Anli Yang

Chemical insight into electroforming of resistive switching manganite heterostructures

We have investigated the role of the electroforming process in the establishment of resistive switching behaviour for Pt/Ti/Pr0.5Ca0.5MnO3/SrRuO3 layered heterostructures (Pt/Ti/PCMO/SRO) acting as non-volatile Resistance Random Access Memories (RRAMs). Electron spectroscopy measurements demonstrate that the higher resistance state resulting from electroforming of as-prepared devices is strictly correlated with the oxidation of the top electrode Ti layer through field-induced electromigration of oxygen ions. Conversely, PCMO exhibits oxygen depletion and downward change of the chemical potential for both resistive states. Impedance spectroscopy analysis, supported by the detailed knowledge of these effects, provides an accurate model description of the device resistive behaviour. The main contributions to the change of resistance from the as-prepared (low resistance) to the electroformed (high resistance) states are respectively due to reduced PCMO at the boundary with the Ti electrode and to the formation of an anisotropic n-p junction between the Ti and the PCMO layers.

The valence band structure of AgxRh1–x alloy nanoparticles

The valence band (VB) structures of face-centered-cubic Ag-Rh alloy nanoparticles (NPs), which are known to have excellent hydrogen-storage properties, were investigated using bulk-sensitive hard x-ray photoelectron spectroscopy. The observed VB spectra profiles of the Ag-Rh alloy NPs do not resemble simple linear combinations of the VB spectra of Ag and Rh NPs. The observed VB hybridization was qualitatively reproduced via a first-principles calculation. The electronic structure of the Ag0.5Rh0.5 alloy NPs near the Fermi edge was strikingly similar to that of Pd NPs, whose superior hydrogen-storage properties are well known.

Size dependence of structural parameters in fcc and hcp Ru nanoparticles, revealed by Rietveld refinement analysis of high-energy X-ray diffraction data.

To reveal the origin of the CO oxidation activity of Ruthenium nanoparticles (Ru NPs), we structurally characterized Ru NPs through Rietveld refinement analysis of high-energy X-ray diffraction data. For hexagonal close-packed (hcp) Ru NPs, the CO oxidation activity decreased with decreasing domain surface area. However, for face-centered cubic (fcc) Ru NPs, the CO oxidation activity became stronger with decreasing domain surface area. In comparing fcc Ru NPs with hcp Ru NPs, we found that the hcp Ru NPs of approximately 2 nm, which had a smaller domain surface area and smaller atomic displacement, showed a higher catalytic activity than that of fcc Ru NPs of the same size. In contrast, fcc Ru NPs larger than 3.5 nm, which had a larger domain surface area, lattice distortion, and larger atomic displacement, exhibited higher catalytic activity than that of hcp Ru NPs of the same size. In addition, the fcc Ru NPs had larger atomic displacements than hcp Ru NPs for diameters ranging from 2.2 to 5.4 nm. Enhancement of the CO oxidation activity in fcc Ru NPs may be caused by an increase in imperfections due to lattice distortions of close-packed planes and static atomic displacements.

Systematic investigation of surface and bulk electronic structure of undoped In-polar InN epilayers by hard X-ray photoelectron spectroscopy

The surface and bulk electronic structure of undoped In-polar InN (u-InN) epilayers with surface electron accumulation (SEA) layer was investigated by soft and hard X-ray photoelectron spectroscopies (SX-PES and HX-PES, respectively). The potential-energy profile was obtained by fitting the N 1s core-level spectra accounting for the probing depth of both SX-PES and HX-PES and the surface downward band bending. In this study, we found that a significant potential-energy bending as large as 1.2 ± 0.05 eV occurred from surface to the depth of ∼20 nm. Taking into account such a large downward bending, the valence band maximum (VBM) with respect to Fermi energy (EF) level in the bulk was determined to be 0.75 ± 0.05 eV. A weak signal with a peak position at 0.3 ± 0.05 eV was reproducibly observed from the VBM to EF level in the HX-PES spectrum. The peak position was in agreement with the calculated energy of the E1 sub-band in the surface quantum well. HX-PES is powerful tool for revealing the intrinsic bulk e...

