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 x 10(14) 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 (kd/ka). We also report the adsorption and desorption rate constants (ka and kd) 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 ka = 2.4 +/- 0.2 cm3/(mol s) and kd = (8.2 +/- 0.5) x 10(-6)(s-1); (2) slow adsorption with ka = 0.8 +/- 0.5 cm3/(mol s) and kd = (3 +/- 2) x 10(-6) s(-1).
We have investigated in situ and in real time vapor-phase self-assembly of 1-decene on Si, using attenuated
total reflection Fourier transform infrared spectroscopy (ATR-FTIRS). The adsorption of 1-decene on
hydrogenated Si(100) results in a decane-terminated hydrophobic surface, indicated by the sessile-drop
water contact angle at 107 ± 2°. This maximum contact angle is achieved at 160 °C under 30 mTorr of
vapor-phase 1-decene. The fractional surface coverage of decane, calculated from the IR absorbance of
C−H stretching vibrational modes near 2900 cm-1, follows a Langmuir isotherm. The absolute surface
coverage calculated from the IR absorbance saturates at 3.2 × 1014 cm-2. On the basis of this isotherm,
the empirical rate constant (
) that governs the rate-limiting step in 1-decene adsorption on HF-treated
Si(100) is (3.3 ± 0.7) × 10-2 min-1. The thickness and cant angle of the decane monolayer at the saturation
coverage are calculated from angle resolved X-ray photoelectron spectroscopy (AR-XPS). The calculated
thickness ranges from 8.4 to 18 Å due to the uncertainty in the attenuation lengths of C(1s) and Si(2p)
photoelectrons through the decane layer. For the same uncertainty, the calculated cant angle ranges from
0 to 55°. Spectroscopic ellipsometry is independently used to approximate the film thickness at 16 Å.
Monitoring the decane monolayer over a period of 50 days using AR-XPS indicates that the Si surface
underneath the decane monolayer gets oxidized with time, leading to the degradation of the decane layer.
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