LAP 2006 International Conference on Laser Probing
September 11 - 15, 2006 Vienna, Austria
STRUCTURE DEPENDENT IRRADIATON-INDUCED DESORPTION OF
BIPHENYL ALKANETHIOL SELF-ASSEMBLED MONOLAYERS
S.Wyczawska1, M.Buck2, P. Cyganik3*, P. Lievens1*, Z. Postawa3, R. E. Silverans1, E. Vandeweert1, and F. Vervaecke1
1
Laboratory of Solid State Physics and Magnetism, K.U.Leuven, Celestijnenlaan 200D, B 3001 Leuven, Belgium
2
School of Chemistry, St Andrews University, North Haugh, St Andrews, KY16 9ST, United Kingdom
3
Institute of Physics, Jagiellonian University, ul. Reymonta 4, PL 30-059 Krakow, Poland
1.
INTRODUCTION
Many nanotechnological applications rely on the
patterned functionalization of surfaces with complex
organic molecules.
In this context, self-assembled
monolayers (SAMs) play an important role [1]. SAMs are
spontaneously formed highly ordered and oriented
molecular assemblies on a solid substrate. Although most
of the insight has been compiled for relatively simple
alkanethiol model systems, SAMs containing aromatic
structures are gaining much attention lately. They are
found to be particularly interesting for applications in
molecular electronics [2] or for use as ultrathin resists in
ion [3] or electron-based [4] lithography. The aromatic
moiety changes drastically the intermolecular interaction,
with important implications for the geometric and
electronic configuration.
2.
EXPERIMENTAL METHOD
Our interest is in investigating the sputtering of selfassembled monolayers with an aromatic group using
resonance enhanced multiphoton ionization (REMPI) in
combination with mass spectrometry. We used this
approach to investigate desorbed neutral molecules,
following sputtering with 15 keV Ar+ ions (pulsed beam,
600 ns) hitting the surface at 45° incident angle and with a
spot of a circular cross section of 8 mm2. The photoionization of molecules containing a phenyl ring can be
enhanced by using one-color two-photon ionization
scheme from the ground state using a wavelength  = 259
nm [5,6]. The laser beam was introduced in the vacuum
vessel parallel to the sample surface at 4 mm distance.
3.
RESULTS AND DISCUSSION
Monolayers of ω-(4’-methyl-biphenyl-4-yl)-alkanethiols
(H3C-C6H4-C6H4-(CH2)n-SH, BPn, n=1-6) adsorbed on
Au(111) demonstrate a strong dependence of their
structural configuration in function of the alkane spacer
[7,8], as well as the temperature of preparation or of
*
annealing following the assembly [9]. For an even
number n of methylene groups in the spacer, two stable
phase of BPn/Au were found [Figure 1]. The α and β/δ
phase are respectively characterized by a different bond
angle φ of the molecules with the substrate.
Figure 1. Schematic illustration of odd and even (-and
-phase) BPn SAMs on Au (111)
Mass spectra of desorbed fragments were recorded during
the irradiation of BPn/Au (n=2, 4, 5) corresponding to the
different phases. Systematic phase-related variations were
observed in the desorption behavior, and can be
unambiguously related to structural differences in the
monolayers. This is interpreted as direct evidence that the
different structural phases of these SAMs are characterized
by markedly different proportions of contributions from
the molecule-substrate bonding and the intra- and
intermolecular interactions to the energy balance which
dictates structure and stability of these SAMs.
Acknowledgments. This work is financially supported by
the Belgian Fund for Scientific Research – Flanders
(FWO), the Flemish Concerted Research Action Program
(GOA), the Belgian Interuniversity Poles of Attraction
Program (IAP), the Leverhulme Trust, and the Scottish
Higher Education Funding Council (SHEFC).
4.
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E-mail: cyganik@if.uj.edu.pl, Peter.Lievens@fys.kuleuven.be
LAP 2006 International Conference on Laser Probing
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September 11 - 15, 2006 Vienna, Austria