Waterproof perovskite-hexagonal boron nitride hybrid nanolasers with

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Waterproof perovskite-hexagonal boron nitride hybrid nanolasers with low lasing thresholds and high operating temperature Haoran Yu, Xing Cheng, Yilun Wang, Yang Liu, Kexiu Rong, Ziling Li, Yi Wan, Wenting Gong, Kenji Watanabe, Takashi Taniguchi, Shufeng Wang, Jianjun Chen, Yu Ye, and Lun Dai ACS Photonics, Just Accepted Manuscript • DOI: 10.1021/acsphotonics.8b00977 • Publication Date (Web): 23 Oct 2018 Downloaded from http://pubs.acs.org on October 25, 2018

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Waterproof perovskite-hexagonal boron nitride hybrid nanolasers with low lasing thresholds and high operating temperature Haoran Yu1, 2, Xing Cheng1, Yilun Wang1, Yang Liu1, Kexiu Rong1, Ziling Li1, Yi Wan1, Wenting Gong1, Kenji Watanabe3, Takashi Taniguchi3, Shufeng Wang1, Jianjun Chen1, Yu Ye1, 2*, and Lun Dai1, 2* 1State

Key Laboratory for Artificial Microstructure &Mesoscopic Physics, School of

Physics, Peking University, Beijing 100871, China 2Collaborative 3National

Innovation Center of Quantum Matter, Beijing 100871, China

Institute for Materials Science, 1-1 Namiki, Tsukuba 305-0044, Japan

*Correspondence and requests for materials should be addressed to Y.Y. (email: [email protected]) and L.D. (email: [email protected])

Abstract Solid-state perovskites have recently emerged as promising coherent light sources, due to its efficient gain. While the continuous-wave (CW) optically pumped perovskite laser has been achieved at low temperatures, the final frontier of an electrical perovskite-based laser diode remains challenging due to the heat management and intrinsic instability of perovskite materials. Here, we demonstrate waterproof perovskite-hexagonal boron nitride (hBN) hybrid nanolasers with low lasing thresholds and high operating 1

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temperature. After capping with the hBN flake which possesses superb and anisotropic thermal conductivity, heat dissipation of the perovskite nanolaser is accelerated and an overheated hot spot is avoided. This results in the significant reduction of lasing thresholds (17.05% to 60.15%) in 16 measured samples, and clear lasing behavior under a temperature as high as 75.6 °C. Moreover, hBN with high environmental stability can effectively protect the perovskite from the polar solvents. The hBN encapsulated CsPbI3 nanolaser can incessantly lase in water for an hour, and the lasing behavior can be retained even after 24 hour immersion in water. The reduction of lasing threshold, improved heat removal, and higher temperature tolerance of the hybrid structure nanolaser marks a major step towards CW-pumped perovskite laser at room temperature, while also allowing perovskites to be integrated into high power density optoelectronic devices and future electrically driven lasers. In addition, as the sandwiching of perovskites with hBN can withstand polar solvents, this method may serve as a reliable method for both the future fabrication of perovskite-based devices and sensor-based applications in solvent systems. TOC Graphic

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Key Words: Hexagonal boron nitride, cesium lead halide perovskite, hybrid structure， nanolaser, heat management, waterproof

Solid-state lead halide perovskites show great promises in photonic sources,1 due to their superior optical properties, such as low non-radiative recombination rate, large and balanced carrier mobility, long carrier lifetime, low defect density, and tunable band gap.2-4 The excellent properties of the perovskites make them exceptional coherent sources, as single-crystalline methylammonium lead halide perovskites (MAPbX3, X= I, Br, Cl) nanowires (NWs) achieved a lasing carrier density as low as 1016 cm−3, two orders of magnitude lower than those for conventional semiconductor NWs.5 Recently, continuous-wave (CW) optically pumped lasing has been realized in MAPbI3 distributed feedback lasers,6 and CsPbBr3 NW2 at low temperatures, a key step towards electrically driven laser diodes. For CW pumped and electrically driven perovskite lasers to operate at room temperature, efficient heat management becomes an urgent demand, due to the feedback between threshold density increasing with temperature and lattice warming under high excitation. Two-dimensional (2D) materials, especially hexagonal boron nitride (hBN), are widely accepted as ideal candidates for thermal management due to their high intrinsic thermal conductivity and high degree of mechanical flexibility.7,8 Moreover, it has recently been proved that encapsulation with hBN flakes can suppress MAPbI3 microplates’ surface degradation, greatly improving their thermal stability.9 3


