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Antiferroelectric materials with double hysteresis loops are attractive for energy storage applications, which are becoming increasingly important for power electronics nowadays. Among them, AgNbO3 based lead-free ceramics have attracted intensive interest as one of promising environmental-friendly candidates. However, most of the AgNbO3 based ceramics suffers from low dielectric breakdown strength (Eb). The limitation of low Eb is broken to some extent in this work. Here, AgNbO3 epitaxial films were fabricated by pulsed laser deposition, which possess high Eb of 624 kV/cm. The (001)AgNbO3 epitaxial film reveals typical antiferroelectric hysteresis loops when the applied electric fields are over 300 kV/cm. A recoverable energy density of 5.8 J/cm3 and an energy efficiency of 55.8% are obtained at 600 kV/cm, which demonstrates the great promise of the AgNbO3 film for energy storage applications.

A series of Ca15(PO4)2(SiO4)6:xCe3+,yTb3+ phosphors have been prepared by a high-temperature solid-state reaction. Under the excitation of near-UV with 371 nm wavelength, Ca15(PO4)2(SiO4)6:xCe3+ phosphors exhibit strong blue emission with a broad peak at 432 nm. Based on the photoluminescence of Ca15(PO4)2(SiO4)6:xCe3+ phosphors, the coordination environment around Ce3+ ions and the concentration quenching mechanism are inferred. With the doping of Tb3+ ions into Ca15(PO4)2(SiO4)6:1.33%Ce3+, the luminescence color from blue to cyan can be well tuned. By measuring the luminescence intensity and lifetime of the as-prepared phosphors, it can be judged that there exists an energy transfer from Ce3+ to Tb3+. To achieve white light, the optimal Ca15(PO4)2(SiO4)6:1.33%Ce3+, 9%Tb3+ phosphors are mixed with commercial SrAlSiN3:Eu2+ powders and finally warm white light emission could be obtained. The results show that Ca15(PO4)2(SiO4)6:xCe3+,yTb3+ phosphors have potential applications in warm white light-emitting diodes.

A gate electrode is usually used to controllably tune the carrier concentrations, further modulating the electrical conductivity and the Seebeck coefficient to obtain the optimum thermoelectric figure of merit (ZT) in two-dimensional materials. On the other hand, it is necessary to investigate how an electric field induced by a gate voltage affects the electronic structures, further determining the thermoelectric properties. Therefore, by using density functional calculations in combination with Boltzmann theory, the thermoelectric properties of bilayer MX2 (M = W, Mo; X = S, Se) with or without a 1 V nm−1 perpendicular electric field are comparatively investigated. First of all, the variations of the electrical conductivity (σ), electron thermal conductivity and Seebeck coefficient (S) with the carrier concentration are studied. Due to the trade-off relationship between S and σ, there is an optimum concentration to obtain the maximum ZT, which increases with the temperature due to the enhancement of the Seebeck coefficient. Moreover, N-type bilayers have larger optimum ZTs than P-type bilayers. In addition, the electric field results in the increase of the Seebeck coefficient in low hole-doped MS2 bilayers and high hole-doped MSe2 bilayers, thus leading to similar variations in ZT. The optimum ZTs are reduced from 2.11 × 10−2, 3.19 × 10−2, 2.47 × 10−2, and 2.58 × 10−2 to 1.57 × 10−2, 1.51 × 10−2, 2.08 × 10−2, and 1.43 × 10−2 for the hole-doped MoS2, MoSe2, and WSe2 bilayers, respectively. For N-type bilayers, the electric field shows a destructive effect, resulting in the obvious reduction of the Seebeck coefficient in the MSe2 layers and the low electron-doped MS2 bilayers. In electron-doped bilayers, the optimum ZTs will decrease from 3.03 × 10−2, 6.64 × 10−2, and 6.69 × 10−2 to 2.81 × 10−2, 3.59 × 10−2, and 4.39 × 10−2 for the MoS2, MoSe2, and WSe2 bilayers, respectively.

