One‑Pot Room‑Temperature Synthesis of 10‑nm 2D Ruddlesden–Popper Perovskite Quantum Dots with Tunable Emission

Background

Developing fluorescent materials that combine a narrow emission band with precise color tuning is essential for modern lighting and display technologies. Colloidal quantum dots (QDs) meet these requirements, offering adjustable emission wavelengths and high PLQYs. While traditional II–VI and III–V semiconductors have dominated this field, the advent of 3D organic–inorganic and inorganic halide perovskite QDs (formula AMX₃) has opened new avenues for bright, tunable emitters. These 3D perovskites deliver light‑emitting diode (LED) and photovoltaic efficiencies across a 400–800 nm range with sharp emissions (FWHM ≈ 20 nm). However, non‑radiative recombination via sub‑band defect states limits their PLQY and electroluminescence performance. Two‑dimensional Ruddlesden–Popper perovskites, constructed by inserting large organic cations (R) into the A‑site of AMX₃ layers, eliminate typical trap states at the layer edges, yielding long PL lifetimes, superior photostability, and chemical robustness. The interlayer separation acts as a quantum‑well barrier, allowing the band gap to be engineered by varying the inorganic layer thickness (n). Consequently, 2D layered perovskite thin films have already shown promise in photovoltaic and LED devices, benefiting from reduced non‑radiative pathways and tunable emission wavelengths. Despite these advances, colloidal 2D perovskite QDs below 10 nm remain underexplored, largely due to challenges in size control and synthesis scalability. Our work addresses this gap by presenting a facile, room‑temperature one‑pot method that yields uniform 10‑nm 2D RP perovskite QDs with highly tunable optical properties.

Results and discussion

The synthesis strategy (Fig. 1a) begins with a precursor solution containing PbX₂ (X = Br or I), methylammonium halide (MAX), butylammonium halide (BAX), octylamine (OLA), and oleic acid (OA) dissolved in dimethylformamide (DMF). The mixture is injected dropwise into chlorobenzene, triggering immediate nucleation and growth of 2D RP QDs at room temperature. By varying the MAX:PbX₂ ratio (Table 1), we obtain QDs spanning a range of n values. OA and OLA serve as capping agents that stabilize the growing nanocrystals and suppress uncontrolled aggregation. Figure 1b and c show the as‑prepared Br and I series QDs dispersed in toluene. Under UV illumination (λ = 365 nm), the emission color evolves from blue to greenish (Br series) and greenish to bright red (I series) as n increases, demonstrating the tunability of the band gap. Notably, the 3D iodide perovskite QDs (n = ∞) exhibit the weakest emission, underscoring the superior optical stability of the 2D variants. Photoluminescence measurements (Fig. 2) reveal that the Br series emits between 410 and 523 nm, while the I series covers 527–761 nm. Both series display a systematic red shift with increasing n and maintain low FWHM values (≈ 11–21 nm), indicative of high crystallinity and monodispersity. PLQYs range from 6.8 % to 48.6 % for the Br series and 1.1 % to 24.8 % for the I series, with the highest value achieved for (BA)₂(MA)₃Pb₄Br₁₁ (n = 4). The pronounced quantum‑confinement effect arises from the discrete inorganic layer thickness, which separates the perovskite slabs via BA spacers. Transmission electron microscopy (TEM) images (Fig. 3) confirm the spherical morphology and narrow size distribution of the QDs, with an average diameter of ≈ 10 nm. High‑resolution TEM (HRTEM) reveals clear lattice fringes: a d‑spacing of 0.27 nm for n = 1 (matching the (0100) plane) and ≈ 0.29 nm for n = 2 (corresponding to the (200) plane). These structural features corroborate the 2D layered architecture. X‑ray diffraction (XRD) patterns (Fig. 4) display characteristic low‑angle peaks that shift with n, reflecting the incremental expansion of the unit cell as the inorganic layer thickness increases. All QDs with n ≥ 2 exhibit (100) peaks at 15.1° (Br) and 14.1° (I), aligning with 3D perovskite references. Broadening of these peaks with increasing n confirms the nanoscale grain size. The presence of additional low‑angle reflections for n = 1–4 further confirms the successful synthesis of discrete 2D phases. Time‑resolved photoluminescence (TRPL) measurements (Fig. 5) show non‑exponential decay profiles with average lifetimes ranging from 1–9 ns for the Br series and 48–75 ns for the I series. The longer lifetimes of the iodide QDs reflect their smaller band gaps and reduced non‑radiative recombination. Compared to exfoliated bulk (BA)₂(MA)ₙ₋₁PbₙI₃ₙ₊₁ crystals (τ < 10 ns), our colloidal QDs exhibit significantly prolonged radiative lifetimes, underscoring the efficacy of the 2D architecture in suppressing trap‑mediated decay.

