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UV polaritons at room temperature in a microcavity containing perovskites G. Lanty, J.S. Lauret, E. Deleporte, S. Bouchoule, X. Lafosse PII: DOI: Reference:

S0022-2313(09)00226-9 doi:10.1016/j.jlumin.2009.04.048 LUMIN 9727 www.elsevier.com/locate/jlumin

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Journal of Luminescence

Cite this article as: G. Lanty, J.S. Lauret, E. Deleporte, S. Bouchoule and X. Lafosse, UV polaritons at room temperature in a microcavity containing perovskites, Journal of Luminescence, doi:10.1016/j.jlumin.2009.04.048 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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UV polaritons at room temperature in a microcavity containing perovskites G. Lanty, J. S. Lauret, and E. Deleporte∗ ´ Laboratoire de Photonique Quantique et Mol´eculaire de l’Ecole normale Sup´erieure de Cachan, 61 avenue du Pr´ esident Wilson, 94235 Cachan Cedex, France

S. Bouchoule and X. Lafosse Laboratoire de Photonique et Nanostructures, Route de Nozay, 91460 Marcoussis, France (Dated: April 29, 2009) The molecular crystal of (C6 H5 C2 H4 − NH3 )2 PbCl4 perovskite presents an excitonic state with an absorption energy in the ultraviolet range (3.64 eV, 341 nm) and a large binding energy (few hundred of meV). We report here on the realization of a P´erot-Fabry λ-microcavity containing a thin film of this material as active layer. Angle resolved reflectivity and photoluminescence measurements demonstrate this microcavity works, at room temperature, in the strong coupling regime: the cavity photon mode and the excitonic state are not eigenmodes of the system any more, the new eigenmodes are a linear and coherent superposition of the exciton and photon states, called cavity polaritons.

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PACS numbers: 78.55.Kz, 42.55.Sa, 71.36.+c, 78.67.Pt

One-dimensional (1D) planar microcavities, consisting of P´erot-Fabry structures containing an optically active region, have proved that they are a powerful tool to study the light-matter interaction [1–3] and may lead to applications in optical devices [4, 5]. In particular, in the strong coupling regime, the cavity photon mode and the exciton of the optically active region are not eigenmodes of the system any more: the new eigenmodes are a linear and coherent superposition of the exciton and photon states, called cavity polaritons. This particular regime is intensively studied due to the interest in coherent and stimulated effects in such systems, which can lead to the realization of low threshold polariton laser [5–7].

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In order to realize new optoelectronic devices based on polaritonic effects, it is crucial to find optically active materials allowing to reach 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, 9]. Alternatively, since a decade, it has been demonstrated that the strong coupling regime can be obtained at 300 K in one-dimensional microcavities containing thin layers of organic materials [10–13], or hybrid organicinorganic materials [14–16]. Because of the relatively large oscillator strengths of the exciton in these materials, Rabi splittings around 150 meV for zinc porphyrin [10], J-aggregates of cyanine dyes [11, 12] or perovskite [14– 16], and even as large as 430 meV for a sigma-conjugated polysilane [13], have been reported at room temperature. Such large Rabi splittings are of particular significance for current efforts to achieve polariton lasing in microcavities at room temperature because it will be possible to maintain the strong coupling at 300 K.

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The wavelength region of the emission is an other characteristic which is important for the realization of

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interesting optoelectronic devices. Lot of new applications are developed in the ultraviolet range. For instance, UV sources are required for advanced chemical and biological sensors, high density optical storage, displays and illumination technologies (as pump sources for phosphors used in solid state white lighting). Inorganic semiconductors emitting in the near UV range are thus intensively studied: electrically injected GaN-based VCSEL’s has recently been obtained [17], studies on Zn0 increase [18]. Up to now, the molecular structures demonstrating the strong coupling regime at room temperature when inserted in a P´erot-Fabry cavity, emit light in the near IR range (J-aggregates of cyanine dyes) or in the visible range (sigma-conjugated polysilane, perovskite): the shortest emitted wavelength is 413 nm [13]. We use here, for the first time to our knowledge, the two-dimensional layered perovskite-type semiconductor (C6 H5 C2 H4 − NH3 )2 PbCl4 to realize a molecule-based 1D microcavity showing the strong-coupling regime at room temperature and emitting light in the UV range.

