Near-surface Electronic Structure Research Articles

Broken Screw Rotational Symmetry in the Near-Surface Electronic Structure of AB-Stacked Crystals.

We investigate the electronic structure of 2H-NbS_{2} and h-BN by angle-resolved photoemission spectroscopy (ARPES) and photoemission intensity calculations. Although in bulk form, these materials are expected to exhibit band degeneracy in the k_{z}=π/c plane due to screw rotation and time-reversal symmetries, we observe gapped band dispersion near the surface. We extract from first-principles calculations the near-surface electronic structure probed by ARPES and find that the calculated photoemission spectra from the near-surface region reproduce the gapped ARPES spectra. Our results show that the near-surface electronic structure can be qualitatively different from the bulk electronic structure due to partially broken nonsymmorphic symmetries.

Preventing Alloy Electrocatalyst Segregation in Air Using Sacrificial Passivating Overlayers.

Many alloy electrocatalysts, including intermetallics, are exceptionally sensitive to segregation in air due to the electronic dissimilarity of the constituent metals. We demonstrate that even alloys with strong cohesive energies rapidly segregate upon air exposure, completely burying the less reactive constituent metal beneath the surface. To circumvent this issue, we develop and validate a new experimental approach for bridging the pressure gap between electronic structure characterization performed under ultrahigh vacuum and electrocatalytic activity testing performed under ambient conditions. This method is based on encapsulation of the alloy surface with a sacrificial passivating overlayer of aluminum oxide. These passivating overlayers protect the underlying material from segregation in the air and can be completely and rapidly removed in an alkaline electrochemical environment under potential control. We demonstrate that alloy surfaces prepared, protected, and introduced into the electrolyte in this manner exhibit near-surface compositions consistent with those of the bulk material despite prior air exposure. We also demonstrate that this protection scheme does not alter the electrocatalytic activity of benchmark electrocatalysts. Implementation of this approach will enable reliable correlations between the electrocatalytic activity measured under ambient conditions and the near-surface electronic structure measured under ultrahigh vacuum.

Near-surface electronic structure in strained Ni-ferrite films: An x-ray absorption spectroscopy study

We report on the x-ray absorption spectra (XAS) and x-ray magnetic circular dichroism (XMCD) of a series of NiFe2O4 (Ni ferrite) films grown on symmetry matched substrates and measured in two geometries: out-of-plane and near in-plane. The Ni ferrite films, grown by pulsed laser deposition, are epitaxial and the substrates used (ZnGa2O4, CoGa2O4, MgGa2O4, and MgAl2O4) introduce a systematic variation in the lattice mismatch between the substrate and the film. Modeling of the XAS and XMCD spectra, both measured with the surface sensitive total electron yield mode, indicates that the Ni2+ cations reside on the octahedrally coordinated lattice sites in the spinel structure. Analyses of the Fe XAS and XMCD spectra are consistent with Fe3+ cations occupying a subset of the octahedral and tetrahedral sites in the spinel oxide lattice with the addition of a small amount of Fe2+ located on octahedral sites. The Ni2+ orbital to spin moment ratio (μℓ/μs), derived from the application of XMCD sum rules, is enhanced for the substrates with a small lattice mismatch relative to NiFe2O4. The results suggest a path for increasing the orbital moment in NiFe2O4 by applying thin film growth techniques that can maintain a highly strained lattice for the NiFe2O4 film.

Realization of interstitial boron ordering and optimal near-surface electronic structure in Pd-B alloy electrocatalysts

The fundamental understanding and precise tuning of interatomic s,p-d orbital hybridization are critical to the design of interstitial alloy catalysts containing light constituent atoms. Here, we present a joint theoretical and experimental study that reveals the importance of distribution, concentration and ordering of interstitial boron atoms in Pd-B alloy’s near-surface electronic structure and catalytic performance. Our theoretical results demonstrate that the sub-surface location of interstitial boron atoms with a Pd:B atomic ratio of 2:1 is necessary to ensure an appropriate degree of interatomic s,p-d orbital hybridization, and thereby an optimal surface electronic structure for the hydrogen evolution reaction (HER). We experimentally achieve this with highly-crystalline intermetallic Pd2B. Due to its optimal electronic structure and ordered arrangement of interstitial boron atoms, the intermetallic Pd2B exhibits Pt-like catalytic activity for HER and has excellent catalytic stability for over seven days.

