Molecular nanostructures and their electrical probing

A brief review is provided of a self-assembly technique based on the specific interaction between the protein streptavidin and its complementary ligand, biotin. This methodology is capable of being developed for fabricating the 2D molecular networks that will be essential to the realisation of a molecular electronic system. Such a system is taken as an example of a prototypical memory array in which data is stored as an electrostatic charge or molecular dipole and the information 'read' using the electric force mode of an atomic force microscope (AFM). Based on a simple, spherical model of the AFM lip, equations are derived for the electrostatic forces and force gradients experienced by the tip as it scans over (a) a point charge and (b) a point dipole. It is shown that the spatial resolution of the probe is governed largely by the tip-to-surface height with the tip radius being less important. It is shown that for operation in the non-contact mode in the thermal vibration limit, the probe should be capable of detecting a single electronic charge at a tip-to-surface distance of similar to 15 nm and a dipole of moment 1 Debye at similar to 1.5 nm. For a typical tip-to-surface distance (similar to 50 nm) which allows electrostatic effects to be decoupled from Van der Waals interactions, the sensitivity of the probe decreases to similar to 10 electronic charges and similar to 10(4) dipoles of moment 1 Debye. Finally, it is shown that application of an external potential to the tip while significantly improving sensitivity leads to a loss of spatial resolution. (C) 1998 Elsevier Science S.A. All rights reserved.