Investigation of the Shockley surface state on clean and air-exposed Au (1 1 1)

Scanning tunneling microscopy (STM) and spectroscopy (STS) carried out in vacuum and air were used to study the electronic structure of the Au (1 1 1) surface in the range of 0.0-0.7 eV below the Fermi level. The STS experiment carried out in UHV showed the existence of the Shockley surface state (SS) located 0.48 eV below the Fermi level. STS carried out in air showed strong local maximum located 0.35 eV below the Fermi level. This maximum was ascribed to the SS shifted toward lower energy due to carbon and oxygen overlayer. To confirm that the SS could exist on the sample exposed to air we did ultraviolet photoemission spectroscopy (UPS) experiment on air-treated and clean Au (1 1 1). Our results suggest that the SS position initially measured at 0.38 eV below the Fermi level was shifted to 0.27 eV after air treatment. Additionally, the level of contamination was measured using X-ray photoelectron spectroscopy (XPS).     

Scanning tunnelling microscopy and spectroscopy of the reduced TiO2(1 0 0) surface

Scanning tunnelling microscopy and current imaging tunnelling spectroscopy were used to study the topographic and electronic structure of a reduced TiO₂(1 0 0) surface. The STM results showed that the TiO₂(1 0 0) surface is capable to form (1 × 7) reconstruction which can transform to (1 × 3) reconstruction due to reoxidation of the surface. The CITS results showed that the (1 × 7) reconstruction is much more metallic in compared to the (1 × 3) reconstruction showing pronounced surface states at energy 1.3 eV and 0.8 eV below the Fermi level and at energy 1.0-1.2 eV above the Fermi level.     

High temperature STM/STS investigations of resonant image states on Au(1 1 1)

Scanning tunneling microscopy (STM) and spectroscopy (STS) were used to study the electronic structure of Au(1 1 1) surface in the range of 2-5.5 eV above the Fermi level. In this paper, we concentrate firstly on the position of the upper band gap edge (BE) existing in [1 1 1] direction in Au(1 1 1) and secondly on the position of the resonant image potential surface state (RIS) located in the bulk states approximately 1.1 eV above BE. The experiment was carried out in UHV at two temperatures 294 K and 580 K. Our high temperature STS (HT-STS) results clearly show the presence of RIS and BE local maxima at both temperatures. What is more, a slight shift towards the Fermi level of BE and RIS was observed. Those shifts were the consequence of the change of [1 1 1] band gap and lowering gold work function due to the thermal extension of interatomic distances. Finally, estimation of the work function was given at 294 K and 580 K.     

Vacancy charging on Si(1 1 1)-(7 × 7) investigated by density functional theory

The structure and energetics of charged vacancies on Si(1 1 1)-(7 × 7) are investigated using density functional theory calculations supplemented by estimates of ionization entropy. The calculations predict multiple possible charge states for the unfaulted edge vacancy in the adatom layer, although the −2 state is most stable on real Si(1 1 1) surfaces for which the Fermi level lies near the middle of the band gap.     

Neutralization of Li ions scattered by the Cu (1 0 0) and (1 1 1) surfaces: A localized picture of the atom-surface interaction

Large and face dependent neutral fractions have been found recently in the scattering of Li⁺ by Cu(1 0 0) and Cu(1 1 1) surfaces. These results for high work function surfaces are unexpected within the ‘traditional’ picture of a Li⁺ ion departing from a jellium surface model. In the present work the Li⁺/Cu(1 0 0) and Li⁺/Cu(1 1 1) interacting systems are described by a previously developed bond-pair model based on the localized interactions between the projectile ion and the atoms of the surface, and on the extended features of the electronic band structure through the surface local density of states. By only including the resonant neutralization to the Li atom ground state we explained the face and energy dependences of the measured neutral fractions for large outgoing energy values. We found that the downward shift of the Li ionization level below the Fermi level caused by the short range chemical interactions, is the main responsible of a high neutralization by the resonant mechanism. The remaining differences between theory and experiment values can be explained in terms of the energy gaps and image potential states appearing in these surfaces. The calculated distance behaviours of the energy levels corresponding to the first excited (Li-1s²2p) and the negative (Li-1s²2s²) atomic configurations indicate that they can also participate in the ion-surface charge exchange process.     

