Electronic Gap States Research Articles

In-situ plasma hydrogenated TiO2 thin films for enhanced photoelectrochemical properties

In this paper, we report the effect of in-situ plasma hydrogenation of TiO2 (iH:TiO2) thin films by the incorporation of known amount of hydrogen in the Ar plasma during rf-sputter deposition of TiO2 films. As compared to pristine TiO2 films (∼0.43mA/cm2 at 0.23V vs Ag/AgCl), hydrogenated TiO2 showed enhanced photoelectrochemical activity in terms of improved photocurrent density of ∼1.08mA/cm2 (at 0.23V vs Ag/AgCl). These results are explained in terms of reduction in band gap energy, shift in valence band maximum away from the Fermi level, improved donor density and more negative flat band potential in iH:TiO2 sample. The presence of Ti2+ states in iH:TiO2 films in addition to Ti3+ states in pristine TiO2 act as additional electronic states in the TiO2 band gap and increases the optical absorption in the visible region. This method of in-situ hydrogenation can be used as a general method for improving the properties of metal oxide thin films for photoelectrochemical and photocatalytic applications.

Narrowing of band gap and effective charge carrier separation in oxygen deficient TiO2 nanotubes with improved visible light photocatalytic activity

Oxygen vacancies are introduced into hydrothermally processed TiO2 nanotube by vacuum calcination. Formation of oxygen vacancies modifies the local coordination in TiO2 as evident from Raman spectroscopy and secondary ion mass spectrometry (SIMS) results. The surface area is increased from 172.5m2/g in pure to 405.1m2/g in defective TiO2 nanotube. The mid-band gap electronic states created by oxygen vacancies are mostly responsible for the effective narrowing of band gap. Charge carrier separation is sufficiently prolonged as the charged oxygen defect states inhibit facile carrier recombination. With high surface area, narrowed band gap and separated charge carriers defective TiO2 nanotube is a suitable candidate in the photodegradation of methylene blue (MB) and phenol under visible light illumination. Photosensitized electron transfer from MB to the conduction band of TiO2 and the photodegradation of MB is facilitated in presence of high density of oxygen vacancies. Unlike MB, phenol absorbs in the UV region and does not easily excited under visible light. Phenol shows activity under visible light by forming charge transfer complex with TiO2. Defect trapped carriers become available at the phenol–TiO2 interface and finally interact with phenol molecule and degrade it.

Selenium modified oxalate chelated titania: Characterization, mechanistic and photocatalytic studies

In order to enhance the visible photocatalytic activity of titanium dioxide (TiO2), selenium (Se) doped TiO2 was prepared by sol–gel method using oxalic acid as a chelating agent. To characterize and describe the effect of Se+4 ions on the structural, optical and photocatalytic properties of TiO2, the powders were characterized by XRD, FTIR, UV-DRS, HR-TEM, PL, Raman, XPS and BET measurements. XRD analysis showed that doped powders consist of anatase phase, while undoped TiO2 is a composite mixture of anatase and rutile at a calcination temperature of 550°C. Williamson–Hill plot results indicated the presence of tensile strain on the addition of 0.02, 0.04 and 0.06mol% Se content, while compressive strain was specified on the addition of 0.08, 0.1 and 0.2mol% Se content in TiO2 lattice. FTIR analysis revealed the chelating behaviour of oxalic acid with titanium ions in forming titanium oxalate stretches. Diffuse reflectance measurements showed that visible light absorption is induced by dopant localized/intermediate electronic states, probably originating from Se 4s and 4p states within the band gap of TiO2. Raman peak broadening and the small frequency shifting for the peaks (1-Eg and B1g) were attributed to oxygen vacancies and crystal lattice distortions. Photoluminescence (PL) analysis showed that electron hole recombination was decreased with the incorporation of Se+4 ions in TiO2, which act as trapping sites for the photogenerated electrons. Moreover, PL analysis demonstrated that Se doping produces Ti+3 centers, oxygen defects and localized electronic states in TiO2 band gap, which become responsible for the TiO2 response towards the visible light. The activity of the prepared photocatalysts powder were tested by investigating the kinectics of the photocatalytic decoloration of methylene blue (MB) under UV irradiations and decomposition of 4-chlorophenol (4-CP) under visible light irradiation. It was observed that the TSe0.08 powder with Brönsted acid sites, calcined at 550°C showed highest photocatalytic activity under both UV and visible light irradiations for the selected pollutants. In the decoloration of MB, TSe0.08 powder attain a rate constant (kapp) value of 0.045min−1 which is higher than Degussa P25 (0.033min−1) as well two times higher than undoped TiO2 (0.021min−1). Based on the obtained results, a UV/vis light irradiated photocatalytic mechanism has been proposed for the Se+4 doped anatase TiO2.

