Growth of emergent simple pseudo-binary ferroelectrics and their potential in neuromorphic computing devices

Ferroelectric memory devices such as ferroelectric memristors, ferroelectric tunnel junctions, and field-effect transistors are considered among the most promising candidates for neuromorphic computing devices. The promise arises from their defect-independent switching mechanism, low energy consumption and high power efficiency, and important properties being aimed for are reliable switching at high speed, excellent endurance, retention, and compatibility with complementary metal-oxide-semiconductor (CMOS) technology. Binary or doped binary materials have emerged over conventional complex-composition ferroelectrics as an optimum solution, particularly in terms of CMOS compatibility. The current state-of-the-art route to achieving superlative ferroelectric performance of binary oxides is to induce ferroelectricity at the nanoscale, e.g., in ultra-thin films of doped HfO2, ZrO2, Zn1-xMgxO, Al-xScxN, and Bi1-xSmxO3. This short review article focuses on the materials science of emerging new ferroelectric materials, including their different properties such as remanent polarization, coercive field, endurance, etc. The potential of these materials is discussed for neuromorphic applications.

Reconfigurable Resistive Switching in VO2/La0.7Sr0.3MnO3/Al2O3 (0001) Memristive Devices for Neuromorphic Computing

The coexistence of nonvolatile and volatile switching modes in a single memristive device provides flexibility to emulate both neuronal and synaptic functions in the brain. Furthermore, such a device structure may eliminate the need for additional circuit elements such as transistor-based selectors, enabling low-power consumption and high-density device integration in fully memristive spiking neural networks. In this work, we report dual resistive switching (RS) modes in VO2/La0.7Sr0.3MnO3 (LSMO) bilayer memristive devices. Specifically, the nonvolatile RS is driven by the movement of oxygen vacancies (Vo) at the VO2/LSMO interface and requires a higher biasing voltage, whereas the volatile RS is controlled by the metal-insulator transition (MIT) of VO2 under a lower biasing voltage. The simple device structure is electrically driven between the two RS modes and thus can operate as a one selector-one resistor (1S1R) cell, which is a desirable feature in memristive crossbar arrays to avoid the sneak-path current issue. The RS modes are found to be stable and repeatable and can be reconfigured by exploiting the interfacial and phase transition properties, and thus, they hold great promise for applications in memristive neural networks and neuromorphic computing.

Hydrogen-Driven Low-Temperature Topotactic Transition in Nanocomb Cobaltite for Ultralow Power Ionic-Magnetic Coupled Applications

We reversibly control ferromagnetic-antiferromagnetic ordering in an insulating ground state by annealing tensile-strained LaCoO3 films in hydrogen. This ionic-magnetic coupling occurs due to the hydrogen-driven topotactic transition between perovskite LaCoO3 and brownmillerite La2Co2O5 at a lower temperature (125-200 °C) and within a shorter time (3-10 min) than the oxygen-driven effect (500 °C, tens of hours). The X-ray and optical spectroscopic analyses reveal that the transition results from hydrogen-driven filling of correlated electrons in the Co 3d-orbitals, which successively releases oxygen by destabilizing the CoO6 octahedra into CoO4 tetrahedra. The transition is accelerated by surface exchange, diffusion of hydrogen in and oxygen out through atomically ordered oxygen vacancy "nanocomb" stripes in the tensile-strained LaCoO3 films. Our ionic-magnetic coupling with fast operation, good reproducibility, and long-term stability is a proof-of-principle demonstration of high-performance ultralow power magnetic switching devices for sensors, energy, and artificial intelligence applications, which are keys for attaining carbon neutrality.

High-performance and self-powered visible light photodetector using multiple coupled synergetic effects

In this work, we demonstrate, for the first time, that coupling together the pyroelectric effect, the photovoltaic effect and the plasmonic effect is a novel method to significantly enhance the performance of self-powered photodetectors in the visible region. Photodetectors based on tri-layered heterojunction of n-Si/p-SnO/n-ZnO through the inclusion of silver (Ag) nanoparticles (NPs) at the SnO/ZnO interface were fabricated. The photo-response of the device, with excitation from a chopped 650 nm wavelength laser, was carefully investigated, and it was shown that the photodetector performance is enhanced the most with the inclusion of spheroidal Ag NPs with ∼70 nm diameter. The Al/Si/SnO/Ag NPs/ZnO/ITO device exhibited an optimum responsivity, detectivity and sensitivity of 210.2 mA W-1, 5.47 × 109 Jones and 15.0 × 104, respectively, together with a rise and fall time of 2.3 and 51.3 μs, respectively, at a laser power density of 317 mW cm-2 and at a chopper frequency of 10 Hz. The present photodetectors are more than twice as responsive as the current best-performing ZnO-based pyro-phototronic photodetectors and they also exhibit other competitive features, such as detectivity, and fall and rise times. Therefore, by exploiting the plasmonic effect of the Ag NPs together with the pyroelectric effect in a ZnO film, and the photovoltaic effect at a Si/SnO junction, all in a single device, photodetectors were developed with state-of-the-art performance for the visible region.

Self-Assembled Au Nanoelectrodes: Enabling Low-Threshold-Voltage HfO2-Based Artificial Neurons

Filamentary-type resistive switching devices, such as conductive bridge random-access memory and valence change memory, have diverse applications in memory and neuromorphic computing. However, the randomness in filament formation poses challenges to device reliability and uniformity. To overcome this issue, various defect engineering methods have been explored, including doping, metal nanoparticle embedding, and extended defect utilization. In this study, we present a simple and effective approach using self-assembled uniform Au nanoelectrodes to controll filament formation in HfO2 resistive switching devices. By concentrating the electric field near the Au nanoelectrodes within the BaTiO3 matrix, we significantly enhanced the device stability and reduced the threshold voltage by up to 45% in HfO2-based artificial neurons compared to the control devices. The threshold voltage reduction is attributed to the uniformly distributed Au nanoelectrodes in the insulating matrix, as confirmed by COMSOL simulation. Our findings highlight the potential of nanostructure design for precise control of filamentary-type resistive switching devices.

La-doped BaSnO3 for electromagnetic shielding transparent conductors

In this work, we find that La-doped BaSnO3 (BLSO) is shown to be a promising electromagnetic shielding transparent conductor. While films grown on industrially practical optoelectronic MgAl2O4 substrates have higher sheet resistance by three orders of magnitude than in previous reports, we show how to recover the sheet resistance close to the single-crystal level by use of an MgO template layer which enables high quality (001)-oriented BLSO epitaxial film growth on (001) MgAl2O4. There is a positive correlation between crystallinity and conductivity; high crystallinity minimizes scattering of free electrons. By applying this design principle to 5-20% doped films, we find that highly crystalline 5% La-doped BLSO films exhibit low sheet resistance of ~ 8.7 Ω ▯ -1, high visible transmittance of ~ 80%, and high X-band electromagnetic shielding effectiveness of ~ 25.9 dB, thus outperforming transparent conducting oxides films of Sn-doped In2O3 and SrMoO3.

