Epitaxial Growth of the Ionic Polymer Fluoroaluminum Phthalocyanine on the Basal Plane of Single Crystal Tin Disulfide

The epitaxial growth of fluoroaluminum-phthalocyanine (FAlPc) thin films on the hep (0001) tin disulfide (SnS₂) surface is reported. Growth conditions can be achieved that lead to FAlPc films of polymer-like aggregates (FAlPc)_n with the long axis of the aggregate apparently standing perpendicular to the surface. When the substrate is held at a temperature of 260 °C during deposition, reflection high-energy electron diffraction of the resultant thin films showed that FAlPc grows epitaxially on SnS₂ in a square lattice structure, starting with the first monolayer, with a unit cell dimension of 13.2 ± 0.8 Å. One axis of the resulting FAlPc square lattice overlayer is oriented along the main axis of the basal plane SnS₂. Scanning tunneling microscopy (STM) showed elongated crystallites growing along the main crystallographic axes of the substrate, and these same STM data indicate that phthalocyanine film growth may be initiated at step sites on the SnS₂ surface. The development of H-aggregate (FAlPc)_n in these thin films is confirmed by the position of the Q-band absorbance maximum observed for these thin films (642 nm), which is blue-shifted from the monomer absorbance maximum at ca. 675 nm. The Q-band spectrum of the epitaxial film is much narrower (fwhm 28 nm) than comparable spectra from less-ordered thin films of (FAlPc)_n aggregates deposited on sapphire (fwhm 55 nm). The polarization dependence of these Q-band spectra was consistent with individual FAlPc molecules in these thin films, on average, lying parallel to the surface plane. Dye sensitization/photoelectrochemistry showed photocurrent action spectra that paralleled the absorbance spectra and made possible the characterization of even monolayer-level coverages. Quantum efficiencies for charge injection from the FAlPc aggregates into the SnS2 conduction band (≤5%) was lower by at least a factor of 10, relative to previously explored phthalocyanine sensitizers which do not form linear cofacial aggregates.