© The Author(s) 2024. Published by Science Press and EDP Sciences.

INTRODUCTION

Semiconductor science and technology have played a transformative role in human civilization’s progress, serving as a significant benchmark of scientific advancement and societal evolution. Over nearly two centuries, scientists have consistently revealed the unique properties of semiconductor materials interacting with heat, light, electricity, magnetism, and stress, giving rise to a series of landmark semiconductor effects. Researchers, through exploration of semiconductor effects such as the thermoelectric effect (1834) and the photovoltaic effect (1839), have developed a range of energy technologies that effectively harness energy from natural environments [1–3]. Thermoelectric generators and photovoltaic power generation play a vital role in capturing and converting ambient energy sources such as illumination and industrial waste heat [4,5]. The effective use of semiconductor energy technologies can convert energy from various physical field environments into electricity, improving energy conversion efficiency and expanding the applicability of power generation devices. In 2012, the triboelectric nanogenerator (TENG) rapidly became widely researched for harvesting energy from mechanical motions in everyday life to natural resources [6,7]. However, conventional TENGs face substantial limitations due to their inherently low current densities (nA cm⁻²–μA cm⁻²) [8,9]. To address these constraints, researchers began exploring semiconductor-based TENG designs in 2018 and found these TENG exhibits direct current output characteristics [10,11]. In 2019, Wang and Wang [12] coined the term “tribovoltaic effect” by analogizing it to the photovoltaic effect based on this new phenomenon. In 2020, Zhang et al. [13] defined the tribovoltaic effect as the generation of direct current voltage and current by the mechanical friction on semiconductor interface. Tribovoltaic nanogenerators (TVNGs) exhibit improved attributes of high current densities (mA cm⁻²–A cm⁻²) and low resistances (Ω–kΩ) [14–16]. The tribovoltaic effect, as an emerging semiconductor energy harvesting technology, has been extensively studied and reviewed, covering the output characteristics and principles of various types of TVNGs. However, semiconductor interfaces are typically sensitive to various physical fields such as illumination, temperature, and humidity. These factors can significantly affect the output performance of the tribovoltaic effect [17–19]. Environmental multi-physics coupled tribovoltaic effect currently requires urgent discussion and investigation. The environmental multi-physics coupled tribovoltaic effect necessitates immediate attention and thorough investigation as an energy harvesting technology due to its significant implications [19].

Under illumination, semiconductor materials generate photogenerated charge carriers, increasing the carrier concentration [20,21]. Under the built-in electric field of the TVNG, both photogenerated and triboelectrically generated charge carriers move directionally, creating an electric signal. TVNGs can harvest both light and mechanical energy, converting both into electrical power. The coupling between illumination and mechanical friction significantly impacts the electric signal, a phenomenon known as the tribo-photovoltaic coupling effect. During the relative sliding motion at the TVNG interface, some energy inevitably dissipates as heat, creating a temperature gradient within the semiconductor. This causes majority of carriers to move from the hot end to the cold end, forming a thermoelectric current [22,23]. Alongside this, triboelectrically generated carriers also contribute to charge transport, resulting in a tribo-thermoelectric coupling effect. Additionally, external high temperatures can increase the excitation of carriers in the semiconductor, while temperatures below room temperature can enhance carrier mobility [24]. These changes affect the tribovoltaic effect by directly influencing the internal carrier dynamics of the material. In high-humidity environments, water molecules at the interface can enhance the semiconductor’s surface states and reduce contact resistance, significantly improving triboelectric performance. Liquids can act both as frictional materials and interfacial media. Adding functional liquid media between TVNG interfaces can boost electrical output while reducing device wear, thereby extending lifespan, and showcasing the versatility of TVNGs [25,26]. Gaseous environments impact the semiconductor friction interface by inducing oxidation or corrosion, influencing interfacial charge transfer through gas ionization, and altering environmental temperature and humidity. These factors affect charge generation and transfer, as well as tribological performance [27,28]. Typically, inert gases like N₂ effectively maintain interface stability during friction. Light and temperature impact triboelectric signals by modifying carrier transport in semiconductors. Additionally, liquids and gases change the triboelectric signals by affecting the semiconductor friction interface. The tribovoltaic effect represents a new paradigm of charge excitation mechanism in semiconductors, arising from the complex coupling of friction with multiple physical fields (Figure 1). According to the current research, several popular energy harvesting technologies are summarized and compared (Table 1). We can see that multi-physics coupled TVNG has more advantages. Therefore, a comprehensive summary of multi-physics coupled energy harvesting based on the tribovoltaic effect is essential.

Figure 1 Outline illustration of the review of tribovoltaic effect coupled energy harvesters based on environmental multi-physics.

This review aims to elucidate the mechanism and response characteristics of the tribovoltaic effect in multi-physics environments, including the application of tribovoltaic devices under multi-physical conditions. Specifically, we emphasize the synergistic coupling of the tribovoltaic effect with other energy harvesting mechanisms, such as the tribo-photovoltaic coupling effect and the tribo-thermoelectric coupling effect. This coupling interaction not only enables the devices to adapt to the environment but also contributes to the enhancement of triboelectric output. The output performance of TVNG in liquid environments shows promising applications, particularly demonstrating enhanced triboelectric performance in high-humidity environments. TVNG exhibits different triboelectric characteristics under different environmental atmospheres. Additionally, we highlight the application of TVNG in capturing various forms of mechanical energy from the surrounding environment under multi-physical conditions, including wind, raindrops, ocean waves, and illumination (Figure 1). Research on the tribovoltaic effect involves multi-physics coupling, integrating interactions across various physical dimensions and energy levels. Such coupling strategies not only enhance overall energy utilization efficiency but also address the issue of unstable power output commonly encountered in single-mode energy conversion processes. This review presents the future challenges and prospects in the field of multi-physics coupled tribovoltaic effects. We hope to contribute to the advancement of this field and the realization of its full potential in addressing the growing demand for sustainable energy solutions.

