Formation and Structure of Self-Assembled Monolayers of Alkanethiolates on Palladium

Abstract

The adsorption of n-alkanethiols onto polycrystalline thin films of palladium containing a strong (111) texture produces well-organized, self-assembled monolayers. The organization of the alkane chains in the monolayer and the nature of the bonding between the palladium and the thiol were studied by contact angle measurements, optical ellipsometry, reflection absorption infrared spectroscopy (RAIRS), and X-ray photoelectron spectroscopy (XPS). The XPS data reveals that a compound palladium-sulfide interphase is present at the surface of the palladium film. The RAIR spectra, ellipsometry data, and wetting properties show that the palladium-sulfide phase is terminated with an organized, methyl-terminated monolayer of alkanethiolates. The local molecular environment of the alkane chains transitions from a conformationally disordered, liquidlike state to a mostly all-trans, crystalline-like structure with increasing chain length (n = 8-26). The intensities and dichroism of the methylene and methyl stretching modes support a model for the average orientation of an ensemble of all-trans-conformer chains with a tilt angle of approximately 14-18 degrees with respect to the surface normal and a twist angle of the CCC plane relative to the tilt plane of approximately 45 degrees. The SAMs are stable in air, although the sulfur present at the surface oxidizes in air over a period of 2-5 days at room temperature. The differences in chain organization between SAMs formed by microcontact printing and by solution deposition are also examined by RAIRS and XPS.

The Porter-Whitesides Discrepancy: Revisiting Odd-Even Effects in Wetting Properties of n-Alkanethiolate SAMs

“…They discussed the results based on the effect of SAMs structure on wetting. These seminal works, and others [8,10,12,23,24], showed that monolayers on silver and gold were more reproducible than on copper in part due to morphological changes that occur due to oxidation and/or subsequent desorption of the alkanethiol. Therefore, most studies of interface structure-property relationships in wetting of monolayers has focused on SAMs formed on silver and gold.…”

“…This self-assembly process is a mechanism by which disordered small building units spontaneously arrange into large, ordered phases [5]. Many self-assembled systems have been investigated, with monolayers of alkanethiolates on gold or silver being the most studied SAM systems to date [6][7][8][9][10][11][12][13][14][15].…”

Integrated nanostructures for direct detection of DNA at attomolar concentrations

“…This is consistent with prior work demonstrating stronger interaction of Pd with thiols compared with Au and Pt. 26 Following these findings, we used electrodeposition through the lithographically defined apertures to reduce palladium ions in solution in order to produce metallic nanostructures, filling the apertures with metal structures that fan out above the top surface of the oxide as shown in the SEM images in Figs. 1͑b͒ and 1͑c͒.…”

Oxygen Attachment on Alkanethiolate SAMs Induced by Low-Energy Electron Irradiation

“…This is somewhat surprising, as ODT is a longer molecule than DDT (a 18 C atom chain versus a 12 C atom chain) and hence forms a thicker SAM than DDT (about 30%). 38,39 On the other hand, the nature (i.e., the length of the molecules) of the SAM can influence the molecular packing. It has been shown by STM that a lack of structure occurs for n-alkanethiolate (C n H 2n+2 ) SAMs of 16 < n < 28 with even n values, (Figure 2).…”