The (2x2) reconstruction of the ZnO(0001) surface has been investigated by X-ray photoelectron diffraction (XPD). Comparing the XPD measurements with multiple-scattering simulations, the single Zn vacancy per (2x2) surface unit cell model is confirmed, and structures with O adatoms are ruled out. The analysis indicates an outward relaxation of the topmost Zn layer, in contrast to the usually reported results by density-functional theory (DFT) calculations. On the basis of DFT, we describe a new stabilizing mechanism of the polar ZnO surface through surface reconstruction where the Zn vacancies are occupied by three hydrogens atoms. The DFT surface relaxation of the proposed model is in excellent agreement with the XPD findings. Our DFT simulations also strongly indicate that the migration of hydrogens atoms to the surface, coming from the bulk, may influence the desorption of the surface Zn atom to create the vacancy.

The synthesis of atomically precise chemically active 2D molecular overlayers may be hindered by chemical interactions with the underlying substrate, especially when based on chlorophenyl porphyrins. At the same time, the chlorination of graphene, i.e., the covalent bonding of chlorine atoms with sp2 carbon atoms, is known to have a significant influence on the electronic properties of pristine graphene. In this study, we deposit a chlorinated porphyrin molecule, namely 5,10,15,20-tetrakis(4chlorophenyl)porphyrin (Cl4TPP), on graphene/Ir(111). Employing a combined experimental and theoretical approach, we demonstrate that the porphyrin layer physisorbed on graphene self-assembles into a periodic square-like arrangement. This carpet like growth is unperturbed by the step edges of the substrate, neither in its periodicity nor in its orientation. In addition, the molecular overlayer is thermally stable and does not alter the electronic properties of graphene. Remarkably, we show that Cl4TPP does not experience a dechlorination reaction with the underlying substrate, even after postdeposition annealing temperatures as high as 550 K. Moreover, postdeposition annealing at 700 K suggests the Cl4TPP molecules desorb intact without affecting graphene's electronic properties. In so doing, we demonstrate the effectiveness of graphene physisorbed on Ir(111) to both promote the formation and preserve the properties of chemically reactive 2D overlayers based on chlorophenyl porphyrins. These results show physisorbed graphene's potential as a general templating material for the formation of highly reactive self-assembled 2D overlayers.

The electronic topology of Bi4Te3, composed of alternating Bi2 and Bi2Te3 layers, is investigated by density functional theory and angle-resolved photoemission spectroscopy. We find, remarkably, that there are three adjacent strong topological gaps with associated protected surface states within a 2-eV range of the Fermi level. The existence of three consecutive Dirac cones in k space gives promise for alternative phenomena and applications, e.g., production of single photons with different energies (in the infrared and visible ranges) for multichannel transport of quantum information as well as multiple degrees of freedom in electron pumping for lasers. Additionally, a surface-state Fermi surface with strong hexagonal warping is observed.

We report on the successful synthesis of a crystal of the strong topological insulator Bi4Te3 and the study of its surface electronic response. A combination of theoretical and experimental techniques allowed for a systematic study of the composition and electronic properties of the sample. These techniques include density functional theory (DFT), scanning tunneling microscopy and spectroscopy (STM-STS). DFT predicts that distinct surface topological states exist for the two surface terminations of Bi4Te3, i.e. Bi-2 and Bi2Te3. These terminations are also clearly distinguished in STS measurements, which allow choosing the main conducting channel through a combination of topography and electronic response. We find that the density of states are similar to those of their parent crystals Bi-2 and Bi2Te3, albeit shifted in energy.

The electronic structure of ultrathin ZnO (000 (1) over bar) films grown on Au (111) was investigated by scanning tunneling microscopy (STM), low-energy electron diffraction (LEED), and X-ray and ultraviolet photoemission spectroscopy (XPS and UPS). Our results show evidence of O-terminated films and formation of bulk-like ZnO (2 x 2) surface reconstruction for films with > 4 monolayers. The measurements indicate that the metal substrate plays a decisive role in the electronic structure of films since p-type doping is obtained as observed from the valence band energy shifts. Moreover, finite-size effects appear to significantly modify the Zn and O core-level energy positions. These electronic effects may account for the role of ZnO catalytic performance in ZnO/metal systems, as well as for their nanostructure optoelectronic properties.

It is known that Sb2Se3 does not exhibit topological insulator behavior due to its orthorhombic structure. The introduction of a small amount of bismuth and tellurium may change its structure to hexagonal, leading to a stable topological insulator compound. We report here the synthesis and the structural, chemical, and electronic properties of the topological insulator BiSbSe2.5Te0.5. Combining X-ray and electron diffraction measurements, we demonstrate the formation of this stable quaternary hexagonal single crystal. We used X-ray photoelectron spectroscopy to determine quantitatively the exact chemical composition of the sample. The topological insulating behavior is similar to that of other bismuth chalcogenides, as probed by angle-resolved photoemission spectroscopy. A p-type doping, leading to a 0.15 eV shift of the Fermi level was found. This value compensates the intrinsically n-type doping produced by selenium vacancies. We also found a smaller effective mass and a higher electron group velocity for the electrons in the topological states compared with Bi2Se3.

