PerovskiteâIon Beam Interactions: Toward Controllable Light Emission

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Functional Inorganic Materials and Devices

Perovskite-Ion Beam Interactions: toward Controllable Light-Emission and Lasing Yue Wang, Zhiyuan Gu, Yinjuan Ren, Ziming Wang, Bingqing Yao, ZhiLi Dong, Giorgio Adamo, Haibo Zeng, and Handong Sun ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b01592 • Publication Date (Web): 10 Apr 2019 Downloaded from http://pubs.acs.org on April 10, 2019

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ACS Applied Materials & Interfaces

Perovskite-Ion Beam Interactions: toward Controllable Light-Emission and Lasing Yue Wang1,2, Zhiyuan Gu1, Yinjuan Ren3, Ziming Wang2, Bingqing Yao4, Zhili Dong4, Giorgio Adamo1,5, Haibo Zeng2*, Handong Sun1,5,6* 1Division

of Physics and Applied Physics, School of Physical and Mathematical Sciences,

Nanyang Technological University, Singapore 637371, Singapore 2MIIT

Key Laboratory of Advanced Display Materials and Devices, Institute of Optoelectronics &

Nanomaterials, College of Materials Science and Engineering, Nanjing University of Science and Technology, Nanjing 210094, China 3Department

of Chemistry, National University of Singapore, 3 Science Drive 3, Singapore

117543, Singapore 4School

of Materials Science and Engineering, Nanyang Technological University, Nanyang

Avenue, Singapore 639798, Singapore 5Centre

for Disruptive Photonic Technologies (CDPT), Nanyang Technological University,

Singapore 637371, Singapore 6MajuLab,

International Joint Research Unit UMI 3654, CNRS, Université Côte d’Azur,

Sorbonne Université, National University of Singapore, Nanyang Technological University, Singapore 637371, Singapore * Authors to whom correspondence should be addressed, electronic email: [email protected]; [email protected]

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KEYWORDS: Inorganic perovskites, focused ion beam, integrated photonics, photopattern, microlaser. ABSTRACT: Achieving controllable coherent and incoherent light sources is crucial to meet the requests of the constantly-developing integrated optics, which, however, remains challenging for the existing semiconductor materials and techniques. All-inorganic lead halide perovskites (ILHPs) are emerging as the promising semiconductors, featuring the defect-tolerant nature and tunable bandgap. Herein, an experimental design, based on the interaction between ILHPs and energetic ions, for achieving controllable light emitters and microlasers is reported. We reveal that the photoluminescence (PL) intensity from ILHPs can be modulated by more than one order of magnitude upon low-dose gallium ions (~1015 ions/cm2) irradiation, which can be attributed to the generation of vacancy/interstitial defects, metallic lead and crystal-to-amorphization transition. Such ion-dependent light emission can be exploited to make the colorful photopatterns and in-situ tailor the lasing behavior from CsPbBr3 microplates. Further, strong sputtering effect is observed with the increase of ion dose (~1017 ions/cm2), which enables the top-down fabrication of microlasers based on ILHPs. These findings represent a significant step toward controllable light sources leveraging on perovskite-ion interactions.

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INTRODUCTION The rising demand for integrated optics has led to intense research on controllable coherent and incoherent light sources with ever-decreasing footprints.1, 2 In recent years, impressive progresses have been made based on the conventional semiconductors and the novel transition-metal dichalcogenide monolayers.3,

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However, these light emitters suffer from either the poor

luminescence efficiency or the limited emission wavelengths (e.g. lacking broadband tunability).4 As a rising star in semiconductor family, all-inorganic cesium lead halide perovskites (ILHPs) feature high photoluminescence (PL) quantum yield and superior optical gain.5-8 Especially, the emission wavelength can be continuously tuned across the whole visible region by manipulating the constituent stoichiometry.7 Therefore, the ILHPs hold great promise for the development of next-generation photonic technologies. Despite that super light-emitting and lasing performance have been demonstrated from ILHPs, it is still challenging to realize controllable microlasers and light-emitters that fit for the integrated photonics based on ILHPs.5,

