ARTICLES PUBLISHED ONLINE: 13 APRIL 2015 | DOI: 10.1038/NMAT4271
Lead halide perovskite nanowire lasers with low lasing thresholds and high quality factors Haiming Zhu1†, Yongping Fu2†, Fei Meng2, Xiaoxi Wu1, Zizhou Gong1, Qi Ding2, Martin V. Gustafsson1, M. Tuan Trinh1, Song Jin2* and X-Y. Zhu1* The remarkable performance of lead halide perovskites in solar cells can be attributed to the long carrier lifetimes and low non-radiative recombination rates, the same physical properties that are ideal for semiconductor lasers. Here, we show room-temperature and wavelength-tunable lasing from single-crystal lead halide perovskite nanowires with very low lasing thresholds (220 nJ cm−2 ) and high quality factors (Q ∼ 3,600). The lasing threshold corresponds to a charge carrier density as low as 1.5 × 1016 cm−3 . Kinetic analysis based on time-resolved fluorescence reveals little charge carrier trapping in these single-crystal nanowires and gives estimated lasing quantum yields approaching 100%. Such lasing performance, coupled with the facile solution growth of single-crystal nanowires and the broad stoichiometry-dependent tunability of emission colour, makes lead halide perovskites ideal materials for the development of nanophotonics, in parallel with the rapid development in photovoltaics from the same materials.
S
emiconductor nanowire (NW) lasers, owing to their ultracompact physical sizes, highly localized coherent output, and efficient waveguiding, are promising building blocks for fully integrated nanoscale photonic and optoelectronic devices1 . Each NW can serve as a waveguide along the axial direction and the two end facets form a Fabry–Perot cavity for optical amplification. Since the pioneering work of Yang and co-workers2 for ultraviolet lasing from ZnO NWs, optically pumped lasing has been demonstrated in a number of NWs in the ultraviolet to near-infrared regions3–11 . One of the main obstacles limiting potential applications of semiconductor NW lasers is the high threshold carrier density required for lasing. The high lasing threshold not only makes key technical advancement such as electrically driven lasing12 and integration into optoelectronic devices difficult, but also imposes fundamental limits due to the onset of Auger recombination losses7,8 . In searching for an ideal material for NW lasing, we turn to a new class of hybrid organic–inorganic semiconductors, methyl ammonium lead halide perovskites (CH3 NH3 PbX3 , X = I, Br, Cl), which is emerging as one of the most promising materials for solution-processable photovoltaic technology13–17 . The exceptional solar cell performance can be attributed to the long carrier lifetimes (101−2 ns) and diffusion lengths17–20 (µm). These properties, along with high fluorescence yield and wavelength tunability20–22 , also make lead halide perovskites ideal materials for lasing. Indeed, perovskite films and nanoplates have been shown to be efficient optical gain media20,22–24 , but lasing thresholds remained high in these reports, probably owing to the polycrystalline nature of the samples used. Here we show the growth of high-quality single-crystal NWs from low-temperature solution processing. We demonstrate room-temperature lasing in these NWs with: low lasing thresholds; high quality factors; near-unity quantum yield; and broad tunability covering the near-infrared to visible wavelength region. The first step towards a perovskite NW laser was the synthesis of high-quality single-crystal NWs. We develop a surface-initiated
solution growth strategy using a lead acetate (PbAc2 ) solid thin film deposited on glass substrate in contact with a high concentration of CH3 NH3 X (X = Cl, Br, or I) solution in isopropanol at room temperature (see Methods). As discussed in detail in Supplementary Figs 1–4, we suggest the following two-step process for the growth of single-crystal NWs and other nanostructures: PbAc2 (s) + 4I− (sol) → PbI4 2− (sol) + 2Ac− (sol) PbI4 2− (sol) + CH3 NH3 + (sol) → CH3 NH3 PbI3 (s) + I− (sol) We found that the key to successful nanostructure growth is the slow release of the low-concentration Pb precursor (PbI4 2− ) from the solid Pb(Ac)2 film on the substrate and the careful tuning of the CH3 NH3 X precursor concentration to maintain a low supersaturation condition for the crystal growth of perovskites25 (see Supplementary Information). Figure 1a,b shows optical and scanning electron microscopy (SEM) images of CH3 NH3 PbI3 NWs (and a few nanoplates) on a glass substrate after 24 h growth. The CH3 NH3 PbI3 NWs typically had lengths up to ∼20 µm, with flat rectangular end facets (Fig. 1d,e and Supplementary Fig. 5). The width of the rectangular cross-section varied from NW to NW and was typically a few hundred nanometres. The corresponding powder X-ray diffraction (PXRD; Fig. 1g, red line) pattern shows a set of strong diffraction peaks that can be assigned to a pure tetragonal CH3 NH3 PbI3 crystal structure (space group I 4/mcm, a = 8.896 Å, c = 12.707 Å, see Supplementary Fig. 6 for simulated PXRD; refs 26,27), without impurities (for example, PbI2 or PbAc2 ). The presence of the small (121) peak and the split of the (220) and (004) peaks confirm that the as-grown CH3 NH3 PbI3 is in the room-temperature tetragonal phase, rather than the higher-temperature cubic phase27 . The appearance of diffraction peaks from high-index lattice planes further suggests high crystalline quality. We also performed transmission electron microscope (TEM) analysis on single CH3 NH3 PbI3 NWs (Fig. 1c,f). The insets show the corresponding selected-area electron diffraction
1 Department of Chemistry, Columbia University, New York, New York 10027, USA. 2 Department of Chemistry, University of Wisconsin—Madison, Madison, Wisconsin 53706, USA. †These authors contributed equally to this work. *e-mail: [email protected]; [email protected]
