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Adsorption kinetics of 1-alkanethiols on hydrogenated Ge(111)
We have investigated the liquid-phase self-assembly of 1-alkanethiols (HS(CH{sub 2}){sub n-1}CH{sub 3}, 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 x 10{sup 14} hexadecanethiolates/cm{sup 2} 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{sub d}/k{sub a}). We also report the adsorption and desorption rate constants (k{sub a} and k{sub 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{sub a} = 2.4 {+-} 0.2 cm{sup 3}/(mol s) and k{sub d} = (8.2 {+-} 0.5) x 10{sup -6} s{sup -1}; (2) slow adsorption with k{sub a} = 0.8 {+-} 0.5 cm{sup 3}/(mol s) and k{sub d} = (3 {+-} 2) x 10{sup -6} s{sup -1}.