Phase Stable Red Emitting CsPbI3 Nanocrystals: Successes and

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Phase Stable Red Emitting CsPbI3 Nanocrystals: Successes and Challenges Anirban Dutta, and Narayan Pradhan ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.9b00138 • Publication Date (Web): 07 Feb 2019 Downloaded from http://pubs.acs.org on February 7, 2019

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ACS Energy Letters

Phase Stable Red Emitting CsPbI3 Nanocrystals: Successes and Challenges Anirban Dutta* and Narayan Pradhan* School of Materials Sciences, Indian Association for the Cultivation of Science, Kolkata, India 700032

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Abstract: Semiconducting bulk -cubic CsPbI3 phase is stable at high temperature. However, recent developments concluded that in nanodimension this phase can also able be stable even at room temperature. The unique feature of these -CsPbI3 nanocrystals is their low energy red color emission which remained an essential part for perovskite family of nanocrystals to cover entire visible spectrum. Even though these were reported stable at room temperature; but under certain conditions. These are mostly phase sensitive and under ambient condition, the phase is transformed to non-emitting phase. Hence, this phase stability in these nanocrystals remained one of the major challenges in current research. In this perspective, the origin of phase instability, observations of change in optical properties along with phase transformation under different environmental conditions, insights of possible modulations in A, B and X sites of the perovskites, the precaution in purification process and the ligand shell chemistry adopted during and post synthesis for obtaining stability of these CsPbI3 nanocrystals were systematically analyzed and reported. In addition, different possible aspects of future research for retaining the phase stability are also discussed. TOC:

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Lead halide perovskite (CsPbX3 X= Cl, Br and I) nanocrystals remain in spotlight due to their wide tunable optical property, high photoluminescence quantum yield (PLQY), low full width half maxima of the spectral emission, defect tolerance and dopant induced intense emission shuttling.1-6 These exciting properties made these nanocrystals ideal for solar cell, light emitting devices, solar concentrator, photodetector etc.7-12 However, the most demanding materials among these is CsPbI3 nanocrystals which show the emission in low energy visible window, and unlike CsPbBr3 or CsPbCl3, these possesses synthesis challenges in obtaining the stable nanocrystals in solution. These are survived with serious phase instability. The bulk cubic phase is stable at high temperature, but several attempts were made to stabilize this semiconducting cubic phase in the nanodimension.

13-15

Keeping the high demand of these materials in mind,

the origin of phase instability, nature of instability, possible crystal engineering for obtaining stability, purification strategies, additive chemistry and surface chemistry are discussed in this perspective. Nature of Instability: For understanding the phase sensitivity of nanocrystals in different environment conditions, a brief study of the bulk phase stability is first discussed. Semiconducting bulk -cubic CsPbI3 phase is known only stable at high temperature. However, this bulk phase is also showed phase-transformation to other forms under different reaction conditions. In recent report, Even and coworkers have shown that this  phase exists only above 645K and at lower temperature this is converted to  then  phase. The  phase remains in metastable state and finally 3



transformed to the non-perovskite  phase.13 However, during heating  phase directly converted to the  phase without transferring via intermediate stages.

Figure 1. (a) Temperature-dependent synchrotron X-ray diffraction pattern of CsPbI3 and (b) Schematic presentation of structural phase transition of various polymorphs of CsPbI3 as a function of temperature. Panel 1a is reprinted with the permission from reference 13. Figure 1a presents the temperature-dependent synchrotron X-ray diffraction pattern of CsPbI3 and Figure 1b shows the schematic presentation of the phase change during heating and the cooling processes. Later, Snaith and coworkers have shown that rapid cooling in presence of dry air led to-CsPbI3 phase; but slow cooling turned to the non-perovskite -CsPbI3.16 These were the developments of typical bulk phase transformation of CsPbI3 and when explored in nanoscale, similar phase instability was also largely observed. However, recent literature reports revealed that this light emitting -phase could be stabilized in nanocrystals at room temperature under certain conditions.14-15 The variation of such bulk property in nanodimension motivated for further exploration in understanding the crystal stabilization 4


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ACS Energy Letters

parameters for making phase stable highly emitting nanocrystals. However, the developed nanocrystals which showed considerable extent of stability, revealed that special strategies were adopted for inducing the phase stability; otherwise, like bulk these nanocrystals were remained vulnerable to phase transformations. Before discussing the successes achieved in retaining optical stability, the phase instability of these nanocrystals in different environments were discussed.

