Electroluminescence from single-wall carbon nanotube network transistors

The electroluminescence (EL) properties from single-wall carbon nanotube network field-effect transistors (NNFETs) and small bundle carbon nanotube field effect transistors (CNFETs) are studied using spectroscopy and imaging in the near-infrared (NIR). At room temperature, NNFETs produce broad (approximately 180 meV) and structured NIR spectra, while they are narrower (approximately 80 meV) for CNFETs. EL emission from NNFETs is located in the vicinity of the minority carrier injecting contact (drain) and the spectrum of the emission is red shifted with respect to the corresponding absorption spectrum. A phenomenological model based on a Fermi-Dirac distribution of carriers in the nanotube network reproduces the spectral features observed. This work supports bipolar (electron-hole) current recombination as the main mechanism of emission and highlights the drastic influence of carrier distribution on the optoelectronic properties of carbon nanotube films.     

Frequency response of top-gated carbon nanotube field-effect transistors

The ac performance of carbon nanotube field-effect transistors (CNFETs) has been characterized using two approaches involving: 1) time- and 2) frequency-domain measurements. A high input impedance measurement system was used to demonstrate time-domain switching of CNFETs at frequencies up to 100 kHz. The low level of signal crosstalk in CNFETs fabricated on quartz substrates enabled frequency-domain measurements of the ac response of CNFETs in the megahertz range, over five orders of magnitude higher in frequency than previously reported ac measurements of CNFET devices.     

High-performance carbon nanotube field-effect transistor with tunable polarities

State-of-the-art carbon nanotube field-effect transistors (CNFETs) behave as Schottky-barrier-modulated transistors. It is known that vertical scaling of the gate oxide significantly improves the performance of these devices. However, decreasing the oxide thickness also results in pronounced ambipolar transistor characteristics and increased drain leakage currents. Using a novel device concept, we have fabricated high-performance enhancement-mode CNFETs exhibiting n- or p-type unipolar behavior, tunable by electrostatic and/or chemical doping, with excellent OFF-state performance and a steep subthreshold swing (S=63 mV/dec). The device design allows for aggressive oxide thickness and gate-length scaling while maintaining the desired device characteristics.     

Novel Local Silicon-Gate Carbon Nanotube Transistors Combining Silicon-on-Insulator Technology for Integration

By taking advantage of the silicon-on-insulator technology and the in situ carbon nanotube (CNT) growth, new local silicon-gate carbon nanotube FETs (CNFETs) have been implemented in this paper. We propose an approach to integrate the CNFET onto the silicon CMOS platform for the first time. Individual device operation, batch fabrication, low parasitic capacitance, and better compatibility to the CMOS process were realized. The characteristics of the CNFETs are comparable to the state-of-the-art devices reported. The scaling effect, ambipolar conductance, Schottky barrier effect, and I–V characteristics noise were analyzed. The physical properties of the CNTs were also characterized.      

A statistical-based material and process guidelines for design of carbon nanotube field-effect transistors in gigascale integrated circuits

Carbon nanotube field-effect transistors (CNFETs) show great promise as building blocks of future integrated circuits. However, synthesizing single-walled carbon nanotubes (CNTs) with accurate chirality and exact positioning control has been widely acknowledged as an exceedingly complex task. Indeed, density and chirality variations in CNT growth can compromise the reliability of CNFET-based circuits. In this paper, we present a novel statistical compact model to estimate the failure probability of CNFETs to provide some material and process guidelines for the design of CNFETs in gigascale integrated circuits. We use measured CNT spacing distributions within the framework of detailed failure analysis to demonstrate that both the CNT density and the ratio of metallic to semiconducting CNTs play dominant roles in defining the failure probability of CNFETs. Besides, it is argued that the large-scale integration of these devices within an integrated circuit will be feasible only if a specific range of CNT density with an acceptable ratio of semiconducting to metallic CNTs can be adjusted in a typical synthesis process.     

