Enhancing upconversion luminescence and thermal sensing properties of Er/Yb co-doped oxysulfide core-shell nanocrystals

The development of noncontact thermal probe based on stable inorganic materials of trivalent lanthanide (Ln³⁺) doped phosphors with nontoxicity is of vital importance for their promising applications in bio-medical fields. Here we explore the upconversion luminescence and thermal sensing properties of Er³⁺, Yb³⁺ co-doped oxysulfide in a broad temperature range of 300-583 K. It was found that constructing an active shell with an optimum concentration of sensitizers is an efficient way to improve both the luminescent intensities and thermal sensitivity. Compared with the core-only sample, the luminescent intensity of the Y₂O₂S: Er³⁺, Yb³⁺@ Y₂O₂S: 5%Yb³⁺ sample is significantly enhanced by 12-fold at excitation of 980 nm. While further increasing the Yb³⁺ concentration in the shell activates new quenching pathways of Er³⁺ → Yb³⁺ → quencher from the core to the shell. Similar quenching mechanisms are also observed at excitation of 1550 nm. These energy transfer processes and luminescence mechanisms are verified in the fluorescence decay measurements. Furthermore, coating the core sample with an active shell doped by 10% Yb³⁺ enhances the thermal sensitivity by 30%, holding a high and stable sensitivity more than 50 × 10⁻⁴ K⁻¹ in a broad temperature range of 423-573 K at 980 nm excitation. In addition, at the much safer excitation wavelength of 1550 nm, this sample achieved the maximum sensitivity of 45 × 10⁻⁴ K⁻¹ at 503 K. Our work contributes a feasible and versatile way to promote the luminescence and thermal sensing properties of Ln³⁺-based materials, combining with the nontoxic oxysulfide host, indicating their potential applications as safe fluorescent and temperature nano-probes in bio-field.     

上转换/半导体核壳结构纳米复合材料光催化研究进展

上转换荧光材料具有将近红外光转换为紫外光和可见光的光学性能,与传统的半导体材料复合,使其光谱响应拓展到近红外区域,充分利用太阳光,从而提高光催化效率。核壳结构除能够有效地防止核(上转换纳米材料)被污染失活外,还可以使其紧密接触,有利于荧光共振能量的传递,提高壳(半导体材料)对光的利用率。系统论述了上转换半导体核壳结构纳米复合材料的制备方法、催化降解效率和催化影响因素,对此类复合材料的结合和催化机理进行分析,指出今后应加强高效率荧光材料和催化活性更强的半导体材料复合及如何在实际中应用的研究。     

Designing and temperature sensing characteristics of upconversion luminescence core-shell structures with negative and positive thermal expansion

Generally, lanthanum ions doped positive expansion and negative expansion materials exhibit thermal quenching and enhancement of upconversion luminescence (UCL), respectively. Combining the UCL characteristics of positive expansion and negative expansion lattices is of importance for developing efficient temperature sensing systems. Here, positive expansion TiO₂:Yb³⁺, Er³⁺ three dimensionally ordered macroporous film was prepared by the template-assisted approach, and the Yb₂W₃O₁₂: Er³⁺ solution was filled into the TiO₂: Yb³⁺, Er³⁺ three dimensionally ordered macroporous film. After secondary sintering, the shell of negative expansion Yb₂W₃O₁₂: Er³⁺was formed on the surface of TiO₂:Yb³⁺/Er³⁺ core. Under 980 nm excitation, the red and green UCL is predominate for the spectra of TiO₂:Yb³⁺/Er³⁺ core and Yb₂W₃O₁₂: Er³⁺ shell, respectively. With the measurement temperature increasing, the green UCL from negative expansion Yb₂W₃O₁₂: Er³⁺ shell increases, while the red UCL from positive expansion TiO₂:Yb³⁺, Er³⁺ core decreases. The performance of temperature sensing was characterized by the monitoring the UCL intensity ratio between 525 nm and 660 nm. The temperature sensitivity is about 1.12% K^?1, which is larger than that of thermally coupled FIR technology. We believed that the present work is instructive for developing new generation temperature sensor.     

