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We measure the mean inner potential (MIP) of hematite, α-Fe2O3, using electron holography and transmission electron microscopy. Since the MIP is sensitive to valence electrons, we propose its use as a chemical bonding parameter for solids. Hematite can test the sensitivity of the MIP as a bonding parameter because of the Morin magnetic phase transition. Across this transition temperature, no change in the corundum crystal structure can be distinguished, while a change in hybridized Fe-3d and O-2p states was reported, affecting ionic bonding. For a given crystallographic phase, the change in the MIP with temperature is expected to be minor due to thermal expansion. Indeed, we measure the temperature dependence in corundum α-Al2O3(112¯0) between 95 and 295 K showing a constant MIP value of ∼16.8 V within the measurement accuracy of 0.45 V. Thus, our objectives are as follows: measure the MIP of hematite as a function of temperature and examine the sensitivity of the MIP as a bonding parameter for crystals. Measured MIPs of α-Fe2O3(112¯0) above the Morin transition are equal, 17.85 ± 0.50 V, 17.93 ± 0.50 V, at 295 K, 230 K, respectively. Below the Morin transition, at 95 K, a significant reduction of ∼1.3 V is measured to 16.56 ± 0.46 V. We show that this reduction follows charge redistribution resulting in increased ionic bonding.

Hematite (α-Fe2O3) is a leading photoanode candidate for photoelectrochemical water splitting. Despite extensive research efforts, the champion hematite photoanodes reported to date have achieved less than half of the maximal photocurrent predicted by its bandgap energy. Here we show that this underachievement arises, to a large extent, because of unproductive optical excitations that give rise to localized electronic transitions that do not generate electron–hole pairs. A comprehensive method for extraction of the photogeneration yield spectrum, the wavelength-dependent fraction of absorbed photons that generate electron–hole pairs, and the spatial charge carrier collection efficiency is presented, and applied for a thin (32 nm) film hematite photoanode. Its photogeneration yield is less than unity across the entire absorption range, limiting the maximal photocurrent that may be attained in an ideal hematite photoanode to about half of the theoretical limit predicted without accounting for this effect.

Optimising the photoelectrochemical performance of hematite photoanodes for solar water splitting requires better understanding of the relationships between dopant distribution, structural defects and photoelectrochemical properties. Here, we use complementary characterisation techniques including electron microscopy, conductive atomic force microscopy (CAFM), Rutherford backscattering spectroscopy (RBS), atom probe tomography (APT) and intensity modulated photocurrent spectroscopy (IMPS) to study this correlation in Ti-doped (1 cat.%) hematite films deposited by pulsed laser deposition (PLD) on F:SnO2 (FTO) coated glass substrates. The deposition was carried out at 300 {\deg}C, followed by annealing at 500 deg C for 2 h. Upon annealing, Ti was observed by APT to segregate to the hematite/FTO interface and into some hematite grains. Since no other pronounced changes in microstructure and chemical composition were observed by electron microscopy and RBS after annealing, the non-uniform Ti redistribution seems to be the reason for a reduced interfacial recombination in the annealed films, as observed by IMPS. This results in a lower onset potential, higher photocurrent and larger fill factor with respect to the as-deposited state. This work provides atomic-scale insights into the microscopic inhomogeneity in Ti-doped hematite thin films and the role of defect segregation in their electrical and photoelectrochemical properties.

Electrolytic hydrogen production faces technological challenges to improve its efficiency, economic value and potential for global integration. In conventional water electrolysis, the water oxidation and reduction reactions are coupled in both time and space, as they occur simultaneously at an anode and a cathode in the same cell. This introduces challenges, such as product separation, and sets strict constraints on material selection and process conditions. Here, we decouple these reactions by dividing the process into two steps: an electrochemical step that reduces water at the cathode and oxidizes the anode, followed by a spontaneous chemical step that is driven faster at higher temperature, which reduces the anode back to its initial state by oxidizing water. This enables overall water splitting at average cell voltages of 1.44–1.60 V with nominal current densities of 10–200 mA cm−2 in a membrane-free, two-electrode cell. This allows us to produce hydrogen at low voltages in a simple, cyclic process with high efficiency, robustness, safety and scale-up potential. Conventionally, the two half reactions involved in water electrolysis occur simultaneously, presenting materials and process challenges. Here, the authors decouple these to split water efficiently in two steps: electrochemical hydrogen evolution, followed by spontaneous oxygen evolution at elevated temperature.

