In Situ Raman Spectroscopic Studies of Thermal Stability of All-Inorganic Cesium Lead Halide (CsPbX3, X = Cl, Br, I) PerovskiteNanocrystalsMengling Liao, Beibei Shan, and Ming Li*

School of Materials Science and Engineering, State Key Laboratory for Power Metallurgy, Central South University, Changsha,Hunan 410083, China

*S Supporting Information

ABSTRACT: Thermal degradation becomes the main obstacle for industrialapplications of all-inorganic cesium lead halide (CsPbX3, X = Cl, Br, I) perovskiteoptoelectronic devices. A complete understanding of thermal degradation of CsPbX3perovskites is required but greatly challenging for achieving optoelectronic devices withlong-term stability, particularly under extreme settings. Herein, we present an in situspectroscopic study of thermal stability of CsPbX3 nanocrystals between the cryogenictemperature and high temperature. The low-frequency Raman signatures of CsPbX3nanocrystals dramatically evolve but differentiate from the halogen atoms at elevatedtemperatures, acting as potent indicators of their crystalline structures and phasetransitions. The merging of doublet Raman bands of CsPbX3 nanocrystals indicatestheir high-temperature phase transitions. CsPbX3 (X = Br, I) nanocrystals undergo astate of high degree of disorder with featureless Raman spectra before being thermallydecomposed. Such understanding is of particular importance for future design andoptimization of high-performance CsPbX3 perovskite devices with long-term stabilityunder extreme settings.

The growing interest in semiconducting lead halideperovskite nanocrystals (NCs) mainly stems from their

outstanding photovoltaic and optoelectronic properties forpromising applications in photovoltaic cells, light-emittingdiodes (LEDs), lasers, and photodetectors.1−7 Recently, all-inorganic cesium lead halide (CsPbX3, X = Cl, Br, I) perovskiteNCs have been the subject of intense research, mainly owingto their salient features of ultrahigh photoluminescence (PL)quantum yields (∼100%), narrow emission bandwidths, size/composition-tunable emission wavelength, broadband absorp-tion, large exciton diffusion length, good electron mobility, andlow-cost fabrication routes.8−10 CsPbX3 NCs are typicallyachieved by a hot injection method or a room-temperatureprecipitation method commonly used in the literature.3,11−15

Despite the impressive improvement in intrinsic long-termstability in comparison with their organic−inorganic hybridperovskite counterparts,16,17 these CsPbX3 NCs are extremelysensitive to the external environment (i.e., moisture, solvents,light, temperature, pressure, surface ligands) and undergodramatic changes in structures and optoelectronic propertiesaccordingly, thereby degrading the device performance.18−25

The structural instability has been one of the most importantobstacles for practical applications of CsPbX3 perovskites.

26,27

CsPbX3 NCs typically exist in orthorhombic, tetragonal, orcubic phases, among which the cubic phase is highly symmetricand thermodynamically more stable than both tetragonal andorthorhombic ones at high temperatures.28 These phases couldtransform into each other by controlling the temperature.

Bulky CsPbCl3 typically displays a thermodynamicallyfavorable orthorhombic phase at ambient temperature, andits phase transition into the tetragonal phase takes place at 42−47 °C and into the cubic phase above 47 °C.29 CsPbBr3possesses the orthorhombic structure at ambient temperature,which is transformed to the tetragonal phase at 100 °C andthen to the cubic phase above 130 °C.28 As for CsPbI3, theblack (cubic) perovskite phase that shows the desirablephotovoltaic and optoelectronic properties is stable onlyabove 300 °C.30,31 Upon cooling, the black perovskite phaseundergoes a phase transition to thermodynamically favorableyellow orthorhombic nonperovskite phase with poor photo-voltaic and optoelectronic properties. In practical applications,CsPbX3 perovskite optoelectronic devices (i.e., LEDs and solarcells) may encounter a temperature between the cryogenictemperature (<−180 °C) and high temperature (>500 °C) inthe field during the fabrication and operation of devices, e.g.,space exploration or deep drilling.27 Field temperature changesmay cause device performance degradation through temper-ature-induced structural changes or thermal decomposition,which seriously hampers their usage in such robust environ-ments.32−38 Studies showed the dramatic performancedegradation of perovskite solar cells upon heating at orabove the operation temperature (>150 °C).32,33,39 Also,

