Doping Effects in CMOS-compatible CoSi Thin Films for Thermoelectric and Sensor Applications

Abstract . We report on semi-metallic cobalt monosilicide (CoSi) as a CMOS-compatible thermoelectric (TE) material and discuss the effect of n- and p-type dopants on its transport properties. Thin films of CoSi are developed using chemical vapor deposition tools and subsequent rapid thermal processing. Film properties such as microstructure, crystallinity and elemental distribution are studied via electron microscopy, X-ray diffraction and time-of-flight secondary ion mass spectroscopy. Doping silicon with boron prior to silicidation impedes the Co-Si diffusion process, while phosphorus atoms distribute uniformly in silicides with no voids or agglomerations. CoSi makes a suitable n-type TE candidate and provides an alternative to Si or SiGe materials. Transport properties of undoped CoSi exhibit a linear dependence within the investigated temperature window, whereas dopants in CoSi increase the number of electron carriers that contribute to charge transport and thereby influence the Seebeck coefficient. Thus, TE characteristics of thin CoSi films can be tuned via (i) the type of dopants used and/or (ii) varying the residual silicon thickness post silicidation.     

Inverse Vulcanization of Styrylethyltrimethoxysilane–Coated Surfaces,Particles, and Crosslinked Materials

Sulfur as a side product of natural gas and oil refining is an underused resource. Converting landfilled sulfur waste into materials merges the ecological imperative of resource efficiency with economic considerations. A strategy to convert sulfur into polymeric materials is the inverse vulcanization reaction of sulfur with alkenes. However, the materials formed are of limited applicability, because they need to be cured at high temperatures (>130 °C) for many hours. Herein, we report the reaction of elemental sulfur with styrylethyltrimethoxysilane. Marrying the inverse vulcanization and silane chemistry yielded high sulfur content polysilanes, which could be cured via room temperature polycondensation to obtain coated surfaces, particles, and crosslinked materials. The polycondensation was triggered by hydrolysis of poly(sulfur-r-styrylethyltrimethoxysilane) (poly(S_n-r-StyTMS) under mild conditions (HCl, pH 4). For the first time, an inverse vulcanization polymer could be conveniently coated and mildly cured via post-polycondensation. Silica microparticles coated with the high sulfur content polymer could improve their Hg²⁺ ion remediation capability.     

O2‐Mediated Oxidation of Aminoboranes through 1,2‐N Migration

In analogy to the classical reaction of C?B bonds with peroxides, the first oxidative functionalization of aminoboranes through a 1,2‐N migration was realized. Readily available aliphatic nitro compounds are thereby transformed into N‐ and O‐functionalized hydroxylamines in a single synthetic operation. Addition of hazardous peroxides is avoided. Instead, the insertion of O₂, as the terminal oxidant, into Zn?C bonds provides the necessary peroxides. The required zinc organyls, in turn, are formed through a boron‐to‐zinc exchange, from an organoboronic ester byproduct of the nitro‐to‐aminoborane transformation.     

Vertex pancyclic graphs

Let G be a graph of order n. A graph G is called pancyclic if it contains a cycle of length k for every 3kn, and it is called vertex pancyclic if every vertex is contained in a cycle of length k for every 3kn. In this paper, we shall present different sufficient conditions for graphs to be vertex pancyclic.     

Stereoselective Synthesis and Reactions of Secondary Alkyllithium Reagents Functionalized at the 3‐Position

Secondary alkyllithium reagents bearing an OTBS group (TBS=tert‐butyldimethylsilyl) at the 3‐position can be prepared stereoconvergently through an I/Li exchange from a diastereomeric mixture of the corresponding secondary alkyl iodides. These lithium reagents react with a range of electrophiles, including carbon electrophiles, with retention of configuration to yield various 1,3‐difunctionalized derivatives with good diastereoselectivities. Kinetic studies show that the 3‐siloxy group strongly accelerates the epimerization at the lithium‐substituted carbon atom. This method offers a new way to construct chiral open‐chain molecules with excellent stereoselectivity.     

Homolytic N−S Bond Cleavage in Vinyl Triflimides Enabled by Triplet–Triplet Energy Transfer

Vinyl triflimides are a new compound class with unknown reactivity. A computational analysis identified homolytic cleavage of the N−Tf bond induced by triplet–triplet energy transfer (EnT) as a highly interesting reaction type that might be accessible. A combination of experimental and mechanistic work verified this hypothesis and proved the generated radicals to be amenable to radical–radical coupling. Thereby, vinyl triflimides were transformed into a range of α-quaternary, β-trifluoromethylated amines in a 1,2-difunctionalization reaction with no need for external CF₃ reagents.     

Crystalline morphologies in segmented copolymers with hard segments of uniform length

The crystal morphological properties of segmented poly(ether ester aramide) elastomers with aromatic hard‐segment amide units of uniform length were studied. Four samples with hard‐segment fractions ranging from 3.4 to 9 wt % were studied by tapping atomic force microscopy (AFM). For one sample, both solution and melt‐processed surfaces were examined, and similar crystal morphologies were found. The semicrystalline morphologies of these polymers had some similarities to other low‐hard‐segment segmented elastomers. The very thin needlelike or ribbonlike crystallites at the surface had a high aspect ratio for all the samples. The main difference observed for the different compositions was a decrease in the surface area density of ribbons with a decrease in the hard‐segment fraction. One sample was chosen for in situ AFM studies during film extension. The details of the crystallite orientation and breakup were studied in increments up to 700% elongation (8× stretch ratio) and after relaxation. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 1783–1792, 2004