2D MXenes: THz spectroscopy and Applications

Lyubov Titova

Department of Physics, Worcester Polytechnic Institute, Worcester, MA 01609, USA

MXenes are 2D transition metal carbides and nitrides, with a general formula M_n+1X_nT_x, where M is a metal, X is carbon or nitrogen, n = 1-4, and T_x denotes surface terminations such as –OH, –O, and/or –F. Metallic-like conductivity, flexibility, high optical damage threshold and ease of processing owing to their hydrophilicity, make MXenes candidates for a host of electronic and optical applications. We use ultrafast THz spectroscopy to investigate the intrinsic and photoexcited charge carrier dynamics in MXenes of different chemistries: Ti₃C₂T_x, Mo₂Ti₂C₃T_x, and Nb₂CT_x (Figure 1). While all studied MXenes are metallic in nature, we find that electronic and optical properties of MXenes can be engineered by choices of the transition metals and their order as well as by controlling the intercalants in the interlayer gaps [1-3]: while the MXenes with relatively low (~ 10²⁰ cm^-3) carrier density such as Mo₂Ti₂C₃T_x, and Nb₂CT_x exhibit a transient increase in photoconductivity following excitation with 800 nm pulses, the conductivity of MXene with the highest (> 10²¹ cm^-3) carrier density (Ti₃C₂T) is suppressed in response to photoexcitation. We discuss the origin of such divergent behavior.

Figure 1: Transient photoconductivity of (a) Ti₃C₂T_x, (b) Mo₂Ti₂C₃T_x, and (c) Nb₂CT_x, following photoexcitation with 800 nm (1.55eV) optical pulses, measured using time-resolved THz spectroscopy.

Furthermore, we demonstrate that MXenes with high free carrier density show promise as polarizers and tunable electromagnetic interference shields in the THz range [4,5], as well as the photothermal energy conversion devices [2].

Acknowledgments
Approved for public release (PR #). This work is supported in part by the US Army DEVCOM Soldier Center AA1 basic research program.

References
[1] G. Li, V. Natu, T. Shi, M.W. Barsoum, L.V. Titova, “Two-Dimensional Mxenes Mo₂Ti₂C₃T_z and Mo₂TiC₂T_z: Microscopic Conductivity and Dynamics of Photoexcited Carriers,” ACS Applied Energy Materials 3, 1530-1539 (2020)
[2] E. Colin‐Ulloa, A. Fitzgerald, K. Montazeri, J. Mann, V. Natu, K. Ngo, J. Uzarski, M.W. Barsoum, L.V. Titova, “Ultrafast Spectroscopy of Plasmons and Free Carriers in 2D MXenes,” Adv. Materials 35, 2208659 (2023)
[3] A.M. Fitzgerald, E. Sutherland, T. A. El-Melegy, M. Qin Hassig, J. Martin, E. Colin-Ulloa, K. Ngo, R. L. Grimm, J. R. Uzarski, M.W. Barsoum, N. A. Deskins, L.V. Titova, and K. Kushnir Friedman, “Photoexcited Charge Carrier Dynamics and Electronic Properties of Two-dimensional MXene, Nb₂CT_x,” in revision, 2D Materials (2024)
[4] G. Li, N. Amer, H.A. Hafez, S. Huang, D. Turchinovich, V.N. Mochalin, F.A. Hegmann, L.V. Titova, “Dynamical Control over Terahertz Electromagnetic Interference Shielding with 2D Ti₃C₂T_y MXene by Ultrafast Optical Pulses,” Nano Lett. 20, 636-643 (2020)
[5] G. Li, K. Montazeri, M.K. Ismail, M.W. Barsoum, B. Nabet, L.V. Titova, Adv. Photonics Research, 2000084 (2020)