Whispering Gallery Mode Lasing from Self-Assembled Hexagonal

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Whispering Gallery Mode Lasing from Self-Assembled Hexagonal Perovskite Single Crystals and Porous Thin Films Decorated by Dielectric Spherical Resonators Packiyaraj Perumal, CihSu Wang, Karunakara Moorthy Boopathi, Golam Haider, Wei-Cheng Liao, and Yang-Fang Chen ACS Photonics, Just Accepted Manuscript • Publication Date (Web): 16 Dec 2016 Downloaded from http://pubs.acs.org on December 16, 2016

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Whispering Gallery Mode Lasing from Self-Assembled Hexagonal Perovskite Single Crystals and Porous Thin Films Decorated by Dielectric Spherical Resonators Packiyaraj Perumal†,‡, Cih-Su Wang†, Karunakara Moorthy Boopathi§, Golam Haider†, WeiCheng Liao† and Yang-Fang Chen*,† †

Department of Physics, National Taiwan University, Taipei 106, Taiwan Nano Science and Technology Program, Taiwan International Graduate Program, Academia Sinica and National Taiwan University § Research Center for Applied Sciences, Academia Sinica, Taipei 115, Taiwan ‡

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ABSTRACT Lasing in self-assembled hybrid organic-inorganic lead halide perovskites semiconductors has attained intensive research for low cost and high performance optoelectronic devices due to their inherent outstanding optical response. However, to achieve the controllable laser action from a small single crystal remains as a challenging issue. Here, we present a novel technique to fabricate self-assembled high-quality hexagonal perovskite single crystals for realizing roomtemperature near-infrared whispering-gallery-mode (WGM) laser action. Quite interestingly, the lasing spectrum for an individual CH3NH3PbI3 hexagonal single crystals encompasses the aspects of high quality factor (Q) and low threshold WGM lasing around 1200 and 26.8 µJ/cm2, respectively. In addition, we demonstrate that when the porous perovskite thin films were decorated with dielectric spheres, the laser oscillation can be achieved through the coupling of WGM with perovskite gain material. We found that the lasing spectra can be well manipulated by the size of hexagonal single crystals and SiO2 spheres. Moreover, the discovered laser action and chemical stability of hexagonal single crystal perovskites not only render them significant practical use in highly efficient near infrared emitting devices for laser photonics, solid-state lighting and display applications, but also provide a potential extension towards various optoelectronic devices.

KEYWORDS: controllable laser action, individual hexagonal perovskite single crystal, solution process, whispering-gallery-mode resonance


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Recently, low-dimensional organic-inorganic semiconductors frameworks have attracted wide-range of scrutiny as a consequence of inherent significance in nanoscience and technology. Lead halide perovskite semiconductors, methyl ammonium lead iodide (MAPbI3), with a bandgap around 1.5-1.6 eV have been used extensively as a light harvester in solar cells over the past several years.1-5 Latter, it has also emerged as a highly-promising material for optoelectronic devices such as light emitting diodes (LEDs),6-7 photodetectors,8-9 lasers,10-14 sensors15 owing to the excellent light emission, high absorption coefficient, balanced mobility between electron and hole, small exciton binding energy (37-75 meV), longer exciton diffusion length and low nonradiative recombination.16 These properties, along with high quantum yield and wavelength tunability make MAPbI3 ideal material for semiconductor lasers. The class of semiconducting perovskites thus provides a potentially useful and complement alternative to fabricate optoelectronic devices. The success of this class of perovskite semiconductors is due in part to well-developed low temperature solution processes, which allows for the fabrication of optoelectronic devices with low cost and convenient to integrate directly with alternative technologies. As a result, these materials can be fashioned into several structured devices by chemical processing. Therefore, MAPbI3 based materials are identified as potential building blocks for opto-electronic devices performed within the near infrared (NIR) spectral ranges. Particularly, the richness of micro and nanostructures of MAPbI3 eases the belief of assorted fascinating optical confinement and laser cavities. Over the past few decades, the optical processes associated with various reasonable micro/nano cavities, such as photonic crystals cavity, Fabry-Perot (F-P) optical cavity, and whispering-gallery-mode (WGM) resonator have been considered as an attracting platform for micro or nanolaser systems.17 Amidst them, the WGM induced laser action is of present scrutiny


