Hi, I want an electronic circuit to construct a "conducting afm". Can you please help me in this?


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orksecurity7 years ago
What is a "conducting afm" supposed to do? Description or citation, please...
Abrasive flow machining ? Nice process.
.  It's amazing what one can find when looking for something else. Serendipity FTW!
sagarjan12 (author)  NachoMahma7 years ago
Sorry for the abbreviation. It's Atomic Force Microscopy only!
A conducting AFM is one operating mode of an AFM.  Most times, you use the AFM to detect the atomic force (eg van der Waals) between the tip and the surface.

In c-AFM mode, the current passes through the tip into the sample.  By monitoring the current, you can detect local variations in conductivity on the sample.

Here are some links on c-AFM, from two of the big players in the field:
http://www.veeco.com/conductive-afm-option-mode-cafm/index.aspx
http://www.asylumresearch.com/Applications/Orca/Orca.shtml

c-AFM and STM are different.  In STM, the tip does not come into contact with the sample.  The electrons pass from the tip to the sample by tunneling across the vacuum space between the tip and the sample.

To answer the original question: Building a c-AFM module is fairly tricky, mainly because of the requirements for low noise amplifiers.  The links above may help, but really, if you're looking for something that can give you publishable data in a reasonable amount of time, you might have to just bite the bullet and get a commercial module.

Disclaimer: I'm not affiliated with any of the companies above. :)
AIUI the tip NEVER touches the sample in either technique, the difference is sensing. Your first link seems pretty clear on that point too.

The tips, being atomically pointy do not like being touched to anything.

c-AFM - dropping into a new jargon, outputs the tunneling current, which has to be a function of spacing and sample conductivity.

Damned clever stuff.

Steve
Well, it depends on how you define touching, as atoms aren't really hard spheres.  A working definition of touching, in the AFM sense, is when the atoms get close enough to feel a force, ie when the electron clouds of the atoms start overlapping.  Because of this overlap, in c-AFM, electrons can simply travel along the overlapping clouds to get from one atom to the next (hence conduction happens).

Incidentally, AFM tips aren't atomically pointy, but actually are rounded, and have radii of curvature of around a few nanometers (ie a few atoms wide).

In STM, the electron clouds of the atoms in the tip and the atoms in the surface don't overlap - there is a distinct barrier between the tip and surface electron clouds.  In this barrier, you can't have any electrons - if you think of them as classical physics sense.  But once you bring in quantum mechanics, the electrons can essentially 'hop' across this barrier, and thereby get a current flow between the tip and the surface.
sagarjan12 (author)  origamiwolf7 years ago

Your explanation is very useful to understand the concept. Thank you.. :-)

sagarjan12 (author)  origamiwolf7 years ago
Really a valuable answer, Thank you very much... And please do consider my actual need which i have given below and help me.
I'd go for Nacho's first suggestion.

How can you have a conducting atomic force microscope ? Isn't that an STM ?
sagarjan12 (author)  steveastrouk7 years ago

First let me thank you people.
Well, let me explain the stuff, (people, please consider this also as my question) actually in order to make a conducting atomic force microscope first I have to make a bias voltage setup to make the current (in the order of nano ampere (nA)) to pass through my sample via the afm tip.
Currently I am seeking for a circuit which gives current in the range of nA to pA (pico ampere). (And my bias voltage would be in the order of 100mV). Can you help me in finding out such circuits? Please…

You want a constant current SOURCE, or you want to apply a bias VOLTAGE and MEASURE the current ? 

The confusion is about "Gives current"

Wish I had time to build an AFM or STM too.

Steve
The usual operating mode for AFM and c-AFM is to have a voltage bias, and measure the conduction current.  The voltage bias is typically in the mV to V range.  Noise and thermal drift would be the main obstacles when measuring nA currents.

Google scholar lists some articles on precision voltage references and nanoamp sensing:

Precision voltage reference
Nanoamp current sensing

Most of the articles would require subscription, but there are some freely downloadable pdfs around.

Good luck. :)

sagarjan12 (author)  origamiwolf7 years ago

Thanks people! From all your comments and ideas, I clearly have understood that I still should have clear idea about the stuff!!! OK, please help me in this, can you tell me how a bias voltage is applied to the afm tip and sample and how to measure the current? Can you please give me a clear picture and texts of this?
After Origami's comments, I stand by my original opinion, and suggest you search on "DIY STM" in Google. There are some references there with everything you need - circuits, BOMs and PCB layouts.

Have fun.

Steve
sagarjan12 (author)  steveastrouk7 years ago
Sure, Thank you! :-)
A good place to start would be the Veeco technical library:

http://www.veeco.com/library/default.aspx

There are many articles and application notes there.  Here's an introductory guide, for example:

http://www.veeco.com/pdfs/library/SPM_Guide_0829_05_166.pdf
(which also explains the difference between an AFM and STM)

Not to sound discouraging, but building an AFM is significantly more complex than building an STM.  Also, the physical properties probed by an AFM are somewhat different than those probed by an STM, so which one you build will depend on what you're trying to measure.


sagarjan12 (author)  origamiwolf7 years ago
Sure, I will check everything. Thank you! :-)
sagarjan12 (author)  steveastrouk7 years ago
yes! I want to apply a bias voltage between the tip and the sample and have to measure the current...