Photoemission measurements on exfoliated 2D heterostructures reveal detailed electronic structure and hybridization effects.

The stacking orders in layered hexagonal boron nitride bulk and bilayers are studied using high-level ab initio theory [local second-order Møller-Plesset perturbation theory (LMP2)]. Our results show that both electrostatic and London dispersion interactions are responsible for interlayer distance and stacking order, with AA' being the most stable one. The minimum energy sliding path includes only the AA' high-symmetry stacking, and the energy barrier is 3.4 meV per atom for the bilayer. State-of-the-art density functionals with and without London dispersion correction fail to correctly describe the interlayer energies with the exception of a Perdew-Burke-Ernzerhof functional intended for solid state and surface systems that agrees very well with our LMP2 results and experiment.

The ability to directly observe electronic band structure in modern nanoscale field-effect devices could transform understanding of their physics and function. One could, for example, visualize local changes in the electrical and chemical potentials as a gate voltage is applied. One could also study intriguing physical phenomena such as electrically induced topological transitions and many-body spectral reconstructions. Here we show that submicron angle-resolved photoemission (-ARPES) applied to two-dimensional (2D) van der Waals heterostructures affords this ability. In graphene devices, we observe a shift of the chemical potential by 0.6 eV across the Dirac point as a gate voltage is applied. In several 2D semiconductors we see the conduction band edge appear as electrons accumulate, establishing its energy and momentum, and observe significant band-gap renormalization at low densities. We also show that -ARPES and optical spectroscopy can be applied to a single device, allowing rigorous study of the relationship between gate-controlled electronic and excitonic properties.Angle resolved photoemission spectroscopy (ARPES), in which the energy and momentum of photoemitted electrons are measured from a sample subjected to a spectrally narrow ultraviolet or X-ray excitation, is a powerful technique that yields the momentum-dependent single-electron band structure and chemical potential in a solid with essentially no assumptions. It probes only electron states near the surface, and so cannot be applied to conventional semiconductor devices. It is, however, very effective when applied to 2D materials and has been used extensively to study the bands in graphene 1 , monolayer transition metal dichalcogenides 2-7 , and others 8,9 . Furthermore, µ-ARPES (with a micron-scale beam spot) can be performed 10 on 2D heterostructures (2DHSs) 11 made of stacked exfoliated 2D materials 12-14 , suggesting the possibility of monitoring electronic structure during actual device operation. We demonstrate here that momentum-resolved electronic spectra can indeed be obtained during reversible electrostatic gating, enabling direct visualization of chemical potential shifts and band structure changes controlled by the gate electric field.A limitation of ARPES is that it probes only occupied electron states, and so a semiconductor must first be electron-doped in order to obtain a signal from the conduction band. The usual approach is to deposit alkali metal atoms 1-7,15 which act as an n-type dopant, but this has several limitations: the density cannot be controlled accurately; it can only be reversed by high-temperature annealing; it introduces disorder through the random positions of the dopants; and it chemically perturbs the electronic structure in ways that are hard to calculate. Electrostatic doping has none of these disadvantages, and the accessible carrier densities are most relevant to practical devices.We first validate our technique using graphene, and then go on to apply it to the 2D transition metal dichalcogenide (TMD) sem...

Black phosphorus (BP) has recently emerged as a promising semiconducting twodimensional material. However, its viability is threatened by its instability in ambient conditions, and by the significant decrease of its band gap in multilayers. We show that one could solve all the aforementioned problems by interfacing BP with hexagonal boron nitride (hBN). To this end, we simulate large, rotated hBN/BP interfaces using linear-scaling density functional theory (DFT). We predict that hBN-encapsulation preserves the main electronic properties of the BP monolayer, while hBN spacers can be used to counteract the band gap reduction in stacked BP. Finally, we propose a model for a tunneling field effect transistor (TFET) based on hBN-spaced BP bilayers.Such BP TFETs would sustain both low-power and fast-switching operations, including negative differential resistance behaviour with peak-to-valley ratios of the same order of magnitude as those encountered in transition metal dichalcogenide TFETs.1 Keywords 2D heterostructures; tunneling transistor; linear-scaling DFT; electric fields Since its inception, the two-dimensional (2D) material community has switched focus several times between different materials, aiming to satisfy different technological requirements. For instance, the lack of a band gap in graphene caused an increased interest in semiconducting transition metal dichalcogenides (TMDCs). While their many qualities have enabled their remarkable success in semiconductor electronics, 1,2 spintronics, 3 and optoelectronics, 2,4 the associated flaws of TMDCs are also of concern. Most notably, they only exhibit a direct band gap when in monolayer form, 5 the carrier mobilities are far smaller than those encountered in graphene, and their large band gap implies that the infrared (IR) spectrum cannot be harnessed by TMDC optoelectronics.In this context, the spotlight has recently shifted to layered black phosphorus (BP), 6,7 a material which rectifies the most important shortcomings of TMDCs. The band gap of BP is direct even in stacked forms, and its value changes significantly with the number of layers: from 0.3 eV in bulk to 1.5-2.0 eV for the monolayer. 8,9 This range of values allows BP to

We present an overview of the ONETEP program for linear-scaling density functional theory (DFT) calculations with large basis set (planewave) accuracy on parallel computers. The DFT energy is computed from the density matrix, which is constructed from spatially localized orbitals we call Non-orthogonal Generalized Wannier Functions (NGWFs), expressed in terms of periodic sinc (psinc) functions. During the calculation, both the density matrix and the NGWFs are optimized with localization constraints. By taking advantage of localization, ONETEP is able to perform calculations including thousands of atoms with computational effort, which scales linearly with the number or atoms. The code has a large and diverse range of capabilities, explored in this paper, including different boundary conditions, various exchangecorrelation functionals (with and without exact exchange), finite electronic temperature methods for metallic systems, methods for strongly correlated systems, molecular dynamics, vibrational calculations, time-dependent DFT, electronic transport, core loss spectroscopy, implicit solvation, quantum mechanical (QM)/molecular mechanical and QM-in-QM embedding, density of states calculations, distributed multipole analysis, and methods for partitioning charges and interactions between fragments. Calculations with ONETEP provide unique insights into large and complex systems that require an accurate atomic-level description, ranging from biomolecular to chemical, to materials, and to physical problems, as we show with a small selection of illustrative examples. ONETEP has always aimed to be at the cutting edge of method and software developments, and it serves as a platform for developing new methods of electronic structure simulation. We therefore conclude by describing some of the challenges and directions for its future developments and applications.