This work is funded by the national science foundation (Award Number: 1756444) via Biological &#38; Environmental Interfaces of Nano Materials, the USDA National Institute of Food and Agriculture, AFRI project [2018-07549] and Assistance Agreement No. 83945101-0 awarded by the U.S. Environmental Protection Agency to New Jersey Institute of Technology. It has not been formally reviewed by EPA. The views expressed in this document are solely those of authors and do not necessarily reflect those of the Agency. EPA does not endorse any products or commercial services mentioned in this publication. The authors also thank Undergraduate Research and Innovation program (URI) Phase-1 &#38; Phase-2 at New Jersey Institute of Technology.

1. Sample preparation
<ol>
	<li>Preparation of faceted Cu2O nanoparticles and deposition on silicon substrate
	<ol>
		<li>Dissolve 0.175 g of CuCl2&#8729;2H2O (99.9%) into 100 mL of deionized (DI) water to generate an aqueous solution of 10 mM CuCl2.</li>
		<li>Add 10.0 mL of 2.0 M NaOH and 10 mL of 0.6 M ascorbic acid dropwise into the CuCl2 solution.</li>
		<li>Heat the solution in a 250 mL round-bottom flask under constant stirring in a 55 &#176;C water bath for 3 h.</li>
		<li>Collect the resulting precipitate by centrifugation (5,000 x g for 15 min), followed by washing with DI water 3 times and ethanol twice to remove the residual inorganic ions and polymers.</li>
		<li>Dry precipitate in vacuum at 60 &#176;C for 5 h53.</li>
		<li>Use the prepared silicon wafer as the substrate to deposit Cu2O nanoparticles as illustrated in Figure 1A using epoxy to ensure the testing. 
		Caution: The silicon wafer (&#216;3&#8221; Silicon wafer, Type P/&#60;111&#62;) was cut into a single piece of 38 mm x 38 mm, followed by washing using ethanol, methanol and DI water to remove organic and inorganic contaminants.</li>
		<li>Directly deposit 10 &#181;L of epoxy on the cleaned silicon wafer using a pipette tip and tile with a clean glass slide. After about 5 min, drop 10 &#181;L of the nanoparticles/water suspension (10 mg L-1) on different epoxy-coated silicon wafer substrates, separately. The four different red spots shown in Figure 1B indicate the potential position of the deposited nanoparticles.</li>
		<li>Vacuum dry the substrate at 40 &#176;C for 6 h.</li>
		<li>Place the sample substrate into the EC sample cell (Figure 4) to be filled with 1.8 mL of a 0.1 M KCl containing 10 mM Ru(NH3)6Cl3(98%).</li>
	</ol>
	</li>
	<li>Preparation of NBs
	<ol>
		<li>Generate oxygen nanobubbles by direct injection of compressed oxygen (purity 99.999%) through a tubular ceramic membrane (100 nm pore size, WFA0.1) into DI water. 
		NOTE: The gas was injected continuously under a pressure of 414 kPa and a flow of 0.45 L&#183;m-1 until reaching stable bubble size distribution as reported elsewhere54.</li>
		<li>Add 1.8 mL of the water suspension of NBs on a gold substrate in the EC sample cell and stabilize for 10 min. 
		NOTE: Fresh 40 mm x 40 mm gold plates (Au on Si) were used as the substrate to immobilize NBs.</li>
		<li>Decant 0.9 mL of NB suspension and replace with 0.9 mL of a 10 mM Ru(NH3)6Cl3 solution in 0.1 M KCl.</li>
	</ol>
	</li>
</ol>
2. Setup of AFM-SECM
NOTE: AFM was used in the presented AFM-SECM measurements. To perform the EC analyses, the AFM was equipped with a bipotentiostat and SECM accessories. As shown in Figure S1, the bipotentiostat was connected to the AFM controller and both the potentiostat and AFM were connected to the same computer. The accessories include an SECM chuck, an SECM probe holder with protective boot, and a strain-release module with a resistance selector (10 M&#937; resistance was used) to limit the maximum current flow55. As shown in Figure 2, the AFM-SECM probes have a tip radius of 25 nm and a tip height of 215 nm. The sample acted as a working electrode, which shares the same pseudo-reference using the Ag wire electrode (25 mm diameter) and the counter electrode of a Pt wire (25 mm in diameter). The probe and the sample could be biased at different potentials (vs the Ag wire pseudo-reference electrode) to enable different redox reactions. In the presented work, the tip reduces the [Ru(NH3)6]3+ to [Ru(NH3)6]2+ at -400 mV versus an Ag wire pseudo-reference electrode.
<ol>
	<li>Replace the existing sample chuck with SECM chuck and screw the chuck in place using two M3 x 6 mm socket head cap screws and a 2.5 mm hex wrench (Figure 3A).</li>
	<li>Connect the temperature control cable to the SECM chuck, and connect the low-noise SECM cables to the spring connector block (color to color) and switch block (Figure 3B). 
	NOTE: The switch needs to be kept on the right side during SECM testing.</li>
	<li>Install the strain-release module onto the AFM scanner and also connect it to the working electrode connector on the spring connector block with extension cable (Figure 3C).</li>
	<li>Assemble the EC sample cell.
	<ol>
		<li>Put the insert onto the top ring (Figure 4A).</li>
		<li>Assemble two O-rings onto the bottom groove and top groove of the insert, respectively (Figure 4B and Figure 4C).</li>
		<li>Put a glass cover onto the top ring top and then tighten by four screws lightly and diagonally (Figure 4D).</li>
		<li>Use a hard sharp wire with a 24 mm diameter (Figure 4E) to poke two holes in the O-ring through two channels of plastic part on top ring (Figure 4F).</li>
		<li>Insert Ag wire and Pt wire through the hole on the O-ring, and curve the Pt wire to a circle in EC sample cell as shown in Figure 4G.</li>
		<li>To seal the EC sample cell top part, press the assembled EC sample cell down on the EC sample cell bottom to make the O-ring fully contact the glass cover (Figure 4H).</li>
		<li>Place the top part of the EC sample cell upside-down and face the test sample (or substrate) downward so that the spring-loaded pins (pogo pins) touch the sample surface as shown in Figure 4I and Figure 4J. The test sample should be covering the O-ring to make EC sample cell bottom part seal.</li>
		<li>Put the EC sample cell bottom on and tighten diagonally with right length screw (Figure 4K).</li>
	</ol>
	</li>
</ol>
3. Operation of AFM-SECM
<ol>
	<li>Initialization of the AFM and bipotentiostat instruments
	<ol>
		<li>Double click the two software icons to initialize the AFM system and the bipotentiostat control interface.</li>
	</ol>
	</li>
	<li>Loading SECM Probe
	<ol>
