1. Introduction

The perovskite has attracted massive attention because of the advantages of high absorption coefficient, long carrier lifetime, high quantum yield, and stoichiometric wavelength tunability as well as its simple synthesis and low-cost technology recently [1–7]. Therefore, it is widely researched and used in solar cells, light emitting diodes (LEDs), nanolasers, photoelectric detectors etc [1–7]. The classical inorganic-organic hybrid perovskite (CH₃NH₃PbX₃, X = I^-, Br^-, Cl^-) will easily decompose when exposed under the light illumination or atmosphere vapor, which severely limit their further application [8–13]. To improve the stability, all inorganic cesium lead halide CsPbX₃ (X = Cl, Br, I) perovskites were developed [14]. All inorganic CsPbX₃ perovskites also possessed excellent photoluminescence quantum yields (PLQY) of ~90%, tunable optical bandgaps, long carrier lifetimes, and narrow full-width-at-half-maxima (FWHM) of the PL peak (~20 nm) [14]. Up to now, the nano-scale inorganic perovskites, such as quantum dots, nanoplates (NPs) and nanowires (NWs), have been utilized as nanolasers, which exhibit great potential applications in integrated optical systems [6,7,15–19].

Ion exchange reaction has been proved to be a simple and versatile way to achieve fine-grained control over the composition of substances and to create new materials and nanostructures [18]. Recently, ion exchange has become an effective way to rapidly regulate the halogen composition of perovskites [18,20,21]. Due to the high mobility of halogen ions and the strict sublattice nature of perovskite cations, the fast anion exchange reaction has been realized for example in CsPbX₃ quantum dots [21,22].

Significantly, self-assembly is an effective method to prepare NWs of organic materials. However, it is rarely used for the preparation of perovskite NWs. In this paper, we propose a one-drop self-assembly method to prepare cesium lead bromide perovskite NWs, by which only very small amounts of perovskite and solvent are required. In order to expand the emission band of the NWs, an ion exchange method is adapted to convert CsPbBr₃ NWs into CsPbCl₃, CsPbI₃, and hybrid versions of these. The obtained NWs demonstrate excellent optical performance and robust stability. 420-560 nm lasing is confirmed individually by use of femtosecond (fs) laser pumping.

2. Synthesis and characterization of perovskite NWs

2.1 One-drop self-assembly CsPbBr₃ NWs

The one-drop self-assembly of CsPbBr₃ NWs can be divided into two steps. Firstly, the CsPbBr₃ nano/microcrystals (NCs/MCs) were dispersed on a hydrophilic substrate by a substrate-supported rapid evaporation crystallization (SSREC) method [23]. The used CsPbBr₃ perovskite NCs/MCs were prepared by the method described in the previous work [14]. One drop (about 35 μL) of the perovskite suspension with concentration of 0.244 mM in toluene was dropped onto a 130 °C hydrophilic glass substrate and baked for 10 s to evaporate the solvent. Secondly, the substrate was then quickly transferred to a petri dish and sealed with 5 drops of methanol to create a saturated methanol atmosphere. After 8 days cultivation, self-assembled NWs were obtained. Under saturated methanol vapour at room temperature, the CsPbBr₃ NCs/MCs underwent anisotropic self-assembly, generating high-quality CsPbBr₃ NWs. The fluorescent images (Zeiss Scope.A1, RETIGA R6) were shown in Fig. 1(a)-1(d), which depicted the evolution process of the perovskite NWs. As shown in Fig. 1(a), the perovskite NCs/MCs were randomly and uniformly distributed on the substrate by the rapid evaporation of the solvent. At this time, no NWs can be observed. Figure 1(b)-1(d) show photographs after 2, 4, and 8 days cultivation. From these, as the NCs/MCs density decreases, the randomly dispersed NCs/MCs self-assembled in an anisotropic manner, forming a configuration of NWs. The NWs continually grow until the depletion of the surrounding NCs/MCs. The typical length range is 20-60 μm, while some of them extend to as long as 200 μm, as the inset of Fig. 1(d). The formation mechanism of the NWs is similar to that outlined in our previous work [24]. Under saturated methanol vapour pressure, the randomly dispersed perovskite NCs/MCs will be partially dissolved, making the surface-wetted crystals easy to move and aggregate together on the glass substrate due to the capillary effect and lubrication effect of the solution. The aggregated NCs/MCs then undergo an anisotropic self-assembling process, finally resulting in fabrication of high-quality perovskite NWs.

