Filling the gap between topological insulator nanomaterials and triboelectric nanogenerators

Surfactant Self-Assembly Enhances Tribopositivity of Stretchable Ionic Conductors for Wearable Energy Harvesting and Motion Sensing

Boosting stretchability and electric output is critical for high-performance wearable triboelectric nanogenerators (TENG). Herein, for the first time, a new approach for tuning the composition of surface functional groups through surfactant self-assembly to improve the tribopositivity, where the assembly increases the transferred charge density and the relative permittivity of water polyurethane (WPU). Incorporating bis(trifluoromethanesulfonyl)imide (TFSI-) and alkali metal ions into a mixture of WPU and the surfactant forms a stretchable film that simultaneously functions as positive tribolayer and electrode, preventing the conventional detachment of tribolayer and electrode in long term usage. Further, the conductivity of the crosslinked film reaches 3.3 × 10-3 mS cm-1 while the elongation at break reaches 362%. Moreover, the surfactant self-assembly impedes the adverse impact of the fluorine-containing groups on tribopositivity. Consequently, the charge density reaches 155 µC m-2, being the highest recorded for WPU and stretchable ionic conductor based TENG. This work introduces a novel approach for boosting the output charge density while avoiding the adverse effect of ionic salts in solid conductors through a universal surfactant self-assembly strategy, which can be extended to other materials. Further, the device is used to monitor and harvest the kinetic energy of human body motion.

Enhancing the Output Performance of Triboelectric Nanogenerator Through Regulation of its Internal Nano-Architecture

Triboelectric Nanogenerator (TENG) has proven highly effective in converting mechanical energy into electrical energy. Previous research on manipulating microstructure for performance enhancement primarily focused on the surface of TENGs. In this study, an innovative bottom-up strategic design to control the internal nano-architecture for the enhanced output of TENG is proposed. This multiscale structural design strategy consists of defect chemistry (angstrom-scale), surface modification (nano-scale), and spatial regulation of nanoparticles (meso-scale), which helps explore the optimal utilization of TENG's internal structure. After fine-tuning the nano-architecture, the output voltage is significantly increased. This optimized TENG serves as a robust platform for developing self-powered systems, including self-powered electrochemical chlorination systems for sterilization. Additionally, through the utilization of multiscale simulations (density functional theory, all-atom molecular dynamics, and dissipative particle dynamics), the underlying mechanisms governing how the optimized nanoparticle-polymer interface and spatial arrangement of nanoparticles influence the storage and transfer of charges are comprehensively elucidated. This study not only demonstrates the effectiveness of manipulating internal nano-architecture to enhance TENG performance for practical applications but also provides invaluable insights into structural engineering for TENG advancement.

Interfacial-Assembly-Induced In Situ Transformation from Aligned 1D Nanowires to Quasi-2D Nanofilms

As the dimensionality of materials generally affects their characteristics, thin films composed of low-dimensional nanomaterials, such as nanowires (NWs) or nanoplates, are of great importance in modern engineering. Among various bottom-up film fabrication strategies, interfacial assembly of nanoscale building blocks holds great promise in constructing large-scale aligned thin films, leading to emergent or enhanced collective properties compared to individual building blocks. As for 1D nanostructures, the interfacial self-assembly causes the morphology orientation, effectively achieving anisotropic electrical, thermal, and optical conduction. However, issues such as defects between each nanoscale building block, crystal orientation, and homogeneity constrain the application of ordered films. The precise control of transdimensional synthesis and the formation mechanism from 1D to 2D are rarely reported. To meet this gap, we introduce an interfacial-assembly-induced interfacial synthesis strategy and successfully synthesize quasi-2D nanofilms via the oriented attachment of 1D NWs on the liquid interface. Theoretical sampling and simulation show that NWs on the liquid interface maintain their lowest interaction energy for the ordered crystal plane (110) orientation and then rearrange and attach to the quasi-2D nanofilm. This quasi-2D nanofilm shows enhanced electric conductivity and unique optical properties compared with its corresponding 1D geometry materials. Uncovering these growth pathways of the 1D-to-2D transition provides opportunities for future material design and synthesis at the interface.

