Light–Matter Interactions in Cesium Lead Halide ... - ACS Publications

Letter pubs.acs.org/JPCL

Light−Matter Interactions in Cesium Lead Halide Perovskite Nanowire Lasers Kidong Park,†,§ Jong Woon Lee,‡,§ Jun Dong Kim,† Noh Soo Han,‡ Dong Myung Jang,† Seonghyun Jeong,‡ Jeunghee Park,*,† and Jae Kyu Song*,‡ †

Department of Chemistry, Korea University, Jochiwon 339-700, Korea Department of Chemistry, Kyung Hee University, Seoul 130-701, Korea

‡

S Supporting Information *

ABSTRACT: Light−matter interactions in inorganic perovskite nanolasers are investigated using single-crystalline cesium lead halide (CsPbX3, X = Cl, Br, and I) nanowires synthesized by the chemical vapor transport method. The perovskite nanowires exhibit a uniform growth direction, smooth surfaces, straight end facets, and homogeneous composition distributions. Lasing occurs in the perovskite nanowires at low thresholds (3 μJ/cm2) with high quality factors (Q = 1200−1400) under ambient atmospheric environments. The wavelengths of the nanowire lasers are tunable by controlling the stoichiometry of the halide, allowing the lasing of the inorganic perovskite nanowires from blue to red. The unusual spacing of the Fabry−Pérot modes suggests strong light−matter interactions in the reduced mode volume of the nanowires, while the polarization of the lasing indicates that the Fabry−Pérot modes belong to the same fundamental transverse mode. The dispersion curve of the exciton−polariton model suggests that the group refractive index of the polariton is significantly enhanced.

H

cation with an inorganic one, such as cesium (CsPbX3), while preserving the lead-halide-perovskite-like structure, which shows long-term thermal and hydrolysis stability without losing superior optical and electronic properties of the hybrid halide perovskites.17,22,25,26 In addition, the use of fully inorganic halide perovskites as laser gain media is supported by the demonstration of amplified spontaneous emission as well as whispering-gallery-mode lasing in the microcrystals and Fabry− Pérot-mode lasing in the nanowires.15,17,22,25,26 Despite the potential of perovskite nanowires for optoelectronic applications, the light−matter interactions in these nanowires are not yet well understood. The nature of waveguiding in nanowires is different from that in bulk cavities because the diameter of the waveguide is smaller than or comparable to the wavelength of light.27−30 The light−matter interactions are expected to be strong in nanowires because the extent of coupling between the photon (light) and the exciton (matter) is strengthened by the decrease in the mode volume and the high oscillator strength in nanowires. Accordingly, electromagnetic waves in nanowires exhibit different characteristics from classical waves in the bulk, which has been explained by the exciton−polariton model.27−30 In the present work, we report low-threshold, high-stability lasing in single-crystalline CsPbX3 (X = Cl, Br, and I) perovskite nanowires, which were synthesized by the vapor transport method. The nanowires were prepared using three

ybrid organic−inorganic halide perovskites have been studied extensively because of their unique properties, such as high optical absorption coefficients, direct band gaps, and long carrier-diffusion lengths.1−7 In particular, hybrid halide perovskite thin films improve the performance of solar cells, which have reached an efficiency of 20.1%.8 Furthermore, the optical and electrical properties of hybrid halide perovskites make them suitable for important optoelectronic devices, such as lasers.9−26 The possibility of these applications is also found in the reduced dimensionality, wherein a sufficiently large gain is observed in two-dimensional crystals with whispering-gallerymode lasing.11,16 An optical gain is also observed in onedimensional nanowires, which can be easily incorporated into the miniaturized circuits of optoelectronic devices as light sources.13,14 Nanowires can act as the gain medium as well as the natural cavity at subwavelength dimensions, where the confinement effect maintains the standing-wave modes of light and thus leads to novel optical properties. Indeed, low lasing thresholds and high quality factors have been reported for hybrid lead halide perovskite nanowires, which also exhibit easy wavelength tunability through the substitution of the halide.13,14 Therefore, perovskite nanowires are expected to play an important role as the building blocks for nanoscale optoelectronic devices. However, hybrid lead halide perovskites, such as CH3NH3PbX3 (X = Cl, Br, and I), exhibit limited stability due to thermal degradation and hydrolysis by atmospheric water.13,20 Their stability can be improved by substituting the methylammonium cation with a less reactive cation such as formamidinium.20 Another approach is to substitute the organic © XXXX American Chemical Society

