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Abstract
We report on a high-temperature vapor drop-casting technique used to fabricate a chemically stable nanomaterial‒CsPbBr3 quantum dot nanoglass that exhibits stable photoluminescence and stimulated emission properties. Such nanomaterial is comprised of two material domains: nano-fractured glassy-phase matrix and embedded CsPbBr3 quantum dots. The nanoglass is compatible with various pump sources to attain a low-threshold stimulated emission via either linear or nonlinear pump schemes. We demonstrate that the pump threshold could be remarkably reduced when the pump wavelength shifts from the peak position of the photoluminescence excitation spectrum to the photoluminescence peak. In addition, the lowest pump threshold achieved in the two-photon pump regime is comparable to that ever-reported lowest value using colloidal CsPbBr3 quantum dots. Cooperative contributions from local disorders on both of the pump beam and the emitted light are proposed to account for the observed stimulated emission properties.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Recently, all-inorganic perovskite quantum dots (PQDs) based on cesium lead tri-halides, with the formula of CsPbX3 (X = Cl, Br, I), have attracted tremendous attention as an emerging family of materials for various optoelectronic applications including solar cells, photodetectors, light-emitting diodes (LEDs), and compact lasers [1–8]. Such PQDs have been viewed as excellent candidates to constitute quantum optical emitters due to their high quantum yield (approximately >70%), narrow spectral linewidth of 70–100 meV (corresponding to 12-40 nm), and tunable optical bandgap (approximately 1.7–3.0 eV) [9–13]. So far, state-of-the-art light-emitting devices, including LEDs and compact lasers, have been demonstrated by utilizing superb optical absorption, emission, and charge transport properties of CsPbX3 PQDs [14–16]. In comparison to traditional zero-dimensional semiconductor materials (e.g. CdSe, PbS, etc.), the optical bandgap of CsPbX3 PQDs can be engineered through a facile anion-exchange process to meet the requirements for various purposes [17–20].
Despite impressive merits of CsPbX3 PQDs, their practical applications have been severely impeded by their poor chemical stability due to their sensitivity to moisture, temperature, and light exposure [14]. Several strategies based on surface modification have been proposed to improve the chemical stability of CsPbX3 PQDs. For instance, encapsulating PQDs within designed organic ligands could passivate the surface of PQDs, consequently improving the chemical stability [21]. However, organic ligands are generally loosely bound to the surface of CsPbX3 PQDs and thus the ligands are easily lost when such PQDs are exposed to purification processes or intense pulsed laser irradiation, leading to the agglomeration of PQDs [22]. Attempts have been made as well to embed CsPbX3 PQDs into passive matrices such as polymer, silica, and alumina [23–25]. The embedding strategy nevertheless suffers from either complex chemical fabrication procedures or non-dense structure of surface protective layer. Recently, a post-annealing procedure was developed to in-situ precipitate CsPbX3 PQDs in parent optical glasses containing the chemical constituents of CsPbX3 [26–29]. Optical glasses are dense structures in three dimensions, thus CsPbX3 PQDs embedded therein exhibit remarkably improved photoluminescence (PL) stability upon moisture and light exposure. For instance, CsPbBr3 PQDs embedded inside phosphosilicate glasses remain bright under UV irradiation after immersion in water for 1 day, while bare CsPbBr3 PQDs exhibit almost entire PL quenching after immersion in water for only 20 seconds [27]. Such excellent PL stability of PQDs embedded inside the glass matrix arises from the sufficient protection of the glass matrix. Therefore, optical glasses embedded with PQDs show great potential in development of LEDs with high performance. As a result, white LEDs have been achieved with such optical glasses [27,28]. In addition to the potential application in the area of LEDs, optical glasses embedded with semiconductor nanocrystals have long been viewed as intriguing optical gain media to achieve solid-state lasers and optical fiber amplifiers [30]. Thus, optical glasses embedded with PQDs are anticipated to serve as potential optical gain media. Nevertheless, it remains a challenge to achieve a high density of PQDs in optical glasses in order to attain sufficient gain for light amplification at room temperature despite of the lasing-like behavior observed at cryogenic temperature [29].
Here, we report on a one-step in-situ nano-crystallization synthetic route to generate CsPbBr3 PQDs embedded in a glass matrix, we entitle such nanocomposites “CsPbBr3 PQD nanoglass”. The as-prepared CsPbBr3 PQD nanoglass exhibited a high stability ‒ no PL degradation was observed when the sample was exposed in ambient conditions over two months. In comparison to previous reports, such CsPbBr3 PQD nanoglass exhibits a much lower photoluminescence quantum yield (PLQY) of ∼0.3%. However, they demonstrate intriguing stimulated emission properties comparable to or beyond traditional colloidal CsPbBr3 PQDs with a much higher PLQY when they are subject to either a single-photon or a two-photon pump (referred to as SPP and TPP, respectively, for simplicity of discussion hereafter) scheme. We also demonstrate a novel wavelength-dependent behavior of the pump threshold, i.e., the pump threshold upon the SPP scheme shows a remarkable decrease when the pump wavelength moves away from the peak wavelength of the photoluminescence excitation (PLE) spectrum. A pump threshold as low as 1.04 mJ/cm2 was achieved via the TPP scheme. The intriguing stimulated emission properties demonstrated in the CsPbBr3 PQD nanoglass are explained in terms of cooperative effects of light scattering on both of the pump beam and the emitted light.
