1,608 research outputs found
Tunable Hybridization Between Electronic States of Graphene and Physisorbed Hexacene
Non-covalent functionalization via physisorption of organic molecules
provides a scalable approach for modifying the electronic structure of graphene
while preserving its excellent carrier mobilities. Here we investigated the
physisorption of long-chain acenes, namely, hexacene and its fluorinated
derivative perfluorohexacene, on bilayer graphene for tunable graphene devices
using first principles methods. We find that the adsorption of these molecules
leads to the formation of localized states in the electronic structure of
graphene close to its Fermi level, which could be readily tuned by an external
electric field. The electric field not only creates a variable band gap as
large as 250 meV in bilayer graphene, but also strongly influences the charge
redistribution within the molecule-graphene system. This charge redistribution
is found to be weak enough not to induce strong surface doping, but strong
enough to help preserve the electronic states near the Dirac point of graphene.Comment: 17 pages, 7 figures, supporting informatio
The interlayer cohesive energy of graphite from thermal desorption of polyaromatic hydrocarbons
We have studied the interaction of polyaromatic hydrocarbons (PAHs) with the
basal plane of graphite using thermal desorption spectroscopy. Desorption
kinetics of benzene, naphthalene, coronene and ovalene at sub-monolayer
coverages yield activation energies of 0.50 eV, 0.85 eV, 1.40 eV and 2.1 eV,
respectively. Benzene and naphthalene follow simple first order desorption
kinetics while coronene and ovalene exhibit fractional order kinetics owing to
the stability of 2-D adsorbate islands up to the desorption temperature.
Pre-exponential frequency factors are found to be in the range
- as obtained from both Falconer--Madix (isothermal
desorption) analysis and Antoine's fit to vapour pressure data. The resulting
binding energy per carbon atom of the PAH is 5 meV and can be identified
with the interlayer cohesive energy of graphite. The resulting cleavage energy
of graphite is ~meV/atom which is considerably larger than previously
reported experimental values.Comment: 8 pages, 4 figures, 2 table
On the physisorption of water on graphene: Sub-chemical accuracy from many-body electronic structure methods
Molecular adsorption on surfaces plays a central role in catalysis,
corrosion, desalination, and many other processes of relevance to industry and
the natural world. Few adsorption systems are more ubiquitous or of more
widespread importance than those involving water and carbon, and for a
molecular level understanding of such interfaces water monomer adsorption on
graphene is a fundamental and representative system. This system is
particularly interesting as it calls for an accurate treatment of electron
correlation effects, as well as posing a practical challenge to experiments.
Here, we employ many-body electronic structure methodologies that can be
rigorously converged and thus provide faithful references for the
molecule-surface interaction. In particular, we use diffusion Monte-Carlo
(DMC), coupled cluster (CCSD(T)), as well as the random phase approximation
(RPA) to calculate the strength of the interaction between water and an
extended graphene surface. We establish excellent, sub-chemical, agreement
between the complementary high-level methodologies, and an adsorption energy
estimate in the most stable configuration of approximately -100\,meV is
obtained. We also find that the adsorption energy is rather insensitive to the
orientation of the water molecule on the surface, despite different binding
motifs involving qualitatively different interfacial charge reorganisation. In
producing the first demonstrably accurate adsorption energies for water on
graphene this work also resolves discrepancies amongst previously reported
values for this widely studied system. It also paves the way for more accurate
and reliable studies of liquid water at carbon interfaces with cheaper
computational methods, such as density functional theory and classical
potentials
Physisorption of Nucleobases on Graphene
We report the results of our first-principles investigation on the
interaction of the nucleobases adenine (A), cytosine (C), guanine (G), thymine
(T), and uracil (U) with graphene, carried out within the density functional
theory framework, with additional calculations utilizing Hartree--Fock plus
second-order Moeller-Plesset perturbation theory. The calculated binding energy