Comprehensive study of Raman optical response of typical substrates for thin-film growth under 633 nm and 785 nm laser excitation

Raman spectroscopy is one of the most efficient and non-destructive techniques for characterizing materials. However, it is challenging to analyze thin films using Raman spectroscopy since the substrates beneath the thin film often obscure its optical response. Here, we evaluate the suitability of fourteen commonly employed single-crystal substrates for Raman spectroscopy of thin films using 633 nm and 785 nm laser excitation systems. We determine the optimal wavenumber ranges for thin-film characterization by identifying the most prominent Raman peaks and their relative intensities for each substrate and across substrates. In addition, we compare the intensity of background signals across substrates, which is essential for establishing their applicability for Raman detection in thin films. The substrates LaAlO3 and Al2O3 have the largest free spectral range for both laser systems, while Al2O3 has the lowest background levels, according to our findings. In contrast, the substrates SrTiO3 and Nb:SrTiO3 have the narrowest free spectral range, while GdScO3, NGO and MgO have the highest background levels, making them unsuitable for optical investigations.

Low Temperature Epitaxial LiMn2O4 Cathodes Enabled by NiCo2O4 Current Collector for High-Performance Microbatteries

Epitaxial cathodes in lithium-ion microbatteries are ideal model systems to understand mass and charge transfer across interfaces, plus interphase degradation processes during cycling. Importantly, if grown at <450 °C, they also offer potential for complementary metal-oxide-semiconductor (CMOS) compatible microbatteries for the Internet of Things, flexible electronics, and MedTech devices. Currently, prominent epitaxial cathodes are grown at high temperatures (>600 °C), which imposes both manufacturing and scale-up challenges. Herein, we report structural and electrochemical studies of epitaxial LiMn2O4 (LMO) thin films grown on a new current collector material, NiCo2O4 (NCO). We achieve this at the low temperature of 360 °C, ∼200 °C lower than existing current collectors SrRuO3 and LaNiO3. Our films achieve a discharge capacity of >100 mAh g-1 for ∼6000 cycles with distinct LMO redox signatures, demonstrating long-term electrochemical stability of our NCO current collector. Hence, we show a route toward high-performance microbatteries for a range of miniaturized electronic devices.

Stable Supercapacity of Binder-Free TiO2(B) Epitaxial Electrodes for All-Solid-State Nanobatteries

Owing to its pseudocapacitive, unidimensional, rapid ion channels, TiO2(B) is a promising material for application to battery electrodes. In this study, we align these channels by epitaxially growing TiO2(B) films with the assistance of an isostructural VO2(B) template layer. In a liquid electrolyte, binder-free TiO2(B) epitaxial electrodes exhibit a supercapacity near the theoretical value of 335 mA h g-1 and an excellent charge-discharge reproducibility for ≥200 cycles, which outperform those of other TiO2(B) nanostructures. For the all-solid-state configuration employing the LiPON solid electrolyte, excellent stability persists. Our findings suggest excellent potential for miniaturizing all-solid-state nanobatteries in self-powered integrated circuits.

Local symmetry breaking drives picosecond spin domain formation in polycrystalline halide perovskite films

Photoinduced spin-charge interconversion in semiconductors with spin-orbit coupling could provide a route to optically addressable spintronics without the use of external magnetic fields. However, in structurally disordered polycrystalline semiconductors, which are being widely explored for device applications, the presence and role of spin-associated charge currents remains unclear. Here, using femtosecond circular-polarization-resolved pump-probe microscopy on polycrystalline halide perovskite thin films, we observe the photoinduced ultrafast formation of spin domains on the micrometre scale formed through lateral spin currents. Micrometre-scale variations in the intensity of optical second-harmonic generation and vertical piezoresponse suggest that the spin-domain formation is driven by the presence of strong local inversion symmetry breaking via structural disorder. We propose that this leads to spatially varying Rashba-like spin textures that drive spin-momentum-locked currents, leading to local spin accumulation. Ultrafast spin-domain formation in polycrystalline halide perovskite films provides an optically addressable platform for nanoscale spin-device physics.

Bandlike Transport and Charge-Carrier Dynamics in BiOI Films

Following the emergence of lead halide perovskites (LHPs) as materials for efficient solar cells, research has progressed to explore stable, abundant, and nontoxic alternatives. However, the performance of such lead-free perovskite-inspired materials (PIMs) still lags significantly behind that of their LHP counterparts. For bismuth-based PIMs, one significant reason is a frequently observed ultrafast charge-carrier localization (or self-trapping), which imposes a fundamental limit on long-range mobility. Here we report the terahertz (THz) photoconductivity dynamics in thin films of BiOI and demonstrate a lack of such self-trapping, with good charge-carrier mobility, reaching ∼3 cm² V^-1 s^-1 at 295 K and increasing gradually to ∼13 cm² V^-1 s^-1 at 5 K, indicative of prevailing bandlike transport. Using a combination of transient photoluminescence and THz- and microwave-conductivity spectroscopy, we further investigate charge-carrier recombination processes, revealing charge-specific trapping of electrons at defects in BiOI over nanoseconds and low bimolecular band-to-band recombination. Subject to the development of passivation protocols, BiOI thus emerges as a superior light-harvesting semiconductor among the family of bismuth-based semiconductors.

Thin-film design of amorphous hafnium oxide nanocomposites enabling strong interfacial resistive switching uniformity

A design concept of phase-separated amorphous nanocomposite thin films is presented that realizes interfacial resistive switching (RS) in hafnium oxide-based devices. The films are formed by incorporating an average of 7% Ba into hafnium oxide during pulsed laser deposition at temperatures ≤400°C. The added Ba prevents the films from crystallizing and leads to ∼20-nm-thin films consisting of an amorphous HfO_x host matrix interspersed with ∼2-nm-wide, ∼5-to-10-nm-pitch Ba-rich amorphous nanocolumns penetrating approximately two-thirds through the films. This restricts the RS to an interfacial Schottky-like energy barrier whose magnitude is tuned by ionic migration under an applied electric field. Resulting devices achieve stable cycle-to-cycle, device-to-device, and sample-to-sample reproducibility with a measured switching endurance of ≥10⁴ cycles for a memory window ≥10 at switching voltages of ±2 V. Each device can be set to multiple intermediate resistance states, which enables synaptic spike-timing-dependent plasticity. The presented concept unlocks additional design variables for RS devices.

