Book Description

Organic thin film devices’ performance depends on their properties, such as the crystallinity of the active layer and the specific arrangement of molecules in it. The morphology of molecular structures can be studied using imaging techniques (such as atomic force microscopy AFM, scanning tunneling microscopy STM, and scanning electron microscopy SEM),but to gain a more complete understanding of their nature, temperature programmed desorption (TPD) may be used. During a TPD experiment, the temperature of the adsorbate-adsorbent system is increased linearly while simultaneously the number of desorbing molecules is measured. The resulting TPD spectrum may be used to determine the type of structure (monolayer, multilayer) and the desorption energy associated with it. Furthermore, the thermal stability of observed structures, as determined by the temperatures at which desorption occurs, is evident from the TPD spectra. Thus, TPD measurements may lead to a better understanding of the growth and characteristics of molecular structures which may be of use for potential applications in organic electronics. The first aim of this thesis was the assembly and calibration of the TPD system. The optimal parameters of the mass spectrometer (ionizer’s emission current and electron energy) were determined. Moreover, the linear heating of the samples using direct resistive heating was established. The optimal mass-to-charge ratio for para-hexaphenyl (6P) was determined based on the mass spectrum of 6P. The final test of the TPD system was a comparison of the measured TPD spectrum of 6P film grown on mica(001) with the one published in the literature. The second aim was the measurement of spectra of 6P films on ion-beam modified (110) surfaces of TiO2. Two substrates of different topographies (determined by STM) were studied: one with deep ripples (height modulation of 2.6nm) and one with shallow ripples (height modulation of 0.76nm).In the case of the deep-rippled sample, the effect of molecular film coverage, and the desorption from the sample holder on the measured spectra was investigated. In contrast, the shallow-rippled sample was only studied in the context of the effect of desorption from the sample holder on the spectra. The morphology of 1.91nm thin film of 6P on the deep-rippled sample was studied using SEM, while the morphology of a 0.57nm thin 6P film on shallow-rippled TiO2(110) was determined using STM and AFM. Additionally, the desorption of 6P powder from an effusion cell was studied for comparison of the bulk and thin-film desorption parameters. The optimal parameters for the spectrometer’s ionizer were determined to be: emission current of 700μA, electron energy of 37eV, and use of two filaments. The chosen mass-to-charge ratio was 229.35, e.i. 6P with 2+ charge. The TPD spectrum of 1.91nm of 6P on mica(001) contained a multilayer peak at temperature 222℃ which was in agreement with published data. Due to desorption from the sample holder, all spectra in the TPD experiments showed additional peaks and an increase in background intensity. This effect was almost completely eliminated by covering parts of the holder with tantalum sheets during the molecule deposition process. The TPD spectra for 6P grown on both samples contained two peaks: one which began at approximately 180℃ and ended at different temperatures depending on coverage and another which began at 300℃(the complete spectrum for the deep-rippled sample was not measured, but the peak ended at 550℃ for the shallow-ripples sample). The first peak is due to multilayered molecular structures, which, based on morphologies of the samples, can be identified as originating from standing-molecule islands and laying-molecule nanoneedles in the case of deep-rippled, and laying-molecule nanoneedles for shallow-rippled TiO2(110). The second peak is probably due to a monolayer of 6P molecules laying directly on the TiO2 surface (such a layer was observed for the shallow-rippled sample).