Except as specifically noted below, all AFM and KPFM imaging was performed in the Boise State University Surface Science Laboratory (SSL), as was the co-localized scanning confocal Raman microscopy, with co-localized SEM/EDS imaging performed in the Boise State Center for Materials Characterization (BSCMC). The glovebox AFM system used in much of this work was purchased under National Science Foundation Major Research Instrumentation (NSF MRI) Grant Number 1727026, which also provided partial support for PHD and OOM, while the Raman microscope was purchased with funding from the Micron Technology Foundation. The authors thank Micron Technology for the use of their glovebox AFM system in securing preliminary data for the MRI grant, including acquiring the inert atmosphere KPFM images of the binary MgLa alloy shown in Figure 3 of this manuscript. Partial support for OOM and MFH was also provided by NSF CAREER Grant Number 1945650, while CME and MFH acknowledge additional funding from the NASA Idaho Space Grant Consortium EPSCoR Seed Grant. FWD was supported by the Center for Integrated Nanotechnologies, a Department of Energy Office of Basic Energy Sciences user facility. Sandia National Laboratories is a multimission laboratory managed and operated by National Technology and Engineering Solutions of Sandia LLC, a wholly owned subsidiary of Honeywell International Inc., for the U.S. Department of Energy National Nuclear Security Administration under contract DE-NA0003525.
The authors thank Jasen B. Nielsen for preparation of the braze samples for KPFM imaging. The binary MgLa alloy (Figure 3) was provided by Nick Birbilis, formerly of Monash University, Australia, with support from the U.S. Army Research Laboratory (Agreement Number W911NF-14-2-0005). Kari (Livingston) Higginbotham is gratefully acknowledged for her KPFM imaging and analysis contributions to the Cu-Ag-Ti braze sample. Nik Hrabe and Jake Benzing of the National Institute of Standards and Technology (NIST) are acknowledged for helpful discussions, as well as their extensive contributions in preparing (including printing, polishing, and creating nanoindentation fiducials) and performing SEM/EBSD analysis at NIST on the AM Ti-6Al-4V sample while Jake Benzing held a National Research Council Postdoctoral Research Associateship.
This paper describes objective technical results and analysis. Any subjective views or opinions that might be expressed in the paper are those of the author(s) and do not necessarily represent the views of the U.S. Department of Energy, National Aeronautics and Space Administration, National Institute of Standards and Technology, National Science Foundation, or the United States Government.

The authors have no conflicts of interest to disclose.

