This paper describes improved methods for both data acquisition and analysis that are pertinent to interpreting the structural composition of single-walled carbon nanotube (SWNT) mixtures and suspensions.

Rapid data acquisition of photoluminescence is made possible with a specially configured spectrofluorometer which uses a liquid nitrogen-cooled InGaAs array detector, imaging spectrograph, excitation reference photodiode and tunable xenon excitation source to collect seamless, instrument-corrected, excitation-emission quantum maps. The preferred instrument configuration with the InGaAs array can generate a quantum map with both high S/N levels and spectral resolution in only seconds to minutes; previous single-channel InGaAs photodiode and photomultiplier detector measurements took hours to days. The standard spectral range for excitation and emission is from 250 nm to 1700 nm with an option to extend to 2200 nm. Robust analysis of the quantum maps is facilitated by a custom global analysis program (US Patent Pending) which incorporates a powerful ‘double-convolution integral’ DCI algorithm to simultaneously model the excitation and emission spectral bands for each SWNT species. The patented DCI algorithm reduces the number of free fitting parameters for the spectral simulation by up to a factor of 1000 compared to conventional 2 D spectral simulators. The global analysis yields quantitative information on the chirality and diameter parameters of SWNT species in a given mixture. Together the acquisition hardware and analysis software constitute significant developments with respect to enhanced sensitivity and statistical significance for quantifying the components of complex SWNT mixtures.

Raman maps of SWNT's were collected from dispersions plated onto substrates such as silicon plates using multiple excitation wavelengths. Identification of the chirality of the tubes was made using the Kataura plot. In many instances single lines in the radial breathing mode region were recorded indicating the presence of a single tube. The implication for this observation is that there is more than adequate sensitivity to detect and identify single-walled tubes. More recently Raman excitation on an AFM using a gold-coated tip to induce surface enhancement has been achieved. In this configuration it will be possible to map the topology of the tubes using detailed information on their structural composition.

Materials science is playing a dramatic role in discovering new materials with tailored physical properties. Cold cathodes/field emitters are one of the examples. Electron field emitting materials are of vital importance for a variety of vacuum microelectronic devices including field emission displays for flat panel displays, electron microscopes, X-ray generators, and vacuum lamps. This is the driving force to investigate the advanced nanostructured carbons as cold cathodes as one of the potential candidates. Recently, they are also being proposed for thermionic power generators. The rationale is that reducing one or more dimensions of a system below some critical length changes the systems' physical properties, where carbon nanotubes (CNTs) in the class of carbon nanostructures serve as a model example. In this paper, synthesis and characterization of vertically aligned multiwall and single-/double-wall carbon nanotube films using a microwave plasma-assisted chemical vapor deposition technique for vacuum microelectronics is presented. Recent advances in their synthesis, processing, and characterization indicate that the above mentioned potential is slowly being realized. Experiments showed that by continuous reduction in the thickness of the catalyst film produces hollow concentric tubes in contrast to bamboo-like multiwalled tubes with larger thickness. To assess the electron field emission properties, besides the traditional field emission (I-V) properties, temperature dependent field electron emission microscopy (T-FEEM) enabling real-time imaging of electron emission providing information on emission site density, temporal variation of the emission intensity, and insight into the role of adsorbates from nanotube films will be discussed. Physics based models (such as negative or low electron affinity, geometric enhancement, surface dipole, tunneling due to adsorbates, structure modification due to doping etc.) will be described to support the experimental observations in addition to weak thermionic field emission contribution. These findings provided a great insight into the field emission mechanism and a contrasting comparison between small and large diameter carbon nanotubes.

The response of vertically oriented multi-walled nanotubes (MWNTs) to fluid flows forms a basis for monitoring shear forces at the nanoscale. We report on the modulation of polarized light transmission through a MWNT mat, as a function of the nanotube axis relative to the laser polarization. Nanotubes are deflected under fluid flow, with fluid pressures corresponding to pico-Newton range forces. This deflection is measured as a function of transmitted laser light intensity. While the response of the CNTs to the flows could be modeled through standard elastic beam deflection theory, their recovery, after the force removal, invokes viscoelastic behavior due to CNT disentanglement. The fluid flow direction, as well as its pressure, can also be determined by monitoring the change in transmitted laser polarization.

Novel Mn-incorporated ZnSe and CdSe 1-dimensionl nanostructures; straight nanowires, zigzagged nanobelts, and nanohooks, were first synthesized using chemical vapor deposition method. The Mn content reaches up to 40%. They all consisted of single-crystalline wurtzite structure for all Mn content. The structure has been thoroughly investigated by high-resolution transmission electron microscopy images as well as energy-dispersive X-ray fluorescence spectroscopy. The X-ray diffraction pattern confirms the formation of the wurtzite structure, even for 40% Mn incorporation. The lattice constants of Mn-doped ZnSe and CdSe 1-D nanostructures are expanded and reduced, respectively, by the Mn doping. The Mn2+ emission at 2.1 eV, originating from the d-d (4T1 → 6A1) transition, confirms the effective paramagnetic Mn2+ doping at tetrahedral coordinate sites. These Mn-incorporated nanostructures exhibit a paramagnetic behavior.

