Synthesis of Ultrathin Few-Layer 2D Nanoplates of Halide Perovskite

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Synthesis of Ultrathin Few-Layer 2D Nanoplates of Halide Perovskite Cs3Bi2I9 and Single-Nanoplate Super-Resolved Fluorescence Microscopy Arka Sarkar,†,§ Paribesh Acharyya,†,§ Ranjan Sasmal,† Provas Pal,† Sarit S. Agasti,† and Kanishka Biswas*,†,‡ New Chemistry Unit and ‡School of Advanced Materials and International Centre of Materials Science, Jawaharlal Nehru Centre for Advanced Scientific Research (JNCASR), Jakkur P.O., Bangalore 560064, India

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S Supporting Information *

ABSTRACT: The discovery of new two-dimensional (2D) perovskite halides has created sensation recently because of their structural diversity and intriguing optical properties. The toxicity of Pb-based perovskite halides led to the development of Pb-free halides. Herein, we have demonstrated a one-pot solution-based synthesis of 2D ultrathin (∼1.78 nm) few-layer (2−4 layers) nanoplates (300−600 nm lateral dimension), nanosheets (0.6−1.5 μm), and nanocrystals of layered Cs3Bi2I9 by varying the reaction temperature from 110 to 180 °C. We have established a mechanistic pathway for the variation of morphology of Cs3Bi2I9 with temperature in the presence of organic capping ligands. Further, we have synthesized the bulk powder of Cs3Bi2I9 by mechanochemical synthesis and liquid-assisted grinding and crystalline ingot by vacuum-sealed tube melting. 2D nanoplates and bulk Cs3Bi2I9 demonstrate optical absorption edge along with excitonic transition. Photoluminescence properties of individual nanoplates were studied by super-resolution fluorescence imaging, which indicated the blinking behavior down to the level of an individual Cs3Bi2I9 nanoplate along with its emission at the far-red region and high photostability.

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INTRODUCTION The interest in layered two-dimensional (2D) materials burgeoned since the discovery of graphene,1 which paved way toward an ever-improving field on 2D materials. Thereafter, the discovery of mono/few-layer MoS2 and hexagonal boron nitride further contributed to this development.2,3 The importance of 2D materials lies in their subtle electronic and optical properties, for example, because of their enriched quantum, dielectric confinements, and interlayer coupling.4 Recently, in the case of perovskite halides, the aforementioned properties of 2D layered solids have intensified the interest in them over their 3D counterparts because of enhanced quantum confinement.5−7 Exotic 2D layered perovskite halides,8 such as Ruddlesden−Popper9,10 and Dion− Jacobson11 phases demonstrate promising optical properties12,13 and have potential applications in photovoltaics and optoelectronic devices.14−17 Perovskite 2D-nanoplatelets have increased exciton binding energy, reduced fluorescence decay time, enhanced absorption cross-section, and notable optical nonlinearity with respect to their bulk counterparts.18 Pb-based perovskite halides have been extensively studied because of their low-defect states that lead to magnificent optoelectronic properties.19−23 However, the issues that need to be addressed for their practical applications are their © XXXX American Chemical Society

tendency to decompose slowly under moisture, heat, air, or light,24,25 and the high toxicity of Pb. To mitigate these issues, several stable, Pb-free perovskites have been improvised in the last few years.5,26,27 Sn and Ge have been tried as replacements for Pb in perovskite halides,28,29 but Sn-perovskites have been reported to possess self-doping effects and more unstable than its Pb-based counterparts,28 while Ge being an expensive rare metal cannot be used for large scale practical applications. Further, the tendency of Sn2+ and Ge2+ to oxidize to their respective +4 states by losing the outermost ns2 electrons affects the stability, transport properties, and carrier mobility of those specific perovskites.30 Pb2+in the [MX6] octahedra can also be replaced by stable trivalent and tetravalent cations by keeping the perovskite-type structure intact. Replacing three Pb2+ ions with two stable trivalent cations (Sb3+/Bi3+) would transform the 3D connectivity into a 2D layered network by aligning along a particular crystallographic direction.27 However, the higher size of the monovalent cation at the Asite causes every two [MX6] octahedra to form isolated facesharing bioctahedra, resulting in a 0D structure with the A-site cations occupying the interstitial spaces, which is observed in Received: October 11, 2018

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DOI: 10.1021/acs.inorgchem.8b02887 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Cs3Bi2I9 (Figure 1a).27,28 Such reduction in the dimensionality of a perovskite structure decreases the band dispersion and

microscopy, which revealed that the luminescence intensity of the NPLs is homogeneous and in the far-red region; importantly, the fluorescence blinking was in the millisecond time scale. The dispersive nature of the blinking kinetics indicated the existence of surface defects present on the large surface of the NPLs that trapped excitons, leading to nonradiative recombination. These trapping sites could alternatively be activated and deactivated by incident photons.

