Facile synthesis of CsPbBr3/PbSe composite clusters

Abstract In this work, CsPbBr3 and PbSe nanocomposites were synthesized to protect perovskite material from self-enlargement during reaction. UV absorption and photoluminescence (PL) spectra indicate that the addition of Se into CsPbBr3 quantum dots modified the electronic structure of CsPbBr3, increasing the band gap from 2.38 to 2.48 eV as the Cs:Se ratio increased to 1:3. Thus, the emission color of CsPbBr3 perovskite quantum dots was modified from green to blue by increasing the Se ratio in composites. According to X-ray diffraction patterns, the structure of CsPbBr3 quantum dots changed from cubic to orthorhombic due to the introduction of PbSe at the surface. Transmission electron microscopy and X-ray photoemission spectroscopy confirmed that the atomic distribution in CsPbBr3/PbSe composite clusters is uniform and the composite materials were well formed. The PL intensity of a CsPbBr3/PbSe sample with a 1:1 Cs:Se ratio maintained 50% of its initial intensity after keeping the sample for 81 h in air, while the PL intensity of CsPbBr3 reduced to 20% of its initial intensity. Therefore, it is considered that low amounts of Se could improve the stability of CsPbBr3 quantum dots.


Introduction
Nowadays, much of the research concerning halide perovskites is focused on the new generation of solar cells, light-emitting diodes, transistors, lasers, and memristors with thin film, nanowire, and nanorod structures [1][2][3][4][5][6]. The power conversion efficiency of lead halide perovskite-based thin film photovoltaic cells has improved fast, from 3.8% to over 20% in a decade [7]. Among organic and inorganic perovskite materials, cesium lead halide has shown better stability than organic perovskite materials, suggesting great potential for use in opto-electronic devices.
Research on the morphology of perovskites shows that nano-size materials display superior properties in comparison with bulk-size materials, such as higher quantum yield, narrower emission bandwidth, and tunable color [8,9]. Recently, it was reported that cesium lead halide quantum dots have promisingly high color purity for light-emitting diodes [10,11]. Furthermore, it was shown that the growth rate, structure and size of CsPbBr 3 dots can be controlled by changing the length of carbon chains of acids and amines in ligands during synthesis [12]. This allows the possibility for finding the optimal structure and size for cesium lead halide OPEN ACCESS quantum dots. However, the thermal and electrical stability of cesium lead halide quantum dots is still limited.
A nanocomposite material of CsPbBr 3 and PbSe was synthesized to protect perovskite material from selfenlargement during reaction. As a core material, CsPbBr 3 quantum dots were chosen because CsPbBr 3 is the most stable among cesium lead halides. It is reported that materials with low lattice mismatch could easily form the core/shell structure such as CdSe/CdS (3.9% lattice mismatch) [28] or CdSe/ZnS (12% lattice mismatch) [29]. The lattice constant of cubic phase CsPbBr 3 is known to be 5.87 Å and that of PbSe is reported to be 6.1 Å [30,31]. Therefore, the lattice mismatch between CsPbBr 3 and PbSe is approximately 4%. The same source of Pb 2+ in an organic solvent could be used to produce CsPbBr 3 and PbSe. Furthermore, trioctylphosphine selenide can be used as an easy source for the PbSe shell layer. It is therefore expected that CsPbBr 3 /PbSe composites could be synthesized to improve the stability of inorganic perovskite quantum dots.

Synthesis of CsPbBr 3 quantum dots
The synthesis of CsPbBr 3 follows the previously reported procedures [32]. 70 mg of PbBr 2 was loaded into a 100 mL flask along with 10 mL of ODE, 1 mL of OA, and 0.5 mL of OAm. The solution was then heated up to 120 °C under N 2 to completely disperse PbBr 2 . Cesium oleate was prepared by loading 0.814 g of Cs 2 CO 3 along with 30 mL of ODE and 2.5 mL of OA into a 100 mL flask. The temperature of the solution was increased to 120 °C for the completion of cesium oleate. After heating the PbBr 2 solution to 190 °C, 0.4 mL of cesium oleate heated at 100 °C was injected to complete the synthesis of CsPbBr 3 quantum dots. The mixture was cooled down in an ice water bath. Quantum dots were precipitated and washed using hexane and butanol with a 1:1 volume ratio. After that, quantum dots were dispersed into toluene for further use.

