Adsorption-controlled growth of BiVO4 by molecular-beam epitaxy

Single-phase epitaxial films of the monoclinic polymorph of BiVO4 were synthesized by reactive molecular-beam epitaxy under adsorption-controlled conditions. The BiVO4 films were grown on (001) yttria-stabilized cubic zirconia (YSZ) substrates. Four-circle x-ray diffraction, scanning transmission electron microscopy (STEM), and Raman spectroscopy confirm the epitaxial growth of monoclinic BiVO4 with an atomically abrupt interface and orientation relationship (001)BiVO4 parallel to (001)(YSZ) with [100]BiVO4 parallel to [100](YSZ). Spectroscopic ellipsometry, STEM electron energy loss spectroscopy (STEM-EELS), and x-ray absorption spectroscopy indicate that the films have a direct band gap of 2.5 +/- 0.1 eV

Semicrystalline Structure in Hydrogenated Norbornene-Methylnorbornene Statistical Copolymers

Crystallinity in polymers is controlled by a variety of factors, one of which is the ability of the chains to align into the regular structures required for crystallization. Studies have shown that the degree of crystallization and the crystallite thickness are highly affected by any defects along the chain. Quantification of these effects has been undertaken in published reports on some polymer systems by copolymerizing two monomer units which differ by the presence of an extra side group substituent on one of the monomers or a difference in this substituent between the two monomers. Hydrogenated polynorbornene-based copolymers have not yet been studied for similar effects from the introduction of side groups. In addition to a melting point, hydrogenated ring-opened polynorbornene exhibits a thermoreversible crystal-crystal transition, a change from one crystal polymorph to another, at a temperature below the melting point. The exact temperature of this transition is highly sensitive to the tacticity of the polymer. Based on these results, the unstudied area of the effects of copolymerization of norbornene with substituted norbornene comonomers is expected to reveal similar sensitive changes in polymer crystallinity and transition temperature. For this study, the effects of copolymerizing norbornene with a small amount of methylnorbornene have been examined. Compositional and kinetic control of the copolymerization are first proven, and reactivity ratios are given. Thermal analysis of hydrogenated copolymers reveals that the melting point and crystal-crystal transition point are reduced by an amount proportional to the content of the methylnorbornene added for dilute comonomer contents (less than 8 mol % methylnorbornene). An examination of the X-ray diffraction patterns of the copolymers shows decreasing crystallinity and expansion of the unit cell with increasing comonomer content. A comparison of analogous copolymers incorporating hexylnorbornene is used to explore the relationship between substituent and copolymer structure, showing that a portion of the methylnorbornene comonomer is incorporated into the crystalline region

Counit Inclusion in Hydrogenated Polynorbornene Copolymer Crystals

Model crystallizable copolymers of norbornene and two 5-alkylnorbornenes were synthesized to investigate the extent and consequences of defect inclusion into hydrogenated polynorbornene (hPN) crystals. Living ring-opening metathesis polymerization yielded narrow-distribution polymers of targeted molecular weights, with modest down-chain compositional gradients controllable through the polymerization conversion; hydrogenation yielded semicrystalline copolymers. When the comonomer was 5-methylnorbornene (MeN), extensive inclusion of MeN units into the hPN crystal was observed; the copolymers showed substantial crystallinities even above 30 mol % MeN, and the dependence of the melting point <i>T</i>_m on crystal thickness followed that for hPN homopolymer. By contrast, when the comonomer was 5-hexylnorbornene, the more usual case of strong exclusion of the counits from the crystal was observed. hPN shows a transition between two crystal polymorphs below <i>T</i>_m, at a temperature <i>T</i>_cc; comonomer incorporation reduces <i>T</i>_cc more rapidly than it reduces <i>T</i>_m, expanding the region over which the high-temperature rotationally disordered polymorph is stable and providing insight into the dependence of the free energy for the two polymorphs on crystal thickness