Article pubs.acs.org/cm
Cite This: Chem. Mater. 2018, 30, 2973−2982
Shape‑, Size‑, and Composition-Controlled Thallium Lead Halide Perovskite Nanowires and Nanocrystals with Tunable Band Gaps Parth Vashishtha,† Dani Z. Metin,‡ Matthew E. Cryer,† Kai Chen,† Justin M. Hodgkiss,† Nicola Gaston,‡ and Jonathan E. Halpert*,†,§
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MacDiarmid Institute for Advanced Materials and Nanotechnology, and School of Chemical and Physical Sciences, Victoria University of Wellington, P.O. Box 600, Wellington, New Zealand ‡ MacDiarmid Institute for Advanced Materials and Nanotechnology, and Department of Physics, University of Auckland, Private Bag 92019, Auckland, New Zealand § Department of Chemistry, Hong Kong University of Science and Technology (HKUST), Kowloon, Hong Kong S Supporting Information *
ABSTRACT: Perovskite nanocrystals have shown themselves to be useful for both absorption- and emission-based applications, including solar cells, photodetectors, and LEDs. Here we present a new class of size-, composition-, and shapetunable nanocrystals made from Tl3PbX5 (X= Cl, Br, I). These can be synthesized via colloidal methods to produce faceted spheroidal nanocrystals, and perovskite TlPbI3 nanowires. Crystal structures for the orthorhombic and tetragonal phase materials, for both pure and mixed halide species, are compared to the literature and also calculated from first-principles in VASP. We show the ability to tune the band gap by halide substitution to create materials that can absorb strongly between 250 and 450 nm. In addition, we show evidence of the confinement effect in pure halide Tl3PbBr5 nanocrystals suggesting size-tuning is possible as well. By tuning the band gap we can create materials with specific absorption spectra suitable for photodetection that display conduction and photoresponse properties similar to previously observed perovskite nanocrystals. We also observe weak emission consistent with indirect bandgap materials. Finally, we are able to demonstrate shape control in these materials, to give some insight into observable phase changes with varying reaction conditions, and to demonstrate the utility of the TlPbI3 perovskite nanowires as wide-band-gap photoconductors. These novel perovskite nanocrystalline materials can be expected to find applications in photodetectors, X-ray detectors, and piezoelectrics.
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groups are working to replace the A, B, and X groups in the ABX3 structure with other feasible atoms.17,18,22 One such candidate for replacing methylammonium as the A group, in the ABX3 perovskite crystal structure, is thallium. Thallium is an abundant monovalent cation with a crystalline radius of roughly 1.61 Å. It is known best for its toxicity, including carcinogenic and teratogenic effects, and has only once been used in a reported quantum dot, as a binary halide salt.34−36 Monovalent Tl+ readily forms insoluble perovskites and analogous crystals in combination with a range of lead halides.37−39 This family of bulk crystalline materials are generally found to be high-energy (2.5−4.0 eV), indirect band gap materials with several known applications, such as X-ray and UV−vis photodetectors, IR photodetectors when doped appropriately,
ecently perovskite nanocrystals (peNCs) have demonstrated an array of novel physical and optical properties for quantum dots.1−6 In addition to the quantum dot properties of size-tunable band gaps, MAPbX3 (MA = methylammonium) and CsPbX3 nanocrystals (NCs) have demonstrated unusual properties such as high quantum yield of emission,1,7 facile interparticle anion exchange,8 relatively high charge mobility and conductivity,9,10 and low-temperature particle fusion to form 1-D and 2-D structures.11−14 These observations have driven a number of studies into alternative NC structures based upon the perovskite model, including FAPbX3, MA3BiX9, Cs3BiX9, CsSnX3, Cs3Sb2Br9 (where X = Cl, Br, I; FA = formamidinium), as well as other perovskite analogues.15−23 These can be said to include double perovskites, alloys, mixed halide, doped perovskites, and other nanocrystalline materials analogous in some way to methylammonium lead iodide (MAPI).24−27 MAPI, of course, is the well-characterized 3-D structured perovskite used in highefficiency thin film solar cells and LEDs.28−33 Finding novel perovskites and related materials with useful, similar properties is currently a major area of nanomaterials research, and many © 2018 American Chemical Society
Received: January 29, 2018 Revised: March 28, 2018 Published: April 3, 2018 2973
DOI: 10.1021/acs.chemmater.8b00421 Chem. Mater. 2018, 30, 2973−2982
Article
Chemistry of Materials
Figure 1. High-resolution transmission electron micrographs (HR-TEMs) for (a) ∼17.2, (b) ∼28.0, and (c) ∼20.5 nm NCs, X-ray diffraction spectra of these same NCs, matched to index spectra (X’Pert High Score Plus), as well as spectra theoretically modeled in VESTA, and their respective periodic crystal structures,47,48 which have been optimized in VASP, for (a) Tl3PbI5, (b) Tl3PbBr5, and (c) Tl3PbCl5.
