Publications
441 results found
Axelsson A-K, Le Goupil F, Valant M, et al., 2016, Electrocaloric effect in lead-free Aurivillius relaxor ferroelectric ceramics, ACTA MATERIALIA, Vol: 124, Pages: 120-126, ISSN: 1359-6454
Liu G, Feng H, Liu B, et al., 2016, Formation of V-grooves in SrRuO3 epitaxial film, Journal of Crystal Growth, Vol: 455, Pages: 13-18, ISSN: 0022-0248
SrRuO3 thin films were epitaxially grown on a (001) SrTiO3 substrate using pulsed laser deposition technique. Various defects such as V-grooves, threading dislocations and dislocation dipoles are observed in the SrRuO3 epitaxial film. It is found that the sidewalls of most V-grooves are {101} facets, and the dominant angle between the sidewalls is 90°. Some threading dislocations end at the apexes of the V-grooves while the others penetrate the entire film. The threading dislocations and V-grooves can partially relieve the strain in the epitaxial SrRuO3 film. During the relaxation process, a two-dimensional growth mode transforms into a three-dimensional one, along with the formation of mesa-like islands separated by V-grooves. The dimensions and distributions of V-grooves are associated with the growth conditions. The control of growth mechanism and surface morphology are very important for the fabrication of novel perovskite oxide devices.
Petrov PK, Zou B, Walker C, et al., 2016, Growth of Epitaxial Oxide Thin Films on Graphene, Scientific Reports, Vol: 6, ISSN: 2045-2322
The transfer process of graphene onto the surface of oxide substrates is well known.However, for many devices, we require high quality oxide thin films on the surface ofgraphene. This step is not understood. It is not clear why the oxide should adopt theepitaxy of the underlying oxide layer when it is deposited on graphene where there isno lattice match. To date there has been no explanation or suggestion of mechanismswhich clarify this step. Here we show a mechanism, supported by first principlessimulation and structural characterisation results, for the growth of oxide thin films ongraphene. We describe the growth of epitaxial SrTiO3 (STO) thin films on a grapheneand show that local defects in the graphene layer (e.g. grain boundaries) act as bridgepillarspots that enable the epitaxial growth of STO thin films on the surface of thegraphene layer. This study, and in particular the suggestion of a mechanism forepitaxial growth of oxides on graphene, offers new directions to exploit thedevelopment of oxide/graphene multilayer structures and devices.
Niedermeier CA, Råsander M, Rhode S, et al., 2016, Band gap bowing in NixMg1-xO., Scientific Reports, Vol: 6, ISSN: 2045-2322
Epitaxial transparent oxide NixMg1-xO (0 ≤ x ≤ 1) thin films were grown on MgO(100) substrates by pulsed laser deposition. High-resolution synchrotron X-ray diffraction and high-resolution transmission electron microscopy analysis indicate that the thin films are compositionally and structurally homogeneous, forming a completely miscible solid solution. Nevertheless, the composition dependence of the NixMg1-xO optical band gap shows a strong non-parabolic bowing with a discontinuity at dilute NiO concentrations of x < 0.037. Density functional calculations of the NixMg1-xO band structure and the density of states demonstrate that deep Ni 3d levels are introduced into the MgO band gap, which significantly reduce the fundamental gap as confirmed by optical absorption spectra. These states broaden into a Ni 3d-derived conduction band for x > 0.074 and account for the anomalously large band gap narrowing in the NixMg1-xO solid solution system.
Liu G, Wang Y, Zou B, et al., 2016, Probing the Electronic Structures of BaTiO3/SrTiO3 Multilayered Film with Spatially Resolved Electron Energy-Loss Spectroscopy, Journal of Physical Chemistry C, Vol: 120, Pages: 16681-16686, ISSN: 1932-7455
A multilayered film of BaTiO3/SrTiO3 was grown on a LaAlO3 substrate using dual-target pulsed laser deposition technique. High-resolution scanning transmission electron microscopy observations show that the nine unit-cell BaTiO3 layer and the three unit-cell SrTiO3 layer are alternatively arranged in the epitaxial film where a sharp interface exists between the BaTiO3 and SrTiO3 layers. Electron energy-loss spectroscopy analysis demonstrates that the O-K edge spectra of SrTiO3 and BaTiO3 layers are quite distinct; in particular, the energy-loss peak at 547 eV in the SrTiO3 spectrum splits into two peaks in the BaTiO3 spectrum. The multiple-scattering calculations of O-K edge spectra for BaTiO3 and SrTiO3 agree well with the experimental results. The low-energy region (<542 eV) of the O-K edge spectra for both BaTiO3 and SrTiO3 is mainly caused by the hybridization of the O 2p with Ti 3d orbitals. The splitting peaks between 542 and 552 eV in the O-K edge spectra of BaTiO3 are attributed to its complex crystal structure including two unequal oxygen sites and low site symmetry.
