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Surfaces, Interfaces, and Catalysis; Physical Properties of Nanomaterials and Materials

Post-Synthesis Spontaneous Coalescence of Mixed Halide Perovskite Nanocubes into Phase Stable Single-Crystalline Uniform Luminescent Nanowires Bapi Pradhan, Aamir Mushtaq, Dipanwita Roy, Sumanta Sain, Bidisa Das, Uttam Kumar Ghorai, Suman Kalyan Pal, and Somobrata Acharya J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.9b00832 • Publication Date (Web): 01 Apr 2019 Downloaded from http://pubs.acs.org on April 1, 2019

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The Journal of Physical Chemistry Letters

Post-synthesis Spontaneous Coalescence of Mixed Halide Perovskite Nanocubes into Phase Stable Single-crystalline Uniform Luminescent Nanowires Bapi Pradhan†, Aamir Mushtaq‡, Dipanwita Roy†, Sumanta Sain§, Bidisa Das¶, Uttam Kumar Ghorai⊥ , Suman Kalyan Pal‡ and Somobrata Acharya*† †

School of Applied and Interdisciplinary Sciences, §School of Materials Science and ¶Technical

Research Center, Indian Association for the Cultivation of Science, Jadavpur, Kolkata 700032 (India) ‡

School of Basic Sciences and Advanced Material Research Center, Indian Institute of Technology Mandi, Kamand, 175005 HP, (India)

⊥ Department

of Industrial & Applied Chemistry, Swami Vivekananda Research Center,

Ramakrishna Mission Vidyamandira, Belur Math, Howrah 711202 (India) AUTHOR INFORMATION Corresponding Author: [email protected] ORCID Somobrata Acharya: 0000-0001-5100-5184 Bapi Pradhan: 0000-0002-6202-7343

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ACS Paragon Plus Environment

The Journal of Physical Chemistry Letters 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT: All inorganic mixed halide perovskite, CsPb(BrxI1−x)3 (0 ≤x≤ 1) nanocrystals possess tunable photoluminescence with high quantum yield in the visible window. However, the photoluminescence degrades rapidly with post-synthetic aging due to the spontaneous ion separation and phase instability. Here we show that the post-synthetic aging of CsPb(BrxI1−x)3 nanocubes spontaneously form highly uniform single-crystalline nanowires with a diameter of 9 ± 0.5 nm and length up to several micrometers. The nanowires show bright photoluminescence with an absolute photoluminescence quantum yield of 41%. Rietveld refinement identifies stable orthorhombic phase of the nanowires implying a phase transition from cubic crystallographic phase of the nanocubes during the morphology evolution. Transient absorption spectroscopy reveals a faster excited state decay dynamic with large exciton delocalization length in onedimensional nanowires. Our findings elucidate the insights of post-synthesis morphology evolution of mixed halide perovskite nanocrystals leading to luminescent nanowires with excellent crystal phase stability for potential optoelectronic applications.

KEYWORDS: Perovskites, nanocubes, nanowires, phase transition, exciton dynamics.

