Publications

2018
Nevers, D. R. ; Williamson, C. B. ; Savitzky, B. H. ; Hadar, I. ; Banin, U. ; Kourkoutis, L. F. ; Hanrath, T. ; Robinson, R. D. . Mesophase Formation Stabilizes High-Purity Magic-Sized Clusters. JOURNAL OF THE AMERICAN CHEMICAL SOCIETY 2018, 140, 3652-3662. Abstract

Magic-sized clusters (MSCs) are renowned for their identical size and closed-shell stability that inhibit conventional nanoparticle (NP) growth processes. Though MSCs have been of increasing interest, understanding the reaction pathways toward their nucleation and stabilization is an outstanding issue. In this work, we demonstrate that high concentration synthesis (1000 mM) promotes a well-defined reaction pathway to form high-purity MSCs (>99.9%). The MSCs are resistant to typical growth and dissolution processes. On the basis of insights from in situ X-ray scattering analysis, we attribute this stability to the accompanying production of a large (>100 nm grain size), hexagonal organic–inorganic mesophase that arrests growth of the MSCs and prevents NP growth. At intermediate concentrations (500 mM), the MSC mesophase forms, but is unstable, resulting in NP growth at the expense of the assemblies. These results provide an alternate explanation for the high stability of MSCs. Whereas the conventional mantra has been that the stability of MSCs derives from the precise arrangement of the inorganic structures (i.e., closed-shell atomic packing), we demonstrate that anisotropic clusters can also be stabilized by self-forming fibrous mesophase assemblies. At lower concentration (<200 mM or >16 acid-to-metal), MSCs are further destabilized and NPs formation dominates that of MSCs. Overall, the high concentration approach intensifies and showcases inherent concentration-dependent surfactant phase behavior that is not accessible in conventional (i.e., dilute) conditions. This work provides not only a robust method to synthesize, stabilize, and study identical MSC products but also uncovers an underappreciated stabilizing interaction between surfactants and clusters.


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He, Y. ; Ke, W. ; Alexander, G. C. B. ; McCall, K. M. ; Chica, D. G. ; Liu, Z. ; Hadar, I. ; Stoumpos, C. C. ; Wessels, B. W. ; Kanatzidis, M. G. . Resolving The Energy Of Γ-Ray Photons With Mapbi3 Single Crystals. ACS PHOTONICS 2018, 5, 4132-4138. Abstract
Halide perovskites exhibit remarkably high-performance as semiconductors compared to conventional materials because of an unusually favorable combination of optoelectronic properties. We demonstrate here that solution-grown single-crystals of organic–inorganic hybrid perovskite CH3NH3PbI3 (MAPbI3), implemented in a Schottky-type device design, can produce outstanding hard radiation detectors with high spectral response and low dark current for the first time. Schottky-type MAPbI3 detector achieves an excellent energy resolution of 6.8% for 57Co 122 keV gamma ray. The high detector performance is achieved due to the balanced charge collection efficiency for both electrons and holes, reflected in the high mobility-lifetime (μτ) products of both carriers (∼0.8 × 10–3 cm2/V). MAPbI3 also demonstrates remarkably long electron and hole lifetimes (τe = 10 μs and τh = 17 μs) and impressive operational stability over time. Furthermore, dual-source detection of α particle (5.5 MeV) and γ-ray (59.5 keV) from the 241Am radiation source is achieved simultaneously by Schottky-type MAPbI3 detector. These results reveal the great potential of MAPbI3 as a high-performance, low-cost radiation detection material.
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Mao, L. ; Guo, P. ; Kepenekian, M. ; Hadar, I. ; Katan, C. ; Even, J. ; Schaller, R. D. ; Stoumpos, C. C. ; Kanatzidis, M. G. . Structural Diversity In White-Light-Emitting Hybrid Lead Bromide Perovskites. JOURNAL OF THE AMERICAN CHEMICAL SOCIETY 2018, 140, 13078-13088. Abstract
Hybrid organic–inorganic halide perovskites are under intense investigations because of their astounding physical properties and promises for optoelectronics. Lead bromide and chloride perovskites exhibit intrinsic white-light emission believed to arise from self-trapped excitons (STEs). Here, we report a series of new structurally diverse hybrid lead bromide perovskites that have broad-band emission at room temperature. They feature Pb/Br structures which vary from 1D face-sharing structures to 3D corner- and edge-sharing structures. Through single-crystal X-ray diffraction and low-frequency Raman spectroscopy, we have identified the local distortion level of the octahedral environments of Pb2+ within the structures. The band gaps of these compounds range from 2.92 to 3.50 eV, following the trend of “corner-sharing < edge-sharing < face-sharing”. Density functional theory calculations suggest that the electronic structure is highly dependent on the connectivity mode of the PbBr6 octahedra, where the edge- and corner-sharing 1D structure of (2,6-dmpz)3Pb2Br10 exhibits more disperse bands and smaller band gap (2.49 eV) than the face-sharing 1D structure of (hep)PbBr3 (3.10 eV). Using photoemission spectroscopy, we measured the energies of the valence band of these compounds and found them to remain almost constant, while the energy of conduction bands varies. Temperature-dependent PL measurements reveal that the 2D and 3D compounds have narrower PL emission at low temperature (∼5 K), whereas the 1D compounds have both free exciton emission and STE emission. The 1D compound (2,6-dmpz)3Pb2Br10 has the highest photoluminescence quantum yield of 12%, owing to its unique structure that allows efficient charge carrier relaxation and light emission.
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Su, X. ; Hao, S. ; Bailey, T. P. ; Wang, S. ; Hadar, I. ; Tan, G. ; Song, T. - B. ; Zhang, Q. ; Uher, C. ; Wolverton, C. ; et al. Weak Electron Phonon Coupling And Deep Level Impurity For High Thermoelectric Performance Pb1-XGaXTe. ADVANCED ENERGY MATERIALS 2018, 8. Abstract

