Amorphous TiO2 Thin Films Research Articles

Photonic Band Gap and Bactericide Performance of Amorphous Sol-Gel Titania: An Alternative to Crystalline TiO₂.

In addition to its traditional application in white pigments, nanocrystalline titania (TiO2) has optoelectronic and photocatalytic properties (strongly dependent on crystallinity, particle size, and surface structure) that grant this naturally occurring oxide new technological applications. Sol-gel is one of the most widely used methods to synthesize TiO2 films and NPs, but the products obtained (mostly oxy-hydrated amorphous phases) require severe heat-treatments to promote crystallization, in which control over size and shape is difficult to achieve. In this work, we obtained new photocatalytic materials based on amorphous titania and measured their electronic band gap. Two case studies are reported that show the enormous potential of amorphous titania as bactericide or photocatalyst. In the first, amorphous sol-gel TiO2 thin films doped with N (TiO2−xNx, x = 0.75) were designed to exhibit a photonic band gap in the visible region. The identification of Ti-O-N and N-Ti-O bindings was achieved by XPS. The photonic band gaps were found to be 3.18 eV for a-TiO2 and 2.99 eV for N-doped a-TiO2. In the second study, amorphous titania and amine-functionalized amorphous titania nanoparticles were synthetized using a novel base-catalysed sol-gel methodology. All the synthesized amorphous TiO2 nanoparticles exhibit bactericide performance (E. coli, ASTME 2149-13).

Effect of anodic oxidation time on resistive switching memory behavior based on amorphous TiO2 thin films device

Resistance random access memory (RRAM) is a promising memory technology in the applications of memory device. Herein, the amorphous TiO2 thin film was grown onto titanium (Ti) foil by anodic oxidation. Further, the Ag/TiO2/Ti sandwich structure device was prepared, which displays a resistive switching memory effect with a high HRS/LRS resistance ratio with ∼27 at room temperature when the TiO2 film was oxidized ∼5 min. Finally, the formation/rupture models of Ag conductive filaments are suggested to explain the resistive switching memory behavior. This work open a new way for preparing the RRAM device for memory applications in the future.

Correlation between photoactivity of TiO2 and diffusion of Na+ ions from soda lime glass

In this study, we report the effects of Na+ diffusion into TiO2 during thermal annealing on TiO2 photocatalysis. After annealing at 500 °C for 1 h, the photoactivity of colloidal sol-based crystalline TiO2 is almost similar regardless of substrates. Interestingly, after the annealing the photoactivity of polymeric sol-based amorphous TiO2 thin film on soda lime glass (SLG) is significantly reduced compared to that on quartz. We observed that Na+ does not easily bond with TiOTi in crystalline TiO2, while large amount of TiONa bonds are formed in the amorphous TiO2 on SLG during crystallization at 500 °C. Our results suggest that the control of TiO2 photoactivity has applications in improving photo-stability of various optoelectronic devices.

Fabrication of TiO2 thin films with both anatase and rutile structures together using the ion-implantation method

The ion-implantation method can fabricate TiO2 thin films of anatase and rutile structures in an alternate sequence together so that a wide range of applications can be realized. By changing the implantation parameters, the thickness of anatase and rutile thin films can be modulated, and films with a thickness of nano-scale can be achieved. An amorphous TiO2 thin film with a thickness of 13nm was formed in a rutile TiO2 single crystal by low energy He+ ions with high fluences, and this amorphous phase transformed to an anatase and even a rutile phase with thermal treatments. SEM and AFM were used to observe the surface morphology of TiO2 thin film. TEM and Raman scattering techniques confirmed the phase transition process in TiO2 thin films with temperature increase. The design of a combination of TiO2 thin films with both anatase and rutile phases together using the ion-implantation method is also explained.

