Rewritable ghost floating gates by tunnelling triboelectrification for two-dimensional electronics

Highly efficient microbial inactivation enabled by tunneling charges injected through two-dimensional electronics

Airborne pathogens retain prolonged infectious activity once attached to the indoor environment, posing a pervasive threat to public health. Conventional air filters suffer from ineffective inactivation of the physics-separated microorganisms, and the chemical-based antimicrobial materials face challenges of poor stability/efficiency and inefficient viral inactivation. We, therefore, developed a rapid, reliable antimicrobial method against the attached indoor bacteria/viruses using a large-scale tunneling charge-motivated disinfection device fabricated by directly dispersing monolayer graphene on insulators. Free charges can be stably immobilized under the monolayer graphene through the tunneling effect. The stored charges can motivate continuous electron loss of attached microorganisms for accelerated disinfection, overcoming the diffusion limitation of chemical disinfectants. Complete (>99.99%) and broad-spectrum disinfection was achieved <1 min of attachment to the scaled-up device (25 square centimeters), reliably for 72 hours at high temperature (60°C) and humidity (90%). This method can be readily applied to high-touch surfaces in indoor environments for pathogen control.

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.

Prospective applications of two-dimensional materials beyond laboratory frontiers: A review

The development of nanotechnology has been advancing for decades and gained acceleration in the 21st century. Two-dimensional (2D) materials are widely available, giving them a wide range of material platforms for technological study and the advancement of atomic-level applications. The design and application of 2D materials are discussed in this review. In order to evaluate the performance of 2D materials, which might lead to greater applications benefiting the electrical and electronics sectors as well as society, the future paradigm of 2D materials needs to be visualized. The development of 2D hybrid materials with better characteristics that will help industry and society at large is anticipated to result from intensive research in 2D materials. This enhanced evaluation might open new opportunities for the synthesis of 2D materials and the creation of devices that are more effective than traditional ones in various sectors of application.

Energy Interplay in Materials: Unlocking Next-Generation Synchronous Multisource Energy Conversion with Layered 2D Crystals

Layered 2D crystals have unique properties and rich chemical and electronic diversity, with over 6000 2D crystals known and, in principle, millions of different stacked hybrid 2D crystals accessible. This diversity provides unique combinations of properties that can profoundly affect the future of energy conversion and harvesting devices. Notably, this includes catalysts, photovoltaics, superconductors, solar-fuel generators, and piezoelectric devices that will receive broad commercial uptake in the near future. However, the unique properties of layered 2D crystals are not limited to individual applications and they can achieve exceptional performance in multiple energy conversion applications synchronously. This synchronous multisource energy conversion (SMEC) has yet to be fully realized but offers a real game-changer in how devices will be produced and utilized in the future. This perspective highlights the energy interplay in materials and its impact on energy conversion, how SMEC devices can be realized, particularly through layered 2D crystals, and provides a vision of the future of effective environmental energy harvesting devices with layered 2D crystals.

Variations in the Effective Work Function of Graphene in a Sliding Electrical Contact Interface under Ambient Conditions

Control of work function (WF) in graphene is crucial for graphene application in electrode material replacement and electrode surface protection in optoelectronic devices. Although efforts have been made to manipulate the effective WF of graphene to optimize its application, most studies have focused on graphene employed in static electrical contact interfaces. In this work, we investigated WF variations of supported single-layer graphene (SLG) in sliding electrical contact under ambient conditions, which was achieved by sliding an electrically biased conductive atomic force microscopy (cAFM) probe on the SLG surface. The effective WF, structural properties, and chemical compositions of rubbed SLG were subsequently measured by Kelvin probe force microscopy, Raman spectroscopy, and X-ray photoelectron spectroscopy, respectively. We found that the effective WF of the rubbed SLG was governed by both the tunneling triboelectric effect (TTE) and tribochemical-induced surface functionalization. The TTE charges generated by the sliding cAFM probe tunneled through the structural defects of the SLG and were trapped underneath the SLG. The SLG will be either p-doped or n-doped depending on the type of TTE charges and the polarity of electric bias applied to the cAFM probe during the rubbing process. However, the applied electric bias also led to the electrolysis of a water meniscus formed at the cAFM probe-SLG contact, resulting in surface oxidation and the increase of SLG WF. Further absorption of ambient water molecules on the oxygenated functional groups gradually reduced the SLG WF. The influence of TTE and surface functionalization on the SLG WF depends on the magnitude and polarity of applied electric biases, relative humidity, and physical properties of the supporting substrates. Our results demonstrate that the effective WF of SLG in a sliding electrical contact interface will vary with time and might need to be considered for related applications.

