  Considering the difference of energy bands in graphene and silicene, we put
forward a new model of the graphene-silicene-graphene (GSG) heterojunction. In
the GSG, we study the valley polarization properties in a zigzag nanoribbon in
the presence of an external electric field. We find the energy range associated
with the bulk gap of silicene has a valley polarization more than 95%. Under
the protection of the topological edge states of the silicene, the valley
polarization remains even the small non-magnetic disorder is introduced. These
results have certain practical significance in applications for future valley
valve.
