[Spm] Tapping in Liquid - Phase vs. Amplitude images

Tue Apr 15 18:41:42 EDT 2008

Hi Don,

As to your first question: Taking into account most modern AFM systems allow
the acquisition of many channels at a time, and hard disks are far below
1$/GB, I'd consider it silly not to record the phase when you have it
available. On older systems (like the Nanoscope IIIa's in our lab) you will
have to make a choice. I don't see why the arguments that go into this
choice should be different for air or liquid. If you feel phase imaging is a
useful technique in air, I am curious to hear why you are uncertain it will
be useful in liquid. Of course, not all samples will show relevant
differences in tip-sample interaction, but if they are present it is more
likely to show up in the phase than in the amplitude, assuming you have a
well-tuned feedback. Under that same assumption the amplitude image will
only show features very similar to the derivative of the topography in the
scanning direction. In the case of feature sizes exceeding the dynamic range
of the topography image (very small structure on top of big features) the
amplitude image may contain additional information not present in the
topography. In any case amplitude variations will show up in the phase
image, too. It is possible to deconvolve this to some extent, but it can be
difficult. 

As to the second question: a good way to find the real resonance frequency
is to get an AFM with some type of direct-drive excitation, such as magnetic
excitation. Using the Brownian motion spectrum of your cantilever is just as
good (at least to find the resonance). Some AFM software allows you to do a
thermal tune, when not imaging, and there's some workarounds for others. But
I can recommend having a spectrum analyzer (we use a Stanford Research
770FFT, but many more options are possible) connected to your photodiode
difference signal all the time during your measurement. Once you've learnt
how to read it, it's amazing how much you can tell about what is happening
to your tip from the oscillation spectrum. Be aware that if you want to see
the thermal spectrum in liquid, using the stiff tips usually used for
tapping in air puts demands on your optical lever detection system that are
not often met by commercially available AFMs. We usually use either AC240
(~2N/m, 30 kHz in liquid) or Biolever (~0.02 N/m, 8 kHz) tips from Olympus,
or the 'F' triangular lever on the Veeco Microlevers (0.5 N/m, 35 kHz) for
tapping in liquid.

Tuning your drive just below the resonance is useful because of the
amplitude behavior, not so much because of the phase. In theory, tuning
slightly below resonance should set you on the net repulsive part of the
tip-sample interaction, tuning slightly above on the net attractive part.
Which of these you'd prefer depends on your goals, but what happens in
practice may depend on strongly on the details of settings and instrument.
In any case, the low quality factor in liquid and the large influence of
squeeze damping make this less of an issue. In contrast to most 'autotuning'
software I do not think that choosing a frequency that coincides with a
resonance of the liquid cell is very useful unless the driving amplitude
demands it.

Hope this helps,

Allard Katan

Post-doctoral researcher

Bio-AFM lab/Interface Physics group

Kamerlingh Onnes Laboratory

Leiden University

  _____  

From: Don Chernoff at ASM [mailto:donc at asmicro.com] 
Sent: Friday, April 11, 2008 13:24
To: SPM List
Subject: [Spm] Tapping in Liquid - Phase vs. Amplitude images

My questions today concern whether phase imaging in liquid is useful and how
to set it up.

1) Is phase imaging in liquid a useful technique, as it is in air?  Does it
have significant advantages over amplitude imaging?

As background, consider this:

A 1997 paper from Helen Hansma and colleagues reports an impressive set of
observations using tapping mode phase imaging to visualize DNA molecules in
liquid.  [see 

http://www.pubmedcentral.nih.gov/picrender.fcgi?artid=147043
<http://www.pubmedcentral.nih.gov/picrender.fcgi?artid=147043&blobtype=pdf>
&blobtype=pdf]

They reported:

Phase imaging is optimal at lower imaging forces than height

imaging (Fig. 3). This is a significant advantage for phase

imaging over height imaging, since there is less damage to

biological samples at lower imaging forces. In addition, phase

images remained clear at a 3- to 6-fold faster scan rate than height

images, 27.5 Hz in these experiments (Fig. 2), corresponding to

a tip speed of 110 mm/s. With faster scan rates images can be

captured more quickly for observing processes in the AFM.

...

 We have not experimented

systematically with the amplitude images of moving DNA, but

those who can do amplitude imaging ... might find that 

amplitude imaging is also useful for imaging moving

DNA at faster scan speeds and lower forces.

===

With the perspective of almost 11 years since that publication, what is the
situation of phase vs amplitude imaging in liquid?

2) When doing phase imaging in air, it is easy to find the fundamental
frequency of the cantilever and (we think) it is important to set the drive
frequency a little bit lower than the resonant frequency.  In liquid, it can
be hard to choose the correct resonance.  Assuming you can choose it, is
there a benefit to adjusting the drive frequency slightly in order to make
the phase response more reproducible?  Are there any other hints the
experienced users can give?

regards,
Don
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