Received: August 12, 2016 Accepted: September 5, 2016

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DOI: 10.1021/acs.jpclett.6b01821 J. Phys. Chem. Lett. 2016, 7, 3703−3710

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Figure 1. (a) SEM images of vertically aligned CsPbBr3 nanowires with a rectangular cross section grown on a substrate. (b) HRTEM and FFT images of a CsPbBr3 nanowire along the [001]C zone axis, showing its single-crystalline nature and the [100]C growth direction. (c) HRTEM images and SAED pattern along the [001]C zone axis of a single-crystalline CsPbCl3 nanowire grown along the [100]C direction. (d) HRTEM and FFT images along the [001]C zone axis of a CsPbI3 nanowire grown along the [100]C direction. HAADF STEM images and EDX line-scanned elemental mapping data showing the composition of the nanowire. The undefined scale bar represents 100 nm.

average length of 10 μm. The cross sections of the nanowires were mostly rectangular, while the diagonal length of the rectangular cross-section (diameter) was 200−500 nm with the average value of 300 nm. High-resolution transmission electron microscopy (HRTEM), the corresponding fast-Fourier-transform (FFT) images, and selected-area electron diffraction (SAED) patterns along the [001]C zone axis revealed that the CsPbCl3, CsPbBr3, and CsPbI3 nanowires consisted of defectfree single-crystalline cubic or pseudocubic phases (Figure 1b− d). Remarkably, all three types of CsPbX3 nanowires had the same growth direction ([100]C). In addition, they exhibited straight, smooth surfaces and end facets without any amorphous outer layers. The d-spacing between the neighboring (100) planes (d100) was determined to be 5.6, 5.8, and 6.4 Å for X = Cl, Br, and I, respectively. High-angle annular dark field scanning transmission electron microscopy (HAADF STEM) images and energy-dispersive X-ray fluorescence spectroscopy (EDX) mappings of the CsPbX3 nanowires confirmed that the nanowires had homogeneous compositions over the entire lengths. The compositions were calculated using the Cs L-shell, Pb L-shell, and Br (Cl, I) K-shell, which indicated [Cs]/[Pb]/ [X] = 1:1:3. The CsPbX3 perovskite nanowires were transferred onto glass substrates by a simple dry contact process to investigate the optical properties of single nanowires under ambient conditions. The diameter of the 355 nm excitation pulses was about 50 μm at the substrate, which was much larger than the length of the nanowires to ensure the homogeneous excitation of the single nanowire (Figure S3a). In the photoluminescence spectrum of a single CsPbBr3 nanowire at a low excitation intensity (0.1 μJ/cm2), a broad peak centered at 525 nm was observed with a full-width at half-maximum (fwhm) of 20 nm (Figure 2a). This energy was similar to the exciton emission energy in the bulk, implying that the quantum confinement effect was not significant in the nanowires15,31,32 because of much larger dimensions than the Bohr radius of CsPbBr3 (3.5

different compositions of halides, which showed that the band gap was successfully tuned over a wide range. The lasing was observed in the nanowires at low thresholds with high quality factors, which was indicative of the high absorbance and excellent crystal quality of the nanowires. The spectral spacing of the Fabry−Pérot lasing modes suggested the occurrence of the strong light−matter interactions in the nanowires, which determined the polaritonic dispersion curve and might explain the lasing phenomena reported previously as well as those observed in this study. The nanowire synthesis and characterization procedures are described in the Supporting Information. In brief, the CsPbX3 perovskite nanowires were grown on a large substrate with an area of 1 cm2. The X-ray diffraction (XRD) patterns of the nanowires (Supporting Information, Figure S1) revealed the presence of an orthorhombic phase CsPbBr3 (a = 8.245 Å, b = 11. 735 Å, and c = 8.198 Å), a tetragonal phase CsPbCl3 (JCPDS 18-0366; a = 5.605 Å, c = 5.623 Å), and a cubic phase CsPbI3 (a = 6.348 Å). Notice that the cell parameters a and c of the orthorhombic unit cell of CsPbBr3 are directed along the diagonals of the faces of the elementary cube (a = 5.8 Å), while parameter b is close to twice the side of the cubic unit cell (Figure S2). A simple index correlation linked the orthorhombic and cubic unit cells, i.e., the [101] direction of the orthorhombic unit cell was equivalent to the [100] direction of the cubic unit cell. Therefore, the crystal structure of orthorhombic phase CsPbBr3 was analyzed using the pseudocubic unit cell with a = 5.8 Å. For CsPbCl3, the cell parameters a and c of the tetragonal unit cell were quite similar, which could be approximated as an elementary cube with a = 5.6 Å. Therefore, the lattices of all three types of CsPbX3 nanowires could be indexed using the cubic unit cell with the subscript “C”. Figure 1a shows scanning electron microscopy (SEM) images of vertically aligned CsPbBr3 nanowires grown on Si substrates. The length of the nanowires was 2−15 μm with an 3704

