Size-Dependent Phase Transition in Perovskite Nanocrystals

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Spectroscopy and Photochemistry; General Theory

Size-Dependent Phase Transition in Perovskite Nanocrystals Lige Liu, Ru Zhao, Changtao Xiao, Feng Zhang, Federico Pevere, Kebin Shi, Houbing Huang, Hai-Zheng Zhong, and Ilya Sychugov J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.9b02058 • Publication Date (Web): 29 Aug 2019 Downloaded from pubs.acs.org on August 29, 2019

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The Journal of Physical Chemistry Letters

Size-Dependent Phase Transition in Perovskite Nanocrystals Lige Liu,†,‡ Ru Zhao,§,∥Changtao Xiao,§ Feng Zhang,§ Federico Pevere,‡ Kebin Shi,† Houbing Huang,*§,∥Haizheng Zhong,§ Ilya Sychugov*‡ †State

Key Laboratory for Mesoscopic Physics, Collaborative Innovation Center of Quantum

Matter, School of Physics, Peking University, Beijing 100871, China ‡Department

of Applied Physics, KTH Royal Institute of Technology, Electrum 229, 16440,

Kista, Sweden §School

of Materials Science & Engineering, Beijing Institute of Technology, 5 South street of

zhongguancun, 100081, Beijing, China ∥ Advanced

Research Institute of Multidisciplinary Science, Beijing Institute of Technology, 5

South street of zhongguancun, 100081, Beijing, China AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] * E-mail: [email protected]

ACS Paragon Plus Environment

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ABSTRACT. Complex structure of halide and oxide perovskites strongly affect their physical properties. Here the effect of reduced dimensions to the nanoscale has been investigated by a combination of single-dot optical experiments with a phase transition theory. Methylammonium lead bromide (CH3NH3PbBr3) nanocrystals with two average particle sizes of ~2 and ~4 nm with blue- and green-photoluminescence, respectively, were spectrally and temporally probed on a single particle level from 5 K to 295 K. The results show that the abrupt blue-shift of the photoluminescence spectra and lifetimes at ~150 K can be attributed to the cubic-to-tetragonal phase transition in the large 4 nm nanocrystals, while this phase transition is completely absent for the small 2 nm particles in the investigated temperature range. Theoretical calculations based on Landau theory reveal strong size-dependent effect on temperature-induced phase transitions in individual CH3NH3PbBr3 nanocrystals, corroborating experimental observations. This effect should be considered in structure-property analysis of ultra-small perovskite crystals.

TOC GRAPHICS

KEYWORDS CH3NH3PbBr3; Single nanocrystal; Phase transition; Ferroelectricity


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Halide perovskite materials reveal different crystal phases. For example, both organometal and inorganic halides possess cubic, tetragonal, and orthorhombic configurations as bulk crystals.1-5 With optical properties of this type of perovskites in focus for photovoltaics and optoelectronics the influence of composition and structure has been thoroughly investigated.6-11 It was found, for instance, that cation plays important role in phase transition temperatures. CsPbX3 perovskites have orthorhombic-to-tetragonal and tetragonal-to-cubic phase transition points at 365 and 407 K, while for CH3NH3PbX3 they are at ~150 K and ~230 K, respectively.2,3 At the same time, quantum dots (nanocrystals with sizes comparable or below Bohr exciton radius) from this class of materials also attract interest because of the established high efficiency and color purity photoluminescence (PL).12-16 Phase transitions for such small nanocrystallites may differ from the bulk, because a large surface-to-volume ratio modifies crystal Gibbs free energy17,18. As a result, optical and other properties may also be affected for perovskite quantum dots, yet these aspects have not been as thoroughly examined as for bulk counterparts.19-22 This is partly due to the fact that most bulk characterization methods are not so straightforward in application to nanocrystals (NCs), which are only a few nanometer in size. X-ray and neutron diffraction peaks suffer from Scherrer broadening, inversely proportional to the nanoparticle diameter. Pair distribution function analysis needs to be applied,23-25 requiring bright sources. Electron beams for direct high-resolution imaging or diffraction may induce local heating or even damage in temperature-dependent experiments. Calorimetry requires nanocrystal solids, where inter-particle distance may vary subject to sample density and surface ligands. On the other hand, optical characterization methods, such as photoluminescence (PL), may also reflect changes in the material structure with temperature.26-30 The stability of particles under the laser beam is an important condition in this case.


