Owing to its outstanding optoelectronic properties, including flexible tunability of emission wavelength, long charge diffusion, high photoluminescence quantum yield (PLQY), large absorption coefficients, and narrow emission spectrum, all-inorganic halide perovskites nanomaterials CsPbX3 (X=Br, Cl, or I) have drawn attention as promising candidates for next-generation photovoltaics [1–6], light emitters [7], photodetectors [8–11], etc., and particularly as an optical gain medium for lasers and amplified spontaneous emission (ASE) [12–17]. Historically, Kovalenko et al. first reported the ASE and lasing performance from CsPbBr3 quantum dots (QDs) under femtosecond and nanosecond laser excitation [18]. Soon afterward, single-photon and two-photon pumped lasers of CsPbBr3 QDs were demonstrated [19–21]. In addition, aiming at enhancing the lasing quality, reducing threshold, and improving the stability of CsPbBr3, a series of studies have been conducted over the past few years [20–23].

Nevertheless, the sensitivity and instability of perovskite to moisture and heat seriously affect the performance of the perovskite lasers. To address these issues, diverse strategies such as coating, surface ligand engineering, and doping/alloying heterogeneous atoms have been implemented [24–28]. For example, our group devised an effective methodology to fabricate perovskite core/shell QDs by capping CsPbBr3 QDs with CdS, resulting in ultrastability and nonblinking performance [29]. Mir et al. reported a doping method by using Mn and Yb to reduce defect density and improve stability [30]. However, the long-chain capping ligands including oleylamine (OAm) and oleic acid could still result in a nonradiative recombination of excitons and photoluminescence emission degradation, which largely affects the quality of perovskite lasers and further hinders their practical applications [31–35]. Therefore, among all these methods, the surface ligand modification in the synthesis process was vigorously promoted and developed. For instance, the surface of CsPbBr3 QDs covered with ligands of different lengths can bring about different physical and chemical properties, including size, defect, stability, and so on [16,36,37]. However, among the previously reported methods, simple and effective strategies to improve not only photochemical properties of CsPbBr3 QDs but also the performance of perovskite lasers remain scarce. Thus, it is necessary to explore a facile and low-cost method to fabricate CsPbBr3 QDs for high-performance perovskite-based microlasers.

In view of previous studies, the weak binding energy between long-chain organic carbon chain ligands and QDs will lead to the aggregation of QDs and poor long-term stability, which has a great impact on the perovskite laser’s performance [16,36–40]. A facile ligand-engineering strategy was proposed to promote properties of CsPbBr3 QDs in our previous work by introducing the short-chain ligand octylamine (OLA) ligand (CsPbBr3-OLA) to replace the traditional long ligands OAm (CsPbBr3-OAm) during the hot injection process [22]. Based on this consequence, herein, we further explore its thermal stability and the evolution of photoluminescence quantum yield (PLQY) under ambient environment. The modified CsPbBr3 QDs film can maintain 33% of initial PL intensity, but only 17% is retained in the case of unmodified QDs after 10 h continuous heating at 60°C. Further, the obtained QDs with higher initial PLQY (91.8%) can maintain PLQY to 39.9% after being continuously exposed in air for 100 days, while the PLQY of original QDs is reduced to 5.5%. Meanwhile, the synthesized CsPbBr3-OLA QDs exhibit longer lifetime (16.90 ns) and no aggregation phenomenon after continuous exposure in air for 100 days. In addition, after coating CsPbBr3 QDs onto the micro SiO2 sphere, we finally succeeded in achieving the highly-efficient micro random lasers, and a lower threshold of 79.81μJ/cm2 and higher-quality factor (Q) of 1312 are presented from CsPbBr3-OLA QDs. All these results indicate that a simple yet effective method to improve the properties of CsPbBr3 QDs and the performance of perovskite microlasers has been realized. Simultaneously, it also provides a good prospect for the practical application of micro-nano semiconductor lasers.

Cs2CO3 (cesium carbonate 81.5 mg, 0.25 mmol), ODE (octadecene 5 mL), and OA (oleic acid 0.5 mL) were mixed in a 100 mL three-neck flask. The mixture was heated to 120°C with magnetic stirring under flowing nitrogen and kept for 60 min until the solution became clear; then, the Cs precursor fluid was obtained. Meanwhile, PbBr2 (lead bromide 138 mg) and ODE (10 mL) was added into a 100 mL three-neck flask, and the mixture were heated to 120°C with magnetic stirring and kept for 60 min, then OA (oleic acid, 1 mL) and OAm (oleylamine, 1 mL) were added in this solution; the temperature was then raised to 150°C and kept for 5 min until the solution became clear. The mixture (0.5 mL) was quickly injected into the Cs precursor fluid as soon as possible, and the temperature was maintained at 120°C for 5 s. The mixture was quickly cooled to room temperature by an ice-water bath after the reaction completed to produce OAm-CsPbBr₃ QDs. Finally, ethyl acetate was added into the crude solution with a volume ratio of 3:1; then, the mixed solution was put into a centrifuge with 6000 r/min for 5 min, and the precipitate was collected separately after centrifugation; finally, the sediment was dissolved into n-hexane. A similar procedure (replacing OAm with OLA, octylamine) was adopted for OLA-CsPbBr3 QDs. All the above chemicals were purchased from Aladdin Chemistry Co., Ltd. (Shanghai, China).

