Measurement of single-molecular conductivity

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Molecular electronics has attracted much attention in the last quarter-century. Investigations on the electronic properties and functions of molecular systems have made remarkable advances since the invention of scanning tunneling microscopy (STM). However, STM measurement is inadequate for the device structure consisting of conductive and insulating parts due to the requirement of conductivity for all over the scanning area. In this study, novel methods of scanning probe microscopy based on atomic force microscopy (AFM). These methods enable us to evaluate electronic properties of molecular nanostructures located on insulating substrates and/or connected with metal electrodes.1.Non-contact atomic force microscopy (NC-AFM) has been employed to observe double-stranded DNA. The simultaneous measurements of frequency-shift and tunneling current indicate that DNA molecules show tunneling conductivity with the. attenuation factor β=1.1Å^<-1>.2.Point-contact current-imaging atomic force micr … More oscopy (PCI-AFM) enables us to obtain topographic images and current-voltage characteristics simultaneously in nanoscale. PCI-AFM is a combined method with two different measurement techniques : tapping-mode scan and point-contact measurement. This new technique will be a good approach to obtain the relationship between topography and electrical properties of individual nanoscale structures.3.Charge transfer force microscopy/ spectroscopy (CTFM/S) detects the energy dissipation of cantilever oscillation with constant tip-sample distance under bias voltage. It enables us to obtain a map of density of states (DOS) and DOS spectra in manometer-scale resolution for the surface consisting of insulating and conductive parts. CTFM images and CTFS spectra are obtained by probing frequency shift of cantilever oscillation. Figure 1 shows CTFS spectra as a function of bias voltage for 5, 10, 15, 20-tetraphenyl-21H, 23H-porphine tetrasulfonic acid (TPPS). HOMO-LUMO gap (1.7eV) is observed clearly on an insulating substrate. Less