Atomistic simulations of low-density nanoporous materials: Carbon nanofoams

Atomistic simulations give new insights into the properties of carbon nanofoams, a low-density nanoporous and nanostructured material. It is shown that agglomeration, crosslinking, and deformation, processes that are often ignored in theoretical descriptions of nanomaterials, have a dramatic effect on their properties. A most striking finding is that nanofoams composed exclusively of semiconducting nanostructures turn out to be metallic with high conductivity and optical absorptance. The underlying mechanism may explain relevant observations in other nanoporous materials. The simulated structures contain trivalent carbon atoms, suggested earlier to be a major source of magnetism in these materials.

Figure 1

(a, b) Models of CNF networks, with periodic boundary conditions, composed of ${\text{C}}_{168}$ and $\text{G}8$ units, respectively. Grey, orange, and magenta denote $s{p}^2$, $s{p}^3$, and $s{p}^1$ bonding, respectively. (c) The three schwarzite fundamental units used in this work. (d) An eight-membered ring in a ${\text{C}}_{168}$ CNF. An atom with significant contribution to the electronic density of states (EDOS) at the Fermi level is highlighted. (e) Site contributions to the wave function of a $\pi$ state at the Fermi level in the CNF of panel (a). (f) A trivalent carbon atom in a heptagon. Thin and thick sticks denote single and double bonds, respectively.

Figure 2

Reduced radial distribution functions $G\left(r\right)$ of ${\text{C}}_{168}$ and $\text{G}8$ foams. Arrows denote intense peaks indicative of medium-range order. The inset shows the $G\left(r\right)$ of a low-density a-C network.

Figure 3

(a) EDOS of ${\text{C}}_{168}$ CNFs with various densities. Vertical lines indicate a typical distribution of localized states from inverse participation ratio (IPR) analysis. The inset shows the EDOS of the periodic ${\text{C}}_{168}$ structure. (b) EDOS of P8 and G8 CNFs and the localized states. The inset shows the EDOS of the respective periodic structures.

Figure 4

(a) Conductivity of two representative foams compared to that of a graphene layer, of a low-density a-C cell, and of a tetrahedral a-C cell. (b) Optical absorption of the above materials.