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In this work we have investigated individual quantum confined CH3NH3PbBr3 NCs with different average size of 1.7±0.4 (blue PL) and 3.7±0.6 nm (green PL). We used a protective polymer with oxygen-scavenging properties, which layer has successfully stabilized NCs, enabling repeatable single-dot measurements.31 The characterization was systematically done in a temperature range from liquid helium to room temperature, where both individual emission spectra and decay transients were recorded. In general, temperature-dependent PL spectra can provide insight on the bandgap and its temperature dependence by monitoring the emission wavelength, as shown in studies of phase transitions in bulk and thin film perovskites20,32. Here single perovskite QDs were examined to avoid averaging effects of an ensemble. PL transients, on the other hand, carry information on the transition oscillator strength, manifested as the radiative lifetime. In this way a non-invasive and non-destructive probing of small perovskite nanocrystallites was achieved. Abrupt changes in optical properties were observed, reflecting possible phase transitions. By combination with theoretical calculations of para- to ferro-electric phase transformations a strong size effect on the phase change temperature for CH3NH3PbBr3 NCs was confirmed. Results clearly show that temperature induced phase transitions are size-dependent, confirming the hypotheses of the perovskite NC Gibbs free energy role in phase transformations. Large CH3NH3PbBr3 NCs with the size of ~ 4 nm reveal a clear cubic-tetragonal phase transition at ~150 K, as deduced from PL spectra and lifetimes, while the phase transition is absent in smaller NCs with the size of ~ 2 nm. The experimental results agree well with the theoretical calculations which predict a cubic-tetragonal phase transition temperature of ~120 K for 4 nm NCs and close to 0 K for 2 nm NCs. Perovskite CH3NH3PbBr3 NCs with two different sizes were prepared using an emulsion process.33,34 According to the quantum confinement effect, the CH3NH3PbBr3 NCs with a smaller


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size (small perovskite nanocrystals, SPNCs, 1.7±0.4 nm) emit blue light (centered at ~450 nm), while CH3NH3PbBr3 NCs with a larger size (large perovskite nanocrystals, LPNCs, 3.7±0.6 nm,) emit green light (centered at ~500 nm). Figures 1a and 1d show the transmission electron microscope (TEM) images of the SPNCs and LPNCs, respectively. Their corresponding size analysis has been shown in Figure S1. Cubic phase at room temperature was confirmed for these nanoparticles by high-resolution TEM (HRTEM) and X-ray diffraction (XRD) characterization, as shown in Figure 2, which coincides with the reported results.33,35 In Figure 2, both the blue- and green-CH3NH3PbBr3 NCs show main diffraction peaks of (100), (110), (200), (210), (220) and (300) at 15.04°, 21.29°, 30.26°, 33.92°, 43.30°, and 45.98°, respectively, which can be well matched with the corresponding bulk CH3NH3PbBr3 in cubic phase (Pm3m)36. The high resolution TEM images of the blue- and green-CH3NH3PbBr3 NC in Figure 2 show the inter-planar distance of 2.93 and 2.10 Å, which is consistent with the (200) and (220) perovskite crystal facets, respectively. For single dot measurements diluted solution was spin-casted on a silicon substrate. Considering the instability of the perovskites NCs in the surrounding air, a layer of off-stoichiometry thiol-enes (OSTE) polymer was spin-coated over the NCs to protect them from degradation under laser exposure.31 Figure 1b shows room temperature PL spectra of the SPNC single nanodots and ensembles. Figure 1c shows the typical PL spectra of single SPNCs as a function of temperature. It can be seen that the PL peak position and linewidth appear to be temperature-dependent. Broader lines result from the stronger phonon interaction of the quantum confined exciton at higher temperatures. For the LPNCs, the PL spectrum of the ensembles (green line) is broad with Gaussian linewidth of 96 meV, while the spectra (black lines) of the three different single CH3NH3PbBr3 NCs exhibit narrower linewidth of 61, 81, and 87 meV as shown in figure 1e. Peak