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NATURE MATERIALS DOI: 10.1038/NMAT4271
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Figure 1 | Structural characterization of single-crystal CH3 NH3 PbX3 NWs. a,b, Optical (a) and SEM (b) images of CH3 NH3 PbI3 nanostructures grown from PbAc2 thin film in a 40 mg ml−1 CH3 NH3 I/isopropanol solution with a reaction time of 24 h. c, Low-resolution TEM image and its selected-area electron diffraction pattern along the [110] zone axis (ZA). d,e, Magnified SEM images of NWs (top view), showing a square or rectangular cross-section and flat end facets perpendicular to the long NW axis. f, High-resolution TEM image and its corresponding fast Fourier transform. g, PXRD patterns of as-grown CH3 NH3 PbX3 (X = I, Br, Cl) NWs, confirming the tetragonal phase (for X = I) and cubic phase (for X = Br, Cl) of the perovskites, without any impurity phases.
and fast Fourier transform patterns. The sharp diffraction spots are indexed to a tetragonal crystal structure with zone axes of [110] or [001] (which are identical directions of h100i in the pseudo-cubic lattice). Quantitative elemental analysis from energydispersive X-ray spectroscopy (EDX) on individual NWs yields an I/Pb ratio of ∼3, as expected from the CH3 NH3 PbI3 stoichiometry (Supplementary Fig. 7). All of these characterizations confirm highquality single-crystal CH3 NH3 PbI3 NWs with smooth end facets, making them ideal Fabry–Perot cavities for lasing. We also successfully synthesized single-crystal NWs of other halide perovskites, CH3 NH3 PbX3 (X = Br, Cl), by replacing CH3 NH3 I with CH3 NH3 Br or CH3 NH3 Cl (see Supplementary Information). The PXRD patterns (Fig. 1g and Supplementary Fig. 10) confirm that the as-grown NWs and nanoplates are the 2
¯ cubic phase CH3 NH3 PbX3 (space group Pm3m) without other impurities. EDX measurements on single NWs also confirm their stoichiometry (Supplementary Figs 8 and 9). It is noteworthy that, in some cases, we observed the formation of single-crystal lead halide perovskite nanotubes (see Supplementary Fig. 11). As template-free, catalyst-free and spontaneous formation of single-crystal hollow tubes is a signature of dislocation-driven crystal growth28 , this observation, together with the effectiveness of controlling the supersaturation to encourage the surface-initiated NW growth (see details in Supplementary Figs 1–4), strongly suggests that the catalyst-free anisotropic growth of these lead halide perovskite NWs is probably driven by screw dislocations25,28,29 . Unfortunately, the instability of these perovskite NWs under prolonged exposure to the electron beam in TEM prevented us from observing the
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NATURE MATERIALS DOI: 10.1038/NMAT4271
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dislocation diffraction contrast29 using TEM to unambiguously confirm this hypothesis30 . To carry out the lasing experiment, we transferred a small number of well-dispersed perovskite NWs from the growth surface to a Si/SiO2 substrate by a simple dry contact process. We performed optically pumped lasing measurements on a home-built far-field epi-fluorescence microscope at room temperature in a dry N2 atmosphere. A 402 nm pulsed laser beam, with the beam waist adjusted to be larger than the length of each NW, was used as a nearly uniform pump source (see Fig. 2a and Methods). Figure 2b shows a twodimensional (2D) pseudo-colour plot of the emission spectra as a function of pump laser fluence (P) for a CH3 NH3 PbI3 NW (8.5 µm length, Fig. 2d). Representative emission spectra near the lasing threshold are shown in Fig. 2c (see Supplementary Fig. 15 for the full spectral range). At low pump laser fluence (P < 600 nJ cm−2 ), each emission spectrum shows a broad peak centred at ∼777 nm with a full-width at half-maximum (FWHM) of δλ = 44 nm; this corresponds to spontaneous emission (SPE). At P ≥ 600 nJ cm−2 , a sharp peak at 787 nm appears and grows rapidly with increasing P, and the intensity of the broad SPE peak (non-lasing) remains almost constant (see Supplementary Fig. 15), indicating singlemode lasing operation. The inset in Fig. 2c shows the light-in– light-out (L–L) data and FWHM plot as a function of P. Fitting the
L–L plot to the expected S-curve model31 gives a lasing threshold of PTh ∼ 595 nJ cm−2 (Supplementary Fig. 15c). The FWHM plot shows a constant value below PTh and a sudden drop by more than two orders of magnitude at P ≥ PTh . Additional representative NW lasing and an L–L plot with a fit to the S-curve can be found in Supplementary Fig. 17b. Further confirming lasing operation, we note that the emission image of the NW below PTh shows uniform intensity from the whole NW (middle image in Fig. 2d) and that above PTh (right image in Fig. 2d) shows strong emission with spatial interference from the two coherent light sources at the two end facets32 . The bright emission localized at the two ends is consistent with a strong waveguiding effect and axial Fabry–Perot cavity modes, as confirmed later. The FWHM at P = 630 nJ cm−2 , at which power the lasing peak dominates, is 0.22 nm (Fig. 2c and Supplementary Fig. 15 for Gaussian fit). This gives a quality factor Q = λ/δλ ∼ 3,600, which is more than an order of magnitude higher than that from the state-of-the-art GaAs–AlGaAs core–shell NW laser operating at a temperature of 4 K (ref. 11). We note a small blueshift (≤0.5 nm) and broadening of the lasing peak with pump power. The blueshift with increasing carrier density has been observed before in NW lasers and could have multiple origins: thermally induced bandgap/refractive index change, band filling, optical density fluctuations, and electron/hole many-body interactions8,33 .
