Yongguang Cheng

Tunable broad-band perfect absorber by exciting of multiple plasmon resonances at optical frequency

A broad-band perfect absorber composing a two-dimensional periodic metal-dielectric-metal sandwiches array on dielectric/metal substrate is designed and numerically investigated. It is shown that the nearly-perfect absorption with a bandwidth of about 50 nm in visible region can be achieved by overlapping of two plasmon resonances: one originating from the coupling of electric dipoles between adjacent unit cells and another arising from magnetic dipole plasmon resonances. A capacitor-inductor circuit description is introduced to explain the dependence of resonance frequencies and band-width on geometrical parameters.

A giant localized field enhancement and high sensitivity in an asymmetric ring by exhibiting Fano resonance

The optical properties of asymmetric ring structures are investigated theoretically by using the discrete dipole approximation method. The numerical results revealed that this kind of structure can achieve a giant localized field enhancement (LFE, 264) and a high LSPR sensitivity (corresponding FOM, 8.28) in the visible spectrum by Fano resonance, whose origin is discussed based on plasmon hybridization theory. Furthermore, the dependence of the Fano resonance on the polarization states of the incident light is also demonstrated. Giant LFE and high LSPR sensitivity enable this structure to be promising for surface enhanced Raman spectroscopy and sensing applications.

Negative thermal expansion and broad band photoluminescence in a novel material of ZrScMo2VO12.

In this paper, we present a novel material with the formula of ZrScMo2VO12 for the first time. It was demonstrated that this material exhibits not only excellent negative thermal expansion (NTE) property over a wide temperature range (at least from 150 to 823 K), but also very intense photoluminescence covering the entire visible region. Structure analysis shows that ZrScMo2VO12 has an orthorhombic structure with the space group Pbcn (No. 60) at room temperature. A phase transition from monoclinic to orthorhombic structure between 70 and 90 K is also revealed. The intense white light emission is tentatively attributed to the n- and p-type like co-doping effect which creates not only the donor- and acceptor-like states in the band gap, but also donor-acceptor pairs and even bound exciton complexes. The excellent NTE property integrated with the intense white-light emission implies a potential application of this material in light emitting diode and other photoelectric devices.

Interaction of crystal water with the building block in Y2Mo3O12 and the effect of Ce3+ doping

Ce(3+) ions are introduced into the lattice of Y2Mo3O12 with a sol-gel method with the aim to reduce its hygroscopicity and pursue the interaction of crystal water molecules with the building block. It is found that Ce(3+) ions occupy the positions of Y(3+) in the lattice and have the function of expelling crystal water molecules in the microchannels so that the number of crystal water molecules decreases significantly as the Ce(3+) content increases and a complete depletion of the crystal water is achieved when the content of Ce(3+) is higher than 8 mol%. Based on the binding energy changes of Mo 3d and Y 3d with and without Ce(3+) in the lattice, the configuration of the crystal water in the building block is deduced, namely, a crystal water serves as a spring with its O(2-) pointing to the Y(3+) in an octahedron and with its H(+) approaching the next nearest O(2-) in the Y-O-Mo bridge. With such a configuration, the effects of the crystal water on the thermal expansion properties of Y2Mo3O12 and the like are explained. It is also shown that the number of crystal water molecules per molecular formula can be quantified by the full width at half maximum of the Raman bands or relative intensity with linear relationships, suggesting that Raman spectroscopy can be a potential tool in quantifying crystal water molecules at room temperature in this or related materials.

Amorphous NiO electrocatalyst overcoated ZnO nanorod photoanodes for enhanced photoelectrochemical performance

Developing high-performance photoanodes is essential for practical applications of photoelectrochemical (PEC) water splitting. In this work, we demonstrated that introducing amorphous NiO electrocatalysts onto the surface of ZnO nanorod (NR) photoanodes can largely improve their PEC performance. The NiO/ZnO core–shell NR arrays were obtained by two step electrodepositions and annealing. The amorphous NiO nanosheet electrocatalyst shell on a single crystalline ZnO NR semiconductor core formed a composite photoanode configuration for PEC water splitting. The NiO/ZnO photoanode yielded a remarkable 260 mV cathodic shift in the onset potential for water oxidation compared to the bare ZnO photoanode. And the highest PEC efficiency of the NiO/ZnO photoanode was found to be 1.81%, which is 30 times higher than that of the ZnO photoanode (0.06%). This research demonstrates that introducing amorphous NiO electrocatalysts can largely improve the PEC performance of ZnO photoanodes.

