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Oct 8, 2018 - Department of Physics, Savitribai Phule Pune University, Pune 411 007, India. §. Synchrotrons Utilization Section, Raja Ramanna Centre ...
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C: Physical Processes in Nanomaterials and Nanostructures

Enhanced Photoluminescence and Stimulated Emission in CsPbCl Nanocrystals at Low Temperature 3

Amruta Ashok Lohar, Aparna Shinde, Richa Gahlaut, Archna Sagdeo, and Shailaja Mahamuni J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b06579 • Publication Date (Web): 08 Oct 2018 Downloaded from http://pubs.acs.org on October 8, 2018

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Enhanced Photoluminescence and Stimulated Emission in CsPbCl3 Nanocrystals at Low Temperature Amruta A. Lohar†, Aparna Shinde†, Richa Gahlaut†, Archna Sagdeo §, ‡ and Shailaja Mahamuni†* †Department of Physics, S.P. Pune University, Pune 411 007, India § Synchrotrons Utilization Section, Raja Ramanna Centre for Advanced Technology, Indore 452017, India ‡ Homi Bhabha National Institute, Training School Complex, Anushakti Nagar, Mumbai 400094, India

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ABSTRACT: CsPbCl3 nanocrystals (NCs) are known to show low photoluminescence (PL) quantum efficiency compared to CsPbBr3 NCs. In the present work, at 20 K, stimulated emission is observed from CsPbCl3 NCs at low excitation intensity. Efforts are made to understand the optical behavior and structural properties of chemically grown, 5.3 ± 1.1 nm sized CsPbCl3 NCs. Bulk CsPbCl3 is known to become cubic at 320 K, while, NCs are cubic or tetragonal at room temperature. Temperature dependent PL studies on CsPbCl3 NCs are carried out in a wide temperature range. The primary experimental finding is, about ninety times rise in PL emission intensity and existence of multiple sharp features at low temperature attributable to free exciton, bound excitons, phonon replica along with the defect related emission and stimulated emission. The optical phonon energy is about 26 meV with coupling constant of 54 meV in ~5.3 nm sized CsPbCl3 NCs. Structural phase transition in CsPbCl3 NCs at about 200 K is also observed.

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INTRODUCTION Hybrid organic-inorganic perovskites as well as inorganic cesium lead halide CsPbX3 (X;Cl,Br,I) perovskites are indeed emerging semiconductors and have shown above 20 % solar cell conversion efficiency1,2 and very high photoluminescence quantum yield. The outstanding optical properties of CsPbX3 perovskites have attracted a great deal of attention. These ionically bonded, cubic or orthorhombic semiconductors are far different than conventional covalent, tetrahedral semiconductors like Si, GaAs, and CdSe. The profound difference is, inorganic perovskite semiconductors are believed to be defect tolerant.3 Most probable defects in these multi-component moieties are vacancies, and related energy levels, that form shallow traps or levels within the bands itself which is essentially a manifestation of antibonding character of the hybrid levels present in valence and conduction band.4 Amongst the conventional semiconductors, CdSe colloidal quantum dots are extensively studied to understand the quantum size effects, and to date, these dots have witnessed application in television screens.5 Bare CsPbX3 perovskite NCs show superior luminescent behavior compared to CdSe based core /shell nanostructures.6

Moreover, the larger inter-particle

attraction in lead halide NCs yields formation of self assembly and super-crystals,7 which is essential for the device fabrication. On the other hand, CdSe core/shell structures are thermally stable as well as photo-stable.6 Limited defect tolerance of CsPbX3 perovskites NCs, even though far better than CdSe and other conventional semiconductors, is noted recently. Halides on the surface of NCs tend to pair and interact with surface amine ligand, which can be removed easily. Fixing of halide defects with Ag(I) complex is demonstrated.8 Further existence of charged NC is evidenced which differs in surface treated and untreated NC.9 The other notable difference between CdSe and