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All-inorganic cesium lead halide perovskite (CsPbX3, X = Cl, Br, I) nanocrystals (NCs) are at the center of current research owing to a range of potential applications in solar cells, lightemitting diodes, photodetectors, and lasers.1–5 Remarkably high photoluminescence quantum yield (PLQY) has been obtained from CsPbBr3, which leads to the fabrication of efficient light emitting diodes.6,7 Complementarily, high carrier mobility, large diffusion length, and ultrafast interfacial charge transfer make CsPbBr3 attractive candidates for solar cell fabrication.2,4,5,8–10 Inspiring power conversion efficiency was obtained from CsPbBr3, however, a relatively large band gap (2.36 eV) limits CsPbBr3-based photovoltaic devices.11 Specifically, CsPbI3, with a narrower band gap (1.73 eV), has shown high solar power conversion efficiency over 17%.1,12 The cubic crystallographic phase of CsPbBr3 is energetically stable exhibiting the highest PLQY.6 On the contrary, cubic CsPbI3 is thermodynamically unstable since it converts into nonphotoactive yellow orthorhombic non-perovskite phase with larger band gap (3.0 eV) at roomtemperature.11,13,14 Such a crystal phase instability is detrimental for photovoltaic applications since orthorhombic phase tends to form during film annealing or upon exposure to ambient conditions.11,15 Strategies to address the phase stability issue involve alloying with halide ions or to use suitable ligands and cations to stabilize the cubic phase.9,16–18 Partial substitution of bulky Iwith the relatively smaller sized Br- results in stable CsPb(BrxI1−x)3 structures that are thermodynamically stable at room temperature.18,19 Such mixed halide composition is attractive since the band gap can be controlled by adjusting the ratio of halide ions.6,18 However, mixed halide perovskites were often reported to be phase-segregated into iodide and bromide-rich phases, which leads to charge carrier trapping into the iodide-rich regions.18,20 Hence, reported literatures on the mixed halide CsPb(BrxI1−x)3 perovskites are counterintuitive and highly depends on the synthesis protocols. Herein, we report for the first time on the in-situ formation of a phase

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stable homogeneous mixed halide CsPb(BrxI1−x)3 (x=0.5, CPBI) luminescent nanowires (NWs) of several micrometers length through spontaneous coalescence of nanocubes in ambient conditions. Nanocubes synthesized by colloidal hot-injection method stored under refrigeration (4°C) in an equal volume mixture of toluene and chloroform undergoes transformation into single-crystalline NWs via oriented-attachment process with aging. The width of the nanocubes are largely preserved in the NWs. Rietveld refinement carefully identifies the orthorhombic phase of the NWs suggesting that a phase transition from cubic crystallographic phase of the nanocubes occurs during the spontaneous coalescence process. Density functional theoretical (DFT) analyses reveal that the oriented-attachment process is driven by the dipolar interaction between the adjacent nanocubes. Transient absorption (TA) spectroscopy reveals a faster excited state dynamic in the NWs in comparison to the nanocubes. A delocalization of charge carriers upon morphology evolution from nanocubes to NWs is observed from the Time-correlated single photon counting (TCSPC) measurements. Our findings reveal a stable orthorhombic crystallographic phase of mixed halide CsPb(BrxI1−x)3 (x=0.5) perovskite luminescent NWs obtained by post-synthetic phase transformation process by aging, which holds importance for optoelectronic applications. Mixed halide CPBI nanocubes were synthesized using hot injection method.6,21 Purified nanocubes was dispersed separately in toluene, chloroform and in a mixture of chloroform and toluene in equal volume and aged in refrigerator (4°C) for several days. Transmission electron microscope (TEM) images show highly monodispersed self-assembled cubic morphology of the as-prepared NCs with an average cube edge dimension of 9 ± 0.5 nm (Figure 1a,b). High resolution TEM images (HRTEM) reveals inter-planar spacing’s of 0.40 ± 0.02 nm and 0.29 ±0.02 nm corresponding to (011) and (002) planes of bulk cubic phase (reference CsPbBr3, COD

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(a)

(b)

(g)

(c)

60 days

6 8 10 12 Dimension (nm)

(f) 10

15

1 day

10 nm

0.5 µm

10 nm

10

20

30

(112)

(002) (111)

(001)

#153-3064

(012)

(011)

Width (nm)

40

(122)

5

7 days

(022)

(e)

200 nm

(003)

(d)

10 nm

30 days

Intensity (a.u.)

100 nm

Counts (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Counts (a.u.)

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50

2θ (Degree) (c,d): 30 days and (e,f): 60 days. HRTEM images at intermediate stages of aging shows the attachment of nanocubes along direction (d). HRTEM at three different position of the nanocubes junctions shows common (002) plane of attachment direction (marked by arrow and spacing’s). (g) XRD pattern of CPBI NCs with aging for different days. Vertical rectangular red box indicates reduction of (011) peak with aging. Inset shows photographs of solid state luminescence of corresponding thin film under 365 nm UV illumination.