Unusual Janus Bi2TeSe2 Topological Insulators Displaying Second-Harmonic Generation, Linear-in-Temperature Resistivity, and Weak Antilocalization

Well-established knowledge about inversion-symmetric Bi2TexSe3-x topological insulators characterizes the promising new-generation quantum device. Noticeably, the inversion asymmetric phase containing different surface electronic structures may create an extra topological phenomenon pointing to a new device paradigm. Herein, Janus Bi2TeSe2 single-crystal nanosheets with an unconventional stacking sequence of Se-Bi-Se-Bi-Te are realized via chemical vapor deposition growth, which is clarified by atomically resolved AC-STEM and elemental mapping. An obvious polarization-dependent second-harmonic generation with a representative 6-fold rotational symmetry is detected due to the broken out-of-plane mirror symmetry in this system. Low-temperature transport measurements display a strange metal-like linear-in-temperature resistivity. Anomalous conductance peaks under low magnetic fields induced by the weak antilocalization effect of topological surface states and the two-dimensional transport-dominated anisotropic magnetoresistance are revealed. These findings correlate the Janus Bi2TeSe2 phase with emerging physics topics, which would inspire fresh thoughts in well-developed Bi3TexSe3-x topological insulators and open up opportunities for exploring hybrid nonlinear optoelectronic topological devices.

Self-Powered and Self-Recoverable Multimodal Force Sensors Based on Trap State and Interfacial Electron Transfer

Multi-dimensional force sensing that combines intensity, location, area and the like could gather a wealth of information from mechanical stimuli. Developing materials with force-induced optical and electrical dual responses would provide unique opportunities to multi-dimensional force sensing, with electrical signals quantifying the force amplitude and the luminescence output providing spatial distribution of force. However, the reliance on external power supply and high-energy excitation source brings significant challenges to the applicability of multi-dimensional force sensors. Here we reported the mechanical energy-driven and sunlight-activated materials with force-induced dual responses, and investigated the underlying mechanisms of self-sustainable force sensing. Theoretical analysis and experimental data unraveled that trap-controlled luminescence and interfacial electron transfer play a major role in force-induced optical and electrical output. These materials were manufactured into pressure sensor with renewable dual-mode output for quantifying and visualization of pressures by electrical and optical output, respectively, without power supply and high-energy irradiation. The quantification of tactile sensation and stimuli localization of mice highlighted the multi-dimensional sensing ability of the sensor. Overall, this self-powered pressure sensor with multimodal output provides more modalities of force sensing, poised to change the way that intelligent devices sense with the world.

Triboelectric Energy Harvesting from Highly Conjugated Fused Aromatic Ladder Structure Under Extreme Environmental Conditions

Practical application of triboelectric nanogenerators (TENGs) has been challenging, particularly, under harsh environmental conditions. This work proposes a novel 3D-fused aromatic ladder (FAL) structure as a tribo-positive material for TENGs, to address these challenges. The 3D-FAL offers a unique materials engineering platform for tailored properties, such as high specific surface area and porosity, good thermal and mechanical stability, and tunable electronic properties. The fabricated 3D-FAL-based TENG reaches a maximum peak power density of 451.2 µW cm-2 at 5 Hz frequency. More importantly, the 3D-FAL-based TENG maintains stable output performance under harsh operating environments, over wide temperature (-45-100 °C) and humidity ranges (8.3-96.7% RH), representing the development of novel FAL for sustainable energy generation under challenging environmental conditions. Furthermore, the 3D-FAL-based TENG proves to be a promising device for a speed monitoring system engaging reconstruction in virtual reality in a snowy environment.