 

Fig. 1 Self-assembly process of CsPbBr₃ NWs. (a) the randomly dispersed NCs/MCs on the glass substrate prepared by the substrate-supported rapid evaporation method; (b-d) the samples after 2, 4, and 8 days cultivation. Inset of (d) is a 200 μm long nanowire grown on the substrate. (All scale bars = 40 μm.)

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The morphology of the NWs was studied by scanning electron microscope (SEM, Hitachi S-4800), sputter-coated with 10 nm gold (Denton Vacuum Desk IV sputtering system), as shown in Fig. 2(a). From the SEM images, we can see that the well crystallized NWs have very smooth surfaces and neat edges (inset), which is essential for light confinement and propagation. The X-ray diffraction (XRD, Bruker AXS D8 Advance diffractometer with a Cu Kα source) patterns of the CsPbBr₃ NWs are shown in Fig. 2(b). The diffraction peaks clearly indicate that the CsPbBr₃ perovskite NW belongs to a orthorhombic crystal structure with a space group of Pnma(62), and lattice constants of a = 8.24 Å, b = 11.73 Å, and c = 8.19 Å. These values are very consistent with previously reported lattice constants [16,17,25–28]. In contrast with the standard XRD patterns of cubic (top panel) and orthorhombic (middle panel) CsPbBr₃, at 2θ ≈30.8°,the single peak splits into two distinct peaks (004 and 220 faces) for orthorhombic phase, which indicates that the as-grown CsPbBr₃ NWs (bottom panel) belong to orthorhombic crystal structure. The diffraction peaks are sharp and intense, further indicates the highly crystallized nature of the sample. The crystal structure of the NWs was further investigated by transmission electron microscopy (TEM, FEI Tecnai G2 F20). The selected area electron diffraction (SAED) pattern shown in Fig. 2(c) suggests that the NWs are single crystalline with a zone axis (ZA) of [001] for the orthorhombic phase, which is consistent with the XRD results. The lattice fringe profile observed in the high-resolution TEM image [Fig. 2(d)] indicates a d-spacing of ~5.8 Å.

 

Fig. 2 Crystal Characteristics of CsPbBr₃ NWs. (a) SEM image of CsPbBr₃ NW. (b) XRD pattern of as-grown CsPbBr₃ (black) with the standard XRD patterns of cubic (red) and orthorhombic (green) CsPbBr₃. (c) the SAED pattern of the NW. (scale bar = 5 nm⁻¹), the inset is low-resolution TEM image of NW. (scale bar = 500 nm), and (d) high-resolution TEM image of CsPbBr₃ NW with d-spacing of 5.8 Å. (scale bar = 3 nm)

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2.2 Ion-exchange for other CsPbX₃ NWs

In order to tune the emission band of the NWs, we used an ion exchange method to control the halogen component in the samples, whereby the fabricated CsPbBr₃ NWs were utilized as wire-templates. As an ion source, 0.13 mmol of PbX₂ (36.15 mg for PbCl₂ or 54.08 mg for PbI₂) and 5 mL 1-octadecene were loaded into a 20 mL flask, and dried at 120 °C for 1 h, then 0.5 mL oleic acid (OA) and 0.5 mL oleylamine (OAm) was injected to dissolve the PbI₂ salt (for PbCl₂ 1 mL OA, 1 mL OAm, along with 1 mL tri-n-octylphosphine were required and temperature increased to 150-170 °C), then cooled to room temperature after the PbX₂ dissolved completely. The prepared CsPbBr₃ NWs were completely immersed into the ion source face-up. After a certain time, the samples were taken out and washed by toluene three times to remove the residual ion source. Fluorescence images of the ion exchanged NWs were shown in Fig. 3(a)-3(d), and the corresponding PLs were shown in Fig. 3(e). Due to different halogen components of the NWs, the emitting band of the NWs can cover the whole visible light range. We studied the kinetics of the ion exchange in the perovskite through both the absorption spectrum and emission spectrum as a function of ion exchange time, taking CsPbBr₃ NW in I^- ion source as an example, shown in Fig. 3(f). The ion exchange process can be simply explained by Fick’s second law of diffusion: $\mathrm{x}=\sqrt{D_{eff}t}$, in which $\mathrm{x}$ is the diffusion depth, $D_{eff}$ is the effective diffusion coefficient and t is the diffusion time [29]. Based on this theory, a qualitative fitting for the cut-off wavelength as a function of time was plotted in Fig. 3(g). Here the cut-off wavelength, derived from the absorption properties, was utilized as a benchmark for diffusion depth. In the first 30 minutes, the rapid ion exchange triggers the absorption cut-off wavelength to markedly red-shift from 544 nm to 696 nm; then the rate of change slows with the cut-off wavelength, further shifting to 714 nm after another 30 minutes. Along with the bathochromic-shift of the absorption, the fluorescence spectrum peak also changes from 526 nm to 704 nm. The broadening of the emission band in the ion exchange process could be attributed to its hybrid nature.

