Growth behavior of carbon nanotubes on multilayered metal catalyst film in chemical vapor deposition

Polymer-functionalization of carbon nanotube by in situ conventional and controlled radical polymerizations

Functionalization of carbon nanotube (CNT) with polymers has drawn much attention due to its wide range of applications. Polymer-functionalized CNT could exhibit variety of properties, such as responsivity to environmental stimuli, ability of complexation with metal ions, increased dispersibility in different solvents, higher compatibility with polymer matrix, etc. Chemical and physical methods have been developed for the preparation of polymer-functionalized CNT. Polymer chains are chemically bonded to the CNT edge or surface in the chemical methods, which results in highly stable CNT/polymer composites. "Grafting to", "grafting from", and "grafting through" methods are the most common chemical methods for polymer-functionalization of CNT. In "grafting to" method, pre-fabricated polymer chains are coupled with the either functionalized or non-functionalized CNT. In "grafting from" and "grafting through" methods, CNT is functionalized by polymers simultaneously synthesized by in situ polymerization methods. Conventional free radical polymerization (FRP) and also controlled radical polymerization (CRP) are the most promising methods for in situ tethering of polymer brushes onto the surface of CNT due to their control over the grafting density, thickness, and functionality of the polymer brushes. The main focus of this review is on the synthesis of polymer-functionalized CNT via both the "grafting from" and "grafting through" methods on the basis of FRP and CRP routs, which is commonly known as in situ polymerizations. Finally, the most important challenges and applications of the in situ polymer grafting methods are discussed, which could be interesting for the future works.

Low-Cost Catalyst Ink for Simple Patterning and Growth of High-Quality Single- and Double-Walled Carbon Nanotubes

Research into carbon nanotubes (CNTs) has been a hot topic for almost 3 decades, and it is now that we are beginning to observe the impact of advanced applictions of this nanomaterial in areas such as electronics. Currently, in order to mass produce CNT devices, either large-scale synthesis, followed by numerous energy-intensive processing steps or photolithography processes, including several sputter-deposition steps, are required to pattern this material to fabricate functional devices. In the work reported here, through the utilization of a universal catalyst precursor (cyclopentadienyl iron dicarbonyl dimer) and the optimization of solution parameters, patterned high-quality vertically aligned arrays of single- and few-walled CNTs have been synthesized via various inexpensive, commercially scalable methods such as inkjet printing, stamp printing, spray painting, and even handwriting. The two-step process of precursor printing, followed immediately by CNT growth, results in CNTs with a Raman I_D/I_G ratio of 0.073, demonstrating very high-quality nanotubes. This process eliminates time-consuming and costly CNT post processing techniques or the deposition of numerous substrate barrier and catalyst layers to achieve device manufacturing. As a result, this method has the potential to provide a route for the large-scale synthesis of high-quality single- and few-walled CNTs that can be applied in industrial settings.

Modulating the height of carbon nanotube forests by controlling the molybdenum thin film reservoir thickness

Many catalyst materials have been tried to synthesize ultra-long carbon nanotubes (CNTs) by extending catalyst lifetime and delaying growth termination. We propose a time-controlled, variable composition iron-molybdenum catalyst system, where the diffusion of molybdenum (as a thin layer reservoir) is mediated by the alumina underlayer, to reach and to slowly alloy with the Fe catalyst on the surface during the thermal process. This technique enhanced both the catalytic activity and the catalytic lifetime to grow CNT carpets with heights up 5 mm, compared to a maximum of approximately 1.5 mm for a regular sample (without Mo reservoir). Moreover, the CNT height increased with the thickness of the Mo thin layer reservoir for thicknesses from 10 nm to 30 nm. We discuss this new growth mechanism using high resolution transmission microscopy (HRTEM) images of cross-section lamellas and Rutherford Back Scattering (RBS) analysis to show the increasing alloying of Mo with Fe. Overall, the proposed technique of mediated diffusion of Mo to the surface with subsequent progressive alloying with the Fe catalyst, besides enhancing CNT height, could allow the one-step synthesis of CNT carpets with regions of different heights based on patterning these regions with different thicknesses of the Mo reservoir during sample preparation.

