Collective quantum coherence and subband redistribution in artificially assembled nanotube arrays

Band Engineering of Carbon Nanotubes for Device Applications

Carbon nanotubes (CNTs) have been proposed as electrical interconnects in microelectronic devices to address such problems as stress, electromigration, and heat removal. For electronic device and packaging applications, chemical vapor deposition (CVD) methods are particularly attractive due to characteristic CNT growth features such as selective spatial growth, large area deposition capability and aligned CNT growth. However, the CVD technique suffers from several drawbacks. One of the main challenges for applying CNTs to circuitry is the high growth temperature (≫600°C). Such temperatures are incompatible with microelectronic processes. To fabricate microelectronics devices that incorporate CNT blocks, the CNTs should be selectively positioned and interconnected to other materials such as metal electrodes or bonding pads. However, the adhesion between CNTs and the substrates is usually very poor, which will result in long term reliability issues and high contact resistance. To overcome these disadvantages, we propose a methodology that we term “CNT transfer technology”. The distinctive CNT-transfer-technology features are separation of CNT growth and CNT device assembly at solder reflow temperature. In this paper, we combined our expertise in growth of well-aligned open-ended CNT bundles with the CNT transfer process to assemble CNT bundles for fine-pitch interconnect applications. The open-ended multi-walled CNT arrays could carry higher current density than close-ended CNTs, since the internal walls can participate in the electrical transport. We for the first time developed an in-situ process to grow well aligned CNT bundles by water-assistant selective etching. The process is very efficient, with CNT growth rate of 80 μm/min. To demonstrate the feasibility of transfer process to assemble the fine-pitch CNT bundles, the CNT bundles with diameter, aspect-ratio and pitch of 25 μm, 4, and 80 μm, respectively, were assembled on the copper substrates. The measured resistivity of the long CNTs is ˜ 2.3×10−4 Ω-cm. Due to the capillary force effects, the Sn/Pb show improved wetting properties on open-ended CNT films. It is desirable for CNT interconnect with metal electrode by solder reflow process. The CNT-solder interfaces were analyzed by the SEM. The results indicated that molten SnPb solder form strong mechanical bonding with CNTs. Overall, the advantages of CNT transfer technology are embodied in the low process temperature, adhesion improvement and the feasibility of transferring CNT bundles to different substrates for fine-pitch interconnect applications.

We report on a novel and facile thermal chemical vapour deposition method for fabricating dense and vertically well-aligned bamboo-like carbon nanotube (CNT) arrays on a copper substrate from ethanol and acetone for the first time. The effect of growth time, temperature and catalysts has been systematically studied. Using ethanol as the carbon source, well-aligned CNTs were produced at 800 °C, and random, long and large carbon fibres/tubes were formed at 900 °C. When acetone was used, mushroom-like carbon nanostructures were formed at 800 °C, and well-aligned CNTs were produced at 850 °C. In contrast, large amounts of random carbon micro-fibres were formed at 800 °C using ethanol as the carbon source when Fe or Ni was employed as the substrate. The electrochemical properties of the novel mushroom-like carbon nanostructures are presented; the growth mechanisms for the formation of the bamboo-like CNTs and the mushroom-like carbon nanostructures are discussed.

Preface.- 1 Improved mechanical performance of CNTs and CNT fibres in nanocomposites through inter-wall and inter-tube coupling - Michael A. McCarthy, Emmett M. Byrne, Nathan P. O'Brien, and Tony Murmu.- 2 A Review on the Application of Nonlocal Elastic Models in Modeling of Carbon Nanotubes and Graphenes - Behrouz Arash and Quan Wang.- 3 A heterogeneous discrete approach of interfacial effects on multi-scale modelling of carbon nanotube and graphene based composites - S.K. Georgantzinos, G.I. Giannopoulos, K.N. Spanos, and N.K. Anifantis.- 4 Effect of Covalent Functionalization on Young's Modulus of a Single-Wall Carbon Nanotube - Priyal H. Shah and Romesh C. Batra.- 5 Multiscale Modeling of Multifunctional Fuzzy Fibers based on Multi-Walled Carbon Nanotubes - Gary Don Seidel, George Chatzigeorgiou, Xiang Ren and Dimitris C. Lagoudas.- 6 Geometry-property relation in corrugated nanocarbon cylinders - Hiroyuki Shima.- 7 Prediction of Mechanical Properties of CNT Based Composites Using Multi-scale Modeling and Stochastic Analysis - Roham Rafiee and Mahmood M. Shokrieh.- 8 Molecular Dynamics Simulation and Continuum Shell Model for Buckling Analysis of Carbon Nanotubes - C. M. Wang, A. N. Roy Chowdhury, S. J. A. Koh, and Y. Y. Zhang.- 9 Influence of Bond Kinematics on the Rupture of Non-Chiral CNTs under Stretching-Twisting - Bruno Faria, Nuno Silvestre, and Jose N. Canongia Lopes.- 10 Finite Element Modeling of the Tensile Behavior of Carbon Nanotubes, Graphene and Their Composites - Konstantinos I. Tserpes, and P. Papanikos.

