Publications

2025
Abarbanel, O. ; Hirzalla, R. ; Aridor, L. ; Michman, E. ; Hadar, I. . Studying The Effect Of Dimensions And Spacer Ligands On The Optical Properties Of 2D Metal Halide Perovskites. Nanoscale 2025.Abstract
In recent years, metal–halide perovskites (MHPs) have emerged as highly promising optoelectronic materials based on their exceptional properties and versatility in applications such as solar cells, light-emitting devices, and radiation detectors. This study investigates the optical properties of two-dimensional (2D) MHPs, with the Ruddlesden–Popper structure, comparing three morphologies–bulk poly-crystals, colloidal nanoplatelets (NPs), and thin films, aiming to bridge between the bulk and nano dimensionalities. By synthesizing bulk 2D MHPs using long alkyl ammonium spacers, typically found in colloidal systems, and NPs using shorter ligands suitable for bulk growth, we elucidate the relationship between these materials’ structural modifications and optical characteristics. We propose the existence of two regions in these 2D MHPs, which differ in their optoelectronic properties and are associated with “bulk” and “surface” regions. Specifically, for poly-crystals, we observe the appearance of a lower energy “bulk” phase associated with the stacking of many 2D sheets, apparent both in absorption and photoluminescence. For NPs, this stacking is hindered, and hence, only the “surface” phase exists. With the elongation of the spacer chain, the poly-crystal becomes more similar to the NPs. For thin films, an interesting phenomenon is observed – the rapid film formation mechanism forces a more colloid-like structure for the shorter ligands and a more poly-crystal-like structure for the longer ones. Overall, this study bridging the different dimensions of 2D MHPs may support new possibilities for future research and development in this innovative field.
Bader, B. ; Michman, E. ; Gkikas, I. N. ; Spanopoulos, I. ; Hadar, I. . Tuning Self-Trapped Exciton Emission In 1D White-Light Emitting Perovskites Through Halide Composition And Synthesis Route. ACS Omega 2025.Abstract
Low-dimensional halide perovskites (LDHPs) are promising candidates for optoelectronic applications, specifically light emission. LDHP’s reduced-dimensional frameworks allow exceptional tunability of the optoelectronic properties. Self-trapped exciton emission (STE) is an intriguing characteristic of some LDHPs, offering a compelling mechanism for white-light emission. In this work, we study the effect of halide substitution and synthetic methods on the optical properties, with a particular focus on STE emission, of one-dimensional LDHPs using (2,5-dmpz)Pb(BrxCl1–x)4 (dmpz = dimethylammonium piperazine) as a model system, exhibiting high emission quantum efficiency and structural consistency. Mechanochemical approaches, including manual grinding and ball milling, were employed to synthesize pure and mixed halide compounds, enabling precise composition control. Structural characterization revealed that ball milling produces materials with improved crystallinity and a homogeneous halide distribution. Optical characterization showed an anticorrelation between the absorption onset and STE emission energies, with STE properties influenced by both halide composition and a synthesis route. Moreover, we found that the STE emission of the mixed halide compounds evolved with time. The post-synthesis evolution of the STE emission spectrum is associated with local lattice distortions rather than long-range structural changes. The emission quantum efficiency of these compounds was measured and reached a value of 35%, among the highest values measured for 1D broad emitters. These findings provide additional insights into the design of next-generation LDHP compounds exhibiting STE emission and development of white-light-emitting devices based on them.
Yin, H. ; Sun, Z. ; Liu, K. ; Li, Z. ; Wibowo, A. Anggara; Chen, J. ; Gu, H. ; Jing, X. ; Chen, Y. - L. ; Macdonald, D. ; et al. Solar-Driven Dehydrogenation And Dehydration Of Formate To Syngas With Near-Zero Co 2 Emission. Journal of Materials Chemistry A 2025, 13, 9144–9151.Abstract
