Publications (20)58.61 Total impact
-
Article: Soft X-ray characterization of Zn(1-x)Sn(x)O(y) electronic structure for thin film photovoltaics.
[show abstract] [hide abstract]
ABSTRACT: Zinc tin oxide (Zn(1-x)Sn(x)O(y)) has been proposed as an alternative buffer layer material to the toxic, and light narrow-bandgap CdS layer in CuIn(1-x),Ga(x)Se(2) thin film solar cell modules. In this present study, synchrotron-based soft X-ray absorption and emission spectroscopies have been employed to probe the densities of states of intrinsic ZnO, Zn(1-x)Sn(x)O(y) and SnO(x) thin films grown by atomic layer deposition. A distinct variation in the bandgap is observed with increasing Sn concentration, which has been confirmed independently by combined ellipsometry-reflectometry measurements. These data correlate directly to the open circuit potentials of corresponding solar cells, indicating that the buffer layer composition is associated with a modification of the band discontinuity at the CIGS interface. Resonantly excited emission spectra, which express the admixture of unoccupied O 2p with Zn 3d, 4s, and 4p states, reveal a strong suppression in the hybridization between the O 2p conduction band and the Zn 3d valence band with increasing Sn concentration.Physical Chemistry Chemical Physics 06/2012; 14(29):10154-9. · 3.57 Impact Factor -
Article: Electronic structure of room-temperature ferromagnetic Mg1−xFexOy thin films
[show abstract] [hide abstract]
ABSTRACT: We present herein a soft x-ray spectroscopy study on the electronic and magnetic properties of Mg1�xFexOy thin films. A distinct shoulder feature on the absorption edge reflecting the unoccupied oxygen 2p states is evident in the intrinsic thin films, which diminishes upon Fe doping, while a pre-edge absorption feature (reflecting the O 2p-Fe 3d acceptor state) evolves with the same. Our findings demonstrate the reduction in the intrinsic holes as a result of charge-transfer hole doping. All the thin films display room-temperature ferromagnetism, and the saturation magnetization is found to increase from ca. 0.70-4.34 emu/cm3 on 7 at. % Fe doping.Applied Physics Letters 01/2012; 101:082411. · 3.84 Impact Factor -
Article: X-ray absorption, photoemission spectroscopy, and Raman scattering analysis of amorphous tantalum oxide with a large extent of oxygen nonstoichiometry.
[show abstract] [hide abstract]
ABSTRACT: The electronic structure and modification of the local interatomic structure of a reactive sputtered amorphous tantalum oxide (a-TaO(x)) thin film with the variation of oxygen nonstoichiometry, x in a-TaO(x) have been investigated by X-ray absorption spectroscopy (XAS), X-ray photoemission spectroscopy (XPS), Raman scattering spectroscopy, and Rutherford back scattering spectroscopy. A parallel chemical shift of Ta4f(7/2) and O1s core levels observed with the variation of x indicates the Fermi level shift by reduction and oxidation in the framework of the rigid band model. Extended X-ray absorption fine structure (EXAFS) suggests both the increase of average coordination number of the first Ta-O shell in polyhedra and a considerable reduction of the average Ta-O bond length with the increase of x. The relative intensity of Raman shift peaks at 670 cm(-1) and 815 cm(-1), corresponding to Ta-O stretching of TaO(6) octahedra and TaO(5) probably with a pyramidal form, respectively, drastically changes between x = 2.47 to 1.86, suggesting the change in the predominant polyhedron from TaO(6) to TaO(5) with a modification in multiplicity of oxygen by the reorganization of the polyhedral network.Physical Chemistry Chemical Physics 08/2011; 13(38):17013-8. · 3.57 Impact Factor -
Article: Electron enrichment in 3d transition metal oxide hetero-nanostructures.