Conclusions

We have demonstrated a scalable, room‑temperature, one‑pot synthesis of highly luminescent 10‑nm 2D RP perovskite QDs. By adjusting the MA/halide ratio, the band gap can be tuned to yield emission across the entire visible spectrum (410–523 nm for the Br series, 527–761 nm for the I series). The Br series reaches a record PLQY of 48.6 %, surpassing conventional 3D perovskite QDs while offering superior thermal and chemical stability. The pronounced quantum‑confinement and reduced trap states inherent to the 2D structure pave the way for robust, solution‑processed perovskite devices in LEDs, photodetectors, and photovoltaic cells.

Methods

Chemicals used

Lead(II) bromide (≥ 98 %), lead(II) iodide (≥ 99 %), methylamine solution (33 wt % in ethanol), n‑butylamine (≥ 99.5 %), hydrobromic acid (48 %), hydroiodic acid (57 wt % aqueous), octylamine (≥ 99 %), oleic acid (SLR grade), N,N‑dimethylformamide (≥ 99.8 %), and toluene (HPLC grade) were purchased from Acros, Fisher, Alfa Aesar, and Macron and used without further purification.

Synthesis of alkylammonium halide

Butylammonium bromide (BABr), methylammonium bromide (MABr), butylammonium iodide (BAI), and methylammonium iodide (MAI) were prepared by reacting HBr or HI (1.1:1 molar ratio) with butylamine or methylamine in ethanol. The reaction mixture was stirred for 2 h at 0 °C, then the solvent was evaporated, and the product was washed with diethyl ether, filtered, and dried at 60 °C under vacuum. The resulting crystals were stored under argon and transferred to a glove box for subsequent use.

2D‑layered nanocrystals (NCs) synthesis

All syntheses were performed at room temperature in ambient conditions. For a given n, BAX, MAX, and PbX₂ (X = Br or I) were mixed in a molar ratio of 2:n − 1:3n + 1 and dissolved in DMF to form a 0.04 mM PbX₂ solution. OA (0.5 mL) and OLA (0.05 mL) were added, then 100 µL of this precursor was injected into 10 mL toluene under vigorous stirring to precipitate 2D NCs. Detailed compositions are shown in Fig. 1b.

Characterizations

Morphology and crystal structure were examined by TEM (JEOL 2100F, 200 kV) and HRTEM (HITACHI HT7700, 120 kV). XRD patterns were recorded on a Rigaku Miniflex 600. Photoluminescence spectra were measured with a HITACHI F‑4500 fluorescence spectrophotometer. PLQYs were determined in toluene using C‑102 and DCJTB as standards (QY = 0.76 and 0.78, respectively). Time‑resolved photoluminescence (TRPL) was collected with a PicoQuant FluoTime 300 TCSPC system, exciting at 375 nm and 466 nm with 70 ps pulses, 90 µW power, and 4 MHz repetition rate.

Abbreviations

(BA)₂(MA)_n-1Pb_nBr_3n+1

Br series

(BA)₂(MA)_n-1Pb_nI_3n+1

I series

2D

Two‑dimensional

3D

Three‑dimensional

BA

1‑Butylammonium

BAX

Butylammonium halogen

CB

Chlorobenzene

DMF

Dimethylformamide

FWHM

Full width at half maximum

HRTEM

High‑resolution TEM

MA

Methylammonium

MAX

Methylammonium halogen

OA

Oleic acid

OLA

Octylamine

PL

Photoluminescence

PLQYs

Photoluminescence quantum yields

QDs

Quantum dots

RP

Ruddlesden–Popper

TEM

Transmission electron microscopy

TRPL

Time‑resolved PL spectroscopy

XRD

X‑ray diffraction