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NATURE MATERIALS DOI: 10.1038/NMAT4271
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Figure 3 | Visible lasing from CH3 NH3 PbBr3 NWs. a, 2D pseudo-colour plots of NW emission spectra as a function of pump fluences for CH3 NH3 PbBr3 NWs of different lengths: top 7.5 µm; middle 13.6 µm; and bottom 23.6 µm. Note the logarithmic colour scale for emission intensity. Inset: Emission images of NWs of different lengths above the lasing threshold (scale bars, 10 µm). b, TRPL decay kinetics after photoexcitation with fluence below (P ∼ 0.82PTh , blue) and above (P ∼ 1.13PTh , red) the threshold, showing a ∼2 ns spontaneous decay process below PTh and a ≤20 ps (instrument limited; see grey dashed curve for instrument response function) lasing process above PTh for the 7.5-µm-long NW. The dots are data points and solid lines are multi-exponential decay fitting. Inset: Emission spectrum above the threshold with a Gaussian fitting. The FWHM is ∼0.23 nm, corresponding to a Q factor of ∼2,400. c, Mode spacing of the lasing peaks as a function of reciprocal NW length (black triangles). The experimental data are well described by a linear function (green) with intercept at zero.
Of the CH3 NH3 PbI3 NWs examined (29 in total), more than 85% showed lasing, which confirms the quality of the single-crystal NWs from our room-temperature solution growth method. In addition to the single-mode lasing shown in Fig. 2, we also observed multiple lasing modes from some NWs (see Supplementary Fig. 17a). In principle, multiple longitudinal modes in a Fabry–Perot cavity are competitive with each other and the one with the highest gain will dominate, but inhomogeneous gain saturation caused by spatial hole burning or crystal/cavity inhomogeneity can sustain multiple lasing modes31,34 . The lasing threshold of 25 NW lasers studied varies between 220 nJ cm−2 and 600 nJ cm−2 . The NW lasing threshold depends on multiple factors, for example, dimensions, end facets and crystalline quality (Supplementary Fig. 17c)35 . The room-temperature lasing threshold values of our single-crystal CH3 NH3 PbI3 NWs are nearly two orders of magnitude lower than those reported recently for near-infrared lasing from lead halide perovskites in polycrystalline thin films or nanoplates20,22–24 and a whole family of III–V near-infrared NW lasers including those with specially engineered core–shell structures and operating at cryogenic temperatures (see Supplementary Table 1). On the basis of the absorption cross-section (∼5 × 10−12 m2 at 402 nm, see Supplementary Information), we calculate a threshold carrier density of ρTh = 1.5–4.5 × 1016 cm−3 for pump power densities of 220–600 nJ cm−2 . 4
Further insight into the remarkable performance of the single-crystal lead iodide perovskite NW laser comes from timeresolved photoluminescence (TRPL) measurements (Fig. 2e). The apparent SPE lifetime was as long as τSPE = 150 ns at low excitation densities (ρ = 1.5 × 1014 cm−3 , black dots and fit in Fig. 2e, see details in Supplementary Fig. 16). The apparent τSPE decreases to 5.5 ns at high excitation density (ρ = 0.85ρTh , blue dots and fit line in Fig. 2e). Kinetic analysis of the ρ-dependent photoluminescence decay dynamics allows us to estimate fluorescence quantum yields: the quantum yield increases from ∼60% at ρ = 1 × 1014 cm−3 to ∼87% at ρ = 1 × 1016 cm−3 (see Supplementary Information). We expect the photoluminescence quantum yield to further increase to near unity (∼100%), because the apparent stimulated emission time constant (τlasing ≤ 20 ps, instrument limited, Fig. 2e and Supplementary Fig. 18) is much shorter than τSPE . For comparison, the quantum yields from CH3 NH3 PbI3 polycrystalline thin films are PTh ) degrades over a few tens of minutes. This stability problem may be overcome with a lower repetition rate and better heat dissipation. The results presented above establish room-temperature lasing from single-crystal perovskite NWs with the lowest lasing thresholds and highest Q factors reported so far for NW lasers. The exceptional lasing performance of lead halide perovskites can be attributed to long carrier lifetimes and low non-radiative recombination rates, the same attributes that enable their remarkable performance in solar cells. In view of the unique rectangular NW geometry, the ease of growing single-crystal perovskites and 6
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Acknowledgements X-Y.Z. acknowledges support by the US Department of Energy under grant No. ER46980 for all lasing and photophysical measurements. S.J. acknowledges support by the Department of Energy under grant No. ER46664 for NW synthesis and characterizations. S.J. is also grateful for the support of an NSF Grant (DMR-1106184) that provided the insights for designing the NW synthesis here. H.Z. thanks C. Nelson for help with experimental set-up and data analysis.