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position variation from dot-to-dot and narrow linewidths are indications of a single nanocrystal luminescence. At low temperature, the PL linewidth of the LPNCs is much narrower than that at high temperature. Compared to blue-emitting particles the PL linewidth of the LPNCs is obviously much sharper than that of the SPNCs (Figure 1f) at low temperature.

Figure 1. TEM images, room-temperature PL spectra of single NCs and ensembles, and the PL spectra of the single NCs at various temperatures of the (a, b, c) smaller sized-CH3NH3PbBr3 and (d, e, f) larger sized-CH3NH3PbBr3 dots, respectively. The insets are the photographs of CH3NH3PbBr3 NC solution taken under room and UV light. Figure 3a shows the histogram of the PL peak position distribution of all studied individual perovskite NCs at various temperatures. It can be seen that for the single LPNCs (green bars) upon cooling an abrupt blue-jump of the peak distribution center appears, as indicated by the red arrow. The distribution peak moves from about 510 to 460 nm in the 140-160 K range. This sudden shift takes place on a week background of anti-Varshni bandgap temperature dependence trend, typical for such semiconductors, as discussed below. The blue-jump is caused by the phase transition from


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cubic to tetragonal phase as clarified by the theoretical calculations presented later on. The observed blue-jump is due to the higher intrinsic bandgap of the tetragonal phase than the cubic phase of the perovskite.37,38 The phase transition from cubic to tetragonal is mainly due to the rotation of the PbBr64+ octahedron around the c-axis, which leads to a larger bandgap.39

Figure 2. High-resolution TEM images of the (a) blue-CH3NH3PbBr3 and (b) greenCH3NH3PbBr3 NCs. (c) X-ray diffraction spectra of the blue- and green-CH3NH3PbBr3 NCs and the corresponding CH3NH3PbBr3 bulk. Bulk CH3NH3PbBr3 perovskites have two phase transition points: at ~237 K between cubic and tetragonal phase and ~150 K between tetragonal and orthorhombic phase as was reported based on XRD, neutron powder diffraction, and calorimetric spectroscopy studies.1,40,41 Therefore in the temperature-dependent PL spectra studies on micro-sized single crystals and polycrystalline thin films of CH3NH3PbBr3 perovskites, the obvious PL spectra blue-jump at ~150 K was usually


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attributed to the tetragonal-to-orthorhombic phase transition, while the cubic-tetragonal phase transition at ~237 K was not evident in bulk PL spectra.9,11,37,42,43 These two phase transitions are not very clear in CH3NH3PbBr3 NC ensembles12,17,44 or in CH(NH2)2PbBr3 NC ensembles.42 For our single perovskite NCs, the pronounced PL blue-jump indicates that the phase transition in single CH3NH3PbBr3 NC is much more obvious than in the bulk micro-sized single crystals, probably due to a stronger effect on the bandgap in quantum confined nanocrystals. It is also much clearer than in NC ensembles, most likely due to the suppressed inhomogeneous broadening in single particle luminescence.

Figure 3. (a) Distribution of PL peak positions and (b) statistical distribution of the linewidth (full width at half-maximum, FWHM) of the PL spectra evolving as a function of temperature for individual NCs. The blue and green bars represent the single smaller and larger sizedCH3NH3PbBr3 dots, respectively.