of the nucleobases shows the following hierarchy: G > T ~ C ~ A > U, with the
equilibrium configuration being very similar for all five of them. Our results
clearly demonstrate that the nucleobases exhibit significantly different
interaction strengths when physisorbed on graphene. The stabilizing factor in
the interaction between the base molecule and graphene sheet is dominated by
the molecular polarizability that allows a weakly attractive dispersion force
to be induced between them. The present study represents a significant step
towards a first-principles understanding of how the base sequence of DNA can
affect its interaction with carbon nanotubes, as observed experimentally.Comment: 7 pages, 3 figure
van der Waals density functionals built upon the electron-gas tradition: Facing the challenge of competing interactions
The theoretical description of sparse matter attracts much interest, in
particular for those ground-state properties that can be described by density
functional theory (DFT). One proposed approach, the van der Waals density
functional (vdW-DF) method, rests on strong physical foundations and offers
simple yet accurate and robust functionals. A very recent functional within
this method called vdW-DF-cx [K. Berland and P. Hyldgaard, Phys. Rev. B 89,
035412] stands out in its attempt to use an exchange energy derived from the
same plasmon-based theory from which the nonlocal correlation energy was
derived. Encouraged by its good performance for solids, layered materials, and
aromatic molecules, we apply it to several systems that are characterized by
competing interactions. These include the ferroelectric response in PbTiO,
the adsorption of small molecules within metal-organic frameworks (MOFs), the
graphite/diamond phase transition, and the adsorption of an aromatic-molecule
on the Ag(111) surface. Our results indicate that vdW-DF-cx is overall well
suited to tackle these challenging systems. In addition to being a competitive
density functional for sparse matter, the vdW-DF-cx construction presents a
more robust general purpose functional that could be applied to a range of
materials problems with a variety of competing interactions
Trapping of electrons near chemisorbed hydrogen on graphene
Chemical adsorption of atomic hydrogen on a negatively charged single layer
graphene sheet has been analyzed with ab-initio Density Functional Theory
calculations. We have simulated both finite clusters and infinite periodic
systems to investigate the effect of different ingredients of the theory, e.g.
exchange and correlation potentials, basis sets, etc. Hydrogen's electron
affinity dominates the energetic balance in the charged systems and the extra
electron is predominantly attracted to a region nearby the chemisorbed atom.
The main consequences are: (i) the cancellation of the unpaired spin resulting
in a singlet ground-state, and (ii) a stronger interaction between hydrogen and
the graphene sheet.Comment: 11 pages, 8 figures, to be published in PR
Methane and carbon dioxide adsorption on edge-functionalized graphene: A comparative DFT study
With a view towards optimizing gas storage and separation in crystalline and
disordered nanoporous carbon-based materials, we use ab initio density
functional theory calculations to explore the effect of chemical
functionalization on gas binding to exposed edges within model carbon
nanostructures. We test the geometry, energetics, and charge distribution of
in-plane and out-of-plane binding of CO2 and CH4 to model zigzag graphene
nanoribbons edge-functionalized with COOH, OH, NH2, H2PO3, NO2, and CH3.
Although different choices for the exchange-correlation functional lead to a
spread of values for the binding energy, trends across the functional groups
are largely preserved for each choice, as are the final orientations of the
adsorbed gas molecules. We find binding of CO2 to exceed that of CH4 by roughly
a factor of two. However, the two gases follow very similar trends with changes
in the attached functional group, despite different molecular symmetries. Our
results indicate that the presence of NH2, H2PO3, NO2, and COOH functional
groups can significantly enhance gas binding with respect to a
hydrogen-passivated edge, making the edges potentially viable binding sites in
materials with high concentrations of edge carbons. To first order, in-plane
binding strength correlates with the larger permanent and induced dipole
moments on these groups. Implications for tailoring carbon structures for
increased gas uptake and improved CO2/CH4 selectivity are discussed.Comment: 12 pages, 7 figure
- …