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EXPERIMENTAL SECTION

Reagents. Caesium carbonate (Cs2CO3, 99.9%, Sigma-Aldrich), bismuth iodide (BiI3, 99.9%, Sigma-Aldrich), oleic acid (OA, tech. 90%, Sigma-Aldrich), oleylamine (OAm, tech. 70%, Sigma-Aldrich), 1-octadecene (ODE, tech. 90%, Sigma Aldrich), and toluene (AR, 99.0%, SDFCL) were used for the syntheses without further purification. Synthesis of Cs-Oleate. In a typical synthesis, 410 mg (1.26 mmol) Cs2CO3 and 20 mL ODE were taken in a 50 mL 3-necked round-bottom flask and heated at 120 °C in vacuum for 1 h to get a homogeneous solution. 1.30 mL of OA was added into the above solution, and the solution was kept for another 30 min at the same condition. The vacuum was replaced with nitrogen atmosphere, and temperature was increased up to 150 °C and kept for 5 min to get a clear Cs-oleate solution. This Cs-oleate solution was stored at room temperature under nitrogen atmosphere for further use. Synthesis of Colloidal Cs3Bi2I9. In order to synthesize the colloidal Cs3Bi2I9 nanostructures of different morphologies (NSs, NPLs and NCs), the reactions were carried out by a hot injection method. 222 mg (0.38 mmol) BiI3 and 10 ml ODE were taken in a 50 mL 3-necked round-bottom flask, and the flask was degassed at 100 °C for 60 min along with magnetic stirring. 1 mL OA and 1 mL OAm were added to the mixture at 100 °C under N2 atmosphere to dissolve BiI3. Then, temperature of the reaction was raised to 110, 150, and 180 °C for three separate reactions to produce different morphology nanostructures. The presynthesized Cs-oleate (0.1 M, 0.6 mL) solution in ODE, preheated at 90 °C, was swiftly injected to the reaction mixture at 110, 150, and 180 °C respectively. Within few seconds, the reaction mixture became turbid red, and then the reaction was quenched by dipping the reaction flask into an ice bath. N2 atmosphere was maintained throughout the reaction process in a Schlenk line. The synthesized Cs3Bi2I9 nanostructures were then washed with tert-butanol and precipitated by centrifugation at 8000 rpm for 10 min, followed by redispersing the products in nonpolar solvents such as toluene for further studies. We obtained Cs3Bi2I9 with NSs and NPLs-like morphology at 110 and 150 °C, respectively; whereas at 180 °C, Cs3Bi2I9 NCs were formed. Solid-State Mechanochemical Synthesis of Cs3Bi2I9. In a typical synthesis of bulk powder of Cs3Bi2I9, 64.5 mg (0.25 mmol) of CsI and 98.25 mg (0.16 mmol) of BiI3 were taken in stoichiometric ratios of 3:2 in a mortar pestle, mixed, and ground mechanically for 2 h in a N2-filled glove bag. The physical mixture of CsI and BiI3 converted from black to deep red in 1 h. The grinding process was still continued for another 1 h to assure pure-phased product. Synthesis of Cs3Bi2I9 by Liquid-Assisted Grinding. In this synthetic process, 64.5 mg (0.25 mmol) of CsI and 98.25 mg (0.16 mmol) of BiI3 were taken in a stoichiometric ratio of 3:2 in a mortar pestle, and few drops of different organic solvents (ethanol, N,Ndimethyl formamide, and N-methyl pyrrolinone) were added in three different reactions. The mixtures were ground mechanically for 2 h in a N2-filled glove bag. The black physical mixture of CsI and BiI3 converted to bright red for the ethanol-assisted reaction and deep red for N,N-dimethylformamide (DMF)- and N-methyl-2-pyrrolidone (NMP)-assisted reactions, respectively. Synthesis of Crystalline Ingot of Cs3Bi2I9. Crystalline ingot (∼1 g) of Cs3Bi2I9 was synthesized by reaction of stoichiometric (3:2) amounts of the starting materials CsI and BiI3 in quartz ampoules. The tube was sealed under vacuum (∼10−6 Torr) and slowly heated to 923 K. It was then left for a time period of 12 h, followed by slow cooling to room temperature over 8 h.

