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Various types of CNTs, i.e. single-wall, double-wall, triple-wall, quadruple-wall and multi-wall carbon nanotubes (CNTs), and fullerites were fluorinated in inductive coupled radio-frequency (RF) CF4 plasma at 13.56 MHz, and their structural and bonding properties were investigated by transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). We have discussed the correlation between the number of graphene sheets in a CNT and the stability against the fluorination. TEM and XPS analysis clearly state that increase of the number leads to the gain of fluorinated stability. The fluorination of CNTs is initiated at outer tubes and proceeds to inner tubes with increasing RF power, but fluorination depth is limited to only surface area. The fluorination of fullerites forms amorphous layer at the surface, and increases the depth of the layer with RF power.
Carbon nanotube (CNT) reinforced polymers are of high interest for various industries due to their unique mechanical and electrical properties. Most research has been done at room temperature (RT), but little is known about properties at cryogenic temperature. This paper presents results on CNT-polycarbonate (PC) composites with respect to mechanical properties at 77 K in comparison with RT. CNT-PC composites with 0wt% (neat), 0.1wt%, and 1.0wt% CNTs have been studied. Results imply that the CNT effects are more obvious at low temperature and are seen in the form of serrations in the stress-strain plot. No significant difference has been noticed between the neat and reinforced samples at either temperature. However, it was determined that the strength increases drastically while the elongation decreases at low temperature as compared to RT. SEM images confirm that the samples at low temperature exhibit brittle failure. Additionally, it can be seen that at low temperature the nanotubes align with the direction of tensile force while the nanotubes at RT align with the sample surface.
We present a high-current reliability study of carbon nanofibers (CNFs) for interconnect applications. In situ scanning transmission electron microscopy (STEM) reveals structural damage to CNFs after current stress. The effect of heat dissipation on the current capacity is also discussed by using different experimental configurations. Long-time reliability tests are performed with a vertical via interconnect structure, showing promising high reliability of CNF interconnects for future electronic devices.
The growth of ZnO nanowire arrays on a variety of substrates using a chemical wet process is presented. ZnO seeds can act as a nucleation layer for wire growth and ZnO nanowire arrays can have direct contact with a variety of substrates. The structural and optical properties of ZnO nanowire arrays grown at 90°C are investigated. FESEM and X-ray diffraction observations reveal that the crystalline ZnO nanowire arrays are preferentially oriented along the c axis. The room temperature photoluminescence (PL) measurements had shown ultraviolet peaks and a deep level peak. After annealing at 900°C for 1 min. by RTA, the deep level peak became much weaker and the band-edge emission PL was much stronger.
We present a method to realize nanoelectromechanical systems (NEMS) resonator structures from carbon nanotubes (CNT)/metal layered composite structures. The method utilizes a self aligning process over a near 2-D CNT network on GaAs to realize resonator structures and a highly selective etchant, a standard citric acid/hydrogen peroxide solution, to achieve release of NEMS resonators from the substrate. We find this method along with critical point drying (CPD) to yield robust hybrid CNT/metal resonator structures with fundamental resonant frequencies in the 10 MHz range. With high reflectivity, conductivity, and bio-compatibility of metals, this fabrication method along with possible benefits of CNT have many application possibilities.
In this research work, a model has been proposed in view of the recent experimental demonstration using Calcium (Ca) as a contact metal to realize the n-type carbon nanotube field effect transistors (CNTFET). In order to fully optimize this proposed device model, effects of different parameters like the work function, oxide thickness, the oxide capacitance and the source velocity limits were studied. Among all the parameters, the work function of the contact metal plays an important role for controlling the flow of carriers through the carbon nanotube channel and to reduce the threshold voltage. A semi-classical simulation of the proposed n-type CNTFET has been performed. Results show an excellent subthreshold swing value of 62.91 mV/decade, close to the International Technology Roadmap for Semiconductor (ITRS) specifications.
