van der Waals epitaxy of transition metal dichalcogenides via molecular beam epitaxy: looking back and moving forward

Abstract

Initiated by the discovery of graphene, tremendous research advances have been witnessed in the field of two-dimensional (2D) transition metal dichalcogenides (TMDCs) in the past few years. The unique optical and electrical properties of these layered TMDCs due to their well-defined low dimensionalities make them promising building blocks for next-generation electronics. Recently, a lot of research interest has been focussed on the growth of high-quality and large-area epilayers of several TMDCs using molecular beam epitaxy (MBE), owing to their ultra-high vacuum environment. The capability to precisely control the composition, thickness, and structural phases of TMDCs by MBE makes it suitable for studying fundamental sciences, investigating technological applications, and exploring new physics, which possibly could lead to fascinating applications. In the current perspective, an overview of the basic properties of TMDCs has been discussed, followed by the recent advancements in this area of the growth of TMDC thin films by MBE. Finally, the authors’ viewpoints highlight this emerging research field's key challenges and opportunities.

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1. Introduction

Over the last few decades, with the research advancements in the established and matured technology of traditional three-dimensional (3D) semiconductors such as III-nitrides, metal oxides, Si, etc.,^1–8 high-performance state-of-the-art electronic devices have been successfully fabricated. However, further progress and developments in these devices are often limited due to certain drawbacks encountered in these 3D semiconductors, for instance, the low charge carrier mobility, presence of dangling bonds at the surface, low light absorption properties, etc.^9,10 Hence, it becomes vital to explore alternatives that can overcome these limitations, for the development of next-generation devices. Therefore, two-dimensional (2D) layered materials have recently become a priority choice for applications in modern-day electronics.^11–15 The successful experimental realization of graphene¹⁶ in 2004 led to a plethora of research in the field of ultrathin layered materials,^9,17–19 and several other graphene alternatives have been recently explored for various applications.^20–34 Among these, a particular class of layered materials, i.e., transition metal dichalcogenides (TMDCs), has garnered significant interest, owing to their rich physics and fascinating potential applications.^35–41 TMDCs, just like graphene, can be scaled down to monolayers, although these TMDCs are three atoms thick instead of one.⁴² TMDC-based devices possess considerable charge carrier mobility and a semiconducting band gap, making them suitable for switching applications,²⁵ and therefore, exhibit significant photoluminescence (PL) and intrinsic on-off current ratios as compared to graphene.^43–45 Additionally, TMDCs have also shown remarkable physical properties such as high tensile strength, making them a promising constituent in advanced device technology.⁴⁶

However, the yield and cost are a major bottleneck in the extensive application of these layered materials. Presently, the most common and widely used technique to produce ultrathin TMDCs is exfoliation;^47–53 however, it presents specific common challenges such as the growth of high-quality thin films, scaling up for wafer-scale production, etc.^54,55 Therefore, considerable efforts have been made to research and optimize different production methodologies of these 2D layered TMDCs,^56–59 to improve their production yield without compromising the growth quality. One promising yet less-explored method to synthesize high-quality, epitaxial, and large area thin films of 2D TMDCs is via molecular beam epitaxy (MBE) due to its several potential advantages over the other growth techniques. The growth in MBE can be highly controlled because of the use of high-purity source materials, limiting contaminations due to growth carried out in an ultrahigh vacuum environment.^60–62

Many reviews exist regarding the growth and applications of TMDCs by various synthesis techniques. McDonnell et al.⁶³ have described the current status of TMDC thin film-based research, highlighting the importance of electronic and photonic applications. Lin et al.⁶⁴ have reported the recent progress in the field of thin-film synthesis and the processing techniques that exhibit excellent controllability and reliability for the substitutional doping of TMDC thin film monolayers. Walsh et al.⁶⁵ have discussed the various growth modes and the advantages of van der Waals (vdW) epitaxy, along with the most significant issues regarding the growth of TMDCs and topological insulators, including defect density, grain size control, electronic transport, doping, etc. Manzeli et al.⁶⁶ have examined the various synthesis methods for TMDCs along with a discussion about their properties, with specific focus on the superconductivity, charge density wave, and topological phases. Moreover, TMCD-based nanoelectronic devices have also been discussed and the strategies to improve and tailor the charge carrier properties have also been explored. However, a comprehensive analysis focussed on the recent advances and current prospects on TMDCs exclusively grown by MBE is still missing. In this perspective, we aim to provide a brief overview of MBE and the basic properties of TMDCs, following which an in-depth deliberation on the recent progress of the MBE-grown TMDCs for various applications is presented. Finally, we have discussed the unresolved issues and future perspectives for this evolving family of materials.

