Solution deposition conditions influence the surface properties of 3-mercaptopropyl(trimethoxysilane) (MPTS) films

The deposition of organosilanes onto semiconducting and metal oxide surfaces is a versatile approach to impart corrosion resistance or provide a site for the subsequent immobilization of a range of molecules. Organosilane deposition is however, quite a complex process, with many factors influencing the film properties. Here, we investigated the deposition of 3-mercaptopropyl(trimethoxysilane) (MPTS) in organic solvents on Si substrates with various post-deposition treatments and subsequently characterize film properties using surface analytical techniques including ellipsometry, X-ray photoelectron spectroscopy (XPS), static water contact angles, and attenuated total reflection Fourier transform infra-red (ATR-FTIR) spectroscopy. Condensation of MPTS onto Si surfaces was most efficient in more anhydrous and hydrophobic solvents such as toluene, with monolayer thickness values (≈0.77 nm) being observed at MPTS concentrations of 0.08–1% (v/v), whereas all thickness values were seen to be below monolayer coverage when deposition was carried out in ethanol, regardless of the concentration used. It was found that the shortcomings of ethanol deposition can be overcome by applying a pre-hydrolysis period at low pH to achieve similar film thicknesses and properties compared to toluene at shorter deposition times - which is important when functionalization needs to be carried out in plastic consumables. Moreover, with minimal post-deposition treatments it was found that controllable film thicknesses can also be achieved with a high-retention of thiol functional groups which is evidenced by a clear S 2p XPS peak at a binding energy of 164.0 eV. However, these films will only be loosely bound to the surface and be lost upon further treatments. Overall, the procedures developed are important for many applications such as biomolecule immobilization, and the functionalization of planar substrates and nanoparticles, where the organosilane thickness, degree of order, reactivity and surface concentration of thiols influence film functionality.