HREELS Study of Self-Assembled Monolayers of Alkylthiols Functionalized with Oligothiophene Moieties:  Extreme Surface Analysis and Evidence for Intermolecular Interactions

• Published in: Langmuir 2003-04, Vol.19 (7), p.2649-2657
• Main Authors: Vilar, M. Rei, Lang, P, Horowitz, G, Nogués, C, Jugnet, Y, Pellegrino, O, Botelho do Rego, A. M
• Format: Article
• Language: English
• ISSN: 0743-7463; 1520-5827
• DOI: 10.1021/la025868c

SAMs (self-assembled monolayers) of alkanethiols functionalized by α-terthienyl (3T-SAMs) and α-quaterthienyl (4T-SAMs) were formed on a gold surface presenting (111) terraces and studied by high-resolution electron energy loss spectroscopy (HREELS). The angular width of the elastic peak leads to a domain size of ca. 90 Å, comparable to that of the Au(111) terraces of the substrate. Enhancement of the spectral resolution by using a new generation spectrometer enables the improvement of the spectral analysis in the energy range of the vibrational losses. A detailed analysis of the differential cross sections of the different vibrational peaks indicates the nature of their mechanisms:  impact mechanisms show that the extreme surface of the monolayers is mainly constituted by thienyl moieties with the thienyl density in 3T-SAMs higher than that in 4T-SAMs; dipolar mechanisms show that molecules are standing up slightly tilted on the substrate; a resonance via a negative ion formation was detected around 4.5 eV. On the other hand, the electronic excitation domain leads to excitonic gap values of 2.62 ± 0.08 and 2.90 ± 0.08 eV for 4T-SAMs and 3T-SAMs, respectively. A sharp feature located at 0.28 ± 0.08 above the electronic threshold was detected in the 3T-SAM spectra. This feature, also appearing in the spectra of the α-oligothiophene evaporated films, is here assigned to a vibronic excitation. Its intensity is always related to the organization degree of the SAMs. An additional band without equivalence in the optical spectra appearing in the 4T-SAM and 3T-SAM HREELS spectra with thresholds at 3.58 ± 0.08 and 4.15 ± 0.08 eV, respectively, is assigned to the electronic gap. The values of the excitonic and electronic gaps are well correlated with those found in the corresponding spectra of evaporated α-quaterthiophene, α-quinquethiophene, and α-sexithiophene films formerly studied. These results lead to the conclusion that thienyl functionalities present strong intermolecular interactions.