Hierarchical Defect Engineering for Spectral Phonon Control in Monolayer MoS2.

Two-dimensional (2D) transition-metal dichalcogenides are attractive for nanoelectronics and energy harvesting, where thermal transport critically impacts device reliability, performance, and energy efficiency. Here, we establish hierarchical defect engineering in monolayer MoS2 by combining growth-programmed mesoscale grain boundaries with He+-irradiation-induced atomic vacancies and quantify how these multiscale defects govern the in-plane thermal conductivity (κ) and its temperature dependence. By integrating opto-thermal Raman thermometry and variance-reduced Monte Carlo simulations, we decouple the distinct, temperature-dependent roles of these multiscale defects in phonon scattering. We reveal that while grain boundaries impose an approximately temperature-insensitive suppression by limiting long-mean-free-path phonons, vacancies specifically target high-frequency phonons, thereby dominating scattering at elevated temperatures. This complementary behavior enables a synergetic 85% suppression of κ at an ambient temperature (Ta) of 50 K, from 42.24 to 6.25 W m-1 K-1, confirming broadband phonon blocking. At Ta = 300 K, vacancy-induced scattering overrides grain boundary effects, causing the κ of highly defective single- and polycrystalline samples to converge. Our work provides fundamental design rules for tuning phonon transport in 2D materials and for deploying them in thermal management and thermoelectric applications.