Internal Heterostructure of Anion-Exchanged Cesium Lead Halide

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Internal Heterostructure of Anion Exchanged Cesium Lead Halide Nanocubes Anamul Haque, Vikash Kumar Ravi, G. Shiva Shanker, Indranil Sarkar, Angshuman Nag, and Pralay K. Santra J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b11118 • Publication Date (Web): 12 Dec 2017 Downloaded from http://pubs.acs.org on December 12, 2017

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The Journal of Physical Chemistry

Internal Heterostructure of Anion Exchanged Cesium Lead Halide Nanocubes Anamul Haque,‡ Vikash Kumar Ravi,¶ G. Shiva Shanker, ¶ Indranil Sarkar,† Angshuman Nag, ¶, ⊥ * Pralay K. Santra‡ *

‡ ¶

Centre for Nano and Soft Matter Sciences, Jalahalli, Bangalore 560013, India

Department of Chemistry, Indian Institute of Science Education and Research (IISER), Pune 411008, India ⊥

Center for Energy Science, Indian Institute of Science Education and Research (IISER), Pune-411008, India

†

Deutsches Elektronen-Synchrotron DESY, Notkestrasse 85, D-22607 Hamburg, Germany

Email address: [email protected]; [email protected]

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Abstract All inorganic cesium lead halide (CsPbX3, X = Cl, Br and I) perovskite nanocubes (NCs) exhibit fascinating optical and optoelectronic properties. Post-synthesis anion exchange by mixing NCs with reactive anion species has emerged as a unique strategy to control their composition and bandgap. For example, we started with CsPbBr3 NCs with intense green colored emission, and then anion exchange with iodide ions yields CsPb(Br/I)3 mixed halides, and CsPbI3 with emission color systematically varying in the green-red region. However, the internal structure of the anion exchanged perovskite NCs is not probed. It is believed that the NCs possess a homogeneous alloyed composition, but X-ray diffraction pattern could not give evidence for such alloy formation, because the crystal structure also varies with anion composition. Here, we elucidate the internal heterostructure of anion exchanged NCs using variable energy hard X-ray photoelectron spectroscopy. The results show that, in contrast to homogenous alloy, there is a significant inhomogeneity in the composition across the radius of NCs. Surface of CsPb(Br/I)3 NCs is rich with exchanged iodide ions, whereas the core is rich with native bromide ions. Even CsPbI3 NCs obtained after assumed complete anion exchange, show a small amount bromide ions in the core. This finding of gradient internal heterostructure inside the anion-exchange NCs will be important for future understanding of electronic properties and stability related issues of CsPbX3 NCs.

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Introduction In recent years, a considerable research interest has been developed in the field of all inorganic perovskite nanomaterials, mainly because of their fascinating optical and optoelectronic properties. Among various all-inorganic perovskite nanomaterials, cesium lead halides (CsPbX3) have attained a huge interest due to their high photoluminescence quantum yield (PLQY) and narrow emission width.1-10 The surface defects of these perovskite nanomaterials do not form significant deep midgap states, thus making them ‘defecttolerant’.11-15 Therefore, CsPbX3 nanocrystals exhibit efficient optical and optoelectronic properties processed without requiring the formation of core/shell nanocrystals, unlike more traditional II-VI or III-V semiconducting nanoparticles.16-21 In addition to controlling shape and size of nanocrystals to achieve tuning of bandgap, tuning the composition of nanocrystals has always been a way to control the bandgap, for a desired application. In this regard, the post-synthesis ion exchange reactions provide an excellent control over composition of nanocrystals, while maintaining the size and shape of nanocrystals in a controllable way. Many such strategies of partial or complete ion exchanges are reported for II-VI semiconducting nanocrystals, and mostly the ions that got exchanged were cations while maintaining the anionic sub-lattice intact.22-26 In this regard, CsPbX3 nanocrystals are again exceptions, where anion exchange is preferred over cation exchange.2729

