TiOx Films Research Articles

Oxidation and Reduction of TiOx Thin Films on Pd(111) and Pd(100).

Thin films of TiOx on Pd(100) and Pd(111) have been investigated with respect to their properties after oxidation and reduction cycles. High-resolution photoemission spectroscopy (HRPES) and low energy electron diffraction (LEED) have been applied to characterize the thin film oxidation states and structure before and after oxidation and reduction under ultrahigh vacuum conditions. Fully oxidized TiO2 films were formed on both surfaces. These structures display Moiré patterns in LEED, in one dimension for Pd(100) and in two dimensions for Pd(111), and they have previously not been reported for TiO2/Pd. The oxidation process causes strong reduction in the interaction between the oxide thin film and the Pd substrate, most significantly for Pd(111). Reversible oxidation/reduction cycling of TiOx thin films on Pd(111) and Pd(100) was possible.

Effective passivation of crystalline silicon surfaces by ultrathin atomic-layer-deposited TiOx layers

We characterize the surface passivation properties of ultrathin titanium oxide (TiOx) films deposited by atomic layer deposition (ALD) on crystalline silicon by means of carrier lifetime measurements. We compare different silicon surface treatments prior to TiOx deposition, such as native silicon oxide (SiOy), chemically grown SiOy and thermally grown SiOy. The best passivation quality is achieved with a native SiOy grown over 4 months and a TiOx layer thickness of 5 nm, resulting in an effective lifetime of 1.2 ms on 1.3 Ωcm p-type float-zone silicon. The measured maximum lifetime corresponds to an implied open-circuit voltage (iVoc) of 710 mV. For thinner TiOx layers the passivation quality is reduced, however, samples passivated with only 2 nm of TiOx still show a lifetime of 612 μs and an iVoc of 694 mV. The contact resistivity of the TiOx including the SiOy interlayer between the silicon wafer and the TiOx is below 0.8 Ωcm2. The combination of excellent surface passivation and low contact resistivity has the potential for silicon solar cells with efficiencies exceeding 26%.

Optical and morphological evolution of black TiO x synthesized in water by Nd:YAG laser

Optical and morphological properties of nanostructured TiOx film formed by 532 nm nanosecond pulsed laser irradiation in water is analysed. We observe three different surface features: blackening, surface pits and sub-surface bubbles. The appearance of these features is allowed once an irradiation fluence of 0.9 J cm−2 is used. Nevertheless, blackening, formation of pits and bubble are delayed until an accumulated fluence threshold of 7.5, 23 and 24 J cm−2 is achieved respectively. A fully evolved complex nano-porous structure is obtained at an accumulated fluence of 100 J cm−2. A possible mechanism of formation of these features was inferred.

Morphology study of nanoporous black TiOx synthesized by laser‐based method

In this work, we analyse and discuss the morphology evolution of black TiOx film formed on the surface of a titanium foil following the 1064 nm wavelength nanosecond pulsed laser irradiation in water. The nanostructure of the film depends on the number of laser pulses N and laser fluence F. We interpreted the experimental observations in terms of accumulated fluence FA, obtained as the product of N and F. We showed that the nanoporous morphology of this material occurs above a specific FA threshold of about 100 J/cm−2.

Low-Temperature Photochemical Conversion of Organometallic Precursor Layers to Titanium(IV) Oxide Thin Films

Thin layers of titanium(IV) ethoxide [Ti(OEt)4] as a metal–organic precursor were spin-coated onto silicon wafers under inert conditions and subsequently photochemically converted to thin titanium(IV) oxide (TiOx) films employing vacuum ultraviolet (VUV) radiation from a xenon excimer lamp. The photochemical conversion was performed below 35 °C and at ambient pressure in a nitrogen atmosphere with an optimized content of oxygen. Ti(OEt)4 decomposition and its kinetics were monitored and analyzed by gas chromatography and infrared spectroscopy. Precursor layers with a thickness between 270 and 1060 nm could be converted into much thinner TiOx films (40–165 nm). The decrease in thin film thickness was found to coincide with the removal of organic side chains and densification to a compact oxide network. For precursor layers with a thickness of up to 550 nm, VUV irradiation with a moderate radiant exposure (He) of 2.3 J cm–2 led to almost carbon-free amorphous layers with a composition close to stoichiometri...

