  Property by design is one appealing idea in material synthesis but hard to
achieve in practice. A recent successful example is the demonstration of van
der Waals (vdW) heterostructures,1-3 in which atomic layers are stacked on each
other and different ingredients can be combined beyond symmetry and lattice
matching. This concept, usually described as a nanoscale Lego blocks, allows to
build sophisticated structures layer by layer. However, this concept has been
so far limited in two dimensional (2D) materials. Here we show a class of new
material where different layers are coaxially (instead of planarly) stacked. As
the structure is in one dimensional (1D) form, we name it "1D vdW
heterostructures". We demonstrate a 5 nm diameter nanotube consisting of three
different materials: an inner conductive carbon nanotube (CNT), a middle
insulating hexagonal boron nitride nanotube (BNNT) and an outside
semiconducting MoS2 nanotube. As the technique is highly applicable to other
materials in the current 2D libraries,4-6 we anticipate our strategy to be a
starting point for discovering a class of new semiconducting nanotube
materials. A plethora of function-designable 1D heterostructures will appear
after the combination of CNTs, BNNTs and semiconducting nanotubes.
