190 research outputs found
Size effects in surface reconstructed and silicon nanowires
The geometrical and electronic structure properties of
silicon nanowires in the absence of surface passivation are studied by means of
density-functional calculations. As we have shown in a recent publication [R.
Rurali and N. Lorente, Phys. Rev. Lett. {\bf 94}, 026805 (2005)] the
reconstruction of facets can give rise to surface metallic states. In this
work, we analyze the dependence of geometric and electronic structure features
on the size of the wire and on the growth direction
On the properties of surface reconstructed silicon nanowires
We study by means of density-functional calculations the role of lateral
surface reconstructions in determining the electrical properties of
silicon nanowires. The different lateral reconstructions are explored by
relaxing all the nanowires with crystalline bulk silicon structure and all
possible ideal facets that correspond to an average diameter of 1.5 nm. We show
that the reconstruction induces the formation of ubiquitous surface states that
turn the wires into semi-metallic or metallic
Metallic and semi-metallic <100> silicon nanowires
Silicon nanowires grown along the -direction with a bulk Si core are
studied with density functional calculations. Two surface reconstructions
prevail after exploration of a large fraction of the phase space of nanowire
reconstructions. Despite their energetical equivalence, one of the
reconstructions is found to be strongly metallic while the other one is
semi-metallic. This electronic-structure behavior is dictated by the particular
surface states of each reconstruction. These results imply that doping is not
required in order to obtain good conducting Si nanowires.Comment: 13 pages, 4 figures; Phys. Rev. Lett., in pres
Ferrodistortive instability at the (001) surface of half-metallic manganites
We present the structure of the fully relaxed (001) surface of the
half-metallic manganite La0.7Sr0.3MnO3, calculated using density functional
theory within the generalized gradient approximation (GGA). Two relevant
ferroelastic order parameters are identified and characterized: The tilting of
the oxygen octahedra, which is present in the bulk phase, oscillates and
decreases towards the surface, and an additional ferrodistortive Mn
off-centering, triggered by the surface, decays monotonically into the bulk.
The narrow d-like energy band that is characteristic of unrelaxed manganite
surfaces is shifted down in energy by these structural distortions, retaining
its uppermost layer localization. The magnitude of the zero-temperature
magnetization is unchanged from its bulk value, but the effective spin-spin
interactions are reduced at the surface.Comment: 4 pages, 2 figure
Electronic transport in Si nanowires: Role of bulk and surface disorder
We calculate the resistance and mean free path in long metallic and
semiconducting silicon nanowires (SiNWs) using two different numerical
approaches: A real space Kubo method and a recursive Green's function method.
We compare the two approaches and find that they are complementary: depending
on the situation a preferable method can be identified. Several numerical
results are presented to illustrate the relative merits of the two methods. Our
calculations of relaxed atomic structures and their conductance properties are
based on density functional theory without introducing adjustable parameters.
Two specific models of disorder are considered: Un-passivated, surface
reconstructed SiNWs are perturbed by random on-site (Anderson) disorder whereas
defects in hydrogen passivated wires are introduced by randomly removed H
atoms. The un-passivated wires are very sensitive to disorder in the surface
whereas bulk disorder has almost no influence. For the passivated wires, the
scattering by the hydrogen vacancies is strongly energy dependent and for
relatively long SiNWs (L>200 nm) the resistance changes from the Ohmic to the
localization regime within a 0.1 eV shift of the Fermi energy. This high
sensitivity might be used for sensor applications.Comment: 9 pages, 7 figures, submitted to Phys. Rev.
Band bending and quasi-2DEG in the metallized -SiC(001) surface
We study the mechanism leading to the metallization of the -SiC(001)
Si-rich surface induced by hydrogen adsorption. We analyze the effects of band
bending and demonstrate the existence of a quasi-2D electron gas, which
originates from the donation of electrons from adsorbed hydrogen to bulk
conduction states. We also provide a simple model that captures the main
features of the results of first-principles calculations, and uncovers the
basic physics of the process.Comment: accepted for publication in physica status solidi - Rapid Research
Letter
Aconsegueixen desplaçar objectes nanomètrics mitjançant canvis de temperatura
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