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CsPbX3 perovskites is that perovskites show multiple phases at different temperature. Specifically, in nano-size regime, phase diagram changes substantially and yet to be explored. The ligand-driven phase transformation in perovskite NCs is also addressed.10 Different modes of deformation due to application of hydrostatic pressure are reported in CsPbX3 perovskites.11 The very first protocol proposed12 for preparation of perovskite NCs is similar to that of the traditional semiconductors. Later, it has been shown13 that the simple anion exchange reaction (by chlorine) with CsPbBr3 NCs can change the emission wavelength in blue region and red region by using iodine salts, thus spanning the complete visible region. In the nanocrystalline regime, an added advantage is ease of preparation of perovskite quantum dots compared to the conventional semiconductors due to strong ionic bonding. CsPbBr3 NCs have shown the highest luminescence efficiency of about 95% at room temperature and are the most studied inorganic perovskite NCs. Nonetheless, CsPbCl3 is known as one of the most photoluminescent14 semiconductors in bulk form having emission wavelength close to 400 nm. On the contrary, the reported PL emission quantum yield of colloidal CsPbCl3 NCs is about 10 %.12 In spite of defect tolerance of perovskites, such a meager quantum yield is rather surprising. It is worth to understand the comparatively lower emission efficiency of perovskite NCs near the blue region. The unique feature of single crystalline perovskite CsPbCl3 is, it undergoes

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successive phase

transitions in a narrow temperature regime of 310 K to 320 K. Consequently, it is referred as crystalline liquid in the literature. It is yet unresolved whether similar phase transitions in the nanocrystalline regime are responsible for generation of trap levels and subsequently lead to the lower value of PL quantum efficiency. In spite of lower PL quantum efficiency of CsPbCl3 NCs, miscibility of it with other perovskites halide NCs is exploited16 to explore the feasibility of the mixture for light emitting devices.

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Substantial difference is noted for single crystal CsPbCl3 and nanocrystals thereof. In single crystal CsPbCl3, at 77 K, under high excitation intensity, six different PL features from free exciton emission, phonon shifted lines of free exciton along with the stimulated emission were reported.17 On the other hand, single excitonic emission peak was also observed18 for mono crystalline CsPbCl3 at low temperature, which was deconvoluted in various features. Nanocrystalline CsPbCl3 grown from amorphous moiety revealed14 PL dominated by free exciton emission in contrast to the single crystal (whose PL shows profoundly trapped luminescence) at 77 K. At higher excitation intensity free exciton luminescence is accompanied by features related to exciton-phonon inelastic interactions and the stimulated emission. Literature survey indicates conflicting results as well. For instance, at 77 K, PL of single crystal CsPbCl3 reveal features19 at 3.01 eV (free exciton), 2.99 eV, 2.98 eV, 2.97 eV (phonon assisted transitions), 2.96 eV (for stimulated emission) and 2.94 eV (phonon replica). Optical phonon energy of 26 meV is noted20 in CsPbCl3. The optical absorption feature of 4.00 nm sized, nanocrystallites is reported at 3.00 eV at 77 K.14 PL spectra comprise of feature at 3.02 eV corresponding to free excitons. At high intensity (70 kW/cm2) and at 77 K, features at 3.02 eV, 3.00 eV and 2.97 eV corresponding to free excitons, phonon replica and stimulated emission are seen. Temperature dependent PL was measured to explore exciton-phonon interaction in CsPbCl3 NCs with size of 10.7 nm21 having optical absorption feature at 3.09 eV and PL emission at 3.04 eV. Single PL emission peak was observed at 300 K through 5 K. The energy of the PL emission feature displayed initial blue shift followed by the red shift. The structural phase change between 175 K and 200 K is conjectured to play a crucial role. Phase transition at 194 K in CsPbCl3 is also reported by others.22,23

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An anomalous red shift observable at low temperature in cesium lead halide in single crystal and nanocrystalline regime is still not fully understood. Strong exciton-phonon coupling is observed in conventional semiconductor NCs. The interaction of electrons with phonons is one of the important parameters in carrier dynamics and is yet to be understood in “novel” ionic semiconductors. Recently, we observed24 stimulated emission from CsPbBr3 NCs at low temperature under very weak pumping intensity. Inspired by this observation, PL properties of ~5 nm sized CsPbCl3 NCs are explored at low temperature. A complex behavior of amplified spontaneous emission as a function of temperature, trap density, and incident light intensity in CsPbBr3 nanocrystalline films is also reported25,26 now. However, the PL efficiency of CsPbBr3 NCs is very high at room temperature compared to that of CsPbCl3 NCs. Most importantly, CsPbCl3 NCs suffer from phase transitions at low temperature which was not observed in CsPbBr3 NCs or in bulk CsPbCl3. The primary experimental finding of the present work is, ninety times rise in PL emission intensity at low temperature and existence of three multiple sharp features which are attributable to the bound excitons, phonon replica along with the stimulated emission from CsPbCl3 NCs.