As KPFM measures surface topography and VPDs with nanoscale resolution, sample preparation is crucial for obtaining high-quality KPFM images. The finely gradated polishing steps discussed in the protocol section are an optimal starting point for achieving a high-quality final surface finish for metal alloys. In addition, examining the surface after each polishing step with an optical microscope can confirm improving surface quality (e.g., reduced number, size, and depth of visible scratches), while finishing with a vibratory polisher will offer the best final surface quality. Finally, one must consider solvent compatibility with the sample and mounting agent when choosing polishing compounds and cleaning methods. In addition to careful sample preparation, co-localizing different characterization techniques requires the use of a common reference (i.e., fiducial mark) to indicate the origin location and XY coordinate axes directions (i.e., sample orientation/rotation) 6,7,32. There are a variety of possible methods to accomplish this. The simplest method is to identify distinct, preexisting features on the surface that can be seen by eye or with the aid of an optical microscope. For this method to work, the feature needs to have a well-defined, easily identifiable origin point (e.g., a corner or protrusion) and exhibit a clear orientation. The CuSil braze sample described here demonstrated micron-scale features meeting these requirements, making co-localization straightforward (Figure 1 and Figure 5) 30. Furthermore, the distinctive visible colors of the two phase-separated regions provided insight into their composition (i.e., copper vs. silver-rich). Perhaps the best, most reproducible method for creating fiducial marks is nanoindentation, although this requires access to a standalone nanoindenter or AFM-integrated nanoindenter system. Nanoindents can be arranged in a variety of ways, but the most obvious is to use one indent as an origin and two additional indents aligned along orthogonal axes to indicate the X and Y directions from the origin, as shown in the AM Ti64 example (Figure 2) 32. Finally, fiducial marks can also be established by scratching or marking the surface (e.g., with a diamond scribe, razor blade, or micromanipulator probe tip; or indelible ink or permanent marker). Scratch fiducials can be beneficial when distinct surface features and/or a nanoindenter are not available; however, these methods can cause issues, particularly when examining corrosion properties (e.g., a scratch can damage the surface, causing it to be susceptible to corrosion). If using a scratch fiducial, one should place the scratch a bit farther from the examined surface to help ensure the scratch does not affect the experimental results. Likewise, contamination from ink may impact corrosion performance, and hence, these methods are better used when studying material properties other than corrosion.
As quantification of the VPD in KPFM depends upon the application of both an AC bias and a DC nulling potential, the path from the sample surface to the AFM chuck must be electrically continuous. Thus, if the sample is somehow electrically insulated from the chuck (e.g., it has a backside oxide coating, is deposited on a non-conducting substrate, or is covered by epoxy), then a connection will need to be made. One solution is to use silver paste (see the Table of Materials) to draw a line from the top surface of the sample to the chuck, ensuring the line has no breaks and is completely dry prior to imaging. Copper tape or conductive carbon tape can also be used to create a similar electrical connection. Regardless of the method used to establish the electrical connection, the chuck-sample continuity should be checked with a multimeter prior to KPFM imaging.
Oxidation or contamination of a metal surface leads to drastic changes in measured VPDs. Minimizing the amount of oxygen the sample comes into contact with can slow down surface passivation or degradation. One way to prevent oxidation is by placing the AFM in an inert atmosphere glovebox. By replacing the oxygen-rich ambient environment with an inert gas such as argon or nitrogen, the sample surface can be maintained in a relatively pristine condition for an extended period (Figure 3). An additional benefit of employing a glovebox is the elimination of surface water, which can introduce dissolved contaminants, accelerate corrosion or passivation, and degrade the resolution due to the need for increased lift heights (see below). Additionally, the measured VPD has been shown to be sensitive to the relative humidity15,23, and it is, therefore, important to monitor (and ideally report) the relative humidity if KPFM experiments are performed under ambient conditions.
Depending on the AFM used (see the Table of Materials) and the KPFM implementation mode employed, the available imaging parameters and nomenclature will vary. However, some general guidelines can be formulated. KPFM combines AFM topography with VPD measurements. Thus, a good topography image is an essential first step, with a setpoint chosen to minimize the tip-sample force (and hence, the potential for tip wear and sample damage) while still maintaining high-fidelity tracking of the topography (through optimizing the interplay of the gains and setpoint). In other words, regardless of the topography imaging mode, the user must determine a balance between sufficient interaction with the surface without damaging the sample or probe (particularly if it is metal-coated). Additionally, if the sample is dirty or not polished well, the probe tip may come into contact with debris, resulting in a broken tip or tip artifacts. It is also imperative to avoid topographical artifacts in the KPFM Volta potential channel, which is more easily achieved in a dual pass KPFM mode such as the one described here. Optimal KPFM imaging requires a balance between lower and higher lift heights, as the lateral resolution of KPFM decreases with increasing lift height, but short-range van der Waals forces (which are responsible for the tip-sample interactions that underpin AFM topography measurements) can produce instabilities that affect the measurement of the longer-range electrostatic interaction at lower lift heights. Working in an inert atmosphere glovebox as described above can be beneficial in this regard, as elimination of the layer of surface water removes its contribution to the tip-sample interaction for improved feedback, thereby enabling lower KPFM lift heights and improved spatial resolution, with the additional benefit of more reproducible VPDs due to constant (essentially zero) humidity and reduced charge screening. Likewise, decreased surface roughness (i.e., better polishing) can enable lower lift heights and result in improved KPFM resolution, as a good rule of thumb to avoid topographical artifacts is to set the lift height approximately equal to the height of the tallest high aspect ratio surface feature(s) present within the scan region. Another factor that comes into play in determining the optimal lift height is the probe oscillation amplitude during the lift mode pass-larger amplitude confers greater sensitivity to small VPDs, but at a cost of necessitating larger lift heights to avoid topographical artifacts or striking the surface (often visible as abrupt spikes in the lift scan phase). Again, the smoother the surface, the lower the lift height that can be achieved for a given oscillation amplitude, thereby improving both spatial resolution and Volta potential sensitivity-good sample preparation is key. Finally, when capturing a KPFM image, one should keep in mind that a larger scan size allows for more sample coverage but at the cost of increased scan time, as slow scan rates are required to allow the accurate measurement of Volta potentials by the detection electronics.
Inference about the relative nobility of microstructures observed on the surface of a conductive material can be made from VPDs measured using KPFM (e.g., microgalvanic couples, intergranular corrosion, pitting corrosion). However, the absolute Volta potentials of materials reported in the literature vary widely18,24,27. This lack of reproducibility has resulted in misinterpretations about different materials systems and their corrosion behavior23,25. As a result, for the determination of absolute Volta potentials (i.e., work functions) or comparison of VPDs measured across labs, probes, or days, calibration of the KPFM probe&#39;s work function relative to an inert material (e.g., gold) is essential25,48. A 2019 study by some of the authors examined different KPFM probes and showed the variability of the resulting measured VPD between those probes and an aluminum-silicon-gold (Al-Si-Au) standard. Differences in work function were even observed for individual probes of the same nominal material and design (Figure 12)25. As a proof of concept, the 316L stainless steel joined together by a CuSil braze previously referenced was used as an exemplary material for measuring absolute VPDs or work functions. The data from the 2016 work by Kvryan et al.30 was compared with KPFM VPDs obtained on the same sample with a variety of probes and used to analyze the inner-braze Volta potentials. By calibrating the probe work function using the Au portion of the Al-Si-Au standard as a reference work function, the repeatability of the measured VPD of the braze phases improved by over an order of magnitude, from several hundred millivolts (Figure 12A) to tens of millivolts (Figure 12C). Further improvements in calibration can be realized by directly measuring the work function of the inert reference (e.g., via photoemission spectroscopy or Auger electron spectroscopy) or calculating the work function using density functional theory25,48.
<img alt="Figure 12" class="xfigimg" src="/files/ftp_upload/64102/64102fig12v2.jpg" /> 
Figure 12: Effect of probe calibration on KPFM Volta potential reproducibility. (A) VPDs for copper-rich and silver-rich regions within the CuSil braze sample obtained relative to three different PFQNE-AL probes. (B) VPDs for the same three probes relative to the gold portion of the Al-Si-Au standard presented on the left ordinate axis, with resulting modified PFQNE-AL work function values presented on the right ordinate axis, as calculated from density functional theory. (C) Absolute VPDs of the copper-rich and silver-rich regions obtained by scaling the measured VPDs relative to the gold of the Al-Si-Au standard imaged prior to imaging of the braze sample. The left ordinate axis (calculated using the equation above panel C) indicates the VPD between the braze sample phases and the gold standard. The right ordinate axis (calculated using the equation below panel C) presents the resultant modified work function for each phase based on the modified work function of the probe calculated in panel B. This figure is reproduced from Efaw et al.25. Abbreviations: KPFM = Kelvin probe force microscopy; VPD = Volta potential difference. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/64102/64102fig12largev2.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
In conclusion, the co-localization of KPFM Volta potential maps with advanced SEM techniques, including SE images, BSE images, EDS elemental composition maps, and EBSD inverse pole figures can provide insight into structure-property-performance relationships. Likewise, other nano- to microscale characterization techniques such as scanning confocal Raman microscopy can be co-localized as well to provide further structural insight. However, when co-localizing multiple characterization tools, sample preparation is crucial, including both minimizing surface roughness and debris, as well as identifying or creating reliable fiducial markers to indicate the sample imaging origin and axes (i.e., orientation or rotation). Additionally, the potential impact of a given characterization technique on subsequent measurements must be taken into consideration, and for this reason, it is preferable that KPFM (which is both nondestructive and highly sensitive to surface contamination) be performed first prior to other characterization methods. Finally, it is important to minimize surface contaminants, take into account and monitor (or better yet, eliminate) the confounding effects of the testing environment (e.g., ambient humidity), and properly calibrate the work function of the KPFM probe to enable the reliable, meaningful comparison of KPFM Volta potential measurements reported in the literature. To this end, the use of an inert atmosphere glovebox to house the AFM system (or, if not available, employing another form of humidity control/low-moisture environment) and a gold or other inert reference material standard with a well-characterized work function for probe calibration are recommended.

