Lian-Pin Hwang

Dynamic effects of pair correlation functions on spin relaxation by translational diffusion in liquids

It is shown how the equilibrium pair correlation function between spin‐bearing molecules in liquids may be incorporated as an effective force in the relative diffusion expressions, and how one may solve for the resulting time correlation functions and spectral densities needed for studies of spin relaxation by translational diffusion. The use of finite difference methods permits the solution no matter how complex the form of the pair correlation function (pcf) utilized. In particular, a Percus–Yevick pcf as well as one corrected from computer dynamics, both for hard spheres, are utilized. Good agreement with the experiments of Harmon and Muller on dipolar relaxation in liquid ethane is obtained from this analysis. Effects of ionic interactions in electrolyte solutions upon dipolar relaxation are also obtained in terms of Debye–Huckel theory for the pcf. Analytic solutions are given which are appropriate for the proper boundary‐value problem for the relative diffusion of molecules (i.e., a distance of mini...

Generalized Einstein relations for rotational and translational diffusion of molecules including spin

In this work the validity of generalized Einstein relations, D (ω) = kT/Iβ (ω), where D (ω) and β (ω) are frequency‐dependent diffusion and damping coefficients, is examined from a general point of view. It is shown that generalized Smoluchowski (S) equations for both translational and rotational diffusion involving the D (ω), which are defined in terms of simple correlation functions of the fluctuating forces and torques, follow from the appropriate generalized Fokker–Planck (FP) expressions when the translational and angular momenta are taken as rapidly relaxing and a coarse graining in time is introduced. It is found useful in this context to distinguish between those forces and torques fluctuating at rates faster than the diffusive‐type motions of the B particle versus those fluctuating more slowly. The appropriate FP equation for rotational diffusion is also derived. Generalized FP and S equations are also derived for the semiclassical case where the molecule(s) examined contains spin degrees of free...

A novel approach to bulk synthesis of carbon nanotubes filled with metal by a catalytic chemical vapor deposition method

Abstract A novel microporous aluminophosphate (AlPO-5) support is reported for the first time as an alternative to the Na–Y zeolite support for large-scale production of carbon nanotubes using a cobalt metal catalyst which makes the separation and purification process of nanotubes easier. The use of an aluminophosphate support results in filling of the nanotubes with metal particles and metal nanowires while zeolite (Na–Y) support yields metal free nanotubes. This difference has been attributed to the difference in the mechanism of nanotube growth from electroneutral aluminophosphate and highly charged zeolite supports due to difference in the metal–support interaction in two cases.

Photoluminescence from mesoporous silica: Similarity of properties to porous silicon

Photoluminescence (PL) from mesoporous silica (MS) with the pore size of ∼6 nm and the thickness of walls among pores of ∼1 nm has been studied at room temperature. The heat pretreatment of MS in air at different temperatures and the variation of the excitation wavelengths allow one to shift the PL peak through the whole visible spectral range. The PL is suggested to originate from nonbridging oxygens (red bands), hydrogen-related species (green bands), and water-carbonyl groups (blue bands). The spectroscopic properties of MS are found to be similar to those of surface-oxidized silicon nanocrystals and porous silicon.

Reconstruction of mass spectra of components of unknown mixtures based on factor analysis

Abstract A method is described for the spectral resolution of combined gas chromatographic—mass spectrometric data. Factor analysis is applied to the identification of a second species in a single gas chromatographic peak. A plot can be constructed from the region of the non-negative values of spectral lines in the factor space. Feasible locations for the spectra of these constituents in the plot can be identified and give recognizable spectra of the separated constituents.