Synthesis of CsPbBr 3 /PbSe nanocomposite
The selenide source was prepared by loading 10 mg of selenium in 0.5 mL of trioctylphosphine (TOP) and 0.5 mL of ODE. The mixture was heated up to 140 °C in 1 h. The 0.2 mL of TOPSe was quickly added into the solution of PbBr 2 with cesium oleate. The solution was kept at 190 °C for 1 h. Then, the color of the solution changed from light green to dark green. The materials were precipitated and washed using hexane and 2-butanol. The synthesized materials were re-dispersed in toluene for further use. In order to investigate the effect of synthesis temperature and the concentration of Se on the properties of the CsPbBr 3 /PbSe nanocomposite, samples were produced at synthesis temperatures of 150, 170, and 190 °C, and using 0.2, 0.4, and 0.6 mL of TOP-Se to match a Cs:Se mol ratio of 1:1, 1:2, and 1:3, respectively.

Characterization
A JASCO V-670 UV-vis spectrophotometer with a xenon arc lamp and PMT-1527 Hamamatsu photomultiplier was used to measure UV-visible absorbance and photoluminescence (PL) of the materials. X-ray diffraction (XRD, D8-Advance/Bruker-AXS), field-emission scanning electron microscopy (FE-SEM, SIGMA/Carl Zeiss), and transmission electron microscopy (TEM, JEOL-2100F, Japan) were applied to measure the structures and sizes of CsPbBr 3 and CsPbBr 3 /PbSe composites. Synchrotron radiation photoemission spectroscopy (SRPES) experiments were also performed in an ultrahigh-vacuum chamber (base pressure of ~10 −10 Torr) at the 4D beam line of the Pohang Acceleration Laboratory. The onset of photoemission, corresponding to the vacuum level at the sample surface, was measured using an incident photon energy of 250 eV with a negative bias on the sample. The results were corrected for charging effects using Au 4f as an internal reference.

