Size-dependent ultrafast carrier dynamics in MXenes via femtosecond transient absorption spectroscopy

MXenes, renowned for their remarkable photothermal properties, have demonstrated significant potential in applications such as solar desalination, photothermal therapy, and flexible photothermal devices. The physical and chemical properties of MXenes are profoundly influenced by their lateral dimensions. However, a comprehensive understanding of how the lateral size affects the photogenerated carrier dynamics in MXenes remains elusive. In this study, Ti₃C₂Tₓ samples with varying lateral sizes were successfully prepared via a centrifugation-based method. Using femtosecond pump-probe spectroscopy, we systematically investigated the ultrafast carrier relaxation processes in MXenes dispersions of different lateral sizes. Our findings reveal that smaller lateral size leads to relatively lower density of photogenerated carriers, strengthened electron-phonon coupling, and increased thermal diffusion rates. Moreover, we observed a redshift of the ground-state bleaching negative peak due to the cooling of hot electrons, with a higher rate in samples with smaller lateral size, further supporting the presence of enhanced electron-phonon coupling. These results provide critical insights into the underlying relationship between lateral dimensions and ultrafast carrier dynamics in MXenes, which shed light on design of MXene-based optoelectronic devices with specific thermal management and photothermal applications.