McEuen group recipes

Aligned nanotube growth

Melina, June 2009

Fabrication:

First: photolithography + etch step to define the alignment marks on a whole wafer.

Second: Another photolith step on the whole wafer to pattern the catalyst stripes. I dice the wafer on our wafer saw immediately after patterning the catalyst pads; I then do a 1-minute oxygen plasma clean on the resultant pieces and evaporate 1-2A of iron using an e-beam evaporator. The tooling factor for the quart crystal monitor is set to 500% to get a more accurate measurement of thickness. Lift-off is an acetone soak for 1 hour, followed by a 5-minute sonication and an IPA rinse.

Anneal:

Heat chips (with catalyst lines) in air to 900 C for 1.5 hours. Cool. This anneal can take place days before growth.

Growth:

Slide sample into growth tube. Use the long metal stick to push the sample to the center of the furnace (optimized growth location is ~1 inch upstream of the center). Connect flow channels, making sure not to move your sample; close the furnace.

Open gas cylinders to give the flow controller access. Adjust setpoints for all gases in the controller menu (Ar=1 SLM, H2=0.3 SLM, CH4=1.9 SLM).

Begin flowing Argon (1 SLM), and check that the oil is bubbling and that the controller reads 1 SLM in “Act. flow.” Use a bottle of Sneak to check for leaks at both ends of the tube.

Turn on the furnace, set to 900ºC.

When furnace reaches 900ºC (~10 minutes), turn on the CH4 at the controller, and turn off the Ar for ~5 seconds.

Turn on H2 at the controller, and turn off the CH4.

Let H2 flow over sample for 5 minutes.

Turn on CH4, and let it grow for 1 hour.

Turn on Ar, turn off H2 and CH4. Lower furnace to 0ºC and prop it open with the provided block. At 600ºC you can open the furnace; at 200ºC you can remove your sample.

Turn off Ar.

PVA nanotube transfer:

Melina, June 2009

Cover NTs with 100nm of gold.

Spin a layer of PVA (I use Emulsitone Laser Scribing Solution 1146, which is PVA in water and can be stored at room temperature) at 3000 RPM, 1000 RMP ramp, for 30s.

Bake the chips for 30 minutes in a 70 C oven.

Using tweezers, peel the NT/Au/PVA PVA film off of the donor substrate and place it on the receiving substrate. Use a gloved finger to smooth the film down and eliminate any bubbles. If you don't get decent adhesion to the surface - i.e. if the film would rather curl up or peel off than stick to the receiving substrate - then your PVA layer is probably too thick. Carefully place the chip in a warm DI water bath for one hour. Do not flow water over the chip or otherwise disturb it, as that can cause the film to release from the surface. Let the chip air-dry. You will notice that the NT/Au film will be mostly free of PVA, and will look as though it was evaporated onto your new surface rather than just being placed on top.

Run the dry chip through an oxygen clean for 10-30 minutes (150 mtorr, 20 sccm, 150W). (I've tried skipping the O2 clean but we get a lot of PVA residue left on the surface, and the gold won't even come off all the way because it's being shielded and you are left with a lot of PVA gunk on your tubes). After removal of PVA the gold usually looks pretty much like an evaporated layer.

Place the clean chip in gold etchant (TFA) for two minutes. Rinse with water.

PDMS nanotube transfer:

Melina, June 2009

As I mentioned, I haven't actually tried this myself; however, here are the details I gave the Park group that apparently worked:

The PI the Rogers group used is Poly(pyromellitic dianhydride-co-4,4′-oxydianiline), amic acid from Sigma Aldrich (http://www.cnfusers.cornell.edu/cnf5_msds.taf?_function=detail&ms_id=50396&_UserReference=E22981DC406E4A5A4A315C31). It needs to be stored in a freezer.

The PDMS recipe is as follows:

Preheat oven to 70 C. In a clean plastic dish, mix 80 g of PDMS mix and 5.3 g of hardener. Using the non-cloth end of a green plastic swab, mix the solution for five minutes. Place the dish in the dessicator and connect the vacuum pump. After five minutes, turn off the pump and vent to prevent the PDMS from bubbling over. Turn the pump back on and let the mixture degas for another 35 minutes or so. Pour the mix into a clean 6“ petri dish. Put the petri dish back into the dessicator and pump on it for another 15 minutes. Cover the petri dish, and bake in the 70 C oven for 70 minutes. Cut the PDMS into the desired size of pieces for transfer using an x-acto knife.


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