Synthesis of CsPbBr₃ QDs and NaYF₄:Yb,Tm UCNP

CsPbBr₃ QDs with regular cubic shape were first synthesized based on Kovalenko’s method³⁰. The size of the CsPbBr₃ QDs was 12.6 nm, and the diffraction peaks at 2θ = 15.2, 21.6, 26.5 30.6, 34.4, 37.8, 43.9, 46.7, 49.4, 54.5, and 59.3° correspond to diffractions from the {100}, {110}, {111}, {200}, {210}, {211}, {220}, {300}, {310}, {222}, and {321} planes of the cubic-phase CsPbBr₃ (PDF# 00-054-0752) (Supplementary Figs. 1 and  2). To analyze the optical properties, the UV−vis absorption spectra, and the PL emission and excitation spectra (Supplementary Fig. 3) was obtained from the colloidal solution of the CsPbBr₃ nanocrystals. The PL emission spectrum showed a peak position at 515 nm and showed a strong green fluorescence under 365 nm excitation (Supplementary Fig. 3A). The absorption onset of CsPbBr₃ nanocrystals was around 510 nm (Supplementary Fig. 3B). The time-resolved PL decay of CsPbBr₃ showed average PL decay lifetimes was 18.4 ns (Supplementary Fig. 3C and Supplementary Table 1). NaYF₄:30%Yb,0.5%Tm nanocrystals were synthesized by the high-temperature co-precipitation method³¹. When the temperature reached 300 °C, NaYF₄:30%Yb and 0.5%Tm nanocrystals with a size of about 5 nm were synthesized, and they emitted a weak fluorescence under 980 nm excitation (Supplementary Fig. 5). At 300 °C for 60 min, uniform NaYF₄:30%Yb,0.5%Tm nanocrystals with a size of 32.5 nm were synthesized, and they emitted a strong blue fluorescence under 980 nm excitation (Supplementary Fig. 6). According to the XRD results (Supplementary Fig. 7), the diffraction peaks at 2θ = 17.2, 30.1, 30.8, 34.6, 39.6, 43.5, 46.5, 52.1, 53.1, 53.7, and 55.2° correspond to diffractions from the {100}, {110}, {101}, {200}, {111}, {201}, {210}, {002}, {300}, {211}, and {102} planes of the hexagonal-phase NaYF₄ (JCPDS: 00-016-0334).

Synthesis of heterostructured nanocrystals

As shown in Fig. 1a, cubic-phase NaYF₄:Yb,Tm UCNP can be epitaxially grown on the surface of cubic-phase CsPbBr₃ QDs due to the similarity in the crystal structure and lattice. When the temperature is increased, UCNP is converted from cubic to hexagonal phase, while cubic-phase CsPbBr₃ QDs remain unchanged so hexagonal-phase UCNP is formed on the surface of cubic-phase CsPbBr₃ QDs. CsPbBr₃-NaYF₄:Yb,Tm hybrid nanocrystals were synthesized using a high-temperature co-precipitation method. Their formation and crystal structures were studied. CsPbBr₃ QDs with regular cubic shape were first synthesized based on Kovalenko’s method (Supplementary Fig. 1)¹¹, and then added to the precursor solution for UCNP synthesis which consists of various lanthanide ions such as Y³⁺, Yb³⁺, and Tm³⁺ and surfactants, such as oleic acid (OA) and octadecylene (ODE). As shown in Fig. 1b–e, heterostructured nanocrystals were formed with the CsPbBr₃ QDs embedded in the NaYF₄:Yb,Tm UCNP, to form a structure similar to that of watermelon with multiple seeds. The average size of the nanocrystals was ~37 nm, and the outer shell was ~3.7-nm thick (Fig. 1c). CsPbBr₃ QDs were found to be in the middle region of the nanocrystals with distinct crystal boundary formed between the CsPbBr₃ QDs and UCNP from the bright- and dark-field TEM images. The heterostructure interface can be clearly seen from the HRTEM diagram (Supplementary Fig. 8), and the area segmented by the arc presents obvious light and dark patterns. Above the arc is the cubic-phase CsPbBr₃ crystals and below the arc is the hexagonal-phase NaYF₄:Yb,Tm crystals. The cubic-phase NaYF₄:Yb,Tm is a metastable crystalline phase, which led to the formation of irregular small cubic-phase CsPbBr₃ crystals within the heterogeneous junction, thus making the interface of heterogeneous junction irregular. From HRTEM, STEM, and elemental imaging results (Supplementary Fig. 9), it can be clearly seen that the perovskites in the heterostructured nanocrystals were small irregular particles with a size of 6–12 nm. At 250 °C, the heterostructured nanocrystals were composed of a cubic-phase UCNPs and cubic-phase perovskites. The cubic-phase UCNPs has a high-temperature metastable crystalline phase, which led to the irregular growth of perovskites in the heterojunction. Subsequently, under the effects of thermodynamics (250–300 °C), the cubic-phase UCNP transited to the hexagonal phase. At this time, the epitaxial growth was mainly due to the hexagonal-phase UCNP, and most of the perovskite stopped growing due to the mismatch of crystal phases. As shown in Fig. 1f, eight elements including Br, Pb, Cs, Na, Y, F, Tm, and Yb were distributed in the nanocrystal structure (Supplementary Fig. 10). The elemental mapping images showed that Na, Y, F, Tm, and Yb ions which constitute the NaYF₄:Yb,Tm UCNP were uniformly distributed throughout the hybrid nanocrystals, while Br, Pb, and Cs ions which make up CsPbBr₃ QDs were located in the middle part. This could also be seen from the images of single-particle element imaging (Supplementary Fig. 11), supporting that the hybrid nanocrystals were formed with the CsPbBr₃ QDs embedded in the NaYF₄:Yb,Tm UCNP. According to the XPS results, the concentration ratio of CsPbBr₃ and NaYF₄:Yb,Tm in the heterostructured nanocrystals was 1:59. (Supplementary Table 2).

