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Magnetic transitions in graphene derivatives

Nano Research 2014年第10期7卷 1507-1518页

作者： Pengzhan Sun kunlin wang Jinquan Wei Minlin Zhong Dehai Wu Hongwei Zhu School of Materials Science and Engineering State Key Laboratory of New Ceramics and Fine Processing Key Laboratory of Materials Processing Technology of MOE Tsinghua University Beijing 100084 China Department of Mechanical Engineering Tsinghua University Beijing 100084 China Center for Nano and Micro Mechanics Tsinghua University Beijing 100084 China

The magnetic transitions in graphene oxide （GO） have been investigated experimentally. Micron-sized GO flakes exhibit dominant diamagnetism accompanied by weak ferromagnetism at room temperature. However, when the lateral dimensions of GO flakes are reduced from micron-size to nano-size, a clear transition from dominant diamagnetism to ferromagnetism is observed. After reducing the GO chemically or thermally, the dominant magnetic properties are not altered markedly except for the gradual enhancement of ferromagnetic components. In contrast, at 2 K, significant paramagnetism is present in both the micron-sized and nano-sized GO sheets. The effects of different functional groups on magnetic transitions in graphene derivatives have been further investigated using on hydroxyl-, carboxyl-, amino- and thiol- functionalized graphene. The results reveal that significant diamagnetism with weak ferromagnetism is present at room temperature in all of these functionalized graphene derivatives and the ability of different functional groups to introduce magnetic moments follows the order -SH 〉 --OH 〉 -COOH, -NH2. Notably, at 5 K, diamagnetism, paramagnetism and ferromagnetism coexist in thiol-, hydroxyl- and carboxyl-functionalized graphene, while amino-graphene exhibits dominant paramagnetism, analogous to the low-temperature magnetism in GO. These results indicate that diamagnetism, paramagnetism and ferromagnetism can coexist in graphene derivatives and magnetic transitions among the three states can be achieved which depend on edge states, vacancies, chemical doping and the attached functional groups. The results obtained may help settle the current controversy about the magnetism of graphene-related materials.

关键词： graphene derivatives defects functionalization magnetic transition

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Interconnected graphene/polymer micro-tube piping composites for liquid sensing

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Nano Research 2014年第6期7卷 869-876页

作者： Tingting Yang Hongze Zhang Yan wang Xiao Li kunlin wang Jinquan Wei Dehai Wu Zhihong Li Hongwei Zhu School of Materials Science and Engineering State Key Laboratory of New Ceramics and Fine Processing Key Laboratory of Materials Processing Technology of MOE Tsinghua University 8eijing 100084 China Center for Nano and Micro Mechanics Tsinghua University Beijing 100084 China National Key Laboratory of Science and Technology on Micro/Nano Fabrication Institute of Microelectronics Peking University Beijing 100871 China Department of Physics and Nanoscience and Nanotechnology Laboratory Institute for Advanced Study Nanchang University Nanchang 330031 China Department of Mechanical Engineering Tsinghua University Beijing 100084 China

Detection and analysis of volatile organic compounds （VOCs） as pollutants in the atmosphere and liquids are of great significance because of the detrimental effects of VOCs. A polymer-coated graphene micro-tube piping （GMP） structure with a cross-linked and interconnected channel network was synthesized for liquid sensing. By virtue of their unique cross-linked and interconnected channel network configuration, polycrystalline conformation, and the support of a polymer layer, the resistivity of the 3D hollow micro-tubing GMPs was sensitive to strain, ensuring high sensitivity of the liquid sensor （R/Ro of -4 × 10^3% for pure acetone and R/Ro of -105% for 0.01 wt.% acetone solution）. Due to the capillary force, the interfaces of the 3D structures can speed up the penetration of solvents into the polymer, thus promote distinct selectivity within seconds and significantly decrease the response time. Owing to their good selectivity, high sensitivity, rapid response and flexibility, and the ease of use of the sensors and the simplicity of the fabrication processes, the GMP/polymer composites should be a good candidate for liquid sensing.

关键词： liquid sensing graphene micro-tube piping flexible devices high sensitivity

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Microwave absorbing properties and magnetic properties of different carbon nanotubes

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Science China(Technological Sciences) 2009年第1期52卷 227-231页

作者： XuChun Gui kunlin wang JinQuan Wei RuiTao Lü QinKe Shu Yi Jia Chen wang HongWei Zhu DeHai Wu Key Laboratory for Advanced Manufacturing by Materials Processing Technology Department of Mechanical Engineering Tsinghua University Beijing 100084 China Department of Materials Science and Engineering Tsinghua University Beijing 100084 China

The microwave absorbing properties and magnetic properties of as-grown Fe-filled carbon nanotubes (CNTs), annealed Fe-filled CNTs, and multi-walled CNTs were studied. Vibrating sample magnetometer results showed that the annealed Fe-filled CNTs have the weakest coercivity and strongest saturation magnetization among the three types CNTs, due to the presence of more ferromagnetic α-Fe nanowires. After annealing, the values increased to 291.0 Oe and 28.0 emu/g and the samples showed excellent microwave absorbing properties. The reflection loss was over 5 dB between 11.6 GHz and 18 GHz with a maximum value of 10.8 dB for annealed Fe-filled CNTs (1.1 wt%)/epoxy composite.

