High‑Performance Dual‑Emissive Mn‑Doped InP/ZnS Quantum Dots with 78 % Photoluminescence Quantum Yield: A Growth‑Doping Approach

Abstract

We report the first synthesis of dual‑emissive, color‑tunable Mn‑doped InP/ZnS quantum dots (Mn:InP/ZnS QDs) that exhibit an absolute photoluminescence quantum yield (PL QY) of up to 78 %. The dual emission—comprising an intrinsic InP core band and a Mn‑doped defect band—can be precisely tuned by adjusting the Mn/In precursor ratio. Increasing Mn incorporation shifts the intrinsic emission red‑wards from 485 to 524 nm, a phenomenon directly linked to size growth and band‑gap narrowing. These Mn:InP/ZnS QDs offer a promising, non‑toxic platform for next‑generation white‑LED technologies.

Background

Quantum dots (QDs) have revolutionized bio‑imaging, sensing, and optoelectronics thanks to their exceptional photostability, large Stokes shifts, and tunable lifetimes. Doping semiconductor QDs with transition‑metal ions introduces new emissive pathways without altering the host absorption profile, yielding dual‑emission QDs that combine broad spectra with high color fidelity—an ideal match for white‑LED design. While cadmium‑based QDs dominate the market, their toxicity restricts widespread deployment. Indium phosphide (InP) QDs have emerged as a leading non‑toxic alternative, yet doped InP systems remain underexplored. Previous reports on Cu‑ or Ag‑doped InP QDs either failed to produce dual emission in the visible window or required complex, non‑scalable syntheses. Our work addresses these gaps by delivering a scalable growth‑doping route that yields high‑yield, dual‑emissive Mn:InP/ZnS QDs.

Methods

Chemicals

Zinc iodide (ZnI₂, ≥ 98 %), tris(dimethylamino)phosphine, manganese chloride (MnCl₂, ≥ 99 %), indium chloride (InCl₃, ≥ 99.995 %), 1‑dodecanethiol, 1‑octadecene, oleylamine, and other solvents were purchased from commercial suppliers and used without further purification.

Synthesis of Mn:InP/ZnS QDs

In a 50‑mL three‑neck flask, 0.7 mmol InCl₃, 2.8 mmol ZnI₂, 6 mL OLA, and 4 mL ODE were degassed at 120 °C for 1 h and then heated to 220 °C under N₂. A 0.25 mL aliquot of tris(dimethylamino)phosphine was injected at 220 °C to initiate InP core growth. After 5 min, the temperature was raised to 240 °C, 3 mL DDT and a pre‑prepared MnCl₂ stock (0.54 mmol MnCl₂ dissolved in 1 mL ODE + 1 mL OLA at 120 °C) were sequentially injected. The mixture was then maintained at 200 °C for 5 h, cooled, and the QDs were purified by hexane‑ethanol precipitation and redispersed in toluene or hexane.

Characterization

UV–vis and PL spectra were recorded on a Shimadzu UV‑3600 and RF‑5301PC, respectively. TEM and HRTEM imaging employed a JEOL 2100F (200 kV). XRD used a Bruker D8 Advance, XPS an ESCALAB 250Xi, and time‑resolved PL a FLSP920. Absolute PL QY was measured in an integrating sphere per the standard equation (Eq. 1).

Results and Discussions

Crystalline Structure and Composition

Figure 1 displays TEM/HRTEM images of Mn:InP/ZnS QDs at Mn/In ratios of 0, 0.4, and 0.6. Average diameters increase from 3.6 nm to 5.0 nm as Mn content rises, confirming size growth with dopant concentration. XRD patterns (Fig. 2) show broad peaks at 28.3°, 47.3°, and 55.8°, corresponding to (111), (220), and (311) planes of a zincblende lattice. No separate ZnS or InP phases are detected, indicating successful core–shell architecture. XPS confirms the presence of Mn²⁺ (Mn 2p 642.2 eV) in doped samples while retaining Zn, In, P, and S signals, confirming dopant incorporation without disrupting crystallinity.

Optical Properties

Absorption spectra (Fig. 4a) reveal an excitonic peak at 445 nm that remains largely unchanged across Mn/In ratios. PL spectra (Fig. 4b) exhibit a dominant intrinsic peak at 485 nm for undoped QDs and a new Mn‑doped emission centered at 590 nm that strengthens with higher Mn loading. The intrinsic peak red‑shifts to 524 nm at Mn/In = 0.6, consistent with the larger particle size and reduced bandgap. Time‑resolved PL (Fig. 4c,d) shows a 217 ns lifetime for the intrinsic emission and a pronounced 5.6 ms decay for the Mn‑doped band, characteristic of d‑d transitions in Mn²⁺. Absolute PL QY curves (Fig. 5) reveal an optimum at Mn/In = 0.6, achieving 78.86 %—the highest reported for non‑toxic dual‑emissive QDs to date. Excessive Mn beyond this ratio quenches the InP core emission and introduces non‑radiative centers, reducing overall QY.

Growth‑Doping Mechanism

During the Mn‑doping step at 240 °C, DDT supplies abundant thiolate anions that promote surface reactions, enabling Mn incorporation into the growing lattice. The resulting dual‑emission arises from (1) band‑to‑band recombination in the InP core and (2) Mn²⁺ d‑state recombination (4T₁→6A₁). The red‑shift of the core emission with increasing Mn reflects band‑gap narrowing due to lattice expansion.

Conclusions

We have demonstrated a scalable growth‑doping strategy that yields Mn‑doped InP/ZnS QDs with dual‑emission and an absolute PL QY of 78 %. The emission spectrum can be tuned by adjusting the Mn/In ratio, offering a non‑toxic, high‑performance material for white‑LED applications.