Thermal desorption of organic molecules
Adolf Winkler
Institute of Solid State Physics, Graz University of Technology, Petersgasse 16, A-8010
Graz, Austria
E-mail: a.winkler@tugraz.at
Abstract. The applicability of thermal desorption spectroscopy (TDS) to differentiate
between a strongly bound wetting layer and the subsequently formed multilayers of organic
thin film is discussed. After describing the fundamentals of TDS, particularly for large
organic molecules, several model systems are presented (p-4P on Au(111), p-6P on
Au(111), mica(0001) and KCl(001)), demonstrating the importance of the substrate
material, surface structure and surface contaminations on the formation of a wetting layer.
The wetting layer acting as a template for the multilayer growth strongly influences the
structure and morphology of the organic thin film.
1.
Introduction
Thermal desorption spectroscopy (TDS) is a powerful and well known
experimental technique to investigate adsorption/desorption kinetics and energetics of small (inorganic) molecules on surfaces [1,2]. The application of this
technique for large organic molecules and ultra-thin organic films is not that
widely acknowledged. In this review I will demonstrate that TDS can be successfully used to obtain information on a number of parameters characterizing organic
films in the sub-monolayer, monolayer and near multilayer regime. In particular,
the frequently put question as to the existence of a wetting layer prior to the
growth of island like films can be answered unambiguously. In addition, the
binding energy of the first monolayer and the heat of evaporation of the multilayer
can be determined. Other kinetic parameters of adsorption and desorption, like the
sticking coefficient and the pre-exponential factor for desorption can be obtained
as well.
In this contribution I will exemplify the power of TDS on some model
systems: p-quaterphenyl (p-4P) on Au(111) and p-hexaphenyl (p-6P) on Au(111),
mica(0001) and KCl(001). I will particularly show that the chemical composition
(carbon contamination) and the morphology (surface roughening by sputtering) of
the substrate surface can have a tremendous effect on the thin film growth.
2.
Fundamentals of TDS
For thermal desorption spectroscopy (TDS) the material is first put onto the
surface and thereafter the sample is heated with a constant heating rate until the
material is desorbed again from the surface. The desorbing particles are detected
with a mass spectrometer and the signal vs. time (or temperature) is called the TD