Octadecanoic Acid Self-Assembled Monolayer Growth at Sapphire Surfaces

Self-assembled monolayer growth of octadecanoic acid on single-crystal C-plane (0001) and R-plane (1102) sapphire (α-Al2O3) has been investigated by ex situ tapping mode atomic force microscopy, contact angle measurements, and Fourier transform infrared spectroscopy. Partial monolayers, interrupted during growth, contain islands of a densely packed well-ordered phase within a lower-lying disordered molecular matrix. The areal fraction of the ordered phase increases as a function of immersion time. The growth kinetics are sensitive to the history of the sapphire substrate; e.g., complete monolayers were formed from 1.5 mM hexadecane solution in ∼1 min on freshly annealed (i.e., dehydrated) substrates in comparison with ∼1 h on substrates aged for 1 day under ambient conditions. The growth is slightly faster on C-sapphire than R-sapphire. Although weakly bound, the complete monolayers are very well-ordered, conformal, and essentially defect-free.