The thermal properties of carbon nanotubes are strongly dependent on their unique structure and size, and show promise as an ideal material for thermal management on the micro- and macro-scale. The specific heat of nanotubes is similar to that of two-dimensional graphene at high temperatures, but is sensitive to the effects of rolling the the graphene sheet into a small cylinder at low temperatures. Specifically, the acoustic phonon modes are stiffened due to the cylindrical geometry, and the phonon spectrum is quantized due to the small diameter of the tube. In bundles of single-walled nanotubes, the specific heat is a sensitive probe of inter-tube mechanical coupling. Measurements of the specific heat show that inter-tube coupling is relatively weak, and show direct evidence for quantum effects. The thermal conductivity of nanotubes should reflect the on-tube phonon structure. Aligned bundles of SWNTs show a high thermal conductivity (>200 W/m-K at room temperature), and possible quantization effects at low temperature.

Well-aligned multiwalled carbon nanotubes (CNTs) were grown by microwave plasma-enhanced chemical vapor deposition using N2 and NH3 as the carrier gases and CH4 as the carbon source. Iron films with 1−5 nm thickness on silicon substrates acted as catalysts. Scanning electron microscopy revealed that the CNTs grew via the base growth mechanism at a rate ∼100 nm/s. Transmission electron microscopy showed that multiwalled CNTs had a “bamboo” structure, and the smallest CNTs of ∼6 nm in diameter were acquired on 1 nm Fe film. These “smallest” CNTs were comprised of amorphous structure, due to the formation of sp3 C−H bonds as proven by Fourier transform infrared spectroscopy and electron energy loss spectroscopy, suggesting hydrogen incorporation during growth of CNTs. Without N2 gas, no CNTs could be grown, while curly CNTs with poor alignment were grown with no NH3. In both cases, high-purity CNTs with no CNx impurities were obtained. Field electron emission revealed that the lowest turn-on and threshold fields were obtained from the CNTs grown on the 1 nm thick iron films on silicon substrate, i.e., 3.5 V/μm and 4.5 V/μm, respectively. Under an applied field of 7.5 V/μm, emission current density of 0.63 A/cm2 can be obtained.

We have used a bias-assisted microwave plasma chemical vapor deposition system to synthesize carbon nanotubes presenting graphitic nanoflakes, named coral-like carbon nanotubes, and well-aligned carbon nanotubes on carbon cloth substrates. Applying an external bias of -100 V led to the growth of well-aligned carbon nanotubes. In the absence of an external bias, the coral-like nanotubes presenting graphite nanoflakes were formed. The specific surface areas of the well-aligned and coral-like carbon nanotubes electrodes were 90.31 and 143.69 m2/g, respectively. In terms of energy storage, we estimated the capacitance of the coral-like carbon nanotube electrode to be ca. 194 F/g in an electrolyte of 1 M H2SO4. This value is almost double that of the well-aligned carbon nanotubes electrode (104 F/g), presumably because the presence of the carbon nanoflakes had a positive influence on the migration and adsorption of ions within the electrode. The fitting results indicated that the coral-like carbon nanotubes electrode behaved as a traditional electrochemical capacitor. Durability tests revealed that the coral-like carbon nanotube electrode was reliable, with a decay of 9% in capacitance over 1000 cycles.

Once fundamental difficulties such as active sites and selectivity are fully resolved, metal-free catalysts such as 3D graphene or carbon nanotubes (CNT) are very cost-effective substitutes for the expensive noble metals used for catalyzing CO2. A viable method for converting environmental wastes into useful energy storage or industrial wealth, and one which also addresses the environmental and energy problems brought on by emissions of CO2, is CO2 hydrogenation into hydrocarbon compounds. The creation of catalytic compounds and knowledge about the reaction mechanisms have received considerable attention. Numerous variables affect the catalytic process, including metal–support interaction, metal particle sizes, and promoters. CO2 hydrogenation into different hydrocarbon compounds like lower olefins, alcoholic composites, long-chain hydrocarbon composites, and fuels, in addition to other categories, have been explained in previous studies. With respect to catalyst design, photocatalytic activity, and the reaction mechanism, recent advances in obtaining oxygenated hydrocarbons from CO2 processing have been made both through experiments and through density functional theory (DFT) simulations. This review highlights the progress made in the use of three-dimensional (3D) nanomaterials and their compounds and methods for their synthesis in the process of hydrogenation of CO2. Recent advances in catalytic performance and the conversion mechanism for CO2 hydrogenation into hydrocarbons that have been made using both experiments and DFT simulations are also discussed. The development of 3D nanomaterials and metal catalysts supported on 3D nanomaterials is important for CO2 conversion because of their stability and the ability to continuously support the catalytic processes, in addition to the ability to reduce CO2 directly and hydrogenate it into oxygenated hydrocarbons.