CHARACTERISTICS AND MECHANISM OF TRIBOVOLTAIC NANOGENERATOR

As we know, traditional TENGs generate alternating current (AC) signals based on contact electrification and electrostatic induction through relative motion between objects. TENGs typically employ insulating polymer materials as triboelectric pairs [12]. When two materials with different electronegativities come into contact, electrostatic induction occurs at the interface. Upon separation, electrons flow through an external circuit, creating an electric signal. This periodic contact-separation process results in AC output. In contrast, TVNGs based on semiconductor interfaces produce direct current (DC) output, exhibiting different operational principles and output characteristics [9,10,18,29,30]. TVNGs are mainly composed of semiconductors or metals semiconductors or semiconductor/metal. When these interfaces experience relative sliding, friction induces the generation of free-charge carriers. Under the influence of a built-in electric field (E_b), these carriers undergo directional separation, forming a current [13]. The typical DC output characteristics of TVNGs are illustrated in Figure 2A and C. Initially, no current is generated when the materials are separated. Upon contact, electrons flow from the material with a higher work function to one with a lower work function due to differences in Fermi levels, creating a E_b with a direction from the higher to the lower Fermi level (Figure 2B and D). As the materials move relative to each other, mechanical friction at the interface continually breaks and forms atomic bonds. The energy released during bond formation, termed “bindington”, excites free electron-hole pairs [31]. These pairs generate direct current under the built-in electric field, with the current direction aligning with the field. Recently, third-generation semiconductor materials like gallium nitride (GaN) and silicon carbide (SiC) have been used as TVNG triboelectric layers [14,15,32,33]. Compared to silicon, these materials have wider band gaps. Chen et al. [34] observed that the output characteristic of GaN-based TVNGs outperforms silicon-based ones, and their electric field direction opposes the built-in electric field (Figure 2B and D). By applying a reverse bias, they demonstrated the presence of an interface electric field (E_in). In wide bandgap semiconductor-based TVNGs, this interface electric field is significantly stronger than the built-in field, thus dominating and determining the direction of the output field (Figure 2B and D). Specifically, the built-in electric field originates from the inherent properties of the semiconductor material, creating an internal potential difference that facilitates the separation and movement of charge carriers (electrons and holes). Simultaneously, the interface electric field, which arises at the contact surfaces between different materials due to charge transfer during friction, further influences the direction and efficiency of carrier transport. These two electric fields ensure effective carrier separation. In 2023, Gong et al. [35] discussed the competing mechanisms between the E_b and E_in in silicon-based TVNGs (Figure 2B and D). The interface oxide layer, resistivity, and applied load of semiconductor triboelectric materials significantly influence both the E_in and the E_b. Due to contact electrification, the oxide layer at the semiconductor interface accumulates charges, leading to the formation of an E_in that is stronger than E_b (Figure 2B). Additionally, reducing the material’s resistivity or increasing the applied load can lower the contact resistance of the TVNG, thereby reducing the amount of charge at the interface and weakening the strength of E_in. In this case, the E_b becomes dominant over the E_in (Figure 2D). Currently, the highest reported output achieves an open-circuit voltage exceeding 130 V and an ultrahigh normalized average power density of 24.6 kW m⁻² Hz⁻¹, capable of powering small devices such as commercial wristbands and calculators. TVNGs show great potential for future micro- and nano-energy harvesting applications [14–16,19,36].

Figure 2 Characteristics and mechanism of TVNG. (A), (C) Device structure of the TVNG. (B), (D) Working mechanism of the TVNG.

MULTI-PHYSICS FIELD ENVIRONMENT

Light

When semiconductor materials are illuminated, photons with energies greater than or equal to the bandgap can be absorbed, promoting electrons from the valence band to the conduction band [37]. The process generates free electron-hole pairs, which in turn elevates the carrier concentration and thereby enhances the semiconductor’s conductivity—an effect referred to as the photoconductive effect. In structures with built-in electric fields, such as PN (P: positive, N: negative) junctions, illumination leads to the separation of photogenerated carriers by the internal field, producing an electromotive force and current. This is the basis of the photovoltaic effect, essential to solar cell functionality. Consequently, the effects of illumination on semiconductor friction interfaces are noteworthy for discussion.

Traditional semiconductor materials

In 2019, Liu’s team [38] demonstrated the tribo-photovoltaic coupling effect using a silicon-based TVNG. This effect was observed at both micro and macro scales (Figure 3A and B), with illumination significantly enhancing the voltage and current outputs. Under illumination at 635 nm, the triboelectric voltage and current were dramatically increased compared to dark conditions. The decay time of the triboelectric signal pulse under illumination (0.22 s) was much longer than in the dark (60 ms) (Figure 3C), indicating that the recombination of friction-induced electron-hole pairs is notably delayed by the photogenerated electric field. This coupling between frictional and photogenerated electric fields enhances the separation and transport of friction- or light-excited electron-hole pairs. Moreover, the tribo-photovoltaic coupling effects were observed at solid-liquid friction interfaces. Zheng et al. [31] demonstrated the power generation characteristics and mechanisms of solid-liquid friction interfaces under illumination (Figure 3D). In the dark, the triboelectric current and voltage were approximately −0.3 μA and −0.3 V, while under illumination, the photoelectric voltage and current were around −0.08 V and −0.3 μA. The electrical signal of the friction component increased significantly (Figure 3E), with enhancements correlating with increased light intensity or decreased wavelength. The triboelectric signals at the interface are attributed to the tribovoltaic effect. When both photon excitation and frictional excitation occur simultaneously at the interface, the concentration of electron-hole pairs significantly increases, leading to an enhanced triboelectric signal (Figure 3F). Sharov et al. [39] observed the tribo-photovoltaic coupling effect in triboelectric layers composed of nInP/Si (Figure 3G). The peak current of the TVNG was approximately 1 μA, while in the dark, the current was only 0.5 nA (Figure 3H). When the friction-induced electric field and the photogenerated electric field were aligned, the output significantly increased, whereas if they were opposed, the triboelectric signal was weakened (Figure 3I). In addition, this coupling effect was also observed in wide-bandgap semiconductor materials. Ren et al. [40] reported that a TVNG composed of Si and GaN exhibited multiple enhancements in current and voltage under illumination (Figure 3J and K). This work further confirmed the coupling effect between the tribovoltaic effect and the photovoltaic effect. The “bindington” causes the generation of free carriers, and under the built-in electric field, it generates triboelectric signals. Additionally, ultraviolet (UV) light stimulation increases the electron-hole pair concentration, resulting in significantly increased direct current and voltage (Figure 3L).