Syngas, a vital H2 and CO mixture, is crucial for industrial applications and advancing the circular carbon economy. Traditional photocatalytic CO2 reduction to syngas relies on sacrificial agents and photosensitizers, limiting scalability and practice. Here, we demonstrate a Co3O4–CdS heterojunction photocatalyst that efficiently converts formate (HCOO−), a stable, easily-handled and accessible CO2 reduction product, into syngas under alkaline conditions (pH ∼ 10). This dual-function catalyst enables CO generation via CdS-mediated dehydration and H2 production via Co3O4-mediated dehydrogenation, achieving a syngas production rate of ∼3300 μmol g−1 h−1. Notably, this system operates without sacrificial agents or noble metals, with near-zero CO2 emissions, surpassing current efficiency benchmarks. By recycling CO2 into formic acid and further converting it to syngas, this approach promotes a closed carbon loop. Its cost-effectiveness, ease of formate storage, direct solar utilization, and low carbon footprint position it as a promising pathway for sustainable syngas production and clean energy solutions.
2024
Zhou, Y. ; Li, J. ; Liu, L. ; Wang, C. ; Lynch, R. P. ; Bai, B. ; Hsu, H. - Y. ; Yin, Z. ; Cabot, A. ; Robinson, R. D. ; et al. Anion-Driven Enabled Functional Nanomaterials From Metal And Metal Oxide Nanoparticles. Materials Today 2024.Abstract
Despite significant progress in the synthesis of nanocrystals (NCs) by conventional wet-chemical synthetic approaches, producing nanostructures with complex architectures tailored to specific applications remains a formidable challenge. Recently, anion-driven synthesis, including oxidation, sulfidation, phosphorization, nitridation, selenization, telluridation, and chlorination have emerged as a versatile approach to produce novel nanostructured materials with tuned size, morphology, crystal structure, and composition from the chemical transformation of template NCs. This chemical conversion can be accompanied by the formation of new NCs architectures, overall modifying the surface chemistry and the mechanical, electronic, optical, and magnetic properties of the material. This strategy can be used to optimize the performance of the material in a range of applications, including energy conversion and storage, catalysis, bioimaging, drug delivery, and sensing. In this review, we first detail the possible anion-driven synthesis and discuss the related underlying mechanisms. Subsequently, we overview the unique nanostructure obtained by this strategy and summarize their functional properties and potential applications. Finally, we provide perspectives and discuss the remaining challenges and the new opportunities in this field.
Li, Y. ; Cotlet, M. ; Hadar, I. ; Guo, P. . Broadband Emission In Alkali Halides Triggered By Sb 3+ Doping. Chemical Communications 2024, 60, 14806–14809.Abstract
Broadband emission in a series of alkali chlorides are achieved by doping NaCl, KCl, and RbCl with Sb3+. These compounds show photoluminescence peaks in the visible range of 536–574 nm with long lifetimes in the microsecond range. Our findings could offer valuable insights for the development of new lead-free phosphors.
Zhou, Y. ; Zhang, S. ; Li, J. ; Liu, L. ; Wang, C. ; Bai, B. ; Hsu, H. - Y. ; Hadar, I. ; Yin, Z. ; Buntine, M. A. ; et al. Rational Entry-Diffusion Induced Kirkendall Effect Towards Au2S Nanotubes. Materials Today Chemistry 2024, 38, 102029.Abstract
emiconductor nanotubes manifesting large surface area, high tensile strength, light weight, fast electron transfer kinetics and high biocompatibility have attracted tremendous attention and find widespread applications. Rational design and preparation of semiconductor nanotubes with such a unique morphology to maximum their performance and therefore, to fulfill their applications is still a challenge. Herein, we report a strategy that is capable of rationalizing entry-diffusion of Au + ions into Cu2-xS nanorods based on the Kirkendall effect, accomplishing consecutive morphology control from solid nanorods to nanotubes, and ultimately to nanorings of Au2S. The structure and composition of Au2S nanotubes were further confirmed through X-ray photoelectron spectroscopy, annular bright-field scanning transmission electron microscopy-energy dispersive spectroscopy and scanning transmission electron microscopy and Aberration-corrected transmission electron microscopy. Compared with nanorods, Au2S nanotubes demonstrated significantly enhanced catalytic activity in electrocatalytic hydrogen evolution reaction, a remarkably low overpotential of 602.0 mV at a current density of −10 mA cm−2 and a relatively low Tafel slope of 36.87 mV dec−1 in 0.5 M H2SO4 solution. It is anticipated that this novel strategy of mediating ion diffusion rate would inspire rational control of unique morphologies and structures of semiconductor nanocrystals, providing a platform for further applications based on nanomaterials
Laing, C. C. ; Kim, D. ; Park, J. ; Shen, J. ; Hadar, I. ; Hoffman, J. M. ; He, J. ; Shin, B. ; Wolverton, C. ; Kanatzidis, M. G. . Solution-Processable Mixed-Anion Cluster Chalcohalide Rb6Re6S8I8 In A Light-Emitting Diode. Nature materials 2024, 23, 230–236.Abstract

