Long life Perovskite Nanoplatelet Lasers with high quality factor enabled through engineering degradation pathways

MAPbI 3 perovskite has attracted widespread interests for developing low-cost near infrared semiconductor gain media. However, it faces the instability issue under operation conditions, which remains a critical challenge. It is found that the instability of the MAPbI 3 nanoplatelet laser comes from the thermal-induced-degradation progressing from the surface defects towards neighboring regions. By using PbI 2 passivation, the defect-initiated degradation is signicantly suppressed and the nanoplatelet degrades in a layer-by-layer way, enabling the MAPbI 3 laser sustain for 4500 s (2.7×10 7 pulses), which is almost 3 times longer than that of the nanoplatelet laser without passivation. Meanwhile, the PbI 2 passivated MAPbI 3 nanoplatelet laser with the nanoplatelet cavity displaying a maximum quality factor up to ~7800, the highest reported for all MAPbI 3 nanoplatelet cavities. Furthermore, a high stability MAPbI 3 nanoplatelet laser that can last for 8500 s (5.1×10 7 pulses) is demonstrated based on a dual passivation strategy, by retarding the defect-initiated degradation and surface-initiated degradation, simultaneously. This work provides in-depth insights for understanding the operating degradation of perovskite lasers and the dual passivation strategy paves the way for developing high stability near infrared semiconductor laser media.

An e cient gain medium is one of the key components of the near infrared nanolasers [4][5][6] . The traditional gain media of near infrared lasers are made of inorganic semiconductors, but their quantum e ciencies are low and the growths require critical conditions 3 . Perovskites have attracted considerable interests and 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][8][9][10] . Among various perovskite materials, the organic-inorganic hybrid materials, with MAPbI 3 as a representative, are of particular interesting in the elds of semiconductor lasers as well as solar cells 11,12 , light emitting diodes 13 , photodetectors 14 , etc., due to their large absorption coe cient, exceptionally low trap-state densities, long charge carrier diffusion lengths, and high charge mobilities 15 .
In recent years, organic-inorganic hybrid perovskite lasers have achieved rapid progresses. Zhang et al.
achieved a room temperature MAPbI 3 nanoplatelet laser with a lasing threshold of 37 µJ cm -2 and a cavity quality factor of 650 through vapor phase deposition method 3 . Zhu et al. demonstrated roomtemperature lasing using solution processed single-crystalline MAPbI 3 nanowires, which showed a lasing threshold down to 220 nJ cm -2 and a cavity quality factor Q as high as 3600 1 . Jia et al. demonstrated continuous wave lasing by an MAPbI 3 distributed feed-back laser at a substrate temperature of 102 K 16 .
In 2020, Qin et al. achieved continuous wave pumped lasing with quais-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 16,18,19 and different lead-free perovskites materials have also been developed [20][21][22] , 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 11,12,23 .
However, organic and inorganic hybrid perovskites suffer from instability under operating conditions. It was reported that the temperature of a distributed feedback MAPbI 3 laser on sapphire increased by 30 K after pumping for 50 ns and then by 90 K for 1 ms 24 . Such temperature increase can result in thermalinduced-degradation of perovskite crystals. Fan et al. found that the crystalline structure gradually evolved from tetragonal MAPbI 3 to trigonal layered 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 pulses. For example, the emission intensity of an inkjet-printed MAPbI 3 laser on a exible PET substrate with a nanoimprinted grating in N 2 atmosphere dropped to 90% of its initial value after ~1×10 6 pulses 26 . The MAPbI 3 laser with a silica microsphere resonator could sustain by 8.6×10 6 pulses 2 . Similarly, the solution-processed FAPbBr 3 microdisk lasers could work stably for 3000 s (3×10 6 pulses) before dropping to 90% of its initial value 27 .
From 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 the outstanding photophysical properties [29][30][31] . The encapsulation strategy has been resorted to improve the perovskite lasing stability. For example, a thin poly-methylmethacrylate (PMMA) encapsulation layer was applied in a MAPbI 3 photonic crystal laser so that the operational stability at a pump intensity of 102.5±6.4 µJ/cm 2 being extended from 600 s (10 5 pulses) to 6000 s (10 6 pulses) 32 . By using a CYTOP encapsulation lm, a MAPbI 3 distributed feedback laser that operated at a pump intensity of 7 µJ/cm 2 could sustain 10 7 pulses before dropping to 90% of its initial value 33 . It was also demonstrated that the stability of MAPbI 3 could be improved by encapsulating with boron nitride akes 25 . Nevertheless, the stability performance of hybrid perovskite lasers is still dissatisfactory, and the microscopic degradation mechanism for the hybrid perovskite during the laser pumping process remains unknown.
