We describe a method for the production of nanoelectrodes at the apex of atomic force microscopy (AFM) probes. The nanoelectrodes are formed from single-walled carbon nanotube AFM tips which act as the template for the formation of nanowire tips through sputter coating with metal. Subsequent deposition of a conformal insulating coating, and cutting of the probe end, yields a disk-shaped nanoelectrode at the AFM tip apex whose diameter is defined by the amount of metal deposited. We demonstrate that these probes are capable of high-resolution combined electrochemical and topographical imaging. The flexibility of this approach will allow the fabrication of nanoelectrodes of controllable size and composition, enabling the study of electrochemical activity at the nanoscale.

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Investigations of the kinetics of molecular transfer across the liquid/gas interface and the effect of a molecular monolayer are of considerable interest as a model for certain biological and environmental processes. In this work, a combined scanning electrochemical microscopy (SECM)-Langmuir trough technique has been used to investigate the effect of the chemical character and mechanical compression of molecular monolayers on the rate of oxygen transfer across the air/water (A/W) interface. Specifically, monolayers comprising the fatty alcohol 1-octadecanol and the phospholipid L-R-dipalmitoyl phosphatidic acid were considered. A mercury hemispherical microelectrode probe has been used to measure interfacial kinetics in SECM, and a numerical model has been developed for mass transport in this configuration to allow quantitative analysis of experimental data. The results obtained suggest that, for both monolayers, the oxygen-transfer rate across the interface decreased compared to that across the clean interface, with the blocking effect becoming more pronounced as the surface pressure of the monolayer increased. A simple energy-barrier model was used successfully to interpret the dependence of the rate constant of oxygen transfer on the surface pressure. The experimental data also provide evidence for the effect of the SECM probe on the deformation of the water surface at very close distances to the A/W interface.

In this paper we demonstrate that the nucleation density of single-walled carbon nanotubes (SWNTs), formed by thermal catalytic chemical vapor deposition, strongly depends on the grain size of Al underlayers covered with a native oxide (Al/Al2O3). By varying the substrate temperature during Al sputter deposition it was possible to investigate the effect of Al grain size on growth without inducing changes in the underlayer thickness, surface chemistry, or any other growth parameter. The resulting SWNT growth structures ranged from low-density 2D nanotube networks that lay across the surface of the substrate to high density 3D nucleation which gave rise to vertical “forest” growth. The height of the SWNT “forest” was observed to increase with increasing Al deposition temperature as follows, 200 > 100 > 60 > 20 °C on Si/Al but in the order 100 > 200 > 60 > 20 °C on SiO2/Al substrates for fixed growth conditions. The differences in the SWNT growth trends on Si and SiO2 substrates are believed to be due to the existence of an optimal Al/Al2O3 underlayer grain size for the formation of active catalytic nanoparticles, with larger Al/Al2O3 grains forming on SiO2 than Si at a fixed substrate temperature. Numerous surface analysis techniques including AFM, XPS, FESEM, TEM, and Raman spectroscopy have been employed to ascertain that the observed changes in nanotube growth for this system are related primarily to changes in underlayer morphology.

The deposition and characterisation of Langmuir-Blodgett (LB) layers of polyaniline (PAN) on solid supports is described. Langmuir films were spread as a mixture of PAN and dodecylbenzenesulfonic acid (DBSA) at the water/air interface and deposited on either glass or indium tin oxide (ITO). Mono- and multi-layer films of PAN/DBSA were characterized by X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), absorption spectroscopy and cyclic voltammetry (CV). The ultrathin films produced were found to be highly uniform and very stable. Further characterisation of the films was accomplished by scanning electrochemical microscopy (SECM) in the feedback mode. It was found that the conductivity depended strongly on the pH of the solution and the number of layers deposited. Values for the pH-dependent lateral conductivity of PAN LB films are reported.

The effect of AFM probe geometry on diffusion to micrometer-scale reactive (electrode) interfaces is considered. A disk-shaped substrate electrode was held at a potential to reduce a species of interest (aqueous Ru(NH 3) 6 (3+)) at a diffusion-controlled rate and the current response during AFM imaging provided information on local mass transport to the interface. This approach reveals how the AFM probe influences diffusion to a reactive surface, which is of importance in more clearly delineating the conditions under which in-situ AFM can be treated as a noninvasive probe of surface processes involving mass transport (e.g., electrode reactions and crystal dissolution and growth). An assessment has been made of three types of probes: V-shaped silicon nitride contact mode probes; single beam silicon probes; and batch-fabricated scanning electrochemical-atomic force microscopy (SECM-AFM) probes. Two disk electrodes, (6.1 microm and 1.6 microm diameter) have been considered as substrates. The results indicate that conventional V-shaped contact mode probes are the most invasive and that the batch-fabricated SECM-AFM probes are the least invasive to diffusion at both of the substrates used herein. The experimental data are complemented by the development of simulations based on a simple 2D model of the AFM probe and active surface site. The importance of probe parameters such as the cantilever size, tip cone height, and cone angle is discussed, and the implications of the results for studies in other areas, such as growth and dissolution processes, are considered briefly.

A new method has been developed for measuring local adsorption rates of metal ions at interfaces based on scanning electrochemical microscopy (SECM). The technique is illustrated with the example of Ag+ binding at Langmuir phospholipid monolayers formed at the water/air interface. Specifically, an inverted 25 microm diameter silver disc ultramicroelectrode (UME) was positioned in the subphase of a Langmuir trough, close to a dipalmitoyl phosphatidic acid (DPPA) monolayer, and used to generate Ag+ via Ag electro-oxidation. The method involved measuring the transient current-time response at the UME when the electrode was switched to a potential to electrogenerate Ag+. Since the Ag+/Ag couple is reversible, the response is highly sensitive to local mass transfer of Ag+ away from the electrode, which, in turn, is governed by the interaction of Ag+ with the monolayer. The methodology has been used to determine the influence of surface pressure on the adsorption of Ag+ ions at a phospholipid (dipalmitoyl phosphatidic acid) Langmuir monolayer. It is shown that the capacity for metal ion adsorption at the monolayer increased as the density of surface adsorption sites increased (by increasing the surface pressure). A model for mass transport and adsorption in this geometry has been developed to explain and characterise the adsorption process.

This paper describes in detail the use of electron beam lithography (EBL) to successfully batch microfabricate combined scanning electrochemical-atomic force microscopy (SECM-AFM) probes. At present, the process produces sixty probes at a time, on a 1/4 of a three-inch wafer. Using EBL, gold triangular-shaped electrodes can be defined at the tip apex, with plasma enhanced chemical vapor deposited silicon nitride serving as an effective insulating layer, at a thickness of 75 nm. The key features of the fabrication technique and the critical steps are discussed. The capability of these probes for SECM-AFM imaging in both tapping and constant distance mode is illustrated with dual topographical-electrochemical scans over an array of closely-spaced 1 microm diameter Pt disc electrodes, held at a suitable potential to generate an electroactive species at a transport-limited rate. As highlighted herein, understanding diffusion to heterogeneous electrode surfaces, including array electrodes, is currently topical and we present preliminary data highlighting the use of SECM-AFM as a valuable tool for the investigation of diffusion and reactivity at high spatial resolution.