Ultrafast Spectroscopy of Plasmons and Free Carriers in 2D MXenes.

Two-dimensional (2D) MXenes have diverse and chemically tunable optical properties that arise from an interplay between free carriers, interband transitions and plasmon resonances. The nature of photoexcitations and their dynamics in three different members of MXene family, Ti 3 C 2 , Mo 2 Ti 2 C 3 , and Nb 2 C, were investigated using two complementary pump-probe techniques, transient optical absorption and time-resolved terahertz (THz) spectroscopy. Measurements reveal pronounced plasmonic effects in the visible and near-IR in all three MXenes. Optical excitation, with either 400 nm or 800 nm pulses, results in rapid increase in lattice temperature, evidenced by a pronounced broadening of the plasmon mode which presents as a plasmon bleach in transient absorption measurements. Observed kinetics of plasmon bleach recovery provide a means to monitor lattice cooling. Remarkably slow cooling, proceeding over hundreds of picoseconds to nanoseconds time scales, implies MXenes have low thermal conductivities. The slowest recovery kinetics were observed in the MXenes with the highest free carrier density which supports phonon scattering by free carriers as a possible mechanism limiting thermal conductivity. These new insights into photoexcitation dynamics can facilitate their applications in photothermal solar energy conversion, plasmonic devices and even photothermal therapy and drug delivery. This article is protected by copyright. All rights reserved.