Dynamics of Long-lived Carriers in Molybdenum Carbide Nanosheets

Molybdenum carbide (MoC) is a promising candidate for substituting expensive platinum-group metals in many applications owing to its low cost and excellent properties. A comprehensive understanding of the carrier dynamics in MoC facilitates its implementations and helps designing synthesis strategies. In this work, the carrier relaxation in MoC nanosheets is investigated by combining femtosecond transient reflection spectroscopy with first-principles calculations. The observed processes of electron-electron, electron-phonon, and phonon-phonon scattering show longer lifetimes compared to those of other transition metal carbides. The nanosecond carrier lifetime is explained by the restricted phonon decay pathways induced by the large mass difference between C and Mo atoms, which is revealed through the analysis of calculated phonon dispersion. The slow cooling of hot carriers in MoC nanosheets offers a simple approach for designing devices that effectively utilize hot carriers, which are expected to improve photothermal and photovoltaic performances.

💡 Research Summary

This work investigates the ultrafast carrier dynamics of two‑dimensional molybdenum carbide (MoC) nanosheets using femtosecond transient‑reflection spectroscopy combined with first‑principles calculations. The authors first confirm that the salt‑assisted synthesis yields high‑quality hexagonal MoC sheets (≈8–9 nm thick, ≈1 µm lateral size) that exhibit strong broadband absorption from the visible to the near‑infrared, characteristic of a metallic electronic structure. Upon excitation with 400 nm and 800 nm pulses, a broad bleach appears within a few picoseconds and decays through three distinct stages. The initial rise (τ₀≈0.5–1 ps) corresponds to electron–electron thermalization, which is slower than in typical metals, allowing hot electrons to persist longer. The second stage (τ₁, tens to hundreds of picoseconds) is attributed to electron–phonon (e‑p) scattering; MoC shows a markedly longer τ₁ than other transition‑metal carbides such as Nb₂C or Ti₃C₂, indicating a reduced e‑p coupling strength. The final stage (τ₂, nanoseconds) reflects phonon‑phonon (p‑p) scattering or heat diffusion. Density‑functional‑theory phonon calculations reveal a large phonon band gap between optical and acoustic branches, caused by the substantial mass difference between Mo and C atoms. This gap suppresses the usual Klemens decay of high‑frequency longitudinal optical phonons into two low‑frequency acoustic phonons, forcing decay through slower multi‑phonon (four‑phonon or higher) processes. Consequently, a phonon bottleneck forms, dramatically extending the carrier lifetime to the nanosecond regime. The authors also demonstrate that higher pump fluence leads to convergence of τ₁ and τ₂, consistent with hot‑phonon bottleneck effects. The study concludes that the combination of slow electron‑phonon coupling and restricted phonon decay pathways makes MoC nanosheets promising for devices that exploit long‑lived hot carriers, such as high‑efficiency photothermal converters and hot‑carrier photovoltaic architectures.