An Introduction to PeakForce SECM: AFM-Based Scanning Electrochemical Microscopy

Table of Contents

Introduction
A Pioneering Nanoelectrode Probe
PeakForce SECM Hardware and Operation
COMSOL Simulation
Electrochemical Performance
Cyclic Voltammetry (CV) and Amperometry
     Approach Curves
PeakForce SECM Imaging
Conductivity Measurements in Liquid
PeakForce SECM Application Examples
     Au-SiO2 Nanomesh Electrode
     Charge Transfer Dynamics
     Self-Assembled Monolayers (SAM)
     Highly Oriented Pyrolytic Graphite (HOPG)
     Nanoparticle (NP) Catalysis
Utilizing PeakForce TUNA for Semiconductor/Metal Junctions in Liquid
Conclusion

Introduction

Electrochemistry refers to the interplay between chemical and electrical energy, where electrons drive chemical changes or chemical reactions to move electric charges.1 There are many electrochemistry applications that affect our day-to-day lives, such as chemical manufacturing, biological systems¸ frontier research and development activities in energy research, surface protection, and materials development2-5, but macroscopic electrochemical behavior is an average of the heterogeneous reactivity over the surface of an electrode. This may comprise of surface defects, crystal-facet-dependent properties, or different active sites.6 In addition, reactivity variation is the result of the heterogeneity in mechanical, structural, electrical, and/or electrochemical properties over the surface of the electrode7,8. Hence, for current highly multidisciplinary research, in situ, localized techniques that simultaneously capture nanoscale multidimensional information and electrochemistry are highly preferred9,10.

The most popular and established method for local electrochemical studies at micro- and nanoscales is scanning electrochemical microscopy (SECM)11,13. In traditional SECM, an ultramicroelectrode (UME, 5-25 µm) is placed in very close to the sample and then scanned across the surface. The properties and nature of the sample area below the probe affect the electrochemical processes at the tip of the electrode. During scanning, the variation of electrochemical properties is imaged by capturing the current and/or possible response of the tip. However, achieving sub-µm resolution with traditional SECM is very difficult. The well-known constant height and constant current modes also suffer from convolution problems between electrochemistry and topography because the tip current is dependent on the surface properties as well as the distance between the tip and sample. Classic SECM can achieve no more than topographical and electrochemical information, which is not adequate for a complex system for example batteries13.

Atomic force microscopy (AFM) has a high-spatial-resolution imaging capability. Since the development of SECM in 1980s14, constant efforts have been made to implement an UME as an AFM probe for integrated AFM-SECM imaging to separate electrochemical and topographic information15. In the interim, parallel efforts were also made to change an AFM probe into a nanoelectrode to obtain sub-µm or even sub-100 nm resolution16,21 but, even after 20 years of development19,22,24, it continues to be a challenge to obtain the batch production of reliable, cost-effective, and stable nanoelectrode probes with a characteristic dimension of sub-100 nm.

For imaging, AFM-SECM typically depends on traditional tapping mode or contact mode. However, in tapping mode, the shear force is minimized and the imaging force is reduced, but since the method deepens on a mechanical resonance, the mode is sensitive to the probe’s working environment. Similarly, contact mode is not suitable for soft and fragile samples because of the high imaging and shear forces.

Bruker developed PeakForce Tapping® mode in 200925 which maintains the advantages of both traditional tapping and contact modes and at the same time avoids the disadvantages from both. In the PeakForce Tapping® mode, the probe is modulated sinusoidally off-resonance, at a low frequency of typically 1-2 kHz with a small amplitude (typically 5-100 nm). In addition, there is no need for cantilever tuning which makes it easy for liquid imaging.

Since the feedback signal is the peak force, or maximum force, between the sample and the tip, PeakForce Tapping conducts a triggered force curve at every tapping cycle. Real-time analysis of these force curves enables quantitative and simultaneous imaging of mechanical properties at a normal AFM scan rate (Bruker’s PeakForce QNM® mode)26. The tip velocity, utilizing sinusoidal modulation, is virtually zero as it comes close to the surface. This enables precise, stable force control, ultralow imaging force (<50 pN) as well as automatic image optimization. PeakForce QNM imaging has been effectively inculpated with a array of sophisticated AFM modes to further improve their capabilities, including surface spreading resistance microscopy (PeakForce SSRM™), electrical mapping (PeakForce TUNA™)27, Kelvin probe force microscopy (PeakForce KPFM™)28, and scanning microwave impedance microscopy (PeakForce sMIM™)29.

High-quality, batch-fabricated, robust PeakForce SECM probes with a characteristic tip dimension of about 50 nm have also been developed by Bruker. With the help of these probes, PeakForce SECM scanning is achieved, which simultaneously offers nanoscale electrical, topographical, and quantitative mechanical maps, together with electrochemical images of sub-100 nm resolution. When used with high-bandwidth electronics, PeakForce SECM scanning also offers exclusive capabilities for high-quality nanoelectrical imaging in liquid.