Berenov AV, Le Goupil F, Alford N, 2016, Effect of ionic radii on the Curie temperature in Ba1-x-ySrxCayTiO3 compounds, Scientific Reports, Vol: 6, ISSN: 2045-2322
A series of Ba1-x-ySrxCayTiO3 compounds were prepared with varying average ionic radii and cation disorder on A-site. All samples showed typical ferroelectric behavior. A simple empirical equation correlated Curie temperature, TC, with the values of ionic radii of A-site cations. This correlation was related to the distortion of TiO6 octahedra observed during neutron diffraction studies. The equation was used for the selection of compounds with predetermined values of TC. The effects of A-site ionic radii on the temperatures of phase transitions in Ba1-x-ySrxCayTiO3 were discussed.
Le Goupil F, McKinnon R, Koval V, et al., 2016, Tuning the electrocaloric enhancement near the morphotropic phase boundary in lead-free ceramics, Scientific Reports, Vol: 6, ISSN: 2045-2322
The need for more energy-efficient and environmentally-friendly alternatives in the refrigeration industry to meet Global emission targets has driven efforts towards materials with a potential for solid state cooling. Adiabatic depolarisation cooling, based on the electrocaloric effect (ECE), is a significant contender for efficient new solid state refrigeration techniques. Some of the highest ECE performances reported are found in compounds close to the morphotropic phase boundary (MPB). This relationship between performance and the MPB makes the ability to tune the position of the MPB an important challenge in electrocaloric research. Here, we report direct ECE measurements performed on MPB tuned NBT-06BT bulk ceramics with a combination of A-site substitutions. We successfully shift the MPB of these lead-free ceramics closer to room temperature, as required for solid state refrigeration, without loss of the criticality of the system and the associated ECE enhancement.
Le Goupil F, Alford NM, 2016, Upper limit of the electrocaloric peak in lead-free ferroelectric relaxor ceramics, APL Materials, Vol: 4, ISSN: 2166-532X
The electrocaloric effect (ECE) of two compositions (x = 0.06 and 0.07) of (1 − x)(Na0.5Bi0.5)TiO3-xKNbO3 in the vicinity of the morphotropic phase boundary is studied by direct measurements. ΔTmax = 1.5 K is measured at 125 °C under 70 kV/cm for NBT-6KN while ΔTmax = 0.8 K is measured at 75 °C under 55 kV/cm for NBT-7KN. We show that the “shoulder,” TS, in the dielectric permittivity, marks the upper limit of the ECE peak under high applied electric fields. These results imply that the range of temperature with high ECE can be quickly identified for a given composition, which will significantly speed up the process of materials selection for ECE cooling.
Zhang X, Wu X, Centeno A, et al., 2016, Significant broadband photocurrent enhancement by Au-CZTS core-shell nanostructured photocathodes, Scientific Reports, Vol: 6, ISSN: 2045-2322
Copper zinc tin sulfide (CZTS) is a promising material for harvesting solar energy due to its abundance and non-toxicity. However, its poor performance hinders their wide application. In this paper gold (Au) nanoparticles are successfully incorporated into CZTS to form Au@CZTS core-shell nanostructures. The photocathode of Au@CZTS nanostructures exhibits enhanced optical absorption characteristics and improved incident photon-to-current efficiency (IPCE) performance. It is demonstrated that using this photocathode there is a significant increase of the power conversion efficiency (PCE) of a photoelectrochemical solar cell of 100% compared to using a CZTS without Au core. More importantly, the PCE of Au@CZTS photocathode improved by 15.8% compared to standard platinum (Pt) counter electrode. The increased efficiency is attributed to plasmon resonance energy transfer (PRET) between the Au nanoparticle core and the CZTS shell at wavelengths shorter than the localized surface plasmon resonance (LSPR) peak of the Au and the semiconductor bandgap.
