Session V5: Surface Electronic & Lattice Properties: Structure, Surface States, & Adsorption

Surface energy anisotropy for FCC metals: Functional forms

The energy of a crystalline surface depends on the angle of the surface normal with respect to the crystalline axes (i.e.~the Miller index). We show that the surface energy as a function of angle is surprisingly easy to describe with a simplistic broken bond model including only a few parameters. In particular, this model is successful at capturing the cusps at high symmetry surfaces. We will use our fitted functional form as a characteristic continuum description of material surface energies for FCC metals. The anisotropy of these surface energies can then be utilized in the study of many material properties- equilibrium shapes (Wolff plots) of crystals and voids, fracture mechanics, cleavage and faceting. We calculate the surface energies using ab initio calculations and various interatomic potentials, providing a measure of the fitness of the potentials for studying physical systems with surfaces. Furthermore, we assemble systematic tables of these results, available to the community through the Knowledgebase of Interatomic Models (KIM, https://openkim.org/).   

Explicit consideration of surface structures for work function calculations

We investigate the effects of atomic structure of surfaces on work functions, using density functional theory (DFT) calculations. Despite the well-known orientation dependence of work functions, the effect of geometric structure of surfaces on work functions is rarely considered. To examine whether the atomic structure of a surface is important to the work function, we perform DFT calculations of thick slabs in the periodic supercell. We then extract the energy levels of the occupied states relative to the vacuum level. Surfaces of oxides, such as BaTiO$_3$ (001), SrTiO$_3$ (001), and TiO$_2$ (110), are especially well-tested because of their various reconstructed structures. Our calculations show that, even for the same surface orientation, the difference in stoichiometry and geometry of the surface gives rise to substantially different work functions. This finding strongly implies that the atomic structures of surfaces must be explicitly taken into account in predicting work functions of materials.   

The role of surface states in inelastic electron tunneling into metal surfaces

Inelastic electron tunneling spectroscopy is a highly capable technique to explore dynamic properties of material surfaces, particularly the phonon spectrum. Presently, many of the so-called ``propensity'' rules that determine the strength of the measured inelastic loss in STM experiments are still unknown or controversial. We have carried out systematic IETS of surface phonons on a Au(111) surface, and spatially-resolved variations in the vibrational spectrum of an adsorbate on the STM tip. In both cases, the IETS intensity markedly dropped at the step edges. At the same time, the IETS intensity exhibits long-range oscillations, the wavelength of which coincided with the Friedel oscillations of the surface state in the vicinity of the defects. All the observations combined, attest to the important role of the surface state in electron-phonon coupling. We will rationalize the observed effects by invoking the symmetry of the tunneling states, with particular emphasis on the effect of the projected band-gap of the gold surface.   

Anisotropic Surface State Mediated RKKY Interaction Between Adatoms on a Hexagonal Lattice

Motivated by recent numerical studies of Ag on Pt(111), we derive a far-field expression for the RKKY interaction mediated by surface states on a (111) FCC surface, considering the effect of anisotropy in the Fermi edge. The main contribution to the interaction comes from electrons whose Fermi velocity {\bf v}$_F$ is parallel to the vector {\bf R} connecting the interacting adatoms; we show that in general, the corresponding Fermi wave-vector {\bf k}$_F$ is not parallel to {\bf R}. The interaction is oscillatory; the amplitude and wavelength of oscillations have angular dependence arising from the anisotropy of the surface state band structure. The wavelength, in particular, is determined by the component of the aforementioned {\bf k}$_F$ that is parallel to {\bf R}. Our analysis is easily generalized to other systems. For Ag on Pt(111), our results indicate that the RKKY interaction between pairs of adatoms should be nearly isotropic and so cannot account for the anisotropy found in the studies motivating our work.   

Inverse photoemission and LEED investigation of the ion-bombarded Ni(110) surface

Ion bombardment of the clean Ni(110) surface is investigated by a combination of Inverse Photoemission Spectroscopy (IPES) with a Geiger-Muller detector and Low Energy Electron Diffraction (LEED) with a homebuilt video capture system. Disorder of the surface is induced by argon ion bombardment with various combinations of argon pressure and sputtering time. The intensity of the unoccupied surface state at $\sim$ 2eV above the Fermi level at the \={Y} point of the surface Brillouin zone decreases with increasing surface bombardment. Simultaneously, intensity profiles of diffraction spots in LEED exhibit broadening and a rising background level for increasing surface disorder. Multiple attempts at correlation between the results of the two techniques are presented.   