Author contributions H.Z., Y.F., S.J. and X-Y.Z. conceived the idea and designed the experiments. Y.F., F.M. and Q.D. synthesized and characterized the samples. H.Z., X.W. and Z.G. conducted the optical measurements. M.V.G. helped with metal-coated sample preparation and M.T.T. with experimental set-up for lasing measurement. H.Z. analysed the data and performed the simulation. H.Z., Y.F., S.J. and X-Y.Z. wrote the manuscript. All authors discussed the results, interpreted the findings, and reviewed the manuscript.
Additional information Supplementary information is available in the online version of the paper. Reprints and permissions information is available online at www.nature.com/reprints. Correspondence and requests for materials should be addressed to S.J. or X-Y.Z.
Competing financial interests The authors declare no competing financial interests.
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NATURE MATERIALS DOI: 10.1038/NMAT4271
ARTICLES Methods Growth of single-crystal perovskite NWs. The single-crystal NWs were synthesized from a PbAc2 thin film immersed in a CH3 NH3 X (X = I, Br, Cl or mixed halide precursor) solution in isopropanol in an ambient environment. The PbAc2 thin film was prepared by drop-casting 100 mg ml−1 PbAc2 · 3H2 O aqueous solution on a glass slide (baked in an oven at 65 ◦ C), and then dried for another 30 min at 65 ◦ C. The mass loading was ∼1 mg cm−2 . For the synthesis of CH3 NH3 PbI3 NWs, a piece of glass slide (∼1–2 cm2 ) coated with PbAc2 was carefully placed in 1 ml 40 mg ml−1 CH3 NH3 I/isopropanol solution, with the PbAc2 -coated side facing up. For the synthesis of CH3 NH3 PbBr3 (or CH3 NH3 PbCl3 or mixed halide) NWs, the PbAc2 glass slide was placed in 1 ml CH3 NH3 Br/isopropanol (or CH3 NH3 Cl/isopropanol or mixed CH3 NH3 -halide/isopropanol) solution with a concentration of 5 mg ml−1 , with the PbAc2 -coated side facing down. After a reaction time of ∼20 h at room temperature (22 ◦ C) or 50 ◦ C for CH3 NH3 PbBry I3−y , the glass slide was taken out, and subsequently dipped into isopropanol to remove the residual salt on the film. The product was then dried under a stream of N2 flow.
Structural characterizations. The optical images of CH3 NH3 PbX3 nanostructures were obtained on an Olympus BX51M optical microscope. The SEM images were collected on a LEO SUPRA 55 VP field-emission SEM operated at 1.5 kV. The PXRD data were acquired on a Siemens STOE diffractometer with Cu Kα radiation. Note that some minor peaks associated with the Cu Kβ radiation not completely filtered out in our instrument were observed in the PXRD (Supplementary Figs 2 and 6), which reflect the high crystalline quality of the samples. The sample for TEM analysis was prepared by dry transfer of as-grown CH3 NH3 PbI3 nanostructures onto a TEM grid (Ted Pella, lacey carbon type-A
support film, 300 mesh, copper, no. 01890F). The TEM images were acquired on a FEI Titan scanning TEM at an accelerating voltage of 200 kV. A large spot size was used to avoid sample damage by the electron beam. EDX was performed on single CH3 NH3 PbX3 NWs transferred onto a SiO2 /Si wafer using a LEO 1530 field-emission SEM equipped with an EDS detector operating at 10.0 kV.
