Chalcohalides consisting of mixed chalcogen and halogen anions and metal cations are an emerging family of semiconductors^[1−3]. The rich composition and crystal structure of chalcohalides has provided significant opportunities for fundamental materials research and energy applications^[4−9]. Among them, the expansion of Pb chalcohalides, as an extension of two classic material systems, namely Pb chalcogenides (such as PbS, PbSe) and lead halide perovskites (such as CH₃NH₃PbI₃, CsPbI₃), has sparked great interests^[10−16]. Pb chalcohalides (PbYX) can be understood as binary phase diagrams composed of two phases, PbY−PbX₂ (Y = S, Se; X = Cl, Br, I), combined in different proportions^[17]. However, early studies found that only a few combinations of phases could exist stably at ambient temperature and pressure, such as Pb₅S₂I₆ and Pb₇S₂Br₁₀^[18]. It was not until recently that the Manna group discovered the synthesis of Pb₃S₂Cl₂, Pb₄S₃Br₂, and Pb₄S₃I₂ nanocrystals (NCs) through chemical methods^[19]. These phases do not conform to stable bulk PbY−PbX₂ diagrams but can exist stably in the form of NCs at ambient temperature and pressure^[17]. Based on the novel material systems, they further find these PbYX exhibits remarkable lattice matching with lead-halide perovskite, making them potential structures to enhance the optoelectronic properties and stability of perovskite materials^[20−23].

Furthermore, halide ions have been identified as the optimal surface passivating agents for classical PbS and PbSe NCs, playing a crucial role in enhancing the performance of near-infrared and mid-infrared optoelectronic devices based on PbY NCs^[24−28]. The formation of Pb−X bonds on the surface can also act as a bridge, connecting them with novel lead halide perovskite materials, thereby achieving superior material and device performance^{[20, 23]}. The successful synthesis of PbXY NCs has deepened our understanding of traditional classical material systems. For instance, passivation with Cl ions significantly improves the luminescence efficiency and stability of PbS and PbSe NCs^{[29, 30]}. In earlier studies, it was speculated that the surface adsorbed only Cl ions or formed a PbCl_x shell^[31−33]. Although not confirmed yet, recent research suggests that the surface layer may be a Pb sulfochloride surface layer based on the similarity of reaction conditions^[32]. Further exploration of the relation of PbXY with classical PbS and PbSe NCs will deepen our understanding of the structure-property relationships in materials, facilitating the design and synthesis of new materials and structures with superior optoelectronic performance.

In this study, we have discovered that within the framework of the classical PbS NC synthesis approach, the straightforward introduction of bis(trimethylsilyl) halide (TMS−X, X = Cl, Br, I) precursors enables the synthesis of a series of materials, starting from PbS NCs and progressing to PbS/Pb₃S₂X₂ core-shell structures, ultimately leading to the formation of Pb₃S₂X₂ NCs. We have conducted a detailed investigation of the evolution process and its impact on the optical properties of the NC materials. Notably, we found that the growth of a Pb₃S₂X₂ shell on the surface of PbS effectively enhances its photoluminescence quantum yield (PLQY) from 49.3% to 72.0%. Furthermore, this strategy enables the synthesis of Pb₃S₂Br₂ NCs, which have not been previously reported.

Lead acetate trihydrate (PbAc₂·3H₂O, 99%), hexamethyldisilathiane (TMS−S, 98%), trimethylsilyl chloride (TMS−Cl, 99%), trimethylsilyl bromide (TMS−Br, 99%), trimethylsilyl iodide (TMS−I, 99%), sulfur powder (S, 99.99%), oleylamine hydrochloride (OLA−Cl, 99.5%), oleic acid (OA, 90%), 1-octadecene (ODE, 90%), hexane (Hex, 95%), isopropanol (IPA, 95%), acetone (95%).

The NC synthesis was performed with a modified recipe previously documented^[34]. All operations were performed under a nitrogen atmosphere using standard air-free Schlenk line techniques. Briefly, 1 mmol of PbAc₂·3H₂O and 0.8 mL of OA were dissolved in 10 mL of ODE in a three-neck flask. The solution was then heated to 100 °C under vacuum for 1 h. The TMS−S (0.25 mmol) and different amounts of TMS−X (X = Cl, Br, I) precursor were dissolved in 1 mL of ODE. The reaction was initiated by rapid injection of the TMS−S and TMS−X precursors into the lead precursor solution at 100 °C under nitrogen. The NCs were grown at this temperature for 10 min and then cooled to room temperature slowly. The solution was then transferred into a nitrogen-filled glove box and purified by precipitation once in hexane/isopropyl alcohol and once in hexane/acetone. The obtained NC solids were stored in a nitrogen-filled glove box.

