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PeakForce Scanning Electrochemical Microscopy with Nanoelectrode Probes

This article describes new batch-fabricated, robust, and easyto-use scanning electrochemical microscopy (SECM) nanoelectrode probes with a characteristic dimension of about 50 nm. The resulting microscopy method, PeakForce SECMTM, provides electrochemical images with sub-100-nm resolution, as well as simultaneously acquired topographical, electrical, and mechanical maps on the nanometerscale. Using Bruker’s existing high-bandwidth electronics, PeakForce SECM also provides the capability for high-quality nanoelectrical imaging in liquid. This article describes several applications of this new PeakForce SECM capability that enable various types of multidisciplinary research. Introduction Electrochemistry deals with the interplay between electrical energy and chemical energy. Many electrochemistry applications impact daily life, from chemical manufacturing to frontier R&D activities in energy research, biological systems, materials development, and surface protection. Macroscopic electrochemical behavior is an average of the heterogeneous reactivity over an electrode surface. This may include different active sites, crystal-facet-dependent properties, or surface defects. Reactivity variation is a result of the heterogeneity in structural, mechanical, electrical, and/or electrochemical properties over the electrode surface. Driven by the recent nanostructural engineering of electrode materials, in situ and localized techniques are needed to map surface reactivity on the nanoscale [1]. To accommodate today’s highly multidisciplinary research, approaches capable of simultaneously capturing correlated microscopic multidimensional information in addition to electrochemistry are needed. Local electrochemical studies require an electrochemical probe with dimensions smaller than the required spatial resolution, along with stable and precise control of the probe position. Ultramicroelectrodes (UME) were first adopted in scanning electrochemical microscopy (SECM) for spatially resolved measurements. In classic SECM, a UME positioned in close proximity to the sample surface is scanned across the sample. The nature and properties of the sample area underneath the probe perturb the electrochemical processes at the electrode tip. By capturing the electrochemical response of the tip during scanning, the variation of electrochemical properties is imaged. Classic SECM generally has limitations in spatial resolution on the micron scale due to probe size, inefficient control of tip position, the convolution between topography and electrochemistry, and obtainable information only limited to electrochemistry and topography [2]. To address these limitations, atomic force microscopy (AFM) based SECM has been developed. This is to take advantage of AFM for its sharp cantilevered probe, its high-spatial-resolution imaging capability, and the ability to probe multidimensional material properties beyond electrochemistry on a fine scale. However, it remains an unmet challenge to batchfabricate stable, reliable, and cost-effective nanoelectrode SECM probes. Recently we have developed batch-fabricated, high-quality, robust PeakForce SECM probes with a characteristic tip dimension of approximately 50 nm. With these probes, PeakForce SECM provides electrochemical images with sub-100-nm resolution along with simultaneous acquisition of topographical, electrical, and quantitative mechanical maps on the nanometer-scale. In addition, this method also provides unique capabilities for high-quality nanoelectrical imaging in liquid. This article describes PeakForce SECM and provides some illustrative examples of applications. Materials and Methods SECM probes and application specimens. The PeakForce SECM probes used here are commercially available from Bruker AFM Probes (Camarillo, CA). A patterned silicon nitride on Au electrode was prepared using a conventional MEMS approach. A nanoelectrode array sample (125 nm diameter by 30 nm height) with a 300 nm electrode spacing on a silicon oxide substrate was prepared using electron beam lithographic approach. A sample of self-assembled monolayers of CH3-thiols was transferred by micro-contact printing to Au substrates. Hexaammineruthenium(III) chloride ([Ru(NH3)6]Cl3, 98%, Aldrich / ABCR), potassium chloride (BioXtra, ≥99.0%, Aldrich), Figure 1: Pictorial illustration of PeakForce SECM accessories with a pre-mounted PeakForce SECM probe loaded.