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

Kelvin probe force microscopy, or KPFM, measures surface topography and differences in surface potential at the nano scale, while scanning electron microscopy, or SEM, can elucidate composition, crystalinity, and crystallographic orientation. Colocalizing SEM or other microscopy techniques with KPFM can enable direct identification material structure, property performance relationships inaccessible via a single technique. Colocalization of SEM or other microscopy techniques with KPFM can provide insight into the effects of nanoscale composition and surface structure on corrosion initiation and propagation mechanisms.

KPFM probe calibration and fiducials marking the region of interest, origin, and orientation are crucial to the success of this method. A glove box to minimize humidity is also highly beneficial. Demonstrating the procedure will be Olivia Maryon, a current doctoral student in Professor Mike Hurley's applied electric chemistry and corrosion laboratory, a former undergraduate AFM researcher from my laboratory.

To begin, prepare samples to meet the dimensional requirements of the AFM and other characterization tools to be employed. Use optical microscopy to determine if the polish is sufficient and ensure that the sample has virtually no visible scratches on the surface. Implement the desired colocalization method to create an origin and axes.

Make sure the sample is smooth enough on the bottom to seal against the AFM stage's sample chuck vacuum, exhibits minimal surface roughness with no loose debris, and provides a conductive path from the base to the top surface. To do this, load the sample on the chuck and turn on the chuck vacuum using the on off lever switch. Apply a thin line of conductive silver paste to provide a continuous electrical path from the sample to the chuck.

Once the silver paste has dried, use a multimeter to ensure the top surface of the sample has good continuity to the sample stage. Open the AFM control software. In the Select Experiment window that opens, select the appropriate experiment category, experiment group, and experiment.

Then click Load Experiment to open the desired workflow. Once the experiment workflow has opened, click on Setup in the workflow. While wearing conductive gloves to prevent electrostatic discharge, carefully mount and secure a conductive AFM probe on the appropriate probe holder.

Install the probe holder on the AFM head, taking care to first discharge any static buildup by touching the side of the AFM enclosure before aligning the holes on the probe holder with the contact pins on the AFM head. In the Probe Setup menu, ensure the probe type being used is displayed. If necessary, click Select Probe and choose the correct probe type from the dropdown menu.

Then click Return and save changes. In the Focus Tip menu, bring the end of the cantilever into focus using the focus control's up and down arrows. Adjust the focus speed, optical zoom, and video illumination as needed.

Align the crosshair over the tip location by clicking on the optical image at the location corresponding to the tip's position beneath the cantilever based on the known setback of the tip from the distal end of the cantilever. Using the laser alignment knobs on the AFM head, optimize the laser alignment by aiming the laser onto the center of the back of the probe cantilever toward the distal end and centering the reflected beam on the position-sensitive detector, or PSD, to maximize the sum voltage while minimizing the vertical and horizontal deflections. Select the Navigate window in the AFM control software workflow, and move the probe over the sample using the stage movement X-Y control arrows.

Bring the sample surface into focus using the scan head up and down arrows. Then use the stage movement X-Y control arrows again to locate the designated origin and move to the region of interest. Use the stage movement X-Y control to position an easily identifiable feature directly beneath the probe tip.

Once over the feature, zoom in and correct for the parallax induced by the side-mounted camera optics by clicking Calibrate in the toolbar then selecting Optical and Optics SPM Axis Colinearity. Walk through the colinearity calibration steps by clicking Next. Align the crosshairs over the same distinctive feature in each of the presented optical images before clicking Finish.

Then click Navigate in the software workflow to continue. Locate the designated origin, and align the X and Y coordinate axes accordingly, centering the probe tip over the origin. To enable repeatable navigation to the desired region of interest and colocalization with other characterization techniques, note the X and Y position values shown at the bottom of the software window.

Click Stage in the toolbar, and select Set References. While over the designated origin, click Mark Point As Origin under Define Origin to zero the X and Y location values. Then move the probe to the desired ROI, and note the distance from the origin to the ROI displayed as the X and Y values at the bottom of the screen.

