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Probing six-fold ring clusters in alloyed sp2 dominated carbon thin films by the means of Raman spectroscopyAbrasonis, G.; Krause, M.; Gago, R.; Kolitsch, A.; Möller, W.
The six-fold ring clustering in sp2-dominated pure carbon (C), nitrogenated carbon (CNx) and nickel alloyed carbon (C:Ni) thin films is investigated by Raman spectroscopy as a function of substrate temperature. Films have been grown by direct and dual ion beam sputtering, ion beam assisted evaporation and magnetron sputtering, resulting in amorphous, graphite-like, fullerene-like, paracyanogenlike, and in the case of C:Ni films nanocomposite - structures.
Raman spectra in the wave-number region of 9002000 cm−1 exhibit usually two main peaks positioned at ~1360 and ~15601590 cm−1 which are denoted conventionally by D and G, respectively, the G peak corresponding to in-plane bond stretching vibrations (it can be present in aromatic clusters as well as in chain structures) and the D peak corresponding to breathing vibrations of aromatic rings (it can be present only in ring structures). The total intensity of these peaks is proportional to the amount of the sp2 phase, while their relative ratio provides the information on the six-fold ring cluster size.
The results show that each type of atomic arrangement results in a characteristic set of the Raman spectra parameters, which describe the degree of aromatic clustering, bond length and angle distortion and order in sixfold structures1.
In the case of pure C films, the atomic structure evolves with substrate temperature from a disordered network to nanocrystalline planar graphitic configurations, with a progressive promotion in size and ordering of sixfold ring clusters1.
Nitrogen incorporation favors the promotion of sixfold rings in highly disordered networks produced at low temperatures, but precludes the formation of extended graphite-like clusters at elevated substrate temperatures (>400°C). In the latter case, N introduces a high degree of disorder in sixfold ring clusters and enhances the formation of a fullerene-like microstructure1.
The presence of Ni slightly favours the six-fold ring clustering at low temperatures (<300°C), while Raman spectra at higher temperatures have similar features for both C and C:Ni films. For deposition temperatures lower than 300°C, the the absolute intensity of the D-G band decreases by more than one order of magnitude despite the similar total amount of deposited material and one additional Raman line at around 1100 cm-1 appear indicating the presence of new carbon structures in the film. This correlates with the Ni segregation into nickel carbide or nickel nanoparticles occurring at T~300°C, thus segregated Ni and C phases may undergo separate ordering paths at higher temperatures. In contrast, a mixed C-Ni phase is formed at lower temperatures and the precipitation C sp2 phase is hindered.
1. G. Abrasonis, R. Gago, M. Vinnichenko, U. Kreissig, A. Kolitsch, and W. Möller, Phys. Rev. B 73, 125427 (2006).
4th International Workshop on Nanoscale Spectroscopy and Nanotechnology, 17.-21.09.2006, Rathen, Germany