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