Toward Fabrication of Devices Based on Graphene/Oxide Multilayers.

Owing to its high electrical conductivity, low density, and flexibility, graphene has great potential for use as a building block in a wide range of applications from nanoelectronics to biosensing and high-frequency devices. For many device applications, it is required to deposit dielectric materials on graphene at high temperatures and in ambient oxygen. This has been proven to be highly challenging because these conditions cause significant degradation in graphene. In this work, we investigate the degradation of graphene at elevated temperatures in an oxygen atmosphere and possible protection mechanisms to enable the growth of oxide thin films on graphene at higher temperatures. We show that coating graphene with self-assembled monolayers of hexamethyldisilazane (HMDS) prior to a high-temperature deposition can significantly reduce the damage induced. Furthermore, a graphene sample treated with HMDS displayed a weaker doping effect due to weak interaction with oxygen species than bare graphene, and a much slower rate of electrical resistance degradation was exhibited during annealing. Thus, it is a promising approach that could enable the deposition of metal oxide materials on graphene at high temperatures without significant degradation in graphene quality, which is critical for a wide range of applications.

Crystalline AuNP-Decorated Strontium Niobate Thin Films: Strain-Controlled AuNP Morphologies and Optical Properties for Plasmonic Applications.

Gold nanoparticle (AuNP) decoration is a commonly used method to enhance the optical responses in many applications such as photocatalysis, biosensing, solar cells, etc. The morphology and structure of AuNPs are essential factors determining the functionality of the sample. However, tailoring the growth mechanism of AuNPs on an identical surface is not straightforward. In this study, AuNPs were deposited on the surface of a perovskite thin film, strontium niobate (SNO), using pulsed laser deposition (PLD). AuNPs exhibited a dramatic variation in their growth mechanisms, depending on whether they were deposited on SNO thin films grown on magnesium oxide (SNO/MgO) or strontium titanate (SNO/STO) substrates. On SNO/MgO, the Au aggregates form large NPs with an average size of up to 3500 nm2. These AuNPs are triangular with sharp edges and corners. The out-of-plane direction of growth is favored, and the surface coverage ratio by AuNPs is low. When deposited on SNO/STO, the average size of AuNPs is much smaller, i.e., ∼250 nm2. This reduction in the average size is accompanied by an increase in the number density of NPs. AuNPs on SNO/STO have a round shape and high coverage ratio. Such an impact from the substrate selection on the AuNP structure is significant when the sandwiched SNO film is below 80 nm thickness and is weakened for 200 nm of SNO films. X-ray diffraction (XRD) and scanning electron microscopy (SEM) were used to characterize all samples. Strain analysis was used to explain the growth mechanism of AuNPs. The average height of AuNPs was measured by using atomic force microscopy (AFM). Ellipsometry in the visible-near-infrared (vis-NIR) region was used to characterize the optical response of all samples. AuNP-decorated SNO/MgO and SNO/STO thin films exhibit different optical properties, with only gold-decorated SNO/MgO samples showing a size-dependent epsilon-near-zero behavior of nanoparticles. These results provide an additional route to control the structure of AuNPs. They can be used for various plasmonic applications like the design and development of strain-engineered gold-nanoparticle-decorated devices for surface-enhanced Raman spectroscopy (SERS) and photocatalysis.

Optimizing Hot Electron Harvesting at Planar Metal-Semiconductor Interfaces with Titanium Oxynitride Thin Films.

Understanding metal-semiconductor interfaces is critical to the advancement of photocatalysis and sub-bandgap solar energy harvesting where electrons in the metal can be excited by sub-bandgap photons and extracted into the semiconductor. In this work, we compare the electron extraction efficiency across Au/TiO2 and titanium oxynitride (TiON)/TiO2-x interfaces, where in the latter case the spontaneously forming oxide layer (TiO2-x) creates a metal-semiconductor contact. Time-resolved pump-probe spectroscopy is used to study the electron recombination rates in both cases. Unlike the nanosecond recombination lifetimes in Au/TiO2, we find a bottleneck in the electron relaxation in the TiON system, which we explain using a trap-mediated recombination model. Using this model, we investigate the tunability of the relaxation dynamics with oxygen content in the parent film. The optimized film (TiO0.5N0.5) exhibits the highest carrier extraction efficiency (NFC ≈ 2.8 × 1019 m-3), slowest trapping, and an appreciable hot electron population reaching the surface oxide (NHE ≈ 1.6 × 1018 m-3). Our results demonstrate the productive role oxygen can play in enhancing electron harvesting and prolonging electron lifetimes, providing an optimized metal-semiconductor interface using only the native oxide of titanium oxynitride.