DOI: 10.1021/acs.jpclett.6b01821 J. Phys. Chem. Lett. 2016, 7, 3703−3710

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To expand the lasing wavelengths of inorganic perovskite nanowires, as in hybrid halide perovskite nanowires, the optical properties of the CsPbCl3 and CsPbI3 nanowires were also investigated. The photoluminescence of single CsPbCl3 nanowire with rectangular facets was centered at 420 nm with a fwhm of 12 nm (Figure 3a). With an increase in the excitation

Figure 2. (a) Emission spectra of a single CsPbBr3 nanowire. The inset shows a superlinear increase in the emission intensity at the lasing wavelength (530 nm) above the lasing threshold (3 μJ/cm2) and a sublinear increase at the nonlasing wavelength (525 nm). (b) Timeresolved emission profiles of the single CsPbBr3 nanowire. The inset shows the time-resolved emission profiles for the lasing and nonlasing wavelengths at an excitation intensity of 5 μJ/cm2.

Figure 3. (a) Wavelength tunability of the inorganic perovskite nanowires by the substitution of the halide. (b) Emission spectra of a single CsPbCl3 nanowire. The inset shows a superlinear increase in the emission intensity at the lasing wavelength above the lasing threshold (7 μJ/cm2). (c) Wavelength tunability of the inorganic perovskite nanowire lasers. The inset shows the images of the emissions of the inorganic perovskite nanowire lasers. The bright lasing emission from the end facets of the CsPbI3 nanowire cannot be observed with clarity because of the limited color range of the optical camera. (d) Emission spectra of a single CsPbI3 nanowire. The inset shows a superlinear increase in the emission intensity at the lasing wavelength above the lasing threshold (6 μJ/cm2).

nm).33,34 To examine the photonic confinement effect in the nanowires, the photoluminescence spectra were obtained with increasing excitation intensities. A sharp peak with a fwhm of 0.4 nm appeared at 530 nm above an excitation intensity of 3 μJ/cm2 (Figure 2a), which grew more rapidly than the spontaneous emission (inset of Figure 2a). The log-scale plot of the emission intensity indicated the S-shaped curve as a function of the excitation intensity, which was attributed to the transition from spontaneous to stimulated emission processes (Figure S3b). Thus, the sharp peak indicated the lasing in the nanowire, while this excitation intensity was similar to the threshold value (Pth) for amplified spontaneous emission in CsPbBr3 thin films and lasing in CsPbBr3 nanowires (5 μJ/ cm2).15,17,22 In rectangular CsPbBr3 nanowires with a diameter of 300 nm, most of the electromagnetic field (∼90%) was confined within the resonator and was amplified by the well-defined pathways inside the built-in Fabry−Pérot resonator.35,36 As a result, the quality factor of lasing in the CsPbBr3 nanowires (Q = λ/Δλ = ∼1300) was much higher than that of amplified spontaneous emission in CsPbBr3 thin films (Q = ∼100).15,17 Indeed, comparable quality factors have been observed at the lasing in CsPbBr3 nanowires,22 confirming that the observed sharp peak was the lasing in the nanowires. Furthermore, the quality factor of the lasing was comparable to that in closely related hybrid halide perovskite (CH3NH3PbBr3) nanowires.13,14 It should be noted that the lasing of the CsPbBr3 nanowires was observed under ambient atmospheric conditions at room temperature, whereas lasing of the CH3NH3PbBr3 nanowires was observed in dry nitrogen conditions presumably because of easy degradation of the nanowires by moisture.13,17

