Force Volume Troubleshooting

If a force volume image is uniform at all Z display values (that is, all slices), check the various display parameters first. The settings may be too high or too low. For example, if the Data Scale is much smaller (or larger) than the actual range of cantilever deflections at any Z position, then all the pixels at the Z position will be to the low (or high) end of the scale. The same effect occurs if Data Center fails to bring the measured forces into the Volume scale range. Adjust Data Scale and Data Center until the variation in forces is displayed.

If FV Channel/Z display is set too high, force curves appear as flat lines; if it is too low, the majority of force curves display flattened against one or the other extreme of Z position.

Check the color bar. If the contrast is too high, only the extremes of deflection show up in the force volume image. Changing the Color Table in Image Mode can reduce contrast (different color tables have different default contrast levels).

If Sensitivity was not set properly in Force Plot Mode, then force plot scaling is inaccurate.

If a trigger is being used, ensure that it is enabled, and vary Trig threshold to assure that the threshold deflection is large enough. If it is too low, only the noncontact portion of the curve is measured. For example, a 320 µm cantilever with a spring constant of 0.01 N/m oscillates with an amplitude of roughly 3 nm at room temperature. These oscillations and a 2 nm Trig threshold initiate the trigger almost immediately after beginning a scan.

All the interesting forces may be mapped to a single pixel of Z data; that is, the Z resolution is too low for the forces of interest. Trade off XY resolution (i.e., Force per line or Samples per line or both) for Z resolution (Number of samples) to alleviate the problem.

If no adjustment to display parameters creates a non-uniform image, perhaps there is no force variability in the scan area. Adjust Z scan start and Ramp size in the Z Scan panel to scan the regions of interest within the force curves. It may be necessary to return to Force Plot Mode to do this. If the probe cantilever is too stiff to respond to the forces present, replace it with a more pliant cantilever or use a higher Trig threshold.

Although drift is a symptom that has a number of causes, it appears in Force Volume in two characteristic ways. The force curves, over time, can drift vertically or horizontally in the force plots region. The shades of the force volume image drift to one end of the color bar, as do the pixel values in the height image. Horizontal, or substrate, drift is due to the substrate or Z piezoelectric actuator drifting. Vertical, or cantilever, drift can result from DC electrical drift in the probe, thermal fluctuations, bubbles attached to the cantilever in fluid or slipping of the O-ring in a fluid cell - to name a few sources. Even small rates of drift can distort a force volume image during the hours needed to collect such a data set.

If the drift rate is low, it can be ignored during data collection and subtracted out offline by a horizontal or vertical cursor and selecting X or Y Translate. A relative trigger keeps the magnitude of probe deflections within a fixed range.

If the drift is slow to moderate (relative to the time scale of the entire collection process), adjust FV Channel/Z display upwards to keep the curves from drifting out of view. Reset the Setpoint and collect the image. Set Force Channel/Center Plot to Enable to zero the force curves as they are taken.

If the drift is fast, check the hardware for mechanical causes. Ensure that the probe is secure in its holder. Thermal fluctuations near and within a probe can cause fast drift. Flexible cantilevers make good thermometers and respond to the slight temperature increases due to laser heating. To minimize thermal drift, bring sample, fluid (if any), probe and probe holder into thermal equilibrium (i.e., the same temperature) at the start of imaging.

If the force curves appear nonsensical, the extending and retracting portions reading maximum force along their entire length, FV scan rate may be set too low for the NanoScope DSP board. A FV scan rate of 6 Hz ensures smooth signal averaging. Too low a FV scan rate and the force curves will appear discretized.

If imaging in fluid, bubbles in the fluid cell or adsorbed to the cantilever can result in fast drift rates. Bubbles can be removed by injecting more solution into the fluid cell and forcing the bubbles out of the cell through a tube. If the O-ring was subjected to torque in making a seal, it can relax and slide across the sample surface. This is remedied by minimizing lateral movement of the O-ring during set-up, or by not using an O-ring at all (though, then the probe is more sensitive to air currents and thermal variation).

CAUTION: Not using an O-ring is not recommended by Bruker. Leaks are hard to avoid and can quickly lead to expensive damage to the high voltage piezoelectric scanner tube.

As a last resort, if fast drift cannot be removed from the system, reset the setpoint (Feedback > Setpoint) during data collection to keep the force curves “in bounds”. Capture must be forced (click on the Capture icon twice in quick succession) to save the image. Another way to compensate for fast drift during imaging is to track the drift with the photodetector adjustment screws during the retrace portion of a line scan. The banding in the resulting height and force volume images can be removed offline by setting Center plot to Enabled.

Excessive adhesion of the tip to the sample results in the absence of the noncontact portion of the force curves as well as, in some cases, a downward curvature in the extending portion of the curve towards and meeting the retracting curve at the end of the noncontact region (see Figure 1). This is caused by the inability of the tip to leave the surface as it is retracted. This can damage the sample during the repositioning of the tip to the next XY location. Adjust Z scan start and Ramp size in the Z Scan panel to ensure that the tip is lifted clear of the surface during retraction. Check that most piezoelectric actuator motion occurs near its center position. Adjust Trig threshold if the tip is being pushed so far into the sample that adhesive forces on the tip are unnecessarily large.

Figure 1: Example of Excessive Adhesion

Occurring only in fluid cells, hydrodynamic drag appears as a vertical separation between the noncontact portions of the extending and retracting curves on the force plot. The magnitude of the separation is dependent on the rate of the scan; reduce Z Scan/Scan Rate until the curves are co-linear in the noncontact region.