Fig. 1

Synthesis of CsPbBr₃-NaYF₄:Yb,Tm hybrid nanocrystals.

a Schematic showing the formation of heterostructured CsPbBr₃–NaYF₄: Yb,Tm nanocrystals, b, c TEM and d, e STEM images of heterostructured CsPbBr₃–NaYF₄:Yb,Tm nanocrystals. f Elemental mapping of heterostructured CsPbBr₃–NaYF₄:Yb,Tm nanocrystals. Scale bars in f, 20 nm.

Nanocrystal heterostructure formation during heat treatment

The formation of the CsPbBr₃-NaYF₄:Yb,Tm hybrid nanocrystals was studied by TEM measurement of the samples collected at different temperatures during the heat-treatment process. From the literature, it was reported that cubic-phase CsPbBr₃ crystals (5.83 Å) and cubic-phase NaYF₄ crystals (5.47 Å) have a small lattice mismatch of ~5.9%, suggesting that theoretically it is possible to grow cubic phase CsPbBr₃ crystals on cubic-phase NaYF₄ crystals epitaxially or vice versa^32–34. In this study, pre-made cubic-phase CsPbBr₃ QDs were added to the precursor solution to synthesize NaYF₄:Yb,Tm UCNP, so it is a high probability that NaYF₄:Yb,Tm crystals were grown on the surface of CsPbBr₃ QDs, evidenced from the dark spots formed on the surface of the cubic-shape nanocrystals at 150 °C and 200 °C, as shown in Fig. 2a, b. This is similar to cubic-phase CsPbX₃–PbS hybrid nanocrystals formed under a similar condition²⁵. The temperature was subsequently increased to 250 °C, resulting in much smaller nanocrystals with a size of ~12 nm (Fig. 2c). Besides poor chemical stability in water, poor thermostability of CsPbBr₃ QDs has also been well reported^35,36. High temperature may cause the oxidation and hydration and eventually the decomposition of CsPbBr₃ QDs. At 250 °C, the size of the nanocrystals was apparently much smaller as compared to a lower temperature, suggesting that CsPbBr₃ QDs started to decompose partially. This is usually considered a problem for perovskite QDs; however, in this study, the smaller size of perovskite QDs caused by partial decomposition makes it possible to synthesize small perovskite–UCNP hybrid nanocrystals. On the other side, the cubic phase is dominant in the NaYF₄ nanocrystals formed at 250 °C, as previously reported^29,37,38. The small hybrid nanocrystals served as the seeds for further growth of the NaYF₄:Yb,Tm UCNP. The average size of the nanocrystals increased to 30.6 nm when the temperature reached 300 °C (Fig. 2d). With the extension of the insulation time to 30 and 60 min, the size continued to increase to 34.5 nm and 37.1 nm, respectively (Fig. 2e, f). Their morphology changed from square or hexagonal to ellipsoidal shape. In addition, multiple cores were clearly observed in the middle region of the perovskite–UCNP hybrid nanocrystals during the thermal insulation process (Supplementary Fig. 12), which was consistent with the previous elemental mapping results.

Fig. 2

Nanocrystal heterostructure formation during heat treatment.

TEM images of heterostructured CsPbBr₃–NaYF₄: Yb,Tm nanocrystals collected during the synthesis at different temperatures. a 150 °C, b 200 °C, c 250 °C, d 300 °C—0 min, e 300 °C-30 min, f 300 °C—60 min.