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Another intrinsic disadvantage of perovskite is the poor stability when in contact with polar solvents, which becomes a major obstacle for its practical fabrication and subsequent applications. In contrast, hBN is extremely stable in most solvents, providing an elegant solution to waterproof perovskite devices. Herein, we report waterproof perovskite-hBN hybrid nanolasers with low lasing thresholds and high operating temperature. Effective heat management of the CsPbI3 nanoplate (NP) is achieved through the capping of hBN, with its superb and anisotropic thermal conductivity,7,10 leading to a significant reduction of the lasing threshold, and clear lasing behavior at a temperature as high as 75.6 °C, higher than the previous reported highest operating temperature of 43.6 °C for MAPbI3 plasmonic nanowire lasers.11 Moreover, we demonstrate the encapsulation of hBN flakes with high environmental stability can effectively protect the perovskite from the polar solvents. The hybrid structure nanolasers incessantly lase in water for an hour, and can still lase even after being immersed in water for 24 hours. In addition, the hBN encapsulated perovskite is resistant to a series of traditional fabrication processes, such as electron beam lithography, development, reaction ionic etching, metallization and subsequent lift-off, opening a new route for perovskite-based nano-devices. Coupled with recently developed techniques of mechanical assembly and edge-contact of an embedded graphene electrode,12 the hBNperovskite NP-hBN hybrid structure may lead to the future realization of the electrically driven perovskite nanolaser. 4


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Results and Discussion Lasing threshold reduction after hBN capping In order to make a perovskite-hBN hybrid nanolaser (Figure 1a), we separately prepare the CsPbI3 NPs and hBN flakes. The CsPbI3 NPs are synthesized via a chemical vapor deposition (CVD) method on muscovite mica substrates3,4 (see SI Section 1). With the quasi square shape and a proper thickness, CsPbI3 NP simultaneously acts as the gain material and resonant cavity for the formation of a high-quality whispering gallery mode (WGM) laser.3,4 The hBN flake is mechanically exfoliated from hBN single crystals, grown under high pressure and temperature using barium boron nitride (Ba3B2N4) as a solvent13. The hBN is transferred onto the selected CsPbI3 NP by all-dry PDMS viscoelastic stamping method14 (see SI Section 2), which guarantees a clean hBN-CsPbI3 interface that avoids contaminations from any polymers or solutions. The threedimensional (3D) atomic force microscopy (AFM) topography clearly shows how the hBN fully seals the CsPbI3 NP, with an atomic top surface roughness (Figure 1b). The line profile indicates the thicknesses of CsPbI3 NP and hBN capping layer are about 177 nm and 57 nm, respectively, and the size of this NP is about 5 μm×4.5 μm (see SI Section 3). Since hBN is transparent over a broadband ranging from visible to the near infrared,15,16 the hBN capping layer negligibly absorbs both the excitation and the light emission from the perovskite. The CsPbI3 NP after hBN capping supports the WGM 5


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mode as well, confirmed by 3D electric field distribution simulation (see SI Section 4).

Figure 1. Optical characteristics of CsPbI3 NP lasers before and after hBN capping. (a) Schematic diagram of the NP laser lying on a muscovite mica substrate under hBN capping. A 532 nm excitation laser beam covers the entire CsPbI3 NP and the NP emits red light. Inset: ball-and-stick model showing hBN and CsPbI3 stacking at the interface of 6


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the hybrid structure. (b) 3D representation of the AFM topographic data of the structure. Different colors indicate height variances of the structure. CsPbI3 NP is fully sealed by hBN. (c) CsPbI3 NP’s emission spectra as a function of pump intensity before and after hBN capping. Left and right insets depict the normalized spectral maps after and before hBN capping, respectively. At pump intensity around lasing threshold, a shoulder appears on the long-wavelength side of the broad spontaneous emission peak. The shoulder occurs at much lower pump intensity after hBN capping. (d) L-L curves of the CsPbI3 NP laser before and after hBN capping. Dashed lines are the linear fitting results of the output intensities, from which the lasing thresholds are extracted. Lasing threshold of the NP reduces by about 50% after hBN capping. (e) FWHM of the NP emission peak as a function of pump intensity. A sudden reduction in FWHM is observed as pump intensity exceeds lasing threshold. (f) Histogram of lasing threshold reduction ratios of sixteen measured samples, clearly showing a reduction centered at around 30%.