Optical characterization. We carried out optically pumped lasing measurements on a home-built far-field epi-fluorescence microscope set-up (Olympus, IX73 inverted microscope). NWs on the as-grown substrate were dry-transferred and dispersed onto a silicon substrate covered with a 300-nm-thick silica layer; each sample was mounted in a N2 -gas-filled cell for optical measurements. The 402 nm excitation light was generated from the second harmonic of the fundamental output (805 nm, 100 fs, 250 kHz) from a regenerative amplifier (Coherent RegA amplifier seeded by a Coherent Mira oscillator). The light was focused onto the sample surface by a ×50, NA 0.5 objective (Olympus LMPLFLN50X) and the pulse duration was broadened to ∼150 fs. We optimized the laser beam size using a lens in front of the microscope to give a beam waist of 34 µm (FWHM) to ensure uniform illumination of each NW. The polarization of the excitation beam was not changed because the absorption anisotropy in these NWs was small (
Lead halide perovskite nanowire lasers with low lasing thresholds and high quality factors Haiming Zhu1†, Yongping Fu2†, Fei Meng2, Xiaoxi Wu1, Zizhou Gong1, Qi Ding2, Martin V. Gustafsson1, M. Tuan Trinh1, Song Jin2* and X-Y. Zhu1* The remarkable performance of lead halide perovskites in solar cells can be attributed to the long carrier lifetimes and low non-radiative recombination rates, the same physical properties that are ideal for semiconductor lasers. Here, we show room-temperature and wavelength-tunable lasing from single-crystal lead halide perovskite nanowires with very low lasing thresholds (220 nJ cm−2 ) and high quality factors (Q ∼ 3,600). The lasing threshold corresponds to a charge carrier density as low as 1.5 × 1016 cm−3 . Kinetic analysis based on time-resolved fluorescence reveals little charge carrier trapping in these single-crystal nanowires and gives estimated lasing quantum yields approaching 100%. Such lasing performance, coupled with the facile solution growth of single-crystal nanowires and the broad stoichiometry-dependent tunability of emission colour, makes lead halide perovskites ideal materials for the development of nanophotonics, in parallel with the rapid development in photovoltaics from the same materials.
S
emiconductor nanowire (NW) lasers, owing to their ultracompact physical sizes, highly localized coherent output, and efficient waveguiding, are promising building blocks for fully integrated nanoscale photonic and optoelectronic devices1 . Each NW can serve as a waveguide along the axial direction and the two end facets form a Fabry–Perot cavity for optical amplification. Since the pioneering work of Yang and co-workers2 for ultraviolet lasing from ZnO NWs, optically pumped lasing has been demonstrated in a number of NWs in the ultraviolet to near-infrared regions3–11 . One of the main obstacles limiting potential applications of semiconductor NW lasers is the high threshold carrier density required for lasing. The high lasing threshold not only makes key technical advancement such as electrically driven lasing12 and integration into optoelectronic devices difficult, but also imposes fundamental limits due to the onset of Auger recombination losses7,8 . In searching for an ideal material for NW lasing, we turn to a new class of hybrid organic–inorganic semiconductors, methyl ammonium lead halide perovskites (CH3 NH3 PbX3 , X = I, Br, Cl), which is emerging as one of the most promising materials for solution-processable photovoltaic technology13–17 . The exceptional solar cell performance can be attributed to the long carrier lifetimes (101−2 ns) and diffusion lengths17–20 (µm). These properties, along with high fluorescence yield and wavelength tunability20–22 , also make lead halide perovskites ideal materials for lasing. Indeed, perovskite films and nanoplates have been shown to be efficient optical gain media20,22–24 , but lasing thresholds remained high in these reports, probably owing to the polycrystalline nature of the samples used. Here we show the growth of high-quality single-crystal NWs from low-temperature solution processing. We demonstrate room-temperature lasing in these NWs with: low lasing thresholds; high quality factors; near-unity quantum yield; and broad tunability covering the near-infrared to visible wavelength region. The first step towards a perovskite NW laser was the synthesis of high-quality single-crystal NWs. We develop a surface-initiated
solution growth strategy using a lead acetate (PbAc2 ) solid thin film deposited on glass substrate in contact with a high concentration of CH3 NH3 X (X = Cl, Br, or I) solution in isopropanol at room temperature (see Methods). As discussed in detail in Supplementary Figs 1–4, we suggest the following two-step process for the growth of single-crystal NWs and other nanostructures: PbAc2 (s) + 4I− (sol) → PbI4 2− (sol) + 2Ac− (sol) PbI4 2− (sol) + CH3 NH3 + (sol) → CH3 NH3 PbI3 (s) + I− (sol) We found that the key to successful nanostructure growth is the slow release of the low-concentration Pb precursor (PbI4 2− ) from the solid Pb(Ac)2 film on the substrate and the careful tuning of the CH3 NH3 X precursor concentration to maintain a low supersaturation condition for the crystal growth of perovskites25 (see Supplementary Information). Figure 1a,b shows optical and scanning electron microscopy (SEM) images of CH3 NH3 PbI3 NWs (and a few nanoplates) on a glass substrate after 24 h growth. The CH3 NH3 PbI3 NWs typically had lengths up to ∼20 µm, with flat rectangular end facets (Fig. 1d,e and Supplementary Fig. 5). The width of the rectangular cross-section varied from NW to NW and was typically a few hundred nanometres. The corresponding powder X-ray diffraction (PXRD; Fig. 1g, red line) pattern shows a set of strong diffraction peaks that can be assigned to a pure tetragonal CH3 NH3 PbI3 crystal structure (space group I 4/mcm, a = 8.896 Å, c = 12.707 Å, see Supplementary Fig. 6 for simulated PXRD; refs 26,27), without impurities (for example, PbI2 or PbAc2 ). The presence of the small (121) peak and the split of the (220) and (004) peaks confirm that the as-grown CH3 NH3 PbI3 is in the room-temperature tetragonal phase, rather than the higher-temperature cubic phase27 . The appearance of diffraction peaks from high-index lattice planes further suggests high crystalline quality. We also performed transmission electron microscope (TEM) analysis on single CH3 NH3 PbI3 NWs (Fig. 1c,f). The insets show the corresponding selected-area electron diffraction