Received: February 6, 2019Accepted: March 1, 2019Published: March 1, 2019

Letter

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changes in PL emission of CsPbX3 perovskite NCs have beenobserved at elevated temperatures, indicating temperature-induced performance degradation of LEDs.35−38 To date,substantial efforts have been made to address the thermalinstability of CsPbX3 perovskites and to understand theirdegradation mechanisms.40−44 Remarkable correlations wereobserved between spectral features of Raman and structuralchanges of CsPbX3 at temperatures from the ambienttemperature to <250 °C.45,46 In addition, Raman spectroscopyrevealed the distinct low-temperature order−disorder tran-sition between organic−inorganic hybrid CH3NH3PbBr3 andall-inorganic CsPbBr3 perovskites.47−49 Unfortunately, thesestudies were performed at either low or high temperatures orin a narrow temperature range. Thus, some questions regardingthe thermal stability of CsPbX3 perovskites remain unclear: (1)How does the thermal stability scale with temperature,particularly at temperatures between the cryogenic temper-atures and high temperatures, such as those usuallyencountered in space exploration and deep drilling? (2)What are the final products of thermally decomposedphotoactive CsPbX3 NCs? (3) What is the difference inthermal stability among CsPbX3 NCs with different halogenatoms?In this work, we address the aforementioned questions

through investigating structural evolution of CsPbX3 NCs overthe broad temperature range from −190 to 500 °C using insitu Raman spectroscopy in combination with in situ X-raydiffraction (XRD) techniques. Our results revealed that Ramansignatures of CsPbX3 NCs underwent dramatic variations with

temperature, which indicates successive phase transitions andintermediate phases unidentifiable by traditional XRDmeasurements. We further unveiled the halogen-dependentthermal stability among CsPbX3 NCs. Both CsPbBr3 andCsPbI3 NCs underwent a state of high-degree of disorder athigh temperatures before finally being decomposed into theirconstituents. These findings provide in-depth insights into thethermal degradation of CsPbX3 NCs and benefit furtherexploration for achieving CsPbX3 NC devices with long-termstability under robust environments.CsPbX3 NCs were first synthesized by a hot-injection

method in which the cesium oleate precursor was injected intoa 1-octadecene (ODE) solution containing lead halide salts(PbCl2, PbBr2, or PbI2), oleic acid (OA), and oleylamine(OAm).11,50 Both OA and OAm were used as organic ligandsto stabilize the CsPbX3 NCs. Optical images show that CsPbX3NCs prepared here display distinct visual colors under ambientlight and emit different PL under UV irradiation (Figure 1A).Under ambient light CsPbX3 NCs appear white, orange, andblack for X = Cl, Br, and I, respectively; they emit blue, green,and red PL under UV light. The absorption spectra show clearband-edge absorption centered at 405 nm for CsPbCl3, 512nm for CsPbBr3, and 681 nm for CsPbI3, and thecorresponding PL peaks are positioned at 408 nm with a fullwidth at half-maximum (fwhm) of 9 nm for CsPbCl3, 519 nmwith a fwhm of 18 nm for CsPbBr3, and 694 nm with a fwhmof 33 nm for CsPbI3 (Figure 1B). The narrow fwhm verifiesthe high homogeneity of the prepared CsPbX3 NCs. XRDpatterns unambiguously substantiate the tetragonal phase for

Figure 1. Optical properties and structural characterization of CsPbX3 NCs. (A) Optical images of CsPbX3 NCs under ambient light (top) or 365nm UV light (bottom). (B) Optical absorption and PL spectra of CsPbX3 NCs in hexane. (C) Ex situ powder XRD patterns of CsPbCl3, CsPbBr3,and CsPbI3 NCs. The standard XRD patterns of all three CsPbX3 NCs are shown as well (JCPDS no. of tetragonal CsPbCl3, 010-9294; JCPDS no.of orthorhombic CsPbBr3, 009-7851; JCPDS no. of cubic CsPbI3, 016-1481). (D) TEM images of (i) CsPbCl3, (ii) CsPbBr3, and (iii) CsPbI3 NCs.The dark spots in the TEM images result from the in situ Pb2+ reduction into Pb metals by the electron beam during the TEM measurements, asreported in the literature.51 All scale bars in panel D are 50 nm.

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the as-prepared CsPbCl3 NCs, orthorhombic for the as-prepared CsPbBr3 NCs, and cubic for the as-prepared CsPbI3NCs without any secondary phase observed (JCPDS no. oftetragonal CsPbCl3, 010-9294; JCPDS no. of orthorhombicCsPbBr3, 009-7851; JCPDS no. of cubic CsPbI3, 016-1481)(Figure 1C). TEM images reveal a cubic shape and narrow sizedistribution for all three CsPbX3 NCs. Their average edgelengths are 22.3 ± 2.6 nm for CsPbCl3 NCs, 24.7 ± 4.3 nm forCsPbBr3 NCs, and 26.6 ± 3.4 nm for CsPbI3 NCs (Figures 1Dand S1). All these results confirmed the successful synthesis ofhigh-quality CsPbX3 NCs.CsPbX3 NCs exhibit three different structural phases, which

are cubic (Pm3m), tetragonal (P4/mbm), and orthorhomobic(Pnma) (Figure 2A). Successive phase transitions amongcubic, tetragonal, and orthorhombic phases could take place bychanging the temperature. These structural changes couldlower the space group symmetry because of condensations ofthe zone boundary phonons associated with rotations of the[PbX6] octahedra around the high-temperature cubic axes andchange the number (Z = 1−4) of CsPbX3 formula per unit

cell.28,50 According to the Lyddane−Sachs−Teller relation,29,45the high-symmetry perovskite-type structure possesses threenondegenerate longitudinal optical (LO) and three doublydegenerate transverse optical (TO) phonon modes grouped as((LOi, TOi), i = 1−3), with ωLO > ωTO, where ωLO and ωTO

are the frequency of the LO and TO phonon modes,respectively. To implement in situ Raman measurements attemperatures between −190 and 500 °C, the as-preparedCsPbX3 NCs were drop-coated onto a cleaned silicon wafer.The temperature profile adopted for the in situ Ramanmeasurements is shown in Figure S2. The starting temperaturewas set at 25 °C followed by the cooling process, allowing us toobserve the structural evolution in the low-temperature region(25 °C to −190 °C) of the as-prepared CsPbX3 NCs withthermodynamically favorable perovskite structures. All CsPbX3

