Gap states in diamond-like amorphous carbon

Outstanding Broadband (532 nm to 2.2 μm) and Very Efficient Optical Limiting Performance of Some Defect-Engineered Graphenes

Graphene derivatives and defect-engineered graphenes have attracted the interest of researchers owing to the excellent and tunable properties they exhibit. In this work the optical limiting performance of two defect-engineered boron- and nitrogen-doped reduced graphene oxides is investigated. Both graphenes are found to exhibit exceptional and broadband optical limiting action ranging from 532 to 2200 nm. Their optical limiting efficiency was found to be superior to that of all the other graphene derivatives studied to date, exhibiting a gradually decreasing optical limiting onset, reaching the record low value of ∼0.002 J cm^-2 at 2200 nm. The results demonstrate the potential of engineering the defects of such reduced graphene oxides, resulting in very broadband and efficient optical limiting graphene derivatives, showing a promising method to further tailor their optical and optoelectronic properties.

Fluence Enhanced Optical Response of Ag Implanted Amorphous Carbon Thin Films

Silver nanoparticles (NPs) are known to exhibit strong interaction with light photons because their surface conduction electrons undergo collective oscillations once photo-excited at specific wavelengths; the so-called surface plasmon resonance (SPR). Their incorporation into carbon-based material is shown to greatly influence the overall optical response of the matrix due to aggregation. In this paper, we studied the optical response of silver-irradiated amorphous carbon films due to varying fluence of 25 keV Ag ions in the range 2.5–3.4 × 1016 ions/cm2. Raman spectroscopy provided an insight into the microstructural details of the Ag:a-C nanocomposites such that access to bond characteristics of the films is enabled by directly linking the Raman information with sp2/sp3 configurations. Atomic force microscopy (AFM) analysis show significant increase in particle grain size and surface roughness of the films with increasing fluence while transmission electron microscopy (TEM) confirmed fluence-induced particle aggregation due to irradiation. Optical absorption studies revealed that the SPR of Ag NPs occurs in the wavelength range 418–395 nm in the irradiated films. The blue shift in plasmonic wavelength response is explained with respect to the fluence-induced increase in the particulate grain size and particle density as confirmed by AFM and TEM. The optical band gap energy ( E g ) of the pristine carbon film decreased from 1.79 eV to 1.41 eV while Urbach parameter ( E u ) increased from 0.01 eV to 12.0 eV, respectively with increasing fluence. These tunable optical parameters can be tailored into applications in surface coatings and as functional materials for solar cell efficiency enhancement.

The Electronic Transport Mechanism in Amorphous Tetrahedrally-Coordinated Carbon Films

The electronic transport mechanism in tetrahedrally-coordinated amorphous carbon was investigated using measurements of stress relaxation, thermal evolution of electrical conductivity, and temperature-dependent conductivity measurements. Stress relaxation measurements were used to determine the change in 3-fold coordinated carbon concentration, and the electrical conductivity was correlated to this change. It was found that the conductivity was exponentially proportional to the change in 3-fold concentration, indicating a tunneling or hopping transport mechanism. It was also found that the activation energy for transport decreased with increasing anneal temperature. The decrease in activation energy was responsible for the observed increase in electrical conductivity. A model is described wherein the transport in this material is described by thermally activated conduction along 3-fold linkages or chains with variable range and variable orientation hopping. Thermal annealing leads to chain ripening and a reduction in the activation energy for transport.

Refinement of Plas Samples by Using Afm Image and First Observation of Plas Signals on Amorphous Carbon Nitridea-CNx Films

The shape of several waveguide end of samples for photoluminescence absorption spectroscopy (PLAS) was studied by atomic force microscope (AFM), because there was an experimental problem where some samples for PLAS did not work. Using the result of AFM, the waveguide end was reshaped by plasma dry etching. The shortening of the etching time was an effective method to improve the structure of the waveguide end. Secondly, the PLAS method was extended to the other materials from a-Si:H. The PLAS signal of amorphous carbon nitride a-CNx was detected for the first time. Amorphous carbon nitride a-CNx film itself and the interface between a-CNx and a-Si02 are found as good as a-Si:H and the interface between a-Si:H and a-Si 3N4+x:H, respectively.

Molecular dynamics simulation of the formation of sp3 hybridized bonds in hydrogenated diamondlike carbon deposition processes

The formation process of sp3 hybridized carbon networks (i.e., diamondlike structures) in hydrogenated diamondlike carbon (DLC) films has been studied with the use of molecular-dynamics simulations. The processes simulated in this study are injections of hydrocarbon (CH3 and CH) beams into amorphous carbon (a-C) substrates. It has been shown that diamondlike sp3 structures are formed predominantly at a subsurface level when the beam energy is relatively high, as in the "subplantation" process for hydrogen-free DLC deposition. However, for hydrogenated DLC deposition, the presence of abundant hydrogen at subsurface levels, together with thermal spikes caused by energetic ion injections, substantially enhances the formation of carbon-to-carbon sp3 bonds. Therefore, the sp3 bond formation process for hydrogenated DLC films essentially differs from that for hydrogen-free DLC films.