40 research outputs found
Atomistic theory for the damping of vibrational modes in mono-atomic gold chains
We develop a computational method for evaluating the damping of vibrational
modes in mono-atomic metallic chains suspended between bulk crystals under
external strain. The damping is due to the coupling between the chain and
contact modes and the phonons in the bulk substrates. The geometry of the atoms
forming the contact is taken into account. The dynamical matrix is computed
with density functional theory in the atomic chain and the contacts using
finite atomic displacements, while an empirical method is employed for the bulk
substrate. As a specific example, we present results for the experimentally
realized case of gold chains in two different crystallographic directions. The
range of the computed damping rates confirm the estimates obtained by fits to
experimental data [Frederiksen et al., Phys. Rev. B, 75, 205413(R)(2007)]. Our
method indicates that an order-of-magnitude variation in the damping is
possible even for relatively small changes in the strain. Such detailed insight
is necessary for a quantitative analysis of damping in metallic atomic chains,
and in explaining the rich phenomenology seen in the experiments.Comment: 11 pages, 13 figure
Atomic-scale model for the contact resistance of the nickel-graphene interface
We perform first-principles calculations of electron transport across a
nickel-graphene interface. Four different geometries are considered, where the
contact area, graphene and nickel surface orientations and the passivation of
the terminating graphene edge are varied. We find covalent bond formation
between the graphene layer and the nickel surface, in agreement with other
theoretical studies. We calculate the energy-dependent electron transmission
for the four systems and find that the systems have very similar edge contact
resistance, independent of the contact area between nickel and graphene, and in
excellent agreement with recent experimental data. A simple model where
graphene is bonded with a metal surface shows that the results are generic for
covalently bonded graphene, and the minimum attainable edge contact resistance
is twice the ideal edge quantum contact resistance of graphene.Comment: 12 pages, 6 figure
Search for a Metallic Dangling-Bond Wire on -doped H-passivated Semiconductor Surfaces
We have theoretically investigated the electronic properties of neutral and
-doped dangling bond (DB) quasi-one-dimensional structures (lines) in the
Si(001):H and Ge(001):H substrates with the aim of identifying atomic-scale
interconnects exhibiting metallic conduction for use in on-surface circuitry.
Whether neutral or doped, DB lines are prone to suffer geometrical distortions
or have magnetic ground-states that render them semiconducting. However, from
our study we have identified one exception -- a dimer row fully stripped of
hydrogen passivation. Such a DB-dimer line shows an electronic band structure
which is remarkably insensitive to the doping level and, thus, it is possible
to manipulate the position of the Fermi level, moving it away from the gap.
Transport calculations demonstrate that the metallic conduction in the DB-dimer
line can survive thermally induced disorder, but is more sensitive to imperfect
patterning. In conclusion, the DB-dimer line shows remarkable stability to
doping and could serve as a one-dimensional metallic conductor on -doped
samples.Comment: 8 pages, 5 figure
Delta Self-Consistent Field as a method to obtain potential energy surfaces of excited molecules on surfaces
We present a modification of the SCF method of calculating energies
of excited states, in order to make it applicable to resonance calculations of
molecules adsorbed on metal surfaces, where the molecular orbitals are highly
hybridized. The SCF approximation is a density functional method
closely resembling standard density functional theory (DFT), the only
difference being that in SCF one or more electrons are placed in higher
lying Kohn-Sham orbitals, instead of placing all electrons in the lowest
possible orbitals as one does when calculating the ground state energy within
standard DFT. We extend the SCF method by allowing excited electrons to
occupy orbitals which are linear combinations of Kohn-Sham orbitals. With this
extra freedom it is possible to place charge locally on adsorbed molecules in
the calculations, such that resonance energies can be estimated. The method is
applied to N, CO and NO adsorbed on different metallic surfaces and
compared to ordinary SCF without our modification, spatially
constrained DFT and inverse-photoemission spectroscopy (IPES) measurements.
This comparison shows that the modified SCF method gives results in
close agreement with experiment, significantly closer than the comparable
methods. For N adsorbed on ruthenium (0001) we map out a 2-dimensional part
of the potential energy surfaces in the ground state and the 2-resonance.
