Influence of nanocrystalline structure and composition on hardness of thin films based on TiO<Subscript>2</Subscript>

In this work, the influence of Tb-doping on structure, and especially hardness of nanocrystalline TiO₂ thin films, has been described. Thin films were formed by a high-energy reactive magnetron sputtering process in a pure oxygen atmosphere. Undoped TiO₂-matrix and TiO₂:Tb (2 at. % and 2.6 at. %) thin films, had rutile structure with crystallite sizes below 10 nm. The high-energy process produces nanocrystalline, homogenous films with a dense and close packed structure, that were confirmed by X-ray diffraction patterns and micrographs from a scanning electron microscope. Investigation of thin film hardness was performed with the aid of a nanoindentation technique. Results of measurements have shown that the hardness of all manufactured nanocrystalline films is above 10 GPa. In the case of undoped TiO₂ matrix, the highest hardness value was obtained (14.3 GPa), while doping with terbium results in hardness decreasing down to 12.7 GPa and 10.8 GPa for TiO₂:(2 at. % Tb) and TiO₂:(2.6 at. % Tb) thin films, respectively. Incorporation of terbium into TiO₂-matrix also allows modification of the elastic properties of the films.     

Physical properties of Ba<Subscript>0.8</Subscript>Sr<Subscript>0.2</Subscript>TiO<Subscript>3</Subscript> thin films

The behavior of material constants in ferroelectric Ba_0.8Sr_0.2TiO₃ thin films is studied depending on the misfit strain at room temperature in the context of nonlinear thermodynamic potential of the phenomenological theory. Some constants are found to undergo drastic changes with the alternating strain at the interfaces. The gathered results allow one to evaluate the material constants for a specific film and to outline the direction in searching the ways to synthesize films with the needed properties.     

Magnetism and optical properties of nanocrystalline Cu<Subscript>2</Subscript>O and TiO<Subscript>2</Subscript> powders

The optical spectra of Cu₂O and TiO₂ nanopowders have been studied, which contain information about structural defects and are of interest in the search for optimum regimes providing the synthesis of ferromagnetic nanocrystals with Curie temperatures above room temperature.     

Preparation and optical properties of nanocrystalline thin films in the ZnO-TiO<Subscript>2</Subscript> system

Nanocrystalline compound thin films of ZnO-TiO₂ with different Zn/Ti atomic ratios were prepared by radio frequency magnetron reactive sputtering. The optical constants and the optical band gap were investigated using spectroscopic ellipsometry and the optical absorption spectrum. It was found that the cubic ZnTiO₃ phase can be obtained with the atomic ratio of Zn to Ti of about 1:1, and transforms to rhombohedral ZnTiO₃ phase and a phase mixture of rhombohedral ZnTiO₃ and ZnO with increasing Zn content. The refractive index decreases with the increase of Zn content, and the extinction coefficient in the visible range is near zero. The optical band gap was derived from the modeling of ellipsometry data and extinction coefficient spectra, and compared with that obtained from optical absorption spectrum, and it was found that the optical band gaps obtained by these three methods are consistent with each other. PACS 42.70.-a; 68.55.Jk; 78.20.Ci; 81.15.Cd     

Optoelectronics properties of TiO<Subscript>2</Subscript>:Cu thin films obtained by sol gel method

In this research, Cu-doped TiO₂ thin films have been successfully deposited onto a glass substrate by Sol–gel technique using dip coating method. The films were annealed at different annealing temperatures (400–500 °C) for 1 h. The structural, optical and electrical properties of the films were investigated and compared using X-ray Diffraction, UV–visible spectrophotometer and 4-point probe method. Optical analysis by mean transmittance T(λ) and absorption A(λ) measurements in the wavelength range between 300 to 800 nm allow us to determine the indirect band gap energy. DRX analysis of our thin films of TiO₂:Cu shows that the intensities of the line characteristic of anatase phase increasing in function of the temperature.     