关键词： carbon nanotubes microwave absorbing properties magnetic properties

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Ultra-sensitive graphene strain sensor for sound signal acquisition and recognition

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Nano Research 2015年第5期8卷 1627-1636页

作者： Yan wang Tingting Yang Junchao Lao Rujing Zhang Yangyang Zhang Miao Zhu Xiao Li Xiaobei Zang kunlin wang Wenjian YU Hu Jin Li wang Hongwei Zhu Department of Physics and Institute for Advanced Study Nanchang University Nanchang 330031 China Center for Nano and Micro Mechanics Tsinghua University Beijing 100084 China School of Materials Science and Engineering State Key Laboratory of New Ceramics and Fine Processing Key Laboratory ot Materials Processing Technology of MOE Tsinghua University Beijing 100084 China Department of ComputerScience and Technology Tsinghua University Beijing 100084 China 2D Carbon Graphene Material Co. Ltd. Changzhou 213149 China

A wearable and high-precision sensor for sound signal acquisition and recognition was fabricated from thin films of specially designed graphene woven fabrics （GWFs）. Upon being stretched, a high density of random cracks appears in the network, which decreases the current pathways, thereby increasing the resistance. Therefore, the film could act as a strain sensor on the human throat in order to measure one＇s speech through muscle movement, regardless of whether or not a sound is produced. The ultra-high sensitivity allows for the realization of rapid and low-frequency speech sampling by extracting the signature characteristics of sound waves. In this study, representative signals of 26 English letters, typical Chinese characters and tones, and even phrases and sentences were tested, revealing obvious and characteristic changes in resistance. Furthermore, resistance changes of the graphene sensor responded perfectly with pre-recorded sounds. By combining artificial intelligence with digital signal processing, we expect that, in the future, this graphene sensor will be able to successfully negotiate complex acoustic systems and large quantities of audio data.

关键词： graphene strain sensors sound detectors high sensitivity

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Luminescence of carbon nanotube bulbs

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Chinese Science Bulletin 2007年第1期52卷 113-117页

作者： LI ChuanGang wang kunlin WEI JinQuan WEI BingQing ZHU HongWei wang ZhiCheng LUO JianBin LIU WenJin ZHENG MingXin WU DeHai Department of Mechanical Engineering Tsinghua University Beijing 100084 China Department of Electrical & Computer Engineering and Center for Computation & Technology Louisiana State University Baton Rouge LA 70803 USA

Carbon nanotube (CNT) bulbs made of decimeter-scale double-walled carbon nanotube (DWCNT) strands and films were fabricated and their luminescence properties, including the lighting efficiency, voltage-current relation and thermal stability were investigated. The results show that the DWCNT bulb has a comparable spectrum of visible light with tungsten bulb and its average efficiency is 40% higher than that of a tungsten filament at the same temperature (1400―2300 K). The nanotube filaments show both resistance and thermal stability over a large temperature region. No obvious damage was found for a nanotube bulb illuminating at 2300 K for more than 24 hours in vacuum.

关键词：碳纳米管灯管发光性电子产品

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Spread of double-walled carbon nanotube membrane

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Chinese Science Bulletin 2007年第7期52卷 997-1000页

作者： JIA Yi WEI JinQuan SHU QinKe CHANG JianGuo wang kunlin wang ZhiCheng LUO JianBin LIU WenJin ZHENG MingXin WU DeHait Key Laboratory for Advanced Materials Processing Technology Ministry of Education of China Department of Mechanical Engineering Tsinghua University Beijing 100084 China

A single-layer double-walled carbon nanotube (DWNT) membrane, with 100 cm2 in area, 50 nm in thickness, was spread by adding a few drops of ethanol (or acetone) on the purified DWNTs. Compared with acetone, ethanol is more efficient in spreading DWNT films. The spreading rate of DWNT mem-brane increases with the ethanol concentration, and it also first increases with temperature (below 60℃), then decreases as the temperature rises to above 60℃.

关键词：双壁碳纳米管薄膜扩展方法乙醇浓度

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Fabrication of large area hexagonal boron nitride thin films for bendable capacitors

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Nano Research 2013年第8期6卷 602-610页

作者： Ning Guo Jinquan Wei Yi Jia Huanhuan Sun Yuhang wang Kehan Zhao Xiaolan Shi Liuwan Zhang Xinming Li Anyuan Cao Hongwei Zhu kunlin wang Dehai Wu Key Lab for Advanced Materials Processing Technology of Education Ministry School of Materials Science and Engineering Tsinghua University Beijing 100084 China Department of Physics Tsinghua University Beijing 100084 China Department of Materials Science and Engineering College of Engineering Peking University Beijing 100871 China

Highly reliable and bendable dielectrics are desired in flexible or bendable electronic devices for future applications. Hexagonal boron nitride （h-BN） can be used as bendable dielectric due to its wide band gap. Here, we fabricate high quality h-BN films with controllable thickness by a low pressure chemical vapor deposition method. We demonstrate a parallel-plate capacitor using h-BN film as the dielectric. The h-BN capacitors are reliable with a high breakdown field strength of -9.0 MV/cm. Tunneling current across the h-BN film is inversely exponential to the thickness of dielectric, which makes the capacitance drop significantly. The h-BN capacitor shows a best specific capacitance of 6.8 F/cm^2, which is one order of magnitude higher than the calculated value.

