Research group of Academician Wang Zhonglin, Beijing Institute of nano energy and system, Chinese Academy of Sciences

The phenomenon of lead friction electrification has been studied for more than 2600 years, but its mechanism is still uncertain In 2012, the research group of Academician Wang Zhonglin invented the friction nano generator by using the friction electrification phenomenon to realize the conversion between mechanical energy and electrical energy Friction nano generator has obvious advantages in energy collection and self driving system applications, which has attracted extensive attention of researchers, and also aroused researchers' interest in the ancient scientific problem of friction electrification In the study of the mechanism of triboelectrification, one of the most important problems is how to determine the type of carriers, that is, whether the triboelectrification is caused by electron transfer or ion transfer To solve this problem, the research group of Academician Wang Zhonglin has studied the attenuation behavior of friction charge at different temperatures at micro and macro scales It is found that the attenuation of friction charge accords with the law of hot electron emission, which indicates that the carrier of friction electrification is electron However, polymer materials will change or even melt at high temperature, so the carrier type on the surface of polymer materials cannot be verified by temperature effect As a common material in friction nano generator, it is very important to design a new method to judge the type of friction charge carrier on the surface of polymer In fact, in addition to heat, light can also excite electrons on the surface of materials, that is, electrons absorb the energy of incident photons, and escape from the surface of materials, that is, photoelectric effect If the friction charge on the surface of the material is electron, the friction charge should be attenuated under the illumination, and the attenuation law accords with the photoemission theory Recently, under the guidance of Wang Zhonglin, chief scientist of Beijing nano energy and System Research Institute of Chinese Academy of Sciences and President Professor of Georgia Institute of technology, Dr Lin Shiquan and others studied the influence of light on the friction charge attenuation of the material surface, and verified that the friction electrification of the polymer surface is also electron through photoelectric effect Relevant research results were published in adv mater (DOI: 10.1002/adma.201901418) Academician Wang Zhonglin is a scientist with important academic influence in the field of international nano science His research is original, prospective and leading Academician Wang Zhonglin first invented piezoelectric nano generator in 2006 and successfully developed friction nano generator in 2012 Nano generator can directly convert mechanical energy into electrical energy output, providing a new mode for effective collection of mechanical energy In addition, nano generator can also be used as self driving sensor to detect mechanical model An important direction of the research group is to explore the mechanism of triboelectrification, mainly focusing on the types of triboelectrification carriers, the factors of triboelectrification and so on At present, the research team has published many papers on the study of triboelectrification mechanism (nano lett 2013, 13, 2771; nano lett 2014, 14, 1567; ACS Nano 2016, 10, 2528; adv mater 2018, 30, 1803968; adv mater 2018, 30, 1706790; adv mater 2019, 31, 1808197; ACS Nano 2019, 13, 2034 et al.) Wang Zhonglin graduated from northwest Telecommunication Engineering College (now known as Xi'an University of Electronic Science and Technology) in 1982, and was admitted to CUSPEA in the same year In 1987, he received his Ph.D in physics from Arizona State University and studied under the international authority of electronic microscopy, Professor John Cowley Dr Wang Zhonglin is now a chair professor of the University of Georgia Institute of technology, a lifetime Chair Professor of Hightower, an outstanding Chair Professor and director of the center for characterization of nanostructures, director and chief scientist of Beijing Institute of nano energy and systems, Chinese Academy of Sciences, foreign academician of Chinese Academy of Sciences and academician of European Academy of Sciences Wang Zhonglin is an internationally recognized leader in the field of nanotechnology He has made great contributions to the preparation and characterization of one-dimensional oxide nanostructures and their applications in energy technology, electronic technology, optoelectronic technology and biotechnology The piezoelectric Nanogenerator and friction Nanogenerator were invented, and the concept of self driving system was put forward at first The research of nanostructured piezoelectric electronics and piezoelectric optoelectronics was pioneered, which opened up a new way for the development of micro nano electronic system Wang Zhonglin has published more than 1300 Journal Papers (31 of which are published in science, nature and nature subjournals), 6 scientific monographs and more than 200 patents in the world-class journals The H-index of this paper is 223, ranking 21st in the world In addition, on July 23, 2018, Academician Wang Zhonglin was awarded the "Eni prize" by the Nobel Prize in energy industry In recognition of his first invention of nano generator, the creation of self driving system and blue energy two original fields, and the application of nano generator in the Internet of things, sensor networks, environmental protection, artificial intelligence and other new era energy fields, he has made significant pioneering contributions Research results: it is very important to study the process of electron transfer in triboelectrification by photoemission to determine whether the triboelectrification carrier is electron or ion in the study of triboelectrification mechanism However, it is a difficult problem to judge the type of carrier on the surface of polymer materials in triboelectrification Recently, the research team of Academician Wang Zhonglin of Beijing Institute of nano energy and system of Chinese Academy of Sciences published a research paper on adv mater Entitled "electron transfer in nanoscale contact electrification: photoexception effect" (adv mater 2019, DOI: 10.1002 / ADMA 201901418) The team found that the friction charge on the surface of SiO2, PVC and PMMA decreased rapidly under UV irradiation The shorter the