Solvent-Assisted Self-Assembly of CsPbBr3 Perovskite Nanocrystals

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Solvent-Assisted Self-Assembly of CsPbBr Perovskite Nanocrystals Into One-Dimensional Superlattice Naiya Soetan, William R. Erwin, Andrew M. Tonigan, Don Greg Walker, and Rizia Bardhan J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b03939 • Publication Date (Web): 04 Aug 2017 Downloaded from http://pubs.acs.org on August 5, 2017

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The Journal of Physical Chemistry

Solvent-Assisted Self-Assembly of CsPbBr3 Perovskite Nanocrystals Into OneDimensional Superlattice Naiya Soetan,† William R. Erwin,† Andrew M. Tonigan‡, D. Greg Walker§, Rizia Bardhan†* †

Department of Chemical and Biomolecular Engineering, ‡Department of Interdisciplinary Materials Science, and §Department of Mechanical Engineering, Vanderbilt University, Nashville, TN 37235, USA RB: E-mail: [email protected]

Abstract Self-assembly of colloidal nanocrystals into ordered architectures has attracted significant interest enabling innovative routes to manipulate the physiochemical properties for targeted applications. This study reports the self-assembly of CsPbBr3 perovskite nanocrystals (NCs) in one-dimensional (1D) superlattice chains mediated by ligand-solvent interactions. CsPbBr3 NCs synthesized at ≥ 170 °C and purified in a nonpolar solvent, hexane, self-assembled into 1D chains whereas when purified in polar solvents including toluene and ethyl acetate they were disordered or showed short-range 2D assemblies. The NCs assembled into 1D chain show a redshift in both the absorbance and photoluminescence spectra relative to the disordered NCs purified in 50/50 hexane/ethylacetate mixture. Microscopy and X-ray diffraction results confirm the formation of polymeric nanostrands in hexane followed by organization of the NCs into 1D chains along the nanostrands.

Our results suggest excess aliphatic ligands remaining after

purification of the NCs complex with ionic Cs+ and Br- species via hydrophobic effect; further the alkyl chains of these ligands interlace with each other via van der Waals forces. Collectively these interactions give rise to the nanostrands and subsequent self-assembly of CsPbBr3 into 1D chains. In polar solvents the minimization of repulsive forces between the solvent and the ligands drives proximal CsPbBr3 NCs together into short-range 2D assemblies or disordered clusters. Our solvent-assisted self-assembly approach provides a general strategy to design 1D superlattice chains of nanocrystals of any geometry, dimension, and composition by simply tuning the ligand-solvent interactions. 1 ACS Paragon Plus Environment


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Introduction Self-assembly is ubiquitous in nature and has now emerged as an approach to engineer and control properties at the nanoscale. The self-assembly of colloidal nanocrystals (NCs) into one-, two- and three-dimensional (1D, 2D, and 3D) ordered superlattices has enabled a host of exciting opportunities due to the striking collective properties that emerge from the interaction of these nanoscale building blocks.1-4 The directed functionality that arises from ordered structures of NCs has been harnessed in a range of technologies from magnetic devices5 to electronic applications.6 The self-assembly of nanocrystals can be (1) driven at interfaces such as liquidliquid or liquid-air, (2) enabled by controlled solvent evaporation on a templated substrate, and (3) directed by solvent-dependent supramolecular interactions of the participating ligands on the nanocrystals surface.7-9 The polarity of the solvent has a strong impact on the nanocrystal arrangement resulting from a delicate balance between ligand-ligand and ligand-solvent interactions.10-11 This solvent-driven assembly process is governed by both specific and noncovalent interactions such as hydrogen bonding, van der Waals, electrostatic, and entropic forces.12 Furthermore, the morphology, and size- and shape-monodispersity of the NCs also controls long-range ordering.13-15 Collectively these parameters have been fine-tuned to achieve both large-domain 2D superlattices16-18 and 3D colloidal supercrystals.19-22 However, the formation of ordered 1D superlattice remains a challenge due to a lack of directional control of isotropic NCs and typically requires external forces to drive ordered organization.23-24 The discovery of organolead halide bulk perovskites as efficient absorbers in lightharvesting and light-emitting technologies have piqued tremendous interest in this class of material.25-32 Recently research efforts have shifted towards the development of perovskite NCs both with organic-inorganic (RNH3PbX3) and all-inorganic (CsPbX3) composition. The CsPbX3 (X = Cl, Br, I) NCs have been extensively synthesized33-39 by varying the halide ions, surface 2 ACS Paragon Plus Environment

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ligands, and reaction temperature, resulting in size- and morphology-controlled optical properties.40-43 that have been harnessed in a range of optoelectronic applications.44-48 Unlike conventional semiconductor NCs, CsPbX3 perovskite nanocrystals are highly ionic and have strong ionic interactions with capping ligands that give rise to interesting surface properties. The characteristics of the CsPbX3 NCs surface ultimately impact the particles’ ability to selfassemble and the material’s resulting structure-property relationship.

