Defect Formation Energies Research Articles

Mechanically Resilient and Highly Efficient Flexible Perovskite Solar Cells with Octylammonium Acetate for Surface Adhesion and Stress Relief.

Flexible perovskite solar cells (FPSCs) have advanced significantly because of their excellent power-per-weight performance and affordable manufacturing costs. The unsatisfactory efficiency and mechanical stability of FPSCs are bottleneck challenges that limit their application. Here, we explore the use of octylammonium acetate (OAAc) with a long, intrinsic, flexible molecular chain on perovskite films for surface adhesion and mechanical releasing. The results showed that OAAc with high structural flexibility and strong molecular interactions can act as a mechanical release layer in releasing residual tensile stress, confirmed by the film and device characterizations as well as finite-element simulation. Moreover, the passivation of the OAAc could increase the formation energy of defects including I vacancy, Pb vacancy, and Pb-I antisite. The experimental results showed that the trap states of perovskites were significantly suppressed after OAAc modification, which is beneficial to the construction of high-quality films. With a high open-circuit voltage of 1.196 V, the efficiency of the OAAc-treated devices increased from 23.14% to 25.47% on a rigid substrate (23.12% on a flexible substrate), yielding superior long-term and mechanical durability. The corresponding flexible device retains 74% of the initial value even after 8000 bending cycles at a bending radius of 5 mm.

Effect of dopants and surface oxides on the electronic properties of dislocations in p-type SiC

Abstract Understanding the electronic properties of dislocations is key to tailoring the properties of silicon carbide (SiC)-based electronic devices. By combining the Kelvin probe force microscopy (KPFM) analysis and first-principles calculations, we investigate the effect of aluminum (Al) dopants and surface oxides on the electronic properties of dislocations in p-type SiC. It is found that dislocations, including threading screw dislocations (TSDs), threading edge dislocations (TEDs), and basal plane dislocations (BPDs) create deep acceptor-like states in p-type SiC. The defect levels move upwards in the order of BPD, TED and TSD, which closely relates with the lattice distortion of dislocations. Interestingly, we find that the defect levels of dislocations are higher than that of Al in p-type SiC. First-principles calculations indicate that the defect formation energies of Al dopants at the cores of dislocations are higher than those in the perfect region of p-type SiC, as a result of the steric effect. This indicates that the concentration of Al acceptors at the dislocation cores is lower than that in the perfect region, resulting in the higher defect level positions of dislocations in p-type SiC. Additionally, we find that surface oxidation reduces the difference of defect-level positions between dislocations and the perfect region of p-type SiC, which paves the way for tailoring the electronic properties of dislocations in p-type SiC.

Ab initio computational study of hydration thermodynamics in cubic yttria-stabilized zirconia

Abstract Hydration is a major process that controls defect equilibria in oxides through the exchange of oxygen and hydrogen species between the solid and its gaseous environment. For yttria-stabilized zirconia (YSZ), the presence of intrinsic oxygen vacancies that provide charge compensation to the acceptor dopants and the inherent structural disorder pose significant problems towards an understanding of how hydration operates at the atomistic level. First-principles calculations and ab-initio thermodynamics are employed in order to study the hydration reaction in cubic YSZ and the two types of defects appearing therein as reactants and products, the oxygen vacancies and protons, respectively, yielding the defect-formation energies, defect-induced deformation tensors and chemical expansion coefficients. The calculations are based on density-functional theory using a semilocal density functional and a screened-exchange functional approach and take into account the intrinsic structural disorder of the YSZ lattice. The various terms to the free energy of the hydration reaction are determined as a function of temperature and water-vapor partial pressure. The calculations provide estimates of the enthalpy and entropy of hydration in cubic YSZ examining how the solid-state and gas-phase contributions affect the free-energy balance. The final results are discussed in connection with experimental observations of hydration effects in YSZ and other oxides.