Nanostructures are considered the critical component in a wide range of potential nanoscale device applications. Yet a procedure to fabricate them with both controllable results and in bulk quantities must be developed in order to achieve their commercialization at reduced cost. In this report, we introduce an improved vapor-liquid-solid method that is capable of preparing high yield, high quality CdS nanowires and nanobelts in a turf-like configuration. To increase yield, we placed gold-coated substrates in a ceramic boat partially covered with a glass slide to form a gas trap. Only a small opening was provided to allow the CdS vapor to escape from the trap. This arrangement increases catalyst exposure to CdS vapor flow in comparison to conventional CVD methods. This allowed the CdS vapor to deposit densely over the substrate at a predetermined temperature range of 501°C-630°C inside the quartz tube. These conditions results in synthesis of various morphologies on both quartz and tungsten substrates including an intertwined-like structure not previously reported. Electron microscopy and microanalysis techniques were utilized in characterizing these morphologies, internal structures and elemental compositions. Electron field emission properties were investigated in an ultra high vacuum chamber set up with a base pressure of ∼1E-9 torr.

Thin film transistor (TFT) with low (1%wt) Ga-doped ZnO (GZO) as an active channel on SiO2/Si substrate has been fabricated at room temperature by rf-magnetron sputtering. The devices show a mobility of 5.7 cm2/V.s at low operation voltage (<5V), a low turn-on voltage of 0.5 V and sub-threshold swing of 85 mV/decade. The TFT device performance is significantly affected by vacuum-level and annealing temperature, which can be attributed to the removal of chemisorped oxygen in the active channel surface. Low doped GZO is a new class of high performance TFT channel material that is easy to process.

A nanocomposite carbon composed with single wall carbon nanotube (SWNT) and nanoporous carbon (NPC) was prepared by grafting a carbonizable polymer, poly(furfuryl alcohol) (PFA) to a SWNT. The SWNT was first functionalized with arylsulfonic acid groups on sidewall (SA-SWNT) and then converted to PFA-functionalized SWNT (PFA-SWNT) by in situ polymerization of furfuryl alcohol (FA). SWNT/NPC nanocomposite carbon was generated by heating PFA-SWNT in argon at 600°C. A continuous phase was formed between SWNT and NPC. The deformation of the nanocomposite carbon at high temperature was studied by heating it at temperatures from 1200 to 2000 °C and characterized with HRTEM and Raman spectra. It was found that NPC tended to graphitize along the axis of neighboring nanotubes at temperature higher than 1400°C. Complete graphitization of NPC and SWNTs was obtained at 2000 °C, when the NPC transformed to graphitic nanoribbon (GNR) and SWNT or DWNT collapsed within the confines of the GNR. Nanocomposite polymer and carbon fibers were prepared by dispersing small amount of SA-SWNT in FA, followed with polymerization, thermosetting and pyrolysis. The composite polymer fibers' Young's modulus was lower than the pure PFA fibers prepared at the same conditions. However, after heated in argon at 300 °C, 400 °C, 500 °C and 600 °C respectively, the Young's modulus of the nanocomposite carbon fibers turned to be higher than the fibers derived from pure PFA and the enhancement by SWNT increase with increasing temperature. For the nanocomposite carbon fibers treated at 600 °C, the fibers had ∼20% increase of Young's modulus over the fibers from pure PFA with only 0.1wt% of SA-SWNT in FA.

An oxidation method has been applied to functionalize multiwalled carbon nanotubes with carboxylic acid (-COOH) group. Functionalized carbon nanotubes (f-MWNT) were used for the fabrication of conducting nanocomposite fibers by electrospinning, in comparison with the composite nanofibers made of un-functionalized carbon nanotubes (u-MWNT). Our results showed that the addition of f-MWNTs into polymer solution could increase the compatibility of MWNTs with the polymer matrix, and thus result in composite nanofibers with uniform diameters. Alignment of the composite nanofibers was achieved by using a rotating drum as the collector. F-MWNTs were found to align parallel to the axis direction of the nanofibers. Temperature-dependent DC electrical properties of a single composite fiber were investigated by a two-probe method. It was shown that the conductivity of the material could be significantly improved above a percolation threshold. The conductivity could be of ten orders of magnitude higher than the pure PVAc.

