The piezoelectric characteristics of nanowires, thin films and bulk crystals have been closely studied for potential applications in sensors, transducers, energy conversion and electronics. With their high crystallinity and ability to withstand enormous strain, two-dimensional materials are of great interest as high-performance piezoelectric materials. Monolayer MoS2 is predicted to be strongly piezoelectric, an effect that disappears in the bulk owing to the opposite orientations of adjacent atomic layers. Here we report the first experimental study of the piezoelectric properties of two-dimensional MoS2 and show that cyclic stretching and releasing of thin MoS2 flakes with an odd number of atomic layers produces oscillating piezoelectric voltage and current outputs, whereas no output is observed for flakes with an even number of layers. A single monolayer flake strained by 0.53% generates a peak output of 15 mV and 20 pA, corresponding to a power density of 2 mW m(-2) and a 5.08% mechanical-to-electrical energy conversion efficiency. In agreement with theoretical predictions, the output increases with decreasing thickness and reverses sign when the strain direction is rotated by 90°. Transport measurements show a strong piezotronic effect in single-layer MoS2, but not in bilayer and bulk MoS2. The coupling between piezoelectricity and semiconducting properties in two-dimensional nanomaterials may enable the development of applications in powering nanodevices, adaptive bioprobes and tunable/stretchable electronics/optoelectronics.

Transparent, flexible and high efficient power sources are important components of organic electronic and optoelectronic devices. In this work, based on the principle of the previously demonstrated triboelectric generator, we demonstrate a new high-output, flexible and transparent nanogenerator by using transparent polymer materials. We have fabricated three types of regular and uniform polymer patterned arrays (line, cube, and pyramid) to improve the efficiency of the nanogenerator. The power generation of the pyramid-featured device far surpassed that exhibited by the unstructured films and gave an output voltage of up to 18 V at a current density of ∼0.13 μA/cm 2 . Furthermore, the as-prepared nanogenerator can be applied as a self-powered pressure sensor for sensing a water droplet (8 mg, ∼3.6 Pa in contact pressure) and a falling feather (20 mg, ∼0.4 Pa in contact pressure) with a low-end detection limit of ∼13 mPa. KEYWORDS: Nanogenerator, transparent, polymer, pressure sensor T he integration of flexible and transparent characteristics is an important component in the new organic electronic and optoelectronic devices 1−3 and has been achieved for various applications, including transistors, 4,5 lithium-ion batteries, 6 supercapacitors, 7,8 pressure sensors, and artificial skins. 9−12 Indeed, building flexible transparent energy conversion and storage units plays a key role in realizing fully flexible and transparent devices. In 2006, our group demonstrated the first piezoelectric ZnO nanogenerator that successfully converted mechanical energy into electric energy. 13 Since then, various nanogenerators (NGs) based on piezoelectric effect have been demonstrated. 14−17 As an important part in this field, some studies on fully integrated flexible and transparent NGs have been reported. 18−21 Almost all of them are based on piezoelectric ZnO nanowires and the entire device requires sophisticated design and a high degree of integration.The general physical process for energy conversion has three important steps: charge generation, charge separation, and charge flow. These steps were accomplished in piezoelectric NGs by employing the piezoelectric potential created under strain. Recently, we have developed a flexible triboelectric generator (TEG) using all-polymer based materials. 22 By stacking two thin polymer films made of Kapton and polyester (PET), a charge generation, separation, and induction process can be achieved through a mechanical deformation of the polymer films as a result of the triboelectric effect. This is a simple, low-cost, readily scalable fabrication process of generator that can convert random mechanical energy in our living environment into electric energy using the well-known triboelectric effect. Furthermore, through rational design, this new mode of power generation can be developed to build a high-output, flexible, and transparent NG.To make the device transparent and improve the power generation density, three approaches were employed in this research: (i) replacing Kapton ...

