Nanowire-Quantum Dot Hybridized Cell for Harvesting Sound and Solar Energies

We have demonstrated sound-wave-driven nanogenerators using both laterally bonded single wires and vertically aligned nanowire arrays for energy harvesting in the frequency range of 35-1000 Hz. The electricity produced by the single wire generator (SWG) is linearly proportional to the input acoustic energy, while the frequency does not affect the performance of the SWG in this study. By infiltrating CdS/CdTe quantum dots among vertical nanowires, we have fabricated a hybrid cell that simultaneously harvests both soundand solar energies. This demonstrates a possible approach for effectively harvesting available energies in our living environment with and without the presence of light. SECTION Energy Conversion and Storage I n addition to solar energy, enormous mechanical vibrational energy exists in our living environmentwith a large variation in amplitude and frequency, such as wind, sonic wave, hydraulic, and even noises. Harvesting this type of “random” energy can be a practical solution to power small mobile electronics for the future sensor network. Great efforts have been made to efficiently convert solar, mechanical, and chemical energies into electricity, but most of the technologies demonstrated are designed to harvest only one type of energy. As a future direction in energy research, simultaneous harvesting of multitype energy by a single device was first demonstrated for solar and mechanical energy as well as chemical and mechanical energy, which are referred to as hybrid energy cells. Acoustic waves, such as various sound noises from any living activities, car traffic, and the construction industry, are one of the most common mechanical vibrations in our surroundings. As for indoor mobile electronics, other types of energies are of equal or even more importance than solar energy. It is important to develop technologies that simultaneously harvest multitype energies and use whatever is available. The previously demonstrated hybrid cell (HC) utilizes an ultrasonic wave with a fixed frequency of ∼41 kHz and irregular biomotion at∼10 Hz or below. Considering that, our living environment has abundant mechanical vibrations with various frequencies ranging from a few to thousands of Hz. A systematic study of the nanogenerator (NG) in various acoustic frequencies is needed for practical applications of the NG. In this Letter, we first present the nanogenerators fabricated using both laterally bonded single wires and vertically aligned nanowire arrays for harvesting sound energy in the frequency range of 35-1000 Hz. The electricity produced by the single wire generator (SWG) is linearly proportional to the input acoustic energy, while the frequency does not affect the performance of the SWG in this study. Then, by infiltrating CdS/CdTe quantum dots among vertical nanowires, we fabricated a “composite” type of HC that harvests both sound and solar energies in a complete single-volume layer rather than a stacked layer structure as we first demonstrated, which is a promising approach for potential driving of portable electronics. This research is expected to inspire new research effort for developing the HC. Lateral Single Wire Generator for Harvesting Sound Energy. Traditional piezoelectric generator is based on a two-layer bender (or bimorph)-mounted cantilever beam, which produces the maximum power when the driving frequency matches the resonance frequency of the cantilever. In practice, a low-resonance-frequency resonator tends to have a large size/mass, which is too stiff to be driven by relatively weak sonic wave, while a small-size resonator has a high resonance frequency that is beyond the sonicwave frequency range, although it can be driven by small mechanical agitation. The cantilever-based piezoelectric generator is not the most effective choice for harvesting random sonic wave energy with a large variation in amplitude and frequency. Recently, piezoelectric ZnO nanowires (NWs) have been shown to be effective for harvesting small-magnitudemechanical energy. The SWG is based on a single piezoelectric ZnO wire with its two ends bonded to a flexible polymer substrate, as reported previously. The ZnO wire is stretched or compressed when the substrate is bent upward anddownward underexternal excitation. The thickness of the substrate is much larger than the diameter of the ZnO NW. Consequently, the ZnONWexperiences solely tensile strain or solely compressive strain, depending on the bending direction of the substrate. The strain within the piezoelectric NW thereafter produces a piezoelectric field along the length of Received Date: August 23, 2010 Accepted Date: September 14, 2010