Energy harvesting for self-powered nanosystems

Developing novel technologies for wireless nanodevices and nanosystems is of critical importance for in situ, real-time and implantable biosensing and defense applications, and even wearable personal electronics. A nanodevice requires a power source, which may be provided directly or indirectly by a battery. But it is highly desirable for wireless devices to be self-powered. That requires exploring innovative nanotechnologies for converting mechanical, vibration, and hydraulic energy into electric energy for battery-free nanodevices.

We have demonstrated an innovative approach for converting mechanical energy into electricity using piezoelectric zinc oxide (ZnO) nanowires that can be grown on any substrate (e.g., metals, ceramics, polymers, and textile fibers) of any shape.^1,2 Measurements were performed by an atomic force microscope (AFM) equipped with a platinum-coated silicon tip. In the AFM contact mode, a constant normal force of 5nN (nanonewtons) was maintained between the tip and the sample surface. The tip scanned over the top of the ZnO nanowires, bending and releasing them (see Figure 1). The operation of the electric generator relies on the unique combination of the piezoelectric and semiconducting properties of ZnO.

Figure 1. (a) Experimental setup and procedures for generating electricity by deforming a piezoelectric nanowire using a conductive AFM tip. (b) Output voltage image map of ZnO nanowire arrays. ZnO: Zinc oxide. Pt: Platinum. Si: Silicon. Ag: Silver. V_L: Voltage. R_L: Load resister. I: Current.

The output power can be increased by using a large number of nanowires to generate electricity. The performance of the nanogenerator can be dramatically improved by mechanically deforming the nanowires to generate power in an adaptable, mobile, and cost-effective way over a larger scale than the AFM can achieve. The nanowires must all generate electricity simultaneously and continuously, and the electricity generated must be collected and output effectively. In addition, the energy to be converted should be provided in the form of waves or vibrations in the environment to enable the nanogenerator to operate `independently’ and wirelessly. We have developed an innovative approach that uses ultrasonic waves to drive a nanogenerator built from an array of vertically aligned ZnO nanowires.^3,4

Figure 2. Schematic diagram showing the DC nanogenerator built using aligned ZnO nanowire arrays with a zigzag top electrode. The nanogenerator is driven by an external ultrasonic wave or mechanical vibration, and the output current is continuous. The lower plot is the output from a nanogenerator with the ultrasonic wave on and off. The output current reached 600nA for a 3mm² nanogenerator.

The experimental setup is shown schematically in Figure 2. An array of aligned ZnO nanowires is covered by a zigzag silicon electrode coated with platinum. The platinum coating not only enhances the conductivity of the electrode but also creates a Schottky contact at the interface with the ZnO that functions like a p-n junction or diode.

The nanowires were grown on gallium nitride substrates, which served as a common electrode for directly connecting with an external circuit. The asymmetric piezoelectric potential across the width of a ZnO nanowire and the Schottky contact between the metal electrode and the nanowire are the two key processes for creating, separating, preserving, and outputting the charges. A top electrode is designed to achieve the coupling process and to replace the role played by the AFM tip, and its zigzag trenches act as an array of aligned AFM tips. The DC nanogenerator is driven by ultrasonic waves. Once the wave is on, a nanogenerator measuring 2mm² delivers a current output of 30nA. The design is new, cost-effective, and meets the stipulated requirements. The approach provides a basis for optimizing and improving the performance of the nanogenerator for its applications in nanotechnology.

The principle and technology demonstrated here have the potential to convert energy from mechanical movement (such as body motion, muscle stretching, and blood pressure), vibrations (such as acoustic and ultrasonic waves), and hydraulic movement (such as flow of body fluid and blood, or contraction of blood vessels) into electrical energy to power nanodevices and nanosystems. Relevant applications include implantable biosensing, wireless and remote sensing, nanorobotics, microelectromechanical systems, and sonic wave detection.

This research was supported by the National Science Foundation, NASA, the Defense Research Projects Agency, and the National Institutes of Health. Thanks to Xudong Wang, Jinhui Song, and Jin Liu for their contribution.