NCs exhibit sharp low-frequency (<250 cm−1) Raman bands atcryogenic temperatures due to the heavy ion (Pb and Cs)effects (Figures 2B and S3−S5). At −190 °C, low-frequencyRaman bands were observed at (52, 72, 89, 110, 121, and 203)cm−1 for CsPbCl3 NCs, (52, 67, 74, 81,87, 101, 129, and 133)

Figure 2. Temperature-dependent Raman profiles of CsPbX3 NCs. (A) Schematic illustration of structures of orthorhombic, tetragonal, and cubicCsPbX3 NCs. (B) Temperature-dependent Raman spectra at representative temperatures of (i) CsPbCl3, (ii) CsPbBr3, and (iii) CsPbI3 NCs. Insitu Raman measurements were started from 25 °C, and then the temperature was reduced to −190 °C to monitor the spectral evolution ofCsPbX3 NCs in low-temperature regions. The marked yellow and green regions in ii and iii represent the decomposition of CsPbBr3 and CsPbI3NCs. More detailed information regarding in situ Raman spectra of CsPbX3 NCs can be found in Figures S3−S5.

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cm−1 for CsPbBr3 NCs and (58, 68, 89, 101, 111, and 119)cm−1 for CsPbI3 NCs. These narrow Raman bands areconsidered as the first-order resonance modes with no or veryweak overlaps at cryogenic temperatures. In general, thedegenerate TOi modes appear as clear doublets at cryogenictemperatures, gradually broaden, and eventually merge intoone broad band in the central positions with the increasedtemperature.29,45,46,49 Thus, we are able to identify (72, 89)cm−1 and (110, 121) cm−1 for CsPbCl3 NCs as the TO2 andTO3 phonon modes, respectively. The absence of theassociated band with the 52 cm−1 band in the doublet TO1mode is because its position is beyond the spectral lower limit(50 cm−1) of our Raman system. Similarly, we conclude thatthree sets of doublet TO modes are situated at (52, 67) cm−1,(74, 81) cm−1, and (129, 133) cm−1 for CsPbBr3 NCs and (58,68) cm−1, (89, 101) cm−1, and (111, 119) cm−1 for CsPbI3NCs. The Raman band at 203 cm−1 in the Raman spectra ofCsPbCl3 NCs is for the nondegenerate LO2 mode. Theabsence of LO1 and LO3 modes for all CsPbX3 NCs is due toeither their positions being out of the measurement range oroverlapping with their strong TO modes. Assignments ofRaman bands are listed in Table 1 as well. The TO2 mode isassigned to the vibrations of [PbX6] octahedra, while both TO1and TO3 modes have important implications for the motion ofCs ions.29

It can be obviously seen that an increase in the temperaturegradually broadens the well-resolved doublet TO Raman bandsand causes their eventual merging into their singlet TO bands(Figures 2B and S3−S5). The merging was completed at 50 °Cfor CsPbCl3 NCs, 100 °C for CsPbBr3 NCs, and 200 °C forCsPbI3 NCs. Increasing the temperature to ca. −70 °C leads todisappearance of the 52 cm−1 Raman band for CsPbCl3, withinthe temperature range for an order−disorder structuraltransition previously reported.45 The order−disorder transitionin CsPbBr3 NCs occurs at the same temperature as theCsPbCl3 NCs. CsPbX3 NCs become more pseudocubic withthe temperature much higher than the order−disordertransition temperature, increasing degeneracies and overlapsof the Raman bands. However, we did not observe thedisappearance of the 58 cm−1 Raman band in CsPbI3 NCsacross the similar low-temperature region, which may be dueto the absence of the order−disorder intermediate phase. Weconclude that these changes of Raman signatures can beattributed to the different symmetry of the [PbX6] octahedra inthe different crystal structures and the distinct influence of theCs ions.A further temperature increase beyond the merging

temperature of doublet TO modes greatly made theseRaman bands weak or disappear and then produced atemperature region of featureless Raman spectra in CsPbX3NCs. New Raman bands at 129 cm−1 beyond 350 °C forCsPbBr3 NCs and at 134 cm

