The controlled synthesis of materials by methods that permit their assembly into functional nanoscale structures lies at the crux of the emerging field of nanotechnology. Although only one of several materials families is of interest, carbon-based nanostructured materials continue to attract a disproportionate share of research effort, in part because of their wide-ranging properties. Additionally, developments of the past decade in the controlled synthesis of carbon nanotubes and nanofibers have opened additional possibilities for their use as functional elements in numerous applications. Vertically aligned carbon nanofibers (VACNFs) are a subclass of carbon nanostructured materials that can be produced with a high degree of control using catalytic plasma-enhanced chemical-vapor deposition (C-PECVD). Using C-PECVD the location, diameter, length, shape, chemical composition, and orientation can be controlled during VACNF synthesis. Here we review the CVD and PECVD systems, growth control mechanisms, catalyst preparation, resultant carbon nanostructures, and VACNF properties. This is followed by a review of many of the application areas for carbon nanotubes and nanofibers including electron field-emission sources, electrochemical probes, functionalized sensor elements, scanning probe microscopy tips, nanoelectromechanical systems (NEMS), hydrogen and charge storage, and catalyst support. We end by noting gaps in the understanding of VACNF growth mechanisms and the challenges remaining in the development of methods for an even more comprehensive control of the carbon nanofiber synthesis process.

Vertically Aligned Carbon Nanofibers and Related Structures: Controlled Synthesis and Directed Assembly

Synthesis of carbon nanotubes by catalytic chemical vapour deposition: A review on carbon sources, catalysts and substrates

We demonstrate that topographically modified substrates can be used as templates to control solid-state dewetting of thin films by creating a periodic variation in the initial curvature of the film. The dewetting of gold films on oxidized silicon surfaces patterned with arrays of inverted pyramid shaped pits is investigated. For specific ranges of relative film thickness and topographic dimension, dewetting results in arrays of submicrometer-scale particles with uniform periodic spacing and nearly uniform size and crystallographic orientation.

Solid-state dewetting for ordered arrays of crystallographically oriented metal particles

Hf0.5Zr0.5O2 thin films are one of the most appealing HfO2-based ferroelectric thin films, which have been researched extensively for their applications in ferroelectric memory devices. In this work, a 1 mol % La-doped Hf0.5Zr0.5O2 thin film was grown by plasma-assisted atomic layer deposition and annealed at temperatures of 450 and 500 °C to crystallize the film into the desired orthorhombic phase. Despite the use of a lower temperature than that used in previous reports, the film showed highly promising ferroelectric properties—a remnant polarization of ∼30 μC/cm2 and switching cycle endurance up to 4 × 1010. The performance was much better than that of undoped Hf0.5Zr0.5O2 thin films, demonstrating the positive influence of La doping. Such improvements were mainly attributed to the decreased coercive field (by ∼30% compared to the undoped film), which allowed for the use of a lower applied field to drive the cycling tests while maintaining a high polarization value. La doping also decreased the leakage...

Improved Ferroelectric Switching Endurance of La-Doped Hf0.5Zr0.5O2 Thin Films

A thin film transistor includes a source electrode, a drain electrode, a semiconducting layer, a first conductive layer, a second conductive layer, an insulating layer and a gate electrode The drain electrode is spaced apart from the source electrode The first conductive layer is sandwiched between the source electrode and the semiconductor layer The second conductive layer is sandwiched between the drain electrode and the semiconductor layer The gate electrode is insulated from the source electrode, the drain electrode, the first conductive layer, the second conductive layer, and the semiconductor layer by the insulating layer A first work-function of a first material of the first conductive layer and the second conductive layer is same as a second work-function of a second material of the semiconductor layer

Thin film transistor

The dielectric stacking effects of Al2O3 and HfO2 thin layers in metal-insulator-metal capacitors are investigated for their leakage current, breakdown voltage, and voltage linearity of the dielectrics. The stacked dielectrics over three layers show the enhancement of dielectric permittivity and voltage linearity of quadratic voltage coefficient compared to one-layer dielectrics. Stacking over five layers attributes to improving leakage current density and breakdown voltage characteristics than three layers.

Dielectric Stacking Effect of $\hbox{Al}_{2} \hbox{O}_{3}$ and $\hbox{HfO}_{2}$ in Metal–Insulator–Metal Capacitor

Carbon nanotubes (CNTs) were grown on silicon (Si) substrates with a native oxide layer using a nickel (Ni) catalyst by the plasma-enhanced chemical vapor deposition method at 550 and 650°C. Multiwall CNTs with a large inside hollow core were observed with samples deposited on Si substrates with a native oxide layer. However, no CNT growth was observed with samples deposited on Si substrates without a native oxide layer due to the formation of Ni-silicide. The native oxide layer was believed to act as the diffusion barrier and to suppress the formation of Ni-silicide. The typical diameter and length of CNTs ranged from about 30 to 100 nm and 2 to 3 µm, respectively. The size of CNTs was more strongly dependent on the thickness of the Ni layer than on the growth temperature. The surface roughness and amount of surface defects decreased with increasing process temperature. We performed a post hydrogen plasma treatment for the purification of raw CNTs. The post hydrogen plasma treatment successfully removed residual graphite particles and/or metallic impurities without significant damage to the CNTs.

Kyoung-min Ryu

Papers

Dielectric Stacking Effect of $\hbox{Al}_{2} \hbox{O}_{3}$ and $\hbox{HfO}_{2}$ in Metal–Insulator–Metal Capacitor

Low-Temperature Growth of Carbon Nanotube by Plasma-Enhanced Chemical Vapor Deposition using Nickel Catalyst