Figure 2. Digital image of the reaction flasks obtained without (a) and with (b) ice bath cooling after Cs precursor injection for 180 oC reaction temperature. (c) and (d) time dependent digital images of the CsPbI3 nanocrystals treated with ethanol in ambient light and UV light respectively. (e) and (g) High angle annular dark field (HAADF) images of the pristine cubic CsPbI3 and ethanol absorbed CsPbI3 nanocrystals respectively, (f) and (h) are the magnified view of (e) and (g) respectively. Panel 2c-2h were reprinted with permission from reference 18. -CsPbI3 Nanocrystals in Reaction Flask: 5



Analysis of the developed synthetic protocols for the perovskites nanocrystals revealed that during the preparation, the reaction flask was ice-cooled soon after the Cs precursor injection.15 However, instead of ice-cooling, if the reaction was annealed, the red color flask having CsPbI3 was turned to yellow. Digital images of the reaction flasks for these two different colored materials are shown in Figure 2a and 2b. Analysis showed that the yellow color was from the orthorhombic phase and hence, it could be stated here that these nanocrystals were susceptible to phase change during annealing and the color change was due to the phase transformation from cubic to orthorhombic.17 Hence, ice-cool approach was adopted for arresting the instantly formed cubic phase of the CsPbI3 nanocrystals though developments were also made in retaining the crystal phase during high temperature annealing (discussed in later section). Nanocrystals during Purifications: After cooling down to room temperature, these nanocrystals were also observed unstable during successive purification process. Being these were hydrophobic nanocrystals, polar solvents were used as non-solvent for precipitation. In most cases, during this process the red color slowly changed to yellow indicating the phase transformation. Wan and coworkers analyzed details of impact of the polar solvent on the stability of the cubic CsPbI3 nanocrystals.18 The authors had shown that the solvent induced lattice distortion to the ligand stabilized cubic phase CsPbI3 nanocrystals and this triggered the phase change in the nanocrystals. Figure 2c and Figure 2d present the digital images of the sample vials containing dispersed CsPbI3 nanocrystals in presence of ethanol under ambient light and UV-light illumination respectively. With time, the red emission observed to be 6


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quenched and this was ascribed due to the phase change of the red emitting -CsPbI3 nanocrystals to non-perovskite -CsPbI3. To visualize the distortions induced by polar molecules, aberration corrected scanning transmission electron microscope (AC-STEM) were used which revealed the structural evolution in atomic scale. High angle annular dark field (HAADF) STEM image of the pristine CsPbI3 nanocrystals presented in Figure 2e and zoomed view presented in Figure 2f clearly showed the atomic structure of the cubic phase (Pb−I columns, Cs atom column, and I column labeled red, cyan and purple dots); but ethanol treated sample showed lattice distortion (Figure 2g and Figure 2h). From these images, it was observed that the Pb-I-Pb unit was initially linear in pristine CsPbI3 nanocrystals and on ethanol treatment it acquired a zig-zag structure indicating distortion in the lattice. This distortion was caused by absorption of the ethanol molecule which was also observed in the image in Figure 2h. For avoiding or minimizing such lattice distortion, in several cases low polar solvents were used (details discussed in later section) for restricting the phase change in these nanocrystals. Nanocrystals under Ambient Exposure: Beyond the high temperature annealing and use of polar non-solvent, it is also revealed that the purified nanocrystals once stored were also lost their stability while exposed to ambient atmosphere.17, 21-22 Apart from all these standard processes, there are also several other experiments carried out which also evidenced the phase sensitivity of these nanocrystals. Recent studies reported that the phase change also triggered with the combined effect of oxygen, moisture and light.22 Further these nanocrystals were observed unstable to high temperature23 and pressure.24 All these developments revealed that the

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stability here remained a major concern and stabilizing the cubic phase became indeed challenging.

Figure 3. (a) A schematic presentation showing the tilting angle (), bond length and Goldschmidt tolerance factor. (b) Octahedral factor () Vs Goldschmidt tolerance factor () plot. This calculation is performed varying one parameter of ABX3 keeping other two constant. For A site, B and X remained Pb and I; for B site, A and X remained Cs and I; for X site A and B remained Cs and Pb respectively. Origin of Instability: Goldschmidt tolerance factor (𝜏) is considered to be the rule of thumb for predicting stability of rA + rX

the perovskite system, which is expressed as, 𝜏 = √2(𝑟𝐵 + 𝑟𝑋) where rA, rB and rx are the effective ionic radios of the A,B and X of a general ABX3 perovskite structure.25 Figure 3a shows schematic presentation of the tilting angle () and bond lengths of a perovskite system. As stated by Nag and co-workers, for ideal cubic perovskite, the value of 𝜏 should be 1 where B-X-B 8


