[Show abstract][Hide abstract] ABSTRACT: Monodispersed strontium titanate nanoparticles were prepared and studied in
detail. It is found that ~10 nm as-prepared stoichiometric nanoparticles are in
a polar structural state (with possibly ferroelectric properties) over a broad
temperature range. A tetragonal structure, with possible reduction of the
electronic hybridization is found as the particle size is reduced. In the 10 nm
particles, no change in the local Ti-off centering is seen between 20 and 300
K. The results indicate that nanoscale motifs of SrTiO3 may be utilized in data
storage as assembled nano-particle arrays in applications where chemical
stability, temperature stability and low toxicity are critical issues.
[Show abstract][Hide abstract] ABSTRACT: Near-edge x-ray absorption fine structure (NEXAFS) spectroscopy provides detailed information about the orientation and alignment of thin films. NEXAFS is a synchrotron based technique - the availability of beam-time per user is typically limited to no more than a few weeks per year. The limited availability is currently a true barrier for using NEXAFS in combinatorial studies of molecular alignment. We have recently demonstrated how large area full field NEXAFS imaging allows users to pursue combinatorial studies of surface chemistry. Now we report an extension of this approach which allows the acquisition of orientation information from a single NEXAFS image. An array with 80 elements (samples) containing eight series of different surface modifications was mounted on a curved substrate allowing the collection of NEXAFS spectra with a range of orientations with respect to the x-ray beam. Images collected from this array show how hyperspectral NEXAFS data collected from curved surfaces can be used for high-throughput molecular orientation analysis.
[Show abstract][Hide abstract] ABSTRACT: An important and fast-growing technology in the realm of cryogenic detectors has recently been applied to beamline science: the superconducting Transition-Edge Sensor (TES). A TES consists of a thin metal film that is held in the superconducting-to-normal transition by a voltage bias. When a single X-ray photon is absorbed, the temperature and resistance of the film increase, which causes a negative-going current pulse with amplitude proportional to the deposited energy. The thin-film thermometer is typically coupled to a photon absorber composed of a high atomic number element such as bismuth or gold, so efficiencies near unity are possible for X-rays up to 10 keV. TESs are an example of a broader class of sensors called microcalorimeters that measure energetic events through a change in temperature. The energy resolution of a well-designed microcalorimeter is set by thermodynamic power fluctuations between the sensor and the local heat bath as well as by additional noise terms such as the Johnson noise in the sensing element. The energy resolution ΔE of a microcalorimeter is proportional to (kbT2C)1/2 where T is the sensor temperature and C is its specific heat. The temperature dependence of ΔE makes it advantageous to operate TESs at the lowest temperatures that can be conveniently reached, typically 0.1 K. For X-ray measurements between 1–10 keV, the dependence of ΔE on C limits the C of individual sensors to about 10−12 J/K, corresponding to device areas near 0.1 mm2 .
[Show abstract][Hide abstract] ABSTRACT: Carbon nanotubes (CNTs) continue to attract significant interests due to their unique combination of extraordinary thermal, electrical, optical, and mechanical properties. The preparation of CNTs coated with magnetically sensitive Fe2O3 nanoparticles has implications to the development of advanced heat transfer nanofluids and high capacity lithium ion batteries. In this paper, various characterization methods confirm that single-walled carbon nanotubes (SWNTs) were uniformly coated with Fe2O3 nanoparticles through solution mixture. Scanning and transmission electron microscopy were used to compare the morphology of pristine SWNTs and as-prepared SWNTs coated with Fe2O3 nanoparticles. Raman spectroscopy and thermo gravimetric analysis presented the extent of defects and the amount of Fe2O3 nanoparticles present in the sample. Near edge X-ray absorption fine structure spectroscopy was used to probe the electronic band structure of as-prepared core-shell structures. Magnetization measurements indicate that the coercive field of SWNTs coated with Fe2O3 nanoparticles was twice that of pristine SWNTs
ECS Journal of Solid State Science and Technology. 06/2014; 3(8):M39-M44.
