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Structure-Tuned Lead Halide Perovskite Nanocrystals Yasser Hassan, Yin Song, Ryan D. Pensack, Ahmed I. Abdelrahman, Yoichi Kobayashi, Mitchell A. Winnik, and Gregory D. Scholes* Since pioneering work in 2009[1] incredible advances have been made in the development of organic–inorganic metal halide perovskite materials for thin film solar cells; devices have been reported with efficiencies exceeding 20%.[2,3] These CH3NH3PbX3 (X = Br, Cl, and I) materials have a broad absorption profile across the visible spectral region and they exhibit intense and narrow-band luminescence. The excitonic properties and long diffusion length of charge carriers[4–6] explain the interplay of efficient energy and charge transport underpinning the efficient photovoltaic properties. However, the nature and dynamics of the excited states that ultimately determine the performance of optoelectronic devices are still being elucidated. Here, we report the preparation and characterization of a colloidally stable suspension of methyl ammonium lead halide perovskite nanocrystals (NCs). The perovskite NCs are seeded from high quality PbX2 NCs (X = I or Br) with a pretargeted size. Subsequent reaction with methyl ammonium iodide (MAI) yields perovskite NCs. According to ligand choice and ratio, these NCs are prepared with different layered multiple quantum well like NC structures, denoted R2(CH3NH3)n−1PbnI3n+1; n = 1, 2, 3 where R is the ligand. The excitonic gap is tuned stepwise; for example, the photoluminescence (PL) peak for 5.5 nm diameter colloids is 505, 565, and
Y. Hassan, Y. Song, Prof. M. A. Winnik, Prof. G. D. Scholes Department of Chemistry University of Toronto 80 St. George Street, Toronto, Ontario M5S 3H6, Canada E-mail: [email protected] Y. Hassan, Prof. M. A. Winnik Department of Chemical Engineering and Applied Chemistry University of Toronto 200 College Street, Toronto M5S 3E5, Canada Y. Hassan Chemistry Department Faculty of Science Zagazig University 44511 Zagazig, Egypt Dr. R. D. Pensack, Prof. G. D. Scholes Department of Chemistry Princeton University NJ 08544, USA Dr. A. I. Abdelrahman SABIC Corporate Research and Innovation Center at KAUST Thuwal 23955, Saudi Arabia Prof. Y. Kobayashi Department of Chemistry School of Science and Engineering Aoyama Gakuin University 5-10-1 Fuchinobe, Chuo-ku, Sagamihara, Kanagawa 252-5258, Japan
DOI: 10.1002/adma.201503461
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600 nm, respectively. The sharp excitonic features make these NCs well suited for systematic investigations of the intrinsic photophysics and spectroscopy of organic–inorganic metal halide perovskites. To date, limited syntheses of CH3NH3PbX3 perovskite materials on the nanoscale have been reported.[7–9] Burschka et al. used a sequential deposition method to infiltrate the perovskite pigment within the pores of porous metal oxide film.[8] This method enabled control over perovskite morphologies with particle sizes that ranged from 50 to 200 nm and led to reproducible, efficient photovoltaic devices.[8] Another recently reported approach utilized a long chain ligand-assisted reprecipitation technique to produce colloidal perovskite material.[9] The quest to prepare perovskite NCs and control their size motivated us to investigate a synthetic approach inspired by colloidal semiconductor quantum dot synthesis.[10,11] The basis of our synthesis is the preparation of colloidal PbI2 NCs. Since the original report of Sandroff et al. on the quantization effects in PbI2 small clusters[12] in films and solution, these NCs have been prepared by various methods that include Langmuir–Blodgett techniques,[13] zeolite cages,[14] silica films,[15] copolymer films,[16] colloidal solutions,[17,18] vapor deposition,[19,20] reverse micelles,[21] or surfactant-assisted hydrothermal routes.[22] Control of the size of colloidally stable PbI2 NCs remains a challenge. In this work we address this issue. The colloidal perovskite NCs reported in this work were synthesized in a two-step process. In the first step, we synthesize nearly monodisperse PbI2 (or PbBr2) NCs using a method inspired by lead chalcogenide semiconductor NCs.[10,23] Subsequently, we react the PbI2 NCs with an alkyl ammonium iodide to produce the corresponding perovskite NCs. A lead oleate precursor reacts with an iodide–organic complex[24] in a noncoordinating solvent such as octadecene (ODE). For instance, to prepare PbI2 NCs, tetrabutylammonium iodide (TBAI, 4 mmol) was dissolved in oleylamine (7 mL) at 200 °C under an inert atmosphere to form the iodide–amine complex. The resulting solution was cooled to 50 °C before it was injected into a hot solution of the lead–oleate complex (2 mmol) in ODE. The solution changed to golden-yellow color after a few seconds, indicating the formation of PbI2 NCs. In this ionic metathesis reaction, nucleation occurs and growth begins over the course of a few seconds.[25] The temperature of the ODE solution (160–200 °C) and the reaction time can be used to control particle size. The reaction was halted after several minutes by removing the heat source and injecting anhydrous toluene into the reaction mixture. This synthetic procedure produces homogeneous and crystalline lead halide (PbX2) NCs with mean sizes in the range of 3–11 nm. Figure 1 presents transmission electron microscopy (TEM), high-resolution TEM (HRTEM), and energy-dispersive X-ray
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COMMUNICATION Figure 1. Structural, optical, and chemical characterization of lead iodide NCs. a–c) TEM images of PbI2 NCs prepared at 200 °C at a growth interval time of a) 5, b) 15, and c) 30 min exhibiting a diameter of 3.4–4.2, 4.7–5.8, and 9.5–10 nm, respectively. d) HRTEM of the PbI2 NCs prepared in this work, that shows a lattice parameter of 0.34 nm. e) Absorption spectra of colloidal PbI2 NCs in toluene solution where the NC size has been varied from 2 to 6 nm diameter and clearly exhibiting a quantum confinement effect. f) EDX spectra obtained from 6 nm sized NCs of PbI2. The Cu signals arise from the TEM grid.
