Occupied Surface States Research Articles

Electrochemistry and Electronic Structure of the Deuterium-Grown Boron-Doped Diamond Interfaces

Integration of the first principles quantum mechanical simulations with electrochemistry represents a difficult research area due to the complexity and large size of real systems and computational limitations. On the one hand it is highly desirable to be able to predict electrochemical properties of materials and systems from simulations to limit the money expenditure for experimental work and accelerate applications [1]. On the other hand, utilization of ab-initio simulations to understand the experimentally observed phenomena is also required to embed them into a solid theoretical framework.To fulfill this task, several multiscale approaches are used with the density functional theory (DFT) being fundamental tool for investigation of electronic structure. DFT results can be further extended to build forcefields used in molecular dynamics (MD) [2] for larger scale simulations and non-equillibrium Green's functions for investigations of electronics and spintronics of nanodevices [3]. Semiconductor electrochemistry is one of the most important areas to explore this experiment-theory interface because of the necessity to develop new materials for photovoltaics, photoelectrochemistry, energy storage and conversion. The purpose of the following talk is to show the power of this paradigm by elucidation of the deuterium-grown boron doped diamond (BDD-D) material [4].A brief introduction to the material synthesis, chemical and physical properties is provided exploiting the synchrotron data of X-Ray absorption spectroscopy and X-Ray photoelectron spectroscopy. Then, a strong emphasis is put into merging the (photo)electrochemical data with the first principle results for better understanding of charge transfer phenomena in the nanoscale. Specifically, DFT calculations of projected local density of states clearly show the presence of highly occupied surface states on the (111) plane of BDD-D in contrast to its standard hydrogen-grown counterpart (BDD-H). The resulting surface states are capable of photocurrent generation in the visible light, which is strongly magnified in BDD-D. Moreover, photoelectrochemical measurements evidenced that photocurrents can be positive or negative depending on the bias - despite BDD being a p-type semiconductor. These confirm a profound role of surface states in the semiconductor electrochemistry and capability of photoinduced charge transfer in the modified materials. The proposed picture shines some new light on the commonly established paradigm of band bending as the key factor driving surface properties of the nanodiamond surfaces [5]. Figure. SEM images of a) BDD@D and b) BDD@H; c) grain size distribution among two samples; d) XRD patterns of the diamond films deposited in the D2/CH4 and H2/CH4 gas mixtures. Reflections at 2θ around 44°, 75°, and 91° correspond to the (111), (220), and (311) diamond lattice planes, respectively. Doubling of the reflections is related to the presence of Kα1 and Kα2 wavelengths in the X-ray radiation [4]. Acknowledgements: M.S., M.B., and M.F. thank Helmholtz-Zentrum Berlin (HZB) for the allocation of synchrotron radiation beamtime at HZB (Germany). M.S. gratefully acknowledges the financial support of these studies fromthe Gdansk University of Technology through the DEC-02/2021/IDUB/ II.1/AMERICIUM grant under the Americium – “Excellence Initiative – Research University” program. R.B. acknowledges the funding from the National Science Centre, Poland under the OPUS call in the Weave programme (Project number: 2021/43/I/ST7/03205).

Honeycomb Sb layers on Cu(111) and parities of the surface states

The performed DFT study of the electronic structure of honeycomb Sb monolayer adsorbed on the Cu(111) surface has confirmed previous observations of significant role of tensile strain and related reduced buckling of adsorbed layer. The simulated STM pictures are perfectly consistent with those observed in experiment. We show that a significant band mixing with the substrate, which leads to an absence of occupied surface states at M point, questions applicability of the method of determination of the topological invariant Z2 by means of estimates of the products of the parity eigenvalues. Hence, since the adsorption interaction with a metal surface is rather strong, the transformations of the bands of free honeycomb Sb layers with increasing lattice constants, which was recently suggested to cause the transition to the topological insulator state for certain range of the tensile strain, do not directly correspond to the changes which occur for adsorbed layers.