Erratum

External quantum efficiency (EQE) of bismuth vanadate thin film photoanodes, measured in a pH 7 potassium phosphate buffer solution with sodium sulfite hole scavenger, was observed to substantially decrease when measured under white light bias (LB). While the EQE exhibited a fast initial decrease across its full spectral range, a ∼3.5 eV (350 nm) feature under front illumination conditions became disproportionally suppressed after being under LB (strongest when it is also incident on the front side of the sample) for several tens of minutes, in spite of this wavelength being outside the spectral range encompassed by the LB source. Applied potential does not have a strong effect on the qualitative behavior. From its different decay time, the wavelength-specific decrease of the 3.5 eV feature, and its responsible mechanism, is distinct from the initial, spectrally uniform decrease of EQE, which happens at a faster timescale and is similar for all illumination conditions. To more closely examine the suppression of the 3.5 eV feature, we compare calculated depth-dependent optical generation profiles and behaviors under different illumination conditions, which imply the involvement of in-gap states and long-lived states deeper into the conduction (or alternatively, valence) band. Possible mechanisms are discussed.

Environmental concerns deriving from fossil fuel dependency are driving an energy transition based on sustainable processes to make fuels and chemicals. Solar hydrogen is the pillar of this new green economy, but the technological readiness level of PV electrolysis and direct photoelectrochemical (PEC) electrolysis are still too low to allow broad commercialization. Direct conversion through PEC technology has more potential in the medium–long term but must be first guided by the scientific enhancements to improve device efficiencies. For this purpose, in situ and operando photoelectrochemistry will guide the discovery of new materials and processes to make solar fuels and chemicals in PEC cells. The use of advanced in situ and operando characterizations under working photoelectrochemical (PEC) conditions is reviewed here and anticipated to be a fundamental tool for achieving a basic understanding of new PEC processes and for enabling the large-scale development of PEC technology by 2050, thus delivering fuels and chemicals having zero (or negative) carbon footprint. Hydrogen from solar water splitting is the most popular solar fuel and can be mainly produced by indirect photovoltaic-driven electrolysis (PV electrolysis) and direct photoelectrochemistry. Although PV electrolysis has already been developed on a level of MW-scale pilot plants, PEC technology, which is much less mature, holds several advantages in the long term over PV-electrolysis systems. The key enabling feature to developing PEC technology is the improvement of the photoelectrode materials which are responsible for the absorption of light, and transport of the photo-generated charge carriers to drive the electrochemical surface reaction. These processes are often complex and multistep, spanning multiple timescales and following the simultaneous detection of photoelectrodes modification and formation of reaction intermediates/products can be achieved using eight well-known characterization techniques here presented.

The photoelectrochemical behavior of a planar 1 cm2 thick Ti-doped hematite film deposited on F:SnO2 coated glass was studied with both front and back illumination. Despite low quantum efficiency, photocurrent was observed upon back illumination with low wavelengths, indicating that some photogenerated holes are able to traverse at least 700 nm across the hematite film and effectively oxidize water. This cannot be accounted for using the commonly accepted hole collection length of hematite based on fitting to the Gartner model. Furthermore, under back illumination, 450 nm excitation resulted in increased photocurrent as compared to 530 nm excitation despite most of the light being absorbed further away from the surface. These results demonstrate that the photocurrent is strongly dependent on the optical excitation wavelength, and related to both delocalized holes with long lifetime and localized excitations rather than only being dependent on the proximity of the absorption to the surface.

Strong interference in ultrathin film semiconductor absorbers on metallic back reflectors has been shown to enhance the light harvesting efficiency of solar cell materials. However, metallic back reflectors are not suitable for tandem cell configurations because photons cannot be transmitted through the device. Here, we introduce a method to implement strong interference in ultrathin film top absorbers in a tandem cell configuration through use of distributed Bragg reflectors (DBRs). We showcase this by designing and fabricating a photoelectrochemical-photovoltaic (PEC-PV) stacked tandem cell in a V-shaped configuration where short wavelength photons are reflected back to the photoanode material (hematite, Fe2O3), whereas long wavelength photons are transmitted to the bottom silicon PV cell. We employ optical simulations to determine the optimal thicknesses of the DBR layers and the V-shape angle to maximize light absorption in the ultrathin (10 nm thick) hematite film. The DBR spectral response can be tailored to allow for a more than threefold enhancement in absorbed photons compared to a layer of the same thickness on transparent current collectors. Using a DBR to couple a bottom silicon PV cell with an ultrathin hematite top PEC cell, we demonstrate unassisted solar water splitting and show that DBRs can be designed to enhance strong interference in ultrathin films while enabling stacked tandem cell configuration.