0.4 µm images of CPBI NCs in mixed solvent at (a,b): 1 day, Figure 1. TEM and HRTEM

#

1533063) respectively (Figure 1b). The fast Fourier transform (FFT) pattern of HRTEM

image shows prominent diffraction spots corresponding to (011), (002) and (012) planes supporting highly crystalline nature of CPBI nanocubes (Figure S1a, SI). Energy dispersive Xray spectroscopy (EDX) analyses show an elemental atomic ratio of Cs:Pb:Br:I ~1:1:1.5:1.5 suggesting CsPb(BrxI1−x)3 (x=0.5, CPBI) composition of nanocubes (Figure S1b, SI, inset). X-ray photoelectron spectroscopy (XPS) was further employed to investigate the elemental composition and valence states of the constituent elements (Figure S2, SI), which reveal presence of monovalent Cs+ ion, divalent Pb2+ ion, monovalent Br and I ions respectively in the −

−

nanocubes.22,23 We have examined the morphology of CPBI nanocubes with aging of colloidal dispersion. We observed that the CPBI nanocubes retains the pristine cubic shape till 7 days of

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aging (Figure S3, SI). Interestingly, a mixture of nanocubes and NWs was observed after 30 days of aging (Figure 1c,d and Figure S4a-d, SI). The ratio of NWs to nanocubes gradually increases with longer aging and most of the nanocubes are converted into NWs after 60 days (Figure 1e). TEM images reveal uniform width of ~9 ± 0.5 nm of the NWs along the entire length, which is of the same dimension of an edge of a nanocube (Figure 1a,e, insets). Careful observation of the TEM images of intermediate stages with aging reveal that NWs are formed by spontaneous alignment and coalescence of nanocubes (Figure 1d, Figure S4, SI). HRTEM images show alignment of (002) lattice planes of the adjacent nanocubes (Figure 1b). This observation is supported by FFT analyses which show predominant diffraction spots corresponding to (002) planes from the adjacent nanocubes (Figure 1d, Figure S4g,h,k-n, SI). Additionally, HRTEM image of NWs reveals an inter-planar distance of 0.29±0.02 nm (Figure 1f) corresponding to (002) planes of bulk cubic phase suggesting single-crystalline nature of the NWs. STEMHAADF EDX elemental mapping suggests co-localization of the Cs, Pb, Br, and I elements within a single CPBI nanowire (Figure S5, SI). In fact, the coalescence of nanocubes into NWs is also reflected from the XRD measurements with aging (Figure 1g). The (001) peak in the XRD pattern at 2θ ~15° is the signature peak for pure cubic phase of CPBI nanocubes with Pm3m space group. A bifurcation of the (001) peak is initiated with aging for 30 days, where a mixture of nanocubes and nanowires was observed in TEM images, suggesting coexistence of orthorhombic nanowires along with cubic nanocubes. The splitting (001) peak is prominent for 60 days of aging suggesting a complete transformation into orthorhombic nanowires. A gradual decrease of the intensity of (011) reflection with time is observed along with the decrease of low intense peaks at higher angles (Figure 1g). Interestingly, only (002) reflections retain after longer aging. A shift of (002) reflection towards higher angle is also observed with aging (Figure S6,

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SI). These observations suggest the growth of NWs along direction by the orientedattachment

of

nanocubes

along

(002)

planes

(Figure

1).

(a)

(b)

+ Figure 2. (a) Schematic presentation of the polar cubic crystals of CsPbX3 leading to formation of nanowires. (b) The model NCs Cs12Pb18I48 with appropriate open facets for attachment are shown with the blue arrow showing the net dipole moment. The formation of NWs via oriented-attachment of NCs has been reported earlier.16,24 The formation mechanism was proposed based on the difference in surface structure and reactivity of specific facets, partial removal of the stabilizing agents from the NCs surfaces and dipolar interactions.16,24 In order to identify the origin of dipole moment in the CPBI nanocubes, we have carried out DFT calculations on model Cs12Pb18Br48, Cs12Pb18I48 and mixed halide clusters consisting of perpendicular (002) and (011) planes following the experimental observations (Figure 2, Table S1, SI). A correct charge balance cluster can be obtained by removing every third Pb layer along direction. Our DFT calculations reveal a strong dipole moment along

