Atomistic Origin of Photoluminescence Quenching in Colloidal MoS2 and WS2 Nanoplatelets

TL;DR

This study uncovers that edge-located hole traps cause ultrafast exciton decay and PL quenching in colloidal MoS2 and WS2 nanoplatelets, with implications for improving their light-emitting properties.

Contribution

It combines spectroscopy and first-principles calculations to identify intrinsic edge trap states as the cause of exciton quenching in TMD nanoplatelets, revealing differences between MoS2 and WS2.

Findings

Edge-located hole traps cause sub-picosecond exciton decay.

WS2 nanostructures have more localized edge states than MoS2.

Zigzag edges have higher trap density than armchair edges.

Abstract

Large chemical tunability and strong light-matter interactions make colloidal transition metal dichalcogenide (TMD) nanostructures particularly suitable for light-emitting applications. However, ultrafast exciton decay and quenched photoluminescence (PL) limit their potential. Combining femtosecond transient absorption spectroscopy with first-principles calculations on MoS2 and WS2 nanoplatelets, we reveal that the observed sub-picosecond exciton decay originates from edge-located optically bright hole traps. These intrinsic trap states stem from the metal d-orbitals and persist even when the sulfur-terminated edges are hydrogen-passivated. Notably, WS2 nanostructures show more localized and optically active edge states than their MoS2 counterparts, and zigzag edges exhibit a higher trap density than armchair edges. The nanoplatelet size dictates the competition between ultrafast edge-trapping and slower core-exciton recombination, and the states responsible for exciton quenching enhance catalytic activity. Our work represents an important step forward in understanding exciton quenching in TMD nanoplatelets and stimulates additional research to refine physicochemical protocols for enhanced PL.