EXPERIMENTAL SECTION Cesium lead chloride (CsPbCl3) NCs were prepared by reported hot injection method with slight modifications.12 Initially, to prepare Cs-oleate, 0.814 g of Cs2CO3 was loaded into 100 ml 3-neck flask along with 40 ml octadecene (ODE) and 2.5 ml oleic acid (OA). This solution was degassed by nitrogen gas flow for half an hour. The solution was dried for 1 hour at 120 ˚C and then heated up to 150 ˚C until all Cs2CO3 reacted with OA (about 10 minutes). The mixture was

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cooled down to room temperature and sealed in 15 ml vial for further use. An equal ratio of oleylamine and oleic acid of 0.5 ml each added into ODE with 0.188 mmol of PbCl2 in 50 ml three neck flask. In this mixture PbCl2 dissolves and stabilizes NCs in colloidal solution form.12 The mixture purged for ½ an hour with N2 gas flow and heated upto 120 ˚C under N2 gas flow. This setup was kept for an hour at 120 ˚C. Temperature was raised upto 150 ˚C followed by addition of 1 ml trioctylphosphine. After 5 minutes, 0.4 ml Cs-oleate solution was swiftly injected and 5 s later the reaction mixture was cooled in ice water bath. The crude solution was centrifuged at 5000 rpm for 3 min. NCs are separated from crude solution and used for further characterization. X-ray diffraction (XRD) measurements were carried out on synchrotron radiation at beamline (BL-12) on Indus-2 synchrotron source; data was recorded using Image plate (Mar 345) area detector. XRD data reduction was carried out using Fit2D software.27 Transmission electron microscopic (TEM) measurements were carried out on FEI, TECHNAI G 20U-TWIN instrument. UV-Visible absorption measurements were carried out on Perkin Elmer Lambda 950 spectrometer, while PL measurements were carried out on home built PL set up. It consists of excitation monochromator (Jobin Yvon triax 180), an emission monochromator (Jobin Yvon iHR 320) and photomultiplier (PMT) as detector for Xenon lamp (450 W). Janis closed cycle cryostat was used for low temperature measurements.

RESULTS AND DISCUSSION Figure 1 (a) shows the optical absorption and PL emission curve of a typical nanocrystalline CsPbCl3. The optical absorption appears at about 3.14 eV (395 nm) and PL emission maximum is located at 3.04 eV (407 nm) at room temperature. Somewhat larger Stokes shift at room

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temperature indicates that the emission from CsPbCl3 NCs is not due to the band edge transition, but shallow traps participate in the PL emission. PL emission at room temperature is not symmetric further confirming presence of low energy features. X-ray diffraction pattern was recorded using synchrotron radiation with incident wavelength of 0.623 Å and analysed by Rietveld refinement [Figure 1 (b)]. A comparison with the standard data [JCPDS pdf-84-0438 file] indicates the cubic perovskite structure with space group Pm3m. It may be noted that, at room temperature, bulk CsPbCl3 crystallizes in the orthorhombic structure. Not only size of NCs, but also organic surface ligands play detrimental role28 in stabilizing the crystal structure. TEM image [Figure 1 (c,d)] shows average size of NCs to be 5.3±1.1 nm. Selective area electron diffraction pattern also confirms cubic structure of CsPbCl3 NCs. It is worth mentioning here that presence of tetragonal phase cannot be denied which cannot be distinctly noted due to broadening of XRD features in nanocrystalline regime [See Table S1]. At 300 K, PL spectra [Figure 2 (a)] were fitted with four features F (3.10 eV), B (3.04 eV), D1 (2.97 eV), and D2 (2.90 eV) having width [full width at half maximum (FWHM)] to be 75 meV, 71 meV, 88 meV, and 96 meV respectively. Comparison of the present work with the published work18 on single crystal CsPbCl3 indicates that the first two features are attributable to free excitons and bound excitons. The broader and less intense features could be due to the defect related transitions associated with halogen vacancies and structural defects (detail explanation is given below). Temperature dependent PL studies on CsPbCl3 NCs are carried out from 20 K through 300 K. Remarkable rise in PL emission intensity is seen at low temperature along with the appearance of multiple sharp features [Figure 2 (b)]. At 20 K, PL feature could be fitted in to five different

