Occupied Surface Research Articles

Band bending at the surface of Bi2Se3 studied from first principles

The band bending (BB) effect on the surface of the second-generation topological insulators implies a serious challenge to design transport devices. The BB is triggered by the effective electric field generated by charged impurities close to the surface and by the inhomogeneous charge distribution of the occupied surface states (SSs). Our self-consistent calculations in the Korringa–Kohn–Rostoker framework showed that in contrast to the bulk bands, the spectrum of the SSs is not bent at the surface. In turn, it is possible to tune the energy level of the Dirac point via the deposited surface dopants. In addition, the electrostatic modifications induced by the charged impurities on the surface induce long range oscillations in the charge density. For dopants located beneath the surface, however, these oscillations become highly suppressed. Our findings are in good agreement with recent experiments, however, our results indicate that the concentration of the surface doping cannot be estimated from the energy shift of the Dirac cone within the scope of the effective continuous model for the protected SSs.

Spin-orbit influence ondz2-type surface state at Ta(110)

The influence of spin-orbit interaction on an occupied surface state at Ta(110) is investigated with spin-and angle-resolved photoemission and electronic structure calculations. The surface state a ...

Is lack of space a limiting factor for the development of aquaculture in EU coastal areas?

This study examines the spatial occupancy of marine finfish aquaculture in the European Union (EU), identifies geographical clusters and administrative areas where cage aquaculture development is particularly significant and provides evidence on the interactions between aquaculture and the touristic use of the coastline.Despite the increasing demand for seafood in the EU, its aquaculture is not expanding at the same rate (FAO, 2014), and the low number of new licences issued in recent years is a clear sign of the difficulties of the sector to expand.In this study, Google Earth satellite images and GIS methods were used to map and analyse spatial properties of marine finfish aquaculture sites in the EU. The analysis covers ten member states (Cyprus, Spain, France, Greece, Croatia, Ireland, Italy, Malta, Slovenia, United Kingdom) representing around 95% of EU marine finfish aquaculture production by volume, and Turkey.The results indicate that existing marine aquaculture sites occupy around 230 hectares (ha) in Greece, and 34 ha in UK, which represent respectively 28% and 44% of EU marine finfish production by volume. Considering these very low figures of occupied surface, it is difficult to imagine that the expansion of marine aquaculture in the EU would be constrained by a lack of space in absolute terms. Limitations to growth may be better explained by the competition for space which takes place at the local level with more established coastal economic activities. To examine in particular the interactions with the touristic use of the coastline, the analysis considered the distribution of hotels around the aquaculture sites and found that there is evidence of strong negative spatial interaction up to a distance of 3 km. These quantitative findings corroborate more qualitative considerations on the conflicts affecting the establishment of marine aquaculture in specific coastal regions in USA, Canada, Australia and New Zealand described in the literature. Another contribution from this study lies in the identification and mapping of geographical clusters and local administrative units where aquaculture production is particularly significant. Since socio-economic data for the individual aquaculture sites in the EU are not easily accessible, the mapping of EU aquaculture clusters is the prerequisite for further research to understand the local enabling conditions apart from bio-physical conditions which favoured the expansion of aquaculture in specific areas and not in others and identifying examples of best practices for the governance of the sector.

Hydrogen adsorbed at N-polar InN: Significant changes in the surface electronic properties

The interaction of atomic hydrogen and ammonia with as-grown N-polar InN surfaces is investigated using in situ photoelectron spectroscopy. Changes in the surface electronic properties, including the band alignment and work function, as well as the chemical bonding states of the substrate and adsorbates are characterized. Ammonia molecules are dissociating at the InN surface, resulting in adsorption of hydrogen species. Consequently, the considerable changes of the chemical and electronic properties of the InN surface during ammonia interaction are almost identical to those found for adsorption of atomic hydrogen. In both cases, hydrogen atoms preferentially bond to surface nitrogen atoms, resulting in the disappearance of the nitrogen dangling-bond-related occupied surface state close to the valence band edge at $\ensuremath{\sim}1.6$ eV binding energy and the formation of new occupied electron states at the conduction band edge. Furthermore, a decrease in work function during adsorption from 4.7 to 3.7--3.8 eV, as well as an increase in the surface downward band bending by 0.3 eV, confirm that hydrogen is acting as electron donor at InN surfaces and therefore has to be considered as one main reason for the surface electron accumulation observed at N-polar InN samples exposed to ambient conditions, for example as the dissociation product of molecules. The measured formation and occupation of electronic states above the conduction band minimum occur in conjunction with the observed increase in surface electron concentration and underline the relationship between the energy position of occupied electron states and surface band alignment for InN as a small-band-gap semiconductor.

