Results and Discussion: Nanopore array

In this section, the performance of the ICF method is assessed for a range of conditions; initially the method is applied to nanopore arrays (Section 4.5.4) so that its performance can be evaluated across multiple samples in one test. However as nanopore arrays consist of a larger effective ion conductance area, they are not able to assess the sensitivity of the ICF method. Hence, Section 4.5.5 repeats the range of tests conducted for the arrays on single pore membranes.

In order to test the ICF method, the ionic current measured during deposition was compared to that measured using cyclic voltammetry (Section 2.6) both prior and subse- quent to deposition. In this case sweeps of ±500 mV were applied across the nanopore and the resulting current-voltage (I-V) characteristics were measured.

However the Pt deposition need not be completed in a single step starting from the initial Au nanopore and ending with the target pore diameter. Instead the deposition process may be stopped and started several times in between, hence achieving a multi-step

deposition. So between each step, I-V measurements were made and compared to those of the ICF. Hence the ICF method was analysed over a wide range of deposition times, using different initial pore sizes. The nanopore arrays were subjected to the ICF deposition approach using the electrode arrangement discussed in Section 4.5.3. The deposition was then performed using both a single-step and a uniform multi-step sequence.

For the single-step deposition sequence, a constant deposition potential of -300 mV was applied to WE1 for 200 s whilst a bias potential of +200 mV was applied between WE2 and CE/RE. I-V measurements were carried out before and after the deposition in 100 mM KCl. A schematic of this approach is illustrated in Figure 4.15 (top panel).

For the uniform multi-step approach, a potential of -300 mV was applied to WE1 for steps of 100 s each. A schematic of this approach is illustrated in Figure 4.15 (bottom panel). During each deposition step a bias potential of +200 mV was applied between WE2 and CRE and the ionic current was monitored through the pores. The I-V measurements were carried out at each deposition interval in 100 mM KCl by sweeping the potential between -500 mV and +500 mV.

Single step deposition

Chronoamperometric measurements

The chronoamperometry technique facilitates the monitoring of the ionic current through the nanopore whilst simultaneously monitoring the resulting current from the applied po- tential for Pt deposition. Figure 4.16 shows current-time transient scans for the deposition current (blue curve) and ionic current feedback (green curve) observed when Pt was de- posited over 200 s using the four electrode configuration. The ionic feedback current show an initial sharp increase which is produced by capacitive charging effects at the electrodes, the current then gradually decreases over the duration of the deposition as a result of the pore shrinking. The ionic feedback current decreases as the Pt deposition current begins to increase (at approximately 80 s) and continues to do so over the deposition duration, this corresponds to the Pt deposition as discussed in Section 3.4.1 for a flat gold surface. A total of -0.0437 C was transferred over the course of the deposition process with a final Pt film thickness of 112.5 nm.

I-V measurements

Figure 4.15: Schematic representation of potentials applied to the ICF system using a bipotentiostat. Top: 1stapproach- for depositing Pt and measuring the ionic current

feedback, where a constant potential Vdep was applied to WE1 for 200 s at -300 mV

(including 2 s at a lower negative potential where no faradaic processes occur). Simultaneously a bias potential Vbias was applied between WE2 and CRE (Ag/AgCl

electrodes), respectively. Bottom: 2nd _{approach- showing the multi-step depositions,}

each lasting for 100 s, between which I-V measurements were carried out (represented by red dotted line) in 100 mM KCl to derive the pore conductances, notations: WE1 (nanoporous membrane); WE2 (Ag/AgCl electrode); CE/RE (Ag/AgCl electrode).

0 20 40 60 80 100 120 140 160 180 200 −2 −1.5 −1 −0.5 0 0.5 1 Current/ µ A Times/ s −500 −400 −300 −200 −100 0 100 200 300 400 500 −50 −40 −30 −20 −10 0 10 20 30 40 50 Current/ µ A Potential/ V

Figure 4.16: Left: Current-time transient curves measured for the deposition of Pt at Vdep of -300 mV; t= 200 s current response for the deposition (blue curve) and ionic

current feedback (green curve). As the Pt is deposited, the ionic current gradually decreased over time until reaching a lower limit of ∼6.32 µA. Right: I-V traces for an array of 16 nanopores, acquired via cyclic voltammetry in 100 mM KCl; before (black

curve) and after (red curve) Pt deposition.

process by applying sweep potentials between ±500 mV across the nanopore array in 100 mM KCl. The I-V measurements for this are shown in Figure 4.16; the gradients of which are the pore conductance, G.

