Atomic-scale visualization of oxide thin-film surfaces.

The interfaces of complex oxide heterostructures exhibit intriguing phenomena not observed in their constituent materials. The oxide thin-film growth of such heterostructures has been successfully controlled with unit-cell precision; however, atomic-scale understandings of oxide thin-film surfaces and interfaces have remained insufficient. We examined, with atomic precision, the surface and electronic structures of oxide thin films and their growth processes using low-temperature scanning tunneling microscopy. Our results reveal that oxide thin-film surface structures are complicated in contrast to the general perception and that atomically ordered surfaces can be achieved with careful attention to the surface preparation. Such atomically ordered oxide thin-film surfaces offer great opportunities not only for investigating the microscopic origins of interfacial phenomena but also for exploring new surface phenomena and for studying the electronic states of complex oxides that are inaccessible using bulk samples.

Lattice and Valence Electronic Structures of Crystalline Octahedral Molybdenum Halide Clusters-Based Compounds, Cs2[Mo6X14] (X = Cl, Br, I), Studied by Density Functional Theory Calculations.

The electronic and crystal structures of Cs2[Mo6X14] (X = Cl, Br, I) cluster-based compounds were investigated by density functional theory (DFT) simulations and experimental methods such as powder X-ray diffraction, ultraviolet-visible spectroscopy, and X-ray photoemission spectroscopy (XPS). The experimentally determined lattice parameters were in good agreement with theoretically optimized ones, indicating the usefulness of DFT calculations for the structural investigation of these clusters. The calculated band gaps of these compounds reproduced those experimentally determined by UV-vis reflectance within an error of a few tenths of an eV. Core-level XPS and effective charge analyses indicated bonding states of the halogens changed according to their sites. The XPS valence spectra were fairly well reproduced by simulations based on the projected electron density of states weighted with cross sections of Al Kα, suggesting that DFT calculations can predict the electronic properties of metal-cluster-based crystals with good accuracy.

Negligible "negative space-charge layer effects" at oxide-electrolyte/electrode interfaces of thin-film batteries.

In this paper, we report the surprisingly low electrolyte/electrode interface resistance of 8.6 Ω cm(2) observed in thin-film batteries. This value is an order of magnitude smaller than that presented in previous reports on all-solid-state lithium batteries. The value is also smaller than that found in a liquid electrolyte-based batteries. The low interface resistance indicates that the negative space-charge layer effects at the Li3PO(4-x)N(x)/LiCoO2 interface are negligible and demonstrates that it is possible to fabricate all-solid state batteries with faster charging/discharging properties.

Imaging the evolution of d states at a strontium titanate surface.

Oxide electronics is a promising alternative to the conventional silicon-based semiconductor technology, owing to the rich functionalities of oxide thin films and heterostructures. In contrast to the silicon surface, however, the electronic structure of the SrTiO3 surface, the most important substrate for oxide thin films growth, is not yet completely understood. Here we report on the electronic states of a reconstructed (001) surface of SrTiO3 determined in real space, with scanning tunneling microscopy/spectroscopy and density functional theory calculations. We found a remarkable energy dependence of the spectroscopic image: Theoretical analysis reveals that symmetry breaking at the surface lifts the degeneracy in the t2g state (dxy, dyz, and dzx) of Ti 3d orbitals, whose anisotropic spatial distribution leads to a sharp transition in the spectroscopic image as a function of energy. The knowledge obtained here could be used to gain further insights into emergent phenomena at the surfaces and interfaces with SrTiO3.

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 × âœ“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.

Ultrafine self-N-doped porous carbon nanofibers with hierarchical pore structure utilizing a biobased chitosan precursor.

Ultrafine porous carbon nanofiber network with ~40 nm fiber diameter is realized for the first time utilizing a biobased polymer as carbon precursor. A simple one-step carbonization procedure is applied to convert the electrospun chitosan/poly(ethylene oxide) nanofibers to self-N-doped ultrafine hierarchically porous carbon nanofiber interconnected web. The pore formation process is governed by the immiscible nature of the two polymers and the sacrificial character of poly(ethylene oxide) with low carbon yield at the carbonization temperature (800 °C). The obtained porous scaffold has a high specific surface area (564 m2 g-1), high micro (0.22 cm3 g-1) as well as meso/macropore volume (0.28 cm3 g-1). Structural analysis indicates high graphitic content and the existence of turbostratic carbon typical for carbon fibers derived from otherwise synthetic polymer precursors. X-ray photoelectron spectroscopy confirms the presence of an N-doped structure with dominating graphitic N, together with a smaller amount of pyridinic N. The prepared electrode exhibits good electrochemical performance as a supercapacitor device. The excellent charge storage characteristics are attributed to the unique ultrafine hierarchical nanoarchitecture and the interconnected N-doped carbon structure. This green material holds great promise for the realization of more sustainable high-performance energy storage devices.

Embedding hexanuclear tantalum bromide cluster {Ta6Br12} into SiO2 nanoparticles by reverse microemulsion method.

Hexanuclear tantalum bromide cluster units [{Ta6Bri12}La6] (i = inner, a = apical, L = ligand OH or H2O) are embedded into SiO2 nanoparticles by a reverse microemulsion (RM) based method. [{Ta6Bri12}Bra2 (H2O)a4]·nH2O (noted TBH) and tetraethyl orthosilicate (TEOS) are used as the starting cluster compound and the precursor of SiO2, respectively. The RM system in this study consists of the n-heptane (oil phase), Brij L4 (surfactants), ethanol, TEOS, ammonia solution and TBH aqueous sol. The size and morphology of the product namely {Ta6Br12}@SiO2 nanoparticles are analyzed by HAADF-STEM and EDS mappings. The presence and integrity of {Ta6Br12} in the SiO2 nanoparticles are evidenced by EDS mapping, ICP-OES/IC and XPS analysis. The optical properties of {Ta6Br12}@SiO2 nanoparticles are analyzed by diffuse reflectance UV-vis spectroscopy, further evidencing the integrity of the embedded {Ta6Br12} and revealing their oxidation state. Both {Ta6Br12}2+ and {Ta6Br12}3+ are found in SiO2 nanoparticles, but the latter is much more stable than the former. The by-products in this RM-based synthesis, as well as their related factors, are also discussed.