spectroscopy (EDX) measurements of a typical sample of PbI2 NCs prepared at 200 °C. The TEM images of this PbI2 NC sample, without any size-selection treatment, reveal an abundance of uniform spherical NCs. By controlling the reaction temperature (120–200 °C) and growth time, particles with a narrow size distribution can be obtained with diameters (d) in the range of 3–10 nm. A quantitative analysis of the TEM images reveals that PbI2 NCs prepared at 200 °C at time intervals of 5, 15, and 30 min are characterized by a diameter range of 3.4–4.2, 4.7–5.8, and 9.5–10 nm, respectively (Figure 1a–c). Figure S2 (Supporting Information) shows TEM images of PbI2 NCs prepared at 160 °C. HRTEM images (Figure 1d) exhibit lattice fringes throughout the entire particle. The lattice fringes of the PbI2 NCs are spaced by 0.344 nm, corresponding to the (101) lattice. HRTEM (Figure 1d) and with the selected area electron diffraction pattern (SAED), Figure S3 (Supporting Information), show the high crystallinity of the PbI2 NCs, assigned to the hexagonal PbI2 phase.[26] TEM diffraction patterns (SAED) were analyzed with a diffraction ring profiler, which was developed for phase identification in complex microstructures.[27] The SAED pattern of the selected area of collection of PbI2 NC particles exhibits diffuse rings that are typical of nanometer-sized particles. The patterns index to the (101), (100), (102), (003), (111), (004), and (104) reflections of the hexagonal structure of PbI2 (PDF card no. 43–1484).[26] More details about simulated SAED are provided in Figure S4 (Supporting Information). In order to investigate the effect of particle diameter on the optical properties of the PbI2 NCs and the corresponding perovskites, we prepared two different sizes of PbI2 NCs (d = 2 and 6 nm, Figure 1e). Figure 1e shows absorption spectra recorded
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at room temperature for aliquots of PbI2 NCs dispersed in toluene, removed from the reaction mixture at 200 °C after reaction for 5 and 15 min of growth. We found that with decreasing the size of the PbI2 NCs from 6 to 2 nm, the exciton energy blueshifts by 0.22 eV (λmax from 413 to 385 nm). This quantum confinement effect (the bulk PbI2 bulk semiconductor has a bandgap of 2.57 eV) is thought to be mainly a 2D exciton quantum confinement along the crystal c axis of the layered hexagonal structure.[17,19] EDX was utilized to determine the elemental composition of our PbI2 NCs. An example is presented in Figure 1f, showing clear evidence for the presence of both Pb and I. The elemental analysis from EDX is in agreement with the atomic ratio calculated from X-ray photoelectron spectroscopy (XPS) measurements (Figure S7, Supporting Information). In order to prepare the lead iodide perovskite NCs, the PbI2 NCs suspended in toluene were diluted with chloroform to a volume ratio of 2:1 (toluene:chloroform). An alkyl ammonium halide, methyl ammonium halide (MAI) alone, or a MAI mixture with a long-chain alkyl ammonium iodide was added at 50 °C over 30 s. It is known that the PbI2 single crystal consists of strongly bonded I Pb I layers, and the interlayer bonding is through van der Waals forces.[28] The conversion of lead halide NCs to the corresponding perovskite takes place by intercalation of the organic MAI moiety between the Pb I Pb layers of the crystalline PbI2 host[29] forming 2D organo-lead halide perovskites with the structure of R2(CH3NH3)n−1MnX3n+1,[30,31] where (R = long-chain amine, M = Pb, X = halide ion, and n = 1, 2, 3). To obtain lamellar 2D lead iodide perovskites R2(CH3NH3)n−1MnX3n+1[30,32] with different n values, we tuned the molar ratio between the long-chain organic ammonium
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iodide (e.g., oleylammonium iodide) and MAI. By adding either oleylammonium ioidide alone, or MAI and oleylammonium iodide mixtures with a molar ratio of (2:1), or with a molar ratio of (1:1), we found the solution immediately changed color to green, orange, or red, respectively, indicating the formation of 2D organo-lead halide perovskites with number of layers n = 1, 2, or 3. It was found that a small quantity of chloroform added to toluene enhances the reaction between the MAI and the lead halides (I or Br), while the addition of any amount of methanol, acetonitrile, or acetone degrades the perovskite (as evidenced by diminished perovskite absorption). Figure 2a–c shows the TEM images of the colloidal 2D organo-lead iodide perovskite (n = 3), (C18H35NH3)2(CH3NH3)2Pb3I10, prepared from three different samples of PbI2 NCs seeds with particles size of 3.5, 6, and 10 nm, respectively. On the nanoscale, the 2D perovskite naturally self-assembles into a stack of lamellae consisting of alternate layers of organic and inorganic components. The inorganic layers comprise semiconducting corner sharing octahedral PbX64− sheets (wells) stacked in the direction of the c-axis, sandwiched between bilayers of low dielectric constant organic barriers consisting of alkyl ammonium chains.[30,31] These self-assembled structures are similar to inorganic multiple quantum wells, characterized by very stable exciton transport in the inorganic layers with enhanced exciton binding energy that can reach four times larger than in the case of 3D perovskite.[31,33]
In Figure 2d, we show the room temperature absorption and PL spectra corresponding to the three 2D, n = 3 samples presented in Figure 2a–c. We found that the colloidal suspension of (C18H35NH3)2(CH3NH3)2Pb3I10 NCs exhibits a quantum confinement due to the particle size compared to the film absorption value reported by Wu et al.[30,34] In the case of our NC suspension, the first exciton λmax is 585, 592, and 599 nm for the sizes of 3.5, 5.5, and 10 nm, respectively. This compares to 608 nm in the case of the film.[30,34] As shown in Figure 3d, the change in particle size has marginal quantum confinement effect as compared to the effect of the 2D confinement. Perovskite material presented in this study ranged between 3.5 and 11 nm which is bigger than the Bohr radius of perovskite, i.e., 2.1 nm.[31] Figure 3 shows the versatility of our method to prepare perovskite NCs with different 2D structures, where n = 1, 2, or 3. To prepare 2D perovskite with one layer (n = 1), we used either butylammonium iodide, octylammonium iodide, or oleylammonium iodide, while these long-chain amines mixed with MAI in a molar ratio of R:MAI = 2:1 and 1:1 yielded n = 2 and 3, respectively. For these 5.5 nm diameter particles, the absorption peaks are 505, 565, and 593–600 nm, respectively for n = 1, 2, and 3. The room temperature absorption and PL spectra of 2D lead iodide perovskite (n = 3), (C18H35NH3)2(CH3NH3)2Pb3I10 (Figures 2d and 3c) exhibit a strong “band-edge exciton” absorption and well-resolved higher energy band-to-band transitions that is indicative of a narrow particle size distribution. The PL
Figure 2. Structural and optical properties of 2D organo-lead iodide perovskite NCs. a–c) TEM images of the 2D organo-lead iodide perovskite (n = 3) NCs prepared from PbI2 NCs that exhibit a diameter of a) 3.5, b) 6, and c) 10 nm, respectively. d) Normalized absorption and PL spectra of 2D lead iodide perovskite (n = 3) NCs in a toluene/chloroform mixture as prepared from PbI2 NCs of three different diameters: 3 (black line), 5 (light gray line), and 10 nm (dark gray line).
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COMMUNICATION Figure 3. 2D layered perovskite NCs. a) Schematic structures of the colloidal 2D organo-lead iodide perovskites NCs (C18H35NH3)2(CH3NH3)n−1[PbnI3n+1] synthesized in the present study, where (from left to right) n = 1, 2, and 3. b) Photograph of colloidal 2D organo-lead iodide perovskite NC solutions of (from left to right) n = 1, 2, and 3, respectively, under ambient light. c) Absorption spectrum of colloidal PbI2 NCs in toluene with a diameter of 4–5 nm compared to the absorption and emission spectra of the resultant 2D lead iodide perovskite (n = 3) colloidal NC solution in toluene/chloroform mixture. All spectra have been normalized to the lowest energy optical transition. The 2D lead iodide perovskite (n = 3) NC absorption exhibits an onset at 593 nm. Energy bandgap estimated from the intersection point of the normalized curve was 2.09 eV. The steady-state PL spectrum (dotted red) consists of a single peak slightly redshifted from the excitonic peak with a maximum at 607 nm. The PL was measured with an excitation wavelength of 480 nm. Absorption spectra of the three 2D organo-lead iodide perovskite samples (blue, green, and red for n = 1, 2, and 3, respectively), and steady-state PL that peaks at 520, 580, and 610 nm (dotted blue, green, and red lines for n = 1, 2, and 3, respectively). The PL was measured with an excitation wavelength of 480 nm. Absorbance is plotted as optical density normalized to the exciton peak.
