High-Resolution Electrochemical and Topographical Imaging Using

Apr 28, 2014 - High-Resolution Electrochemical and Topographical Imaging Using Batch-Fabricated Cantilever Probes ... *E-mail: [email protected]. ...
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High-Resolution Electrochemical and Topographical Imaging Using Batch-Fabricated Cantilever Probes Andrew J. Wain,*,† Andrew J. Pollard,† and Christoph Richter‡ †

National Physical Laboratory, Hampton Road, Teddington, TW11 0LW United Kingdom NanoWorld Services GmbH, Schottkystraße 10, Erlangen, Bavaria 91058, Germany

‡

S Supporting Information *

ABSTRACT: New cantilever probes for combined scanning electrochemical microscopy−atomic force microscopy (SECMAFM) have been batch-fabricated, and their application to high resolution electrochemical-topographical imaging has been demonstrated. The conical probes yield outstanding quality Faradaic current maps alongside subnm level topographical information as exemplified by the electrochemical imaging of exfoliated graphene and graphite samples. Current mapping reveals significant heterogeneities in the electroactivity of these carbon surfaces that do not directly correlate to topographical features, suggesting the presence of adsorbed chemical contaminants or intrinsic impurities.

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been developed,8,12,18,19,22,23 scalability has often come at the cost of electrode size or imaging performance. High-resolution electrochemical and topographical imaging was achieved by Frederix et al., who used conical SECM-AFM probes to study various substrates.19 However, these authors undertook Faradaic current imaging in contact mode, rather than Lift Mode, and therefore did not control the electrode-surface separation during the electrochemical measurement, making SECM-AFM image interpretation more challenging. More recently, Wandlowski et al. reported the modification of commercial AFM probes using focused ion beam (FIB) and reactive ion etching (RIE) to yield conical probes for current sensing applications.13 Although a thorough characterization was presented, the application of these probes to Faradaic current SECM-AFM imaging was not fully realized. In this work, we exploit developments in the wafer scale fabrication of conical probes for SECM-AFM and report their application to high resolution electrochemical and topographical imaging of heterogeneous graphene and graphite surfaces.

t is well established that the local heterogeneity of electrode surfaces can have a profound effect on their macroscopic interfacial behavior.1 This inherent trait impacts a wealth of electrochemical applications, such as sensing and energy conversion, in which optimizing materials performance is central to commercial viability. While great strides have been made in measuring heterogeneous electron transfer (HET) kinetics of nonuniform materials through bulk voltammetry and modeling,2 localized approaches are a valuable addition to the electrochemist’s toolkit. Scanning electrochemical microscopy (SECM) is one such technique in which a micro- or nanoelectrode probe is used to examine Faradaic processes in the electrolyte solution immediately adjacent to a surface, providing a unique route to characterizing the electroactivity of solid−liquid interfaces with spatial resolution.3 Of the various advances in high resolution electrochemical imaging that have emerged over the past decade,4,5 the combination of SECM with atomic force microscopy (SECM-AFM) offers potentially the most promising route to correlating electrochemical heterogeneities with nanoscopic topography. This requires integration of a nanoelectrode at, or close to, the apex of an AFM probe such that local electrochemical measurements can be undertaken at short and controllable working distances from the surface of interest. Since the seminal work of MacPherson6−9 and Kranz,10−12 a major bottleneck to the widespread uptake of this technique has been the reliable fabrication of suitable dual function probes. Advances in semiconductor microprocessing and nanofabrication technologies have prompted a more recent surge in interest,13−21 but in few of the latest examples have the topographical imaging strengths of combined SECM-AFM been truly harnessed. While batch-fabrication approaches have © 2014 American Chemical Society

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EXPERIMENTAL SECTION Reagents and Solutions. Electrolyte solutions were prepared by dissolving ferrocenemethanol (Aldrich) and potassium nitrate (Fluka) in deionized water (resistivity of 18.2 MΩ cm) taken from a Millipore Advantage purification system. Exfoliated graphene/graphite samples were fabricated by the conventional “Scotch tape” method,24 in which highly Received: March 14, 2014 Accepted: April 28, 2014 Published: April 28, 2014 5143

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Figure 1. Conical probe fabrication process: (a) silicon wafer, (b) first stage of probe body etching, (c) silicon surface oxidation (oxide layer: yellow), (d) separation of cantilever from wafer, (e) deposition of Pt (red) and Si3N4 (green) layers, (f) deposition of masking sacrificial Cr layer (blue), (g) ion beam etching of mask, (h) wet chemical etching of exposed Si3N4 regions, and (i) removal of chromium mask.