 

Rhenium chalcohalide cluster compounds are a photoluminescent family of mixed-anion chalcohalide cluster materials. Here we report the new material RbRe₆S₈I₈, which crystallizes in the cubic space group Fm3̅m and contains isolated [Re₆S₈I₆]⁴⁻ clusters. Rb₆Re₆S₈I₈ has a band gap of 2.06(5) eV and an ionization energy of 5.51(3) eV, and exhibits broad photoluminescence (PL) ranging from 1.01 eV to 2.12 eV. The room-temperature PL exhibits a PL quantum yield of 42.7% and a PL lifetime of 77 μs (99 μs at 77 K). Rb₆Re₆S₈I₈ is found to be soluble in multiple polar solvents including N,N-dimethylformamide, which enables solution processing of the material into films with thickness under 150 nm. Light-emitting diodes based on films of Rb₆Re₆S₈I₈ were fabricated, demonstrating the potential for this family of materials in optoelectronic devices.

2023
Bai, B. ; Zhang, C. ; Dou, Y. ; Kong, L. ; Wang, L. ; Wang, S. ; Li, J. ; Zhou, Y. ; Liu, L. ; Liu, B. ; et al. Atomically Flat Semiconductor Nanoplatelets For Light-Emitting Applications. Chemical Society Reviews 2023, 52, 318–360.Abstract
The last decade has witnessed extensive breakthroughs and significant progress in atomically flat two-dimensional (2D) semiconductor nanoplatelets (NPLs) in terms of synthesis, growth mechanisms, optical and electronic properties and practical applications. Such NPLs have electronic structures similar to those of quantum wells in which excitons are predominantly confined along the vertical direction, while electrons are free to move in the lateral directions, resulting in unique optical properties, such as extremely narrow emission line width, short photoluminescence (PL) lifetime, high gain coefficient, and giant oscillator strength transition (GOST). These unique optical properties make NPLs favorable for high color purity light-emitting applications, in particular in light-emitting diodes (LEDs), backlights for liquid crystal displays (LCDs) and lasers. This review article first introduces the intrinsic characteristics of 2D semiconductor NPLs with atomic flatness. Subsequently, the approaches and mechanisms for the controlled synthesis of atomically flat NPLs are summarized followed by an insight on recent progress in the mediation of core/shell, core/crown and core/crown@shell structures by selective epitaxial growth of passivation layers on different planes of NPLs. Moreover, an overview of the unique optical properties and the associated light-emitting applications is elaborated. Despite great progress in this research field, there are some issues relating to heavy metal elements such as Cd2+ in NPLs, and the ambiguous gain mechanisms of NPLs and others are the main obstacles that prevent NPLs from widespread applications. Therefore, a perspective is included at the end of this review article, in which the current challenges in this stimulating research field are discussed and possible solutions to tackle these challenges are proposed.
2022
Hoffman, J. M. ; Hadar, I. ; Li, X. ; Ke, W. ; Vasileiadou, E. S. ; Strzalka, J. ; Chen, L. X. ; Kanatzidis, M. G. . Film Formation Mechanisms In Mixed-Dimensional 2D/3D Halide Perovskite Films Revealed By In Situ Grazing-Incidence Wide-Angle X-Ray Scattering. Chem 2022, 8, 1067-1082. Publisher's VersionAbstract

Summary Mixed-dimensional 2D/3D hybrid halide perovskites retain the stability of 2D perovskites (formula (A′)2(A)n−1PbnI3n+1) and long diffusion lengths of the 3D materials (AMX3), thereby affording devices with extended stability as well as state-of-the art efficiencies approaching those of the 3D materials. These films are made by spin-coating precursor solutions with an arbitrarily large average layer thickness n (⟨n⟩ > 7) to give films with both 2D and 3D phases. Although the 2D and 3D perovskite film formation mechanisms have been studied, little is understood about composite 2D/3D film formation. We used in-situ grazing-incidence wide-angle scattering with synchrotron radiation to characterize the films fabricated from precursor solutions with stoichiometries of (BA)2(MA)n−1PbnI3n+1 (⟨n⟩ = 3, 4, 5, 7, 12, 50, and ∞ (MAPbI3)). Four different mechanisms are seen depending on the stoichiometry in the precursor solution. Kinetic analysis shows faster and earlier growth of the solvate with increasing ⟨n⟩.


