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Single-Mode Lasers Based on Cesium Lead Halide Perovskite Submicron Spheres Bing Tang,†,‡ Hongxing Dong,*,† Liaoxin Sun,§ Weihao Zheng,⊥ Qi Wang,§ Fangfang Sun,§ Xiongwei Jiang,† Anlian Pan,⊥ and Long Zhang*,†,∥ †

Key Laboratory of Materials for High-Power Laser, Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Shanghai 201800, China ‡ University of Chinese Academy of Sciences, Beijing 100049, China § National Lab for Infrared Physics, Shanghai Institute of Technical Physics, Chinese Academy of Sciences, Shanghai 200083, China ⊥ Key Laboratory for Micro-Nano Physics and Technology of Hunan Province, State Key Laboratory of Chemo/Biosensing and Chemometrics, and School of Physics and Electronics, Hunan University, Changsha 410082, China ∥ IFSA Collaborative Innovation Center, Shanghai Jiao Tong University, Shanghai 200240, China S Supporting Information *

ABSTRACT: Single-mode laser is realized in a cesium lead halide perovskite submicron sphere at room temperature. All-inorganic cesium lead halide (CsPbX3, X = Cl, Br, I) microspheres with tunable sizes (0.2−10 μm) are first fabricated by a dual-source chemical vapor deposition method. Due to smooth surface and regular geometry structure of microspheres, whispering gallery resonant modes make a single-mode laser realized in a submicron sphere. Surprisingly, a single-mode laser with a very narrow line width (∼0.09 nm) was achieved successfully in the CsPbX3 spherical cavity at low threshold (∼0.42 μJ cm−2) with a high cavity quality factor (∼6100), which are the best specifications of lasing modes in all natural nano/microcavities ever reported. By modulating the halide composition and sizes of the microspheres, the wavelength of a single-mode laser can be continuously tuned from red to violet (425−715 nm). This work illustrates that the well-controlled synthesis of metal cesium lead halide perovskite nano/microspheres may offer an alternative route to produce a widely tunable and greatly miniaturized single-mode laser. KEYWORDS: single-mode laser, cesium lead halide, whispering gallery mode, microsphere, wavelength tunable

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realized in this way; others still stay in the theoretical simulation due to the difficulties in the fabrication of such multinanostructures.9 Additionally, single-mode lasing can also be observed through the Vernier effect in two or more coupled laser cavities, which mainly relies on the sophisticated nanomanipulation and fabrication techniques.10−13 To address these problems, the straightforward way to obtain a singlemode laser is reducing the optical path of the laser cavity to keep only one mode surviving in the bandwidth of the optical gain. In one representative report, Li et al. obtained single-mode ZnO lasing from an individual sub-micron-sized rod with a diameter of about 600 nm.14 However, the reduction of cavity size will usually increase the optical loss, resulting in high threshold and low cavity quality factor for lasing action. To compensate the loss caused by the shortening optical path,

iniaturized semiconductor lasers have been a significant research subject due to their potential for the integration of nanoscale photonic and optoelectronic circuits and devices.1−3 In the past two decades, tremendous research efforts have been devoted to the development of high-quality semiconductor lasers with broadband tunability and controllable output wavelength. In particular, semiconductor lasers oscillating at a single frequency, such as single-mode lasers, have attracted great interest because of their practical applications in on-chip optical processing, communication, and data storage. However, most of the reported semiconductor lasers are subject to random fluctuations and instabilities due to lack of a mode selection mechanism.4−7 The realization of single-mode lasing with small mode volume, narrow spectral line width, and a broad-band tunability is still a great challenge. Generally, a single-mode laser could be obtained through the use of distributed Bragg reflector (DBR) mirrors or distributed feedback (DFB) structures on nanostructures.8 Unfortunately, only Zhang et al. reported single-mode CdS nanoribbon laser © 2017 American Chemical Society

Received: June 27, 2017 Accepted: October 9, 2017 Published: October 9, 2017 10681

