Ultrathin Titanium Oxide Research Articles

Fabrication of Inverted Bulk-Heterojunction Organic Solar Cell with Ultrathin Titanium Oxide Nanosheet as an Electron-Extracting Buffer Layer

The contributions and deposition conditions of ultrathin titania nanosheet (TN) crystallites were studied in an inverted bulk-heterojunction (BHJ) cell in indium tin oxide (ITO)/titania nanosheet/poly(3-hexylthiophene) (P3HT):phenyl-C61-butyric acid methylester (PCBM) active layer/MoOx/Ag multilayered photovoltaic devices. Only one or two layers of poly(diallyldimethylammonium chloride) (PDDA) and TN multilayered film deposited by the layer-by-layer deposition technique effectively decreased the leakage current and increased both open circuit voltage (VOC) and fill factor (FF), and power conversion efficiency (η) was increased nearly twofold by the insertion of two TN layers. The deposition of additional TN layers caused the reduction in FF, and the abnormal S-shaped curves above VOC for the devices with three and four TN layers were ascribed to the interfacial potential barrier at the ITO/TN interface and the series resistance across the multilayers of TN and PDDA. The performance of the BHJ cell with TN was markedly improved, and the S-shaped curves were eliminated following the the insertion of anatase-phase titanium dioxide between the ITO and TN layers owing to the decrease in the interfacial potential barrier.

Fabrication of Inverted Bulk-Heterojunction Organic Solar Cell with Ultrathin Titanium Oxide Nanosheet as an Electron-Extracting Buffer Layer

Fabrication of Inverted Bulk-Heterojunction Organic Solar Cell with Ultrathin Titanium Oxide Nanosheet as an Electron-Extracting Buffer Layer

Growth of Ultra-thin Titanium Dioxide Films by Complete Anodic Oxidation of Titanium Layers on Conductive Substrates

ABSTRACTThe growth of thin and ultra-thin titanium dioxide layers was investigated. Oxide films were grown by galvanostatic and potentiodynamic anodisation of evaporated titanium layers on conductive substrates. It is shown that thin-film oxidation differs significantly from anodic oxidation of solid foils or plates, due to the sudden stop of anodisation process before complete oxidation of the thick films. Depending on the pH value and the potential sweep rate, the effective defect density and the dielectric constant of the anodized layers vary from 3·1019 cm-3 to 1020 cm-3and from 16 to 27, respectively, whereas the electrolyte temperature plays only a minor role.

Adsorption and Diffusion of Fe on a Titania Ultrathin Film

The phenomenology of the adsorption of Fe adatoms and addimers and the diffusion of Fe adatoms on an ultrathin titanium oxide monolayer grown on the Pt(111) surface is investigated via density functional simulations. It is found that (i) Fe atoms adsorb on 3-fold hollow sites or defective holes of the oxide slab with interaction energies ranging from 2.3 to 4.7 eV; (ii) Fe dimers adsorb with a proportional energetics and a clear preference for horizontal rather than vertical configurations; (iii) Fe atoms diffuse over the oxide surface with energy barriers ranging from approximately 0.7-1.1 eV on the compact regions to approximately 1.6-2.0 eV on the defective regions of the film. All this points to a great importance of kinetic effects in the growth of Fe clusters on this surface, much greater than in the previously investigated cases of noble or quasi-noble metals such as Au and Pd.

Low‐Temperature Atomic Layer‐Deposited TiO2 Films with Low Photoactivity

Atomic layer deposition (ALD) has been successfully utilized for the conformal and uniform deposition of ultrathin titanium dioxide (TiO2) films on high‐density polyethylene (HDPE) particles. The deposition was carried out by alternating reactions of titanium tetraisopropoxide and H2O2 (50 wt% in H2O) at 77°C in a fluidized bed reactor. X‐ray photoelectron spectroscopy confirmed the deposition of TiO2 and scanning transmission electron microscopy showed the conformal TiO2 films deposited on polymer particle surfaces. The TiO2 ALD process yielded a growth rate of 0.15 nm/cycle at 77°C. The results of inductively coupled plasma atomic emission spectroscopy suggested that there was a nucleation period, which showed the reaction mechanism of TiO2 ALD on HDPE particles without chemical functional groups. TiO2 ALD films deposited at such a low temperature had an amorphous structure and showed a much weaker photoactivity intensity than common pigment‐grade anatase TiO2 particles.