Phase Transition and Negative Thermal Expansion Property of ZrMnMo3O12

A novel material of ZrMnMo3O12 with negative thermal expansion is presented. The phase transition temperature and coefficient of thermal expansion (CTE) are investigated by temperature-dependent x-ray diffraction and Raman spectra. It is shown that ZrMnMo3O12 adopts monoclinic structure with space group P21/a (No. 14) from 298 to 358 K and transforms to orthorhombic with space group Pnma (No. 62) above 363 K. The linear CTE obtained from the results of XRD refinement is −2.80 × 10−6 K−1 from 363 to 873 K. The CTE of the bulk cylinder ceramic measured by a thermal dilatometer is −4.7 × 10−6 K−1 from 373 to 773 K approximatively.

Plasmonic Nanostructure for Enhanced Light Absorption in Ultrathin Silicon Solar Cells

The performances of thin film solar cells are considerably limited by the low light absorption. Plasmonic nanostructures have been introduced in the thin film solar cells as a possible solution around this issue in recent years. Here, we propose a solar cell design, in which an ultrathin Si film covered by a periodic array of Ag strips is placed on a metallic nanograting substrate. The simulation results demonstrate that the designed structure gives rise to 170% light absorption enhancement over the full solar spectrum with respect to the bared Si thin film. The excited multiple resonant modes, including optical waveguide modes within the Si layer, localized surface plasmon resonance (LSPR) of Ag stripes, and surface plasmon polaritons (SPP) arising from the bottom grating, and the coupling effect between LSPR and SPP modes through an optimization of the array periods are considered to contribute to the significant absorption enhancement. This plasmonic solar cell design paves a promising way to increase light absorption for thin film solar cell applications.

Negative thermal expansion and photoluminescence properties in a novel material ZrScW2PO12

A novel material, ZrScW2PO12, with negative thermal expansion (NTE) behavior, at least from 138 to 1300 K, and intense photoluminescence (PL) property is first reported in this paper. Temperature dependent Raman and PL spectral studies indicate that the material holds an orthorhombic structure down to about 74 K and exhibits NTE property in the temperature range. The intense PL covering nearly the whole visible region was observed and can be deconvoluted into four bands, which present different shifts with elevation of temperature. The abundant optical property may be attributed to n- and p-type like co-doping effect and the specific structure with the abnormal thermal expansion property of the material. The integrated properties might suggest potential applications of this material in light emitting diodes and other light emitting devices.

Near-zero thermal expansion of In2(1−x)(HfMg) x Mo3O12 with tailored phase transition*

Solid solutions of In2(1−x)(HfMg) x Mo3O12 are synthesized by solid state reaction with the aim to reduce the phase transition temperature of In2Mo3O12 and improve its thermal expansion property. The effects of (HfMg)6+ incorporation on the phase transition and thermal expansion are investigated. It is shown that the monoclinic-to-orthorhombic phase transition temperature obviously decreases and the coefficient of thermal expansion (CTE) of the orthorhombic becomes less negative and approaches to zero with increasing the content of (HfMg)6+. A near zero thermal expansion covering the case at room temperature (RT) is achieved for the solid solutions with x ≥ 0.85, implying potential applications of this material in many fields.

Zero and controllable thermal expansion in

HfMgMo W x O12 with , 1.0, 1.5, 2.0, and 2.5 are developed with a simple solid state method. With increasing the content of W, solid solutions of HfMgMo W x O12 crystallize in an orthorhombic structure for and a monoclinic structure for . A near-zero thermal expansion (ZTE) is realized for HfMgMo W O12 and negative coefficients of thermal expansion (NCTE) are achieved for other compositions with different values. The ZTE and variation of NCTE are attributed to the difference in electronegativity between W and Mo and incorporation of a different amount of W, which cause variable distortion of the octahedra and softening of the MoO4 tetrahedra, and hence an enhanced NCTE in the a- and c-axis and reduced CTE in the b-axis as revealed by Raman spectroscopy and x-ray diffraction.