Local Electronic Density Of States Research Articles

Quantum-Chemical Calculations on the Mechanism of the Water–Gas Shift Reaction on Nanosized Gold Cluster

We have studied the mechanism of the water–gas shift reaction (WGS, CO + H2O → CO2 + H2) catalyzed by nanosized gold particles by using density functional theory calculations. The molecular structures and adsorbate/substrate interaction energies of H2O/Au38, CO/Au38, HO/Au38, and H/Au38 configurations were predicted. Several adsorption sites on the Au38 nanoparticle were considered in this study and characterized as top, bridge, hollow, and hcp sites. A potential energy surface for WGS reaction on the Au38 nanoparticle has been constructed using the nudged elastic band method. It was found that water dissociation (H2O → H + OH) is the rate-limiting step, with an energy barrier of 31.41 kcal/mol. The overall reaction CO + H2O + Au38 → CO2 + H2 + Au38 is exothermic by 16.18 kcal/mol. To gain insights into the high catalytic activity of the gold nanoparticles, the nature of the interaction between adsorbate and substrate is also analyzed by the detailed electronic local density of states.

Study of local density of electron states in InGaAs/GaAs quantum rings by combined STM/AFM

The electronic structure of self-assembled InGaAs/GaAs(001) quantum rings grown by atmospheric pressure metal-organic vapor phase epitaxy has been investigated by combined scanning tunneling/atomic force microscopy (STM/AFM) in ultrahigh vacuum (UHV) for the first time. The current images and the tunnel spectra of contact of Pt-coated Si AFM probe to the quantum ring heterostructure surface revealing the spatial distribution of the local density of states and the electron size quantization spectra in the quantum rings have been obtained.

Internal Electron Diffraction from Atomically Ordered Subsurface Nanostructures in Metals

We demonstrate that a part of interface at a subsurface nanocavity in Cu(110) can efficiently induce electron scattering back to the surface even if it is inclined with respect to the surface, if the condition for electron diffraction is fulfilled. This backscattering induces oscillations of electron local density of states at the surface versus electron energy. In agreement with our model calculations, the diffraction is assigned to a specific atomic structure at the interface, and is found to be significantly enhanced by focussing of electron waves for propagation along the [110] direction.

Electronic properties of Cs-intercalated single-walled carbon nanotubes derived from nuclear magnetic resonance

We report on the electronic properties of Cs-intercalated single-walled carbon nanotubes (SWNTs). A detailed analysis of the 13C and 133Cs nuclear magnetic resonance (NMR) spectra reveals an increased metallization of the pristine SWNTs under Cs intercalation. The ‘metallization’ of CsxC materials where x=0–0.144 is evidenced from the increased local electronic density of states (DOS) n(EF) at the Fermi level of the SWNTs as determined from spin–lattice relaxation measurements. In particular, there are two distinct electronic phases called α and β and the transition between these occurs around x=0.05. The electronic DOS at the Fermi level increases monotonically at low intercalation levels x<0.05 (α-phase), whereas it reaches a plateau in the range 0.05⩽x⩽0.143 at high intercalation levels (β-phase). The new β-phase is accompanied by a hybridization of Cs(6s) orbitals with C(sp2) orbitals of the SWNTs. In both phases, two types of metallic nanotubes are found with a low and a high local n(EF), corresponding to different local electronic band structures of the SWNTs.

Nano-Scale Strain-Induced Giant Pseudo-Magnetic Fields and Charging Effects in CVD-Grown Graphene on Copper

Scanning tunneling microscopic and spectroscopic (STM/STS) studies of graphene grown by chemical vapor deposition (CVD) on copper reveal that the monolayer carbon structures remaining on copper are strongly strained and rippled, with different regions exhibiting different lattice structures and local electronic density of states (LDOS). The large and non-uniform strain induces pseudo-magnetic field up to ∼ 50 Tesla, as manifested by the integer and fractional pseudo-magnetic field quantum Hall effects (IQHE and FQHE) in the LDOS of graphene. Additionally, ridges appear along the boundaries of different lattice structures, which exhibit excess charging effects. For graphene transferred from copper to SiO2 substrates after the CVD growth, the average strain and the corresponding charging effects and pseudo-magnetic fields become much reduced. These findings suggest the feasibility of strain-engineering of graphene-based nano-electronics.