Luminescence emission profile

The fluorescence emission of nanocrystals is dependent on the crystal structure and phases. Fluorescence emission spectra of the perovskite–UCNP hybrid nanocrystals collected at different temperatures were measured under excitation at 980 nm and 365 nm, respectively, and used to study the crystal phase formation and transition during the synthesis of the hybrid nanocrystals. As shown in Fig. 3a, the hybrid nanocrystals had three different states of luminescence: it emitted blue and cyan fluorescence under 980 nm excitation and green fluorescence under 365 nm excitation. When the temperature was increased from 150 °C to 250 °C, upconversion luminescence was weak and there was no characteristic green fluorescence emission of CsPbBr₃ QDs at 515 nm under 980 nm excitation, indicating that there is no energy transfer from the NaYF₄:Yb,Tm UCNP to CsPbBr₃ QDs due to the low luminescence efficiency of cubic-phase NaYF₄:Yb,Tm UCNP formed at low temperature^38,39. The intensity of the green fluorescence of CsPbBr₃ QDs under 365 nm excitation decreased due to the partial decomposition of the QDs. When the temperature was further increased to 300 °C, the hybrid nanocrystals exhibited UV/blue upconversion fluorescence under 980 nm excitation (Fig. 3b), with emission peaks located at 292 nm (¹I₆ → ³F₄), 347 nm (¹I₆ → ³F₄), 362 nm (¹D₂ → ³H₆), 450 nm (¹D₂ → ³F₄), and 478 nm (¹G₄ → ³H₆), and green fluorescence at 522 nm corresponding to the emission of CsPbBr₃ QDs, suggesting a significant energy transfer from the NaYF₄:Yb,Tm UCNP to CsPbBr₃ QDs and formation of hexagonal-phase NaYF₄:Yb,Tm UCNP. It has been reported that hexagonal-phase NaYF₄:Yb,Tm UCNP emits much stronger upconversion fluorescence as compared to its cubic phase^38–40. During the process of maintaining the temperature at 300 °C for 0 to 60 min, the intensity of the green fluorescence emission peak increased, suggesting an increased energy transfer from the NaYF₄:Yb,Tm UCNP to CsPbBr₃ QDs. Meanwhile, at 300 °C, the hybrid nanocrystals emitted strong green fluorescence at 525 nm under 365 nm excitation which was corresponding to CsPbBr₃ QDs (Fig. 3c). The absorption of the heterostructure composite of CsPbBr₃–NaYF₄:Yb,Tm nanocrystals were around 510 nm. The excitation spectrum showed that the heterostructure composite of CsPbBr₃–NaYF₄:Yb,Tm nanocrystals exhibited the strongest peak at 475 nm monitored at the emission wavelength of 525 nm, which is in good agreement with the result from the UV–vis absorption spectrum (Supplementary Fig. 13). During the process of nanocrystals formation and growth, the heterostructured nanocrystal at 250 °C was composed of cubic-phase UCNPs and cubic-phase CsPbBr₃. When the temperature increased to 300 °C and incubated for 60 min, the cubic-phase UCNPs changes to the hexagonal phase and resulted in the heterostructured nanocrystals of the hexagonal-phase UCNPs and cubic-phase CsPbBr₃ formation. The PLQY of these heterostructured nanocrystals under excitation at 980 nm were as follows: 150 °C (0%), 200 °C (0%), 250 °C (0.02%), 300 °C—0 min (0.15%), and 300 °C—60 min (0.25%) (Supplementary Table 3). Similarly, the PLQY of these heterostructured nanocrystals increased from 0.02% to 0.139% under the 980 nm excitation. This PLQY increase was a result of the NaYF₄ phase transition from cubic to hexagonal. The hexagonal-phase NaYF₄ has a much higher fluorescence efficiency as compared to its cubic phase. The PLQY of these heterostructured nanocrystals under 365 nm excitation were as follows: 150 °C (53%), 200 °C (35%), 250 °C (12%), 300 °C—0 min (23%), and 300 °C—60 min (21%) (Supplementary Table 3). Under the excitation of 365 nm, the peak position of heterostructured nanocrystals was 525 nm. The PLQY increased from 12% to 23% during the heterostructured nanocrystal phase-transition process. The fluorescence quantum efficiency of the conventional CsPbBr₃ nanocrystals was 65%, and fluorescence quenching occurred after CsPbBr₃ nanocrystals was kept at 300 °C for 60 min (Supplementary Table 3 and Supplementary Table 4). On the other hand, the PLQY of the perovskite within the heterostructured nanocrystals excited by 365 nm was about 21% after being kept at 300 °C for 60 min far higher than that of the conventional CsPbBr₃ nanocrystals at 300 °C for 60 min (Supplementary Table 4). The conventional CsPbBr₃ nanocrystals were kept at 300 °C for 60 min, resulting in very small-sized particles and very weak fluorescence (Supplementary Fig. 4). This is because the conventional CsPbBr₃ nanocrystals shrank in size, dissolved, and their crystal structure was destroyed at 300 °C after 60 min. The heterojunctured nanocrystals have strong fluorescence at the 525 nm peak under 365 nm excitation, which fully indicated that the cubic-phase and hexagonal-phase UCNP have an obvious protective effect on CsPbBr₃ nanocrystals at high temperature. The hybrid nanocrystals were reheated reheated to different temperatures of 25 °C, 40 °C, 50 °C, 60 °C, 70 °C, 80 °C, 90 °C, 100 °C, 150 °C, and 200 °C, and compared to pure CsPbBr₃ perovskite QDs. When the temperature was increased to 200 °C, the intensity of CsPbBr₃ QDs quickly reduced to 2% while the intensity of the hybrid nanocrystals dropped to 71% only, showing the protection of the CsPbBr₃ QDs by the UCNP at high temperature (Fig. 3d and Supplementary Fig. 14). Relative fluorescence intensity of the heterostructured CsPbBr₃–NaYF₄:Yb,Tm hybrid nanocrystals still maintained around 90% during the heating process under 980 nm excitation (Supplementary Fig. 15). The hybrid nanocrystals maintained the luminescent properties of both perovskite QDs and UCNP and improved the stability of perovskite QDs at high temperatures.

