Bias Modulated Scanning Ion Conductance Microscopy

Abstract

Nanopipets are versatile tools for nanoscience, particularly when used in scanning ion conductance microscopy (SICM) to determine, in a noncontact manner, the topography of a sample. We present a new method, applying an oscillating bias between a quasi-reference counter electrode (QRCE) in the SICM nanopipet probe and a second QRCE in the bulk solution, to generate a feedback signal to control the distance between the end of a nanopipet and a surface. Both the amplitude and phase of the oscillating ion current, induced by the oscillating bias and extracted using a phase-sensitive detector, are shown to be sensitive to the probe-surface distance and are used to provide stable feedback signals. The phase signal is particularly sensitive at high frequencies of the oscillating bias (up to 30 kHz herein). This development eliminates the need to physically oscillate the probe to generate an oscillating ion current feedback signal, as needed for conventional SICM modes. Moreover, bias modulation allows a feedback signal to be generated without any net ion current flow, ensuring that any polarization of the quasi reference counter electrodes, electro-osmotic effects, and perturbations of the supporting electrolyte composition are minimized. Both feedback signals, magnitude and phase, are analyzed through approach curve measurements to different surfaces at a range of distinct frequencies and via impedance measurements at different distances from a surface. The bias modulated response is readily understood via a simple equivalent circuit model. Bias modulated (BM)-SICM is compared to conventional SICM imaging through measurements of substrates with distinct topographical features and yields equivalent results. Finally, BM-SICM with both amplitude and phase feedback is used for topographical imaging of subtle etch features in a calcite crystal surface. The 2 modes yield similar results, but phase-detection opens up the prospect of faster imaging.

Simultaneous Nanoscale Surface Charge and Topographical Mapping

“…In a BM-SICM configuration, a small harmonic oscillation of potential is applied to induce an AC ionic current component, which can be used for vertical probe positioning even in the absence of mean bias applied between two QRCEs. 45 Additionally, by applying an additional bias, V, we show herein that one can control the extent to which the SICM current response is sensitive (or not) to surface charge. In essence for V = 0, the BM-SICM response faithfully maps topography (Figure 1b), due to minimal surface induced rectification about 0 V, while for V  0 the SICM response becomes surface charge sensitive.…”

“…The basic instrumentation has been described elsewhere. 45,67 Briefly, movement of the SICM probe in the direction normal to the substrate was controlled using a piezoelectric positioning stage of range 38 µm (P-753-3CD, Physik…”

Multifunctional scanning ion conductance microscopy

“…At a higher bias, the effect of surface charge is more pronounced, while at a lower bias the convolution is negligible, making 0 V the ideal bias at which to approach a surface and determine topography [41]. This brings BM-SICM [58] to the fore, as zero net bias can be applied. The effect of SIR on the SICM tip current response, which can then be converted to surface charge density, can be elucidated in the following hopping regime: (i) the topography of the sample, essentially free from surface charge effects, is extracted at zero net bias, using the change in the AC phase between the bulk and surface as feedback (set point) during the approach of the probe; (ii) at each pixel in the topographical image, at the point of closest approach, surface charge is then elucidated by sweeping the bias between the two QRCEs; and (iii) this voltammetric response is then compared with a current−voltage curve recorded in bulk solution to reveal any effects of the substrate surface charge.…”

In-Phase Bias Modulation Mode of Scanning Ion Conductance Microscopy With Capacitance Compensation

“…Recently, two similar SICM imaging modes have been independently introduced by two research groups, namely, the bias-modulated (BM) mode [23], which was proposed by the Unwin group, and the in-phase bias modulation (IPBM) mode [24], which was proposed by our group. In both of these modes, instead of vibrating the tip or the stage, an ac current is induced by an alternative voltage applied between the electrodes.…”