Defect Engineering of WO3 by Rapid Flame Reduction for Efficient Photoelectrochemical Conversion of Methane into Liquid Oxygenates.

Photoelectrochemical (PEC) conversion is a promising way to use methane (CH4) as a chemical building block without harsh conditions. However, the PEC conversion of CH4 to value-added chemicals remains challenging due to the thermodynamically favorable overoxidation of CH4. Here, we report WO3 nanotube (NT) photoelectrocatalysts for PEC CH4 conversion with high liquid product selectivity through defect engineering. By tuning the flame reduction treatment, we carefully controlled the oxygen vacancies of WO3 NTs. The optimally reduced WO3 NTs suppressed overoxidation of CH4 showing a high total C1 liquid selectivity of 69.4% and a production rate of 0.174 µmol cm-2 h-1. Scanning electrochemical microscopy revealed that oxygen vacancies can restrain the production of hydroxyl radicals, which, in excess, could further oxidize C1 intermediates to CO2. Additionally, band diagram analysis and computational studies elucidated that oxygen vacancies thermodynamically suppress overoxidation. This work introduces a strategy for understanding and controlling the selectivity of photoelectrocatalysts for direct conversion of CH4 to liquids.

Versailles Project on Advanced Materials and Standards interlaboratory study on intensity calibration for x-ray photoelectron spectroscopy instruments using low-density polyethylene.

We report the results of a Versailles Project on Advanced Materials and Standards interlaboratory study on the intensity scale calibration of x-ray photoelectron spectrometers using low-density polyethylene (LDPE) as an alternative material to gold, silver, and copper. An improved set of LDPE reference spectra, corrected for different instrument geometries using a quartz-monochromated Al Kα x-ray source, was developed using data provided by participants in this study. Using these new reference spectra, a transmission function was calculated for each dataset that participants provided. When compared to a similar calibration procedure using the NPL reference spectra for gold, the LDPE intensity calibration method achieves an absolute offset of ∼3.0% and a systematic deviation of ±6.5% on average across all participants. For spectra recorded at high pass energies (≥90 eV), values of absolute offset and systematic deviation are ∼5.8% and ±5.7%, respectively, whereas for spectra collected at lower pass energies (<90 eV), values of absolute offset and systematic deviation are ∼4.9% and ±8.8%, respectively; low pass energy spectra perform worse than the global average, in terms of systematic deviations, due to diminished count rates and signal-to-noise ratio. Differences in absolute offset are attributed to the surface roughness of the LDPE induced by sample preparation. We further assess the usability of LDPE as a secondary reference material and comment on its performance in the presence of issues such as variable dark noise, x-ray warm up times, inaccuracy at low count rates, and underlying spectrometer problems. In response to participant feedback and the results of the study, we provide an updated LDPE intensity calibration protocol to address the issues highlighted in the interlaboratory study. We also comment on the lack of implementation of a consistent and traceable intensity calibration method across the community of x-ray photoelectron spectroscopy (XPS) users and, therefore, propose a route to achieving this with the assistance of instrument manufacturers, metrology laboratories, and experts leading to an international standard for XPS intensity scale calibration.

Evolutionary selection growth of two-dimensional materials on polycrystalline substrates.

There is a demand for the manufacture of two-dimensional (2D) materials with high-quality single crystals of large size. Usually, epitaxial growth is considered the method of choice 1 in preparing single-crystalline thin films, but it requires single-crystal substrates for deposition. Here we present a different approach and report the synthesis of single-crystal-like monolayer graphene films on polycrystalline substrates. The technological realization of the proposed method resembles the Czochralski process and is based on the evolutionary selection 2 approach, which is now realized in 2D geometry. The method relies on 'self-selection' of the fastest-growing domain orientation, which eventually overwhelms the slower-growing domains and yields a single-crystal continuous 2D film. Here we have used it to synthesize foot-long graphene films at rates up to 2.5 cm h-1 that possess the quality of a single crystal. We anticipate that the proposed approach could be readily adopted for the synthesis of other 2D materials and heterostructures.

Detection of the Spin-Chemical Potential in Topological Insulators Using Spin-Polarized Four-Probe STM.

We demonstrate a new method for the detection of the spin-chemical potential in topological insulators using spin-polarized four-probe scanning tunneling microscopy on in situ cleaved Bi_{2}Te_{2}Se surfaces. Two-dimensional (2D) surface and 3D bulk conductions are separated quantitatively via variable probe-spacing measurements, enabling the isolation of the nonvanishing spin-dependent electrochemical potential from the Ohmic contribution. This component is identified as the spin-chemical potential arising from the 2D charge current through the spin momentum locked topological surface states (TSS). This method provides a direct measurement of spin current generation efficiency and opens a new avenue to access the intrinsic spin transport associated with pristine TSS.

