Rutile Thin Films Research Articles

Role of charge carriers for ferromagnetism in cobalt-doped rutile TiO2

Electric and magnetic properties of a high-temperature ferromagnetic oxide semiconductor, cobalt-doped rutile TiO2, are summarized. Cobalt-doped rutile TiO2 epitaxial thin films with different electron densities and cobalt contents were grown on r-sapphire substrates with laser molecular beam epitaxy. Results of magnetization, magnetic circular dichroism and anomalous Hall-effect measurements were examined for samples with systematically varied electron densities and cobalt contents. The samples with high electron densities and cobalt contents show high-temperature ferromagnetism, suggesting that charge carriers induce the ferromagnetism.

Growth of high quality rutile TiO 2 thin film using ZnO buffer layer on Si(100) substrate

The TiO 2 rutile thin films were grown on Si(100) substrates with ZnO buffer layer at a substrate temperature of 500 °C by radio frequency (rf) magnetron sputtering. In order to investigate the effect of buffer layer thickness on the growth of the TiO 2 film, the ZnO buffer layers were deposited in the thickness range of 70–150 nm. The thickness of the TiO 2 films was about 200 nm identical for all the samples. The crystal structure of the buffer layers and the TiO 2 thin films was characterized by X-ray diffractometer (XRD). The XRD spectra confirmed that TiO 2 rutile film with high crystalline quality was achieved on the ZnO buffer layer, which had a great relation to the improvement of the crystalline quality of the ZnO buffer layer. The surface morphologies of the TiO 2 rutile thin films were evaluated by atomic force microscopy (AFM). The roughness of the TiO 2 thin films became smoother as the thickness of the ZnO buffer layer increased.

Role of oxygen vacancies on ferromagnetism in Fe-doped TiO2 thin films

Fe-doped TiO2 rutile thin films were fabricated by reactive magnetron sputtering on SiO2 substrates under different growth conditions, and the structure and magnetism have been systematically investigated. X-ray diffraction data, atomic force microscopy, and magnetic force microscopy confirm the absence of metallic Fe clusters or any other impurity phase. The divalent Fe ion was identified by x-ray photoelectron spectra. Room-temperature ferromagnetism has been observed for all the FexTi1−xO2 (x=3%–15%) rutile films. However, the saturation magnetization (Ms) varied drastically for different pressures, doping concentrations, and annealing processes. The results show that ferromagnetism originates from the doped matrix rather than any type of magnetic cluster and the ferromagnetism is strongly correlated with the oxygen vacancies in TiO2.

The electrochemistry of transparent quantum size rutile nanowire thin films prepared by one-step low temperature chemical bath deposition

We performed a spectro- and photoelectrochemical study of electrodes consisting of oriented rutile TiO 2 nanowires with a diameter of ∼2 nm prepared directly by chemical bath deposition on conducting glass. A significant increase (around 0.25 eV with respect to bulk rutile) of the band gap energy for the nanowire film is observed and attributed to quantum size effects, pointing to the presence of individual monocrystalline nanowires with no significant electronic communication among them. This allows for the investigation of morphologically well defined electrodes in the two dimensional quantum confinement regime, which are characterized by particularly good photoelectrocatalytic and electrochromic properties.

Pulsed laser deposition of anatase and rutile TiO 2 thin films

The investigation deals with the preparation of both anatase and rutile thin films from a sintered rutile target of TiO 2 by pulsed laser ablation technique. Microstructural characterization of the sintered target was carried out using X-ray diffraction and AC impedance spectroscopy. Thin films of titania were deposited on (111) Si substrates at 673 K in the laser energy range 200–600 mJ/pulse at two different conditions: (i) deposition at 3.5 × 10 − 5 mbar of oxygen, and (ii) deposition at an oxygen partial pressure of 0.1 mbar. The influence of laser energy and oxygen addition on the film growth has been studied. X-ray diffraction analysis of the films indicated that the films are single phasic and nano crystalline. Titania films deposited in the energy range 200–600 mJ/pulse at a base pressure of 5 × 10 − 5 mbar are rutile with particle sizes in the range 5–10 nm, whereas the films formed at the oxygen partial pressure 0.1 mbar are anatase with particle sizes in the range 10–24 nm. In addition, at higher energies, a significant amount of particulates of titania are found on the surface of the films. The change in the microstructural features of the films as a function of laser energy and oxygen addition is discussed in relation with the interaction of the ablated species with the background gas.

