NetNotes
I agree with what Steve had to say and have some additional comments. A sample of a square centimeter or so is probably not too large. T e conductive media should make a continuous electrical connection between the bare metal stub and the upper surface and continuing onto the upper surface of the sample. I oſt en use Leit-C carbon adhesive from my friendly microscopy supply house because it combines electrical conductivity with excellent mechanical strength that materials like colloidal graphite (DAG) and silver paint don’t have. Silver paint can also be problematic in EDS analysis. — T e power that fi eld emission SEM off ers is incredibly bright source combined with a stable low voltage beam. Secondary electron imaging (SEI) at low voltage provides the best topographical analysis of surface roughness. Charge control in SEI is achieved by balancing the number of incident electrons entering the sample and electrons leaving as secondary and backscattered electrons. I highly recommend the book “Scanning Electron Microscopy and X-ray Microanalysis”, 3rd Edition by Goldstein et al., Springer, 2013. T e chapter on Procedures for Elimination of Charging in Nonconducting Specimens should be very helpful. — My 25 years of experience in low voltage, fi eld emission SEM of polymers leads me to believe that some of the imaging problems experienced with leather may be similar to those observed in polymers. Both are insulators and consist of low atomic weight elements. Like Steve, I suggest you start at around 1 kV accelerating voltage. Use the spot size that the service engineers use to check your spatial resolution at 1 kV: this spot size is a good place to start because the manufacturer knows it gives an excellence balance of resolution and signal-to-noise. For best results, I suggest you work with an uncoated sample. You might start with a coated sample then move to the bare, uncoated sample. Gold and gold-palladium coatings will obscure possibly important surface information. Faster scan rates produce less charging than longer scan rates. If a single scan allows you to control charging but is somewhat noisy, average several scans recorded using shorter scan rates. Do a few quick tests on fresh areas of the sample to see how long a scan rate you can use without producing charging. Be aware that your default image acquisition scan rate may be much longer than the ideal scan rate for your sample. If this happens, record your images at the preferred rate. Fortunately, sub-micrometer surface texture and porosity (as I would expect to see in collage-based leather surfaces) dissipate charging thus making imaging easier. Gary Brown
microscopy.gmb@
gmail.com Wed Oct 7
LM: demonstrating the fi eld and aperture
For those of you teaching microscopy courses, it can be challenging to explain the fi eld and aperture diaphragm and how they aff ect the cone of light emerging from the condenser. On a whim, I fi lled a cuvette with fl uorescein in order to visualize the cone of light. T e demonstration proved quite successful, so I made a movie of the demonstration, showing how the fi eld diaphragm aff ects the width of the cone without impacting its angle, and the aperture diaphragm impacts the angle of the cone without aff ecting its width. Having the students see the cone also went a long way in helping them understand why axial resolution goes down with a lower NA. Here is the link to the movie:
https://youtu.be/06CQ6IIaDWs T e cuvette also allowed me to show the hollow cone generated in phase contrast and darkfi eld:
http://imgur.com/hswMRjH http://imgur.com/Zl87dcr T e sectored ray in oblique illumination:
http://imgur.com/oerMkk0 As well as the solid cone in brightfi eld:
http://imgur.com/KZP3Sv0 http://imgur. com/akeiI0T Small disclaimer, the oblique was a “poor man’s” oblique done my misaligning the darkfi eld annulus and partially closing the aperture diaphragm, but I also like to show “poor man’s” oblique to
2016 January •
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students to show how thinking a little outside the box can allow you to get the most out of your microscope. Ben Smith
benjamin.smith@
ou.edu Wed Sep 23 Well done, Ben. T ese are such valuable lessons to learn. Your movie was especially eff ective. When we are on the road, we oſt en use a business card for this demonstration, placed perpendicular to the stage. Also, I have a piece of screwdriver handle stock (about 1“ in diameter), polished on one end, and placed on a slide with a drop of oil under it. In the old days, we used to use uranium glass. I understand that Jerry Sedgewick (
jerry@imagingandanalysis.com) has a new supplier, so that approach may also come back into vogue. As for “poor man’s oblique”: unless one has a condenser with an Abbe slider, I doubt that any of us could have done better. No apologies necessary! So here’s question to put to your students: What impact does off -setting the zero order have on resolution? Barbara Foster
bfoster@the-mip.com T u Sep 24
EM: converting diffusion pumps to Fomblin Could someone please share, or point me to good resource on
choosing fl uids for diff usion pumps? Maybe even cross-reference table for Fomblin and hydrocarbon oils? I am trying to return an old Electroscan SEM back to life and considering charging Fomblin instead of mineral oil into its Varian M-2 diff usion pumps. Valery Ray vray@partbeam-
systech.com Tue Sep 15
Chapter 5 of my book “Vacuum Methods in Electron Microscopy” (Wilbur C. Bigelow. Portland Press, 1994) contains a rather extensive discussion of the characteristics of various common diff usion pump fl uids, plus a discussion of the factors to consider in choosing a fl uid. Wil Bigelow
bigelow@umich.edu Tue Sep 15
Fomblin is great if you are pumping something reactive like
oxygen or fl uorine. But for an EM you just want something that is low backstreaming. Look at something like Santovac 5, that is what is used in JEOL scopes. It is pretty spendy. Also look at adding an alumina- media fore line trap, this will help stop mechanical pump oil from back streaming into the scope during roughing and contaminating the Santovac. I did the expensive route and replaced the diff usion pump with a Varian turbo backed with an Edwards oil-less scroll pump. I think those two pumps are worth more than my SEM. Jerry Biehler
jerry.biehler@
gmail.com Tue Sep 15
TEM: fl uctuating room temperature
We noticed changes of temperature in the TEM room, so I installed a thermometer. T e temperature fl uctuates by approx. 3°C (21.4-24.1°C) in the day or overnight. We have a Tecnai G20 and a powerful air conditioning system. I know that temperature should be as constant as possible but I don’t know how much it is important. What inconve- nience should I expect with such a temperature fl uctuation? What are the specifi cations of this instrument? Stephane Nizets nizets2@yahoo. com T u Sep 3 T e specifi cations of your microscope allow a maximum temperature change of 1ºC/24 h, 0.5ºC/h, or 0.1ºC/min. One problem you might experience because of “very” fast temperature changes is specimen driſt . Guenter Resch
lists@nexperion.net T u Sep 3 Sure the room temperature may have an eff ect on TEM perfor- mance over time, but that mainly relates to the power supplies and electronics. Provided your room temperature falls within the manufacturer’s specifi cations you should not have a problem. I have worked with instruments in hot countries well outside the recommen- dations and still not had problems. If we consider the average exposure time, then temperature stability over that time is not a problem.
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