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Two-dimensional layered perovskite compounds such as (R − NH3 )2 MX4 (R = alkyl chain, M = metal, X = 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]. Because the band gap of MX4 layers is smaller than that of organic layers, the lowest exciton is confined in the MX4 layer, so the quantum wells consist of MX4 inorganic monolayers (thickness around 0.5 nm) and the barriers consist of the organic alkylammonium layers (thickness around 1.0 nm). By virtue to the high contrast in dielectric constants between the organic layers and the MX4 layers, the Coulomb interaction in the well layer is hardly screened

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by the presence of the barrier layers, so the interaction between an electron and a hole in an exciton is strengthened resulting in very large exciton binding energies of a few hundred of meV and huge oscillator strengths: the order of magnitude of the oscillator strength per quantum well in (C6 H5 C2 H4 − NH3 )2 PbI4 is 4x1013 cm−3 [14], which is one order of magnitude higher than in conventional inorganic semiconductor quantum wells. Because of this strong binding energy of the exciton, the optical features can be observed at room temperature: absorption spectra exhibiting sharp resonances and strong photoluminescence corresponding to the excitonic transitions can be observed [20]. Since the excitons are associated with the bandgap of the inorganic framework, the spectral position of the excitonic transitions can be tailored by substituting different metal cations or halides within the inorganic framework. Let us focus on the family (C6 H5 C2 H4 − NH3 )2 PbX4 to illustrate the flexibility of such molecules: it is possible to tune the energy of the exciton by changing only the nature of the halogen ion [21]: a (C6 H5 C2 H4 − NH3 )2 PbI4 layer absorbs at 2.40 eV (517 nm), a (C6 H5 C2 H4 − NH3 )2 PbBr4 layer absorbs at 3.06 eV (405 nm), and a (C6 H5 C2 H4 − NH3 )2 PbCl4 layer absorbs at 3.64 eV (341 nm).). Figure 1 shows the absorption and photoluminescence spectra of a spin-coated (C6 H5 C2 H4 − NH3 )2 PbCl4 layer (thickness around 30 nm) on a quartz substrate: a relatively sharp absorption feature (width  100 meV) is observed, and photoluminescence can be observed at 3.61 eV when excited at 3.82 eV from a He-Cd laser. In this work, we have embedded the perovskite molecule (C6 H5 C2 H4 − NH3 )2 PbCl4 , emitting in the UV range, in a λ P´erot-Fabry microcavity constituted with a dielectric mirror and a metallic one, as shown in figure 2a. The bottom dielectric Bragg mirror (centered at 3.6 eV under 40o incidence) is deposited onto a fused silica substrate by plasma enhanced chemical vapor deposition (PECVD) and is composed of 7.5 λ/4 pairs of silicon oxide (d = 64 nm, n = 1.49) and silicon nitride (d = 46 nm, n = 1.96). The normal incidence reflectivity of the PECVD mirror is shown in figure 2b. It can be seen that the PECVD mirror is centered at 3.4 eV at normal incidence, presents a maximal reflectivity of 96 %, the stop-band extends from 3.1 eV to 3.7 eV. The dielectric mirror ends with a thin film (21 nm) of SiO2 in order the perovskite layer to lay, in the cavity, near the maximum of the electric field intensity (calculated with a transfer matrix model). A thin film of (C6 H5 C2 H4 − NH3 )2 PbCl4 [bis(phenethylammonium) tetrachloroplumbate] perovskite is deposited on top of this dielectric mirror by spincoating a 5 wt% solution of C6 H5 C2 H4 − NH3 Cl and PbCl2 dissolved in stoechiometric amounts in DMF (N,N-DiMethylFormamide), the thickness of the film is around 30 nm. Then a PMMA (PolyMethylMetAcrylate) layer is spin-coated acting as a spacer layer in or-

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FIG. 1: Absorption and photoluminescence spectra of a 30 nm thick (C6 H5 C2 H4 − NH3 )2 PbCl4 layer deposited by spin-coating on a quartz substrate. The excitation of the photoluminescence is provided by a He-Cd laser.

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der to tune the cavity photon mode energy at 40o incidence close to that of the exciton, the thickness of the PMMA layer has been calibrated as a function of the spin-coating parameters. The top mirror of the microcavity is then produced by electron-beam evaporation of aluminium (thickness of 13 nm) on the PMMA layer.