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by the reason of inherently high quality (Q) factor, small mode volume, low-lasing threshold, and relatively simple fabrication.18 One of the advantages of WGM oscillations arises from the underlying working principle of the strong coupling between confined light beam and the cavity. Attributable to the total internal reflections (TIRs) of light at the hexagonal or circular boundary, there exists possibility of high-quality factor (Q) of WGMs, which enables low threshold microcavity lasers.19-20 In spherical microcavities whose size is several times larger than the emission wavelength, WGM lasing with a series of sharp peaks has been demonstrated.21 In small optical cavities with dimensions approaching to the wavelength of light, lasers with ultralow threshold and low operating power can be perceived.22 The coherent light generation on the micro-scale cavity has recently attracted significant attention due to numerous application in the field of lasers. Since the pioneering work of T.C.Sum and co-workers11 for wavelength tunable perovskites lasing, it has inspired a great deal of attention in this field of research. Efficient optically pumped perovskite lasing has been demonstrated in the form of vertical microcavities,10 random lasing,12-13 spherical resonator,14 microdisck,23-25 microplates,26 nanoplates27 and nanowire laser28 etc… Several promising perovskite materials29-33 have been successfully identified to be highly potential in practical applications, such as light emitters and other optoelectronic devices. However, so far the room temperature WGM lasing in a small single crystal has not been reported, which is greatly desired owing to reduce the dimensions of the devices for practical application, although CH3NH3PbI3 hexagonal single crystals with size varies from ~ 50 µm upto ~ 100 µm are often easily acquired. In addition, the reported high lasing threshold is not only a drawback for application, but also imposes fundamental limits such as auger recombination losses. Furthermore, it is still a challenging task to realize lasers with tunable wavelength.


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In this work, we focus on one-step spin coating process of mixed perovskite solution (CH3NH3I and PbI2) with different concentration to serve as gain materials. We synthesized high quality MAPbI3 hexagonal single crystals by self-assembled solution processed techniques using horizontal tube reaction chamber. It is found that the MAPbI3 single crystals possess an ideal hexagonal configuration with several different sizes, which permit the analysis of the cavity size dependence of WGM. By using room temperature confocal micro-photoluminescence (µ-PL) system, we observed very sharp WGM lasing modes from MAPbI3 hexagonal single crystals, which shows the aspects of high quality factor (Q) and low threshold around 1200 and 26.8 µJ/cm2, respectively. Size-dependent investigation of the WGM lasing characteristics was carried out systematically. Furthermore, to couple perovskite gain material with dielectric spherical resonators, SiO2 spheres was used as cavities synthesized by the Stöber process. Stimulated emission of porous MAPbI3 perovskite thin films deposited on SiO2 spheres spin-coated glass substrate was achieved. With the coupling of SiO2 spheres, we have observed a low lasing threshold at 85.5 µJ/cm2 and controllable laser modes. The underlying origin of the laser with large-scale, extremely intensed, controllable wavelength, and narrow-linewidth (~ 0.8 nm) mode resonances can be realized in terms of the waveguiding and scattering media within the dielectric sphere network. These outstanding results provide the fact that the fabricated MAPbI3 hexagonal single crystals and porous thin films with high quality crystalline can serve as feasible building blocks for highly promising opto-electronic devices.


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RESULTS AND DISCUSSION

X-ray diffraction spectra were used to confirm the phase, purity and crystal structure as well as the stoichiometric perovskite formation. Figure 1a exhibits the formation of ~150 nm thickness of MAPbI3 on glass substrate using one-step spin coating process. Figure 1b shows the evidence for tetragonal-structured perovskite with strong diffraction peaks at values of 2θ of 14.08 (110), 28.5 (220), 31.8° (310) and 40.6° (224), which is in good agreement with literature data.34 The film formation is confirmed primarily at the value of 2θ of 14.08°, where the signal intensity increases upon increasing the relative ratio of PbI2 from 0.65 to 1, which shows the phase of crystalline nature (Figure S1a). To investigate the dependence of the morphology of perovskite film on different molar weight ratio of MAI and PbI2 film, SEM images were recorded and shown in Figure 1c. Morphology and grain size of perovskite film can be controlled by the volume of CH3NH3I. It is found that the grain size increases with the stoichiometric ratio of PbI2 and CH3NH3I. The structure is identical to that of widely studied perovskite, showing that increasing concentration does not affect the overall crystal structure of the material. For comparison, we also shown SEM images of spin coated mixed perovskite solution with completely different molar weight ratios (CH3NH3I:PbI2 (1:0.65, 1:0.75, 1:0.85)) as shown in Figure 2a-c. The optical characterization of the grown materials is shown in Figure 1d. It clearly indicates that MAPbI3 film contains a robust and broad absorbance from UV to near-infrared (800 nm) region with two separate peaks centered at 480 nm and 750 nm, which are accordant with previous reports.35-36 The broad and strong absorbance is a good indication of its 6 ACS Paragon Plus Environment

