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Surfaces for molecular recognition at the atomic level

Final Report Summary - SMALL (Surfaces for molecular recognition at the atomic level)


The overarching aim of the SMALL ITN project is to train early stage researchers in the field of molecular recognition at surfaces from fundamental science to novel applications, through an integrated program drawing on surface science, nanotechnology, theory, chemical synthesis, physics, biology and industry. The researchers work within a well-structured scientific programme aimed at molecular recognition, underpinning the next generation of molecular sensors, catalysis, biomimetics, and molecular electronics. The programme fosters scientists who, in addition to being specialists in particular branches of molecular nanotechnology, have broad interdisciplinary experience in the experimental and theoretical techniques of molecular nanotechnology. By combining cutting edge experimental and theoretical techniques, the research projects explore the nature of the interactions responsible for molecular and atomic recognition and the role that these play in the massively parallel self-assembly of supramolecular nanostructures, how to achieve chemical selectivity at surfaces, enantioselective recognition, and by molecular and atomic surface modification develop routes to novel catalysis and nanoscale sensors.

The work on the SMALL was based on the three key areas of ‘Supramolecular structures at surfaces’, ‘Chemical selection at surfaces’ and ‘Devices’. Within the theme of supramolecular structures work has been carried out to form covalently linked surface networks. This was achieved experimentally by the coadsorption of alkyne molecules and azide molecules on a copper surface at room temperature and the formation of covalent bonds between them by the so called ‘click’ reaction (cycloaddition reaction of the functional groups). We also achieved the formation of surface covalent organic frameworks (SCOF) through the reaction of two benzene derivatives (TMC and TPB) on gold surfaces. Also explored in this workpackage was the charge transfer induced structural changes that occur in the adsorbed molecules that play a key role in their self-assembly. The experimental work was supported by novel implementations of density functional theory (DFT) that through the work of the SMALL project was extended to the accurate treatment of the non-covalent van der Waals bonding that so often plays a key role in the initial self-assembly of molecules on surfaces prior to covalent network formation. The second workpackage in this theme was the study of supramolecular structures in realistic environments. Here work was carried out to study the behaviour of water on strontium titanate surfaces, finding that on the TiO2 regions of the surface is was associated but on the SrO regions was dissociated, and also on pure titania. This work is relevant to dye-sensitised solar cell surfaces in environmental conditions and was supported in the network by our theoretical work on ab-initio molecular dynamics (AIMD). This technique was used to investigate the structure of water at transition metal oxides, graphene and boro-nitride surfaces, with direct relevance to surface-based devices in realistic environments. The project also saw the development of a core-hole clock implementation of resonant inelastic x-ray scattering that can be used to probe interfaces under ambient conditions. The third workpackge in this theme was harnessing the intrinsic functionality of complex molecules at surfaces. The experimental work was based on complex aromatic molecules such as TCNQ and their ability to undergo conformational changes in response to charge transfer from metal surfaces, and the adsorption of intrinsically functional molecules such as porphyrin nanorings and single molecule magnets by the novel deposition technique of in-situ UHV electrospray deposition. This enabled techniques such as STM and synchrotron-based electron spectroscopy to determine the structure of the nanorings (thus verifying the synthetic route) and the magnetic properties of the SMMs on metal and oxide surfaces with applications in light-harvesting and information processing respectively. In addition, the design and synthesis of sublimable spin transition compounds and state of the art organic molecular wires was achieved with applications in spintronics and magnetic molecular switches respectively. The theory work in this workpackage focussed on the calculation and interpretation of STM images of organometallic molecules on metal surfaces, leading to the development of a new Java program coupled to the VASP code.

Within the theme of chemical selection at surfaces, work has been carried out on recognising chirality through molecular interactions. This workpackage determined the bonding, organisation and chirality of complex organic molecules on copper surfaces, showing temperature-dependent organisation and chiral surface arrays of biomolecules. Closely related to this was further theoretical work that developed the DFT+vdWsurf method that exetends pairwise vdW corrections to the modelling of adsorbates on surfaces, yielding remarkable agreement with experiment for PTCDA molecules on metal surfaces. In addition, we addressed the question of what drives the formation of chiral structures at surfaces, studying a range of chiral and achiral molecules on surfaces in the presence of chiral precursors (chiral induction) and adsorption on intrinsically chiral cut surfaces, leading to new understanding of these effects and also how the conformation of interacting molecules influence each other under dynamic conditions. The second workpackage in this theme dealt with enhancing chemical specificity at heterogeneous catalyst surfaces. This work included the successful development of catalytic reactions for direct activation of unreactive carbon-hydrogen bonds, through the immobilization of PEPPSI-type palladium complexes on mesoporous silica. In addition, the electronic, structural and magnetic properties of metal phthalocyanines were studied on surfaces, leading to key insights into the use of organometallics in surface catalytic reactions.

Within the devices theme the single workpackage was based on developing surfaces that could specifically harness molecular recognition as the bases of sensors and devices. This work resulted in the development of a microfluidic technique for the formation of chemical gradients on surfaces that were then studied in the context of protein gradients for variable cell adhesion and mobility. Also, achieved was the design and synthesis of molecular tweezers that bind to specific molecules through the interaction of their pi orbitals. These were not only synthesised in the project but deposited on surfaces for synchrotron studies and also by in-situ UHV-electrospray deposition, resulting in high resolution STM imaging of the resulting macrocycle structures. These molecules have potential applications in molecular information storage and readout in addition to sensing. In the context of molecularly imprinted polymers (MIP) for the formation of molecular recognition surfaces the project investigated silane chemistry and its use in linkage and support of MIP particles on surfaces and a groundbreaking study of silicon oxide CVD on titanium dioxide with applications from sensing to photovoltaics.

The impact of the individual research projects is the new scientific knowledge and tools that have arisen. However, the overiding impact of the training network is clearly the 22 fellows that have been trained in this burgeoning scientific area and the skills to drive it forwards. Through the workshops, masterclasses and hands-on training, the fellows have gained skills in cutting-edge theoretical and experimental techniques. Through their presentations at conferences, workshops and summer schools they have developed the skills to talk about their research with colleagues and others, and through secondments to work together to solve challenging scientific problems and learn new skills.