Microencapsulation of TiO2 Nanoparticles with Polymer by Rapid Expansion of Supercritical Solution

A novel preparation method is reported for the microencapsulation of TiO₂ nanoparticles by rapid expansion of supercritical solution with a nonsolvent. A suspension of TiO₂ nanoparticles in carbon dioxide containing a cosolvent and dissolved polymer is sprayed through a nozzle to atmospheric pressure. After rapid expansion, polymer microspheres were obtained. The microspheres do not tend to agglomerate, since the pure cosolvent is a nonsolvent for the polymer. The structure and morphology of microspheres were investigated by SEM, TEM and XRD. The obtained polymer microspheres are microcapsules of TiO₂ nanoparticles. The mean particle diameter and particle size distribution of microcapsules, could be controlled by changing the polymer concentration, pre-expansion pressure, temperature and injection distance. The polymer feed compositions are more effective than other factors on the control of particle size.     

Quantum statistical hierarchy equation in nonequilibrium systems

An extension is given for the Fourier expansion method with the contraction technique, which was introduced by Balescu for quantum statistical systems. This is attained by introducing a diagrammatic method with a concept of moving contraction. Then the hierarchy equation for the Contracted Fourier coefficient of the Wigner distribution function is obtained. As an application, a generalized master equation involvingn-body collision effects and quantum statistical effects is also derived.     

Enols of 2‐nitro‐ and related 2‐substituted malonamides

The structures of 2‐substituted malonamides, YCH(CONR¹R²)CONR³R⁴ (Y = Br, SO₂Me, CONH₂, COMe, and NO₂) were investigated. When Y = Br, R¹R² = R³R⁴ = HEt; Y = SO₂Me, R¹–R⁴ = H and for Y = CONH₂ or CONHPh, R¹–R⁴ = Me, the structure in solution is that of the amide tautomer. X‐ray crystallography shows solid‐state amide structures for Y = SO₂Me or CONH₂, R¹–R⁴ = H. Nitromalonamide displays an enol structure in the solid state with a strong hydrogen bond (O^…O distance = 2.3730 Å at 100 K) and d(OH) ≠ d(O^…H). An apparently symmetric enol was observed in solution, even in appreciable percentages in highly polar solvents such as DMSO‐d₆, but K_enol values decrease on increasing the solvent polarity. The N,N′‐dimethyl derivative is less enolic. Acetylmalonamides display a mixture of enol on the acetyl group and amide in non‐polar solvents, and only the amide in DMSO‐d₆. DFT calculations gave the following order of pK_enol values for Y: H > CONH₂ > COMe ≥ COMe (on acetyl) ≥ MeSO₂ > CN > NO₂ in the gas phase, CHCl₃, and DMSO. The enol on the C?O group is preferred to the aci‐nitro compound, and the N? O? H^…O?C is less favored than the C?O? H^…O?C hydrogen bond. Copyright © 2009 John Wiley & Sons, Ltd.     

Structural phase transitions at clean and metal-covered Si(111) surfaces investigated by RHEED spot analysis

Structural phase transitions between various kinds of superlattice structures formed on a Si(111) surface have been investigated by spot analysis of reflection high-energy electron diffraction (RHEED). Reversible transitions induced by temperature changes and irreversible ones induced by metal depositions were observed. Detailed discussions on the dynamics of the phase transitions are made by quantitative analyses of integrated spot intensity and profile. For a phase transition of 7′7  1′1 structures on a clean Si(111) surface, a hysteresis with temperature difference of 5°C. between in heating and cooling processes was found in the spot intensity change, indicating a first-order transition. Hysteresis was hardly recognized, on the other hand, for transitions of Au-induced superstructures (5×2-Au or ×-Au)  1×1-Au. The spot profiles were found to be broadened during the transition of Si(111)-×-Au  1×1-Au, which was a signature of a continuous transition, while the profiles remained unchanged during the transitions of the 7×7  1×1 and 5×2-Au  1×1-Au phases. Structural conversions induced by In adsorption on the Si(111) surface kept at constant temperatures were also analyzed. The conversions at room temperature were totally dependent on the initial substrate surface structures; the 7×7 surface did not show any structural conversion with In adsorption, while the ×-In surface successively converted to a 2×2 and a × phase with coverage increase. The structural transitions at elevated temperatures were sensitively dependent on the temperatures. Sequences of transitions among the 7×7, 4×1, ×, 1×, and ×4 were quantitatively revealed as changes in RHEED spot intensity.     

Coherent transfer of light polarization to electron spins in a semiconductor

We demonstrate that the superposition of light polarization states is coherently transferred to electron spins in a semiconductor quantum well. By using time-resolved Kerr rotation, we observe the initial phase of Larmor precession of electron spins whose coherence is transferred from light. To break the electron-hole spin entanglement, we utilized the big discrepancy between the transverse g factors of electrons and light-holes. The result encourages us to make a quantum media converter between flying photon qubits and stationary electron-spin qubits in semiconductors.     

Morphological fluctuations in discharge-produced μc-Si:H studied by small angle X-ray scattering

Structural inhomogeneities of μc-Si:H in the range 20≈500 A are observed for the first time by small angle X-ray scattering. Inhomogeneous distributions of bonded hydrogen in the amorphous tissue are suggested to be a possible origin of the SAXS yield. Such inhomogeneities can be reduced when the average grain size and volume fraction of microcrystallites are simultaneously increased.     

Stacking of multilayer InAs quantum dots with combination capping of InAlGaAs and high temperature grown GaAs

We are reporting the growth of multilayer stacks of quantum dots (10 periods) with a combination capping of In_0.21Al_0.21Ga_0.58As (30 Å) and GaAs (70–180 Å) grown by solid source molecular beam epitaxy (MBE). Reflection high energy electron diffraction (RHEED) has been used for the insitu monitoring of quantum dot (QD) formation in heterostructure samples. The samples were also characterized by other exsitu techniques like cross sectional transmission electron microscopy (XTEM) and photoluminescence measurements (PL). For a heterostructure sample with thin barrier thickness (<100 Å), an XTEM image showed the stacking of QDs only up to the 5th layer and in the upper layers there was hardly any formation of dots. We presume the stoppage of dot formation is due to the uneven surface of the InAlGaAs alloy overgrown on the InAs QDs, as a result of the local compositional deviations of the Group-III atoms. Samples grown with thicker barriers (>100 Å of GaAs) showed good stacking of islands until the tenth layer. The thick GaAs layer overgrown on the InAlGaAs at 590 ^°C is believed to remove the surface modifications of the quaternary layer thereby creating a smoother surface front for the growth of subsequent QD layers.