Reliability of ultra-thin titanium dioxide (TiO 2) films on strained-Si

Ultra-thin high-k titanium oxide (equivalent oxide thickness ~ 2.2 nm) films have been deposited on strained-Si/relaxed-Si 0.8Ge 0.2 heterolayers using titanium tetrakis iso-propoxides (TTIP) as an organometallic source at low temperature (< 200 °C) by plasma enhanced chemical vapor deposition (PECVD) technique in a microwave (700 W, 2.45 GHz) plasma cavity discharge system at a pressure of 66.67 Pa. The trapping/detrapping behavior of charge carriers in ultra-thin TiO 2 gate dielectric during constant current (CCS) and voltage stressing (CVS) has been investigated. Normalized trapping centroid and trapped charge density variation with injected fluences have been investigated and also empirically modeled. Oxide lifetime is predicted using empirical reliability model developed. Dielectric breakdown and reliability of the dielectric films have been studied using constant voltage stressing. A high time-dependent dielectric breakdown (TDDB, t bd > 1000 s) is observed under high constant voltage stress.

Sum Frequency Generation Spectroscopic Investigation of TiOx/Pt(111): Surface Active Sites and Reaction Paths Probed by Formate

The catalytic behavior of ultrathin titanium oxide films deposited on platinum (111) surfaces is investigated through sum frequency generation (SFG) spectroscopy of the adsorption and decomposition of formate. TiOx films of 7 Å or less in thickness are prepared on Pt(111) substrates by evaporation of titanium under oxygen, and samples are examined as-prepared or after annealing at 873 or 973 K. Formate is adsorbed to the samples by exposure to 1000 langmuir of formic acid at 250 K. The frequency of the stretching vibration (ν(C−D)) of the deuterated formate C−D bond is found to be sensitive to the interaction between the adsorption sites and formate. SFG observations under different polarization combinations reveal that most formate is adsorbed to the surface with a bridge configuration, and that only a small amount of monodentate formate is present on the surface of TiOx/Pt(111) annealed at 973 K. Investigation of the effect of annealing temperature, time, and atmosphere on formate ν(C−D) frequency and intensity, in combination with low-energy electron diffraction (LEED) observations, reveals three kinds of surface active sites: defect sites, disordered titanium ion sites, and ordered 5-fold coordinated Ti4+ sites. These adsorption sites are responsible for ν(C−D) responses at frequencies of 2134−2184, 2195, and 2210−2226 cm−1, respectively. The defect sites are suggested to be located at the boundary between the disordered phase and the ordered phase of titanium oxide. Upon heating, formate adsorbed to defect sites undergoes decomposition at temperatures below 250 K, while formate adsorbed to ordered 5-fold coordinated Ti4+ sites decomposes via reaction paths: dissociation via monodentate formate as an intermediate, and diffusion onto defect sites before dissociation. The temperature of direct decomposition of bridge formate adsorbed to Ti4+ sites appears to be related to the formate ν(C−D) frequency, with lower frequencies corresponding to higher decomposition temperatures.

Preparation of Titanium Oxide Pillars on Glass Substrates and Ultrathin Titanium Oxide Layer using PMMA/PS Blend Films

In this paper, a simple and inexpensive method of preparing nanosized titanium oxide (TiO2) pillars on glass substrates and ultrathin TiO2 layers is presented, utilizing a blend of commercially available polystyrene (PS) and poly(methyl methacrylate) (PMMA). The surface morphology of PS/PMMA blend films is investigated in terms of the processing parameters including solution concentration, blending ratio, and spin-coating speed. For the first time, a phase inversion was revealed for the PS/PMMA blend films spun coated on conducting substrates (ITO and TiO2), with increasing solution concentration. Atomic force microscopy studies show that PMMA forms vertical cylindrical structures in the matrix of PS on glass substrates and on ultrathin TiO2 layers deposited on indium tin oxide (ITO) substrates. After the PMMA phase is etched away by ultraviolet irradiation and acetic acid, TiO2 pillar structures are successfully created on both types of substrates by infiltration of the sol−gel mixture into the nanosized...

