Organic–inorganic hybrid metal halide perovskites have emerged as a highly promising class of light emitters, which can be used as phosphors for optically pumped white light-emitting diodes (WLEDs). ...By controlling the structural dimensionality, metal halide perovskites can exhibit tunable narrow and broadband emissions from the free-exciton and self-trapped excited states, respectively. Here, we report a highly efficient broadband yellow light emitter based on zero-dimensional tin mixed-halide perovskite (C4N2H14Br)4SnBr x I6–x (x = 3). This rare-earth-free ionically bonded crystalline material possesses a perfect host-dopant structure, in which the light-emitting metal halide species (SnBr x I6–x 4–, x = 3) are completely isolated from each other and embedded in the wide band gap organic matrix composed of C4N2H14Br–. The strongly Stokes-shifted broadband yellow emission that peaked at 582 nm from this phosphor, which is a result of excited state structural reorganization, has an extremely large full width at half-maximum of 126 nm and a high photoluminescence quantum efficiency of ∼85% at room temperature. UV-pumped WLEDs fabricated using this yellow emitter together with a commercial europium-doped barium magnesium aluminate blue phosphor (BaMgAl10O17:Eu2+) can exhibit high color rendering indexes of up to 85.
Tin halide perovskites and perovskite-related materials have emerged as promising lead-free hybrid materials for various optoelectronic applications. While remarkable progress has been achieved in ...the development of organic tin halide hybrids with diverse structures and controlled dimensionalities at the molecular level, some controversial results that have been reported recently need to be addressed. For instance, different photophysical properties have been reported for two-dimensional (2D) (PEA)2SnBr4 (PEA = phenylethylammonium) by several groups with distinct emission peaks at around 468 and 550 nm. Here we report our efforts in the synthesis of phenylethylammonium tin bromide hybrids with zero-dimensional (0D) and 2D structures, and characterizations of their structural and photophysical properties. 0D (PEA)4SnBr6(PEA)Br2CCl2H22 was found to exhibit strong yellow emission peak at 566 nm with a photoluminescence quantum efficiency (PLQE) of ~90%, while 2D (PEA)2SnBr4 had weak emission peak at 470 nm with a PLQE of <0.1%. Interestingly, 0D (PEA)4SnBr6(PEA)Br2CCl2H22 can be converted into 2D (PEA)2SnBr4 upon drying, which would return to 0D (PEA)4SnBr6(PEA)Br2CCl2H22 upon addition of dichloromethane. Powder X-ray diffraction results confirmed the reversible transformation between 0D and 2D structures. Density functional theory calculations showed that excitons in 0D (PEA)4SnBr6(PEA)Br2CCl2H22 are highly localized, resulting in a strongly Stokes shifted broadband emission, while delocalized electronic states in 2D (PEA)2SnBr4 result in weaker exciton binding, a higher exciton mobility, and a higher nonradiative decay.
We have tested the ability of N-heterocyclic carbene (NHC)-modified ligands to coordinate and stabilize luminescent CdSe–ZnS core–shell quantum dot (QD) dispersions in hydrophilic media. In ...particular, we probed the effects of ligand structure and coordination number on the coating affinity to the nanocrystals. We find that such NHC-based ligands rapidly coordinate onto the QDs (requiring ∼5–10 min of reaction time), which reflects the soft Lewis base nature of the NHC groups, with its two electrons sharing capacity. Removal of the hydrophobic cap and promotion of carbene-driven coordination on the nanocrystals have been verified by 1H NMR spectroscopy, while 13C NMR was used to identify the formation of carbene–Zn complexes. The newly coated QD dispersions exhibit great long-term colloidal stability over a wide range of conditions. Additionally, we find that coordination onto the QD surfaces affects the optical and spectroscopic properties of the nanocrystals. These include a size-dependent red-shift of the absorption and fluorescence spectra and a pronounced increase in the measured fluorescence intensity when the samples are stored under white light exposure compared to those stored in the dark.