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components. The energy of various components is 3.04 eV (F), 3.02 eV (B), 2.99 eV (P), 2.97 eV (S), and 2.96 eV (D2), while the width is 32 meV, 22 meV, 21 meV, 8 meV and 22 meV respectively. Substantially different PL emission spectra of CsPbCl3 NCs compared to CsPbBr3 NCs are primarily due to the possibility of different structural phases present in CsPbCl3 NCs. At low temperature, ninety fold improvement in the PL emission is observed, which is remarkably larger than CsPbBr3 NCs (only about two fold improvement in PL intensity is observed in CsPbBr3 NCs)17. This observation indicates CsPbCl3 NCs are prone to have shallow defects compared to CsPbBr3 NCs along with the faults that could arise due to stabilizing multiple phases. It may be noted that very commonly observable defect in CsPbBr3 single crystal is Br vacancies.18 On the similar lines, Cl vacancy formation requires the least energy in CsPbCl3. Additionally, in case of CsPbCl3, Pb2+ cation defects are reported.29 Recently, intrinsic point defects are observed in CsPbBr3. It is found that PbBr, BrPb, and Pbi introduces deep trap states, while most of other intrinsic point defects induce shallow states.30 Five intrinsic point defects, namely VCs, VBr, Csi, Pbi, and PbBr antisites are confirmed by thermally stimulated current measurements on melt grown CsPbBr3 crystals.31 In CsPbCl3 NCs, surface defects are predominantly observed, which arise due to Cl ion vacancies32 and Pb rich surface.33 The structural defects due to chlorine vacancies34 also affect short range ordering. The low PL efficiency of CsPbCl3 NCs is associated34 with structural disorder introduced by VCl. Presence of different polymorphs like tetragonal and orthorhombic phases in CsPbCl3 NCs cannot be denied and could also be the prime origin of defect levels. Moreover, the strong electron-phonon interaction could be expected in CsPbCl3 NCs due to Fröhlich interactions. At low temperature, phonon contribution would be minimal, further causing an improvement in PL emission intensity.

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Furthermore, integrated PL intensity as a function of temperature clearly shows three different regimes as seen from Figure 3(a). From room temperature to about 200 K, the intensity of PL emission increases with lowering temperature. At about 200 K, it takes sudden jump, which is followed by slight decrease in the intensity which remains more or less constant up to about 125 K. Further decrease in temperature from 125 K to 20 K leads to substantial rise in the intensity. The PL data suggests structural phase transitions at about 200 K which leads to the spike in PL intensity. The typical trend in PL intensity as a function of temperature is repeatedly seen in the different set of samples. Similar, phase transition in CsPbCl3 is predicted23 on the basis of Raman studies. Such a phase transition could be responsible for sudden rise in the intensity at 200 K. At about 125 K, the rise in the intensity is a manifestation of diminishing phonon perturbation. As seen from Figure 2(b), a sharp peak, named S emerges at 2.97 eV (417 nm). Initial broader feature D1, which was seen at room temperature, cannot be fitted in PL spectra. Feature S shadows the presence of D1. Notably, the difference between B and F features is equal to 22 meV while difference between B and S is 50 meV which is about twice of that for the bound exciton. In CsPbX3 quantum structures, biexciton-driven stimulated emission is reported to be red shifted by 50 meV.35 These experimental facts lead us to presume that the sharp feature at 2.97 eV is due to the stimulated emission. The stimulated emission is also reported6 at 2.97 eV for 4.00 nm-sized CsPbCl3 NCs and at 2.6 eV for bulk CsPbCl3 at 77 K. At higher incident intensity (ps pulsed diode laser at 405 nm), profound rise in stimulated emission is observed, further corroborating the assignment of the feature at about 2.97 eV [Figure S1]. Temperature dependent X-ray diffraction (XRD) patterns [Figure S2] reconfirm structural phase transition at about 200 K. At lower temperature, XRD patterns are complicated mixture of orthorhombic distortion along with the presence of tetragonal and cubic phase. Due to the

