Self-Assembled Monolayers of BPS Control the Molecular Aggregation of Polymer-Dispersed Liquid-Crystal Films of 5CB
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exposed to 0.1 Å/s Si flux for 30 s. The final structure consisted of a 2-µm-thick Si0.89Ge0.10C0.01/Si superlattice (100 periods with each sublayer 10 nm in thickness) grown at 500°C and lattice matched to the Si substrate. This structure was doped with Sb to approximately 2 × 1019 cm-3, and the sample was capped with a 100-nm Si layer. The cross-plane thermal conductivity of the superlattice was found to be 0.085 W/(cmK), or over one order lower than that of Si (1.5 W/[cmK]). The researchers grew SiGeC/Si superlattice microcoolers with dimensions ranging from 40 × 40 µm2 to 100 × 100 µm2. The coolers were tested on a constant-temperature heatsink by measuring the device cooling as a function of electrical current. Maximum cooling temperatures of 2.8 K and 6.9 K were obtained at heatsink temperatures of 25°C and 100°C, respectively, indicating better microcooler performance at higher temperatures. The maximum cooling temperature was also found to increase with decreasing device size. These cooling temperature data correspond to maximum cooling power densities of the order of 1000 W/cm2. The SiGeC/Si microcoolers were compared with Si microcoolers, and a threefold improvement in maximum cooling was observed for the SiGeC/Si coolers. The addition of carbon to SiGe enables the design of a SiGeC layer which is lattice matched to silicon. This allows the SiGeC layer to be grown directly on a Si substrate without strain which reduces the cost of material growth and simplifies integration of the cooler with Si-based devices. Furthermore, the addition of C also enables the use of thermionic emission to enhance TE cooling for SiGeC/Si materials due to the large conduction band offset between SiGeC and Si. According to Fan, “Solid-state coolers that are monolithically integrated with microelectronic and optoelectronic devices are an attractive way to achieve compact and efficient cooling. Our results indicate that efficient refrigeration is possible with coolers made of conventional semiconductor materials.” STEFFEN K. KALDOR
Self-Assembled Monolayers of BPS Control the Molecular Aggregation of Polymer-Dispersed Liquid-Crystal Films of 5CB S. Kato and C. Pac of the Kawamura Institute of Chemical Research have controlled the molecular aggregation of polymer-dispersed liquid-crystal (PDLC) films by varying the hydroxyl densities of selfassembled monolayers (SAMs) of BPS, a 276
boronate-terminated silane compound, on the substrates. Previously the researchers demonstrated that in PDLC films of 4cyano-4’-pentylbiphenyl (5CB), a typical nematic LC, the LC molecules and the polymers form an interface layer at the substrate with different molecular alignment than the interior. In the current work, fluorescence analysis shows that the presence of the SAM on the substrate affects the properties of the interface layer and allows control of the molecular alignment. As reported in the March issue of Chemistry of Materials, PDLC films were made by photopolymerization-induced phase separation (PIPS) of a mixture of 30 wt% 5CB and diacr
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