Photoconductivity studies on amorphous and crystalline TiO<Subscript>2</Subscript> films doped with gold nanoparticles

In this work, amorphous and crystalline TiO₂ films were synthesized by the sol–gel process at room temperature. The TiO₂ films were doped with gold nanoparticles. The films were spin-coated on glass wafers. The crystalline samples were annealed at 100°C for 30 minutes and sintered at 520°C for 2 h. All films were characterized using X-ray diffraction, transmission electronic microscopy and UV-Vis absorption spectroscopy. Two crystalline phases, anatase and rutile, were formed in the matrix TiO₂ and TiO₂/Au. An absorption peak was located at 570 nm (amorphous) and 645 nm (anatase). Photoconductivity studies were performed on these films. The experimental data were fitted with straight lines at darkness and under illumination at 515 nm and 645 nm. This indicates an ohmic behavior. Crystalline TiO₂/Au films are more photoconductive than the amorphous ones.     

Optical and electrical properties of nanocrystalline TiO<Subscript>2</Subscript>:Pd semiconducting oxides

Electrical and optical properties of TiO₂:Pd thin films deposited from Ti-Pd mosaic targets sputtered in reactive oxygen plasma have been studied. The properties were investigated for thin films with the Pd amount of 5.5 at. %, 8.4 at. % and 23 at. %. Based on resistivity measurements a drop from 10³ down to almost 10⁻³Ωcm has been recorded when the Pd amount was varied from 5.5 at. % to 23 at. %, respectively. Moreover, it was shown that doping with different amounts of Pd results in the possibility of obtaining both types of electrical conduction: n-type for the TiO₂ with 5.5 at. % and 8.4 at. % of Pd and p-type for the TiO₂ with 23 at. % of Pd thin films. From optical measurements it has been found that as the Pd amount was increased the transmission through the thin films was reduced and position of the fundamental absorption edge was shifted toward a longer wavelength range of up to 600 nm. The optical band gap was calculated for direct and indirect transitions from optical absorption spectra. Structural properties were characterized by X-ray diffraction (XRD) and atomic force microscopy (AFM). The XRD patterns displayed occurrence of the crystalline, TiO₂-rutile for lower Pd amounts (5.5 at. %, 8.4 at. %), while the TiO₂:Pd (23 at. %) thin films displayed XRD-amorphous behaviour. Images obtained from AFM displayed dense, nanocrystalline structure with homogenous distribution of crystallites. Additionally performed secondary ion mass spectroscopy investigation confirmed homogenous distribution of Pd in the whole thickness of the prepared thin films.     

Anomalous magnetism of the nanocrystalline oxide TiO<Subscript>2</Subscript> surface

The magnetic properties of an oxygen-deficient nanocrystalline undoped titanium dioxide synthesized by the gas-phase, electric-explosion, and chemical method have been studied. The defect state was controlled using reduction treatments in vacuum or in a hydrogen atmosphere. It is shown that the defect state of the surface of nanocrystalline oxides (for example, the existence of vacancies in the anion sublattice and other defects) has a dominant influence on the formation of the magnetic properties of the samples under study. The main contributions to the magnetism of TiO₂ nanoparticles after the reduction treatments are the paramagnetic contribution of the matrix, the paramagnetic Curie–Weiss contribution, and the contribution of the spontaneous magnetic moment provided by the existence of regions with different spin ordering. A heterogeneous magnetic state is found to exist in the TiO₂ nanopowders; for example, at low temperatures, shifted hysteresis loops are observed as a result of a possible set of magnetic states with different spin orders. It is shown that a soft compaction or grinding of nanopowders in an agate mortar lead to substantial increase in the magnetization, sometimes, by a factor of more than two, regardless of the nanopowder synthesis method and the initial phase state of TiO₂ (anatase or rutile structures). This experimental fact proves the key role of the surface defects and the magnetic moment carriers with different spin configurations localized mainly on the nanoparticle surface. The compaction changes the magnetization only in the case when the initial magnetic state has a nonlinear “quasi-superparamagnetic” character of the magnetization curve. As a result of predominant exchange interaction between the nanoparticles with a frustrated character of spin ordering on the nanoparticles surface, the ferromagnetic contribution increases as nanoparticles contact.     

