Smart watches worn on millions of wrists today pack vastly more processing power than the computers that helped Apollo astronauts land on the moon. 

But the scientific drive to go tiny has come at a cost: It means heat, electricity and energy now have much less room to flow through devices.

"We will eventually reach a bottleneck to design the next generation of electronic and photonic devices," said Longji Cui, an assistant professor in the Paul M. Rady Department of Mechanical Engineering at CU Boulder. "Thermal management inside electronic devices is extremely important, but to do it more effectively, we need to understand how heat is flowing through these nanoscale devices."

Longji Cui

Building devices at the tiny nanoscale level will require engineers to better understand how the materials inside them, like glass, behave when they are shrunk to such a small size. 

In a new paper, published in the journal Nature Nanotechnology, Cui and his team discovered that at that extremely small atomic scale, heat easily flows through glass. It’s counterintuitive because glass is known for being a poor thermal conductor. In fact, the heat flowed in discrete steps, each equal to 40% of the maximum amount a single channel can carry under the laws of quantum physics—the first time such "quantized" heat flow carried by atomic vibrations has been observed.

"Glass is what we put in windows to keep heat in," said Guilherme de Vilhena, a researcher at the Materials Science Institute of Madrid and a co-author of the study. "But stretch it into a thread one atom wide and it does the opposite: it lets heat through with an efficiency we had never seen in an insulator."

In ordinary glass, the jumbled arrangement of atoms scatters the vibrations that carry heat. In a thread one atom wide, there is no room for that disorder, so the vibrations pass straight through.

The findings could have implications for how engineers design smaller nanoscale electronics in the future.  Glass, in the form of silicon dioxide, is used throughout today’s computer chips as an insulating layer. 

Scientists are using a new tool that enables them to observe how the tiniest particles of material transfer heat. Using it, the researchers have discovered that, at the atomic level, materials behave in surprising ways.

A tiny fingertip

The tool is called a scanning thermal probe microscope (SThM). It uses a glass tip and a built-in thermometer to feel the motion of tiny atoms and molecules to measure how heat flows through atomic-scale contacts. Cui compares it to an extremely small fingertip. 

"When you put your own finger to touch a surface, you can feel the texture and the heat," he said. "With this microscope, we shrink this finger to nanoscale and it can read the temperature and heat flow through a single atom."

In their research, the team used this fingertip to observe two pieces of glass being pressed together. They then slowly pulled them apart, stretching the contact until it narrowed to a thread just one atom wide before snapping.

Measuring the unknown

Cui said this counterintuitive behavior they discovered in tiny glass particles could also happen for other materials. 

"We will now be able to study any material we want with this microscope, and there could be constant surprises that come from this instrument," he said. 

The research is all part of Cui’s quest to develop more accurate sensors to measure things we can't see.

"I want to push the world record for thermal sensing, which means I need to build more and more sensitive platforms," he said. "And people will use it to study fundamental thermal problems."

The microscope can help scientists study the atomic-scale thermal transport in virtually any material, both metals and insulators. 

"The materials we are particularly interested in are semiconductors that are the building blocks of nanoelectronics devices," Cui said. "This new microscope we developed makes a great diagnostic tool to explore issues associated with energy efficiency and reliability of electronic devices at the nanoscale."

The team also included researchers from the Autonomous University of Madrid and the Materials Science Institute of Madrid in Spain. The research was supported by the U.S. National Science Foundation.