Capacitive nanoelectromechanical switch based on suspended carbon nanotube array

We present the fabrication and high frequency characterization of a capacitive nanoelectromechanical system (NEMS) switch using a dense array of horizontally aligned single-wall carbon nanotubes (CNTs). The nanotubes are directly grown onto metal layers with prepatterned catalysts with horizontal alignment in the gas flow direction. Subsequent wetting-induced compaction by isopropanol increases the nanotube density by one order of magnitude. The actuation voltage of 6 V is low for a NEMS device, and corresponds to CNT arrays with an equivalent Young's modulus of 4.5-8.5 GPa, and resistivity of under 0.0077 Ω·cm. The high frequency characterization shows an isolation of -10 dB at 5 GHz. © 2010 American Institute of Physics.

Carbon nanotube array: a new MIP platform.

Here we demonstrate that a free-standing carbon nanotube (CNT) array can be used as a large surface area and high porosity 3D platform for molecular imprinted polymer (MIP), especially for surface imprinting. The thickness of polymer grafted around each CNT can be fine-tuned to imprint different sizes of target molecules, and yet it can be thin enough to expose every imprint site to the target molecules in solution without sacrificing the capacity of binding sites. The performance of this new CNT-MIP architecture was first assessed with a caffeine-imprinted polypyrrole (PPy) coating on two types of CNT arrays: sparse and dense CNTs. Real-time pulsed amperometric detection was used to study the rebinding of the caffeine molecules onto these CNT-MIPPy sensors. The dense CNT-MIPPy sensor presented the highest sensitivity, about 15 times better when compared to the conventional thin film, whereas an improvement of 3.6 times was recorded on the sparse CNT. However, due to the small tube-to-tube spacing in the dense CNT array, electrode fouling was observed during the detection of concentrated caffeine in phosphate buffer solution. A new I-V characterization method using pulsed amperometry was introduced to investigate the electrical characterization of these new devices. The resistance value derived from the I-V plot provides insight into the electrical conductivity of the CNT transducer and also the effective surface area for caffeine imprinting.

Bouncing Water Droplet on a Superhydrophobic Carbon Nanotube Array

Over the past few decades, superhydrophobic materials have attaracted a lot of interests, due to their numerous practical applications. Among various superhydrophobic materials, carbon nanotube arrays have gained enormous attentions simply because of their outstanding properties. The impact dynamic of water droplet on a superhydrophobic carbon nanotube array is shown in this fluid dynamics video.

Use of vertically-aligned carbon nanotube array to enhance the performance of electrochemical capacitors

In the domain of energy storage, electrochemical capacitors have numerous applications ranging from hybrid vehicles to consumer electronics, with very high power density at the cost of relatively low energy storage. Here, we report an approach that uses vertically aligned carbon nanotube arrays as electrodes in electrochemical capacitors. Different electrolytes were used and multiple parameters of carbon nanotube array were compared: carbon nanotube arrays were shown to be two to three times better than graphite in term of specific capacitance, while the surface functionalization was demonstrated to be a critical factor in both aqueous and nonaqueous solutions to increase the specific capacitance. We found that a maximum energy density of 21 Wh/kg at a power density of 1.1 kW/kg for a hydrophilic electrode, could be easily achieved by using tetraethylammonium tetrafluoroborate in propylene carbonate. These are encouraging results in the path of energy-storage devices with both high energy density and power density, using only carbon-based materials for the electrodes with a very long lifetime, of tens of thousands of cycles. © 2011 IEEE.

Reversible tuning of the wettability of carbon nanotube arrays: the effect of ultraviolet/ozone and vacuum pyrolysis treatments.

Among diverse types of synthetic materials, arrays of vertically aligned carbon nanotubes have attracted the most attention, mainly because of their exceptional mechanical, electrical, optical, and thermal properties. However, their wetting properties are yet to be understood. In this present study, oxygenated surface functional groups have been identified as a vital factor in controlling the wetting properties of carbon nanotube arrays. The results presented herein indeed show that a combination of ultraviolet/ozone and vacuum pyrolysis treatments can be used to vary the surface concentration of these functional groups such that the carbon nanotube array can be repeatedly switched between hydrophilic and hydrophobic.

Optical waveguides and switches based on periodic arrays of carbon nanotubes

This document presents the modeling and characterization of novel optical devices based on periodic arrays of multiwalled carbon nanotubes. Vertically aligned carbon nanotubes can be grown in the arrangement of two-dimensional arrays of precisely determined dimensions. Having their dimensions comparable to the wavelength of light makes carbon nanotubes good candidates for utilization in nano-scale optical devices. We report that highly dense periodic arrays of multiwalled carbon nanotubes can be utilized as sub-wavelength structures for establishing advanced optical materials, such as metamaterials and photonic crystals. We demonstrate that when carbon nanotubes are grown close together at spacing of the order of few hundred nanometers, they display artificial optical properties towards the incident light, acting as metamaterials. By utilizing these properties we have established micro-scaled plasmonic high pass filter which operates in the optical domain. Highly dense arrays of multiwalled also offer a periodic dielectric constant to the incident light and display interesting photonic band gaps, which are frequency domains within which on wave propagation can take place. We have utilized these band gaps displayed by a periodic nanotube array, having 400 nm spacing, to construct photonic crystals based optical waveguides and switches. © 2011 IEEE.

