Growth Of Aligned Carbon Nanotubes Research Articles

Optimizing the growth of aligned carbon nanotubes by literature mining and high-throughput experiments

Optimizing the growth of aligned carbon nanotubes by literature mining and high-throughput experiments

The effect of fuzzy fiber-reinforced composite skin on bending stiffness and debonding growth between skin and polymeric foam core of sandwich structures

Progressive innovations in the field of nanotechnology demonstrate that through the radial growth of aligned carbon nanotubes (CNTs) on the circumferential surfaces of fibers known as fuzzy fibers, the transverse elastic properties of fiber-reinforced composites are notably improved. The main objective of the present work is to numerically examine the effects of using fuzzy fiber-reinforced composite (FFRC) skins on the flexural behavior of sandwich structures. Based on the considered two-stage analysis, in the first instance, the mechanical properties of FFRC are determined by the simplified unit cell (SUC) micromechanical model. Afterward, by simulating the three-point bending test via the finite element method (FEM), major assessments are accomplished to reflect the effects of microstructure-level features, including fiber and CNT volume fractions, CNT/polymer interphase, and CNT diameter, on the bending stiffness and debonding growth between the FFRC skin and polyvinyl chloride (PVC) foam core. The results reveal the satisfactory influence of employing FFRC skins on improving the flexural behavior of sandwich structures.

Gas flow–directed growth of aligned carbon nanotubes from nonmetallic seeds

Kite growth is a process that utilizes laminar gas flow in chemical vapor deposition to grow long, well-aligned carbon nanotubes (CNTs) for electronic application. This process uses metal nanoparticles (NPs) as catalytic seeds for CNT growth. However, these NPs remain as impurities in the grown CNT. In this study, nanodiamonds (NDs) with negligible catalytic activity were utilized as nonmetallic seeds instead of metal catalysts because they are stable at high temperatures and facilitate the growth of low-defect CNTs without residual metal impurities. Results demonstrate the successful growth of over 100-μm-long CNTs by carefully controlling the growth conditions. Importantly, we developed an analysis method that utilizes secondary electron (SE) yield to distinguish whether or not CNTs grown from metal impurities. The absence of metallic NPs at the CNT tips was revealed by the SE yield mapping, whereas the presence of some kind of NPs at the same locations was confirmed by atomic force microscopy (AFM). These results suggest that most of the aligned CNTs were grown from nonmetallic seeds, most likely ND-derived NPs, via the tip-growth mode. Structural characterizations revealed the high crystallinity of CNTs, with relatively small diameters. This study presents the first successful use of nonmetallic seeds for kite growth and provides a convincing alternative for starting materials to prepare long, aligned CNTs without metal impurities. The findings of this study pave the way for more convenient fabrication of aligned CNT-based devices, potentially simplifying the production process by avoiding the need for the removal of metal impurities.

Large-area synthesis of vertically aligned carbon nanotubes growth on the aluminum foil via FCCVD method

In this study, VA-CNTs were synthesized homogeneously on Al foil at 610 oC temperature by Floating Catalyst Chemical Vapor Deposition (FCCVD) method in a horizontal tube furnace. While ethanol was used as the carbon source, ferrocene was used as the catalyst. VA-CNTs with diameters in the range of ~10-15 nm and lengths in the range of ~30-35 µm were obtained. Structural and morphological analyzes of VA-CNTs were determined using X-Ray Diffraction (XRD), Field Emission Scanning Electron Microscopy (FESEM), Energy Dispersion X-Ray Spectroscopy (EDS), Raman Spectroscopy and X-Ray Photoelectron Spectroscopy (XPS). These VA-CNTs, synthesized by the FCCVD method on an Al foil in a large area, have the potential applications to be used especially in energy storage, optoelectronic, and sensor.

Analysis on the synthesis of vertically aligned carbon nanotubes: growth mechanism and techniques

This review article deals with orientation controlled growth of Carbon Nanotubes (CNTs). Vertically aligned CNTs (VACNTs) are considered as the best type of CNTs among all known forms. The present article covered various aspects required for the successful growth of orientation controlled CNTs especially vertically aligned form. We discussed in details about the important properties of VACNTs and the importance of VACNTs in materials science with relative applications. It included different types of CVD techniques which support the vertically aligned CNTs growth on the substrate and also supports its mass production with proper growth mechanism. We made a detailed discussion about catalyst engineering and different type of carbon sources. The catalyst lifetime is a key factor for growing long CNTs, so, the reason of catalyst poisoning and the protection of catalyst from poisoning is also a part of our article. We also explained in detail about the types of catalyst protector elements with their suitability on different temperature as well for different gases used for continuous growth of VACNTs, yarns of CNTs as well as bulk production of CNTs. We covered a broad study about the various catalysts, catalyst thickness and catalyst deposition techniques. The suitable supporting layers for different catalysts with their impacts on morphology as well as on the structure of as grown CNTs are also discussed. For reducing cost of VACNTs production, we included eco-friendly natural wastage materials for catalysts and as carbon precursor. It also included the process by which different designs and structures of VACNTs can be made specifically by bending of VACNTs during growth running time through the utilization of different catalyst reservoirs. The discussion made in present article on the growth of oriented CNTs will help to grow selective vertically aligned CNTs in the convenient way with economical rate and thereby helps in the mass production of CNTs.