In this work we

demonstrate that CsPbBr3 perovskite NCs self-assemble into 1D chains when purified in hexane, a nonpolar solvent, but are disordered or show short-range 2D assemblies when polar solvents are used including toluene and ethyl acetate. We provide a mechanistic overview of the driving forces that result in self-assembly into 1D superlattice in low polarity solvents, and demonstrate the impact on optical properties that arise from the ordered organization.

Experimental Methods Preparation of Cesium Oleate (Cs-oleate): Cs-oleate was prepared as described by Protesescu et. al.

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A 100-mL 3-neck flask was loaded

with 0.814 g of Cs2CO3, 40 mL octadecene (ODE), and 2.5 mL of oleic acid (OA). The flask was heated under vacuum at 120 ˚C for 1 h followed by heating at 150 ˚C under N2 until the reaction was complete. Note: the internal temperature of the reaction flask was different from the oil bath, therefore the oil bath was set to 175 ˚C prior to use. Synthesis of CsPbBr3 Nanocrystals: CsPbBr3 nanocrystals were synthesized as detailed by Protesescu et. al.43 Oleylamine (OLA) was dried for at least 3 hrs on a Schlenk line in an oil bath on a hot plate set to 175 ˚C. In an N2 glove box, 69 mg of PbBr2, 5 mL of ODE, and a stir bar were loaded into a 25-mL flask and sealed.

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This flask was removed from the glove box and dried for at least 1 hr in an oil bath on a hot plate set to 175 ˚C. After drying on the Schlenk line for 3 hrs, OLA was put under N2 immediately before it was used; after use, it was put back under vacuum. To synthesize the cubes, 0.5 mL of OLA was added to the 25-mL flask using a 1 mL luer pipette with a 6-in 20-gauge stainless steel needle. Then, 0.5 mL of OA was added to the 25-mL flask using a 1.25-in 23-guage disposable needle with a 1 mL disposable luer pipette. The 25-mL flask containing all the reagents was stirred under vacuum until the PbBr2 was fully dissolved. Then, the 25-mLflask was placed in an oil bath heated to 135, 150, 170, or 190 ˚C. Cs-oleate (0.4 mL) was injected into the 25-mLflask, and the reaction was quenched in an ice bath after 10 s. There was no stirring during the reaction. Following the quenching of the reaction, the crude solutions were centrifuged for 5 – 10 min at 8500 rpm, until a pellet formed at the bottom of the tube. Then, the supernatant was removed and the pellet was dispersed in hexane or toluene. This solution was stored in an N2 glove box for future use or briefly chilled and centrifuged again. Chilling aided with the precipitation of the NCs and reduced the temperature that the solution reached during centrifugation. Note: ethyl acetate was added to the crude solution (135 ˚C) or mixed with hexane or toluene in the second centrifugation step (135 and 150 ˚C) at a 1:1 (v:v) ratio to precipitate the NCs. After the second centrifugation step, the supernatant was removed and the pellet was dispersed in a small amount of solvent (hexane or toluene). All samples used for characterization were prepared following the exact same procedure where a droplet of nanocrystals at the same concentration dispersed in the respective solvents (toluene, hexane, and hexane/ethyl acetate 50/50 mixture) was deposited on glass substrate for SEM and copper grids for TEM and dried under vacuum with no heating. All glass microscope slides used were sonicated for 30 min in a 1:1 (v:v) solution of acetone and isopropanol, rinsed with isopropanol, and dried with air. Then, the glass slides were plasma


