Surface electronic structure

Demand for high-performance rechargeable batteries has driven the development of novel electrodes with high reversibility and energy density. Tracking the change in electronic and local structure is crucial to investigate and improve the stability and performance of these batteries. Advanced characterization techniques allow the user to monitor electronic and local atomic structure changes to better understand the role and behavior of different elements during cycling of electrode material for rechargeable batteries.X-ray Absorption and Emission Spectroscopy allow us to explore such changes in 3d transition metal elements through the process of generation and quenching of core holes in the sample. Operando measurements enable real-time monitoring of electronic structure changes in electrode materials which enable us to capture phenomena which are only kinetically accessible.In this study, the change in electronic structure of transition metals present in electrode materials is explored as a function of lithium removal through XES Kβ1,3feature along with K-edge XAFS using a lab-scale instrument. The choice of electrodes explored in this study considers the degree of oxidation, structure, and mechanism for ion storage. Change in peak and edge positions of Kβ1,3and K-edge feature is utilized to monitor oxidation/spin changes in electrode materials during charge/discharge. Further the magnetic behavior for ex-situ samples prepared by electrochemical delithiation of LCO in the range of 2-10% delithiation is investigated using Kα and Kβ fluorescence (spin sensitive) followed by K-edge XAFS for Cobalt and Oxygen. Figure 1

Surface energies and work functions of the transition metal carbides

Functionalization of Ge(1 0 0) surface by adsorption of phenylthiol

Electron-surface interaction and metallization of the CaF₂(111)-surface studied by photothermal techniques

ZnO(1010)/O2 was chosen as a model system to study characteristic interaction mechanisms of reactive gases with surfaces of compound semiconductors. Reactions between 120 and 1000 K depend on thermodynamically controlled surface structures. – Reconstruction and point defects characterize the geometric and electronic structure of clean surfaces. – Reversible physisorption data car be determined at 120 ≤ T ≤ 250 K (coverages θ < 10−10 for P < 10−4 Pa) characterized by adsorption isotherms and isosteric heats of adsorption. – Chemisorption of O2 is found in the range of 300 to 650 K (θ<2.5 · 10−4) on crystals with negligible amount of point defeats at the surface. The energy of chemisorption and reaction rates can be described quantitatively by electron transfer models. – From crystal sublimation measurements at T > 700 K desorption rates for zinc and oxygen are derived. The latter depend strongly on the concentration of point defects (most probably oxygen vacancies) at the surface, which can be determined quantitatively as a function of pressure and temperature and may be removed during reactions at lower temperatures (T < 700 K). Under these conditions a strong influence of oxygen vacancies on sticking coefficients and rates of electron transfer during oxygen exposure is found. – Investigations with various independent techniques (such as AES, LEED, EPR, thermal desorption spectroscopy, isotopic exchange as well as changes in the surface conductivity and work function) are shown to be necessary for a unique interpretation of experimental results. This enables the separation of chemisorption processes and reactions with point defects during oxygen interaction which is of importance to understand the elementary steps during oxidation catalysis on ZnO.

We have computed the surface energies, work functions, and interlayer surface relaxations of clean (111), (100), and (110) surfaces of Al, Cu, Ru, Rh, Pd, Ag, Pt, and Au. We interpret the surface energy from liquid metal measurements as the mean of the solid-state surface energies over these three lowest-index crystal faces. We compare experimental (and random phase approximation) reference values to those of a family of nonempirical semilocal density functionals, from the basic local density approximation (LDA) to our most advanced general purpose meta-generalized gradient approximation, strongly constrained and appropriately normed (SCAN). The closest agreement is achieved by the simplest density functional LDA, and by the most sophisticated one, SCAN+rVV10 (Vydrov-Van Voorhis 2010). The long-range van der Waals interaction, incorporated through rVV10, increases the surface energies by about 10%, and increases the work functions by about 3%. LDA works for metal surfaces through two known error cancellations. The Perdew-Burke-Ernzerhof generalized gradient approximation tends to underestimate both surface energies (by about 24%) and work functions (by about 4%), yielding the least-accurate results. The amount by which a functional underestimates these surface properties correlates with the extent to which it neglects van der Waals attraction at intermediate and long range. Qualitative arguments are given for the signs of the van der Waals contributions to the surface energy and work function. A standard expression for the work function in Kohn-Sham (KS) theory is shown to be valid in generalized KS theory. Interlayer relaxations from different functionals are in reasonable agreement with one another, and usually with experiment.