CsPbBr3 QDs were redispersed in n-hexane. 10 mg of commercial SiO2 spheres was added in CsPbBr3 QDs solution; the solution was then drop-coated onto the cleaned glass substrate for 60 min.

The QDs solution was dropped to the glass (15mm×15mm) to study the properties of CsPbBr3 QDs; the X-ray diffraction (XRD) pattern was collected by Cu Ka radiation (XRD-6100, Shimadzu, Japan). A Zeiss LIBRA 200FE microscope was used for TEM and high-resolution TEM images. The XPS profiles were tested by an ESCA Lab220I-XL. The FTIR spectra were obtained by an IRPrestige-21 spectrophotometer. The material absorption spectra were measured by a scan ultraviolet–visible spectrophotometer (ultraviolet-2100, ranging from 300 to 900 nm). The photoluminescence (PL) spectrum was obtained by a fluorescence spectrophotometer (Agilent Cary Eclipse, Australia). PL lifetime was obtained by a fluorescence spectrometer from Edinburgh Instruments (FS5-TCSPC). The PLQY was measured by an Edinburgh fluorescence spectrometer.

The fundamental pulse at 800 nm from a Ti:sapphire laser (repetition rate: 1 kHz, pulse-width: 35 fs, Solstice, Spectra-Physics) was used as a pump source. All the lasing experiments were carried out at room temperature.

Traditional hot injection was adopted to synthesize CsPbBr3 QDs, and lead bromide and cesium oleate were used as precursors [41]. In general, the long-chain ligand OAm is widely used during the synthesis process, which will result in intrinsic instability of CsPbBr3 QDs [42–44]. Thus, OLA with the short carbon chain was used to replace the regular OAm ligand. The synthesis scheme of the engineered CsPbBr3 QDs is schematically illustrated in Fig. 1(a). The X-ray diffraction (XRD) patterns of CsPbBr3-OAm and CsPbBr3-OLA QDs are shown in Fig. 1(b), confirming the cubic phase of CsPbBr3 [45]. The main peaks are located at 2θ=15.2°, 21.66°, and 30.69°, which correspond to the (100), (110), and (200) crystal planes of perovskite, respectively, indicating the excellent crystallization and pure phase of perovskite QDs. To investigate the surface ligand, the corresponding Fourier transform infrared (FTIR) spectra were measured, as shown in Fig. 1(c), and the peaks of CH3 and ─CH2─ reveal that the CsPbBr3 QDs are well capped with the OAm/OLA ligand after a purification process. There is no difference between CsPbBr3-OAm and CsPbBr3-OLA QDs, indicating that both have been successfully synthesized and well identified. In order to further study the morphological characteristics of perovskite QDs, transmission electron microscopy (TEM) was used to inspect the morphology and size distribution of QDs. High-resolution TEM (HRTEM) images for CsPbBr3-OAm and CsPbBr3-OLA QDs are displayed in Fig. 1(d) and Fig. 1(e), respectively. All QDs exhibit a cubic perovskite structure, high crystallinity, and good monodispersity. The average particle diameters of CsPbBr3-OAm QDs and CsPbBr3-OLA QDs are about 14 and 8 nm, respectively. The smaller size of CsPbBr3-OLA QDs could be ascribed to the short allylic ligands, which would result in weaker attractive van der Waals (VDW) interactions with each other than the long ones [46]. Meanwhile, Fig. 2(a) displays an X-ray photoelectron spectroscopy (XPS) profile of perovskite QDs, and the Cs 3d peaks at 724 and 738 eV, Pb 4f peaks at 138 and 143 eV, and Br 3d peaks at 68 eV can be observed in the Figs. 2(b)–2(d). All the above experimental results confirm the successful synthesis of the CsPbBr3 QDs.

As shown in Fig. 3(d), the PL intensity of CsPbBr3-OAm QDs film decreases quickly for 10 h under the annealing at 60°C, while the PL intensity of CsPbBr3-OLA QDs film displays a slow decrease under the same condition in Fig. 3(e). The overall tendency is summarized in the Fig. 3(f). Although PL intensity of both types of the QDs films exhibits a downward trend over time, the CsPbBr3-OLA QDs film can maintain 33% of initial PL intensity, but only 17% is retained in the case of CsPbBr3-OLA QDs after 10 h continuous heating, illustrating that the CsPbBr3-OLA QDs film possesses better heat tolerance. Figure 3(g) shows the photographs of CsPbBr3-OAm and CsPbBr3-OLA QDs in a hexane solution for 100 days under daylight and an ultraviolet (UV) lamp. The initial solutions of two types QDs are clear and highly bright. After stored in ambient for 100 days, the CsPbBr3-OAm QDs solution has aggregated and degraded, while the solution of CsPbBr3-OLA QDs still exhibits high luminescent brightness after 100 days. Meanwhile, we investigated the change of PLQY during 100 days; as shown in Fig. 3(h), the initial PLQY of CsPbBr3-OLA QDs is as high as 91.3%. The PLQY of CsPbBr3-OAm QDs has decreased to only 5.5% after being continuously exposed in air for more than 100 days, while the PLQY still remains 39.9% for CsPbBr3-OLA QDs, indicating better chemical stability in air, and it is beneficial to practical applications in future.