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magnificent light harvesting application in optoelectronic devices. The primary absorption peak at 480 nm can be attributed to the transition from lower valence band to the conduction band minimum.16 The second absorption peak at 750 nm is due to the direct gap transition from the first valence band maximum to the conduction band minimum. The narrow band edge PL emission peak at 780 nm is also shown in Figure 1d. The steady-state PL studies for the MAPbI3 samples with the morphology as shown in Figure 3a have been performed under different pumping energy of 266 nm pulsed laser excitation. Figure 3b shows the emission spectra of spin coated MAPbI3 (1:0.85) on glass substrate. We observe only a broad spontaneous emission (SE) spectra centered at 774 nm with a full width at half-maximum (FWHM) of 28 nm, which is similar to the previous report.11 At low pumping power, the broad SE increases linearly with increasing pumping energy. Above the threshold fluence (~ 9.0 µJ/cm2), the emission intensity increases dramatically. Under the same process, the emission band yields a sharp peak at 784 nm with a FWHM of 3.5 nm as shown in Figure 3c. The observation of sharp peaks within the MAPbI3 gain medium, indicates the amplified spontaneous emission (ASE), because the linewidth of the sharp peak is far larger than that of the standard lasers.37 These behaviors thus serve as an additional evidence to support the optically excited transition from SE to ASE. We have noticed that the ASE transition tends to show a red shift owing to the balance between optical gain and self-absorption, which is located near the tail of the absorption edge.38-39 On the other hand, Figure S2b shows the emission spectra of the spin coated MAPbI3 (1:0.75), obtained with the similar experimental conditions as described above. Interestingly, here we found that the peak of the ASE is also centered around 784 nm, which is similar to that of MAPbI3 with the molar weight ratio of 1:0.85. The other relevant optical properties at this concentration of perovskite are shown in Figure S2.


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In order to derive the laser action, the active gain medium (MAPbI3) has to be capable of sustaining population inversions; in the meantime, the resonant cavity needs to partially trap the light. Once the total gain within the cavity is larger than the losses, the system reaches a threshold and the laser action occurs. Here, we demonstrate controllable laser actions arising from a hexagonal structured MAPbI3 single crystalline microcavity and porous MAPbI3 thin films decorated with dielectric SiO2 spheres as shown below. Figure 4 describes a schematic illustration for the growth procedures of MAPbI3 hexagonal single crystals, in conjugation with the corresponding structural and optical characteristics. To synthesize perovskite single crystals, two totally different steps are promptly established throughout the growth of MAPbI3 hexagonal single crystals. As presented above, the prepared MAPbI3 solution was spin-coated on glass substrate by one step method (as shown in Figure 4a) and then kept it into horizontal tube reaction chamber instead of glow box as shown in Figure 4b. It is found that a large amount of spatially isolated hexagonal-like MAPbI3 grains of a few micrometers are organized or self-assembeld on the glass substrate. Surprisingly, we further discovered that the hexagonal structured morphology of the transformed MAPbI3 can be changed significantly. The elaborate synthesis procedure and different parameters of MAPbI3 hexagonal single crystals can be found within the experimental section. Figure 4c shows the FE-SEM image of self-assembled MAPbI3 hexagonal single crystals at a reaction time of 90 min (for other FESEM images under different reaction times are shown in Figure S3). It is evidently that the single crystal has a hexagonal configuration with high quality single crystalline domain. It was further supported by the single crystal X-ray diffraction pattern of MAPbI3 samples as shown in Figure S4. The strong X-ray diffraction pattern exhibits only two sharp and symmetric peaks, which can be assigned to the (110) and (220) facets of MAPbI3,34 indicating the MAPbI3 single crystals are


