Ordered ultrathin layers of organic semiconductors are of great interest since they offer a alternative technology for building electronic devices. Charge transport mechanisms or electroluminescence efficiencies can be varied, for instance, by modification of the structure or by mixing different molecules. In our work the of growth of a mixed monolayer covered with N-N´-dimethylperylene-3,4,9,10 -dicarboximide (DiMe-PTCDI) and 3,4,9,10 perylenetetracaboxylic dianhydrid (PTCDA) on Cu(111) has been investigated for different preparations.
Experimental:
Preparation and analysis have been carried out in an ultra high vacuum (UHV)-chamber at a base pressure of 2*10-10 mbar. About 0.4 ML DiMe-PTCDI and 0.4 ML PTCDA have been evaporated consecutively from an oven at a constant rate, controlled by a quadrupol mass spectrometer. In a first experimental session the sample was held at room temperature during the preparation and analyse. In a second session the sample was cooled down to 200K during the evaporation and annealed to 520K afterwards. The STM-data were acquired in situ at room temperature with a self-built STM optimized for low tunnelling currents of 1..2pA.
Results:
DiMe-PTCDI and PTCDA show both highly ordered monolayer growth on Cu(111) [1]. We have been able to distinguish at least four different phases for DiMe-PTCDI and two for PTCDA for the first monolayer on Cu(111). Independed of the preparation method we observe only separated phases of the both molecules.
Consecutively evaporation at room temperature leads to the known unit cells for both molecules. For DiMe-PTCDI these are a herringbone like structure, two oblique unit cells and the star phase, which occurs rarely. For PTCDA two modifications of the herringbone structure were observed. The second preparation session resulted in a different layer. Again, PTCDA is ordered in the herringbone structure. DiMe-PTCDI is ordered in the Star Phase to 80 percent. About 10 percent growth in the herringbone structure, which is in contrast to the layer grown with the first preparation method.
[1] Th. Wagner, A. Bannani, C. Bobisch, H. Karacuban, R. Möller, J. Phys.: Condens. Matter 19 |