固态van der Waals C60膜晶格的Coulomb膨胀

利用扫描隧道显微镜观测到生长在GaAs(001)2×4表面上的固态van der Waals C60膜在室温下较之C60晶体具有13%的晶格膨胀.这种膨胀是由从GaAs衬底饱和的As悬挂键上转移到C60分子上的电荷之间的Coulomb作用引起的.理论计算表明,平均约有1.76个电子转移到每个C60分子上.有趣的是,该固态C60膜为(110)取向,这明显不同于生长于其他半导体或金属表面上的具有(111)取向的六角密排C60膜.这种反常取向的薄膜是由GaAs衬底的各向异性产生的一维限制作用导致的.通过选择不同的衬底,可以控制C60薄膜的晶体生长.     

Scattering and Focusing Effect of Stepped Index Luneberg Lens

Focusing effect of the Luneberg lens for wide bandwidth is studied with emphasis on experiment. A model experiment was performed using a stepped index (six-layer) spherical lens. The total electric field amplitude along the propagation axis has the maximum value (focal point) on the surface of the lens. As the size parameter ka (k: wave number, a: radius of the sphere) is larger, the electric field amplitude at the focal point becomes larger. These results correspond with the power of convergence (focusing effect) by the Luneberg lens. The experimental values are in a good agreement with theoretical ones that are calculated using homogeneous multilayered approximate method. They are also in a good agreement with the results using exact solutions for the inhomogeneous dielectric sphere (the ideal Luneberg lens). Therefore, it is found that the six-layer spherical lens has the optimum variation of dielectric constants for the Luneberg lens and maintains focusing effect for wide bandwidth.     

Atomic structures of two-dimensional strained InAs epitaxial layers on a GaAs(001) surface: in situ observation of quantum dot growth

Scanning tunneling microscopy and reflection high-energy electron diffraction under ultrahigh vacuum conditions were used to make an in situ study of atomic structures at the surface of an InAs/GaAs heterostructure grown by molecular-beam epitaxy. It was observed that the deposition of approximately 0.3 ML of indium on an arsenic-enriched GaAs(001)-2 × 4 surface leads to the formation of the 4 × 2 phase while the deposition of 0.6 ML indium leads to the appearance of a new 6 × 2 reconstruction. It is shown that layer-by-layer two-dimensional epitaxial growth of InAs on GaAs(001) as far as 13 monolayers can only be achieved if the growth front reproduces the 4 × 2 or 6 × 2 symmetry of the substrate and models of 4 × 2 and 6 × 2 reconstructions are proposed. Atomic-resolution images of faceted planes on the surface of three-dimensional islands in an InAs/GaAs(001) system were obtained for the first time and structural models of these were developed.     

Synthesis and Characterization of the Dilithium Salt of the Octasilyl[4]radialene Dianion: Evidence for a Lithium Walk on the Eight-Center,Ten-Electron π-Electron System

A highly symmetric structure is displayed in toluene by 1 , which was obtained by reduction of octasilyl[4]radialene with lithium in THF. The two Li⁺ ions of 1 are not fixed to the π-electron system in toluene, but are fluxional, giving rise to a bis-contact ion pair. In a solvating medium such as THF, one of the Li⁺ ions dissociates in such a way that 1 exists as a contact ion pair and a solvent-separated ion pair.     

GROWTH MODE AND SURFACE RECONSTRUCTION OF GaN(0001) THIN FILMS ON 6H-SiC(0001)

Two-dimensional growth of GaN thin films on an atomically flat C-face 6H-SiC(0001) surface prepared by ultra-high vacuum Si-etching is observed when using an AlN buffer layer in N plasma-assisted molecular beam epitaxy. Scanning tunneling microscopy and reflection high energy electron diffraction observations reveal a series of Ga-stabilized reconstructions which are consistent with those reported for an N-polar GaN(0001) film. The result, including the effect observed previously for GaN thin film on Si-terminated 6H-SiC(0001), agrees with the polarity assignment of heteroepitaxial wurtzite GaN films on polar 6H-SiC substrates, i.e., GaN film grown on SiC(0001) is <0001> oriented (N-face) while that on SiC(0001) is <0001> oriented (Ga-face).     

Nitroxide‐Mediated Controlled/Living Free Radical Copolymerization of Styrene and Divinylbenzene in Aqueous Miniemulsion

Summary: The nitroxide‐mediated controlled/living free radical copolymerization of styrene and divinylbenzene using a polystyrene‐TEMPO macroinitiator in aqueous miniemulsion and in bulk have been investigated. The crosslink densities were estimated based on the content of pendant vinyl groups as determined by ¹H NMR. Considerably lower crosslink densities were revealed in the miniemulsion than in the corresponding bulk system. The rate of polymerization in the miniemulsion increased with decreasing particle size, and was significantly higher than in bulk.

Crosslink density for the TEMPO‐mediated free radical copolymerization of S(1) and DVB(2) (f = 0.99, f = 0.01) at 125 °C in bulk (□) and in miniemulsions with d_n = 585 nm (○) and 53.3 nm (•).     

Microphase‐separated structure of 1,3‐cyclohexadiene/butadiene triblock copolymers and its effect on mechanical and thermal properties

We report the effect of microphase‐separated structure on the mechanical and thermal properties of several poly(1,3‐cyclohexadiene‐block‐butadiene‐block‐1,3‐cyclohexadiene) triblock copolymers (PCHD‐block‐PBd‐block‐PCHD) and of their hydrogenated derivatives: poly(cyclohexene‐block‐ethylene/butylene‐block‐cyclohexene) triblock copolymers (PCHE‐block‐PEB‐block‐PCHE). Both mechanical strength and heat‐resistant temperature (ex. Vicat Softening Temperature: VSPT) tended to increase with an increase in the 1,3‐cyclohexadiene (CHD)/butadiene ratio. On the other hand, heat resistance of the hydrogenated block copolymer was found to be higher than that of the unhydrogenated block copolymer. However, the mechanical strength was lower than those of the unhydrogenated block copolymer with the same ratio of CHD to butadiene. To clarify the relationship between the higher order structures of those block copolymers and their properties, we observed the microphase‐separated structure by transmission electron microscope (TEM). Hydrogenated block copolymers were found to have more finely dispersed microphase‐separated structures than those of the unhydrogenated block copolymers with the same CHD/Bd ratios through the use of TEM and the small‐angle X‐ray scattering (SAXS) technique. Those results indicated that the segregation strength between the PCHE block sequence and the PEB block sequence increased, depending on hydrogenation of the unhydrogenated precursor. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 39: 13–22, 2001