Ultra-thin Titanium Oxide Films on Mo(112), Measured by XPS

Two preparation methods were employed to grow a well-ordered ultra-thin titanium oxide film on Mo(112). The first method consisted of growing the films by direct deposition of Ti by chemical vapor deposition, followed by oxidation. Better film quality was obtained by the second method which involved deposition of Ti onto monolayer SiO2/Mo(112), as described in our prior work. By using x-ray photoelectron spectroscopy, Ti was determined to be in +3 oxidation state.

Spectroscopy of ultrathin epitaxial rutile TiO2(110) films grown on W(100)

Epitaxial ultrathin titanium dioxide films of 0.3 to approximately 7 nm thickness on a metal single crystal substrate have been investigated by high resolution vibrational and electron spectroscopies. The data complement previous morphological data provided by scanned probe microscopy and low energy electron diffraction to provide very complete characterization of this system. The thicker films display electronic structure consistent with a stoichiometric TiO(2) phase. The thinner films appear nonstoichiometric due to band bending and charge transfer from the metal substrate, while work function measurements also show a marked thickness dependence. The vibrational spectroscopy shows three clear phonon bands at 368, 438, and 829 cm(-1) (at 273 K), which confirms a rutile structure. The phonon band intensity scales linearly with film thickness and shift slightly to lower frequencies with increasing temperature, in accord with results for single crystals.

Charge trapping properties of ultra-thin TiO2 films on strained-Si

Ultra-thin high-k titanium oxide (equivalent oxide thickness ∼2.2 nm) films have been deposited on strained-Si/relaxed-SiGe heterolayers using microwave plasma at <200 °C. The dielectric breakdown and reliability of the dielectric films have been studied using constant voltage stressing. An improvement in the leakage current characteristics is observed after post-deposition annealing. During constant voltage stressing of MIS capacitors, a time-dependent gate current increase and an anomalous current variation have been observed. The amplitude of the anomalous current is found to decrease with increase in annealing temperature. A time-dependent defect density variation in the dielectric layers for different annealing temperature has been observed. The effect of transient response and dielectric relaxation on annealing conditions has been investigated under high voltage pulse stressing. The stress-induced trap charge density and its spatial distribution have been studied. A high time-dependent dielectric breakdown (TDDB, tbd > 1000 s) is observed for as-deposited TiO2 under high constant voltage stress.

Facile Fabrication of Gold Nanoparticle–Titanium Oxide Alternate Assemblies by Surface Sol–Gel Process

Multistructured assemblies consisting of gold nanoparticles and titanium oxide were alternately fabricated by a surface sol–gel process. First, a quartz glass substrate was immersed into an organic solution of titanium butoxide [Ti(OBu)4]. Then, the substrate was rinsed with water, and dried in air, giving ultra-thin titanium oxide [Ti(O)]-modified quartz glass substrate. This modified glass substrate was immersed into an aqueous colloidal solution of gold nanoparticles (AuPs) that were stabilized with citrate ions, giving a AuP–Ti(O)-modified glass substrate. By repeating these surface sol–gel processes, the multistructured assemblies of AuPs and Ti(O), [AuP/Ti(O)]n/Glass (n=1–4) were fabricated. Plasmon band intensity increased with the number of surface sol–gel process cycles. The resultant assemblies were stable even after 11 days, or after treatment with an aqueous electrolyte solution. The alternate assembling of AuPs and Ti(O) was confirmed by quartz crystal microbalance measurements and absorption and X-ray photoelectron spectroscopies. Accordingly, we have succeeded in the preparation of stable AuP–Ti(O) composite films on the substrates.

