| AVS 55th International Symposium & Exhibition | |
| Applied Surface Science | Tuesday Sessions |
| Session AS-TuA |
| Session: | Practical Surface Analysis |
| Presenter: | J.E. Baio, University of Washington |
| Authors: | J.E. Baio, University of Washington T. Weidner, University of Washington D.J. Graham, Asemblon L.J. Gamble, University of Washington D.G. Castner, University of Washington |
| Correspondent: | Click to Email |
Self-assembled monolayers (SAMs) of alkanethiols on metal surfaces have been extensively used as model systems. In particular, amine terminated SAMs have been used to model positively charged surfaces. However, unlike simple methyl terminated SAMs, the quality of amine SAMs is harder to control. This difficulty is illustrated by the previously reported x-ray photoelectron spectroscopy (XPS) results from amine terminated SAMs, all of which describe an unusually high concentration of oxygen within the monolayer. Across a range of protocols, the amount of oxygen detected by XPS varies from 5-8 atomic %. There are two hypotheses that could explain the presence of this oxygen. The first is that the SAM is covered with oxygen containing coadsorbates, while the second assumes that the nitrogen, carbon, and sulfur species are oxidizing. To explore these hypotheses, we characterized amine terminated SAMs on gold substrates by XPS, time-of-flight secondary ion mass spectroscopy (ToF-SIMS), and sum-frequency generation (SFG) vibrational spectroscopy. XPS characterization of our amine SAMs yielded oxygen concentrations as low as 2 atomic %. High-resolution XPS results from the S(2p), C(1s) and N(1s) regions did not detect any oxidized species. However, small amounts of oxidized fragments did appear in both the positive and negative secondary ion spectra. Some of the more prominent oxidized fragments detected included SO2H+ and C2H5NO+ in the positive secondary ion spectra and SO3- in the negative secondary ion spectra. There were two distinct stretches present in the SFG vibrational spectra: one at 3150 cm-1 that corresponds to structured water, and a broad peak at 2850 cm-1 indicating a high number of gauche defects within the monolayer. Nonetheless, the lack of N-O, S-O, and C-O stretches in the SFG spectra are consistent with the XPS and SIMS results that show oxidation alone cannot completely account for all of the oxygen detected by XPS on the amine SAM surfaces. Therefore, the majority of the oxygen found within these amine SAMs most likely arises from the presence of coadsorbates.