		<li>Prepare the ESD field service package including antistatic pad, electrostatic discharge (ESD) protective probe stand, wearable anti-static gloves and wrist strap (Figure 5A). Figure 5B shows the connection of the ESD monitor with wrist strap. 
		NOTE: The ESD monitor beeps when the red pad is connected with ground. The beep will stop when user wears the wrist strap.</li>
		<li>To prevent AFM scanner from exposure to liquid, use a protective boot (Figure 6A) during AFM-SECM testing. Put the probe holder onto the ESD protective probe stand (Figure 6B). Use a pair of plastic tweezers to attach the protective boot to the tip holder (Figure 6C). Then, align the small cut in the protective boot to the notch in the probe holder as illustrated in Figure 6D.</li>
		<li>Open the box of AFM-SECM probes (Figure 7A) using a tip tweezer (green color) to grab the probe from both sides of the grooves (Figure 7B). While using the disk gripper (silver color) to hold the probe holder on the stand, put the probe wire into the hole of the stand, and then slide the probe into the slot of the probe holder (Figure 7C). After the probe is inside the slot, use the flat end of the tweezer to push it in. Make sure the probe is completely in the tip holder (Figure 7D).</li>
		<li>As shown in Figure 8A, attach the whole probe holder (including the holder-boot) to the scanner.</li>
		<li>Use the Teflon tip tweezer to grab the wire right below the copper ring and connect it to the module (Figure 8B).</li>
		<li>Put the scanner back to the dovetail.</li>
	</ol>
	</li>
	<li>Loading the sample cell
	<ol>
		<li>After assembling the test sample (or substrate) in the EC sample cell, which was mentioned in Section 2.4, put the EC sample cell on the central point of the SECM chuck and the pseudo-reference electrode (Ag wire) and connect the counterelectrode (Pt wire) to the spring connector block (Figure 3). The EC sample cell is magnetically attached to the chuck.</li>
	</ol>
	</li>
	<li>SECM software preparation before imaging
	<ol>
		<li>In the AFM-SECM software, select SECM- PeakForce QNM to load the workspace (Figure S2).</li>
		<li>In Setup, load the SECM probe, and then align a laser on the tip using an alignment station.</li>
		<li>Go to Navigation (Figure S3). Move the scanner downwards slowly to focus on the sample surface. Adjust the position of EC sample cell slightly to make sure the scanner would not touch the glass cover of EC sample cell while moving. After focusing on the sample, click Update Blind Engage Position. 
		Caution: Different samples have different heights, so it is necessary to update the blind engage position after changing a sample.</li>
		<li>Click Move to Add Fluid Position.</li>
		<li>Add ~1.8 mL of the buffer solution into the EC sample cell, to make sure the level of the solution is lower than the glass cover. If the water level is over the glass cover, water can creep to the scanner and cause an electric short and break the scanner. Wait for another 5 minutes and use a pipette to agitate the solution to remove bubbles. 
		NOTE: The buffer solution (10 mM [Ru(NH3)6]3+ with supporting electrolyte of 0.1 M KCl) should be constantly stored in a refrigerator after preparation. Use a syringe with filter (no larger than 1 &#181;m pore size) to filter the solution before using it.</li>
		<li>Click Move to Blind Engage Position. The tip will move back into the buffer solution. Adjust the laser slightly to make sure the laser is aligned on the tip.</li>
		<li>Open CHI software. As shown in Figure S4, click on the Technique command on the toolbar to open up the tech selector and select Open Circuit Potential &#8211; Time. Use the default setting (Run time as 400 s) for OCP measurement and run the OCP measurement. 
		NOTE: The potential showed in OCP test should be near zero stably.</li>
		<li>Click the Technique command again and run Cyclic Voltammetry (CV), as shown in Figure S5 and Figure S6. 
		NOTE: Set up the parameters as below. Set &#8220;sweep segments&#8221; to a larger number if needed. The &#8220;init E/Final E&#8221; should be as the same as the potential value from OCP measurement and &#8220;High E&#8221; and &#8220;Low E&#8221; could be +0.3 V or &#8722;0.3 V of &#8220; init E/Final E&#8221;, respectively. Here we use 0 V as initial and high E and -0.4 V as Low and Final E. The scan rate was 0.05 V/s and the sensitivity was 1 e-009. Run the CV test, the highest current (i) measured here should be 0.3-1.2 nA for 10 mM [Ru(NH3)6]3+.</li>
	</ol>
	</li>
	<li>SECM Imaging
	<ol>
		<li>Go back to the AFM-SECM software. Since the tip is already in the liquid, click Engage.</li>
		<li>After scanning, turn on lift mode (Lift by Sensor) with a lift height of 100 nm and adjust the lift height based on the sample roughness.</li>
		<li>In CHI software, run a chronoamperometry with parameters shown in Figure S7. Set the initial E as -0.4 V, the pulse width as 1000 seconds (which is the maximum number accepted by the system), and the sensitivity the same with CV scan. 
		NOTE: The chronoamperometry technique was chosen because of the absence of amperometric i-t technique in the presented bi-potentiostat.</li>
		<li>With the CHI program running, go back to AFM-SECM software, check the real-time reading on the strip chart and click on Start (Figure S8). The reading will be updated in real-time. Then both topography imaging and current imaging process will begin. Save images in the AFM-SECM software.</li>
	</ol>
	</li>
	<li>Check approach curve
	<ol>
		<li>Engage the tip on sample or substrate region with a scan size of 1 &#181;m.</li>
		<li>Run the Chronoamperometry as mentioned in 3.5.3.</li>
		<li>Go back to AFM-SECM Software and select the command Go To Ramp.</li>
		<li>Click Ramp. An approach curve would be recorded in the AFM-SECM software.</li>
	</ol>
	</li>
	<li>Tip cleaning
	<ol>
		<li>Using the EC sample cell as a clean water container. Move the tip in and out of the liquid using blind engage functions in the navigation panel. Change the clean water three times. After this three-time cleaning, using clean wipes to carefully remove residual water from the probe holder and put the probe back in the probe box. 
		Caution: After imaging, the AFM-SECM probe needs to be carefully cleaned. Never use water coming out from the wash bottle to clean the probe as the electrostatic charge might damage the probe.</li>
	</ol>
	</li>
</ol>

