Complex Oxide Surfaces Research Articles

Vacancy Ordering in Ultrathin Copper Oxide Films on Cu(111).

Oxide thin films grown on metal surfaces have wide applications in catalysis and beyond owing to their unique surface structures compared to their bulk counterparts. Despite extensive studies, the atomic structures of copper surface oxides on Cu(111), commonly referred to as "44" and "29", have remained elusive. In this work, we demonstrated an approach for the structural determination of oxide surfaces using element-specific scanning tunneling microscopy (STM) imaging enhanced by functionalized tips. This approach enabled us to resolve the atomic structures of "44" and "29" surface oxides, which were further corroborated by noncontact atomic force microscopy (nc-AFM) measurements and Monte Carlo (MC) simulations. The stoichiometry of the "44" and "29" frameworks was identified as Cu23O16 and Cu16O11, respectively. Contrary to the conventional hypothesis, we observed ordered Cu vacancies within the "44" structure manifesting as peanut-shaped cavities in the hexagonal lattice. Similarly, a combination of Cu and O vacancies within the "29" structure leads to bean-shaped cavities within the pentagonal lattice. Our study has thus resolved the decades-long controversy on the atomic structures of "44" and "29" surface oxides, advancing our understanding of copper oxidation processes and introducing a robust framework for the analysis of complex oxide surfaces.

Distinct differences in surface transport between electrolyte-gated (110) and (001) SrTiO3

Electrolyte gating technology has been widely used as an effective tool to study novel physics at complex oxide surfaces and interfaces. Certain emergent phenomena reported in electro-gated SrTiO3 (STO) surfaces are particularly interesting. Here, we report on the disparate electron transport behaviors of electrolyte-gated STO (001) and (110) oriented surfaces. In contrast to the anomalous transport from an insulating state to a Kondo-like state and then metallic states on the (001) surface, with increasing carrier density, the (110) surface always behaves as an insulating state. A comparison study suggests that the oxygen vacancies and localized Ti3+ ions on the (001) surface are scatter carriers, but no such defects are found on the (110) surface. This suggests that these surfaces are intrinsically insulating, and the observed anomalous effects are likely induced by the Ti3+ ions and oxygen vacancies introduced during electrolyte gating. This work sheds light on complicated phenomena in electrolyte-gated STO-based systems.

Coexistence of Both Localized Electronic States and Electron Gas at Rutile TiO2 Surfaces.

Localized electron polarons formed through the coupling of excess electrons and ionic vibrations play a key role in the functionalities of materials. However, the mechanism of the coexistence of delocalized electrons and localized polarons remains underexplored. Here, the discovery of high-mobility 2D electron gas at the rutile TiO2 surfaces through argon ion irradiation induced oxygen vacancies is reported. Strikingly, the electron gas forms localized electronic states at lower temperatures, resulting in an abrupt metal-insulator transition. Moreover, it is found that the low-temperature conductivity in the insulating state is dominated by excess free electrons with a high mobility of ≈103 cm2 V-1 s-1 , whereas the carrier density is dramatically suppressed with decreasing temperature. Remarkably, it reveals that the application of an electric field can lead to a collapse of the localized states, resulting in a metallic state. These results reveal the strongly correlated/coupled nature between the localized electrons and high-mobility electrons and offer a new pathway to probe and harvest the exotic electron states at the complex oxide surfaces.

Probing active species for CO hydrogenation over ZnCr2O4 catalysts

Oxide catalysts are increasingly employed for hydrogenation reactions, among which ZnCrO x is a major catalyst for the oxide-zeolite (OXZEO) process and for the hydrogenation of C1 molecules in general. Owing to the complex nature of ternary oxides, the surface and catalytic properties of ZnCr 2 O 4 spinel have remained controversial for CO hydrogenation. Combining in-situ Fourier-transformed infrared spectroscopy and X-ray photoelectron spectroscopy, we examined the adsorption and reaction of CO/H 2 on the ZnCr 2 O 4 catalysts, which were pre-treated under oxidative or reductive conditions. The reduced ZnCr 2 O 4 catalyst was found to expose more surface sites for CO adsorption/reaction than the oxidized ZnCr 2 O 4 catalyst. Exposing the reduced ZnCr 2 O 4 to H 2 at room temperature led to the formation of surface hydride species, which would transform into hydroxyl species at elevated temperatures. The reduced ZnCr 2 O 4 surface exhibited much stronger interaction with CO and H 2 than ZnO and Cr 2 O 3 . Exposing the reduced ZnCr 2 O 4 to the CO and H 2 (1:1) mixture gas led to the hydrogenation of CO. However, CO was oxidized by the hydroxyl species via the water-gas-shift reaction, whereas the hydrogenation of CO could only be achieved by surface hydride species on the reduced ZnCr 2 O 4 to formyl or formate species at 373–473 K. Our study has thus shed light on the active species that control elementary reaction process of CO hydrogenation on complex oxide surfaces. Surface hydroxyl species are consumed by CO via the route of water-gas-shift reaction, while surface hydride species would hydrogenate CO to form formate. Surface hydride species could transform into hydroxyl species on ZnCr 2 O 4 at elevated temperatures.