spectra are symmetric and narrow (full-width at half maximum of 0.12 eV) and the quantum yield of the perovskite NCs in toluene was measured to be ≈20%, estimated by comparison with the dye cresyl violet.[35] In the case of n = 1 NCs, we find that increasing the alkyl chain length from R = butyl to oleylammonium has an effect on the first exciton absorption peak energy (Figure S5a, Supporting Information). In addition, increasing the volume fraction of chloroform in the sample changes the peak position, which might indicate a modification of the dielectric confinement effect[31] (Figure S5b, Supporting Information). The conversion of lead halide NCs to the corresponding perovskite NCs was confirmed by investigating their optical properties along with a structural analysis by electron microscopy, X-ray diffraction (XRD) and X-ray photoelectron spectroscopy. For instance, treating the PbI2 NCs with MAI fully converts the PbI2 NCs to perovskites, as is clear from the disappearance of the PbI2 absorption peak at 413 nm (see the dashed line for comparison Figure 3a). The powder XRD (PXRD) pattern of the 2D organo-lead halide perovskite NCs (R2(CH3NH3)n−1PbnI3n+1) with number of layers n = 1, 2, or 3 (and where we used R = butylammonium as the long-chain ligand) are shown in Figure 4. The diffractogram of the 2D perovskite NCs in the case of n = 1 and 2 was obtained on a powder of NCs, while the diffractogram of the 2D perovskite NCs with n = 3 was performed on a film of the material prepared by spin coating. More details about the film
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preparation for the XRD measurements are presented in the Supporting Information. The XRD data show all expected diffraction peaks of the corresponding orthorhombic structure for the three samples suggesting perfect layer periodicity along the out-of-plane direction (002), c-axis. The (002) peak shifts toward the low diffraction angle with increasing the number of layers. Using Bragg’s law, one can calculate the interplanar distance, i.e., d-spacing, at the (002) diffraction.[36] The d-spacing value was measured to be 13.8, 19.7, and 24.8 Å for our 2D perovskites n = 1, 2, and 3, in a good agreement with literature values of the corresponding bulk phase of the same materials.[37] Then lattice constant of the c-axis was evaluated to be 27.9, 39.4, and 49.6 Å, respectively, where c = 2d002 (Supporting information for more details). The PXRD pattern of the lead iodide NCs and corresponding 3D perovskite NCs (CH3NH3PbI3) are presented in Figure S6 (Supporting Information). The XRD shows the phase purity (tetragonal) of organo-lead iodide perovskite NCs with the exception of a small amount of residual MAI ligands, which is in very good agreement with the calculated XRD powder pattern for the tetragonal phase of lead halide perovskite.[6] Figure S7 (Supporting Information) shows high-resolution XPS spectra taken of the Pb and I regions of perovskite NC samples prepared by depositing a film through evaporating solvent from the NP solution. Several features can be assigned by analysis of the binding energy measured in these regions. Typically, one can distinguish the chemical environment of each
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Figure 4. PXRD measurements of 2D organo-lead iodide perovskite NCs (n = 1, 2, and 3). X-ray diffractogram of the 2D lead halide perovskite (n = 1 bottom line, n = 2 middle line, and n = 3 top line). The XRD results for 2D perovskite (n = 1 and 2) powder, which was deposited on a silicon substrate at room temperature from a concentrated colloidal NCs solution without any further treatment, while 2D perovskite (n = 3) was spin coated on a glass. Perovskite reflection peaks are assigned to the orthorhombic perovskite crystal lattice which in agreement with ref. (37). The plot displays the X-ray intensity as a function of twice the diffraction angle (2θ).
species as well as determine their stoichiometric ratios. XPS analysis shows a binding energy of symmetric core peak shapes of Pb 4f7/2 level at a binding energy of 138.48 eV; assigned for Pb(II), and I 3d5/2 peak at 619.12 eV (Figure S7, Supporting Information), which is in agreement with values of perovskite in literature.[38] Pb 4f7/2 binding energy of perovskite has a binding energy lower shift of 0.4 eV from the Pb 4f7/2 binding energy of PbI2 NCs (Figure S8, Supporting Information). Also, the absence of metallic Pb atoms in the measurements is obvious. In order to determine the stoichiometry and chemical environments present in the NCs, the peaks were
fitted according to a Gaussian–Lorentzian peak shape along with a "Smart" background subtraction. Quantifying the values of the area under the peaks, we calculated the ratio of Pb:I and found it to be 3:10 corresponding to 2D (n = 3) OL2(CH3NH3)2[ Pb3I10] perovskite (where OL = oleylamine) and molar ratio 2:7 in case of 2D (n = 2) OL2CH3NH3[Pb2I7] and these results are consistent in all samples measured. To characterize the photophysical properties and exciton dynamics of the perovskite NCs (diameter 5.5 nm, n = 3), we employed TA spectroscopy. We used an excitation and probe pulse with the same spectrum spanning from 525 to 720 nm and a temporal full-width at half maximum of ≈20 fs. Figure 5a displays TA spectra of the sample recorded at different population times. We observe two main features: a strong, positive feature peaked at ≈599 nm and a broad, negative feature extending toward shorter wavelengths. The peak position of the positive feature is consistent with the excitonic peak of 2D organo-lead iodide perovskites as shown in prior reports and is therefore assigned as the ground-state bleach (GSB) of the excitonic peak.[30,31,34] On the basis of previous studies, the broad, negative feature is assigned to the photoinduced absorption (PIA) of excitons/charge carriers.[4,39,40] To examine the time evolution of the photogenerated populations, we performed a global analysis of the transient data, Figure 5b,c. The global analysis (Figure 5b) reveals three decayassociated spectra with time constants (amplitudes) of 250 fs (22%), 69 ps (12%), and 5.63 ns (67%). The short- and longlived species are the dominant species. The spectral profile of the short- and long-lived species match those observed in previous reports[39,41] on perovskite films. The short-lived species exhibits a derivative feature slightly lower in energy than the excitonic peak position (i.e., 599 nm). It is likely to arise from electroabsorption and/or a charge-induced “Stark-effect.[39,42] The long-lived species decays with a time constant of 5.63 ns, which is generally consistent with the short decay component recovered from time-correlated single photon counting measurements on our NCs (Figure S9, Supporting Information) as well as with luminescence lifetime measurements on perovskite films.[41] We attribute the decay of this species to exciton recombination.
Figure 5. TA dynamics of perovskite NCs. a) TA spectra of perovskite NCs in toluene:chloroform mixture (2:1). The corresponding pump–probe time delays are indicated in the legend. b) Decay-associated spectra obtained from a global analysis of TA data. The spectra have been normalized to the most intense bleach feature. The inset displays the decay-associated spectra without normalization. c) Dynamics of GSB and PIA at 599 and 530 nm, respectively. The square’s and x’s represent the experimental data, while the lines represent the fits obtained from the global analysis.
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Experimental Section All procedures were carried on a Schlenk line under oxygen-free conditions and nitrogen flow. Materials: All chemicals were used as received without further purification. Lead (II) oxide (99.99%), tetrabutyl ammomium iodide (98.0%), tetrabutyl ammomium bromide (98.0%,), 1-octadecene (90%), oleic acid (99.0%), oleylamine (70%), HBr (48% in water), HI (57% in water), and methylamine solution (33% in absolute ethanol) were purchased from Sigma-Aldrich and used without further purification. All solvents such as chloroform and toluene were anhydrous and were purchased from Sigma-Aldrich.
mixture temperature was adjusted to 120–200 °C for the subsequent growth of the PbI2 semiconductor NCs. The reaction solution color changed, after the first few seconds, to a yellow-gold color indicating the formation of PbI2 NCs. The reaction was stopped after several minutes (1–30 min) by removing the heating mantle and injecting anhydrous toluene or chloroform. Synthesis of Methyl Ammonium Iodide: The precursors CH3NH3I (MAI) and CH3NH3Br (MABr) were synthesized from HI (45% in water) and HBr (40% in water), respectively, by reaction with methylamine solution (33% in absolute ethanol) according to previously reported procedures.[3,43] Typically hydroiodic acid (30 mL, 0.227 mol, 57 wt% in water) was added to methylamine (24 mL, 33% in absolute ethanol) in a 250 mL round-bottom flask. Absolute ethanol (50 mL) was added to the mixture and stirred, while the solution was maintained at 0 °C in an ice bath for 2 h under nitrogen atmosphere (caution: this is an exothermic reaction). The resulting solution was evaporated at 40 °C via rotary evaporation to half of the volume quantity. A white precipitate of MAI crystals was obtained by washing the solution three times with diethyl ether. The MAI was dissolved in ethanol, recrystallized from diethyl ether, and finally dried at 60 °C for 24 h and used without any further purification. The same method used for preparing alkyl (olyel, octyl, or butyl) ammonium iodide from the corresponding amine with HI (45% in water), but at room temperature. MABr were prepared following the same procedure except using HBr acid instead of HI acid in a quantity of (44 mL) of (HBr, 48% in water). Synthesis of Lead Halide Perovskite: A solution of lead bromide or lead iodide-based perovskite NCs was prepared by mixing the PbBr2 or PbI2 NCs and the synthesized powder of MABr or MAI, respectively, in toluene:chloroform mixture at 30–50 °C. Stirring this mixture immediately formed the perovskites. The solution of perovskite NCs was filtered twice using a PTFF syringe filter (Whatman, 0.2 µm) and then diluted in toluene, toluene:chloroform, or toluene:chlorobenzene mixture for optical measurements.