First-principles study on electronic and optical properties of perovskite light-emitting diodes CsPb(Br1− xIx)3

CsPb(Br1−xIx)3, a mixed-halide all-inorganic perovskite, is a promising light-emitting diode (LED) material due to its impressive performance. It has been demonstrated that the mixing parameter x of halogen composition significantly influences the luminescence efficiency of CsPb(Br1−xIx)3. However, the underlying microscopic mechanisms remain unclear. Using first-principles calculations, we investigate the effects of anion mixing on the radiative and non-radiative recombination properties of perovskite materials. Simulations on the carrier mobility, exciton binding energy, affinity energy, and defect formation energy of the materials in the CsPb(Br1−xIx)3 system collaboratively revealed that a high ratio off Br is associated with enhanced luminescence efficiency. Specifically, CsPb(Br1−xIx)3 exhibits optimal luminescent performance with a 2:1 bromine-to-iodine ratio, while it shows the performance degradation with a 1:1 ratio. The results demonstrate that the ratio of halogen atoms (Br and I) has a significant influence on the LED properties of cesium-based all-inorganic perovskites CsPb(Br1−xIx)3, providing a valuable guide for the experimental preparation of cesium-based all-inorganic perovskites.

Flexible molecules dedicate to release strain of inverted inorganic perovskite solar cell

The tensile strain in inorganic perovskite films induced by thermal annealing is one of the primary factors contributing to the inefficiency and instability of inorganic perovskite solar cells (IPSCs), which reduces the defect formation energy. Here, a flexible molecule 5-maleimidovaleric acid (5-MVA) was introduced as a strain buffer to release the residual strain of CsPbI2.85Br0.15 perovskite. Maleic anhydride and carboxyl groups in 5-MVA interact strongly with the uncoordinated Pb2+ through Lewis acid-base reaction, thus tightly “pull” the perovskite lattice. The in-between soft carbon chain increased the structural flexibility of CsPbI2.85Br0.15 perovskite materials, which effectively relieved the intrinsic internal strain of CsPbI2.85Br0.15, resisted the corrosion of external strain, and also reduced the formation of defects such as VI and Pb0. In addition, the introduction of 5-MVA improved crystal quality, passivated residual defects, and narrowed energy level barriers. Eventually, power conversion efficiency (PCE) of NiOx-based inverted IPSCs increased from 19.25% to 20.82% with the open-circuit voltage enhanced from 1.164 V to 1.230 V. The release of strain also improved the stability of CsPbI2.85Br0.15 perovskite films and devices.

Exploring the radiation stability and electronic properties of bcc Zr1−xNbx (0 ≤ x ≤ 1) alloy from first-principles

First-principles studies on the radiation stability of Zr and Zr-based alloys are very popular in nuclear material development research. The present work is performed by utilizing the density functional theory for investigating the radiation stability and electronic properties of Zr1-xNbx alloys (0 ≤ x ≤ 1). The simulation study is performed with a structural phase in which Zr and Nb remain distributed in solid solution ratio with bcc structure which resembles the phase as obtained at high temperature. The calculation of formation enthalpy for various structures exhibits that the solid solution is thermodynamically most stable with the 50% Nb concentration. The defect formation energy of the structures possessing defects via Zr or Nb vacancy in the structures is also found to be positive maximum, which suggests that the creation of defect is tougher in the Zr1-xNbx alloy for x = 0.5. Additionally, the observation of electronic properties in terms of electronic density of state reveals that introducing vacancy affected the electronic properties of the perfect structures. Notably, Zr0.5 Nb0.5 shows unique electronic behavior compared to other compositions of Zr-Nb alloy, making it the most stable composition in a harsh radiation environment.