Figure 2. SECM-AFM probe characterization. (a) SEM image of tip with conical electrode apex. (b) Cyclic voltammogram (20 mV s−1) of a typical probe immersed in 1 mM FcMeOH/0.1 M KNO3. (c) Approach curves toward insulating (SiO2/Si) and conducting (Au/Si) substrates (L = ratio of distance/cone radius).

First, a silicon wafer (Figure 1a) was etched to yield rectangular probe bodies with diving board cantilevers of dimensions 50 μm × 450 μm (Figure 1b). The surface of the silicon probes was insulated by thermal oxidation in an oxygen atmosphere (Figure 1c) and, after separation of the cantilever from the connecting wafer (Figure 1d), the tip side of the probe was subsequently coated with an evaporated layer of 70 nm Pt followed by a 130 nm thick layer of Si3N4, by plasma-enhanced chemical vapor deposition (Figure 1e). Exposure of a platinum cone electrode at the tip apex was achieved using a patented procedure,25 in a similar fashion to the process described previously.13 A sacrificial masking layer (120 nm) of chromium was applied by sputter coating (Figure 1f), and then, a Ga+ ion

ordered pyrolytic graphite (HOPG, ZYA grade, NT-MDT, Moscow) was repeatedly contacted with the adhesive side of the tape and the carbon peelings subsequently transferred onto 1 × 1 cm2 pieces of oxidized silicon wafer. Probe Fabrication. The cantilever probes employed were fabricated via a unique process from wafer level, depicted schematically in Figure 1. The probes consist of a pyramidal tip situated at the end of a diving board cantilever, the apex of which comprises an addressable platinum nanoelectrode, elsewhere surrounded by a layer of silicon nitride. Similar to the work of Wandlowski,13 the electrode apexes were exposed by a combination of FIB and selective chemical etching, using a sacrificial chromium layer to control the exposure. 5144

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Figure 3. Feedback mode SECM-AFM images of gold patterned silicon oxide substrate immersed in 1 mM FcMeOH/0.1 M KNO3. (a) Topographical scan acquired in contact mode and (b) electrochemical scan recorded at a lift height of 150 nm. Tip bias 0.3 V vs Ag, substrate bias −0.2 V vs Ag, line scan frequency 0.5 Hz (total scan time ∼20 min), bulk tip current ∼100 pA.

2 μm. By monitoring the deflection of the tip, the z-position of the tip was then adjusted so that tip−surface contact was made only at the far limit of the displacement. The tip current was measured continuously throughout this displacement to generate multiple SECM approach curves. Electrochemical imaging was typically undertaken in Lift Mode, employing a lift height of 150 nm and a line scan rate of 0.5 Hz.

beam (Helios NanoLab Dual Beam FIB) was used to locally remove the masking material from the tip apex (Figure 1g). Wet chemical etching using a buffered oxide etch was then employed to remove the Si3N4 from the exposed regions (Figure 1h). Finally, the remaining chromium masking layer was removed by wet etching using using (NH4)2Ce(NO3)6 (Figure 1i). A fabrication success rate of approximately 80% was typically achieved, and the majority of probes were found to function electrochemically. However, the imaging quality and resolution varied considerably depending on the size of the conical electrode, and naturally, the smallest of probes exhibited the poorest current mapping success rates. As with all AFM probes, imaging quality deteriorated with extended use, but high quality current maps could typically be acquired continuously over several hours before this became problematic. Probe Testing. Probe characterization and SECM-AFM measurements were undertaken by loading the SECM-AFM probe into a modified Bruker fluid AFM cell containing integrated reference (Ag wire) and counter (Pt wire) electrodes. Probes were fabricated with an exposed platinum contact pad on the tip side of the probe body so that the electrode could be electrically addressed. Electrical connection between the contact pad and a stripped enamel copper wire (36 AWG, Belden, USA) was achieved using silver loaded epoxy (RS Components, UK) which was left for several hours to cure. Subsequent insulation of the probe body and the electrical contact was ensured using two separate coats of insulating varnish (RS). The AFM cell was fitted into a Bruker Multimode AFM system, with the head and scanner section housed in a custom built Faraday cage. Electrochemical measurements were undertaken using a CHI 760C potentiostat combined with a picoamp booster connected to the side of the Faraday cage. Data signals from the potentiostat were fed into the Multimode software using a Bruker Signal Access Module. Probe approach curves were measured by engaging the tip to the surface in contact mode and measuring a continuous force-displacement curve at a frequency of 0.1 Hz over a distance of approximately

