Publications (181)853.72 Total impact
- [Show abstract] [Hide abstract] ABSTRACT: Metamaterials made of nanoscale inclusions or artificial unit cells exhibit exotic optical properties that do not exist in natural materials. Promising applications, such as super-resolution imaging, cloaking, hyperbolic propagation, and ultrafast phase velocities have been demonstrated based on mostly micrometer-scale metamaterials and few nanoscale metamaterials. To date, most metamaterials are created using costly and tedious fabrication techniques, with limited paths towards reliable large-scale fabrication. In this work, we demonstrate the one-step direct growth of self-assembled epitaxial metal-oxide nanocomposites as a drastically different approach to fabricating large-area nanostructured metamaterials. Using pulsed laser deposition, we fabricated nanocomposite films with vertically aligned Au nanopillars (~ 20 nm in diameter) embedded in various oxide matrices with high epitaxial quality. Strong, broad absorption features in the measured absorbance spectrum are clear signatures of plasmon resonances of Au nanopillars. By tuning their densities on selected substrates, anisotropic optical properties are demonstrated via angular dependent and polarization resolved reflectivity measurements and reproduced by full-wave simulations and effective medium theory. Our model predicts exotic properties, such as zero permittivity responses and topological transitions. Our studies suggest that these self-assembled metal-oxide nanostructures provide an exciting new material platform to control and enhance optical response at nanometer scale.
- [Show abstract] [Hide abstract] ABSTRACT: We report on spontaneously phase ordered heteroepitaxial SrTiO3 (STO):ZnFe2O4 (ZFO) nanocomposite films that give rise to strongly enhanced photoelectrochemical solar water oxidation, consistent with enhanced photoinduced charge separation. The STO:ZFO nanocomposite yielded an enhanced photocurrent density of 0.188 mA/cm2 at 1.23 V vs a reversible hydrogen electrode, which was 7.9- and 2.6-fold higher than that of the plain STO film and ZFO film cases under 1-sun illumination, respectively. The photoelectrode also produced stable photocurrent and Faradaic efficiencies of H2 and O2 formation that were more than 90%. Incident-photon-to-current-conversion efficiency measurements, Tauc plots, Mott–Schottky plots, and electrochemical impedance spectroscopy measurements proved that the strongly enhanced photogenerated charge separation resulted from vertically aligned pseudosingle crystalline components, epitaxial heterojunctions, and a staggered band alignment of the components of the nanocomposite films. This study presents a completely new avenue for efficient solar energy conversion applications.
- [Show abstract] [Hide abstract] ABSTRACT: Vertically aligned nanocomposite (VAN) (La0.7Sr0.3MnO3)0.5(CeO2)0.5 and pure La0.7Sr0.3MnO3 layers were incorporated into YBa2Cu3O7-δ (YBCO) thin films as bilayer stacks for magnetic flux pinning enhancement. The films show high epitaxial quality, suggested by XRD and TEM study. The critical temperature T c of the bilayers is about 90K, which is close to that of pure YBCO films, while both the self-field J csf and in-field critical current density J cin-field are largely enhanced. Among all samples, the film with VAN cap layer shows the highest J c values in all field ranges. This study demonstrates an effective way towards the tunable pinning effect for YBCO coated conductors by both defect and magnetic pinning.
- [Show abstract] [Hide abstract] ABSTRACT: The dielectric properties of epitaxial BaFe12O19 hexaferrite thin film have been investigated as a function of frequency (50 Hz − 2 MHz) and temperature (100–375 K). The frequency dependent permittivity, impedance () and modulus () spectra show that the dielectric responses of BaFe12O19 thin film are thermally activated. The activation energy of BaFe12O19 film (E a) is much smaller than that of the polycrystalline bulk BaFe12O19. In addition, E a increases with increasing temperature and there is a distribution of relaxation time in the sample. The scaling behavior of and spectra of the sample further suggest that the distribution of relaxation time is temperature independent at low temperatures (<250 K) and temperature dependent at high temperatures. The temperature dependent dc conductivity shows that small polaron hopping is the most probable conduction mechanism for BaFe12O19 film.
