Tuning the Size of CsPbBr3 Nanocrystals: All at One

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Tuning Size of CsPbBr3 Nanocrystals: All at One Constant Temperature Anirban Dutta, Sumit K Dutta, SAMRAT DAS ADHIKARI, and Narayan Pradhan ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.7b01226 • Publication Date (Web): 02 Jan 2018 Downloaded from http://pubs.acs.org on January 2, 2018

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ACS Energy Letters

Tuning Size of CsPbBr3 Nanocrystals: All at One Constant Temperature

Anirban Dutta, Sumit Kumar Dutta, Samrat Das Adhikari and Narayan Pradhan* Department of Materials Science, Indian Association for the Cultivation of Science, Kolkata, India 700032

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ABSTRACT: For varying the size of perovskite nanocrystals, variation in the reaction temperature or tuning the ligand chain lengths are established as the key parameters for high temperature solution processed synthesis. These also need sharp cooling for obtaining desired dimensions and optical stability. In contrary, using preformed alkylammonium bromide salt as the precise dimension controlling reagent, wide window size tuneable CsPbBr3 nanocrystals were reported without varying the reaction temperature of changing the ligands. The size tunability even with ~1 nm step growth regimes was achieved just as a function of the concentration of added alkylammonium bromide salt. Not only for the cube shape; but the width also varied in sheet structures. Since these nanostructures loss their optical stability and crystal phase on prolonged annealing, stabilizing these in high temperature synthesis for all inorganic lead halide perovskites is important and remained challenging. In this aspect, this method turned out to be more facile which does not need sharp cooling and the nanocrystals retained their phase and optical properties even on prolonged annealing. TOC


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Lead halide perovskites which have emerged as a new class of functional energy materials for both light emitting and photovoltaic applications, remained in forefront of current research.1-16 Enormous efforts have been put forwarded in designing these nanocrystals and understanding their formation chemistry.4,

6, 8, 17-57

The high temperature synthesis of tunable monodisperse lead halide perovskite

nanocrystals are typically achieved by modifying the surface ligands, ligands ratio, reaction temperature and precursors ratio etc; 24, 27, 37, 48, 52, 57 but not in unique reaction set up with similar reagents and in a concise pre-programmable manner. Moreover, all such approaches required instant cooling for protecting the emission intensity, phase and also the dimensions of these nanocrystals. This indicates that these synthesis methods are indeed sensitive and demands more flexible processes for obtaining the phase stable highly emissive nanocrystals. One of the most widely studied all inorganic perovskite system is CsPbBr3 nanocrystals which have room temperature air stable highly emissive crystal phase. A close look into the literature reports on high temperature synthesis of lead halide perovskite nanocrystals reveals that their surfaces are coordinated/bonded with alkylamines (or ammonium ions) and acids(or carboxylate ions).24,

27, 58-59

Recent study also suggested that ammonium ions replaced surface Cs ions to passivate/stabilize the perovskite nanocrystals.60 Inspiring from this concept, herein, alkylammonium bromide salt was introduced as a size controlling additive reagent which successfully controlled the size of cubes from the quantum confinement regime to bulk size just as a function of the amount of added salt without varying the reaction temperature or any other reaction parameters. The synthetic control remained robust which could predict precisely the size of the nanocubes even in 1 nm step growth regimes. This approach does not require any sharp cooling strategy and retained the stable optical emission even on prolonged annealing. Not only cube shapes, but also the widths of sheets are tuned with varying the concentration of added salts.