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Angle-resolved reflectivity measurements of the microcavity are perfomed using a Xenon lamp as the excitation source, between 0o and 80o , at room temperature (angle-resolved transmission measurements were not possible because the transmission signal is too low). Varying the incidence angle relative to the surface normal allows to tune the energy separation (and hence the degree of interaction) between the exciton (which is dispersionless, and so angle-independent) and the cavity photon mode [2]. Figure 3 shows a series of reflectivity spectra at room temperature as a function of the incident angle. Two dips, whose energy position, intensity and linewidth are angle-dependant, can be seen. A clear anticrossing between the two transitions can be seen. For low incidence angles, the lower energy minimum is the stronger one (it allows us to evaluate the quality factor of the microcavity: Q  15). As the angle increases, the intensity of the low energy minimum progressively decreases whereas the intensity of the high energy one increases. For 40o both transitions have the same intensity and for larger incidence angles, the upper energy minimum becomes the stronger one, and the lower energy minimum

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intensity decreases. This anticrossing is the signature of the strong coupling between the perovskite exciton and the cavity mode. The energy of the two minima observed in figure 3, are reported in figure 4. The experimental results are fitted to the dispersion using a standard twolevel model [22] (solid lines in figure 4): EUPB,LPB = (Eph + Eper )/2 ± V2 + (Eph − Eper )2 /4. This relation is the same as the relation obtained for two coupled oscillators with a coupling energy of V. The cavity photon mode energy is related to θ by:  Eph (θ) = E0 / 1 − (sin2 (θ))/n2eff where E0 and neff are respectively the photon mode energy at normal incidence and the effective refractive index of the entire cavity. The fitting parameters are V (assumed to be constant at all angles), neff and E0 . The very good agreement between the experimental and calculated results shows unambiguously that strong coupling leads to an anticrossing between the exciton and the photon modes, with a value of the Rabi splitting (2V) equal to 230 meV. The two transitions observed in figure 3 are then identifed as the lower polariton branch (LPB) and the upper polariton branch (UPB). 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 [23].The 325 nm He-Cd laser beam is focused on the microcavity through the dielectric mirror, at normal incidence, and the photolumines-

cence spectra are recorded for various detection angles. Figure 5 shows a series of photoluminescence spectra obtained for different collection angles, ranging from 5o to 50o . 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 more and more intense, suggesting a bottleneck effect [24]. The energy positions of the luminescence peaks have been reported as stars in figure 4), superimposed to the fitted dispersion curves and reflectivity positions of the upper and lower polaritonic branches. The dispersionless data correspond to the non-coupled part of the perovskite exciton, since the energy position of this peak corresponds to the one of the photoluminescence spectrum of the perovskite layer (see figure 1). The variation of the low energy photoluminescence peak as a function of θ coincides with the dispersion relation of the low energy polaritonic branch. This clearly indicates that this photoluminescence arises from the polaritonic emission and confirms the demonstration of the strong coupling regime at room temperature in the UV range. The emission of the upper polaritonic branch has not been observed, probably because of the relaxation

FIG. 3: Reflectivity spectra of the perovskite microcavity for different incidence angles. The spectra are vertically shifted for clarity. The dotted lines are guides to the eyes showing the angular dispersion of the upper (UPB) and lower (LPB) polariton branches. The spectrum displayed using a bold line corresponds to resonance. Eper ( 3.64 eV) is the excitonic absorption energy of the (C6 H5 C2 H4 − NH3 )2 PbCl4 thin film extracted from figure 1.

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[8] FIG. 4: Polariton dispersions (UPB: upper polariton branch, LPB: lower polariton branch) measured from reflectivity spectra (squares) and calculated (solid lines) . The dispersions of the uncoupled perovskite exciton Eper (dotted line) and cavity photon mode Eph (dashed line) are also shown. The stars represent the energy positions of the photoluminescence peaks observed in figure 5.

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Acknowledgements: 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 (grant PNANO MICHRY) and by CNANO from ’r´egion Ile-de-France’ (grant MICRORG). ’Laboratoire de Photonique Quantique et Mol´eculaire’ is a ’Unit´e mixte de recherche associ´ee au CNRS’ (UMR8537).

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In summary, we have realized a 1D-microcavity emitting in the UV range (around 3.5 eV), 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: (C6 H5 C2 H4 − NH3 )2 PbCl4 . This work opens the way to opto-electronic 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 [26].

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FIG. 5: Photoluminescence spectra of the microcavity for detection angles from 5o to 50o (excitation at 325 nm, i.e. 3.82 eV, through the dielectric mirror at normal incidence).

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