DotâWireâPlateletâCube: Step Growth and Structural Transformations

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Dot-Wire-Platelet-Cube: The Step Growth and Structural Transformations in CsPbBr3 Perovskite Nanocrystals Lucheng Peng, Anirban Dutta, Renguo Xie, Wensheng Yang, and Narayan Pradhan ACS Energy Lett., Just Accepted Manuscript • Publication Date (Web): 27 Jul 2018 Downloaded from http://pubs.acs.org on July 27, 2018

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Dot-Wire-Platelet-Cube: The Step Growth and Structural Transformations in CsPbBr3 Perovskite Nanocrystals

Lucheng Peng,† Anirban Dutta,# Renguo Xie,†,* Wensheng Yang,† and Narayan Pradhan#,* †

State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of Chemistry Jilin University, Changchun 130012, China.

#

Department of Materials Science, Indian Association for the Cultivation of Science, Kolkata, 700032, India.

Abstract: While the classical mechanism for the growth of colloidal chalcogenide nanocrystals is largely understood; the fundamental insights for the growth of perovskite nanocrystals still remained elusive. Using nanoclusters of ~0.6 nm diameter as monomers and growing to more than 25 nm in a single reaction, herein, the step growth process of perovskite CsPbBr3 nanocrystals is reported. This is performed in step-rise of reaction temperature with correlating the annealing time. The growth was so precise that ~0.6 nm (nearly one unit cell) increments were successively monitored in parallel to the conversions of clusters to nanowires and then to thickness tunable platelets and finally to size tunable cube shaped nanostructures. The entire reaction was monitored optically and microscopically, and their step growths were correlated. From these observations, the possible growth mechanism for perovskite nanocrystals along with their shape transformations was proposed.

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TOC:


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How do perovskite nanocrystals grow in reaction flask? In spite of significant successes have been achieved in fabricating highly emissive CsPbX3 nanocrystals following different reaction protocols; but understanding their crystal growth in step processes still remained elusive.1-7 In comparison to well-developed classical growth mechanism of chalcogenide nanocrystals,8-14 growth patterns for perovskite crystals in reaction flask are little explored. The most successful approach for these nanocrystals typically followed a sharp or ice bath cooling process intending to obtain phase pure highly emissive nanocrystals.1 However, this mostly led to one-reactionfor-one-size approach and does not focus on one size to another size or one shape to other shape transformation. Unfortunately, even though reports with nucleations,15 growths,15 oriented attachments,16-17 shape conversions,18-21 hetero-structures formation,22 effects of ligands,19, 21, 23-33 effect of solvent polarity,34 reaction temperature on size/shape effects,1, 12, 24, 35

the phase change,19, 27-28 cation exchange,36-37 anion exchange,38-43 thermal stability,16, 44

doping36-37, 45-52 etc. were established; but step by step crystal growths in a single reaction leading to wide window optical and size tunable perovskite nanocrystals has not been yet established. Being these nanocrystals are emerged as one of the most efficient energy materials,4, 53-54 understanding the fundamental aspects of crystal growth and the impact of thermodynamic and kinetic factors for perovskite nanocrystals synthesized in reaction flask indeed remained challenging. To address these issues, a new synthetic approach of a single reaction leading to successive changes in dimensions of CsPbBr3 nanocrystals is reported. Using smaller nanoclusters as monomers and restricting ligands concentration in the reaction medium, the sequential step growth of ~0.6 nm starting from unit cell dimension to beyond the quantum 3 ACS Paragon Plus Environment


confinements (>25 nm) was achieved. This growth process was also observed with the shape transformation of wire-to-platelet-to-cube shapes of CsPbBr3 nanostructures; but all in one reaction. The entire process was monitored optically and microscopically, and the intermaterials transfer induced successive change in size or shape was established as the major driving mechanism involved in these perovskite nanocrystals. This step growths study of these nanostructures and their correlation of successive spectral evolutions in a single reaction remained unique, and fundamentally, these also inducted new insights in the field of crystal growth of perovskite nanocrystals. The all-in-one reaction for observing the step-growth of CsPbBr3 perovskites was studied in non-polar solvent (ODE) as a function of step-rise of reaction temperature with variations in reaction time. Initially, smaller size clusters of CsPbBr3 were prepared (see experimental section), purified with high-speed centrifuge for removing excess precursors, amines, acids and stored in freezing temperature for their long term stability. The absorption peak position of this cluster solution resembled with 0.6 nm CsPbBr3 (assumed from the plot, shown later section).55 Being these materials were used for the growth of CsPbBr3, these were termed as cluster monomers. For observing the step-growths, required amount of these purified cluster monomers were mixed with ODE at room temperature and the reaction was heated with step increasing the temperature. Figure 1a and Figure 1b show the successive absorption and emission spectra obtained during the step-growth process in a typical reaction. As observed, the absorption peak continued to red shift from ~402 nm to ~515 nm and then remained fixed. This nearly 100 nm tuning was within the confinement regime before the nanocrystals attained the bulk size (>7.0 nm). We believe that the precise confinement tuning for such a large 4 ACS Paragon Plus Environment

