Excitons imaging in hybrid organic-inorganic films

Excitons imaging in hybrid organic-inorganic films Amani Trigui, Adnen Mlayah, Younes Abid, Antoine Zwick, and Habib Boughzala Citation: J. Appl. Phys. 112, 093105 (2012); doi: 10.1063/1.4761983 View online: http://dx.doi.org/10.1063/1.4761983 View Table of Contents: http://jap.aip.org/resource/1/JAPIAU/v112/i9 Published by the American Institute of Physics.

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JOURNAL OF APPLIED PHYSICS 112, 093105 (2012)

Excitons imaging in hybrid organic-inorganic films Amani Trigui,1 Adnen Mlayah,2 Younes Abid,1 Antoine Zwick,2 and Habib Boughzala3 1

Laboratoire de Physique Appliqu ee (LPA), Facult e des Sciences, Universit e de Sfax, BP 802, 3018 Sfax, Tunisie 2 Centre d’Elaboration de Mat eriaux et d’Etudes Structurales CEMES-CNRS-Universit e de Toulouse, 29 rue Jeanne Marvig, 31055 Toulouse, France 3 Laboratoire de Cristallochimie et des Mat eriaux, Facult e des Sciences de Tunis, 1092 El Manar, Tunisie

(Received 9 May 2012; accepted 3 October 2012; published online 5 November 2012) In this work we investigate the excitonic properties of (4-FC6H4C2H4NH3)2PbI4 hybrid organic/ inorganic thin films. We first use a standard point-by-point photoluminescence mapping. The maps formed using the photoluminescence intensity, line width, and broadening reveal the presence of structural defects. Using a statistical treatment of the data we found that the spatial fluctuations of the photoluminescence peak wavelengths are rather small compared to the photoluminescence line width. Moreover, we report the first direct observation of spatially resolved excitonic photoluminescence in this type of materials using dark-field imaging of white-light pumped photoluminescence. Owing to the rapid acquisition time of the dark-field images, their temperature dependence was studied, and the thermal behavior of the photoluminescence was investigated using this technique. We show that photoluminescence mapping combined with dark-field imaging and spectroscopy provides valuable information on the excitonic properties of hybrid organic/inorganic C 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4761983] thin films. V INTRODUCTION

Hybrid organic-inorganic materials have emerged as highly promising systems for applications in optoelectronic devices opening up new horizons to nanotechnology.1–4 In particular, the family of metal halide-based hybrid organicinorganic perovskites has attracted much attention because of their unique crystal structures and functional physical properties.5,6 The two-dimensional perovskites are of special interest, mostly in the form (R-NH3)2MX4 (R, CnH2nþ1; X, halogen I, Br, Cl; and M, Pb, Cd, Sn, etc.), which consist in self-assembled multilayers where the organic spacer layers are sandwiched between the inorganic semiconductors layers forming the quantum wells.5–9 Because of the strong confinement of the electronic states in the nanometer-thick quantum wells and the high organic/inorganic dielectric mismatch, the exciton binding energies can be of few hundreds of meV.10–17 In particular, lead halides based compounds form robust excitons with binding energies up to 540 meV in the case of (4-FC6H4C2H4NH3)2PbI4.18–24 Owing to this noticeable property, optical transitions between interacting electrons and holes are characterized by extremely high oscillator strengths. Therefore, PbI-based perovskites exhibit intense photoluminescence,25,26 bright electroluminescence,27,28 large thirdorder optical nonlinearity,29,30 and Rabi-type plasmon-exciton hybridization.30–33 Furthermore, these materials can be processed in films by conventional methods such as thermal ablation,8,34 dip coating,35,36 and spin coating.23,24 In contrast with the numerous published works on the optical properties of hybrid organic/inorganic single crystals, only few works address the relation between the morphology of thin films and their excitonic properties.37–41 Indeed, spatially resolved optical spectroscopy and imaging of excitonic photoluminescence (PL) in hybrid organic/inorganic thin films have rarely been reported.24,42–47 Using PL mapping, Pradeesh 0021-8979/2012/112(9)/093105/7/$30.00

