[Show abstract][Hide abstract] ABSTRACT: Nanocrystalline silicon carbide (SiC) represents an excellent model system for a fundamental
study of interfacial (grain boundary) processes under nuclear radiation, which are critical to
the understanding of the response of nanostructured materials to high-dose irradiation. This
study reports on a comparison of irradiation effects in cubic phase SiC (3C-SiC) grains of a
few nanometres in size and single-crystal 3C-SiC films under identical Au ion irradiation to a
range of doses at 700 K. In contrast to the latter, in which the lattice disorder is accumulated
to a saturation level without full amorphization, the average grain size of the former increases
with dose following a power-law trend. In addition to coalescence, the grain grows through
atomic jumps and mass transport, where irradiation-induced vacancies at grain boundaries
assist the processes. It is found that a higher irradiation temperature leads to slower grain
growth and a faster approach to a saturation size of SiC nanograins. This unusual behaviour
could be associated with irradiation-induced grain nucleation and growth in amorphous SiC
matrix in which the 3C-SiC grains are embedded. The results could potentially have a positive
impact on structural components of advanced nuclear energy systems.
Full-text · Article · Jan 2016 · Journal of Physics D Applied Physics
[Show abstract][Hide abstract] ABSTRACT: Copper and copper oxide thin films have been synthesized on (001) SrTiO3 substrates using pulsed laser deposition. Three kinds of thin films can be achieved by changing deposition temperature and laser energy density: pure Cu, Cu:Cu2O composite, and pure Cu2O thin films. A phase diagram about the compositions of thin films has been represented and explained by redox reaction at the substrate surface during the deposition process. The present work represents a novel approach to further understand the surface reaction during deposition process and to fabricate oxide thin films with variable valence states using pulsed laser deposition.
No preview · Article · Nov 2015 · Journal of Applied Physics
[Show abstract][Hide abstract] ABSTRACT: Enhancement of oxygen ion conductivity in oxides is important for low-temperature (o500 °C) operation of solid oxide fuel cells, sensors and other ionotronic devices. While huge ion conductivity has been demonstrated in planar heterostructure films, there has been considerable debate over the origin of the conductivity enhancement, in part because of the difficulties of probing buried ion transport channels. Here we create a practical geometry for device miniaturization, consisting of highly crystalline micrometre-thick vertical nanocolumns of Sm-doped CeO 2 embedded in supporting matrices of SrTiO 3. The ionic conductivity is higher by one order of magnitude than plain Sm-doped CeO 2 films. By using scanning probe microscopy, we show that the fast ion-conducting channels are not exclusively restricted to the interface but also are localized at the Sm-doped CeO 2 nanopillars. This work offers a pathway to realize spatially localized fast ion transport in oxides of micrometre thickness.
Full-text · Article · Oct 2015 · Nature Communications
[Show abstract][Hide abstract] ABSTRACT: Heterointerfaces in manganite-based heterostructures in either layered or vertical geometry are considered as one of the keys to manipulate and improve the magnetotransport properties. Instead of using the spin polarized tunneling across the interface, a unique approach based on the magnetic exchange coupling along the vertical interface to control magnetotransport properties has been demonstrated. By coupling ferromagnetic La0.7Sr0.3MnO3 and antiferromagnetic NiO in an epitaxial vertically aligned nanocomposite (VAN) architecture, a dynamic and reversible switch of the resistivity between two distinct exchange biased states has been achieved. This study explores the use of vertical interfacial exchange coupling to tailor magnetotransport properties, which may serve as a viable route for spintronic applications.
[Show abstract][Hide abstract] ABSTRACT: Complex oxides provide an ideal playground for exploring the interplay among the fundamental
degrees of freedom: structural (lattice), electronic (orbital and charge), and magnetic (spin). In
thin films and heterostructures, new states of matter can emerge as a consequence of such
interactions. Over the past decade, the ability to synthesize self-assembled nanocomposite
thin films of metal oxides has provided another pathway for creating new interfaces and,
thus, new physical phenomena. In this article, we describe examples of such materials
systems explored to date and highlight the fascinating multifunctional properties achieved.
These include enhanced flux pinning in superconductors, strain-enhanced ferroelectricity,
strain- and charge-coupled magnetoelectrics, tunable magnetotransport, novel electrical/
ionic transport, memristors, and tunable dielectrics.
