know the phase diagram of the system. Unfortunately, there is limited information (phase diagram) available for most of TMDCs. Hence, building a thermodynamic database, possibly via combining first principle with mesoscale calculation for a set of possible molecular combinations, to construct a phase diagram will be crucial for future TMDCs development. Such computational-assisted phase diagram database would offer monumental potential for the discovery and innovation of new materials.

Development of energy-efficient methods to alloy elements in TMDCs
In literature, the isomorphs substitution in alloy TMDCs explores in greater detail. They are relatively easy to synthesize owing to extended solubility in each other. On the other hand, the non-isomorphous elements are difficult to alloy, as they need extreme conditions. Taking inspiration from nature, the alloying happens with minimum energy input. In the future, new synthesis routes to alloy elements in energy-efficient methods will be crucial. It can be bio-assisted synthesis methods or energetically stabilized alloys systems, or anything else.

Improving mechanical properties and environmental stability
The recent reports show limited stability of the 2D TMDCs, which can be further improved with the help of alloying elements. In the materials development program of structural alloys (e.g., Ni-based superalloys),

OPEN ACCESS
iScience 24, 103532, December 17, 2021 11 iScience Review appropriate alloying elements are added to improve its oxidation/corrosion resistance. The idea can be extended to improve the environmental stability of the TMDCs. Alloying improves/tunes the mechanical properties of materials, in general. The similar concept can be extended in 2D materials as well. There are recent work utilizing such aspect, but most of the mechanical testing is indirect. The in-situ mechanical testing either with Raman or electron microscope will be helpful. Quantifying the mechanical properties will be crucial for future automation in device fabrication or large-scale transfer.

Utilization of machine learning
Alloying in 2D materials provides enormous opportunity to design new 2D materials consisting of new structure or phases. It also changes the physical and chemical properties drastically. In such case, selection of required 2D materials with specific structure/phase with certain required physical and chemical properties becomes a difficult task. Synthesizing multiple 2D materials and measuring the properties are challenging. In such case, machine learning (ML) can be utilized. Employing machine learning tools to materials design or optimizing materials processes, etc., critically depend on the accuracy of the machine learning parameters used. Inaccuracy or unreliable parameter values would invariably ingress errors in the prediction and lead to unreliable prediction. Thus, care must be given in reliably selecting the parameters carefully. Physics-based machine learning approach as compared to curve fitting or empirical approach would yield higher reliability of using machine learning methodologies in materials discovery and design. There are few efforts to utilized ML for synthesis of single component 2D or single phase TMDs. In future, it can be utilized for design of alloy 2D materials with specific composition and physical/chemical properties.