et al.47 have recently pointed out a spectral shift between the optical emissions from spin-coated films and single crystals of di-ammonium-alkyl based (NH3(CH2)12NH3)PbI4. PL scans were however performed with low spatial resolution (20 lm step-size). The present work is devoted to the study of spatially resolved photoluminescence, in (4-FC6H4C2H4NH3)2PbI4 thin films, performed with a 1 lm resolution. In addition to the standard point-by-point mapping technique used in this work, we report a novel approach that allows for direct and rapid PL imaging. This approach makes use of dark-field imaging of white-light pumped PL and is reported here for the first time. We show that at low-temperatures, the excitonic photoluminescence can be visualized directly with a micrometer spatial resolution. A quantitative analysis of the dark-field images allows for determining the temperature behavior of the PL intensity. Both point-by-point PL mapping and dark-field imaging allow for a full characterization and a better understanding of the optical properties of organic-inorganic perovskites thin films. EXPERIMENTS

Single crystals of (4-FC6H4C2H4NH3)2PbI4 (hereafter abbreviated 4FPbI) were grown by slow evaporation of a solution containing lead iodide and (4-FC6H4C2H4NH3I) salts. An aqueous solution of HI was added to the 4-fluorophenethylammonium to synthesize the precursor 4-FC6H4C2H4NH3I. Stoichiometric amounts of 4-FC6H4C2 H4NH3I and PbI2, with HI excess, were mixed in dimethylformamide (DMF). The mixture was stirred till it became clear and was stored at room temperature. After few days, orange needle-like crystals were formed. The crystal structure of 4FPbI has been published elsewhere.18 Briefly, 4FPbI crystallizes in the P21/a space group;

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C 2012 American Institute of Physics V


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˚, the dimensions of the primitive unit cell are a ¼ 16.713 A ˚ ˚ b ¼ 8.625 A, c ¼ 8.800 A. The crystal packing is shown in the inset of Figure 1. It consists in two-dimensional (2D) anionic networks of corner-sharing PbI6 octahedra sandwiched between 4-fluorophenethylammonium cationic layers. The layers extend infinitely in the bc plane and form a stacking along the a-(100) direction. Each lead atom at the center of the octahedron is connected to six iodine atoms at the corners. The organic cations are hydrogen-bonded to the iodine atoms through NH3 groups. Thin films of 4FPbI were formed as follows: single crystals were first dissolved in DMF solvent at a concentration of 200 mg/ml. A drop (1 ll) was then deposited onto a glass substrate which was first cleaned with acetone in an ultrasonic bath. The drop was coated onto the substrate by sliding a thin glass cover on the deposit. The so-formed film was kept in a vacuum chamber at room-temperature. We found that this simple film processing method allows for obtaining thin films (few microns thick) with a rather good reproducibility. A typical film consists in a continuous layer of close packed 4FPbI domains with sizes typically ranging from 1 to 80 lm. In this work, our aim is to study the optical properties of the so-obtained thin films rather than to improve the film homogeneity. The photoluminescence maps were generated using a DILOR XY set-up equipped with a step-by-step displacement stage and a confocal optical microscope. The photoluminescence emission was excited at room-temperature using the 488 nm wavelength of an Argon laser. The laser was focused with a 100 , NA ¼ 0.9 objective leading to a spot size around 0.6 lm2. The laser power was kept as low as 10 lW in order to avoid sample heating. A typical PL spectrum recorded from a 4FPbI thin film is shown in Figure 1. The intense emission around 527 nm is characteristic of radiative excitonic recombinations in 4FPbI.18 High resolution PL maps were generated with a 1 lm step-size. PL maps with lower resolution (2 lm step-size) were also used.

FIG. 1. Typical room-temperature photoluminescence spectrum recorded from a 4FPbI thin film excited with 488 nm laser. The inset shows a sketch of the 4FPbI crystal structure.

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The dark-field imaging and spectroscopy were performed using an XploRA HORIBA Jobin-Yvon setup. The images were acquired using a cooled CCD color camera and illumination by a halogen white lamp through a dark-field 50 microscope objective. The color balance of the camera was adjusted using a white paper as a reference, and no postprocessing of the images was realized. For the lowtemperature measurements, the sample was placed on a cold finger micro-cryostat. A special attention has been paid to maintain a dry environment in order to avoid ice formation. For spectroscopy, the light collected through the 50 darkfield objective was dispersed using a 300 grids/mm grating. The spectra were then corrected for the white lamp emission spectrum and spectrometer/detector response. RESULTS AND DISCUSSION