and as piezoelectric materials.38,40−44 Thallium lead triiodide (TlPbI3), for example, is a perovskite type material structurally similar to CsPbBr3 and a wide-band-gap semiconducting material that has been predicted to have piezoelectric properties.38 It could be a candidate for X-ray detection due to its high atomic number (Z), wide band gap, and high density (6.6 g cm−3). It is known to have a higher absorption cross section than CdTe and other commonly reported quantum dot (QD) materials.42,44 Having a higher band gap (Eg > 1.4 eV) also prevents the thermal generation of charge carriers, improving signal-to-noise at room temperature.42,44 Other thallium lead halide species, Tl3PbBr5 and Tl3PbCl5, have been used as mid-infrared (MIR) nonlinear optics and for near-IR nonlinear optical effects.38,40,43 For example, rare-earth-doped Tl3PbBr5 is considered a promising material for mid-IR solid-state lasers; quantum dots of this material could be an even more attractive material for that application.41,45,46 Here we present a facile synthesis of thallium lead halide nanocrystals and nanowires as novel analogues to other nanocrystalline inorganic lead halide perovskites.1,20,21,23 Orthorhombic nanocrystals with the chemical formula Tl3PbX5 [X = I, Br] and tetragonal nanocrystals of Tl3PbCl5 were synthesized
with a wide range of direct transition peaks across the UV−blue spectrum, from 440 to 280 nm (Figure 3). Band-edge-tuning was demonstrated both by tuning the halide mixture in mixed-halide Tl3PbX5−xBrx NCs [X = I, Cl] and, to a lesser degree, by sizetuning. A size series of Tl3PbBr5 nanocrystals were used to demonstrate the confinement effect in these materials, and weak photoluminescence is observed in Tl3PbI5 NCs. Furthermore, we demonstrate control over the shape and crystal structure of thallium lead iodide by altering the reaction conditions to produce TlPbX3 perovskite nanowires with high aspect ratios and lengths greater than 4 μm. We find that these materials are air-stable for several weeks.
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RESULTS AND DISCUSSION Synthesis. Thallium lead halide NCs were synthesized in a two-step reaction in which thallium acetate was heated with oleic acid and 1-octadecene to produce thallium oleate, which was then stored under N2. In the second step, Tl-oleate was injected into a heated pot under N2, containing pure or mixed lead halides PbX2 (X = I, I/Br, Br, Cl/Br, Cl) in oleylamine, oleic acid, and 1octadecene to form nanocrystals. The NCs were grown for 30− 45 s at high temperature before cooling to room temperature. 2974
DOI: 10.1021/acs.chemmater.8b00421 Chem. Mater. 2018, 30, 2973−2982
Article
Chemistry of Materials
Figure 2. Histogram with the TEM micrograph of (a) Tl3Pb(I0.43/Br0.57)5 NCs synthesized at 140 °C. NCs have 15.2 nm diameter with 2.2 nm standard deviation. Particles were dissolved in hexane before the TEM sample preparation. (c) Tl3Pb(Cl0.36/ Br 0.64)5 NCs synthesized at 130 °C. NCs have 19.2 nm diameter with 1.7 nm standard deviation. Particles were dissolved in hexane before the TEM sample preparation. (b, d) X-ray diffraction spectra of the same NCs, matched to the theoretically modeled crystal structure with two different halogen arrangements (see sections S9 and S10).