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treated immediately before drop casting the NCs onto them. Samples used for optical characterization as well as a few of the substrates characterized by x-ray diffraction were coated with a 10 mg/mL solution of PMMA dissolved in chlorobenzene by accelerating to 1000 rpm over 5 s followed by dwelling at 1000 rpm for 45 s. Synthesis of Ligand Polymeric Nanostrands OLA was dried as described previously. CsBr (75 mg), was combined with 5 mL of ODE, put under vacuum using a Schlenk line and stirred on a hot plate set to 175 ˚C for several hours. After the mixture dried, 0.5 mL of dried OLA and 0.5 mL of OA were added as described previously. The solution was left to stir for ~20 h at 190 oC. After 20 h, CsBr crystals remained, but had become a fine powder. The solution had a red tint. The solution was then chilled and centrifuged. A gel-like white precipitate appeared at the bottom of the Eppendorf tube in addition to the CsBr powder. This pellet was suspended in hexane or toluene and centrifuged again. The supernatant was removed from the pellet and the precipitate was once again suspended in either hexane or toluene before being deposited on a glass slide. Characterization The images of the NCs were taken using an Osiris Transmission Electron Microscope (TEM) at 200 keV or a Zeiss Merlin Scanning Electron Microscope between 3 and 10 kV. Samples prepared for TEM/SEM were not coated with PMMA. Fast Fourier transformation (FFT) of the transmission electron microscopy (TEM) images was used to provide information on crystalline domains of the NCs. A Scintag XGEN-4000 X-ray diffractometer with CuKa providing the radiation at a wavelength of λ = 0.154 nm obtained X-ray Diffraction (XRD) spectra for the NCs. Absorbance spectra of the samples were taken using a Varian Cary 5000 UV-vis-NIR spectrophotometer

with

dual-beam

capabilities.

To

obtain

photoluminescence

(PL)



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measurements, we used a ~380 nm, broad-linewidth, uv-excitation source and an Avantes AvaSpec-2048 spectrometer for the spectrum analysis under dark conditions. To account for background noise, before each measurement, a dark reference was taken and subtracted from the subsequent measurement. Following excitation, the fluorescence of the sample was channeled via an armored bifurcated fiber optic cable to the spectrometer. A sufficient PL peak height was obtained for the samples with an integration time of 1.05 ms.

Results and Discussion The CsPbBr3 nanocrystals were synthesized by following a method previously reported by Protesecu et.al.43 by reacting Cs-oleate (Cs(OOCR)) with lead bromide (PbBr2) in the presence of oleylamine and oleic acid as ligands and octadecene as solvent. The formation of CsPbBr3 is given by the reaction: 2Cs(OOCR) + 3PbBr2 → 2CsPbBr3 + Pb(OOCR)2. The reaction temperature and the solvents used to purify the nanocrystals post-synthesis govern the self-assembly of the nanocrystals. The reaction temperatures studied were 135 ˚C, 150 ˚C, 170 ˚C, and 190 ˚C (Figure 1); the NCs were subsequently purified and re-dispersed in hexane or toluene. For the nanocrystals synthesized at 135 ˚C and 150 ˚C, a 50/50 mixture of ethyl acetate and hexane was utilized to precipitate the NCs out of solution (see Methods for more details). Surprisingly CsPbBr3 NCs synthesized at 170 ˚C and 190 ˚C re-dispersed in hexane selfassembled into 1D superlattice chains (Fig. 1c, d) where the distance between two adjacent chains varied between 80 ± 20 nm at 170 ˚C to 150 ± 30 nm at 190 ˚C. We hypothesized the space between two adjacent chains are polymeric nanostrands composed of excess organic ligands, remaining after purification, that crosslink with the capping ligands on the CsPbBr3 NCs surface. This process is mediated by ligand-solvent interactions. This is supported by additional


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TEM and SEM images provided in the supporting information (Figure S1) which show low contrast polymeric nanostrands. High magnification transmission electron micrographs and FFT images (Figure 2a,b and S2) reveal that the NCs are single crystalline with a lattice spacing of ~5.8 and ~4.1 Å that is characteristic of the [100] plane and [110] planes of CsPbBr3 respectively.37 High angle annular dark field (HAADF) STEM micrographs and elemental mapping (Figure 2c-f, Figure S3) confirm the composition of the perovskite NCs. We have provided additional STEM-EDS micrographs in Figure S4 as well as estimated elemental composition shown in Table S1 that confirms that excess ligands form the polymeric nanostrands decorated with 1D assemblies of NCs. X-ray diffraction (XRD) of the CsPbBr3 NCs demonstrate that the NCs have orthorhombic and monoclinic phases (Figure 3 and Figure S6) as reported previously for CsPbBr3.49-50

The XRD diffractograms also show that the organic ligands

separating adjacent perovskite chains have characteristic peaks at low angles at 2θ = 2.5 and 7 degrees. It has been reported previously by Lu et. al.51 and Wang et. al.52 that the presence of oleylamine in the reaction mixture results in the formation of nanoscale polymeric nanostrands which have XRD peaks at low diffraction angles.



