The bonds

TMDCs are a subset of the layered materials family, where the intra-layer bonding is covalent and the inter-layer bonding consists of van der Waals (vdW) forces. This type of bond structure leads to highly anisotropic properties, where in-plane mechanical, thermal, and electrical properties significantly outperform the out-of-plane properties.^{Reference Lévy6}

Within a single plane, there are two types of crystal structures that dictate the electronic properties of the layers—trigonal prismatic (2H) and octahedral (1T). The trigonal prismatic structure is thermodynamically favorable for the majority of Mo and W-based TMDCs and leads to semiconducting behavior in the layers, while the octahedral phase is more stable for Hf and Zr-based TMDCs.^{Reference Gong, Zhang, Wang, Colombo, Wallace and Cho7} While Hf and Zr-based TMDCs are semiconducting, many octahedrally bound TMDC layers (for example, 1T-TaS₂) exhibit metallic or semi-metallic behavior, and each phase (trigonal or octahedral) can be transformed to the other via atomic gliding of the layers.

A brief history

Naturally occurring TMDCs (specifically MoS₂) have been around for billions of years,^{Reference Golden, McMillan, Downs, Hystad, Goldstein, Stein, Zimmerman, Sverjensky, Armstrong and Hazen8} but scientific publications on synthetic TMDCs did not begin to appear until the 1950s.^{Reference Eggertsen and Roberts9} The first reports of MoS₂ synthesis were from the reduction of molybdenum trisulfide (MoS₃)^{Reference Eggertsen and Roberts9,Reference Yagoda and Fales10} and high temperature solution-based techniques.^{Reference Arutyuny and Khurshud11} These initial works led to a wave of TMDC research from the 1960s to the 1980s, which primarily focused on the utilization of TMDCs for catalysis. During this time, there were hundreds of papers, books, and review articles written on the chemical synthesis (chemical vapor transport or deposition) and properties of TMDCs (too many to properly cite all).^{Reference Lévy6,Reference Wilson and Yoffe12–Reference Grasso17} The first report of a single-layer TMDC was reported in 1986,^{Reference Joensen, Frindt and Morrison18} but this was followed by stagnation of the field until 2005 when Geim and Novoselov demonstrated mechanical exfoliation of graphite, boron nitride, and molybdenum disulfide (among other) layered materials to form monolayers,^{Reference Novoselov, Jiang, Schedin, Booth, Khotkevich, Morozov and Geim4} which exhibited very different properties than their bulk counterparts. Since then, the field of 2D materials has been a primary focus of many nanoscience and nanotechnology journals, with thousands of publications on the subject.¹⁹

Synthesis of transition-metal dichalcogenides

The earliest methods of synthesizing single layers of TMDCs include solution-based synthesis and chemical exfoliation of naturally occurring TMDCs; however, it was only in the 1970s when thin films of TMDCs were synthesized. The primary synthesis method was the conversion of a thin transition-metal M (Mo, W, etc.) or transition-metal oxide (MoO₃, WO₃, etc.) film into MX₂ via exposure to a chalcogen vapor X (S(g), Se(g), H₂S, etc.),^{Reference Khelil, Essaidi, Bernede, Pouzet and Mebarki20–Reference Hadouda, Pouzet, Bernede and Barreau22} or reaction with solid chalcogen layers (Figure 1a). While this technique is quite successful at achieving continuous films of TMDs, it often results in nanocrystalline structures, though recent works indicate that this can be overcome to achieve high-quality multilayer films.^{Reference Tarasov, Campbell, Tsai, Hesabi, Feirer, Graham, Ready and Vogel23} On the other hand, many of the mechanically exfoliated flakes used today^{Reference Novoselov, Jiang, Schedin, Booth, Khotkevich, Morozov and Geim4,Reference Salvatore, Münzenrieder, Barraud, Petti, Zysset, Büthe, Ensslin and Tröster24–Reference Terrones, Del Corro, Feng, Poumirol, Rhodes, Smirnov, Pradhan, Lin, Nguyen, Elías, Mallouk, Balicas, Pimenta and Terrones32} often come from bulk crystals that are synthesized via chemical vapor transport (CVT).^{Reference Mercier33}

Figure 1. Various popular routes for the synthesis of 2D materials beyond graphene. (a) Thin-film conversion, where a solid film of metal or metal oxide is converted to an MX₂ structure by exposing it to chalcogen vapors; (b) chemical vapor transport, where constituent powders of M and X are mixed with a transport agent (i.e., Br) that assists the elements in vaporizing on the hot side of the growth vessel and recondensing on the cold side; (c) powder vaporization and transport, where powders of the constituent M and X are vaporized in a dynamic flow system to react and grow thin films of 2D materials; and (d) chemical vapor deposition, where chemical vapor precursors are used to synthesize the 2D materials. Adapted with permission from Reference 48. Note: M, transition metal; X, chalcogen; TA, transport agent; P, pressure.