The self-initiation of the thiol–ene coupling reaction of tetravinyl monomers containing main group elements and trivinyl heterocycles with alkyl and aryl dithiols resulted in the formation of highly ...cross-linked prepolymer gels which upon final curing at 120 °C yielded hard, monolithic polymeric materials. Because of the presence of highly polarizable main group elements such as Si, Ge, Sn, and S and the relative absence of highly electronegative elements, the resulting polymers exhibited high refractive indices ranging from 1.590 to 1.703 and Abbe numbers between 24.3 and 45.0. All of the polymers were highly transparent over the UV–vis region of the spectrum. Moreover, due to the high cross-linked density achievable in specific compositions, very hard materials capable of being ground and polished could be produced. The range of compositions produced yields important structure–property relationships, indicating the effect of monomer structure on mechanical and optical properties.
Thin films of amorphous alumina of thickness 350nm were deposited on fused silica substrates by electron beam evaporation. Amorphous films were annealed at several temperatures in the range: ...400–1130°C and changes in film crystallinity, short-range structure, optical and mechanical properties were studied. X-ray diffraction studies found that crystallization starts at 800°C and produces γ and δ-alumina, the latter phase grows with heat treatment and the sample was mostly δ and θ-alumina after annealing at 1130°C. The as-deposited amorphous alumina films have low hardness of 5 to 8GPa, which increases to 11 to 12GPa in crystalline sample. 27Al Magic Angle Spinning Nuclear Magnetic Resonance was used to study the short-range order of amorphous and crystalline alumina films and it was found that amorphous alumina film contains AlO5 and AlO4 structural units in the ratio of 1:2. The concentration of AlO5 was significantly suppressed in crystalline film, which contains 48% of Al3+ ions in AlO6, 7% in AlO5 and 45% in AlO4 units.
•Structure–property correlations in alumina films grown by electron-beam evaporation•Amorphous films crystallize into γ and δ-alumina on annealing in air at 800°C.•δ and θ-alumina films are stable up to 1130°C and do not transform to α-phase.•Amorphous alumina films contain 5Al and 4Al structural units in the ratio of 1:2.•5Al decreases whereas 6Al concentration increases on crystallization.
Several Si and Co substituted AlPO
4-based molecular sieves with AFI, AEL and CHA structures including SAPO-
n and CoAPO-
n (where
n
=
5, 11 and 34) have been synthesized under dry gel conversion ...(DGC) conditions by using the pre-prepared AlPO-based precursors which have been demonstrated previously to be structurally related to the target molecular sieves. We show that highly crystalline SAPO-
n and CoAPO-
n can be prepared by this approach. Unlike conventional DGC method where the Si source is mixed with P and Al sources at the very beginning to form an initial amorphous dry gel, in this work the Si is slowly introduced to the surface of the dry powder of the semi-crystalline or crystalline AlPO-based precursor via vapor phase, and subsequently incorporated into the framework gradually. The Si environments in the SAPOs prepared by this approach appear to be more diversified compared to those in the corresponding sieves made by the normal hydrothermal synthesis (HTS) method. One feature of this synthesis is that the Si substitution in the lattice may only occur at some specific positions. This is due to the fact that since certain P sites in the precursors are directly carried into the framework of the final sieves, they are not available for Si incorporation. This finding may have implication to the locations of the catalytic sites in the SAPO-based catalysts. Similarly, for CoAPO-
n, the Co source was mixed with the preformed AlPO-based precursor before DGC treatment. CoAPO-5 has a higher degree of the Co incorporation than that of the CoAPO-5 made under normal HTS conditions.
Crystallization of AlPO4-5 (AFI), a representative aluminophosphate-based molecular sieve with AFI topology, was examined under dry gel conversion, in particular, vapor phase transport (VPT) ...conditions. The VPT approach allows one to “freeze” crystallization at different stages and, therefore, to capture several reactive intermediates, which provide “snapshots” of the crystallization process. These intermediates were characterized by powder X-ray diffraction, Raman spectroscopy, and a combination of several solid-state NMR techniques. The results show that the reactive intermediates have two key features: (1) They bear certain structural characteristics of the target framework. (2) They are fragile and held together by weak bonding interactions. Hydrogen bonding apparently plays a key role in the formation of intermediates and their subsequent transformation to the final AFI structure. An AlPO4-5 crystallization mechanism under VPT conditions was proposed.
