1. INTRODUCTION

Owing to the outstanding optoelectronic properties, such as high photoluminescence quantum yield (PLQY), flexible tunability of emission wavelength, and narrow emission spectrum, all-inorganic CsPbX3 (X=Cl, Br, I) quantum dots (QDs) have received attentions as promising candidates for the next generation optoelectronic devices [13" target="_self" style="display: inline;">–3]. The outstanding optical properties make them potentially suitable for using in light-emitting diodes (LEDs) [46" target="_self" style="display: inline;">–6], solar cells [7,8], photodetectors [9,10], and most importantly as optical gain materials [11–14" target="_self" style="display: inline;">–14]. Lasers and amplified spontaneous emission (ASE) from perovskite QDs have been reported recently [1517" target="_self" style="display: inline;">–17]. Yakunin et al. first reported the ASE performance of CsPbBr3 QDs under a single-photon-pumped laser [18]. Pan et al. showed the ASE threshold of CsPbBr3 QDs film to be 192μJ/cm2 and 12mJ/cm2 under single-photon and two-photon pumped laser, respectively [19]. Most recently, Song et al. embedded CsPbBr3 QDs into an Au@SiO2 core–shell nanostructure and showed a reduced threshold of 5mJ/cm2 under one-photon exciton [20]. In addition, a number of studies were performed to reduce the threshold of ASE and laser [21–24" target="_self" style="display: inline;">–24]. However, their practical applications are still limited owing to the poor stability resulting from the high sensitivity to moisture and heat [2527" target="_self" style="display: inline;">–27].

Therefore, various strategies, including coating, doping/alloying heteroatoms, and ligand modification have been studied to improve the stability of CsPbBr3 QDs [28–33" target="_self" style="display: inline;">–33]. Our group reported a ligand modification method to synthesize CsPbBr3 QDs by using 2-hexyldecanoic acid (DA), resulting in an excellent stability [34]. Nag et al. reported a doping method by using Mn and Yb with a reduced-defect density and enhanced stability [35]. However, ligand modification and elemental doping are not effectively protecting QDs from humidity and heat. Among all these methods, coating is an effective and practical strategy to control the stability and suppress the nonradiative auger recombination [36–39" target="_self" style="display: inline;">–39]. Chen et al. reported a two-photon-pumped ASE performance by embedding the CsPbBr3 QDs with dual-mesoporous silica [40]. Tang et al. showed that by capping CsPbBr3 QDs core with a CdS shell, the chemical stability and two-photon-pumped ASE performance could be improved [41]. Zhang et al. embedded CsPbBr3 QDs into silica by tetraethyl orthosilicate (TEOS) hydrolysis, which improved the moisture resistance and enhanced the stability [42]. However, such methods are generally complicated because it requires the use of extra gases during the synthesis and is not time efficient. The high processing temperature and prolonged stirring might also impede further research of stable and efficient gain materials. Therefore, it is quite urgent to exploit a room-temperature synthesis technique, which could synthesize CsPbBr3 QDs with high PLQY and high stability for ASE.

In this work, we synthesized the CsPbBr3 QDs with a high thermal stability by a facile one-step at room temperature method. The QD films were coated with SiO2 by adding 3-aminopropyl-triethoxysilane (APTES). The PLQY of CsPbBr3@SiO2 QDs reached 71.6%, while it was only 46% in those with pure CsPbBr3 QDs. In addition, the CsPbBr3@SiO2 QDs exhibited an excellent stability under heat. An enhanced two-photon pumped ASE that operated in an ambient air condition was also demonstrated. Compared with those using CsPbBr3 QDs, the ASE threshold of CsPbBr3@SiO2 QDs films was reduced by 70μJ/cm2 under a two-photon pumped laser excitation. Such a simple and yet effective method to coat shell onto CsPbBr3 QDs might have potential applications in fields such as room-temperature-operated frequency up-conversion lasers.

2. EXPERIMENT

Chemicals and reagents: PbBr2 (99.99%) and CsBr (99.9%) were purchased from Xi’an Polymer Light Technology Corp. DMF (99.9%, Sigma-Aldrich). OA (90%), OAm (80%–90%), toluene (99.5%), and APTES (99.5%) were obtained from Adamas. All these reagents were used without further purification.

