Near infrared semiconductor nanolasers are of great significance for integrated optoelectronic chips [1–3]. An efficient gain medium is one of the key components of near infrared nanolasers [4–6]. The traditional gain media of near infrared lasers are made of inorganic semiconductors, but their quantum efficiencies are low and the growths require critical conditions [3]. Perovskites have attracted considerable interests and have been considered as leading-candidate gain media for next generation on-chip optical sources, thanks to their outstanding photophysical properties as well as low cost and promise for electrically driven lasing [7–10]. Among various perovskite materials, organic–inorganic hybrid materials, with ${\mathrm{MAPbI}}_3$ as a representative, are of particular interest in the fields of semiconductor lasers as well as solar cells [11,12], light emitting diodes [13], photodetectors [14], etc., due to their large absorption coefficients, exceptionally low trap-state densities, large charge carrier diffusion lengths, and high charge mobilities [15].

In recent years, organic–inorganic hybrid perovskite lasers have achieved rapid progress. Zhang et al. achieved a room-temperature ${\mathrm{MAPbI}}_3$ nanoplatelet laser with a lasing threshold of $37\mathrm{\mu J}{\mathrm{cm}}^{-2}$ and a cavity quality factor of 650 through the vapor phase deposition method [3]. Zhu et al. demonstrated room-temperature lasing using solution-processed single-crystalline ${\mathrm{MAPbI}}_3$ nanowires, which showed a lasing threshold down to $220\mathrm{nJ}{\mathrm{cm}}^{-2}$ and a cavity quality factor $Q$ as high as 3600 [1]. Jia et al. demonstrated continuous wave lasing by an ${\mathrm{MAPbI}}_3$ distributed feedback laser at a substrate temperature of 102 K [16]. In 2020, Qin et al. achieved continuous wave pumped lasing with quasi-2D phenylethylammonium bromide and 1-naphthylmethylamibe bromide based perovskite media in air at room temperature [17]. Although various inorganic cations have been proposed to replace the organic cation [18–20] and different lead-free perovskite materials have also been developed [21–23], their lasing characteristics including the lasing threshold and cavity quality factor performed well below those of the organic–inorganic Pb based counterparts up to now [11].

However, organic and inorganic hybrid perovskites suffer from instability under operating conditions. It was reported that the temperature of a distributed feedback ${\mathrm{MAPbI}}_3$ laser on sapphire increased by 30 K after pumping for 50 ns and then by 90 K for 1 ms [24]. Such a temperature increase can result in thermal-induced degradation of perovskite crystals. Fan et al. found that the crystalline structure gradually evolved from tetragonal ${\mathrm{MAPbI}}_3$ to trigonal layered ${\mathrm{PbI}}_2$ after the temperature increased to 358 K [25]. Ascribed to the detrimental temperature increase, most of the organic and inorganic hybrid perovskite lasers could not sustain more than ${10}^7\text{pulses}$. For example, the emission intensity of an inkjet-printed ${\mathrm{MAPbI}}_3$ laser on a flexible PET substrate with a nanoimprinted grating in ${\mathrm{N}}_2$ atmosphere dropped to 90% of its initial value after $\sim 1\times {10}^6\text{pulses}$ [26]. The ${\mathrm{MAPbI}}_3$ laser with a silica microsphere resonator could sustain by $8.6\times {10}^6\text{pulses}$ [2]. Similarly, solution-processed ${\mathrm{FAPbBr}}_3$ microdisk lasers could work stably for 3000 s ($3\times {10}^6\text{pulses}$) before dropping to 90% of the initial value [27].

On the one hand, promoting the operating stability of lasers is one of the constant tasks of laser technology [28]. Although room-temperature continuous wave perovskite lasers have been reported [17], one of the major hurdles towards electrically pumped lasers is resistive heating under current injection [7]. On the other hand, improving the thermal stability is of critical importance for achieving electrically pumped perovskite lasers. Until now, great efforts have been made to improve the stability of organic–inorganic perovskites while maintaining their outstanding photophysical properties [29–31]. Working at cryogenic temperatures to keep perovskites below thermal degradation temperature is helpful to promote the stability of perovskite lasers. For example, CW amplified spontaneous emission (ASE) in a phase-stable perovskite has been demonstrated at temperatures up to 120 K [32]. However, room-temperature operating lasers are preferred in most applications [28]. The encapsulation strategy has been resorted to improve the perovskite lasing stability. For example, a thin poly-methyl-methacrylate (PMMA) encapsulation layer was applied in an ${\mathrm{MAPbI}}_3$ photonic crystal laser so that the operational stability at a pump intensity of $102.5\pm 6.4\mathrm{\mu J}/{\mathrm{cm}}^2$ was extended from 600 s (${10}^5\text{pulses}$) to 6000 s (${10}^6\text{pulses}$) [33]. By using a CYTOP encapsulation film, an ${\mathrm{MAPbI}}_3$ distributed feedback laser that operated at a pump intensity of $7{\mathrm{\mu J}/\mathrm{cm}}^2$ could sustain ${10}^7\text{pulses}$ before dropping to 90% of its initial value [34]. It was also demonstrated that the stability of ${\mathrm{MAPbI}}_3$ could be improved by encapsulating with boron nitride flakes [25]. Nevertheless, the stability performance of hybrid perovskite lasers is still dissatisfactory. Improving their stability is still one of the major tasks in this field, which is what has been done in the organic display industry [35]. For example, perovskite microlasers with longer lifetime can produce stronger signal and sustain for longer measurement times, which will generate a better signal to noise ratio for sensors [36]. The ultimate goal is to achieve perovskite laser diodes with lifetimes of many thousands of hours that are capable of supporting many commercial applications [7,37].

