  Persistent electrical polarized states are fundamentally important to the
electric industry as they can be used in the non-volatile memory, the
artificial neuromorphic network, and negative capacitors, making ultralow
energy consumption electronic devises possible. With the recent development in
low dimensional ferroelectric materials, emerging 2D out-of-plane ferroelectric
materials like MoTe2 have great potential for future development. Despite
previous phenomenological studies, the underlying microscopic origin of
ferroelectricity is still missing. Here, using density functional theory and
Wannier function methods, we reveal that the origin of ferroelectricity of
these transition metal dichalcogenides comes from the Jahn taller effect and
followed by a covalent bonding between transition metal atoms. Moreover, the
atypical electronic behavior of these monolayer AB2 (A=Cr, Mo, W; B=S, Se, Te)
TMDs compare to traditional improper ferroelectrics hints a strong electronic
origin of the polarization, suitable for future industrial applications.