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While a phase transition was indicated by the sudden shift of the PL spectra for the LPNC samples, the SPNC samples appear to be different. We note that no sudden jump was observed in smaller-sized CH3NH3PbBr3 single dots with temperature decreasing from 295 to 5 K. These observations indicate that the PL spectra most likely originates from the same phase. It is the first direct observation of the size-dependent temperature-driven phase transitions in lead halide perovskite NCs. A possible explanation is the size-dependent NC free energy, which makes phase transition less favorable in smaller nanocrystallites. The size-dependent temperature induced phase transition may explain previous inconsistent PL results for ensembles of NCs, where a broad size distribution was typically probed.12,17,44,45 A second important difference from the LPNC samples in SPNC is the lack of the “anti-Varshni” trend. An anti-“Varshni” trend, i.e. decreasing of the bandgap at lower temperature, can be seen for both phases of the LPNC samples, and it is especially obvious for the tetragonal phase. The PL emission red-shift at lower temperature, reflecting decreasing bandgap, is often observed in lead salt46-48 and copper halides semiconductors.49 As opposite to the green LPNC samples, for the blue SPNC sample results show only a weakly varying peak position distribution, remaining roughly the same across the whole temperature range. The blue SPNC ensembles also show the same PL behavior between 295 and 80 K as can be seen in Figure S2. It may reflect a subtle interplay between the opposite effects of a standard bandgap narrowing (blue-shift, Varshni trend), and perovskite-specific band-edge reordering (red-shift, anti-Varshni trend) for the same phase in these ultra-small NCs. The small size effect can be understood from the analysis of temperaturedependent PL linewidth (Figure 3a). As summarized in Figure 3b, with increasing temperature, the Lorentzian FWHM shows a progressive growth for these two samples studied here. This is a commonly observed feature in other semiconductor NCs.50 It also shows that the low-temperature


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linewidth is much broader for the SPNCs (45±9 meV) than the linewidth (8±3 meV) for the LPNCs. Broader PL linewidth in SPNC may stem from higher sensitivity of the exciton to surrounding charges, the so called quantum Stark effect.51 The lack of phase transition in the SPNCs can also be demonstrated by their PL decay transients. Temperature-dependent time-resolved individual PL decays were acquired for all investigated samples (raw data in Figure S3). At room temperature, single SPNC- and LPNC-CH3NH3PbBr3 show decay with short- and long-lived components (Figure 4). The bi-exponential kinetics may originate from different physical scenarios. Spin-flip process27,52 can be ruled out since twocomponent decays persist all the way to high temperatures (see derivations in the Supplementary). Therefore charge trapping and delayed luminescence, common in nanocrystals53-55, is a more plausible explanation in this case. In comparing between decays of different phases we note that for the LPNCs, both components reduced after the conjectured phase transition from cubic to tetragonal phase (from 160 to 140 K). Since the radiative rate lies strictly between these two in the trapping scenario53 we conclude that the radiative lifetime reduces for LPNC, reflecting the phase-dependent transition oscillator strength. At low temperature the lifetime increase for both sizes is a result of the lower-lying partly forbidden state population. At high temperature, where the singlet-triplet manifold population is thermalized, the lifetime should be constant, as observed for blue-emitting dots. Indeed, there is no lifetime change at 140-160 K for the SPNC, again indicating no phase transition in this temperature range (cubic phase only), which is consistent with the observed PL peak position evolution (Figure 3a).


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Figure 4. Histograms of the PL decay times of single NCs at various temperatures. (a) Smallersized CH3NH3PbBr3 and (b) larger-sized CH3NH3PbBr3 NCs. Different background colors represent different crystal phases: green–cubic and orange–tetragonal phase. The above results indicate size-dependent phase transition in CH3NH3PbBr3 NC perovskites. In order to investigate further the size effect in cubic-tetragonal phase transition, we studied the influence of size on the paraelectric-ferroelectric phase transition temperature, based on the Ginzburg-Landau-Devonshire theory. It directly invokes the paraelectric and ferroelectric property of the cubic and tetragonal phase, respectively.56-59 The calculation model is used to illustrate the relationship between the Curie temperature and the diameter of ferroelectric nanoparticles. Considering the innovative application of this model on the metal organic perovskite material, we tested our model on the classical ferroelectric BaTiO3 material first. It proved that the size effect has a great influence on cubic-tetragonal transition temperature, as experiments on BaTiO3 also indicate (Figure S5). The detailed calculations can be seen in the supporting information.