Figure 3 TVNG based on traditional semiconductor materials. (A)‒(C) Schematic diagram of metal/silicon point contact system structure. Adapted with permission from Ref. [38], Copyright©2019, Elsevier. (D)‒(F) Schematic diagram of interface diagram of DI water and Si crystal. Adapted with permission from Ref. [31]. Copyright©2021, Elsevier. (G)‒(I) Schematic diagram of metal-oxide-semiconductor interface. Adapted with permission from Ref. [39]. Copyright©2019, American Chemical Society. (J)‒(L) Schematic diagram of nGaN and pSi interface. Adapted with permission from Ref. [40]. Copyright©2021, American Chemical Society.

Perovskite semiconductor materials

Perovskite materials are known for their efficient light absorption, superior charge carrier transport properties, low cost, and tunable structures, making them outstanding candidates in photovoltaic applications. This sets a promising research avenue for TVNGs based on perovskite materials.

Hao et al. [41] introduced perovskite-based TVNGs and discovered the tribo-photovoltaic coupling effect (Figure 4A). Under illumination, the output current of the dynamic metal/perovskite junction was tripled compared to dark conditions. This TVNG can harvest both light and mechanical energy simultaneously, showcasing a nonlinear light-enhanced effect later termed the tribo-photovoltaic coupling effect (Figure 4C). This effect arises because light excitation generates photoinduced carriers, and the perovskite film has a broader absorption range and higher absorption coefficient compared to traditional semiconductors. Additionally, Kim’s team [42] incorporated a charge transport layer with perovskite in the TVNG (Figure 4D). They concluded that the electrical performance depends on band alignment and a high work function difference, thus choosing FAPbI₃ and spiro as the TVNG friction layer materials. Under sunlight, the current was hundreds of times higher than that in the dark (Figure 4F). This high current and voltage output is due to the coupling between the tribovoltaic effect and photovoltaic effects. FAPbI₃’s narrow bandgap (~1.5 eV) generates more carriers under illumination and effectively separates them at the perovskite/spiro interface, enhancing current and voltage output. Furthermore, Yuan et al. [43] proposed a rolling mode metal/perovskite TVNG (Figure 4G). The output current under illumination was 4.7 times that under dark conditions (Figure 4H). This device also showed excellent stability (Figure 4I) and potential applications in temperature and humidity sensing and direct capacitor charging. Yin et al. [44] introduced a novel light-enhanced TVNG based on a dynamic n-type perovskite/p-type perovskite homojunction (Figure 4J). It achieved a voltage of ~8.72 V and a current of ~30.84 μA under illumination (Figure 4K) and could operate continuously for over 1000 s. In the depletion region, additional electron-hole pairs are excited by light (Figure 4L), increasing carrier density in the dynamic perovskite homojunction. Thanks to this friction-photovoltaic coupling effect, the sliding perovskite homojunction’s DC output increased significantly under illumination.

Figure 4 TVNG based on perovskite semiconductor materials (A)‒(C) Schematic diagram of metal/ perovskite Schottky junction. Adapted with permission from ref. [41], Copyright©2019, Elsevier. (D)‒(F) Schematic diagram of spiro/ perovskite Schottky junction. Adapted with permission from ref. [42], Copyright©2021, Royal Society of Chemistry. (G)‒(I) Schematic diagram of rolling-mode perovskite Schottky junction. Adapted with permission from ref. [43], Copyright©2022, John Wiley and Sons. (J)‒(L) Schematic diagram nMAPbI3 and pMAPbI3 perovskite Schottky junction. Adapted with permission from ref. [44], Copyright©2023, John Wiley and Sons.

Tribo-photovoltaic coupled devices can simultaneously harvest mechanical energy and light energy from the environment, enhancing the overall efficiency of energy collection systems, especially in scenarios where both light and dynamic conditions coexist [45]. Current challenges include improving energy conversion efficiency, enhancing device stability, gaining a deeper understanding of the coupling mechanisms, and optimizing materials and structural designs. Future research will focus on developing theoretical models, discovering new materials, fine-tuning interface effects, and integrating systems for practical applications.

Temperature

The Seebeck effect in semiconductors, a type of thermoelectric effect, generates a voltage difference between the ends of a material when a temperature gradient exists [46]. The magnitude of this electrical signal is directly influenced by the temperature variation in the material. In the operation of TVNGs, some energy is lost as heat, and this thermoelectric effect can impact the output characteristics of the tribovoltaic effect by contributing additional carriers transport due to the generated thermal gradients.

Tribo-thermoelectric coupling effect

Zhang et al. [47] first identified the coupling between the triboelectric effect and the thermoelectric effect. Using a TVNG composed of copper and silicon (Figure 5A), they observed that relative sliding at the interface generated heat, creating an uneven temperature distribution within the semiconductor. This causes the majority of charge carriers to migrate from the hot end to the cold end. Consequently, the DC output of the TVNG consists of a stable component from the triboelectric thermoelectric effect (I_h) and a fluctuating component from the tribovoltaic effect (I_t) (Figure 5B–E). Differences in the work functions of the materials result in a built-in electric field at the interface (Figure 5E). Both I_h and I_t contribute to the signal in the same direction, leading to an enhanced electrical output. When the relative sliding stops, I_t disappears immediately, while I_h gradually decreases as the temperature gradient diminishes (Figure 5B and C). This study suggests that the tribovoltaic effect is an interface effect linked to the built-in electric field at the interface, whereas the tribo-thermoelectric coupling effect is a bulk effect related to the internal temperature gradient of the material [47]. Unlike the instantaneous tribovoltaic effect, the thermoelectric effect has a cumulative nature. In subsequent research, Šutka et al. [48] constructed a TVNG using TiO₂ thin films and metal. Their experiments revealed that the current density of the TVNG is nearly independent of the metal’s work function (Figure 5F) but strongly correlates with the metal’s thermoelectric coefficient (Figure 5G). The normalized thermoelectric voltage U_TE and current density show a significant correlation for different triboelectric metal pairs (Figure 5H). Incorporating the thermoelectric effect provides a more comprehensive understanding of the tribovoltaic effect and the tribo-thermoelectric coupling effect.

Figure 5 Tribo-thermoelectric coupling effect. (A) Three-dimensional (3D) schematic for the experiment setup and structure of the TVNG. (B) Open-circuit voltage output, (C) short-circuit current output, and (D) temperature of the TVNG under a steady state. (E) Energy band diagram of n-type silicon and metal. Adapted with permission from Ref. [47]. Copyright©2020, Elsevier. (F) The current density of semiconductor materials with different work functions and the schematic image of the TVNG. (G) Respective average temperature difference distributions. (H) Normalized Seebeck effect voltage. Adapted with permission from Ref. [48]. Copyright©2023, American Chemical Society.