The authors have nothing to disclose.

A combined AFM-SECM technique that enables high-resolution multimodal imaging has been described in this protocol. This technique allows for topography to be mapped simultaneously with the SECM current collected or mapped on single nanoparticles or nanobubbles. Experiments were performed using commercial probes. These probes were designed to provide chemical compatibility with a wide range of electrochemical environments, electrochemical performance, mechanical stability, and multiple-cycle handling18</...

Characterization of electrochemical (EC) behavior can provide critical insights into the kinetics and mechanisms of interfacial reactions in diverse fields, such as biology1,2, energy3,4, material synthesis5,6,7, and chemical process8,9. Traditional EC measurements including electrochemical impedance spectroscopy10, electrochemical noise methods11, galvanostatic intermittent titration12, and cyclic voltammetry13 are usually performed at macroscopic scale and provide a surface-average response. Thus, it is difficult to extract information on how electrochemical-activity is distributed across a surface, but local scale surface properties in nanoscale are especially important where nanomaterials are widely used. Therefore, new techniques capable of simultaneously capturing both nanoscale multidimensional information and electrochemistry are highly desirable.
Scanning electrochemical microscopy (SECM) is a widely used technique for measuring the localized electrochemical activity of materials at micro- and nanoscales14. Typically, SECM uses an ultra-microelectrode as a probe for detecting electroactive chemical species as it scans a sample surface to spatially resolve local electrochemical properties15. The measured current at the probe is produced by reduction (or oxidation) of the mediator species, and this current is an indicator of the electrochemical reactivity at the surface of the sample. SECM has evolved significantly after its first inception in 198916,17 but it is still challenged by two main limitations. Since EC signals are typically sensitive to tip-substrate interaction characteristics, one limitation of SECM is that keeping the probe at a constant height prevents a direct correlation of electrochemical activity with the surface landscape, due to the convolution of topography with the collected EC information18. Second, it is difficult for a commercial SECM system to obtain sub-micrometer (&#181;m) image resolution as the spatial resolution is partially determined by the probe dimensions, which is on the micrometer scale19. Therefore, nanoelectrodes, the electrodes with a diameter in the nanometer range, are increasingly used in SECM to achieve a resolution below the sub-micrometer scale20,21,22,23.
To provide a constant tip-substrate distance control and obtain a higher spatial electrochemical resolution, several hybrid techniques of SECM have been used, such as ion conductance positioning24, shear force positioning25, alternating current SECM26, and atomic force microscopy (AFM) positioning. Among these instrumentations, SECM integrating AFM positioning (AFM-SECM) has become a highly promising approach. As AFM can provide fixed tip-substrate distances, the integrated AFM-SECM technique enables simultaneous acquisition of nanoscale surface structural and electrochemical information through mapping or sample sweeping with the sharp AFM tips. Since the first successful operation of AFM-SECM by MacPherson and Unwin in 199627, significant improvements have been achieved on probe design and fabrication, as well as its applications in various research fields such as electrochemistry in chemical and biological processes. For example, AFM-SECM has been implemented for imaging composite material surfaces, such as noble metal nanoparticles28, functionalized or dimensionally stable electrodes29,30, and electronic devices31. AFM-SECM can map the electrochemically active sites from the tip current image.
Simultaneous topographical and electrochemical measurements could also be achieved by other techniques such as conductive AFM32,33,34,35, electrochemical AFM (EC-AFM)36,37,38,39, scanning ion conductance microscopy-scanning electrochemical microscopy (SICM-SECM)24,40, and scanning electrochemical cell microscopy (SECCM)41,42 The comparison between these techniques has been discussed in a review paper1. The aim of the present work was to employ SECM-AFM to demonstrate the electrochemical mapping and measurement on faceted crystalline cuprous oxide nanomaterials and nanobubbles in water. Faceted nanomaterials are widely synthesized for metal oxide catalysts in clean energy applications because the facets with distinctive crystallographic features have distinctive surface atomic structures and further dominate their catalytic properties. Moreover, we also measured and compared the electrochemical behavior at the liquid/gas interfaces for surface nanobubbles (NBs) on gold substrates. NBs are bubbles with a diameter of &#60;1 &#956;m (also known as ultrafine bubbles)43, and they elicit many intriguing properties44,45, including long residence times in the solutions46,47 and high&#160;efficiency of gas mass transfer46,48. Furthermore, the collapse of NBs creates shock waves and the formation of hydroxyl radicals (&#8226;OH)49,50,51,52. We measured the electrochemical reactivity of oxygen NBs in the solution to better understand the fundamental chemical properties of NBs.