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presence of mixed states in NCs, and in absence of the standard JCPDS data for orthorhombic CsPbCl3, attribution of XRD features is not feasible in the present case. The detailed study of different CsPbCl3 phases is a topic of our future work. CsPbCl3 NCs are susceptible to structural deformation compared to its bromide counterparts. The possibility of presence of multiple phases in CsPbCl3 NCs could be responsible to generate defect levels and hence causing variation in the PL intensity. The temperature-dependent PL peak width broadening of bound exciton peak is plotted in Figure 3(b) and fitted using Boson model36 𝜞𝒐𝒑

𝜞(𝑻) = 𝜞𝒐 + 𝝈𝑻 +

(2)

(𝑬𝒑𝒉

𝒆

𝒌𝑩 𝑻)

―𝟏

where the first term Γo is the inhomogeneous broadening constant due to exciton-exciton scattering. The second term σ is the exciton-acoustic phonon coupling coefficient, and the third term Γop is the exciton-longitudinal optical phonon coupling coefficient or the Fröhlich coupling coefficient with Eph being optical phonon energy. These coupling coefficients contribute to the line width broadening. It can be observed that the FWHM has a significant temperature dependence implying strong exciton–phonon interactions in CsPbCl3 NCs.21 This leads to wide thermal broadening of emission as the temperature is increased from 20 to 300 K. It was found that inhomogeneous broadening is Γo = 22.3 ± 2.5 meV, acoustic phonon coupling is σ = 81.3 ± 16.9 μeV/K, optical phonon coupling is Γop = 54.3 meV, and optical phonon energy is Eph = 25.96 meV. These results are consistent with Raman scattering experiment on CsPbCl3, which show optical phonon energy to be 27.65 meV.20,23 Energy of PL emission features further indicates interesting behavior. Energy of free exciton feature shows continuous red shift with decreasing temperature [Figure 4]. Such a red shift in the

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forbidden gap is unique in perovskite lead halide semiconductors in contrary to the conventional semiconductors. The anomalous red shift in band gap is due to dominating effect of thermal expansion of lattice over electron phonon interaction.5-8 On the contrary, bound exciton feature B, displays subtle blue shift, followed by the red shift with decreasing temperature. Binding energy involved in the formation of bound exciton being independent of the lattice contraction or expansion, its energy does not change appreciably. Feature P is phonon replica of bound excitons and its energy varies weakly with temperature. It may also be noted that ninety fold rise in PL intensity of CsPbCl3 NCs is primarily due to the enhancement [Figure S4] in bound exciton emission. Intensity of free exciton emission improves at lower temperature in a comparatively subtle way. PL quantum yield (PLQY) recorded at room temperature with respect to anthracene in cyclohexane is 7 %. Such low PLQY is due to carrier traps which give rise to non-radiative recombination32–34 Kondo et al.14,37–39 have reported PL of CsPbCl3 in various forms. The interesting result is observed in microcrystals. The large optical gain due to exciton-exciton superradiance is observed. It is possible that in the present work, such a phenomenon would be instrumental for peculiarly intense PL emission. Excitonic superradiance is a cooperative radiation process of a coherently delocalized exciton in a crystal, leading to a very short radiative decay time depending on the coherence length (the extent of the coherently delocalized exciton).40 Since the coherently delocalized exciton state is a superposition of all the lowest electronic excited states in individual unit cells of the crystal, this phenomenon has been observed41 in microcrystalline matter. In the bulk, due to the lattice imperfections and lattice vibrations even at low temperature, such a phenomenon is not observable. In case of very small crystallites (microcrystal) the exciton coherence length is

