The next generation of semiconductors
Professor Malika Jeffries-EL looks to one of the most abundant elements to solve one of the world’s most pressing problems
Chemistry professor Malika Jeffries-EL studies carbon-based polymers. She’s looking for new materials for the next generation of semiconductors. Photos by Cydney Scott
For Malika Jeffries-EL, the love affair began at science camp. She was young, and like many young scientists before her, she fell hard, smitten by the elegance and beauty of the periodic table. “Back then, I was like, ‘Oh my God, there are all these different atoms!’” she recalls. “You can mix them together and make all these different molecules! I could be all day with this.”
Decades later, the Boston University associate professor of chemistry is still mixing molecules, looking for new materials that could fill a pressing need for cheaper, simpler, flexible semiconductors. Semiconductors are materials with distinct electrical properties—electricity flows through them better than it flows through, say, wood or plastic, but not as well as it flows through metal. By altering a semiconductor’s chemistry, scientists can tweak its characteristics; this versatility makes them indispensible for electronics. And as the world grows ever hungrier for laptops, iPhones, solar cells, and lights, the race for next-generation semiconductors is on. Jeffries-EL is betting on organic polymers—long molecules comprised mostly of carbon.
BU Research spoke to Jeffries-EL about a life in chemistry, her favorite element, and seeing her work in lights. A condensed and edited version of the conversation follows:
BU Research: What is a polymer, anyway?
Jeffries-EL: A polymer is what we call a macromolecule, with molecules being some of the smallest things that make up, well, everything.
Wait. But no, that’s not true, is it?
Well, okay, yeah, if you want to talk physics, you can break things into particles. But if we go chemistry-style into the periodic table, we have our atoms; and if you can bind atoms together in various fashions, you can make molecules. So H2O is a molecule. It’s composed of three atoms: two hydrogens and one oxygen. A polymer is the type of molecule you get if you string a bunch of smaller molecules together. If you think about a pearl necklace, a pearl would be your molecule but the necklace would be your polymer.
A lot of people use the words ‘polymer’ and ‘plastic’ interchangeably. But the way you describe it, it doesn’t sound like the same thing at all.
Plastics were one of the earliest industrial applications of polymers, and so people tend to equate polymers and plastics. And some of the most well-known polymers are also plastic-y. Things like nylon, for example. But there are tons of polymers out there that I wouldn’t say are plastics. And the types of polymers that my group works with are actually semiconductors—so they’re lousy plastics.
So why do we care about polymers?
Because we can manipulate both their molecular structure and atomic structure to get properties that we may not be able to get from naturally occurring materials.
Can you give me an example?
Okay. Some of the early research in polymers was trying to make materials that mimic silk. But if you think about things like polyester and nylon, which were some of the original synthetic polymers, they’re not as silky as silk. They’re not as good as silk. And so many would view that as a failure. But they have properties that silk does not have. Maybe we can’t get the exact properties of nature, but we can get properties nature can’t do, like flame retardancy or stiffness and strength.
And so in my materials research, we kind of do the same thing but we’re doing it in semiconductors. Everyone wants a mobile phone, everyone wants a laptop, everyone wants a flat-screen TV, and we need semiconductors to do this. We look at our periodic table, and there are several elements that are well-known semiconductors, and our technology is being driven by these elements—silicon being the most prevalent. Everyone knows silicon—Silicon Valley—that drives our computer industry.
What’s wrong with silicon?
The manufacturing of silicon is not trivial. Most of the costs of your computer are not the plastic and stuff that’s the outside; it’s the inside. It’s the chips and the processing of silicon.
But there are also other types of semiconductors that are widely used in our handheld devices. Some scientists call these “rare-earth” metals, because they’re not as abundant as other elements. If we’re all depending on these rare-earths, somebody’s not going to be able to have a phone.
And then the world will collapse!
Carbon alone is not a natural semiconductor. But certain arrangements of carbon atoms allow charge to flow through the system. In this sample, energy from the flashlight is re-emitted as blue light.
But Earth is rich in carbon, and so what my group does is make polymers that are carbon-based, which mimic the properties of these semiconducting materials. So now we’re taking our most abundant element and using it to do something that we’re currently doing with the rare-earth elements.
The advantage of organic-based materials—carbon-based materials—is that we can make a lot of them into solutions. So instead of taking a big chip and machining it down into smaller chips, maybe we can take a strip of plastic and inkjet-print all the circuitry, and then cut it out. It’s a different process, and can be a lot cheaper and a lot less labor-intensive.
Cool! So why doesn’t the technology exist yet?
People are working on it. There are some prototypes, both in academic labs and in research labs, but one of the biggest unknowns is the lifetime. Some of the newer stuff, the organic stuff, isn’t lasting as long as the conventional stuff, so that’s a drawback.
And I don’t think these materials necessarily can replace existing technology. It’s kind of like the fabrics. Could nylon replace silk? Probably not. But is there a niche for nylon? Yes. And that, I think, is the opportunity—to go into new markets and to change things. One of the big things about plastic-based materials is they’re flexible. If you have an irregular surface and you want to cover it with, say, solar panels, then maybe you can get an organic that can bend and fit around it.
How did you get interested in chemistry in the first place?
I was always curious. I was that kid who said, “Why? Why? Why?” And I was fortunate enough one summer to do a science camp, when I was probably about 11 or 12 years old. And I came back with my periodic table, and I looked at my mother and said, “I want to be a chemist.” And she said, “Okay, that’s nice, dear. Why?” And I said, “Because a chemist’s job is to mix, stir, heat, and figure out what happens.” She said, “I think you’d be good at that.”
Do you have a favorite element?
Do I have a favorite element? That’s a great question.
I bet one came right into your head but you didn’t want to say it, because you felt bad for the other elements.
You’re right! Gold! [laughter]
That is so funny. Just because it’s gold?
Yeah. Well, you know what? Platinum is a little bit more double service. Platinum is great in jewelry, and platinum can actually do some awesome chemistry, so we’ll go with platinum.
You said that carbon is abundant but it’s not a natural semiconductor, right? So why try to force it to be something that it’s not?
We’re not using pure carbon; we’re actually using molecules that contain carbon—hydrocarbons, largely containing hydrogen and carbon. If you have certain arrangements of carbon atoms, you could get electron transfer in the system, and that’s the beginnings of what you need for a semiconductor. And so one carbon on its own can’t do it, but if you have a couple of them arranged correctly, it actually moves quite freely.
What is the hardest part of your research?
One of the weird things about polymers is that the properties are not just a function of the actual chemical structure, but also its history. How was it processed? With semiconductors, how you process it changes everything. You can do a bad processing technique and your devices don’t work, and you’re like, “Oh my God, this is a bad polymer.” And then you spend some time, play with your processing, and all of a sudden you’ve got a top-class device.
And so we struggle to make sure that every material is realizing its full potential, so that the molecules arrange themselves in the most optimal fashion for what we need it to do. And it often is hard to predict exactly how the molecules are going to interact with each other.
I like that. Every molecule can be its best molecule! Come on, guys! You can do it!
That’s right! You can be your best version of yourself!
What’s your dream for where you might see your research in the world?
I’m a New York girl, so I would love to see my molecules being used on a billboard in Times Square. The LEDs lighting it up, or my solar material powering it. That would be really cool.
Chemistry
By Barbara Moran
Thursday, May 5, 2016
http://www.bu.edu/research/articles/malika-jeffries-el-organic-polymers-semiconductor/