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Figure 1: TEM micrographs of CsPbBr3 nanocrystals synthesized at various temperatures: (a) 135 ˚C, (b) 150 ˚C, (c) 170 ˚C, and (d) 190 ˚C. Scale bar in inset of (c) and (d) is 100 nm.

Self-assembly is most favorable between NCs of similar morphologies. Therefore we measured the sizes of the CsPbBr3 NCs synthesized at different temperatures, which ranged from ~10 to 23 nm (Table S2) and showed narrow size-distributions. Further, the interparticle spacing controlled by the surface ligands, also plays a critical role in the alignment of the NCs. The interparticle spacing varied between 1.5 ± 0.3 nm and 2.6 ± 0.6 nm with no correlation with the temperature of the reaction or the solvent used for purifying the NCs. While all samples were dried under the same conditions, it is possible that small variations in evaporation rate of the 8 ACS Paragon Plus Environment

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solvents or asymmetric ligand distributions53 may have influenced the interparticle spacing. Details on interparticle spacing between the NCs resulting from various reaction conditions are provided in Table S3. Since the approximate chain lengths of oleic acid and oleylamine are 1.5 nm54 and 2.1 nm55 respectively, we conclude that a mixture of these ligands is likely present on the NCs surface, all of which participate in directing the self-assembly process by intercalation of the alkyl chains. Since the observed assembly is directed by ligand-solvent interactions we also performed thermogravimetric analysis (TGA) and from the 1st derivative of TGA we show that the % mass of ligand present after purification is directly correlated to the nature of the solvent (Figure S5 and Table S4). The high amount of ligand remaining in hexane as observable in TGA data suggests that the self-assembly of NCs may be coordinated by London forces in nonpolar solvents. Since the CsPbBr3 NCs self-assembled along the polymeric nanostrands at 170 °C and 190 °C, we initially hypothesized that the higher temperatures induced the formation of the organic polymeric nanostrands by providing excess cesium ions to complex with oleylamine as Cs-oleate begins to decompose between 150 and 200 ˚C.56-57 However, our control experiments suggest that the solvent polarity plays a stronger role than reaction temperature in the self-assembly process where van der Waals and hydrophobic interactions in nonpolar solvents drive self-assembly.12,

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As a control study, the CsPbBr3 NCs were

synthesized at 190 ˚C; half of the NCs precipitate was purified and re-dispersed in hexane and the other half in a 50/50 mixture of ethyl acetate/hexane. XRD clearly shows the NCs sample rinsed with ethyl acetate/hexane did not have the presence of the organic polymeric nanostrands (Figure S3) as indicated by the lack of peaks at low angles at 2θ = 2.5 and 7 degrees (Figure 3b) whereas NCs purified in hexane self-assemble into 1D chains (Figure 1c,d and 2c).



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Figure 2: TEM micrographs of the CsPbBr3 nanocrystals 1D superlattice at (a) low magnification and (b) high magnification with an FFT image provided as inset. The lattice spacing of the [100] plane is ~5.8 Å. (c-f) HAADF STEM micrographs confirming the composition of the nanocrystals. The scale bar in panels c-f is 100 nm. 10 ACS Paragon Plus Environment

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Figure 3. (a) X-ray diffractograms of CsPbBr3 nanocrystals purified in hexane, purified in 50/50 ethyl acetate/hexane (EA) mixture, and ligand polymeric nanostrands control sample with no nanocrystals. (b) Magnified view of the peaks arising from the ligand polymeric nanostrands at lower diffraction angles.

We also investigated the role of the solvent in directing the self-assembly and subsequent formation of 1D superlattice chains of NCs. The synthesis was performed at the temperatures mentioned previously, but the NCs were purified and dispersed in toluene (170 ˚C and 190 ˚C) or a mixture of toluene/ethyl acetate for NCs synthesized at 135 ˚C and 150 ˚C. The polarity indices of hexane, toluene, and ethyl acetate are 0.6, 2.4, and 4.4, respectively.

TEM

micrographs (Figure 4) indicate that polar solvents do not induce self-assembly into 1D chains; NCs were either disordered or self-assembled into short-range 2D assemblies.