Machine learned interatomic potentials for gas-metal interactions

Abstract Developing interatomic potentials for gas-metal systems is difficult due to the wide range of chemical compositions that the potential must be able to reproduce. There is a need for these types of potentials for studying plasma-material interactions in fusion reactors where gaseous plasma species will implant in metallic reactor components. The challenges presented by these material systems make them suitable candidates for treatment by a machine learning approach, such as that of the spectral neighbor analysis potential (SNAP). However, constraining the dynamics with these more flexible potentials is difficult. In this work, we have developed a SNAP potential for W-N and W-H in order to study the material degradation due to ion implantation in tungsten. We have developed a large set of density functional theory training data spanning multiple chemical environments including gas phase, surface, bulk, and gas-metal configurations. Additional methodologies for developing training data and optimizing the potential for accurately describing fast diffusing impurity species are detailed. The SNAP potential well-reproduces key material properties relevant for modeling plasma-material interactions including defect formation energies, surface adsorption energies, dimer binding energies, and tungsten nitride formation energies. In addition to testing on static energetic properties, the SNAP potential was also used to simulate thermal and dynamic gas-metal interactions, including bulk diffusion, molecular gas adsorption isotherms, and ion implantation. The SNAP potentials are demonstrated to well-reproduce behavior in the wide range of chemical environments investigated, demonstrating the suitability of these machine learned interatomic potentials for future studies of plasma material interactions.

Minimizing vacancy defects with Eu passivation strategy to enable highly efficient perovskite photovoltaics

The intrinsic ionic nature of metal halide perovskites facilitates the formation of abundant defects at grain boundaries (GBs) and surfaces, consequently diminishing the stability and photovoltaic performance of perovskite solar cells (PSCs). Due to the low formation energies and instability of Pb-I bonds, iodine (I) and lead (Pb) vacancies are prevalent high-density point defects. In this study, we explore the use of europium (Eu) as a novel passivator to concurrently address I and Pb vacancies through first-principles calculations. Our results demonstrate that Eu significantly lowers the defect formation energies and adjusts the bandgap, enlarged by vacancy defects, to align with the ideal Shockley-Queisser limit, indicating a strong potential for achieving high power conversion efficiency (PCE). Furthermore, Eu passivation markedly increases the activation energy barriers for ion migration, resulting in substantially reduced migration rates. This suggests a minimal likelihood of I and Pb ion migration on defected surfaces, underscoring Eu’s potential to mitigate hysteresis effects and enhance stability. This work advances our understanding of the passivation effects of rare earth elements and establishes a foundation for designing highly effective passivation strategies to achieve stable and efficient PSCs.

Impact of strain and doping on Li-ion transport in Li3OBr0.5Cl0.5 anti-perovskite solid lithium-ion electrolytes: Insights from DFT and AIMD calculations

The impact of applying strain on the Li-ion transport characteristics of Li3OBr0.5Cl0.5 anti-perovskite solid lithium-ion electrolytes with different doping positions have been systematically studied within the framework of density functional theory. We have examined the changes in bandgap, defect formation energy, and Li-ion migration barrier under triaxial compressive and tensile strains, spanning from −4% to 4 %. Following, by calculating of the diffusion and conductivity of different structures of Li3OBr0.5Cl0.5, we have screened out one structure (Br-ions and Cl-ions are diagonally distributed at each vertex of the lattice) with the highest conductivity at room temperature from six structures of Li3OBr0.5Cl0.5. Afterward, the changes of diffusion, conductivity and MSD of the structure with the highest conductivity at room temperature under applying −4% and 4 % triaxial strains have been studied. The results reveal intriguing implications: Tensile strains increase the defect formation energy for both Li-ion vacancies and Li interstitials, while simultaneously lowering the migration barrier of Li-ion. The influences of the formation energy and migration barrier significantly enhance the conductivity of Li-ion. This consequence is confirmed by AIMD simulation that both diffusivity and conductivity are enhanced under applying tensile strains, while an opposing effect observed under applying compressive strains. These findings unequivocally demonstrate that strains exert a profound influence on Li-ion conductivities in Li3OBr0.5Cl0.5 anti-perovskite electrolytes. Specifically, the application of tensile strain emerges as a promising strategy to augment lithium-ion transport.