A bright-field confocal microscope image of a typical 4FPbI thin film studied in this work is shown in Figure 2. The film consists in a continuous layer of close packed 4FPbI domains with sizes ranging from roughly 1 to 80 lm (Figure 2(a)). A zoomed view of one of these domains shows an irregular and complex texture (Figure 2(b)). The latter is more clearly seen in the dark-field images (Figures 2(c)– 2(f)) in which the domain texture resembles that of a leaf

FIG. 2. (a) Bright-field confocal microscope image of a typical 4FPbI film acquired with a 10 objective. (b) Zoom (50 objective) inside square shown in (a). (c) and (d) Dark-field images (50 objective) of (b) at room temperature and close to liquid nitrogen temperature, respectively. (e) and (f) Numerically zoomed views of (c) and (d) inside square shown in (c).


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with principal and secondary veins visible as bright regions. This texture reveals the polycrystalline nature of the 4FPbI domain certainly due to the uncontrolled rapid growth initiated at the small crystal seed located at the center of the domain. The non-uniformity of the 4FPbI domain (Figures 2(c)–2(f)) occurs at the micrometer scale. In the following, we shall discuss the maps generated for the excitonic PL intensity, peak wavelength, and broadening and their relation with the non-uniformity of the 4FPbI domains. On the other hand, by comparing Figures 2(c) and 2(e) with Figures 2(d) and 2(f) one can clearly see that the lowtemperature images are much brighter than the roomtemperature ones. Moreover, their color is green as the photoluminescence emission. In order to understand the origin of the dark-field images and their relation to the excitonic properties of the 4FPbI domains, we have systematically acquired dark-field images for various sample temperatures and analyzed their spectral content. PHOTOLUMINESCENCE MAPPING

A small region (45 45 lm2) of a typical 4FPbI domain has been selected for PL mapping. The bright-field image (Figure 3(a)) shows the non-uniformity of this domain which exhibits a quite regular structure similar to the one observed in Figures 2(e) and 2(f). The photoluminescence has been mapped using a 1 lm step-size. Each PL spectrum was fitted using a gaussian/lorentzian peak lineshape in order to extract the PL intensity, peak wavelength, and its full width at half maximum (FWHM). The corresponding maps are shown in Figures 3(b)–3(d). First, the PL intensity map clearly reflects the 4FPbI domain structure observed in the bright-field image. In 4FPbI, the PL is excitonic in nature even at room-temperature since the exciton binding energy (around 540 meV) is much higher than the thermal energy (25 meV at room temperature). It is very sensitive to the presence of defects. The low intensity regions in the PL map (Figure 3(b)) correspond to regions

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with high density of defects. These are structural defects such as grain boundaries and micro-scale cracks because of the uncontrolled growth of the polycrystalline 4FPbI domains. Second, despite the weak variations (only few nanometers) in the PL peak wavelength and FWHM, the corresponding maps (Figures 3(c) and 3(d)) also reflect the bright-field image (Figure 3(a)). This is due to the fact that the local variations of the dielectric environment around the grain boundaries and micro-cracks are responsible for a systematic red-shift and broadening of the PL emission. By comparing Figures 3(b)– 3(d) one can notice a quite good correlation between the PL intensity, peak wavelength, and broadening: in the regions where the PL intensity is low, the PL peak is red-shifted and broadened. In order to investigate more precisely the impact of the defects on the broadening of the PL emission we have performed a statistical analysis of the data of Figure 3(c). Figure 4 presents a typical PL spectrum and the histogram of the wavelengths forming the map in Figure 3(c). The PL wavelength fluctuates from point to point (Figure 3(c)). As mentioned above, these fluctuations may be caused by local variations of the dielectric environment associated with the grain boundaries and micro-cracks observed in the PL maps and the dark-field images (Figures 2 and 3). The wavelength distribution is peaking at 527 nm, and its FWHM is around 4 nm. This value is an estimation of the inhomogeneous contribution to the broadening of the PL emission one would measure for a spatially integrated PL signal (i.e., for a macroscopic excitation of the film). On the other hand, the FWHM of the typical PL peak is around 16 nm. The latter arises not only from the homogeneous contribution, i.e., the finite exciton lifetime (which may also fluctuate spatially) but also from an inhomogeneous contribution that may occur at a scale smaller than the one investigated in the PL maps. Indeed, since our spatial resolution is around 1 lm, we are able to probe inhomogeneities that occur on that scale only. Possible exciton localization on a sub-micrometer scale may contribute to the 16 nm PL linewidth.

