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Hi, I work with a Veeco Bioscope II microscope. I measured a force curve on a flat surface. Also I observed "Friction" and "Sum Signal" signals at the same point. I repeated measurements with different probes and different types of cantilevers (short, long, triangular, or straight beam). Always I observed lateral deflection down as it is shown below on the image. How could I explain such behavior of the lateral deflection data? Thank you.
Beat regards,
Arturas
It's hard to see your data clearly, but it doesn't look like your lateral deflection data is random from what I can see. I think you will find that it can be understood in terms of how the cantilever experiences a torque when pressed against surfaces with different slopes. This issue has been looked at quite a bit as it relates to calibrating LFM. A nice reference that I found that illustrates the principles is Asay and Kim's 2006 paper in RSI (http://dx.doi.org/10.1063/1.2190210). I'll borrow a few figures to help explain here, but please take a look at the original paper for more details.
As the paper discusses, there can be several contributions to lateral deflection. On a sloped surface, a simple force balance quickly shows how there will be both a vertical and a lateral component to the force. As I mentioned in my previous post, any offset of the tip from the centerline of the cantilever will also result in lateral deflection, even if the force curve is perfectly normal to the substrate. Less obvious is that the photodetector can also be slightly misaligned relative to the cantilever.
Figure 8 from the above reference nicely shows the force balance, including the contribution from an off-axis tip:
Their figures 3 and 4 show examples of how sloped surfaces will general different lateral force curves:
This is the ideal case where the lateral deflection is dominated by the sloped surface. But if the contributions from tip offset or detector misalignment are large then you could see lateral force curves like the ones you show where the lateral force goes in the same direction in each case, just to a different degree.
The curves that you show actually look quite nice. The BioScope II was frankly not our best AFM for force curves because the unusual laser path sometimes resulted in baseline tilt in the force curves. It's certainly possible that detector misalignment could be contributing to your measurements, but that should not interfere with interpreting them.
Regards,
-Ben
It's normal to get some torsional bending of the cantilever during a Z-ramp force curve. All it takes is some small offset between the tip location and the centerline of the cantilever to generate a torque when the tip presses on the sample. There can also be imperfections in the alignment of the photodetector that would cause some coupling. But looking at the relative sizes of the vertical deflection versus lateral deflection (and sum too) you see that these combined effects are quite small here. It looks like your vertical deflection must be roughly 5-6V and the lateral deflection change is only about 150mV, or about 3% of your vertical deflection. This is well within the range that could arise from a tip offset.
Note that the sign of the lateral deflection channel is fairly arbitrary since the cantilever can bend either direction. If the tips are uniformly offset to one side of the cantilever then you would expect to consistently see the same sign on the lateral deflection signal change. But a different wafer of probes could have a different offset, which could change both the sign and magnitude of the lateral deflection.
Hi, thank for your reply. I expected to use lateral deflection of a cantilever to explain force imaging results. Please find enclosed image with measurements.
Shortly, there is an sample with nearly spherical symmetry and I have measured force curves at different points. Results on the first plot show nice symmetrical spreading of force curves. The second image illustrates how points have been chosen and the position of a probe with x, y measurements directions. The hardness of the sample must be the same through all surface, but I have got nice deflection as shown by blue (upper points on the image) and green (lower points on the image) colors. Red points correspond to the central line.
Fresh measurements are shown at the bottom (force and friction measurements). I expected that due to slope the cantilever could be deflected laterally. Though as shown at bottom it seems friction data shows that the cantilever deflects more accidentally while force data again shows quite predictable deflection from the central part.
Please could you explain why it happens? Can be this effect due to a equipment ? The microscope is Bioscope II.
Best regards,
Thank you a lot. I think the article explains my results. Best regards, Arturas