Modelling of deposition processes on the TiO2 rutile (1 1 0) surface

Deposition of Ti_xO_y clusters onto the rutile TiO₂ (1 1 0) surface has been modelled using empirical potential based molecular dynamics. Deposition energies in the range 10-40 eV have been considered so as to model typical deposition energies of magnetron sputtering. Defects formed as a function of both the deposition energy and deposition species have been studied.The results show that in the majority of cases Ti interstitial atoms are formed, irrespective of whether Ti was contained within the deposited cluster. Furthermore that the majority of these interstitials are formed by displacing a surface Ti atom into the interstitial site. O surface atoms are also relatively common, with Ti and TiO₂ surface units often occurring when the deposited cluster contains Ti but becoming less frequent as the deposition energy is increased. Structures that would give rise to the growth of further layers of rutile are not observed and in the majority of the simulations the energy barriers for diffusion of the end-products is high.     

N-TiO2 nanoparticles embedded in silica prepared by Ti ion implantation and annealing in nitrogen

Room-temperature Ti ion implantation and subsequent thermal annealing in N₂ ambience have been used to fabricate the anatase and rutile structured N-doped TiO₂ particles embedded in the surface region of fused silica. The Stopping and Range of Ions in Matter (SRIM) code simulation indicates a Gaussian distribution of implanted Ti, peaked at ∼75 nm with a full width at half maximum of ∼80 nm. However, the transmission electron microscopy image shows a much shallower distribution to depth of ∼70 nm. Significant sputtering loss of silica substrates has occurred during implantation. Nanoparticles with size of 10-20 nm in diameter have formed after implantation. X-ray photoelectron spectroscopy indicates the coexistence of TiO₂ and metallic Ti in the as-implanted samples. Metallic Ti is oxidized to anatase TiO₂ after annealing at 600 °C, while rutile TiO₂ forms by phase transformation after annealing at 900 °C. At the same time, N-Ti-O, Ti-O-N and/or Ti-N-O linkages have formed in the lattice of TiO₂. A red shift of 0.34 eV in the absorption edge is obtained for N-doped anatase TiO₂ after annealing at 600 °C for 6 h. The absorbance increases in the ultraviolet and visible waveband.     

Atomistic properties of helium in hcp titanium: A first-principles study

First-principles calculations based on density functional theory have been performed to investigate the behaviors of He in hcp-type Ti. The most favorable interstitial site for He is not an ordinary octahedral or tetrahedral site, but a novel interstitial site (called FC) with a formation energy as low as 2.67 eV, locating the center of the face shared by two adjacent octahedrons. The origin was further analyzed by composition of formation energy of interstitial He defects and charge density of defect-free hcp Ti. It has also been found that an interstitial He atom can easily migrate along 〈0 0 1〉 direction with an activation energy of 0.34 eV and be trapped by another interstitial He atom with a high binding energy of 0.66 eV. In addition, the small He clusters with/without Ti vacancy have been compared in details and the formation energies of He_nV clusters with a pre-existing Ti vacancy are even higher than those of He_n clusters until n ? 3.     

Oxygen deficiency and excess of rutile titania (1 1 0) surfaces analyzed by ion scattering coupled with elastic recoil detection

Oxygen deficiency and excess of rutile titania (TiO₂) surfaces are important factors for catalytic activities of metal nano-particles on the TiO₂ supports. Medium energy ion scattering (MEIS; 80 keV He⁺) coupled with elastic recoil detection analysis (ERD; 150 keV Ne⁺) can determine the numbers of bridging O (O_br) vacancies (V_O) and excess O atoms adsorbed on the 5-fold Ti rows of TiO₂(1 1 0) surfaces. The amounts of V_O and adsorbed O were derived by H₂O and ¹⁸O₂ exposure followed by ERD and MEIS analyses, respectively. The present analysis revealed that only about a half of V_O are filled and a comparable amount of O atoms are adsorbed on the reduced TiO₂(1 1 0) surface after exposure to O₂ (1000 L; 1 L = 1 × 10⁻⁶ Torr s) at room temperature (RT). We also detected the adsorbed O for the hydroxylated TiO₂(1 1 0) after ¹⁸O₂ exposure at RT. Finally, it is shown that the O adsorbed on the Ti rows reacts with CO probably to form CO₂ at RT. Based on the results obtained here, we clarify the reason why only a half of V_O are filled by exposing reduced surface to O₂ at RT and what is the primary source of subsurface excess electronic charge, which acts as a leading part of the surface electrochemistry and gives the defect state in the band gap seen in the valence band spectra for reduced and hydroxylated TiO₂(1 1 0) surfaces.     