Construction of Triboelectric Series and Chirality Detection of Amino Acids Using Triboelectric Nanogenerator

Triboelectrification necessitates a frictional interaction between two materials, and their contact electrification is characteristically based on the polarity variance in the triboelectric series. Utilizing this fundamental advantage of the triboelectric phenomenon, different materials can be identified according to their contact electrification capability. Herein, an in-depth analysis of the amino acids present in the stratum corneum of human skin is performed and these are quantified regarding triboelectric polarization. The principal focus of this study lies in analyzing and identifying the amino acids present in copious amounts in the stratum corneum to explain their positive behavior during the contact electrification process. Thus, an augmented triboelectric series of amino acids with quantified triboelectric charging polarity by scrutinizing the transfer charge, work function, and atomic percentage is presented. Furthermore, the chirality of aspartic acid as it is most susceptible to racemization with clear consequences on the human skin is detected. The study is expected to accelerate research exploiting triboelectrification and provide valuable information on the surface properties and biological activities of these important biomolecules.

Revamping Triboelectric Output by Deep Trap Construction

High output performance is critical for building triboelectric nanogenerators (TENGs) for future multifunctional applications. Unfortunately, the high triboelectric charge dissipation rate has a significant negative impact on its electrical output performance. Herein, a new tribolayer is designed through introducing self-assembled molecules with large energy gaps on commercial PET fibric to form carrier deep traps, which improve charge retention while decreasing dissipation rates. The deep trap density of the PET increases by two orders of magnitude, resulting in an 86% reduction in the rate of charge dissipation and a significant increase in the charge density that can be accumulated on tribolayer during physical contact. The key explanation is that increasing the density of deep traps improves the dielectric's ability to store charges, making it more difficult for the triboelectric charges trapped by the tribolayer to escape from the deep traps, lowering the rate of charge dissipation. This TENG has a 1300% increase in output power density as a result of altering the deep trap density, demonstrating a significant improvement. This work describes a simple yet efficient method for building TENGs with ultra-high electrical output and promotes their practical implementation in the sphere of the Internet of Things.

A Natural Gradient Biological-Enabled Multimodal Triboelectric Nanogenerator for Driving Safety Monitoring

A key element to ensuring driving safety is to provide a sufficient braking distance. Inspired by the nature triply periodic minimal surface (TPMS), a gradient and multimodal triboelectric nanogenerator (GM-TENG) is proposed with high sensitivity and excellent multimodal monitoring. The gradient TPMS structure exhibits the multi-stage stress-strain properties of typical porous metamaterials. Significantly, the multimodal monitoring capability depends on the implicit function of the defined level constant c, which directly contributes to the multimodal driving safety monitoring. The mechanical and electrical responsive behavior of the GM-TENG is analyzed to identify the applied speed, load, and working mode. In addition, optimized peak open-circuit voltage (Voc) is demonstrated for self-awareness of the braking condition. The braking distance factor (L) is conceived to construct the self-aware equation of the friction coefficient based on the integration of Voc with respect to time. Importantly, R-squared up to 94.29 % can be obtained, which improves self-aware accuracy and real-time capabilities. This natural structure and self-aware device provide an effective strategy to improve driving safety, which contributes to the improvement of road safety and presents self-powered sensing with potential applications in an intelligent transportation system.

Conversion of Dielectric Surface Effect into Volume Effect for High Output Energy

Improving the output energy and durability of triboelectric nanogenerators (TENGs) remains a considerable challenge for their practical applications. Owing to the interface effect of triboelectrification and electrostatic induction, thinner films with higher dielectric constants yield a higher output; however, they are not durable for practical applications. Herein, the dielectric surface effect is changed into a volume effect by adopting a millimeter-thick dielectric film with an inner porous network structure so that charges can hop in the surface state of the network. Charge migration inside the dielectric film is the key factor affecting the output of the triboelectric nanogenerator (TENG) with a thick film, based on which each working stage follows the energy-maximization principle in the voltage-charge plot. The maximum peak and average power densities of the TENG with polyurethane foam film in 1 mm thickness reach 40.9 and 20.7 W m-2  Hz-1 , respectively, under environmental conditions, and the output charge density is 5.14 times that of TENGs with a poly(tetrafluoroethylene) film of the same thickness. Superdurability is achieved in the rotary-mode TENG after 200 000 operation cycles. This study identifies the physical mechanism of the thick dielectric film used in TENGs and provides a new approach to promote the output and durability of TENGs.