CVT (Figure 1b) consists of mixing stoichiometric amounts of the transition metal with the chalcogen and a transport agent in a sealed vacuum tube. This system is then placed in a high-temperature furnace (700–1000°C), where a temperature gradient across the sealed tube provides a driving force for transport of the elemental materials to the cool end, forming crystals of MX₂.^{Reference Liu, Ang, Lu, Song, Li and Sun34–Reference Al-Hilli and Evans36} While this technique provides a facile route to mechanical exfoliation of monolayer materials, it was not until the development of in situ powder vaporization (Figure 1c) that single crystal monolayer TMD materials were directly synthesized on insulating substrates.^{Reference Lin, Zhang, Huang, Liu, Lee, Liang, Chu and Li37} Most studies utilizing this method focus on vaporization of metal oxide (MoO₃, WO₃, etc.) and chalcogen (S, Se) powders in a horizontal tube furnace, where the chalcogen powder is placed upstream from the hot zone and just outside the tube furnace.^{Reference Zhan, Liu, Najmaei, Ajayan and Lou38–Reference Gutiérrez, Perea-López, Elías, Berkdemir, Wang, Lv, López-Urías, Crespi, Terrones and Terrones42} Upon heating, the metal oxide and chalcogen powders are vaporized and transported downstream in the furnace, coating the substrate material and forming MX₂ (Figure 1c).

Finally, there are a variety of more sophisticated, industrially compatible, methods of synthesizing monolayer TMDCs as well. For example, molecular beam epitaxy has yielded a variety of monolayer TMDC films, with unprecedented ability for in situ characterization of the films during synthesis. Additionally, chemical vapor deposition (CVD), and more specifically, metal–organic CVD (Figure 1d) shows significant promise for a truly scalable synthesis technique for TMDCs.^{Reference Eichfeld, Hossain, Lin, Piasecki, Kupp, Birdwell, Burke, Lu, Peng, Li, Azcatl, McDonnell, Wallace, Kim, Mayer, Redwing and Robinson43} This technique provides a means to quantitatively control the metal and chalcogen precursor introduction, can be scaled to >200 mm diameter wafers, and provides a way to strictly control the vapor-phase chemistry during synthesis—critical when tuning the properties of TMDCs for electronic and optical applications.

While the synthesis of these materials could lead to high impact science and advancements in technology, we still have a long way to go for controlling the stoichiometry of molybdenum-based (and other) TMDCs. Many reports discuss the synthesis of MoS₂ and related TMDCs, however, MoS₂ can exist over a wide range of sulfur atomic concentrations (66 → 69 at.%; see Figure 2) based on synthesis temperature.^{Reference Brewer, Lamoreaux and Massalski44–Reference Brewer, Lamoreaux and Massalski46} This translates to the possibility of growing the tetragonally bounded (2H) phase of MoS₂ with stoichiometry up to MoS_2.22 while still maintaining the octahedral structure. The story is quite similar for MoSe₂ and MoTe₂ (Figure 2), and this excess chalcogen (S, Se, Te) can significantly impact the physico-chemical properties of the layers and will undoubtedly affect one’s ability to achieve controlled doping of such layers for electronic applications. Importantly, tungsten-based TMDCs do not have this same problem. Each W-based TMDC is stoiciometrically pure (a line compound), which is favorable for controlling the stoichiometry, but these materials are often more difficult to synthesize, requiring higher temperatures.

Figure 2. Partial phase diagram of molybdenum-based transition-metal dichalcogenide materials showing the wide miscibility ranges of each compound. This miscibility must be considered for the synthesis of two-dimensional transition-metal dichalcogenides.^{Reference Brewer, Lamoreaux and Massalski44–Reference Brewer, Lamoreaux and Massalski46}

The possibilities

“What could we do with layered structures with just the right layers? What would the properties of materials be if we could really arrange the atoms the way we want them ...”^{Reference Feynman47} These words from Richard Feynman started the scientific world down the path to discover 2D materials. Like their three-dimensional material counterparts (metals, silicon, In_xGa_1-xAs, Al_xGa_1-xN etc.), monolayer 2D TMDCs have a wide range of properties from metallic and superconducting to semiconducting to magnetic with varying direct and indirect bandgaps.^{Reference Fiori, Bonaccorso, Iannaccone, Palacios, Neumaier, Seabaugh, Banerjee and Colombo2,Reference Wang, Kalantar-Zadeh, Kis, Coleman and Strano3,Reference Das, Robinson, Dubey, Terrones and Terrones48,Reference Chhowalla, Shin, Eda, Li, Loh and Zhang49} Furthermore, alloying or straining of TMDCs can be used to vary the bandgap. Figure 3 shows the theoretical bandgap for 2H-MX_2(1–x)X′_2x as a function of concentration (M = Mo, W and X, X′ = S, Se, Te)^{Reference Kang, Tongay, Li and Wu50} and Figure 4 shows the theoretical bandgap of several 2H-TMDs as a function of lattice constant modified through application of –5% to +5% biaxial strain.^{Reference Guzman and Strachan51} While experimental verification of these theoretical bandgaps is continuing, the results suggest that a combination of alloying and strain can provide bandgaps in these TMDCs ranging from 0 eV to as high as ∼2.0 eV.