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RESULTS AND DISCUSSION

Probe Characterization. Batch fabricated probes were first assessed using scanning electron microscopy (SEM). Precise electrode dimensions varied between probes but the conical base radius was typically of the order of 200 nm (Figure 2a). Electrochemical characterization was undertaken using the single electron oxidation of aqueous ferrocenemethanol (FcMeOH/FcMeOH+) as an outer sphere redox probe. A typical cyclic voltammogram (CV) of a mounted probe immersed in 1 mM FcMeOH/0.1 M KNO3 is depicted in Figure 2b. In this case, a diffusion limited plateau of approximately 100 pA is observed. There is no simple analytical expression for the steady state current at such a conical electrode but, by employing numerical simulation, Denuault et al. derived an empirical polynomial expression relating the limiting current to the aspect ratio of the cone (i.e., height/base radius) and the Rg parameter (i.e., the ratio of the total insulation sheath radius to the electrode base radius).26 For an aspect ratio of 2, an Rg of 1.1, and a diffusion coefficient of 7.8 × 10−6 cm2 s−1,27 a 100 pA diffusion limited current yields a theoretical electrode base radius of approximately 150 nm, which is in good agreement with the electron microscopy. We note the capacitive charging current present in the voltammogram in Figure 2b, which is attributed to a small degree of ionic porosity in the insulating varnish used to seal the probe in place. While not ideal, this charging did not prevent the successful application of the probes. Further probe characterization was achieved by undertaking approach curves at insulating and conducting surfaces in 5145

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Figure 4. SECM-AFM line scans, showing the Faradaic current at various lift heights and the corresponding topography profile of gold patterned silicon oxide substrate immersed in 1 mM FcMeOH/0.1 M KNO3 (line scan frequency 0.5 Hz). (a) Feedback mode (tip bias 0.3 V, substrate bias 0.2 V); (b) substrate generation tip collection mode (tip bias −0.2 V vs Ag, substrate bias 0.3 V vs Ag).

resolved both topographically and electrochemically. Further experiments with sharpened nonelectrochemical AFM probes (not shown) confirmed that the topographical ridges observed along the gold bands are real surface features, as opposed to tipshape artifacts. Examination of the imaging resolution was achieved by undertaking line scans over a single gold band at various lift heights (Figure 4). In SECM feedback mode (Figure 4a), the lateral resolution of the electrochemical response is notably high at a lift height of 100 nm, with diffusional broadening being limited to within approximately 200 nm of the edge of the gold band. Naturally, the magnitude of the feedback current weakens rapidly and the diffusion field broadening becomes more significant at higher lift heights; however, current contrast is still discernible at a distance of 500 nm. The line scan response was also measured in generator-collector mode, in which the substrate was instead biased to oxidize FcMeOH, while the tip was poised to collect the electrogenerated FcMeOH+ cations (Figure 4b). This mode is known to be less sensitive to tip−substrate separation than feedback mode, as indicated by the less rapid tail-off in current contrast with increasing lift height. In both cases, submicrometer variations in activity across the gold band are evident, which suggest local nonuniformities in the gold surface that are not obvious from the topographical scan. Imaging of Graphene and Graphite Substrates. Having verified the basic imaging performance of the SECM-AFM probes, we now turn our attention to their application to the high resolution imaging of exfoliated graphene and graphite surfaces. The emergence of graphene as a functional nanomaterial has been met with great excitement by the scientific community, and it has received significant attention as a potential electrode material.29,30 Electrochemical imaging has previously been undertaken at graphene surfaces,31,32 but correlating localized interfacial behavior with nanoscale top-

