Thermoelectrics Session, MIT Memorial
Thank you. I joined Millie's group in 1999, towards the tail end of the group of students that worked on thermoelectrics, starting with Lyndon and Son and Zhibo and Taka. And, in my time, I overlapped with Steve Cronin, who is here, and Marcie Black, who I see there, and Yu-Ming, unfortunately not here, and Min Tang.
And, together, we really were not individual students. We were a team that Millie organized, and we supported each other with our strengths and our expertises — some of us were electrical engineers, some were physicists, some chemists were involved, as well.
It was a way for me for us to gain both a great insight into the science and the physics, as well as a lot of really good friends. And this support network helped us carry along on the great ideas that Millie had and the research that we had going on — even when Millie was not around, supporting each other. Of course, we always had help from Laura and Gene. They were there, for us, every day.
Now, Millie was there for us whenever she could be, because she always put us first. As people have mentioned, she had piles of papers and work — but she told us, always, that if she's in her office, she's happy to let us talk to her, especially if we came in before 8:00 in the morning.
The other great thing about working in Millie's group was that she, as has already been mentioned, made sure that we connected with other people. And having Millie's reputation behind us was a great door opener for our careers. At that time, the MURI was already established, so I was very lucky to work with Gang Chen's group — even before they moved to MIT, and a lot more after they moved to MIT, where our two labs were basically connected through an imaginary tunnel — and with other people on the West Coast and internationally.
What we were trying to do was to improve the thermoelectric efficiency. The plot at the top shows the ZT values that were there, at the time. Millie chose for us to tackle bismuth-antimony, which was the material that offered the best chance for improvement, since it was not so good, and also because she had a lot of insight into what the future would bring.
Below it, you can see a phase diagram that we developed, which showed that this system had a very complex and very interesting solid state behavior, where you could change it from semi-metal to semiconductor, and different types of semiconductors. And the error in the middle, there, shows the holy grail that we were trying to achieve — nanowires with a certain composition and without such a small diameter. Remember that this happened in the days when nanoscience was in its infancy, so making nanowires, then, was not as trivial as it is, today.
So we would go across MIT, to other labs, make the nanowires in one room, put them together in another room, take SCM pictures in another room, and pass them from Yu-Ming to Stephen, back to me, and to Marcie to characterize them. And so you see, here, on the slide, some of the arrays that we were making, and you can see the nanowires in and on the top.
There were very few electrical and thermoelectric properties that we were able to consistently give, because, as you can see, not all the cores were filled with nanowires. Contact resistance was very hard to control. But, eventually, the trends that Millie predicted were there. They were slowly, slowly uncovered. And we, basically, show all that this data represents — just these four points in the phase diagram, but it was showing that we were moving in the right direction. And, years later, other people arrived with much more sophisticated equipment and ideas, and continued to prove that bismuth nanowires and silicon nanowires are good potential thermoelectrics. I’ll show you, here, some pictures from Arun Majumdar's lab, and from James Heath’s.
I just want to finish with an anecdote. While looking for pictures, we found old pictures from Millie's lab. You can see those pictures, here. There is a lot of very important equipment, here, that we used — particularly Dust-off to remove dust, and silver paint to connect our devices, and those yellow sticky notes. The sticky notes were like a time capsule, wherein graduate students from previous generations passed along very important information to future graduate students.
One more thing. I was asked to speak a little on the impact Millie had on her thermoelectrics students. I believe that most of us have found interests in other fields, although, occasionally, we all come back to thermoelectrics. And what we can say is that Millie contributed to the way that we, now, approach problems. We learned to be critical and creative thinkers, and we look for the truth in our data and look for creative models to explain it. And, in fact, my last comeback to thermoelectrics was a new model to connect the Hicks and Dresselhaus model to more real materials, and, through it, we gained a new insight into just how smart Millie had been when she chose bismuth-antimony for the nanowires.
Thank you very much.