A Pioneering Nanoelectrode Probe

A considerable amount of time and effort was spent by Bruker to fabricate a nanoelectrode probe that could facilitate consistent SECM capability. To ensure high consistency during probe fabrication, the SECM nanoelectrode probe was developed using a wafer-based MEMS approach. Figure 1a shows the pre-mounted PeakForce SECM probe30. Components of this probe assembly were strictly tested for chemical compatibility, exhibiting ≤1% change in mass over 5 days of immersion in solvent/solutions (Table 1). These solutions/solvent include aqueous solutions of pH 1 and pH 13; liquid generally used in Li-ion battery systems; and solvents that are often used in non-aqueous electrochemistry. Compared to a regular AFM probe, the nanoelectrode probe mounted has a much larger size (11.7 x 6.1 x 3.7 mm versus 3.4 x 1.6 x 0.3 mm), and handling grooves are provided in the glass packaging. These two features offer ease of use and increased safety. Figure 1b shows the rectangular cantilever. A Pt conductive path pf 11 µm wide runs between the base and the tip and offers electrical conductivity, as shown in Figure 1c. This method has been previously used to reduce the potential formation of pin holes in the passivation layer on the surface of Pte24,31+ Shown in Figure 1d is the apex of the tip, which has a Pt coated area of ~200 nm tip height and ~50 nm in diameter. Except for this small electrochemically active region, the probe is completely insulated by a SiO2 layer to control leakage, which is important for all nanoelectrical and nanoelectrochemical measurements in liquid.

(a) Pre-mounted PeakForce SECM probe; (b) SEM side view image of the cantilever; (c) SEM top view image of the cantilever showing the 15-µm-wide Pt conductive path; (d) SEM image revealing exposed Pt-coated tip apex with ~50 nm endtip diameter and ~200 nm tip height (adapted from Nellist et al., Nanotechnology, 2017, 28(9), 095711, IOP Publishing30).

Figure 1. (a) Pre-mounted PeakForce SECM probe; (b) SEM side view image of the cantilever; (c) SEM top view image of the cantilever showing the 15-µm-wide Pt conductive path; (d) SEM image revealing exposed Pt-coated tip apex with ~50 nm endtip diameter and ~200 nm tip height (adapted from Nellist et al., Nanotechnology, 2017, 28(9), 095711, IOP Publishing30).

Table 1. Solvents and solutions for chemical compatibility test

Aqueous solution (0.1 M) NaOH, KOH, HCI, H2SO4,HNO3, and H2O2
Organic solvents Acetonitrile, 1-methyl-2-pyrrolidinone, ethyl acetate, toluene, methanol, ethanol, and acetone
Solvents/solutions for Li-ion batteries Diethyl carbonate, dimethyl carbonate, ethyl methyl carbonate; 1 M LiPF6 in ethylene carbonate : dimethyl carbonate (1:1 in volume)

PeakForce SECM Hardware and Operation

PeakForce SECM hardware comes with a protective Kalrez® boot, PeakForce SECM probe holder, and a strain-released module with resistance selector (0, 1, and 10 MΩ) to restrict the maximum flow of current (Figure 2)32. The module prevents direct electrical connection to the probe, which often produces mechanical noises. The 10 MΩ current-limiting resistor is typically chosen, but this 10 MΩ resistor limits the current to <100 nA, while the SECM probe can handle currents above 1 µA in PeakForce Tapping® mode, and 600 nA when in continuous contact with a conductive substrate30. The PeakForce SECM hardware operates in tandem with the standard Bruker EC-AFM kit with temperature control from ambient to 65 °C.

PeakForce SECM accessories with a pre-mounted nanoelectrode probe loaded. (adapted from Huang, Z. et al.32).

Figure 2. PeakForce SECM accessories with a pre-mounted nanoelectrode probe loaded. (adapted from Huang, Z. et al.32).

PeakForce SECM combines PeakForce QNM® mode with AFM-SECM functions (Figure 3). The sample and the probe serve as working electrodes and share the same counter and reference electrodes. During SECM measurement, both the sample and the probe are biased at different potentials to allow a wide range of chemical reactions; for instance, the probe reduces the [Ru(NH3)6]3+ to [Ru(NH3)6]2+ at -350 to -500 mV against a Ag/AgCl or a Ag/AgCl-quasi reference electrode (AgQRE), and the sample is biased at 0 to -100 mV for [Ru(NH3)6]3+ regeneration. PeakForce SECM can leverage the benefits of LiftMode™ for imaging (Figure 3b). PeakForce QNM imaging is carried out during the main scan. The contact current is also imaged, apart from mechanical and topography properties. The probe follows the stored topographic profile during the lift scan, and at the same time keeps a constant tip-sample distance defined by the lift height. Electrochemical information is captured as the probe is lifted. In this manner, PeakForce SECM enables simultaneous mapping of multi-dimensional properties, such as electrochemistry, topography, conductivity and mechanics. This capability is versatile and has never been realized before.

a) Schematic illustration of a PeakForce SECM system showing the major mechanical and electronic components; and (b) an illustration of interleaved scanning mode.

