Bio-nano complexes: DNA/surfactant/single-walled carbon nanotube interactions in electric field

Bio-nano complexes of calf-thymus DNA with a cationic surfactant (DNA/DTAB) and with surfactant wrapped single-walled carbon nanotubes (DNA/SD) were prepared and investigated. The focus of this study was on the molecular interactions, dynamics and binding characteristics as evaluated by a battery of experimental techniques. The major binding sites on DNA and their binding affinities toward the surfactant (DTAB) are identified by Fourier Transform InfraRed (FTIR) spectroscopy. DTAB molecules display higher binding affinity toward DNA when hydrophobically attached to the single-walled carbon nanotube (SWNT) surface. Dielectric spectroscopy of the neat DNA and the two complexes revealed the presence of a process assigned to the motions of counterions distributed along the DNA backbone. The time scale of this process is a function of the composition of the complex and is shortest at a particular DNA/DTAB or DNA/SD ratio. The effect of SWNT on the zeta potential and the hydrodynamic diameter is manifested by the speed-up of charge reversal and a large complex size at the iso-electric point (IEP). The secondary structure of DNA is altered by the presence of SWNTs; however, there is no evidence of the transition of DNA in either complex from type B to other forms.     

Resist-assisted assembly of single-walled carbon nanotube devices with nanoscale precision

We report a novel resist-assisted dielectrophoresis method for single-walled carbon nanotube (SWCNT) assembly. It provides nanoscale control of the location, density, orientation and shape of individual SWCNTs. Sub-50 nm accuracy and a yield higher than 85% have been achieved. Using the method, we demonstrate suspended-body SWCNT field-effect transistors (FETs) with back-gate and sub-100 nm air-gap lateral-gate configurations. The suspended-body SWCNT FETs show excellent electrical characteristics with I_on/I_off ～ 10⁷, ultra-low off currents ～10^?14 A and small subthreshold swings. The technique contributes to the ultimate solution for bottom-up fabrication of a broad field of CNT-based devices, such as: complementary metal–oxide-semiconductor and nano-electrical–mechanical-system devices for sensing and radio-frequency applications. Moreover, the versatile method could be applied to the assembly of many other promising materials, such as: nanowires and graphene flakes.     

Enhanced field emission properties of a reduced graphene oxide/carbon nanotube hybrid film

A facile vacuum filtration method for the preparation of hybrid films to achieve superior field emission properties from carbon nanotubes (CNTs) using reduced graphene oxide (rGO) as a bi-functional filler has been proposed. In the hybrid films, CNTs serve as electron emitters, while rGO helps to control the density of the CNT-emitters and reduce electrical resistance of the films. Via controlling volumes of CNTs and rGO dispersions, electron field emission properties of the hybrid films can be easily tailored. Higher weight ratio of rGO:CNT results in better electrical properties and the best field emission property is achieved when a rGO:CNT weight ratio of 1:3 is employed. The hybrid film reveals a significant improvement in field emission properties, as compared with the CNT film without adding rGO. Decreases in sheet resistance, turn-on field, and threshold field are attributed to the formation of extended conjugated network between CNTs and rGO in association with the reduction of screening effect through the optimization of density of CNT-emitters. The concept that rGO can be employed to control the density of CNT emitters will be of special interest for field emission enhancement.     

Horizontally aligned single-walled carbon nanotube field emitters fabricated on vertically aligned multi-walled carbon nanotube electrode arrays

Vertically aligned multi-walled carbon nanotube (MWCNT) arrays were fabricated on an anodic aluminum oxide membrane bonded to a Si wafer. After obtaining a protruding tip for the MWCNTs by etching away some oxide, they were used as electrodes in the fabrication of carbon nanotube field emitters. Long single-walled carbon nanotubes (SWCNTs) were spin coated on the MWCNT arrays of uniform height. Clean SWCNTs were suspended by attaching them to the tips of the vertically aligned MWCNT arrays. The spin coated SWCNTs function as emitters, while the MWCNT arrays function as electrodes. The field emission was greatly improved by coating gold on the MWCNT arrays and annealing at 400 °C. Our field emitter exhibits good field emission properties such as a low turn-on field (1.4 V/μm), high current density (122 mA/cm²), and good stability (55 h for 10% degradation of current density from 400 μA/cm²).     