Sanlialp M, Shvartsman VV, Acosta M, et al., 2016, Electrocaloric Effect in Ba(Zr,Ti)O<inf>3</inf>–(Ba,Ca)TiO<inf>3</inf> Ceramics Measured Directly, Journal of the American Ceramic Society, Vol: 99, Pages: 4022-4030, ISSN: 0002-7820
In this paper, we report on studies of the electrocaloric (EC) effect in lead-free (1−x)Ba(Zr0.2Ti0.8)O3–x(Ba0.7Ca0.3)TiO3 ceramics with compositions range between 0.32 ≤ x ≤ 0.45. The EC effect was measured directly using a modified differential scanning calorimeter. The maximum EC temperature change, ΔTdirect = 0.33 K under an electric field of 2 kV/mm, was observed for the composition with x = 0.32 at ~63°C. We found that the EC effect peaks not only around the Curie temperature but also at the transition between the ferroelectric phases with different symmetries. A strong discrepancy observed between the results of the direct measurements and indirect estimations points out that using Maxwell's equations is invalid for the thermodynamic nonequilibrium conditions that accompany only partial (incomplete) poling of ceramics. We also observe a nonlinearity of the EC effect above the Curie temperature and in the temperature range corresponding to the tetragonal ferroelectric phase.
Breeze J, 2016, Temperature and frequency dependence of complex permittivity in metal oxide dielectrics: theory, modelling and measurement, Publisher: Springer International Publishing, ISBN: 9783319445458
Le Goupil F, Bennett J, Axelsson A-K, et al., 2015, Electrocaloric enhancement near the morphotropic phase boundary in lead-free NBT-KBT ceramics, Applied Physics Letters, Vol: 107, ISSN: 1077-3118
The electrocaloric effects (ECEs) of the morphotropic phase boundary (MPB) composition 0.82(Na0.5Bi0.5)TiO3-0.18(K0.5Bi0.5)TiO3 (NBT-18KBT) are studied by direct measurements. The maximum ECE ΔTmax = 0.73 K is measured at 160 °C under 22 kV/cm. This corresponds to an ECE responsivity (ΔT/ΔE) of 0.33 × 10−6 K m/V, which is comparable with the best reported values for lead-free ceramics. A comparison between the direct and indirect ECE measurements shows significant discrepancies. The direct measurement of both positive and negative electrocaloric effect confirms the presence of numerous polar phases near the MPB of NBT-based materials and highlights their potential for solid-state cooling based on high field-induced entropy changes.
Wu X, Zhang X, Price D, et al., 2015, Broadband plasmon photocurrent generation from Au nanoparticles/ mesoporous TiO2 nanotube electrodes, Solar Energy Materials and Solar Cells, Vol: 138, Pages: 80-85, ISSN: 0927-0248
There has been an increasing interest in plasmon-induced enhancement of solar cells and more recently in the direct generation of photocurrent using noble metal nanoparticles with their Localised Surface Plasmon Resonance (LSPR) in the visual part of the spectrum. In this paper we report broadband plasmon photocurrent generation using novel Au nanoparticle incorporated mesoporous TiO2 nanotube electrodes. Plasmonic induced photocurrent due to hot electrons is observed over a broad wavelength range (~500 to 1000 nm). Incident photon-to-electron conversion efficiency (IPCE) measurements undertaken showed a maximum photocurrent enhancement of 200 fold around 700–730 nm wavelength.
Braic L, Vasilantonakis N, Zou B, et al., 2015, Optimizing strontium ruthenate thin films for near-infrared plasmonic applications, Scientific Reports, Vol: 5, ISSN: 2045-2322
Several new plasmonic materials have recently been introduced in order to achieve better temperature stability than conventional plasmonic metals and control field localization with a choice of plasma frequencies in a wide spectral range. Here, epitaxial SrRuO3 thin films with low surface roughness fabricated by pulsed laser deposition are studied. The influence of the oxygen deposition pressure (20–300 mTorr) on the charge carrier dynamics and optical constants of the thin films in the near-infrared spectral range is elucidated. It is demonstrated that SrRuO3 thin films exhibit plasmonic behavior of the thin films in the near-infrared spectral range with the plasma frequency in 3.16–3.86 eV range and epsilon-near-zero wavelength in 1.11–1.47 μm range that could be controlled by the deposition conditions. The possible applications of these films range from the heat-generating nanostructures in the near-infrared spectral range, to metamaterial-based ideal absorbers and epsilon-near-zero components, where the interplay between real and imaginary parts of the permittivity in a given spectral range is needed for optimizing the spectral performance.