[Show abstract][Hide abstract] ABSTRACT: Lithium–sulfur batteries have drawn much attention in advanced energy storage development due to their high theoretical specific capacity; however, several obstacles hinder their applications, including rapid capacity loss due to dissolution of polysulfide into the electrolyte. Nitrogen-doped mesoporous carbon cathode materials were found to effectively immobilize sulfur species and minimize the sulfur loss. In this work, we use X-ray absorption near-edge structure (XANES) spectroscopy to probe the coordination structures of C, O, and N in a carbon cathode before and after the sulfur loading in order to better understand the effects of nitrogen doping. A significant change in oxygen coordination structure is observed, whereas the carbon and nitrogen chemical environments remain unaltered. In addition, the significant change in S K-edge XANES spectra is also observed after sulfur was loaded on nitrogen-doped carbon cathode material. These observations reveal that strong interaction between the nitrogen-doped carbon and sulfur is through oxygen functional groups, and nitrogen doping probably makes oxygen functional groups more reactive toward sulfur.
The Journal of Physical Chemistry C. 04/2014; 118(15):7765–7771.
[Show abstract][Hide abstract] ABSTRACT: NEXAFS spectroscopy was used to investigate the temperature dependence of thermally active ethylene-vinyl acetate | multiwall carbon nanotube (EVA|MWCNT) films. The data shows systematic variations of intensities with increasing temperature. Molecular orbital assignment of interplaying intensities identified the 1s → π*C=C and 1s → π*C=O transitions as the main actors during temperature variation. Furthermore, enhanced near-edge interplay was observed in prestrained composites. Because macroscopic observations confirmed enhanced thermal-mechanical actuation in prestrained composites, our findings suggest that the interplay of C=C and C=O π orbitals may be instrumental to actuation.
The Journal of Physical Chemistry C 02/2014; 118(7):3733-3741. · 4.84 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We explore structural changes of the carbon in MnO2/reduced graphene oxide (RGO) hybrid materials prepared by the direct redox reaction between carbon and permanganate ions (MnO4–) to reach better understanding for the effects of carbon corrosion on carbon loss and its bonding nature during the hybrid material synthesis. In particular, we carried out near-edge X-ray absorption fine structure spectroscopy at the C K-edge (284.2 eV) to show the changes in the electronic structure of RGO. Significantly, the redox reaction between carbon and MnO4– causes both quantitative carbon loss and electronic structural changes upon MnO2 deposition. Such disruptions of carbon bonding have a detrimental effect on the initial electrical properties of the RGO and thus lead to a significant decrease in electrical conductivity. Electrochemical measurements of the MnO2/reduced graphene oxide hybrid materials using a cavity microelectrode revealed unfavorable electrochemical properties that were mainly due to the poor electrical conductivity of the hybrid materials. The results of this study should serve as a useful guide to rationally approaching the syntheses of metal/RGO and metal oxide/RGO hybrid materials.
The Journal of Physical Chemistry C 01/2014; · 4.84 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: This work describes the near conduction band edge structure of electrospun mats of MWCNT-PDMS-PMMA by near edge X-Ray absorption fine structure (NEXAFS) spectroscopy. Effects of adding nanofillers of different sizes were addressed. Despite observed morphological variations and inhomogeneous carbon nanotube distribution, spun mats appeared homogeneous under NEXAFS analysis. Spectra revealed differences in emissions from glancing and normal spectra; which may evidence phase separation within the bulk of the micron-size fibers. Further, dicroic ratios show polymer chains did not align, even in the presence of nanofillers. Addition of nanofillers affected emissions in the C-H, C=O and C-C regimes, suggesting their involvement in interfacial matrix-carbon nanotube bonding. Spectral differences at glancing angles between pristine and composite mats suggest that geometric conformational configurations are taking place between polymeric chains and carbon nanotubes. These differences appear to be carbon nanotube-dimension dependent, and are promoted upon room-temperature mixing and shear flow during electrospinning. CH- π bonding between polymer chains and graphitic walls, as well as H-bonds between impurities in the as-grown CNTs and polymer pendant groups are proposed bonding mechanisms promoting matrix conformation.