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highly crystallized and well oriented. The self-assembled MAPbI3 single crystals manifest welldefined hexagonal structures with the length of an outsize varying from ~ 50 µm to ~ 100 µm, that facilitates us to investigate the lasing phenomena of an isolated hexagonal single crystals with various sizes. In addition, the impact of nucleation and following development of MAPbI3 hexagonal single crystals was also examined. In all cases, the synthesis was carried throughout at 110oC under argon atmosphere. Finally, we found that the size of those MAPbI3 hexagonal single crystals continues to extend when the reaction duration increased steadily. We then utilize these single crystals for the initial studies to determine the corresponding absorption and emission spectra. Figure 4d shows a broad PL emission peak at ~775 nm. Figure 4e shows the emission spectra of MAPbI3 hexagonal single crystals deposited on glass substrate. We observed a broad SE spectra at low pumping power, and a sharp peak at 784 nm above the threshold fluence (9.8 µJ/cm2). Figure 4f is a plot of PL intensity and FWHM as a function of excitation pumping energy, showing a clear ASE threshold at PASE = 9.8 µJ/cm2, which is much lower than widely reported values.10,14 This observation further confirms the crystallinity of self-assembled MAPbI3 hexagonal single crystals, which is a very vital parameter for realising amplification of photons in these spontaneously organized hexagonal microcavities. To further study the hexagonal microcavity effect, we investigated an individual MAPbI3 hexagonal single crystals using a home-built confocal µ-PL system. Figure 5a schematically outline the home-built experimental setup of confocal µ-PL system to investigate the optical properties of an individual MAPbI3 hexagonal single crystals at room temperature. To ensure the uniform excitation, the pumping source was guided into a optical microscope objective and then focused with a beam diameter of 0.5 mm at a glancing angle of 45˚ surface normal to the (along the c-axis) MAPbI3 single crystal. To ensure the position of the single


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crystal, the as-synthesized sample was placed on a sample holder ascended on the associated degree by a XYZ adjustable stand. The emitted signal from an independent single crystal was converged with an objective lens and carried into an optical fiber and guided into a monochromator integrated with charge coupled device (CCD). Optical studies of the separated MAPbI3 hexagonal single crystals are done by exciting the sample by a confocal µ-PL pulsed laser as shown in Figure 5a. Figure 5b presents the direct confirmation of room temperature confocal µ-PL induced laser action from MAPbI3 hexagonal single crystal with a size of 100 µm (inset Figure 5b) at a pumping fluence of 40.0 µJ/cm2. It can be seen distinctly that the selfassembled crystal has a hexagonal structure. Figure 5c shows the power-dependent µ-PL emission spectra from MAPbI3 hexagonal single crystal, at low power illumination ( < 26.8 µJ/cm2), a broad weak photoluminescence emission spectra has been observed. On increasing the pumping energy density to ~ 30.0 µJ/cm2, a series of sharp emission peaks in the emission spectra at λ ≈ 781.5, 783.2,784.1, and 785.4 nm with FWHM of ~ 0.65 nm emerges over the SE spectra, indicating the prevalence of lasing behavior derived from the WGM cavity. Each peak is equivalent to one WGM cavity mode. The interval of lasing mode between adjacent peaks is nearly identical, suggesting the origin of the waveguide. With the pumping intensity increased further, the emission peaks intensity increases sharply and the modes reveal in the same position. The elaborative cavity type and the mechanism of lasing was scrutinized subsequently. The pronounced emission spectra of 100 µm MAPbI3 hexagonal single crystal above and below threshold pumping energy density are shown in Figure S5. A plot of integrated emission intensity and FWHM as a function of pumping energy is shown in Figure 5d. It can be clearly seen that there is an abrupt change of the slope, which is a signature for the occurrence of lasing emission. Additionally, the slope of output intensity can be excellently fitted by two non-