[show abstract] [hide abstract]
ABSTRACT: Direct experimental observation of spontaneous electron enrichment of metal d orbitals in a new transition metal oxide heterostructure with nanoscale dimensionality is reported. Aqueous chemical synthesis and vapor phase deposition are combined to fabricate oriented arrays of high-interfacial-area hetero-nanostructures comprised of titanium oxide and iron oxide nanomaterials. Synchrotron-based soft X-ray spectroscopy techniques with high spectral resolution are utilized to directly probe the titanium and oxygen orbital character of the interfacial region's occupied and unoccupied densities of states. These data demonstrate the interface to possess electrons in Ti 3d bands and an emergent degree of orbital hybridization that is absent in parent oxide reference crystals. The carrier dynamics of the hetero-nanostructures are studied by ultrafast transient absorption spectroscopy, which reveals the presence of a dense manifold of states, the relaxations from which exhibit multiple exponential decays whose magnitudes depend on their energetic positions within the electronic structure.Nano Letters 08/2011; 11(9):3855-61. · 13.20 Impact Factor -
Article: Mechanism of light emission and electronic properties of a Eu3+-doped Bi2SrTa2O9 system determined by coupled X-ray absorption and emission spectroscopy
Journal of Materials Chemistry 07/2011; · 5.97 Impact Factor -
Article: In-situ X-ray absorption study of evolution of oxidation states and structure of cobalt in Co and CoPt bimetallic nanoparticles (4 nm) under reducing (H2) and oxidizing (O2) environments.
[show abstract] [hide abstract]
ABSTRACT: In-situ near edge X-ray absorption fine structure spectroscopy was performed to monitor the oxidation states of Co and CoPt nanoparticles (NPs) of 4 nm size in the presence of H(2) and O(2) in the pressure range of 1 bar and 36 Torr respectively. Platinum helps the rapid reduction of cobalt oxides in hydrogen at a rather low temperature (38 °C). In addition, reversible changes of the oxidation states of cobalt in the Co and CoPt NPs as a function of cycling oxygen pressure (in the range of millitorr to 36 Torr) are quantified and compared. The role of Pt in the process of Co reducing and oxidizing was explored. Our findings permit the prediction of the cobalt oxidation states as the reaction conditions are altered. The experimental results also suggest the presence of tetrahedral structure of Cobalt oxide that differs from the Co(3)O(4) spinel structure.Nano Letters 02/2011; 11(2):847-53. · 13.20 Impact Factor -
Chapter: Soft X‐Ray Absorption and Emission Spectroscopy in the Studies of Nanomaterials
11/2010: pages 211 - 254; , ISBN: 9783527632282 -
Chapter: Electronic Structure Study of Nanostructured Transition Metal Oxides Using Soft X‐Ray Spectroscopy
[show abstract] [hide abstract]
ABSTRACT: In this chapter, we introduce the basics of soft x-ray absorption (XAS) and emission spectroscopy (XES), and resonant inelastic soft x-ray scattering (RIXS) followed by description of instrumentation including beamline, ensdtation, and spectrometer. Chemical cells are designed for in-situ electronic structure study of samples in gas or liquid environment. The application of XAS, XES, and RIXS on TiO2 crystals of rutile and anatase phases have yielded characteristic fingerprints that provide the information on geometric structure, bandgap, doping effects. A number of in-situ electronic structure studies are also addressed.03/2010: pages 123 - 142; , ISBN: 9780470823996 -
Article: Thickness-dependent electronic structure of intermetallic CeCo2 nanothin films studied by X-ray absorption spectroscopy.
