Control of the interlayer twist angle in two-dimensional (2D) van der Waals (vdW) heterostructures enables one to engineer a quasiperiodic moire superlattice of tunable length scale. In twisted bilayer graphene (TBG), the simple moire superlattice band description suggests that the electronic band width can be tuned to be comparable to the vdW interlayer interaction at a magic angle, exhibiting strongly correlated behavior. However, the vdW interlayer interaction can also cause significant structural reconstruction at the interface by favoring interlayer commensurability, which competes with the intralayer lattice distortion. Here we report the atomic scale reconstruction in TBG and its effect on the electronic structure. We find a gradual transition from incommensurate moire structure to an array of commensurate domain structures as we decrease the twist angle across the characteristic crossover angle, $theta_c$ ~1deg. In the twist regime smaller than $theta_c$ where the atomic and electronic reconstruction become significant, a simple moire band description breaks down. Upon applying a transverse electric field, we observe electronic transport along the network of one-dimensional (1D) topological channels that surround the alternating triangular gapped domains, providing a new pathway to engineer the system with continuous tunability.
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