To describe how matter works at infinitesimal scales, researchers designate collective behaviors with single concepts — like calling a group of birds flying in sync a “flock” or “murmuration.” Known as quasiparticles, the phenomena these concepts refer to could be the key to next-generation technologies.
In a recent study published in Science Advances, a team of researchers led by Shengxi Huang, associate professor of electrical and computer engineering and materials science and nanoengineering at Rice, describe how one such type of quasiparticle — polarons — behaves in tellurene, a nanomaterial first synthesized in 2017 that is made up of tiny chains of tellurium atoms and has properties useful in sensing, electronic, optical and energy devices.
“Tellurene exhibits dramatic changes in its electronic and optical properties when its thickness is reduced to a few nanometers compared to its bulk form,” said Kunyan Zhang, a Rice doctoral alumna who is a first author on the study. “Specifically, these changes alter how electricity flows and how the material vibrates, which we traced back to the transformation of polarons as tellurene becomes thinner.”
A polaron forms when charge-carrying particles such as electrons interact with vibrations in the atomic or molecular lattice of a material. Imagine a phone ringing in a packed auditorium during a lecture: Just as the audience shifts their gaze collectively to the source of the interruption, so do the lattice vibrations adjust their orientation in response to charge carriers, organizing themselves around an aura of polarization — hence the name of the quasiparticle.
Depending on the thinness of the layer of tellurene, the magnitude of this response — i.e., the span of the aura — can vary significantly. Understanding this polaron transition is important because it reveals how fundamental interactions between electrons and vibrations can influence the behavior of materials, particularly in low dimensions.
“This knowledge could inform the design of advanced technologies like more efficient electronic devices or novel sensors and help us understand the physics of materials at the smallest scales,” said Huang, who is a corresponding author of the paper.
The researchers hypothesized that as tellurene transitions from bulk to nanometer thickness, polarons change from large, spread-out electron-vibration interactions to smaller, localized interactions. Computations and experimental measurements backed up this scenario.
“We analyzed how the vibration frequencies and linewidths varied with thickness and correlated these with changes in electrical transport properties, complemented by the structural distortions observed in X-ray absorption spectroscopy,” Zhang said. “Furthermore, we developed a field theory to explain the effects of enhanced electron-vibration coupling in thinner layers.”
The team’s comprehensive approach yielded deeper insight into thickness-dependent polaron dynamics in tellurene than previously available. This was possible due to both improvements in the advanced research techniques deployed and the recent development of high-quality tellurene samples.
“Our findings highlight how polarons impact electrical transport and optical properties in tellurene as it becomes thinner,” Zhang said. “In thinner layers, polarons localize charge carriers, leading to reduced charge carrier mobility. This phenomenon is crucial for designing modern devices, which are continually becoming smaller and rely on thinner materials for functionality.”
On the one hand, reduced charge mobility can limit the efficiency of electronic components, especially for applications that require high conductivity such as power transmission lines or high-performance computing hardware. On the other hand, this localization effect could guide the design and development of high-sensitivity sensors and phase-change, ferroelectric, thermoelectric and certain quantum devices.
“Our study provides a foundation for engineering materials like tellurene to balance these trade-offs,” Huang said. “It offers valuable insights into designing thinner, more efficient devices while addressing the challenges that arise from the unique behaviors of low-dimensional materials, which is vital for the development of next-generation electronics and sensors.”
The research was supported by the National Science Foundation (2246564, 1943895, 2230400, 2329111, 2118448, 2046936), the Air Force Office of Scientific Research (FA9550-22-1-0408, FA2386-21-1-4064), the Welch Foundation (C-2144) and the U.S. Department of Energy (DE-SC0020148, DE-AC02-06CH11357, DE-AC02-05CH11231, BES-ERCAP0024568, DE-AC05-00OR22725). The content herein is solely the responsibility of the authors and does not necessarily represent the official views of the funding organizations and institutions.