Layered Material Heterostructures: A new twist on 2D materials

Speaker: Dr Nick Hine (Warwick)

A 2D material that is incorporated into any kind of device automatically becomes in effect part of a heterostructure, since the atomic-scale widths of these novel materials also mean their `bulk' regions are never really independent of their surroundings. Furthermore, novel physics often emerges from the hybridisation of the bandstructure that results when materials are combined. Controlling the properties of layered material heterostructures is therefore crucial to the success of devices based on the novel capabilities of 2D materials.

However, while monolayer 2D materials are very well-studied, theoretical insight into heterostructures has been limited by the large system sizes required to study the interfaces between pairs of materials, which in general will be incommensurate and may be rotated with respect to one another. My group develops and uses a code called ONETEP which can to perform linear-scaling DFT calculations with non-local vdW functionals, and exploit bandstructure unfolding for insight into electronic properties. We have used it to explore large-scale models of heterostructures of interest for device applications.

I will present results for heterostructures including MoS2/MoSe2, MoSe2/WSe2, hBN/Phosphorene and InSe/graphene. Band-structure changes caused by stacking and rotation of the layers are obtained by unfolding the supercell spectral function into the primitive cells, incorporating spin-orbit coupling. Changes in spectral weight and band-structure between the monolayers and heterostructured interfaces show how lattice mismatch (MoS2/MoSe2) or spacer layers (Phosphorene/hBN/Phosphorene) can allow the component monolayers to retain more independence in heterostructures than in homo-stacks. Applying electric fields allows the behaviour of gated structures to be predicted and explained. Finally, we find that in the case of InSe, the choice of substrate can be used to dramatically tune the nature of the bandgap, potentially driving transitions from indirect to direct gap in the monolayer.

P. V. Nguyen, [...], G. C. Constantinescu, N. Yeung, NDMH, X. Xu, D. H. Cobden, N. R. Wilson, Visualizing electrostatic gating effects in two-dimensional heterostructures, Nature 572, 220 (2019).

N. R. Wilson, ..., NDMH, X. Xu, D. H. Cobden, Determination of band offsets, hybridization and exciton binding in 2D semiconductor heterostructures, Science Advances 3, e1601832 (2017).

G. C. Constantinescu, NDMH, Multi-purpose Black-Phosphorus/hBN heterostructures
Nano Letters, 16, 2586 (2016).

G. C. Constantinescu and NDMH, Energy landscape and band-structure tuning in realistic MoS2/MoSe2 heterostructures, Phys. Rev. B. 91, 195416 (2015).