  We investigate van der Waals (vdW) heterostructures made of germanene,
stanene or silicene with hexagonal Boron Nitride (h-BN). The intriguing
topological properties of these buckled honeycomb materials can be maintained
and further engineered in the heterostructures, where the competition between
the substrate effect and external electric fields can be used to control the
tunable topological phase transitions. Using such heterostructures as building
blocks, various vdW topological domain walls (DW) are designed, along which
there exist valley polarized quantum spin Hall edge states or
valley-contrasting edge states which are protected by valley(spin)- resolved
topological charges and can be tailored by the patterning of the
heterojunctions and by external fields.