Different semiconductor materials

Temperature significantly influences the physical properties of semiconductor materials, resulting in variations in the triboelectric signals of TVNGs when operating conditions change.

Zheng et al. [49] investigated a TVNG composed of Cu and Si (Figure 6A). At low temperature (77 K), the TVNG operated effectively, with voltage and current outputs increasing to 1.21 V and11.38 μA compared to 0.76 V and 4.86 μA at room temperature, respectively (Figure 6B and C). This enhancement is attributed to reduced lattice and ionized impurity scattering in semiconductors at lower temperatures, leading to improved carrier mobility and extended mean free path. Consequently, the carrier collection efficiency at the electrodes improves, enhancing the electrical performance. Additionally, Lin’s team [50] further explored the impact of high temperatures on TVNG performance. Their findings indicate that rising temperatures (0–90 °C) increase the triboelectric voltage and current at water/Si interfaces during sliding (Figure 6D and E). They proposed a novel energy band model to explain the tribovoltaic effect, attributing energy generation to the formation of chemical bonds during liquid-solid contact (“bindington”). As the temperature rises, more “bindington” form, exciting additional electron-hole pairs and increasing carrier concentration, thereby enhancing the triboelectric signal (Figure 6F). Deng et al. [51] raised the operating temperature of a TVNG from room temperature to 373 K. This increase led to a significant boost in peak power density for the TVNG composed of Cu and p-Si (Figure 6G). Elevated temperatures enhance the formation rate of Si–O–Cu bonds and generate more electron-hole pairs at the interface (Figure 6H). However, the narrowing of the semiconductor depletion region and the presence of an opposing thermoelectric potential due to the thermoelectric effect reduce the triboelectric voltage. However, the output characteristics of TVNG devices based on wide-bandgap semiconductor materials exhibit distinct responses to temperature variations. Zhu and co-workers [32] tested a Cu/4H-SiC TVNG at high temperatures (Figure 6I). The results show that from room temperature to 180 °C, voltage initially increases and then decreases, while current continuously rises (Figure 6J). Higher temperatures further ionize 4H-SiC, increasing carrier density and reducing resistivity, which boosts current despite decreasing voltage. Compared to silicon-based TVNGs, 4H-SiC exhibits superior wear resistance. Perovskite materials, celebrated for their outstanding optoelectronic properties, nonetheless exhibit pronounced sensitivities to temperature fluctuations, which can significantly influence their structural integrity, carrier recombination dynamics, and optical characteristics. Yin et al. [44] used a homojunction perovskite material in their TVNG experiments (Figure 6K). With rising temperatures, the Fermi level of the semiconductor moves toward the center of the bandgap, weakening the electron-hole separation field and increasing charge recombination rates. Moreover, thermal energy accelerates the transfer and release of induced charges on the material’s surface, exacerbating charge and energy loss. As a result, the voltage continuously decreases (Figure 6L).

Figure 6 Temperature dependence of TVNG based on different semiconductor materials. (A)–(C) The TVNG of Cu and pSi at low temperatures. Adapted with permission from Ref. [49]. Copyright©2021, John Wiley and Sons. (D)–(F) The TVNG of deionized water and Si at high temperatures. Adapted with permission from Ref. [50]. Copyright©2021, John Wiley and Sons. (G), (H) The TVNG of Cu and pSi at high temperatures. Adapted with permission from Ref. [51]. Copyright©2023, Elsevier. (I), (J) The TVNG of Cu and SiC at high temperatures. Adapted with permission from Ref. [32]. Copyright©2022, American Chemical Society. (K), (L) The TVNG-based perovskite materials at high temperatures. Adapted with permission from Ref. [44]. Copyright©2023, John Wiley and Sons.

The heat generated under frictional conditions not only influences the physical and chemical properties of contact interfaces but can also stimulate or enhance the electric output characteristics of materials, leading to the generation or variation of potential differences. As an emerging field of research, the tribo-thermoelectricity coupling effect shows great potential in energy conversion and self-powered systems, offering new approaches for energy harvesting at the micro and nano scales.

Liquid

It is well-known that the electrical performance of traditional TENGs often degrades in liquid environments. However, in the field of TVNGs, these devices not only harvest frictional energy from liquid droplets but also operate effectively in liquid environments, with enhanced output. This unique power generation capability of TVNGs, driven by the distinctive mechanisms of the tribovoltaic effect, broadens their potential applications significantly.

Water

Water is the most abundant natural substance on Earth, making the exploration of its energy-harvesting potential crucial for the future application market of TVNGs. Firstly, water can be utilized as a friction layer material for power generation. Lin et al. [52] were pioneers in using water as the friction layer in TVNG experiments, where frictional voltage was generated by sliding water droplets on a Si surface (Figure 7A and B). This presents a novel approach for liquid-solid TVNGs. The interaction between water molecules and the semiconductor surface generates an electric signal due to contact electrification and charge transfer, and also through electron-hole pair excitation facilitated by bond formation at the interface, which is driven by the built-in electric field (Figure 7C). Additionally, TVNGs perform exceptionally well in high-humidity environments, contrasting with traditional TENGs. Wang et al. [53] introduced a humidity-enhanced TVNG (Figure 7D), where the charge transfer and peak power at high humidity (RH 90%; RH refers to relative humidity) reached 12.6 mC m⁻²and 1.6 mW m⁻², respectively—about 88 and 100 times higher than those at low humidity (RH 30%) (Figure 7E). XPS analysis suggested that high humidity enhances the surface states of Si, leading to increased carrier excitation and directional separation under the interface electric field, producing Direct current. Water molecules fill gaps at the friction interface, significantly reducing contact resistance. This results in a coupling effect between enhanced surface states of Si, friction-induced carrier excitation, and reduced contact resistance in high-humidity environments (Figure 7F). Moreover, Xia et al. [32,33] observed excellent output characteristics for 4H-SiC and Cu in high humidity (Figure 7G). As RH increases from 80% to 98%, the output signal decreases and reverses direction above RH 90% (Figure 7H). This is due to differences in the coupling of built-in and triboelectric electric fields induced by solid-liquid triboelectrification under high RH. Water can also serve as a medium in the friction layer contact interface to boost triboelectric output and reduce wear, extending operational life. Qiao et al. [54,55] demonstrated that adding aqueous graphene oxide (GO) or transition metal carbides or nitrides (MXenes) solutions between the Si and Cu friction layers enabled TVNGs to achieve a current density of 700 mA m⁻², maintaining over 90% electrical output after 90,000 cycles (Figure 7I and J). Using conductive polar liquid lubricants can effectively fill the gaps at the interface, reducing the contact resistance between surfaces. This reduction in contact resistance enhances the transfer efficiency of electron-hole pairs, thereby increasing the triboelectric signal. Recently, water has been explored for synergistic coupling with the tribovoltaic effect for power generation. Sohn et al. [56] achieved tribovoltaic and moisture-coupled power generation in layered double hydroxides (LDH) (Figure 7K). LDHs have strong water absorbency and undergo spontaneous water dissociation. By leveraging the concentration gradient of water molecules, power generation occurs. OH⁻ produced from dissociation selectively recombines with holes from the tribovoltaic effect, suppressing electron-hole pair recombination and enhancing output performance (Figure 7L).

