8 research outputs found
Molecular Ordering and Dipole Alignment of Vanadyl Phthalocyanine Monolayer on Metals: The Effects of Interfacial Interactions
We present an <i>in situ</i> low-temperature scanning
tunneling microscopy (LT-STM) study to elucidate the effects of interfacial
interactions on the molecular ordering and dipole alignment of dipolar
vanadyl phthalocyanine (VOPc) monolayer on metal surfaces, including
Cu(111), Ag(111), Au(111), and graphite. The adsorption of VOPc on
the relatively inert graphite surface leads to the formation of well-ordered
molecular dipole monolayer with unidirectionally aligned O-up configuration.
In contrast, VOPc on Cu(111), Ag(111), and Au(111) adopts both O-up
and O-down configurations. The VOPc strongly chemisorbs on Cu(111),
leading to the formation of one-dimensional molecular chains, and
two-dimensional molecular islands comprising pure O-down adsorbed
VOPc molecules at low and high coverage, respectively. In contrast,
VOPc physisorbs on Au(111) and results in an orientation transition
from flat-lying to inclined molecular islands. Regarding the interfacial
interaction strength, the Ag(111) represents an intermediate case
(weak chemisorption), which enables the formation of disordered phase
and ordered islands, as well as the orientation transition within
the disordered phase
Growth Intermediates for CVD Graphene on Cu(111): Carbon Clusters and Defective Graphene
Graphene
growth on metal films via chemical vapor deposition (CVD)
represents one of the most promising methods for graphene production.
The realization of the wafer scale production of single crystalline
graphene films requires an atomic scale understanding of the growth
mechanism and the growth intermediates of CVD graphene on metal films.
Here, we use <i>in situ</i> low-temperature scanning tunneling
microscopy (LT-STM) to reveal the graphene growth intermediates at
different stages via thermal decomposition of methane on Cu(111).
We clearly demonstrate that various carbon clusters, including carbon
dimers, carbon rectangles, and āzigzagā and āarmchairā-like
carbon chains, are the actual growth intermediates prior to the graphene
formation. Upon the saturation of these carbon clusters, they can
transform into defective graphene possessing pseudoperiodic corrugations
and vacancies. These vacancy-defects can only be effectively healed
in the presence of methane via high temperature annealing at 800 Ā°C
and result in the formation of vacancy-free monolayer graphene on
Cu(111)
Dipole Orientation Dependent Symmetry Reduction of Chloroaluminum Phthalocyanine on Cu(111)
We demonstrate a dipole orientation dependent symmetry
reduction
of 4-fold symmetric chloroaluminum phthalocyanine (ClAlPc) molecules
on a Cu(111) surface by combined low temperature scanning tunneling
microscopy (LT-STM) and density functional theory (DFT) calculations.
Unexpected symmetry reduction from 4-fold (C4) to 2-fold (C2) was
observed for Cl-down (dipole up) adsorbed ClAlPc, while molecules
adopted Cl-up (dipole down) configuration reserved the C4 symmetry.
DFT calculations indicated strong charge accumulation at the interface
region between Cu surface and the Cl atom in Cl-down adsorbed ClAlPc
due to the electron transfer from the bonded Cu atoms. This can result
in charge redistribution within the phthalocyanine (Pc) macrocycle,
and the formation of anionic Pc with an uptake of 1.3 e, which can
be subjected to JahnāTeller distortion. The inequivalent charge
distribution onto the four lobes would be further enlarged due to
the conformational distortion. The two down-bended lobes with more
electrons interact stronger with the substrate and are much closer
to the surface, leading to the C2 symmetry with one pair of up-bended
lobes brighter and longer than their perpendicular counterparts for
Cl-down adsorbed ClAlPc
Halogen-Adatom Mediated Phase Transition of Two-Dimensional Molecular Self-Assembly on a Metal Surface
Construction of tunable
and robust two-dimensional (2D) molecular
arrays with desirable lattices and functionalities over a macroscopic
scale relies on spontaneous and reversible noncovalent interactions
between suitable molecules as building blocks. Halogen bonding, with
active tunability of direction, strength, and length, is ideal for
tailoring supramolecular structures. Herein, by combining low-temperature
scanning tunneling microscopy and systematic first-principles calculations,
we demonstrate novel halogen bonding involving single halogen atoms
and phase engineering in 2D molecular self-assembly. On the Au(111)
surface, we observed catalyzed dehalogenation of hexabromobenzene
(HBB) molecules, during which negatively charged bromine adatoms (Br<sup>Ī“ā</sup>) were generated and participated in assembly
via unique CāBr<sup>Ī“+</sup>Ā·Ā·Ā·Br<sup>Ī“ā</sup> interaction, drastically different from HBB
assembly on a chemically inert graphene substrate. We successfully
mapped out different phases of the assembled superstructure, including
densely packed hexagonal, tetragonal, dimer chain, and expanded hexagonal
lattices at room temperature, 60 Ā°C, 90 Ā°C, and 110 Ā°C,
respectively, and the critical role of Br<sup>Ī“ā</sup> in regulating lattice characteristics was highlighted. Our results
show promise for manipulating the interplay between noncovalent interactions
and catalytic reactions for future development of molecular nanoelectronics
and 2D crystal engineering
Anisotropic Strain-Mediated Growth of Monatomic Co Chains on Unreconstructed Regions of the Au(111) Surface
Single-metal-atom chains, the ultimate one-dimensional
(1D) structure,
have intriguing physical and chemical properties. However, their controllable
and massive production remains challenging as it requires the positioning
of individual atoms with atomic precision. Here, by using a two-step
molecular beam epitaxy method, we successfully fabricate single-cobalt-atom
chains on a metal surface, where organic molecules are first sublimated
onto heated Au(111), followed by deposition of Co atoms. Adsorption
of 8OH-TPB (octahydroxyl tetraphenylbenzene) on Au(111) induces surface
reconstruction transition from a herringbone to a triangular pattern.
