Selenophosphate Pb2P2Se6 Single Crystals Growth by Chemical Vapor Transport (CVT) Method for Radiation Detection.

The heavy metal selenophosphate Pb2P2Se6 emerges as a promising room-temperature X-ray/γ-ray detectors due to its high resistivity, robust radiation-blocking capability, and outstanding carrier mobility-lifetime product, etc. However, the high activity of phosphides poses significant impediment to the synthesis and single crystal growth. In this work, we have prepared high-quality Pb2P2Se6 single crystals with using the chemical vapor transport (CVT) method. The XRD analysis combined with EDS result confirmed the uniform composition of the resulting as-grown single crystals, while UV-Vis-NIR transmittance spectra revealed the bandgap of 1.89 eV. Selected area electron diffraction patterns indicated the crystal belonged to the P21/c(14) space group. Additionally, the Au/Pb2P2Se6/Au device is fabricated, which exhibits a robust X-ray response with a sensitivity of 648.61 µC Gy-1 cm-2 at 400 V mm-1 under 50 kVp. Notably, the device also excels in alpha particle detection, boasting a resolution of ~14.48 % under a bias of 400 V bias. The hole mobility-lifetime product (µτ)h of Pb2P2Se6 is estimated to be ~2.58×10-5 cm2 V-1. The results underscore potential applications of Pb2P2Se6 crystal is in the field of the semiconductor radiation detectors.

Effects of interlayer interactions on the nanoindentation response of freely suspended multilayer gallium telluride.

Freestanding indentation is a widely used method to characterise the elastic properties of two-dimensional (2D) materials. However, many controversies and confusion remain in this field due to the lack of appropriate theoretical models in describing the indentation responses of 2D materials. Taking the multilayer gallium telluride (GaTe) as an example, in this paper we conduct a series of experiments and simulations to achieve a comprehensive understanding of its freestanding indentation behaviours. Specifically, the freestanding indentation experiments show that the elastic properties of the present multilayer GaTe with a relatively large thickness can only be extracted from the bending stage in the indentation process rather than the stretching stage widely utilised in the previous studies on thin 2D materials, since the stretching stage of thick 2D materials is inevitably accompanied with severe plastic deformations. In combination with existing continuum mechanical models and finite element simulations, an extremely small Young's modulus of multilayer GaTe is obtained from the nanoindentation experiments, which is two orders of magnitude smaller than the value obtained from first principles calculations. Our molecular dynamics (MD) simulations reveal that this small Young's modulus can be attributed to the significant elastic softening in the multilayer GaTe with increasing thickness and decreasing length. It is further revealed in MD simulations that this size-induced elastic softening originates from the synergistic effects of interlayer compression and interlayer shearing in the multilayer GaTe, both of which, however, are ignored in the existing indentation models. To consider these effects of interlayer interactions in the theoretical modelling of the freestanding indentation of multilayer GaTe, we propose here novel multiple-beam and multiple-plate models, which are found to agree well with MD results without any additional parameters fitting and thus can be treated as more precise theoretical models in characterising the freestanding indentation behaviours of 2D materials.

Anisotropic dielectric behavior of layered perovskite-like Cs3Bi2I9 crystals in the terahertz region.

The ternary metal halide perovskites have gradually attracted attention for application in the optoelectronic field, owing to their tunable crystal structure and appropriate bandgap. Lead free Cs3Bi2I9 perovskite, with a 0D layered structure containing molecular [Bi2I9]3- dimers, exhibits prominent optical and electrical anisotropies. Here, the anisotropic properties of the Cs3Bi2I9 crystals were evaluated using terahertz time-domain spectroscopy (THz-TDS); meanwhile, the effect of phonon vibration on the THz transmission was confirmed using density functional perturbation theory (DFPT). Accordingly, the refractive index and extinction coefficient are estimated using THz-TDS, thanks to the high transmission in the range of 0.2-0.9 THz. The anisotropic refractive index was observed for the Cs3Bi2I9 crystals, and was found to be 3.2-3.7 for the (100) plane (CBI(100)) in contrast to 2.8-3.2 for the (001) plane (CBI(001)). Furthermore, the Lorentz model was employed to extract the dielectric constant of Cs3Bi2I9, in which anisotropy is obviously indicated by the static dielectric constant and the high-frequency dielectric constant. These anisotropic behaviors are determined by the dipole moment, which is attributed to the anisotropic packing density of [Bi2I9]3- dimers. This study is significant and provides a deeper insight into the anisotropic photoelectric properties of Cs3Bi2I9, thus contributing to the development of metal halide perovskites in the field of optoelectronics.

