Maximum In Density Of States Research Articles

Bias-Dependent Scanning Tunneling Microscopy Signature of Bridging-Oxygen Vacancies on Rutile TiO2(110).

The rutile TiO2(110) surface has long-served as a well-characterized, prototypical transition-metal oxide surface used in heterogeneous catalysis and photocatalytic water splitting. Naturally occurring defects on this surface, called bridging-oxygen (BO) vacancies, are important as they determine the overall reactivity of the surface. Herein, we report a bias-dependent, scanning tunneling microscopy (STM) signature of the BO vacancies on TiO2(110): for sample bias voltages past a threshold of +3 V, the bright vacancies are flanked on either side (along the oxygen row) by two dark spots approximately shaped like half-moons. The BO vacancies have a bright aspect below the threshold bias also but are not surrounded by half-moon dark depressions. Using generalized gradient approximation calculations with Hubbard correction (GGA + U) for projected density of states (DOS) and simulated STM images, we find that the bias-dependent STM signature originates from (i) local DOS maxima of all BOs (lighter background that occurs above the threshold bias) and (ii) the increased separation between the first and second BO atoms neighboring the vacancy which leads to an apparent dip between these neighboring oxygens. These results offer a new striking example of the STM signature that appears without switching the polarity of the bias. Similar approaches can be employed for seeking distinguishing features on the surfaces of other large band gap semiconductors and insulators.

First-Principles Investigation on Electronic Transport Properties of Tungsten Nitride Nanoribbon Based Molecular Device

The electronic transport property of tungsten nitride nanoribbon-based molecular device is investigated using density functional theory. The study on electronic transport properties are discussed in terms of device density of states and transmission spectrum of the nanoribbon. The scattering region of the nanoribbon is placed between the electrodes. For different bias voltages, the transport properties are studied. The contribution due to various orbitals leads to the peak maximum in device density of states. The transmission spectrum and transmission pathway provides insight to the transmission of charge under various bias conditions. The maximum peak in the transmission spectrum corresponds to the variation in transmission of charge with different bias voltage. The reported results will pave the way to improve the performance in the field of nanoelectronics.

Electronic Properties ofTiO2Nanoparticles Films and the Effect on Apatite-Forming Ability

Nanoparticle-covered electrodes have altered properties as compared to conventional electrodes with same chemical composition. The changes originate from the large surface area and enhanced conduction. To test the mineralization capacity of such materials, TiO2 nanoparticles were deposited on titanium and gold substrates. The electrochemical properties were investigated using cyclic voltammetry and impedance spectroscopy while the mineralization was tested by immersion in simulated body fluid. Two types of nucleation and growth behaviours were observed. For smooth nanoparticle surfaces, the initial nucleation is fast with the formation of few small nuclei of hydroxyapatite. With time, an amorphous 2D film develops with a Ca/P ratio close to 1.5. For the rougher surfaces, the nucleation is delayed but once it starts, thick layers are formed. Also the electronic properties of the oxides were shown to be important. Both density of states (DOS) in the bandgap of TiO2 and the active area were determined. The maximum in DOS was found to correlate with the donor density (N d) and the active surface area. The results clearly show that a rough surface with high conductivity is beneficial for formation of thick apatite layers, while the nanoparticle covered electrodes show early nucleation but limited apatite formation.

Integral and local density of states of InAs quantum dots in GaAs/AlGaAs heterostructure observed by ballistic electron emission spectroscopy near one-electron ground state

Density of states is studied by a ballistic electron emission microscopy/spectroscopy on self-assembled InAs quantum dots embedded in GaAs/AlGaAs heterostructure prepared by metal–organic vapor phase epitaxy. An example of integral quantum dot density of states which is proportional to superposition of a derivative of ballistic current–voltage characteristics measured at every pixel (1.05 nm×1.05nm) of quantum dot is presented. For the two lowest observed energy levels of quantum dot (the maxima in density of states) the density of states is mapped and correlated with the shape of quantum dot. It was found that prepared quantum dots have a few peaks on their flatter top and a split of the lowest energy level can be observed. This effect can be explained by inhomogeneous (nonuniform) stress distribution in the examined quantum dot.

