Contribution To Density Of States Research Articles

Fine-Tuning the Photocatalytic Activity of the Anatase {1 0 1} Facet through Dopant-Controlled Reduction of the Spontaneously Present Donor State Density.

The present study highlights the importance of the net density of charge carriers at the ground state on photocatalytic activity of the faceted particles, which can be seen as a highly underexplored problem. To investigate it in detail, we have systematically doped {1 0 1} enclosed anatase nanoparticles with Gd3+ ions to manipulate the charge carrier concentration. Furthermore, control experiments using an analogical Nb5+ doped sample were performed to discuss photocatalytic activity in the increased range of free electrons. Overall results showed significant enhancement of phenol degradation rate and coumarin hydroxylation, together with an increase of the designed Gd/Ti ratio up to 0.5 at. %. Simultaneously, the mineralization efficiency, measured as a TOC reduction, was controlled between the samples. The observed activity enhancement is connected with the controlled decrease of the donor state density within the materials, being the net effect of the spontaneously present defects and introduced dopants, witch reduce hydroxylation and the hole trapping ability of the {1 0 1} facets. This allows to fine-tune multi-/single-electron processes occurring over the prepared samples, leading to clear activity maxima for 4-nitrophenol reduction, H2O2 generation, and ·OH formation observed for different donor densities. The optimized material exceeds the activity of the TiO2 P25 for phenol degradation by 52% (377% after surface normalization), showing its suitable design for water treatment. These results present a promising approach to boost photocatalyst activity as the combined result of the exposed crystal facet and dopant-optimized density of ground-state charge carriers.

Highly Conductive Nitrogen‐Doped sp2/sp3 Hybrid Carbon as a Conductor‐Free Charge Storage Host

AbstractIt is commonly accepted that the increased degree of graphitization leads to a higher electrical conductivity of carbon materials. However, more and more evidence reveals that heteroatom doping on carbon host can also improve the conductivity, owing to the dopant atoms contributing to higher charge delocalization and density of donor states near Fermi level. The reality is, such conductivity improvement from doping is often overwhelmed by graphitized carbon. Although heteroatom‐doped carbon is widely used as active materials in the fields of energy storage and electrocatalysis, which still requires extra carbon‐based conductive additives to enhance the overall conductivity. In this stu, it is demonstrated that the electrical conductivity of finely designed nitrogen‐doped carbon is even beyond the commercialized carbon conductors over 3.5 times, endowing such conductive agent‐free electrode material an excellent performance in an all‐solid‐state flexible supercapacitor. The theoretical simulation further demonstrates that N‐doped sp2/sp3 hybrid carbon can migrate the Fermi level to the conduction band, leading to an n‐type conductivity due to the additional electrons caused by the N dopant.

Fluctuational anomalous Hall and Nernst effects in superconductors

In this work we develop microscopic kinetic theory of the anomalous Hall and Nernst effects in superconductors induced by fluctuations in the vicinity of the critical transition temperature. The technical analysis is carried out within the Matsubara diagrammatic technique and linear response Kubo formula with disorder averaging and an analytical continuation. It is shown that asymmetric skew-scattering due to spin–orbit interaction gives rise to a new anomalous Hall conductance promoted by Maki–Thompson interference and density of states fluctuational effects. The side-jump mechanism of the anomalous Hall effect is present only in the density of states contributions. The anomalous Nernst effect is found due to a special nonlinear Aslamazov–Larkin term from the quantum-crossing of interacting fluctuations in a Hikami box with the skew-scattering process. In addition to calculations in the weak-coupling BCS model we also sketch an approach for the account of fluctuational transport effects in the strong-coupling limit. In particular we explore an example of how Keldysh version of the Eliashberg theory can in principle be constructed and estimate fluctuation-induced conductivity within conventional electron–phonon coupling scenario. These ideas may pave the way for further developments of strong coupling theories in applications to fluctuations in unconventional superconductors and anomalous transport responses in particular.