In this work, by continuously monitoring the emission properties of a MAPbI 3 nanoplatelet laser, we nd that the gradual degradation of tetragonal MAPbI 3 starts from the surface defects and the laser output intensity drops to 90% after ~1200 s (7.2×10 6 pulses). Those surface defects on the MAPbI 3 nanoplatelets can be effectively passivated by introducing excess PbI 2 . As a result, the evolution from tetragonal MAPbI 3 to 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 (2.7×10 7 pulses). On the basis of the PbI 2 passivated nanoplatelet, we further introduce an additional DBP (C 64 H 36 ) protection lm, which can suppress the surface initiated degradation by passivating the surface dangling bonds, thereby dramatically improving the operational stability of the MAPbI 3 laser to up to 8500 s (5.1×10 7 pulses), which is around 1.89 times as long as that of the MAPbI 3 nanoplatelet with only PbI 2 passivation. Compared with the initial MAPbI 3 nanoplatelets with surface defects, the dual passivation strategy with both PbI 2 and DBP enables the MAPbI 3 laser sustain 6 times longer, promoting the stability performances of MAPbI 3 perovskite lasers signi cantly. The present passivation strategy of improving the perovskite laser stability paves the way on developing high stability near infrared gain media. In addition, our rst attempt on 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.

Results And Discussions
The MAPbI 3 nanoplatelets used in our study were synthesized by the two-step chemical vapor deposition method, that includes the rst step of growing PbI 2 nanoplatelets and the second step of converting PbI 2 nanoplatelets into MAPbI 3 nanoplatelets 34 . During the operational stability measurement, we continuously monitored the emission properties and the spectra of the MAPbI 3 nanoplatelet laser in ambient air condition with a pumping density of 26.1 µJ/cm 2 (1.1P th ).
As can be seen in Fig. 1a, the emission intensity was almost uniform on the surface of the nanoplatelet laser during the rst 600 s. After operating for 800 s, the emission intensity on the left side of the nanoplatelet laser started to decrease. Since the measurement takes long time, the emission intensity of the laser operating at a pump density of 1.1P th (26.1 µJ/cm 2 ) during the operating time were measured using an ideaoptics PG2000-Pro spectrometer(See stability characteristic section for more information) as shown in Fig. 1b. As can be seen, the laser output intensity as a whole does not change, because more pumping energy can reach lower MAPbI 3 layer, that keeps the population inversion ΔN required for maintaining the output intensity I∝ΔN almost unchanged as the upper layer of MAPbI 3 degrades; see supporting information for more details. After operating for 1100 s, the emission intensity on a small area on the left side of the nanoplatelet laser decreases dramatically and the area almost becomes dark. After operating for 1200 s (7.2×10 6 pulses), the dark area increases as shown in Fig. 1a and the output intensity of the nanoplatelet laser decreases to 90% of the initial intensity as can be seen in Fig. 1b. The operational stability data is in agreement with most of the reported MAPbI 3 lasers 1,2,35 . After operating for 1400 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 another two unpassivated nanoplatelet lasers has also been measured. The two nanoplatelet lasers can sustain for 1170 s (Fig. S1a) and 1200 s (Fig. S1b) before output intensity decreases to 90% of their initial value which are consistent with that of the rst nanoplatelet laser.
The emission spectrum evolutions of the laser operating at a pump density of 1.1P th (26.1 µJ/cm 2 ) during the operating time were also measured by using an ideaoptics PG2000-Pro spectrometer (See stability characteristic section for more information). From the emission spectrum as shown in Fig. 1c, 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 1700 s, the spontaneous emission intensity drops down to 50% of its initial intensity and the laser line almost disappears at the same time (see in Fig. S2). As can be seen in the microscopic image ( Fig. 1d) of the MAPbI 3 nanoplatelet after operating for 1800 s, some parts of the nanoplatelet have faster degradations and the color of these parts have changed to brown as compared with the yellow color of the rest regions.
