Due to the unique narrow-band red emission and broadband blue light excitation, as well as milder synthesis conditions, Mn4+ ion activated fluoride red phosphors show great promise for white light emitting diode (W-LED) applications. However, as the Mn4+ emission belongs to a spin-forbidden transition (2Eg → 4A2), it is a fundamental challenge to synthesize these phosphors with a high external quantum efficiency (EQE) above 60%. Herein, a highly efficient and thermally stable red fluoride phosphor, Cs2SiF6:Mn4+, with a high internal quantum efficiency (IQE) of 89% and ultrahigh EQE of 71% is demonstrated. Furthermore, nearly 95% of the room-temperature IQE and EQE are maintained at 150 °C. The static and dynamic spectral measurements, as well as density functional theory (DFT) calculations, show that the excellent performance of Cs2SiF6:Mn4+ is due to the Mn4+ ions being evenly distributed in the host lattice Cs2SiF6. By employing Cs2SiF6:Mn4+ as a red light component, stable 10 W high-power warm W-LEDs with a luminous efficiency of ∼110 lm/W could be obtained. These findings indicate that red phosphor Cs2SiF6:Mn4+ may be a highly suitable candidate for fabricating high-performance high-power warm white LEDs.Keywords: Mn4+; narrow-band; red phosphor; stable; ultrahigh quantum efficiency; warm white LED;

Due to the electron deficiency, boron clusters evolve strikingly with the increasing size as confirmed by experimentalists and theorists. However, it is still a challenge to propose a model potential to describe the stabilities of boron. On the basis of the 2c-2e and 3c-2e bond models, we have found the constraints for stable boron clusters, which can be used for determining the vacancy concentration and screening the candidates. Among numerous tubular structures and quasi-planar structures, we have verified that the stable clusters with lower formation energies bounded by the constraints, indicating the competition of tubular and planar structures. Notably, we have found a tubular cluster of B76 which is more stable than the B80 cage. We show that the vacancies, as well as the edge, are necessary for the 2c-2e bonds, which will stabilize the boron nanostructures. Therefore, the quasi-planar and tubular boron nanostructures could be as stable as the cages, which have no edge atoms. Our finding has shed light on understanding the complicated electron distributions of boron clusters and enhancing the efficiency of searching stable B nanostructures.

We have investigated the mechanism of tunable luminescence in a bismuth doped ScVO4 matrix by using the first-principles calculations. It is found that some intrinsic defects generally exist in the matrix due to their low formation energies, while they have no remarkable effect in the optical band gap of the compound. The calculated formation energy of substitutional Bi at the Sc site is 2.0–3.3 eV lower than that of replacing V by Bi in various chemical environments. Yet, once there is an oxygen defect (vacancy) around a Bi atom, it is energetically preferable to form a defect complex, BiSc + VacO. Based on the calculated formation energy and the imaginary part of the dielectric function, the defect complex transfers between the neutral charge state and the 1+ charge state, which is ascribed to be responsible for the red bismuth photoemission observed in experiment. With the defect complex, tunable bismuth photoemission could be achieved by selectively controlling the content of hydrogen. Our calculations have shown that there is a passivation effect resulting from the re-padding of the oxygen defect by the hydrogen atom. This confirms the experimental observation of tunable bismuth luminescence due to the defect complex, leading to a potential facile design of other defect-controllable, micro- and nano-sized luminescent materials ranging from the visible to the near- and far-infrared spectrum.

The energy gap of graphene nanoflakes is important for their potential application in nano-devices; however, it is still a challenge to perform a systemic search of systems with large gaps due to the presence of numerous candidates. Herein, we showed an ideal feasible approach that involved structural recognition, simplified effective evaluation, and successive optimization strategy. Considering the local bonding environment of carbon atoms, we first proposed a tight-binding model with the parameters fitted from the first-principles calculations of possible GNFs; this model provided an ideal avenue to screen the candidates with high accuracy and efficiency. Via combining the Monte Carlo tree search method and the congruence check, we determined the correlation between structures and the gap distributions according to the carbon numbers, and the results were confirmed via the first-principles calculations. The structural stabilities of the candidates with different numbers of hydrogen atoms might be modulated by the chemical potential of hydrogen, whereas the candidates with larger gaps might be more stable for the isomers with the same number of C and H atoms. Note that the gap variation is dominated by the structural features despite the quantum confinement effect since the gap maximum fluctuates rather than gradually decreasing with the increase in size. Our finding shows the gap variety of GNFs due to the configuration diversity, which may help explore the potential application of GNFs in nano-devices and fluorescence labeling in biomedicine.