Enhanced dispersion of carbon nanotube in silicone rubber assisted by graphene

We report a novel, scalable and inexpensive approach to fully disperse carbon nanotubes in silicone rubber by the addition of graphene. In comparison to graphene, the dispersion of multi-walled carbon nanotubes (MWNTs) in silicone rubber matrix is extremely difficult although both of them possess similar physical structure. The different dispersion behavior of graphene and MWNTs could be contributed to the difference in their interaction with polymer matrix and their geometry. Based on SEM, TEM and XRD analysis, we find that the dispersion of MWNTs in silicone rubber is dramatically improved by the addition of graphene. Graphene acts as a compatilizer since it shows strong interaction with both polymer matrix and MWNTs. This method provides a simple route to enhance the dispersion of carbon nanotubes and improve the electrical property of the polymer composites. The synergic effect of the hybrid materials may not to be limited to the applications in polymer composites.     

Influence of electric field and emission current on the configuration of nanotubes in carbon nanotube layers

The influence of applied electric field (E_av) and emission current (I_FE) on the configuration of conical layers carbon nanotubes (CLNTs) grown by CVD on the edge of Ni foil has been investigated. TEM profile imaging revealed a high concentration of nanotubes near the foil edge surface, whereas on the nanotube layers’ outer surfaces single, non-oriented nanotubes with open ends free of catalytic particles, were observed. After sufficient electric field application many nanotubes became oriented towards the anode, but one or two of them were found to be always a few microns more extended. In situ SEM investigation showed that below E_av = 3.2–3.9 V/μm, emission was achieved at the expense of originally existing free nanotube ends. Configuration changes began at larger electric fields. On the observed foil edge length (14.6–17.8 μm, with an edge thickness of 200 μm) one or two nanotubes extended towards the anode and probably became the main emitters. Upon further increasing the field to E_av = 5.7–8 V/μm and at an emission current I_FE = 2 × 10⁻⁵ A these tubes disappeared (or essentially shortened). At E_av = 8 V/μm and higher and at an exposure time up to 40 min, several tens of extended nanotubes appeared, with one or two extended well beyond the others. This nanotube configuration pattern is connected with electrostatic screening between the nanotubes. Our interpretation of the data suggests that in the investigated range of E_av and I_FE, a limited number of nanotubes are emitting and these nanotubes are constantly changing as E_av, I_FE and exposure time increase.     

High-performance field emission from a carbon nanotube carpet

We have created a field emitter composed of a carbon nanotube (CNT) yarn, which was prepared by direct spinning through chemical vapor deposition and then formed into a carpet structure by tying the yarn to a conductive substrate before cutting it. The structure of the carpet is arranged to induce the tips of the CNT yarn to protrude toward the anode for maximum electron emission. The turn-on field, threshold field, and field enhancement factor of the device are 0.33, 0.48 V/μm, and 19,141, respectively. Extremely low operating electric fields and a high field enhancement factor result from the high density of CNT emitters with high crystallinity, the electrically good contact between the emitters and the substrate, and the effects of the multistage structure. The emission is stable even at a high current density of 2.13 mA/cm², attributed to the strong adhesion between the emitters and the substrate. The emission performance is found to be customizable by adjusting the structure, for example, the CNT pile density. These results are relevant for practical applications, such as large-area flat-panel displays, large-area low-voltage lamps, and X-ray sources.     