Surface passivation dictated site-selective growth of aligned carbon nanotubes.

Surface defects play a significant role in the nucleation and growth of metal particles. Site-selective nucleation of metal catalyst particles, and the subsequent growth of nanostructures, could thus be accomplished by defect engineering. This paper demonstrates the switching of growth sites of vertically aligned multiwall carbon nanotubes (MW-CNTs) by manipulation of surface passivation of the substrate and discusses the possible mechanism behind this selectivity. A complementary growth pattern of CNTs is observed for pre-treatment of identically patterned SiO2/Si substrates under a reducing and non-reducing atmosphere. Variation in the number density of oxygen vacancies on the silicon dioxide surface and the presence of native oxide on the silicon face are believed to dictate the observed selectivity. The CNT architectures mimic the substrate pattern meticulously, exhibiting sharp edges, illustrating a high degree of site selectivity. The chemical state of the substrate surface and catalyst particles has been studied using Auger electron spectroscopy. Electron microscopy and Raman spectroscopy were employed to characterize the synthesized CNTs. The Hermans orientation factor was calculated to quantify the degree of alignment of the MWCNTs. Such facile control over the growth site of aligned carbon nanotubes on a substrate is a desirable aspect of synthesis for easy integration with existing silicon fabrication technology.

Future Dielectric Materials for CNT Interconnects - Possibilities and Challenges

Carbon nanotube (CNT) interconnects are emerging as the ultimate choice for next generation ultra large scale integrated (ULSI) circuits. Significant progress in precise growth of aligned CNTs and integration of multiwalled CNT interconnects into a test chip make them promising candidates for future nanoelectronic chips. Tremendous research efforts were made on silicon based ultra-low-k dielectrics for Cu interconnects, but, the most recent advancements in polymer based composites as dielectric materials open up fresh challenges in the use of low-k dielectrics for CNT interconnects. This paper reviews the emerging polymer composites like Boron Nitride Nanotubes, Graphene/Polyimide composites, Metal Organic Frameworks and small diameter CNTs. Many reviews are already exists on the synthesis, fabrication, dielectric, mechanical, chemical and thermal properties of these materials. In this review, we have explained the specific properties of these materials and the necessities for integrating them into CNT interconnects to meet the requirements of future IC designers.Keywords: low-k dielectric materials, ultra low-k dielectrics, carbon nanotubes, interconnects, dielectric constant,

An effect of gas-phase reactions on the vertically aligned CNT growth by temperature gradient chemical vapor deposition

Gas-phase reactions among carbon precursors could have a significant effect on the catalytic synthesis of vertically aligned (VA-) carbon nanotubes (CNTs). State-of-the-art adjustment of the gas-phase reaction conditions is done ad hoc with low reproducibility, ascribed partly to an incomplete understanding of the mechanism how the gas-phase reaction influences the growth kinetics of VA-CNTs. Here, we investigate this mechanism and propose that in the presence of prominent gas-phase reaction, the growth kinetics characterization shifts from a single rate-limiting reaction to a multi-route reaction with each pathway having its own rate-limiting step. The rate of the gas-phase reaction could be tailored by the partial pressure and thermal history of a carbon feedstock (e.g., acetylene). According to in-situ mass spectrometry and numerical simulation, a rich set of thermally rearranged secondary species (e.g., C4H4 and C6H6) are found above the catalyst surface. It appears that these secondary species, along with a chief precursor (C2H2), can give variation to a yield of CNTs through various reaction pathways with altering the overall growth rate of VA-CNTs. Our findings would further the current understanding of the VA-CNT growth from thermally rearranged precursors, instrumental to applications such as diameter-controlled growth and CMOS-compatible low-temperature growth.

