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Principles & Protocols

Protocols - Solid Carbon Film

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Using Mica (Best for flat films)

Solutions Required:-

1. Carbon rod, either 1/8" or 1/4" depending on the gun provided with your vacuum evaporator. Alternatively, a carbon thread can also be used.
2. Mica sheets (EMS or Ted Pella)
3. Crystallizing dish (190x100mm) (VWR)
4. small test tube rack, tygon tubing, 5-10 ml glass pipette, hemostats, plastic tray, 2 pennies
5. 90 mm plastic petri dishes, 90mm Grade 4 Whatman filter paper
6. 300 mesh Cu EM grids (EMS or Ted Pella)
7. stainless steel forceps (e.g. #5 Dumont)
8. Vacuum evaporator (e.g. Edwards Auto306)

Instructions:

1. Sharpen carbon rod, remove dust remnants and mount in vacuum evaporator. Handle the carbon rod with gloves to prevent transfer of body oil.
2. Cut 4x2 cm piece of mica with scissors. Cleave in half longitudinally and place on filter paper in petri dish with freshly cleaved surface face up. Cut triangular piece of filter paper, fold in half such that one vertex overlaps the opposite side of the triangle and place in petri dish alongside the mica. Place petri dish into vacuum evaporator and pump to achieve a vacuum of at least 1x10-6 torr.
3. If available, use retractable shield to cover mica sheets. Increase current through carbon rod until vacuum begins to degrade. Reduce current until vacuum recovers. Repeat until degradation is minimal. This step is designed to burn contamination off of the carbon rod prior to evaporation.
4. Retract shield from mica sheets. Gradually increase current until carbon begins to evaporate. Maintain lowest possible current during evaporation to prevent sparking. After 15-30 sec, turn down current and inspect shadow produced by deposited carbon visible on the folded triangle of filter paper. Repeat evaporation as required to obtain a light, but distinct gray shadow. (See Note 1)
5. Vent vacuum evaporator, remove petri dishes and store covered at room temperature. (See Note 2).
6. Fill crystallization dish with water and place on a tray. Place test tube rack under water. Cut 3x5 cm piece of filter paper and place on top of the test tube rack. Weigh down the filter paper by placing pennies on the edges.
7. Individually place 300 mesh EM grids on top of the filter paper with forceps in a close packed array that matches the size of the mica. (See Notes 3 and 4).
8. Carefully add water until crystallization dish is almost overflowing. Wipe surface with 10ml glass pipette to remove dust from the surface. Insert 40-50 cm length of tygon tubing into bottom of crystallization dish and suck on end to create siphon. Clamp tygon tubing to stop siphon until later.
9. Pick up mica with forceps. "Huff" on the mica to humidify the carbon-coated surface (don't blow, say "hot" without pronouncing the "t"). Hold mica at 10-20° with the water surface and gently lower it into the water. The carbon should float off of the mica and remain at the surface. Drop the mica into the bottom of the crystallization dish.
10. Use the glass pipette and/or the forceps to maneuver the carbon over the filter paper. Release the hemostats to allow the siphon to remove water gradually from the crystallization tray. The carbon should land on top of the grids as the water recedes below the filter paper. (See note 5).
11. Remove the filter paper and grids and place on dry filter paper in a petri dish. Store at room temperature until ready for use.

Notes:

1. Some report that multiple evaporations of carbon produces a stronger, flatter and more conductive carbon film. Some prefer graphite rods rather than amorphous carbon, but in each case the rods should be the highest possible purity.

2. Storage conditions affect the ability of the carbon film to separate from the mica in subsequent step 10. Generally, the carbon will not separate immediately after evaporation and should be stored for at least a couple of days. Separation may be helped by equilibrating the mica in a humid chamber immediately before floating. The carbon film can also be baked for 60ºC for 1hr to 2d prior to floating in order to increase strength and conductivity.

3. Grids can be cleaned if desired by bath sonication in a small beaker filled with acetone. This removes traces of machine oil that may be present. After sonication, remove bulk acetone, then turn the beaker upside down on a piece of filter paper. As the grids dry, they tend to drop onto the filter paper, especially after tapping the bottom of the beaker.

4. Grids should be placed on the filter paper in a consistent way, usually with the polished (shiny) side up. This removes ambiguity about which side the carbon is on. Grids can be purchased with a Rh coating on one side, which makes it very obvious.

5. During water removal, the siphon can be paused just before the carbon contacts the filter paper in order to perfect the positioning of the carbon over the grids.

-- DavidStokes - 16 Jul 2008