Investigation on X-Ray Photocurrent Response of CdZnTe Photon Counting Detectors.

Counting rate is an important factor for CdZnTe photon counting detectors as high-flux devices. Until recently, there has been a lack of knowledge on the relationship between X-ray photocurrent response and the photon counting performance of CdZnTe detectors. In this paper, the performance of linear array 1 × 16-pixel CdZnTe photon counting detectors operated under different applied biases is investigated. The relation between experimental critical flux and applied bias show an approximate quadratic dependence, which agrees well the theoretical prediction. The underlying relationship among X-ray photocurrents, carrier transport properties, and photon counting performance was obtained by analyzing X-ray current-voltage and time current curves. The typical X-ray photocurrent curve can be divided into three regions, which may be explained by the photoconductive gain mechanism and electric field distortion characteristics. To keep CdZnTe photon counting detectors working in a "non-polarized state", the applied bias should be set on the left side of the "valley region" (high bias direction) in the X-ray I-V curves. This provides an effective measurement for determining the proper working bias of CdZnTe detectors and screening photon counting detector crystals.

Effect of Deep-Level Defects on the Performance of CdZnTe Photon Counting Detectors.

The effect of deep-level defects is a key issue for the applications of CdZnTe high-flux photon counting devices of X-ray irradiations. However, the major trap energy levels and their quantitive relationship with the device's performance are not yet clearly understood. In this study, a 16-pixel CdZnTe X-ray photon counting detector with a non-uniform counting performance is investigated. The deep-level defect characteristics of each pixel region are analyzed by the current-voltage curves (I-V), infrared (IR) optical microscope photography, photoluminescence (PL) and thermally stimulated current (TSC) measurements, which indicate that the difference in counting performance is caused by the non-uniformly distributed deep-level defects in the CdZnTe crystals. Based on these results, we conclude that the CdZnTe detectors with a good photon counting performance should have a larger Te cd 2 + and Cd vacancy-related defect concentration and a lower A-center and Tei concentration. We consider the deep hole trap Tei, with the activation energy of 0.638-0.642 eV, to be the key deep-level trap affecting the photon counting performance. In addition, a theoretical model of the native defect reaction is proposed to understand the underlying relationships of resistivity, deep-level defect characteristics and photon counting performance.

Improvement to the Carrier Transport Properties of CdZnTe Detector Using Sub-Band-Gap Light Radiation.

The effects of sub-band-gap light radiation on the performance of CdZnTe photon-counting X-ray detectors were studied using infrared light with different wavelengths in the region of 980⁻1550 nm. The performance of the detectors for X-ray detection was improved by the radiation of infrared light with the wavelengths of 1200 nm and 1300 nm. This was because the increase of the electron indirect transition, and the weakening of the built-in electric field induced by the trapped holes, reduced the drift time of the carrier, and increased the charge collection efficiency. To further analyze the intrinsic behavior of the trapped charge, the deep-level defects of CdZnTe crystal were measured by thermally stimulated current spectroscopy (TSC). The deep-level defect indicated by the trap named T4 in TSC spectra with the ionization energy of 0.43 eV should be responsible for the performance deterioration of CdZnTe detectors.

Effects of Ga-Te interface layer on the potential barrier height of CdTe/GaAs heterointerface.

