Extreme γ-Ray Radiation Tolerance of Spectrometer-Grade CsPbBr3 Perovskite Detectors.

The perovskite compound CsPbBr3 has recently been discovered as a promising room-temperature semiconductor radiation detector, offering an inexpensive and easy-to-manufacture alternative to the current benchmark material Cd1-x Znx Te (CZT). The performance of CsPbBr3 sensors is evaluated under harsh conditions, such as high radiation doses often found in industrial settings and extreme radiation in space. Results show minimal degradation in detector performance after exposure to 1 Mrad of Co-60 gamma radiation, with no significant change to energy resolution or hole mobility and lifetime. Additionally, many of the devices are still functional after being exposed to a 10 Mrad dose over 3 days, and those that do not survive can still be refabricated into working detectors. These results suggest that the failure mode in these devices is likely related to the interface between the electrode and material and their reaction, or the electrode itself and not the material itself. Overall, the study suggests that CsPbBr3 has high potential as a reliable and efficient radiation detector in various applications, including those involving extreme fluxes and energies of gamma-ray radiation.

Laser Scribing for Electrode Patterning of Perovskite Spectrometer-Grade CsPbBr3 Gamma-ray Detectors.

Making semiconductor radiation detectors that work at room temperature relies heavily on the deposition and pixelation of electrodes. Electrode patterning of perovskite solar cells widely implements laser scribing techniques, which is a convenient, scalable, and inexpensive technique. However, this method has not found its application in radiation detector patterning yet, and the question whether laser scribing can achieve high-quality patterns with minimum damage to a detector crystal and low interpixel cross-talk remains largely unanswered. To prove that laser scribing is a practical method for electrode patterning on perovskite CsPbBr3 detectors, we use the material to create a variety of patterns. A very low lateral leakage current (60 nA at 10 V) and high mobility-lifetime product (9.7(3) × 10-4 cm2/V) were observed between the pixel and the guard ring in tests of single-pixel devices with a separation of 200 or 100 µm between the central electrode and the guard ring. The 122 and 136 keV photopeaks in 57Co gamma-ray spectra were very well resolved with an energy resolution of up to 6.1% at 122 keV. A further reduction in gap size to 50 µm is conceivable, but more process optimization is needed.

Ultrahigh-Flux X-ray Detection by a Solution-Grown Perovskite CsPbBr3 Single-Crystal Semiconductor Detector.

Solution-processed perovskites are promising for hard X-ray and gamma-ray detection, but there are limited reports on their performance under extremely intense X-rays. Here, a solution-grown all-inorganic perovskite CsPbBr3 single-crystal semiconductor detector capable of operating at ultrahigh X-ray flux of 1010 photons s-1 mm-2 is reported. High-quality solution-grown CsPbBr3 single crystals are fabricated into detectors with a Schottky diode structure of eutectic gallium indium/CsPbBr3 /Au. A high reverse-bias voltage of 1000 V (435 V mm- 1 ) can be applied with a small and stable dark current of ≈60-70 nA (≈9-10 nA mm- 2 ), which enables a high sensitivity larger than 10 000 µC Gyair -1 cm- 2 and a simultaneous low detection limit of 22 nGyair s- 1 . The CsPbBr3 semiconductor detector shows an excellent photocurrent linearity and reproducibility under 58.61 keV synchrotron X-rays with flux from 106 to 1010 photons s- 1 mm- 2 . Defect characterization by thermally stimulated current spectroscopy shows a similar low defect density of a synchrotron X-ray and a lab X-ray irradiated device. Solid-state nuclear magnetic resonance spectroscopy suggests that the excellent performance of the solution-grown CsPbBr3 single crystal may be associated with its good short-range order, comparable to the spectrometer-grade melt-grown CsPbBr3 .

Perovskite CsPbBr3 Single Crystal Detector for High Flux X-Ray Photon Counting.