DOI: 10.1021/acsnano.7b04496 ACS Nano 2017, 11, 10681−10688

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Figure 1. Characterizations of cesium lead halide MSs. (a) Typical SEM image of the CsPbI3 MSs. Top inset: magnified image of an individual CsPbI3 MS. Bottom inset: PL image of as-grown CsPbI3 MSs excited by 457 nm continuous-wave laser at room temperature. (b) TEM image of an individual CsPbI3 MS. HRTEM image (c) and the corresponding FFT pattern (d) from the individual CsPbI3 MS. EDS elemental mapping for CsPbI3 (e), CsPbBr3 (f), and CsPbCl3 (g), showing the composition of the MSs. (h) PL spectra of CsPbCl3, CsPbBr3, and CsPbI3 MSs. Inset: PL images of CsPbCl3, CsPbBr3, and CsPbI3 MS (left to right).

threshold (∼0.42 μJ cm−2) with a high cavity quality factor (∼6100), even in submicron spheres. Additionally, when element modulation and size control are combined, highquality single-mode lasing can be extended to the whole visible spectra region.

some materials with high optical gain and nano/microcavities with good optical confinement are highly desirable to achieve single-mode lasing with good performance. Recently, lead halide perovskites have gained great attention for their success in photovoltaics.15−17 As a direct band gap semiconductor, the long carrier lifetimes and diffusion lengths, the low nonradiative recombination rates, and the high photoluminescence quantum yield make this kind of material particularly attractive for laser applications.18−21 Compared to organic−inorganic hybrid perovskites, all-inorganic perovskite (such as cesium lead halide) exhibits better stability without losing its superior optoelectronic properties.22,23 Indeed, perovskite lasers have been achieved in many nano/microstructures with different geometries, such as nanowires,24−26 nanoplatelets,27−29 nanorods,30 and quantum dots,31,32 but a low threshold, narrow laser width and wide-band tunable single-mode laser spanning the full visible region has never been reported. As we mentioned above, besides the high optical gain, a tightly confined optical mode in the submicron cavity is crucial to the performance of single-mode laser. Here, we note that the light confined in whispering gallery mode (WGM) microcavities (cross section of microwire, nanoplatelets) usually presents a quality factor better than that of Fabry-Pérot-type nanowire or nanorod cavities because of total internal reflection effect in WGM microcavities.33,34 With this in mind, the synthesis of the perovskite submicron structures as WGMs could be a promising route to achieve a single-mode laser with good performance. In this work, we first report a tunable single-mode lasing in all-inorganic cesium lead halide (CsPbX3, X = Cl, Br, I) submicron spheres at room temperature, which were synthesized by a simple-step vapor transport process. The single-mode laser with very narrow line width (∼0.09 nm) is realized in the perovskite submicron spherical cavity at low

RESULTS AND DISCUSSION Cesium lead halide CsPbX3 (X = Cl, Br, I) microspheres (MSs) were grown by a simple dual-source chemical vapor deposition method using CsX and PbX2 powders as source material and bare silicon wafer as a substrate in a horizontal tube furnace. Similar to the growth of other semiconductor microspheres, the formation of the spherical shape is mainly due to the surface tension forces at high temperature.35−37 Figure 1a shows a typical scanning electron microscopy (SEM) image of synthesized samples. It can be seen that the as-grown CsPbI3 MSs dispersed on a piece of silicon wafer with diameters ranging from 0.2 to 1.0 μm, placing them in an ideal size regime for single-mode lasing as confirmed later. The top-right inset of Figure 1a clearly shows perfectly spherical morphology and very smooth surface of the CsPbI3 MSs. Moreover, varying the deposition temperature and/or the position of silicon substrate, the diameters of the CsPbI3 MSs can be further increased to several micrometers, as shown in Figure S1a. For a widely tunable single-mode laser, similar MS structures of CsPbX3 (X = Cl, Br) were also obtained by halogen substitution, with diameter controlled around 1.0 μm, as shown in Figure S1b,c. Photoluminescent (PL) images of CsPbX3 MSs are shown in Figure S2. The preparation details are presented in Methods section and Table S1. Figure 1b presents a typical transmission electron microscopy (TEM) image of an individual CsPbI3 MS, confirming its spherical structure inferred from the relatively uniform contrast of the MS. Furthermore, the high-resolution TEM (HRTEM) image in Figure 1c, showing continuous 10682

