Flexoelectricity, Triboelectricity, and Free Interfacial Charges.

Triboelectricity has been a topic of some confusion for many years, probably because it is very diverse and some of the fundamental science has not been clear. This is now starting to change. A few years ago, the importance of flexoelectricity at asperities is pointed out. That paper exploited the established physics of compensation of bound surface or interfacial charges without going into detail. The purpose of this paper is to expand further on this, mapping from the established physics of electrostatics with contact potentials and Maxwell's displacement field to the underlying fundamentals of charge transfer in triboelectricity. Examples from the published literature are used to illustrate this. In the discussion, some of the open questions and challenges to the community are mentioned.

Self-Powered Colorful Dynamic Electrowetting Display Systems Based on Triboelectricity.

Electrowetting displays (EWDs) based on microfluidics are highly sought after in the fields of electronic devices, smart homes, and information communication. However, the power supply of the EWD systems for visually engaging multi-color displays remains a big challenge. Herein, self-powered colorful dynamic display systems are developed by integrating the triboelectric nanogenerator (TENG) with the EWD device. The TENG is designed with a nanotube-patterned surface and can generate open-circuit voltages ranging from 30 to 295 V by controlling the contact area. The wetting property of the micro-droplet exhibits a response to the applied voltage, enabling the triboelectricity-triggered electrowetting-on-dielectric. Driven by the voltage of 160 V, the monochromatic EWD exhibits bright color switching from magenta to transparent with a pixel aperture ratio of 78%, and the recovery process can be rapidly completed. Furthermore, the self-powered colorful dynamic EWD system can be achieved. By selectively applying the voltage to the pixels in the three monochromatic layers that constitute the colorful EWD, the wetting properties of the fluids can be controlled, allowing for colorful dynamic display. This work contributes to the advancement of color display technology for portable and wearable electronic ink displays, indoor and outdoor sports equipment, and information communication.

Interfacial Triboelectricity Lights Up Phosphor-Polymer Elastic Composites: Unraveling the Mechanism of Mechanoluminescence in Zinc Sulfide Microparticle-Embedded Polydimethylsiloxane Films.

Composites comprising copper-doped zinc sulfide phosphor microparticles embedded in polydimethylsiloxane (ZnS:Cu-PDMS) have received significant attention over the past decade because of their bright and durable mechanoluminescence (ML); however, the underlying mechanism of this unique ML remains unclear. This study reports empirical and theoretical findings that confirm this ML is an electroluminescence (EL) of the ZnS:Cu phosphor induced by the triboelectricity generated at the ZnS:Cu microparticle-PDMS matrix interface. ZnS:Cu microparticles that exhibit bright ML are coated with alumina, an oxide with strong positive triboelectric properties; the contact separation between this oxide coating and PDMS, a polymer with strong negative triboelectric properties, produces sufficient interfacial triboelectricity to induce EL in ZnS:Cu microparticles. The ML of ZnS:Cu-PDMS composites varies on changing the coating material, exhibiting an intensity that is proportional to the amount of interfacial triboelectricity generated in the system. Finally, based on these findings, a mechanism that explains the ML of phosphor-polymer elastic composites (interfacial triboelectric field-driven alternating-current EL model) is proposed in this study. It is believed that understanding this mechanism will enable the development of new materials (beyond ZnS:Cu-PDMS systems) with bright and durable ML.

Band Bending and Ratcheting Explain Triboelectricity in a Flexoelectric Contact Diode.

Triboelectricity was recognized millennia ago, but the fundamental mechanism of charge transfer is still not understood. We have recently proposed a model where flexoelectric band bending due to local asperity contacts drives triboelectric charge transfer in non-metals. While this ab initio model is consistent with a wide range of observed phenomena, to date there have been no quantitative analyses of the proposed band bending. In this work we use a Pt0.8Ir0.2 conductive atomic force microscope probe to simultaneously deform a Nb-doped SrTiO3 sample and collect current-bias data. The current that one expects based upon an analysis including the relevant flexoelectric band bending for a deformed semiconductor quantitively agrees with the experiments. The analysis indicates a general ratcheting mechanism for triboelectric transfer and strong experimental evidence that flexoelectric band bending is of fundamental importance for triboelectric contacts.

When Flexoelectricity Drives Triboelectricity.

Triboelectricity has been known since antiquity, but the fundamental science underlying this phenomenon lacks consensus. We present a flexoelectric model for triboelectricity where contact deformation induced band bending at the nanoscale is the driving force for charge transfer. This framework is combined with first-principles and finite element calculations to explore charge transfer implications for different contact geometry and materials combinations. We demonstrate that our ab initio based formulation is compatible with existing empirical models and experimental observations including charge transfer between similar materials and size/pressure dependencies associated with triboelectricity.

