Photocurrent response of the carbon nanotube-silicon heterojunction array

A highly ordered array of parallel, identical carbon nanotubes is grown non- lithographically in a bottom-up fabrication approach to form a heterojunction with a silicon substrate. Evidence of a space-charge separated region at the nanotube-silicon interface is present in the form of diode rectification and a closed-circuit zero-bias photocurrent in response to infrared light. Because carbon nanotubes are narrow bandgap semiconductors, their heterojunction with silicon was analysed spectrally via Fourier transform infrared photocurrent spectroscopy with the aim of investigating the suitability of this structure for infrared (IR) detector applications. IR photoresponse shows signs of temperature-dependent activation that is complex but consistent with estimates of the heterojunction barrier height. Considering the many interesting benefits and properties of carbon nanotubes, these results despite their earliness suggest that nanotube-silicon heterojunction systems could form the foundation for a new kind of infrared detection device.     

Electrical and optical transport of GaAs/carbon nanotube heterojunctions

Heterojunctions consisting of nanotubes and an industrialized semiconductor-GaAs have been produced, and their transport properties were studied. We found that the p-doped GaAs forms an ohmic contact with a nanotube but the n-doped GaAs/nanotube heterojunction is rectifying. Analysis of measurement results at various temperatures shows that tunneling transport plays an important role. We also observed photovoltaic effects in n-GaAs/nanotube junction with the illumination of a green laser or desk lamp.     

Precise control of the number of walls formed during carbon nanotube growth using chemical vapor deposition

We demonstrate a one-step approach for selecting the number of walls formed during carbon nanotube (CNT) growth by catalytic decomposition of CH(4) over Fe-Mo/MgO catalysts. Scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM), thermal gravimetric analysis (TGA) and Raman spectroscopy analyses indicate that high purity single-walled, double-walled and triple-walled carbon nanotubes can be synthesized by tuning the Fe:Mo atomic ratio of catalysts. The results reveal that the concentration of Mo in the catalyst plays an important role in the size of catalyst particles and in the deposition rate of carbon atoms during CNT growth. Thus, the wall numbers of CNTs can be controlled precisely.     

Template synthesis and growth mechanism of metal nanowire/carbon nanotube heterojunctions

Copper (Cu) and Cobalt (Co) with remarkable difference in the catalytic activity for the growth of carbon nanotubes (CNTs) have been used to prepare metal-nanowire/CNT heterojunctions. The ordered arrays of Cu nanowire/CNT (CuNW/CNT) and Co nanowire/CNT (CoNW/CNT) heterojunctions were prepared by combining electrochemical deposition and chemical vapor deposition. The interfaces between CNTs and Cu or Co nanowires have been examined and compared. At the interface of CuNW/CNT heterojunction, the tip of CuNW is encapsulated by carbon material (named "cap") and connected the CNT consisting of amorphous carbon (a-C). Two-segment CuNW/amorphous CNT (CuNW/a-CNT) hybrid nanostructure was obtained for the CuNW/CNT heterojunctions due to low catalytic activity of Cu. It is also interesting that a hollow gap was observed between the "cap" and the CuNW. By contrast with the case of Cu, multi-walled CNT (MWCNT) was achieved and no hollow gap was observed at the interface of CoNW/CNT heterojunctions. Three-segment CoNW/MWCNT/a-CNT hybrid nanostructure was observed for the CoNW/CNT heterojunctions because of high catalytic activity of Co. Because no stable copper carbides are observed, we infer that the growth mechanism of CuNW/CNT heterojunctions is different from that of CoNW/CNT. Possible growth models of CuNW/CNT and CoNW/CNT heterojunctions are proposed based on experimental results, respectively.     

Multiwalled carbon nanotube film for strain sensing

We have studied the possibility of using multiwalled carbon nanotube (MWCNT) films as strain sensors. The MWCNT films were prepared by a solution/filtration method and were bonded directly onto specimens by a nonconductive adhesive. For comparison, conventional foil strain gages were also bonded to the structure on the opposite side. The specimens then underwent a uniaxial tensile load-unload cycle to evaluate them as strain sensors. To ensure good electrical contact between carbon nanotube film and the wires, a thin layer of copper was thermally deposited on both ends of the film as electrodes, and the wires were connected to the electrodes by silver ink. Wheatstone bridges were used to convert the resistance changes of the MWCNTs to voltage output. Results indicated that the output voltages were proportional to the strain readings from the stain indicator. The effect of temperature on the resistance was measured and the MWCNT film resistance was found to be independent of temperature over the range 273-363?K. The optimal film dimension for strain sensing was evaluated as well. Dynamic tests suggest that the MWCNTs were able to extract the structural signature. Our results indicate that MWCNT film is potentially useful for structural health monitoring and vibration control applications.     