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For example, in order to acquire spatially controllable emission

from ILHPs, the selective ion exchange and photolithography had been investigated.10,

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However, they exclusively rely on the multi-step and mask-assisted processes. It is noted that the alignment between the mask and ILHPs within micro/nanometer precision is extremely difficult,11 making these strategies inefficient for the integrated systems. In order to achieve ILHP-based microlasers serving as the compact coherent light sources, the bottom-up synthetic methods, such

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as the vapor transport chemical vapor deposition (CVD) and low-temperature solution-phase processes, were developed.12-14 However, these bottom-up grown active microcrystals typically exhibit random shapes and sizes, rendering the lasing performance uncontrollable.15 As such, the lasing characteristics of one particular ILHP-microlaser are difficult to be reproduced in another, which fails to meet the demands of on-chip integration. Energetic ions are well-known to be able to tailor the material structures and properties, resulting in a large number of important applications including microelectronics, radiotherapy treatment, and engineering of nanomaterials.16,

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Nowadays, the focused ion beam (FIB)

technology has become mature and popular, providing an enabling tool to control the optical and electrical properties as well as the morphologies of semiconductors in a programmable and duplicable manner.16,

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The direct writing nature of FIB renders the simple one-step operation

without the need of any masks and the spatial resolution of FIB can reach as high as nanometer scale. Therefore, it would be promising to investigate the FIB treatment on ILHPs, which may provide a new channel to achieve customizable light emission and lasing from ILHPs. In this work, we for the first time probe the interaction between ILHPs and high-energy gallium ions (Ga+ ions at 30 keV) in a broad range of ion doses provided by a commercial FIB system. We found that the optical properties of CsPbX3 are highly sensitive to the energetic Ga+ ions due to the relatively vulnerable ionic bonding. Specially, even low-dose Ga+ irradiation (~1 1015 ions/cm2) can lead to more than one-order-of-magnitude reduction in PL intensity, which can be attributed to the generation of vacancy/interstitial defects, metallic Pb (Pb0) and crystal-to-amorphization transition. Such PL quenching phenomenon renders the spatially controllable emission across the full visible regime and fabrication of various colorful

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micro-/nano-photopatterns. Besides, it is demonstrated that the low-dose FIB treatment can in-situ tailor the lasing behavior from CsPbBr3 microplate via purposely introducing optical loss, which is desirable for system-on-a-chip applications. Furthermore, we observed strong ion sputtering effect in ILHPs under high-dose (~1017 ions/cm2) FIB processing. Leveraging on the mature FIB technique and through optimizing the ion parameters, we demonstrate that the ILHP crystals can be sculpted into precisely controlled shapes and sizes, enabling the first demonstration of the customizable microlasers by top-down approach. Notably, the lasing characteristics of the newly-developed microlasers including the lasing wavelength, free spectral range and Q-factor are highly reproducible, which is essential for on-chip integration. Our results pave the way for developing controllable light sources based on perovskite-ion interactions and may infiltrate fresh blood in integrated optics. RESULTS AND DISCUSSION The CsPbX3 microcrystals with lateral dimensions of several tens to hundreds of micrometers and thickness of hundreds of nanometers were fabricated by the chemical vapor deposition (CVD) method (see the detailed fabrication process in Supporting Information).19 The elemental mapping analysis by the energy dispersive X-ray spectroscopy (EDX) coupled with scanning electron microscopy (SEM) on individual CsPbBr3 microplate manifests the uniform spatial distribution of Cs, Pb, and Br and confirms the formation of CsPbBr3 compound (Figure S1, Supporting Information). The X-ray diffraction (XRD) measurement reveals the cubic phase of the CsPbBr3 crystals (Figure S2, Supporting Information), which is consistent with previous reports.12, 19 The Helios 600 NanoLab FIB was employed to deliver the energetic gallium ions (Ga+ at 30 keV) and the sample was placed in the SEM/FIB dual beam system consisted of both electron- and

