Modification of the electronic properties of the TiO2 (1 1 0) surface upon deposition of the ultrathin conjugated organic layers

A few nm thick 3,4,9,10-perylenetetracarboxylic acid dianhydride (PTCDA) and Cu-phthalocyanine (CuPc) overlayers were thermally deposited in situ in UHV onto TiO₂ (1 1 0) surface. Atomic composition of the surfaces under study was monitored using Auger electron spectroscopy (AES). The formation of the interfacial potential barrier and the structure of the unoccupied electronic states located 5-25 eV above the Fermi level (E_F) was monitored using a probing beam of low-energy electrons according to the total current electron spectroscopy (TCS) method. The work function values upon the overlayer deposition changed from 4.6 to 4.9 eV at the PTCDA/TiO₂ (1 1 0) interface and from 4.6 to 4.3 eV at the CuPc/TiO₂ (1 1 0) interface. Band bending in the TiO₂ substrate, molecular polarization in the organic film and changes in the work function due to the change in the surface composition were found to contribute to the formation of the interfacial potential barriers. Oxygen admixture related peaks were observed in the AES and in the TCS spectra of the CuPc overlayers. A mechanism of the transformations in the PTCDA and CuPc overlayers on the TiO₂ (1 1 0) upon elevating temperature from 25 to 400 °C was suggested.     

Atomic layer deposited HfO2 based metal insulator semiconductor GaN ultraviolet photodetectors

A report on GaN based metal insulator semiconductor (MIS) ultraviolet (UV) photodetectors (PDs) with atomic layer deposited (ALD) 5-nm-thick HfO₂ insulating layer is presented. Very low dark current of 2.24 × 10⁻¹¹ A and increased photo to dark current contrast ratio was achieved at 10 V. It was found that the dark current was drastically reduced by seven orders of magnitude at 10 V compared to samples without HfO₂ insulating layer. The observed decrease in dark current is attributed to the large barrier height which is due to introduction of HfO₂ insulating layer and the calculated barrier height was obtained as 0.95 eV. The peak responsivity of HfO₂ inserted device was 0.44 mA/W at bias voltage of 15 V.     

A study of the adsorption of oxygen on Ni(111) using auger electron spectroscopy: Chemical shifts and valence spectra

Auger electron spectra have been recorded when oxygen is adsorbed on a Ni(111) single crystal surface. For the coverage range θ < 1, an analysis of the plot of the peak to peak height (H) of the oxygen KVV (516 eV) transition versus the total number of molecules cm^2? impinging on the surface (molecular beam dosing) shows agreement with the kinetic mechanism proposed by Morgan and King [Surface Sci. 23 (1970) 259] for the adsorption of oxygen on polycrystalline nickel films. In this coverage range, no energy shifts of the nickel or oxygen Auger peaks were recorded.At coverages θ > 1 (standard dosing procedure) shifts in the valence spectra M_{2, 3}VV (61 eV) and L₃M_{2, 3}V (782 eV) of ?2.3 eV and ?1.8eV respectively are recorded at 1.4 × 10^?2 torr-sec. Up to these coverages no shift of the L₃VV transition (849 eV) is observed. A chemical shift of ?2.1 eV is recorded in the L₃M_{2, 3}M_{2, 3} Auger transition (716 eV) at 1.4 × 10^?2 torr-sec.In the coverage range θ > 1, shifts in the energy of the oxygen Auger peaks are observed. At 5.8 × 10^?3 torr-sec. the KVV (516 eV) and KL₁V (495.2 ± 0.3 eV) transitions show shifts of ?1.5 eV and ?(1.0 ±0.3) eV respectively. No shift up to this coverage is recorded in the KL₁L₁ (480.6 ± 0.3 eV) transition.     

Role of sweep direction and substrate work function on resistive switching of titanium-di-oxide [TiO2] nanoparticles

The resistive switching mechanism in titanium-dioxide nanoparticles (TiO₂ NP) is studied using the current-voltage (I–V) measurements. The TiO₂ NP are spin-coated on different substrates like FTO, ITO, Gold, and p-Silicon. The I–V measurements are carried out by changing the initial potential of the substrates to either 0 V (sweep1) or −1 V (sweep2). Resistive switching (RS) was observed only for FTO/TiO₂ NP and ITO/TiO₂NP devices in sweep1 direction. Whereas, in sweep2 direction, no such RS was observed in any of the devices. The detailed I–V analysis infers the Ohmic conduction followed by space charge limited conduction (SCLC) during the RS forming process for FTO/TiO₂ NP and ITO/TiO₂NP devices. The Au and p-Si substrates act as blocking contact for TiO₂ and exhibit Schottky/thermionic emission at lower voltages and SCLC at higher voltages. The TiO₂ NP coated on p-Si substrate exhibits rectifying behaviour with a current ratio of 3 orders of magnitude.     

