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 fil ms on the hcp (0001) tin disulfide (SnS2) surface is reported. Growth conditions can be achieved that lead to FAlPc films of polymer-like ag gregates (FAlPc), with the long axis of the aggregate apparently stand ing perpendicular to the surface. When the substrate is held at a temp erature of 260 degrees C during deposition, reflection high-energy ele ctron diffraction of the resultant thin films showed that FAlPc grows epitaxially on SnS2 in a square lattice structure, starting with the f irst monolayer, with a unit cell dimension of 13.2 +/- 0.8 Angstrom. O ne axis of the resulting FAlPc square lattice overlayer is oriented al ong the main axis of the basal plane SnS2. Scanning tunneling microsco py (STM) showed elongated crystallites growing along the main crystall ographic 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 observe d for these thin films (642 nm), which is blue-shifted from the monome r absorbance maximum at ca. 675 nm. The Q-band spectrum of the epitaxi al film is much narrower (fwhm 28 nm) than comparable spectra from les s-ordered thin films of (FAlPc)(n) aggregates deposited on sapphire (f whm 55 nm). The polarization dependence of these Q-band spectra was co nsistent with individual FAlPc molecules in these thin films, on avera ge, lying parallel to the surface plane. Dye sensitization/photoelectr ochemistry showed photocurrent action spectra that paralleled the abso rbance spectra and made possible the characterization of even monolaye r-level coverages. Quantum efficiencies for charge injection from the FAlPc aggregates into the SnS2 conduction band (less than or equal to 5%) was lower by at least a factor of 10, relative to previously explo red phthalocyanine sensitizers which do not form linear cofacial aggre gates.