Figure 7 The electric output characteristics of TVNGs based on water-solid interface and humidity environment dependence of TVNG. (A)–(C) Water-Si interface. Adapted with permission from Ref. [52]. Copyright©2020, Elsevier. (D)–(F) Humidity at the interface between mental and nSi. Adapted with permission from Ref. [53]. Copyright©2022, Royal Society of Chemistry. (G), (H) Humidity at the interface between mental and 4H-SiC. Adapted with permission from Ref. [32,33]. Copyright©2022, American Chemical Society, Copyright©2023, John Wiley and Sons. (I), (J) Lubricated interface of Cu and pSi. (K), (L) A coupling of tribovoltaic and moisture-enabled electricity. Adapted with permission from Ref. [54,56]. Copyright©2023, Springer Nature; Copyright©2024, John Wiley and Sons.

Other liquid

Recent studies have investigated the energy generation characteristics by replacing the friction layer materials at solid-liquid interfaces with various organic and inorganic solvents, including acids and bases. Zheng et al. [50] employed alkaline solution/Si as the triboelectric pair material for TVNGs, finding that electrical performance improves with increasing alkalinity. For a given pH value, the triboelectric signal also increases with temperature (Figure 8B). The energy from the redox reactions between OH⁻ and the Si interface, termed “bindington”, increases with higher OH⁻ concentrations or temperatures, accelerating “bindington” generation and carrier excitation. Further, Yan et al. [57] investigated power generation by introducing various solutions between graphene-Si friction layers (Figure 8C). The solution can move freely between the two solid interfaces. Introducing nonpolar nonelectrolyte liquids such as ethanol (C₂H₅OH) and methanol (CH₃OH) produced 0.63 and 0.54 V of DC voltage, respectively, whereas nonpolar nonelectrolyte liquids like carbon tetrachloride (CCl₄) and n-hexane (C₆H₁₄) produced no voltage (Figure 8D). In the presence of an interfacial electric field, polar molecules undergo a depolarization process, which drives the movement of charge carriers and generates an electrical output. The voltage magnitude and direction are determined by the Fermi level difference between the friction layers. Li et al. [58] demonstrated power generation by sliding acid and alkali droplets across semiconductor interfaces (Figure 8E). Replacing neutral water droplets with pH 14 droplets boosted the output current from 4.2 to 428.9 μA (Figure 8F). Jiang et al. [59] further explored the effect of adding polar liquids and ionic solutions to semiconductor-liquid-semiconductor structures in TVNGs (Figure 8G and H). A 100 g L⁻¹ NaCl solution yielded the highest output, with a current of 4.2 μA and a voltage of 0.05 V, compared to just 0.03 μA without the medium (Figure 8I). This concludes that ionic solutions enhance output more effectively than polar liquids, while nonpolar liquids suppress output.

Figure 8 TVNGs of liquid-solid interface based on other liquid. (A), (B) pH (liquid-Si). Adapted with permission from Ref. [50]. Copyright©2021, John Wiley and Sons. (C), (D) Organic liquid (graphene-liquid-Si). Adapted with permission from Ref. [57]. Copyright©2021, American Chemical Society. (E), (F) Different pH (liquid-Si). Adapted with permission from Ref. [58]. Copyright©2023, Royal Society of Chemistry. (G)‒(I) Solution concentration (pSi-liquid-nWS2). Adapted with permission from Ref. [59]. Copyright©2024, American Chemical Society.

TVNG technology for energy harvesting in liquid environments is breaking traditional constraints, offering a promising direction for self-sustaining energy systems in humid and underwater conditions. Potential applications include powering underwater devices, biomedical fields, and marine environmental monitoring [60–62].

Gas

Different gases interact uniquely with the TVNG interface. For instance, surface oxidation reactions, the ionization of gases affecting interfacial charge transfer, and variations in ambient temperature and humidity can influence the tribological performance of the material. Moreover, gases may cause interfacial corrosion, leading to the formation of surface layers that subsequently alter the efficiency of charge generation. Understanding these interactions is crucial for optimizing the design and functionality of TVNGs in various gas environments.

Zhang and colleagues [63] investigated the tribovoltaic effect under different atmospheric conditions (Figure 9A). They found that TVNGs exhibit more stable and higher electrical output in a vacuum compared to N₂ or air atmospheres (Figure 9B). Further experiments showed that conducting tests in a nitrogen atmosphere led to a higher output—18 μA of stable triboelectric signals, unlike in air (Figure 9C–E). This improved performance in N₂ is attributed to its low humidity, which prevents oxidation of the semiconductor surface induced by triboelectricity [51]. Recently, Zhang’s team [64] achieved macroscopic solid superlubricity based on the tribovoltaic effect in a nitrogen atmosphere (Figure 9F). In dry, inert N₂, the current signal reached 14 nA, with the coefficient of friction (COF) below 0.01. However, when ambient air was introduced, the steady-state current dropped to −80 nA and the COF increased to about 0.05 (Figure 9G). The energy released from atomic bonding at the interface generates electron-hole pairs, resulting in a direct current under the built-in electric field. The negative current in the air might result from friction between the steel ball and diamond-like carbon (DLC). The ultra-low friction of DLC is primarily due to the highly passivated hydrogen-terminated surface (Figure 9H). The high passivation level of the C‒H bonds on the DLC surface means less mechanical work is dissipated as heat and more is converted into other forms of energy, such as electricity. Furthermore, friction in the air involves the adsorption of water molecules and oxygen, which leads to interfacial oxidation. This hinders the formation of hydrogen-terminated surfaces, resulting in increased wear at the interface.

