Atomistic Origin of Photoluminescence Quenching in Colloidal MoS2 and WS2 Nanoplatelets

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.

💡 Research Summary

This paper investigates the origin of the pronounced photoluminescence (PL) quenching observed in colloidal transition‑metal dichalcogenide (TMD) nanoplatelets (NPLs) of MoS₂ and WS₂. The authors combine femtosecond transient absorption (TA) spectroscopy with density‑functional theory (DFT) calculations to elucidate the ultrafast exciton dynamics and the electronic structure of these nanostructures.

Synthesis and structural characterization: MoS₂ and WS₂ nanostructures are produced by a hot‑injection method in oleylamine (OlAm) using MoCl₅/WCl₆, elemental sulfur, and hexamethyldisilazane (HMDS). By varying the injection time and sulfur excess, the authors obtain small NPLs (average lateral size ≈3.7 nm) and larger nanosheets (NSs, ≈20 nm). High‑resolution TEM, FFT, XPS, Raman and second‑harmonic generation confirm that all samples are monolayer, 2H‑phase, and free of metallic polymorphs.

Steady‑state optical properties: Lateral confinement shifts the A‑exciton of MoS₂ by ~130 meV (1.91 → 2.04 eV) and reduces its intensity, whereas WS₂ shows only a ~20 meV shift. The shift is larger for MoS₂, indicating a stronger size‑dependent modification of its electronic structure.

Transient absorption results: TA measurements are performed with pump excitation resonant with the B‑exciton at low fluence (4.7 µJ cm⁻²). Global analysis using a sequential bi‑/tri‑exponential model yields three characteristic times: (i) an initial sub‑picosecond decay (t₁ ≈ 200–300 fs) attributed to carrier trapping, (ii) a slower component (t₂ ≈ 10–40 ps) linked to band‑gap renormalization, and (iii) a long‑lived component (t₃ > ns) representing radiative recombination from trapped states. NPLs exhibit markedly faster and more dominant t₁ than NSs, indicating a higher density of mid‑gap traps that promote non‑radiative decay.

Hole‑trapping identification: To determine whether electrons or holes dominate the initial trap, the authors add ascorbic acid, a known hole scavenger. The presence of the scavenger dramatically extends the excited‑state absorption lifetime and reduces the contribution of the fast component from ~48 % to ~10 %. This experiment confirms that the sub‑picosecond trapping is hole‑mediated.

First‑principles calculations: Theoretical models consist of hydrogen‑passivated monolayer MoS₂ and WS₂ NPLs with lateral dimensions of 0.9, 1.5, and 2.1 nm, featuring both zigzag (ZZ) and armchair (AC) sulfur‑terminated edges. Formation energies decrease with size, and WS₂ NPLs are consistently more stable than MoS₂ NPLs due to stronger W–S bonds.

Electronic structure analysis: Inverse participation ratio (IPR) calculations reveal that states near the Fermi level (EF) that are occupied (hole states) are highly localized across all sizes, especially in the smallest NPLs. Electron (unoccupied) states remain largely delocalized. WS₂ exhibits higher IPR values for hole states than MoS₂, correlating with its experimentally observed smaller excitonic shift and stronger PL quenching.

Bader charge analysis shows that, despite sulfur termination and hydrogen passivation, the edge metal atoms retain significant charge deviations and dominate the localized states. The disrupted bonding at the edges reduces charge sharing, creating spatially confined mid‑gap states derived primarily from metal d‑orbitals.

Core‑edge partitioning demonstrates that zigzag edges contribute more strongly to the edge‑localized states than armchair edges, consistent with the higher trap density observed experimentally for zigzag terminations.

Size dependence: Because NPLs have a high edge‑to‑core atom ratio, the density of these hole traps scales inversely with lateral size. Consequently, ultrafast hole trapping outcompetes radiative recombination, leading to the near‑absence of PL in NPLs. In contrast, larger NSs possess fewer edge sites, allowing a measurable radiative component (t₃) and observable PL.

Catalytic relevance: The authors note that the same edge‑localized d‑orbital states responsible for PL quenching are known to be active sites for catalytic reactions (e.g., hydrogen evolution). Thus, the “defect” that degrades optical performance may be harnessed to improve catalytic functionality.

Overall conclusions: (1) Sub‑picosecond PL quenching in MoS₂ and WS₂ NPLs originates from intrinsic, optically bright hole traps located at the edges, especially zigzag terminations. (2) These traps are metal d‑orbital in nature and persist even after hydrogen passivation of sulfur edges. (3) WS₂ exhibits more localized and optically active edge states than MoS₂, explaining its stronger PL suppression. (4) The competition between ultrafast edge trapping and slower core‑exciton recombination is governed by nanoplatelet size. (5) Edge trap states, while detrimental to PL, enhance catalytic activity, suggesting a pathway to engineer multifunctional TMD nanostructures.

The study provides a comprehensive atomistic picture linking morphology, ultrafast photophysics, and functional performance, and it offers clear guidance for future synthetic strategies aimed at either suppressing edge traps to boost PL or exploiting them for catalytic applications.