Breeze J, Tan K-J, Richards B, et al., 2015, Enhanced magnetic Purcell effect in room-temperature masers, Nature Communications, Vol: 6, Pages: 1-6, ISSN: 2041-1723
Recently, the world’s first room-temperature maser was demonstrated. The maser consisted of a sapphire ring housing a crystal of pentacene-doped p-terphenyl, pumped by a pulsed rhodamine-dye laser. Stimulated emission of microwaves was aided by the high quality factor and small magnetic mode volume of the maser cavity yet the peak optical pumping power was 1.4 kW. Here we report dramatic miniaturization and 2 orders of magnitude reduction in optical pumping power for a room-temperature maser by coupling a strontium titanate resonator with the spin-polarized population inversion provided by triplet states in an optically excited pentacene-doped p-terphenyl crystal. We observe maser emission in a thimble-sized resonator using a xenon flash lamp as an optical pump source with peak optical power of 70 W. This is a significant step towards the goal of continuous maser operation.
Roqan IS, Venkatesh S, Zhang Z, et al., 2015, Obtaining strong ferromagnetism in diluted Gd-doped ZnO thin films through controlled Gd-defect complexes, Journal of Applied Physics, Vol: 117, ISSN: 1089-7550
We demonstrate the fabrication of reproducible long-range ferromagnetism (FM) in highly crystalline Gdx Zn 1−xO thin films by controlling the defects. Films are grown on lattice-matched substrates by pulsed laser deposition at low oxygen pressures (≤25 mTorr) and low Gd concentrations (x ≤ 0.009). These films feature strong FM (10 μB per Gd atom) at room temperature. While films deposited at higher oxygen pressure do not exhibit FM, FM is recovered by post-annealing these films under vacuum. These findings reveal the contribution of oxygen deficiency defects to the long-range FM. We demonstrate the possible FM mechanisms, which are confirmed by density functional theory study, and show that Gd dopants are essential for establishing FM that is induced by intrinsic defects in these films.
Venkatesh S, Franklin JB, Ryan MP, et al., 2015, Defect-band mediated ferromagnetism in Gd-doped ZnO thin films, JOURNAL OF APPLIED PHYSICS, Vol: 117, ISSN: 0021-8979
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- Citations: 39
Donchev E, Pang JS, Gammon PM, et al., 2014, Erratum to: The rectenna device: From theory to practice (a review) — CORRIGENDUM (10.1007/10.1557/mre.2014.6), MRS Energy and Sustainability, Vol: 1
The article contains the following errors: Page 23, left column, line 19. The sentence should read: Another requirement for the insulator is a small dielectric constant since for small area devices the capacitance is reduced, however due to the requirements for tunneling the ultra-thin thicknesses in turn increase capacitance, which has to be kept to a minimum as per Eq. (2). Page 25, left column, line 1. The sentence should read: If the thickness of the higher electron affinity insulator is increased relative to the other insulator, then the QW will be wide enough to form resonant energy levels. The author regrets these errors.
Donchev E, Pang JS, Gammon PM, et al., 2014, The rectenna device: From theory to practice (a review), MRS Energy and Sustainability, Vol: 1
This review article provides the state-of-art research and developments of the rectenna device and its two main components–the antenna and the rectifier. Furthermore, the history, efficiency trends, and socioeconomic impact of its research are also featured. The rectenna (RECTifying antENNA), which was first demonstrated by William C. Brown in 1964 as a receiver for microwave power transmission, is now increasingly researched as a means of harvesting solar radiation. Tapping into the growing photovoltaic market, the attraction of the rectenna concept is the potential for devices that, in theory, are not limited in efficiency by the Shockley–Queisser limit. In this review, the history and operation of this 40-year old device concept are explored in the context of power transmission and the ever increasing interest in its potential applications at terahertz frequencies, through the infrared and visible spectra. Recent modeling approaches that have predicted controversially high efficiency values at these frequencies are critically examined. It is proposed that to unlock any of the promised potential in the solar rectenna concept, there is a need for each constituent part to be improved beyond the current best performance, with the existing nanometer scale antennas, the rectification and the impedance matching solutions all falling short of the necessary efficiencies at terahertz frequencies. Advances in the fabrication, characterization, and understanding of the antenna and the rectifier are reviewed, and common solar rectenna design approaches are summarized. Finally, the socioeconomic impact of success in this field is discussed and future work is proposed.