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ACS Energy Letters

bond angle remains 180ᵒ.26 However, 𝜏 value between 0.8 to 1 favors perovskite formation; but in lower range these are distorted due to tilting of the BX6 octahedra (B-X-B bond angel less than 180ᵒ) and symmetry is also lost.27 On the other hand, for 𝜏 value greater than 1 strongly disfavors perovskite formation due to larger A site cation. Importantly, this approach of predicting stability of perovskite lattice was originated for oxide and fluorides, but in comparison to halides especially iodide which is less electronegative and more polarizable. Hence, the B-X bond will be more covalent and the model of hard sphere will be less applicable. Palgrave and coworkers19 have shown that the traditional tolerance factor calculation using Shannon radii28-29 was not in well agreement with the several iodide systems.Correlating with different experimental results, they have modified the calculation and observed the change in the radii, e.g. the Pb2+ ionic radii in chloride, bromide and iodide are 0.99Å (deviation of nearly 0.1 Å), 0.98 Å and 1.03 Å respectively which remained significantly shorter than Shannon ionic radii (1.19 Å).19 However, the tolerance factor mostly predicts the suitability of the A site cation to be placed in the cubo-octahedral void resulted by eight corner sharing BX62- units of acube. However, for understanding the suitability of the B site cation in the anionic sub lattice, volume of the octahedral void would be under consideration which could be described as 𝑟ℎ𝑜𝑙𝑒 = 0.41𝑟𝑥. For iodides, the radius of the octahedral hole formed by six iodide ions is 0.9 Å (0.412.2). Hence, any B site cation should have radius less than 0.9 Å. Further, the fitting of the B site cation in the octahedral anionic sublattice is described by Octahedral factor (𝜇), which rB

is defined as 𝜇 = rx . For a typical stable perovskite structure the value of τ should be greater 0.875 and 𝝁 should be greater than 0.41. Interestingly, CsPbI3 falls in the border line has  9



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value 0.8472 with 𝜇 value 0.47 and hence they are in a metastable state. Importantly, the above discussion was based on theoretical study to predict the bulk phase stability and this idea can also be extended to metastable CsPbI3 nanocrystals. Table 1. Size of possible A, B and X site ions19-20 for calculation of Figure 3b.

A-Site

Size (Å)

B-Site

Ammonium Rubidium Cesium Hydroxylammonium Methylammonium Hydrazinium Azetidinium Formamidinium Imidazolium Dimethylammonium 3-Pyrollinium Ethyalammonium Guanidinium Tetramethylammonium Thiazolium Piperazinium Tropylium Dabconium

1.46 1.72 1.88 2.16 2.17 2.17 2.50 2.53 2.58 2.72 2.72 2.74 2.78 2.92 3.2 3.22 3.33 3.39

Ni(II) Hg(II) Ti(II) V(II) Cr(II)hs Fe(II)hs Mn(II) hs Mg(II) Ge(II) Cd(II) Ca(II) Yb(II) Tm(II) Sn(II) Dy(II) Pb(II) Sm(II) Sr(II)

Size (Å) 0.57 0.61 0.66 0.68 0.68 0.68 0.72 0.75 0.77 0.81 0.92 0.93 0.95 0.97 0.97 1.03 1.11 1.18

X-Site Chloride Bromide Iodide

Size (Å) 1.87 1.96 2.2

To stabilize these metastable phase of the nanocrystals the tolerance factor needs to be increased for preventing excessive octahedral tilting. This can be achieved by doping/alloying A, B and X compositions. However, tuning any of the compositions might change in the stability along with the optical property. Even though success has been achieved to considerable extent; but to have wider possibilities, a plot of octahedral factor Vs tolerance factor, popularly known 𝜇 - 𝜏 plot, for various possible A, B and X constituents is provided in Figure 3b. This plot opens 10


