Transparent Infrared-Emitting CeF₃:Yb−Er Polymer Nanocomposites for Optical Applications

Bright infrared-emitting nanocomposites of unmodified CeF3:Yb−Er with polymethyl-methacrylate (PMMA) and polystyrene (PS), which offer a vast range of potential applications, which include optical amplifiers, waveguides, laser materials, and implantable medical devices, were developed. For the optical application of these nanocomposites, it is critical to obtain highly transparent composites to minimize absorption and scattering losses. Preparation of transparent composites typically requires powder processing approaches that include sophisticated particle size control, deagglomeration, and dispersion stabilization methods leading to an increase in process complexity and processing steps. This work seeks to prepare transparent composites with high solids loading (>5 vol%) by matching the refractive index of the inorganic particle with low cost polymers like PMMA and PS, so as to circumvent the use of any complex processing techniques or particle surface modification. PS nanocomposites were found to exhibit better transparency than the PMMA nanocomposites, especially at high solids loading (≥10 vol%). It was found that the optical transparency of PMMA nanocomposites was more significantly affected by the increase in solids loading and inorganic particle size because of the larger refractive index mismatch of the PMMA nanocomposites compared to that of PS nanocomposites. Rayleigh scattering theory was used to provide a theoretical estimate of the scattering losses in these ceramic-polymer nanocomposites

Thermochemistry of 3D and 2D Rare Earth Oxychlorides (REOCls)

The thermodynamic stability of rare earth (RE) materials plays a key role in the design of separation and recycling processes for RE elements. Thermodynamic stability is fundamentally influenced by the lanthanide contraction, as observed in the systematic reduction of unit cell volumes with increasing atomic number. RE materials are found in the form of solids having primary bonds in three dimensions (3D materials) as well as ones with primary bonds in two dimensions (2D materials) whose layers are held together by weak van der Waals (vdW) forces. While studies of synthesis, structure, and physical properties of 2D RE materials are numerous, no systematic research has compared their thermodynamic stability to that of 3D materials. In the present work, RE oxychlorides (REOCls), which display a structural transition from a 3D-polyhedral network (PbFCl-type) to a vdW-bonded layered one (SmSI-type) as the RE size decreases, were all synthesized by the flux method. High-temperature oxide melt solution calorimetry was used to determine their formation enthalpies to enable Born–Haber cycles to calculate lattice energies. Our results indicate that REOCl compounds are thermodynamically stable when compared to their binary oxides and chlorides. The lattice energies of 3D REOCls increase with decreasing RE size yet are insensitive to unit cell volumes for 2D REOCls. This is caused by interatomic interactions parallel and perpendicular to layers in the SmSI-type REOCls, causing a different structure response to the lanthanide contraction than 3D RE materials

Comprehensive Study on the Size Effects of the Optical Properties of NaYF₄:Yb,Er Nanocrystals

Monodisperse β-NaYF₄:Yb,Er nanocrystals with mean sizes of 11, 40, and 110 nm were synthesized by a thermal decomposition solvothermal process to better understand the relationship between particle size and optical properties. A systematic study of luminescence intensity versus size revealed that both visible upconversion and infrared downconversion emission intensities decrease with decreasing nanocrystal size. The intrinsic quantum efficiency of the infrared ⁴<i>I</i>_13/2 → ⁴<i>I</i>_15/2 downconversion transition was studied in great detail since this specific transition allows us to quantify the contribution of nonradiative losses more easily than the observed upconversion transitions. The intrinsic quantum efficiency of the ⁴<i>I</i>_13/2→⁴<i>I</i>_15/2 transition decreased from 50% (110 nm) to 15% (11 nm). Multiphonon relaxation and −OH quenching was studied in these materials by measuring the vibrational characteristics of β-NaYF₄:Yb,Er nanospheres. While multiphonon relaxation exhibited increased contribution to nonradiative decay, −OH quenching rates were calculated to be ∼4 orders of magnitude higher than that of the multiphonon relaxation. Therefore, surface −OH quenching effects were concluded to be primarily responsible for the observed dependence of emission intensity versus particle size