Electronic structure of the indium-adsorbed Au/Si(111)-$\sqrt{3}\times\sqrt{3}$ surface: a first-principles study

Electronic structures of the indium-adsorbed Au/Si(111)-$\sqrt{3}\times\sqrt{3}$ surface were examined using first-principles calculations at In coverages range from 0.15 ML to 1 ML. The band structures of the various proposed models were analyzed in detail. Our results show that the calculated bands for the identified atomic models for indium-adsorbed on conjugate honeycomb-chained-trimer model at In coverages of 1/3 ML and 2/3 ML are in agreement with the identified bands in the angle-resolved photoemission study [Phys. Rev. B \textbf{80} 075312 (2009)].   

Lattice relaxation as the origin of the insulating nature of the alkali/Si(111):B surface

\emph{Ab initio} density-functional theory calculations, photoemission spectroscopy (PES), scanning tunneling microscopy, and spectroscopy (STM, STS) have been used to solve the $2\sqrt{3} \times2\sqrt{3}R30$ surface reconstruction observed previously by LEED on 0.5 ML K/Si:B. It is found that the large K-induced vertical lattice relaxation obtain in the calculations and occurring only for $3/4$ of Si adatoms is shown to quantitatively explain both the chemical shift of $1.14$ eV and the ratio $1/3$ measured on the two distinct B 1s core levels. A gap is observed between valence and conduction surface bands by ARPES and STS which is shown to have mainly a Si-B character using the ab initio calculations. Finally, the calculated STM images agree with our experimental results. Therefore, the insulating character of alkali/Si:B interfaces has been captured to an excellent accuracy, from the low lying 1s state of boron, to the unoccupied states above the gap, within a one-electron approach. This work solves the controversy about the origin of the insulating ground state of alkali-metal/Si(111):B semiconducting interfaces which were believed previously to be related to many-body effects   

First-Principles Study of One-Dimensional Metal-Molecule Hybrid Chains Self-Assembled on Ag Substrate

We explore the formation mechanism and the structural and electronic properties of one-dimensional metal-molecule hybrid chain self-assembled on Ag(111) substrate using {\em{ab initio}} density functional theory. It is observed that such a hybrid chain is formed by spontaneous transformation from 4,4"-dibromo-{\em{p}}-terphenyl (DBTP) molecules when deposited on Ag(111) substrate at room temperature. We find that the chain is composed of {\em{p}}-terphenyl (TP) connected through an Ag atom to form (TP-Ag)$_n$. Our study shows that Ag(111) surface plays a catalytic role removing two Br atoms (DB) from DBTP and connecting the remaining TP through an Ag atom, which is a similar phenomenon as the Ullmann cross coupling reaction occurred on Cu surface. We find that those Br atoms detached from DBTP play important roles for spontaneous formation of well-aligned pattern. Our calculated electronic structures and simulated scanning tunneling microscopy (STM) images of (TP-Ag)$_n$ hybrid chain exhibit remarkably different charge distributions depending on the energy values, which are related to the tip voltage in STM experiments. We also investigate the end state of the hybrid chain, which can be spatially resolved.   

Quantum Monte Carlo studies of surface adsorptions

Surface adsorption is the first step to the study of surface catalytic reaction. The most common used tool is the Density Functional Theory (DFT) based on exchange-correlation approximations and the accuracy usually has not been checked carefully by highly accurate quantum many-body approaches. We have performed calculations of the surface adsorptions using the state-of-the-art diffusion quantum Monte Carlo (QMC) method to examine the accuracy of LDA and GGA (PBE) functionals in the study of surface adsorptions. The systems examined include the H$_{2}$O and OH adsorptions on various types of surfaces such as NaCl(100), MgO(100), TiO$_{2}$(110), graphene, Si(100)-(2x2) and Al(100). By comparing GGA (PBE) results with DMC, our results indicate that (i) for the H$_{2}$O adsorption, PBE predicts the correct adsorption energies; (ii) for the OH adsorption, PBE has predicted a large over-binding effect except on graphene and Si(100) surfaces. This fact indicates that one needs to be cautious when using DFT to study the surface adsorptions of OH free radical.   