This work proposes the use of the composite carbon felt/carbon nanotube/Polyaniline as an alternative for applications as a pH sensor device. The carbon felt/carbon nanotube is an electronic conductivity material that was obtained from polymer felt (poliacrilonitrile felt) using oxidation and carbonization processes. The cup-stacked and bamboo-like tubes were grown on the fibers of carbon felt by chemical vapor decomposition method. The sensor was obtained by incorporating polyaniline (Pani) on the nanotubes present on the fibers of carbon felt/carbon nanotubes composite. The measuring process uses an EGFET (Extended Gate Field Effect Transistors) configuration, which is a derivation of the ISFET (Ion Sensitive Field Effect Transistor) - that is basically a chemical semiconductor sensor. The drain-current versus source-drain voltage is presented for varying pH concentrations from 2 up to 12.
PECVD-grown carbon nanotubes on (100) silicon membranes have been realized and exploited for electron and ion emission applications. The growth of CNT's is achieved by a mixture of hydrogen and acetylene gases in a CVD reactor and a 5-10nm thick nickel is used as the seed for the growth. The presence of a DC-plasma yields a vertical growth of carbon nanotubes. The as-grown nanotubes are encapsulated by means of an insulating TiO2 layer. The formation of a thin membrane is possible by means of a chemical anisotropic etching technique. The membrane is then removed from the back side to fully suspend the CNT-holding TiO2 layer. Upon exposure to a plasma ashing step the nanotubes are partially removed and a both-end opened hollow nanostructure is formed which can be used as a miniaturized ion source. The CNT-holding substrate can be exploited as a grid to extract the ions from an ionization chamber just underneath the membrane. Applying a proper accelerating electric field, positive ions made inside a DC discharge cavity can form a beam-shape emission of ions towards the opposite negative electrode. The beam is well suited for a source of ion lithography. In which, the emission has the ability of direct writing on a photo-resist coated substrates. Preliminary nano-scale dots have been created with sizes between 50 and 80nm. Scanning electron microscopy has been used to investigate the results.
In this invited presentation, we report (1) the assembly of polymer surfactants at the single-walled carbon nanotube (SWNT) and water interface, which not only results in the dispersion of SWNTs in aqueous environments but also provides opportunities for controlled assembly of ternary complexes and the introduction of external stimuli-responsive SWNT dispersions; (2) the first experimental measurement of dc polarization of individual carbon nanotubes using modified scanning force microscopy techniques. The transverse dielectric constant of carbon nanotube is about 10, and this method could be used to distinguish metallic from semiconducting nanotubes.
This paper describes the growth mechanism of silicon whisker on a silicon substrate using hot filament CVD reactor. Only hydrogen is used as source gas. The particle layer could be obtained at high filament current condition under hydrogen ambient. XPS analysis result suggests that the particle is composed of tungsten silicide. The deposition condition of the particle layer is much depended on the substrate size, surface condition and the distance between the substrate and the filament. The experimental results suggest that the silicon hydride, which generated at the silicon surface by hydrogen radical etching, react with the tungsten filament material around the filament, depositing on the silicon substrate. The silicon surface is etched by hydrogen radical and its resultant surface morphology is much depended on the particle deposition pattern. Many silicon whiskers, which diameter is varied from 10 to 50 nm, are observed on the textured silicon surface when the residence time of the source gas in the reactor is long. Each whisker has a silicon particle on their tip. The silicon hydride generated by the hydrogen radical etching is much absorbed to the silicide particle when the source gas residence time is long, enabling the silicon whisker growth from the particle. The results suggest that nm size whisker structure is much stable compare to the bulk silicon against etching reaction.
