Cherno Jaye

New Jersey Institute of Technology, Newark, NJ, United States

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Publications (75)239.61 Total impact

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    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. · 4.81 Impact Factor
  • The Journal of Physical Chemistry C 01/2014; · 4.81 Impact Factor
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    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.
    Langmuir 12/2013; · 4.19 Impact Factor
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    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.
    Nano Letters 09/2013; · 13.03 Impact Factor
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    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.
    Journal of Physical Chemistry Letters 08/2013; 4(18):3144. · 6.59 Impact Factor
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    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.
    J. Mater. Chem. C. 06/2013; 1(26):4114-4121.
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    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.
    Analytical Chemistry 04/2013; · 5.70 Impact Factor
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    ABSTRACT: Single-phase [Ca2CoO3][CoO2]1.61 (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).
    03/2013;
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    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.
    03/2013;
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    ABSTRACT: Self-assembled monolayers (SAMs) of alkyl and fluorinated thiols were investigated after ozonolysis shadow mask treatments with varying areal domains and after exposure to displacing thiols. Imaging near edge X-ray absorption fine structure (NEXAFS) spectroscopy was used as a novel method for investigating the resulting heterogeneous SAM films. Composition and work function were characterized via X-ray and ultraviolet photoelectron spectroscopy, respectively. Imaging NEXAFS characterization of these heterogeneous films clearly shows the displacement of oxidized thiols after ozonolysis patterning. Changes in the SAM after patterning, and after displacement from back-filling thiols, can be inferred from changes in the effective work function. Larger changes in work function were observed for cases where thiols were deposited after ozonolysis suggesting that the displacing SAM was denser and/or more ordered than as-deposited films. These results highlight one of the first demonstrations of imaging NEXAFS and present measurements of effective work function on intentionally heterogeneous SAM films. Copyright © 2013 John Wiley & Sons, Ltd.
    Surface and Interface Analysis 03/2013; · 1.22 Impact Factor
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    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.
    Langmuir 01/2013; · 4.19 Impact Factor
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    Macromolecules 01/2013; 46(1):103-112. · 5.52 Impact Factor
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    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.
    Topics in Catalysis 01/2013; · 2.61 Impact Factor
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    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. · 1.27 Impact Factor
  • Laser Physics Review 01/2013; 3(12). · 10.04 Impact Factor
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    ABSTRACT: The nature of chemical bonding at graphene–metal interfaces is intriguing from a fundamental perspective and has great relevance for contacts to novel spintronics and high-frequency electronic devices. Here, we use near-edge X-ray absorption fine structure (NEXAFS) spectroscopy in conjunction with Raman spectroscopy and first-principles density functional theory to examine chemical bonding and perturbation of the π-electron cloud at graphene–metal interfaces. Graphene–metal bonding has been contrasted for graphene interfaced with single-crystalline metals, polycrystalline metal foils, and with evaporated metal overlayers and is seen to be strongest at the last noted interface. Strong covalent metal-d-graphene-π hybridization and hole doping of graphene is observed upon deposition of Ni and Co metal contacts onto graphene/SiO2 and is significantly stronger for these metals in comparison to Cu. Of single-crystalline substrates, the most commensurate (111) facets exhibit the strongest interactions with the graphene lattice. First-principles electronic structure simulations, validated by direct comparison of simulated spectra with NEXAFS measurements, suggest that metal deposition induces a loss of degeneracy between the α- and β-graphene sublattices and that spin-majority and spin-minority channels are distinctly coupled to graphene, contributing to splitting of the characteristic π* resonance. Finally, the electronic structure of graphene is found to be far less perturbed by metal deposition when the π cloud is pinned to an underlying substrate; this remarkable behaviour of “sandwich” structures has been attributed to electronic accessibility of only one face of graphene and illustrates the potential for anisotropic functionalization.