Electropositive Metal Doping into Lanthanum Hydride for H− Conducting Solid Electrolyte Use at Room Temperature

Hydride ion (H–) conductors have made remarkable progress in recent years; in particular, the fluorite-type LaH3–δ series exhibits high conductivity around room temperature. However, its intrinsic character of hydrogen non-stoichiometry still makes its application as a solid electrolyte challenging, for which high electronic insulation is essential. Here, Sr-substituted LaH3–δ with slight O2– incorporation, represented as La1–xSrxH3–x–2yOy (0.1 ≤ x ≤ 0.6, y ≤ 0.171), is synthesized, which exhibits H– conductivity of 10–4 - 10–5 S cm–1 at room temperature. The galvanostatic discharge reaction using an all-solid-state cell composed of Ti|La1-xSrxH3-x-2yOy|LaH3–δ shows that the Ti electrode is completely hydrogenated to TiH2 for x ≥ 0.2, whereas a short circuit occurs for x = 0.1. These experimental observations, together with calculation studies on the density of states and the defect formation energy, provide clear evidence that electropositive cation, such as Sr, doping critically suppresses the electron conduction in LaH3–δ. Achieving a superior H–conducting solid electrolyte is a novel milestone in the development of electrochemical devices that utilize its strong reducing ability (Eº(H–/H2) = –2.25 V vs. SHE), such as batteries with high energy density and electrolysis/fuel cells with high efficiency.

Construction of Perovskites Thin Films of Black Orthorhombic Phased Cesium Tin Iodide Using Pulsed Laser Deposition

Introduction: Perovskite solar cells have the potential to achieve higher efficiency than current mainstream silicon-based solar cells. Black orthorhombic (B-γ) phased cesium tin iodide (CsSnI3) perovskite, which contains no lead and organic content leading to be environmentally friendly and thermally stable, respectively, has attracted much more attention than methylammonium lead iodide (MAPbI3), which is the most commonly used photosensitive layer in the perovskite solar cells. However, its photoelectric efficiency is much lower than that of MAPbI3. This low efficiency is attributed to the high defect density in CsSnI3, such as Cs or Sn vacancies which have very low defect formation energies. Thus, it is essential to control the atomic arrangement at the interface and the crystal growth process. The CsSnI3 perovskite exists in two orthorhombic polymorphs at room temperature: one is a yellow 1D double-chain structure and the other is a black 3D perovskite structure (B-γ). The B-γ phase has a band gap of ca. 1.3 eV, which is desirable for the photosensitive layer of the solar cells, in contrast to the Y phase with ca. 2.6 eV band gap. However, the orthorhombic B-γ phase rapidly transitions to the Y phase when some external perturbation (oxygen, moisture, solvent) is applied, and subsequently transfers to photoinactive Cs2SnI6 because of self-doping from Sn2+ to form Sn4+ under ambient-air conditions [1]. Therefore, it is necessary to construct defect-free B-γ CsSnI3 layers on solid substrates. In this study, we attempted to construct defect-free CsSnI3 perovskite thin films on various solid substrates at room temperature using the pulsed laser deposition (PLD) method, which enables the control of crystal growth process at an atomic level. Experiment:To prepare the precursor target for the PLD, stoichiometric amounts of SnI2 and CsI powders were mixed and pressed to a tablet shape. Then it heated to 300ºC and held for 18 h under Ar atmosphere. The target was loaded into the PLD chamber, and CsSnI3 thin films were deposited onto n-typed Si(100), TiO2(100), (110), (001) single crystal and glass substrates at room temperature using a Nd:YAG laser. An Ar working pressure of 1.0 × 10-3 Torr was kept constant during deposition. Following CsSnI3 deposition, an amorphous Tin-doped Indium oxide (ITO) capping layer was applied by PLD to prevent the film oxidation. Result and Discussion: X-ray diffraction (XRD) analysis was conducted for the precursor target and the obtained thin films. It revealed that the precursor target contains only pure B-γ CsSnI3 polycrystals. For the obtained thin films, B-γ CsSnI3 was obtained and it oriented mainly in the [010] direction normal to the surface regardless of the substrate type. In addition, the 2D diffraction images indicated that the orientation ranged within ±10º, also regardless of the substrate type. In the previous study by Kiyek et al. [2], polycrystalline films were obtained by the PLD method with the precursor containing the mixture of the raw materials (CsI and SnI2). On the other hand, in this study, the oriented films were obtained with B-γ CsSnI3 precursor, suggesting that the precursor condition may affect the orientation because the laser plume constantly contains strictly Cs:Sn:I = 1:1:3 composition during ablation. We will discuss this phenomenon in detail.

