Scanning Tunneling Microscopy and Spectroscopy Studies of 4‐Methyl‐ 4′‐(n‐mercaptoalkyl)biphenyls on Au(111)‐(1×1)

Abstract

4-methyl-4'-(n-mercaptoalkyl)biphenyl (CH3-C6H4-C6H4-(CH2)n-SH, n=3-6, BPn) monolayers assembled on Au(111)-(1x1) in 1,3,5,-trimethylbenzene (TMB) at various temperatures are studied by scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS). High resolution STM images reveal that BP3 and BP5 form a (sqrt 3x2sqrt 3) repeating motif superimposed on a temperature-dependent Moire pattern. BP4 and BP6 adlayers are characterized by a coexisting (2sqrt 3x5sqrt 3) majority phase and a temperature-dependent (3xpsqrt 3) minority phase. Assembly at 60 degrees C or 90 degrees C leads to p=5. Compression of the adlayer was found at higher temperatures. Combined with high-resolution structure experiments, the electronic characteristics of BP3 and BP4 self-assembled monolayers (SAMs) were studied by monitoring current-distance (iT-Deltaz) and current-voltage (iT-Ebias) characteristics in TMB employing a gold STM tip|BPn|Au(111)-(1x1) configuration. The semilogarithmic (iT-Deltaz) plots yielded three linear regions in the range 10 pA<or=iT<or=20 microA, which were attributed to the different positions of the STM tip with respect to the molecular adlayer (I outside, II in contact with and/or within the adlayer, and III just before the formation of Au-Au nanocontacts). The effective decay constants kappaeff and the corresponding barrier heights phi(eff) decrease with decreasing tip-substrate distance. The iT-Ebias curves at low bias voltages are represented by the Simmons tunneling model. Based on measurements in the high current limit, the electric field strength of the dielectric breakdown was estimated to range between |0.85+/-0.05|x10(9) V m-1 and |1.5+/-0.1|x10(9) V m-1.