Ring-shaped field emission patterns from carbon nanotube films

Highly symmetric ring-shaped field emission patterns were observed from broad-area flat cathodes prepared by growing a film of vertically aligned carbon nanotubes (CNTs) on TiN coated Si substrates. The images were obtained utilizing a luminescent screen of a specially designed triode cell composed of parallel electrodes. The emission rings sporadically appeared during voltage scans in which the emission patterns and cathode currents were recorded. The fine structure and stability of the rings suggests that their formation is due to an emission state of an individual CNT. The observed patterns are consistent with models that predict the formation of emission rings produced by the inhomogeneous electron emission from CNTs. The macroscopic value of the electric field when the rings were observed was between 0.7 and 2.5 V/μm, and the emission current corresponding to individual rings was estimated to be in the range of 2–4 μA. Numerical simulation of electron trajectories for sidewall emission from similar shaped metallic structures is in qualitative and quantitative agreement with the experimentally observed ring-shaped field emission patterns. The results also appear consistent with a recent model [Marchand M, Journet C, Adessi C, Purcell ST. Phys Rev B 2009;80:245425] based on thermal-field emission due to Joule heating.     

Micropatterning of single-walled carbon nanotube forest

In this work, high-aligned single-walled carbon nanotube (SWCNT) forest have been grown using a high-density plasma chemical vapor deposition technique (at room temperature) and patterned into micro-structures by photolithographic techniques, that are commonly used for silicon integrated circuit fabrication. The SWCNTs were obtained using pure methane plasma and iron as precursor material (seed). For the growth carbon SWCNT forest the process pressure was 15 mTorr, the RF power was 250 W and the total time of the deposition process was 3 h. The micropatterning processes of the SWCNT forest included conventional photolithography and magnetron sputtering for growing an iron layer (precursor material). In this situation, the iron layer is patterned and high-aligned SWCNTs are grown in the where iron is present, and DLC is formed in the regions where the iron precursor is not present. The results can be proven by Scanning Electronic Microscopy and Raman Spectroscopy. Thus, it is possible to fabricate SWCNT forest-based electronic and optoelectronic devices.     

Focused-ion-beam assisted fabrication of individual multiwall carbon nanotube field emitter

We report the fabrication of an individual carbon nanotube (CNT) electron field emitter using a focused-ion-beam (FIB) technique. The monolithic multiwall CNT with a graphitic shield is synthesized using chemical vapor deposition technique. The FIB technique is applied to attach the monolithic multiwall CNT on an etched tungsten tip. Field emission measurements are carried out in a vacuum of 10⁻⁷ Torr. Threshold voltage as low as 120 V has been obtained.     

Electron field emission from transparent multiwalled carbon nanotube sheets for inverted field emission displays

Strong, conducting, transparent carbon nanotube sheets were prepared by solid-state draw from well-ordered, aligned multiwalled carbon nanotube (MWCNT) forests [Zhang et al., 2005] [1]. Study of electron field emission from such transparent MWCNT sheets shows threshold fields of less than 0.5 V/μm with current densities high enough for display applications. Step-like field emission current increase and hysteresis behavior in I-V curves has been observed. The origin of such behavior is discussed in terms of mechanical rearrangement of the nanotube network in high electric field. Studied MWCNT transparent sheet field emission cathodes have several advantages when used as multi-functional electrodes. They are high current, high stability, transparent, and flexible field emission sources and can be used in an inverted geometry, with cathode being in front of the light emitting plate. At the same time transparent CNT sheets may serve as a transparent conducting electrode for electrical connection and pixel addressing in field emission displays (FEDs). Also, these sheets can be used as an optical polarizer in FEDs.     

Controlled growth of carbon nanotube films for high-current field emission

Carbon nanotubes (CNTs) offer great potential for numerous cold-cathode field emission applications. A less studied need is for high-current cathodes. While work to date has focused on the use of tangled webs of single-wall CNTs, much understanding about field emission has occurred from studies using multi-wall CNTs with controlled geometries. However, the crystalline nature of these multi-wall CNTs typically is far inferior to that of single-wall CNTs. We use high-resolution transmission electron microscopy to demonstrate that growth at temperatures ≤ 630 °C via thermal chemical vapor deposition can produce highly crystalline multi-wall CNTs, with structures consisting entirely of concentric graphene cylinders. Conversely, growth at temperatures ≥ 650 °C results in crystalline CNTs embedded in a nanocrystalline graphite, or glassy carbon, sheath. This sheath material is likely a poor electrical conductor, due to phonon scattering, and will have deleterious effects on field emission. Field emission measurements taken from such films are consistent with the best field-emitting multi-wall CNT films in the literature, in terms of total current for a given applied field, but without the benefit of the preferred perpendicular orientation. These results are promising toward the development of reliable high-current field emission cathodes.     