H2O2 is a sacrificial reductant that is often used as a hole scavenger to gain insight into photoanode properties. Here we show a distinct mechanism of H2O2 photo-oxidation on haematite (α-Fe2O3) photoanodes. We found that the photocurrent voltammograms display non-monotonous behaviour upon varying the H2O2 concentration, which is not in accord with a linear surface reaction mechanism that involves a single reaction site as in Eley–Rideal reactions. We postulate a nonlinear kinetic mechanism that involves concerted interaction between adions induced by H2O2 deprotonation in the alkaline solution with adjacent intermediate species of the water photo-oxidation reaction, thereby involving two reaction sites as in Langmuir–Hinshelwood reactions. The devised kinetic model reproduces our main observations and predicts coexistence of two surface reaction paths (bi-stability) in a certain range of potentials and H2O2 concentrations. This prediction is confirmed experimentally by observing a hysteresis loop in the photocurrent voltammogram measured in the predicted coexistence range.

Light absorption in strongly correlated electron materials can excite electrons and holes into a variety of different states. Some of these excitations yield mobile charge carriers, whereas others result in localized states that cannot contribute to photocurrent. The photogeneration yield spectrum, ξ(λ), represents the wavelength-dependent ratio between the contributing absorption that ultimately generates mobile charge carriers and the overall absorption. Despite being a vital material property, it is not trivial to characterize. Here, we present an empirical method to extract ξ(λ) through optical and external quantum efficiency measurements of ultrathin films. We applied this method to haematite photoanodes for water photo-oxidation, and observed that it is self-consistent for different illumination conditions and applied potentials. We found agreement between the extracted ξ(λ) spectrum and the photoconductivity spectrum measured by time-resolved microwave conductivity. These measurements revealed that mobile charge carrier generation increases with increasing energy across haematite's absorption spectrum. Low-energy non-contributing absorption fundamentally limits the photoconversion efficiency of haematite photoanodes and provides an upper limit to the achievable photocurrent that is substantially lower than that predicted based solely on absorption above the bandgap. We extended our analysis to TiO

Transparent Fe1-xNixOOH overlayers (~2 nm thick) were deposited photoelectrochemically on (001) oriented heteroepitaxial Sn- and Zn-doped hematite (Fe2O3) thin film photoanodes. In both cases, the water photo-oxidation performance was improved by the co-catalyst overlayers. Intensity modulated photocurrent spectroscopy (IMPS) was applied to study the changes in the hole current and recombination current induced by the overlayers. For the Sn-doped hematite photoanode, the improvement in performance after deposition of the Fe1-xNixOOH overlayer was entirely due to reduction in the recombination current, leading to a cathodic shift in the onset potential. For the Zn-doped hematite photoanode, in addition to a reduction in recombination current, an increase in the hole current to the surface was also observed after the overlayer deposition, leading to a cathodic shift in the onset potential as well as an enhancement in the plateau photocurrent. These results demonstrate that Fe1-xNixOOH co-catalysts can play different roles depending on the underlying hematite photoanode. The effect of the co-catalyst is not always limited to changes in the surface properties, but also to an increase in hole current from the bulk to the surface that indicates a possible crosslink between surface and bulk processes.

