Stabilization of Eu2+ in KLuS2 crystalline host: an EPR and optical study

Electron paramagnetic resonance and luminescence spectroscopies were applied to study the incorporation and charge stability of Eu²⁺ luminescent ions in single crystals of KLuS₂:Eu found in an earlier optical study [Jary et al., Chem. Phys. Lett. 574 , 61 (2013)]. The location of Eu²⁺ in the structure was unambiguously determined and three different centers were identified and described. Two of these centers correspond to substitution of Eu²⁺ for K⁺ and Lu³⁺ ions providing thus effective mechanism for Eu²⁺ incorporation due to the charge self‐compensation in the lattice. The observed luminescence spectra are consistent with the results of electron paramagnetic resonance experiment and can be decomposed accordingly. (© 2014 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Spectroscopic ellipsometry of layer by layer deposited colloidal HgTe nanocrystals exhibiting quantum confinement

We have explored the optical properties of bilayers of Mercury telluride (HgTe) nanocrystals (NCs) embedded in polymer which were prepared from a colloidal solution. These NCs show strong luminescence in the near infrared at room temperature, which makes them an interesting material for the telecommunication area. The emission wavelength can efficiently be tuned by controlling the size of the NCs. We report spectroscopic ellipsometry measurements, which clearly show an energy shift of the critical points (CPs) in the dielectric function to higher energies compared to the HgTe bulk properties. This is caused by quantum confinement in the crystals. The exact peak energies of the transitions are fitted with line-shape models for CPs. Surprisingly, concepts coming from semiconductor bulk optics, as CPs, can be applied to NCs with a diameter of less than 5 unit cells.     

Synthesis and Post-Synthesis Transformation of Germanosilicate Zeolites

Zeolites are one of the most important heterogeneous catalysts, with a high number of large-scale industrial applications. While the synthesis of new zeolites remain rather limited, introduction of germanium has substantially increased our ability to not only direct the synthesis of zeolites but also to convert them into new materials post-synthetically. The smaller Ge-O-Ge angles (vs. Si-O-Si) and lability of the Ge−O bonds in aqueous solutions account for this behaviour. This Minireview discusses critical aspects of germanosilicate synthesis and their post-synthesis transformations to porous materials.     

Colloidal HgTe nanocrystals with widely tunable narrow band gap energies: from telecommunications to molecular vibrations

A convenient, aqueous-based synthesis of stable HgTe nanocrystals with widely size-tunable room temperature emission between wavelengths of 1.2 to 3.7 mum is demonstrated. By the choice of the thiols, applied as stabilizers, we optimized the growth dynamics, the luminescence quantum yields (up to 40%), and a ligand-exchange procedure, required to transfer the nanocrystals from water to nonpolar organic solvents. The latter is greatly improved and facilitated by the use of mercaptoethylamine as initial stabilizer. The possibility to tune the HgTe nanocrystal sizes from 3 to 12 nm and to control their surface functionalities (hydrophobic and hydrophilic) makes them very promising for the development of infrared optical devices, emitting in the wavelength region between the telecommunications and the molecular vibrations.     

Scaling the microrheology of living cells

We report a scaling law that governs both the elastic and frictional properties of a wide variety of living cell types, over a wide range of time scales and under a variety of biological interventions. This scaling identifies these cells as soft glassy materials existing close to a glass transition, and implies that cytoskeletal proteins may regulate cell mechanical properties mainly by modulating the effective noise temperature of the matrix. The practical implications are that the effective noise temperature is an easily quantified measure of the ability of the cytoskeleton to deform, flow, and reorganize.     

On polygons circumscribing a closed convex curve

Length and area formulas for closed polygonal curves are derived, as functions of the vertex angles and the distances to the lines containing the sides. Applications of the formulas are made to the class of polygons which circumscribe a given convex curve and have a prescribed sequence of vertex angles. Geometric conditions are given for polygons in the class which have extremal perimeter or area.     

Electron-acceptor luminescence from a non-Maxwellian distribution of photoexcited electrons in GaAs

Distribution functions are calculated for photoexcited electrons in GaAs, under conditions of continuous, monochromatic excitation. The lattice temperature is taken to be 1.2 K and the excitation intensity such that the density of photoexcited carriers is insufficient for the distribution to be affected by intercarrier scattering. A Boltzmann equation approach is used to take account of the effects of, injection of electrons into the conduction band, at an energy below the threshold for longitudinal optical phonon emission, scattering by acoustic phonons, via the deformation potential and piezoelectric interactions, and recombination. The equation is solved numerically using an iterative technique and the distribution functions are found to differ significantly from a Maxwellian form. Emission spectra due to conduction band to neutral acceptor transitions are derived from the computed distribution functions and are compared with recent experimental results.     