[Show abstract][Hide abstract] ABSTRACT: Solid solutions have been widely investigated for solar energy conversion because of the ease to control properties (e.g., band edge positions, charge carrier transport, and chemical stability). In this study, we introduce a new method to investigate intrinsic solar energy conversion properties of solid solutions through fabricating high-quality single-crystalline solid solution films by pulsed laser deposition. This method rules out external factors, such as morphology, crystalline grain size, orientation, density and distribution, surface area, and particle-particle or particle-conducting layer connection, that have plagued previous studies on solid solution photoelectrodes. Perovskite BiFeO3 (BFO) and SrTiO3 (STO) were chosen as "end" members of the solid solutions (i.e., (BFO)x(STO)1-x (0 ≤ x ≤ 1)). Optical and photoelectrochemical (PEC) properties of the solid solutions significantly varied with changing compositions. Among the six studied compositions, BFO:STO (3:1 molar ratio) exhibited the highest photocurrent density with the photovoltage of 1.08 V. The photoelectrode also produced stable photocurrent for 12 h. Faradaic efficiencies of H2 and O2 formation close to 100% were measured.
No preview · Article · Sep 2015 · Chemistry of Materials
[Show abstract][Hide abstract] ABSTRACT: We design and create a unique cell geometry of templated micrometer-thick epitaxial nanocomposite films which contain ~20-nm diameter yttria-stabilized ZrO2 (YSZ) nanocolumns, strain coupled to a SrTiO3 matrix. The ionic conductivity of these nanocolumns is enhanced by over two orders of magnitude compared to plain YSZ films. Concomitant with the higher ionic conduction is the finding that the YSZ nanocolumns in the films have much higher crystallinity and orientation, compared to plain YSZ films. Hence, "oxygen migration highways" are formed in the desired out-of-plane direction. This improved structure is shown to originate from the epitaxial coupling of the YSZ nanocolumns to the SrTiO3 film matrix and from nucleation of the YSZ nanocolumns on an intermediate nanocomposite base layer of highly aligned Sm-doped CeO2 nanocolumns within the SrTiO3 matrix. This intermediate layer reduces the lattice mismatch between the YSZ nanocolumns and the substrate. Vertical ionic conduction values as high as 10(-2) Ω(-1)cm(-1) were demonstrated at 360(o)C (300(o)C lower than plain YSZ films), showing the strong practical potential of these nanostructured films for use in much lower operation temperature ionic devices.
[Show abstract][Hide abstract] ABSTRACT: The dielectric properties of Z-type hexaferrite Sr3Co2Fe24O41 (SCFO) have been investigated as a function of temperature from 153 to 503 K between 1 and 2 GHz. The dielectric responses of SCFO are found to be frequency dependent and thermally activated. The relaxation-type dielectric behavior is observed to be dominating in the low frequency region and resonance-type dielectric behavior is found to be dominating above 10(8) Hz. This frequency dependence of dielectric behavior is explained by the damped harmonic oscillator model with temperature dependent coefficients. The imaginary part of impedance (Z″) and modulus (M″) spectra show that there is a distribution of relaxation times. The scaling behaviors of Z″ and M″ spectra further suggest that the distribution of relaxation times is temperature independent at low frequencies. The dielectric loss spectra at different temperatures have not shown a scaling behavior above 10(8) Hz. A comparison between the Z″ and the M″ spectra indicates that the short-range charges motion dominates at low temperatures and the long-range charges motion dominates at high temperatures. The above results indicate that the dielectric dispersion mechanism in SCFO is temperature independent at low frequencies and temperature dependent at high frequencies due to the domination of resonance behavior.
Full-text · Article · Aug 2015 · Scientific Reports
[Show abstract][Hide abstract] ABSTRACT: Vertically aligned nanocomposite (VAN) (La0.7Sr0.3MnO3)1−x:(CeO2)x (LSMO:CeO2) thin films have been grown on SrTiO3 (001) substrates by pulsed laser deposition. Tunable magnetoresistance
properties as well as microstructures are demonstrated in these VAN films by modulating the film composition (x = 0, 0.3, 0.4, 0.45, 0.5, and 0.55). The sample of x = 0.3 shows a large low-field magnetoresistance (LFMR) in a high temperature range, i.e., over 10% at the range of 280 K to 320 K under 1 T and with a peak value of ∼13.5% at 310 K. In addition, a vast enhancement of LFMR in a low temperature range of 20–150 K with peak of ≈34.3% at 45 K for 1 T could be achieved with x = 0.5. The enhanced LFMR properties can be attributed to both the phase boundary induced spin fluctuation and the magnetic tunneling effect through vertical ferromagnetic/insulator/ferromagnetic structures. The observed enhanced LFMR performance, especially at high temperatures, as well as its simple growth method, offers a great potential for LSMO:CeO2
nanocomposites to be used in spintronic devices in a large temperature range.
