The vibrational Stark effect is observed in the surface-enhanced Raman scattering spectra of self-assembled monolayers functionalized with pendant nitrile groups. Stark tuning of the nitrile-stretching frequency serves as a localized probe of the electric field in the diffuse double layer of a SAM-modified electrochemical interface. Stark-tuning rates at low ionic strength correspond to reasonable values of the local electric (E) field in the double layer. The nitrile-stretching frequency converges on its isotropic value at applied potentials approaching the PZC, which indicates that Stark-tuning of the frequency is a direct probe of the E field at the interface. Loss of the local electric field at high ionic strengths shows that the probe responds to changes in the Debye length of the double layer. The results demonstrate the applicability of this electric-field probe for characterizing the diffuse double-layer region.

Potential-dependent surface-enhanced Raman scattering (SERS) spectra of the nitrile stretching mode were acquired from a series of monolayers composed of alkanethiols (HS(CH 2 ) x CH 3 , where 6 e x e 10) and mercaptododecanenitrile (HS(CH 2 ) 11 CN). These spectra were used to investigate the diffuse double layer at a silver electrode interface modified with mixed self-assembled monolayers (SAMs). The alkanethiol species acts to dilute the nitrile-terminated thiol to isolate the nitrile reporter group within the diffuse double-layer region. Nitrile groups co-immobilized with shorter diluent alkanethiol chains are placed more deeply into the diffuse double layer (relative to the methyl terminus of the surrounding alkanethiol). Interfacial electric fields, measured using observed Stark tuning rates of the nitrile stretching frequency, were examined as a function of SAM composition to map the structure of the diffuse double-layer region versus distance from the SAM/ solution interface. The trends in the experimental data are largely consistent with Gouy-Chapman theory, in which Stark tuning rates, and the interfacial electric fields from which they originate, depend on both distance of the probe from the electrode surface and the ionic strength of the aqueous phase. For measurements at the highest ionic strengths, the experimentally observed double layer appeared to extend further into solution than predicted by Guoy-Chapman theory, which is consistent with the finite size of hydrated ions and theoretical predictions of the effect of a hydrophobic interface on the structure of the adjacent water layer. The results demonstrate the ability of this spectroelectrochemical experiment to characterize diffuse double-layer structure at electrochemical interfaces on a subnanometer distance scale.

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Single-ion detection has, for many years, been the domain of large devices such as the Geiger counter, and studies on interactions of ionized gasses with materials have been limited to large systems. To date, there have been no reports on single gaseous ion interaction with microelectronic devices, and single neutral atom detection techniques have shown only small, barely detectable responses. Here we report the observation of single gaseous ion adsorption on individual carbon nanotubes (CNTs), which, because of the severely restricted one-dimensional current path, experience discrete, quantized resistance increases of over two orders of magnitude. Only positive ions cause changes, by the mechanism of ion potential-induced carrier depletion, which is supported by density functional and Landauer transport theory. Our observations reveal a new single-ion/CNT heterostructure with novel electronic properties, and demonstrate that as electronics are ultimately scaled towards the one-dimensional limit, atomic-scale effects become increasingly important.