Carbon nanotube electronics

Presents experimental results on single-wall carbon nanotube field-effect transistors (CNFETs) operating at gate and drain voltages below 1V. Taking into account the extremely small diameter of the semiconducting tubes used as active components, electrical characteristics are comparable with state-of-the-art metal oxide semiconductor field-effect transistors (MOSFETs). While output as well as subthreshold characteristics resemble those of conventional MOSFETs, we find that CNFET operation is actually controlled by Schottky barriers (SBs) in the source and drain region instead of by the nanotube itself. Due to the small size of the contact region between the electrode and the nanotube, these barriers can be extremely thin, enabling good performance of SB-CNFETs.     

Synthesis of carbon nanotube-TiO(2) nanotubular material for reversible hydrogen storage

A material consisting of multi-walled carbon nanotubes (MWCNTs) and larger titania (TiO(2)) nanotube arrays has been produced and found to be efficient for reversible hydrogen (H(2)) storage. The TiO(2) nanotube arrays (diameter ～60?nm and length ～2-3?μm) are grown on a Ti substrate, and?MWCNTs a few μm in length and ～30-60?nm in diameter are grown inside these TiO(2) nanotubes using chemical vapor deposition with cobalt as a catalyst. The resulting material has been used in H(2) storage experiments based on a volumetric method using the pressure, composition, and temperature relationship of the storage media. This material can store up to 2.5?wt% of H(2) at 77?K under 25?bar with more than 90% reversibility.     

Functionality of C(4,4) carbon nanotube as molecular detector

Alterations in the electronic transport properties of C(4,4) single walled carbon nanotube when an agent is introduced to the outer surface are investigated theoretically. Several chemical agents in this context are investigated. The calculations are performed in two steps: First an optimized geometry for the functionalized carbon nanotube is obtained using semi-empirical calculations at the PM3 level, and then the transport relations are obtained using non equilibrium green-function approach. Gaussian and Transiesta-C simulation packages are used in the calculations correspondingly. The "electrodes" are chosen to be ideal geometry of the particular carbon nanotube, eliminating current quantization effects due to contact region. By varying chemical potential in the electrode regions, an I-V curve is traced for each particular functionalisation. Conductance in carbon nanotubes show a strong dependence on the geometry and aromaticity, both are which altered when the suitable agent is introduced. This dependence results in rather dramatic response in the I-V trace, the current is reduced significantly, and quantization effects are observed, even for a single molecule. However due to chemically stable nature, not all agents form a chemical bond to the surface. Overall, the material is a promising candidate for detector equipment.     

Threshold Voltage and On–Off Ratio Tuning for Multiple-Tube Carbon Nanotube FETs

In this paper, we demonstrate postprocessing techniques to adjust the threshold voltage (V_t) and on-off ratio (I_ON/I_OFF) of multiple-tube carbon nanotube field effect transistors (CNFETs). These postprocessing techniques open up an additional degree of freedom to further tune individual CNFETs in addition to various device synthesis and processing techniques. We demonstrate proof-of-concept experiments and fully characterize their design spaces and tradeoffs. The techniques, threshold voltage setting and on-off ratio tuning, were able to adjust the threshold by as much as 2 V and tune the on-off ratio across 5times10³ to times10⁵. In addition,V_t setting could be used as an analysis tool to infer the V_t distribution of grown carbon nanotubes (CNTs). These tuning techniques, combined with processes such as doping, will enable high-performance multiple-nanotube devices.     

Chemical optimization of self-assembled carbon nanotube transistors

We present the improvement of carbon nanotube field effects transistors (CNTFETs) performances by chemical tuning of the nanotube/substrate and nanotube/electrode interfaces. Our work is based on a method of selective placement of individual single walled carbon nanotubes (SWNTs) by patterned aminosilane monolayer and its use for the fabrication of self-assembled nanotube transistors. This method brings a relevant solution to the problem of systematic connection of self-organized nanotubes. The aminosilane monolayer reactivity can be used to improve carrier injection and doping level of the SWNT. We show that the Schottky barrier height at the nanotube/metal interface can be diminished in a continuous fashion down to an almost ohmic contact through these chemical treatments. Moreover, sensitivity to 20 ppb of triethylamine is demonstrated for self-assembled CNTFETs, thus opening new prospects for gas sensors taking advantages of the chemical functionality of the aminosilane used for assembling the CNTFETs.     