Microscopic characterization of materials is fundamentally important for understanding and developing new materials. Numerous microscopy methods provide maps of material surfaces and their properties, including topography, elasticity, strain, electrical and thermal conductivity, surface potential, elemental composition, and crystal orientation. However, the information provided by one microscopy modality is often insufficient to fully understand the collection of properties that may be contributing to the material behavior of interest. In some cases, advanced microscopes have been constructed with combined characterization capabilities, such as an inverted optical microscope platform that incorporates an atomic force microscope (AFM) or utilizing multiple scanning probe modalities (e.g., Kelvin probe force microscopy [KPFM] or intermodulation electrostatic force microscopy [ImEFM1], surface potential measurements, and magnetic force microscopy [MFM])2,3,4,5 to characterize a sample on the same AFM. More generally, one would like to combine the information from two separate microscopes to obtain structure-property correlations6,7. The co-localization of scanning Kelvin probe force microscopy with scanning electron and Raman-based microscopies and spectroscopies is presented here to illustrate a process for correlating information obtained from two or more separate microscopes by way of a specific application example, namely, multi-modal characterization of metal alloys to understand corrosion behavior.
Corrosion is the process by which materials react chemically and electrochemically with their environment8. Electrochemical corrosion is a spontaneous (i.e., thermodynamically favorable, driven by a net decrease in free energy) process involving electron and charge transfer that occurs between an anode and a cathode in the presence of an electrolyte. When corrosion occurs on a metal or alloy surface, anodic and cathodic regions develop based on variations in the composition of microstructural features in a process known as micro-galvanic corrosion9. Through the use of co-localized, nanoscale characterization techniques, the methods described here provide an experimental route to identify likely micro-galvanic couples between a wide variety of alloy microstructural features, providing potentially helpful insight for corrosion mitigation and the development of new materials. The results of these experiments can determine which microstructural features at the alloy surface are likely to serve as local anode sites (i.e., sites of oxidation) or cathodes (i.e., sites of reduction) during active corrosion, as well as provide new insight into the nanoscale features of corrosion initiation and reactions.
KPFM is an AFM-based scanning probe microscopy (SPM) characterization technique that can generate simultaneous (or line-by-line sequential) topography and Volta potential difference (VPD) maps of a sample surface with resolutions in the order of 10 nanometers and millivolts, respectively10. To accomplish this, KPFM utilizes a conductive AFM probe with a nanoscale tip. Typically, the probe first tracks the topographical variations in the sample surface, then lifts to a user-defined height above the sample surface before retracing the topography line to measure the VPD between the probe and the sample (i.e., the relative Volta potential of the sample surface). Although there are multiple ways of practically implementing KPFM measurements, fundamentally, the determination of the VPD is carried out by simultaneously applying both an AC bias (in the presented implementation, to the probe) and a variable DC bias (in the presented implementation, to the sample) to null the tip-sample potential difference as indicated by nulling the oscillation of the probe at the applied AC bias frequency (or its heterodyne-amplified sum and difference frequencies on either side of the probe&#39;s natural mechanical resonance frequency) 11. Regardless of the implementation method, KPFM produces correlated high lateral spatial resolution topography and VPD maps across a metallic surface12.
The VPD measured via KPFM is directly correlated to the difference in work function between the sample and probe, and furthermore, the VPD (generally) trends with the electrode potential in solution13,14,15. This relationship can be used to determine the expected (local) electrode behavior of microstructural features based on the VPD and has been explored for a number of metal alloy corroding systems15,16,17,18,19,20,21,22. Additionally, the measured VPD is sensitive to local composition, surface layers, and grain/crystal/defect structure, and, hence, provides nanoscale elucidation of the features that are expected to initiate and drive corrosion reactions on a metal surface. It should be noted that the VPD (&#936;) is related to, but distinct from, the (non-measurable) surface potential (&#967;), as described in greater detail in the literature13,14, including helpful diagrams and precise definitions of correct electrochemistry terminology23. Recent advancements in the application of KPFM to corrosion studies have greatly increased the quality and repeatability of acquired data through careful consideration of the influence of sample preparation, measurement parameters, probe type, and external environment24,25,26,27.
One drawback of KPFM is that, while it generates a nanoscale resolution map of the surface VPD, it provides no direct information regarding composition, and, thus, the correlation of variations in VPD to differences in elemental composition must be provided by co-localization with complementary characterization techniques. By co-localizing KPFM with SEM, energy dispersive spectroscopy (EDS), electron backscattered diffraction (EBSD), and/or Raman spectroscopy, such compositional and/or structural information can be determined. However, co-localizing nanoscale techniques can be difficult due to the extreme magnification of the imaging, differences in field of view and resolution, and sample interactions during characterization28. Obtaining nano- to microscale images of the same region of a sample on different instruments requires high precision and careful planning to co-localize techniques and minimize artifacts due to possible cross-contamination during sequential characterization18,28.
The aim of this article is to define a systematic method for co-localizing KPFM and SEM imaging, the latter of which can be substituted for by other characterization techniques such as EDS, EBSD, or Raman spectroscopy. It is necessary to understand the proper ordering of characterization steps, the environmental effects on KPFM resolution and measured VPDs, KPFM probe calibration, and various strategies that can be employed to successfully co-localize SEM or other advanced microscopy and spectroscopy techniques with KPFM. Accordingly, a step-by-step generalized procedure for co-localizing SEM with KPFM is provided, followed by exemplary works of such co-localization along with helpful tips and tricks to obtain meaningful results. More generally, the procedure described here should serve to outline a broadly applicable process for co-localizing images/property maps obtained from other microscopy modalities with KPFM and other AFM modes to obtain useful structure-property relationships in a variety of material systems6,7,29,30,31,32.