Resistivity of thin gold films on mica induced by electron-surface scattering from a self-affine fractal surface

We present a rigorous comparison between resistivity data and theoretical predictions involving the theory of Palasantzas [G. Palasantzas et al., \textit{Phys. Rev. }\textbf{B 56 }7726 (1997)], and the mSXW-fractal theory [R. C. Munoz et al., \textit{Phys. Rev. }\textbf{B 66 }205401 (2002)], regarding the resistivity arising from electron scattering by a self affine fractal surface on gold films \textit{using no adjustable parameters}. We find that both theories lead to an approximate description of the temperature dependence of the resistivity data. However, the description of charge transport based upon fractal scaling seems oversimplified, and the predicted increase in resistivity arising from electron-surface scattering seems at variance with other experimental results. If the samples are made up of grains such that the mean grain diameter D $>$ L(300), the electronic mean free path in the bulk at 300 K, then the predicted increase in resistivity at 4 K is of the order of a few percent. This contradicts published measurements of magnetomorphic effects arising from size effects where \textit{electron-surface scattering} \textit{is the dominant electron scattering mechanism at 4 K }. On the contrary, if the samples are made out of grains such that D $<$ L(300), then \textit{the dominant electron scattering mechanism controlling the resistivity is not electron-surface scattering but rather electron-grain boundary scattering}, \textit{and the latter electron scattering mechanism is not included in either theory.}   

Electronic confinement imposed by a nanoporous network: Band formation from coupled quantum dots

The electronic and optical properties of crystalline solids exhibit characteristics that derive to a large extent from the periodic arrangement and interactions of their component quantum systems, such as atoms or molecules. Quantum effects due to confinement of electronic states have been extensively studied for surface states of noble metals which are characterized by a quasi 2D electron gas. The design of such periodic 2D structures on surfaces is more readily achieved using molecular self-assembly from building blocks instead of atom by atom manipulation. We chose a perylene derivative (DPDI) as organic building block which is known to form a highly ordered porous network on Cu(111) upon thermal dehydrogenation. To study the interaction between the electronic surface state and our porous network structure, scanning tunneling spectroscopy (STS) and angle-resolved photoemission (ARPES) was used. Each pore of our porous network confines the surface state of the Cu substrate what can be described as a 0D quantum dot. This work can lead to artifically created electronic structures by modification of the dimensions of the molecular network periodicities together with the appropriate choice of the substrate and the building block.   

First-Principles Investigations of Oxygen Vacancies on SnO2 Nanofilms

In recent years multiple critical advances in nanofabrication have allowed for the well-controlled formation of nanocrystals of the n-type semiconductor tin oxide ($SnO_2$). Because gas sensing in $SnO_2$ involves changes in surface resistivity as a function of gas concentration, the high surface-to volume ratio of $SnO_2$ nanocrystals could be leveraged to produce a gas sensor with significantly enhanced sensitivity. A key feature of the sensing mechanism is the facile formation and destruction of oxygen vacancies at (or near) the surface. In this talk I will discuss our ongoing first-principles investigations of surface oxygen vacancies in $SnO_2$ nanofilms. We have focused on vacancy formation among the so-called bridging oxygen atoms on the (110) surface of rutile $SnO_2$ as a function of vacancy concentration and film thickness and have studied the effect on local atomic and electronic structure. From a set of first-principles Density Functional Theory calculations on ordered vacancy structures, we have parametrized and tested a lattice-gas model describing vacancy-vacancy interactions. Using this model we have conducted extensive Monte Carlo simulations to investigate the oxygen vacancy phases on $SnO_2$ (110) as a function of temperature and oxygen vapor pressure.   

Orbital tomography: Deconvoluting photoemission spectra of organic molecules

We study the interface of an organic monolayer with a metallic surface, \emph{i.~e.}, PTCDA (3,4,9,10-perylene-tetracarboxylic-dianhydride) on Ag(110), by means of angle-resolved photoemission spectroscopy (ARPES) and \emph{ab initio} electronic structure calculations. We present a tomographic method which uses the energy and momentum dependence of ARPES data to deconvolute spectra into individual orbital contributions beyond the limits of energy resolution. This provides an orbital-by-orbital characterization of large adsorbate systems without the need to invoke sophisticated theory of photoemission, allowing us to directly estimate the effects of bonding on individual orbitals. Moreover, this experimental data serves as a most stringent test necessary for the further development of \emph{ab initio} electronic structure theory.   

Two-Dimensional Band Structure Study of Bi$_{1-x}$Sbx Thin Films

Alloys of Bi$_{1-x}$Sb$_{x}$, are considered as one of the best thermoelectric materials for low temperature applications below 200 K. The band structure of Bi$_{1-x}$Sb$_{x}$ varies as a function of stoichiometry. At a temperature below 77 K, it does not change with temperature. At a certain Sb composition (x=0.04), the conduction band and the valence band touch each other at the $L$ point, and the band-crossing occurs. The electronic dispersion relation becomes linear at the $L$ point, which implies that a Dirac cone is formed at each of the three $L$ points. By making the alloys of Bi$_{1-x}$Sb$_{x}$ into thin films, we have two more parameters to vary the band structure, namely film thickness and growth orientation. In our present work, the rich variety of band structure configurations, as well as various phases, of Bi$_{1-x}$Sb$_{x}$ thin films has been revealed.