Density and size effects on the thermal conductivity of atomic layer deposited TiO2 and Al2O3 thin films

We report on the room temperature thermal conductivity of atomic layer deposition-grown amorphous TiO2 and Al2O3 thin films as a function of film thickness and atomic density. For films thinner than 50 nm, we measure an effective thermal conductivity that is reduced with decreasing film thickness. This dependence is attributed to the increased influence of thermal boundary resistances as film thickness is reduced. In addition, we fit for a thickness-independent intrinsic thermal conductivity using a series-resistor model. For films thicker than ∼50 nm, there is no significant dependence on thickness or substrate. We observe a dependence of the thermal conductivity on density, which agrees well with a differential effective-medium approximation modified minimum limit model.

Amorphous TiO2-Based Thin-Film Phototransistor

This letter presents amorphous TiO2-based thin-film phototransistors whose structures are designed for front-side and back-side illumination. The channel and gate dielectric layers are deposited by ultrasonic spray pyrolysis deposition, which is a non-vacuum, cost-effective, thin-film deposition technique similar to chemical vapor deposition. The material structure, chemical composition, and optical characteristics of the amorphous TiO2 thin film are analyzed. The phototransistor shows the typical current–voltage ( ${I}$ – ${V}$ ) characteristics of field-effect transistors. It demonstrates a UV-to-visible rejection ratio of approximately 70 –80, a high responsivity of 13.64 A/W, and a high detectivity of $1.64\times 10^{12}$ Jones. The phototransistor can be operated in the high-photo-responsivity and high-detectivity modes by biasing at different quiescent points.

Optical and electrical responses of magnetron-sputtered amorphous Nb-doped TiO2 thin films annealed at low temperature

Nb-doped TiO2 (TNO) thin films were prepared by annealing at 300°C for 30min after a magnetron-sputter process. A laser-irradiated post-annealing Raman scattering analysis indirectly showed the possible formation of small size anatase TNO clusters within the thin film matrix Although the TNO thin films were not crystallized, oxygen vacancies were created by adding H2 into the sputter gas during the deposition process. This improved the conductivity and carrier concentration of the thin films. As the ratio of H2 in sputter gas is f(H2) = [H2/Ar+H2] = 10%, the carrier concentration of the amorphous TNO thin film reached 1022 (cm−3) with the resistivity being about 10−2 (Ω.cm). Even though a new methodology to decrease the fabrication temperature is not presented; this study demonstrates an efficient approach to shorten the annealing process, which ends prior to the crystallization of the thin films. Besides, in situ H2 addition into the sputter atmosphere is proven to be a good solution to enhance the electrical conductivity of semiconductor thin films like TNOs, despite the fact that they are not well crystallized.

Variation in the density, optical polarizabilities, and crystallinity of TiO2 thin films deposited via atomic layer deposition from 38 to 150 °C using the titanium tetrachloride-water reaction

Refractive index, density, and optical polarizabilities as a function of atomic layer deposition (ALD) growth temperature for titanium dioxide (TiO2) thin films are reported for the first time between 38 and 150 °C using the titanium tetrachloride-water reaction. Consistent with prior reports, Raman spectroscopy and x-ray diffraction indicate that the films are amorphous below 150 °C and form the crystalline anatase phase at deposition temperatures at or above 150 °C. Despite this change in atomic structure, mass densities of the TiO2 films are found to increase smoothly between 38 and 125 °C (3.25–3.68 g cm−3), as measured by both x-ray reflectometry (XRR) and optical ellipsometry. Molecular polarizabilities were calculated from the XRR mass densities, and ellipsometric refractive indices and were found to vary from 5.26–5.83 × 10−24 cm−3, which is consistent with prior reports of chemical vapor deposition of amorphous TiO2 thin films. Here, the authors demonstrate that the greater precision of ALD provides better control over the atomic structure and molecular polarizability of amorphous TiO2 thin films than other growth approaches, providing new opportunities for precisely studying the structure of amorphous oxides.

