Direct Growth of 1D SWCNT/2D MoS2 Mixed-Dimensional Heterostructures and Their Charge Transfer Property

A unique mixed-dimensional van der Waals heterostructure can be formed by integrating one-dimensional (1D) and two-dimensional (2D) materials. Such a 1D/2D mixed-dimensional heterostructure will not only inherit the unique properties of 2D/2D heterostructures, but also has a variety of stacking configurations, offering a new platform to adjust its structure and properties. The combination of p-type 1D single-walled carbon nanotubes (SWCNTs) and n-type 2D molybdenum disulfide (MoS2) is one such example, possessing tunable properties. In situ chemical vapor deposition (CVD) is one of the most effective methods to construct 1D SWCNT/2D MoS2 mixed-dimensional heterostructures. There are several reports of successfully grown SWCNT/MoS2 heterostructures. The reports indicate that these heterostructures exhibit strong electrical and mechanical couplings between the SWCNTs and MoS2, making it suitable for the construction of high-performance electronic and optoelectronic devices. However, there are still several problems associated with the in situ CVD growth of SWCNT/MoS2 heterostructures. First, the growth mechanism of the 1D SWCNT/2D MoS2 heterostructure is unclear. We still do not know how the existence of small-diameter SWCNTs will affect the nucleation and growth process of MoS2. It is undetermined whether MoS2 flakes will grow above the preexisting SWCNTs or under them. Second, current studies all report the growth of MoS2 on a substrate sparsely covered by SWCNTs, which have a wide chirality distribution. Since the chirality of SWCNTs determines their physical properties and the density of SWCNTs significantly affects its performance in electronic devices, both the low density and wide chirality distribution of SWCNTs reported in these studies impose negative impacts on the interface behavior of SWCNT/MoS2 heterostructures and their performance in devices. Herein, we report the preparation of high-quality 1D SWCNT/2D MoS2 heterostructures by directly growing MoS2 on dense and narrow-chirality distributed SWCNTs on a silicon substrate. To achieve this goal, high-purity semiconducting SWCNTs with narrow chirality distributions were sorted from the raw arc-discharged SWCNTs, and then high-density SWCNT arrays or networks were formed on a silicon substrate by dip-coating. Through in-depth analyses of the surface morphology and structure of the nuclei, we found that MoS2 may prefer to grow under the SWCNTs and will grow much faster in the grooves between the SWCNTs to form a growth front. Therefore, an interesting "absorption-diffusion-absorption" growth mechanism has been proposed to explain the nucleation and growth process of SWCNT/MoS2 heterostructures. In addition, we confirm the presence of strong charge coupling in the mixed-dimensional heterostructure through Raman analysis. Carriers can be quickly transferred through the interface between the SWCNTs and MoS2, paving a way for the future design and fabrication of novel electronic and optoelectronic devices based on 1D/2D heterostructures.