Formation of free-standing carbon nanotube array on super-aligned carbon nanotube film and its field emission properties

Flexible and free-standing well-aligned carbon nanotube arrays have been synthesized on super-aligned carbon nanotube films. The combined structure of the carbon nanotube array and carbon nanotube film was formed during chemical vapor deposition on a quartz substrate which had previously been covered with a super-aligned carbon nanotube film. It was found that the growing carbon nanotube array could support up the super-aligned carbon nanotube film entirely, and the top of the array became densely entangled with the super-aligned carbon nanotube film. The carbon nanotube array with the super-aligned carbon nanotube film could be easily peeled off from the quartz substrate as a whole, giving a flexible and free-standing structure with good mechanical properties. The bottom of the array was also exposed after being peeled off and was used as a field emitter. The combined structure of the carbon nanotube array with the carbon nanotube film allowed adsorbent-free field emission by passing a heating current through it. Furthermore, due to the fast thermal response of the structure and the long time needed for re-adsorption of adsorbates in vacuum, it was found that pulsed heating with a 10% duty ratio was sufficient for adsorbent-free field emission. The heating power necessary to sustain the adsorbent-free state can be lowered in this way.

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Semiconducting carbon nanotube/fullerene blended heterojunctions for photovoltaic near-infrared photon harvesting

We demonstrate that the near-infrared (NIR) absorptivity of semiconducting single-walled carbon nanotubes (s-SWCNTs) can be harnessed in blended heterojunctions with the fullerene derivative [6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM). Photogenerated charge separation is efficiently driven by the ultrahigh interfacial area of the blends and the favorable energy offsets between the two materials. NIR-sensitive photovoltaic and photodetector devices utilizing the stack (indium tin oxide/ca. 10 nm s-SWCNT:PCBM/100 nm C₆₀/10 nm 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP)/Ag) were fabricated with NIR power conversion efficiencies >1.3% and peak, zero bias external quantum efficiency of 18% at λ = 1205 nm.      

Building desired heterojunctions of semiconductor CdS nanowire and carbon nanotube via AAO template-based approach

Heterojunctions of CdS nanowire (CdSNW) and carbon nanotube (CNT) have been achieved in the nanochannels of anodic aluminum oxide (AAO) templates via sequentially electrodepositing CdSNWs and chemical vapor depositing CNTs. Transport measurements reveal that Ohmic-like behavior has been achieved, which may result from a very low energy barrier in the junction of CdSNW/CNT. Furthermore, three-segment heterostructures of CNT/CdSNW/CNT have also been obtained by adding a procedure of selectively etching part of the deposited CdSNWs before chemical vapor depositing CNTs. The approach could be exploited to build nanodevices and functional networks consisting of well-interconnected two- or three-segment nanoheterostructures.     

碳纳米管薄膜电极的制备及电容吸附性能

采用低压CVD法制备得到的金属镍基碳纳米管薄膜直接作为电容去离子器（CDD）的电极材料，并对碳纳米管薄膜进行了扫描电镜观察和比表面与孔径分析，探讨了该电极材料的电容吸附性能。NaF溶液的吸附实验结果表明：该电极材料的去离子效果明显、且可再生和重复使用。     

Activated carbon–carbon nanotube composite porous film for supercapacitor applications

Activated carbon/carbon nanotube composite electrodes have been assembled and tested in organic electrolyte (NEt₄BF₄ 1.5 M in acetonitrile). The performances of such cells have been compared with pure activated carbon-based electrodes. CNTs content of 15 wt.% seems to be a good compromise between power and energy, with a cell series resistance of 0.6 Ω cm² and an active material capacitance as high as 88 F g⁻¹.     

Photovoltaic device performance of single-walled carbon nanotube and polyaniline films on n-Si: device structure analysis

In this paper, we explore the use of two organic materials that have been touted for use as photovoltaic (PV) materials: inherently conducting polymers (ICPs) and carbon nanotubes (CNTs). Due to these materials' attractive features, such as environmental stability and tunable electrical properties, our focus here is to evaluate the use of polyaniline (PANI) and single wall carbon nanotube (SWNT) films in heterojunction diode devices. The devices are characterized by electron microscopy (film morphology), current-voltage characteristics (photovoltaic behavior), and UV/visible/NIR spectroscopy (light absorption). We have found that both PANI and SWNT can be utilized as photovoltaic materials in a simple bilayer configuration with n-type Silicon: n-Si/PANI and n-Si/SWNT. It was our aim to determine how photovoltaic performance was affected utilizing both PANI and SWNT layers in multilayer devices: n-Si/PANI/SWNT and n-Si/SWNT/PANI. The short-circuit current density increased from 4.91 mA/cm(2) (n-Si/PANI) to 12.41 mA/cm(2) (n-Si/PANI/SWNT), while an increase in power conversion efficiency by ~91% was also observed. In the case of n-Si/SWNT/PANI and its corresponding device control (n-Si/SWNT), the short-circuit current density was decreased by an order of magnitude. The characteristics of the device were affected by the architecture and the findings have been attributed to the more effective transport of holes from the PANI to SWNT and less effective transport of holes from PANI to SWNT in the respective multilayer devices.     