Structural phase instability,mixed-phase,and energy band gap change in BiFeO3 under lattice strain effect from first-principles investigation

Lattice strain effects drive a variety of novel functional responses in epitaxial BiFeO₃ thin films and have attracted significant interest and attention from researchers in experimental and theoretical studies. However, the difficulty in designing experimental techniques in addition to facing problems in the first principles approach, such as output accuracy and high computational costs, constitute the discovery of new functional responses in epitaxial BiFeO₃ thin films not entirely understood. Therefore, in this study, we perform a first principles calculation based on the less expensive LDA+U method to investigate the structural phase instability and electronic properties change in BiFeO₃ under the lattice strain effect. The structural phase transformation of BiFeO₃ under volumetric and compressive/tensile lattice strain was examined established on the calculated lower energy phases. Importantly, we demonstrated that the change of crystal structure phases of BiFeO₃ was extremely sensitive to the volumetric and compressive/tensile lattice strain, comparable with various experiment data, as reported in the literature. Moreover, we revealed for the first time from the first principles prediction the coexistence of mixed R-T phases in the region of moderate compressive ζ_in-plane of ?2.9% (e.g. LaAlO₃ substrates with ɑ = 3.79 Å). From the prediction of electronic properties obtained by the LDA+U and PBE0 methods, we found that the energy band gap increased when the compressive in-plane lattice strain is increased while, in contrast, the energy band gap decreased when BiFeO₃ was under the tensile in-plane lattice strain effect. We also demonstrate that our computational technique based on the first principles study was sufficiently accurate enough, helping to speed up the process of designing new materials having an excellent multifunctional response (piezoelectric, magnetic, photovoltaic, spintronic).     

Effects of LiF additive on crystal structures,lattice vibrational characteristics and dielectric properties of CaWO4 microwave dielectric ceramics for LTCC applications

In this paper, CaWO₄-x wt.% LiF (x = 0.5–3.0) microwave dielectric ceramics are prepared by conventional solid-state reaction at 850 °C. All the X-ray diffraction peaks are attributed to CaWO₄, combined with the results of X-ray photoelectron spectroscopy and scanning electron microscope. They indicate that LiF forms liquid phase at high temperature, exists in amorphous phase, and the CaWO₄-x wt.% LiF samples belong to the composite ceramics. With the increase of the LiF content, the liquid phase fills the pores of the ceramics and increases the compactness. Therefore, the quality factor increases from 59,055 GHz (x = 0.5) to 77,855 GHz (x = 3.0), and the dielectric constant decreases from 9.26 (x = 0.5) to 8.41 (x = 3.0). The lattice vibration characteristics of the CaWO₄-x wt.% LiF samples are studied by Raman and infrared spectroscopy. The external modes associated with the vibration of the Ca–O bonds have the greatest influence on the dielectric response, as shown by the fitting results based on the four-parameter semi-quantum (FPSQ) model. In addition, the simulated intrinsic properties and measured values are in general agreement. When the x value increases, the dielectric constant decreases with the decrease of the bond length. The quality factor has a negative correlation with the FWHM value of the B_g mode and a positive correlation with the packing fraction. Finally, the CaWO₄-3 wt.% LiF sample has good compatibility with the silver powders at 850 °C, and high application value in the LTCC field.     

Synthesis, crystal structures and photoluminescence properties of Sm-based enantiomeric pair

Novel Sm-based enantiomeric pair, generally formulated Sm(DBM)₃L (L_R,R in 1, L_S,S in 2, DBM = dibenzoylmethanate) have been successfully prepared via the reaction of Sm(DBM)₃·2H₂O with chiral ligands L_R,R (−)-4,5-pinene bipyridine and L_S,S (+)-4,5-pinene bipyridine (Scheme 1), respectively. The crystal structure analysis of 1 and 2 reveal that they crystallize in chiral space group P2₁ of monoclinic system. The central Sm(III) ion is octacoordinate with six β-diketonate oxygen atoms and two chiral pinene bipyridine nitrogen atoms, forming a coordination polyhedron best described as the distorted square antiprism (SA). The CD spectra (Fig. S1) further confirm that 1 and 2 are enantiomers. The photoluminescence investigations of 1 and 2 demonstrate that they display deep-red luminescence characteristic of the Sm³⁺_.     

