Intelligent materials with adaptive response to external stimulation lay foundation to integrate functional systems at the material level. Here, with experimental observation and numerical simulation, we report a delicate nano-electro-mechanical-opto-system naturally embedded in individual multiwall tungsten disulfide nanotubes, which generates a distinct form of in-plane van der Waals sliding ferroelectricity from the unique combination of superlubricity and piezoelectricity. The sliding ferroelectricity enables programmable photovoltaic effect using the multiwall tungsten disulfide nanotube as photovoltaic random-access memory. A complete “four-in-one” artificial vision system that synchronously achieves full functions of detecting, processing, memorizing, and powering is integrated into the nanotube devices. Both labeled supervised learning and unlabeled reinforcement learning algorithms are executable in the artificial vision system to achieve self-driven image recognition. This work provides a distinct strategy to create ferroelectricity in van der Waals materials, and demonstrates how intelligent materials can push electronic system integration at the material level.

The synthesis of high-quality WS₂ and more so of MoS₂ (multiwall) nanotubes in substantial amounts from oxide precursors is a very challenging and important undertaking. While progress has been offered by a recent report, the present work presents another step forward in the synthesis of MoS₂ nanotubes with a narrow size distribution and better crystallinity than before. Williamson–Hall analysis of the X-ray diffraction data is used to analyze the crystallinity and strain in the nanotubes. This analysis shows that the crystallinity and average diameter of the WS₂ and MoS₂ nanotubes reported here (type II) are better than those obtained according to the previous methods (type I). Size selection by centrifugation reported by others is used here to prepare several fractions of WS₂ and MoS₂ nanotubes according to their average diameter. The high refractive index of WS₂ and more so MoS₂ enables the nanotubes to trap light by total internal reflection, turning them into nanocylindrical resonators and thereby supporting cavity-mode resonances. The extinction, net absorption, and transient absorption of suspensions of MoS₂ and WS₂ nanotubes of different (average) diameters were investigated. A strong coupling effect between optical cavity modes and the A and B excitons was observed for the WS₂ and MoS₂ nanotubes with diameters above 80 and 60 nm, respectively. These conclusions are also supported by transient absorption measurements. Finite-difference time-domain (FDTD) calculations support the experimental findings, confirming the strong coupling effect in the WS₂ and MoS₂ nanotubes. These results are important not only for their own sake but also because they may bear on the new photocatalytic applications of such nanotubes.

Due to their favourable and rich electronic and optical properties, group-VI-B transition-metal dichalcogenides (TMDs) have attracted considerable interest. They have earned their position in the materials portfolio of the spintronics and valleytronics communities. The electrical performance of TMDs is enhanced by rolling up the two-dimensional (2D) sheets to form quasi-one-dimensional (1D) tubular structures. The fabrication of p-n junctions out of these tubular TMDs would boost their potential for optoelectronic devices as such junctions represent a fundamental building block. Here, we report the realization of a p-n junction out of a single, isolated WS ₂ -nanotube (WS ₂ -NT). Light-emitting diode operation and photovoltaic behaviour were observed based on such p-n junctions. The emitted light as well as the photovoltaic effect exhibit strong linear polarization characteristics due to the quasi-1D nature. The external quantum efficiency for the photovoltaic effect reaches a value as high as 4.8%, exceeding by far that of 2D TMDs and even approaching the internal quantum efficiency of the 2D TMDs. This efficiency improvement indicates that TMD nanotubes are superior candidates over 2D TMDs for optoelectronic applications.