The dp type π-bond and chiral charge density waves in 1T-TiSe2

Based on the atomic electronic configuration and Ti-Se coordination, a valence bond model for the layered transition metal dichalcogenide (TMDC) 1T-TiSe2 is proposed. 1T-TiSe2 is viewed as being composed of edge-sharing TiSe4-plaquettes as TiSe2-ribbon chains in each layer via a directional valence shell electron distribution as chemical bonds, in contrast to the conventional layer view of face-sharing TiSe6-octahedra. The four valence electrons per Ti in the hybridized dsp(2)-orbitals of square coordination form sigma-bonds with the four nearest neighbor Se atoms in the chain. The electrons in the lone pair of the Se-4p(z) orbital are proposed to form a dp type -bond via side-to-side orbital overlap with the empty Ti-3d(xz)/3d(yz) orbitals within each chain, which is positively supported by quantum chemistry calculations. A study of electron energy loss spectroscopy (EELS) with transmission electron microscopy (TEM) for 1T-TiSe2 is presented to show an energy loss near approximate to 7 and approximate to 20 eV, which confirms the existence of collective plasmon oscillations with the predicted effective electron numbers for the - and ( + sigma)-bond electrons, respectively.