Size Tunable Cesium Antimony Chloride Perovskite Nanowires and

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Size Tunable Cesium Antimony Chloride Perovskite Nanowires and Nanorods Bapi Pradhan, Gundam Sandeep Kumar, Sumanta Sain, Amit Dalui, Uttam Kumar Ghorai, Swapan Kumar Pradhan, and Somobrata Acharya Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.8b00427 • Publication Date (Web): 26 Feb 2018 Downloaded from http://pubs.acs.org on February 26, 2018

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Chemistry of Materials

Size Tunable Cesium Antimony Chloride Perovskite Nanowires and Nanorods Bapi Pradhan†, Gundam Sandeep Kumar†, Sumanta Sain‡, Amit Dalui†,§, Uttam Kumar Ghorai¶, Swapan Kumar Pradhan# and Somobrata Acharya*,† †

Centre for Advanced Materials, Indian Association for the Cultivation of Science, Jadavpur, Kolkata 700032, India

‡

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

Department of Chemistry, Jogamaya Devi College, Kolkata 700026, India

¶

Department of Industrial & Applied Chemistry, Swami Vivekananda Research Center, Ramakrishna Mission Vidyamandira, Belur Math, Howrah 711202, India #

Department of Physics, University of Burdwan, Burdwan 713104, West Bengal, India

ABSTRACT: All-inorganic perovskite nanocrystals are emergent alternative of organolead halide perovskites. Cesium antimony halide (Cs3Sb2X9, X = Cl, Br, I) all-inorganic perovskites nanocrystals possessing analogous electronic configuration to the organolead halide perovskites are promising materials for optoelectronic applications. We report on a colloidal route to synthesis uniform Cs3Sb2Cl9 perovskite nanowires with lengths up to several microns. We have synthesized aspect ratio controlled nanorods with the same ~ 20 nm diameter of nanowires by tuning the precursors and ligands in the reaction. The crystallinity of the nanocrystals is significantly altered from the pristine bulk trigonal and orthorhombic phases owing to the one-dimensional shape of the nanocrystals. Rietveld refinement carefully separates out orthorhombic phase from the trigonal phase revealing a coexistence of both the phases in a minor and major ratio in the nanocrystals. The functionality in the form of fast photodetector demonstrates Cs3Sb2Cl9 nanocrystals as promising materials for optoelectronic applications.

organic23 and all-inorganic perovskite NCs using colloidal route have been reported recently.15,24–30 Antimonycontaining thin films and anti-solvent vapor-assisted bulk crystals of (CH3NH3)3Sb2I9, (NH4)3Sb2IxBr9-x and Cs3Sb2I9 perovskites were developed using non-colloidal routes.13,14,31 Recently, solution phase synthesis of Cs3Sb2X9 (X = Cl, Br, I) quantum dots have been reported.32 Advantageously, Cs3Sb2X9 perovskite structure is favourable by Goldschmidt tolerance factor suggesting an enhanced stability.33–35 Additionally, the similarity of electronic configuration of Cs3Sb2X9 with the organolead halide perovskites NCs implies important advantages of this class of all-inorganic materials.13 Although trigonal and orthorhombic phases of bulk Cs3Sb2Cl9 (CSC) have been reported more than 40 years ago,36,37 the NCs of this specific elemental composition has never been explored. Here we report on the microns long ambient stable all-inorganic CSC perovskite NWs using colloidal synthesis route. Monodispersed NRs have been synthesized by changing the ligands using the same reaction conditions of NWs. Fine kinetic control over the aspect ratio of NRs have been achieved by adjusting reactant precursor concentrations. Rietveld refinement reveals coherent existence

