February 3, 2010 – Researchers at Rice U. have figured out a way to transfer patterns of carbon nanotubes from a substrate to any other surface in a single dry room-temperature step, and then reuse the substrate with intact catalyst particles to grow more.

The research, published in ACS Nano, started with first-year postgrad Cary Pint "playing around with water vapor" to clean up amorphous carbons on some single-walled carbon nanotubes (SWNT), and discovering that the nanotubes he was extracting stuck to the tweezers — this led to investigating how the process could transfer CNTs to other surfaces. In his work, CNTs are grown via chemical vapor deposition (CVD) and etched with a mix of hydrogen gas and water vapor to weaken the bonds formed with the metal catalyst. Once stamped, the CNTs lay down and adhere via van der Waals forces to the new surface, leaving all traces of the catalyst behind.


A potassium bromide window covered by a film of single-walled carbon nanotubes, transferred from the growth substrate, which serves as a template, at right. (Source: Rice U.)


Among the results of the work: a crisscross film of nanotubes made by stamping one set of lines onto a surface and then reusing the catalyst to grow more tubes and stamping them again over the first pattern at a 90-degree angle. The process took about 15 minutes.

Eventually Pint sees the technique, which he says can scaled up "easily," can be used to embed nanotube circuitry into electronic devices. Future steps for the process are to make highly efficient optical sensing devices, and look at doping techniques to enable more precise growth of metallic (conducting) or semiconducting SWNTs.

His own goal is to develop the process to make a range of highly efficient optical-sensing devices. He’s also investigating doping techniques that will take the guesswork out of growing metallic (conducting) or semiconducting SWNTs.

The paper also describes a process for quickly and easily termining the range of diameters in a batch of nanotubes grown through chemical vapor deposition, something many spectroscopic techniques can’t do for structures >2nm in diameter. "This is important since all of the properties of the nanotubes — electrical, thermal and mechanical — change with diameter," Pint said. The good news: the method involves a Fourier transform infrared (FTIR) spectrometer, which "nearly every university has …sitting around that can do these measurements," he added.

From the ACS Nano paper abstract:

Utilizing this transfer approach, anisotropic optical properties of the SWNT films are probed via polarized absorption, Raman, and photoluminescence spectroscopies. Using a simple model to describe optical transitions in the large SWNT species present in the aligned samples, polarized absorption data are demonstrated as an effective tool for accurate assignment of the diameter distribution from broad absorption features located in the infrared. This can be performed on either well-aligned samples or unaligned doped samples, allowing simple and rapid feedback of the SWNT diameter distribution that can be challenging and time-consuming to obtain in other optical methods. Furthermore, we discuss challenges in accurately characterizing alignment in structures of long versus short carbon nanotubes through optical techniques, where SWNT length makes a difference in the information obtained in such measurements. This work provides new insight to the efficient transfer and optical properties of an emerging class of long, large diameter SWNT species typically produced in the CVD process.