Fig. 3

Fluorescence of CsPbBr₃–NaYF₄:Yb,Tm hybrid nanocrystals.

a Photographs showing different color fluorescence emitted from the heterostructured CsPbBr₃-NaYF₄:Yb,Tm nanocrystals collected during the synthesis under different conditions (state 1, 300 °C—0 min, state 2, 300 °C—60 min, both under 980 nm excitation, state 3, 300 °C—60 min, under 365 nm excitation). b, c Fluorescence emission spectra of heterostructured CsPbBr₃–NaYF₄:Yb,Tm nanocrystals at different temperatures under 980 nm and 365 nm excitations, respectively. d Fluorescence intensity of CsPbBr₃ nanocrystals (red) and heterostructured CsPbBr₃–NaYF₄:Yb,Tm nanocrystals (black) collected at different temperatures under 365 nm excitation.

Exploration of the energy-transfer mechanism

The Förster resonance energy transfer (FRET) and the emission reabsorption (ERA) were the two main energy-transfer processes. The changes in fluorescence lifetime are directly associated with FRET, but not with ERA. Efficient FRET between NaYF₄:Yb,Tm donors and CsPbBr₃ acceptors will only take place at short distances. Indeed, the fluorescence average lifetime reduction of the blue emission at 478 nm from 0.66 ms of the NaYF₄:30%Yb,0.5%Tm nanocrystals to 0.29 ms of the CsPbBr₃–NaYF₄:Yb,Tm nanocrystals (Supplementary Fig. 16 and Supplementary Table 5) suggested that the energy transfer from NaYF₄:Yb,Tm to CsPbBr₃ in the heterostructure follows a typical FRET process^27,41. The FRET efficiency can be calculated with the equation Eff = 1 − τ_D–A/τ_D, where Eff is the energy transfer efficiency, τ_D–A is the effective lifetime of a donor conjugated with an acceptor, and τ_D is the effective lifetime of a donor in the absence of an acceptor. The FRET efficiency of the CsPbBr₃–NaYF₄:Yb,Tm nanocrystals calculated was 56%, which justifies our design for an efficient energy transport strategy. The fluorescence lifetime of the mixed CsPbBr₃ and NaYF₄:Yb,Tm is 0.59 ms, which only reduces 0.07 ms compared with the NaYF₄:30%Yb,0.5%Tm NPs fluorescence lifetime of 0.66 ms, and the efficiency of FRET is calculated to be 11%, far lower than the energy transfer efficiency of FRET in the heterogeneous junction (Supplementary Figs. 16, 17 and Supplementary Tables 5, 6). Therefore, the energy transfer process of the mixed CsPbBr₃ and NaYF₄:Yb,Tm is mainly EAU, which is different from that of FRET in heterostructured CsPbBr₃-NaYF₄:Yb,Tm hybrid nanocrystals. In addition, the fluorescence lifetime of the perovskites has increased from 18 ns to 0.37 ms (Supplementary Fig. 18 and Supplementary Table 7).