Metallic versus Semiconducting SWCNT Chemiresistors: A Case for Separated SWCNTs Wrapped by a Metallosupramolecular Polymer.

As-synthesized single-walled carbon nanotubes (SWCNTs) are a mixture of metallic and semiconducting tubes, and separation is essential to improve the performances of SWCNT-based electric devices. Our chemical sensor monitors the conductivity of an SWCNT network, wherein each tube is wrapped by an insulating metallosupramolecular polymer (MSP). Vapors of strong electrophiles such as diethyl chlorophosphate (DECP), a nerve agent simulant, can trigger the disassembly of MSPs, resulting in conductive SWCNT pathways. Herein, we report that separated SWCNTs have a large impact on the sensitivity and selectivity of chemical sensors. Semiconducting SWCNT (S-SWCNT) sensors are the most sensitive to DECP (up to 10000% increase in conductivity). By contrast, the responses of metallic SWCNT (M-SWCNT) sensors were smaller but less susceptible to interfering signals. For saturated water vapor, increasing and decreasing conductivities were observed for S- and M-SWCNT sensors, respectively. Mixtures of M- and S-SWCNTs revealed reduced responses to saturated water vapor as a result of canceling effects. Our results reveal that S- and M-SWCNTs compensate sensitivity and selectivity, and the combined use of separated SWCNTs, either in arrays or in single sensors, offers advantages in sensing systems.

A Single-Atom-Thick TiO2 Nanomesh on an Insulating Oxide.

The electronic structures and macroscopic functionalities of two-dimensional (2D) materials are often controlled according to their size, atomic structures, and associated defects. This controllability is particularly important in ultrathin 2D nanosheets of transition-metal oxides because these materials exhibit extraordinary multifunctionalities that cannot be realized in their bulk constituents. To expand the variety of materials with exotic properties that can be used in 2D transition-metal-oxide nanosheets, it is essential to investigate fabrication processes for 2D materials. However, it remains challenging to fabricate such 2D nanosheets, as they are often forbidden because of the crystal structure and nature of their host oxides. In this study, we demonstrate the synthesis of a single-atom-thick TiO2 2D nanosheet with a periodic array of holes, that is, a TiO2 nanomesh, by depositing a LaAlO3 thin film on a SrTiO3(001)-(√13×√13)-R33.7° reconstructed substrate. In-depth investigations of the detailed structures, local density of states, and Ti valency of the TiO2 nanomesh using scanning tunneling microscopy/spectroscopy, scanning transmission electron microscopy, and density functional theory calculations reveal an unexpected upward migration of the Ti atoms of the substrate surface onto the LaAlO3 surface. These results indicate that the truncated TiO5 octahedra on the surface of perovskite oxides are very stable, leading to semiconducting TiO2 nanomesh formation. This nanomesh material can be potentially used to control the physical and chemical properties of the surfaces of perovskite oxides. Furthermore, this study provides an avenue for building functional atomic-scale oxide 2D structures and reveals the thin-film growth processes of complex oxides.

Visualizing atomistic formation process of SrOx thin films on SrTiO3.

Metallic conductivity observed in the heterostructure of LaAlO3/SrTiO3 has attracted great attention, triggering a debate over whether the origin is an intrinsic electronic effect or a defect-related phenomenon. One of the issues to be solved is the role of SrO layer, which turns the conductive interface into an insulator when inserted between LaAlO3 and SrTiO3. To understand the origins of this oxide interface phenomenon and to further explore unconventional functionalities, it is necessary to elucidate how SrO layers are formed during the initial growth process at the atomic level. Here, we atomically resolve growth processes of heteroepitaxial SrOx films on SrTiO3(001)-(√13×√13)-R33.7° substrate using scanning tunneling microscopy/spectroscopy. On the sub-unit-cell SrOx film surface, no periodic structure was observed as a result of random Ti incorporation into the SrOx islands, indicating the importance of the control of excess Ti atoms on the substrate prior to deposition. This random arrangement of Ti atoms is a marked contrast to the homoepitaxy on SrTiO3(001)-(√13×√13)-R33.7°. Furthermore, the formation of SrOx islands introduced defects in the surrounding SrTiO3 substrate surface. Such atom-by-atom engineering and characterizations of oxide heterostructures not only provide microscopic understanding of formation process of interfaces in metal-oxides, but also would lead to the creation of exotic electronic phenomena and novel functionalities at these interfaces.

Atomic-scale visualization of initial growth of homoepitaxial SrTiO3 thin film on an atomically ordered substrate.

The initial homoepitaxial growth of SrTiO(3) on a (√13 × âˆš13)-R33.7° SrTiO(3)(001) substrate surface, which can be prepared under oxide growth conditions, is atomically resolved by scanning tunneling microscopy. The identical (√13 × âˆš13) atomic structure is clearly visualized on the deposited SrTiO(3) film surface as well as on the substrate. This result indicates the transfer of the topmost Ti-rich (√13 × âˆš13) structure to the film surface and atomic-scale coherent epitaxy at the film/substrate interface. Such atomically ordered SrTiO(3) substrates can be applied to the fabrication of atom-by-atom controlled oxide epitaxial films and heterostructures.