Phthalocyanines (Pcs) form crystals whose structure and morphology depend on the growth conditions,
leading to changes in the physical properties which are still little understood. Pc thin films and
nanostructures have already been exploited in optoelectronic applications and could form the basis of
spintronic devices but little or contradictory structural information is available because they are
challenging systems to study. Hence the precise determination of the molecular order in these systems is
of considerable interest both from a fundamental and technological point of view but requires a
combination of complementary techniques.
Crystalline powders of α-copper phthalocyanine (CuPc), α-metal-free phthalocyanine (H2Pc) and their
mixtures are studied using powder X-ray diffraction (XRD) and found to be isomorphous and adopt a
triclinic structure first proposed for α-CuPc (Hoshino et al., 2003). This information is used to study highly
textured crystalline α-Pc thin films. The texture reduces the available crystallographic information but
allows for the manipulation of the anisotropic physical properties. The Pc molecular plane lies 82±11° to
the substrate when deposited on a weakly interacting substrate but at 7 or 9±5° when templated by a
layer of perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA). Such an interpretation is different to all
those previously given.
The change in the texture is confirmed by high resolution transmission electron microscopy (HRTEM) of
ultramicrotomed cross-sections of the films. The optimum TEM operating conditions were first determined
on sections of CuPc single crystals which demonstrated an information limit of ~5Å with HRTEM. The
technique was then applied to the films and the morphology, crystallinity and texturing of the layers is
largely retained by the sectioning process. With further refinements it is hoped that this technique could
be used to study the properties of interfaces and individual domains in multilayers and blends of organic
thin films.
Lastly the crystal structure of a new CuPc phase designated as η which forms nanowires as thin as 10nm
and shows enhanced absorption in the infra-red (IR) is proposed. XRD, transmission electron diffraction
(TED) and lattice potential energy (LPE) minimisation were used to determine the crystal structure:
monoclinic P21/a, Z = 2, a = 24.8±0.2Å, b = 3.77±0.02Å, c = 13.2±0.1Å and β = 106±1°. The LPE
minimisation was validated by correctly predicting the atomic coordinates of β-CuPc to within 0.05Å.