Film formation mechanisms

He, Y. ; Hadar, I. ; De Siena, M. C. ; Klepov, V. V. ; Pan, L. ; Chung, D. Y. ; Kanatzidis, M. G. . Sensitivity And Detection Limit Of Spectroscopic-Grade Perovskite Cspbbr3 Crystal For Hard X-Ray Detection. Advanced Functional Materials 2022, 2112925. Publisher's VersionAbstract

Spectroscopic-grade single crystal detectors can register the energies of individual X-ray interactions enabling photon-counting systems with superior resolution over traditional photoconductive X-ray detection systems. Current technical challenges have limited the preparation of perovskite semiconductors for energy-discrimination X-ray photon-counting detection. Here, this work reports the deployment of a spectroscopic-grade CsPbBr3 Schottky detector under reverse bias for continuum hard X-ray detection in both the photocurrent and spectroscopic schemes. High surface barriers of ≈1 eV are formed by depositing solid bismuth and gold contacts. The spectroscopic response under a hard X-ray source is assessed in resolving the characteristic X-ray peak. The methodology in enhancing X-ray sensitivity by controlling the X-ray energies and flux, and voltage, is described. The X-ray sensitivity varies between a few tens to over 8000 μC Gyair−1 cm−2. The detectable dose rate of the CsPbBr3 detectors is as low as 0.02 nGyair s−1 in the energy discrimination configuration. Finally, the unbiased CsPbBr3 device forms a spontaneous contact potential difference of about 0.7 V enabling high quality of the CsPbBr3 single crystals to operate in “passive” self-powered X-ray detection mode and the X-ray sensitivity is estimated as 14 μC Gyair−1 cm−2. The great potential of spectroscopic-grade CsPbBr3 devices for X-ray photon-counting systems is anticipated in this work.


Spectroscopic‐Grade Perovskite Crystal for Hard X‐Ray Detection

He, Y. ; Hadar, I. ; Kanatzidis, M. G. . Detecting Ionizing Radiation Using Halide Perovskite Semiconductors Processed Through Solution And Alternative Methods. Nature Photonics 2022, 16, 14-26. Publisher's VersionAbstract

The direct detection of high-energy radiation such as X-rays and γ-rays by semiconductors at room temperature is a challenging proposition that requires remarkably pure and nearly perfect crystals. The emergence of metal halide perovskites, defect-tolerant semiconductors, is reviving hope for new materials in this field after an almost 20 year hiatus. Metal halide perovskites, which combine exceptional optoelectronic properties, versatile chemistry and simple synthesis, are challenging traditional approaches for the development of novel semiconductors for detecting hard radiation. We discuss the relevant physical properties, promising materials, fabrication techniques and device architectures for high-performance, low-cost detectors by targeting next-generation semiconductors for radiation detection. We also present a perspective on the impact of such advances in future medical imaging applications.


X-ray semiconductor detection modes

2021
He, Y. ; Petryk, M. ; Liu, Z. ; Chica, D. G. ; Hadar, I. ; Leak, C. ; Ke, W. ; Spanopoulos, I. ; Lin, W. ; Chung, D. Y. ; et al. Cspbbr3 Perovskite Detectors With 1.4% Energy Resolution For High-Energy Γ-Rays. NATURE PHOTONICS 2021, 15, 36-42. Publisher's VersionAbstract

Halide perovskite semiconductors are poised to revitalize the field of ionizing radiation detection as they have done to solar photovoltaics. We show that all-inorganic perovskite CsPbBr3 devices resolve 137Cs 662-keV γ-rays with 1.4% energy resolution, as well as other X- and γ-rays with energies ranging from tens of keV to over 1 MeV in ambipolar sensing and unipolar hole-only sensing modes with crystal volumes of 6.65 mm3 and 297 mm3, respectively. We report the scale-up of CsPbBr3 ingots to up to 1.5 inches in diameter with an excellent hole mobility–lifetime product of 8 × 10−3 cm2 V−1 and a long hole lifetime of up to 296 μs. CsPbBr3 detectors demonstrate a wide temperature region from ~2 °C to ~70 °C for stable operation. Detectors protected with suitable encapsulants show a uniform response for over 18 months. Consequently, we identify perovskite CsPbBr3 semiconductor as an exceptional candidate for new-generation high-energy γ-ray detection.