Results and discussion
The expected structure of CsPbBr 3 /PbSe composite is schematically drawn in Figure 1(a). Figure 1 The synthesis of perovskite quantum dots via solution methods is quite sensitive. A large size was simple to achieve due to the easy bonding of anion and cation. According to previous reports, the CsPbBr 3 quantum dot solution was cooled down immediately after finishing the reaction [32,33]. In our case, the CsPbBr 3 quantum dot solution was heated for 1 h after adding TOPSe to make CsPbBr 3 /PbSe composites. However, luminescence was detected under UV illumination as shown in Figure 1. This means that the crystal structure of CsPbBr 3 was maintained even after a longer heating process and CsPbBr 3 was protected by PbSe.
To investigate the effects of the Se content of CsPbBr 3 / PbSe on its optical properties, UV-vis and PL spectra were measured. Figure 2 shows (a) UV-vis absorption spectra and (b) PL spectra of CsPbBr 3 quantum dots and composites of CsPbBr 3 /PbSe with different Cs:Se ratios (1:1, 1:2, and 1:3). The synthesis temperature of the CsPbBr 3 /PbSe composites was fixed at 190 °C. The  appeared, which are assigned to the (1 0 0) and (1 0 1) planes of orthorhombic CsPbBr 3 [35]. This means that the structure of CsPbBr 3 quantum dots changed from cubic to orthorhombic due to the introduction of PbSe at the surface. In the case of the CsPbBr 3 /PbSe composite with a 1:2 Cs:Se ratio, the peaks corresponding to PbSe (1 0 0) and (2 0 0) planes shifted to higher angles of 13 and 29°, respectively, suggesting that the plane distance of PbSe increased following the increase in Se. It is reported that PbSe has a cubic rocksalt structure with a lattice parameter of 6.12 Å and CsPbBr 3 has perovskite structure with a lattice parameter of 5.87 Å [30,31]. Therefore, it is considered that PbSe was compressed due to the smaller lattice parameter of CsPbBr 3 and that the lattice parameter of PbSe was restored as Se content increased. It is reported that orthorhombic phase has higher band gap than cubic phase. However, the photoluminescence is inactive in cubic-to-orthorhombic crystal transformation [26]. But, PL peak shift was found as shown in Figure 2. Therefore, band gap enlargement could be explained by Se doping. The (2 0 0) peak of CsPbBr 3 was absent in XRD spectra. However, the (1 0 0) peak of PbSe and peaks of orthorhombic CsPbBr 3 and (1 0 0) cubic CsPbBr 3 appeared in XRD patterns with weak intensity. The full width at half maximum of PbSe and CsPbBr 3 peaks also increased, indicating the size becomes smaller. Therefore, it is considered that the structure of cubic CsPbBr 3 is broken as the ratio of Se increased. This result agrees with the data of absorbance and photoluminescence as shown in Figure 2. Figure 4 shows SRPES spectra of CsPbBr 3 quantum dots and CsPbBr 3 /PbSe composite synthesized at 190 °C with a Cs:Se ratio of 1:3. The two peaks of Cs 3d 5/2 and Cs 3d 3/2 are shown at 726.1 and 740 eV, respectively. The Br 3d 5/2 and Br 3d 3/2 peaks are also shown at 69.3 and 70.4 eV, respectively. No peak change was observed in Cs 3d and Br 3d after the formation of PbSe at the surface absorption peak appeared at 455 nm in CsPbBr 3 quantum dots and in CsPbBr 3 /PbSe composites with a 1:1 Cs:Se ratio, which corresponds to an optical band gap of 2.38 eV as shown in the inset of Figure 2(a). In the case of CsPbBr 3 /PbSe composites with a 1:2 Cs:Se ratio, the absorption peak shifts to 429 nm, indicating that the band gap is increased to 2.42 eV. A Cs:Se ratio of 1:3 shifts the absorption peak to 414 nm, further increasing the band gap to 2.48 eV. These results indicate that the addition of Se into CsPbBr 3 quantum dots modifies the electronic structure of CsPbBr 3 . This result is in agreement with the PL spectra as shown in Figure  2(b). The CsPbBr 3 quantum dots emit green color with a wavelength between 500 and 550 nm. The PL spectra of CsPbBr 3 /PbSe composites with a 1:1 Cs:Se ratio indicate that their cyan color consists of a blue peak at 478 nm and a green peak at 516 nm. As the Cs:Se ratio increased from 1:1 to 1:3, the PL peak shifted to 462 nm. These results indicate that the emission color of CsPbBr 3 perovskite quantum dots could be modified from green to blue by adjusting the Se ratio in composites.
In order to confirm the formation of PbSe on the surface of CsPbBr 3 quantum dots, XRD measurements were performed. Figure 3 shows the XRD patterns of CsPbBr 3 quantum dots and CsPbBr 3 /PbSe composites with different Cs:Se ratios (1:1, 1:2, and 1:3). The synthesis temperature of the CsPbBr 3 /PbSe composites was fixed at 190 °C. The CsPbBr 3 quantum dots show two peaks at 15.16 and 30.6°, indicating that the synthesized CsPbBr 3 quantum dots have a cubic structure with (1 0 0) and (2 0 0) planes. This result agrees well with previous reports on CsPbBr 3 quantum dots synthesized at above 140 °C [32,34,35]. In the CsPbBr 3 /PbSe composite with a 1:1 Cs:Se ratio, new peaks appeared at 12.8 and 29.1° which correspond to the (1 0 0) and (2 0 0) planes of PbSe [36,37]. Furthermore, the (1 0 0) and (2 0 0) peaks of cubic CsPbBr 3 disappeared and new peaks at 14 and 16° FE-SEM ( Figure 5) and TEM ( Figure 6). It is shown that the average size of CsPbBr 3 quantum dots is approximately 15 nm and that of CsPbBr 3 /PbSe composite is approximately 20-25 nm. This suggests that PbSe is attached to the surface of CsPbBr 3 , increasing the size by approximately 5-8 nm. The magnified images shown in the insets of Figure 5(a) and (b) indicate that CsPbBr 3 is mono-dispersed well and CsPbBr 3 /PbSe forms a cluster. Figure 5(c) shows the size distribution of CsPbBr 3 /PbSe composite clusters according to synthesis temperature. The average size of particles synthesized at 150, 170, and 190 °C is 6-8, 8-10, and 16-22 nm, respectively. These results support the conclusion that the emission wavelength of CsPbBr 3 /PbSe composites under UV illumination increased as the synthesis temperature increased (see Figures 1 and 2). The CsPbBr 3 /PbSe composite clusters were examined by TEM as shown in Figure 6. The (0 2 0) plane distance of CsPbBr 3 was measured as 0.29 nm, and the (2 0 0) plane distance of PbSe had a similar value of 0.30 nm. Therefore, it is considered that the (0 2 0) plane of CsPbBr 3 connects with the (2 0 0) plane of PbSe, forming the CsPbBr 3 /PbSe composite clusters. Energy dispersive X-ray spectroscopy (EDS) mapping of Pb, Se, Br, and Cs atoms confirmed that the atomic distribution in CsPbBr 3 /PbSe composite clusters is uniform and the composite materials were well-formed.
In order to test their stability, CsPbBr 3 quantum dots and CsPbBr 3 /PbSe composites synthesized at 190 °C were dispersed in hexane and kept in air. Figure 7(a) shows the change in normalized PL as a function of time. The PL was measured every 24 h from the starting point of measurement. After 33 h, the PL intensity had degraded to around 70-80% of its initial intensity regardless of samples. After 57 h, PL values of CsPbBr 3 /PbSe samples with Cs:Se ratios of 1:2 and 1:3 dramatically decreased to of CsPbBr 3 . In the case of the Pb 4f peak, two peaks are shown at 138.1 and 143 eV in CsPbBr 3 quantum dots, which correspond to Pb 4f 7/2 and Pb 4f 5/2 , respectively. Additional peaks appeared at 137.5 and 142.4 eV in the CsPbBr 3 /PbSe composite, which indicates the formation of Pb-Se bonds. These values are compatible with previous research on CsPbBr 3 and PbSe nanocrystals [38][39][40]. These results indicate that PbSe was formed well on the surface of CsPbBr 3 quantum dots.
The size of CsPbBr 3 quantum dots and CsPbBr 3 /PbSe composite with a 1:3 Cs:Se ratio was measured using  with a 1:1 Cs:Se ratio still maintained 50% of its initial intensity even after 81 h, suggesting that low amounts of Se could improve the stability of CsPbBr 3 quantum dots. Figure 7(b) shows normalized PL spectra after storage approximately 20% of its initial intensity, suggesting that PbSe could not elongate CsPbBr 3 stability. In the case of CsPbBr 3 quantum dots, the PL intensity reduced to 20% of its initial value after 81 h. However, CsPbBr 3 /PbSe  20% of their initial intensity. These results suggested that low amounts of Se could improve the stability of CsPbBr 3 quantum dots. Therefore, it is considered that this synthesis method for CsPbBr 3 /PbSe composites could be used for other perovskite materials to increase the stability.

