The electrical properties and charge transport mechanism of MXenes

The electrical properties and charge transport mechanism of MXenes

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Bowen Song^a, Chenxu Shi^a, Yinghao Qu^a, Hongjun Liu^a, Hui Yang^*^,^a, Xiaoming Wu^*^,^a, Xijun Liu^*^,^b

Chinese Chemical Letters | 2025, 36(6) : 110823

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Chinese Chemical Letters | 2025, 36(6): 110823

• Editorial •

The electrical properties and charge transport mechanism of MXenes

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Bowen Song^a, Chenxu Shi^a, Yinghao Qu^a, Hongjun Liu^a, Hui Yang^*^,^a, Xiaoming Wu^*^,^a, Xijun Liu^*^,^b

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^aKey Laboratory of Display Materials and Photoelectric Devices (Ministry of Education), Tianjin Key Laboratory for optoelectronic Materials and Devices, Tianjin Key Laboratory of Functional Crystal Materials, Institute of Functional Crystals, School of Materials Science and Engineering, Tianjin University of Technology, Tianjin 300384, China

^bGuangxi Key Laboratory of Processing for Non-ferrous Metals and Featured Materials, MOE Key Laboratory of New Processing Technology for Nonferrous Metals and Materials, School of Resources, Environment and Materials, Guangxi University, Nanning 530004, China

Published: 2025-06-15 doi: 10.1016/j.cclet.2025.110823

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Bowen Song, Chenxu Shi, Yinghao Qu, Hongjun Liu, Hui Yang, Xiaoming Wu, Xijun Liu. The electrical properties and charge transport mechanism of MXenes[J]. Chinese Chemical Letters, 2025 , 36 (6) : 110823 - . DOI: 10.1016/j.cclet.2025.110823

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Two-dimensional (2D) transition metal carbides and/or nitrides (MXenes) have exhibited many outstanding merits, including good conductivity, tunable bandgap, high electric capacity and optical transparency [1,2]. In the past several years, MXenes have shown promising advantages in the fields of energy storage, electrocatalysis, electromagnetic shielding, and (opto-)electronic devices. These excellent properties can be tuned by controlling the chemical composition, shape and size of the nanosheets, defects, boundaries, and surface functional groups, etc.

2D MXene nanosheets can be obtained by selectively etching away the A elements from their parent ternary phase precursors and always terminate with surface functional groups such as -OH, -F and -O. Generally, non-terminated MXenes are normally metallic with a high density of states near the Fermi surface, and the presence of surface functional groups makes MXenes the narrow-band semiconductors. Friedman et al. [3] revealed that Nb₂CT_x had an intrinsic conductivity of ~60 Ω⁻¹ cm^-1, and an intrinsic carrier density of 10²⁰ cm^-3 with the carriers strongly localized due to the disorder and nanoflake boundaries. Short-range, intra-flake free carrier mobility was found to be 30±4 cm² V^-1 s^-1, while the flake-boundaries and the disorder suppressed the long-range (inter-flake) mobility to 2.4 ± 0.4 cm² V^-1 s^-1. Furthermore, theoretical studies have predicted the efficient band-like transport in MXenes, while device measurements revealed that the conductivity increased at elevated temperature, which was attributed to the thermally activated hopping-type transport in MXenes [4]. This debate between the theoretical and experimental results has sparked issues regarding the nature of the charge carriers and charge transport mechanism of MXenes. Understanding the charge transfer mechanism of MXenes can help to optimize the electrical properties of materials. Clarifying the charge transfer mechanism of MXenes at different temperatures can guide their application in fields such as electronic, optoelectronic devices, energy storage, and electromagnetic shielding, ensuring the stability and performance of the devices. Therefore, the fundamental understanding of electronic properties and charge transport mechanism in MXenes is essential for the fundamental studies and a variety of applications.

