Research

Our recent research is focused on:

 1. Nanogenerators and self-powered nanosystems (2005 – today)

 2. Piezotronics for smart systems (2006 – today)

 3. Piezo-phototronics for energy science and optoelectronics (2009 – today)

 4. Hybrid cells for energy harvesting (2008 – today)

1. Nanogenerators and self-powered nanosystems (2005 – today).

    Ever since the wide range applications of laptop computers and cell phones, seeking of power sources for driving portable electronics is becoming increasingly important. The current technology mainly relies on rechargeable batteries. But for the near future, micro/nano-systems will be widely used in health monitoring, infrastructure and environmental monitoring, internet of things and defense technologies; the traditional batteries may not meet or may not be the choice as power sources for the following reasons. First, with the increasing shrinkage in size, the size of the total micro/nano-systems could be largely dominated by the size of the battery rather than the devices. Second, the number and density of micro/nano-systems to be used for sensor network could be large, thus, replacing batteries for these mobile devices are becoming challenging and even impractical. Lastly, the power needed to drive a micro/nano-system is rather small, in the range of micro- to milli-Watt range. To meet these technological challenges, Wang proposed the self-powering nanotechnology in 2005, aiming at harvesting energy from the environment to power the micro/nano-systems based sensor network. After 6 years of effort, we have extensively developed the science, engineering and technology related to nanogenerator as a sustainable self-sufficient power source for micro/nano-systems by harvesting energy from our body and living environment. We initiated the research for self-powered system in 2005, which is now a very attractive field of research worldwide. Our research aims at solving the power needs for small electronics, with applications in personal/mobile electronics, medical care/sciences, environmental/infrastructure monitoring and other related fields.

Key publications:

[1] Zhong Lin Wang and Jinhui Song, "Piezoelectric Nanogenerators Based on Zinc Oxide Nanowire Arrays", Science, 312 (2006) 242-246.

[2] Xudong Wang, Jinhui Song Jin Liu, and Zhong Lin Wang, "Direct-Current Nanogenerator Driven by Ultrasonic Waves", Science, 316 (2007) 102-105.

[3] Yong Qin, Xudong Wang and Zhong Lin Wang, "Microfibre–nanowire hybrid structure for energy scavenging", Nature, 451 (2008) 809-813.

[4] Rusen Yang, Yong Qin, Liming Dai and Zhong Lin Wang, "Power generation with laterally-packaged piezoelectric fine wires", Nature Nanotechnology, 4 (2009) 34-39.

[5] Sheng Xu, Yong Qin, Chen Xu, Yaguang Wei, Rusen Yang, Zhong Lin Wang, "Self-powered nanowire devices", Nature Nanotechnology, 5 (2010) 366-373

[6] Youfan Hu, Yan Zhang, Chen Xu, Guang Zhu and Zhong Lin Wang, "High output nanogenerator by rational unipolar-assembly of conical-nanowires and its application for driving a small liquid crystal display", Nano Lett., 10 (2010) 5025-5031.

[7] Guang Zhu, Rusen Yang, Sihong Wang, Zhong Lin Wang, "Flexible high-output nanogenerator based on lateral ZnO nanowire array", Nano Lett., 10 (2010) 3151–3155.

[8] Youfan Hu, Yan Zhang, Chen Xu, Long Lin, Robert L. Snyder, and Zhong Lin Wang, "Self-Powered System with Wireless Data Transmission", Nano Lett., 11 (2011) 2572-2577.

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2. Piezotronics for smart systems (2006 – today)

     Piezoelectricity, a phenomenon known for centuries, is an effect that is about the production of electrical potential in a substance as the pressure on it changes. The most well-known material that has piezoelectric effect is the perovskite structured Pb(Zr, Ti)O3 (PZT), which has found huge applications in electromechanical sensors, actuators and energy generators. But PZT is an electric insulator and it is less useful for building electronic devices. Wurtzite structures, such as ZnO, GaN, InN and ZnS, also have piezoelectric properties but they are not extensively used as much as PZT in piezoelectric sensors and actuators due to their small piezoelectric coefficients. In fact, due to the polarization of ions in a crystal that has non-central symmetry, a piezoelectric potential (piezopotential) is created in the crystal by applying a stress. For materials such as ZnO, GaN, InN in the wurtzite structure family, the effect of piezopotential to the transport behavior of charge carriers is significant due to their multiple functionalities of piezoelectricity, semiconductor and photon excitation. By utilizing the advantages offered by these properties, a few new fields have been created. Electronics fabricated by using inner-crystal piezopotential as a “gate” voltage to tune/control the charge transport behavior is named piezotronics, which was first coined by Wang in 2006. Devices fabricated using the piezotronic effect are distinctly different from those realized through traditional CMOS technologies in principle, design and applications. Piezotronics will have important applications in human-CMOS interfacing, micro/nano-electromechanical systems, nanorobotics, next generation of sensor and transducers, smart electronics, flexible electronics and many more.

Key publications:

[1] Zhong Lin Wang, "Nanopiezotronics", Adv. Mater., 19 (2007) 889-992.