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direction of the nanocubes (Table S1, Figure S7, SI). We observed different energies of the clusters, however, the dipole moment is minimally affected for the clusters with different sizes and with mixed halide ion ratios (Table S1, Figure S7, SI). Hence, the adjacent nanocubes (Figure 2a) in solution medium can reorient, align and attach along appropriate facets through strong dipolar interaction, which can lead to the fusion of the nanocubes along direction to yield one-dimensional (1D) NWs (Figure 2b, Figure S7, SI). We have observed the coalescence of nanocubes in different solvents, however, complete coalescence of nanocubes into NWs is obtained using a mixed solvent of toluene and chloroform (1:1 by volume) only. When the nanocubes are aged in pure toluene for 60 days, NWs were observed along with nanocubes (Figure S8a, SI). On the other hand, a very less numbers of NWs were formed when nanocubes are aged in pure chloroform (Figure S8b, SI). These control experiments suggest that oriented-attachment mechanism depends on the polarity of the solvents and reactivity of different facets.16,24 The (002) crystal facets of the nanocubes consist of mixture of I-/Br- and Pb2+ ions which are passivated by OLAc (as oleate) and OLAm via [X···H−N+] Hbonding interactions.25 Partial removal of these ligands occur from the nanocubes surfaces with aging facilitate growth of NWs unidirectionally along direction.25 We have analyzed surface passivation of the NCs by the ligands with aging by Fourier transform infrared spectroscopy (FTIR) (Figure S9, SI). The NCs display characteristic CH2 symmetric and CH3 asymmetric stretching in the range of 2840–2950 cm−1 and bending vibrations at 1466 cm−1 which are the characteristics of aliphatic chains of OLAc and OLAm.23 The presence of amine (N-H bending mode at 1576 cm-1; NH3+ at 1735 cm-1, N-H stretching mode at 3100 cm-1) and carboxylic acid (-COOH at 1524 cm-1, 1624 cm-1) moieties of OLAm and OLAc indicate significant amount of ligands are bound to the surface of the nanocubes.10,26 The intensity of N-H

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and C-O bending modes gradually decreases with aging, suggesting partial removal of surface ligands which is favorable for the observed oriented-attachment mechanism into NWs (Figure S9b, SI).27 Since the crystal structure of the mixed anionic CPBI NCs is unknown, we have determined the actual crystal structure by using Rietveld refinement technique. Rietveld refinement of the XRD patterns confirms pure cubic phase of CPBI nanocubes with Pm3m space group (Figure 3a) resembling the bulk CsPbBr3 (COD# 1533063, a =5.8700 Å), however, with a larger lattice constant (a= 6.0255 Å).6 An increase in lattice parameter originates due to the presence of larger sized iodide along with smaller sized bromide in the crystal lattice of mixed anionic CPBI nanocubes. The unit cell of the nanocubes consists of Cs atoms at the corners (0,0,0), Pb at the body centre (0.5,0.5,0.5), while Br/I atoms occupy face centre (0,0.5,0.5) positions respectively (Figure 3b). Evidently, Cs−Cs bond length represents the lattice constant (a= 6.0255Å). A regular undistorted Pb(Br/I)6 octahedron is shown in Figure 3b where Pb atoms remain at the centre of the octahedral void constituted by six (Br/I) atoms. We found that average Pb−(Br/I) bond lengths associated with Pb(Br/I)6 octahedron increase from 2.94 Å to 3.01 Å compared to the bulk counterpart (Figure 3b). Literatures suggest that CsPbX3 perovskites crystallizes into cubic, orthorhombic, and tetragonal crystal phases and phase transformation among these phases occurs depending upon the energetics of the crystal.6,28 Apparently, the XRD pattern for CPBI NWs is identical to the nanocubes in terms of major peak positions (Figure 1g). However, Rietveld analysis confirms the orthorhombic phase of CPBI NWs (COD# 4510745, Pnma, a= 8.2440 Å,b=11.7351 Å, c= 8.1982 Å) where all the reflections are unambiguously identified and indexed. The lattice parameters of NWs increase (a= 8.9801 Å, b= 12.6637 Å, c= 8.2350 Å) from their bulk counterpart. Hence, a crystallographic phase transition occurs during the oriented-attachment of