Figure 1. (a) Crystal structures of Cs3Bi2I9. (b) Photograph of hot injection synthesis of Cs3Bi2I9, (c) solid-state mechanochemical synthesis of Cs3Bi2I9 from BiI3 and CsI. Photographs of Cs3Bi2I9 synthesized by LAG with (d) ethanol and (e) NMP. (f) Picture of Cs3Bi2I9 ingot synthesized by sealed-tube melting reaction. (g) PXRD patterns of Cs3Bi2I9 obtained by hot injection at different temperatures.

raises the effective masses of charge carriers.31−33 Cs3Bi2I9, a bismuth-based inorganic lead-free perovskite halide, is a wellknown layered perovskite that has found immense interest for its optical and electronic properties.34−46 The Bi3+ oxidation state being stable because of its inert lone pair effect leads to its long-term durability, potentially better transport properties, and carrier mobility. It is more stable and less toxic than many of its Pb-based counterparts. The layered Cs3Bi2I9 perovskite in the form of nanostructures, bulk solid, and single crystals are generally synthesized by spin coating, chemical vapor deposition, Bridgman crystal growth, and others.34−44 Nonetheless, any simple synthesis without any support or template as well as the study of the optical properties of the freestanding few-layer ultrathin Cs3Bi2I9 2D nanoplates (NPLs) is still rare. Here, we have demonstrated the low-temperature colloidal synthesis of free-standing ultrathin (∼1.78 nm) few-layer (∼2 layers) NPLs (300−600 nm lateral dimension) of Cs3Bi2I9 without using any support or template with high yield and reproducibility and studied their optical properties. We have also synthesized nanosheets (NSs; 0.6−1.5 μm) and nanocrystals (NCs) of layered Cs3Bi2I9 by varying the reaction temperature and provided intriguing chemical insight to the formation mechanism. Further, bulk powder and crystalline ingot of Cs3Bi2I9 have also been synthesized by mechanochemistry, liquid-assisted grinding (LAG), and vacuum-sealed tube melting reaction. In ultrathin NPLs, the optical property is controlled by lower energy states associated with the local intrinsic electronic structure of the edges of the perovskite layers. We have studied the single-particle fluorescence properties of Cs3Bi2I9 NPLs by super-resolution fluorescence B