Bioclimatology and Vegetation Series in Sicily: A Geostatistical Approach

Tackling the Sicilian woody vegetation as a case-study, this work aims to verify the correspondence between Rivas-Martinez's bioclimatic units and the main vegetation series in the Mediterranean region. Following this approach, one macrobioclimate and 25 bioclimatic type belts can be recognized in Sicily. By means of a geostatistical analysis based on WorldClim data sets, cartographic models of the distribution range of each single bioclimatic unit were obtained and combined with vegetation data, in order to develop a new regional spatial framework, integrating climatic and vegetation data. Fidelity of each vegetation unit to a given climatic range was then evaluated as percent distribution of the occupied surface within a given bioclimatic unit, while the predictive power of the WorldClim data sets was tested by using half of the spatial data of the processed vegetation units as independent variables. Our results suggest that: (1) any kind of numerical threshold used to define bioclimatic units is not ef...

Spin-dependent lifetime and exchange splitting of surface states on Ni(1 1 1)

We report on a spin-resolved two-photon photoemission study of the Ni(1 1 1) surface states. Nickel thin films were grown by molecular beam epitaxy on a W(1 1 0) substrate. The first image-potential state is used as a sensor to map the spin polarization of the occupied surface states. This allows us to identify the majority spin component of the Shockley surface state as well as a majority and minority d-derived surface resonance. The n = 1 image-potential state is found to be exchange split by 14 ± 3 meV. In spite of the fact that the band structure at the Fermi level exhibits a strongly discerned density of states in both spin channels, we observe low spin asymmetries in the decay and dephasing rates of the photoexcited electrons. Varying the sample preparation reveals that the Shockley surface state contributes about 40% to the spin-dependent decay rate.

Twisty itsy bitsy topological field theory

We extend the topological field theory (itsy bitsy topological field theory) of our previous work from mod 2 to twisted coefficients. This topological field theory is derived from sutured Floer homology (SFH) but described purely in terms of surfaces with signed points on their boundary (occupied surfaces) and curves on those surfaces respecting signs (sutures). It has information-theoretic (itsy) and quantum-field-theoretic (bitsy) aspects. In the process we extend some results of SFH, consider associated ribbon graph structures, and construct explicit admissible Heegaard decompositions.

INDICADORES AMBIENTALES EN LOS MUNICIPIOS DE MONTELÍBANO, PUERTO LIBERTADOR Y SAN JOSÉ DE URÉ, PARA EL RECURSO FORESTAL.

Environmental indicators are important tools for the periodic monitoring of a system’s components, allowing to execute controls and to apply politics that improve unfavorable situations in the system itself. The present investigation work has as objective designing environmental indicators in the municipalities of Montelíbano, Puerto Libertador and San José de Uré, for forest resource, using the model PSR (Pressure-State-Response). To its development, a bibliographical review was made related to the study area and environmental indicators to achieve the identification and selection of the potential tools to be adjusted to the regional context with the structuring of specific methodological sheets. As a result, three indicators were obtained in the biotic system for the terrestrial vegetation component and the occupied surface factor, under the supervision of professionals with experience in the field. It must be taken into account that the conceptual development and the sources of information are a key input to design an indicator to monitor any environmental component, so its absence limits the structuring of these tools in a specific context.