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(121)

IC

(021) (002) (211)

(b)

IO-IC

(022)

1 day Pm3m

(202)

IO

(020) (101)

(a)

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(001) (011) (111)


(c)

60 days Pnma

15 20 25 30 35 40 45 50

2θ (degree)

Figure 3. (a) Simulated output patterns employing Rietveld profile fitting of the XRD patterns of CPBI NCs in mixed solvent for 1 day (top panel) and 60 days (bottom panel) respectively. Difference between observed (Io) and calculated (Ic) intensities, (Io−Ic) represents residue of refinement. Red dotted and black continuous curves represent observed and calculated patterns respectively. Green curves, qualitatively representing goodness of fitting (GoF), are plotted against each simulated output spectrum. Atomic arrangement of Cs (red), Pb (black), Br/I (white) atoms, unit cell, bond lengths and bond angels of Pb(Br/I)6 octahedron in cubic (b) and orthorhombic (c) CPBI NCs. the nanocubes into NWs. The intermediate XRD patterns of the aged NCs for 7 days and 30 days reflect combined characteristics of cubic and orthorhombic phases (Figure 1g). Two sets of

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Pb−(Br/I) bonds with lengths of 3.15 Å and 3.07 Å respectively are observed in the equatorial plane of Pb(Br/I)6 octahedron (Figure 3c). Bond angles are deviated from the regular bond angles of 90°. This asymmetric change in both bond lengths and bond angles causes Jahn-Teller distortion of Pb(Br/I)6 octahedra.29 Interestingly, (002) planes dominate over (011) planes for both the CPBI NWs and nanocubes. This implies existence of preferred orientation (texturing effect) of crystal planes along direction both for the nanocubes and NWs (Figure S10, SI). From Rietveld analyses, the amount of texturing is quantified for both nanocubes and NWs to be 0.418 and 0.104 respectively, where 0 suggests complete texturing and 1 suggests no texturing. A lower order texturing of NWs signifies growth along direction.30,31 Additionally, the coherently diffracting crystalline size and the r.m.s. lattice strain of the nanocubes (28.32 nm, 71.8×10−4) is smaller than the NWs (150.40 nm, 119.1×10−4). The increase in crystallite size and

(b)

(c) Counts (norm.)

(a)

Intensity (a.u.)

r.m.s. lattice strain manifests the formation of the NWs by the nanocubes.

Absorbance (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


60 days

30 days

7 days

60 days

1 day

1 day

3.1

2.5

2.1

0

1.8

Energy (eV)

600

1200

Time (ns)

1800

Figure 4. (a) UV-vis absorption and PL spectra of CPBI NCs from day 1 to day 60. (b) Photographs of CPBI NCs in dispersion at respective time period under room light (left) and under 365 nm UV irradiation (right). (b) TCSPC decay profile of CPBI NCs with aging, λex=410 nm.

