Ultrafast optoelectronic processes in 1D radial van der Waals heterostructures: carbon, boron nitride and MoS2 nanotubes with coexisting excitons and highly mobile charges.

Heterostructures built from 2D, atomically-thin crystals are bound by the van der Waals force, and exhibit unique optoelectronic properties. Here, we report the structure, composition and optoelectronic properties of 1D van der Waals heterostructures comprising carbon nanotubes wrapped by atomically-thin nanotubes of boron nitride and molybdenum disulfide (MoS2). The high quality of the composite was directly evidenced on the atomic scale by transmission electron microscopy, and on the macroscopic scale by a study of the heterostructure's equilibrium and ultrafast optoelectronics. Ultrafast pump-probe spectroscopy across the visible and terahertz frequency ranges identified that, in the MoS2 nanotubes, excitons coexisted with a prominent population of free charges. The electron mobility was comparable to that found in high-quality atomically-thin crystals. The high mobility of the MoS2 nanotubes highlights the potential of 1D van der Waals heterostructures for nanoscale optoelectronic devices.