synthesized at higher temperature over longer reaction times. The agreement between XRD calculated from the periodic 3D lattice and the experimental XRD pattern suggests that this is indeed the correct structure. Mixed halides, shown in Figure 2, display intermediate XRDs with some peaks shifted slightly between those of the two pure materials (see sections S9 and S10 for detailed discussion). Tl3Pb(Br/I)5 and Tl3Pb(Br/Cl)5 mixed halide XRD spectra were fitted to simulated spectra produced in VESTA57 using structures descended from the bulk lowtemperature orthorhombic crystal structure of Tl3PbBr5,47 which has been optimized using the Vienna Ab-Initio Simulation Package (VASP)51−53 with two variations in halide positions (Figures S9.2 and S10.2). Both structures fit reasonably well, suggesting that the distribution of Br and I is not well-ordered since higher-order peaks could not be observed. Optical Properties. Absorption spectra are shown in Figure 3. The pure halide Tl3PbX5 NCs show evidence of a strong first direct transition peak, typical of semiconductor nanocrystals under weak and intermediate confinement,54 where the width of the peak is related to the size distribution. In addition to the first transition peak, a weak, lower-energy indirect transition is also observed. This is in agreement with theoretical and experimental observations of bulk Tl3PbI5, Tl3PbBr5, and Tl3PbCl5.38,39,55 The energy of the band gaps, direct and indirect, increases as you go up the halide series from I to Cl, as would be expected for semiconducting halides with similar crystal structures. Tauc plots are shown in Figure S5, with extrapolated band edges for both
They were then purified by between two and four solvent/ antisolvent cycles before being finally redispersed in hexane. The results of several variations on this procedure are summarized in Table S1, to produce both pure and mixed halides as well as perovskite structured nanowires from the iodide precursor. Crystal Structure. The NCs produced by this method, shown in Figure 1, appear as faceted spheroidal particles, with XRDs matching those of the bulk orthorhombic phases of Tl3PbI5 and Tl3PbBr5 and the tetragonal phase of Tl3PbCl5, respectively. NCs of all species can be seen to be highly crystalline in high-resolution transmission electron microscope (HR-TEM) images, with visible lattice fringes matched to the (312), (021), and (213) crystal planes (Figure S4) of the lattice structures with symmetry P212121, P212121, and P41, respectively. XRD peaks for NCs are broadened compared to the bulk index peaks according to the Scherrer equation. Crystallographic structures of the tetragonal Tl3PbBr5 and the orthorhombic Tl3PbBr5 and Tl3PbI5 are based on the work of Keller47,48 but are fully optimized using first-principles calculations. The Goldschmidt tolerance factors for each perovskite were calculated to be 0.772, 0.778, and 0.780 for I, Br, and Cl, respectively, and are all in the range of stable orthorhombic perovskite structures.49 Despite this, bulk ABX3 perovskites of thallium lead bromide and chloride have not been reported at atmospheric pressures.50 In this work, low-dimensional NCs all formed the preferred reduced symmetry structure, A3BX5, while TlPbI3 nanowires (NWs) were able to be 2975
DOI: 10.1021/acs.chemmater.8b00421 Chem. Mater. 2018, 30, 2973−2982
Article
Chemistry of Materials
Figure 3. (a) Absorption spectra of TlPbI3 nanowires and Tl3PbI5, Tl3PbBr5, and Tl3PbCl5 nanocrystals along with mixed halides I/Br and Cl/Br synthesized via a 1:1 reaction mixture, with a resulting 57% and 64% Br content, respectively. (b) Emission spectra observed at high pump excitation density for Tl3PbI5 NCs display the wide emission across the visible spectrum. (c) Chart of the effect of composition on the direct band gap, as measured by the first strong absorption peak.
direct and indirect band-edge transition peaks as estimates of the band-gap energy. Mixed halide NCs displayed larger full-width half maxima (fwhm) for their first strong transition peak. The direct band-gap energy of these materials was found to be an intermediate value between their pure halide parent NCs, allowing tuning by halide composition across the visible blue to UV regions as shown in Figure 3c. This data is summarized in Table S2, along with the peak wavelength and estimated fwhm of the first optical transition as determined by a simple Gaussian fit (Figure 3, dashed line). The first strong absorption peaks were found to range from 281 to 440 nm, indicating the feasibility of using these materials for detecting deep blue to moderate UV photons. Emission from Tl3PbI5. Weak emission was observed in Tl3PbI5 NCs excited at high pump excitation density (∼42 μJ cm−2) and is shown in Figure 3b, along with the photoluminescence (PL) lifetimes in Figure S2. Emission could not be observed in other samples (see the Methods section). The broad emission peak across the white light spectrum from 450 to 600 nm does not fit the profile of trap emission in NCs but rather can be decomposed into two Gaussian curves (Figure 3b). There is additionally some small intensity observable beyond 650 nm,
which is indicative of trap emission. The presence of a visible shoulder to the curve, and the poor fit of a single Gaussian profile, further justifies this approach.56−58 The first fitted peak at 554 nm (2.24 eV) accounts for 55% of the integrated emission signal. This value agrees with the literature indirect band-gap energy of 2.29 eV for emission in bulk Tl3PbI5 at 300 K.38 It is notable that direct/indirect emission cocontributions have been recently reported in other similar perovskite analogues.58 However, due to the discrepancy in the long decay lifetimes (τ2), the fitted peak at 498 nm is more likely to arise from high-energy trap sites, from smaller NCs with blue-shifted band gaps, or from recombination from the first strong (direct) transition observed in the absorbance spectrum. Neither peak matches the reported emission from binary thallium halide salt NCs, which was reported to occur at ∼2.7 eV.36 The PL peaks were measured using time-resolved photoluminescence, and each peak was fitted to a double exponential decay (Figure S2). These decays showed sub-nanosecond decay pathways, shorter than the instrumental limit (