The more rigid nature of the cationic sub-lattice of the halide perovskites limits the cation

exchange,30-31 whereas anions are more mobile.32-33 Consequently, anion exchange has emerged as a simple yet highly effective method to control the composition of CsPbX3 perovskite nanocrystals with X = Cl, Cl/Br mixed halide, Br, Br/I mixed halide, and I. This post-synthesis tunability of halide composition can tune the band gap of CsPbX3 nanocrystals throughout the visible region. Furthermore, other relevant optoelectronic parameters such as dielectric constant of the material, excitonic binding energies, and optical transition 3



probabilities can be controlled.34 Since the valence band maxima (VBM) has major contribution from halide ions, the VBM can be tuned a systematic way with great accuracy in a device interface, just by tuning the halide compositions.13 Clearly, the post-synthesis anion exchange reaction of CsPbX3 nanocrystals is an important strategy to achieve desired material properties. Therefore, understanding such anion exchange reactions achieving different halides and mixed halide perovskite is essential for both fundamental understanding and optimized applications. Two pertaining questions addressed here are: (a) does a partial anion exchange process form a homogeneous alloy and (b) does the complete anion exchange replace all the native anions from CsPbX3 nanocrystals? These questions are important as they reveal information about the electronic properties as well as the structural stability of the nanocrystals. It is generally assumed that the anion exchanged nanocrystals possess a homogenous composition, for example CsPb(Br/I)3 mixed halide has same composition across the nanocrystals, forming an homogenous alloy. Such interpretation has been drawn from the qualitative monotonous change in lattice parameters and optical bandgap with composition. However, we want to note here that a gradient alloy, or core-shell structures for II-VI nanocrystals forming an inhomogeneous alloy across the interface, also show such qualitative variation in lattice parameters and bandgap.35-38 For example, in the case of addition of multiple anions (S2-, Se2- etc.) to the cationic precursor (Cd2+), the final CdSeS nanoparticles form a gradient alloy, with CdSe-rich core and CdS-rich outermost shell.39 In this work, we studied the distribution of anions inside the NCs after partial and complete anion exchange of CsPbBr3 NCs with iodide ions, using variable energy hard X-ray photoemission spectroscopy (HAXPES). The distribution of anion provides the internal heterostructure of the NCs. Our results show that the anion exchange forms an inhomogeneously alloyed heterostructure with a higher concentration of the exchanged iodide

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ions on the surface of the NCs. No shifts in emission peak position indicate a complete anion exchange in the presence of excess iodide ions, however we find that a significant amount of bromide ions are still present at the core of the anion exchanged NCs. These findings were not possible without the use of HAXPES.

Experimental Methods: Chemicals: Cesium carbonate (Cs2CO3, 99.9%, Sigma-Aldrich), lead (II) bromide (PbBr2, 99.999%, Sigma-Aldrich), lead (II) iodide (PbI2, 99.99%, Sigma-Aldrich), oleic acid (OA, 90%, Sigma-Aldrich), oleylamine (OLA, technical grade 70%, Sigma-Aldrich), 1octadecene (ODE, technical grade 90%, Sigma-Aldrich), toluene (anhydrous 99.8%, Aldrich), ethyl acetate (99.5%, Rankem), ethanol (99.9% AR, S D Fine Chem), TiCl4 (Merck), and F-doped SnO2 coated glass (FTO, ~7 ohm/sq, Sigma-Aldrich). Synthesis of CsPbBr3 NCs and its anion exchanged counterparts 1. Preparation of cesium precursor: A mixture of 0.4 g (1.2 mmol) Cs2CO3, 2 mL OA and 20 mL ODE was taken in 50 mL 3-necked round bottom flask. The mixture was alternatively kept under vacuum and purged with nitrogen (N2) at 120 ˚C, to ensure the removal of moisture and dissolved oxygen. After that, the temperature of the reaction mixture was increased to 150 ˚C, to completely dissolve the Cs2CO3 giving a clear solution of Csoleate. It was then transferred to a sealed vial purged with N2 and then stored in glove box. 2. Synthesis of CsPbBr3 NCs: Colloidal CsPbBr3 NCs were synthesized by modifying the methods earlier reported.1 15mL ODE and 0.207 g (0.564 mmol) PbBr2 were taken in a 50 mL 3-necked round bottom flask. The above mixture was then kept under vacuum and N2 alternately at 120 ˚C along with magnetic stirring for complete removal of oxygen and moisture. Dried OA and OLA, each 1.5 mL, was added to the mixture at 120 ˚C having N2 5