By changing the number of perovskites, we have changed the ratio between the perovskite and UCNP in the heterostructured nanocrystals, and obtained 0.2-fold and fivefold CsPbBr₃–NaYF₄:Yb,Tm nanocrystals. The fluorescence lifetime of the onefold heterostructured nanocrystal is 0.29 ms (Supplementary Fig. 19 and Supplementary Table 8). The fluorescence lifetime of the 0.2-fold heterostructured nanocrystal increased to 0.49 ms, while that of the fivefold heterostructured nanocrystal decreased to 0.25 ms (Supplementary Table 8). As compared to the FRET efficiency of the onefold heterostructured nanocrystal, 56%, the FRET efficiency of the 0.2-fold heterostructured nanocrystal was reduced to 25.8%. On the other hand, the FRET efficiency of the fivefold heterostructured nanocrystal increased to 62.1% (Supplementary Table 9). Therefore, it shows that the decrease in the perovskite/UCNP ratio resulted in increased fluorescence lifetime and decreased FRET efficiency. Compared to the fluorescence spectra of pure UCNP under 980 nm excitation, the absorption of the fluorescence emitted from UCNP increases with the increase of the perovskite amount (Supplementary Fig. 20).

Regulation of optical properties

The heterojunction nanocrystals synthesized by us could undergo anion exchange with Cl⁻ and I⁻ to control the energy transfer and luminescence performance. Under the 980 nm excitation, the 525 nm fluorescence peak emitted by the perovskites within the heterogeneous crystals was shifted to 578 nm after the addition of I⁻. On the other hand, this same peak was shifted to 478 nm after the addition of Cl⁻ (Supplementary Fig. 21A). During the anion exchange in CsPbBr₃ nanocrystals with the introduction of I⁻ into the heterogeneous crystal, and the fluorescence lifetime of CsPbBr₃ nanocrystals in the heterojunction became longer. However, the introduction of Cl⁻ into the heterojunction shortened the fluorescence lifetime of CsPbBr₃ nanocrystals in the heterogeneous crystal⁴², thus affecting the energy transfer efficiency. Under the 365 nm excitation, the 525 nm fluorescence peak emitted from the heterogeneous crystal was blue shifted to 479 nm and red shifted to 578 nm, respectively (Supplementary Fig. 21B). Therefore, this heterojunction nanocrystal structure was able to regulate the energy-transfer process and optical properties from NaYF₄:Yb,Tm to CsPbBr₃ nanocrystals via anion exchange. Compared with pure UCNPs, heterogeneous crystals can not only regulate their optical properties through anion exchange but also change the composition of CsPbX₃ nanocrystals in heterogeneous crystals. With the ratios of perovskites to UCNPs kept unchanged and only the perovskite composition was changed, we have obtained the heterostructured CsPbBr₂/Cl₁-NaYF₄:Yb,Tm nanocrystals and the heterostructured CsPbBr₂/I₁-NaYF₄:Yb,Tm nanocrystals. The size of the heterostructured CsPbBr₂/Cl₁-NaYF₄:Yb,Tm and CsPbBr₂/I₁-NaYF₄:Yb,Tm nanocrystals were 42.3 nm and 39.6 nm, respectively (Supplementary Fig. 24A, B). The irregular small particles of perovskite in the heterostructured nanocrystals were also observed. According to the fluorescence spectrum under the excitation of 980 nm (Supplementary Fig. 24C, D), the emission peak of the perovskites in the heterostructured CsPbBr₂/Cl₁-NaYF₄:Yb,Tm nanocrystals was shifted left to 477 nm, while the emission peak of the perovskites in the heterostructured CsPbBr₂/I₁-NaYF₄:Yb,Tm nanocrystals was shifted right to 542 nm. Under 365 nm excitation, the fluorescence peaks of the heterostructured CsPbBr₂/Cl₁-NaYF₄:Yb,Tm and CsPbBr₂/I₁-NaYF₄:Yb,Tm nanocrystals were 478 nm and 542 nm (Supplementary Fig. 24E, F), respectively. These peaks matched with the upconversion fluorescence peaks of CsPbX₃ in the heterostructured nanocrystals which indicated that the NaYF₄:Yb,Tm has a protective effect on the structure of perovskite at high temperature. The fluorescence lifetimes of the heterostructured CsPbBr₂/Cl₁-NaYF₄:Yb,Tm and CsPbBr₂/I₁-NaYF₄:Yb,Tm nanocrystals at the 478 nm peak were 0.23 ms and 0.48 ms, respectively (Supplementary Fig. 24G-H and Supplementary Table 10). Therefore, we have shown that our synthesis strategy is compatible to synthesize heterogeneous junctions composed of different types of UCNPs and perovskites. If the aforementioned three conditions were met, different types of heterogeneous junction products such as PbS-UCNPs, CdS-UCNPs can be obtained.