Tunable Three-Dimensional Plasmonic Arrays for Large Near-Infrared Fluorescence Enhancement.

Metal-enhanced fluorescence (MEF), resulting from the near-field interaction of fluorophores with metallic nanostructures, has emerged as a powerful tool for dramatically improving the performance of fluorescence-based biomedical applications. Allowing for lower autofluorescence and minimal photoinduced damage, the development of multifunctional and multiplexed MEF platforms in the near-infrared (NIR) windows is particularly desirable. Here, a low-cost fabrication method based on nanosphere lithography is applied to produce tunable three-dimensional (3D) gold (Au) nanohole-disc arrays (Au-NHDAs). The arrays consist of nanoscale glass pillars atop nanoholes in a Au thin film: the top surfaces of the pillars are Au-covered (effectively nanodiscs), and small Au nanoparticles (nanodots) are located on the sidewalls of the pillars. This 3D hole-disc (and possibly nanodot) construct is critical to the properties of the device. The versatility of our approach is illustrated through the production of uniform and highly reproducible Au-NHDAs with controlled structural properties and tunable optical features in the NIR windows. Au-NHDAs allow for a very large NIR fluorescence enhancement (more than 400 times), which is attributed to the 3D plasmonic structure of the arrays that allows strong surface plasmon polariton and localized surface plasmon resonance coupling through glass nanogaps. By considering arrays with the same resonance peak and the same nanodisc separation distance, we show that the enhancement factor varies with nanodisc diameter. Using computational electromagnetic modeling, the electric field enhancement at 790 nm was calculated to provide insights into excitation enhancement, which occurs due to an increase in the intensity of the electric field. Fluorescence lifetime measurements indicate that the total fluorescence enhancement may depend on controlling excitation enhancement and therefore the array morphology. Our findings provide important insights into the mechanism of MEF from 3D plasmonic arrays and establish a low-cost versatile approach that could pave the way for novel NIR-MEF bioapplications.

Temperature stability of thin film refractory plasmonic materials.

Materials such as W, TiN, and SrRuO3 (SRO) have been suggested as promising alternatives to Au and Ag in plasmonic applications owing to their stability at high operational temperatures. However, investigation of the reproducibility of the optical properties after thermal cycling between room and elevated temperatures is so far lacking. Here, thin films of W, Mo, Ti, TiN, TiON, Ag, Au, SrRuO3 and SrNbO3 are investigated to assess their viability for robust refractory plasmonic applications. These results are further compared to the performance of SrMoO3 reported in literature. Films ranging in thickness from 50 to 105 nm are deposited on MgO, SrTiO3 and Si substrates by e-beam evaporation, RF magnetron sputtering and pulsed laser deposition, prior to characterisation by means of AFM, XRD, spectroscopic ellipsometry, and DC resistivity. Measurements are conducted before and after annealing in air at temperatures ranging from 300 to 1000° C for one hour, to establish the maximum cycling temperature and potential longevity at elevated temperatures for each material. It is found that SrRuO3 retains metallic behaviour after annealing at 800° C, while SrNbO3 undergoes a phase transition resulting in a loss of metallic behaviour after annealing at 400° C. Importantly, the optical properties of TiN and TiON are degraded as a result of oxidation and show a loss of metallic behaviour after annealing at 500° C, while the same is not observed in Au until annealing at 600° C. Nevertheless, both TiN and TiON may be better suited than Au or SRO for high temperature applications operating under vacuum conditions.

Continuous-wave room-temperature diamond maser.