2. Transition metal dichalcogenides

TMDCs are represented by the formula MX₂, where M is a d block metal (Mo, W, Pd, Ti, etc.), and X represents a chalcogen (S, Te, and Se), respectively. The groups IV–X in the periodic table usually belong to these transition metals, containing a different number of valence d-electrons, giving rise to different electronic properties of these elements such as metallic, superconducting, semiconducting, etc.⁶⁶ TMDCs exist in a layered structure at the atomic level, consisting of one or a few monolayers.⁶⁷ A representative schematic depicting the layered structure of MoS₂ is shown in Fig. 1(a). TMDCs usually occur in several crystal structures due to the difference in the coordination environments of the transition metal. The two most common crystal structures of the TMDCs are the trigonal prismatic structure (2H) and the octahedral phase (1T). One can understand these structural phases in terms of the different stacking arrangements of the constituent atoms. Fig. 1(b) shows the different atomic structures of MoS₂. The three atoms, i.e., chalcogen–transition metal–chalcogen, form the individual monolayers of TMDCs. The ABA stacking characterizes the 2H crystal phase, while the 1T octahedral phase corresponds to the ABC stacking sequence. Most of the TMDCs (MoS₂, WS₂, WSe₂, MoSe₂, MoTe₂, etc.) are thermodynamically more stable in their 2H phase than the metastable 1T octahedral phase.⁶⁷ In the 2H phase, TMDCs show semiconducting characteristics that accentuate TMDCs for potential next-generation electronic applications. The metastable or intermediate state for the 2H phase of TMDCs is the orthorhombic phase (1T′ phase). For WTe₂, the most stable crystal phase is the 1T′ phase at room temperature.⁶⁷

Fig. 1 (a) Schematic depicting multiple layers of MoS₂. Atoms of Mo are bonded via covalent bonds with S atoms. The individual layers are stacked together and held by weak vdW forces to form multi-layered or bulk structures. The figure has been reproduced from ref. 10. (b) Different crystal structures of MX₂, showing different stacking sequences. The two most common phases 2H and 1T, as well as the intermediate 1T′ phase, are shown. The figure has been reproduced from ref. 75. (c) Schematic diagram showing the various components of an MBE system. The figure has been adapted and reproduced from ref. 62.

Through the persevering efforts of researchers over the past few years, various techniques have been evolved to produce atomically ultrathin TMDCs.^9,10 In general, these synthesis techniques can be classified into two groups: the first method, known as the top-down technique, produces TMDC flakes by thinning their bulk crystals. Different kinds of exfoliations fall in this group.^9,54,68,69 Since different monolayers of 2D TMDCs are held together by weak inter-layer vdW forces, under external perturbation, bulk 2D TMDC crystals readily get reduced into their few-layered flakes. The second approach is to grow layered TMDCs via bottom-up techniques, wherein the constituent species assemble to form continuous thin films. The main techniques under this category are atomic layer deposition,⁷⁰ magnetron sputtering,⁷¹ chemical vapor deposition (CVD),^42,56 pulsed laser deposition (PLD),³⁶ and MBE.⁷²

Among these methods, MBE is a very promising synthesis technique used to grow epitaxial thin films and heterostructures, where fine control over the layer thickness and ultra-high purity of the material components are required.^73,74 MBE involves the production of molecular/atomic beams of constituent source materials and/or doping species, which then react on the surface of a substrate to form an ordered thin film in an ultrahigh vacuum environment. The composition of this epitaxial thin film and its doping (if any) depends upon the arrival rate of the molecules/atoms and dopants, respectively. A typical MBE system consists of a stainless-steel ultrahigh vacuum chamber and various components such as vacuum pumps, substrate heater, effusion cells, etc. The source materials, usually of ultra-high purity (99.99%), are kept inside effusion cells (known as the Knudsen cells or K-cells). In the case of TMDCs, the transition metals are generally supplied from an e-beam evaporator, while the chalcogenides are placed inside these Knudsen cells.⁷² Appropriate substrates are heated to the desired growth temperature and usually rotated to improve the uniformity and homogeneity of the final product. These Knudsen cells are then heated to high temperatures, depending on the required growth rates, the composition of the product, and the doping concentrations. The schematic of a typical MBE system is shown in Fig. 1(c), depicting the different components and the three zones where various physical phenomena occur. The details of the various processes in these zones have been described elsewhere.⁶² The usual growth rates in an MBE process are a few hundred nm h⁻¹ in contrast with other techniques where the growth rates are in the range of mm h⁻¹, thereby allowing the grown interfaces to be precisely controlled in their compositions. In addition, due to lower arrival rates of the species on the substrates, the growth is highly controlled along with the negligible probability of back-sputtering of the deposited film, which results into very smooth films with high control over the surface damage.