The role of metal-nanotube contact in the performance of carbon nanotube field-effect transistors

Single-wall carbon nanotube field-effect transistors (CNFETs) have been shown to behave as Schottky barrier (SB) devices. It is not clear, however, what factors control the SB size. Here we present the first statistical analysis of this issue. We show that a large data set of more than 100 devices can be consistently accounted by a model that relates the on-current of a CNFET to a tunneling barrier whose height is determined by the nanotube diameter and the nature of the source/drain metal contacts. Our study permits identification of the desired combination of tube diameter and type of metal that provides the optimum performance of a CNFET.     

钛酸纳米管的AFM和STM研究

使用AFM 和STM研究了钛酸纳米管.得到了纳米管表面的精面结构.在纳米管表面观察到一个约0.8nm高的大台阶,这是由于纳米管的螺旋特性造成的.还观察到表面各处有楞边平行于管轴的小台阶.这些小台阶与钛酸片层上的相同,是Ti-O八面体的连接方式造成.这些结果与先前在TEM研究基础上提出的钛酸纳米管模型相符合.     

Functional Covalent Chemistry of Carbon Nanotube Surfaces

In this Progress Report, we update covalent chemical strategies commonly used for the focused functionalization of single‐walled carbon nanotube (SWNT) surfaces. In recent years, SWNTs have been treated as legitimate nanoscale chemical reagents. Hence, herein we seek to understand, from a structural and mechanistic perspective, the breadth and types of controlled covalent reactions SWNTs can undergo in solution phase, not only at ends and defect sites but also along sidewalls. We explore advances in the formation of nanotube derivatives that essentially maintain and even enhance their performance metrics after precise chemical modification. We especially highlight molecular insights (and corresponding correlation with properties) into the binding of functional moieties onto carbon nanotube surfaces. Controllable chemical functionalization suggests that the unique optical, electronic, and mechanical properties of SWNTs can be much more readily tuned than ever before, with key implications for the generation of truly functional nanoscale working devices.     

Linear increases in carbon nanotube density through multiple transfer technique

We present a technique to increase carbon nanotube (CNT) density beyond the as-grown CNT density. We perform multiple transfers, whereby we transfer CNTs from several growth wafers onto the same target surface, thereby linearly increasing CNT density on the target substrate. This process, called transfer of nanotubes through multiple sacrificial layers, is highly scalable, and we demonstrate linear CNT density scaling up to 5 transfers. We also demonstrate that this linear CNT density increase results in an ideal linear increase in drain-source currents of carbon nanotube field effect transistors (CNFETs). Experimental results demonstrate that CNT density can be improved from 2 to 8 CNTs/μm, accompanied by an increase in drain-source CNFET current from 4.3 to 17.4 μA/μm.     

Electrochemical hydrogen storage in single-walled carbon nanotube paper

Single-walled carbon nanotube (SWNT) papers were successfully prepared by dispersing SWNTs in Triton X-100 solution, then filtered by PVDF membrane (0.22 microm pore size). The electrochemical behavior and the reversible hydrogen storage capacity of single-walled carbon nanotube (SWNT) papers have been investigated in alkaline electrolytic solutions (6 N KOH) by cyclic voltammetry, linear micropolarization, and constant current charge/discharge measurements. The effect of thickness and the addition of carbon black on hydrogen adsorption/desorption were also investigated. It was found that the electrochemical charge-discharge mechanism occurring in SWNT paper electrodes is somewhere between that of carbon nanotubes (physical process) and that of metal hydride electrodes (chemical process), and consists of a charge-transfer reaction (Reduction/Oxidation) and a diffusion step (Diffusion).     