flow. After around 30 min, PbBr2 get dissolved in ODE having a pale yellow color and then the temperature was increased to 180 ˚C. Then the Cs-oleate solution in ODE, pre-heated at 100 ˚C, was swiftly injected to the reaction mixture. Instantly the reaction mixture became greenish and the reaction was stopped after ~ 5 s by an ice bath. The crude solution was then centrifuged at 7000 rpm to remove excess unreacted precursor and solvent. The washing of the NCs were done by toluene/ethyl acetate (1:1 v/v) mixture and finally, the wet pellet of the NCs were re-dissolved in 15 mL toluene and was used as stock solution for anion exchange reaction. 3. Anion Exchange of CsPbBr3 NCs: 15 mL of colloidal CsPbBr3 NCs were divided into 3 parts having 5 mL solution each. PbI2 was used as iodide sources for anion exchange reaction. 0.09 mmol of PbI2 for CsPb(Br/I)3 and 0.22 mmol of PbI2 for CsPbI3 were mixed with ODE (5mL) in 25 mL 3- neck round bottom flask and was degassed and dried by applying alternate vacuum and N2 flow for 1 hr. Dried OA and OLA (0.2 mL each) were injected at 120 ˚C under N2 flow to completely solubilize PbI2. The solution was then allowed to cool at room temperature and then CsPbBr3 NCs solution (5 mL) was injected to give anion exchanged CsPb(Br/I)3 and CsPbI3 NCs. The NCs were precipitated by adding ethyl acetate (1:1 v/v) and then centrifuged at 7000 rpm for 10 min. The supernatant was discarded and the wet pellet was re-dissolved in 5 mL toluene and was used for further characterization. 4. Thin film preparation for HAXPES study: FTO glasses were cleaned sequentially with soap solution, distil water and ethanol (about 30 minutes each) using ultra sonication bath followed by heating at 450 oC for 10 minutes in order to get rid of the organic contaminants. It was then allowed to cool down to room temperature naturally. After that, a thin layer of TiO2 was made on the top of the conducting surface of FTO coated glass by treating it with 0.04 M TiCl4 aqueous solution at 80 oC for 30 minutes and then washing it with distilled water and ethanol. The films of CsPbBr3, CsPb(Br/I)3 and CsPbI3 NCs were 6


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made by spin coating the dispersion of these NCs in toluene (20 mg/mL) onto the above FTO coated glasses. Characterization of Nanocrystals: Perkin Elmer, Lambda-45 UV/Vis spectrometer was used for recording UV-visible absorption spectra. Steady state photoluminescence (PL) measurements were carried out using FLS 980 (Edinburgh Instruments). Powder X-ray diffraction (XRD) data were recorded using a Bruker D8 Advance X-ray diffractometer using Cu Kα radiation (1.54 Å). Transmission electron microscopy (TEM) studies were carried out using a JEOL JEM 2100F field emission transmission electron microscope at 200 kV. The X-ray photoemission spectroscopy was performed at HAXPES end station of P09 beamline at PETRA-III synchrotron, DESY, Hamburg, Germany (Ref) using photon energies between 3500 – 6000 eV.40-41 The base pressure of the measurement chamber was ~ 6 × 10-9 mbar range during measurements. We calibrated the data with the binding energy of Au 4f7/2 (84.0 eV) from gold foil mounted on the sample holder and Ti 3s core level from the TiO2 substrate as an internal calibration. The HAXPES data were deconvoluted using XPST (Xray Photoelectron Spectroscopy Tool) program package developed for IGOR Pro by Dr. Martin Schmid, Philipps University, Marburg. The total instrumental resolution varied between 390 to 800 meV during these measurements. The intensity ratio between the spin orbit split peaks was kept fixed to 0.667 and 0.75 for “ d ” and “ f ” core levels. The core levels peaks were considered to have pseudo-Voigt functional form and the background was considered as a combination of Shirley and parabolic function.

