2D Layered Transition Metal Dichalcogenide Semiconductors via Non-Aqueous Electrodeposition

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Transition metal dichalcogenides (TMDCs) are inorganic materials of formula ME2 (M = metal; E = chalcogen = sulfur, selenium or tellurium). They form 2-dimensional layered hexagonal structures related to that of cadmium diiodide, in which the metal-chalcogen bonding within the layers is very strong, whilst that between the layers is much weaker (van der Waals interactions) - i.e. inorganic analogues of graphite. They form a class of extremely important functional semiconductors, and by changing the metal or chalcogen type, the semiconductor band gap can be tuned, making them useful for a wide range of applications. As a result of both their structures and their semiconducting properties, these materials are widely considered to have the potential to revolutionalise next generation electronics, e.g. allowing the mass manufacture of 2D nanotransistors, leading to more powerful and faster devices. Controlling their dimensionality to produce individual layers of highly anisotropic ME2, leads to a number of remarkable properties, including strong spin splitting. Hence 2D thin films of materials such as molybdenum sulfide/selenide (MoE2) are highly promising candidates in a variety of applications. Amongst the most technologically important application is in next generation 2D transistors. Their lack of 'dangling' bonds and structural stability make them the primary candidate for post-Si CMOS (Complementary Metal-Oxide-Semiconductor) transistors, particularly in low power electronics. Only as recently as 2012, the first field effect transistors (FETs) based solely upon 2D TMDCs were reported - using molybdenum or tungsten disulfide obtained by exfoliation of individual layers from crystals, combined with boron nitride gate dielectric and graphene electrodes. Advances in scalable and controllable sample preparation to make large amounts of atomically thin and uniform TMDC layers is the key breakthrough required. Our proposal addresses these issues in a unique way. Our vision is to pioneer the development of a versatile platform for the non-aqueous electrodeposition of high quality 2D layered TMDC thin films from custom-made single molecular compounds that can act as the source of both the metal and the chalcogen. Our priorities are to demonstrate electrodeposition of MoE2, WE2 and the magnetically interesting NbE2 films with good control of the M:E ratio present in the deposited films and their morphology. We will benchmark their functional properties (electrical and magnetic). Since atom-by-atom growth away from a conducting electrode surface is an intrinsic feature of electrodeposition, but is not typical of other alternative (vapour) deposition methods, we will seek to exploit this unique opportunity by: (i) using specially designed recessed-line electrodes to create an electrical contact directly from the conducting surface of the electrode to the edge of the 2D layer (where the M-E bonding is strongest),(ii) electrodepositing the 2D TMDC directly into a fabricated back-gate transistor structure to create a demonstrator device; this approach would eliminate several expensive and inconvenient processing steps, such as exfoliation to form individual TMDC layers, transfer and electrical contacting onto the top of the van der Waals layer of the TMDC, and, more speculatively,(iii) directly depositing a p-n-p type junction based on sequentially depositing p-type and n-type TMDC semiconductors, by changing the precursor source during the experiment. In this way we will establish the viability of electrodeposition as an alternative low-cost processing method for the production of next generation devices incorporating these important materials. This is a high-risk/high-gain project that has the potential to have significant impact, opening up many opportunities for academic researchers in the short-medium term, and which could have very significant commercial impact in the longer term.