Molecular dynamics study of the effect of substrate temperature and Ar ion assisted deposition on the deposition of amorphous TiO2 films

This paper has investigated the impact of the substrate temperature and Ar ion assisted deposition on the surface structure formation mechanism and the film properties during the amorphous TiO2 thin film deposition process with the molecular dynamics simulation method. The results show that the reduction of the surface roughness happen when the energy of Ar ions assisted is increased or the substrate temperature rises, and also the film density on surface is changed with the increasing of Ar ions energy and substrate temperature. It is also found that the Volmer-Weber (island) growth mode of films is promoted by the lower Ar ion energy and higher substrate temperature when the substrate has an island structure. The assisted Ar ion has power of making a flat surface and increasing the local temperature. Besides, it will influence the growth mode with the change of surface atom mobility. With a high assisted Ar ion energy the Volmer-Weber (island) growth mode is inhibited, which will be conducive to the formation of more smooth film surface.

Coexistence of bipolar and unipolar resistive switching characteristics of thin TiO2 film grown on Cu foil substrate for flexible nonvolatile memory device

This work reports the coexistence of bipolar and unipolar resistive switching behaviors in the flexible Ni/TiO2/Cu device. The amorphous TiO2 thin film was deposited by radio frequency magnetron sputtering method on flexible copper (Cu) foil substrate at room temperature. The resistive switching behaviors of the flexible device were slightly degraded upon different bending states, indicating the huge potential applications in the flexible electronics. The switching mechanism based on conducting filaments is proposed to explain the resistive switching behaviors.

Optical constants of e-beam evaporated titanium dioxide thin films in the 25.5- to 612-eV energy region

The optical constants of titanium dioxide (TiO2) have been experimentally determined at energies in the extreme ultraviolet and soft x-ray spectral regions, from 25.5 to 612 eV. Measuring angle-dependent reflectance of amorphous TiO2 thin films with synchrotron radiation at the BEAR beamline of Synchrotron ELETTRA. The experimental reflectivity profiles were fitted to the Fresnel equations using a genetic algorithm applied to a least-square curve fitting method, obtaining values for δ and β. We compared our measurements with tabulated data. All samples were grown on Si (100) substrates by the electron-beam evaporation technique, with a substrate temperature of 150°C and deposition rates of 0.3 to 0.5 Å/s. Complete films characterization have been carried out with structural (XRD, ellipsometry, and profilometry), compositional (x-ray photoelectron spectroscopy), and morphological (atomic force microscopy) analyses.

Electron-Selective TiO2 Contact for Cu(In,Ga)Se2 Solar Cells.

The non-toxic and wide bandgap material TiO2 is explored as an n-type buffer layer on p-type Cu(In,Ga)Se2 (CIGS) absorber layer for thin film solar cells. The amorphous TiO2 thin film deposited by atomic layer deposition process at low temperatures shows conformal coverage on the CIGS absorber layer. Solar cells from non-vacuum deposited CIGS absorbers with TiO2 buffer layer result in a high short-circuit current density of 38.9 mA/cm2 as compared to 36.9 mA/cm2 measured in the reference cell with CdS buffer layer, without compromising open-circuit voltage. The significant photocurrent gain, mainly in the UV part of the spectrum, can be attributed to the low parasitic absorption loss in the ultrathin TiO2 layer (~10 nm) with a larger bandgap of 3.4 eV compared to 2.4 eV of the traditionally used CdS. Overall the solar cell conversion efficiency was improved from 9.5% to 9.9% by substituting the CdS by TiO2 on an active cell area of 10.5 mm2. Optimized TiO2/CIGS solar cells show excellent long-term stability. The results imply that TiO2 is a promising buffer layer material for CIGS solar cells, avoiding the toxic CdS buffer layer with added performance advantage.

Synthesis and Characterization of Nb-Doped TiO<sub>2</sub> Thin Films Prepared by RF Magnetron Sputtering

Amorphous Nb-doped TiO2 thin films were deposited on (100) Si and glass substrates at room temperature by RF magnetron sputtering and a mosaic-type Nb2O5-TiO2 sputtering target. To adjust the amount of the niobium dopant in the film samples, appropriate numbers of Nb2O5 pellets were placed on the circular area of the magnetron target with intensive sputtering. By adjusting the discharge conditions and the number of niobium oxide pellets, films with dopant content varying between 0 and 16.2 at.% were prepared, as demonstrated by X-ray photoelectron spectroscopy data. The X-ray diffraction patterns of the as-deposited samples showed the lack of crystalline ordering in the samples. Surfaces roughness and energy band gap values increase with dopant concentration, as showed by atomic force microscopy and UV-Vis spectroscopy measurements.