Dielectric capacitors have been receiving increasing attention due to their high power density. Among the various possibilities, Bi-based perovskite ferroelectrics, such as BiFeO3 (BFO) and Na0.5Bi0.5TiO3 (NBT), are considered as potential candidates due to their superior ferroelectric properties. In this work, NBT and 0.9NBT-0.1BFO thin films have been fabricated on (0 0 1) SrTiO3 substrate, in which 0.9NBT-0.1BFO film was deposited by magnetron sputtering and pulsed laser deposition hybrid technique. The X-ray diffraction confirms that the (0 0 1) oriented 0.9NBT-0.1BFO thin film is epitaxial grown on SrTiO3 substrate. Enhanced energy storage performance, with recoverable energy density of 44 J/cm3 and high thermal stability of the energy storage density (with minimal variation of ≤6%) over 40–180 °C, could be achieved in in 0.9NBT-0.1BFO film. It is revealed that the introduction of BFO leads to increased polarization up to 141μC/cm2 vs 76.4μC/cm2 for pure NBT film, which accounts for the high energy storage density. The results indicated that the 0.9NBT-0.1BFO thin-films might be promising environmentally friendly lead-free materials for energy storage applications.

This study is built on density functional calculations in combination with the non-equilibrium Green's function, and we probe the thermoelectric transport mechanisms through C60 molecules anchored to Al nano-electrodes in three different ways, such as, the planar, pyramidal, and asymmetric surfaces. When the electrode is switched from the planar and pyramidal surfaces, the electrical conductance (σ) and electron's thermal conductance (κel) decrease almost two orders of magnitude due to the reduction of the molecule–electrode contact coupling, whereas the Seebeck coefficients (S) are reduced by ∼55%. Furthermore, the maximum electron's thermoelectric figure of merit (ZelT = S2σT/κel, assuming a vanishing phonon's thermal conductance) is about 0.12 in the asymmetric junction. In particular, all σ, S, κel, and ZelT increase along with the average temperature (T) in all C60-junctions, although their growth is really quite negligible in the pyramidal junction because the Fermi level is far away from the frontier orbitals. In addition, when the strain increases from the compressive (−1.0 A) to tensile (1.0 A) strain, the Seebeck coefficient in the planar junction increases drastically, while the Seebeck coefficients in the asymmetric and pyramidal junctions reach their maximum values at 0.2 A tensile and −0.4 A compressive strains, respectively. This is because the Seebeck coefficient is inversely proportional to the magnitudes and proportional to the slopes of the transmission spectrum around the Fermi level. Finally, it is found that the shift of the Fermi level is an effective scheme to obtain the maximum ZelT of any molecular junction, including fullerene-based junctions.

Mechanical properties of two-dimensional (2D) transition-metal dichalcogenides (TMDCs) are of vital importance in any practical applications to flexible devices and nano-electromechanical systems. Thus, the mechanical properties of monolayer TMDCs, a stoichiometric formula MX2 in which M = Mo, W and X = S, Se, Te, are investigated by using density functional theory. More importantly, based on the different atomic arrangement, all three chemical isomers, such as 1T, 1T′, and 1H phases, are compared in detail. We found that their 2D Young’s moduli and Poisson’s ratios display a strong dependence not only on the atomic species but also on the atomic arrangements. For the same structural phase, monolayer TMDCs with the W (S) atom are found to be much stiffer in each chalcogenide (metal) group. Due to the threefold rotation symmetry of the hexagonal lattice, 1T- and 1H-TMDC monolayers belong to the isotropic structures, while the strong anisotropic Young’s moduli and Poisson’s ratios are observed in the 1T′ phase, i.e., 2D Young’s moduli along the armchair direction are nearly 50% larger than those along the zigzag direction for tellurides. Interestingly, 1T-TMDC monolayers show negative Poisson’s ratios. Furthermore, their in-plane 1H/1T′ heterostructures could be constructed, and the corresponding mechanical properties are explored. We found that the influence of the 1H/1T′ interface on the mechanical behavior is detrimental, which reduces the in-plane stiffness normal to the 1H/1T′ interface as compared with 1H and 1T′ structures. However, in comparison with the 1T′ phase, a remarkable strength of these novel heterostructures is along the 1H/1T′ interface direction. In brief, the present first-principles results constitute a useful picture for the mechanical properties of 2D TMDCs and their in-plane heterostructures.

