  One-dimensional (1D) metallic mirror-twin boundaries (MTBs) in monolayer
transition metal dichalcogenides (TMDCs) exhibit a periodic charge modulation
and provide an ideal platform for exploring collective electron behavior in the
confined system. The underlying mechanism of the charge modulation and how the
electrons travel in 1D structures remain controversial. Here, for the first
time, we observed atomic-scale structures of the charge distribution within one
period in MTB of monolayer MoTe2 by using scanning tunneling
microscopy/spectroscopy (STM/STS). The coexisting apparent periodic lattice
distortions and U-shaped energy gap clearly demonstrate a Peierls-type charge
density wave (CDW). Equidistant quantized energy levels with varied periodicity
are further discovered outside the CDW gap along the metallic MTB. Density
functional theory (DFT) calculations are in good agreement with the gapped
electronic structures and reveal they originate mainly from Mo 4d orbital. Our
work presents hallmark evidence of the 1D Peierls-type CDW on the metallic MTBs
and offers opportunities to study the underlying physics of 1D charge
modulation.
