Optical Thermometry in Monolayer WS2: Raman and Photoluminescence Spectroscopy from 5 K to 1273 K

Imagine a world made of materials so thin they are only a single layer of atoms thick. These are two-dimensional crystals, a class of matter that behaves differently from the solid blocks we touch every day. Because they are so thin, they are incredibly sensitive to their surroundings, reacting strongly to heat, light, and the materials they sit upon. Among these, a material called tungsten disulfide, which looks like a shimmering sheet of metal, has become a favorite for scientists building future electronic devices. To understand how these tiny sheets work, researchers must know their temperature with great precision. However, measuring the heat of something only a few atoms thick is a paradox: you cannot stick a thermometer on it without covering it up or changing its behavior. For years, scientists have used light to peek inside these materials, watching how their colors or vibrations change as they warm up. But these optical tricks have usually worked only within a narrow range of temperatures, failing when things get too cold or too hot.

A team of researchers in France and Japan has now solved this problem for tungsten disulfide, creating a reliable way to measure its temperature from the freezing cold of deep space to the scorching heat of a furnace. They demonstrated that by shining a laser on a single layer of this material, they could read its temperature simply by analyzing the light that bounces back or glows out. This method works across a massive range, from 5 Kelvin, which is just a few degrees above absolute zero, all the way up to 1273 Kelvin, a temperature where most materials would melt or break apart. The key to their success was not just finding a new trick, but proving that two different ways of reading the light—one based on how the atoms vibrate and another based on how the material glows—could work together to cover the entire spectrum of heat.

The researchers began by preparing two samples of this single-layer material. One was left bare, sitting alone on a special membrane that could be heated by electricity. The other was sandwiched between two layers of a different material called hexagonal boron nitride, which acts like a protective shield. They placed these samples inside a machine that could cool them down to near absolute zero and then heat them up again, all while a laser beam focused on a tiny spot on the surface. As they changed the temperature, they recorded the light coming from the samples. For the bare sample, they could only measure the vibrations of the atoms, known as Raman scattering, because the intense laser light would otherwise disrupt the delicate electronic states needed for glowing. For the protected sample, the shield prevented this disruption, allowing them to measure both the vibrations and the glowing light simultaneously.

The first discovery was that the vibrations of the atoms in the bare material changed in a perfectly straight line as the temperature rose from room temperature up to 1223 Kelvin. Specifically, a specific vibration mode shifted its pitch steadily downward as the material got hotter. This shift happened at a consistent rate, allowing the researchers to use it as a ruler for temperature. They found that the bare material was slightly more sensitive to heat than the protected one, which makes sense because the protective layers in the second sample held the atoms in place, preventing them from expanding as freely. This difference was crucial: it meant that a temperature reading taken from a bare sheet could not be directly applied to a protected sheet without adjustment. The researchers established that this linear relationship held true over a range of more than 900 degrees, far wider than any previous study had managed for this material.

The second part of the study focused on the glowing light, or photoluminescence, which could only be seen clearly in the protected sample. At very low temperatures, the material glows with a complex mix of colors, but as it warms up, these colors merge into a single, bright line of light. The researchers tracked the exact energy of this light as the temperature climbed from 5 Kelvin to 1273 Kelvin. They found that the light shifted toward the red end of the spectrum in a smooth, predictable curve that matched a known physical model describing how heat affects the energy gaps in atoms. This glow provided a second, independent way to measure temperature. At lower temperatures, where the vibrations were too subtle to measure accurately, the glow was sharp and precise. At higher temperatures, the glow became broader and harder to pinpoint, but it remained measurable all the way to the upper limit of their experiment.

By combining these two methods, the team created a complete toolkit for measuring heat in this material. The vibration method works best when things are hot, while the glowing method is excellent for the cold and the middle range. Together, they cover the entire span from 5 to 1273 Kelvin. The researchers also discovered that the protective layers did more than just allow them to see the glow; they actually helped the material survive higher temperatures. The bare sample began to fall apart and lose its signal around 1223 Kelvin, likely because the sulfur atoms started to evaporate. The protected sample, however, remained intact and measurable even at 1273 Kelvin, suggesting that the shield not only improved the quality of the data but also extended the life of the material under extreme heat.

This work matters because it removes a major obstacle in the development of future nanodevices. Engineers designing circuits with these ultra-thin materials need to know exactly how hot they are getting to prevent damage or malfunction. Before this study, there was no reliable way to check the temperature of a single layer of material across such a wide range without touching it. Now, scientists can simply shine a laser and read the result, whether they are studying the material in a cryogenic lab or a high-temperature oven. The study confirms that these optical signals are robust and predictable, offering a contact-free thermometer that works from the coldest depths of space to the hottest industrial processes. While the upper limit of 1273 Kelvin was set by the equipment used in the experiment rather than the material itself, the results suggest that with the right protection, these materials could withstand even more extreme conditions, opening the door for new applications in high-temperature electronics and sensors.