From the microscopic image of the initial MAPbI 3 nanoplatelet as shown in Fig. 1e, it is seen that the nanoplatelet with a thickness of ~ 130 nm (see in Fig. S3) initially has a uniform surface and the color of the whole surface is almost the same. However, from the scanning electron microscopy (SEM) images of the MAPbI 3 nanoplatelets, some surface defects are found on the surface as can be seen Fig. 1e. The atomic force microscopy (AFM) images ( Fig. 1f) of the nanoplatelets shows that the RMS roughness of the surface is ~2.1 nm. Therefore, the MAPbI 3 nanoplatelet under operating condition starts to degrade from the surface defects and progress gradually to neighboring areas as shown in Fig. 1a. The corresponding X-ray diffraction (XRD) pattern shows that initially the perovskites nanoplatelets has a pure tetragonal MAPbI 3 crystal structure without impurities such as PbI 2 (see in Fig. S4). The existence of the small (202), (112), (210), and (221) peaks indicate that the MAPbI 3 nanoplatelets are in the roomtemperature tetragonal phase 36 . After operating for 1800 s, more than a half of the surface has changed from yellow to brown as can be seen in Fig. 1d. The corresponding XRD pattern shows that (001), (003), and (004) peaks of PbI 2 appears after the nanoplatelets operating for 1800 s, con rming that some part of the tetragonal phase MAPbI 3 nanoplatelet degrades to PbI 2 36 .
The observed phenomenon of 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 MAPbI 3 as a result of dangling bonds, structure relaxation and charge redistribution on the surface and happens in a sequential layer-by-layer style 25 37 , these bonds tend to break rst under external stimulus and form dangling bonds. The region with more defects 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 making the nanoplatelet lasers operating for longer times.
In contrast to fully converting PbI 2 to MAPbI 3 during the second step of chemical vapor deposition, a certain amount of PbI 2 was intentionally reserved to passivate the defects in fabrication of new perovskite nanoplatelets. As shown in Fig. 2a, MAPbI 3 nanoplatelets with well-de ned triangular and hexagonal shape and 100-200 nm thickness and tens of micrometers edge lengths were synthesized 34 .
As can be seen in the XRD pattern (Fig. 2b), there also exist (001), (003) and (004) peaks of the PbI 2 structure in addition to the tetragonal phase MAPbI 3 peaks, con rming the excess PbI 2 being reserved in the perovskites nanoplatelets. Fig. 2c shows the microscopic image of the MAPbI 3 nanoplatelet for carrying out the following lasing operation. The perovskite nanoplatelet has a thickness of ~ 180 nm (see in Fig. S7). The SEM image in Fig. 2d re ects 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 38 . According to the XRD pattern as shown in Fig. 2b, the species should be PbI 2 , while the darker lms are considered to be perovskite. Here, the unreacted PbI 2 , without destroying the perovskite crystal structure, is of great helpful for reducing the surface defects in the MAPbI 3 nanoplatelets. The AFM image in Fig. 2e indicates a RMS roughness of ~ 0.7 nm, con rming that the nanoplatelets have much smoother surfaces supporting the whispering-gallery-mode cavity after passivation.
The in uence of excess PbI 2 on the laser performance are investigated in the following. The light-in-lightout curve in Fig. 2f shows that the emission intensity grows slowly with the increasing pump density below the pump density of ~14.98 µJ/cm 2 , and then the emission intensity grows very quickly. At a pump intensity of 15.87 µJ/cm 2 , the emission intensity saturate due to blue shift of center wavelength of the laser 1 . Lasing death did not happen in the measurement. Here, the lasing threshold of ~14.98 µJ/cm 2 is lower than that of the MAPbI 3 nanoplatelet laser without passivation. Since the spectrum has a narrow linewidth which cannot resolved by ideaoptics PG2000-Pro spectrometer, the emission spectrum evolutions of the laser operating at a different pump density were measured by using a Horiba iHR 550 spectrometer(See optical spectrum characterization section for more information). The spectra of the emission light in Fig. 2g show that there exists only spontaneous emission below 14.98 µJ/cm 2 . Above the threshold, a narrow laser peak appears and the laser peak increases rapidly with the increasing of pump density. As shown in Fig. 2h, separation between adjacent modes is ~0.3 nm which is in agreement with the theoretical value (~0.3 nm) calculated with edge length of the cavity 34 . Lorentz t of the laser peak at the pump density of 14.98 µJ/cm 2 shows that the full-width at half-maximum (FWHM) is ~0.1 nm which corresponds to a cavity quality factor Q of 7810, much superior to the values of all reported MAPbI 3 nano-laser (the highest reported Q of 3600 belonged to the state-of-the-art MAPbI 3 nanowire laser cavity 1 ).