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identical linear parts (sublinear and superlinear for below and above threshold, respectively) with a knee at (lasing threshold) Pth ≈ 26.8 µJ/cm2. Interestingly, the value of the lasing threshold of 26.8 µJ/cm2 implies that the laser action can be easily achieved in a MAPbI3 hexagonal single crystals compared with previous reports.23 The phenomena of laser action could not take place without favorable resonance cavity of optical feedback meachanism. Miscellaneous feedback processes can serve as possible reasons for the induced laser action, such an F-P cavity, random cavity, or WGM cavity resonances. In our demonstration, since the induced laser action is perceived from a MAPbI3 hexagonal single crystals, it cannot be attributed to random lasing and F-P cavity originated by multiple scatterings in an exceedingly disordered gain media and two opposite boundary surfaces of the MAPbI3 hexagonal single crystal, respectively. It is known that the hexagonal structure can serve as an excellent WGM cavity, where the incident light is completely reflected with respect to the normal of the given six lateral facet of the MAPbI3 single crystal as a result of the incidence angle of TIR at the MAPbI3 single crystal / air interface. Due to the WGMs, a light wave trapped among the hexagonal single crystal is believed to propagate continuously inside the hexagonal closed system. Owing to the high refractive index of MAPbI3 (n ~ 2.4) in the NIR spectral range, multiple TIRs in the hexagonal single crystal cavity were certainly attained.40 The quality factor (Q) is a vital parameter to justify the optical cavity of a laser. Experimentally, the cavity Q value can be estimated by the ratio of peak wavelength to its FWHM, which is expressed as Q = λ⁄Δλ, where λ is the peak wavelength and ∆λ is the FWHM of the peak. From the turn-on curve and the linewidth narrowing data as shown in Figure 5c, the Q value is estimated to be approximately 1200. The regular and unwrinkled facet structure and fence width are essential for prime qualities of waveguides whereas not large dissipation from


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the surface, thereby resulting in a high Q factor and also low threshold of the resonant cavity for the induced lasing action. The Q factor for a hexagonal structured WGM cavity is often expressed as:41 πDmnR

m/4

2 λ (1 − R

m/2

Q=

* Sin (2π / m )

(1)

)

where D, m and R are the diameter, the number of facets, and reflectivity of the sides that circumscribes the MAPbI3 hexagonal single crystal, respectively. Based on the induced laser action spectra by WGM cavity, the reflectivity is estimated ~ 94.70 %, which is in accordance with the condition of the total internal reflection. Thus, the above result provides an excellent evidence that the WGM resonance can serve a strong optical confinement and induced lasing action. For an F-P cavity lasing mode, it can be fashioned by two opposite sides of the hexagonal single crystal. The Q factor for an F-B cavity can be simply calculated by the following equation.42 Q = 2πnL / λ (1 − R )

(2)

where n is the refractive index, R is the reflectivity and L is the cavity length. The calculated Q factor is about ~ 350, which is far smaller (four times smaller than the experimental value of about 1200) than the experimental value. Furthermore, if different losses like the scattering within the cavity are considered, the Q factor might be even smaller. Therefore, it is accomplished that an F-P cavity yields a very low Q factor, which is absent in the hexagonal single crystal.24 In the interest to additionally make sure the above presented methods for the lasing characteristics from the WGM cavity, the influence of different sizes of MAPbI3 hexagonal 12 ACS Paragon Plus Environment

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single crystals on the mode confinement and consequently the lasing threshold have been examined at room temperature. Figure 6a-c shows the typical lasing spectra of MAPbI3 hexagonal single crystals (inset shows the size of ~ 100 to 53 µm) after they are optically pumped by a pulsed laser. At low power illumination, the emission spectra are broad. However, after the pumping power is greater than the threshold, sharp peaks appeared. Figure 6d shows the transition from spontaneous to stimulated emission from a plot of the emission intensity versus pumping enegy. It is found that the lasing threshold increases with decreasing the size of hexagonal single crystal, originating more optical loss from the hexagonal WGM cavity.24 As the diameter of the MAPbI3 hexagonal single crystal decreases from 100 to 72, to 53 µm, the shift of sharp peaks can be seen. In addition, the mode spacing increases from 1.32 to 3.65, to 6.28 nm, when the diameter decreases from 100, 72 and 53 µm, respectively. For a WGM cavity, the relationship ∆λ = λ2 /6nL is applicable,41 where ∆λ, λ, n, and L are the mode spacing, wavelength, refractive index and edge length, respectively. The behavior of ∆λ is indeed inversely proportional to L as illustrated in Figure 6e, which is in good agreement with the theoritical prediction. In addition, we have carried out systematically the intensity-dependent emission properties of porous MAPbI3 films conformally deposited on SiO2 spheres with different size. For the case of SiO2 microspheres with diameter around 50 ± 7 µm, under low power pulsed laser illumination, a broad PL signal centered at 784 nm is observed as shown in Figure 7a. Quite interestingly, when the pumping energy increases above the threshold fluence (85.5 µJ/cm2), several laser-like emission peaks with FWHM of central peak ~ 0.8 nm were observed. The line widths of these peaks are comparable with the previously reported room temperature WGM spherical cavity lasers,14,38,39,43-45 suggesting the occurrence of laser action. We also found that


