Publications

Vibration is one of the most common energy sources in ambient environment. Harvesting vibration energy is a promising route to sustainably drive small electronics. This work introduces an approach to scavenge vibrational energy over a wide frequency range as an exclusive power source for continuous operation of electronics. An elastic multiunit triboelectric nanogenerator (TENG) is rationally designed to efficiently harvest low-frequency vibration energy, which can provide a maximum instantaneous output power density of 102 W·m–3 at as low as 7 Hz and maintain its stable current outputs from 5 to 25 Hz. A self-charging power unit (SCPU) combining the TENG and a 10 mF supercapacitor gives a continuous direct current (DC) power delivery of 1.14 mW at a power management efficiency of 45.6% at 20 Hz. The performance of the SCPU can be further enhanced by a specially designed power management circuit, with a continuous DC power of 2 mW and power management efficiency of 60% at 7 Hz. Electronics such as a thermometer, hygrometer, and speedometer can be sustainably powered solely by the harvested vibration energy from a machine or riding bicycle. This approach has potential applications in self-powered systems for environment monitoring, machine safety, and transportation.

ABSTRACT The flexoelectric effect refers to the electromechanical coupling between electric polarization and mechanical strain gradient. It universally exists in a variety of materials in any space group, such as liquid crystals, dielectrics, biological materials, and semiconductors. Because of its unique size effect, nanoscale flexoelectricity has shown novel phenomena and promising applications in electronics, optronics, mechatronics, and photovoltaics. In this review, we provide a succinct report on the discovery and development of the flexoelectric effect, focusing on flexoelectric materials and related applications. Finally, we discuss recent flexoelectric research progress and still‐unsolved problems.

Kelvin probe force microscopy (KPFM), a characterization method that could image surface potentials of materials at the nanoscale, has extensive applications in characterizing the electric and electronic properties of metal, semiconductor, and insulator materials. However, it requires deep understanding of the physics of the measuring process and being able to rule out factors that may cause artifacts to obtain accurate results. In the most commonly used dual-pass KPFM, the probe works in tapping mode to obtain surface topography information in a first pass before lifting to a certain height to measure the surface potential. In this paper, we have demonstrated that the tapping-mode topography scan pass during the typical dual-pass KPFM measurement may trigger contact electrification between the probe and the sample, which leads to a charged sample surface and thus can introduce a significant error to the surface potential measurement. Contact electrification will happen when the probe enters into the repulsive force regime of a tip-sample interaction, and this can be detected by the phase shift of the probe vibration. In addition, the influences of scanning parameters, sample properties, and the probe's attributes have also been examined, in which lower free cantilever vibration amplitude, larger adhesion between the probe tip and the sample, and lower cantilever spring constant of the probe are less likely to trigger contact electrification. Finally, we have put forward a guideline to rationally decouple contact electrification from the surface potential measurement. They are decreasing the free amplitude, increasing the set-point amplitude, and using probes with a lower spring constant.

Abstract For Abstract see ChemInform Abstract in Full Text.

Herein, we introduce an ultra-thin self-powered artificial skin (SPAS) based on a piezoelectric nanogenerator, which harvests stored elastic deformation energy produced by the bending and stretching actions of the skin. This finding is an important step toward building a self-powered "smart skin".

Electronic skin based on a multimodal sensing array is ready to detect various stimuli in different categories by utilizing highly sensitive materials, sophisticated geometry designs, and integration of multifunctional sensors. However, it is still difficult to distinguish multiple and complex mechanical stimuli in a local position by conventional multimodal E-skin, which is significantly important in the signals' feedback of robotic fine motions and human-machine interactions. Here, we present a transparent, flexible, and self-powered multistage sensation matrix based on piezoelectric nanogenerators constructed in a crossbar design. Each sensor cell in the matrix comprises a layer of piezoelectric polymer sandwiched between two graphene electrodes. The simple lamination design allows sequential multistage sensation in one sensing cell, including compressive/tensile strain and detaching/releasing area. Further structure engineering on PDMS substrate allows the sensor cell to be highly sensitive to the applied pressures, representing the minimum sensing pressure below 800 Pa. As the basic combinations of compressive/tensile strains or detaching/releasing represent individual output signals, the proposed multistage sensors are capable of decoding to distinguish external complex motions. The proposed self-powering multistage sensation matrix can be used universally as an autonomous invisible sensory system to detect complex motions of the human body in local position, which has promising potential in movement monitoring, human-computer interaction, humanoid robots, and E-skins.

Abstract Contact electrification (CE) has been known for over 25 centuries, but the origin of the CE remains mysterious. Recent theoretical studies suggest that flexoelectricity may drive the CE, but experimental evidence is lacking. Here, the CE between a nanoscale tip and flat polymers is studied by using atomic force microscopy. The contributions of flexoelectricity to the CE are analyzed. We focus on the effect of the load, which is coupled to the strain gradient at the contact region. It is revealed that the flexoelectric polarization in general polymers can drive electron transfer, and even reverse the intrinsic polarity of electron transfer in the CE. It implies that the flexoelectricity is one of the driving forces for the CE. The flexoelectricity induced electric field is measured by applying a voltage between the tip and the sample, which counteracts the flexocoupling voltage. Further, a band structure model is proposed, in which the surface states of the solid are suggested to be shifted by the flexoelectric polarization.

By integrating a triboelectric nanogenerator (TENG) and a simple circuit board, the motion of water droplets can be controlled by the output of the TENG, which demonstrates a self-powered microfluidic system toward various practical applications in the fields of microfluidic system and soft robotics. This paper describes a method to construct a physical model for this self-powered system on the basis of electrostatic induction theory. The model can precisely simulate the detailed motion behavior of the droplet under driving of TENG, and it can also reveal the influences of surface hydrophobicity on the motion of the droplet, which can help us to better understand the key parameters that decide the performance of the system. The experimental observation of the dynamic performance of the droplet has also been done with a high speed camera system. A comparison between simulation results and real measurements confirms that the proposed model can predict the velocity and position of the water droplet driven by high voltage source as well as TENG. Hence, the proposed model in this work could serve as a guidance for optimizing the self-powered systems in future studies.