Scanning Ion Conductance Microscopy: a Model for Experimentally Realistic Conditions and Image Interpretation

Abstract

Scanning ion conductance microscopy (SICM) is a scanned probe microscopy technique in which the probe is a fine glass pipet filled with a contact (reference) electrode and an electrolyte solution. The current flow between the reference electrode and a second reference electrode positioned in bulk solution when the two electrodes are biased externally can be used as a feedback signal to maintain a constant separation between the tip and a surface during imaging. In usual practice the tip position is modulated over a small amplitude perpendicular to the surface, and the resulting alternating current (AC) is used as the feedback signal, although the direct current can also be used. A comprehensive model for the current response is reported. Laplace's equation has been solved for the electrolyte solution for a range of tip geometries, enabling the factors controlling the tip current to be identified. The approach developed is shown to represent an improvement over earlier semiempirical treatments. To explore the influence of surface topography on the (AC) current response, various surfaces have been considered, including a tip moved toward a planar surface (in the normal direction) and tips scanned over a pit and a step in the surface. The results have allowed a critical assessment of the SICM response as a means of probing surface topography. Features identified through simulation have been found in experiments through studies of two model substrates for which imaging results are reported. In typical experimental practice, the response of the SICM tip to surface features occurs over much greater lateral distances than the size of the pipet aperture.

Simultaneous Nanoscale Surface Charge and Topographical Mapping

“…and 2d) make it clear that with V = 0, the phase shift can be used as a set point for determining topography and being able to position the probe at close tip-to-substrate separations, which is important for enhancing the resolution of SICM. 56,57 In a similar way to the experimental approaches at non-zero bias (Figure 2), the model predicts a dramatic change of the phase-distance behavior for V  0.…”

Fabrication and Characterization of Dual Function Nanoscale pH-Scanning Ion Conductance Microscopy (SICM) Probes for High Resolution pH Mapping

“…100 nm, corresponding to the set-point employed. 36 Surface features resembling the edge of a calcite hillock 40,41 was observed in the topographical map demonstrating the feasibility of high resolution topography imaging, using these pH-SICM probes. To quantify the dissolution rate, two successive scans were performed on the calcite microcrystal surface and the change in crystal height between each (Figure 4 (panels A and B)) was determined.…”

“…Field was used to achieve close positioning of the probe to the sample and to maintain the height of the electrode from the surface as is standard in SICM; 29,30,36 this was set based on the probe size. For the probe sizes mentioned herein, an i AC of 9 pA was used as the set point for constant distance 31 …”

Voltammetric Scanning Electrochemical Cell Microscopy: Dynamic Imaging of Hydrazine Electro-oxidation on Platinum Electrodes

“…Briefly, a dual barrel pipet with a small opening (in this study ~1 µm diameter) was used to make a series of constant tip-to-substrate distance was maintained by using the magnitude of i AC as feedback. [7][8][9][10][11][12][13] In previous SECCM and related studies, the substrate was scanned by the probe using a continuous scanning 13 (for flat substrates) or approach-hold-withdraw (hopping) mode (for rough substrates and/or dissolution studies), 36,37 while the substrate, if a conductor, was held at a constant potential (E s ). To directly extract data for a wide potential range, this latter hopping mode SECCM was combined with a potential sweep at each position, with the associated current response measured.…”