L.L. Kesmodel

Chemically homogeneous and thermally reversible oxidation of epitaxial graphene

With its exceptional charge mobility, graphene holds great promise for applications in next-generation electronics. In an effort to tailor its properties and interfacial characteristics, the chemical functionalization of graphene is being actively pursued. The oxidation of graphene via the Hummers method is most widely used in current studies, although the chemical inhomogeneity and irreversibility of the resulting graphene oxide compromises its use in high-performance devices. Here, we present an alternative approach for oxidizing epitaxial graphene using atomic oxygen in ultrahigh vacuum. Atomic-resolution characterization with scanning tunnelling microscopy is quantitatively compared to density functional theory, showing that ultrahigh-vacuum oxidization results in uniform epoxy functionalization. Furthermore, this oxidation is shown to be fully reversible at temperatures as low as 260 °C using scanning tunnelling microscopy and spectroscopic techniques. In this manner, ultrahigh-vacuum oxidation overcomes the limitations of Hummers-method graphene oxide, thus creating new opportunities for the study and application of chemically functionalized graphene.

Vibrational Spectroscopy of Acetylene Decomposition on Palladium (111) and (100) Surfaces.

Abstract Vibrational spectra for thermal decomposition of acetylene (C 2 H 2 ) on (111) and (100) surfaces of palladium obtained with high-resolution electron energy loss spectroscopy (EELS) are described. Of particular interest is the formation of CCH species on both surfaces, which are identified spectroscopically in this context for the first time. On Pd(110), CCH forms with thermal processing of chemisorbed acetylene to ∼ 400 K but on Pd(111) is co-adsorbed with a CCH 3 species following 300 K C 2 H 2 adsorption. With further annealing to 450 K, CCH is the dominant species on both surfaces. The EEL spectra indicate that both carbon atoms are involved in the CCH interaction with the surface as implied by a C-C bond order between 1 and 2. In view of recent reports of acetylene trimerization to benzene (C 6 H 6 ) on Pd surfaces, the EEL spectra of C 6 H 6 and isotopic mixtures were also analyzed but no evidence for this reaction was found.

Ethylene adsorption and reaction on Pd(111): An angle-dependent EELS analysis

Abstract : The chemisorption and surface reactivity of ethylene on a palladium (111) surface near room temperature has been studied by high-resolution electron energy loss spectroscopy (EELS). The C2H4 molecule reacts with the Pd(111) surface to form a rather stable surface species which is identified as ethylidyne (C(3+)-CH3) by analysis of the EELS vibrational spectra. This molecule is also formed on Pt(111) and Rh(111) and is believed to be symmetrically bonded at threefold surface sites. Angle-dependent EELS spectra are reported and show the relative importance of impact and dipole excitation mechanisms Off-specular data is found to be of critical importance in drawing conclusions about surface bonding symmetry from EELS spectra.

EELS analysis of the low temperature phase of ethylene chemisorbed on Pd(111)

The chemisorption of C2H4 and C2D4 on Pd(111) at 150 K has been studied by high resolution electron energy loss spectroscopy. Analysis of the vibrational spectra indicates that (i) C2H4 is more weakly bound on Pd(111) than on Ni(111) and Pt(111) and (ii) softened and broadened CH stretching frequencies suggest hydrogen bond-like interactions between the molecule and the metal surface.

Adsorption and bonding of first layer and bilayer terephthalic acid on the Cu(100) surface by high-resolution electron energy loss spectroscopy.

The self-assembled and highly ordered first layer of terephthalic acid on Cu(100) as well as its bilayer on the same surface are studied here using high-resolution electron energy loss spectroscopy (HREELS), Auger electron spectroscopy, and low energy electron diffraction. These experiments show completion of the first layer before growth of the second layer. HREELS measurements show that the first layer of the acid deprotonates, which is seen in the absence of the OH stretching mode for the acid groups. However, this mode is present in the bilayer structure, confirming that the deprotonation is due to a reaction with the Cu surface and suggesting that there is little mixing of the layers. It has been suggested previously that the TPA monolayer structure is stabilized by an intermolecular hydrogen bonding interaction, but we are not able to resolve any distortion of the CH stretching mode for such an interaction, but instead see evidence for direct bonding to the Cu surface.

High‐resolution electron energy loss spectroscopy and infrared spectroscopy of polymer surfaces: High‐resolution and orientation effects of polytetrafluoroethylene films

The vibrational structure of oriented polytetrafluoroethylene has been studied using high‐resolution electron energy loss spectroscopy (HREELS) and infrared spectroscopy. Record‐high resolutions of 6–15 cm−1 in HREELS with a state‐of‐the‐art spectrometer are reported. Selection rules and orientation effects probed by the two different spectroscopies are discussed and comparisons are made between surface and bulk structure. Resonance electron scattering in HREELS has been used to study orientation effects of an intense Raman‐active ν(C–C) mode at 730 cm−1. The resonance seen in HREELS is identified as the σ*(C–C) state. HREELS results have been related to near‐edge x‐ray absorption fine structure results. We discuss several future improvements that need to be addressed if ultrahigh resolution EELS is to become a more applicable and powerful tool for the study of polymer surfaces.

High resolution EELS measurements of CO adsorption on Pt/Al2O3 model catalysts via impact scattering

Abstract The use of high resolution electron energy loss spectroscopy (EELS) to study adsorption on model supported metal catalysts is investigated, using CO adsorption on Pt/Al2O3 as a simple test case. The model catalysts are formed by evaporating Pt onto an oxidized, polycrystalline, aluminum foil. Transmission electron microscope pictures show that the model catalyst consists of metal particles of average diameter 1.1–1.3 nm covering 18%–25% of the surface area of the support. EELS spectra recorded for specular and off-specular electron scattering reveal that the strong loss peaks due to vibrations of the smooth substrate occur via inelastic dipole scattering, but that the loss peaks due to CO adsorbed on the rough particles occur via the inelastic impact scattering mechanism. This has important implications for the use of EELS to study model supported metal catalysts.

Vibrational HREELS spectra of atomic nitrogen and oxygen on Cu(100)

Abstract Vibrational HREELS spectra for a c(2×2) overlayer of atomic nitrogen on Cu(100) are reported for a nominal energy resolution of 5 meV at low primary beam energy (0

A HREELS investigation of ethylene on Pt model catalysts

Abstract The adsorption of C2H4 on supported Pt model catalysts is investigated with high resolution electron energy loss spectroscopy for the cases of Pt vapor deposited on an oxidized Al foil and a single crystal of TiO2. At 160 K the HREELS spectra show evidence of the di-σ bonded ethylene species present on the supported Pt clusters, but upon warming to 325 K, only the TiO2 supported model catalyst shows evidence of forming the ethylidyne species commonly seen on Pt(111) single crystals. The oxidized Al supported species begins to decompose by 250 K, and no spectra characteristic of ethylidyne has been seen. In the case of the TiO2 supported model catalyst, we believe this is the first HREELS observation of ethylidyne on supported metal clusters.

Surface phonon dispersion along ΓX on Pd(100)

Abstract High resolution electron energy loss spectroscopy was used to measure the dispersion of the Rayleigh wave along the 〈110〉 direction on the Pd(100) surface. It was found that the phonon frequency (2.7 THz) at zone boundary was lower than that calculated by the slab method using bulk force constants (3.0 THz). The result is anomalous from that investigated at X on other fcc transition metal surfaces. The experimental data can be fit by a considerable softening (40%) of the surface interlayer force constant. The phonon softening is qualitatively in agreement with the LEED analysis which revealed a surface expansion by 3% instead of the inward relaxation found on most fcc (100) transition metal surfaces.