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window in a single reaction is unique for perovskite nanocrystals and this strongly reflected the involvement of both thermodynamics and kinetics parameters in crystal growth similar tothe classical growth.This observation was alsodifferent from several reported one-reaction-for-onedimension ice-cool approaches.1, 56-57

Figure 1. (a) Successive UV-visible and (b) respective PL spectra obtained from the samples collected at different reaction temperatures and time variations. Inset shows the superimposed time dependent PL spectra. Excitation wavelength is 360 nm and for all cases OD is corrected at the excitation wavelength. (c) Absorption spectra of samples collected at different time



intervals in a 50-110 oC step rise of reaction temperature. Inset shows the enlarged spectral portions showing dual absorption peaks. A 3D plot of time-temperature-dimension of this reaction is provided in TOC. (d) Successive absorption spectra of samples collected from a reaction at different time intervals in a fixed reaction temperature (40 ᵒC). For all cases approximately equal amount of aliquots of samples were collected for the optical measurements. Further, a close look at these absorption spectra reflected that at the beginning, apart from the perovskite clusters, each spectra had multiple peaks. With gradual increase of temperature and/time, the intensities of the blue side peaks reduced and that of red endpeaks enhanced. This might be related to dissolutions of one dimenison and formation of another dimension of nanocrystals or transformation of one form of nanostructures to other. However, once the peak position reached at ~ 460 nm, then only one peak retained which also continued to be red shifted untill the size reached the bulk dimension. In contrary, for one-reaction-forone-size, tunable optical spectra were obtained from different set of reactions.While bandgap vs size (obtained from the absorption spectra and TEM images, plot provided in later section), it was observed that the differences of two consecutive peak positions corresponds to ~0.6 nm dimension which was close to an unit cell of cubic CsPbBr3. This concluded that these nanostructures grew stepwise with 0.6 nm dimension at a step for more than 100 nm optical window and all in a single reaction. In association to the absorption spectra, the photoluminesence (PL) spectra of respective samples were also measured and presented in Figure 1b. Like the trend of absorption spectra,


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corresponding emisison peaks were also red shifted and then fixed at ~525 nm (bulk dimension).2 The FWHM for initial spectra were also broad reflecting dual peaks obtained from dual absorptions. The shifting of the peak was clearly indicating the contribution of kinetically driven crystal growth of these new class of nanostructures.

Figure 2. (a-b) TEM images of fragmented nanowire like 1D structures having diameter ~1.2 and ~1.8 nm. (c-d) Platelets like nanostructures with wide distributions of their lengths. (e) TEM images of mixture of platelets and cubes in different resolutions. (f-g) TEM images of ~15 nm and ~25 nm nanocubes. The reaction conditions and size of each set of nanocrystals are depicted in respective panels. More TEM images in each case are provided in supporting information (Figure S1, S2, S3 and S4). (h) Powder XRD patterns of samples collected at different time intervals for the 50-200 oC step rise reaction. The absorption peak positions are marked near each pattern.