FcMeOH solution, with the tip biased at a sufficient potential to oxidize the FcMeOH at the limit of diffusion control (Figure 2c). The two measured curves, in which the tip current is normalized to that measured in bulk solution and the tip− surface separation is normalized to the cone radius, are qualitatively as expected. Negative feedback is observed upon approach to the insulating surface, wherein the tip current diminishes due to diffusional hindrance. Conversely, the tip current is augmented upon approach to the conducting surface (positive feedback) as the tip-generated FcMeOH+ cation is reduced by the substrate, locally replenishing the depleted FcMeOH concentration. The approach curves also compare reasonably well with theory derived recently for such electrode geometries (see Supporting Information, SI),28 although we found that the inherent experimental variability of these probes limited the rigorous interpretation of the approach curves generated. Nevertheless, the new SECM-AFM probes exhibit the necessary sensitivity toward surface activity required for relative electrochemical analysis. SECM-AFM Measurements. The true advantage of SECM lies in its efficacy for spatially localized measurement. Assessment of the SECM-AFM imaging quality of the probes was achieved using a patterned silicon oxide substrate, consisting of 50 nm high, 2 μm wide gold bands, immersed in a supported aqueous solution of 1 mM FcMeOH. A dual pass imaging approach was adopted (Lift Mode) in which surface topography is recorded in contact mode on the first scan, and the electrochemical response is recorded at a specified lift height during a second scan. Typical images are shown in Figure 3 which depict the topography (Figure 3a) and tip current measured at a lift height of 150 nm (Figure 3b). The tip was biased to oxidize FcMeOH, while the substrate was poised at a sufficiently cathodic potential so as to ensure positive feedback at the electroactive gold bands. The images generated are of a high quality, with micro- and nanoscale features well 5146

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Figure 5. Feedback mode SECM-AFM images of exfoliated graphene/graphite flakes immersed in 1 mM FcMeOH/0.1 M KNO3 solution: Topography is shown in (a), (c), (e), and (g) and the corresponding electrochemical scans are depicted in (b), (d), (f), and (h), respectively. Line scan profiles for parts (g) and (h) are shown in (i) (shaded areas highlight regions of different graphene thickness: single-layer (SL), multilayer (ML), and few-layer (FL)). Tip bias 0.3 V vs Ag, line scan frequency 0.5 Hz, lift height 150 nm, bulk tip current typically ∼200 pA.

In spite of the above thermodynamic driving force and the relatively low feedback efficiencies compared to conventional disk electrodes, we observe considerable nonuniformities in the feedback current across the samples, with retarded HET kinetics exhibited at a highly localized level. In particular, numerous circular regions of low electroactivity are present on graphitic areas that do not clearly correlate to features resolved in the topographical image. An example of this is depicted in Figure 5a (topography) and Figure 5b (current), in which a low spot in electroactivity is present close to the center of this ∼20 nm high graphite flake, with no obvious sign of a resolved topographical feature at the same location. Closer inspection reveals that the topography varies by less than 1 nm across this region (see SI). Similar behavior was observed, albeit to a lesser extent, on highly ordered pyrolytic graphite (HOPG) by Frederix et al., who employed SECM-AFM in a continuous contact mode.19 The exact nature of these low activity spots is unclear but they suggest the presence of blocking surface adsorbates or intrinsic chemical impurities in the carbon that are beyond the topographical resolution of the AFM. The HET kinetics of carbon surfaces is known to be highly sensitive to contamination and aging effects,33 and hence, local variations in electroactivity might be anticipated for such mechanically exfoliated samples. Nevertheless, it is noteworthy that such impurities or contaminants could have such a dramatically inhibitive effect on outer-sphere HET at such a highly localized level. Further electrochemical heterogeneities are shown in Figure 5c,d, which depicts an isolated graphite flake that exhibits a number of interesting localized features. Once again, we observe a circular region of low activity (bottom right quadrant), which in this case exhibits close to pure negative feedback. As in Figure 5b, the majority of current variations across Figure 5d do not appear to be linked to the topography

ography remains a challenge. For example, submicrometer resolution electrochemical imaging of graphene was achieved using scanning electrochemical cell microscopy (SECCM),32 but this technique does not have the topographical resolution that AFM affords. Hence, this two-dimensional material, and indeed its parent material, graphite, represent ideal test candidates for demonstrating the sensitivity of electrochemical-topographical imaging using the batch fabricated SECMAFM probes. Substrates of graphene/graphite immobilized on a 300 nm SiO2/Si wafer were fabricated using the common mechanical exfoliation (“Scotch tape”) approach,24 which yielded surfaces consisting of a distribution of isolated graphite flakes surrounded by associated regions of single- and multilayer graphene. Confocal-Raman spectroscopy performed at various regions of the substrate was consistent with a largely defect-free mixed graphene/graphite sample, as expected from mechanical exfoliation (see SI). A selection of SECM-AFM images recorded at such a sample is presented in Figure 5, from which a number of observations are apparent. First, we predominantly record a positive feedback current over the carbon regions; normalized tip currents (surface current/bulk current) up to 1.1 were measured over the graphene/graphite, indicating facile HET between the tipgenerated FcMeOH+ cations and the majority of the carbon surface. It is important to note that no bias was applied to the substrate and so, assuming flakes are sufficiently large and conducting, the carbon electrode potential is held fixed by the surrounding FcMeOH solution. Given the rapid nature of this classical outer-sphere redox process, and in light of previous observations made by Unwin et al.,32 we cannot exclude the possibility that the highest currents are driven close to diffusion limit. 5147