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We found no evidence that these materials degraded significantly under photoexcitation. Specifically, we measured the absorption spectra before and after the TA measurements and found very minor changes. In summary, we reported the preparation and characterization of methyl ammonium lead halide perovskite NCs seeded from high quality PbX2 NCs (X = I or Br). According to ligand choice and ratio, these NCs were prepared in different layered crystal structures denoted as R2(CH3NH)3n−1PbnI3n+1, n = 1, 2, 3, where R is the ligand (butyl, octyl, or oleylammonium) and the excitonic gap was thus tuned stepwise. Our femtosecond TA measurements indicate that the photophysical properties and excited-state dynamics of the perovskite NCs are very similar to those observed in perovskite films.[39] Sharp excitonic features make these NCs well suited for systematic investigations of the intrinsic photophysics and spectroscopy of organic–inorganic metal halide perovskites.
Characterization Synthesis PbI2 NCs 3–11 nm size: These were synthesized by mixing the halide and lead precursors in a three-neck round-bottom flask with magnetic stirrer attached to a condenser and connected to a Schlenk line. For temperature control and measurement, a heating mantle and thermocouple were used. Halide precursor preparation: TBAI or tetrabutylammomium bromide was dissolved in a mixture of oleylamine (OLA) and 1-octadecene (ODE) in a three-neck round-bottom flask (flask I) connected to Schlenk line. Typically for the iodide precursor: (0.735 g, 2 mmol) of TBAI was mixed with a mixture of OLA (5 or 7 mL) of and ODE (5 or 3 mL). The total volume of OLA and ODE was (10 mL). The mixture was put under vacuum (pumping) for 1.5 h at 150 °C to remove any traces of water and oxygen. The solution was subsequently heated at 200 °C for 1 h under N2 to reach a colorless or (pale yellow) solution. The heating mantle was removed and the solution then kept at 50 °C to avoid solidification. A similar recipe was used for the preparation of bromide precursor solutions. As reported by Zhijun Ning et al.,[24] heating the halide salts in OLA solution under N2 atmosphere leads to the formation of N-butyloctadec-9-en-1-aminium iodide and tributylamine and it is essential for dissolving the iodide salts in OLA solution. Lead precursor preparation: Lead oleate precursor was synthesized according the method used to synthesize Pb-based quantum dots.[23,24] In another flask (flask II), PbO (0.225 g, 1.00 mmol), oleic acid (1 mL, 3.00 mmol), and ODE (15 mL) were mixed. The mixture was held under vacuum at 100 °C for 1.5 h before the flask was allowed to return to 80 °C under nitrogen gas flow for another 1 h. At that point, the solution turned colorless indicating the complete formation of lead oleate complex. Lead iodide (PbI2) NCs synthesis: “Flask (II)” of lead oleate solution was then heated to around 160–200 °C before the content of “flask (I)” was swiftly injected into this hot solution; the temperature of the reaction
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TEM, HRTEM, and EDX: Size distributions and energy-dispersive X-ray spectra of the PbI2 and lead halide perovskite NCs were determined by TEM using a JEOL JEM-2010 TEM with a LaB6 filament operating at 200 kV for both low-magnification (TEM) and HRTEM images. UV–vis absorption and PL: These spectra were recorded using a Varian Cary 100 and Varian Eclipse fluorescence spectrometer, respectively, in a 1 cm cuvette. PXRD: PXRD patterns for phase purity were recorded on a Rigaku MiniFlex Benchtop X-ray diffractometer (equipped with Cu Kα X-ray tube) operating at 600 watts (tube voltage 40 kV and 15 mA) with a time per step of 3 s. Samples were rotated during data collection. All the samples were prepared by drying nanoparticle solutions. XPS: XPS data were acquired on a ThermoFisher Scientific Kα spectrometer with a monochromatic Al Kα X-ray radiation source generating X-ray photons of 1486.7 eV in energy. This was performed in an ultrahigh-vacuum chamber with base pressure of 10−9 Torr. The samples for XPS analysis were prepared by drop-casting the samples onto Si(100). Both survey and regional spectra were acquired from the samples. All data analyzes were carried out using the Avantage software (provided from the vendor of the instrument) fitting program to confirm the incorporation of Pb and I in the PbI2 and lead halide perovskite NCs. We calculated the elemental composition and mathematically modeled the experimental data to deconvolute the peak shapes and investigate the chemical state(s) present. In both cases, a smart background function was used to approximate the experimental backgrounds. Surface elemental compositions were calculated from backgroundsubtracted peak areas derived from transmission function corrected regional spectra. Scofield Al Kα sensitivity factors were used to calculate the relative atomic percentages. The peak shapes were deconvoluted utilizing Lorentzian–Gaussian peak shapes. The binding energies were referenced to the National Institute of Standards XPS database.