Furthermore, to confirm these interesting step-growth patterns as a generic one, the reaction was also carried out at a different step-rise reaction temperature and varying the sample collection time. Figure 1c presents the successive absorption spectra of a different reaction with 50 to 110 oC with random time variations and the pattern was observed similar to that presented in Figure 1a. Inset reflected the enlarge view of the spectra of the clusters and few initial dual peaks spectrum reflecting presence of mixed dimensions nanostructures. To support further, the reaction was also carried out at a fixed temperature (40 oC) with prolonged annealing. Interestingly, this led the nanostructures having dual and then triple peaks in one spectrum (Figure 1d). The three peak positions in one spectrum correspond to 0.6 nm, 1.2 nm and 1.8 nm respectively. This low and fixed temperature reaction reflected strong kinetic factors involved for driving the growth of the nanocrystals. The most exciting feature in these step-growths is the nanostructures seen under electron microscope. The clusters which were expected to be 0.6 nm (as per absorption peak discussed later) could not be obtained in our system (mostly turned to Pb(0)). However, for ~1.2 nm and ~1.8 nm cases, connected or broken nanowires assembled like organic mesostructures (Figure 2a-2b, Figure S1) were observed. These were expected from the alignments of CsPbBr3 nanoclusters. Because of low temperature, their alignments could only be achieved in one dimension as pulling out more ligands and allowing the growth in multiple facets were difficult at this temperature. Further rise of temperature, these were again fragmented and transformed to platelet shaped nanostructures with different dimensions (Figure 2c and 2d, Figure S2, S3). In the next step, the sample showed the mixture of platelets and cubes (Figure 2e, Figure S4), and finally all turned to cubes above 150 oC and retained the cube shape during 8 ACS Paragon Plus Environment

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further annealing (Figure 2f and 2g). Samples having wire like structures, platelets or mixture of platelets and cubes always showed dual absorptions confirming the presence of at least two confinement dimensions and also within the confinement regime. Size reaching 7 nm had only cubes and further collected samples retained similar absorptions which were corresponding to bulk CsPbBr3. Powder X-ray diffraction patterns for all these samples (Figure 2h) resembled with bulk cubic phase of CsPbBr3 though these nanomaterials are in debate over the cubic and orthorhombic phases.1, 58 Importantly, no phase change was noticed during the entire reaction suggesting the adopted protocol as robust and insensitive to phase change. This might be because of the absence of excess PbBr2 and ligands or any of the reactant precursors, and also the medium was in balanced polarity which helped for phase retaining. As the optical tunability observed here remained the widest tunable system for CsPbBr3 carried out in a single pot reaction; their optical band positions were correlated with the confinement dimensions of the nanostructures obtained from the microscopic images. Accordingly, the confinement diameters versus bang gaps are plotted (Figure 3a) and also corresponding theoretical plot obtained from the literature method was compared.59-61 The optical band positions (both absorption peak and band edge) with diameter is provided in Table S1 in supporting information. Only for the initial reaction mixture (clusters), the size was calculated from the extrapolated experimental plot (square box, Figure 3a) as it was difficult to obtain from the microscope and this represented ~0.6 nm size which remained nearly equal to the unit cell length (0.59 nm). From their size and optical positions, the change in QY (Figure S5) with the progress of reaction is also correlated. Standard QY for the cube samples varied within 60-65%. 9 ACS Paragon Plus Environment


Figure 3. (a) Diameter confinement versus change in band gaps. This has been correlated with optical absorption peak positions, provided in supporting information. The theoretical values were calculated following Brus equation (provided in supporting information). The hollow square is obtained for the nanoclusters after extrapolating the fitted plot. (b) Absorption spectra of purified ~12 nm nanocubes dispersed in ODE and sample collected at 210 oC, 20 min annealing. TEM images of initial and the final samples are shown in Figure S6.

Figure 4. TEM images of nearly monodisperse (a) nanowires, (b) platelets, (c) cubes of 9 nm and (d) cubes of 15 nm obtained with prolonged annealing at different reaction temperature. e) Absorption spectra of all these nanostructures. Annealing time and reaction temperature are mentioned in each panel.