[show abstract] [hide abstract]
ABSTRACT: We report the electronic structure study of intermetallic CeCo2 nanothin films of various thicknesses by X-ray absorption near-edge structure (XANES) spectroscopy at Ce L3-, Co K-, and L2,3-edges. The Ce L3-edge absorption spectra reveal that the contribution of tetravalent Ce component increases with the film thickness, and all investigated nanothin films exhibit intermediate valence nature. Variation of the spectral intensities observed at the Co K-edge threshold implies modification in the Co 3d states and the enhancement of 3d-4f-5d hybridization. The Co 3d and Ce 4f occupation numbers were estimated from these spectroscopic results. The present study brings out how the surface-to-bulk ratio and the charge transfer between Ce and Co ions affect the electronic structure of nanothin films.Langmuir 07/2009; 25(13):7568-72. · 4.19 Impact Factor -
Article: A Self-Templated Route to Hollow Silica Microspheres
[show abstract] [hide abstract]
ABSTRACT: A simple, mild, and effective self-templated approach has been developed to directly convert solid SiO2 microspheres into hollow structures. The reaction involves initial partial dissolution of silica cores in a NaBH4 solution and subsequent shell formation due to the redeposition of the silicate species back onto the colloid surfaces. The increasing concentration of NaBO2 as the result of the slow decomposition of NaBH4 in water is found to be responsible for the regrowth of the silica shell. This method allows the production of hollow silica spheres with sizes ranging from 70 nanometers to several micrometers, largely determined by the size of the starting silica colloids. The solid-to-hollow transformation mechanism is investigated in detail by transmission electron microscopy (TEM), scanning electron microscopy (SEM), Fourier Transform Infrared (FTIR) spectrometry, X-ray absorption spectroscopy (XAS), N2 adsorption−desorption, and X-ray diffraction (XRD). We also study the reaction conditions that allow control over the wall thickness, surface morphology, and shell porosity.02/2009; -
Article: Structural conformation in a poly(ethylene oxide) film determined by X-ray emission spectroscopy.
[show abstract] [hide abstract]
ABSTRACT: The electronic structure of pol(ethylene oxide) (PEO) in a thin (<1 mu) film sample was experimentally probed by X-ray emission spectroscopy. Both nonresonant and resonant X-ray emission spectra were simulated by using density functional theory (DFT) applied to four different models representing different conformations in the polymer. Calculated spectra were compared with experimental results for the PEO film. It was found that the best fit was obtained with the polymer conformation in PEO electrolytes from which the salt (LiMF6, M = P, As, or Sb) had been removed. This conformation is different from the crystalline bulk polymer and implies that film casting, commonly used to form electrolytes for Li polymer batteries, induces the same conformation in the polymer not depending upon the presence of salt.The Journal of Physical Chemistry B 10/2007; 111(40):11658-61. · 3.70 Impact Factor -
Article: High Resolution Photon Counting Detection System for Advanced Inelastic X-Ray Scattering Studies
[show abstract] [hide abstract]
ABSTRACT: High brilliance and high spectral resolution of synchrotron sources and associated optics enable a large number of soft-X-ray spectroscopic studies such as inelastic X-ray scattering, which is becoming a technique of choice for the investigation of the electronic properties of complex materials. The resolution of the detection system in such experiments has to match the accuracy of the probe beam in order to take the full advantage of the performance of modern synchrotron sources. In this paper we describe our advanced photon counting detection system capable of simultaneously registering both position and time of individual photons with 2-dimensional spatial accuracy of < 50 mum and timing accuracy of < 130 ps FWHM. The open face, 25 mm active area detector consists of a Z-stack of microchannel plates and a cross delay line readout and has a dark count rate of only a few counts per second. The associated electronics allows event counting rates up to ~400 KHz with 10% dead time for randomly events. We present the results of our first measurements of delayed fluorescence from different materials performed at the Advanced Light Source. Time and angular resolved fluorescence measurements allowed us to separate images for the prompt elastically scattered and the delayed photons. The detector can also distinguish registration of electrons, ions or photons by variation of the potential on its input mesh. These results demonstrate the capabilities of our detection system, which is currently being integrated into an advanced time resolved X-ray emission spectroscopy system.IEEE Transactions on Nuclear Science 07/2007; · 1.45 Impact Factor -
Article: One-step synthesis of highly water-soluble magnetite colloidal nanocrystals.