If using an ambient system, close and lock the acoustic hood in closing the AFM. Select the Check Parameters workflow window, and ensure the default initial imaging parameters are acceptable. Go to the microscope settings in the toolbar.

Select Engage Settings, and ensure the default engage parameters are acceptable, modifying them if desired. Click the Engage button in the workflow to engage on the surface. Monitor the engage process to ensure that the tip engages properly.

Once engaged, switch the display type of the force curve from force versus time to force versus Z by right clicking on the curve and selecting Switch Display Type. Optimize the AFM topography and KPFM parameters in the Parameters window of the scan interface. After defining an appropriate directory path and file name under Capture, click Capture File Name.

Click the capture icon to set up the capture the desired next complete image. Then click Withdraw in the workflow once the image has been captured. Ensure that the sample inhibits charging.

If the sample is insufficiently conductive, consider carbon coating prior to imaging. Load the sample into the SEM chamber. Close and pump down the chamber.

Turn the electron beam on using the Beam On button, and zoom out optically using the magnification knob to obtain the maximum field of view of the sample surface. Locate the designated origin, then zoom in using the magnification knob. Orient the X and Y axes according to the fiducial markers by inputting values into the stage rotation in Tilt Options.

Zoom in as needed, and capture the desired images of the designated ROI, and save the files. Use appropriate software for each characterization tool to process the raw data as needed. Save and export the acquired KPFM and SEM images in the desired file format.

After opening the KPFM data file, apply a first-order plane fit to the AFM topography channel of the KPFM images to remove the sample tip and tilt, as well as a first-order flatten if needed to compensate for any line-to-line offsets due to probe wear or picking up debris on the probe tip. Select the desired color scheme or gradient for the KPFM images by first selecting the potential channel thumbnail on the left of the AFM topography image then double-clicking on the color scale bar on the right of the KPFM Volta potential difference map to open the Image Color Scale Adjust window to the Choose Color Table tab. In the Modified Data Scale tab of the Image Color Scale Adjust window, enter appropriate minimum and maximum values in the scale bar range for the KPFM VPD image.

Repeat this process for the AFM topography image after first reselecting the height sensor channel thumbnail image. Save journal-quality exports of the processed AFM topography image and KPFMV VPD map as image files. Open the processed AFM topography image and KPFM VPD map, along with the raw SEM image, in the image manipulation software of choice.

Identify the specified origin in both the AFM KPFM data and SEM images. Overlay the origins in the two images. Then rotationally align the images using the X and Y coordinate axes designated by the chosen fiducial markers or characteristic features.

Scale the images as needed. An asymmetric pattern of three nano indents was created and used as fiducial markers to enable colocalization of KPFM and SEM EBSD. The origin indent is indicated in the SEM images by a triangle with the two axes'indents indicated by circles.

High-resolution colocalized imaging was then performed on the region outlined by the solid rectangle. Inclusion of one of the fiducial indents marked by a circle allowed precise overlap of the backscattered electron SEM and AFM topography images. The resultant EBSD crystallographic orientation and KPFM Volta potential maps could then be colocalized as well.

As indicated by the arrows, line scans across the same sample regions in the EBSD and KPFM maps enabled correlation of differences in crystallographic orientation with small changes in measured Volta potential. Confocal Raman microscopy showed that the tetragonal-rich zirconium oxide was preferentially located near the metal oxide interface. Colocalized KPFM found this tetragonal-rich oxide to be significantly more active than the adjacent more noble bulk monoclinic-rich zirconium oxide region.

Similarly, KPFM mapping across the bright cathodic particle embedded in the zirconium metal showed a large increase in the relative Volta potential, which also correlated with a significant change in the Raman spectrum. Easily identifiable fiducial marks in step 2.2 are key for colocalization. To avoid potential sample damage or contamination, KPFM should typically be performed before other characterization methods in step four.

In addition to electron and Raman microscopies, other complementary micro to nanoscale characterization techniques, including fluorescence-based super resolution microscopy, can be colocalized with KPFM or other advanced scanning probe microscopy modes. Conducting KPFM in a low-moisture inert-atmosphere glove box to control humidity and surface moisture can improve KPFM's spatial resolution and reproducibility of measured Volta potentials.