Femtosecond laser irradiation-induced phase transformation on titanium dioxide crystal surface

Titanium dioxide (TiO₂) rutile single crystal was irradiated by infrared femtosecond (fs) laser pulses with repetition rate of 250 kHz and phase transformation of rutile TiO₂ was observed. Micro-Raman spectra show that the intensity of E_g Raman vibrating mode of rutile phase increases and that of A_1g Raman vibrating mode decreases apparently within the ablation crater after fs laser irradiation. With increasing of irradiation time at laser average power of 300 mW, the Raman vibrating modes of anatase phase emerged. Rutile phase of TiO₂ single crystal is partly transformed into anatase phase. The anatase phase content transformed from rutile phase increased to a maximum and then decreased with increasing of fs pulse laser irradiation time. The line mapping of micro-Raman spectrum at the crater, irradiated by fs pulse laser for 100 s at average power of 300 mW, indicates more rutile phase are transformed into anatase phase at the center.     

Alkali ion scattering from Ag(0 0 1) and Ag thin films at low and hyperthermal energies

We have investigated the scattering of K⁺ and Cs⁺ ions from a single crystal Ag(0 0 1) surface and from a Ag-Si(1 0 0) Schottky diode structure. For the K⁺ ions, incident energies of 25 eV to 1 keV were used to obtain energy-resolved spectra of scattered ions at θ_i = θ_f = 45°. These results are compared to the classical trajectory simulation safari and show features indicative of light atom-surface scattering where sequential binary collisions can describe the observed energy loss spectra. Energy-resolved spectra obtained for Cs⁺ ions at incident energies of 75 eV and 200 eV also show features consistent with binary collisions. However, for this heavy atom-surface scattering system, the dominant trajectory type involves at least two surface atoms, as large angular deflections are not classically allowed for any single scattering event. In addition, a significant deviation from the classical double-collision prediction is observed for incident energies around 100 eV, and molecular dynamics studies are proposed to investigate the role of collective lattice effects. Data are also presented for the scattering of K⁺ ions from a Schottky diode structure, which is a prototype device for the development of active targets to probe energy loss at a surface.     

Dependence of cluster ranges on target cohesive energy: Molecular-dynamics study of energetic Au402 cluster impacts

It has long been known that the stopping and ranges of atoms and clusters depends on the projectile-target atom mass ratio. Recently, Carroll et al. [S.J. Carroll, P.D. Nellist, R.E. Palmer, S. Hobday, R. Smith, Phys. Rev. Lett. 84 (2000) 2654] proposed that the stopping of clusters also depends on the cohesive energy of the target. We investigate this dependence using a series of molecular-dynamics simulations, in which we systematically change the target cohesive energy, while keeping all other parameters fixed. We focus on the specific case of Au₄₀₂ cluster impact on van-der-Waals bonded targets. As target, we employ Lennard-Jones materials based on the parameters of Ar, but for which we vary the cohesive energy artificially up to a factor of 20. We show that for small impact energies, E₀ ? 100 eV/atom, the range D depends on the target cohesive energy U, D ∝ U^−β. The exponent β increases with decreasing projectile energy and assumes values up to β = 0.25 for E₀ = 10 eV/atom. For higher impact energies, the cluster range becomes independent of the target cohesive energy. These results have their origin in the so-called ‘clearing-the way’ effect of the heavy Au₄₀₂ cluster; this effect is strongly reduced for E₀ ? 100 eV/atom when projectile fragmentation sets in, and the fragments are stopped independently of each other. These results are relevant for studies of cluster stopping and ranges in soft matter.     

6 MeV electron irradiation effects on electrical properties of Al/TiO2/n-Si MOS capacitors

The irradiation effects of 6 MeV electrons on the electrical properties of Al/TiO₂/n-Si metal-oxide-semiconductor capacitors have been investigated. Nine Al/TiO₂/n-Si capacitors were fabricated using radio frequency magnetron sputtering and divided into three groups. Groups were irradiated with 6 MeV electrons at 10, 20, and 30 kGy doses, respectively, keeping the dose rate ∼1 kGy/min. The variations in the capacitance-voltage and leakage current-voltage characteristics, in addition to the electrical parameters, such as conductance (G/ω), flat-band voltage, interface trap density and the surface charge density with electron dose were studied. The Poole-Frenkel coefficient of the MOS capacitors was determined from current-voltage characteristics. Possible mechanisms for the enhanced leakage current in the electron irradiated MOS capacitors are discussed.     

