Self-organized twist-heterostructures via aligned van der Waals epitaxy and solid-state transformations

Peter Sutter,Rina Ibragimova,Hannu-Pekka Komsa,Eli Sutter,Bruce A Parkinson

doi:10.1038/s41467-019-13488-5

Peter Sutter, Rina Ibragimova + Show 3 more

Open Access

https://doi.org/10.1038/s41467-019-13488-5

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Abstract

Vertical van der Waals (vdW) heterostructures of 2D crystals with defined interlayer twist are of interest for band-structure engineering via twist moiré superlattice potentials. To date, twist-heterostructures have been realized by micromechanical stacking. Direct synthesis is hindered by the tendency toward equilibrium stacking without interlayer twist. Here, we demonstrate that growing a 2D crystal with fixed azimuthal alignment to the substrate followed by transformation of this intermediate enables a potentially scalable synthesis of twisted heterostructures. Microscopy during growth of ultrathin orthorhombic SnS on trigonal SnS2 shows that vdW epitaxy yields azimuthal order even for non-isotypic 2D crystals. Excess sulfur drives a spontaneous transformation of the few-layer SnS to SnS2, whose orientation – rotated 30° against the underlying SnS2 crystal – is defined by the SnS intermediate rather than the substrate. Preferential nucleation of additional SnS on such twisted domains repeats the process, promising the realization of complex twisted stacks by bottom-up synthesis.

Highlights

Vertical van der Waals heterostructures of 2D crystals with defined interlayer twist are of interest for band-structure engineering via twist moiré superlattice potentials
We showed that architectures that depart from the usual 2D van der Waals (vdW) heterostructure geometry— layered nanowires with Eshelby twist—can spontaneously yield selforganized twist moirés with small twist angles that are tunable via the nanowire diameter[30]
The weak interaction between layered crystals can enable a different avenue toward the bottomup synthesis of twist heterostructures via a two-step process in which the synthesis of an intermediate 2D crystalline phase (B) on a layered substrate (A) is followed by the conversion of the intermediate to a final phase Arot with a defined azimuthal rotation relative to the substrate

Summary

Introduction

Vertical van der Waals (vdW) heterostructures of 2D crystals with defined interlayer twist are of interest for band-structure engineering via twist moiré superlattice potentials. There are two fundamentally interesting regimes for which synthesis approaches need to be developed: (i) Precise small-angle interlayer twists, which cause moiré patterns that give rise to a modulation of the electronic structure within large unit cells whose lateral dimensions can exceed 100 nm; and (ii) high-angle twists In both cases, the realization of a controlled interlayer orientation during growth is hindered by the strong tendency of 2D crystals to stack in their equilibrium registry, i.e., azimuthally aligned without any twist, both for homo-stacks of the same or hetero-stacks of different but isostructural 2D materials[20,21,22,23,24,25,26,27,28,29]. In situ low-energy electron microscopy (LEEM) during growth allows us to identify the sequence of azimuthally aligned vdW epitaxy and spontaneous solid-state transformations that gives rise to self-organized SnS2 twist heterostructures and incipient complex architectures, such as periodic vertical stacks with multiple twisted vdW interfaces

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