Synergetic effect in RuxMo(1-x)S2/SBA-15 hydrodesulfurization catalysts: Comparative experimental and DFT studies

The effect of introducing Ru impurities into the MoS2 crystalline structure of the sulfided RuxMo(1-x)S2/SBA-15 catalysts have been investigated using density functional theory (DFT) calculations and the catalyst characterization by different techniques (chemical analysis (ICP-AES), temperature-programmed reduction (TPR), X-ray diffraction (XRD), N2 physisorption, DRIFTS of adsorbed pyridine (DRIFTS-Py) and X-ray photoelectron spectroscopy (XPS)). The catalyst activity was tested in the hydrodesulfurization (HDS) of dibenzothiophene (DBT) reaction carried out in a batch reactor, T = 320 °C and total H2 pressure of 5.5 MPa. From electronic structure DFT calculations is was concluded that the 4d orbitals of both Mo and Ru played an important role in the catalyst optimization being the processes of transport and charge transference the most important ones. It was found that the enrichment with Ru, promotes a greater electronic participation (DOS at the Fermi level) of the different atoms in the RuxMo(1-x)S2 phase leading to metallization of the Mo ions. The catalyst activity in HDS of DBT reaction demonstrated a similar behavior to that of theoretical density of states (DOS) calculated via DFT. All bimetallic systems presented the synergetic effect between Ru and Mo in the HDS of DBT reaction over RuxMo(1-x)S2/SBA-15 catalysts. The highest activity observed for Ru content of x = 0.4 was consistent with theoretical results predicting that the optimum DOS contributions should be around x = 0.44. The most active Ru0.4Mo0.6S2/SBA-15 exhibit the best hydrogenation properties linked with the Ru-induced metallization of Mo ions in the Ru0.4Mo0.6S2 phase. This catalyst showed two-fold higher hydrogenation properties than CoMoS/γ-Al2O3 reference catalyst. The linear dependencies of initial activity on Brønsted-to-Lewis acidities ratio (from DRIFT-Py) and total metal surface exposure (from XPS) were observed.

Structural evolution of (Ti0.5V0.5)n+1GeCn (n = 1–4) under pressure from first principles

The elastic properties and structural evolution of (Ti0.5V0.5)n+1GeCn (n=1–4) are studied under pressure from first principles. Many general evolution trends are concluded for the six structures, including the lattice parameters (a, c), elastic constants (cij) and the degree of the isotropy (A, A1, A2), and the axial shrinkage tendency. The (Ti0.5V0.5)2GeC behaves the largest compressive anisotropy in (c/a)/(c0/a0) but (Ti0.5V0.5)5GeC4 behaves opposite compressive anisotropy which shows a value below 1 between 0 and 40GPa, whereas (Ti0.5V0.5)4GeC3 exists a softening behavior between 20 and 40GPa. The lowest volume contraction of (Ti0.5V0.5)4GeC3 is confirmed by the density of states, together with the observation of its minimum c12 and c13 among the same type of compounds. The density of states analysis observed the smaller contribution of Ti at Fermi level with respect to that of V. A common phenomenon is observed from the six structures, namely, for any considered heavy atoms including Ti or V, once the heavy atom is surrounded by one C layer and one Ge layer, its density of states contribution at Fermi level is larger than that of atom surrounded by two C layers originating mainly from the electronegativity difference of C and Ge. The bond length of C/GeTi is always slightly longer than that of C/GeV except for (Ti0.5V0.5)2GeC which shows totally equivalent length. A smaller shear modulus of (Ti0.5V0.5)2GeC is observed relative to its two end members. The electron density difference analysis reveals the diverse charge transfer directions and bonding features in these compounds.

Synthesis and Stabilization of Blue-Black TiO2 Nanotube Arrays for Electrochemical Oxidant Generation and Wastewater Treatment.

Efficient, inexpensive, and stable electrode materials are key components of commercially viable electrochemical wastewater treatment system. In this study, blue-black TiO2 nanotube array (BNTA) electrodes are prepared by electrochemical self-doping. The 1-D structure, donor state density, and Fermi energy level position are critical for maintaining the semimetallic functionality of the BNTA. The structural strength of the BNTA is enhanced by surface crack minimization, reinforcement of the BNTA-Ti metal interface, and stabilized by a protective overcoating with nanoparticulate TiO2 (Ti/EBNTA). Ti/EBNTA electrodes are employed as both anodes and cathodes with polarity switching at a set frequency. Oxidants are generated at the anode, while the doping levels are regenerated along with byproduct reduction at the cathode. The estimated maximum electrode lifetime is 16 895 h. Ti/EBNTA has comparable hydroxyl radical production activity (6.6 × 10-14 M) with boron-doped diamond (BDD, 7.4 × 10-14 M) electrodes. The chlorine production rate follows a trend with respective to electrode type of Ti/EBNTA > BDD > IrO2. Ti/EBNTA electrodes operated in a bipolar mode have a minimum energy consumption of 62 kWh/kg COD, reduced foam formation due to less gas bubble production, minimum scale formation, and lower chlorate production levels (6 mM vs 18 mM for BDD) during electrolytic wastewater treatment.

Density of States for Warped Energy Bands.

