A model illustration depicts a connected quantum dot solid. The bright blue line represents an electron moving through the structure.

Image courtesy of Prof. Tobias Hanrath

A model illustration depicts a connected quantum dot solid. The bright blue line represents an electron moving through the structure.

March 22, 2016

Connecting The Quantum Dots: Cornell Researchers Step Towards Better Electronics

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When the first silicon chip was made, few envisioned that it would lead to smart phones. So pointed out Prof. Tobias Hanrath, material sciences and engineering, when discussing his and graduate student Kevin Whitham’s, work that could have applications ranging from improved electronic devices to helping solve the world’s energy crisis.

What’s helping to potentially solve such big issues? The answer may not be big at all. Hanrath and Whitham’s work revolves around crystals called ‘quantum dots,’ which are so tiny that it would take about 200,000 dots to fit the width of a human hair.

“Solar cells are made out of silicon now, which works fine but is expensive to make, whereas quantum dots can be made in a beaker very cheaply,” Whitham said.

The process of growing crystals of quantum dots is like that of growing rock candy, “except instead of sugar and water, you have lead and selenium,” Whitham said.

Quantum dots are special because scientists can readily control what wavelength of light the dots absorb or emit by simply changing the size of the dot. This is useful for potentially creating optoelectronic devices or even low-powered computing devices, according to Hanrath.

Another advantage of quantum dots is that whereas silicon must be manufactured in a factory, quantum dots can be printed and are thus much easier to make. According to Whitham, this property of quantum dots could make them useful in creating flexible solar cells.

Discovered about 40 years ago, quantum dots are a relatively new find. Research has only started picking up in the last 15 years.

One major hurdle has stopped quantum dots from being used in electronic devices.

In a similar way to how batteries must be connected to each other in order to produce a current, so must quantum dots be directly connected.

“If quantum dots are floating around in a liquid, they cannot make electricity flow well,” Whitham said.

While ‘connecting the dots’ may sound easy, the process requires working at the molecular-level, and has remained elusive for many years.

“Imagine putting a bunch of particles in a beaker, shaking it up, and hoping that they connect in just the right way — serendipitously we found the right ‘recipe’ to connect the particles,” Hanrath said. “It had been a dream of ours to get this kind of uniformity.”

Of course, making sure you have connected such tiny particles requires seeing them. And no basic lab microscope can do that. Luckily, Cornell had exactly the resources needed to complete this work.

“There aren’t many places in the world where you have access to the kinds of tools to do something like what we have done here,” Hanrath said.

Hanrath and Whitham used electron microscopes at the Cornell Center for Material Research and x-ray beams at the Cornell High Energy Syncrotron to visualize their work.

Since connecting the dots, Hanrath and Whitham have worked to test whether the connected quantum dots actually do conduct electricity better than quantum dots that essentially “float” freely.

Their findings, published in the February issue of Nature Materials, showed that the dots were able to conduct electricity better. According to Hanrath, their work is “the current upper limit” to how well these quantum dot structures can be connected.

Drawing from the expertise of Prof. Lena Kourkoutis and Prof. Frank Weiss, applied engineering physics, they were also able to answer the question: how much closer do the dots need to be in order to get better current flow?

According to Hanrath, whereas the ideal connectivity between dots might be at least 95 percent, the current connectivity is about 80 percent.

Another goal is to control not just how much the quantum dots connect, but also how they connect. While there is work to do, the research so far provides a guide for not just the Hanrath lab but also other researchers to better understand the properties of these tiny materials.

“When people first made single crystal silicon, they weren’t envisioning microelectronics for iPhones and iPads to come out of it,” Hanrath said, “Access to high quality materials provides new understanding and in turn enables new technologies. It’s still a long road before we can make good devices, but what we’re excited about is the fundamental scientific insights we can get from this. Once you get the fundamentals, the technology can come after.”

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