In recent years, the use of micro-energy has attracted our attention as a fundamental technology of IoT and as a means of solving energy saving problems. Elementary processes of dissipation of minute energy are related to various mechanical-, electronic-, and optical-processes. This special issue introduces the state of the art in advanced research on the control and observation of micro-energy generated and dissipated on surfaces and interfaces.
Electret or permanent electrical charge is used as a part of mechano-electric energy conversion system for vibrational energy harvesting device. Microscopic capacitors are produced by silicon micromachining or MEMS (microelectromechanical systems) fabrication process, and the surfaces are turned into impurity-rich silicon oxide, which is later processed to make an electret by displacing the impurity ions by the same mechanism as the anodic bonding. The built-in potential of the electret is used to produce electrical current through the electrostatic induction when the movable electrode is periodically shaken by the external vibrations. In this article, the fabrication processes for the electret as well as the MEMS vibrational energy harvester are discussed. The fundamental characteristics of the energy harvesters are reported, along with a demonstration result as an autonomous powerpack for an IoT (internet-of-things) type wireless sensor node.
We proposed a cavity-free planar device architecture of a Si-nanowire (NW) thermoelectric (TE) power generator with high affinity for Si-LSI process technology. In this generator, an external temperature difference is induced between a front-side and backside of the substrate without cavity. Thus, most of induced heat energy is wasted to the substrate. However, a diffused heat flow to in-plane direction maintains a temperature difference in the Si nanowires. The exuded heat flux formed a steep temperature gradient in Si-NWs. Then, the 10-µW/cm2 class high power is generated by our TE generator. In this paper, the design concept of the new Si-NW based TE generator is presented. A numerical and experimental demonstration of test devices is reviewed. The possibility of large-scale integration is discussed by designing the structure of our TE generator.
We introduce the Tribo-Phonon Spectroscopy (TPS) developed to study phonons producing and dissipating at a tip-surface, which consists of an atomic force microscope (AFM) and a quartz-crystal microbalance (QCM). An AFM tip used to apply stresses generated lattice strain on an oscillating MoS2(0001) surface, which dissipated via acoustic phonons. The dissipation energy of the phonons strongly depended on the size of the lattice strain. The motion of the acoustic phonons consisted of a longitudinal mode and a transverse mode, but the occurrence of their phonon modes depended on the crystallographic direction, which reflects the atomic arrangement of the MoS2(0001) surface. In addition, we report that the acoustic phonons produced in MoS2 islands are confined within MoS2 islands smaller than the mean free path (MFP) of the phonons.
The thermoelectric generator (TEG) is expected to be an energy harvester for the supply of electricity to wearable or Internet-of-Things devices. For this purpose, organic-based thermoelectric materials have been extensively studied. In this report, the results of a survey study on organic-based thermoelectric materials by the author's group are presented, and two novel and interesting material groups introduced. The first is a group of pure organic semiconductors. Giant Seebeck coefficients (＞0.1 V/K) are found to appear in such materials near room temperature. Owing to the extremely large Seebeck coefficient, the device structure can be ultimately simple without connecting many p/n semiconductor blocks as in conventional TEGs. The second is a group containing two different material phases, such as carbon nanotube (CNT) composites. They are suitable for fabric-type TEGs. A key issue is how to suppress their high thermal conductivity. A protein molecular junction is used to suppress the phonon propagation while maintaining the electron transport.
Nanoscale observation of charge migration is crucial for understanding and controlling functional materials and devices. We developed tip-synchronized time-resolved electrostatic force microscopy. The analysis of cantilever motion provides the temporal resolution with the timescale of cantilever vibration cycle. The observation of sub-microsecond charge migration is achieved for photovoltaic bilayer and conductive polymer thin films by a movie with 0.3 microsecond frame step time resolution.