UV-Driven Oxygen Surface Exchange and Stoichiometry Changes in a Thin-Film, Nondilute Mixed Ionic Electronic Conductor, Sr(Ti,Fe)O3-d.

Enabling light-controlled ionic devices requires insight into photoionic responses in technologically relevant materials. Mixed-conducting perovskites containing nondilute Fe─serving as electrodes, catalysts, and sensors─can support large, electronically accommodated excursions in oxygen content, typically controlled by temperature, bias, and gas atmosphere. Instead, we investigated the ability of low-fluence, above-bandgap illumination to adjust oxygen stoichiometry and drive oxygen fluxes in nondilute Sr(Ti1-xFex)O3-x/2+δ (x = 0.07, 0.35) thin films with high baseline hole concentrations. Films' optical transmission at 2.8 eV was used as a probe of oxygen stoichiometry in the range ∼100-500 °C. We compared pO2-step-driven and UV (3.4 eV)-step-driven visible optical transmission relaxations in films, finding that the time constants and activation energies of the relaxations were consistent with each other and thus with oxygen-surface-exchange-limited kinetics. Blocking oxygen exchange at the solid-gas interface with a UV-transparent capping layer resulted in no UV-induced optical relaxations. These results demonstrate that above-bandgap illumination can increase oxygen content in nondilute compositions through oxygen flux into the solid from the gas. First-principles simulations of defect formation enthalpies indicate that oxygen vacancies are energetically less favorable under steady-state illumination owing to shifts in quasi-Fermi levels. A larger 2.8 eV-optical response to UV illumination in x = 0.07 vs x = 0.35 samples was further investigated through ultrafast transient spectroscopy, where it was found that the x = 0.07 sample exhibits a slower carrier recombination. Together, these results suggest potential design principles for materials supporting large stoichiometry changes under above-gap illumination: (1) long excited carrier lifetimes and (2) highly charged, rather than neutral, defects/associates.

Formation and impact of nanoscopic oriented phase domains in electrochemical crystalline electrodes.

Electrochemical phase transformation in ion-insertion crystalline electrodes is accompanied by compositional and structural changes, including the microstructural development of oriented phase domains. Previous studies have identified prevailingly transformation heterogeneities associated with diffusion- or reaction-limited mechanisms. In comparison, transformation-induced domains and their microstructure resulting from the loss of symmetry elements remain unexplored, despite their general importance in alloys and ceramics. Here, we map the formation of oriented phase domains and the development of strain gradient quantitatively during the electrochemical ion-insertion process. A collocated four-dimensional scanning transmission electron microscopy and electron energy loss spectroscopy approach, coupled with data mining, enables the study. Results show that in our model system of cubic spinel MnO2 nanoparticles their phase transformation upon Mg2+ insertion leads to the formation of domains of similar chemical identity but different orientations at nanometre length scale, following the nucleation, growth and coalescence process. Electrolytes have a substantial impact on the transformation microstructure ('island' versus 'archipelago'). Further, large strain gradients build up from the development of phase domains across their boundaries with high impact on the chemical diffusion coefficient by a factor of ten or more. Our findings thus provide critical insights into the microstructure formation mechanism and its impact on the ion-insertion process, suggesting new rules of transformation structure control for energy storage materials.

Revealing the role of the cathode-electrolyte interface on solid-state batteries.

Interfaces have crucial, but still poorly understood, roles in the performance of secondary solid-state batteries. Here, using crystallographically oriented and highly faceted thick cathodes, we directly assess the impact of cathode crystallography and morphology on the long-term performance of solid-state batteries. The controlled interface crystallography, area and microstructure of these cathodes enables an understanding of interface instabilities unknown (hidden) in conventional thin-film and composite solid-state electrodes. A generic and direct correlation between cell performance and interface stability is revealed for a variety of both lithium- and sodium-based cathodes and solid electrolytes. Our findings highlight that minimizing interfacial area, rather than its expansion as is the case in conventional composite cathodes, is key to both understanding the nature of interface instabilities and improving cell performance. Our findings also point to the use of dense and thick cathodes as a way of increasing the energy density and stability of solid-state batteries.