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Figure 2. Single-mode lasing from an individual CsPbBr3 MS. (a) Schematic of an individual CsPbBr3 MS on silicon substrate pumped by a 400 nm laser excitation (∼40 fs, 10 kHz). The green circle indicates the light propagation inside the spherical WGM cavity. (b) Excitation power-dependent lasing spectra from one single CsPbBr3 MS. Inset: PL image of CsPbBr3 MS above the lasing threshold. (c) Integrated emission intensity as a function of pump density showing the lasing threshold at ∼0.42 μJ cm−2. (d) Lorentzian fitting of a lasing oscillation mode. The fwhm of the lasing peak (δλ) is 0.09 nm, corresponding to a Q factor ∼6100.

Optically pumped lasing experiments were performed in a vacuum atmosphere at room temperature. As schematically presented in Figure 2a, a 400 nm femtosecond laser was used as the excitation light source. The laser spot covered the whole MS to ensure its homogeneous excitation. The green circle indicates the light propagation inside the MS WGM cavity. Figure 2b shows typical excitation-power-dependent PL spectra of one single CsPbBr3 MS with a diameter of about 780 nm. It can be seen that a single broad emission peak resulting from spontaneous emission was observed at ∼530 nm with a fwhm of δλ = 16.6 nm, as pump density was below the threshold of 0.42 μJ cm−2. When pump density exceeded the threshold, one single sharp peak abruptly appeared above the spontaneous emission background and grew drastically with further increasing pump density. Importantly, no other resonant peaks were found in this process. A clear evolvement from spontaneous emission to stimulated emission occurs in the CsPbBr3 MS, indicating the achievement of a single-mode laser. Simultaneously, it can be noticed that the broadening of the fwhm of the lasing peak (0.09−0.62 nm) occurs with increasing of the excitation power due to highly injected carrier density, which may induce variation of the refractive index of materials and thus the shift of cavity modes during the dynamic process of stimulated emission for pulsing lasers.38,39 The PL microscopy optical image of this corresponding MS above the lasing threshold is shown in the top inset of Figure 2b. The interference pattern was produced by the coherent light emission in the MS WGM microcavity, confirming the generation of stimulated radiation. A slight blue shift (∼0.5 nm) could also be observed in CsPbBr3 MSs with pump density increasing, which may be caused by the band-filling effect and/

lattice fringe along the whole surface without apparent grain boundaries, reveals the cubic structure of CsPbI3 MS, in good agreement with the corresponding fast Fourier transform (FFT) pattern in Figure 1d. The microsphere with high crystalline quality can be confirmed by similar selected area electron diffraction patterns in Figure S3 taken from different positions on an individual CsPbBr3 MS. Additionally, the chemical composition and uniform spatial distribution of corresponding elements over the whole CsPbX3 MSs were confirmed by energy-dispersive X-ray spectroscopy (EDS) mapping (Figure 1e−g). Optical properties of the as-prepared CsPbX3 MSs were characterized with a confocal micro-photoluminescence spectrometer. As the band gap of CsPbX3 perovskites is largely determined by the hybridization states of Pb and X orbitals, band gap engineering can be achieved via halogen substitution, especially by controlling the Cl/Br or Br/I ratio to continuously tune emission wavelength covering the entire visible spectrum. The PL spectra were measured on the CsPbX3 MSs at room temperature (Figure 1h), from which three strong, narrow peaks at 427, 527, and 702 nm were obtained. The top insets of Figure 1h clearly present the PL images of these synthesized CsPbX3 MSs recorded at low excitation density, showing uniform blue, green, and red emissions from MS bodies. Furthermore, the excellent optical properties of these CsPbX3 microcrystals could be inferred from their PL spectra, exhibiting relatively small full width at half-maximum (fwhm) at 11, 13, and 20 nm. All these characterizations confirm high-quality CsPbX3 microspheres with perfectly spherical shape, highly smooth surface, and controllable size, making them maybe ideal WGM cavity candidates for single-mode lasers. 10683