A route to truly realize the chirality-specific growth of aligned carbon nanotubes

A route to truly realize the chirality-specific growth of aligned carbon nanotubes

Surface Interaction Between Carbon Patches and Catalyst Nanoparticle as the Key Factor in Aligned Carbon Nanotube Growth Using Alcohol Catalytic CVD

Aligned carbon nanotubes (ACNTs) have very unique properties that made them applicable as electrode material for energy storage devices. ACNTs were grown directly on substrates by catalytic chemical vapor deposition (CVD). Cobalt, thermally-oxidized aluminum and ethanol were used as catalyst, catalyst-support and carbon feedstock, respectively. Optimum catalytic and reaction conditions were investigated by varying the CVD processing temperature and time. CVD processing temperature of 725[Formula: see text]C and processing time at 10[Formula: see text]min showed the optimum CNT alignment with thickness of above 2[Formula: see text][Formula: see text]m. Other than the effect of catalyst particle diameter, formation of amorphous carbon at the top of CNTs observed by SEM is suggested as another factor that influences the growth mechanism of ACNTs. The CVD processing parameter and the interaction between small carbon patches are supposed to be built after dehydrogenation of the first carbonaceous molecules and the catalyst nanoparticle surface considered as the hidden factors behind the growth results.

Differential preheating of hydrocarbon decomposition and water vapor formation shows that single ring aromatic hydrocarbons enhance vertically aligned carbon nanotubes growth

Using a system of multi-zone thermal chemical vapor deposition (CVD) furnaces in parallel and in series, we performed differential preheating of the incoming gases to independently control water vapor formation (from oxygen and hydrogen) and hydrocarbon decomposition to grow vertically aligned multi-wall carbon nanotube (MWCNT) carpets. We identified specific process parameter sweet spots for water vapor formation and for hydrocarbon decomposition that led to over three times taller CNT carpets compared to the reference process. For instance, we performed a kinetic study that showed that additional external preheating of oxygen and hydrogen to form water vapor significantly extended catalyst life, leading to a terminal height approaching 5 mm, while external preheating of the hydrocarbon precursors dramatically hindered CNT growth at high thermal doses.Using gas chromatography – mass spectrometry, we correlated specific carbon species resulting from ethylene decomposition from specific process points to reveal which species favored and which hindered CNT growth. We identified that hydrocarbons composed of one benzene ring connected to different substituents favored CNT growth while hydrocarbons composed of multiple aromatic rings hindered CNT growth. This study can further our understanding of CNT growth mechanisms and help designing complex CVD systems for more efficient synthesis of CNTs.

Internal geometry of woven composite laminates with “fuzzy” carbon nanotube grafted fibers

Radially aligned carbon nanotubes (A-CNTs) grown on micron-scale fibers promise structural composites with high mechanical performance and multi-functional properties. Changes in the internal structure of woven composite laminates after A-CNT growth are studied here utilizing micro-computed tomography. The laminates are produced by vacuum impregnation in a closed mold with and without clamping pressure. Two A-CNT lengths are investigated: 4–6μm and 17–19μm. A-CNTs were found to increase the distance between fibers, scaled by the A-CNT length. This “swelling” resulted in an increased cross-sectional area, crimp and in-plane misalignment of the yarns. The laminate thickness doubled for laminates with long A-CNTs compared to shorter ones. The laminate with long A-CNTs produced under pressure showed a remarkable alteration of the internal structure. Fibers migrated within the fabric plane, filling almost completely the resin rich pockets. Further research is needed to understand the effect of these changes on the composite mechanical performance.

Film fabrication of Fe or Fe3O4 nanoparticles mixed with palmitic acid for vertically aligned carbon nanotube growth using Langmuir–Blodgett technique

Vertically aligned carbon nanotubes (VA-CNTs) were studied as a new catalyst support for polymer electrolyte fuel cells (PEFCs). Controlling the number density and the diameter of VA-CNTs may be necessary to optimize PEFC performance. As the catalyst for CNT growth, we fabricated Fe or Fe3O4 nanoparticle (NP) films by the Langmuir–Blodgett (LB) technique. The catalyst Fe or Fe3O4 NPs were widely separated by mixing with filler molecules [palmitic acid (C16)]. The number density of VA-CNTs was controlled by varying the ratio of catalyst NPs to C16 filler molecules. The VA-CNTs were synthesized from the catalyst NP–C16 LB films by thermal chemical vapor deposition (CVD) using acetylene gas as the carbon source. The developing solvents used in the LB technique and the hydrogen reduction conditions of CVD were optimized to improve the VA-CNT growth rate. We demonstrate that the proposed method can independently control both the density and the diameter of VA-CNTs.

Nanoring Arrays on Fe Coated Substrate: Formation and Guidance for the Growth of Hierarchical CNTs.