3. Recent advances in MBE-grown transition metal dichalcogenides

In this section, an overview is presented describing the recent progress in the field of MBE-grown TMDCs. Thin-film growth by MBE possesses the potential to isolate monolayers of TMDCs and synthesize large-area ultrathin films, which otherwise cannot be easily produced by exfoliation or other growth techniques. The growth of epitaxial TMDC films by MBE dates back to the mid-1980s.^76–78 These pioneering works have been done by Koma et al.⁷⁷ where they have demonstrated the growth of epitaxial NbSe₂ thin films, for the first time, on a cleaved 2H MoS₂ surface, following which, Koma et al.⁷⁸ successfully grew epitaxial MoSe₂ thin films on a CaF₂(111) substrate. In both reports, the in-plane crystallographic axes of the grown thin films showed a high alignment towards the axes of the substrates used for their growth, despite the large lattice mismatches (∼20%). Thus, the authors coined the term as “vdW epitaxy,” although extensive characterizations on the properties of these thin films were not carried out at that time. However, after almost three decades, recent research on different TMDCs has resurrected the concept of vdW epitaxy, and indeed there is an increasing number of research articles in this area being published in the last few years.^63–65,79 For instance, similar to the works carried out by Koma et al.,^77,78 Kreis et al.⁸⁰ have applied scanning tunnelling microscopy (STM) and angle-resolved photoemission spectroscopy (ARPES) to evaluate the electronic valence band spectra of HfS₂, during different stages of its epitaxial growth on WSe₂. In another report, Kreis et al.⁸¹ have investigated the unoccupied and occupied electronic structures of HfS₂ ultrathin films, by employing combined angle-resolved photoemission and inverse photoemission, along with the band structure calculations. They have studied the impact of the thickness of the film on the two- and three-dimensional conduction and valence band states. Thus, a lot of research is being done in the field of MBE-grown TMDCs and in the coming subsections, the recent progress has been discussed.

3.1 Growth of selenides

In one of these early works, Barton et al.⁸² have reported the growth of monolayer HfSe₂ on different substrates using MBE and have shown that this vdW epitaxy method allows the growth of high-quality heterostructures, without any strain or misfit dislocations in the thin films, in spite of the large lattice mismatch. Fig. 2(a) shows the high angle annular dark-field (HAADF) scanning transmission electron microscopy (STEM) image of HfSe₂ on MoS₂ grown by MBE. Peng et al.⁸³ have shown a nearly layer-by-layer growth of TiSe₂ ultrathin films on graphitized SiC(0001) substrate via MBE and have analyzed the defect structures of these films. The chemical inertness of graphene ensures a sharp interface between TiSe₂ thin films and SiC. The STM measurements reveal the identification of two dominant types of Se vacancies and interstitial defects (Fig. 2(b–d)).

Fig. 2 (a) HAADF-STEM image of HfSe₂ grown on MoS₂ by MBE. The figure has been reproduced from ref. 82. (b) STM micrograph of TiSe₂ thin film (17 nm × 17 nm, I = 100 pA, and V_S = 0.24 V), showing the two kinds of Se defects: bright Se interstitials and dark Se vacancies. Zoomed-in STM micrographs of a single (c) Se vacancy and (d) interstitial (2 nm × 2 nm, I = 100 pA, and V_s = 0.24V). (e) STM micrograph of a charge density waves (CDW)-related 2 × 2 superstructure in single-layer TiSe₂ thin film (20 nm × 20 nm, I = 100 pA, and V_S = 0.1 V). The figures have been reproduced from ref. 83. (f) Schematic of the band alignment at the WSe₂/SnSe₂ heterojunction as revealed by X-ray photoelectron spectroscopy (XPS) core level analysis. E_F and χ denote the Fermi energy and electron affinity, respectively. E_g is the band gap, whereas VBO and CBO denote the valence and conduction band offsets, respectively. The figure has been reproduced from ref. 84. (g) Schematic of the top-gated electric-double-layer transistor (EDLT) based on the WSe₂ epitaxial thin film grown on the sapphire substrate. (h) Chemical structure of the electrolyte used in the electrolyte-gated transistor. (i and j) show typical transfer characteristics of WSe₂-EDLT carried out at T = 220 K, indicating clear ambipolar operation. The figures have been reproduced from ref. 79.