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Results and Discussion We started with the CsPbBr3 NCs and performed anion exchange reactions different extent to get a CsPb(Br/I)3 mixed halide NCs, and complete anion-exchanged CsPbI3 NCs following previous literature.28 The anion exchange was monitored through absorption and emission spectrum. In case of CsPbI3, we used an excess of iodide precursor to ensure complete exchange of bromide ions by iodide ions. We found that there was no shift in both

Figure 1: a) UV-visible absorption and emission spectra of CsPbBr3, CsPb(Br/I)3 and CsPbI3 NCs dispersed in toluene. The solid line is the absorption spectra and shaded areas represent the corresponding emission spectra. The inset figures show the NCs dispersion under UV light excitation at 365 nm. b) Powder X-ray diffractogram (PXRD) of CsPbBr3, CsPb(Br/I)3 and CsPbI3 NCs along with the bulk reference of orthorhombic phase of CsPbBr3 and cubic phase of CsPbI3. c) TEM image of parent CsPbBr3 NCs of size ~10.1 ± 0.5 nm and, (d) its anion (iodide) exchanged nanocrystals of CsPbI3 having size ~ 11.3 ± 0.8 nm. The scale bars in both the images are 5 nm.

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absorption and emission peaks above a particular concentration of iodide precursor, confirming the limit of the anion exchange. The absorption and emission spectra of the parent CsPbBr3, mixed halide (CsPb(Br/I)3), and CsPbI3 NCs are shown in Figure 1a. We observe a systematic red-shift in the absorption spectra with the anion exchange. The emission peaks from these three samples overlap with their absorption edge, suggesting the emissions to be band-edge or excitonic emission. We did not observe any surface states emission in the anion exchanged NCs. The powder X-ray diffraction (XRD) patterns for three samples are shown in Figure 1b. It is evident that the CsPbBr3 NCs formed in the orthorhombic phase. The diffraction peaks for the mixed halide system shift to lower angles suggesting an increase in lattice parameter; thus confirming the incorporation of the iodide ions into the lattice. The converted CsPbI3 forms in the cubic phase. This change in crystal phase with halide exchange makes it more difficult to verify the alloy formation in these NCs using the Vegard’s law. No other impurity phases were found after the complete anion exchange. TEM images show the average edge length of CsPbBr3 (Figure 1c) and anion exchanged CsPbI3 NCs (Figure 1d) is to be ~ 10.1 ± 0.5 nm and ~11.3 ± 0.8 nm, respectively. The size distribution histograms are shown in Figure S1 in supporting information. The slight increase in the average size is due to the incorporation of the bigger radius of iodide ions into the lattice, further evidencing the anion exchange.27-28

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We performed HAXPES to probe any internal heterostructure within the inorganic part of the NCs. Survey scans of the three samples are shown in Figure S2 of supporting information. The data were collected with a photon energy of 3500 eV and presented after normalizing with the Cs 3d signal intensity at 725.2 eV. All the essential elemental peaks are present in the spectrum as marked in the figure. The signals corresponding to Ti, O, and Sn, are originated from the substrate - TiO2 compact layer on FTO. We found that the relative intensity of Br 3p signal at 182.2 eV decreases and I 3d signal at 620.4 eV increases with anion exchange. There is hardly any Br signal for CsPbI3 suggesting a successful anion exchange in our reactions.

