Re-ordering chaotic carbon: origins and application of textured carbon

Electronic Transport and Corrosion Mechanisms of Graphite-Like Nanocrystalline Carbon Films Used on Metallic Bipolar Plates in Proton-Exchange Membrane Fuel Cells

Nanocrystalline carbon films, which consist of graphite-like nanocrystals within an amorphous carbon matrix, have recently attracted extensive theoretical and experimental attention. Understanding the electronic transport and corrosion mechanisms of graphite-like nanocrystalline carbon films (GNCFs) is essential for their application in proton-exchange membrane fuel cells (PEMFCs). So far, limited progress has been made on the electronic or atomistic understanding of how the degree of structural order and grain boundaries affect the electronic transport and corrosion behaviors of GNCFs. In this work, using the Landauer-Büttiker formula merged with first-principles density functional theory, the conductance of GNCFs is presented as a function of their crystallinity. As the crystallinity decreases, the electron states around the Fermi level are found to be more spatially localized, thus hindering the electronic transport of GNCFs. Additionally, a systemic picture of the chemical reactivity of nanostructured surface in GNCFs toward typical particles existing in PEMFCs is drawn by ab initio molecular dynamics simulations. Systemic experimental investigations on the corrosion mechanisms of GNCFs used in PEMFCs have been conducted in this work. Compared with pure amorphous carbon films, the GNCFs exhibit higher corrosion current densities due to the preferential corrosion in the larger slit pores at the grain boundaries, but their stability in interfacial contact resistance is significantly improved by the embedded graphite-like nanocrystals, which have high levels of resistance to oxygen chemical adsorptions and act as high-speed ways to transport electrons.

Fabrication of field-effect transistors with transfer-free nanostructured carbon as the semiconducting channel material

Carbon nanostructures used as the active channel material in field effect transistors (FETs) are appealing in microelectronics for their improved performance, such as their high speed and low energy dissipation. However, these devices require the incorporation of nanostructure transfer steps in the fabrication process flow, which makes their application difficult in large scale integrated circuits. Here we present a novel method for the fabrication of FETs with nanostructured carbon in the channel with p-type semiconducting properties and intermediate drain-source current (I_DS ) on/off ratio. The method is based on the use of Ni nanoparticles in the source-drain gap region as the seed material for the formation of carbon nanostructures in the FET channel. FETs without Ni nanoparticles in the channel showed no modulation of I_DS as a function of gate voltage. The device fabrication process does not require any carbon nanostructure transfer steps since it directly forms carbon nanostructures electrically connected to the device's source and drain electrodes via electron-beam evaporation of carbon and conventional lithographic processes. Since all device fabrication steps are compatible with existing Si technology processes, they are capable of being further optimized following process development protocols practiced by the semiconductor industry.

Stress Writing Textured Graphite Conducting Wires/Patterns in Insulating Amorphous Carbon Matrix as Interconnects

This study reports a mechanical stress-based technique that involves scratching or imprinting to write textured graphite conducting wires/patterns in an insulating amorphous carbon matrix for potential use as interconnects in future carbonaceous circuits. With low-energy post-annealing below the temperature that is required for the thermal graphitization of amorphous carbon, the amorphous carbon phase only in the mechanically stressed regions transforms into a well aligned crystalline graphite structure with a low electrical resistivity of 420 μΩ-cm, while the surrounding amorphous carbon matrix remains insulating. Micro-Raman spectra with obvious graphitic peaks and high-resolution transmission electron microscopic observations of clear graphitic lattice verified the localized phase transformation of amorphous carbon into textured graphite exactly in the stressed regions. The stress-induced reconstruction of carbon bonds to generate oriented graphitic nuclei is believed to assist in the pseudo-self-formation of textured graphite during low-temperature post annealing.

Control of Nanoplane Orientation in voBN for High Thermal Anisotropy in a Dielectric Thin Film: A New Solution for Thermal Hotspot Mitigation in Electronics

High anisotropic thermal materials, which allow heat to dissipate in a preferential direction, are of interest as a prospective material for electronics as an effective thermal management solution for hot spots. However, due to their preferential heat propagation in the in-plane direction, the heat spreads laterally instead of vertically. This limitation makes these materials ineffective as the density of hot spots increases. Here, we produce a new dielectric thin film material at room temperature, named vertically ordered nanocrystalline h-BN (voBN). It is produced such that its preferential thermally conductive direction is aligned in the vertical axis, which facilitates direct thermal extraction, thereby addressing the increasing challenge of thermal crosstalk. The uniqueness of voBN comes from its h-BN nanocrystals where all their basal planes are aligned in the direction normal to the substrate plane. Using the 3ω method, we show that voBN exhibits high anisotropic thermal conductivity (TC) with a 16-fold difference between through-film TC and in-plane TC (respectively 4.26 and 0.26 W·m^-1·K^-1). Molecular dynamics simulations also concurred with the experimental data, showing that the origin of this anisotropic behavior is due to the nature of voBN's plane ordering. While the consistent vertical ordering provides an uninterrupted and preferred propagation path for phonons in the through-film direction, discontinuity in the lateral direction leads to a reduced in-plane TC. In addition, we also use COMSOL to simulate how the dielectric and thermal properties of voBN enable an increase in hot spot density up to 295% compared with SiO₂, without any temperature increase.

Oxidation-Based Continuous Laser Writing in Vertical Nano-Crystalline Graphite Thin Films

Nano and femtosecond laser writing are becoming very popular techniques for patterning carbon-based materials, as they are single-step processes enabling the drawing of complex shapes without photoresist. However, pulsed laser writing requires costly laser sources and is known to cause damages to the surrounding material. By comparison, continuous-wave lasers are cheap, stable and provide energy at a more moderate rate. Here, we show that a continuous-wave laser may be used to pattern vertical nano-crystalline graphite thin films with very few macroscale defects. Moreover, a spatially resolved study of the impact of the annealing to the crystalline structure and to the oxygen ingress in the film is provided: amorphization, matter removal and high oxygen content at the center of the beam; sp² clustering and low oxygen content at its periphery. These data strongly suggest that amorphization and matter removal are controlled by carbon oxidation. The simultaneous occurrence of oxidation and amorphization results in a unique evolution of the Raman spectra as a function of annealing time, with a decrease of the I(D)/I(G) values but an upshift of the G peak frequency.

Vertically self-ordered orientation of nanocrystalline hexagonal boron nitride thin films for enhanced thermal characteristics

Vertically self-ordered hexagonal boron nitride (ordered h-BN) is a highly ordered turbostratic BN (t-BN) material similar to hexagonal BN, with its planar structure perpendicularly oriented to the substrate. The ordered h-BN thin films were grown using a High Power Impulse Magnetron Sputtering (HiPIMS) system with a lanthanum hexaboride (LaB6) target reactively sputtered in nitrogen gas. The best vertical alignment was obtained at room temperature, with a grounded bias and a HiPIMS peak power density of 60 W cm(-2). Even though the film contains up to 7.5 at% lanthanum, it retains its highly insulative properties and it was observed that an increase in compressive stress is correlated to an increase in film ordering quality. Importantly, the thermal conductivity of vertically ordered h-BN is considerably high at 5.1 W m(-1) K(-1). The favourable thermal conductivity coupled with the dielectric properties of this novel material and the low temperature growth could outperform SiO2 in high power density electronic applications.