Highly Occupied Surface States at Deuterium-Grown Boron-Doped Diamond Interfaces for Efficient Photoelectrochemistry.

Polycrystalline boron-doped diamond is a promising material for high-power aqueous electrochemical applications in bioanalytics, catalysis, and energy storage. The chemical vapor deposition (CVD) process of diamond formation and doping is totally diversified by using high kinetic energies of deuterium substituting habitually applied hydrogen. The high concentration of deuterium in plasma induces atomic arrangements and steric hindrance during synthesis reactions, which in consequence leads to a preferential (111) texture and more effective boron incorporation into the lattice, reaching a one order of magnitude higher density of charge carriers. This provides the surface reconstruction impacting surficial populations of CC dimers, CH, CO groups, and COOH termination along with enhanced kinetics of their abstraction, as revealed by high-resolution core-level spectroscopies. A series of local densities of stateswerecomputed, showing a rich set of highly occupied and localized surface states for samples deposited in deuterium, negating the connotations of band bending. The introduction of enhanced incorporation of boron into (111) facet of diamond leads to the manifestation of surface electronic states below the Fermi level and above the bulk valence band edge. This unique electronic band structure affects the charge transfer kinetics, electron affinity, and diffusion field geometry critical for efficient electrolysis, electrocatalysis, and photoelectrochemistry.

Unoccupied topological surface state in MnBi2Te4

The unoccupied part of the band structure in the magnetic topological insulator MnBi$_2$Te$_4$ is studied by first-principles calculations. We find a second, unoccupied topological surface state with similar electronic structure to the celebrated occupied topological surface state. This state is energetically located approximate $1.6$ eV above the occupied Dirac surface state around $\Gamma$ point, which permit it to be directly observed by the two-photon angle-resolved photoemission spectroscopy. We propose a unified effective model for the occupied and unoccupied surface states. Due to the direct optical coupling between these two surface states, we further propose two optical effects to detect the unoccupied surface state. One is the polar Kerr effect in odd layer from nonvanishing ac Hall conductance $\sigma_{xy}(\omega)$, and the other is higher-order terahertz-sideband generation in even layer, where the non-vanishining Berry curvature of the unoccupied surface state is directly observed from the giant Faraday rotation of optical emission.

Rashba-type splitting of the Au(110) surface state: A combined inverse and direct photoemission study

The Shockley surface state located at $\overline{\mathrm{Y}}$ on the $(1\ifmmode\times\else\texttimes\fi{}2)$-reconstructed Au(110) surface is predicted to exhibit a Rashba-type spin splitting. Previous photoemission experiments searched for this splitting but it could not be resolved yet. In order to uncover a possible splitting, the unoccupied surface state on Au(110) is examined with spin- and angle-resolved inverse photoemission, whereas Na-covered Au(110) allows for investigation of the now occupied surface state by means of spin- and angle-resolved direct photoemission. Our data show clear spin splittings in the order of $100\phantom{\rule{0.16em}{0ex}}\mathrm{meV}$ with a sign reversal at $\overline{\mathrm{Y}}$ in the surface state's in-plane spin components which is characteristic for a Rashba-type behavior. Furthermore, we deduce an effective mass of ${m}^{*}=(0.27\ifmmode\pm\else\textpm\fi{}0.02){m}_{e}$ and a Rashba parameter of ${\ensuremath{\alpha}}_{R}=(0.46\ifmmode\pm\else\textpm\fi{}0.04)\phantom{\rule{0.16em}{0ex}}\mathrm{eV}\phantom{\rule{0.16em}{0ex}}\AA{}$ from direct photoemission measurements.

Effect of Redox Atmosphere on Contact Electrification of Polymers.