Understanding the degradation mechanisms is of significant importance for improving the operating stability of perovskite lasers. ${\mathrm{MAPbI}}_3$ perovskite is reported to evolve from tetragonal ${\mathrm{MAPbI}}_3$ to trigonal lead iodide layered crystal layer by layer due to the fact that a surface-initiated layer by layer degradation path exhibits the lowest energy barrier for crystal transition under moderate heating at 358 K in 2017 [25]. With the rapid development of the perovskite solar cell, the degradation mechanisms of operating perovskite solar cells have received widespread interest in recent years. The degradation behavior of perovskite solar cells was found to be profoundly influenced by macroscopic operation conditions in 2018 [38]. In perovskite solar cells, the performance degradation after hundreds of hours of operation in ${\mathrm{N}}_2$ atmosphere was found to be induced by cation-dependent phase segregation during device operation in 2020 [39]. Degradation of operating perovskite solar cells under vacuum was considered to be caused by a large degree of lattice shrinkage and a spontaneous process for phase segregation in 2021 [40]. Compared with standard one sun illumination ($1.5{\mathrm{kW}/\mathrm{m}}^2$) in PSC, perovskite lasers are illuminated by much stronger laser light (peak intensity up to $650{\mathrm{GW}/\mathrm{m}}^2$) and degrade after outputting ${10}^7\text{pulses}$. Pumped by a femtosecond laser with a repetition rate of 250 kHz, perovskite lasers will degrade after tens of minutes, which is much shorter than that of PSCs. Therefore, the faster degradation process of perovskite lasers cannot be explained by lattice shrinkage and phase segregation, which are responsible for the degradation of PSCs. To direct scientific progress towards more applications, the microscopic degradation mechanism for hybrid perovskite during the laser pumping process needs to be fully understood.

In this work, by continuously monitoring the emission properties of an ${\mathrm{MAPbI}}_3$ nanoplatelet laser, we find that the gradual degradation of tetragonal ${\mathrm{MAPbI}}_3$ starts from the surface defects and the laser output intensity drops to 90% after $\sim 1200\mathrm{s}$ ($7.2\times {10}^6\text{pulses}$). Those surface defects on the ${\mathrm{MAPbI}}_3$ nanoplatelets can be effectively passivated by introducing excess ${\mathrm{PbI}}_2$. As a result, the evolution from tetragonal ${\mathrm{MAPbI}}_3$ to ${\mathrm{PbI}}_2$ launches from the crystal surface and the nanoplatelet degrades layer by layer, bringing forward the operational stability being extended from 1200 s to 4500 s ($2.7\times {10}^7\text{pulses}$). On the basis of the ${\mathrm{PbI}}_2$ passivated nanoplatelet, we further introduce an additional DBP (${\mathrm{C}}_{64}{\mathrm{H}}_{36}$) protection film, which can suppress the surface-initiated degradation by passivating the surface dangling bonds, thereby dramatically improving the operational stability of the ${\mathrm{MAPbI}}_3$ laser to up to 8500 s ($5.1\times {10}^7\text{pulses}$), which is around 1.89 times as long as that of the ${\mathrm{MAPbI}}_3$ nanoplatelet with only ${\mathrm{PbI}}_2$ passivation. Compared with the initial ${\mathrm{MAPbI}}_3$ nanoplatelets with surface defects, the dual passivation strategy with both ${\mathrm{PbI}}_2$ and DBP enables the ${\mathrm{MAPbI}}_3$ laser to sustain for six times longer, promoting the stability performance of ${\mathrm{MAPbI}}_3$ perovskite lasers significantly. The present passivation strategy of improving the perovskite laser stability paves the way for developing high stability near infrared gain media. In addition, our first attempt at demonstrating the degradation mechanism of the hybrid perovskite crystals under laser pumping might provide in-depth insights for resolving the critical stability hurdle in practical applications of perovskite lasers.

