Understanding properties of engineered catalyst supports using contact angle measurements and X-ray reflectivity.

There is significant interest in broadening the type of catalyst substrates that support the growth of high-quality carbon nanotube (CNT) carpets. In this study, ion beam bombardment has been utilized to modify catalyst substrates for CNT carpet growth. Using a combination of contact angle measurements (CAMs) and X-ray reflectivity (XRR) for the first time, new correlations between the physicochemical properties of pristine and engineered catalyst substrates and CNT growth behavior have been established. The engineered surfaces obtained after exposure to different degrees of ion beam damage have distinct physicochemical properties (porosity, layer thickness, and acid-base properties). The CAM data were analyzed using the van Oss-Chaudhury-Good model, enabling the determination of the acid-base properties of the substrate surfaces. For the XRR data, a Fourier analysis of the interference patterns enabled extraction of layer thickness, while the atomic density and interfacial roughness were extracted by analyzing the amplitude of the interference oscillations. The dramatic transformation of the substrate from "inactive" to "active" is attributed to a combined effect of substrate porosity or damage depth and Lewis basicity. The results reveal that the efficiency of catalyst substrates can be further improved by increasing the substrate basicity, if the minimum surface porosity is established. This study advances the use of a non-thermochemical approach for catalyst substrate engineering, as well as demonstrates the combined utility of CAM and XRR as a powerful, nondestructive, and reliable tool for rational catalyst design.

Epitaxial superlattices with titanium nitride as a plasmonic component for optical hyperbolic metamaterials.

Titanium nitride (TiN) is a plasmonic material having optical properties resembling gold. Unlike gold, however, TiN is complementary metal oxide semiconductor-compatible, mechanically strong, and thermally stable at higher temperatures. Additionally, TiN exhibits low-index surfaces with surface energies that are lower than those of the noble metals which facilitates the growth of smooth, ultrathin crystalline films. Such films are crucial in constructing low-loss, high-performance plasmonic and metamaterial devices including hyperbolic metamaterials (HMMs). HMMs have been shown to exhibit exotic optical properties, including extremely high broadband photonic densities of states (PDOS), which are useful in quantum plasmonic applications. However, the extent to which the exotic properties of HMMs can be realized has been seriously limited by fabrication constraints and material properties. Here, we address these issues by realizing an epitaxial superlattice as an HMM. The superlattice consists of ultrasmooth layers as thin as 5 nm and exhibits sharp interfaces which are essential for high-quality HMM devices. Our study reveals that such a TiN-based superlattice HMM provides a higher PDOS enhancement than gold- or silver-based HMMs.