The oxygen evolution reaction (OER) at the surface of semiconductor photoanodes is critical for photoelectrochemical water splitting. This reaction involves photo-generated holes that oxidize water via charge transfer at the photoanode/electrolyte interface. However, a certain fraction of the holes that reach the surface recombine with electrons from the conduction band, giving rise to the surface recombination loss. The charge transfer efficiency, Zt, defined as the ratio between the flux of holes that contribute to the water oxidation reaction and the total flux of holes that reach the surface, is an important parameter that helps to distinguish between bulk and surface recombination losses. However, accurate determination of Zt by conventional voltammetry measurements is complicated because only the total current is measured and it is difficult to discern between different contributions to the current. Chopped light measurement (CLM) and hole scavenger measurement (HSM) techniques are widely employed to determine Zt, but they often lead to errors resulting from instrumental as well as fundamental limitations. Intensity modulated photocurrent spectroscopy (IMPS) is better suited for accurate determination of Zt because it provides direct information on both the total photocurrent and the surface recombination current. However, careful analysis of IMPS measurements at different light intensities is required to account for nonlinear effects. This work compares the Zt values obtained by these methods using heteroepitaxial thinfilm hematite photoanodes as a case study. We show that a wide spread of Zt values is obtained by different analysis methods, and even within the same method different values may be obtained depending on instrumental and experimental conditions such as the light source and light intensity. Statistical analysis of the results obtained for our model hematite photoanode show good correlation between different methods for measurements carried out with the same light source, light intensity and potential. However, there is a considerable spread in the results obtained by different methods. For accurate determination of Zt, we recommend IMPS measurements in operando with a bias light intensity such that the irradiance is as close as possible to the AM1.5 Global solar spectrum.

Spintronics seeks to functionalize antiferromagnetic materials to develop memory and logic devices operating at terahertz speed and robust against external magnetic field perturbations. To be useful, such functionality needs to be developed in thin film devices. The key functionality of long-distance spin-transport has, however, so far only been reported in bulk single crystal antiferromagnets, while in thin films, transport has so far been limited to a few nanometers. In this work, we electrically achieve a long-distance propagation of spin-information in thin films of the insulating antiferromagnet hematite. Through transport and magnetic imaging, we demonstrate a strong correlation between the efficiency of the transport of magnons, which carry spin-information, and the magnetic domain structure of the films. In thin films with large domains, magnons propagate over micrometer distances whilst they attenuate over much shorter distances in multidomain thin films. The governing factor of the attenuation is related to scattering at domain walls, and we demonstrate that we can reduce this through training by field cyclings. For the appropriate crystalline orientation of the films, spin-transport is achieved in zero applied field across micrometers, as required for integration with devices.

Optical interference is used to enhance light-matter interaction and harvest broadband light in ultrathin semiconductor absorber films on specular back-reflectors. However, the high-temperature processing in oxygen atmosphere required for oxide absorbers often degrades metallic back-reflectors and their specular reflectance. In order to overcome this problem, a newly developed film flip and transfer process is presented that enables high-temperature processing without degradation of the metallic back-reflector and without the need of passivation interlayers. The film flip and transfer process improves the performance of photoanodes for photoelectrochemical water splitting comprising ultrathin (

H2O2 is a sacrificial reductant that is often used as a hole scavenger to gain insight into photoanode properties. Here we show a distinct mechanism of H2O2 photo-oxidation on haematite (α-Fe2O3) photoanodes. We found that the photocurrent voltammograms display non-monotonous behaviour upon varying the H2O2 concentration, which is not in accord with a linear surface reaction mechanism that involves a single reaction site as in Eley–Rideal reactions. We postulate a nonlinear kinetic mechanism that involves concerted interaction between adions induced by H2O2 deprotonation in the alkaline solution with adjacent intermediate species of the water photo-oxidation reaction, thereby involving two reaction sites as in Langmuir–Hinshelwood reactions. The devised kinetic model reproduces our main observations and predicts coexistence of two surface reaction paths (bi-stability) in a certain range of potentials and H2O2 concentrations. This prediction is confirmed experimentally by observing a hysteresis loop in the photocurrent voltammogram measured in the predicted coexistence range., Understanding fundamental processes that occur using solar-to-fuel conversion materials is crucial for constructing effective renewable energy collection. Here, authors find the hydrogen peroxide light-driven hole-scavenging mechanism over haematite to proceed with two active sites rather than one

Photoelectrochemical impedance spectroscopy (PEIS) is a useful tool for the characterization of photoelectrodes for solar water splitting. However, the analysis of PEIS spectra often involves a priori assumptions that might bias the results. This work puts forward an empirical method that analyzes the distribution of relaxation times (DRT), obtained directly from the measured PEIS spectra of a model hematite photoanode. By following how the DRT evolves as a function of control parameters such as the applied potential and composition of the electrolyte solution, we obtain unbiased insights into the underlying mechanisms that shape the photocurrent. In a subsequent step, we fit the data to a process-oriented equivalent circuit model (ECM) whose makeup is derived from the DRT analysis in the first step. This yields consistent quantitative trends of the dominant polarization processes observed. Our observations reveal a common step for the photo-oxidation reactions of water and H2O2 in alkaline solution.

