Molecular Mechanisms of Solvent-Controlled Assembly of Phosphonate Monolayers on Oxide Surfaces

The explicit role of a series of solvents in octadecylphosphonic acid (ODPA) association and monolayer formation on the (0001) surface of α-aluminum oxide is explored from molecular dynamics simulations. For this purpose, molecule-by-molecule attachment and subsequent relaxation is studied in hexane, THF, and 2-propanol, respectively. From a purely structural viewpoint, the simulations show that the packing and ordering of the resulting SAMs closely resembles that of monolayers initially grown <i>in vacuo</i>, followed by immersion into the solvent afterward. In terms of the formation kinetics, we however find significant dissimilarities which result from solvent structuring at the interface and considerable hindering of surfactant association to the template surface at later stages of SAM growth. This leads to drastic deviation from diffusion-controlled kinetics and calls for a time-dependent picture of SAM formation mechanisms

Self-Assembled Monolayers Get Their Final Finish via a Quasi-Langmuir–Blodgett Transfer

The growth of self-assembled monolayers (SAMs) of octadecylphosphonic acid (ODPA) molecules on α-Al₂O₃(0001) and subsequent dewetting of the SAMs were studied with a combination of in situ sum-frequency generation (SFG) and molecular dynamics (MD) simulations. Although SAM growth after deposition times >8 h reduces to nearly negligible values, the resultant ODPA SAMs in solution are still not in a well-ordered state with the alkyl chains in all-trans configurations. In fact, in situ SFG spectroscopy revealed a comparatively high concentration of gauche defects of the SAM in the ODPA 2-propanol solution even after a growth time of 16 h. Here, results of the MD simulations strongly suggest that defects can be caused by ODPA molecules which are not attached to the substrate but are incorporated into the SAM layer with the polar headgroup oriented into the 2-propanol solvent. This inverted adsorption geometry of additional ODPA molecules blocks adsorption sites and thus stabilizes the SAM without improving ordering to an extent that all molecules are in the all-trans configuration. While persistent in solution, the observed defects can be healed out when the SAMs are transferred from the solvent to a gas phase. During this process, a quasi-Langmuir–Blodgett transfer of molecules takes place which drives the SAM into a higher conformational state and significantly improves its quality

Indentation and Self-Healing Mechanisms of a Self-Assembled MonolayerA Combined Experimental and Modeling Study

A combination of <i>in situ</i> 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 <i>gauche-</i>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 <i>gauche-</i>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 <i>gauche-</i>defects

Improving the Charge Transport in Self-Assembled Monolayer Field-Effect Transistors: From Theory to Devices

A three-pronged approach has been used to design rational improvements in self-assembled monolayer field-effect transistors: classical molecular dynamics (MD) simulations to investigate atomistic structure, large-scale quantum mechanical (QM) calculations for electronic properties, and device fabrication and characterization as the ultimate goal. The MD simulations reveal the effect of using two-component monolayers to achieve intact dielectric insulating layers and a well-defined semiconductor channel. The QM calculations identify improved conduction paths in the monolayers that consist of an optimum mixing ratio of the components. These results have been used both to confirm the predictions of the calculations and to optimize real devices. Monolayers were characterized with X-ray reflectivity measurements and by electronic characterization of complete devices