FIG. 3. (a) Bright-field confocal optical microscope view (100 objective) of the investigated region. (b)-(d) High resolution (1 lm step-size) maps of the integrated PL intensity, emission wavelength, and full width at half maximum, respectively. These data were extracted using a fit of a gaussian/lorentzian peak line shape and linear baseline to the PL spectrum measured at each point.


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FIG. 4. Normalized histogram of the PL peak wavelengths, corresponding to map (c) in Figure 3, compared to a typical photoluminescence emission spectrum (full line). The FWHM of the PL peak is around 16 nm whereas that of the histogram is around 4 nm.

DARK-FIELD IMAGING AND SPECTROSCOPY

In order to investigate more deeply the impact of the defects on the excitonic properties, we have performed darkfield imaging of the 4FPbI film at temperatures ranging from 95 K to 295 K. The results are presented in Figure 5. The bright-field image of the 4FPbI domain investigated here and the corresponding room-temperature PL intensity map are shown in Figures 5(a) and 5(b), respectively. The PL map reflects the domain structure and exhibits the same trends discussed in the previous section. More interestingly, the dark-field images (Figures 5(c)–5(g)) also reflect the domain structure (as already noticed in Figures 2(c)–2(f)) and show a good correlation with the PL intensity map (Figure 5(b)). In these images bright points located at the dark regions or at the boundaries between dark and bright regions of the 4FPbI domain (Figure 5(a)) can be seen. The intensity and number of points clearly increase with decreasing temperature (see also Figures 2(c) and 2(f)). In order to analyze quantitatively this effect, a 10 lm2 area (shown in Figure 5(c)) of the dark-field images was selected, and the intensity of the pixels located inside this area was integrated. As shown in Figure 5, the integrated intensity increases linearly with decreasing temperature, and no signature of an exciton mobility edge is observed. Another interesting information is the green color of the dark-field images (Figures 2(c)–2(f) and 5(c)–5(g)). In order to understand the origin of this color we have performed a spectroscopic analysis of the light collected from the small region indicated in Figure 5(c). The spectra measured at high, low, and intermediate temperatures are shown in Figure 6(a). First, in the yellow-red region, the room-temperature spectrum clearly follows the k4 Rayleigh law characteristic of non-resonant elastic light scattering. The spectrum exhibits an abrupt cut-off around 520 nm which corresponds to the ma-

FIG. 5. (a) Bright-field confocal optical microscope view (50 objective) of the investigated 4FPbI domain and (b) corresponding room-temperature photoluminescence intensity map (low resolution: 2 lm step-size). (c)-(g) The corresponding dark-field images recorded at 275, 255, 215, 175, and 95 K, respectively. The scale in these images is the same as in (a). For each temperature a 10 lm2 square region (shown in (c)) has been selected, and the pixels intensities inside this region integrated IT. The bottom-right plot shows IT/Imax as a function of temperature. Imax being the maximum value of the integrated intensity obtained at the lowest temperature. The full line is a linear least square fit to the experimental data.

terial band gap.18 This cut-off arises because incident photons with energies above the material band gap are strongly absorbed and are therefore not present in the scattered light. These missing blue components combined with the k4 law are responsible for the green color of the dark-field images acquired at high temperatures (Figures 2(c), 2(e), and 5(c)). Moreover, this explains why the color of the dark-field images is very similar to that of the photoluminescence emission (Figure 1). Second, in addition to the non-resonant Rayleigh scattering, an intense and narrow peak clearly comes out with


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FIG. 6. (a) Spectra of the elastically scattered light measured at different sample temperatures. The dashed line shows the k4 law valid for non-resonant Rayleigh scattering. (b) Difference between the spectra measured at temperatures ranging from 95 K to 255 K, and the spectrum recorded at room temperature (295 K). The so-obtained difference spectra show only the narrow peak that comes out with decreasing temperature. The intensity of each difference spectrum has been integrated spectrally and normalized to the maximum value Imax; the inset shows the ratio IT/Imax versus temperature. The full line in the inset is a linear least square fit to the experimental data.