Growth of TiO 2 thin film by reactive RF magnetron sputtering using oxygen radical

TiO 2 thin films were deposited on MgO substrates by reactive RF magnetron sputtering using oxygen radicals. In order to perform the low temperature crystallization and control of crystal structure, the TiO 2 thin film was irradiated by highly reactive oxygen radicals during the deposition. When the distance between the substrate and Ti-metal target (S–T distance) was 90 mm and substrate temperature were kept at 125 °C, the radical irradiated TiO 2 film exhibited a (1 1 0) orientation with rutile structure. The radical irradiated TiO 2 thin film prepared at 300 °C was changed from rutile to anatase structure by the adjustment of S–T distance. When the S–T distance was fixed at 110 mm, the TiO 2 film exhibited a high a-axis orientation with anatase structure. The TiO 2 rutile and anatase thin films consisted of small grains with very smooth surface.

Hetero-nanoscale Surface Structures of TiO2 and Their Applications to the Development of New Functional Devices

TiO2 is one of the most important oxides in surface science and its surface/interface properties have been intensively studied in view of photochemistry and catalysis. Furthermore, TiO2 has attracted a growing attention from the viewpoint of electronics applications today. The state of the art of oxide epitaxy now enables us to design the nano-scale TiO2-based heterostructures. Development of atomically flat TiO2 (rutile) single crystal substrates is a key to a good reproducible growth of rutile thin films by the layer-by-layer method. The resultant high-quality of the films can meet very well with the requirements for the studies of well-defined surface/interface properties of TiO2. In the present review, we would like to report new physics and chemistry in TiO2-based epitaxial heterostructures whose composition and structure are atomically well controlled.

Preparation of strain-included rutile titanium oxide thin films and influence of the strain upon optical properties

Rutile type titanium dioxide (TiO 2) thin films are prepared by pulse laser deposition (PLD) with controlled oxygen pressure. Transmission electron microscopy (TEM) analysis clarified that crystalline structures, surface structures and electric states of the TiO 2 films almost correspond to those of the bulk rutile. We observed crystalline domain including strain in the PLD film. The strain of the TiO 2 film was measured by nano-beam diffraction. The strain-included domains affected properties of the film as optical absorption. The obtained optical band gap energy value was 3.30 eV, which was larger than that of bulk. TEM results such as crystalline grain sizes and distribution were used to consider quantum size effect in order to explain the larger band gap value. Moreover the influence of strain in rutile crystalline grains upon optical properties was suggested in the present study. The difference of band gap energy between experimental and theoretically calculated ones was considered to come from the strain effects.

Influence of anatase-rutile phase transformation on dielectric properties of sol-gel derived TiO2 thin films

The phase transformation behavior and resulting dielectric properties of sol-gel derived TiO 2 thin films were investigated. Thin films showed a typical behavior of mixture systems during the phase transformation; a kinetic investigation on the isothermal curve of pre-crystallized thin films revealed that the phase transformation was a first-order reaction with an Avrami time component of 1. Dielectric constants of TiO 2 thin films increased with the increasing amount of the rutile phase while the dielectric losses showed the opposite relationship. From a fitting process using the parallel mixing rule, dielectric constants of two end members of the mixture system were calculated to be 41.4 and 145.2 for the pure anatase and rutile phase thin films, respectively.

Crystallographically orientated fcc Co nanocrystals in rutile TiO2 thin films

Cobalt nanocrystals dispersed in rutile thin films on (0001) α-Al2O3 substrates are grown by pulsed-laser deposition. Their microstructure is investigated by transmission electron microscopy and their magnetic properties measured at temperatures from 5to350K. The Co nanocrystals have fcc structure with no multi-twin defects and are crystallographically orientated with their (111) planes parallel to the substrate surface having an orientation relationship to TiO2 of (111)Co‖(100)rutile, and in-plane orientations of [112¯]Co‖[010]rutile and [1¯10]Co‖[001]rutile. The diameter of the Co nanocrystals is 4.4±0.15nm. The samples show superparamagnetic behavior at low temperatures. The zero-field-cooled magnetization versus temperature curve has a peak at about 105K. There is strong magnetic dipolar interaction between the particles which might have resulted in the observed hysteresis at room temperature.