In order to analyze the influence of the hBN capping, optical characterizations of the same CsPbI3 NP before and after hBN capping are conducted. The NP is pumped by a 532 nm nanosecond laser (4.5 ns pulse duration, 1.15 kHz repetition rate) at room temperature (see Materials and Methods). The excitation laser beam is focused (spot size, ~21 µm) by an objective (Zeiss Epiplan 20×/0.40na HD) and covers the entire NP. Emission spectra, as a function of pump intensity, are collected by the same objective 7


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(Figure 1c). At low pump intensities, we observe a broad spontaneous emission from the perovskite. At higher pump intensity, a shoulder appears on the long-wavelength side of the broad peak, amplified by the optical feedback in the NP cavity. Note that the shoulder occurs at much lower pump intensity after hBN capping. As pump intensity further increases, the resonant peak goes up sharply, as lasing is achieved. The far-field interference emission pattern and the strong polarized beam emission of the NP above the lasing threshold further confirm the lasing oscillation (see SI Section 5). There exists an hBN capping induced wavelength-shift of the resonant peak, as the hBN’s refractive index influences mode selection by the NP cavity. We also note a slight blue-shift of the lasing peak from 721.1 to 720.9 nm (from 719.1 to 718.0 nm) as the pump intensity increases above the lasing threshold before (after) hBN capping. The blue-shift with increasing carrier density can have multiple origins, such as thermally induced bandgap/refractive index change, band filling, optical density fluctuations, and electron/hole many-body interactions,5 etc. From both of the pump intensity dependent light emission curves (L-L curves) measured before and after hBN capping (Figure 1d), we observe a distinct kink with a super-linear increase in the emission output. Apparently, the normalized spectral maps (insets in Figure 1c) and the L-L curves (Figure 1d) of the NP show a significant threshold reduction after hBN capping. For the representative sample (Figure 1b), the lasing threshold is lowered by more than 50%, from 276.99 to 130.09 μJ/cm2 before and after hBN capping (Figure 1d). In addition, we 8


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observe abrupt spectral narrowing above lasing threshold, as well as spectral broadening with increasing excitation intensity (Figure 1e). The reduction of the lasing threshold indicates that the carrier excitation required to attain gain threshold is reduced after the hBN capping. However, the hBN capping layer affect the quality of the optical cavity, since the full-width at half-maximum (FWHM) of the resonant peak under identical excitation intensity above lasing threshold is larger after the NP is covered by hBN flake (for example, under the pump intensity of 375.44 μJ/cm2, the FWHM of the resonant peak increases from 0.63 nm to 1.17 nm, see SI Section 6). Using a rate equation analysis, the experimental data can be best fitted with a spontaneous emission factor, β, of 0.0203 (0.0163) before (after) hBN capping (see SI Section 7). Usually, for a cavity with high quality factor (Q), the Purcell effect enhances the β factor, resulting in a higher percentage of the spontaneous emission that can directly couple to the desired lasing mode. Therefore, the threshold of laser decreases as the β and Q factor increases.17,18 In contrast, our hybrid nanolaser shows decreased β factor and increased FWHM of the resonant peak, but it gives a significant reduction in the lasing threshold. The observation is further confirmed by 16 different samples, with the reduction of lasing threshold ranging from 17.05% to 60.15%, due to the different morphologies, thicknesses of the measured NPs, as well as the capping condition in different samples. Therefore, there must exist some other crucial factors which affect the lasing threshold of the CsPbI3 NPhBN hybrid nanolasers. 9