Determination of the mechanism and extent of surface degradation in Ni-based cathode materials after repeated electrochemical cycling

We take advantage of scanning transmission electron microscopy and electron energy loss spectroscopy to investigate the changes in near-surface electronic structure and quantify the degree of local degradation of Ni-based cathode materials with the layered structure (LiNi0.8Mn0.1Co0.1O2 and LiNi0.4Mn0.3Co0.3O2) after 20 cycles of delithiation and lithiation. Reduction of transition metals occurs in the near-surface region of cathode materials: Mn is the major element to be reduced in the case of relatively Mn-rich composition, while reduction of Ni ions is dominant in Ni-rich materials. The valences of Ni and Mn ions are complementary, i.e., when one is reduced, the other is oxidized in order to maintain charge neutrality. The depth of degradation zone is found to be much deeper in Ni-rich materials. This comparative analysis provides important insights needed for the devising of new cathode materials with high capacity as well as long lifetime.

An Investigation of Electrodeposited Platinum Metal Monolayer Catalysts By X-Ray Photoelectron Spectroscopy and Cyclic Voltammetry

Platinum group metals (PGM) are the catalyst of choice in a wide variety of catalytic reactions, including the oxygen reduction reaction. We present here our research on tailoring the near-surface electronic structure of core/shell catalytic systems under low-loading limits of PGM overlayers fabricated via surface-limited redox replacement. Synchrotron-based XPS allowed us to profile the transitions in the electronic structure from the surface down to the adlayer/support interface and beyond. Catalyst durability is also a significant aspect for further consideration, with catalyst instability oftentimes caused by metal dissolution or corrosion. It is has been shown that Au as a supporting metal can have a stabilizing effect on Pt even under high oxidizing conditions and thus can suppress Pt dissolution, resulting in improved durability of the Pt catalysts. This study looks at the durability of Pt monolayer catalysts by subjecting them to aggressive cyclic voltammetry cycling and by examining the activity of the surfaces towards the ORR during potential cycling in an oxygen environment. Samples with an overlayer thickness of 2 monolayers or less show a more dramatic decay in electrochemically active surface area under aggressive cycling, indicating poor durability. However, Pt surface retention is significantly improved once it is at least 3 ML, indicating increased durability due to chemical state and thickness of the surface. The cycling in oxygen-rich media show markedly enhanced currents for the ORR once a 2 monolayer Pt overlayer thickness is achieved, showing the highest activity towards the ORR relative to the amount of platinum present.

Structure and Architecture Driven Metallic Bonding and Ligand Effects in Low-Dimensional Platinum Group Metal Catalyst

Whether grown as clusters or ultrathin films, extremely small quantities of active elements exhibit changes in catalytic activity that arise from both size effects and electron-transfer effects. These size and transfer effects can be related to increased propensity for oxidation of the metallic deposit, as well as to various changes in electrochemical performance such as durability or required overpotential for a given reaction. A review of our recent work on layer-by-layer near-surface effects of atomic and electronic structure on catalytically relevant properties will be presented. We use electrochemical atomic layer deposition for layer-by-layer growth of platinum group metals (PGM), and measure the evolution of electronic and atomic structure using synchrotron-based X-ray Photoelectron Spectroscopy (XPS) and X-ray absorption spectroscopy. XPS using a tunable energy synchrotron X-ray source allows us to profile the transitions in the electronic structure from the surface down to the adlayer/support interface and beyond. In addition to depth profile studies of these layered metal architectures, the effects of thermally and electrochemically activated near-surface alloying in the PMG systems will be presented here. We observe undulations of near-surface electronic structure brought on by the low dimensionality of the PMG adlayer as well as the adlayer-support interactions, effects that can explain the resulting surface.