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For the synthesis of CsPbBr3 nanocrystals, stock solution of oleylamine-HBr (OLA-HBr) was first prepared (see experimental in supporting information), and for controlling the size or shape, required amount of this salt was introduced into the reaction system. Cs-oleate solution was injected at desired reaction temperature for triggering the reactions following the procedure reported by Protesescu et al.4 Optimizing reactions at different reaction temperatures, 140-180 oC was observed the ideal temperature window for obtaining the wide size tunable nanocrystals. Figure 1(a-h) (and Figure S1-8) presents the TEM images of eight different size CsPbBr3 nanocubes with introduction of different amount of OLA-HBr at 160 oC. The average size of ~17 nm in standard case obtained without salt addition was reduced to ~3.5 nm with increasing the amount of OLA-HBr from 0 to 0.6 ml (0 to 0.52 mmol). The size tuning window of these nanocubes further represented in their size distribution histograms shown in Figure 1i which clearly suggested the effect of the concentration of OLA-HBr on the particles size. Representative high resolution TEM image for ~9 nm size nanocubes is shown in Figure S9. Similarly, nanocrystals obtained at 180 oC reactions also showed that with changing the amount of added salt, the size was tuned. TEM images and corresponding size distribution histograms of these nanocrystals are presented in Figure S10. Interestingly, with increase in OLA-HBr amount, monodispersity of the nanocubes were also enhanced. Smaller size nanocubes which were obtained with higher amount of salt were highly monodisperse and these were reflected from their 2D self-assemblies on the TEM grid (Figure 1a-d and Figure S1-3). For correlating further, size of nanocubes as a function of added ammonium salts, a plot of size Vs the amount of introduced salt is presented in Figure 2a (squares). These size variations were also highly reproducible with ~6-10% error range. The size versus the amount of salt for this temperature is also plotted in Figure 2a (Triangles).


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Figure 1.(a-h) TEM images of CsPbBr3 nanocubes obtained from the reaction with the variations of amount of OLA-HBr stock solution. The size of these cubes varied from ~3.5 nm to ~17 nm with introduction of 0.52, 0.44, 0.35, 0.26, 0.17, 0.09, 0.04 and 0.0 mmol of the salts respectively. Scale bar in all cases is 100 nm. (i) Size distribution histograms of all the above right set of nanocrystals (from a to h and Figure S1-S8). These are calculated considering 200 numbers of nanocrystals.

Figure 3a shows the powder X-ray diffraction (XRD) patterns of four different size nanocubes obtained from four different concentration of the salt. The peak positions were resembled with the orthorhombic phases of CsPbBr3. However, several studies showed that the orthorhombic phase is the stable phase and we have also found the same phase (COD: 4510745).54, 61-65 Figure 3b presents the absorption spectra of all eight samples where the tunability is clearly observed from below the quantum confinement regime to bulk size. Corresponding photoluminescence (PL) spectra of all these samples are shown in Figure 3c where the emissions were observed tunable from 465 nm to 510 nm. The quantum yield of these nanocrystals (varied from 70 to 80 %) retained similar to its best reports and the excited state decay lifetime also observed in nanoseconds (4.9 to 5.24 ns, See Figure S11).24



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Figure 2. Plot of amount of added OLA-HBr Vs obtained size of nanocubes carried out at 160 oC (squares plot) and 180 oC (triangles plot). Reaction conditions for all cases remain identical in both temperatures. The error bars remains ~6-10%.

Figure 3.(a) XRD patterns of CsPbBr3 nanocrystals with variation of amount of OLA-HBr. (b) Absorption and (c) corresponding PL spectra of CsPbBr3 nanocrystals with variations of OLA-HBr salts. (d) Annealing time dependent successive PL spectra of nanocrystals obtained at 160 ᵒC and 180 ᵒC reactions in presence and absence of the salt. In each case the amount of salt is written along side of each spectrum. Excitation wavelength is at 350 nm.


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The interesting observation noticed here is the PL stability of these nanocrystals (Figure 3d). For the standard reaction without the salt carried out at 160 oC, the PL intensity is significantly reduced and completely quenched for 180 oC reaction while annealing. However, in presence of salt, the intensity remained unchanged. The comparison PL spectra are shown in Figure 3d and corresponding UV-visible spectra presented in Figure S12. The decrease in optical property might be due to the possible phase change as reported in the literature.66 This suggests that the nanocrystals obtained in our approach not only result precise tunable size, but also remained robust and thermally stable. However, here also observed a limiting stage for the density of OLA-HBr for retaining the cube shape at 160 oC reaction. Introducing excess salt changed the shape to square platelets. UV-Vis, PL spectra, powdered XRD pattern and TEM images of these platelets are shown in Figure S13. Hence, our entire study was limited within the permitted amount of OLA-HBr for retaining the cube shape.