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From all of the above results, it could be established here that the size/shape of perovskite nanocrystals would be controllably tuned in a single reaction. However, the bigger question remained here with the driving forces which supply monomers and trigger the growth or shape transformation. Further, as the monodispersity is compromised with the progress of a reaction, this also cast doubt whether this would be able to compete with several optimized reports on the synthesis of highly monodisperse perovskite nanocrystals. For understanding these issues, one representative control reaction was performed where nanocubes of certain size obtained in this method were purified, again dispersed in ODE and continued the step growths following the protocol exactly similar to the adopted one for Figure 1a (and also Figure 2). Interestingly, it was observed that the size continued to grow further as expected even though no cluster monomers or no excess reactant precursors were present in the system. The optical absorption spectra and TEM images of the purified (12 nm cubes) and the final (28 nm cubes) nanostructures were shown in Figure 3b and Figure S6. In addition, a drastic reduction in OD was noticed confirming significant decrease of number of particles in the reaction. Hence, the growth in this process could be assumed here as inter-particles materials transfer induced ripening like growth; a new concept for perovskite nanocrystals. This can also be related to decreasing in the PL intensity in Figure 1b because of the decrease in particle counts. The materials transfer process was further varied with addition of additional acids and amines which were expected for helping as carriers for the transfer process. These organic ligands were expected (1) to cap the nanostructures, (2) to provide dynamicity at the surface to control the growth and also (3) act as carriers for the materials transfer for diffusion controlled growth. Figure S9 presents the absorption spectra and also the TEM images of the nanocrystals



obtained with addition of additional acid and amine to the reaction system. These showed the nanocrystals were grown much faster than the ligand free system. For perovskite nanocrystals, it was already established presence of excess acid or amine triggers the acid-amine equilibrium to the right side of the ammonium and carboxylate ion pair formation, which acted as carriers or transporting agent for facilitating the growth.62 As these accelerated the materials transportation process, the reaction quickly led to the final cube shape. Further, the question remained about the efficacy of this cluster mediated method for obtaining the high quality nanocrystals and to investigate this, the reaction was allowed to continue with annealing at different reaction temperatures. Importantly, fragmented wires turned to well packed wires, platelets having different lengths and widths (a ≠ b) turned to square platelets (a = b) with long range self-assembly and cubes of different sizes turned to nearly monodisperse cubes with prolonged annealing. Representative images of these nanostructures obtained from the same reaction protocol (as followed for Figure 1a and Figure 2) by ceasing the step-rise of reaction temperature are shown in Figure 4. Wide view TEM images are provided in supporting Figure S7 and S8. This strongly suggested that this method might also be a robust approach and could lead to high quality perovskite nanostructures. This size focusing during annealing is expected due to the kinetically driven materials dissolution and diffusion within the nanocrystals. Accordingly, a schematic presentation for the reaction temperature induced size defocusing and annealing induced size focusing of all such nanostructures is presented in Figure 5. Being these are materials diffusion induced reactions though controlled differently for predominated thermodynamically or kinetically governed reactions; but these findings have correlation with Ostwald ripening type mechanism. However, 12 ACS Paragon Plus Environment

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for further confirmation factors like number of particles present in each step, the size/shape variation exciton coefficients, the confinement tuning in one shape of the nanostructures etc. are indeed essential for establishing the crystal growth process of these perovskite nanocrystals.

Figure 5. Schematic presentation of the step growth controlled wide and narrow size distributions during heating and annealing of the nanostructures in the reaction medium. In conclusion, a step growth approach for tuning the dimensions of CsPbBr3 in quantum confinement regime is reported. The precise step growth of 0.6 nm was observed with the formation of 1D nanowires to platelets with different thickness to nanocubes with variable sizes. From different observations, it could be concluded here that; (1) the growth of perovskites could be followed with progress of reaction time and increase of reaction temperature; (2) it could proceed with step growths preferably with increase of a unit cell at a time; (3) the growth occurred as function of materials transfer induced dissolutions and diffusion; (4) decoupling the step-rise of reaction time and reaction temperatures, size dispersity could be controlled. While the growth mechanism of perovskite nanocrystals in 13 ACS Paragon Plus Environment


reaction flask is still unclear even though large numbers of reports were established; this finding of step growth would certainly help understanding more on the growth mechanism of these fabulous nanocrystals. However, these findings are restricted to the change in quantum confinement in a simultaneous shape variable reaction system; but more investigations are required with planned reactions for establishing the growth in size tuning in a particular shape of the nanocrystals. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI:xx Experimental section, additional TEM images, optical spectra, Bandgap calculations, table showing the bandedge and dimension of CsPbBr3 nanocrystals.

AUTHOR INFORMATION Corresponding Authors (NP) [email protected], (RX) [email protected]

ACKNOWLEDGEMENTS This work was supported by the National Natural Science Foundation of China (21373097 and 51072067) and DST, Govt. of India (SERB/F/7159/2016-17) AD acknowledged CSIR India for fellowship. Notes The authors declare no competing financial interest 14 ACS Paragon Plus Environment

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