2. Materials and experimental methods
The CsPbBr3 PQD nanoglass was fabricated by a high-temperature vapor drop-casting technique, as illustrated in Fig. 1(a). Analytical-grade chemicals of B2O3, SiO2, ZnO, Al2O3, Cs2CO3, PbBr2, and NaBr powders were chosen as the starting materials. They were mixed thoroughly in an agate mortar and loaded into an alumina crucible. The crucible was then capped, put in an electrical furnace, and heated up to 1050 °C at a heating rate of ∼6 °C min−1. The crucible was kept at 1050 °C for 20 min, leading to vapor phases accumulated inside the alumina crucible. The vapor phases were drop-casted onto a stainless-steel plate, forming fragments of the CsPbBr3 PQD nanoglass.
Fig. 1. (a) Schematics of the fabrication process for the CsPbBr3 PQD nanoglass. (b) Configuration of the resultant sample comprising of a glassy matrix embedded with nanocrystals of PQDs. (c) Digital photograph of the CsPbBr3 PQD nanoglass sample.
X-ray diffraction (XRD) patterns of the CsPbBr3 PQD nanoglass were measured on a Bruker D8 advanced X-ray diffractometer equipped with graphite-monochromatized Cu Kα radiation (λ = 1.5418 Å). Surface morphologies of the CsPbBr3 PQD nanoglass were examined with a scanning electron microscope (SEM, Carl Zeiss Gemini SEM500). Transmission electron microscope (TEM) and high-resolution TEM (HR-TEM) images were recorded on a JEM 2100 TEM setup with an accelerating voltage of 200 KV. The chemical components of the samples were analyzed by X-ray photoelectron spectroscopy (XPS, ESCALAB Xi+) using a focused monochromatized Al Kα radiation (hν = 1486.6 eV).
The diffuse-reflectance spectrum was recorded on a UV–Vis–NIR spectrophotometer (UV-3600, SHIMADZU). PL spectra were measured with a fluorescence spectrophotometer (Fluorolog-3, HORIBA JOVIN YVON INC.). The PLQY of the CsPbBr3 PQD nanoglass was evaluated with an absolute method using Varian FLR025 spectrometer equipped with an integrating sphere. In lasing experiments, the pump source is an optical parametric generator (OPG, EKSPLA, PG-401, 420–800 nm) pumped by the third-harmonic output of a picosecond Nd: YLF laser (EKSPLA, PL-2250, 355 nm, 30 ps, and 1 Hz). The pump laser was focused onto the front surface of the sample by a focus lens (focal length: f = 4 cm) to form an 88 µm-diametered circular spot. The PL signals were collected by a focus lens (f = 4 cm) onto the entrance slit of a monochromator (Princeton Instruments, SP-2500) which is equipped with a CCD (Princeton Instruments, PIXIS 256).
3. Results and discussions
The sample obtained via the high-temperature vapor drop-casting approach appears as fragments that are composed of a glassy matrix embedded with nanocrystals, as illustrated in Fig. 1(b). From the photograph of the sample shown in Fig. 1(c), the nanoglass fragment looks rather opaque, suggesting a strong capability to scatter light.
XPS characterizations were performed to quantitatively analyze the chemical components of the drop-casted product, as shown in Fig. 2. One can clearly identify the existence of Cs, Pb and Br elements. In addition, the existence of Zn, Na, and O could be well-distinguished as well, which forms the nanoglass phase with the chemical composition of Zn4Na10O9. The formation of CsPbBr3 nanocrystals inside the glassy matrix should be related to a self-crystallization process that occurs during the quenching process [31]. It should be noted that the chemical elements of Si and Al introduced in the starting materials are rarely found in the drop-casted PQD nanoglass. Instead, they are residual at the bottom of the crucible to constitute another type of glassy phase. Although this glassy phase is out of the scope of this work, we would like to mention that CsPbB3 PQDs could be precipitated inside the glassy matrix when a post-annealing process was applied in line with that adopted in [26–29]. However, we are unable to achieve stimulated emission from this glassy phase, which will be discussed later.
Fig. 2. XPS analyses of (a) Cs 3d; (b) Pb 4f; (c) Br 3d; and (d) the survey spectrum of the CsPbBr3 PQD nanoglass.
