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 ...

The reliable production of two-dimensional crystals is essential for the development of new technologies based on 2D materials. However, current synthesis methods suffer from a variety of drawbacks, including limitations in crystal size and stability. Here, we report the fabrication of large-area, high-quality 2D tellurium (tellurene) using a substrate-free solution process. Our approach can create crystals with a process-tunable thickness, from monolayer to tens of nanometres, and with lateral sizes of up to 100 µm. The chiral-chain van der Waals structure of tellurene gives rise to strong in-plane anisotropic properties and large thicknessdependent shifts in Raman vibrational modes, which is not observed in other 2D layered materials. We also fabricate tellurene field-effect transistors, which exhibit air-stable performance at room temperature for over two months, on/off ratios on the order of 10 6 and field-effect mobilities of around 700 cm 2 /Vs. Furthermore, by scaling down the channel length and integrating with high-k dielectrics, transistors with a significant on-state current density of 1 A/mm are demonstrated. MainThe continuing development of two-dimensional materials, be it the exploration of new science 1-3 or the implementation of new technologies 4-8 , requires reliable methods of synthesising 2D crystals. Whether current approaches can be scaled up though remains uncertain 9,10 and are restricted by factors such as growth substrates and conditions 11-13 , small crystal sizes 14 and the instability of the synthesized materials 11,15,16 .

Designing, fabricating, and integrating arrays of nanodevices into a functional system are the key to transferring nanoscale science into applicable nanotechnology. We report large-array three-dimensional (3D) circuitry integration of piezotronic transistors based on vertical zinc oxide nanowires as an active taxel-addressable pressure/force sensor matrix for tactile imaging. Using the piezoelectric polarization charges created at a metal-semiconductor interface under strain to gate/modulate the transport process of local charge carriers, we designed independently addressable two-terminal transistor arrays, which convert mechanical stimuli applied to the devices into local electronic controlling signals. The device matrix can achieve shape-adaptive high-resolution tactile imaging and self-powered, multidimensional active sensing. The 3D piezotronic transistor array may have applications in human-electronics interfacing, smart skin, and micro- and nanoelectromechanical systems.

Health, infrastructure, and environmental monitoring as well as networking and defense technologies are only some of the potential areas of application of micro-/nanosystems (MNSs). It is highly desirable that these MNSs operate without an external electricity source and instead draw the energy they require from the environment in which they are used. This Review covers various approaches for energy harvesting to meet the future demand for self-powered MNSs.