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.

Summary Mixed-dimensional 2D/3D hybrid halide perovskites retain the stability of 2D perovskites (formula (A′)₂(A)_n−1Pb_nI3_n+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−1Pb_nI_3n+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⟩.

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.

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.

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.

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.

State-of-the-art p-i-n-based 3D perovskite solar cells (PSCs) use nickel oxide (NiO_x) 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 NiO_x 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 NiO_x 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 NiO_x 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.

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 K_α X-rays (22.4 keV), and detects 122 keV γ-rays from ⁵⁷Co radiation source. The electron mobility-lifetime product µ_eτ_e 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.

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-1Pb_nI_3n+1 (n = 2–6) and (HA)₂(MA)_n-1Pb_nI_3n+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-1Pb_nI_3n+1 (n = 2–5) and (4AMP)(MA)_n-1Pb_nI_3n+1 (n = 2–4) have a positive enthalpy of formation. In addition, for the (PA)₂(MA)_n−1Pb_nI_3n+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.

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 (C_mH_2m+1NH₃)₂(CH₃NH₃)Pb₂Br₇ (m = 6–8) that belong to the Ruddlesden–Popper structure type, incorporating long chain alkyl-monoammonium cations (C_mH_2m+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 (C_mH_2m+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 (C_mH_2m+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.

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 x > 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.

We report a novel hierarchical microstructure in the PbSe–CdSe system, which collectively contributes to significant enhancement in thermoelectric performance, with ZT_ave ∼ 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 Pb_0.98K_0.02Se–6%CdSe.

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.