Emergent and robust ferromagnetic-insulating state in highly strained ferroelastic LaCoO3 thin films

Transition metal oxides are promising candidates for the next generation of spintronic devices due to their fascinating properties that can be effectively engineered by strain, defects, and microstructure. An excellent example can be found in ferroelastic LaCoO₃ with paramagnetism in bulk. In contrast, unexpected ferromagnetism is observed in tensile-strained LaCoO₃ films, however, its origin remains controversial. Here we simultaneously reveal the formation of ordered oxygen vacancies and previously unreported long-range suppression of CoO₆ octahedral rotations throughout LaCoO₃ films. Supported by density functional theory calculations, we find that the strong modification of Co 3d-O 2p hybridization associated with the increase of both Co-O-Co bond angle and Co-O bond length weakens the crystal-field splitting and facilitates an ordered high-spin state of Co ions, inducing an emergent ferromagnetic-insulating state. Our work provides unique insights into underlying mechanisms driving the ferromagnetic-insulating state in tensile-strained ferroelastic LaCoO₃ films while suggesting potential applications toward low-power spintronic devices.

Ferroelectric Orthorhombic ZrO2 Thin Films Achieved Through Nanosecond Laser Annealing

A new approach for the stabilization of the ferroelectric orthorhombic ZrO₂ films is demonstrated through nanosecond laser annealing (NLA) of as-deposited Si/SiO_x /W(14 nm)/ZrO₂ (8 nm)/W(22 nm), grown by ion beam sputtering at low temperatures. The NLA process optimization is guided by COMSOL multiphysics simulations. The films annealed under the optimized conditions reveal the presence of the orthorhombic phase, as confirmed by X-ray diffraction, electron backscatter diffraction, and transmission electron microscopy. Macroscopic polarization-electric field hysteresis loops show ferroelectric behavior, with saturation polarization of 12.8 µC cm^-2 , remnant polarization of 12.7 µC cm^-2 and coercive field of 1.2 MV cm^-1 . The films exhibit a wake-up effect that is attributed to the migration of point defects, such as oxygen vacancies, and/or a transition from nonferroelectric (monoclinic and tetragonal phase) to the ferroelectric orthorhombic phase. The capacitors demonstrate a stable polarization with an endurance of 6.0 × 10⁵ cycles, demonstrating the potential of the NLA process for the fabrication of ferroelectric memory devices with high polarization, low coercive field, and high cycling stability.

Layered BiOI single crystals capable of detecting low dose rates of X-rays

Detecting low dose rates of X-rays is critical for making safer radiology instruments, but is limited by the absorber materials available. Here, we develop bismuth oxyiodide (BiOI) single crystals into effective X-ray detectors. BiOI features complex lattice dynamics, owing to the ionic character of the lattice and weak van der Waals interactions between layers. Through use of ultrafast spectroscopy, first-principles computations and detailed optical and structural characterisation, we show that photoexcited charge-carriers in BiOI couple to intralayer breathing phonon modes, forming large polarons, thus enabling longer drift lengths for the photoexcited carriers than would be expected if self-trapping occurred. This, combined with the low and stable dark currents and high linear X-ray attenuation coefficients, leads to strong detector performance. High sensitivities reaching 1.1  × 10³ μC Gy_air^-1 cm^-2 are achieved, and the lowest dose rate directly measured by the detectors was 22 nGy_air s^-1. The photophysical principles discussed herein offer new design avenues for novel materials with heavy elements and low-dimensional electronic structures for (opto)electronic applications.

Interface-related phenomena in epitaxial complex oxide ferroics across different thin film platforms: opportunities and challenges

Interfaces in complex oxides give rise to fascinating new physical phenomena arising from the interconnected spin, lattice, charge and orbital degrees of freedom. Most commonly, interfaces are engineered in epitaxial superlattice films. Of growing interest also are epitaxial vertically aligned nanocomposite films where interfaces form by self-assembly. These two thin film forms offer different capabilities for materials tuning and have been explored largely separately from one another. Ferroics (ferroelectric, ferromagnetic, multiferroic) are among the most fascinating phenomena to be manipulated using interface effects. Hence, in this review we compare and contrast the ferroic properties that arise in these two different film forms, highlighting exemplary materials combinations which demonstrate novel, enhanced and/or emergent ferroic functionalities. We discuss the origins of the observed functionalities and propose where knowledge can be translated from one materials form to another, to potentially produce new functionalities. Finally, for the two different film forms we present a perspective on underexplored/emerging research directions.

Charge transport in mixed metal halide perovskite semiconductors

Investigation of the inherent field-driven charge transport behaviour of three-dimensional lead halide perovskites has largely remained challenging, owing to undesirable ionic migration effects near room temperature and dipolar disorder instabilities prevalent specifically in methylammonium-and-lead-based high-performing three-dimensional perovskite compositions. Here, we address both these challenges and demonstrate that field-effect transistors based on methylammonium-free, mixed metal (Pb/Sn) perovskite compositions do not suffer from ion migration effects as notably as their pure-Pb counterparts and reliably exhibit hysteresis-free p-type transport with a mobility reaching 5.4 cm² V^-1 s^-1. The reduced ion migration is visualized through photoluminescence microscopy under bias and is manifested as an activated temperature dependence of the field-effect mobility with a low activation energy (~48 meV) consistent with the presence of the shallow defects present in these materials. An understanding of the long-range electronic charge transport in these inherently doped mixed metal halide perovskites will contribute immensely towards high-performance optoelectronic devices.

Floating perovskite-BiVO4 devices for scalable solar fuel production

Photoelectrochemical (PEC) artificial leaves hold the potential to lower the costs of sustainable solar fuel production by integrating light harvesting and catalysis within one compact device. However, current deposition techniques limit their scalability¹, whereas fragile and heavy bulk materials can affect their transport and deployment. Here we demonstrate the fabrication of lightweight artificial leaves by employing thin, flexible substrates and carbonaceous protection layers. Lead halide perovskite photocathodes deposited onto indium tin oxide-coated polyethylene terephthalate achieved an activity of 4,266 µmol H₂ g^-1 h^-1 using a platinum catalyst, whereas photocathodes with a molecular Co catalyst for CO₂ reduction attained a high CO:H₂ selectivity of 7.2 under lower (0.1 sun) irradiation. The corresponding lightweight perovskite-BiVO₄ PEC devices showed unassisted solar-to-fuel efficiencies of 0.58% (H₂) and 0.053% (CO), respectively. Their potential for scalability is demonstrated by 100 cm² stand-alone artificial leaves, which sustained a comparable performance and stability (of approximately 24 h) to their 1.7 cm² counterparts. Bubbles formed under operation further enabled 30-100 mg cm^-2 devices to float, while lightweight reactors facilitated gas collection during outdoor testing on a river. This leaf-like PEC device bridges the gulf in weight between traditional solar fuel approaches, showcasing activities per gram comparable to those of photocatalytic suspensions and plant leaves. The presented lightweight, floating systems may enable open-water applications, thus avoiding competition with land use.

