Sum frequency generation study of immobilized 3‐aminopropyltriethoxysilane self‐assembled layer on Si(111) substrates

Distribution and orientation of 3‐aminopropyltriethoxysilane (APTES) on a sensing surface play critical role on biosensor, but have not been fully investigated. In this work, the quality of APTES SAM immobilized on the Si(111) substrate was studied by observing sum frequency generation (SFG) response. The polarization, azimuthal angle, and position‐dependent SFG spectra of APTES coverage were obtained. In ppp and psp‐SFG spectra, the CH2 and CH3 vibrational and NH stretching modes were clearly seen around 3000 cm−1 and around 3300 cm−1 regions, respectively. The spp and ssp‐SFG spectra presented too weak signals that the vibrational regions were not clear. The positional and azimuthal angle dependence of the SFG spectra was observed from 0o to 360o with the step of 45o rotation around the surface normal. The 0° is defined as the [ 1¯1¯2] direction of Si(111) crystal lying in the incident plane and opposite to the incident k vectors. The appearance of the NH mode was observed to be strongly dependent on the position. The results indicate that the distribution and orientation of APTES molecules were not well ordered on the Si surfaces, suggesting that SFG is an efficient method to check the quality of APTES coverage immobilized on substrates.