Sol−Gel Synthesis and Spectroscopic Properties of Thick Nanocrystalline CdSe Films

A Non-Hydrolytic Sol–Gel Route to Organic-Inorganic Hybrid Polymers: Linearly Expanded Silica and Silsesquioxanes

Condensation reactions of chlorosilanes (SiCl4 and CH3SiCl3) and bis(trimethylsilyl)ethers of rigid, quasi-linear diols (CH3)3SiO–AR–OSi(CH3)3 (AR = 4,4′-biphenylene (1) and 2,6-naphthylene (2)), with release of (CH3)3SiCl as a volatile byproduct, afforded novel hybrid materials that feature Si–O–C bridges. The precursors 1 and 2 were characterized using FTIR and multinuclear (1H, 13C, 29Si) NMR spectroscopy as well as single-crystal X-ray diffraction analysis in case of 2. Pyridine-catalyzed and non-catalyzed transformations were performed in THF at room temperature and at 60 °C. In most cases, soluble oligomers were obtained. The progress of these transsilylations was monitored in solution with 29Si NMR spectroscopy. Pyridine-catalyzed reactions with CH3SiCl3 proceeded until complete substitution of all chlorine atoms; however, no gelation or precipitation was found. In case of pyridine-catalyzed reactions of 1 and 2 with SiCl4, a Sol–Gel transition was observed. Ageing and syneresis yielded xerogels 1A and 2A, which exhibited large linear shrinkage of 57–59% and consequently low BET surface area of 10 m2⋅g−1. The xerogels were analyzed using powder-XRD, solid state 29Si NMR and FTIR spectroscopy, SEM/EDX, elemental analysis, and thermal gravimetric analysis. The SiCl4-derived amorphous xerogels consist of hydrolytically sensitive three-dimensional networks of SiO4-units linked by the arylene groups. The non-hydrolytic approach to hybrid materials may be applied to other silylated precursors, if the reactivity of the corresponding chlorine compound is sufficient.

A ground-state-dominated magnetic field effect on the luminescence of stable organic radicals

Organic radicals are an emerging class of luminophores possessing multiplet spin states and potentially showing spin-luminescence correlated properties. We investigated the mechanism of recently reported magnetic field sensitivity in the emission of a photostable luminescent radical, (3,5-dichloro-4-pyridyl)bis(2,4,6-trichlorophenyl)methyl radical (PyBTM) doped into host αH-PyBTM molecular crystals. The magnetic field (0-14 T), temperature (4.2-20 K), and the doping concentration (0.1, 4, 10, and 22 wt%) dependence on the time-resolved emission were examined by measuring emission decays of the monomer and excimer. Quantum mechanical simulations on the decay curves disclosed the role of the magnetic field; it dominantly affects the spin sublevel population of radical dimers in the ground states. This situation is distinctly different from that in conventional closed-shell luminophores, where the magnetic field modulates their excited-state spin multiplicity. Namely, the spin degree of freedom of ground-state open-shell molecules is a new key for achieving magnetic-field-controlled molecular photofunctions.

Modifying Titania Using the Molten-Salt-Assisted Self-Assembly Process for Cadmium Selenide-Quantum Dot-Sensitized Photoanodes

Sensitizing titania with semiconducting quantum dots (QDs) is an important field for the development of third-generation photovoltaics. Many methods have been developed to effectively incorporate QDs over the surface of mesoporous titania, assembled from the 20-25 nm titania nanoparticles. Here, we introduce a molten-salt-assisted self-assembly (MASA) method to fabricate CdSe-modified mesoporous titania photoanodes. A mixture of ethanol, two surfactants (cetyltrimethylammonium bromide and 10-lauryl ether), silica (tetramethyl orthosilicate) or titania source (Ti(OC₄H₉)₄, acid (HNO₃), and cadmium nitrate solution was infiltrated into the pores of mesoporous titania (assembled using Degussa 25, P25) and immediately calcined at 450 °C to obtain mesoporous cadmium oxide-silica-titania (meso-CdO-SiO₂-P25) or cadmium titanate-titania (meso-CdTiO₃-P25) films. The MASA process is a simple method to smoothly coat or fill the pores of titania with mesoporous CdO-SiO₂ or CdTiO₃ that can be reacted under an H₂Se atmosphere to convert cadmium species to CdSe at 100 °C. Etching of the silica films with a very dilute hydrogen fluoride solution produces mesoporous CdSe-titania (meso-CdSe-P25) electrodes. The method is flexible to adjust the CdSe/TiO₂ mole ratio over a very broad range in the films. The films were characterized at every stage of the preparation to demonstrate the effectiveness of the method. The electrodes were also tested in a simple two-electrode solar cell to demonstrate the performance of the electrodes that have a power conversion efficiency of 3.35%.

