The technology has always been an integral part of our lives whether it’s a simple light bulb in our homes and buildings or the latest smartphones with advanced front cameras for taking selfies. Our inclination towards overcoming our own limitations has gone to such heights that we have defined ages based on technology created – Stone age, Bronze age, Iron age, Industrial age and now the Information age.
Various researchers and scientists have gone to extreme lengths to uncover what different materials have to offer. Metals, non-metals, alloys, composites, polymers, and ceramics have all successfully made to commercial and industrial applications since large-scale manufacturing of these materials is highly feasible in the current era.
The present and relatively new class of materials under investigation also envisioned as materials of future, are atomically thick which makes them truly two-dimensional (having length and breadth only) and thus are termed “two-dimensional (2D) materials”.
The story of 2D materials started with the discovery of the first 2D material, graphene, in 2004 by Geim and Novoselov [1,2] who showcased graphene’s fascinating optoelectronic properties. This was followed by studying various graphene-like 2D materials.
Transition metal dichalcogenides (TMDCs), particularly, have shown great interest due to their wide range of electronic , optical , mechanical , chemical, and thermal properties . They are a class of materials with formula MX2 where M is transition metal atom (e.g. Mo, W, Ti, V, and Nb) and X is chalcogen atom (S, Se or Te). The TMDCs have X-M-X type layered structures where two planes of chalcogen atoms are separated by a plane of transition metal atoms.
TMDCs – potential superconductors
Superconductivity or conductivity without the interference of resistance is among the hot topics in the field of science and technology since the first discovery of the phenomenon in 1911 in mercury metal by Dutch physicist Heike Kamerlingh Onnes at Leiden University who showed near zero resistance of mercury metal at 4 K temperature.
Superconductors are materials that can conduct electricity without resistance when cooled below a certain temperature (transition temperature, Tc). Many elements including Pb, V, Al, Bi, Cd, and Mo and many compounds including MgB2, InN, In2O3, NbO, NbN and TiN also showed superconductivity at different transition temperatures. The most recent materials sought for superconductivity are iron-based chemical compounds called “pnictides” whose Tc is well above 50K.
However, large-scale production of many elements and complex compounds for superconducting applications is not feasible to date due to shear lack of their presence in generous quantities and overly high production costs.
TMDCs are now seen as a potential escape route for this scenario. Niobium diselenides and disulfides (NbSe2 and NbS2) are most sought after TMDCs for this role as they are easily available and can be easily manufactured. NbSe2 has reported Tc of ~ 7.2 K and NbS2 has reported Tc of ~ 6 K. The attempts are made to increase their Tc to expand their applications range in higher temperature regime and truly see them as future superconductors.
TMDCs – next semiconductors
TMDCs are also explored as a possible replacement for conventional semiconductors. Any material which shows electrical conductivity only when external energy is provided in form of heat, incident photons, or electrical means, is called a “semiconductor”.
Conventional semiconductors like Si, Ge, GaAs and InGaAs are usually opaque, lack effective light absorption, and are brittle in nature. These limitations restrict their usage to rigid substrates only. However, there may be certain exceptions.
Sizeable band gap (1 to 2 eV) and transparent nature of some of the TMDCs enables them to exhibit strong light-matter interactions, flexibility, and ease of processing which further empowers them to overcome limitations associated with the conventional choice of semiconductors.
Molybdenum disulfide (MoS2) and Tungsten disulfide (WS2) have caught recent attention in this arena owing to indirect-to-direct band gap transitions when exfoliated from their bulk state to their monolayer forms. The ease of processing, availability in significant quantities, and flexibility gives them a huge potential market as next-generation semiconductors, especially for flexible electronics.
Mechanical exfoliation, i.e., repeated peeling of TMDC layers from bulk crystals using scotch tape, is a most widely used method for producing high-quality 2-D TMDC nanosheets. However, uniform and controlled production of nanosheets are not possible with this approach.
Chemical vapour deposition (CVD) process is another approach where metal oxides and chalcogen precursors are evaporated to undergo vapour phase reaction leading to stable 2-D TMDC film formation over a suitable substrate. This method is used for the controlled, scalable and reliable production of TMDC nanosheets but the quality of the deposited TMDC films is relatively poor.
Atomic layer deposition (ALD) process is a thin film deposition technique used to deposit atomically thin films layer by layer by making precursors react with the surface of the material one at a time in a progressive fashion. The process can be carried out at relatively lower substrate temperatures and is highly scalable with precise control of thickness. However, high cost and sensitivity of precursors remain a set back for this process for large-scale production purposes.
Metal-organic chemical vapour deposition (MOCVD) utilises metal-organic compound precursors. The atoms to be deposited are fused with complex organic molecules and are made to flow over the substrate while maintaining adequate heat flow to decompose molecules and deposit desired atoms on the substrate atom by atom. The MOCVD process is highly scalable and can be significantly controlled. However, the use of toxic precursors, slow rate of film deposition, and high production costs limit the MOCVD process from making it big to significant production levels.
The probable future
To bring the TMDCs into actual practical use, we need a method that can be controlled to enable large area growth of atomically thin 2-D TMDCs while not compromising on uniformity and quality and which can be carried out at lower production costs.
The efforts are made to study new and different TMDC materials for any superconductive or semiconductive properties while also enhancing the properties of existing ones. The TMDCs are full of potential to make their move and take electronics on a whole new level.
 Novoselov, K. S., et al., Electric field effect in atomically thin carbon films, Science, 666-669, 2004.
 Novoselov, K. S., et al., Two-dimensional atomic crystals, Proceedings of the National Academy of Sciences, 10451-10453, 2005.
 Choi, W., et al., Recent development of two-dimensional transition metal dichalcogenides and their applications, Materials Today, 116-130, 2017.
 Staley, N. E., et al., Electric field effect on superconductivity in atomically thin flakes of NbSe2.” Physical Review B, 184505, 2009.
 Yue, Q., et al., Mechanical and electronic properties of monolayer MoS2 under elastic strain, Physics Letters A, 1166-1170, 2012.
 Shi, J., et al., Temperature-mediated selective growth of MoS2/WS2 and WS2/MoS2 vertical stacks on Au foils for direct photocatalytic applications, Advanced Materials, 10664-10672, 2016.
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