Figure 9 The influence of gas environmental factors on the electrical output. (A), (B) The interface between nSi and pSi. Adapted with permission from Ref. [63]. Copyright©2019, Elsevier. (C)‒(E) The interface between nSi and pSi. Adapted with permission from Ref. [51]. Copyright©2023, Elsevier. (F)‒(H) The super lubrication interface between steel ball and DLC. Adapted with permission from Ref. [64]. Copyright©2022, Elsevier.

The efficiency and stability of TVNG energy harvesting are significantly influenced by gaseous environments. Improving encapsulation techniques to regulate the gaseous conditions at the triboelectric interface is essential. Controlling gas composition and pressure is crucial for guiding the design and optimization of materials and devices within TVNG systems. Such measures can greatly enhance the efficiency and reliability of these energy conversion systems, leading to more effective and stable performance in various environmental conditions.

APPLICATIONS OF TVNG UNDER MULTI-PHYSICS

TVNGs are characterized by their lower impedance and higher current density compared with TENG. So they have a wider range of applications that mainly include two aspects: harvesting energy and sensing. TVNGs can power numerous light-emitting diodes (LEDs), commercial temperature, and humidity meters, and have been developed into self-powered tribovoltaic bearings for monitoring temperature and pressure in industrial Internet of Things (IoT) applications, sustaining over 1,000,000 cycles. They can also track breathing, detect motion parameters, and monitor lubrication states. Here, we review the typical applications of the environmental multi-physics coupled tribovoltaic effect.

Device and application for harvesting energy

Huang et al. [65] designed a flexible TVNG based on a metal-liquid-semiconductor structure using indium gallium zinc oxide (IGZO), which can conform to the skin (Figure 10A). Connecting six of these layers in series easily powered a red LED. A team has successfully integrated TVNGs into wind turbines through structural design. Yu et al. [66] introduced a wind-driven TVNG capable of producing a continuous DC output averaging 4.4 mA over 740 s, with a peak of 8.4 mA (Figure 10B). This device can also drive a graphene UV photodetector with a responsivity of 35.8 A W⁻¹ at 365 nm UV (Figure 10C). You et al. [67] developed a TVNG for wind energy harvesting using poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT) and aluminum foil as the triboelectric layers, generating a maximum open-circuit voltage of 0.6 V at 7 m s⁻¹ wind speed (Figure 10D and E). The capacitor charging curves under lubricated and dry friction conditions are shown (Figure 10F). Additionally, portable solid-liquid structured devices are gradually entering the public view. Lin’s team [61] proposed a method for harvesting droplet energy using integrated TVNGs, achieving 4.5 V DC voltage sufficient to power commercial LEDs (Figure 10G). These devices exhibit significant potential for harvesting energy from environmental sources such as raindrops and ocean waves. Demonstrated is a pSi-water-nSi layered structure (Figure 10H) that generates 0.11 V continuous DC voltage as water droplets slide freely between the semiconductors with plastic mold agitation, making it suitable for wearable droplet energy harvesting (Figure 10I) [68].

Figure 10 Device and application for harvesting energy. (A) Wearable flexible power generation device. Adapted with permission from Ref. [65]. Copyright©2022, John Wiley and Sons. (B), (C) Harvesting wind energy drives UV detectors. Adapted with permission from Ref. [66]. Copyright©2021, the Royal Society of Chemistry. (D)‒(F) Harvesting wind energy to charge the capacitor. Adapted with permission from Ref. [67]. Copyright©2023, Elsevier. (G) The liquid-solid interface powers a LED. Adapted with permission from Ref. [61]. Copyright©2021, John Wiley and Sons. (H), (I) A portable energy storage device. Adapted with permission from Ref. [68]. Copyright©2021, The Author(s).

Device and application for sensing

Xia et al. [32] designed a TVNG-based worker monitoring device (Figure 11A). This technology is well-suited for deployment in harsh environments characterized by high temperatures and humidity. It operates when a person steps on it, with different weights generating unique electrical signals that are collected and amplified (Figure 11B). Thanks to the robust output of the 4H-SiC-based TVNG in extreme conditions, this device can be used in high-humidity, high-temperature environments like underground mines. Huang et al. [60] introduced a TVNG using graphite paper to harvest energy from flowing water and monitor water speed in aquariums, generating 250 nA cm⁻² when the pump is activated (Figure 11C and D). TVNGs also show promise in capturing wave and ocean energy, offering sustainable solutions for marine applications. Huang’s team [65] also demonstrated a flexible, wearable liquid-interlayer TVNG (Figure 10A) that produces varying outputs during different body movements, such as falling or standing (Figure 11E and F), showing potential for detecting falls in elderly or patients. Yu et al. [66] developed a wind-driven TVNG (Figure 11G), which can be used as a revolution counter due to its stable output (Figure 11H). Additionally, extensive research is being conducted to accelerate the application of TVNGs, particularly focusing on developing sensor devices for industrial IoT applications. Wang’s team [55] proposed a self-powered vibration sensor system based on TVNG for monitoring building structural health. It generates AC signals within safe zones and DC signals with an alarm light outside them (Figure 11I), and can also act as a weight sensor to monitor pressure. Corresponding signals show that placing a 50 g weight on the sensor generates an AC signal indicating a qualified product, while a 100 g weight produces a higher DC signal indicating an unqualified product with a red alarm light (Figure 11J). Zhang’s team [64] proposed a method for achieving superlubricity on DLC interfaces to monitor friction states under different atmospheres (Figure 11K). In dry inert nitrogen, they observed a positive current of approximately 14 nA with a COF below 0.01 in a superlubric state. In atmospheric conditions, the friction produced a −80 nA negative current signal, with higher friction and wear indicating lubrication failure (Figure 11L).

Figure 11 Device and application for sensing. (A), (B) A device for high-temperature and humidity environments. Adapted with permission from Ref. [32]. Copyright©2022, American Chemical Society. (C), (D) Monitoring water velocity. Adapted with permission from Ref. [60]. Copyright©2021, Elsevier. (E), (F) Monitor body posture. Adapted with permission from Ref. [65]. Copyright©2022, John Wiley and Sons. (G), (H) Monitoring wind speed. Adapted with permission from Ref. [66]. Copyright©2021, the Royal Society of Chemistry. (I), (J) Weight monitoring alarm. Adapted with permission from Ref. [55]. Copyright©2022, John Wiley and Sons. (K), (L) Monitor for super lubrication. Adapted with permission from Ref. [64]. Copyright©2022, Elsevier.