Finally we compare the SCF approach on gas-phase N and CO, to
higher accuracy methods. Excitation energies are approximated with accuracy
close to that of time-dependent density functional theory, and we see very good
agreement in the minimum shift of the potential energy surfaces in the excited
state compared to the ground state.Comment: 11 pages, 7 figure
On-surface synthesis of nanographenes and graphene nanoribbons on titanium dioxide
The formation of two types of nanographenes from custom designed and synthesized molecular precursors has been achieved through thermally induced intramolecular cyclodehydrogenation reactions on the semiconducting TiO(110)-(1×1) surface, confirmed by the combination of high-resolution scanning tunneling microscopy (STM) and spectroscopy (STS) measurements, and corroborated by theoretical modeling. The application of this protocol on differently shaped molecular precursors demonstrates the ability to induce a highly efficient planarization reaction both within strained pentahelicenes as well as between vicinal phenyl rings. Additionally, by the combination of successive Ullmann-type polymerization and cyclodehydrogenation reactions, the archetypic 7-armchair graphene nanoribbons (7-AGNRs) have also been fabricated on the titanium dioxide surface from the standard 10,10′-dibromo-9,9′-bianthryl (DBBA) molecular precursors. These examples of the effective cyclodehydrogenative planarization processes provide perspectives for the rational design and synthesis of molecular nanostructures on semiconductors
A tunable electronic beam splitter realized with crossed graphene nanoribbons
Graphene nanoribbons (GNRs) are promising components in future nanoelectronics due to the large mobility of graphene electrons and their tunable electronic band gap in combination with recent experimental developments of on-surface chemistry strategies for their growth. Here, we explore a prototype 4-terminal semiconducting device formed by two crossed armchair GNRs (AGNRs) using state-of-the-art first-principles transport methods. We analyze in detail the roles of intersection angle, stacking order, inter-GNR separation, GNR width, and finite voltages on the transport characteristics. Interestingly, when the AGNRs intersect at θ=60°, electrons injected from one terminal can be split into two outgoing waves with a tunable ratio around 50% and with almost negligible back-reflection. The split electron wave is found to propagate partly straight across the intersection region in one ribbon and partly in one direction of the other ribbon, i.e., in analogy with an optical beam splitter. Our simulations further identify realistic conditions for which this semiconducting device can act as a mechanically controllable electronic beam splitter with possible applications in carbon-based quantum electronic circuits and electron optics. We rationalize our findings with a simple model suggesting that electronic beam splitters can generally be realized with crossed GNRs
Tunneling spectroscopy of close-spaced dangling-bond pairs in Si(001):H
We present a combined experimental and theoretical study of the electronic properties of close-spaced dangling-bond (DB) pairs in a hydrogen-passivated Si(001):H p-doped surface. Two types of DB pairs are considered, called “cross” and “line” structures. Our scanning tunneling spectroscopy (STS) data show that, although the spectra taken over different DBs in each pair exhibit a remarkable resemblance, they appear shifted by a constant energy that depends on the DB-pair type. This spontaneous asymmetry persists after repeated STS measurements. By comparison with density functional theory (DFT) calculations, we demonstrate that the magnitude of this shift and the relative position of the STS peaks can be explained by distinct charge states for each DB in the pair. We also explain how the charge state is modified by the presence of the scanning tunneling microscopy (STM) tip and the applied bias. Our results indicate that, using the STM tip, it is possible to control the charge state of individual DBs in complex structures, even if they are in close proximity. This observation might have important consequences for the design of electronic circuits and logic gates based on DBs in passivated silicon surfaces
Density functional theory based screening of ternary alkali-transition metal borohydrides: A computational material design project
The dissociation of molecules, even the most simple hydrogen molecule, cannot be described accurately within density functional theory because none of the currently available functionals accounts for strong on-site correlation. This problem led to a discussion of properties that the local Kohn-Sham potential has to satisfy in order to correctly describe strongly correlated systems. We derive an analytic expression for the nontrivial form of the Kohn-Sham potential in between the two fragments for the dissociation of a single bond. We show that the numerical calculations for a one-dimensional two-electron model system indeed approach and reach this limit. It is shown that the functional form of the potential is universal, i.e., independent of the details of the two fragments.We acknowledge funding by the Spanish MEC (Grant No. FIS2007-65702-C02-01), “Grupos Consolidados UPV/EHU del Gobierno Vasco” (Grant No. IT-319-07), and the European Community through e-I3 ETSF project (Grant Agreement No. 211956).Peer reviewe