The ${\mathrm{MAPbI}}_3$ nanoplatelets used in our study were synthesized by the two-step chemical vapor deposition method (see Appendix A for more details), which includes a first step of growing ${\mathrm{PbI}}_2$ nanoplatelets and a second step of converting ${\mathrm{PbI}}_2$ nanoplatelets into ${\mathrm{MAPbI}}_3$ nanoplatelets. The lasing threshold of an unpassivated nanoplatelet laser can be as low as $18.0{\mathrm{\mu J}/\mathrm{cm}}^2$ (see Appendix B). We measured the time-resolved photoluminescence (TRPL) of an ${\mathrm{MAPbI}}_3$ nanoplatelet laser without passivation (see Appendix B). Since ${\mathrm{MAPbI}}_3$ crystals show both fast dynamics and slow dynamics, biexponential fitting was performed to quantify the carrier dynamics. Here, the slow decay component reveals the lifetime of carriers [41]. At a pump intensity of $17.87{\mathrm{\mu J}/\mathrm{cm}}^2$ (below threshold), the PL decay curve shows a long average lifetime of $\sim 1.74\mathrm{ns}$. At a pump intensity of $26.81{\mathrm{\mu J}/\mathrm{cm}}^2$ (above the threshold), the PL decay curve shows a short average lifetime of $\sim 0.86\mathrm{ns}$. More information of ${\mathrm{MAPbI}}_3$ nanoplatelet lasers without passivation can be found in our previous work [42]. As ${\mathrm{MAPbI}}_3$ perovskite is very sensitive to electron-beam irradiation and begins to decompose into ${\mathrm{PbI}}_2$ under $151\mathrm{e}{Å}^{-2}$ total dose irradiation [43], monitoring the degradation process continuously with scanning electron microscopy (SEM) will introduce radiation damage. Therefore, only one SEM measurement is made on a nanoplatelet and the time is controlled within 1 min to avoid total dose irradiation exceeding $151\mathrm{e}{Å}^{-2}$. We continuously monitored the emission properties and the spectra of the ${\mathrm{MAPbI}}_3$ nanoplatelet laser in ambient air conditions with a home-built microscopic imaging and excitation system (see Appendix A for more details) in the operational stability measurement.

The degradation evolution of an ${\mathrm{MAPbI}}_3$ nanoplatelet laser operating at a pumping intensity of $26.1{\mathrm{\mu J}/\mathrm{cm}}^2$ ($1.1P_{\mathrm{th}}$) is shown in Fig. 1(a). In order to illustrate the degradation process better, the degradation region is marked with dashed lines. As can be seen in microscopic images, the emission intensity was almost uniform during the first 300 s. After operating for 500 s, the emission intensity on the left side of the laser started to decrease, which indicates that parts of the ${\mathrm{MAPbI}}_3$ molecules degrade to molecules that do not emit light in the monitoring spectral region. After operating for 700 s, the dark area expands to neighboring regions, which is different from the layer-to-layer degradation demonstrated in a previous report [25]. After operating for 900 s, the dark area continues to expand to neighboring regions. After operating for 1100 s, the dark area expands to the edge of the nanoplatelet. Therefore, we can conclude that the degradation propagated to the surrounding areas during the operating process. Since the measurement takes a long time, the emission intensity of the laser operating at a pump intensity of $1.1P_{\mathrm{th}}$ ($26.1{\mathrm{\mu J}/\mathrm{cm}}^2$) during the operating time was measured using an ideaoptics PG2000-Pro spectrometer (see Appendix A for more details) as shown in Fig. 1(b). As can be seen, the laser output intensity as a whole does not change, because more pumping energy can reach a lower ${\mathrm{MAPbI}}_3$ layer, which keeps the population inversion $\mathrm{\Delta }N$ required for maintaining the output intensity $I\propto \mathrm{\Delta }N$ almost unchanged as the upper layer of ${\mathrm{MAPbI}}_3$ degrades (see Appendix C for more details). After operating for 1200 s ($7.2\times {10}^6\text{pulses}$), the output intensity of the nanoplatelet laser decreases to 90% of the initial intensity as can be seen in Fig. 1(b). The operational stability data are in agreement with most of the reported ${\mathrm{MAPbI}}_3$ lasers [1,34]. After operating for 1100 s, the dark area keeps increasing and the output intensity of the nanoplatelet laser decreases dramatically. The laser dies after working for 1750 s. Besides this nanoplatelet laser, the operational stability of two other unpassivated nanoplatelet lasers has also been measured. The two nanoplatelet lasers can sustain for 1170 s and 1200 s (see Appendix B) before the output intensity decreases to 90% of its initial values, which are consistent with that of the first nanoplatelet laser.

