Pedro de Andres
Pedro de Andres
Welcome. My field of interest is Theoretical Physics and Chemistry
(Condensed Matter and Surface Science). I have been working on/with
LEED, STM, BEEM, DFT, chemisorption on surfaces, surface electrodynamics, etc.
I shall be pleased to hear your comments:
do not hesitate to contact me if you need any assistance.
My email can be found by clicking the following link:
Correspondence to Pedro de Andres
Instituto de Ciencia de Materiales de Madrid (CSIC)
E-28049 Cantoblanco (Madrid) SPAIN
Tfno: +34 91 334 9049; Mobile: +34 669 633 037; FAX: +34 91 372 0623
FaceBook
Updated on the 23th of July, 2008.
You will find information about my research interests and references
to some selected recent publications following the link above.
You are welcome to contact me anytime for more information.
Graphene layers are stable, hard, and relatively inert. We study how tensile
stress affects and bonds and the resulting change in the chemical activity.
Stress affects more strongly bonds that can become chemically active and bind
to adsorbed species more strongly. Upon stretch, single C bonds are activated
in a geometry mixing 120° and 90°, an intermediate state between sp2 and sp3
bonding. We use ab initio density functional theory to study the adsorption of
hydrogen on large clusters and two-dimensional periodic models for graphene.
The influence of the exchange-correlation functional on the adsorption energy
is discussed.
©2008 American Institute of Physics
PDF
We report on a new type of carbon extended structure formed by two graphene
layers stacked directly on top of each other (stacking AA). This polymorphic
form of a graphene bi-layer is meta-stable with a distance between planes of
0.156 nm, denoting single covalent carbon-carbon bonding accross the layers.
The size of the 2D hexagonal unit cell is streched from 0.243 nm to 0.267 nm
length, weakening the $sp^{2}$ in-plane bonds.
Depending on the separation between layers, the electronic
structure of the bi-layer changes from semi-metal, to metal, to wide-gap
semiconductor. We describe the electronic structure of these bi-layers
and their behaviour under external anisotropic stresses.
Carbon shows one of the richest chemistry in the periodic table and
it is often found in allotropic forms. In molecules it is the basis
for organic compounds, being central to different fields from biology to
electronics in new materials. In solid state it shows very different
properties drifting from a soft metal (graphite, the most stable
configuration at P=0 GPa, T= 0 K) to a hard wide-gap
semiconductor (diamond). New forms like fullerenes and nano-tubes have
raised even more the interest in carbon for their potential applications.
Recently, the realization of two-dimensional periodic systems made by the
stacking of few graphene layers (FGL), going down to the single layer,
has attracted much interest as the basis for new electronic
devices\cite{novoselov04,berge06}.
The peculiar linear dispersion found in the electronic band structure near
the charge neutrality point (Dirac Point), where the carriers behave like
mass-less chiral relativistic particles, translates in all sort of new
phenomena related to transport properties on these
systems\cite{heersche07,altland06}.
We have found that two graphene layers stack directly on top of each other
to make strong covalent bonds at short distances, unlike the standard weak
van der Waals interaction between layers in graphite (Fig. 1).
This polymorphic form of a graphene bi-layer is meta-stable with a distance
between planes of 0.156 nm and a 0.267 nm length for the hexagonal
two-dimensional (2D) unit cell ($a=b$, $\gamma=120^{o}$;
four carbon atoms). Each carbon in this structure is bonded to the four
nearest carbon neighbours, at 0.154 and 0.156 nm for in-plane and
out-of-plane bonds respectively.
Under these conditions, the bi-layer is a wide gap semiconductor.
As a function of the separation between layers, transport properties of the
AA stacking are rich: at large distances between planes (e.g., as found
in graphite) the system behaves like a low density of states 2D metal or
even a semi-metal, being mostly dominated by the single graphene layer
properties. As the distance between layers decreases it is possible
to find interlayer distances and/or different 2D unit cell sizes where
the bi-layer becomes metallic under their own internal stresses,
opening a new route to understand and control transport experiments on FGL.
These configurations sit in between the meta-stable local energy minimum
reported in this paper (Fig. 2, label "A") and the global one
(graphite-like, "G").
©2008 American Physical Society
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First-principles density-functional theory has been used to
investigate equilibrium geometries, total energies, and diffusion
barriers for H as an interstitial impurity in alpha-Fe. Internal
strain/stress upon hydrogen absorption are a crucial factor to
understand preferred absorption sites and diffusion. For high
concentrations H absorbs near the octahedral site favoring a large
tetrahedral distortion in the bcc lattice. Diffusion paths depend
on the concentration regime too; hydrogen diffuses about ten times
faster in the distorted bct lattice. External stresses of serveral
GPa modify barriers by 10% and diffusion rates by 30%
©2008 American Physical Society
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Clare Hall is a College for Advanced Studies in the University of Cambridge.
It hosts scholars and alumni in many different fields and from a wide variety
of countries over the world.
The President, Prof. E. Salje, has followed and active policy to promote the
development of national societies of people related to the college and a group
of Spanish scholars and alumni interested in that initiative created in 2005
the Clare Hall Spanish Society. Our goal is to help Spanish people interested
to work in the University of Cambridge to find the right environment provided
by such a college as Clare Hall. We have been meeting regularly once per year,
starting in Madrid (2006), followed by Salamanca (2007) and our next meeting
will be in Barcelona (2008). Everyone interested is welcome to come and meet us
(more details in the Clare Hall website that you can reach following the link
above).
Estrella Morente canta a la muerte de su padre:
ESTRELLA