Study of Insulating Properties of Alkanethiol Self-Assembled Monolayers Formed Under Prolonged Incubation Using Electrochemical Impedance Spectroscopy

Abstract

The electrochemical interfacial properties of a well-ordered self-assembled monolayer (SAM) of 1-undecanethiol (UDT) on evaporated gold surface have been investigated by electrochemical impedance spectroscopy (EIS) in electrolytes without a redox couple. Using a constant-phase element (CPE) series resistance model, prolonged incubation times (up to 120 h) show decreasing monolayer capacitance approaching the theoretical value for 1-undecanethiol. Using the CPE exponent α as a measure of ideality, it was found that the monolayer approaches an ideal dielectric (α = 0.992) under prolonged incubation, which is attributed to the reduction of pinholes and defects in the monolayer during coalescence and annealing of SAM chains. The SAMs behave as insulators until a critical potential, Vc, is exceeded in both cathodic and anodic regimes, where electrolyte ions are believed to penetrate the monolayers. Using a Randles circuit model for these cases, the variation of the capacitance and charge transfer resistance with applied dc potential shows decreased permeability to ionic species with prolonged incubation time. The EIS data show that UDT (methylene chain length n = 10), incubated for 120 h, forms a monolayer whose critical voltage range extends from −0.3 to 0.5 V versus Ag/AgCl, previously attained only for alkanethiol at n = 15. At low frequencies where ion diffusion occurs, almost pure capacitive phase (−89 deg) was attained with lengthy incubation.

Substrate-Dependent Study of Chain Orientation and Order in Alkylphosphonic Acid Self-Assembled Monolayers for ALD Blocking

“…The decrease in capacitance (Table S5) for the ODPA-treated surfaces is proof of the SAM layer on the surface. 6 Cu and Co-ODPA show capacitances near 1 μF/cm 2 which is consistent with what is expected for well-ordered monolayers. 6 All of the substrate-specific data including the previous ALD blocking data are summarized in Table 1.…”

“…6 Cu and Co-ODPA show capacitances near 1 μF/cm 2 which is consistent with what is expected for well-ordered monolayers. 6 All of the substrate-specific data including the previous ALD blocking data are summarized in Table 1. These results show three key findings.…”

“…Both of these techniques are powerful surface analytical tools that are commonly used for evaluating SAMs. 6,9,26 With regard to SAM formation, the results of these studies show that, in agreement with WCA measurements, highly oriented phosphonic acid SAMs can be formed on Cu, Co, and W surfaces, while only poor adsorption occurs on Ru as evidenced by its lack of angle dependence in the NEXAFS. Furthermore, FTIR spectroscopy suggests that ODPA SAMs formed on Cu and Co are "crystal-like" while those on Ru and W are more "liquid-like", with W showing the most disorder.…”

“…The resulting data were modeled using a Randles circuit model. 6,46 NEXAFS Studies. M-ODPA, Δ/M-ODPA, and ZnO/M-ODPA samples were evaluated by NEXAFS.…”

“…It measures the resistive and capacitive behavior of a system over a range of frequencies. 6 The data collected can be represented in a Bode phase plot. The technique has been used in previous studies to investigate the quality of SAMs on several surfaces.…”