A simple and low temperature process for super-hydrophilic rutile TiO2 thin films growth

We investigate an environmentally friendly aqueous solution system for rutile TiO2 violet color nanocrystalline thin films growth on ITO substrate at room temperature. Film shows considerable absorption in visible region with excitonic maxima at 434nm. X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), selected area electron diffraction (SAED), UV–vis, water surface contact angle and energy dispersive X-ray analysis (EDX) techniques in addition to actual photo-image that shows purely rutile phase of TiO2 with violet color, super-hydrophilic and densely packed nanometer-sized spherical grains of approximate diameter 3.15±0.4nm, characterize the films. Band gap energy of 4.61eV for direct transition was obtained for the rutile TiO2 films. Film surface shows super-hydrophilic behavior, as exhibited water contact angle was 7°. Strong visible absorption (not due to chlorine) leaves future challenge to use these films in extremely thin absorber (ETA) solar cells.

Structural and magnetic properties of cobalt-doped rutile thin films deposited on TiN–Si

Highly oriented Co-doped rutile TiO2 thin film on silicon has been achieved by using a TiN buffer layer. The direct deposition of TiO2:Co film on TiN–Si yields (110)-oriented film structure having two equivalent structural domains which are perpendicular to each other. Interestingly, no ferromagnetism is observed in this film structure down to 10K, the reason for which is attributed to the formation of cobalt silicide in the silicon. On the other hand, when the TiO2:Co film is deposited on TiO2, formed by in situ oxidation of TiN–Si, cobalt appears highly mobile leading to segregation at the TiO2–Si interface along with the formation of cobalt clusters in the TiO2:Co film region. This is responsible for the ferromagnetism in this case.

Cobalt valence states and origins of ferromagnetism in Co doped TiO2 rutile thin films

Co doped rutile thin films were fabricated on α-Al2O3 (10-12) substrates by laser molecular beam epitaxy. X-ray diffraction, scanning electron microscopy, and transmission electron microscopy indicated that the rutile thin films are (101) oriented and have smooth surfaces with no impurity phases. Co K-edge x-ray absorption near-edge structure, extended x-ray absorption fine structure, and x-ray photoelectron spectroscopy revealed the coexistence of different valence states of Co in the film. Magnetic circular dichroism studies suggest that the observed ferromagnetism is uniform and is related to the electron band structure of TiO2 rutile. Contribution of oxidized (Co2+) and metallic (Co0) cobalt to the ferromagnetism is discussed.

Microstructure characterisation of ALD-grown epitaxial SnO 2 thin films

The microstructures of epitaxial SnO 2 (rutile) thin films deposited by atomic layer deposition on α-Al 2O 3 (0 1 2) substrates at 600°C using either SnCl 4 or SnI 4 as tin precursor have been investigated by X-ray diffraction and transmission electron microscopy. It is shown that the film/substrate interface is flat without any visible additional phase, while the film structure and surface morphology depend on the metal precursor. Typical of the SnCl 4-process films is a rough surface and a high density of defects, including (0 1 1) and (1 0 1) twins and anti-phase boundaries (APBs). In contrast, the SnI 4-process films with their uniform thickness, flat surface and low density of APBs approach the perfect single crystal. Irrespective of the processing, all films have (1 0 1) texture, parallel to the substrate surface, with the epitaxial relationships [0 1 0] SnO 2 ∥ [1 0 0] α-Al 2 O 3 and [1 0 1 ̄ ] SnO 2 ∥[ 1 ̄ 2 ̄ 1] α-Al 2 O 3 .

Photo-induced surface charge separation of highly oriented TiO 2 anatase and rutile thin films

Surface charge separation behavior of photo-generated carriers in highly oriented TiO 2 anatase and rutile films was investigated using a technique in which the transient surface charge is observed by laser pulse irradiation without metal contacts and an externally applied field. According to the measurements, the quantum efficiency of photo-generated holes transported toward the surface was determined as a function of incident laser energy. The photo-generated holes in anatase can be transported toward the surface for irradiation at the photon energy of its bandgap. The holes transported toward the rutile surface, however, were generated close to the surface for irradiation at the photon energy much higher than its bandgap.