From surface-viewed SEM image presented in Fig. 3(a), one can clearly see the existence of nano-fractures with dimensions of ∼100 nm. The TEM image reveals that there are a large amount of nanocrystals dispersed in the glass (Fig. 3(b)). The HR-TEM image of an individual nanocrystal reveals an inter-planar distance of ∼2.85 Å that corresponds to the (220) lattice plane of CsPbBr3 with a monoclinic phase (inset of Fig. 3(b)) [27]. The average size of the nanocrystals is determined to be ∼3.2 nm from the size distribution (Fig. 3(c)). As shown in the XRD patterns (Fig. 3(d)), the majority of the diffraction peaks, as marked with solid squares located at 2θ = 15.2, 21.5, 30.7, 37.7, and 43.7°, are well consistent with the standard patterns of (100), (110), (200), (−121) and (202) lattice planes of the monoclinic CsPbBr3 (PDF#: 18-0364). In addition, there is a trace amount of other patterns that can be ascribed to Cs4PbBr6 (PDF#: 73-2478), as marked with solid triangles in Fig. 3(d). The crystalline structures of CsPbBr3 and Cs4PbBr6 are illustrated in Fig. 3(e) and 3(f), respectively. Apparently, the crystalline structures of CsPbBr3 and Cs4PbBr6 are based on corner-shared and isolated PbBr64− octahedra, respectively.
Fig. 3. (a) SEM and (b) TEM images of the CsPbBr3 PQD nanoglass, respectively. The red circles in (a) indicate some nano-fractures. The inset in (b) is a HR-TEM image of the nanocrystal marked in the white square. (c) Histogram for the size distribution of the CsPbBr3 PQDs. (d) Experimentally measured XRD patterns (top) of the CsPbBr3 PQD nanoglass as well as standard XRD patterns of CsPbBr3 (middle, PDF#: 18-0364) and Cs4PbBr6 (bottom, PDF#: 73-2478). The patterns marked with solid squares are ascribed to those of CsPbBr3 (PDF#: 18-0364), while the patterns marked with solid triangles are assigned to those of Cs4PbBr6 (PDF#: 73-2478). (e, f) Crystalline structures of CsPbBr3 (e) and Cs4PbBr6 (f).
The diffuse-reflectance spectrum was examined to evaluate the optical band gap (Eg) of the CsPbBr3 PQD nanoglass. The optical absorption coefficient (α), derived from the diffuse-reflectance spectrum, is related to Eg through the formula developed by Pankove:
Here, A is a constant, h is Plank constant, ν represents the optical frequency, and m defines whether the optical transition is direct (m = 0.5) or indirect (m = 2) [32]. For optical glasses with indirect transitions, we adopt m = 2 according to Tauc equation [33]. Thus, we could evaluate the Eg of the CsPbBr3 PQD nanoglass to be ∼2.21 eV from the (αhν)1/2 ‒ hν plot at the zero absorption location, as shown in Fig. 4(a). The value of Eg of the CsPbBr3 PQD nanoglass is comparable to that of CsPbBr3 PQDs. Figure 4(b) displays the PLE and PL spectra, respectively. The PL spectrum is dominated with a peak centered at ∼520 nm along with a shoulder at the longer-wavelength side. To better illustrate the spectral feature of the PL spectrum, we decompose the PL spectrum into two Gaussian-profiled components that are centered at ∼520 (Peak 1) and 530 nm (Peak 2), respectively. Peak 1 has a narrower spectral linewidth of ∼13.3 nm, while Peak 2 is stronger with a much broader linewidth of ∼34.6 nm. The PLE spectrum of Peak 1 is composed of two peaks with comparable magnitude that are centered at ∼344 and 360 nm, respectively. In comparison to Peak 1, the PLE spectrum of Peak 2 is composed of two PLE bands as well, but the longer-wavelength component is more dominant over the shorter-wavelength one. Two-dimensional PLE-PL mapping indicates that the spectral position and profile of the PL band are loosely dependent on the PLE wavelength (320–400 nm) (Fig. 4(c)). The loose dependence of the spectral position and profile of the PL band suggests that the PL arises from intrinsic excitonic transitions [29], which benefits in achieving stimulated emission. Figure 4(d) presents the time-resolved PL spectrum of the CsPbBr3 PQD nanoglass. The results show that the decay of the PL at ∼520 nm significantly deviates from the single-exponential curve but could be fitted with tri-exponential decay components as depicted by Eq. (2),Fig. 4. (a) (αhν)1/2 ‒ hν plot derived from the linear optical absorption spectrum of the CsPbBr3 PQD nanoglass (black line) and the corresponding tangential line (red line) utilized to determine the bandgap of the nanoglass. (b) PLE spectra obtained by monitoring the PL peaks at 520 (black) and 530 nm (gray), as well as the PL spectrum (red circles) recorded upon excitation at 360 nm for the CsPbBr3 PQD nanoglass. The PL spectrum is decomposed into two components centered at ∼520 (solid red line marked with 1) and ∼530 nm (solid green line marked with 2) in line with Gaussian functions, respectively. The merged spectrum (solid blue line) of the above two components is shown as well in comparison to the measured spectrum (red circles). (c) Two-dimensional PLE-PL mapping of the CsPbBr3 PQD nanoglass. (d) Time-resolved PL spectra of the PL peak centered at ∼520 (discrete black circles) and 530 nm (discrete red circles) from the CsPbBr3 PQD nanoglass as well as the fit curves (black & red lines), respectively.