    Chemical Science 11/2012; 4(1):494-502. · 8.31 Impact Factor
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    ABSTRACT: Temperature dependent electrical resistivity, crystal structure and heat capacity measurements reveal a resistivity drop and electrical transport behavior change corresponding to a structural change near 400 K in Ca(3)Co(4)O(9). The lattice parameter c varies smoothly with increasing temperature while anomalies in a, b(1) and b(2) lattice parameters occur near 400 K. The Ca site in the Ca(2)CoO(3) block becomes distorted and a change in electrical transport behavior is found above 400 K. Resistivity and heat capacity measurements as a function of temperature under magnetic field combined with Co L-edge x-ray absorption spectra reveal only a weak spin contribution to this change. Reduced resistivity associated with the structural change enhances the thermoelectric properties at moderately high temperatures and points to the electrical transport behavior change as a mechanism for improved ZT in this thermoelectric oxide.
    Journal of Physics Condensed Matter 10/2012; 24(45):455602. · 2.36 Impact Factor
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    ABSTRACT: A quartz crystal microbalance (QCM) with a graphene/Ni(111) electrode has been used to probe frictional heating effects in Kr monolayers sliding on the microbalance electrode in response to its oscillatory motion. The temperatures of the sliding Kr monolayers are observed to rise approximately 13 K higher than their static counterparts, but show surprisingly little dependence on oscillation amplitude. Although counterintuitive, the observation can be explained by noting that the Kr surface residence times are limited, which effectively caps how much the temperature can rise.
    Journal of Physics Condensed Matter 10/2012; 24(42):424201. · 2.36 Impact Factor
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    ABSTRACT: In this work, we investigate the ability to tune the quantity of surface amine functional groups in the interfacial region of epoxy-diamine composites using NEXAFS, a technique that is extremely sensitive to surface composition. Thereby, we employ a model surface (silicon wafer with the native oxide present) and, after deposition of an epoxy functionalized silane, we immersed the wafers in various diamines, followed by reaction with a diepoxy acting as a molecular probe. These results show that the number of available surface amines depends on the diamine chosen, wherein smaller molecular weight diamines provide more reaction sites. Subsequent experiments with mixtures of diamines undergoing competitive adsorption show that the amine quantity can be tailored by choice of the diamine mixture. Further experiments of diamine treated 3-(glycidoxypropyl) trimethoxysilane layers in a reacting epoxy/diamine showed that the surface reaction site density differences observed for adsorption experiments persisted in the reacting epoxy, implying that the surface reaction rate (and by extension, the surface amine concentration) dictate interfacial cross-link density up to the point of gelation.
    Langmuir 09/2012; · 4.19 Impact Factor
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    ABSTRACT: Robust methods to tune the unique electronic properties of graphene by chemical modification are in great demand due to the potential of the two dimensional material to impact a range of device applications. Here we show that carbon and nitrogen core-level resonant X-ray spectroscopy is a sensitive probe of chemical bonding and electronic structure of chemical dopants introduced in single-sheet graphene films. In conjunction with density functional theory based calculations, we are able to obtain a detailed picture of bond types and electronic structure in graphene doped with nitrogen at the sub-percent level. We show that different N-bond types, including graphitic, pyridinic, and nitrilic, can exist in a single, dilutely N-doped graphene sheet. We show that these various bond types have profoundly different effects on the carrier concentration, indicating that control over the dopant bond type is a crucial requirement in advancing graphene electronics.
    Nano Letters 06/2012; 12(8):4025-31. · 13.03 Impact Factor

Publication Stats

158 Citations
239.61 Total Impact Points

Institutions

  • 2012
    • New Jersey Institute of Technology
      • Department of Physics
      Newark, NJ, United States
  • 2008–2012
    • National Institute of Standards and Technology
      • • Material Measurement Laboratory (MML)
      • • Materials Science and Engineering Division
      Maryland, United States
  • 2009–2011
    • University at Buffalo, The State University of New York
      • Department of Chemistry
      Buffalo, NY, United States
    • Brookhaven National Laboratory
      New York City, New York, United States
  • 2004–2007
    • North Carolina State University
      Raleigh, North Carolina, United States