RESEARCH

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RESEARCH

• Nano-engineering of novel thermoelectric
  materials
• Chemical bonding in periodic systems
• Topological analysis of scalar fields from
  periodic wfs
• Developments of QTAIM the Source Function
• Chemical bonding at surfaces

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The Source Function

• Several descriptors providing useful chemical
  insight may be extracted from the wave function.
  Among these, particularly interesting are those
  obtained also from experiment.
• In this line, we recently introduced the
  source function (SF) enabling one to retrieve the
  non local character of the electron density -
  which is manifest from its expression in terms of
  the N-electron wave-function or of the pair
  density - using information derived from the
  electron density (ED) only.
• The SF can be thus obtained either
  theoretically or experimentally, using, for
  instance, EDs derived from X-ray diffraction
  and/or quantitative convergent beam electron
  diffraction (QCBED) data.

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The Source Function

• The SF is a model-independent, quantitative
  measure of the relative importance of an atoms
  or groups contribution to the ED at any point in
  a system and it represents an interesting tool to
  provide chemical insight.
• It discloses the non-local character of the
  electron density and quantifies such a
  non-locality in terms of a physically sound and
  appealing chemical partitioning

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The Source Function Theory
The electron density ?(r) at any point r within a
molecule may be viewed as consisting of
contributions from a source G(r,) operating at
all other points . By evaluating the source over
regions bounded by surfaces that satisfy the
topological definition of an atom, the electron
density at r, may be equated to a sum of atomic
source contributions S(r,?)
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The Source Function
Basic paper R.F.W. Bader, C. Gatti, A Greens
function for the density, Chem. Phys. Lett. 287,
233-238 (1998) The Source Function in action
C. Gatti, F. Cargnoni, L. Bertini, Chemical
Information from the Source Function, J. Comput.
Chem. 24, 422-436, (2003) The Local Form of the
Source Function C. Gatti, L. Bertini, 'The
Local form of the Source Function as a
fingerprint of strong and weak intra- and
inter-molecular interactions', Acta Cryst. A60,
438-449 (2004).
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The Source Function

• The SF enables one to classify hydrogen bonds in
  terms of characteristic source contributions to
  the density at the H-bond critical point.
• These contributions come from the H involved in
  the H-bond, the H-donor, the H-acceptor and the
  remaining atoms in the molecular complex.
• As shown in the figure on the next page, the SF
  from the H appears as the most distinctive marker
  of the H-bond strength, being highly negative for
  isolated H-bonds, slightly negative for polarized
  assisted H-bonds, close to zero for
  resonance-assisted H-bonds and largely positive
  for charge-assisted H-bonds. The contributions
  from atoms other than those directly involved in
  the H-bond strongly increase with decreasing
  H-bond strength, consistently with the parallel
  increased electrostatic character of the
  interaction.
• An interesting correspondence between the
  classification provided by the SF and the
  Electron Localization Function (ELF) topological
  approach was also highlighted.  

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The Source Function
Source contributions to the electron density at
the H-bond critical point in 1 , H5O2 2
formic acid-formate anion 3 malonaldeyde, Cs
equilibrium form 4 malonaldeyde, C2v transition
state for H-migration 5 cyclic homodromic water
trimer 6 water dimer at equilibrium geometry 7
acetylene-water complex. Sources are displayed
as circles whose size is proportional to the
percentage contribution from each atom, with
positive sources in blue and negative sources in
yellow. C. Gatti, F. Cargnoni, L. Bertini, J.
Comput. Chem. 24, 422-436, (2003)
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Nano-engineering of novel thermoelectric
materials
Thermoelectric (TE) materials have a unique
position for their dual purpose electrical
generation on one side and cooling/heating on the
other side. Power generation is achieved by
applying a temperature difference between two
ends of the TE material, while cooling or heating
is obtained by applying electrical current. TE
devices have very attractive features, such a
small size, simplicity, reliability, no need of
maintenance and have important terrestrial and
space applications. They will also play an
important role in the development of clean and
efficient cooling and energy conversion systems.
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Nano-engineering of novel thermoelectric
materials

• Cars, as an example, loose about 1/3 of the
  energy directly as heat in the exhaust, which can
  partly be recovered with an efficient TE device
  and converted to useable electricity resulting in
  overall vehicle operation economy and sizeable
  impact on CO2 emission. TE coolers would bring
  about a reduction in ozone depletion due the
  fluorocarbons used in standard vapour compression
  systems and would yield a quieter environment,
  since they operate without moving parts.
• Yet, to economically compete with conventional
  cooling and energy conversion systems, the
  efficiency of current TE devices must be largely
  increased, mostly through an improvement of the
  properties of the kernel of the device, the two
  TE materials forming each TE element.

Semiconductors are cool C. Vining, Nature, 413,
557 (2001)
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Nano-engineering of novel thermoelectric
materials
The device efficiency depends on the material
properties through the dimensionless
thermoelectric figure of merit ZT of the material
where ? and k are the electrical and thermal
conductivity (with an electronic ke and a kl
lattice contribution), S the thermopower and T
the absolute temperature.
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NanoThermel
Our group at ISTM is one of the seven European
partners, from six member states, involved in a
joint effort aimed at developing nano-engineered
high performance thermoelectric materials and
devices. Such a team project, named NanoThermel,
is funded by the European Community within the
Vth Framework Program.
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NanoThermel
Our project has developed a viable strategy for
improving ZT, based on the use of nanotechnology
as well as on the introduction of structural
modifications of new classes of host-guest TE
materials like the filled skutterudites and the
inorganic clathrates. Materials containing
amorphous-like interstitial metal atoms within a
semi-regular semiconductor framework, like the
zinc antimonide, have also been investigated.
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NanoThermel
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Nano-engineering of novel thermoelectric
materials
Nanostructuring of cobalt skutterudite led to a
dramatic decrease of k, one order of magnitude
lower than conventional skutterudite, due to much
higher density of grain boundaries. Such a
promising result was partly offset by the
concomitant decrease of S and ?. The ongoing
search for optimum doping and for synthetic
pathways preventing oxidation of grains surfaces,
is aimed at opposing such a decrease. Filling the
cubic voids of skutterudites or the cages of
clathrates with atoms that are small enough to
rattle and create dynamical disorder also
reduces k, without much adverse the electronic
transport. By combining such a void filling with
suitable substitutions (doping) into the host
framework, a good compromise between decrease of
k and concomitant decrease of S and ? may be
hopefully achieved.
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The role of the theoretical modelling
Kel?T-1(L2 L1L0-1L1)

• Upon doping, we can predict
• and rationalize changes in
• cell parameters and fractional coordinates
• Electronic structure (band gap, EDDs topology,
  DOSs
• Electron transport properties
• If different structures are present in the
  material, theory indicates which one is the most
  beneficial, directing the synthethic efforts to
  increase the of such a structure in the TM.

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Comprehensive review on Chemical bonding in
crystals new directionshttp//www.zkristallogr.d
eC. Gatti, Zeitschrift für Kristallographie,
220, 399-457 (2005) special issue
onComputational Crystallography, Artem Oganov
Editor, ETH Zürich, CH