Local Electronic Density Of States Research Articles

A Theoretical Study of the Water–Gas-Shift Reaction on Cu6TM (TM = Co, Ni, Cu, Rh, Pd, Ag, Ir, Pt, Au) Clusters

We perform density-functional theory calculations to investigate the water–gas-shift (WGS) reaction on Cu6TM (TM = Co, Ni, Cu, Rh, Pd, Ag, Ir, Pt, Au) clusters through redox, carboxyl, and formate mechanisms, which correspond to CO* + O* → CO2 (g), CO* + OH* → COOH* → CO2 (g) + H*, CO* + H* + O* → CHO* + O* → HCOO** → CO2(g) + H* respectively. An energetic span model is used to estimate the efficiency of the three mechanisms of different Cu6TM. It finds that for groups 9 and 10, carboxyl mechanism is the predominant mechanism in the three. While for Cu6TM (Cu, Ag, Au), it finds that the formate mechanism form the TDI and TDTS. Furthermore, the turnover frequency calculations are done for every Cu6TM cluster. The results show that Cu6Co is the best catalyst for WGS reaction. Finally, to understand the high catalytic activity of the Cu6Co cluster, the nature of the interaction between adsorbate and substrate is also analyzed by the detailed electronic local density of states. These findings enrich the applications of Cu-based materials to the high activity catalytic field.

Direct observation of many-body charge density oscillations in a two-dimensional electron gas

Quantum interference is a striking manifestation of one of the basic concepts of quantum mechanics: the particle-wave duality. A spectacular visualization of this effect is the standing wave pattern produced by elastic scattering of surface electrons around defects, which corresponds to a modulation of the electronic local density of states and can be imaged using a scanning tunnelling microscope. To date, quantum-interference measurements were mainly interpreted in terms of interfering electrons or holes of the underlying band-structure description. Here, by imaging energy-dependent standing-wave patterns at noble metal surfaces, we reveal, in addition to the conventional surface-state band, the existence of an 'anomalous' energy band with a well-defined dispersion. Its origin is explained by the presence of a satellite in the structure of the many-body spectral function, which is related to the acoustic surface plasmon. Visualizing the corresponding charge oscillations provides thus direct access to many-body interactions at the atomic scale.

Linear augmented cylindrical wave method for nanotubes electronic structure

In this review paper, the main ideas and results of application of the linearized augmented cylindrical wave (LACW) method for the electron properties of the single‐walled, double‐walled, embedded, and intercalated nanotubes are summarized. We start with the simplest case of the achiral single‐walled (n, 0) and (n, n) tubules having small translational unit cells. Then, the electron properties of chiral (n, m) nanotubes having very large translational cells are discussed with account of tubules possessing rotational and screw symmetries. Based on the LACW and Green's function techniques, the ab initio numerical approach to calculating the electron local densities of states of the substitutional impurities in the nanotubes is presented. The relativistic version of LACW theory is described and applied to calculating the effects of spin–orbit coupling on π‐bands of the cumulenic (C)n and polyynic (C2)n carbynes and armchair tubules. The approach of the cylindrical waves applied for the description of nanotubes permits to reproduce their geometry in an explicit form that offers the great advantages. © 2015 Wiley Periodicals, Inc.

Adsorption and Reaction of C₂H₄ and O₂ on a Nanosized Gold Cluster: A Computational Study.

We have investigated the adsorption and reaction mechanisms of C2H4 and O2 catalyzed by a Au38 nanoparticle based on periodic density-functional theory (DFT) calculations. The configurations of the adsorption of C2H4/Au38, O2/Au38, and O/Au38 as well as the coadsorption of C2H4-O2/Au38 were predicted. The calculation results show that C2H4, O2, and O are preferably bound at top (T), bridge (B), and hexagonal (h) sites with adsorption energies of -0.66, -0.99, and -3.93 eV, respectively. The detailed reaction mechanisms for ethylene epoxidation on the Au38 nanoparticle has been illustrated using the nudged elastic band (NEB) method. The oxidation process takes place via the Langmuir-Hinshelwood (LH) mechanism to generate ethylene oxide and acetaldehyde. The overall reaction of C2H4 + O2 + Au38 → ethylene oxide + O/Au38 is exothermic by 2.20-2.40 eV whereas those are 3.03-3.08 eV for the production of acetaldehyde and O/Au38. The nature of the interaction between the adsorbate and gold nanocluster has been analyzed by the detailed electronic local density of states (LDOS) to understand the high catalytic activity of the gold nanoclusters.