−1 beyond 300 °C for CsPbI3 NCsappeared, but no new Raman band was observed even up to500 °C for CsPbCl3 NCs. A great intensity increase was also

seen at 370 °C both for the 58, 68, and 80 cm−1 Raman bandsof CsPbBr3 NCs and for the 57 and 69 cm−1 Raman bands ofCsPbI3 NCs. These new Raman bands become intense withthe temperature increasing to 500 °C. The Raman bands at129 and 134 cm−1 are assigned to the decomposed CsBr andCsI3 from CsPbBr3 and CsPbI3 NCs, respectively (Figure S6).All the above results suggest that CsPbX3 NCs undergo a stateof high degree of disorder with increased temperature beyondthe merging temperature of doublet TO modes and were thenthermally decomposed at much higher temperatures, followedby

→ +CsPbX (s) PbX (s) CsX(s) (or CsX (s))3 2 3

The thermal decomposition temperature of CsPbX3 NCsfollows the order CsPbCl3> CsPbBr3> CsPbI3. Furthermore,the temperature dependence of Raman shifts of CsPbX3 NCsexhibits a regular red-shift with the increased temperature,except the new produced 129 and 134 cm−1 Raman bandsfrom thermal decomposition of CsPbBr3 and CsPbI3 NCs(Figure 3). Raman shifts follow a nonlinear decrease and an

increase in line width to follow with the increased temperature.In CsPbCl3 NCs, the doublet TO2 Raman band (72, 89) cm−1

at −190 °C merges into ∼72 cm−1 at −70 °C andsubsequently red shifts to 58 cm−1; the TO3 Raman band(110, 121) cm−1 at −190 °C merges into ∼111 cm−1 at 50 °Cand then red shifts to 107 cm−1. Similar results can be observedfor CsPbBr3 and CsPbI3 NCs with different Raman shifts andmerging temperatures (Figure 3B,C). These changes areattributed to the strain from the volume expansion and the

Table 1. Assignments of Raman Bands of CsPbX3 (X = Cl,Br, I) at Cryogenic Temperatures

TO1 (cm−1) TO2 (cm

−1) TO3 (cm−1)

CsPbCl3 (−, 52) (72,89) (110,121)CsPbBr3 (52, 67) (74, 81) (129, 133)CsPbI3 (58,68) (89, 101) (111, 119)

Figure 3. Temperature-dependent evolution of Raman shifts as afunction of temperature of (A) CsPbCl3, (B) CsPbBr3, and (C)CsPbI3 NCs. Note: a and b in panels A−C indicate the temperaturesof disappearance of TO1 modes and merging point of all doublet TOmodes into their singlet TO modes, respectively; regions marked withpink and light green indicate the temperature regions of featurelessRaman spectra and thermal decomposition of CsPbX3 NCs,respectively. Decomposition of CsPbCl3 NCs was not observed attemperatures investigated in this work, which may be due to itsrelatively high thermal stability.

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increasing disorder of crystal lattices. With the continuingincrease in the temperature beyond the decompositiontemperature, both 129 and 134 cm−1 Raman bands becomemuch sharper, which may be due to the increasingcrystallization of CsBr and CsI3.

19,52−54

To further understand the structural evolution of CsPbX3NCs at elevated temperatures, in situ XRD measurements wereperformed between −190 and 500 °C as well, and thecorresponding temperature profile is presented in Figure S7. Itshould be noted that the distinct XRD intensities wereobserved at each temperature investigated for all three CsPbX3NCs, which may originate from different crystal structures andcrystallinity. At 25 °C, the as-prepared CsPbCl3, CsPbBr3, andCsPbI3 NCs are indexed as tetragonal, orthorhombic, andcubic phases, respectively (Figures 1C and 4). Typically, thetetragonal phase of CsPbCl3 NCs has two XRD peaks at 2θ =31.76° and 31.99°, which are assigned to the crystal facets(002) and (200), respectively, while its cubic phase has only a

single peak at 2θ = 31.91° belonging to the crystal facet (200)(Figure 4A). The double XRD peaks at 2θ = 31.76° and 31.99°of the tetragonal phase disappear at 50 °C, and the single peakat 2θ = 31.91° of the cubic phase appears at 100 °C (FigureS8). Thus, we clearly see the transition of the tetragonal intocubic phases in CsPbCl3 NCs at above 50 °C, and theorthorhombic−cubic phase transition temperature in CsPbBr3NCs is beyond 100 °C (Figure 4A). These phase transitiontemperatures are consistent with those reported in theliterature.28,30 However, although the thermodynamicallystable polymorph of CsPbI3 NCs is the cubic phase at hightemperatures, the cubic−orthorhombic phase transition wasobviously observed at above 200 °C (Figure 4A). Theincreasing temperature causes Bragg reflections shifting towardlower 2θ angles, which is due to the thermal volume expansionof crystal lattice. XRD results evidenced again that bothCsPbBr3 and CsPbI3 NCs were decomposed into theirconstituents PbX2 and CsX (or CsX3) at above ca. 400 and

Figure 4. In situ XRD patterns of CsPbX3 NCs at various temperatures from −190 to 500 °C. (A) In situ powder XRD patterns of (i) CsPbCl3, (ii)CsPbBr3, and (iii) CsPbI3 NCs. Standard JCPDS card data of cubic, orthorhombic, and (or) tetragonal phases of CsPbX3 NCs are shown as well.The red circles (●) and blue diamonds (◊) indicate the decomposed products PbX2 and CsX (or CsX3). The in situ XRD measurement wasstarted at 25 °C, and then the measurement was performed at a temperature interval as the temperature decreased from 25 °C to −190 °C,followed by returning to 25 °C and implementing the measurement at a temperature interval as the temperature increased from 25 to 500 °C. TheXRD measurement was performed after the sample was cooled back to 25 °C as well. (B) Color changes of (i) CsPbCl3 NCs, (ii) CsPbBr3 NCs,and (iii) CsPbI3 NCs after in situ XRD measurements.

