Predicted crystal structures of superhard, low-density carbon polymers. (a–i) 3D carbon nanotubes polymers.^[80] (j) and (k) 3D graphyne polymers.^[83] (l) yne-diamond polymer.^[82] The structure of 3D-(2,2) carbon (a) is the same as bct-C₄,^[27] and (b) 3D-(3,0) carbon is actually the lonsdaleite structure. (a, b) sp³ carbons; (c–k): sp²-sp³ carbons. (l) sp-sp³ carbon. Due to the bonding difference, the carbon polymers possess distinct conductivity, such as insulating or semi-conducting characteristics with tunable band gaps (a–c) and novel (g) 1D, (h–i) 2D, and (d–f) 3D metallicities. The orange shadows represent insulating sp³-hybridized carbon layers, which interrupt the electronic conduction in 3D.