We also measured the time-resolved photo-luminescence as shown in Fig. 2i. Since MAPbI 3 crystals show both fast dynamics and slow dynamics, biexponential tting were performed to quantify the carrier dynamics. Here, the slow decay component reveals the lifetime of carriers 39 . At a pump density of 11.12 µJ/cm 2 (below threshold), the PL decay curve shows a long average lifetime of ~6.62 ns. At a pump density of 24.8 µJ/cm 2 (above the threshold), the PL decay curve shows a short average lifetime of ~0.67 ns. It can be concluded that the lasing threshold has been reduced and quality factor of nanoplatelet cavities has been improved signi cantly thanks to the reduced surface defects by PbI 2 passivation.
Operational stability of the PbI 2 passivated MAPbI 3 nanoplatelet laser has also been tested under continuous laser pumping with a pumping density of 16.5 µJ/cm 2 (P = 1.1P th ). As can be seen in Fig. 3a, the laser emission intensity of the PbI 2 passivated laser is very stable for 4600 s. After 4600 s, the laser output intensity decreases very rapidly and the emission from surface becomes weak as a whole. After operating for 5600 s, its surface color was still uniform as shown by the microscopic image of the nanoplatelet in Fig. 3b. Thanks to PbI 2 passivation, the surface defects are reduced signi cantly and thereby the surface defects induced degradation are 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; see supporting information for more details. Since the measurement takes long time, the emission intensity of the laser operating at a pump density of 1.1P th (16.48 µJ/cm 2 ) during the operating time were also measured by using an ideaoptics PG2000-Pro spectrometer which is capable of long-time measurement (See stability characteristic section for more information). As can be seen in Fig. 3c, the monitoring of the laser emission intensity shows that the laser can maintain 90% of the initial intensity after 4500 s (2.7×10 7 pulses), which is nearly 3 times longer than that of the MAPbI 3 nanoplatelet laser without passivation, and is 2.7 times longer than that of the state of the art MAPbI 3 nanowire laser 1 . Besides this PbI 2 passivated nanoplatelet laser, the operational stability of another two PbI 2 passivated nanoplatelet lasers has also been measured. The two nanoplatelet lasers can sustain for 4400 s (Fig. S8a) and 4300 s (Fig.  S8b) before output intensity decreases to 90% of their initial value which are consistent with that of the rst PbI 2 passivated nanoplatelet laser.
Next, we optimized the operational stability of PbI 2 passivated MAPbI 3 nanoplatelet laser by introducing an additional encapsulation layer to passivate the surface of the nanoplatelet. The surface-imitated layerby-layer degradation of MAPbI 3 is considered to be caused by Pb and I dangling bonds on the MAPbI 3 surface. 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 40,41 . DBP (C 64 H 36 ) is a promising material for improving the performances of perovskite optoelectronic devices such as solar cells and light emitting diodes 42,43 .
To suppress the surface-imitated degradation of perovskite nanoplatelets, we employed a thin DBP lm as the encapsulation layer on newly synthesized PbI 2 passivated MAPbI 3 nanoplatelet surface to form a DBP-MAPbI 3 -mica heterostructure as shown in Fig. 4a. The DBP lm was spin-coated on the surface of MAPbI 3 nanoplatelets on mica substrate as shown in Fig. 4b. After coating the DBP lm, the MAPbI 3 nanoplatelets on mica substrate (Fig. S9a) become darker as compared with the uncoated MAPbI 3 nanoplatelets on mica substrate (Fig. S9b). Without passivation, the MAPbI 3 surface with Pb and I dangling bonds is more susceptible to degrade. As shown in Fig. S10, the yellow nanoplatelet degrades severely for 48 h in ambient air condition. Instead, the DBP encapsulated nanoplatelet can keep in ambient air condition for more than 120 h as can be seen in Fig. S11. That is because, with DBP encapsulation, the Hin the C 64 H 36 interact with perovskite surface dangling bonds, which effectively reduces the surface activity and enables a highly stable MAPbI 3 nanoplatelet.
The lasing performances of the DBP encapsulated MAPbI 3 nanoplatelet laser are shown in Fig. 4. It is found that the lasing threshold (~16.32 µJ/cm 2 ) of MAPbI 3 nanoplatelet laser is slightly increased by DBP encapsulation as shown in Fig. 4c, which might be induced by light absorption of DBP. Since the spectrum has a narrow linewidth which cannot resolved by ideaoptics PG2000-Pro spectrometer, the emission spectrum evolutions of the laser operating at a different pump density were also measured by using a Horiba iHR 550 spectrometer (See optical spectrum characterization section for more information). The spectra of the emission light in Fig. 4d show that there exists only spontaneous emission below 16.32 µJ/cm 2 . Above the threshold, a narrow laser peak appears and the laser peak increases rapidly with the increasing of pump density. As can be seen in Fig. 4e, Lorentz t of the laser peak at the pump density of 16.47 µJ/cm 2 shows that the FWHM is ~0.1 nm which corresponds to a cavity quality factor Q of ~ 7799.
