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V Encuentro Sud Americano de Colisiones Inelásticas en la Materia
Secondary Ions emission from Alkanethiol-SAMs due to highly charged ions
bombardment
M Flores 1 , V. Esaulov 2 and Y. Yamazaki 3
1 Depto. de Física, Facultad de ciencias físicas y matemáticas, Universidad de Chile, casilla 110-V, Santiago, Chile.
2 Laboratoire des Collisions Atomiques et Moleculaires, Universite Paris-Sud, Orsay Cedex, France.
3 Atomic Physics Laboratory, Riken Institute, Wako, Saitama, Japan.
email address corresponding author: mflorescarra@ing.uchile.cl
Self-assembled monolayers (SAMs) are
ordered molecular assemblies formed by the adsorption
of an active surfactant on a solid surface.
SAMs provide a convenient, flexible, and
simple system with which to tailor the interfacial
properties of metal, metal oxides and semiconductors
[1]. The alkanethiols are a common type
of molecules used to build SAMs.
Several techniques have been used to
study and characterize SAMs, for example SPM,
optical and ion spectroscopies. In the latter case,
the interaction of ions with SAM surfaces leads
to the sputtering of molecules from the surface,
with the detection of such molecules giving information
on both the chemical and structural
composition of the SAM. For this reason much
experimental and theoretical effort has been directed
towards ion-SAM collisions. Special case
is the highly charged ions (HCI).
For slow HCI, the potential energy stored
in the projectile can far exceed its kinetic energy.
In contrast to the kinetic sputtering process,
which is due to momentum transfer from the
ion to the surface, HCI may also transfer significant
potential energy, removing ions and molecules
from the surface in a process called potential
sputtering.
SAMs of the alkanethiol molecules Undecanethiol
(UDT), HS(CH 2 ) 10 CH 3 , and Dodecanethiol
(DDT), HS(CH 2 ) 11 CH 3 , were prepared
on atomically flat Au(111) which was deposited
as a thin film on freshly cleaved mica.
The quality of the SAMs was confirmed by
STM observations [2]. The samples were bombardment
with Ar q+ ions.
The q-dependence of the proton sputtering
yield from hydrogen contaminated/covered surfaces
has been found to follow a power law dependence
and this was explained by the classical
over barrier model. In this model a HCI approaching
an atom/molecule induces multielectron
transfer. In the case of our SAM it
would induce electron transfer from the alkanethiol
molecule terminal functional group.
Removal of two electrons from the most external
part of the SAM, would create a doubly charged
chemical bond (C-H) 2+ . Because the molecule is
a poor conductor, the reneutralization probability
in the molecular layer should be lower than
on a metal, and a proton may be released in the
bond direction by Coulomb repulsion [3]. In
general, in the above model the proton yield is
then dependent on the probabilities of removal
of the first and second electron and on reneutralization
as the ion moves away from the surface,
given a power law, Fig 1(a,b), and in the
case under study the probabilities are strong dependent
of the orientation of the terminal functional
group, Fig. 1(c,d).
Figure 1. Proton Yields from (a)UDT and (b)DDT
under bombardment with Ar q+ ions. Top surface
scheme of the SAMs corresponding to (c)UDT and
(d)DDT.
References
[1] Love et al, Chem. Rev. 105 (2005) 1103.
[2] O’Rourke et al, Appl. Phys. Lett., submitted.
[3] Flores et al, Phys. Rev. A79 (2009) 022902.
54 Valparaíso, Chile