According to the required properties, the nano-conductive material can be modified with functional molecular materials by reasonable design. It can convert various external input signals into electrical output signals that are easy to monitor. It has broad potential application value in the field of switching, detection, and sensing. It is one of the important means to simulate the life phenomena in nature, such as the visual process and photosynthesis in biological systems. In particular, the preparation of one-dimensional (1D) nano-conducting materials, such as nanowires, nanotubes, etc., into functional electronic devices has attracted extensive attention from researchers in the field of micro / nanoelectronics. 1D nanomaterials can be used as excellent backup materials for switches and sensor devices mainly due to their reduced dimensional characteristics. For example, reducing the dimensions of the material to the nanoscale can greatly enhance the important role of the surface chemistry of the material. At this time, the size of the surface and the analyte are matched. At the same time, the low dimension also brings ultra-small structure and large specific surface area. The synergistic effect of the above two aspects brought about by the reduction of dimensions gives this type of material unique properties and has an ultra-high sensitivity to changes in the local chemical environment. On the other hand, miniaturizing the size of the device to the nanometer level also meets the urgent needs of the development of traditional silicon-based complementary metal oxide semiconductor (CMOS) devices in industry. Therefore, it is of great significance to develop different nanomaterials and their corresponding devices, especially functionalized nano / molecular devices that respond to external stimuli. In this regard, carbon nanomaterials such as single-walled carbon nanotubes (SWNT), because of their unique structure and physicochemical properties, and with the continuous improvement of SWNT synthesis technology and purification methods, have been widely used by researchers in the past 10 years To build functionalized nano devices, and made a lot of exploration. SWNT-based devices include: single-electron transistors, molecular diodes, memory elements and logic gates; SWNT's super tensile strength makes it possible to manufacture SWNT reinforcing fibers and polymer additives; in the field of analytical chemistry, carbon nanotubes The surface is easy to combine functional molecules, biomolecules or nanoprobe molecules through self-assembly, suitable for the development of various specific bio / chemical sensors and nanoprobes; high specific surface area and strong adsorption can make carbon Nanotubes are used as hydrogen storage and energy storage materials. The functional modification of single-walled carbon nanotubes can effectively adjust the energy band structure of carbon nanotubes and enhance the signal response to external stimuli. It is expected to break through the structural types of simple devices and prepare carbon nanotube electronic circuits with new structures and integration. Circuit. Therefore, the functionalization of single-walled carbon nanotube devices has been one of the hot topics in this research field.
2 Structural characteristics of single-walled carbon nanotubes Nanotubes as applied materials have attracted great attention in many fields. Carbon nanotubes have the simplest atomic composition and bonding method, but they are the nanomaterials with the most abundant and diverse structure and structure-property relationship. Carbon nanotubes are cylindrical materials with unique structure, mechanical and electrical properties, with diameters ranging from 1nm to tens of nm and lengths up to centimeters. According to its (super) structure, carbon nanotubes can be divided into two categories, namely multi-walled carbon nanotubes and single-walled carbon nanotubes. Multi-walled carbon nanotubes are composed of multiple layers of concentric cylinders with a spacing of 0.34nm between adjacent layers, while single-walled carbon nanotubes can be regarded as a seamless cylindrical structure made of a single layer of graphene curled. SWNT's 1D nanostructure, monolayer atomic surface, and extended curved n-conjugated system give it exceptional tensile strength, excellent thermal conductivity, electronic ballistic transport characteristics, and ultra-high elasticity. The arrangement of carbon atoms in the honeycomb hexagonal grid on the surface of SWNT has helicity. This helicity and diameter bring about significant differences in the electronic states of carbon nanotubes, and therefore bring various electrical characteristics of SWNT. In other words, SWNT may be metallic or semiconductor. Metallic carbon tubes have become a model system for studying various quantum phenomena in quasi-one-dimensional solid systems, including single electron conduction, Ratingen liquids, weak delocalization effects, ballistic transmission, and quantum interference, while semiconductor tubes are used to construct New-type nanoelectronic devices, including transistors, logic devices, memory devices and sensor devices. At present, carbon nanotubes can be effectively prepared and synthesized using a variety of different methods.
3 There are a large number of single-molecule devices based on single-walled carbon nanotube electrodes. In the same bipolar carbon nanotube device, when a negative gate voltage is applied, we observe a rapid and significant current reduction under ultraviolet light; as mentioned above It is predicted that when a forward gate voltage is applied, a significant increase in current is seen. This is the first optical switch effect with mirror symmetry on the same device. Further experiments prove that this kind of mirrored optical switching phenomenon has wide universality, and can be realized through reasonable device structure design using different nano materials.