DOI: 10.1021/acs.inorgchem.8b02887 Inorg. Chem. XXXX, XXX, XXX−XXX

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Powder X-ray Diffraction. Powder X-ray diffraction (PXRD) patterns of the samples were collected using Cu Kα (λ = 1.5406 Å) radiation on a Bruker D8 diffractometer at room temperature. Field-Emission Scanning Electron Microscopy. Field-emission scanning electron microscopy (FESEM) experiments were performed using NOVANANO SEM 600 (FEI, Germany) operated at 15 kV. Energy dispersive X-ray (EDAX) compositional analysis was performed during FESEM imaging. Thermogravimetric Analysis. Thermogravimetric analysis (TGA) was performed by using a 2 STAR TGA instrument. Samples were heated at a rate of 5 °C min−1 in N2 atmosphere (40 mL min−1) throughout a temperature range of 30−750 °C. The TGA curve of assynthesized Cs3Bi2I9 NPLs has been shown in Figure S3, Supporting Information. The TGA data shows an initial weight loss of 10% in the temperature range of 50−410 °C because of the decomposition of the capping agents. Subsequently, the second weight loss of 35% is observed within a temperature range of 410−650 °C because of the decomposition of Cs3Bi2I9 into CsI and BiI3. Evaporation of BiI3 accounts for the major weight loss. Atomic Force Microscopy. Atomic force microscopy (AFM) was performed on a Bruker Innova Microscope in the tapping mode with 10 nm diameter containing an antimony-doped Silicon tip. Transmission Electron Microscopy. The transmission electron microscopy (TEM) images of the synthesized Cs3Bi2I9 were taken using a JEOL (JEM3010) TEM instrument (300 kV accelerating voltage) fitted with a Gatan CCD camera and also with a FEI TECNlAI G2 20 STWIN TEM instrument (operating at 200 kV). Electronic Spectroscopy. UV−visible absorption spectra were recorded using a PerkinElmer, Lambda-900 UV/vis/near-IR spectrometer. To estimate the optical band gap in the solid state sample, diffuse reflectance measurements were carried out in the range of λ = 250−800 nm by using a PerkinElmer Lambda 900 UV/vis/near-IR spectrometer in the reflectance mode. Absorption (α/S) data were calculated from reflectance data by using the Kubelka−Munk equation: α/S = (1 − R)2/(2R), in which R is the reflectance, α and S are the absorption and scattering coefficients, respectively. The energy band gaps were derived from α/S versus E (nm) plots. Electronic absorption spectroscopy is also performed in the solution phase and compared with solid state spectra. The emission spectra were recorded on PerkinElmer LS 55 luminescence spectrometer in the solid state and in solution phase. Luminescence spectra of were recorded at different excitation wavelengths. Fluorescent Lifetime Measurement. Time-resolved decay experiments were recorded on a Horiba Delta Flex time-correlated single-photon-counting instrument. A 373 nm nano-LED with a pulse repetition rate of 1 MHz was used as the light source. The instrument response function was collected by using a scatterer (Ludox AS40 colloidal silica, Sigma-Aldrich). For the 373 nm LED light source, the instrumental full-width at half-maximum including the detector response was 0.2 ns. The excited state decay of the sample was collected by fixing the emission wavelength at a particular wavelength. The decay was fitted using IBH software (DAS6) Super-Resolved Fluorescence Imaging. Cs3Bi2I9 NPLs were dispersed in toluene and dried over a cover-slip for imaging. Bright field images were captured first using lower magnification objective (EC plan-Neofluar 10×/0.3 M27) and the resulted image showed a distribution of tiny NPLs. The fluorescence images of the same crystals revealed the photoluminescent properties of the materials. Next, we further investigated the properties of these materials after looking at higher magnification objective (Alpha Plan-Apochromat 100×/1.46 Oil DIC M27 Elyra) using high energy laser for excitation. Confocal Microscopy Imaging. Confocal microscopy imaging was done at room temperature using a Zeiss LSM 880 laser scanning confocal microscope. The microscope objective of 63× (NA 1.4) was employed. The sample was prepared by sealing cover slip on top of the glass slide. Laser was used at 488 and 543 nm for excitation.