The emission spectrum evolutions of the laser operating at a pump intensity of $1.1P_{\mathrm{th}}$ ($26.1\mathrm{\mu J}/{\mathrm{cm}}^2$) during the operating time were also measured by using an ideaoptics PG2000-Pro spectrometer (see Appendix A for more details). From the emission spectrum as shown in Fig. 1(c), we can see that the intensity of the laser line after operating for 1000 s starts to decrease with the decreasing spontaneous emission intensity. After 1800 s, the spontaneous emission intensity drops down to 50% of its initial value and the laser line almost disappears at the same time (see Appendix B). As can be seen in the microscopic image [Fig. 1(d)] of the ${\mathrm{MAPbI}}_3$ nanoplatelet after operating for 1800 s, part of the nanoplatelet marked with a dashed line has faster degradations, and the color of this part has changed to brown as compared with the yellow color of the other regions, which is similar to the initial color of the ${\mathrm{MAPbI}}_3$ nanoplatelet as shown in Fig. 1(e).

From the microscopic image of the initial ${\mathrm{MAPbI}}_3$ nanoplatelet as shown in Fig. 1(e), it is seen that the nanoplatelet with a thickness of $\sim 130\mathrm{nm}$ (see Appendix B) initially has a uniform surface and the whole surface is almost the same yellow color. However, from the SEM image of the ${\mathrm{MAPbI}}_3$ nanoplatelets, some surface defects are found on the surface, as can be seen in Fig. 1(e). Defects are mainly located at perovskite grain boundaries, which is in good agreement with previous results [44]. Atomic force microscopy (AFM) image [Fig. 1(f)] of the nanoplatelets shows that the RMS roughness of the surface is $\sim 2.1\mathrm{nm}$. As can be seen in Fig. 1(f), there is a trench that almost covers the whole image in the $600\mathrm{nm}\times 600\mathrm{nm}$ region. Its depth is $\sim 8\mathrm{nm}$. Therefore, the ${\mathrm{MAPbI}}_3$ nanoplatelet under operating condition starts to degrade from surface defects and progresses gradually to neighboring areas as shown in Fig. 1(a). The corresponding X-ray diffraction (XRD) pattern shows that the perovskite nanoplatelets initially have a pure tetragonal ${\mathrm{MAPbI}}_3$ crystal structure without impurities such as ${\mathrm{PbI}}_2$ (see Appendix B). The existence of the small (202), (112), (210), and (221) peaks indicates that the ${\mathrm{MAPbI}}_3$ nanoplatelets are in the room-temperature tetragonal phase [45]. After operating for 1800 s, more than a half of the surface has changed from yellow to brown as can be seen in Fig. 1(d). The corresponding XRD pattern shows that (001), (003), and (004) peaks of ${\mathrm{PbI}}_2$ appear after the nanoplatelets operate for 1800 s, confirming that some part of the tetragonal phase ${\mathrm{MAPbI}}_3$ nanoplatelet degrades to ${\mathrm{PbI}}_2$ [45].

The observed phenomenon of ${\mathrm{MAPbI}}_3$ degradation launching from the surface defects deviates from the layer-by-layer degradation theory, which expresses that the thermal-induced degradation starts from the surface of ${\mathrm{MAPbI}}_3$ as a result of dangling bonds, structure relaxation, and charge redistribution on the surface and occurs in a sequential layer-by-layer style [25]. A calculation of the transient thermal response of an ${\mathrm{MAPbI}}_3$ nanoplatelet shows that, with a moderate laser pump intensity of $\sim 17{\mathrm{\mu J}/\mathrm{cm}}^2$, the transient temperature at the nanoplatelet (see Appendix D) far exceeds the thermal degradation threshold temperature [25]. It is unquestionable that the ${\mathrm{MAPbI}}_3$ nanoplatelet suffers detrimental thermal-induced degradation in the experiment. In reality, with respect to the smooth flat surface, the surface defect regions on the surface of the nanoplatelet can form extra dangling bonds on their walls, which initiate new degradation pathways. Since the longer Pb–I–Pb bonds along the [001] direction of ${\mathrm{MAPbI}}_3$ are less resistant to bond breakage than those in the (001) plane [46], these bonds tend to break first under an external stimulus and form dangling bonds. The more defects there are on the nanoplatelet, the faster the speed of the thermal-induced degradation. Under laser operating conditions, the expansion of the defect region would accelerate the degradation, so a snowball effect is produced. Therefore, ascribed to the existence of surface defects, the degradation proceeds from the inner part to the edge rather than following the layer-by-layer degradation theory. It is plausible to suppose that reducing the defects can suppress the degradation and make the nanoplatelet lasers operate for longer times.