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Figure 2. Time-resolved photoluminescence (TRPL) measurements. (a) Typical TRPL decay transients following photo-excitation of pump fluence below and above lasing threshold, before and after hBN capping. The instrument response function is plotted with the gray dashed line. Below lasing threshold, the measured spontaneous emission lifetime is consistent with the timescale of surface traps induced non-radiative recombination. The lifetime increases from 908 ps to 1075 ps after hBN capping, indicating a lower surface recombination rate as a result of the surface passivation by hBN. Above lasing threshold, there is a new fast decay component (~20 ps limited by the instrument response), corresponding to the stimulated emission process. The measured TRPL decay curves before and after hBN capping overlap with each other, indicating the photo-excited carriers deplete in a similar way. (b-e) Images of spectrum vs. time (collected over a time window of ~1.34 ns) taken under different conditions. (b) Below lasing threshold, before hBN capping. (c) Below lasing threshold (identical pump intensity with b), after hBN 10


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capping. The reduction of the surface trap-mediated non-radiative recombination enhances the room-temperature PLQY of the CsPbI3 NP, resulting in an increase of the PL output after hBN capping under the identical pump intensity. (d) Above lasing threshold, before hBN capping. (e) Above lasing threshold (identical pump intensity with d) after hBN capping. Consistent with the reduction of the lasing threshold, there is a significant increase of the lasing emission intensity after hBN capping under the identical pump intensity above threshold.

Carrier dynamics of the nanolasers In order to attain further insight into the remarkable influences of hBN capping, timeresolved photoluminescence (TRPL) measurements are performed on a same CsPbI3 NP before and after hBN capping (Figure 2). At low excitation density, where Auger recombination is not dominant, the excited charge carriers can either relax through bandedge radiative emission or trap-mediated non-radiative pathways, including bulk defects with fast trapping time in the timescale of fs to ps, and surface traps which typically require >100 ps for carriers to diffuse through a few tens of nanometers of the material.19 Due to the measured low photoluminescence quantum yield (PLQY ~8.3%20) of CsPbI3 at room temperature, trap-mediated non-radiative pathways give rise to spontaneous emission with a lifetime of 908 ps for the bare NP (Figure 2a). Due to single crystalline nature of the synthesized NP, the bulk traps are probably absent.21 Therefore, the 11


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measured spontaneous emission lifetime is consistent with the timescale of surface traps induced non-radiative recombination. After hBN capping, the spontaneous emission lifetime increases to 1075 ps, indicating a lower surface recombination rate as a result of the surface passivation brought by hBN.9 The reduction of the surface trap-mediated nonradiative recombination enhances the room-temperature PLQY of the CsPbI3 NP, resulting in an increase of the PL output after hBN capping under the identical low pump intensity below lasing threshold (Figure 2b and 2c). Such efficient PL enhancement and lifetime increase after hBN capping are also observed in another CsPbI3 NP (see SI Section 8). When the excitation intensity is increased above lasing threshold, a new fast decay component appears both before (21.55 ps, 87.07%) and after (20.65 ps, 85.67%) hBN capping, which decays more rapidly than the other component (445.55 ps, 12.93% before hBN capping; 419.66 ps, 14.33% after hBN capping). The measured fast decay lifetime (~20 ps) is limited by the instrument response (dashed gray line in Figure 2a). Concurrently, the emission band collapses to yield a sharp peak at 722.5 nm (723.1 nm) before (after) hBN capping (Figure 2d and 2e). This rapid decay corresponds to the stimulated emission process, which has a much shorter timescale compared with the nonradiative recombination process at the surface traps. Thus, the surface states will not affect the stimulated processes. Above the lasing threshold, the normalized TRPL decay curves measured before and after hBN capping overlap with each other (Figure 2a), indicating the photo-excited carriers deplete in a similar way. However, we observe a 12


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significant increase of the lasing emission intensity after hBN capping under the identical high pump intensity (Figure 2d and 2e), consistent with the reduction of the lasing threshold. Thus, the carrier dynamics alone cannot explain why the lasing thresholds of the hybrid nanolasers are reduced compared to the bare perovskites.

Figure 3. Simulation of thermal dissipation process. (a) Time evolution of the nanolasers’ temperature distribution (left column: before hBN capping; right column: after hBN capping). Cyan lines outline the NP and hBN layer. Non-radiative recombination induces heating in the gain region, leading to a temperature increase in the NP. For bare CsPbI3 NP, heat accumulates perpendicularly, since the pathway for thermal dissipation is limited to the NP crystal and the substrate. After hBN capping, a significant portion of the heat generated in the NP is dissipated through the hBN laterally due to its 13


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highly anisotropic thermal conductivity, showing a broader temperature distribution. (b) Evolution of the average temperature inside CsPbI3 NP in a short time scale. The temperature of the hBN capped NP rises slower within the pump pulse duration. The temperature rises 13.13 °C in bare CsPbI3 NP at the end of the 4.5 ns laser pulse duration, while that in the hBN capped one rises only 11.88 °C. (c) Temperature evolution over a long timescale. The temperature of the hBN capped NP decreases much faster when the laser is off. It drops back to room temperature in a time of about 1,000 ns, while the bare NP still has a finite temperature of 3.96 °C remaining.

