Wavelength-tunable and highly stable perovskite-quantum-dots

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Functional Inorganic Materials and Devices

Wavelength-tunable and highly stable perovskite-quantumdots-doped lasers with liquid crystal lasing cavities Lin-Jer Chen, Jia-Heng Dai, Jia-De Lin, Ting-Shan Mo, Hong-Ping Lin, Hui-Chen Yeh, Yu-Chou Chuang, Shun An Jiang, and Chia-Rong Lee ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b08474 • Publication Date (Web): 10 Sep 2018 Downloaded from http://pubs.acs.org on September 10, 2018

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ACS Applied Materials & Interfaces

Wavelength-tunable

and

highly

stable

perovskite-quantum-dots-doped lasers with liquid crystal lasing cavities Lin-Jer Chen,† Jia-Heng Dai,† Jia-De Lin,† Ting-Shan Mo,‡ Hong-Ping Lin,§ Hui-Chen Yeh,∥ Yu-Chou Chuang,† Shun-An Jiang,† and Chia-Rong Lee*,†

†Department of Photonics, National Cheng Kung University, Tainan 701, Taiwan ‡

Department of Electronic Engineering, Kun Shan University of Technology, Tainan 710, Taiwan

§

Department of Chemistry, National Cheng Kung University, Tainan 701, Taiwan

∥

Graduate Institute of Electrical Engineering, National Kaohsiung University of Science and

Technology, Kaohsiung 824, Taiwan. Correspondence and requests for materials should be addressed to Chia-Rong Lee (*email: [email protected])

Keywords: perovskite, quantum dots, cholesteric liquid crystal, laser, solvothermal

Abstract This study applies a low-cost solvothermal method to synthesize all-inorganic (lead-free cesium tin halide) perovskite quantum dots (AIPQDs) and to fabricate AIPQD-doped lasers with cholesteric liquid crystal (CLC) lasing cavities. The lasers present highly qualified lasing features of low threshold (150 nJ/pulse) and narrow linewidth (0.20 nm) that are attributed to the 1

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conjunction of the suppression of photoluminescence (PL) loss caused by the quantum confinement of AIPQDs and the amplification of PL caused by the band-edge effect of the CLC-distributed feedback resonator. In addition, the lasers possess highly flexible lasing-wavelength tuning features and a long-term stability under storage at room temperature and under high humidity given the protective role of CLC. These advantages are difficult to confer to typical light-emitting perovskite devices. Given these merits, the AIPQD doped CLC laser device has considerable potential applications in optoelectronics and photonic devices, including lighting, displays, and lasers. Keywords: perovskite, quantum dots, cholesteric liquid crystal, laser, solvothermal

1. INTRODUCTION Over the past several years, perovskites have recently emerged as promising candidate materials for optoelectronic devices, such as light-emitting diodes (LEDs),1 solar cells,2 transistors,3 and photodetectors,4

because

of

their

high

light

absorption,

narrow

emission,

high

photoluminescence (PL) efficiency, and low cost. Recent advances in halide-perovskite-based laser devices have been mainly limited to hybrid organic–inorganic-perovskite-based devices.5, 6 Unfortunately, hybrid organic–inorganic perovskite materials and their associated devices have significant drawbacks stemming from their poor stability.7–9 Hence, all-inorganic perovskites 2


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(AIP) are potential replacements for organic–inorganic perovskites because of their relatively high stability. The unique physical–chemical properties and the high optical efficiency of AIPs promise to promote the development of novel light-emitting materials for use in optical or laser devices. The nanoscaling of materials has become increasingly important because some extraordinary phenomena can only observed on the nanoscale. For example, exerting precise control over synthesis conditions may induce the emergence of optical properties from quantum size effects10 and anisotropic particle growth.11 The optical properties of quantum-dot (QD) nanomaterials are more remarkable than those of their bulk counterparts. Kovalenko and coworkers fabricated cesium lead halide (CsPbX3, X = Cl, Br, and I) all-inorganic perovskite quantum dots (AIPQDs)12 with excellent optical properties, such as tunability (by changing halide element) and high quantum-yield PL, as well as high stability with quantum confinement effect. Liu and coworkers demonstrated that the PL performance of the CsPbBr3 AIPQD-based LED device is superior to that of CdSe QD-based devices and provided insight into the role of AIP in light-emitting devices.13 However, the PL performance of QDs is often limited by a serious aggregation effect, which results in self-quenching.14 The alleviation of aggregation in QDs system remains an open and challenging topic for future investigation. Liquid crystals (LCs) are soft materials with several unique properties, such as high optical 3