INTRODUCTION Organolead halide perovskites nanocrystals (NCs) possesses remarkable properties leading to a variety of optoelectronic applications.1–4 However, the stability of the organolead halide perovskites NCs remains a major technical hurdle5–8 owing to the sensitivity towards oxygen and moisture, which often disassociates under environmental stress.9 Hence, there has been a swift surge of alternate all-inorganic perovskite NCs negating volatile organic component. In recent years, a variety of inorganic perovskite materials have been reported with promising optoelectronic properties.10–14 However, most of these materials are developed in the form of thin films. In comparison, the investigation on onedimensional (1D) perovskite nanowires (NWs) and nanorods (NRs) is rather limited.2,15–20 The dimension dependent properties of 1D NCs is expected to show improved carrier migration, significant light trapping and enhanced mechanical properties, in comparison to the 3D structures adding advantages in the functions of optoelectronics devices.21,22 However, colloidal synthesis of 1D perovskite NWs or NRs has remain challenging, although both hybrid inorganic-

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of major trigonal and minor orthorhombic phases in the NWs and NRs. We demonstrated fabrication of photodetector using the perovskite NWs with excellent sensitivity, fast time response and repeatability.

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creased to 120°C (ramp 5°C/min) and annealing was carried out for 30 min under N2 gas. Color of the solution changes from turbid to light transparent indicating Sb-OLAc-HDA complex formation with annealing time. After 30 min, temperature of the reaction was set to 160°C (ramp 5°C/min). When reaction temperature was reached to 160°C, preheated solution (at 100°C) of 0.65 mL of Cs-OLAc-ODE was swiftly injected into the reaction mixture and reaction was annealed for different time. To check the morphology of the formed NCs, aliquots were collected for different time interval. The reaction was quenched immediately by cooling on ice water bath. Isolation and purification of Cs3Sb2Cl9 NRs and NWs The crude NCs formed in the reaction flask were collected in a centrifuge tube followed by the addition of equal volume of tBuOH. The mixture was centrifuged at 8500 rpm for 2 min. After centrifugation, the supernatant part was discarded and the precipitate NCs were re-dispersed in a small amount of toluene. Excess tBuOH was added to the NCs toluene dispersion and re-centrifuged at 8500 rpm for 2 min. The procedure was repeated for three times to remove unreacted precursors and ligands. After final wash, the precipitate was redispersed in toluene to form stable colloidal suspension of NCs and stored under 4°C refrigeration for further characterization, device fabrication. Characterization Absorption spectra were measured in a Varian Carry 5000 UV-Vis-NIR spectrophotometer. The photoluminescence (PL) spectra were collected by using a Nanolog spectrofluorometer from HORIBA Jobin Yvon. The photoluminescence quantum yield (QY) was recorded in calibrated integrated sphere attached with the Edinburgh FLS 980 spectrofluorometer. The FTIR spectra were taken in solid state KBr pellet form in a Perkin Elmer 100 FTIR spectrometer. X-rayelectron-diffraction (XRD) measurements were performed on a drop casted thin film of CSC NCs in a Bruker D8 advance powder diffractometer, using Cu Kα (λ=1.54 Å) as the incident radiation. Scanning rate of 0.02°/sec was employed in 2θ range of 10° to 70°. Transmission electron microscopy was carried out using JEOL JEM-2010 electron microscopy operating at 200 kV electron source. A dilute CSC NCs toluene dispersion were drop casted on a carbon-coated copper grid followed by vacuum drying before imaging. For chemical mapping of CSC NCs, a FEI, Tecnai G2 F30, S-Twin microscope operating at 300 kV equipped with a GATAN Orius B CCD camera was used. High-angle annular dark-field scanning transmission electron microscopy (STEM-HAADF) is employed in the same microscope which is equipped with a scanning unit and a HAADF detector from Fischione (model 3000). X-ray photoelectron spectroscopy (XPS) was performed using PerkinElmer Phi 5500 ESCA spectrometer with an Al Kα X-ray source generating X-ray photons of 1486.7 eV in energy in an ultrahigh-vacuum chamber with base pressure of 10–9 Torr. The spectra were obtained with an analyzer pass-energy of 23.5 eV and a scan speed of 0.05 eV s– 1. For XPS measurements, a thin film of the NCs were prepared on a silicon wafer substrate, followed by heating at 85°C for 80 min under N2 atmosphere. The film was then cooled to 25°C and stored under vacuum before measurement. Raman spectra were collected on a drop casted thin film of NCs on silicon wafer substrate in a J-Y HORIBA