Reflectance anisotropy spectroscopy of clean and Sb covered Ge(001) surfaces and comparison with clean Si(001) surfaces

Reflectance anisotropy (RA) spectra of clean, dimerized Si and Ge (001) surfaces with (2×1), p(2×2), and c(4×2) surface unit cells and Sb covered Ge(001) with a (2×1) unit cell are calculated using a local orbital hybrid density functional theory approach. The weight of Fock exchange in the DFT Hamiltonian is chosen so that bulk dielectric function critical point transition energies and dielectric constants for Si and Ge agree with experimental values. The RA spectrum calculated for the c(4×2) phase of Si(001) is in excellent agreement with experiment. RA spectra calculated for Ge(001) clean surface phases show two prominent minima below 2 eV. The RA spectrum of the clean Ge(001)–(2×1) surface was measured at room temperature down to 0.4 eV and found to be in agreement with currently available experimental data above 1.5 eV. These RA spectra show a single minimum in the range up to 2 eV, but do not show a second minimum where another minimum is predicted by calculation. Wavefunction amplitudes at the Γ‾ point of the surface Brillouin zone for the two occupied surface states of Si(001)’c(4×2).

Electronic and atomic structures of the Ag induced [formula omitted] superstructure on Ge(111)

The Ag/Ge(111) 3×3 surface together with Ag/Si(111) 3×3 constitutes a set of surfaces that is ideally suited for fundamental studies related to low dimensional physics. We here focus on the atomic and electronic structures of the two-dimensional 3×3 superstructure induced by Ag on Ge(111), a surface that is significantly less studied than the Si counterpart. Extensive information on the surface band structure obtained by angle resolved photoelectron spectroscopy (ARPES) is presented, complemented by atomic information from scanning tunneling microscopy (STM). The results reveal new findings that are important for the understanding of the Ag induced 3×3 structure, acting as a prototype for semiconductor/metal interfaces. i) We have identified a new occupied surface band near the M¯-point of the 3×3 surface Brillouin zone. ii) The Ag/Ge(111) 3×3 surface exhibits a partially occupied surface band, S1, with a parabolic-like shape at Γ¯. At low temperature (LT) this band splits into two bands, S1U and S1D. The identification of two bands is significantly different from the case of Ag/Si(111) 3×3 for which just one band has been reported. Besides these specific results, our extensive ARPES study reveals four surface bands at room temperature (RT), while five surface bands were identified at ≈100K (LT). Room temperature empty state STM images show, depending on the tunneling bias, both honeycomb and hexagonal periodicities which are consistent with the honeycomb chained trimer and the in-equivalent trimer models, respectively.

Image-potential states of graphene on noble-metal surfaces

Time-, angle-, and energy-resolved two-photon photoelectron spectroscopy is used to study image-potential states of weakly bound monolayer graphene on the noble-metal surfaces Pt(111), Au/Ir(111), Au/Ni(111), and Ir(100). Binding energies are increased relative to the substrates, and lifetimes are similar for all systems under investigation. The properties of the image-potential states are mainly determined by the graphene layer with reduced influence of the substrate. Substrate-related occupied surface states close to the Fermi level are observed for graphene on fcc(111) substrates.

Factors that determine and limit the resistivity of high-quality individual ZnO nanowires

Knowing and controlling the resistivity of an individual nanowire (NW) is crucial for the production of new sensors and devices. For ZnO NWs this is poorly understood; a 108 variation in resistivity has previously been reported, making the production of reproducible devices almost impossible. Here, we provide accurate resistivity measurements of individual NWs, using a four-probe scanning tunnelling microscope (STM), revealing a dependence on the NW dimensions. To correctly interpret this behaviour, an atomic level transmission electron microscopy technique was employed to study the structural properties of the NWs in relation to three growth techniques: hydrothermal, catalytic and non-catalytic vapour phase. All NWs were found to be defect free and structurally equivalent; those grown with a metallic catalyst were free from Au contamination. The resistivity measurements showed a distinct increase with decreasing NW diameter, independent of growth technique. The increasing resistivity at small NW diameters was attributed to the dominance of surface states removing electrons from the bulk. However, a fundamental variance in resistivity (102) was observed and attributed to changes in occupied surface state density, an effect which is not seen with other NW materials such as Si. This is examined by a model to predict the effect of surface state occupancy on the measured resistivity and is confirmed with measurements after passivating the ZnO surface. Our results provide an understanding of the primary influence of the reactive nature of the surface and its dramatic effect on the electrical properties of ZnO NWs.