Moreover, the observation of charge density waves (CDW) persisting down to monolayer TiSe₂, as shown in Fig. 2(e), favors the mechanism of the excitonic insulator in TiSe₂ thin films. These were a few early reports demonstrating the potential of the MBE as a growth technique for the production of TMDCs. In another report, Aretouli et al.⁸⁴ have shown the growth of high-quality vdW heterostructures of SnSe₂/WSe₂ by MBE on AlN/Si(111) substrates using Bi₂Se₃ as a buffer layer. 1T polytype of SnSe₂ is obtained having an indirect band gap of ∼0.8 eV with a strong and unintentionally doped n-type behavior. In contrast, the MBE-grown WSe₂ stabilizes in the 2H phase, exhibiting a p-type behavior. Band offsets analysis of this heterojunction revealed a nearly broken gap heterostructure as depicted in Fig. 2(f), which can be suitable for designing tunneling field-effect electronic devices. It must be noted that other groups have reported mid-gap or even slightly above mid-gap Fermi levels for MBE-grown epitaxial WSe₂ thin films, which correspond to an n-type behavior. For instance, Zhang et al.⁸⁵ have shown the successful synthesis of WSe₂ ultrathin films on an epitaxial bilayer graphene substrate with controlled film thickness at the atomic level. Employing in situ ARPES measurements, they have demonstrated the evolution of the electronic structure of the epitaxial WSe₂ thin films, which confirms a direct band gap for the single layer and bilayer WSe₂, and an indirect band gap for three or more layers of WSe₂. From the scanning tunnelling spectroscopic (STS) measurements, it is observed that an asymmetry exists between the valence band maximum and conduction band maximum, with respect to the Fermi level, revealing slightly n-type doping of the grown WSe₂ films. In another report, Blades et al.⁸⁶ have comprehensively studied the electronic and geometric structure of defects in WSe₂ thin films grown on highly oriented pyrolytic graphite (HOPG) with STS and STM, and the effect of temperature on the nature of these defects. In 2018, Wang et al.⁸⁷ reported the transport properties of highly crystalline epitaxial TiSe₂ thin films on a sapphire substrate grown by MBE. Robust CDW transitions have been demonstrated, down to 5 monolayers of TiSe₂ thin films, highlighting an exciting phenomenon of in-plane self-rotational growth of the epitaxial thin film with respect to the substrate. Yan et al.⁸⁸ have shown the successful growth of high-quality epitaxial PtSe₂ thin films with controlled thickness by MBE for the first time. In another work, Nakano et al.⁷⁹ have presented a study that demonstrates epitaxial and layer-by-layer growth of WSe₂ thin films on sapphire by MBE, utilizing an optimized growth recipe. They have also shown the ambipolar transistor operation of these thin films by an electrolyte gating technique (Fig. 2(g–j)). The enhancement in the drain current (I_D) is observed to be larger at the negative gate bias (V_G), which corresponds to the hole-doped regime compared to the positive gate voltages, corresponding to the electron-doped regime. Consequently, the four-probe sheet resistance (R_S) is found to be lower at the hole-doped side. The obtained results provide a benchmark towards the growth of epitaxial and large area TMDC thin films for scalable devices. In a recent report by Freedy et al.,⁸⁹ the interfacial electrical resistance, interface chemistry, Seebeck coefficient, and thermal conductance of Ti, TiO_x, and Ti/TiO_x contacts to the MBE-grown WSe₂ thin films have been carried out. Metallic Ti exhibits a strong chemical reactivity with WSe₂ and results in W–Se bond scission, producing Ti–Se chemical states and metallic W. In addition, a high thermal boundary conductance along with low electrical resistance is also observed. On the other hand, TiO_x exhibits low reactivity, lower thermal boundary conductance, and higher electrical resistance. Due to the extremely small thermal conductance obtained and the Ohmic nature of the metal contacts, this interface may be promising for thermionic and thermoelectric applications. So, these works on MBE-grown TMDCs have shown promise to fabricate a variety of wafer-scale TMDC thin films and heterostructures in a carefully controlled manner, thereby, opening a route towards intensive fundamental and applied science research on TMDC-based structures.

One of the TMDCs that has been readily explored by MBE is MoSe₂.^90,91 Roy et al.⁹² have reported the growth of MoTe₂ and MoSe₂ thin films on sapphire substrates and studied the temperature-dependent transport measurements of these thin films. The distinct layered structure of MoTe₂ thin films is apparent from the high-resolution cross-sectional transmission electron microscopy (HR-XTEM), along with the abrupt interface formed with the substrate, as shown in Fig. 3(a). The hexagonal lattice arrangement of the atoms has been shown in the plan-view TEM image in Fig. 3(b), and a lattice constant of 3.5 Å is extracted, which matches with that of bulk 2H MoTe₂. Fig. 3(c) shows the plan-view TEM micrograph of the MoSe₂ thin film, depicting a hexagonal lattice arrangement of atoms. The derived lattice constant of 3.3 Å is in good agreement with that of the bulk 2H MoSe₂. Moreover, an insulating behavior is observed for the grown thin films, which is in good agreement with 2D-variable range hopping (VRH), suggesting that the charge transport in these thin films is governed by the localized charge-carrier states (Fig. 3(d and e)). In another report, Chen et al.⁷² have grown atomically thin, highly oriented, and ambipolar monolayer MoSe₂ on GaAs(111)B substrates. They have realized an electrolytically-gated transistor based upon the transferred MoSe₂ thin film (Fig. 3(f and g)), and it is perceived that the electrical transport characteristics of the device also follow the 2D-VRH (Fig. 3(h and i)) because of the disorders in the thin film. Ma et al.⁹³ have shown that twin grain boundaries in MBE-grown MoSe₂ thin films on different substrates are metallic in nature, substrate-independent, and undergo a Peierls metal to insulator transition at low temperatures. By utilizing STS measurements, substantial band gap narrowing of the MoSe₂ thin film is observed in the vicinity of these grain boundaries, which might be due to the stress induced within the film. It has been demonstrated in other reports that these twin boundaries in MoSe₂ have excess incorporation of Mo-atoms, which is a possible method for modifying MoSe₂ basal planes and consequently, altering their properties to potentially enhance their application in selective water adsorption.⁹⁴ Various other groups have also demonstrated the growth of MoSe₂ thin films on different 2D substrates, such as hexagonal boron nitride.⁹⁵ In a recent report, He et al.⁹⁶ have shown the gate-tuneable phonon properties of 2H MoSe₂ grown via MBE. The study demonstrates the electrostatically enhanced electron–phonon interactions in monolayer MoSe₂ thin films. It focuses on the correlation between the phonon properties and multiple valleys of monolayer and bilayer MoSe₂ thin films at different doping levels. The monolayer MoSe₂-based ion-gated devices also exhibit typical n-type transport characteristics with a high on/off ratio of ∼10⁵, along with a mobility of ∼31 cm² V⁻¹ s⁻¹.