Warping of energy bands can affect the density of states (DOS) in ways that can be large or subtle. Despite their potential for significant practical impacts on materials properties, these effects have not been rigorously demonstrated previously. Here we rectify this using an angular effective mass formalism that we have developed. To clarify the often confusing terminology in this field, “band warping” is precisely defined as pertaining to any multivariate energy function E(k) that does not admit a second-order differential at an isolated critical point in k-space, which we clearly distinguish from band non-parabolicity. We further describe band “corrugation” as a qualitative form of band warping that increasingly deviates from being twice differentiable at an isolated critical point. These features affect the density-of-states and other parameters ascribed to band warping in various ways. We demonstrate these effects, providing explicit calculations of DOS and their effective masses for warped energy dispersions originally derived by Kittel and others. Other physical and mathematical examples are provided to demonstrate fundamental distinctions that must be drawn between DOS contributions that originate from band warping and contributions that derive from band non-parabolicity. For some non-degenerate bands in thermoelectric materials, this may have profound consequences of practical interest.

Germanium-related deep electron traps in ALD-grown HfO2 insulators studied through Exhaustive PhotoDepopulation Spectroscopy

Energy distribution of deep gap states related to incorporation of Ge impurity into insulating HfO2 is characterized using Exhaustive Photo-Depopulation Spectroscopy technique. While the undoped (Ge-free) HfO2 layers exhibit a 1-eV broad distribution of acceptor states (electron traps) centered at 2eV below the oxide conduction band, incorporation of Ge gives rise to additional acceptor levels with the energy depth of about 3eV. At the same time, a measurable density of donor states (hole traps) is found neither in the Ge-free nor in the Ge-doped HfO2 layers.

(Invited) Spectroscopy of Deep Gap States in High-k Insulators

The work describes the physical principles of the exhaustive photodepopulation spectroscopy. This method allows one to determine the energy distribution of gap states in insulating materials using the observation of optically-assisted electron removal from the gap states in incremental spectral windows. Examples of the inferred energy distributions of acceptor-type electron states in several high-k oxide insulators (Al2O3, HfO2, GdxAl2-xO3) indicate that these are »1-2 eV broad. At the same time, no detectable density of donor states, expected to be present due to oxygen vacancies, is found in the studied oxides.

Spectroscopy of Deep Gap States in High-k Insulators

Spectroscopy of Deep Gap States in High-k Insulators

Zinc Oxide Doped Red Sea Egyptian Clay as a Varistor

Clay consists of about 50% SiO2 + 25% Al2O3 plus some other oxides with low abundance ratio like CaO, TiO2, Fe2O3, MgO and Mn2O3. Burning the clay at 700°C removes out the organic compounds and the mentioned oxides only remain. Our aim is to dope ZnO by the remaining oxides for varistors fabrication. Six types of clay were collected from Egyptian Red Sea Coast mines, after burning for 2 hrs the remaining oxides were mixed and ball milled with ZnO for 10 hrs according to the formula (100-Xn) ZnO +Xn, where n is the clay type and X is the ratio of the clay in grams, X takes the values 0.525 in 15 steps. Samples were pressed and sintered at three different temperatures (Tsin) 1200°C, 1300°C and 1400°C for 1 hr, and then studied via XRD, SEM, EDAX, J-E and C-V measurements. The obtained results were discussed in terms of the microstructure of the samples and the formation of Schottky barriers. Barrier height Ф, width W, interface state density Ns and donor state density Nd were calculated for the samples with highest nonlinearity α. It was found that the breakdown electric field Eo is related exponentially with the ratio of Zinc silicate phase whereas α scales with ratio of ZnO phase and the later increases linearly with Tsin. The electrical characteristics of the most promising sample were compared with those of ZnO samples doped pure SiO2 and Al2O3 obtained at the same preparation condition. This sample is comparable with the Japanese commercial varistor 5N220k Dc JVR.

Pairing fluctuation ac conductivity of disordered thin films

We study temperature T and frequency ω dependence of the in-plane fluctuation conductivity of a disordered superconducting film above the critical temperature. Our calculation is based on the nonlinear sigma model within the Keldysh technique. The fluctuation contributions of different physical origin are found and analyzed in a wide frequency range. In the low-frequency range, ω ≪ T, we reproduce the known leading terms and find additional subleading ones in the Aslamazov–Larkin and the Maki–Thompson contributions to the ac conductivity. We also calculate the density of states ac correction. In the dc case these contributions logarithmically depend on the Ginzburg–Landau rate and are considerably smaller that the leading ones. However, in the ac case an external finite-frequency electromagnetic field strongly suppresses the known Aslamazov–Larkin and Maki–Thompson ac contributions, while the corresponding new terms and the density of states contribution are weakly suppressed and therefore become relevant at finite frequencies.

