Kento Kariya

Piezoelectric Vibrational Energy Harvester Using Lead-Free Ferroelectric BiFeO3 Films

We have proposed that BiFeO3 films are suitable for piezoelectric vibrational energy harvester (VEH) applications, because BiFeO3 has high spontaneous polarization and low dielectric permittivity. We demonstrated that energy can be harvested by a micromachined VEH using a BiFeO3 film deposited using a sol–gel process. A VEH with a resonant frequency of ~98 Hz produced an output voltage of 1.5 VG-1 and electrical power of 2.8 µWmm-3G-2 (G=9.8 m/s2) at a load resistance of 1 MΩ. Using the analytical model for VEH, the generalized electromechanical coupling factor was estimated to be 0.41%. These results were comparable to those of the best-performing VEHs using other piezoelectric films.

Development of Piezoelectric MEMS Vibration Energy Harvester Using (100) Oriented BiFeO3 Ferroelectric Film

Piezoelectric vibration energy harvesters (VEHs) with unimorph structure have been developed using Si micro-electrical mechanical systems (MEMS) technology. Since we revealed that (100) epitaxial BiFeO3 (BFO) piezoelectric films have high figure-of-merit on energy conversion, (100)-oriented BFO films have been prepared on (100)-oriented LaNiO3 bottom electrodes by the sol-gel method. We fabricated the piezoelectric VEHs using BFO films with resonance frequencies of ~100 Hz. The maximum output power density of these VEHs was determined to be 10.5 μWmm−3G−2 (G=9.8 ms−2) at a load resistance of 1 MΩ, which exceeds or is comparable to those of the best-performing VEHs using other piezoelectric films.

Enhancement of Direct Piezoelectric Properties of Domain-Engineered (100) BiFeO3 Films

The improvement of the e31,f coefficient of domain-engineered BiFeO3 films, which is one of the important parameters on piezoelectric MEMS application, is discussed. The (100) BiFeO3 epitaxial films grown on SrRuO3/SrTiO3 substrates have a rhombohedral structure with tetragonal distortion. It was found that the BiFeO3 film deposited at the highest temperature (650 °C) has the lowest tetragonality and smallest domain size. The d33(AFM) coefficients characterized by scanning probe microscopy indicated that the intrinsic contribution of the piezoelectric response increases with increasing tetragonality. On the other hand, the film with the lowest tetragonality exhibited the highest e31,f coefficient. The results of the analysis of the domain structure by piezoelectric force microscopy revealed that a 71° domain wall has a large contribution to the increase in e31,f coefficient.

Thickness dependence of piezoelectric properties of BiFeO3 films fabricated using rf magnetron sputtering system

The piezoelectric property of BiFeO3 films prepared on a (100) LaNiO3/Si(100) substrate using an rf magnetron sputtering system was investigated for their applications in MEMS vibration energy harvesters. The X-ray diffraction profiles indicate that (100)-oriented BiFeO3 films with thicknesses from 450 to 1750 nm were obtained at a deposition temperature of 510 °C. All the films showed well-defined ferroelectric hysteresis loops at room temperature. The thickness dependence of crystallinity and electrical properties indicated that the films have a bottom layer with a high defect density. The e 31,f piezoelectric coefficient and electromechanical coupling factor () increase with increasing film thickness and reach −3.2 C/m2 and 3.3%, respectively, at a thickness of 1750 nm, which is considered to be caused by the decrease in defect density.

Enhancement of piezoelectric properties of (100)-orientated BiFeO3 films on (100)LaNiO3/Si

The piezoelectric properties of (100)-orientated BiFeO3 thin films grown on (100)LaNiO3/SiO2/(100)Si were investigated. 200-nm-thick LaNiO3 and 250-nm-thick BiFeO3 was deposited by the rf magnetron sputtering method and sol–gel method, respectively. The (100)-oriented BiFeO3 films were distorted to the in-plane tensile direction owing to the low thermal expansion coefficient of the Si substrate. While no significant dependence of the in-plane lattice distortion on the dielectric and ferroelectric properties and d33,AFM piezoelectric coefficient measured by scanning probe microscopy was observed, it was found that the e31,f piezoelectric coefficient increases with increasing the lattice distortion. The maximum e31,f and figure of merit (FOM) were −4.0 C/m and 14 GPa, respectively, which are comparable to those of epitaxially grown (100)BiFeO3 films.

Output power of piezoelectric MEMS vibration energy harvesters under random oscillations

Environmental vibrations include random oscillations of different frequencies and amplitudes. Energy harvesters recover the energy associated with these vibrations. Properties of the vibrations and output power are characterized for cantilever-type piezoelectric vibration energy harvesters using (100)-orientated BiFeO3 films subject to both ideal and random oscillations. The displacement and output power under random oscillations were smaller than those under ideal oscillations. This decrease originates with the decreasing acceleration of the fundamental wave with the spurious component having little influence on the resonance response.

Reaction of N,N’-dimethylformamide and divalent viologen molecule to generate an organic dopant for molybdenum disulfide

Tuning the carrier concentration is essential for semiconducting materials to apply optoelectronic devices. Molybdenum disulfide (MoS2) is a semiconducting material composed of atomically thin (∼0.7 nm thickness) layers. To dope thin MoS2, instead of using conventional atom/ion injection processes, a surface charge transfer method was successfully applied. In this study, we report a simple preparation method of a molecular dopant applicable to the doping process. The method follows a previous report for producing a molecular dopant, benzyl viologen (BV) which shows electron doping to MoS2. To prepare dopant BV molecules, a reduction process with a commercially available divalent BV by sodium borohydride (NaBH4) is required; however, the reaction requires a large consumption of NaBH4. NaBH4 drastically reacts with the solvent water itself. We found a reaction process of BV in an organic solvent, N,N’-dimethylformamide (DMF), by adding a small amount of water dissolving the divalent BV. The reaction is mild (at room temperature) and is autonomous once DMF comes into contact with the divalent BV aqueous solution. The reaction can be monitored with a UV-Vis spectrometer, and kinetic analysis indicates two reaction steps between divalent/monovalent/neutral viologen isomers. The product was soluble in toluene and did not dissolve in water, indicating it is similar to the reported dopant BV. The synthesized molecule was found to act as a dopant for MoS2 by applying a metal-oxide-semiconductor field-effect-transistor (MOSFET) structure. The process is a general method and applicable to other viologen-related dopants to tune the electronic structure of 2D materials to facilitate generating atomically thin devices.

Piezoelectric properties of (100) orientated BiFeO3 thin films on LaNiO3

To improve the piezoelectric properties of sol?gel-derived BiFeO3 films for MEMS application, (100) oriented films were prepared by using LaNiO3 bottom electrodes, which have preferential (100) orientation on Si substrates. The microstructure of the films crystalized at 450 and 500 ?C was a layered granular structure and that crystalized at 550 and 600 ?C changed to a columnar structure. The increase in the domain size with increasing the annealing temperature was also observed by piezoelectric force microscopy. Most of the film characteristics including the dielectric constant, remanent polarization, and d33,AFM coefficient were enhanced gradually with increasing annealing temperature. In contrast, the e31,f coefficient showed a much larger increase than the others, which suggests that the e31,f coefficient is strongly influenced by the domain size. The largest e31,f coefficient of the (100) BiFeO3 films was ?4.1 C/m2, which is about 4 times larger than that of the polycrystalline films.