Thermal Stability and Interlayer Exchange Processes in Heterolayers of CuPc and PTCDA on Ag(111)

The structural properties and thermal evolution of heterolayers comprising copper-phthalocyanine (CuPc) and 3,4,9,10-perylene-tetracarboxylic-dianhydride (PTCDA) on Ag(111) were investigated using FT-IRAS, SPA-LEED, and TDS. The bilayer systems have been prepared at low temperatures (T ≃ 80 K) and annealed to successively higher T. The layered arrangement of CuPc deposited onto PTCDA/Ag(111) displays unusual thermal stability up to 450–500 K when intermixing sets in. For layers with a reversed stacking sequence, exchange processes of first- and second-layer molecules take place already at 200–300 K. PTCDA shows a stronger tendency to occupy first-layer sites, displacing CuPc from the direct contact with the metal substrate; continued annealing, however, leads to a depletion of PTCDA within the deposited organic film. This apparent paradoxon is resolved by comparing two related quantities, the adsorption energy per area (for parallel adsorbed molecular species) and the adsorption energy per molecule. As a consequence of the dissimilar “footprint” areas of CuPc and PTCDA, and the larger overall size of CuPc, the former quantity is larger for PTCDA, while the latter quantity is larger for CuPc. The experimental observations regarding layer exchange, intermixing, and modifications in thin film composition and structure are then readily explained.