<table><tbody><tr><td>Atomic force microscope</td><td>Bruker</td><td>Dimension Icon</td><td>Uses Nanoscope control software, PF-KPFM module/key enabled</td></tr><tr><td>Colloidal silica polish</td><td>Leco</td><td>812-121-300</td><td>Abrasive: 0.08 &amp;mu;m (80 nm). Used as a finishing polish for metals. Great when preparing samples for performing high resolution EBSD.</td></tr><tr><td>Conductive silver paint, Pelco</td><td>Ted Pella</td><td>16062</td><td>Other products with similar conductivity can be used (e.g., Pelco #16031 or 16034), but this product combines fast ambient drying, low VOC, high mechanical strength, easy cleanup/removal, and relatively low sheet resistance: https://www.tedpella.com/adhesive_html/Adhesive-Comparison.aspx</td></tr><tr><td>Diamond slurry</td><td>Buehler</td><td>MetaDi Supreme, Polycrystalline Diamon Suspension&amp;nbsp;</td><td>Final steps in polishing the sample. Start with 1 &amp;mu;m, then move to 0.05 &amp;mu;m (50 nm).</td></tr><tr><td>Digital Multimeter</td><td>Fluke</td><td>Fluke 21 Multimeter</td><td>For checking continuity from the AFM stage/chuck to the sample surface, confirming proper grounding and biasing, etc.</td></tr><tr><td>Epoxy</td><td>Buehler</td><td>EpoThin 2</td><td>4:1 ratio of resin to hardener. Mixed together and used for mounting samples to help with polishing and experiments.&amp;nbsp;</td></tr><tr><td>Ethanol</td><td>Sigma Aldrich</td><td>459828</td><td>200 proof, spectrophotometric grade. Used to clean samples after polishing and/or prior to imaging.&amp;nbsp;</td></tr><tr><td>Glovebox, inert atmosphere</td><td>MBraun</td><td>LabMaster Pro MB200B + MB20G gas purification unit</td><td>Custom design (leaktight electrical feedthroughs, vibration isolation, acoustical noise and air current minimization, etc.) and depth for use with Bruker Dimension Icon AFM, 3 gloves, argon atmosphere</td></tr><tr><td>Image overlap software</td><td>Microsoft</td><td>PowerPoint</td><td>Other software products can be used as desired depending upon user knowledge. The essential software capabilities needed are translation, rotation, and scaling of images, as well as ideally adjustment of image transparency during overlay of KPFM/other microscopy images.</td></tr><tr><td>KPFM probe</td><td>Bruker</td><td>PFQNE-AL</td><td>Have also tried Bruker SCM-PIT and SCM-PIC probes, as well as solid Pt probes from Rocky Mountain Nanotechnology, but have found PFQNE-AL probes to provide superior performance</td></tr><tr><td>KPFM standard</td><td>Bruker</td><td>PFKPFM-SMPL</td><td>8 mm x 8 mm silicon wafer patterned with a 3 x 9 array of rectangular islands of aluminum (50 nm thick) surrounded by gold (50 nm thick). Mounted on a 15 mm steel disk with top surface gold layer electrically connected to disk.</td></tr><tr><td>Nanoindenter</td><td>Hysitron</td><td>TS 75</td><td>Nanoindented additively manufactured Ti-6Al-4V samples in a right triangle pattern to create an origin and XY axes for co-localized imaging.</td></tr><tr><td>Nanscope Analysis</td><td>Bruker</td><td>Version 2.0</td><td>Free AFM image processing and analysis software package, but proprietary, designed for, and limited to Bruker AFMs; similar functionality is available from free, platform-independent AFM image processing and analysis software packages such as Gwyddion, WSxM, and others</td></tr><tr><td>Polisher</td><td>Allied</td><td>MetPrep 3</td><td>Used during slurry polishing</td></tr><tr><td>Probe holder</td><td>Bruker</td><td>DAFMCH</td><td>Specific to the particular AFM used, but must provide a direct electrical path from the probe to the instrument; DAFMCH is the standard contact and tapping mode probe holder for the Dimension Icon AFM, suitable for KPFM</td></tr><tr><td>Raman microscope, scanning confocal</td><td>Horiba</td><td>LabRAM HR Evolution</td><td>Scanning confocal Raman microscope with 442 nm, 532 nm, and 633 nm excitation wavelengths/lasers (used 532 nm doubled Nd:YAG); 10x, 20x, 50x, and 100x Olympus objectives; 50-250 mm adjustable confocal pinhole, 0.8 m imaging spectrometer with 600 and 1800 line/mm gratings; TE cooled 256 x 1024 CCD array detector; and 80 mm x 100 mm Marzhauser motorized XYZ stage plus DuoScan mirror capabilities for scanning</td></tr><tr><td>Sample Puck</td><td>Ted Pella</td><td>16218</td><td>Product number is for 15 mm diameter stainless steel sample puck. Also available in 6 mm, 10 mm, 12 mm, and 20 mm diameters at https://www.tedpella.com/AFM_html/AFM.aspx#anchor842459</td></tr><tr><td>Scanning electron microscope</td><td>Hitachi</td><td>S-3400N-II</td><td>Located at Boise State. Used to perform co-localized SEM/EDS on all samples except additively manufactured (AM) Ti-6Al-4V.</td></tr><tr><td>Scanning electron microscope</td><td>Zeiss</td><td>Leo</td><td>Field Emission SEM. Located at NIST&#039;s Boulder, CO, campus. Used to provide co-localized SEM/EBSD on the AM Ti-6Al-4V samples.</td></tr><tr><td>Silicon carbide grit paper (abrasive discs)</td><td>Allied</td><td>120 grit: 50-10005, 400 grit: 50-10025, 800 grit: 50-10035, 1200 grit: 50-10040</td><td>Polished samples progressively from ANSI standard 120 grit to 1200 grit prior to employing any slurries. Note that ANSI standard 120 grit corresponds to P120 (European), while ANSI standard 1200 grit corresponds to P4000 (European) - i.e., the ANSI (US Industrial Grit) and European FEPA (P-Grading) abrasives characterization standards agree at coarse grits, but diverge numerically for finer abrasives.</td></tr><tr><td>Sonicator</td><td>VWR (part of Avantor)</td><td>97043-992</td><td>Used to clean samples via sonication after polishing.</td></tr><tr><td>Ultrahigh purity nitrogen (UHP N&lt;sub&gt;2&lt;/sub&gt;), 99.999%</td><td>Norco</td><td>SPG TUHPNI - T</td><td>T size compressed gas cylinder of ultrahigh purity (99.999%) nitrogen for drying samples</td></tr><tr><td>Variable Speed Grinder</td><td>Buehler</td><td>EcoMet 3000</td><td>Used with silicon carbide grit papers during hand polishing.</td></tr><tr><td>Vibratory polisher</td><td>Buehler</td><td>AutoMet 250 Grinder Polisher</td><td>Used to polish samples for longer periods of time. Automatic polishing.</td></tr></tbody></table>