<table><tbody><tr><td>&lt;strong&gt;Equipment&lt;/strong&gt;</td><td></td><td></td><td></td></tr><tr><td>Atomic force microsopy</td><td>Bruker, CA</td><td>Dimenison Icon</td><td></td></tr><tr><td>Bipotentiostat</td><td>CH Instruments, Inc.</td><td>CHI 700E</td><td></td></tr><tr><td>&lt;strong&gt;Materials&lt;/strong&gt;</td><td></td><td></td><td></td></tr><tr><td>Silicon wafer</td><td>TED PELLA, Inc.</td><td>16013</td><td></td></tr><tr><td>Fresh gold plates</td><td>Bruker, CA</td><td>model 119-017-307</td><td></td></tr><tr><td>PF-SECM-AFM probes</td><td>Bruker, CA</td><td>990-050138</td><td></td></tr><tr><td>PF-SECM strain-release module</td><td>Bruker, CA</td><td>840-012-724</td><td></td></tr><tr><td>PF-SECM Probe Holder</td><td>Bruker, CA</td><td>900-050121</td><td></td></tr><tr><td>PF-SECM Chuck</td><td>Bruker, CA</td><td>PF-SECM Chuck</td><td></td></tr><tr><td>PF-SECM O-ring</td><td>Bruker, CA</td><td>598-000-106</td><td></td></tr><tr><td>PF-SECM cover glass, SECM Cell</td><td>Bruker, CA</td><td>900-050137</td><td></td></tr><tr><td>EC Cell Assy</td><td>Bruker, CA</td><td>932-017-300</td><td></td></tr><tr><td>ESD Field Service</td><td>Bruker, CA</td><td>490-000-066</td><td></td></tr><tr><td>PF-SECM Boot</td><td>Bruker, CA</td><td>900-050136</td><td></td></tr><tr><td>Spring connector block</td><td>Bruker, CA</td><td>900-050524</td><td></td></tr><tr><td>PFSECM Tweezers</td><td>Bruker, CA</td><td></td><td></td></tr><tr><td>Cable, SECM Tip module</td><td>Bruker, CA</td><td>468-050171</td><td></td></tr><tr><td>Ag wire</td><td>Bruker, CA</td><td>249-000-056</td><td></td></tr><tr><td>Pt wire</td><td>Bruker, CA</td><td>248-000-004</td><td></td></tr><tr><td>Hard sharp wire</td><td>Bruker, CA</td><td>TT-ECM10</td><td></td></tr><tr><td>Tubular ceramic membrane</td><td>Refracton</td><td>WFA0.1</td><td></td></tr><tr><td>&lt;strong&gt;Chemicals&lt;/strong&gt;</td><td></td><td></td><td></td></tr><tr><td>Copper(II) chloride dihydrate</td><td>ACROS Organics</td><td>AC315281000</td><td></td></tr><tr><td>Sodium Hydroxide</td><td>Fisher Chemical</td><td>S318-100</td><td></td></tr><tr><td>Ascorbic Acid</td><td>Fisher Chemical</td><td>A61-25</td><td></td></tr><tr><td>Epoxy</td><td>Loctite</td><td>Instant Mix</td><td></td></tr><tr><td>Potassium Chloride</td><td>Fisher Chemical</td><td>P217-500</td><td></td></tr><tr><td>Hexaammineruthenium(III) chloride</td><td>ACROS Organics</td><td>AC363342500</td><td></td></tr></tbody></table>

probing surface electrochemical activity of nanomaterials using a hybrid atomic force microscope-scanning electrochemical microscope (afm-secm)

Scanning electrochemical microscopy (SECM) is used to measure the local electrochemical behavior of liquid/solid, liquid/gas and liquid/liquid interfaces. Atomic force microscopy (AFM) is a versatile tool to characterize micro- and nanostructure in terms of topography and mechanical properties. However, conventional SECM or AFM provides limited laterally resolved information on electrical or electrochemical properties at nanoscale. For instance, the activity of a nanomaterial surface at crystal facet levels is difficult to resolve by conventional electrochemistry methods. This paper reports the application of a combination of AFM and SECM, namely, AFM-SECM, to probe nanoscale surface electrochemical activity while acquiring high-resolution topographical data. Such measurements are critical to understanding the relationship between nanostructure and reaction activity, which is relevant to a wide range of applications in material science, life science and chemical processes. The versatility of the combined AFM-SECM is demonstrated by mapping topographical and electrochemical properties of faceted nanoparticles (NPs) and nanobubbles (NBs), respectively. Compared to previously reported SECM imaging of nanostructures, this AFM-SECM enables quantitative assessment of local surface activity or reactivity with higher resolution of surface mapping.