The maser-the microwave progenitor of the optical laser-has been confined to relative obscurity owing to its reliance on cryogenic refrigeration and high-vacuum systems. Despite this, it has found application in deep-space communications and radio astronomy owing to its unparalleled performance as a low-noise amplifier and oscillator. The recent demonstration of a room-temperature solid-state maser that utilizes polarized electron populations within the triplet states of photo-excited pentacene molecules in a p-terphenyl host paves the way for a new class of maser. However, p-terphenyl has poor thermal and mechanical properties, and the decay rates of the triplet sublevel of pentacene mean that only pulsed maser operation has been observed in this system. Alternative materials are therefore required to achieve continuous emission: inorganic materials that contain spin defects, such as diamond and silicon carbide, have been proposed. Here we report a continuous-wave room-temperature maser oscillator using optically pumped nitrogen-vacancy defect centres in diamond. This demonstration highlights the potential of room-temperature solid-state masers for use in a new generation of microwave devices that could find application in medicine, security, sensing and quantum technologies.

Ferroelectric thin film acoustic devices with electrical multiband switching ability.

Design principles of a new class of microwave thin film bulk acoustic resonators with multiband resonance frequency switching ability are presented. The theory of the excitation of acoustic eigenmodes in multilayer ferroelectric structures is considered, and the principle of selectivity for resonator with an arbitrary number of ferroelectric layers is formulated. A so called "criterion function" is suggested that allows to determine the conditions for effective excitation at one selected resonance mode with suppression of other modes. The proposed theoretical approach is verifiedusing thepreexisting experimental data published elsewhere. Finally, the possible application of the two ferroelectric layers structures for switchable microwave overtone resonators, binary and quadrature phase-shift keying modulators are discussed. These devices could play a pivotal role in the miniaturization of microwave front-end antenna circuits.

Titanium Oxynitride Thin Films with Tunable Double Epsilon-Near-Zero Behavior for Nanophotonic Applications.

Titanium oxynitride (TiOxNy) thin films are fabricated using reactive magnetron sputtering. The mechanism of their growth formation is explained, and their optical properties are presented. The films grown when the level of residual oxygen in the background vacuum was between 5 nTorr to 20 nTorr exhibit double epsilon-near-Zero (2-ENZ) behavior with ENZ1 and ENZ2 wavelengths tunable in the 700-850 and 1100-1350 nm spectral ranges, respectively. Samples fabricated when the level of residual oxygen in the background vacuum was above 2 × 10-8 Torr exhibit nonmetallic behavior, while the layers deposited when the level of residual oxygen in the background vacuum was below 5 × 10-9 Torr show metallic behavior with a single ENZ value. The double ENZ phenomenon is related to the level of residual oxygen in the background vacuum and is attributed to the mixture of TiN and TiOxNy and TiOx phases in the films. Varying the partial pressure of nitrogen during the deposition can further control the amount of TiN, TiOx, and TiOxNy compounds in the films and, therefore, tune the screened plasma wavelengths. A good approximation of the ellipsometric behavior is achieved with Maxwell-Garnett theory for a composite film formed by a mixture of TiO2 and TiN phases suggesting that double ENZ TiOxNy films are formed by inclusions of TiN within a TiO2 matrix. These oxynitride compounds could be considered as new materials exhibiting double ENZ in the visible and near-IR spectral ranges. Materials with ENZ properties are advantageous for designing the enhanced nonlinear optical response, metasurfaces, and nonreciprocal behavior.

Solid-state source of intense yellow light based on a Ce:YAG luminescent concentrator.

A luminescent concentrator functioning as a bright source of yellow light is reported. It comprises a waveguide made of cerium-doped YAG crystal, in the form of a long-thin rectangular strip, surrounded by flowing air and optically pumped from both sides with blue light from arrays of high-efficiency InGaN LEDs. Phosphor-converted yellow light, generated within the strip, is guided to a glass taper that is butt-coupled to one of the strip's end faces. Up to 20 W of optical power, centered on 575 nm with a linewidth of 76 nm, can be continuously radiated into air from the taper's 1.67 mm × 1.67 mm square output aperture. The intensity of the outputted light is significantly greater than what any yellow (AlGaInP) LED can directly produce (either singly or arrayed), with only a modest increase in linewidth. Furthermore, the wall-plug efficiency of the source exceeds that of any yellow laser. The concept allows for further substantial increases in intensity, total output power and wall-plug efficiency through scaling-up and engineering refinements.

Nanosecond time-resolved characterization of a pentacene-based room-temperature MASER.