Figure 4. (a) Absorption spectra, (b) PL spectra and (c) powder XRD patterns of CsPbBr3 nanosheets obtained with introduction of 0.44mmol and without OCTA-HBr salt. (d-e) and (f-g) TEM images of nanosheets in different resolutions obtained in absence and presence of OCTA-HBr respectively. It is widely established that the surface is bound with both ammonium and carboxylate ions. Though, carboxylate binds strongly to the nanocrystal surface, ammonium ion is more effective in 7 Environment ACS Paragon Plus


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tuning the growth of anisotropic platelet structure.24, 48, 62 While oleylamine was leading to exclusive nanocubes, octylamine (OCTA) was forming nanosheets under identical reaction conditions.4, 48 These nanosheets formation, while carried out in presence of octylamine-HBr (OCTA-HBr), interestingly, the thickness of the sheets was observed reduced. Figure 4 presents the optical spectra, XRD and TEM images obtained with and without OCTA-HBr. The appearance of a new absorption peak (~440 nm) (Figure 4a) suggested that the length of one of the confinement direction (thickness) of the sheets were reduced. PL spectra (Figure 4b) further supported with dual emissions where one was controlled by quantum confinement regime and another by bulk. These are typically observed for sheet like structures for their dual mode of confinements.48 Powder XRD pattern confirmed the same orthorhombic phase of these nanosheets. Figure 4d-e and Figure 4f-g presents TEM images obtained without and with OCTAHBr (0.5 ml) respectively. The contrast differences clearly indicate sheets obtained with OCTA-HBr were thinner. From Atomic Force Microscopic (AFM) images, the widths of sheets obtained with and without OCTA-HBr were observed ~5 nm and ~15 nm (also visible from the TEM image of marked area in Figure 4e) respectively (Figure S14). This confirmed that alkylamine-HBr not only reduced the size of the nanocubes of CsPbBr3; but also reduced the thickness of sheets obtained in different amine medium. All the above results either for the size of the cubes or width of sheets, suggested that the preformed ammonium bromide salt played the important role for such dimension variations. From NMR and FTIR spectroscopic data analysis, it is observed that ammonium ions are present on the surfaces (Figure S15 and S16). The results also overlapped with previous reports in oleylamine and oleic acid mixed systems.24, 60 These ions typically replace surface Cs+ ions and acts as integrated bonded ions on the surface of perovskite crystals.60 Accordingly, with increase of their density or concentration on the reaction mixture, the dimension of the nanocrystals became smaller due to instant capping. Schematic presentations for the surface bonding with ammonium ions in replacement of Cs+ ions are shown in 8 Environment ACS Paragon Plus

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Figure S17, which is the most plausible condition expected here. Further, it is also observed that PbBr2 which is widely known as bromide source does not control the dimensions. While reactions were carried out using PbO for the synthesis of CsPbBr3, similar results were obtained (Figure S18-S19 and supporting information). The bromide source of this reaction was also the ammonium bromide. In typical case under this reaction condition, more amount of the precursor should result larger size particle; but the case remained opposite. Hence, the sole reason for the size restrictions with more alkyl ammonium bromide is assumed here is the strong interface binding which restrict the size. This alkyl ammonium halide approach was extended to chloride system, at 160 oC, a mixture of different shaped particles were formed probably due to less solubility of PbCl2 at this temperature. However, wide size tunable window was observed for CsPbCl3 at 180 oC ranging from 25 nm to 6 nm. The TEM images of the nanocubes obtained on varying amount of salt and corresponding size histogram presented in Figure S20. The method also did not function for iodides, possibly due to the instant phase change. In conclusion, a fixed reaction temperature synthetic approach for wide window size tunable CsPbBr3

perovskite

nanocrystals

is

reported.

This

has

been

achieved

by

introducing

alkylammoniumbromide salts and with increase of its concentration, the size of the cubes and width of sheets was reduced. The size window tunability also varied while the reaction temperature is varied. Importantly, this approach requires no instant cooling, and the stable emission, size, shape and phase of the nanocrystals were remained intact even on prolonged annealing, which were typically not observed in standard synthetic routes. However, even this would wide spread the synthesis of these materials still the chemistry of controlling the dimensions of ionic perovskites is long way to go for clear understanding and better control on the synthesis of these nanostructures. ASSOCIATED CONTENT Supporting Information 9 Environment ACS Paragon Plus


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Experimental methods, supporting figures (Lifetime decay plots, more TEM images, AFM images etc.) AUTHOR INFORMATION Corresponding Author Email: (NP) [email protected] Authors AD and SKD contributed equally ACKNOWLEDGMENT AD, SKD and SDA acknowledged CSIR for fellowship. DST (SR/NM/NS-1383/2014(G) of India is acknowledged for Funding. REFERENCES (1)

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