Topography and current imaging of ONBs by AFM-SECM
Previous studies that characterized NBs with AFM only reported topography images to reveal the size and distribution of NBs immobilized on a solid substrate56,57. The experiments here revealed both morphological and electrochemical information. Individual oxygen nanobubbles (ONBs) can be clearly identi&#64257;ed in Figure 9, which provides...

Watch this Scientific Journal Video about Probing Surface Electrochemical Activity of Nanomaterials using a Hybrid Atomic Force Microscope-Scanning Electrochemical Microscope AFM-SECM at JoVE.com

Probing Surface Electrochemical Activity of Nanomaterials using a Hybrid Atomic Force Microscope-Scanning Electrochemical Microscope AFM-SECM

Atomic force microscopy (AFM) combined with scanning electrochemical microscopy (SECM), namely, AFM-SECM, can be used to simultaneously acquire high-resolution topographical and electrochemical information on material surfaces at nanoscale. Such information is critical to understanding heterogeneous properties (e.g., reactivity, defects, and reaction sites) on local surfaces of nanomaterials, electrodes and biomaterials.

Probing Surface Electrochemical Activity of Nanomaterials using a Hybrid Atomic Force Microscope-Scanning Electrochemical Microscope (AFM-SECM)

Scanning electrochemical microscopy (SECM) is used to measure the local electrochemical behavior of liquid/solid, liquid/gas and liquid/liquid interfaces. ...

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Research

JoVE Journal

Chemistry

6.8K Views.  New Jersey Institute of Technology. Dit protocol bevat een krachtige en innovatieve technologie, de zogenaamde Atomic Force Microscope, gekoppeld aan de Electro-chemicals Scanning Microscope, afm-SECM om de morfologische en elektrochemische informatie over gefacetteerd nanomateriaal en nanobubbels in water te scannen. AFM-SECM is in staat om elektrochemisch reactief oppervlak in kaart te brengen op basis van tipstroombeelden en maakt ook gelijktijdige verwerving van nanoschaaloppervlakstructuren en elektrochemie-informatie op m...

Video: Probing Surface Electrochemical Activity of Nanomaterials using a Hybrid Atomic Force Microscope-Scanning Electrochemical Microscope AFM-SECM

Probing and Mapping Electrode Surfaces in Solid Oxide Fuel Cells

Solid oxide fuel cells (SOFCs) are potentially the most efficient and cost-effective solution to utilization of a wide variety of fuels beyond hydrogen 1-7. The performance of SOFCs and the rates of many chemical and energy transformation processes in energy storage and conversion devices in general are limited primarily by charge and mass transfer along electrode surfaces and across interfaces. Unfortunately, the mechanistic understanding of these processes is still lacking, due largely to the difficulty of characterizing these processes under in situ conditions. This knowledge gap is a chief obstacle to SOFC commercialization. The development of tools for probing and mapping surface chemistries relevant to electrode reactions is vital to unraveling the mechanisms of surface processes and to achieving rational design of new electrode materials for more efficient energy storage and conversion2. Among the relatively few in situ surface analysis methods, Raman spectroscopy can be performed even with high temperatures and harsh atmospheres, making it ideal for characterizing chemical processes relevant to SOFC anode performance and degradation8-12. It can also be used alongside electrochemical measurements, potentially allowing direct correlation of electrochemistry to surface chemistry in an operating cell. Proper in situ Raman mapping measurements would be useful for pin-pointing important anode reaction mechanisms because of its sensitivity to the relevant species, including anode performance degradation through carbon deposition8, 10, 13, 14 ("coking") and sulfur poisoning11, 15 and the manner in which surface modifications stave off this degradation16. The current work demonstrates significant progress towards this capability. In addition, the family of scanning probe microscopy (SPM) techniques provides a special approach to interrogate the electrode surface with nanoscale resolution. Besides the surface topography that is routinely collected by AFM and STM, other properties such as local electronic states, ion diffusion coefficient and surface potential can also be investigated17-22. In this work, electrochemical measurements, Raman spectroscopy, and SPM were used in conjunction with a novel test electrode platform that consists of a Ni mesh electrode embedded in an yttria-stabilized zirconia (YSZ) electrolyte. Cell performance testing and impedance spectroscopy under fuel containing H2S was characterized, and Raman mapping was used to further elucidate the nature of sulfur poisoning. In situ Raman monitoring was used to investigate coking behavior. Finally, atomic force microscopy (AFM) and electrostatic force microscopy (EFM) were used to further visualize carbon deposition on the nanoscale. From this research, we desire to produce a more complete picture of the SOFC anode.

We present a unique platform for characterizing electrode surfaces in solid oxide fuel cells (SOFCs) that allows simultaneous performance of multiple characterization techniques (e.g. in situ Raman spectroscopy and scanning probe microscopy alongside electrochemical measurements). Complementary information from these analyses may help to advance toward a more profound understanding of electrode reaction and degradation mechanisms, providing insights into rational design of better materials for SOFCs.

Solid oxide fuel cells (SOFCs) are potentially the most efficient and cost-effective solution to utilization of a wide variety of fuels beyond hydrogen ...