Long-term solar water and CO2 splitting with photoelectrochemical BiOI-BiVO4 tandems

Photoelectrochemical (PEC) devices have been developed for direct solar fuel production but the limited stability of submerged light absorbers can hamper their commercial prospects.^1,2 Here, we demonstrate photocathodes with an operational H₂ evolution activity over weeks, by integrating a BiOI light absorber into a robust, oxide-based architecture with a graphite paste conductive encapsulant. In this case, the activity towards proton and CO₂ reduction is mainly limited by catalyst degradation. We also introduce multiple-pixel devices as an innovative design principle for PEC systems, displaying superior photocurrents, onset biases and stability over corresponding conventional single-pixel devices. Accordingly, PEC tandem devices comprising multiple-pixel BiOI photocathodes and BiVO₄ photoanodes can sustain bias-free water splitting for 240 h, while devices with a Cu₉₂In₈ alloy catalyst demonstrate unassisted syngas production from CO₂.

Ultrahigh energy storage in superparaelectric relaxor ferroelectrics

[Figure: see text].

Tetrafluoroborate-Induced Reduction in Defect Density in Hybrid Perovskites through Halide Management

Hybrid-perovskite-based optoelectronic devices are demonstrating unprecedented growth in performance, and defect passivation approaches are highly promising routes to further improve properties. Here, the effect of the molecular ion BF₄ ^- , introduced via methylammonium tetrafluoroborate (MABF₄ ) in a surface treatment for MAPbI₃ perovskite, is reported. Optical spectroscopy characterization shows that the introduction of tetrafluoroborate leads to reduced non-radiative charge-carrier recombination with a reduction in first-order recombination rate from 6.5 × 10⁶ to 2.5 × 10⁵ s^-1 in BF₄ ^- -treated samples, and a consequent increase in photoluminescence quantum yield by an order of magnitude (from 0.5 to 10.4%). ¹⁹ F, ¹¹ B, and ¹⁴ N solid-state NMR is used to elucidate the atomic-level mechanism of the BF₄ ^- additive-induced improvements, revealing that the BF₄ ^- acts as a scavenger of excess MAI by forming MAI-MABF₄ cocrystals. This shifts the equilibrium of iodide concentration in the perovskite phase, thereby reducing the concentration of interstitial iodide defects that act as deep traps and non-radiative recombination centers. These collective results allow us to elucidate the microscopic mechanism of action of BF₄ ^- .

Facilitating the Deprotonation of OH to O through Fe4+ -Induced States in Perovskite LaNiO3 Enables a Fast Oxygen Evolution Reaction

Aliovalent doping is widely adopted to tune the electronic structure of transition-metal oxides for design of low-cost, active electrocatalysts. Here, using single-crystalline thin films as model electrocatalysts, the structure-activity relationship of Fe states doping in perovskite LaNiO₃ for oxygen evolution reaction (OER) is studied. Fe⁴⁺ state is found to be crucial for enhancing the OER activity of LaNiO₃ , dramatically increasing the activity by six times, while Fe³⁺ has negligible effect. Spectroscopic studies and DFT calculations indicate Fe⁴⁺ states enhance the degree of Ni/Fe 3d and O 2p hybridization, and meanwhile produce down-shift of the unoccupied density of states towards lower energies. Such electronic features reduce the energy barrier for interfacial electron transfer for water oxidization by 0.2 eV. Further theoretical calculations and H/D isotope experiments reveal the electronic states associated with Fe⁴⁺ -O^2- -Ni³⁺ configuration accelerate the deprotonation of *OH to *O (rate-determining step), and thus facilitate fast OER kinetics.

Creating Ferromagnetic Insulating La0.9Ba0.1MnO3 Thin Films by Tuning Lateral Coherence Length

In this work, heteroepitaxial vertically aligned nanocomposite (VAN) La_0.9Ba_0.1MnO₃ (LBMO)-CeO₂ films are engineered to produce ferromagnetic insulating (FMI) films. From combined X-ray photoelectron spectroscopy, X-ray diffraction, and electron microscopy, the elimination of the insulator-metal (I-M) transition is shown to result from the creation of very small lateral coherence lengths (with the corresponding lateral size ∼ 3 nm (∼7 u.c.)) in the LBMO matrix, achieved by engineering a high density of CeO₂ nanocolumns in the matrix. The small lateral coherence length leads to a shift in the valence band maximum and reduction of the double exchange (DE) coupling. There is no "dead layer" effect at the smallest achieved lateral coherence length of ∼3 nm. The FMI behavior obtained by lateral dimensional tuning is independent of substrate interactions, thus intrinsic to the film itself and hence not related to film thickness. The unique properties of VAN films give the possibility for multilayer spintronic devices that can be made without interface degradation effects between the layers.

Route to High-Performance Micro-solid Oxide Fuel Cells on Metallic Substrates

Micro-solid oxide fuel cells based on thin films have strong potential for use in portable power devices. However, devices based on silicon substrates typically involve thin-film metallic electrodes which are unstable at high temperatures. Devices based on bulk metal substrates overcome these limitations, though performance is hindered by the challenge of growing state-of-the-art epitaxial materials on metals. Here, we demonstrate for the first time the growth of epitaxial cathode materials on metal substrates (stainless steel) commercially supplied with epitaxial electrolyte layers (1.5 μm (Y₂O₃)_0.15(ZrO₂)_0.85 (YSZ) + 50 nm CeO₂). We create epitaxial mesoporous cathodes of (La_0.60Sr_0.40)_0.95Co_0.20Fe_0.80O₃ (LSCF) on the substrate by growing LSCF/MgO vertically aligned nanocomposite films by pulsed laser deposition, followed by selectively etching out the MgO. To enable valid comparison with the literature, the cathodes are also grown on single-crystal substrates, confirming state-of-the-art performance with an area specific resistance of 100 Ω cm² at 500 °C and activation energy down to 0.97 eV. The work marks an important step toward the commercialization of high-performance micro-solid oxide fuel cells for portable power applications.