EXPERIMENTAL SECTION Materials Antimony Chloride (SbCl3, 98%, Sigma), Oleic acid (OLAc, 90%, Aldrich), 1-Octadecene (ODE, 90%, Aldrich), Hexadecylamine (HDA, 70%, Aldrich), Oleylamine (OLAm, 70%, Aldrich), Hydrochloric acid (HCl, 37%, Merck), Cesium carbonate (Cs2CO3, 98%, Merck) acetone, toluene, t-butanol (tBuOH) were used without further purification. All reactions were carried out under N2 gas flow using Schlenk techniques unless otherwise stated. Gold patterned electrodes used for photoconductivity measurements were purchased form Qudos Technology LTD. Methods Cesium oleate preparation Cesium-oleate (Cs-OLAc) was prepared by dissolving 0.8 gm of Cs2CO3 in 8 mL of OLAc at 150°C for 30 min under N2 atmosphere until all Cs2CO3 is dissolved. Then, the reaction was cooled to 100°C and stored under N2 atmosphere for further use. Cesium-oleate-octadecene (Cs-OLAc-ODE) was prepared by dissolving 0.4 g Cs2CO3 in 1.75 ml OLAc and 15 ml ODE by heating the mixture under vacuum at 120°C for 30 min followed by heating up to 150°C until clear solution was obtained. Then the solution was cooled to room temperature and stored under N2.10,38,39 HDA.HCl preparation Hexadecylamine hydrochloride (HDA.HCl) was prepared by the reacting of HDA with HCl. A total of 4.83 g (20 mmol) of HDA was dissolved in 150 mL of acetone, and 2.46 mL (30 mmol) of HCl (37% in water) was added dropwise to the solution. White precipitate was stirred overnight to complete the reaction. White precipitate of HDA·HCl was filtered and washed with Milli-Q water and dried under vacuum desiccator. Cs3Sb2Cl9 NWs synthesis In a three neck round bottom flask, 60 mg SbCl3 salt, 0.5 mL OLAc, 440 mg HDA.HCl and 4 mL ODE was loaded and N2 gas was purged at 120°C (ramp 5°C/min) for 30 min. Color of the solution changes from turbid to transparent yellow indicating Sb-OLAc-HDA complex formation with annealing time. After 30 min, the temperature of the reaction solution was set to 170°C (ramp 5°C/min). When the reaction temperature was reached to 170°C, 0.65 mL of Cs-OLAc was swiftly injected into the reaction mixture and reaction was annealed for 20 min. The reaction was quenched immediately by cooling on ice water bath. Pure trigonal phase Cs3Sb2Cl9 NCs was obtained by using a similar methodology by injecting 0.15 ml Cs-OLAc at 155°C keeping all other parameter unchanged. Pure orthorhombic phase Cs3Sb2Cl9 NCs was obtained using similar methodology by injecting 0.65 ml Cs-OLAc at 200°C and annealing for 5 min keeping all other parameter unchanged. Cs3Sb2Cl9 NRs synthesis Synthesis of CSC NRs was carried out by modifying previously reported methods10,38,39. A three neck round bottom flask was loaded with 45 mg SbCl3 salt, 0.5 mL OLAc, 500 mg HDA and 4 mL ODE. The reaction mixture temperature was in-


