Local Probe Studies of Organised Monolayers

Masahiko Hara, Jaegeun Noh, Keita Mitsui and Ken Nakajima

Local Spatio-Temporal Functions Laboratory, Frontier Research System, RIKEN
Wako, Saitama 351-0198, Japan

1. STM for Self-Assembled Monolayers
Growth processes of self-assembled monolayers (SAMs) were studied by scanning tunneling microscopy (STM), surface plasmon spectroscopy (SPS), thermal desorption spectroscopy (TDS) and x-ray photoelectron spectroscopy (XPS). A new equilibrium state for alkanethiol SAMs different from previously reported phase was confirmed for the first time. While the conventional final equilibrium state was interpreted as the homogeneous adsorption sites on three-fold hollow on Au(111), the existence of a new equilibrium state shows that there are three adsorption sites: hollow, bridge and on-top sites on Au(111), suggesting that the previously accepted phase is still under nonequilibrium. As a result of this study, temporal development of interfacial reaction and dimerization problems were discussed in more detail.

2. AFM for Single Molecule Nano-Rheology
We have studied protein stretching event using force curve measurement mode of atomic force microscopy (AFM). Protein molecules were sandwiched between Au(111) and a gold-coated AFM tip through thiol-gold chemisorption. Thiol groups were introduced by adding cystein residues at both N- and C-terminal to facilitate chemisorption. We have established a newly assembled piezo controller of AFM system and programmed a software to unfold and fold protein molecules. Although the length of the protein molecule responded in-phase to the applied movement in most case, we found a novel out-of-phase response around a certain stretching length where the second domain unfolded. Applying the measured force in quasi-static experiment, the out-of-phase response was reproduced in simple calculation, which suggested that the unfolding and the refolding of the protein were taking place repeatedly.

3. SNOM for Molecular Proximity Effect
The hybridization of scanning near-field optical microscopy (SNOM) with scanning tunneling microscopy (STM) was realized by introducing a doubly metal-coated optical fiber tip with an extremely small aperture (<100 nm), where the second metal-coating to the aperture was performed to obtain a half-transparent conducting tip after the fabrication of aperture probe. A simultaneous SNOM/STM observation and the identical channel transport for both electrons and photons were achieved. For the life time measurement of dye molecules, the measured luminescence decay time was variable against the intrinsic (macroscopic) decay time, mainly due to the existence of the metal layer as an optical boundary in the proximity of the luminescent center.

References
(1) "Nanoscopic Evidence for Dissociative Adsorption of Asymmetric Disulfide Self-Assembled Monolayers on Au(111)" J. Noh and M. Hara: Langmuir, 16 (2000) 2045-2048.; "Nanoscopic Investigation of Self-Assembly Processes of Dialkyl Disulfides and Dialkyl Sulfides on Au(111)" J. Noh, T. Murase, K. Nakajima, H. Lee and M. Hara: J. Phys. Chem. B, 104 (2000) 7411-7416.
(2) "Development of a Hybrid Scanning Near-Field Optical/Tunneling Microscope (SNOM/STM) System" K. Nakajima, R. Micheletto, K. Mitsui, T. Isoshima, M. Hara, T. Wada, H. Sasabe and W. Knoll: Jpn. J. Appl. Phys., 38 (1999) 3949-3953.
(3) "Dynamic Measurement of Single Protein's Mechanical Properties" K. Mitsui, K. Nakajima, H. Arakawa, M. Hara and A. Ikai: Biochem. Biophys. Res. Comm., 272 (2000) 55-63.