Atomic layer deposition derived amorphous TiO2 thin film decorating graphene nanosheets with superior rate capability

Composites consisting of graphene nanosheets (GNSs) decorated with amorphous TiO2 thin films were synthesized by an atomic layer deposition (ALD) technique. It was revealed that the TiO2 thin films were uniformly deposited onto the worm-like GNSs. When used as anode materials in lithium ion batteries (LIBs), the TiO2–GNS composites deliver a stable capacity of ~ 140 mAh g− 1 after 100 cycles at a specific current of 100 mA g− 1 as well as superior rate capability, accounting for a sustainable 95 mAh g− 1 capacity at a specific current of 1200 mA g− 1. It is believed that the remarkable electrochemical performance lied in the unique features of the composites, i.e., the amorphous nature of the TiO2 films and the large surface area of GNSs. The former rendered short Li+ diffusion pathways in TiO2 thin film, while the latter offered excellent electronic conductivity as well as a large intimate contact area between the electrolyte and TiO2. This work laid a venue for designing new electrodes for high-performance LIBs.

Rapid hydrogenation of amorphous TiO2 to produce efficient H-doped anatase for photocatalytic water splitting

Hydrogen-doped anatase titanium dioxide (H:TiO2) thin films were synthesized by a mild annealing in hydrogen atmosphere of amorphous TiO2 thin films deposited on ITO by RF-magnetron sputtering. Two kinds of TiO2 electrodes (amorphous and anatase) were tested for the H-doping in different annealing conditions, but H-doped anatase was obtained exclusively by a rapid heat treatment in hydrogen atmosphere of amorphous TiO2 at 300°C. Instead, anatase thin films required enforced annealing conditions to incorporate H, leading to an undesirable conversion to rutile. XPS analysis reveals that in H-doped anatase thin films H atoms are bonded to O. DFT calculations, performed to investigate the density of states, predict a slight band-gap narrowing, in agreement with spectroscopic evidence. Photoelectrochemical measurements on H:TiO2 showed an elevated current density of 0.99mA/cm2 at 1.23V vs. RHE in oxygen evolution reaction, revealing better performance than pure anatase TiO2. This enhancement in catalytic activity is accounted to the effect of dangling bonds passivation performed by highly reactive H atoms, which reduce the density of carrier recombination centers. The used method for the conversion of amorphous TiO2 thin film in H:doped anatase TiO2 can be extended to amorphous TiO2 prepared by various techniques.

Molecular dynamics study of the effect of titanium ion energy on surface structure during the amorphous TiO2 films deposition

This paper has investigated the impact of the incident titanium ion energy on the surface structure formation mechanism and the film properties during the amorphous TiO2 thin film deposition process with the molecular dynamics simulation method. The results show that the increase of film density and the reduction of the surface roughness happen when the energy of incident titanium ions is increased. It is also found that the film growth mode is the Volmer-Weber (island) growth mode under the influence of the surface potential. The effect of surface potential is more significant if the incident ions energy becomes smaller. That will make the Volmer-Weber (island) growth mode promoted and the surface roughness of the deposited films become larger. Conversely, the Volmer-Weber growth mode is inhibited when the incident ion energy is higher. It will be conducive to the formation of more smooth film surface.

Fabrication of porous and amorphous TiO2 thin films on flexible textile substrates

Radio Frequency (RF) magnetron sputtered TiO2 thin films have been deposited on flexible textile substrates such as polyester and cotton fabrics, yielding a flexible composite architecture. The structure of the composites was systematically investigated by Scanning Electron Microscopy (SEM), X-ray Diffraction (XRD), and Contact Angle tester. The deposited TiO2 thin films featured unique high porosity and amorphous structure which combined with the flexibility of the textile base show promise for photovoltaic applications.