Carbon quantum dots as modulators of hydroxyapatite c-axis orientation and mechanical reinforcement in dentin: Unlocking quantum entanglement and coherence on biomineralization.

Carbon nanotubes were synthesized by hot filament assisted direct current plasma enhanced chemical vapor deposition (HF/DC-P CVD) in CH4/H2 plasmas. Well-aligned and high density carbon nanotubes were grown. By I-V characteristics and SEM images, it was found that there is a close relation between well-aligned carbon nanotubes and field emission properties. It was also found by the F-N plot of I-V characteristics that the current is due to the tunnelling effect and there are no relations between the feld enhancement factor and the diameter of carbon nanotubes for the grown carbon nanotubes.

The exceptional mechanical properties of carbon nanotubes (CNTs) make them highly attractive as potential reinforcing constituents in next generation composites. CNTs can be used individually or in small bundles as toughening agents in matrices, or large, aligned bundles can be twisted into fibres (Cheng 2007; Zhang et al. 2007). However, in both applications a major drawback is the weak van der Waals forces between the walls of multi-walled CNTs (MWCNTs) and between individual tubes in CNT bundles. This makes for easy sliding between CNT walls and between CNTs in bundles, which drastically reduces their effective shear, bending, tensile and compressive properties. In this chapter we discuss the potential for addressing this deficiency through creation of inter-wall and/or inter-tube covalent bonds via irradiation with electrons or ions. The topic is addressed through an extensive series of Molecular Dynamics simulations as well as an analytical shear-lag model. We show that both inter-wall and inter-tube bonding can have highly beneficial effects on the mechanical properties of CNT-based nanocomposites. The benefits can significantly outweigh the detrimental effects of induced defects from the irradiation process.KeywordsMulti-wall carbon nanotubeNanotube fibre, nanocompositeInter-wall and inter-tube bondingLoad transferStructural propertiesMolecular dynamics

We calculate the structural and electronic properties of an ordered ``bundle'' of (10,10) carbon nanotubes. Our results indicate that intertube coupling causes an additional band dispersion of $\ensuremath{\lesssim}0.2 \mathrm{eV}$ and opens up a pseudogap of the same magnitude at ${E}_{F}.$ Soft librations at $\ensuremath{\nu}\ensuremath{\approx}12 {\mathrm{cm}}^{\ensuremath{-}1}$ are predicted to occur below the orientational melting temperature which marks the onset of tube rotations about their axis. Whereas the density of states near ${E}_{F}$ increases by 7% due to intertube coupling and by one order of magnitude due to K doping in ${\mathrm{KC}}_{8},$ these states do not couple to tube librations.

Hydrogen adsorption in bundles of well-aligned carbon nanotubes at room temperature

We study the thermal conductivity of single-walled carbon nanotube bundles and multi-walled carbon nanotubes. It is shown that, for an individual single-walled carbon nanotube, its thermal conductivity is both diameter and chirality dependent. In defect-free bundles and multi-walled carbon nanotubes, the phonon essentially does not transcend the bonds between the constituent walls due to the weak intertube interaction. If, however, the intertube coupling is strong, a substantial reduction in the thermal conductivity maybe implied. Such a low thermal conductivity can be found in several thermal transport experiments of carbon nanotube mats.

We study theoretically the in-plane electromagnetic response and the exciton–plasmon interactions for an experimentally feasible carbon nanotube (CN) film system composed of parallel aligned periodic semiconducting CN arrays embedded in an ultrathin finite-thickness dielectric. For homogeneous single-CN films, the intertube coupling and thermal broadening bring the exciton and interband plasmon resonances closer together. They can even overlap due to the inhomogeneous broadening for films composed of array mixtures with a slight CN diameter distribution. In such systems, the real part of the response function is negative for a broad range of energies (negative refraction band), and the CN film behaves as a hyperbolic metamaterial. We also show that for a properly fabricated two-component CN film, by varying the relative weights of the two constituent CN array components, one can tune the optical absorption profile to make the film transmit or absorb light in the neighborhood of an exciton absorption resonance on-demand.