The electronic structure of defect chalcopyrite CdGa2Se4 as determined from first principles calculations and X-ray spectroscopy studies

The total and partial densities of states of all components of CdGa2Se4 are calculated using the modified ab initio augmented plane wave (APW) method implemented in the WIEN2k software package. The results of the APW calculations indicate that in the CdGa2Se4 compound, the Sep states are the main contributions to the valence band, and their contribution is maximum at the top of the valence band, meanwhile at the bottom of the conduction band the contributions of the Gas* states dominate. According to the theoretical APW-calculations, the Cdd and Gap electronic states also contribute sufficiently to the valence band (mostly at the bottom and at the top of the band respectively) in CdGa2Se4. The joint alignment of CdLβ2,15, GaKβ2 and SeKβ2 X-ray emission bands and of the X-ray photoelectron spectrum of the valence electrons obtained for the CdGa2Se4 single crystal, in a common energy scale, shows good agreement of the obtained theoretical and experimental data referring to the characteristic properties of the electronic structure of the CdGa2Se4 compound.

Soft X-ray spectroscopic studies of the electronic structure of M:BiVO4(M = Mo, W) single crystals

X-ray spectroscopy reveals the presence of inter-band gap electronic states of egsymmetry in M:BiVO4(M = Mo, W) PEC anodes.

Thermodynamically driven one-dimensional evolution of anatase TiO2 nanorods: one-step hydrothermal synthesis for emerging intrinsic superiority of dimensionality.

In photoelectrochemical cells, there exists a competition between transport of electrons through the porous semiconductor electrode toward the conducting substrate and back-reaction of electrons to recombine with oxidized species on the semiconductor-electrolyte interface, which determines the charge collection efficiency and is strongly influenced by the density and distribution of electronic states in band gap and architectures of the semiconductor electrodes. One-dimensional (1D) anatase TiO2 nanostructures are promising to improve charge transport in photoelectrochemical devices. However, the conventional preparation of 1D anatase nanostructures usually steps via a titanic acid intermediate (e.g., H2Ti3O7), which unavoidably introduces electronic defects into the host lattice, resulting in undesired shielding of the intrinsic role of dimensionality. Here, we manage to promote the 1D growth of anatase TiO2 nanostructures by adjusting the growth kinetics, which allows us to grow single-crystalline anatase TiO2 nanorods through a one-step hydrothermal reaction. The synthesized anatase nanorods possess a lower density of trap states and thus can simultaneously facilitate the diffusion-driven charge transport and suppress the electron recombination. Moreover, the electronically boundary free nanostructures significantly enhance the trap-free charge diffusion coefficient of the anatase nanorods, which enables the emergence of the intrinsic superiority of dimensionality. By virtue of these merits, the anatase nanorods synthesized in this work take obvious advantages over the conventional anatase counterparts in photoelectrochemical systems (e.g., dye-sensitized solar cells) by showing more efficient charge transport and collection and higher energy conversion efficiency.

Performance of solid oxide fuel cells approaching the two-dimensional limit

We model electrochemical kinetics and physical conduction mechanisms for carrier transport in electrolyte membranes to determine the limits of dimensionality reduction (down to 1 nm) on maximum power output of solid oxide fuel cells with symmetric Pt electrodes. Using Y-doped ZrO2, we find a minimum thickness of ∼6 nm to realize near ideal chemical potential in such fuel cells, which is limited by electronic breakdown when approaching the dielectric breakdown strength. For larger electrolyte thicknesses, the greatest source of electronic leakage influencing power loss is from Ohmic transport of minority carriers and emission of trapped carriers. For porous metallic electrodes, an ideal microstructure with the particle size comparable to particle spacing dimensions is found to accurately model experimental results. The role of electronic trap states in the electrolyte band gap on power density characteristics is highlighted.

Passivation of the GaP(111) surface by treatment in selenium vapors

Results of examination of structural phase transitions on the GaP(111) surface after heat treatment in selenium vapors in a vacuum chamber with a quasi-closed volume are described. The electrophysical characteristics of Schottky-barrier diodes on GaP(111) are studied before and after treatment in selenium vapors by measuring the current-voltage characteristics and by deep-level transient spectroscopy. It is found that, after treatment in selenium vapors, the Schottky-barrier height becomes dependent on the work function of the metal in accordance with the Schottky-Mott rule for an ideal diode. It is shown that a decrease in the density of surface electronic states in GaP(111) results from the formation of a Ga2Se3(111)(√3 × √3)-R30° surface phase with ordered stoichiometric gallium vacancies.