Synthesis of CsPbBr3 QDs: PbBr2 (0.4 mmol) and CsBr (0.4 mmol) were first added in 10 mL of DMF and stirred for 1 h to obtain a clear solution. Then OA (0.6 mL) and OAm (0.2 mL) were added into the precursor solution followed by a stirring for another 30 min. After that, 0.5 mL of the precursor solution was quickly added in a beaker containing 10 mL of toluene under vigorous stirring at 1500 r/min for 10 s.

Synthesis of CsPbBr3@SiO2 QDs: 0.5 mL precursor solution was quickly added into 10 mL toluene containing 0.69 μL APTES under vigorous stirring at 1500 r/min for 10 s. All the above experiments were carried out at room temperature.

Characterizations: The X-ray diffraction (XRD) characterization was performed using XRD-6100 (Shimadzu, Japan). Fourier transform infrared (FTIR) spectra of the samples were recorded with a Nicolet iS50 (Thermo Fisher Scientific, Waltham, MA, USA). X-ray photoelectron spectroscopy (XPS) profiles were measured on an ESCA Lab220I-XL. The transmission electron microscopy (TEM) was performed using an electron microscope (Libra 200 FE, Zeiss, Germany). Absorption spectrum was enforced by a UV-2100 (Shimadzu, Japan). The photoluminescence spectrum was obtained by a fluorescence spectrophotometer (Agilent Cary Eclipse, Australia) equipped with a Xe lamp. Time-resolved fluorescence spectra were recorded with a GL-3300 (Photon Technology International Inc., USA). The PLQY was investigated by an FLSP920 (Edinburgh Instruments Ltd., UK). The ASE measurements were performed using a pumping source of a Ti:sapphire amplifier system (wavelength: 800 nm, repetition rate: 1 kHz, pulse-width: 50 fs, Solstice, Spectra-Physics). The ASE was detected by a fiber spectrometer (Ocean Optics) with a spectral resolution of 1 nm.

3. RESULTS AND DISCUSSION

CsPbBr3@SiO2 QDs were obtained through a modified supersaturated recrystallization method at room temperature, and the whole process only took 10 s. As shown in Fig. 1, first CsBr, PbBr2, OA, and OAm (OA:OAm=3:1, volume ratio) were all mixed in N, N-dimethylformamide (DMF). Precursors were then rapidly injected into the toluene solution, which contained a certain amount of 3-aminopropyl-triethoxysilane (APTES). Upon the injection into a poor solvent of toluene, CsPbBr3 QDs were formed immediately. Meanwhile, APTES gradually linked to the surface of QDs and then reacted with the trace of water from open air to hydrolysis [42]. Finally, silica (SiO2) was formed and coated onto the CsPbBr3 QDs.

Fig. 1. Reaction process schematics of the formation of CsPbBr3@SiO2 QDs.

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The crystal structure of CsPbBr3 QDs was first studied using X-ray diffraction. As shown in Fig. 2(a), black and red lines correspond to CsPbBr3 and CsPbBr3@SiO2 QDs, respectively. The CsPbBr3 QDs exhibit diffraction peaks at 15.66°, 21.96°, 31.01°, 34.87°, 38.15°, and 44.30°, which correspond to the (100), (110), (200), (210), (211), and (202) planes of CsPbBr3 QDs (JCPDS 18-0364), respectively. The XRD patterns of CsPbBr3@SiO2 are in good agreement with those of CsPbBr3 QDs, indicating that the in situ growth of the SiO2 does not affect the cubic crystal structure of CsPbBr3. Surface functional groups were then determined using Fourier transform infrared spectra of CsPbBr3 and CsPbBr3@SiO2 QDs shown in Fig. 2(b). While the weak peaks located at 1113, 1013, and 751cm−1 indicate the presence of Si–O–Si bonds, the one located at 925cm−1 corresponds to the existence of Si–OH bonds, both suggesting the presence of SiO2 in CsPbBr3@SiO2 QDs. To determine the chemical composition of CsPbBr3@SiO2 QDs, X-ray photoelectron spectroscopy was also carried out with Fig. 2(c) showing the full scan of CsPbBr3@SiO2 QDs. While the peaks of Cs 3d [737.5 and 723.8 eV, Fig. 2(d)], Pb 4f [142.8 and 137.9 eV, Fig. 2(e)], and Br 3d [68 eV, Fig. 2(f)] clearly demonstrate the formation of CsPbBr3 [43,44], Si 2p peak [102.4 eV, Fig. 2(g)] and O 1s peak [531.7 eV, Fig. 2(h)] suggest the formation of Si–O–Si bonds [45] and are in agreement with the results obtained from FTIR spectra.