Figure 2. (a) Core level photoemission spectra of Pb 4f from CsPbBr3, CsPb(Br/I)3 and CsPbI3 NCs at 3500 eV. The experimental data are shown by the solid circles and the total fit is shown by the black solid line, which is the sum of the two independent species of Pb in “2+” and “0” oxidation states, as highlighted in shaded blue and solid magenta line, respectively. The vertical lines indicate the binding energy of the two species. (b) High resolution core level spectra of Cs 4d, Br 3d, and I 4d from CsPbBr3, CsPb(Br/I)3 and CsPbI3 at 3500 eV.

Figure 2a shows the core level spectra of Pb 4f for CsPbBr3, CsPb(Br/I)3 and CsPbI3 NCs samples collected at 3500 eV. Evidently, two different species of Pb are present in all three samples. Peaks at 138.8 eV and 143.7 eV separated with 4.9 eV correspond to Pb 4f7/2 and 4f5/2 core levels of the first specie. The binding energies of these peaks suggest the oxidation state of this Pb specie is “2+” and is originated from cesium lead halide perovskite.42-43 The peaks at lower binding energies 136.9 eV and 141.8 eV correspond to Pb 4f7/2 and 4f5/2 core levels for metallic Pb with an oxidation state of “0”.44 To determine the relative concentration 10


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of metallic Pb0, we deconvoluted the core level spectra. The higher oxidation specie is highlighted in blue shades, and the peaks for lower oxidation state are shown by the solid magenta line in the same figure. The relative concentration of metallic Pb0 is found to be 4.6 % in CsPbBr3, and this is similar to the values reported recently for a similar system.45 The metallic Pb0 concentration increases to 16.6 % for the case of CsPb(Br/I)3 mixed halide NC, and 14.3 % for the CsPbI3 NCs. These experimental data suggest that the anion exchanged halide systems are prone to have more metallic Pb0 compared to the CsPbBr3 NCs. The observation of metallic Pb0 has earlier been reported for methyl ammonium lead mixed halide system.42, 46 The origin of metallic Pb0 can be due to different reasons. The high X-ray photon energy being absorbed by the material may generate high thermal energy, which may convert Pb2+ to Pb0. The fact anion exchanged samples are more prone to generate Pb0 suggest the mixed-halide coordination around the Pb2+ might make the PbX6 octahedral less stable. However, further studies are required to understand the effect of mixed-halide coordination on the generation of Pb0. The core levels of Cs 4d, Br 3d, and I 4d from the three samples at 3500 eV photon energy are shown in Figure 2b. We collected these core levels to examine their respective oxidation states. The binding energies and spin orbit splitting energies of the different core levels are mentioned in Table S1 in the supporting information. The spectra presented in Figure 2b are normalized with the corresponding Cs 4d signal intensity. The features of Cs 4d spectra are similar for all three samples. There is only one type of Cs, and the binding energy for Cs 4d5/2 is 75.9 eV with spin orbit splitting energy of 2.3 eV, which matches well with earlier literature.43, 45, 47 For CsPbBr3, we did not collect the I 4d spectrum region as there are no possibilities of having iodine in this sample (also evident from the survey scans from I 3d region). The binding energy of I 4d5/2 is 49.5 eV with a spin orbit splitting of 1.7 eV