Phase transition

At 250 °C, the heterojunction nanocrystals mainly contained cubic-phase CsPbBr₃ crystals and cubic-phase NaYF₄:Yb,Tm crystals according to the standard data of JCPDS# 00-054-0752 and JCPDS# 00-016-0334 (Supplementary Fig. 25). The 2θ of the {100} crystal plane of the cubic-phase CsPbBr₃ crystals has increased from 15.18° to 15.3°, and the lattice spacing has decreased. On the other hand, the 2θ of the {100} crystal plane of the cubic-phase NaYF₄:Yb,Tm crystals has decreased from 28.2° to 27.8°, which further indicated that the lattice strain was generated by the heterojunction nanocrystals at 250 °C. In order to understand the phase transition during the process of maintaining the temperature at 300 °C for 0 to 60 min, lattice spacing and XRD measurements for the above samples were performed. In this process, as seen in Fig. 4a–i, the lattice distance on the surface of the hybrid nanocrystals was 0.3 nm in accordance with the {110} lattice spacing of hexagonal-phase NaYF₄ UCNP (JCPDS: 00-016-0334), while the lattice distance in the middle was 0.34 nm corresponding to the {111} lattice spacing of cubic-phase CsPbBr₃ QDs (JCPDS: 00-054-0752). The high-resolution TEM images confirmed that the two crystals were grown together and well combined which is very important to ensure a highly efficient energy transfer between them when excited at 980 nm. At 300 °C, XRD results in Fig. 4j showed that the nanocrystals mainly contained cubic-phase CsPbBr₃ crystals and hexagonal-phase NaYF₄ crystals, according to the standard data of JCPDS# 00-054-0752 and JCPDS# 00-016-0334. Very weak peaks of cubic-phase NaYF₄ crystals (JCPDS: 01-077-2042) were also observed. With the extension of the thermal insulation time, the cubic-phase NaYF₄ crystals transformed to hexagonal-phase NaYF₄ crystals and finally disappeared, resulting in an increase in the relative proportion of hexagonal NaYF₄ crystals (Supplementary Fig. 26), so only cubic-phase CsPbBr₃ crystals and hexagonal-phase NaYF₄ crystals co-existed in the hybrid nanocrystals. The process of phase transition led to the formation of crystal boundary in the nanocrystal structure, which was confirmed with HRTEM results. When cubic-phase NaYF₄ crystals transformed to the hexagonal phase with high fluorescence efficiency, upconversion fluorescence was enhanced, which was also consistent with the results of fluorescence measurement.

Fig. 4

Structural analysis of CsPbBr₃-NaYF₄:Yb,Tm hybrid nanocrystals.

HRTEM images of heterostructured CsPbBr₃-NaYF₄:Yb,Tm nanocrystals collected during the synthesis after heated at 300 °C for different time: a 0 min, b 30 min, c 60 min. d–f Enlarged images of region 1 in a, b, and c, respectively. g–i Enlarged images of region 2 in a, b, and c, respectively. j X-ray diffraction (XRD) patterns of the three samples. Hexagonal NaYF₄, red color (JCPDF#00-016-0334), cubic NaYF₄, blue color (JCPDF #01-077-2042), cubic CsPbBr₃, green color (JCPDF #00-054-0752).

