Dioxide Single Crystals Research Articles

Towards wafer-scale growth of two-dimensional cerium dioxide single crystal with high dielectric performance

Towards wafer-scale growth of two-dimensional cerium dioxide single crystal with high dielectric performance

Near-infrared study of radiation damage in cerium dioxide

Radiation damage is studied in electron-irradiated cerium dioxide (CeO2) single crystals. Near Infra-red (NIR) spectra are recorded at room temperature between 2800 and 11,000 cm−1 (i.e. ∼0.9–3.57 µm in wavelength and ∼0.35–1.36 eV in photon energy). Measurements were carried out in the transmission mode for the single crystals and diffuse reflectivity mode for sintered CeO2 and (Ce, Gd)O2-x reference un-irradiated samples. The ceria single crystals were irradiated with 1.4-MeV and 2.5-MeV electrons for high fluences up to 3 × 1016 cm−2. Five broad absorption bands centered at 3600, 4100, 4600, 6000, and 7100 cm−1 (i. e. ∼0.44, 0.51, 0.57, 0.74, and 0.88 eV) are deduced from fits for the 2.5-MeV electron irradiation, whereas no absorption band is recorded for the 1.4-MeV electron energy. Those bands are consistent with the broad 0.9-eV band (7260 cm−1, 1.37 µm) in the UV–visible range associated with the change in color from light blue to deep green for the 2.5-MeV energy. Optical absorption arises from point-defect formation by elastic collisions for electron irradiation above the threshold energy near 1.4 MeV corresponding to the onset of cerium atom displacement. No defect bleaching occurs after isothermal annealing at 700 °C. No such bands are recorded for the virgin (Ce, Gd)O2-x samples for 5, 10, and 15 mol% Gd2O3 with an increasing amount of oxygen vacancies. The origin of absorption bands in the NIR range is discussed in relation to the available experimental data and ab-initio calculations of point defects in CeO2 and other dioxides with the cubic fluorite-like structure. Absorption bands are tentatively assigned to electronic transitions involving cerium vacancy levels in the band gap.

The influence of lattice defects, recombination, and clustering on thermal transport in single crystal thorium dioxide

Thermal transport is a key performance metric for thorium dioxide in many applications where defect-generating radiation fields are present. An understanding of the effect of nanoscale lattice defects on thermal transport in this material is currently unavailable due to the lack of a single crystal material from which unit processes may be investigated. In this work, a series of high-quality thorium dioxide single crystals are exposed to 2 MeV proton irradiation at room temperature and 600 °C to create microscale regions with varying densities and types of point and extended defects. Defected regions are investigated using spatial domain thermoreflectance to quantify the change in thermal conductivity as a function of ion fluence as well as transmission electron microscopy and Raman spectroscopy to interrogate the structure of the generated defects. Together, this combination of methods provides important initial insight into defect formation, recombination, and clustering in thorium dioxide and the effect of those defects on thermal transport. These methods also provide a promising pathway for the quantification of the smallest-scale defects that cannot be captured using traditional microscopy techniques and play an outsized role in degrading thermal performance.

Direct growth of 2D MoO2 single crystal on SiO2/Si substrate by atmospheric pressure chemical vapor deposition

Due to its distinct properties, two-dimensional (2D) materials had attracted much attention and plenty of 2D materials had been synthesized and studied. Metal oxides have been widely used in lithium-ion battery, supercapacitor, catalyst, etc, but the corresponding 2D material of metal oxides has not been sufficiently researched. Here, micrometer-scale 2D Molybdenum dioxide (MoO2) single crystals have been successfully synthesized directly on silicon dioxide/silicon (SiO2/Si) substrate by atmospheric pressure chemical vapor deposition, through the reduction by hydrogen. The growth temperature is 690 °C, and the reduction reaction lasted with 90 minutes. Raman spectra and x-ray photoelectronic spectroscopy are used to confirm the chemical component of MoO2. The shape of two-dimensional MoO2 single crystals has three types, including standard parallelogram, parallelogram with two angles misses and hexagonal. The growth model of three shape types is discussed, and the difference in growth velocity between two growth directions is the key factor of the final shape. The theoretical calculation confirmed the metallic property of as-synthesized 2D MoO2 and its resistivity is 2 × 10−4 Ω*cm, indicating it has excellent conductivity and may have potential application for conducting films.

