Scanning Electrochemical Cell Microscopy for Substrate Mapping

All experiments were carried out at room temperature (22 ± 2 °C). Conventional electrochemical experiments were performed in a three electrode format with an Ag/AgCl QRCE (preparation described above) and platinum wire (Goodfellow, U.K.) auxiliary electrode on a CHI-730A potentiostat (CH instruments, U.S.A.). All other electrochemical experiments were carried out in the SECCM format on a home-built electrochemical workstation.^{34 ,45 (link)} In this setup, a dual-barreled nanopipet probe was filled with electrolyte solution (5 or 100 mM HClO₄) and mounted on a z-piezoelectric positioner (P-753.3CD, PhysikInstrumente). The tip of the nanopipet probes were elliptical in shape, with major (r_a) and minor (r_b) radii of approximately 250 nm and 130 nm, respectively, as shown in Fig. S2a.† Ag/AgCl wire placed in each barrel served as QRCEs (detailed above). A bias potential (E_b) of either +0.05 V (100 mM HClO₄) or +0.2 V (5 mM HClO₄) was applied between the QRCEs in order to generate an ion conductance current, which was used as a feedback signal during positioning of the nanopipet probe (see below). The nanopipet was positioned above the surface of interest using micropositioners for coarse movement and an xy-piezoelectric positioner (P-622.2CD, PhysikInstrumente) for fine movement. The nanopipet was oscillated normal to the surface of interest (f ≈ 280 Hz, Δz ≈ 30 nm peak-to-peak) by an ac signal generated by a lock-in amplifier (SR830, Stanford Research Systems, U.S.A.) applied to the z-piezoelectric positioner. During approach, the magnitude of the ac ion conductance current generated by distance modulation (measured using the same lock-in amplifier) was used as feedback to detect when the meniscus at the end of the nanopipet had made contact with the working electrode surface.^{34 ,45 (link)} The nanopipet itself did not contact the substrate. Electrochemical (voltammetric) measurements were performed in the confined area defined by the meniscus (droplet cell) created between the tip and substrate. The size of the confined area (i.e., working electrode area) was determined by (SEM) imaging the droplet “footprint” left after electrochemical measurements, as demonstrated in Fig. S2b.†Electrochemical measurements at the substrate (working electrode) were made using a linear-sweep voltammetric “hopping” regime, as described previously.^{37 (link),39 (link),40 (link)} In brief, as shown schematically in Fig. 1a, the nanopipet was approached to the surface of interest at a series of predefined locations in a grid and, upon each landing, a linear sweep voltammetric experiment was carried out, building up an voltammetric ‘map’ of the substrate. In other words, in the resulting “electrochemical map” (equipotential image), each pixel corresponds to an individual LSV. The hopping distance between each pixel was 1 μm to avoid overlap of the probed areas. Note that in the images and movies presented, there is no interpolation of the data.
The SECCM cell and all piezoelectric positioners were placed in an aluminum Faraday cage equipped with heat sinks and vacuum panels to block out all light (important in the study of semiconducting materials) and minimize noise and thermal drift. The QRCE potentials were controlled (with respect to ground) with a home-built bipotentiostat and the substrate (working electrode, common ground) current was measured using a home-built electrometer with variable data acquisition times. A home-built 16th order (low-pass) brick-wall filter unit (time constant = 2 ms) was utilized during data (current) collection. Data acquisition and fine control of all the instruments was achieved using an FPGA card (PCIe-7852R) controlled by a LabVIEW 2016 (National Instruments, U.S.A.) interface. Data treatment and analysis was carried out using the Matlab R2015b (8.6.0.267246, Mathworks, U.S.A.) and OriginPro 2016 64bit (b9.3.226, OriginLab, U.S.A.) software packages.
The dual-barrelled nanopipets were pulled from quartz filamented theta-capillaries (QTF120-90-100, Friedrich & Dimmock Inc., U.S.A.) using a CO₂-laser puller (P-2000, Sutter Instruments, U.S.A.). Following pulling, the outer walls of nanopipet tips were silanized with dichlorodimethylsilane to aid meniscus confinement (and stability) when coming into contact with the substrate of interest. After the nanopipet tips were filled with the solution of interest using a MicroFil syringe (World Precision Instruments Inc., U.S.A.), a layer of silicone oil (DC 200, Sigma-Aldrich) was added on top in order to minimize evaporation (exacerbated by the filament, shown schematically in Fig. S3†). The QRCEs were then inserted through the oil layer, into the solution of interest, and mounted on the z-piezoelectric positioner, as described above.