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Figure 5. Dependence of the Curie temperature and Gibbs free energy on the diameter of the CH3NH3PbBr3 NC. The inset is the schematic diagram of the size-dependent phase transition. The calculation results show that the paraelectric-ferroelectric phase transition in CH3NH3PbBr3 exhibits strong dependence on the diameter of nanoparticles. As for other nanoparticles it grows with the nanoparticle size17-18 until the saturation at the bulk value. As shown in Figure 5, the cubic-tetragonal phase transition temperature decreases with the particle diameter, dropping in the calculated diameter range from 100 to 2 nm. Especially this effect is pronounced as an abrupt decrease for sizes below 20 nm. The calculated cubic-tetragonal phase transition temperature is predicted to be 120 K for the LPNC with size of 4 nm, which is in good agreement with the above experimental results (although the phase transition temperature is ~30 K lower than ~150 K from the experiment, tentatively due to the ligand attachment to the NC surface). More importantly is when the particle size further reduces to 2 nm, the phase transition temperature becomes close to 0 K. This means that the perovskite maintains the cubic paraelectric phase at least at the investigated here liquid helium and nitrogen temperature range. It is also in good agreement with the above experimental results that the ultra-small perovskite CH3NH3PbBr3 NCs feature no phase


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transitions. When the particle size is above 20 nm, the theoretical curve does not show significant variations and the cubic-tetragonal phase transition temperature saturates at about 225 K. This value is very close to the cubic-tetragonal phase transition temperature of the corresponding reported bulk CH3NH3PbBr3 perovskite (237 K).3 The Gibbs free energy (blue line in figure 5) increases with decreasing diameter, especially showing an abrupt change below 20 nm, corresponding to the high energy of the small sized NCs. We have also calculated the electrical polarization of the CH3NH3PbBr3 perovskite, and it also shows the size effect (Figure S6). The polarization is almost constant with the value about 0.14 𝜇𝐶/ 𝑐𝑚2 when the diameter is greater than 20 nm, while it reduces to 0.065 𝜇𝐶/𝑐𝑚2 for NCs with diameter of 4 nm. Further reducing the diameter to 2 nm, the value of the electrical polarization decreases to 0 𝜇𝐶/𝑐𝑚2, which means that nanoparticles have transformed to the centrosymmetric paraelectric cubic phase. The above calculation results indicate a strong size-dependent phase transition in CH3NH3PbBr3 perovskite, and have good overall agreement with the experimental results. The inset in Figure 5 schematically describes the size effect in phase transition. For the larger-sized 4 nm green-perovskite NCs, the electrical polarization has an abrupt change at 120 K from calculation results and the bandgap at about 150 K from the experimental results, which indicates a phase transition between cubic and tetragonal phase. For the smaller-sized 2 nm blueperovskite NCs, the electrical polarization maintains the same value during the investigated temperature range, indicating no phase transition. We should note that a little difference between the longer linear organic ligand of ndodecylamine for the SPNCs than n-octylamine for the LPNCs has almost no effect on the particle surface free energy. This is in contrast to the branched ligands shown to make strong contributions to the perovskite surface free energy.60,61 In fact, both of the investigated here spherical perovskite


