Structure-Dependent Electronic Relaxation Dynamics of Two-Dimensional Silver Monolayers

The electronic relaxation dynamics of two-dimensional silver polar metal heterostructures (2D-PMets), isolated with two different Ag lattice structures, were studied with femtosecond transient absorption (fs-TA) spectroscopy. The two 2D Ag phases, called Ag₍₁₎and Ag₍₂₎, differ in atomic packing density, which leads to phase-specific ultralow frequency (ULF) phonon modes and visible electronic absorption transitions. Time-resolved kinetic traces for both phases were fit to a biexponential decay function, with the first decay component pertaining to ultrafast electronic relaxation and the second corresponding to carrier-phonon scattering. The first decay time constant τ₁is <400 fs for both phases. In contrast, carrier-phonon scattering exhibited lattice-specific and excitation wavelength-independent relaxation time constants; τ₂∼ 2 ps for Ag₍₁₎and ∼ 1 ps for Ag₍₂₎. The shorter τ₂in Ag₍₂₎is attributed to increased carrier-phonon scattering probability in more close-packed lateral structures. The results indicate that atomic-level structure controls energy flow in spatially confined 2D materials.