anisotropy, long-length-scaled softness and elasticity, and high flexible manipulability by externality.15 Among different kinds of LCs, cholesteric LC (CLC) is particularly important given its twisting structure with high one-dimensional (1D) periodic modulation of refractive index. The planar structure of CLC is formed through the assembly and rotation of rod-like LC molecules around a helical axis via interaction with doped chiral materials. Therefore, a planar CLC can be considered as a 1D photonic crystal with photonic bandgaps or reflection bands.15, 16 Due to the abovementioned unique properties, the planar CLC has considerably advanced the development of displays and tunable photonic devices, such as tunable filters or reflectors and laser devices.17-19 Among the different devices that have been developed in the field of soft-matter photonics, LC lasers have received particular attention because of their numerous remarkable features, including wide-band tunability, large coherence area, and potential for multidirectional emission.19 When the fluorescence photons spontaneously emit in all directions after the excitation of external pump source on the active medium (e.g., fluorescence dyes) in the CLC resonator, the fluorescence spectrum can be significantly modified by the CLC photonic bandgap — the fluorescence can be suppressed inside the bandgaps and enhanced at the band-edges. At the band-edges, the fluorescence can propagate and survive for a long time by way of multi-reflection in the CLC resonator, resulting in a very low group velocity and a very large density of photonic state (DOS) for the fluorescence. Due to the distributed feedback of the 4


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active CLC PC structure in the multi-reflection process, the spontaneous and stimulated emission rates of the fluorescence photons at the band-edges are both effectively amplified such that a high gain beyond loss can be achieved to induce a low-threshold lasing emission.19, 20 However, the development of LC lasers for commercial application has been limited because the doped fluorescent dyes used as gain media in LC lasers are associated with drawbacks, such as chemical and photo-instabilities, strong light-induced thermal effect, and intrinsic three-level system with triplet state.21−26 Additionally, previous reports have shown that ASE and lasing characteristics are exhibited by the optical gain materials of hybrid organic–inorganic perovskite, including thin films,27 spherical resonators,28 nanoplatelets,29 nanowires,30 and distributed Bragg reflector with CLC polymer layer.31 However, the drawbacks of these systems involve inflexibility and short device lift time. Considering that the CLC microresonator and AIPQD gain medium have the individual merits of flexible tunability, quantum size effect, and high stability, combining these two technologies may lead to the development of novel optoelectronic or photonic devices with superior properties and high reliability, as well as inspire and expand light-emitting applications in the future. This work is the first to demonstrate an AIPQD-doped CLC (AIPQD-CLC) laser with highly flexible tunability and long-term stability. The AIPQD material, which is employed as an efficient optical gain medium in the optical resonator of the CLC planar structure, can be 5



obtained through a low-cost solvothermal pretreatment process. The line width and energy threshold of the lasing emission from the AIPQD-CLC laser can be as narrow as 0.20 nm and as low as 0.15 µJ/pulse, respectively, and are comparable with (or even better than) those of traditional dye-doped CLC lasers. The AIPQD-CLC laser can retain approximately 87% of its initial lasing efficiency after half a year of storage under room temperature and high humidity of 60%. Additionally, the lasing wavelength of the AIPQD-CLC laser can be tuned through various approaches, such as changing the compositional ratio of the chiral dopant in NLC, adjusting the ambient temperature, or subjecting the sample to different AC voltages. The AIPQD-CLC device with high stability and tuning flexibility has potential applications in optoelectronic and photonic devices, particularly in light-emitting devices.