For the synthesis of large PbS NCs and Pb₃S₂X₂ NCs, 1.2 mmol of PbAc₂·3H₂O and 12 mL of OA were added in a three-neck flask by heating the mixture to 100 °C under vacuum for 1 h. The TMS−S (0.2 mmol) and different amounts of TMS−X (X = Cl, Br, I) precursor were dissolved in 1 mL of ODE. The reaction was initiated by rapid injection of the TMS−S and TMS−X precursor into the lead precursor solution at 150 °C under nitrogen. The NCs were grown at this temperature for 5 s and then cooled to room temperature quickly. The solution was then transferred into a nitrogen-filled glove box and purified by precipitation once in hexane/isopropyl alcohol and once in hexane/acetone. The obtained NC solids were stored in a nitrogen-filled glove box.

Briefly, 1 mmol of PbAc₂·3H₂O and 0.8 mL of OA were dissolved in 10 mL of ODE in a three-neck flask by heating the mixture to 100 °C under vacuum for 1 h. The anion precursors were prepared by sonicating 0.5 mmol sulfur powder and 0.8 mmol OLA−Cl in 2 mL dried ODE. The reaction was initiated by rapid injection of the anion precursors into the lead precursor solution at 150 °C under nitrogen. The NCs were grown at this temperature for 30 min and then cooled to room temperature slowly. The solution was then transferred into a nitrogen-filled glove box and purified by precipitation twice in hexane/isopropyl alcohol and once in hexane/acetone. The obtained NC solids were stored in a nitrogen-filled glove box.

The absorption spectra of NCs in hexane were recorded at room temperature using an ultraviolet−visible−near infrared (UV−VIS−NIR) spectrophotometer (Lambda950, PerkinElmer). Photoluminescence (PL) spectra and PL lifetime were measured by a FluoroMax-4 spectrofluorometer (HORIBA Scientific). The PLQY was measured using an integrating sphere equipped on FLS1000 fluorescence spectrometer. The excitation wavelength was 750 nm. The PbS NC films for the XPS test were made by spin-coating ~100 nm thick film onto a silicon substrate. The X-ray diffraction (XRD) spectra were obtained on an X-ray diffractometer with a Cu Kα source (PANalytical B.V. Empyrean). TEM measurements were performed by a Tecnai G2 F20 S-Twin system.

The NCs were synthesized based on the typical hot injection method with lead-oleate as the Pb precursor^[34]. The reaction was initiated by injecting mixtures of TMS−S and TMS−X with different molar ratios (represented by X : S). Taking TMS−Cl as an example, at low Cl : S values, the reaction proceeded similarly to the synthesis of conventional PbS NCs, where the colorless reaction solution turned red and then rapidly black. As the Cl : S value increased, the transition from red to black gradually slowed down. The obtained XRD pattern of the NCs is presented in Fig. 1(a). The graph reveals that the crystal structure of the NCs obtained at Cl : S values below 0.6 is nearly identical to the cubic phase of PbS, with only a slight shift observed in the (111) plane (Fig. 1(b)). When the Cl : S value exceeded 0.6, the intensity of the (220) peak of PbS NCs gradually decreased and eventually disappeared. Simultaneously, the (111) plane at 25.9° shifted to lower angles, and the (200) plane at 29.7° shifted to higher angles. When the Cl : S value reached 1.8, the two planes settled at 24.4° and 31.8°, respectively, corresponding to the (112) and (013) planes of Pb₃S₂Cl₂. Additionally, when the Cl : S value reached 0.8, a broad peak started to appear around ~35°, and with an increasing Cl : S value, it continued to shift to higher angles, ultimately reaching ~37°, corresponding to the (123) plane of Pb₃S₂Cl₂. This phenomenon is highly reminiscent of the typical evolution from core to core/shell NC structures^[35−37]. Furthermore, the above XRD diffraction behavior differs significantly from that of a simple mixture of PbS and Pb₃S₂Cl₂ phases (Fig. S1). Therefore, we hypothesize that a PbS/Pb₃S₂Cl₂ core/shell structure is formed under intermediate Cl : S ratios of 0.8−1.6 (with further evidence presented in the subsequent text). When the Cl : S ratio reaches 1.8, pure-phase Pb₃S₂Cl₂ NCs can be obtained. The crystal structure of PbS and Pb₃S₂Cl₂ is shown in Fig. 1(c) and 1(d). The cubic PbS exhibits a regular Pb and S octahedral structure, with the Pb atoms coordinated by six S anions. In contrast, for the Pb₃S₂Cl₂, the Pb is of different chemical environments compared with the PbS. All the Pb atoms occupy the 12a site, which has a point group symmetry of 4 bar and is coordinated by eight anions.