Thermochemical Investigation of the Stability and Conversion of Nanocrystalline and High-Temperature Phases in Sodium Neodymium Fluorides

As important upconversion materials, sodium rare-earth fluorides (nominally NaREF4 in composition but actually often harboring sodium deficiency, especially in nanophase materials) have been subjected to intensive studies, particularly in the synthesis and applications of nanocrystals. However, the mechanisms of the conversion between the two phases (α and β) of NaREF4 nanocrystals during the synthesis are still controversial and lack thermodynamic investigations, which limit the rational design, synthesis, and processing of these materials. In this work, aiming at NaREF4 with light rare-earth elements, the thermochemistry of the NaF–NdF3 system, including the α and β phases in nanocrystalline/nanophase and bulk stoichiometric samples, is systematically studied by thermogravimetry and differential scanning calorimetry and high-temperature oxide melt solution calorimetry. With the help of compositional analysis and structural characterization, a strong Na deficiency is found in nanocrystals with small crystal sizes, which leads to the formation of cubic (α) crystallographic polymorphs at the nucleation stage, possibly because of the relative thermodynamic stability of the α phase compared to the β phase in such compositions. After converting to the hexagonal (β) structure, the crystal growth is accompanied by an increase of Na content in nanocrystals with increasing energetic stability until the formation of the stoichiometric compound (β-NaNdF4). On the contrary, the stoichiometric α phase (α-NaNdF4) is metastable at room temperature but is the intermediate phase as the temperature increases. We show that the α → β phase conversion in aqueous solution synthesis is distinct from the β → α transition driven by temperature because of composition differences

Lanthanide Clusters with Internal Ln Ions: Highly Emissive Molecules with Solid-State Cores

“Er(SePh)2.5I0.5” reacts with elemental S to give (THF)10Er6S6I6, a double cubane cluster with one face of the Er4S4 cube capped by an additional Er2S2. Reactions with a mixture of elemental S/Se results in the formation of (THF)14Er10S6(Se2)6I6, a cluster composed of an Er6S6 double cubane core, with two “Er2(Se2)3” units condensed onto opposing rectangular sides of the Er6S6 fragment. This deposition of Er2Se6 totally encapsulates the two central Er with chalcogen atoms (4 S, 4 Se) and excludes neutral THF donors or iodides from the two primary coordination spheres. The Er10 compound is the first lanthanide cluster to contain internal, chalcogen encapsulated Ln. This cluster shows strong fluorescence at 1544 nm with a measured decay time of 3 ms and an estimated quantum efficiency of 78%, which is comparable to Er doped solid-state materials. The unusual fluorescence spectral properties of (THF)14Er10S6Se12I6 are unprecedented for a molecular Er complex and are attributed to the low phonon energy host environment provided by the I-, S2-, and Se22- ligands

Thiolate-Bound Thulium Compounds: Synthesis, Structure, and NIR Emission

Molecular (DME)2Tm(SC6F5)3 and tetrametallic (DME)4(μ2-SPh)8Tm4(SPh)4 have been isolated in high yield and characterized by conventional methods. The fluorinated thiolate has an eight-coordinate Tm(III) ion bound to three sulfur atoms from the thiolates, four oxygen donors from the THF ligands, and a relatively weak dative Tm−F interaction with a 2.842(3)Å Tm−F separation. The benzenethiolate compound crystallizes as a tetramer with an alternating number (3, 2, 3) of thiolates connecting the nearly linear array of Tm(III) ions. Emission properties of both thiolates and the chalcogen-rich cluster (THF)6Tm4Se9(SC6F5)2 have been established. The Ph compound is significantly more emissive than either of the fluorinated species