Study of vacancy on diamond (1 0 0) (2 × 1) surface from first-principles

Structure and energy related properties of neutral and charged vacancies on relaxed diamond (1 0 0) (2 × 1) surface were investigated by means of density functional theory. Calculations indicate that the diffusion of a single vacancy from the top surface layer to the second layer is not energetically favored. Analysis of energies in charged system shows that neutral state is most stable on diamond (1 0 0) (2 × 1) surface. The multiplicity of possible states can exist on diamond (1 0 0) surface in dependence on the surface Fermi level, which supports that surface diffusion of a vacancy is mediated by the change of vacancy charge states. Analysis of density of states shows surface vacancy can be effectively measured by photoelectricity technology.     

Comparative study of the GaAs(1 0 0) surface cleaned by atomic hydrogen

In attempt to correlate electronic properties and chemical composition of atomic hydrogen cleaned GaAs(1 0 0) surface, high-resolution photoemission yield spectroscopy (PYS) combined with Auger electron spectroscopy (AES) and mass spectrometry has been used. Our room temperature investigation clearly shows that the variations of surface composition and the electronic properties of a space charge layer as a function of atomic hydrogen dose display three successive interaction stages. There exists a contamination etching stage which is observed up to around 250 L of atomic hydrogen dose followed by a transition stage and a degradation stage which is observed beyond 700 L of exposure. In the first stage, a linear shift in the surface Fermi level is observed towards the conduction band by 0.14 eV, in agreement to the observed restoration of the surface stoichiometry and contamination removal. The next stage is characterized by a drop in ionization energy and work function, which quantitatively agrees with the observed Ga-enrichment as well as the tail of the electronic states attributed to the breaking As-dimers. As a result of the strong hydrogenation, the interface Fermi level E_F − E_v has been pinned at the value of 0.75 eV what corresponds to the degradation stage of the GaAs(1 0 0) surface that exhibits metallic density of states associated with Ga_As antisites defects. The results are discussed quantitatively in terms of the surface molecule approach and compared to those obtained by other groups.     

The influence of thermal fluctuations on electronic and structural properties of Ge(0 0 1) surface

We present a theoretical study (within the density functional theory) of the influence of thermal fluctuations of the Ge(0 0 1) structure on its electronic properties, mainly in the context of possible metallization of this surface observed in experiment. Analysis is based on the results of three series of long-time molecular dynamics (MD) simulations performed for p(1 × 2), c(2 × 4) and p(2 × 4) surface unit cells. We have found that for the p(1 × 2) unit cell thermal fluctuations lead to the appearance of a sharp peak in the density-of-states distribution at the Fermi level, and consequently, to metallization of the germanium surface, while the energy structure of the c(2 × 4) reconstructed surface remains non-metallic even at high temperatures. This result is in an agreement with recently published scanning tunneling spectroscopy data. Our MD simulations show that, depending on the translational symmetry of the surface, thermal fluctuations may modify its electronic structure in quite a distinct way. The obtained results also indicate that the observed Ge(0 0 1) surface metallization is an averaged effect of the variations of the local density of states caused by thermal fluctuations.     

Investigation of STM tip influence on the recorded position of the Shockley surface state on Au(1 1 1)

Current imaging tunneling spectroscopy (CITS) carried out in UHV at room temperature was used to study the electronic structure of the Au(1 1 1) surface in the range of −1.0-1.0 eV. The CITS experiment showed the existence of the Shockley surface state (SS) located in range 0.3-0.7 eV below the Fermi level. A wide range of SS locations is explained by the influence of the electronic structure of the tip on the measurement of the tunneling current. This hypothesis is supported by a simple theoretical model. Finally, a new method of CITS analysis is suggested. The method is related to counting local maxima locations on each tunneling spectroscopy curve and building a histogram. This method of data analysis allows finding statistically the most frequently appearing maxima. Using the histogram method the location of the SS is estimated to 0.47 eV below the Fermi level.     