Point contact Andreev reflection from semimetallic bismuth—The roles of the minority carriers and the large spin-orbit coupling

The Point Contact Andreev Reflection (PCAR) technique has been traditionally and predominantly applied to either the characterisation of superconductors or of spin-polarised metals. The sufficiently large and slowly varying, on the scale of tens of meV around the Fermi level (EF), Density of States (DOS) in metals and highly degenerate semiconductors significantly simplifies the interpretation and quantitative analysis of the measured Differential Conductance Spectra (DCS). Semimetals in general, and particularly the heavy semimetals, because of the presence of several carrier bands and the larger Spin-Orbit coupling, exhibit significantly higher variations in the DOS, close to EF, which can be observed and evaluated with PCAR. Here, DCS of bismuth/niobium point contacts are examined to reveal the relative energy offsets [0.12(3), 2.32(5), and 5.7(2)] meV and the approximate relative DOS contributions of the three main carrier bands (16:5:4). Apart from the bulk, some surface states are also expected to contribute to the observed DOS and be potentially spin-split. The spin polarisation due to all conducting states is evaluated at P≤ 0.09(7).

The role of relativity and dispersion controlled inter-chain interaction on the band gap of thiophene, selenophene, and tellurophene oligomers

Ab initio relativistic density functional theoretical calculations have been carried out on π-conjugated oligomers of increasing length with S, Se, and Te as heteroatoms. The band gap of the corresponding polymers has been obtained by plotting lowest unoccupied molecular orbital (LUMO)-highest occupied molecular orbital (HOMO)gap against the reciprocal of the number of monomer units (1/N) and extrapolating the curve to 1/N = 0. With B3LYP functional, we predict that role of relativistic correction terms is not very significant in the determination of final band gap of thiophene, selenophene, and tellurophene polymer. The origin of this observation is provided through the density of states (DOS) analysis which manifests that DOS contribution across the Fermi level of these polymers is mostly governed by C atoms and as a consequence relativistic correction terms due to heavy heteroatom remain insignificant to the band gap modification. We also inspected the role of inter-chain interaction in determining the net LUMO-HOMO gap of π-stacked double chain oligomers of increasing length. We have found that due to the exciton splitting in the stacked configurations, the LUMO-HOMO gap decreases steadily. Furthermore, we have noticed that dispersion force has important role in the reduction of the LUMO-HOMO gap of the oligomers studied.

Single crystalline quasi aligned one dimensional P-type Cu2O nanowire for improving Schottky barrier characteristics

Schottky diodes usually exhibit non-ideal characteristics. The departure from an ideal junction may result from a thick interfacial layer between the semiconductor and metal, or the presence of series resistance from the bulk semiconductor below the depletion region and back side Ohmic contact resistance. In the present work an attempt was made to avoid most of these factors in order to obtain Schottky diode with better characteristics. For such purpose, single crystalline vertically aligned P-type Cu2O nanowires are deposited on a silicon substrate using solid-vapor technique, without using a catalyst or pre-deposited buffer layers. The structure and morphology of the as-synthesized nanowires are characterized using X-ray diffraction, scanning and transmission electron microscopy. The results showed that the use of CuCl2 is critical for the formation of Cu2O nanowires. The (I–V) and (C–V) characteristic curves of Au/p-Cu2O Schottky diode were measured. The results showed that the ideality factor, barrier height and donor state density states equal 1.05, 1.32eV and 3.65×1018cm−3, respectively.

Method for Efficient Computation of the Density of States in Water-Explicit Biopolymer Simulations on a Lattice

We present a method for fast computation of the density of states of binary systems. The contributions of each of the components to the density of states can be separated based on the conditional independence of the individual components' degrees of freedom. The conditions establishing independence are the degrees of freedom of the interfacial region between the two components. The separate contributions of the components to the density of states can then be calculated using the Wang-Landau algorithm [Wang, F.; Landau, D. P. Phys. Rev. Lett. 2001, 86, 2050]. We apply this method to a 2D lattice model of a hydrophobic homopolymer in water that exhibits protein-like cold, pressure, and thermal unfolding. The separate computation of the protein and water density of states contributions is faster and more accurate than the combined simulation of both components and allows for the investigation of larger systems.