Level density and shape changes in excited sd shell nuclei

In the present calculation we have used the Monte Carlo method of generating collective spin and total energy of the nucleus for various configurations of the system with N0 single particle states available for n number of particles. The different configurations (arrangements of occupied single particle states) leading to a particular energy E and spin J are then collected to get the density of states for the given energy E and spin J. We find that if we use the cranked Nilsson model single particle states for the rotational frequency Ω = 0.0ħω, 0.05ħω and 0.1ħω there is a shift in the maximum density of states Wmax with a tendency for the system to become more oblate or prolate depending on the shift in the maximum density of states as the angular momentum decreases or increases. The change in nuclear level density with collectivity, i.e. with the use of cranked Nilsson model single particle levels has been noticed.

A Particle Detector Based on a Double Barrier Josephson Junction

We analyze the performance of a particle detector based on a double-barrier Nb/Al/AlOx/Al/AlOx/Al/Nb junction where the Al trapping layer is embedded between two insulating AlO/sub x/ barriers. Within this layer, a very sharp Andreev bound state (ABS) level E/sub 0/ is formed. Because of a very long recombination time, the charge amplification factor is expected to be 10-100 times larger in the double-barrier detector compared to ordinary single-barrier devices. The double barrier detector also has much higher idle-state subgap resistance at low temperatures. Because the middle Al quasiparticle trap layer is separated from the adjacent layers by two insulating tunneling barriers, the proximity effect is minimized. This provides much better alignment between the maximum of nonequilibrium quasiparticle distribution function and the maximum in the electron density of states. All the factors listed result in much higher sensitivity and the energy resolution of the double barrier Nb-based STJ as compared to their single-barrier analogs.

Magnetic and electronic properties of binary alloy monolayers:FexMn1−xandCo1−xFexon W(110)

Pseudomorphic binary alloy monolayers Fe x Mn 1 - x and Co 1 - x Fe x were grown on W(110) by molecular-beam epitaxy in ultrahigh vacuum. Fe atoms are randomly replaced by Mn or Co atoms. The pseudomorphic monolayers form extended areas of monolayer coverages at elevated temperatures in the step flow growth mode. For Fe x Mn 1 - x /W(1100) monolayers the Kerr rotation decreases with decreasing Fe concentration, corresponding to a decrease of the magnetization of Δμ/Δx=-(8′2)μ B /atom. The Curie temperature decreases with a rate of [ΔT C (x)/Δx]T C (Fe)=-(1.8′0.2)×10 - 2 /at. %. Scanning tunneling spectroscopy reveals characteristic maxima in the unoccupied local density of states near the Fermi level, which shift towards E F with decreasing Fe concentration for both alloys investigated here. The reduction of magnetization with decreasing Fe concentration can be explained by a simple model of d-band filling, similar to the explanation of the Slater-Pauling curve for binary alloys. We observe a maximum value for T C and for the Kerr rotation for the pure Fe/W(110) monolayer, in contrast to the behavior of bulk alloys where maximum values are obtained for Co-Fe alloys.

Unoccupied states evolution with oxidation of ultrathin Mg, Zn and Cd layers on SrTiO3(100) surfaces

An unoccupied density of states (DOS) evolution under oxidation of ultrathin Mg, Zn and Cd layers on SrTiO3(100)-(2×2) substrates are investigated. The metal deposition and subsequent oxidation were studied in situ under UHV conditions by means of LEED, AES, TCS and work function measurements. An appearance of metal- and oxygen-derived bands of the unoccupied DOS was traced in the TC spectra. Metal oxidation was followed by the hybridization of metal and oxygen orbitals and by the appearance of the mainly O 2p oxygen-derived bands, which are characterized by a set of DOS maxima, located at the following energies (above the Fermi level): 8.0, 11.5, 14.3, 15.0 and 19.0eV for MgO; 6.0, 8.1, 16.8 and 20.5eV for ZnO and 7.5, 10.0 and 12.7eV for CdO. The results obtained are discussed in comparison with previously published experimental and theoretical data on unoccupied DOS in metal oxides.