In contrast to fully converting ${\mathrm{PbI}}_2$ to ${\mathrm{MAPbI}}_3$ during the second step of chemical vapor deposition, a certain amount of ${\mathrm{PbI}}_2$ was intentionally reserved to passivate the defects in the fabrication of new perovskite nanoplatelets. As shown in Fig. 2(a), ${\mathrm{MAPbI}}_3$ nanoplatelets with well-defined triangular and hexagonal shape, 100–200 nm thickness, and tens of micrometers edge lengths were synthesized. As can be seen in the XRD pattern [Fig. 2(b)], there also exist (001), (003), and (004) peaks of the ${\mathrm{PbI}}_2$ structure in addition to the tetragonal phase ${\mathrm{MAPbI}}_3$ peaks, confirming the excess ${\mathrm{PbI}}_2$ being reserved in the perovskite nanoplatelets. Figure 2(c) shows the microscopic image of the ${\mathrm{MAPbI}}_3$ nanoplatelet for carrying out the following lasing operation. The perovskite nanoplatelet has a thickness of $\sim 180\mathrm{nm}$ (see Appendix E). The SEM image in Fig. 2(d) reflects that the surface defects were successfully passivated to a large extent. As can be seen, a newly formed species appeared on the nanoplatelet surface, and the new species displayed brighter color as compared with neighboring species as a result of poorer conductivity [47]. According to the XRD pattern as shown in Fig. 2(b), the species should be ${\mathrm{PbI}}_2$, while the darker films are considered to be perovskite. Because the formation energies of defects are generally related to the chemical potentials of the perovskite constituent element, defects can be controlled by adjusting the ratio of I/Pb in perovskite films [47]. It has been reported that the trap density can be reduced through a moderate excess ${\mathrm{PbI}}_2$ passivation [44]. During the chemical vapor deposition process, ${\mathrm{PbI}}_2$ is squeezed to grain boundaries by perovskite grain growth. Thus, the rich defect regions can be passivated by ${\mathrm{PbI}}_2$ and defects can be reduced [Fig. 2(d)]. From the SEM images, it can be clearly observed that the excess ${\mathrm{PbI}}_2$ was mainly distributed on the perovskite grain boundaries. Benefiting from reduced defects, the ${\mathrm{PbI}}_2$ passivated perovskite nanoplatelets have smoother surfaces. The AFM image in Fig. 2(e) indicates an RMS roughness of $\sim 0.7\mathrm{nm}$, confirming that the nanoplatelets have much smoother surfaces supporting the whispering-gallery-mode cavity after passivation. After ${\mathrm{PbI}}_2$ passivation, there are only two tiny pinholes with diameter of $\sim 100\mathrm{nm}$ and depth of less than 3 nm in a $600\mathrm{nm}\times 600\mathrm{nm}$ region as shown in Fig. 2(e).

The influence of excess ${\mathrm{PbI}}_2$ on the laser performance is investigated in the following. The light-in–light-out curve in Fig. 2(f) shows that the emission intensity grows slowly with the increasing pump intensity below the pump intensity of $\sim 14.98{\mathrm{\mu J}/\mathrm{cm}}^2$, and then the emission intensity grows very quickly. At a pump intensity of $15.87{\mathrm{\mu J}/\mathrm{cm}}^2$, the emission intensity saturates due to blue shift of the center wavelength of the laser [1]. Lasing death did not happen in the measurement. Here, the lasing threshold of $\sim 14.98{\mathrm{\mu J}/\mathrm{cm}}^2$ is lower than that of the ${\mathrm{MAPbI}}_3$ nanoplatelet laser without passivation, which can be found in our previous work [42]. Since the spectrum has a narrow linewidth that cannot be resolved by an ideaoptics PG2000-Pro spectrometer, the emission spectrum evolution of the laser operating at different pump intensities was measured by using a Horiba iHR 550 spectrometer (see Appendix A for more information). The spectra of the emission light in Fig. 2(g) show that there exists only spontaneous emission below $14.98{\mathrm{\mu J}/\mathrm{cm}}^2$. Above the threshold, a narrow laser peak appears, and the laser peak increases rapidly with the increasing pump intensity. As shown in Fig. 2(h), the separation between adjacent modes is $\sim 0.3\mathrm{nm}$, which is in agreement with the theoretical value ($\sim 0.3\mathrm{nm}$) calculated with the edge length of the cavity [42]. A Lorentz fit of the laser peak at the pump intensity of $14.98{\mathrm{\mu J}/\mathrm{cm}}^2$ shows that the full-width at half-maximum (FWHM) is $\sim 0.1\mathrm{nm}$, which corresponds to a cavity quality factor $Q$ of 7810, far superior to the unpassivated ${\mathrm{MAPbI}}_3$ nanoplatelet laser, which shows a cavity quality factor $Q$ of 2600 [42].