First-principles study on predicting the crystal structures,mechanical properties and electronic structures of HfCxN1-x

The crystal structures, mechanical properties and electronic structures of HfC_xN_1-x have been predicted by using evolutionary structure search followed by the first-principles calculations in this study. The crystal structures predicted indicate that there are 10 thermodynamic stable phases for HfC_xN_1-x, of which 8 are newly discovered crystal structures and 2 are already known. We investigated the mechanical properties, including the bulk modulus, shear modulus, Young’s modulus, Poisson’s ratio and Vickers hardness, of all 10 stable phases, and further established the relationship between such properties and the ratio of nitrogen to carbon content. Besides, the Fermi energy level and electronic density of states of these 10 stable phases are calculated as well, and the results reveal the fundamental reason why the mechanical properties change with the ratio of nitrogen to carbon. The predictions of this study agree well with both the experimental data and the previous theoretical evaluations.     

Ligand substitution in sandwich-type complexes of germanotungstates: Syntheses,crystal structures and magnetic properties

Two new sandwich-type germanotungstates K₂H₂{Zn(en)₂}{Zn(en)₂}₂{(GeW₉O₃₄)₂Zn₄(Hen)₂}·17H₂O (1) and K₂H₂{Ni(en)₂}{Ni(en)₂}₂{(GeW₉O₃₄)₂Ni₄(Hen)₂}·12H₂O (2) (en = ethylenediamine) have been hydrothermally synthesized. The polyanions of the two compounds are composed of a [Ge₂W₁₈M₄(Hen)₂O₆₈]^10? (M = Zn, Ni) group decorated by two [M(en)₂(H₂O)]²⁺ units. The luminescence of compound 1 exhibits an intense UV radiation emission maximum at λ ≈ 449 nm upon excitation of 372 nm. The magnetic investigation of compound 2 demonstrates the presence of ferromagnetic interactions between the Ni²⁺ ions of the Ni₄O₁₄(Hen)₂ clusters in 2.     

Lanthanide-doped nanocrystals: synthesis, optical-magnetic properties, and applications

Because of the potential applications of lanthanide-doped nanocrystals in display devices, optical communication, solid-state lasers, catalysis, and biological labeling, the controlled synthesis of these new nanomaterials has sparked considerable interest. Nanosized phosphorescent or optoelectronic devices usually exhibit novel properties, depending on their structures, shapes, and sizes, such as tunable wavelengths, rapid responses, and high efficiencies. Thus, the development of facile synthetic methods towards high-quality lanthanide-doped nanocrystals with uniform size and shape appears to be of key importance both for the exploration of their materials properties and for potential applications. This Account focuses on the recent development in our laboratory of the synthesis and applications of lanthanide-doped nanocrystals. Since 2005, when we proposed a general strategy for nanocrystal synthesis via a liquid-solid-solution process, a range of monodisperse and colloidal lanthanide-doped fluoride, oxide, hydroxide, orthovanadate, thiooxide, borate, and phosphate nanocrystals have been successfully prepared. By rationally tuning the reaction conditions, we have readily synthesized nanostructures, such as hollow microspheres, nanorods, nanowires, hexagonal nanoplates, and nanobelts. By adjusting the different colloidal nanocrystal mixtures, we fabricated unique binary nanostructures with novel dual-mode luminescence properties through a facile ultrasonic method. By tridoping with lanthanide ions that had different electronic structures, we successfully achieved β-NaYF(4) nanorods that were paramagnetic with tuned upconversion luminescence. We have also used NaYF(4):Yb(3+)/Er(3+) conbined with magnetite nanoparticles as a sensitive detection system for DNA: NaYF(4):Yb(3+)/Er(3+) and Fe(3)O(4) nanoparticles were modified with two different DNA sequences. Then, the modified NaYF(4):Yb(3+)/Er(3+) nanoparticles were conjugated to the modified Fe(3)O(4) nanoparticles. These binary nanoparticles can be hybridized with a third DNA (target DNA) molecule and separated with the assistance of a magnetic field. In addition, a novel fluorescence resonance energy transfer (FRET) method for nonenzymatic glucose determination has been developed by using the glucose-modified LaF(3):Ce(3+)/Tb(3+) nanocrystals. By using bioconjugated NaYF(4):Yb(3+)/Er(3+) nanoparticles as the energy donor and bioconjugated gold nanoparticles as the energy acceptor, we successfully developed a simple and sensitive fluorescence resonance energy transfer (FRET) biosensor for avidin. Meanwhile, we also carried out preliminary studies to investigate possible applications of lanthanide-doped nanocrystals in catalysis and in dye-sensitized solar cells.     

