Nanocrystalline Graphitic Carbon Research Articles

Hydrogen-Type Binding Sites in Carbonaceous Electrodes for Rapid Lithium Insertion.

Direct pyrolysis of coronene at 800 °C produces low-surface-area, nanocrystalline graphitic carbon containing a uniquely high content of a class of lithium binding sites referred to herein as "hydrogen-type" sites. Correspondingly, this material exhibits a distinct redox couple under electrochemical lithiation that is characterized as intermediate-strength, capacitive lithium binding, centered at ∼0.5 V vs Li/Li+. Lithiation of hydrogen-type sites is reversible and electrochemically distinct from capacitive lithium adsorption and from intercalation-type binding between graphitic layers. Hydrogen-type site lithiation can be fully retained even up to ultrafast current rates (e.g., 15 A g-1, ∼40 C) where intercalation is severely hampered by ion desolvation kinetics; at the same time, the bulk nature of these sites does not require a large surface area, and only minimal electrolyte decomposition occurs during the first charge/discharge cycle, making coronene-derived carbon an exceptional candidate for high-energy-density battery applications.

Natural Porous Carbon Derived from Popped Rice as Anode Materials for Lithium-Ion Batteries

Popped rice carbons (PC) were derived from popped rice by using a facile and low-cost technique. PC was then activated by different kinds of activating agents, such as potassium hydroxide (KOH), zinc chloride (ZnCl2), iron (III) chloride (FeCl3), and magnesium (Mg), in order to increase the number of pores and specific surface area. The phase formation of porous activated carbon (PAC) products after the activation process suggested that all samples showed mainly graphitic, amorphous carbon, or nanocrystalline graphitic carbon. Microstructure observations showed the interconnected macropore in all samples. Moreover, additional micropores and mesopores were also found in all PAC products. The PAC, which was activated by KOH (PAC-KOH), possessed the largest surface area and pore volume. This contributed to excellent electrochemical performance, as evidenced by the highest capacity value (383 mAh g−1 for 150 cycles at a current density of 100 mA g−1). In addition, the preparation used in this work was very simple and cost-effective, as compared to the graphite preparation. Experimental results demonstrated that the PAC architectures from natural popped rice, which were activated by an optimal agent, are promising materials for use as anodes in LIBs.

Fabrication and Electrochemical Properties of Three-Dimensional (3D) Porous Graphitic and Graphenelike Electrodes Obtained by Low-Cost Direct Laser Writing Methods

The development of three-dimensional (3D) porous graphitic structures is of great interest for electrochemical sensing applications as they can support fast charge transfer and mass transport through their extended, large surface area networks. In this work, we present the facile fabrication of conductive and porous graphitic electrodes by direct laser writing techniques. Irradiation of commercial polyimide sheets (Kapton tape) was performed using a low-cost laser engraving machine with visible excitation wavelength (405 nm) at low power (500 mW), leading to formation of 3D laser-induced graphene (LIG) structures. Systematic correlation between applied laser dwell time per pixel (“dwell time”) and morphological/structural properties of fabricated electrodes showed that conductive and highly 3D porous structures with spectral signatures of nanocrystalline graphitic carbon materials were obtained at laser dwell times between 20 and 110 ms/pix, with graphenelike carbon produced at 50 ms/pix dwell time, with comparable properties to LIG obtained with high cost CO2 lasers. Electrochemical characterization with inner and outer sphere mediators showed fast electron transfer rates, comparable to previously reported 2D/3D graphene-based materials and other graphitic carbon electrodes. This work opens the way to the facile fabrication of low-cost, disposable electrochemical sensor platforms for decentralized assays.

Electrical properties of FCVA deposited nano-crystalline graphitic carbon thin films with in situ treatment techniques

In this report, detailed studies of the nano-crystalline graphitic (NCG) carbon thin films deposited by filtered cathodic vacuum arc (FCVA) following its first discovery and first electrical characterization have been performed. The microstructure of carbon thin film can be modified by applying in situ treatment techniques (such as altering the thermal and carbon ion deposition energy). It was found that highly electrical conductive carbon thin films with graphitic planes can also be deposited at a low deposition temperature instead of the previously reported high deposition temperature with a 20% improvement in electrical characteristic. The detailed electrical characterization comparison between amorphous carbon and NCG has been conducted. To conclude, NCG carbon can be fabricated at deposition temperature above 400 °C and exhibits increasing through film Ohmic electrical conductivity with increasing deposition temperature. At low temperatures, NCG carbon can be produced by increasing the negative substrate bias above 300 V (impinging carbon ion energy). Higher negative substrate bias leads to thinner amorphous interface layer hence better electrical conductivity.

Stoichiometry and structure driven optical properties of carbon incorporated titanium oxide thin films

This study shows that the optical properties of titanium oxide films depend on their chemical composition as well as structure properties. The chemical composition as well as the structure of the sputter-deposited titanium oxide films on fused quartz glass substrates was modulated by annealing them in Ar/H2 atmosphere, experimentally investigated by various spectroscopic methods including XRD, Raman, SIMS, and UV-Vis spectroscopy and simulated by Bruggeman effective medium theory (BR-EMT) and full wave finite difference time domain (FDTD) approach. The as-deposited films are amorphous titanium oxide films with oxygen deficiency, become closer to titanium dioxide in stoichiometry, and are converted to crystalline anatase TiO2 nano-particles with particle size of ~21 nm by increasing the annealing temperature from 100 °C to 450 °C. The annealing process also allows the titanium oxide films to be spontaneously doped with environmental carbon species or coated with nano-crystalline graphitic carbon. Further the FDTD simulation results indicate that the annealed titanium oxide films become porous. The chemical composition modulation, anatase crystalline formation, carbon doping and graphitic carbon coating are simultaneously achieved by the simply annealing the sputter-deposited amorphous titanium oxide thin films on fused quartz substrates at relatively low temperatures.