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www.MaterialsViews.com TA experiment: The experimental setup has been described in detail previously.[44] Briefly, 800 nm, 100 fs laser light generated at a repetition rate of 5 kHz by a Spectra-Physics Spitfire Pro laser amplifier was used as the light source in the experiment. The output was directed into a home-built nonlinear optical parametric amplifier to convert the light to the visible with a spectral range from 525 to 720 nm for pump–probe measurements. Pulse compression was achieved with a combination of a grating compressor and a prism compressor. The pulse was compressed to less than 20 fs (estimated by polarization-gated frequency-resolved optical gating).[45] The compressed broadband pulse was split into two beams—pump and probe beam—and the time delay between them was controlled by a motorized translation stage. The pump pulse was chopped at a frequency of 625 Hz. Thus the pump–probe signal captured by the camera was averaged over the signals generated by four continuous pulses. The pump and probe beams were crossed at the sample position to generate the signal. The polarizations of the pump and the probe beams were at the magic angle. The pump–probe signal was detected by a Newton camera (Andor DU971N-FI Newton). The signal was balanced by a photodiode recording the intensity fluctuations of the probe to compensate for laser intensity fluctuations. Global target analysis was performed through Glotaran.[46]
Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements The Natural Science and Engineering Research Council of Canada (NSERC) is gratefully acknowledged for financial support. The authors thank D. Seferos for generous use of the fluorometer. The authors are grateful to M. Grätzel, T. Berkelbach, and N. Soheilnia for helpful discussions. The authors are grateful to P. Brodersen for carrying out the XPS experiments, N. Coombs for help with electron microscopy, and A. Danaei for carrying out the PXRD measurements. Y.H. had the idea for, conceived and directed the project, designed the experiments, carried out the synthesis of materials, and performed data analysis. Y.S. and R.D.P. performed the transient absorption measurements and data analysis. A.I.A. and Y.H. carried out the electron microscope measurements. Y.K. contributed to the morphology analysis of the materials. Y.H., R.D.P., Y.S., and G.D.S. wrote the manuscript. All authors discussed the results and commented on the manuscript. G.D.S. and M.A.W. supervised the project. Received: July 17, 2015 Revised: October 2, 2015 Published online: November 24, 2015
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Structure-Tuned Lead Halide Perovskite Nanocrystals Yasser Hassan, Yin Song, Ryan D. Pensack, Ahmed I. Abdelrahman, Yoichi Kobayashi, Mitchell A. Winnik, and Gregory D. Scholes* Since pioneering work in 2009[1] incredible advances have been made in the development of organic–inorganic metal halide perovskite materials for thin film solar cells; devices have been reported with efficiencies exceeding 20%.[2,3] These CH3NH3PbX3 (X = Br, Cl, and I) materials have a broad absorption profile across the visible spectral region and they exhibit intense and narrow-band luminescence. The excitonic properties and long diffusion length of charge carriers[4–6] explain the interplay of efficient energy and charge transport underpinning the efficient photovoltaic properties. However, the nature and dynamics of the excited states that ultimately determine the performance of optoelectronic devices are still being elucidated. Here, we report the preparation and characterization of a colloidally stable suspension of methyl ammonium lead halide perovskite nanocrystals (NCs). The perovskite NCs are seeded from high quality PbX2 NCs (X = I or Br) with a pretargeted size. Subsequent reaction with methyl ammonium iodide (MAI) yields perovskite NCs. According to ligand choice and ratio, these NCs are prepared with different layered multiple quantum well like NC structures, denoted R2(CH3NH3)n−1PbnI3n+1; n = 1, 2, 3 where R is the ligand. The excitonic gap is tuned stepwise; for example, the photoluminescence (PL) peak for 5.5 nm diameter colloids is 505, 565, and
Y. Hassan, Y. Song, Prof. M. A. Winnik, Prof. G. D. Scholes Department of Chemistry University of Toronto 80 St. George Street, Toronto, Ontario M5S 3H6, Canada E-mail: [email protected] Y. Hassan, Prof. M. A. Winnik Department of Chemical Engineering and Applied Chemistry University of Toronto 200 College Street, Toronto M5S 3E5, Canada Y. Hassan Chemistry Department Faculty of Science Zagazig University 44511 Zagazig, Egypt Dr. R. D. Pensack, Prof. G. D. Scholes Department of Chemistry Princeton University NJ 08544, USA Dr. A. I. Abdelrahman SABIC Corporate Research and Innovation Center at KAUST Thuwal 23955, Saudi Arabia Prof. Y. Kobayashi Department of Chemistry School of Science and Engineering Aoyama Gakuin University 5-10-1 Fuchinobe, Chuo-ku, Sagamihara, Kanagawa 252-5258, Japan
DOI: 10.1002/adma.201503461
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600 nm, respectively. The sharp excitonic features make these NCs well suited for systematic investigations of the intrinsic photophysics and spectroscopy of organic–inorganic metal halide perovskites. To date, limited syntheses of CH3NH3PbX3 perovskite materials on the nanoscale have been reported.[7–9] Burschka et al. used a sequential deposition method to infiltrate the perovskite pigment within the pores of porous metal oxide film.[8] This method enabled control over perovskite morphologies with particle sizes that ranged from 50 to 200 nm and led to reproducible, efficient photovoltaic devices.[8] Another recently reported approach utilized a long chain ligand-assisted reprecipitation technique to produce colloidal perovskite material.[9] The quest to prepare perovskite NCs and control their size motivated us to investigate a synthetic approach inspired by colloidal semiconductor quantum dot synthesis.[10,11] The basis of our synthesis is the preparation of colloidal PbI2 NCs. Since the original report of Sandroff et al. on the quantization effects in PbI2 small clusters[12] in films and solution, these NCs have been prepared by various methods that include Langmuir–Blodgett techniques,[13] zeolite cages,[14] silica films,[15] copolymer films,[16] colloidal solutions,[17,18] vapor deposition,[19,20] reverse micelles,[21] or surfactant-assisted hydrothermal routes.[22] Control of the size of colloidally stable PbI2 NCs remains a challenge. In this work we address this issue. The colloidal perovskite NCs reported in this work were synthesized in a two-step process. In the first step, we synthesize nearly monodisperse PbI2 (or PbBr2) NCs using a method inspired by lead chalcogenide semiconductor NCs.[10,23] Subsequently, we react the PbI2 NCs with an alkyl ammonium iodide to produce the corresponding perovskite NCs. A lead oleate precursor reacts with an iodide–organic complex[24] in a noncoordinating solvent such as octadecene (ODE). For instance, to prepare PbI2 NCs, tetrabutylammonium iodide (TBAI, 4 mmol) was dissolved in oleylamine (7 mL) at 200 °C under an inert atmosphere to form the iodide–amine complex. The resulting solution was cooled to 50 °C before it was injected into a hot solution of the lead–oleate complex (2 mmol) in ODE. The solution changed to golden-yellow color after a few seconds, indicating the formation of PbI2 NCs. In this ionic metathesis reaction, nucleation occurs and growth begins over the course of a few seconds.[25] The temperature of the ODE solution (160–200 °C) and the reaction time can be used to control particle size. The reaction was halted after several minutes by removing the heat source and injecting anhydrous toluene into the reaction mixture. This synthetic procedure produces homogeneous and crystalline lead halide (PbX2) NCs with mean sizes in the range of 3–11 nm. Figure 1 presents transmission electron microscopy (TEM), high-resolution TEM (HRTEM), and energy-dispersive X-ray
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COMMUNICATION Figure 1. Structural, optical, and chemical characterization of lead iodide NCs. a–c) TEM images of PbI2 NCs prepared at 200 °C at a growth interval time of a) 5, b) 15, and c) 30 min exhibiting a diameter of 3.4–4.2, 4.7–5.8, and 9.5–10 nm, respectively. d) HRTEM of the PbI2 NCs prepared in this work, that shows a lattice parameter of 0.34 nm. e) Absorption spectra of colloidal PbI2 NCs in toluene solution where the NC size has been varied from 2 to 6 nm diameter and clearly exhibiting a quantum confinement effect. f) EDX spectra obtained from 6 nm sized NCs of PbI2. The Cu signals arise from the TEM grid.