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200 °C, respectively (Figures 5A). However, we did notobserve the thermal decomposition of CsPbCl3 NCs even at500 °C, and the orthorhombic phase of CsPbI3 NCs did notreturn to the thermodynamically stable cubic phase as thetemperature further increased from 200 to 500 °C. Theseresults show the halogen atom-dependent thermal stability ofCsPbX3 NCs with the thermal decomposition temperature isCl > Br > I, in agreement with in situ Raman measurements.The state of a high degree of disorder shown in high-temperature Raman measurements was not observed in the insitu XRD measurements. Also, we did not observe theintermediate order−disorder transition at −70 °C forCsPbX3 NCs in the in situ XRD measurements. In addition,we found that CsPbCl3 NCs maintained their initial crystallinestructures after the cooling−heating process (25 °C → −190°C → 25 °C → 500 °C); both CsPbBr3 and CsPbI3 NCsmaintained their initial crystalline structures during the coolingprocess (25 °C → −190 °C → 25 °C), while the thermaldecomposition induced by the high-temperatures is irreversible(Figure S9). In addition, significant color changes after in situXRD measurements were observed: white to black for CsPbCl3NCs, orange to black for CsPbBr3 NCs, and black to yellow forCsPbI3 NCs (Figure 4B). The thermogravimetric analysis−differential scanning calorimetry (TGA-DSC) analysis per-formed under inert atmosphere revealed a mass loss of lessthan 6% over the 30−500 °C temperature range, which may bedue to removal of surface organic ligands (ODE, OAm, andOA). TGA-DSC results again confirmed the high-temperaturephase transitions and their thermal decomposition of CsPbX3(X = Br, I) NCs (Figure S10). These findings can be furtherconfirmed by changes of Raman signatures of CsPbX3 NCsafter the cooling−heating process, as shown in Figure 5B.We for the first time employed in situ Raman spectroscopy

in combination with in situ XRD measurements to understandthe thermal stability of CsPbX3 NCs over a broad temperature

range from −190 to 500 °C. CsPbX3 NCs undergo successivephase transitions between −190 and 500 °C, along withchanges of Raman signatures that have important implicationsfor structures and phase transitions. At cryogenic temperatures,CsPbX3 NCs exhibit sharp low-frequency Raman bands, whichare assigned to the TO and LO phonon modes. Temperature-induced spectral changes include disappearance, broadening,and merging of Raman bands, implying the order−disorderstate, phase transition, and thermal decomposition of CsPbX3NCs. These CsPbX3 NCs possess significantly distinct thermaldecomposition temperatures, tightly related to the halogenatom type. We did not observe the thermal decomposition ofCsPbCl3 NCs even at the highest temperature adopted in thiswork, while both CsPbBr3 and CsPbI3 NCs begin todecompose at above 350 and 300 °C, respectively.Furthermore, a state of high degree of disorder with featurelessRaman spectra was observed before thermal decomposition ofCsPbX3 NCs. We found a remarkable agreement between themerging temperature of all doublet TO modes and thetemperature of the high-temperature phase transition (tetrag-onal/orthorhombic−cubic transition) in these CsPbX3 NCs.Thus, we suggest the strong correlation of the merging ofdoublet TO Raman bands with the high-temperature phasetransition in CsPbX3 NCs. In situ Raman spectroscopycombined with in situ XRD measurement presents newinsights into the structural evolution of CsPbX3 NCs, whichbenefits better understanding of the degradation of CsPbX3perovskite optoelectronic devices and forms foundations ofnovel strategies for their performance improvement.In summary, this work presents a systematic study on the

structural stability of all-inorganic CsPbX3 NCs (X = Cl, Br, I)over a broad temperature range from −190 to 500 °C, using insitu Raman spectroscopy combined with in situ XRDmeasurements. Results demonstrate that Raman signatures ofCsPbX3 NCs have important implications for their structural

Figure 5. XRD patterns of degraded CsPbX3 (X = Br, I) NCs and changes of ambient-temperature Raman spectra after in situ Ramanmeasurements. (A) In situ XRD patterns of degraded (i) CsPbBr3 NCs and (ii) CsPbI3 NCs at 450 °C. Standard JCPDS card data of cubic CsBr,orthorhombic PbBr2, hexagonal PbI2, and orthorhombic CsI3 are shown as well. It is worth noting that the 2θ angle difference between themeasured XRD patterns at 450 °C and the standard JCPDS card data of CsPbX3 NCs is due to the thermal expansion-induced shift of Braggreflection angles. (B) Raman spectra of (i) CsPbCl3 NCs, (ii) CsPbBr3 NCs, and (iii) CsPbI3 NCs measured at (1) 25 °C before the coolingprocess, (2) 25 °C returning from cryogenic temperatures, and (3) 25 °C after cooling back from 500 °C.

