Transition-metal dichalcogenides (TMDs) are renowned for their rich and varied bulk properties, while their single-layer variants have become one of the most prominent examples of two-dimensional materials beyond graphene. Their disparate ground states largely depend on transition metal d-electron-derived electronic states, on which the vast majority of attention has been concentrated to date. Here, we focus on the chalcogen-derived states. From density-functional theory calculations together with spin-and angle-resolved photoemission, we find that these generically host a coexistence of type-I and type-II three-dimensional bulk Dirac fermions as well as ladders of topological surface states and surface resonances. We demonstrate how these naturally arise within a single p-orbital manifold as a general consequence of a trigonal crystal field, and as such can be expected across a large number of compounds. Already, we demonstrate their existence in six separate TMDs, opening routes to tune, and ultimately exploit, their topological physics.The classification of electronic structures based on their topological properties has opened powerful routes for understanding solid state materials. 1 The nowfamiliar Z 2 topological insulators are most renowned for their spin-polarised Dirac surface states residing in inverted bulk band gaps. 1 In systems with rotational invariance, a band inversion on the rotation axis can generate protected Dirac cones with a point-like Fermi surface of the bulk electronic structure. 2-8 If either inversion or time-reversal symmetry is broken, a bulk Dirac point can split into a pair of spin-polarised Weyl points. 9-15 Unlike for elementary particles, Lorentz-violating Weyl fermions can also exist in the solid state, manifested as a tilting of the Weyl cone. If this tilt is sufficiently large, so-called type-II Weyl points can occur, now formed at the touching of open electron and hole pockets. [15][16][17][18][19][20][21][22] Realising such phases in solid-state materials not only offers unique environments and opportunities for studying the fundamental properties of fermions, but also holds potential for applications exploiting their exotic surface excitations and bulk electric and thermal transport properties. [23][24][25][26][27] Consequently, there is an intense current effort focused on identifying compounds which host the requisite band inversions. In many cases, however, this arXiv:1702.08177v2 [cond-mat.mtrl-sci]

Surface sensitive synchrotron x-ray photoelectron spectroscopy (XPS) and real-time in situ XPS were used to study the thermal stability of the hydroxyl termination and downward band bending on the polar surfaces of ZnO single crystals. On the O-polar face, the position of the Fermi level could be reversibly cycled between the conduction band and the band gap over an energetic distance of approximately 0.8 eV (similar to 1/4 of the band gap) by controlling the surface H coverage using simple ultrahigh vacuum (UHV) heat treatments up to 750 degrees C, dosing with H2O/H-2 and atmospheric exposure. A metallic to semiconductorlike transition in the electronic nature of the O-polar face was observed at an H coverage of approximately 0.9 monolayers. For H coverage less than this, semiconducting (depleted) O-polar surfaces were created that were reasonably stable in UHV conditions. In contrast, the downward band bending on the Zn-polar face was significantly more resilient, and depleted surfaces could not be prepared by heat treatment alone.preprintPeer reviewe

The stability of the ubiquitous hydroxyl termination and downward band bending on the m-plane () and a-plane () faces of ZnO single crystals was investigated using synchrotron and real-time x-ray photoelectron spectroscopy. On these non-polar surfaces, a strong correlation was found between the surface band bending and surface OH coverage, both of which could be modified via heat treatment in ultra high vacuum (UHV). On the m-plane () face, a threshold temperature of ~400 o C was observed, after which there was a sudden increase in OH desorption and upwards movement of the near-surface bands, leading to a metallic-to-semiconductor transition in the electronic nature of the surface, and a change from surface electron accumulation to depletion. This loss of surface metallicity is associated with the disruption of a stable monolayer of chemisorbed hydroxyl groups that form a closed hydrogen-bonded network, across the rows of Zn-O dimers, on the m-plane () face. The downward band bending and surface electron accumulation layers on both the m-plane () and a-plane () faces could be modified and eventually removed by simple UHV heat treatment, with important implications for the processing and electrical performance of ZnO nanostructures and catalytic ZnO nanopowders, which usually contain a high proportion of these non-polar surfaces.

Metallic transition-metal dichalcogenides (TMDCs) are benchmark systems for studying and controlling intertwined electronic orders in solids, with superconductivity developing from a charge-density wave state. The interplay between such phases is thought to play a critical role in the unconventional superconductivity of cuprates, Fe-based and heavy-fermion systems, yet even for the more moderately-correlated TMDCs, their nature and origins have proved controversial. Here, we study a prototypical example, 2H-NbSe2, by spin- and angle-resolved photoemission and first-principles theory. We find that the normal state, from which its hallmark collective phases emerge, is characterized by quasiparticles whose spin is locked to their valley pseudospin. This results from a combination of strong spin–orbit interactions and local inversion symmetry breaking, while interlayer coupling further drives a rich three-dimensional momentum dependence of the underlying Fermi-surface spin texture. These findings necessitate a re-investigation of the nature of charge order and superconducting pairing in NbSe2 and related TMDCs.

Nanoscale topographies and chemical patterns can be used as synthetic cell interfaces with a range of applications including study and control of cellular processes. Herein, we describe the fabrication of high aspect ratio nanostructures using electron beam lithography in the epoxy-based polymer SU-8. We show how nanostructure geometry, position and fluorescent properties can be tuned, allowing flexible device design. Further, thiol-epoxide reactions were developed to give effective and specific modification of SU-8 surface chemistry. SU-8 nanostructures were made directly on glass cover slips, enabling the use of high resolution optical techniques such as live-cell confocal, total internal reflection and 3D structured illumination microscopy to investigate cell interactions with the nanostructures. Details of cell adherence and spreading, plasma membrane conformation and actin organization in response to high aspect ratio nanopillars and nanolines were investigated. The versatile structural and chemical properties combined with high resolution cell imaging capabilities of this system are an important step towards better understanding and controlling cell interactions with nanomaterials.