Thermogravimetric and electrical study of non-stoichiometric titanium dioxide TiO2−x, between 800 and 1100°C

By means of thermogravimetrie and electrical measurements, it has been possible to give accurate informations on the nature of the defects produced by reduction of TiO₂ between 800 and 1100°C. For small partial pressures of oxygen P_o2. interstitial titanium Ti⁴_i prevails at temperature higher than 900°C. When P_O2 increases, oxygen vacancies are produced at first in the doubly ionized form V″_o and a progressive transition to singly ionized vacancies V″_o can be assumed. The formation enthalpies associated with these defects as calculated from our experimental results are 10.1 eV for Ti⁴_i, 4.6 eV for V″_o and 3.6–4.0 eV for V′_o. The electronic drift mobility μ is independent both of nature and concentration of the defects. The shape of its temperature dependence leads to conclude that the conduction in spite of the low μ value (0.06 cm² V^?1 s^?1 at 1100°C) is of the classical type for wide band semiconductors and not a hopping process.     

Structure formation and mechanisms of DC conduction in thermally evaporated nanocrystallite structure ZnIn2Se4 thin films

ZnIn₂Se₄ is of polycrystalline structure in as synthesized condition. It transforms to nanocrystallite structure of ZnIn₂Se₄ film upon thermal evaporation. Annealing temperatures influenced crystallite size, dislocation density and internal strain. The hot probe test showed that ZnIn₂Se₄ thin films are n-type semiconductor. The dark electrical resistivity versus reciprocal temperature for planar structure of Au/ZnIn₂Se₄/Au showed existence of two operating conduction mechanisms depending on temperature. At temperatures >365 K, intrinsic conduction operates with activation energy of 0.837 eV. At temperatures <365 K, extrinsic conduction takes place with activation energy of 0.18 eV. The operating conduction mechanism in extrinsic region is variable range hopping. The parameters such as density of states at Fermi level, hopping distance and average hopping energy have been determined and it was found that they depend on film thickness. The dark current–voltage characteristics of Au/n-ZnIn₂Se₄/p-Si/Al diode at different temperatures ranging from 293–353 K have been investigated. Results showed rectification behavior. At forward bias potential <0.2 V, thermionic emission of electrons from ZnIn₂Se₄ film over a potential barrier of 0.28 V takes place. At forward bias potential >0.2 V, single trap space charge limited current is operating. The trap concentration and trap energy level have been determined as 3.12×10¹⁹ cm⁻³ and 0.24 eV, respectively.     

Effect of substrate bias voltage on the structure, electric and dielectric properties of TiO2 thin films by DC magnetron sputtering

Titanium dioxide (TiO₂) films have been deposited on glass and p-silicon (1 0 0) substrates by DC magnetron sputtering technique to investigate their structural, electrical and optical properties. The surface composition of the TiO₂ films has been analyzed by X-ray photoelectron spectroscopy. The TiO₂ films formed on unbiased substrates were amorphous. Application of negative bias voltage to the substrate transformed the amorphous TiO₂ into polycrystalline as confirmed by Raman spectroscopic studies. Thin film capacitors with configuration of Al/TiO₂/p-Si have been fabricated. The leakage current density of unbiased films was 1 × 10⁻⁶ A/cm² at a gate bias voltage of 1.5 V and it was decreased to 1.41 × 10⁻⁷ A/cm² with the increase of substrate bias voltage to −150 V owing to the increase in thickness of interfacial layer of SiO₂. Dielectric properties and AC electrical conductivity of the films were studied at various frequencies for unbiased and biased at −150 V. The capacitance at 1 MHz for unbiased films was 2.42 × 10⁻¹⁰ F and it increased to 5.8 × 10⁻¹⁰ F in the films formed at substrate bias voltage of −150 V. Dielectric constant of TiO₂ films were calculated from capacitance–voltage measurements at 1 MHz frequency. The dielectric constant of unbiased films was 6.2 while those formed at −150 V it increased to 19. The optical band gap of the films decreased from 3.50 to 3.42 eV with the increase of substrate bias voltage from 0 to −150 V.     

