The development of molecular nanostructures plays a major role in emerging organic electronic applications, as it leads to improved performance and is compatible with our increasing need for miniaturisation. In particular, nanowires have been obtained from solution or vapour phase and have displayed high conductivity 1, 2 or large interfacial areas in solar cells. 3 In all cases however, the crystal structure remains as in films or bulk, and the exploitation of wires requires extensive post-growth manipulation as their orientations are random. Here we report copper phthalocyanine (CuPc) nanowires with diameters of 10-100 nm, high directionality and unprecedented aspect ratios. We demonstrate that they adopt a new crystal phase, designated η-CuPc, where the molecules stack along the long axis. The resulting high electronic overlap along the centimetre length stacks achieved in our wires mediates antiferromagnetic couplings and broadens the optical absorption spectrum. The ability to fabricate ultralong, flexible metal phthalocyanine nanowires opens new possibilities for applications of these simple molecules.Keywords: nanowire, phthalocyanine, organic vapour phase deposition, polymorph, molecular magnetism, molecular optoelectronics.Organic vapour phase deposition (OVPD) has been very successful at generating a wide range of film morphologies with a number of functional molecular materials, including smooth amorphous films, 4 textured islands, 5 and, more recently, nanowires . 6 The latter have been investigated for a variety of applications including sensors, 7 field effect transistors 1-2, 8 and photovoltaic cells. 3 So far, with the exception of 2 some very short nanobrushes, 9 only wires randomly oriented on a substrate have been produced. Their implementation on a device therefore requires micromanipulation, which is severely limiting for future applications where texture and length are desirable.Metal phthalocyanines (MPcs) are planar aromatic macrocyles which crystallise as a range of at least fifteen distinct polymorphs. 10,11 Typically, thin films exist as either α or β-phases, which have also been observed in relatively thick ( > 100 nm in diameter) wires. 6 The optoelectronic 12 and magnetic 13 properties of Pc materials are strongly dependent on their structure, and the creation of a new polymorph can have important consequences both for devices and in industry where α, β and ε-CuPc forms are routinely used as pigments.Through the optimisation of growth parameters in the OVPD we have obtained CuPc nanowires with diameters of 10-100 nm for lengths up to 1.4 cm. They adopt a new structure, with novel electronic absorption and magnetic properties. Their high directionality could significantly facilitate parallel integration into devices, and leads to confined antiferromagnetically coupled spin chains along the long wire axis. Figure 1.a shows a photograph of the CuPc branches after a growth time of typically 120 minutes. Those branches nucleate in a region of the sample tube which is ~ 10 cm outside the...

Nanostructure and molecular orientation play a crucial role in determining the functionality of organic thin films. In practical devices, such as organic solar cells consisting of donor-acceptor mixtures, crystallinity is poor and these qualities cannot be readily determined by conventional diffraction techniques, while common microscopy only reveals surface morphology. Using a simple nondestructive technique, namely, continuous-wave electron paramagnetic resonance spectroscopy, which exploits the well-understood angular dependence of the g-factor and hyperfine tensors, we show that in the solar cell blend of C(60) and copper phthalocyanine (CuPc)-for which X-ray diffraction gives no information-the CuPc, and by implication the C(60), molecules form nanoclusters, with the planes of the CuPc molecules oriented perpendicular to the film surface. This information demonstrates that the current nanostructure in CuPc:C(60) solar cells is far from optimal and suggests that their efficiency could be considerably increased by alternative film growth algorithms.

Subsurface mapping is crucial to understanding many biological systems as well as structured thin films for (opto)electronic or photonic applications. A non-invasive method is presented to map subsurface nanostructures from scanning near-field optical microscopy images. The Bethe-Bouwkamp model is used to simulate imaging of buried nano-objects or subsurface slanted planar interfaces, and it is shown how to determine their depth and size, or the interface inclination, from just one image. It is shown that the steep optical field gradient makes near-field microscopy a particularly sensitive depth probe for thin films.

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