Effect of the final annealing of cold rolled stainless steels sheets on the electronic properties and pit nucleation resistance of passive films

Semiconducting properties of passive films formed on AISI 304 stainless steel grade were investigated by capacitances measurements in chloride containing aqueous solutions for different surface finishes: BA (bright annealing in hydrogen containing atmospheres) and 2B (standard annealing in oxidising atmospheres followed by pickling in acid, then water rinsing). Mott–Schottky analysis shows that for high enough electrode potential, and whatever the surface finish, the films behave like n-type semiconductors. 2B passive film appears to be more donor-doped than BA one and the density of donor states increases with chloride concentration. The electron donor levels are assumed to be generated by negatively charged cations vacancies produced by the chloride ions reaction with the outer passive film. This reaction looks easier for 2B than BA condition, which explains why BA resists better than 2B to pit nucleation.

Relaxation Dynamics of Highly Vibrationally Excited Picoline Isomers (Evib= 38 300 cm-1) with CO2: The Role of State Density in Impulsive Collisions

Strong collisions of highly vibrationally excited picoline isomers and CO2 (00(0)0) were investigated using high resolution transient IR absorption probing to investigate the role of donor state density. Vibrationally excited 3-picoline and 4-picoline (3-methylpyridine and 4 methylpyridine) with E(vib) = 38300 cm(-1) were prepared by 266 nm excitation followed by rapid internal conversion. Transient IR probe measurements of the nascent rotational and translational energy gain in CO2 (00(0)0) show that large DeltaE collisions for 3- and 4-picoline are similar to those for excited 2-picoline. The probability distributions for the large DeltaE energy transfer of the three isomers have similar dependence on DeltaE. The results are compared with other earlier results demonstrating that the shape of the large DeltaE probability distribution correlates with the DeltaE dependence of the donor vibrational state density. The results are discussed in terms of the GRETCHEN model for collisional relaxation.

Influence of the Distribution of Tail States in a-Si:H on the Field Dependence of Carrier Drift Mobilities

ABSTRACTExponential distributions of tail states have been able, within the framework of a multiple-trapping transport model, to account rather well for the time-of-flight photoconductivity transients that are measured with ‘standard’ a-Si:H, i.e. material prepared by plasma-enhanced chemical vapor deposition at ∼250°C. A field-dependent carrier mobility in the dispersive transport regime is part of the observations. However, samples prepared in an expanding thermal plasma, although still exhibiting the dispersive transients, fail to show this field dependence. The presence of a Gaussian component in the density of valence-band tail states can account for such behavior for the hole transients. Nanoscale ordered inclusions in the amorphous matrix are thought to be responsible for the Gaussian density of states contribution.

Superconducting fluctuation probed by in-plane and out-of-plane conductivities inTl2Ba2CaCuO8+ysingle crystals

Extensive measurements of the in-plane and the out-of-plane zero field resistivities $[{\ensuremath{\rho}}_{\mathrm{ab}}(T,0),{\ensuremath{\rho}}_{c}(T,0)]$ and the magnetoresistivities $[{\ensuremath{\rho}}_{\mathrm{ab}}(T,H),{\ensuremath{\rho}}_{c}(T,H)]$ of high-quality ${\mathrm{Tl}}_{2}{\mathrm{Ba}}_{2}{\mathrm{CaCu}}_{2}{\mathrm{O}}_{8+y}$ single crystals were carried out. The obtained zero field fluctuation conductivities $[\ensuremath{\Delta}{\ensuremath{\sigma}}_{\mathrm{ab}}(T,0),\ensuremath{\Delta}{\ensuremath{\sigma}}_{c}(T,0)]$ and fluctuation-induced magnetoconductivities $[\ensuremath{\Delta}{\ensuremath{\sigma}}_{\mathrm{ab}}(T,H)$ and $\ensuremath{\Delta}{\ensuremath{\sigma}}_{c}(T,H)]$ were analyzed based on the theory of thermal fluctuations of the superconducting order parameters, originated from fluctuations of the quasiparticle density of states as well as the Aslamazov-Lakin and Maki-Thompson contributions. We observed that $\ensuremath{\Delta}{\ensuremath{\sigma}}_{\mathrm{ab}}(T,H)$ and the sign change in $\ensuremath{\Delta}{\ensuremath{\sigma}}_{c}(T,H)$ near ${T}_{c}$ could be described adequately only if the density of states contribution was included. The important physical parameters, such as the coherence length $(\ensuremath{\xi}),$ the in-plane scattering time $(\ensuremath{\tau}),$ and the hopping integral $(J),$ obtained during these analyses are compared with the corresponding parameters for other $\mathrm{h}\mathrm{i}\mathrm{g}\mathrm{h}\ensuremath{-}{T}_{c}$ materials.