book of abstracts - IM2NP

A B S T R A C T S THURSDAY, JULY 1 N A N O S E A 2 0 1 0
Room Port-Pin
9H00-9H40
Polarity effects in ultra-thin oxide films.
Jacek Goniakowski and Claudine Noguera (Institut des Nanosciences de Paris, France).
Jacek.Goniakowski @insp.jussieu.fr, Claudine.Noguera @insp.jussieu.fr
Polar surfaces of extended objects present an electrostatic instability and require substantial modifications of
their characteristics in order to become stable. Surface processes related to polarity compensation have been
extensively studied in the last years and are seen as a potential tool for tuning surface properties [1].
With the reduction of the object sizes, as in the case of ultra-thin films, polarity effects gain an additional
dimension and result in specific, thickness-dependent film properties. On one hand, for the thinnest films, the
electrostatic instability may not occur at all. We have predicted that such films may sustain a considerable
dipole moment and thus, contrary to extended systems, may exist in an uncompensated polar state [2]. On
the other hand, at low thickness, polarity effects may extend beyond the surface region. We have predicted
that they may lead to a structural transformation of the entire film, resulting in novel structures, different
from those observed in thicker samples [3]. Additionally, we have shown that the energetic cost of polarity
compensation may be substantially lowered by non-stoichiometry [4]. Oxide ultra-thin films are often
synthesized on metal substrates. As result, their polarity characteristics are further modified by the covalent
and electrostatic couplings which exist at the interface. They induce an interfacial charge transfer and
electrostatic forces, responsible for a non-vanishing polarisation (rumpling) in the oxide film [5]. When
species (molecules or metal atoms) are adsorbed on such ultra-thin films, two qualitatively different
adsorption modes may take place, associated to a local polaronic-like distortion of the film (modification of
rumpling) [6]. Contrary to the macroscopic dipole compensation characteristic of polar surfaces, the
compensation of electrostatic dipoles which takes place at metal-supported oxide films is only partial and
occurs along both polar and nonpolar orientations.
We will exemplify several of these effects, which suggest that an adequate choice of the oxide/substrate
electronic characteristics may be used to tailor surface structural characteristics and thus to tune the
electronic and reactivity properties of such supports.
1. J. Goniakowski, F. Finocchi and C. Noguera, Rep. Prog. Phys. 71, 016501 (2008).
2. J. Goniakowski, C. Noguera and L. Giordano, Phys. Rev. Lett. 98, 205701 (2007).
3. J. Goniakowski, C. Noguera and L. Giordano, Phys. Rev. Lett. 93, 215702 (2004).
4. C. Noguera and J. Goniakowski, J. Phys.: Condens. Matt. 20, 264003 (2008).
5. J. Goniakowski and C. Noguera, Phys. Rev. B 79, 155433 (2009).
6. J. Goniakowski, C. Noguera, L. Giordano, and G. Pacchioni, Phys. Rev. B 80, 125403 (2009).
105