Due to the complexity of the interaction between boron and 3d transition metals, stable two-dimensional (2D) iron borides FeBx compounds have attracted tremendous attention in recent years. Combining the evolutionary algorithm with first-principles calculations, we have systematically investigated the structural stabilities and electronic properties of 2D iron borides FeBx (x = 2–10) alloys. It is found that the multilayer iron borides FeBx (x = 4, 6, 8, 10) are wide-band-gap semiconductors, which are more stable than the corresponding monolayers. Furthermore, the electronic and optical properties of these semiconductors may be modulated by biaxial strains, indicating their potential application for advanced blue/UV light optoelectronic devices.

Using first-principles calculations, we investigate the structural, electronic and superconducting properties of Mg intercalated bilayer borophenes BxMgBx (x = 2–5). Remarkably, B2MgB2 and B4MgB4 are predicted to exhibit good phonon-mediated superconductivity with a high transition temperature (Tc) of 23.2 K and 13.3 K, respectively, while B4MgB4 is confirmed to be more practical based on the analyses of its stability. The densities of states of in-plane orbitals at the Fermi level are found to be dominant at the superconducting transition temperature in Mg intercalated bilayer borophenes, providing an effective avenue to explore Mg–B systems with high Tcs.

Monolayer hexagonal arsenene (hAs), a typical two-dimensional semiconducting material with a wide band gap and high stability, has attracted increasing research interest due to its potential applications in optoelectronics. Using first-principles calculations, we have investigated the electronic and magnetic properties of x-substituted hAs (x = B, C, N, O, Ga, Ge, Se, and monovacancy) and x-adsorbed hAs (x = As). Our results show that the B-, N-, and Ga-substituted hAs have spin-unpolarized semiconducting characters like pristine hAs, and indirect–direct band gap transitions are induced in the B- and N-substituted systems. In contrast, the O-, Se-, and monovacancy-substituted hAs are metallic, and the C- and Ge-substituted hAs show spin-polarized semiconducting characters with band gaps of 1.1 and 1.3 eV for the spin-up channels and 1.0 and 0.7 eV for the spin-down channels, respectively. For the As-adsorbed hAs, the Fermi level crosses the spin-up states, yielding metallic behavior, while the spin-down channel retains semiconducting character. Detailed analysis of electronic structures for the C-substituted, Ge-substituted, and As-adsorbed hAs shows that strong hybridizations between the doping atoms and As atoms lead to energy splitting near the Fermi level and consequently induce magnetic moments. By selective doping, hAs can be transformed from a spin-nonpolarized semiconductor to a spin-polarized semiconductor, to a half-metal, or even to a metal, which indicates that the doped hAs will have promising potential in future electronics, spintronics, and optoelectronics.

Alkali-metal intercalated graphite and graphene have been intensively studied for decades, where alkali-metal atoms are found to form ordered structures at the hollow sites of hexagonal carbon rings. Using first-principles calculations, we have predicted various stable structures of high-coverage 3d transition metal (TM) intercalated bilayer graphene (BLG) stabilized by the strain. Specifically, with reference to the bulk metal, Sc and Ti can form stable TM-intercalated BLG without strain, while the stabilization of Fe, Co, and Ni intercalated BLG requires the biaxial strain of over 7%. Under the biaxial strain ranging from 0% to 10%, there are four ordered sandwich structures for Sc with the coverage of 0.25, 0.571, 0.684, and 0.75, in which the Sc atoms are all distributed homogenously instead of locating at the hollow sites. According to the phase diagram, a homogenous configuration of C8Ti3C8 with the coverage of 0.75 and another inhomogeneous structure with the coverage of 0.692 were found. The electronic and magnetic properties as a function of strain were also analyzed to indicate that the strain was important for the stabilities of the high-coverage TM-intercalated BLG.

B sheets have been intently studied, and various candidates with vacancies have been reported in theoretical investigations, including their possible growth on metal surfaces. However, a recent experiment reported that the borophene formed on a Ag (111) surface consisted of a buckled triangular lattice without vacancies. Our calculations propose a novel nucleation mechanism of B clusters and emphasize the B–Ag interaction in the growth process of borophene, demonstrating the structural evolution of triangular fragments with various profiles and vacancy distributions. Compared with the triangular lattice without vacancies, we have confirmed that the sheet energetically favored during the nucleation and growth is that containing 1/6 vacancies in a stripe pattern, whose scanning tunneling microscopy image is in better agreement with the experimental observation.

•Ti atoms can be dispersed on defective graphene under various strains.•Hydrogen uptake can be modulated as a function of chemical potential and strain.•The control of strain is dominant to the hydrogen uptake.Hydrogen storage with Ti decorated nano-materials is attributed to the d levels of Ti with unsaturated bonding, whose configuration significantly affects the system's stability and activity. Using first-principles calculations, we have investigated hydrogen adsorption and desorption on the Ti decorated defective graphene under various strains (from 0% to 15%), in which Ti atoms' dispersing are energetically stable. According to the phase diagram, we showed that hydrogen uptake can be modulated as a function of chemical potential and strain, since the strain modifies the configuration of d levels, and consequently affects the binding between H2 and Ti atom. Remarkably, Ti decorated defective graphene under 15% strain could be considered as an ideal media of hydrogen storage, in which the desorption temperature of H2 is expected to be ∼300 K at 0.5 atm. The control of strain is found to be dominant to the H2 uptake, besides the temperature and pressure.

