PFM on a microsope
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Basically, any microscope working in the contact mode can be converted into PFM provided it is equipped with conducting cantilever, external lock-in amplifier and dc/ac voltage source. The ac signal from the photodiode is imaged along with the regular topography. Electrical cross-talk between input and output signals should be avoided.
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The easiest way to proceed is to make an alternative tip holder which will allow direct biasing of the tip (this will avoid capacitive cross-talk in older microscopes). But you still need to tap into photodiode signal (so you need a break box or equivalent).
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Has anyone had success using 'all metal' cantilevers (such at the ones made by Clayton Williams company) for PFM?
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sundamey: break box = signal access module
dave: to use the max. dynamic range of the LIA, I think you just want to use the smallest sensitivity value that doesn't overload, and then increase by one or two steps to give yourself some headroom so you don't saturate at a particularly active region during the scan. -
Matt: Actually what i was talking about was if you put a 5V signal onto the tip and into the "ref in", it detects the frequency and uses that frequency but then you can control the phase and amplitude for the reference signal and i'm wondering how the amplitude setting on the reference signal affects the output?
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Difficult to say. In most cases, intrinsic noise for commercial AFM system is already pretty low (factor of 10 from thermomechnical limit). External noise depends on building, air handling system, etc.
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So you use an external FG, and run that into the LIA "Ref IN" and what to know what you should spin the wheel in that panel to when you set the Amplitude? I think that particular amplitude is irrelevant when you use the Ref In BNC -- the Amplitude dial only controls the "Sine Out." When you use Ref In, the LIA decodes it and generates a digital internal sine wave of fixed amplitude (that the microscope never sees) which it then mixes with the with experimental input signal.
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KKathan,
We measure the crosstalk (one important form of noise) by driving the potential of the tip when it is far from any surface and watching the deflection. Ideally, far from the surface you should see no signal at the drive frequency. If you do, this will be a background from which you will need to extract the small PFM signal. Since PFM signals are often on the order of picometers, even a small crosstalk can be a serious problem.
The group at ORNL has worked very hard at eliminating this issue in a few other commercial microscopes, Sergei will know the references off the top of his head.
Crosstalk can originate from many sources, including the drive signal coupling into the deflection measurement electronics and/or the "shake" piezo commonly used to mechanically excite vibrations in many microscopes. -
Sergei,
From your experience what are the primary differences in the way PFM is implemented on the most recent microscopes from different vendors? In terms of the electronics specifically. -
ok, anybody.
Is PFM implemented the same way on all commerical microscopes in terms of electronics? Is it simply a matter of getting a signal to the tip and having internal lock-ins and functional generators?
(this is a different question than who does the frequency sweeps and spectroscopy.