wavelength of incident light, the stronger the light intensity, the faster the deceleration rate of friction charge on the surface of material When the wavelength of light is greater than a certain critical value, light can not induce the attenuation of friction charge on the surface of materials, and the corresponding wavelength critical values of different materials are different These experimental phenomena are in accordance with the theory of photoelectron emission In the theory of photoemission, the greater the intensity of light and the number of photons, the more electrons on the surface of the material are excited, and the faster the decay rate of friction charge is; the shorter the wavelength of light, the greater the photon energy, the greater the probability of excited electrons, and the faster the decay rate of friction charge is When the wavelength of light is larger than a certain value, the photon energy is not enough to excite the electrons trapped in the energy state of the material surface, and the friction charge can not be attenuated According to the attenuation law of friction charge in light, based on Spicer photoemission theory, the photoemission model of friction electron on insulator surface is proposed for the first time After the hot electron emission, this study provides a strong evidence of electron transfer in triboelectrification In addition, according to the characteristics of the response of friction electrons on the surface of the material to the incident light, a new idea is proposed to characterize the electronic structure of the material by using the phenomena of photoemission and friction luminescence in friction electrification Firstly, the friction charge is generated on the insulator surface by atomic force microscopy (AFM), and then the Kelvin probe force microscopy (KPFM) is used to measure the friction charge density on the material surface After the surface charge density of the material is tested, the light with different wavelength and intensity is used to irradiate the electrified area of the sample surface, and the decay rate of the surface charge density is observed under the light In the experiment, the light is produced by a xenon lamp, and through filters and focal lenses, a light source with a specific wavelength and intensity is produced (Fig 1) Fig 1 Experimental design of light induced friction charge decay (source: adv mater.) when a mixed light source of 200 nm ~ 2500 nm is used to irradiate the SiO2 sample, the friction charge on the sample surface decays rapidly However, the friction charge on the surface of the SiO2 sample remained unchanged in 40 minutes when the sample was irradiated with a 400 nm ~ 2500 nm mixed light source In the process of illumination, the maximum temperature rise of SiO2 surface is only 18 degrees, which is not enough to produce hot electron emission This result shows that the light with the wavelength of 200 nm ~ 400 nm can excite the friction charge on the surface of SiO2 and make it escape from the material surface (Fig 2) Figure 2 The influence of light on the surface charge attenuation and the temperature rise of the material surface (source: adv mater.) further experiments show that the wavelength can affect the rate of the friction charge attenuation on the surface of SiO2 The shorter the wavelength of light is, the faster the surface charge decay rate is In the photoelectric effect, the shorter the wavelength of light, the greater the energy of light, and the greater the probability of electrons being excited out of the material surface This shows that the experimental phenomenon is in accordance with the photoelectron emission model (Fig 3) Fig 3 The influence of UV wavelength on the decay rate of friction charge on the surface of SiO2 (source: adv mater.) similarly, the higher the intensity of light, the more photons, the higher the decay rate of friction charge on the surface of material (Fig 4) Figure 4 The influence of UV light intensity on the decay rate of friction charge on the surface of SiO2 (source: adv mater.) an important advantage of using photoelectric effect to judge carrier type is that it can be used to judge carrier type on the surface of polymer materials Fig 5 shows the attenuation behavior of friction charge on PVC and PMMA surfaces under different intensities and wavelengths of light It is also found that the deceleration rate of charge decay increases with the decrease of wavelength The results show that the friction charge decay behavior of PVC and PMMA is also in accordance with the photoelectric effect Fig 5 Influence of UV light intensity and wavelength on the decay rate of friction charge on PVC and PMMA surfaces (source: adv mater.) in the photoelectric induced charge decay experiment, the decay curve is linear first, and then the decay rate becomes smaller This phenomenon should be slightly different from the photoelectric excitation effect of semiconductor The model of photoelectric effect on insulator surface is established to explain this phenomenon For semiconductors, electrons are excited out of the surface from the valence band, that is to say, the potential barrier of electrons escaping from the surface is certain, which will not increase or decrease with the escape of electrons The electrons produced by surface friction are trapped in the surface energy state With the increase of electrons, the highest occupied energy level of the surface energy state will increase, and the potential barrier of electrons escaping from the surface will decrease On the contrary, when the friction electron is excited out of the surface, the occupied state of the highest energy level of the electron decreases, the potential barrier from which the electron escapes increases, and the rate of charge decay decreases (Fig 6) Figure 6 Schematic diagram of photoelectric effect of friction charge on insulator surface (source: adv mater.) this work first proposed a photoelectric emission model for friction electron on insulator surface, and after hot electron emission, it also proposed a strong evidence for friction electron transfer In addition, according to the characteristics of the response of friction electrons on the surface of the material to the incident light, a new idea is proposed to characterize the electronic structure of the material by using the phenomena of photoemission and friction luminescence in friction electrification Today, people and scientific research are getting more and more attention in the economic life, China has ushered in the "explosion of science and technology"