Figure 3. (a) Schematic illustration of a PeakForce SECM system showing the major mechanical and electronic components; and (b) an illustration of interleaved scanning mode.

COMSOL Simulation

Shown in Figure 4 is the COMSOL simulation of the concentration profile when the probe is subjected to a [Ru(NH3)6]2+/3+ solution, and executes a reduction reaction33. This type of nanoelectrode has a radial, three-dimensional, convergent diffusive transport, which results in a steady-state diffusion layer as shown in Figure 4a. This is a highly compact layer. The normalized concentration profile from the surface of the electrode following the center axis (Figure 4b) displays 60% recovery of the [Ru(NH3)6]3+ concentration at 50 nm away from the electrode surface. Since the perturbation to the diffusion layer helps sense electrochemical imaging, such a compact layer is needed for sub-100 nm spatial resolution. Also, the steady-state diffusive transport leads to a steady-state diffusion-limited current.

Simulation carried out for a probe in 0.1 M KCl solution and 10 mM [Ru(NH3)6]3+ produce a diffusion-limited current of 720 pA, which is consistent with experimental results (0.75 ± 0.45 nA)30.

The time for establishing such a steady state reduces with the probe dimension and are only <10 µs for a nanoelectrode34. The three premises for high-resolution electrochemical imaging in PeakForce SECM are rapid response to external perturbations, the steady-state probing current, and small characteristic dimension with a tight diffusion layer.

(a) COMSOL simulation of the [Ru(NH3)6]3+ concentration profile; (b) a normalized concentration profile from the electrode surface following the center axis. Cd is the [Ru(NH3)6]3+ concentration at a distance, d, from the electrode surface, and C0 is the bulk [Ru(NH3)6]3+ concentration. Simulation conditions are 10 mM [Ru(NH3)6]3+ and 10 mM [Ru(NH3)6]2+ in the bulk, and the probe is 1 mm away from an insulating substrate.

Figure 4. (a) COMSOL simulation of the [Ru(NH3)6]3+ concentration profile; (b) a normalized concentration profile from the electrode surface following the center axis. Cd is the [Ru(NH3)6]3+ concentration at a distance, d, from the electrode surface, and C0 is the bulk [Ru(NH3)6]3+ concentration. Simulation conditions are 10 mM [Ru(NH3)6]3+ and 10 mM [Ru(NH3)6]2+ in the bulk, and the probe is 1 mm away from an insulating substrate.

Electrochemical Performance

Cyclic Voltammetry (CV) and Amperometry

Figure 5a superimposes the 1st, 25th and 50th cycles of a nanoelectrode probe from 50 continuous cyclic scans. There is no performance degradation. The sigmoidal shape of the CV curves is predicted for a nanoelectrode as a result of the radial diffusion process. The capacitive charging current is ~5 pA at 20 mV/s scanning rate. Then, the probe was cleaned for a successive amperometric test (Figure 5b), which also shows stable performance of the nanoelectrode probe for over 2 hours. The inset in Figure 5b also demonstrates the current drift of <2 pA and more than 30 minutes sub-pA noise level in this measurement30.

(a) The 1st, 25th and 50th CVs selected from 50 continuous scans at a scan rate of 20 mV/s; and (b) 2-hour amperometric test at -0.1 V vs Ag/AgCl. The inset is magnification from 70 to 120 minutes. Solution: 5 mM [Fe(CN)6]4-, 5 mM[Fe(CN)6]3- and 0.1 M KNO3.

Figure 5. (a) The 1st, 25th and 50th CVs selected from 50 continuous scans at a scan rate of 20 mV/s; and (b) 2-hour amperometric test at -0.1 V vs Ag/AgCl. The inset is magnification from 70 to 120 minutes. Solution: 5 mM [Fe(CN)6]4-, 5 mM[Fe(CN)6]3- and 0.1 M KNO3.

Approach Curves

Figure 6 shows a typical approach curves captured from an insulating (red dotted) and a conductive (blue dashed) surface, respectively. The approach curves are normalized to the tip current at 1-µm tip-sample distance.

On an insulator, the diffusion of [Ru(NH3)6]3+ to the electrode surface is progressively blocked as the tip-sample distance reduces. Current reduction of ~25% on an insulating surface is consistent with the COMSOL simulation (open circles), while a conducting surface is biased for oxidation of [Ru(NH3)6]2+ to regenerate [Ru(NH3)6]3+, thereby increasing the local concentration of [Ru(NH3)6]3+ at the tip. This positive feedback contends with the negative blocking effect, resulting in current enhancement (black dashed).