Formation, Migration, and Reactivity of Au-CO Complexes on Gold Surfaces.

We report experimental as well as theoretical evidence that suggests Au-CO complex formation upon the exposure of CO to active sites (step edges and threading dislocations) on a Au(111) surface. Room-temperature scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy, transmission infrared spectroscopy, and density functional theory calculations point to Au-CO complex formation and migration. Room-temperature STM of the Au(111) surface at CO pressures in the range from 10(-8) to 10(-4) Torr (dosage up to 10(6) langmuir) indicates Au atom extraction from dislocation sites of the herringbone reconstruction, mobile Au-CO complex formation and diffusion, and Au adatom cluster formation on both elbows and step edges on the Au surface. The formation and mobility of the Au-CO complex result from the reduced Au-Au bonding at elbows and step edges leading to stronger Au-CO bonding and to the formation of a more positively charged CO (CO(δ+)) on Au. Our studies indicate that the mobile Au-CO complex is involved in the Au nanoparticle formation and reactivity, and that the positive charge on CO increases due to the stronger adsorption of CO at Au sites with lower coordination numbers.

Dimensionality Controlled Octahedral Symmetry-Mismatch and Functionalities in Epitaxial LaCoO3/SrTiO3 Heterostructures.

Epitaxial strain provides a powerful approach to manipulate physical properties of materials through rigid compression or extension of their chemical bonds via lattice-mismatch. Although symmetry-mismatch can lead to new physics by stabilizing novel interfacial structures, challenges in obtaining atomic-level structural information as well as lack of a suitable approach to separate it from the parasitical lattice-mismatch have limited the development of this field. Here, we present unambiguous experimental evidence that the symmetry-mismatch can be strongly controlled by dimensionality and significantly impact the collective electronic and magnetic functionalities in ultrathin perovskite LaCoO3/SrTiO3 heterojunctions. State-of-art diffraction and microscopy reveal that symmetry breaking dramatically modifies the interfacial structure of CoO6 octahedral building-blocks, resulting in expanded octahedron volume, reduced covalent screening, and stronger electron correlations. Such phenomena fundamentally alter the electronic and magnetic behaviors of LaCoO3 thin-films. We conclude that for epitaxial systems, correlation strength can be tuned by changing orbital hybridization, thus affecting the Coulomb repulsion, U, instead of by changing the band structure as the common paradigm in bulks. These results clarify the origin of magnetic ordering for epitaxial LaCoO3 and provide a route to manipulate electron correlation and magnetic functionality by orbital engineering at oxide heterojunctions.

The Ehrlich-Schwoebel barrier on an oxide surface: a combined Monte-Carlo and in situ scanning tunneling microscopy approach.

The controlled growth of epitaxial films of complex oxides requires an atomistic understanding of key parameters determining final film morphology, such as termination dependence on adatom diffusion, and height of the Ehrlich-Schwoebel (ES) barrier. Here, through an in situ scanning tunneling microscopy study of mixed-terminated La5/8Ca3/8MnO3 (LCMO) films, we image adatoms and observe pile-up at island edges. Image analysis allows determination of the population of adatoms at the edge of islands and fractions on A-site and B-site terminations. A simple Monte-Carlo model, simulating the random walk of adatoms on a sinusoidal potential landscape using Boltzmann statistics is used to reproduce the experimental data, and provides an estimate of the ES barrier as ∼0.18 ± 0.04 eV at T = 1023 K, similar to those of metal adatoms on metallic surfaces. These studies highlight the utility of in situ imaging, in combination with basic Monte-Carlo methods, in elucidating the factors which control the final film growth in complex oxides.

Water-mediated electrochemical nano-writing on thin ceria films.

Bias dependent mechanisms of irreversible cathodic and anodic processes on a pure CeO2 film are studied using modified atomic force microscopy (AFM). For a moderate positive bias applied to the AFM tip an irreversible electrochemical reduction reaction is found, associated with significant local volume expansion. By changing the experimental conditions we are able to deduce the possible role of water in this process. Simultaneous detection of tip height and current allows the onset of conductivity and the electrochemical charge transfer process to be separated, further elucidating the reaction mechanism. The standard anodic/cathodic behavior is recovered in the high bias regime, where a sizable transport current flows between the tip and the film. These studies give insight into the mechanisms of the tip-induced electrochemical reactions as mediated by electronic currents, and into the role of water in these processes, as well as providing a different approach for electrochemical nano-writing.

Local crystallography analysis for atomically resolved scanning tunneling microscopy images.

