In nanoscale devices—such as the nanometer-sized transistors that make up computer microprocessors—tiny heat currents can have a big effect on those devices’ performance.
To measure heat currents, researchers use a technique called calorimetry. Due to advances in calorimeter technology, it’s now possible to measure very small heat flow rates—down to picowatt, or even sub-picowatt, levels—and this has enabled researchers to investigate energy transport processes that happen at the nanoscale.
“The issue is that even though we can achieve very high sensitivity with these calorimetric techniques, the time response—the fastest change in heat flow rate that can be detected—of these techniques is slow,” says Dakotah Thompson, an assistant professor of mechanical engineering at UW-Madison. “So we can’t measure dynamic processes that occur at very fast time scales—like nanoseconds or below.”
Thompson says the ability to measure rapidly changing heat currents within nanomaterials is important for understanding the underlying physics of energy conversion in nanoscale devices. These insights could open the door to more efficient solar cells, new classes of sensors, and computer processors with higher bandwidth and lower power consumption.
But first, researchers need a faster calorimeter device—one that doesn’t sacrifice sensitivity for speed.
“There’s a tradeoff. A calorimeter with a really high sensitivity will typically have a poor time response, and vice versa,” Thompson says.
With funding from a National Science Foundation CAREER Award, Thompson will develop a new calorimeter technique that is both highly sensitive and fast. “With the new calorimeter device I’m making, I want to achieve picowatt sensitivity and nanosecond time response, which has never been done before,” he says.
Ultimately, Thompson wants to use this new technique to understand the specific energy conversion processes that lead to heat generation in nanomaterials.
“Researchers can excite certain materials to a higher energy level using an external stimulus, such as light,” Thompson says. “But it won’t stay at that energy level for long. The material will ultimately fall back to its original equilibrium state, and that process is called relaxation. Relaxation in nanomaterials is a complicated process that typically generates heat depending on the specific relaxation pathways involved. The hope is to identify these pathways by probing the transient heat flow rates in real-time using my new technique.”
That’s important, Thompson says, because the heat generated during the relaxation process affects the performance of a device made from nanomaterials. For example, relaxation processes in certain semiconductors can limit their efficiency in applications such as solar cells or photodetectors.
For the project, Thompson will use thermal modeling to determine an ideal design for the nanosized calorimeter structure, which he will then fabricate using the College of Engineering cleanroom. Central to this design is an optical scheme for temperature sensing, which he is currently developing in his lab in the Mechanical Engineering Building.
“The plan is to focus a laser beam onto our nanosized calorimeter structure to detect how much its temperature changes during the relaxation of an attached nanomaterial sample,” Thompson says. “Basically, this laser optics system and the calorimeter structure will work together to allow us to measure heat flow rates in various nanomaterials with high sensitivity and speed.”
The CAREER award also has an education element that Thompson hopes will create a pipeline for future researchers from Wisconsin to become leaders in the field of thermal nanoscience and the larger STEM community. He is planning educational initiatives targeted at three levels: graduate school, undergraduate and K-12 level. He will design a graduate-level UW-Madison course on precision measurements. He also plans to involve undergraduate students in creating a series of educational demonstrations, which can then be used for K-12 outreach to engage students and expose them to thermal nanoscience concepts.
Author: Adam Malecek