A systematic study of the bulk and surface geometrical and electronic properties of a series of transition-metal carbides (TMC with TM = Ti, V, Zr, Nb, Mo, Hf, Ta, and W) by first-principles methods is presented. It is shown that in these materials the chemical bonding is strongly covalent, the cohesive energies being directly related to the bonding-antibonding gap although the shift of the center of the C(2s) band related peak in the density of states with respect to diamond indicates that some metal to carbon charge transfer does also take place. The (001) face of these metal carbides exhibits a noticeable surface rumpling which grows along the series. It is shown that neglecting surface relaxation results in very large errors on the surface energy and work function. The surface formation induces a significant shift of electronic energy levels with respect to the corresponding values in the bulk. The extent and nature of the shift can be understood from simple bonding-antibonding arguments and is enhanced by the structural rippling of this surface.

The measurement of the electric quadrupole interaction with nuclear techniques allows the study of monolayer coverages or even isolated atoms at surfaces and interfaces due to the inherent sensitivity of these methods. Recently electric field gradients at surface atoms have been investigated with Mößbauer Spectroscopy, Polarized Beam NMR , and Perturbed Angular Correlation (PAC). The highly sensitive PAC technique with 111In and 111mCd sources, in particular, has been applied to several metal and semiconductor surfaces. The use of flat as well as stepped surfaces has allowed the assignment of the measured quadrupole interaction frequencies to specific surface sites. For the Pd(111) surface a particularly complete description is possible, since for five different sites (terrace adatoms, ledge adatoms, kink adatoms, ledge and terrace atoms) the field gradients and their orientation could be determined. They represent a challenging problem for understanding the electronic structure of metallic surfaces. Through the study of the temperature dependence of the different components the diffusion of the radioactive probe atoms can be followed. The data from 57Fe Mößbauer spectroscopy on iron surfaces with monolayer coverage and NMR surface studies of 7Li and 23Na at high temperatures are reviewed. An outlook for further developments of the nuclear techniques is presented, particularly in view of the variety of radioactive isotope beams available from on-line isotope separators like the ISOLDE facility at CERN.

A charge-neutral electron-gas system with a surface is studied over a wide range of densities including the metallic range, by application of certain theoretical and physical constraints to a model surface effective potential. The properties derived are metallic surface dipole barriers, work functions, and surface energies, the behavior of the work function for densities higher than those existing in metals, and low-density surface energies. The finite-linear-potential model employed is one which varies linearly over a finite region and is constant beyond some point, the parameters of the system being the edge of the neutralizing positive charge (jellium) background, the field strength, and the barrier height. For this model effective potential the determination of all properties is primarily analytic. The jellium edge and barrier height are determined by the condition of charge neutrality and requirement of self-consistency of the surface dipole barrier, respectively, whereas the field strength is either adjusted so as to satisfy the constraint set on the electrostatic potential by the Budd-Vannimenus theorem (BVT) or varied in an application of the variational principle for the energy. The results obtained on use of the BVT constraint indicate that although the work function decreases with increasing density, it does not saturate at some finite value in the high-density limit as concluded by Peuckert, but rather that it vanishes at a density corresponding to ${r}_{s}\ensuremath{\simeq}0.4$. For metallic densities, the surface dipole barriers and work functions thus determined lie within three tenths of an electron volt of the fully self-consistent calculations of Lang and Kohn (LK). The upper bounds obtained by a variational-self-consistent calculation of metallic surface energies as determined within the local-density approximation (LDA) are within 5 ergs/${\mathrm{cm}}^{2}$ of the LK values for ${r}_{s}\ensuremath{\ge}2.5$ and differ by less than 3% for ${r}_{s}=2$. Surface energies for metallic and lower densities are also determined by use of the Vannimenus-Budd theorem in conjunction with the physical criterion of the vanishing of the surface energy in the low-density limit. For the same choice of single-particle Hamiltonian, determined by the BVT sum rule, these results lie significantly below those of a LDA calculation over the entire range of densities considered.

The surface-embedded Green-function method has been successfully used to perform self-consistent calculations of the electronic structure of various metal surfaces. Although the method is quite general, the original implementation treats only surfaces having two-dimensional inversion symmetry. We have now modified the technique to remove this restriction. As a test, the electronic structure of Al(111) has been calculated. The calculated work function of 4.22 eV and the energy and dispersion of the calculated surface state/surface resonance bands along high-symmetry directions in the surface Brillouin zone are all in excellent agreement with experiment. Results are compared with other theoretical work.