Triton assisted fabrication of uniform semiconducting single-walled carbon nanotube networks for highly sensitive gas sensors

Surfactants are vital for the dispersion and separation of single-walled carbon nanotubes (SWCNTs) when using chromatographic methods. However, excessive surfactant impurities left on the sorted tube surface and in solution may dramatically affect the fabrication and performance of various SWCNT-based thin-film devices. We demonstrated that compared to the widely used anionic surfactants, including sodium dodecyl sulfate, sodium dodecyl benzene sulfonate and sodium deoxycholate, the nonionic Triton surfactant was more effective not only for the enrichment of semiconducting (s-) SWCNTs, but also for preventing the tubes from severe aggregation upon network formation. Simply by spreading and rinsing treatment, a clean and homogenous network of s-SWCNTs was favorably formed between electrode channels, in which intertube contacts or junctions were well built up. The resultant sensor device showed an obviously improved electrical response to H₂S with the detection level down to 50 ppb even at room temperature, compared to those fabricated from the pristine SWCNTs and s-SWCNTs eluted with anionic surfactants. Our work provides an easy route for the sorting and application of s-SWCNTs at a low cost.     

Enhanced field emission properties from titanium-coated carbon nanotubes

The effect of titanium (Ti) coating over the surface of carbon nanotubes (CNTs) on field emission characteristics was investigated. Vertically aligned CNTs were grown by inductively-coupled plasma-enhanced chemical vapor deposition (ICP-CVD). In order to reduce the screening effect of electric field due to densely packed CNTs, as-grown CNTs were partly etched back by DC plasma of N₂. Ti with various thicknesses from 5 nm to 150 nm was coated on CNTs by a sputtering method. Since thick Ti coating with thickness of 100 nm or more resulted in the shape of a metal post by merging an individual CNT in a bundle, it was inadequate to a field emission application. On the other hand, thin Ti-coated CNTs with thickness of 10 nm or less showed a lower turn-on field, a higher emission current density, and improved emission uniformity compared with pristine CNTs. The improved emission performance was mainly attributed to the low work function of Ti and the reliable and lower resistance contact between CNTs and substrates.     

Simple strategy to tune the charge transport properties of conjugated polymer/carbon nanotube composites using an electric field assisted deposition technique

Work concerning the incorporation of carbon nanotubes (CNTs) in organic semiconducting polymers have now been reported by many research groups, and the electrical properties of polymer/CNT nanocomposites have been extensively studied. In this work, we present a simple procedure to tune the charge transport properties of planar organic polymer films based on poly(3‐hexylthiophene) (P3HT). The polymer/CNT composites are simultaneously processed and oriented from solution using an electric field assisted orientation technique. We first study the behavior of CNTs alone during the alignment procedure and emphasize the main experimental parameters that drive their final orientation on the substrate. By quantitatively analyzing the CNT angular distribution on the substrate, we show that the dielectric constant of the solvent used to disperse and deposit the CNTs is crucial to ensure an efficient orientation, and that a dielectrophoresis‐like orientation procedure occurs. The transposition of this approach to planar P3HT/CNT composites is made by investigating the electric properties in ambient conditions of aligned and non‐aligned devices. Current–voltage characteristics show a drastic increase of the composite conductivity upon addition and alignment of CNTs. Field‐effect transistor charge mobilities are improved by an order of magnitude upon addition of CNT (1 wt%) in P3HT, and another decade is gained using the optimized alignment parameters, without any additional annealing. These results demonstrate the strong potentialities of our approach in the field of printed electronics and organic optoelectronics. © 2013 Society of Chemical Industry     

A monolithic electron beam amplified carbon nanotube field emission cell

This article reports the fabrication and characterization of a CNT field emission cell with a built-in electron beam source for electron excited amplified field emission. A monolithic lateral field emission cell (FEC) with integrated metallic anode was fabricated. Then the field emission behaviors with and without activation of the built-in electron beam were characterized in diode configuration. A high voltage of 1.8 kV was applied to generate the bombarding electron beam on the FEC. The emission current of the FEC increases markedly with the activation of the electron beam source due to impact ionization and direct interaction with the FEC CNT cathode. The emission behaviors were confirmed by F–N plots. It was found that almost 10 times current amplification was achieved. These results demonstrate the feasibility of an electron beam amplified field emission using carbon nanotube emitters.     