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suggesting iodine is present in “1-” oxidation state.48 The relative intensity of I 4d signal increases with the extent of the anion exchange. We observed only one type of Br with a binding energy of 68.8 eV for 3d5/2 core level and spin orbit splitting of 1.0 eV. However, it was shown earlier45 that there are two different types of Br present for CsPbBr3 for similar sizes of NCs and these were assigned to Br species present on the surface and the core of the NCs. In this case, the binding energy of Br 3d core level matches with the Br species found in the core of the NCs of the previous work.45 We detected only the core of the NCs due to the higher photon energy used in these experiment compared to the previous experiments, which makes these experiments more bulk sensitive and we could not detect the bromide ions present on the surface of the NCs in these experiments. In case of the mixed halide system, CsPb(Br/I)3 NCs, we observed a decrease in Br signal intensity, suggesting that the anion exchange replaced the native bromide ions from the NCs. The peak features for Br 3d core level for the mixed halide system is similar to that of CsPbBr3 NCs, and there is no change in oxidation state of bromide ions upon anion exchange. It is interesting to note here that at the same photon energy, we observed a significant amount of metallic Pb0 in the sample. If Pb2+ is converting to Pb0, there must be a change in the oxidation state of Br as PbBr6 forms the octahedral in these perovskite materials. However, we do not observe any change in the oxidation state of Br. Similar observations were earlier reported for mixed halide CH3NH3PbI3-xClx.42 There is a possibility that while Pb2+ converts to Pb0, Br- ions will convert to Br2 in "0" oxidation state. Being a gas, Br2 may diffuse out from the system, and we would not be able to detect the corresponding Br2 signal due to the extremely low concentration inside the chamber. To our surprise, we observed minute amount of Br 3d signal intensity from the complete anion exchanged sample, CsPbI3 NCs. The intensity of the signal is just above the noise level

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of the data. There are possibilities that the anion exchange is not complete or some CsPbBr3 did not undergo the anion exchange. There are also possibilities of formation of internally heterostructured systems during anion exchange. However, the present data is not enough to confirm any of these possibilities.

Figure 3. Schematic representation of different possibilities of anion exchange of CsPbBr3 NCs in presence of iodide ions. Path A describes a homogeneous anion exchange all throughout the NC thus forming a homogeneous alloy. In presence of excess iodide ions, this homogeneously alloyed system converts to CsPbI3 through Path C. Path B shows a gradient anion exchange where the surface of NCs undergo the anion exchange. In presence of excess iodide ions, this gradient alloy gradually become homogeneous from outer side (Path D1) and finally forming CsPbI3 (Path D2).

While undergoing anion exchange, the internal structure can be very different. We explained this schematically in Figure 3. The anion exchange on CsPbBr3 can occur uniformly throughout the system (Path A) forming a homogeneous alloy. It can even happen in a gradient fashion (Path B) with the highest concentration of the iodide ions on the surface. This type of anion exchange will lead to an inhomogeneous internal structure. In the presence of excess iodide ions, either the homogenous alloy will convert evenly to CsPbI3 (Path C) or the inhomogeneous alloy system will undergo the anion exchange from the surface to the core of the NC and finally converting to CsPbI3 (Path D1 and D2). Li et al.49 have studied the evolution of the in-situ PL during the anion exchange and suggest that the exchange process starts almost immediately after the addition of the halide 13



precursor. They have observed two distinct emission peaks during the initial stage of the reaction (< 10 s) after the addition of PbI2 precursor solution to CsPbBr3 nanoparticles, suggesting that the reaction is inhomogeneous at the beginning and results in two or more different compositions. As the halide exchange process is very fast, the system attains an equilibrium and finally results one emission peak at the end of the reaction. However, their studies could not shed any light on the internal structure of the final NCs. To probe the internal heterostructure of nanomaterials, variable energy X-ray photoelectron spectroscopy has been used in the past.36, 50-52 This is possible as the inelastic mean free path of the photoelectron is comparable to the size of the nanoparticles. The inelastic mean free path depends on the kinetic energy of the photoelectron. By changing the photon energy, one can easily tune the inelastic mean free path of the photoelectron, thus the probe depth in X-ray photoelectron spectroscopy. To understand the internal structure of such anion exchange halide systems, we employed HAXPES with variable photon energies (3500 eV – 6000 eV) to achieve the inelastic mean free path (~ 29 Å – 40 Å) so that they are comparable to the edge length of the CsPbX3 NCs. We collected the core levels of Cs 4d, Br 3d, and I 4d at three different photon energies from the anion exchanged samples as shown in Figure 4. We purposely choose these regions for the following reasons; (a) all the orbitals are “d” orbitals so that the orbital anisotropy will be identical, and (b) the kinetic energies (binding energies) of these relevant core levels of different elements are similar, which makes the typical inelastic mean free path similar for the relevant core levels (as mentioned in Table S2 in the supporting information). This will also ensure the photoelectrons from these three different core levels originate at the same depth from the surface of the NCs.

