Layered WS₂ Nanotubes/Graphene Nanocomposite as Lightweight and Flexible Optoelectronic Device

Wearable electronics and optoelectronics are topics that have recently been gathering more and more attention for the development of, for example, portable health monitoring devices or portable solar cells.
Here we explore a novel stretchable optoelectronic device with at the core a photoactive junction made by a film of semiconducting WS₂ nanotubes (NTs) and a layer of semimetal graphene. The NTs are used to take advantage of their optical properties whereas graphene is mainly used as a transparent electrode. The device can be encapsulated by two flexible and transparent polyethylene (PE) layers that guarantee protection.
Firstly, we investigate the mechanical properties of WS₂ nanotube/graphene nanocomposite by using molecular dynamics simulations. In particular, we show that the presence of WS₂ nanotubes slightly affect the mechanical properties of the graphene layer by reducing the fracture strength and Young's modulus by 15% and 53% respectively, Yet, the overall mechanical properties of the composite still guarantee enough flexibility to the device to make it a good candidate material for flexible devices [Wu F. et al. J. Phys.: Condens. Matter 36 (2024) 225301].
Secondly, we experimentally characterize the ability of a single WS₂ NT to act as photodetector. Notably, we find self-powered photoconduction driven by the photovoltaic effect, with a photoresponsivity R ≈ 10 mAW⁻¹ at 2 V drain bias and room temperature. Moreover, we show that the device can be used for data storage applications when gate and light pulses are combined to demonstrate an optoelectronic memory with four well-separated states [Pelella A. et al. Small (2024) 2403965]. Building on this knowledge, next, we move to a film of WS₂ NTs to test them as active material in interdigitated photodetectors, reporting fast response time and high sensitivity that increase with laser power [Durante O. et al. 2023 IEEE Nanotechnology Materials and Devices Conference, 676-680].
Finally, we study a photovoltaic device made of a film of randomly oriented WS₂ NTs sandwiched by two graphene layers. The whole device is encapsulated by two transparent PE films. The photovoltaic response of the device is tested by using three different laser wavelengths (405 nm, 520 nm, 630 nm) and incident power (15.9 mW, 4.2 mW, 2.6 mW). Our results show a photovoltaic behaviour, with an open circuit voltage and a short circuit current that both increase with the increasing incident power. The proposed device offers three main advantages over current flexible solar cell technologies. Firstly, owing to its solid-state nature and making use only of inorganic materials for its core elements, performance of our device will not degrade in hot or humid environments. Secondly, this technology is greener than, for example, the perovskite solar cells including Pb, which makes difficult their disposal once the device lifetime is over. Thirdly, because WS₂ is sensitive to several harmful gases such as H₂S, NO₂, volatile organic compounds and ammonia [Tang H. et al. ACS Appl. Mater. Interfaces 11, 40850 (2019); Asres G.A. et al. Nano Res. 11, 4215 (2018); Li X. et al. Sens. Act. B 240, 273 (2017); Gu D. Sens. Act. B 256, 992 (2018)] as well as to UV light [Zeng L. et al. Scientific Rep. 6, 20343 (2016); Jia C. et al. Nanoscale 12, 4435 (2020)] the technology we propose could be developed even further in the future. For example, this device could work as a self-powered gas and/or light sensor.
This research is the result of a multi-year project (MYP) sponsored by the Science for Peace and Security program under Grant id. G5936.