The Triboelectric Nanogenerator has demonstrated broad applications in energy, environmental, and electronic fields, as well as huge potential in the mechanism study of contact electrification, since 2012. Herein, we employed a Triboelectric Nanogenerator working in vertical contact-separation mode to study the electrification performance of the polymer under redox atmosphere. The results show that the electron-withdrawing ability of the polymer is weakened with increasing O3 concentration. Considering that O3 is typically one of the strongest oxidants, we further studied the electrification performance under H2, CO, and O2 atmosphere. It is found that the electron-withdrawing ability was predictably weakened under O2 atmosphere similar to the case of O3. On the contrary, the electron-withdrawing ability was enhanced under H2 and CO atmosphere. Accordingly, a theoretical mechanism involving the highest occupied surface state level is proposed to explain the effect of redox atmosphere on contact electrification. These results clarify that contact electrification can be varied by redox agents. Conversely, it also suggests the possibility to manipulate the redox reactions through the modification of contact electrification.

Structural stability and electronic properties of Sr induced (5×4) reconstruction on Si(111) surface

Honeycomb-chain-channel (HCC) and π-bond Seiwatz chain (SC) are the well-known reconstructions induced by alkaline earth metal on Si(111) surface. Here we identify by ab initio calculations a stable intermediate phase of Sr/Si(111)-(5×4) HCC+SC. This new reconstruction satisfies the electron counting rule with the Sr coverage of 3/10 monolayer. In each (5×4) surface unit cell, six Sr atoms are distributed as evenly as possible in the two channels. One of the channels contains two Sr atoms on the top site T4 and one Sr atom on the hollow site H3, while another channel contains two Sr atoms on site H3 and one on site T4. Electronic band structures and band-decomposed charge density distributions show that there are seven occupied surface states (S1–S5, Σ1 and Σ2) and two unoccupied surface states (S6 and S7) near the Fermi level. Moreover, we find that this new (5×4) surface structure is a semiconductor with a surface band gap of 0.62 eV, determined by the S2 and S6 surface states in honeycomb Si chains.

Numerical modelling of the potential inside the cylindrical shaped nanocrystallite metal oxide semiconductors and its effect on gas sensor response

The surface of metal oxide semiconductors exhibits great gas sensing response when the size of the grains becomes much smaller, i.e. in the range of the Debye length. In the presented work, the gas sensing properties of nanocrystalline cylindrical shaped metal oxide semiconductors have been studied. The effect of crystal size and surface states on gas sensor response has been investigated. The electric potential inside the grains has been calculated by solving the Poisson’s equation coupled with electroneutrality condition. The relationship between the electric potential and the size of the nanograins has been established as a function of available surface states, surface temperature and occupied surface states. It has been found that the critical value of available surface state density increases with an increase in the size of nanograins. Total carrier concentration inside the grain volume has been simulated as a function of available surface state density and grain size. Eventually, the sensitivity of the gas sensor has been simulated and compared for different sizes of nanocrystals. From the simulated studies, it is clear that the smaller cylindrical-shaped nanostructured grains could result in better gas sensing performances. Presented model has universal application for all metal oxides (both n-type and p-type) based semiconductors and can be extended for both oxidising and reducing gases with essential modifications.

Relationship of the electric potential inside the nano crystallite with the chemisorbed gas molecules at metal oxide semiconductor surface and its effect on gas sensor response

The metal oxide semiconductors surface exhibit great gas sensing response when the size of the grains becomes much smaller i.e. in the range of debye length. In the presented study, the effect of crystal size and surface states has been studied on gas sensing properties for cylindrical shaped nanostructures of metal oxide semiconductors. To calculate the grain potential Poisson’s equation coupled with electroneutrality condition in cylindrical shaped nanostructures has been solved. The shape of the potential profile inside the nanograins has been calculated as a function of available surface states, surface temperature and occupied surface states. Eventually, the sensitivity of the gas sensor has been simulated and compared for different sizes of nanocrystals.