spectroscopy (EDX) measurements of a typical sample of PbI2 NCs prepared at 200 °C. The TEM images of this PbI2 NC sample, without any size-selection treatment, reveal an abundance of uniform spherical NCs. By controlling the reaction temperature (120–200 °C) and growth time, particles with a narrow size distribution can be obtained with diameters (d) in the range of 3–10 nm. A quantitative analysis of the TEM images reveals that PbI2 NCs prepared at 200 °C at time intervals of 5, 15, and 30 min are characterized by a diameter range of 3.4–4.2, 4.7–5.8, and 9.5–10 nm, respectively (Figure 1a–c). Figure S2 (Supporting Information) shows TEM images of PbI2 NCs prepared at 160 °C. HRTEM images (Figure 1d) exhibit lattice fringes throughout the entire particle. The lattice fringes of the PbI2 NCs are spaced by 0.344 nm, corresponding to the (101) lattice. HRTEM (Figure 1d) and with the selected area electron diffraction pattern (SAED), Figure S3 (Supporting Information), show the high crystallinity of the PbI2 NCs, assigned to the hexagonal PbI2 phase.[26] TEM diffraction patterns (SAED) were analyzed with a diffraction ring profiler, which was developed for phase identification in complex microstructures.[27] The SAED pattern of the selected area of collection of PbI2 NC particles exhibits diffuse rings that are typical of nanometer-sized particles. The patterns index to the (101), (100), (102), (003), (111), (004), and (104) reflections of the hexagonal structure of PbI2 (PDF card no. 43–1484).[26] More details about simulated SAED are provided in Figure S4 (Supporting Information). In order to investigate the effect of particle diameter on the optical properties of the PbI2 NCs and the corresponding perovskites, we prepared two different sizes of PbI2 NCs (d = 2 and 6 nm, Figure 1e). Figure 1e shows absorption spectra recorded
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at room temperature for aliquots of PbI2 NCs dispersed in toluene, removed from the reaction mixture at 200 °C after reaction for 5 and 15 min of growth. We found that with decreasing the size of the PbI2 NCs from 6 to 2 nm, the exciton energy blueshifts by 0.22 eV (λmax from 413 to 385 nm). This quantum confinement effect (the bulk PbI2 bulk semiconductor has a bandgap of 2.57 eV) is thought to be mainly a 2D exciton quantum confinement along the crystal c axis of the layered hexagonal structure.[17,19] EDX was utilized to determine the elemental composition of our PbI2 NCs. An example is presented in Figure 1f, showing clear evidence for the presence of both Pb and I. The elemental analysis from EDX is in agreement with the atomic ratio calculated from X-ray photoelectron spectroscopy (XPS) measurements (Figure S7, Supporting Information). In order to prepare the lead iodide perovskite NCs, the PbI2 NCs suspended in toluene were diluted with chloroform to a volume ratio of 2:1 (toluene:chloroform). An alkyl ammonium halide, methyl ammonium halide (MAI) alone, or a MAI mixture with a long-chain alkyl ammonium iodide was added at 50 °C over 30 s. It is known that the PbI2 single crystal consists of strongly bonded I Pb I layers, and the interlayer bonding is through van der Waals forces.[28] The conversion of lead halide NCs to the corresponding perovskite takes place by intercalation of the organic MAI moiety between the Pb I Pb layers of the crystalline PbI2 host[29] forming 2D organo-lead halide perovskites with the structure of R2(CH3NH3)n−1MnX3n+1,[30,31] where (R = long-chain amine, M = Pb, X = halide ion, and n = 1, 2, 3). To obtain lamellar 2D lead iodide perovskites R2(CH3NH3)n−1MnX3n+1[30,32] with different n values, we tuned the molar ratio between the long-chain organic ammonium
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iodide (e.g., oleylammonium iodide) and MAI. By adding either oleylammonium ioidide alone, or MAI and oleylammonium iodide mixtures with a molar ratio of (2:1), or with a molar ratio of (1:1), we found the solution immediately changed color to green, orange, or red, respectively, indicating the formation of 2D organo-lead halide perovskites with number of layers n = 1, 2, or 3. It was found that a small quantity of chloroform added to toluene enhances the reaction between the MAI and the lead halides (I or Br), while the addition of any amount of methanol, acetonitrile, or acetone degrades the perovskite (as evidenced by diminished perovskite absorption). Figure 2a–c shows the TEM images of the colloidal 2D organo-lead iodide perovskite (n = 3), (C18H35NH3)2(CH3NH3)2Pb3I10, prepared from three different samples of PbI2 NCs seeds with particles size of 3.5, 6, and 10 nm, respectively. On the nanoscale, the 2D perovskite naturally self-assembles into a stack of lamellae consisting of alternate layers of organic and inorganic components. The inorganic layers comprise semiconducting corner sharing octahedral PbX64− sheets (wells) stacked in the direction of the c-axis, sandwiched between bilayers of low dielectric constant organic barriers consisting of alkyl ammonium chains.[30,31] These self-assembled structures are similar to inorganic multiple quantum wells, characterized by very stable exciton transport in the inorganic layers with enhanced exciton binding energy that can reach four times larger than in the case of 3D perovskite.[31,33]
In Figure 2d, we show the room temperature absorption and PL spectra corresponding to the three 2D, n = 3 samples presented in Figure 2a–c. We found that the colloidal suspension of (C18H35NH3)2(CH3NH3)2Pb3I10 NCs exhibits a quantum confinement due to the particle size compared to the film absorption value reported by Wu et al.[30,34] In the case of our NC suspension, the first exciton λmax is 585, 592, and 599 nm for the sizes of 3.5, 5.5, and 10 nm, respectively. This compares to 608 nm in the case of the film.[30,34] As shown in Figure 3d, the change in particle size has marginal quantum confinement effect as compared to the effect of the 2D confinement. Perovskite material presented in this study ranged between 3.5 and 11 nm which is bigger than the Bohr radius of perovskite, i.e., 2.1 nm.[31] Figure 3 shows the versatility of our method to prepare perovskite NCs with different 2D structures, where n = 1, 2, or 3. To prepare 2D perovskite with one layer (n = 1), we used either butylammonium iodide, octylammonium iodide, or oleylammonium iodide, while these long-chain amines mixed with MAI in a molar ratio of R:MAI = 2:1 and 1:1 yielded n = 2 and 3, respectively. For these 5.5 nm diameter particles, the absorption peaks are 505, 565, and 593–600 nm, respectively for n = 1, 2, and 3. The room temperature absorption and PL spectra of 2D lead iodide perovskite (n = 3), (C18H35NH3)2(CH3NH3)2Pb3I10 (Figures 2d and 3c) exhibit a strong “band-edge exciton” absorption and well-resolved higher energy band-to-band transitions that is indicative of a narrow particle size distribution. The PL
Figure 2. Structural and optical properties of 2D organo-lead iodide perovskite NCs. a–c) TEM images of the 2D organo-lead iodide perovskite (n = 3) NCs prepared from PbI2 NCs that exhibit a diameter of a) 3.5, b) 6, and c) 10 nm, respectively. d) Normalized absorption and PL spectra of 2D lead iodide perovskite (n = 3) NCs in a toluene/chloroform mixture as prepared from PbI2 NCs of three different diameters: 3 (black line), 5 (light gray line), and 10 nm (dark gray line).
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COMMUNICATION Figure 3. 2D layered perovskite NCs. a) Schematic structures of the colloidal 2D organo-lead iodide perovskites NCs (C18H35NH3)2(CH3NH3)n−1[PbnI3n+1] synthesized in the present study, where (from left to right) n = 1, 2, and 3. b) Photograph of colloidal 2D organo-lead iodide perovskite NC solutions of (from left to right) n = 1, 2, and 3, respectively, under ambient light. c) Absorption spectrum of colloidal PbI2 NCs in toluene with a diameter of 4–5 nm compared to the absorption and emission spectra of the resultant 2D lead iodide perovskite (n = 3) colloidal NC solution in toluene/chloroform mixture. All spectra have been normalized to the lowest energy optical transition. The 2D lead iodide perovskite (n = 3) NC absorption exhibits an onset at 593 nm. Energy bandgap estimated from the intersection point of the normalized curve was 2.09 eV. The steady-state PL spectrum (dotted red) consists of a single peak slightly redshifted from the excitonic peak with a maximum at 607 nm. The PL was measured with an excitation wavelength of 480 nm. Absorption spectra of the three 2D organo-lead iodide perovskite samples (blue, green, and red for n = 1, 2, and 3, respectively), and steady-state PL that peaks at 520, 580, and 610 nm (dotted blue, green, and red lines for n = 1, 2, and 3, respectively). The PL was measured with an excitation wavelength of 480 nm. Absorbance is plotted as optical density normalized to the exciton peak.