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T64000 triple Raman spectrophotometer. Thermogravimetric (TGA) experiment was carried out in a Q600, TA Instruments. Atomic force microscopy (AFM) imaging of NCs on a mica substrate were performed with an Asylum Research MFP-3D AFM in tapping mode using AC160TS silicon probes, with nominal tip radii ˂10 nm. Thin phosphorous doped silicon cantilever with resistance 1-10 Ωcm has been used for scanning. XRD analyses Rietveld’s whole profile fitting method40,41 is regarded as one of the best methods to obtain different structure and microstructure parameters like lattice parameters, phase volume fractions, coherently diffracting domain (crystallite) size, r.m.s. lattice strain, etc. In the present study, we have adopted this method to evaluate different structure and microstructure parameters using the software MAUD42 which allows to refine different structure and microstructure parameters simultaneously. Detail of this method has been discussed elaborately elsewhere.43,44 Photoconductivity measurements The device was fabricated by simple drop casting method. For the preparation of film, Cs3Sb2Cl9 NWs and NRs solution was prepared in toluene (20 mg/ml) and few microliters of that solution was drop casted on the pre-patterned gold electrodes on silicon wafer. The device was annealed at 80°C for 10 mins in N2 atmosphere to dry the film. The photoconductivity I-V measurements (dark and light) were carried out by illuminating the fabricated photodetector device with Newport white light source (mercury lamp-one sun–100 mW/cm2; model 69911) connected to a Keithley 4200 semiconductor characterization system. The photoswitching measurements were done by using a slab, which is used to illuminate or block the incoming light manually on the device. The monochromatic light (~ 70 µW power) of wavelength ~ 410 nm is generated by using a monochromator (Newport, model 74125). The figure of merits of the photodetector like responsivity (R), detectivity (D) and external quantum efficiency (EQE) are calculated by measuring the dark current (Id) and photocurrent (Ip) at ~ 410 nm illumination. The following formulas are used for the calculation of responsivity, detectivity, and EQE of photodetector device: Responsivity (R) R = ΔI/A*P = Ip-Id/A*P; where Ip, Id represent the current at an applied bias voltage in the dark and under illumination of light respectively, A is the effective active area of the photodetector device and P is input power per unit area. Detectivity (D) D = R/√2qJd; where R is Responsivity, q is charge of an electron and Jd is dark current density. External Quantum Efficiency (EQE) EQE = hcRλ/qλ; where h is planks constant, c is velocity of light, Rλ is responsivity measured at a specific wavelength of monochromatic light source, q is charge of an electron and λ is wavelength of monochromatic light source.

ylammonium chloride (HDA.HCl) at 170°C under N2 atmosphere (see experimental section). We added Cs-OLAc as Cs+ source in a solution of SbCl3 in high boiling solvent 1Octadecene (ODE) containing a mixture of OLAc and HDA.HCl ligands in a molar ratio of 3:2. In this reaction, SbCl3 serves as the source of both Sb3+ and Cl− ions and HDA.HCl supplies additional Cl− ions required for the charge balance of the resultant NWs. Low resolution transmission electron microscope (TEM) image shows microns long (3.4±0.6 µm) CSC NWs with a diameter of ~20±6 nm (Figure 1a, Figure S1a,b, SI). AFM topographic image shows the uniform diameter of 20±2 nm (Figure 1b,c) along the length of a NW with a thickness of ∼3±0.5 nm (Figure 1d). High resolution TEM (HRTEM) image of NWs reveals well-resolved lattice planes implying crystalline nature of the NWs (Figure 1e). An interplanar spacing of ~0.29±0.05 nm corresponding to the planes of trigonal crystal structure (ICSD 22075) is observed. However, selected area electron diffraction (SAED) pattern shows the existence of orthorhombic phase in addition to the trigonal phase (Figure 1f). Energy dispersive X-ray (EDX) in TEM shows the atomic ratio of Cs:Sb:Cl3:2:8, which matches closely with Cs3Sb2Cl9 chemical composition (Figure S1c, SI). UV-vis absorption spectrum of NWs reveals an optical band gap of 3.4 eV (Figure S2a, SI). Reported band gap of bulk Cs3Sb2Cl9 is ~3 eV.47 An increase in the band gap of the NWs in comparison to the bulk is attributed to the quantum confinement effect. In addition to the reduced size effect, the 1D shape of the NWs also contributes to the quantum confinement effect since the charge carriers are confined along the diameter and allowed to move along the length of

Figure 1. (a) TEM images of CSC NWs. Inset shows NWs width distribution histogram. (b) Tapping mode AFM image of a single NW. (c) Cross section profile along the black line in Figure b. (d) Reconstruction of the AFM image showing the thickness of the NW. (e) HRTEM image of CSC NWs showing interplanar lattice spacing’s. (f) SAED pattern of NWs showing coexistence of trigonal and orthorhombic phases.