The electronic properties of bilayer strained boronitrenes are investigated under an external electric field using density functional methods. Our result is just the same as the previous conclusion: ie, that the electric field will reduce their band gaps. Except for the decrease of their band gaps, the degeneracy of π valence bands at K points will be lifted and the degenerate gap will increase with the electric field increasing. Moreover, the widths of π valence bands are nearly robust and increase a little. In addition, a simple tight-binding model, where different electrostatic potentials are applied to boronitrene layers, can be sufficient to describe the variations of their band gaps. It is found that the interlayer hopping interaction increases while the intralayer hopping parameter changes little with increasing the electric field. Furthermore, a band gap phase diagram is determined within the in-plane strain [-0.2, 0.2] and the interlayer bias [0, 10] V nm(-1). The strain could make the bottom of conduction bands shift from K to M, then to Γ in the Brillouin zone, while the top of valence bands shifts from K to Γ. Thus, a direct-gap semiconductor at K points is changed into an indirect-gap semiconductor, and then a semiconductor with the direct band gap at Γ points. When bilayer boronitrene is a semiconductor with a direct gap at K points, the electric field and strain are inverse proportional relationships. Particularly, when the compressive strain exceeds -0.194, there is an insulator-metal transition and the system becomes metallic with sizable pocket Fermi surfaces.

Quantum interference is a well-known phenomenon which results in unique features of the transmission spectra of molecular junctions at the nanoscale. We investigate and compare the thermoelectric properties of three types of junctions like the anti, Breit-Wigner, and Fano resonances. Due to its asymmetric line-shaped transmission function, Fano resonances lead to a larger thermoelectric figure of merit (ZT) than the symmetric anti and Breit-Wigner resonances. The occurrence of quantum interference in molecular and other nanoscale junctions is independent of contact couplings between the sandwiched molecules and left/right electrodes. However, it is found that the contact couplings determine the electric and thermoelectric performances of quantum interference junctions. In anti-resonant junctions, the Seebeck coefficient is enhanced by strong contact couplings. By contrast, for Breit-Wigner resonant junctions, this same property will increase in the weak contact coupling regime. Contrary to what is observe...

The great enhancement of light-induced transverse thermoelectric effect in c-axis inclined BiCuSeO thin films has been achieved by Pb doping. Large open-circuit thermoelectric voltage signals are all observed in the inclined Bi1-xPbxCuSeO (x = 0, 0.04, 0.08) films when the film surface is irradiated by a 308nm pulsed laser. As the Pb doping content increases, the magnitude Vp of the induced voltage signal increases greatly while the response time τ decrease obviously. A large figure of merit Vp/τ of about 236.7 mV/ns is obtained in the 8% Pb-doped film sample under the 308 nm pulsed irradiation with energy density of 0.5mJ/mm2, which is about 25 times larger than that in the undoped film. Possible mechanism is proposed to explain the experiment results. This work might pave the way for the practical application of BiCuSeO-based thin films for high sensitive and fast response ultraviolet pulsed photodectors.

We investigate the spatial redistribution of the electron density in bilayer graphene in the presence of an interlayer bias within density functional theory. It is found that the interlayer charge redistribution is inhomogeneous between the upper and bottom layers and the transferred charge from the upper layer to the bottom layer linearly increases with the external voltage which further makes the gap at K point linearly increase. However, the band gap will saturate to 0.29 eV in the strong-field regime, but it displays a linear field dependence at the weak-field limit. Due to the AB-stacked way, two carbon atoms per unit cell in the same layer are different and there is also a charge transfer between them, making the widths of π valence bands reduced. In the bottom layer, the charge transfers from the direct atoms which directly face another carbon atom to the indirect atoms facing the center of the hexagon on the opposite layer, while the charge transfers from the indirect atoms to the direct atoms in the upper layer. Furthermore, there is a diploe between the upper and bottom layers which results in the reduction of the interlayer hopping interaction.