Fig. 3 (a) HR-XTEM image of a layered structure of MoTe₂. Inset shows the fast Fourier transform (FFT) of the area marked by the red square. The plan-view TEM image depicts the hexagonal arrangement of atoms in the (b) MoTe₂ thin film and (c) MoSe₂ lattice. Inset shows the zoomed-in view for 2 × 2 nm². Electrical transport measurements show the conductivity variation with T^−1/3 for (d) MoTe₂ and (e) MoSe₂. Insets of (d, e) show the variations of the measured resistivities with temperature. The figures have been reproduced from ref. 67. (f) The schematic device structure of the MoSe₂ thin film-based EDLT. (g) Variation of the channel current with polymer electrolyte voltage (VPE) depicting ambipolar behavior. (h and i) Sheet conductivity (G_SH) as a function of T^−1/3 on the p and n sides for different values of V_PE. The figure has been reproduced from ref. 45. The HAADF-STEM images for the (j) bilayered WSe₂/MoSe₂ and (k) four-layered MoSe₂/WSe₂ heterostructures grown on GaAs(111)B. The figure has been reproduced from ref. 71. (l) Optical image of epitaxially grown MoSe₂. (m) Peak energy of the A_1g mode obtained through Raman mapping. (n) Typical Raman spectrum of MoSe₂. (o) Distribution of the peak energy of the A1g mode indicating the mean value and standard deviation. The figure has been reproduced from ref. 73.

Recently, Ohtake et al.⁹⁷ have successfully fabricated large-area 2D highly oriented WSe₂/MoSe₂ and MoSe₂/WSe₂ heterostructures using MBE, comprising consecutively stacked epitaxial WSe₂ and MoSe₂ monolayers on GaAs(111)B substrates, which have been pre-treated with Se. Fig. 3(j) displays the HAADF-STEM image of the MBE-grown bilayered WSe₂/MoSe₂ heterostructure, depicting the distinct layered structure. TMDC bilayered heterostructures can have several basic stacking geometries. In the most fundamental stacking geometry, the metal/chalcogen atoms of the first TMDC layer are on top of metal/chalcogen atoms of the second TMDC layer, where the T stacking geometry is obtained by the translation of the fundamental stacking sequence in such a way that the metal atoms of one of the TMDC layers are on top of the hexagonal centers of the other TMDC layer. In contrast, the C7 stacking geometry can be obtained from the T stacking via rotation of one TMDC layer concerning the other layer by an angle of 180° about the axis, which crosses a pair of overlapped chalcogen and metal atoms of the two TMDC layers (the metal atoms go on top of the chalcogen atoms and the chalcogen atoms go on top of the metal atoms).⁹⁸ The magnified image (shown on the right in Fig. 3(j)) confirms the formation of a C7-stacking configuration. In contrast, in the four-layered MoSe₂/WSe₂ heterostructure, as shown in Fig. 3(k), two kinds of stacking sequences, i.e., C7 and T, are observed. The simultaneous formation of the T stacking configuration could be attributed to the small difference in the energies between the T and C7 stacking sequences for the MoSe₂/WSe₂ heterostructure. In other reports, wafer-scale growth of MoSe₂ has been achieved on a h-BN/sapphire substrate at a low growth temperature by combining the techniques of metal-organic vapor phase epitaxy (MOVPE) and MBE, revealing a large uniformity and homogeneity of the grown heterostructure on the whole wafer through an in-depth examination of the optical studies (Fig. 3(l–o)).⁹⁹ Recently, Wei et al.¹⁰⁰ have shown the complex nature of the nucleation and growth processes of MoSe₂ monolayers grown via MBE on mica. It has been observed that the 1T phase of MoSe₂ forms along with a comparable quantity of the 2H phase of MoSe₂, where the 1T phase converts gradually into the stable 2H phase before the complete growth of the first monolayer. These results demonstrate the possibility of stabilizing the metastable phases of atomically thin TMDCs by MBE.

