Preparing the Nanotubes

Suitable Solvents

DI Water

not very well suited to make nanotube solutions. Can however be a good choice when used with detergents

As CNTs are hydrophobic they are best dissolved in organic solvents such as DMF.

Making Nanotube Samples

Preparations

The Substrate

Preparing chips from a wafer

Important: Open wafer boxes only inside an operating flowbox!

• Spin a layer of protective resist on the entire wafer and bake it thoroughly.
• Use a ruler and a diamond pen to scribe the wafer. Make a clean scratch 4 to 5 mm long, parallel to the edge. Cover the wafer with cleanroom paper and break the wafer over an edge.
• Keeping the strip, store the rest of the wafer securely for future use.
• Scribe the resulting strip in rectangular pieces.
• Tear off a strip of clear adhesive tape a little longer than the substrate. Glue it to the resist side. Break the substrate by pulling it over an edge holding the ends of the tape.
• Carefully remove the wafer pieces from the tape with a tweezer.
• Wash the chips in IPA (not in acetone, which would remove the protective resist layer).

Preparing alignment marks on the surface

• Remove protective resist layer with Acetone and wash the chip in IPA
• Spin on your preferred resist system and bake the wafer
• EBL pattern the chips with a 3x3 matrix of alignment marks
• Evaporate a sufficiently thick (>35 nm) Cr/Au or Ti/Au metal layer

Applying Nanotubes

This section covers methods for random deposition of tubes. Instructions on dielectrophoretic trapping can be found in this chapter.

Tubes in SDS (aq.) Solution

The tubes are encapsulated in SDS micells and form a stable solution. Thus there is no need for sonication before applying.

• The substrate chip is placed on the hotplate (level 1-2; approx 100 deg C)
• One drop of nanotube solution is applied and left to dry in. This procedure is repeated two more times or until a uniform staining of the chip is visible
• The chip is put into a beaker with water. The beaker is put on the hotplate and heated until the water starts to boil
• The chip is flushed in Popanol and blown dry with nitrogen

This procedure should result in an acceptably even distribution of single tubes on the chip. If there are to many regions of aggregated tubes visible in the SEM repeat the cooking step.

Special Techniques

Cleaning the Substrate

Wet Cleaning

An effective method for removing nanotubes from a wafer without destroying other structures is to use surfactants. Here a simple recipe with SDS:

1. Sonicate samples in 1 to 10 % SDS (aq.) solution for about 5 min
2. Rinse with DI water
3. Rinse in IPA
4. Blow dry with Nitrogen

Very fragile samples can be cooked in the SDS solution instead of sonication.

Annex