First principles study of electronic structure of x-form phthalocyanine crystals doped with one-dimensional iodine atomic chains

The x-form phthalocyanine (Pc) crystal is composed of a square-lattice arrangement of one-dimensional and double-period molecular chains with molecular planes normal to the stacking direction, and doped iodine (I) atomic chains between these molecular chains are known to induce the insulator-metal transition. Using the van der Waals density functional method, we investigate the electronic structure of a single x-form silicon Pc (x-SiPc) chain and the x-SiPc crystal undoped and doped with I atomic chains in a comparative manner. Although a SiPc molecule has a Si pz derived orbital just above the LUMO, the aligned Si atoms in x-crystals, each of which is at the center of the molecular plane, dimerize in the stacking direction, which prevents formation of a Si metallic band. In a single SiPc chain, two molecules in each primitive unit cell are stacked face-to-face with a staggering angle of 45°. However, when these molecular chains aggregate to create x-crystals, the staggering angle deviates from 45° to about 40° to form H–H bonding orbitals like H2 molecules between neighboring molecular planes in the lateral direction. Doping of the I atomic chains converts half-filling of the doubly degenerate bands to a lower band occupancy, which corresponds to the insulator-metal transition observed experimentally. The equally spaced I atomic chains create a metallic band due to pz-orbital overlapping with an effective-mass ratio of 0.15. Although the SiPc chains operate to create equally spaced I atomic chains, the effect of I atoms trying to trimerize is larger. This trimerization prevents pz orbitals of I atoms from making a metallic band.