Behavior of UV-generated carriers and local structure around doped aluminum in anatase titanium dioxide

Al-doped anatase titanium dioxide single crystals and highly Al-doped powders were prepared by chemical vapor transport and by sol-gel process, respectively. Electron paramagnetic resonance (EPR) measurement on the single crystals and powders revealed the characteristic behavior of the UV-induced carriers. The UV-induced EPR sextuplet signals originating from photogenerated holes interacting with Al were observed in both the single crystals and powders. Simulation of the powder pattern of the sextuplet signals observed in EPR measurement indicates that the local structure around the trapped hole in the powder is identical to that of the single crystals. The signal intensity under the UV irradiation and the lifetime of the sextuplets after the UV irradiation decreased with increasing Al content. Rietveld analysis of the X-ray diffraction patterns of the powders revealed that the doped Al substitutes Ti, and that the lattice volume decreases with increasing Al content. The Raman bands of the powders shifted to a higher frequency with increasing Al content. On the basis of these experimental results, the mechanism and the stability of the photogenerated hole are discussed in terms of the dependence of the local structural changes around the doped Al on the Al content.

Crystal viscoplastic modeling of UO2 single crystal

The viscoplastic behavior of uranium dioxide (UO2) single crystal is of great interest to perform predictive multiscale modeling of the nuclear fuel. Here, a viscoplastic model is built considering dislocation glide in ½<110>{100} and ½<110>{110} slip systems. The constitutive law parameters are determined adjusting the temperature dependency of the experimental critical resolved shear stress for both principal slip modes. Crystal plasticity finite element simulations of single crystal compression tests show a reasonable agreement with experimental viscoplastic anisotropy of UO2. However, for specific orientations where ½<110>{111} slip is observed experimentally, significant differences remain between experimental and computed compression stresses. Therefore, the role of ½<110>{111} slip is investigated based on a parametric study that provides new insights on UO2 plastic deformation. Several parameterizations of ½<110>{111} slip are tested highlighting the complexity of UO2 viscoplastic behavior. Significant improvements are still required to explain all simulation-experiment gaps.

Defects induced in cerium dioxide single crystals by electron irradiation

Micro-Raman spectroscopy, X-band electron paramagnetic resonance (EPR) spectroscopy, and UV-visible optical absorption spectroscopy were used to study the damage production in cerium dioxide (CeO2) single crystals by electron irradiation for three energies (1.0, 1.4, and 2.5 MeV). The Raman-active T2g peak was left unchanged after 2.5-MeV electron irradiation at a high fluence. This shows that no structural modifications occurred for the cubic fluorite structure. UV-visible optical absorption spectra exhibited a characteristic sub band-gap tail for 1.4-MeV and 2.5-MeV energies, but not for 1.0 MeV. Narrow EPR lines were recorded near liquid-helium temperature after 2.5-MeV electron irradiation; whereas no such signal was found for the virgin un-irradiated crystal or after 1.0-MeV irradiation for the same fluence. The angular variation of these lines in the {111} plane revealed a weak g-factor anisotropy assigned to Ce3+ ions (with the 4f1 configuration) in a high-symmetry local environment. It is concluded that Ce3+ ions may be produced by a reduction resulting from the displacement damage process. However, no evidence of F+ or F0 center or hole center formation due to irradiation was found from the present EPR and optical absorption spectra.

Geometric constraints on phase coexistence in vanadium dioxide single crystals

The appearance of stripe phases is a characteristic signature of strongly correlated quantum materials, and its origin in phase-changing materials has only recently been recognized as the result of the delicate balance between atomic and mesoscopic materials properties. A vanadium dioxide (VO2) single crystal is one such strongly correlated material with stripe phases. Infrared nano-imaging on low-aspect-ratio, single-crystal VO2 microbeams decorated with resonant plasmonic nanoantennas reveals a novel herringbone pattern of coexisting metallic and insulating domains intercepted and altered by ferroelastic domains, unlike previous reports on high-aspect-ratio VO2 crystals where the coexisting metal/insulator domains appear as alternating stripe phases perpendicular to the growth axis. The metallic domains nucleate below the crystal surface and grow towards the surface with increasing temperature as suggested by the near-field plasmonic response of the gold nanorod antennas.

