Seeded growth of AlN on N- and Al-polar 〈0001〉 AlN seeds by physical vapor transport

We demonstrated seeded growth of AlN on large-area Al- and N-polar <0 0 0 1>-oriented AlN seeds using the physical vapor transport method (PVT). In both cases, crystals having a diameter of 15 mm were obtained from 5 mm seeds. Based on growth step and terrace width analyses, it was found that the N-polar face was suitable for growth within a large window of growth parameters while the Al-polar seeds yielded high-quality crystals only at low supersaturation.     

Interaction of –CClx (x=1–3) with Ru2 and RuSn dimers: a density functional study

Density functional calculation were performed on the Ru₂ and RuSn metal dimers and the species formed from their interaction with –CCl_x (x=1–3) fragments. The importance of these fragments in the hydrodechlorination of carbon tetrachloride has been motivated this study aiming to contribute to understand the effect of the tin in the performance of the noble metals based catalysts. We have observed that the carbon tetrachloride does not form a precursor with the metal dimers. The CCl₄ readily dissociates forming adsorbed chlorine and –CCl₃ complexes. The chlorine atoms prefer to adsorb on the bridge sites and the RuSn–Cl binding energy is about 5 kcal mol⁻¹ larger than the Ru₂–Cl binding energy. The Ru₂–CCl_x (x=1–3) binding energies are larger than the respective RuSn–CCl_x (x=1–3) binding energies. However, the reaction energy of the dechlorination of CCl₄ leading to adsorbed –CCl_x (x=1–3) and chlorine is thermodynamically more favorable for RuSn than the Ru₂. The differences between Ru₂ and RuSn systems have been discussed based on the different interaction mechanism due to the presence of the tin center and its affinity for the chlorine atoms.     

Spectroscopic study of molecular associations between flavins FAD and RFN and some indole derivatives

The formation of complexes between two flavins [flavine adenine dinucleotide (FAD) and riboflavin (RFN)] and some indole derivatives has been studied in aqueous solution. The molecular associations have been examined by means of electronic absorption spectra, since in each, a new charge-transfer like band has been located, and also by observing the variation of the fluorescence emission of FAD or RFN on the solutions. The formation constants for the molecular complexes were determined from data of absorption using the Foster—Hammick—Wardley method. The quenching fluorescence phenomena observed for the FAD and RFN were related to the concentration of the indole derivatives and the corresponding quenching constants have been determined. Thermodynamic parameters have been derived from the values of association constants for the molecular complexes at several temperatures. Clear evidence is found for the influence on the stability of these complexes of the different substituent groups in the indole derivatives and the molecular structure of both flavins.     

Exciton linewidth due to scattering from free carriers in semiconducting quantum well structures

The contribution to the exciton linewidth in semiconducting quantum well structures due to the scattering of excitons by free carriers is calculated. It is found that this contribution becomes very important in limiting the exciton linewidth when a high density of free carriers is present or at low temperatures where the scattering of the excitons by optical and acoustic phonons is reduced. This contribution to the linewidth in quantum well structures is found to increase with the free carrier concentration and to extremely broaden and exciton peak at high carrier concentrations. At lower carrier concentrations, where the carriers behave as a nondegenerate gas of particles, the contribution to the exciton linewidth due to scattering by free carriers increases with temperature.     

Fe3O4/PEI/Au@CdSe/CdS多功能复合材料的制备与表征

以共沉淀法制备出Fe₃O₄纳米粒子,通过聚乙烯亚胺(PEI)修饰Fe₃O₄纳米粒子,再原位复合上Au纳米粒子,制得Fe₃O₄/PEI/Au纳米颗粒微球。再将Fe₃O₄/PEI/Au纳米颗粒与巯基乙酸修饰的量子点CdSe/CdS连接,成功制备了Fe₃O₄/PEI/Au@CdSe/CdS多功能复合微球。经过傅里叶变换红外光谱仪(FTIR)、荧光分光光度计、荧光显微镜、X射线衍射(XRD)、透射电子显微镜(TEM)及振动样品磁强计(VSM)的表征。结果表明:多功能复合微球的粒径在40 nm左右,具有超顺磁性,剩磁,矫顽力近似等于零,饱和磁化强度为28.83 A·m²·kg^-1,同时兼有优越的荧光性能和金纳米粒子的特性。     

分散液液微萃取-气相色谱-质谱法测定水样中4种嗅味物质

以四氯乙烯作萃取剂,以丙酮为分散剂对水样中4种嗅味物质,二甲基异莰醇、土臭素、β-环柠檬醛和β-紫罗兰酮进行分散液液微萃取,提取液供气相色谱-质谱仪分析。在选择离子监测模式下,4种嗅味物质的线性范围均为0.05～20μg.L-1。二甲基异莰醇、土臭素、β-环柠檬醛和β-紫罗兰酮的检出限(3S/N)分别为0.03,0.01,0.02,0.01μg.L-1。方法用于自来水和河水样品分析,4种嗅味物质的回收率在87.7%～102%之间,测定值的相对标准偏差(n=6)在1.8%～7.8%之间。     

The Shackling Effect in Cyclic Azobenzene Liquid Crystal

We demonstrate here a novel method for the design of liquid crystals (LCs) via the cyclization of mesogens by flexible chains. For two azobenzene-4,4′-dicarboxylate derivatives, the cyclic dimer, cyclic bis(tetraethylene glycol azobenzene-4,4′-dicarboxylate) (CBTAD), shows LC properties with smectic A phase, while its linear counterpart, bis(2-(2′-hydroxyethyloxy)ethyl azobenzene-4,4′-dicarboxylate (BHAD), has no LC phase. The difference is ascribed to the shackling effect from the cyclic topology, which leads to the much smaller entropy change during phase transitions and increases the isotropic temperature greatly for cyclics. In addition, the trans-to-cis isomerization of azobenzene groups under UV-light is also limited in CBTAD. With the reversible isomerization of azobenzene groups, CBTAD showed interesting isothermal phase transition behaviors, where the LC phase disappeared upon photoirradiation of 365 nm UV-light, and recovered when the UV-light was off. Combined with the smectic LC nature, a novel UV-light tuned visible light regulator was designed, by simply placing CBTAD in two glass plates. The scattered phase of smectic LC was utilized as the “OFF” state for light passage, while the UV-light induced isotropic phase was utilized as the “ON” state. The shackling effect outlined here should be applicable for the design of cyclic LC oligomers/polymers with special properties.

     

CeO2 quantum dots doped Ni-Co hydroxide nanosheets for ultrahigh energy density asymmetric supercapacitors

By integrating the merits of lanthanide elements and quantum dots, we firstly design CeO₂ quantum dots doped Ni-Co hydroxide nanosheet via a controllable synthetic strategy, which exhibits a large specific capacitance (1370.7 F/g at 1.0 A/g) and a good cyclic stability (90.6% retention after 4000 cycles). Moreover, we assemble an aqueous asymmetric supercapacitor with the obtained material, which has an extremely high energy density (108.9 Wh/kg at 378 W/kg) and outstanding cycle stability (retaining 88.1% capacitance at 2.0 A/g after 4000 cycles).     

How probable is inflation?

The Henneaux-Gibbons-Hawking-Stewart canonical measure on the set of classical universes is applied to a Friedmann-Robertson-Walker model containing a massive scalar field. Although a uniform probability distribution in this measure would solve the flatness problem, it gives an ambiguous probability for inflation, since both the set of inflationary solutions and the set of noninflationary solutions have infinite measure.