Fig. 2. (a) XRD patterns, (b) FTIR spectra, (c) XPS full scan of CsPbBr3@SiO2 QDs and the high-resolution XPS profiles of (d) Cs 3d, (e) Pb 4f, (f) Br 3d, (g) Si 2p, and (h) O 1s.

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Figures 3(a)–3(d) show the transmission electron microscopy (TEM) and high resolution TEM (HRTEM) images of the obtained CsPbBr3 QDs and CsPbBr3@SiO2 QDs, respectively. It is found that the CsPbBr3@SiO2 QDs maintain an orthorhombic morphology, a good dispersity, and the same lattice plane distance of 0.58 nm as pure CsPbBr3 QDs, indicating that the SiO2 coating has no effect on the crystal structure of CsPbBr3. In addition, it was found that CsPbBr3 QDs were all dispersed and embedded inside the SiO2. The average size of CsPbBr3 QDs is 13.4 nm [inset of Fig. 3(a)], while a shrinking and narrow size distribution with an average QD size of 12.3 nm is observed in CsPbBr3@SiO2 QDs [inset of Fig. 3(c)]. This phenomenon might be due to the hydrolysis of APTES, which in turn leads to the presence of silica around the lead ions and binds the growth of perovskite crystals [46]. Figure 3(e) shows the energy-dispersive spectroscopy (EDS) mapping images of CsPbBr3@SiO2 QDs, from which a uniform distribution of the Cs, Pb, and Br components, and the presence of SiO2 were observed in the films with CsPbBr3@SiO2.

Fig. 3. (a), (c) TEM images and (b), (d) HRTEM images of pure CsPbBr3 and CsPbBr3@SiO2 QDs. (e) Element distribution of CsPbBr3@SiO2 QDs.

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As shown in Fig. 4(a), although both CsPbBr3 and CsPbBr3@SiO2 QDs show green emission colors, the luminescence of CsPbBr3@SiO2 QDs is greener. In Fig. 4(b), we show the PL emission spectra of both CsPbBr3 QD films with the same quantity, which were measured under the same condition. The PL intensity of the films with CsPbBr3@SiO2 QDs is almost doubled compared with those using CsPbBr3 QDs. In particular, the PLQY of CsPbBr3@SiO2 QDs is as high as 71.6%, while it is only 46% in the CsPbBr3 case. As shown in Fig. 4(c), the absorption and PL peaks of CsPbBr3 and CsPbBr3@SiO2 QDs are located at 529/526 nm and 517/508 nm, respectively. For CsPbBr3@SiO2 QDs films, a blue shift is observed both in the absorption and PL results, which might be due to the decreased QDs size [47]. To understand the carrier dynamics, a PL lifetime measurement was carried out with the results shown in Fig. 4(d). These PL decay curves are fitted with a biexponential function consisting of a fast-decay component (τ1) and a slow-decay component (τ2) [48]. The fast-decay component (τ1) is speculated to be the trap-assisted nonradiative recombination, while the slow-decay component (τ2) is speculated to be the free-charge carrier radiative recombination [49]. Clearly, the CsPbBr3@SiO2 QDs exhibit a longer decay time [7.6 ns (τ1) and 36.6 ns (τ2)] than CsPbBr3 QDs [4.9 ns (τ1) and 22.5 ns (τ2)] [in inset of Fig. 4(d)]. SiO2 might reduce the surface defect/trap state density of the perovskite QDs, resulting in a suppression of the defect-assisted nonradiative recombination [45]. Therefore, it is believed that such an enhanced PLQY and elongated PL lifetime of CsPbBr3@SiO2 QDs are mainly due to the passivation effects of SiO2, and the decrease of the surface defects could contribute to the improvement of the QDs luminescent performance.