spectra are symmetric and narrow (full-width at half maximum of 0.12 eV) and the quantum yield of the perovskite NCs in toluene was measured to be ≈20%, estimated by comparison with the dye cresyl violet.[35] In the case of n = 1 NCs, we find that increasing the alkyl chain length from R = butyl to oleylammonium has an effect on the first exciton absorption peak energy (Figure S5a, Supporting Information). In addition, increasing the volume fraction of chloroform in the sample changes the peak position, which might indicate a modification of the dielectric confinement effect[31] (Figure S5b, Supporting Information). The conversion of lead halide NCs to the corresponding perovskite NCs was confirmed by investigating their optical properties along with a structural analysis by electron microscopy, X-ray diffraction (XRD) and X-ray photoelectron spectroscopy. For instance, treating the PbI2 NCs with MAI fully converts the PbI2 NCs to perovskites, as is clear from the disappearance of the PbI2 absorption peak at 413 nm (see the dashed line for comparison Figure 3a). The powder XRD (PXRD) pattern of the 2D organo-lead halide perovskite NCs (R2(CH3NH3)n−1PbnI3n+1) with number of layers n = 1, 2, or 3 (and where we used R = butylammonium as the long-chain ligand) are shown in Figure 4. The diffractogram of the 2D perovskite NCs in the case of n = 1 and 2 was obtained on a powder of NCs, while the diffractogram of the 2D perovskite NCs with n = 3 was performed on a film of the material prepared by spin coating. More details about the film
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preparation for the XRD measurements are presented in the Supporting Information. The XRD data show all expected diffraction peaks of the corresponding orthorhombic structure for the three samples suggesting perfect layer periodicity along the out-of-plane direction (002), c-axis. The (002) peak shifts toward the low diffraction angle with increasing the number of layers. Using Bragg’s law, one can calculate the interplanar distance, i.e., d-spacing, at the (002) diffraction.[36] The d-spacing value was measured to be 13.8, 19.7, and 24.8 Å for our 2D perovskites n = 1, 2, and 3, in a good agreement with literature values of the corresponding bulk phase of the same materials.[37] Then lattice constant of the c-axis was evaluated to be 27.9, 39.4, and 49.6 Å, respectively, where c = 2d002 (Supporting information for more details). The PXRD pattern of the lead iodide NCs and corresponding 3D perovskite NCs (CH3NH3PbI3) are presented in Figure S6 (Supporting Information). The XRD shows the phase purity (tetragonal) of organo-lead iodide perovskite NCs with the exception of a small amount of residual MAI ligands, which is in very good agreement with the calculated XRD powder pattern for the tetragonal phase of lead halide perovskite.[6] Figure S7 (Supporting Information) shows high-resolution XPS spectra taken of the Pb and I regions of perovskite NC samples prepared by depositing a film through evaporating solvent from the NP solution. Several features can be assigned by analysis of the binding energy measured in these regions. Typically, one can distinguish the chemical environment of each
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Figure 4. PXRD measurements of 2D organo-lead iodide perovskite NCs (n = 1, 2, and 3). X-ray diffractogram of the 2D lead halide perovskite (n = 1 bottom line, n = 2 middle line, and n = 3 top line). The XRD results for 2D perovskite (n = 1 and 2) powder, which was deposited on a silicon substrate at room temperature from a concentrated colloidal NCs solution without any further treatment, while 2D perovskite (n = 3) was spin coated on a glass. Perovskite reflection peaks are assigned to the orthorhombic perovskite crystal lattice which in agreement with ref. (37). The plot displays the X-ray intensity as a function of twice the diffraction angle (2θ).
species as well as determine their stoichiometric ratios. XPS analysis shows a binding energy of symmetric core peak shapes of Pb 4f7/2 level at a binding energy of 138.48 eV; assigned for Pb(II), and I 3d5/2 peak at 619.12 eV (Figure S7, Supporting Information), which is in agreement with values of perovskite in literature.[38] Pb 4f7/2 binding energy of perovskite has a binding energy lower shift of 0.4 eV from the Pb 4f7/2 binding energy of PbI2 NCs (Figure S8, Supporting Information). Also, the absence of metallic Pb atoms in the measurements is obvious. In order to determine the stoichiometry and chemical environments present in the NCs, the peaks were
fitted according to a Gaussian–Lorentzian peak shape along with a "Smart" background subtraction. Quantifying the values of the area under the peaks, we calculated the ratio of Pb:I and found it to be 3:10 corresponding to 2D (n = 3) OL2(CH3NH3)2[ Pb3I10] perovskite (where OL = oleylamine) and molar ratio 2:7 in case of 2D (n = 2) OL2CH3NH3[Pb2I7] and these results are consistent in all samples measured. To characterize the photophysical properties and exciton dynamics of the perovskite NCs (diameter 5.5 nm, n = 3), we employed TA spectroscopy. We used an excitation and probe pulse with the same spectrum spanning from 525 to 720 nm and a temporal full-width at half maximum of ≈20 fs. Figure 5a displays TA spectra of the sample recorded at different population times. We observe two main features: a strong, positive feature peaked at ≈599 nm and a broad, negative feature extending toward shorter wavelengths. The peak position of the positive feature is consistent with the excitonic peak of 2D organo-lead iodide perovskites as shown in prior reports and is therefore assigned as the ground-state bleach (GSB) of the excitonic peak.[30,31,34] On the basis of previous studies, the broad, negative feature is assigned to the photoinduced absorption (PIA) of excitons/charge carriers.[4,39,40] To examine the time evolution of the photogenerated populations, we performed a global analysis of the transient data, Figure 5b,c. The global analysis (Figure 5b) reveals three decayassociated spectra with time constants (amplitudes) of 250 fs (22%), 69 ps (12%), and 5.63 ns (67%). The short- and longlived species are the dominant species. The spectral profile of the short- and long-lived species match those observed in previous reports[39,41] on perovskite films. The short-lived species exhibits a derivative feature slightly lower in energy than the excitonic peak position (i.e., 599 nm). It is likely to arise from electroabsorption and/or a charge-induced “Stark-effect.[39,42] The long-lived species decays with a time constant of 5.63 ns, which is generally consistent with the short decay component recovered from time-correlated single photon counting measurements on our NCs (Figure S9, Supporting Information) as well as with luminescence lifetime measurements on perovskite films.[41] We attribute the decay of this species to exciton recombination.
Figure 5. TA dynamics of perovskite NCs. a) TA spectra of perovskite NCs in toluene:chloroform mixture (2:1). The corresponding pump–probe time delays are indicated in the legend. b) Decay-associated spectra obtained from a global analysis of TA data. The spectra have been normalized to the most intense bleach feature. The inset displays the decay-associated spectra without normalization. c) Dynamics of GSB and PIA at 599 and 530 nm, respectively. The square’s and x’s represent the experimental data, while the lines represent the fits obtained from the global analysis.
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Experimental Section All procedures were carried on a Schlenk line under oxygen-free conditions and nitrogen flow. Materials: All chemicals were used as received without further purification. Lead (II) oxide (99.99%), tetrabutyl ammomium iodide (98.0%), tetrabutyl ammomium bromide (98.0%,), 1-octadecene (90%), oleic acid (99.0%), oleylamine (70%), HBr (48% in water), HI (57% in water), and methylamine solution (33% in absolute ethanol) were purchased from Sigma-Aldrich and used without further purification. All solvents such as chloroform and toluene were anhydrous and were purchased from Sigma-Aldrich.
mixture temperature was adjusted to 120–200 °C for the subsequent growth of the PbI2 semiconductor NCs. The reaction solution color changed, after the first few seconds, to a yellow-gold color indicating the formation of PbI2 NCs. The reaction was stopped after several minutes (1–30 min) by removing the heating mantle and injecting anhydrous toluene or chloroform. Synthesis of Methyl Ammonium Iodide: The precursors CH3NH3I (MAI) and CH3NH3Br (MABr) were synthesized from HI (45% in water) and HBr (40% in water), respectively, by reaction with methylamine solution (33% in absolute ethanol) according to previously reported procedures.[3,43] Typically hydroiodic acid (30 mL, 0.227 mol, 57 wt% in water) was added to methylamine (24 mL, 33% in absolute ethanol) in a 250 mL round-bottom flask. Absolute ethanol (50 mL) was added to the mixture and stirred, while the solution was maintained at 0 °C in an ice bath for 2 h under nitrogen atmosphere (caution: this is an exothermic reaction). The resulting solution was evaporated at 40 °C via rotary evaporation to half of the volume quantity. A white precipitate of MAI crystals was obtained by washing the solution three times with diethyl ether. The MAI was dissolved in ethanol, recrystallized from diethyl ether, and finally dried at 60 °C for 24 h and used without any further purification. The same method used for preparing alkyl (olyel, octyl, or butyl) ammonium iodide from the corresponding amine with HI (45% in water), but at room temperature. MABr were prepared following the same procedure except using HBr acid instead of HI acid in a quantity of (44 mL) of (HBr, 48% in water). Synthesis of Lead Halide Perovskite: A solution of lead bromide or lead iodide-based perovskite NCs was prepared by mixing the PbBr2 or PbI2 NCs and the synthesized powder of MABr or MAI, respectively, in toluene:chloroform mixture at 30–50 °C. Stirring this mixture immediately formed the perovskites. The solution of perovskite NCs was filtered twice using a PTFF syringe filter (Whatman, 0.2 µm) and then diluted in toluene, toluene:chloroform, or toluene:chlorobenzene mixture for optical measurements.