Titanium oxide thin films obtained with physical and chemical vapour deposition methods for optical biosensing purposes

This work discusses an application of titanium oxide (TiOx) thin films deposited using physical (reactive magnetron sputtering, RMS) and chemical (atomic layer deposition, ALD) vapour deposition methods as a functional coating for label-free optical biosensors. The films were applied as a coating for two types of sensors based on the localised surface plasmon resonance (LSPR) of gold nanoparticles deposited on a glass plate and on a long-period grating (LPG) induced in an optical fibre. Optical and structural properties of the TiOx thin films were investigated and discussed. It has been found that deposition method has a significant influence on optical properties and composition of the films, but negligible impact on TiOx surface silanization effectiveness. A higher content of oxygen with lower Ti content in the ALD films leads to the formation of layers with higher refractive index and slightly higher extinction coefficient than for the RMS TiOx. Moreover, application of the TiOx film independently on deposition method enables not only for tuning of the spectral response of the investigated biosensors, but also in case of LSPR for enhancing the ability for biofunctionalization, i.e., TiOx film mechanically protects the nanoparticles and induces change in the biofunctionalization procedure to the one typical for oxides. TiOx coated LSPR and LPG sensors with refractive index sensitivity of close to 30 and 3400nm/RIU, respectively, were investigated. The ability for molecular recognition was evaluated with the well-known complex formation between avidin and biotin as a model system. The shift in resonance wavelength reached 3 and 13.2nm in case of LSPR and LPG sensors, respectively. Any modification in TiOx properties resulting from the biofunctionalization process can be also clearly detected.

Structural and Hydrophilic Characteristics of N-doped TiOx Films Deposited by RF-Magnetron Sputtering

Nitrogen doped titanium oxides (N-doped TiO x ) films were deposited by sputtering the TiO 2 target in N 2 /Ar gas mixture to control the doping amount of nitrogen accurately. In order to modify the resultant films, the samples were annealed in air in a temperature range of 300 ∼ 600 ° C, and then they were irradiated under visible-light (VIS) after being placed in dark. X-ray photoelectron spectroscopy (XPS) spectra show that N-Ti-O ( β -N) and hydroxyl form in the films due to nitrogen doped into TiO x lattice. And the content of hydroxyl increases with annealing temperatures increasing, which leads to the better hydrophilicity. X-ray diffraction (XRD) was used to investigate the crystallinity of the films annealed at different temperatures and the results indicate that the amorphous films transform into crystalline phase as a consequence of annealing. Scanning electron microscopy (SEM) results reveal that the particle size becomes bigger at the higher temperature. Hydrophilicity was tested by contact angle meter. The results show that the water contact angle decreases with raising heat treatment temperature due to the change of particle size and the content of −OH. Hydrophilicity is also influenced by the storing process, and the water contact angles increase with time. Furthermore, the hydrophilicity of N-doped TiO x films increases under visible light (VIS) irradiation.

IR emission and electrical conductivity of Nd/Nb-codoped TiOx (1.5 < x < 2) thin films grown by pulsed-laser deposition

The effect of the co-doping with Nd and Nb on electrical and optical properties of TiOx films is reported. The role of oxygen vacancies on the physical properties is also evidenced. The films are grown by pulsed-laser deposition onto (001) sapphire and (100) silicon substrates. The substrate temperature was fixed at 700°C. To obtain either stoichiometric (TiO2) or highly oxygen deficient (TiOx with x<1.6) thin films, the oxygen partial pressure was adjusted at 10−1 and 10−6mbar, respectively. 1%Nd-1%Nb, 1%Nd-5%Nb and 5%Nd-1%Nb co-doped TiO2 were used as bulk ceramic target. Composition, structural and morphological properties of films determined by Rutherford backscattering spectroscopy, X-ray diffraction and scanning electron microscopy, are correlated to their optical (UV–vis transmission and photoluminescence) and electrical properties (resistivity at room temperature). The most intense Nd3+ emission in the IR domain is obtained for stoichiometric films. Codoping Nd-TiOx films by Nb5+ ions is found to decrease the photoluminescence efficiency. The oxygen pressure during the growth allows to tune the optical and electrical properties: insulating and highly transparent (80% in the visible range) Nd/Nb codoped TiO2 films are obtained at high oxygen pressure, while conductive and absorbent films are grown under low oxygen pressure (10−6mbar).