RESULT AND DISCUSSION Synthesis of CSC NWs was carried out through ionic metathesis process10,15,45,46 using antimony chloride (SbCl3) and cesium oleate (Cs-OLAc) in presence of mixture of long chain capping ligands oleic acid (OLAc) and hexadec-



the NWs.48 Photoluminescence (PL) spectrum shows a peak maximum at 436 nm with quantum yield of 4% (Figure S2bd, SI). Raman spectrum (Figure S3, SI) shows major peaks at 253 cm-1 and 308 cm-1 corresponding to B1g mode of SbCl3 and A1g, E2g, T2g modes of CsCl respectively.49,50 We have carried out XPS measurements to perceive the chemical states of the elements in the NWs (Figure S4, SI). XPS confirms Cs (+1), Sb (+3) and Cl (-1) oxidation states within the CSC NWs. Details analysis of XPS spectra shows a relative ratio of Cs:Sb:Cl3:2:8 which matches closely with the EDX measurements. Fourier transform infrared (FTIR) spectroscopy shows the presence of OLAc, HDA and HDA.HCl on the surface of CSC NWs (Figure S5, SI).30,51,52 Presence of C-H stretching frequencies (2914 and 2847 cm-1) in Cs3Sb2Cl9 NWs are similar to free HDA and HDA.HCl since nonpolar C-H bonds do not interact with the surface of the NCs. N-H bending vibrations appear at 1587 cm-1 for NWs which is shifted from free HDA, HDA.HCl ligands N-H vibration (1567 cm-1). N-H stretching vibration for free HDA and HDA.HCl appears in the range of 3000-3500 cm-1. The N-H stretching vibrations

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as observed for Cs3Sb2Cl9 NWs overlaps with the free HDA and HDA.HCl ligands. C-O stretching frequency of carboxylate group observed at 1450-1470 cm-1 for unbound OLAc ligand. Presence of C-O stretching frequency peaks in NWs (1471 cm-1 and 1495 cm-1) indicate binding of OLAc in oleate form on the surface of NWs. TGA analysis shows that CSC NWs are stable up to 600°C (Figure S6, SI), though the first weight loss occurs at ~ 69°C due to the decomposition of organic ligands.53 Powder X-ray diffraction (XRD, Figure 2a) pattern of CSC NWs shows reflections primarily corresponding to trigonal phase (ICSD 22075; space group: ; a=7.61Å, c=9.32Å). A careful observation using Rietveld analysis identifies low intensity peaks at 11.9°, 19.9° and 30.9° corresponding to (012), (020) and (043) planes of orthorhombic phase (space group: Pmcn; a=7.63Å, b=13.079Å, c=18.663Å, ICSD 2066) respectively (Figure 2b). Additional isolated reflection from the orthorhombic phase is not observed, instead, several overlapped reflections with the trigonal phase is evidenced (Figure 2b). Notably, Rietveld analysis using solely trigonal

Figure 2. (a) XRD pattern of CSC NWs showing trigonal phase. (b) Rietveld analysis confirming the presence of orthorhombic phase along with trigonal phase. Red curve represents the observed intensity (IO) and black curve represents calculated intensity (IC) and blue curve represents the residue (IO−IC) of the fitted pattern. (c) Refined output pattern using only trigonal phase (red curve) showing absence of orthorhombic phase (black curve). Enlarged sections are shown in the insets. (d) Individual contribution of trigonal and orthorhombic phases obtained from Rietveld refinement. (e) Atomic arrangements, unit cell (top-inset) and SbCl6 octahedra (bottom-inset) of trigonal phase. (f) Atomic arrangements, irregular triangular planar geometry of Sb and Cl (top-inset) and polyhedron (bottom-inset) of orthorhombic phase. Black spheres correspond to Sb, red spheres represent Cs and white spheres correspond to Cl.


