Photoluminescence Path Bifurcations by Spin Flip in Two-Dimensional CrPS₄

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 due to long-range spin correlation and efficient transition between high- and low-spin excited states of Cr3+ ions. In this study, we report on the 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 groundbreaking magneto-optic functions and devices.

Nanomechanical absorption spectroscopy of 2D materials with femtowatt sensitivity

“…4 A 2g to 4 T 1g ). This spectrum measured above the antiferromagnet's Néel temperature is expected and in line with literature reports [33][34][35]. Thin CrPS 4 is rather sensitive to photodamage [33,35], which can be problematic for classical optical approaches.…”

Nanomechanical absorption spectroscopy of 2D materials with femtowatt sensitivity

“…This spectrum measured above the antiferromagnet's Néel temperature is expected and in line with literature reports. [33][34][35]. Thin CrPS 4 is rather sensitive to photodamage [33,35], which can be problematic for classical optical approaches.…”

“…[33][34][35]. Thin CrPS 4 is rather sensitive to photodamage [33,35], which can be problematic for classical optical approaches. For NMS however small laser powers (P inc ≈1 µW) are sufficient to induce sizeable frequency shifts (comp.…”

Single-molecule acceptor rise time (smART) FRET for nanoscale distance sensitivity

“…The timescale of energy transfer is also encoded in the rise of the acceptor fluorescence, a readout spectroscopically separated from photophysical fluctuations in intensity or lifetime and from background emission. Single-molecule measurements of the rise were not possible until the recent development of fast single-photon detectors 21 . Using this technology, we introduce single-molecule acceptor rise time (smART) FRET, a method that extracts the efficiency from the acceptor rise.…”