We report on the electrochemical properties of ultra long aligned multiwalled carbon nanotube (MWNT) bundles synthesized using water-assisted chemical vapor deposition process. Cyclic voltammogram with diffusion-controlled-reversible reaction obtained at MWNT electrodes in 10 mM K3(Fe(CN)6) /0.1 M KCl solution with varying scan rates indicates that radial diffusion mass transport is dominant at these electrodes. We further show that these electrodes can detect very low concentrations of ascorbic acid (AA) and dopamine (DA) (0.7 μM for AA and 1.87 μM for DA ). The excellent electrochemical properties along with nice performance for single species detection suggest that these MWNTs are promising electrode materials for developing high sensitive chemical and/or biological sensors.

We demonstrate patterning of SWNT films down to 200 nm lateral dimensions using e-beam lithography and reactive ion etching with good selectivity and directionality and then we fabricate standard four-point-probe structures to characterize the resistivity of these films as a function of device geometry. The resistivity is found to be independent of device length for a given width and thickness, while increasing over three orders of magnitude compared to bulk films, as the width and the thickness of the films shrink. In particular, the resistivity of SWNT films starts to increase with decreasing device width below 20 μm, exhibiting an inverse power law dependence on device width at sub-micron dimensions. We explain this behavior by a purely geometrical argument. Although the “top-down” patterning of nanotube films allows for their use in sub-micron device structures and perhaps their integration into standard silicon fabrication technology, the resistivity scaling is an important effect that needs to be taken into account when fabricating small devices in which nanotube film transport characteristics play a vital role.

The incorporation of chemically modified multi-walled carbon nanotubes (MWNTs) have demonstrated, as an additive material to enhance the electrical conductivity, into the electrode gives rise to a highly improved rate capability of a super-capacitor. The preferential attack of potassium hydroxide on less disordered carbon results in the formation of high-performance MWNTs with increased surface area, electrical conductivity and sustained long tube morphology, simultaneously. It is envisaged that newly developed functional MWNTs will be utilized in various electrochemical systems, where high current is critically required.

Carbon nanotubes (CNTs) are of great interest because of several unsurpassable physical (mechanical, electrical, thermal, and chemical) properties. Especially their large elastic modulus and breaking strength make them highly attractive for their use as reinforced agents in forming a new class of multifunctional advanced materials - onanocomposites, in addition to high conductivity (either in semiconducting or metallic regimes) achieved through lower percolation thresholds for several electronic applications. Among the known conducting polymers, polyaniline (PANI) has a high potential due to its ease of synthesis, excellent environmental, and thermal stability and reversible control of its electrical/electronic properties. In this work, PANI-single-/multiwalled NTs composites films containing different nanotube content of both kinds were synthesized by spin-cast preceded by ultrasonic mixing of the constituents. They were characterized using complementary techniques including scanning electron microscopy, X-ray diffraction, infrared and Raman spectroscopy, and conductivity revealing their microscopic structure and physical properties thus helping in establishing process-structure-property correlations. The present work will discuss some of these findings in terms of a) self-alignment of nanotubes in conducting polymer b) their optical and electrical properties, and c) their design with a view to electronic and sensor applications, all ascribed due to long range π-π interaction between the constituents.

A helical nanowire can be defined as being a nanoscopic rod whose axis follows a helical curve in space. In the case of a nanowire with asymmetric cross section, the helical nanostructure can be classified as normal or binormal helix, according to the orientation of the cross section with respect to the helical axis of the structure. In this work, we present a simple model to study the elastic properties of a helical nanowire with asymmetric cross section. We use the framework of the Kirchhoff rod model to obtain an expression relating the Hooke's constant, h, of normal and binormal nanohelices to their geometric features. We also obtain the Young's modulus values. These relations can be used by experimentalists to evaluate the elastic properties of helical nanostructures. We showed that the Hooke's constant of a normal nanohelix is higher than that of a binormal one. We illustrate our results using experimentally obtained nanohelices reported in the literature.

The present work compares two different organic electrolytes (glycerol and ethylene glycol) for the growth of self-organized TiO2 nanotubes by anodic oxidation of Ti. In both electrolytes these self-organized layers of TiO2 nanotubes can be grown to considerable thickness (several 10 μm). It is found that except for the detailed electrochemical conditions also the water content has a dramatic influence on the tube dimensions, mainly length and tube morphology. The best result in terms of length is achieved in “water-free” electrolytes.

