Force-Distance Measurements and Simulations for Alignment and Registration

Young-Kyun Kwon, Physics; James Whitten, Chemistry

The ultimate goal of this work is to develop "molecular guides" that can be used for the final stage of aligning a template and substrate for nanoscale manufacturing applications. It is desirable to find self-assembling molecules that exhibit strong, but reversible, intermolecular attraction when adsorbed on the template and substrate regions. By functionalization of an atomic force microscope (AFM) tip, it is possible to measure forces of attraction/repulsion between the functionalized tip and a substrate.

AFM has been used to measure adhesion between a probe tip and a substrate modified with a fluorinated alkanethiol in different media. Force vs. distance curves recorded in air, hexane and water show attractive forces. Ab initio calculations have been carried out to understand attractive forces between CF3-terminated monolayers. Long-range attraction up to several hundreds nanometers has been observed in water at different pH values. Ion-dipole interaction (CF3/NH3+) was also explored, and the adhesive forces are being compared with dipole-dipole interactions (CF3/CF3).

Thermal deposition of gold on a monolayer of 3-mercaptopropyltrimethoxysilane (MPTS) in ultrahigh vacuum has been studied, and gold-thiolate bond formation has been confirmed by appearance of a new peak in X-ray photoelectron spectroscopy (XPS) of the S2p region at 162.4 eV. Force-distance curves were also recorded for MPTS and octadecyl trichlorosilane (ODTS) - modified silicon surfaces using gold tips in dry air environments. Due to gold-thiolate bond formation, the MPTS monolayer shows larger pull-off forces as compared to ODTS monolayers.