Quantifying electronic band interactions in van der Waals materials using angle-resolved reflected-electron spectroscopy

High electron mobility is one of graphene's key properties, exploited for applications and fundamental research alike. Highest mobility values are found in heterostructures of graphene and hexagonal boron nitride, which consequently are widely used. However, surprisingly little is known about the interaction between the electronic states of these layered systems. Rather pragmatically, it is assumed that these do not couple significantly. Here we study the unoccupied band structure of graphite, boron nitride and their heterostructures using angle-resolved reflected-electron spectroscopy. We demonstrate that graphene and boron nitride bands do not interact over a wide energy range, despite their very similar dispersions. The method we use can be generally applied to study interactions in van der Waals systems, that is, artificial stacks of layered materials. With this we can quantitatively understand the 'chemistry of layers' by which novel materials are created via electronic coupling between the layers they are composed of.We are grateful to Marcel Hesselberth, Daan Boltje and Ruud van Egmond for technical assistance. We thank Charles Kane for fruitful discussions and Kenji Watanabe for supplying the hBN base crystal. This work was supported by the Spanish Ministry of Economy and Competitiveness MINECO (project number FIS2013-48286-C2-1-P) and the Netherlands Organization for Scientific Research (NWO) via an NWO-Groot grant ('ESCHER'), a VIDI grant (680-47-502, S.J. v.d.M.), a VENI grant (680-47-447, J.J.) and by the FOM foundation via the 'Physics in 1D' programme. C.R.D. acknowledges support from NSF grant DMR-1463465

Understanding the catalyst-free transformation of amorphous carbon into graphene by current-induced annealing

We shed light on the catalyst-free growth of graphene from amorphous carbon (a–C) by current induced annealing by witnessing the mechanism both with in-situ transmission electron microscopy and with molecular dynamics simulations. Both in experiment and in simulation, we observe that small a–C clusters on top of a graphene substrate rearrange and crystallize into graphene patches. The process is aided by the high temperatures involved and by the van der Waals interactions with the substrate. Furthermore, in the presence of a–C, graphene can grow from the borders of holes and form a seamless graphene sheet, a novel finding that has not been reported before and that is reproduced by the simulations as well. These findings open up new avenues for bottom-up engineering of graphene-based devices.QN/Quantum NanoscienceApplied Science

Synthesis of large single-crystal hexagonal boron nitride grains on Cu-Ni alloy

Hexagonal boron nitride (h-BN) has attracted significant attention because of its superior properties as well as its potential as an ideal dielectric layer for graphene-based devices. The h-BN films obtained via chemical vapour deposition in earlier reports are always polycrystalline with small grains because of high nucleation density on substrates. Here we report the successful synthesis of large single-crystal h-BN grains on rational designed Cu-Ni alloy foils. It is found that the nucleation density can be greatly reduced to 60 per mm(2) by optimizing Ni ratio in substrates. The strategy enables the growth of single-crystal h-BN grains up to 7,500 mu m(2), approximately two orders larger than that in previous reports. This work not only provides valuable information for understanding h-BN nucleation and growth mechanisms, but also gives an effective alternative to exfoliated h-BN as a high-quality dielectric layer for large-scale nanoelectronic applications