Self-assembled monolayers (SAMs) are promising candidates as active components in molecular electronics. A major disadvantage in organic molecule-based electronic devices is that the stability and robustness of organic systems, in general, are less than their inorganic counterparts. The development of nanoscale insulators and dielectrics, molecular switches, rectifiers, and field effect transistors, require that organic molecules have sufficient stability against electrical breakdowns. STM provides a rapid and precise means to systematically survey the electronic properties as well as the structure and stability of SAMs prior to device fabrication. In addition to high-resolution structural characterization, STM can be used to mimic the interfacial junctions in electronic devices, by regulating tip-surface interactions. We have developed a new method, by combining STM structural characterization with spectroscopy measurements, to investigate the electronic properties of single organic molecules and nanoparticles.

Scanning tunneling spectroscopy of decanethiol SAMs on Au(111). (A) A typical current-distance curve of decanethiol SAMs measured from the setpoint (I = 50 pA and V = 1.0 V). The contact between the tip and the monolayer surface is defined as the first kink point on the lnI-Z curve, indicated by the dash line. (B) STM tip is approached at contact with the methyl group of decanethiols. (C) I-V characteristics of decanethiol SAMs when the tip is at contact with the SAM surface. At 2.4 V (sample positive), a peak with negative differential resistance is present.

Current-induced desorption by STM. Images of a decanethiol SAM were taken before (A) and after (B) fabrication. A 1 nm hole was fabricated at +1.0 V and 200 pA, and a hole of 3 nm formed at 300 pA. (C) Schematic diagram of the desorption procedure using tunneling electrons.