Brian J. Schultz

Interactions of Aqueous Ag+ with Fulvic Acids: Mechanisms of Silver Nanoparticle Formation and Investigation of Stability

This study investigated the possible natural formation of silver nanoparticles (AgNPs) in Ag(+)-fulvic acid (FA) solutions under various environmentally relevant conditions (temperature, pH, and UV light). Increase in temperature (24-90 °C) and pH (6.1-9.0) of Ag(+)-Suwannee River fulvic acid (SRFA) solutions accelerated the appearance of the characteristic surface plasmon resonance (SPR) of AgNPs. The rate of AgNP formation via reduction of Ag(+) in the presence of different FAs (SRFA, Pahokee Peat fulvic acid, PPFA, Nordic lake fulvic acid, NLFA) and Suwannee River humic acid (SRHA) followed the order NLFA > SRHA > PPFA > SRFA. This order was found to be related to the free radical content of the acids, which was consistent with the proposed mechanism. The same order of AgNP growth was seen upon UV light illumination of Ag(+)-FA and Ag(+)-HA mixtures in moderately hard reconstituted water (MHRW). Stability studies of AgNPs, formed from the interactions of Ag(+)-SRFA, over a period of several months showed that these AgNPs were highly stable with SPR peak reductions of only ~15%. Transmission electron microscopy (TEM) and dynamic light scattering (DLS) measurements revealed bimodal particle size distributions of aged AgNPs. The stable AgNPs formed through the reduction of Ag(+) by fulvic and humic acid fractions of natural organic matter in the environment may be transported over significant distances and might also influence the overall bioavailability and ecotoxicity of AgNPs.

Imaging local electronic corrugations and doped regions in graphene

Electronic structure heterogeneities are ubiquitous in two-dimensional graphene and profoundly impact the transport properties of this material. Here we show the mapping of discrete electronic domains within a single graphene sheet using scanning transmission X-ray microscopy in conjunction with ab initio density functional theory calculations. Scanning transmission X-ray microscopy imaging provides a wealth of detail regarding the extent to which the unoccupied levels of graphene are modified by corrugation, doping and adventitious impurities, as a result of synthesis and processing. Local electronic corrugations, visualized as distortions of the π*cloud, have been imaged alongside inhomogeneously doped regions characterized by distinctive spectral signatures of altered unoccupied density of states. The combination of density functional theory calculations, scanning transmission X-ray microscopy imaging, and in situ near-edge X-ray absorption fine structure spectroscopy experiments also provide resolution of a longstanding debate in the literature regarding the spectral assignments of pre-edge and interlayer states.

An electronic structure perspective of graphene interfaces

The unusual electronic structure of graphene characterized by linear energy dispersion of bands adjacent to the Fermi level underpins its remarkable transport properties. However, for practical device integration, graphene will need to be interfaced with other materials: 2D layered structures, metals (as ad-atoms, nanoparticles, extended surfaces, and patterned metamaterial geometries), dielectrics, organics, or hybrid structures that in turn are constituted from various inorganic or organic components. The structural complexity at these nanoscale interfaces holds much promise for manifestation of novel emergent phenomena and provides a means to modulate the electronic structure of graphene. In this feature article, we review the modifications to the electronic structure of graphene induced upon interfacing with disparate types of materials with an emphasis on iterative learnings from theoretical calculations and electronic spectroscopy (X-ray absorption fine structure (XAFS) spectroscopy, scanning transmission X-ray microscopy (STXM), angle-resolved photoemission spectroscopy (ARPES), and X-ray magnetic circular dichroism (XMCD)). We discuss approaches for engineering and modulating a bandgap in graphene through interfacial hybridization, outline experimental methods for examining electronic structure at interfaces, and overview device implications of engineered interfaces. A unified view of how geometric and electronic structure are correlated at interfaces will provide a rational means for designing heterostructures exhibiting emergent physical phenomena with implications for plasmonics, photonics, spintronics, and engineered polymer and metal matrix composites.

On chemical bonding and electronic structure of graphene–metal contacts

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.