[2] Zhong Lin Wang, "Piezopotential gated nanowire devices: Piezotronics and piezo-phototronics", Nano Today, 5 (2010) 540-552.

[3] Xudong Wang, Jun Zhou, Jinhui Song Jin Liu, Ningsheng Xu and Zhong Lin Wang, "Piezoelectric-Field Effect Transistor and Nano-Force-Sensor Based on a Single ZnO Nanowire", Nano Lett., 6 (2006) 2768-2772.

[4] Jun Zhou, Peng Fei, Yudong Gu, Wenjie Mai, Yifan Gao, Rusen Yang, Gang Bao, Zhong Lin Wang, "Piezoelectric-potential-controlled polarity-reversible Schottky diodes and switches of ZnO wires", Nano Lett., 8 (2008) 3973-3977.

[5] Wenzhuo Wu, Yaguang Wei and Zhong Lin Wang, "Strain-gated piezotronic logic nanodevices", Adv. Mater., 22 (2010) 4711-4715.

[6] Wenzhuo Wu and Zhong Lin Wang, "Piezotronic nanowire based resistive switches as programmable electromechanical memories", Nano Lett., 11 (2011) 2779-2885.

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3. Piezo-phototronics for energy science and optoelectronics (2009 – today)

    Due to the polarization of ions in a crystal that has non-central symmetry, a piezoelectric potential (piezopotential) is created in the crystal under stress. Piezopotential can effectively raise the Schottky barrier height at a metal-semiconductor interface or change the transport at a p-n junction, while laser excitation and effectively low the Schottky barrier height. Therefore, we can use the coupling between piezoelectric effect and laser excitation to introduce new optoelectronic devices. Piezo-phototronics effect is a result of three-way coupling among piezoelectricity, photonic excitation and semiconductor transport, which allows tuning and controlling of electro-optical processes by strain induced piezopotential. The piezo-phototronic effect was first coined by Wang in 2009. Recently, we have applied this effect for fabricating highly sensitive UV sensors, LED with largely enhanced efficiency and high performance solar cells. The development of piezo-phototronics will have great impact to the energy science and optoelectronic devices fabricated using ZnO and GaN materials.

Key publications:

[1] Youfan Hu, Yanling Chang, Peng Fei, Robert L. Snyder, and Zhong Lin Wang, "Designing the Electric Transport Characteristics of ZnO Micro/Nanowire Devices by Coupling Piezoelectric and Photoexcitation Effects", ACS Nano, 4 (2010) 1234-1240.

[2] Zhong Lin Wang, "Piezopotential gated nanowire devices: Piezotronics and piezo-phototronics", Nano Today, 5 (2010) 540-552.

[3] Qing Yang, Xin Guo, Wenhui Wang, Yan Zhang, Sheng Xu, Der Hsien Lien, Zhong Lin Wang, "Enhancing sensitivity of a single ZnO micro/nanowire photodetector by piezo-phototronic effect", ACS Nano, 4 (2010) 6285-6291.

[4] Qing Yang, Wenhui Wang, Sheng Xu and Zhong Lin Wang, "Enhancing light emission of ZnO microwire-based diodes by piezo-phototronic effect", Nano Lett., DOI: 10.1021/nl202619d.

4. Hybrid cells for energy harvesting (2008 – today)

    Our living environment has an abundance of energies in the forms of light, thermal, mechanical (such as vibration, sonic wave, wind and hydraulic), magnetic, chemical and biological. Harvesting these types of energies is of critical importance for long-term energy needs and sustainable development of the world. Over the years, rationally designed materials and technologies have been developed for converting solar and mechanical energies into electricity. Photovoltaic relies on approaches such as inorganic p-n junctions, organic thin films, and organic-inorganic heterojunctions. Mechanical energy generators have been designed based on principles of electromagnetic induction and piezoelectric effect. Innovative approaches have to be developed for conjunctional harvesting of multiple types of energies using an integrated structure/material so that the energy resources can be effectively and complimentarily utilized whenever and wherever one or all of them are available. We have been developing hybrid cells that are designed for simultaneously harvesting solar and mechanical, and chemical and mechanical energies using nanotechnology. The two energy harvesting approaches can work simultaneously or individually, and they can be integrated in parallel and serial for raising the output current and voltage, respectively. This study is to demonstrate an innovative approach for developing integrated technologies for effectively scavenging available energies in our environment around the clock.

Key publications:

[1] Chen Xu, Xudong Wang and Zhong Lin Wang, "Nanowire Structured Hybrid Cell for Concurrently Scavenging Solar and Mechanical Energies", J. Am. Chem. Soc., 131(2009) 5866-5872.

[2] Chen Xu and Zhong Lin Wang, "Compacted hybrid cell made by nanowire convoluted structure for harvesting solar and mechanical energies", Adv. Mater. 23 (2011) 873-877.

[3] Benjamin J. Hansen, Ying Liu, Rusen Yang, and Zhong Lin Wang, "Hybrid Nanogenerator for Concurrently Harvesting Biomechanical and Biochemical Energy", ACS Nano, 4 (2010) 3647-3652.

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