intensity, narrow lasing peaks were observed at 420−430 nm with FWHMs of 0.3 nm (Figure 3b), which demonstrated the blue regime of inorganic perovskite nanowire lasers (Figure 3c). Although the quality factor of the lasing of the CsPbCl3 (Q = ∼1400) nanowires was comparable to that of CsPbBr 3 nanowires, Pth of the CsPbCl3 nanowires (7 μJ/cm2) was higher than that of the CsPbBr3 nanowires. Nevertheless, Pth of our CsPbCl3 nanowires was 1 order of magnitude lower than that reported for CsPbCl3 nanowires (∼86 μJ/cm2).22 Lasing in the red regime was also observed in the CsPbI3 nanowires (Figure 3d). The emission spectrum of single CsPbI3 nanowire showed sharp peaks at 720−730 nm with FWHMs of 0.6 nm (Q = ∼1200) at an excitation intensity of 6 μJ/cm2. This lasing from the CsPbI3 nanowire completed the lasing range of inorganic perovskite nanowires from blue to red (Figure 3c). Lasing was also observed in a very short nanowire (2.4 μm) as well as a long nanowire (14.7 μm) at Pth of 3−14 μJ/cm2 (Figure S4). However, the relationship was not clearly found between the nanowire length and Pth,13,22 because the amplification efficiency in the nanowire resonators depended on several factors, such as the reflectivity of the end facets as well as the nanowire length, width, and crystal quality.13,37,38 Although the primary determining factors need to be investigated further, the low threshold was attributable to the characteristics of the CsPbBr3 nanowires. First, the high crystal quality of the nanowires, which also had a low defect density, 3705

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inorganic halide perovskites. On the other hand, the decay rate of lasing in the nanowires became similar for excitation intensities higher than Pth (Figure S6), which suggested that the lasing process was faster than the competing decay processes in the inorganic perovskite nanowires and the nonradiative loss of carriers did not affect the lasing process significantly. Multiple lasing modes in the single nanowire were assigned to Fabry−Pérot-type modes (Figure 4d), because the spatial interference of the coherent emission from the two end facets was observed in the emission image at an excitation intensity higher than Pth (Figure 4c), whereas the intensity of the emission was uniform over the entire nanowire at an excitation intensity lower than Pth (Figure 4b). The length of the lasing nanowire (L) was estimated using the classical cavity model, Δv