The vibrational density of states revisited in the archetypical icosahedral quasicrystal Al62Cu25.5Fe12.5

We have studied the phonon dynamics of icosahedral i-Al62Cu25.5Fe12.5 using inelastic neutron scattering (INS) in isotope-doped samples, and have combined this with previous inelastic nuclear-resonant absorption (INA) of synchrotron radiation measurements. We are able to show that the maximum in the vibrational density of states (VDOS) at 14 meV is heavily weighted by copper. The INA results have shown that the upper peak at 27 meV is dominated by iron. We are able to explain previous lattice specific heat results from the low-energy region of the neutron-weighted VDOS, including the non-integer power behaviour.

A total current spectroscopy study of metal oxide surfaces: I. Unoccupied electronic states of ZnO and MgO

Unoccupied density of states (DOS) for the polar (0001) Zn, (000) O and non-polar (100) ZnO surfaces and for a MgO(001)-c(2 × 2) thin film were studied using total current spectroscopy (TCS). TC spectra of the ZnO surfaces reveal the Zn 4s-O 2p band (6.2 eV) and two DOS maxima in the Zn 4p-O 2p band which exhibited different energy positions on the Zn-terminated (8.1 eV) and O-terminated (11.0 eV) surfaces. In the TC spectra of the MgO thin film, the energy location of Mg-derived (8.0, 14.8 and 21.0 eV) and O-derived (11.6, 14.2 and 18.5 eV) maxima of DOS were determined. Previously reported data on DOS of these materials is discussed, and it is shown that additional detailed information on the energy structure of unoccupied DOS in ZnO and MgO in the 5-25 eV range above EF has been found.

Study of the electronic bonding of Cl–Si(100) by synchrotron radiation photoemission spectroscopy and many-body calculations

The electronic structure of chlorine chemisorbed on a Si(100)-(2×1) surface is investigated by means of synchrotron radiation photoemission spectroscopy and many-body calculations of the one-hole density of states in order to study the effect of strong correlations in the adsorbate layer. The experimental spectrum stands in close agreement with previous experimental work. Three adsorption geometries were considered in the density of states (DOS) calculations: (i) adsorption on both ends of symmetric Si dimers; (ii) adsorption on only the upper Si atoms of buckled dimers; and (iii) a combination of both. Although the energy of the DOS maxima was similar in the three cases, and in almost quantitative agreement with the experimental peaks below the ionisation threshold (13 eV), their intensities and widths were strongly different. Only when structures (i) and (ii) are mixed in the ratio 1–2.5, as suggested by electron-stimulated desorption experiments, does the DOS strongly resemble the photoemission spectrum. These structural effects are also evident in the Cl chemisorption on Si vicinal surfaces, and subsequent annealing of the chlorinated surface, which leads to similar electronic states but with different intensities. The evolution of the experimental spectra with temperature suggests that weaker phases are desorbed, more bound species remaining only at the step edges and, after annealing, at the terraces.

From amorphous to polycrystalline thin films: dependence on annealing time of structural and electronic properties

Some new results about the amorphous to polycrystalline transition of silicon thin films obtained by low pressure chemical vapor deposition (LPCVD) at 550°C are presented. From in situ (monitored during the crystallization annealing) conductance, electron spin resonance, photoluminescence and modulated photocurrent experiments, the density of states is shown to increase in the so-called nucleation regime and reach a maximum just before crystal grain growth starts. These results indicate that crystallization needs the creation of a defected material with large dangling bond densities and wide band-tails. It appears that the electronic quality of the final polycrystalline material is linked to the maximum density of states reached just at the end of the nucleation phase. This level may be associated to a viscous structure.