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NCs have essentially the same long oil acid chains attached to prevent agglomeration among NCs.12 In a word, it is the small particle size of the perovskite NCs that contributes high surface free energy to stabilize the cubic phase of the blue SPNCs. Perovskite QDs have potential for applications as single photon sources62 and light converters33. Their operating temperature spans from the liquid helium for the former to the ambient for the latter. The found here strong temperature effect on the crystal phase implies that a correct phase cannot simply be assumed from the corresponding bulk property. This should be considered for theoretical description of these nanoparticles. Some QDs of the perovskite family, such as CsPbX3, possess phase transition points in bulk above room temperature (365 K and 407 K). Reduction of the transition temperature in smaller QDs signifies that even at room temperature the crystal phase of QDs may exhibit size effect. So it is revealed here that in addition to the size, composition, passivating ligands, etc. the size-dependent crystal phase also affects perovskite QD properties, such as the bandgap and the recombination time. In conclusion, it is known that in bulk halides different crystal phases exist, and those may exhibit deviations from the ideal symmetries, such as tilting or rotation of the octahedra and metal atom displacement, also subject to temperature. Here we have shown that as a result of the crystal size reduction to the zero-dimension level another important correction to the free energy appears. The effect of size-dependent phase transition has been confirmed in individual perovskite NCs experimentally and theoretically. By investigating CH3NH3PbBr3 NCs with two different average sizes of 2 and 4 nm by single-dot PL spectroscopy and lifetime measurements from 295 to 5 K, we found obvious jump in optical properties at ~150 K for the large size CH3NH3PbBr3 NCs, while this was absent in the smaller counterparts. The theoretical calculation results show a clear change of electrical polarization at ~120 K in the large size NCs, corresponding to a cubic-tetragonal phase


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transition, while the phase transition is completely absent in small NCs. The good agreement between the experimental and calculation results demonstrate size-dependent phase transition in individual CH3NH3PbBr3 perovskite NCs and reflect high free energy of small nanoparticles. ASSOCIATED CONTENT Supporting Information. The following files are available free of charge. The fabrication and size characterization of smaller-sized blue- and larger-sized greenCH3NH3PbBr3 NCs. The PL spectra of blue-dots ensemble and PL decay curves. The details of single-dot measurement and theoretical calculations of size-effect of BaTiO3 and CH3NH3PbBr3. (PDF) AUTHOR INFORMATION Present Addresses §Dr.

Feng Zhang, Department of Chemistry, Tsinghua University, Haidian district, 100084 ，

Beijing, China §Changtao

Xiao, Yangtze River Storage Technology Co., Ltd. Gaoxin 4th Road, Donghu

Development Zone, 430000, Wuhan, China Notes The authors declare no competing financial interest. ACKNOWLEDGMENT We would like to thank Pei Zhang for providing the OSTE polymer precursor. We acknowledge the support from the Swedish Foundation for International Cooperation in Research and Higher Education (STINT) and from the China Scholarship Council (CSC).


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Figure 1. TEM images, room-temperature PL spectra of single NCs and ensembles, and the PL spectra of the single NCs at various temperatures of the (a, b, c) smaller sized-CH3NH3PbBr3 and (d, e, f) larger sizedCH3NH3PbBr3 dots, respectively. The insets are the photographs of CH3NH3PbBr3 NC solution taken under room and UV light. 140x73mm (300 x 300 DPI)



Figure 2. High-resolution TEM images of the (a) blue-CH3NH3PbBr3 and (b) green-CH3NH3PbBr3 NCs. (c) X-ray diffraction spectra of the blue- and green-CH3NH3PbBr3 NCs and the corresponding CH3NH3PbBr3 bulk. 85x103mm (300 x 300 DPI)


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Figure 3. (a) Distribution of PL peak positions and (b) statistical distribution of the linewidth (full width at half-maximum, FWHM) of the PL spectra evolving as a function of temperature for individual NCs. The blue and green bars represent the single smaller and larger sized-CH3NH3PbBr3 dots, respectively. 84x68mm (300 x 300 DPI)



Figure 4. Histograms of the PL decay times of single NCs at various temperatures. (a) Smaller-sized CH3NH3PbBr3 and (b) larger-sized CH3NH3PbBr3 NCs. Different background colors represent different crystal phases: green–cubic and orange–tetragonal phase. 83x49mm (300 x 300 DPI)


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Figure 5. Dependence of the Curie temperature and Gibbs free energy on the diameter of the CH3NH3PbBr3 NC. The inset is the schematic diagram of the size-dependent phase transition. 84x60mm (300 x 300 DPI)



TOC 50x45mm (300 x 300 DPI)


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Size-Dependent Phase Transition in Perovskite Nanocrystals

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