The performance of a room temperature, zero-field MASER operating at 1.45 GHz has been examined. Nanosecond laser pulses, which are essentially instantaneous on the timescale of the spin dynamics, allow the visible-to-microwave conversion efficiency and temporal response of the MASER to be measured as a function of excitation energy. It is observed that the timing and amplitude of the MASER output pulse are correlated with the laser excitation energy: at higher laser energy, the microwave pulses have larger amplitude and appear after shorter delay than those recorded at lower laser energy. Seeding experiments demonstrate that the output variation may be stabilized by an external source and establish the minimum seeding power required. The dynamics of the MASER emission may be modeled by a pair of first order, non-linear differential equations, derived from the Lotka-Volterra model (Predator-Prey), where by the microwave mode of the resonator is the predator and the spin polarization in the triplet state of pentacene is the prey. Simulations allowed the Einstein coefficient of stimulated emission, the spin-lattice relaxation and the number of triplets contributing to the MASER emission to be estimated. These are essential parameters for the rational improvement of a MASER based on a spin-polarized triplet molecule.

Growth of Epitaxial Oxide Thin Films on Graphene.

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 of graphene. This step is not understood. It is not clear why the oxide should adopt the epitaxy of the underlying oxide layer when it is deposited on graphene where there is no lattice match. To date there has been no explanation or suggestion of mechanisms which clarify this step. Here we show a mechanism, supported by first principles simulation and structural characterisation results, for the growth of oxide thin films on graphene. We describe the growth of epitaxial SrTiO3 (STO) thin films on a graphene and show that local defects in the graphene layer (e.g. grain boundaries) act as bridge-pillar spots that enable the epitaxial growth of STO thin films on the surface of the graphene layer. This study, and in particular the suggestion of a mechanism for epitaxial growth of oxides on graphene, offers new directions to exploit the development of oxide/graphene multilayer structures and devices.

Band gap bowing in NixMg1-xO.

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.

Tuning the electrocaloric enhancement near the morphotropic phase boundary in lead-free ceramics.

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.

Significant Broadband Photocurrent Enhancement by Au-CZTS Core-Shell Nanostructured Photocathodes.

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.

Optimizing strontium ruthenate thin films for near-infrared plasmonic applications.

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.

Enhanced magnetic Purcell effect in room-temperature masers.

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.

On stoichiometry and intermixing at the spinel/perovskite interface in CoFe2O4/BaTiO3 thin films.

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.

Experimental observation of negative capacitance in ferroelectrics at room temperature.

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.

Predicting the fluorescent enhancement rate by gold and silver nanospheres using finite-difference time-domain analysis.

Metal-induced fluorescence enhancement (MIFE) is a promising strategy for increasing the sensitivity of fluorophores used in biological sensors. This study uses the finite-difference time-domain technique to predict the fluorescent enhancement rate of a fluorophore molecule in close proximity to a gold or silver spherical nanoparticle. By considering commercially available fluorescent dyes the computed results are compared with the published experimental data. The results show that MIFE is a complex coupling process between the fluorophore molecule and the metal nanoparticle. Nevertheless using computational electromagnetic techniques to perform calculations it is possible to calculate, with reasonable accuracy, the fluorescent enhancement. Using this methodology it will be possible to consider different shaped metal nanoparticles and any supporting substrate material in the future, an important step in building reliable biosensors capable of detecting low levels of proteins tagged with fluorescence molecules.

Tunable synthesis of ordered zinc oxide nanoflower-like arrays.

A novel approach to fabricate an ordered array of ZnO nanoflowers, consisting of uniform polymer cores of 100s of nanometer diameter decorated with ZnO nanorods of 10s of nanometer diameter, is presented. The 2-stage method combines the formation of ZnO seed layer by pulsed laser deposition (PLD) onto a colloidally assembled polystyrene sphere monolayer and the subsequent hydrothermal growth of ZnO nanowires (NWs). The main advantages of this methodology are low cost and the large area scalability of perfectly ordered hierarchical structures. More importantly, the process enables a versatile control of dimensions and morphologies of ZnO NWs as well as control of the core diameter by changing the polystyrene sphere diameter. A strong improvement of light scattering by such arrays is observed, offering promise as building blocks in different types of solar cells and potentially useful for a wide variety of applications in optoelectronic devices.