co-localizing kelvin probe force microscopy with other microscopies and spectroscopies: selected applications in corrosion characterization of alloys

Kelvin probe force microscopy (KPFM), sometimes referred to as surface potential microscopy, is the nanoscale version of the venerable scanning Kelvin probe, both of which measure the Volta potential difference (VPD) between an oscillating probe tip and a sample surface by applying a nulling voltage equal in magnitude but opposite in sign to the tip-sample potential difference. By scanning a conductive KPFM probe over a sample surface, nanoscale variations in surface topography and potential can be mapped, identifying likely anodic and cathodic regions, as well as quantifying the inherent material driving force for galvanic corrosion.
Subsequent co-localization of KPFM Volta potential maps with advanced scanning electron microscopy (SEM) techniques, including back scattered electron (BSE) images, energy dispersive spectroscopy (EDS) elemental composition maps, and electron backscattered diffraction (EBSD) inverse pole figures can provide further insight into structure-property-performance relationships. Here, the results of several studies co-localizing KPFM with SEM on a wide variety of alloys of technological interest are presented, demonstrating the utility of combining these techniques at the nanoscale to elucidate corrosion initiation and propagation.
Important points to consider and potential pitfalls to avoid in such investigations are also highlighted: in particular, probe calibration and the potential confounding effects on the measured VPDs of the testing environment and sample surface, including ambient humidity (i.e., adsorbed water), surface reactions/oxidation, and polishing debris or other contaminants. Additionally, an example is provided of co-localizing a third technique, scanning confocal Raman microscopy, to demonstrate the general applicability and utility of the co-localization method to provide further structural insight beyond that afforded by electron microscopy-based techniques.

Watch this Scientific Journal Video about Co-localizing Kelvin Probe Force Microscopy with Other Microscopies and Spectroscopies: Selected Applications in Corrosion Characterization of Alloys at JoVE.

Co-localizing Kelvin Probe Force Microscopy with Other Microscopies and Spectroscopies: Selected Applications in Corrosion Characterization of Alloys

Kelvin probe force microscopy (KPFM) measures surface topography and differences in surface potential, while scanning electron microscopy (SEM) and associated spectroscopies can elucidate surface morphology, composition, crystallinity, and crystallographic orientation. Accordingly, the co-localization of SEM with KPFM can provide insight into the effects of nanoscale composition and surface structure on corrosion.

