Two-dimensional Niobium Carbide MXene, Nb2CTx: intrinsic and photoexcited carrier dynamics

Garnering attention for high conductivity, nonlinear optical properties, and more, MXenes are water-processable 2D materials that are considered candidates for applications in electromagnetic interference shielding, optoelectronic and photonic devices among others. Herein we investigate the intrinsic and photoexcited conductivity in Nb
2
CT
x
, a MXene with reported high photothermal conversion efficiency. DFT calculations show that hydroxyl and/or fluorine-terminated $\mathrm{Nb}_{2} \mathrm{CT}_{x}\left(\mathrm{~T}_{x}=\mathrm{OH}\right.$ or $\left.\mathrm{F}\right)$ is metallic, in agreement with THz spectroscopy, which reveals the presence of free charge carriers that are highly localized over mesoscopic length scales. Photoexcitation of Nb
2
CT
x
, known to result in rapid heating of the crystal lattice, is found to produce additional free carriers and a transient enhancement of photoconductivity. Most photoexcited carriers decay over the sub-picosecond time scales while a small fraction remain for much longer, sub-nanoseconds, times.