Fabrication of Nanopore Array Electrodes by Focused Ion Beam Milling

Abstract

Single nanopore electrodes and nanopore electrode arrays have been fabricated using a focused ion beam (FIB) method. High aspect ratio pores (approximately 150-400-nm diameter and 500-nm depth) were fabricated using direct-write local ion milling of a silicon nitride layer over a buried platinum electrode. This local milling results in formation of a recessed platinum electrode at the base of each nanopore. The electrochemical properties of these nanopore metal electrodes have been characterized by voltammetry. Steady-state voltammograms were obtained for a range of array sizes as well as for single nanopore electrodes. High-resolution scanning electron microscopy imaging of the arrays showed that the pores had truncated cone, rather than cylindrical, conformations. A mathematical model describing diffusion to an electrode located at the base of a truncated conical pore was developed and applied to the analysis of the electrode geometries. The results imply that diffusion to the pore mouth is the dominant mass transport process rather than diffusion to the electrode surface at the base of the truncated cone. FIB milling thus represents a simple and convenient method for fabrication of prototype nanopore electrode arrays, with scope for applications in sensing and fundamental electrochemical studies.

Electroanalysis at discrete arrays of gold nanowire electrodes

“…Despite this diffusional overlap, at low scan rates (<100 mV s --1 ) sigmoidal voltammograms were observed for all arrays with all achieving steady-state currents see Fig 5(a). This is in agreement with previous reports using nanoscale electrode arrays [8,16,30] and arises from partial and complete diffusional overlap, where the nanowire array it behaves as ultra-microelectrode of the same size of the total array, i.e., the width of a six nanowire array is 6.6 m. Such ultramicroelectrodes also experience enhanced mass transport and exhibit sigmoidal behaviour and steady-state currents.…”

“…In these works, it was found that the underlying metallisation dominated to the overall capacitance, such that the creation of extra nanopore electrodes did not affect the measured capacitance. [8,20,35] We also found this to be true for the nanowire electrode arrays, whereby increasing the number of nanowires present in the array did not affect the measured capacitive response.…”

“…To analyse these capacitive contributions at both single nanowires and nanowire arrays, we undertook cyclic voltammetry in deaereated 10 mM PBS buffer in the absence of a redox active species. [8,31,34] Fig 7(b) where the average capacitive currents (n=30) for both a single nanowire array and six nanowire arrays are displayed (R 2 =0.999, and 0.998, respectively). This strongly suggests that the measured capacitance may not be solely due to the double layer charging at the nanowire electrodes but also have a dominant contribution from elsewhere.…”

“…[6] Finally, smaller electrodes dimensions should enable nanosensors to be fabricated at high density, at technologically relevant substrates, such as at a silicon chip, thus yielding much higher information-generating capability per device. [7,8] In electrochemistry, work at very small (sub-micrometre) electrode sizes at silicon substrates, has mainly been restricted to electrochemically favourable geometries, such as nanodisks, nanopores and short inlaid nanobands. [5,[9][10][11][12][13] However, when employed as discrete electrodes, these structures can have extremely low measurable signals (typically < 100 pA) [8,14,15] and require large electrode numbers in array format (> 10 x 10) to realise reasonable measureable currents, i.e.…”

“…in the nanoAmpere range. [8,16,17] To address this challenge, researchers have recently begun to explore high aspect ratio 1-dimensional discrete nanoelectrodes such as nanobands and nanowires. [18][19][20][21][22][23] This newer class of nanoelectrode benefits from radial diffusion due to their nanometre scale critical dimension while achieving relatively large currents (nA) due to the long length of the electrode, typically ≥40 µm.…”