Spray pyrolysis-deposited TiO<Subscript>2</Subscript> thin films as high-performance lithium ion battery anodes

Focusing on additive-free electrodes, thin films are of typical interest as electrodes for lithium ion battery application. Herein, we report the fabrication of TiO₂ thin films by spray pyrolysis deposition technique. X-ray diffraction and transmission electron microscopic analysis confirms the formation of anatase TiO₂. Electrochemical evaluation of these sub-micron TiO₂ thin films exhibits high-rate performance and long cycling stability. At 1C rate (1C?=?335 mA/g), the electrode delivered discharge capacity of 247 mAh/g allowing about 0.74 lithium into the structure. The electrodes also delivered specific capacities of 122 and 72 mAh/g at 10 and 30C rates, respectively. Without conductive additives, this excellent performance can be attributed to the nanosize effect of TiO₂ particles combined with the uniform porous architecture of the electrode. Upon cycling at high rates (10 and 30C), the electrode exhibited excellent cycling stability and retention, specifically only <?0.6% capacity loss per cycle over 2500 cycles.     

Low-macroscopic field emission from nanocrystalline Al doped SnO<Subscript>2</Subscript> thin films synthesized by sol–gel technique

We have observed low-macroscopic field electron emission from wide bandgap nanocrystalline Al doped SnO₂ thin films deposited on glass substrates. The emission properties have been studied for different anode-sample spacings and for different Al concentrations in the films. The turn-on field and approximate work function were calculated and we have tried to explain the emission mechanism from this. The turn-on field was found to vary in the range 5.6–7.5 V/μm for a variation of anode sample spacing from 80–120 μm. The turn-on field was also found to vary from 4.6–5.68 V/μm for a fixed anode-sample separation of 80 μm with a variation of Al concentration in the films 8.16–2.31%. The Al concentrations in the films have been measured by energy dispersive X-ray analysis. Optical transmittance measurement of the films showed a high transparency with a direct bandgap ∼3.98 eV. Due to the wide bandgap, the electron affinity of the film decreased. This, along with the nanocrystalline nature of the films, enhanced the field emission properties. PACS 81.20.Fw; 61.10.-i; 79.70.+q     

Synthesis and nanostructural investigation of TiO<Subscript>2</Subscript> nanorods doped by SiO<Subscript>2</Subscript>

TiO₂ Nano rods can be used as dye-sensitized solar cells, various sensors and photocatalysts. These nanorods are synthesized by a hydrothermal corrosion process in NaOH solution at 200°C using TiO₂ powder as the source material. In the present work, the synthesis of TiO₂ nanorods in anatase, rutile and Ti₇O₁₃ phases and synthesis of TiO₂ nanorods by incorporating SiO₂ dopant, using the sol–gel method and alkaline corrosion are reported. The morphologies and crystal structures of the TiO₂ nanorods are characterized using field emission scanning electron microscopy (FE-SEM), atomic force microscopy (AFM) and X-ray diffraction (XRD) study. The obtained results show not only an aggregation structure at high calcination temperatures with spherical particles but also Ti–O–Si bonds having four-fold coordination with oxygen in SiO^4 −.     

