Professor Cheng Gang received his PhD in Jilin University in 2008 and is a professor in the Key Laboratory of Special Functional Materials of the Ministry of Education of Henan University. 2006-2007, Research Assistant of the Chinese University of Hong Kong, Cooperating mentors: Professor Huang Jiawei, Professor Du Ruoxu. 2013-2016, Visiting Scholar, Georgia Institute of Technology, USA, Cooperating mentor: Academician Wang Zhonglin.

In 2015, it won the National Excellent Youth Award. Professor Cheng is committed to the "development of new high-performance optoelectronic nano-devices" with the specific capabilities of nanostructures as the goal. By designing and constructing specific nanostructures, revealing and then controlling the special physical properties of nanostructures, a variety of high-performance nanodevices have been developed. Systematic research work has been carried out in the construction of nanostructures, the characterization and regulation of surface-surface photoelectric properties, and self-driven photovoltaic nanodevices. So far, Professor Cheng has published more than 50 SCI papers and cited more than 1,200 times.

Link to the webpage of the research group: http://lab.henu.edu.cn/index/ktz/c___g_Cheng_G1/jj.htm

Recently, Professor Cheng Gang‘s team developed surface ion grid control technology using gas ionization caused by friction nano-generators, and developed a variety of new optoelectronic devices through the control of low-dimensional semiconductor transmission characteristics. The following summarizes the latest research results of Professor Cheng Gang and his research team on surface ion grid regulation, so that everyone can fully understand the research progress.

1 ) Gas ionization characteristics caused by friction nano-generator

Figure 1. Schematic of gas ionization using a friction nano-generator

Taking advantage of the high output voltage of the horizontal sliding friction nano-generator, the air can be ionized by tip discharge, as shown in Figure 1. The air-discharge switch is composed of two discharge electrodes. Only when the voltage of the friction nano-generator is high enough to cause air discharge, the switch will be closed and produce a transient pulse output. By adjusting the distance ( d ) between the two discharge electrodes, the working mode of the switch can be changed: arc discharge and corona discharge, as shown in Figure 2. In the arc discharge mode, the output performance of the friction nano-generator has been greatly improved. In addition, in a certain range of ultraviolet light intensity, the irradiation of ultraviolet light can change the working mode of the switch and the output current of TENG-ADS. Based on this, a self-driven UV light detector was developed in this work. Its photoelectric switch ratio reached 18.6, and the lowest detectable ultraviolet light intensity was 26.2 μW / cm ² , as shown in FIG. 3. Related research work was published in Nano Energy (2018, 44, 208-216).

Figure 2. The output voltage and current of the friction nanogenerator as a function of the tip-electrode distance d .

(a) d = 0, that is, the needle tip is in direct contact with the electrode, and no discharge occurs; (b) d = 0.03 mm, and (c) is an enlarged view thereof; (d) d = 0.2 mm; (e) d = 0.6 mm ; (F) d = 0.72 mm; (g) d = 1.10 mm; (h) d = 5 mm; (i) d = 20 mm.

Figure 3. Performance test of a friction nano-generator-driven air discharge switch as a self-acting UV detector

(a) The output current curve of TENG-ADS when ultraviolet light is turned on or off for multiple cycles; (b) The schematic diagram of the principle of changing the working mode of the air discharge switch from arc discharge to corona discharge under ultraviolet light; c) the relationship between photoelectric switch ratio and d ; (d) the relationship between photoelectric switch ratio and light intensity.

Article link: https://www.sciencedirect.com/science/article/pii/S2211285517307516?via%3Dihub

2 ) Production of active negative ions in the process of gas ionization of friction nano-generator

During the gas ionization process caused by the friction nano-generator, gases with high electron affinity, such as O ₂ and CO ₂ , can capture electrons and form active negative ions. As shown in Figure 4, after passing a certain concentration of CO ₂ into N ₂ , the negative ions formed by the electrons of the CO ₂ molecule consume a part of the electrons, hindering the acceleration process of the electrons, and the active negative ions generated will occur with positive ions. complex. Therefore, the generation process of the plasma is suppressed, and the threshold voltage of the gas discharge is increased. As shown in FIG. 5, as the concentration of CO ₂ gas increases, the number of active negative ions increases, the threshold voltage gradually increases, and the number of discharge peaks in a single cycle decreases. Based on this phenomenon, self-driven sensing of CO ₂ is achieved .