Tuning p-Si(111) Photovoltage via Molecule|Semiconductor Electronic Coupling.

Photoelectrochemical (PEC) device efficiency depends heavily on the energetics and band alignment of the semiconductor|overlayer junction. Exerting energetic control over these junctions via molecular functionalization is an extremely attractive strategy. Herein we report a study of the structure-function relationship between chemically functionalized pSi(111) and the resulting solar fuels performance. Specifically, we highlight the interplay of chemical structure and electronic coupling between the attached molecule and the underlying semiconductor. Covalent attachment of aryl surface modifiers (phenyl, Ph; nitrophenyl, PhNO2; anthracene, Anth; and nitroanthracene, AnthNO2) resulted in high-fidelity surfaces with low defect densities (S < 50 cm/s). Electrochemical characterization of these surfaces in contact with methyl viologen resulted in systematically shifted band edges (up to 0.99 V barrier height) and correspondingly high photoelectrochemical performance (Voc up to 0.43 V vs MV2+) consistent with the introduction of a positive interfacial dipole. We extend this functionalization to HER conditions and demonstrate systematic tuning of the HER Voc using pSi(111)-R|TiO2|Pt architecture. Correlation of the shifts in barrier height with the photovoltage provides evidence for nonideality despite low surface recombination. Critically, DFT calculations of the electronic structure of the organic-functionalized interfaces show that the molecule-based electronic states effectively hybridized with the silicon band edges. A comparison of these interfacial states with their isolated molecular analogues further confirms electronic coupling between the attached molecule and the underlying semiconductor, providing an induced density of interfacial states (IDIS) which decreases the potential drop across the semiconductor. These results demonstrate the delicate interplay between interfacial chemical structure, interfacial dipole, and electronic structure.

Ultrasoft slip-mediated bending in few-layer graphene.

Continuum scaling laws often break down when materials approach atomic length scales, reflecting changes in their underlying physics and the opportunities to access unconventional properties. These continuum limits are evident in two-dimensional materials, where there is no consensus on their bending stiffnesses or how they scale with thickness. Through combined computational and electron microscopy experiments, we measure the bending stiffness of graphene, obtaining 1.2-1.7 eV for a monolayer. Moreover, we find that the bending stiffness of few-layer graphene decreases sharply as a function of bending angle, tuning by almost 400% for trilayer graphene. This softening results from shear, slip and the onset of superlubricity between the atomic layers and corresponds with a gradual change in scaling power from cubic to linear. Our results provide a unified model for bending in two-dimensional materials and show that their multilayers can be orders of magnitude softer than previously thought, among the most flexible electronic materials currently known.

Author Correction: Ultrasoft slip-mediated bending in few-layer graphene.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

Mechanism of creation and destruction of oxygen interstitial atoms by nonpolar zinc oxide(101[combining macron]0) surfaces.

Oxygen vacancies (VO) influence many properties of ZnO in semiconductor devices, yet synthesis methods leave behind variable and unpredictable VO concentrations. Oxygen interstitials (Oi) move far more rapidly, so post-synthesis introduction of Oi to control the VO concentration would be desirable. Free surfaces offer such an introduction mechanism if they are free of poisoning foreign adsorbates. Here, isotopic exchange experiments between nonpolar ZnO(101[combining macron]0) and O2 gas, together with mesoscale modeling and first-principles calculations, point to an activation barrier for injection only 0.1-0.2 eV higher than for bulk site hopping. The modest barrier for hopping in turn enables diffusion lengths of tens to hundreds of nanometers only slightly above room temperature, which should facilitate defect engineering under very modest conditions. In addition, low hopping barriers coupled with statistical considerations lead to important qualitative manifestations in diffusion via an interstitialcy mechanism that does not occur for vacancies.