Control of laser pointer radiation by means of tunable acousto-optic filter

Abstract We measured optical radiation parameters of commercial laser pointers controlled by stabilized external sources of electric power and also by autonomous built-in alkaline batteries slowly discharging during a continuous operation of the pointers. In particular, variations of optical intensity and wavelength of the radiation generated by the pointers at the wavelengths close to 405 nm and 650 nm were recorded by the acousto-optic method. We used for the purpose, a tunable acousto-optic filter based on tellurium dioxide single crystal. Light diffraction on ultrasound was provided in the crystal by a slow shear acoustic wave propagating at the angle 9° with respect to the axis [1 1 0] in the ( 1 1 ¯ 0 ) plane of the material. Dependences of the output intensity and the lasing wavelengths on time and driving voltage were recorded in the carried out experiments. We also examined drifts of the wavelengths and the intensities in case of stabilized external as well as discharging built-in sources of electrical power. When used with the built-in batteries, the violet pointer demonstrated drifts of the wavelengths up to 3.2 nm during the first 50 min of the laser operation. Similar measurements in the red pointer resulted in the drift about 0.35 nm. Advantages of the used acousto-optic method of the radiation control are briefly discussed in the paper.

Defective titanium dioxide single crystals exposed by high-energy {001} facets for efficient oxygen reduction.

The cathodic material plays an essential role in oxygen reduction reaction for energy conversion and storage systems. Titanium dioxide, as a semiconductor material, is usually not recognized as an efficient oxygen reduction electrocatalyst owning to its low conductivity and poor reactivity. Here we demonstrate that nano-structured titanium dioxide, self-doped by oxygen vacancies and selectively exposed with the high-energy {001} facets, exhibits a surprisingly competitive oxygen reduction activity, excellent durability and superior tolerance to methanol. Combining the electrochemical tests with density-functional calculations, we elucidate the defect-centred oxygen reduction reaction mechanism for the superiority of the reductive {001}-TiO2−x nanocrystals. Our findings may provide an opportunity to develop a simple, efficient, cost-effective and promising catalyst for oxygen reduction reaction in energy conversion and storage technologies.

Calculation and fabrication of two-dimensional complete photonic bandgap structures composed of rutile TiO2 single crystals in air/liquid

Photoelectrochemical applications of photonic crystals are gathering great interests both from physicists and chemists. Here, we theoretically and experimentally present two-dimensional photonic bandgap (2D-PBG) structures based on rutile titanium dioxide (TiO2) single crystal that is a famous material because of the photoelectrochemical ability. The structures were the arrays of hollow hexagonal rutile TiO2 pillars in contact with air or a typical nonaqueous electrolyte solution, acetonitrile. Since the TiO2 refractive indices exhibit a strong dispersive behavior, the bandgap width was discussed from the viewpoint of the refractive index map that would be helpful for the real application of this structure. The 2D-PBG structures for both infrared light and visible light were fabricated by our established lithography technique for rutile TiO2 with and without Nb doping, i.e., photocatalytic TiO2 and high electron conductive TiO2, respectively. These structures show characteristic absorbance peaks or reflectance dips at wavelengths predicted by our theoretical calculations.

Lattice location and annealing behaviour of helium atoms implanted in uranium dioxide single crystals

Helium behaviour in irradiated uranium dioxide may play an important role in the mechanical stability of nuclear fuels during and after its use in nuclear power plants. Helium migration mechanisms in bulk UO2 have already been the subject of theoretical studies but there is a lack of experimental data relating to the most stable location in the crystal. To this end, we have studied uranium dioxide samples implanted with helium ions at low fluence before and after thermal annealing in the range 600 and 800 °C. UO2 single crystals were implanted with 50 keV−3He ions at the fluence of 1 × 1015 at cm−2 and the location in the lattice of helium atoms was investigated using NRA (Nuclear Reaction Analysis) based on the reaction of 3He with deuterons (3He (d,p) 4He) in a channelling mode, recording angular scans across axes and planes. Furthermore, the uranium sub-lattice was analysed by the classical RBS method. After implantation, the experimental angular scans recorded across the main crystallographic axes and along major planes show that the helium atoms in their large majority occupy octahedral interstitial sites. No modification of the occupied crystallographic site was found after annealing at 600 °C. Conversely, no crystallographic relationship between matrix and helium signals was revealed following annealing at 800 °C. The latter feature is likely related to the clustering of implanted helium atoms into gas-filled bubbles. These experimental results have been quantified and interpreted using Monte Carlo simulations with the McChasy code.