Excimers from stable and persistent supramolecular radical-pairs in red/NIR-emitting organic nanoparticles and polymeric films

For the first time, using a carbon free-radical, excimeric emission from stable and persistent supramolecular radical-pairs has been observed.

Structural and optical studies of pHEMA encapsulated ZnS:Ni²⁺ nanoparticles

In this study, ZnS:Ni(2+) nanostructures have been synthesized through chemical precipitation method using poly (2-hydroxyethyl methacrylate) (pHEMA) as capping agent. The structural, morphological and optical properties at different pHEMA concentration of ZnS:Ni(2+) were studied by using X-ray diffraction (XRD), high resolution transmission electron microscopy (HR-TEM), UV-Vis Spectroscopy (UV-Vis), fourier transform infrared spectroscopy (FT-IR), Photoluminescence (PL). The Average crystalline size of the nanoparticles was found to be in the range of ∼3.59-4.36 nm. The surface morphological analysis reveals that the pHEMA capped nanoparticles showed homogeneous smooth surface. HR-TEM analysis reminds the original size of pHEMA capped nanoparticles. The band gap investigation revealed the size dependent of quantum confined nanoparticles. The immobilized nanoparticles in pHEMA matrix were verified by FT-IR studies. Novel luminescence properties have been observed for uncapped and pHEMA capped ZnS nanoparticles. The optimal capping concentration was successfully determined and its influence on photoluminescence behavior has been thoroughly analyzed.

Stoichiometric effect on electrical, optical, and structural properties of composition-tunable In(x)Ga(1-x)As nanowires

Ternary InGaAs nanowires have recently attracted extensive attention due to their superior electron mobility as well as the ability to tune the band gap for technological applications ranging from high-performance electronics to high-efficiency photovoltaics. However, due to the difficulties in synthesis, there are still considerable challenges to assess the correlation among electrical, optical, and structural properties of this material system across the entire range of compositions. Here, utilizing a simple two-step growth method, we demonstrate the successful synthesis of composition and band gap tunable In(x)Ga(1-x)As alloy nanowires (average diameter = 25-30 nm) by manipulating the source powder mixture ratio and growth parameters. The lattice constants of each NW composition have been well correlated with the chemical stoichiometry and confirmed by high-resolution transmission electron microscopy and X-ray diffraction. Importantly, the as-grown NWs exhibit well-controlled surface morphology and low defect concentration without any phase segregation in all stoichiometric compositions. Moreover, it is found that the electrical nanowire device performances such as the turn-off and I(ON)/I(OFF) ratios are improved when the In concentration decreases at a cost of mobility degradation. More generally, this work suggests that a careful stoichiometric design is required for achieving optimal nanowire device performances.

Controlling the Particle Size of Quantum Dots Incorporated in Hybrid Materials

Semiconductor PbS quantum dot-doped Ormocers were successfully prepared by the sol-gel technique. Ormocers preparation was based on the use of trifunctional silane precursors at the solution stage. Formation of PbS particles took place in the pores of the Ormocers through lead precursor reaction with H2S gas. It was observed that temperature was an important factor in the reaction leading to the first appearance of PbS particles. The dot size of PbS was controlled through chemical interaction with the non-hydrolyzed groups of the trifunctional silane precursors. These groups prevent uncontrolled nucleation and aggregation processes during the particle formation and growth. The control of particle size was studied at different conditions for nucleation and aggregation. Determination of the average particle size was done by XR-Diffraction. Optical absorption spectra were also measured at the UV-VIS wavelength range. Absorption edge blue shifts show the quantum confinement effect in these materials.

Effect of PVA surrounding medium on ZnSe nanoparticles: size, optical, and electrical properties

Polyvinyl alcohol (PVA) matrix was used to confine the particle size of ZnSe nanocrystallites as well as the variation of zinc (Zn) to selenium (Se) ion ratio which showed a remarkable decrease on the particle size as this ratio increased. The particle size decrease was monitored from the UV-vis absorption measurement as well as photoluminescence which suffered a blue shift with particle size decrease. The particle size was characterized with the aid of X-ray diffraction (XRD). The Raman spectra showed that, as the particle size increases, the peak position of the line centers (LO) mode were found to be red shifted from 239 to 234 cm(-1), accompanied by an increase in the full-width at half-maximum (FWHM). The electrical measurements and FT-IR spectra (overtone and normal) band vibration were used to study the effect of ZnSe NPs size on the PVA matrix.