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Steady state UV-vis absorption spectra show a blue shift with different aging time (Figure 4a). A bright PL with peak maximum at 2.03 eV is observed for the nanocubes at the initial stage (1 day, Figure 4a,b) with absolute PLQY of 64% (Figure S11, SI). The PL maximum is blue shifted from 2.03 eV to 2.16 eV for the NWs with 60 days aging followed by a decrease of PLQY, however, still maintaining a high PLQY of 41% (Figure 4a,b). The observed blue shift of the PL maximum is attributed to the change in crystallographic cubic phase to orthorhombic phase with aging.16 Importantly, both the nanocubes and NWs show solid state luminescence under UV light illumination (Figure 1g, insets) and under fluorescence microscope (Figure S12, SI). We observed that the colour of the thin films gradually changes from red to yellow upon transformation of the nanocubes to the NWs with aging for 60 days. Notably, the yellow orthorhombic phase is known to be non-luminescent non-perovskite double chain phase for pristine CsPbI3.11,16 However, luminescence still persists in our NWs because of the compositional gradient CsPb(BrxI1−x)3 structure and perovskite stability in orthorhombic phase. We have compared the PL and PLQY for the pristine CsPbBr3, CsPbI3 and CsPb(BrxI1−x)3 NCs (Figure S13, SI). CsPb(BrxI1−x)3 nanocubes show lower PLQY than CsPbBr3, however, higer than CsPbI3. Furthermore, we have studied the phase stability of CPBI NWs with further aging (Figure S14, SI). A gradual decrease in the PL is observed after 60 days of aging; however, the PL still persists for 180 days. We have carried out TCSPC measurements of nanocubes to NWs with aging. TCSPC measurements suggest a slower decay rate with aging from 1 day to 60 days owing to the slower exciton radiative recombination dynamics in the NWs in comparison to the nanocubes (Figure 4c, Table S2, SI). An increase in the PL lifetime with aging is attributed to the change in the morphology from nanocubes to 1D NWs allowing delocalization of charge carriers along the

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length. Furthermore, we have estimated the radiative recombination rate from the correlationship of PL lifetime and PLQY (Table S3, SI).32 A decrease in the radiative recombination rate is observed for the NWs in comparison to the nanocubes, which supports the delocalization of charge carriers along the length of 1D NWs. To elucidate the influence of the morphology evolution on the observed PLQY, we measured excited state dynamics of CPBI nanocubes and NWs using femtosecond TA spectroscopy at different pump-probe delays (∆t). The TA spectra of the nanocubes exhibit an intense negative absorption band at 2.13 eV (Δt=0.2 ps), which grows till 2 ps followed by a fast recovery (Figure 5a). This band is a consequence of the ground state bleach which arises due to state filling via Pauli blocking.33 A closer inspection of the TA spectra of nanocubes at early times (∆t < 1ps) reveals an asymmetric derivative feature near 2.13 eV with small induced absorption band at lower energy 2.03 eV (Figure 5b). A red shift in the peak position of the bleach signal is observed with increasing pump-probe delay time. The perturbed spectral shape of TA spectra is attributed to the biexcitonic Stark effect.34,35 We have carried out the global fitting of TA spectra using Gaussian parameters obtained from the absorption spectrum (Figure S15a, SI) to quantify the magnitude of the spectral red shift induced by the biexcitonic stark effect at the early time. We model the derivative feature of TA assuming that in presence of hot excitons by the pump beam, the transition energies of the probe excitons are reduced by 𝜹, where 𝜹 is the red shift in the energy due to exciton –exciton interactions. The difference between excitonic signals centered at 𝑬 − 𝛅 and 𝑬𝟏 enable us to write TA at each pump-probe delay using Gaussian functions in the following equation. ∆𝑨 = 𝑨𝟏𝒆 𝒆𝒙𝒑 −

𝑬!𝑬𝟏 !𝜹 𝒘

𝟐

− 𝑨𝟏𝒈 𝒆𝒙𝒑 −

𝑬!𝑬𝟏 𝒘

𝟐

(1)

Where, A is the amplitude of the resonant exciton transition, subscripts g and e refer to ground and excited states, respectively. The spectral fit in Figure 5b suggests that the red shift is largest

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(b) 0.005

-0.012 -0.016 1.9

2.0

2.1

2.2

2.3

2.4

(c)

0.20 0.15 0.10 0.05 0.00

0

5

10

15

20

Shift δ (meV)

25

-0.015

2ps 1ps 0.5ps 0.2ps Fitting

-0.020 1.9

2.0

-0.010

(e)

1.00

ΔA (Norm.)

Energy (eV) 0.25

T< 3 ps

-0.005

0.75

𝜹 𝑬𝒈

2.1

2.2

2.3

Energy (eV)

2.4

2.03 eV Fitting

0.50

(f)

-0.002

0.00

-0.25 0.0

0.2

0.4

0.6

0.8

Time delay (ps)

1.0

-0.003

State filling

(g) 0.000

0.000

-0.001

0.25

pump

-0.008

T< 1 ps

ΔA (Norm.)