reduced the lasing threshold. The lifetime of the exciton states was 7.2 ns at a low excitation intensity (Figure 2b), which was longer than the lifetime of the CsPbBr3 nanocrystals (3.6−4.5 ns),17,33 whose quantum yield was as high as 80% with the trivial decay to defect states. Thus, the excitation intensity needed to reach the minimum carrier density for lasing was lowered, because the loss of carriers by the nonradiative decay to the defect states was minimized in the high-crystal-quality nanowires. Second, the fast lasing process was responsible for the low threshold. With an increase in the excitation intensity, the decay rate of the exciton states increased because of multiparticle processes such as the Auger process.13,15,17 Above Pth, the decrease in the lifetime became more substantial to the instrumental response time of the system (50 ps),39 suggesting that the lasing process was faster than the Auger process. Indeed, for the same excitation intensity, the decay rate at the lasing wavelength was considerably higher than that at the nonlasing wavelength, where the spontaneous emission and the Auger process occurred (inset of Figure 2b). This difference in the decay profiles confirmed that the Auger recombination process did not disturb the lasing process significantly, because the lasing occurred quickly in a short period before the nonradiative loss of the carriers. Furthermore, the high absorbance led to the threshold carrier density at a low excitation intensity, which was a common property of perovskite materials. 9,13 To estimate the absorbance, the photoluminescence intensity was examined, which was proportional to the density of the excited states.15,17 The generation probability of the excited states (1 − P0) follows the Poisson distribution, P0 = exp(−σI/hv), where P0 is the density of the unexcited states, σ the absorption crosssection, I the excitation intensity, and hv the photon energy.17,40 The emission intensity was fitted with a σ of 1.2 × 10−13 cm2 (Figure S5), which was much larger than the σ of CdSe quantum dots,41 because of the intrinsically high absorbance of halide perovskites. Indeed, the σ of CsPbBr3 was estimated to be 0.25−1.23 × 10−13 cm2 in the cube-shaped nanocrystals with the mean sizes of 9−10 nm,15,17,42 which indicated the high absorbance of the CsPbBr3. In addition, a comparable value of σ was reported in the CH3NH3PbBr3 nanowires (1.8 × 10−13 cm2),9 which supported the high absorbance of halide perovskites. On the other hand, the relationship between the dimension of the nanowires and σ was not clearly found, which suggested that the absorbance of halide perovskites was not mainly affected by the dimension of the nanowires, although the primary factors need to be investigated further. Therefore, the high crystal quality, fast lasing process, and high absorbance were responsible for the low lasing threshold of the CsPbBr3 nanowires. The Pth of the CsPbCl3 and CsPbI3 nanowires was higher than that of the CsPbBr3 nanowires, which might be related to the crystal qualities of the nanowires. Indeed, the lifetimes of the CsPbCl3 (1.1 ns) and CsPbI3 (3.7 ns) nanowires were shorter than that of the CsPbBr3 (7.2 ns) nanowires at an excitation intensity of 0.1 μJ/cm2 (Figure S6). These results indicated that the long lifetimes of the nanowires (CsPbBr3 > CsPbI3 > CsPbCl3) were responsible for the low Pth (CsPbBr3 < CsPbI3 < CsPbCl3), which confirmed that the crystal quality of the nanowires was at play in Pth. In addition, the absorption cross sections of the CsPbCl3 and CsPbI3 nanowires were slightly lower than that of the CsPbBr3 nanowires (Figure S5), which were also responsible for the higher Pth of the CsPbCl3 and CsPbI3 nanowires, despite the superior absorbance of

Figure 4. (a) Optical image of a single CsPbBr3 nanowire with a length of 6.2 μm. Emission images at excitation intensities (b) below and (c) above the lasing threshold with the scale bar of 5 μm. (d) The lasing emission spectra of a single nanowire with increasing excitation intensity. (e) The lasing emission spectrum at a high excitation intensity (left) matches the dispersion curve in the energy−wavevector diagram (center) of the exciton−polariton model with the reduced oscillator strength, compared to the lasing emission spectrum at a low excitation intensity (right). 3706

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The Journal of Physical Chemistry Letters = c/2nL, where Δv is the difference in the frequencies of the Fabry−Pérot modes, c the speed of light in vacuum, and n the refractive index of the cavity material (n = 2.3 for CsPbBr3).15 From the mode spacing (Δv = 1.7 THz), the cavity length was calculated to be 38 μm, which was much longer than the length of the nanowire length of 6.2 μm (Figure 4a). Even when it was assumed that total internal reflection occurred in the waveguide,11,15,16 the travel length was only as long as 14 μm for a 6.2-μm nanowire, which was still shorter than the calculated cavity length. Indeed, the mode spacing of the Fabry−Pérot modes in all the lasing CsPbBr3 nanowires could not account for the cavity lengths, even though the mode spacing changed linearly with the inverse of the nanowire length (Figure S7).13,22 Upon close examination, it was found that the mode spacings of the Fabry−Pérot modes were not identical. The energy difference between the modes in the low-energy regime was 7.0 meV (Δv = 1.7 THz), which was not the same as the 6.3 meV (Δv = 1.5 THz) between the modes in the high-energy regime (Figure 4d). This nonclassical and nonidentical spacing of the lasing modes suggested that the general photonic model was inadequate for explaining the Fabry−Pérot modes in the CsPbBr3 nanowires, which could be alternatively attributed to the strong light−matter interactions in the nanowires.27−30 At present, it remains unclear whether the emission of CsPbBr3 originates from excitons or free carriers,31−34 while this question also occurs regarding the hybrid halide perovskite materials.10,11 The free carriers (electron−hole pairs) are generated by excitation above the band gap of CsPbBr3, which are efficiently thermalized through the carrier−carrier and carrier−phonon scattering in a short time. This thermal equilibrium leads to the formation of exciton from free carriers, while the formation rate depends on the carrier density. Accordingly, the fraction of excitons is predominant over that of free carriers at a high carrier density, such as lasing conditions, because of the quadratic dependence of exciton formation on the carrier density,43 although the formation of excitons is in equilibrium with the dissociation to free carriers. Furthermore, the exciton binding energy of CsPbBr3 (35−40 meV) is larger than the room-temperature thermal energy (26 meV),32,33 which implies the viable exciton states against free carriers at room temperature, even though the exciton binding energy should be carefully compared to the thermal energy.43 Indeed, the exciton gain process was recently suggested as a possible explanation in the CsPbBr3 nanocrystals at the high carrier density,33 while a biexcitonic process was also reported.17 Therefore, it was expected that exciton states would be maintainable in nanowires at the high carrier density above the lasing threshold. Consequently, the strong exciton−photon coupling is achievable in the nanowires because the light−matter interactions depend on the oscillator strength and mode volume.44 In other words, the exciton−polariton would be formed in the CsPbBr3 nanowires by the strong exciton− photon coupling due to the high oscillator strength and the reduced mode volume, as found in the ZnO and CdS nanowires.27−30 In the exciton−polariton model, the energy of the polariton is described by E(ω , k) = ℏω = ℏck / ε(ω) where ω is the angular frequency and ε(ω) is the dielectric function of the medium (see the Supporting Information for details).28,29 Thus, the wave traveling in the nanowires is affected by ε(ω), which is not a constant but changes with the