In this case, a current enhancement of >25% is achieved, which is consistent with the simulation (open squares). The current enhancement/reduction mainly occurs at <100 nm tip-sample distance, as shown in Figure 6. This is consistent with the diffusion layer thickness shown in Figure 4a, and such a high-spatial sensitivity is preferred for high-resolution electrochemical imaging.

Approach curves captured on insulating (red dotted) and conducting (black dashed) surfaces. The tip and the substrate were biased at 0 and -0.5 V vs. AgQRE, respectively. The solution was 10 mM [Ru(NH3)6]3+ with 0.1 M KCl supporting electrolyte. Symbol plots are COMSOL-simulated results. These results are normalized at the tip-sample distance of 1 µm.

Figure 6. Approach curves captured on insulating (red dotted) and conducting (black dashed) surfaces. The tip and the substrate were biased at 0 and -0.5 V vs. AgQRE, respectively. The solution was 10 mM [Ru(NH3)6]3+ with 0.1 M KCl supporting electrolyte. Symbol plots are COMSOL-simulated results. These results are normalized at the tip-sample distance of 1 µm.

PeakForce SECM Imaging

PeakForce SECM was used to test a sample with a 50-nm-thick patterned silicon nitride layer deposited onto an Au substrate. The sample potential was -0.1 V for the regeneration of [Ru(NH3)6]3+, while the tip voltage was -0.4 V against Ag/AgCl for reducing [Ru(NH3)6]3+. During the PeakForce Tapping cycle, when the tip comes into contact with the Au surface, the 300 mV difference between the sample and the tip results in a current upon contact of sample and tip (Figure 7).

During the lift scan, the electrochemical current distinctly differentiates between the nitride and Au regions (Figure 7b). The contact current is considerably larger than the noncontact electrochemical current, as expected32.

PeakForce SECM measurement of an electrode with a 50-nm-thick patterned silicon nitride layer deposited on an Au substrate: (a) Map of current response from the PeakForce Tapping scan; (b) electrochemical current map at a lift height of 100 nm (dashed line indicates location for cross-sectional analysis); (c) approach curves on Au and nitride regions plotted with respect to the probe movement, respectively; (d) line profiles of tip current during the lift scan (solid-red: left y-axis) and surface topography (dashed-green: right y-axis). Solution, 10 mM [Ru(NH3)6]3+ and 0.1 M KCl (figures a and b were adopted from Huang, Z. et al.32).

Figure 7. PeakForce SECM measurement of an electrode with a 50-nm-thick patterned silicon nitride layer deposited on an Au substrate: (a) Map of current response from the PeakForce Tapping scan; (b) electrochemical current map at a lift height of 100 nm (dashed line indicates location for cross-sectional analysis); (c) approach curves on Au and nitride regions plotted with respect to the probe movement, respectively; (d) line profiles of tip current during the lift scan (solid-red: left y-axis) and surface topography (dashed-green: right y-axis). Solution, 10 mM [Ru(NH3)6]3+ and 0.1 M KCl (figures a and b were adopted from Huang, Z. et al.32).

Approach curves were captured on both the nitride and Au regions, and then compared in Figure 7c. Both these curves are plotted with respect to the distance between the tip and the sample. As the probe approaches the nitride surface, the diffusion of [Ru(NH3)6]3+ toward the probe is progressively blocked, leading to reduction in current. However, in the case of the Au surface, the regeneration of [Ru(NH3)6]3+ on the surface increases the current and outperform the blocking effect. When the tip is about 100 nm away from the sample surface, the current variation between the Au and nitride is ~100 pA, which corresponds to the current contrast shown in Figure 7b. The line profiles at the same location for the electrochemical current and the topographical height were compared (Figure 7d) to correlate the electrochemical activity with the surface topography. The difference in the electrochemical current tracks the surface features with a sub-micron resolution.

Even though the nitride surface is topographically 50 nm higher than the Au substrate, the current over the nitride surface is ~90 pA lower than the Au surface. This suggests that the electrochemical information is suitably decoupled from the topographic features. Moreover, the ~90 pA difference is found to be consistent with that demonstrated on the approaching curves in Figure 7c.32

Conductivity Measurements in Liquid

With the SECM probe, high-quality electrical measurements can be performed in liquids. Except for its apex, the tip is completely insulated. The coating dramatically reduces both stray capacitance caused by a stray current from electrochemical reactions of chemical impurities and the small electrically exposed area. For conductivity measurement with PeakForce Tapping, the tip occasionally contacts the surface for ~100 µs, so that electronics with a bandwidth of about 15 kHz is needed. This capability is provided by the PeakForce tunneling AFM (PeakForce TUNA™) hardware, which also maintains low noise (<70 fA). In this manner, nanoelectrical imaging in liquid with sub-pA sensitivity and with simultaneous nanomechanical imaging can be achieved is possible.