- [Show abstract] [Hide abstract] ABSTRACT: Ferromagnetic insulating thin films of Sm0.34Sr0.66MnO3 (SSMO) on (001) SrTiO3 substrates with TC of 140 K were formed in self-assembled epitaxial nanocomposite thin films. The high TC ferromagnetism was enabled through vertical epitaxy of the SSMO matrix with embedded, stiff, ~40 nm Sm2O3 nanopillars giving a c/a ratio close to 1 in the SSMO. In contrast, bulk and single phase SSMO films of the same composition have much stronger tetragonal distortion, the bulk having c/a >1 and the films having c/a <1, both of which give rise to antiferromagnetic coupling. The work demonstrates a unique and simple route to creating ferromagnetic insulators for spintronics applications where currently available ferromagnetic insulators are either hard to grow and/or have very low TC.
- [Show abstract] [Hide abstract] ABSTRACT: This research was funded by the Engineering and Physical Sciences Research Council, (EP/P50385X/1), the European Research Council (ERC-2009-AdG 247276 NOVOX). The work at Texas A&M was funded by the U.S. National Science Foundation (DMR-1401266). The work at Los Alamos was supported by the U.S. Department of Energy through the LANL/LDRD program and was performed, in part, at the Center for Integrated Nanotechnologies, a U.S. Department of Energy, Office of Basic Energy Sciences user facility. Use of the National Synchrotron Light Source, Brookhaven National Laboratory, was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-98CH10886.
- [Show abstract] [Hide abstract] ABSTRACT: The transition temperatures of epitaxial films of Fe(Te0:9Se0:1) are remarkably insensitive to applied magnetic field, leading to predictions of upper critical fields Bc2(T = 0) in excess of 100 T. Using pulsed magnetic fields, we find Bc2(0) to be on the order of 45 T, similar to values in bulk material and still in excess of the paramagnetic limit. The same films show strong magnetoresistance in fields above Bc2(T), consistent with the observed Kondo minimum seen above Tc. Fits to the temperature dependence in the context of the WHH model, using the experimental value of the Maki parameter, require an effective spin-orbit relaxation parameter of order unity. We suggest that Kondo localization plays a similar role to spin-orbit pair breaking in making WHH fits to the data.
- [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.
- [Show abstract] [Hide abstract] ABSTRACT: Magnetic epitaxial (CoFe2O4)0.1(CeO2)0.9 nanocomposite layers were incorporated into superconducting FeSe0.1Te0.9 thin films as either a cap layer or a buffer layer. Both capped and buffered samples show an enhancement of the superconducting property compared to the reference sample without the incorporated layer, while the capped one shows the best pinning properties of all the samples. Specifically for the capped sample, the critical temperature [Formula: see text] is ~12.5 K, while the self-field critical current density [Formula: see text] increases to as high as 1.20 MA cm(-2) at 4 K. Its [Formula: see text] value shows a slower decrease with increasing applied magnetic field, with the lowest power-law exponent α values (derived following [Formula: see text] by the [Formula: see text] plot) of 0.20, 0.23 and 0.33 at 2 K, 4 K and 8 K, respectively. This nanocomposite capped sample also exhibits a high upper critical field [Formula: see text] of 166 T, which indicates its potential in high field applications. This pinning method provides an effective way of enhancing the superconducting property of iron chalcogenide thin film.
- [Show abstract] [Hide abstract] ABSTRACT: We describe a study of the temperature dependent deformation behavior of a multilayered NiFe-60 wt%Fe alloy with a layer thickness of 5 μm fabricated by electrodeposition. The structure of adjacent layers alternates between a nanocrystalline and a coarse grained. Uniaxial tensile tests at temperature between 20 °C and 400 °C and strain rate of 10−4–10−2 were used to determine the mechanical behavior. Microstructure observations via transmission electron microscopy and fractography were performed to provide insight into the underlying deformation mechanism. The mechanical behavior is discussed in the context of the bimodal microstructure of multilayered samples and the contribution of each sub-layer to strength and ductility. The results reveal that even at higher temperatures the nanocrystalline layer determines the mechanical performance of multilayered materials.