Low

magnification SEM micrographs (Figure S7) also support that toluene does not favor the formation of polymeric nanostrands. However CsPbBr3 NCs purified in toluene maintained a cubic morphology (Figure 4), an orthorhombic phase as seen from XRD (Figure S6), and are bound by the [110] and [100] crystal planes as observed in FFT images (Figure S8). In another control study we wanted to determine if the nucleation of the CsPbBr3 NCs in solution drives the formation of the polymeric nanostrands and subsequent self-assembly into 1D chains. We 11 ACS Paragon Plus Environment


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therefore mixed CsBr precursor in octadecene, oleic acid, and oleylamine overnight at 190 °C and purified the white precipitate obtained after centrifugation in either hexane or toluene. SEM micrographs shown in Figure S9 clearly show that the polymeric nanostrands composed of organic ligands were formed when purified in nonpolar hexane but not in polar toluene. In a final control study we wanted to determine if the polymeric nanostrands would serve as a template for template-assisted 1D self-assembly if mixed with a separate dispersion of NCs. We synthesized NCs at 135 °C and purified them in toluene to remove all excess ligands, and also synthesized polymeric nanostrands in hexane without any nanocrystals. When the nanostrands were combined with the NCs dispersion, we observed that the NCs were aligned along the polymeric nanostrands. This control experiment suggests that (1) the polymeric nanostrands can provide a generalized template for directed 1D self-assembly of NCs and can likely be extended to metal, metal oxide, and other semiconductor NCs, and (2) the self-assembly is strongly NC concentration-dependent as shown in Figure S10. These control experiments suggest that the solvent polarity plays a dominant role in the self-assembly process and is not dependent on the presence of CsPbBr3 nuclei. Since the NCs synthesized at 135 °C and 150 °C could not be precipitated with hexane alone, it is unclear if reaction temperature controls the ordered organization of the NCs.


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Figure 4: TEM micrographs of CsPbBr3 nanocrystals synthesized at (a) 135 ˚C, (b) 150 ˚C, (c) 170 ˚C, and (d) 190 ˚C and purified in toluene.

Based on our observations, we propose a mechanistic understanding of the formation of the polymeric nanostrands and the subsequent self-assembly of CsPbBr3 NCs along the nanostrands into 1D superlattice chains (Figure 5). The CsPbBr3 NCs are formed at an elevated temperature following a reaction between Cs-oleate and lead bromide in the presence of oleic acid and oleylamine (Figure 5a). Oleic acid stabilizes the NCs in solution by binding to Pb2+ ions on the surface of the NCs while oleylamine, which weakly binds to Pb2+, diffuses to the surface.39, 59 The surfaces of CsPbBr3 NCs are negatively charged since they terminate in PbBr6-; this is also related to the surfaces having an excess of Pb2+ ions but a deficiency of Cs+ ions.59-60 The polarity of the solvent used during the purification process determines the amount of excess and weakly bound ligands that are eliminated from the nanocrystal surface (Figure 5b). More ligands are expected to be removed in a higher polarity solvent due to an increase in the ligands’ diffusion coefficient.

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This indicates CsPbBr3 NCs purified in hexane, a nonpolar solvent 13 ACS Paragon Plus Environment


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(polarity index 0.6), has an excess of ligands remaining which results in the formation of the polymeric nanostrands (Figure 5c). The nanostrands are likely formed from the complexation of the ligands, with Br- and Cs+ in hexane in part due to hydrophobic interactions between the polar heads of the ligands and the solvent. The hydrophobic interactions are expected to be weaker in higher polarity solvents due to reduced repulsive forces between the polar head groups and the solvent. The width of the nanostrands is controlled by the amount of ligands remaining after purification, and the temperature of the reaction mixture providing the thermal energy for the complexation of ligands and ions. However, the number of layers of ligand/ion complex that ultimately results in the nanostrands is difficult to determine. When the interparticle spacing is less than twice the ligand length (~4 nm), the solvent polarity can drive self-assembly in distinct ways during a gradual evaporation of the solvent. Polar solvents preferentially drive adjacent CsPbBr3 NCs closer together to minimize repulsive forces between polar groups of the solvent and nonpolar groups of the ligands. This supports the short-range 2D assemblies we observed in polar solvents (Figure 4, S3, S8). However in nonpolar solvents, aliphatic ligands complex with ionic species via hydrophobic interactions, as well as the alkyl chains of the ligands interconnect via van der Waals interactions. These forces collectively result in the formation of the polymeric nanostrands followed by self-assembly of the CsPbBr3 NCs along the nanostrands into 1D superlattice chains (Figure 5d).