Electron density of states in terahertz driven two-dimensional electron gases

In this paper, a detailed theoretical study of the density of states (DOS) is presented for free electrons in terahertz (THz) driven two-dimensional electron gases (2DEGs). Using the Green's function approach and including the electron-photon interaction exactly, we have derived the electronic DOS in a THz driven 2DEG. The results obtained show that: (1) in the presence of intense THz electromagnetic radiations, the maximum DOS in the system will be shifted to the high-energy regime; (2) a stronger effect of the radiation on electron DOS and Fermi energy can be observed at relatively low-frequency and/or high-intensity radiations where the energy shift induced by the radiation field plays an important role in determining the DOS and the Fermi energy; (3) the processes of the multiphoton absorption and emission will lead to a small increase in DOS; and (4) as a consequence of (1) and (2), an intense THz radiation will drive electrons in a 2DEG to occupy higher electronic subbands.

Study on photoelectric properties of a TiO2 nanoparticle

Here we report a study of surface states in nanometer-sized TiO2 particles (P1) prepared in our lab with the aid of surface photovoltage spectroscopy, electric field-induced photovoltage spectroscopy (EFISPS), and fluorescence spectrum. The diameter determined by x-ray diffraction is 3.5 nm. The EFISPS of P1 shows several subband gap absorption bands at 350, 375, and 460 nm, respectively. The fluorescence peaks of dry powder and an aqueous suspension of P1 at 361, 391, and 469 nm were detected. The above results are concluded to be relative with the located surface states within the band gap. The surface states were deduced to be oxygen vacancies, i.e., Ti3+, Ti2+, Ti+, whose maximum densities lie at 0.32, 0.50, and 1.02 eV below the maximum density of states in the conduction band, respectively.

On competition between s- and d-symmetry types of the order parameter in high-temperature superconductors

It is shown that strong anisotropy of the quasi-two-dimensional electron spectrum in high-temperature superconductors and the competition between attraction and repulsion in the electron–electron interaction can lead either to anisotropic s-, or d-type of Cooper pairing depending on the system parameters. In the case when attraction prevails on the entire Fermi surface (e.g., due to electron–phonon interaction), the sxy- or s*-symmetry in the superconducting order parameter is advantageous from the energy point of view. At the same time, the dx2−y2-symmetry of the gap is observed in the case of repulsion in the entire Brillouin zone (which is typical, for example, of the electron–magnon interaction) as well as in the case of effective attraction on the regions of Fermi surface with the maximum density of states. In the latter case, the superconducting transition temperature Tc is always higher than in the former case, indicating the important role of additional attraction in the mechanism of high-temperature superconductivity.

Determination of transparency in modulated structures

In order to obtain a high yield in impact ionization in a superlattice structure, the transition rate of the corresponding process has to be optimized. Since this transition rate is, apart from the magnitude of the pair creation matrix element, determined by the densities of states of the electrons and the holes, it is of importance to operate the device at, or near, the maximum density of states. In this paper we develop a general theory for the calculation and prediction of high densities of states. It is pointed out that the energy at which the density of states is maximum lies close to the energy at which the superstructure is transparent.

Segmented polyacetylene, (CHD y) x: Electrochemical n-doping studies

Defects involving sp 3 hybridized carbon atoms in the form of (HCD) units have been controllably introduced into trans-(CH) x by chemical methods to produce polymers of the type trans-(CHD y ) x , i.e., [(CH) 1- y − (HCD) y ] x ( y = 0, 0.15 and 0.17), in order to study the nature of these defects and their effect on the electronic and magnetic properties of trans-(CH) x . The effects of these defects on the electrochemical reduction (n-doping) of trans-(CHD y ) x up to 4.0 mol % reduction levels have been studied. The results are consistent with optical and Raman spectra of trans-(CHD y ) x species and are interpreted as showing that: (i) the band gap in trans-(CHD y ) x is essentially unchanged from that in trans-(CH) x because of a small but significant amount of long conjugated sequences remaining in the deuterated polymer; (ii) the maximum density of states near the conduction band edge has moved upwards from the band edge; and (iii) the (HCD) defects are clustered rather than randomly spaced in a given (CHD y ) x chain.

