Infrared spectroscopic ellipsometry of n-alkylthiol (C5-C18) self-assembled monolayers on gold

The infrared spectroscopic ellipsometry (IRSE) of n-alkylthiol (CH3(CH2)xSH, x = 4, 6, 7, 8, 10, 13, 15, and 17, self-assembled monolayers (SAMs), with 5-18 carbon atoms (C5-C18), grown on gold-coated Si(100) substrates) was investigated at room temperature. The C-H stretching vibrations could be resolved even for pentathiol, the shortest chain studied. The symmetric and asymmetric stretching vibrations of the CH2 groups are located at about 2850 and 2920 cm(-1), and those of CH3 are at about 2877 and 2962 cm(-1), respectively; they show a slight shift with the number of CH2 units. In addition, Fermi resonance of the symmetric CH3 stretching vibration at 2940 cm(-1) appears with decreasing chain length due to weak coupling with the asymmetric CH2 stretching vibration. The "odd-even effect" of the n-alkylthiol SAMs with varying CH2 units could be distinguished by the two interactive IRSE parameters. The relative ellipsometric spectra for the four longest chains could be reproduced quite well by using a Lorentz multioscillator model with a three-phase optical model (air/SAMs/gold). On the basis of the theoretical calculations, the vibrational strength of these oscillators is very weak, its magnitude being 10(-4)-10(-5). The full width at half-maximum (fwhm) of the peaks varies from 7 to 33 cm(-1). Moreover, the intensity of the C-H vibrations increases with the number of methylene units, due to strong lateral interactions and ordering effects occurring for longer chains.