Thermal dissipation simulations In a nanolaser, all energy lost through non-radiative pathways will ultimately generate heat, including the excess energy of excitation photons with respect to emitted photons, the so-called quantum defect, and non-radiative recombination.22 Efficient thermal dissipation from gain materials with limited radiative efficiency remains a critical point for realizing nanolasers with low-threshold and high working temperature. Ideally, 2D materials can potentially address many of the heat dissipation issues which arise in nanoscale devices, due to their superior thermal properties.7,8 In contrast to graphene, thermally conductive hBN is electrically insulating,7,10,15 and transparent from visible to the near infrared region,15,16 which make it a superb heat dissipater in a nanolaser without additional screening of the fluorescence from the gain materials. To investigate the heat 14


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conveying capability of the hBN in our hybrid structure nanolaser, we conduct numerical simulations of the transient temperature distribution, where the thicknesses of the structure’s components are extracted from the AFM measurement and the initial temperature is set to 25 °C for both of the device and the environment (see SI Section 9). We consider the gain region as a heat source due to non-radiative recombination and assume that all the energy in the non-radiative pathway is converted to lattice vibration. Under a pump intensity of 200 kW/cm2 above the lasing threshold (with a duration of 4.5 ns), the temperature distribution shows clear distinction before and after hBN capping (Figure 3a). Firstly, non-radiative recombination induces heating in the gain region, leading to a temperature increase in the NP. There is a temperature rise of 13.13 °C in bare CsPbI3 NP at the end of the 4.5 ns laser pulse. In contrast, only a temperature rise of 11.88 °C is observed in the hBN capped case (Figure 3b). Secondly, the large in-plane thermal conductivity of hBN provides a large lateral pathway for heat dissipation. For bare CsPbI3 NP, heat accumulates perpendicularly, since the pathway for thermal dissipation is limited to the CsPbI3 NP crystal and the mica substrate. After hBN capping, a significant portion of the heat generated in the NP is dissipated through the hBN laterally due to its highly anisotropic thermal conductivity.10 Therefore, the lateral temperature distribution of CsPbI3 NP-hBN hybrid structure is broader than that of bare NP (Figure 3a), indicating an accelerated thermal dissipation in the hBN capped case. Magnitude and direction of the heat flux inside the hybrid structure clearly illustrate the 15


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large and anisotropic heat dissipation through the hBN layer (see SI Section 10). Thirdly, compared with that of the bare CsPbI3 NP, the temperature of the hBN capped NP rises much slower within the pump pulse (4.5 ns), and decreases much faster when the laser is off (Figure 3b and 3c). We can see that the temperature of hBN capped CsPbI3 NP drops back to room temperature in a time of about 1,000 ns, while the bare NP still remains a finite temperature of 3.96 °C (Figure 3c). As material gain decreases when the temperature increases,23 lasing is more difficult to achieve in the bare CsPbI3 NP, with all else equal. The decrease in material gain can be compensated by increasing pumping, but at the expense of a broadened gain spectrum and higher lasing threshold. Considering the feedback between a threshold density increasing with temperature and lattice warming under high excitation in CsPbI3 NPs, the existence of highly thermally conductive hBN successfully solves the issue of heat runaway, and therefore results in a significant reduction of the lasing threshold.