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We found no evidence that these materials degraded significantly under photoexcitation. Specifically, we measured the absorption spectra before and after the TA measurements and found very minor changes. In summary, we reported the preparation and characterization of methyl ammonium lead halide perovskite NCs seeded from high quality PbX2 NCs (X = I or Br). According to ligand choice and ratio, these NCs were prepared in different layered crystal structures denoted as R2(CH3NH)3n−1PbnI3n+1, n = 1, 2, 3, where R is the ligand (butyl, octyl, or oleylammonium) and the excitonic gap was thus tuned stepwise. Our femtosecond TA measurements indicate that the photophysical properties and excited-state dynamics of the perovskite NCs are very similar to those observed in perovskite films.[39] Sharp excitonic features make these NCs well suited for systematic investigations of the intrinsic photophysics and spectroscopy of organic–inorganic metal halide perovskites.
Characterization Synthesis PbI2 NCs 3–11 nm size: These were synthesized by mixing the halide and lead precursors in a three-neck round-bottom flask with magnetic stirrer attached to a condenser and connected to a Schlenk line. For temperature control and measurement, a heating mantle and thermocouple were used. Halide precursor preparation: TBAI or tetrabutylammomium bromide was dissolved in a mixture of oleylamine (OLA) and 1-octadecene (ODE) in a three-neck round-bottom flask (flask I) connected to Schlenk line. Typically for the iodide precursor: (0.735 g, 2 mmol) of TBAI was mixed with a mixture of OLA (5 or 7 mL) of and ODE (5 or 3 mL). The total volume of OLA and ODE was (10 mL). The mixture was put under vacuum (pumping) for 1.5 h at 150 °C to remove any traces of water and oxygen. The solution was subsequently heated at 200 °C for 1 h under N2 to reach a colorless or (pale yellow) solution. The heating mantle was removed and the solution then kept at 50 °C to avoid solidification. A similar recipe was used for the preparation of bromide precursor solutions. As reported by Zhijun Ning et al.,[24] heating the halide salts in OLA solution under N2 atmosphere leads to the formation of N-butyloctadec-9-en-1-aminium iodide and tributylamine and it is essential for dissolving the iodide salts in OLA solution. Lead precursor preparation: Lead oleate precursor was synthesized according the method used to synthesize Pb-based quantum dots.[23,24] In another flask (flask II), PbO (0.225 g, 1.00 mmol), oleic acid (1 mL, 3.00 mmol), and ODE (15 mL) were mixed. The mixture was held under vacuum at 100 °C for 1.5 h before the flask was allowed to return to 80 °C under nitrogen gas flow for another 1 h. At that point, the solution turned colorless indicating the complete formation of lead oleate complex. Lead iodide (PbI2) NCs synthesis: “Flask (II)” of lead oleate solution was then heated to around 160–200 °C before the content of “flask (I)” was swiftly injected into this hot solution; the temperature of the reaction
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TEM, HRTEM, and EDX: Size distributions and energy-dispersive X-ray spectra of the PbI2 and lead halide perovskite NCs were determined by TEM using a JEOL JEM-2010 TEM with a LaB6 filament operating at 200 kV for both low-magnification (TEM) and HRTEM images. UV–vis absorption and PL: These spectra were recorded using a Varian Cary 100 and Varian Eclipse fluorescence spectrometer, respectively, in a 1 cm cuvette. PXRD: PXRD patterns for phase purity were recorded on a Rigaku MiniFlex Benchtop X-ray diffractometer (equipped with Cu Kα X-ray tube) operating at 600 watts (tube voltage 40 kV and 15 mA) with a time per step of 3 s. Samples were rotated during data collection. All the samples were prepared by drying nanoparticle solutions. XPS: XPS data were acquired on a ThermoFisher Scientific Kα spectrometer with a monochromatic Al Kα X-ray radiation source generating X-ray photons of 1486.7 eV in energy. This was performed in an ultrahigh-vacuum chamber with base pressure of 10−9 Torr. The samples for XPS analysis were prepared by drop-casting the samples onto Si(100). Both survey and regional spectra were acquired from the samples. All data analyzes were carried out using the Avantage software (provided from the vendor of the instrument) fitting program to confirm the incorporation of Pb and I in the PbI2 and lead halide perovskite NCs. We calculated the elemental composition and mathematically modeled the experimental data to deconvolute the peak shapes and investigate the chemical state(s) present. In both cases, a smart background function was used to approximate the experimental backgrounds. Surface elemental compositions were calculated from backgroundsubtracted peak areas derived from transmission function corrected regional spectra. Scofield Al Kα sensitivity factors were used to calculate the relative atomic percentages. The peak shapes were deconvoluted utilizing Lorentzian–Gaussian peak shapes. The binding energies were referenced to the National Institute of Standards XPS database.
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www.MaterialsViews.com TA experiment: The experimental setup has been described in detail previously.[44] Briefly, 800 nm, 100 fs laser light generated at a repetition rate of 5 kHz by a Spectra-Physics Spitfire Pro laser amplifier was used as the light source in the experiment. The output was directed into a home-built nonlinear optical parametric amplifier to convert the light to the visible with a spectral range from 525 to 720 nm for pump–probe measurements. Pulse compression was achieved with a combination of a grating compressor and a prism compressor. The pulse was compressed to less than 20 fs (estimated by polarization-gated frequency-resolved optical gating).[45] The compressed broadband pulse was split into two beams—pump and probe beam—and the time delay between them was controlled by a motorized translation stage. The pump pulse was chopped at a frequency of 625 Hz. Thus the pump–probe signal captured by the camera was averaged over the signals generated by four continuous pulses. The pump and probe beams were crossed at the sample position to generate the signal. The polarizations of the pump and the probe beams were at the magic angle. The pump–probe signal was detected by a Newton camera (Andor DU971N-FI Newton). The signal was balanced by a photodiode recording the intensity fluctuations of the probe to compensate for laser intensity fluctuations. Global target analysis was performed through Glotaran.[46]
Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements The Natural Science and Engineering Research Council of Canada (NSERC) is gratefully acknowledged for financial support. The authors thank D. Seferos for generous use of the fluorometer. The authors are grateful to M. Grätzel, T. Berkelbach, and N. Soheilnia for helpful discussions. The authors are grateful to P. Brodersen for carrying out the XPS experiments, N. Coombs for help with electron microscopy, and A. Danaei for carrying out the PXRD measurements. Y.H. had the idea for, conceived and directed the project, designed the experiments, carried out the synthesis of materials, and performed data analysis. Y.S. and R.D.P. performed the transient absorption measurements and data analysis. A.I.A. and Y.H. carried out the electron microscope measurements. Y.K. contributed to the morphology analysis of the materials. Y.H., R.D.P., Y.S., and G.D.S. wrote the manuscript. All authors discussed the results and commented on the manuscript. G.D.S. and M.A.W. supervised the project. Received: July 17, 2015 Revised: October 2, 2015 Published online: November 24, 2015
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