3.2 Growth of sulfides

Recently, extensive research has also been focused on the growth of sulfides, such as MoS₂ thin films, utilizing MBE. Fu et al.¹⁰¹ have performed the growth of MoS₂ on h-BN under thermodynamically controlled conditions, which allow MoS₂ grains to align epitaxially on an h-BN substrate having the same orientation, thus, merging perfectly without the formation of grain boundaries. In another report, El Kazzi et al.¹⁰² have presented large-area growth of MoS₂ on 200 mm SiO₂/Si and Al₂O₃/Si templates using gas source MBE and have analysed the effect of the underneath surface template on the crystallinity of the MoS₂ layers. The HRTEM and HAADF-STEM images of MoS₂ on Al₂O₃/Si and MoS₂ on SiO₂/Si are shown in Fig. 4(a and b). The HRTEM images show the presence of a strong disorder in the MoS₂ thin film with clear crossing and overlapping of the atomic planes, which is because of the formation of small grains (∼10 nm diameter). The energy dispersive spectroscopy (EDS) line profile shown in Fig. 4(b) reveals a small concentration of atomic Mo beneath the native oxide layer, indicating that Mo diffusion takes place through the thin native SiO₂ layer during the MoS₂ film deposition. On the other hand, this diffusion does not occur when an Al₂O₃ template is used (Fig. 4(a)), thereby efficiently blocking the diffusion of both Mo and S through the oxide layer. Xu et al.¹⁰³ have performed growth of 1H and 1T′ phases of nanosized MoS₂ islands on an Au(111) substrate by MBE and have analyzed the polymorph evolution by modulating the growth fluxes of Mo and S. A band gap opening of 80 meV at cryogenic temperatures (∼4.5 K) for the 1T′ phase is noticed, which is in agreement with the predicted quantum spin Hall characteristics for the 1T′ phase of MoS₂. Fig. 4(c) shows the STS measurements of the 1T′ MoS₂ islands taken at the center, depicting a small band gap instead of the sharp V shape. The zoomed-in dI/dV curves taken at the edge and center of 1T′ islands are shown in Fig. 4(d), clearly indicating an energy gap opening of 75 ± 5 meV at the center. In another report, Ehlen et al.¹⁰⁴ have demonstrated the growth of epitaxial MoS₂ on a graphene/Ir(111) substrate and have reported a detailed spectroscopical analysis of this epitaxial system. A photoluminescence spectrum with a narrow peak width (∼18 meV) has been observed. It was explained based on weak MoS₂–graphene interaction, preventing PL quenching, usually expected for metallic substrates. Mortelmans et al.¹⁰⁵ have studied the epitaxial growth of MoS₂ on sapphire and exfoliated MoS₂ flakes via MBE and MOVPE, and the analysis reveals that the technique of MBE yields superior epitaxial MoS₂ quality as compared with MOVPE. However, it was noticed that MBE provides greater control on the epitaxial registry in both cases. Recently, Ermolaev et al.¹⁰⁶ have presented a detailed study on the structural and optical properties of a monolayer of MoS₂ thin films grown by MBE on sapphire. The analysis has shown that the MBE-grown MoS₂ film exhibits a two-fold greater quantum yield of PL and lower photobleaching when compared with the CVD-synthesized MoS₂ films (Fig. 4(e)), thereby, making it a promising candidate for photonic applications.

Fig. 4 The HR-TEM and HAADF-STEM images and the corresponding EDS mappings of (a) MoS₂ on Al₂O₃/Si and (b) MoS₂ on SiO₂/Si. The line profiles (red in color) in the EDS maps probe the concentrations of Mo and S atoms along the film/substrate interface. The figures have been reproduced from ref. 102. The electronic structure of the MBE-grown 1T′-MoS₂ nanocrystals at 4.5 K. (c) STS measurements taken at the bulk (center) of the 1T′-MoS₂ nanocrystals. (d) Zoomed-in views of the STS measurements of the 1T′-MoS₂ nanocrystals taken at the bulk and edge. The figure has been reproduced from ref. 103. (e) The PL spectra of the CVD and MBE-grown single-layer MoS₂ on Al₂O₃ substrates. Dotted and dashed lines indicate the deconvolution of the PL spectra into Gaussian peaks corresponding to defects and A-excitons, respectively. The figure has been reproduced from ref. 106.