Mechanism of CO preferential oxidation catalyzed by Cu n Pt (n = 3–12): a DFT study

The CO preferential oxidation reaction (PROX) is particularly well suited for hydrogen purification for proton exchange membrane fuel cell applications. In this work, the mechanism of CO-PROX catalyzed by Cu n Pt (n = 3–12) clusters has been studied by density functional theory calculations. The calculated results indicate that the most favored adsorption site of H2 for all clusters is on the Pt sites, and O2 prefers to bind on Cu sites and CO bind on Pt sites. The lowest energy barrier for hydrogen dissociation is 0.02 eV. Smaller H–Pt bond length of Cu n PtH2 corresponds to larger H–H bond length. CO-PROX occurs via the main intermediates of COOH and OH. Cu6Pt is proposed as the most effective catalyst for CO-PROX. To understand the high catalytic activity of Cu n Pt clusters, the nature of the interaction between adsorbate and substrate is also analyzed by detailed electronic local density of states. These findings enrich applications of Cu-based materials to the field of high-activity catalysts.

On the mechanism of the preferential oxidation of carbon monoxide over Cu n Pd (n = 3–12) catalysts

Preferential oxidation of CO (CO-PROX) is an important practical process for the purification of H2 for use in proton exchange membrane fuel cells. In order to clarify the mechanism of CO-PROX, the reaction promoted by CunPd (n = 3–12) clusters has been studied by density functional theory calculations. The CO-PROX reaction via carboxylic and hydroxyl intermediates is found to be the most likely mechanism. The theoretical results show that CO oxidation is promoted by the attack of surface OH groups, which is different from the usual oxidation of CO with O2. The Cu6Pd cluster is predicted to have a lower activation barrier compared with other CunPd clusters and is therefore proposed as the most effective nanocatalyst. To gain insights into the high catalytic activity of the CunPd nanoparticles, the nature of the interaction between adsorbate and substrate has been analyzed by the detailed electronic local density of states. The results should be helpful in developing efficient catalysts for the CO-PROX reaction.

Effect of oxygen vacancy on lattice and electronic properties of HfO2 by means of density function theory study

HfO2, as a gate dielectric material for the charge trapping memory, has been studied extensively due to its merits such as high k value, good thermal stability, and conduction band offset relative to Si, etc.. In order to understand the reason why the charge trapping efficiency is improved by high k capture layer with respect to charge trapping type memory, the variation of HfO2 crystal texture induced by oxygen vacancy and the influences of it are investigated using the first principle calculation based on density functional theory. Results show that the distance of the nearest neighbor oxygen atom from oxygen vacancy is markedly reduced after optimization, whereas the decrease of distances between the next nearest neighbor oxygen atom from oxygen vacancy and hafnium is less. The change of local crystal lattice is caused by optimized oxygen vacancy for it significantly changes the local lattice, but rarely influences the far lattice. Deep energy level and density of electron states in conduction band are contributed by Hf atoms, while the density of electron states in valence band is contributed by O atoms. The local density of electron states in each element and the total density of electron states in the optimization system are all larger than those in the system without optimization, and the sum of the local densities of electron states is less than the total density of electron states. The trapped charges are moving mainly around the oxygen vacancy and the adjacent atoms of oxygen in the optimization system, but the charges are without optimization throughout the system. The local energy of charge is increased in optimized defect system, while the local energy of charge is conspicuously reduced in the system without optimization, i.e. lattice variation without saturation characteristic has a large effect on the local energy of charge. Results further prove that the change of crystal lattice induced by oxygen vacancy has strong ability to capture charge, which helps improve the features of memory.

The structure and properties of graphene on gold nanoparticles.