Catalytic growth of multi-walled carbon nanotubes using NiFe2O4 nanoparticles and incorporation into epoxy matrix for enhanced mechanical properties

Mechanical properties of multi-walled carbon nanotubes (CNT) reinforced epoxy nanocomposites, with and without any structural defect, were investigated using different weight percent values of pristine and covalently functionalized CNT. First, nickel ferrite (NiFe2O4) catalyst nanoparticles were prepared using the co-precipitate method followed by CNT growth via chemical vapor deposition, using acetylene as carbon feedstock. Through a combination of magnetic stirring and ultrasound vibration treatments in acetone, pristine, COOH-, or NH2-functionalized CNTs at 0.15, 0.60, 1.10 and 1.50 wt% were added to the Epon 828 epoxy. During each stage, extensive materials characterization was carried out using scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), Fourier transform infrared (FTIR) and thermogravimetric analysis (TGA)/differential thermal analysis (DTA) techniques. Tensile testing of the specimens revealed an increase in the elastic modulus and tensile strength values with maximum increase registered in the case of nanocomposites made from 1.1 wt% CNT-NH2 (+73%) or CNT-COOH (67%) addition. The energy absorbed during impact testing also increased by 86% upon addition of 1.50 wt% CNT-NH2. The presence of a small notch in the nanocomposite specimens yielded superior mechanical properties to those of the neat epoxy. Such enhancement in the mechanical properties can be attributed to better CNT dispersion in the nanocomposites and good interfacial bonding, as confirmed from microstructural examination of the fractured surfaces.

Low-Temperature Growth of Carbon Nanotube Forests Consisting of Tubes with Narrow Inner Spacing Using Co/Al/Mo Catalyst on Conductive Supports

We grow dense carbon nanotube forests at 450 °C on Cu support using Co/Al/Mo multilayer catalyst. As a partial barrier layer for the diffusion of Co into Mo, we apply very thin Al layer with the nominal thickness of 0.50 nm between Co and Mo. This Al layer plays an important role in the growth of dense CNT forests, partially preventing the Co-Mo interaction. The forests have an average height of ∼300 nm and a mass density of 1.2 g cm(-3) with tubes exhibiting extremely narrow inner spacing. An ohmic behavior is confirmed between the forest and Cu support with the lowest resistance of ∼8 kΩ. The forest shows a high thermal effusivity of 1840 J s(-0.5) m(-2) K(-1), and a thermal conductivity of 4.0 J s(-1) m(-1) K(-1), suggesting that these forests are useful for heat dissipation devices.

Performance of hybrid nanostructured conductive cotton materials as wearable devices: an overview of materials, fabrication, properties and applications

Recent advances and overview of hybrid nanostructured cotton materials will boost an essential encouragement for the development of next generation smart textiles and flexible devices which could be worn by human beings.

Carbon atoms in ethanol do not contribute equally to formation of single-walled carbon nanotubes

We propose a unique experimental technique in which isotopically labeled ethanol, e.g., 12CH3-13CH2-OH, is used to trace the carbon atoms during the formation of single-walled carbon nanotubes (SWNTs) by chemical vapor deposition (CVD). The proportion of 13C is determined from Raman spectra of the obtained SWNTs, yielding the respective contribution of ethanol's two different carbon atoms to SWNT formation. Surprisingly, the carbon away from the hydroxyl group is preferably incorporated into the SWNT structure, and this preference is significantly affected by growth temperature, presence of secondary catalyst metal species such as Mo, and even by the substrate material. These experiments provide solid evidence confirming that the active carbon source is not limited to products of gas-phase decomposition such as ethylene and acetylene, but ethanol itself is arriving at and reacting with the metal catalyst particles. Furthermore, even the substrate or other catalytically inactive species directly influences the formation of SWNTs, possibly by changing the local environment around the catalyst or even the reaction pathway of SWNT formation. These unexpected effects, which are inaccessible by conventional techniques, paint a clearer picture regarding the decomposition and bond breaking process of the ethanol precursor during the entire CVD process and how this might influence the quality of the obtained SWNTs.

