Incredible carbon

Carbon is one of the most abundant elements at the Earth, providing eighteen percent of the mass in flora and  fauna. Different forms of carbon have been known since ancient times, but nowadays the importance of carbon materials in our lives is constantly increasing and difficult to overstate. The existence of numerous allotropic forms of carbon in the solid state gives rise to a wide variety of physical properties. Diamond and graphite are the most well-known types of crystalline carbon. In ideal graphite, carbon atoms are joined in planar atomic layers with a honeycomb structure. While the interactions between neighboring atoms in each layer demonstrate record strength, the interlayer connections via van der Waals interaction are very weak. The atomic arrangement in diamond may be considered a result of a perturbation to the graphite structure leading to the creation of additional links between the atomic layers. As a result of these links, diamond is the most rigid material despite some reduction of the interatomic bonding in comparison to graphite. There is also a wide variety of amorphous carbons with graphite-like (soot, carbon black, amorphous carbon films, etc.) or diamond-like (diamond-like carbon, nanocrystalline diamond, tetrahedral amorphous carbon, etc.) interatomic links.

During the last two decades a few previously unknown forms of well-ordered carbon were discovered. These new members of the carbon family, including fullerenes, carbon nanotubes, nanocones and nanowalls, mostly possess the graphite type of interatomic bond. The curved shapes of the atomic layers in these materials make their structures closed and provide unique properties. Together with nanometrically small diamond crystallites all these materials may be assigned to the class of nanocarbons.

One of the most usual and convenient ways to produce nanocarbon with controllable properties is chemical vapor deposition (CVD) from gas mixtures containing activated carbonaceous components. The remarkable flexibility of the CVD method allows one to employ the same technology for the production of materials with dramatically different properties [1] by changing the amount of carbonaceous gas in the mixture, activation power, etc. The availability of carbons with extremely different properties provides a diverse range of possible applications. For example, nanographite films produced by the CVD method exhibit excellent electron field emission properties and are attractive as efficient cold cathodes in vacuum electronics devices [2,3]. On the other hand, nanocarbons with the diamond type of interatomic bond demonstrate excellent mechanical, thermal, and optical properties. Recently, we have developed a method for producing micrometer sized single crystal diamonds with a regular pyramidal shape by using a combination of CVD growth and selective oxidation of textured diamond films [4]. The method allows for the mass production of diamond needles, which are attractive for various advanced applications; from superior AFM probes to quantum computers [5].

This month’s cover image shows a forest-like nanographite species inspected using a Zeiss Leo 1550 scanning electron microscope in backscattering mode operated with 5 kV accelerating voltage. The sample was prepared via CVD from a methanehydrogen gas mixture activated by a DC discharge in two steps: in the first stage nanodiamond globules were obtained on a Si substrate, and after that the CVD parameters were switched to those corresponding to nanographite formation. The carbon finally obtained exhibits long and sharp, nanometrically scaled tips and flakes with a graphite type structure covering the nanodiamond globules. This work represents a part of our efforts to combine nanographite and nanodiamond in the same carbon material to exploit advantages of their merging.

Further Reading
[1] A.N. Obraztsov, et al. Carbon 41 (2003) 836.
[2] A.N. Obraztsov, et al. Appl. Surf. Sci. 215 (2003) 214.
[3] P. Janhunen, et al. Rev. Sci. Instrum. 81 (2010) 111301.
[4] A.A. Zolotukhin, et al. Diamond Relat. Mater. 19 (2010) 1007.
[5] A.N. Obraztsov, et al. Rev. Sci. Instrum. 81 (2010) 013703.