© 2017 Elsevier Inc. The spatial collection efficiency portrays the driving forces and loss mechanisms in photovoltaic and photoelectrochemical devices. It is defined as the fraction of photogenerated charge carriers created at a specific point within the device that contribute to the photocurrent. In stratified planar structures, the spatial collection efficiency can be extracted out of photocurrent action spectra measurements empirically, with few a priori assumptions. Although this method was applied to photovoltaic cells made of well-understood materials, it has never been used to study unconventional materials such as metal-oxide semiconductors that are often employed in photoelectrochemical cells. This perspective shows the opportunities that this method has to offer for investigating new materials and devices with unknown properties. The relative simplicity of the method, and its applicability to operando performance characterization, makes it an important tool for analysis and design of new photovoltaic and photoelectrochemical materials and devices. Understanding the optoelectronic and transport properties of semiconductors is essential for producing high-efficiency photovoltaic and photoelectrochemical cells. To this end, empirical extraction of the spatial collection efficiency (i.e., the fraction of photogenerated charge carriers created at a specific point within the device that contribute to the photocurrent) is a useful, nondestructive, analytical tool to study new materials, junctions, and devices. This perspective describes how the spatial collection efficiency can be extracted by combining photocurrent action spectra with optical absorption profiles. The result is high-resolution depth profiles of device functionality with very few assumptions, which paves the way to operando semiconductor tomography. The challenges and opportunities that this method offers for analysis of complex materials are discussed. Since the method is based on widely used spectral response measurements, it can be an important addition to the toolbox of analytical methods for material research for future solar energy conversion systems. Quantifying transport and loss mechanisms is a key step in producing high-efficiency solar energy conversion and storage devices. Empirical extraction of the spatial collection efficiency under real operating conditions is a useful analytical tool for studying such processes. We describe how the spatial collection efficiency can be extracted and apply the method to a α-Fe2O3water splitting photoanode. The extracted spatial collection efficiency profiles provide significant insights on driving forces and the advanced optoelectronic properties of α-Fe2O3.

In recent years, hematite's potential as a photoanode material for solar hydrogen production has ignited a renewed interest in its physical and interfacial properties, which continues to be an active field of research. Research on hematite photoanodes provides new insights on the correlations between electronic structure, transport properties, excited state dynamics, and charge transfer phenomena, and expands our knowledge on solar cell materials into correlated electron systems. This research news article presents a snapshot of selected theoretical and experimental developments linking the electronic structure to the photoelectrochemical performance, with particular focus on optoelectronic properties and charge carrier dynamics.

One micron thick Gd2O3 films were grown on GaN/AlGaN heterostructures by reactive electron beam physical vapor deposition. The films were of cubic bixbyite phase with strong (222) out-of-plane and in-plane textures. The films showed a columnar microstructure with feather-like growth. Transmission electron microscopy analysis and selected area diffraction showed highly oriented single crystal like growth near the film interface which degraded as the film thickness increased. Capacitance–voltage (C–V) characteristics show that the Gd2O3 device results in a negative threshold shift of approximately 1.9 V. Hysteresis of 0.9 V was extracted from the C–V curve corresponding to a trapped charge density of 6.9 × 1010 cm− 2. The conduction mechanisms were found to be dominated by Poole–Frenkel conduction between 50 and 100 °C and Schottky emission between 125 and 200 °C. The trap height for Poole–Frenkel conduction was 0.46 eV and the Schottky barrier height was 0.79 eV.