M13 Virus Triboelectricity and Energy Harvesting.

Triboelectrification is a phenomenon that generates electric potential upon contact. Here, we report a viral particle capable of generating triboelectric potential. M13 bacteriophage is exploited to fabricate precisely defined chemical and physical structures. By genetically engineering the charged structures, we observe that more negatively charged phages can generate higher triboelectric potentials and can diffuse the electric charges faster than less negatively charged phages can. The computational results show that the glutamate-engineered phages lower the LUMO energy level so that they can easily accept electrons from other materials upon contact. A phage-based triboelectric nanogenerator is fabricated and it could produce ∼76 V and ∼5.1 µA, enough to power 30 light-emitting diodes upon a mechanical force application. Our biotechnological approach will be useful to understand the electrical behavior of biomaterials, harvest mechanical energy, and provide a novel modality to detect desired viruses in the future.

Bumblebee electric charge stimulates floral volatile emissions in Petunia integrifolia but not in Antirrhinum majus.

The timing of volatile organic compound (VOC) emission by flowering plants often coincides with pollinator foraging activity. Volatile emission is often considered to be paced by environmental variables, such as light intensity, and/or by circadian rhythmicity. The question arises as to what extent pollinators themselves provide information about their presence, in keeping with their long co-evolution with flowering plants. Bumblebees are electrically charged and provide electrical stimulation when visiting plants, as measured via the depolarisation of electric potential in the stem of flowers. Here we test the hypothesis that the electric charge of foraging bumblebees increases the floral volatile emissions of bee pollinated plants. We investigate the change in VOC emissions of two bee-pollinated plants (Petunia integrifolia and Antirrhinum majus) exposed to the electric charge typical of foraging bumblebees. P. integrifolia slightly increases its emissions of a behaviorally and physiologically active compound in response to visits by foraging bumblebees, presenting on average 121 pC of electric charge. We show that for P. integrifolia, strong electrical stimulation (600-700 pC) promotes increased volatile emissions, but this is not found when using weaker electrical charges more representative of flying pollinators (100 pC). Floral volatile emissions of A. majus were not affected by either strong (600-700 pC) or weak electric charges (100 pC). This study opens a new area of research whereby the electrical charge of flying insects may provide information to plants on the presence and phenology of their pollinators. As a form of electroreception, this sensory process would bear adaptive value, enabling plants to better ensure that their attractive chemical messages are released when a potential recipient is present.

A ferroelectric nanocomposite-film-based device for harvesting energy from water droplets using both piezoelectric and triboelectric effects.

In this paper, we have demonstrated a novel design of a liquid-solid interface triboelectric nanogenerator based on a ZnO- polyvinylidene fluoride (PVDF) flexible ferroelectric film that employs both piezoelectric and triboelectric properties to produce more electricity from water droplets. The present device gives an output voltage of ∼1.32 V and a short-circuit current of ∼0.32µA from the conventional liquid-solid triboelectric nanogenerator (LSTENG), while an additional open-circuit voltage of ∼2.72 mV and short-circuit current of ∼20 nA is generated from the piezoelectric effect. The mechanism for generating energy in both the piezoelectric and triboelectric components is also discussed. Furthermore, we explored the effect of ions in water on the performance of the LSTENG, and the results were confirmed by Kelvin probe force microscopy measurements. The current work reveals a new LSTENG design and the benefit of employing a ferroelectric polymer as the contacting material rather than other non-piezoelectric materials for the LSTENG.

Household Materials Selection for Homemade Cloth Face Coverings and Their Filtration Efficiency Enhancement with Triboelectric Charging.

The COVID-19 pandemic is currently causing a severe disruption and shortage in the global supply chain of necessary personal protective equipment (e.g., N95 respirators). The U.S. CDC has recommended use of household cloth by the general public to make cloth face coverings as a method of source control. We evaluated the filtration properties of natural and synthetic materials using a modified procedure for N95 respirator approval. Common fabrics of cotton, polyester, nylon, and silk had filtration efficiency of 5-25%, polypropylene spunbond had filtration efficiency 6-10%, and paper-based products had filtration efficiency of 10-20%. An advantage of polypropylene spunbond is that it can be simply triboelectrically charged to enhance the filtration efficiency (from 6 to >10%) without any increase in pressure (stable overnight and in humid environments). Using the filtration quality factor, fabric microstructure, and charging ability, we are able to provide an assessment of suggested fabric materials for homemade facial coverings.