Effects of Mg-ion implantation in α-Al2O3 and α-Al2O3:Mg crystals: Electrical conductivity and electronic structure changes

Undoped and Mg-doped α-Al₂O₃ single crystals were implanted with Mg ions, with an energy of 90 keV and a fluence of 10¹⁷ ions/cm². DC electrical measurements using the four-point probe method, between 295 and 428 K, were used to characterize the electrical conductivity of the implanted area. Measurements in this temperature range indicate that the electrical conductivity after implantation is thermally activated with an activation energy of about 0.03 eV both in undoped and in reduced Mg-doped α-Al₂O₃ crystals, whereas the activation energy in oxidized Mg-doped α-Al₂O₃ crystals remains close to that before implantation. The I-V characteristics of the latter samples reveal a blocking behavior of the electrical contacts on the implanted area in contrast to the ohmic contacts observed in α-Al₂O₃ single crystals with the c-axis perpendicular to the broad face, where the Mg ions were implanted. We conclude that the enhancement in conductivity observed in the implanted regions is related to the intrinsic defects created by the implantation, rather than to the implanted Mg ions. The relationship between the oxygen vacancy concentrations at different stages of etching and the changes in the electronic structure, the chemical bonding, and the Al³⁺(2p)/O²⁻(1s) and Mg²⁺(1s)/O²⁻(1s) relative intensities was studied by X-ray Photoemission Spectroscopy.     

Simulating radiation damage in Ga stabilised δ-Pu

Radiation events in Ga stablised δ-Pu are investigated by means of Molecular Dynamics simulations. Pu 5 at.% Ga is considered using the Modified Embedded Atom Method to govern the atomic interactions. Cascades were initiated with Primary Knock-on Atom (PKA) energies in the range of 0.4-10 keV, with trajectories deduced through comprehensive sampling of a representative set of directions, combined with different Ga atomic positions. The displacement threshold energy, E_d, for Pu and Ga atoms was also determined through similar extensive studies to aid the understanding and interpretation of the cascade results.Values of E_d between 5 and 40 eV were determined for Pu, with Ga PKAs requiring generally more energy to create a defect with E_d between 8 and 70 eV. Low energy collision cascades, initiated with energies in the range of 0.4-1 keV, show that the cascades form in a similar manner to other fcc metals with a vacancy rich zone surrounded by isolated interstitial defects. A feature of these cascades is that the displaced Ga atoms return to lattice sites during the ballistic phase, leading to a lack of Ga-type residual defects. Higher energy cascades show similar features but with the development of an amorphous region at the cascade core of around 5 nm diameter at 5 keV. Quantitatively, the residual number of defects found shows no distinct variation to that for previous work on pure Pu, suggesting the inclusion of Ga does not significantly effect the susceptibility or resistance of Pu to initial cascade development.     

Computer simulation of primary damage creation in displacement cascades in copper. I. Defect creation and cluster statistics

Atomic-scale computer simulation has been used to investigate the primary damage created by displacement cascades in copper over a wide range of temperature (100 K ? T ? 900 K) and primary knock-on atom energy (5 keV ? E_PKA ? 25 keV). A technique was introduced to improve computational efficiency and at least 20 cascades for each (E_PKA, T) pair were simulated in order to provide statistical reliability of the results. The total of almost 450 simulated cascades is the largest yet reported for this metal. The mean number of surviving point defects per cascade is only 15-20% of the NRT model value. It decreases with increasing T at fixed E_PKA and is proportional to (E_PKA)^1.1 at fixed T. A high proportion (60-80%) of self-interstitial atoms (SIAs) form clusters during the cascade process. The proportion of clustered vacancies is smaller and sensitive to T, falling from 30% to 60% for T ? 600 K to less than 20% when T = 900 K. The structure of clusters has been examined in detail. Vacancies cluster predominantly in stacking-fault-tetrahedron-type configurations. SIAs tend to form either glissile dislocation loops with Burgers vector b = 1/2<1 1 0> or sessile faulted Frank loops with b = 1/3<1 1 1>. Despite the fact that cascades at a given E_PKA and T exhibit a wide range of defect numbers and clustered fractions, there appears to be a correlation in the formation of vacancy clusters and SIA clusters in the same cascade. The size and spatial aspects of this are analysed in detail in part II [unpublished], where the stability of clusters when another cascade overlaps them is also investigated.     