Indium-free transparent TiOx/Ag/WO3 stacked composite electrode with improved moisture resistance

Amorphous TiOx, WO3 and Ag mid-layer films were deposited on polyethersulfone (PES) and soda-lime glass via electron beam evaporation at room temperature. The crystallinity, microstructure and surface morphology of asymmetric TiOx/Ag/WO3 multilayers (TAW) and symmetric WO3/Ag/WO3 (WAW) multilayers were investigated. The polygrains oriented along the (111) planes in the inserted Ag thin films were adopted to supply carriers into both of the dielectric layers and reduce the sheet resistance of the multilayer. The TAW multilayer with a sufficiently large Ag thickness (>15 nm) exhibited low resistance, and highly visible transmittance, low infra-red transmittance, to serve as a transparent conductive electrode and a good heat mirror. The dependence of the Ag and TiOx thickness on the optical and electrical properties of TAW multilayers was also explored. A figure of merit (FOM) was used to find an optimal layer structure for a TAW multilayer with superior conductivity and highly visible transparency. A high FOM of 1.2 × 10−1 (Ω−1) was achieved at the visible wavelength of 550 nm for a asymmetric TAW stacked layer on PES with a 15-nm-thick Ag layer, a 40-nm-thick TiOx and a 40 nm-thick WO3 layer. The moisture resistances of the TAW stacking layer showed more robustness than that of the WAW sample after damp heat exposure.

The appearance of Ti3+ states in solution-processed TiOx buffer layers in inverted organic photovoltaics

We study the low-temperature solution processed TiOx films and device structures using core level and valence X-ray photoelectron spectroscopy (XPS) and electronic structure calculations. We are able to correlate the fraction of Ti3+ present as obtained from Ti 2p core level XPS with the intensity of the defect states that appear within the band gap as observed with our valence XPS. Constructing an operating inverted organic photovoltaic (OPV) using the TiOx film as an electron selective contact may increase the fraction of Ti3+ present. We provide evidence that the number of charge carriers in TiOx can be significantly varied and this might influence the performance of inverted OPVs.

Black TiOx photocatalyst obtained by laser irradiation in water

In this work, we present the synthesis of a nanostructured TiOx film on a Ti foil by using a scalable and environmental friendly methodology such as the laser irradiation in liquid. The obtained black-TiOx film is composed by amorphous titanates and shows a high absorption in both visible and UV range. The introduction of a thin layer of Pt nanoparticles (PtNps) on the rear side of the Ti foil, gave rise to a stacked layered structure (TiOx/Ti/PtNps) that boosted the quantum photocatalytical efficiency, according to ISO 10678:2010. Such enhancement is discussed in terms of hole trapping by surface states and electron scavenging by the Pt Nps.

Atomic layer deposition synthesized TiOx thin films and their application as microbolometer active materials

This paper demonstrates the possible usage of TiOx thin films synthesized by atomic layer deposition as a microbolometer active material. Thin film electrical resistance is investigated as a function of thermal annealing. It is found that the temperature coefficient of resistance values can be controlled by coating/annealing processes, and the value as high as −9%/K near room temperature is obtained. The noise properties of TiOx films are characterized. It is shown that TiOx films grown by atomic layer deposition technique could have a significant potential to be used as a new active material for microbolometer-based applications.

Dual-functional Memory and Threshold Resistive Switching Based on the Push-Pull Mechanism of Oxygen Ions.