Toroidal plasmoid generation via extreme hydrodynamic shear.

Saint Elmo's fire and lightning are two known forms of naturally occurring atmospheric pressure plasmas. As a technology, nonthermal plasmas are induced from artificially created electromagnetic or electrostatic fields. Here we report the observation of arguably a unique case of a naturally formed such plasma, created in air at room temperature without external electromagnetic action, by impinging a high-speed microjet of deionized water on a dielectric solid surface. We demonstrate that tribo-electrification from extreme and focused hydrodynamic shear is the driving mechanism for the generation of energetic free electrons. Air ionization results in a plasma that, unlike the general family, is topologically well defined in the form of a coherent toroidal structure. Possibly confined through its self-induced electromagnetic field, this plasmoid is shown to emit strong luminescence and discrete-frequency radio waves. Our experimental study suggests the discovery of a unique platform to support experimentation in low-temperature plasma science.

Nanoscale Modulation of Friction and Triboelectrification via Surface Nanotexturing.

Nanoscale contact electrification (CE) of elastomer surfaces and the resulting tribocharge formation are important in many branches of nanotechnology but their mechanism is not fully clarified. In this Letter, we investigate the mechanism using the recently discovered phenomenon of replica molding-induced nanoscale CE. By generating tribocharge distributions patterned in close correlation with the interfacial nanotextures, the phenomenon provides well-defined targets for the investigation. By applying a variety of scanning probe microscopy techniques (AFM/KPFM/EFM) and finite element modeling (FEM) to the tribocharge distributions, we extract a process model that can explain how their patterns are formed and affected by the interfacial nanotexture's morphology. It turns out that the cumulative distance of the elastomer's tangential sliding during the interfacial separation plays the key role in shaping the tribocharge's distribution pattern. The model proves remarkably universal, staying valid to nanotextures all the way down in the sub-10 nm regime. This replica molding-induced CE also turns out to be an effective tool for sculpting nanoscale tribocharge distributions into unconventional forms, such as rings, partial eclipses, and dumbbells. Both the model and the technique will prove useful in many areas of nanotechnology.

Energy Harvesting from the Animal/Human Body for Self-Powered Electronics.

Living subjects (i.e., humans and animals) have abundant sources of energy in chemical, thermal, and mechanical forms. The use of these energies presents a viable way to overcome the battery capacity limitation that constrains the long-term operation of wearable/implantable devices. The intersection of novel materials and fabrication techniques offers boundless possibilities for the benefit of human health and well-being via various types of energy harvesters. This review summarizes the existing approaches that have been demonstrated to harvest energy from the bodies of living subjects for self-powered electronics. We present material choices, device layouts, and operation principles of these energy harvesters with a focus on in vivo applications. We discuss a broad range of energy harvesters placed in or on various body parts of human and animal models. We conclude with an outlook of future research in which the integration of various energy harvesters with advanced electronics can provide a new platform for the development of novel technologies for disease diagnostics, treatment, and prevention.

Schottky Diode Leakage Current Fluctuations: Electrostatically Induced Flexoelectricity in Silicon.

Nearly four decades have passed since IBM scientists pioneered atomic force microscopy (AFM) by merging the principles of a scanning tunneling microscope with the features of a stylus profilometer. Today, electrical AFM modes are an indispensable asset within the semiconductor and nanotechnology industries, enabling the characterization and manipulation of electrical properties at the nanoscale. However, electrical AFM measurements suffer from reproducibility issues caused, for example, by surface contaminations, Joule heating, and hard-to-minimize tip drift and tilt. Using as experimental system nanoscale Schottky diodes assembled on oxide-free silicon crystals of precisely defined surface chemistry, it is revealed that voltage-dependent adhesion forces lead to significant rotation of the AFM platinum tip. The electrostatics-driven tip rotation causes a strain gradient on the silicon surface, which induces a flexoelectric reverse bias term. This directional flexoelectric internal-bias term adds to the external (instrumental) bias, causing both an increased diode leakage as well as a shift of the diode knee voltage to larger forward biases. These findings will aid the design and characterization of silicon-based devices, especially those that are deliberately operated under large strain or shear, such as in emerging energy harvesting technologies including Schottky-based triboelectric nanogenerators (TENGs).

Understanding and Controlling Electrostatic Discharge in Triboelectric Nanogenerators.