CsPbBr3 Single Crystal Detector

He, Y. ; Stoumpos, C. C. ; Hadar, I. ; Luo, Z. ; McCall, K. M. ; Liu, Z. ; Chung, D. Y. ; Wessels, B. W. ; Kanatzidis, M. G. . Demonstration Of Energy-Resolved Γ-Ray Detection At Room Temperature By The Cspbcl3 Perovskite Semiconductor. JOURNAL OF THE AMERICAN CHEMICAL SOCIETY 2021, 143, 2068-2077. Publisher's VersionAbstract

The detection of γ-rays at room temperature with high-energy resolution using semiconductors is one of the most challenging applications. The presence of even the smallest amount of defects is sufficient to kill the signal generated from γ-rays which makes the availability of semiconductors detectors a rarity. Lead halide perovskite semiconductors exhibit unusually high defect tolerance leading to outstanding and unique optoelectronic properties and are poised to strongly impact applications in photoelectric conversion/detection. Here we demonstrate for the first time that large size single crystals of the all-inorganic perovskite CsPbCl3 semiconductor can function as a high-performance detector for γ-ray nuclear radiation at room temperature. CsPbCl3 is a wide-gap semiconductor with a bandgap of 3.03 eV and possesses a high effective atomic number of 69.8. We identified the two distinct phase transitions in CsPbCl3, from cubic (Pm-3m) to tetragonal (P4/mbm) at 325 K and finally to orthorhombic (Pbnm) at 316 K. Despite crystal twinning induced by phase transitions, CsPbCl3 crystals in detector grade can be obtained with high electrical resistivity of ~1.7 X 109 Ω·cm. The crystals were grown from the melt with volume over several cubic centimeters and have a low thermal conductivity of 0.6 W m-1 K-1. The mobilities for electron and hole carriers were determined to ~30 cm2/(V s). Using photoemission yield spectroscopy in air (PYSA), we determined the valence band maximum at 5.66 ± 0.05 eV. Under gamma-ray exposure, our Schottky-type planar CsPbCl3 detector achieved an excellent energy resolution (~16% at 122 keV) accompanied by a high figure-of-merit hole mobility-lifetime product 3.2 x 10-4 cm2/V and a long hole lifetime (16 μs). The results demonstrate considerable defect tolerance of CsPbCl3 and suggest its strong potential for γ-radiation and X-ray detection at room temperature and above.


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Sidhik, S. ; Wang, Y. ; Li, W. ; Zhang, H. ; Zhong, X. ; Agrawal, A. ; Hadar, I. ; Spanopoulos, I. ; Mishra, A. ; Traore, B. ; et al. High-Phase Purity Two-Dimensional Perovskites With 17.3% Efficiency Enabled By Interface Engineering Of Hole Transport Layer. CELL REPORTS PHYSICAL SCIENCE 2021, 2. Publisher's VersionAbstract

State-of-the-art p-i-n-based 3D perovskite solar cells (PSCs) use nickel oxide (NiOx) as an efficient hole transport layer (HTL), achieving efficiencies >22%. However, translating this to phase-pure 2D perovskites has been unsuccessful. Here, we report 2D phase-pure Ruddlesden-Popper BA2MA3Pb4I13 perovskites with 17.3% efficiency enabled by doping the NiOx with Li. Our results show that progressively increasing the doping concentration transforms the photoresistor behavior to a typical diode curve, with an increase in the average efficiency from 2.53% to 16.03% with a high open-circuit voltage of 1.22 V. Analysis reveals that Li doping of NiOx significantly improves the morphology, crystallinity, and orientation of 2D perovskite films and also affords a superior band alignment, facilitating efficient charge extraction. Finally, we demonstrate that 2D PSCs with Li-doped NiOx exhibit excellent photostability, with T99 = 400 h at 1 sun and T90 of 100 h at 5 suns measured at relative humidity of 60% ± 5% without the need for external thermal management.