PeakForce TUNA measurement of a Pt surface partially covered by Si3N4: (a) topography; (b) TUNA currents at a sample bias of 10 mV; and (c) point-and-shoot I-V spectroscopy at selected locations in 8b (adapted from Nellist et al., Nanotechnology, 2017, 28(9), 095711, IOP Publishing30).

Figure 8. PeakForce TUNA measurement of a Pt surface partially covered by Si3N4: (a) topography; (b) TUNA currents at a sample bias of 10 mV; and (c) point-and-shoot I-V spectroscopy at selected locations in 8b (adapted from Nellist et al., Nanotechnology, 2017, 28(9), 095711, IOP Publishing30).

Shown in Figure 8 is a PeakForce TUNA measurement using a nanoelectrode probe in dimethyl carbonate solvent conducted in an Ar-filled glovebox with <1 ppm H2O and O2 contents. The surface topography indicating four Si3N4 corners is shown in Figure 8a. Among the Si3N4 islands is a Pt substrate that is partly covered by other Si3N4. Figure 8a shows the exposed Pt area, which is more clearly resolved in the conductivity map shown in Figure 8b. In addict, PeakForce TUNA can perform point-and-shoot measurements to identify the target location for current-voltage (I-V) spectroscopy (Figure 8c). The numerically labeled plots correspond to the locations shown in Figure 8b. The blue solid line in Figure 8b is a plot of current captured when the tip was landed on the nitride island. ~5 pA is the capacitive current which is hardly observed on the figure. The plateau on plot #1 and plot #2 are the saturation current at the 1 nA/V sensitivity setting. Figure 8c shows that the background current is insignificant for the measurement on the Pt surface. Higher conductivity locations display I-V behaviors with larger slopes.30

PeakForce SECM Application Examples

Au-SiO2 Nanomesh Electrode

Similar to many important components in such optoelectronic devices as solar fuels devices and photovoltaic cells, Au nanomesh is a transparent conducting electrode that can be stretched and folded35,36. Electrode materials such as Au and Pt are also excellent catalysts for fuel-generating photoelectrochemical reactions, such as solar-driven hydrogen evolution37.

Shown in Figure 9a is an Au nanomesh electrode supported on a SiO2 substrate prepared by nanosphere lithography.38. This mesh features through-hole patterns of a hexagonal lattice with a period of 1 µm. The hole has a 750-nm diameter with an inter-hole spacing of 250 nm. The depth of the hole is roughly 80 nm and fabrication imperfections result in exposed SiO2 within the holes of ~400 x 500 nm. Figure 9b shows the electrochemical image, demonstrating the predicted difference of the tip current across the nanomesh electrode. The top Au surface has improved tip current as a result of the positive feedback, and current reduction from a surface contaminant is also demonstrated. The tip current within the hole is reduced by the blocking effect that results from the hole feature as well as the exposed inactive SiO2 substrate. The sizes of the low current features (brown dots) are found to be consistent with the ~400 x 500 nm elliptical regions within the holes (Figure 9b). Figure 9c shows a cross-sectional analysis comparing the electrochemical and topographic line profiles at the same location, as indicated by the dashed lines in Figure 9a and 9b. Also, the tip current profile closely tracks the sample’s height variation, indicating a <100 nm spatial resolution for electrochemical imaging using the PeakForce SECM method30.

As demonstrated on the approach curves in Figure 6 and Figure 7c, and confirmed here, the variation in tip current between insulator and conductor decreases with increasing distances between the tip and the sample and is most sensitive when the tip-sample distance is similar to the thickness of the diffusion layer. Shown in Figure 9d is the variations in tip current across a hole feature of the nanomesh electrode captured at different lift heights while the same line position was scanned numerous times. The lift heights were slowly increased from 50 nm to 400 nm and subsequently decreased back to 75 nm. The electrochemical map clearly shows the color contrast among different regions at a lift height of 50 nm while the contrast slowly disappears for increasing lift heights. There is no major contrast at tip-sample separations of 400 nm. COMSOL simulations (Figure 4a) predict the [Ru(NH3)6]3+ concentration 400 nm away from the surface to be greater than 90% of the bulk concentration. As a result, a much lower spatial sensitivity is expected. Figure 9e shows the cross-sectional analysis, clearly illustrating the tip current contrast. On the Au region, as the lift height increases the tip current decreases. With increasing tip-sample separation, the current enhancing effect caused by redox-cycling of the Ru-complex is reduced until the diffusion-limited current in bulk solution is reached. On the other hand, on the SiO2 regions, a larger separation between the tip and the sample reduces the blocking of the Ru-complex diffusion to the tip electrode, leading to an increased diffusion-limited current. Due to these two effects on Au and SiO2, respectively, the tip current contrast decreases with increasing lift height.