To ascertain the exact nature of surface chemistry after introducing TMS−Cl, we conducted X-ray photoelectron spectroscopy (XPS) measurement. The well-defined Cl 2p peaks in the XPS spectra clearly demonstrate the incorporation of Cl in the obtained NCs (Fig. 2(a)). The Pb 4f^7/2 spectra were deconvoluted by fitting the sum of the Lorentz−Gaussian functions. They were found to contain two chemical states, observed at 137.7 and 138.6 eV, respectively (Fig. 2(b)). The main peak at 137.7 eV was assigned to the Pb−S bond, while the peak at 138.6 eV corresponded to Pb−Cl bonds^[29]. The Pb−Cl peak becomes stronger as increasing the Cl : S input during the synthesis, providing further evidence for the incorporation of Cl in the NCs. The S 2p spectra become broader as increasing Cl : S ratio, due to the more complicated coordination environment in Pb₃S₂Cl₂ compared to that in PbS crystal (Fig. 2(c)). The element ratio of the NCs is extracted by fitting the peak areas in XPS spectra and presented in Fig. 2(d). The molar ratio of Cl in the final NCs increases with the increment of the Cl : S ratio. When the input Cl : S ratio reaches 1.8, the obtained Pb : S : Cl = 50.3 : 23.6 : 26.1, which is close to the ratio in the chemical formula (Pb₃S₂Cl₂). XPS is a surface-sensitive technique because of the shallow escape depth of photoelectrons, whereas the sampling depth of SEM-EDX measurement is typically above 1 µm. The Cl concentrations measured by XPS for different Cl : S were found to be consistently higher than the values obtained from SEM-EDX analysis, indicative of the presence of a Cl-rich near-surface region within NCs (Fig. 2(e)). The extent of Cl surface enrichment can be estimated by taking the ratio between atomic percent values obtained from XPS and EDX measurements. As shown in Fig. 2(f), the Cl(EDX)/Cl(XPS) ratio monotonically increases and approaches 1 with the increment of the input Cl : S ratio, which indicates that the NCs gradually transform into a homogeneous composition (e.g. Pb₃S₂Cl₂)^[38]. The above observation aligns with the previous XRD results, confirming the structural evolution from PbS to PbS/Pb₃S₂Cl₂ core/shell and eventually to pure-phase Pb₃S₂Cl₂ as increasing the input Cl : S ratio.

We further investigated the influence of the aforementioned structural evolution on the optical properties of the NCs. As depicted in Fig. 3(a) and 3(b), a slight red shift is observed in the absorbance and PL peaks when the input Cl : S ratio is 0.2. Subsequently, as the molar ratio of Cl : S increased, the absorbance and PL peaks blueshifted, PLQY gradually increased and Stokes shift decreased, reaching a maximum at Cl : S = 0.8, with a minimum Stokes shift of 104 nm and a PLQY of 72.0% (Fig. 3(c), 3(e) and Fig. S2). Further increase in the Cl : S ratio leads to a decrease in PLQY and an increase in Stokes shift. Interestingly, as the Cl : S ratio reaches 1.2, a new peak located at ~680 nm appears, which can be assigned to the emission from the Pb₃S₂Cl₂ phase (Fig. S3). As the Cl : S ratio reaches 1.6, the exciton absorption peak blue shifts to ~710 nm from the ~906 nm of initial PbS NCs. The color of the obtained NC solution gradually transitions from brownish to red (Fig. 3(d)). Meanwhile, the NCs lose PL entirely in the near-infrared region. Further increasing the Cl : S ratio to 1.8 results in the vanishing of both PL and exciton absorption peaks. To understand the variation of optical properties, we further tested their size and morphology through transmission electron microscopy (TEM) characterization. As shown in Fig. S4 and Fig. S5, with the increase of input Cl/S ratio, the size of NCs gradually decreases. Under low concentration (Cl : S = 0.2), the size reduction effect of Cl may not be significant, but the excitons in the PbS core can delocalize to the shell, resulting in a slight redshift of absorption and PL^[39]. As Cl content increases, the binding of Cl and Pb precursors limits the growth of NCs and causes a decrease in the size of the PbS core, resulting in a gradual blueshift of the absorption and PL peaks. The simultaneous appearance of PL peaks from both PbS and Pb₃S₂Cl₂ confirms the formation of a PbS/Pb₃S₂Cl₂ core/shell structure^{[40, 41]}. The time-resolved PL (TRPL) data shows that increasing the amount of Cl can enhance the PL lifetime of the NCs when the Cl : S ratio is below 0.8 (2.01 to 3.70 μs), (Fig. 3(f) and Table S1), which is consistent with the increase of PLQY, demonstrating the passivation effect at low input Cl : S ratios. Further increasing the Cl : S ratio can significantly reduce PL lifetime, with the value as low as 8.68 ns for pure-phase Pb₃S₂Cl₂ NCs (input Cl : S = 1.8), which is also consistent with PL lifetime reported previously (Fig. 3(g), Table S2). Similar structural and optical evolutions can be observed with large PbS NCs (first exciton peak at 1627 nm) as starting materials (Fig. S6−Fig. S8). The dominant size of the finial pure Pb₃S₂Cl₂ can reach 3.2 nm, which was larger than the size of pure Pb₃S₂Cl₂ NCs (1.5 nm) triggered from small PbS NCs. Additionally, the NCs absorbance band edge of these two methods were all around 600 nm, indicating the size of Pb₃S₂Cl₂ NCs has little effect on their optical characterization due to their negligible quantum confinement effects, which is also consistent with the previous report^[19].