Stochastic Stress Jumps Due to Soliton Dynamics in Two-Dimensional van der Waals Interfaces.

The creation and movement of dislocations determine the nonlinear mechanics of materials. At the nanoscale, the number of dislocations in structures become countable, and even single defects impact material properties. While the impact of solitons on electronic properties is well studied, the impact of solitons on mechanics is less understood. In this study, we construct nanoelectromechanical drumhead resonators from Bernal stacked bilayer graphene and observe stochastic jumps in frequency. Similar frequency jumps occur in few-layer but not twisted bilayer or monolayer graphene. Using atomistic simulations, we show that the measured shifts are a result of changes in stress due to the creation and annihilation of individual solitons. We develop a simple model relating the magnitude of the stress induced by soliton dynamics across length scales, ranging from <0.01 N/m for the measured 5 µm diameter to ∼1.2 N/m for the 38.7 nm simulations. These results demonstrate the sensitivity of 2D resonators are sufficient to probe the nonlinear mechanics of single dislocations in an atomic membrane and provide a model to understand the interfacial mechanics of different kinds of van der Waals structures under stress, which is important to many emerging applications such as engineering quantum states through electromechanical manipulation and mechanical devices like highly tunable nanoelectromechanical systems, stretchable electronics, and origami nanomachines.

Kinetic Control of Oxygen Interstitial Interaction with TiO2(110) via the Surface Fermi Energy.

Atomically clean surfaces of semiconducting oxides efficiently mediate the interconversion of gas-phase O2 and solid-phase oxygen interstitial atoms (Oi). First-principles calculations together with mesoscale microkinetic modeling are employed for TiO2(110) to determine reaction pathways, assess appropriate rate expressions, and obtain corresponding activation energies and pre-exponential factors. The Fermi energy (EF) at the surface influences the rate-determining step for both injection and annihilation of Oi. The barriers range between 0.72-0.82 eV for injection and 0.60-2.34 eV for annihilation and may be manipulated through intentional control of EF. At equilibrium, the microkinetic model and first-principles calculations indicate that interconversion of Oi species in the first and second sublayers limits the rate. The effective pre-exponential factors for injection and annihilation are surprisingly low, probably resulting from the use of simple Langmuir-like rate expressions to describe a complicated kinetic sequence.

Fermi level dependence of gas-solid oxygen defect exchange mechanism on TiO2 (110) by first-principles calculations.

In the same way that gases interact with oxide semiconductor surfaces from above, point defects interact from below. Previous experiments have described defect-surface reactions for TiO2(110), but an atomistic picture of the mechanism remains unknown. The present work employs computations by density functional theory of the thermodynamic stabilities of metastable states to elucidate possible reaction pathways for oxygen interstitial atoms at TiO2(110). The simulations uncover unexpected metastable states including dumbbell and split configurations in the surface plane that resemble analogous interstitial species in the deep bulk. Comparison of the energy landscapes involving neutral (unionized) and charged intermediates shows that the Fermi energy EF exerts a strong influence on the identity of the most likely pathway. The largest elementary-step thermodynamic barrier for interstitial injection trends mostly downward by 2.1 eV as EF increases between the valence and conduction band edges, while that for annihilation trends upward by 2.1 eV. Several charged intermediates become stabilized for most values of EF upon receiving conduction band electrons from TiO2, and the behavior of these species governs much of the overall energy landscape.

Asynchronous Photoexcited Electronic and Structural Relaxation in Lead-Free Perovskites.

