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CNT-Carbon nano tubes-A magical view

Single wall carbon nanotube (SWCNT)

Single wall carbon nanotube (SWCNT)is a promising one dimensional (1D) material to fabricate high performance infrared (IR) detectors owing to its unique electrical and physical properties. The 1D Schottky barrier between metal and CNT can separate the photon-generated electron-hole pairs so as to produce photocurrent for quantification and detection. However, the theory developed for the planar metal-semiconductor contact is not compatible with the 1D Schottky barrier within the CNT, thus the optimized structure for a CNT detector is unknown. Our understanding can be improved by using the capacitance-coupled electrostatic doping from a gate of a CNT transistor, which will find out the role of the CNT energy level. A standard back gate CNT transistor based photodetector was fabricated, which showed that positive gate voltages could improve the performance by widening the Schottky barriers. However, the back gate geometry will modulate two Schottky barriers simultaneously with applied bias, severely degrading the detector performance. In order to optimize gate structure for the CNT IR detector, we propose a detector integrated with three different gate structures: side gates for source and drain, and middle gates for the bulk of CNT. The side gates next to the source and drain control the carrier injection at the junctions independently, while the middle gates can block the fringing field from the other gates, and modulate the Fermi level of the CNT channel. We found that opposite gate voltages at source and drain terminals can optimize the performance of the detector by widening one barrier, but eliminating the other. The optimized structure can lead to a high performance nano-scale photon harvest device. This will pave the way for the CNT as a significant building block for future nano-optoelectronics. Index Terms—Carbo

'''Improving the Detectability of CNT based Infrared Sensors using Multi-gate Field Effect Transistor''' Carbon nanotube (CNT) is a novel one dimensional (1D) material that has unique electrical and optoelectronic properties. Photo-sensors using CNT can sense infrared signals by using Schottky barriers between metal and nanotube, which are able to separate photo-generated electron-hole pairs in order to generate photocurrent or photovoltage for detection and quantification. It has been demonstrated that both asymmetric metal structure and electrical field can improve the performance of the sensors by manipulating the energy alignment between metal and CNT. However, it is not clear how to optimize the design of the CNT photo-sensors. An asymmetric multi-gate field effect transistor based infrared detector was fabricated, integrating with asymmetric metal structure (Au-CNT-Al) and multiple gates, which allow for controlling the doping level at source, drain and channel independently. It was found that dark current was suppressed and photocurrent was enhanced by applying negative gate voltages, thus improving sensor’s performance. The CNT detector exhibited similar photo-response when modulating the doping level of CNT segments at source, drain and bulk. We ascribe this to the charge distribution that has a long tail extending over the whole tube.

CNT-MAGICS
Transistor based Infrared Detector a CNT IR detector with back gate structure was fabricated. It was found that negative gate voltages increased dark current and reduced photocurrent due to the shrinkage of the Schottky barrier, on the other hand positive gate voltage improved its performance by suppressing dark current and enhance photocurrent due to the widened depletion regions. However, the performance of the CNT photodetector was not optimized, since the back gate modulated two Schottky barriers simultaneously. We proposed a multi-gate photodetector, which can control source, drain, and CNT channel independently. The external potential simulation uisng Ansoft Maxwell 3D shows that optimal performance can be obtained by applying opposite gate voltages to the source and drain. This work will help researchers design high performance

How CNTs are made
Arc discharge CNTs Can be found in the carbon soot of graphite electrodes during an arc discharge involving high current. This process yields CNTs with lengths up to 50 microns. Laser Ablation In the laser ablation process, a pulsed laser vaporizes a graphite target in a high-temperature reactor while an inert gas is inserted into the reactor. Nanotubes develop on the cooler surfaces of the reactor as the vaporized carbon condenses.

Other methods where CNTs are created: - Chemical Vapor Decomposition - Natural, incidental, and controlled flame environments

DEFECTS OF CNT
Defects can occur in the form of atomic vacancies. High levels of such defects can lower the tensile strength by up to 85%.

Because of the very small structure of CNTs, the tensile strength of the tube is dependent on its weakest segment in a similar manner to a chain, where the strength of the weakest link becomes the maximum strength of the chain.

Carbon Nanotubes Carbon nanotubes (CNTs) have been investigated widely since they were discovered in 1990 by Iijima.1 The structure of a carbon nanotube comprises two regions: tube and cap both made of sp2 carbons. The tube may be considered as rolledup graphene sheets that form a hollow cylinder. Both ends of the tube are often capped by a fullerene type hemisphere; the diameter and the length of a carbon nanotube can be up to several nanometers and several millimeters respectively.2 There are two groups of carbon nanotubes: single-wall carbon nanotubes (SWCNTs) can be thought of a single sheet of graphene rolled up, and they are usually 1~2 nm in diameter. Multi-wall carbon nanotubes (MWCNTs) can be considered as concentric graphene sheets rolled up with sheet-sheet spacings of 0.34 nm and diameters from 2~25 nm.3 Carbon nanotubes are synthesized by mainly three methods: electrical arch discharge, laser ablation and chemical vapor deposition (CVD).4

Carbon Nanotubes: theory and applications
Carbon Nanotubes: theory and applications Abstract. A brief review is given of electronic and transport properties of carbon nanotubes mainly from a theoretical point of view. The topics cover an effective-mass description of electronic states, optical absorption including interaction effects on the band gap and excitonic effects, and the absence of backward scattering except for scatterers with a potential range smaller than the lattice constant which causes a metallic nanotube to be a ballistic conductor even at room temperature

Carbon nanotubes (CNs) are quasi-one-dimensional materials made of sp2-hybridized carbon networks [1] and have been a subject of an extensive study. In particular, the electronic structure of a single CN has been studied theoretically, which predicted that CN becomes either metallic or semiconducting depending on its chiral vector, i.e., boundary conditions in the circumference direction. [2, 3, 4, 5, 6, 7, 8, 9, 10, 11]. These predictions have been confirmed by Raman experiments [12] and direct measurements of local density of states by scanning tunneling spectroscopy [13, 14, 15]. The purpose of this paper is to give a brief review of recent theoretical study on electronic and transport properties of carbon nanotubes. Transport properties are particularly interesting because of their unique topological structures [16]. For potential scattering, it was shown theoretically that there is no backscattering for impurity potentials with a range larger than the lattice spacing in metallic CNs [17]. The absence of backscattering was related to Berry’s phase acquired by a rotation in the wave-vector space in the system described by a k¢p Hamiltonian [18]. It has been confirmed by numerical calculations in a tight-binding model [19]. There have been some reports on experiments which seem to support this theoretical prediction [20, 21]. In understanding electronic properties of nanotubes, a k¢p method or an effective-mass approximation [11] has been quite useful. It has been used successfully in the study of wide varieties of electronic properties of CN. Some of such examples are magnetic properties [22] including the Aharonov-Bohm effect on the band gap, optical absorption spectra [23], exciton effects [24], lattice instabilities in the absence [25, 26] and presence of a magnetic field [27], magnetic properties of ensembles of nanotubes [28], effects of spin-orbit interaction [29], effects of lattice vacancies [30, 31], and electronic properties of nanotube caps [32]. In this paper, we shall first discuss electronic states, then optical spectra, and finally transport properties, obtained mainly in the k¢p scheme.