PeakForce SECM images of a Au-SiO2 nanomesh electrode prepared by nanosphere lithography: (a) topography of the Au pattern on the SiO2 substrate; (b) electrochemical map captured in the lift scan, while following the sample topography at a separation of 75 nm; (c) line profiles of both the topographic height and electrochemical current at the same location, labeled by the yellow dashed lines in (a) and (b); (d) tip current at sequentially varied lift heights as indicated in the image (tip and sample potentials were -0.1 V and -0.4 V vs Ag/AgCl, respectively); (e) the tip current contrast is clearly illustrated by cross-sectional analysis (adapted from Nellist et al., Nanotechnology, 2017, 28(9), 095711, IOP Publishing30).

Figure 9. PeakForce SECM images of a Au-SiO2 nanomesh electrode prepared by nanosphere lithography: (a) topography of the Au pattern on the SiO2 substrate; (b) electrochemical map captured in the lift scan, while following the sample topography at a separation of 75 nm; (c) line profiles of both the topographic height and electrochemical current at the same location, labeled by the yellow dashed lines in (a) and (b); (d) tip current at sequentially varied lift heights as indicated in the image (tip and sample potentials were -0.1 V and -0.4 V vs Ag/AgCl, respectively); (e) the tip current contrast is clearly illustrated by cross-sectional analysis (adapted from Nellist et al., Nanotechnology, 2017, 28(9), 095711, IOP Publishing30).

Charge Transfer Dynamics

By capturing and examining the approach curves, the SECM serves as a suitable method to quantify the dynamics of local interfacial charge transfer14,39. A Si3N4 patterned sample on a Pt substrate is shown in Figure 10a. The nitride pattern features 2 x 2 µm squares in a simple square lattice with a period of 3 µm, and the gap between two nitride squares measures 1 µm. However, the etching of nitride to achieve this pattern was incomplete, which results in limited exposed regions of the Pt substrate. Figure 10b is the map of contact current during the main scan of PeakForce SECM imaging. The Pt grid can be clearly observed by the higher current against the nitride squares.

Also, the inhomogeneous contact current over the Pt area confirms the incomplete etching. Electrochemical current is captured during the lift scan, and these currents which can be directly correlated with the conductivity map. Areas that possess higher conductivity are also more active in electrochemistry, as shown in Figure 10c. In Figure 10d, a comparison is made on approach curves that were captured from locations of various activities. These plots are normalized to the current at a tip-sample distance of 1 µm. The black curve was then captured on the nitride square, which reveals 22% of current reduction at the surface of nitride. Other curves were captured on the Pt grid areas where the positive feedback improves the tip current, leading to less current reduction on the approach curves when compared to the one from the nitride surface. The shapes of this set of approach curves are dependent on the surface electrochemical activities.

When on a conductive surface, a contact current can also be obtained as demonstrated by the red and cyan plots. Aided with simulation, quantitative data regarding interfacial charge transfer dynamics can be derived from these approach curves.

(a) A Si3N4 patterned sample on Pt substrate; (b) tip current during the main scan of PeakForce SECM imaging; (c) the electrochemical current map; and (d) approach curves captured from locations of different activities. All these plots are normalized to the current at 1 µm tip-sample distance. The black curve was captured on the nitride square. Others were captured on the Pt grid regions.

Figure 10. (a) A Si3N4 patterned sample on Pt substrate; (b) tip current during the main scan of PeakForce SECM imaging; (c) the electrochemical current map; and (d) approach curves captured from locations of different activities. All these plots are normalized to the current at 1 µm tip-sample distance. The black curve was captured on the nitride square. Others were captured on the Pt grid regions.

Self-Assembled Monolayers (SAM)

SAMs are employed as etch resist barriers to charge transfer for organic/molecular electronics and electrochemistry, and as platforms for sensors and biological surfaces.40 An ideal SAM should have controlled conductivity, topographical homogeneity (compactness and flatness without surface defects), chemical stability, and mechanical stability. Its electrochemical properties should also be customizable when utilized in electrochemical sensors. PeakForce SECM fulfils the needs for characterizing SAM samples to obtain high-resolution, quantitative, and multidimensional information.

In this case study, investigation was made on an ultra-flat gold electrode that was chemically patterned by micro-contact printing of CH3-thiol SAMs. In Figure 11a, the topography shows smaller than 1 nm height variations which rules out topographic convolution on other imaged properties, but the donut structure is clearly seen by the quantitative adhesion force signal acquired through PeakForce QNM, a function in PeakForce SECM. A difference of ~1 nN in adhesion forces between the tip-Au and tip-SAM interactions is shown in Figure 11b. The adhesion force is very sensitive to the surface chemistry as indicted by defects with <100 nm width on the SAM (Figure 11b). For electrochemistry, the thiol layer serves as a non-ideal insulator, leading to a reduced interfacial charge transfer, and thus to decreased electrochemical current. This behavior is shown in Figure 11c. The tip current on the thiol SAM layer is reduced by 110 pA compared to that on the bare Au-electrode.32

PeakForce SECM images of micro-contact printed CH3-thiol self-assembled monolayer (SAM) on an Au substrate: (a) topography; (b) adhesion; and (c) electrochemical activity. Solution, 5 mM [Ru(NH3)6]3+ and 0.1 M KNO3 (adapted and modified from Huang, Z. et al.32).

