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EPSRC Reference: EP/V047515/1
Title: Self-limiting Growth Mechanisms for Stable Monolayer Films of Non-van-der-Waals Oxides
Principal Investigator: Hofmann, Professor S
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Department: Engineering
Organisation: University of Cambridge
Scheme: Standard Research - NR1
Starts: 01 April 2021 Ends: 31 March 2023 Value (£): 202,154
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Summary on Grant Application Form
Progress and breakthroughs in high-tech applications, ranging from ICT, lighting to energy storage/generation and catalysis, builds on material deposition technology, which over the last decades has moved from bulk, to thin films to nanotechnology. A monolayer of a material represents the ultimate thinnest film possible, and the isolation and bottom-up growth of atomically-thin monolayer from layered van-der-Waals (vdW) materials highlight the huge impact both on fundamental research and applications that material design at the monolayer level can have. While the catalogue of such experimentally isolated 2D layers has been expanding, the focus has been almost exclusively on vdW solids for which the natural phase is layered. There remain large gaps in accessible properties and functionalities, such as the lack of electronic band gaps between 2.3-6 eV.

The motivation for this proposal is to explore new science and approaches how to achieve selective nucleation and anisotropic crystal growth that is truly self-limiting to a monolayer for non-vdW materials, and thus open a new horizon of possibilities for materials design and tailoring of properties at the atomic level that hitherto have been limited to vdW materials. Reported approaches for this to date generally fall into two categories: (1) forcing a 2D phase of the material by strong interaction with the substrate (epitaxy) or with two interfaces (confined growth), (2) achieving a 2D layering by selective etching of planes in a specific crystal structure, followed by liquid exfoliation (e.g. MAXene to MXene). A number of surface science studies have explored approach (1) with the state-of-the-art being the formation of small (<50 nm) quasi-2D domains due to strong support interactions. Such growth modes are neither fundamentally self-limiting, nor do they offer viable routes to remove the as-formed film from the substrate, with the material phase not stable in isolation. All literature on approach (2) is based on flake production, which remain multi-layered and of limited quality.

Given their industrial importance, this proposal will focus on high-k oxide materials as model system, not only on the fundamental studies of such materials, such as crystal growth and relaxation routes of non-vdW monolayers, but also on discovering scalable production approaches that are technologically relevant for future generations of electronics. In-operando methodology will be used to directly explore the nucleation, phase changes and domain growth behaviour of oxide crystal films during new hybrid atomic layer/chemical vapour deposition (ALD/CVD) processes, followed by their transfer and free-standing isolation through remote epitaxy. The identified mechanisms for the formation of self-limiting mono-layers of intrinsically non-vdW materials will open up a new playground for targeted structure and property design for a whole new class of materials, including inter-doping and inter-alloying strategies. The proposed novel oxide mono-layers can be exploited in a rich variety of applications, including in heterogeneous catalysis such as automotive converters, ultrathin barriers for spintronics, quantum-well phosphors, integrated gas sensors, optical coatings and high-k dielectrics in electronics.

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