Small molecule organic semiconductors like para-sexiphenyl (6P) can be used for a wide range of applications. 6P in particular has shown a high potential as optical active layer in organic light emitting diodes, lasers [1] and other light emitting or harvesting devices because of it's 3.1 eV band gap, which makes it a blue emitter. On crystalline substrates 6P normally exhibits a highly anisotropic growth morphology [2,3] and can therefore be adapted for wave guiding applications[4]. However, the high mobility of up to 14 cm2/(Vs) [5] makes it also an ideal candidate for organic thin film transistors. For all these applications the control of molecular orientation and a fundamental understanding of growth dynamics in the thin films is crucial. For inorganic epitaxy a number of models exist that accurately describe the processes during growth. However, due to the intrinsic anisotropy of organic molecules a transfer of these models to organic growth is not always possible.
Here, we present an atomic-force microscopy investigation of 6P thin films grown on mica(001) after surface modification. Ion bombardment prior to deposition or the use of carbon as a surfactant allows the growth of upright standing molecules[6]. The resulting island morphology is discussed qualitatively and quantitatively. The film is characterized by growth mounds separated by deep grooves. Such a morphology is well known from inorganic growth [7]. Quantitative analysis reveals an almost Poisson distribution of the layer fractions indicating a high step edge barrier for interlayer diffusion.
This work was financially supported by the FWF National Research Network "Interface controlled and Functionalised Organic Films" (S9707-N08)
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