Local Partial Densities Of States Research Articles

Scanning gate microscopy in graphene nanostructures

The conductance of graphene nanoribbons and nanoconstrictions under the effect of a scanning gate microscopy tip is systematically studied. Using a scattering approach for noninvasive probes, the first- and second-order conductance corrections caused by the tip potential disturbance are expressed explicitly in terms of the scattering states of the unperturbed structure. Numerical calculations confirm the perturbative results, showing that the second-order term prevails in the conductance plateaus, exhibiting a universal scaling law for armchair graphene strips. For stronger tips, at specific probe potential widths and strengths beyond the perturbative regime, the conductance corrections reveal the appearance of resonances originated from states trapped below the tip. The zero-transverse-energy mode of an armchair metallic strip is shown to be insensitive to the long-range electrostatic potential of the probe. For nanoconstrictions defined on a strip, scanning gate microscopy allows to get insight into the breakdown of conductance quantization. The first-order correction generically dominates at low tip strength, while for Fermi energies associated with faint conductance plateaus, the second-order correction becomes dominant for relatively small potential tip strengths. In accordance with the spatial dependence of the partial local density of states, the largest tip effect occurs in the central part of the constriction, close to the edges. Nanoribbons and nanoconstrictions with zigzag edges exhibit a similar response as in the case of armchair nanostructures, except when the intervalley coupling induced by the tip potential destroys the chiral edge states.

Simulation of Supercell Defect Structure and Transfer Phenomena in TlInTe2

The local environment of atoms in a semiconductor compound TlInTe2 with tetragonal syngony is studied by the density functional theory (DFT). The introduction of a point defect (indium vacancies) into the TlInTe2 lattice is modeled using supercells. The DFT electronic properties (total and local partial densities of states (PDOS) of electrons) are modeled for the primitive TlInTe2 cell (16 atoms per unit cell) and for the defective TlInTe2 cell (where is the vacancy In) consisting of 32 atoms. The DFT-GGA calculations of the TlInTe2 band structure show that the band gap ( ) is = 1.21 eV. This value is significantly dif-ferent from the experimental value. The Hubbard model is used to correct the interaction of particles in the lattice. The DFT-GGA + U (U is the Hubbard potential) calculated by the TlInTe2 band gap is 0.97 eV. For the TlInTe2 supercell, the energies of the formation of a vacancy, the chemical potential of indium, and the standard enthalpy of the formation of TlInTe2 are calculated. When explaining the effect of various factors on the transport phenomena in TlInTe2, their thermal and electrical conductivity, both the DFT-calculated data and experimental data, are used. Taking into account the experimental data for the p-TlInTe2 crystals, the mechanism of conduction in the direction of structural chains (c axis of the crystal) is established. From the experimental data in the temperature range = 148–430 K, the band gap = 0.94 eV and the activation energy of impurity conduction = 0.1 eV (at 210–300 K) are estimated. At temperatures of ≤ 210 K, DC hopping conduction takes place in the p-TlInTe2 crystals. With this in mind, the following physical parameters are calculated for p-TlInTe2: the density of states localized near the Fermi level, their energy spread, and the average hopping distance.

Resonant inelastic soft x-ray scattering on LaPt2Si2

X-ray absorption and resonant inelastic x-ray scattering spectra of LaPt2Si2 single crystal at the Si 2p and La 4d edges are presented. The data are interpreted in terms of density functional theory, showing that the Si spectra can be described in terms of Si s and d local partial density of states (LPDOS), and the La spectra are due to quasi-atomic local 4f excitations. Calculations show that Pt d-LPDOS dominates the occupied states, and a sharp localized La f state is found in the unoccupied states, in line with the observations.

Effect of impurities from deposition precursors on the electronic properties of Si/SiO2 interfaces

In this study, we investigated the effect of impurities—carbon (C), nitrogen (N), and fluorine—on the electronic properties of a Si/SiO2 interface by measuring the atomic layer-resolved partial local density of states using first-principles simulations. Our results show that C and N impurity atoms with dangling bonds (DBs) induce midgap states at the interface. If DBs are terminated by hydrogen atoms, the formation of midgap states is effectively suppressed even in the presence of impurity atoms at the interface. If DBs are terminated by an oxygen (O) atom, both π bonding and π∗ antibonding orbitals of C=O are occupied and such occupied antibonding orbital forms a midgap state at the interface, increasing the leakage current. Based on these observations, we propose that preparation of the SiO2 substrate with oxygen-rich conditions prior to Si deposition is important in realizing a defect-tolerant interface. In addition, hydrogen treatment after Si deposition recovers the pure band structure by terminating the midgap states, thereby preventing leakage current.

