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AFM - Atomic Force Microscopy: NANO161, Lab Report 2

1) The document discusses calculating the force constant of an atomic force microscopy cantilever based on its length, width, and thickness using provided equations. The force constant is most affected by thickness and length. 2) An AFM experiment was conducted to investigate how the proportional gain (Ki) parameter impacts images and to determine an optimal Ki value. Force-distance curves were also collected. 3) A contact force of 2.2 μN was calculated from the force-distance curve using the provided equation and estimated cantilever spring constant. Too small a Ki value resulted in overshooting ridges while too large resulted in undershooting. An optimal Ki value produced the best image quality.

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89 views6 pages

AFM - Atomic Force Microscopy: NANO161, Lab Report 2

1) The document discusses calculating the force constant of an atomic force microscopy cantilever based on its length, width, and thickness using provided equations. The force constant is most affected by thickness and length. 2) An AFM experiment was conducted to investigate how the proportional gain (Ki) parameter impacts images and to determine an optimal Ki value. Force-distance curves were also collected. 3) A contact force of 2.2 μN was calculated from the force-distance curve using the provided equation and estimated cantilever spring constant. Too small a Ki value resulted in overshooting ridges while too large resulted in undershooting. An optimal Ki value produced the best image quality.

Uploaded by

wer809
Copyright
© © All Rights Reserved
We take content rights seriously. If you suspect this is your content, claim it here.
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Download as PDF, TXT or read online on Scribd
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NANO161, lab report 2

AFM - Atomic
Force Microscopy
Lab partners: Galina Yavasheva and Sturla Stokken Grøndal

Einar Kjellback
INTRODUCTION AND BACKGROUND

- Calculate the force constant in dependence of l, w and d.

The designer of the experiment ask that one calculates the force constant in dependence of l, w and
d provided the equations

3𝐸𝐼
𝑘= (1)
𝑙3

𝑤 𝑑
𝐼 = ∫ 2𝑤 ∫2𝑑 𝑧 2 𝑑𝑧 𝑑𝑥 (2)
− −
2 2

Solving equation 2 and substituting the result into equation 1 it leads to equation 3 underneath.

𝐸𝑑 3 𝑤
𝑘= 4𝑙 3
(3)

Thus, equation 3 reveals that the force constant, or the stiffness of the cantilever, is drastically
affected by its thickness d and its length l and less by its width w.

1
MATERIALS AND METHOD

In the first part of this experiment, different Ki values were picked in an atomic force microscopy
(AFM) scan to investigate what impact this parameter has on a picture. Furthermore, an optimal
value for Ki is to be chosen based on the quality of the picture that the various Ki values produce. In
the second part, the force-distance curve of the cantilever is investigated to find the contact force
between the sample and the tip. For a detailed description of the experiment please refer to the
booklet of the experiment.

2
RESULTS

Figure 1 – A 10 μm * 10 μm T-B scan done while Figure 2 – A 10 μm * 10 μm topo scan done


varying Ki while varying Ki

Figure 3 – A 10 μm * 10 μm T-B scan done with Figure 4 – A 10 μm * 10 μm topo scan done


optimal Ki value. with optimal Ki value.

3
Figure 5 – A picture scanned in dynamic mode and
projected as a 3D model.

Figure 6 – Force-distance curve of contact mode with a gain of


-1,2383 mV/nm, TB0 of -161,85 mV and TBset 600,17 mV.

4
DISCUSSION AND CONCLUSION

- Calculate the contact the force


The data procured from figure 5 can be put into equation 4, which outputs as the contact
force between the tip and the specimen, provided from the handout describing the
experiment.

𝑘(𝑇𝐵𝑠𝑒𝑡 −𝑇𝐵0 )
𝐹= |𝑔𝑎𝑖𝑛|
(4)

Which gives that F = 2,2 μm given a k value of 3,5 N/m. The reason this value was chosen is because
on the day of the experiment, the type of cantilever used and its force constant was not noted. Thus,
the author used the force constant for a NSC18 cantilever provided by the handout.

The top-most parts of Figure 1 and 2 visibly illustrates the effect of choosing Ki too small. As can be
seen in figure 1, the first lines scanned follow the trend of being dark and gradually becoming
brighter, but without contradicting the notion that this is indeed a grating. This trend becomes less
and less apparent as Ki is increased. A possible explanation for this is as follows: When the cantilever
meets the first ridge it is raised by the PI signal, however the signal is so strong that the cantilever
grossly over-shoots. The remainder of the scan for that line is used to correct this over-shoot, thus
the cantilever inevitably oscillates to an under-shoot. As the Ki value increases over-shooting is only
visible at the edges of the ridges. From the last arrow and down in figure 1 one can observe that the
edges of the ridges gradually become more pronounced. This is due to less and less overshooting.
Caution must be taken though not to set the Ki value too high which can result in an under-shoot for
which the consequences can be a deleterious crash between the tip, or cantilever, and the sample.
The same applies to Kp.

- Comment on commenting on picture processing.

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