Structural and electronic properties of β-In2X3 (X = O, S, Se, Te) using ab initio calculations

Several III-VI body-centered tetragonal layered compounds belonging to space group I41/amd have been a subject of interest recently because of their potential applications in high efficiency and environmentally friendly copper–indium–gallium–selenide solar cells and molecules. Here we have studied the structural, energetic, and electronic properties of four compounds β-In2X3 (X=O, S, Se, Te), in this space group. Using first principles computations, we have fully determined the lattice constants a and c, as well as 10 internal parameters that define this unique structure of primitive unit cells of 40 atoms. For β-In2S3 our computed values are found to be consistent with experimental measurements. The bulk modulus B, local electronic density of states, total density of states, and band gap Eg of these phases have been investigated.

Spin-Density Wave near the Vortex Cores in the High-Temperature SuperconductorBi2Sr2CaCu2O8+y

Competition with magnetism is at the heart of high-temperature superconductivity, most intensely felt near a vortex core. To investigate vortex magnetism we have developed a spatially resolved probe based upon NMR spin-lattice-relaxation spectroscopy. With this approach we have found a spin-density wave associated with the vortex core in Bi(2)Sr(2)CaCu(2)O(8+y), similar to checkerboard patterns in the local density of electronic states reported from scanning tunneling microscope experiments. We have determined both the spin-modulation amplitude and decay length from the vortex core in fields up to H=30 T.

Anisotropic scattering of surface state electrons at a point defect on Bi(111)

Scanning tunneling microscopy was applied to study the lateral variation of the local density of electronic states on the Bi(111) surface in the vicinity of a point defect. At an energy close to the Fermi level a characteristic pattern with a threefold symmetry is found. The pattern can be attributed to the scattering between two electronic surface states which are split by spin orbit coupling. The observation is well described by the superposition of three monochromatic waves. The phase of the waves relative to the center of the defect leads to a reduction to a threefold symmetry.

Structures and Local Electronic States of Dislocation Loop in 4H-SiC via a Linear-Scaling Tight-Binding Study

The atomic- and electronic-level structures of a dislocation loop and a stacking fault in 4H-SiC crystal are investigated by using large-scale tight-binding (TB) molecular-dynamics simulation. We employ a linear-scaling TB method implemented on a parallel computer in order to accelerate the 9,600-atoms calculation which is required for such a nanoscale simulation. We find that the initial configuration that involves unstable C-C networks around the dislocation loop is relaxed to a structure having six-membered rings, and that the distribution of electron populations is inhomogeneous on the loop. The local electronic density of states shows two peaks in the bulk band gap, where one of these peaks may correspond to the defect state observed in EBIC and CL experiments.

Variable-height scanning tunneling spectroscopy for local density of states recovery based on the one-dimensional WKB approximation

We introduce a variable-height scanning tunneling spectroscopy (VH-STS) method that provides a good level of recovery of the combined surface and tip density of states (DOS) in any bias range without the complexity of previous methods. The combined local electron DOS (LDOS) recovery was performed using a simplified algorithm based on the one-dimensional Wentzel-Kramers-Brillouin approximation already known in the literature. On the basis of this VH-STS approach we derive three separate methods for the determination of tunnel barrier height and absolute tip-surface separation, and which are critical to enabling LDOS recovery. We report experimental results on polycrystalline-Pt and Si(100) surfaces using this scheme. Experimental and simulated spectra are compared to investigate the limits of the various methods and to demonstrate that this approach can be applied successfully to all kind of probes and surfaces.

The new high-temperature surface structure on reducedTiO2(001)

Scanning tunnelling microscopy, ultraviolet photoelectron spectroscopy and currentimaging tunnelling spectroscopy (STM/UPS/CITS) were used to study thetopographic and electronic structure of a high-temperature structure formed on theTiO2(001) surface after heating at 1173 K. The STM images revealed different domain-like orderingand periodicity on the surface in comparison to those observed previously. The UPS studiesshowed the presence of a surface state at energy about 1.1 eV below the Fermilevel. This result was confirmed by the CITS data showing pronounced periodicmaxima of the electron local density of states at energy around 1.1–1.2 eV belowthe Fermi level and located on top of every row of the new high-temperaturestructure. The CITS results recorded for small grains, which coexist with theobserved structure, showed that their chemical composition is closer to theTi2O3 materialthan to TiO2 − x for .