Red and near-infrared photoluminescence from silica-based nanoscale materials: Experimental investigation and quantum-chemical modeling

Experimental study of room temperature photoluminescence (PL) from silica nanoparticles and mesoporous silicas induced by ultraviolet and visible laser light (λEXC=266 and 532 nm or 4.66 and 2.33 eV, respectively) reveals several well-defined PL bands in the red- and near-infrared spectral range, which are peaked at 1.905, 1.78, 1.61, 1.40, 1.27, and 1.14 eV. The relative intensities of the bands depend on the specimen heat pretreatment temperature and excitation wavelength. The band at 1.905 eV shows all conceivable characteristics of nonbridging oxygen (NBO) defects in bulk silica, so it can be assigned to the same species in nanometer-sized SiO2 fragments. The more slowly decayed 1.78-eV band was assigned to NBOs incorporated into distorted SiO4 tetrahedrons on the surface. The 1.14–1.61 eV PL bands can be observed mainly with 2.33-eV excitation for heat-pretreated specimens (Tht=873 and 1173 K) and can also be associated with NBOs. The red shift of these bands is explained in terms of formation of com...

Dynamics of supercooled water in various mesopore sizes

Double-quantum-filtered NMR and T(1) inversion-recovery spectroscopy were employed to exploit the temperature-dependent dynamics of D(2)O confined in MCM-41. Samples with three pore sizes of 1.58, 2.03, and 2.34 nm and two D(2)O contents were investigated. The reorientation correlation times of confined D(2)O in variously sized pores exhibit different temperature dependencies. The results reveal that the D(2)O molecules at fast motion site remain mobile below approximately 225 K and a liquid-liquid phase transition occurs around this temperature for all samples studied. This temperature is thought to be unreachable for supercooled D(2)O. Particularly, in 20 wt % D(2)O loaded samples with pore diameters of 1.58 and 2.03 nm, the reorientational correlation times of D(2)O at fast motion site exhibit Arrhenius behavior between 225 and 290 K, while other samples show power law dependency. Thus, a liquid phase of the fragile type in bigger pores changes to the strong type in samples with smaller pores.

Photoluminescence from mesoporous silica akin to that from nanoscale silicon: the nature of light-emitters

Abstract Photoluminescence (PL) from mesoporous silica is evidenced to originate from non-bridging oxygens (NBOs) located inside ∼1 nm pore walls and on the pore internal surfaces. Based on density functional calculations, we showed that the PL band diversity in the range of 1.0–2.1 eV results from both isolated bulk and surface NBOs and those combined with variously placed pore wall oxygen vacancies. The spectroscopic features are found to be similar to those of surface-oxidized silicon nanocrystals and porous silicon. The results obtained are expected to be useful to clarify the nature of light-emitters in silicon nanoscale materials.

Ferromagnetic cluster behaviors and magnetoresistance in Ni-doped LaSrMnO3 systems

We have studied the magnetic and magnetotransport properties of the perovskites La8Sr0.2Mn8Ni0.2O3 and La0.7Sr0.3Mn0.8Ni0.2O3. Ferromagnetic cluster behaviors have been observed. The most interesting feature is that the cluster-glass state exhibits both macroscopic insulator-like transport behavior and largely enhanced magnetoresistance (MR) at low temperature. Such a low-temperature MR is weakly temperature dependent. In particular, a low-temperature magnetoresistance ratio (Δρ/ρ0) as high as 60% has been found in La0.7Sr0.3Mn0.8Ni0.2O3. We attribute the low-temperature magnetotransport to the spin-dependent scattering inside the grains.

Uniform carbon spheres of high purity prepared on kaolin by CCVD

Abstract Carbon spheres with diameters of 400–2000 nm were synthesized using transition metal (TM) supported on kaolin catalysts by CCVD in large quantities. The synthetic reaction was carried out at temperatures higher than 650 °C. The as-synthesized carbon spheres can be purified and separated by the flow of N 2 and C 2 H 2 gases at 200 and 50 sccm, respectively. The carbon spheres are composed of unclosed graphene layers with interlayer distances of 0.33–0.35 nm between the layers. The carbon spheres were found to be thermally stable in air and no endothermic or exothermic phenomena were found in N 2 atmosphere at temperatures lower than 500 °C.