The electrical and optical response of thin films of
surface functionalized nanoparticles upon exposure to various
chemical vapours has been studied. It was found that the
electrical response to chemical vapours adsorbed on the various
nanoparticle films varied markedly and was determined by the
surface functional groups. Ellipsometric studies revealed that
the film thickness increased during exposure to the
chemical vapours. These thickness changes of the films
correlate with the changes in electrical conductivity. Two
physical effects are believed to play a role in determining
these conductivity changes. Under high partial pressure, the
change in nanoparticle core-core separation is the main
contribution to the change in conductivity and generally leads
to a reduction in the conductivity. However, for relatively low
partial pressures the adsorption of vapour molecules leads to
permittivity changes that tend to increase the conductivity.
A series of gold nanoparticles stabilized by the adsorption of heterocyclic mercaptan derivatives, including 2-mercaptopyridine (2MPy), 2-mercaptopyrimidine (2MPm) and 4-mercaptopyridine (4MPy), were synthesized and characterized by TEM, FTIR, UV-vis, and XPS. The adsorption of 2MPy molecules led to the most stable and uniform nanoparticles. In contrast, the 4MPy-coated nanoparticles showed a strong tendency to form 3D aggregates, which is attributed to cross linking between sulfur groups on neighboring particles. FTIR and UV-vis spectroscopy were employed to study the chemical conformation of the heterocyclic molecules on the gold surface. Significant differences between the IR spectra of the functionalized nanoparticles and the free mercaptan molecules were observed. Interpretations of these IR spectra were achieved with the aid of ab initio calculations. We found that the "thione form" is predominant when the molecules are either in a polycrystalline state or in methanol solution; however, they are stabilized in the "thiol-like form" when adsorbed on the particles. The XPS spectra of the 2MPy nanoparticles showed narrow peaks with the expected peak position. In contrast, the peaks in the spectra of the 2MPm-and 4MPy-passivated nanoparticles were strongly shifted. Oxidization of the sulfur group was detected in the 4MPy-modified nanoparticles, indicating that some molecules were adsorbed onto the gold surface via their nitrogen groups.
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