Carbon nanotube growth for through silicon via application

Through silicon via (TSV) technology is key for next generation three-dimensional integrated circuits, and carbon nanotubes (CNT) provide a promising alternative to metal for filling the TSV. Three catalyst preparation methods for achieving CNT growth from the bottom of the TSV are investigated. Compared with sputtering and evaporation, catalyst deposition using dip-coating in a FeCl2 solution is found to be a more efficient method for realizing a bottom-up filling of the TSV (aspect ratio 5 or 10) with CNT. The CNT bundles grown in 5 min exceed the 50 μm length of the TSV and are multi-wall CNT with three to eight walls. The CNT bundles inside the TSV were electrically characterized by creating a direct contact using a four-point nanoprober setup. A low resistance of the CNT bundle of 69.7 Ω (297 Ω) was measured when the CNT bundle was contacted midway along (over the full length of) the 25 μm deep TSV. The electrical characterization in combination with the good filling of the TSV demonstrates the potential use of CNT in fully integrated TSV applications.

Local synthesis of aligned carbon nanotube bundle arrays by using integrated micro-heaters for interconnect applications

A rapid synthesis process for local growth of the aligned carbon nanotube (CNT) bundle arrays in a quartz tube operated at room temperature is reported. The CNT bundles of 105 microm in length and 130 microm in diameter were thermally and locally synthesized on the wafer areas where the micro-heaters were integrated on the opposite side. The micro-heaters can reach a temperature of about 720 degrees C in the localized areas for the CNT growth while the temperature in the non-heating areas remains below 400 degrees C, which prevents damage to the integrated circuits (ICs) located in those areas. Furthermore, exact control of the CNT growth length is possible by varying the voltage supply time to the micro-heater since the processing wafer can be heated up and cooled down very rapidly due to its small mass. The resistivity of CNT bundles was measured to be 0.009 57 Omega cm, obtained from a nearly linear current-voltage characteristic. Hence, these locally grown CNT bundle arrays could find potential applications in three-dimensional (3D) IC wafer-level packaging.

Patterning of metallic nanoparticles for the growth of carbon nanotubes

We report a straightforward method of patterning large ordered arrays of metallic nanoparticles using existing semiconductor processing technology. The topographies of contact holes created on full 200 mm wafers serve as templates to pattern Ni or Co nanoparticles. Over a large range of synthesis conditions, these patterned nanoparticles are demonstrated to successfully catalyse the growth of carbon nanotubes (CNTs) selectively in the patterned areas of the wafer. This approach to catalyst deposition is scalable and fully compatible with existing semiconductor processing technology. Thus, it can be exploited for a variety of applications where confinement of materials in nanometric domains is necessary. These results represent a further step towards the integration of CNTs into conventional Si-based technology.

Growth of CNTs on Fe-Si catalyst prepared on Si and Al coated Si substrates

In this paper we report the effect of Al interlayers on the growth characteristics of carbon nanotubes (CNTs) using as-deposited and plasma etched Fe-Si catalyst films as the catalysts. Al interlayers having various thicknesses ranging from 2 to 42 nm were deposited on Si substrates prior to the deposition of Fe-Si catalysts. It was found that the Al interlayer diffuses into the Fe-Si catalyst during the plasma etching prior to the CNT growth, leading to the swelling and amorphization of the catalyst. This allows enhanced carbon diffusion in the catalyst and therefore a faster growth rate of the resulting CNTs. It was also found that use of an Al interlayer having a thickness of ∼3 ± 1 nm is most effective. Due to the effectiveness of this, the normally required catalyst etching is no longer needed for the growth of CNTs.

Imperfect surface order and functionalization in vertical carbon nanotube arrays probed by near edge X-ray absorption fine structure spectroscopy (NEXAFS)

Probing surface order as well as the degree of structural modification in carbon nanotube systems is of fundamental importance for incorporation of these materials into practical functional devices. The current study pertains to the analysis of the surface order of vertically-aligned single-walled and multi-walled carbon nanotube arrays of varying length and composition by means of near-edge X-ray fine structure spectroscopy (NEXAFS). Both NEXAFS and scanning electron microscopy (SEM) studies concluded that the nanotubes in these samples were oriented vertically to the plane of the surface. However, NEXAFS polarization analysis provided a more quantitative and nuanced description of the surface structure, indicative of far less localized surface order, an observation partially attributed to misalignment and bending of the tubes. Moreover, it was demonstrated by NEXAFS that the surface order of the arrays was imperfect and relatively independent of the height of the nanotube arrays. In addition, we have shown that NEXAFS can be used to correlate the extent of chemical functionalization and oxygenation with disruption of the electronic and physical structure of nanotubes embedded in array motifs.