The spin states at the surface of epitaxial thin films of hematite, both undoped and doped with 1% Ti, Sn, or Zn, respectively, were probed with x-ray magnetic linear dichroism (XMLD) spectroscopy. Morin transitions were observed for the undoped $({T}_{M}\ensuremath{\approx}200$ K) and Sn-doped $({T}_{M}\ensuremath{\approx}300$ K) cases, while Zn- and Ti-doped samples were always in the high- and low-temperature phases, respectively. In contrast to what has been reported for bulk hematite doped with the tetravalent ions ${\mathrm{Sn}}^{4+}$ and ${\mathrm{Ti}}^{4+}$, for which ${T}_{M}$ dramatically decreases, these dopants substantially increase ${T}_{M}$ in thin films, far exceeding the bulk values. The normalized Fe ${L}_{II}$-edge dichroism for $Tl{T}_{M}$ does not strongly depend on doping or temperature, except for an apparent increase of the peak amplitudes for $Tl100$ K. We observed magnetic field-induced inversions of the dichroism peaks. By applying a magnetic field of 6.5 T on the Ti-doped sample, a transition into the $Tg{T}_{M}$ state was achieved. The temperature dependence of the critical field for the Sn-doped sample was characterized in detail. It was demonstrated the sample-to-sample variations of the Fe ${L}_{III}$-edge spectra were, for the most part, determined solely by the spin orientation state. Calculations of the polarization-dependent spectra based on a spin-multiplet model were in reasonable agreement with the experiment and showed a mixed excitation character of the peak structures.

The structural evolution of thick polycrystalline gadolinium oxide (Gd 2 O 3 ) films deposited by reactive electron beam-physical vapor deposition (EB-PVD) is investigated. High deposition rates (> 5 A/s) lead to the growth of mixed phase films which are of the cubic phase near the film/substrate interface before forming monoclinic phase as distance from the interface increases. By decreasing the deposition rate to 2 O 3 films.

Ti-doped, undoped, and Zn-doped hematite (α-Fe2O3) thick (∼1 μm) films were found to be n-type, weak n-type, and p-type, respectively. Heterogeneous doping profiles were generated in 30 nm thick hematite stacks on F:SnO2-coated glass substrates with 25 nm thick SnO2 underlayers in order to investigate the effect of different doping profiles on photoelectrochemical performance and compare with homogeneously doped counterpart photoelectrodes. Among the homogeneously doped photoelectrodes, the Ti-doped sample displayed the highest plateau photocurrent but also the highest onset potential, whereas the Zn-doped one had the lowest onset potential and the lowest plateau photocurrent. Heterogeneously doped photoelectrodes displayed both high plateau photocurrent and low onset potential, with the highest performance achieved for the specimen with Ti-doped, undoped, and Zn-doped layers at the bottom, center, and top parts of the stack, respectively. This demonstrates the potential of heterogeneous doping to improve...

Thick polycrystalline gadolinium oxide (Gd 2 O 3 ) films up to 11 μm in thickness were deposited via reactive electron beam-physical vapor deposition (EB-PVD) on silicon (111) substrates for use in neutron radiation detection. The effects of coating thickness, substrate temperature, and oxygen flow on film structural, electrical and optical properties were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), atomic force microscopy (AFM), transmission electron microscopy (TEM), capacitance–voltage (C–V) measurements, and ultraviolet–visible (UV–Vis) spectroscopy. Films were characterized as either monoclinic or mixed monoclinic and cubic phase depending on deposition parameters. Increasing the deposition temperature resulted in increased film crystallinity and cubic phase volume while decreasing the O 2 flow rate resulted in increased volume of the monoclinic phase. Evidence of a thickness dependent crystallography is also presented. Electrical property measurements showed thin film dielectric constant could be tailored between 12 and 20 at 1 MHz frequency by decreasing the oxygen flow rate at deposition temperatures of 250 °C which is attributed to an increased presence of the monoclinic phase and increased film density. Band gap values were calculated from transmission measurements and ranged between 5.44 and 5.96 eV.