Here, I0, A1, A2, and A3 are constants. From the fitting curve, the values of τ1, τ2, and τ3 could be extracted to be 3.06, 0.42, and 36.05 ns, respectively. The tri-exponential decay behavior from lead-based halide PQDs has ever been reported [34]. The short (3.06, 0.42 ns) and long (36.05 ns) components might originate from bound and free excitons, respectively [35]. In contrast, the decay of the PL at ∼530 nm could be described with a bi-exponential curve and the fitted lifetime components are 1.32 and 9.19 ns, respectively (Eq. (3)).
The large difference in the spectral profile and decay behavior between Peak 1 and Peak 2 suggests that the two PL peaks originate from different light-emitting centers. Saidaminov et al. has ever reported on similar observations of a dual-peak PL band in mixed solids of CsPbBr3 and Cs4PbBr6, where the short- and long-wavelength components were ascribed to Cs4PbBr6 and CsPbBr3, respectively [36]. Given that Cs4PbBr6 and CsPbBr3 coexist in the nanoglass as identified in XRD patterns (Fig. 3(d)), we suppose that Peak 1 and Peak 2 might be ascribed to Cs4PbBr6 and CsPbBr3 as well, respectively.The PL stability of PQDs in ambient conditions is an important factor for practical applications. We have measured the PL stability of our developed nanoglass and compared with that of bare CsPbBr3 PQDs. As depicted in Fig. 5(a) and 5(b), the PL spectrum of the CsPbBr3 PQD nanoglass remains almost unchanged when exposed in ambient conditions for 2 months, while the bare CsPbBr3 PQDs exhibit rapid degradation within one week (Fig. 5(c) and 5(d)). The rapid PL degradation is common to CsPbBr3 PQDs which are rather sensitive to moisture and light [37]. The results suggest that the developed nanoglass exhibits excellent PL stability.
Fig. 5. (a) PL spectra of the CsPbBr3 PQD nanoglass recorded at the beginning (black) and after 60 days (red). (b) Plot of the PL peak intensity as a function of the recording time for the CsPbBr3 PQD nanoglass. (c) PL spectra of pure CsPbBr3 quantum dot film fabricated via spin-coating commercial quantum dots (Nanjing MKNANO Tech. Co., Ltd., Nanjing). The spectra were recorded at the beginning (black) and after 5 days (red). (d) Plot of the PL peak intensity as a function of the recording time for the CsPbBr3 quantum dot film. During measurements, all the samples were exposed in ambient conditions with humidity of ∼24%. The PLQY of the commercial quantum dots is ∼80%.
The optical absorption cross section (σabs) of semiconductor quantum dots is at the scale of 10−14 cm2, which is ∼2 orders that of dye molecules, and ∼6 orders that of rare-earth ions [38,39]. Therefore, semiconductor quantum dots are very promising media to achieve intense light sources based on stimulated emission. We examined the potential of the CsPbBr3 PQD nanoglass in achieving stimulated emission. Figure 6(a) illustrates the experimental excitation-detection scheme on investigation of stimulated emission properties. Figure 6(b) shows the evolution of the PL spectral profile with the pump fluence under optical pumping at 490 nm. The PL spectrum of the CsPbBr3 PQD nanoglass recorded at low pump fluences appears as a broad band, which is centered at ∼555 nm with the spectral linewidth of ∼30 nm, and such emission is characteristic of spontaneous emission (SE). When the pump fluence is increased up to 0.51 mJ/cm2, a narrow peak centered at ∼544 nm began to emerge on the blue side of the broad SE band. With further increment of the pump fluence, the PL intensity of the new peak increases more rapidly than the initial SE band. In accordance with previous reports, the pump fluence under which the new narrow peak began to emerge is defined as the pump threshold [15,16]. The pump threshold behavior can be further identified in the dependence of the PL intensity on the pump fluence (Fig. 6(c)), where one can find a critical value of ∼0.51 mJ/cm2 above which the PL signal increases nonlinearly and more rapidly. Thus, we can determine that stimulated emission occurred in the CsPbBr3 PQD nanoglass. The spectral linewidth of the stimulated emission is ∼4 nm when the pump energy is beyond the pump threshold. Thus the stimulated emission can be ascribed to amplified spontaneous emission (ASE) instead of lasing (typically with spectral linewidth less than 1 nm). The number of photons absorbed to achieve stimulated emission could be derived from the following formula:
Here, I, P, and N represent the PL intensity, the pump power below the pump threshold, and the number of photons involved in the excitation [40]. Take the logarithmic form, Eq. (4) can be written as:Fig. 6. (a) Schematics of the excitation-detection configuration. The sample is pumped by picosecond pulses emitted from an OPG driven by the third harmonic output of a Nd: YLF laser (λ = 355 nm). The PL signals were collected and analyzed with a monochromator equipped with a CCD. (b) Evolution of the PL spectral profile with the pump fluence for the CsPbBr3 PQD nanoglass under excitation at 490 nm. (c) Plots of the PL intensity (solid squares) as a function of the pump fluence below and above the threshold, showing the transition from SE to ASE at ∼0.51 mJ/cm2. The red solid lines are for convenience of eye vision, showing that the PL intensity starts to increase more rapidly when the pump energy is above 0.51 mJ/cm2. (d) Log-log plot of integrated PL intensity as a function of the pump fluence for the CsPbBr3 PQD nanoglass pumped below the pump threshold. The solid circles correspond to experimental data while the solid line is a linear fit with a slope of 0.97.