Ever since the first report of the triboelectric nanogenerator (TENG) in January 2012, its output area power density reaches 500 Wm -2 , an instantaneous conversion efficiency of ~70% and total energy conversion efficiency of up to 85% have been demonstrated. We provide a comprehensive review about the four modes, their theoretical modelling, and the applications of TENGs for harvesting energy from human motion, walking, vibration, mechanical triggering, rotating tire, wind, flowing water and more as well as self-powered sensors. REVIEWThis journal is © The Royal Society of Chemistry 2014 Energy Environ.Sci., 2015, 00, 1-10 | 3 walking, 33,36,37 biomedical system. 38 And it was also developed to build up self-powered sensor systems, including magnetic sensor, 39 pressure sensor, 40 vibration sensor, 31 mercury ion sensor, 41 catechin detection sensor, 42 acoustic sensors 43, 44 . In-plane sliding mode.As shown in Fig. 2b, when two materials with opposite triboelectric polarities, for instance, PTFE and aluminum, are brought into contact, surface charge transfer takes place due to the triboelectrification effect. Since PTFE holds a higher electron affinity than aluminum, electrons are injected from aluminum into PTFE. When the PTFE and Al are fully aligned, the electric field created by the triboelectric charges does not produce a potential drop, because the positive charges on aluminum are fully compensated by the negative ones on PTFE. Once a relative displacement is introduced by an externally applied force in the direction parallel to the interface, triboelectric charges are not fully compensated at the displaced/mismatched areas, resulting in the creation of an effective dipole polarization in parallel to the direction of the displacement.Therefore, a potential difference across the two electrodes is generated. A sliding back and forth between the two will result in a periodical change of the electric potential difference, which will

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Harvesting energy from our living environment is an effective approach for sustainable, maintenance-free, and green power source for wireless, portable, or implanted electronics. Mechanical energy scavenging based on triboelectric effect has been proven to be simple, cost-effective, and robust. However, its output is still insufficient for sustainably driving electronic devices/systems. Here, we demonstrated a rationally designed arch-shaped triboelectric nanogenerator (TENG) by utilizing the contact electrification between a polymer thin film and a metal thin foil. The working mechanism of the TENG was studied by finite element simulation. The output voltage, current density, and energy volume density reached 230 V, 15.5 μA/cm 2 , and 128 mW/cm 3 , respectively, and an energy conversion efficiency as high as 10−39% has been demonstrated. The TENG was systematically studied and demonstrated as a sustainable power source that can not only drive instantaneous operation of light-emitting diodes (LEDs) but also charge a lithium ion battery as a regulated power module for powering a wireless sensor system and a commercial cell phone, which is the first demonstration of the nanogenerator for driving personal mobile electronics, opening the chapter of impacting general people's life by nanogenerators. KEYWORDS: Energy harvesting, triboelectric nanogenerator, self-powered system, lithium ion battery A rapid expansion of electronic devices 1−4 toward wireless, portability, and multifunction desperately needs the development of independent and maintenance-free power sources. 5−7 The emerging technologies for mechanical energy harvesting 8−10 are effective and promising approaches for building self-powered systems because of a great abundance of mechanical energy existing in our living environment and human body. Since 2006, piezoelectric nanogenerators (PNGs) 11−14 have been developed to efficiently convert tinyscale mechanical energy into electricity. Recently, another creative invention is the cost-effective and robust triboelectric nanogenerators (TENGs) 15−17 based on the universally known contact electrification effect. 18,19 TENG harvests mechanical energy through a periodic contact and separation of two polymer plates. However, in order to realize sustainable driving of electronic devices/systems, the output of TENG must be significantly improved through a rational design.The two different types of nanogenerators presented above have a similar underlying physical process 12,17 for producing electricity: generation of immobile charges (ionic charges for PNG or electrostatic charges on insulators for TENG), and a periodic separation and contact of the oppositely charged surfaces to change the induced potential across the electrodes, which will drive the flow of free electrons through an external load. The electrical output and efficiency are radically determined by the effectiveness of the above two processes.As for the charge generation in TENG, maximizing the generation of electrostatic charges on opposit...