Transparent nano composite PVA–TiO2 and PMMA–TiO2 thin films: Optical and dielectric properties

Transparent nano composite PVA–TiO₂ and PMMA–TiO₂ thin films were prepared by an easy and cost effective dip coating method. Al/PVA–TiO₂/Al and Al/PMMA–TiO₂/Al sandwich structures were prepared to study the dielectric behavior. The presence of metal–oxide (Ti–O) bond in the prepared films was confirmed by Fourier transform infrared spectroscopy. X-ray diffraction pattern indicated that the prepared films were predominantly amorphous in nature. Scanning electron micrographs showed cluster of TiO₂ nanoparticles distributed over the film surface and also there were no cracks and pin holes on the surface. The transmittance of the films was above 80% in the visible region and the optical band gap was estimated to be about 3.77 eV and 3.78 eV respectively for PVA–TiO₂ and PMMA–TiO₂ films by using Tauc's plot. The determined refractive index (n) values were between 1.6 and 2.3. High value of dielectric constant (?′ = 24.6 and ?′ = 26.8) was obtained for the prepared composite films. The conduction in the composite films was found to be due to electrons. The observed amorphous structure, good optical properties and dielectric behavior of the prepared nano composite thin films indicated that these films could be used in opto-electronic devices and in thin film transistors.     

Optical properties of TiO2-MnO2 thin films prepared by electron-beam evaporation

The optical constants and thickness of TiO₂-MnO₂ films (with MnO₂ concentration of 0, 1, and 5%) prepared by electron-beam evaporation are determined. A considerable dependence of the optical properties of thin TiO₂ films on the manganese concentration is observed. It is found that thin films are indirect gap semiconductors with gap width E _g = 3.43 eV (TiO₂), 2.89 eV (TiO₂-MnO₂ (1%)), and 2.73 eV (TiO₂-MnO₂ (5%)).     

Optical properties of ITO/TiO2 single and double layer thin films deposited by RPLAD

Indium tin oxide (ITO) and titanium dioxide (TiO₂) single layer and double layer ITO/TiO₂ films were prepared using reactive pulsed laser ablation deposition (RPLAD) with an ArF excimer laser for applications in dye-sensitized solar cells (DSSCs). The films were deposited on SiO₂ substrates either at room temperatures (RT) or heated to 200-400 °C. Under optimized conditions, transmission of ITO films in the visible (vis) range was above 89% for films produced at RT and 93% for the ones deposited at higher temperatures. Increasing the substrate temperature from RT to 400 °C enhances the transmission of TiO₂ films in the vis-NIR from about 70% to 92%. High transmission (≈90%) was observed for the double layer ITO/TiO₂ with a transmission cut-off above 900 nm. From the transmission data, the energies gaps (E_g), as well as the refractive indexes (n) for the films were estimated. n ≈ 2.03 and 2.04, respectively for ITO films and TiO₂ film deposited at 400 °C in the visible region. Post-annealing of the TiO₂ films for 3 h at 300 and 500 °C was performed to enhance n. The refractive index of the TiO₂ films increases with the post-annealing temperature. The direct band gap is 3.6, 3.74 and 3.82 eV for ITO films deposited at RT, 200, and 400 °C, respectively. The TiO₂ films present a direct band gap of 3.51 and 3.37 eV for as deposited TiO₂ films and when annealed at 400 °C, respectively. There is a shift of about 0.1 eV between ITO and ITO/TiO₂ films deposited at 200 °C. The shift decreases by half when the TiO₂ film was deposited at 400 °C. Post-annealing was also performed on double layer ITO/TiO₂.     

Stability of gold nanostructures on rutile TiO2(110) surface

The adsorption of gold atoms and formation of nanostructures on the rutile TiO₂(110) surface with different degree of oxygen reduction was studied from first principles. The Au atoms adsorb strongest at oxygen vacancy sites. Starting from a very low coverage limit the potential energy profiles or diffusion paths of the adsorbed Au monomers and dimers were calculated. Stable structures of two to nine Au atoms arranged in finite and infinite rows and in the shape of finite-size clusters were determined. All these structures are found to bind to the reduced surface stronger than 2 eV/atom. The elongated Au row-like structures bind by about 0.1 eV stronger than 3D clusters, suggesting a preference for the 1D-like Au growth mode on the missing-row reconstructed TiO₂(110).     