We also measured the TRPL as shown in Fig. 2(i). At a pump intensity of $11.12{\mathrm{\mu J}/\mathrm{cm}}^2$ (below threshold), the PL decay curve shows a long average lifetime of $\sim 6.62\mathrm{ns}$, which is longer than that of an unpassivated ${\mathrm{MAPbI}}_3$ nanoplatelet. At a pump intensity of $24.8{\mathrm{\mu J}/\mathrm{cm}}^2$ (above the threshold), the PL decay curve shows a short average lifetime of $\sim 0.67\mathrm{ns}$, which is slightly shorter than that of an unpassivated ${\mathrm{MAPbI}}_3$ nanoplatelet. It can be concluded that the lasing threshold has been reduced and the quality factor of nanoplatelet cavities has been improved significantly thanks to the reduced surface defects by ${\mathrm{PbI}}_2$ passivation.

The operational stability of the ${\mathrm{PbI}}_2$ passivated ${\mathrm{MAPbI}}_3$ nanoplatelet laser has also been tested under continuous laser pumping with a pumping intensity of $16.5{\mathrm{\mu J}/\mathrm{cm}}^2$ ($P=1.1P_{\mathrm{th}}$). As can be seen in Fig. 3(a), the laser emission intensity of the ${\mathrm{PbI}}_2$ passivated laser is very stable for 4600 s. During the 4600 s operation, the microscopic images captured at different times show that the emission intensity on the surface of the ${\mathrm{PbI}}_2$ passivated laser is uniform. A region with a faster degradation rate than the unpassivated nanoplatelet laser cannot be found, which indicates that degradation in the ${\mathrm{PbI}}_2$ passivated laser is different from defect-initiated degradation in the unpassivated laser. After 4600 s, the laser output intensity decreases very rapidly and the emission from the surface becomes weak as a whole, confirming that there is no defect-initiated degradation in the ${\mathrm{PbI}}_2$ passivated laser. According to density functional theory calculation, decomposition starting with the surface is kinetically preferred compared with bulk degradation and the next surface layer underneath will be exposed [25]. The decomposition will progress sequentially throughout the entire bulk in a layer-by-layer fashion, eventually leading to the degradation of ${\mathrm{MAPbI}}_3$ bulk. Therefore, the whole surface of the ${\mathrm{PbI}}_2$ passivated laser degrades at a similar rate from top layer to inner layer. After operating for 5600 s, its surface color was still uniform as shown by the microscopic image of the nanoplatelet in Fig. 3(b), which confirms that the ${\mathrm{PbI}}_2$ passivated laser degrades layer by layer. Thanks to ${\mathrm{PbI}}_2$ passivation, the surface defects are reduced significantly and thereby the surface-defect-induced degradation is effectively suppressed. Therefore, on the surface of the nanoplatelet, there only exists the dangling bonds triggered thermal decomposition, and, correspondingly, the degradation starts from the surface and proceeds layer by layer. Since the measurement takes a long time, the emission intensity of the laser operating at a pump intensity of $1.1P_{\mathrm{th}}$ ($16.48{\mathrm{\mu J}/\mathrm{cm}}^2$) during the operating time was also measured by using an ideaoptics PG2000-Pro spectrometer, which is capable of long-time measurement (see Appendix A for more details). As can be seen in Fig. 3(c), the monitoring of the laser emission intensity shows that the laser can maintain 90% of the initial intensity after 4500 s ($2.7\times {10}^7\text{pulses}$), which is nearly 3 times longer than that of the ${\mathrm{MAPbI}}_3$ nanoplatelet laser without passivation, and is 2.7 times longer than that of the state-of-the-art ${\mathrm{MAPbI}}_3$ nanowire laser [1]. Besides this ${\mathrm{PbI}}_2$ passivated nanoplatelet laser, the operational stability of two other ${\mathrm{PbI}}_2$ passivated nanoplatelet lasers has also been measured. The two nanoplatelet lasers can sustain for 4400 s and 4300 s (see Appendix E) before the output intensity decreases to 90% of the initial values, which are consistent with that of the first ${\mathrm{PbI}}_2$ passivated nanoplatelet laser.