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limited40,41 to the size of the crystallite and thus exciton can be coherently delocalized throughout the crystallite. Further increase in the size of crystallite will show quenched excitonic superradiance. In large sized crystallites weak luminescence has been observed41 due to dephasing-induced quenching of excitonic superradiance. The main cause of the dephasing in the excitonic superradiance may be exciton-phonon scattering rather than lattice imperfections in the microcrystals. In case of CsPbCl3 NCs at room temperature, PL intensity is very low, while ninety times increase is observed at 20 K. The drastic change in PL intensity can be explained by excitonic superradiance. At room temperature, non-radiative decay rate is profound than radiative decay rate leading to very low quantum yield of CsPbCl3. Non-radiative decay might be due to the large exciton-phonon scattering and defects associated with distortions or multiple phases in CsPbCl3 NCs. Very recent report indicates an increase in PLQY (96 %) for CsPbCl3 NCs with nickel doping caused by improved local structural ordering by eliminating Cl vacancies.34 The activation energy corresponding to the bound exciton is determined [Figure S5] to be 23 meV. Further, the difference between features F and B is about 20 meV, which is close to that determined from the intensity as a function of temperature, however, it does not match with the reported value of 75 meV in case of single crystal. The discrepancy in the results is due to the fact that the present work is carried out on ~5.3 nm sized CsPbCl3 NCs with large number of defects and which also show phase transition around 200 K.

CONCLUSIONS In summary, low PL quantum efficiency along with the large value of Stokes shift provides an evidence of defects in CsPbCl3 NCs at room temperature. In contrary to the traditional

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semiconductors, lead halide perovskites show decrease in the forbidden gap at low temperature. In nanosize regime, the same behavior persists. It is believed42,43 that due to thermal expansion of lattice at high temperature, the interaction between valence s and p bands reduce, further narrowing the valence band width and hence increasing the value of the forbidden gap. The optical phonon energy is found to be Eph = 25.96 meV in about 5 nm sized CsPbCl3 NCs. Interesting experimental finding is, ninety times rise in the PL intensity at low temperature which is due to excitonic superradiance. Rather low value of quantum efficiency of CsPbCl3 NCs compared to bromide analogue is due to the defects and deformation present. Three sharp PL emission features are observed at low temperature similar to the bulk CsPbCl3. Proper fitting of the data indicates presence of free excitons, bound excitons, its phonon replica, stimulated emission and defect related emission in CsPbCl3 NCs. The variation in the relative intensity of the features was seen in the samples prepared under the identical conditions, thereby re-affirming existence of multiphase structure.

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FIGURES

Figure 1. (a) Optical absorption and photoluminescence emission spectra recorded with the excitation wavelength of 375 nm (3.3 eV), (b) Rietveld refined X-ray diffraction pattern recorded using synchrotron radiation with incident wavelength of 0.623 Å, (c) Transmission electron microscopic image of CsPbCl3 NCs with SAED pattern and (d) statistical analysis of particle size distribution.

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Figure 2. PL spectra recorded with incident wavelength of 375 nm at (a) 300 K and (b) 20 K.

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Figure 3. (a) Integrated PL emission intensity and (b) FWHM as a function of temperature.

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Figure 4. Variation in the energy of PL emission features with temperature.

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ASSOCIATED CONTENT Supporting Information PL spectrum of CsPbCl3 NCs with pulsed laser at 15 K, Temperature dependent XRD, Rietveld analysis for tetragonal phase, Variation in PL intensity with temperature, Intensity as a function of temperature for bound exciton, and Low temperature PL of PbCl2. AUTHOR INFORMATION Corresponding Author *E-mail id: [email protected] ORCID Shailaja Mahamuni: 0000-0002-4668-4730 Present Addresses †Department of Physics, Savitribai Phule Pune University, Pune, India. ACKNOWLEDGMENTS A.A.L. thanks Savitribai Phule Pune University, Pune for the financial support as a fellowship to M.Phil. student and from UPE grant. R.G. thanks Dr. D.S.Kothari Postdoctoral Fellowship Scheme, UGC, New Delhi. A.S. thanks to Council of Scientific and Industrial Research for SRF. S.M. thanks the Department of Science and Technology for the research grant. Dr. A. K. Sinha, RRCAT Indore has generously allowed us to record XRD measurements at the synchrotron facility. Thanks to M. N. Singh for helping in low temperature synchrotron XRD measurements.

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