High-Efficiency Photo-Assisted Large Current-Density Water Splitting with Mott-Schottky Heterojunctions.

The development of bifunctional photogenerated carrier-assisted electrocatalytic (PCA-EC) electrodes that operate with stability at large current-density remains a significant challenge. Herein, we demonstrate a simple sputtering-deposition process to synthesize a novel MnWO4/FeCoNi Mott-Schottky heterojunction coating and deposit it on a pure Ti substrate to prepare high-performance PCA-EC electrodes, which exhibits enhanced light absorption range/intensity and rapidly separated photogenerated electron-hole pairs. This design allows photogenerated electrons to directly participate in the hydrogen evolution reaction (HER), while the strong oxidation of photogenerated holes significantly reduces the defect formation energy of active metals, thereby facilitating the rapid reconstruction of highly active Ni(FeCo)OOH/MnOOH species for the oxygen evolution reaction (OER). As expected, the as-prepared electrode demonstrates the overpotentials of 64 mV for the HER and 204 mV for the OER at 10 mA cm-2 under illumination. Benefiting from the stable interface with Fe/Co/Ni-O-Mn/W bonding units, the dual-electrode photoassisted electrolytic cell achieves long-term stability at current densities of 500 and 1000 mA cm-2. This work provides detailed insights into the enhancement mechanism of PCA-EC and contributes to the development of photo-assisted water splitting electrodes for large current-density applications.

High‐Efficiency Photo‐Assisted Large Current‐Density Water Splitting with Mott‐Schottky Heterojunctions

AbstractThe development of bifunctional photogenerated carrier‐assisted electrocatalytic (PCA‐EC) electrodes that operate with stability at large current‐density remains a significant challenge. Herein, we demonstrate a simple sputtering‐deposition process to synthesize a novel MnWO4/FeCoNi Mott‐Schottky heterojunction coating and deposit it on a pure Ti substrate to prepare high‐performance PCA‐EC electrodes, which exhibits enhanced light absorption range/intensity and rapidly separated photogenerated electron‐hole pairs. This design allows photogenerated electrons to directly participate in the hydrogen evolution reaction (HER), while the strong oxidation of photogenerated holes significantly reduces the defect formation energy of active metals, thereby facilitating the rapid reconstruction of highly active Ni(FeCo)OOH/MnOOH species for the oxygen evolution reaction (OER). As expected, the as‐prepared electrode demonstrates the overpotentials of 64 mV for the HER and 204 mV for the OER at 10 mA cm−2 under illumination. Benefiting from the stable interface with Fe/Co/Ni‐O−Mn/W bonding units, the dual‐electrode photoassisted electrolytic cell achieves long‐term stability at current densities of 500 and 1000 mA cm−2. This work provides detailed insights into the enhancement mechanism of PCA‐EC and contributes to the development of photo‐assisted water splitting electrodes for large current‐density applications.

Longitudinal Piezoelectricity and Polarization-Insensitive Oxidation in Janus vdWs Nb3SeI7.

As a newly discovered Janus van der Waals (vdW) material, semiconducting Nb3SeI7 offers several notable advantages, including spontaneous out-of-plane polarization, facile exfoliation to the monolayer limit, and significant out-of-plane emission dipole in second harmonic generation. These properties make it a promising candidate for piezoelectric and piezophototronic applications in highly efficient energy conversion. However, Nb3SeI7 is prone to oxidation when exposed to oxygen, which can severely limit the exploration and utilization of these intriguing physical properties. Therefore, understanding the oxidation mechanism of pristine Nb3SeI7 and its correlation with electrical polarization-an area that remains largely unexplored-is highly significant. In this study, the out-of-plane piezoelectricity of Nb3SeI7 is experimentally demonstrated, with a piezoelectric coefficient (|d33 eff|) of 0.76nmV-1. Furthermore, by combining near ambient-pressure X-ray photoelectron spectroscopy (NAP-XPS), Time-of-Flight secondary ion mass spectrometry (ToF-SIMS), and Density functional theory (DFT)calculations, it is revealed that the oxidation of Nb3SeI7 is self-limiting and independent of its electrical polarization, owing to the similar defect formation energies of Se and I atoms. This self-limiting and polarization-insensitive oxidation provides valuable insights into the stabilization mechanisms and expands the potential applications of out-of-plane piezoelectricity and other intriguing physical properties in Janus vdW Nb3SeI7.