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Lin, W. ; He, J. ; McCall, K. M. ; Stoumpos, C. C. ; Liu, Z. ; Hadar, I. ; Das, S. ; Wang, H. - H. ; Wang, B. - X. ; Chung, D. Y. ; et al. Inorganic Halide Perovskitoid Tlpbi3 For Ionizing Radiation Detection. ADVANCED FUNCTIONAL MATERIALS 2021, 31. Publisher's VersionAbstract

Room temperature semiconductor detector (RTSD) materials for γ-ray and X-ray radiation are in great demand for the nonproliferation of nuclear materials as well as for biomedical imaging applications. Halide perovskites have attracted great attention as emerging and promising RTSD materials. In this contribution, the material synthesis, purification, crystal growth, crystal structure, photoluminescence properties, ionizing radiation detection performance, and electronic structure of the inorganic halide perovskitoid compound TlPbI3 are reported on. This compound crystallizes in the ABX3 non-perovskite crystal structure with a high density of d = 6.488 g·cm–3, has a wide bandgap of 2.25 eV, and melts congruently at a low temperature of 360 °C without phase transitions, which allows for facile growth of high quality crystals with few thermally-activated defects. High-quality TlPbI3 single crystals of centimeter-size are grown using the vertical Bridgman method using purified raw materials. A high electrical resistivity of ~1012 Ω·cm is readily obtainable, and detectors made of TlPbI3 single crystals are highly photoresponsive to Ag Kα X-rays (22.4 keV), and detects 122 keV γ-rays from 57Co radiation source. The electron mobility-lifetime product µeτe was estimated at 1.8 x 10-5 cm2·V–1. A high relative static dielectric constant of 35.0 indicates strong capability in screening carrier scattering and charged defects in TlPbI3.


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Vasileiadou, E. S. ; Wang, B. ; Spanopoulos, I. ; Hadar, I. ; Navrotsky, A. ; Kanatzidis, M. G. . Insight On The Stability Of Thick Layers In 2D Ruddlesden-Popper And Dion-Jacobson Lead Iodide Perovskites. JOURNAL OF THE AMERICAN CHEMICAL SOCIETY 2021, 143, 2523-2536. Publisher's VersionAbstract

Two-dimensional (2D) hybrid organic–inorganic halide perovskites are a preeminent class of low-cost semiconductors whose inherent structural tunability and attractive photophysical properties have led to the successful fabrication of solar cells with high power conversion efficiencies. Despite the observed superior stability of 2D lead iodide perovskites over their 3D parent structures, an understanding of their thermochemical profile is missing. Herein, the calorimetric studies reveal that the Ruddlesden–Popper (RP) series, incorporating the monovalent-monoammonium spacer cations of pentylammonium (PA) and hexylammonium (HA): (PA)2(MA)n-1PbnI3n+1 (n = 2–6) and (HA)2(MA)n-1PbnI3n+1 (n = 2–4) have a negative enthalpy of formation, relative to their binary iodides. In contrast, the enthalpy of formation for the Dion–Jacobson (DJ) series, incorporating the divalent and cyclic diammonium cations of 3- and 4-(aminomethyl)piperidinium (3AMP and 4AMP respectively): (3AMP)(MA)n-1PbnI3n+1 (n = 2–5) and (4AMP)(MA)n-1PbnI3n+1 (n = 2–4) have a positive enthalpy of formation. In addition, for the (PA)2(MA)n−1PbnI3n+1 family of materials, we report the phase-pure synthesis and single crystal structure of the next member of the series (PA)2(MA)5Pb6I19 (n = 6), and its optical properties, marking this the second n = 6, bulk member published to date. Particularly, (PA)2(MA)5Pb6I19 (n = 6) has negative enthalpy of formation as well. Additionally, the analysis of the structural parameters and optical properties between the examined RP and DJ series offers guiding principles for the targeted design and synthesis of 2D perovskites for efficient solar cell fabrication. Although the distortions of the Pb–I–Pb equatorial angles are larger in the DJ series, the significantly smaller I···I interlayer distances lead to overall smaller band gap values, in comparison with the RP series. Our film stability studies on the RP and DJ perovskites series reveal consistent observations with the thermochemical findings, pointing out to the lower extrinsic stability of the DJ materials in ambient air.