Stability study

The stability in a mixture solution of cyclohexane and ethanol with a ratio of 9:1 or 1:1 has significantly improved as compared to the conventional CsPbBr₃ nanocrystals. We have studied the stability of heterostructure composite of CsPbBr₃-NaYF₄:Yb,Tm nanocrystals in a mixture solution of cyclohexane and ethanol with a ratio of 9:1 (Fig. 5a and Supplementary Fig. 27) also. The relative fluorescence intensity of the CsPbBr₃ nanocrystals decreased to 12% at 10 min and 5% at 120 min in the mixture solution. On the other hand, the heterostructure composite of CsPbBr₃-NaYF₄:Yb,Tm nanocrystals has managed to maintain a relative fluorescence intensity of 93% at 10 min and 85% at 120 min. This result suggests good stability in a mixture of cyclohexane and ethanol with a ratio of 9:1. Furthermore, the relative fluorescence intensity of the CsPbBr₃ nanocrystals quickly decreased to 10% at 1 min and 5% at 120 min in a mixture of cyclohexane and ethanol with a ratio of 1:1 (Fig. 5b and Supplementary Fig. 27), whereas the heterostructure composite of CsPbBr₃-NaYF₄:Yb,Tm nanocrystals has maintained a relative fluorescence intensity of 95% at 1 min and slowly decrease to 80% after 2 h. This result suggests excellent fluorescence stability in a mixture of cyclohexane and ethanol with a ratio of 1:1. The stability of the heterostructure composite of CsPbBr₃–NaYF₄:Yb,Tm nanocrystals in water was still less than ideal. As shown in Fig. 5c, the relative fluorescence intensity of CsPbBr₃ nanocrystals in water solution has decreased to <1% after 30 s, while the relative fluorescence intensity of the heterostructure composite of CsPbBr₃-NaYF₄:Yb,Tm nanocrystals was able to remain at 46% in water solution (Supplementary Fig. 28). Even though the relative fluorescence intensity decreased to about 4% in 5 min eventually, the stability of these heterostructured nanocrystals in the water has shown slight improvement as compared to the conventional CsPbBr₃ nanocrystals. From the results of solvent stability experiments, it can be inferred that there were bare perovskite crystals on the heterogeneous crystal surface that were not completely covered by NaYF₄:Yb,Tm nanocrystals. Upon contact with the water molecules, the water molecules infiltrated into the heterogeneous crystal structure along with the perovskite found on the crystal surface. Thus, the perovskite crystal structure in the heterogeneous junction was destroyed and reduced the fluorescence significantly. In addition, we have studied light stability. Under continuous ultraviolet light, the CsPbBr₃ nanocrystals only have a 40% relative fluorescence intensity after 360 min, while heterostructure composite of CsPbBr₃–NaYF₄:Yb,Tm nanocrystals maintained an 80% fluorescence intensity (Fig. 5d and Supplementary Fig. 29). This result suggests that the heterostructured nanocrystals have good light stability.

Fig. 5

Stability analysis of CsPbBr₃–NaYF₄:Yb,Tm hybrid nanocrystals.

PL intensity test was used to monitor the stabilities of CsPbBr₃ (red) and the heterostructure composite of CsPbBr₃–NaYF₄:Yb,Tm nanocrystals (black) in cyclohexane and ethanol (v/v = 9: 1) mixed solvent (a), cyclohexane and ethanol (v/v = 1: 1) mixed solvent (b), water solvent (c), and continuous ultraviolet light irradiation (d) under 365 nm excitation.

Chemicals and materials

Cesium carbonate (Cs₂CO₃, 99.9%), lead bromide (PbBr₂, 99.9%), lead chloride (PbCl₂, 99.9%), lead iodide (PbI₂, 99.9%), yttrium acetate ((CH₃CO₂)₃Y·xH₂O, 99.9%), ytterbium acetate ((CH₃CO₂)₃Yb·4H₂O, 99.9%), thulium(III) acetate ((CH₃CO₂)₃Tm·xH₂O, 99.9%), sodium hydroxide (NaOH, 99%), ammonium fluoride (NH₄F, 98%), octadecene (ODE, 95%), oleic acid (OA, 90%), oleylamine (OAm, 98%), oleylamine chloride (OAmCl, 99%), oleylamine iodide (OAmI, 99%), cyclohexane (99.9%), and toluene (99.9%) were purchased from Sigma-Aldrich and used as received without further purification.

Synthesis of CsPbBr₃ perovskite nanocrystals

Cs-oleate was synthesized by reacting Cs₂CO₃ (0.2 g) with OA (0.6 mL) in octadecene (7.5 mL) and pre-heated to 120 °C before injection. PbBr₂ (0.188 mmol) and 5 mL of ODE were loaded into a 100 mL 3-neck flask, dried under vacuum at 120 °C for 1 h, and mixed with vacuum-dried OAm (0.5 mL) and OA (0.5 mL) under a N₂ atmosphere. The temperature was raised to 150 °C, and 0.6 mL of Cs-oleate solution was rapidly injected. After 5 s, the reaction mixture was cooled in an ice-water bath. Nanocrystals were precipitated by centrifugation at 12,000 rpm and re-dispersed in 10 mL of cyclohexane. Other samples (CsPbBr₂Cl₁ and CsPbBr₂I₁) with different ratios of Br/Cl or I = 2/1 were synthesized by the same strategy. The other conditions for synthesis and purification remain the same.