First principles theoretical study of complex magnetic order in transition‐metal nanowires

AbstractThe electronic and magnetic properties of one‐dimensional (1D) transition‐metal (TM) systems are investigated from a first principles theoretical perspective. The development of local magnetic moments and the stability of various collinear and non‐collinear spin arrangements are determined in the framework of a generalized gradient approximation to density‐functional theory (DFT). Two specific complementary problems are considered. The first one concerns the local moment formation in Cu wires doped with Co and Ni impurities. Recent results as a function of wire length, interatomic distances, impurity position within the wire, and total spin polarization $S_{z} $ are reviewed. It is shown that both Co and Ni impurities preserve their magnetic degree of freedom in monoatomic Cu wires by developing almost saturated local moments in all low‐lying total spin configurations ($S_{z} \leq 5/2$). The impurity moments are largely dominated by the d‐electron contributions. In the ground state they couple ferromagnetically with the moments induced at the Cu host atoms. The changes in the spin‐density distribution and in the local densities of electronic states are quantified as a function of the position of the impurity position within the wire. In addition, recent results on the ground‐state magnetic order in V nanowires are reviewed. The stability of ferromagnetic (FM), antiferromagnetic (AF), and spiral non‐collinear (NC) orders is analyzed in terms of the frozen‐magnon spectrum. A remarkable transition from FM to NC order is observed as a function of the nearest‐neighbor (NN) distance a. The dependence of the single‐particle electronic structure of the wire on the wave‐vector of the spin‐density wave (SDW) is demonstrated. Finally, the role of magnetic anisotropy on the stability of NC order is discussed in the framework of a simple classical spin model.

Evidence for surface states in a single 3 nm diameter Co3O4 nanowire

The electronic local density of states of a single Co3O4 semiconductor nanowire with the diameter of 3 nm is explored using low-temperature scanning tunneling microscopy and spectroscopy. The energy gap between the conduction band and valence band of the nanowire is about 1.7 eV, which is slightly enhanced compared to the bulk value, ∼1.5 eV, due to the quantum confinement effect. Two surface states are observed locating near the Fermi level in the band gap.

ChemInform Abstract: NMR Spectroscopy as a Probe of Surfaces of Supported Metal Catalysts

The nuclear magnetic resonance (NMR) properties of bulk metals are usually dominated by a magnetic coupling between the nuclear spin and the spin of the conduction electrons. The coupling is proportional to the density of one-electron-states at the Fermi energy and results in NMR shifts that are large in comparison with the usual chemical shift range and also in a characteristic temperature dependence of the nuclear spin lattice relaxation rate. In systems that lack the translational invariance of the extended solid, these properties depend on the local electronic environment of the resonating nucleus (the local density of states). Examples of such systems are bulk random alloys and small metal particles, such as those in supported metal catalysts. In certain cases an adsorbate on a metal also acquires such metallic NMR properties; this often happens with carbon monoxide and probably also with hydrogen, the only adsorbates considered in this review. These metallic NMR characteristics are of interest in catalysis for several reasons. In investigations of the metal (typically 195 Pt), the local density of states on the surface is different from that in the bulk so that NMR spectroscopy can be used to determine the metal dispersion. The 1 H NMR shift of adsorbed hydrogen induced by the metal surface can be used to distinguish hydrogen on the metal from that on the support, e.g., in investigations of hydrogen spillover. The spin lattice relaxation rate of adsorbed 13 CO shows that the chemisorption bond has metallic character. On a theoretical level, the local density of electron states at the Fermi energy on metal surface sites plays a role in frontier orbital theories of chemisorption. This quantity can be measured by NMR of the metal for catalysts that have undergone various surface treatments, not necessarily under vacuum conditions, and the results can be qualitatively interpreted in a much simplified version of such a theoretical framework. This review provides a brief presentation of the relevant theory of metals and a thorough discussion of the metallic characteristics observed in the NMR spectroscopy of hydrogen, carbon monoxide, and platinum.