X-ray photon-counting detectors capable of resolving the energies of single X-ray photons are critical in medical imaging, and a high count rate is essential for photon-counting detectors. Here, we report the performance of the perovskite CsPbBr3 single-crystal semiconductor detector for X-ray photon counting. The CsPbBr3 detector noise floor, energy response linearity, energy resolution, count rate, and polarization were evaluated. By fine-tuning the detector working conditions, our CsPbBr3 detector with a planar electrode can count an incident X-ray photon rate of ~0.099 and ~0.336 Mcps/pixel at 10% and 30% deadtime loss, respectively, with corresponding energy resolutions of ~18% at 59.5 keV and ~12% at 122 keV at same pulse processing conditions. We also demonstrated that our CsPbBr3 detectors show negligible polarization under an X-ray flux of ~0.45 M photons/s/mm2 for the typical timescale of multiple clinical X-ray scans, such as 1 s- 100 s. Our evaluation demonstrates the high potential of CsPbBr3 detectors for X-ray photon-counting applications in medical and industrial diagnostics with a lower cost than the current state-of-the-art.

Thick-Layer Lead Iodide Perovskites with Bifunctional Organic Spacers Allylammonium and Iodopropylammonium Exhibiting Trap-State Emission.

The nature of the organic cation in two-dimensional (2D) hybrid lead iodide perovskites tailors the structural and technological features of the resultant material. Herein, we present three new homologous series of (100) lead iodide perovskites with the organic cations allylammonium (AA) containing an unsaturated C═C group and iodopropylammonium (IdPA) containing iodine on the organic chain: (AA)2MAn-1PbnI3n+1 (n = 3-4), [(AA)x(IdPA)1-x]2MAn-1PbnI3n+1 (n = 1-4), and (IdPA)2MAn-1PbnI3n+1 (n = 1-4), as well as their perovskite-related substructures. We report the in situ transformation of AA organic layers into IdPA and the incorporation of these cations simultaneously into the 2D perovskite structure. Single-crystal X-ray diffraction shows that (AA)2MA2Pb3I10 crystallizes in the space group P21/c with a unique inorganic layer offset (0, <1/2), comprising the first example of n = 3 halide perovskite with a monoammonium cation that deviates from the Ruddlesden-Popper (RP) halide structure type. (IdPA)2MA2Pb3I10 and the alloyed [(AA)x(IdPA)1-x]2MA2Pb3I10 crystallize in the RP structure, both in space group P21/c. The adjacent I···I interlayer distance in (AA)2MA2Pb3I10 is ∼5.6 Å, drawing the [Pb3I10]4- layers closer together among all reported n = 3 RP lead iodides. (AA)2MA2Pb3I10 presents band-edge absorption and photoluminescence (PL) emission at around 2.0 eV that is slightly red-shifted in comparison to (IdPA)2MA2Pb3I10. The band structure calculations suggest that both (AA)2MA2Pb3I10 and (IdPA)2MA2Pb3I10 have in-plane effective masses around 0.04m0 and 0.08m0, respectively. IdPA cations have a greater dielectric contribution than AA. The excited-state dynamics investigated by transient absorption (TA) spectroscopy reveal a long-lived (∼100 ps) trap state ensemble with broad-band emission; our evidence suggests that these states appear due to lattice distortions induced by the incorporation of IdPA cations.

Giant band splittings in EuS and EuSe magnetic semiconductor nanocrystals.

Using magnetic circular dichroism (MCD) spectroscopy, we demonstrate giant temperature- and field-dependent conduction-band splittings in colloidal EuS and EuSe nanocrystals.

Two-Dimensional van der Waals Nanoplatelets with Robust Ferromagnetism.

We have synthesized unique colloidal nanoplatelets of the two-dimensional (2D) van der Waals ferromagnet CrI3 and have characterized these nanoplatelets structurally, magnetically, and by magnetic circular dichroism spectroscopy. The CrI3 nanoplatelets have lateral dimensions of ∼25 nm and thicknesses of only ∼4 nm, corresponding to just a few CrI3 monolayers. Magnetic and magneto-optical measurements demonstrate robust 2D ferromagnetic ordering with Curie temperatures similar to bulk CrI3, despite their small size. These data also show magnetization steps akin to those observed in micron-sized few-layer 2D sheets associated with concerted spin-reversal of individual CrI3 layers within few-layer van der Waals stacks. Similar data have also been obtained for CrBr3 and anion-alloyed Cr(I1-xBrx)3 nanoplatelets. These results represent the first example of lateral nanostructures of 2D van der Waals ferromagnets of any composition. The demonstration of robust ferromagnetism at nanometer lateral dimensions opens new doors for miniaturization in spintronics devices based on van der Waals ferromagnets.

