Jun Zhu
Assistant professor of physics
321 Davey Lab
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Dr. Jun Zhu, assistant professor of physics, studies the low-temperature properties of carbon nanotubes.
Workmen are digging a hole in the concrete basement floor beneath Osmond Lab where a new piece of microscopy equipment will be placed on a concrete slab detached from the rest of the building to isolate it from vibrations. Jun Zhu, an experimental physicist, who arrived this January from Cornell, visits the work site every day or two to check on the progress. She hopes to see the equipment, an atomic force microscope probe built inside a cryogenic system to keep the temperature of her samples hovering just above absolute zero, in place this June.
"It’s really coming together nicely now," she says about the ragged pit in front of her, but she might just as well be talking about her new appointment as assistant professor in the department of physics. "I very much enjoy being here at Penn State. The department is wonderful. This is such an exciting environment. You look in every direction and there is a colleague doing something interesting. If you don’t know about a particular material, or if you don’t know a particular technique, you can always find someone to ask. With the material science, electrical engineering and chemistry departments surrounding you, there are just endless opportunities to collaborate."
Her current collaboration began by e-mail even before leaving Cornell, where she worked as a postdoctoral scientist with Paul McEuen, one of the pioneers of carbon nanotube research. Several faculty were putting together a proposal to NSF to study graphene systems, the parent material of the carbon nanotube. Jun Zhu’s name was brought up as someone with expertise in 2-dimensional systems, and she was invited to join the group, which includes a chemist, an electrical engineer, and three theoretical and experimental physicists.
"Graphene is a hot topic right now," Zhu says. "People have looked at the material before, but in the past it had always been as bulk carbon. A little more than a year ago a group at U. Manchester in the UK discovered a simple way of exfoliating graphene sheets out of bulk graphite, putting them on a substrate, hooking them up with electrodes, and looking at their physical properties. This is quite new, and it has so excited the physics community that papers on graphene are popping up on a daily basis on condmat, the physics archive server."
The reason for the excitement is that graphene has a peculiar band structure that is very different from conventional semiconductor materials, such as gallium arsenide and silicon, the present day electronic materials. According to Zhu, graphene has many of the best features of the widely studied carbon nanotube (CNT), which is essentially a rolled up sheet of graphene. In addition to exotic 2-D physics, the unusual band structure gives graphene desirable electronic properties, such as long mean free path, which means there is very little electron scattering. And because of its free edges, graphene is easier to functionalize than CNT, which has few places on which to hang molecules.
"The idea now is to learn how to cut graphene sheets in different angles to engineer the bandgap of the material; how to functionalize the edges to recognize a bio or chemical molecule you want to detect; and how to link pieces together to make a device. We experimentalists are very excited about getting our hands dirty and doing all this."
The Holland Tunnel Effect
At Cornell, Zhu concentrated on looking at the one-dimensional behavior of carbon nanotubes. CNTs have a very high aspect ratio. They can grow to a very long length, but the diameter of the tube is only a few nanometers, making them essentially one dimensional.
Zhu explains some of the theory behind 1-D effects: "When you reduce the dimension of systems from 3-D to 2-D to 1-D, you observe different behavior from the electrons, so a good analogy is to think of a 1-D system as the Holland Tunnel. If there is one car stuck in the middle everybody is stuck, because you have no way of passing that car. This tells you that the interaction among all the carriers in a 1-D system is so important that they will always feel each other’s existence. In 2-D systems, you have a better situation because you can take a different path to avoid that dead car in the middle. In 3-D you have up and down to escape as well. In 1-D, the theory was worked out 50 years ago by Tomonaga and Luttinger. Much interesting physics has been predicted since. For example, there is an exotic phenomenon called spin charge separation. The charge and spin excitations would have different speeds. You have two waves propagating independently, with the electron wave going in one direction and the spin wave going the other direction, and they don’t have to have the same speed."
Frequency shift (FS) imaging of quantum dots formed on carbon nanotubes (CNTs). (a) A CNT contacted by two electrodes. (b) Coulomb oscillations of two neighboring quantum dots formed on a CNT. Each contour represents the addition of one electron onto the enclosed dot. Location of CNT indicated by a white line. (c) FS detection of charge hopping among quantum dots formed on an isolated CNT. Figure taken from Zhu et al, APL 87, 242102 (2005)
To study the unusual properties of one-dimensional systems, Zhu and her colleagues at Cornell used a home-made atomic force microscope built inside a cryostat capable of 1/3 degree Kelvin. At that temperature more of the quantum physics comes into play.
"What we see is that instead of being a whole 1-D system, the carbon nanotube breaks down into a series of puddles that we call quantum dots. Electrons are confined to those puddles because the temperature is very low. They don’t have enough energy to hop around, so they get stuck. In order to put a charge onto this puddle, this island of electrons, you have to pay an energy cost, that’s the charging energy of the system. The smaller the dimension of the system, the larger the charging energy. If you have to pay an energy cost you cannot charge it whenever you want – the energy level of the quantum dot has to match the energy level of the outside system, and that only happens discretely. This is what’s called the Coulomb Blockade and Coulomb Oscillation phenomena. We see that effect in the carbon nanotube because it breaks down into a series of quantum dots.
"What we can do with an AFM probe is to park the tip on top of a quantum dot and actually sense the charging events of the quantum dot. Our instrument is so sensitive that every time an electron hops on we see a spike in our force measurement. You are literally watching the quantum dot get charged electron by electron. Here at Penn State, we plan to continue the quest of Luttinger liquid behavior with improved sample designs that will preserve the 1-D nature of carbon nanotubes.
When the hole in the basement is filled and her equipment set up, Zhu’s low-temperature scan probe expertise will be at the service of her new colleagues at Penn State, a list of collaborators she has already compiled in her mind.
Education:
B. S. in Physics, June 1996, University of Science and Technology of China (USTC), Hefei, Anhui, China
M. A. in Physics, May 1998
M. Phil. in Physics, May 1999
Ph. D. in Physics, February 2003, Columbia University
Postdoctoral scientist, Cornell University, 2003-2005