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measurements will have critical impact on the potential application of graphene as an electrode material.

although there is subtle evidence of higher electroactivity along step edges. An example is highlighted by the red box, wherein a straight line of marginally enhanced current crossing from the bottom left to the top right of the box is discernible, which correlates to a ∼1.5 nm high step edge in Figure 5c. Increased feedback currents at step edges is to be expected on the basis of enhanced intrinsic heterogeneous kinetics,34 although in this case we cannot exclude the possibility that the step edges are simply less prone to surface contamination. We note that many of the other step edges clearly visible elsewhere in the topographical scan do not appear to exhibit enhanced activity, an observation that has been reported previously for HOPG on the basis of SECM-AFM data.14,19 One previously proposed argument is the presence of thin overlayers of carbon blocking efficient HET without fully masking the topography.19 Moving onto the graphene regions, Figure 5e,f depicts an example of few-layer graphene (∼1 nm in height) attached to the edge of the larger (∼50 nm high) graphite flake. Importantly, these two regions appear to be indistinguishable in the electrochemical image indicating that the tip-generated FcMeOH+ impacting the surface is reduced at a similar rate at the few-layer graphene as the graphite. This behavior is demonstrated further in Figure 5g,h. The central region of this graphene flake appears to consist of up to approximately 10 layers and again exhibits various step edges in topography, as highlighted by the topographical cross section shown in Figure 5i. Close inspection indicates a region of single-layer graphene, approximately 400 pm in height, located between the 6 and 7 μm x-coordinate positions. Despite the different graphene thicknesses, we observe a positive feedback response across the entire flake, which fully extends over the single-layer graphene region. Given that the potential of the graphene surface was not controlled in our experiments, it is possible that the HET rate for this redox process is high enough over some regions that the current is close to transport limited. Experiments under sample bias would alleviate this issue, but the use of an addressable substrate was avoided in this preliminary study in order to (i) circumvent large decay currents between contact and lift scan and (ii) demonstrate the current contrast between active and inactive surfaces. It is therefore difficult to make assertions about the intrinsic HET kinetics without further investigation. Nevertheless, in light of the low activity spots observed elsewhere, it can be concluded that the graphene regions do not exhibit considerably lower activity than graphite. Furthermore, the images presented demonstrate the topographical sensitivity of these probes and highlight their efficacy for high resolution electrochemical mapping in the 10−100 pA range.

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ASSOCIATED CONTENT

S Supporting Information *

Theoretical approach curves; Raman spectroscopy; and graphite contaminant imaging. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +44 (0)20 8943 6243. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the UK National Measurement System and the European Metrology Research Programme (EMRP, Ind 15 - SurfChem). The EMRP is jointly funded by the EMRP participating countries within EURAMET and the European Union. The authors wish to thank NPL colleagues H. Stec, for fabricating the exfoliated graphene samples, and M. O’Connell and A. Turnbull for their technical support in this project.

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REFERENCES

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CONCLUSIONS We have demonstrated the application of new batch-fabricated SECM-AFM cantilever probes to high-resolution combined electrochemical-topographical imaging. The probes yield excellent-quality Faradaic current maps, collected in Lift Mode, alongside topographical images with subnm height resolution, and these valuable attributes have been exploited in the imaging of exfoliated graphene and graphite surfaces. This application has gleaned new insights into the interfacial behavior of such exfoliated samples, most notably in their high sensitivity to localized surface impurities with subnm topography, which is unexpected for an outer-sphere redox mediator such as FcMeOH. Further work employing slower redox couples under sample bias is underway to investigate the effect of graphene thickness on intrinsic HET kinetics. Such 5148

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