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Figure 4. Lasing of the hybrid nanolaser under high temperature. (a) Temperature and pump intensity dependent emission spectra. In a representative hybrid nanolaser, with 55% reduction in lasing threshold after hBN capping, the lasing behavior can be clearly observed as the temperature reaches as high as 75.6 °C. (b) Lasing threshold of the CsPbI3 NP extracted from L-L curves under various temperatures. The lasing threshold increases from 140.62 μJ/cm2 to 534.09 μJ/cm2 as the temperature increases from 25.0 °C to 69.5 °C. Uncertainties of thresholds are from the fitting of L-L curves, indicated by the error bars. Due to the intensity fluctuation around the lasing threshold, the fitting error 17


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becomes larger at high temperature. (c) Blue shift of the lasing emission peak with temperature and pump intensity. By increasing the temperature from 25.0 °C to 75.6 °C, the peak wavelength of the lasing spectrum decreases from 726.05 nm to 722.13 nm, corresponding to a wavelength shift of ~0.077 nm/°C.

Lasing under high temperature In general, the direct application of solid state lasers demands reliable operation under high temperature.24 However, so far, the high-temperature performance of perovskite is limited to 43.6 °C,11 as the lasing threshold increases significantly with temperature (due to the decrease in material gain at high temperature, and the low thermal stability of perovskites11,25). In our case, as the operating temperature increases, CsPbI3 nanolasers perform poorly or fail, resulting in no bare NP lasing at 60 °C (see SI Section 10). Given that highly thermally conductive hBN capping can improve heat dissipation, reduce the lasing threshold of CsPbI3 NP, suppress the surface degradation, and improve the overall thermal stability of the material,9 the CsPbI3 NP-hBN hybrid structure shows a promise for high temperature lasing operation. In a representative hybrid nanolaser, with 55% reduction in lasing threshold after hBN capping, clear lasing behavior can still be observed as the operating temperature reaches as high as 75.6 °C (Figure 4a). CsPbI3 is known to be unstable in the black perovskite phase in ambient atmospheric conditions and easily converts to the non-perovskite yellow phase.25 Due to their poor thermal 18


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stability, the reliability and performance data of perovskite material based laser, especially at high operation temperature, is rarely reported. Lasing threshold is one of the important physical parameters, which decides the minimum external pumping required for lasing from the sample. Herein, lasing thresholds are extracted from L-L curves of the CsPbI3 NP-hBN hybrid nanolaser under various temperatures (Figure 4b). It can be clearly seen that the lasing threshold increases gradually with temperature. For instance, lasing threshold increases from 140.62 μJ/cm2 to 534.09 μJ/cm2 as temperature increases from 25.0 °C to 69.5 °C. The fitting error becomes larger at high temperature, due to the higher intensity fluctuation around the lasing threshold. For example, from 25 ℃ to 30 ℃, the lasing threshold of the representative hybrid nanolaser increases from 140.62 μJ/cm2 to 218.94 μJ/cm2. Correspondingly, a 5 ℃ temperature difference leads to a 35.76% reduction of the lasing threshold, which is in consistent with our statistical lasing threshold reduction of 30% among 16 measured samples (Figure 1f). In addition, the variation of the spectral characteristics, such as peak wavelength, are another important parameter for lasers, as these variations can be catastrophic to system operation. Here, we observe that the wavelength of the lasing peak blue-shifts with increasing temperature and pump intensity (Figure 4c), which agrees well with the gain spectrum shift with temperature.26 With temperature increasing from 25.0 °C to 75.6 °C, the peak wavelength of the lasing spectrum decreases from 726.05 nm to 722.13 nm, corresponding to a wavelength blue shift of about 0.077 nm/°C, which is three-fold smaller than that of 19


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GaAs diode laser.27 Due to the high in-plane thermal conductivity of hBN used in the hybrid structure, heat can be effectively conveyed laterally. Concurrently, the low out-ofplane thermal conductivity of hBN prevents the formation of an overheated hot spot on the CsPbI3 NP, which greatly improves the lasing performance of the perovskite laser at high operating temperature.

Figure 5. Lasing of hBN encapsulated CsPbI3 NP in water. (a) Optical image of a dry transferred CsPbI3 NP on a pre-prepared hBN flake. The bottom hBN is large enough, and the CsPbI3 NP is transferred onto the center of the flake. (b) Optical image of CsPbI3 NP encapsulated by two pieces of hBN flakes. The top hBN is mechanically exfoliated onto a PDMS stamp, followed by a dry transfer. The CsPbI3 NP is fully sealed by two pieces of hBN flakes. Length of the scale bars in a and b is 10 μm. (c) Normalized lasing spectral map of the hBN encapsulated CsPbI3 NP. It incessantly lases in water for an hour with negligible lasing peak variations. (d) Normalized lasing spectra of hBN encapsulated CsPbI3 NP after being immersed in water for 0, 3, 5, and 24 hours.