Glasses of the system:
xBi
2O
3–(100−
x)B
2O
3 (
x
=
20 to 66
mol%) were prepared and characterized by density, DSC, UV–visible absorption and
11B MAS-NMR spectroscopy. Glass molar volume increases ...while the glass transition temperature decreases with Bi
2O
3 concentration. Densities of some bismuth borate glasses are found to be greater or very close to those of single crystal phases with equal composition. B
11 MAS-NMR studies determined that the fraction of tetrahedrally coordinated borons (
N
4) is maximum at 42
mol% of Bi
2O
3 and that there is a local maxima in
N
4 at Bi
2O
3 concentration of 50
mol%. Glasses containing Bi
2O
3 concentration of 33
mol% and higher show an unusual, intense absorption band just below the optical band gap. Two crystalline phases: Bi
3B
5O
12 and Bi
4B
2O
9 were prepared by devitrification of glasses and characterized by X-ray diffraction, FTIR and
11B MAS-NMR studies. Both crystalline phases contained significantly lower
N
4 than glasses with equal composition.
1H nuclear magnetic resonance (NMR) spectroscopy was used to study hydroxylic surface species on ceria nanocubes, a crystalline, high-surface-area CeO2 that presents mostly (100) facets. Water ...adsorption and desorption experiments in combination with fast magic angle spinning (MAS, 20–40 kHz) 1H NMR provide high-resolution 1H spectra that allow the observation of ten resonance bands (water or hydroxyl) on or under the (100) surface. Assignments were made using a combination of adsorption and temperature-programmed desorption, quantitative spin counting, deuterium exchange, spin–lattice (T 1) and spin–spin (T 2) relaxation, and DFT calculations. In air, the (100) surface exists as a fully hydroxylated surface. Water adsorption and dissociation on dry ceria surfaces occur first at oxygen vacancies, but Ce3+ centers are not required since water dissociation is barrier-less on the fully oxidized surface. Surface −OH functionality occurs in two resolved bands representing isolated −OH (1 ppm) and hydrogen-bonded −OH (9 ppm), the latter being dominant. Deuterium exchange of surface hydroxyls with D2O does not occur under mild or forcing conditions. Despite large differences in the T 1 of surface hydroxyls and physisorbed water, surface hydroxyl T 1 values are independent of the presence or absence of physisorbed water, demonstrating that the protons within these two functional group pools are not in intimate contact. These observations show that, once hydroxylated, the surface −OH functionality preferentially forms hydrogen bonds with surface lattice oxygen, i.e., the hydroxylated (100) surface of ceria is hydrophobic. Near this surface it is energetically more favorable for physisorbed water to hydrogen bond to itself rather than to the surface. DFT calculations support this notion. Impurity Na+ remaining in incompletely washed ceria nanocubes increases the surface hydrophilicity. Sharp, low-field resonances observed in spectra of noncalcined nanocubes arise from kinetically trapped subsurface −OH.
Glasses of bismuth borosilicates, aluminoborates and aluminoborosilicates were prepared by melt quenching and characterized by density measurements, 11B and 27Al Magic Angle Spinning (MAS) Nuclear ...Magnetic Resonance (NMR) spectroscopy, and Differential Scanning Calorimetry (DSC). Increase in silica concentration increases density without any change in the fraction of tetrahedral borons (N4) in the borosilicate glass network, whereas increase in alumina concentration in aluminoborate and aluminoborosilicate glasses drastically reduces density and N4. 27Al MAS NMR measurements revealed that Al ions are in four, five and six fold coordination with oxygen and about 47% of these are in 5Al and 6Al structural units, the concentration of the latter species decreases by adding silica in the glass network. Higher cation field strength of Bi3+ generates very large concentration of non-bridging oxygens, which act as charge balancing centers in the glass network. DSC measurements found that alumina and silica incorporation in bismuth borate glasses do not produce any systematic changes in the glass transition temperature but the tendency towards crystallization decreases steadily.
•Study of bismuth borosilicate, aluminoborate and aluminoborosilicate glasses•Alumina decreases density and N4 in glasses.•High concentration of 5Al and 6Al units in aluminoborate glasses•Silica suppresses the concentration of 5Al and 6Al in aluminoborosilicate glasses.•Alumina and silica increase the thermal stability of glasses against crystallization.
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