•We propose a simple and practical algorithm to generate the spatially correlated random field.•It is obtained by the weighted average of random fields without spatial correlation.•A required microstructure can be obtained in a much faster way.•An application study on the effective elastic behavior materials is given.The properties of material strongly depend on the microstructure, and the development of microstructure is closely related to the phase transition with the temperature-dependent spatial correlation. To consider more realistic microstructures, we have proposed an efficient and simple algorithm for generating the spatially-correlated random field, which is obtained by the weighted average of random fields without spatial correlation according to the spatially-correlated length and anisotropy parameter. By using a mesoscale finite element model with the microstructures generated by our algorithm, an application study on the effective elastic behavior of Al2O3–NiAl composite materials is given. Our numerical results are in agreement with the experimental measurements. The proposed method is general and robust, which can be extended to the multi-phase materials.

Alkali-metal intercalated graphite and graphene have been intensively studied for decades, where alkali-metal atoms are found to form ordered structures at the hollow sites of hexagonal carbon rings. Using first-principles calculations, we have predicted various stable structures of high-coverage 3d transition metal (TM) intercalated bilayer graphene (BLG) stabilized by the strain. Specifically, with reference to the bulk metal, Sc and Ti can form stable TM-intercalated BLG without strain, while the stabilization of Fe, Co, and Ni intercalated BLG requires the biaxial strain of over 7%. Under the biaxial strain ranging from 0% to 10%, there are four ordered sandwich structures for Sc with the coverage of 0.25, 0.571, 0.684, and 0.75, in which the Sc atoms are all distributed homogenously instead of locating at the hollow sites. According to the phase diagram, a homogenous configuration of C8Ti3C8 with the coverage of 0.75 and another inhomogeneous structure with the coverage of 0.692 were found. The electronic and magnetic properties as a function of strain were also analyzed to indicate that the strain was important for the stabilities of the high-coverage TM-intercalated BLG.

Monolayer hexagonal arsenene (hAs), a typical two-dimensional semiconducting material with a wide band gap and high stability, has attracted increasing research interest due to its potential applications in optoelectronics. Using first-principles calculations, we have investigated the electronic and magnetic properties of x-substituted hAs (x = B, C, N, O, Ga, Ge, Se, and monovacancy) and x-adsorbed hAs (x = As). Our results show that the B-, N-, and Ga-substituted hAs have spin-unpolarized semiconducting characters like pristine hAs, and indirect–direct band gap transitions are induced in the B- and N-substituted systems. In contrast, the O-, Se-, and monovacancy-substituted hAs are metallic, and the C- and Ge-substituted hAs show spin-polarized semiconducting characters with band gaps of 1.1 and 1.3 eV for the spin-up channels and 1.0 and 0.7 eV for the spin-down channels, respectively. For the As-adsorbed hAs, the Fermi level crosses the spin-up states, yielding metallic behavior, while the spin-down channel retains semiconducting character. Detailed analysis of electronic structures for the C-substituted, Ge-substituted, and As-adsorbed hAs shows that strong hybridizations between the doping atoms and As atoms lead to energy splitting near the Fermi level and consequently induce magnetic moments. By selective doping, hAs can be transformed from a spin-nonpolarized semiconductor to a spin-polarized semiconductor, to a half-metal, or even to a metal, which indicates that the doped hAs will have promising potential in future electronics, spintronics, and optoelectronics.

We have investigated the mechanism of tunable luminescence in a bismuth doped ScVO4 matrix by using the first-principles calculations. It is found that some intrinsic defects generally exist in the matrix due to their low formation energies, while they have no remarkable effect in the optical band gap of the compound. The calculated formation energy of substitutional Bi at the Sc site is 2.0–3.3 eV lower than that of replacing V by Bi in various chemical environments. Yet, once there is an oxygen defect (vacancy) around a Bi atom, it is energetically preferable to form a defect complex, BiSc + VacO. Based on the calculated formation energy and the imaginary part of the dielectric function, the defect complex transfers between the neutral charge state and the 1+ charge state, which is ascribed to be responsible for the red bismuth photoemission observed in experiment. With the defect complex, tunable bismuth photoemission could be achieved by selectively controlling the content of hydrogen. Our calculations have shown that there is a passivation effect resulting from the re-padding of the oxygen defect by the hydrogen atom. This confirms the experimental observation of tunable bismuth luminescence due to the defect complex, leading to a potential facile design of other defect-controllable, micro- and nano-sized luminescent materials ranging from the visible to the near- and far-infrared spectrum.