Engineering

Concurrent Quantitative Conductivity and Mechanical Properties Measurements of Organic Photovoltaic Materials using AFM

Organic photovoltaic (OPV) materials are inherently inhomogeneous at the nanometer scale. Nanoscale inhomogeneity of OPV materials affects performance of photovoltaic devices. Thus, understanding of spatial variations in composition as well as electrical properties of OPV materials is of paramount importance for moving PV technology forward.1,2 In this paper, we describe a protocol for quantitative measurements of electrical and mechanical properties of OPV materials with sub-100 nm resolution. Currently, materials properties measurements performed using commercially available AFM-based techniques (PeakForce, conductive AFM) generally provide only qualitative information. The values for resistance as well as Young's modulus measured using our method on the prototypical ITO/PEDOT:PSS/P3HT:PC61BM system correspond well with literature data. The P3HT:PC61BM blend separates onto PC61BM-rich and P3HT-rich domains. Mechanical properties of PC61BM-rich and P3HT-rich domains are different, which allows for domain attribution on the surface of the film. Importantly, combining mechanical and electrical data allows for correlation of the domain structure on the surface of the film with electrical properties variation measured through the thickness of the film.

Organic photovoltaic (OPV) materials are inherently inhomogeneous at the nanometer scale. Nanoscale inhomogeneity of OPV materials affects performance of photovoltaic devices. In this paper, we describe a protocol for quantitative measurements of electrical and mechanical properties of OPV materials with sub-100 nm resolution.

Organic photovoltaic  (OPV) materials are inherently inhomogeneous at the nanometer scale. Nanoscale  inhomogeneity of OPV materials affects performance ...

Ohmic Contact Fabrication Using a Focused-ion Beam Technique and Electrical Characterization for Layer Semiconductor Nanostructures

Layer semiconductors with easily processed two-dimensional (2D) structures exhibit indirect-to-direct bandgap transitions and superior transistor performance, which suggest a new direction for the development of next-generation ultrathin and flexible photonic and electronic devices. Enhanced luminescence quantum efficiency has been widely observed in these atomically thin 2D crystals. However, dimension effects beyond quantum confinement thicknesses or even at the micrometer scale are not expected and have rarely been observed. In this study, molybdenum diselenide (MoSe2) layer crystals with a thickness range of 6-2,700 nm were fabricated as two- or four-terminal devices. Ohmic contact formation was successfully achieved by the focused-ion beam (FIB) deposition method using platinum (Pt) as a contact metal. Layer crystals with various thicknesses were prepared through simple mechanical exfoliation by using dicing tape. Current-voltage curve measurements were performed to determine the conductivity value of the layer nanocrystals. In addition, high-resolution transmission electron microscopy, selected-area electron diffractometry, and energy-dispersive X-ray spectroscopy were used to characterize the interface of the metal&#8211;semiconductor contact of the FIB-fabricated MoSe2 devices. After applying the approaches, the substantial thickness-dependent electrical conductivity in a wide thickness range for the MoSe2-layer semiconductor was observed. The conductivity increased by over two orders of magnitude from 4.6 to 1,500 &#937;&#8722;1 cm&#8722;1, with a decrease in the thickness from 2,700 to 6 nm. In addition, the temperature-dependent conductivity indicated that the thin MoSe2 multilayers exhibited considerably weak semiconducting behavior with activation energies of 3.5-8.5 meV, which are considerably smaller than those (36-38 meV) of the bulk. Probable surface-dominant transport properties and the presence of a high surface electron concentration in MoSe2 are proposed. Similar results can be obtained for other layer semiconductor materials such as MoS2 and WS2.

We describe the approaches for the device fabrication and electrical characterization of molybdenum diselenide (MoSe2) layer semiconductor nanostructures with different thicknesses. In addition, the fabrication of ohmic contacts for MoSe2-layer nanocrystals by the focused-ion beam deposition method using platinum (Pt) as a contact metal is described.

Layer semiconductors with easily processed two-dimensional (2D) structures exhibit indirect-to-direct bandgap transitions and superior transistor ...

A Novel Method for In Situ Electromechanical Characterization of Nanoscale Specimens

Electrically assisted deformation (EAD) is increasingly being used to improve the formability of metals during processes such as sheet metal rolling and forging. Adoption of this technique is proceeding despite disagreement concerning the underlying mechanism responsible for EAD. The experimental procedure described herein enables a more explicit study compared to previous EAD research by removing thermal effects, which are responsible for disagreement in interpreting previous EAD results. Furthermore, as the procedure described here enables EAD observation in situ and in real time in a transmission electron microscope (TEM), it is superior to existing post-mortem methods that observe EAD effects post-test. Test samples consist of a single crystal copper (SCC) foil having a free-standing tensile test section of nanoscale thickness, fabricated using a combination of laser and ion beam milling. The SCC is mounted to an etched silicon base that provides mechanical support and electrical isolation while serving as a heat sink. Using this geometry, even at high current density (~3,500 A/mm2), the test section experiences a negligible temperature increase (&#60;0.02 &#176;C), thus eliminating Joule heating effects. Monitoring material deformation and identifying the corresponding changes to microstructures, e.g. dislocations, are accomplished by acquiring and analyzing a series of TEM images. Our sample preparation and in situ experiment procedures are robust and versatile as they can be readily utilized to test materials with different microstructures, e.g., single and polycrystalline copper.

A Novel Method for In Situ Electromechanical Characterization of Nanoscale Specimens

Isolating electrical and thermal effects on electrically assisted deformation (EAD) is very difficult using macroscopic samples. Metallic sample micro- and nanostructures together with a custom test procedure have been developed to evaluate the impact of applied current on the formation without joule heating and evolution of dislocations on these samples.

Electrically assisted deformation (EAD) is increasingly being used to improve the formability of metals during processes such as sheet metal rolling and ...