X-ray absorption spectroscopy studies of electronic structure recovery and nitrogen local structure upon thermal reduction of graphene oxide in an ammonia environment

Annealing graphene oxide under an ammonia environment provides a facile approach to defunctionalise this material while simultaneously enabling nitrogen incorporation en route to the preparation of chemically derived graphene. Here, we use X-ray photoemission spectroscopy (XPS) in conjunction with near-edge X-ray absorption fine-structure (NEXAFS) spectroscopy to probe both the global recovery of electronic structure in this material as well as to monitor evolution of the local structure of incorporated nitrogen atoms when graphene oxide is reduced under an ammonia gas environment at ambient and low pressures in the temperature range between 250 and 1000 °C. The local structure and extent of recovery of the π-conjugated framework is correlated to electrical conductivity measurements. Angle-resolved C K-edge NEXAFS spectra along with O K-edge NEXAFS and C 1s high-resolution XPS spectra suggest that hydroxyl and epoxide functional groups on the basal plane of graphene oxide are eliminated upon annealing to a temperature of 250 °C, bringing about substantial restoration of the π-conjugated framework of graphene. Furthermore, an increase in the in-plane orientation of constituent graphene oxide flakes is observed up to a temperature of 750 °C for annealing under both sets of conditions and is manifested as a greater spread in the intensity of the C K-edge π* resonance as a function of angle of incidence of the X-ray beam. Angle-resolved N K-edge NEXAFS spectra and high-resolution N 1s XPS spectra supplement the global view of recovery of π-conjugation with a local perspective of the chemical bonding environments of incorporated nitrogen atoms. Three distinct modes of nitrogen incorporation are evidenced: amine or nitrile like (N1), pyridinic (N2), and substitutional/graphitic (N3). The data suggest that nitrogen is initially incorporated as nitrile like functionalities at lower temperatures with these moieties protruding above and below the graphene basal plane; however, the nitrile and amine groups are subsequently transformed at higher temperatures through the elimination of oxygenated functional groups and reconstitution of the sp2-hybridized network to in-plane pyridinic and graphitic moieties. The latter two configurations are seen to substantially enhance the conductivity of reduced graphene oxide.

Near-edge x-ray absorption fine structure spectroscopy study of nitrogen incorporation in chemically reduced graphene oxide

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.

Two-Dimensional Graphene as a Matrix for MALDI Imaging Mass Spectrometry

AbstractHere, a matrix using two-dimensional (2D) graphene is demonstrated for the first time in the context of MALDI IMS using a Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometer. Although graphene flakes have been used previously in MALDI, it is described here how a single 2D layer of graphene is applied directly on top of rat brain sections and soybean leaves. Several classes of molecules are desorbed and ionized off of the surface of the tissues examined using 2D graphene, with minimal background interference from the matrix. Moreover, no solvents are employed in application of 2D graphene, eliminating the potential for analyte diffusion in liquid droplets during matrix application. Because 2D graphene is an elemental form of carbon, an additional advantage is its high compatibility with the long duration needed for many IMS experiments. Graphical Abstractᅟ

Near-edge x-ray absorption fine structure spectroscopy studies of charge redistribution at graphene/dielectric interfaces

Charge redistribution at graphene/dielectric interfaces is predicated upon the relative positioning of the graphene Fermi level and the charge neutralization level of the dielectric. The authors present an angle-resolved near-edge x-ray absorption fine structure (NEXAFS) spectroscopy investigation of single-layered graphene transferred to 300 nm SiO2/Si with subsequent deposition of ultrathin high-κ dielectric layers to form graphene/dielectric interfaces. The authors’ NEXAFS studies indicate the appearance of a distinct pre-edge absorption for graphene/HfO2 heterostructures (but not for comparable TiO2 and ZrO2 constructs). The hole doping of graphene with substantial redistribution of electron density to the interfacial region is proposed as the origin of the pre-edge feature as electron depletion renders part of the initially occupied density of states accessible for observation via NEXAFS spectroscopy. The spectral assignment is validated by calculating the NEXAFS spectra of electron- and hole-doped g...

Microwave-induced nucleation of conducting graphitic domains on silicon carbide surfaces

Microwave irradiation of the C-rich (0001¯) surface of 6H-SiC is seen to rapidly induce the nucleation of conductive nanoscopic graphitic grains. Discrete graphitic islands are observed and Raman spectroscopy suggests turbostratic stacking with minimal electronic coupling between adjacent graphene layers. Ensemble Raman and near-edge x-ray absorption fine structure (NEXAFS) spectroscopies are used in conjunction with spatially resolved atomic force microscopy, scanning Kelvin probe microscopy (SKPM), and colocalized Raman imaging to characterize the topography and electronic structure of the obtained graphitic domains and to develop a mechanistic description of the nucleation process. SKPM provides a direct spatially resolved means to differentiate conductive graphitic grains from the wide-bandgap SiC semiconductor. NEXAFS spectroscopy allows for evaluation of the planar alignment of the graphitic nuclei. The microwave processing method demonstrated here provides a facile route to patterning conductive do...