Graphene covered metal nanoparticles constitute a novel type of hybrid material, which provides a unique platform to study plasmonic effects, surface-enhanced Raman scattering (SERS), and metal-graphene interactions at the nanoscale. Such a hybrid material is fabricated by transferring graphene grown by chemical vapor deposition onto closely spaced gold nanoparticles produced on a silica wafer. The morphology and physical properties of nanoparticle-supported graphene are investigated by atomic force microscopy, optical reflectance spectroscopy, scanning tunneling microscopy and spectroscopy (STM/STS), and confocal Raman spectroscopy. This study shows that the graphene Raman peaks are enhanced by a factor which depends on the excitation wavelength, in accordance with the surface plasmon resonance of the gold nanoparticles, and also on the graphene-nanoparticle distance which is tuned by annealing at moderate temperatures. The observed SERS activity is correlated with the nanoscale corrugation of graphene. STM and STS measurements show that the local density of electronic states in graphene is modulated by the underlying gold nanoparticles.

Scanning tunneling microscopy and spectroscopy at very low temperatures

During past decades, an increasing number of laboratories is using cryogenic scanning tunneling microscopy and spectroscopy (STM/S) to probe different kinds of electronic systems. Measurements in a dilution refrigerator are particularly useful to study superconductors, because temperatures of order of 100 mK are well below most critical temperatures and effectively reduce thermally excited quasiparticles. The local electronic density of states is then obtained at atomic level with a resolution in energy of some tens of μeV. Visualizing spatial variations of the local density of states allows characterizing vortex cores and the vortex lattice. Vortex core electronic features provide the anisotropy of the superconducting properties, and help understanding the influence of competing orders such as charge density waves. Here we will review results in dichalcogenide superconductors, in the magnetic borocarbide TmNi2B2C and in thin films, discussing in some detail a few relevant aspects of thermal depinning and melting in thin films.

Modified gap states in Fe/MgO/SrTiO3 interfaces studied with scanning tunneling microscopy

The geometric and electronic structures of Fe islands on MgO film layers were studied with scanning tunneling microscopy and spectroscopy. The MgO layers were grown on a Nb-doped single crystal SrTiO3 (100) surface. Deposited Fe atoms aggregate into islands, the height and diameter of which are about 2.5 and 9.4 nm respectively. Fe islands modify the electronic structure of MgO surface; a ring type depression in the scanning tunneling microscope topography appears by lowered local electron density of states around Fe islands. We find that adsorbed Fe atoms reduce the gap states of MgO layers around Fe islands, which is attributed to the reason for the depletion of the electronic density of states.

Anisotropic Fabry-Pérot resonant states confined within nano-steps on the topological insulator surface.

The peculiar nature of topological surface states, such as absence of backscattering, weak anti-localization, and quantum anomalous Hall effect, has been demonstrated mainly in bulk and film of topological insulator (TI), using surface sensitive probes and bulk transport probes. However, it is equally important and experimentally challenging to confine massless Dirac fermions with nano-steps on TI surfaces. This potential structure has similar ground with linearly-dispersed photons in Fabry-Pérot resonators, while reserving fundamental differences from well-studied Fabry-Pérot resonators and quantum corrals on noble metal surfaces. In this paper, we study the massless Dirac fermions confined within steps along the x (Γ–K) or y (Γ–M) direction on the TI surface, and the Fabry-Pérot-like resonances in the electronic local density of states (LDOS) between the steps are found. Due to the remarkable warping effect in the topological surface states, the LDOS confined in the step-well running along Γ-M direction exhibit anisotropic resonance patterns as compared to those in the step-well along Γ-K direction. The transmittance properties and spin orientation of Dirac fermion in both cases are also anisotropic in the presence of warping effect.

DFT simulations of water adsorption and activation on low-index α-Ga2O3 surfaces.

Density functional theory (DFT) calculations are used to explore water adsorption and activation on different α-Ga2O3 surfaces, namely (001), (100), (110), and (012). The geometries and binding energies of molecular and dissociative adsorption are studied as a function of coverage. The simulations reveal that dissociative water adsorption on all the studied low-index surfaces are thermodynamically favorable. Analysis of surface energies suggests that the most preferentially exposed surface is (012). The contribution of surface relaxation to the respective surface energies is significant. Calculations of electron local density of states indicate that the electron-energy band gaps for the four investigated surfaces appears to be less related to the difference in coordinative unsaturation of the surface atoms, but rather to changes in the ionicity of the surface chemical bonds. The electrochemical computation is used to investigate the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER) on α-Ga2O3 surfaces. Our results indicate that the (100) and (110) surfaces, which have low stability, are the most favorable ones for HER and OER, respectively.

