Water-resistant perovskite nanodots enable robust two-photon lasing in aqueous environment

Lead halide perovskite quantum dots (PQDs) are promising materials for optoelectronic devices such as LEDs, solar cells, scintillators, and lasers due to their large absorption cross-sections and high photoluminescence quantum yields. Recent demonstrations show amplified spontaneous emission (ASE) and lasing at low thresholds under one- and multi-photon pumping. Two-photon lasing is particularly attractive for in-vivo applications because near-infrared/infrared excitation penetrates deeper into tissue. A major obstacle, however, is the poor stability of PQDs in aqueous media and humid air, where they lose structural integrity and emission. Previous attempts to synthesize PQDs directly in water have yielded limited stability. The study aims to solve the water-instability problem without significantly increasing particle size or compromising optical properties, enabling robust two-photon lasing in aqueous environments.

Multiple strategies have been explored to enhance PQD stability: (i) functionalization with moisture-tolerant molecules, (ii) passivation layers on PQD films, (iii) metal shell coatings, and (iv) encapsulation into polymer microspheres. While these approaches improve stability in humid environments to some extent, each has drawbacks limiting practical use. Surfactant-based modifications and passivation layers do not provide long-term persistence in water; metal shell coatings require high temperatures that degrade optical properties; polymer microsphere encapsulation increases size significantly, complicating integration with optical cavities for lasers. Previous PQD–silica nanocomposites using TEOS/TMOS showed only reasonable stability in humid air and often formed phase-separated heteroparticles due to unfavorable interfacial energies.

Synthesis of CsPbBr3 PQDs: A Cs-oleate precursor was prepared by reacting 0.267 g Cs2CO3 with 0.833 mL oleic acid (OA) in 10 mL octadecene (ODE) under argon at 130 °C for 10 min, then 150 °C for 10 min. Separately, 10 mL ODE, 1 mL OA, 1 mL oleylamine (OAm), and 0.138 g PbBr2 were mixed and degassed in N2 at 130 °C for 1 h, heated to 180 °C for 10 min; 1 mL Cs-precursor was swiftly injected, and the reaction was quenched after 5 s using an ice bath. The crude solution was centrifuged, precipitate discarded; PQDs from the supernatant were precipitated with methyl acetate by centrifugation at 13,000 rpm for 5 min, washed with toluene/methyl acetate, and re-dispersed in 4 mL toluene.

Silica encapsulation to form water-resistant perovskite nanodots (wr-PNDs): 2 mL of PQDs in toluene (~0.046 g mL−1) was mixed with 5 µL (3-mercaptopropyl)trimethoxysilane (MPTMS), sonicated and stirred for 1 h (silanization). The mixture was transferred to 15 mL toluene with an additional 20 µL MPTMS; 2 µL deionized water was slowly added to initiate hydrolysis. The solution was stirred at 45 °C for 32 h to promote hydrolysis and condensation, then wr-PNDs were collected by centrifugation, dried, and dispersed in water (for water-stability tests) or toluene (control). Mechanistically, MPTMS binds strongly to PQDs via Pb–S bonds, reduces interfacial energy, and prevents phase separation. During encapsulation, thin silica layers form within ~1 h; after ~6 h, larger SiO2 nanoparticles develop embedding more PQDs; after 32 h, nanodots with average size ~170 nm result, containing multiple cubic PQDs uniformly embedded in an amorphous SiO2–SH matrix.

Film and device fabrication: For ASE films, PQDs or wr-PNDs in toluene (with 5% PMMA) were spin-coated on plasma-cleaned glass/quartz, yielding ~450 nm thick films (from cross-sectional SEM). For whispering-gallery-mode (WGM) laser devices, a glass capillary (inner radius ~40 µm) was immersed in concentrated PQD or wr-PND solution; after filling, tubes were placed under vacuum overnight to evaporate solvent, leaving a thin wr-PND layer on the inner wall to form a tubular microcavity.

Characterization: Structural analyses included TEM (JEM-2100, 100 kV), HRTEM, STEM/HAADF (JEM-2100F, 200 kV), SEM (JEOL FE-SEM). Elemental distribution was probed by EDX mapping. XRD (Bruker D2 Phaser, Cu Kα, λ=1.54 Å, 40 kV, 30 mA) identified cubic CsPbBr3 and amorphous silica. FTIR (BRUKER Vertex) detected Si–O–Si modes (~800, 1085 cm−1) and Pb–S (~880 cm−1). XPS (VG ESCALAB 2201-XL) deconvoluted Pb 4f (Pb–Br at 138.6/143.5 eV; Pb–S at 138.3/143.2 eV) and S 4p (Pb–S at 163.5 eV; C–S at 164.7 eV). Optical properties: PLQY was measured in an integrating sphere. UV–vis absorption and PL spectra were recorded for PQDs in toluene and wr-PNDs in water. Time-resolved PL in toluene was fit to a biexponential model to extract radiative and trap-assisted components. Two-photon absorption coefficients were measured by open-aperture Z-scan. ASE was studied using 800 nm femtosecond pumping (Libra Coherent, 1 kHz, 50 fs) in a stripe-pumping configuration. Two-photon lasing in the WGM capillary device was excited at 800 nm (same laser), with emission collected via an optical fiber; polarization dependence was measured. Random lasing was evaluated on wr-PND powder dispersed in water for 15 days, pumped at 800 nm at 173 K, monitoring spectral spikes indicative of coherent feedback.