We then performed the operational stability test of the obtained stable MAPbI 3 nanoplatelet at a pump density of 1.1P th (~17.95 µJ/cm 2 ) at room temperature in ambient air condition. Since the measurement takes long time, the emission intensity of the laser was measured by using an ideaoptics PG2000-Pro spectrometer (See stability characteristic section for more information). As can be seen in Fig. 4f, it shows that the dual passivation processed MAPbI 3 nanoplatelet laser have considerably improved operational stability. The output intensity of the dual passivation processed laser keeps 9 % of the initial value for longer than 8500 s (5.1×10 7 pulses), which is around 1.89 times as long as that of the MAPbI 3 nanoplatelet with only PbI 2 passivation. Compared with the initial unpassivated MAPbI 3 nanoplatelets with surface defects, the dual passivation strategy enables the MAPbI 3 laser sustains 6 times longer, outperforming the performances of all reported hybrid perovskite lasers. Its operational stability is even better than some of the all inorganic CsPbBr 3 lasers 16,44 . This result con rms that the rich hydrogen atoms contained in the DBP molecules can provide effective passivation of the electron in the dangling bonds on the surface of MAPbI 3 nanoplatelets. Besides this dual passivation processed nanoplatelet laser, the operational stability of another two dual passivation processed nanoplatelet lasers has also been measured. The two nanoplatelet lasers can sustain for 8290 s (Fig. S12a) and 8390 s (Fig. S12b) before output intensity decreases to 90% of their initial value which are consistent with that of the rst dual passivation processed nanoplatelet laser.
The average operation time of unpassivated (sample A), 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 condition are 1190 s, 4400 s and 8450 s (see Fig. S13), respectively. It can be seen that the average operation time of PbI 2 passivated nanoplatelet lasers is more than three times longer than that of the 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 MAPbI 3 nanoplatelet laser has been demonstrated based on a dual passivation strategy, in which excess PbI 2 and a DBP encapsulation lm were utilized to passivate the defect-initiated degradation and the surface-initiated degradation, respectively. The continuous monitoring of the emission intensity of the initial MAPbI 3 nanoplatelet laser re ects that the laser instability stems from the thermal-induced-degradation which starts at the surface defects on the surface of MAPbI 3 and then progresses towards the neighboring regions. Unreacted PbI 2 has been employed to successfully suppress the defect-induced-degradation, therefore the nanoplatelet degrades in a layer-bylayer way. As a result, the PbI 2 passivated nanoplatelet laser can sustain for 4500 s (2.7×10 7 pulses), which is more than 3 times longer than the nanoplatelet laser without passivation. It has been demonstrated that the PbI 2 passivated nanoplate laser has a threshold as low as 14.98 µJ/cm 2 and a cavity quality factor up to ~7810. To further retard the surface-initiated degradation, an additional DBP lm has been utilized as a protection layer on the PbI 2 passivated MAPbI 3 nanoplatelet. The DBP encapsulated nanoplatelet show considerably improved operational stability which can last for 8500 s (5.1×10 7 pulses) until it falls to 90% of its initial intensity. Our results demonstrate the microscopic degradation mechanism of a 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.

Methods
Synthesis of Perovskite NPLs: PbI 2 (99.999%, Alfa) was used as a single source and placed into a quartz tube mounted on a single zone furnace (CY scienti c instrument, CY-O1200-1L) at a room temperature of 18°C. The fresh-cleaved muscovite mica substrate was pre-cleaned with acetone and placed in the downstream region inside the quartz tube. The quartz tube was rst evacuated to 0. holographic diffraction grating and the entrance slit of 50 µm were used in the measurement. The spectral resolution of the spectrometer is ~ 0.04 nm. The emission was time-resolved by using a TCSPC module (Picoquant, PicoHarp 300) and a SPAD detector (MPD, PD-100-CTE) with an instrument response function of 30 ps (FWHM).
Stability characterization. The emission intensity from a single nanoplatelet was monitored using an ideaoptics PG2000-Pro spectrometer with a wavelength resolution (FWHM) of 0.3 nm in the range 700-900 nm. Since the spectrometer does not require cooling liquid, it can work stably for longer times. For spectral range of 200-1100 nm, the ideaoptics PG2000 spectrometer with a wavelength resolution (FWHM) of 1.3 nm was used.