Two-Dimensional Hybrid Organohalide Perovskites from Ultrathin PbS

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Two-Dimensional Hybrid Organohalide Perovskites from Ultra-Thin PbS Nanocrystals as Template Jayita Pradhan, Soham Mukherjee, Ali Hossain Khan, Amit Dalui, Biswarup Satpati, Carlo U. Segre, D. D. Sarma, and Somobrata Acharya J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b01236 • Publication Date (Web): 28 Feb 2017 Downloaded from http://pubs.acs.org on March 2, 2017

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The Journal of Physical Chemistry

Two-dimensional Hybrid Organohalide Perovskites from Ultra-thin PbS Nanocrystals as Template Jayita Pradhan,† Soham Mukherjee,§ Ali Hossain Khan,† Amit Dalui,† Biswarup Satpati,⊥ Carlo U. Segre,∥ D. D. Sarma†, §,# and Somobrata Acharya*, † †

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

Kolkata 700032, India; §Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore 560012, India; ⊥Surface Physics and Material Science Division, Saha Institute of Nuclear Physics, 1/AF Bidhannagar, Kolkata 700064, India; ∥Department of Physics & CSRRI, Illinois Institute of Technology, Chicago, Illinois 60616, United States; #Council of Scientific and Industrial Research-Network of Institutes for Solar Energy (CSIR-NISE), New Delhi 110001, India.

ABSTRACT

Direct conversion of pre-processed binary semiconductor NCs as template holds the key towards the shape control of hybrid perovskites. Here we report on an innovative route for realizing shape controlled hybrid organohalide perovskite NCs from two-dimensional (2D) PbS NCs on solid substrates. Rectangular PbI2 NCs are first synthesized by iodination of PbS NCs. Resultant PbI2 NCs are subsequently transformed into the well-defined rectangular hybrid perovskite NCs upon controlled CH3NH3Br exposure. Structural analyses using X-ray absorption

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fine structure (XAFS) reveal transition of cubic lattice of PbS to hybrid perovskites with a mixture of cubic and tetragonal phases exhibiting a bimodal distribution of shorter Pb-Br and longer Pb-I bonds around an immediate neighboring lead absorber within first coordination shell. This direct all anion exchange reaction route opens up new strategies for the fabrication of shape controlled perovskite NCs on flexible substrates from suitable existing binary NCs as template for optoelectronic applications.

INTRODUCTION Organohalide metal perovskite nanocrystals (NCs) are rapidly emerging as promising materials for high efficiency optoelectronic applications.1–7 The majority of perovskite NCs possess large absorption coefficients over a broad spectral range, high charge carrier mobilities, 8 small exciton binding energies9 and long exciton diffusion lengths.5,10 As a result, photovoltaic performance with high power conversion efficiencies has been achieved within an unprecedentedly short span.11 Complimentarily, perovskite NCs also exhibit promising applications in lasers with low thresholds and as efficient light emitting diodes.11,12 Organicinorganic trihalide perovskites are generally fabricated with ABX3 structure, where A represents methylammonium (CH3NH3+) (MA), B is commonly a metal (Pb2+) cation and X is a halide (Cl-, Br-, or I-).4,5 Hybrid perovskite halides such as MAPbI3-xBrx or MAPbI3-xClx exhibit better capabilities for high-efficiency solar cells as active light absorber because of easy tailoring of the broad range band gaps (1.6 eV to 3.1 eV) compared to pure trihalide perovskites.1,4,7,13-15 Conventionally, organic–inorganic perovskite NCs are fabricated using sequential deposition of solutions of lead halide (PbX2) and MAX or by direct mixing of PbX2 and MAX.3,16 However, such solution-processed chemical reactions contain barrier over controlling the resultant shapes,