Recent Progress of Advanced Materials for Triboelectric Nanogenerators

Triboelectric nanogenerators (TENGs) have received intense attention due to their broad application prospects in the new era of internet of things (IoTs) as distributed power sources and self-powered sensors. Advanced materials are vital components for TENGs, which decide their comprehensive performance and application scenarios, opening up the opportunity to develop efficient TENGs and expand their potential applications. In this review, a systematic and comprehensive overview of the advanced materials for TENGs is presented, including materials classifications, fabrication methods, and the properties required for applications. In particular, the triboelectric, friction, and dielectric performance of advanced materials is focused upon and their roles in designing the TENGs are analyzed. The recent progress of advanced materials used in TENGs for mechanical energy harvesting and self-powered sensors is also summarized. Finally, an overview of the emerging challenges, strategies, and opportunities for research and development of advanced materials for TENGs is provided.

Recent Advances in Triboelectric Nanogenerators: From Technological Progress to Commercial Applications

Serious climate changes and energy-related environmental problems are currently critical issues in the world. In order to reduce carbon emissions and save our environment, renewable energy harvesting technologies will serve as a key solution in the near future. Among them, triboelectric nanogenerators (TENGs), which is one of the most promising mechanical energy harvesters by means of contact electrification phenomenon, are explosively developing due to abundant wasting mechanical energy sources and a number of superior advantages in a wide availability and selection of materials, relatively simple device configurations, and low-cost processing. Significant experimental and theoretical efforts have been achieved toward understanding fundamental behaviors and a wide range of demonstrations since its report in 2012. As a result, considerable technological advancement has been exhibited and it advances the timeline of achievement in the proposed roadmap. Now, the technology has reached the stage of prototype development with verification of performance beyond the lab scale environment toward its commercialization. In this review, distinguished authors in the world worked together to summarize the state of the art in theory, materials, devices, systems, circuits, and applications in TENG fields. The great research achievements of researchers in this field around the world over the past decade are expected to play a major role in coming to fruition of unexpectedly accelerated technological advances over the next decade.

Interfacial Roughness Enhanced Gel/Elastomer Interfacial Bonding Enables Robust and Stretchable Triboelectric Nanogenerator for Reliable Energy Harvesting

Gel-based triboelectric nanogenerator (TENG) has demonstrated promising potentials in stretchable electronics owing to gel electrodes' intrinsic softness, stretchability, and conductivity. However, delamination between gel and elastomer layers in deformations remains a considerable challenge for gel-based TENG, which most often induces structure failure. Herein, gels are regarded as adhesives and further effectively enhances interfacial bonding strength by a rough interface in adhesives' view, which exploits gels' liquid-to-solid transformation. This method just needs surface roughness of elastomer, which avoids chemical modification. Moreover, this method is effective to both organogel with good stickiness and hydrogel with weak stickiness, demonstrating wide applicability to different gels. Owing to the tough gel/elastomer interfacial bonding, TENG-Rough largely solves delamination problem under various deformations and the corresponding output performances of TENG-Rough are also maintained, implying a robust stretchable TENG device for reliable energy harvesting. This work demonstrates a general and facile method to enhance interfacial bonding in an adhesives' way, which provides a view for addressing delamination problem in gel-based TENGs and other kinds of gel-based devices.

A Dual-Functional Triboelectric Nanogenerator Based on the Comprehensive Integration and Synergetic Utilization of Triboelectrification, Electrostatic Induction, and Electrostatic Discharge to Achieve Alternating Current/Direct Current Convertible Outputs