Manipulation of electronic states and photonic states in nanosilicon

On different size hierarchy, period symmetry provides energy band structure, and symmetry breaking produces localized states in gap, for example nanostructures open electronic band gap by confining electrons, but defects in symmetry system produce localized electronic states in gap. The experimental results demonstrate that controlling localized states in gap by changing passivation environment can manipulate emission wavelength, such as stimulated emission at 700nm due to oxygen passivation and enhanced electroluminescence near 1600nm due to ytterbium passivation on nanosilicon. In same way, modulating filling fraction and period parameters in photonic crystal enlarges width of photonic band gap (PBG) by confining photons. Symmetry breaking due to defects is effective in manipulating photonic states. New applications for selecting modes in nanolaser and for building single photon source in quantum information are explored by manipulating and coupling between electronic states and photonic states.

Conversion of multilayer graphene into continuous ultrathin sp3-bonded carbon films on metal surfaces

The conversion of multilayer graphenes into sp3-bonded carbon films on metal surfaces (through hydrogenation or fluorination of the outer surface of the top graphene layer) is indicated through first-principles computations. The main driving force for this conversion is the hybridization between sp3 orbitals and metal surface dz2 orbitals. The induced electronic gap states and spin moments in the carbon layers are confined in a region within 0.5 nm of the metal surface. Whether the conversion occurs depend on the fraction of hydrogenated (fluorinated) C atoms at the outer surface and on the number of stacked graphene layers. In the analysis of the Eliashberg spectral functions for the sp3 carbon films on a metal surface that is diamagnetic, the strong covalent metal-sp3 carbon bonds induce soft phonon modes that predominantly contribute to large electron-phonon couplings, suggesting the possibility of phonon-mediated superconductivity. Our computational results suggest a route to experimental realization of large-area ultrathin sp3-bonded carbon films on metal surfaces.

Quantification of electronic band gap and surface states on FeS2(100)

The interfacial electronic properties and charge transfer characteristics of pyrite, FeS2, are greatly influenced by the presence of electronic states at the crystal free surface. We investigate the surface electronic structure of FeS2 (100) using scanning tunneling spectroscopy (STS) and interpret the results using tunneling current simulations informed by density functional theory. Intrinsic, dangling bond surface states located at the band edges reduce the fundamental band gap Eg from 0.95eV in bulk FeS2 to 0.4±0.1eV at the surface. Extrinsic surface states from sulfur and iron defects contribute to Fermi level pinning but, due to their relatively low density of states, no detectable tunneling current was measured at energies within the intrinsic surface Eg. These findings help elucidate the nature of energy alignment for electron transfer processes at pyrite surfaces, which are relevant to evaluation of electrochemical processes including corrosion and solar energy conversion.

A DFT + U study of (Rh, Nb)-codoped rutile TiO2

A systematic study of electronic structure and band gap states is conducted to analyze the monodoping and charge compensated codoping of rutile TiO2 with Rh and Nb, using the DFT + U approach. Doping of rutile TiO2 with Rh atoms induces hybridized O 2p and Rh 4d band gap states leading to a red shift of the optical absorption edge, consistent with previous experimental studies. Since Rh monodoping may induce recombination centers, charge compensated codoping with Rh and Nb is also explored. This codoping induces an electron transfer from Nb induced states to Rh 4d states, which suppresses the formation of Rh4+, thereby leading to a reduction in recombination centers and to the formation of more stable Rh3+. A combination of band gap reduction by 0.5 eV and the elimination of band gap states that account for recombination centers makes (Rh, Nb)-codoped TiO2 a more efficient and stable photocatalyst.

Shape and curved surface effect on silicon quantum dots

Curviform surface breaks the symmetrical shape of silicon quantum dots on which some bonds can produce localized electronic states in band gap. The calculation results show that the bonding energy and electronic states of silicon quantum dots are different on various curved surfaces, for example, an Si–O–Si bridge bond on curved surface provides the localized levels in band gap and its bonding energy is shallower than that on facet. The red-shifting of PL spectrum on smaller silicon quantum dots can be explained by curved surface effect. Experiments demonstrate that silicon quantum dots are activated for emission due to the localized levels formed in the band gap.

Electronic states and curved surface effect of silicon quantum dots

The calculation results show that the bonding energy and electronic states of silicon quantum dots (Si QDs) are different on various curved surfaces (CS), for example, a Si-O-Si bridge bond on curved surface provides the localized levels in band gap and its bonding energy is shallower than that on facet. Curved surface breaks symmetrical shape of silicon quantum dots on which some bonds can produce localized electronic states in band gap. The red-shifting of photoluminescence spectra on smaller silicon quantum dots can be explained by CS effect. In CS effect, surface curvature is determined by the shape of Si QDs or silicon nanostructures, which is independent of their sizes. The CS effect has the interesting fundamental physical properties in nanophysics as that of quantum confinement effect.