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Table 1. Comparisons of Main Laser Parameters among Reported Semiconductor Nano/Microlasers on Natural Nano/ Microcavitiesa material

peak (nm)

line width (nm)

Q factor

threshold at RT (μJ cm−2)

excitation source

ref

ZnSe NW GaN NP ZnO NW ZnO ND CdS NW MAPbIaCl3−a NW CsPbBr3 NW CsPbBr3 NR CsPbCl3 NW MAPbI3 NW CsPbBr3 NPL FAPbI3 NPL FAPbI3 NW CsPbBr3 MS

461 369 387 389 512 744.4 538 543 420 787 536 837 824 545.2

0.72 2.20 0.80 0.70 0.40 1.60 0.26 0.155 0.30 0.22 0.15 0.49 0.53 0.09

640 170 484 556 1280 372 2069 3500 1400 3600 3600 1700 1554 6100

∼340 ∼40000 ∼400 ∼750 ∼14 ∼60 ∼6.2 ∼14.1 ∼7 ∼0.6 ∼2.2 ∼25 ∼6.2 ∼0.42

150 fs, 1 kHz 0.5 ns, 1 kHz 8 ns, 10 Hz 8 ns, 10 Hz 120 fs, 1 kHz 50 fs, 1 kHz 100 fs, 250 kHz 100 fs, 1 kHz 150 fs, 100 kHz 100 fs, 250 kHz 50 fs, 1 kHz 100 fs, 250 kHz 100 fs, 250 kHz 40 fs, 10 kHz

49 50 51 42 52 53 24 30 22 20 27 28 26 this work

a MA, FA, NW, NP, ND, NR, NPL, and RT denote CH3NH3, CH(NH2)2, nanowire, nanopillar, nanodisk, nanorod, nanoplatelet, and room temperature, respectively.

Figure 3. Lifetime and stability measurements of one single CsPbBr3 MS. (a) Streak camera image of an isolated CsPbBr3 MS at the pump density of 0.1 PTh (top inset) and 1.2 PTh (bottom inset). (b) Typical PL decay curve obtained at three different excitation densities (0.1 PTh, 0.9 PTh, 1.2 PTh). (c) Integrated emission intensity of a CsPbBr3 MS under 400 nm femtosecond laser excitation at a constant pump density of 1.2 PTh while exposed to ambient atmosphere.

or the reduction of refractive index.40,41 Figure 2c shows the dependence of emission intensity on the excitation power. A typical “S”-shaped curve can be observed, confirming the evolution from random spontaneous emission to amplified spontaneous emission at ∼0.42 μJ cm−2, then finally simulated emission taking over at ∼0.77 μJ cm−2. To further analyze the characteristics of single-mode lasing, a representative full lasing spectrum taken at ∼0.42 μJ cm−2 is presented in Figure 2d. The peak was well fitted by a Lorentzian function with a fwhm of 0.09 nm. Using the relationship Q = λ/δλ, where λ is the peak center wavelength and δλ is the peak width, the cavity quality factor Q is calculated to be ∼6100. Interestingly, we found that lasing line width, lasing threshold, and the cavity quality factor Q in such a submicron sphere are better than those of other nano/microsized lasers reported in previous literature (Table 1). Generally, the Q factor of a traditional dielectric microcavity decreases drastically due to the greater radiation loss in a smaller cavity, which also leads to a higher lasing threshold.14,42 In our work, the good performance of such small CsPbBr3 MSs can be attributed to a nearly perfect naturally formed spherical WGM cavity and high optical gain material. In our regular