500ps 100ps 50ps 20ps 2ps 1ps 0.5ps 0.2ps

-0.004

T< 0 ps

(d)

0.000

ΔΑ (OD)

ΔA (OD)

ΔΑ (OD)

0.000

𝑬𝒈 − 𝜹

(a)0.004

Am/Bm

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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500ps 50ps 20ps 6ps 0.1ps

2.2

-0.200 -0.400 -0.600 Nanocubes Nanowires Fitting

-0.800 -1.000

2.3

2.4

Energy (eV)

0

2.5

100

200

300

400

Time delay (ps)

500

Figure 5. (a) TA spectra of CPBI nanocubes at various time delays. (b) Early time TA spectra of CPBI nanocubes showing asymmetric feature and time dependent red shift of the bleach signal (c) Variation of Am/Bm ratio with δ. (d) Schematic of the ultrafast excitate state processes. (e) Normalized TA kinetics at 2.03 eV. (f) TA spectra of CPBI NWs at various delay times. (g) Comparison of representative TA kinetics of the bleach bands of NWs (blue dots) and NCs (pink dots) measured at 2.38 eV and 2.13 eV, respectively. Solid curves are the fitting of experimental data. at the shortest time (~200 fs) and amounts to 14 meV. Further, we have estimated the value of δ from the amplitudes of the TA to verify that δ obtained from global fit is certainly correct.33 We have numerically simulated the ratio of the amplitudes of early time absorption (Am) and the later time bleach (Bm) maxima (Figure S15b,c, SI) using equations (2) and (3). ∆𝑨 𝚫𝒕 < 1𝒑𝒔 = 𝑨

𝜹 𝟐𝑬!𝜹𝒘 𝟐𝑬!𝜹𝒘 𝟐 !𝟏 𝑬𝟐 !𝟏

𝑨

𝑬𝟐 !𝟏!𝟐 𝑬!𝜹𝒘 𝟐

𝟐

𝟐𝑬!𝜹𝒘 𝟐 !𝟏 𝑬𝟐 !𝟏

∆𝑨 𝚫𝒕 > 1𝒑𝒔 =

Where A is the absorption amplitude and 𝑬 =

ℏ𝝎!𝑬𝒆 𝒘

(2) (3)

. Ee and w are the exciton energy and

half width measured from optical spectrum. Equations (2) and (3) elucidate the early time absorption and later time bleaching signal due to state filling as a result of hot carrier relaxation into low energy states of the band edges. It is apparent from Figure 5c that the ratio Am/Bm varies directly with δ and the value of δ corresponding to Am/Bm ≈ 0.16 (obtained value of Am/Bm from

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experimental data is 0.16, Figure S15d, SI) appears to be 13 meV, which is close to that estimated from equation 1. For ∆t > 1 ps, this effect disappears completely resulting in strong bleach signal at the excitonic transition. Carriers from excited states relax to the low energy states near the band edge with longer delay leading to state filling36,37 which accords to bleaching via Pauli blocking (Figure 5d).38 Such relaxation of the excited state carrier replaces the band edge absorption by the bleach signal at 2.13 eV.39 We have fitted the early time TA signal at 2.03 eV using single exponential function to determine the intraband cooling time (Figure 5e) of 692 fs. In contrast, TA spectra of NWs show a broad negative absorption band at ~2.38 eV, which is assigned to ground state bleach (Figure 5f). This bleach feature arises due to the removal of occupied valance band population upon excitation, which is consistent with the excitonic peak position in the absorption spectra (Figure 4a). Next, we carried out TA kinetic measurements near the spectral maxima of bleach bands to monitor the change in exciton dynamics while nanocubes are converted into NWs. Normalized TA kinetics clearly show that the bleach recovery is faster in NWs compared to the nanocubes (Figure 5g). The TA kinetics of both the nanocubes and NWs were described by a bi-exponential function (Figure 5g). The biexponential fitting yields decay time components of 0.6 ps (32%) and 71 ps (68%) for nanocubes in comparison to 0.2 ps (51%) and 28.6 ps (49%) of the NWs. Hence, the average bleach recovery time in nanocubes (48 ps) is higher than NWs (~ 14 ps). Apparently, this observation contradicts with the TCSPC measurements, which showed a slower PL decay in NWs in comparison to nanocubes (Figure 4c). However, in the time-scale of TA measurements (few hundred ps), fast nonradiative processes dominates whereas radiative recombination prevails in the nanosecond time-scale to determine PL kinetics. Therefore, the fast bleach recovery in NWs is most presumable due to enhanced nonradiative recombination. Excitons are sufficiently

