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Micro-/ Nano-scale Mechanical Energy Harvesting
Vibration-based mechanical energy is the most ubiquitous and accessible energy source in the environment. Examples include random vibrations, acoustic waves, air flows, and so forth. Harvesting this type of energy offers great potential for remote/wireless sensing, battery charging, and powering electronic devices. Compared to conventional film-based piezoelectric cantilever transducers, using piezoelectric nanowires (NWs) for mechanical energy harvesting offers three unique advantages:

•  Enhanced piezoelectric effect - Theory has predicted l arge enhancement of the piezoelectric effect due to the flexoelectric effect, i.e. when a strain gradient is experienced by a ferroelectric NW with a thickness of a few tens of nanometers..

•  Superior mechanical properties - The perfection of NWs' atomic lattice enables much larger critical strain, greater flexibility, and longer operational lifetime.

•  High sensitivity to small forces - High aspect ratio and small thickness allow NWs to undergo significant strain under a force at the nano Newton or even pico Newton level.

We are working to develop uniform ferroelectric nanowire arrays to understand electromechanical coupling properties at the nanometer scale, and to develop micro-/ nano-scale piezoelectric devices (nanogenerators) for harvesting mechanical energy from low-level vibrations, low-speed wind, and body movement.

3D Nanowire Architectures

To fully capture nanowires' promising surface and transport properties in practical devices or systems, researchers must effectively translate nanowires' extraordinary 1D characteristics into a 3D space. How to grow uniform nanowires inside confined 3D spaces is a serious challenge for current synthesis techniques due to coupling between the nanowire growth rate and the precursor concentration. We developed a surface-reaction limited chemical vapor deposition process that successfully addressed this challenge, and grew uniform TiO2 nanorod arrays along anodic aluminum oxide (AAO) nanochannels. In this process, growth is dictated by the number of precursor molecules that are absorbed on the nanowire surface. Thus our method sufficiently decouples the nanowire growth rate from the precursor vapor concentration so that growing uniform nanowire arrays inside sub-micron sized 3D confined spaces is possible. The 3D nanowire architecture potentially can improve the performance or efficiency of many electrical and electrochemical devices, such as sensors, detoxification filters, hydrogen storage systems, lithium-ion batteries, fuel cells, photovoltaic devices, photocatalysis, and supercapacitors..

We are currently studying the nucleation step for a full understanding of the nanorod formation process, which we can use to developnanowire-based 3D nanoarchitectures from a variety of functional materials. We are also practicing this technique on silicon nanowire templates and exploring the applications as solar cell electrodes.

Self-Catalyzed Growth of ZnO Nanostructures

An in-depth understanding of the kinetics of the vapor deposition process is essential to advance this bottom-up nanostructure synthesis approach into a large-scale nanomanufacturing technology. We are interested in the growth behavior of ZnO nanostructures without using catalysts. We observed Zn cluster drift at high deposition temperatures, which provides evidence a for self-catalyzed vapor-liquid-solid nanowire growth process. These clusters seeded formation of the ZnO nanoflower structure. This result illustrated an association between ZnO morphologies are and the discrepancy between oxidation rate and condensation rate of Zn. The experimental observation and analysis united the vapor-solid and self-catalyzed growth mechanisms into one coherent system and illustrated the interaction between these two mechanisms via kinetics control.

We are currently working to obtain in-depth understanding of the physical properties associated with the self-catalyzed Zn-rich growth conditions.

Our thanks to Jenny Morber for text editing jennymorber.com

Professor Xudong Wang's Nanoscience and Nanotechnology Group
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Department of Materials Science and Engineering • University of Wisconsin – Madison
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