Spin-correlated electronic state on the surface of a spin-orbit Mott system

We use angle-resolved photoemission spectroscopy to show that the near-surface electronic structure of a bulk insulating iridate ${\mathrm{Sr}}_{3}{\mathrm{Ir}}_{2}{\mathrm{O}}_{7}$ lying near a metal-Mott insulator transition exhibits weak metallicity signified by finite electronic spectral weight at the Fermi level. The surface electrons exhibit a spin structure resulting from an interplay of spin-orbit, Coulomb interaction and surface quantum magnetism. Our results shed light on understanding the exotic quantum entanglement and transport phenomena in iridate-based oxide devices.

Corrosion of UO 2 and ThO 2: A quantum-mechanical investigation

The addition of Th to U-based fuels increases resistance to corrosion due to differences in redox-chemistry and electronic properties between UO 2 and ThO 2. Quantum-mechanical techniques were used to calculate surface energy trends for ThO 2, resulting in (1 1 1) < (1 1 0) < (1 0 0). Adsorption energy trends were calculated for water and oxygen on the stable (1 1 1) surface of UO 2 and ThO 2, and the effect of model set-up on these trends was evaluated. Molecular water is more stable than dissociated water on both binary oxides. Oxidation rates for atomic oxygen interacting with defect-free UO 2(1 1 1) were calculated to be extremely slow if no water is present, but nearly instantaneous if water is present. The semi-conducting nature of UO 2 is found to enhance the adsorption of oxygen in the presence of water through changes in near-surface electronic structure; the same effect is not observed on the insulating surface of ThO 2.

A Comparative Study on the Electronic Structure of the 1-Ethyl-3-Methylimidazolium Bis(trifluoromethylsulfonyl)amide RT-Ionic Liquid by Electron Spectroscopy and First Principles Calculations

The near-surface electronic structure of the room-temperature ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide has been investigated with ultraviolet and X-ray photoelectron spectroscopy as well as metastable induced electron spectroscopy. The results have been compared with density functional theory calculations. The good agreement between the experimental and theoretical data provides detailed insight into the origin of the observed spectral features. In particular, we found that a simple composition of the spectra of the isolated ions does not suffice to fit to the experimental results, but interionic interactions have to be considered.

Electronic Structure of the Surface of the Ionic Liquid [EMIM][Tf2N] Studied by Metastable Impact Electron Spectroscopy (MIES), UPS, and XPS

The near-surface electronic structure of the room-temperature ionic liquid (RT-IL) 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([EMIM][Tf(2)N]) has been investigated with the combination of the electron spectroscopies metastable impact electron spectroscopy (MIES), ultraviolet photoelectron spectroscopy (UPS (HeI and HeII)), and monochromatized X-ray photoelectron spectroscopy (XPS). We find that the top of the valence band states originates from states of the cation (see also ref 1). The ultimately surface-sensitive technique MIES proves that the surface layer consists of both cations and anions. The temperature dependence of the spectra has been measured between about 160 and 610 K. Information on the glass transition and the possibility for low-temperature distillation of [EMIM][Tf(2)N] at reduced pressures is derived from the present results.

Angle-resolved photoemission study of the near-surface electronic structure of Cr(001)

Normal-emission photoelectron spectra of a clean Cr(001) were measured at room temperature with photon energies from 24 to 70 eV. The periodicity of the energy-band dispersion along [001] direction normal to the surface is compatible with that of an antiferromagnetic structure rather than a paramagnetic one. One of the two peaks just below the Fermi level shows energy dispersion, which reveals the generally accepted simple picture that these peaks are exchange-split surface states is no longer valid, though the appearance of surface ferromagnetism of Cr(001) is still undeniable.