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Lasing in water For many applications of lasers, reliable operation, especially under harsh environmental conditions, is desired. The poor stability of lead halide perovskites, especially in contact with polar solvents, is a barrier to its practical applications. In addition to high thermal conductivity, hBN has many other advantages such as high thermal stability (1000 °C in air, and 1400 °C in vacuum) as well as chemical and oxidation resistance,7 providing an elegant solution to waterproofing the perovskite nanolaser. Encapsulated by hBN flakes on both sides (Figure 5a and 5b), the stability of the CsPbI3 NP laser immersed in water is investigated as a function of time at room temperature (see SI Section 11). Under the protection of hBN flakes, CsPbI3 NP in water shows stable lasing with negligible variations of lasing peak position for an hour under incessant excitation (Figure 5c). It can still lase after being immersed in water for as long as 24 hours (Figure 5d). Moreover, lasing behavior from this sandwich structure can also be observed in isopropanol, glycol, and 38% dextrose in water (D38W), indicating the CsPbI3 NP has been protected successfully from these polar solvents (see SI Section 11). The waterproof hBN-CsPbI3 NP-hBN hybrid nanolaser benefits from the interfacial van der Waals interactions between the two inert hBN flakes,28-30 preventing the molecules of polar solvents from entering. Inspiringly, the hybrid structure can survive and even lase after going through a series of traditional fabrication processes, such as electron beam lithography, development, reaction ionic etching, metallization, and lift-off (see SI Section 11). 21


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Coupled with recently developed techniques of mechanical assembly and edge contact of the embedded graphene electrode,12 the perovskite hBN-NP-hBN hybrid structure paves the way for perovskite-based nano-devices, and even the electrically driven perovskite nanolaser.

Conclusion In summary, by effective heat management of CsPbI3 NP through hBN capping, the lasing threshold of a CsPbI3 NP-hBN hybrid nanolaser can be remarkably reduced, and clear lasing behavior can be observed at a high temperature of 75.6 °C. Moreover, the waterproof perovskite nanolaser is realized from hBN encapsulated CsPbI3 NP, which can incessantly lase in water for an hour, and can still lase after being immersed in water for 24 hours. Our finding that the hBN-CsPbI3 NP-hBN hybrid structure can survive and still lase after going through a series of traditional fabrication processes opens a new route to fabricate perovskite-based nano-devices, which might also work in solvent systems. In addition, the improved thermal dissipation and temperature tolerance of our hybrid structure perovskite nanolaser promise a bright future for the realization of roomtemperature CW optically pumped and even electrically driven perovskite lasers.

Materials and Methods Growth of CsPbI3 NPs. Chemical vapor deposition (CVD) is employed to synthesize 22


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CsPbI3 NPs. The precursor is the mixed powder of PbI2 and CsI with a molar ratio of 1:1, and is placed at the center of the tube furnace. The substrates of muscovite mica are placed at the downstream of the furnace. The furnace temperature is set at a target temperature of 540 °C. The growth time is 10 mins. During the whole growth process, pressure inside the quartz tube is maintained at about 1.1×104 Pa, with 50 sccm flow rate of high purity argon carrier gas to transport the reactant vapor. At the end of the growth, the furnace is cooled down naturally. Optical pumping characterizations. A frequency-doubled solid-state laser (EO-W-532600 mW-11748) is used to pump the NP individually (wavelength of 532 nm, repetition rate of 1.15 kHz, pulse width of 4.5 ns). The excitation beam is focused by a microscopy objective (Zeiss Epiplan 20×/0.40na HD) with a spot size of about 21 μm in diameter. The emission light is collected by the same objective. A 550 nm long pass filter and a 532 nm notch filter are placed before the Andor spectrometer (SR-500i-D2-R) equipped with a Newton CCD (DU920P-BEX2-DD) to remove the excitation laser wavelength. Time-resolved photoluminescence measurements. A 517 nm fs laser with a repetition rate of 1 kHz and a pulse energy ranging from 1 mJ/cm2 to 50 mJ/cm2 is used as the excitation light. A long pass filter with a cutoff edge at 600 nm is used to separate the emission of the NPs from the scattered excitation light. The emission images of the NPs are taken by a home-built microscope-CCD system. By placing a small aperture with a diameter of 100 μm on the object plane of the CCD, which coincides with the image 23


