Low-Threshold Amplified Spontaneous Emission and Lasing from Colloidal

ARTICLE Received 21 Apr 2015 | Accepted 13 Jul 2015 | Published 20 Aug 2015 DOI: 10.1038/ncomms9056 OPEN Low-threshold amplified spontaneous emission and lasing from colloidal nanocrystals of caesium lead halide perovskites Sergii Yakunin1,2, Loredana Protesescu1,2, Franziska Krieg1,2, Maryna I. Bodnarchuk1,2, Georgian Nedelcu1,2, Markus Humer3, Gabriele De Luca4, Manfred Fiebig4, Wolfgang Heiss3,5,6 & Maksym V. Kovalenko1,2 Metal halide semiconductors with perovskite crystal structures have recently emerged as highly promising optoelectronic materials. Despite the recent surge of reports on microcrystalline, thin-film and bulk single-crystalline metal halides, very little is known about the photophysics of metal halides in the form of uniform, size-tunable nanocrystals. Here we report low-threshold amplified spontaneous emission and lasing from B10 nm monodisperse colloidal nanocrystals of caesium lead halide perovskites CsPbX3 (X ¼ Cl, Br or I, or mixed Cl/Br and Br/I systems). We find that room-temperature optical amplification can be obtained in the entire visible spectral range (440–700 nm) with low pump thresholds down to 5±1 mJcmÀ 2 and high values of modal net gain of at least 450±30 cm À 1.Two kinds of lasing modes are successfully observed: whispering-gallery-mode lasing using silica microspheres as high-finesse resonators, conformally coated with CsPbX3 nanocrystals and random lasing in films of CsPbX3 nanocrystals. 1 Department of Chemistry and Applied Biosciences, Laboratory of Inorganic Chemistry, ETH Zu¨rich, Vladimir-Prelog-Weg 1, CH-8093 Zu¨rich, Switzerland. 2 Laboratory for Thin Films and Photovoltaics, Empa—Swiss Federal Laboratories for Materials Science and Technology, U¨berlandstrasse 129, CH-8600 Du¨bendorf, Switzerland. 3 Institute of Semiconductor and Solid State Physics, University Linz, Altenbergerstrae 69, 4040 Linz, Austria. 4 Department of Materials, Laboratory for Multifunctional Ferroic Materials, ETH Zu¨rich, Vladimir-Prelog-Weg 4, CH-8093 Zu¨rich, Switzerland. 5 Materials for Electronics and Energy Technology (i-MEET), Friedrich-Alexander-Universita¨tErlangen-Nu¨rnberg, Martensstrae7,91058Erlangen,Germany.6 Energie Campus Nu¨rnberg (EnCN), Fu¨rther Strae 250, 90429 Nu¨rnberg, Germany. Correspondence and requests for materials should be addressed to M.V.K. (email: [email protected]). NATURE COMMUNICATIONS | 6:8056 | DOI: 10.1038/ncomms9056 | www.nature.com/naturecommunications 1 & 2015 Macmillan Publishers Limited. All rights reserved. ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/ncomms9056 ecent years have seen multiple reports demonstrating need to be extremely small (r5 nm) to emit in the blue–green, outstanding optoelectronic characteristics of metal halide and as-synthesized they exhibit rather low PL QYs of r5% due to Rsemiconductors with perovskite crystal structures, in the mid-gap trap states. In addition, they are chemically and form of thin films, microcrystals and bulk single crystals1–11.In photochemically unstable, and require coating with an epitaxial particular, hybrid organic–inorganic lead halide perovskites such layer of a more chemically robust, wider-gap semiconductor, as MAPbX3 (where MA ¼ methyl ammonium and X ¼ Cl, Br or I, such as CdS. On the other hand, narrow emission linewidths of or mixed Cl/Br and Br/I systems) have shown great potential ca. 100 meV, high PL QYs of up 90% and high photochemical as both light-absorbing and light-emitting direct-bandgap stability have been achieved for Cd-chalcogenide NCs as the solution-deposited semiconductors. As absorber layers, result of two decades of research efforts to precisely engineer MAPbX3 materials have enabled inexpensive solar cells with core-shell morphologies with independent control of the core and certified power conversion efficiencies of up to 20% (NREL shell compositions and thicknesses (for example, CdSecore/ 12 27–30 efficiency chart, www.nrel.gov) and highly sensitive solution- ZnCdSshell or ‘giant-shell’ CdSecore/CdSshell) and anisotropic cast photodetectors operating in the visible13, ultraviolet14 and CdSe–CdS dot-in-rod and platelet-like morphologies31,32. X-ray15 spectra regions. Owing to their bright photoluminescence Overall, each of these two families of colloidal semi- (PL), MAPbX3 thin films and nanowires have been used in conductors—CsPbX3 NCs and Cd-chalcogenide nano- electrically driven light-emitting diodes16 and as optical gain structures—feature their respective advantages. 