Kelvin probe force microscopy (KPFM), sometimes referred to as surface potential microscopy, is the nanoscale version of the venerable scanning Kelvin ...

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2.5K Views.  Boise State University. Kelvin-sondekrachtmicroscopie, of KPFM, meet oppervlaktetopografie en verschillen in oppervlaktepotentiaal op nanoschaal, terwijl scanning-elektronenmicroscopie, of SEM, samenstelling, kristaliniteit en kristallografische oriëntatie kan ophelderen. Het colocaliseren van SEM of andere microscopietechnieken met KPFM kan directe identificatiemateriaalstructuur mogelijk maken, eigenschapsprestatierelaties die ontoegankelijk zijn via een enkele techniek. Colocalisatie van SEM of andere microscopi...

Video: Co-localizing Kelvin Probe Force Microscopy with Other Microscopies and Spectroscopies: Selected Applications in Corrosion Characterization of Alloys

Scanning-probe Single-electron Capacitance Spectroscopy

The integration of low-temperature scanning-probe techniques and single-electron capacitance spectroscopy represents a powerful tool to study the electronic quantum structure of small systems - including individual atomic dopants in semiconductors. Here we present a capacitance-based method, known as Subsurface Charge Accumulation (SCA) imaging, which is capable of resolving single-electron charging while achieving sufficient spatial resolution to image individual atomic dopants. The use of a capacitance technique enables observation of subsurface features, such as dopants buried many nanometers beneath the surface of a semiconductor material1,2,3. In principle, this technique can be applied to any system to resolve electron motion below an insulating surface.
As in other electric-field-sensitive scanned-probe techniques4, the lateral spatial resolution of the measurement depends in part on the radius of curvature of the probe tip. Using tips with a small radius of curvature can enable spatial resolution of a few tens of nanometers. This fine spatial resolution allows investigations of small numbers (down to one) of subsurface dopants1,2. The charge resolution depends greatly on the sensitivity of the charge detection circuitry; using high electron mobility transistors (HEMT) in such circuits at cryogenic temperatures enables a sensitivity of approximately 0.01 electrons/Hz&frac12; at 0.3 K 5.

Scanning-probe single-electron capacitance spectroscopy facilitates the study of single-electron motion in localized subsurface regions. A sensitive charge-detection circuit is incorporated into a cryogenic scanning probe microscope to investigate small systems of dopant atoms beneath the surface of semiconductor samples.

The integration of low-temperature scanning-probe techniques and single-electron capacitance spectroscopy represents a powerful tool to study the ...

Quantitative and Qualitative Examination of Particle-particle Interactions Using Colloidal Probe Nanoscopy

Colloidal Probe Nanoscopy (CPN), the study of the nano-scale interactive forces between a specifically prepared colloidal probe and any chosen substrate using the Atomic Force Microscope (AFM), can provide key insights into physical interactions present within colloidal systems. Colloidal systems are widely existent in several applications including, pharmaceuticals, foods, paints, paper, soil and minerals, detergents, printing and much more.1-3 Furthermore, colloids can exist in many states such as emulsions, foams and suspensions. Using colloidal probe nanoscopy one can obtain key information on the adhesive properties, binding energies and even gain insight into the physical stability and coagulation kinetics of the colloids present within. Additionally, colloidal probe nanoscopy can be used with biological cells to aid in drug discovery and formulation development. In this paper we describe a method for conducting colloidal probe nanoscopy, discuss key factors that are important to consider during the measurement, and show that both quantitative and qualitative data that can be obtained from such measurements.

Colloidal probe nanoscopy can be used within a variety of fields to gain insight into the physical stability and coagulation kinetics of colloidal systems and aid in drug discovery and formulation sciences using biological systems. The method described within provides a quantitative and qualitative means to study such systems.

Colloidal Probe Nanoscopy (CPN), the study of the nano-scale interactive forces between a specifically prepared colloidal probe and any chosen substrate ...

Chemistry

Potentiodynamic Corrosion Testing

Different metallic materials have different polarization characteristics as dictated by the open circuit potential, breakdown potential, and passivation potential of the material. The detection of these electrochemical parameters identifies the corrosion factors of a material. A reliable and well-functioning corrosion system is required to achieve this.
Corrosion of the samples was achieved via a potentiodynamic polarization technique employing a three-electrode configuration, consisting of reference, counter, and working electrodes. Prior to commencement a baseline potential is obtained. Following the stabilization of the corrosion potential (Ecorr), the applied potential is ramped at a slow rate in the positive direction relative to the reference electrode. The working electrode was a stainless steel screw. The reference electrode was a standard Ag/AgCl. The counter electrode used was a platinum mesh. Having a reliable and well-functioning in vitro corrosion system to test biomaterials provides an in-expensive technique that allows for the systematic characterization of the material by determining the breakdown potential, to further understand the material's response to corrosion. The goal of the protocol is to set up and run an in vitro potentiodynamic corrosion system to analyze pitting corrosion for small metallic medical devices.

Here, we present a protocol to set up and run an in vitro potentiodynamic corrosion system to analyze pitting corrosion for small metallic medical devices.

Different metallic materials have different polarization characteristics as dictated by the open circuit potential, breakdown potential, and passivation ...