Room and high-temperature scanning tunnelling microscopy and spectroscopy (HT-STM/STS) investigations of surface nanomodifications created on the TiO2(1 1 0) surface

High-temperature scanning tunnelling microscopy, scanning tunnelling spectroscopy and current imaging tunnelling spectroscopy (HT-STM/STS/CITS) were used to study the topographic and electronic structures changes due to surface modifications of the TiO₂(1 1 0) surface caused by the STM tip. In situ high-temperature STM results showed that the created modifications were stable even at elevated temperatures. The STS/CITS results showed the presence of energy gap below the Fermi level on the untreated regions. The disappearance of energy gap below the Fermi level on the modifications created by the tip was observed. It is assumed that the presence of the tip can change the chemical stoichiometry of the surface from TiO_2−x towards Ti₂O₃.     

High resolution scanning tunneling spectroscopy of ultrathin Pb on Si(1 1 1)-(6 × 6) substrate

The electronic structure of Si(1 1 1)-(6 × 6)Au surface covered with submonolayer amount of Pb is investigated using scanning tunneling spectroscopy. Already in small islands of Pb with thickness of 1 ML Pb_(1 1 1) and with the diameter of only about 2 nm we detected the quantized electronic state with energy 0.55 eV below the Fermi level. Similarly, the I(V) characteristics made for the Si(1 1 1)-(6 × 6)Au surface reveal a localized energy state 0.3 eV below the Fermi level. These energies result from fitting of the theoretical curves to the experimental data. The calculations are based on tight binding Hubbard model. The theoretical calculations clearly show prominent modification of the I(V) curve due to variation of electronic and topographic properties of the STM tip apex.     

Charge transfer in the atomic structure of Ge (1 0 5)

The atomic structure and charge transfer on the Ge (1 0 5) surface formed on Si substrates are studied using scanning tunneling microscopy and spectroscopy (STM and STS). The bias-dependent STM images of the whole Ge (1 0 5) facets formed on a Ge “hut” structure on Si (0 0 1) are observed, which are well explained by the recently confirmed structure model. The local surface density of states on the Ge (1 0 5) surface is measured by STS. The localization of the electronic states expected from charge transfer mechanism is observed in the dI/dV spectra. The surface band gap is estimated as 0.8-0.9 eV, which is even wider than the bulk bandgap of Ge, indicating the strong charge transfer effect to make the dangling bonds stable. The shape of normalized tunnel conductance agrees with the theoretical band structure published recently by Hashimoto et al.     

STS investigations of temperature dependence of Au(1 1 1) surface state energy position

High temperature scanning tunneling spectroscopy (HT-STS) was used to investigate the electronic structure of Au(1 1 1) at different temperatures in the energy range 0-1 eV below the Fermi level. We concentrated on the influence of temperature on the Shockley surface state (SS) appearing on noble metals surface due to a surface projected bulk bang gap in [1 1 1] direction. The influence of temperature on the projected band gap edge (BE) was also investigated. The experiment was carried out in the temperature range 294-580 K. As the result of the experiment a delicate shift of the SS and the BE in direction of the Fermi level was reported.     

Electron-stimulated desorption from an unexpected source: Internal hot electrons for Br-Si(1 0 0)-(2 × 1)

The desorption of Br adatoms from Br-saturated Si(1 0 0)-(2 × 1) was studied with scanning tunneling microscopy as a function of dopant type, dopant concentration, and temperature for 620-775 K. Analysis yields the activation energies and prefactors for desorption, and the former correspond to the energy separation between the Fermi level and Si-Br antibonding states. Thus, electron capture in long-lived states results in Br expulsion via a Franck-Condon transition. Analysis of the prefactors reveals that optical phonons provide the energy needed for the electronic excitation. These results show that desorption induced by an electronic transition can occur in closed system without external stimulus, and they indicate that thermally-excited charge carriers may play a general role in surface reactions.     