Preparation of anatase and rutile thin films by controlling oxygen partial pressure

Titanium dioxide (TiO 2) films on glass substrates were prepared by pulsed laser deposition (PLD) technique. The dependence of crystal structure, morphology and photocatalytic activity on oxygen partial pressure were investigated using X-ray diffraction (XRD), atomic force microscope (AFM) and UV–VIS spectrophotometer. By increasing oxygen partial pressure, the surface morphology changed from smooth to rough, and the crystal structure changed from rutile to anatase at fixed substrate temperature of 773 K. The change of the surface morphology and the formation of anatase structure by variation of oxygen partial pressure resulted in an increase of photocatalytic activity.

High-resolution transmission electron microscopy study of defects and interfaces in epitaxial TiO2films on sapphire and LaAlO3

Abstract TiO2 thin films grown on sapphire (α-Al2O3) (1102) and on LaAlO3 (001) at 800° C by pulsed-laser deposition have been characterized by transmission electron microscopy. The TiO2 thin films grown on LaAlO3 (001) crystallize in anatase, while those on sapphire (1102) crystallize in rutile. The epitaxial relationship between the anatase thin films and the LaAlO3 substrate is (004)[110]A//(001)[110]L, where the subscripts A and L indicate anatase and LaAlO3 respectively, and that between the rutile thin films and the sapphire substrate is (011)[111]R//(1102)[202 1]S, where the subscripts R and S indicate rutile and sapphire respectively. The rutile thin films contain a high density of growth twins with (101) and (011)T being the twin planes and [101] being the twinning direction, where the subscript T represents a twinning plane. The atomic structure of the twin boundary, the anatase-LaAlO3 and the rutile-sapphire interfaces were investigated by high-resolution transmission electron microscopy in com...

Transmission Electron Microscopy Study of Defect Structure in Epitaxial SnO2 Rutile Thin Film.

SnO2 thin film heteroepitaxially grown on a rutile (100) TiO2 single crystal substrate exhibits high-density interfacial misfit dislocations and related planar defects. The misfit dislocation network formed on the heterointerface consists mainly of two types of partial edge dislocations with Burgers vectors of 1/2[101] and 1/2[110], to fully relieve the lattice mismatch, and the dislocations inevitably involve inclined (101) and vertical (010) planar defects extending toward the film surface, respectively. Some of the (101) planar defects form wedge-shaped defects, and frequently accompany a single or multiply stacked (101) nanotwin lamellae. Another (011) type of planar defect originating at fused threading dislocations with 1/2[011] Burgers vectors is also detected by plan-view observation, showing the same crystallographic feature as (101) planar defects observed in the cross-sectional (010) projection. The dominant relaxation mechanism through partial edge dislocations suggests the low interfacial energy of the defect plane in the rutile structure, hindering the control of high crystallinity in rutile oxide thin films.

Effects of annealing and quenching treatments on reconstruction of rutile thin films on sapphire substrates

Morphology change in rutile TiO 2 thin films on sapphire substrates prepared by pulsed laser deposition under reduced oxygen environment was investigated as a function of film thickness, temperature and cooling treatments with atomic force microscopy, X-ray diffraction and scanning electron microscopy equipped with X-ray spectroscopy (SEM/EDX). The deposited TiO 2 was determined as epitaxially grown rutile films whose crystallographic correlation with substrates was (1 0 0) rutile//(0 0 0 1) sapphire. As increasing thickness of TiO 2 films, smooth surface changed to island structure. In addition, the morphology of TiO 2 film on α-Al 2O 3(0 0 0 1) varied drastically by annealing treatment from 973 to 1123 K. In case of ∼5 nm thickness films, morphology strongly depended on annealing and cooling treatments. We found interesting order structure of TiO 2 islands at annealing temperature (∼1073 K) and subsequent quenching (∼1.3 K/s). Formation process of TiO 2 particles on α-Al 2O 3(0 0 0 1) substrates is modeled based on instability of substrate at elevated temperatures.

Ion-beam modification of TiO 2 film to multilayered photocatalyst

We report a dry process to produce a multilayered TiO 2 film which has the rutile phase on an anatase substrate, for highly activated photocatalysis. Ar ion beam irradiation changes the anatase surface into rutile at 500°C, which is less than the crystallization temperature of rutile from anatase (600°C). The ion beam modification makes it possible to form rutile thin film on anatase. The multilayered structure should be a promising photocatalyst, theoretically.