Growth and surface morphology of ion-beam sputtered Ti-Ni thin films

Titanium-nickel thin films have been deposited on float glass substrates by ion beam sputtering in 100% pure argon atmosphere. Sputtering is predominant at energy region of incident ions, 1000 eV to 100 keV. The as-deposited films were investigated by X-ray photoelectron spectroscopy (XPS) and atomic force microscope (AFM). In this paper we attempted to study the surface morphology and elemental composition through AFM and XPS, respectively. Core level as well as valence band spectra of ion-beam sputtered Ti-Ni thin films at various Ar gas rates (5, 7 and 12 sccm) show that the thin film deposited at 3 sccm possess two distinct peaks at binding energies 458.55 eV and 464.36 eV mainly due to TiO₂. Upon increasing Ar rate oxidation of Ti-Ni is reduced and the Ti-2p peaks begin approaching those of pure elemental Ti. Here Ti-2p peaks are observed at binding energy positions of 454.7 eV and 460.5 eV. AFM results show that the average grain size and roughness decrease, upon increasing Ar gas rate, from 2.90 μm to 0.096 μm and from 16.285 nm to 1.169 nm, respectively.     

Theory of He trapping, diffusion, and clustering in UO2

We have performed ab initio total energy calculations to investigate the behavior of helium and its diffusion properties in uranium dioxide (UO₂). Our investigations are based on the density functional theory within the generalized gradient approximation (GGA). The trapping behavior of He in UO₂ has been modeled with a supercell containing 96-atoms as well as uranium and oxygen vacancy trapping sites. The calculated incorporation energies show that for He a uranium vacancy is more stable than an oxygen vacancy or an octahedral interstitial site (OIS). Interstitial site hopping is found to be the rate-determining mechanism of the He diffusion process and the corresponding migration energy is computed as 2.79 eV at 0 K (with the spin-orbit coupling (SOC) included), and as 2.09 eV by using the thermally expanded lattice parameter of UO₂ at 1200 K, which is relatively close to the experimental value of 2.0 eV. The lattice expansion coefficient of He-induced swelling of UO₂ is calculated as 9 × 10⁻². For two He atoms, we have found that they form a dumbbell configuration if they are close enough to each other, and that the lattice expansion induced by a dumbbell is larger than by two distant interstitial He atoms. The clustering tendency of He has been studied for small clusters of up to six He atoms. We find that He strongly tends to cluster in the vicinity of an OIS, and that the collective action of the He atoms is sufficient to spontaneously create additional point defects around the He cluster in the UO₂ lattice.     

Molecular dynamics study on carbon film deposition

Hydrogen-free carbon film deposition is studied through molecular dynamics (MD) simulation. MD simulation is performed for low-energy carbon particle impacts on carbon surfaces at room temperature. Deposition energy is considered up to E = 100 eV per carbon atom. Deposited surface is set into amorphous, and (1 1 1) surface of diamond crystal is considered in order to compare. For (1 1 1) surface, we found three characteristic regions. The deposition frequently results in the shallow trapping in E ? 20 eV. In E ? 30 eV, the deep trapping occurs about normal incidence, and the reflection appears about grazing incidence. For the amorphous surface, however, the reflection and the deep trapping are suppressed.     

Predicting the probability for fission gas resolution into uranium dioxide

Classical molecular dynamics simulations, using a set of previously established pair potentials, have been used to predict the minimum energy needed for krypton and xenon atoms to be resolved into uranium dioxide across a perfect (1 1 1) surface. The absolute minimum energy, E_min, is 53 eV for krypton and 56 eV for xenon atoms, significantly less than the 300 eV value often assumed in fuel modelling as the minimum energy required for gas resolution. The present values are, however, still sufficient to preclude thermal resolution at normal reactor temperatures. The discrepancies between the present and previous resolution energies are due to the significant variation in probabilities of absorption at different impact points on the crystal surface; we have mapped out the probability distribution for various impact sites across the crystal surface. The value of 300 eV corresponds to an 85% chance of resolution.     