The interface layer has great significance on the potential barrier height of the CdTe/GaAs heterointerface. In this study, the electronic properties of the CdTe/GaAs heterostructure prepared by molecular beam epitaxy was investigated in situ by synchrotron radiation photoemission spectroscopy for CdTe thicknesses ranging from 3.5 to 74.6 Å. During CdTe deposition, an As-Te and Ga-Te interface reaction occurred, which caused the out diffusion of Ga. As a result a stable GaTe interface dipole layer (more than 30 Å) was formed, which reduced the potential barrier height by 0.38 eV. The potential barrier height was in proportion to the chemical bonding density and thickness of the Ga-Te interface layer. These results provide a more fundamental understanding of the influencing mechanism of the interface layer on the potential barrier height of the CdTe/GaAs heterointerface.

Correction: Effects of Ga-Te interface layer on the potential barrier height of CdTe/GaAs heterointerface.

Correction for 'Effects of Ga-Te interface layer on the potential barrier height of CdTe/GaAs heterointerface' by Shouzhi Xi et al., Phys. Chem. Chem. Phys., 2016, 18, 2639-2645.

Study on the Low Temperature Photoluminescence Spectra of II-VI Group Telluride Bulk Crystals.

The dominant point defects in II-VI group telluride bulk crystals grown from melt usually varied due to different growth conditions and cooling history, in turn affect the electrical and optical behaviors of corresponding single crystals and devices. Low temperature photoluminescence (PL) spectra acts as a contact-less and non-destructive technique, can be used to evaluate the behaviors of point defects and impurities in the as-grown telluride bulk crystals. With the purpose of comparing the defect structures in un-doped ZnTe and CdTe crystals grown under Te-rich condition, 8. 6 K PL spectra were obtained. The conductivity type and resistivity were investigated by Hall-effect measurements at room temperature (RT). For p-type low resistivity ZnTe crystal, the intensity of. free electron to neutral acceptor (e, A(0)) transition is higher than the donor-acceptor pair (DAP) transition, which predominates in the PL spectra. However, in the contrary, DAP peak dominates the PL emissions for n-type high resistivity CdTe. This difference is mainly attributed to the distinct properties of the grown-in point defects due to different growth. velocities and cooling processes. In terms of the un-doped CdZnTe crystal grown under stoichiometry, neutral donor bound exciton (D(0), X) emission is predominated in the 9.2 K PL spectra, with the intensity of (e, A(0)) peak is higher than DAP peak, which then overlaps to each other when the temperature higher then 15 K. In the case of In-doped CdZnTe crystal grown by Te-rich situation, A-center emission is clearly observed, which introduces an energy level approximately of 0.15 eV, with the intensity proportional to the concentration of indium dopant. This defect is seemingly related to the complex of [In(Cd)+V(Cd)2-]- formed by a shallow donor In(Cd) and Cd vacancy.

High energy resolution CsPbBr3 alpha particle detector with a full-customized readout application specific integrated circuit.

α particles must be monitored to be managed as radioactive diagnostic agents or nuclear activity indicators. The new generation of perovskite detectors suffer from limited energy resolution, which affects spectroscopy and imaging applications. Here, we report that the solution-grown CsPbBr3 crystal exhibits a low and stable dark current (34.6 nA·cm-2 at 200 V) by thinning the as-grown crystal to decrease the high concentration CsPb2Br5 phase near the surface. The introduction of the Schottky electrode for the CsPbBr3 detector further reduces the dark current and improves the high-temperature stability. An energy resolution of 6.9% is achieved with the commercial electronic system, while the effects of air scattering and absorption are investigated. Moreover, 1.1% energy resolution is recognized by a full-customized readout application-specific integrated circuit without any additional signal processing, which matches well with the given parameters of the CsPbBr3 detector by reducing the parasitic capacitance and electronic noise.

Heavy-to-light electron transition enabling real-time spectra detection of charged particles by a biocompatible semiconductor.