2. EXPERIMENTAL SECTION 2.1. Materials. The CLC materials used in this work included nematic LC (MDA-03-3970 from Merck) and chiral dopant (S811 in left-handed form from Merck). The materials used for the synthesis of perovskite QDs included anhydrous tin (II) iodide (SnI2, 99.999%), cesium carbonate (Cs2CO3, reagent Plus, 99.99%), octadecene (ODE, technical grade, 90%), oleylamine (OLAM, 90%), oleic acid (OA, 90%), absolute ethanol (Alfa Aesar, 99.8%), anhydrous toluene (Sigma-Aldrich, 99.8%), and anhydrous hexane (Sigma Aesar, 99%). 6


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Poly(vinyl alcohol)(PVA, Mw 89,000-124,000, Alfa Aesar, 99%) was used as a homogeneously-aligned material and was precoated on the ITO substrates of the CLC cell for inducing a planar texture. 2.2. Synthesis of CsSnI3 QDs. In a typical synthesis, 0.05 g of Cs2CO3 (1.2 mmol; 325.8), 0.5 g of SnI2 (1.3 mmol; 372.5), 30 mL of ODE, 3 mL of OA, and 2.5 mL of OLAM were loaded in a 50 mL flask with stirring at 120 °C under continuous nitrogen purging for 1 h. The mixture was degassed for 30 minutes at 120 °C under alternating vacuum and nitrogen with magnetic stirring. The reagents were loaded into a 50 mL Teflon lined autoclave, which was then filled with anhydrous ODE up to 80% of the total volume. The autoclave was sealed, and its temperature was maintained for 1 h at 170 °C. The QD solution was then quickly cooled down to room temperature in an ice bath (approximately −2°C), and the products were transferred to a glove box. After this step, the solution was filtered and alternately washed several times with toluene and hexane through centrifugation. Finally, the products were dried in a vacuum at 80 °C for 3 h. The dried products exhibited a black appearance. The obtained black products were redispersed in toluene for further use.

2.3. Fabrication of the all-inorganic CsSnI3-QD-doped CLC device. In this work, each empty planar cell was pre-constructed by piling up two ITO glass substrates. 7



Two spacer stripes with identical thicknesses of 23 µm were placed between the substrates. The inner surfaces of the substrates were precoated with PVA films and rubbed along the same direction. Prepared AIPQD-doped CLC (marked as AIPQD-CLC) mixtures are injected into these identical empty cells in isotropic state at approximately 95 °C. Then, the cells were slowly cooled down to room temperature to produce a planar AIPQD-CLC cells. The lasing performance of the AIPQD-doped CLC (marked as AIPQD-CLC) laser may be significantly dependent on the concentration of the QDs. Therefore, the lasing spectra of the AIPQD-CLC cells with identical CLC mixture of 22.81 wt% chiral dopant and 75.22 wt% nematic LC and six different concentrations of AIPQD (0.56, 1.1, 1.56, 1.96, 2.62, and 3.14 wt%) were pre-investigated at pumped energy E = 0.25 µJ/pulse. Figures S1a and S1b (Supporting Information) show the lasing spectra and peak intensities of the AIPQD-CLC laser with different QD concentrations, respectively. Apparently, the strongest lasing intensity for the six lasers at the same pumped energy can be obtained with an optimum QD concentration of 1.96 wt%. This is reasonable because a higher QD concentration results in a stronger fluorescence emission, thereby causing a stronger lasing output. However, a too high concentration of QD will lead to an obvious aggregation effect of QDs, which decreases the lasing performance of the AIPQD-CLC laser. The QD concentration will be fixed at the optimum value (1.96 wt%) in all the following lasing experiments. 8


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Five CLC mixtures containing different weight ratios of NLC and chiral dopant were further prepared (see Table S1 for the prescriptions in Supporting Information). The five CLC mixtures were individually doped with 1.96 wt%-AIPQD as a gain medium. Five AIPQD-CLC mixtures were obtained. To form AIPQD-CLC Cells 1–5, the five uniform AIPQD-CLC mixtures were individually injected into five identical empty cells and underwent the above-mentioned cooling process to form a planar texture in each cell. The measured wavelengths at the long- and short-wavelength-edges (λLWE and λSWE, respectively) of each reflection band of each cell are marked in Table S1.