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Vasileiadou, E. S. ; Hadar, I. ; Kepenekian, M. ; Even, J. ; Tu, Q. ; Malliakas, C. D. ; Friedrich, D. ; Spanopoulos, I. ; Hoffman, J. M. ; Dravid, V. P. ; et al. Shedding Light On The Stability And Structure-Property Relationships Of Two-Dimensional Hybrid Lead Bromide Perovskites. CHEMISTRY OF MATERIALS 2021, 33, 5085-5107. Publisher's VersionAbstract

Two-dimensional (2D) hybrid lead iodide perovskites have gained prominence due to their remarkable structural tunability, optoelectronic features, and moisture stability, which have rendered them as attractive alternatives to 3D MAPbI3 for optoelectronic devices. 2D multilayer lead bromide perovskites remain an unfathomed phase space with the lack of systematic studies to establish the structure, photophysical properties and stability behavior of this family of 2D halide perovskites. Herein, we present new members of bilayer lead bromide perovskites (CmH2m+1NH3)2(CH3NH3)Pb2Br7 (m = 6–8) that belong to the Ruddlesden–Popper structure type, incorporating long chain alkyl-monoammonium cations (CmH2m+1NH3) of hexylammonium (m = 6), heptylammonium (m = 7), and octylammonium (m = 8). A universal solution synthetic methodology for bulk multilayer lead bromide perovskites is presented with all structures solved and refined using single crystal X-ray diffraction. The studied bilayer lead bromide perovskites demonstrate a decrease in the lattice rigidity and lattice match of the inorganic perovskite layer–organic layer, as the alkyl-monoammonium chain length increases. In comparison to their iodide analogues, the bilayer lead bromide compounds exhibit elongation of their stacking axis despite the smaller dimensions of the [PbBr6]4− lattice, while their internal lattice strain was calculated to be reduced, inferring a greater lattice match between the inorganic [PbBr6]4− perovskite layer and organic layer. The (CmH2m+1NH3)2(CH3NH3)Pb2Br7 (m = 4, 6–8) compounds exhibit narrow-band emission near 2.5 eV. Time-resolved photoluminescence (PL) displays longer carrier lifetimes on the nanosecond time scale comparing to their iodide analogues, where electronic structure calculations indicate that the increase of the alkyl chain length and, thus, lattice softness enhances nonradiative recombinations. A complete set of air, light, and heat stability tests on unencapsulated thin films of (CmH2m+1NH3)2(CH3NH3)Pb2Br7 (m = 4, 6–8) and MAPbBr3 show they are stable in ambient air for at least 5 months, exhibiting greater extrinsic stability than the 2D lead iodide congeners. Extraordinarily, 3D MAPbBr3 films prove to be more stable than films of 2D lead bromide perovskites, in contrast to MAPbI3 which is less stable than the 2D lead iodide perovskites.


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Spanopoulos, I. ; Hadar, I. ; Ke, W. ; Guo, P. ; Mozur, E. M. ; Morgan, E. ; Wang, S. ; Zheng, D. ; Padgaonkar, S. ; Reddy, G. N. M. ; et al. Tunable Broad Light Emission From 3D ``Hollow'' Bromide Perovskites Through Defect Engineering. JOURNAL OF THE AMERICAN CHEMICAL SOCIETY 2021, 143, 7069-7080. Publisher's VersionAbstract

Hybrid halide perovskites consisting of corner-sharing metal halide octahedra and small cuboctahedral cages filled with counter cations have proven to be prominent candidates for many high-performance optoelectronic devices. The stability limits of their three-dimensional perovskite framework are defined by the size range of the cations present in the cages of the structure. In some cases, the stability of the perovskite-type structure can be extended even when the counterions violate the size and shape requirements, as is the case in the so-called “hollow” perovskites. In this work, we engineered a new family of 3D highly defective yet crystalline “hollow” bromide perovskites with general formula (FA)1–x(en)x(Pb)1–0.7x(Br)3–0.4x (FA = formamidinium (FA+), en = ethylenediammonium (en2+), x = 0–0.44). Pair distribution function analysis shed light on the local structural coherence, revealing a wide distribution of Pb–Pb distances in the crystal structure as a consequence of the Pb/Br-deficient nature and en inclusion in the lattice. By manipulating the number of Pb/Br vacancies, we finely tune the optical properties of the pristine FAPbBr3 by blue shifting the band gap from 2.20 to 2.60 eV for the x = 0.42 en sample. A most unexpected outcome was that at > 0.33 en incorporation, the material exhibits strong broad light emission (1% photoluminescence quantum yield (PLQY)) that is maintained after exposure to air for more than a year. This is the first example of strong broad light emission from a 3D hybrid halide perovskite, demonstrating that meticulous defect engineering is an excellent tool for customizing the optical properties of these semiconductors.