1 Department of Chemistry, Columbia University, New York, New York 10027, USA. 2 Department of Chemistry, University of Wisconsin—Madison, Madison, Wisconsin 53706, USA. †These authors contributed equally to this work. *e-mail: [email protected]; [email protected]
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and fast Fourier transform patterns. The sharp diffraction spots are indexed to a tetragonal crystal structure with zone axes of [110] or [001] (which are identical directions of h100i in the pseudo-cubic lattice). Quantitative elemental analysis from energydispersive X-ray spectroscopy (EDX) on individual NWs yields an I/Pb ratio of ∼3, as expected from the CH3 NH3 PbI3 stoichiometry (Supplementary Fig. 7). All of these characterizations confirm highquality single-crystal CH3 NH3 PbI3 NWs with smooth end facets, making them ideal Fabry–Perot cavities for lasing. We also successfully synthesized single-crystal NWs of other halide perovskites, CH3 NH3 PbX3 (X = Br, Cl), by replacing CH3 NH3 I with CH3 NH3 Br or CH3 NH3 Cl (see Supplementary Information). The PXRD patterns (Fig. 1g and Supplementary Fig. 10) confirm that the as-grown NWs and nanoplates are the 2
¯ cubic phase CH3 NH3 PbX3 (space group Pm3m) without other impurities. EDX measurements on single NWs also confirm their stoichiometry (Supplementary Figs 8 and 9). It is noteworthy that, in some cases, we observed the formation of single-crystal lead halide perovskite nanotubes (see Supplementary Fig. 11). As template-free, catalyst-free and spontaneous formation of single-crystal hollow tubes is a signature of dislocation-driven crystal growth28 , this observation, together with the effectiveness of controlling the supersaturation to encourage the surface-initiated NW growth (see details in Supplementary Figs 1–4), strongly suggests that the catalyst-free anisotropic growth of these lead halide perovskite NWs is probably driven by screw dislocations25,28,29 . Unfortunately, the instability of these perovskite NWs under prolonged exposure to the electron beam in TEM prevented us from observing the
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dislocation diffraction contrast29 using TEM to unambiguously confirm this hypothesis30 . To carry out the lasing experiment, we transferred a small number of well-dispersed perovskite NWs from the growth surface to a Si/SiO2 substrate by a simple dry contact process. We performed optically pumped lasing measurements on a home-built far-field epi-fluorescence microscope at room temperature in a dry N2 atmosphere. A 402 nm pulsed laser beam, with the beam waist adjusted to be larger than the length of each NW, was used as a nearly uniform pump source (see Fig. 2a and Methods). Figure 2b shows a twodimensional (2D) pseudo-colour plot of the emission spectra as a function of pump laser fluence (P) for a CH3 NH3 PbI3 NW (8.5 µm length, Fig. 2d). Representative emission spectra near the lasing threshold are shown in Fig. 2c (see Supplementary Fig. 15 for the full spectral range). At low pump laser fluence (P < 600 nJ cm−2 ), each emission spectrum shows a broad peak centred at ∼777 nm with a full-width at half-maximum (FWHM) of δλ = 44 nm; this corresponds to spontaneous emission (SPE). At P ≥ 600 nJ cm−2 , a sharp peak at 787 nm appears and grows rapidly with increasing P, and the intensity of the broad SPE peak (non-lasing) remains almost constant (see Supplementary Fig. 15), indicating singlemode lasing operation. The inset in Fig. 2c shows the light-in– light-out (L–L) data and FWHM plot as a function of P. Fitting the
L–L plot to the expected S-curve model31 gives a lasing threshold of PTh ∼ 595 nJ cm−2 (Supplementary Fig. 15c). The FWHM plot shows a constant value below PTh and a sudden drop by more than two orders of magnitude at P ≥ PTh . Additional representative NW lasing and an L–L plot with a fit to the S-curve can be found in Supplementary Fig. 17b. Further confirming lasing operation, we note that the emission image of the NW below PTh shows uniform intensity from the whole NW (middle image in Fig. 2d) and that above PTh (right image in Fig. 2d) shows strong emission with spatial interference from the two coherent light sources at the two end facets32 . The bright emission localized at the two ends is consistent with a strong waveguiding effect and axial Fabry–Perot cavity modes, as confirmed later. The FWHM at P = 630 nJ cm−2 , at which power the lasing peak dominates, is 0.22 nm (Fig. 2c and Supplementary Fig. 15 for Gaussian fit). This gives a quality factor Q = λ/δλ ∼ 3,600, which is more than an order of magnitude higher than that from the state-of-the-art GaAs–AlGaAs core–shell NW laser operating at a temperature of 4 K (ref. 11). We note a small blueshift (≤0.5 nm) and broadening of the lasing peak with pump power. The blueshift with increasing carrier density has been observed before in NW lasers and could have multiple origins: thermally induced bandgap/refractive index change, band filling, optical density fluctuations, and electron/hole many-body interactions8,33 .
