Sp3 Matrix Research Articles

Role of sp2 phase in field emission from nanostructured carbons

It is shown that sp2 phase organization plays an important role in the field emission from nanostructured carbons. Emission is found to depend on the cluster size, anisotropy, and mesoscale bonding of the sp2 phase, and the electronic disorder. It is found by Raman spectroscopy that increasing the size of sp2 clusters in the 1–10 nm range improves emission. Anisotropy in the sp2 phase orientation can help or inhibit the emission. sp2 clusters embedded in the sp3 matrix or electronic disorder induced by localized defects oriented in the field direction can provide a local field enhancement to facilitate the emission.

Nanostructure of thin amorphous hydrogenated carbon films studied by positron annihilation and photoluminescence

We deposited polymer-like a-C:H films using the plasma-enhanced chemical vapor deposition technique and characterized film microstructure by variable-energy positron lifetime spectroscopy, photoluminescence (PL), and UV-visible absorption spectroscopy. It was confirmed that PL occurs from a chromophore in a sp2 cluster as a result of fast recombination of a photoexcited electron-hole pair. Positron annihilation lifetime spectroscopy showed that positronium (Ps) formation takes place via electron-positron recombination in the sp3 matrix. The lifetime of ortho-positronium (o-Ps) in our a-C:H films was similar to that in polyethylene, indicating their polymer-like nature. The relative PL efficiency increased by about an order of magnitude with increasing film band gap from 1.3 to 3.4 eV, which can be related to the decreasing concentration of nonradiative centers. On the other hand, Ps formation was much less influenced by the band gap and nonradiative centers. Comparison of this result with that for polyethylene mixed with carbon-black nanoparticles, where a considerable reduction in Ps formation was observed, showed that nonradiative centers were of a different nature from the defects on the carbon nanoparticle surface. This work demonstrated the usefulness of positron lifetime spectroscopy combined with optical measurements to study the nanostructure of a-C:H.

Origin of electric field enhancement in field emission from amorphous carbon thin films

The observation of electron emission from amorphous carbon thin films at low applied electric fields is explained in terms of an enhancement of the field brought about by dielectric inhomogeneities within the film. These inhomogeneities originate from the differences between conductive, spatially localized sp2 C clusters surrounded by a more insulating sp3 matrix. By a more complete understanding of the concentration and distribution of the clusters, a generic model for field emission from amorphous carbon thin films can be developed. Extensions of this model to explain the emission properties of carbon nanotubes and carbon nanocomposite materials are also presented.

Effect of work function and surface microstructure on field emission of tetrahedral amorphous carbon

The work function of tetrahedral amorphous carbon (ta-C) has been measured by Kelvin probe to lie in the range 4–5 eV, irrespective of its sp3 content or nitrogen addition. This implies that the surface barrier to emission is dominant and that emission changes caused by sp3 bonding or nitrogen addition are not directly due to changes in work function. Hydrogen, oxygen, and argon plasma treatments are all found to increase the emission of a-C, but hydrogen and argon treatments are found to reduce the work function while oxygen treatment increases it. Detailed studies of the surface with varying plasma treatment conditions suggest that the changes in emission arise mainly from changes in the surface microstructure, such as the formation of sp2 regions within the sp3 bulk. The need for local field enhancement mechanisms to account for emission over the sizeable barrier is emphasized, which may arise from local chemical nonhomogeneity, or formation of nanometer-size sp2 clusters embedded in an sp3 matrix.

Effect of sp2-phase nanostructure on field emission from amorphous carbons

Electron field emission from amorphous carbon is found to depend on the clustering of the sp2 phase. The size of the sp2 phase is varied by thermal annealing and it dominates the effect of other parameters, such as chemical composition, surface termination, sp3 content, or conductivity. The optimum size of the sp2 phase is determined by Raman spectroscopy and is of the order of 1 nm. The field emission originates from the sp2 regions and is facilitated by the large field enhancement from more conductive sp2 clusters in an insulating sp3 matrix.

Mechanical properties and Raman spectra of tetrahedral amorphous carbon films with high sp3 fraction deposited using a filtered cathodic arc

Abstract The mechanical and structural properties of the tetrahedral amorphous carbon (ta-C) films deposited by the filtered cathodic vacuum arc technique on silicon at room temperature have been studied over the carbon ion energy range 15–200 eV. High (about 80% or higher) sp3 bond fractions were obtained for almost all ion energies investigated (E > 50 eV), with a maximum of about 87% at about 95 eV. The variations in the mechanical properties have been correlated to the sp3 fraction and show an almost linear dependence on the small variation in sp3 content. The maximum hardness, Young's modulus, stress critical load and minimum friction coefficient all coincide with the highest sp3 fraction as determined by electron-energy-loss spectroscopy. All films are atomically smooth. The Raman spectra of these films when fitted with a skewed Lorentzian, a parameter Q, which measures the degree of skewness, is noticeably dependent on films with sp2 content below 30% and can also be used to verify the ion energy corresponding to the maximum sp3 content. The change observed in the mechanical properties as well as the skewness parameter Q is attributed to a change in the local ordering of the sp2 bonded atoms in the ta-C sp3 matrix.

Electrical conductivity of amorphous carbon and amorphous hydrogenated carbon

Abstract Amorphous carbon (a-C) thin films have been prepared by r.f. magnetron sputtering of graphite targets in an Ar atmosphere and their d.c. electrical dark conductivity has been measured as a function of temperature. The results are compared with those obtained by earlier workers for a-C and hydrogenated a-C. A detailed analysis leads us to the conclusion that conductivity of amorphous C with or without H is determined mostly by the amount and distribution of sp2 sites in the material. Near room temperature the experimental results suggest hopping between neighbouring sp2 (graphitic) islands embedded in an sp3 (diamond-like) matrix if the sp2:sp3 ratio is below its percolation threshold value and hopping in the band tails if this ratio is above such a threshold. In the temperature region above 500 K hopping in the band tails is shown to be the only effective mechanism.