A new class of nanoscale materials that act like microscopic mood rings, changing color with temperature, could help measure temperature at the tiniest scales, with potential applications in electronics, biology, and beyond. Published in Advanced Materials, this research from scientists at the University of California, Irvine involves a one-dimensional nanoscale material known as indium selenium iodide (InSeI).

“We found that we can make really small and sensitive thermometers,” explained Maxx Arguilla, UC Irvine professor of chemistry, in a press release. “It’s one of the most applied and translatable works to come out of our lab.”

Helical ‘slinkies’

The breakthrough happened when postdoctoral scholar Dmitri Cordova and colleagues grew InSeI crystals in their lab and noticed that, at nanometer length scales, they resemble helical “slinkies.”

They scientists first grew the crystals to subject them to heat stress and explore at what temperatures the crystals would disintegrate.

Cordova and undergraduate researcher Leo Cheng then noticed that the colors of the InSeI crystals systematically shifted from light yellow to red-orange, depending on the temperature. Light yellow corresponded to temperatures around –190°C, while red-orange corresponded to temperatures around 200°C.

The team took measurements to calibrate the color changes with temperature. They found InSeI has a large electron-phonon coupling constant (S = 7.32) and a high temperature sensitivity of the bandgap, with (dEg/dT)max = 1.26 × 10−3 eV K−1, significantly larger than most optically active semiconductors in the visible region.

To obtain nanoscale samples of InSeI, the researchers stuck adhesive tape to the bulk crystals, peeled it back, and transferred the adhered nanoscale structures onto transparent substrates. “We can peel off these structures, and we can use them as nanoscale thermometers that can be transferred, reconfigured and coupled with other materials or surfaces,” said Arguilla.

The development of InSeI nanothermometers opens up new possibilities in a range of fields. In biomedicine, these tiny thermometers could be used to monitor temperature fluctuations within individual cells. “The need to measure temperature is important because a lot of biological and industrial processes depend on tracking minute changes in temperature,” Arguilla said. “We may now have thermometers that we could try poking into the cells.”

InSeI could also be integrated into microchips and data storage devices to monitor temperature and prevent overheating. Already, the semiconductor industry uses optical thermometers when fabricating computer components, but the researchers’ new material is “at least an order of magnitude more sensitive,” noted Dmitri Cordova in the press release.

A closer look at how it works

Indium Selenium Iodide (InSeI) is a van der Waals solid, meaning it’s made of layers held together by weak forces. It’s composed of weakly bound helical chains that exhibit significant visible thermochromism, shifting its optical absorption edge from 450 to 530 nm over a temperature range of 380 K.

Think of it like this: the arrangement of atoms in InSeI changes slightly with temperature, altering how it absorbs and reflects light, leading to the visible color change. This significant shift is a result of strong interactions between electrons and vibrations within the material (electron-phonon interactions) and the way these vibrations behave at different temperatures (anharmonic phonons). That is the trait that makes InSeI a promising candidate for highly sensitive optical thermometry at the nanoscale.