Chemical vapour deposition of single walled carbon nanotubes freely suspended over nanotube supports

Single walled carbon nanotubes (SWNTs) suspended above the substrate can be fabricated simply and rapidly by chemical vapour deposition growth over pre-grown multi-walled carbon nanotubes (MWNTs). SWNTs are suspended either on a randomly organized carbon nanotube network on an unpatterned substrate, or between organized pillars made from vertically aligned nanotube forests on a patterned substrate. All nanotubes are produced during a single growth run using a two step growth technique. This approach enables the fabrication of laterally suspended SWNT networks which are well suited for optical applications.     

Incremental growth of short SWNT arrays by pulsed chemical vapor deposition

Very short arrays of continuous single-wall carbon nanotubes (SWNTs) are grown incrementally in steps as small as 25 nm using pulsed chemical vapor deposition (CVD). In-situ optical extinction measurements indicate that over 98% of the nanotubes reinitiate growth on successive gas pulses, and high-resolution transmission electron microscopy (HR-TEM) images show that the SWNTs do not exhibit segments, caps, or noticeable sidewall defects resulting from repeatedly stopping and restarting growth. Time-resolved laser reflectivity (3-ms temporal resolution) is used to record the nucleation and growth kinetics for each fast (0.2 s) gas pulse and to measure the height increase of the array in situ, providing a method to incrementally grow short nanotube arrays to precise heights. Derivatives of the optical reflectivity signal reveal distinct temporal signatures for both nucleation and growth kinetics, with their amplitude ratio on the first gas pulse serving as a good predictor for the evolution of the growth of the nanotube ensemble into a coordinated array. Incremental growth by pulsed CVD is interpreted in the context of autocatalytic kinetic models as a special processing window in which a sufficiently high flux of feedstock gas drives the nucleation and rapid growth phases of a catalyst nanoparticle ensemble to occur within the temporal period of the gas pulse, but without inducing growth termination.     

Superior prospect of chemically activated electrospun carbon fibers for hydrogen storage

In this study, the capacity of hydrogen storage was evaluated by using electrospun activated carbon fibers prepared by electrospinning and chemical activation based on the comparison with other carbon materials such as active carbon, single walled carbon nanotube, and graphite. For an improved hydrogen storage system, the optimized conditions of carbon materials were investigated with studying their specific surface area, pore volume, size, and shape. The hydrogen adsorption capacity of chemically activated electrospun carbon fiber itself is better than that of other porous carbon materials. This is attributed to the optimized pore structure of electrospun activated carbon fibers that might provide better sites for hydrogen adsorption than other carbon materials.     

On‐Chip Magnetic Platform for Single‐Particle Manipulation with Integrated Electrical Feedback

Methods for the manipulation of single magnetic particles have become very interesting, in particular for in vitro biological studies. Most of these studies require an external microscope to provide the operator with feedback for controlling the particle motion, thus preventing the use of magnetic particles in high‐throughput experiments. In this paper, a simple and compact system with integrated electrical feedback is presented, implementing in the very same device both the manipulation and detection of the transit of single particles. The proposed platform is based on zig‐zag shaped magnetic nanostructures, where transverse magnetic domain walls are pinned at the corners and attract magnetic particles in suspension. By applying suitable external magnetic fields, the domain walls move to the nearest corner, thus causing the step by step displacement of the particles along the nanostructure. The very same structure is also employed for detecting the bead transit. Indeed, the presence of the magnetic particle in suspension over the domain wall affects the depinning field required for its displacement. This characteristic field can be monitored through anisotropic magnetoresistance measurements, thus implementing an integrated electrical feedback of the bead transit. In particular, the individual manipulation and detection of single 1‐μm sized beads is demonstrated.     

Silicon adsorption in defective carbon nanotubes: a first principles study

The electronic and structural properties of an (8, 0) single-walled carbon nanotube (SWNT) with a single vacancy and interacting with a Si atom are studied using first principles calculations based on the density-functional theory. Initially, the Si atom is positioned in the site above the vacancy, with its position fixed until the nanotube geometry is fully relaxed. After that, the Si atom approaches the tube and it is shown that one C atom is displaced outwards forming a bump. The final configuration, as well as each step of the process, is studied in detail and the resulting band structures and the total charge densities are systematically analysed.