UV polaritonic emission from a perovskite-based microcavity

an electron and a hole in an exciton is strengthened resulting in very large exciton binding energies of a few hundreds of. meV and huge oscillator strengths.
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APPLIED PHYSICS LETTERS 93, 081101 共2008兲

UV polaritonic emission from a perovskite-based microcavity G. Lanty,1 J. S. Lauret,1 E. Deleporte,1,a兲 S. Bouchoule,2 and X. Lafosse2 1

Laboratoire de Photonique Quantique et Moléculaire de l’École normale Supérieure de Cachan, 61 avenue du Président Wilson, 94235 Cachan Cedex, France 2 Laboratoire de Photonique et Nanostructures, Route de Nozay, 91460 Marcoussis, France

共Received 12 June 2008; accepted 28 July 2008; published online 25 August 2008兲 We report on the realization of a molecule-based one-dimensional microcavity emitting in the near UV range at room temperature. The active material is a thin film of the two-dimensional perovskite 共C6H5C2H4 – NH3兲2PbCl4, a molecular compound absorbing and emitting light around 3.6 eV. Angle-resolved reflectivity and photoluminescence measurements show that this microcavity works in the strong coupling regime. The emitting UV polariton is a mixed state between the photon cavity mode and the exciton of the perovskite-type semiconductor. © 2008 American Institute of Physics. 关DOI: 10.1063/1.2971206兴 One-dimensional 共1D兲 planar microcavities, consisting of Fabry–Pérot structures containing an optically active region, have proved that they are powerful tools to study the light-matter interaction,1–3 and may lead to applications.4 In particular, the strong coupling regime is intensively studied due to the interest in coherent and stimulated effects in such systems, which can lead to Bose–Einstein condensate5 and the realization of low threshold polariton laser.6,7 In this regime, the cavity photon mode and the exciton of the optically active region are not eigenmodes of the system anymore. The new eigenmodes are a linear and coherent superposition of the exciton and photon states, called cavity polaritons. In order to build new optoelectronic devices based on polaritonic effects, it is crucial to find optically active materials allowing to achieve the strong coupling regime at room temperature. In the field of inorganic semiconductors, the strong coupling regime has been observed recently at room temperature in GaN 1D microcavities.8–10 Alternatively, in the past decade, it has been demonstrated that the strong coupling regime can be obtained at 300 K in 1D microcavities containing thin layers of organic materials,11–13 or hybrid organic-inorganic materials.14–16 Because of the relatively large oscillator strengths of the exciton in these materials, Rabi splittings of several 100 meV 共Refs. 11–16兲 have been reported at room temperature. Among the optoelectronic devices, ultraviolet sources are important because they are required for advanced chemical and biological sensors, high density optical storage, displays, and illumination technologies 共as pump sources for phosphors used in solid state white lighting兲. It is the reason why inorganic semiconductors emitting in the near UV range are intensively studied. An electrically injected GaN-based vertical cavity surface emitting laser has recently been obtained17 and studies on ZnO increase.18 The near UV range is difficult to reach with molecular compounds. All the organic microcavities cited above emit light in the near IR range or in the visible range. In this letter, we use the two-dimensional layered perovskite-type semiconductor 共C6H5C2H4 – NH3兲2PbCl4 to realize a molecule-based 1D microcavity showing the strong coupling regime at room temperature and emitting light in the UV range. a兲

Electronic mail: [email protected].

0003-6951/2008/93共8兲/081101/3/$23.00

Two-dimensional layered perovskite compounds such as 共C6H5C2H4 – NH3兲2PbX4, where X is a halogen, have been shown to have a self-organized multiple quantum well structure when the organic solution is deposited by spin coating on a substrate. The inorganic wells of thickness around 0.5 nm alternate with organic barriers of thickness around 1.0 nm.19,20 In this structure, the Coulomb interaction between an electron and a hole in an exciton is strengthened resulting in very large exciton binding energies of a few hundreds of meV and huge oscillator strengths. For instance, the order of magnitude of the oscillator strength per quantum well in 共C6H5C2H4 – NH3兲2PbI4 is 4 ⫻ 1013 cm−3,14 which is one order of magnitude higher than in conventional inorganic semiconductor quantum wells. Because of its strong binding energy, the excitonic state can be observed at room temperature. The spectral position of the excitonic transitions can be tailored by substituting different halides X, leading to a great flexibility of these molecules.21,22 In particular, a 共C6H5C2H4 – NH3兲2PbCl4 关bis共phenethylammonium兲 tetrachloroplumbate兴 layer exhibits a sharp absorption peak 共full width at half maximum ⯝90 meV兲 at 3.64 eV 共341 nm兲 and a photoluminescence emission in the near UV range at 3.61 eV 共343 nm兲,21,22 whereas the 共C6H5C2H4 – NH3兲2PbI4 perovskite 共used in Ref. 14兲 emits in the visible range. In this work, we have embedded the perovskite molecule 共C6H5C2H4 – NH3兲2PbCl4 in a ␭ Fabry–Pérot microcavity. A bottom dielectric Bragg mirror 共centered at 3.6 eV under 40° incidence兲 is deposited onto a fused silica substrate by plasma enhanced chemical vapor deposition 共PECVD兲 and is composed of 7.5␭ / 4 pairs of silicon nitride 共d = 46 nm, n = 1.96兲 and silicon oxide 共d = 64 nm, n = 1.49兲. The normal incidence reflectivity of the PECVD mirror is centered at 3.4 eV, and presents a maximal reflectivity of 96 %. The stop band extends from 3.1 to 3.7 eV. The dielectric mirror ends with a thin film 共21 nm兲 of SiO2 in order for the perovskite layer to be centered near an antinode of the stationary electric field in the cavity 共calculated with a transfer matrix model兲. A 30 nm thin film of 共C6H5C2H4 – NH3兲2PbCl4 perovskite is deposited on top of this dielectric mirror by spin coating a 5 wt% solution of C6H5C2H4 – NH3Cl and PbCl2 dissolved in stoechiometric amounts in N , N-dimethylformamide. Then a polymethylmetacrylate 共PMMA兲 spacer layer is spin coated. The targeted PMMA