Whereas in this study, solvent-ligand interactions and the

amount of ligands present in the solution plays a strong role in self-assembly, NCs alignment can also be induced by the rate of solvent evaporation, and the affinity of the solvent for the substrate. Since the NCs were deposited following the exact same procedure on the substrates utilized in this study (glass, copper grids, quartz) and self-assembly was observed in each case, substrate-affinity do not play a role here. A more in-depth study will be needed, which is beyond


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the scope of this work, to understand the role of substrates and rate of evaporation of solvents towards 1D assembly of perovskite NCs. We note that while short-range 2D assemblies were observed for samples purified in polar solvents, long-range 2D assemblies can likely be achieved with better size monodispersity of the NCs as shown previously for metal and metal oxide NCs.61-63

Figure 5: Schematic describing the mechanism behind CsPbBr3 nanocrystal self-assembly into 1D superlattice chains. (a) The nanocrystals are initially formed in solution and stabilized by oleylammonium ions and oleic acid, and (b) during purification excess and weakly bound ligands are eliminated from the surface. (c) The excess ligands and some metal ions crosslink in nonpolar solvents such as hexane to form polymeric stands, and subsequently (d) CsPbBr3 nanocrystals self-assemble along the nanostrands into 1D chains. 15 ACS Paragon Plus Environment


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The alignment of the CsPbBr3 NCs into 1D chains is also accompanied by an alteration in their optical properties. We observe the NCs purified in a 50/50 hexane/ethyl acetate mixture that do not self-assemble have a band edge at 515 nm which is characteristic of CsPbBr3.43, 64 NCs purified in hexane assembled in 1D chains have a redshifted band edge at 520 nm (Figure 6a). The photoluminescence (PL) of the CsPbBr3 NCs (Figure 6b) also shows a spectral redshift once the NCs self-assemble. The observed redshift in both absorbance and PL could be attributable to a change in the dielectric constant of the surrounding medium, electronic coupling of excitons in the assembled NCs system, or energy transfer via nonradiative pathways between adjacent CsPbBr3 NCs.65-67 The absorbance and PL of all the samples synthesized at different temperatures and purified in hexane are shown in Figure S11.

Figure 6. (a) Absorbance and (b) PL spectra of CsPbBr3 nanocrystals synthesized at 190 °C and deposited on glass substrates which self-assembled into 1D chains (NCs_hexane) or remained disordered (NCs_EA). EA indicates nanocrystals are purified in a 50/50 mixture of ethylacetate/hexane.

Conclusions This work demonstrates the formation of 1D superlattice chains with CsPbBr3 perovskite NCs driven by solvent-ligand interactions between alkyl ligands on NCs surface and nonpolar 16 ACS Paragon Plus Environment

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solvent, hexane. We observe the formation of polymeric nanostrands in hexane and subsequent organization of the CsPbBr3 NCs along the nanostrands into 1D superlattice chains. Our results show a combination of van der Waals and hydrophobic interactions between solvent and ligands drive the perovskite NCs into 1D chains. Control experiments show that polar solvents, including toluene and ethyl acetate, do not favor 1D assemblies rather NCs are in either disordered clusters or in short-range 2D assemblies. Our results also indicate that optical properties of CsPbBr3 NCs can be tailored and tuned by the formation of 1D chains where a redshift was observed in both absorbance and PL. While solvent-ligand mediated self-assembly occurring in a similar manner has been reported for other semiconductor NCs (for example ZnS nanorods, CdSe NCs),68, 53 the findings of this work extend this process to perovskite NCs. We envision our approach could also be utilized with shorter chain length alkylamines, allowing us to control the hydrophobic and van der Waals forces resulting in different chain length 1D assemblies. We expect better size control of the perovskite NCs could also result in large 2D assemblies in polar solvents by finetuning the ligand/solvent ratios. These controlled self-assembled systems could ultimately be harnessed in a range of applications with targeted optical and electronic properties.66, 69-70

ASSOCIATED CONTENT Supporting Information Additional SEM images, TEM images, absorbance spectra, photoluminescence spectra, x-ray diffractograms, and nanocrystal sizes are provided. The Supporting Information is available free of charge on the ACS Publications website at DOI:

AUTHOR INFORMATION



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Corresponding Author *Email: [email protected] Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS N.S was supported by Vanderbilt University Startup Funds and NSF EPS 1004083. WRE acknowledges support from the National Science Foundation Graduate Research Fellowship Program under Grant Number 1445197. References 1.

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