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changes and thermal decomposition. The TO Raman bandssituated in low-frequency regions (<250 cm−1) appear asdoublets at cryogenic temperatures, broaden gradually, andmerge into their singlet TO Raman bands with increasingtemperature. Both CsPbCl3 and CsPbBr3 NCs show an order−disorder intermediate state at ca. −70 °C, manifested bydisappearance of the ∼52 cm−1 Raman band. There is aremarkable agreement between the merging temperature of alldoublet TO modes into their respective singlet TO modes andthe temperature of high-temperature phase transitions. Thissuggests that the merging of the doublet TO bands is anindicator of the corresponding phase transition. A furtherincrease in the temperature produces a temperature region offeatureless Raman spectra and finally causes the completedegradation of CsPbBr3 or CsPbI3 NCs into PbX2 and CsX (orCsX3). However, we did not observe the decomposition ofCsPbCl3 NCs at temperatures covered in this work, which maybe due to its high thermal stability. Thus, thermal stability ofCsPbX3 NCs relies on the halogen atom type and follows theorder of CsPbCl3 > CsPbBr3 > CsPbI3. The present workclearly unveils the structural evolution of all-inorganic CsPbX3NCs at elevated temperatures and extends our understandingof the degradation mechanism of CsPbX3 NCs. Although all-inorganic CsPbX3 NCs may have crystal structures similar toorganic−inorganic hybrid lead halide (e.g., CH3NH3PbX3, X =Cl, Br, I) NCs, hybrid lead halide NCs exhibit different Ramansignatures and distinct thermal stability. The in-depthunderstanding of thermal stability of hybrid lead halide NCsover the broad temperature range from −190 to 500 °C needsto be carefully examined. We believe that this study will greatlybenefit the performance improvement of optoelectronicdevices in practical fields over a broad operating temperaturerange, particularly under extreme settings such as spaceexploration and deep drilling.

■ ASSOCIATED CONTENT

*S Supporting InformationThe Supporting Information is available free of charge on theACS Publications website at DOI: 10.1021/acs.jp-clett.9b00344.

Distribution of edge lengths of CsPbCl3, CsPbBr3, andCsPbI3 NCs; temperature profile for in situ Ramanmeasurements; evolution of Raman spectra of CsPbCl3,CsPbBr3, and CsPbI3 NCs at elevated temperatures from−190 to 500 °C; Raman spectra at 450 °C and aftercooling back to 25 °C from 500 °C; temperature profilesfor in situ XRD measurements; magnified view of in situXRD patterns of CsPbCl3 nanocrystals at 50 and 100°C; in situ powder XRD patterns at 25 °C; TGA-DSCanalysis (PDF)

■ AUTHOR INFORMATION

Corresponding Author*E-mail: liming0823@csu.edu.cn, liming0823@gmail.com.

ORCID

Ming Li: 0000-0002-2289-0222NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTS

The authors acknowledge financial support by the NationalThousand Young Talents Program of China, National NaturalScience Foundation of China (No. 51871246), Innovation-Driven Project of Central South University (No. 2018CX002),and Hunan Provincial Science & Technology Program (No.2017XK2027). We thank Professor J. W. Huang of the Schoolof Materials Science and Engineering, Central South Universityfor help with in situ XRD measurements.

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S1

Supporting Information

In Situ Raman Spectroscopic Studies of Thermal

Stability of All-inorganic Cesium Lead Halide

(CsPbX3, X=Cl, Br, I) Perovskite Nanocrystals

Mengling Liao, Beibei Shan and Ming Li*

School of Materials Science and Engineering, State Key Laboratory for Power Metallurgy,

Central South University, Changsha, Hunan 410083, China

*To whom the correspondence should be addressed. E-mail: liming0823@csu.edu.cn and

liming0823@gmail.com

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MATERIALS AND METHODS

Chemicals and Materials. Cesium carbonate (Cs2CO3, 99.90%), 1-octadecene (ODE, ≥90%),

oleylamine (OAm, 80-90%), cesium chloride (CsCl, 99.90% metal basis), cesium bromide (CsBr, 99.50%

metal basis), lead(II) bromide (PbBr2, 99.00% metal basis), lead(II) chloride (PbCl2, 99.99% metal basis)

and lead(II) iodide (PbI2, 99.99% metal basis) were purchased from Aladdin Chemistry Co. Ltd.