[Show abstract][Hide abstract] ABSTRACT: We use scanning tunneling microscopy and x-ray spectroscopy to characterize the atomic and electronic structure of boron-doped and nitrogen-doped graphene created by chemical vapor deposition on copper substrates. Microscopic measurements show that boron, like nitrogen, incorporates into the carbon lattice primarily in the graphitic form and contributes ~0.5 carriers into the graphene sheet per dopant. Density functional theory calculations indicate that boron dopants interact strongly with the underlying substrate while nitrogen dopants do not. The local bonding differences between graphitic boron and nitrogen dopants lead to large scale differences in dopant distribution. The distribution of dopants is observed to be completely random in the case of boron, while nitrogen displays strong sublattice clustering. Structurally, nitrogen-doped graphene is relatively defect-free while boron-doped graphene films show a large number of Stone-Wales defects. These defects create local electronic resonances and cause electronic scattering, but do not electronically dope the graphene film.
[Show abstract][Hide abstract] ABSTRACT: The oxidative chemistry of graphite has been investigated for over 150 years and has attracted renewed interest given the importance of exfoliated graphene oxide as a precursor to chemically derived graphene. However, the bond connectivities, steric orientations, and spatial distribution of functional groups remain to be unequivocally determined for this highly inhomogeneous nonstoichiometric material. Here, we demonstrate the application of principal component analysis to scanning transmission X-ray microscopy data for the construction of detailed real space chemical maps of graphene oxide. These chemical maps indicate very distinct functionalization motifs at the edges and interiors and, in conjunction with angle-resolved near-edge X-ray absorption fine structure spectroscopy, enable determination of the spatial location and orientations of functional groups. Chemical imaging of graphene oxide provides experimental validation of the modified Lerf–Klinowski structural model. Specifically, we note increased contributions from carboxylic acid moieties at edge sites with epoxide and hydroxyl species dominant within the interior domains.
[Show abstract][Hide abstract] ABSTRACT: Resistivity and Seebeck coefficient measurements on Ca3Co4−xFexO9 (x = 0, 0.05, 0.1, 0.2 and 0.25) reveal enhanced thermoelectric performance with an optimal x value of 0.2. X-ray diffraction measurements show continuous Fe doping into the host lattice, while X-ray absorption experiments reveal that Fe substitutes for Co in the Ca2CoO3 (rock salt) block. The Fe substitution for Co produces electron doping. The local structure around Fe in the Ca2CoO3 block becomes disordered, while the structure in the conducting CoO2 layer becomes more ordered. The structural change in the CoO2 layer plays the key role to enhance the electron transport. The highest ordered structure is achieved at x = 0.2 with the lowest resistivity. Soft X-ray absorption measurements find no Co site spin-state change with Fe doping. Thermoelectric property enhancement associated with doping induced structural change points to a new approach for creating materials with improved ZT in complex oxide systems.
[Show abstract][Hide abstract] ABSTRACT: Near-edge x-ray absorption fine structure (NEXAFS) spectroscopy, as a technique, offers detailed information about the bonding environment of molecules at a surface. However, because it is a synchrotron based method beam-time is limited and users must typically prioritize and narrow the scopes of experiments. In this study we demonstrate a novel method that opens up the possibility of using large area NEXAFS imaging to pursue combinatorial studies. To explore the capabilities of the NIST full field NEXAFS microscope available at the National Synchrotron Light Source as a high throughput imaging instrument, we collected NEXAFS images from a sample array consisting of 144 different elements with a periodic sequence of different surface modifications. NEXAFS images collected from this model system illustrate how hyperspectral NEXAFS data can be used for parallel analysis of large numbers of samples either directly from the overall image or by extracting spectra from regions of interest.
[Show abstract][Hide abstract] ABSTRACT: Single-phase
(Ca3Co4O9) materials doped by
transition metals were prepared by solid state reaction followed by
annealing under oxygen. The temperature dependent thermoelectric
properties, including resistivity (ρ), Seebeck coefficient (S) and
thermal conductivity (κ), were measured. In order to understand
the origin of the changes in ZT with doping, local (XAS) and long range
(XRD) structural measurements as a function of doping were conducted.