Crystallization and electrical properties of V2O5 thin films prepared by RF sputtering

V₂O₅ thin films were prepared under various conditions by using reactive RF sputtering technique. The microstructure and electrical properties of the films are have been investigated. X-ray diffraction data revealed the films deposited at low O₂/Ar ratio are amorphous. The orthorhombic structure of film improved after post annealing at 873 K. The microstructure parameters (crystallite/domain size and macrostrain) have been evaluated by using a single order Voigt profile method. Using the two-point probe technique, the dark conductivity as a function of the condition parameters such as film thickness, oxygen content and temperature are discussed. It was also found that, the behaviour of ρd versus d was found to fit properly with the Fuchs-Sondheimer relation with the parameters: ρ_o = 2.14 × 10⁷ Ω cm and ?_o = 112 ± 2 nm. At high temperature, the electrical conductivity is dominated by grain boundaries, the values of activation energy and potential barrier height were 0.90 ± 0.02 eV and 0.92 ± 0.02 V, respectively.     

Oxidation of TiNi surface with hyperthermal oxygen molecular beams

We report results of our detailed studies on the initial oxidation process of TiNi with a 2 eV hyperthermal oxygen molecular beam (HOMB) and thermal O₂ in the backfilling. The oxidation processes are monitored by X-ray photoemission spectroscopy (XPS) measurements in conjunction with synchrotron radiation (SR). In the early stage of oxidation, the precursor mediated dissociative adsorption is the dominant reaction mechanism. In the oxide formation process at higher O coverage, HOMB has the advantage in the dissociation process of O₂ molecule and can grow TiO₂ layers with the underlying TiO_x-rich and/or Ni-rich layers. We succeeded in fabricating thick Ni-free TiO₂ layer, possibly blue colored rutile TiO₂, combining HOMB and surface annealing.     

Pd–Mn<Subscript>3</Subscript>O<Subscript>4</Subscript> on 3D hierarchical porous graphene-like carbon for oxygen evolution reaction

A 3D hierarchical porous graphene-like carbon (3D HPG) has been studied as conducting support for Pd and Mn₃O₄ nanoparticles. The Pd–Mn₃O₄ supported on 3D HPG demonstrates as an excellent catalyst for oxygen evolution reaction (OER) in alkaline medium. The Pd–Mn₃O₄(wt 2:1)/HPG catalyst shows a low onset potential of 0.497 V and achieves a high current density of 5.3 mA cm^?2 at 0.7 V (vs. SCE). The outstanding electrocatalytic activity is attributed to a synergistic effect between Pd and Mn₃O₄, and the 3D HPG enhances conductivity for charge transport and gives more active sites for the OER reaction.     

Structure, magnetic and optical properties of polycrystalline Co-doped TiO2 films

Co-doped TiO₂ films were fabricated under different conditions using reactive facing-target magnetron sputtering. Co doping improves the transformation of TiO₂ from anatase phase to rutile phase. The chemical valence of doped Co in the films is +2. All the films are ferromagnetic with a Curie temperature above 340 K. The average room-temperature moment per Co of the Co-doped TiO₂ films fabricated at 1.86 Pa decreases from 0.74 μ_B at x=0.03 to 0.02 μ_B at x=0.312, and decreases from 0.54 to 0.04 μ_B as x increases from 0.026 to 0.169 for the Co-doped TiO₂ films fabricated at 0.27 Pa. The ferromagnetism originates from the oxygen vacancies created by Co²⁺ dopants at Ti⁴⁺ cations. The optical band gaps value (E_g) of the Co-doped TiO₂ films fabricated at 1.86 Pa decreases linearly from 3.35 to 2.62 eV with the increasing x from 0 to 0.312. For the Co-doped TiO₂ films fabricated at 1.86 Pa, the E_g decreases linearly from 3.26 to 2.53 eV with increasing x from 0 to 0.350.     

