C.A. Achete

Quantifying Defects in Graphene via Raman Spectroscopy at Different Excitation Energies

We present a Raman study of Ar(+)-bombarded graphene samples with increasing ion doses. This allows us to have a controlled, increasing, amount of defects. We find that the ratio between the D and G peak intensities, for a given defect density, strongly depends on the laser excitation energy. We quantify this effect and present a simple equation for the determination of the point defect density in graphene via Raman spectroscopy for any visible excitation energy. We note that, for all excitations, the D to G intensity ratio reaches a maximum for an interdefect distance ∼3 nm. Thus, a given ratio could correspond to two different defect densities, above or below the maximum. The analysis of the G peak width and its dispersion with excitation energy solves this ambiguity.

Raman Signature of Graphene Superlattices

When two identical two-dimensional periodic structures are superposed, a mismatch rotation angle between the structures generates a superlattice. This effect is commonly observed in graphite, where the rotation between graphene layers generates Moiré patterns in scanning tunneling microscopy images. Here, a study of intravalley and intervalley double-resonance Raman processes mediated by static potentials in rotationally stacked bilayer graphene is presented. The peak properties depend on the mismatch rotation angle and can be used as an optical signature for superlattices in bilayer graphene. An atomic force microscopy system is used to produce and identify specific rotationally stacked bilayer graphenes that demonstrate the validity of our model.

Raman spectroscopy on nitrogen-incorporated amorphous hydrogenated carbon films

Abstract Nitrogenated a-C:H films, a-C:H(N), were obtained by plasma decomposition of methane-nitrogen mixtures. The samples were thermally annealed in vacuum for 30 min at fixed temperatures between 300 and 700°C. Undoped films were implanted at room temperature with 70 keV-N+ at fluences of 6, 10 and 20 × 1016 N cm−2. The induced structural modifications were studied by Raman spectroscopy throygh the evolution of the D and G bands. The spectral evolution observed on a-C:H(N) samples shows evidence that a progressive graphitization follows the nitrogen incorporation in these films. Micro-Raman depth profile analysis indicates that the structural and modifications observed on implanted samples are due to the energy deposited by the incident ions. Thermal annealing induces the formation of graphitic domains in a-C:H(N) films.

Raman study of ion-induced defects in N-layer graphene

Raman scattering is used to study the effect of low energy (90 eV) Ar(+) ion bombardment in graphene samples as a function of the number of layers N. The evolution of the intensity ratio between the G band (1585 cm(-1)) and the disorder-induced D band (1345 cm(-1)) with ion fluence is determined for mono-, bi-, tri- and ∼50-layer graphene samples, providing a spectroscopy-based method to study the penetration of these low energy Ar(+) ions in AB Bernal stacked graphite, and how they affect the graphene sheets. The results clearly depend on the number of layers. We also analyze the evolution of the overall integrated Raman intensity and the integrated intensity for disorder-induced versus Raman-allowed peaks.

Characterization of ultra-hard silicon carbide coatings deposited by RF magnetron sputtering

Abstract Silicon carbide films were deposited onto crystalline silicon substrates from a sintered SiC target using a RF magnetron sputtering system. The influence of substrate temperature (150–500°C) and polarization (0−100 V), Ar pressure (0.05–4 Pa) and RF power (50–400 W) on the mechanical properties (hardness and stress) of the resulting films was studied. Films with hardness values larger than 40 GPa could be obtained, provided that Si and C sputtered atoms can reach the surface of the growing film with sufficient high energy and low deposition rates in order to guarantee a high surface mobility. At high deposition rates the surface mobility is limited, but the increase in substrate temperature can contribute to stress relief. Upon thermal annealing at high temperatures, completely stress-free films could be produced without affecting the material hardness. This effect is accompanied by an increased structural and chemical order. Substrate bias was found not to be beneficial to the film properties, since it leads to substantial argon incorporation into the material.

Magnetron sputtered SiC coatings as corrosion protection barriers for steels

Silicon carbide films between 2.3 and 3.0 μm were deposited on AISI 304 stainless steel (SS), carbon steel (CS) and crystalline silicon from a SiC target in a magnetron sputtering system. Good mechanical properties were obtained for the films by carefully controlling the deposition parameters. Results of scratch tests revealed that adhesion of the films is a function of deposition parameters and substrate type. Additionally, an influence of substrate preparation prior to deposition was also observed. Critical loads of 20 N, 8 N and 5 N were obtained in case of Si, SS and CS substrates, respectively. Vickers micro-hardness values were between 10 and 30 GPa, films on SS being harder than films on CS. The behavior of the films as corrosion protection barriers in aggressive environments was evaluated by immersion tests and electrochemical impedance spectroscopy measurements. Films on SS exhibited a better corrosion resistance than those on CS. Their adhesion to the SS was outstanding, even after a long time of immersion in a HCl 0.8 M solution, showing that they are efficient protection barriers. The corrosion process of the substrates starts at micro-pores present in the films so that corrosion pits all over the surface of the samples can be observed.