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phase shows a clear mismatch in the intensity pattern at the peak positions of the minor orthorhombic phase (Figure 2c). This observation implies the simultaneous presence of trigonal and orthorhombic phases within the NWs. We have quantified the crystal phases of the NWs using Rietveld’s structure refinement method.40,41 Relative volume fractions of crystalline phases are extracted from scale factors of the respective calculated intensities (Figure 2d). Rietveld analysis quantifies the volume fractions of major trigonal and minor orthorhombic phases to be ~79% and ~21% respectively. This observation implies that the size of the coherently diffracting domains of trigonal phase is greater than the orthorhombic phase in the NWs. Based on the experimental observation, we have modelled the atomic arrangements of trigonal and orthorhombic phases within the NWs. Both trigonal and orthorhombic CSC shows a close-packed arrangement of Cs and Cl atoms with Sb atoms (Figure 2e,f). The trigonal unit

cell consists of a Sb atom surrounded by six Cl atoms forming SbCl6 octahedron (Figure 2e, top-inset). The bond lengths of elements in the trigonal unit cell are calculated to be ~5.38Å (Cs−Cs), ~4.66Å (Cs−Sb), ~3.81Å (Cs−Cl) and ~2.69Å (Sb−Cl) respectively. A change in bond angles of the SbCl6 octahedron from their ideal value of 90° is observed (Figure 2e, bottom-inset). The orthorhombic CSC unit cell consists of a Sb atom bonded with three nearest Cl atoms in an irregular triangular planar geometry (Figure 2f). Different Sb−Cl bond lengths and Cl−Sb−Cl bond angles of such arrangement is observed (top-inset, Figure 2f). Each Cs atom in the unit cell constitutes an irregular twelve coordination numbered convex polyhedron with nearest twelve Cl atoms (bottom-inset, Figure 2f). The Cs atom remains at the center while Cl atoms occupy the corner positions of the polyhedron. Out of twelve Cl atoms, six Cl atoms remain in an equatorial plane within the polyhedron forming a

Figure 3. (a) TEM image, (b) HRTEM image, (c) SAED pattern of CSC NRs with 30 min of annealing time. (d-g) STEM-HAADFEDX elemental mapping of NRs showing homogeneous distribution of Cs, Sb and Cl. (h-k) TEM images of NRs with different annealing time. (l) TEM image of elongated CSC NRs with SbCl3 amount variation for 30 min of annealing time. The bottom panels represent respective width and length distribution histograms for (h-l).