Electron transfer in nano-scale contact electrification: Atmosphere effect on the surface states of dielectrics

Contact electrification (CE) is a common phenomenon, in which the identity of the charge carriers has been debated for decades. Recently, the temperature and photoexcitation effects in CE reveal that the electron in the surface states of dielectrics is the charge carriers, which are responsible for the creation of electrostatic charges. Therefore, the electron transfer in CE may be predicted and controlled by measuring and changing the surface states of dielectrics. Here, a new method to measure the surface states of dielectric based on bias effect in CE is proposed. The CE between metals and dielectrics is performed in different atmospheres, and the surface state density (SSD) and highest occupied surface state level (HOSL) of the dielectric are measured by the method. The results suggest that both the SSD and HOSL of the dielectric can be changed by the adsorption of gas molecules. Particularly, the O2 molecule is found to shift the HOSL of the dielectric to a lower energy level and make the dielectric more likely to be negatively charged. The findings provide a method to control charge transfer characteristics in CE by changing the O2 concentration in the atmosphere, and the surface states measurement method has implications for the characterization of the CE properties of materials.

Valence band structure and effective masses of GaN(101¯0)

The electronic structure of the clean and stoichiometric $\mathrm{GaN}(10\overline{1}0)$ cleavage surface is investigated in a comprehensive angle-resolved photoelectron spectroscopy study. A clear distinction between surface and bulk related features allows us to measure the dispersions of the occupied surface state band from the N dangling bonds as well as of the uppermost bulk valence bands and to extract the effective hole masses directly with high precision. This is performed along various directions in ${k}_{\ensuremath{\parallel}}$ space providing a detailed picture of the electronic band dispersion. The obtained results show three separated bulk bands without indications of a crossing of these bands as commonly predicted in theoretical works. Moreover, from the observed Fermi-level pinning we determine the position of the minimum of the empty Ga-derived surface state band, which is found deep within the fundamental band gap at $\ensuremath{\sim}2.3$ eV above the valence band maximum.

Plasmon-polaron of the topological metallic surface states

We report a plasmon-polaron mode of a two-dimensional electron gas occupying surface states of a three-dimensional topological crystal. This low-frequency, acoustic plasmon mode splits off from the conventional spin-plasmon mode as a result of strong interactions of the surface electrons with bulk phonons. We show that like in the case of the conventional spin plasmon, the scattering of this mode is strongly suppressed in some regions of the phase space. This signature of topological protection leads to an umklapp-free mode dispersion at the Brillouin zone boundary. Such a plasmon-polaron mode has indeed been recently observed in the topological metal ${\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$.

Charging ain't all bad: Complex physics in DyScO3

Although charging is ubiquitous in electron microscopy, its effects are typically avoided or ignored. However, avoiding charging is not possible in some materials, e.g. lanthanide scandates with well-ordered surfaces positively charge immensely under electron beam illumination because of their electronic structure, and ignoring charging can leave new science undiscovered. In this work, a combination of rapidly acquired electron energy loss spectra and cross-correlation were used to understand and overcome charging effects in DyScO3. A 5.4 eV band gap was extracted from the charging-corrected loss spectrum, in good agreement with previously reported band gaps, and a 3.8 eV in-gap peak was attributed to surface states via comparison with density functional theory calculations. Additionally, ultraviolet photoelectron spectroscopy measurements indicated that under some conditions well-annealed DyScO3 surfaces charge negatively causing upward band bending associated with occupied surface states in the gap. As was previously found in the case of positive charging under electron beam illumination with in-situ flexoelectric bending observations, the magnitude of negative charging under ultraviolet illumination is Zener tunneling limited in well-annealed DyScO3.

