Spectroscopic Characterization and Transport Properties of Aromatic Monolayers Covalently Attached to Si(111) Surfaces

We fabricated self-assembled monolayers (SAMs) composed of aromatic molecules with different anchor groups on Si(111) surfaces by wet chemical reactions. We investigated the bonding structures and transport properties by spectroscopic and electrical measurements, respectively. By using simple aromatic molecules (phenol, styrene, and phenylacetylene) as initial precursors, we successfully fabricated aromatic SAMs covalently bonded to Si(111) surfaces through different anchor structures (Si–O–, Si–CH₂–CH₂–, and Si–CHCH−). Transmission infrared spectroscopy clarify that the phenyl rings in the SAMs are oriented almost perpendicular to the Si surfaces. High-resolution X-ray photoelectron spectroscopy reveals that the aromatic molecules attach to the Si surface with the surface coverage of ∼0.5. The experimental results of these spectroscopies lead to a conclusion that the aromatic SAMs form densely packed monolayers on Si(111). Current density–voltage measurements of Hg/aromatic SAM–Si(111) sandwiched structures revealed that the “Si­(111)–O–Ph” (SAM from phenol) show higher conductivity compared with the long-chain alkyl SAM on Si(111)

Mechanism of Olefin Hydrogenation Catalysis Driven by Palladium-Dissolved Hydrogen

The Pd-catalyzed hydrogenation of CC double bonds is one of the most important synthetic routes in organic chemistry. This catalytic surface reaction is known to require hydrogen in the interior of the Pd catalyst, but the mechanistic role of the Pd-dissolved H has remained elusive. To shed new light into this fundamental problem, we visualized the H distribution near a Pd single crystal surface charged with absorbed hydrogen during a typical catalytic conversion of butene (C₄H₈) to butane (C₄H₁₀), using H depth profiling via nuclear reaction analysis. This has revealed that the catalytic butene hydrogenation (1) occurs between 160 and 250 K on a H-saturated Pd surface, (2) is triggered by the emergence of Pd bulk-dissolved hydrogen onto this surface, but (3) does not necessarily require large stationary H concentrations in subsurface sites. Even deeply bulk-absorbed hydrogen proves to be reactive, suggesting that Pd-dissolved hydrogen chiefly acts by directly providing reactive H species to the surface after bulk diffusion rather than by indirectly activating surface H through modifying the surface electronic structure. The chemisorbed surface hydrogen is found to promote hydrogenation reactivity by weakening the butene-Pd interaction and by significantly reducing the decomposition of the olefin

Monolayer Selective Methylation of Epitaxial Graphene on SiC(0001) through Two-Step Chlorination–Alkylation Reactions

One of the real challenges in realization of many of graphene’s anticipated applications is the development of a common chemical route for modifying graphene with varieties of functionalities. Here, we successfully demonstrate the organic modification of epitaxial graphene (EG) grown on the Si-face of SiC substrate through two-step chlorination–alkylation reactions. Pristine and chemically modified graphene are characterized by scanning tunneling microscope and spectroscopy, X-ray photoelectron spectroscopy, and Raman measurements. The first-step photochlorination is found to occur very selectively on the monolayer graphene region leaving the bi- and trilayer graphene regions clean. Consequently, the CH₃-functionalized graphene is observed only in the monolayer graphene regions after the chlorinated EG was treated with CH₃MgBr in air-free condition. Both Cl and CH₃ are observed to be chemically bonded to the basal plane of the graphene. The CH₃-functionalized graphene is thermally more stable than that of the chlorinated graphene. The present two-step chlorination–methylation procedure is expected to open a new route for organic modification of graphene with different functional groups using a variety of Grignard reagents

Aqueous-Phase Oxidation of Epitaxial Graphene on the Silicon Face of SiC(0001)

To explore the chemical and electronic states of oxidized epitaxial graphene (EG) grown on the Si face of SiC(0001), we employ the Hummers oxidizing agents (H₂SO₄ + NaNO₃ + KMnO₄) under different reaction conditions that oxidize the graphene layer. The resulting material is characterized with scanning tunneling microscopy (STM), scanning tunneling spectroscopy (STS), Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS). A mild “drop-cast” procedure at 60 °C is found to be equally effective at oxidizing EG as the conventional Hummers procedure. This aqueous-phase oxidation reaction appears to proceed in an autocatalytic manner as indicated by the concurrent observation of patches of oxidized and clean graphene areas in atomically resolved STM images on partially oxidized EG. STS further reveals substantial changes in electronic structure for oxidized EG including the opening of a local band gap of ∼0.4 eV. The oxidation is confined to the graphene layers as verified by XPS characterization of the underlying SiC substrate. In contrast to EG oxidized in ultrahigh vacuum that contains only epoxy groups and can be fully reverted back to pristine EG following annealing at 260 °C, aqueous-phase oxidized EG possesses carbonyl and hydroxyl groups in addition to the dominant epoxy groups and thus remains partially oxidized even following annealing at 1000 °C

Adsorption of CO₂ on Graphene: A Combined TPD, XPS, and vdW-DF Study

The adsorption of CO₂ molecules on monolayer epitaxial graphene on a SiC(0001) surface at 30 K was investigated by temperature-programmed desorption and X-ray photoelectron spectroscopy. The desorption energy of CO₂ on graphene was determined to be (30.1–25.1) ± 1.5 kJ/mol at low coverages and approached the sublimation energy of dry ice (27–25 kJ/mol) with increasing the coverage. The adsorption of CO₂ on graphene was thus categorized into physisorption, which was further supported by the binding energies of CO₂ in core-level spectra. The adsorption states of CO₂ on graphene were theoretically examined by means of the van der Waals density functional (vdW-DF) method that includes nonlocal correlation. The experimental desorption energy was successfully reproduced with high accuracy using vdW-DF calculations; the optB86b-vdW functional was found to be most appropriate to reproduce the desorption energy in the present system