Syntheses, characterization, and energy transfer properties of benzothiadiazole-based hyperbranched polyfluorenes

A series of benzothiadiazole-based (BT) hyperbranched polyfluorene copolymers with various branching degrees (5-40%) were designed and synthesized. TGA and film annealing tests showed the substantial thermal stability of these highly branched polymers. The optical performance of the polymers in solutions and as films, and their electrochemical properties were characterized. The energy transfer (ET) processes in these hyperbranched conjugated polymers, both in solutions and in the solid state, were also investigated. With the change of the solution concentration and the branching degree, the energy transfer efficiency of the polymers varied in solutions and the main photoluminescence (PL) peaks changed from blue to green region. As films, only green light emitted from BT units. In addition, the PL efficiency of the films decreased dramatically with the increase of branching degrees. All these features demonstrated that highly branched structure would effectively impede the intra- and interchain energy migration, especially in solutions, and remarkably influence the ET process in the solid state, which resulted in low PL efficiency.     

Two novel lead-carboxylate complexes based on nicotinic acid N-oxide: Synthesis,crystal structures and luminescent properties

Two new lead-carboxylate complexes based on nicotinic acid N-oxide ligand, [Pb(NNO)(FA)_0.5] (1) (HNNO = nicotinic acid N-oxide, FA = fumaric acid) and [Pb₂(NNO)(BTC)] (2) (H₃BTC = 1,3,5-benzenetricarboxylic acid) were hydrothermally synthesized and characterized by the elemental analyses, IR spectra and single crystal X-ray diffraction. Significant phosphorescent properties of two compounds were observed at room temperature.     

2D lanthanide-based pyridine-substituted triazole benzoate coordination polymers: Structure,optical and magnetic properties

A series of novel lanthanide-based coordination polymers, [Ln(L)(atpa)(H₂O)₂]_n·1.5nH₂O (HL = 4-[3-methyl-5-(pyridin-4-yl)-1,2,4-triazol-4-yl]benzoic acid, H₂atpa = 2-aminoterephthalic acid and Ln = Tb(1), Dy(2), Ho(3), Er(4), Tm(5), Yb(6)) have been hydrothermally synthesized and characterized by single-crystal X-ray diffraction, optical and magnetic measurements. Compounds 1–6 are isomorphous, which can be described as a 2D network based on Ln₂(CO₂)₂ units. The magnetic properties of compound 1 (Tb) show typical antiferromagnetic interaction, while compound 2 (Dy) has ferromagnetic behavior. The room-temperature photoluminescence spectra of 1 exhibit strong characteristic emissions in the visible region, whereas compounds of Dy(2), Er(4) and Yb(6) display NIR luminescence upon irradiation at the ligand band.     

Effect of crystal structures on energy transfer behavior from Bi3+ to Eu3+ in alkaline earth metalstannates

A single phased color-tunable phosphor via energy transfer has drawn much attention, whereas it is still a question that how to choose a suitable host to realize an efficient energy transfer. In this work, novel single Bi³⁺ or Eu³⁺ doped and Bi³⁺, Eu³⁺ co-activated Ca₂SnO₄ (CSO) and Sr₂SnO₄ (SSO) phosphors have been prepared through a high temperature solid state reaction. The energy transfer happening from hosts to Eu³⁺ has been confirmed. By co-doping of Bi³⁺, a brighter red light is realized which is attributed to the much more efficient energy transfer from Bi³⁺ to Eu³⁺. After comparison, the energy transfer efficiency from Bi³⁺ to Eu³⁺ is much higher in CSO:Bi³⁺, Eu³⁺ than that in SSO:Bi³⁺, Eu³⁺. Furthermore, the experiment reveals that the single-phased color-tunable phosphors exhibit excellent resistance against thermal quenching, suggesting that they can be potential candidates in UV-pumped white light emitting diodes (LEDs).     