UV-induced formation of oxygen-derived dangling bonds on hydroxyl-terminated SiC

A combined theoretical and multi-technique experimental study was employed to comprehensively determine the electronic structure of 6H–SiC(0 0 0 1) surfaces upon hydroxyl and oxygen termination. We demonstrate the UV-induced formation of single-coordinated oxygen radicals in on-top sites above the atoms of the uppermost silicon layer of the substrate on initially hydroxyl-terminated SiC. Such a configuration of oxygen radicals represents an unprecedented adsorbate-derived system of unpaired electrons, bearing analogy to silicon and carbon dangling bonds on clean, unreconstructed SiC surfaces. We evidence the presence of adsorbate-derived surface states within the fundamental band gap for both hydroxyl- and oxygen-terminated SiC. For hydroxyl termination, a hydrogen-induced unoccupied surface state is revealed consistently by inverse photoemission spectroscopy and density-functional theory calculations employing self-interaction-corrected pseudopotentials (DFT-SIC). The formation of oxygen dangling bonds is accompanied by the occurrence of an occupied surface state derived from px- and py-orbitals associated with the unpaired electrons as proven by both ultraviolet photoemission spectroscopy and DFT-SIC.

Large tunneling anisotropic magnetoresistance mediated by surface states

We investigated the tunneling anisotropic magnetoresistance (TAMR) in thick hcp Co films at cryogenic temperatures using scanning tunneling microscopy. At around -350 mV, a strong TAMR up to 30\% is found with a characteristic voltage dependence and a reversal of sign. With the help of \textit{ab initio} calculations the TAMR can be traced back to a spin-polarized occupied surface states that experience a strong spin-orbit interaction leading to a magnetization direction depending hybridization with bulk states.

Tunable Ultraviolet Photorefractive Effect in LiNbO3/TiO2‐X (X = Pd or N): Theoretically Prediction and Experimental Validation

In this study, the introduction of unoccupied or partially occupied surface states energy levels into TiO2 upon Pd or N modification, respectively are theoretically predicted, suggesting a tunable carriers behavior in LiNbO3/TiO2‐X (X = Pd or N) composites. Inspired by the theoretical simulation, a new low‐cost method of compositing LiNbO3 with TiO2‐X (X = Pd or N) to mediate the ultraviolet photorefraction of LiNbO3 is proposed. The experimental results indicate that LiNbO3/TiO2 and LiNbO3/TiO2‐Pd show enhanced photorefraction due to reduced recombination of photoinduced carriers or increased free holes in LiNbO3. While LiNbO3/TiO2‐N exhibits photorefraction resistance since the photoinduced holes in LiNbO3 would get trapped by NOX species at the interface. This study will not only provide a simple way to fabricate tailored photorefractive material, but also offer new insights into the mechanism of photorefraction and photorefractive material design method.

The surface electronic structure of silicon terminated (100) diamond

A combination of synchrotron-based x-ray spectroscopy and contact potential difference measurements have been used to examine the electronic structure of the (3 × 1) silicon terminated (100) diamond surface under ultra high vacuum conditions. An occupied surface state which sits 1.75 eV below the valence band maximum has been identified, and indications of mid-gap unoccupied surface states have been found. Additionally, the pristine silicon terminated surface is shown to possess a negative electron affinity of −0.86 ± 0.1 eV.

The Origin of the Giant Hall Effect in Metal-Insulator Composites

Near the metal-insulator transition, the Hall coefficient R of metal-insulator composites (M-I composite) can be up to 104 times larger than that in the pure metal called Giant Hall effect. Applying the physical model for alloys with phase separation developed in [1] [2], we conclude that the Giant Hall effect is caused by an electron transfer away from the metallic phase to the insulating phase occupying surface states. These surface states are the reason for the granular structure typical for M-I composites. This electron transfer can be described by [1] [2], provided that long-range diffusion does not happen during film production (n is the electron density in the phase A. uA and uB are the volume fractions of the phase A (metallic phase) and phase B (insulator phase). β is a measure for the average potential difference between the phases A and B). A formula for calculation of R of composites is derived and applied to experimental data of granular Cu1-y(SiO2)y and Ni1-y(SiO2)y films.

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 ...