Imaging signatures of the local density of states in an electronic cavity

We use Scanning Gate Microscopy to study electron transport through an open, gate-defined resonator in a Ga(Al)As heterostructure. Raster-scanning the voltage-biased metallic tip above the resonator, we observe distinct conductance modulations as a function of the tip-position and voltage. Quantum mechanical simulations reproduce these conductance modulations and reveal their relation to the partial local density of states in the resonator. Our measurements illustrate the current frontier between possibilities and limitations in imaging the local density of states in buried electron systems using scanning gate microscopy.

Partial local density of states from scanning gate microscopy

Scanning gate microscopy images from measurements made in the vicinity of quantum point contacts were originally interpreted in terms of current flow. Some recent work has analytically connected the local density of states to conductance changes in cases of perfect transmission, and at least qualitatively for a broader range of circumstances. In the present paper, we show analytically that in any time-reversal invariant system there are important deviations that are highly sensitive to imperfect transmission. Nevertheless, the unperturbed partial local density of states can be extracted from a weakly invasive scanning gate microscopy experiment, provided the quantum point contact is tuned anywhere on a conductance plateau. A perturbative treatment in the reflection coefficient shows just how sensitive this correspondence is to the departure from the quantized conductance value and reveals the necessity of local averaging over the tip position. It is also shown that the quality of the extracted partial local density of states decreases with increasing tip radius.

Ab initio calculation and synchrotron X-ray spectroscopy investigations of tin oxides near the Sn L 3 absorption edges

The results of theoretical and experimental investigations of the electron-energy structure of the conduction band of tin oxides have been presented. The Sn L3 X-ray absorption near-edge fine structure (XANES) has been calculated for the first time for single crystals of metallic tin and tin monoxide, as well as for the orthorhombic and tetragonal phases of tin dioxide, using the linearized augmented plane wave (LAPW) method. The fine structure of the XANES spectra has been compared with the specific features of the energy distribution of the local partial densities of states of the tin compounds under investigation. A joint analysis of the results of the simulation and the experimental X-ray synchrotron Sn L3 XANES spectra of commercial bulk samples of metallic tin and tin oxides SnO and SnO2 has been carried out.

Electronic structure of 2H-SnSe2: ab initio modeling and comparison with experiment

Energy band structure, total and local partial densities of states, spatial distribution of electronic density of 2H-SnSe2 have been calculated using the density functional theory method in LDA and LDA+U approximations both with and without consideration of spin-orbit interaction. From the band structure calculation results, it follows that 2H-SnSe2 is an indirect-gap semiconductor. The calculated band structure is compared with the dispersion curves E(k) plotted using the known measurement results of angular dependent photoemission spectra. It has been observed the good agreement between theoretical and experimental dispersion curves. The calculated total and local partial densities of states have been compared with the known experimental data obtained using XPS, UPS, ARXPS, BIS methods.

Current Delivered by an STM Tip in Landauer-Büttiker Formalism

When an STM tip is brought close to a nano sized sample then it can deliver (or draw) a current that is determined by the Landauer-Büttiker formalism in terms of the scattering matrix that gives partial local density of states. We show that, very paradoxically, the interference related term in this formula vanish in a quantum regime making semi-classical formula for injectance exact in some regime. We explicitly show how evanescent modes are responsible for this. This can have useful implications to experimentalists as semi-classical formulas are much simpler.

Electronic structure of polycrystalline cadmium dichloride studied by X-ray spectroscopies and ab initio calculations

The electronic structure of cadmium dichloride has been studied by X-ray absorption near edge structure (XANES) and, for the first time, by resonant inelastic X-ray scattering (RIXS) at the Cl K edge. Good agreement was obtained between the non-resonant X-ray emission (XES) along with XANES experimental spectra and the calculated Cl 3p local partial density of states (DOS). The calculations were performed using the full-potential linearized-augmented-plane-wave with the local orbitals (FP-(L)APW_lo) method utilized in the WIEN2k code. It was shown that the position of the RIXS band in CdCl2 follows a linear dispersion according to the Raman–Stokes law if the excitation energy is tuned below the absorption threshold. The situation changes for core excitation above the photoabsorption threshold where the dispersion relation is split into two branches. The position of the resonant contribution does not depend on the excitation energy, while the excitonic sideband follows the Raman–Stoke law. Combined XANES and RIXS measurements compared to calculated band structure allowed us to determine the direct band gap of CdCl2 to be at 5.7±0.05 eV.