Controlled oxide removal for the preparation of damage-free InAs(110) surfaces

Controlled oxide removal from InAs(110) surfaces using atomic hydrogen (H*) has been achieved by monitoring the contaminant vibrational modes with high resolution electron energy loss spectroscopy (HREELS). The contributing oxide vibrational modes of the partially H* cleaned surface have been identified. Following hydrocarbon desorption during preliminary annealing at 360 °C, exposure to atomic hydrogen at 400 °C initially removes the arsenic oxides and indium suboxides; complete indium oxide removal requires significantly higher hydrogen doses. After a total molecular hydrogen dose of 120 kL, a clean, ordered surface, exhibiting a sharp (1×1) pattern, was confirmed by low energy electron diffraction and x-ray photoelectron spectroscopy. Energy dependent HREELS studies of the near-surface electronic structure indicate that no residual electronic damage or dopant passivation results from the cleaning process.

Near-surface electronic structure on InAs(100) modified with self-assembled monolayers of alkanethiols

Surface chemical modification is used to eliminate the problem of high surface recombination velocity and control surface band bending on InAs(100). Alkanethiols, RSH; R=CH3(CH2)n, both neat and in ethanolic solutions, are used to passivate this surface against oxidation, as characterized by Raman scattering and x-ray photoelectron spectroscopy of the adsorbate-covered and bare surfaces. The magnitude of the interfacial band bending is obtained by analysis of Raman scattering from the unscreened longitudinal optical phonon, which arises from the near-surface charge accumulation region. Removing the native oxide with a Br2:CH3OH chemomechanical etch reduces the surface band bending, but atmospheric oxidation increases band bending to its original level over several hours. In contrast, alkanethiol passivation of InAs(100) prevents band bending for periods of up to several weeks.

Raman scattering as a probe of the superconducting proximity effect

Temperature-dependent Raman spectroscopy is used to investigate the effect of superconductivity on the near-surface electronic structure of a semiconductor in good electrical contact with a superconductor. The light scattering is performed through a high-quality thin (60–100A) film of Nb, grown directly ontoin-situ Ar+-etched (100)-n+InAs. Below Tc, the LO mode, associated with the surface charge accumulation layer in the InAs, is enhanced by ∼40% in comparison with the nearby L_bulk phonon mode. This change, reversible upon temperature cycling, is observed only when the Nb is in good electrical contact with the InAs. Preliminary results show a similar effect on NbN/InAs. Our results constitute the first optical detection of the superconducting proximity effect.

Near-surface electronic structure in GaAs (100) modified with self-assembled monolayers of octadecylthiol

Passivation of the GaAs (100) surface by self-assembled monolayers of octadecylthiol (ODT) has been studied using inelastic light scattering as a probe of the near-surface electronic structure. Application of the ODT self-assembled monolayers reduces the width of the depletion region at the surface of GaAs resulting in a reduction of the surface band bending, and the electron scattering time is increased as well. The ODT passivated surfaces are more stable to environmental degradation, over time and under temperature stress, than inorganic sulfide treated surfaces which have been reported. Organic thiol passivation may provide an attractive alternative to inorganic sulfide protocols for reduction of surface recombination velocities in III–V devices.

Surface and bulk electronic structure of Ge(111) c(2 x 8) and Ge(111):As 1 x 1.