- [Show abstract] [Hide abstract] ABSTRACT: The ability to control the morphology of heterointerfaces with coupled functionalities is fascinating from both fundamental and technological perspectives. Here, using BiFeO3:CoFe2O4 vertically aligned nanocomposite (VAN) films as a model system, we demonstrate a simple and effective method to modulate the heterointerface and its morphology in nanocomposite films with pulsed laser deposition. By tuning the deposition frequency through thickness during film growth, both vertically straight and gradient heterointerfaces have been achieved. The modulated heterointerface is strongly correlated with strain tuning and interface coupling, and thus modifies the magnetic anisotropy, coercive fields, and ferroelectric switching behavior. This study provides a viable approach for tailoring the interface strain and coupling in VAN and achieving tunable physical properties.
- [Show abstract] [Hide abstract] ABSTRACT: We examined the thermal stability of amorphous silicon oxycarbide (SiOC) and crystalline Fe composite by in situ and ex situ annealing. The Fe/SiOC multilayer thin films were grown via magnetron sputtering with controlled length scales on a surface-oxidized Si (100) substrate. These Fe/SiOC multilayers were in situ or ex situ annealed at temperature of 600 °C or lower. The thin multilayer sample (~10 nm) was observed to have a layer breakdown after 600 °C annealing. Diffusion starts from low groove angle triple junctions in Fe layers. In contrast, the thick multilayer structure (~70 nm) was found to be stable and an intermixed layer (FexSiyOz) was observed after 600 °C annealing. The thickness of the intermixed layer does not vary as annealing time goes up. The results suggest that the FexSiyOz layer can impede further Fe, Si and O diffusion, and assists in maintaining morphological stability.
- [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.
- [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.
- [Show abstract] [Hide abstract] ABSTRACT: It is well adopted that a majority of ceramic materials have low ductility and the plastic yielding in crystalline ceramics occurs succeeding elastic deformation. Here by using in situ nanoindentation we show, however, that in nanocrystalline TiN, massive non-elastic events, such as grain rotation and grain boundary (GB) sliding, occur at ultra-low stresses (<1 GPa), well below the yielding strength of the material (6–8 GPa). Furthermore, by performing in situ nanoindentations on TiN with different average grain sizes of 110 nm, 35 nm and 20 nm, we observed a prominent grain size effect: that is specimens with smaller average grain sizes possess greater plastic deformation than the ones with larger grains. This in situ nanoindentation study thus provides new perspectives on understanding plasticity of hard nanocrystalline ceramics at room temperature.
- [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: Vanadium dioxide (VO2) thin films with controlled grain sizes are deposited on amorphous glass substrates by pulsed laser deposition. The grain boundaries (GBs) are found as the dominating defects in the thin films. The semiconductor to metal transition (SMT) properties of VO2 thin films are characterized and correlated to the GB density. The VO2 films with lower GB density exhibit a sharper SMT with a larger transition amplitude. A high resolution TEM study at GB area reveals the disordered atomic structures along the boundaries and the distorted crystal lattices near the boundaries. The VO2 SMT amplitude and sharpness could be directly related to these defects at and near the boundaries.
- [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.
Texas A&M University
College Station, Texas, United States
- Materials Science and Engineering Program
Alabama A & M UniversityHuntsville, Alabama, United States
Texas A&M University - KingsvilleKingsville, Texas, United States
Los Alamos National Laboratory
Лос-Аламос, California, United States
- Superconductivity Technology Center
North Carolina State University
Raleigh, NC, United States
- Department of Materials Science and Engineering