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Band transport by large Fröhlich polarons in MXenes

In MXenes, the charge carrier (generated e.g. by light excitations) is “dressed” by the local lattice distortion induced by the charge itself. @ Wenhao Zheng | Max Planck Institute for Polymer Research

MXenes 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 free charges and their transport mechanism in MXenes remains elusive, despite their excellent performance. In fact, strongly conflicting charge transport mechanisms have previously been proposed. New research shows that a free electron in MXenes displaces the atoms in the material: the electron is “dressed” by a local lattice deformation, extending over several lattice constants. This transforms the electron into a polaron, which plays a crucial role in determining the electrical conductivity of MXenes. In general, electron transport can take place via coherent, band-like transport in delocalized states or/and incoherent thermally-activated hopping transport between localized states. Notably, there is an ongoing debate whether band-like or hooping transport prevails in MXenes: while theoretical studies have indeed predicted band transport, recent device measurements revealed thermally activated, hopping-type transport.

Now, scientists from the Max Planck Institute for Polymer Research, together with colleagues from the Helmholtz-Zentrum Dresden-Rossendorf (HZDR), the Center for Advancing Electronics Dresden (cfaed) at Technische Universität Dresden, the Max Planck Institute of Microstructure Physics in Halle as well as research institutes in 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 the intrinsic transport properties in MXenes: the oscillating electrical field in the transient THz probe pulse drives the electrons locally over tens of nanometer, thereby providing local charge transport properties. Combined with electrical transport measurements performed by the colleagues at HZDR's Institute of Ion Beam Physics and Materials Research, they reveal that band-like transport dominates short-range, intra-flake charge conduction in MXenes, whereas long-range, inter-flake transport occurs through thermally activated hopping, and limits charge percolation across the MXene flakes. The study therefore reconciles the debate related to the charge transport mechanism in MXenes, and provides a guide for improving the electrical transport properties in MXene flake networks.

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 large distance. Large polaron formation is expected to affect the charge transport and carrier lifetime in a wide range of MXene materials, as these all have similar lattice properties. The study therefore provides insight into the polaronic nature of free charges in MXenes, and unveils intra- and inter-flake transport mechanisms in the MXene materials, which are relevant for both fundamental studies and applications. The team’s findings have been published in the journal “Nature Physics”. Reference Band transport by large Fröhlich polarons in MXenes

Wenhao Zheng, Boya Sun, Dongqi Li, Sai Manoj Gali, Heng Zhang, Shuai Fu, Lucia Di Virgilio, Zichao Li, Sheng Yang, Shengqiang Zhou, David Beljonne, Minghao Yu, Xinliang Feng, Hai I. Wang & Mischa Bonn


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