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ACS Energy Letters

up guidelines for understanding the crystal manipulations for obtaining the stability of these nanocrystals and has been discussed sequentially in this perspective. Crystal Modulations Via Doping: A-Site Modulation: As discussed above, high tolerance factor could be achieved by placing a larger cation on A sites of the crystal (Figure 3b). The largest cation Cs in the periodic table has already been explored; but, still this showed moderate tolerance. Hence, theoretically cations having ionic radii below Cs should not favor stable perovskite structure (e.g. (NH4)PbI3 or RbPbI3). Attempts were expedited by alloying with other large size organic cations and Figure 3b (Black squares) showed the prediction of tolerance factors of some of the hybrid perovskites. As per the plot, any intermediate tolerance factor between two A site cations could be achieved by simply alloying with the other organic cation and this also could lead to entropic stabilization of the cubic phase.30-33 However, there are certain limitations here, and the major one is the miscibility of the two cations in the host lattice.27, 31, 34 The widely explored organic cation for this aspect is the Formamidinium ion (FA). Methylammonium ion (MA) was also helped in stabilizing the hybrid perovskites; but its volatile nature limited its wide exploration. Kovalenko and co-workers have shown that 10 % of FA with respect to Cs could stabilize the lattice and retained the stable emission.35 However, this part is least explored in comparison to the change in other parameters of the crystals and need more exploration.

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Figure 4. (a) Absorbance and PL spectra of CsPbI3 and CsPb0.9Mn0.1I3 nanocrystals, (b-c) time dependent powder XRD patterns of CsPbI3 and CsPb0.9Mn0.1I3 nanocrystals respectively. (d-f) Time dependent digital images of CsSnI3 nanocrystals in presence of air, CsSnI3 nanocrystals in presence of N2 and CsSn0.6Pb0.4I3 nanocrystals in presence of air respectively. Panels 4a-4c and panels 4d-4f were reprinted with permission from reference 22 and 21 respectively. B-Site Modulation: Unlike A-site modulation, B-site modulation has direct impact on both tolerance factor and octahedral factor. Figure 3b (red dots) shows some expected ionic species which could sit in B sites replacing Pb. However, metals beyond the boundary line as shown in Figure 3b cannot form perovskite structure or would lead tounstable perovskite structure (e.g. CsSnI3 or CsMnI3). However, these could again be explored by alloying with Pb (CsPb(1-x)MxI3) for providing stability to the nanocrystals. From literature reports, it is revealed that the progress on B site modulation is limited, and only Sn, Mn and Sr were studied for the same.21-22, 36-39 The 12


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major issue in using these metal ions is their chemical reactivity which has high possibility of formation of cross nucleations. Though several reports are there to modulate the B site cation by post synthesis modification/cation exchange, were established for CsPbBr3,40-41 but to our best knowledge such post synthesis cation replacements were not yet established for CsPbI3. However, there are reports on post synthesis anion assisted cation exchange on CsPbI3 which led to mix cation and mix halide CsPb(1-x)MxBr(3-x)Ix perovskites.39 The B site cation doping was initiated with Mn for obtaining the spin polarized Mn d-d emission;42 but later it was observed that Mn dopant stabilized all CsPbX3 nanocrystals though the study was mostly confined with the bromide system.37 Later, Manna and co-worker has shown the impact of Mn doping in phase and optical stability of CsPbI3 nanocrystals.22 Authors have shown that nearly 10% of Mn with respect to Pb was enough in the CsPbI3 nanocrystals to stabilize these metastable nanocrystals for months, whereas the pristine CsPbI3 nanocrystals degraded within 5 days. Figure 4a presents the absorbance and corresponding PL spectra of the pristine CsPbI3 and CsPb0.9Mn0.1I3 nanocrystals. Figure 4b-4c show the time dependent powder X-ray diffraction (XRD) patterns of the pristine and Mn alloyed nanocrystals. The stability was tested in toluene dispersion and also in the crude reaction mixture. Importantly, Mn alloy does not alter the bandgap and the optical properties of the parent nanocrystals. Theoretical study 22 showed that the valence and conduction bands of CsPbI3 are dominated by the p and s orbitals of I and Pb respectively and Mn d-states retained below the conduction band. Hence, incorporation of Mn did not contribute to the band gap. Further, Sn alloying was also observed to stabilize this metastable phase of CsPbI3nanocrystals. Shen and co-workers had shown that the CsSnI3 is 13