Direct growth of nanocrystalline graphitic carbon films on BaF2 by alcohol CVD

Multilayered nanocrystalline graphitic carbon films were directly formed on BaF2 substrates by the alcohol chemical vapor deposition method using ethanol. Domain size was typically 46 nm as estimated from the peak intensity ratios of G- and D-band of the Raman spectra, which were higher than currently reported values for various dielectric substrates. Sheet resistance measured by the four-probe method was 900 Ω/sq. A possible formation mechanism of the graphitic carbon layer was discussed referring to the results of X-ray photoelectron spectroscopy (XPS) and energy dispersive X-ray spectroscopy (EDX) for a composition analysis. It was found that the graphitic carbon layers were formed on the BaO surface, which was produced by the oxidization of the BaF2 substrate by the ethanol source.

Ni-induced graphitization for enhanced long-term stability of ohmic contact to polycrystalline 3C-SiC

Stable metal contacts for SiC-based devices are required for high temperature microelectronics and microsensor devices. Ni-induced nanocrystalline graphitic carbon is introduced between Pt/Ti and n-type polycrystalline 3C-SiC as a means of forming contacts that are stable at high temperature. With the addition of an alumina protection layer, this metallization scheme is further improved and can maintain low contact resistivity after 500 h at 450 °C in air. The role of the graphitic layer in both the formation and long-term stability of the contact is investigated. Although the formation of an ohmic contact between Pt/Ti and polycrystalline 3C-SiC does not require the graphitic carbon, this interfacial layer is necessary for maintaining low contact resistivity during long-term exposure to elevated temperature.

Graphitization of n-type polycrystalline silicon carbide for on-chip supercapacitor application

Synthesis of silicon carbide-derived carbon films with excellent supercapacitor characteristics is demonstrated by a process that is fully compatible with standard microfabrication technology. NiTi alloy deposited on nitrogen-doped polycrystalline SiC films is shown to result in the growth of a rough, porous, high conductivity, nanocrystalline graphitic carbon film upon rapid thermal annealing to 1050 °C. Electrodes fabricated in this manner exhibit high charge/discharge rates with a time constant of about 0.062 s. Analysis shows that the incorporated nitrogen in the carbon electrode may induce pseudo-capacitance, and the electrodes exhibit the capacitance/area values comparable to those reported on carbon nanotube-based supercapacitors.

Enhanced Ohmic contact via graphitization of polycrystalline silicon carbide

Electrical contact to silicon carbide with low contact resistivity and high stability is a critical requirement for SiC-based microsystem and nanosystem technology for harsh environment applications. In this letter, nanocrystalline graphitic carbon is grown at the interface between SiC and Pt to lower the Ohmic contact resistivity and enhance the stability of Pt contacts to polycrystalline 3C-SiC operated at elevated temperatures. Analysis shows that reduced barrier height, oxide-free surface, reduced density of vacancy defects, and suppressed reactivity between Pt and SiC are likely responsible for the reduced Ohmic contact resistivity and high thermal stability of Pt contacts to graphitized SiC.

Microwave plasma chemical vapor deposition of nano-composite C/Pt thin-films

Composite carbon–platinum thin-films of nano-crystalline graphitic carbon decorated with uniformly-dispersed 2–4 nm Pt nano-particles have been synthesized from a solid organic precursor by a one-step microwave plasma chemical vapor deposition (MPCVD). The fast Ar-plasma discharge and the presence of microwave radiation accelerate the formation of sites suitable for in situ heterogeneous nucleation, and consequently, the fine dispersion of metal in the carbonaceous matrix. The electrochemical response of the 2 μm C/Pt thin-film electrode displays electrochemical activity, which is attributed to the high ca. 18 m 2/g effective surface area of Pt nano-particles.

Mechanistic study of ion-induced diamond nucleation

Abstract The effect of low-energy ion bombardment of silicon on diamond nucleation was investigated. By bombarding 100 eV ions of methane and hydrogen on a silicon substrate prior to diamond growth by chemical vapor deposition, diamond nucleation can be immensely enhanced. The ion beam treatment deposited a layer of nano-crystalline graphitic carbon embedded with amorphous SiC. Diamond then nucleated on the graphite overlayer; the nucleation density increased with increasing ion dose. At 1×1019 ions cm−2, a nuclei density of 4×108 cm−2 was obtained. These results show that ion bombardment of the substrate enhances diamond nucleation.

Hard a-C(N):H films obtained from plasma decomposition of methylamine-containing mixtures

Hard amorphous hydrogenated carbon-nitrogen films were obtained by PECVD of pure methylamine and methylamine-acetylene mixtures. The chemical composition of the films was determined by nuclear techniques, and their structure examined by infrared and Raman spectroscopies. The film obtained from pure methylamine have a NC atomic ratio of 0.64, and a Raman spectrum that is characteristic of nanocrystalline graphitic carbon films. The films obtained from acetylene-methylamine mixtures show the same dependence of the structure details on the nitrogen content as already observed in a-C(N):H films deposited using other gas mixtures, i.e. a progressive increase in sp2 character of the carbon atoms when the nitrogen content in the film increases.