Self-assembled monolayers (SAMs) consisting of alkanethiols and similar sulfurcontaining molecules on noble metal substrates are extensively used and explored for various chemical and biological surface-functionalization in the scientific community. SAMs consisting of thiol-or disulfide-containing molecules adsorbed on gold are commonly used due to their ease of preparation and stability. However, the gold-thiolate bond is easily and rapidly oxidized under ambient conditions, adversely affecting SAM quality and structure. Here, the oxidation of dodecanethiol on gold is explored for various 12-hour exposures to ambient laboratory air and light. SAM samples are freshly prepared, air-exposed, and stored in small, capped vials. X-ray photoelectron spectroscopy (XPS) reveals nearly complete oxidation of the thiolate in air-exposed samples, and a decrease in carbon signal on the surface. Near-edge X-ray absorption fine structure spectroscopy (NEXAFS) at the Carbon K-edge shows a loss of upright orientational order upon air-exposure. Alternatively,
We present a nanoscale color detector based on a single-walled carbon nanotube functionalized with azobenzene chromophores, where the chromophores serve as photoabsorbers and the nanotube as the electronic read-out. By synthesizing chromophores with specific absorption windows in the visible spectrum and anchoring them to the nanotube surface, we demonstrate the controlled detection of visible light of low intensity in narrow ranges of wavelengths. Our measurements suggest that upon photoabsorption, the chromophores isomerize from the ground state trans configuration to the excited state cis configuration, accompanied by a large change in dipole moment, changing the electrostatic environment of the nanotube. All-electron ab initio calculations are used to study the chromophore-nanotube hybrids and show that the chromophores bind strongly to the nanotubes without disturbing the electronic structure of either species. Calculated values of the dipole moments support the notion of dipole changes as the optical detection mechanism.
X-ray absorption spectroscopy and X-ray photoemission spectroscopy have been used to examine sulfur−gold bond formation in monolayers derived from a dithiol monomer and related disulfide-containing polyamides. These compounds were designed to allow the molecules to adsorb to gold through two terminal sulfurs, forming surface-attached loops. Element and site-specific density of unoccupied electronic states were probed by X-ray absorption spectroscopy at the C 1s, N 1s, O 1s (K-edge), and S 2p (L2,3-edge) absorption edges. Photoemission measurements of the C 1s, N 1s, O 1s, and S 2p core lines were also used to estimate relative coverage, to confirm layer formation, and to evaluate chemical bonding of the monomer and polyamide to the gold-coated substrates. In the case of the dithiol monomer, the spectroscopic evidence clearly shows that most of the molecules adsorb through a single thiol end. The disulfide-containing precursors, in contrast to the monomer, attach to the surface through both sulfurs to form the anticipated surface-attached loop.
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