Atomistic Origin of Photoluminescence Quenching in Colloidal MoS2 and WS2 Nanoplatelets

Technical Summary: Atomistic Origin of Photoluminescence Quenching in Colloidal MoS₂ and WS₂ Nanoplatelets

Problem Statement
Colloidal transition metal dichalcogenide (TMD) nanostructures, specifically MoS₂ and WS₂, possess large chemical tunability and strong light-matter interactions, making them promising candidates for light-emitting applications. However, their potential is severely limited by ultrafast exciton decay and quenched photoluminescence (PL). While PL quenching in colloidal systems is generally attributed to surface defects, charge carrier trapping, or insufficient passivation, the specific atomistic origin of these phenomena in colloidal TMD nanoplatelets (NPLs) has remained unclear. The disordered nature of colloidal samples complicates the isolation of specific physical contributions, leaving a gap in understanding why NPLs exhibit significantly weaker PL compared to larger nanosheets (NSs) or exfoliated monolayers.

Methodology
The authors employed a combined experimental and theoretical approach to investigate the exciton dynamics in colloidal MoS₂ and WS₂ NPLs and NSs.

• Synthesis and Characterization: NPLs and NSs were synthesized via substrate-free colloidal methods using oleylamine (OlAm) as a solvent/ligand. NPLs were produced with short addition times and high sulfur excess, resulting in lateral sizes of 3.7 nm, while NSs were larger (20 nm). Structural purity (2H-phase) and monolayer nature were confirmed via X-ray photoelectron spectroscopy, X-ray diffraction, Raman spectroscopy, and second-harmonic generation.
• Ultrafast Spectroscopy: Femtosecond transient absorption (TA) spectroscopy was used to probe carrier dynamics immediately after photoexcitation. Global analysis of sequential bi/tri-exponential models was applied to resolve sub-picosecond decay components. To identify the carrier type involved in trapping, experiments were conducted with ascorbic acid as a hole scavenger.
• First-Principles Calculations: Density Functional Theory (DFT) calculations were performed on model structures of hydrogen-passivated MoS₂ and WS₂ NPLs with varying lateral dimensions (0.9, 1.5, and 2.1 nm). The study utilized inverse participation ratio (IPR) analysis to assess wavefunction localization, Bader charge analysis to examine charge fluctuations, and projected density of states (PDOS) to identify orbital contributions. Oscillator strengths were calculated within the independent-particle approximation to assess optical activity.

Key Results

1. Ultrafast Decay in NPLs: TA measurements revealed that NPLs exhibit significantly faster A-exciton bleaching decay compared to larger NSs. NPLs show a dominant sub-picosecond trapping component (270–290 fs), whereas NSs display a slower component (410–740 fs) and a long-lived radiative recombination tail.
2. Hole-Mediated Trapping: The introduction of a hole scavenger (ascorbic acid) significantly extended the decay lifetime of WS₂ NPLs and reduced the initial trapping contribution from ~48% to ~10%. This confirms that the ultrafast quenching is driven primarily by hole trapping.
3. Edge-Localized Trap States: DFT calculations identified that the mid-gap states responsible for trapping are located at the edges of the nanoplatelets. These states are dominated by metal d-orbitals (Mo or W) and persist even when sulfur-terminated edges are hydrogen-passivated.
4. Localization and Size Dependence:
   • MoS₂ vs. WS₂: WS₂ NPLs exhibit more localized and optically active edge states compared to MoS₂. Consequently, the optical transitions in WS₂ are less sensitive to size changes (20 meV shift) compared to MoS₂ (130 meV shift), which follows a "particle-in-a-box" behavior.
   • Edge Geometry: Zigzag edges exhibit a higher density of trap states than armchair edges.
5. Mechanism of Quenching: In NPLs, the high edge-to-core ratio ensures that the density of edge-localized hole traps is sufficient to outcompete radiative recombination. The excitons are captured by these optically bright edge states on a sub-picosecond timescale, leading to non-radiative decay before PL can be emitted. In larger NSs, the core dominates, allowing for longer-lived radiative recombination.

Significance and Claims
The paper claims to establish the atomistic origin of PL quenching in colloidal TMD NPLs, identifying edge-located metal d-orbitals as intrinsic, optically bright hole traps. The work demonstrates that:

• The quenching is an intrinsic property of the edge geometry and electronic structure, not solely a result of sulfur vacancies or poor passivation.
• There is a direct link between the material's morphology (size and edge type), fundamental photophysics (ultrafast quenching), and functional performance (catalytic activity). The same edge states that quench PL are responsible for the high catalytic activity of these nanostructures.
• The study provides a rational basis for designing wet-chemically synthesized TMD nanostructures. It suggests that while edge states are detrimental to PL in small NPLs, understanding their nature (metal d-orbitals) allows for future strategies, such as post-synthesis chemical treatment or functionalization, to potentially refine physicochemical protocols for enhanced PL.

The authors maintain a modest stance, noting that while they have identified the trap states, future work involving the Bethe-Salpeter equation and explicit ligand modeling is required for a fully quantitative characterization of excited states and dynamics.