[show abstract] [hide abstract]
ABSTRACT: A high-temperature solution-phase hydrolysis approach has been developed for the synthesis of colloidal magnetite nanocrystals with well-controlled size and size distribution, high crystallinity, and high water solubility. The synthesis was accomplished by the hydrolysis and reduction of iron(III) cations in diethylene glycol with a rapidly injected solution of sodium hydroxide at an elevated temperature. The high reaction temperature allows for control over size and size distribution and yields highly crystalline products. The superior water solubility is achieved by using a polyelectrolyte, that is, poly(acrylic acid) as the capping agent, the carboxylate groups of which partially bind to the nanocrystal surface and partially extend into the surrounding water. The direct synthesis of water-soluble nanocrystals eliminates the need for additional surface modification steps which are usually required for treating hydrophobic nanocrystals produced in nonpolar solvents through the widely recognized pyrolysis route. The abundant carboxylate groups on the nanocrystal surface allow further modifications, such as bioconjugation, as demonstrated by linking cysteamine to the particle surface. The monodisperse, highly water-soluble, superparamagnetic, and biocompatible magnetite nanocrystals should find immediate important biomedical applications.Chemistry 02/2007; 13(25):7153-61. · 5.93 Impact Factor -
Article: Soft-x-ray spectroscopy probes nanomaterial-based devices
Spie Newsroom. 01/2007; -
Conference Proceeding: High Resolution Photon Counting Detection System for Advanced Inelastic X-Ray Scattering Studies
[show abstract] [hide abstract]
ABSTRACT: High brilliance and high spectral resolution of synchrotron sources enable a large number of soft-X-ray spectroscopic studies such as inelastic X-ray scattering, which is becoming a technique of choice for the investigation of the electronic properties of complex materials. The resolution of the detection system in such experiments has to match the accuracy of the probe beam in order to take the full advantage of the performance of modern synchrotron sources. In this paper we describe our advanced photon counting detection system capable of simultaneously registering both position and time of individual photons with 2-dimensional spatial accuracy of <50 mum and timing accuracy of <130 ps FWHM. The open face, 25 mm active area detector consists of a Z-stack of microchannel plates and a cross delay line readout and has dark count of only a few counts per second. The associated electronics allows event counting rates up to ~400 KHz with 10% dead time for randomly distributed events. We present the results of our first measurements of delayed fluorescence from different materials performed at the Advanced Light Source. Time and angular resolved fluorescence measurements allowed us to separate images for the prompt elastically scattered and the delayed photons. The detector can also distinguish registration of electrons, ions or photons by variation of the potential on its input mesh. These results demonstrate the capabilities of our detection system, which is currently being integrated into an advanced time resolved X-ray emission spectroscopy system.Nuclear Science Symposium Conference Record, 2006. IEEE; 12/2006 -
Article: 3D Highly Oriented Nanoparticulate and Microparticulate Array of Metal Oxide Materials
[show abstract] [hide abstract]
ABSTRACT: Advanced nano and micro particulate thin films of 3d transition and post-transition metal oxides consisting of nanorods and microrods with parallel and perpendicular orientation with respect to the substrate normal, have been successfully grown onto various substrates by heteronucleation, without template and/or surfactant, from the aqueous condensation of solution of metal salts or metal complexes (aqueous chemical growth). Three-dimensional arrays of iron oxide nanorods and zinc oxide nanorods with parallel and perpendicular orientation are presented as well as the oxygen K-edge polarization dependent x-ray absorption spectroscopy (XAS) study of anisotropic perpendicularly oriented microrod array of ZnO performed at synchrotron radiation source facility.MRS Proceedings. 12/2000; 704. -