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ion-columns. To test the impacts of the Ga+ ions on CsPbBr3 microcrystals, we start with low dose ion irradiation on the CsPbBr3 sample in range of ~1014-1015 ions/cm2. Figure 1b shows the SEM image of the FIB-treated CsPbBr3 surface. The dark squares correspond to the FIB-treated area, 1014 ions/cm2 to 1.5

where the ion dose gradually increases from 1.8

1015 ions/cm2 with equal

intervals from 1 to 8 as labeled in Figure S3, Supporting Information, and the square becomes more and more darker with the increase of ion dose. The corresponding optical image of the FIB-treated sample is presented in Figure 1a, which does not show any observable change on the surface due to the low ion-dose treatment. Interestingly, the fluorescent image (Figure 1c) under laser excitation (wavelength: 442 nm) clearly manifests the FIB-treated squares as being much less emissive than the pristine parts, indicating that the PL was quenched by the ion irradiation. In order to quantitatively analyze the PL change in CsPbBr3 by ion irradiation, we performed the ion-dose dependent steady-state PL measurements. As shown in Figure 2a, the PL intensity keeps decreasing with the increase of ion dose and the PL intensity with ion irradiation of ~1

1015

ions/cm2 reduces by more than one order of magnitude (inset in Figure 2a). Simultaneously, the time-resolved PL measurement reveals that the PL decay rate rises with the increase of ion dose (Figure 2b), which suggests that more carrier trap defects are formed under higher ion dose irradiation. As the binding energy of the atoms in the CsPbBr3 target sample (several eV)20 is much smaller than the kinetic energy of Ga+ ion at 30 keV, the atoms in CsPbBr3 will be readily displaced from their lattice positions by the elastic core collision resulting from energy and momentum exchanges when the Ga+ ion beam is directed onto the target sample.16, 21 Moreover, the incident ions after collision and the target atoms knocked free from the lattice sites may still have enough energy to displace more target atoms, which in turn continue to dislocate yet other

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atoms.16,

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As a result, a “displacement cascade” is formed upon Ga+ ion irradiation, which

generates plenty of defects including vacancies and interstitials, leading to the PL quenching phenomenon. In general, the penetration depth of the Ga+ ion beam lies in the range of several to tens of nanometers, so that the irradiation induced quenching effects primarily occurs on the surface of sample.16 To investigate the composition change upon Ga+ ion irradiation, we performed the surface-sensitive X-ray photoelectron spectroscopy (XPS) measurements on the CsPbBr3 sample before and after FIB treatment. The presence of Cs, Pb and Br element is consistent with the material composition of CsPbBr3 (Figure S4, Supporting Information). Whilst, upon the FIB treatment (1.5

1015 ions/cm2), new peaks locating near Pb 4f7/2 and Pb 4f5/2 peaks

were observed (Figure 2c), indicating the formation of metallic Pb (Pb0).6, 23 The presence of Pb0 can effectively serve as the nonradiative recombination centers,23 which may further contribute to the PL reduction. It is noted that we do not observe the signal related to the gallium element, indicating that gallium is not present above 0.1% concentration (the detection limit for XPS instrument) in the FIB-treated CsPbBr3 surface.24 We also performed the transmission electron microscopy (TEM) to access the change of crystal structure by the Ga+ ion irradiation. To do so, we scratched the CsPbBr3 microcrystals and collected the tiny fragments onto the copper grids. Afterwards, the edge of the CsPbBr3 fragment was treated by FIB (1.5

1015 ions/cm2) and then examined by TEM. As displayed in Figure 2e,

clear lattice fringes can be observed for the CsPbBr3 fragment before FIB treatment, indicating the high crystalline nature. Whilst, many nanometer-sized particles are formed and the lattice fringes disappear after the FIB treatment, indicating that the ion irradiation can induce the transition from crystal to amorphous phase.