Environment

Force Spectroscopy of Single Protein Molecules Using an Atomic Force Microscope

The determination of the folding process of proteins from their amino acid sequence to their native 3D structure is an important problem in biology. Atomic force microscopy (AFM) can address this problem by enabling stretching and relaxation of single protein molecules, which gives direct evidence of specific unfolding and refolding characteristics. AFM-based single-molecule force-spectroscopy (AFM-SMFS) provides a means to consistently measure high-energy conformations in proteins that are not possible in traditional bulk (biochemical) measurements. Although numerous papers were published to show principles of AFM-SMFS, it is not easy to conduct SMFS experiments due to a lack of an exhaustively complete protocol. In this study, we briefly illustrate the principles of AFM and extensively detail the protocols, procedures, and data analysis as a guideline to achieve good results from SMFS experiments. We demonstrate representative SMFS results of single protein mechanical unfolding measurements and we provide troubleshooting strategies for some commonly encountered problems.

We describe the detailed procedures and strategies to measure the mechanical properties and mechanical unfolding pathways of single protein molecules using an atomic force microscope. We also show representative results as a reference for selection and justification of good single protein molecule recordings.

The determination of the folding process of proteins from their amino acid sequence to their native 3D structure is an important problem in biology. ...

Biochemistry

Surface Potential Measurement of Bacteria Using Kelvin Probe Force Microscopy

Surface potential is a commonly overlooked physical characteristic that plays a dominant role in the adhesion of microorganisms to substrate surfaces. Kelvin probe force microscopy (KPFM) is a module of atomic force microscopy (AFM) that measures the contact potential difference between surfaces at the nano-scale. The combination of KPFM with AFM allows for the simultaneous generation of surface potential and topographical maps of biological samples such as bacterial cells. Here, we employ KPFM to examine the effects of surface potential on microbial adhesion to medically relevant surfaces such as stainless steel and gold. Surface potential maps revealed differences in surface potential for microbial membranes on different material substrates. A step-height graph was generated to show the difference in surface potential at a boundary area between the substrate surface and microorganisms. Changes in cellular membrane surface potential have been linked with changes in cellular metabolism and motility. Therefore, KPFM represents a powerful tool that can be utilized to examine the changes of microbial membrane surface potential upon adhesion to various substrate surfaces. In this study, we demonstrate the procedure to characterize the surface potential of individual methicillin-resistant Staphylococcus aureus USA100 cells on stainless steel and gold using KPFM.

Here, we present a protocol explaining the use of Kelvin probe force microscopy as a tool for generating high resolution nano-scale surface potential maps. This tool was applied to assess the role of surface potential on the binding capacity of microorganisms to substrate surfaces.

Surface potential is a commonly overlooked physical characteristic that plays a dominant role in the adhesion of microorganisms to substrate surfaces. ...

Bioengineering

Imaging Corrosion at the Metal-Paint Interface Using Time-of-Flight Secondary Ion Mass Spectrometry

Corrosion developed at the paint and aluminum (Al) metal-paint interface of an aluminum alloy is analyzed using time-of-flight secondary ion mass spectrometry (ToF-SIMS), illustrating that SIMS is a suitable technique to study the chemical distribution at a metal-paint interface. The painted Al alloy coupons are immersed in a salt solution or exposed to air only. SIMS provides chemical mapping and 2D molecular imaging of the interface, allowing direct visualization of the morphology of the corrosion products formed at the metal-paint interface and mapping of the chemical after corrosion occurs. The experimental procedure of this method is presented to provide technical details to facilitate similar research and highlight pitfalls that may be encountered during such experiments.

Time-of-flight secondary ion mass spectrometry is applied to demonstrate the chemical mapping and corrosion morphology at the metal-paint interface of an aluminum alloy after being exposed to a salt solution compared with a specimen exposed to air.

Corrosion developed at the paint and aluminum (Al) metal-paint interface of an aluminum alloy is analyzed using time-of-flight secondary ion mass ...

Covalent Attachment of Single Molecules for AFM-based Force Spectroscopy

Atomic force microscopy (AFM)-based single molecule force spectroscopy is an ideal tool for investigating the interactions between a single polymer and surfaces. For a true single molecule experiment, covalent attachment of the probe molecule is essential because only then can hundreds of force-extension traces with one and the same single molecule be obtained. Many traces are in turn necessary to prove that a single molecule alone is probed. Additionally, passivation is crucial for preventing unwanted interactions between the single probe molecule and the AFM cantilever tip as well as between the AFM cantilever tip and the underlying surface. The functionalization protocol presented here is reliable and can easily be applied to a variety of polymers. Characteristic single molecule events (i.e., stretches and plateaus) are detected in the force-extension traces. From these events, physical parameters such as stretching force, desorption force and desorption length can be obtained. This is particularly important for the precise investigation of stimuli-responsive systems at the single molecule level. As exemplary systems poly(ethylene glycol) (PEG), poly(N-isopropylacrylamide) (PNiPAM) and polystyrene (PS) are stretched and desorbed from SiOx (for PEG and PNiPAM) and from hydrophobic self-assembled monolayer surfaces (for PS) in aqueous environment.

Covalent attachment of probe molecules to atomic force microscopy (AFM) cantilever tips is an essential technique for the investigation of their physical properties. This allows us to determine the stretching force, desorption force and length of polymers via AFM-based single molecule force spectroscopy with high reproducibility.

Atomic force microscopy (AFM)-based single molecule force spectroscopy is an ideal tool for investigating the interactions between a single polymer and ...