Next, we optimized the operational stability of a ${\mathrm{PbI}}_2$ passivated ${\mathrm{MAPbI}}_3$ nanoplatelet laser by introducing an additional encapsulation layer to passivate the surface of the nanoplatelet. Pb–I–Pb bonds along the [001] direction tend to break first under an external stimulus due to weaker bond strengths as compared with those in the [001] plane, which forms ${\mathrm{Pb}}^{+}$ and ${\mathrm{I}}^{-}$ dangling bonds on the ${\mathrm{MAPbI}}_3$ surface. Surface-initiated layer-by-layer degradation of ${\mathrm{MAPbI}}_3$ is considered to be caused by the ${\mathrm{MAPbI}}_3$ surface ${\mathrm{Pb}}^{+}$ and ${\mathrm{I}}^{-}$ dangling bonds where atoms are no longer stabilized by the ${\mathrm{PbI}}_2$ layer as in the bulk [25]. Therefore, the surface atoms are more susceptible to rearrange under even moderate thermal excitation. Hydrogen and pseudo-hydrogen atoms are supposed to provide an ideal passivation to pair the electron in the dangling bonds on the surface of semiconductor nano-structures [48,49]. DBP (${\mathrm{C}}_{64}{\mathrm{H}}_{36}$) is a promising material for improving the performance of perovskite optoelectronic devices such as solar cells and light emitting diodes [50,51].

To suppress the surface-initiated degradation of perovskite nanoplatelets, we employed a thin DBP film as the encapsulation layer on a newly synthesized ${\mathrm{PbI}}_2$ passivated ${\mathrm{MAPbI}}_3$ nanoplatelet surface to form a $\mathrm{DBP}–{\mathrm{MAPbI}}_3–\mathrm{m}\mathrm{ica}$ heterostructure as shown in Fig. 4(a). The DBP film was spin-coated on the surface of ${\mathrm{MAPbI}}_3$ nanoplatelets on the mica substrate as shown in Fig. 4(b). After coating the DBP film, the ${\mathrm{MAPbI}}_3$ nanoplatelets on the mica substrate become darker as compared with the uncoated ${\mathrm{MAPbI}}_3$ nanoplatelets on the mica substrate (see Appendix F). The peak wavelength of a DBP passivated nanoplatelet laser is redshifted as compared with that of the nanoplatelet laser before passivation due to a change of the effective refractive index after DBP coating (see Appendix F). Without passivation, the ${\mathrm{MAPbI}}_3$ surface with Pb and I dangling bonds is more susceptible to degradation. As shown in Appendix F, the yellow nanoplatelet degrades severely for 48 h in ambient air conditions. Instead, the DBP encapsulated nanoplatelet can remain in ambient air conditions for more than 120 h as can be seen in Appendix F. This is because, with DBP encapsulation, the ${\mathrm{H}}^{+}$ in the ${\mathrm{C}}_{64}{\mathrm{H}}_{36}$ pairs the electron in perovskite surface dangling bonds, which effectively reduces the surface activity and enables a highly stable ${\mathrm{MAPbI}}_3$ nanoplatelet.

The lasing performance of the DBP encapsulated ${\mathrm{MAPbI}}_3$ nanoplatelet laser is shown in Fig. 4. It is found that the lasing threshold ($\sim 16.32{\mathrm{\mu J}/\mathrm{cm}}^2$) of the ${\mathrm{MAPbI}}_3$ nanoplatelet laser is slightly increased by DBP encapsulation as shown in Fig. 4(c), which might be induced by light absorption of DBP. Since the spectrum has a narrow linewidth that cannot be resolved by the ideaoptics PG2000-Pro spectrometer, the emission spectrum evolutions of the laser operating at a different pump intensity were also measured by using a Horiba iHR 550 spectrometer (see Appendix A for more details). The spectra of the emission light in Fig. 4(d) show that there exists only spontaneous emission below $16.32{\mathrm{\mu J}/\mathrm{cm}}^2$. Above the threshold, a narrow laser peak appears and the laser peak increases rapidly with the increase of the pump intensity. As can be seen in Fig. 4(e), a Lorentz fit of the laser peak at the pump intensity of $16.47{\mathrm{\mu J}/\mathrm{cm}}^2$ shows that the FWHM is $\sim 0.1\mathrm{nm}$, which corresponds to a cavity quality factor $Q$ of $\sim 7799$.