Molecular dynamics simulations of large fluorine cluster impact on silicon with supersonic velocity

The etching process by very large reactive gas cluster impact was investigated by molecular dynamics (MD) simulations. Fluorine-molecule clusters with the size up to 100,000 atoms (50,000 F₂ molecules) were irradiated on silicon (1 0 0) targets at supersonic velocity regime (0.1-1 eV/atom, 1.0-3.2 km/s). The MD simulations revealed that the existence of threshold energy-per-atom around 0.3 eV/atom (1.75 km/s) to cause surface deformation and enhancement of Si desorption. When the incident energy-per-atom is less than the threshold, the incident cluster breaks up itself on the target without surface deformation. The fluorine molecules in the cluster spread in the lateral direction along the target surface, and some part of them decompose and adsorbs on the target to form silicon fluoride composites. On the other hand, the clusters penetrate the surface of silicon target when the energy-per-atom is larger than 0.3 eV/atom. In these collisional processes, the target surface is deformed to create shallow crater shape. The incident fluorine molecules are preferentially concentrated at the bottom of the crater, which resulted in high desorption yield of silicon as in the form of SiF₂, SiF₃ and SiF₄.     

Depth profiling of Al2O3 + TiO2 nanolaminates by means of a time-of-flight energy spectrometer

Atomic layer deposition (ALD) is currently a widespread method to grow conformal thin films with a sub-nm thickness control. By using ALD for nanolaminate oxides, it is possible to fine tune the electrical, optical and mechanical properties of thin films. In this study the elemental depth profiles and surface roughnesses were determined for Al₂O₃ + TiO₂ nanolaminates with nominal single-layer thicknesses of 1, 2, 5, 10 and 20 nm and total thickness between 40 nm and 60 nm. The depth profiles were measured by means of a time-of-flight elastic recoil detection analysis (ToF-ERDA) spectrometer recently installed at the University of Jyväskylä. In TOF-E measurements ⁶³Cu, ³⁵Cl, ¹²C and ⁴He ions with energies ranging from 0.5 to 10 MeV, were used and depth profiles of the whole nanolaminate film could be analyzed down to 5 nm individual layer thickness.     

Photoconduction investigations in 75 MeV oxygen ion irradiated kapton-H polyimide

Photoconduction behaviour of 75 MeV oxygen ion irradiated (Fluences: 1.8 × 10¹¹, 1.8 × 10¹² and 1.8 × 10¹³ ions/cm²) kapton-H polyimide film in the visible region has been investigated at different temperatures ranging 400-2500 °C and at various electric fields ranging 40-600 kV/cm. A photoinduced exciton formation is the major source for providing charge carriers through thermolization and field-assisted dissociation processes. An attempt has been made to fit the field dependence of the steady state photocurrent to one of the several possible conduction mechanisms. In the high and low fluence (1.8 × 10¹³ and 1.8 × 10¹¹ ions/cm²) irradiated samples there exists a possibility of Poole-Frankel type of photoconduction mechanism, whereas at intermediate fluence (1.8 × 10¹² ions/cm²) a Schottky type photoconduction mechanism may be operative. The log I_ps versus 1/T plots consist of two straight lines with a knee point around 800-1000 °C. The activation energy estimated from the slope of these lines is field dependent varying from 0.40 to 0.73 eV and 0.18 to 0.23 eV above and below the knee point, respectively. This indicates the presence of more than one type of trapping levels in irradiated kapton-H polyimide.     

Formation and properties of defects and small vacancy clusters in SiC: Ab initio calculations

Large-scale ab initio simulation methods have been employed to investigate the configurations and properties of defects in SiC. Atomic structures, formation energies and binding energies of small vacancy clusters have also been studied as a function of cluster size, and their relative stabilities are determined. The calculated formation energies of point defects are in good agreement with previously theoretical calculations. The results show that the di-vacancy cluster consists of two C vacancies located at the second nearest neighbor sites is stable up to 1300 K, while a di-vacancy with two Si vacancies is not stable and may dissociate at room temperature. In general, the formation energies of small vacancy clusters increase with size, but the formation energies for clusters with a Si vacancy and nC vacancies (V_Si-nV_C) are much smaller than those with a C vacancy and nSi vacancies (V_C-nV_Si). These results demonstrate that the V_Si-nV_C clusters are more stable than the V_C-nV_Si clusters in SiC, and provide possible nucleation sites for larger vacancy clusters or voids to grow. For these small vacancy clusters, the binding energy decreases with increasing cluster size, and ranges from 2.5 to 4.6 eV. These results indicate that the small vacancy clusters in SiC are stable at temperatures up to 1900 K, which is consistent with experimental observations.