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In-House 2DCC Research Highlights 2019

In-House 2DCC Research Highlights 2019

2-dimensional materials hold promise for next-generation electronics. However, in order to realize 2D-based technologies, key milestones must be identified and met. This article identifies areas of research which are fundamentally required for achieving electronic grade 2D materials and brings together experts in these respective areas to discuss key challenges. As a result, this article serves as a roadmap for the realization of 2D materials which can be used in the development of technology toward industries including healthcare, internet of things, high-performance computing, and economical solar energy.

 

This article can serve as a guide, and a source of inspiration and motivation for researchers and members of the 2D community. Additionally, it communicates the importance of 2D research to the greater scientific community, as well as industry players, demonstrating the  current and potential impact of these materials on technology and society.

A graphene lattice remains the same under a rotation of 180º. This special symmetry is lost in monolayer transition metal dichalcogenides (TMDs), e.g. every horizontal Mo-S bond would end up as S-Mo bonds. While this lack of inversion symmetry brings out fascinating properties such as spin-coupled valley degrees of freedom, it also means that when two grains related by 180º rotation merge, the two lattices cannot smoothly stitch together, leaving an unavoidable scar – an inversion domain boundary (IDB) – that degrades transport properties.

 

In two recent 2DCC publications, joint experimental and theory efforts identified a general method to achieve orientational selectivity, originating from a localized defect pair that amplifies the energetic distinction between the two orientations. For monolayer MoS2 or WSe2 grown on hexagonal boron nitride (hBN), the defect pair is a generalization of a so-called Frenkel pair – a interstitial-vacancy pair shared between adjacent layers of a 2D heterostack. MoS2 and WSe2 domains initiated from such defects exhibit an orientation uniformity of over 80%, leading to suppression IDBs upon coalescence, as confirmed by aberration-corrected scanning/transmission electron microscopy. Fully coalesced 2D chalcogenide monolayer films synthesized using this approach demonstrated superior optical and electrical properties compared to films grown on sapphire under similar conditions. The results motivate further efforts focused on the development of single crystal hBN substrates and epilayers for synthesis of wafer-scale single crystal TMD films.

The strongly spin-momentum coupled electronic states on the surfaces of topological insulators (TIs) exist because of time-reversal symmetry. The theoretical description of these states is fundamentally analogous to a picture used to describe particles known as ‘axions’ theoretically postulated to exist in Nature but never observed. Demonstrating and understanding this conceptual analogy is important for gaining new insights into how our universe works. By interfacing the two opposite surfaces of a TI thin film with different magnetic materials and measuring the magnetic field dependence of the electrical conductance,  we show that the ‘axion’ analogy is robust and leads to a new phase of matter known as an ‘axion insulator.’

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