Role of ALD Al2O3 Surface Passivation on the Performance of p-Type Cu2O Thin Film Transistors

High-performance p-type oxide thin film transistors (TFTs) have great potential for many semiconductor applications. However, these devices typically suffer from low hole mobility and high off-state currents. We fabricated p-type TFTs with a phase-pure polycrystalline Cu₂O semiconductor channel grown by atomic layer deposition (ALD). The TFT switching characteristics were improved by applying a thin ALD Al₂O₃ passivation layer on the Cu₂O channel, followed by vacuum annealing at 300 °C. Detailed characterization by transmission electron microscopy-energy dispersive X-ray analysis and X-ray photoelectron spectroscopy shows that the surface of Cu₂O is reduced following Al₂O₃ deposition and indicates the formation of a 1-2 nm thick CuAlO₂ interfacial layer. This, together with field-effect passivation caused by the high negative fixed charge of the ALD Al₂O₃, leads to an improvement in the TFT performance by reducing the density of deep trap states as well as by reducing the accumulation of electrons in the semiconducting layer in the device off-state.

High Yield Transfer of Clean Large-Area Epitaxial Oxide Thin Films

In this work, we have developed a new method for manipulating and transferring up to 5 mm × 10 mm epitaxial oxide thin films. The method involves fixing a PET frame onto a PMMA attachment film, enabling transfer of epitaxial films lifted-off by wet chemical etching of a Sr₃Al₂O₆ sacrificial layer. The crystallinity, surface morphology, continuity, and purity of the films are all preserved in the transfer process. We demonstrate the applicability of our method for three different film compositions and structures of thickness ~ 100 nm. Furthermore, we show that by using epitaxial nanocomposite films, lift-off yield is improved by ~ 50% compared to plain epitaxial films and we ascribe this effect to the higher fracture toughness of the composites. This work shows important steps towards large-scale perovskite thin-film-based electronic device applications.

Colloidal Synthesis and Optical Properties of Perovskite-Inspired Cesium Zirconium Halide Nanocrystals

Optoelectronic devices based on lead halide perovskites are processed in facile ways, yet are remarkably efficient. There are extensive research efforts investigating lead-free perovskite and perovskite-related compounds, yet there are challenges to synthesize these materials in forms that can be directly integrated into thin film devices rather than as bulk powders. Here, we report on the colloidal synthesis and characterization of lead-free, antifluorite Cs₂ZrX₆ (X = Cl, Br) nanocrystals that are readily processed into thin films. We use transmission electron microscopy and powder X-ray diffraction measurements to determine their size and structural properties, and solid-state nuclear magnetic resonance measurements reveal the presence of oleate ligand, together with a disordered distribution of Cs surface sites. Density functional theory calculations reveal the band structure and fundamental band gaps of 5.06 and 3.91 eV for Cs₂ZrCl₆ and Cs₂ZrBr₆, respectively, consistent with experimental values. Finally, we demonstrate that the Cs₂ZrCl₆ and Cs₂ZrBr₆ nanocrystal thin films exhibit tunable, broad white photoluminescence with quantum yields of 45% for the latter, with respective peaks in the blue and green spectral regions and mixed systems exhibiting properties between them. Our work represents a critical step toward the application of lead-free Cs₂ZrX₆ nanocrystal thin films into next-generation light-emitting applications.

Dielectric films for high performance capacitive energy storage: multiscale engineering

Dielectric capacitors are fundamental components in electronic and electrical systems due to their high-rate charging/discharging character and ultrahigh power density. Film dielectrics possess larger breakdown strength and higher energy density than their bulk counterparts, holding great promise for compact and efficient power systems. In this article, we review the very recent advances in dielectric films, in the framework of engineering at multiple scales to improve energy storage performance. Strategies are summarized including atomic-scale defect control, nanoscale domain and grain engineering, as well as mesoscale composite design. Challenges and remaining concerns are also discussed for further performance improvement and practical application of dielectric films.

Revealing the Structure and Oxygen Transport at Interfaces in Complex Oxide Heterostructures via 17O NMR Spectroscopy

Vertically aligned nanocomposite (VAN) films, comprising nanopillars of one phase embedded in a matrix of another, have shown great promise for a range of applications due to their high interfacial areas oriented perpendicular to the substrate. In particular, oxide VANs show enhanced oxide-ion conductivity in directions that are orthogonal to those found in more conventional thin-film heterostructures; however, the structure of the interfaces and its influence on conductivity remain unclear. In this work, ¹⁷O NMR spectroscopy is used to study CeO₂-SrTiO₃ VAN thin films: selective isotopic enrichment is combined with a lift-off technique to remove the substrate, facilitating detection of the ¹⁷O NMR signal from single atomic layer interfaces. By performing the isotopic enrichment at variable temperatures, the superior oxide-ion conductivity of the VAN films compared to the bulk materials is shown to arise from enhanced oxygen mobility at this interface; oxygen motion at the interface is further identified from ¹⁷O relaxometry experiments. The structure of this interface is solved by calculating the NMR parameters using density functional theory combined with random structure searching, allowing the chemistry underpinning the enhanced oxide-ion transport to be proposed. Finally, a comparison is made with 1% Gd-doped CeO₂-SrTiO₃ VAN films, for which greater NMR signal can be obtained due to paramagnetic relaxation enhancement, while the relative oxide-ion conductivities of the phases remain similar. These results highlight the information that can be obtained on interfacial structure and dynamics with solid-state NMR spectroscopy, in this and other nanostructured systems, our methodology being generally applicable to overcome sensitivity limitations in thin-film studies.

Evidence of Rotational Fröhlich Coupling in Polaronic Trions

Electrons commonly couple through Fröhlich interactions with longitudinal optical phonons to form polarons. However, trions possess a finite angular momentum and should therefore couple instead to rotational optical phonons. This creates a polaronic trion whose binding energy is determined by the crystallographic orientation of the lattice. Here, we demonstrate theoretically within the Fröhlich approach and experimentally by photoluminescence emission that the bare trion binding energy (20 meV) is significantly enhanced by the phonons at the interface between the two-dimensional semiconductor MoS_{2} and the bulk transition metal oxide SrTiO_{3}. The low-temperature binding energy changes from 60 meV in [001]-oriented substrates to 90 meV for [111] orientation, as a result of the counterintuitive interplay between the rotational axis of the MoS_{2} trion and that of the SrTiO_{3} phonon mode.