Origin of the catalytic activity of face-centered-cubic ruthenium nanoparticles determined from an atomic-scale structure

The 3-dimensional (3D) atomic-scale structure of newly discovered face-centered cubic (fcc) and conventional hexagonal close packed (hcp) type ruthenium (Ru) nanoparticles (NPs) of 2.2 to 5.4 nm diameter were studied using X-ray pair distribution function (PDF) analysis and reverse Monte Carlo (RMC) modeling. Atomic PDF based high-energy X-ray diffraction measurements show highly diffuse X-ray diffraction patterns for fcc- and hcp-type Ru NPs. We here report the atomic-scale structure of Ru NPs in terms of the total structure factor and Fourier-transformed PDF. It is found that the respective NPs have substantial structural disorder over short- to medium-range order atomic distances from the PDF analysis. The first-nearest-neighbor peak analyses show a significant size dependence for the fcc-type Ru NPs demonstrating the increase in the peak height due to an increase in the number density as a function of particle size. The bond angle and coordination number (CN) distribution for the RMC-simulated fcc- and hcp-type Ru NP models indicated inherited structural features from their bulk counterparts. The CN analysis of the whole NP and surface of each RMC model of Ru NPs show the low activation energy packing sites on the fcc-type Ru NP surface atoms. Finally, our newly defined order parameters for RMC simulated Ru NP models suggested that the enhancement of the CO oxidation activity of fcc-type NPs was due to a decrease in the close packing ordering that resulted from the increased NP size. These structural findings could be positively supported for synthesized low-cost and high performance nano-sized catalysts and have potential application in fuel-cell systems and organic synthesis.

Impact of Mg concentration on energy-band-depth profile of Mg-doped InN epilayers analyzed by hard X-ray photoelectron spectroscopy

The electronic structures of Mg-doped InN (Mg-InN) epilayers with the Mg concentration, [Mg], ranging from 1 × 1019 to 5 × 1019 cm−3 were systematically investigated by soft and hard X-ray photoelectron spectroscopies. The angle-resolved results on the core-level and valence band photoelectron spectra as a function of [Mg] revealed that the energy band of Mg-InN showed downward bending due to the n+ surface electron accumulation and p type layers formed in the bulk. With an increase in [Mg], the energy-band changed from monotonic to two-step n+p homojunction structures. The oxygen concentration rapidly increased at the middle-bulk region (∼4.5 to ∼7.5 nm) from the surface, which was one of the reasons of the transformation of two-step energy band.

Strong Correlation Between Oxygen Donor and Near-Surface Electron Accumulation in Undoped and Mg-Doped In-Polar InN Films

A strong electron accumulation was observed in near-surface regions of undoped and Mg-doped In-polar InN films by analyzing the valence band maximum dependent on the take-off angles. The amount of oxygen correlated with electron carrier concentration drastically increased in both near-surface regions, suggesting that the oxygen atoms in both near-surface regions act as donors. For Mg-doped InN, the amount of oxygen in the near-surface region was almost twice that of undoped InN, indicating that much more oxygen atoms in the near-surface region were incorporated to compensate the Mg acceptors to contribute to the electron accumulation surface layer.

Investigation of the near-surface structures of polar InN films by chemical-state-discriminated hard X-ray photoelectron diffraction

Near-surface structures of polar InN films were investigated by laboratory-based hard X-ray photoelectron diffraction (HXPD) with chemical-state-discrimination. HXPD patterns from In 3d5/2 and N 1s core levels of the In-polar and N-polar InN films were different from each other and compared with the simulation results using a multiple-scattering cluster model. It was found that the near-surface structure of the In-polar InN film was close to the ideal wurtzite structure. On the other hand, on the N-polar InN film, defects-rich surface was formed. In addition, the existence of the In-polar domains was observed in the HXPD patterns.

Structural studies of metal nanoparticles using high-energy x-ray diffraction

The XRD patterns of nanoparticles exhibit broad Bragg peaks because of small size, where the contribution of diffuse component provides us with inherent structural information. Therefore, pair distribution function obtained from a Fourier transformation of high-energy XRD data and structure modeling on the basis of diffraction data becomes an essential tool to understand the structure of nanoparticles. This promising tool was utilized to obtain structural information of Pd/Pt bimetallic core/shell and solid-solution nanoparticles, which show much attention due to their improved hydrogen storage capacity and catalytic activity.

Investigation of the Effect of Oxygen on the Near-Surface Electron Accumulation in Nonpolar m-Plane (101̄0) InN Film by Hard X-ray Photoelectron Spectroscopy

A strong electron accumulation was observed in a near-surface region of an as-grown nonpolar m-plane (100) InN film by analyzing the valence band hard X-ray photoelectron spectra as a function of the take-off angle. In addition, two oxygen chemical states correlated with electron carrier concentration were observed in the O 1s core-level spectra. By comparing with the oxygen concentration in a bulk-like region, the amount of oxygen drastically increased in a near-surface region, suggesting that the oxygen atoms in the near-surface region act as donors to contribute to the near-surface electron accumulation layer.