Deep level defect spectroscopies of complex oxide surfaces and interfaces

Intrinsic point defects are commonly present in and can strongly affect the electronic properties of complex oxides and their interfaces. The near- and subsurface characterization techniques, depth-resolved cathodoluminescence spectroscopy and surface photovoltage spectroscopy, can measure the density distributions, energy levels, and optical transitions of intrinsic point defects in complex oxides on a near-nanometer scale. These measurements on SrTiO3, BaTiO3, and related materials reveal the sensitivity of intrinsic point defects to growth temperature, mechanical strain, crystal orientation, and chemical interactions. Spatial redistribution of these defects can vary significantly near surfaces and interfaces and can have strong electronic effects. The combination of these deep level spectroscopies along with other advanced characterization techniques provides an avenue to further expand the understanding and control of complex oxide defects in general.

In situ Auger electron spectroscopy of complex oxide surfaces grown by pulsed laser deposition

The authors report in situ Auger electron spectroscopy (AES) of the surfaces of complex oxide thin films grown by pulsed laser deposition (PLD). The authors demonstrate the utility of the technique in studying elemental composition by collecting characteristic Auger spectra of elements from samples such as complex oxide thin films and single crystals. In the case of thin films, AES studies can be performed with single unit cell precision by monitoring thickness during deposition with reflection high energy electron diffraction. The authors address some of the challenges in achieving in situ and real-time AES studies on complex oxide thin films grown by PLD. Sustained layer-by-layer PLD growth of a CaTiO3/LaMnO3 superlattice allows depth-resolved elemental composition analysis during the growth process. The evolution of the Auger spectra of elements from individual layers was used to perform compositional analysis with monolayer-depth resolution.

Origin of H2 Formation on Perfect SrTiO3 (001) Surface: A First-principles Study

We systematically investigate saturated adsorption of H atoms on the SrTiO3 (001) surface to reveal the origin of H2 molecule formation using density functional theory methods. We find that H atoms prefer to adsorb on O sites at low coverage, while adsorbing on Ti sites at higher coverage. Interestingly, H on O sites and H on neighboring Ti sites (HTi) dimerize to form H2 molecules provided that enough electrons are doped in the conduction bands of SrTiO3. Bader charges and electronic structures show that negatively charged HTi plays a decisive role in the formation of H2. Both calculated hydrogen coverage and electron density at saturated adsorption are in good agreement with experimental values. Our results provide an illuminating instance of H2 formation on perfect oxide surface, which shall pave the way to further understanding the detailed mechanism of H2 evolution on more complex oxide surfaces.

Scanning tunneling microscopy/spectroscopy on perovskite oxide thin films deposited in situ.

Complex oxide surfaces and interfaces, consisting of two or more cations and oxygen anions, have attracted a great deal of attention because their properties are crucial factors in the performance of catalysts, fuel cells, and Li-ion batteries. However, atomic-scale investigations of these oxide surfaces have been hindered because of the difficulties in surface preparation. Here, we demonstrate atomic-scale surface studies of complex perovskite oxides and the initial growth processes in oxide epitaxial films deposited on (✓13 × ✓13)-R33.7° reconstructed SrTiO3 (001) substrates using a scanning tunneling microscope integrated with a pulsed laser deposition system. The atomically ordered, reconstructed SrTiO3 (001) surface is stable under the typical conditions necessary for the growth of oxide thin films, and hence is considered suitable for the study of the initial growth processes in oxide films. The atomic-scale microscopic/spectroscopic characterizations performed here shed light on the microscopic origin of electronic properties observed in complex oxides and their heterostructures.

