Disorder in Alkylsilane Monolayers Assembled on Surfaces with Nanoscopic Curvature

Assembly of molecular layers on surfaces with nanoscopic curvature play an important role in nanoparticle functionalization and friction modification of surfaces with nanoscale roughness such as those found in microelectromechanical systems (MEMS) devices. Here we have investigated the assembly of alkylsilane monolayers on silica surfaces with nanoscopic curvature using FTIR spectroscopy. It was observed that the degree of order of alkylsilane self-assembled monolayers (SAMs) on curved surfaces such as silica nanoparticles with sub-40 nm radius of curvature varies, depending on surface curvature (particle diameter), chain length, and the self-assembly reaction conditions. FTIR data show that, as the radius of curvature decreases, the predominately trans conformations of molecules typically found for silanes assembled on smooth surfaces, such as the oxide of Si(100), begin to disappear with more radically curved surfaces exhibiting greater numbers of gauche defects. The extent of disorder can be followed experimentally as a shift toward higher wavenumber in the CH2 asymmetric and symmetric stretch frequencies in the FTIR spectrum, which is known to occur as. the number of gauche defect sites in the film increase. Such studies of SAM assembly on silica nanoparticles also provide a convenient platform for the spectroscopic characterization of surfaces with controlled nanoscopic roughness.