more unstable due to low  and  values and facile oxidation of Sn2+ to Sn4+, compared to CsPbI3nanocrystals; but alloying Sn with Pb in CsPbI3 lattice enhanced the stability of the nanocrystals.21 Figure 4d-4f show the digital images of the solution of CsSnI3 nanocrystals in air and nitrogen atmospheres, and CsSn0.4Pb0.6I in air respectively. In both cases, the CsSnI3 nanocrystals were observed unstable; but the Sn alloyed sample remained stable for 150 days in solution. Later, Yang and co-workers have shown stable CsPb(1-x)SnxI3 nanowires and they have also observed similar optical change as stated earlier.36 The stability was also tested in films. It is reported that while the CsPbI3 nanocrystals were completely transformed to the nonemissive orthorhombic phase, but the film of CsSn0.6Pb0.4I3 nanocrystals remained stable upto 35 days. However, unlike the Mn alloying, Sn incorporation tunes the bandgap of the alloyed nanocrystals. While CsPbI3 has band gap of 1.73 eV14 and CsSnI3 has 1.3 eV; but their alloyed here observed having intermediate bandgap. This is also reflected in red shifting the absorbance and PL position in comparison to the CsPbI3, but remained blue shifted compared to CsSnI3. The alloying further increased the intrinsic defects and reduced the quantum yield drastically. From these observations, Mn was observed to be a better B-site (than Sn) alloying agent in terms of restoring original intense red emission of CsPbI3 as this does not contribute the bandgap like Sn. Beyond these dopants/alloys, recently Sr2+ ions were also explored for stabilizing the CsPbI3 nanocrystals.38 These results suggest that B site cation replacements indeed provided stability to these nanocrystals though more explorations in this respect are required.

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X-Site Modulation: The X-site (halide) modulation remained more feasible and well established compared to A or B site modulations.43-45 This tuned the optical properties of the nanocrystals as halides has major influence in the conduction band of the perovskite system. Hence, with changing their composition, the band gap significantly tunes. Even though the optical emission position is tuned; but importantly it also tunes the octahedral factor and induced crystal stability. This is also reflected in the plot shown in Figure 3b where the blue triangles referred todifferent τ and  values for CsPbX3 and any of the intermediate value could be obtained by tuning the halide compositions. Hence, this halide modulation can bring stability to the metastable cubic CsPbI3 nanocrystals and this can be achieved either taking different ratio of lead halide precursors at the beginning or by anion exchange.44-48 However, in bulk phase these mix halide systems has shown light induced phase segregation which limits their device performance.49 Interestingly, mix halide nanoparticles did not show any such phase segregation under certain device fabrication conditions. However, this has other demerit as annealing the nanocrystals film allowed agglomeration and the phase segregation.48 Hence, more optimizations and developments of new protocols in developing such nanocrystals film are essentially required. Ligand Shell on Nanocrystals Stability: The ligand shell of nanocrystals mostly contributes for the solution dispersion, colloidal stability, interacts with surface states and also helps in retaining some of the properties of the material. The interface chemistry in the ligand shell of the perovskite nanocrystals also 15



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contributed for the phase stability of CsPbI3 nanocrystals. However, surface characterization in perovskites is mostly studied in CsPbBr3 nanocrystals and not much explored for CsPbI3. However, as all have similar crystallographic interface and composition, there might have similar interface chemistry for all three lead halide perovskites. Kovalenko and co-workers50 suggested that the ligand shells of perovskite nanocrystals were constituted with both oleate and oleyalammonium ion (for oleic acid and oleylamine pair); but oleic acids were present in unbounded state. Later, Nag and co-workers had shown that the oleylammonium ions were bonded through H-bonding at the interface and occupied the surface Cs positions on the crystal surfaces.51 Luther and co-workers predicted that these interactions could be explained by hardsoft acid base principle.14 Since, chlorides are more electronegative acted as a hard acid and bound strongly with the hard base ammonium ion; but, such interactions were weaker for iodide

systems

owing

to

their

soft

nature.

Hence,

thesurface

of

the

iodide

perovskitenanocrystals remained more sensitive and remained vulnerable for the phase instability even onto the exposure to ambient atmospheres or adding polar solvents during purifications. The Purification Strategies and Stability of Nanocrystals: Common non-solvents used for purifying hydrophobic nanocrystals are methanol, ethanol, acetone etc. and their high polarity helped in precipitating the hydrophobic nanocrystals. Unfortunately, these could not serve as ideal non-solvent for perovskite nanocrystals as their excess use and particularly during successive purifications, triggered the phase change. This remained more sensitive for CsPbI3 though CsPbBr3 as well as CsPbCl3 also showed the phase change during rigorous purification 16