Twin-Wire Networks for Zero Interconnect, High-Density 4-Wire Electrical Characterizations of Materials

Four-wire measurements have been introduced by Lord Kelvin in 1861 and have since become the standard technique for characterizing small resistances and impedances. However, high-density 4-wire measurements are generally complex, time-consuming, and inefficient because of constraints on interconnects, pads, external wires, and mechanical contacts, thus reducing reproducibility, statistical significance, and throughput. Here, we introduce, systematically design, analyze, and experimentally validate zero interconnect networks interfaced to external instrumentation by couples of twin wire. 3D-printed holders with magnets, interconnects, nonadhesive layers, and spacers can effortlessly establish excellent electrical connections with tunable or minimum contact forces and enable accurate measurements even for delicate devices, such as thin metals on soft polymers. As an example, we measured all the resistances of a twin-wire 29-resistor network made of silver-nanoparticle ink printed on polyimide, paper, or photo paper, including during sintering or temperature calibration, resulting in an unprecedentedly easy and accurate characterization of both resistivity and its temperature coefficient. The theoretical framework and experimental strategies reported here represent a breakthrough toward zero interconnect, simple, and efficient high-density 4-wire characterizations, can be generalized to other 4-wire measurements (impedances, sensors) and can open the way to more statistically meaningful and reproducible analyses of materials, high-throughput measurements, and minimally invasive characterizations of biomaterials.

Manipulating Edge Current in Hexagonal Boron Nitride via Doping and Friction

We map spatially correlated electrical current on the stacking boundaries of pristine and doped hexagonal boron nitride (hBN) to distinguish from its insulating bulk via conductive atomic force microscopy (CAFM). While the pristine edges of hBN show an insulating nature, the O-doped edges reveal a current 2 orders of higher even for bulk layers where the direct transmission through tunnel barrier is implausible. Instead, the nonlinear current-voltage characteristics (I-V) at the edges of O-doped hBN can be explained by trap-assisted lowering of the tunnel barrier by adopting a Poole-Frenkel (PF) model. However, in the stacked heterostructure with multilayer graphene (MLG) on top, the buried edge of pristine hBN shows a signature of electron conduction in the scanning mode which contradicts the first-principle calculation of spatial distribution of local density of states (LDOS) data. Enhancement of friction between the Pt-tip and MLG at the step-edge of the heterostructure while scanning in the contact mode has prompted us to construct a phenomenological model where the localization of opposite surface charges on two conducting plates (MLG and Si substrate) containing a dielectric film (hBN) with negatively charged defects creates an internal electric field opposite to the external electric field due to the applied voltage bias in the CAFM setup. An equivalent circuit with a parallel resistor network based on a vertical conducting channel through the MLG/hBN edge and an in-plane surface carrier transport through MLG can successfully analyze the current maps on pristine/doped hBN and the related heterostructures. These results yield fundamental insight into the emerging field of insulatronics in which defect-induced electron transport along the edge can be manipulated in an 1D-2D synergized insulator.

Mixed Triboelectric and Flexoelectric Charge Transfer at the Nanoscale

The triboelectric effect is a ubiquitous phenomenon in which the surfaces of two materials are easily charged during the contact-separation process. Despite the widespread consequences and applications, the charging mechanisms are not sufficiently understood. Here, the authors report that, in the presence of a strain gradient, the charge transfer is a result of competition between flexoelectricity and triboelectricity, which could enhance charge transfer during triboelectric measurements when the charge transfers of both effects are in the same direction. When they are in the opposite directions, the direction and amount of charge transfer could be modulated by the competition between flexoelectric and triboelectric effects, which leads to a distinctive phenomenon, that is, the charge transfer is reversed with varying forces. The subsequent results on the electrical power output signals from the triboelectrification support the proposed mechanism. Therefore, the present study emphasizes the key role of the flexoelectric effect through experimental approaches, and suggests that both the amount and direction of charge transfer can be modulated by manipulating the mixed triboelectric and flexoelectric effects. This finding may provide important information on the triboelectric effect and can be further extended to serve as a guideline for material selection during a nanopatterned device design.

Flexible Piezoelectricity of Two-Dimensional Materials Governed by Effective Berry Curvature

Two-dimensional piezoelectric materials have been regarded as ideal candidates for flexible and versatile nanoelectromechanical systems, yet their fundamental piezoelectric mechanisms remain to be fully understood. Employing joint theoretical-statistical analyses, we develop universal models for quantifying the piezoelectricity of three-coordinated honeycomb-like monolayers at the atomistic level. The theoretical model within the framework of modern polarization theory suggests that the distribution of effective Berry curvature is essential for interpreting the relation between microscopic/electronic structures and piezoelectric properties. The statistical model based on DFT high-throughput calculation reveals that 2D piezoelectricity crucially depends on the effective mass, bandgap, and atomic distance along the rotation axis. Implementing stress and doping is demonstrated to be effective for optimizing piezoelectricity. Such findings provide valuable guidelines for designing 2D piezoelectric materials.