frequency. As a result, the dispersion curve of the traveling wave deviates from the classical models, when the energy of the polariton is near the resonance energy (Figure S8). The polariton model could explain the abnormal spectral spacing between the polariton eigenmodes, because the dispersion curve became flattened with approaching the exciton energy (Figure 4e). In other words, the cavity length was estimated to be 6.2 μm from the Fabry−Pérot modes, when the group refractive index of the confined light was assumed to be n = 14−17. This was 6−7 times larger than the refractive index of CsPbBr3 (n = 2.3), which was in good agreement with the enhancement observed in the CdS nanowires in the exciton− polariton regime.29,30 With increasing excitation intensity, the lasing peaks were slightly blue-shifted (Figure 4d), which has been attributed to several factors such as the band filling and thermally induced change of band gap.13,15,35 In addition, the oscillator strength was responsible for the shift,27 because the lasing peaks at the high excitation intensity (9 μJ/cm2) matched the dispersion curve obtained with the reduced oscillator strength (Figure 4e), compared to the dispersion curve at the low excitation intensity (5 μJ/cm2). At high carrier densities, the exciton binding energy was reduced by carrier screening,45 which altered the character of exciton and weakened the oscillator strength enhancement in nanowires.27−30 Therefore, the peak shift was also related to the characteristics of the exciton, which weakened the exciton−photon coupling at high carrier densities and thus changed the group index. The polariton eigenmodes in other lasing CsPbBr3 nanowires were also explained by the flattened dispersion curve (Figure S9). In addition, the mode spacings in all lasing CsPbBr3 nanowires were fitted with n = 17 (Figure S7), which confirmed the enhanced group index in the nanowires. Likewise, the Fabry−Pérot modes in the CsPbCl3 and CsPbI3 nanowires were related to the strong light−matter interactions with the 6−7-fold increase in the group refractive index (Figure S9), as in the CsPbBr3 nanowires. It is possible that the unusual spectral spacing between the Fabry−Pér ot modes was caused by the simultaneous appearance of two transverse modes,11,13 instead of the light−matter interactions. Hence, the lasing emission was observed as a function of the detection angle in order to distinguish the transverse modes (see the Supporting Information for details), where the mode spacing was not classical (Figure 5a). When the electric field parallel (Ex) to the substrate (x−z) plane was separated from the perpendicular one (Ey) by the detection angle (Figure S10),35,45 the polarization of the lasing modes exhibited a predominant Ex component with minor contribution of the Ey component (Figure 5b). Indeed, the fundamental mode in waveguides with rectangular cross-section was simulated to be transverseelectric-like with a dominant Ex component.13 Quantitatively, the degree of polarization (DOP) is described by (I Ex − I Ey)/(I Ex + I Ey) where IEx and IEy are the intensities determined by the polarizer when aligned to detect the Ex and Ey components, respectively. The DOP of each mode was similarly fitted using a value of ∼0.95 (Figure S11), which supported the transverse-electric-like modes in the waveguides. In addition, the similar DOP indicated that the observed modes belonged to the identical transverse mode, i.e., the fundamental waveguide mode with the lowest gain threshold. Therefore, the nonclassical spectral spacing could be attributed to the strong 3707