Article

RESULTS AND DISCUSSION A solution-based synthetic approach under an inert atmosphere Schlenk-line was followed here for the synthesis of 2D few-layer NPLs with thickness in the range of 2−3 unit cells (∼2 nm) and also for the syntheses of NSs and NC of Cs3Bi2I9. Typically, BiI3 was dissolved in ODE using OA and OAm, followed by hot injection of presynthesized Cs-oleate quickly into the BiI3 solution at different temperatures (110−180 °C) (Figure 1b). The orange/red reaction mixture was then quenched by ice-water bath after 10 min of reaction and precipitated by tert-butanol, which was further thoroughly washed with hot toluene several times. The final products varied in morphologies such as NSs, NPLs, and NCs depending on reaction temperatures that is 110, 150, and 180 °C, respectively. Moreover, we have synthesized bulk powder Cs3Bi2I9 by simple mechanochemistry (Figure 1c)47 and LAG in three different solvents, namely DMF, NMP, and ethanol (LAG, Figure 1d−e) with stoichiometric CsI and BiI3 at room temperature. Further, we have synthesized crystalline ingot (Figure 1f) of Cs3Bi2I9 under vacuum (∼10−5 Torr) via a sealed quartz tube melting reaction at 650 °C. PXRD patterns of NSs, NPLs, and NCs (Figure 1g) synthesized by the hot injection method, bulk compounds synthesized by mechanical grinding (Figure S1a, Supporting Information) and the solid-state sealed tube melting process (Figure S1a, Supporting Information); and the bulk powders by LAG (Figure S1b, Supporting Information) could be indexed based on the pure-phase hexagonal Cs3Bi2I9 (space group P63 /mmc). The Rietveld-refined PXRD pattern indicated the formation of no other phases (Figure S2, Supporting Information), and the measured crystallite sizes are listed in Table S1, Supporting Information. Cs3Bi2I9 possesses 0D connectivity in the structure consisting of two face-sharing Bi2I93− bioctahedra, which are separated by the independent monovalent Cs+ occupying the interstitial spaces (Figure 1a). The face-sharing Bi2I93− bioctahedra form the layers which are stacked in crystallographic b-axis with the thickness of each layer being 0.84 nm (Figure 1a). Further, as-synthesized Cs3Bi2I9 powders possessed good thermal stability as seen in the TGA (Figure S3, Supporting Information). NPL (Figure 2a) and large-area thin NS (Figure S4a, Supporting Information) morphologies of Cs3Bi2I9 synthesized by the hot injection method at 150 and 110 °C, respectively, were observed in FESEM. 2D NPLs of Cs3Bi2I9 possess lateral dimensions of 300−600 nm with a thickness of few nanometers (Figure 2a). EDAX analysis of Cs3Bi2I9 NPLs showed the presence of Cs, Bi, and I being consistent with their stoichiometric ratio (Figure S4d, Supporting Information). Small NCs (Figure S4c, Supporting Information) resulted in the hot injection synthesis at 180 °C, while agglomerated bulk particles with large grain size in the range 0.6−1.5 μm of Cs3Bi2I9 were observed in the mechanochemical synthesis and LAG. The thickness of the NPLs was measured with AFM to be ∼1.78 nm, which matches closely with total thickness of two successive layers (i.e. 1.68 nm) stacked along the crystallographic b-direction consisting of face-sharing metal-halide bioctahedral (Figure 2b). However, we have also observed the presence of few relatively thicker NPLs with thicknesses of ∼3.55 nm (Figure S5, Supporting Information), which consists of four layers of Bi2I93− bioctahedra. C

DOI: 10.1021/acs.inorgchem.8b02887 Inorg. Chem. XXXX, XXX, XXX−XXX

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size of the Cs3Bi2I9 nanostructures. Thickness of perovskite NPLs are generally controlled by the concentration of OA in the reaction mixture, where thinner and larger (lateral dimension) NSs/NPLs are resulted at higher OA concentration and NCs are formed with lower OA concentration.49 We assumed that at low temperature (110 °C), OA binds through the carboxylate group strongly to the selective surface of Cs3Bi2I9.50 Hence, adsorbed OA prevents the growth in the crystallographic b-direction and Cs3Bi2I9 grows asymmetrically along the ac-plane, resulting in large (2−5 μm) NSs with the exposed (101) lattice planes (Figure S6b, Supporting Information). However, at higher temperature (150 °C), the solubility of OA in ODE increases (i.e. effective OA capping agent concentration decreases), which gives rise to the formation of 2D NPLs with lateral dimension of 300−600 nm (Figure S8, Supporting Information). When the reaction was conducted at further higher temperature (180 °C), the surface−ligand interactions were not controlled because of the enhanced solubility of OA in ODE, resulting in the synthesis of NCs (Figures S6c and S8, Supporting Information). A similar temperature-controlled study of the formation of different morphologies was reported for CsPbBr348 and wurtzite CdSe.51 We first studied the optical properties of Cs3Bi2I9 NPLs, NSs, and NCs in the solid state (Figure 3) and then compared

Figure 2. (a) FESEM image of the Cs3Bi2I9 nanoplates (b) AFM image and height profile (inset) of the Cs3Bi2I9 nanoplates (c) TEM image of the Cs3Bi2I9 nanoplates (d) HRTEM image with lattice plane (101) and SAED pattern (inset) of the Cs3Bi2I9 nanoplates (e) TEM image of Cs3Bi2I9 nanosheets, SAED pattern (inset) (f) TEM and HRTEM image (inset) of Cs3Bi2I9 nanocrystals.