The use of carbon nanotubes for tissue engineering has become one of the most fascinating applications. The exquisite electronic and mechanical properties of carbon nanotubes may provide a three-dimensional (3D) microenvironment that closely mimics in vivo situation for facilitating the use of stem cells in the tissue regeneration. Therefore, it is important to know whether carbon nanotubes enhance the adhesion, proliferation, and differentiation of stem cells. Here, we hypothesized that the carbon nanotubes promote the differentiation of osteoblast progenitors into mature osteoblasts. To test this hypothesis, we quantified the differentiation of murine osteoblast progenitors, with and without pro-differentiating growth factor Bone Morphogenetic Protein-2 (BMP-2), cultured on the 3D scaffolds made by single-walled carbon nanotubes (SWNTs). Three types of SWNT samples, chitosan functionalized SWNTs, acid-oxidized SWNTs, and surfactant-free pristine SWNTs were used for the construction of these 3D microarchitectures. Osteoblast progenitors were harvested from calvariae from 3∼5-day-old mice, and cultured on the 3D scaffolds made by carbon nanotubes until ∼ 80% confluent. Then the cells were treated with BMP-2 (100 ng/ml) for 5 days. It was found that osteoblast progenitors cultured on the SWNTs dramatically increased the level of mature osteoblastic marker osteocalcin in either the absence or the presence of BMP-2, as compared to the cells cultured on the regulate tissue culture plastic plates. The results suggested that SWNTs highly promote osteoblast progenitor differentiation into mature osteoblasts. The finding indicates that SWNTs may provide an ideal scaffold for facilitating the differentiation of osteoblast progenitors in the repair of bone defects.
Conventional fuel cell architecture on one side of the membrane electrode assembly consists of a carbon backing layer, hydrophobic microporous layer (MPL), and a catalyst layer, which is in contact with the solid proton exchange membrane. Pt nanoparticles are deposited onto multi-walled carbon nanotubes (Pt/MWNTs) and a free-standing film of Pt/MWNTs is fabricated to act as the MPL and the catalyst layer in hydrogen fuel cells. The free-standing film of Pt/MWNTs condenses two functions into one bifunctional layer that simplifies the fuel cell fabrication procedure. Fuel cell polarization performance improves when using the free-standing film of Pt/MWNTs without the MPL resulting in a higher peak performance of 1.2 W/cm2 in comparison with 1.0 W/cm2 when in the presence of a MPL.
This article reports forward and reverse biased emission in vertical ZnO nanowire/p-GaN heterojunction light emitting diodes (LEDs) grown out of solution on Mg-doped p-GaN films. The electroluminescence spectra under forward and reverse bias are distinctly different. Forward bias showed two peaks centered around 390 nm and 585 nm, while reverse bias showed a single peak at 510 nm. Analysis of the current-voltage characteristics and electroluminescence spectra is presented to determine the transport mechanism and location of electron hole recombination. Reverse bias transport and luminescence are attributed to hot-hole injection from the ZnO nanowires into the GaN film through tunneling breakdown. Forward bias transport and luminescence are attributed to hole injection from the GaN into the ZnO and recombination at defect states inside the ZnO yielding distinct color variations between individual wires. Major resistive losses occurred in the GaN lateral thin film connecting to the vertical ZnO nanowires.
While nanowires and nanotubes have been shown to be electrically sensitive to various chemicals, not enough is known about the underlying mechanisms to control or tailor this sensitivity. By limiting the chemically sensitive region of a nanostructure to a single binding site, single molecule precision can be obtained in order to study the chemoresistive response. We have developed techniques using single-walled-carbon-nanotube (SWCNT) circuits that enable single-site experimentation and illuminate the dynamics of chemical interactions. Discrete changes in the circuit conductance reveal chemical processes happening in real-time and allow SWCNT sidewalls to be deterministically broken, reformed, and conjugated to target species.
Bismuth telluride (Bi2Te3) and its alloys have long been held as the best bulk commercial thermoelectric (TE) materials. In recent years, significant enhancement of the TE figure of merit (ZT) of these traditional TE materials has been predicted through reduction of dimensions (i.e., nanostructures and nanoengineering). We are particularly interested investigating electrolyte composition variations to control the composition of nanowires to enable large ZT enhancement. We report here the constant current electrochemical deposition of BixTey nanowires of diameters of 35, 55, 73 and 200 nm and lengths up to 50 microns. We are able to obtain controlled, uniform growth of high quality n-type bismuth telluride nanowires. A design of experimental matrix investigating the effects of current density and solution pH values on the overall growth rate and nanowire crystalline quality has been performed. The effects of growth conditions on materials and structural characteristics of BixTey nanowires have been studied by SEM, High Resolution TEM, EDX, concentric beam electron diffraction patterns, and ICP. The TE properties of individual BixTey nanowires are currently being evaluated using micro/nano fabricated devices and UHV Scanning Thermoelectric Microscopy.