Adsorption of Poly(rA) on the Carbon Nanotube Surface and its Hybridization with Poly(rU)

Adsorption of poly(rA) on a single‐walled carbon nanotube surface in aqueous suspension and the subsequent hybridization of this polymer with free poly(rU) is studied. A comparison of the temperature dependence of the absorbance of free poly(rA) and poly(rA) adsorbed on the nanotube surface [poly(rA)^NT] at ν_max=38 500 cm^?1 shows that the thermostability of the adsorbed polymer is higher. Molecular dynamics simulations demonstrate that more than half of the adenines are not stacked on the tube surface and some of them undergo self‐stacking. After addition of a complementary poly(rU) to the poly(rA)^NT suspension, a double‐stranded polymer is formed as confirmed by the characteristic S‐like form of its melting curve. However, the melting temperature of this polymer is lower than that of the free poly(rA)?poly(rU) duplex. This result indicates that poly(rU) hybridization with poly(rA)^NT occurs with defects along the whole length of the polymer because of π–π stacking between nitrogen bases and the nanotube surface, which hinders the usual hybridization process. Computer modeling demonstrates different possible structures of hybridized polymers on the nanotube surface.     

Highly Efficient and Flexible Scintillation Screen Based on Organic Mn(II) Halide Hybrids toward Planar and Nonplanar X-Ray Imaging

The urgency for cost-effective, high-resolution, flexible X-ray imaging detectors is generating great demand for scintillators with low-temperature processability, high scintillation yield, and negligible afterglow. X-ray imaging materials are currently dominated by inorganic scintillators in the form of rigid films and bulk crystals, which have inherent limitations including high-temperature, complex synthesis, and considerable challenges toward advanced nonplanar imaging. Here, high-performance and flexible X-ray scintillators based on novel zero-dimensional (0D) (BTPP)₂MnX₄ (BTPP = benzyltriphenylphosphonium; X = Cl, Br) halides are reported. They emit bright green light originating from the ⁴T₁-⁶A₁ transition of Mn²⁺ under X-ray excitation. In particular, (BTPP)₂MnBr₄ single crystals exhibit excellent air- and radiation-stability, a high scintillation yield of 53 000 photons MeV⁻¹, a low detection limit of 89.9 nGy_air s⁻¹, and an ultralow afterglow comparable to commercial Bi₄Ge₃O₁₂ (BGO) single crystals. Moreover, the (BTPP)₂MnCl₄@polydimethylsiloxane (PDMS) flexible scintillation screens achieve a high spatial resolution of 14.1 lp mm⁻¹ and realize high-quality imaging results of nonplanar objects. This study demonstrates that the flexible scintillation screens based on low-dimensional Mn(II) hybrid halides have significant potential for low-dose and high-resolution X-ray imaging applications.     

Amplified Spontaneous Emission Threshold Dependence on Determination Method in Dye-Doped Polymer and Lead Halide Perovskite Waveguides

Nowadays, the search for novel active materials for laser devices is proceeding faster and faster thanks to the development of innovative materials able to combine excellent stimulated emission properties with low-cost synthesis and processing techniques. In this context, amplified spontaneous emission (ASE) properties are typically investigated to characterize the potentiality of a novel material for lasers, and a low ASE threshold is used as the key parameter to select the best candidate. However, several different methods are currently used to define the ASE threshold, hindering meaningful comparisons among various materials. In this work, we quantitatively investigate the ASE threshold dependence on the method used to determine it in thin films of dye-polymer blends and lead halide perovskites. We observe a systematic ASE threshold dependence on the method for all the different tested materials, and demonstrate that the best method choice depends on the kind of information one wants to extract. In particular, the methods that provide the lowest ASE threshold values are able to detect the excitation regime of early-stage ASE, whereas methods that are mostly spread in the literature return higher thresholds, detecting the excitation regime in which ASE becomes the dominant process in the sample emission. Finally, we propose a standard procedure to properly characterize the ASE threshold, in order to allow comparisons between different materials.