Scanning probe microscopy has emerged as a powerful and flexible tool for atomically resolved imaging of surface structures. However, due to the amount of information extracted, in many cases the interpretation of such data is limited to being qualitative and semi-quantitative in nature. At the same time, much can be learned from local atom parameters, such as distances and angles, that can be analyzed and interpreted as variations of local chemical bonding, or order parameter fields. Here, we demonstrate an iterative algorithm for indexing and determining atomic positions that allows the analysis of inhomogeneous surfaces. This approach is further illustrated by local crystallographic analysis of several real surfaces, including highly ordered pyrolytic graphite and an Fe-based superconductor FeTe0.55Se0.45. This study provides a new pathway to extract and quantify local properties for scanning probe microscopy images.

Tunable metallic conductance in ferroelectric nanodomains.

Metallic conductance in charged ferroelectric domain walls was predicted more than 40 years ago as the first example of an electronically active homointerface in a nonconductive material. Despite decades of research on oxide interfaces and ferroic systems, the metal-insulator transition induced solely by polarization charges without any additional chemical modification has consistently eluded the experimental realm. Here we show that a localized insulator-metal transition can be repeatedly induced within an insulating ferroelectric lead-zirconate titanate, merely by switching its polarization at the nanoscale. This surprising effect is traced to tilted boundaries of ferroelectric nanodomains, that act as localized homointerfaces within the perovskite lattice, with inherently tunable carrier density. Metallic conductance is unique to nanodomains, while the conductivity of extended domain walls and domain surfaces is thermally activated. Foreseeing future applications, we demonstrate that a continuum of nonvolatile metallic states across decades of conductance can be encoded in the size of ferroelectric nanodomains using electric field.

Correlating electronic transport to atomic structures in self-assembled quantum wires.

Quantum wires, as a smallest electronic conductor, are expected to be a fundamental component in all quantum architectures. The electronic conductance in quantum wires, however, is often dictated by structural instabilities and electron localization at the atomic scale. Here we report on the evolutions of electronic transport as a function of temperature and interwire coupling as the quantum wires of GdSi(2) are self-assembled on Si(100) wire-by-wire. The correlation between structure, electronic properties, and electronic transport are examined by combining nanotransport measurements, scanning tunneling microscopy, and density functional theory calculations. A metal-insulator transition is revealed in isolated nanowires, while a robust metallic state is obtained in wire bundles at low temperature. The atomic defects lead to electron localizations in isolated nanowire, and interwire coupling stabilizes the structure and promotes the metallic states in wire bundles. This illustrates how the conductance nature of a one-dimensional system can be dramatically modified by the environmental change on the atomic scale.

Identifying the secondary electron cutoff in ultraviolet photoemission spectra for work function measurements of non-ideal surfaces.

Absolute values of work functions can be determined in ultraviolet photoemission spectroscopy (UPS) by measuring the minimum kinetic energy of secondary electrons generated by a known photon energy. However, some samples can produce spectra that are difficult to interpret due to additional intensity below the true secondary electron cutoff. Disordered absorbates on elemental metals add small intensity below the onset for the transition metal surfaces studied, which can be attributed to energy losses after photoelectrons are generated. In contrast, spectra from WO3-x films can produce multiple onsets with comparable intensity which do not fit this model. False onsets (in the context of work function measurements) can be minimized by optimizing experimental detection parameters including limiting analyzer acceptance angles and pass energy. True work functions can be identified by examining the onsets as the sample bias is varied.

Dynamic conductivity of ferroelectric domain walls in BiFeO3.

Topological walls separating domains of continuous polarization, magnetization, and strain in ferroic materials hold promise of novel electronic properties, that are intrinsically localized on the nanoscale and that can be patterned on demand without change of material volume or elemental composition. We have revealed that ferroelectric domain walls in multiferroic BiFeO(3) are inherently dynamic electronic conductors, closely mimicking memristive behavior and contrary to the usual assumption of rigid conductivity. Applied electric field can cause a localized transition between insulating and conducting domain walls, tune domain wall conductance by over an order of magnitude, and create a quasicontinuous spectrum of metastable conductance states. Our measurements identified that subtle and microscopically reversible distortion of the polarization structure at the domain wall is at the origin of the dynamic conductivity. The latter is therefore likely to be a universal property of topological defects in ferroelectric semiconductors.

Scaling and disorder analysis of local I-V curves from ferroelectric thin films of lead zirconate titanate.

