Stable and metastable phases of PTCDA on epitaxial NaCl films on Ag(100)

Thin films of the model molecule perylene-3,4,9,10-tetracarboxylic acid dianhydride (PTCDA) were vacuum deposited on epitaxial NaCl films on Ag(100) and investigated by spot-profile-analysis low-energy electron diffraction (SPA-LEED). At growth temperatures between 193 K and room temperature (RT), the first layer (monolayer) exhibits a commensurate (3 root 2x3 root 2)R45 degrees superstructure with a quadratic unit cell, which is identical to that found for PTCDA growth on bulk NaCl crystals [Burke et al., Phys. Rev. Lett. 100, 186104 (2008)]. At RT, dewetting occurs and complicates the formation of multilayers. However, at low surface temperatures (223 K), closed PTCDA multilayers grow. The corresponding unit cell [19.69(4) angstrom x 12.23(3) angstrom] is rectangular and similar to the herringbone structure of the (102) bulk plane of PTCDA. This growth form is metastable and the surface dewets upon annealing at 300 K. Submonolayers deposited at low temperatures (20-100 K) are disordered; upon annealing, they transform into the (3 root 2x3 root 2)R45 degrees structure via a third metastable phase with a herringbone-type rectangular unit cell.