[nanoPost] Measuring and Manipulating Molecular Organisation at the Nanometre Scale

University UK

Measuring molecular organisation is the first theme. We have carried out extensive studies of polymer surface morphology by atomic force microscopy (AFM). Poly(ethylene terephthalate) (PET) has been a particular interest. We have imaged, for the first time, the nanocrystallites in biaxially oriented PET films, using phase imaging. We have also explored the influence of processing parameters on surface morphology. However, our principal objective has been to develop quantitative methods for the measurement of surface properties based on AFM. Many of these have been based on studying tribological phenomena. Friction force microscopy (FFM) has proved especially powerful. We have carried out detailed studies of model heterogeneous systems (self-assembled monolayers, or SAMs, formed by the adsorption of alkanethiols onto gold) using FFM and have established that quantitative relationships may be established between frictional data and surface composition and molecular organisation. A key methodological step has been the use of AFM probes that have specific chemical functionality (chemical force microscopy, or CFM) to enable specific interactions (eg hydrogen bonding or dispersion forces) to be measured in a selective way. Details of surface phase morphology down to a few nm may be explored using a combination of CFM and FFM. FFM enables the measurement of variations in surface chemistry with nm spatial resolution, and because of the ability that exists for quantification, has enabled us to measure the kinetics of surface reactions. Studies of tip-induced wear processes have also led to valuable insights into molecular organisation, enabling, for example, the comparison of plasma polymer films that exhibit different degrees of cross-linking because of variations in process parameters.

Nanometre scale patterning of organic and biological molecules is our second major area of activity. Here we are particularly interested in the development of photochemical methods for surface patterning. We have been studying SAM photopatterning for ten years. The process involves the selective oxidation of a region of a SAM, by exposure to UV light through a mask, and then replacement of the oxidation products by immersion in a solution of a contrasting, second thiol. At the micron scale, this is readily accomplished to generate clean, well-defined patterns that we have used to control cellular attachment. At the nanometre scale, the obstacle, as is the case with all photolithographic processes, is the diffraction limit at /2, which places a lower bound on the resolution of a mask-based process. We have solved this problem, however, using a near-field scanning optical microscope (NSOM) coupled to a UV laser. We call this approach scanning near-field photolithography (SNP). Our best performance to date, 13 nm, is equivalent to /20, by a substantial margin the best result ever by a photolithographic technique. Routinely, it is possible to rival the performance of electron beam lithography using SNP, but unlike e-beam methods, SNP may be implemented under ambient conditions. Recently we have also demonstrated that liquid-phase nanolithography is possible using SNP. We have also demonstrated that the process may be extended to substrates other than gold, by patterning molecules on hydrogen passivated silicon and on the native oxide of silicon.