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As a result of the high energy of Ga+ ions, some of the target atoms can be ejected from the surface of the sample, the process is known as ion sputtering.16 Notably, the surface morphology of the CsPbBr3 sample can be dramatically transformed due to ion sputtering effect. Figure 2d shows the atomic force microscopy (AFM) image of CsPbBr3 after FIB treatment with relatively low ion dose (2.5

1014 ions/cm2). It is found that the concave square with depth of ~5 nm was

formed. Deeper concave with microscale depth can be obtained by the high-dose FIB treatment. Figure 1b displays the SEM image of the representative deep concave square fabricated by high-dose FIB treatment (1.25

1017 ions/cm2) as numbered P7S (Figure S3, Supporting

Information). In the following part, we will show that the high-dose FIB can be adopted to process the CsPbBr3 for highly controllable microlasers which are unattainable by the bottom-up approaches. The ability to spatially control the PL in a programmable fashion allows the direct writing of patterned emission from CsPbX3.25 As a proof-of-concept experiment, we designed a QR Code whose mirror image corresponds to the text “perovskite” and then encode it on CsPbBr3 microcrystals by low-dose FIB treatment (see the SEM image in Figure 3a). When the CsPbBr3 microcrystals are exposed to the laser excitation, the QR Code clearly emerges (Figure 3b). The corresponding optical image of the CsPbBr3 microcrystals under microscope was shown in Figure S5, Supporting Information, where the QR Code is invisible due to small topographical modification of the surface by the low-dose ion treatment (2.5

1014 ions/cm2). The size of the

QR Code can be easily controlled by the FIB technique as demonstrated in Figures 3b and 3c. Moreover, leveraging on the facile emission wavelength tunability of CsPbX3 by tailoring the composition, the blue and red QR Code can be fabricated based on the CsPb(Cl/Br)3 (Figure 3d)

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and CsPb(Br/I)3 microcrystals (Figure 3e), respectively. Besides the QR Code, the text information can also be encoded by simply pre-defining it in the FIB system. As an example, the word “encrytion” was written on the CsPbBr3 sample and the word was clearly resolved by the laser excitation (Figure 3f). It is well-known that FIB can reach nanometer spatial resolution, thus photopatterns with subwavelength scale can be achieved. Figure S6 shows the stripes fabricated by FIB treatment, where the width of the stripe can be made as small as several tens of nanometers. Figure S7 presents the code “0101” in nanometer scale written on the sample, demonstrating the feasibility of subwavelength photopatterns. Note that the photopattern with such tiny dimension cannot be distinguished by the common optical imaging system due to the diffraction limit. Nevertheless, the more advanced imaging techniques, such as the scanning near-field optical microscopy (SNOM),26 shall be feasible. In addition to achieving the photopatterns, low-dose FIB processing can also be employed to tailor the lasing emission from CsPbBr3, which is beneficial for on-chip integration. Figure 4a shows the typical lasing spectra from the individual CsPbBr3 microplate (see details of micro-PL (µ-PL) measurement in Supporting Information). The plot of the integrated intensity over the sharp peak spectral range as a function of the pump fluence (inset in Figure 4a) discloses the low lasing threshold (Pth) of ~16.0 µJ cm-2 from the microplate.27 The fluorescent image of CsPbBr3 microplate exhibits nearly uniform green emission when the excitation fluence is below threshold (Figure 4b). While, a bright periphery of the microplate is observed as the pump fluence exceeds the threshold (Figure 4c), indicating the whispering gallery mode (WGM) lasing occurred in the perovskite microplates.28 In the cubic WGM cavity, the free spectral range (FSR) is given by: 2

=2

2

,27 where

is the lasing wavelength and L is the edge length of the microplate.

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Accordingly, the group refractive index

is derived to be ~4.2, which agrees well with previous

reports.12 In fact, it is not surprising to observe WGM lasing from the CsPbBr3 miroplates since the cubic microcrystal naturally behalves as the high-Q WGM resonator.12, 29 To tune the lasing emission by FIB, we partially treat the perovskite microplate by Ga+ ion irradiation with low dose of ~3

1015 ions/cm2. As shown in Figure 4e, the fluorescent image of the perovskite microplate

under low excitation fluence (~5 µJ cm-2) exhibits obvious PL quenching phenomenon in the FIB-treated area. The corresponding µ-PL spectra from the FIB-treated microplate are presented in Figure 4h. Under relatively low pump fluence (< ~24 µJ cm-2), the PL spectra are dominated by the broad spontaneous emission. As pump fluence further increases, sharp spikes emerge, and the plot of the integrated PL intensity over pump fluence exhibits the threshold-like behavior, which suggests the occurrence of lasing action in the FIB-treated CsPbBr3 microplate. Notably, in sharp contrast to the pristine CsPbBr3 microplate, only a pair of bright edges in the untreated area were observed for the FIB-treated microplate (Figure 4f), which indicates the development of Fabry– Pérot (F-P) lasing instead of WGM analog. By taking the measured FSR of 1.33 nm into the 2

formula describing F-P oscillation:

=

, the group refractive index is derived to be ~4.1,

further confirming the F-P lasing mechanism in the FIB-treated microplate. In principle, by purposely introducing optical loss through selective FIB treatment, the CsPbBr3 microplate fails to provide sufficient optical gain for WGM oscillation, thereby leading to the transition from WGM to F-P lasing in the FIB-treated microplate. The corresponding resonant oscillations in the pristine and FIB-treated CsPbBr3 microplate can be well retrieved by numerical simulation using finite element method (FEM) (see details in Supporting Information) as shown in Figures 4d and 4g, respectively. It is noted that the threshold increases slightly as the lasing behavior transformed

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from WGM to F-P lasing, which can be mainly attributed to the relatively high optical loss for the F-P oscillation. Until now, almost all of the ILHP-based microlasers are developed from the bottom-up synthetic methods. However, these bottom-up synthesized perovskite microcrystals typically have random shapes and sizes,12,

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making the lasing characteristics uncontrollable. As mentioned

above, the high-dose ion irradiation could dramatically alter the morphology of the pervoskites, such finding triggers us to explore the fabrication of CsPbX3 microlasers which are customizable by top-down approach via high-dose FIB processing. Figure 5b shows the SEM image of the microdisk (Diameter: 27.1 µm) fabricated by high-dose FIB sputtering (1.25 × 1017 ions/cm2). By optimizing the parameters including current (22 pA), beam diameter (50 nm) and ion incident angle (90o), smooth surface of the perovskite microdisks can be obtained (Figure S8, Supporting Information). It is known that the smooth surface can reduce the optical loss and enable the high-Q microcavities.30 Moreover, the vertical incidence of ions onto the sample can suppress the ion-induced optical loss on the surface, thus the bright green-color microdisk (Figure 5c) can be observed under low excitation fluence (~5 µJ cm-2). To check the lasing performance, we performed the pump-fluence dependent µ-PL measurement on the individual microdisk (inset of Figure 5a). Figure 5a shows the µ-PL spectra from the microdisk with diameter of 27.1 µm. The PL spectra are dominated by the broad spontaneous emission under low excitation intensities (< ~20 µJ cm-2). As the pump fluence keeps increasing, evenly-spaced sharp peaks appear, and the plot of the integrated PL intensity over pump fluence exhibits nonlinear behavior (Figure 5f), indicating the development of lasing action in the CsPbBr3 microdisk. To get deeper insights, we carried out the time-resolved µ-PL measurements on the individual CsPbBr3 microdisk with

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different excitation fluences (Figure 5h). It is seen that the spontaneous emission lifetime is ~460 ps at low pump fluence (~0.2 Pth). As the pump flence increase to ~0.8 Pth, the lifetime decreases to ~240 ps. Notably, when the pump fluence exceeds the threshold (~1.2 Pth), a much faster decay channel with lifetime of < ~50 ps (limited by the temporal response of the streak camera system) is revealed, which corresponds to the stimulated emission process and further confirms the occurrence of lasing action. To the best of our knowledge, it represents the first report of the laser emission from circular-shaped CsPbBr3 microdisk. The corresponding fluorescent image of the CsPbBr3 microdisk above lasing threshold (Figure 5d) exhibits bright ring feature around the periphery, which suggests the WGM lasing mechanism. Hence, we tentatively examined the lasing peaks using WGM model. Considering the first radial mode order (q = 1), the resonant condition is given by: are the angular mode number and the resonant wavelength at