2.4. Experimental setup. The lasing characteristics of each AIPQD-CLC cell were measured after excitation with a pumped-pulse laser beam using the experimental setup presented in Figure S2 (Supporting Information). A second harmonic pumped-pulse laser beam generated by a Q-switch Nd: YAG laser (LAB-130-10, Spectra-Physics) with a wavelength, pulse width, and pulse repetition frequency of 532 nm, 8 ns, and 10 Hz, respectively, was used as a pump beam in the lasing experiment on AIPQD-CLC cells. The incident beam was focused on the cell by a lens (focal length: 5 cm) at an incident angle of 0° from the normal of the cell. A fiber-based spectrometer was used to collect the induced fluorescence or lasing emission spectra (Jaz-combo-2 with a resolution of ~0.9 nm or HR4000 with a resolution of 0.02 nm, Ocean 9



Optics) along the normal direction of the cell. A notch filter (for 532 nm) was placed behind the cell to prevent the strong transmitted light of the pumped pulses from directly impacting the spectrometer. A half-wave plate (for 532 nm) and a polarizer were placed behind the exit hole of the laser to adjust the incident pumped energy (E) of the pumped-pulse laser beam on the cell. The energy of the pulse laser beam from a 50/50 beam splitter (BS1) was measured using a energy meter (Ophiropt mobile PD10-C). The lasing output from the pumped sample was collected by a 40× objective (Olympus) to successively pass through BS2 and filter (for 532 nm), and then was focused by a 10× objective onto the spectrometer. To identify the corresponding photonic bandgap structure of each cell, the reflection spectra of the incident white light, which was illuminated at normal incidence on each AIPQD-CLC cell, were obtained using a fiber-based spectrometer (Jaz-combo-2).

2.5. Characterization. HRTEM and XRD measurements were collected to characterize the structural properties of the obtained samples. XRD was performed on a D8 DISCOVER with GADDS system with Cu Kɑ radiation (ë = 1.5418 Å; Bruker AXS Gmbh, Karlsruhe, Germany). A scanning rate of 0.02° was applied to record patterns in the 2θ range of 10° ~ 60°. The XRD data showed that the sample had a well-defined perovskite structure, thus confirming the formation of CsSnI3. TEM characterization was conducted on a TEM-2100F system using an 10


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acceleration voltage of 200 kV. Absorption spectra were recorded at room temperature on a UV-VIS spectrophotometer (HITACHI U4100). Average diameter and size distribution were calculated from the data collected by Particle Size and Zeta Potential Analyzer (Malvern, Zetasizer Nano).

3. RESULTS AND DISCUSSION The absorption and PL spectra of CsSnI3QDs in toluene are shown in Figure 1a. The PL spectrum is obtained during the excitation of a CW diode-pumped solid-state laser beam with a wavelength of 532 nm at the excitation intensity (Iex) of 0.8 mW/cm2.Similar to that of the CsSnI3 bulk system,32 the absorption spectrum of CsSnI3 QDs shows a maxima at 555 nm, and the PL spectrum presents a narrow emission peak at 594 nm with a full-width at half-maximum (FWHM) of about 35 nm. As shown in Figure1b, at Iex = 10.5 mW/cm2, the obtained PL efficiency peak of the AIPQD-CLC system (Cell 1) is 24-fold higher than that of the CsSnI3 film. This result is attributed to the conjunction of two reasons: the suppression of PL loss as a result of the quantum confinement of AIPQDs33 and the amplification of PL caused by the band-edge effect of the CLC distributed feedback (DFB) optical resonator.34

11



Figure 1 (a) Absorption spectrum (black curve) and photoluminescence spectrum (blue curve) of CsSnI3QDs in toluene. (b) Measured PL spectra of AIPQD-CLC (Cell 1)(red curve) and CsSnI3 film (black curve) at Iex = 10.5 mW/cm2.

The high-resolution TEM (HRTEM) images of the CsSnI3 QDs are shown in Figure 2a. As shown in Figure 2b, the lattice fringes of a single QD have an interplanar spacing of 0.58 nm. This spacing corresponds to the distance between the adjacent (202) lattice planes of the black orthorhombic perovskite phase with further confirmations through energy-dispersive X-ray spectroscopy (EDS), X-ray photoelectron spectra (XPS), and X-ray diffraction (XRD). These measured results will be presented later. As inferred from the detailed size distribution analysis shown in Figure 2c, the particle sizes of the synthesized QDs are mostly distributed in the range of 3~5 nm. The existence of Cs, Sn, and I elements in a single QD is proven by TEM images (Figure 2b) and elemental mapping analyses (Figures 2d–2f), in which the chemical composition of CsSnI3 nanocrystals on QDs is quantitatively demonstrated and experimentally mapped out. To quantitatively confirm the chemical composition of the QDs, EDS and XPS analysis were 12


