L.-K. Chau

Orientation and structure of monolayer →→ multilayer phthalocyanine thin films on layered semiconductor (MoS2 and SnS2) surfaces

Thin films of both chloroindium and copper pthalocyanines have been vacuum deposited onto metal dichalcogenide surfaces such as MoS2 and SnS2, with ordering achieved for these four‐fold symmetric molecules ranging from below monolayer to multilayers. Reflection high‐energy electron diffraction suggests that square lattice geometries are adopted for low coverages of each phthalocyanine (Pc), but with multiple domains. Low‐energy electron diffraction confirms the presence of three square lattice domains, each domain rotated by 60° with respect to the other. Basal plane defects, and especially terrace sites in the metal dichalcogenide surface, are implicated as the nucleation sites for the growth of these square lattice domains. Optical spectroscopies have been used to characterize submonolayer to multilayer deposits of chloroindium phthalocyanine on SnS2 thin films, where the packing geometries of the adjacent Pcs cause perceptible changes in the position and width of the absorbance band in the visible/near...

RHEED and optical characterization of ordered multilayers of phthalocyanine⧸C60 and phthalocyanine/perylene-tetracarboxylicdianhydride (PTCDA)

Ordered monolayers → multilayers of trivalent metal phthalocyanines, C60, and the perylenetetracarboxylic dianhydride (PTCDA) have been formed by molecular beam epitaxy processes on both single crystal MoS2 and SnS2, and on MBE-deposited SnS2 thin films. The bulk packing structures for the trivalent metal chloride phthalocyanines lend themselves to layered growth during deposition, as do the structures for C60 and PTCDA. RHEED data collected during the formation of [(InPc-Cl)1−4 ML/(C60)1−4 ML]1−20/SnS2/mica multilayers suggests that the structures observed during the formation of single component thin films can be sustained during multilayer formation, up to several lattice periods. Absorbance spectra of Pc/C60 and Pc/PTCDA multilayers show Pc Q-bands with maxima and full widths consistent with the formation of ordered Pc layers, provided that the Pc layer thickness is kept at 1–2 monolayers throughout the formation of the multilayer assembly.

Dye aggregates and organic superlattices formed by organic-inorganic molecular beam epitaxy

Abstract We describe the formation of ordered ultrathin films of chloroindium phthalocyanine (InPcCl) on thin films of the layered semiconductor SnS 2 , both of which are deposited by a molecular beam epitaxy process to form InPcCl/ SnS 2 /mica assemblies. The resultant thin films show markedly narrowed linear absorbance spectra versus those seen previously for Pc thin films. Low energy electron diffraction and reflection high energy electron diffraction data collected for the first few monolayers of InPcCl deposited on the SnS 2 substrate suggest that three (equally populated) ordered Pc domains are formed, with the Pc rings parallel to the SnS 2 surface, aligned along the principle axes of this hcp SnS 2 substrate. The red-shift and peak shape of the absorbance spectra of these thin films (about 758 nm, full width at half maximum of about 60 nm) are consistent with the type of splitting of the exciton energies expected of Pc aggregates in which the adjacent Pc rings are cofacial, but staggered in both x and y directions by about half a molecular diameter. This is the structure determined from related trivalent metal Pcs by single crystal X-ray diffraction. Loss of the flat-lying configuration of Pc in these ultrathin films is immediately apparent from the surface electron diffraction data.

Organic/Inorganic Molecular Beam Epitaxy (O/I-MBE): Formation and Characterization of Ordered Phthalocyanine Thin Films – Photoelectrochemical Processes

Abstract – Photoelectrochemical processes are described for epitaxial phthalocyanine thin films on SnS2 single crystal photoelectrodes. Square lattice formation is indicated during the nucleation of the ordered monolayers of the trivalent metal Pcs (e.g. InPc-CI) and the tetravalent metal Pcs (e.g. VOPc). Multilayer growth leads to packing structures which strongly red-shift (VOPc, InPc-CI) or blue-shift (AIPc-F) the Q-band photocurrent spectra, in accordance with established models for exciton coupling effects. Quantum yields for photocurrent production versus Pc coverage are consistent with the requirement for exciton diffusion from the point of excitation in the Pc multilayer to the SnS2 interface, where charge injection occurs.

O/I-MBE: formation of highly ordered phthalocyanine/semiconductor junctions by molecular-beam epitaxy: photoelectrochemical characterization

Highly ordered phthalocyanine (Pc)/inorganic semiconductor heterojunctions have been created by a process of molecular beam epitaxy in ultrahigh vacuum environments (O/I-MBE). The layered metal dichalcogenides, MoS2, and SnS2, have been used as substrates, either as melt grown crystals or as thin films, created by the MBE process, immediately prior to the deposition of the organic layer. Surface electron diffraction (RHEED) is used to ascertain the type and extent of ordering of the Pc layer. Absorption or reflection spectroscopies are used in the visible wavelength region to characterize the shape and position of the Q-band absorbance of the ordered dye layers, which confirm the ordering seen by RHEED. When deposited on doped SnS2 crystals, and immersed in aqueous electrolytes, the photocurrent yield of these dye layers can also be measured, which provides an alternative means of obtaining the Q-band spectrum for ultrathin dye layers.