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in addition to the stability, defects and uniformity.3,4 Recently, Sutherland et al. have reported on the synthesis of CH3NH3PbI3.17 They have first formed PbS seeds on a glass substrate using ALD technique. The entire film is then converted into PbI2 through exposure to iodine gas, which is subsequently converted into CH3NH3PbI3 by dipping the film in methylammonium iodide. This work shows that PbS NCs can be converted into halide perovskite NCs, however, controlling the shapes of the resultant perovskite structures remains a major challenge. Hence, efforts are underway to control the shapes and improve the crystallinity of perovskite NCs which results in the recent development of shape controlled perovskite NCs by direct synthesis routes.18-21 In this context, direct conversion of pre-processed shape controlled binary NCs as template into high quality shape controlled perovskite NCs on solid substrates containing large planar areas may provide an attractive approach towards planar optoelectronic device fabrications. Here we report on the direct conversion of ultra-thin 2D PbS NCs into uniform rectangular shaped hybrid perovskite NCs by all anion exchange reaction route directly on solid substrates. More specifically, 2D PbS NCs were converted into PbI2 NCs upon iodination retaining the same shape. Subsequently, PbI2 NCs were exposed for different times to MABr to obtain hybrid perovskite NCs retaining the 2D morphology of the pristine PbS NCs. Structural phase transition of the cubic lattice of PbS NCs leads to 2D shaped hybrid perovskites containing cubic and tetragonal phases. The coordination numbers and bond lengths within the first coordination shell are extracted from X-ray absorption fine structure (XAFS) spectroscopy, which are corroborated with the global structure determined by X-ray diffraction (XRD) and energy dispersive X-ray spectroscopy (EDX) techniques. Transition from the tetragonal to a higher symmetric cubic phase is observed for hybrid perovskites upon longer annealing time after MABr loading. Hybrid


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perovskite NCs show composition dependent band gaps useful in fabricating efficient optoelectronic devices on flexible substrates. RESULTS AND DISCUSSIONS

We synthesized 2D NCs of PbS using an one-pot reaction route by decomposing Pbhexadecylxanthates (PbHdX) in trioctylamine (TOA) in a single step at 80 C (see supporting information for synthesis details).22-25 TEM images reveal a flat rectangular morphology of the NCs with lengths of 200  50 nm and widths of 50  10 nm (Figure 1a and Figure S1a). AFM height profile analysis of the PbS NCs shows the overall thickness is ~4.8 nm including the TOA capping layers on the top and bottom surfaces (Figure S1b,c). The actual thickness of the inorganic PbS 2D NCs is estimated by accounting the geometric upright lengths of TOA molecules.26 The calculated length of an upright TOA molecule is 1.12 nm, suggesting an average thickness of ∼2.24 nm of TOA bilayer at the top and bottom surfaces of a 2D NC. Taken together, we can estimate a thickness less than ∼2.6 nm of a 2D NC (Figure S1d). SAED analysis of NCs shows one set of diffraction spots with lattice planes indexed to the rock-salt cubic structure of bulk PbS (Figure 1b). High resolution TEM (HRTEM, Figure 1c) reveals that the NCs are single-crystalline in nature with well-resolved lattice planes with interplanar spacings of 0.29 ± 0.02 nm corresponding to hkl (200) of bulk PbS rock-salt crystal structure (JCPDS #05-0592). Elemental analysis using EDX suggests the presence of Pb to S in a stoichiometric ratio of ~1:1 (Pb-51%, S-49%) (Figure S2). We carried out chemical mapping of 2D NCs to detect distribution of constituent elements Pb and S using EDX imaging technique. We have collected the Pb-L and S-K edges within 2D NCs which show a uniform distribution of the lead and sulfur in different locations within the NCs (Figure 1d,e). Powder X-ray diffraction


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Figure 1. (a) TEM image of 2D NCs of PbS. (b) SAED pattern from 2D NCs of PbS shows diffraction spots corresponding to (200), and (220) planes of bulk cubic rock-salt PbS. The zone axis is indicated in the figure. (c) HRTEM image of 2D NCs of PbS showing (200) lattice planes. HRTEM image is taken from the area marked by yellow box in Figure a. (d,e) Chemical mapping of 2D NCs of PbS by EDX technique shows elemental distribution of lead and sulfur respectively. (f) XRD pattern from 2D NCs of PbS showing prominent (200) reflection. Standard diffraction pattern (JCPDS #05-0592) for bulk cubic rock-salt PbS is indexed by the vertical red lines.