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process.14, 52 In a strategic development, Luther and coworkers used low polar methylacetate as the non-solvent and successfully obtained stable CsPbI3 nanocrystals even at room temperature.14 This indeed motivated for investigating more interface chemistry and the insight mechanism of the role of ligand shells for stabilizing the nanocrystals. After this pioneer work, methyl acetate was extensively used as ideal non-solvent for the purification of CsPbI3 nanocrystals. Effect of Phosphine and Phosphonic Acid: Phosphine and phosphonic acid which were considered as strong binding ligands for chalcogenide nanocrystals, had also extended for passivating perovskite nanocrystals. Shen and coworkers introduced Trioctylphosphine (TOP)PbI2 complex as a reactive precursor for improving the stability of CsPbI3 QDs.53 Authors investigated this TOP-PbI2 adduct by several spectroscopic techniques and concluded that the adduct remained highly reactive which accelerated nucleation and growth of CsPbI3 nanocrystals. This is also shown improved the crystallinity as well as yields of the product. The obtained nanocrystals remain highly emissive and the high PLQY could be achieved for all size of the particles. This high PLQY was also observed more stable compared to the traditional synthesis. However, the exact role of the TOP on the surface is largely unknown; as no detectable

31P

Nuclear Magnetic Resonance (NMR) signal was observed after second round

purification. Like TOP, phosphonic acid was also observed to stabilize the cubic CsPbI3nanocrystals. Jasieniak and coworkers has shown that the replacement of oleic acid by bis-(2,2,4-trimethylpentyl) phosphonic acid (TMPPA) increased the stability of the cubic CsPbI3 nanocrystals.54-55 The OA 17



treated nanocrystals emission was quenched within 3 days; but TMPPA treated nanocrystals were observed stable more than 20 days. However, the exact origin of the stabilization remained still largely unknown; but it is stated that TMPPA formed ion pair with the oleyalammonium ion in the ligand shell though direct evidence of their binding was not established.54

Figure 5.(a) Annealing time dependent absorbance and PL spectra. (b-c) TEM image of the CsPbI3 nanocrystals obtained after 1 min and 1 h annealing in presence of OLA-HI. These panels were reprinted from reference 17. Copyright was obtained from Wiley VCH. Effect of Pre-formed Ammonium Ions as Binding Agent: As discussed above ammonium ions and carboxylate remain on the ligand shell of perovskite nanocrystals and passivate the nanocrystals. Ammonium ions are mostly generated insitu in the reaction system. In a recent report of our group, preformed ammonium halide salts were used in the reaction process for stabilizing the nanocrystals. This helped for controlling the size of nanocrystals, enhanced the doping efficiency and also provided stability to the nanocrystals.5, 52, 56-57 Presence of these salts also provided thermal stability of all three halide perovskite nanocrystals in reaction flask. 18


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Interestingly, these also helped stabilizing the cubic phase of CsPbI3 synthesized at higher reaction temperature. Figure 5a present the annealing time dependent absorbance and PL spectra in the presence of adequate amount of the oleylammonium iodide (OLA-HI) salt. As discussed earlier, the perovskite nanocrystals were unstable on annealing; but in presence of OLA-HI, the emitting cubic phase became stable even with prolonged annealing. TEM images also confirmed the size and shape of the particles remain unchanged (Figure 5b and Figure 5c). Once synthesized at high temperature reactions, these CsPbI3 nanocrystals were also stable in ambient condition retaining their high PLQY.17 Later several other reports on the stabilization of these cubic CsPbI3 nanocrystals also emphasized on the ammonium ion binding.58-59

Figure 6. Schematic presentation of hydrolysis of methylacetate (Sheme I), anionic ligand (Scheme II) and cationic ligand (Scheme III) exchange on the surface of CsPbI3 nanocrystals. These panels were reprinted from reference 60. 19



Figure 7. (a-b) Surface charge redistributions on CsPbI3(PbI2 rich surface) surfaces with OA and IDA ligand modification respectively.(c) Normalized PL intensity of CsPbI3 nanocrystals with time for both IDA treated and untreated nanocrystals. (d) Schematic presentation of possible surface passivation mechanism of GR on CsPbI3 nanocrystals. (e) Digital image of CsPbI3 nanocrystals in both ambient and UV light. The harvested nanocrystals became optically dead after 15 days and retrieved emission on addition of various amount of TOP. (f-g) PL spectra of ethanol treated nanocrystals with and without presence of TOP respectively. Panels 7a-7c, 7d and 7e-7g were reprinted with permission from references 9, 61 and 64 respectively. Post Synthesis Surface Modification: As discussed earlier, iodide remained the most unstable system with most sensitive surface, and hence, certainly this demands suitable passivating agent for improving the photophysical properties and device performances. Further, the common ligands which encapsulate the nanocrystals create an insulating layer which hinders 20


