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

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.

Beneficiaries from the new knowledge and scientific advances arising from this project fall into the following main categories:

1) High Tech Sector - companies working on advanced materials for high tech applications, including '21st century products' and 'internet of things' applications will benefit from having access to a new and generic processing method for the production of thin film transition metal dichalcogenide (TMDC) materials. Electrodeposition offers room temperature deposition and access to a range of different TMDCs (allowing easy tuning of the band gap), potentially bringing significant energy and cost savings (reducing the number of processing steps) and also allowing deposition of these materials onto a wide range of flexible substrates (e.g. polymers) for next generation devices. Further benefits could arise by taking advantage of the unique features of electrodeposition to allow atom-by-atom growth of the TMDC away from the conducting surface (electrode), and the stronger in-plane bonding characteristic of the TMDCs which will favour 2D growth in a way that other deposition methods (e.g. physical vapour deposition or chemical vapour deposition) do not. Lateral growth of the TMDC from an electrical contact point would also reduce (and potentially eliminate completely) the challenges associated with electrically contacting the 2D TMDC semiconductor, since the contact is made as the first point of nucleation (and is preferentially formed on the edge of the 2D sheet). If successful, this method would enable growth directly onto the device structure, removing the need for transfer of the 2D material onto the device substrate, potentially making mass production of 2D transistor devices much simpler. Successful demonstration of these new capabilities has the potential to bring significant societal (access to faster and smaller personal electronic devices) and economic benefits to the UK in the medium to long term.

2) General Public and School Children - we plan to develop props and activities to convey the scientific principles behind the project. This will benefit school children and the general public who will have opportunities to engage with cutting-edge research, both through publically accessible demonstrators to explain how transistors, the basic component of a microprocessor, work (e.g. the 'water transistor' to be hosted in the Winchester Science Centre), as well as facilitating direct conversations with researchers through talks, school visits and science festivals. These activities will increase the Science Capital of both the public (including UK tax payers) by informing them of the key role that (interdisciplinary) scientific research plays in improving the quality of life in modern society, as well as informing and inspiring the next generation of scientists and engineers, encouraging increased uptake of these disciplines through schools and universities, and in the longer term increasing the UK's scientific skills base.

3) Academic Researchers - materials scientists and electronic engineers working on layered metal chalcogenide materials for electronics and other application areas (catalysis, environmental sensing, spintronics, magnetism) will also benefit from the ability to readily access a wide range of TMDCs via a new and generally applicable deposition method. The novel concept of developing 'single source electrochemical precursors' (single molecular reagents that act as the source of both the metal and the chalcogen) will also benefit inorganic synthesis chemists and electrochemists by stimulating new work both to exploit this concept more widely to create new reagents and to electrodeposit TMDCs for other applications and to target other important categories of materials (e.g. III-V, II-VI materials) and coatings.

4) The training of researchers in advanced synthesis, electrodeposition and nanoelectronics will equip UK industry to compete better in these areas in future.