Strong-Segregation Theory of Bicontinuous Phases in Block Copolymers

We compute phase diagrams for $A_nB_m$ starblock copolymers in the strong-segregation regime as a function of volume fraction $\phi$, including bicontinuous phases related to minimal surfaces (G, D, and P surfaces) as candidate structures. We present the details of a general method to compute free energies in the strong segregation limit, and demonstrate that the gyroid G phase is the most nearly stable among the bicontinuous phases considered. We explore some effects of conformational asymmetry on the topology of the phase diagram.Comment: 14 pages, latex, 21 figures, to appear in Macromolecule

Interfaces in Diblocks: A Study of Miktoarm Star Copolymers

We study AB$_n$ miktoarm star block copolymers in the strong segregation limit, focussing on the role that the AB interface plays in determining the phase behavior. We develop an extension of the kinked-path approach which allows us to explore the energetic dependence on interfacial shape. We consider a one-parameter family of interfaces to study the columnar to lamellar transition in asymmetric stars. We compare with recent experimental results. We discuss the stability of the A15 lattice of sphere-like micelles in the context of interfacial energy minimization. We corroborate our theory by implementing a numerically exact self-consistent field theory to probe the phase diagram and the shape of the AB interface.Comment: 12 pages, 11 included figure

Initial investigations into the MOS interface of freestanding 3C-SiC layers for device applications

This letter reports on initial investigation results on the material quality and device suitability of a homo-epitaxial 3C-SiC growth process. Atomic force microscopy surface investigations revealed root-mean square surface roughness levels of 163.21 nm, which was shown to be caused by pits (35 μm width and 450 nm depth) with a density of 1.09 × 105 cm−2 which had formed during material growth. On wider scan areas, the formation of these were seen to be caused by step bunching, revealing the need for further epitaxial process improvement. X-ray diffraction showed good average crystalline qualities with a full width of half-maximum of 160 arcseconds for the 3C-SiC (002) being lower than for the 3C-on-Si material (210 arcseconds). The analysis of C–V curves then revealed similar interface-trapped charge levels for freestanding 3C-SiC, 3C-SiC on Si and 4H-SiC, with forming gas post-deposition annealed freestanding 3C-SiC devices showing DIT levels of 3.3 × 1011 cm−2 eV−1 at EC−ET = 0.2 eV. The homo-epitaxially grown 3C-SiC material's suitability for MOS applications could also be confirmed by leakage current measurements

Comparing MRI metrics to quantify white matter microstructural damage in multiple sclerosis

Quantifying white matter damage in vivo is becoming increasingly important for investigating the effects of neuroprotective and repair strategies in multiple sclerosis (MS). While various approaches are available, the relationship between MRI‐based metrics of white matter microstructure in the disease, that is, to what extent the metrics provide complementary versus redundant information, remains largely unexplored. We obtained four microstructural metrics from 123 MS patients: fractional anisotropy (FA), radial diffusivity (RD), myelin water fraction (MWF), and magnetisation transfer ratio (MTR). Coregistration of maps of these four indices allowed quantification of microstructural damage through voxel‐wise damage scores relative to healthy tissue, as assessed in a group of 27 controls. We considered three white matter tissue‐states, which were expected to vary in microstructural damage: normal appearing white matter (NAWM), T2‐weighted hyperintense lesional tissue without T1‐weighted hypointensity (T2L), and T1‐weighted hypointense lesional tissue with corresponding T2‐weighted hyperintensity (T1L). All MRI indices suggested significant damage in all three tissue‐states, the greatest damage being in T1L. The correlations between indices ranged from r = 0.18 to r = 0.87. MWF was most sensitive when differentiating T2L from NAWM, while MTR was most sensitive when differentiating T1L from NAWM and from T2L. Combining the four metrics into one, through a principal component analysis, did not yield a measure more sensitive to damage than any single measure. Our findings suggest that the metrics are (at least partially) correlated with each other, but sensitive to the different aspects of pathology. Leveraging these differences could be beneficial in clinical trials testing the effects of therapeutic interventions

An energy transfer study of homopolymer localization in block copolymers

We have investigated the localization of homopolystyrene (hPS) in its blends with poly(styrene-b-butyl methacrylate) (PS-b-PBMA), at 140 degreesC, measuring the quantum efficiency of direct energy transfer (Phi(ET) of DET) between hPS randomly labeled with phenanthrene moieties and PS-b-PBMA labeled at the junction with anthracene. The molecular weight of hPS was similar to the molecular weight of the PS block of PS-b-PBMA, and the hPS content was kept to a maximum of 26 vol %, to preserve the lamellar morphology of PS-b-PBMA. We have found very low rates of DET, which implies that hPS is almost totally localized between the PS brushes of PS-b-PBMA. In addition to the experimental measurements, we also calculated Phi(ET) values by simulating fluorescence decay curves, based on Vavasour and Whitmore's numerical self-consistent-field (NSCF) formalism. The latter theory confirmed that we were dealing with a "dry brush" situation, but the NSCF results suggested a higher level of penetration of hPS than the experiments. We attribute this discrepancy to uncertainties about inputs to the simulations, particularly the Flory-Huggins chi parameter and the Kuhn length for the PBMA block

The improved reliability performance of post-deposition annealed ALD-SiO2

A systematic capacitance-voltage (C-V) and time-dependent dielectric breakdown (TDDB) study on silicon carbide (SiC) metal-oxide-semiconductor capacitors (MOSCAPs) that use silicon dioxide (SiO2) is shown in this paper. Oxides were formed using atomic layer deposition (ALD), low-pressure chemical vapour deposition (LPCVD) or direct thermal growth in nitrous oxide (N2O) ambient, where both deposited oxides were post-deposition annealed in N2O ambient, too. The electrical characterisation results reveal that the ALD-deposited and N2O-annealed oxides show the best capacitance-voltage (C-V) characteristics, with flatband and hysteresis voltages (VFB) averaging 1.44 V and 0.41 V, respectively. When measuring the leakage current levels at 175°C, the ALD-deposited MOSCAPs’ breakdown electric fields are averaging similar to their counterparts at 9.71 MV/cm. MOSCAPs which utilized ALD-deposited SiO2 also showed 29% and 345% increased average injected charge-to 63% failure (QBD,63%) at 9 MV/cm and 9.6 MV/cm, respectively, when comparing these devices to their direct thermally grown SiO2 counterparts