Tileli V, Duchamp M, Axelsson A-K, et al., 2014, On stoichiometry and intermixing at the spinel/perovskite interface in CoFe2O4/BaTiO3 thin films, Nanoscale, Vol: 7, Pages: 218-224, ISSN: 2040-3372
The performance of complex oxide heterostructures depends primarily on the interfacial coupling of the two component structures. This interface character inherently varies with the synthesis method and conditions used since even small composition variations can alter the electronic, ferroelectric, or magnetic functional properties of the system. The focus of this article is placed on the interface character of a pulsed laser deposited CoFe2O4/BaTiO3 thin film. Using a range of state-of-the-art transmission electron microscopy methodologies, the roles of substrate morphology, interface stoichiometry, and cation intermixing are determined on the atomic level. The results reveal a surprisingly uneven BaTiO3 substrate surface formed after the film deposition and Fe atom incorporation in the top few monolayers inside the unit cell of the BaTiO3 crystal. Towards the CoFe2O4 side, a disordered region extending several nanometers from the interface was revealed and both Ba and Ti from the substrate were found to diffuse into the spinel layer. The analysis also shows that within this somehow incompatible composite interface, a different phase is formed corresponding to the compound Ba2Fe3Ti5O15, which belongs to the ilmenite crystal structure of FeTiO3 type. The results suggest a chemical activity between these two oxides, which could lead to the synthesis of complex engineered interfaces.
Kozyrev A, Mikhailov A, Ptashnik S, et al., 2014, Selective normal mode excitation in multilayer thin film bulk acoustic wave resonators, Applied Physics Letters, Vol: 105, ISSN: 1077-3118
A method for selective normal mode excitation in thin film bulk acoustic wave resonators, based on multilayer structures with any number of ferroelectric films in the paraelectric phase, is presented. The possibility to control the excitation of thin film bulk acoustic resonators' normal modes by simultaneous manipulating both the polarities and the magnitudes of the dc bias voltages applied to the ferroelectric layers is demonstrated. The proposed method was verified using the Lakin's model, modified to describe the electro-mechanical behavior of a structure with four active ferroelectric layers.
Appleby DJR, Ponon NK, Kwa KSK, et al., 2014, Ferroelectric properties in thin film barium titanate grown using pulsed laser deposition, JOURNAL OF APPLIED PHYSICS, Vol: 116, ISSN: 0021-8979
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- Citations: 21
Appleby DJR, Ponon NK, Kwa KSK, et al., 2014, Experimental observation of negative capacitance in ferroelectrics at room temperature, Nano Letters: a journal dedicated to nanoscience and nanotechnology, Vol: 14, Pages: 3864-3868, ISSN: 1530-6984
Effective negative capacitance has been postulated in ferroelectrics because there is a hysteresis in plots of polarization-electric field. Compelling experimental evidence of effective negative capacitance is presented here at room temperature in engineered devices, where it is stabilized by the presence of a paraelectric material. In future integrated circuits, the incorporation of such negative capacitance into MOSFET gate stacks would reduce the subthreshold slope, enabling low power operation and reduced self-heating.
Le Goupil F, Axelsson A-K, Valant M, et al., 2014, Effect of Ce doping on the electrocaloric effect of Sr<sub>x</sub>Ba<sub>1-x</sub>Nb<sub>2</sub>O<sub>6</sub> single crystals, APPLIED PHYSICS LETTERS, Vol: 104, ISSN: 0003-6951
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- Citations: 15
Le Goupil F, Axelsson A-K, Dunne LJ, et al., 2014, Anisotropy of the Electrocaloric Effect in Lead-Free Relaxor Ferroelectrics, ADVANCED ENERGY MATERIALS, Vol: 4, ISSN: 1614-6832
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- Citations: 31
Petrov PK, Zou B, Wang Y, et al., 2014, STO/BTO Modulated Superlattice Multilayer Structures with Atomically Sharp Interfaces, ADVANCED MATERIALS INTERFACES, Vol: 1, ISSN: 2196-7350
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- Citations: 7
Altynnikov AG, Gagarin AG, Gaidukov MM, et al., 2014, Suppression of slow capacitance relaxation phenomenon in Pt/Ba<sub>0.3</sub>Sr<sub>0.7</sub>TiO<sub>3</sub>/Pt thin film ferroelectric structures by annealing in oxygen atmosphere, APPLIED PHYSICS LETTERS, Vol: 104, ISSN: 0003-6951
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Wu X, Scott K, Xie F, et al., 2014, A reversible water electrolyser with porous PTFE based OH− conductive membrane as energy storage cells, Journal of Power Sources, Vol: 246, Pages: 225-231, ISSN: 0378-7753
, 2014, Electrocaloric Materials: New Generation of Coolers, Publisher: SPRINGER-VERLAG BERLIN, ISBN: 978-3-642-40263-0
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Donchev E, Gammon PM, Pang JS, et al., 2014, Systematic study of metal-insulator-metaldiodes with a native oxide, Conference on Thin Films for Solar and Energy Technology VI, Publisher: SPIE-INT SOC OPTICAL ENGINEERING, ISSN: 0277-786X
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