DNA Origami-Mediated Substrate Nanopatterning of Inorganic Structures for Sensing Applications

Structural DNA nanotechnology provides a viable route for building from the bottom-up using DNA as construction material. The most common DNA nanofabrication technique is called DNA origami, and it allows high-throughput synthesis of accurate and highly versatile structures with nanometer-level precision. Here, it is shown how the spatial information of DNA origami can be transferred to metallic nanostructures by combining the bottom-up DNA origami with the conventionally used top-down lithography approaches. This allows fabrication of billions of tiny nanostructures in one step onto selected substrates. The method is demonstrated using bowtie DNA origami to create metallic bowtie-shaped antenna structures on silicon nitride or sapphire substrates. The method relies on the selective growth of a silicon oxide layer on top of the origami deposition substrate, thus resulting in a patterning mask for following lithographic steps. These nanostructure-equipped surfaces can be further used as molecular sensors (e.g., surface-enhanced Raman spectroscopy (SERS)) and in various other optical applications at the visible wavelength range owing to the small feature sizes (sub-10 nm). The technique can be extended to other materials through methodological modifications; therefore, the resulting optically active surfaces may find use in development of metamaterials and metasurfaces.

Here, we describe a protocol to create discrete and accurate inorganic nanostructures on substrates using DNA origami shapes as guiding templates. The method is demonstrated by creating plasmonic gold bowtie-shaped antennas on a transparent substrate (sapphire).

Structural DNA nanotechnology provides a viable route for building from the bottom-up using DNA as construction material. The most common DNA ...

Nanoscale Characterization of Liquid-Solid Interfaces by Coupling Cryo-Focused Ion Beam Milling with Scanning Electron Microscopy and Spectroscopy

Physical and chemical processes at solid-liquid interfaces play a crucial role in many natural and technological phenomena, including catalysis, solar energy and fuel generation, and electrochemical energy storage. Nanoscale characterization of such interfaces has recently been achieved using cryogenic electron microscopy, thereby providing a new path to advancing our fundamental understanding of interface processes.
This contribution provides a practical guide to mapping the structure and chemistry of solid-liquid interfaces in materials and devices using an integrated cryogenic electron microscopy approach. In this approach, we pair cryogenic sample preparation which allows stabilization of solid-liquid interfaces with cryogenic focused ion beam (cryo-FIB) milling to create cross-sections through these complex buried structures. Cryogenic scanning electron microscopy (cryo-SEM) techniques performed in a dual-beam FIB/SEM enable direct imaging as well as chemical mapping at the nanoscale. We discuss practical challenges, strategies to overcome them, as well as protocols for obtaining optimal results. While we focus in our discussion on interfaces in energy storage devices, the methods outlined are broadly applicable to a range of fields where solid-liquid interface play a key role.

Cryogenic Focused Ion Beam (FIB) and Scanning Electron Microscopy (SEM) techniques can provide key insights into the chemistry and morphology of intact solid-liquid interfaces. Methods for preparing high quality Energy Dispersive X-ray (EDX) spectroscopic maps of such interfaces are detailed, with a focus on energy storage devices.

Physical and chemical processes at solid-liquid interfaces play a crucial role in many natural and technological phenomena, including catalysis, solar ...

Microscopic Visualization of Porous Nanographenes Synthesized through a Combination of Solution and On-Surface Chemistry

On-surface synthesis has recently been regarded as a promising approach for the generation of new molecular structures. It has been particularly successful in the synthesis of graphene nanoribbons, nanographenes and intrinsically reactive and instable, yet attractive species. It is based on the combination of solution chemistry aimed at preparation of appropriate molecular precursors for further ultra-high vacuum surface assisted transformations. This approach also owes its success to an incredible development of characterization techniques, such as scanning tunneling/atomic force microscopy and related methods, which allow detailed, local characterization at atomic scale. While the surface-assisted synthesis can provide molecular nanostructures with outstanding precision, down to single atoms, it suffers from basing on metallic surfaces and often limited yield. Therefore, the extension of the approach away from metals and the struggle to increase productivity seem to be significant challenges toward wider applications. Herein, we demonstrate the on-surface synthesis approach for generation of non-planar nanographenes, which are synthesized through a combination of solution chemistry and sequential surface-assisted processes, together with the detailed characterization by scanning probe microscopy methods.

A combination of solution and surface assisted synthesis opens new directions in the atomically precise synthesis of nanostructures. Scanning tunneling microscopy (STM) supplemented by non-contact atomic force microscopy (nc-AFM) enables detailed characterization of newly designed and generated carbon-based nano-objects.

On-surface synthesis has recently been regarded as a promising approach for the generation of new molecular structures. It has been particularly ...

Optimizing Magnetic Force Microscopy Resolution and Sensitivity to Visualize Nanoscale Magnetic Domains

Magnetic force microscopy (MFM) enables mapping local magnetic fields across a sample surface with nanoscale resolution. To perform MFM, an atomic force microscopy (AFM) probe whose tip has been magnetized vertically (i.e., perpendicular to the probe cantilever) is oscillated at a fixed height above the sample surface. The resultant shifts in the oscillation phase or frequency, which are proportional to the magnitude and sign of the vertical magnetic force gradient at each pixel location, are then tracked and mapped. Although the spatial resolution and sensitivity of the technique increases with decreasing lift height above the surface, this seemingly straightforward path to improved MFM images is complicated by considerations such as minimizing topographical artifacts due to shorter range van der Waals forces, increasing the oscillation amplitude to further improve sensitivity, and the presence of surface contaminants (in particular water due to humidity under ambient conditions). In addition, due to the orientation of the probe's magnetic dipole moment, MFM is intrinsically more sensitive to samples with an out-of-plane magnetization vector. Here, high-resolution topographical and magnetic phase images of single and bicomponent nanomagnet artificial spin-ice (ASI) arrays obtained in an inert (argon) atmosphere glovebox with &lt;0.1 ppm O2 and H2O are reported. Optimization of lift height and drive amplitude for high resolution and sensitivity while simultaneously avoiding the introduction of topographical artifacts is discussed, and detection of the stray magnetic fields emanating from either end of the nanoscale bar magnets (~250 nm long and &lt;100 nm wide) aligned in the plane of the ASI sample surface is shown. Likewise, using the example of a Ni-Mn-Ga magnetic shape memory alloy (MSMA), MFM is demonstrated in an inert atmosphere with magnetic phase sensitivity capable of resolving a series of adjacent magnetic domains each ~200 nm wide.