Recently, a unified understanding of conductivity and charge transport mechanism in MXenes by employing ultrafast terahertz and static electrical transport measurements has been proposed [5]. This work revealed that the short-range, intra-flake charge conduction in MXenes was band-like transport, while the long-range, inter-flake transport occurred via the thermally activated hopping process (Fig. 1a) which limited the charge percolation across the MXene flakes. Due to the presence of charge carriers, it can induce lattice distortion through coulomb interaction, known as polarons. The carrier-phonon coupling constant α characterizes the strength of the coupling between the charge carriers and longitudinal optical (LO) phonons. For the weak coupling (α < 6), large polarons are formed. Considering the Fröhlich polaron theory for MXene materials, different types of MXenes have the similar dielectric constants, which leads to the weak coupling between the charge carriers and phonons in MXenes, thereby leading to the formation of large polarons (Fig. 1b). The formation of large polarons in the MXene fundamentally affects the intrinsic charge transport. Despite high density of defects existing in MXenes, the charge carriers may be protected by large polarons, which effectively screens the defect potential and reduces the charge scattering, thus improving charge transfer efficiency. For the semiconducting Nb₄C₃T_x, the selected-area electron diffraction pattern demonstrates the high crystallinity and hexagonal symmetry (P6₃/mmc), in line with typical MXene structures (Fig. 1c). The photoexcited electrons of Nb₄C₃T_x result in a positive photoconductivity. While for the metallic Ti₃C₂T_x, the transient photoconductivity decreases and gives rise to a negative conductivity (Fig. 1d). The photoconductivity of Nb₄C₃T_x (Fig. 1e) decays swiftly within several picoseconds and then slowly decays, which is attributed to the trapped free carriers at defects. The transient photoconductivity decrease in Ti₃C₂T_x confirms the metallic nature of Ti₃C₂T_x (Fig. 1f). The LO-phonon scattering dominates the charge transport over the entire temperature range, while the impurity scattering becomes notable only at the lower temperatures (T < 150 K). The transient, picosecond-duration terahertz field drives the charge carrier over approximately tens of nanometers. This provides the short-range, intra-flake charge transport information. Furthermore, the intra-flake charge transport is dominated by the band-like transport, which is mainly influenced by the LO phonons scattering for both semiconducting and metallic MXenes. Moreover, static electrical transport studies provide information on the long-range charge carrier conductance over macroscopic distances. It was investigated that the long-range, inter-flake charge transport was relevant with thermally activated hopping, which was the rate-limiting step for charge percolation through devices consisting of many MXene flakes.

In summary, MXenes exhibit the amazing prospects in various fields due to their excellent merits. The electronic properties, particularly the conductivity and the presence of large polarons in MXenes have been elucidated, and the intra-flake and inter-flake charge transport mechanism of MXene network was also clearly revealed. This is of great significance for the further fundamental research on 2D MXenes and various applications in electronic, optoelectronic, and energy devices.

Declaration of competing interest

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The authors declare no conflict of interest exists and state that the article is original, unpublished, and not being considered for publication elsewhere.All authors have participated sufficiently in this work to take public responsibility for it.

All authors have reviewed the final version of the manuscript and approved it for publication.

CRediT authorship contribution statement

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Bowen Song: Resources. Chenxu Shi: Data curation. Yinghao Qu: Formal analysis. Hongjun Liu: Conceptualization. Hui Yang: Conceptualization. Xiaoming Wu: Supervision. Xijun Liu: Supervision.

References

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[1]

L. Hou, X. Peng, S. Lyu, et al., Chin. Chem. Lett. 36 (2025) 110392.

[2]

M. Shang, Y. Sun, China Powder Sci. Technol. 30 (2024) 91–101.

[3]

A.M. Fitzgerald, E. Sutherland, T.A. El-Melegy, et al., 2D Mater. 11 (2024) 035028.

[4]

M. Han, K. Maleski, C.E. Shuck, et al., J. Am. Chem. Soc. 142 (2020) 19110–19118.

[5]

W. Zheng, B. Sun, D. Li, et al., Nat. Phys. 18 (2022) 544–550.

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Year 2025 volume 36 Issue 6

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doi: 10.1016/j.cclet.2025.110823

Receive Date：2024-11-08
Online Date：2025-10-29
Published：2025-06-15

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Received：2024-11-08
Revised：2024-12-24
Accepted：2025-01-06

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表12种不同金属材料的力学参数

科 Family	属数 Number of genus	种数 Number of species	占总种数比例 Percentage of total species (%)	属 Genus	种数 Number of species	占总种数比例 Percentage of total species (%)
鹅膏菌科Amanitaceae	2	11	5.26	鹅膏菌属 Amanita	10	4.78
小菇科 Mycenaceae	2	12	5.74	丝盖伞属 Inocybe	5	2.39
多孔菌科 Polyporaceae	8	14	6.70	蜡蘑属 Laccaria	5	2.39
红菇科 Russulaceae	3	23	11.00	小皮伞属 Marasmius	6	2.87
				小菇属 Mycena	11	5.26
				光柄菇属 Pluteus	5	2.39
				红菇属 Russula	17	8.13
				栓菌属 Trametes	5	2.39

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