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FIG. 1. Reflectivity spectra of the perovskite microcavity, for different angles of incidence. The spectra are vertically shifted for clarity. The dotted lines are guides to the eyes showing the angular dispersion of the UPB and LPB. The spectrum displayed using a bold line corresponds to the resonance. Eper is the absorption energy of the 共C6H5C2H4 – NH3兲2PbCl4 thin film.

thickness of 180 nm is calculated to maintain the optical mode position around 3.6 eV at 40°. The top mirror of the microcavity is then produced by electrobeam evaporation of aluminum 共thickness of 13 nm兲 on the PMMA layer. Angle-resolved reflectivity measurements of the microcavity are performed using a Xenon lamp as the excitation source, between 0° and 80°, at room temperature. Varying the incidence angle ␪ relative to the surface normal allows tuning of the cavity photon mode energy. By varying ␪, it is thus possible to vary the relative energy separation 共and hence the degree of interaction兲 between the exciton 共which is dispersionless, and so angle independent兲 and the cavity photon mode.3,14,16 Figure 1 shows a series of reflectivity spectra at room temperature for several different incident angles ␪. Two dips, whose energy position, intensity, and linewidth are angle dependent, can be seen. A clear anticrossing between the two transitions can be seen, which is the signature of the strong coupling between the perovskite exciton and the cavity mode. The energy of the two minima observed in Fig. 1 are reported in Fig. 2 as a function of k储 = 共E / បc兲sin ␪, the wave vector parallel to the surface. The experimental results are fitted to the dispersion using a standard two-level model23 共solid lines in Fig. 2兲, EUPB,LPB = 共Eph + Eper兲 / 2 ⫾ 冑V2 + 共Eph − Eper兲2 / 4. This relation is the same as the relation obtained for two coupled oscilla-

FIG. 2. Polariton dispersions 共UPB and LPB兲 measured from reflectivity spectra uncoupled perovskite exciton Eper 共dotted line兲 and cavity photon mode Eph 共dashed line兲 are also shown. The stars represent the energy positions of the photoluminescence peaks observed in Fig. 3.

Appl. Phys. Lett. 93, 081101 共2008兲

FIG. 3. 共a兲 Photoluminescence spectra of a thin film of 共C6H5C2H4 – NH3兲2PbCl4. 共b兲 Photoluminescence spectra of the microcavity for detection angles from 5° to 50° 共excitation at 325 nm through the dielectric mirror at normal incidence兲.

tors with a coupling energy of V. V is a fitting parameter, assumed to be constant at all angles. A good agreement between the experimental results and the two-level model 共see Fig. 2兲 is attained with a value of the Rabi splitting 共2V兲 equal to 230 meV. The two transitions observed in Fig. 1 are then identified as the lower polariton branch 共LPB兲 and the upper polariton branch 共UPB兲, respectively. In order to confirm this result, photoluminescence experiments have been performed; the energy position of the photoluminescence signal being directly linked to the polariton dispersions.24 The 325 nm He-Cd laser beam is focused on the microcavity through the dielectric mirror, at normal incidence. The photoluminescence spectra are recorded for various detection angles. Figure 3 shows a series of photoluminescence spectra obtained for different angles of detection, ranging from 5° to 50°. For the lower angles, two peaks are present in the spectra. The position of the high energy peak is independent of the detection angle, whereas the position of the lower energy peak varies as the detection angle is tuned. For the higher angles, the lower energy peak becomes predominant, suggesting a bottleneck effect.25 The energy positions of the luminescence peaks have been reported as stars in Fig. 2, superimposed to the fitted dispersion curves and reflectivity positions of the upper and LPBs. The dispersionless data correspond to the noncoupled part of the perovskite exciton, since the energy position of this peak corresponds to one of the photoluminescence spectra of the perovskite layer. The variation in the low energy photoluminescence peak as a function of k储 coincides with the dispersion relation of the low energy polaritonic branch. This clearly indicates that this photoluminescence arises from the emission of the LPB and confirms the demonstration of the strong coupling regime at room temperature in the UV range. The emission of the UPB has not been observed, probably because of the relaxation toward uncoupled excitonic states26 or because of the fast emission of optical phonons between the upper and the lower branches.23 In summary, we have realized a 1D microcavity emitting in the UV range 共⯝3.5 eV兲, working in the strong coupling regime at room temperature. The emitting UV polariton is a mixed state between the photon cavity mode and the exciton of the two-dimensional perovskite-type semiconductor,

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共C6H5C2H4 – NH3兲2PbCl4. This work opens the way to optoelectronic devices emitting in the UV range and to more complex structures where the perovskite UV excitons can be coupled via the cavity photon to GaN excitons, following the suggestion of Agranovich et al.27 The authors thank D. Byrne and A. Vella for their helpful work on the angle-resolved reflectivity experiment. This work is supported by Agence Nationale pour la Recherche 共grant PNANO MICHRY兲 and by CNANO from “région Ilede-France” 共grant MICRORG兲. “Laboratoire de Photonique Quantique et Moléculaire” is a “Unité mixte de recherche associée au CNRS” 共UMR8537兲. 1

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