Efficient channel of the associative oxygen desorption from Pt(1 1 1)

Mechanism of the associative desorption of oxygen from the Pt(1 1 1) surface has been studied on atomic level by means of density-functional calculations. Key to the association of two oxygen adatoms into the O₂ molecule is the excitation of one of the adatoms to on-top site, where it becomes essentially neutral. The related redistribution of the electronic density at the O adatom leads to the appearance of the lateral attraction with the other O atom, thus providing an efficient channel for associative desorption. Calculated local densities of states resemble the transformation of the electronic structure of adsorbed O adatoms from the reactive to bound state in the course of association.     

Surface electronic structure of the (3 × 2) reconstruction induced by Yb on a Si(1 1 1) surface

We have investigated the electronic structure of the Yb/Si(1 1 1)-(3 × 2) surface using angle-resolved photoelectron spectroscopy. Five surface states have been identified in the gap of the bulk band projection. Among these five surface state, the dispersions of three of them agree well with those of the surface states of monovalent atom adsorbed Si(1 1 1)-(3 × 1) surfaces. The dispersions of the two other surface states agree well with those observed on the Ca/Si(1 1 1)-(3 × 2) surface, whose basic structure is the same as that of monovalent atom adsorbed Si(1 1 1)-(3 × 1) surfaces. Taking these results into account, we conclude that the five surface states observed in the band gap originate from the orbitals of Si atoms that form a honeycomb-chain-channel structure.     

A density functional theory study of H2S decomposition on the (1 1 1) surfaces of model Pd-alloys

In this work, we report density functional theory calculations exploring H₂S dissociation on the (1 1 1) surfaces of Pd, Cu, Ag, Au, and various bimetallic surfaces consisting of those metals. To understand the contributions of lattice strain and electronic ligand effects, the thermodynamics of each elementary dissociation step were explored on model bimetallic surfaces, including PdMPd sandwiches and Pd pseudomorphic overlayers, as well as strained Pd(1 1 1) surfaces and homogeneous Pd₃M alloys. Sulfuric (H₂S, SH, and S) adsorption energies were found to correlate very well with lattice constant, which can be explained by the strong correlation of the lattice constant with d-band center, Fermi energy, and density of states at the Fermi level for strained Pd(1 1 1) surfaces. Compressing the Pd lattice shifts the d-band center away from the Fermi level, lowers the Fermi energy, and reduces the density of d-states at the Fermi level. All three effects likely contribute to the destabilization of sulfuric adsorption on Pd alloys. Introducing ligand effects was found to alter the distribution of the d-states and shift the Fermi level, which eliminates the correlation of the d-band center with the density of states at the Fermi level and the Fermi energy. As a result, the d-band center by itself is a poor metric of the H₂S reaction energetics for bimetallic surfaces. Furthermore, combining strain with ligand effects was found to lead to unpredictable alterations of the d-band. Therefore, adsorption of H₂S, SH, and S on PdMPd surfaces do not accurately predict adsorption on Pd₃M surfaces.     

Theoretical simulation of butane isomers adsorption on a Pt(1 0 0) surface

The adsorption of the two butane isomers on Pt(1 0 0) has been characterised with use of density functional simulations. The adsorption energies corresponding to various adsorption configurations were evaluated in good agreement with experimental values. Limited changes of the molecular structure were evidenced. The C-H bond length increases at a degree depending on the surface-hydrogen distance, while the C-C bond length remains similar to that of the free molecule. The surface on-top Pt sites exert a preferential attraction on the molecule, probably through the interaction with the H atoms. The local density of states curves around H as well as C of the adsorbed molecules show dispersed states below the metal Fermi level indicating a molecule-Pt mixing demonstrating a chemical interaction.     

Spin polarization in photoelectron spectroscopy from antiferromagnets: Cr films on Fe(1 1 0) from first principles

The spin- and angle-resolved photoelectron spectroscopy from ultrathin Cr films on Fe(1 1 0) is investigated by means of first-principles electronic structure and photoemission calculations. The antiferromagnetic ordering in the Cr films leads in dependence on film thickness to a rapidly decreasing and oscillating photoelectron spin polarization, in reasonable agreement with recent experiments (Dedkov (2007) [1]). The oscillation period is explained by quantum-well states in the Cr film and by a Fermi surface nesting vector. The importance of transition matrix elements is highlighted. The findings point to a noncollinear magnetic structure at the Fe/Cr interface.