Infrared spectroscopic studies of self-assembled monolayers (SAMs) on gold: structural characterization and water adsorption measurements
Research in the Martin laboratory focuses on taking vibrational spectroscopy measurements at solid-solid and gas-solid interfaces. These measurements are made through via a polarization modulation infrared reflection-absorption spectrometer (PM-IRRAS), a modified Fourier transform infrared (FT-IR) spectrometer that has sufficient sensitivity to measure films of thicknesses less than 200 nm.1, 2 In addition to this sensitivity, advantages of this technique include non-destructive interrogation of the sample by the IR probe, high spectral resolution of the FT-IR spectrometer, and a range of possible temperature and pressure conditions during experiments allowing for in situ measurements.3 Polymer thin-films adsorbed on metal surfaces can be grown via self assembly, where molecules organize themselves into crystal-like structures called self-assembled monolayers (SAMs). The characterization of these SAMs is difficult due to the small amount of material present and the presence of the metal substrate. The PM-IRRAS measurements provide a direct probe of the composition, conformation, and orientation of the polymer thin films. The system is shown in Figure 1 below.
Figure 1. Agilent Cary 680 FTIR with PM-IRRAS accessory; overhead view of optical layout.
The PM-IRRAS external setup was then modified to accommodate gas adsorption studies on SAMs by installing into the infrared beam path a sample reactor that gives precise control of the sample temperature and pressure.4 SAMs of octadecanethiol (ODT) and 16-mercaptohexadecanoic acid (MHA) were formed on gold. In the presence of humidified nitrogen, an adsorbed hydration layer was observed on the carboxyl-terminated MHA SAMs, shown in Figure 2, but not on the methyl-terminated ODT. We continue to explore the relationship between the wettability of the film and the growth of hydration layers.
Figure 2. MHA SAM with and without hydration layer, centered at 3450 cm-1.
1 Polarization-modulation approaches to reflection-absorption spectroscopy, Frey, B.; Corn, R.; Weibel, S., in: J. Chalmers and P. R. Griffiths, (Eds.), Handbook of Vibrational Spectroscopy, John Wiley & Sons, Vol. 2, (2001) pp. 1042-1056.
2 Polarization modulation Fourier transform infrared reflectance measurements of thin films and monolayers at metal surfaces utilizing real-time sampling electronics, Barner, B.; M. Green, M.; E. Saez, E.; Corn, R. Analyt. Chem. 1991, 63, 55.
3 PM-IRRAS spectroscopy for the characterization of polymer nanofilms: chains conformation, anisotropy and crystallinity, Elzein, T.; Bistac, S.; Brogly, M.; Schultz, J. Macromolecular Symposia 2004, 205, 181.
4 Water adsorption isotherms on CH3-, OH-, and COOH-terminated organic surfaces at ambient conditions measured with PM-RAIRS, Tu, A.; Kwag, H.; Barnette, A.; Kim, S. Langmuir, 2012, 28, 15263.