decreasing temperature. This peak occurs close to the excitonic PL emission (Figure 1). In order to isolate its contribution we have computed the difference between the spectra recorded at each temperature and the room-temperature spectrum which is supposed to include only elastically scattered light. The results are shown in Figure 6(b). As can be seen, the peak contribution is very weak at high temperatures but clearly comes out with decreasing temperature. In the inset of Figure 6(b), the spectral integral of the peak intensity versus temperature is plotted. It increases linearly with decreasing temperature in agreement with the evolution shown in Figure 5. As a matter of fact, the slopes of the linear intensity variations in Figures 5 and 6 are very similar: 3.3 103 and 4.4 103 K1, respectively, which indicates that the overall intensity increase observed in the dark-field images

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(Figures 5(c)–5(g)) is indeed due to the narrow peak (Figure 6(b)). Notice that the slight difference in the slopes is due to the fact that the spatially integrated intensities, used in Figure 5, include elastic light scattering (dominant at room-temperature) whereas the spectrally integrated intensities (inset of Figure 6) are due only to the narrow peak contribution. Let’s now discuss the origin of this signal. The light collected by the dark-field microscope objective is due either to the light scattered out of the specular direction by the sample inhomogeneities and/or to the photoluminescence emission excited by the incident white light beam. At room-temperature we observe nonresonant elastic light scattering only. But with decreasing temperature, excitonic mediated light scattering may be reinforced and could thus lead to resonant light scattering (i.e., to the narrow peak observed in Figure 6). On the other hand, it is well known that the excitonic photoluminescence emission strongly increases with decreasing temperature18 and could also explain the behavior observed in Figure 6. Both photoluminescence and resonant Rayleigh scattering peaks are expected to occur at nearly the same wavelength since the Stokes shift is very small. It is therefore very difficult to distinguish between both types of processes. In order to address this issue, we have estimated the contribution of the photoluminescence emission using a calibration procedure. We have first measured the photoluminescence intensity excited with the 488 nm laser line at 1 lW/lm2. Then we have converted this PL intensity by taking into account the incident white light intensity (1 nW/lm2) used for dark-field imaging, as well as the sample absorption coefficient. In that way we can estimate the intensity of the white light pumped photoluminescence that may contribute to the dark-field images. We found that the white light pumped PL emission fully accounts for the intensity of the narrow peak intensity observed in Figure 6, thus elucidating its origin. The fact that the PL could be observed even with white light pumping of only 1 nW/lm2 is due to the high excitonic PL efficiency characteristic of 4FPbI. Both photoluminescence mapping and dark-field imaging are complementary techniques. PL mapping reveals the presence of defects even at room-temperature (Figures 3 and 5(b)). However, at room-temperature, the dark-field imaging is mainly due to non-resonant Rayleigh scattering and consequently brings no interesting information on the excitonic properties. However, at low-temperatures, whitelight pumped PL emission dominates and can be visualized directly using dark-field imaging. Obviously, point-by-point PL mapping could also be performed at low-temperature but that is very difficult to achieve in practice. Indeed, around 14 h are needed to record a PL map as the one shown in Figure 3; during such an experiment one needs to maintain a good temperature stability of the sample and avoid ice formation which is very difficult. However, a dark-field image is formed directly on the CCD camera within few seconds. Owing to this very rapid image formation, the dark-field imaging and spectroscopy of the whitelight pumped PL proved to be a reliable technique for studying the optical properties of thin films.


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CONCLUSION

Up to now, exciton localization in hybrid organic/inorganic perovskites has been characterized in terms of Stokes shift48,49 between optical absorption and emission or by measuring the band-gap variation with temperature and comparison with Varshni’s law.18,50 Here we have reported PL mapping of a hybrid organic/inorganic film that addresses the influence of the local fluctuations of the exciton environment on the photoluminescence emission. Using a statistical treatment of these data, we have shown that the PL wavelength fluctuations are within a gaussian-like histogram with a HWHM of 4 nm. This value is nearly four times smaller than the PL linewidth. Moreover, we have reported the first dark-field imaging of white light pumped excitonic PL in this type of materials. Owing to the large oscillator strength of the excitonic transition in 4FPbI, the PL emission is very strong even under moderate white light pumping and can therefore be visualized directly on a CCD camera. This technique is complementary with point-by-point PL mapping and is very useful for investigating the optical properties of hybrid organic/inorganic materials thin films that exhibit inhomogeneities and defects at the micrometer scale. 1

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