Vacancy-ordered lead-free perovskites with more-stable crystalline structures have been intensively explored as the alternatives for resolving the toxic and long-term stability issues of lead halide perovskites (LHPs). The dispersive energy bands produced by the closely packed halide octahedral sublattice in these perovskites are meanwhile anticipated to facility the mobility of charge carriers. However, these perovskites suffer from unexpectedly poor charge carrier transport. To tackle this issue, we have employed the ultrafast, elemental-specific X-ray transient absorption (XTA) spectroscopy to directly probe the photoexcited electronic and structural dynamics of a prototypical vacancy-ordered lead-free perovskite (Cs3Bi2Br9). We have discovered that the photogenerated holes quickly self-trapped at Br centers, simultaneously distorting the local lattice structure, likely forming small polarons in the configuration of Vk center (Br2- dimer). More significantly, we have found a surprisingly long-lived, structural distorted state with a lifetime of ∼59 µs, which is ∼3 orders of magnitude slower than that of the charge carrier recombination. Such long-lived structural distortion may produce a transient "background" under continuous light illumination, influencing the charge carrier transport along the lattice framework.

Multiscale Computational Design of Functionalized Photocathodes for H2 Generation.

We present an integrated computational approach combining first-principles density functional theory (DFT) calculations with wxAMPS, a solid-state drift/diffusion device modeling software, to design functionalized photocathodes for high-efficiency H2 generation. As a case study, we have analyzed the performance of p-type Si(111) photocathodes functionalized with a set of 20 mixed aryl/methyl monolayers, which have a known synthetic route for attachment to Si(111). DFT is used to screen for high-performing monolayers by calculating the surface dipole induced by the functionalization. The trend in the calculated surface dipoles was validated using previously published experimental measurements. We find that the molecular dipole moment is a descriptor of the surface dipole. wxAMPS is used to predict the open-circuit voltage (efficiency) of the photocathode by calculating the photocurrent versus voltage behavior using the DFT surface dipole calculations as inputs to the simulation. We find that Voc saturates beyond a surface dipole of ∼0.3 eV, suggesting an upper limit for achievable device performance. This computational approach provides a possibility for the rational design of functionalized photocathodes for enhanced H2 generation by combining the angstrom-scale results obtained using DFT with the micron-to-nanometer scale capabilities of wxAMPS.

Computational Analysis of the Interplay between Deep Level Traps and Perovskite Solar Cell Efficiency.

New deposition methods of halide perovskites are being developed with the aim of improving solar cell power conversion efficiency by controlling the physiochemical properties of the perovskite film. In the case of methylammonium lead iodide (MAPbI3), deep level traps limit efficiency by participating in charge carrier recombination. Prior work has shown that the solar cell efficiency of MAPbI3 solar cells varied significantly with deposition method; specifically, efficiencies of 13.5 and 17.7% were observed for MAPbI3 processed with a one- and two-step method, respectively. However, the origin of the difference in efficiency remains unclear. In this study, we analyze the interplay between deep level traps and efficiency by simulating the photoexcited charge carrier pathway across solar cells processed via the one- and two-step method using finite-element drift-diffusion modeling. We determined that in the case of one-step processing, the traps propagate throughout the bulk, while for two-step, the traps congregate at the interface where the MAPbI3 was grown (mesoporous TiO2). Composition and structural analysis are used to propose a plausible explanation as to why the difference in processing changes the spatial distribution of the traps.

Identifying Charge Transfer Mechanisms across Semiconductor Heterostructures via Surface Dipole Modulation and Multiscale Modeling.

The design and fabrication of stable and efficient photoelectrochemical devices requires the use of multifunctional structures with complex heterojunctions composed of semiconducting, protecting, and catalytic layers. Understanding charge transport across such devices is challenging due to the interplay of bulk and interfacial properties. In this work, we analyze hole transfer across n-Si(111)- R|TiO2 photoanodes where - R is a series of mixed aryl/methyl monolayers containing an increasing number of methoxy units (mono, di, and tri). In the dimethoxy case, triethylene glycol units were also appended to substantially enhance the dipolar character of the surface. We find that hole transport is limited at the n-Si(111)- R|TiO2 interface and occurs by two processes- thermionic emission and/or intraband tunneling-where the interplay between them is regulated by the interfacial molecular dipole. This was determined by characterizing the photoanode experimentally (X-ray photoelectron spectroscopy, voltammetry, impedance) with increasingly thick TiO2 films and complementing the characterization with a multiscale computational approach (first-principles density functional theory (DFT) and finite-element device modeling). The tested theoretical model that successfully distinguished thermionic emission and intraband tunneling was then used to predict the effect of solution potential on charge transport. This prediction was then experimentally validated using a series of nonaqueous redox couples (ferrocence derivatives spanning 800 mV). As a result, this work provides a fundamental understanding of charge transport across TiO2-protected electrodes, a widely used semiconductor passivation scheme, and demonstrates the predictive capability of the combined DFT/device-modeling approach.

