Role of Nitrogen and Oxygen in Capacitance Formation of Carbon Nanowalls.

Supercapacitors based on carbon nanomaterials are attracting much attention because of their high capacitance enabled by large specific surface area. The introduction of heteroatoms such as N or O enhances the specific capacitance of these materials. However, the mechanisms that lead to the increase in the specific capacitance are not yet well-studied. In this Letter, we demonstrate an effective method for modification of the surface of carbon nanowalls (CNWs) using DC plasma in atmospheres of O2, N2, and their mixture. Processing in the plasma leads to the incorporation of ∼4 atom % nitrogen and ∼10 atom % oxygen atoms. Electrochemical measurements reveal that CNWs functionalized with oxygen groups are characterized by higher capacitance. The specific capacitance for samples with oxygen reaches 8.9 F cm-3 at a scan rate of 20 mV s-1. In contrast, the nitrogen-doped samples demonstrate a specific capacitance of 4.4 F cm-3 at the same scan rate. The mechanism of heteroatom incorporation into the carbon lattice is explained using density functional theory calculations.

Damage-free plasma etching of porous organo-silicate low-k using micro-capillary condensation above -50 °C.

The micro-capillary condensation of a new high boiling point organic reagent (HBPO), is studied in a periodic mesoporous oxide (PMO) with ∼34 % porosity and k-value ∼2.3. At a partial pressure of 3 mT, the onset of micro-capillary condensation occurs around +20 °C and the low-k matrix is filled at -20 °C. The condensed phase shows high stability from -50 < T ≤-35 °C, and persists in the pores when the low-k is exposed to a SF6-based plasma discharge. The etching properties of a SF6-based 150W-biased plasma discharge, using as additive this new HBPO gas, shows that negligible damage can be achieved at -50 °C, with acceptable etch rates. The evolution of the damage depth as a function of time was studied without bias and indicates that Si-CH3 loss occurs principally through Si-C dissociation by VUV photons.

Optical Emission from C2- Anions in Microwave-Activated CH4/H2 Plasmas for Chemical Vapor Deposition of Diamond.

Visible emission from C2-(B2Σu+) anions has been identified underlying the much stronger Swan band emission from neutral C2(d3Πg) radicals (henceforth C2-* and C2*, respectively) in MW-activated C/H/(Ar) plasmas operating under conditions appropriate for the chemical vapor deposition (CVD) of diamond. Spatially resolved measurements of the C2-* and C2* emissions as functions of the C/H/(Ar) ratio in the input gas mixture, the total pressure, and the applied MW power, together with complementary 2-D(r, z) plasma modeling, identifies dissociative electron attachment (DEA) to C2H radicals in the hot plasma as the dominant source of the observed C2-* emission. Modeling not only indicates substantially higher concentrations of C2H- anions (from analogous DEA to C2H2) in the near-substrate region but also suggests that the anion number densities will typically be 3-4 orders of magnitude lower than those of the electrons and partner cations, i.e., mainly C2H2+ and C2H3+. The identification of negatively charged carbon-containing species in diamond CVD plasmas offers a possible rationale for previous reports that nucleation densities and growth rates can be enhanced by applying a positive bias to the substrate.

Simulation of diffuse, constricted-stratified, and constricted modes of a dc discharge in argon: Hysteresis transition between diffuse and constricted-stratified modes.

This paper presents the results of theoretical studies of high-pressure p (tens and hundreds of Torr) direct current (dc) discharges in argon. The diffuse (D), constricted-stratified (CS), and constricted (C) discharge modes are studied using a developed one-dimensional (radial) model. The model includes the conservation equations for electrons, ions (Ar+ and Ar2+), and excited atoms (metastable and resonant states) for mean electron energy and for the temperature of the high-energy part of the electron-energy distribution function (EEDF), the heat conduction equation for the neutral gas, and Poisson's equation for the radial electric field. The developed model of a dc discharge allowed us, without any artificial assumptions, to obtain periodic oscillations of plasma parameters for the CS mode and to describe a hysteresis transition between the D mode and the CS mode. Direct transition from the D to the CS mode is accompanied by an increase of several orders of magnitude in the electron density at the discharge axis and the appearance of moving striations. It was shown that the experimentally observed hysteresis of the current-voltage characteristic at the transition between the CS and D modes deals with the nonlocal formation of the EEDF, namely, the diffusion of high-energy electrons from the central constricted region. The effect of the nonlocal formation of the EEDF is taken into account by introducing the effective temperature of the high-energy part of the EEDF and solving the equation for the radial profile of this temperature. The transition from the CS mode to the C mode occurs smoothly, without any jumps of the plasma parameters. Plasma parameters and characteristics of all three modes and transitions between these modes are calculated and compared with experimental data.