5 research outputs found
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–CH<sub>2</sub>–CH<sub>2</sub>–, and Si–CHCH−).
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)
Strong Hydrogen Bonds at the Interface between Proton-Donating and -Accepting Self-Assembled Monolayers on Au(111)
Hydrogen-bonding heterogeneous bilayers
on substrates have been
studied as a base for new functions of molecular adlayers by means
of atomic force microscopy (AFM), X-ray photoelectron spectroscopy
(XPS), infrared reflection absorption spectroscopy (IRAS), and density
functional theory (DFT) calculations. Here, we report the formation
of the catechol-fused bis(methylthio)tetrathiafulvalene
(H<sub>2</sub>Cat-BMT-TTF) adlayer hydrogen bonding with an imidazole-terminated
alkanethiolate self-assembled monolayer (Im-SAM) on Au(111). The heterogeneous
bilayer is realized by sequential two-step immersions in solutions
for the individual Im-SAM and H<sub>2</sub>Cat-BMT-TTF adlayer formations.
In the measurements by AFM, a grained H<sub>2</sub>Cat-BMT-TTF adlayer
on Im-SAM is revealed. The coverage and the chemical states of H<sub>2</sub>Cat-BMT-TTF on Im-SAM are specified by XPS. On the vibrational
spectrum measured by IRAS, the strong hydrogen bonds between H<sub>2</sub>Cat-BMT-TTF and Im-SAM are characterized by the remarkably
red-shifted OH stretching mode at 3140 cm<sup>–1</sup>, which
is much lower than that for hydrogen-bonding water (typically ∼3300
cm<sup>–1</sup>). The OH stretching mode frequency and the
adsorption strength for the H<sub>2</sub>Cat-BMT-TTF molecule hydrogen
bonding with imidazole groups are quantitatively examined on the basis
of DFT calculations
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 (H<sub>2</sub>SO<sub>4</sub> + NaNO<sub>3</sub> +
KMnO<sub>4</sub>) 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
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 CH<sub>3</sub>-functionalized graphene is observed only in the
monolayer graphene regions after the chlorinated EG was treated with
CH<sub>3</sub>MgBr in air-free condition. Both Cl and CH<sub>3</sub> are observed to be chemically bonded to the basal plane of the graphene.
The CH<sub>3</sub>-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
Adsorption of CO<sub>2</sub> on Graphene: A Combined TPD, XPS, and vdW-DF Study
The
adsorption of CO<sub>2</sub> 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 CO<sub>2</sub> 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 CO<sub>2</sub> on graphene was thus categorized
into physisorption, which was further supported by the binding energies
of CO<sub>2</sub> in core-level spectra. The adsorption states of
CO<sub>2</sub> 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