Seeking to clarify the impact of the Pb₃S₂Cl₂ shell on ensemble photophysics, we further investigate the consequences of Pb₃S₂Cl₂ shelling on PbS NCs by using time-resolved emission spectroscopy (TRES). To avoid the Förster resonant energy transfer and self-absorbance phenomenon, we dilute the PbS NCs solution (1 mg/mL) in hexene. The PbS NC solution displays transient emission spectra with a dynamic spectral red-shift without TMSCl introduction. (Fig. 4(a) and 4(c), monitored via the emission peak). Conversely, the shelled NCs with Pb₃S₂Cl₂ display much less overall spectral red-shifting after photoexcitation and the early (<2 μs) component is effectively eliminated (Fig. 4(b) and 4(d)), which reveals the Pb₃S₂Cl₂ can well passivate PbS NCs^[42].

We also find this phase and structural evolution from PbS NCs to Pb₃S₂Cl₂ can be extended to Br and I systems. The XRD pattern and optical evolution Br system show similar behavior as that in the Cl case (Fig. S9). The solution of Pb₃S₂Br₂ and Pb₃S₂I₂ NCs also exhibits a red color (Fig. 5(a)). The extracted bandgap values are 2.2, 2.1, and 1.9 eV for Pb₃S₂Cl₂, Pb₃S₂Br₂ and Pb₃S₂I₂, respectively. To validate the crystal structure, we compared the XRD patterns of Pb₃S₂Cl₂ and Pb₃S₂Br₂ NCs with those of the bulk structure published by Ni et al. The XRD analysis confirmed a good match between the NCs and the bulk structures (Fig. 5(b))^{[43, 44]}. It should be noted that we have discovered that Pb₃S₂Br₂ NCs have not yet been reported. However, this method cannot deliver pure-phase Pb₃S₂I₂ NCs, the XRD pattern from the PbS phase can also be observed even at input I : S ratio up to 1.0 in large PbS NCs (Fig. S10). The obtained Pb₃S₂X₂ all exhibit sphere shapes with sizes around 3 nm (Fig. 5(c)−5(e)). Additionally, we found that by reducing the reactivity of the anion precursors, using ODE−S and oleylamine hydrochloride (OLACl) to replace TMS−S and TMSCl, larger-sized Pb₃S₂Cl₂ NCs can be synthesized, with size up to ~10 nm (Fig. S11).

In this work, we reported the synthesis of PbS/Pb₃S₂X₂ (X = Cl, Br, I) core-shell structures for the first time. Compared to pure PbS NCs, this core-shell structure effectively enhances the optical properties, increasing the PLQY from 49.8% to 72.0% and the fluorescence lifetime from 2.01 to 3.70 μs. Under conditions of high halogen incorporation, we obtained pure-phase Pb₃S₂Cl₂ and Pb₃S₂Br₂ NCs. Notably, our approach enables the synthesis of stable Pb₃S₂Br₂ that had not yet been reported. This work provides a novel synthetic approach to achieve heterogeneous integration of lead chalcohalides (PbYX, X = Cl, Br, I; Y = S, Se) with lead chalcogenides (PbY) and explores their impact on the structure and properties, potentially encouraging further exploration of other metal chalcohalides and heterostructure synthesis and applications.

Supplementary materials to this article can be found online at https://doi.org/10.1088/1674-4926/24050026.