Figure 4. Schematic diagram of detection principle of self-driven CO2 gas sensor

(a) A schematic diagram of the discharge principle in N ₂ ; (b) A schematic diagram of the discharge principle after CO _{2 is introduced} in N ₂ .

Figure 5. Concentration response curve of a self-driven CO2 gas sensor

The curve of discharge current with CO ₂ gas concentration when the tip-electrode distance is 0.15 mm .

This study shows that the high-voltage characteristics of friction nano-generators can achieve gas ionization at room temperature and generate a large number of active negative ions. This research laid the foundation for the realization of surface ion regulation technology. Related research work was published on Nano Energy (2018, 5, 898-905) and applied for national invention patent.

Article link: https://www.sciencedirect.com/science/article/pii/S2211285518307018?via%3Dihub

3 ) Establishment of surface ion control technology

Combining friction nano-generators, discharge probes and nano-devices, we have established a surface ion regulation technology as shown in Figure 6a. The voltage output terminal of the friction nano-generator is connected to the input terminal of the full-wave rectifier bridge, and the alternating voltage generated by it is converted into a unipolar output voltage. The negative end of the rectifier bridge is connected to a tungsten needle tip with a radius of curvature of 5 μm, which serves as the cathode of the gas discharge. The positive end of the rectifier bridge is connected to the electrode of the ZnO nanodevice and serves as the anode of the gas discharge. In order to avoid damage to the ZnO nanowire devices during the discharge process, we use a more moderate negative corona discharge mode in the experiment.

Figure 6. Schematic diagram of surface ion grid control technology based on frictional nano-generator-induced gas discharge

(a) Schematic diagram of surface ion control technology; (b) Schematic diagram of gas ionization and generation of active negative ions.

A schematic of ion generation and migration during negative corona discharge is shown in Figure 6b. The negative corona discharge can be divided into two regions, namely the plasma region and the single ion region. A plasma region is formed near the needle tip (cathode), and a single ion region is formed in a region far from the needle tip. In the single ion region, the kinetic energy of electrons is reduced, and gas molecules cannot be ionized to generate positive ions. The electrons will be captured by highly charged neutral gas molecules to produce negative ions. The negative charges that eventually reach the anode include negative ions and electrons. In the air, oxygen molecules have a high electronegativity. Therefore, the negative ions formed in the single ion region are mainly O ₂ ^- ions. Under the action of an electric field, positive ions in the plasma region migrate to the cathode (needle tip), and O ₂ ^- and electrons in the single ion region migrate to the anode. Finally, O ₂ ^- and electrons reach the surface of the semiconductor nanostructure, form surface local ions, and act as a gate to regulate the band structure and electrical transmission characteristics of the nanostructure. This method is an effective method for in-situ regulation of the surface state of semiconductor nanostructures in a gas environment, and has potential applications in developing new devices and improving sensing performance. At present, this technology has applied for a national invention patent.

4 ) Application of surface ion grid control in one-dimensional semiconductor optoelectronic devices

ZnO nanowires have excellent performance in the detection of ultraviolet light, and have been deeply studied by everyone. However, the photodetection process is controlled by the surface state of the ZnO nanowires, resulting in long photocurrent recovery time and slow photodetection switching speed, which limits the practical application of ZnO nanowire photodetectors.

Using surface ion grid control technology, the height of the Schottky barrier of Ag / ZnO nanowires and the electrical transmission characteristics can be controlled by effectively controlling the surface state.

Fig. 7. Regulation curve of ZnO nanowires by surface ion grid in different atmospheres

(a) Current-voltage curves and SEM images of ZnO nanowire Schottky barriers in the dark state; current-time characteristics of surface ion grids regulating ZnO nanowires in different atmospheres, (b) air, (c ) Oxygen, (d) nitrogen.

The test results in different atmospheres show that the adsorption of oxygen negative ions generated on the surface by the discharge and the surface local negative charge formed by the surface electrons trapped by the surface defects are the main ways for the surface ion grid to regulate the surface state.