Influence of MoS2-metal interface on charge injection: a comparison between various metal contacts

Achieving good contacts is vital for harnessing the fascinating properties of two-dimensional (2D) materials. However, unsatisfactory 2D material-metal interfaces remain a problem that hinders the successful application of 2D materials for fabricating nanodevices. In this study, Kelvin probe force microscopy (KPFM) and other high-resolution microscopy techniques are utilized to characterize the surface morphology and contact interface between MoS₂ and common metals including Au, Ti, Pd, and Ni. Surface potential information, including the contact potential difference ([Formula: see text]) and surface potential difference ([Formula: see text]) of each MoS₂-metal contact, is obtained. By comparing the surface potential distribution mappings with and without illumination, non-zero surface photovoltage (SPV) values and evident shift with amplitudes of 32 mV and 44 mV are observed for MoS₂-Au and Ti, but not for MoS₂-Pd and Ni. The Schottky barrier heights of MoS₂-Au, Ti, Pd, and Ni are roughly evaluated from their I-V curves. Raman spectroscopy is also carried out to ensure more convincing results. All the results suggest that a smoother MoS₂-metal interface results in better charge transport behaviors. Our analysis of the underlying mechanism and experimental findings offer a new perspective to better understand MoS₂-metal contacts and underscore the fundamental importance of interface morphology for MoS₂-based devices.

Nonvolatile and Neuromorphic Memory Devices Using Interfacial Traps in Two-Dimensional WSe2/MoTe2 Stack Channel

Very recently, stacked two-dimensional materials have been studied, focusing on the van der Waals interaction at their stack junction interface. Here, we report field effect transistors (FETs) with stacked transition metal dichalcogenide (TMD) channels, where the heterojunction interface between two TMDs appears useful for nonvolatile or neuromorphic memory FETs. A few nanometer-thin WSe₂ and MoTe₂ flakes are vertically stacked on the gate dielectric, and bottom p-MoTe₂ performs as a channel for hole transport. Interestingly, the WSe₂/MoTe₂ stack interface functions as a hole trapping site where traps behave in a nonvolatile manner, although trapping/detrapping can be controlled by gate voltage (V_GS). Memory retention after high V_GS pulse appears longer than 10000 s, and the Program/Erase ratio in a drain current is higher than 200. Moreover, the traps are delicately controllable even with small V_GS, which indicates that a neuromorphic memory is also possible with our heterojunction stack FETs. Our stack channel FET demonstrates neuromorphic memory behavior of ∼94% recognition accuracy.

Doping of MoTe2 via Surface Charge Transfer in Air

Doping is a key process by which the concentration and type of majority carriers can be tuned to achieve desired conduction properties. The common way of doping is via bulk impurities, as in the case of silicon. For van der Waals bonded semiconductors, control over bulk impurities is not as well developed, because they may either migrate between the layers or bond with the surfaces or interfaces becoming undesired scattering centers for carriers. Herein, we investigate by means of Kelvin probe force microscopy (KPFM) and density functional theory calculations (DFT) the doping of MoTe2 via surface charge transfer occurring in air. Using DFT, we show that oxygen molecules physisorb on the surface and increase its work function (compared to pristine surfaces) toward p-type behavior, which is consistent with our KPFM measurements. The surface charge transfer doping (SCTD) driven by adsorbed oxygen molecules can be easily controlled or reversed through thermal annealing of the entire sample. Furthermore, we also demonstrate local control of the doping by contact electrification. As a reversible and controllable nanoscale physisorption process, SCTD can thus open new avenues for the emerging field of 2D electronics.

Intrinsically Stretchable Organic-Tribotronic-Transistor for Tactile Sensing

Stretchable electronics are of great significance for the development of the next-generation smart interactive systems. Here, we propose an intrinsically stretchable organic tribotronic transistor (SOTT) without a top gate electrode, which is composed of a stretchable substrate, silver nanowire electrodes, semiconductor blends, and a nonpolar elastomer dielectric. The drain-source current of the SOTT can be modulated by external contact electrification with the dielectric layer. Under 0-50% stretching both parallel and perpendicular to the channel directions, the SOTT retains great output performance. After being stretched to 50% for thousands of cycles, the SOTT can survive with excellent stability. Moreover, the SOTT can be conformably attached to the human hand, which can be used for tactile signal perception in human-machine interaction and for controlling smart home devices and robots. This work has realized a stretchable tribotronic transistor as the tactile sensor for smart interaction, which has extended the application of tribotronics in the human-machine interface, wearable electronics, and robotics.