Outdoor and diurnal performance of large conformal flexible metal/plastic dye solar cells

We carried out an investigation on the performance of a conformal curved large area, flexible, metal/plastic dye solar cells in outdoor conditions. We calculated the energy produced by the flexible cells over the course of a diurnal cycle utilizing their peculiar property of being able to be conformed to curved surfaces. The total electrical energy E produced and energy produced per unit of occupied surface (i.e. footprint) E∗ over the course of a whole day were extracted from flat cells placed facing South with a 60° tilt angle and also for cells conformal to a cylindrical curve surface. For RCURV=13.5cm an enhancement of 3.3% and 7.9% respectively for E and E∗ was observed with respect to the cell positioned flat. The enhancement in E∗ was even greater (9.1%) when RCURV=5.5cm. Mechanisms induced by curving the device and leading to this enhancement were analyzed. These results that show up to ∼10% enhancement of energy produced (per unit of occupied projected area) compared to flat devices demonstrate the attractiveness and possibilities of DSC technology for all those applications where devices handiness, flexibility and conformability are required.

Structural and electronic properties of Sb-doped SnO2 (1 1 0) surface: A first principles study

The structural and electronic properties of stoichiometric and Sb-doped SnO2 (110) surfaces have been investigated using effective core pseudopotential (ECP) first-principle approach in the density functional theory at the B3LYP level. The supercells Sn48−2nO96Sb2n in which n=2, 3, 4 and 5 with symmetric and homogeneous distribution of Sb were employed to adjust the atomic Sb/Sn ratio in Sb-doped SnO2 (110) surfaces over a range of atomic Sb/Sn ratio from 9.09 to 26.31% which are closed to experimental values of the Sb doped SnO2. We find that substitution of Sb for Sn yields significant structural relaxation and modification of the charge around Sb in Sb-doped SnO2 (110) surfaces, resulting in very large displacements of five-fold Sb compared to that of Sn atoms in stoichiometric SnO2 (110) surface. Sb doping introduces occupied surface states of hybridized 5s and 5p orbitals of Sb atoms at the high energy side of the top of the valence band (VB) within the band gap, and they contribute to the band gap narrowing from 2.82eV to 2.33eV. It is found that the Sb substituted for Sn at the surface acts as donor as well as acceptor centers. The role of Sb in improving the surface conductivity of Sb-doped SnO2 (110) surfaces is discussed and compared to that reported in literature for Sb-doped SnO2 bulk and nanostructure.

Three‐dimensional Modelling of Archaeological Sites Using Close‐range Automatic Correlation Photogrammetry and Low‐altitude Imagery

ABSTRACTPhotogrammetry is a commonly used technique in three‐dimensional image‐based modelling for cultural heritage documentation. Three‐dimensional image‐based techniques that combine close‐range photogrammetry and automatic image matching are presently attracting a great deal of interest. This paper evaluates the feasibility of the application of photogrammetric three‐dimensional modelling of an archaeological site with close‐range automatic image‐matching software and a consumer‐grade digital camera. In order to correctly model upward‐facing surfaces using this technique, the photographs were taken from the air with the camera in low‐oblique position. A mast pole was used as camera platform to obtain low‐altitude imagery. To verify the usefulness of this method it was applied to the hill fort ‘Castro de Formigueiros’, located in Lugo, Spain. In this case, low‐cost automatic image‐correlation photogrammetry has proved to be a powerful, cost‐effective and versatile technique for the documentation, analysis and cultural dissemination of archaeological mapping. The mast used is inexpensive, easily transported and handled on site, and allows oblique photographs of the ground and wall‐tops to be obtained at distances similar to photographs of vertical wall faces taken from the ground. As a result, the resolution and accuracy of the point clouds of all the surfaces of the final model are similar. Given the geometric peculiarities of hill forts (the occupied surface area, height, volume and distribution of the objects) and the relief (they are generally located in areas difficult to access and often on high ground), the mast is particularly suitable for photographing low‐altitude imagery at archaeological sites, as the final accuracy of the three‐dimensional model demonstrates. The tools that have been applied to the final digital model of the hill fort show the information analysis potential that can be obtained for archaeological work. Copyright © 2013 John Wiley & Sons, Ltd.