Heteroepitaxial multilayer Pt(111)/Fe2O3(0001) films were deposited on sapphirec-plane (0001) substrates by RF magnetron sputtering and pulsed laser deposition, respectively. The films were highly crystalline, displaying an in-plane mosaic spread of less than 1° and a homogenous surface morphology with roughness of ∼3 Å. Ellipsometry and UV-vis spectroscopy measurements were shown to be in excellent agreement with modelling, demonstrating that the optics of the system including absorption in the hematite layer are well described. For polycrystalline hematite photoanodes deposited on platinum, full characterization of the system is hampered by the inability to make measurements in alkaline electrolyte containing hydrogen peroxide (H2O2) due to spontaneous decomposition of H2O2by the exposed platinum. The pin-hole free high quality of the heteroepitaxial films is demonstrated by the ability to make stable and reproducible measurements in H2O2containing electrolyte allowing for accurate extraction of charge separation and injection efficiency. The combination of excellent crystalline quality in addition to the well characterized optics and electrochemical properties of the heteroepitaxial hematite photoanodes demonstrate that Al2O3(0001)/Pt(111)/Fe2O3(0001) is a powerful model system for systematic investigation into solar water splitting photoanodes.

A new method for studying the dynamics of a sessile drop by atomic force microscopy (AFM) is demonstrated. A hydrophobic microsphere (radius, r ∼ 20-30 μm) is brought into contact with a small sessile water drop resting on a polytetrafluoroethylene (PTFE) surface. When the microsphere touches the liquid surface, the meniscus rises onto it because of capillary forces. Although the microsphere volume is 6 orders of magnitude smaller than the drop, it excites the normal resonance modes of the liquid interface. The sphere is pinned at the interface, whose small (100 nm) oscillations are readily measured with AFM. Resonance oscillation frequencies were measured for drop volumes between 5 and 200 μL. The results for the two lowest normal modes are quantitatively consistent with continuum calculations for the natural frequency of hemispherical drops with no adjustable parameters. The method may enable sensitive measurements of volume, surface tension, and viscosity of small drops.

The effects of growth temperature, film thickness, and oxygen flux on the microstructure, phase transition, and interfacial chemistry of gadolinium oxide (Gd2O3) films grown on Si(111) substrates by electron-beam physical vapor deposition were investigated using a combination of transmission electron microscopy (TEM), electron diffraction, scanning TEM, x-ray energy dispersive spectrometry, and electron energy loss spectrometry. The authors find that a low growth temperature (250 °C) and a high oxygen flux (200 sccm) led to a small grain size and a high porosity of the Gd2O3 film. Lowering the oxygen flux to 50 sccm led to reduced film porosity, presumably due to the increased diffusion length of the Gd atoms on the surface. Increasing the growth temperature to 650 °C resulted in a film with large columnar grains and elongated pores at the grain boundaries. Thin films grown at 250 °C consisted of cubic Gd2O3, but thermodynamically less stable monoclinic phase formed as the film thickness increased. Loweri...

We report room temperature long-distance spin transport of magnons in antiferromagnetic thin film hematite doped with Zn. The additional dopants significantly alter the magnetic anisotropies, resulting in a complex equilibrium spin structure that is capable of efficiently transporting spin angular momentum at room temperature without the need for a well-defined, pure easy-axis or easy-plane anisotropy. We find intrinsic magnon spin-diffusion lengths of up to 1.5 {\mu}m, and magnetic domain governed decay lengths of 175 nm for the low frequency magnons, through electrical transport measurements demonstrating that the introduction of non-magnetic dopants does not strongly reduce the transport length scale showing that the magnetic damping of hematite is not significantly increased. We observe a complex field dependence of the non-local signal independent of the magnetic state visible in the local magnetoresistance and direct magnetic imaging of the antiferromagnetic domain structure. We explain our results in terms of a varying and applied-field-dependent ellipticity of the magnon modes reaching the detector electrode allowing us to tune the spin transport.

The Dzyaloshinskii-Moriya interaction (DMI) is at the heart of many modern developments in the research field of spintronics. DMI is known to generate noncollinear magnetic textures, and can take two forms in antiferromagnets: homogeneous or inter-sublattice, leading to small, canted moments and inhomogeneous or intra-sublattice, leading to formation of chiral structures. In this work, we first determine the strength of the effective field created by the DMI, using SQUID based magnetometry and transport measurements, in thin films of the antiferromagnetic iron oxide hematite, $\alpha$-Fe$_2$O$_3$. We demonstrate that DMI additionally introduces reconfigurability in the long distance magnon transport in these films under different orientations of a magnetic field. This arises as a hysteresis centred around the easy-axis direction for an external field rotated in opposing directions whose width decreases with increasing magnetic field as the Zeeman energy competes with the effective field created by the DMI.