Figure 11. PeakForce SECM images of micro-contact printed CH3-thiol self-assembled monolayer (SAM) on an Au substrate: (a) topography; (b) adhesion; and (c) electrochemical activity. Solution, 5 mM [Ru(NH3)6]3+ and 0.1 M KNO3 (adapted and modified from Huang, Z. et al.32).

Highly Oriented Pyrolytic Graphite (HOPG)

HOPG is a vital electrode or electrode material used in many applications, such as electrocatalysis, functionalized interfaces, and electrochemical sensors. For rational designs of related devices, knowledge about the step edges, the electrochemistry at the basal surface, and the HOPG defects is important. HOPG’s electrochemical characteristics have been attracting a great deal of interest and the study of HOPG on the nanoscale by SECM continues to be an active research area41,43. Traditionally, researchers can only obtain electrochemical maps by SECM or topographic information by AFM. This constraint can be now be resolved with PeakForce SECM.

In Figure 12a, a ~600 x 900 nm defect area on the HOPG surface is seen on the height image, and this defect area is 0.4 nm higher than the surrounding terrace. The defect area has a faradic current ~55 pA or 10% less than the basal area (485 pA versus 540 pA, Figure 12b). Also shown in Figure 12b are slightly enhanced tip currents at the step edges, which is roughly 2–5 pA higher than the terraces30.

PeakForce SECM images of an HOPG sample: (a) topography; (b) electrochemistry (adapted from Nellist et al., Nanotechnology, 2017, 28(9), 095711, IOP Publishing30).

Figure 12. PeakForce SECM images of an HOPG sample: (a) topography; (b) electrochemistry (adapted from Nellist et al., Nanotechnology, 2017, 28(9), 095711, IOP Publishing30).

Nanoparticle (NP) Catalysis

Nanoparticle catalysts deposited onto and dispersed over numerous supports are now being used in many different fields, including environmental remediation, energy-related applications and chemical manufacturing44,45. The sizes, shapes, compositions, patterns, structures, and interfaces with the substrates are all important parameters for the resulting activity, stability, and selectivity of these catalysts. The heterogeneous natures of these catalysts generally have spatial variation down to a single-particle level at the nanoscale10. Further, the electrolyte or surrounding media can have significant impacts on performance36. Hence, high-resolution in situ studies that can obtain multi-dimensional information simultaneously are highly preferred.

The probe and sample are biased at -0.4 V and -0.1 versus AgQRE, respectively for PeakForce SECM measurements on a Pt NP/p+-Si electrode (Figure 13). A 700 pN imaging force was applied to avoid moving the particles, which are loosely fixed to the substrate. The surface topography is illustrated in Figure 13a, showing particles with different sizes (~50 to ~200 nm).

For comparison purposes, some representative particles and an area are numerically labeled. An image of the tip current captured from the main scan during the SECM imaging is shown in Figure 13b, and an image of the tip current captured during the lift scan at 100 nm lift height is shown in Figure 13c. Both these images clearly resolve the four particles, showing sub-100 nm spatial resolving power of the electrochemical imaging. From these correlated maps, the tip faradic current, contact current, and surface topography among different particles can be compared in detail. As demonstrated on the contact current in the main scan, even particles that are loosely fixed to the Si surface can carry a large current density (e.g., particle 3, >4 nA, or ~2.5 mA cm-2). The interfacial conductivity, however, is not homogeneous among particle (Figure 13b). Particles 3 and 4 also exhibit similar activities, although with a big difference in interfacial conductivity. This indicates that the rate-limiting step is the surface electrochemical reaction. The variation in electrochemical activities can occur from multiple factors, such as shapes, sizes, compositions, structures, and so on. Deconvolution of these contributions continues to be a challenge10.

PeakForce SECM images of a semiconductor electrode decorated with nanoparticle catalysts; (a) topography; (b) tip current from the main scan; and (c) electrochemistry from the lift scan. The particles are numerically labelled for comparison.

Figure 13. PeakForce SECM images of a semiconductor electrode decorated with nanoparticle catalysts; (a) topography; (b) tip current from the main scan; and (c) electrochemistry from the lift scan. The particles are numerically labelled for comparison.