Laser flash and self-heating 3ω techniques were employed to determine the anisotropic thermal conductivity and thermal diffusivity of highly oriented free standing multiwalled carbon nanotube (MWNT) sheet drawn from a sidewall of a MWNT forest that was grown by chemical-vapor deposition. The thermal conductivity and the thermal diffusivity along the alignment are 50±5 W/m·K and 45±5 mm2/s, respectively, and are mostly limited by intrinsic defects of individual nanotubes and phonon-phonon interaction within bundles which form the supporting matrix of the MWNT sheet. The long tube-tube overlapping substantially decreases the electrical and thermal interconnection resistances which are usually dominate in randomly deposited mat-like nanotube assemblies. The extremely large surface area of the MWNT sheet leads to excessive heat radiation that dose not allow to transfer the heat energy by means of phonons to distances > 2 mm.

The metal nanocrystal (NC) based carbon nanotube (CNT) memory device has been probed with tunneling rate measurements. Firstly, tunneling behavior at two temperatures (300K and 10K) is reported here to demonstrate the distinct charge tunneling behavior for traps versus NCs and understand their relative contributions to program operations. Low temperature measurements show clear differentiation for two regimes of quantum transport. The FN tunneling regime exhibits strong bias dependence and dominates at high electric fields producing larger tunneling rates than the direct tunneling regime. In comparison to traps, the metal NCs repel potential contours and hence produce higher electric fields that enhance tunneling. The FN tunneling diminishes when the charging of the nanocrystal or traps decreases (relaxes) the electric field in the tunnel dielectric (TD) enough for the low field direct tunneling to dominate. The direct tunneling occurs at low fields, and is less sensitive to electric fields. The NCs demonstrate faster tunneling which can be ascribed to their large tunneling cross-section compared to traps. This is despite the relative proximity of traps to the channel in our structure. Secondly, the tunneling rates for two different TDs of similar EOT (under linear approximation) have been characterized and compared. They are a homogenous evaporated SiO2 and layered dielectric consisting of an evaporated SiO2 and ALD Al2O3 stack. While the evaporated SiO2 based TD demonstrates the distinct NC versus trap tunnel rate performance, the layered TD demonstrates stronger resistance to tunneling to the NCs. This result is consistent with the low tunneling rates demonstrated in Al2O3 elsewhere. Finally, the program performance of the NC-CNT memory device is evaluated as 0.5 V threshold voltage (VT) shift for a charging pulse of 9V and 100 μs. Combining with previous results, this indicates that NC-CNT memory is a promising candidate for low voltage, fast, multi-level cell (MLC) operation with sub-lithographic (self-assembled) features for sub 30 nm FLASH memory node. From the device physics perspective, these measurements may serve as the calibration and validation for advanced tunneling calculations and device modeling for promising nanoscale charge-based non-volatile memories.

We report here a facile solution method to synthesis of C60 nanorods. A very simple and effective solvent, neat m-xylene, from which C60 can rapidly grow into individual novel nanorods with widths and thicknesses of the order of nanometers was found as a shape controller. These unusual nanorods can easily grow on various substrates such as glass and silicon, and nanorods with different diameters and length-to-diameter ratios can be synthesized under different growth conditions. The smallest nanorods we obtained have widths smaller than 30 nm. The nanorods obtained are highly crystalline and single phase.

Scanning electron microscopy (SEM) for imaging the interface between carbon nanofibers (CNFs) and the underlying substrate is presented. By irradiating the electron beam perpendicular to the substrate, bright contrast is observed at the region where a small gap exists between the CNF and substrate. The energy-diameter diagram for the observation of the bright contrast is derived, which can be understood by using the theory of electron penetration into solid. Monte Carlo simulation is performed to reproduce the experimental observation based on our model, and the contrast sensitivity to the gap height is discussed.

Carbon nanotubes (CNTs) have been investigated as field emission electron sources for a number of applications such as x-ray source and microwave power amplifier. These applications often require that nanotubes provide a stable field emission at a high emission current. In this paper, we investigated the emission stability of CNTs as functions of hydrogen post-treatment and duty factor. The experimental results show that the hydrogen plasma treatment and a reduced duty cycle from 100% to 35% can noticeably improve the emission stability of CNTs at a high current value of 4 mA. Our study also shows that emission induced anode heating at high duty factor and high emission current level degrades vacuum level and thus the emission stability of CNTs.

Geometrically controlled single-walled carbon nanotube (SWNT) and multi-walled carbon nanotube (MWNT) networks were fabricated by a width confinement technique to characterize their electrical characteristics. The results demonstrated non-linear resistance decay with the number of conducting channels. The current-voltage characteristics at high field were studied until the electrical breakdown took place. Large current (∼2 mA), low resistance (∼5 KΩ) and current densities exceeding ∼108 A/cm2 were demonstrated from multi-channel MWNT networks confined in a 10 μm × 15 μm trench. Additionally, chronological SEM imaging was used to identify the breakdown sequences in the carbon nanotube networks, which revealed a strong tendency for CNT breakdown to occur in the vicinity of CNT-CNT intersections. Our results offer insights for interconnect applications using CNT networks.