Crystal growth of tetraphenylporphyrin thin films

The crystal growth and the orientation of thin films of 5,10,15,20-tetraphenylporphyrin (H 2 TPP) vacuum-evaporated onto substrates of a KCl (001) surface and a semitransparent A1 electrode film have been investigated by electron microscopy and electron diffraction. The films deposited on a KCI (001) surface show epitaxial growth in which the crystals take a four-directional orientation, and the molecular plane of HzTPP stands perpendicular to the substrate surface. The H 2 TPP crystals with a morphology of rectangular islands grow on the KCI surface kept at 200 ° C, while a continuous thin film totally covers the KCI surface kept at 20 ° C with an identical epitaxial orientation. When H 2 TPP is deposited on a KCI (001) surface coated partially with a small number of Al particles, the crystals take the same orientation. This epitaxial orientation decreases with increasing the coverage of the A1 particles on the KCI surface, and the H 2 TPP film deposited on a semitransparent A1 film shows a random orientation of needle-like crystals. The growth of the thin film with oriented crystals depends on the electrostatic interaction of the H 2 TPP molecules with ionic charges on the KCI surface.