Disclosure statement
No potential conflict of interest was reported by the authors.
Funding for 9 and 81 h. The reduction of photoluminescence after keeping samples in room condition was found even though the peak position was maintained. However, the shift peak to higher wavelength was found in CsPbBr 3 / PbSe composite synthesized at 190 °C with a Cs:Se ratio of 1:3. This result indicated that large amount of PbSe with CsPbBr 3 could not protect perovskite quantum dots. We suppose the effect of PbSe with CsPbBr 3 due to the stress of lattice mismatch. CsPbBr 3 bonded due to the halide bonding that lead easily degrading in an ionic solvent or a humidity condition [32,41]. The high concentration of Se could cause the high stretch leading the degradation of CsPbBr 3 . With low concentration, the low stretch is not effect on CsPbBr 3 and also protect the itself enlargement then improve the stability.

Conclusions
CsPbBr 3 /PbSe composite clusters were synthesized by adding TOPSe into a solution of PbBr 2 with cesium oleate. As the ratio of Cs:Se increased from 1:1 to 1:3, CsPbBr 3 partially transformed from cubic structure to orthorhombic structure and bonded with PbSe crystal, forming the composite cluster. The addition of Se into CsPbBr 3 quantum dots modified the electronic structure of CsPbBr 3 , increasing the band gap from 2.38 to 2.48 eV as the Cs:Se ratio increased to 1:3. Therefore, the emission color of CsPbBr 3 perovskite quantum dots was modified from green to blue by increasing the Se ratio in composites. The average size of CsPbBr 3 quantum dots was approximately 15 nm and that of CsPbBr 3 /PbSe composite with a 1:3 Cs:Se ratio was approximately 20-23 nm. According to SEM and TEM images, CsPbBr 3 /PbSe composite exists in cluster form and each atom is uniformly distributed in the whole composite. The PL intensity of a CsPbBr 3 /PbSe sample with a 1:1 Cs:Se ratio maintained 50% of its initial intensity after keeping the sample for 81 h in air, while the PL intensity of CsPbBr 3 quantum dots reduced to