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the positions of the sharp peaks are fixed and independent of the increasing pumping energy, various excited regions and time. These behaviors thus indicate that the laser action does not arise from the transaction of coherent feedback close-loop due to the light scattering in a random medium. The value of the lasing threshold is approximately 85.5 µJ/cm2, implying that the laser action can be easily achieved as compared with previous reports.14,18,43 A plot of integrated emission intensity and FWHM as a function of pumping energy density is shown in Figure 7b. We can clearly see the occurrence of threshold behaviors, which provides a firm signature of laser action. The differential quantum efficiency (ηd), which is an important parameter defined as photons emitted per radiative electron-hole pair recombination above the threshold, can be determined by ηd = PO/PI,46 where PO and PI are the output and input pumping power, respectively. It is found that the differential quantum efficiency (ηd) of MAPbI3 on SiO2 of spheres is about 6 times larger than the efficiency without SiO2 spheres. Note that the emission spectra collected herein were accumulated from multiple spheres with different sizes, which can induce multiple peaks shown in Figure 7a. In order to furthur confirm the underlying mechanism responsible for the laser action, additional characteristics features of nanosphere induced laser action can be found in Figure S6. It shows that due to the small size of dielectric spheres, the energy seperation between the WGMs is very large. Therefore, we can observe much less sharp peaks in the emission range. In addition to the lasing action, we now shift our focus to explain elementary mechanisms for the orginated laser action and high ηd as observed above. There are two main mechanisms for the improved lasing properties emerging from the SiO2 spheres coupled with MAPbI3. (1) The SiO2 spheres located on glass substrate provide a good opportunity for the optical confinement to occur. For spherical geometry, light that travels almost at the edge and continuously reflect back


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interior by virtue of TIR at the cavity surface. The light beam can be strongly scattered and confined inside SiO2 spheres. Therefore, the controlled laser action can be easily sustained and achieved, while the threshold energy is relatively reduced. (2) Due to the continuous TIRs of light at the smooth edges of the circular boundary, the spherical-shaped dielectric cavity could support WGM specific resonance. We believe that WGM modes can construct optical feedback through numerous pathways on the boundary of the cavity.14 In principle, once the electron-hole pairs in MAPbI3 were excited, the emitted light can propagate into the SiO2 microsphere. The TIRs of light at the circular boundary are possible due to the large contrast in the refractive index at the boundary of the composite of MAPbI3 coated on SiO2 spheres and air. After the occurrence of WGM resonance due to SiO2 sphere, the ratio of the emission between the resonant and non-resonant frequencies will be greatly enhanced with a reduced linewidth. Therefore, the stimulated emission can be much easier to achieve when the light with resonant frequency arising from MAPbI3 and coupled into the dielectric spheres with a closed-loop path. The high quality WGM resonance is often clearly ascertained around the circular boundary of the SiO2 sphere, which is in agreement with our experimental results. The lasing action arising from the coupling between the cavity of SiO2 sphere and MAPbI3 suggests a good control of attributable to dielectric spheres. Finally, we compare the results presented above with other perovskite based lasers in the form of nanowires, nanoplates, microcavities and random lasing.10-12 One of the main obstacles limiting the application of nanowires, nanoplates and microcavity lasings is the high threshold energy requirement and complicate fabrication process of optoelectronic devices. In addition, the random lasing could only be achieved in disordered microcrystal networks with a broad range of lasing peaks and fluctuations in wavelength and peak intensity.12 Our work demonstrates a facile


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solution process to make halide perovskites ideal material for laser devices with high performance. The method used in the present study provides a low cost alternative for the fabrication of WGM lasing, which can be very useful for a wide range of applications in optoelectronic devices.

