21 research outputs found
Graphene as a Long-Term Metal Oxidation Barrier: Worse Than Nothing
Anticorrosion and antioxidation surface treatments such as paint or anodization are a foundational component in nearly all industries. Graphene, a single-atom-thick sheet of carbon with impressive impermeability to gases, seems to hold promise as an effective anticorrosion barrier, and recent work supports this hope. We perform a complete study of the short- and long-term performance of graphene coatings for Cu and Si substrates. Our work reveals that although graphene indeed offers effective short-term oxidation protection, over long time scales it promotes more extensive wet corrosion than that seen for an initially bare, unprotected Cu surface. This surprising result has important implications for future scientific studies and industrial applications. In addition to informing any future work on graphene as a protective coating, the results presented here have implications for grapheneās performance in a wide range of applications
Conserved Atomic Bonding Sequences and Strain Organization of Graphene Grain Boundaries
The
bulk properties of polycrystalline materials are directly influenced
by the atomic structure at the grain boundaries that join neighboring
crystallites. In this work, we show that graphene grain boundaries
are comprised of structural building blocks of conserved atomic bonding
sequences using aberration corrected high-resolution transmission
electron microscopy. These sequences appear as stretches of identically
arranged periodic or aperiodic regions of dislocations. Atomic scale
strain and lattice rotation of these interfaces is derived by mapping
the exact positions of every carbon atom at the boundary with ultrahigh
precision. Strain fields are organized into local tensile and compressive
dipoles in both periodic and aperiodic dislocation regions. Using
molecular dynamics tension simulations, we find that experimental
grain boundary structures maintain strengths that are comparable to
idealized periodic boundaries despite the presence of local aperiodic
dislocation sequences
Tuning the Band Gap of Graphene Nanoribbons Synthesized from Molecular Precursors
A prerequisite for future graphene nanoribbon (GNR) applications is the ability to fine-tune the electronic band gap of GNRs. Such control requires the development of fabrication tools capable of precisely controlling width and edge geometry of GNRs at the atomic scale. Here we report a technique for modifying GNR band gaps <i>via</i> covalent self-assembly of a new species of molecular precursors that yields <i>n</i> = 13 armchair GNRs, a wider GNR than those previously synthesized using bottom-up molecular techniques. Scanning tunneling microscopy and spectroscopy reveal that these <i>n</i> = 13 armchair GNRs have a band gap of 1.4 eV, 1.2 eV smaller than the gap determined previously for <i>n</i> = 7 armchair GNRs. Furthermore, we observe a localized electronic state near the end of <i>n</i> = 13 armchair GNRs that is associated with hydrogen-terminated sp<sup>2</sup>-hybridized carbon atoms at the zigzag termini
Direct Growth of Single- and Few-Layer MoS<sub>2</sub> on hāBN with Preferred Relative Rotation Angles
Monolayer
molybdenum disulfide (MoS<sub>2</sub>) is a promising two-dimensional
direct-bandgap semiconductor with potential applications in atomically
thin and flexible electronics. An attractive insulating substrate
or mate for MoS<sub>2</sub> (and related materials such as graphene)
is hexagonal boron nitride (h-BN). Stacked heterostructures of MoS<sub>2</sub> and h-BN have been produced by manual transfer methods, but
a more efficient and scalable assembly method is needed. Here we demonstrate
the direct growth of single- and few-layer MoS<sub>2</sub> on h-BN
by chemical vapor deposition (CVD) method, which is scalable with
suitably structured substrates. The growth mechanisms for single-layer
and few-layer samples are found to be distinct, and for single-layer
samples low relative rotation angles (<5Ā°) between the MoS<sub>2</sub> and h-BN lattices prevail. Moreover, MoS<sub>2</sub> directly
grown on h-BN maintains its intrinsic 1.89 eV bandgap. Our CVD synthesis
method presents an important advancement toward controllable and scalable
MoS<sub>2</sub>-based electronic devices
Iodine versus Bromine Functionalization for Bottom-Up Graphene Nanoribbon Growth: Role of Diffusion
Deterministic bottom-up
approaches for synthesizing atomically well-defined graphene nanoribbons
(GNRs) largely rely on the surface-catalyzed activation of selected
labile bonds in a molecular precursor followed by step-growth polymerization
and cyclodehydrogenation. While the majority of successful GNR precursors
rely on the homolytic cleavage of thermally labile CāBr bonds,
the introduction of weaker CāI bonds provides access to monomers
that can be polymerized at significantly lower temperatures, thus
helping to increase the flexibility of the GNR synthesis process.
