Band transport by large Fröhlich polarons in MXenes
MXenes, firstly described in 2011, are a relatively new class of layered materials, each layer consisting of a few atoms of transition metal carbides and/or nitrides, e.g. Ti3C2Tx. MXenes have attracted considerable research attention for electronics and electrochemical applications, benefiting from their outstanding electrical and ionic transport properties. However, the nature of charges which can move freely in the material – so-called “free electrons” - and their transport mechanism in MXenes have remained elusive. In the past, strongly conflicting charge transport mechanisms have been proposed. New research shows that a free electron in MXenes displaces the atoms in the material's lattice: the electron is “dressed” by a local lattice deformation, extending over several lattice constants. This transforms the electron into a polaron, a quasi-particle, which plays a crucial role in determining the electrical conductivity of MXenes.
In general, electron transport can take place via two different physical processes: coherent, band-like transport in delocalized states or/and incoherent thermally-activated hopping transport between localized states. For MXenes, there is an ongoing debate whether band-like or hopping transport prevails in the material: Theoretical studies have indeed predicted band transport, however, recent device measurements revealed thermally activated, hopping-type transport.
Now, scientists from the Max Planck Institute for Polymer Research, together with partner institutes in Germany, Belgium, and China, have reconciled the debate between previous theoretical and experimental studies on the charge transport mechanism and propose a unifying picture of charge transport in MXenes. The Mainz team employed ultrafast conductivity measurements using terahertz (THz) spectroscopy to study intrinsic transport properties: the oscillating electric field in the transient THz probe pulse drives the electrons locally over tens of nanometer, thereby providing insights into short-range charge transport properties. Combined with electrical transport measurements, the researchers reveal that band-like transport dominates the short-range charge conduction, whereas long-range transport occurs through thermally activated hopping, and limits charge transport across the MXene device. The study, therefore, reconciles the debate related to the charge transport mechanism in MXenes, and provides a guide for improving the electrical transport properties.
Furthermore, the researchers reveal that electrons interact with the atoms in the material by displacing them, i.e., the electron is dressed with a lattice deformation. This was concluded from the temperature dependence of the conductivity, in conjunction with a model developed by Richard Feynman in the 1950s. The researchers show that electrons are present as large polarons in MXenes: polarons because the electrons deform the lattice around them, and large because they do so over a relatively long distance. Large polaron formation is expected to affect the charge transport and carrier lifetime in a wide range of MXene materials, as many of them share similar lattice properties. The study provides insight into the polaronic nature of free charges in MXenes, and unveils transport mechanisms in the MXene materials, relevant for both fundamental studies and applications.
The team’s findings have been published in the journal “Nature Physics”.