title: Indentation and self-healing mechanisms of a self-assembled monolayer--a combined experimental and modeling study.
abstract: A combination of in situ vibrational sum-frequency generation (SFG) spectroscopy and molecular-dynamics (MD) simulations has allowed us to study the effects of indentation of self-assembled octadecylphosphonic acid (ODPA) monolayers on α-Al2O3(0001). Stress-induced changes in the vibrational signatures of C-H stretching vibrations in SFG spectra and the results of MD simulations provide clear evidence for an increase in gauche-defect density in the monolayer as a response to indentation. A stress-dependent analysis indicates that the defect density reaches saturation at approximately 155 MPa. After stress is released, the MD simulations show an almost instantaneous healing of pressure-induced defects in good agreement with experimental results. The lateral extent of the contact areas was studied with colocalized SFG spectroscopy and compared to theoretical predictions for pressure gradients from Hertzian contact theory. SFG experiments reveal a gradual increase in gauche-defect density with pressure before saturation close to the contact center. Furthermore, our MD simulations show a spatial anisotropy of pressure-induced effects within ODPA domains: molecules tilted in the direction of the pressure gradient increase in tilt angle while those on the opposite side form gauche-defects.
authors: Institute of Particle Technology (LFG), Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU) , Cauerstrasse 4, 91058 Erlangen, Germany.
authors: Christian
authors: C
authors: Meltzer
authors: Jonas
authors: J
authors: Paul
authors: Hanno
authors: H
authors: Dietrich
authors: Christof M
authors: CM
authors: Jäger
authors: Timothy
authors: T
authors: Clark
authors: Dirk
authors: D
authors: Zahn
authors: Björn
authors: B
authors: Braunschweig
authors: Wolfgang
authors: W
authors: Peukert