Surface states and electronic structure of polar and nonpolar InN – An in situ photoelectron spectroscopy study

Valence band structure and surface states of InN with (0001), (000-1), (1-100), and (11-20) orientation were investigated in situ after growth using photoelectron spectroscopy. Depending on surface orientation, different occupied surface states are identified and differentiated from bulk contributions. For N-polar, m-plane, and a-plane InN, the surface states are located at the valence band maximum, while In-polar InN features surface states close to the Fermi level. The surface band alignment correlates with the position of surface states. For InN(0001), a much larger surface downward band bending is observed compared to N-polar, m-plane, and a-plane InN, where almost flat band conditions occur.

The surface states of lithium tetraborate

The different low index surface terminations of lithium tetraborate, Li2B4O7, are dominated by electronic states that fall within the band gap of the projection of the bulk states. As a pyroelectric material, lithium tetraborate, Li2B4O7, is a wide band gap dielectric, yet the (110) surface has a much smaller band gap because of occupied surface states that fall at binding energies between the valence band maximum and the Fermi level. The (100) surface is dominated, however, by unoccupied surface states that also fall in the gap between the conduction band minimum and the valence band maximum, but at binding energies just below the conduction band minimum. These very different types of surface states are consistent with the assignment of occupied surface states to states that are observed near the Fermi level in photoemission studies of Li2B4O7(110). The unoccupied surface states are also seen to be consistent with the inverse photoemission spectra of Li2B4O7(100). Clearly, the different surface terminations of lithium tetraborate yield very different types of surface electronic states.

Malcolmia littorea: The isolated Italian population in the European context

We compared the coenological information of the only Italian population of Malcolmia littorea (L.) R. Br. with published phytosociological relevés, including ones of this species, throughout its European range. With the aim of highlighting the main climatic features influencing the distribution patterns of M. littorea, we integrated coenological data with some climatic variables and considered major drivers for plant distribution at the European scale. Finally, we analysed the population extent of M. littorea in Italy, in order to assess its conservation status at regional level. The DCA analysis, performed on a matrix 139 relevés×183 species, separated the relevés according to their floristic composition, showing a geographic gradient from Portugal to Mediterranean coasts, until Italy; with Mediterranean relevés clearly separated from Atlantic ones as well. Along the beach-inland gradient, the analysis highlights that the species is typical of fixed dunes habitats (more inland, mainly stabilised dunes), although in the Atlantic it can also be found in mobile dunes. The analysis of climatic variables in relation to M. littorea distribution, suggested that the species is sensitive to low winter temperature and to summer drought. The only Italian population of M. littorea is subjected to many threats, due to its small dimensions (<1ha), isolation from the rest of its distribution area, that ranges from Portugal to France (until the Camargue region) and intensive human disturbances. Using both field and remote sensing information, we showed a considerable decrease of the occupied surface in Italy, leading us to suggest that the IUCN threat category of M. littorea in Italy should be reassessed from endangered to critically endangered.

Charge transport at the interface of n-GaAs (100) with an aqueous HCl solution: Electrochemical impedance spectroscopy study

Charge transport processes at the interface of n-GaAs (100) with an aqueous HCl solution are studied by electrochemical impedance spectroscopy. It is found that when open-circuit potential and anodic potentials are applied to the semiconductor the impedance spectra contain two capacitive semicircles corresponding to the capacitances of the space charge layer and surface states. In the case of open-circuit potential, semiconductor band bending at the interface with the solution is about 0.7 eV and the density of occupied surface states in the dark and under daylight conditions is 1.6 and 2.8 × 1012 cm2 eV−1, respectively. When cathode potentials are applied to GaAs, hydrogen evolution begins at the semiconductor/electrolyte interface and an additional inductive loop appears in the impedance spectra. At the same time, the density of occupied surface states increases considerably both due to a straightening of the semiconductor bands and to the appearance of As-H bonds. Thus, charge transport through the n-GaAs (100)/aqueous HCl solution interface is always mediated by semiconductor surface states.

Method for calculating the contribution of inelastic electron-electron scattering lifetimes of electronic states on (110) noble metal surfaces

A method for calculating the decay rate of surface states on the (110) noble metal surfaces within the GW approximation is described in detail. The band structure is described by a two-dimensional pseudo-potential model. As an example, the calculated lifetimes of surface states at the $ \bar{Y} $ point on Cu(110) are presented. The calculated lifetime of the occupied state is approximately 2 times greater than that of the unoccupied state. The result obtained for the occupied surface state is in good agreement with the photoemission data.