Effect of fast Joule heating on the properties of carbon films formed by methane plasma deposition

Fast Joule heating of amorphous carbon films formed by deposition in methane plasma was performed by electric discharge of a capacitor bank with a total capacity of 180 mF charged to a voltage of 100 to 300 V. The methods of Raman spectroscopy, scanning electron microscopy, X-ray energy-dispersive spectroscopy and current-voltage characteristics were used for the  study. The Raman spectra of the samples after fast Joule heating clearly show G-, 2D- and  D-peaks characteristic of graphene structures with sp2-hybridized bonds. Analysis of the  spectra showed that the intensity ratio of the 2D- and G-peaks and the D- and G-peaks  indicates a high quality of the formed flakes of turbostratic graphene structures. The most  effective, in terms of the formation of single-layer graphenes, was fast Joule heating, carried  out as a result of the discharge current of capacitors charged to a voltage of U = 160 V.  Electron microscopy methods have established that fast thermal heating during electric  discharge leads to a significant transformation of the state of the atomically smooth surface of the amorphous carbon film. As a result, spherical particles of about 1 μm in size are formed on the surface, which have a granular structure with grain sizes of about 100 nm. On the other hand, spherical particles are collected in agglomerations of up to several micrometers in size. Elemental analysis carried out by energy-dispersive spectroscopy, in addition to carbon,  showed a high oxygen content in spherical particles. The most likely cause of this phenomenon  may be the absorption of oxygen by the formed graphene flakes. Studies of the wettability  of the surface of the samples before and after Joule heating showed an increase in  hydrophobicity. The reason for the emergence of water-repellent properties may be the  “lotus effect” caused by the formation of spherical particles up to 1 μm in size and their larger conglomerates on the film surface. A radical decrease in the electrical resistance of the  original amorphous film from values corresponding to an insulator (R> 1 TΩ) to units of kΩ  per square of surface was found. The increase in electrical conductivity is explained by the transition of carbon from an amorphous state to an electrically conductive graphene-like structure.

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