In environmental monitoring, health surveillance, and similar fields, multi-physics coupled TVNGs can provide a continuous and stable energy source for sensors and microelectronic devices. With their efficient operation in liquid environments, TVNGs hold immense potential for applications in extreme conditions such as marine exploration, deep-sea resource extraction, and space exploration, making them valuable for powering equipment and devices in challenging settings.

CONCLUSIONS AND OUTLOOK

Conclusions

This review elucidates the mechanisms and response characteristics of the environmental multi-physics coupled tribovoltaic effect, enhancing the understanding of how various physical fields influence the tribovoltaic effect.

In high humidity or liquid environments, the output of TVNGs is enhanced due to the reduction in contact resistance and the improved surface states of the semiconductor. Additionally, liquids serve as lubricants at the interface, reducing wear and thereby extending the device’s lifespan. An inert gas atmosphere ensures the chemical stability of the friction interface, enhancing tribological performance. Illumination and temperature significantly influence the triboelectric signals by affecting carrier transport within the semiconductor. Additionally, liquids and gaseous environments modify the friction interface properties, further impacting the triboelectric signals. This capability allows TVNGs to not only operate effectively in extreme and complex environments such as liquid media, high humidity, and elevated temperature but also to simultaneously harvest energy from multi-physics fields.

The ability of TVNGs to maintain stable energy output across different conditions underlines their versatility. This significantly broadens the practical applications of TVNGs. These interactions enhance the adaptability and performance of TVNGs in diverse environmental conditions, making them suitable for harvesting mechanical energy from sources such as marine exploration, deep-sea mining, and space missions.

Outlook

The sensitivity of semiconductor materials to environmental changes increases their diverse application potential for TVNGs. The environmental multi-physics coupled tribovoltaic effect significantly enhances energy harvesting efficiency and broadens practical application scenarios. However, several challenges and shortcomings need to be addressed for wider adoption and optimization.

(1) Coupling mechanisms. Future research should deepen the understanding of coupling mechanisms in tribovoltaic effect in multi-physics environments. Investigating the interplay between mechanical friction and semiconductor properties can reveal how different physical fields influence carrier excitation. This includes the impact of lattice vibrations, interface bonding, and surface states on charge generation and transport. Especially under varying physical fields such as illumination, temperature, and pressure, understanding how these fields affect bonding and electron-hole pair generation can optimize TVNG efficiency in diverse conditions. Additionally, future work should explore the coupling of tribovoltaic effect and other multiple effects like thermoelectric and photovoltaic coupling. This strategy enables the simultaneous or independent harvesting of different types of mechanical, thermal, and solar energy from the surrounding environment. Understanding the interactions among different effects, such as thermoelectric, tribovoltaic, and photoexcitation, can reveal and optimize the impact on carrier behavior, ensuring that these physical fields do not negatively influence each other. This can guide device performance improvements and promote efficient energy conversion.

(2) Fabrication methods improvement and device structural design. In practical applications of TVNGs, extreme conditions such as high temperatures and humidity can significantly impact the physicochemical properties of material interfaces. These challenges necessitate rigorous requirements for the electrical and mechanical properties of the materials used in TVNGs. Semiconductor doping techniques can be employed to modify the carrier concentration and resistivity, enhancing the electrical performance of functional semiconductor films in extreme environments. Additionally, techniques such as high-temperature annealing and surface modifications can improve the mechanical properties of these films, thereby increasing energy conversion efficiency and extending the device lifespan. Such adaptations ensure that the materials maintain high performance, stability, and compatibility with different types of physical fields, even under harsh conditions, by enhancing their ability to handle various environmental stresses effectively. Additionally, We can introduce the “π” structure of the thermoelectric generator into the design of the TVNG device structure, two types of doped semiconductors can be connected in series using metal electrodes to form a “π” unit. Multiple such units can be further integrated into series, leading to a cumulative electromotive force. This configuration allows for more efficient structural coupling of thermoelectric and triboelectric and enables the simultaneous harvesting of energy from multi-physics fields. Additionally, integrating diverse energy harvesting components, such as photovoltaic cells and thermoelectric materials, within the TVNG can enhance the overall energy collection efficiency. This multifaceted approach allows TVNGs to effectively harness light, heat, and mechanical energy, optimizing energy conversion in diverse environments.

(3) Device applications. Beyond light, heat, wind, and ocean energy, other forms of energy in nature, such as chemical energy, thermophotovoltaic energy, and electrochemical energy, are worth exploring for their coupling with the tribovoltaic effect. TVNG devices coupled with multi-physics, are characterized by their simple structure, direct current output, and strong environmental adaptability. There are numerous future application opportunities. They can collect energy from ocean waves, raindrops, and wind to power sensors and devices in harsh environments like the deep sea and space. Due to their strong adaptability to various environments, multi-physics coupled energy harvesting devices can also power high-temperature and high-humidity smart industrial systems and environmental sensors. In everyday life, they can make gadgets self-charging and help create cleaner, renewable energy systems. They are also effective in powering sensors and monitoring systems for agriculture and environmental monitoring in remote outdoor areas. Incorporating energy management modules can improve energy utilization efficiency and ensure system stability. Enhanced reliability and energy collection efficiency through system integration and packaging can better adapt TVNGs to multi-physics environments.

In conclusion, the multi-physics coupling in TVNGs offers a promising avenue for future research and practical energy solutions. This integration of traditional and emerging energy technologies promotes their synergy and mutual development.

Data availability

The original data is available from corresponding authors upon reasonable request.

Funding

This work was supported by the Beijing Natural Science Foundation (3232019) and the National Natural Science Foundation of China (62104020 and 52450006).

Author contributions

C.Z. and Z.Z. supervised the project. Y.F., Z.Z and C.Z. organized the structure of the manuscript. Y.F. wrote the original draft. Y.F., Z.Z., L.G., R.L., Z.W., S.D., and C.Z. reviewed and edited the manuscript.

Conflict of interest

The authors declare no conflict of interest.