Thiolate-Bound Thulium Compounds: Synthesis, Structure, and NIR Emission

Molecular (DME)2Tm(SC6F5)3 and tetrametallic (DME)4(μ2-SPh)8Tm4(SPh)4 have been isolated in high yield and characterized by conventional methods. The fluorinated thiolate has an eight-coordinate Tm(III) ion bound to three sulfur atoms from the thiolates, four oxygen donors from the THF ligands, and a relatively weak dative Tm−F interaction with a 2.842(3)Å Tm−F separation. The benzenethiolate compound crystallizes as a tetramer with an alternating number (3, 2, 3) of thiolates connecting the nearly linear array of Tm(III) ions. Emission properties of both thiolates and the chalcogen-rich cluster (THF)6Tm4Se9(SC6F5)2 have been established. The Ph compound is significantly more emissive than either of the fluorinated species

Lanthanide Clusters with Internal Ln Ions: Highly Emissive Molecules with Solid-State Cores

“Er(SePh)2.5I0.5” reacts with elemental S to give (THF)10Er6S6I6, a double cubane cluster with one face of the Er4S4 cube capped by an additional Er2S2. Reactions with a mixture of elemental S/Se results in the formation of (THF)14Er10S6(Se2)6I6, a cluster composed of an Er6S6 double cubane core, with two “Er2(Se2)3” units condensed onto opposing rectangular sides of the Er6S6 fragment. This deposition of Er2Se6 totally encapsulates the two central Er with chalcogen atoms (4 S, 4 Se) and excludes neutral THF donors or iodides from the two primary coordination spheres. The Er10 compound is the first lanthanide cluster to contain internal, chalcogen encapsulated Ln. This cluster shows strong fluorescence at 1544 nm with a measured decay time of 3 ms and an estimated quantum efficiency of 78%, which is comparable to Er doped solid-state materials. The unusual fluorescence spectral properties of (THF)14Er10S6Se12I6 are unprecedented for a molecular Er complex and are attributed to the low phonon energy host environment provided by the I-, S2-, and Se22- ligands

Lanthanide Clusters with Internal Ln Ions: Highly Emissive Molecules with Solid-State Cores

“Er(SePh)2.5I0.5” reacts with elemental S to give (THF)10Er6S6I6, a double cubane cluster with one face of the Er4S4 cube capped by an additional Er2S2. Reactions with a mixture of elemental S/Se results in the formation of (THF)14Er10S6(Se2)6I6, a cluster composed of an Er6S6 double cubane core, with two “Er2(Se2)3” units condensed onto opposing rectangular sides of the Er6S6 fragment. This deposition of Er2Se6 totally encapsulates the two central Er with chalcogen atoms (4 S, 4 Se) and excludes neutral THF donors or iodides from the two primary coordination spheres. The Er10 compound is the first lanthanide cluster to contain internal, chalcogen encapsulated Ln. This cluster shows strong fluorescence at 1544 nm with a measured decay time of 3 ms and an estimated quantum efficiency of 78%, which is comparable to Er doped solid-state materials. The unusual fluorescence spectral properties of (THF)14Er10S6Se12I6 are unprecedented for a molecular Er complex and are attributed to the low phonon energy host environment provided by the I-, S2-, and Se22- ligands

Heterometallic Chalcogenido Clusters Containing Lanthanides and Main Group Metals: Emissive Precursors to Ternary Solid-State Compounds

Heterometallic clusters containing lanthanides and the group 12 metals can be isolated as crystalline compounds in high yields. These products [(py)8Ln4M2Se6(SePh)4 (Ln = Er, Yb, Lu; M = Cd, Hg)] adopt a double cubane structure with the covalent M occupying an opposing pair of external metal sites. Both Er/M compounds are strongly emissive materials, with emission lifetimes of 1.41 ms (Er/Cd) and 0.71 ms (Er/Hg) and with the Er/Cd radiative quantum efficiency twice that of the Er/Hg compound. Thermal decomposition of the Er/Cd and Yb/Cd compounds at 650 °C give the ternary solid-state materials CdLn2Se4