3.3 Growth of tellurides

In addition, apart from the transition metal sulfides and selenides, researchers have also started exploring transition metal tellurides grown by MBE. Yu et al.¹⁰⁷ have reported the growth of continuous MoTe₂ thin films (atomically thin heterophase, homojunctions) across graphene substrates through MBE and have obtained the stoichiometric 2H and 1T′ phases of MoTe₂ simultaneously, showing potential for novel phase-patterning applications. The STS measurements performed on the thin films (from points A to G as shown in Fig. 5(a)) reveal “V”-type curves that gradually transform into “U”-type curves from point A to point G (differential conductance spectra shown in Fig. 5(b)). These results suggest the coherent phase transformation taking place from the semimetallic 1T′-MoTe₂ into the semiconducting 2H-MoTe₂. Walsh et al.¹⁰⁸ have reported the first-ever growth and characterization of WTe₂ thin films by MBE on several 2D substrates, including Bi₂Te₃, MoS₂, and graphite, and have enabled the growth of WTe₂ thin films in the distorted octahedral phase via interrupting the beam of the metal source. In another report, Li et al.¹⁰⁹ have discussed the growth of high-quality and epitaxial PdTe₂ thin film on a bilayer graphene/SiC(0001) substrate by MBE and have characterized this atomically thin film using STM. PdTe₂ has recently stimulated considerable research interest because of the underlying physics associated with this material system, as it exhibits the coexistence of both type-II Dirac fermions and superconductivity. ARPES measurements of a six-layered PdTe₂ thin film reveal a type-I Dirac cone contributing from Z2 topological surface states and their metallicity. Similarly, PtTe₂ is a type-II Dirac semimetal and exhibits unique photodetection capability in the mid infrared (MIR) region and robust ambient stability. Wei et al.¹¹⁰ have recently shown large area and high-quality 1T-PtTe₂ monolayer thin films grown by MBE with excellent air stability. It is observed that a PtTe₂-based photodetector displays a photoresponse in a broad spectral range with high photoresponsivity and specific detectivity. The transmission electron microscopy (TEM) images of the grown PtTe₂ depict the hexagonal arrangement of the Pt and Te atoms with lattice spacings of 0.20 and 0.35 nm, corresponding to the (110) and (100) planes of the hexagonal phase of PtTe₂, respectively, as shown in Fig. 5(c–e). The PtTe₂-based photodetector shows a broadband photoresponse (bias voltage of 0.1 V) in the wavelength range of 420 nm–10.7 μm, exhibiting responsivities of 1.6 and 0.2 mA W⁻¹ at 4.7 and 10.7 μm, respectively, as shown in Fig. 5(f and g). The three photoresponse cycles (Fig. 5(f)) maintain similar responsivities and noise levels, indicating stable and repeatable photodetection. In addition, these photodetectors also exhibit high sensitivity in the near infrared and visible regions.

Fig. 5 (a) Atomic-resolution STM image (I_t = 300 pA, V_b = 500 mV) and the corresponding (b) STS curves of the 2H-1T′ MoTe₂/graphene ultrathin films. The figures have been reproduced from ref. 107. (c) The HRTEM image of the 1T-PtTe₂ thin film. (d) The zoomed-in image of the marked area in (c). (e) FFT of the layered 1T-PtTe₂ structure is shown in (d). (f) Spectral photoresponse of the PtTe₂. (g) Broadband photoresponse at different wavelengths (as indicated in the figure). The magnitudes of low responsivities (420 nm and 10.7 μm) have been multiplied for better visualization. The figure has been reproduced from ref. 110. (h) The STM image of monolayer VTe₂ measured at 77 K (150 × 150 nm²; tunnelling current = 68 pA, tip bias = −0.89 V). Residual caps have been marked by yellow arrows. (i) A step height of ∼8.5 Å indicates monolayer VTe₂ as shown in the line profile in (h). (j) The STM image showing atomic-resolution (10 × 10 nm²; tunnelling current = 150 pA, tip bias = +0.1 V). The figure has been reproduced from ref. 112.

Other transition metal di-tellurides to be successfully grown by MBE include monolayers of VTe₂, TiTe₂, and CrTe₂. In one such report, Lasek et al.¹¹¹ have shown that monolayer synthesis of 1T-TMDCs could be realized for all three early transition metal (Ti, V, and Cr) ditellurides via MBE, including the growth of CrTe₂, which is metastable only in its bulk form. In another report, Wong et al.¹¹² have presented direct spectroscopic and microscopic evidence of d¹ electronic configuration and metallic phase (1T) in monolayer VTe₂ grown by MBE on HOPG and a (4 × 4) CDW reconstruction pattern is obtained (Fig. 5(h–j)), different from the double zigzag chains observed in the bulk structure. Regarding the magnetic properties of the monolayer VTe₂, the X-ray magnetic circular dichroism (XMCD) measurements exclude the presence of an inherent ferromagnetic ordering, which contradicts the previous theoretical predictions.

3.4 Growth of chalcogenide-based alloys

Recently, 2D material-based alloys have also been investigated as they can represent a versatile platform that can extend the properties of the existing atomically thin TMDCs to the next level. Xie et al.¹¹³ have realized the MBE growth of monolayer Mo_xW_1−xSe₂ alloys with controllable values of stoichiometric ratio. Utilizing in situ ARPES and XPS, the modifications in the valence band dispersion and the size of the spin-splitting have been determined by modulating the Mo/W ratios, as illustrated in Fig. 6(a–h). The growth of monolayers of these Mo_xW_1−xSe₂ alloys by MBE and the growth recipe for controlling the stoichiometric ratio paves an effective method for band structure engineering and spin-splitting in the 2D AB₂ family. Similarly, Zhang et al.¹¹⁴ have investigated the synthesis of V_xMo_1−xSe₂ alloys by MBE on HOPG. A systematic study on the evolution of magnetism, electronic structure, and the thermal stability of V_xMo_1−xSe₂ alloys has been presented. Essentially, a critical value of x (∼0.44) has been observed, above which the system remains as a homogeneous metallic phase and below which, separation of phases is favoured. Low vanadium concentrations (x ≤ ∼0.05) effectually increase the density of mirror twin boundaries in MoSe₂. Fig. 6(i–m) depict the evolution of the morphology of the 2D V_xMo_1−xSe₂ alloys and as the value of x increases, the crystalline domains gradually evolve from leaf-like (MoSe₂) into the triangular (VSe₂) form. The XMCD measurements show the absence of ferromagnetism down to 65 K, both in V_xMo_1−xSe₂ alloys and monolayer VSe₂. Thus, these works provide a comprehensive understanding of the electronic structures and properties of 2D TMDC alloys, which can be useful for designing futuristic 2D electronic devices.