ChemInform Abstract: Chemical Bonding of Water to Metal Surfaces Studied with Core‐Level Spectroscopies

AbstractReview: 106 refs.

Chemical bonding of water to metal surfaces studied with core-level spectroscopies

The nature of the contact layer of water on surfaces is of relevance for many practical fields, including corrosion, electrochemistry, environmental science and heterogeneous catalysis. Here we focus on the geometric and electronic structure of the water contact layer on transition metal surfaces and the interaction between the water monolayer and the surface. By combining synchrotron radiation-based X-ray photoelectron spectroscopy (XPS), X-ray absorption spectroscopy (XAS) and X-ray emission spectroscopy (XES) techniques with density functional theory (DFT) computational methods we obtain element-specific information on the partial local density of states, local atomic structure, geometrical parameters and molecular orientation, allowing general principles for water–metal interaction to be derived.

Evolution of nanoporous silicon phase composition and electron energy structure under natural ageing

AbstractNanoporous silicon layers were obtained on different types of single crystalline silicon substrates by standard electrochemical etching in HF solution. Investigations of electron structure, surface layers phase composition and photoluminescence evolution in nanoporous silicon during natural ageing for the time period for up to one year were performed with the use of the ultrasoft X‐ray emission spectroscopy (USXES), X‐ray absorption near‐edge structure (XANES) spectroscopy and photoluminescence. USXES and XANES spectra represent local partial density of states in valence and conduction band, correspondingly. The presence of the elementary silicon together with different silicon suboxides in surface layers is shown. Considerable differences in the kinetics of ageing can explain different photoluminescence properties of nanoporous silicon doped with P and Sb. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Cation-Specific Interactions with Carboxylate in Amino Acid and Acetate Aqueous Solutions: X-ray Absorption and ab initio Calculations

Relative interaction strengths between cations (X = Li (+), Na (+), K (+), NH 4 (+)) and anionic carboxylate groups of acetate and glycine in aqueous solution are determined. These model systems mimic ion pairing of biologically relevant cations with negatively charged groups at protein surfaces. With oxygen 1s X-ray absorption spectroscopy, we can distinguish between spectral contributions from H 2O and carboxylate, which allows us to probe the electronic structure changes of the atomic site of the carboxylate group being closest to the countercation. From the intensity variations of the COO (-) aq O 1s X-ray absorption peak, which quantitatively correlate with the change in the local partial density of states from the carboxylic site, interactions are found to decrease in the sequence Na (+) > Li (+) > K (+) > NH 4 (+). This ordering, as well as the observed bidental nature of the -COO (-) aq and X (+) aq interaction, is supported by combined ab initio and molecular dynamics calculations.

The local electronic structure of tin phthalocyanine studied by resonant soft X-ray emission spectroscopies

The electronic structure of thin films of the organic semiconductor tin phthalocyanine (SnPc) has been investigated by resonant and non-resonant soft X-ray emission (RXES and XES) at the carbon and nitrogen K-edges with excitation energies determined from near edge X-ray absorption fine structure (NEXAFS) spectra. The resultant NEXAFS and RXES spectra measure the unoccupied and occupied C and N 2p projected density of states, respectively. In particular, RXES results in site-specific C 2p and N 2p local partial density of states (LPDOS) being measured. An angular dependence of C 2p and N 2p RXES spectra of SnPc was observed. The observed angular dependence, the measured LPDOS and their correspondence to the results of density functional theory calculations are discussed. Observed differences on the same site-specific features between resonant (non-ionising) and non-resonant (ionising) NXES spectra are attributed to symmetry selection and screening.