The current challenge of wearable/implantable personal dosimeters for medical diagnosis and radiotherapy applications is lack of suitable detector materials possessing both excellent detection performance and biocompatibility. Here, we report a solution-grown biocompatible organic single crystalline semiconductor (OSCS), 4-Hydroxyphenylacetic acid (4HPA), achieving real-time spectral detection of charged particles with single-particle sensitivity. Along in-plane direction, two-dimensional anisotropic 4HPA exhibits a large electron drift velocity of 5 × 105 cm s-1 at "radiation-mode" while maintaining a high resistivity of (1.28 ± 0.003) × 1012 Ω·cm at "dark-mode" due to influence of dense π-π overlaps and high-energy L1 level. Therefore, 4HPA detectors exhibit the record spectra detection of charged particles among their organic counterparts, with energy resolution of 36%, (µt)e of (4.91 ± 0.07) × 10-5 cm2 V-1, and detection time down to 3 ms. These detectors also show high X-ray detection sensitivity of 16,612 µC Gyabs-1 cm-3, detection of limit of 20 nGyair s-1, and long-term stability after 690 Gyair irradiation.

High-Performance Industrial-Grade CsPbBr3 Single Crystal by Solid-Liquid Interface Engineering.

All-inorganic metal halide perovskite CsPbBr3 crystal is regarded as an attractive alternative to high purity Ge and CdZnTe for room temperature γ-ray detection. However, high γ-ray resolution is only observable in small CsPbBr3 crystal; more practical and deployable large crystal exhibits very low, and even no detection efficiency, thereby thwarting prospects for cost-effective room temperature γ-ray detection. The poor performance of large crystal is attributed to the unexpected secondary phase inclusion during crystal growth, which traps the generated carriers. Here, the solid-liquid interface during crystal growth is engineered by optimizing the temperature gradient and growth velocity. This minimizes the unfavorable formation of the secondary phase, leading to industrial-grade crystals with a diameter of 30 mm. This excellent-quality crystal exhibits remarkably high carrier mobility of 35.4 cm2 V-1 s-1 and resolves the peak of 137 Cs@ 662 keV γ-ray at an energy resolution of 9.91%. These values are the highest among previously reported large crystals.

Oriented preparation of Large-Area uniform Cs2TeI6 perovskite film for high performance X-ray detector.

Large-area flexible perovskite films are attracting widespread research interest for applications in wearable solar cells, portable photodetectors, bendable X-ray imaging detectors and other implantable optoelectronic devices. In this work, a facile mobile platform assisted electrospray method is developed to prepare large-area (100 cm2) lead-free Cs2TeI6 film on flexible polyimide substrate. The spraying parameters are coupled with the growth temperature to achieve a dynamic balance. The as-prepared film by optimized process shows high uniformity in grain size, thickness and X-ray response without pinholes and cracks. Moreover, oriented nucleation is more likely to occur on the flexible organic substrates for less growth stress and mismatch stress, leading to preferred (222) plane orientation. X-ray detectors prepared with the films exhibit a resistivity of 1.9 × 1011 Ω·cm, an X-ray sensitivity of 226.8 µC⋅Gyair-1⋅cm-2 and a transient response rise time as fast as 42 ms under 50 kV X-ray at an electrical field of 6.67 × 103 V·mm-1. The modified electrospray method shows great potential applications for large-area devices of radiography, solar cell and other optoelectronic devices.

Effects of Si and Sr elements on solidification microstructure and thermal conductivity of Al-Si-based alloys.

Effects of Si and Sr on solidification microstructure and thermal conductivity of Al-Si binary alloys and Al-9Si-Sr ternary were investigated, respectively, with a special focus on the relationship between solidification microstructure and thermal conductivity. It was found that (i) in Al-Si binary alloys, with increasing Si content, α-Al grain size increases and then decreases when Si content is over 7 wt%, while the percentage of eutectic Si continuously increases, which significantly decreases the thermal conductivity and electrical conductivity, and (ii) in Al-9Si-Sr ternary alloys, the presence of Sr has no significant effect on α-Al grain, but effectively modifies eutectic Si and significantly improves the thermal and electrical conductivity. On this basis, two theoretical calculation models [the Maxwell model and the Hashin-Shtrikman (H-S) model] were used to elucidate the relationship between solidification microstructure and thermal conductivity. Compared with the Maxwell model, the H-S model fits better with the measured values. The obtained results are very helpful to the precise composition control during alloy design and recycling of Al-Si-based alloys with the aim to further improve the thermal conductivity of Al-Si-based alloys. Supplementary Information: The online version contains supplementary material available at 10.1007/s10853-022-07045-7.