The electronic properties were probed by x-ray spectroscopic methods.
Identification of the locations of the dopant sites and the impact on ZT
will be discussed. This work is supported by DOE Grant
DE-FG02-07ER46402. The Physical Properties Measurements System was
acquired under NSF MRI Grant DMR-0923032 (ARRA award).
[Show abstract][Hide abstract] ABSTRACT: Bias stress, during which a reduction in source-drain current is
observed under continuous application of gate voltage in organic
thin-film transistors, originates from trapped mobile charges. Organic
semiconductors often exhibit tail states that extend into their band
gap; these tail states can act as traps to immobilize charge.
Alternatively, defects at the organic semiconductor-dielectric interface
can also trap charge. Whether bias stress originates from impurities or
defects in the bulk of the organic semiconductor or at the organic
semiconductor-dielectric interface, however, remains unclear. By
building and testing organic single-carrier diodes having different
active layer thicknesses, we can infer the trapping contributions in the
bulk of the organic semiconductor relative to those at the organic
semiconductor-electrode interface. In conjunction with device
characteristics of organic thin-film transistors having different
dielectrics, we found that the broad distribution of tail states that is
present in poly(3-hexyl thiophene), P3HT, is responsible for bias stress
in P3HT-comprising devices. On the other hand, traps at the
[6,6]-phenyl-C61-butyric acid methyl ester, PCBM,-dielectric interface
are more dominant than those in the bulk in PCBM-containing devices.
[Show abstract][Hide abstract] ABSTRACT: We have used flip chip lamination (FCL) to form monolayer and bilayer molecular junctions of carboxylic acid-containing molecules with Cu atom incorporation. Carboxylic acid-terminated monolayers are self-assembled onto ultrasmooth Au by using thiol chemistry and grafted onto n-type Si. Prior to junction formation, monolayers are physically characterized by using polarized infrared absorption spectroscopy, X-ray photoelectron spectroscopy, and near-edge X-ray absorption fine structure spectroscopy, confirming the molecular quality and functional group termination. FCL was used to form monolayer junctions onto H-terminated Si or bilayer junctions of carboxylic acid monolayers on Au and Si. From the electrical measurements, we find that the current through the junction is attenuated as the effective molecular length within the junction increases, indicating that molecules are electrically active within the junction. We find that the electronic transport through the bilayer junction saturates at very thick effective distances possibly because of another electron-transport mechanism that is not nonresonant tunneling as a result of trapped defects or sequential tunneling. In addition, bilayer junctions are fabricated with and without Cu atoms, and we find that the electron transport is not distinguishably different when Cu atoms are within the bilayer.
[Show abstract][Hide abstract] ABSTRACT: A series of well-defined perfluoroalkyl end-functionalized poly(3-hexylthiophenes) (P3HT) were synthesized by Stille coupling of stannylated 2-perfluoralkylthiophene with the bromine end of P3HT. The length of the perfluoroalkyl end group was varied from −C4F13 to −C8F17. These polymers were fully characterized and tested in bulk heterojunction solar cells with phenyl-C61-butyric acid methyl ester (PCBM) as the acceptor. Performance of the solar cells was highest for the unmodified P3HT and decreased as the length of the perfluoroalkyl end increased. The most affected device parameters were the short-circuit current density (Jsc) and series resistance, pointing to lower charge carrier mobility and poor morphology as the cause for the lower performance. While the morphology of blends did not significantly change with perfluoroalkyl end modification, analysis of blended films by energy-filtered transmission electron microscopy (EF-TEM) revealed wider P3HT domains, consistent with the perfluorinated end groups segregating to the edge or exterior of P3HT domains, causing two domains to join. This study demonstrates that the perfluoroalkyl end group can be detrimental to polymer solar cell device performance, and further work toward understanding the interface between the donor and acceptor phases is required to fully understand this effect.