Aiming at harvesting ambient mechanical energy for self-powered systems, triboelectric nanogenerators (TENGs) have been recently developed as a highly efficient, cost-effective and robust approach to generate electricity from mechanical movements and vibrations on the basis of the coupling between triboelectrification and electrostatic induction. However, all of the previously demonstrated TENGs are based on vertical separation of triboelectriccharged planes, which requires sophisticated device structures to ensure enough resilience for the charge separation, otherwise there is no output current. In this paper, we demonstrated a newly designed TENG based on an in-plane charge separation process using the relative sliding between two contacting surfaces. Using Polyamide 6,6 (Nylon) and polytetrafluoroethylene (PTFE) films with surface etched nanowires, the two polymers at the opposite ends of the triboelectric series, the newly invented TENG produces an open-circuit voltage up to ∼1300 V and a short-circuit current density of 4.1 mA/m 2 with a peak power density of 5.3 W/m 2 , which can be used as a direct power source for instantaneously driving hundreds of serially connected light-emitting diodes (LEDs). The working principle and the relationships between electrical outputs and the sliding motion are fully elaborated and systematically studied, providing a new mode of TENGs with diverse applications. Compared to the existing vertical-touching based TENGs, this planar-sliding TENG has a high efficiency, easy fabrication, and suitability for many types of mechanical triggering. Furthermore, with the relationship between the electrical output and the sliding motion being calibrated, the sliding-based TENG could potentially be used as a self-powered displacement/speed/acceleration sensor. KEYWORDS: Mechanical energy harvesting, triboelectric nanogenerators, in-plane charge separation, self-powered systems T he fundamental science and practical applicable technologies for harvesting ambient environmental energy 1−6 are not only essential in realizing the self-powered electronic devices and systems 7,8 but also tremendously helpful in meeting the rapid-growing worldwide energy consumptions. Mechanical energy is one of the most universally existing, diversely presenting but usually wasted energies in the natural environment, which has attracted a lot of effort in developing the energy harvesting techniques based on different effects and mechanisms, such as electrostatics, 9,10 piezoelectricity, 11−13 and electromagnetics. 14,15 The triboelectric effect 16,17 is a wellknown phenomenon that can generate electrostatic charges from mechanical contact. However, it has always been deemed as an undesirable effect for electronic systems. The recent invention of triboelectric nanogenerators (TENGs) has made use of this effect to generate electricity through scavenging mechanical energy, which is proven to be extremely efficient, reliable and cost-effective. 18−21 This type of nanogenerator is based on the coupling...

The development of internet of things and the related sensor technology have been a key driving force for the rapid development of industry and information technology. The requirement of wireless, sustainable and independent operation is becoming increasingly important for sensor networks that currently could include thousands even to millions of sensor nodes with different functionalities. For these purposes, developing technologies of self-powered sensors that can utilize the ambient environmental energy to drive the operation themselves is highly desirable and mandatory. The realization of self-powered sensors generally has two approaches: the first approach is to develop environmental energy harvesting devices for driving the traditional sensors; the other is to develop a new category of sensors-self-powered active sensors-that can actively generate electrical signal itself as a response to a stimulation/triggering from the ambient environment. The recent invention and intensive development of triboelectric nanogenerators (TENGs) as a new technology for mechanical energy harvesting can be utilized as self-powered active mechanical sensors, because the parameters (magnitude, frequency, number of periods, etc.) of the generated electrical signal are directly determined by input mechanical behaviors. In this review paper, we first briefly introduce the fundamentals of TENGs, including the four basic working modes. Then, the most updated progress of developing TENGs as self-powered active sensors is reviewed. TENGs with different working modes and rationally designed structures have been developed as self-powered active sensors for a variety of mechanical motions, including pressure change, physical touching, vibrations, acoustic waves, linear displacement, rotation, tracking of moving objects, and acceleration detection. Through combining the opencircuit voltage and the short-circuit current, the detection of both static and dynamic processes has been enabled. The integration of individual sensor elements into arrays or matrixes helps to

For versatile mechanical energy harvesting from arbitrary moving objects such as humans, a new mode of triboelectric nanogenerator is developed based on the sliding of a freestanding triboelectric-layer between two stationary electrodes on the same plane. With two electrodes alternatively approached by the tribo-charges on the sliding layer, electricity is effectively generated due to electrostatic induction. A unique feature of this nanogenerator is that it can operate in non-contact sliding mode, which greatly increases the lifetime and the efficiency of such devices.