Mechanical and optical manipulation of porphyrin rings at the submicrometre scale

Scanning probe microscopes (SPMs) and especially the atomic force microscope (AFM) can be used as tools for modifying surface structures on the submicrometre and even nanometre scale. For this purpose an advanced interface has been developed to facilitate these manipulations and greatly increase the number of possible applications. In this paper this interface (the nanoManipulator, developed at the University of North Carolina at Chapel Hill) is implemented on a combined AFM-confocal microscope. This setup allows AFM imaging, manipulations and fluorescence imaging of the same area on the sample.

The new setup is tested on ringlike structures of a porphyrin derivative (BP6). A small amount of the fluorescent material could be displaced with the AFM tip. A special tool (sweep mode) allowed a modification of around 130 nm, which was afterwards detectable with the confocal microscope. The resolution attainable in these kind of experiments could go down below 100 nm and is primarily determined by the tip and sample geometry.

Comparable with this experiment is the application of a near-field scanning optical microscope (NSOM) to make photochemical modifications. Using the excitation power coming from the NSOM probe the fluorescence can be quenched by bleaching a selected area instead of displacing the material. Application on the BP6 rings led to a modification of 280 nm wide.

AFM can perform modifications on a smaller scale but is less selective than NSOM. Optical investigation of the changes after AFM manipulation can give more elaborate information on the modifications. This will extend the possible applications of the techniques and may ultimately go down to the single-molecule level.