Previously, all-inorganic CsPbX3 perovskites have been reported as having excellent potential as candidates for nanolasers [39,45]. To explore the lasing performance before and after modification, a commercial silica sphere (SiO2) was adopted. CsPbBr3-OAm QDs and CsPbBr3-OLA QD were coated on the surface of the SiO2 to form composites (CsPbBr3-OAm/SiO2 and CsPbBr3-OLA/SiO2), respectively. Specific experimental methods can be seen in the experimental section. Figure 4(a) presents the TEM image of a micro SiO2 sphere covered with CsPbBr3-OAm QDs; the average diameter of the SiO2 sphere can be observed to be about 200 nm. The HRTEM image is displayed in Fig. 4(b); the lattice fringe of the QDs can be clearly observed, indicating the existence of QDs on the surface of the SiO2 sphere. To further study the chemical composition of CsPbBr3-OAm/SiO2, energy-dispersive spectrometer (EDS) mapping was performed, as shown in Fig. 4(c), which illustrates uniform and effective distribution of the Cs, Pb, Br, and Si atoms coated on SiO2 sphere. Moreover, the corresponding TEM, HRTEM, and EDS mapping analyses based on the CsPbBr3-OLA/SiO2 composite are shown in Figs. 4(d)–4(f), respectively. All the above experimental results demonstrate that a composite structure between the perovskite QDs and silica beads is successfully formed, providing a good foundation for further laser output.

To study the lasing properties, the well-dispersed CsPbBr3/SiO2 composite was transferred from the solution to the glass substrates. Subsequently, the close-packed thin film of CsPbBr3/SiO2 composite was pumped by an 800 nm femtosecond laser with 35 fs pulses at a repetition rate of 1 kHz under ambient conditions. The schematic of random lasing from CsPbBr3/SiO2 composite film is depicted in Fig. 5(a). The typical excitation fluence-dependent PL spectra of the CsPbBr3-OAm/SiO2 composite are displayed in Fig. 5(b), and the lasing behavior has been achieved at various pump excitation. Below the lasing threshold, only a broad spontaneous emission centered at ∼525nm with a full-width at half-maximum (FWHM) of ∼25nm can be observed. Strikingly, as the pumping intensity increases to exceed a threshold of 257.34μJ/cm2, multiple sharp peaks emerge at the low-energy shoulders of the spontaneous emission spectrum and the FWHM decreases dramatically, elucidating that the lasing action was occurring. Additionally, the laser peak position is irregularly changed, and the spacing between adjacent peaks is also not fixed, which is evidence of random laser generation [44]. Figure 5(c) shows the slopes of the output intensity versus pump fluence from CsPbBr3-OAm/SiO2 composite, and the lasing threshold value is determined to be 257.34μJ/cm2, which also reveals the transition from spontaneous emission to stimulated emission as the pump intensity increases [13,18]. Similarly, the laser behavior with analogous characteristic from the CsPbBr3-OLA QDs layer onto SiO2 can be observed in Fig. 5(d). Furthermore, as shown in Fig. 5(e), the lasing threshold of CsPbBr3-OLA/SiO2 composite is only 79.81μJ/cm2, which is less than one-third of the CsPbBr3-OAm/SiO2 composite. To further analyze the performance of a random laser, Q-factor analysis of CsPbBr3/SiO2 composite film was conducted. The relationship Q=λδλ is used to estimate the Q-factor in our experiment, where λ is the center wavelength of lasing and δλ is the FWHM value. The calculated results can be seen in Fig. 6; the Q-factor of CsPbBr3-OAm/SiO2 is only 990, while the Q-factor of CsPbBr3-OAm/SiO2 is as high as 1312. All these aforementioned consequences illustrate a promising development of low-threshold random upconverted laser with all-inorganic perovskite QDs.

In summary, a facile and valid ligand modification strategy is proposed to synthesize CsPbBr3 QDs with better photochemical properties. The obtained perovskite QDs exhibit longer lifetime, higher PLQY, and better stability to moisture and heat. In addition, we coat CsPbBr3 QDs on the surface of the SiO2 to form composites and realize a random laser from CsPbBr3/SiO2 composite under ambient conditions. In comparison with CsPbBr3-OAm/SiO2 composites, the laser of CsPbBr3-OLA/SiO2 composites presents lower threshold (79.81μJ/cm2) and higher Q-factor (1312) under two-photon (800 nm) excitation. This work would provide a novel and feasible strategy for the remarkable upconversion random lasers and promote the development of microlasers with all-inorganic perovskite QDs.