microsphere WGM microcavities, there are no such corner optical losses, and three-dimensional confinement of light due to the total internal reflection is successfully achieved. Compared to other nano/microstructure optical cavities, of which most parts lay on the substrate, the coupling between CsPbX3 MS cavity and the substrate is comparatively very weak, resulting in less optical losses.43 Perhaps, as with these properties, the spherical microcavities can exhibit a better quality factor. Further investigations about the dynamics of single-mode lasing in CsPbBr3 MSs were conducted by time-resolved photoluminescence (TRPL) using a 400 nm femtosecond laser and a streak camera system. Figure 3a shows the typical streak camera images of CsPbBr3 MS recorded at the pump density of 0.1 and 1.2 PTh. It can be clearly seen that a typical wide spontaneous emission decay with a broad emission band and long decay time changes into a narrow emission peak with a short decay time when the pump density increases from 0.1 to 1.2 PTh. As shown in Figure 3b, at low excitation density (0.1 and 0.9 PTh), a deconvolution biexponential decay function with rapid and slow components can be used to simulate the 10684

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Figure 4. Single-mode lasing characterization and theoretical simulation on CsPbBr3 MSs with a diameter of around 780 nm. (a) Single-mode lasing spectra of five typical CsPbBr3 MSs with different diameters. (b) Resonant mode is extracted and plotted as a function of the diameter of CsPbBr3 MS. Top inset: simulated electric-field distribution under a transverse magnetic resonant mode at 545.2 nm.

simulation using finite domain time difference (FDTD solution) package was performed, where the dispersion of refractive index of CsPbBr3 was employed.46 Considering the diameters (∼780 nm) of MSs, the simulated transverse magnetic-polarized (TM-polarized) whispering gallery modes can well match the experimental results; here, we note that the preferred TM-polarized lasing modes are consistent with the results from cesium lead halide perovskite nanoplatelets.27 The single-mode laser peaks with a diameter dependence are also plotted, as shown in Figure 4b. The lasing emission peaks show a linear red shift with increasing of the diameters of the CsPbBr3 MS cavities, as we expected from eq 1. The electricfield distributions of the laser (λ = 545.2 nm) in MS is also shown in the inset of Figure 4b; the typical WGM electric-field pattern can be clearly observed. In addition, the electric-field distributions for lasing peaks (λ = 533.8, 537, 539, 542.4 nm) are presented in Figure S4. The size of the microcavity not only is critical to the wavelength of laser resonance but also affects the number of resonant peaks. As shown in Figure S5, multimode lasing could also be observed around 535 nm when the size of the microcavity increases to 12.9 μm. Single-mode lasing emission with high cavity quality factor Q and a stable and spectrally narrow peak in the entire visible region are first achieved through element modulation of cesium lead halide MSs. Figure 5 shows the typical lasing spectra and corresponding PL images. It can be clearly seen that a tunable single-mode laser has been made ranging from 425 to 715 nm. Clearly, spatial interference patterns of the PL images (top insets of Figure 5) above the lasing threshold show the

decay curves. Based on previous reports, rapid and slow components can be attributed to surface state and bulk recombination, respectively.23 Worth noting, a slight difference of lifetime was observed between 0.1 PTh (195.2 ± 2.5 ps, 60.4%; 749.1 ± 6.1 ps, 39.6%) and 0.9 PTh (143.3 ± 1.3 ps, 77.6%; 669.3 ± 7.6 ps, 22.4%), indicating that there is only a fraction of nonradiative recombination at such low excitation density. As the pump density increases to 1.2 PTh, an additional decay component is observed (30.5 ± 0.9 ps, 95.3%), which contributes far more than the other two components (125.0 ± 0.9 ps, 3.9%; 608.0 ± 12.6 ps, 0.8%), implying the occurrence of an effective stimulated emission process. Considering the blue shift (∼0.5 nm) of the CsPbBr3 laser in Figure 2b, the EHP mechanism is responsible for simulated emission, similar to other CsPbBr3 nanostructure lasing.23,44,45 Additionally, to probe the stability of lasing output, the CsPbBr3 MS was constantly pumped by a pulsed excitation source. One single CsPbBr3 MS was pumped constantly by a 400 nm pulsed laser (∼40 fs, 10 kHz) under ambient conditions (21 °C, 45% relative humidity) at an excitation density of 1.2 PTh. The integrated emission intensity of CsPbBr3 MS was recorded and plotted as a function of excitation time in Figure 3c, in which stable lasing output can be observed for over 50 min (∼3 × 107 laser shots). The emission intensity of the lasing peak is observed to be very weak until 4.2 × 107 excitation cycles. The results illustrated that the CsPbBr3 MSs have significant operating lifetimes under ambient conditions. To further investigate the modulation of the single-mode laser with different microcavity diameters, we performed laser emission measurements for a set of CsPbBr3 MSs with different diameters varied about 780 nm. As shown in Figure 4a, the single-mode lasing emission can be tuned from 533.8 to 545.2 nm. In general, the resonant modes of a spherical WGM microcavity can be simply expressed as33,34 πD/(λ /N ) ≈ ν