Copper sulfide is a material with immense potential for applications in photovoltaics. Particularly, copper sulfide 1-D nanostructures (i.e. nanowires, nanorods) with well-defined morphologies may enable new types of applications or may enhance the performance of existing photoelectric devices with quantum confinement effects. In this work, we report the synthesis of copper sulfide nanorods by simple, yet very effective template assisted electrochemical deposition. Before synthesizing 1-D copper sulfide nanorods, a detailed study was conducted on electrodeposited 2-D copper sulfide films to ascertain the right parameters for electrodeposition including; electrolyte composition, temperature, deposition potential and membrane type. Excellent structural properties of these resultant nanorods make them desirable for applications in the future nano-opto-electronic devices.
In this paper, we have reported the fabrication and characterization of nanowire electromechanical switches consisting of chemical-vapor-deposition grown silicon nanowires suspended over metal electrodes. The devices operate as transistors with the suspended part of the nanowire bent to touch metal electrode via electromechanical force by applying voltage. The reversible switching, large on/off current ratio, small subthreshold slope and low switching energy compared to current CMOSFET make the switches very attractive for logic device application. In addition, we have developed a physical model to investigate the switching characteristics and extract the material properties.
Carbonaceous purity assessment methods are being sought after for all types of carbon nanotubes as a means to standardize the material metrology. Our most recent work has evaluated chemical vapor synthesized multi-walled carbon nanotubes (MWNTs). This effort included a protocol for assessment involving qualitative information from scanning electron microscopy (SEM) images and quantitative information from thermogravimetric analysis (TGA) and Raman spectroscopy. Presently, the analysis using Raman spectroscopy on a constructed sample set has been extended to a second excitation energy (HeNe laser at 1.96 eV) and the similar trends in the relative Raman peak ratios have been measured. In contrast to the G-band, the D and G' peaks demonstrate a Raman shift that is excitation energy-dependent, consistent with the double resonance theory. However, the Raman ratio of IG'/ID is independent of excitation energy and is observed to be the most sensitive to MWNT carbonaceous purity. Application of this approach to MWNT arrays grown on SiO2 is compared to conventional bulk powders synthesized under similar conditions. The MWNT arrays show a high degree of vertical alignment based upon SEM and a measured carbonaceous purity using the IG'/ID ratio of 75% w/w.
Intra-connects (bridges spanning across in plane electrodes), which were made of carbon nanotube (CNT), were fabricated by CO Plasma Enhanced Chemical Vapor Deposition (PECVD), ethanol CVD and pyrolitic CO CVD. CO PECVD has been used with CO/H2 mixture at relatively low temperatures. Its yield was relatively low though and the quality of CNT intra-connect was not to par. Ethanol CVD resulted in many more multi-wall carbon nanotube (MWCNT) than single-wall carbon nanotube (SWCNT) intra-connects. CO CVD was the most effective and simplest way to grow CNT interconnects among the three methods, yielding well-aligned and straight SWCNT bridges.
We report on a method for direct measurement of site density of vertically-aligned carbon nanotubes (CNTs). Site density is an important parameter of vertically-aligned carbon nanotube forests for various applications. By freezing the CNT forests in a polymer matrix and exposing the CNT ends, we obtained the site density of vertically aligned multi-walled CNTs through SEM observation and particle counting. Site densities of multi-walled CNTs grown by two different CVD processes, ferrocene/xylene process and Fe-Al/ethylene process, were measured to be ∼10 tubes/Ým2 and ∼53 tubes/Ým2, respectively. The results of site density distributions indicate non-uniform growth of carbon nanotubes at the micrometer scale in both processes.