Lattice Mode Study of La2CuO4-y Implanted with Sr

ABSTRACTDramatic implantation-induced changes in the Raman and infrared spectra of polycrystalline samples of La2CuO4-y are reported for implantation of Sr in the fluence range 0.5 to 1.5 × 1016/cm2. The Raman and ir-reflectance spectra of the Sr implanted material reveal disorder-induced lines corresponding to breakdown of selection rules and to non-zone-center phonon modes. Whereas the main features in the Raman spectra of the implanted sample are broad disorder-induced lines centered at ∼ 560 ± 5 cm−1, reststrahlen lineshapes are observed above ∼ 800 cm−1 in the ir-reflectance spectra. After thermal annealing (550°C), the multiphonon lines in the ir spectra diminish, and the disorder-induced lines in the Raman spectra split to form well-resolved, sharper lines at 530 and 580 cm−1, identified with maxima in the phonon density of states. The sensitivity and complementarity of Raman and infrared spectroscopy to implantation-induced disorder and subsequent partial recovery by thermal annealing suggest that these techniques can be used to characterize sensitively disorder in ion implanted La2CuO4-y high Tc, materials.

Photoemission study of the electronic structure of Mo and Mo oxides

The electronic structure of Mo and Mo oxides has been studied using ultraviolet and X-ray photoelectron spectroscopy. The valence band spectrum of Mo(100) is compared with calculations of the density of states (DOS). A fairly good agreement of the characteristic peak positions with the DOS maxima is obtained. The Mo 3d core lines indicate well-defined MoO2 and MoO3 oxidation states with binding energies respectively 0.9 to 5.0 eV higher than that of the Mo metal. By comparing the atomic photoionisation cross sections and electron state occupation numbers with the experimental band intensities the highly ionic character of MoO2 has been estimated. The XPS valence band of MoO2 can be qualitatively described using the molecular orbital model based on a distorted rutile structure. The MoO3 valence band is composed of the O 2p derived band with a characteristic structure around 5 eV reflecting the existence of cationic d interactions. A small peak at about 1 eV below EF is observed in the photoemission of blue substoichiometric MoO3-x (in contrast to transparent MoO3). This is assigned to a small occupied defect band of trapped electrons in oxygen vacancies. Ion bombardment of transparent MoO3 results in a drastic change of the valence band indicating the MoO3 to MoO2 surface modification due to oxygen depletion.

Stoichiometric and oxygen deficient MoO 3(010) surfaces

The (010) surface of single crystal MoO 3 has been prepared and examined using LEED, XPS, UPS, and ELS. Three methods yield the stoichiometric surface: scraping in UHV and annealing, ion etching followed by reoxidation (770 K, 10 2 Pa O 2), or oxygen treatment to remove carbon contamination. LEED shows the surface periodicity is the same as that of the bulk (010). The MoO 3 valence band is 7 eV wide with density of states maxima at 1.5, 3.6, and 5.6 eV below the top of the valence band. Heating MoO 3 in vacuum reduces the surface region. XPS indicates the O/Mo atomic ratio decreases to 2.85 ± 0.12 on heating to 600 K. Ar ion bombardment disorders the surface and reduces the surface O/Mo atomic ratio to 1.6. Annealing of reduced surfaces at > 770 K incompletely reoxidizes them by diffusion of oxygen from the bulk. UPS of reduced and annealed MoO 3 exhibits two new emission features in the bandgap at 0.9 and 2.0 eV above the top of the valence band. These features originate from Mo derived states of a defect involving two or more Mo atoms, such as crystallographic shear planes. Because of the insulating nature of MoO 3, surface charging and electron beam induced damage were substantial hindrances to electron spectroscopic examination.