关键词： hexagonal boron nitride capacitor breakdown strength tunneling effect

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Light emission of double-walled carbon nanotube filaments doped with yttrium and europium

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Science China(Technological Sciences) 2009年第1期52卷 252-255页

作者： SHU QinKe wang kunlin WEI JinQuan ZHU HongWei LI XinMing CHEN Xi JIA Yi GUI XuChun XU ErYang WU DeHai Key Laboratory for Advanced Manufacturing by Materials Processing Technology Department of Mechanical EngineeringTsinghua UniversityBeijing 100084China

As the potential applications of carbon nanotubes in the field of electroluminescence, elements yttrium and europium were introduced to modify the emission properties of double-walled carbon nanotubes (DWNTs) to obtain higher efficacy and other properties. The light emission spectrum of the Y-Eu-doped DWNT filament is suppressed in the near-infrared range, while enhanced in the mid-infrared range. The doped DWNT filament can reach higher efficacy than that of the pure DWNT filament at the same input power and can work stably as long as 5000 h at 12 V. These filaments could be useful for the light sources with special functions, such as infrared light sources operated at low input power.

关键词： carbon nanotubes electroluminescence efficacy yttrium europium

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Three-dimensional porous graphene sponges assembled with the combination of surfactant and freeze-drying

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Nano Research 2014年第10期7卷 1477-1487页

作者： Rujing Zhang Yachang Cao Peixu Li Xiaobei Zang Pengzhan Sun kunlin wang Minlin Zhong Jinquan Wei Dehai Wu Feiyu Kang Hongwei Zhu School of Materials Science and Engineering State Key Laboratory of New Ceramics and Fine Processing Key Laboratory of Materials Processing Technology of MOE Tsinghua University Beijing 100084 China Department of Mechanical Engineering Tsinghua University Beijing 100084 China Advanced Materials Institute Graduate School at Shenzhen Tsinghua University Shenzhen 518055 China Center for Nano and Micro Mechanics Tsinghua University Beijing 100084 China

With the combination of surfactant and freeze-drying, we have developed two kinds of graphene spongy structures. On the one hand, using foams of soap bubbles as templates, three-dimensional porous graphene sponges with rich hierarchical pores have been synthesized. Pores of the material contain three levels of length scales, including millimeter, micrometer and nanometer. The structure can be tuned by changing the freezing media, adjusting the stirring rate or adding functional additives. On the other hand, by direct freeze-drying of a graphene oxide/surfactant suspension, a porous framework with directionally aligned pores is prepared. The surfactant gives a better dispersion of graphene oxide sheets, resulting in a high specific surface area. Both of the obtained materials exhibit excellent absorption capacity and good compression performance, providing a broad range of possible applications, such as absorbents, storage media, and carriers.

关键词： graphene sponge hierarchical freezing media porous foams

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Carbon nanotube-polypyrrole core-shell sponge and its application as highly compressible supercapacitor electrode

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Nano Research 2014年第2期7卷 209-218页

作者： Peixu Li Enzheng Shi Yanbing Yang Yuanyuan Shang Qingyu Peng Shiting Wu Jinquan Wei kunlin wang Hongwei Zhu Quan Yuan Anyuan Cao Dehai Wu Department of Mechanical Engineering Tsinghua University Beijing 100084 China Department of Materials Science and Engineering College of Engineering Peking University Beijing 100871 China Key Laboratory of Analytical Chemistry for Biology and Medicine (Ministry of Education) College of Chemistry and Molecular Sciences Wuhan University Wuhan 430072 China Key Laboratory for Advanced Materials Processing Technology and School of Materials Science and Engineering Tsinghua University Beijing 100084 China

A carbon nanotube （CNT） sponge contains a three-dimensional conductive nano- tube network, and can be used as a porous electrode for various energy devices. We present here a rational strategy to fabricate a unique CNT@polypyrrole （PPy） core-shell sponge, and demonstrate its application as a highly compressible supercapacitor electrode with high performance. A PPy layer with optimal thickness was coated uniformly on individual CNTs and inter-CNT contact points by electrochemical deposition and crosslinking of pyrrole monomers, resulting in a core-shell configuration. The PPy coating significantly improves specific capacitance of the CNT sponge to above 300 F/g, and simultaneously reinforces the porous structure to achieve better strength and fully elastic structural recovery after compression. The CNT@PPy sponge can sustain 1,000 compression cycles at a strain of 50% while maintaining a stable capacitance （〉 90% of initial value）. Our CNT@PPy core-shell sponges with a highly porous network structure may serve as compressible, robust electrodes for supercapacitors and many other energy devices.

关键词： carbon nanotube sponge polypyrrole core-shell configuration compressible electrode supercapacitor

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