(XRD) pattern of 2D NCs of PbS shows bulk cubic PbS rock-salt crystal structure with a prominent (200) reflection suggesting a preferential growth of the NCs along the direction (Figure 1f). In the next step, as-synthesized 2D NCs of PbS dispersed in chloroform were spin casted on


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Figure 2. (a) TEM image of PbI2 NCs. (b,c) Chemical mapping of 2D NCs by EDX technique showing uniform distribution of lead and iodine within the NCs. (d) XRD pattern during PbI2 NCs formation upon iodine exposure to 2D NCs of PbS for different times. The standard diffraction pattern (JCPDS #07-0235) for PbI2 is indexed by vertical green lines. (e) HRTEM image of PbI2 NC showing (001) and (002) lattice planes respectively. (f) SAED pattern of PbI2 NCs showing diffraction spots which are indexed to (002), (003), (004), (104) and (10 ) planes of the hexagonal 2H polytype crystal structure. The zone axis is indicated in the figure. (g) Optical absorption spectra of PbI2 NCs measured at different iodination times. The absorption spectrum of PbS NCs (black solid line) is included for comparison with the PbI2 NCs.

a clean solid substrate (glass, ITO or Si wafers) to prepare thin films. The resultant PbS NCcovered substrates were exposed to iodine vapor within an enclosed dark chamber for different exposure times at 65 °C. Exposure of PbS NCs to iodine vapor changes the color from black to yellow indicating the formation of PbI2. Figure 2a and Figure S3 show TEM images of the resultant PbI2 NCs after 2 hr exposure to iodine vapor. A 2D rectangular morphology of PbI2


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similar to PbS NCs is evidenced. EDX analysis suggests the presence of lead to iodine in a ratio of ~1:2 (Pb-36%, I-64%) in accordance with PbI2 composition (Figure S4). Chemical mapping carried out by EDX technique reveals a uniform distribution of lead and iodine throughout the 2D NCs (Figure 2b,c). We monitored PbI2 NCs formation process by XRD measurements at different iodination times (Figure 2d). XRD pattern shows diffraction peaks corresponding to mainly (001), (002), (003) and (004) lattice planes indicating that PbI2 NCs crystallize in the form of the hexagonal 2H polytype structure (Figure 2d).3,27 No noticeable diffraction peaks corresponding to PbS are observed indicating phase purity of PbI2 NCs. Prolonged heating critically above 1 hr leads to a reduction of additional polytype peaks of PbI2 indicating an improvement in the crystallinity (Figure 2d).28 HRTEM shows lattice spacings correspond to (001) and (002) planes of PbI2 (Figure 2e) (JCPDS #07-0235). SAED measurement shows diffraction patterns dominated by 00l (where l= 2, 3, 4) (Figure 2f), which is consistent with the XRD and HRTEM observa i

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absorption spectra measured at different iodination time reveals gradual formation process of PbI2 from PbS NCs (Figure 2g). The absorption features of PbI2 NCs appear different from PbS NCs. An increase in the step-like absorbance at 430 nm and 500 nm is observed upon the iodine exposure. Retention of the peak positions throughout the iodine exposure time indicates that the iodination process is completed within a quick span after exposing PbS NCs to iodine vapor. Earlier reports show easy insertion of external molecular fragments through interlayer configuration.29˗ 2D NCs of PbS containing inherent atomically layered structures favour the formation of PbI2 upon iodine vapor exposure at elevated temperature due to the greater affinity of iodine to lead than sulfur. Similar transformation of lead oxide (PbO) into PbI2 was reported earlier upon reaction with iodine vapor involving bond dissociation energy of 94.8 Kcal/mol at