Pulsed r.f.‐glow‐discharge time‐of‐flight mass spectrometry for fast surface and interface analysis of conductive and non‐conductive materials

AbstractGlow discharge (GD) is a highly specialised source that especially meets the requirements for accuracy, simplicity and speed for content depth profiling and bulk analysis in both optical emission (OES) and mass spectrometry (MS). The pulsed radio frequency GD source has the potential for both elemental and molecular analysis of conductive and non‐conductive materials. To exploit the information delivered by pulsed radio frequency (r.f.)‐GD sources, fast sampling is required, and is available only through time‐of‐flight mass spectrometry (ToF‐MS). Compared to optical glow discharge (GD‐OES) instrumentation, a GD‐ToF‐MS system shows much simpler spectra, lower background signals and lower detection limits. The presented new r.f.‐GD‐ToF‐MS system is a successful combination of a commercial high‐end glow discharge instrument and an extremely fast and high‐resolution time‐of‐flight mass spectrometer. This new instrument was applied to analyse conductive and non‐conductive materials like anodic thin films. We could resolve 2‐nm Cr makers in aluminium oxide layers and measure trace elements in ultra thin titanium oxide films. Furthermore, we show the potential of the pulsed mode to separate analyte species from elements originating from residual gas. Copyright © 2006 John Wiley &amp; Sons, Ltd.

Synthesis of well-ordered ultra-thin titanium oxide films on Mo(1 1 2)

Well-ordered Mo(1 1 2)–(8 × 2)-TiO x films were grown on the Mo(1 1 2) surface. Two preparation methods, direct growth on the Mo(1 1 2) surface and indirect growth by deposition of Ti onto monolayer SiO 2/Mo(1 1 2), were used. The latter method was found to be more reproducible with respect to film quality as determined by low-energy electron diffraction and scanning tunneling microscopy. The well-ordered (8 × 2)-TiO x film, for which a structure is proposed, was determined to be 1 ML and the Ti to be in an oxidation state of +3 using Auger spectroscopy, high-resolution electron energy loss spectroscopy and X-ray photoelectron spectroscopy.

Surface modification of surface sol–gel derived titanium oxide films by self-assembled monolayers (SAMs) and non-specific protein adsorption studies

Biological events occurring at the implant–host interface, including protein adsorption are mainly influenced by surface properties of the implant. Titanium alloys, one of the most widely used implants, has shown good biocompatibility primarily through its surface oxide. In this study, a surface sol–gel process based on the surface reaction of metal alkoxides with a hydroxylated surface was used to prepare ultrathin titanium oxide (TiO x ) coatings on silicon wafers. The oxide deposited on the surface was then modified by self-assembled monolayers (SAMs) of silanes with different functional groups. Interesting surface morphology trends and protein adhesion properties of the modified titanium oxide surfaces were observed as studied by non-specific protein binding of serum albumin. The surface properties were investigated systematically using water contact angle, ellipsometry, X-ray photoelectron spectroscopy (XPS), and atomic force microscopy (AFM) measurements. Results showed that the surface sol–gel process predominantly formed homogeneous, but rough and porous titanium oxide layers. The protein adsorption was dependent primarily on the silane chemistry, packing of the alkyl chains (extent of van der Waals interaction), morphology (porosity and roughness), and wettability of the sol–gel oxide. Comparison was made with a thermally evaporated TiO x –Ti/Si-wafer substrate (control). This method further extends the functionalization of surface sol–gel derived TiO x layers for possible titanium alloy bioimplant surface modification.

Structure and growth of ultrathin titanium oxide films on Ru(0 0 0 1)

Structure, chemical composition and thermal stability of TiO x films with a thickness of up to 3 ML grown on a Ru(0 0 0 1) substrate were investigated by scanning tunneling microscopy, X-ray photoelectron spectroscopy and Auger electron spectroscopy. The films were prepared by evaporation of Ti in 10 −7 mbar O 2 onto Ru(0 0 0 1) at 640 K, followed by annealing in 10 −7 mbar O 2 or in UHV at temperatures between 700 K and 1000 K. Depending on the deposition and post-annealing conditions, we find several different structures of the Ti oxide, with the layers post-annealed in an O 2 environment being generally better defined than those post-annealed in vacuum. The layers consist of triangular units which coalesce upon annealing. O 2 annealing of monolayer films leads to an oxygen deficient TiO 2 layer. Annealing to 900–1000 K changes the structure and composition of the film, a coincidence structure with a Moiré pattern is observed upon O 2 annealing.

