Polytype-Dependent Upconversion Photoluminescence in 3R-MoS2

TL;DR

This study reveals that upconversion photoluminescence in rhombohedral MoS2 is strongly dependent on polytype, enabling new optical probing of stacking order and dark exciton dynamics with potential for ferroelectric optoelectronic applications.

Contribution

First observation of polytype-dependent upconversion photoluminescence in 3R-MoS2, linking stacking order to nonlinear optical processes and dark exciton behavior.

Findings

UPL intensity varies with polytype, distinguishing ABA and BAB structures.

Dark exciton energy separation doubles due to annihilation process.

Dark excitons are converted into bright emission, revealing valley-specific responses.

Abstract

Ferroelectric van der Waals materials offer switchable polarization states, yet optical readout of their stacking configurations remains challenging. Building on the resonant exciton-exciton annihilation (EEA) mechanism in 2H-phase TMDs, we report the first observation of upconversion photoluminescence (UPL) in rhombohedral MoS2 and demonstrate that this many-body process is strongly polytype-dependent. Using low-temperature spectroscopy, we observe anti-Stokes emission with superlinear power dependence. Beyond serving as a layer-number sensor, UPL provides a sensitive probe of stacking order. Trilayer ABA and BAB polytypes, indistinguishable by surface potential measurements and second harmonic generation, exhibit markedly different UPL intensities, and this persists in thicker samples. First-principles calculations attribute this polytype dependence to modulation of the Gamma-point conduction manifold, which controls energy-matching conditions for the annihilation process. Power-dependent spectroscopy further disentangles two distinct annihilation channels originating from different dark exciton valleys, identified through their contrasting intensity scaling and opposite density-induced energy shifts. Crucially, the annihilation process doubles the energy separation of nearly degenerate dark excitons while converting their weak emission into bright signal, providing experimental access to valley-specific responses that are obscured in direct dark-exciton spectroscopy. Our findings demonstrate that ferroelectric configurations provide a new degree of freedom for controlling nonlinear optical processes, with implications for all-optical ferroelectric readout and electrically switchable wavelength conversion in two-dimensional materials.