Temporal evolution of photothermal-induced Rayleigh wave and plate deformation as an interference in the transient kinetics of photoinduced carrier recombination of a rutile titanium dioxide single crystal.

The dynamic properties of photothermal processes occurring after the initial photogenerated carrier recombination in the flash photolysis of a semiconductor plate sample probed with a reflected infrared beam are reported in this work. Transient kinetics pertaining to the photothermal processes always appear as interfering signals in that of the photogenerated carrier recombination and should be distinguished and excluded. We observed that the photothermal-induced Rayleigh wave occurs immediately after the photogenerated carrier recombination that is then followed by the photothermal-induced flexural vibration of the sample substrate with a set of intrinsic frequencies as reported in our previous work (Appl. Spectrosc. 2013. 67(5): 506-512). When these two faster types of waves decay, the transient decaying signal from the plate deformation due to the inhomogeneous temperature field remains for much longer than 22 ms. Thus, all three types of the photothermal dynamic processes of different temporal properties induced by the pulsed laser on an absorbing semiconductor thin plate are clearly identified.

Bonding xenon and krypton on the surface of uranium dioxide single crystal

Abstract We present density functional theory (DFT) calculation results of krypton and xenon atoms interaction on the surface of uranium dioxide single crystal. A pseudo-potential approach in the generalised gradient approximation (GGA) was applied using the ABINIT program package. To compute the unit cell parameters, the 25 atom super-cell was chosen. It has been revealed that close to the surface of a potential well is formed for xenon and krypton atom due to its interaction with the atoms of oxygen and uranium. Depth and shape of the well is the subject of ab initio calculations in adiabatic approximation. The calculations were performed both for the case of oxygenic and metallic surfaces. It has been shown that the potential well for the oxygenic surface is deeper than for the metallic surface. The thermal stability of immobilising the atoms of krypton and xenon in the potential wells were evaluated. The results are shown in graphs.

Anisotropic thermal conductivity in uranium dioxide.

The thermal conductivity of uranium dioxide has been studied for over half a century, as uranium dioxide is the fuel used in a majority of operating nuclear reactors and thermal conductivity controls the conversion of heat produced by fission events to electricity. Because uranium dioxide is a cubic compound and thermal conductivity is a second-rank tensor, it has always been assumed to be isotropic. We report thermal conductivity measurements on oriented uranium dioxide single crystals that show anisotropy from 4 K to above 300 K. Our results indicate that phonon-spin scattering is important for understanding the general thermal conductivity behaviour, and also explains the anisotropy by coupling to the applied temperature gradient and breaking cubic symmetry.

Investigating the impurity structure for Fe3+ ions doped into rutile titanium dioxide single crystal

The local environment around Fe3+ centers in rutile TiO2 crystals is studied by employing fourth-order perturbation theory formula based on the dominant spin–orbit coupling mechanism. The zero-field splitting parameters (ZFSPs) D and E and crystal field parameters are modeled for the Fe3+ ions not only at the substitution Ti4+ site, but also at the interstitial site with local symmetry D2h. In order to acquire the best agreement between the calculated ZFSPs and those measured by electron magnetic resonance, the model parameters are adjusted on the basis of several approaches. This enables us to determine the feasible values of the structural distortions resulting from dopant Fe3+ ions. Consequently, it is confirmed that Fe3+ ions substitute for Ti4+ sites in rutile TiO2 crystals.

