Hao-Hong Chen

An Eu3+ post-functionalized nanosized metal–organic framework for cation exchange-based Fe3+-sensing in an aqueous environment

A novel strategy was demonstrated to fabricate a luminescent lanthanide functionalized MOF by encapsulating Eu3+ cations in the pores of MIL-53–COOH (Al) nanocrystals. The Eu3+ incorporated sample shows excellent luminescence and good fluorescence stability in water due to the sensitization and protection provided by the parented framework. Subsequently, Eu3+ incorporated nanocrystals were developed as a highly selective and sensitive probe for detection of Fe3+ in aqueous solutions. In addition, the possible sensing mechanism based on cation exchange between Fe3+ and the framework Al3+ in MIL-53–COOH (Al) was discussed in detail. This is the first example for detecting Fe3+ in aqueous solutions based on a lanthanide functionalized nanoscale MOF. The good fluorescence stability in an aqueous environment, the low detection limit and the broad linear range, together with the nanoscale nature of this probe suggest it has potential for intracellular sensing and imaging of Fe3+.

Energy transfer mechanisms in Tb3+, Yb3+ codoped Y2O3 downconversion phosphor

Tb3+ and Yb3+ co-activated luminescent material that can cut one photon of around 483 nm into two NIR photons of around 1000 nm could be used as a downconversion luminescent convertor in front of crystalline silicon solar cell panels to reduce thermalization loss of the solar cell. The Tb3+ → Yb3+ energy transfer mechanisms in the UV–blue region in Y2O3 phosphor were studied by PL excitation spectra and time-resolved luminescence, from which the charge transfer mechanism and the cooperative transfer mechanism were identified. Tb3+ ions in the 4f75d1 state relax down to the 5D4 level and cooperatively transfer energy to two Yb3+ ions, which is followed by the emission of two photons (λ ~ 1000 nm). It was found in the (Y0.79Tb0.01Yb0.20)2O3 sample that 37% of the Tb3+ ions at the 5D4 level transfer energy to two neighbouring Yb3+ ions by the cooperative energy transfer mechanism Tb3+ (5D4) → 2Yb3+ (2F5/2). Unfortunately, the high Yb3+ concentration leads to severe concentration quenching that significantly reduces the external quantum efficiency. Moreover, the energy of the Tb3+ 4f75d1 state can also be lost non-radiatively or transferred to the Yb3+ 2F5/2 state via the charge transfer state Tb4+–Yb2+. In conclusion, RE3+ (RE = Ce, Pr, Tb) with strong absorption in the UV region is not an appropriate sensitizer of Tb3+ in Tb3+–Yb3+ codoped downconversion phosphor.

Synthesis and high thermoelectric efficiency of Zintl phase YbCd2-xZnxSb2

We synthesized a series of polycrystalline YbCd2−xZnxSb2 (x=0, 0.4, 0.8, 1, 1.2, 1.6, and 2) samples and measured their thermoelectric properties. Thermoelectric figure of merit ZT at 700K is higher than 1.0 for Cd-rich samples (x=0, 0.4, 0.8, and 1.0) and Zn substitute of Cd in YbCd2Sb2 can easily tune carrier concentration and reduce thermal conductivity. When x=0.4, sample exhibits the highest power factor (12–20μWcm−1K−2), the lowest lattice thermal conductivity (1.0Wm−1K−1 at 300K), highest ZT (1.2 at 700K), and best “self-compatibility.” The first principles calculations were performed to study the influences of bonding and electronic structures on physical properties.

A new type of thermoelectric material, EuZn2Sb2.

Polycrystalline EuZn(2)Sb(2) is prepared by direct reaction of the elements. Its composition, structure, magnetism, heat capacity, and thermoelectric properties have been investigated. EuZn(2)Sb(2) crystallizes in p3m space group with a=4.4932(7) A and c=7.6170(10) A. Antiferromagnetic ordering is detected at the Neel temperature of 13.06 K, and the saturation magnetization reaches 6.87mu(B)Eu at 2 K and 7 T. Eu ion has +2 valence. Its Hall effects are characterized by a high positive Hall coefficient of +0.226 cm(3)C, proper carrier concentration of 2.77x10(19)cm(3), and high carrier mobility of 257 cm(2)V s at 300 K. This compound shows high p-type Seebeck coefficient (+122 to +181 muVK), low lattice thermal conductivity (1.60-0.40 Wm K), and high electrical conductivity (1137-524 Scm). The obtained figure of merit and powder factor reach 0.92 and 20.72 muWcm K(2), respectively. The thermoelectric properties of EuZn(2)Sb(2) are encouraging.

Thermoelectric properties and electronic structure of Zintl compound BaZn2Sb2

Polycrystalline sample of the title compound was prepared and its thermoelectric properties from 2to675K were investigated. This Zintl compound shows rather low thermal conductivity, 1.6Wm−1K−1, at room temperature. The value of its thermoelectric figure of merit ZT reaches 0.31 at 675K. Its electronic structure, calculated by ab initio methods, suggests that the electrical transport are mainly ascribe to [Zn2Sb2] framework for p-type BaZn2Sb2. The heat capacity curve at low temperature was fitted lineally to obtain Debye temperature (about 208K). It provides the authors with a host lattice for modification and optimization the thermoelectric properties through substitution and/or doping.

