Photoluminescence Path Bifurcations by Spin Flip in Two-Dimensional CrPS4

TL;DR

This paper investigates dual-band near-infrared photoluminescence in ultrathin CrPS4 crystals, revealing thickness-dependent bifurcations between fluorescence and phosphorescence driven by spin state transitions.

Contribution

It demonstrates the thickness, temperature, and defect influence on PL bifurcations in 2D CrPS4 and models the process with a three-level kinetic framework.

Findings

PL emission bifurcates into fluorescence and phosphorescence.

Biexponential decay of PL transients depends on thickness, temperature, and defects.

Transition barriers lower in thinner samples due to surface defects.

Abstract

Ultrathin layered crystals of coordinated chromium(III) are promising not only as two-dimensional (2D) magnets but also as 2D near-infrared (NIR) emitters owing to long-range spin correlation and efficient transition between high and low-spin excited states of Cr3+ ions. In this study, we report on dual-band NIR photoluminescence (PL) of CrPS4 and show that its excitonic emission bifurcates into fluorescence and phosphorescence depending on thickness, temperature and defect density. In addition to the spectral branching, the biexponential decay of PL transients, also affected by the three factors, could be well described within a three-level kinetic model for Cr(III). In essence, the PL bifurcations are governed by activated reverse intersystem crossing from the low to high-spin states, and the transition barrier becomes lower for thinner 2D samples because of surface-localized defects. Our findings can be generalized to 2D solids of coordinated metals and will be valuable in realizing novel magneto-optic functions and devices.