Room temperature plasma oxidation mechanism to obtain ultrathin silicon oxide and titanium oxide layers

Scaling rules for sub-micrometric MOS devices have led to the necessity of ultrathin dielectric films and high- k dielectric layers. In this paper we present first results of room temperature plasma oxidation to obtain ultrathin layers of SiO 2 and TiO 2. The oxidation process in O 2 and N 2O shows a power law dependence with time and inverse proportionality with pressure. The oxidation rate is inversely proportional to pressure for both high and medium resistivities substrates. An oxidation model is proposed to explain this behavior. Ellipsometric and C– V characterization show complete oxidation of titanium verifying that a dielectric layer is formed.

Ultra-thin titanium oxide film with a rutile-type structure

A titanium oxide film was deposited on a silicon substrate by RF-magnetron sputtering using a sintered oxide target in an argon-gas atmosphere. Phase transition and crystallization during the post-heat treatment were investigated. The thermodynamic stability of phase, oxygen diffusion through the film to form an interfacial SiO 2 layer, and the reaction process in which the rutile-type TiO 2 is crystallized were investigated. Rock-salt-type TiO with large deficiency of titanium ions is quenched in the as-deposited film in non-equilibrium. The following post-heat treatment in nitrogen gas transforms the film to Ti 3O 5, while post-oxidation transforms the film into rutile-type TiO 2, even in an ultra-thin film of 5 nm, at the expense of the growth of interfacial SiO 2 with a thickness of 2.5 nm. The growth of the interfacial oxide is explained in terms of three origins: oxygen defects incorporated into the rutile-type TiO 2, fast oxygen diffusion via the defects, and the fact that non-stoichiometric TiO 2 − x is less stable than SiO 2. We also discuss the main criteria that must be satisfied in order to apply the high-permittivity oxide to a gate insulator.

Growth of ultra-thin titanium oxide on Cu(100), Fe/Cu(100) and ordered ultra-thin iron oxide studied by low-energy electron diffraction and X-ray photoelectron spectroscopy

The growth, structure and chemical state of ultra-thin titanium oxide prepared on Cu(100), Fe/Cu(100) and on ultra-thin iron oxide have been investigated using X-ray photoelectron spectroscopy (XPS) and low-energy electron diffraction (LEED). The titanium oxide on Cu(100) was prepared by evaporating titanium atoms on the oxygen chemisorbed substrate, followed by heating at 623 K in 1×10 −5 Pa O 2. Up to one layer coverage the titanium oxide showed a chemical shift of the Ti 2p 3/2 peak corresponding to Ti 4+ and a LEED pattern of a hexagonal structure with two domains rotated 90° with respect to each other. The oxide has been proposed to consist of oxygen–titanium–oxygen trilayers with the stoichiometry of TiO 2. The surface structure was unstable at higher coverage. The same structure of titanium oxide overlayer was prepared by evaporating titanium atoms on 1.7 ML Fe/Cu(100), followed by oxidation under the condition to give the FeO(111) underlayers consisting of two iron–oxygen bilayers. The ordered titanium oxide on FeO(111)/Cu(100) was prepared also by evaporating titanium atoms on an ordered Fe 3O 4/Cu(100) surface, followed by heating at 623 K in vacuum. The ordered structure of the titanium oxide was not stable on the iron oxide at higher oxidation state.

Surface Sol–gel Synthesis of Ultrathin Titanium and Tantalum Oxide Films

TiO2, Ta2O5, and mixed TiO2/Ta2O5 ultrathin films were grown layer-by-layer using a surface sol–gel deposition technique. The technique allows one to prepare smooth films of mixed composition with angstrom-level control of thickness. The thickness increases per adsorption/hydrolysis cycle were 4.5, 2.1, and 2.5 A, respectively, for TiO2, Ta2O5, and mixed TiO2/Ta2O5 (1.6sol;1) films. By combining X-ray diffraction and ellipsometric analysis, it was determined that the as-deposited TiO2 films are less dense than bulk TiO2, and do not adhere persistently to Si/SiO2 or Au/2-mercaptoethanol substrates. These ‘green’ films were annealed at 400°C to produce denser and highly adherent films. Patterning the surface by microcontact printing of siloxane polymer thin films allows one to prepare patterned sol–gel oxide thin films.