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Two-dimensional, 2D, niobium carbide MXene, Nb2CTx, has attracted attention due to its extraordinarily high photothermal conversion efficiency that has applications ranging from medicine, for tumor ablation, to solar energy conversion. Here, we characterize its electronic properties and investigate the ultrafast dynamics of its photoexcitations with a goal of shedding light onto the origins of its unique properties. Through density functional theory, DFT, calculations, we find that Nb2CTx is metallic, with a small but finite DOS at the Fermi level for all experimentally relevant terminations that can be achieved using HF or molten salt etching of the parent MAX phase, including –OH, –O, –F, –Cl, –Br, –I. In agreement with this prediction, THz spectroscopy reveals an intrinsic long-range conductivity of ∼60 Ω−1 cm−1, with significant charge carrier localization and a charge carrier density (∼1020 cm−3) comparable to Mo-based MXenes. Excitation with 800 nm pulses results in a rapid enhancement in photoconductivity, which decays to less than 25% of its peak value within several picoseconds, underlying efficient photothermal conversion. At the same time, a small fraction of photoinjected excess carriers persists for hundreds of picoseconds and can potentially be utilized in photocatalysis or other energy conversion applications. 

Germanium sulfide (GeS) and germanium selenide (GeSe) are layered 2D van der Waals materials that belong to a family of group-IV monochalcogenides. These semiconductors have high carrier mobilities and moderate band gaps in the near infrared. Additionally, we have demonstrated that above gap photoexcitation results in ultrafast surface photocurrents and emission of THz pulses due to a spontaneous ferroelectric polarization that breaks inversion symmetry in the monolayer. Beyond the sub-picosecond time scales of shift currents, photoexcited carriers in both materials result in long-lived transient conductivity. We find that 800 nm excitation results in longer lived free photocarriers, persisting for hundreds of picoseconds to several nanoseconds, compared to tens to hundreds of picoseconds lifetimes for 400 nm excitation. Here, we report on tailoring the free photoexcited carrier lifetimes by intercalation of zero-valent Cu into the van der Waals gaps of GeS and GeSe. Density functional theory calculations predict that Cu atoms introduce mid-gap states. We demonstrate that intercalating only ∼3 atomic % of zero-valent Cu reduces the carrier lifetime by as much as two-to-four-fold, raising the prospects of these materials being used for high-speed optoelectronics. 

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 or 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. 

We use transient optical absorption and time-resolved terahertz THz spectroscopy to investigate photoexcitations in Ti₃C₂, Mo₂Ti₂C₃, and Nb₂C. Measurements reveal pronounced plasmonic effects. Monitoring them provides insights into thermal relaxation processes and low thermal conductivity.