Triboelectric nanogenerators (TENGs) have been widely used to harness various forms of mechanical energy for conversion to electrical energy. However, the contentious challenge in characterising TENGs is the lack of standard protocols for assessing mechanical-to-electrical energy conversion processes. Herein, macroscopic signal analysis is used to identify three key charging events within triboelectric signals: charge induction (CI), contact electrification (CE), and electrostatic discharge (ESD). By considering two phases of motion during contact-separation (approach and departure of the contact materials), CI arising from the motion of bound surface charge (varying electric field) between opposing contact materials is shown to dominate the measured displacement current signal, rather than the process of CE itself. Furthermore, the conventional signal (i. e., voltage, current, charge) interpretation of CE and CI during approach and departure phases is re-assessed, to indicate that the sudden spike of current often observed immediately prior to contact (or after separation) arises from polarity inverting electrostatic discharge (ESD). This aspect of the measured triboelectric effect, which is often ignored, is crucial for the design of TENGs and hence, techniques to enhance the understanding and control over the stochastic occurrence of ESDs is explored. The methods proposed for the deconvolution of the macroscopic signal components of TENGs, and mitigation of ESD occurrences, will allow for precise quantification of the associated charging events. The applications of this study will template the design and development of future super-TENGs with optimised energy conversion capabilities.

An All-Protein Multisensory Highly Bionic Skin.

To achieve a highly realistic robot, closely mimicking human skin in terms of materials and functionality is essential. This paper presents an all-protein silk fibroin bionic skin (SFBS) that emulates both fast-adapting (FA) and slow-adapting (SA) receptors. The mechanically different silk film and hydrogel, which exhibited skin-like properties, such as stretchability (>140%), elasticity, low modulus (<10 kPa), biocompatibility, and degradability, were prepared through mesoscopic reconstruction engineering to mimic the epidermis and dermis. Our SFBS, incorporating SA and FA sensors, demonstrated a highly sensitive (1.083 kPa-1) static pressure sensing performance (in vitro and in vivo), showed the ability to sense high-frequency vibrations (50-400 Hz), could discriminate materials and sliding, and could even identify the fine morphological differences between objects. As proof of concept, an SFBS-integrated rehabilitation glove was synthesized, which could help stroke patients regain sensory feedback. In conclusion, this work provides a practical approach for developing skin equivalents, prostheses, and smart robots.

Analytical Investigation of Replica-Molding-Enabled Nanopatterned Tribocharging Process on Soft-Material Surfaces.

Nanopatterned tribocharge can be generated on the surface of elastomers through their replica molding with nanotextured molds. Despite its vast application potential, the physical conditions enabling the phenomenon have not been clarified in the framework of analytical mechanics. Here, we explain the final tribocharge pattern by separately applying two models, namely cohesive zone failure and cumulative fracture energy, as a function of the mold nanotexture's aspect ratio. These models deepen our understanding of the triboelectrification phenomenon.

Fiber-in-Tube Electrifiable Structure for Virus Filtration Self-Generated Static Electricity by Vibration/Sound.

Fiber has been considered as an ideal material for virus insulation due to the readily available electrostatic adsorption. However, restricted by the electrostatic attenuation and filtration performance decline, their long-lasting applications are unable to satisfy the requirements of medical protective equipment for major medical and health emergencies such as global epidemics, which results in both a waste of resources and environmental pollution. We overcame these issues by constructing a fiber-in-tube structure, achieving the robust reusability of fibrous membranes. Core fibers within the hollow could form generators with tube walls of shell fibers to provide persistent, renewable static electricity via piezoelectricity and triboelectricity. The PM0.3 insulation efficiency achieved 98% even after 72 h of humidity and heat aging, through beating and acoustic waves, which is greatly improved compared with that of traditional nonwoven fabric (∼10% insulation). A mask spun with our fiber also has a low breathing resistance (differential pressure <24.4 Pa/cm2). We offer an approach to enrich multifunctional fiber for developing electrifiable filters, which make the fiber-in-tube filtration membrane able to durably maintain a higher level of protective performance to reduce the replacement and provide a new train of thought for the preparation of other high-performance protective products.

Influence of MXene Composition on Triboelectricity of MXene-Alginate Nanocomposites.