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performed. The results shown in Figures S3 and S4 (Supporting Information) indicate that the QDs have Cs:Sn:I atomic ratios of 20.4:20.1:59.5 (≅1.01:1:2.96) and 1:0.93:2.97.35-37 The negligible deviation from the 1:1:3 ratios expected for perovskites is in good agreement with an orthorhombic-phase perovskite stoichiometry (Table S2). To study the crystal structure of the prepared CsSnI3 perovskite, its XRD pattern was collected (Figure 2g). The pattern exhibits distinct peaks at approximately 16.5°, 26.6°, 29.6°, and 43.3° that correspond to diffractions from (101), (220), (202), and (242) planes, respectively.38−40 Characteristics of impurities are not observed, indicating the high purity of the synthesized CsSnI3 nanocrystals. All the XRD peaks can be perfectly indexed to the black orthorhombic phase of CsSnI3 perovskite (JCPDS file no. 43-1162).

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Figure 2 (a) High-resolution TEM image of the CsSnI3 QDs. (b) TEM image of single CsSnI3 QD (scale bar: 2 nm). (c) Particle size distribution of the synthesized CsSnI3 QDs. Elemental mapping images of (d) Cs, (e) Sn, and (f) I (scale bar: 2 nm) corresponding to the single CsSnI3 QD shown in (b). (g) Measured XRD pattern of CsSnI3 perovskite NCs. The associated results of the lasing experiments are as follows. Figure 3a presents the PL peaks centered at ~594 nm observed at different pumped energies with the measured and simulated reflection spectra (represented by black and red curves, respectively) of AIPQD-CLC 14


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(Cell 2). To recognize these narrow PL peaks, they are magnified, as presented in Figure S5a. This FWHM is considerably narrower than the 35 nm FWHM of the PL of CsSnI3 QDs in toluene. The measured PL intensity and FWHM versus the pumped energy are shown in Figure 3b. The results clearly indicate a lasing threshold at approximately Eth= 0.15 µJ/pulse (corresponding threshold energy density of ~0.8 mJ/cm2 per pulse for the pumped diameter of ~150 µm), which is a standard signature of lasing occurrence. When the threshold is exceeded, the intensity of the fluorescence output dramatically increases with a corresponding abrupt decrease in the FWHM, as shown in Figure 3b. In addition, a lasing peak centered at approximately 579 nm occurs at the band-edge of the CLC PBG (Cell 1) at E = 0.40 µJ/pulse, with a FWHM of as narrow as 0.20 nm (shown in Figure S5b). This FWHM is considerably narrower than the 35 nm FWHM of the PL of CsSnI3 QDs in toluene. The highly qualified lasing features of low threshold and narrow linewidth presented in the AIPQD-CLC laser are attributed to the suppression of PL loss due to the quantum confinement of AIPQDs and the amplification of PL due to the band-edge effect of the CLC-distributed feedback resonator. The quality factor, Q, which is defined as Q = ν0/∆νc, can be obtained to be ~2000 in the present CLC cavity based on Schawlow-Townes formula,41 where ν0 and ∆νc are the resonant frequency and the full-width of the cavity resonance, respectively. To the best of our knowledge, this result is the first demonstration of the achievement of lasing occurrence in a perovskite material embedded LC composite system (AIPQDs doped in a planar 15



CLC in the present study). Obviously, the present AIPQD-CLC laser possesses high qualities of lasing, which are comparable (or even better than) those of traditional DDCLC laser. A microscopy image of the planar texture of the AIPQD-CLC cell observed under polarizing optical microscopy (POM) without an analyzer (reflection mode) is presented in Figure 3c. Generally, the intrinsic serious aggregation effect of doped nanoparticles in LC cell is inevitable such that the cell has a poor properties. However, Figure 3c shows that only a few QDs tend to gather as the black aggregates in the defect lines (oily streaks) but not in other regions where large perfect planar texture domains are retained for lasing excitation in the present AIPQD-CLC sample. As shown in Figure 3a, highly qualified lasing peaks can be generated in these domains. The measured PL spectra of the AIPQD-CLC (Cell 2) at E = 0.4 µJ/pulse, which were measured when a premade right- or left-handed CLC filter (R-CLC or L-CLC filter) was previously inserted between the cell and the spectrometer, are shown in Figure 3d. The transmitted intensity ratio of the lasing emission through the R-CLC and L-CLC filter is approximately 15.6:1. This result indicates that the fluorescence intensity of the left-circular lasing component is significantly stronger than that of the right-circular one. On the basis of the definition of the light circular-polarization dissymmetry factor ge,42 ݃௘ = 2

୍ై ି୍౎ ୍ై ା୍౎

, (1)

Where IL and IR are the intensities of the left- and right-circular components, respectively, the 16


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value of ge obtained in the present case is as high as approximately 1.83. This result is indicative of a nearly perfect left-circular property for the lasing emission of the AIPQD-CLC, which is consistent with the handedness property for the band-edge lasing mechanism for typical left-handed CLC lasers.