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25 °C.32 Expectedly, PbS with lower bond dissociation energy of 74 Kcal/mol at 25 °C facilitated transformation into PbI2 upon iodine vapor exposure.33 Such PbI2 layered structure consists of spatially repeating planes of I–Pb–I along the direction where Pb ions are sandwiched between two close packed layers of iodide ions.28 Bonding within the sandwich structure is largely ionic, though, adjacent inter layers stack together by weak van der Waals forces. Our results demonstrate direct conversion of 2D PbS NCs into PbI2 NCs retaining the morphology upon iodine vapor exposure. Subsequently, MABr in isopropanol (10 mg/mL, 600 µL) was spin coated on PbI2 NCs at room temperature and the temperature was then increased to 65 °C where annealing was carried out for different time intervals (see supporting information for synthesis details). TEM image of NCs after 4 hr of annealing reveals a 2D morphology (Figure 3a). Rectangular shaped NCs with 200  50 nm in length and 50  10 nm in width similar to the PbS NCs are evidenced. Subsequent to MABr loading, we observed radiation damage of the NCs while being exposed to the electron beam of the microscope similar to other hybrid perovskite systems.21 We examined the composition and structure of NCs at different stages of annealing after MABr loading (Figure S5-S8). SAED analysis of NCs immediately after MABr loading reveals sets of diffraction spots corresponding to a major tetragonal phase and a minor cubic phase suggesting coexistence of both the crystal phases within NCs before annealing (Figure S5a). SAED analyses of NCs with annealing show a gradual dominance of diffraction spots corresponding to the cubic crystallographic phase over the tetragonal phase (Figure 3b and Figure S5b-d). Chemical mapping of 2D NCs by using EDX imaging technique shows uniform distribution of lead, iodine and bromine at different locations within the NCs (Figure S6,S7). We have determined the overall bromine to iodine atomic percentage from the EDX analyses collected at different stages


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Figure 3. (a) TEM image of hybrid NCs for 4 hr of annealing after MABr loading. (b) SAED pattern of the NCs for 4 hr of annealing after MABr loading showing dominance of diffraction spots corresponding to cubic crystallographic phase over tetragonal phase. The zone axis for cubic reflections is indicated in the figure. (c) Bromine to iodine atomic ratio calculated from the EDX analyses at different stages of annealing upon MABr. (d) UV-vis absorption spectra of hybrid NCs measured at different annealing times upon MABr loading.

of annealing upon MABr loading. EDX measurements immediately after MABr loading and before annealing suggest the presence of lead, iodine and bromine in a ratio of ~1:2:1 (Pb-33%, I-44%, Br-23%) suggesting possible MAPbI3-xBrx composition (inset, Figure S8a). An increase in bromine to iodine atomic ratio is observed with prolonged annealing suggesting insertion of bromine atom into hybrid perovskite NCs (Figure 3c and Figure S8b-e). These observations


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imply introduction of bromide ions into PbI2 NCs upon MABr loading. The change in the atomic composition with loading time of the MABr into the PbI2 at 65 C is also reflected from the UVvis absorption spectroscopy (Figure 3d). At the beginning of MABr loading, a new peak appears at 750 nm which finally merges to the peak at 550 nm resulting in a broad absorption feature containing signature of both iodine and bromine perovskite components (Figure 3d). A systematic shift of the absorption peaks to longer wavelengths with annealing time indicates that the band gap of the hybrid perovskite NCs can be tuned by varying the annealing time after MABr loading. XRD patterns at different annealing time after MABr loading into the PbI2 reflect concomitant changes in the crystal structure (Figure 4a). The XRD patterns show that both tetragonal and cubic perovskite crystal phases coexists at room temperature, that are in agreement with the reported hybrid perovskite structures.13,34 Notably, MAPbI3 crystallizes in the tetragonal I4/mcm space group while MAPbBr3 assumes cubic symmetry with Pm m space group at room temperature.13,34 Structural differences in these perovskites stem from the difference in the iodide and bromide ionic radii with 6-fold coordination around the lead ion. C

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4c) reveal the dominance of tetragonal phase at the beginning of annealing immediately after M B

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of annealing, which suggests introduction of bromine by substitution of the larger sized iodine thereby decreasing the resultant lattice spacings. EDX measurements also showed an increase in bromine atoms with annealing time (Figure 3c). Prolonged heating leads to the gradual splitting of the peaks at 14.4° and 14.73° corresponding to tetragonal (110) and cubic (100) planes (Figure 4b). A similar splitting of the peaks at 29.33° and 29.92° for higher order tetragonal (220) and


