Oxide Nanofilms from Nanoparticle Suspensions Deposited on Functionalized Surfaces

Single-crystal silicon wafers, with and without self-assembled organic monolayersurface treatments (amine or sulfonate) were immersed in commercially available oxidenanoparticle suspensions (alumina, ceria, or titania). These specimens allowed the studyof the interactions of oxide nanoparticles with functionalized substrates in the presence ofa non-reactive aqueous liquid. They therefore represented models of chemical bathdeposition (CBD) and liquid-phase deposition (LPD) processes, with the difference thatthe model systems lacked the continually changing pH, concentration, and particlenumber and size distributions typical of most CBD and LPD processes.Electrostatic arguments were used to predict the ability of the nanoparticlesuspensions to deposit continuous films on functionalized substrates. These predictionswere based on the simple assumption that a pH value of the suspension between theisoelectric point (IEP) of the particle and the IEP of the functionalized substrate wouldprovide surfaces of opposite charge and provide the necessary attractive force to initiatefilm growth. These predictions were correct in 15 of 18 specimens studied, includingpositive and negative outcomes over the three different oxides and three differentsubstrates.All sample depositions were investigated with x-ray photoelectron spectroscopy(XPS) to determine elemental composition at the surface, atomic force microscopy(AFM) for topographic observation, nanoscratch for thickness determination andscanning electron microscopy (SEM) for morphology. Successful film thicknesses rangedfrom 6.1 nm to 53 nm over deposition times of 15 and 30 h. Results were compared withliterature data from chemical bath deposition.While the nanoparticle suspensions may serve as a suitable model for early CBDfilm deposition, it is conjectured that the non-reacting suspensions do not possess themeans to sustain continued film growth.6