Effects of thickness on the infrared optical properties of Ba<Subscript>0.9</Subscript>Sr<Subscript>0.1</Subscript>TiO<Subscript>3</Subscript> ferroelectric thin films

Using infrared spectroscopic ellipsometry (IRSE), the optical properties of the Ba_0.9Sr_0.1TiO₃ (BST) ferroelectric thin films with different film thicknesses on Pt/Ti/SiO₂/Si substrates prepared by a modified sol-gel method have been investigated in the 2.5–12.6 m wavelength range. By fitting the measured ellipsometric parameter ( and ) data with a three-phase model (Air/BST/Pt) and the classical dispersion relation for the BST thin films, the optical constants and thicknesses of the thin films have been obtained. The average thickness of the single layer decreases with increasing film thickness. The refractive index of the BST films decreases with increasing thickness in the wavelength range 2.5–11 m, and increases with increasing thickness in the wavelength range 11–12.6 m. However, the extinction coefficient of the BST films monotonously decreases with increasing thickness. It is closely associated with the crystallinity of the thin films, the crystalline size effect and the influence of the interface layer. The absorption coefficient of the BST films with different thicknesses decreases with increasing thickness. PACS 77.55.+f; 78.20.Ci; 78.30.Am; 81.70.Fy; 81.40.Tv     

Nonlinear dielectric response of epitaxial Ba<Subscript>0.6</Subscript>Sr<Subscript>0.4</Subscript> TiO<Subscript> 3</Subscript> thin films

Based on Landau-Devonshire (LD)-type phenomenological thermodynamic theory, the electric field dependence of the dielectric properties of tetragonal single-domain barium strontium titanate(Ba_1-xSr_xTiO₃) films on cubic substrates is theoretically investigated by taking into account the high order terms of the polarization. At room temperature, the nonlinear dielectric responses of epitaxial Ba_0.6Sr_0.4TiO₃ films are provided by adjusting the film thickness and growth temperature. The strong nonlinearity of relative dielectric constant and pyroelectric coefficient are attained around critical film thickness on MgO (69 nm) and LaAlO₃ (132 nm) substrates or critical growth temperature on MgO (337 °C) substrate with respect to epitaxy-induced lattice misfit and thermal stresses during deposition. This can be explained that small compressive stresses are effective to support high nonlinearity of dielectric constant and pyroelectric coefficient for Ba_0.6Sr_0.4TiO₃ films irrespective of whether they are on compressive substrate or tensile substrate. It is also predicted that a large tunability may be achieved by altering processing conditions, such as the film thickness and growth temperature for different substrates. Our theoretical results are in good agreement with the experimental data reported in literature.     

Structural properties of amorphous TiO<Subscript>2</Subscript> nanoparticles

Structural properties of amorphous TiO₂ spherical nanoparticles have been studied in models with different sizes of 2 nm, 3 nm, 4 nm and 5 nm under non-periodic boundary conditions. We use the pairwise interatomic potentials proposed by Matsui and Akaogi. Models have been obtained by cooling from the melt via molecular dynamics (MD) simulation. Structural properties of an amorphous nanoparticle obtained at 350 K have been analyzed in detail through the partial radial distribution functions (PRDFs), coordination number distributions, bond-angle distributions and interatomic distances. Moreover, we show the radial density profile in a nanoparticle. Calculations show that size effects on structure of a model are significant and that if the size is larger than 3 nm, amorphous TiO₂ nanoparticles have a distorted octahedral network structure with the mean coordination number Z_Ti-O ≈6.0 and Z_O-Ti ≈3.0 like those observed in the bulk. Surface structure and surface energy of nanoparticles have been obtained and presented.     

Influence of substrate temperature on certain physical properties and antibacterial activity of nanocrystalline Ag-doped In<Subscript>2</Subscript>O<Subscript>3</Subscript> thin films

Nanocrystalline Ag-doped indium oxide (AIO) thin films, by employing a much simplified spray pyrolysis technique in different substrate temperatures (300, 350, 400 and 450°C), were fabricated for the first time. The deposited films were subjected to various characterization studies, to explore certain features like the influence of various deposition temperatures on physical and antibacterial properties. XRD results showed that all the samples exhibited preferential orientation along the (2 2 2) plane. The variation in the crystalline size with increasing substrate temperature was explained on the basis of the Zener pinning effect. The electrical sheet resistance (R _sh) was found to decrease sharply with increasing substrate temperature and attained a minimum value \((62{\Omega } /\Box \)) at 400°C and then started increasing for higher deposition temperatures. Further, PL emission spectra of the samples in the visible range ascertained the possibility of applicability of the same in nanoscale optoelectronic devices. From the studies, it was found that at 400°C deposition temperature, one could expect better antibacterial efficiency against Escherichia coli. The influence of the shape and size of AIO nanograins on the antibacterial activity was analysed using scanning electron microscopy images.     

