Study of the conductivity of nitrogen doped tetrahedral amorphous carbon films

The conductivity of nitrogen incorporated tetrahedral amorphous carbon (taC:N) films prepared by filtered cathodic vacuum arc (FCVA) system was studied. The film resistivity varied as a function of nitrogen content in a wide nitrogen content range from 1 to 23 at.%. An interesting phenomenon of the photoconductivity for the films was found, that the resistivity of those films with low nitrogen content (<5 at.%) increased due to the light irradiation. This kind of behavior of photoconductivity had not been discussed in previous papers on taC:N films.     

RTT presentation of ortho-symplectic super Yangians. Staggered integrable spin chains

We give a RTT presentation of ortho-symplectic super Yangians that encompasses the orthogonal and symplectic cases. In a second part, we construct an integrable ladder chain model and study its quantum symmetry. Presented at the 11th Colloquium “Quantum Groups and Integrable Systems”, Prague, 20–22 June 2002.     

Laser vaporization synthesis of polyhedral graphite

Polyhedral graphite (PG) particles have been synthesized by CO₂ laser vaporization of graphite in high-pressure Ar gas (8×10⁵ Pa). Faceted PG particles, ranging in size from 110 to 500 nm, have a turbostratic structure. The yield of PG is more than 90%. This synthesis is based on the condensation of hot carbon species confined by an Ar-gas atmosphere. Received: 22 January 2003 / Accepted: 24 January 2003 / Published online: 11 April 2003 RID="*" ID="*"Corresponding author. Fax: +81-59/231-9471, E-mail: kokai@chem.mie-u.ac.jp Permanent address: Chemistry Department for Materials, Mie University, 1515 Kamihama, Tsu, Mie 514-8507, Japan     

Space-time dispersion of graphene conductivity

We present an analytic calculation of the conductivity of pure graphene as a function of frequency ω, wave-vector k, and temperature for the range where the energies related to all these parameters are small in comparison with the band parameter γ≃3 eV, but much larger than the collision rate τ^-1. The simple asymptotic expressions are given in various limiting cases. For instance, the conductivity for kv₀≪ T≪ω is equal to σ(ω,k)=e²/4ħ and independent of the band structure parameters γ and v₀. Our results are also used to explain the known dependence of the graphite conductivity on temperature and pressure.     

A New Empirical Model for Estimation of sp^3 Fraction in Diamond-Like Carbon Films

A new empirical model to estimate the content of sp^3 in diamond-like carbon （DLC） films is presented, based on the conventional Raman spectra excited by 488nm or 514nm visible light for different carbons. It is found that bandwidth of the G peak is related to the sp^3 fraction. A wider bandwidth of the G peak shows a higher sp^3 fraction in DLC films.     

Real-time monitoring of diamond nucleation and growth using field-enhanced thermionic emission current

Diamond nucleation and growth in the combustion-flame method were monitored in real time using thermionic emission current from the deposited diamond films. It was observed that the emission current evolved over three periods, the incubation, the fast increase, and the saturation periods. Ball-shaped diamond particles, faceted diamond crystals, and diamond films with well-faceted crystals were formed in the three periods. The current from a diamond-seeded substrate started to increase immediately without an incubation period, confirming that the current is from the diamond. Therefore, the current could be used for real-time monitoring of the diamond nucleation and growth.     

The approach to diamond growth on levitating seed particles

We demonstrate the approach of diamond growth on levitating seed particles in a rf plasma. We introduce a hot filament chemical vapor deposition (CVD) technique into the rf plasma chamber in order to obtain improved crystal growth. Firstly, we confirmed diamond nucleation on seed particles placed on a Si substrate using the hot filament CVD. The deposition conditions, namely the total pressure and the rf power, were chosen so that they correspond to particles levitation conditions. We observe that a hydrogen pre-treatment on the seed particles improves the nucleation. Secondly, we confirm the levitation of particles at high temperatures. Fine particles levitated in a plasma are particularly sensitive to thermophoretic effects due to inhomogeneities in the gas heating. Therefore, proper heating procedures are required for successful particles levitation.     

Surface diffusion of carbon atom and carbon dimer on Si(0 0 1) surface

Carbon (C) atom and carbon dimer (C2) are known to be the main projectiles in the deposition of diamond-like carbon (DLC) films. The adsorption and diffusion of the C adatom and addimer (C2) on the fully relaxed Si(0 0 1)-(2 × 1) surface was studied by a combination of the molecular dynamics (MD) and Monte Carlo (MC) simulation. The adsorption sites of the C and C2 on the surface and the potential barriers between these sites were first determined using the semi-empirical many-body Brenner and Tersoff potential. We then estimated their hopping rates and traced their pathways. It is found that the diffusion of both C and C2 is strongly anisotropic in nature. In addition, the C adatom can diffuse a long distance on the surface while the adsorbed C2 is more likely to be confined in a local region. Thus we can expect that smoother films will be formed on the Si(0 0 1) surface with single C atoms as projectile at moderate temperature, while with C2 the films will grow in two-dimensional islands. In addition, relatively higher kinetic energy of the projectile, say, a few tens of eV, is needed to grow DLC films of higher quality. This is consistent with experimental findings.     

Effect of nanostructured AlN coatings on the oxidation-resistant properties of optical diamond films

Diamond film is an ultra-durable optical material with high thermal conductivity and good transmission in near-infrared and far-IR (8-14 μm) wavebands. CVD diamond is subjected to oxidation at temperature higher than 780 °C bared in air for 3 min, while it can be protected from oxidation for extended exposure in air at temperature up to 900 °C by a coating of aluminum nitride. Highly oriented AlN coatings were prepared for infrared windows on diamond films by reactive sputtering method and the average surface roughness (R_a) of the coatings was about 10 nm. The deposited films were characterized by X-ray diffraction (XRD) and atom force microscope (AFM). XRD confirmed the preferential orientation nature and AFM showed nanostructures. Optical properties of diamond films coated AlN thin film was investigated using infrared spectrum (IR) compared with that for as-grown diamond films.     

Nano-graphene structures deposited by N-IR pulsed laser ablation of graphite on Si

Thin nano-structured carbon films have been deposited in vacuum by pulsed laser ablation, from a rotating polycrystalline graphite target, on Si 〈1 0 0〉 substrates, kept at temperatures ranging from RT to 800 °C. The laser ablation was performed by a Nd:YAG laser, operating in the near IR (λ = 1064 nm).X-ray diffraction analysis, performed at grazing incidence angle, both in-plane (ip-gid) and out-of-plane (op-gid), has shown the growth of oriented nano-sized graphene particles, characterised by high inter-planar stacking distance (d_? ∼ 0.39 nm), compared to graphite. The film structure and texturing are strongly related both to laser wavelength and substrate temperature: the low energy associated to the IR laser radiation (1.17 eV) generates activated carbon species of large dimensions that, also at low T (∼400 °C), easy evolve toward more stable sp² aromatic bonds, in the plume direction. Increasing temperature the nano-structure formation increases, causing a further aggregation of aromatic planes, voids formation, and a related density (by X-ray reflectivity) drop to very low values. SEM and STM show for these samples a strongly increased macroscopic roughness. The whole process, mainly at higher temperatures, is characterised by a fast kinetic mode, far from equilibrium and without any structural or spatial rearrangement.