Mechanistic Insight into the (NHC)copper(I)-Catalyzed Hydrosilylation of Ketones

(NHC)­copper­(I) hydride catalyzed ketone hydrosilylation is an efficient method for the enantioselective synthesis of secondary alcohols. Herein, we represent a computational study of this reaction using density functional theory (DFT) on realistic model systems. This study is supported by kinetic investigations, using in situ FTIR measurements. The calculations validate the previously proposed reaction mechanism and explain the high activity of (OR¹)_<i>x</i>R²_3–<i>x</i>Si–H types of silanes. Experimental evidence in favor of the monomeric (NHC)­CuH form of the catalyst is also given. Combining experimental and theoretical results furthermore highlights a Lewis base activation of the hydrosilane, leading to a modified suggestion for the mechanistic scheme of the catalytic cycle

Low-Temperature Atomic Layer Deposition of Low-Resistivity Copper Thin Films Using Cu(dmap)₂ and Tertiary Butyl Hydrazine

Herein, we describe a process for the low-temperature atomic layer deposition of copper using Cu­(dmap)₂ (dmap = dimethylamino-2-propoxide). The use of tertiary butyl hydrazine (TBH) as the reducing agent was found to have a significant improvement on the purity and the resistivity of the Cu films compared to previous processes. Our process was studied at low temperatures of 80–140 °C on native oxide terminated Si. At 120 °C, self-limiting Cu deposition was demonstrated with respect to both Cu­(dmap)₂ and TBH pulse lengths. During the initial stages of the deposition (125–1000 cycles), a growth rate of 0.17 Å/cycle was measured. Once the substrate surface was completely covered, deposition proceeded with a more moderate growth rate of 0.05 Å/cycle. According to X-ray diffraction, the films were crystalline cubic Cu with a slight preference toward (111) orientation. Based on scanning electron micrographs, the Cu films were relatively smooth with the roughness increasing as a function of both increasing temperature and thickness. A 54 nm film deposited at the low temperature of 120 °C exhibited a low resistivity of 1.9 μΩ·cm. Composition analysis on this film showed a remarkably high purity of approximately 99.4 at.%, with the rest being hydrogen and oxygen. The films could be deposited also on hydrogen terminated Si, glass, Al₂O₃, TiN, and Ru, extending the suitability of the process to a wide range of applications

Au/ε-Fe₂O₃ Nanocomposites as Selective NO₂ Gas Sensors

A combined chemical vapor deposition (CVD)/radio frequency (rf) sputtering approach to Au/Fe₂O₃ nanocomposites based on the scarcely investigated ε-iron­(III) oxide polymorph is reported. The developed materials, analyzed by field emission-scanning electron microscopy (FE-SEM), energy dispersive X-ray spectroscopy (EDXS), X-ray photoelectron spectroscopy (XPS), and secondary ion mass spectrometry (SIMS), consisted of iron oxide nanorods decorated by gold nanoparticles (NPs), whose content and distribution could be tailored as a function of sputtering time. Interestingly, the intimate Au/ε-Fe₂O₃ interfacial contact along with iron oxide one-dimensional (1D) morphology resulted in promising performances for the selective detection of gaseous NO₂ at moderate working temperatures. At variance with the other iron­(III) oxide polymorphs (α-, β-, and γ-Fe₂O₃), that display an <i>n</i>-type semiconducting behavior, ε-Fe₂O₃ exhibited a <i>p</i>-type response, clearly enhanced by Au introduction. As a whole, the obtained results indicate that the sensitization of <i>p</i>-type materials with metal NPs could be a valuable tool for the fabrication of advanced sensing devices

Integrating AlN with GdN Thin Films in an in Situ CVD Process: Influence on the Oxidation and Crystallinity of GdN

The application potential of rare earth nitride (REN) materials has been limited due to their high sensitivity to air and moisture leading to facile oxidation upon exposure to ambient conditions. For the growth of device quality films, physical vapor deposition methods, such as molecular beam epitaxy, have been established in the past. In this regard, aluminum nitride (AlN) has been employed as a capping layer to protect the functional gadolinium nitride (GdN) from interaction with the atmosphere. In addition, an AlN buffer was employed between a silicon substrate and GdN serving as a seeding layer for epitaxial growth. In pursuit to grow high-quality GdN thin films by chemical vapor deposition (CVD), this successful concept is transferred to an in situ CVD process. Thereby, AlN thin films are included step-wise in the stack starting with Si/GdN/AlN structures to realize long-term stability of the oxophilic GdN layer. As a second strategy, a Si/AlN/GdN/AlN stacked structure was grown, where the additional buffer layer serves as the seeding layer to promote crystalline GdN growth. In addition, chemical interaction between GdN and the Si substrate can be prevented by spatial segregation. The stacked structures grown for the first time with a continuous CVD process were subjected to a detailed investigation in terms of structure, morphology, and composition, revealing an improved GdN purity with respect to earlier grown CVD thin films. Employing thin AlN buffer layers, the crystallinity of the GdN films on Si(100) could additionally be significantly enhanced. Finally, the magnetic properties of the fabricated stacks were evaluated by performing superconducting quantum interference device measurements, both of the as-deposited films and after exposure to ambient conditions, suggesting superparamagnetism of ferromagnetic GdN grains. The consistency of the magnetic properties precludes oxidation of the REN material due to the amorphous AlN capping layer

Photoactive Zinc Ferrites Fabricated via Conventional CVD Approach

Owing to its narrow band gap and promising magnetic and photocatalytic properties, thin films of zinc ferrite (ZFO, ZnFe₂O₄) are appealing for fabrication of devices in magnetic recording media and photoelectrochemical cells. Herein we report for the first time the fabrication of photactive zinc ferrites via a solvent free, conventional CVD approach, and the resulting ZFO layers show promise as a photocatalyst in PEC water-splitting. For large scale applications, chemical vapor deposition (CVD) routes are appealing for thin film deposition; however, very little is known about ZFO synthesis following CVD processes. The challenge in precisely controlling the composition for multicomponent material systems, such as ZFO, via conventional thermal CVD is an issue that is caused mainly by the mismatch in thermal properties of the precursors. The approach of using two different classes of precursors for zinc and iron with a close match in thermal windows led to the formation of polycrystalline spinel type ZFO. Under the optimized process conditions, it was possible to fabricate solely ZFO in the desired phase. This work demonstrates the potential of employing CVD to obtain photoactive ternary material systems in the right composition. For the first time, the application of CVD grown ZFO films for photoelectrochemical applications is being demonstrated, showing a direct band gap of 2.3 eV and exhibiting activity for visible light driven photoelectrochemical water splitting