Article: Purpose-Built Anisotropic Metal Oxide Nanomaterials
[show abstract] [hide abstract]
ABSTRACT: Large arrays of perpendicularly oriented anisotropic nanoparticles of ferric oxyhydroxide (Akaganeite, β-FeOOH) and oxide (Hematite, α-Fe2O3) of typically 3-5 nm in diameter, self-assembled as bundles of about 50 nm in diameter and of up to 1 μm in length have been successfully grown onto polycrystalline substrates without template and/or surfactant by heteronucleation from an aqueous solution of ferric salts and their optical and electronic properties investigated.MRS Proceedings. 12/1999; 635. -
Article: Electron delocalization in cyanide-bridged coordination polymer electrodes for Li-ion batteries studied by soft x-ray absorption spectroscopy
[show abstract] [hide abstract]
ABSTRACT: The electronic structure change during the reversible Li-ion storage reaction in a bimetallic MnFe-Prussian blue analogue (Li(x)K(0.14)Mn(1.43)[Fe(CN)(6)] center dot 6H(2)O) was investigated by soft x-ray absorption spectroscopy. The Mn L(2,3)-edgespectra revealed the unchanged Mn(2+) high-spin state regardless of Li-ion concentration (x). On the other hand, the Fe L(2,3)-edge spectra clearly revealed a reversible redox behavior as Fe(3+) <-> Fe(2+) states with Li-ion insertion/extraction. Experimental findings suggested strong metal-to-ligand charge transfer in conjunction with the ligand-to-metal one. The resulting charge delocalization between the Fe and CN is considered to contribute to the high reversibility of the Li-ion storage process.PHYSICAL REVIEW B. 84(4). -
Article: Electronic structure of cobalt nanocrystals suspended in liquid
[show abstract] [hide abstract]
ABSTRACT: The electronic structure of cobalt nanocrystals suspended in liquid as a function of size has been investigated using in situ X-ray absorption and emission spectroscopy. A sharp absorption peak associated with the ligand molecules is found that increases in intensity upon reducing the nanocrystal size. X-ray Raman features due to d-d and to charge-transfer excitations of ligand molecules are identified. The study reveals the local symmetry of the surface of epsilon-Co phase nanocrystals, which originates from a dynamic interaction between Co nanocrystals and surfactant + solvent molecules.NANO LETTERS. 7(7):1919-1922. -
Article: TiO_ {2}-SnO_ {2}: F interfacial electronic structure investigated by soft x-ray absorption spectroscopy
[show abstract] [hide abstract]
ABSTRACT: The electronic structure of the titanium dioxide (TiO2)-fluorine-doped tin dioxide (SnO2:F) interface is investigated by soft x-ray absorption spectroscopy using synchrotron radiation. The measurements probe the site- and symmetry-selected unoccupied density of states and reflect the interaction between an early transition-metal-oxide (d0) semiconductor and a post-transition-metal-oxide (d10) degenerate semiconductor. The distinct interfacial electronic structure of TiO2-SnO2:F is established by contrasting spectra with those for anatase and rutile TiO2, SnO2:F, and ZnO-SnO2:F and CdO-SnO2:F interfaces. Oxygen 1s absorption spectra, which relate to the O 2p partial density of states of the conduction band, indicate that the interface is associated with a reduction in Ti d-O p orbital hybridization and an alteration of the TiO2 crystal field. These observations are consistent with measured titanium 2p absorption spectra, which in addition provide evidence for distortion of long-range order around the cation site in the interfacial TiO2. The TiO2-SnO2:F interface is a functional component of a number of optoelectronic devices, perhaps most notably within the anode structure of solar cell architectures. In nonequilibrium conditions, such as those found in operating solar cells, interfacial electronic structure directly influences performance by modifying, for instance, the quasi-Fermi level electrons and the potential distribution at the transparent electrode.Phys. Rev. B. 85(12).
Top co-authors
Top Journals
Institutions
-
2007–2012
-
Lawrence Berkeley National Laboratory
- Advanced Light Source Facility
Berkeley, CA, USA
-
-
2010
-
National Institute for Materials Science
Tsukuba, Ibaraki-ken, Japan
-
-
2009
-
Academia Sinica
- Institute of Physics
Taipei, Taipei, Taiwan
-
-
1999–2000
-
Uppsala University
Uppsala, Uppsala, Sweden
-
