  Layered van der Waals transition metal dichalcogenides (TMDCs), generally
composed of three atomic X-M-X planes in each layer (M = transition metal, X =
chalcogen), provide versatile platforms for exploring diverse quantum
phenomena. In each MX2 layer, the M-X bonds are predominantly covalent in
nature, as a result, the cleavage of TMDC crystals always occurring between the
layers. Here we report the controllable realization of fractional-layer WTe2
via an in-situ scanning tunnelling microscopy (STM) tip manipulation technique.
By applying STM tip pulses, hundreds of the topmost Te atoms are removed to
form a nanoscale monolayer Te pit in the 1T'-WTe2, thus realizing a brand-new
2/3-layer WTe2. Such a unique configuration undergoes a spontaneous atomic
reconstruction, yielding an energy-dependent unidirectional charge-density-wave
state with the wavevector and geometry quite distinct from that of pristine
1T'-WTe2. Our results expand the conventional understanding of the TMDCs and
are expected to stimulate the research on extraordinary structures and
properties based on fractional-layer TMDCs.