CONCLUSION In summary, a completely unique approach to synthesis high-quality MAPbI3 hexagonal single crystals with size ranges from ~ 50 µm upto ~ 100 µm fabricated by self-assembled solution process techniques using horizontal tube reaction chamber has been demonstrated. Under room temperature, using a confocal µ-PL laser excitation, laser action is perceived from an individual MAPbI3 hexagonal single crystals. It shows the aspects of high quality factor (Q) and low threshold WGM laser action from an individual CH3NH3PbI3 hexagonal single crystal around 1200 and 26.8 µJ/cm2, respectively.The lasing features manifest that the WGM mechanism is solely responsible for the discovered lasing spectra. Experimental studies indicate that the threshold for induced laser action increases steadily with decreasing the size of MAPbI3


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hexagonal single crystal. It is established that the lasing properties can be improved considerably, including lower lasing threshold, narrower emission spectra, sharper lasing peaks, well controlled lasing action and better differential quantum efficiency. These features yield precise lasing action of WGM from MAPbI3 hexagonal single crystals, which shows favourable for low threshold NIR lasers applications. In addition, laser action based on the coupling between WGM resonances due to SiO2 microspheres and porous MAPbI3 thin films has been demonstrated. Through the experimental investigation, we have firmly established that the discovered laser action in MAPbI3 decorated on SiO2 spheres emerges from the support of WGM resonances. These distinctive characteristics reveal an enhanced WGM laser action with controllable performance. In general, our approach could be extended to several different composites consisting of dielectric spheres. It therefore can open up a intersting avenues route for the development of high performance optoelectronic devices.

EXPERIMENTAL SECTION Preparation of MAI: CH3NH3I (MAI) powder was synthesized following technique represented by K. M. Boopathy et al.47-48 Concentrated aqueous solution of hydroiodic acid (HI) (57 wt% in water, Alfa Aesar; 15.0 mL) was reacted with methylamine (CH3NH2) (40 wt% in aqueous solution, 13.5 mL) in a very three-neck flask at 0°C (ice bath) for two hours with constant stirring underneath a nitrogen atmosphere. MAI was crystallized as a white precipitation upon continuous rotary evaporation of the solvent. As prepared white powder was collected once, washed three times with Et2O (diethyl ether) and dried below vacuum at 60 °C nightlong. The dried powder was accumulated in a glove box for further use.


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Synthesis of MAPbI3 Hexagonal Single crystals: The MAPbI3 single crystals utilized herein were grown on glass substrate by a horizontal vapor-solid tube furnace using solution of MAPbI3 as the source. In a typical experiment, spin coated MAPbI3 on a glass substrate was loaded into a small quartz boat and inserted in the middle of the horizontal quartz tube furnace. The each ends of the tube were mounted by versatile plastic that ensured the suitable pressure fashioned within the tube throughout the process. Then, the quartz tube was placed at the middle of the horizontal tube chamber. Throughout the synthesis, the temperature of the substrate was maintained at 110˚C for 90 min and the pressure level was maintained underneath a continuing flow of Ar. The synthesis was accomplished within 120 min, then the chamber was cooled all the way down to room temperature. An outsized quantity of white colored crystal-like creation was formed within the glass substrate. Further, structural and morphological characterization of hexagonal structured MAPbI3 single crystals were performed using a JEOL JSM-7001F field emission scanning electron microscope operating at 15kV. Synthesis of SiO2 microspheres: The SiO2 microspheres was synthesized from tetraethyl orthosilicate (TEOS) using liq. ammoina (liq.NH3) as catalyst. First, 70 mL of ethanol and 30 mL of doubled distilled water were taken in an Erlenmeyer flask and 1.2 mL of TEOS was added to the solution and stirred for 30 min. Then, 6−7 mL of liq. ammonia was added drop by drop to this solution and stirred vigorously. The solution was clear at first, after 15−20 min the solution turned cloudy. The ultimate solution was seen opaque when aging overnight, and also the solution was filtered and washed several times. The obtained SiO2 spheres with diameter about ~ 50 ± 7 µm were finally achieved. Device fabrication and characterization: Glass substrates were cleaned thorough sonication in dissolving agent such as acetone, isopropyl alcohol and then deionized water, each


