3,4,9,10 perylenetetracaboxylic dianhydrid (PTCDA) is an interesting organic molecule discussed for applications in optical devices like sensors or displays. The geometric structure formed by thin films is of major importance for the conductivity or the photosensitivity. The present work focuses on the initial growth of PTCDA on Cu(111). The films have been preparated in an ultra high vacuum system by means of organic beam epitaxy. Films with different coverage (up to 20 monolayer) were studied in situ by thermal desorption spectroscopy (TDS) and scanning tunnelling microscopy (STM).
Both for close-packed layers and for crystallites, herringbone structures (with different sizes) are imaged by STM 1,2. The structures consist of rectangular unit cells with two molecules which are rotated by about 90 degree. Only if more than three closed packed layers have been prepared and the sample has been heated up to 520 K nanocrystallites are formed. They were fully characterised in their threedimensional structure2.
To get a closer insight into the energetic of the growth thermal desorption measurements have been carried out. The spectra reveal three peaks, which can be attributed to the second layer, a bulk phase (third and higher layers) and a crystallite phase. There was no desorption from the first layer. The first two peaks can be described by a zero order desorption kinetic, the third seems to be of higher order. The signal which is attributed to the crystallites appears at temperatures which are higher than for the other two peaks.
A simple and reliable algorithm for the analysis of the energy of desorption was developed. Furthermore numerical calculations have been carried out to model the spectra. Especially the shape of the peak assigned to the crystallites can be ascribed to their distribution of size (height and area). To conclude we succeed to explain the complex desorption spectra and to correlate them to the STM findings.
1Th. Wagner, A. Bannani, C. Bobisch, H. Karacuban, R. Möller, J. Phys.: Condens. Matter 19 (2007) 056009.
2Th. Wagner, A. Bannani, C. Bobisch, H. Karacuban, M. Stöhr, M. Gabriel, and R. Möller, Organic Electronics 5, 35-43 (2004). |