Dresden 2014 – wissenschaftliches Programm
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O: Fachverband Oberflächenphysik
O 84: Nanotribology
O 84.2: Vortrag
Donnerstag, 3. April 2014, 16:15–16:30, WIL B321
Self-Assembled Monolayers under Mechanical Stress — Christian Meltzer, Jonas Paul, Wolfgang Peukert, and •Björn Braunschweig — University of Erlangen-Nuremberg; Institute of Particle Technology (LFG), Cauerstrasse 4, 91058 Erlangen
The molecular structure of mechanically compressed octadecylphosphonic acid self-assembled monolayers (ODPA SAMs) on α-Al2O3 is studied with co-localized vibrational sum-frequency generation (SFG). The excellent stability of our custom-built manipulation device and SFG spectrometer has allowed us to apply defined normal forces and to probe the contact area in-situ with SFG over hours without significant drift which has also facilitated time resolved studies of compressed SAMs. Normal pressures up to 208 MPa are applied and load dependent SFG spectra are recorded as a function of pressure and lateral position relative to the center of contact. These co-localized SFG spectra allow for a comparison of measured contact areas with theoretical predictions from Hertzian theory. A detailed analysis of vibrational SFG spectra shows that both intensity and relative contributions of methylene and methyl vibrational bands to the SFG intensity are excellent indicators for pressure dependent changes in the molecular structure of ODPA SAMs. Analysis of SFG spectra provides strong evidence for the formation of gauche defects. In fact, after load release healing of defects occurs on two different time scales - a fast recovery and a slow process over a time period of several hours. The former is indicative for healing of shallow defects close to the head group while the latter is dominated by high energy barrier defects close to the anchor group.