The importance of an extensive elemental analysis of single-walled carbon nanotube soot

Few manufacturers provide elemental analysis information on the certificates of analysis of their single-walled carbon nanotube (SWCNT) soot products, and those who do primarily perform surface sensitive analyses that may not accurately represent the bulk properties of heterogeneous soot samples. Since the accurate elemental analysis of SWCNT soot is a requisite for exacting assessments of product quality and environmental health and safety (EH&S) risk, the purpose of this work was to develop a routine laboratory procedure for an extensive elemental analysis of SWCNT soot using bulk methods of analyses. Herein, a combination of carbon, hydrogen, nitrogen, sulfur, and oxygen (CHNS/O) combustion analyses, oxygen flask combustion/anion chromatography (OFC/AC), graphite furnace-atomic absorption spectroscopy (GF-AAS), and inductively coupled plasma-mass spectroscopy (ICP-MS) were used to generate a 77-element analysis of two as-received CoMoCAT® SWCNT soot products. Fourteen elements were detected in one product, nineteen in the other, and each data set was compared to its respective certificate of analysis. The addition of the OFC/AC results improved the accuracy of elements detected by GF-AAS and ICP-MS, and an assessment was performed on the results that concluded that the trace elemental impurities should not pose an EH&S concern if these soot products became airborne.     

Oxygen plasma resurrection of triode type carbon nanotube field emission cathodes

The field electron emission of carbon nanotubes has been heavily studied over the past two decades for various applications, such as in display technologies, microwave amplifiers, and spacecraft propulsion. However, a commercializable lightweight and internally gated electron source has yet to be realized. Electrical shorting of the gate to the substrate is a common and problematic failure mode for Spindt type carbon nanotube electron sources, severely limiting their manufacturability. This work explores the novel use of an oxygen plasma etch to reverse this shorting. Plasma treatments on CNTs are commonly used to improve FE performance, but no work presents the use of a plasma etch to reverse shorting. The oxygen plasma etch is shown to be a simple and highly effective method to reverse shorting and increase yield of open circuit Spindt type CNT electron sources by over 70%.     

Surfactant-free assembling of functionalized single-walled carbon nanotube buckypapers

The electrical and textural properties of single-walled carbon nanotube buckypapers were tunned through chemical functionalization processes. Single-walled carbon nanotubes (SWCNTs) were covalently functionalized with three different chemical groups: Carboxylic acids (-COOH), benzylamine (-Ph-CH₂-NH₂), and perfluorooctylaniline (-Ph-(CF₂)₇-CF₃). Functionalized SWCNTs were dispersed in water or dimethylformamide (DMF) by sonication treatments without the addition of surfactants or polymers. Carbon nanotube sheets (buckypapers) were prepared by vacuum filtration of the functionalized SWCNT dispersions. The electrical conductivity, textural properties, and processability of the functionalized buckypapers were studied in terms of SWCNT purity, functionalization, and assembling conditions. Carboxylated buckypapers demonstrated very low specific surface areas (< 1 m²/g) and roughness factor (R_a = 14 nm), while aminated and fluorinated buckypapers exhibited roughness factors of around 70 nm and specific surface areas of 160-180 m²/g. Electrical conductivity for carboxylated buckypapers was higher than for as-grown SWCNTs, but for aminated and fluorinated SWCNTs it was lower than for as-grown SWCNTs. This could be interpreted as a chemical inhibition of metallic SWCNTs due to the specificity of the diazonium salts reaction used to prepare the aminated and fluorinated SWCNTs. The utilization of high purity as-grown SWCNTs positively influenced the mechanical characteristics and the electrical conductivity of functionalized buckypapers.