Colloidal Nanocrystals of Lead-Free Double-Perovskite (Elpasolite) Semiconductors: Synthesis and Anion Exchange To Access New Materials.

Concerns about the toxicity and instability of lead-halide perovskites have driven a recent surge in research toward alternative lead-free perovskite materials, including lead-free double perovskites with the elpasolite structure and visible bandgaps. Synthetic approaches to this class of materials remain limited, however, and no examples of heterometallic elpasolites as nanomaterials have been reported. Here, we report the synthesis and characterization of colloidal nanocrystals of Cs2AgBiX6 (X = Cl, Br) elpasolites using a hot-injection approach. We further show that postsynthetic modification through anion exchange and cation extraction can be used to convert these nanocrystals to new materials including Cs2AgBiI6, which was previously unknown experimentally. Nanocrystals of Cs2AgBiI6, synthesized via a novel anion-exchange protocol using trimethylsilyl iodide, have strong absorption throughout the visible region, confirming theoretical predictions that this material could be a promising photovoltaic absorber. The synthetic methodologies presented here are expected to be broadly generalizable. This work demonstrates that nanocrystal ion-exchange reactivity can be used to discover and develop new lead-free halide perovskite materials that may be difficult or impossible to access through direct synthesis.

Strong Dependence of Quantum-Dot Delayed Luminescence on Excitation Pulse Width.

Delayed luminescence involving charge-carrier trapping and detrapping has recently been identified as a widespread and possibly universal phenomenon in colloidal quantum dots. Its near-power-law decay suggests a relationship with blinking. Here, using colloidal CuInS2 and CdSe quantum dots as model systems, we show that short (nanosecond) excitation pulses yield less delayed luminescence intensity and faster delayed luminescence decay than observed with long (millisecond) square-wave excitation pulses. Increasing the excitation power also affects the delayed luminescence intensity, but the delayed luminescence decay kinetics appear much less sensitive to excitation power than to excitation pulse width. An idealized four-state kinetic model reproduces the major experimental trends and highlights the very slow approach to steady state during photoexcitation, stemming from extremely slow detrapping of the metastable charge-separated state responsible for delayed luminescence. The impacts of these findings on proposed relationships between delayed luminescence and blinking are discussed.

A Selective Cation Exchange Strategy for the Synthesis of Colloidal Yb3+-Doped Chalcogenide Nanocrystals with Strong Broadband Visible Absorption and Long-Lived Near-Infrared Emission.

Doping lanthanide ions into colloidal semiconductor nanocrystals is a promising strategy for combining their sharp and efficient 4f-4f emission with the strong broadband absorption and low-phonon-energy crystalline environment of semiconductors to make new solution-processable spectral-conversion nanophosphors, but synthesis of this class of materials has proven extraordinarily challenging because of fundamental chemical incompatibilities between lanthanides and most intermediate-gap semiconductors. Here, we present a new strategy for accessing lanthanide-doped visible-light-absorbing semiconductor nanocrystals by demonstrating selective cation exchange to convert precursor Yb3+-doped NaInS2 nanocrystals into Yb3+-doped PbIn2S4 nanocrystals. Excitation spectra and time-resolved photoluminescence measurements confirm that Yb3+ is both incorporated within the PbIn2S4 nanocrystals and sensitized by visible-light photoexcitation of these nanocrystals. This combination of strong broadband visible absorption, sharp near-infrared emission, and long (>400 µs) emission lifetimes in a colloidal nanocrystal system opens promising new opportunities for both fundamental-science and next-generation spectral-conversion applications such as luminescent solar concentrators.