Physical and optical waveguiding properties of sol-gel deposited nano-structured TiO<SUB align="right">2 thin films: a study on the effect of synthesis parameters

In this paper, amorphous and polycrystalline nano-structured TiO2 thin films were prepared by sol-gel dip-coating process. The effects of different fabrication parameters (e.g., withdrawal speed, annealing temperature) on the structural, morphological and optical properties of the synthesised films were investigated by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), atomic force microscopy (AFM) and UV-visible Spectrophotometry. Waveguiding properties such as propagating modes and optical loss were measured at 632.8 nm by M-lines spectroscopy (MLS). The results indicate that all thin films annealed at 500oC exhibit XRD patterns and FTIR data consistent with the TiO2 anatase phase. AFM images have revealed that morphology and surface roughness of deposited thin films depend on withdrawal speed and heat treatment temperature. The UV-visible transmittance results show that all thin films were transparent with an average transmittance higher than 70% in the visible region. The results have also demonstrated from MLS measurements that TiO2 thin film slab waveguides deposited at withdrawal speed of 2 cm/min and annealed at 500°C exhibit not only better light confinement but also lower optical loss than those dip-coated at the speed of 1 and 3 cm/min.

Amorphous Ultrathin TiO2 Atomic Layer Deposition Films on Carbon Nanotubes as Anodes for Lithium Ion Batteries

Amorphous TiO2 (a-TiO2) thin films were conformally coated onto the surface of hydroxyl functionalized multi-walled carbon nanotubes (CNTs) using atomic layer deposition (ALD). The electrochemical characteristics of the a-TiO2/CNT nanocomposites were then determined using cyclic voltammetry and galvanostatic charge/discharge curves. The ultrathin TiO2 ALD films displayed high specific capacity and high rate capability. The specific capacities of the a-TiO2/CNT nanocomposites after 50 and 100 TiO2 ALD cycles at 100 mA/g were 220 mAh/g and 240 mAh/g, respectively. For CNTs coated with 100 TiO2 ALD cycles, 88% of the capacity at 100 mA/g could be maintained at 1 A/g. When the voltage window for the a-TiO2/CNT nanocomposites was extended down to 0.5 V versus Li/Li+, the CNTs coated with 50 and 100 TiO2 ALD cycles exhibited specific capacities at 100 mA/g of 275 mAh/g and 312 mAh/g, respectively. These high capacities are higher than the bulk theoretical values and are attributed to additional interfacial charge storage resulting from the high surface area of the a-TiO2/CNT nanocomposites. Free-standing TiO2-CNT electrodes were also fabricated and displayed excellent capacity and rate capability. These results demonstrate that TiO2 ALD on high surface area CNT substrates can provide high power and high capacity anodes for lithium ion batteries.

Excellent resistive switching property and physical mechanism of amorphous TiO2 thin films fabricated by a low-temperature photochemical solution deposition method

Abstract High-performance resistive switching Pt/TiO2/Pt memory cells were fabricated. The amorphous TiO2 active layer was prepared by using a low-temperature photochemical solution deposition method—a simple preparation process combining first chemical solution deposition of the TiO2 film layer and subsequent ultraviolet (UV) irradiation treatment. The obtained Pt/TiO2/Pt memory cells exhibited excellent resistive switching parameters, such as centralized distribution of set and reset voltages, stable current values at high and low resistance states, and long retention time. The conductive mechanisms of high resistance state and low resistance state were Schottky emission and Ohmic conduction, respectively. The X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy, and scanning electron microscopy characterization of the TiO2 thin films indicated that the UV irradiation treatment can lead to decomposition of the residual organics and the formation of enhanced metal-oxide bonds in the thin films. On the basis of the analysis of current–voltage characteristics and the temperature dependence of resistance, we explained the resistive switching phenomenon for Pt/TiO2/Pt devices by using the model of formation/rupture of conductive filaments. Our study also suggested that the simple photochemical solution deposition method can be used for preparing some other oxide thin films with good resistive switching properties at low processing temperature which is promising to be extended to flexible resistive switching devices.