[Show abstract][Hide abstract] ABSTRACT: As the loadings of precious metals in surface-chemical systems continue to decrease for photo-and electro-catalysts for energy and environmental applications, the study of near-surface electronic and atomic structure in functional materials becomes critically important. Extremely small quantities of active elements, whether grown as clusters or ultrathin films, exhibit changes in catalytic activity that arise from both size effects and electron-transfer effects. These size and transfer effects can be related to increased propensity for oxidation of the metallic deposit, as well as to various changes in electrochemical performance such as durability or required overpotential for a given reaction. This work establishes a minimum threshold for Pt loading beyond which bulk-type electronic behavior may be expected. By iteratively growing atomic monolayers and multilayers using self-limited electrodeposition and studying these films using core-electron spectroscopy (X-ray absorption and X-ray photoelectron spectroscopy), electrochemical methods and DFT-based computations the fundamental interactions that govern oxidation state and electron transfer near the surface of a Pt–Au bimetallic system have been explored. It has been shown that the Pt–Au system exhibits increased tendency for the Pt layer to remain cationic below a minimum threshold film thickness of two monolayers. At monodispersed levels of submonolayer coverage Pt exhibits deviated electronic structure, reactivity, and metal stability compared to films in excess of this minimum threshold thickness. At three monolayers Pt is thick enough to avoid the preference for cationicity and the resulting higher rates of metal dissolution, but thin enough to benefit from electron transfers from Au that assist in lowering the overpotentials for CO oxidation. This study shows the efficacy of a concerted method for the investigation of near-surface phenomena in multicomponent systems. By combining electrochemical and vacuum studies of solute-derived samples with advanced computational techniques, a multifaceted understanding of these architectures has been achieved.
[Show abstract][Hide abstract] ABSTRACT: The chemical reduction of exfoliated graphene oxide (GO) has gained widespread acceptance as a scalable route for the preparation of chemically derived graphene albeit with remnant topological defects and residual functional groups that preclude realization of the conductance of single-layered graphene. Reduction of GO with hydrazine is substantially effective in restoring the π-conjugated framework of graphene and leads to about a five-to-six orders of magnitude decrease of sheet resistance, but has also been found to result in incidental nitrogen incorporation. Here, the authors use a combination of x-ray photoelectron spectroscopy (XPS) and C, O, and N K-edge near-edge x-ray absorption fine structure (NEXAFS) spectroscopy to examine the local geometric and electronic structure of the incorporated nitrogen species. Both NEXAFS and XPS data suggest substantial recovery of the sp2-hybridized graphene framework upon chemical reduction and removal of epoxide, ketone, hydroxyl, and carboxylic acid species. Two distinct types of nitrogen atoms with pyridinic and pyrrolic character are identified in reduced graphene oxide. The N K-edge NEXAFS spectra suggest that the nitrogen atoms are stabilized within aromatic heterocycles such as pyrazole rings, which has been further corroborated by comparison to standards. The pyrazole fragments are thought to be stabilized by reaction of diketo groups on the edges of graphene sheets with hydrazine. The incorporation of nitrogen within reduced graphene oxide thus leads to local bonding configurations very distinct from substitutional doping observed for graphene grown by chemical vapor deposition in the presence of NH3.
Journal of vacuum science & technology. B, Microelectronics and nanometer structures: processing, measurement, and phenomena: an official journal of the American Vacuum Society 01/2013; 31(4):041204-041204-9. · 1.36 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The systematic insertion of thin films of P3HT and PCBM at the electron- and hole-collecting interfaces, respectively, in bulk-heterojunction polymer solar cells results in different extents of reduction in device characteristics, with the insertion of P3HT at the electron-collecting interface being less disruptive to the output currents compared to the insertion of PCBM at the hole-collecting interface. This asymmetry is attributed to differences in the tail state-assisted charge injection and recombination at the active layer-electrode interfaces. P3HT exhibits a higher density of tail states compared to PCBM; holes in these tail states can thus easily recombine with electrons at the electron-collection interface during device operation. This process is subsequently compensated by the injection of holes from the cathode into these tail states, which collectively enables net current flow through the polymer solar cell. The study presented herein thus provides a plausible explanation for why preferential segregation of P3HT to the cathode interface is inconsequential to device characteristics in P3HT:PCBM bulk-heterojunction solar cells.