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Figure 4. (a) X-

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°  60°) of the hybrid perovskite NCs for

different annealing times at 65 °C. (b) XRD pattern reveals tetragonal (110) and cubic (100) reflections in the zoomed region (13.5  15.5°). (c) XRD pattern reveals tetragonal (220) and cubic (200) reflections in the zoomed region (28  31).

cubic (200) reflections respectively was also observed (Figure 4c). Evidently, both the cubic and tetragonal phases coexist in our hybrid halide perovskites at different annealing stages. Cubic


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phase dominates over the tetragonal phase in the final stage of annealing. The structural difference within the hybrid perovskite NCs appears to originate from the differences in the ionic radii of iodide and bromide within the PbX6 octahedron.13 Deconvolution of the XRD peaks reveals that tetragonal phase is the majority phase at the beginning of the MABr loading, while the cubic phase becomes dominant at longer annealing times for both lower and higher order reflections (Figure S9a,b). Its worth to mention that a wide variety of synthesis techniques are reported till date using template routes.35 General strategy for the template synthesis involves template preparation, synthesis of target materials within the template and template removal. Advantageously, our method deals with the direct conversion of the existing binary NCs into perovskite structure where the template removal is not necessary. We have probed the local bonding arrangements around Pb atoms by performing X-Ray Absorption Fine Structure (XAFS) spectroscopy at Pb-LIII edge for PbI2 and hybrid halide perovskite NCs. X-ray Absorption Near Edge Structure (XANES), which reflects the local symmetry of the nearest neighboring atoms, shows identical absorption edge positions for PbI2 and hybrid perovskite NCs (Figure 5a). Identical edge energy positions point out a single and stable oxidation state (+2) of lead both in PbI2 and hybrid perovskite NCs. Corresponding first derivative spectra reveal a series of isosbectic points (inset, Figure 5a). We attribute the origin of these isosbectic points to the changes in the local symmetry of PbX6 octahedron (X = Br, I) due to substitution of larger sized iodide (2.2 Å) ions by smaller bromide (1.96 Å) ions. Such large difference in ionic radii of the halides distorts the PbX6 octahedral units within the hybrid halide perovskite structure leading to a phase transition from tetragonal I4/mcm to higher symmetry cubic Pm m phase.13 Notably, ideal tetragonal and cubic symmetry of a perovskite lattice can be realized by extent of cooperative tilt between the adjacent PbX6 octahedrons along the


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Figure 5. (a) Normalized XANES features for PbL-III edge for samples A-D (codes are given below) measured at 300 K showing identical absorption edge positions (marked by black vertical dotted line). Inset, corresponding first derivative spectra over a zoomed energy window. Isobectic points are indicated by red arrows. (b) Overlay plots of k2-weighted (k) oscillations for Pb-LIII edge for samples A-D (codes are given below). (c) Overlay plots of magnitude of Fourier transforms of k2-weighted (k) oscillations for Pb-L III edge showing PbX correlations of the PbX6 octahedra. A bimodal distribution of nearest neighbors with a significant variation of their relative intensities is shown for the hybrid perovskites in comparison to PbI2. SAMPLE_A denotes PbS NCs exposed to iodine for 1 hr; SAMPLE_B denotes PbS NCs exposed to iodine for 1 hr and then annealed for 1 hr after MABr loading, thin film; SAMPLE_C


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denotes PbS NCs exposed to iodine for 1 hr and then annealed for 1 hr after MABr loading, thick film; SAMPLE_D denotes PbS NCs exposed to iodine for 2 hr and then annealed for 3 hr after MABr loading.

axis on the (00l) plane maintaining corner-sharing connectivities.13 Ionic substitution at lattice sites necessarily introduces distortions within perovskite structure and alters the tolerance factor (t), thereby resulting in coexistence of tetragonal and cubic crystallographic phases. Our XRD measurements indeed show

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with gradual replacement of iodide ions by smaller bromide ions in the hybrid halide, which reduces the lattice spacing. Stability of a perovskite structure can be estimated from t, where formability is expected in the range 0.813

Two-Dimensional Hybrid Organohalide Perovskites from Ultrathin PbS

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