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How to use a supersharp probe to image nanowire

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Albert posted on Mon, Oct 29 2012 10:45 PM

Hi, All, we have a multimode 8 and we have purchased some DLCS probes from Bruker to image our nanowire samples. We expected to see a 2-3nm nanowire with this 1nm radius tip but the results showed much larger as 6-8nm. I was told by Bruker engineer we prob has got a blunt tip already upon engage and we need to tune engage parameters to preserve the sharpness of the supersharp tips. Can anyone provide a detailed procedure and tips how we should do when using a supersharp probe if it's so easy to get blunt even before we start to image? great thanks in advance!

 

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Mikromasch produce similar probes, and have a set of engage parameters here (approx half way down the page):

http://www.spmtips.com/howto/res/hr

I use similar parameters on a multimode with sharp tips, however I use a lower engage gain (0.1) and also set the pre-engage setpoint to 95%. The most important parameter is "sew tip" this MUST be set to "yes". See the multimode or command reference manual for the nanoscope software for an explanation of the engage parameters and hints for optimizing them. Bear in mind that the engage will be a lot slower than normal with more cautious engage parameters.

Be aware that these tips are extremely fragile, so scanning can damage them too. For best results, you should tune to a relatively small free amplitude (this will depend on the sample and cantilever properties, but ~0.5V is a good starting point) and control at the highest setpoint that still allows you to track the surface. Keep the gains high, and scan slowly. Avoid large scans (more than a few hundred nm) and large amounts of topography - it can be helpful to scan a large area with a regular tip first to find a clean, flat area with features of interest, and then switch to a sharp probe, engaging straight onto the area of interest.

Also, as the carbon spikes are extremely hydrophobic, they are likely to pick up hydrophobic molecules from the surface, so your sample prep must be very clean.

Hope that helps.

Best wishes,

Nic

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Albert replied on Fri, Nov 2 2012 10:23 AM

Many thanks, Nic, your tips very helpful.

Can any Bruker specialist explain the difference between soft tapping and standard tapping? and I tried small amlitude tapping as someone suggested it would help in terms of resolution, but i could not get it to work when I tuned the amplitude below 2nm, anyone have some tips on that? Thanks very much in advance!

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  1. In soft tapping you start with a small free amplitude (Afree) and you use a relatively large setpoint to free amplitude ratio (Asp/Afree as large as possible, but usually greater than about 90%).  Note that Afree should be measured within a few um of the sample since the cantilever is damped during the approach by air confined between the cantilever and sample. 
  2. The drive frequency should be set close to the thermal peak (either directly on the peak, or a little to the left).  It is a good idea to use the thermal tune on surface function to check the location of the peak after engage at a lift height of 100-500nm -- especially if you think there might be long range forces such as electrostatics present. 
  3. Since there is little damping of the cantilever, you usually have to scan slower than usual in order to track the surface.
  4. If the sample is sticky or there are long range attractive forces, it may be impossible to use a very small free amplitude. The symptom is that the feedback will not work correctly and the height trace will look like a sawtooth (tip touches surface and amp goes to zero, causing piezo to retract. Then amp comes back and feedback drives tip into surface, and repeat). This can be helped by using a stiffer cantilever, but using a stiffer cantilever also increases the peak force between tip and sample. One must find a balance where the spring constant is as low as possible, but Afree is as low as possible as well.

I hope that helps!

--Bede

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