 

Fig. 3 Ion exchange of CsPbBr₃. (a-d) PL images of CsPbCl_xBr_3-x (x = 0-3), CsPbBr_yI_3-y (y = 0-3) perovskite NWs prepared by an ion exchange method. (All scale bars = 20 μm); (e) The emission spectrum of the NWs obtained by the ion exchange method; (f) Absorption and corresponding PL spectrum of the CsPbBr₃ NWs as a function of ion exchanging time in PbI₂ ion source, the solid lines represent the absorptions of the ion exchanged NWs and the dashed lines show the corresponding fluorescent spectrum; (g) relationship between the cut-off wavelength and ion exchange time of the NWs, black squares represented the experiment data and the red solid line represented the fitted curve.

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3. Lasing performances of perovskite NWs

3.1 Lasing of CsPbBr₃ NWs

High-quality NWs/NRs can act as both gain medium and laser resonator, therefore are suitable for nanolaser applications [30–32]. The optical setup for the perovskite NWs lasing is shown in Fig. 4(d). A 400 nm pumping source was generated by a femtosecond (fs) pulse lase (800 nm, 500 Hz, 30-50 fs) via a BBO crystal. A graduated neutral-density filter was used to control the excitation intensity. The position of the convex lens was altered to adjust the size of the laser spot (2 mm) onto the sample to ensure that the whole NW was excited under illumination. Emission spectra from the NWs were collected by a fiber spectrometer with a resolution of 1.5 nm (Idea optics PG2000pro). The fluorescent images irradiated by mercury lamp and ultrafast laser were shown in Fig. 4 (a) and 4(b). When the excitation intensity surpasses the lasing threshold, stimulated radiation is generated, and we can observe high intensity spots at both ends of the NW [Fig. 4(b)]. As the excitation intensity increases, bright interference fringes can in addition be observed [Fig. 4(c)]. Figure 4(d) shows the relationship between the fluorescence spectra and excitation intensities; below the lasing threshold (P_th = 63.86 μJ/cm²), spontaneous emission dominates, with a FWHM of ~16 nm and peak centred at ~516 nm. The fluorescence intensity increases slowly with increasing excitation intensity. Above the lasing threshold, a few sharp lasing peaks appear, and the emission intensity increases quickly with the excitation intensity.

 

Fig. 4 Lasing in CsPbBr₃ NW. (a) Fluorescence image of CsPbBr₃ stimulated by mercury lamp at 365 nm; (b) Above lasing threshold, laser emits from both ends of CsPbBr₃ NW; (c) interference fringe of two laser emissions from CsPbBr₃ NW. (All scale bars = 20 μm); (d) Emission spectrum of CsPbBr₃ NW, inset is optical pump schematic diagram.

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The strongest peak value has red-shifts by 9 nm, stabilizing at 525 nm [Fig. 5(a)]. This red-shift is caused by the reabsorption of band-edge emission within the NW cavity [16]. As for the Quality (Q) factor of 701 is calculated by $\mathrm{Q}=\mathrm{\lambda }/\mathrm{\delta }\mathrm{\lambda }$, where λ is the peak wavelength at 525.49 nm and δλ is 0.75 nm, obtained by a Gaussian fitting for the strongest peak in Fig. 4 (d). Here, we have to note that the low resolution (1.5 nm) of the fiber spectrometer greatly restricts the measurement of the exact lasing performance, therefore the Q-factor is slightly inferior when compared with the other reported values [6,7,18,33,34]. The log-log scale light-light curve between the output intensities and excitation intensities can be fitted into an S-type curve as shown in Fig. 5(b). Three PL mechanisms can be seen clearly from this figure: firstly, below the excitation threshold, spontaneous emission dominates and the fluorescence intensity increases slowly; secondly, when the excitation intensity is above the threshold, amplified spontaneous emission takes over and the output intensity increases rapidly; finally, gain-pining and the emergence of lasing occurs.