There is plenty of room for new structures at the bottom

Since the surface is the termination of a bulk crystal, surface atoms have fewer neighbors than bulk atoms; part of the chemical bonds which constitute the bulk-crystal structure are broken at the surface. These bonds have to be broken to create the surface and thus the formation of a surface costs energy (surface energy) (Sect. 3.1). In comparison with the bulk properties, therefore, the electronic structure near to the surface is markedly different. Even an ideal surface with its atoms at bulk-like positions (called truncated bulk) displays new electronic levels and modified many-body effects due to the change in chemical bonding. Many macroscopic effects and phenomena on surfaces are related to this change in electronic structure, for example, the surface free energy, the adhesion forces, and the specific chemical reactivity of particular surfaces. A central topic in modern surface physics is therefore the development of a detailed understanding of the surface electronic structure. On the theoretical side, the general approach is similar to that for the bulk crystal: In essence the one-electron approximation is used and one tries to solve the Schrodinger equation for an electron near the surface. A variety of approximation methods may then be used to take into account many-body effects.

Research during the last year has led to a better understanding of the electronic and atomic structure of the (110) surfaces of III–V semiconductors. In this paper we will briefly review these new developments as well as point out areas where agreement has been found between various experimental results presented in the literature. It is now generally agreed that there are no intrinsic surface states in the band gap on GaAs and the smaller band-gap materials (e.g., GaSb, InAs, and GaSb) and that Schottky barrier pinning must be due to states produced when the metal adlayer is applied. Particular attention is focused in this paper on the large surface rearrangement which takes place on the (110) GaAs surface and effects of the strain which may be produced in joining this rearranged surface layer to the rest of GaAs crystal. It is pointed out that this may lead to variations in the surface rearrangement which can produce variations in the valence electronic structure at the surface. Such variations are shown in experimental energy distribution curves obtained by the photoemission technique which samples principally the last two molecular layers. It is further shown that surprisingly small amounts of chemisorbed oxygen can produce first-order effects in the valence-band electronic structure. On all GaAs (110) surfaces studied, a phaselike transformation was observed with a few hundredths of a monolayer coverage of chemisorbed oxygen. Near this coverage, the Ga 3d exciton structure disappears and the oxygen uptake increases significantly. On certain samples, first-order changes in the valence-band electronic structure were observed at a coverage of a hundredth of a monolayer or lower. These transformations are discussed in terms of the electronic and atomic configurations at the surface. Experimental data showing As and Ga 3d chemical shifts for oxidation as well as chemisorption are also presented and used to point out difficulties to be expected in passivating practical surfaces. In particular, the effect of mixed As and Ga oxides, the desirability of bonding passivating layers to the GaAs through As bonds, and the effect of strain-induced interface states are discussed.

δ-Pu has been widely used due to its outstanding material properties, and surface stability is essential for studying its surface corrosion mechanism. In this work, δ-Pu and its surface stability were analyzed based on first-principles calculations. The third-order Birch–Murnaghan equation of state fitted the calculated data for the δ-Pu cell, and the lattice constant of equilibrium structure was obtained. By constructing the surface models of three conventional crystal directions, (100), (110), and (111), the surface structure was optimized based upon the Quasi-Newton method, and the surface energy and electronic structure were calculated. The results show that the (110) surface energy of δ-Pu was 0.103 eV/Å2, and the (111) surface energy of δ-Pu was 0.052 eV/Å2. Thus, the d-band center of the (111) plane is farther away from the Fermi level than (100) and (110). Among the three typical crystal orientation planes, the surface energy and electronic structure show that the (111) plane was the most stable crystal orientation plane of δ-Pu.

The $d$-band shape of a metal site, governed by the local geometry and composition of materials, plays an important role in determining trends of the surface reactivity of transition-metal alloys. We discuss this phenomenon using the chemisorption of various adsorbates such as C, N, O, and their hydrogenated species on Pd bimetallic alloys as an example. For many alloys, the $d$-band center, even with consideration of the $d$-band width and $\mathit{sp}$ electrons, can not describe variations in reactivity from one surface to another. We investigate the effect of the $d$-band shape, represented by higher moments of the $d$ band, on the local electronic structure of adsorbates, e.g., energy and filling of adsorbate-metal antibonding states. The upper $d$-band edge ${\ensuremath{\varepsilon}}_{u}$, defined as the highest peak position of the Hilbert transform of the density of states projected onto $d$ orbitals of an active metal site, is identified as an electronic descriptor for the surface reactivity of transition metals and their alloys, regardless of variations in the $d$-band shape. The utilization of the upper $d$-band edge with scaling relations enables a considerable reduction of the parameter space in search of improved alloy catalysts and further extends our understanding of the relationship between the electronic structure and chemical reactivity of metal surfaces.