Transmission electron micrographs of the sample synthesized at 150 °C by hot injection method revealed that the square-shaped ultrathin NPLs (Figure 2c) possessed lateral dimensions ranging between 300 and 600 nm. The highresolution TEM (HRTEM) image of a typical ultrathin Cs3Bi2I9 NPL demonstrates the lattice fringes with the spacing of 0.68 nm (Figure 2d,) corresponding to the (101) planes of hexagonal Cs3Bi2I9. The selected-area electron diffraction (SAED) pattern confirmed that the Cs3Bi2I9 NPLs were single crystalline in nature (Figure 2d inset). Ultrathin NSs synthesized at 110 °C are single crystalline in nature (Figure 2e inset), possessed lateral dimensions of 2−5 μm, and thickness of few nanometers (Figure 2e and S6a, Supporting Information).However, the NCs synthesized at 180 °C had dimensions of 30−50 nm (Figure 2f and S6c, Supporting Information). In the NCs, the d-spacing value was measured as 0.68 nm, which indicated the (101) plane of Cs3Bi2I9 (Figure 2f inset). From the HRTEM analysis, the crystals were shown to have grown along (101) planes for NPLs, NSs, and NCs.48 TEM and HR-TEM images of LAG materials have been shown in Figure S7, Supporting Information. Hot injection at lower temperature favored the formation of large 2D NSs/NPLs, whereas higher temperature reaction favored the formation of NCs, which indicates that temperature played a critical role in determining the morphology and

Figure 3. Solid-state electronic absorption (α/S) (black) and PL (blue) spectra of (a) Cs3Bi2I9 nanoplates, (b) Cs3Bi2I9 nanosheets, (c) Cs3Bi2I9 nanocrystals, and (d) Cs3Bi2I9 bulk powders (mechanochemistry).

with that in the solution phase (Figure S12, Supporting Information). Electronic absorption spectroscopy of the Cs3Bi2I9 NPLs showed an optical absorption edge at 611 nm (2.03 eV, Figure 3a), whereas optical absorption edge for the BiI3 was observed at 720 nm (1.77 eV, Figure S9, Supporting Information), which exclude the formation of BiI3 in the present synthesis. Photoluminescence (PL) studies at the solid state revealed that a broad emission peak (Figure 3a) was deconvoluted to 623 nm (1.99 eV) and 645 nm (1.93 eV) peaks, when excited at 480 nm (Figure S10a, Supporting Information). These two PL bands are probably due to the direct band gap transition (1.99 eV) and phonon-assisted transition (1.93 eV) in Cs3Bi2I9 that are earlier proposed by Zhang et al.,34 which needed further investigation. However, we observe a prominent peak in absorption spectra of NPLs, NCs, and bulk samples of Cs3Bi2I9 around ∼2.55 eV (Figures 3 D

DOI: 10.1021/acs.inorgchem.8b02887 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 4. Fluorescence microscopy images and blinking behavior of Cs3Bi2I9 nanoplatelets: (a) bright-field image, (b) fluorescence microscopy image shows Cs3Bi2I9 NPLs are fluorescent in nature, (c) molecular density of individual plate for multiple blinking events of Cs3Bi2I9 NPLs after excitation using 488 nm laser (630 W cm−2). Fluorescence blinking events were recorded over 10 000 frames with 100 ms exposure time. The zoomed-in region from (c) shows repetitive ON and OFF cycle for each Cs3Bi2I9 NPLs (i,ii), and (d) systematic plot of fluorescence ON and OFF of Cs3Bi2I9 NPLs upon irradiation with 488 nm laser and collecting the emission from a band pass emission filter set (BP 570−650 + LP 750).

presence of more defect states compared to that in solution. The PL decays of NPLs were well fit by a triexponential function with a short-lived component (τ1 = 0.3 ns) with relative amplitudes of 25%, a middle component (τ2 = 0.9 ns) with relative amplitudes of 64%, and a long-lived component (τ3 = 9.1 ns) with relative amplitudes of 11% (Figure S13, Supporting Information).52 We have subsequently investigated the PL behavior of individual Cs3Bi2I9 NPL via super-resolution fluorescence microscopy. The bright-field optical image (Figure 4a) showed a distribution of NPLs. Those bright-field images of Cs3Bi2I9 NPLs showed that the average diameter of NPLs was around ∼450 nm (Figure S14, Supporting Information), which is supported by TEM and SEM measurements. The fluorescence image (Figure 4b) of the NPLs revealed the photoluminescent emission with high signal-to-noise ratio. We further investigated the properties of Cs3Bi2I9 NPLs under higher magnification objective using high-intensity laser for excitation. Super resolution localization microscopy was initially performed on each Cs3Bi2I9 NPL using 488 nm laser excitation, where emission was collected from a band pass emission filter (BP 570−650 + LP 750). In the total internal reflection fluorescence illumination mode, these crystals showed systematic fluorescence ON and OFF behavior because of trap states resulting from surface defects of the material.53,54 This blinking of the crystalline material was recorded for few minutes, and it was observed that each crystal repeatedly transformed to its ON and OFF states (Figure 4d and Supporting Information Video V3), which indicated high photostability of the NPLs. This was further confirmed by looking at the molecular density profile of any particular crystal blinking region (Figure 4c-i,ii), which indicated the number of multiple fluorescence blinking events in a particular region. The high fluorescence intensity of the crystalline material was supported by calculating the photon counts from the blinking events. Photon count was found to be on an average of 400−600 for Cs3Bi2I9 NPLs (Figure S15, Supporting Information).