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or pseudocubic phase with a uniform [100]C growth direction and straight surfaces. The compositions of the nanowires were homogeneous over the entire lengths with [Cs]/[Pb]/[X] = 1:1:3. Lasing was observed at low thresholds with high quality factors even under ambient atmospheric conditions, which was attributable to the high crystal quality, the fast lasing process, and the high absorbance of the nanowires. The wavelengths of the nanowire lasers could be tuned by halide substitution, which completed the lasing color range of the inorganic perovskite materials from blue in the CsPbCl3 nanowires to green in the CsPbBr 3 nanowires to red in the CsPbI3 nanowires. The nonclassical and nonidentical spacing of the Fabry−Pérot modes could be explained using the exciton− polariton model, which implied that strong light−matter interactions occurred in the confined volume of the nanowires. The dispersion curve of the polariton indicated that the group refractive index of the polariton was several times enhanced because of the strong light−matter interactions. The polarization of the lasing suggested the presence of transverseelectric-like modes, which confirmed that the Fabry−Pérot modes belonged to the identical fundamental transverse mode. The method proposed in this study is advantageous for fabricating inorganic halide perovskite nanowire lasers that exhibit good crystal qualities, low thresholds, and strong light− matter interactions.

Figure 5. (a) Collected intensities of the lasing emission with increasing degree of detection polarization. The polarization angle of the parallel electric field (Ex) with respect to the substrate (x−z) plane is denoted to be 0°. The inset shows the definition of the coordinates. (b) DOPs of the individual Fabry−Pérot modes (A, B, and C). The inset compares the experimental data (circles) with the ideal polarization ratio of the Ex mode (line).

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpclett.6b01821. Experimental details, exciton−polariton model, degree of polarization, and Figures S1−S12 (PDF)

light−matter interactions in the nanowires, rather than the simultaneous appearance of different transverse modes. It has been reported that inorganic perovskite nanomaterials exhibit long-term stabilities to preserve the superior optical properties.17,22 Our CsPbBr3 nanowires were also stable against hydrolysis under ambient conditions, because lasing was observed at a slightly higher Pth after the nanowires had been aged for several months at ambient temperature and humidity. Furthermore, the nanowires remained thermally stable even when subjected to pulsed excitation at a high repetition rate (1 × 105 Hz), because the lasing intensities remained at 90% of the initial values until 3.6 × 108 excitation (60 min) at 1.2 Pth under atmospheric conditions (Figure S12). Such duration was in good agreement with that reported recently for lasing in CsPbBr3 nanowires (4.4 × 108 excitation),22 which lasted longer than the amplified spontaneous emission in CsPbBr3 (1.6 × 107 excitation) and the whispering-gallery-mode lasing in CH3NH3PbI3 (8.6 × 106 excitation).12,17 On the other hand, the intensity of lasing in the CsPbCl3 and CsPbI3 nanowires decreased in 5 × 107 excitation (Figure S12), suggesting that the thermal stabilities of the CsPbCl3 and the CsPbI3 nanowires were lower than that of the CsPbBr3 nanowires. The thermal stabilities were attributed to the crystal quality of the CsPbCl3 and CsPbI3 nanowires, as the lasing thresholds and lifetimes of the CsPbCl3 and CsPbI3 nanowires were. Moreover, the thermal stabilities were not identical in the examined lasing nanowires, which implied that the thermal stabilities depended on the crystal quality of the nanowires. In summary, the inorganic perovskite nanowires of CsPbCl3, CsPbBr3, and CsPbI3 were synthesized by the vapor transport method. The nanowires consisted of a single-crystalline cubic

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Author Contributions §

K.P. and J.W.L. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study was supported by NRF (2015R1A2A2A01002805, 2014R1A1A3051259, 2009-0082580) and the Functional Districts of the Science Belt support program, Ministry of Science, ICT and Future Planning (2015K000287). The HVEM (Daejeon) measurement was performed at the KBSI. The experiments at PLS were partially supported by MOST and POSTECH.

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REFERENCES

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