No preview · Article · Aug 2015 · Journal of Applied Physics
[Show abstract][Hide abstract] ABSTRACT: Exchange bias effect with perpendicular anisotropy is of great interest for potential applications such as read heads in magnetic storage devices with high thermal stability and reduced dimension. Here we report a novel approach to achieving perpendicular exchange bias by orienting the ferromagnetic/antiferromagnetic coupling in the vertical geometry through a unique vertically aligned nanocomposite (VAN) design. Our results demonstrate robust perpendicular exchange bias phenomena in micrometer-thick films employing a prototype material system of antiferromagnetic BiFeO3 and ferromagnetic La0.7Sr0.3MnO3. The unique response of exchange bias to perpendicular magnetic field reveals the existence of exchange coupling along their vertical heterointerfaces, which exhibits strong dependence on their strain states. This VAN approach enables a large selection of material systems for achieving perpendicular exchange bias, which could lead to advanced spintronic devices.
[Show abstract][Hide abstract] ABSTRACT: Vertical interfaces in vertically aligned nanocomposite thin films have been approved to be an effective method to manipulate functionalities. However, several challenges with regard to the understanding on the physical process underlying the manipulation still remain. In this work, because of the ordered interfaces and large interfacial area, heteroepitaxial (BaTiO3)1-x:(Sm2O3)x thin films have been fabricated and used as a model system to investigate the relationship between vertical interfaces and dielectric properties. Due to a relatively large strain generated at the interfaces, vertical interfaces between BaTiO3 and Sm2O3 are revealed to become the sinks to attract oxygen vacancies. The movement of oxygen vacancies is confined at the interfaces and hampered by the misfit dislocations, which contributed to a relaxation behavior in (BaTiO3)1-x:(Sm2O3)x thin films. This work represents an approach to further understand that how interfaces influence on dielectric properties in oxide thin films.
Full-text · Article · Jun 2015 · Scientific Reports
[Show abstract][Hide abstract] ABSTRACT: Auxetic-like strain states were generated in self-assembled nanocomposite
thin films of (Ba0.6Sr0.4TiO3)1−x
(BSTO − SmO). A switch from auxetic-like to elastic-like strain behavior was observed for x > 0.50, when the SmO switched from being nanopillars in the BSTO matrix to being the matrix with BSTO nanopillars embedded in it. A simple model was adopted to explain how in-plane strain varies with x. At high x (0.75), strongly enhanced ferroelectric properties were obtained compared to pure BSTO films. The nanocomposite method represents a powerful new way to tune the properties of a wide range of strongly correlated metal oxides whose properties are very sensitive to strain.
[Show abstract][Hide abstract] ABSTRACT: Addition of second-phase nanosize defects to YBa2Cu3O7-δ (YBCO) superconductor thin films is known to enhance flux pinning and increase current densities (Jc). The addition of Y2BaCuO5 (Y211) was previously studied in (Y211/ YBCO)N multilayer structures and in Y211 + YBCO films deposited from pie-shaped targets. This research systematically studies the effect of Y211 addition in thin films deposited by pulsed laser deposition from YBCO1-xY211x(x = 0-15 vol.%) single targets, at temperatures of 785°C-840°C. Interestingly, the resulting size of Y211 particles is 20-40 nm, in contrast to 10-15 nm in previous studies of Y211 and 5-10 nm for other second-phase defect additions, and the number density is reduced. A slight increase of Jc(H, T) was achieved, compared with previous optimization studies. Results and comparisons of flux pinning, intrinsic stresses imaged by TEM, current densities, critical temperatures, and microstructures will be presented. The overall low intrinsic stress on YBCO from Y211 lattice mismatch is smaller than previously studied second-phase defect additions known, which is hypothesized to be the driving force in achieving the unusually large second-phase nanoparticle size and volume fraction thus far in YBCO thin films.
No preview · Article · Jun 2015 · IEEE Transactions on Applied Superconductivity
[Show abstract][Hide abstract] ABSTRACT: Iron-based superconductors have attracted great research interests from both the intriguing fundamental superconducting mechanism aspects and their potential applications in high fields owing to their high critical field Hc2 and low field anisotropy. However, one critical factor limiting the commercial applications of superconducting coated conductors is the significant manufacturing costs involved in the processing of the complex layered buffers and the subsequent epitaxial growth of superconducting coated conductors. Here we demonstrate a much simplified superconducting coated conductor design for Fe-based superconductor on glass and metallic substrates without bi-axial texturing buffers. Using this design, FeSe0.1Te0.9 thin films on glass show superconducting properties of critical temperatures Tczero of 10 K, and Tconset of 12.5 K, self-field critical current density (Jc) of 2.1×104 A/cm2 at 4 K, and upper critical field (Hc2) as high as 126 T. This work could lay a critical foundation toward future practical applications of Fe-based superconductor coated conductors.