References

All Tables

All Figures

	Figure 1 Outline illustration of the review of tribovoltaic effect coupled energy harvesters based on environmental multi-physics.
In the text

	Figure 2 Characteristics and mechanism of TVNG. (A), (C) Device structure of the TVNG. (B), (D) Working mechanism of the TVNG.
In the text

Figure 3 TVNG based on traditional semiconductor materials. (A)‒(C) Schematic diagram of metal/silicon point contact system structure. Adapted with permission from Ref. [38], Copyright©2019, Elsevier. (D)‒(F) Schematic diagram of interface diagram of DI water and Si crystal. Adapted with permission from Ref. [31]. Copyright©2021, Elsevier. (G)‒(I) Schematic diagram of metal-oxide-semiconductor interface. Adapted with permission from Ref. [39]. Copyright©2019, American Chemical Society. (J)‒(L) Schematic diagram of nGaN and pSi interface. Adapted with permission from Ref. [40]. Copyright©2021, American Chemical Society.

In the text

Figure 4 TVNG based on perovskite semiconductor materials (A)‒(C) Schematic diagram of metal/ perovskite Schottky junction. Adapted with permission from ref. [41], Copyright©2019, Elsevier. (D)‒(F) Schematic diagram of spiro/ perovskite Schottky junction. Adapted with permission from ref. [42], Copyright©2021, Royal Society of Chemistry. (G)‒(I) Schematic diagram of rolling-mode perovskite Schottky junction. Adapted with permission from ref. [43], Copyright©2022, John Wiley and Sons. (J)‒(L) Schematic diagram nMAPbI3 and pMAPbI3 perovskite Schottky junction. Adapted with permission from ref. [44], Copyright©2023, John Wiley and Sons.

In the text

Figure 5 Tribo-thermoelectric coupling effect. (A) Three-dimensional (3D) schematic for the experiment setup and structure of the TVNG. (B) Open-circuit voltage output, (C) short-circuit current output, and (D) temperature of the TVNG under a steady state. (E) Energy band diagram of n-type silicon and metal. Adapted with permission from Ref. [47]. Copyright©2020, Elsevier. (F) The current density of semiconductor materials with different work functions and the schematic image of the TVNG. (G) Respective average temperature difference distributions. (H) Normalized Seebeck effect voltage. Adapted with permission from Ref. [48]. Copyright©2023, American Chemical Society.

In the text

Figure 6 Temperature dependence of TVNG based on different semiconductor materials. (A)–(C) The TVNG of Cu and pSi at low temperatures. Adapted with permission from Ref. [49]. Copyright©2021, John Wiley and Sons. (D)–(F) The TVNG of deionized water and Si at high temperatures. Adapted with permission from Ref. [50]. Copyright©2021, John Wiley and Sons. (G), (H) The TVNG of Cu and pSi at high temperatures. Adapted with permission from Ref. [51]. Copyright©2023, Elsevier. (I), (J) The TVNG of Cu and SiC at high temperatures. Adapted with permission from Ref. [32]. Copyright©2022, American Chemical Society. (K), (L) The TVNG-based perovskite materials at high temperatures. Adapted with permission from Ref. [44]. Copyright©2023, John Wiley and Sons.

In the text

Figure 7 The electric output characteristics of TVNGs based on water-solid interface and humidity environment dependence of TVNG. (A)–(C) Water-Si interface. Adapted with permission from Ref. [52]. Copyright©2020, Elsevier. (D)–(F) Humidity at the interface between mental and nSi. Adapted with permission from Ref. [53]. Copyright©2022, Royal Society of Chemistry. (G), (H) Humidity at the interface between mental and 4H-SiC. Adapted with permission from Ref. [32,33]. Copyright©2022, American Chemical Society, Copyright©2023, John Wiley and Sons. (I), (J) Lubricated interface of Cu and pSi. (K), (L) A coupling of tribovoltaic and moisture-enabled electricity. Adapted with permission from Ref. [54,56]. Copyright©2023, Springer Nature; Copyright©2024, John Wiley and Sons.

In the text

Figure 8 TVNGs of liquid-solid interface based on other liquid. (A), (B) pH (liquid-Si). Adapted with permission from Ref. [50]. Copyright©2021, John Wiley and Sons. (C), (D) Organic liquid (graphene-liquid-Si). Adapted with permission from Ref. [57]. Copyright©2021, American Chemical Society. (E), (F) Different pH (liquid-Si). Adapted with permission from Ref. [58]. Copyright©2023, Royal Society of Chemistry. (G)‒(I) Solution concentration (pSi-liquid-nWS2). Adapted with permission from Ref. [59]. Copyright©2024, American Chemical Society.

In the text

Figure 9 The influence of gas environmental factors on the electrical output. (A), (B) The interface between nSi and pSi. Adapted with permission from Ref. [63]. Copyright©2019, Elsevier. (C)‒(E) The interface between nSi and pSi. Adapted with permission from Ref. [51]. Copyright©2023, Elsevier. (F)‒(H) The super lubrication interface between steel ball and DLC. Adapted with permission from Ref. [64]. Copyright©2022, Elsevier.

In the text

Figure 10 Device and application for harvesting energy. (A) Wearable flexible power generation device. Adapted with permission from Ref. [65]. Copyright©2022, John Wiley and Sons. (B), (C) Harvesting wind energy drives UV detectors. Adapted with permission from Ref. [66]. Copyright©2021, the Royal Society of Chemistry. (D)‒(F) Harvesting wind energy to charge the capacitor. Adapted with permission from Ref. [67]. Copyright©2023, Elsevier. (G) The liquid-solid interface powers a LED. Adapted with permission from Ref. [61]. Copyright©2021, John Wiley and Sons. (H), (I) A portable energy storage device. Adapted with permission from Ref. [68]. Copyright©2021, The Author(s).

In the text

Figure 11 Device and application for sensing. (A), (B) A device for high-temperature and humidity environments. Adapted with permission from Ref. [32]. Copyright©2022, American Chemical Society. (C), (D) Monitoring water velocity. Adapted with permission from Ref. [60]. Copyright©2021, Elsevier. (E), (F) Monitor body posture. Adapted with permission from Ref. [65]. Copyright©2022, John Wiley and Sons. (G), (H) Monitoring wind speed. Adapted with permission from Ref. [66]. Copyright©2021, the Royal Society of Chemistry. (I), (J) Weight monitoring alarm. Adapted with permission from Ref. [55]. Copyright©2022, John Wiley and Sons. (K), (L) Monitor for super lubrication. Adapted with permission from Ref. [64]. Copyright©2022, Elsevier.

In the text