Nanoengineering room temperature ferroelectricity into orthorhombic SmMnO3 films

Orthorhombic RMnO3 (R = rare-earth cation) compounds are type-II multiferroics induced by inversion-symmetry-breaking of spin order. They hold promise for magneto-electric devices. However, no spontaneous room-temperature ferroic property has been observed to date in orthorhombic RMnO3. Here, using 3D straining in nanocomposite films of (SmMnO3)0.5((Bi,Sm)2O3)0.5, we demonstrate room temperature ferroelectricity and ferromagnetism with TC,FM ~ 90 K, matching exactly with theoretical predictions for the induced strain levels. Large in-plane compressive and out-of-plane tensile strains (-3.6% and +4.9%, respectively) were induced by the stiff (Bi,Sm)2O3 nanopillars embedded. The room temperature electric polarization is comparable to other spin-driven ferroelectric RMnO3 films. Also, while bulk SmMnO3 is antiferromagnetic, ferromagnetism was induced in the composite films. The Mn-O bond angles and lengths determined from density functional theory explain the origin of the ferroelectricity, i.e. modification of the exchange coupling. Our structural tuning method gives a route to designing multiferroics.

Achieving ferromagnetic insulating properties in La0.9Ba0.1MnO3 thin films through nanoengineering

Strongly correlated manganites have a wide range of fascinating magnetic and electronic properties, one example being the coexistence of ferromagnetic and insulating properties in lightly-doped bulk. However, it is difficult to translate bulk properties to films. Here, this problem is overcome by thin film nanoengineering of the test case system, La_0.9Ba_0.1MnO₃ (LBMO). This was achieved by using vertically aligned nanocomposite (VAN) thin films of LBMO + CeO₂ in which CeO₂ nanocolumns form embedded in a LBMO matrix. The CeO₂ columns produce uniform tensile straining of the LBMO. Also light Ce doping of intrinsic cation vacancies in the LBMO occurs. Together, these factors strongly reduced the double exchange coupling and metallicity. Hence, while standard plain reference films showed an insulator-to-metal transition at >200 K, originating from defects and complex structural relaxation, the VAN LBMO films exhibited ferromagnetic insulating properties (while maintaining a T_c of 188 K). This is the first time that a combined strain + doping method is used in a VAN system to realise exemplary properties which cannot be realised in plain films. This work represents an important step in engineering high performance spintronic and multiferroic thin film devices.

Over 20% Efficiency in Methylammonium Lead Iodide Perovskite Solar Cells with Enhanced Stability via "in Situ Solidification" of the TiO2 Compact Layer

In methylammonium lead iodide (MAPbI₃) perovskite solar cells (PSCs), the device performance is strongly influenced by the TiO₂ electron transport layer (ETL). Typically, the ETL needs to simultaneously be thin and pinhole-free to have high transmittance and avoid shunting. In this work, we develop an "in situ solidification" process following spin coating in which the titanium-based precursor (titanium(diisopropoxide) bis(2,4-pentanedionate)) is dried under vacuum to rapidly achieve continuous TiO₂ layers. We refer to this as "gas-phase quenching". This results in thin (60 ± 10 nm), uniform, and pinhole-free TiO₂ films. The PSCs based on the gas-phase quenched TiO₂ exhibits improved power conversion efficiency, with a median value of 18.23% (champion value of 20.43%), compared to 9.03 and 14.09% for the untreated devices. Gas-phase quenching is further shown to be effective in enabling efficient charge transfer at the MAPbI₃/TiO₂ heterointerface. Furthermore, the stability of the gas-phase quenched devices is enhanced in ambient air as well as under 1 sun illumination. In addition, we achieve 12.1% efficiency in upscaled devices (1.1 cm² active area).

Magnetic signatures of 120 K superconductivity at interfaces in La2CuO4+δ

In self-assembled vertically aligned nanocomposite (VAN) thin films of La2CuO4+δ + LaCuO3, we find from DC magnetic susceptibility measurements, weak signatures of superconductivity at ∼120 K. This compares to a maximum TC of 40 K in bulk La2CuO4+δ. The 120 K signature occurs only when both c-axis and a-axis oriented La2CuO4+δ grains are present in the films. The superconductivity was lost after 3 months of storage but was recovered by annealing in oxygen. From lattice parameter analyses undertaken close to the c/a grain boundaries, it was determined that expansion of the La perovskite block in c-La2CuO4+δ enables the differently oriented grains to join at the interface. This expansion is consistent with the higher TC interfacial region. The work shows a new direction for increasing TC in cuprates - namely careful strain engineering of the crystal structure independently in-plane and out-of-plane.

Growth of Doped SrTiO3 Ferroelectric Nanoporous Thin Films and Tuning of Photoelectrochemical Properties with Switchable Ferroelectric Polarization

Ferroelectric polarization is an intriguing physical phenomenon for tuning charge-transport properties and finds application in a wide range of optoelectronic devices. So far, ferroelectric materials in a planar geometry or chemically grown nanostructures have been used. However, these structural architectures possess serious disadvantages such as small surface areas and structural defects, respectively, leading to reduced performance. Herein, the growth of room-temperature ferroelectric nanoporous/nanocolumnar structure of Ag,Nb-codoped SrTiO₃ (Ag/Nb:STO) using pulsed laser deposition is reported and demonstrated to have enhanced photoelectrochemical (PEC) properties using ferroelectric polarization. By manipulating the external electrical bias, ∼3-fold enhancement in the photocurrent from 40 to 130 μA·cm^-2 of film area is obtained. Concurrently, the flat-band potential is decreased from -0.55 to -1.13 V, revealing a giant ferroelectric tuning of the band alignment at the semiconductor surface and enhanced charge transfer. In addition, an electrochemical impedance spectroscopy study confirmed the tuning of the charge transfer with ferroelectric polarization. Our nanoporous ferroelectric-semiconductor approach offers a new platform with great potential for achieving highly efficient PEC devices for renewable energy applications.

Identifying and Reducing Interfacial Losses to Enhance Color-Pure Electroluminescence in Blue-Emitting Perovskite Nanoplatelet Light-Emitting Diodes

Perovskite nanoplatelets (NPls) hold promise for light-emitting applications, having achieved photoluminescence quantum efficiencies approaching unity in the blue wavelength range, where other metal-halide perovskites have typically been ineffective. However, the external quantum efficiencies (EQEs) of blue-emitting NPl light-emitting diodes (LEDs) have reached only 0.12%. In this work, we show that NPl LEDs are primarily limited by a poor electronic interface between the emitter and hole injector. We show that the NPls have remarkably deep ionization potentials (≥6.5 eV), leading to large barriers for hole injection, as well as substantial nonradiative decay at the NPl/hole-injector interface. We find that an effective way to reduce these nonradiative losses is by using poly(triarylamine) interlayers, which lead to an increase in the  EQE of the blue (464 nm emission wavelength) and sky-blue (489 nm emission wavelength) LEDs to 0.3% and 0.55%, respectively. Our work also identifies the key challenges for further efficiency increases.