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Figure 3 | Visible lasing from CH3 NH3 PbBr3 NWs. a, 2D pseudo-colour plots of NW emission spectra as a function of pump fluences for CH3 NH3 PbBr3 NWs of different lengths: top 7.5 µm; middle 13.6 µm; and bottom 23.6 µm. Note the logarithmic colour scale for emission intensity. Inset: Emission images of NWs of different lengths above the lasing threshold (scale bars, 10 µm). b, TRPL decay kinetics after photoexcitation with fluence below (P ∼ 0.82PTh , blue) and above (P ∼ 1.13PTh , red) the threshold, showing a ∼2 ns spontaneous decay process below PTh and a ≤20 ps (instrument limited; see grey dashed curve for instrument response function) lasing process above PTh for the 7.5-µm-long NW. The dots are data points and solid lines are multi-exponential decay fitting. Inset: Emission spectrum above the threshold with a Gaussian fitting. The FWHM is ∼0.23 nm, corresponding to a Q factor of ∼2,400. c, Mode spacing of the lasing peaks as a function of reciprocal NW length (black triangles). The experimental data are well described by a linear function (green) with intercept at zero.
Of the CH3 NH3 PbI3 NWs examined (29 in total), more than 85% showed lasing, which confirms the quality of the single-crystal NWs from our room-temperature solution growth method. In addition to the single-mode lasing shown in Fig. 2, we also observed multiple lasing modes from some NWs (see Supplementary Fig. 17a). In principle, multiple longitudinal modes in a Fabry–Perot cavity are competitive with each other and the one with the highest gain will dominate, but inhomogeneous gain saturation caused by spatial hole burning or crystal/cavity inhomogeneity can sustain multiple lasing modes31,34 . The lasing threshold of 25 NW lasers studied varies between 220 nJ cm−2 and 600 nJ cm−2 . The NW lasing threshold depends on multiple factors, for example, dimensions, end facets and crystalline quality (Supplementary Fig. 17c)35 . The room-temperature lasing threshold values of our single-crystal CH3 NH3 PbI3 NWs are nearly two orders of magnitude lower than those reported recently for near-infrared lasing from lead halide perovskites in polycrystalline thin films or nanoplates20,22–24 and a whole family of III–V near-infrared NW lasers including those with specially engineered core–shell structures and operating at cryogenic temperatures (see Supplementary Table 1). On the basis of the absorption cross-section (∼5 × 10−12 m2 at 402 nm, see Supplementary Information), we calculate a threshold carrier density of ρTh = 1.5–4.5 × 1016 cm−3 for pump power densities of 220–600 nJ cm−2 . 4
Further insight into the remarkable performance of the single-crystal lead iodide perovskite NW laser comes from timeresolved photoluminescence (TRPL) measurements (Fig. 2e). The apparent SPE lifetime was as long as τSPE = 150 ns at low excitation densities (ρ = 1.5 × 1014 cm−3 , black dots and fit in Fig. 2e, see details in Supplementary Fig. 16). The apparent τSPE decreases to 5.5 ns at high excitation density (ρ = 0.85ρTh , blue dots and fit line in Fig. 2e). Kinetic analysis of the ρ-dependent photoluminescence decay dynamics allows us to estimate fluorescence quantum yields: the quantum yield increases from ∼60% at ρ = 1 × 1014 cm−3 to ∼87% at ρ = 1 × 1016 cm−3 (see Supplementary Information). We expect the photoluminescence quantum yield to further increase to near unity (∼100%), because the apparent stimulated emission time constant (τlasing ≤ 20 ps, instrument limited, Fig. 2e and Supplementary Fig. 18) is much shorter than τSPE . For comparison, the quantum yields from CH3 NH3 PbI3 polycrystalline thin films are PTh ) degrades over a few tens of minutes. This stability problem may be overcome with a lower repetition rate and better heat dissipation. The results presented above establish room-temperature lasing from single-crystal perovskite NWs with the lowest lasing thresholds and highest Q factors reported so far for NW lasers. The exceptional lasing performance of lead halide perovskites can be attributed to long carrier lifetimes and low non-radiative recombination rates, the same attributes that enable their remarkable performance in solar cells. In view of the unique rectangular NW geometry, the ease of growing single-crystal perovskites and 6
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Acknowledgements X-Y.Z. acknowledges support by the US Department of Energy under grant No. ER46980 for all lasing and photophysical measurements. S.J. acknowledges support by the Department of Energy under grant No. ER46664 for NW synthesis and characterizations. S.J. is also grateful for the support of an NSF Grant (DMR-1106184) that provided the insights for designing the NW synthesis here. H.Z. thanks C. Nelson for help with experimental set-up and data analysis.