Zintl phase Yb1−xCaxCd2Sb2 with tunable thermoelectric properties induced by cation substitution

It has been shown previously that the thermoelectric properties of the Zintl phase compound YbCd2Sb2 can be finely tuned via Zn substitution at the Cd-site in the anionic (Cd2Sb2)2− framework. Here we report the results of the investigation of isoelectronic substitution of Yb by Ca. The p-type Yb1−xCaxCd2Sb2 (0.2≤x≤0.8) samples have been synthesized via a solid-state reaction followed by suitable cooling, annealing, grinding, and spark plasma sintering densification processes. In samples with x=0.2, 0.4, 0.5, 0.6, 0.8, the electrical conductivity, Seebeck coefficient, and thermal conductivity measurements have been carried out in the temperature range from 300 to 650 K. It is found that the Ca substitution effectively lowers the thermal conductivity for all samples at high temperature, while it significantly increases the Seebeck coefficient. As a result, the dimensionless figure of merit ZT of 0.96 has been attained at 650 K for samples with x=0.4, while the value is 0.78 for the unsubstituted YbCd2Sb2

Luminescent metastable Y2WO6:Ln3+ (Ln = Eu, Er, Sm, and Dy) microspheres with controllable morphology via self-assembly

Three-dimensional (3D) Y2WO6 and Y2WO6: Ln3+ microspheres with novel controllable morphology have been obtained by the hierarchical self-assembly via a hydrothermal synthesis route with a subsequent heat treatment. The amount of citric acid and polyvinylpyrrolidone (PVP) play crucial roles in the controlling of the morphologies. Hollow, core/shell as well as waxberry-shape Y2WO6 microspheres can be prepared in different reaction systems under the same facile hydrothermal conditions. Metastable phases of the samples form when they are crystallized at low temperature, which transfer to the thermodynamically stable counterpart above 1100 °C. This phase transformation is irreversible. The mechanism of reaction and self-assembly evolution process are proposed. The as-prepared Y2WO6:Ln3+ (Ln = Eu, Er, Sm, and Dy) samples show strong multi-color visible light emission under ultraviolet-visible light excitation. Compared with the monoclinic one, this metastable Y2WO6: Eu3+ phase exhibits different luminescent properties. Due to multi-color luminescent properties, these Ln3+-doped (Ln = Eu, Er, Sm, and Dy) metastable Y2WO6 morophologies controllable microspherical samples may be promising for further fundamental research and find applications in color displays.

Luminescence properties of CaZr(PO4)2:RE (RE = Eu3+, Tb3+, Tm3+) under x-ray and VUV–UV excitation

Novel phosphors of Eu3+, Tb3+ and Tm3+ doped CaZr(PO4)2 were synthesized by solid-state reactions and their x-ray and vacuum ultraviolet–ultraviolet (VUV–UV) spectroscopic properties were investigated. The bands near 147 or 185 nm in the VUV excitation spectra of those doped samples are attributed to the PO4 absorption and O to Zr charge transfer transition in the host. The excitation and emission spectra indicate that the phosphors can be effectively excited by x-ray or 147 nm and 172 nm and exhibit a satisfactory red, green and blue light performance, respectively. Considering the high luminescent intensity, excellent colour purity and chemical stability, CaZr(PO4)2:RE (RE = Eu3+ Tb3+, Tm3+) are attractive red, green and blue emitting x-ray and plasma display panel phosphors.

The spin and orbital moment of Fen (n = 2–20) clusters

Complementary to the recent experimental finding that the orbital magnetic moment is strongly quenched in small Fe clusters [M. Niemeyer, K. Hirsch, V. Zamudio-Bayer, A. Langenberg, M. Vogel, M. Kossick, C. Ebrecht, K. Egashira, A. Terasaki, T. Möller, B. v. Issendorff, and J. T. Lau, Phys. Rev. Lett. 108, 057201 (2012)], we provide the theoretical understanding of the spin and orbital moments as well as the electronic properties of neutral and cation Fen clusters (n = 2-20) by taking into account the effects of strong electronic correlation, spin-orbit coupling, and noncollinearity of inter-atomic magnetization. The generalized gradient approximation (GGA)+U method is used and its effluence on the magnetic moment is emphasized. We find that without inclusion of the Coulomb interaction U, the spin (orbital) moments have an average value between 2.69 and 3.50 μB/atom (0.04 and 0.08 μB/atom). With inclusion of U, the magnetic value is between 2.75 and 3.80 μB/atom (0.10 and 0.30 μB/atom), which provide an excellent agreement with the experimental measurements. Our results confirm that the spin moments are less quenched, while the orbital moments are strongly quenched in small Fe clusters. Both GGA and GGA+U functionals always yield collinear magnetic ground-state solutions for the fully relaxed Fe structures. Geometrical evolution, as a function of cluster size, illustrates that the icosahedral morphology competes with the hexagonal-antiprism morphology for large Fe clusters. In addition, the calculated trends of ionization potentials, electron affinities, fragment energies, and polarizabilities generally agree with respective experimental observations.

Interface engineering for lattice-matched epitaxy of ZnO on (La,Sr)(Al,Ta)O3(111) substrate

ZnO∕(La,Sr)(Al,Ta)O3(LSAT) heterointerface is engineered to control the crystallographic orientation of ZnO films grown by plasmas-assisted molecular beam epitaxy. Lattice-matched in-plane alignment of [112¯0]ZnO‖[112¯]LSAT has been realized using Mg modification of the substrate surface, which is confirmed with in situ reflection high-energy electron diffraction observation, and ex situ characterization of x-ray diffraction and transmission electron microscopy. The low-temperature deposition and high-temperature treatment of the Mg layer on the oxygen-terminated LSAT(111) surface results in selective nucleation of a MgO interface layer which serves as a template for single-domain epitaxy of ZnO. Oxygen-polar ZnO film with an atomically smooth surface has been obtained, which is favorable for metal-ZnO Schottky contact with high barrier height.