Langenhorst and Campione report that carbon is found in nature in a huge variety of allotropic forms and recent research in materials science has encouraged the development of technological materials based on nanocarbon. Carbon atoms with sp2 or sp3 hybridization can be thought of as building blocks. Following a bottom-up approach, the authors show how graphene and diamond molecules are built up and how their properties vary with size, reaching an upper limit with bulk graphite and diamond.
Carbon atoms with sp2 hybridization give rise to an impressive number of different materials, such as carbon nanotubes, graphene nanoribbons, porous carbon and fullerene. As in any crystalline phase, the crystal structures of natural carbon allotropes (i.e. graphite and diamond) contain various types of imperfections. These so-called lattice defects are classified by their dimensions into 0D ( point), 1D (line), 2D ( planar) and 3D (volume) defects.
Lattice defects control the physical properties of crystals and are often a fingerprint of the geological environment in which they formed and were modified. Direct observations of lattice defects are commonly accomplished by transmission electron microscopy. The authors present and discuss the ideal and real structures of carbon allotropes, the energetics of lattice defects and their significance in understanding geological processes and conditions.
Notwithstanding its abundance on Earth, which classifies it as a rare element (concentration c. 0.01 wt%) carbon is found in a huge variety of forms, spanning beyond all experimentally accessible stability fields. This variety of forms is a result of its electronic structure, which allows the formation of stable chemical bonds in various configurations. The four valence electrons (two s and two p electrons) of carbon occupy the L shell, while the inner K shell is occupied by two s electrons. Figure 1 shows that the binding energies of the p electrons in carbon are very close to one another, a fundamental assumption in the interpretation of chemical bonds in the framework of hybrid atomic orbitals. The linear combination of valence carbon orbitals results in the formation of hybrid atomic orbitals with a symmetry that can give rise to 1D, 2D and 3D polymeric structures. The possibility of forming 1D chains (referred to as polymers) has been exploited in nature to build the tissues of living beings and by humans to develop plastics. 2D carbon structures (e.g. polycyclic aromatic hydrocarbons) dominate the world of aromatic compounds, which have a fundamental role in both the geosphere and in the development of functional materials.
3D carbon structures (e.g. diamondoids) are distinguished by their inertness and stability, making such carbon forms ubiquitous throughout the universe. The topic of carbon allotropes is so overwhelmingly broad that it is beyond the scope of this paper to provide a comprehensive review on all aspects of carbon materials. The following sections provide some insight into the principles of the bonding, polymerization and structural arrangements of carbon allotropes. Following a bottom-up approach, we show how the progressive assembly of carbon atoms and low-dimensional building blocks brings about the formation of allotropic forms with different dimensionalities. The authors discuss their relative stability, including a description of the defects found in the crystalline phases of carbon and their significance in identifying their geological origin.
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