hexagon and the remaining six Cl atoms remain equally above and below this equatorial plane. Different Cs−Cl bond lengths suggest a distorted hexagon within the polyhedron (Figure S7, SI). Pristine trigonal and orthorhombic crystal phases of bulk CSC have been reported earlier.36,37 We have synthesized phase pure trigonal CSC NCs using the same reaction procedure at lower reaction temperature than NWs synthesis (Figure S8a-c, SI). TEM images show irregular agglomerated morphology of pure trigonal NCs (Figure S8b,c, SI). Phase pure orthorhombic CSC NCs have been synthesized at an elevated reaction temperature retaining the same reaction procedure of NWs synthesis (Figure S9a-e, SI). TEM images reveal NWs-like morphology of the NCs (Figure S9be, SI). Notably, shape controlled NWs are obtained at an intermediate reaction temperature when both trigonal and orthorhombic phases coexist in the NCs. Simultaneous presence of trigonal and orthorhombic phases in the NWs appears to originate from the fact that the lattice parameter ‘a’ of the trigonal phase remains in close proximity with that of the orthorhombic phase which enables the coherent growth of the two phases (Figure 2e,f). The lattice parameter ‘a’ of the trigonal phase increases and ‘c’ decreases marginally from their bulk counterpart (ICSD 22075) (Table S1, SI). For trigonal phase, the lattice volume of the unit cell marginally increases from 467.43 Å3 to 468.55 Å3 with respect to its bulk counterpart. In case of orthorhombic phase, all three lattice parameters decrease in comparison to their bulk counterpart (ICSD 2066) which reduces the lattice volume from 1862.43 Å3 to 1839.83 Å3 (Table S1, SI). This observation implies that the trigonal phase remains in a more compact form compared to the orthorohmbic phase within the NWs. Such distortion in the unit cells presumably occurs to incorporate the 1D NWs shape instead of phase pure bulk CSC. A suitable theoretical calculation of a NW containing Cs3Sb2Cl9 orthorhombic and trigonal phases may establish this observation to a greater extent.54,55 Perovskite CSC NWs are stable at ambient conditions since the crystal structure remains unchanged for more than two months as indicated by the XRD analyses (Figure S10, SI). Our attempt to synthesize CSC NCs following the synthetic route of all-inorganic CsPbX3 NCs by dissolving SbCl3 in OLAc, oleylamine (OLAm) and ODE followed by the injection of cesium-oleate-octadecene (Cs-OLAc-ODE) resulted in uncontrolled morphologies of NCs (Figure S11, SI).10 Hydrolysis of SbCl3 salt in presence of mixture of OLAc and OLAm yields Sb4O5Cl2 before addition of Cs-OLAc-ODE causing uncontrolled morphologies.56 However, the use of HDA instead of OLAm results in NRs with the average length of 165±12 nm and diameter of 20±5 nm (Figure 3a). HRTEM image of the NRs (Figure 3b) reveals well-resolved lattice planes with interplanar spacing of 0.29±0.05 nm corresponding to plane of bulk trigonal phase (ICSD 22075). SAED pattern of NRs features diffraction spots corresponding to the trigonal and orthorhombic bulk phases of Cs3Sb2Cl9 (Figure 3c). STEM-HAADF EDX elemental mapping shows homogeneous distribution of the constituting elements in the NRs (Figure 3d-g). We have investigated the effect of annealing time on the growth evolution of NRs. We found fast formation of the NRs within 1 min of reaction time with length

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of 165±12 nm and diameter of 20±5 nm (Figure 3h). A systematic increase in the reaction annealing time reveals that the size of the NRs remains the same (Figure 3h-k) suggesting fast formation of NRs within 1 min annealing after injection of Cs-OLAc-ODE to the SbCl3 precursor. Notably, the growth mechanism of Sb2S3, Sb2Se3 NCs showed the formation of NRs and NWs with a slower reaction kinetics.57,58 Hence, CSC NRs follow a different reaction mechanism from the reported antimony-chalcogenides, exhibiting a faster reaction kinetics.57,58 Size tunability of the NRs is further achieved by decreasing the amount of SbCl3 from 0.2 mmol to 0.12 mmol retaining other reaction parameters unchanged. TEM image shows NRs with lengths of 290±12 nm and widths of 20±5 nm suggesting an increased aspect ratio with decreased SbCl3 amount (Figure 3l). UV-vis absorption spectra of CSC NRs show the same peak positions with annealing time variation (Figure S12, SI). FTIR spectroscopy shows the presence of OLAc, HDA and HDA.HCl on the surface of CSC NRs (Figure S5). XRD patterns remain identical with the annealing time (Figure S13a). Rietveld’s analysis of XRD shows the simultaneous presence of trigonal and orthorhombic phases in the NRs (Figure S13b,c, SI). Raman spectrum of NRs replicates the peaks positions of NWs (Figure S14, SI). One unique feature of NWs based photodetector is that NWs can be deposited from the solution phase. CSC NWs based photodetectors are fabricated on pre-patterned gold electrodes with a channel length of ~ 100 nm. A schematic representation of the device structure is shown in Figure 4a. The current–voltage (I-V) characteristics show semiconducting behaviour both in the dark and under the illumination (Figure 4b). A significant enhancement in the current is observed upon illumination (Figure 4b). Further, to check the

Figure 4. (a) Schematic representation of photodetector device. (b) Current versus voltage characteristics of CSC NWs in dark and under illumination. Inset shows the optical image of the device. (c) Photoswitching behaviour at ~ 0.9 V revealing fast switching ON and OFF states with time interval of ~ 500 ms. (d) Rise (tr) and decay (td) time of a single ON-OFF cycle.