Dielectric properties of thin PbTiO<Subscript>3</Subscript> films

The electrophysical properties and phase composition of thin lead titanate films prepared on various substrates by layer-by-layer magnetron sputtering of metals followed by annealing have been studied. The main parameters of the metal-ferroelectric film-metal multilayer structures, namely, the spontaneous polarization, coercive field, and permittivity, were studied for various substrate types and electrode materials. The conditions favorable for the formation of PbTiO₃ films that are similar in stoichiometry and phase composition were established.     

Optical characterization of sol gel TiO<Subscript>2</Subscript> monoliths doped with Brilliant Green

Amorphous titanium dioxide monoliths doped with brilliant green (BG) were synthesized by the sol-gel process. The optical properties of the monoliths were characterized by Photoacoustic Spectroscopy and Photoluminescence Spectroscopy. The absorption spectra for the BG-doped TiO₂ monoliths exhibited two well defined absorption regions: a band below 400 nm corresponding to TiO₂ absorption and three absorption bands centered at 424 nm, 588 nm, and 632 nm due to brilliant green. While the undoped TiO₂ monoliths showed no luminescence, the doped samples showed a strong luminescence band at 673 nm, which increased its intensity for increasing organic dye doping.     

Influence of a dopant source on the structural and optical properties of Mn doped ZnGa<Subscript>2</Subscript>O<Subscript>4</Subscript> thin films

Thin films of ZnGa₂O₄:Mn²⁺ were deposited on quartz substrates using an rf magnetron sputtering technique. The sputtering target, ZnGa₂O₄ doped with 2 at. % manganese, was synthesized by a high temperature solid state reaction. Two different dopant sources were used to incorporate the dopant ions into the target, namely, manganese acetate and manganese oxide. The structural and optical properties of the thin films were studied using XRD, PL and transmission spectra. Polycrystalline ZnGa₂O₄:Mn with a spinel structure could be grown at an optimized substrate–target distance even at room temperature. No luminescence was observed in the as-deposited films grown using (CH₃COO)₂Mn as the dopant source in the target. Substrate heating or post-deposition annealing in the reducing ambient didn’t impart any luminescence to the films, ruling out the possibility of Mn²⁺ incorporation in the films. However, when using MnO as the manganese source in the target, the as-deposited films exhibited green photoluminescent emission (peak maximum at 508 nm) for substrate temperatures at and above 500 °C. This suggests that, in thin films, Mn incorporation and subsequent luminescent outcome is strongly influenced by the dopant source, which is quite different from the bulk phosphor behavior. PACS 81.15.Cd; 78.55.-m; 85.60.-q     

Photoaccumulating film systems based on TiO<Subscript>2</Subscript>/MoO<Subscript>3</Subscript> and TiO<Subscript>2</Subscript>/MoO<Subscript>3</Subscript>:V<Subscript>2</Subscript>O<Subscript>5</Subscript> nanoheterostructures

The thin-film photocatalysts TiO₂/MoO₃ and TiO₂/MoO₃:V₂O₅ obtained by a combination of sol–gel and sintering techniques were studied using the photooxidation of probing dyes, EPR spectroscopy, X-ray diffraction analysis, and electron microscopy. It was shown that due to charge accumulation caused by UV irradiation, these photocatalysts retain their oxidative activity and ability for self-sterilization in the dark for a long time after irradiation was terminated (up to 5 h for TiO₂/MoO₃:V₂O₅).