Influence of Adsorption Site and Wavelength on the Photodesorption of NO from the (Fe,Cr)3O4(111) Mixed Oxide Surface

The photochemical properties of nitric oxide on a mixed oxide single crystal surface was examined in ultrahigh vacuum (UHV) using temperature programmed desorption (TPD), photon stimulated desorption (PSD) and low energy electron diffraction (LEED). The mixed oxide was a 75% Fe and 25% Cr corundum (0001) oxide film prepared on an α-Al2O3(0001) crystal, however its surface became terminated with a magnetite-like (111) structure after sputter/anneal cleaning, leading to a surface designated of (Fe,Cr)3O4(111). TPD of NO from the (Fe,Cr)3O4(111) surface revealed three chemisorbed states at 220, ~315 and 370 K assigned to NO binding at Fe3+, Cr3+ and Fe2+ sites, respectively. No significant thermal chemistry of NO was detected. NO photodesorption, the primary photochemical pathway in UHV, was sensitive to the adsorption site, with rates at the three adsorption sites following the trend: Fe3+ > Fe2+ > Cr3+. Multiexponential rate behavior seen in the overall NO PSD spectra was linked directly to site heterogeneity being manifested as a convolution of the individual NO photodesorption rates at the three types of surface sites. The photodesorption rate with UV light (365 nm) was ~10 times greater than that in the visible, but the per-photon rates across the visible spectrum (from 460 tomore » 630 nm) were independent of the wavelength, which is suggestive of localized photon absorption at the adsorption site. Results in this study demonstrate that the adsorption site plays a critical role in determining photochemical rates on complex oxide surfaces. This work was supported by the US Department of Energy, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences & Biosciences. Pacific Northwest National Laboratory (PNNL) is a multi-program national laboratory operated for DOE by Battelle. The research was performed using EMSL, a national scientific user facility sponsored by the Department of Energy's Office of Biological and Environmental Research and located at Pacific Northwest National Laboratory.« less

Ultrahigh vacuum and low-temperature cleaning of oxide surfaces using a low-concentration ozone beam.

We present a novel method of delivering a low-concentration (<15%) ozone beam to an ultra-high vacuum environment for the purpose of cleaning and dosing experimental samples through oxidation processing. The system described is safe, low-cost, and practical and overcomes the limitations of ozone transport in the molecular flow environment of high or ultrahigh vacuum whilst circumventing the use of pure ozone gas which is potentially highly explosive. The effectiveness of this method in removing surface contamination is demonstrated through comparison of high-temperature annealing of a simple oxide (MgO) in ozone and oxygen environments as monitored using quadrupole mass spectroscopy and Auger electron spectroscopy. Additionally, we demonstrate the potential of ozone for obtaining clean complex oxide surfaces without the need for high-temperature annealing which may significantly alter surface structure.

Atomically resolved force microscopy images of complex surface unit cells: Ultrathin alumina film on NiAl(110)

In this Brief Report we present atomically resolved images of the ultrathin alumina film on NiAl 110 . For the first time detailed images of the complex microstructure for both reflection domains have been obtained by frequency modulation dynamic force microscopy using a very stable, custom built, dual mode scanning force and scanning tunneling microscope. Measurements have been performed under ultrahigh vacuum conditions at 5 K with a quartz tuning fork as a force sensor. The high spatial resolution allows to derive 28 atomic positions in real space in the surface unit cell by simple graphical analysis. This has been successful without application of filtering or correlation methods, emphasizing the potential of this force microscopy method on complex oxide surfaces. With respect to topographic height even quantitative agreement with theory could be achieved, here shown for selected structural elements within the unit cell. Furthermore, deeper insight into a wavelike morphological feature could be obtained. Consistency with a published density-functional theory model and connections to other data from the literature are discussed.

Critical effects of substrate terraces and steps morphology on the growth mode of epitaxial SrRuO3 films

We report on the controlled fabrication of the arrays of self-organized fingerlike nanostructures of SrRuO3 that form on SrTiO3(001). We show that the size (width and height) of the one-dimensional nanometric units can be tuned or suppressed by using appropriate substrate vicinality θv and surface termination. Critical effects on the fingers height are observed when θv is below an angle as small as ∼0.1°. The value of θv also determines if the final two-dimensional growth is by a step flow (θv=0.5°), by layer by layer (θv=0.04°), or by coexistence of both mechanisms (θv=0.1°). The fingers are suppressed when TiO2-terminated low-miscut substrates are used, as film nucleation takes place on the terraces and then substrate steps are not active as templates for the one-dimensional structure formation. These findings shall contribute to progress toward lateral nanostructuration of complex oxide surfaces.

First principles studies of complex oxide surfaces and interfaces

Oxides enter our everyday life and exhibit an impressive variety of physical and chemicalproperties. The understanding of their behaviour, which is often determined by theelectronic and atomic structures of their surfaces and interfaces, is a key question inmany fields, such as geology, environmental chemistry, catalysis, thermal coatings,microelectronics, and bioengineering. In the last decade, first principles methods, mainlythose based on the density functional theory, have been frequently applied to studycomplex oxide surfaces and interfaces, complementing the experimental observations. Inthis work, we discuss some of these contributions, with emphasis on several issues that areespecially important when dealing with oxides: the local electronic structure at interfaces,and its connection with chemical reactivity; the charge redistribution and the bondingvariations, in relation to screening properties; and the possibility of bridging the gapbetween model and real systems by taking into account the chemical environments and theeffect of finite temperatures, and by performing simulations on systems of an adequate(large) size.

