Adsorption Kinetics of 1-Alkanethiols on Hydrogenated Ge(111)

We have investigated the liquid-phase self-assembly of 1-alkanethiols (HS(CH2) n - 1CH3, n = 8, 16, and 18) on hydrogenated Ge(111), using attenuated total reflection Fourier transform infrared spectroscopy as well as water contact angle measurements. The infrared absorbance of C−H stretching modes of alkanethiolates on Ge, in conjunction with water contact angle measurements, demonstrates that the final packing density is a function of alkanethiol concentration in 2-propanol and its chain length. High concentration and long alkyl chain increase the steady-state surface coverage of alkanethiolates. A critical chain length exists between n = 8 and 16, above which the adsorption kinetics is comparable for all long alkyl chain 1-alkanethiols. The steady-state coverage of hexadecanethiolates, representing long-chain alkanethiolates, reaches a maximum at approximately 5.9 × 1014 hexadecanethiolates/cm2 in 1 M solution. The characteristic time constant to reach a steady state also decreases with increasing chain length. This chain length dependence is attributed to the attractive chain-to-chain interaction in long-alkyl-chain self-assembled monolayers, which reduces the desorption-to-adsorption rate ratio (k d/k a). We also report the adsorption and desorption rate constants (k a and k d) of 1-hexadecanethiol on hydrogenated Ge(111) at room temperature. The alkanethiol adsorption is a two-step process following a first-order Langmuir isotherm: (1) fast adsorption with k a = 2.4 ± 0.2 cm3/(mol s) and k d = (8.2 ± 0.5) × 10-6 s-1; (2) slow adsorption with k a = 0.8 ± 0.5 cm3/(mol s) and k d = (3 ± 2) × 10-6 s-1.