Plasmon-enhanced photocurrent generation and water oxidation from visible to near-infrared wavelengths

This paper presents recent investigations of plasmon-enhanced photoelectric conversion and water oxidation by visible and near-infrared light irradiation. Since the discovery of the Honda–Fujishima effect in 1972, significant efforts have been devoted to lengthening the light–energy conversion wavelength. In this context, plasmonic photoelectric conversion has been recently demonstrated at visible-to-near-infrared wavelengths without deteriorating photoelectric conversion by employing titanium dioxide (TiO2) single-crystal photoelectrodes, in which gold nanorods are elaborately arrayed on the surface. A potassium perchlorate aqueous solution was employed as an electrolyte solution without additional electron donors; thus, water molecules provided the electrons. The stoichiometric evolution of oxygen and hydrogen peroxide as a result of the four- or two-electron oxidation of water molecules, respectively, was accomplished with near-infrared light irradiation using the plasmonic optical antenna effect. As there is very little overpotential for water oxidation, these results constitute a significant advancement in this field. In addition, this photoelectric conversion system could potentially be employed in artificial photosynthesis systems that exceed the photosynthetic capabilities of plants by allowing for photoconversion over a wide range of wavelengths. Plasmonic photoelectric conversion from visible to near-infrared wavelengths has been successfully demonstrated using electrodes in which gold nanorods are elaborately arrayed on a titanium dioxide (TiO2) single crystal. Importantly, water molecules serve as the electron donor in the plasmonic photoelectric conversion system. Therefore, the stoichiometric evolution of oxygen and hydrogen peroxide as a result of the four- or two-electron oxidation of water molecules was accomplished even at near-infrared wavelengths, enabling the application of this plasmonic photoelectric system in artificial photosynthesis processes responding to a wide range of solar wavelengths. Sunlight is a promising source of renewable energy, and a considerable amount of research is invested in converting solar energy into electric or chemical energy. As the sunlight that reaches the Earth's surface comprises mostly visible and infrared radiation, it is crucial to be able to capture and convert these longer wavelengths. Metallic nanoparticles that exhibit localized surface plasmon resonance are particularly attractive for this application — incident light induces a collective oscillation of electrons at resonant wavelengths that can be adjusted by tailoring the nanoparticles' size, shape and order within an array. In this Review, Kosei Ueno and Hiroaki Misawa describe recent advances made using a photoconversion system composed of an array of gold nanorods arranged on a single-crystal titanium dioxide electrode. Incoming sunlight excites titanium dioxide through these nanorods — which act as plasmonic antennas — rather than directly, generating photocurrent and promoting water oxidation by means of visible and near-infrared light.

Two regimes of wide angle acousto-optic interaction in tellurium dioxide single crystals

The paper presents results of investigation of wide angle acousto-optic (AO) diffraction based on light beams propagating in birefringent crystals far away from optical axes and also close to the optical axes. A comparison of the two cases of wide angle interaction in tellurium dioxide crystal is carried out. Advantages and drawbacks of the far-off axis (FOA) diffraction are examined in the paper. Possibility of application of the FOA diffraction in imaging AO filters is discussed. Results of theoretical consideration of the problem are experimentally confirmed in a paratellurite filter using a slow shear acoustic wave propagating in the (11¯0) plane of the crystal at the angle α=10° relative to the axis [110].

Monte Carlo simulation of phonon transport in UO2 single crystals

The Monte Carlo (MC) method is applied to solve the Boltzmann transport equation for phonons in uranium dioxide single crystals, with the objective of understanding thermal transport in this material at the mesoscale. The overall solution scheme tracks the phonon density as it evolves in space and time due to phonon drift and phonon–phonon scattering by normal and Umklapp processes. Unlike most previous works on solving the Boltzmann transport equation for phonons by the MC technique, our scheme for calculating phonon lifetime, based on normal and Umklapp scattering, eliminates the need for using many fitting parameters. Instead, the Grüneisen parameter, which is a well-characterized material property, is the only parameter of the problem. The results elucidate the simulation domain size over which the computed conductivity is size dependent; this helps to determine the minimum domain size required to simulate bulk thermal transport and hence calculate the thermal conductivity. The latter is computed over the temperature range 300–1000 K. The computed conductivity values are in good agreement with the previously published experimental and molecular dynamics simulation results.

Persistent Trapping of Photogenerated Carriers in Colorless Anatase TiO2Single Crystals

Ultraviolet (UV) light irradiation to colorless anatase titanium dioxide single crystals at 100–30 K gives rise to the appearance of sextuplets in electron paramagnetic resonance (EPR) spectra. The...