Near-Infrared Mechanoluminescence from Cr3+-Doped Spinel Nanoparticles for Potential Oral Diseases Detection.

Mechanoluminescence (ML) phosphors have found various promising utilizations such as in non-destructive stress sensing, anti-counterfeiting, and bio stress imaging. However, the reported NIR MLs have predominantly been limited to bulky particle size and weak ML intensity, hindering the further practical applications. For this regard, a nano-sized ZnGa2O4: Cr3+ NIR ML phosphor is synthesized by hydrothermal method. By improving the synthesis method and regulating the chemical composition, the NIR ML (600-1000nm) intensity of such nano-materials has been further enhanced about four times. The reasons for the ML performance difference between micro-/nano- sized phosphors also have been preliminarily analyzed. Additionally, this work probes into the ML mechanism deeply in traps' aspect from band structure and defect formation energy, which can supply significant references for a new approach to develop efficient NIR ML nanoparticles. Finally, due to excellent tissue penetration capability, nano-sized ZnGa2O4:Cr3+ NIR ML phosphor shows great potential applications in biomedical fields such as for the detection of clinical oral diseases.

Stepwise Optimization of Thermoelectric Performance in n‐Type SnS

AbstractTin sulfide (SnS) has been developed as an earth‐abundant and eco‐friendly compound that exhibits promising thermoelectric (TE) performance. While significant achievements are achieved in p‐type SnS, the development of its n‐type counterpart remains at a nascent stage, making it rather essential for developing n‐type SnS to ensure good compatibility in SnS‐based TE devices. In this study, Br is selected as the aliovalent dopant for realizing n‐type transport in SnS. Subsequently, isoelectronic Se alloying and vacancy compensation are utilized to further promote the TE performance for n‐type SnS. Se alloying can effectively narrow the bandgap of SnS for enhancing carrier concentration and diminishing thermal conductivity by strain and mass field fluctuations, while the excess Sn further compensates intrinsic Sn vacancies, thus optimizing carrier concentration and mobility simultaneously. These results are well verified by microstructure characterization and defect formation energy calculations. Consequently, the maximum ZT value of ≈0.7 and quality factor B of ≈1.02 at 823 K can be obtained after stepwise optimization, which is currently the highest value for n‐type SnS thermoelectrics. This study presents a significant progress for n‐type SnS and lays an important foundation for constructing all‐SnS‐based TE devices.

Realizing Ultrahigh Conversion Efficiency of ≈9.0% in YbCd2Sb2/Mg3Sb2 Zintl Module for Thermoelectric Power Generation.

Recently, YbCd2Sb2-based Zintl compounds have been widely investigated owing to their extraordinary thermoelectric (TE) performance. However, its p orbitals of anions that determined the valence band structure are split due to crystal field splitting that provides a good platform for band manipulation by doping/alloying and, more importantly, the YbCd2Sb2-based device has yet to be reported. In this work, single-phase YbCd1.5Zn0.5Sb2 is successfully obtained through precise chemical composition control. Then, YbMg2Sb2-alloying increases the cationic vacancy defect formation energy and further optimizes carrier concentration. Moreover, the band structure of YbCd1.5Zn0.5Sb2 is subtly manipulated, and the underlying mechanism is experimentally explored. Combined with the reduced lattice thermal conductivity, a high peak ZT value of ∼1.43 at 700K is obtained for YbCd1.425Zn0.475Mg0.1Sb2. Subsequently, choosing Fe90Sb10 as the diffusion barrier layer and adopting the transient liquid phase bonding technique, for the first time, it is demonstrated that YbCd2Sb2/Mg3(Sb, Bi)2 TE module with an ultrahigh conversion efficiency of ≈9.0% at a heat difference of 430K. More importantly, this module displays good thermal stability. This work paves the way for YbCd2Sb2 materials and devices in mid-temperature heat recovery.