Characterisation of Surface Properties, Adsorbates and Functional Groups on Polycrystalline Oxides by Ultraviolet Photoelectron Spectroscopy (UPS)

Ultraviolet photoelectron spectroscopy (UPS), which is known as a powerful technique in the surface science of single crystals, loses much of its analytical potential when applied to rough, non-ordered complex surfaces of real materials. The paper discusses the question if there are relevant applications of UPS in the analysis of real complex oxide surfaces. It is shown that this technique provides high surface sensitivity combined with chemical sensitivity, which allows to differentiate compounds and adsorbates on surfaces. It is described how UPS contributed to an investigation of depth profiles of the reduction degree in partly reduced V 2 O 5 . Studies of oxide segregation on mixed-oxide surfaces (Bi-Mo-O, Ca-Ce-O) and on titania-supported oxides (MoO 3 , V 2 O 5 ) and of adsorbates and functional groups (H 2 O, OH, CO 2 ) on Ca-Ce-O mixed-oxide surfaces are reported. It is demonstrated that UPS is a highly useful complementary technique in surface-analytical studies performed with X-ray photoelectron spectroscopy and Ion scattering spectroscopy. Its application is, however, limited by its high sensitivity to surface charging, which requires to measure spectra at elevated temperatures and excludes wide bandgap insulators.

Determination of the TiO2 (110) (2×3) surface structure via a parametric approach to STM image simulation

Scanning tunneling microscopy (STM) image simulations for transition-metal oxide surfaces have been compared to STM images to determine the atomic structure of the ${\mathrm{TiO}}_{2}$ (110) $(2\ifmmode\times\else\texttimes\fi{}3)$ reconstruction. The calculation of simulated image contrast is based on fitting Slater-type orbital functions to first-principles empty state contours for an unreconstructed ${\mathrm{TiO}}_{2}$ (110). The calculations are extended to arbitrary surface structures using a parametric approach. For the case of ${\mathrm{TiO}}_{2}$ (110), simulations of empty conduction-band edge state densities are compared with STM images to distinguish between two possible atomic terminations of the $2\ifmmode\times\else\texttimes\fi{}3$ surface. The method is proposed as a general approach that allows a first-order interpretation of features in STM images of complex oxide surfaces. The structures of these surfaces are often quite complex and exhibit a mixture of ionic and covalent bonding, often making image interpretation difficult.

Oxide surfaces and metal/oxide interfaces studied by grazing incidence X-ray scattering

Experimental determinations of the atomic structure of insulating oxide surfaces and metal/oxide interfaces are scarce, because surface science techniques are often limited by the insulating character of the substrate. Grazing incidence X-ray scattering (GIXS), which is not subject to charge effects, can provide very precise information on the atomic structure of oxide surfaces: roughness, relaxation and reconstruction. It is also well adapted to analyze the atomic structure, the registry, the misfit relaxation, elastic or plastic, the growth mode and the morphology of metal/oxide interfaces during their growth, performed in situ. GIXS also allows the analysis of thin films and buried interfaces, in a non-destructive way, yielding the epitaxial relationships, and, by variation of the grazing incidence angle, the lattice parameter relaxation along the growth direction. On semi-coherent interfaces, the existence of an ordered network of interfacial misfit dislocations can be demonstrated, its Burger's vector determined, its ordering during in situ annealing cycles followed, and sometimes even its atomic structure can be addressed. Careful analysis during growth allows the modeling of the dislocation nucleation process. This review emphasizes the new information that GIXS can bring to oxide surfaces and metal/oxide interfaces by comparison with other surface science techniques. The principles of X-ray diffraction by surfaces and interfaces are recalled, together with the advantages and properties of grazing angles. The specific experimental requirements are discussed. Recent results are presented on the determination of the atomic structure of relaxed or reconstructed oxide surfaces. A description of results obtained during the in situ growth of metal on oxide surfaces is also given, as well as investigations of thick metal films on oxide surfaces, with lattice parameter misfit relaxed by an array of dislocations. Recent work performed on oxide thin films having important physical properties such as superconductivity or magnetism is also briefly reviewed. The strengths and limitations of the technique, such as the need for single crystals and surfaces of high crystalline quality are discussed. Finally, an outlook of future prospects in the field is given, such as the study of more complex oxide surfaces, vicinal surfaces, reactive metal/oxide interfaces, metal oxidation processes, the use of surfactants to promote wetting of a metal deposited on an oxide surface or the study of oxide/liquid interfaces in a non-UHV environment.