• wr-PNDs exhibited unprecedented water resistance: dried powder readily dispersed in water with strong green emission persisting for six weeks; in quantitative tracking, wr-PNDs retained ~50% of initial PL intensity after >20 days in water, while pristine PQDs dropped to nearly zero rapidly.
• PLQY after silica coating remained high: wr-PNDs ~78% vs pristine PQDs ~89%, indicating preserved optical quality.
• Spectroscopy confirmed successful encapsulation and Pb–S bonding: XRD showed cubic CsPbBr3 pre- and post-coating with a broad amorphous SiO2 peak (15–35° 2θ); FTIR showed Si–O–Si (~800, 1085 cm−1) and Pb–S (~880 cm−1); XPS resolved Pb–Br and Pb–S components in Pb 4f, and Pb–S (163.5 eV) and C–S (164.7 eV) in S 4p; STEM-EDX mapping showed homogeneous Cs, Pb, Br, S, Si distribution across nanodots.
• Optical spectra: wr-PNDs in water had a slight bandgap red-shift relative to pristine PQDs in toluene (2.38 eV vs 2.4 eV); both showed emission FWHM ~21 nm under identical excitation.
• Time-resolved PL (toluene): non-radiative lifetime (τ2) increased from 8.1 ns (pristine) to 12.4 ns (wr-PNDs), and its proportion decreased from 12% to 6%, evidencing suppressed shallow-level trap-assisted recombination due to silica passivation.
• Two-photon absorption coefficients from Z-scan: ~0.092 cm/GW (pristine PQDs) vs ~0.074 cm/GW (wr-PNDs), comparable to literature values for CsPbBr3 PQDs.
• ASE in films under 800 nm pumping: wr-PNDs film showed ASE onset at ~1.29 mJ cm−2 with FWHM narrowing to ~5 nm (from spontaneous emission FWHM ~18.5 nm at 527 nm); pristine PQDs had a similar threshold (~1.16 mJ cm−2), indicating retained gain characteristics.
• Two-photon WGM laser device (capillary microcavity): in water, wr-PNDs-based device retained ~80% of initial PLQY after 13 h immersion; an analogous pristine PQD device fell below 10% after 3 h. Lasing displayed sharp modes (0.3–0.5 nm linewidth) superimposed on narrowed ASE, with a clear threshold at ~1.12 mJ cm−2. Extracted quality factors Q ~1070–1700, surpassing earlier CH3NH3PbBr3 microdisks (~430) and microwires (~682). Mode indices assigned to 484–491 by WGM theory; emission was linearly polarized.
• Random laser: wr-PNDs powder after 15 days in water exhibited random lasing under 800 nm fs pumping at 173 K, with spectrum-to-spectrum varying spikes characteristic of coherent optical feedback in disordered media.
• Morphology/size: wr-PNDs averaged ~170 nm after 32 h encapsulation, with multiple cubic PQDs embedded in a uniform silica matrix; thin silica shells formed initially (~1 h), evolving to larger SiO2 nanoparticles embedding PQDs by ~6 h.

The study addresses the central challenge of PQD instability in aqueous environments by exploiting strong Pb–S bonding to anchor thiol-functionalized silanes (MPTMS) on CsPbBr3 PQDs, enabling in situ hydrolysis/condensation to form a protective SiO2–SH matrix. This encapsulation maintains the cubic crystallinity and preserves high PLQY while dramatically enhancing water resistance. Suppression of shallow trap-mediated non-radiative pathways (longer τ2 and reduced non-radiative proportion) and retention of large two-photon absorption underpin efficient two-photon-pumped gain. Integrating wr-PNDs into a capillary microcavity couples their amplified spontaneous emission to whispering-gallery modes, yielding low-threshold, high-Q lasing that remains operational following prolonged immersion in water. Furthermore, the wr-PNDs powder supports coherent random lasing after extended dispersion in water, indicating robustness of the gain medium and scattering features. Collectively, these results show that chemically engineered silica encapsulation via Pb–S bonding effectively overcomes water-instability without compromising key optical properties, enabling applications of PQD lasers and imaging agents in aqueous and biological settings.

The authors developed water-resistant perovskite@silica nanodots (wr-PNDs) by embedding CsPbBr3 PQDs into a SiO2–SH matrix via Pb–S bonding with MPTMS. wr-PNDs retain bright emission in water for up to six weeks and maintain high PLQY close to pristine values. Leveraging their stability and strong nonlinear absorption, two-photon-pumped devices were realized: a WGM capillary laser with low threshold (~1.12 mJ cm−2), high Q (1070–1700), linear polarization, and 80% PLQY retention after 13 h in water; and a random laser operational after the powder had been dispersed in water for 15 days (demonstrated at 173 K). This synthetic approach broadens PQD utility in aqueous media, suggesting potential in optoelectronics, bioimaging, and biosensing. Future work could optimize encapsulation to further minimize any trade-offs in nonlinear absorption, explore room-temperature random lasing in water, and integrate these materials into biocompatible platforms and device architectures.

The authors do not explicitly discuss limitations. Noted observations include: (i) the two-photon absorption coefficient of wr-PNDs is slightly reduced compared to pristine PQDs (~0.074 vs ~0.092 cm/GW); (ii) random lasing characterization was performed at 173 K rather than room temperature; (iii) wr-PNDs increase to ~170 nm composite nanodots (multi-QD-in-silica), which, while suitable for device integration, may influence certain applications requiring smaller hydrodynamic sizes.