Amorphous Hydrogenated Carbon Films as Barrier for Gas Permeation through Polymer Films

Abstract The influence of amorphous hydrogenated carbon (a-C:H) coatings on the gas permeation through polymer films was investigated. a-C:H films were deposited from a 13.56-MHz RF glow discharge in methane or acetylene atmosphere. Thin poly(ethylene terephthalate) and polyimide foils were used as substrates. The permeation of the gases H 2 , N 2 , O 2 and CO 2 was measured and the reduction of the permeability coefficient was correlated to composition and density of the a-C:H films. The stoichiometry of the layers was analyzed using ion-beam techniques on films deposited onto silicon samples. The a-C:H/PET surfaces were analyzed using optical microscopy and atomic force microscopy (AFM). Multilayer structures comprising different types of a-C:H films were also investigated. A reduction of the permeability coefficient by 80% for hard, dense and 94% for soft, polymer-like layers was found. Surprisingly, the barrier efficacy of the coating decreases with increasing a-C:H film density. This unexpected result is attributed to the appearance of a network of deep cracks spread out over the whole coating.

Structural, chemical, mechanical and corrosion resistance characterization of TiCN coatings prepared by magnetron sputtering

Abstract A range of different chemical compositions of TiCxNy coatings were deposited by a magnetron sputter ion plating technique onto highspeed steel (M2) and a medium-carbon steel (1045). Variations of the reactive gas mixture, deposition temperature and substrate bias were carried out for each series of deposition, in order to observe the influence of these deposition parameters on chemical composition and on the X-ray diffraction and mechanical properties of the coatings. Rutherford backscattering spectrometry (RBS) and glow discharge optical spectroscopy (GDOS) techniques were used to analyze and characterize chemically the TiCxNy, layers, which could be under-stoichiometric, stoichiometric or over-stoichiometric, depending on the methane content in the gas mixture. Moreover, there was a composition variation within the film thickness for over-stoichiometric coatings. The residual stress characterization was carried out with grazing angle X-ray diffraction, using lattice distance versus sin2ψ plots. The coating hardness and adherence were observed by ultra-microhardness and scratch test analyses, respectively. The coatings were hard and presented a good adherence to the substrates. Coating stoichiometry affected the intensity of 111 and 200 X-ray diffraction lines, although the characteristic Ti(C,N) diffraction lines were able to be distinguished. The film structure was directly affected by its thickness, and the residual stress values were higher for thinner coatings (1 μm). Finally, the coating resistance to aggressive environments was observed by using electrochemical impedance spectroscopy (EIS). This result showed that an increasing carbon content in the coating decreased the corrosion resistance.

Growth and characterization of OLED with samarium complex as emitting and electron transporting layer

In this work, the growth and the characterization of new orange emitting triple-layer electroluminescent organic devices using vacuum deposited trivalent samarium complex [Sm(TTA)3(TPPO)2] as emission layer is described. The electroluminescence (EL) spectra of the devices show narrow bands arising from the 5G5/2→6HJ transitions (J=5/2, 7/2 and 9/2) of the Sm3+ ion with the hypersensitive 5G5/2→6HJ transitions as the prominent group. The hole transporting layer (HTL) was obtained using a thin film of 1-(3-methylphenyl)-1,2,3,4 tetrahydroquinoline-6-carboxyaldehyde-1,1′-diphenylhydrazone (MTCD), while the tris(8-hydroxyquinoline aluminum) (Alq3) is used as electron transport layer (ETL). Moreover, in order to use the Sm complex, two different kinds of OLEDs were prepared: the first one with a typical three layers architecture, MTCD/[Sm(TTA)3(TPPO)2]/Alq3, while the second one was a bi-layer device with an MTCD/[Sm(TTA)3(TPPO)2] design without the Alq3 ETL layer. In the last case, the EL emission was also observed, which indicates that the [Sm(TTA)3(TPPO)2] complex may be used as an electron transporting layer also.

Hard amorphous hydrogenated carbon-nitrogen films obtained by PECVD in methane-ammonia atmospheres

Abstract Hard a-C(N):H films were deposited onto Si(100) substrates by r.f. self-bias glow discharge in CH 4 NH 3 atmospheres (NH 3 content varying from 0% to 12.5%). The chemical composition of the films was determined by nuclear techniques, and the film structure was monitored by IR and Raman spectroscopies. The nitrogen incorporation into the films was found to be more than four times greater than that previously obtained with N 2 gas as the nitrogen source at the same partial pressure. Nitrogen incorporation up to levels of 11 at.% resulted in a 50% decrease in the internal compressive stress. Raman spectra showed that nitrogen incorporation increased the size or number of graphitic domains in the film, while exhibiting smaller changes in the I D / I G ratio. IR spectra showed the same trend as observed in N 2 -derived films, with an increasing presence of nitrogen-containing network terminating groups at the expense of C H groups.