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Hierarchical self-assembly of electroactive supramolecular systems on pRe-patterned surfaces: multifunctional architectures for organic FETs

Final Report Summary - HESPERUS (Hierarchical self-assembly of electroactive supramolecular systems on pre-patterned surfaces: multifunctional architectures for organic FETs)

The HESPERUS project was aimed at exploring the use of principles of supramolecular chemistry at surfaces to achieve a high level of control over the packing of organic molecules on the surface forming pre-programmed assemblies with specific electronic properties at the supramolecular level and at optimising the charge transport across the metal-organic interface based on such supramolecular architecture.

Within this frame, HESPERUS looked into supramolecularly engineered nanostructures (SENs), consisting of organic semiconductors, with tailor-made interfaces to textured solid substrates and electrodes, for fabricating prototypes of three-terminal devices (field-effect transistors). Surface texturing of electrically conductive solid substrates and metallic nanostructures was combined with supramolecular chemistry at surfaces. We chose for our studies perylenebis(dicarboximide) (PDI). The self-assembly and electrical properties of PDI based films were investigated, in particular bicomponent films consisting of a monomeric M-PDI derivative blended with its polymeric P-PDI derivative in which the monomeric units are exposed on the side chain of a rigid polymeric scaffold, i.e. functionalised poly(isocyanopeptide).

These bicomponent films were characterised by atomic force microscopy (AFM) and Kelvin probe force microscopy (KPFM), revealed the relationship between architecture and function for various supramolecular nanocrystalline arrangements at a nanometre spatial resolution. In particular KPFM provided direct insight, on the nanoscale, on the electronic properties of the organic components within the film.

We found that the monomer-polymer interactions can be controlled by varying solvent and/or substrate polarity, so that either the monomer packing dictates the polymer morphology or vice versa, leading to a morphology exhibiting M-PDI nanocrystals connected with each other by P-PDI polymer wires. Compared to pure M-PDI or P-PDI films, the bicomponent films that possess polymer interconnections between crystallites of the monomer display a significant improvement in electrical connectivity and a two orders of magnitude increase in charge carrier mobility within the film, as measured in FET devices. Of a more fundamental interest, our technique allows the bridging of semiconducting crystals, without the formation of injection barriers at the connection points.

Furthermore, a novel dip coating procedure to form highly crystalline and macroscopic conjugated architectures on solid surfaces has been devised. The approach was used with [6, 6]-phenyl C61 butyric acid methyl ester molecule (PCBM), which is the most commonly used electron-acceptor in organic photovoltaics. Highly ordered, hexagonal shaped crystals of PCBM, ranging between 1 to 80 micrometer in diameter and from 20 to 500 nm in thickness, were grown.

These crystals have been found to possess a monocrystalline character, to exhibit a hexagonal symmetry and to display micron sized molecularly flat terraces. The crystals have been prepared on a wide variety of surfaces such as SiOx, silanised SiOx, Au, graphite, amorphous carbon-copper grids and ITO. To test the stability of these electron accepting PCBM crystals, they were coated with a complementary, electron donor hexa-peri-hexabenzocoronene (HBC) derivative by solution processing from acetone and chloroform-methanol blends.

The HBC self-assembles in a well-defined network of nanofibres on the PCBM substrate, and the two materials can be clearly resolved by AFM and KPFM. Due to its structural precision on the macroscopic scale, the PCBM crystals appear as ideal interface to perform fundamental photophysical studies in electron-acceptor and -donor blends, as well as workbench for unravelling the architecture vs. function relationship in organic cell prototypes. Quantum computing is a new research field at the frontier of both computer science and physics. It studies how to apply quantum mechanics to solve problems in computer science and information processing. The principles of quantum mechanics are radically different from those of conventional (classical) physics and these differences can be very useful for computer science.

A quantum computer is a computer that functions according to the principles of quantum mechanics. It has been shown that quantum computers will be able to solve problems that are thought to be hard for classical computers (for example, factoring and the discrete logarithm problem). This discovery was of great importance for cryptography because the security of today's systems for data encryption (e.g. RSA and Diffie-Hellman) is based on the assumption that factoring and discrete logarithm are hard. Thus, building a quantum computer would make today's systems for data encryption insecure.

Quantum computers would also be very useful for search problems. In 1996, Lov Grover invented an algorithm for quantum computers that solves a generic search problem quadratically faster than any classical computer. This generic search algorithm can be applied to any search problem.

The goal of this International Reintegration Grant (IRG) is to support the return of Andris Ambainis to University of Latvia, after studying and working in USA and Canada for nine years. While working in USA and Canada, Andris Ambainis has become one of world's leading experts in the theory of quantum computing. The plan was that Andris Ambainis will start a new research group in quantum computing at University of Latvia. The research of this group will focus on three directions:
- design of new quantum algorithms;
- understanding the limits of quantum computing;
- understanding the fundamental properties of quantum states.

In the first two years of the Marie Curie IRG, Andris Ambainis has established a research group consisting of himself and five PhD students. The most important scientific results of the group are as follows:
- a new quantum algorithm for finding certificates for logic formulas;
- a new method for proving the optimality of quantum algorithms;
- a quantum generalisation of Lovasz local lemma, an important result in combinatorics with many applications to theoretical computer science.