(Shanghai, China). Cesium triiodide（CsI3, 99.9%）was purchased from Sigma-Aldrich (Sigma-Aldrich

Shanghai Trading Co. Ltd., Shanghai, China). Oleic acid (OA, AR), hexane (AR) and toluene (AR) were

obtained from Sinopharm (Beijing, China). Anhydrous ethanol (AR, ≥99.7%) was ordered from Shanghai

Titan Scientific Co. Lld. (Shanghai, China). Ultrapure water (18.2 MΩ•cm) was produced with a Millipore

Direct-Q3 UV system (Millopore Corporation, Molshein, France) and used throughout the experiments.

All chemicals and solvents were of analytical grade and used as received.

Preparation of Cesium Oleate. 0.203 g (0.625 mmol) of Cs2CO3 was added into a three-neck flask

containing a mixture of 10 mL of ODE and 1.0 mL of OA. The reaction mixture was alternately purged

with N2 and pumped under vacuum three times, followed by being kept under vacuum at 120 oC for 1 h

to remove moisture and oxygen from the reaction mixture. Then, the reaction temperature was increased

to 150 oC under N2 atmosphere. The reaction was continued until the reaction solution became clear,

indicating the complete dissolution of Cs2CO3 and formation of cesium oleate. The cesium oleate

precursor could precipitate out of the ODE solvent upon cooling down to room temperature, but it can be

re-dissolved to achieve a clear solution of cesium oleate by being pre-heated to 100 oC before use.

Synthesis of CsPbX3 Perovskite NCs. The CsPbX3 (X=Cl, Br or I) perovskite NCs were synthesized

using a hot-injection method as described in previous reports with a slight modification.S1,S2 Lead halide

salts (PbX2, 0.0278 g (0.1 mmol) for PbCl2, 0.0373 g (0.1 mmol) for PbBr2, 0.0461 g (0.1 mmol) for PbI2)

were added into 6 mL of ODE and then degassed under alternate vacuum and N2 at 40 oC. After heated

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at 120 oC for 30 min, a solution consisting of 0.5 mL of OA and 1.5 mL of OAm was injected. The reaction

was continued until a clear solution was obtained, indicating the complete dissolution of PbX2. The

reaction temperature was then raised to 180 oC, followed by the rapid injection of 0.5 mL of cesium oleate

precursor in ODE. After a designed time (5 min for CsPbCl3, 1 min for CsPbBr3, and 0.5 min for CsPbI3),

the reaction was quenched by cooling down in an ice-water bath. After 7.5 mL of toluene was added, the

reaction products were centrifuged at 9000 rpm for 10 min. After centrifugation, the supernatant was

discarded, and the precipitate was re-dispersed in hexane. This purification process was repeated twice.

The resulting precipitate was the CsPbX3 NCs, re-dispersed in hexane, and stored in a glovebox for further

use.

Characterization. Optical absorption spectra were recorded on an Agilent Cary 5000 UV-vis-NIR

spectrometer (Agilent Technology, USA). Steady-state fluorescence spectra were taken using a Shimadzu

RF-6000 spectrofluorimeter. Both absorption and fluorescence spectra were done in a transmission mode.

Transmission electron microscopy (TEM) images were taken on a Tecnai G2 F20 transmission electron

microscope operating at an acceleration voltage of 200 kV. The TEM samples were prepared by dropping

the colloidal solution on a thin carbon-coated 300 mesh copper grid (Beijing Zhongjingkeyi Technology

Co. Lld., China) and then being dried in the air. Edge length distribution of CsPbX3 NCs was determined

from the TEM data using the ImageJ analysis software. At least 100 particles were measured for each

type of CsPbX3 NCs. Thermogravimetric analysis (TGA) and differential scanning calorimeter (DSC)

were simultaneously performed using a Netzsch Simultaneous Thermal Analyzer (STA449C Jupiter)

equipped with a type-S (Pt/PtRh) TG-DSC sample carrier supporting a PtPh10-Pt thermocouple (Netzsch-

Gerätebau GmbH, Germany). Samples were placed in a A12O3 crucible, and then heated in Ar gas from

ambient to 500 oC at a heating rate of 10 oC/min.

in situ Raman and XRD Measurements. In situ Raman measurement was performed on a Renishaw

inVia Raman microscope system equipped with a Leica DM2700M Ren RL/TL microscope. The

excitation source was the Renishaw high power diode laser of 785 nm emission wavelength. The CCD

S4

detector has holographic grating with 1200 lines/mm. A Linkam THMS 600 heating and freezing stage

system consisting of a THMS 600 stage, a T95 system controller and a LNP95 liquid nitrogen cooling

pump was used to control the temperature. In a typical experiment, CsPbX3 perovskite NCs were placed

in a quartz crucible, and then Raman spectra were collected through a ×50L objectives (NA 0.5) with the

laser power of ~4 mW and acquisition time of 1 s in the temperature range from -190 oC to 500 oC with a

ramp rate of 2.0 oC/min.

In situ XRD measurement was carried out on a SmartLab3kW X-ray diffractiometer (Rigaku, Japan)

using Cu Kα (1.5406 Å) radiation operating at 40 kV and 30 mA. Diffraction data were collected with a

step size of 0.02o in the 2θ range of 10-60o. An Anton Paar TTK 600 chamber was used to control the

temperature for the in situ temperature-dependent XRD measurement in the temperature range from -190

oC to 500 oC.