Author contributions H.Z., Y.F., S.J. and X-Y.Z. conceived the idea and designed the experiments. Y.F., F.M. and Q.D. synthesized and characterized the samples. H.Z., X.W. and Z.G. conducted the optical measurements. M.V.G. helped with metal-coated sample preparation and M.T.T. with experimental set-up for lasing measurement. H.Z. analysed the data and performed the simulation. H.Z., Y.F., S.J. and X-Y.Z. wrote the manuscript. All authors discussed the results, interpreted the findings, and reviewed the manuscript.
Additional information Supplementary information is available in the online version of the paper. Reprints and permissions information is available online at www.nature.com/reprints. Correspondence and requests for materials should be addressed to S.J. or X-Y.Z.
Competing financial interests The authors declare no competing financial interests.
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NATURE MATERIALS DOI: 10.1038/NMAT4271
ARTICLES Methods Growth of single-crystal perovskite NWs. The single-crystal NWs were synthesized from a PbAc2 thin film immersed in a CH3 NH3 X (X = I, Br, Cl or mixed halide precursor) solution in isopropanol in an ambient environment. The PbAc2 thin film was prepared by drop-casting 100 mg ml−1 PbAc2 · 3H2 O aqueous solution on a glass slide (baked in an oven at 65 ◦ C), and then dried for another 30 min at 65 ◦ C. The mass loading was ∼1 mg cm−2 . For the synthesis of CH3 NH3 PbI3 NWs, a piece of glass slide (∼1–2 cm2 ) coated with PbAc2 was carefully placed in 1 ml 40 mg ml−1 CH3 NH3 I/isopropanol solution, with the PbAc2 -coated side facing up. For the synthesis of CH3 NH3 PbBr3 (or CH3 NH3 PbCl3 or mixed halide) NWs, the PbAc2 glass slide was placed in 1 ml CH3 NH3 Br/isopropanol (or CH3 NH3 Cl/isopropanol or mixed CH3 NH3 -halide/isopropanol) solution with a concentration of 5 mg ml−1 , with the PbAc2 -coated side facing down. After a reaction time of ∼20 h at room temperature (22 ◦ C) or 50 ◦ C for CH3 NH3 PbBry I3−y , the glass slide was taken out, and subsequently dipped into isopropanol to remove the residual salt on the film. The product was then dried under a stream of N2 flow.
Structural characterizations. The optical images of CH3 NH3 PbX3 nanostructures were obtained on an Olympus BX51M optical microscope. The SEM images were collected on a LEO SUPRA 55 VP field-emission SEM operated at 1.5 kV. The PXRD data were acquired on a Siemens STOE diffractometer with Cu Kα radiation. Note that some minor peaks associated with the Cu Kβ radiation not completely filtered out in our instrument were observed in the PXRD (Supplementary Figs 2 and 6), which reflect the high crystalline quality of the samples. The sample for TEM analysis was prepared by dry transfer of as-grown CH3 NH3 PbI3 nanostructures onto a TEM grid (Ted Pella, lacey carbon type-A
support film, 300 mesh, copper, no. 01890F). The TEM images were acquired on a FEI Titan scanning TEM at an accelerating voltage of 200 kV. A large spot size was used to avoid sample damage by the electron beam. EDX was performed on single CH3 NH3 PbX3 NWs transferred onto a SiO2 /Si wafer using a LEO 1530 field-emission SEM equipped with an EDS detector operating at 10.0 kV.
Optical characterization. We carried out optically pumped lasing measurements on a home-built far-field epi-fluorescence microscope set-up (Olympus, IX73 inverted microscope). NWs on the as-grown substrate were dry-transferred and dispersed onto a silicon substrate covered with a 300-nm-thick silica layer; each sample was mounted in a N2 -gas-filled cell for optical measurements. The 402 nm excitation light was generated from the second harmonic of the fundamental output (805 nm, 100 fs, 250 kHz) from a regenerative amplifier (Coherent RegA amplifier seeded by a Coherent Mira oscillator). The light was focused onto the sample surface by a ×50, NA 0.5 objective (Olympus LMPLFLN50X) and the pulse duration was broadened to ∼150 fs. We optimized the laser beam size using a lens in front of the microscope to give a beam waist of 34 µm (FWHM) to ensure uniform illumination of each NW. The polarization of the excitation beam was not changed because the absorption anisotropy in these NWs was small (