Emerging Devices Based on Two-Dimensional Monolayer Materials for Energy Harvesting

Two-dimensional (2-D) materials of atomic thickness have attracted considerable interest due to their excellent electrical, optoelectronic, mechanical, and thermal properties, which make them attractive for electronic devices, sensors, and energy systems. Scavenging the otherwise wasted energy from the ambient environment into electrical power holds promise to address the emerging energy needs, in particular for the portable and wearable devices. The versatile properties of 2-D materials together with their atomically thin body create diverse possibilities for the conversion of ambient energy. The present review focuses on the recent key advances in emerging energy-harvesting devices based on monolayer 2-D materials through various mechanisms such as photovoltaic, thermoelectric, piezoelectric, triboelectric, and hydrovoltaic devices, as well as progress for harvesting the osmotic pressure and Wi-Fi wireless energy. The representative achievements regarding the monolayer heterostructures and hybrid devices are also discussed. Finally, we provide a discussion of the challenges and opportunities for 2-D monolayer material-based energy-harvesting devices in the development of self-powered electronics and wearable technologies.

Triboelectrification of Two-Dimensional Chemical Vapor Deposited WS2 at Nanoscale

Triboelectric properties of chemical vapor deposited WS₂ nanoflakes have been characterized in nano-range by atomic force microscopy (AFM) and Kelvin force microscopy (KFM). The triboelectric process is dependent on the thickness of WS₂ nanoflakes, and it is sensitive to the adsorbates like water molecules, as well as transferred Pt from the tip on the sample. The density of tribo-charge can be modified by applying various biases to the conductive Pt-coated tip during the frictional process. Tunneling of the tribo-charge into the gap between WS₂ and the underlying substrate results in a long lifetime, which is about 100 times longer than conventional triboelectric charges. Moreover, we observe a positive correlation between the layer number and resistance to charge dissipation. Our finding can become the driving force for a new category of two-dimensional (2D) WS₂ triboelectrically controllable nanodevices.

Electron Transfer in Nanoscale Contact Electrification: Photon Excitation Effect

Contact electrification (CE) (or triboelectrification) is a well-known phenomenon, and the identity of the charge carriers and their transfer mechanism have been discussed for decades. Recently, the species of transferred charges in the CE between a metal and a ceramic was revealed as electron transfer and its subsequent release is dominated by the thermionic emission process. Here, the release of CE-induced electrostatic charges on a dielectric surface under photon excitation is studied by varying the light intensity and wavelength, but under no significant raise in temperature. The results suggest that there exists a threshold photon energy for releasing the triboelectric charges from the surface, which is 4.1 eV (light wavelength at 300 nm) for SiO₂ and 3.4 eV (light wavelength at 360 nm) for PVC; photons with energy smaller than this cannot effectively excite the surface electrostatic charges. This process is attributed to the photoelectron emission of the charges trapped in the surface states of the dielectric material. Further, a photoelectron emission model is proposed to describe light-induced charge decay on a dielectric surface. The findings provide an additional strong evidence about the electron transfer process in the CE between metals and dielectrics as well as polymers.

Recent progress in piezotronics and tribotronics

As the electronic technology is approaching its limits of materials and processing, a smart interaction between functional device and environment is a promising way for future electronic technology above the Moore's law. The mechanical signal triggering is the most common and natural way for the smart interactions, which has realized direct interaction between human/ambient and electronics and artificial intelligence. In 2006, the piezotronic effect, as a novel effect, was first proposed by Wang to achieve the effective, adaptive and seamless interactions between electronic devices and the external stress, which utilizes the piezoelectric polarization potential as the virtual gate to tune/control the carriers' transportation in the electronic device. Since then, this new effect has been widely observed in many low-dimensional semiconductors such as ZnO, GaN, CdS nanowires, and 2D MoS₂. In extension, tribotronics was first proposed in 2014 by Wang, which is about the devices manufactured using the electrostatic potential created by triboelectrification as a 'gate' voltage to tune/control energy transformation and electrical transport in semiconductors for the smart interaction between device and environment. Tribotronics has made rapid research progress and many tribotronic functional devices have been studied with a variety of materials, such as tribotronic tactile switch, memory, hydrogen sensor and phototransistor. This review highlights advances in piezotronics and tribotronics with focus on fundamental theories, nanoscale materials, functional devices and simulations. Our emphasis is mainly about their application for third-generation semiconductor. The concepts and results presented in this review show that the piezotronics and tribotronics will facilitate the development of MEMS/NEMS, self-powered sensing, man-computer interfacing, and active wearable electronics.