The surface band dispersion of the annealed (111) surface of germanium has been determined experimentally using angle-resolved photoemission. The c(2\ifmmode\times\else\texttimes\fi{}8) symmetry seen in low-energy electron diffraction is not seen in the surface-state dispersion, which exhibits a 2\ifmmode\times\else\texttimes\fi{}2 periodicity. This result is interpreted as indicating that the Ge(111) c(2\ifmmode\times\else\texttimes\fi{}8) surface unit cell contains 2\ifmmode\times\else\texttimes\fi{}2 subunits. The surface-state dispersion and bandwidth are consistent with a 2\ifmmode\times\else\texttimes\fi{}2 adatom model for the surface. An examination is made of the effect of the surface on the near-surface electronic structure. Results for the heavily reconstructed Ge(111) c(2\ifmmode\times\else\texttimes\fi{}8) surface are compared with those for the Ge(111):As 1\ifmmode\times\else\texttimes\fi{}1 surface. This latter surface was chosen for comparison because the arsenic termination removes the reconstruction and leads to an extremely stable surface with a geometry close to that of an ideal (111) surface. Spectral features corresponding to bulk initial states were compared with calculated values and found to agree throughout most of the surface Brillouin zone. Discrepancies were found near K\ifmmode\bar\else\textasciimacron\fi{} and attributed to the effect of back-bond formation. At finite values of ${k}_{?}$, a free-electron parabola with its zero at 9.70 eV below the top of the bulk valence band is the final state which best explains the bulk data. At normal emission, on the other hand, a Bloch conduction-band state was found to be more appropriate. The effect of secondary cone emission on the photoemission is discussed.

Spin polarized inverse photoemission studies of surface magnetism and electronic structure

Abstract Spin polarized inverse photoelectron spectroscopy (SPIPES) is shown to be a powerful new technique to study surface and near-surface electronic structure and magnetism. The process, the information obtained, and the apparatus required in a spin polarized inverse photoemission measurement are compared to the complementary spin polarized photoemission measurement. In SPIPES spectra of a clean Ni(110) surface, transitions to the minority spin 3d holes that give rise to ferromagnetism in Ni can be directly observed, as can sp states which are spin slit as an indirect consequence (via s-d hybridization) of the exchange splitting of the d bands. The adsorption of oxygen or carbon monoxide on Ni(110) causes a dramatic decrease in the number of minority 3d holes, that is, in the magnetic moment of the Ni atoms. The data allow us to severely limit the possible models of how chemisorption induces changes in surface magnetism. Other SPIPES studies, such as the temperature dependent behavior of empty bands in ferromagnetic Fe, and future directions and applications of SPIPES will be reviewed.

Investigation of the near-surface electronic structure of Cr(001).

An angle-resolved photoelectron spectroscopy (ARPES) study of Cr(001) near-surface electronic structure is presented. Measurements are reported for energy-band dispersions along the [010] direction parallel to the crystal surface. The periodicity of these band dispersions indicates that the valence electrons experience and self-consistently establish antiferromagnetism in the near-surface layers of Cr(001). We also present highly-surface-sensitive ARPES measurements of the energy-band dispersions along the [001] direction normal to the surface. The results suggest that the surface magnetic moments, which couple ferromagnetically to each other within the surface layer, couple antiferromagnetically to the moments of the atoms in the second layer. Temperature-dependent studies are presented that reveal the persistence of near-surface antiferromagnetic order for temperatures up to 2.5 times the bulk N\'eel temperature. The temperature dependence of this antiferromagnetic order suggests that its thermal stability derives in part from the stability of the Cr(001) ferromagnetic surface phase.

Electronic states of interstitial ions in alkali halide crystals

The fundamental understanding and precise tuning of interatomic s,p-d orbital hybridization are critical to the design of interstitial alloy catalysts containing light constituent atoms. Here, we present a joint theoretical and experimental study that reveals the importance of distribution, concentration and ordering of interstitial boron atoms in Pd-B alloy’s near-surface electronic structure and catalytic performance. Our theoretical results demonstrate that the sub-surface location of interstitial boron atoms with a Pd:B atomic ratio of 2:1 is necessary to ensure an appropriate degree of interatomic s,p-d orbital hybridization, and thereby an optimal surface electronic structure for the hydrogen evolution reaction (HER). We experimentally achieve this with highly-crystalline intermetallic Pd2B. Due to its optimal electronic structure and ordered arrangement of interstitial boron atoms, the intermetallic Pd2B exhibits Pt-like catalytic activity for HER and has excellent catalytic stability for over seven days.