Combining surface ion grid technology with UV detection, we have achieved rapid detection of ZnO nanowire UV detectors. At the moment when the ultraviolet light is turned off, surface ion grid regulation is implemented, and electrons and oxygen negative ions generated by gas discharge quickly fill the surface state of the ZnO nanowires, increasing the Schottky barrier height, rapidly reducing the photocurrent of ZnO, and realizing light detection Fast recovery of the device.

Figure 8. Photoresponse curve of ZnO nanowires and photocurrent switching curve under surface ion grid control

(a) Current-voltage curve of ZnO nanowire Schottky barrier under dark and ultraviolet light; (b) Photocurrent switching curve of ZnO nanowire Schottky barrier under different atmospheres; (c), ( d): Surface current grid controls the photocurrent switching curve of Schottky barrier of ZnO nanowires in air and pure oxygen atmosphere.

Using surface ion grid control technology, the recovery time of Schottky barrier UV detection of ZnO nanowires has been reduced from 87 s to 0.3 s, making it possible to achieve highly sensitive and fast UV detection in different environments such as air, oxygen and nitrogen. Universal approach. Related research work was published in Nano Energy (2019, 60, 680-688).

Article link: https://www.sciencedirect.com/science/article/pii/S2211285519303167?dgcid=author

5 ) Application of surface ion regulation in two-dimensional semiconductor optoelectronic devices

Transition metal sulfide is an important member of two-dimensional semiconductor materials. Among them, MoS ₂ is the most common transition metal sulfide. The single-layer MoS ₂ not only has a direct band gap of 1.8 eV, but also maintains flexibility, strong photoelectric interaction, Atomic-level thickness and other advantages are expected to develop into the next generation of new semiconductor materials that can replace silicon, and have important application prospects in the fields of optoelectronic devices, adsorption and separation, and catalysis.

We applied the surface ion control technology to MoS ₂ in a single layer . Figures 9 (d)-(f) are the current-time characteristic curves of surface ion grid control MoS ₂ devices under N ₂ , O ₂ and air . The test results under different atmospheres show that the surface local negative charges formed by the oxygen negative ions and electrons generated by the capture of MoS ₂ surface defects are the main reason for the surface ion grid to regulate the device performance.

Figure 9: Surface ion grid regulates the electrical performance of MoS2 devices and the current-time characteristics of the devices in different atmospheres

(A) Schematic diagram of surface ion grid regulation of MoS ₂ devices. (B) The IV characteristic curve of the surface ion grid-regulated MoS ₂ device. The inset is an optical image of the controlled device. The scale is 20 μm. (C) a plurality of times using surface ion gate technology regulation MoS ₂ IV characteristic curve of the device, the device current and the frequency are illustrations of Ion curve corresponding to a gate-control device, V _DS is 1V. (D)-(f) are the current-time characteristic curves of surface ion grid regulating MoS ₂ devices under the conditions of N ₂ , O ₂ and air .

Figure 10. Optical response curves of single-layer MoS2 devices modulated by unused and surface ion grids.

(A) IT characteristic curve of MoS ₂ device under unregulated multiple irradiation cycles . (B) An enlarged view of a single photocurrent cycle in (a) in linear coordinates. (C) IT characteristic curve of MoS ₂ device under multiple irradiation cycles under the control of surface ion grid . (D) An enlarged view of a single photocurrent cycle in (c) in linear coordinates. V _ds is 1V.

We have studied the application of surface ion grid control technology in single-layer MoS ₂ photodetectors, as shown in Figure 10. Using surface plasmon gate control technology, the development of a single-layer MoS ₂ new transistor and the photoelectric detector to obtain a 10 ^{. 4} Maximum ratio of the switching current and the optical response time is reduced from 6.64 s to 74 ms, and discusses the new transistor and a photodetector Working mechanism.

The research shows that the surface ion control technology can control the photoelectric transmission characteristics and surface band structure of two-dimensional materials in real time and in situ, which provides new ideas for the development of new two-dimensional electronic and optoelectronic devices, and has broad application prospect . Related research work was published in Nano Energy (2019, 62, 38-45).

Article link: https://www.sciencedirect.com/science/article/pii/S2211285519304185?via%3Dihub