Controllable Tunneling Triboelectrification of Two-Dimensional Chemical Vapor Deposited MoS2

Tunneling triboelectrification of chemical vapor deposited monolayer MoS2 has been characterized at nanoscale with contact-mode atomic force microscopy (AFM) and Kelvin force microscopy (KFM). Although charges can be trapped on insulators like SiO2 by conventional triboelectrification, triboelectric charges tunneling through MoS2 and localized at the underlying substrate exhibit more than two orders of magnitude longer lifetime. Their polarity and density can be modified by triboelectric process with various bias voltages applied to Pt-coated AFM tips, and the saturated density is almost 30 times higher than the reported result of SiO2. Thus, the controllable tunneling triboelectric properties of MoS2 on insulating substrates can provide guidance to build a new class of two-dimensional (2D) MoS2-based nanoelectronic devices.

A New Facile Route to Flexible and Semi-Transparent Electrodes Based on Water Exfoliated Graphene and their Single-Electrode Triboelectric Nanogenerator

Wearable technologies are driving current research efforts to self-powered electronics, for which novel high-performance materials such as graphene and low-cost fabrication processes are highly sought.The integration of high-quality graphene films obtained from scalable water processing approaches in emerging applications for flexible and wearable electronics is demonstrated. A novel method for the assembly of shear exfoliated graphene in water, comprising a direct transfer process assisted by evaporation of isopropyl alcohol is developed. It is shown that graphene films can be easily transferred to any target substrate such as paper, flexible polymeric sheets and fibers, glass, and Si substrates. By combining graphene as the electrode and poly(dimethylsiloxane) as the active layer, a flexible and semi-transparent triboelectric nanogenerator (TENG) is demonstrated for harvesting energy. The results constitute a new step toward the realization of energy harvesting devices that could be integrated with a wide range of wearable and flexible technologies, and opens new possibilities for the use of TENGs in many applications such as electronic skin and wearable electronics.

Triboelectric Series of 2D Layered Materials

Recently, as applications based on triboelectricity have expanded, understanding the triboelectric charging behavior of various materials has become essential. This study investigates the triboelectric charging behaviors of various 2D layered materials, including MoS₂ , MoSe₂ , WS₂ , WSe₂ , graphene, and graphene oxide in a triboelectric series using the concept of a triboelectric nanogenerator, and confirms the position of 2D materials in the triboelectric series. It is also demonstrated that the results are obviously related to the effective work functions. The charging polarity indicates the similar behavior regardless of the synthetic method and film thickness ranging from a few hundred nanometers (for chemically exfoliated and restacked films) to a few nanometers (for chemical vapor deposited films). Further, the triboelectric charging characteristics could be successfully modified via chemical doping. This study provides new insights to utilize 2D materials in triboelectric devices, allowing thin and flexible device fabrication.

Nanoscale charge transfer and diffusion at the MoS2/SiO2 interface by atomic force microscopy: contact injection versus triboelectrification

Understanding the process of charge generation, transfer, and diffusion between two-dimensional (2D) materials and their supporting substrates is very important for potential applications of 2D materials. Compared with the systematic studies of triboelectric charging in a bulk sample, a fundamental understanding of the triboelectrification of the 2D material/insulator system is rather limited. Here, the charge transfer and diffusion of both the SiO₂ surface and MoS₂/SiO₂ interface through contact electrification and frictional electrification are investigated systematically in situ by scanning Kelvin probe microscopy and dual-harmonic electrostatic force microscopy. Different from the simple static charge transfer between SiO₂ and the PtSi alloy atomic force microscope (AFM) tip, the charge transfer between the tip and the MoS₂/SiO₂ system is complicated. Triboelectric charges, generated by contact or frictional electrification with the AFM tip, are trapped at the MoS₂/SiO₂ interface and act as floating gates. The local charge discharge processes can be obtained by monitoring the surface potential. The charge decay time (τ) of the MoS₂/SiO₂ interface is one (or two) orders of magnitude larger than the decay time τ of the SiO₂ surface. This work facilitates an understanding of the triboelectric and de-electrification of the interface between 2D materials and substrates. In addition to the charge transfer and diffusion, we demonstrate the nanopatterns of surface and interfacial charges, which have great potential for the application of self-assembly of charged nanostructures.