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The core level spectra at different photon energies for the partial anion exchanged mixed halide sample, CsPb(Br/I)3 NCs, are shown in Figure 4a. The relative intensity ratio of Br 3d to I 4d changes with photon energy. The change in the ratio as a function of photon energy

Figure 4. Core level photoemission spectra of Cs 4d, Br 3d, and I 4d from (a) CsPb(Br/I)3, and (b) CsPbI3 at three different photon energies. The spectra are normalized with the corresponding Cs 4d signal intensities. Inset shows the model of internal heterostructures. Details of the deconvolution can be found in Figure S4 and S5 in the supporting information.

may arise from a higher abundance of iodide on the surface of the NCs or higher abundance of bromide near the core region. This may as well arise from a change in photoemission cross section of I 4d relative to that of Br 3d as a function of photon energy. To distinguish between these two effects, we have normalized the relative intensities of Br 3d and I 4d with their corresponding photoemission cross section.53 The photoemission cross sections of the corresponding core levels at different photon energies are mentioned in Table S3 in the supporting information. The ratio of Br/I is found to be 4.64 ± 0.15, 4.98 ± 0.25 and 8.64 ± 0.72 at 3500 eV, 4750 eV and 5946 eV, respectively. There is a continuous increase in the Br 3d relative signal intensity with the photon energy beyond the change in photoemission cross section, thereby signaling a strongly inhomogeneous distribution of the anions in these anion exchanged samples with more iodide ions on the surface and bromide ions relatively more in the core of the NCs. This quantitative analyses suggest that the anion exchange follows Path B rather than Path A as shown schematically in Figure 3.

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For the complete anion exchanges sample, i.e., CsPbI3 NCs, the signal intensity of Br 3d increases with photon energy as shown in Figure 4b. The increase in Br 3d signal intensity cannot be due to change in photoemission cross section as the photoemission cross section decreases with photon energy.53 This suggests that this Br 3d signal is originating from the core of the NCs. The Br/I ratio is found to be 0.42 ± 0.11 at 5946 eV. We could not determine the Br 3d intensity at two other photon energies for low signal intensity. This experimental evidence indicates that the anion exchange of CsPbBr3 is not complete all throughout the NCs even in the presence of excess iodide ions, although the absorption and emission spectra suggest the anion exchange is complete. It may happen that the anion exchange process stops beyond near complete anion exchange or the process becomes extremely slow, and the final anion exchanged NCs stops after Path D1 as shown schematically in Figure 3. In order to understand the origin of metallic Pb0, we collected Pb 4f core levels at three different photon energies. We observed metallic Pb0 for all the samples and the relative amounts are mentioned in Table S4 in the supporting information. For CsPbBr3, there is hardly any change in the concentration of metallic Pb0 as shown in Figure 5a. However, the partial anion exchanged CsPb(Br/I)3 contain much higher concentration of metallic Pb0 as shown in Figure 5b and also mentioned earlier in the manuscript. Interestingly, the relative metallic Pb0 concentration increases with increase in photon energy. Similar observations of increase in metallic Pb0 concentration were observed for the complete anion exchanged CsPbI3 NCs (Table S4 and Figure S8 in supporting information). It is possible that higher photon energy have more prominent effect on the stability of the NCs. We crosschecked by collecting Pb 4f from the partial anion exchanged mixed halide CsPb(Br/I)3 sample over a period of time at the sample position and the individual spectrum are shown in Figure S9. Clearly, the individual spectra look identical within the time period of 11 minutes of data collection. This suggests that either beam damage does not have any prominent effect on the 16