We then performed the operational stability test of the obtained stable ${\mathrm{MAPbI}}_3$ nanoplatelet at a pump intensity of $1.1P_{\mathrm{th}}$ ($\sim 17.95{\mathrm{\mu J}/\mathrm{cm}}^2$) at room temperature in ambient air conditions. Since the measurement takes a long time, the emission intensity of the laser was measured by using an ideaoptics PG2000-Pro spectrometer (see Appendix A for more details). As can be seen in Fig. 4(f), it shows that the dual passivation processed ${\mathrm{MAPbI}}_3$ nanoplatelet laser has considerably improved operational stability. The output intensity of the dual passivation processed laser retains 90% of the initial value for longer than 8500 s ($5.1\times {10}^7\text{pulses}$), which is around 1.89 times as long as that of the ${\mathrm{MAPbI}}_3$ nanoplatelet with only ${\mathrm{PbI}}_2$ passivation. Compared with the initial unpassivated ${\mathrm{MAPbI}}_3$ nanoplatelets with surface defects, the dual passivation strategy enables the ${\mathrm{MAPbI}}_3$ laser to sustain for six times longer, outperforming all reported hybrid perovskite lasers. Its operational stability is even better than that of some of the all-inorganic ${\text{CsPbBr}}_3$ lasers [19,52]. This result confirms that the rich hydrogen atoms contained in the DBP molecules can provide effective passivation of dangling bonds on the surface of ${\mathrm{MAPbI}}_3$ nanoplatelets. By coating the ${\mathrm{MAPbI}}_3$ surface with DBP film, the ${\mathrm{H}}^{+}$ could pair with the electron of perovskite surface dangling bonds as demonstrated in passivation of GaAs quantum dots [48]. Such interaction between charges in DBP and perovskite surface dangling bonds slows down the surface degradation and promotes operational stability. Besides this dual passivation processed nanoplatelet laser, the operational stability of two other dual passivation processed nanoplatelet lasers has also been measured. The two nanoplatelet lasers can sustain for 8290 s and 8390 s (see Appendix E) before the output intensity decreases to 90% of its initial value, which is consistent with that of the first dual passivation processed nanoplatelet laser.

The average operation times of unpassivated (sample A), ${\mathrm{PbI}}_2$ passivated (sample B), and dual passivation processed nanoplatelet lasers (sample C) under femtosecond laser pumping with a repetition rate of 6 kHz in ambient air conditions are 1190 s, 4400 s, and 8450 s (see Appendix G), respectively. It can be seen that the average operation time of ${\mathrm{PbI}}_2$ passivated nanoplatelet lasers is more than three times longer than that of unpassivated nanoplatelet lasers. Through dual passivation processing, the average operation time of nanoplatelet lasers is improved more than seven times as compared with that of the unpassivated nanoplatelet lasers.

In conclusion, a high stability ${\mathrm{MAPbI}}_3$ nanoplatelet laser has been demonstrated based on a dual passivation strategy, in which excess ${\mathrm{PbI}}_2$ and a DBP encapsulation film were utilized to passivate the defect-initiated degradation and the surface-initiated degradation, respectively. The continuous monitoring of the emission intensity of the initial ${\mathrm{MAPbI}}_3$ nanoplatelet laser reflects that the laser instability stems from thermal-induced degradation, which starts at the surface defects on the surface of ${\mathrm{MAPbI}}_3$ and then progresses towards the neighboring regions. Unreacted ${\mathrm{PbI}}_2$ has been employed to successfully suppress the defect-induced-degradation; therefore the nanoplatelet degrades in a layer-by-layer way. As a result, the ${\mathrm{PbI}}_2$ passivated nanoplatelet laser can sustain for 4500 s ($2.7\times {10}^7\text{pulses}$), which is more than three times longer than that of the nanoplatelet laser without passivation. It has been demonstrated that the ${\mathrm{PbI}}_2$ passivated nanoplate laser has a threshold as low as $14.98{\mathrm{\mu J}/\mathrm{cm}}^2$ and a cavity quality factor up to $\sim 7810$. To further retard the surface-initiated degradation, an additional DBP film has been utilized as a protection layer on the ${\mathrm{PbI}}_2$ passivated ${\mathrm{MAPbI}}_3$ nanoplatelet. The DBP encapsulated nanoplatelet shows considerably improved operational stability that can last for 8500 s ($5.1\times {10}^7\text{pulses}$) until it falls to 90% of its initial intensity. Our results demonstrate the microscopic degradation mechanism of an ${\mathrm{MAPbI}}_3$ nanoplatelet laser and show the critical importance of managing the defects and dangling bonds of the surface in developing stable perovskite near infrared lasers. Challenges remain in the commercialization of perovskite lasers. It is believed that the operational stability will be improved quickly by collective efforts in the future.

Acknowledgment. YC also acknowledges support from Key Research and Development, Henry Fok Education Foundation Young Teachers Fund, and Platform and Base Special Project of Shanxi Province. HZ also acknowledges support from the Natural Science Foundation of Guangdong Province, Shenzhen Nanshan District Pilotage Team Program, and the Science and Technology Innovation Commission of Shenzhen.