MXenes are highly versatile and conductive 2D materials that can significantly enhance the triboelectric properties of polymer nanocomposites. Despite the growing interest in the tunable chemistry of MXenes for energy applications, the effect of their chemical composition on triboelectric power generation has yet to be thoroughly studied. Here, we investigate the impact of the chemical composition of MXenes, specifically the Ti3CNTx carbonitride vs the most studied carbide, Ti3C2Tx, on their interactions with sodium alginate biopolymer and, ultimately, the performance of a triboelectric nanogenerator (TENG) device. Our results show that adding 2 wt % of Ti3CNTx to alginate produces a synergistic effect that generates a higher triboelectric output than the Ti3C2Tx system. Spectroscopic analyses suggest that a higher oxygen and fluorine content on the surface of Ti3CNTx enhances hydrogen bonding with the alginate matrix, thereby increasing the surface charge density of the alginate oxygen atoms. This was further supported by Kelvin probe force microscopy, which revealed a more negative surface potential on Ti3CNTx-alginate, facilitating high charge transfer between the TENG electrodes. The optimized Ti3CNTx-alginate nanogenerator delivered an output of 670 V, 15 µA, and 0.28 W/m2. Additionally, we demonstrate that plasma oxidation of the MXene surface further enhances triboelectric performance. Due to the diverse surface terminations of MXene, we show that Ti3CNTx-alginate can function as either tribopositive or tribonegative material, depending on the counter-contacting material. Our findings provide a deeper understanding of how MXene composition affects their interaction with biopolymers and resulting tunable triboelectrification behavior. This opens up new avenues for developing flexible and efficient MXene-based TENG devices.

Improved Energy Harvesting Ability of Single-Layer Binary Fiber Nanocomposite Membrane for Multifunctional Wearable Hybrid Piezoelectric and Triboelectric Nanogenerator and Self-Powered Sensors.

While wearable self-powered electronic devices have shown promising improvements, substantial challenges persist in enhancing their electrical output and structural performance. In this work, a working mechanism involving simultaneous piezoelectric and triboelectric conversion within a monolayer-structured membrane is proposed. Single-layer binary fiber nanocomposite membranes (SBFNMs) (PVDF/CNTX@PAN/CNTX, DPCPCX) with two distinct interpenetrating nanocomposite fibers were created through co-electrospinning, incorporating multiwalled carbon nanotubes (CNTs) into polyvinylidene fluoride (PVDF) and polyacrylonitrile (PAN), respectively. The resulting membrane demonstrated an exceptional synergistic effect of piezoelectricity and triboelectricity along with a high machine-to-electric conversion capability. The addition of CNTs increased the PVDF ß-phase and the PAN planar zigzag conformation. As a result, the DPCPC0.5-SBFNMs-based piezoelectric nanogenerator exhibited excellent electrical output (187 V, 8.0 µA, and 1.52 W m-2), maintaining an exceptionally high level of output voltage compared with other piezoelectric nanogenerators. It successfully illuminated 50 commercial light-emitting diodes simultaneously. The output voltage of DPCPC0.5-SBFNMs was 5.1 and 4.6 times higher than that of PAN or PVDF single-fiber membranes, respectively. Furthermore, the peak voltage of DPCPC0.5-SBFNMs exceeded that of co-electrospinning PVDF/CNT1.0@PAN (DPCP1.0) and PVDF@PAN/CNT1.0 (DPPC1.0) by 20 and 10 V, respectively. The piezoelectric sensor made of DPCPC0.5-SBFNMs accurately sensed human movement, ranging from tiny to large, and demonstrated utility as an alarm in medical treatment, fire fighting, and monitoring. Endogenous triboelectricity is proposed in SBFNM piezoelectric materials, enhancing electromechanical conversion and electrical output capacity, thereby promising a wide application potential in self-powered wearable electronic devices.

Closely Packed Stretchable Ultrasound Array Fabricated with Surface Charge Engineering for Contactless Gesture and Materials Detection.

Communication with hand gestures plays a significant role in human-computer interaction by providing an intuitive and natural way for humans to communicate with machines. Ultrasound-based devices have shown promising results in contactless hand gesture recognition without requiring physical contact. However, it is challenging to fabricate a densely packed wearable ultrasound array. Here, a stretchable ultrasound array is demonstrated with closely packed transducer elements fabricated using surface charge engineering between pre-charged 1-3 Lead Zirconate Titanate (PZT) composite and thin polyimide film without using a microscope. The array exhibits excellent ultrasound properties with a wide bandwidth (≈57.1%) and high electromechanical coefficient (≈0.75). The ultrasound array can decipher gestures up to 10 cm in distance by using a contactless triboelectric module and identify materials from the time constant of the exponentially decaying impedance based on their triboelectric properties by utilizing the electrostatic induction phase. The newly proposed metric of the areal-time constant is material-specific and decreases monotonically from a highly positive human body (1.13 m2 s) to negatively charged polydimethylsiloxane (PDMS) (0.02 m2 s) in the triboelectric series. The capability of the closely packed ultrasound array to detect material along with hand gesture interpretation provides an additional dimension in the next-generation human-robot interaction.