No preview · Article · Jun 2015 · Journal of Alloys and Compounds
[Show abstract][Hide abstract] ABSTRACT: Fast ion transport channels at interfaces in thin films have attracted great attention due to a range of potential applications for energy materials and devices, for, solid oxide fuel cells, sensors, and memories. Here, it is shown that in vertical nanocomposite heteroepitaxial films of SrZrO3–RE2O3 (RE = Sm, Eu, Gd, Dy, and Er) the ionic conductivity of the composite can be tuned and strongly enhanced using embedded, stiff, and vertical nanopillars of RE2O3. With increasing lattice constant of RE2O3 from Er2O3 to Sm2O3, it is found that the tensile strain in the SrZrO3 increases proportionately, and the ionic conductivity of the composite increases accordingly, by an order of magnitude. The results here conclusively show, for the first time, that strain in films can be effectively used to tune the ionic conductivity of the materials.
Full-text · Article · Jun 2015 · Advanced Functional Materials
[Show abstract][Hide abstract] ABSTRACT: Bi2FeMnO6 (BFMO) thin films with both conventional pseudo-cubic structure and novel supercell structure have been grown on SrTiO3 (001) substrates with different thicknesses of CeO2 buffer layers (ranging from 6.7 nm to 50.0 nm) using pulsed laser deposition. The correlation between the thickness of the CeO2 buffer layer and the structure of the BFMO films shows that the CeO2 buffer layer, as thin as 6.7 nm, is sufficient in triggering the novel BFMO supercell structure. This may be ascribed to the interfacial strain between the BFMO supercell structure and the CeO2 buffer layer which also serves as a seed layer. The buffer layer thickness is found to be critical to control the microstructure and magnetism of the formed BFMO supercell structures. Thin seed layers can produce a smoother interface between the BFMO film and the CeO2 buffer layer, and therefore better ferrimagnetic properties. Our results have demonstrated that strain and interface could be utilized to generate novel thin film structures and to tune the functionalities of the thin films.
[Show abstract][Hide abstract] ABSTRACT: Recent studies have shown that chemical immiscibility is important to achieve enhanced radiation tolerance in metallic multilayers as immiscible layer interfaces are more stable against radiation induced mixing than miscible interfaces. However, as most of these immiscible systems have incoherent interfaces, the influence of coherency on radiation resistance of immiscible systems remains poorly understood. Here, we report on radiation response of immiscible Cu/Fe multilayers, with individual layer thickness h varying from 0.75 to 100 nm, subjected to He ion irradiation. When interface is incoherent, the peak bubble density decreases with decreasing h and reaches a minimum when h is 5 nm. At even smaller h when interface is increasingly coherent, the peak bubble density increases again. However, void swelling in coherent multilayers with smaller h remains less than those in incoherent multilayers. Our study suggests that the coherent immiscible interface is also effective to alleviate radiation induced damage.
Full-text · Article · May 2015 · Journal of Materials Research
[Show abstract][Hide abstract] ABSTRACT: NiO effectively promotes the sintering of highly refractory Y-doped BaZrO3 (BZY) through the formation of BaY2NiO5, providing a simple and cost-effective method for the fabrication of dense BZY electrolyte and Ni–BZY hydrogen separation membrane at ∼1400 oC. Unfortunately, insulating BaCO3 and Y2O3 phases formed on the surface of BZY and Ni–BZY prepared by solid state reaction method with NiO after annealing in wet CO2. Ni–BZY membranes prepared from different methods suffered different degree of performance loss in wet H2 at 900 °C. The chemical instability of Ni–BZY is attributed to the formation of a secondary phase (BaY2O4) generated from the reduction of BaY2NiO5 in H2 during the sintering process. Both BaY2O4 and BaY2NiO5 react with H2O, and CO2 at elevated temperatures, generating insulating Ba(OH)2 and BaCO3 phases, respectively. The less BaY2O4 is formed in the fabrication process, the better chemical stability the Ni–BZY membranes possess. Therefore, a new Ni–BZY membrane is prepared through a judicial combination of BZY powders prepared from combined EDTA-citric and solid state reaction methods, and demonstrates exceptional chemical stability in H2O and CO2, enabling stable and even improved hydrogen flux in wet 50% CO2 at 900 °C.
Full-text · Article · Mar 2015 · Journal of Power Sources
[Show abstract][Hide abstract] ABSTRACT: High energy particles can introduce severe radiation damage in metallic materials especially those with low stacking fault energy. Twin boundary (TB) has recently been shown to enable the reduction of defect density in heavy ion irradiated nanotwinned Ag. However, the defect-twin boundary interaction mechanisms in nanotwinned metals remain poorly understood. Here we report on the study of TB affected zone wherein time accumulative defect density and defect diffusivity are substantially different from those in twin interior. In situ studies also reveal surprising resilience of TBs in response to radiation: TBs continue to change their geometry to facilitate the capture, transportation and removal of defect clusters and can recover by absorbing opposite type of defects. This study provides further support for the implementation of TBs as effective defect sinks for the design of radiation tolerant nanostructured metallic materials.