3D strain-induced superconductivity in La2CuO4+δ using a simple vertically aligned nanocomposite approach

A long-term goal for superconductors is to increase the superconducting transition temperature, T _C. In cuprates, T _C depends strongly on the out-of-plane Cu-apical oxygen distance and the in-plane Cu-O distance, but there has been little attention paid to tuning them independently. Here, in simply grown, self-assembled, vertically aligned nanocomposite thin films of La₂CuO_4+δ + LaCuO₃, by strongly increasing out-of-plane distances without reducing in-plane distances (three-dimensional strain engineering), we achieve superconductivity up to 50 K in the vertical interface regions, spaced ~50 nm apart. No additional process to supply excess oxygen, e.g., by ozone or high-pressure oxygen annealing, was required, as is normally the case for plain La₂CuO_4+δ films. Our proof-of-concept work represents an entirely new approach to increasing T _C in cuprates or other superconductors.

All-Oxide Nanocomposites to Yield Large, Tunable Perpendicular Exchange Bias above Room Temperature

In all-oxide-based spintronic devices, large exchange bias effect with robustness against temperature fluctuation and compatibility with perpendicular magnetic recording is highly desired. In this work, rock-salt antiferromagnetic NiO with a Néel temperature ( T_N) of ∼525 K and spinel ferrimagnetic NiFe₂O₄ with a high Curie temperature, T_C, ≈ 790 K and T_C > T_N were chosen as compatible materials to form a well-phase-separated, vertically aligned nanocomposite thin film. In this nanoengineered thin film, an exchange bias effect with a blocking temperature far above room temperature has been achieved. A large perpendicular exchange bias field of up to 0.91 kOe with an interfacial exchange energy density of 0.11-0.34 erg/cm² was obtained at room temperature. It was also demonstrated that the exchange bias effect can be easily tuned by changing the alignment of the magnetic moments in the NiO phase using substrates of different crystalline orientations and by changing the microstructure of the film with substrates of different lattice parameters. The results demonstrate that proper choice of the phases (including use of nonperovskite compositions) and careful strain engineering and nanostructure engineering makes oxide nanocomposites strong potential candidate systems for next generation spintronic devices.

In Situ Atmospheric Deposition of Ultrasmooth Nickel Oxide for Efficient Perovskite Solar Cells

Organic-inorganic perovskite solar cells have attracted significant attention due to their remarkable performance. The use of alternative metal-oxide charge-transport layers is a strategy to improving device reliability for large-scale fabrication and long-term applications. Here, we report solution-processed perovskite solar cells employing nickel oxide hole-extraction layers produced in situ using an atmospheric pressure spatial atomic-layer deposition system, which is compatible with high-throughput processing of electronic devices from solution. Our sub-nanometer smooth (average roughness of ≤0.6 nm) oxide films enable the efficient collection of holes and the formation of perovskite absorbers with high electronic quality. Initial solar-cell experiments show a power-conversion efficiency of 17.1%, near-unity ideality factors, and a fill factor of >80% with negligible hysteresis. Transient measurements reveal that a key contributor to this performance is the reduced luminescence quenching trap density in the perovskite/nickel oxide structure.

Design of a Vertical Composite Thin Film System with Ultralow Leakage To Yield Large Converse Magnetoelectric Effect

Electric field control of magnetism is a critical future technology for low-power, ultrahigh density memory. However, despite intensive research efforts, no practical material systems have emerged. Interface-coupled, composite systems containing ferroelectric and ferri-/ferromagnetic elements have been widely explored, but they have a range of problems, for example, substrate clamping, large leakage, and inability to miniaturize. In this work, through careful material selection, design, and nanoengineering, a high-performance room-temperature magnetoelectric system is demonstrated. The clamping problem is overcome by using a vertically aligned nanocomposite structure in which the strain coupling is independent of the substrate. To overcome the leakage problem, three key novel advances are introduced: a low leakage ferroelectric, Na_0.5Bi_0.5TiO₃; ferroelectric-ferrimagnetic vertical interfaces which are not conducting; and current blockage via a rectifying interface between the film and the Nb-doped SrTiO₃ substrate. The new multiferroic nanocomposite (Na_0.5Bi_0.5TiO₃-CoFe₂O₄) thin-film system enables, for the first time, large-scale in situ electric field control of magnetic anisotropy at room temperature in a system applicable for magnetoelectric random access memory, with a magnetoelectric coefficient of 1.25 × 10^-9 s m^-1.

Pushing the limits of applicability of REBCO coated conductor films through fine chemical tuning and nanoengineering of inclusions

An outstanding current carrying performance (namely critical current density, Jc) over a broad temperature range of 10-77 K for magnetic fields up to 12 T is reported for films of YBa2Cu3O7-x with Ba2Y(Nb,Ta)O6 inclusion pinning centres (YBCO-BYNTO) and thicknesses in the range of 220-500 nm. Jc values of 10 MA cm-2 were measured at 30 K - 5 T and 10 K - 9 T with a corresponding maximum of the pinning force density at 10 K close to 1 TN m-3. The system is very flexible regarding properties and microstructure tuning, and the growth window for achieving a particular microstructure is wide, which is very important for industrial processing. Hence, the dependence of Jc on the magnetic field angle was readily controlled by fine tuning the pinning microstructure. Transmission electron microscopy (TEM) analysis highlighted that higher growth rates induce more splayed and denser BYNTO nanocolumns with a matching field as high as 5.2 T. Correspondingly, a strong peak at the B||c-axis is noticed when the density of vortices is lower than the nanocolumn density. YBCO-BYNTO is a very robust and reproducible composite system for high-current coated conductors over an extended range of magnetic fields and temperatures.

Very high commutation quality factor and dielectric tunability in nanocomposite SrTiO3 thin films with Tc enhanced to >300 °C

We report on nanoengineered SrTiO3-Sm2O3 nanocomposite thin films with the highest reported values of commutation quality factor (CQF or K-factor) of >2800 in SrTiO3 at room temperature. The films also had a large tunability of dielectric constant (49%), low tangent loss (tan δ = 0.01) and a Curie temperature for SrTiO3 > 300 °C, making them very attractive for tunable RF applications. The enhanced properties originate from the unique nanostructure in the films, with <20 nm diameter strain-controlling Sm2O3 nanocolumns embedded in a SrTiO3 matrix. Very large out-of-plane strains (up to 2.6%) and high tetragonality (c/a) (up to 1.013) were induced in the SrTiO3. The K-factor was further enhanced by adding 1 at% Sc3+ (acceptor) dopant in SrTiO3 to a value of 3300 with the tangent loss being ≤0.01 up to 1000 kV cm-1.

