Supramolecular network formation from solution-based deposition techniques

• Main Author: Russell, James Christopher
• Published: University of Nottingham 2011
• Subjects: 547.6; TK7800 Electronics
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.539203

The spontaneous formation of supramolecular assemblies has been viewed as a potential route to the creation of functional nano-scale architectures for a number applications in electronics. In this thesis a number of assemblies formed from molecular constituents deposited from the solution phase have been studied. The structures formed by two carboxylic acid derivatives on the highly oriented pyrolytic graphite (HOPG) surface from nonanoic acid solutions are presented. Quaterphenyl-tetracarboxylic acid (QPTC) molecules are observed to form a supramolecular network where all the constituents lay parallel to one another on the surface. The network is stabilised by four carboxylic acid dimer bonds per molecule in addition to admolecule-substrate interactions. Terphenyl-tetracarboxylic acid (TPTC) molecules form a much more complex structure with individuals orientating themselves in one of three directions to form a network with hexagonal symmetry but no translational order. To characterise such an unusual supramolecular morphology we introduce a rhombus tiling representation of the network where each molecule is schematically replaced with a lozenge rhombus producing a tiling. Such tilings have been studied previously in the literature and utilising this we are able to determine that the morphology is stabilised by entropic contributions to the free energy. In addition to this we present the tip-induced manipulation of the TPTC supramolecular network. The manipulation is performed by imaging the structure within a specific voltage bias range resulting the TPTC molecules reordering into a close packed structure. Returning the voltage to that conventional used for imaging causes the network to relax back into the open structure although with a different morphology. We then discuss the changes induced in these supramolecular networks when additional molecular species are introduced to the system. First, coronene and perylene are separately codeposited with QPTC resulting in the formation of a hexagonally ordered network with coronene or perylene located at the vertices of six QPTC molecules. This new structure is observed to form even when QPTC is deposited first. Second, the adsorption of coronene into the porous TPTC network is presented. When the TPTC network forms before the introduction of coronene we note little effect on the network morphology. However, when the molecules are mixed in the solution phase and deposited simultaneously we observe the non-uniform adsorption of coronene into the TPTC structure. At higher coronene concentrations we note the network forms with a different morphology shifted towards a more ordered state suggesting that when the molecules are deposited sequentially the system is kinetically trapped in the originally formed structure. We then present a series of studies of molecular adsorption on the Au (111) surface. First, hexaazatrinaphthylene (HATNA) molecules are observed to form stable supramolecular structures when deposited from ethanol solutions. A core hydrogen bonding junction is identified. The network switches between two domain orientations and we identify a linear defect where the two domains meet. Second, we report the adsorption of Tri( 4-bromophenyl) benzene (TBPB) on the Au (111) surface. TBPB forms three different structures at room temperature. When samples are heated during the deposition stage we observe the covalent coupling of pairs of molecules to form dimers. This reaction is confirmed by ToFSIMS experiments. The substrate is confirmed to play a significant role in the coupling process as subsequent experiments on HOPG failed to yield dimer formation. Finally we demonstrate the potential of a UHV-prepared sample by templating the adsorption of adamantanethiols. Finally, we demonstrate the adsorption of a solubilised derivative of perylene tetracarboxylic dianhydride (PTCDA). PTCDA molecules have poor solubility in most solvents commonly used for liquid deposition. The addition of alkane chains attached to the sides of the perylene core promotes the solubility of the molecule in these solvents whilst leaving the anhydride functionality intact. Deposition is performed from 1-phenyloctance solutions on HOPG. The molecules form an ordered structure characterised by a single molecule unit cell. The results presented in this thesis show that the understanding of supramolecular networks has progressed to the point where changes in the morphology can be induced via a variety of processes.