References

(S1) Protesescu, L.; Yakunin, S.; Bodnarchuk, M.I.; Krieg, F.; Caputo, R.; Hendon, C.H.; Yang, R.X.;

Walsh, A.; Kovalenko, M.V. Nanocrystals of cesium lead halide perovskites (CsPbX3, X= Cl, Br,

and I): novel optoelectronic materials showing bright emission with wide color gamut. Nano Lett.

2015, 15(6), 3692-3696.

(S2) Su, Y.; Chen, X.; Ji, W.; Zeng, Q.; Ren, Z.; Su, Z.; Liu, L. Highly controllable and efficient

synthesis of mixed-halide CsPbX3 (X= Cl, Br, I) perovskite QDs toward the tunability of entire

visible light. ACS Appl. Mater. Interfaces 2017, 9(38), 33020-33028.

S5

Figure S1. Distribution of edge lengths of (A) CsPbCl3 NCs, (B) CsPbBr3 NCs and (C) CsPbI3

NCs statistically analyzed from TEM images shown in Figure 1D. At least 100 particles were

measured for each type of CsPbX3 NCs. The average edge lengths are 22.3±2.6 nm for CsPbCl3

NCs, 24.7±4.3 nm for CsPbBr3 NCs, and 26.6±3.4 nm CsPbI3 NCs.

S6

Figure S2. Temperature profile for in situ Raman measurements used in this work. In situ Raman

measurements were started from 25 oC and then the measurement was performed at a

temperature interval as the temperature decreasing from 25 oC to -190 oC, followed by returning

to 25 oC and implementing the measurement at a temperature interval as the temperature

increasing from 25 oC to 500 oC.

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Figure S3. Evolution of Raman spectra of CsPbCl3 NCs at elevated temperatures from -190 oC

to 500 oC.

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Figure S4. Evolution of Raman spectra of CsPbBr3 NCs at elevated temperatures from -190 oC

to 500 oC.

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Figure S5. Evolution of Raman spectra of CsPbI3 NCs at elevated temperatures from -190 oC to

500 oC.

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Figure S6. Raman spectra at 450 oC and after cooling back to 25 oC from 500 oC of (A)

CsPbCl3 NCs, (B) CsPbBr3 NCs and (C) CsPbI3 NCs. Raman spectra of CsX (or CsX3) and

PbX2 are shown as well. No Raman band associated with both PbCl2 and CsCl are observed

in CsPbCl3 NCs both at 450 oC and after cooling back to 25 oC from 500 oC. However, Raman

bands associated with PbX2 and CsX (or CsX3) are observed in CsPbBr3 and CsPbI3 NCs both

at 450 oC and after cooling back to 25 oC from 500 oC. These results indicate both CsPbBr3

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and CsPbI3 NCs are thermally decomposed via the high-temperature heating, but CsPbCl3

NCs are not decomposed even at 500 oC.

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Figure S7. Temperature profiles for in situ XRD measurements. Dots represent the

measurement temperature at set time. The in situ XRD measurement was started at 25 oC, and

then the measurement was performed at a temperature interval as the temperature decreasing

from 25 oC to -190 oC, followed by returning to 25 oC and implementing the measurement at a

temperature interval as the temperature increasing from 25 oC to 500 oC.

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Figure S8. Zoom-in in situ XRD patterns of CsPbCl3 nanocrystals at 50 oC and 100 oC. We can clearly

see two peaks at 2θ=31.76o and 31.99o at 50 oC, which are assigned to the crystal facets (002) and (200)

of the tetragonal phase, but only one peak at 2θ=31.91o is observed at 100 oC, which is assigned to the

crystal facet (200) of the cubic phase. Thus, we conclude that the phase transition of the tetragonal into

cubic phases occurs beyond 50 oC.

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Figure S9. In situ powder XRD patterns of (A) CsPbCl3 NCs, (B) CsPbBr3 NCs and (C) CsPbI3

NCs at 25 oC (i) before the cooling process, (ii) returning from the cryogenic temperatures, and

(iii) after cooling back from 500 oC.

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Figure S10. TGA-DSC analysis of (A) CsPbCl3 NCs, (B) CsPbBr3 NCs and (C) CsPbI3 NCs in

the temperature range of 30-500 oC. The mass loss in the 30-280 oC range is due to removal

of surface organic ligands (ODE, OAm and OA), accompanied by the exothermic process. The

exothermic peaks at 134 oC in (B) and 156 oC in (C) correspond to the tetragonal-cubic phase

transition of CsPbBr3 NCs and the cubic-orthorhombic phase transition of CsPbI3 NCs,

respectively. The endothermic peaks at 342 oC and 324 oC in (B,C) are associated with the

decomposition of CsPbBr3 and CsPbI3 NCs. However, we did not observe the endothermic peak

for the decomposition of CsPbCl3 NCs. These results are consistent with results from in situ

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Raman and XRD measurements. The endothermic peaks at 471/476 oC and 483 oC in (B,C)

represent the melting of the decomposed components.