Preparation of N-doped TiO<Subscript>2</Subscript> photocatalyst by atmospheric pressure plasma process for VOCs decomposition under UV and visible light sources

The nitrogen doped (N-doped) titanium dioxide (TiO₂) photocatalyst was prepared by the atmospheric-pressure plasma-enhanced nanoparticles synthesis (APPENS) process operated under normal temperature, i.e. the dielectric barrier discharge plasma process. The N₂ carrier gas is dissociated in the AC powered nonthermal plasma environment and subsequently doped into the TiO₂ photocatalyst that was capable of being induced by visible light sources. The APPENS process for producing N-doped TiO₂ showed a higher film deposition rate in the range of 60–94 nm/min while consuming less power (<100 W) as compared to other plasma processes reported in literatures. And the photocatalytic activity of the N-doped TiO₂ photocatalyst was higher than the commercial ST01 and P25 photocatalysts in terms of toluene removals in a continuous flow reactor. The XPS measurement data indicated that the active N doping states exhibited N 1s binding energies were centered at 400 and 402 eV instead of the TiN binding at 396 eV commonly observed in the literature. The light absorption in the visible light range for N-doped TiO₂ was also confirmed by a clear red shift of the UV-visible spectra.     

Optimal levels of oxygen deficiency in the visible light photocatalyst TiO2−x and long-term stability of catalytic performance

The dependence of the visible light-responsive photocatalytic activity of oxygen deficient TiO₂ (TiO_2−x) prepared by Ar/H₂ plasma surface treatment on the degree of oxygen deficiency (x) was assessed to determine the deficiency region associated with highest performance. The highest activity was obtained at x=0.06 (TiO_1.94). The maximum visible light activity for this material, estimated from the formaldehyde (HCHO) removal rate, was three times higher than that exhibited by nitrogen-doped TiO₂ (TiO_2−xN_x). The catalytic ability was found to decrease over the first week after fabrication of the material, after which it became stable, and the performance of TiO_2−x at this point was found to be nearly equal to that of TiO_2−xN_x. The results of ab initio calculations of density of states for TiO_2−x suggest that new oxygen deficiency states emerge at almost the exact center between the valence and conduction bands when x>0.06, which increases the recombination rate between electrons and holes. Therefore the declining performance of TiO_2−x at larger x values is attributed to the emergence of new oxygen deficient states.     

Anodic TiO2 nanotubes as anode electrode in Li–air and Li-ion batteries

We investigate the possibility of using a TiO₂ anode as an alternative to the Li electrode in Li–air and Li-ion rechargeable batteries. TiO₂ nanotube layer is fabricated by the anodization method and optional thermal treatment is conducted. The electrochemical charge/discharge profile of the TiO₂/liquid electrolyte/LiCoO₂ structured cell is measured under the flowing of O₂, N₂ and Ar, respectively. The elevation of the upper cut-off voltage from 3 to 4.5 V leads to an increase in the specific capacity by a factor of more than three. We suppose this to be a novel mechanism in which the TiO₂/LiCoO₂ system under the oxygen atmosphere works in Li–air battery mode up to 3 V and then works in Li-ion battery mode from 3 V to 4.5 V. This idea is confirmed by ICP-OES analysis.     

Toward sustainable energy: photocatalysis of Cr-doped TiO<Subscript>2</Subscript>: 1. electronic structure

The present chain of five papers considers the concept of defect engineering in processing TiO₂-based photosensitive semiconductors for solar-to-chemical energy conversion. The papers report the effect of chromium on the key performance-related properties of polycrystalline TiO₂ (rutile), including (i) electronic structure, (ii) chromium-related photocatalytic properties, (iii) oxygen-related photocatalytic properties, (iv) electrochemical coupling and (v) surface versus bulk properties. The present work reports the effect of chromium on defect disorder and the related electronic structure of TiO₂, including the band gap and mid-gap energy levels. It is shown that chromium incorporation into the TiO₂ lattice results in a decrease of the band gap from 3.04 eV for pure TiO₂ to 1.4 and 1.3 eV, for Cr-doped TiO₂ (1.365 at% Cr) after annealing at 1373 K in the gas phase of controlled oxygen activity, 21 kPa and 10^?10 Pa, respectively. The optical properties determined using the ultraviolet-vis spectroscopy (in the reflectance mode) indicate that chromium incorporation results in the formation of mid-band energy levels. In this work, we show that chromium at and above the concentrations levels of 0.04 and 0.376 at% results in the formation of acceptor-type energy levels at 0.57 and 1.16 eV (above the valence band), respectively, which are related to tri-valent chromium in titanium sites and titanium vacancies, respectively. Application of well-defined protocol leads to the determination of data that are well reproducible. The new insight involves the determination of the band gap of TiO₂ processed in the gas phase of controlled oxygen activity.

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