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for 15 min and then dried under a stream of N2. The substrates were treated with ultraviolet (UV)/ozone for 15 min to scrub the surfaces and additionally to enhance the surface adhesion. Samples to be used for lasing studies employed SiO2 spheres suspended in ethanol were dipcoated onto the surface of the glass. Excess ethanol was dried by heating on a hot plate at 120°C for 10 min. The substrates were transferred to a glove box for spin-coating of the perovskite. MAPbI3 perovskite films were coated on these substrates by the subsequent methodology. The precursors MAI and PbI2 (99%, Alfa Aesar) were dissolved in anhydrous N,Ndimethylformamide (DMF) at various molar weight ratios (1:0.6, 1:0.7, 1:0.75, 1:0.8 and 1:1) at a concentration of approximately 1 wt.%. Each solution was stirred on a hot plate at 70 °C overnight. The mixture solutions of various molar weight ratios were spin-coated onto the substrate. After spin coating, the sample was kept on the hot plate at 100 °C for 60 min to make crystalline perovskite film. Powder X-ray diffraction (XRD) patterns were recorded at room temperature Bruker D8 X-ray diffractometer with a diffracted beam monochromator set for Cu Kα radiation (λ = 1.54056 Å) within the 2θ range 10–80° with a step size of 0.03939° and step time of 9945 sec. Scanning electron microscopy (SEM) was performed using an FEI Noval 200 scanning electron microscope (15 kV). The absorption spectra of these films were recorded by a UV–Vis spectrophotometer (Jobin-Yvon H10). To investigate spontaneous emission (SE), amplified spontaneous emission (ASE) and laser action, the samples were optically excited by a Qswitched 4th harmonic Nd:YAG laser (266nm, 3-5 ns pulse, 10Hz) beam, which was focused to a diameter of about 300 µm by a Jobin Yvon iHR550 imaging spectrometer system. All measurements were performed at room temperature unless otherwise noted.


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ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: X-ray diffraction pattern and photoluminescence spectra of different stoichiometric ratio of perovskites, amplified spontaneous emission of MAPbI3 with ratio of 1:0.75%, FESEM images of hexagonal single crystals, X-ray diffraction pattern of single crystal, pronounced emission spectra with below and above threshold, emission spectra coupled with SiO2 nanosphere, and theoritical explanation (PDF).

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGEMENTS This work was supported by the Ministry of Science and Technology and Ministry of Education of the Republic of China.

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Figure 1. (a) Schematic illustration of MAPbI3 (150 nm) on a glass substrate using one-step spin coating process. (b) X-ray diffraction pattern of MAPbI3. (c) Scanning electron microscopy image of perovskite formation. (d) Comparison of absorption and photoluminescence spectra for the film deposited on glass substrate.


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Figure 2. Comparison of scanning electron microscopy images of spin coated film of perovskite formation on glass substrate for the precursor of MAI and PbI2 with the ratio of (a) 1:0.65 (b) 1:0.75 (c) 1: 0.85.


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Figure 3. Amplified spontaneous emission (ASE) of MAPbI3 (1: 0.85 %) on glass substrate under 266 nm pulsed laser excitation. (a) SEM image of MAPbI3 (b) Development of ASE with increasing pumping power. (c) Integrated emission intensity and FWHM of SE and ASE as functions of pumping energy density.


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Figure 4. Growth procedures of MAPbI3 hexagonal single crystals. (a) MAPbI3 solution was spin-coated on glass substrate by one step method. (b) Horizontal tube reaction chamber. (c) FESEM image of MAPbI3 hexagonal single crystals. (d) Photoluminescence emission spectra of MAPbI3 hexagonal single crystal. (e) Development of amplified spontaneous emission (ASE) with increasing pumping power. (f) Integrated emission intensity of SE and ASE as functions of pumping energy density and corresponding FWHM. 30 ACS Paragon Plus Environment

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Figure 5. (a) Schematic diagram of home-built experimental setup of confocal µ-PL system. (b) Emission spectra of MAPbI3 hexagonal single crystal (inset shows 100 µm single crystal). (c) Power dependent emission spectra of 100 µm MAPbI3 hexagonal single crystal. (d) Integrated emission intensity and FWHM as functions of pumping energy density.


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Figure 6. Size dependent emission spectra of MAPbI3 hexagonal single crystal. (a-c) Lasing and their corresponding PL spectra of 100, 72, 53 µm size of hexagonal single crystal, respectively. (d) Integrated emission intensity as functions of pumping energy density. (e) Mode spacing as a function of the reciprocal size of hexagonal single crystal.


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Figure 7. Emission spectra of MAPbI3 coupled with SiO2 microsphere with different pumping energy density under pulsed laser excitation. Inset shows the enlarged specta of MAPbI3 coupled with SiO2 microspheres.


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Table of contents Self-assembled MAPbI3 high-quality hexagonal perovskite single crystals with various sizes were used to serve as WGM resonant cavities for realizing laser action with excellent high quality factor (Q) and low threshold around 1200 and 26.8 µJ/cm2, respectively. The novel performance makes it a promissing candidate for next-generation optoelectronic devices. ToC figure


Whispering Gallery Mode Lasing from Self-Assembled Hexagonal

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