Thus, the value of N corresponds to the slope of the logI ‒ logP plot (Fig. 6(d)). We can thus confirm that the PLE is a single-photon absorption process (N = 1). We would like to mention that the ASE peak should originate from Peak 2 exhibited in the PL spectrum (Fig. 4(b)) based on two facts. Firstly, as shown in the PL spectrum, Peak 2 is dominant over Peak 1. Secondly, the wavelength of Peak 2 is closer to that of ASE. In other words, the CsPbBr3 phase should mainly contribute to light amplification, which is reasonable since the CsPbBr3 phase is dominant in the nanoglass (Fig. 3(d)).
Up to date, stimulated emission properties of CsPbBr3 PQDs have been widely studied [15,16,41]. The material configuration adopted thus far for stimulated emission is usually in the form of thin films fabricated by spin coating or drop coating of PQDs. The optical feedback for light amplification was typically introduced by exciting the sample with a striped pump beam which acts as a waveguide structure. The pump threshold reported thus far from such sample configurations ranges from µJ/cm2 to mJ/cm2, and the lowest pump threshold is ∼5 µJ/cm2 [15]. In contrast to previous reports, the CsPbBr3 PQD nanoglass was pumped through a circular spot configuration which is usually supposed to provide much weaker optical feedback for light amplification when compared to the striped pump scheme. Nevertheless, the pump threshold of ∼0.51 mJ/cm2 is still located at a moderate level of those ever reported so far. It is also noteworthy to mention that the PLQY of the CsPbBr3 PQD nanoglass is ∼0.3%, which is ∼2 orders of magnitude lower than those typically employed to achieve ASE. The large contrast between the moderate level of the pump threshold and a rather low magnitude of PLQY suggests that an intense optical feedback for light amplification exists inherently in the CsPbBr3 PQD nanoglass.
In comparison to SPP, a multi-photon pump scheme merits in a large penetration depth to attain a large gain volume, a high spatial resolution to better identify the targets, and loose damage to tissues [42,43]. To examine the potential of the CsPbBr3 PQD nanoglass in achieving stimulated emission upon a multi-photon pump scheme, we firstly investigated PL properties by pumping the sample at ∼800 nm. Similar to the tendency of the above-described SPP scheme, when the pump fluence is increased up to a critical value of ∼1.04 mJ/cm2, the onset of stimulated emission in the CsPbBr3 PQD nanoglass was observed at ∼544 nm, accompanied by a more rapid increase in the PL intensity with further increasing the pump fluence (Fig. 7(a)). The peak wavelength of the newly emerged PL band under pumping at 800 nm is consistent with that of the SPP scheme. The dependence of the PL intensity on the pump fluence shows that the emission intensity increases more rapidly when the pump energy is beyond 1.04 mJ/cm2. The nonlinear increase of the emission intensity with the pump energy suggests the occurrence of stimulated emission. Note, the change in the slope below and above the threshold in the TPP scheme is not as obvious as that shown in the SPP scheme (Fig. 6(c)). To provide further evidence that stimulated emission occurs at ∼1.04 mJ/cm2, the newly emerged peak at ∼544 nm was derived from the broad SE background through decomposition in line with the Lorenz functions [38]. Consequently, the spectral linewidth (full width at half maximum, FWHM) of the PL spectra as a function of the pump fluence could be plotted, as shown in Fig. 7(b). The FWHM of the newly emerged peak (centered at ∼544 nm) is determined to be ∼9.1 nm at 1.04 mJ/cm2, approximately half that of the SE band. Thus, the dependence of FWHM on the pump fluence provides strong evidence that stimulated emission occurs at ∼1.04 mJ/cm2. The spectral linewidth of the PL spectrum obtained under high pump fluences is ∼5 nm, thus the obtained stimulated emission can be ascribed to ASE as well. To find the number of photons absorbed by the CsPbBr3 PQD nanoglass to obtain ASE when pumped at 800 nm, we examined the pump-dependent PL properties when the sample was pumped below the pump threshold at 800 nm. Figure 7(c) depicts the PL spectrum recorded at different energies below the threshold, from which the dependence of the PL intensity on the pump fluence could be extracted. As shown in Fig. 7(d), the slope of the logI ‒ logP plot is determined to be ∼1.97, which is close to 2. Thus, the pump at ∼800 nm is a two-photon absorption process.