 

Fig. 5 Characterization of laser from CsPbBr₃ NW. (a) Fluorescence spectrum peak positions as a function of excitation intensities, the position changes abruptly when the excitation intensity exceeds lasing threshold; (b) the relationship between output intensity and pump intensity; (c) mode spacing is proportional to the inverse of the nanowire length; (d) laser light emitted from NWs shows robust stability under the continuous illumination of the pump laser at 2.63 × P_th for 80 minutes, the laser signal intensity remains ~92%.

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CsPbBr₃ NWs with different lengths (4.8−27.3 μm) were investigated, and we found that their mode spacing Δν were proportional to the inverse of the lengths of the NWs ($∆\mathrm{\nu }=\mathrm{c}/2\mathrm{n}\mathrm{L}$, where Δν is the difference in the frequencies between two laser modes, c is the speed of light in vacuum, L is the length of NW, and n is the refraction index of the NW (n = 2.3 for CsPbBr₃) [Fig. 5(c)] [15]. This result further confirms that the NWs are in essence Fabry−Pérot cavities. The stability was tested at room temperature under an open air environment. After continuously pumping with a 400 nm laser at an excitation intensity of ~2.63 × P_th for 80 min, the laser signal intensity remained around 92% [Fig. 5(d)].

3.2 Lasing of CsPbCl₃, CsPbBr_xCl_3-x (x = 0-3) and CsPbBr_yI_3-y (y = 0-3) NWs

Lasing performances of the CsPbCl₃ NWs are shown in Fig. 6. From the experiment of Fig. 6(a), we confirmed that the CsPbCl₃ NW was successfully pumped by 400 nm laser with a threshold P_th of 68.2 μJ/cm². The threshold is a little higher than that of the CsPbBr₃ NWs, which could be attributed to the shorter fluorescent life time and lower absorption coefficient of CsPbCl₃ [17]. The laser has a Q factor of 456, which is lower than that for CsPbBr₃ NWs. A strong interference can be seen at both ends of the NW, as shown in Fig. 6(a) inset. Similar to CsPbBr₃ NW, the relationship between the integrated output intensity and the excitation intensity can be fitted with the S-type curve [Fig. 6(b)]. The stability of the CsPbCl₃ NWs was also tested by exposing the sample under the laser irradiation with an intensity of 3.1 × P_th at room temperature in an open atmosphere environment. The CsPbCl₃ NW also shows robust stability for 80 minutes (the laser signal intensity remains around 93%), Fig. 6(c). The hybrid NWs obtained by ion exchange method, such as CsPbBrxCl_3-x and CsPbBryI_3-y NWs can also be excited. As shown in Fig. 6(d), the laser peaks cover the range of 420-560 nm. Due to the instability and temporary fluorescence quenching, for NWs with more iodine portion, no lasing was observed.

 

Fig. 6 Lasing in CsPbCl₃ NW. (a) Emission spectrum of CsPbCl₃ NW produced using different excitation intensities. The inset is a laser image of the CsPbCl₃ NW. (scale bar = 10 μm); (b) the relationship between output intensity and excitation intensity; (c) Lasing stability of CsPbCl₃ NWs under the continuous pumping of 3.1 × P_th; (d) Lasing of CsPbBr_xCl_3-x and CsPbBr_yI_3-y NWs

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4. Conclusions

In summary, a one-drop self-assembly method was developed for the fabrication of perovskite NWs. With an extremely small amount of starting materials, high-quality CsPbBr₃ NWs were successfully prepared. By an ion exchange process, the CsPbBr₃ NWs can be further converted to other hybridized perovskite NWs covering the visible range (420-710 nm). The obtained NWs with various halide components achieved lasing in the range 420-560 nm. The thresholds for CsPbBr₃ and CsPbCl₃ NWs were 63.86 μJ/cm² and 68.2 μJ/cm² respectively and the Q factors were about 450-700. All the NWs retained robust lasing properties in ambient atmosphere at room temperature.