Friction study of alkanethiol self-assembled monolayers: Influence of heat and humidity
MetadataShow full item record
Self-assembled monolayers (SAMs) are well-organized molecular layer formed by the adsorption of molecules on the surface of a suitable substrate. Thermal stability of SAMs is important for applications in various surface science applications. As a model material, 16-mercaptohexadecanoic acid (MHA) on template stripped gold surfaces was investigated to determine the effect of temperature on the change of lateral force signal using atomic force microscopy (AFM). Friction force signals were obtained at various temperatures in order to determine whether it was possible to correlate the friction signal with desorption of the thiol molecules from the surface. Samples were heated for up to 10 h ranging from 40 to 80 °C in air and scanned every hour. A kinetic model was introduced to correlate the lateral force signal to the activation energy of desorption of the SAM from gold surface with heating. The activation energy of the detachment using this technique is 25.4 kcal/mol, which is consistent with other more complex techniques. The effect of cyclic heating on the lateral force of MHA SAMs patterned on gold surfaces was also investigated by AFM. The results show a distinctive pattern for the response of lateral force dependent on the heating temperature. Two distinct phases are observed and are postulated to correspond to molecular rearrangement of the SAM surface. The friction properties of two distinctive alkanethiol SAMs, MHA and 1-octadecanethiol (ODT) on gold substrates in various humidity conditions were examined by AFM. The results suggest that hydrophobic ODT SAM is insensitive to humidity. The difference of lateral force signal is within ±10% regardless of humidity. The lateral force signal of hydrophilic MHA SAMs has a significant decrease in signal in humid environments. The influence of bulk water was also investigated by LFM. By imaging under water, the capillary force is eliminated on ODT SAMs, which leads to a lower lateral force. However, the lateral force image was reversed on MHA SAMs, which suggested that hydrophobic forces dominated in water.