Visualizing and Modulating the Surface Carrier Dynamics of Tellurium–Selenium Nanocomposites

A comprehensive investigation of surface carrier dynamics is indispensable for advancing high-performance optoelectronic and energy-conversion devices, where interfaces critically govern the efficiency, stability, and robustness. In this work, we present an integrated study of tellurium–selenium (Te–Se) nanocomposites using X-ray photoelectron spectroscopy (XPS), femtosecond transient absorption spectroscopy (fs-TA), scanning ultrafast electron microscopy (SUEM), and density functional theory (DFT) to disentangle surface-specific and bulk relaxation pathways. XPS reveals that varying the Te:Se ratio alters the balance between surface oxidation and vacancy density, defining the interfacial landscape. Consistent with this, SUEM uncovers striking composition-dependent surface carrier dynamics: TeSe 1:1 exhibits dark contrast with localized carriers (τ ≈ 378 ps, 4.12 ns), whereas TeSe 1:0.5 displays bright contrast and long-lived diffusion (τrise ≈ 315 ps, τ2 > 6 ns), despite stronger oxidation. In contrast, fs-TA shows nearly identical bulk relaxation in both compositions, underscoring that the observed differences originate exclusively at the surface. Supported by DFT, these results demonstrate that composition-driven interface engineering provides an effective route to tailor surface carrier lifetimes, offering actionable design guidelines for Te–Se systems in advanced optoelectronic and energy applications.