Magnetic force microscopy (MFM) employs a vertically magnetized atomic force microscopy probe to measure sample topography and local magnetic field strength with nanoscale resolution. Optimizing MFM spatial resolution and sensitivity requires balancing decreasing lift height against increasing drive (oscillation) amplitude, and benefits from operating in an inert atmosphere glovebox.

Magnetic force microscopy (MFM) enables mapping local magnetic fields across a sample surface with nanoscale resolution. To perform MFM, an atomic force ...

﻿Analyzing the Heterogeneous Performance of Single Nanoparticles with Electrochemical Methods

Tracking Electrochemistry on Single Nanoparticles with Surface-Enhanced Raman Scattering Spectroscopy and Microscopy

Studying electrochemical reactions on single nanoparticles is important to understand the heterogeneous performance of individual nanoparticles. This nanoscale heterogeneity remains hidden during the ensemble-averaged characterization of nanoparticles. Electrochemical techniques have been developed to measure currents from single nanoparticles but do not provide information about the structure and identity of the molecules that undergo reactions at the electrode surface. Optical techniques such as surface-enhanced Raman scattering (SERS) microscopy and spectroscopy can detect electrochemical events on individual nanoparticles while simultaneously providing information on the vibrational modes of electrode surface species. In this paper, a protocol to track the electrochemical oxidation-reduction of Nile Blue (NB) on single Ag nanoparticles using SERS microscopy and spectroscopy is demonstrated. First, a detailed protocol for fabricating Ag nanoparticles on a smooth and semi-transparent Ag film is described. A dipolar plasmon mode aligned along the optical axis is formed between a single Ag nanoparticle and Ag film. The SERS emission from NB fixed between the nanoparticle and the film is coupled into the plasmon mode, and the high-angle emission is collected by a microscope objective to form a donut-shaped emission pattern. These donut-shaped SERS emission patterns allow for the unambiguous identification of single nanoparticles on the substrate, from which the SERS spectra can be collected. In this work, a method for employing the SERS substrate as a working electrode in an electrochemical cell compatible with an inverted optical microscope is provided. Finally, tracking the electrochemical oxidation-reduction of NB molecules on an individual Ag nanoparticle is shown. The setup and the protocol described here can be modified to study various electrochemical reactions on individual nanoparticles.

Author Spotlight: Tracking Electrochemistry on Single Nanoparticles with Surface-Enhanced Raman Scattering Spectroscopy and Microscopy  

The protocol describes how to monitor electrochemical events on single nanoparticles using surface-enhanced Raman scattering spectroscopy and imaging.

Studying electrochemical reactions on single nanoparticles is important to understand the heterogeneous performance of individual nanoparticles. This ...

Precise Electrochemical Sizing of Individual Electro-Inactive Particles

Nanoimpact electrochemistry enables the time-resolved in situ characterization (e.g., size, catalytic activity) of single nanomaterial units, providing a means of elucidating heterogeneities that would be masked in ensemble studies. To implement this technique with redox inactive particles, a solution-phase redox reaction is used to produce a steady-state background current on a disk ultramicroelectrode. When a particle adsorbs onto the electrode, it produces a stepwise reduction in the exposed electrode area, which produces, in turn, a stepwise decrease in the current commensurate with the size of the adsorbing species. Historically, however, nanoimpact electrochemistry has suffered from &#34;edge effects,&#34; in which the radial diffusion layer formed at the circumference of the ultramicroelectrodes renders the step size dependent not only on the size of the particle but also on where it lands on the electrode. The introduction of electrocatalytic current generation, however, mitigates the heterogeneity caused by edge effects, thus improving the measurement precision. In this approach, termed &#34;electrocatalytic interruption,&#34; a substrate that regenerates the redox probe at the diffusion layer is introduced. This shifts the rate-limiting step of the current generation from diffusion to the homogeneous reaction rate constant, thus reducing flux heterogeneity and increasing the precision of particle sizing by an order of magnitude. The protocol described here explains the set-up and data collection employed in nanoimpact experiments implementing this effect for improved precision in the sizing of redox in-active materials.

As an analytical technique, nanoimpact electrochemistry, an increasingly important approach to counting and characterizing nanometer-scale, electro-inactive particles, suffers from poor precision due to the heterogeneous current distributions that arise from its use of ultramicroelectrodes. Outlined here is a generalized approach, termed "electrocatalytic interruption," that enhances precision in such measurements.

Nanoimpact electrochemistry enables the time-resolved in situ characterization (e.g., size, catalytic activity) of single nanomaterial units, providing a ...

Probing Surface Electrochemical Activity of Nanomaterials using a Hybrid Atomic Force Microscope-Scanning Electrochemical Microscope (AFM-SECM)

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