Polarons are quantum entities that arise in crystalline solids due to interactions between electrons and quantized lattice vibrations (phonons). Characterizing polaron behavior is important to scientists because they can play an important role in solid-state phenomena such as thermoelectricity, ferroelectricity, magnetoresistance, and high-temperature superconductivity. While polarons have been extensively investigated in bulk (3D) lattices, few investigations have probed polarons in one- and two-dimensional crystalline structures.
In this research, scientists probed flakes of tellurene with thicknesses of less than 20 nanometers, using a technique called extended X-ray absorption fine structure (EXAFS) spectroscopy. This work was carried out at beamline 20-BM of the Advanced Photon Source, a U.S. Department of Energy (DOE) Office of Science user facility at DOE’s Argonne National Laboratory.
The EXAFS measurements characterized the structural changes in tellurene as flake thickness decreased, suggesting a transition from large-to-small polarons at a thickness of 10 nanometers. The experimental results gleaned from EXAFS, along with Raman spectroscopy data, were buttressed by theoretical insights and quantitative modeling that together provide a highly developed picture of polaron behavior as tellurene thickness varies.
These new findings will aid in developing the significant potential of tellurene for various technological applications, such as use in advanced transistors and sensing devices, and as a superconducting material. More broadly, these findings will also contribute to a deeper understanding of polaron behavior in other thin-film materials.
Tellurene is a thin-film semiconductor composed of helical chains of tellurium (Te) atoms. Because these helical chains interact through weak forces, it is sometimes referred to as a “quasi-one-dimensional” material. Tellurene is appealing for use in a variety of electronic applications due to its P-type semiconductor properties, which make it suitable for creating PN junctions when paired with N-type materials.
Tellurene samples were synthesized using a hydrothermal method that immerses the source materials in a closed bath of water-based solution subjected to high heat and pressure. Tellurium atoms subsequently precipitate out of the aqueous solution onto a substrate, forming tellurene flakes of varying thicknesses. Figure 1A shows a typical flake about 10 micrometers across and 9 nanometers thick. Fig. 1B is an electron microscope image of tellurene.
Phonons can exist in thin films as well as bulk 3D crystals. Just as a photon of light is a discrete unit (quantum) of electromagnetic energy, a phonon is the quantum of vibrational energy of a crystalline lattice. In tellurene, phonons can be polarized, meaning they vibrate along a particular direction, due to tellurene’s crystalline structure (Fig. 1C).
When a phonon strongly interacts with an electron in a crystalline lattice, a quasiparticle called a polaron is formed. A quasiparticle is not an actual particle, like an atom or electron, but rather a collective excitation. However, since the interaction between an electron and phonon is quite complex, treating polarons as quasiparticles makes them easier to describe both mathematically and conceptually.
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Image: Optical image (A) of a tellurene flake. Superimposed lines indicate lattice structure, while inset indicates the depth profile. A scanning transmission electron microscopy (STEM) image of tellurene (B), with lavender lines highlighting lattice structure. Red arrows in panel (C) indicate lattice vibrations of individual tellurium atoms (purple spheres). These unbalanced vibrations produce polarized phonons. Plot of polaron size versus flake thickness in (D) shows that smaller polarons (with higher vibration frequency) arise in thinner flakes. Gray squares represent experimental data, while blue and red spheres are calculated from theory. Panel (E) illustrates small and large polarons. Arrows in magnified inset depict attractive (red) and repulsive (blue) electrical forces.