Fig. 7. (a) Evolution of the PL spectrum with the pump fluence for the CsPbBr3 PQD nanoglass. (b) Plots of the PL intensity (solid triangles) and FWHM (solid squares) as a function of the pump fluence under excitation at 800 nm below and above the pump threshold, showing the transition from SE to ASE at ∼1.04 mJ/cm2. The arrow indicates the threshold. (c) PL spectra recorded below the pump threshold. (d) Log-log plot of the integrated PL intensity as a function of the pump fluence for the CsPbBr3 PQD nanoglass pumped below the threshold. The solid circles correspond to experimental data while the solid line is a linear fit with a slope of 1.97.
From Figs. 6 and 7, we have noticed a fundamentally interesting phenomenon that ASE occurred at the blue side of the SE peak from the CsPbBr3 PQD nanoglass for both of the SPP and TPP schemes. This is in contrast to previous reports that ASE usually occurred at a longer wavelength side of SE for CsPbBr3 PQDs [4,15,16]. We consider that the location of ASE with respect to SE is related to the type of optical feedback responsible for stimulated emission. In the sample configuration of thin films studied before, the optical feedback was offered by the waveguide effect along the pump stripe, where the strength of the optical feedback is loosely wavelength-dependent. In that case, the re-absorption of light emission is considered to be the primary factor that modulates the spectral position of ASE with respect to SE. In contrast, the blue shift of ASE with respect to SE observed in our sample suggests a distinct optical feedback mechanism which is strongly dependent on the wavelength, which will be discussed in detail later.
We have measured the ASE stability for the SPP and TPP schemes performed at 490 and 800 nm, respectively. As shown in Fig. 8(a), the ASE spectral profile remains almost unchanged when the sample was pumped at 490 nm for 300 min (corresponding to 1.8 × 105 laser pulses with a pump fluence of 0.7 mJ/cm2). Similarly, the ASE signal shows a very slight degradation (< 5% reduction in the peak intensity) when the sample was pumped at 800 nm for 300 min (pump influence: 1.7 mJ/cm2), as shown in Fig. 8(b). It is reasonable that the sample will exhibit degradation with the increase of the pump energy. The above results indicate the high stability of ASE from the CsPbBr3 PQD nanoglass.
Fig. 8. (a) Plot of the ASE peak intensity as a function of the irradiation time when the sample was pumped at 490 nm. The pump fluence is 0.7 mJ/cm2. (a) Plot of the ASE peak intensity as a function of the irradiation time when the sample was pumped at 800 mm. The pump fluence is 1.7 mJ/cm2.
The wavelength-dependent stimulated emission properties have been examined systematically with both of SPP and TPP schemes. In the SPP regime shown in Fig. 9(a), our experiments indicate that stimulated emission cannot be obtained for the pump wavelength below 420 nm, which is dramatically different from existing reports where the laser output at 400 nm is usually used to pump PQDs to achieve stimulated emission [11,12]. The available pulse energy below 420 nm from our pump source is up to ∼5 µJ. As shown in Fig. 9(a), the pump threshold monotonically decreases from 6.19 to 0.39 mJ/cm2 when the pump wavelength increases from 420 to 520 nm. The experimentally observed tendency of the wavelength-dependent pump threshold is distinct from the information indicated in the PLE spectrum. In accordance with the PLE spectrum, it is expected to attain an optimal pump efficiency at the peak wavelength position of the PLE spectrum. Subsequently, the pump efficiency should decrease monotonically when the pump laser shifts to the longer wavelength regime with respect to the PLE maximum. In the PLE spectrum, the signal intensity at 500 nm is ∼50 times lower than that at the PLE peak (360 nm). Therefore, the pump efficiency at ∼500 nm is generally expected to be much lower than that at ∼360 nm, which is distinct from the experimental observations in Fig. 9(a). On the other hand, in the TPP regime, there exists a lowest pump threshold of ∼1.04 mJ/cm2 at the pump wavelength of 800 nm, as depicted in Fig. 9(b).
Fig. 9. Dependence of the pump threshold on the pump wavelength for the SPP (a) and TPP (b) schemes, respectively.