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the carrier mobility and reduce the performance. Luther and coworkers have shown that the CsPbI3 quantum dots films when treated with AX (FA+,MA+ and Cs+) type of salts during surface ligand exchange, greatly increased the carrier mobility and performance of the solar cell. Interestingly, best power conversion efficiency was observed in case of FAI treatment. The authors considered all the possible interactions between FAI and CsPbI3 nanocrystals and finally concluded that FAI helped in removing native ligands (oleate and oleylammonium ion) from the surface which reduced the inter-particle gap.12 Later, the same group had shown that the ligand exchange process with FAI was observed very time specific. Initially both ammonium ion and carboxylate ions were removed from the surface and finally FA incorporated into the crystal resulting Fa(1-x)CsxPbI3 alloyed structure.60 It is established that methyl acetate in presence of moisture got hydrolyzed to acetic acid and methanol (Figure 6, scheme I). Now, on treatment of methyl acetate, the acetic acid replaced oleate from the surface by protonating oleate leaving acetate binding on the surface (Figure 6 scheme II) and FAI treatment removed the surface bound ammonium ion (Figure 6 scheme III). Detailed of the ligand exchange procedure was monitored through Furrier Transformed Infra Red (FTIR) and NMR spectroscopy. However, trace amount of moisture in MeOAc is certainly required to get ligand exchanged nanocrystals, but higher moisture leads to more hydrolysis of MeOAc which destabilize the surface and transformed to the non perovskite phase. The authors have also shown that the popularly used Pb(NO3)2 during ligand exchange has no role, rather the moisture associated with the salt helps in ligand exchange.

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In another approach, using 2,2′-iminodibenzoic acid (IDA) as bidentate passivation agent, Bakr and coworkers successfully stabilized these red emitting nanocrystals.9 It was observed that IDA possess higher binding energy (1.4 eV) compared to the oleic acid (1.14 eV), helped for better passivation for stabilizing the cubic phase of the nanocrystals (Figure 7a,b). Figure 7c shows the stability plots for as synthesized and IDA treated nanocrystals and the PL observed stable for 15 days. Further, using μ-graphene (μGR) as a binder, Liu and coworkers showed the stability gained for the cubic CsPbI3 nanocrystals.61 μGR was used for crosslinking the nanocrystals form μGR/CsPbI3 film which showed better stability against moisture, humidity, and at high temperature. A schematic presentation of the surface binding of μGR with CsPbI3 is shown in Figure 7d. Tüysüz and coworkers has shown that the cubic CsPbI3 nanocrystals can act as a catalyst to promote polymerization of 2,2′,5′,2″-ter-3,4-ethylenedioxythiophene (TerEDOT) and resulted CsPbI3 nanocrystals encapsulated in the TerPEDOT polymer network. This encapsulated CsPbI3 nanocrystals has shown significant stability enhancement compared to the pristine nanocrystals.62 Further, trioctylphosphine which had significant role in stabilizing the cubic phase of the CsPbI3 nanocrystals duringsynthesis also found as an efficient surface passivating agent in post synthesized nanocrystals.53, 63 Yu and coworkers have shown that the post synthesis addition of TOP even retrieved emission from the optically dead nanocrystals.64 This also reduced surface traps, enhancedthe emission intensity, and increased thermal stability, photostability and also the tolerance towards polar solvent. Figure 7e shows digital image of CsPbI3 nanocrystals in presence of visible and UV light. The optical emission of the freshly prepared sample was dead; 22


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but regenerated on addition of TOP and the obtained emission intensity was also found directly proportional to the amount of TOP added. As discussed earlier, the cubic phase of CsPbI3 nanocrystals remained highly unstable towards polar solvent like ethanol, but the post synthesis TOP treated samples has shown moderate amount of stability towards ethanol. Figure 7f and Figure 7g show the impact of ethanol treatment on the PLQY of the freshly prepared and TOP treated CsPbI3 nanocrystals respectively. It was observed that the PL intensity remained almost unchanged with ethanol treatment; but the untreated sample was quenched.All these results conclude that surface treatment with specific ligands remained also a key issue in providing optical as well as phase stability to these nanocrystals. Current Challenges and Future Prospects Even though successes have been achieved in stabilizing these red emitting CsPbI3 nanocrystals by manipulating the crystal compositions and using several supporting reagents; but these still remained in embryo stage in comparison to other halide perovskites or the chalcogenide nanocrystals. Hence, deeper investigations are warranted for both understanding the fundamental insights of the crystal design and developing new synthetic methods in obtaining the phase stable nanocrystals. Some of the challenges in front are summarized here. 