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plane of microscope objective lens, the fluorescence signal of individual NP can be selectively coupled into an optical fiber connected with a streak camera system with a response time of about 30 ps. The transient photoluminescence spectra of the individual NP thus can be acquired by the streak camera system. Numerical Calculations. 3D electric field distribution of the hBN capped CsPbI3 NP on mica, and the comparison of thermal dissipation in CsPbI3 NP before and after hBN capping are performed using finite element method (COMSOL Multiphysics). A 3D model is built up for the simulation of electric field distribution, with the dimensional parameters of the hybrid structure duplicated strictly from the AFM measurement. To simplify the calculation for thermal dissipation, we take the CsPbI3 NP as a uniformly heated circular disk, since the shape of the NP will not change the nature of heat dissipation. In this case, with rotation symmetry, the system can be simplified from 3D to 2D, and only the temperature distribution of the cross section along the radius direction is needed. Other parameters used for numerical calculation are provided in SI Section 9.

Corresponding Author Correspondence and requests for materials should be addressed to Y.Y. (email: [email protected]) and L.D. (email: [email protected]) Author contributions Y.Y. conceived the project. H.Y grew the CsPbI3 NP, fabricated the devices, and 24


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conducted the lasing characterizations. Y.W. helped on the optical measurements. X.C. did numerical calculations and simulations. Y.L., S.W. and H.Y. performed the TRPL measurements. Y.W. performed the AFM characterizations. K.R., J.C. and H.Y. measured emission polarization. K.R. took radiation patterns. H.Y., X.C. and Z.L. performed data analysis. H.Y plotted the figures assisted by W.G.. T.T. and K.W. provided boron nitride bulk crystals. Y.Y. and L.D. supervised this research. H.Y., Y.Y. and L.D. wrote the manuscript. All authors contributed to discussions. Acknowledgements This work was supported by National Natural Science Foundation of China (Nos. 61875001, 61521004, 61874003, and 11474007), the Nation Key R&D Program of China (Grant Nos. 2018YFA0306900 and 2017YFA0206301), the National Basic Research Program of China (No. 2013CB921901), and the “1000 Youth Talent Plan” Fund. K.W. and T.T. acknowledge support from the Elemental Strategy Initiative conducted by the MEXT, Japan and the CREST (JPMJCR15F3), JST. Y.Y. thanks Q. Zhang from Peking University and X. Liu from National Center for Nanoscience and Technology, China, for the helpful discussions.

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For Table of Contents Use Only

Waterproof perovskite-hexagonal boron nitride hybrid nanolasers with low lasing thresholds and high operating temperature Haoran Yu, Xing Cheng, Yang Liu, Kexiu Rong, Yilun Wang, Ziling Li, Yi Wan, Wenting Gong, Kenji Watanabe, Takashi Taniguchi, Shufeng Wang, Jianjun Chen, Yu Ye* and Lun Dai*

*Corresponding authors, E-mails: [email protected], [email protected]

In this work, we demonstrate waterproof perovskite-hexagonal boron nitride (hBN) hybrid nanolasers with low lasing thresholds and high operating temperature. After capping with the hBN flake which possesses superb and anisotropic thermal conductivity, heat dissipation of the perovskite nanolaser is accelerated and an overheated hot spot is avoided. This results in the significant reduction of lasing thresholds (17.05% to 60.15%) in 16 measured samples, and clear lasing behavior under a temperature as high as 31


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75.6 °C. Moreover, hBN with high environmental stability can effectively protect the perovskite from the polar solvents. The hBN encapsulated CsPbI3 nanolaser can incessantly lase in water for an hour, and the lasing behavior can be retained even after 24 hour immersion in water. The reduction of lasing threshold, improved heat removal, and higher temperature tolerance of the hybrid structure nanolaser marks a major step towards CW-pumped perovskite laser at room temperature, while also allowing perovskites to be integrated into high power density optoelectronic devices and future electrically driven lasers. In addition, as the sandwiching of perovskites with hBN can withstand polar solvents, this method may serve as a reliable method for both the future fabrication of perovskite-based devices and sensor-based applications in solvent systems.

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