The formation of hydrogen overlayers on the Zn-terminated ZnO(0001) surface has been reexamined by angle-resolved photoemission spectroscopy (ARPES). While low-energy electron diffraction patterns display the same (1 x 1) symmetry for different surface preparations, the electronic structure feature close to the Fermi level shows the formation of electron pockets, compatible with hydrogen-induced metallic states. Using ARPES and density functional theory (DFT) calculations, we show that hydrogen adspecies can also lead to metallization of this zinc-oxide surface in a similar manner as observed previously on ZnO(10 (1) over bar0) and O-terminated ZnO(000 (1) over bar). Importantly, our DFT calculations indicate that these electron pockets are formed by sp hybridized states and therefore the angular distribution of the emitted photoelectron is significantly suppressed at the normal emission.

The surface structure of the prototypical topological insulator Bi${}_{2}$Se${}_{3}$ is determined by low-energy electron diffraction and surface x-ray diffraction at room temperature. Both approaches show that the crystal is terminated by an intact quintuple layer. Specifically, an alternative termination by a bismuth bilayer is ruled out. Surface relaxations obtained by both techniques are in good agreement with each other and found to be small. This includes the relaxation of the van der Waals gap between the first two quintuple layers.

Topological insulator surfaces support metallic surface states with closed Fermi contours, encircling an odd number of Dirac points. Experimental studies have so far concentrated on surfaces with only one Dirac point, but three Dirac points can be expected for certain surface orientations of several topological insulator materials. Here we experimentally realize the Bi1-xSbx(110) surface for which an electronic structure with three Dirac points has been predicted (Teo et al 2008 Phys. Rev. B 78 045426), in contrast to the closed-packed (111) surface of the same material that supports only one Dirac point. We study the electronic structure of Bi1-xSbx(110) with angle-resolved photoemission and tight-binding calculations. We observe several metallic surface states, confirming not only the expectation that a topological insulator should be enclosed by metallic surfaces on all faces, but also the prediction of the surface state topology. Tight-binding calculations of the electronic structure are found to reproduce the expected topology of the surface states but they show one Dirac point that is not observed in the experiment, in the mirror line of the surface Brillouin zone. As in the case of Bi1-xSbx (111), this can be ascribed to an incorrect value of the mirror Chern number in the tight-binding parameters employed for the calculation. The quantitative agreement of the tight-binding calculation and the experiment is poorer than in the case of the (111) surface, something that is ascribed to the existence of dangling bonds on the (110) surface.

Topological insulators such as Bi2Se3 and Bi2Te3 have extremely promising transport properties, due to their unique electronic behavior: they are insulators in the bulk and conducting at the surface. Recently, the coexistence of two types of surface conducting channels has been observed for Bi2Se3, one being Dirac electrons from the topological state and the other electrons from a conventional two-dimensional gas. As an explanation for this effect, a possible structural modification of the surface of these materials has been hypothesized. Using scanning tunneling microscopy we have directly observed the coexistence of a conducting bilayer and the bare surface of bulk-terminated Bi2Te3. X-ray crystal truncation rod scattering was used to directly show the stabilization of this epitaxial bilayer which is primarily composed of bismuth. Using this information, we have performed density functional theory calculations to determine the electronic properties of the possible surface terminations. They can be used to understand recent angular resolved photoemission data which have revealed this dual surface electronic behavior.

Three dimensional topological insulator crystals consist of an insulating bulk enclosed by metallic surfaces, and detailed theoretical predictions about the surface state band topology and spin texture are available. While several topological insulator materials are currently known, the existence and topology of these metallic states have only ever been probed for one particular surface orientation of a given material. For most topological insulators, such as Bi$_{1-x}$Sb$_{x}$ and Bi$_2$Se$_3$, this surface is the closed-packed (111) surface and it supports one topologically guaranteed surface state Dirac cone. Here we experimentally realise a non closed-packed surface of a topological insulator, Bi$_{1-x}$Sb$_{x}$(110), and probe the surface state topology by angle-resolved photoemission. As expected, this surface also supports metallic states but the change in surface orientation drastically modifies the band topology, leading to three Dirac cones instead of one, in excellent agreement with the theoretical predictions but in contrast to any other experimentally studied TI surface. This illustrates the possibility to tailor the basic topological properties of the surface via its crystallographic direction. Here it introduces a valley degree of freedom not previously achieved for topological insulator systems.

Surface structural determination by low energy electron diffraction (LEED) requires a fitting procedure between the theoretical and experimental I(V) curves. This fitting procedure is quantified through an R-factor methodology. However, the R-factor space topology presents a large number of local minima. Thus, the task of identifying the global minimum, i.e. the task of finding the correct surface structure, requires a global optimization method that is able to determine the surface structure of complex systems. In this work we present the results of the application of genetic algorithms to three different systems, including performance tests and a comparison with another optimization method previously applied to the LEED problem, simulated annealing. We also present a scaling relationship of the computational effort versus the number of parameters to be fitted for the genetic algorithm method.