No abstract

No abstract

While global energy consumption has steadily increased in the past decades due to industrialization and population growth, [ 1 ] society is facing a problem with the depletion of fossil energy resources as well as environmental problems (such as global warming, carbon dioxide emissions, and damage to the ozone layer). [ 2 ] These challenges can be addressed by renewable energy resources, which are always available everywhere. [ 1 , 2 ] Outdoor renewable energy sources such as solar energy (15 000 μ W/cm 3 ), [ 3 , 4 ] wind energy (380 μ W/cm 3 ), [ 5 ] and wave energy (1 000 W/cm of wave crest length) [ 6 , 7 ] can provide largescale needs of power. However, for driving small electronics in indoor or concealed environments [ 3 , 8 ] (such as in tunnels, clothes, and artifi cial skin) and implantable biomedical devices, innovative approaches have to be developed.One way of energy harvesting without such restraints is to utilize piezoelectric materials that can convert vibrational and mechanical energy sources from human activities such as pressure, bending, and stretching motions into electrical energy. [9][10][11] Wang and co-workers [ 9 , 10 , 12-15 ] have used piezoelectric ZnO nanowire arrays to develop a nanogenerator technologies, who have demonstrated the feasibility using this type of generator to power commercial light-emitting diodes (LEDs), [ 13 ] liquid crystal displays, [ 14 ] and wireless data transmission. [ 15 ] These nanogenerators can also convert tiny bits of biomechanical energy (from sources such as the movement of the diaphragm, the relaxation and contraction of muscle, heartbeat, and the circulation of blood) into power sources. [ 16 , 17 ] Recently, there have been attempts to fabricate thin fi lmtype nanogenerators [ 11 , 18 ] with perovskite ceramic materials (PbZr x Ti 1-x O 3 and BaTiO 3 ), which have a high level of inherent piezoelectric properties. The BaTiO 3 thin fi lm nanogenerator has demonstrated by the authors [ 11 ] using the transfer process [19][20][21][22] of high temperature annealed perovskite thin fi lm from bulk substrates onto fl exible substrates; it generates a much higher level of power density than other devices with a similar structure. [ 10 ] Herein, we report the nanocomposite generator (NCG) achieving a simple, low-cost, and large area fabrication based on BaTiO 3 nanoparticles (NPs) synthesized via a hydrothermal reaction (see Method S1) [ 23 ] and graphitic carbons, such as single-walled and multi-walled carbon nanotubes (SW/MW-CNTs), and reduced graphene oxide (RGO). The BaTiO 3 NPs and carbon nanomaterials are dispersed in polydimethylsiloxane (PDMS) by mechanical agitation to produce a piezoelectric nanocomposite (p-NC). The p-NC is spin-casted onto metalcoated plastic substrates and cured in an oven. Under periodic external mechanical deformation by bending stage or biomechanical movements from fi nger/feet of human body, electric signals are repeatedly generated from the NCG device and used to operate a commercial red LED.The schematic diagrams ...

The piezoelectric generation of perovskite BaTiO 3 thin films on a flexible substrate has been applied to convert mechanical energy to electrical energy for the first time. Ferroelectric BaTiO 3 thin films were deposited by radio frequency magnetron sputtering on a Pt/Ti/SiO 2 /(100) Si substrate and poled under an electric field of 100 kV/cm. The metal-insulator (BaTiO 3 )-metal-structured ribbons were successfully transferred onto a flexible substrate and connected by interdigitated electrodes. When periodically deformed by a bending stage, a flexible BaTiO 3 nanogenerator can generate an output voltage of up to 1.0 V. The fabricated nanogenerator produced an output current density of 0.19 µA/cm 2 and a power density of ∼7 mW/cm 3 . The results show that a nanogenerator can be used to power flexible displays by means of mechanical agitations for future touchable display technologies.KEYWORDS BaTiO 3 , thin film, piezoelectric, flexible electronics, nanogenerator, energy harvesting E nergy harvesting technologies that convert existing sources of energies, such as thermal energy as well as vibrational and mechanical energy from the natural sources of wind, waves, or animal movements into electrical energy, is attracting immense interest in the scientific community. [1][2][3][4][5][6] The fabrication of nanogenerators is particularly interesting because it can even scavenge the biomechanical energy from inside the human body, such as the heart beat, blood flow, muscle stretching, or eye blinking, and turn it into electricity to power implantable biodevices. [7][8][9] One way of harvesting electrical energy from the mechanical energy of ambient vibrations is to utilize the piezoelectric properties of ferroelectric materials. Piezoelectric harvesting has been proposed and investigated by many researchers.10-14 Chen et al. 12 reported on the fabrication of a nanogenerator that involves the use of lead zirconate titanate (PbZr x Ti 1-x O 3 , PZT) nanofibers on a bulk Si substrate. The PZT nanofibers were connected to interdigitated electrodes (IDEs) and, when pressure was applied perpendicularly to the nanogenerator surface, the nanogenerator producedanoutstandingoutputvoltage.Wangandco-workers 13,14 used piezoelectric ZnO nanowires to develop a multiple lateral-nanowire-array integrated nanogenerator (LING) 13 and a high-output nanogenerator (HONG) 14 on plastic substrates. They also demonstrated the feasibility of harvesting energy from the breath and heartbeat of animals. 9 As of today, the nanogenerator has an output voltage of 2 V, and the power generated can be used to power a commercial light-emitting diode (LED).14 Recently, there have been attempts to transfer flexible perovskite materials and capacitors onto flexible substrates for the purpose of utilizing the high inherent piezo-properties of ferroelectric materials from bulk substrates. 15,16 In those attempts, perovskite thin films (PZT and BaTiO 3 ) deposited on bulk substrates were annealed at high temperatures and transferred onto plastic sub...

Due to polarization of ions in crystals with noncentral symmetry, such as ZnO, GaN, and InN, a piezoelectric potential (piezopotential) is created in the crystal when stress is applied. Electronics fabricated using the inner-crystal piezopotential as a gate voltage to tune or control the charge transport behavior across a metal/semiconductor interface or a p-n junction are called piezotronics. This is different from the basic design of complimentary metal oxide semiconductor (CMOS) field-effect transistors and has applications in force and pressure triggered or controlled electronic devices, sensors, microelectromechanical systems (MEMS), human-computer interfacing, nanorobotics, and touch-pad technologies. Here, the theory of charge transport in piezotronic devices is investigated. In addition to presenting the formal theoretical frame work, analytical solutions are presented for cases including metal-semiconductor contact and p-n junctions under simplified conditions. Numerical calculations are given for predicting the current-voltage characteristics of a general piezotronic transistor: metal-ZnO nanowire-metal device. This study provides important insight into the working principles and characteristics of piezotronic devices, as well as providing guidance for device design.