In practical applications, people are trying to improve the sensitivity, response time and selectivity of UV light detectors. For this purpose, a tunable composite UV light detector can be prepared by modifying high crystal zinc oxide (ZnO) nanoparticles on the outer surface of SWNT (responsivity of this detector is as high as 105). Here, the large specific surface area of ​​ZnO nanoparticles can be used to increase the number of surface capture sites and the reduced dimension to define the characteristics of the active region of charge carriers. Based on the mechanism of light-induced adsorption and oxygen desorption on the surface of nanoparticles, the device exhibits a significant optical switching effect, and at the same time has good reversibility and repeatability. Since ZnO nanoparticles have wavelength-dependent photosensitivity, the device conductivity also exhibits wavelength-dependent characteristics. This result can accelerate the application of ZnO nanoparticles in the field of UV photodetectors.
Schematic diagram of the effect of ZnO nanoparticles on the device properties under UV irradiation and darkness (in order to better show the mechanism, the dodecanoic acid molecules used to connect the nanoparticles to the SWNT sidewalls are ignored in the figure) Looking to the above, we mainly introduce two strategies for developing functional SWNT transistor devices: nano-etching method and surface chemical modification method. Through nano-etching method, we can accurately cut SWNT to form nano-gap, and use conductive molecules to achieve the re-connection of SWNT. This provides a new way to prepare stable molecular devices, which can truly study the intrinsic physical properties of molecules and biological systems at the single molecule level. Through a clear chemical reaction process and specific molecular recognition, this method can be used to prepare functional devices that respond to external stimuli and are used in the fields of biology, materials science, and organic chemistry. On the other hand, surface chemical modification strategies can combine the advantages of modified molecules and SWNT, design and develop specific new functions, and have broad application prospects in the study of molecular devices that respond to stimuli. The research of surface functionalized SWNT devices will also promote their applications in bio / chemical sensing, memory devices, field emission devices, etc.
However, it should also be noted that the use of SWNT to construct functional devices also poses great challenges. Due to the diversity of the chirality and diameter of SWNT itself, the measurement differences between devices cause some difficulties in the analysis and comparison of results. In addition, the nano-etching process has the randomness of the micro-cutting process, and it is difficult to accurately control the method at the atomic level, resulting in a low success rate of device preparation. Therefore, while developing SWNT functional devices, we should also focus on the application research of new materials. Graphene is a new material widely concerned by researchers at this stage. Due to its two-dimensional structural characteristics and excellent electrical conductivity, graphene is another star material with great potential application value in the carbon material family. The large-area graphene grown by CVD is conducive to device preparation and performance research, and the devices show the metalloid conductive properties, and the differences between the devices are small, so the research of SWNT functionalized devices can be migrated to graphene devices in. In response to the above problems, we recently developed a simple dotted line etching method to prepare an array of zigzag graphene point electrode pairs, thereby greatly improving the success rate of device fabrication. This constitutes the second generation of carbon-based molecular electronic devices and provides a reliable platform for further single-molecule scientific research. It is believed that in the near future, the research of carbon material functional devices will be more abundant and deeper, thus greatly promoting the rapid development of carbon-based molecular electronic devices.
Guo Xuefeng, Peking University's "100 people" special researcher. In July 2004, he obtained a doctorate degree from the Institute of Chemistry, Chinese Academy of Sciences. From August 2004 to December 2007, he was engaged in postdoctoral research at the Nano Center of Columbia University. In January 2008, he officially worked in the School of Chemistry and Molecular Engineering of Peking University. He has won 100 outstanding doctoral dissertations nationwide and the Young Chemistry Award of the Chinese Chemical Society, and was awarded the National Outstanding Youth Fund in 2012. The main research directions are: (1) functional nano / molecular electronic devices; (2) chemical and biomolecular sensors; (3) organic / inorganic composite semiconductor materials synthesis and photoelectric performance research. He has published many research papers on nano / molecular electronic devices in Science, Nature Nanotechnology and Proceedingsofthe National Academy of Sciences, USA, etc., and has aroused widespread concern in science and industry. Scientific reports on his research work.
Liu Song was born in Tianjin in 1983. In 2006, he graduated from the Department of Materials Chemistry, School of Chemistry, Nankai University. Later, he entered the School of Chemistry and Molecular Engineering of Peking University to pursue a doctorate degree. His main research direction is the design and application of functionalized single-walled carbon nanotube devices. After graduating in 2011, he went to Case Western Reserve University for postdoctoral research.
The project is supported by the National Natural Science Foundation of China (Nos. 21253311, 51121091, 2112016), the National Key Basic Research and Development Plan (973) (No. 2012CB921404), the Disciplinary Innovation and Introducing Intelligence Program of Higher Education (No. B08001), and Beijing Science and Technology Rising Star (No. .2009A01) funding.
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