The combination of nonvolatile memory switching and volatile threshold switching functions of transition metal oxides in crossbar memory arrays is of great potential for replacing charge-based flash memory in very-large-scale integration. Here, we show that the resistive switching material structure, (amorphous TiOx)/(Ag nanoparticles)/(polycrystalline TiOx), fabricated on the textured-FTO substrate with ITO as the top electrode exhibits both the memory switching and threshold switching functions. When the device is used for resistive switching, it is forming-free for resistive memory applications with low operation voltage (<±1 V) and self-compliance to current up to 50 μA. When it is used for threshold switching, the low threshold current is beneficial for improving the device selectivity. The variation of oxygen distribution measured by energy dispersive X-ray spectroscopy and scanning transmission electron microscopy indicates the formation or rupture of conducting filaments in the device at different resistance states. It is therefore suggested that the push and pull actions of oxygen ions in the amorphous TiOx and polycrystalline TiOx films during the voltage sweep account for the memory switching and threshold switching properties in the device.

Comparative study of RF reactive magnetron sputtering and sol-gel deposition of UV induced superhydrophilic TiOx thin films

TiOx and TiOx-like thin films were deposited on PEEK (Polyether ether ketone) substrates by low-temperature RF reactive magnetron sputtering and the sol-gel method. The resulting films were compared in terms of their properties and photoinduced hydrophilicity. Both techniques resulted in uniform films with good adhesion that can be switched to superhydrophilic after exposure to UVA radiation for similar time periods. In addition, the sputtered films can also be activated and switched to superhydrophilic by natural sunlight due to the higher absorption in the visible spectrum compared to the sol-gel films. On the other hand, the as deposited sol-films remain relatively hydrophilic for a longer time in dark compared to the sputtered film due to the differences in the morphology and the porosity of the two materials. Thus, depending on the application, either method can be used in order to achieve the desirable TiOx properties.

Silicon heterojunction solar cells with electron selective TiOx contact

Silicon solar cells featuring carrier selective contacts have been demonstrated to reach ultra-high conversion efficiency. In this work, the electron-selective contact characteristics of ultrathin TiOx films deposited by atomic layer deposition on silicon are investigated via simultaneous consideration of the surface passivation quality and the contact resistivity. Thin TiOx films are demonstrated to provide not only good passivation to silicon surfaces, but also allow a relative low contact resistivity at the TiOx/Si heterojunction. A maximum implied open-circuit voltage (iVoc) of ~703mV is achieved with the passivation of a 4.5nm TiOx film, and a relatively low contact resistivity of (~0.25Ωcm2 is obtained at the TiOx/n-Si heterojunction simultaneously. N-type silicon solar cell with the champion efficiency of 20.5% is achieved by the implementation of a full-area electron-selective TiOx contacts. A simulated efficiency of up to 23.7% is achieved on the n-type solar cell with a full-area TiOx contact. The efficient, low cost electron-transporting/hole-blocking TiOx layer enables the fabrication of high efficiency silicon solar cells with a simplified process flow.

TiOx thin films grown on Pd(100) and Pd(111) by chemical vapor deposition

The growth of ultrathin TiOx (0≤x≤2) films on Pd(100) and Pd(111) surfaces by chemical vapor deposition (CVD), using Titanium(IV)isopropoxide (TTIP) as precursor, has been investigated by high resolution photoelectron spectroscopy, low energy electron diffraction and scanning tunneling microscopy. Three different TiOx phases and one Pd–Ti alloy phase have been identified for both surfaces. The Pd–Ti alloy phase is observed at the initial stages of film growth. Density functional theory (DFT) calculations for Pd(100) and Pd(111) suggest that Ti is alloyed into the second layer of the substrate. Increasing the TTIP dose yields a wetting layer comprising Ti2+ species (TiOx, x ∼0.75). On Pd(100), this phase exhibits a mixture of structures with (3×5) and (4×5) periodicity with respect to the Pd(100) substrate, while an incommensurate structure is formed on Pd(111). Most importantly, on both surfaces this phase consists of a zigzag pattern similar to observations on other reactive metal surfaces. Further increase in coverage results in growth of a fully oxidized (TiO2) phase on top of the partially oxidized layer. Preliminary investigations indicate that the fully oxidized phase on both Pd(100) and Pd(111) may be the TiO2(B) phase.