Traditional alternating current (AC) and direct current (DC) triboelectric nanogenerators (TENGs), which are implemented via the pairwise coupling of triboelectrification, electrostatic induction, and electrostatic discharge, have been widely explored in various fields. In this work, the comprehensive integration and synergetic utilization of triboelectrification, electrostatic induction, and electrostatic discharge in a single device for the first time is realized, achieving a dual-functional TENG (DF-TENG) to produce an AC/DC convertible output. Distinguishing from the conventional TENGs, the coupling of triboelectrification and electrostatic discharge enables charge circulation between the dielectric tribo-layers, while electrostatic induction realizes charge transfer in the external circuit. This novel energy conversion mechanism has been proven to be applicable to a variety of materials, including polymers, fabrics, and semiconductors. The output mode of the DF-TENG can be tuned by adjusting the slider motion state, and its constant output current and power density can reach 1.51 mA m^-2 Hz^-1 and 398 mW m^-2 Hz^-1 , respectively, which are the highest records reported for constant DC-TENGs to date. This work not only provides a paradigm shift to achieve AC/DC convertible output, but it also exhibits high potential for extending the TENG design philosophy.

Effect of Ag nanoparticle size on triboelectric nanogenerator for mechanical energy harvesting

Triboelectric nanogenerators (TENG) are generally utilized on the grounds that they can catch low-recurrence mechanical energy from various types of movement and convert it into electricity. It has been proved that the adulteration of conductive particles in the triboelectric layer can improve its output performance, but metal nanomaterials have different properties at different scales. In this paper, the triboelectric layer of TENG is a composite film made of silver nanoparticles (AgNPs) with different particle sizes (20 nm, 50 nm, 200 nm and 500 nm) that were dispersed and mixed with two-component liquid silica gel step by step. The open circuit voltage (Voc) and short circuit current (Isc) of the 20 nm component of the AgNPs-dispersed/two-component liquid silica gel TENG(At-TENG) are 102.8 V and 4.42μA, which are higher than the result execution of the other components. Smaller size nanoparticles have more number of nanoparticles when the mass fraction is the same. AgNPs form micro-capacitance structures in the insulating polymer layer and enhance the dielectric properties of the composite films through an interfacial polarization mechanism. At-TENG can light up 53 commercial LEDs and power calculators or wristband electronic watches, proving its utility as a self-powered power source. An extensive experiment proves the advantage of small size using comparison and theoretical analysis and provides suggestions for the selection of TENG dopants.

Direct Observation of the Epitaxial Growth of Bismuth Telluride Topological Insulators from One-Dimensional Heterostructured Nanowires

As extraordinary topological insulators, 2D bismuth telluride (Bi₂Te₃) nanosheets have been synthesized and controlled with a few-layer structure by a facile and fast solvothermal process. The detail-oriented growth evolution of 2D Bi₂Te₃ in an ethylene glycol reducing solution is discovered and recorded for direct observation of the liquid–solid interactions through the use of environmental SEM. At the initial synthesis stage, Te nanowires are rapidly synthesized and observed in solution. In the next stage, Bi nanoclusters slowly adhere to the Te nanowires and react to form hierarchical Te-Bi₂Te₃ nanostructured materials. Additionally, the Te nanowires shorten in-plane in an orderly manner, while the Bi₂Te₃ nanosheets exhibit directional out-of-plane epitaxial growth. In the last procedure, Bi₂Te₃ nanosheets with a clear hexagonal appearance can be largely obtained. Experiments performed under these rigorous conditions require careful consideration of the temperature, time, and alkaline environment for each reaction process. In addition, the yield of a wider and thinner Bi₂Te₃ nanosheet is synthesized by manipulating the crystal growth with an optimal alkaline concentration, which is found through statistical analysis of the AFM results. In the UV–Vis–NIR spectroscopy results, the main peak in the spectrum tends to redshift, while the other peak in the ultraviolet range decreases during Bi₂Te₃ nanosheet synthesis, facilitating a rapid understanding of the trends in the morphological evolution of the Bi₂Te₃ materials in solution. By rationalizing the above observations, we are the first to report the success of environmental SEM, HAADF-STEM, and UV–Vis–NIR spectroscopy for confirming the Bi₂Te₃ nanosheet formation mechanism and the physical properties in the solvent media. This research promotes the future optimization of promising Bi₂Te₃ nanomaterials that can be used in the fabrication of thermoelectric and topological components.