Before the start of the experiment, the conductance per pore gave a value of ∼2.81 µS, where the average initial pore diameter was 150 nm. After depositing the Pt on the Au nanoporous membrane, we observed that the current passing through the pore dropped and a final pore conductance of ∼2.13 µS was obtained. These values were compared to the conductance calculated from the chronoamperometry measurements in Section 4.5.4; by using the applied bias potential of +200 mV and its resulting current (green curve), the conductance (G=I/V) was found to be ∼1.97 µS at 200 s.

The pores were characterised using SEM. Figure 4.17 shows a typical SEM image of a nanopore milled in the thin Au/Si3N4 membrane (Au/Ti 100 nm; Si3N4 240 nm), with a

diameter of 150 nm ± 5 nm. The SEM image of the nanopore following electrochemical deposition of Pt (200 s). Figure 4.17 (right panel) reveals a highly symmetrical final pore, with a diameter of 93 nm ±10 nm; the Pt deposits around the pore in a circular manner without distorting the shape of the pore. The electrodeposited Pt was comparable to

results achieved in the characterisation of Pt films in Chapter 3 and exhibited similar structures with spherical formed particles of platinum between 30-40 nm in size.

Figure 4.17: Scanning electron micrographs of a single nanopore from an array of 16 pores. Left: before and Right: after Pt deposition. Vdep = -300 mV, t= 200 s. Final

pore diameter was 93 nm. Scale bar= 200 nm.

Multiple step depositions Chronoamperometric measurements

As mentioned previously, the multi-step deposition process consisted of four deposition steps each lasting for 100 s. Figure 4.18 shows the deposition current obtained over the course of 400 s. The main characteristics observed for each step of the multi-step deposition procedure correspond to those of the single-step results. Hence, Figure 4.18 (left panel), which presents the deposition current for each step shows a gradual increase in the current as the Pt layer grows on the surface of the membrane. The deposited charge and thickness values calculated for each step using Equation 3.4.1 are provided in Table 4.2.

Correspondingly, the ionic current plotted in Figure 4.18 (right panel) is seen to grad- ually decrease over the duration of each deposition step. Starting from an ionic current value of ∼22.45 µA for the first step (green curve) as the deposition steps are performed, a gradual decrease in ionic current is observed, reaching a lower limit of 2.76 µA (blue curve- fourth step).

I-V measurements

I-V measurements for the multi-step approach were taken between each deposition step by sweeping the nanoporous membrane between ±500 mV in 100 mM KCl. The con- ductance calculated from these sweeps were compared to the conductance values obtained

Step Conductance/ Deposited charge/ Pt thickness/ µS C nm 1 112.25 6.131 x10−5 _{28 nm} 2 93.11 5.720 x10−5 27 nm 3 39.45 3.834 x10−5 _{18 nm} 4 13.80 2.901 x10−5 13 nm

Table 4.2: Summary of results obtained for the multi-step deposition studies (Section 4.5.4) carried out at an array of 16 nanopores with initial diameters of 100 nm.

from current-time transient curves during measurements – as done for the single-step case in Section 4.5.4. A typical I-V measurement for all four steps in this experiment is shown in Figure 4.19 and the conductance values of each step calculated from the ICF and I-V measurements are plotted in Figure 4.19 (right panel).

It can be seen from comparisons of the I-V and ICF measurements, that there is a variance in pore conductances obtained during measurements. Although the ICF curves are quantitatively larger, qualitatively the results of both measurements show a decrease in pore conductance with each subsequent deposition step. The quantitative discrepancy between the two series’ may be a result of the different solutions used during the measure- ments, however further work is necessary to verify this.