Figure 3. Theoretical bandgap of MX_2(1–x)X′_2x monolayer alloys as a function of concentration. Alloying can be used to provide a continuous range of possible bandgaps and band offsets. Reprinted with permission from Reference 50. © 2013 AIP Publishing LLC. Note: M, transition metal; X, X′, chalcogen.

Figure 4. Theoretical bandgap as a function of the lattice constant for the different MX₂ monolayers studied. Each monolayer was strained biaxially in the range of +/–5%. The red arrows indicate the equilibrium lattice constant for each material. Strain can be used to provide a continuous range of possible bandgaps and band offsets. Reprinted with permission from Reference 51. © 2014 AIP Publishing LLC. Note: M, transition metal; X, chalcogen.

Stacking of 2D TMDC monolayers also strongly impacts their properties. For example, monolayer MoS₂ has a direct bandgap of ∼1.8 eV, whereas multilayers have a smaller indirect bandgap saturating at ∼1.2 eV for bulk (greater than ∼5 layers) MoS₂.^{Reference Kang, Tongay, Zhou, Li and Wu52,Reference Padilha, Peelaers, Janotti and Van de Walle53} The combination of materials used in a stack and associated interactions are also important.^{Reference Komsa and Krasheninnikov54} Although monolayer MoS₂ and WS₂ both have direct bandgaps, the bandgap of a MoS₂/WS₂ stack is indirect. On the other hand, a MoS₂/MoSe₂ stack is a direct gap with the valence band maximum associated with the MoSe₂ and the conduction band minimum associated with the MoS₂. Researchers have also developed a range of processes for doping 2D materials including assembling molecular monolayers onto the TMDC, which donate/accept electrons to/from the TMDC resulting in n-type/p-type doping,^{Reference Kiriya, Tosun, Zhao, Kang and Javey55–Reference Tarasov, Zhang, Tsai, Campbell, Graham, Barlow, Marder and Vogel59} doping based on ionic gating,^{Reference Ubrig, Jo, Berger, Morpurgo and Kuzmenko60,Reference Fathipour, Xu, Kinder, Fullerton-Shirey and Seabaugh61} and using conventional dopants that substitute for either the metal or chalcogen in the TMDC.^{Reference Suh, Park, Lin, Fu, Park, Jung, Chen, Ko, Jang, Sun, Sinclair, Chang, Tongay and Wu62–Reference Komsa, Berseneva, Krasheninnikov and Nieminen64}

While this “toolbox” of 2D TMDC properties and associated applications is interesting, conventional three-dimensional (3D) materials provide a similar parameter space. The properties of conventional 3D metals and semiconductors can also be modified using alloying and strain, and vertical heterostructures can be fabricated using epitaxy. So, the question is: What makes 2D TMDCs unique? The asymmetry of the in-plane covalent bonds compared to the out-of-plane vdW bonds for TMDCs results in properties not easily obtainable using 3D crystals. First of all, the unique bonding of TMDCs results in an absence of surface dangling bonds (surface states). For the case of very thin 3D semiconductors (<∼10 nm), control of these surface states through hydrogenation, oxidation, or lattice-matched epitaxy is crucial to ensure that surface states do not degrade the electronic/optical properties of the semiconductor. Also, the need for lattice matching during epitaxy of 3D materials limits the range of material combinations and achievable band offsets. This is critical, for example, in III–V or III-N resonant tunneling heterostructures where the low band offset between a quantum well and tunneling barrier results in thermionic emissions, which strongly contributes to the unwanted valley current. The vdW bond between 2D materials in vertical heterostructures allows for a wider range of materials that can be used. For example, hexagonal boron nitride (h-BN) with a large bandgap can be stacked with a monolayer TMD. Monolayer TMDCs are also an ideal quantum well in that they have only one longitudinal sub-band (one allowed energy level perpendicular to the material). Quantum wells of 3D semiconductors typically have multiple sub-bands associated with confinement effects, which strongly impact their electronic and optoelectronic properties.