Utilizing PeakForce TUNA for Semiconductor/Metal Junctions in Liquid

Correct junction behaviors, such as Ohmic or tunneling, are important for semiconductor devices that depend on the metal-semiconductor contacts. Schottky barrier height is the key parameter for these junctions and relies on the Fermi levels of both materials, and the interfacial chemistry, for example Fermi level pinning. Semiconductor photoelectrodes, decorated with metal nanoparticle catalysts (usually Ag, Au or Pt), are used for artificial photosynthesis, solar- or light-driven decomposition of pollutants in aqueous solution, and water purification to remove organic contaminants. In such conditions, the photoelectrode is exposed to the electrolyte solution, which has a major effect on interfacial energetics of the metal/semiconductor junction. This is especially the case when nanoparticle metal catalysts are used47.

An example of PeakForce TUNA using a nanoelectrode probe in deionized H2O is shown in Figure 14. The sample used is an Au nanoelectrode array on a semiconductor substrate. On applying a +0.3 V sample bias, the nanoelectrode dots had current responses that were distinctly different from the surrounding oxide. The contact currents range from hundreds of pA up to few nA. In order to gain a better insight into the sources of the current signals and junction behaviors, the I-V characteristics of these nanoelectrodes were compared to the oxide areas in deionized H2O (Figure 14c). For each measurement, the voltage for backward and forward ramping was cycled at 400 mV/s and both curves were plotted together. When the tip landed on an oxide area in liquid, the only thing that was detected was the background capacitive charging current (grey curves). This capacitive charging current can be more clearly observed in the inset of Figure 14c, showing a charging current of ~150 pA. While on the Au dot, the I-V curves exhibit non-linear behaviors that deviate from a diode junction (green-red curves) or an Ohmic contact. The inset of Figure 14c clearly presents the slight hysteresis of the cyclic ramping. This hysteresis leads to a ~300 pA current difference between the backward and forward scans. This difference is found to be consistent with the capacitive charging current when the tip is placed on the oxide area. Thus, the I-V behavior of the red-green plot proves to be a true reflection of the junction properties of the metal/semiconductor contact on the nanoelectrode. Interestingly, this junction behavior in liquid is quite different from that in air, where it shows standard rectifying characteristics indicating a diode junction (blue curves)32.

High-resolution PeakForce TUNA in de-ionized H2O measurement for conductivity mapping of an Au nanoelectrode array sample: (A) topography; (B) current map at +0.3 V; and (C) comparison of I-V characteristics of the semiconductor/metal junction in air (blue) and liquid (green-red). The background current in liquid is captured by landing the probe at the oxide region (grey). Voltage ramp rates of 400 mV/s were employed. For each measurement, the voltage was cycled for both the forward and backward ramping and the curves were plotted together (adapted and modified from Huang, Z. et al.32).

Figure 14. High-resolution PeakForce TUNA in de-ionized H2O measurement for conductivity mapping of an Au nanoelectrode array sample: (A) topography; (B) current map at +0.3 V; and (C) comparison of I-V characteristics of the semiconductor/metal junction in air (blue) and liquid (green-red). The background current in liquid is captured by landing the probe at the oxide region (grey). Voltage ramp rates of 400 mV/s were employed. For each measurement, the voltage was cycled for both the forward and backward ramping and the curves were plotted together (adapted and modified from Huang, Z. et al.32).

Conclusion

This article shows Bruker’s success in the batch production fabrication of robust, high-quality, and user-friendly PeakForce SECM probes with an exposed Pt-coated tip apex having an end tip diameter of ~50 nm and a ~200 nm height. The PeakForce SECM probes are adopted for the new PeakForce SECM mode, which combines PeakForce QNM and conductive AFM with AFM-SECM measurements. This PeakForce SECM mode allows simultaneous capture of mechanical, topographical, and electrical maps with nanometer-scale and for electrochemical images with less than <100 nm resolution has been effectively developed.

PeakForce SECM also offers capabilities for high-quality nanoelectric imaging in liquid environments. All these features were developed to meet the requirements of current highly multidisciplinary research fields, as shown by a range of application examples in this study.

Acknowledgements

GP and MR acknowledge the support from the German Research Foundation (DFG) in the framework of the Collaborative Research Center (SFB 840). SWB and MRM acknowledge the support from the Department of Energy, Basic Energy Sciences, award number DE-SC0014279. CX and JJ acknowledge support from the Joint Center for Artificial Photosynthesis, a DOE Energy Innovation Hub, supported through the Office of Science of the U.S. Department of Energy under Award Number DE-SC0004993, BSB acknowledges support from NSF under the NSF Center CHE-1305124. Research was in part carried out at the Molecular Materials Research Center of the Beckman Institute of the California Institute of Technology.

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This information has been sourced, reviewed and adapted from materials originally authored by Z Huang, P. DeWolf, C. Li, R. Poddar, I. Yermolenko, A. Mark, S. Godrich, C. Stelling, M. Nellist, Y. Chen, J. Jiang, J. Thompson, G. Papastavrou, M. Retsch, S. Boettcher, C. Xiang, and B. Brunschwig, provided by Bruker Nano Surfaces.

For more information on this source, please visit Bruker Nano Surfaces.

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