Differential analysis of current-voltage characteristics, obtained on the surface of epitaxial films of ferroelectric lead zirconate titanate (Pb(Zr(0.2)Ti(0.8))O(3)) using scanning probe microscopy, was combined with spatially resolved mapping of variations in local conductance to differentiate between candidate mechanisms of local electronic transport and the origin of disorder. Within the assumed approximations, electron transport was inferred to be determined by two mechanisms depending on the magnitude of applied bias, with the low-bias range dominated by the trap-assisted Fowler-Nordheim tunneling through the interface and the high-bias range limited by the hopping conduction through the bulk. Phenomenological analysis of the I-V curves has further revealed that the transition between the low- and high-bias regimes is manifested both in the strength of variations within the I-V curves sampled across the surface, as well as the spatial distribution of conductance. Spatial variations were concluded to originate primarily from the heterogeneity of the interfacial electronic barrier height with an additional small contribution from random changes in the tip-contact geometry.

Nanoscale switching characteristics of nearly tetragonal BiFeO3 thin films.

We have investigated the nanoscale switching properties of strain-engineered BiFeO(3) thin films deposited on LaAlO(3) substrates using a combination of scanning probe techniques. Polarized Raman spectral analysis indicates that the nearly tetragonal films have monoclinic (Cc) rather than P4mm tetragonal symmetry. Through local switching-spectroscopy measurements and piezoresponse force microscopy, we provide clear evidence of ferroelectric switching of the tetragonal phase, but the polarization direction, and therefore its switching, deviates strongly from the expected (001) tetragonal axis. We also demonstrate a large and reversible, electrically driven structural phase transition from the tetragonal to the rhombohedral polymorph in this material, which is promising for a plethora of applications.

Exotic Long-Range Surface Reconstruction on La0.7Sr0.3MnO3 Thin Films.

Due to an extremely diverse phase space, La1-xSrxMnO3, as with other manganites, offers a wide range of tunability and applications including colossal magnetoresistance and use as spin-polarized electrodes. Here, we study an unprecedented, exotic surface reconstruction (6 × 6) in La1-xSrxMnO3 (x = 0.3) observed via low-energy electron diffraction (LEED). Scanning tunneling microscopy (STM) shows the surface is relatively flat, with unit-cell step heights, and X-ray photoelectron spectroscopy (XPS) reveals a strong degree of Sr segregation at the surface. By combining electron diffraction and first-principles computations, we propose that the long-range surface reconstruction consists of a Sr-segregated surface with La (6 × 6) ordering. This study expands our understanding of manganite systems and underscores their ability to form interesting surface reconstructions, driven largely by cation segregation that can potentially be controlled for tuning surface ordering.

Atomistic screening mechanism of ferroelectric surfaces: an in situ study of the polar phase in ultrathin BaTiO3 films exposed to H2O.

The polarization screening mechanism and ferroelectric phase stability of ultrathin BaTiO(3) films exposed to water molecules is determined by first principles theory and in situ experiment. Surface crystallography data from electron diffraction combined with density functional theory calculations demonstrate that small water vapor exposures do not affect surface structure or polarization. Large exposures result in surface hydroxylation and rippling, formation of surface oxygen vacancies, and reversal of the polarization direction. Understanding interplay between ferroelectric phase stability, screening, and atomistic processes at surfaces is a key to control low-dimensional ferroelectricity.

Direct imaging of the spatial and energy distribution of nucleation centres in ferroelectric materials.

Macroscopic ferroelectric polarization switching, similar to other first-order phase transitions, is controlled by nucleation centres. Despite 50 years of extensive theoretical and experimental effort, the microstructural origins of the Landauer paradox, that is, the experimentally observed low values of coercive fields in ferroelectrics corresponding to implausibly large nucleation activation energies, are still a mystery. Here, we develop an approach to visualize the nucleation centres controlling polarization switching processes with nanometre resolution, determine their spatial and energy distribution and correlate them to local microstructure. The random-bond and random-field components of the disorder potential are extracted from positive and negative nucleation biases. Observation of enhanced nucleation activity at the 90 composite function domain wall boundaries and intersections combined with phase-field modelling identifies them as a class of nucleation centres that control switching in structural-defect-free materials.

Learning from Imperfections: Predicting Structure and Thermodynamics from Atomic Imaging of Fluctuations.

In materials characterization, traditionally a single experimental sample is used to derive information about a single point in the composition space, while the imperfections, impurities, and stochastic details of material structure are deemed irrelevant or complicating factors in the analysis. Here we demonstrate that atomic-scale studies of a single nominal composition can provide information about microstructures and thermodynamic response over a finite area of chemical space. Using the principles of statistical inference, we develop a framework for incorporating structural fluctuations into statistical mechanical models and use it to solve the inverse problem of deriving effective interatomic interactions responsible for elemental segregation in a La5/8Ca3/8MnO3 thin film. The results are further analyzed by a variational autoencoder to detect anomalous behavior in the composition phase diagram. This study provides a framework for creating generative models from a combination of multiple experimental data and provides direct insight into the driving forces for cation segregation in manganites.