Design Strategy for the Molecular Functionalization of Semiconductor Photoelectrodes: A Case Study of p-Si(111) Photocathodes for H2 Generation.

Functionalization of semiconductor photoelectrodes is actively pursued as an approach to improve the efficiency of photoelectrochemical reactions by modulating the semiconductor's barrier height, but the selection of molecules for functionalization remains largely empirical. We propose a simple but effective design strategy for the organic functionalization of photocathodes for high-efficiency hydrogen generation based on first-principles density functional theory (DFT) calculations. The surface dipole of the functionalized photocathode determines its barrier height, which can be optimized to enhance charge separation at the semiconductor-electrolyte interface. Focusing on p-Si(111) photocathodes functionalized with different mixed aryl/methyl monolayers, we use DFT to systematically investigate the effect of - the presence and distribution of pi bonds, binding atom (the atom in the functional group that bonds with the Si surface), functional group length, and electrophilic substituent groups - on the surface dipole and charge rearrangement at the functionalized surface. We find that the most important factors affecting the surface dipole are the intrinsic molecular dipole moment of the organic moiety, the presence of electrophilic substituent groups, and the binding atom. Using these findings, a three-step design strategy is proposed for the experimental realization of high-performing functionalized p-Si(111) photocathodes by maximizing the surface dipole.

First-principles description of oxygen self-diffusion in rutile TiO2: assessment of uncertainties due to enthalpy and entropy contributions.

Properties related to transport such as self-diffusion coefficients are relevant to fuel cells, electrolysis cells, and chemical/gas sensors. Prediction of self-diffusion coefficients from first-principles involves precise determination of both enthalpy and entropy contributions for point defect formation and migration. We use first-principles density functional theory to estimate the self-diffusion coefficient for neutral O0i and doubly ionized Oi2- interstitial oxygen in rutile TiO2 and compare the results to prior isotope diffusion experiments. In addition to formation and migration energy, detailed estimates of formation and migration entropy incorporating both vibrational and ionization components are included. Distinct migration pathways, both based on an interstitialcy mechanism, are identified for O0i and Oi2-. These result in self-diffusion coefficients that differ by several orders of magnitude, sufficient to resolve the charge state of the diffusing species to be Oi2- in experiment. The main sources of error when comparing computed parameters to those obtained from experiment are considered, demonstrating that uncertainties due to computed defect formation and migration entropies are comparable in magnitude to those due to computed defect formation and migration energies. Even so, the composite uncertainty seems to limit the accuracy of first-principles calculations to within a factor of ±103, demonstrating that direct connections between computation and experiment are now increasingly possible.

A Cocatalyst that Stabilizes a Hydride Intermediate during Photocatalytic Hydrogen Evolution over a Rhodium-Doped TiO2 Nanosheet.

The hydrogen evolution reaction using semiconductor photocatalysts has been significantly improved by cocatalyst loading. However, there are still many speculations regarding the actual role of the cocatalyst. Now a photocatalytic hydrogen evolution reaction pathway is reported on a cocatalyst site using TiO2 nanosheets doped with Rh at Ti sites as one-atom cocatalysts. A hydride species adsorbed on the one-atom Rh dopant cocatalyst site was confirmed experimentally as the intermediate state for hydrogen evolution, which was consistent with the results of density functional theory (DFT) calculations. In this system, the role of the cocatalyst in photocatalytic hydrogen evolution is related to the withdrawal of photo-excited electrons and stabilization of the hydride intermediate species; the presence of oxygen vacancies induced by Rh facilitate the withdrawal of electrons and stabilization of the hydride.

