Dr. Mohan Sankaran Receives 2011 Peter Mark Memorial Award for Work in CNT-Focused Plasma
Dr. Mohan Sankaran, 34, of Case Western Reserve University, is the recipient of the 2011 Peter Mark Memorial Award, given by the American Vacuum Society, “for the development of a tandem plasma synthesis method to grow carbon nanotubes with unprecedented control over the nanotube properties and chirality.”
NanoScienceWorks.org speaks with Dr. Sankaran about his work, his award and his upcoming book.
Nanoresearchers contend that plasma technologies face a number of challenges before they be successfully used at the nanoscale. How can your work with the tandem plasma synthesis method benefit the industry?
Dr. Sankaran: Plasma processing is already critical to semiconductor technology like in integrated circuits which is nanoscale. But, you’re right! There is a big challenge ahead for plasma processing. And mostly that is because currently, plasma processing can’t get down below 100 nm. Today, researchers are interested in making nanostructured materials at 10s of nms (including nanoparticles, nanowires and CNTs).
But, as plasma technologies try to reach this lower threshold, chemists also see their opportunity to consistently and cost-effectively ‘grow’ nanomaterials in this size-range.
Normally, when plasmas are used, you have a ‘top-down’ fabrication process, such as start with film and a substrate and using lithography you would create or cut our structures. Chemical approaches traditionally are known as a ‘bottom-up’ techniques where you build up materials from chemical [components].
Our work with tandem plasma synthesis uses plasma chemistry and represents a hybrid approach [combining aspects of both top-down and bottom-up].
Some long-time users of plasmas have even lost faith in applying plasma techniques to nanomaterials. Look at Applied Materials, one of the biggest makers of plasma etching tools for ICs. They recently shifted out of plasma tools for ICs because they found they had reached the [small] size limit or outsourced the development of those tools. Today, they’ve largely shifted to solar. And, they are not the only ones. But I remain a big believer in plasmas and their ability to have a great impact on nanomaterials.
Visit Dr. Sankaran's research website at Case Western
Your upcoming book, Plasma Processing of Nanomaterials, devotes a lot of time to explaining what you call “the art and science” of plasma-based chemical processes. Where do you see the most exciting possibilities for using plasmas in nanomaterials?
Dr. Sankaran: There is an enormous opportunity for the plasma community, and much to be done. Today, there’s no doubt we’re in the minority in believing that plasmas still can play a large role [at the nanoscale], But, I believe we in the plasma community can offer approaches that will help chemists, who are growing materials – and address challenges that haven’t been met yet by this community.
It all sounds like a dynamic area for research, attacking some fascinating problem. Can you share some of the details and results of your research with carbon nanotubes?
Dr. Sankaran: Sure. In the first step, we nucleate small metal particles or clusters by vapor nucleation – the vapor is “activated” in the plasma. We try to control that process to form nanoparticles of a certain size, hopefully in a predictive fashion. In the next step, we flow those nanoparticles into another reactor to catalyze carbon nanotubes. With our technique, we have demonstrated we can control the diameter of nanotubes. Of the [useable] product we can create with this process, 75% is single-walled CNTs
Our work with carbon nanotubes is also aimed at addressing another problem, chirality. With CNTs, researchers have not been able to do predictive things – controlling chirality is the Holy Grail.
Chirality refers to the way carbon nanotubes are rolled up at different diameters and different angles. The problem is that today, we can’t control the chirality and so the result is a mixture of different types that have to be separated after growth. This can be difficult and expensive.
We tried using our process to control chirality and we were able to make single-walled CNTs of which 90% were semiconducting. That’s not good enough, but it’s a great first step. And I hope this will encourage others to explore plasmas approaches for nanoscale [fabrication efforts].
Very interesting. You describe the field as a sort of ‘jump ball’ between plasma and chemistry techniques. How do you see that playing out for the future of nanoscale fabrication?
Dr. Sankaran: That’s a good question. I’m in the plasma camp, so I’m hopeful there is a future where there will be balance between plasma (dry) and chemical approaches (wet). Others may say that plasma methods have reached the end of their useful life and we might move on to soft methods like ink jet printing or stamping, but I definitely disagree with that.
What has driven your interest in this nano-related field? What keeps you excited about your focused area of work?
Dr. Sankaran: Nanotechnology has been an interest to me for a long time. I’ve worked with photoluminescent materials or quantum dots since my undergraduate days at UCLA. That work made it real easy for me to get excited because those materials have some beautiful properties – bright, room temperature fluorescence, for example. In grad school, I first started working on plasmas and got very interested in combining plasmas and nanomaterials. I then went on and after receiving my Ph.D. from Caltech, started a research group at Case Western that is focused on merging plasmas with nanomaterials. We’re not as interested in studying the plasmas themselves – our research is really aimed at making useful nanomaterials.
Where did you get the ideas and inspiration for your research, especially as so many experienced with plasma were becoming less interested in it for nano – not more interested?
Dr. Sankaran: One of my grad students was inspired by work done by Michael Zachariah at the University of Maryland who showed that CNTs could be grown and characterized in the gas phase. We thought it would be easy for us because we were already making beautiful metal particles in our plasma. But when we initially tried it, all we got was junk. So, we looked further and realized we were using iron particles and Zachariah was using nickel. That was exciting! That told us that that the characteristics of iron and nickel were not the same when it came to making carbon nanotubes.
After that, the whole project clicked! We got the nickel and the process worked! And, after that, we just had so many ideas, different combinations. It was really, really exciting. We’re continuing to experiment with metals that will react with carbon to produce new types of nanotubes. Right now, we’re focusing on copper.
Let’s talk about your award. What does winning the Peter Mark Memorial Award mean to you? Tell us about your lectureship plans?
Dr. Sankaran: This award is given to a young person, and I’m 34, so it’s a great privilege to be recognized so early in my career and to start off my career this way. I look at some of the previous winners and some of them are now mentors to me, like Eray Aydil at the University of Minnesota, and it’s wonderful to be associated with them. The lectureship is part of the annual AVS (American Vacuum Society) meeting Oct 30th-Nov 4th. I plan to lecture on the work we did that got this award, as well as more recent happenings in our lab.
Your book also will include contributions from “world-renowned leaders in the area of plasma processing of nanomaterials.” Who else, besides yourself, has made contributions?
Dr. Sankaran: Meyya Meyyapan, one of the scientists at the forefront of development in the field of CNTs, was a key person to influence this book, and helped me develop this book with CRC Press. I wouldn’t have had the opportunity for the book without him. For years he had been thinking about a book on plasma processing of nanomaterials, and was nice enough to choose me to take on this project.
The book is also an outcome from my relationship with Kostya Ostrikov at CSIRO lab in Sydney, Australia. He is the [founder] of the international iPlasmaNano conference, which grew out of his hosting a workshop meeting to try to get people in plasma to work together and create a community. I’ve been attending that meeting for several years and the meeting proved a valuable resource for me, as many of the contributors to the book are people that I met at that conference.
We have experts from around the world – Australia, North America, Asia, Europe - as contributors. I’m very proud of the award and the book.
Thank you for your time, and congratulations again Dr. Sankaran on your award and your forthcoming book, which is coming out when?
Dr. Sankaran: My pleasure, and the book is scheduled for publication this November.