Thickness-Dependent Relaxation of Near-IR Excitons in 2D CrCl₃

Two-dimensional (2D) chromium(III) chloride (CrCl₃) is promising for optoelectronic devices and magnetic sensors because of its intrinsic magnetism in the monolayer limit and strong visible absorption of its bulk form.[1] In this work, we studied the excitonic behaviors of few-layer to bulk CrCl₃ using absorption, photoluminescence (PL) spectroscopy and time-correlated single-photon counting (TCSPC). There was no significant spectral change with the number of layers in absorption and PL signals, which may originate from a localized nature of the near-IR excitons. However, the PL lifetime showed a drastic reduction from 4.5 μs to 1 ns with decreasing thickness from bulk to 1.4 nm. Control experiments with ozone-generated defects and encapsulation with hexagonal BN sandwich support that structural defects are responsible for the accelerated relaxation in 2D forms. In addition, the transition from indirect to direct bandgap may play some role in the observed change. Temperature-dependent spectral and kinetic changes will also be discussed regarding the nature of the excitons.

[1] Michael A. McGuire, Genevieve Clark, Santosh KC, W. Michael Chance, Gerald E. Jellison, Jr., Valentino R. Cooper, Xiaodong Xu, and Brian C. Sales, Phys. Rev. Materials 1, 014001 (2017).