  A serial of two dimensional titanium and zirconium trichalcogenides
nanosheets MX3 (M=Ti, Zr; X=S, Se, Te) are investigated based on
first-principles calculations. The evaluated low cleavage energy indicates that
stable two dimensional monolayers can be exfoliated from their bulk crystals in
experiment. Electronic studies reveal very rich electronic properties in these
monolayers, including metallic TiTe3 and ZrTe3, direct band gap semiconductor
TiS3 and indirect band gap semiconductors TiSe3, ZrS3 and ZrSe3. The band gaps
of all the semiconductors are between 0.57~1.90 eV, which implies their
potential applications in nano-electronics. And the calculated effective masses
demonstrate highly anisotropic conduction properties for all the
semiconductors. Optically, TiS3 and TiSe3 monolayers exhibit good light
absorption in the visible and near-infrared region respectively, indicating
their potential applications in optical devices. In particular, the highly
anisotropic optical absorption of TiS3 monolayer suggests it could be used in
designing nano optical waveguide polarizers.