Synthesis of heterostructured CsPbBr₃- NaYF₄: Yb,Tm nanocrystals

First, 0.695 mmol Y(CH₃CO₂)₃, 0.30 mmol Yb(CH₃CO₂)₃, and 0.005 mmol Tm(CH₃CO₂)₃ were mixed with 6 mL of oleic acid and 15 mL of octadecene in a 50-mL flask and heated to 150 °C. After cooled down to room temperature, 10 mL of methanol solution (100 mg NaOH and 150 mg NH₄F) were slowly added into the flask, and the solution was stirred for 10 min. The solution was subsequently heated to remove methanol and degassed at 120 °C for 20 min. In all, 1 mL of CsPbBr₃ solution was hot-injected into the precursor solution and then heated to 300 °C for 1 h under N₂ environment. The product was washed twice with cyclohexane by centrifugation for 10 min and dispersed in 20 mL of cyclohexane for further use. The ratio between CsPbBr₃ and NaYF₄: Yb,Tm nanocrystals could be adjusted.

Synthesis of heterostructured CsPbBr₂Cl₁- NaYF₄: Yb,Tm and heterostructured CsPbBr₂I₁- NaYF₄:Yb,Tm nanocrystals

In total, 0.695 mmol (CH₃CO₂)₃Y, 0.30 mmol (CH₃CO₂)₃Yb, and 0.005 mmol (CH₃CO₂)₃Tm were mixed with 6 mL of oleic acid and 15 mL of octadecene in a 50-mL flask and heated to 150 °C. After cooled down to room temperature, 10 mL of methanol solution (100 mg NaOH and 150 mg NH₄F) was slowly added into the flask, and the solution was stirred for 10 min. The solution was subsequently heated to remove methanol and degassed at 120 °C for 20 min. In all, 1 mL of CsPbBr₂I₁ or CsPbBr₂I₁ solution was hot-injected into the precursor solution and then heated to 300 °C for 1 h under N₂ environment. The product was washed twice with cyclohexane by centrifugation for 10 min and dispersed in 20 mL of cyclohexane for further use.

Anion exchange of heterostructured CsPbBr₃- NaYF₄: Yb,Tm nanocrystals

The anion-exchange reactions were performed in a 10-mL glass bottle. The heterostructured CsPbBr₃- NaYF₄: Yb,Tm nanocrystals (5 mL, 0.05 mol/L) were used as the precursor. For anion exchange, OAmI or OAmCl (20 mg) was dissolved in cyclohexane (20 mL) as the anion source, and add 850 μL of OAmI or 700 μL of OAmCl solution into the heterostructured nanocrystals solution (5 mL) to achieve anion exchange. Anion exchange could be completed in a time period ranging from tens of seconds to a few minutes. After the reaction, the emission peaks of the solution were detected by a spectrofluorometer.

Characterization

TEM was performed on an FEI Tecnai G2 F20 electron microscope operating at 200 kV. Low-voltage high-resolution STEM measurement was carried out on a double-aberration-corrected Titan^TM cubed G2 60−300 S/TEM equipped with Super-XTM technology. The available point resolution is ~0.1 nm at an operating voltage of 60 kV. XRD was measured with a Bruker AXS D8 X-ray diffractometer equipped with monochromatized Cu Kα radiation (λ = 1.5418 Å). XPS was performed using an achromatic Al Kα source (1486.6 eV) and a double-pass cylindrical mirror analyzer (ULVACPHI 5000 Versa Probe) (Japan). Heterostructured nanocrystals were dissolved in cyclohexane solution at a concentration of 0.05 mol/L, and the upconversion fluorescence spectrum, down-conversion fluorescence spectrum, absorption spectrum, excitation spectrum, and fluorescence lifetime were tested and characterized. Ultraviolet and visible absorption (UV–vis) spectra were recorded with a Shimadzu UV-3600 plus spectrophotometer equipped with an integrating sphere under ambient conditions. Photoluminescence excitation and emission spectra and fluorescence decays were recorded on a FLS980 spectrometer (Edinburgh) equipped with both continuous xenon (450 W) and pulsed flash lamps. Upconversion fluorescence emission spectra were acquired under 980 nm excitation with a CW diode laser (2 W/cm²). Measurement of absolute up- and down-conversion PLQY of nanocrystals was performed using a standard barium sulfate-coated integrating sphere (Edinburgh). The sample chamber was mounted to a FLS980 spectrophotometer. Lifetime was measured with a customized UV to mid-infrared steady-state and phosphorescence lifetime spectrometer (FSP920-C, Edinburgh) equipped with a digital oscilloscope (TDS3052B, Tektronix) and a tunable mid-band Optical Parametric Oscillator pulsed laser as the excitation source (410–2400 nm, 10 Hz, pulse width ≤5 ns, Vibrant 355II, OPOTEK). Fluorescence decay was measured on a Nikon Ni-U Microfluorescence Lifetime system with a 375 nm picosecond laser and a time-correlated single-photon counting system at room temperature.