With the rapid development of nanotechnology and microfabrication technology, miniaturized sensors and devices are constantly emerging in a vast number of applications in the fields of Internet of Things, sensor networks, big data, personal health systems, and artificial intelligence1–5. To date, these sensors and devices have mostly been powered by line cords, or by batteries and external chargers, and thus have had limited applications6,7 particularly in independent, sustainable, maintenance-free operations of implantable biosensors, remote and mobile environmental sensors, nano-/micro-scale robots, and portable/wearable personal electronics7–10. For continued use, these devices require small, wireless, portable, and sustainable power supplies. As a promising solution to these challenges, nanogenerators were first proposed in 20061 to convert weak and random mechanical energy into electricity. Since then, many types of nanogenerators have been proposed, such as piezoelectric nanogenerators (PENGs)1,11, triboelectric nanogenerators (TENGs)12, and electret-based microgenerators (EBMGs)13. More recently, the principle of generating direct current based on relatively sliding Schottky junctions is proposed3,14–19, namely Schottky generators (S-Gs) was proposed. Compared with previously proposed nanogenerators, S-Gs have a simpler structure, and some show higher current densities3,15,20. However, to the best of our knowledge, all S-G reported so far have failed to simultaneously achieve sufficiently high current density and sufficiently long lifetime for real-world applications3,15.
Fig. 1 Output current and friction measurement of Schottky superlubric generator (S-SLG) and comparison with an ordinary Schottky generator (S-G). a Structure of a graphite/n-Si S-SLG. b Optical microscopic image of a graphite/n-Si S-SLG. c Current maps in the first 2,000 cycles of a graphite/n-Si S-SLG with speed of under a controlled normal force of . d Relationship between friction (red) and current (blue) with sliding cycles of a graphite/n-Si S-SLG at different speeds. e Current maps in the first 56 cycles of an atomic force microscope (AFM) tip/n-Si ordinary S-G, and an illustration of the experimental setup. f Relationship between friction (red) and average current (blue) with sliding cycles of an AFM tip/n-Si ordinary S-G.
Our work highlight
The fundamental challenge of most reported S-Gs is rooted in the mechanism of generating current from friction-induced excitation3,19,22,23. However, there may exist another mechanism of generating current in a sliding Schottky joint, namely the mechanism of depletion layer establishment and destruction (DLED)15,16,24,25. This fundamental challenge can be solved and the DLED mechanism can be validated using a specific Schottky joint, which is in a state of structural superlubricity (SSL)—a state of ultralow friction and wearless between two solid surfaces26. Since the first realization of microscale SSL in the atmospheric environment in 201227, in addition to the realizations of high-speed SSL (25−293m/s)28,29, SSL has attracted wide-ranging interest in academic studies and practical applications for obtaining a revolutionary solution for friction and wear probems26. For example, as an application of SSL, several types of superlubric generators (SLGs) based on capacitors, triboelectric or electrets have been proposed30. SLGs are designed to achieve the maximum electric current density allowed by the dielectric material, which is three orders of magnitude higher than the maximum density of all reported TENGs31 and PENGs32, with nearly 100% conversion efficiency and long lifetime. In this study, we demonstrate a Schottky superlubric generator (S-SLG) as the physical prototype of superlubric generators that can not only generate a stable and high current density of ~210 Am−2 and power density of ~7 Wm−2, but more importantly, achieve a long lifetime of at least 5,000 cycles while maintaining stable high electrical current density (~119 Am−2). By excluding the mechanism of friction-induced excitation in our S-SLG, it is revealed that there must be other mechanism(s) of generating currents in Schottky generators.We further demonstrate through finite element simulations that DLED is the most likely mechanism of generating current in S-SLGs.
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