Two-Dimensional Dion-Jacobson Perovskite (NH3C4H8NH3)CsPb2Br7 with High X-ray Sensitivity and Peak Discrimination of α-Particles.

Two-dimensional (2D) halide perovskites have attracted extensive interest because of their excellent optoelectronic properties, structural diversity, and promising stability. Herein, we grow a novel 2D Dion-Jacobson halide perovskite, (BDA)CsPb2Br7 (BDA = 1,4-butanediamine, NH3C4H8NH32+), which exhibits a large bandgap (∼2.76 eV), high resistivity (∼4.35 × 1010 Ω·cm), and considerable switching ratio (>700), indicating great potential for radiation detection. Both experimental and calculated results demonstrate that (BDA)CsPb2Br7 has a significantly improved mobility compared to those of Ruddlesden-Popper perovskites (BA)2CsPb2Br7 and (i-BA)2CsPb2Br7, which is attributed to the shorter interlayer distance leading to the enhanced orbital interactions. The resulting (BDA)CsPb2Br7 detector along the out-of-plane direction achieves a high X-ray sensitivity of 725.5 µC·Gy-1·cm-2. Another fascinating attribute is that the detector exhibits good peak discrimination with an energy resolution of ∼37% when illuminated by the 241Am@5.48 MeV α-particles under a negative bias of 260 V. These results provide a broad prospect for 2D Dion-Jacobson perovskites for future radiation detection applications.

Improved Crystallization Quality of FAPbBr3 Single Crystals by a Seeded Solution Method.

Solution-grown hybrid perovskite, FAPbBr3, has attracted great attentions recently due to its inspiring optoelectronic properties and low-cost preparation method. However, challenges of solution growth for FAPbBr3 bulk crystals remain yet, such as uncontrollable crystalline morphologies, irregular shapes, and limited crystal sizes, which are attributed to the dense crystallization nucleus. In this work, we investigate the effects of growth conditions and seed behaviors on the crystallization quality and the yield of FAPbBr3 single crystals. First, the spontaneous nucleation is tailored by optimizing the precursor concentration and heating rate. Furthermore, the seeded crystals are introduced to solve the issues related to the morphology and the yield of single crystals. Based on the above-mentioned investigations, an optimized growth method, a seeded solution method, under a heating rate of 0.1 °C/h is proposed, and centimeter-scale FAPbBr3 single crystals with a very narrow FWHM of high-resolution X-ray diffraction rocking curves and a high yield of ∼100% of single crystals are obtained. The resulting FAPbBr3 single crystal exhibits a bulk resistivity of 3.42 × 109 Ω·cm and a superior ION/IOFF ratio over 104 under 405 nm light at a bias of 10 V. Finally, the pulse height spectra with an energy resolution of ∼21.4% are also achieved based on an AZO/FAPbBr3/Au detector, illuminated using an uncollimated 241Am@5.49 MeV α-particle source at room temperature. This modified seeded solution method shows great potential in preparing high-quality and high-yield perovskite single crystals.

Unusual Deformation and Fracture in Gallium Telluride Multilayers.

The deformation and fracture mechanism of two-dimensional (2D) materials are still unclear and not thoroughly investigated. Given this, mechanical properties and mechanisms are explored on example of gallium telluride (GaTe), a promising 2D semiconductor with an ultrahigh photoresponsivity and a high flexibility. Hereby, the mechanical properties of both substrate-supported and suspended GaTe multilayers were investigated through Berkovich-tip nanoindentation instead of the commonly used AFM-based nanoindentation method. An unusual concurrence of multiple pop-in and load-drop events in loading curve was observed. Theoretical calculations unveiled this concurrence originating from the interlayer-sliding mediated layers-by-layers fracture mechanism in GaTe multilayers. The van der Waals force dominated interlayer interactions between GaTe and substrates was revealed much stronger than that between GaTe interlayers, resulting in the easy sliding and fracture of multilayers within GaTe. This work introduces new insights into the deformation and fracture of GaTe and other 2D materials in flexible electronics applications.