It is essential to understand the underlying mechanism responsible for stimulated emission occurring in the CsPbBr3 PQD nanoglass in order to interpret the above-mentioned stimulated emission properties and eventually engineer light amplification of PQDs. The ever-reported studies on stimulated emission from CsPbBr3 PQDs are mostly focused on a waveguide structure where the thin film of CsPbBr3 PQDs was pumped with a striped pump beam. A waveguide structure is formed along the pump stripe, along which light propagates and gains optical feedback for light amplification via back-and-forth reflection between two end faces of the pump stripe. In case of the CsPbBr3 PQD nanoglass, we have found that the sample has a rough surface decorated with fractures having dimensions of ∼100 nm that is related to the prompt quenching of the vapor phase, so a circularly-spotted pump configuration was adopted to pump the sample. Thus, the waveguide effect responsible for light amplification can be excluded. Instead, the opaque appearance of the sample suggests a high degree of local disorder which is naturally expected to modify the propagating direction of light. As disclosed previously, the optical path of the emitted light could be prolongated by light scattering before escaping the sample, consequently resulting in light amplification of the emitted light [44–46]. The optical feedback provided by light scattering is believed to contribute to the low-threshold ASE observed in this work. In previous reports, the role of light scattering on the emitted light has received much attention, while the role of the light scattering on the pump beam has been ignored. It has been reported that the coupling of the pump light with the scattering medium could significantly affect the pump threshold [47]. We believe that the effect of light scattering on the pump laser should be taken into account in order to interpret the aforementioned wavelength-dependent behavior of the pump threshold revealed in Fig. 9, given that the nanoglass show obvious nano-fractures on the surface. The optical processes involved in light amplification are depicted and schemed in Fig. 10. When the pump laser hits the nanoglass, it will be split into two portions. That is, a portion of the pump beam will be scattered off the nanoglass by the nano-fractured surface, as depicted by Process 1 in Fig. 10. Since light scattering shows a strong dependence on the wavelength [48], we suppose that the scattering of the pump laser determines the wavelength-dependent behavior shown in Fig. 9, which will be further discussed below. In addition to the portion scattered off the sample, the rest portion of the pump laser will excite the excitons of the CsPbBr3 PQDs to the upper level, consequently giving rise to light emission, as depicted in Process 2 in Fig. 10. The emitted light signals then randomly walk in the active medium and gain amplification due to the optical feedback provided by light scattering, as depicted in Process 3 in Fig. 10.
Fig. 10. Illustration of light-matter interactions involved in light amplification of the CsPbBr3 PQD nanoglass upon excitation with laser pulses. The pump laser experiences two processes: (1) get scattered off the sample and (2) excite the excitons to the upper level to trigger light emission. The emitted light randomly walks in the disordered medium so that it gains optical feedback due to prolongated optical path (3).
To further qualitatively understand how light scattering affects the pump threshold, we have performed calculations based on the Mie theory [49]. The simulations were performed in Mieplot v4.6 which is a non-commercial computer program and proven efficient to compute light scattering behaviors [46]. Figure 11 depicts the two-dimensional angular-wavelength mapping of the scattering power. In the calculation, we have set the radius of the nano-fracture to be R = 50 nm corresponding to the size of the fractures identified in the SEM image (Fig. 3(a)), the refractive index of the nano-fracture to be n = 1.6-1.9, the surrounding medium as air (n = 1), and the incident light to be an un-polarized beam. According to Fig. 11(a)–11(d), one can see an obvious wavelength dependence of light scattering. That is, the scattering at the shorter wavelength is stronger than that at the longer wavelength side. In the above setting, we have considered the uncertainty in the refractive index of the CsPbBr3 PQD nanoglass, since the refractive index of the nano-fracture cannot be determined at this moment. Our additional calculations suggest that the above-mentioned principle based on light scattering works as well when the size and the refractive index of the scattering particle is varied. Therefore, when the sample is pumped at the shorter wavelength, the pump light is experiencing more scattering losses to free space. This could presumably account for the observation that the pump threshold monotonically decreases with the pump wavelength for the SPP scheme. On the other hand, due to stronger optical feedback offered by stronger light scattering at the shorter wavelength portion of the emitted light, it is natural to understand the phenomenon that ASE occurs at the blue side of SE (Fig. 6(b) and Fig. 7(a)). The occurrence of ASE at the blue side of SE in scattering media has been reported previously, which was ascribed to the stronger feedback at the shorter wavelength region as well [50].
Fig. 11. Two-dimensional angular-wavelength mapping of the scattering power for a nanoparticle with R = 50 nm. The refractive index of the nanoparticle is set as n = 1.6 (a), 1.7 (b), 1.8 (c), and 1.9 (d), respectively. The surrounding medium is set as air (n = 1), and the incident light is set as un-polarized.