High ZT of 1.34 at 766 K and a record high average ZT above 1 in the temperature range of 300-864 K are attained in n-type PbTe by engineering the temperature-dependent carrier concentration and weakening electron–phonon coupling upon Ga doping. The experimental studies and first principles band structure calculations show that doping with Ga introduces a shallow level impurity contributing extrinsic carriers and imparts a deeper impurity level that ionizes at higher temperatures. This adjusts the carrier concentration closer to the temperature-dependent optimum and thus maximizes the power factor in a wide temperature range. The maximum power factor of 35 µW cm−1 K−2 is achieved for the Pb0.98Ga0.02Te compound, and is maintained over 20 µWcm−1 K−2 from 300 to 767 K. Band structure calculations and X-ray photoelectron spectroscopy corroborate the amphoteric role of Ga in PbTe as the origin of shallow and deep levels. Additionally, Ga doping weakens the electron–phonon coupling, leading to high carrier mobilities in excess of 1200 cm2 V−1 s−1. Enhanced point defect phonon scattering yields a reduced lattice thermal conductivity. This work provides a new avenue, beyond the conventional shallow level doping, for further improving the average ZT in thermoelectric materials.


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McCall, K. M. ; Liu, Z. ; Trimarchi, G. ; Stoumpos, C. C. ; Lin, W. ; He, Y. ; Hadar, I. ; Kanatzidis, M. G. ; Wessels, B. W. . Α-Particle Detection And Charge Transport Characteristics In The A3M2I9 Defect Perovskites (A = Cs, Rb; M = Bi, Sb). ACS PHOTONICS 2018, 5, 3748-3762. Abstract
We have investigated the defect perovskites A3M2I9 (A = Cs, Rb; M = Bi, Sb) as materials for radiation detection. The phase purity of Bridgman-grown A3M2I9 single crystals was confirmed via high-resolution synchrotron X-ray diffraction, while density functional theory calculations (DFT) show surprisingly dispersive bands in the out-of-plane direction for these layered materials, with low effective masses for both holes and electrons. Accordingly, each of the four A3M2I9 defect perovskites showed response to 241Am α-particle irradiation for hole and electron electrode configurations, a remarkable ambipolar response that resembles the 3D halide perovskites. The electron response spectra were used to estimate the mobility–lifetime product (μτ)e for electrons in these materials, with Rb3Bi2I9 showing the lowest (μτ)e value of 1.7 × 10–6 cm2 V–1 and Cs3Bi2I9 the highest (μτ)e of 5.4 × 10–5 cm2 V–1. The rise time of the α-particle-generated pulse was used to estimate the electron mobility μe of the A3M2I9 defect perovskites, which ranged from 0.32 cm2 V–1s–1 for Rb3Sb2I9 to 4.3 cm2 V–1s–1 in Cs3Bi2I9. Similar analysis of the hole response spectra yielded (μτ)h values for each A3M2I9 compound, with Cs3Bi2I9 again showing the highest (μτ)h value of 1.8 × 10–5 cm2 V–1, while Rb3Bi2I9 showed the lowest (μτ)h with 2.0 × 10–6 cm2 V–1. Rise time analysis gave hole mobilities ranging from 1.7 cm2 V–1 s–1 for Cs3Bi2I9 to 0.14 cm2 V–1 s–1 for Cs3Sb2I9. Comparing the experimental electron and hole mobilities to the effective masses obtained from DFT calculations revealed sizable discrepancies, possibly indicating self-trapping of charge carriers due to electron–phonon interactions. The α-particle response of the A3M2I9 defect perovskites demonstrates their potential as semiconductor radiation detectors, with Cs3Bi2I9 and Cs3Sb2I9 showing the most promise.
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