Mario Hofmann
Millie’s student 2005-2011, now Nat’l Technical Univ, Taiwan

Energy dispersive Raman…

• Raman intensity increases dramatically if excitation energy matches band structure

• “resonance windows” can reveal details about electronic structure and phonon interactions

• energy dispersion is a fingerprint for each nanotube and can be applied in nanotube metrology

…is hard…

• to obtain information on resonance windows, Raman spectra at many different laser excitation wavelengths have to be collected

• laser has to be tuned to each wavelength and its intensity optimized

• organic dyes are used as excitation sources

• dirty, tedious, and complicated process

…but Millie helped.

• Millie heard that our project was not progressing and helped us approach collaborators

• she put together a proposal and helped with finances

A trip to UFMG…

• 3 of Millie’s students got to travel to Belo Horizonte, Brazil, and overall spent 2 months at UFMG in Prof. Marcos Pimenta’s lab

• during that time we used the colortunable Raman system for energy dispersive Raman investigation of individual nanotubes

• the UFMG team changed the dye 6 times for us

• we learned a lot about the operation and optimization of the dye laser (it’s all in the dye circulation system)

• it’s still messy and tedious

But the nice company helped in forgetting the countless hours staring at the spectrograph.

…leads to discoveries…

• our results were the first systematic study of the resonance windows of individual nanotubes

• we observed that the RBM window was surprisingly narrow

Also, there seemed to be a clear difference between metallic and semiconducting nanotube families.

The G-band resonance window remained incomplete.

…and new ideas!

• we learned that the dye circulation system was the bottleneck for efficient operation

• so we updated our system to have 3 separate dye circulators that could be switched quickly

• it just took 3 pumps and a few connectors from a plumbing store

Following generations of MIT students had access to more laser lines and dye types for important studies on double and triple-wall nanotubes.