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delocalized in NWs owing to the one-dimensional morphology in comparison to the nanocubes where excitons are confined in all directions. The large exciton delocalization length in onedimensional NWs increases the PL lifetime and reduces PLQY by slower radiative recombination. On the other hand, it enhances the probability of exciton to encounter surface trap in NW for fast nonradiative recombination and lower PLQY.40 In conclusion, we showed for the first time a post-synthesis spontaneous coalescence process of all-inorganic mixed halide cubic CPBI nanocubes into several micrometers long NWs in solution phase with aging. The width of the nanocubes is largely preserved in the NWs suggesting an oriented-attachment of the nanocubes into NWs. Our DFT calculations confirm that the inherent dipole moment of the nanocubes leads to the single-crystalline NWs through oriented-attachment process along direction. Rietveld analyses reveal a crystallographic phase transformation of the cubic phase of nanocubes into orthorhombic phase of the NWs. A high absolute PLQY of 41% is obtained from the NWs while both NWs and nanocubes showed solid state luminescence. Insight mechanism of the luminescence processes in the nanocubes and NWs using TA spectroscopy reveals a faster excited state decay dynamic with large exciton delocalization length in 1D NWs while the TCSPC measurements reveal a delocalization of charge carriers upon morphology evolution from nanocubes to NWs. Strong two-dimensional quantum confinement owing to the narrow width of the NWs originates the bright PL even after delocalization of charge carriers. These results explore the post-synthesis solution phase dynamics of mixed halide perovskite nanocubes leading to highly luminescent NWs for potential optoelectronic applications.

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ASSOCIATED CONTENT Supporting Information. Synthesis of CPBI NCs and experimental methods, Characterization of CPBI NCs, elemental analyses by EDX, chemical mapping in TEM, UV-vis absorption spectroscopy, PL spectroscopy, FFT, XRD, TEM images at different aging stages, Rietveld analyses, oxidation states of the elements using XPS, FTIR analyses, PLQYs of CPBI NCs, fluorescence microscopy images, details of computational methods using DFT, comparison of PL lifetimes and recombination rate and transient absorption spectra. ACKNOWLEDGMENT We acknowledge SERB grant EMR/2014/000664, DST, India, for financial support. D.R. acknowledges UGC-SRF fellowship. REFERENCES: (1) Mir, W. J.; Mahor Y.; Lohar A.; Jagadeeswararao M.; Das S.; Mahamuni, S.; Nag, A. Postsynthesis Doping of Mn and Yb into CsPbX3 (X = Cl, Br, or I) Perovskite Nanocrystals for Downconversion Emission. Chem. Mater., 2018, 30, 8170–8178. (2) Udayabhaskararao, T.; Houben, L.; Cohen, H.; Menahem, M.; Pinkas, I.; Avram, L.; Wolf, T.; Teitelboim, A.; Leskes, M.; Yaffe, O.; et al. A Mechanistic Study of Phase Transformation in Perovskite Nanocrystals Driven by Ligand Passivation. Chem. Mater. 2018, 30, 84–93. (3) Wang, Y.; Zhi, M.; Chang, Y.-Q.; Zhang, J.-P.; Chan, Y. Stable, Ultralow Threshold Amplified Spontaneous Emission from CsPbBr3 Nanoparticles Exhibiting Trion Gain. Nano Lett. 2018, 18, 4976–4984.

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Postsynthesis Spontaneous Coalescence of Mixed ... - ACS Publications

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