(1)

where D denotes the diameter of CsPbBr3 MS; λ is the wavelength of the resonant peak; ν and N are the mode orders and the refractive index of the medium, respectively. In our experiment, the size of the submicron sphere varies very little and only one lasing mode survives in the gain region, so the mode order is unchanged and thus the resonant wavelength is linearly proportional to the diameter of the spherical WGM microcavity. To further understand the single-mode laser behavior of the spherical WGM microcavity, theoretical

Figure 5. Multicolor single-mode lasers and the corresponding emission images of one CsPbX3 MS. 10685

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field-emission scanning electron microscopy (Auriga S40, Zeiss, Oberkochen, Germany) and HRTEM (JEOL-2010). Optical Characterizations and Numerical Calculation. The optical properties of the CsPbX3 MSs were measured at room temperature in a vacuum atmosphere using a confocal microphotoluminescence system (LabRAM HR Evolution). A multichannel air-cooled (−60 °C) CCD detector (Syncerity OE) with a resolution of ∼0.0166 nm was used for lasing measurement. The room temperature photoluminescence and lasing emission spectra were all collected by the same objective (50×, 0.5 NA) and spectrally resolved using a monochromator. To further explore the characteristic of singlemode lasing, an isolated CsPbBr3 MS was selectively excited by a 400 nm femtosecond laser, which was obtained through a BBO crystal from the amplifier laser source (Libra, Coherent, ∼40 fs, 10 kHz) and a solid state diode laser (532/355 nm switch, 15 kHz, 1.1 ns, crystal, 2Q-STA-IL, Germany). A 400 nm femtosecond laser (repetition rate of 1 kHz, pulse width of 80 fs) equipped with a streak camera (C10910, Hamamatsu) was used to perform TRPL measurements. A BBO crystal was used to generate 400 nm output from the 800 nm laser of a regenerative amplifier (SPTF-100F-1K-ACE, SpectraPhysics). The TRPL decay profile can be fitted by a deconvolution multiexponential decay function with a impulse response function of 180 ps. Additionally, the lasing mode properties including mode orders and electric-field distributions of MSs were calculated using finite element method (FDTD solution). During the simulation process, a dipole source is used, and perfectly matched layer is chosen as the boundary condition; the refractive index of materials is 2.4, the maximum mesh step is set at 4 nm, and the monitored wavelength is from 450 to 600 nm.

predominance of WGM lasing in spherical microcavities. Although the size of the MS was reduced to about 1 μm to obtain the single-mode laser, the laser line width, threshold, and cavity quality factor exhibit surprisingly good performance. Moreover, to further investigate the single-mode lasing properties, we use the nanosecond laser (532/355 nm switch, 15 kHz, 1.1 ns) as the excitation source. Considering the possible thermal effects of nanosecond lasers,47 our laser experiments are performed at liquid nitrogen temperature. The corresponding results are shown in Figure S6. It can be seen that a tunable single-mode laser with narrow line width (∼0.1 nm) also can be obtained in the entire visible region (450−720 nm). Interestingly, the cavity quality factor can be as high as ∼6200. The lasing threshold is 1.65 μJ cm−2. Interestingly, we found that the threshold (0.22 μJ cm−2) obtained under femtosecond excitation (532 nm, 40 fs) is much lower than that under nanosecond excitation (532 nm, 1.1 ns), as shown in Figure S7. Similar results that a decrease of lasing threshold with a decrease of pulse duration were also found in other studies. The reason may be that the evolution of the density of excited states and consequently the gain profiles will be different under various pulse excitations.48 These results further illustrate that the achieved cesium lead halide MSs may be good candidates for the development of single-mode laser devices.

