Chemists Uncover a New Way Heat Travels Between Molecules

A new model developed by Penn Arts and Sciences chemists could be the first step towards better harnessing heat energy to power nanoscale devices. 

Scientists have long understood that heat travels through vibrations. Molecules vibrate faster and faster as they heat up, and their vibrations cause other molecules around them to vibrate as well, warming cooler nearby molecules. For decades this was the only known way heat could be transferred in organic molecules. Only recently have researchers had the ability to take a closer look at what actually happens at the molecular scale during heat transfer.

Abraham Nitzan, Professor of Chemistry, and Galen Craven, a postdoc in his lab, used new information about how to measure temperature on a nanoscale to revisit the mechanism of heat transfer. They created a model to find out how a temperature gradient affects molecular interaction, focusing in on the process of electron transfer. 

Their findings show that heat transfer occurs when the electron moves between two molecules that are at different temperatures. Electron transfer is possibly the most important process in chemistry, according to Nitzan. “Half of chemistry is electron transfer processes,” he says. “It has been investigated for 100 years on the molecular scale.”

Electrons, the negatively charged component of atoms, orbit a positively charged nucleus. In metals, electrons can move freely from molecule to molecule, producing an electric current. Electron transfer in organic molecules, however, requires more energy. When a molecule is energized, an electron will “jump” from one molecule to orbit another. This electron transfer process is essential for many common chemical reactions, especially ones that occur in biological processes.

While electron transfer has been meticulously studied, only recently have scientists been able to look at temperature on the scale of atoms and electrons. Today, scientists can detect temperature differences on the scale of a few nanometers, allowing them to see how differences across individual molecules affect their behavior.

This innovation is what inspired Nitzan and Craven to investigate how heat transfer occurs at the molecular level. They were able to create a theory of how electrons jump to molecules with less heat energy. Their model shows that heat transfer does in fact occur when an electron transfers to a lower temperature molecule. They also observed that, compared to heat transfer via vibration, electron transfer could move heat as much as a million times faster.

Craven believes this could be a discovery that is key to improving the efficiency of nanotechnology devices that rely on small-scale interactions to operate. On the nanoscale, the movement of energy from a molecule with more heat to one with less could be harnessed to power emergent technologies and devices.

While the researchers say there is still much work to be done before this knowledge can be applied, their model is a new discovery of a fundamental process that will change our understanding of how heat transfer works on a molecular level.

“Eventually what we envision in nanotechnology is energy flow and charge transfer on the nanoscale,” says Nitzan, “so it is very important to properly know and understand how molecules interact.”