Co deposition leads to the formation of 1D ā3R30Ā° chains with a cross section of only one atom propagating
along the [11Ģ
0] direction, which are separated from each other
by a lateral spacing of 7ā10 times the lattice constant of
Au(111). The growth mechanism lies in the surface strain anisotropy
induced by the strong CoāAu bonding, where the distance between
the two Au atoms bridged via a Co adatom is significantly enlarged,
while the AuāAu distance along the Co chain remains almost
intact. The observed chain length distribution can be interpreted
in terms of electronic scattering vectors at the Fermi surface of
the Au(111) surface states
Low-Temperature, Bottom-Up Synthesis of Graphene via a Radical-Coupling Reaction
In
this article, we demonstrated a method to synthesize graphene
films at low temperature via a mild radical-coupling reaction. During
the deposition process, with the effectively breaking of the CāBr
bonds of hexabromobenzene (HBB) precursors, the generated HBB radicals
couple efficiently to form graphene films at the low temperature of
220ā250 Ā°C. In situ low-temperature scanning tunneling
microscopy was used to provide atomic scale investigation of the graphene
growth mechanism using HBB as precursor. The chemical structure evolution
during the graphene growth process was further corroborated by in
situ X-ray photoelectron spectroscopy measurements. The charge carrier
mobility of the graphene film grown at low temperature is at 1000ā4200
cm<sup>2</sup> V<sup>ā1Ā </sup>s<sup>ā1</sup>, as evaluated
in a field-effect transistor device configuration on SiO<sub>2</sub> substrates, indicating the high quality of the films
Self-Assembly of Polar Phthalocyanine Molecules on Graphene Grown by Chemical Vapor Deposition
Integration of functional organic
molecules with graphene is expected
to promote the development of graphene-based flexible electronics
with novel properties. Here, the self-assembled structure of dipole
phthalocyanine molecules, chloro-aluminum phthalocyanine (ClAlPc),
on single-layer graphene grown by chemical vapor deposition (CVD)
over a Cu film was characterized by low-temperature scanning tunneling
microscopy (LT-STM). The phthalocyanine molecules show highly ordered
assembled structures on the CVD graphene, and these molecular layers
extend continuously over the steps of the Cu film. We also observe
specific boundaries in the self-assembled molecule arrays, which can
be explained by the presence of domain boundaries in the graphene.
The STM results suggest that CVD graphene is as a good molecular assembly
template for surface functionalization and that these molecular arrays
facilitate the study of domain structures in CVD graphene
Oxygen-Promoted Methane Activation on Copper
The
role of oxygen in the activation of CāH bonds in methane
on clean and oxygen-precovered Cu(111) and Cu<sub>2</sub>OĀ(111) surfaces
was studied with combined in situ near-ambient-pressure scanning tunneling
microscopy and X-ray photoelectron spectroscopy. Activation of methane
at 300 K and āmoderate pressuresā was only observed
on oxygen-precovered Cu(111) surfaces. Density functional theory calculations
reveal that the lowest activation energy barrier of CāH on
Cu(111) in the presence of chemisorbed oxygen is related to a two-active-site,
four-centered mechanism, which stabilizes the required transition-state
intermediate by dipoleādipole attraction of OāH and
CuāCH<sub>3</sub> species. The CāH bond activation barriers
on Cu<sub>2</sub>OĀ(111) surfaces are large due to the weak stabilization
of H and CH<sub>3</sub> fragments