Electrical resistivity anomaly: A consequence of nanometric particles in a metal matrix

The additament of an impurity to a metal matrix generates a shift in the local electron density of states due to a change in the local electric potential so that an electron or electron beam will be scattered upon interaction with the ensuing potential, causing its wave function measured at a distance far removed from the source to show a quantum phase change. This phenomenon, reflected in Friedel's law, permits to relate the quantum phase shifts with both the density of states at the Fermi level and the transport properties of the system. This work calculates the electric resistivity of nanometric clusters in an aluminiumsupercell, emulating the formation of Guinier-Preston zones during pre-precipitation in an Al-Zn alloy. The density of states is evaluated each time a new atom of solute is added to the Al supercell using the ab initio method of the Density Functional Theory or DFT in its GGA and LDA approximations via the Dmol^{3} package of Materials Studio Software. A resistivity anomaly is obtained when, in concordance with literature reports for diluted AlZn alloys, the supercell contains 25 atoms, thus confirming that the electric resistivity anomaly during cluster formation is a consequence of both density of states variations and quantum transitions at the Fermi Level for a nanometric size in particular. The results are contrasted with those obtained via the transport equation.

Tunneling into the Mott insulatorSr2IrO4

The single-layer Mott insulator strontium iridate Sr$_2$IrO$_4$ was studied using a scanning tunneling microscope. This measurement technique is unique due to the transport properties of this Mott insulator allowing tunneling measurements to be performed, even at cryogenic temperatures. We obtained high-resolution images of the sample surface and the differential tunneling conductance at different cryogenic temperatures. The differential conductance is a direct measurement of the local electronic density of states which provided an insulating gap consistent with optical conductivity, angle resolved photoemission spectroscopy and resonant inelastic x-ray scattering (RIXS) experiments. The observed widths of these features is broader than predicted by the Slater approximation and narrower than predicted by dynamical mean field theory. Additionally, the observed density of states due to magnetic fluctuations is found in the derivative of the differential conductance and is consistent with results from Raman scattering and RIXS. At low temperatures, additional low-energy features were observed, suggesting a change in the dispersion of the collective magnetic excitations, which is consistent with the magnetic susceptibility.

Temperature-dependent scanning tunneling spectroscopy on the Si(557)-Au surface

Room-temperature and low-temperature (77 K) scanning tunneling spectroscopy (STS) and voltage-dependent scanning tunneling microscopy (STM) data are used to study the local electronic properties of the quasi-one-dimensional Si(557)-Au surface in real space. A gapped local electron density of states near the Gamma-point is observed at different positions of the surface, i.e., at protrusions arising from Si adatoms and step-edge atoms. Within the gap region, two distinct peaks are observed on the chain of localized protrusions attributed to Si adatoms. The energy gap widens on both types of protrusions after cooling from room temperature to T = 77 K. The temperature dependence of the local electronic properties can therefore not be attributed to a Peierls transition occurring for the step edge only. We suggest that more attention should be paid to finite-size effects on the one-dimensional segments.

Concluding remarks: there's nowt so queer as carbon electrodes.

This contribution provides a personal overview and summary of Faraday Discussion 172 on "Carbon in Electrochemistry", covering some of the key points made at the meeting within the broader context of other recent developments on carbon materials for electrochemical applications. Although carbon electrodes have a long history of use in electrochemistry, methods and techniques are only just becoming available that can test long-established models and identify key features for further exploration. This Discussion has highlighted the need for a better understanding of the impact of surface structure, defects, local density of electronic states, and surface functionality and contamination, in order to advance fundamental knowledge of various electrochemical processes and phenomena at carbon electrodes. These developments cut across important materials such as graphene, carbon nanotubes, conducting diamond and high surface area carbon materials. With more detailed pictures of structural and electronic controls of electrochemistry at carbon electrodes (and electrodes generally), will come rational advances in various technological applications, from sensors to energy technology (particularly batteries, supercapacitors and fuel cells), that have been well-illustrated at this Discussion.