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Figure 5. X-ray photoemission spectra of Pb 4f core level at three different photon energies from (a) CsPbBr3 0 and (b) CsPb(Br/I)3 system. The inset shows the variation of the relative concentration of the metallic Pb at different photon energies. Details of the deconvolution can be found in Figure S6 and S7 in supporting information.

formation of metallic Pb0 or the effect can be very fast and saturates immediately with the exposure of the high energy X-ray. As the metallic Pb0 concentration increases with photon energy, it indicates that metallic Pb0 is present deeper from the surface as suggested in earlier studies for mixed halide perovskite.42 From these observations, we propose that the anion exchanged CsPbX3 are more prone to the formation of metallic Pb0 and it can due to the instability of the PbX6 octahedral. However, a more detailed and systematic studies are necessary to understand the exact reason of the metallic Pb0 formation. In case of the lead halide perovskite, the valence band is comprised of the antibonding orbital of Pb 6s and halide p orbitals with a major contribution from the halide p orbital, and the conduction band comprises of the Pb 6p orbital.7, 54 It is possible to change the valence band position by changing the anion, while the conduction band position remains almost unchanged. In case of a graded internal heterostructure with iodide ions more in the surface of the NC, it is expected that the hole will be localized near the surface of the NC, whereas the electron will be spread all throughout the NC. Further computational and experimental studies of radial dependence of electronic structure of these mixed halide NCs are required for a better understanding of optoelectronic properties of these NCs.

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Conclusion Variable energy HAXPES data reveal the internal heterostructure of partially and completely anion exchanged cesium lead halide NCs. The anion exchange reactions were performed with iodide ions starting from CsPbBr3 NCs. CsPb(Br/I)3 mixed halide NCs are formed after partial anion exchange, and CsPbI3 NCs are obtained after complete anion exchange. HAXPES shows the composition of the NCs are not homogeneous, rather there is a systematic gradient in composition from the surface toward the core of anion-exchanged NCs. Interestingly, for CsPbI3 NCs, the saturation of red-shift in PL peak with addition of iodide ions indicated a complete anion exchange of bromide ions with iodide. But our HAXPES results reveal that the anion exchange is not complete even in the CsPbI3 NCs. In reality, the composition of CsPbI3 could not been achieved employing anion exchange reaction, and a tiny amount of bromide ions are still present deep within the core of the NCs, which has been termed so far as CsPbI3 NCs. In addition to internal compositional structure, HAXPES results also show the formation of metallic Pb0 impurities, and its concentration is more for anion-exchanged NCs. These new insights regarding the internal heterostructure of anion-exchanged CsPbX3 NCs will help in understanding the optical properties and structural stability of these NCs. Supporting Information The Supporting Information is available free of charge on the ACS Publications website. XPS survey spectra, table of binding energies, spin orbit splitting energies, mean free path and photoemission cross sections of different core levels, table of different amount of metallic Pb0 concentration, deconvolution of Cs 4d, Br 3d, I 4d, and Pb 4f core levels, Pb 4f core level spectra at different photon energies for CsPbI3 NCs and time dependent individual scans of Pb 4f from CsPb(Br/I)3 NCs 18


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Acknowledgement Portions of this research were carried out at the P09 beamline at the light source PETRA III of DESY, a member of the Helmholtz Association (HGF). Financial support by the Department of Science & Technology (Government of India) provided within the framework of the India@DESY collaboration is gratefully acknowledged. P.K.S. acknowledges the start up funding at CeNS, Bengaluru. A.H. acknowledges student fellowship from CeNS, Bengaluru. A.N. thanks Science and Engineering Board (SERB) for Ramanujan Fellowship (SR/S2/RJN-61/2012) and DST Nanomission Thematic Unit (SR/NM/TP-13/2016). V.K.R. and G.S.S. thank IISER Pune and UGC respectively, for student fellowships.

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Internal Heterostructure of Anion-Exchanged Cesium Lead Halide

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