Transition-Metal Phthalocyanines on Transition-Metal Oxides: Iron and Cobalt Phthalocyanine on Epitaxial MnO and TiOx Films

The interaction at interfaces between transition-metal phthalocyanines and ultrathin transition-metal oxide films is studied by means of photoemission (PES) and X-ray absorption spectroscopy (XAS). Our results are compared to the recently investigated system CoPc on MnO. A flat-lying adsorption geometry of iron and cobalt phthalocyanines (FePc and CoPc) on the different oxide substrates was observed: ultrathin epitaxially grown MnO films and ultrathin TiOx films. A charge transfer from the MnO, in particular, to the Fe atom of the FePc molecule is observed by both PES and XAS. X-ray absorption spectra of the N K-edge of FePc do not hint at a nitrogen involvement in the interaction process. As a consequence of the charge transfer, a shift of the Fermi level of the semiconducting MnO films is observed, which is visible as a shift of MnO related core levels to lower binding energy. In contrast, CoPc and FePc deposited on TiOx show no hints for a charge transfer, although the flat-lying adsorption geometry al...

Absorbing TiOx thin film enabling laser welding of polyurethane membranes and polyamide fibers

We report on the optical properties of thin titanium suboxide (TiOx) films for applications in laser transmission welding of polymers. Non-absorbing fibers were coated with TiOx coatings by reactive magnetron sputtering. Plasma process parameters influencing the chemical composition and morphology of the deposited thin films were investigated in order to optimize their absorption properties. Optical absorption spectroscopy showed that the oxygen content of the TiOx coatings is the main parameter influencing the optical absorbance. Overtreatment (high power plasma input) of the fiber surface leads to high surface roughness and loss of mechanical stability of the fiber. The study shows that thin substoichiometric TiOx films enable the welding of very thin polyurethane membranes and polyamide fibers with improved adhesion properties.

Role of oxygen vacancies in TiOx films in electronic structure at interface with an α-NPD layer

Metal oxide buffer layers are a promising method for improving the efficiency and stability of organic photovoltaic cells and organic light-emitting diodes. To design organic electronic devices intelligently, it is necessary to understand the electronic structure at organic/metal-oxide interfaces. In this study, the electronic structure of films of a well-known hole-transporting material, N,N′-diphenyl-N,N′-bis(1-naphthyl)-1,1′-biphenyl-4,4′-diamine (α-NPD) on TiOx underlayers was studied via ultraviolet photoemission spectroscopy and inverse photoemission spectroscopy. New electronic states were found near the Fermi level (EF) upon adsorption of α-NPD when the TiOx underlayer was highly oxygen defective. The observed spectra were not fully explained by density functional theory (DFT) calculations for simple ionic α-NPD molecules. In addition, the vacuum level shift at the interfaces was also different from the shift predicted by several theoretical models that do not consider chemical reactions at the interface. These results suggest that the adsorbate–substrate interaction is complicated when a highly defective metal oxide is used. The stoichiometry of the metal oxide buffer layer may have a large effect on carrier injection or extraction properties at the interface.

A quality photocatalytic activity of an amorphous titanium oxide film achieved using the selectively photochemical etching and its application for the organic light emitting diode encapsulation

In this study, a rapid, cost-effective, and room-temperature achieved selectively photochemical etching (SPCE) process was developed to improve the photocatalytic activity of a PECVD-deposited amorphous titanium oxide (a-TiOx) film. The a-TiOx film treated by the SPCE process showed a photocatalytic activity comparable to that of a high-temperature prepared TiOx film. As examined from the surface roughness and morphologies, optical transmittance, and chemical bond configurations, the mechanism responsible for the significant enhancement on the photocatalytic activity was ascribed to the formation of the nano-textures with the increase in the surface acidity due to the incorporating of fluorine ions. The SPCE-treated a-TiOx film with the quality photocatalytic activity was then applied to encapsulate the blue organic light emitting diode (OLED) to result in the surface with self-cleaning function. A super-hydrophilic surface was available using the internal light irradiation. In addition, the OLED encapsulated by the SPCE-treated a-TiOx film also was beneficial for enhancing the light-extraction efficiency as a consequence of its notably nano-textured surface.