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sensitivity, stability and repeatability of the device, we have conducted photoswitching measurements of the CSC based photodetectors using white light (Figure S15, SI). Highly sensitive photodetecting response is observed over repetitive light ON and OFF cycles of illumination (Figure 4c, Figure S16, SI). The rise time (tr) and decay time (td) are ∼0.13 s and ∼0.23 s respectively at a bias voltage of 0.9 V (Figure 4d), which is similar to reported perovskite based photodetectors.19,59,60 The photoswitching properties are stable and repeatable suggesting CSC as a potential optoelectronic material. We have calculated the figure of merits like responsivity, detectivity, and EQE of the photodetector using a specific wavelength of light (~ 410 nm), which was obtained using a monochromator. Schematic diagram of the photocurrent measurements using a monochromator is shown in the supporting information (Figure S15, SI). The responsivity, detectivity and EQE of the CSC NWs photodetector under the illumination of monochromatic light source ~ 410 nm with 70 µW are found to be ~ 3616 A/W, 1.25×106 Jones, and 10959 % respectively suggesting CSC NWs as a robust photodetector (Table S2, SI) in the UV-vis region following the band gap of CSC NWs (3.4 eV).61–64

ACKNOWLEDGMENT G.S.K. acknowledges DST INSPIRE, India for fellowship. We thank B. Satpati, Saha Institute of Nuclear Physics, India and A. H. Khan, Istituto Italiano di Tecnologia, Italy for TEM analyses and useful discussions. SERB, DST is gratefully acknowledged for the financial support.

ABBREVIATIONS NCs, nanocrystals; NWs, nanowires; NRs, nanorods; CSC, Cs3Sb2Cl9; OLAc, oleic acid, HDA, hexadecylamine; HDA.HCl, hexadecylammonium chloride; OLAm, oleylamine; TGA, thermogravimetric analysis.

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CONCLUSIONS In conclusion, we have reported on a new class of allinorganic cesium antimony chloride perovskite NCs using colloidal synthesis route. Uniform NWs of several microns length and aspect ratio controlled NRs have been obtained by tuning precursors and ligands. Rietveld analysis observantly separates the trigonal phase from the orthorhombic phase and suggests coherent existence of these phases in major and minor ratios. The ratio of trigonal to orthorhombic phases is expected to be dependent with the size of the perovskite NCs. Fast photoresponsive properties suggest these perovskite NCs as prospective materials for optoelectronic applications. This work provides an approach for the fabrication of environmental friendly analogous Cs3Sb2X9 (X = Br, I) perovskites NCs. These new NCs may find applications in perovskite NCs based devices such photodetectors and solar cells.

ASSOCIATED CONTENT Supporting Information EDX, UV, PL, XPS, FTIR, Raman, XRD of NRs, TEM images of uncontrolled morphologies, TEM images of NWs, TGA of NWs, photoswitching of NRs. Tables summarizing Rietveld refinement data, comparison table showing photodetector figure of merits.

AUTHOR INFORMATION Corresponding Author *Email: [email protected] (Prof. Somobrata Acharya) ORCID Bapi Pradhan: 0000-0002-6202-7343 Somobrata Acharya: 0000-0001-5100-5184 Notes The authors declare no competing financial interests.



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One-dimensional all-inorganic cesium antimony chloride perovskite nanowires and nanorods have been synthesized using colloidal synthesis technique. Rietveld refinement reveals coherent existence of major trigonal and minor orthorhombic phases in the nanocrystals. Excellent photosensitivity, fast switching speed and repeatability suggest promises for optoelectronic applications.


Size Tunable Cesium Antimony Chloride Perovskite Nanowires and

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