and S11, Supporting Information), which was proposed to be excitonic transition by recent reports.40,44 Optical absorption edge for the NSs were observed at 622 nm (1.99 eV) with two distinct PL emissions taking place at 626 nm (1.98 eV) and 659 nm (1.88 eV) respectively similar to NSs, when excited at 465 nm (Figures 3b and S10b, Supporting Information). This slight red shift of absorption spectra of NSs compared to that of the NPLs can be attributed to the large lateral dimension of NSs. The optical absorption edge for the NCs were observed at 634 nm (1.96 eV) with two PL bands measured at 637 nm (1.95 eV) and 649 nm (1.91 eV) respectively, when excited at 475 nm (Figure 3c and S10c, Supporting Information). However, the optical absorption edge for the bulk powder synthesized by mechanochemical grinding was found to be at 645 nm (1.92 eV), and two PL peaks appeared at 656 nm (1.89 eV) and 668 nm (1.85 eV), when excited at a wavelength of 460 nm (Figures 3d and S10d, Supporting Information). The blue shift in electronic absorption spectra of ultrathin NPLs compared to that of bulk powder as well as of NCs due to possible quantum confinement in ultrathin 2D NPLs. The studies for optical properties were also carried out in the solution phase (Figure S12, Supporting Information). In solution phase absorption spectra, the blue shift of the excitonic peak is prominent for NPLs compared to NCs. The excitonic peak exhibits at 480 nm for NPLs, whereas in case of NCs, the position of this excitonic peak is 497 nm. The blue shift in electronic absorption spectra of ultrathin NPLs compared to that of NCs is probably due to quantum confinement. PL studies in the solution phase revealed that the position of the emission band is at lower wavelength and sharper compared to solid state PL band. The band positions appeared at 544 nm (Figure S12a, Supporting Information), 535 nm (Figure S12c, Supporting Information), and 520 nm (Figure S12e, Supporting Information) for NPLs, NSs, and NCs, respectively, when excited at a wavelength of 450 nm. Similar PL emission in lower wavelength in solution phase small NCs by Nag and co-workers.44 In the solid state optical spectra, broadening in emission band is mainly due to the E

DOI: 10.1021/acs.inorgchem.8b02887 Inorg. Chem. XXXX, XXX, XXX−XXX

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time appeared to be higher in the case of excitation at 560 nm (Figure 5f). Further, the fluorescence properties of the NPLs were studied by confocal microscopy at different wavelengths and emission windows. The confocal microscopy images (Figure S16b,c, Supporting Information) showed that Cs3Bi2I9 NPLs emit fluorescence upon excitation with both 488 and 543 nm lasers. It is shown in colocalized image of red (excitation at 543) and green (excitation at 488 nm) fluorescence of the sample (Figure S16f, Supporting Information), where red emission can be visualized from the entire NPLs (or clusters of NPLs), but green emission is specially restricted toward the middle of the clusters. The emission spectra of the NPLs were also recorded from the spectral scanning confocal microscopy images. It was observed that the emission of the NPLs highly depended upon the size of the sample when excited at 488 nm (Figure S17, Supporting Information). The bigger NPL showed high intensity emission at 620 nm. However, for the smaller NPL, the excitation at 488 nm resulted in multiple emission bands at 580, 620, and 670 nm with similar fluorescence intensity; the 620 nm band is consistent with solid-state PL spectra Cs3Bi2I9 NPLs (Figure 3a), while the other emission bands are observed probably due to the surface defects. On the other hand, excitation at 543 nm gave rise to emission majorly at 620 nm (Figure S18, Supporting Information).