Structural defect regulation in corundum-type medium-entropy oxides for enhanced infrared emissivity performance at high temperature

With the increasing demand in industry and hypersonic vehicles, advanced materials with high-performance infrared radiation need to be developed. Herein, two corundum-type medium-entropy oxide ceramics M3 (Al1/3Cr1/3Fe1/3)2O3 and M4 (Al0.25Cr0.25Fe0.25Ga0.25)2O3 were prepared by solid-state reaction method at different temperatures. Their chemical structure and infrared emissivity performance were analyzed through systematic experiments and theoretical analysis. Compared to the M3, M4 prepared at 1773 K possessed more preeminent emissivity performance, exhibiting emissivity values of 0.917, 0.912 and 0.90 in the wavelength of 0.2–0.78 μm, 0.78–2.50 μm and 2.5–14 μm, respectively. Meanwhile, the infrared emissivity of M4 still could exceed 0.85 at 800 °C in the wavelength of 1.5–25 μm, displaying excellent emissivity and stability under high temperature. As a consequence of lower bandgap and defects formation energy, more oxygen vacancy defects were generated in M4, resulting in better infrared emissivity performance. The results demonstrates that M4 as coating has great potential to be applied in energy-related and thermal protection of hypersonic vehicles.

Enhancing low-energy X-ray excited afterglow in lanthanide-doped fluoride core@shell nanoparticles for autofluorescence-free imaging

Lanthanide-doped fluoride nanoparticles (NPs) exhibit tunable X-ray excited afterglow (XEA), holding great promise for autofluorescence-free flexible X-ray imaging. However, materials with low atomic numbers, while sensitive to low-energy X-ray photons, suffer from weak X-ray absorption coefficients. This poses a significant challenge in achieving high-performance XEA with minimized radiation risk. In this study, we enhance the low-energy XEA of lanthanide activators by engineering the interfacial defect formation energy (Ef) in CaF2-based heterogeneous core/shell nanoarchitectures. Mechanistic investigations demonstrate that a large lattice misfit between the core and shell reduces the interfacial defect Ef, facilitating the formation of Frenkel defects and significantly increasing XEA intensity after low-energy X-ray irradiation. Specifically, the heterogeneous CaF2:Tb@CaLuF5 core/shell NPs, with a misfit of 3.382 %, exhibit approximately ∼ 8.9 times higher XEA intensity compared to the CaF2:Tb@CaF2 homogeneous core/shell NPs at 10 kV. Furthermore, integrating the CaF2:Tb@CaLuF5 NPs into a flexible scintillation screen enables XEA-based delayed imaging with high spatial resolution, up to approximately 14.2 lp mm−1, and an autofluorescence-free background. These findings advance the development of superior low-energy XEA materials and pave the way for autofluorescence-free three-dimensional X-ray imaging.

An investigation on the surface properties of B4C for advancing its nuclear applications

B4C is an important material in diverse nuclear applications. However, a systematic examination of its surface properties is still missing. In this work, we employ first-principles simulations to investigate the energetic stability of 16 distinct slab models representing (001), (100), (101), (110), and (111) surfaces, which are constructed by minimizing dangling bonds. Our results show that C-terminated (001) surface exhibits significantly greater stability than other surfaces under both the carbon and boron-rich conditions. Besides, we also study the defect formation energies on the C-terminated (001) surface and compare them with the cases in bulk. The high formation energies of the defects suggest a low likelihood of their occurrence on this surface, despite their formation energies being lower compared to bulk cases. Furthermore, mid-gap surface states are revealed for the top atomic layers of the C-terminated (001) surface, which are deduced at the deeper layers, and the band structures of the middle layers of this slab recover to the bulk band gap. These surface mid-gap states allow electron excitation from the valence band to these states, resulting in a reduced optical band gap compared to the bulk band gap of B4C. This provides a plausible explanation for the significantly smaller band gap observed in experiments compared to the larger gap predicted by theoretical models. Our study not only sheds light on the surface properties of B4C but also lays the groundwork for advancing this material for more advanced nuclear applications.