Fig. 4. (a) Photographs of CsPbBr3 and CsPbBr3@SiO2 QDs solution with/without UV light and films with UV light. (b) PL intensity, (c) Absorption and PL spectra, and (d) PL decay curves of CsPbBr3 and CsPbBr3@SiO2 QDs.

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Subsequently, the thermal effects on the PL performance of CsPbBr3 and CsPbBr3@SiO2 QDs films were tested, as shown in Fig. 5. While the PL intensity of CsPbBr3 QDs film decreases quickly under the heating at 60°C [Fig. 5(a)], the CsPbBr3@SiO2 QDs film exhibits a slow decrease in PL intensity [Fig. 5(b)]. Although both types of the films show a decrease of PL intensity over time, the films with CsPbBr3@SiO2 QDs maintain 84% of their initial PL intensity, while only 24% remains in the case with pure CsPbBr3 after 80 min continuous heating, indicating that the CsPbBr3@SiO2 QDs film is fairly resistant to heat and possesses a good chemical stability. This result confirms that coating the CsPbBr3 QDs film with SiO2 could significantly enhance the thermal stability of perovskite, which is critical for optoelectronics in future practical applications.

Fig. 5. PL spectra of (a) CsPbBr3 and (b) CsPbBr3@SiO2. (c) Normalized PL intensity of CsPbBr3 and CsPbBr3@SiO2 under continuous heating at 60°C.

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All-inorganic CsPbX3 perovskites were previously reported to exhibit excellent potentials as candidates for lasers and two-photon-excited up-conversion devices [50,51]. To further study the potential of our materials, CsPbBr3 and CsPbBr3@SiO2 QDs were deposited on glass to obtain ASE performance under a two-photon (800 nm) excitation at room temperature. At a relatively low pump excitation, a broad spontaneous emission (SE) with a peak centered at 529 nm and an FWHM of 23 nm are found for CsPbBr3 QDs film, as shown in Fig. 6(a). With the increase of pump density, a peak located at 535 nm emerges and quickly becomes dominant. Meanwhile, the FWHM of the emission spectra narrows sharply to 4.9 nm [Fig. 6(b)], suggesting the transition from SE to ASE regimes. As shown in Fig. 6(c), the emission of CsPbBr3@SiO2 QDs films has similar features: initially a broad SE shows at low pump and quickly transforms to an obvious ASE phenomenon with the increase of pump output power [Fig. 6(c)]. Also, the FWHM of the emission spectra narrows sharply from 25 to 3 nm, as shown in Fig. 6(d). The thresholds (Pth) of two-photon pumped ASE are found to be about 6.9 and 6.2mJ/cm2 for CsPbBr3 QDs and CsPbBr3@SiO2 QDs films, respectively, under excitation of 800 nm and 35 fs laser pulses. A clear decrease of ASE threshold for 70μJ/cm2 is found here owing to the effective capping with SiO2, suggesting such a method is promising for depositing laser devices.

Fig. 6. (a) Pump-intensity dependence of the emission from the CsPbBr3 film under an 800 nm femtosecond laser excitation. (b) Output intensity (red) and linewidth (black) as functions of pump energy density for CsPbBr3 film under a femtosecond laser excitation of 800 nm. (c) Pump-intensity dependence of the emission from the CsPbBr3@SiO2 film under an 800 nm femtosecond laser excitation. (d) Output intensity (red) and linewidth (black) as functions of pump energy density for CsPbBr3@SiO2 film under a femtosecond laser excitation of 800 nm.

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4. CONCLUSION

CsPbBr3@SiO2 QDs were synthesized by a one-step in situ method at room temperature in air. By coating with SiO2, surface defects of CsPbBr3 QDs are passivated, which suppresses the defect-assisted nonradiative recombination. As a result, the PLQY of CsPbBr3 QDs increases from 46% to 71.6%, and the thermal stability significantly improves. In addition, under the two-photon (800 nm) pump laser excitations, the ASE threshold of CsPbBr3@SiO2 QDs film is 70μJ/cm2 lower than that of the CsPbBr3 QDs film owing to the effective SiO2 passivation. The results demonstrate a simple synthesis method to coat SiO2 with CsPbBr3 QDs at room temperature, which also provides an industry-compatible deposition method for lasers.