Fig. 6 The ARPES spectra of monolayer Mo_xW_1−xSe₂ thin films measured along the Γ–K direction with different Mo ratios (as indicated). The Mo ratios (x) in Mo_xW_1−xSe₂ from (a–h) can be stoichiometrically displayed as: (a) WSe₂, (b) Mo_0.10W_0.90Se₂, (c) Mo_0.14W_0.86Se₂, (d) Mo_0.19W_0.81Se₂, (e) Mo_0.40W_0.60Se₂, (f) Mo_0.68W_0.32Se₂, (g) Mo_0.76W_0.24Se₂, and (h) MoSe₂. The, green, yellow, and red lines indicate the energy levels at the K point, Fermi level, and C point, respectively. The energy difference between the K and Γ points, and the spin-splitting at the K point are also shown in (a–h). The figures have been reproduced from ref. 113. The STM and STS images of V_xMo_1−xSe₂ alloys (250 × 250 nm²): (i) MoSe₂, (j) V_0.26Mo_0.74Se₂, (k) V_0.55Mo_0.45Se₂, (l) V_0.78Mo_0.22Se₂, and (m) VSe₂ (set points: (i) −3.8 V, 10 pA; (j) 1.6 V, 17 pA; (k) 1.6 V, 23 pA; (l) −1.6 V, 5 pA; (m) −0.5 V, 44 pA). The figure has been reproduced from ref. 114.

4. Summary and outlook

Recent years have witnessed remarkable progress in the MBE growth of a variety of TMDC thin films. In this article, we have discussed the fundamental properties of TMDCs, along with a brief review of the standard synthesis methods used for their fabrication. A short discussion on MBE has followed this, and finally, a progressive conversation regarding the MBE-growth of various TMDCs has been presented. These results unquestionably show the potential of MBE as one of the most promising and reliable techniques for the growth of high-quality TMDCs and contribute towards the advancement in the applications based on TMDCs in the future.

The published literature in this field indubitably indicates the substantial progress of TMDCs grown by MBE; however, the potential of this special class of materials still needs to be fully unearthed. Hence, a perspective and the associated follow-up research have been given below and illustrated in Fig. 7.

Fig. 7 Schematic illustrating the current prospects and opportunities to be explored in the field of MBE-grown TMDCs.

• The investigations on the MBE-fabricated TMDCs are still in the nascent phase, because of the recent popularity of MBE for the synthesis of TMDCs, in comparison to the other conventional synthesis techniques such as exfoliation and CVD. Thus, there still exists plenty of room for the enhancement of the crystal quality of these thin films by utilizing appropriate substrates for their growth and by further correlating the unexplored or much less explored growth parameters, for instance, the beam flux of the sources, annealing time, growth temperature, etc.^{86,111,115–117}

• To date, most of the research in this field is limited to the growth of TMDC thin films and heterostructures. It is high time that focus needs to be given to fabricating MBE-grown TMDC-based devices, which can be promising in various applications such as photodetection, gas sensing, thermoelectrics, photocatalysis, energy generation, photovoltaics, etc.^89,118–121

• Doping of MBE-grown TMDC thin films is another crucial area that needs to be explored in the future, which will help in enhancing the performance of TMDC-based devices. In addition, there is a great necessity to optimize p-type doping of TMDCs, which will be beneficial for the further growth of wide band gap n-type III-nitride semiconductors such as GaN or AlGaN to be utilized in p–n diodes, Esaki diodes, etc.^122–124

• So far, promising results have been obtained in synthesizing semi-metallic phases of TMDCs (1T), which can be used in advanced optoelectronic applications, as they possess outstanding light absorption and electronic properties.^{75,103,110,112,125–127}

• More research should be devoted to the study of TMDC-based alloys. Moreover, Janus TMDCs, which refer to transition metal layers with dissimilar surfaces, have engrossed intensive interest these days owing to their unique characteristics induced due to symmetry breaking and must be explored by MBE.^113,114,128

• Even though the surfaces of TMDCs are more inert than those of non-layered materials, surface oxidation is still thermodynamically favored for TMDCs. Thus, the issue of surface oxidation needs to be solved and therefore, the study of the oxidation kinetics of the MBE-grown TMDCs can be an exciting area of investigation.^129,130

As a concluding remark, MBE has emerged as a promising growth technique for large-scale TMDC thin films, and hence, holds the potential for use in next-generation device applications.

Conflicts of interest

The authors declare no conflicts of interest.

Acknowledgements

The authors would like to convey sincere gratitude to the Director, CSIR-NPL, New Delhi, India for his constant support. D. K. S. is thankful to the SERB National Postdoctoral Fellowship (PDF/2021/002469) for providing financial support.

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