Optical holograms based on carbon nanotubes

The size of pixels is one of the key limiting features in the state of the art of holographic displays systems. The resolution and field of view in these systems are dictated by the size of the pixel (the smallest light scattering element). We have demonstrated the utilization of carbon nanotubes (nanostructures) as the smallest possible scattering element for diffracting light in a highly controlled manner to produce a two dimensional image. An array of carbon nanotubes was elegantly patterned to produce a high resolution hologram. In response to the incident light on the hologram a high contrast image was produced. Due to the nanoscale dimension of the carbon nanotube array the image presented a wide field of view and high resolution. These results pave way towards the utilization of nanostructures for producing 3D holograms with wide field of view and high resolution. © 2013 IEEE.

The influence of 1D, meso- and crystal structures on charge transport and recombination in solid-state dye-sensitized solar cells

We have prepared single crystalline SnO2 and ZnO nanowires and polycrystalline TiO2 nanotubes (1D networks) as well as nanoparticle-based films (3D networks) from the same materials to be used as photoanodes for solid-state dye-sensitized solar cells. In general, superior photovoltaic performance can be achieved from devices based on 3-dimensional networks, mostly due to their higher short circuit currents. To further characterize the fabricated devices, the electronic properties of the different networks were measured via the transient photocurrent and photovoltage decay techniques. Nanowire-based devices exhibit extremely high, light independent electron transport rates while recombination dynamics remain unchanged. This indicates, contrary to expectations, a decoupling of transport and recombination dynamics. For typical nanoparticle-based photoanodes, the devices are usually considered electron-limited due to the poor electron transport through nanocrystalline titania networks. In the case of the nanowire-based devices, the system becomes limited by the organic hole transporter used. In the case of polycrystalline TiO2 nanotube-based devices, we observe lower transport rates and higher recombination dynamics than their nanoparticle-based counterparts, suggesting that in order to improve the electron transport properties of solid-state dye-sensitized solar cells, single crystalline structures should be used. These findings should aid future design of photoanodes based on nanowires or porous semiconductors with extended crystallinity to be used in dye-sensitized solar cells. © 2013 The Royal Society of Chemistry.

Characterization of a liquid crystal microlens array using multiwalled carbon nanotube electrodes.

Reconfigurable liquid crystal microlenses employing arrays of multiwalled carbon nanotubes (MWNTs) have been designed and fabricated. The cells consist of arrays of 2 microm high MWNTs grown by plasma-enhanced chemical vapor deposition on silicon with a top electrode of indium tin oxide coated glass positioned 20 microm above the silicon and the gap filled with the nematic liquid crystal BLO48. Simulations have found that, while its nematic liquid crystal aligns with MWNTs within a distance of 10nm, this distance is greatly enhanced by the application of an external electric field. Polarized light experiments show that light is focused with focal lengths ranging from approximately 7 microm to 12 microm.

Advantages of top-gate, high-k dielectric carbon nanotube field-effect transistors

The subthreshold slope, transconductance, threshold voltage, and hysteresis of a carbon nanotube field-effect transistor (CNT FET) were examined as its configuration was changed from bottom-gate exposed channel, bottom-gate covered channel to top-gate FET. An individual single wall CNT was grown by chemical vapor deposition and its gate configuration was changed while determining its transistor characteristics to ensure that the measurements were not a function of different chirality or diameter CNTs. The bottom-gate exposed CNT FET utilized 900 nm SiO2 as the gate insulator. This CNT FET was then covered with TiO2 to form the bottom-gate covered channel CNT FET. Finally, the top-gate CNT FET was fabricated and the device utilized TiO 2 (K ∼ 80, equivalent oxide thickness=0.25 nm) as the gate insulator. Of the three configurations investigated, the top-gate device exhibited best subthreshold slope (67-70 mV/dec), highest transconductance (1.3 μS), and negligible hysteresis in terms of threshold voltage shift. © 2006 American Institute of Physics.

Self-aligned, gated arrays of individual nanotube and nanowire emitters

We demonstrate the production of integrated-gate nanocathodes which have a single carbon nanotube or silicon nanowire/whisker per gate aperture. The fabrication is based on a technologically scalable, self-alignment process in which a single lithographic step is used to define the gate, insulator, and emitter. The nanotube-based gated nanocathode array has a low turn-on voltage of 25 V and a peak current of 5 μA at 46 V, with a gate current of 10 nA (i.e., 99% transparency). These low operating voltage cathodes are potentially useful as electron sources for field emission displays or miniaturizing electron-based instrumentation.

GHz modulation of carbon nanotube cathodes for microwave amplifiers

Cold cathodes based on carbon nanotubes (CNs) allow to produce a pulsed/directly modulated electron beam. Using an array of vertically aligned CNs that exhibit an aspect ratio of around 200, we demonstrated the modulation of a 1.5 A/cm2 beam at 1.5 GHz frequency. Such CN cathodes are very promising for their use in a new generation of compact and low cost microwave amplifiers that operates between 30 and 100 GHz. ©2005 IEEE.