Doping dependent crystal structures and optoelectronic properties of n-type CdSe:Ga nanowries

Although CdSe nanostructures possess excellent electrical and optical properties, efforts to make nano-optoelectronic devices from CdSe nanostructures have been hampered by the lack of efficient methods to rationally control their structural and electrical characteristics. Here, we report CdSe nanowires (NWs) with doping dependent crystal structures and optoelectronic properties by using gallium (Ga) as the efficient n-type dopant via a simple thermal co-evaporation method. The phase change of CdSe NWs from wurtzite to zinc blende with increased doping level is observed. Systematical measurements on the transport properties of the CdSe:Ga NWs reveal that the NW conductivity could be tuned in a wide range of near nine orders of magnitude by adjusting the Ga doping level and a high electron concentration up to 4.5 × 10(19) cm(-3) is obtained. Moreover, high-performance top-gate field-effect transistors are constructed based on the individual CdSe:Ga NWs by using high-κ HfO(2) as the gate dielectric. The great potential of the CdSe:Ga NWs as high-sensitive photodetectors and nanoscale light emitters is also exploited, revealing the promising applications of the CdSe:Ga NWs in new-generation nano-optoelectronics.     

Divalent cobalt,zinc, and copper coordination polymers based on a new bifunctional ligand: Syntheses,crystal structures,and properties

Three new coordination polymers, namely, [ML(H₂O)]_n (M = Co (1), Zn (2)) and [CuL(DMF)]_n (3), have been hydrothermally synthesized through the reaction of bifunctional ligand 5-((2-methyl-1H-imidazol-1-yl)methyl)benzene-1,3-dioic acid (H₂L) with divalent cobalt, zinc and copper salts, and structurally characterized by elemental analysis, IR and X-ray diffraction. Both complexes 1 and 2 are two-fold interpenetration 3D frameworks. Complex 3 exhibits (3,6)-connected framework with (4.6²)₂(4².6¹⁰.8³) topology. Furthermore, UV–vis absorption spectra and powder X-ray diffraction (PXRD) were investigated as well.     

Antiferroelectrics: History,fundamentals, crystal chemistry,crystal structures,size effects,and applications

Antiferroelectric (AFE) materials are of great interest owing to their scientific richness and their utility in high-energy density capacitors. Here, the history of AFEs is reviewed, and the characteristics of antiferroelectricity and the phase transition of an AFE material are described. AFEs are energetically close to ferroelectric (FE) phases, and thus both the electric field strength and applied stress (pressure) influence the nature of the transition. With the comparable energetics between the AFE and FE phases, there can be a competition and frustration of these phases, and either incommensurate and/or a glassy (relaxor) structures may be observed. The phase transition in AFEs can also be influenced by the crystal/grain size, particularly at nanometric dimensions, and may be tuned through the formation of solid solutions. There have been extensive studies on the perovskite family of AFE materials, but many other crystal structures host AFE behavior, such as CuBiP₂Se₆. AFE applications include DC-link capacitors for power electronics, defibrillator capacitors, pulse power devices, and electromechanical actuators. The paper concludes with a perspective on the future needs and opportunities with respect to discovery, science, and applications of AFE.     

Influence of crystal lattice defectiveness on the properties of solid solutions based on ZrO2

Conclusions It is shown that the parameter of the crystal lattice of cubic solid solutions based on zirconium dioxide, stabilized with oxides of yttrium, ytterbium, and calcium, is increased with a rise in their concentrations, and hardly alters for the solid solution ZrO₂-Sc₂O₃.In the solid solutions of Y₂O₃, Sc₂O₃, and CaO in zirconium dioxide we note a reduction in the x-ray densities with an increase in the concentration of stabilizing additives. For the solid solutions ZrO₂-Yb₂O₃ we note an increase in the x-ray density with rise in the concentration of ytterbium oxide.The introduction into the zirconia of 9–13% stabilizing oxides ensures the highest density characteristics for the ceramics.Translated from Ogneupory, No. 1, pp. 12–15, January, 1987.     