Strongly Enhanced Photovoltaic Performance and Defect Physics of Air-Stable Bismuth Oxyiodide (BiOI)

Bismuth-based compounds have recently gained increasing attention as potentially nontoxic and defect-tolerant solar absorbers. However, many of the new materials recently investigated show limited photovoltaic performance. Herein, one such compound is explored in detail through theory and experiment: bismuth oxyiodide (BiOI). BiOI thin films are grown by chemical vapor transport and found to maintain the same tetragonal phase in ambient air for at least 197 d. The computations suggest BiOI to be tolerant to antisite and vacancy defects. All-inorganic solar cells (ITO|NiO_x |BiOI|ZnO|Al) with negligible hysteresis and up to 80% external quantum efficiency under select monochromatic excitation are demonstrated. The short-circuit current densities and power conversion efficiencies under AM 1.5G illumination are nearly double those of previously reported BiOI solar cells, as well as other bismuth halide and chalcohalide photovoltaics recently explored by many groups. Through a detailed loss analysis using optical characterization, photoemission spectroscopy, and device modeling, direction for future improvements in efficiency is provided. This work demonstrates that BiOI, previously considered to be a poor photocatalyst, is promising for photovoltaics.

Electronic Structure and Band Alignment at the NiO and SrTiO3 p-n Heterojunctions

Understanding the energetics at the interface, including the alignment of valence and conduction bands, built-in potentials, and ionic and electronic reconstructions, is an important challenge in designing oxide interfaces that have controllable multifunctionalities for novel (opto-)electronic devices. In this work, we report detailed investigations on the heterointerface of wide-band-gap p-type NiO and n-type SrTiO₃ (STO). We show that despite a large lattice mismatch (∼7%) and dissimilar crystal structure, high-quality NiO and Li-doped NiO (LNO) thin films can be epitaxially grown on STO(001) substrates through a domain-matching epitaxy mechanism. X-ray photoelectron spectroscopy studies indicate that NiO/STO heterojunctions form a type II "staggered" band alignment. In addition, a large built-in potential of up to 0.97 eV was observed at the interface of LNO and Nb-doped STO (NbSTO). The LNO/NbSTO p-n heterojunctions exhibit not only a large rectification ratio of 2 × 10³ but also a large ideality factor of 4.3. The NiO/STO p-n heterojunctions have important implications for applications in photocatalysis and photodetectors as the interface provides favorable energetics for facile separation and transport of photogenerated electrons and holes.

Hidden Interface Driven Exchange Coupling in Oxide Heterostructures

A variety of emergent phenomena have been enabled by interface engineering in complex oxides. The existence of an intrinsic interfacial layer has often been found at oxide heterointerfaces. However, the role of such an interlayerin controlling functionalities is not fully explored. Here, we report the control of the exchange bias (EB) in single-phase manganite thin films with nominallyuniform chemical composition across the interfaces. The sign of EB depends on the magnitude of the cooling field. A pinned layer, confirmed by polarized neutron reflectometry, provides the source of unidirectional anisotropy. The origin of the exchange bias coupling is discussed in terms of magnetic interactions between the interfacial ferromagnetically reduced layer and the bulk ferromagnetic region. The sign of EB is related to the frustration of antiferromagnetic coupling between the ferromagnetic region and the pinned layer. Our results shed new light on using oxide interfaces to design functional spintronic devices.

Colossal Terahertz Magnetoresistance at Room Temperature in Epitaxial La0.7Sr0.3MnO3 Nanocomposites and Single-Phase Thin Films

Colossal magnetoresistance (CMR) is demonstrated at terahertz (THz) frequencies by using terahertz time-domain magnetospectroscopy to examine vertically aligned nanocomposites (VANs) and planar thin films of La_0.7Sr_0.3MnO₃. At the Curie temperature (room temperature), the THz conductivity of the VAN was dramatically enhanced by over 2 orders of magnitude under the application of a magnetic field with a non-Drude THz conductivity that increased with frequency. The direct current (dc) CMR of the VAN is controlled by extrinsic magnetotransport mechanisms such as spin-polarized tunneling between nanograins. In contrast, we find that THz CMR is dominated by intrinsic, intragrain transport: the mean free path was smaller than the nanocolumn size, and the planar thin-film exhibited similar THz CMR to the VAN. Surprisingly, the observed colossal THz magnetoresistance suggests that the magnetoresistance can be large for alternating current motion on nanometer length scales, even when the magnetoresistance is negligible on the macroscopic length scales probed by dc transport. This suggests that colossal magnetoresistance at THz frequencies may find use in nanoelectronics and in THz optical components controlled by magnetic fields. The VAN can be scaled in thickness while retaining a high structural quality and offers a larger THz CMR at room temperature than the planar film.

Efficient Triplet Exciton Fusion in Molecularly Doped Polymer Light-Emitting Diodes

Solution-processed polymer organic light-emitting diodes (OLEDs) doped with triplet-triplet annihilation (TTA)-upconversion molecules, including 9,10-diphenylanthracene, perylene, rubrene and TIPS-pentacene, are reported. The fraction of triplet-generated electroluminescence approaches the theoretical limit. Record-high efficiencies in solution-processed OLEDs based on these materials are achieved. Unprecedented solid-state TTA-upconversion quantum yield of 23% (TTA-upconversion reaction efficiency of 70%) at electrical excitation well below one-sun equivalent is observed.

Rapid open-air deposition of uniform, nanoscale, functional coatings on nanorod arrays

Coating of high-aspect-ratio nanostructures has previously been achieved using batch processes poorly suited for high-throughput manufacturing. It is demonstrated that uniform, nanoscale coatings can be rapidly deposited on zinc oxide nanorod arrays in open-air using an atmospheric pressure spatial deposition system. The morphology of the metal oxide coatings is examined and good electrical contact with the underlying nanorods is observed. The functionality of the coatings is demonstrated in colloidal quantum dot and hybrid solar cells.

Very High Surface Area Mesoporous Thin Films of SrTiO3 Grown by Pulsed Laser Deposition and Application to Efficient Photoelectrochemical Water Splitting

Very high surface area, self-assembled, highly crystalline mesoporous SrTiO₃ (STO) thin films were developed for photoelectrochemical water splitting. Much improved performance of these mesoporous films compared to planar STO thin films and any other form of STO such as single crystal samples and nanostructures was demonstrated. The high performance resulted from very large surface area films and optimization of carrier concentration.