In comparison to SPP, the TPP scheme is generally of a lower pump efficiency to attain stimulated emissions from all-inorganic lead-based halide PQDs. The low pump efficiency arises from the fact that PQDs generally own a small magnitude of two-photon σabs at the scale of 105 GM (1 GM = 10−50 cm4 s). In attempts to achieving two-photon excited ASE, a striped pump configuration is generally adopted to promote optical feedback for stimulated emission. In the first report on two-photon excited ASE, the pump threshold is ∼2.4 mJ/cm2 for CsPbBr3 PQDs with PLQY ∼90% and σabs ∼1.2 × 105 GM [51]. Later, a lower pump threshold of ∼0.8 mJ/cm2 was reported by Xiao et al. by using CsPbBr3 PQDs with an even higher σabs ∼2.7 × 106 GM [52]. To the best of our knowledge, this is the lowest threshold in two-photon excited ASE ever reported from CsPbBr3 PQDs thin-film waveguides. The CsPbBr3 PQDs utilized above generally have a very high PLQY (> 80%) and a thin-film waveguide structure was employed to provide intense optical feedback. The thin films exposed to air generally suffers from a poor chemical stability. To overcome the stability issue, two-photon excited ASE from air-stable surface-passivated PQDs has been reported as well, but the threshold is as high as ∼11 mJ/cm2 [53]. In comparison to material platforms investigated thus far, the CsPbBr3 PQD nanoglass is of the merits in both of high stability and low threshold of ASE. As depicted in Fig. 9(b), the two-photon excited ASE from the CsPbBr3 PQD nanoglass has a lowest threshold of ∼1.04 mJ/cm2 when the sample is excited at 800 nm, which is slightly higher than the lowest value ever-reported (∼0.8 mJ/cm2), but significantly lower than those in other reports. The slightly higher threshold exhibited by the CsPbBr3 PQD nanoglass is considered to originate from at least but not limited to the following factors. First, the CsPbBr3 PQD nanoglass has a PLQY of ∼0.3% that is remarkably lower than those reported previously, which might affect the pump efficiency. On the other hand, we notice that the low PLQY could be caused by the existence of dark excitonic states which can be turned into bright states upon intense pumping [15]. This phenomenon suggests that the low PQLY obtained upon weak light excitation is not a key factor to affect the pump threshold. Second, the CsPbBr3 PQD nanoglass has a lower effective optical gain volume since the PQDs are embedded in a glassy matrix. Third, the optical feedback might not be as strong as that offered in the waveguide structure. Fourth, femtosecond laser pulses were usually utilized as the pump source in previous reports, and shorter laser pulses benefits in occurrence of ASE with lower threshold [54,55]. The pump threshold is expected to be remarkably reduced by improving the quality of the sample in accordance with the afore-mentioned aspects, which will be our future research endeavor. In case of the TPP scheme, our observation of the lowest pump threshold at ∼800 nm is consistent with that recently reported on lasing from CsPbBr3 nanorods [56]. We surmise that the magnitude of two-photon σabs might play a vital role in determining the pump threshold, as reported in [56].
We have fabricated conventional CsPbBr3 PQD glasses by using the two-step post-annealing process and examined their preliminary light amplification behavior. Herein, CsPbBr3 PQD borosilicate glasses, as reported in [26], were examined due to their high chemical stability and high PLQY (∼45%). The CsPbBr3 PQD borosilicate glass exhibits intense PL signals upon excitation with UV light. However, ASE cannot be observed in such samples even when the pump energy is rather high. We have attempted to increase the density of PQDs in those optical glasses by increasing the annealing temperature with an attempt to increase the amount of optical gain. Nevertheless, the resultant samples were still unable to produce ASE. The results suggest that the CsPbBr3 PQD nanoglass developed via the high-temperature vapor-casting technique in this work provides a strong capability to enable intense optical feedback for light amplification.
4. Conclusions
To conclude, we have developed a one-step in-situ nano-crystallization process to obtain air-stable CsPbBr3 PQD nanoglass as a new form of the perovskite family of materials for optoelectronic applications. Both of linear and nonlinear optical properties of the CsPbBr3 PQD nanoglass have been investigated to achieve stimulated emission. The dependence of the pump threshold on the pump wavelength exhibits a remarkably declined tendency when driving the pump beam to a longer wavelength for the single-photon pump scheme. The results suggest that the developed material is compatible with various pump sources, in particular neodymium ion-based commercial lasers. A pump threshold as low as ∼1.04 mJ/cm2 was achieved for the two-photon pump scheme. Cooperative light scattering effects of the pump light and the emitted light arising from local disorders were proposed to account for the observed phenomena. The findings suggest that the developed perovskite nanoglass holds potential in creating highly efficient light-emitting devices.
Funding
National Natural Science Foundation of China (NSFC) (11774188); Incubation Program of Universities' Preponderant Discipline of Shandong Province (03010304); Mountain Tai Young Scholarship (23170504); Excellent Youth Foundation of Shandong's Natural Scientific Committee (JQ201802).
Acknowledgements
We acknowledge the funding support listed above.
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