PHTHALOCYANINE AGGREGATES ON METAL DICHALCOGENIDE SURFACES - DYE SENSITIZATION ON SNS2 SEMICONDUCTOR ELECTRODES BY ORDERED AND DISORDERED INPC-CL THIN-FILMS

Photoelectrochemical dye sensitization processes have been explored for various coverages of ultrathin films of chloroindium phthalocyanine (InPc-Cl), on single crystal metal dichalcogenide (SnS2) surfaces. Films were prepared by conventional vacuum deposition and by a process which led mainly to flat-lying, epitaxial Pc layers, exhibiting a ''layer-by-layer'' growth mode. These experiments were carried out in parallel with optical characterization of InPc-Cl layers grown on nonconductive transparent SnS2 thin films, where both the Pc and SnS2 thin films were grown by a molecular beam epitaxy (MBE) process on freshly cleaved mica. Surface electron diffraction data were collected during the organic/inorganic-molecular beam epitaxy (O/I-MBE) experiments, on both bulk and MBE-grown SnS2 and on bulk MoS2. These data suggest that (a) InPc-Cl can deposit in a flat-lying ordered monolayer, consisting of one or two sets of 3-fold rotations of coincident 3 x 2 square lattice domains aligned along the principal axes of the metal dichalcogenide, the ordering of which can be extended to multilayers of InPc-Cl, or (b) the InPc-Cl can deposit in a strict three-dimensional growth mode, which produces less ordered multilayers. Low coverages of both types of InPc-Cl ultrathin films sensitize the SnS2 photoelectrochemical response with quantum yields for charge injection per absorbed photon (QYAP) greater-than-or-equal-to 10%. Low coverage (below monolayer) InPc-Cl deposits produce photocurrent yield spectra nearly as narrow as the solution absorption spectra (fwhm 35 nm). Both the highly ordered and less ordered InPc-Cl multilayers continue to produce sizeable QYAP values out to coverages of tens of monolayers. The ''epitaxial'' deposits show both absorbance and photocurrent yield spectra which are much narrower (fwhm less-than-or-equal-to 60 nm) than those for polycrystalline deposits, consistent with the more homogeneous Pc environment in these ultrathin films.