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2020
Cai, S. ; Hao, S. ; Luo, Z. - Z. ; Li, X. ; Hadar, I. ; Bailey, T. ; Hu, X. ; Uher, C. ; Hu, Y. - Y. ; Wolverton, C. ; et al. Discordant Nature Of Cd In Pbse: Off-Centering And Core-Shell Nanoscale Cdse Precipitates Lead To High Thermoelectric Performance. ENERGY & ENVIRONMENTAL SCIENCE 2020, 13, 200-211. Publisher's VersionAbstract

We report a novel hierarchical microstructure in the PbSe–CdSe system, which collectively contributes to significant enhancement in thermoelectric performance, with ZTave ∼ 0.83 across the 400–923 K temperature range, the highest reported for p-type, Te-free PbSe systems. We have investigated the local atomic structure as well as the microstructure of a series of PbSe–xCdSe materials, up to x = 10%. We find that the behavior of the Cd atoms in the octahedral rock salt sites is discordant and results in off-center displacement and distortion. Such off-centered Cd in the PbSe matrix creates (1) L–Σ electronic energy band convergence, (2) a flattened L band, both contributing to higher Seebeck coefficients, and (3) enhanced phonon scattering, which leads to lower thermal conductivity. These conclusions are supported by photoemission yield spectroscopy in air (PYSA), solid state 111Cd, 77Se NMR spectroscopy and DFT calculations. Above the solubility limit (>6%CdSe), we also observe endotaxial CdSe nano-precipitates with core–shell architecture formed in PbSe, whose size, distribution and structure gradually change with the Cd content. The nano-precipitates exhibit a zinc blende crystal structure and a tetrahedral shape with significant local strain, but are covered with a thin wurtzite layer along the precipitate/matrix interface, creating a core–shell structure embedded in PbSe. This newly discovered architecture causes a further reduction in lattice thermal conductivity. Moreover, potassium is found to be an effective p-type dopant in the PbSe–CdSe system, leading to an enhanced power factor, a maximum ZT of ∼1.4 at 923 K for Pb0.98K0.02Se–6%CdSe.


 off-centering and core–shell nanoscale CdSe precipitates lead to high thermoelectric performance

Luo, Y. ; Cai, S. ; Hao, S. ; Pielnhofer, F. ; Hadar, I. ; Luo, Z. - Z. ; Xu, J. ; Wolverton, C. ; Dravid, V. P. ; Pfitzner, A. ; et al. High-Performance Thermoelectrics From Cellular Nanostructured Sb2Si2Te6. JOULE 2020, 4, 159-175. Publisher's VersionAbstract

We introduce Sb2Si2Te6 as a high-performance thermoelectric material. Single-crystal X-ray diffraction analysis indicates that Sb2Si2Te6 has a layered two-dimensional structure with Sb3+ cations and [Si2Te6]6− units as building blocks adopting the Fe2P2Se6 structure type. Sb2Si2Te6 is a direct-band-gap semiconductor with valence-band maximum and conduction-band minimum at the Z point in the Brillouin zone, where the band is doubly degenerate. Polycrystalline bulk pellets of Sb2Si2Te6 with randomly packed grains exhibit an intrinsically high thermoelectric figure of merit ZT of ∼1.08 at 823 K. We then create a cellular nanostructure with ultrathin Si2Te3 nanosheets covering the Sb2Si2Te6 grains, which act as a hole-transmitting electron-blocking filter and at the same time cause extra phonon scattering. This dual function of the cellular nanostructure achieves an ultralow thermal conductivity value of ∼0.29 Wm−1K−1 and a high ZT value of ∼1.65 at 823 K for Sb2Si2Te6, along with a high average ZT value of ∼0.98.