Selecting of modes in nano-laser of silicon quantum dots

In a nano-laser of Si quantum dots (QD), the smaller QD fabricated by nanosecond pulse laser can form the pumping level tuned by the quantum confinement (QC) effect. Coupling between the active centers formed by localized states of surface bonds and the two-dimensional (2D) photonic crystal used to select model can produce a sharp peak at 2.076eV in the nano-laser. It is interesting to make a comparison between the localized electronic states in gap due to defect formed by surface bonds and the localized photonic states in gap of photonic band due to defect of 2D photonic crystal.

High performance organic light emitting diodes using substoichiometric tungsten oxide as efficient hole injection layer

In this work we demonstrate the unique hole injection and transport properties of a substoichiometric tungsten oxide with precise stoichiometry, in particular WO2.5, obtained after the controlled hydrogen reduction during growth of tungsten oxide, using a simple hot-wire vapor deposition technique. We present clear evidence that tungsten suboxide exhibits metallic character and that an almost zero hole injection barrier exists at the anode/polymer interface due to the formation/occupation of electronic gap states near the Fermi level after oxide’s reduction. These states greatly facilitate hole injection and charge generation/electron extraction enabling the demonstration of extremely efficient hole only devices. WO2.5 films exhibit metallic-like conductivity and, thus, can also enhance charge transport at both anode and cathode interfaces. Electroluminescent devices using WO2.5 as both, hole and electron injection layer, and poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-(1,4-benzo-{2,1′,3}-thiadiazole)] (F8BT) as the emissive layer exhibited high efficiencies up to 7cd/A and 4.5lm/W, while, stability studies revealed that these devices were extremely stable, since they were operating without encapsulation in air for more than 700h.

First-principles study of electronic structures and optical properties of Cu, Ag, and Au-doped anatase TiO2

We perform first-principles calculations to investigate the band structure, density of states, optical absorption, and the imaginary part of dielectric function of Cu, Ag, and Au-doped anatase TiO2 in 72 atoms systems. The electronic structure results show that the Cu incorporation can lead to the enhancement of d states near the uppermost of valence band, while the Ag and Au doping cause some new electronic states in band gap of TiO2. Meanwhile, it is found that the visible optical absorptions of Cu, Ag, and Au-doped TiO2, are observed by analyzing the results of optical properties, which locate in the region of 400–1000nm. The absorption band edges of Cu, Ag, and Au-doped TiO2 shift to the long wavelength region compared with the pure TiO2. Furthermore, according to the calculated results, we propose the optical transition mechanisms of Cu, Ag, and Au-doped TiO2. Our results show that the visible light response of TiO2 can be modulated by substitutional doping of Cu, Ag, and Au.

An all-silicon laser by coupling between electronic localized states and defect states of photonic crystal

In a nano-laser of Si quantum dots (QD), the smaller QD fabricated by nanosecond pulse laser can form the pumping level tuned by the quantum confinement (QC) effect. Coupling between the active centers formed by localized states of surface bonds and the two-dimensional (2D) photonic crystal is used to select model in the nano-laser. The experimental demonstration is reported in which the peaks of stimulated emission at about 600nm and 700nm were observed on the Si QD prepared in oxygen after annealing which improves the stimulated emission. It is interesting to make a comparison between the localized electronic states in gap due to defect formed by surface bonds and the localized photonic states in gap of photonic band due to defect of 2D photonic crystal.

Self-consistent calculations of deep Sn and Se vacancy levels in SnSe by the Green’s function method

593.293.011 In the context of the Green’s function theory, the electronic structure of local defects-vacancies is considered self-consistently in the basis of localized orbitals. The origin and orbital structure of the electronic states in the gap, resonances and antiresonances in the valence band, and defect-induced changes in the electron density are discussed.

DFT + U study of defects in bulk rutile TiO2

We present a systematic study of electronic gap states in defected titania using our implementation of the Hubbard-U approximation in the grid-based projector-augmented wave density functional theory code, GPAW. The defects considered are Ti interstitials, O vacancies, and H dopants in the rutile phase of bulk titanium dioxide. We find that by applying a sufficiently large value for the Hubbard-U parameter of the Ti 3d states, the excess electrons localize spatially at the Ti sites and appear as states in the band gap. At U=2.5 eV, the position in energy of these gap states are in fair agreement with the experimental observations. In calculations with several excess electrons and U=2.5 eV, all of these end up in gap states that are spatially localized around specific Ti atoms, thus effectively creating one Ti(3+) ion per excess electron. An important result of this investigation is that regardless of which structural defect is the origin of the gap states, at U=2.5 eV, these states are found to have their mean energies within a few hundredths of an eV from 0.94 eV below the conduction band minimum.