Asymmetric response of ferroelectric/metal oxide heterojunctions for catalysis arising from interfacial chemistry.

Recently there has been interest in the use of ferroelectricity to modify a material's surface chemistry and enhance its catalytic properties. When a metal oxide catalyst is supported by a switchable ferroelectric underlayer, modifications to the free surface electronic structure can induce changes to the free energy profile of a gas-surface catalytic reaction that either promote or suppress the reaction. The modification of surface properties results from a combination of interface chemistry, surface reconstructions involving adsorbates, and complex interactions between the two, although these interactions are often not characterized in detail. Using the oxygen evolution reaction (OER) on barium titanate/anatase (BTO/TiO2) heterostructures as a case study, we use density functional theory to determine how the OER Gibbs free energy profile depends on the polarization of the ferroelectric and the number of TiO2 monolayers. For positive polarizations, the profile is found to be sensitive to the number of TiO2 monolayers, shows extended finite size effects, and deviates substantially from that of unsupported anatase. For negative polarizations, the monolayer dependence is suppressed and the OER profiles remain similar to that of unsupported anatase independent of the number of TiO2 monolayers. To understand the origin of the differences, we analyze in detail the layer-by-layer rumpling, interface chemistry, surface reconstructions, and interface and surface dipoles. The unbalanced response to negative and positive polarizations is shown to arise from the underlying chemistry of the BTO/TiO2 interface. While the interfacial bonding is largely fixed for negatively polarized systems, it is variable and can be tuned by the presence of nearby surface adsorbates for positively polarized systems. The asymmetry limits the effectiveness of the heterojunction. While positively polarized systems are good candidates for the selective enhancement of catalytic reactions involving negatively charged species at the metal oxide free surface, the negatively polarized systems do not effectively enhance catalytic reactions involving positively charged species. Our work points to interface design rules to overcome this limitation and realize heterojunctions with enhanced catalytic functionality.

Computational insights into charge transfer across functionalized semiconductor surfaces.

Photoelectrochemical water-splitting is a promising carbon-free fuel production method for producing H2 and O2 gas from liquid water. These cells are typically composed of at least one semiconductor photoelectrode which is prone to degradation and/or oxidation. Various surface modifications are known for stabilizing semiconductor photoelectrodes, yet stabilization techniques are often accompanied by a decrease in photoelectrode performance. However, the impact of surface modification on charge transport and its consequence on performance is still lacking, creating a roadblock for further improvements. In this review, we discuss how density functional theory and finite-element device simulations are reliable tools for providing insight into charge transport across modified photoelectrodes.

Phonons, Localization, and Thermal Conductivity of Diamond Nanothreads and Amorphous Graphene.

Recently, the domains of low-dimensional (low-D) materials and disordered materials have been brought together by the demonstration of several new low-D, disordered systems. The thermal transport properties of these systems are not well-understood. Using amorphous graphene and glassy diamond nanothreads as prototype systems, we establish how structural disorder affects vibrational energy transport in low-dimensional, but disordered, materials. Modal localization analysis, molecular dynamics simulations, and a generalized model together demonstrate that the thermal transport properties of these materials exhibit both similarities and differences from disordered 3D materials. In analogy with 3D, the low-D disordered systems exhibit both propagating and diffusive vibrational modes. In contrast to 3D, however, the diffuson contribution to thermal transport in these low-D systems is shown to be negligible, which may be a result of inherent differences in the nature of random walks in lower dimensions. Despite the lack of diffusons, the suppression of thermal conductivity due to disorder in low-D systems is shown to be mild or comparable to 3D. The mild suppression originates from the presence of low-frequency vibrational modes that maintain a well-defined polarization and help preserve the thermal conductivity in the presence of disorder.