# 摘要

We present the experimental synthesis of one-dimensional (1D) van der Waals heterostructures, a class of materials where different atomic layers are coaxially stacked. We demonstrate the growth of single-crystal layers of hexagonal boron nitride (BN) and molybdenum disulfide (MoS2) crystals on single-walled carbon nanotubes (SWCNTs). For the latter, larger-diameter nanotubes that overcome strain effect were more readily synthesized. We also report a 5-nanometer–diameter heterostructure consisting of an inner SWCNT, a middle three-layer BN nanotube, and an outer MoS2 nanotube. Electron diffraction verifies that all shells in the heterostructures are single crystals. This work suggests that all of the materials in the current 2D library could be rolled into their 1D counterparts and a plethora of function-designable 1D heterostructures could be realized.

# 前言

The demonstration of two-dimensional (2D) van der Waals (vdW) heterostructures[1–3]—in which atomic layers are stacked on each other and different 2D crystals are combined beyond symmetry and latticematching—represents a way of manipulating crystals to enable both the exploration of physics not observable in conventional materials and device applications.[4–8] These 2D heterostructures have been fabricated by transferring preprepared layers (transfer approach)[5, 9] or by synthesizing layers onto a base layer (synthesis approach).[10] Whether such artificial materials and interfaces can be fabricated in other dimensions remains an open question. In 1D materials, for example, an ideal vdW heterostructure would be a coaxial structure with different types of nanotubes. Such ideal structures have been investigated in theoretical studies[11, 12] and would appear to require a synthesis approach. However, experimental attempts to fabricate coaxial nanotube structures have yielded only amorphous or very poorly crystallized coatings.[13, 14]

We demonstrate the experimental discovery and controlled fabrication of true 1D vdW heteronanotubes. A typical structure was $4$ to $5\ \rm{nm}$ in diameter but contained three different shells: an inner carbon nanotube (CNT), a middle hexagonal boron nitride nanotube (BNNT), and an outer molybdenum disulfide (MoS2) nanotube. Electron diffraction (ED) and many other characterizations were used to confirm that each shell in this structure was a seamless, perfect nanotube that realized the heteronanotubes studied in theoretical models. The heterostructures formed through an open-end growth mode that has rarely been observed in previous 1D nanostructure growth. We outline some basic geometric principles that governed the formation of these 1D vdW heterostructure nanotubes, including the absence of structural correlation between inner and outer shells and the requirement of a threshold diameter for MoS2 nanotubes.

In this study, the base structure, a singlewalled carbon nanotube (SWCNT),[15] was chosen as the starting material for several reasons. It is, so far, the best-studied 1D material and can be synthesized in many controlled geometries. Also, a SWCNT can be metallic or semiconducting, which means it could serve as the electrode or channel material for a heteronanotube device. The typical SWCNTs used in this study were $1$ to $2\ \rm{nm}$ in diameter and a few micrometers in length and were self-suspended as a random network.[16] Schematics comparing the 2D and 1D vdW heterostructures are presented in Fig. 1, A and B.

# 结论

We have extended the concept of vdW heterostructures to 1D materials. In these coaxial heteronanotubes, both cores and shells are single crystalline and form a seamless structure. We showed the controlled fabrication of SWCNT-BNNT and SWCNT-BNNT-MoS2 coaxial structures with diameters $\lt 5\ \rm{nm}$. We also developed some basic rules governing the fabrication of 1D heterostructures, including the absence of shell-shell epitaxial structure correlation and the requirement of a threshold diameter for MoS2 nanotubes. This approach is likely extendable to other layered materials[35–37] and yields a large number of combinations, and 1D vdW heterostructures could host distinctive physics arising from curvature and diameter confinement.