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NetNotes


well as for resin-embedded specimens. What’s your opinion? Miriam Susanna Lucas miriam.lucas@scopem.ethz.ch


When using molecular sieves, we mix them directly in a


bottle with ethanol (i.e., free floating). Once capped, invert/ shake the bottle a few times to mix the sieves/ethanol and let the ethanol solution sit for at least a day or more before using it. Te sieves and any particulate matter should settle to the bottom of the bottle. Aſterwards, when pipetting or pour- ing the pure ethanol, do not invert or shake the bottle. I’ve never had any issues with contamination by molecular sieve dust in resin-embedded samples, which would have been very noticeable when sectioning with diamond knives. Kim Gibson kimberley.gibson@yale.edu


I tend to avoid the use of molecular sieves in solvents for


freeze substitution. (Fun fact, in the old book Cryotechniques in Biological Electron Microscopy, Hans Moor warns against the use of molecular sieve with freeze substitution). Te main problem (besides the chance of small particles in your sample) is that any trace of the molecular sieve will directly react with any osmium used during the freeze substitution turning it black within a few hours. I’ve successfully used glass distilled solvents (EMS), VWR anhydrous solvents and Merck Secco- Solv solvents (though the latter were discontinued last year). I keep them dry by flushing the bottle with dry nitrogen gas and storing the bottle in a container with silica pearls under dry nitrogen with all lids taped off with parafilm. Alternatively, if you do not use water in the freeze-substitution mix you can also use acidified 2,2-dimethoxypropane (1–2 drops of 37% HCl in 100 ml dimethoxypropane) and add 2 mg of this to 98 ml of 96% ethanol. Any water present will be converted to acetone and methanol. In contrast to other methods of mak- ing dry ethanol this doesn’t react with osmium. If you’re not using osmium, then the only issue will be the dust particles. You could opt for using the “porous containers” EMS sells for critical point drying (catalog nr. 70187–20) instead of dialysis tubing. Rob Mesman r.mesman@science.ru.nl


For freeze substitution with ethanol we use molecular sieves


and, as Kim wrote, we let it settle, neither stir nor shake, and pipette from the upper layer of ethanol. Ethanol is mostly used to replace acetone aſter dehydration is completed and before Lowic- ryl embedding (though lately we use acetone all the way through and we detect no adverse effects on polymerization). As far as I know, one should NOT use molecular sieves with acetone, at least not for prolonged incubation, due to aldol reactions, as you can read here: https://curlyarrow.blogspot.com/2010/04/anhydrous- solvents-part-3-acetone-and.html. Terefore, we are using CaCl2 We have no issues with molecular sieves or with CaCl2 do not see blackening in the presence of OsO4


. and we . However, when


we need to detect calcium in our samples using EDS, we refrain from using CaCl2


and use freshly opened bottles. For SEM we try


to avoid the use of molecular sieves due to the sensitivity of the method to any contamination in the medium used. Note that in order to be fully effective, molecular sieves need to be activated by heating. We use a vacuum oven at 200–250°C overnight. As


72


we are using a LN2 cold trap we see a few ml of frozen water in


the morning. Tere are protocols that do not require vacuum: https://www.sigmaaldrich.com/chemistry/chemical-synthesis/ learning-center/technical-bulletins/al-1430/molecular-sieves. html Eyal Shimoni shimoni.eyal@gmail.com


Focused Ion Beam Sample Preparation Microscopy Listserver What are the best ways to avoid damaging or outright destroy-


ing an epitaxial thin film (∼20 nm thick) using Ga ion FIB for atomic resolution STEM imaging? Aubrey Penn anpenn@ncsu.edu


Avoiding surface damage during FIB sample preparation


is straight-forward: prior to exposing a sample to an ion beam it should be coated with sufficient thickness (∼100 nm optimal, but >30 nm is a minimal requirement) of some protective layer that will “absorb” the ion beam damage. Te following coatings may typically be used for such a protective layer, depending on the nature of the sample: (a) e-beam deposition of C, Pt, Mo, W, SiOx; (b) evaporated carbon or metal; (c) sputter coating by Au, Au/Pd, TiO, Cr, Ir (d) conductive polymer coating by spinning or ultrasonic nozzle dispensing; (e) ink coating (i.e., “Sharpie trick”). For atomic resolution TEM, an amorphous layer on the sides of the lamella also need to be cleaned, but that is already another question. Valery Ray vray@partbeamsystech.com


I’m going to answer this in several parts. You don’t say


what the material or the substrate is. I assume that it is a semi- conductor. If it is and the sample doesn’t have to be site-spe- cific, then the best way to prepare a 20 nm thick epitaxial film and avoid Ga damage is not to use the FIB at all. 1. Te absolute best way to prepare your sample with no


amorphous damage is the Small Angle Cleavage Technique originally developed by John McCaffrey and then modified quite a bit by John and myself. A major disadvantage of this technique is that there is no amorphous damage so it can be difficult to focus and stigmate in a field emission TEM. I sometimes put a thin layer of carbon on the top surface just to have something amorphous that I can use in the FFT to help focus and stigmate. I can send you a detailed presen- tation on how to do the technique (Scott.d.walck2.ctr@mail. mil). 2. Te next best way to prepare a sample is low angle, low energy Ar ion milling as developed by Arpad Barna. All man- ufacturers of ion mills have low angle, low energy capabili- ties. Arpad has several publications showing the amorphous damage at different energies and angles. 3. OK, so you have a FIB and you don’t want to learn how to make samples the old- fashioned way. Protect the top layer from ion damage before putting on the ion beam Pt (or W) protective layer by e-beam deposition of 200–250 nm thickness to stop ions from hitting the top surface. Just don’t let the ion beam hit the surface with too high of a current or for very long (minimize the dosage to the surface or you will erode the protective layer.) If the sample is not site specific and you don’t want to have a high Z material next to the very thin layer, consider putting carbon down. An easy way to do this is to use a Sharpie® pen and coat the surface before you put it in the FIB. Tis works but can lead to open areas. A better way is to use a carbon coater


www.microscopy-today.com • 2020 May


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