17–21 media for lasing . We have recently shown that similarly high Inspired by the highly efficient PL of CsPbX3 NCs, in this study optoelectronic quality is also accessible in fully inorganic CsPbX3 we investigate the possibility of using CsPbX3 NCs as an analogues, when these compounds are synthesized in the form of inexpensive optical gain medium. First, for thin films of 22 colloidal nanocrystals (NCs) . In particular, CsPbX3 NCs exhibit CsPbX3 NCs, we report the observation of amplified spontaneous bright emission with PL quantum yields (QYs) reaching 90% and emission (ASE), tunable over most of the visible range narrow emission linewidths of 70–100 meV (12–40 nm, for PL (440–700 nm) with low pump thresholds down to 5±1 mJcmÀ 2 peaks from 410 to 700 nm, correspondingly). Precise and and high values of modal net gain of at least 450 cm À 1. Among continuous tuning of bandgap energies over the entire visible other colloidal semiconductor materials, such low-threshold spectral region is achievable foremost via compositional control pump fluencies have only been previously demonstrated for (mixed halide Cl/Br and Br/I systems), but also through colloidal CdSe and CdSe/CdS nanoplatelets33–35, whereas À 2 quantum-size effects. CsPbX3 NCs appear to be largely free CdSe/ZnCdS NCs exhibit higher thresholds (from 800 mJcm 23 À 2 28 from mid-gap trap states, similar to their MAPbX3 cousins . in the blue to 90 mJcm in the red) presumably due to 36 Both molecular solutions of MAPbX3 and colloidal solutions of stronger Auger recombination . We then realize two different CsPbX3 NCs share the common feature of facile solution lasing regimes for CsPbX3 NCs depending on the resonator deposition on arbitrary substrates. Further, CsPbX3 NCs are configuration: whispering-gallery-mode (WGM) lasing using readily miscible with other optoelectronic materials (polymers, single silica microsphere resonators, conformally coated with fullerenes and other nanomaterials) and feature surface-capping CsPbX3 NCs, and random lasing in CsPbX3 NC films. ligands for further adjustments of the electronic and optical properties, and solubility in various media. We also note that CsPbX3 NCs are formed in the pure cubic perovskite phase, in Results which PbX6 octahedra are three-dimensionally interconnected by Basic characteristics of CsPbX3 NCs in solutions and in films. corner-sharing. In contrast, their bulk counterparts exist As-synthesized CsPbX3 NCs, capped with oleylamine and oleic exclusively in wider-bandgap one-dimensional orthorhombic acid as surface ligands, form stable colloidal dispersions in phases at ambient conditions24–26. This disparity is most typical nonpolar solvents such as toluene (Fig. 1a)22. For the pronounced for red-emitting CsPbI3 NCs that exhibit a spectroscopic studies in this work, we selected monodisperse narrow gap of down to 1.75 eV, whereas the corresponding samples of cubic-shaped NCs with mean sizes of ca. 9–10 nm bulk material has a ca. 1 eV larger bandgap and is yellow-colored (Fig. 1b). These NCs readily form uniform, compact films of and non-luminescent. A key practical advantage of CsPbX3 sub-micron thickness on drop-casting onto a glass substrate. NCs is the facile access to the blue–green spectral region of The bandgap of CsPbX3 NCs is controlled via compositional 410–530 nm via one-pot synthesis22. In comparison, common modulations, for example, by altering the Cl/Br ratio for the metal chalcogenide colloidal quantum dots such as CdSe NCs 410–530 nm range, and Br/I ratio for the 530–700 nm range a b d CsPbBr NC film 4 3 ) 1 –1 m 3 Abs µ PL c CsPbX NC colloids 3 2 PL (a.u.) Absorption ( 1 0 0 400 500 600 700 450 500 550 600 Wavelength (nm) Wavelength (nm) Figure 1 | Colloidal caesium lead halide perovskite NCs. (a) Stable dispersions in toluene under excitation by a ultraviolet lamp (l ¼ 365 nm). (b)Low- and high-resolution transmission electron microscopy images of CsPbBr3 NCs; corresponding scale bars are 100 and 5 nm. (c) PL spectra of the solutions shown in (a). (d) Optical absorption and PL spectra of a ca. 400-nm-CsPbBr3 NC film. 2 NATURE COMMUNICATIONS | 6:8056 | DOI: 10.1038/ncomms9056 | www.nature.com/naturecommunications & 2015 Macmillan Publishers Limited. All rights reserved. NATURE COMMUNICATIONS | DOI: 10.1038/ncomms9056 ARTICLE (Fig. 1c). A pronounced excitonic peak is preserved in the a wider range of pumping intensities). ASE is spectrally different absorption spectrum of the CsPbBr3 NC film (Fig. 1d). from PL emission; it has a narrower bandwidth of 4–9 nm (full The absorption coefficients of the densely packed films are in width at half maximum, FWHM; see Supplementary Fig. 5) due to the range of (3.6–4.0) Á 104 cm À 1 (or 3.6–4.0 mm À 1), indicating a narrow gain in bandwidth and is red-shifted by ca. 10 nm with that up to 70–80% of the pumping laser light (l ¼ 400 nm) is respect to the PL maximum. When the ASE spectrum is overlaid absorbed by 300–400-nm thick films. The refractive index of a with the Tauc plot of the direct-bandgap absorption CsPbBr3 NC film is estimated to be 1.85–2.30 at 400–530 nm (Supplementary Fig. 3), the ASE spectral maxima coincide with the from the optical reflectance and absorption spectra end of the shallow absorption tail (Urbach tail). This (Supplementary Fig. 1). The PL from this NC film exhibits a red-shifted ASE may have its origins in re-absorbance during peak with a narrow linewidth of 25 nm (110 meV), Stokes-shifted single-exciton lasing28,29 or in the excitonic binding energies in the by 13 nm (57 meV) with respect to the excitonic absorption bi-excitonic optical gain mechanism34,37.

Load more