Figure 3 (a) PL spectrum of AIPQD-CLC (Cell 2) measured as a function of pumped energy and corresponding reflection spectrum. The black and red curve indicates the experimental and simulated reflection spectra of the device, respectively. (b) Variation in PL intensity with pumped energy and FWHM. (c) Microscopic texture of the AIPQD-CLC cell observed under polarizing optical microscopy without an analyzer (reflection mode). QDs tend to aggregate in the 17



oily streaks but not in perfect planar texture domains. (d)The measured lasing emission spectra of AIPQD-CLC (Cell 2) at E = 0.4 µJ/pulse if a pre-made right-or left-handed CLC filter (R-CLC or L-CLC filter) is inserted between the cell and the spectrometer.

The following shows that the lasing wavelength of the AIPQD-CLC laser can be tuned by changing the chiral dopant content of the CLC mixture. As displayed in Figure 4a, five (normalized) lasing peaks with different wavelengths can be obtained for the five AIPQD-CLC cells (Cells 1–5) at E = 0.7 µJ/pulse. The corresponding reflection bands of CLC for the five cells were collected and are shown in Figure 4b. Apparently, the lasing peak that occurs at the long- or short-wavelength-edge of the corresponding reflection band can gradually red-shift from 582 nm to 606 nm (tunable range is approximately 24 nm) as the chiral dopant content of the CLC decreases. Beyond this spectral range, lasing is difficult to generate given the dominance of the strong reabsorption effect of QDs at λ < 582 nm or low fluorescence emission of QDs at λ > 606 nm.

18


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Figure 4 (a) Tuning of the lasing emission of AIPQD-CLC laser by changing the chiral dopant content in the CLC (Cells 1~5) at E = 0.7 µJ/pulse and (b) corresponding reflection spectra of the AIPQD-CLCs (Cells 1~5).

Recent investigations have presented that the efficiency of perovskite-based devices can be drastically improved (e.g., >20% for solar cells). Nevertheless, the future commercialization of these devices is hindered by their poor long-term stability.43, 44 To confirm that the AIPQD-CLC device is stable, the lasing efficiencies of the AIPQD-CLC laser (Cell 1) at E = 0.6 µJ/pulse were measured at one day and at six months after open storage under room temperature (23 °C) and 19



high humidity of 60% (±5%). The results are shown in Figure 5a. The lasing emission intensity only slightly decreased by nearly 13% after six months of storage in a general environment. This result indicates that the stability of the AIPQD-CLC device is considerably better than that of a conventional perovskite-based device without special materials or structure designs. The high stability of the AIPQD-CLC device is attributed to the protection provided by the CLC, which isolates the QDs from moisture and oxidization. In addition, this work also measured the short-term stability of the AIPQD-CLC device during the continuing excitation of the pumped pulse laser in 10 min at room temperature (23 °C) and high humidity of 60% (±5%), as shown in Figure 5b. Apparently, the AIPQD-CLC device still sustain a lasing efficiency higher than 80% of initial value (excited at t = 0) after the continuing excitation for 10 min. The decrease of the lasing efficiency of the AIPQD-CLC device may be attributed to the heat accumulation after the continuing excitation of the pumped pulses, which can disturb the CLC resonator and then degrade the device performance.

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Figure 5 (a) Lasing spectra of the AIPQD-CLC (Cell 1) at E = 0.6 µJ/pulse after 1 day and six months of storage (black and red peaks, respectively) under room temperature (23 °C) and high humidity of 60% (±5%). (b) Variation of the measured lasing intensity of the AIPQD-CLC device under continuing excitation of the pumped pulses for 10 min. The measurement was performed under room temperature (23 °C) and high humidity of 60% (±5%).