The ability to define the layer property in a heterostructure based on the combination of a metal and chalcogen atom, which can be tuned from >40 binary combinations,^{Reference Chhowalla, Shin, Eda, Li, Loh and Zhang49} and nearly countless alloy possibilities opens up an extremely broad range of electronic, optical, and other properties and applications. The presence of a bandgap (as compared to graphene), the ability to intimately control the band structure of the material, and the ability to inherently control the thickness of TMDCs down to a monolayer creates a wide variety of possible electronic, optoelectronic, and other applications. TMDCs can enhance the performance of conventional field-effect transistors for digital electronics since the monolayer thickness and lack of surface dangling bonds improves performance, electrostatics, and associated lateral scaling.^{Reference Fiori, Bonaccorso, Iannaccone, Palacios, Neumaier, Seabaugh, Banerjee and Colombo2,Reference Salvatore, Münzenrieder, Barraud, Petti, Zysset, Büthe, Ensslin and Tröster24,Reference Yoon, Park, Bae, Kim, Jang, Hyun, Lim, Kahng, Hong, Lee and Ko27,Reference Das, Chen, Penumatcha and Appenzeller29,Reference Liu, Kang, Sarkar, Khatami, Jena and Banerjee31,Reference Roy, Tosun, Kang, Sachid, Desai, Hettick, Hu and Javey65–Reference Jariwala, Sangwan, Lauhon, Marks and Hersam67} Tuning the band structure of TMDCs permits development of novel low-power devices based on tunneling.^{Reference Lam, Cao, Guo and Member68,Reference Yamaguchi, Moriya, Inoue, Morikawa, Masubuchi, Watanabe, Taniguchi and Machida69} As monolayer TMDCs have a direct bandgap, efficient optoelectronic devices such as light-emitting diodes and solar cells can be fabricated.^{Reference Wang, Kalantar-Zadeh, Kis, Coleman and Strano3,Reference Nan, Wang, Wang, Liang, Lu, Chen, He, Tan, Miao, Wang, Wang and Ni26,Reference Baugher, Churchill, Yang and Jarillo-Herrero58,Reference Jariwala, Sangwan, Lauhon, Marks and Hersam67,Reference Tsai, Su, Chang, Tsai, Chen, Wu, Li, Chen and He70} The large surface-to-volume ratio of these materials enables a variety of highly sensitive chemical and biological sensors.^{Reference Jariwala, Sangwan, Lauhon, Marks and Hersam67,Reference Donarelli, Prezioso, Perrozzi, Bisti, Nardone, Giancaterini, Cantalini and Ottaviano71–Reference Sarkar, Liu, Xie, Anselmo, Mitragotri and Banerjee73} TMDCs also show large electroactivity, which is critical, for example, in catalysis.^{Reference Eng, Ambrosi, Sofer, Simek and Pumera74–Reference Voiry, Salehi, Silva, Fujita, Chen, Asefa, Shenoy, Eda and Chhowalla77}

The relationship between electron energy and momentum in TMDCs is also different than that for many conventional semiconductors. Because of the lack of spatial symmetry and the presence of d-orbitals associated with the transition-metal atoms, many TMDCs exhibit strong spin–orbit coupling, which is an interaction between the electron spin and orbital angular momentum.^{Reference Zibouche, Philipsen, Kuc and Heine78–Reference Guinea, Katsnelson and Wehling81} Spin–orbit coupling causes the energy of electrons with different spins to split and is useful in spintronic devices where the spin of the electron is used to carry information. The lack of symmetry also allows electrons or holes at the same energy but with different momentum (different band “valleys”) to be independently populated. The use of spin or valley to carry information can potentially result in much lower-power electronic devices because the electron does not necessarily have to move through the material to carry information; information can instead be encoded using the electron’s spin or valley.

We are now at the point where researchers can answer the questions originally posed by Feynman. We are now able to cleave and stack arbitrary atomic layers together to form complex material architectures, which could ultimately lead to the ability to “invent” a wealth of new properties based on the application at hand.

In this issue

The articles in this issue of MRS Bulletin provide a wide overview of the fundamental materials properties of TMDCs and their relationship to applications in electronics, optoelectronics, and energy. Shi et al. review the bulk and interfacial structure of TMDCs in detail. The relative merits of processes for synthesizing and modifying TMDCs are reviewed, including exfoliation, intercalation, vapor phase deposition, doping, and stacking. Kuc et al. review quantum calculations of the TMDC band structure to demonstrate the impact of electric field, strain, and stacking of layers on electronic properties. The application of TMDCs to transistors, circuits, and flexible devices is also presented. Chhowalla et al. describe the use of physical processes such as intercalation, doping, and heating to change the structural phase of TMDCs. The impact of these TMDCs phases on applications such as hydrogen evolution reactions, battery electrodes, and electronics is then discussed. Finally, Cao describes the properties of excitons in TMDCs including their binding energies, absorption, emission, and other dynamic properties. The engineering of these excitons to achieve interesting light–matter interactions and associated optoelectronic applications are described.