The photophysical properties of Cs3Bi2I9 NPLs were also studied using different off-resonance wavelengths. Upon excitation using 405 nm laser, emission spectra were monitored in two different emission windows (BP 495−575 + LP 750 and BP 570−650 + LP 750). Fluorescence blinking was not observed from Cs3Bi2I9 NPLs when it was excited using 405 nm laser, as we do not see any fluorescence emission with this excitation wavelength from the solid sample. Figure 5a−d and in the Supporting Information Videos V1, V2 are the

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CONCLUSIONS 2D ultrathin NPLs of a Pb-free layered Cs3Bi2I9 have been synthesized in a one-step hot injection process at 150 °C with good yield. The morphologies of the Cs3Bi2I9 were varied from NSs to NCs with changing the reaction temperature in the range 110−180 °C. Further, bulk powder and crystalline ingot of Cs3Bi2I9 were also synthesized following mechanochemical grinding and LAG at ambient condition and vacuum-sealed tube solid-state melting. Mechanochemical solid state grinding was an inexpensive, solvent free, and room temperature synthetic process. Possible quantum confinement effect was observed for the ultrathin 2D NPLs of Cs3Bi2I9, which is confirmed by the blue shift in the absorption edge. However, the large surface area for the 2D NPLs gives rise to surface defects that in turn resulted in long-lived photoblinking of the NPLs with good fluorescence properties, indicating high photostability of the product, which is investigated by superresolution fluorescence microscopy.

Figure 5. Single-molecule blinking analysis of Cs3Bi2I9 nanoplates (a) fluorescence images were recorded after excitation using 405 nm laser and emission of nanoplates were monitored in 495−575 nm wavelength, (c) excitation using 405 nm laser and emission of nanoplates were collected in 570−650 nm wavelength, (e) excitation using 560 nm laser and emission of nanoplates were monitored in 570−650 nm wavelength. Arbitrary fluorescence spots were picked to show single nanoplate traces. (b,d) Nonemissive/nonblinking nature of nanoplates at 405 excitation and (f) blinking nature of nanoplates at 560 nm excitation. The scale bar is 2.5 μm (a,c,e).

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b02887. Experimental Sections, PXRD and Rietveld Refinement of as-synthesized Cs3Bi2I9, TGA, additional FESEM, AFM and TEM images, formation mechanism, UV/vis and PL spectra, PL decay curve, bright-field images, photon count calculation, fluorescence microscopy images, confocal microscopy images, and table for crystallite sizes (PDF) Blinking videos of Cs3Bi2I9 NPLs by super-resolution fluorescence microscopy V1: excited at 405, emission BP: 495−575 (AVI) V2: excited at 405, emission BP: 570−650 (AVI)

single NPL fluorescence frames from two emission windows. Subsequently, the fluorescence blinking properties of the NPLs were also recorded using 560 nm laser (emission filter BP 570−650 + LP 750). The NPLs were emitting fluorescence in this excitation and emission window, and the fluorescence ON/OFF switching was observed from this excitation wavelength. In Figure 5e and in the Supporting Information Video V4, it clearly suggests the occurrence of the above phenomenon. In comparison to excitation at 488 nm, the blinking behavior is slightly different and the fluorescence ON F

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V3: excited at 488, emission BP: 570−650 (AVI) V4: excited at 560, emission BP: 570−650 (AVI)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Sarit S. Agasti: 0000-0001-7216-0490 Kanishka Biswas: 0000-0001-9119-2455 Author Contributions §

A.S. and P.A. contribute equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was partially supported by Nanomission, DST (SR/ NM/TP-25/2016) and Sheikh Saqr Laboratory, JNCASR. S.S.A. acknowledges a SERB Early Career Research Award (ECR/2016/002052) and a DBT Innovative Young Biotechnologist Award (IYBA) (BT/09/IYBA/2015/15). P.P. acknowledges National Postdoctoral Fellowship, SERB (PDF/ 2016/001790). P.A. and R.S. acknowledges support from UGC and CSIR Graduate Research Fellowships, respectively. We acknowledge Prof. Ranjani Vishwanatha, JNCASR, for helping in fluorescence lifetime measurement.

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