Scanning tunneling microscopy imaging of molecular precursors, activated
intermediates, and polymers resulting from stepwise thermal annealing
of both Br and I substituted precursors for chevron GNRs reveals that
the polymerization of both precursors proceeds at similar temperatures
on Au(111). This surprising observation is consistent with diffusion-controlled
polymerization of the surface-stabilized radical intermediates that
emerge from homolytic cleavage of either the CāBr or the CāI
bonds
Site-Specific Substitutional Boron Doping of Semiconducting Armchair Graphene Nanoribbons
A fundamental
requirement for the development of advanced electronic
device architectures based on graphene nanoribbon (GNR) technology
is the ability to modulate the band structure and charge carrier concentration
by substituting specific carbon atoms in the hexagonal graphene lattice
with p- or n-type dopant heteroatoms. Here we report the atomically
precise introduction of group III dopant atoms into bottom-up fabricated
semiconducting armchair GNRs (AGNRs). Trigonal-planar B atoms along
the backbone of the GNR share an empty p-orbital with the extended
Ļ-band for dopant functionality. Scanning tunneling microscopy
(STM) topography reveals a characteristic modulation of the local
density of states along the backbone of the GNR that is superimposable
with the expected position and concentration of dopant B atoms. First-principles
calculations support the experimental findings and provide additional
insight into the band structure of B-doped 7-AGNRs
Local Electronic Structure of a Single-Layer Porphyrin-Containing Covalent Organic Framework
We have characterized
the local electronic structure of a porphyrin-containing
single-layer covalent organic framework (COF) exhibiting a square
lattice. The COF monolayer was obtained by the deposition of 2,5-dimethoxybenzene-1,4-dicarboxaldehyde
(DMA) and 5,10,15,20-tetrakisĀ(4-aminophenyl) porphyrin (TAPP) onto
a Au(111) surface in ultrahigh vacuum followed by annealing to facilitate
Schiff-base condensations between monomers. Scanning tunneling spectroscopy
(STS) experiments conducted on isolated TAPP precursor molecules and
the covalently linked COF networks yield similar transport (HOMOāLUMO)
gaps of 1.85 Ā± 0.05 eV and 1.98 Ā± 0.04 eV, respectively.
The COF orbital energy alignment, however, undergoes a significant
downward shift compared to isolated TAPP molecules due to the electron-withdrawing
nature of the imine bond formed during COF synthesis. Direct imaging
of the COF local density of states (LDOS) <i>via</i> d<i>I</i>/d<i>V</i> mapping reveals that the COF HOMO
and LUMO states are localized mainly on the porphyrin cores and that
the HOMO displays reduced symmetry. DFT calculations reproduce the
imine-induced negative shift in orbital energies and reveal that the
origin of the reduced COF wave function symmetry is a saddle-like
structure adopted by the porphyrin macrocycle due to its interactions
with the Au(111) substrate
Adsorption and Stability of ĻāBonded Ethylene on GaP(110)
We
have investigated the structural and electronic properties of
individual ethylene molecules on the GaP(110) surface by combining
low-temperature scanning tunneling microscopy and spectroscopy (LT-STM/STS)
with density functional theory (DFT) calculations. Isolated molecules
were adsorbed on in situ cleaved GaP(110) surfaces through ethylene
exposures at 300 K and 15 K. DFT calculations suggest two possible
stable adsorption geometries for a single ethylene molecule on GaP(110)
at low temperature. High-resolution STM images, however, reveal only
one adsorption geometry for this system, consistent with the site
having the largest computed binding energy. Unlike adsorption of ethylene
on other metallic and semiconducting surfaces, ethylene physisorbs
to GaP(110) through a weak hybridization of molecular Ļ-states
with substrate surface states, leaving the frontier molecular orbitals
largely unperturbed. Differential conductivity spectra acquired on
single molecules are consistent with self-energy corrected DFT calculations
Noncovalent Dimerization after Enediyne Cyclization on Au(111)
We
investigate the thermally induced cyclization of 1,2-bisĀ(2-phenylethynyl)Ābenzene
on Au(111) using scanning tunneling microscopy and computer simulations.
Cyclization of sterically hindered enediynes is known to proceed via
two competing mechanisms in solution: a classic C<sup>1</sup>āC<sup>6</sup> (Bergman) or a C<sup>1</sup>āC<sup>5</sup> cyclization
pathway. On Au(111), we find that the C<sup>1</sup>āC<sup>5</sup> cyclization is suppressed and that the C<sup>1</sup>āC<sup>6</sup> cyclization yields a highly strained bicyclic olefin whose
surface chemistry was hitherto unknown. The C<sup>1</sup>āC<sup>6</sup> product self-assembles into discrete noncovalently bound
dimers on the surface. The reaction mechanism and driving forces behind
noncovalent association are discussed in light of density functional
theory calculations
Hierarchical On-Surface Synthesis of Graphene Nanoribbon Heterojunctions
Bottom-up
graphene nanoribbon (GNR) heterojunctions are nanoscale
strips of graphene whose electronic structure abruptly changes across
a covalently bonded interface. Their rational design offers opportunities
for profound technological advancements enabled by their extraordinary
structural and electronic properties. Thus far, the most critical
aspect of their synthesis, the control over sequence and position
of heterojunctions along the length of a ribbon, has been plagued
by randomness in monomer sequences emerging from step-growth copolymerization
of distinct monomers. All bottom-up GNR heterojunction structures
created so far have exhibited random sequences of heterojunctions
and, while useful for fundamental scientific studies, are difficult
to incorporate into functional nanodevices as a result. In contrast,
we describe a hierarchical fabrication strategy that allows the growth
of bottom-up GNRs that preferentially exhibit a single heterojunction
interface rather than a random statistical sequence of junctions along
the ribbon. Such heterojunctions provide a viable platform that could
be directly used in functional GNR-based device applications at the
molecular scale. Our hierarchical GNR fabrication strategy is based
on differences in the dissociation energies of CāBr and CāI
bonds that allow control over the growth sequence of the block copolymers
from which GNRs are formed and consequently yields a significantly
higher proportion of single-junction GNR heterostructures. Scanning
tunneling spectroscopy and density functional theory calculations
confirm that hierarchically grown heterojunctions between chevron
GNR (cGNR) and binaphthyl-cGNR segments exhibit straddling Type I
band alignment in structures that are only one atomic layer thick
and 3 nm in width