Peculiarities of the Electron-Phonon Interaction in Graphite Containing Metallic Intercalated Layers

The calculation of the local density of electronic states of graphene with vacancies, using the method of Jacobi matrix, was performed. It was shown that for atoms in the sublattice with a vacancy the local density of electronic states conserves the Dirac singularity, similarly as in an ideal graphene. A quasi-Dirac singularity was observed also in the phonon spectra of graphite for the atom displacements in the direction perpendicular to layers. Changes of phonon spectra of graphite intercalated with various metals were analyzed. On the basis of our results and using the BCS theory and Eliashberg equation we proposed what dynamic properties an intercalated graphite system should show to obtain an increased Tc.

Defects of SiC nanowires studied by STM and STS

For the first time the scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS) are employed to investigate the morphology and the surface electronic structure of the defective silicon carbide nanowires (SiCNWs). The SiCNWs produced via combustion synthesis route are studied. The STS measurements are performed in the current imaging tunneling spectroscopy mode (CITS) that allows us to determine the correlation between STM topography and the local density of electronic states (LDOS) around the bend of an isolated SiCNW. The measurements reveal fluctuations of LDOS in the vicinity of the defect. The local graphitisation and the inhomogeneous concentration of doping impurities (e.g. nitrogen, oxygen) are considered to explain these fluctuations of metallic-like LDOS in the vicinity of the SiCNW's deformation.

Experimental observation of two-dimensional charge polarization in unisized platinum cluster disk bonded to silicon(1 1 1) surface

A local density of electronic states and a local HOMO–LUMO gap of a monatomic-layered platinum cluster with a size of 30 (Pt 30 cluster disk) on a silicon(1 1 1) surface were measured by space-resolved tunneling spectroscopy. It has been found that (1) both occupied and unoccupied electronic orbitals are delocalized due to Pt–Pt metallic interaction in the cluster disk and (2) electrons in the occupied orbitals of the cluster disk are transferred to the adjacent silicon atoms so as to induce two-dimensional charge polarization of the cluster disk. The center of the cluster disk is charged positively, while the boundary between the cluster disk and the silicon surface is charged negatively.

Low-temperature scanning tunneling microscopy and spectroscopy of spatial oscillations in the density of states near domain boundaries at theGe(111)2×1surface

By means of low-temperature scanning tunneling microscopy and scanning tunneling spectroscopy we demonstrate the existence of spatial oscillations in the local electron density of states of clean Ge11121 surfaces. The oscillations appear exclusively in the vicinity of boundaries between domains with different atomic arrangements and are present only within a limited range of tunneling voltages approximately between 0.2 and 0.8 V. From the spectroscopy measurements we are able to extract the energy versus wave-vector dispersion relation of the spatial oscillations in the local electron density of states. Relying on a tight-binding based model, we are able to link the observed phenomena to two-dimensional Tamm surface states that are formed within the semiconductor band gap and that are scattered at domain boundaries where translational symmetry is broken.

Mapping the Local Density of States by Very-Low-Energy Scanning Electron Microscope

Reflection of very slow electrons from solid surfaces has been reported to be inversely proportional to the local density of electronic states coupled to the incident electron wave. The reflected electron flux at units of eV used as the image signal in a scanning electron microscope allows mapping of the local density of states at high spatial resolution. Good performance of the microscope at very low energies is enabled by introducing the beam-retarding immersion lens (the cathode lens) with a biased specimen serving as the cathode. Results of demonstration experiments on aluminum are provided.

The local effect of magnetic impurities on superconductivity inCoxNbSe2 andMnxNbSe2 single crystals

We investigate the effect of individual atomic impurities on the superconductingstate that they are embedded in. Using low temperature scanning tunnelingmicroscopy and spectroscopy we could identify Co and Mn atoms in theCoxNbSe2 andMnxNbSe2 single crystals and observe the influence on the local electronic density of states (LDOS) at0.4 K. We find that Co is in the weak scattering limit. In this case the LDOS is quitehomogeneous on the sample surface, despite the number of defects, and retains sharpcoherent superconducting peaks. This is in strong contrast to the effects of Mn impurities,which locally destroy superconductivity. In this case the LDOS shows a strongenhancement of spectral weight inside the superconducting gap even far from the Mnatoms. Moreover, two impurity bound states are found within the superconducting gap atE/Δ0 = 0.18 and 0.36 at locations close to defects.