CONCLUSION We have realized a single-mode laser in individual all-inorganic cesium lead halide CsPbX3 submicron spheres working as WGM microcavities at room temperature. The perovskite MSs of CsPbX3 were prepared by a simple vapor transport method. The perfect smooth surface and regular geometry structure ensure that the as-grown perovskite MSs can be formed as ideal WGM optical cavities. Significantly, a single-mode laser was successfully achieved in one single MS with low threshold (∼0.42 μJ cm−2), and by modulating the halide composition and the sizes of the MSs, the single-mode lasing emission can be continuously tuned over the entire visible range. More importantly, a single-mode laser with narrow line width (∼0.09 nm) can be obtained in such a small diameter (D < 1 μm) cavity with a high cavity quality factor (∼6100), which is the best performance in all the reported natural nano/microcavity lasers.

ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.7b04496. SEM images of CsPbI3, CsPbBr3, CsPbCl3 MSs; PL images of CsPbI3, CsPbBr3, CsPbCl3 MSs; TEM analysis of an individual CsPbBr3 MS; electric-field intensity distribution simulation study of different CsPbBr3 MSs; multimode lasing spectra for CsPbBr3 MS; single-mode lasing from CsPbX3MS at 77 K; single-mode lasing from CsPbI3 MS at 77 K excited by a 532 nm femtosecond laser; detailed reaction conditions for the synthesis of CsPbX3 MSs (PDF)

AUTHOR INFORMATION Corresponding Authors

METHODS

*E-mail: [email protected]. *E-mail: [email protected].

Synthesis of CsPbX3 Perovskite MSs and Structural Characterizations. In our experiment, CsPbX3 microspheres were synthesized in a conventional horizontal tube furnace. All reagents were used without further purification, which were directly purchased from Sigma-Aldrich. The composition of the vapor source is CsX and PbX2 powders with a molar ratio of 1:2, of which the sum of the total weight is controlled to be around 0.15 g. At first, the lead(II) halide powder (PbX2, 99.999%, Sigma-Aldrich) was placed in the upper stream position of the quartz furnace, which was approximately 4 cm from the center of the quartz tube. Second, the cesium halide powder (CsX, Aldrich, 99.999%, Sigma-Aldrich) was placed in the center of the furnace. Then, three silicon substrates (1 cm × 1.4 cm) were placed 8 cm away from the center in the downstream, which were cleaned by distilled water and acetone before. The quartz tube was sealed by vacuum grease and first pumped to 0.5 Torr, followed by a 60 sccm flow of argon for 15 min to exhaust the atmosphere. The temperature of the tube furnace was increased to 900 K (870 for CsPbI3, 890 K for CsPbBr3, 920 K for CsPbCl3) for over 20 min, with argon flow controlled at 30 sccm. After 30−60 min of reaction, the system was cooled to room temperature naturally. The morphologies, structures, and composition of the products were characterized by

ORCID

Anlian Pan: 0000-0003-3335-3067 Long Zhang: 0000-0001-7978-8774 Author Contributions

B.T. and H.D. designed and performed the experiments during the whole process. L.S., Q.W., and F.S. helped to solve some technical problems. W.Z. and A.P. helped to finish the experiment of time-resolved photoluminescence. B.T., H.D., and L.S. wrote and revised the paper. X.J. and L.Z. supervised the project and conceived the study. Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was supported financially by the NSFC (61378074, 61675219, 61475173, 11474297, 11674343, 51525202). H.D. 10686

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and L.S. acknowledge the Youth Innovation Promotion Association CAS and Shanghai Science and Technology Foundation (Grant No. 17ZR1444000).

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DOI: 10.1021/acsnano.7b04496 ACS Nano 2017, 11, 10681−10688

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DOI: 10.1021/acsnano.7b04496 ACS Nano 2017, 11, 10681−10688