Synthesis,crystal structures and NIR luminescence properties of binuclear lanthanide Schiff Base complexes

Three binuclear lanthanide complexes [Nd₂(L¹)₃(MeOH)]·MeOH·H₂O (1) and [Ln₂(H₂L²)₂(OAc)₄]·2(CF₃SO₃)·MeOH·EtOH (Ln = Nd (2) and Ho (3)) were prepared using two Schiff base ligands. Interestingly, 1 has a triple-decker structure with two lanthanide ions enclosed by three rigid conjugated Schiff base ligands (H₂L¹), while 2 and 3 show nanoscale ring structures (8 × 12 × 12 Å) formed by flexible long-chain Schiff base ligands (H₂L², ~ 23 Å) with the lanthanide ions located in the center. Upon excitation of the ligand-centered absorption bands, 1–2 and 3 show typical NIR emission spectra for Nd^3 + and Ho^3 + ions, respectively. In 1, the Nd(III) centers are shielded within the decker-like structure and surrounded by chromogenic Schiff base ligands (energy transfer donors). Luminescence studies show that the NIR emission lifetime of 1 is longer than that of 2 in solution.     

The LSPR regulation of TiO2: W nanocrystals and its application in enhanced upconversion luminescence

The localized surface plasmon resonance (LSPR) absorption peaks of semiconductor nanocrystals are mainly concentrated in the infrared band, and the absorption characteristics can be controlled by the amount of element doping. The coupling of upconversion nanocrystals (UCNPs) and semiconductor nanocrystals can improve the upconversion luminescence (UCL) of rare-earth ions. Here, the LSPR absorption and morphology of the semiconductor nanocrystalline TiO₂: W were adjusted by using ammonium fluoride during synthesis. Significant absorption enhancement of TiO₂: W in the near-infrared region was obtained to enhance the UCL of NaYF₄: Yb³⁺, Er³⁺. The Glass/NaYF₄: Yb³⁺, Er³⁺/TiO₂: W@SiO₂ layered structure films were fabricated through spin coating. Compared with Glass/NaYF₄: Yb³⁺, Er³⁺, the green and red lights of the Glass/NaYF₄: Yb³⁺, Er³⁺/TiO₂: W@SiO₂ films were enhanced by 15.9 and 17.8 times, respectively. The UCL enhancement of Glass/NaYF₄: Yb³⁺, Er³⁺/TiO₂: W@SiO₂ was derived from the LSPR property of TiO₂: W through the enhancement of the excitation. The present work is important for possible applications of these layered structures as biomarkers, photocatalysts, flexible materials, and photoluminescence display panels.     

Preparation of MGF phosphor by O2 postannealing and impact on luminescence properties and crystal lattice

Mn⁴⁺-activated phosphors, Mg₂₈Ge₁₀O_48-δF_δ:Mn⁴⁺ (MGFs), can be obtained through an oxygen postannealing process. Analyses of the crystal structure and elemental composition by powder X-ray diffraction (XRD) and electron probe microanalysis (EPMA), respectively, indicated that under an O₂ atmosphere, oxygen atoms were substituted with fluorine atoms in the original MGF structure to leach the fluorine atoms with germanium atoms as GeF₄ by oxygen postannealing. The MGF phosphor annealed in O₂ exhibited ~1.3 times higher quantum efficiency (QE) than that annealed in ambient air. The Raman spectroscopy results suggested that an increase in the content of the [Mn⁴⁺O₆] octahedron led to an increase in the QE values. Additionally, the relaxation of lattice defects in the lattice interior and on the surface observed by XRD and X-ray photoelectron spectroscopy (XPS) measurements could explain the change in thermal quenching behavior between the different atmospheres, and the decrease in lattice defects increased the QE. The investigation of MGF phosphors prepared by different processes provides insight into the relationships among the surface and local structures, chemical composition, and photoluminescence properties. The optimized synthetic procedure increases the Mn⁴⁺ content and decreases the Mn²⁺ and Mn³⁺ contents in the phosphor, which drastically increases the luminescence efficiency.