Electronic structure of single-crystal rocksalt CdO studied by soft x-ray spectroscopies andab initiocalculations

Soft x-ray emission spectroscopy (XES) and x-ray absorption spectroscopy (XAS) are employed to investigate the occupied and unoccupied electronic structures in rocksalt-phase single-crystal CdO. Resonant XES at the $\mathrm{O}K$ edge reveals a clear $\mathrm{Cd}\phantom{\rule{0.2em}{0ex}}4d--\mathrm{O}\phantom{\rule{0.2em}{0ex}}2p$ hybridized peak and momentum-dependent coherent contributions to the resonant emission spectra. Good agreement is obtained between the above-threshold XES and XAS spectra, and the calculated $\mathrm{O}\phantom{\rule{0.2em}{0ex}}2p$ local partial density of states (PDOS). Calculation of the $\mathrm{O}\phantom{\rule{0.2em}{0ex}}2p$ PDOS was performed within the $GW$ framework of many-body perturbation theory.

ELECTRON STRUCTURE INVESTIGATIONS OF InGaP/GaAs(100) HETEROSTRUCTURES WITH InP QUANTUM DOTS

Ultrasoft X-ray emission spectra (USXES) and X-ray absorption near-edge structure (XANES) spectra with the use of synchrotron radiation in the range of P L2,3-edges were obtained for the first time for nanostructures with InP quantum dots grown on GaAs 〈100〉 substrates by vapor-phase epitaxy from metal–organic compounds. These spectra represent local partial density of states in the valence and conduction bands. The additional XANES peak is detected; its intensity depends on the number of monolayers forming quantum dots. Assumptions are made on the band-to-band origin of luminescence spectra in the studied nanostructures.

Investigations of the electron energy structure and phase composition of porous silicon with different porosity

Energy structure of the valence and conduction bands together with phase composition of porous silicon (por-Si) obtained at different etching times were investigated using ultrasoft X-ray emission spectroscopy (USXES) and X-ray absorption near-edge structure (XANES) techniques. USXES and XANES spectra represent local partial density of states in valence and conduction band, correspondingly. XANES data were obtained with the use of synchrotron radiation at synchrotron BESSY II. Porous silicon layers were obtained on different types of single-crystalline silicon (1 0 0) and (1 1 1) substrates by standard electrochemical etching in HF solution with etching times from 1 to 10 min. USXES and XANES investigations show formation of Si nanocrystals covered by amorphous phase together with nonstoichiometric silicon sub-oxide SiO x ( x ∼ 1.3) in the surface layers. XANES investigations show that the increase of sample porosity leads to the shift of conduction band edge to the higher energy range that corresponds to increase of the band gap width in porous silicon up to ∼1.4 eV.

Electronic structure of CdTe probed by Cd and Te M 4,5 X-ray emission spectra

We have investigated the bulk electronic structure of CdTe focusing on the Cd 5p and Te 5p valence states by X-ray emission spectroscopy (XES). Despite the very low fluorescence yields the Cd and Te M 4,5 (5p → 3d 3/2,5/2) spectra have been recorded successfully. A good correspondence has been found between the valence band XES and X-ray photoelectron spectra (XPS) by comparison on a common binding energy scale. We also performed a density functional theory calculation of the CdTe valence band, obtaining the Cd 4d, 5s, 5p and Te 5s, 5p local partial densities of states. The experimental Cd 5p and Te 5p derived from the X-ray emission spectra are in good agreement with the calculation. The intensity ratio of the Cd M 4,5 to the Te M 4,5 spectrum is obtained to be 0.25, in agreement with the ratio of the calculated Cd 5p to the Te 5p density of states in the CdTe upper valence band (0.22).

Excited and ground state properties ofLaSrMnO4: A combined x-ray spectroscopic study

The electronic properties of the parent compound of the single layered manganites ${\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{Sr}}_{1+x}{\mathrm{MnO}}_{4}$, namely ${\mathrm{LaSrMnO}}_{4}$ have been investigated in a detailed spectroscopic study. We apply different complementary x-ray spectroscopic techniques in the soft x-ray regime. X-ray photoelectron spectroscopy and normal-like x-ray emission spectroscopy were used to reveal a detailed picture of the total and partial densities of states in this compound. Furthermore we apply resonant x-ray emission spectroscopy to the $\mathrm{Mn}\phantom{\rule{0.3em}{0ex}}{L}_{2,3}$ edges. The spectra exhibit a rich multiplet structure. Resonant x-ray emission spectroscopy at the O $K$ edge is used to study the local partial densities of states of the in plane and out of plane oxygen atoms. The results are discussed along available band structure calculations as well as charge transfer multiplet calculations.