Nanotubes and Quantum Dots Give Up Their Secrets to ECE Researchers

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Nick Vamivakas and Anna Swan
Nick Vamivakas and Anna Swan

Nanosystems, miniscule bits of matter or devices on the scale of atoms and molecules, hold substantial challenges for researchers trying to understand their inner workings. Recent publications from ECE faculty delve into this world to uncover new findings that will help two nanosystems– quantum dots and nanotubes– find use in practical applications such as telecommunications and computing. By studying these systems with laser light, the researchers can also find overlaps in the two fields, speeding research in both. 

Quantum dots are specks of matter composed of many atoms but small enough to behave like a single atom. Quantum dots may be key building blocks for a future technology using quantum information processing. 

To study the dots, researchers snare them inside another material, which holds them in place and preserves them. One challenge with this method, however, is that the casing trapping the dots also shields them from researchers’ attempts to manipulate the dots by poking and prodding them with light. 

“You want to be able to communicate with the quantum dots. When you shine light on it you need to have the light interact as much as possible with the dot. You want to see some kind of effect,” said Associate Professor Anna Swan (ECE), an author on the paper. 

To interact with quantum dots, Swan and colleagues — including Professor Selim Ünlü (ECE), Physics Professor Bennett Goldberg (CAS) and former BU graduate students Nick Vamivakas (ECE) and Mete Atatüre (Physics) — used an apparatus developed by Ünlü that narrowly focuses light on quantum dots buried in another material.

Their results show 120 times more light reaching and interacting with the quantum dot than without this device, a promising step towards the use of quantum dots in more complex organizational schemes. 

In addition, because the fields of quantum dots and carbon nanotubes are related, the work already done on quantum dots may help accelerate research in the decade-younger field of nanotubes.

“What we’d like to do next is to apply a lot of the types of experiments and thinking that people have used on quantum dots to nanotube research,” said Swan. “Whenever you have crossing of these fields, it’s great; you can leap frog the progress instead of reinventing the wheel.” She and the group are developing more projects to study the two fields in concert.

In another publication, Swan and colleagues helped advance this younger field of carbon nanotube research, revealing unknown electronic properties of the tubes. Nanotubes, hollow cylinders about one nanometer in diameter, are made of a honey comb-like lattice of carbon atoms.  One reason for the great interest in nanotube research is that minor changes in tube diameter or spiral pattern produce dramatic differences in electrical properties. Changes in tube structure also change the configuration of electrons hovering around them, which give tubes the properties of metal or “all different flavors of semiconductors,” said Swan. This versatility makes nanotubes appealing for such applications as dissipating heat in car tires, and in the developing area of nanotube electronics devices such as solar cells, computer memory and integrated circuits devices like nanotube diodes and transistors.

To date, however, researchers have noted a surprising insensitivity to change in nanotubes’ electronic energy levels when their surrounding conditions change.

Researchers would expect electrons’ behavior as they orbit around nanotubes to depend on the medium through which they travel, just as a person’s motions change if swimming through water or walking on land. But, when Swan, Ünlü, Goldberg and colleagues including Andrew Walsh, a BU graduate student in Physics, surrounded their nanotubes with dry nitrogen, high humidity nitrogen or water, it seemed electron energy levels barely budged.

The group revealed that, underneath this placid façade, the electron energy of the nanotubes changed enormously, but several types of electron interactions occurring in and around the tubes screened these changes, giving the false appearance that energy changes were very small in magnitude.

In future high-tech applications of carbon nanotubes, it will be important to know these underlying electron energy changes, so researchers can match the tubes’ electronic properties to those of other nanotubes or to different materials they must interact with.

The two articles were published in recent issues of Nano Letters. The quantum dots research was also featured in Nature Photonics. See the BU Optical Characterization and Nanophotonics Laboratory website for more information.