=

, where

, respectively, and

and is the

diameter of the microdisk. Following the procedure reported by Ta et al.,28 the lasing peaks are found to match well with the mode numbers indexed as 756–763 (Figure 6a). The Q-factor (calculated by Q = YJZY where Y and ZY are the lasing peak wavelength and the corresponding linewidth of the lasing peak, respectively) is derived to be as high as ~2700. The resonances in circular-shaped microdisks can be retrieved via numerical studies as shown in Figure 5e and the electric field is found to be effectively confined close to the inner boundary of CsPbBr3 microdisk (Figure 5g), which is beneficial for the development of high-Q laser emission. By means of the mature FIB technology, the top-down fabricated perovskite microlasers are highly controllable and reproducible. The size of CsPbBr3 microdisks can be readily manipulated by FIB method. Figure 6b displays the SEM images of CsPbBr3 microdisks with different sizes and the corresponding lasing images. The size dependent lasing spectra discloses that the FSR

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increases with the decrease of the microdisk diameter (Figure 6c), which agrees with WGM lasing behavior.9 It is known that single mode lasing is desired for practical applications. In principle, the single-mode operation can be achieved by reducing the cavity size when the FSR exceeds the width of the optical gain region. Following this idea, we achieved the single mode lasing in FIB-fabricated microdisk with small diameter of ~5 µm as can be seen in Figure 6c. Figure 6d shows the lasing spectra from another FIB-fabricated 27.1 µm-sized CsPbBr3 microdisk (see the full PL spectra in Figure S9, Supporting Information). It is found that the lasing wavelength, FSR and Q-factor are nearly the same as those of the one displayed in Figure 5a, indicating that the lasing characteristics can be duplicated by the FIB approach, which is essential for applications including on-chip integrated optoelectronics. Besides, other resonator shapes, such as the deformed CsPbBr3 microdisk (Figure S10, Supporting Information), can also be fabricated and the lasing emission with nonuniform angular distribution is achieved, which provides an excellent platform to explore light-matter interaction in customizable laser resonators. CONCLUSIONS In summary, we investigate the interaction between CsPbX3 and high-energy gallium ions (Ga+ ions at 30 keV) in a broad range of ion doses for the first time. We reveal that both low-dose (~1014 ions/cm2) and high-dose (~1017 ions/cm2) ion irradiations can serve as effective tools to process CsPbX3 for customizable light emission and lasing, where the underlying physical mechanisms have been examined in detail.

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ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website: Materials and methods, composition analysis of the sample by energy dispersive X-ray spectroscopy; X-ray diffraction (XRD) pattern of the sample; X-ray photoelectron spectroscopy of the sample; SEM images of the original sample and ion-treated sample; Lasing spectra of the ion-treated microdisks. Notes The authors declare no competing financial interest. Author Contributions Y.W., Z.Y.G. and Y.J.R. contribute equally to this work. ACKNOWLEDGEMENTS This

work

is

supported

by

the

Singapore

Ministry

of

Education

through

the

Academic Research Fund under Projects MOE2016-T2-1-054, Tier 1-RG105/16, Tier 1-RG92/15 and Tier 1-RG189/17 (S). H.Z. thanks the National Science Fund for Distinguished Young Scholars of China (61604074). G.A. thanks the MOE Tier3 grant [MOE2016-T3-1-006 (S)].

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a

764

b 28.9 J cm-2

Mode number

762 760 758

Intensity (a.u.) Intensity (a.u.)

756

537

538

539

540

541

542

543

Wavelength (nm)

c

d 5

Lasing spectra

15 m 3

17.2 m 2

27.1 m 1

Normalized PL Intensity (a.u.)

5 m

4

PL intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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27.1 m 2th

27.1 m 1th

30.2 m 0 537

540

543

530

546

540

550

Wavelength (nm)

Wavelength (nm)

Figure 6. a) Lasing modes are well assigned according to 2D WGM model. (b) SEM images of different sized CsPbBr3 microdisks (30.2 and 5.0 µm from left to right) and the corresponding above-threshold fluorescent images. (c) Size dependent lasing spectra of CsPbBr3 microdisks, showing that FSR increases with the decrease of diameter. (d) Plot of lasing spectra from two different 27.1 µm-sized microdisks, which shows that the lasing peaks, FSR and Q-factor are nearly the same.

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