Atomic Force Microscopy Combined with Infrared Spectroscopy as a Tool to Probe Single Bacterium Chemistry

Atomic Force Microscopy-Infrared Spectroscopy (AFM-IR) is a novel combinatory technique, enabling simultaneous characterization of physical properties and chemical composition of sample with nanoscale resolution. By combining AFM with IR, the spatial resolution limitation of conventional IR is overcome, enabling a resolution of 20&#8211;100 nm to be achieved. This opens the door for a broad array of new applications of IR toward probing samples smaller than several micrometers, previously unachievable by means of conventional IR microscopy. AFM-IR is eminently suited for bacterial research, providing both spectral and spatial information at the single cell and intracellular level. The increasing global health concerns and unfavorable future prediction regarding bacterial infections, and especially, rapid development of antimicrobial resistance, has created an urgent need for a research tool capable of phenotypic probing at the single cell and subcellular level. AFM-IR offers the potential to address this need, by enabling detail characterization of chemical composition of a single bacterium. Here, we provide a complete protocol for sample preparation and data acquisition of single spectra and mapping modality, for the application of AFM-IR toward bacterial studies.

Atomic Force Microscopy-Infrared Spectroscopy (AFM-IR) provides a powerful platform for bacterial studies, enabling to achieve nanoscale resolution. Both, mapping of subcellular changes (e.g., upon cell division) as well as comparative studies of chemical composition (e.g., arising from drug resistance) can be conducted at a single cell level in bacteria.

Atomic Force Microscopy-Infrared Spectroscopy (AFM-IR) is a novel combinatory technique, enabling simultaneous characterization of physical properties and ...

Parallel Active Cantilever Arrays in AFMS to Enable High-Throughput Inspections

Active Probe Atomic Force Microscopy with Quattro-Parallel Cantilever Arrays for High-Throughput Large-Scale Sample Inspection

An Atomic Force Microscope&#160;(AFM) is&#160;a powerful and versatile tool for nanoscale surface studies to capture 3D topography images of samples. However, due to their limited imaging throughput, AFMs have not been widely adopted for large-scale inspection purposes. Researchers have developed high-speed AFM systems to record dynamic process videos in chemical and biological reactions at tens of frames per second, at the cost of a small imaging area of up to several square micrometers. In contrast, inspecting large-scale nanofabricated structures, such as semiconductor wafers, requires nanoscale spatial resolution imaging of a static sample over hundreds of square centimeters with high productivity. Conventional AFMs use a single passive cantilever probe with an optical beam deflection system, which can only collect one pixel at a time during AFM imaging, resulting in low imaging throughput. This work utilizes an array of active cantilevers with embedded piezoresistive sensors and thermomechanical actuators, which allows simultaneous multi-cantilever operation in parallel operation for increased imaging throughput. When combined with large-range nano-positioners and proper control algorithms, each cantilever can be individually controlled to capture multiple AFM images. With data-driven post-processing algorithms, the images can be stitched together, and defect detection can be performed by comparing them to the desired geometry. This paper introduces principles of the custom AFM using the active cantilever arrays, followed by a discussion on practical experiment considerations for inspection applications. Selected example images of silicon calibration grating, highly-oriented pyrolytic graphite, and extreme ultraviolet lithography masks are captured using an array of four active cantilevers (&#34;Quattro&#34;) with a 125 &#181;m tip separation distance. With more engineering integration, this high-throughput, large-scale imaging tool can provide 3D metrological data for extreme ultraviolet (EUV) masks, chemical mechanical planarization (CMP) inspection, failure analysis, displays, thin-film step measurements, roughness measurement dies, and laser-engraved dry gas seal grooves.

Author Spotlight: Introduction to Active Probe Atomic Force Microscopy with Quattro-Parallel Cantilever Arrays

Large-scale sample inspection with nanoscale resolution has a wide range of applications, especially for nanofabricated semiconductor wafers. Atomic force microscopes can be a great tool for this purpose, but are limited by their imaging speed. This work utilizes parallel active cantilever arrays in AFMs to enable high-throughput and large-scale inspections.

An Atomic Force Microscope&#160;(AFM) is&#160;a powerful and versatile tool for nanoscale surface studies to capture 3D topography images of samples. ...

Near Simultaneous Laser Scanning Confocal and Atomic Force Microscopy (Conpokal) on Live Cells

Techniques available for micro- and nano-scale mechanical characterization have exploded in the last few decades. From further development of the scanning and transmission electron microscope, to the invention of atomic force microscopy, and advances in fluorescent imaging, there have been substantial gains in technologies that enable the study of small materials. Conpokal is a portmanteau that combines confocal microscopy with atomic force microscopy (AFM), where a probe &#8220;pokes&#8221; the surface. Although each technique is extremely effective for the qualitative and/or quantitative image collection on their own, Conpokal provides the capability to test with blended fluorescence imaging and mechanical characterization. Designed for near simultaneous confocal imaging and atomic force probing, Conpokal facilitates experimentation on live microbiological samples. The added insight from paired instrumentation provides co-localization of measured mechanical properties (e.g., elastic modulus, adhesion, surface roughness) by AFM with subcellular components or activity observable through confocal microscopy. This work provides a step by step protocol for the operation of laser scanning confocal and atomic force microscopy, simultaneously, to achieve same cell, same region, confocal imaging, and mechanical characterization.

Near Simultaneous Laser Scanning Confocal and Atomic Force Microscopy Conpokal on Live Cells

Presented here is the protocol of the Conpokal technique, which combines confocal and atomic force microscopy into a single instrument platform. Conpokal provides same cell, same region, near simultaneous confocal imaging and mechanical characterization of live biological samples.

Techniques available for micro- and nano-scale mechanical characterization have exploded in the last few decades. From further development of the scanning ...

Co-localizing Kelvin Probe Force Microscopy with Other Microscopies and Spectroscopies: Selected Applications in Corrosion Characterization of Alloys

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