The thermal and electrical tunabilities of the AIPQD-CLC laser based on Cells 1 and 2 are illustrated in Figures 6a and 6b, respectively. When temperature increases from 20 °C to 40 °C, the lasing peak of the AIPQD-CLC laser can be tuned continuously from 576 nm to 592 nm at (E = 5.0 µJ/pulse). The result is attributed to the red-shift of the photonic bandgap or its LWE due to the pitch sensitivity of the CLC to temperature variation. At the same time, the order parameter of LCs decreases due to the thermal-disturbance-induced instability of the CLC resonator as increasing from 20 °C to 40 °C. The result will induce the lasing intensity to drop massively. Additionally, the lasing peak of the AIPQD-CLC laser can be continuously tuned from 590 nm to 584 nm under the application of an AC voltage (1 kHz) of 0 V to 20 V (at E = 4.0 µJ/pulse). 21



The result is attributed to the blue-shift of the PBG or its LWE due to the local tilting of the helical axis from the normal of the cell, leading to the electrical tunability of the lasing emission. Once the applied voltage increases to a certain degree (>17 V), a focal conic texture with a disordered distribution of helical axis will form and the lasing emission will cease. The insets in Figures 6a and 6b show the corresponding microscopy images of the cell observed under POM without an analyzer (reflection mode) when the temperature and voltage increase, respectively. These images present the beginning of the apparent disturbance of the CLC texture at approximately 40 °C and 17 V. At above 40 °C or 17 V, the reflectivity of the photonic bandgap decreases. Again, this behavior verifies that lasing tuning has diminished. The stability and flexible tunability demonstrated by AIPQD-CLC lasers indicate that these lasers are superior to most light-emitting perovskite devices.

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Figure 6 (a) Thermal tunability of the AIPQD-CLC laser (Cell 1) at E = 5.0 µJ/pulse at 20 °C, 25 °C, 30 °C, 40 °C, and 45 °C, respectively. (b) Electrical tunability of the AIPQD-CLC laser (Cell 2) at E = 4.0 µJ/pulse under AC voltages of 0~20 V (1 kHz). The insets below the tuning plots of the lasing spectrum in (a) and (b) are the corresponding images observed under the POM without analyzer (reflection mode) at 20~45 °C and 0~20 V, respectively. 23



4. CONCLUSIONS In summary, this study uses a low-cost solvothermal method to fabricate AIPQDs and to investigate AIPQD-CLC composite lasers. Given the conjunction of the suppression of the PL loss induced by the strong quantum confinement effect of the AIPQDs with the amplification of PL contributed by the CLC DFB resonator, the AIPQD-CLC lasers exhibit highly qualified lasing properties that are comparable to (or even better than) those of traditional DDCLC lasers. In addition, the present lasers possess high tuning flexibility and long-term stability. These characteristics are absent from currently available light-emitting perovskite devices without special materials or structure designs. With the booming development of the perovskite and QD industries in recent years,45 the results of this work highlighting the important engineering potential of the quantum confinement effects of AIPQDs demonstrated that AIP CsSnI3 QDs may grow as a new family of gain material for optoelectronic devices, including not only lasers but also other lighting-emitting devices and displays.

ASSOCIATED CONTENT Supporting Information Detailed experimental procedures, Experimental setup, Mixture precursor by weight ratios, Photoluminescence spectra, Energy Dispersive Spectrometer (EDS), X-ray photoelectron spectra 24


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(XPS) and measured lasing emission spectra. This material is available free of charge via the Internet at http:// pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Notes The authors declare no competing financial interest.

Author contributions C.R.L. supervised the project and conceived the idea; L.J.C. led the performance of the experiments in this work; J.H.D. carried out the sample fabrication and optical characterizations measurements; J.D.L and T.S.M both gave important discussions for the performance of the experiments; H.P.L. contributed to sample wash treatment; H.C.Y. processed reflection band simulations; Y.C.C and S.A.J provided assistances in experimental setup and sample fabrication. All the authors discussed the results, commented on and revised the manuscript.

ACKNOWLEDGMENTS The authors would like to thank the Ministry of Science and Technology of Taiwan (Contract numbers: MOST 106-2628-M-006-007-MY3 and MOST 106-2112-M-006-003-MY3) for

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Wavelength-tunable and highly stable perovskite-quantum-dots

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