Preparation of parallel arrayed carbon nanotube film and its structure research

In an atmosphere of hydrogen and methane mixture, we have grown well-aligned carbon nanotube film on a Si slice by microwave plasma enhanced chemical vapor deposition. Observing and analyzing through SEM, TEM, and Raman spectra, we found that carbon nanotubes were vertically arrayed in the film. Their diameters are approximately 40–60 nm, and the lengths are about 4.0 μm. This well-aligned carbon nanotube film will be a very promising material for electron emitters in field emission displays. The text was submitted by the authors in English.     

Microfabrication and characterization of spray-coated single-wall carbon nanotube film strain gauges

We present the design, fabrication, and characterization results of single-wall carbon nanotube (SWCNT) film strain gauges for potential applications as highly sensitive strain, weight, or pressure sensors on the macro-scale. A batch microfabrication process was developed for practical device construction and packaging using spray-coated SWCNTs and a conventional semiconductor process. The prototype was characterized using a commercial metal foil gauge with tensile and compressive testing on a binocular load cell. Our test results demonstrated that the proposed SWCNT film gauges have a linear relationship between resistance changes and externally applied strain. The gauge factor ranged from 7.0 to 16.4 for four different micro-grid configurations, indicating that the maximum strain sensitivity of the prototype was approximately eight times greater than that of commercial gauges.     

碳纳米管压阻效应及其应用研究

利用了三点弯曲法研究了碳纳米管膜压阻效应.研究中所用的碳纳米管是用化学气相沉积法合成的.实验结果表明,碘掺杂和未掺杂的碳纳米管膜的压阻因子在500微形变和室温下分别为350和65,超过了单晶硅和多晶硅的压阻因子.同时还发现化学处理还能进一步增强碳纳米管膜的压阻效应.碘掺杂碳纳米管膜的压阻效应也许是由于在形变时带隙变化所致,而未掺杂碳纳米管压阻效应主要是由管之间接触电阻变化引起的.碳纳米管压阻效应可用于旋转速度的测量.     

碳纳米管与活性炭超级离子电容器的频率响应

分别采用碳纳米管和活性炭作用超级离子电容器的电极材料，应用交流阻抗频谱法，研究了两类超级离子电容器的频率响应特性。结果表明，用碳纳米管作电极，超级离子电容器地频率250mHz以下出现“电荷饱和”；而用活性炭作电极，超级离子电容器在频率为100mHz时仍未出现“电荷饱和”，说明碳纳米管超级离子电容器的频率响应特性优于活性炭超级离子电容器的频率响应特性，但是上述两类超级离子电容器的频率响应特性均比传统介质电容器的频率响应特性差。     

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.     

Quantum dot/carbon nanotube/silicon double heterojunctions for multi-band room temperature infrared detection

We report on the observation of room temperature multi-band photocurrent response from a novel structure formed by interior loading of PbS quantum dots into carbon nanotubes grown on silicon. In addition to a mid infrared photoresponse band at ～ 0.22 eV due to the carbon nanotubes and one band at 1.1 eV corresponding to silicon, two bands in the mid/near infrared at 0.63 and 0.82 eV, corresponding to the first and second exciton bands of the PbS quantum dots, are observed in photocurrents measured at room temperature, in uncooled operation. We have also observed a red shift of the 0.63 and 0.82 eV bands with cooling that reflects a behavior typical for PbS and supports the hypothesis that these photoresponse bands are due to absorption in the PbS quantum dots.     

The effect of phonon scattering on the switching response of carbon nanotube field-effect transistors

The performance of carbon nanotube field-effect transistors is analyzed numerically, using the non-equilibrium Green's function formalism. The effect of electron-phonon scattering on both the DC and switching response of these devices is studied. For the calculation of the switching response, the quasi-static approximation is assumed. The role of the electron-phonon coupling strength and phonon energy are investigated. Our results indicate that scattering with high-energy phonons reduces the on-current only weakly, but can increase the switching time considerably due to charge pile-up in the channel. Conversely, scattering with low-energy phonons reduces the on-current more effectively, but has a weaker effect on the switching time.