Van der Waals heterostructures

TL;DR

With steady improvement in fabrication techniques and using graphene’s springboard, van der Waals heterostructures should develop into a large field of their own.

Abstract

Research on graphene and other two-dimensional atomic crystals is intense and likely to remain one of the hottest topics in condensed matter physics and materials science for many years. Looking beyond this field, isolated atomic planes can also be reassembled into designer heterostructures made layer by layer in a precisely chosen sequence. The first - already remarkably complex - such heterostructures (referred to as 'van der Waals') have recently been fabricated and investigated revealing unusual properties and new phenomena. Here we review this emerging research area and attempt to identify future directions. With steady improvement in fabrication techniques, van der Waals heterostructures promise a new gold rush, rather than a graphene aftershock.

Figures (3)

• Figure 1: Building vdW heterostructures. If one considers 2D crystals as Lego blocks (right panel), construction of a huge variety of layered structures becomes possible. Conceptually, this atomic-scale Lego resembles molecular beam epitaxy but employs different 'construction' rules and a distinct set of materials.
• Figure 2: State of the art vdW structures and devices. a - Graphene-hBN superlattice consisting of 6 stacked bilayers. Right: Its cross-section and intensity profile as seen by scanning transmission electron microscopy. Left: schematic view of the layer sequence. The topmost hBN bilayer is not visible, being merged with the metallic contact. Scale bar, 2 nm . Adapted from ref. 16. b,c- Double-layer graphene heterostructures${ }^{18}$. An optical image of a working device (b) and its schematics (c). Two graphene Hall bars are accurately aligned, separated by a trilayer hBN crystal and encapsulated between relatively thick hBN crystals. Colours in (b) indicate the top (blue) and bottom (orange) Hall bars and their overlapping region in violet. The graphene areas are invisible in the final device image because of the top Au gate outlined by dashes. The scale is given by the width of the Hall bars, $1.5 \mu \mathrm{~m}$.
• Figure 3: Early harvest in vdW fields. a - Magnetic focusing in graphene on hBN. Pronounced resonances are observed if the size of a cyclotron orbit becomes commensurate with the distance between narrow graphene leads used as an injector and detector (adapted from ref. 36 ). Bright colours show maxima in conductivity as a function of carrier density and$B$. The upper panel illustrates the corresponding trajectories. $\mathbf{b}$ - Quantum capacitance of encapsulated graphene as a function of gate voltage and $B$. In this device, spin and valley degeneracies are lifted above 10T. Adapted from ref. 68. c,d Importance of crystallographic alignment. The standard Dirac-like spectrum is strongly reconstructed for graphene on hBN, and new Dirac cones appear in both valence and conduction bands [inset in (c)]. This leads to pronounced peaks in resistivity (c) and the Hall effect changes sign (d). Inset in (d) shows the moiré patterns that lead to the spectral reconstruction. Adapted from ref. 76