Theoretical Investigation of the Mechanism of the Water–Gas Shift Reaction on Cobalt@Gold Core–Shell Nanocluster

We studied the mechanism of the water–gas shift reaction (WGSR; CO + H2O → CO2 + H2) catalyzed by Co6@Au32 core–shell nanoalloy using density-functional theory (DFT) calculations to investigate the bimetallic effects on the catalytic activation. The molecular structures and adsorbate/substrate interaction energies were predicted, along with the potential energy surface constructed using the nudged elastic band (NEB) method. Our results indicated that the energetic barriers of the two hydrogen dissociation reactions are lower on the core–shell nanoalloy than on Au38. Furthermore, all of the related chemical species of the WGSR can adsorb stably on Co6@Au32 to allow the reactions to take place under ambient pressure. To gain insight into the synergistic effect in the catalytic activity of the Co6@Au32 nanoalloy, the nature of the interaction between the adsorbate and substrate was analyzed by detailed electronic local densities of states (LDOS) as well as molecular structures.

Adsorption and dissociation of H2S on Mo(1 0 0) surface by first-principles study

Density-functional theory calculations had been used to investigate the adsorption and dissociation of H2S on Mo(100) surface. Adsorption mechanisms of H2S, HS, S and H on the Mo(100) surface were analyzed. H2S was found to be adsorbed at bridge, hollow and top sites with adsorption energies of −1.25, −1.03 and −0.92eV, respectively. HS was strongly chemically absorbed at hollow, bridge and top sites with adsorption energies of −4.51, −4.08 and −3.45eV, respectively, and sulfur and hydrogen preferred to be absorbed at hollow and bridge sites, respectively. In addition, potential energy profiles of H2S dissociation on Mo(100) had been constructed by a climbing image nudged elastic band method. Four possible dissociation pathways of the first H2S dehydrogenation were examined with reaction barriers of 0.28, 0.37, 0.075, and 0.21eV, respectively, while the energy barrier to break the SH bond of HS with or without hydrogen co-adsorption was almost the same low. This work showed that the decomposition of H2S on the molybdenum surface was kinetically and thermodynamically facile. Local densities of electronic states were further used to characterize the interaction between H2S and substrate.

How Contacting Electrodes Affect Single π-Conjugated Molecular Electronic States: Local Density of States of Phthalocyanine Nanomolecules on MgO(001), Cu(111), Ag(001), Fe(001), and Mn(001)

Single molecules have attracted much interest as new materials for future spin electronic devices; however, many open questions still remain. One of them is how the electronic local density of states (LDOS) of single molecules is affected when they are in contact with electrodes. We show a systematic study of the LDOS of π-conjugated phthalocyanine (H2Pc) nanomolecules adsorbed on various electrodes, namely, (1) MgO(001) thin films grown on Ag(001), (2) noble metals of Cu(111) and Ag(001), and (3) 3d magnetic metals of Fe(001) and Mn(001), adupting scanning tunneling spectroscopy techniques with an ultrahigh-vacuum scanning tunneling microscopy setup at room temperature. Since MgO thin films cut the electronic coupling from the substrate Ag(001), we could observe H2Pc molecular states at -1.5 and +1.0 eV. H2Pc molecules on the noble metal substrates form a pattern with a square unit cell of about 1.5×1.5 nm2 and have similar LDOS peaks near the Fermi energy. Strong hybridizations between the substrate 3d spin-polarized states and the molecular π orbitals produce new molecular states of H2Pc molecules adsorbed on Fe(001) and Mn(001) near the energy positions of the Fe(001) minority spin state and the Mn(001) majority spin state, respectively.

Atomic-scale fingerprint of Mn dopant at the surface of Sr₃(Ru₁-xMnx)₂O₇.

Chemical doping in materials is known to give rise to emergent phenomena. These phenomena are extremely difficult to predict a priori, because electron-electron interactions are entangled with local environment of assembled atoms. Scanning tunneling microscopy and low energy electron diffraction are combined to investigate how the local electronic structure is correlated with lattice distortion on the surface of Sr3(Ru1−xMnx)2O7, which has double-layer building blocks formed by (Ru/Mn)O6 octahedra with rotational distortion. The presence of doping-dependent tilt distortion of (Ru/Mn)O6 octahedra at the surface results in a C2v broken symmetry in contrast with the bulk C4v counterpart. It also enables us to observe two Mn sites associated with the octahedral rotation in the bulk through the “chirality” of local electronic density of states surrounding Mn, which is randomly distributed. These results serve as fingerprint of chemical doping on the atomic scale.