Solution-Grown Formamidinium Hybrid Perovskite (FAPbBr3) Single Crystals for α-Particle and γ-Ray Detection at Room Temperature.

Compared with the widely reported MAPbBr3 single crystals, formamidinium-based (FA-based) hybrid perovskites FAPbBr3 (FPB) with superior chemical and structure stability are expected to be more efficient and perform as more reliable radiation detectors at room temperature. Here, we employ an improved inverse temperature crystallization method to grow FPB bulk single crystals, where issues associated with the retrograde solubility behavior are resolved. A crystal growth phase diagram has been proposed, and accordingly, growth parameters are optimized to avoid the formation of NH4Pb2Br5 secondary phase. The resulting FPB crystals exhibit a high resistivity of 2.8 × 109 Ω·cm and high electron and hole mobility-lifetime products (µτ) of 8.0 × 10-4 and 1.1 × 10-3 cm2·V-1, respectively. Simultaneously, the electron and hole mobilities (µ) are evaluated to be 22.2 and 66.1 cm2·V-1·s-1, respectively, based on the time-of-flight technique. Furthermore, a Au/FPB SC/Au detector is constructed that demonstrates a resolvable gamma peak from 59.5 keV 241Am γ-rays at room temperature for the first time. An energy resolution of 40.1% is obtained at 30 V by collecting the hole signals. These results demonstrate the great potential of FAPbBr3 as a hybrid material for γ-ray spectroscopy and imaging.

Enhanced Transmission from Visible to Terahertz in ZnTe Crystals with Scalable Subwavelength Structures.

The zinc blend nonlinear crystal of zinc telluride (ZnTe) is currently one of the most commonly used electro-optical material for terahertz (THz) probe and imaging. We report herein how to engineer the surface behavior of a ZnTe single crystal to design subwavelength structures (SWSs) for enhancing ultrabroadband transmission. Polystyrene (PS) nanoparticle monolayers with a maximum coverage of 85.2% were produced on the ZnTe crystal by an eccentric spin-coating technique combined with surface wettability engineering. Subsequently, the well-defined conical SWS arrays were fabricated on the ZnTe crystal by reactive ion etching over the PS monolayer template, with the size of the SWS arrays customized by optimizing the etching process. Finally, we demonstrated ultrabroadband antireflection on the surface structured ZnTe crystals in the visible-near-infrared, infrared, and terahertz regions with transmittance increase of 11.6%, 10.0%, and 24.8%, which are attributed to the decrease of surface Fresnel reflection by SWS. Notably, in 0.2-1.0 THz, the transmittance reached over 70%. Our work provides a new strategy to enhance the THz generation efficiency and detection sensitivity based on ZnTe crystals by surface engineering.

High-Stability Flexible X-ray Detectors Based on Lead-Free Halide Perovskite Cs2TeI6 Films.

Normal flat panel X-ray detectors are confined in imaging of curved surfaces and three-dimensional objects. Except that, their rigid panels provide uncomfortable user experience in medical diagnosis. Here, we report a flexible X-ray detector fabricated by the combination of a lead-free Cs2TeI6 perovskite film and a polyimide (PI) substrate. High-quality Cs2TeI6 polycrystalline films are prepared by a low-temperature electrospraying method. The resistivity even remained at the level of 1011 Ω·cm after 100 cycles of bending tests with a low bending radius of 10 mm. The resulting flexible Cs2TeI6 detectors exhibit better response stability than those based on rigid SnO2:F glass (FTO), which is attributed to the superior crystallization of films and the growth stress relief of flexible substrates. Furthermore, an X-ray sensitivity of 76.27 µC·Gyair-1·cm-2 and a detection limit of 0.17 µGyair·s-1 are achieved. A series of distortion-free clear X-ray images are obtained for objects with different materials and densities. These findings provide insights into flexible X-ray detectors based on perovskite films and motivate research in wearable X-ray detectors for medical radiography and dose monitoring.