A possible list of both A site cation and B site cation is presented in Table 1 and could be

chosen as the corresponding site alloying agent to tune the and. However, this modulation strategy has possessed possibility of altering the red emission. Hence, careful selection of the constituent required more theoretical approaches. 23





Apart from the list several M3+ system also observe to replace the Pb in the bulk phase

and these are yet to be tested in the nanodimension.65-67 Importantly, the effect of a M(III) replacement in the place of Pb(II) can change the charge neutrality of the system and its consequent impact on the shape, size and optical property need to be investigated. 

For the X site modulation only halides were used till now but there are several other

anions (e.g. thiocyanate) can be used which have shown to stabilize this phase in bulk dimension.68-69 

Ammonium ion is crucial in stabilizing the perovskite nanocrystals. However several

short chain ammonium ions were also employed to stabilize the bulk -CsPbI3 phase. Hence the nanocrystals could be stabilized with these small chain ammonium ions to facilitate their device fabrication.70-71 

Several multidentate ligand need to design to passivate these nanocrystals for better

stability and ligand exchange process for the device fabrication. 

Till now all the cubic CsPbI3 nanocrystals were prepared through high temperature hot

injection technique or by anion exchange at room temperature. However, direct room temperature synthesis is not reported and needs to be explored. Summary In summary, the recent developments of room temperature phase stable cubic CsPbI3 nanocrystals were reported. The origin of phase instability and the possible remedies in crystal composition as well as the ligand shell chemistry also discussed. Chronological developments in achieving the phase stability including the crystal modulations in A, B and X sites, addition of 24


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protective ligands like ammonium ions,alkyl phosphine, phosphonic acids etc., using less polar solvent during purifications and also varying the reaction parameters were systematically presented. At the end the challenges remained still ahead in obtaining the stable and robust nanocrystals were also summarized. ASSOCIATED CONTENTS AUTHORS INFORMATION Contact authors email: (NP) [email protected], (AD) [email protected] Biography Anirban Dutta received his B.Sc. degree from Suri Vidyasagar College, West Bengal, India and his M.Sc. from the Indian Institute of Technology, Delhi; currently, he is a senior research fellow in the School of Materials Sciences, Indian Association for the Cultivation of Science, Kolkata and working on synthesis and photo-physics of the doped and undoped nanocrystals. Biography Narayan Pradhan is Professor in School of Materials Sciences, Indian Association for the Cultivation of Science, Kolkata. He has obtained his Ph.D. degree from IIT Kharagpur and carried out his postdoctoral research work in Israel and the U.S.A. He joined IACS in 2007. His major research interests are the synthesis, surface functionalization, and photophysical properties of colloidal semiconductor and transition metal-doped semiconductor nanomaterials. Visit http://iacs.res.in/faculty-profile.html?id=102 for further details.

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ACKNOWLEDGEMENTS DST-SERB of India (EMR/2016/001795) is acknowledged for funding. AD acknowledge CSIR, India for fellowship. Note The authors declare no conflict of interest. REFERENCES (1) Huang, H.; Bodnarchuk, M. I.; Kershaw, S. V.; Kovalenko, M. V.; Rogach, A. L. Lead Halide Perovskite Nanocrystals in the Research Spotlight: Stability and Defect Tolerance. ACS Energy Lett. 2017, 2, 2071-2083. (2) Huang, H.; Polavarapu, L.; Sichert, J. A.; Susha, A. S.; Urban, A. S.; Rogach, A. L. Colloidal Lead Halide Perovskite Nanocrystals: Synthesis, Optical Properties and Applications. NPG Asia Mater. 2016, 8, e328. (3) Zhang, F.; Zhong, H.; Chen, C.; Wu, X.-g.; Hu, X.; Huang, H.; Han, J.; Zou, B.; Dong, Y. Brightly Luminescent and Color-Tunable Colloidal CH3NH3PbX3 (X = Br, I, Cl) Quantum Dots: Potential Alternatives for Display Technology. ACS Nano 2015, 9, 4533-4542. (4) Niezgoda, J. S.; Foley, B. J.; Chen, A. Z.; Choi, J. J. Improved Charge Collection in Highly Efficient CsPbBrI2 Solar Cells with Light-Induced Dealloying. ACS Energy Letters 2017, 2, 1043-1049. (5) Das Adhikari, S.; Dutta Sumit, K.; Dutta, A.; Guria Amit, K.; Pradhan, N. Chemically Tailoring the Dopant Emission in Manganese-Doped CsPbCl3 Perovskite Nanocrystals. Angew Chem Int Ed Engl 2017, 56, 8746-8750. 26


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