In this article, we report the formation of nanoring structures on Fe coated substrate and their application in guiding the growth of carbon nanotube (CNT) patterns with hierarchical structures. The formation of nanorings involves the etching of polystyrene (PS) monolayer colloidal crystals (MCCs) under reactive ion etching (RIE), and the redeposition and cross-linkage of the active degradation products at the contact line between the MCCs and the substrate. After washing out the MCCs, insoluble nanorings with hexagonal order on the substrate are developed. The RIE process can control the morphology of the nanorings, as well as the distribution of the Fe element on the substrate; thus, a continuous Fe layer and separated Fe discs on the substrate are created on substrate after washing, depending on the etching time and the shield of MCCs. The surviving Fe element can work as the catalyst to initiate the in situ growth of aligned CNTs in the following chemical vapor deposition (CVD) process, while the Fe element underneath the nanorings keep its inactivity. Eventually, CNT patterns with hierarchical structures are formed. One level originates from the surviving Fe layer; the other level is templated from the nanoring structures, which cause the blank area in the CNT bundles.

Preloading catalysts in the reactor for repeated growth of horizontally aligned carbon nanotube arrays

This work presents a highly efficient and reliable strategy to grow horizontally aligned carbon nanotube (CNT) arrays on surfaces via preliminarily loading catalysts into the reactor. With the heat treatment before the growth of CNTs, the catalysts would migrate from the inner surface of the reactor to the surface of the targeted substrate to induce the growth of aligned CNTs. With this “in-situ catalyst loading” approach, aligned CNTs could be grown on different bare substrates, such as silicon, quartz and sapphire. The growth of CNTs could be repeated up to 50 times with reliable results via only one-time catalyst preloading process, avoiding the deposition of catalysts before each round of growth and the related randomicity, and thus benefiting the mass production of aligned CNT arrays on surfaces.

Electrically conductive aluminum oxide thin film used as cobalt catalyst-support layer in vertically aligned carbon nanotube growth

This paper will present the unique characteristics of aluminum oxide (Al–O) and cobalt catalyst included in aligned carbon nanotube (CNT) electrode system of energy storage device, namely electrochemical capacitor. Electrical conductivity and nanostructure of the thermally oxidized Al–O used as catalyst-support layer in vertically grown single-walled CNTs were studied. Al–O films were characterized by means of current–voltage measurement and high resolution transmission electron microscopy analysis. The Al–O support layer was found to be conductive, with a relatively low resistance and, approximately 20 nm film thickness of Al–O is suggested to be too thin to form insulating barrier. The scanning TEM—annular dark field analysis confirmed that the nanosized cobalt catalyst particles distributed on Al–O surfaces and also embedded inside the Al–O film structure.

Hierarchical carbon nanotube carbon fiber unidirectional composites with preserved tensile and interfacial properties

Hybrid hierarchical carbon-nanotube (CNT)-based composites, such as radially-aligned CNT arrays grown onto microfiber filaments, have significant potential to expand the performance and functionality of fiber reinforced composites. Here, a novel method for high-yield growth of aligned CNTs on aerospace-grade carbon fibers (CFs) is demonstrated at the composite level for the first time. Fuzzy carbon fiber reinforced plastics (fuzzy CFRP) unidirectional composites with >60% microfiber volume fraction are fabricated via vacuum-assisted resin infusion of CNT-grafted tows using an unmodified aerospace-grade epoxy. Preservation of microfiber tensile modulus and strength are demonstrated by longitudinal composite tensile testing, consistent with single-fiber tensile tests. Fiber–matrix interface strength is also unchanged by the CNT growth as revealed through continuously-monitored fiber fragmentation tests. Taken together, the results provide needed new composite-level understanding of hierarchical structural composite laminates and motivate future work on structural CF composite laminates with integrated multifunctionality and improved interlaminar and intralaminar performance.

Al2O3 + FeCo and Al2O3 + FeNi coatings for multiwalled carbon nanotubes growth

In this work the preparation of growth of aligned multiwall carbon nanotubes (MWCNTs) is presented using FeCo and FeNi alumina-based catalysts. The catalysts were prepared using the sol–gel process and some stainless steel substrates were coated using the dip coating technique. These coatings were used for the MWCNT in a Chemical Vapor Deposition (CVD) reactor. X-ray diffraction (XRD) studies of the coating solutions demonstrate the formation of metallic phases rich in iron in both samples. Raman spectroscopy study depicted the characteristic bands of MWCNT and the scanning electron microscopy (SEM) analysis showing the growth of aligned MWCNTs with a length of about 10μm and 20 to 80nm of diameter on both coatings. These results contribute to establishing a new route for the preparation of aligned and high purity MWCNT using catalysts prepared by a very fast and simple way.

Unraveling the growth of vertically aligned multi-walled carbon nanotubes by chemical vapor deposition

The interaction between the main operational variables during the growth of vertically aligned multiwalled carbon nanotubes (VA-MWCNTs) by catalytic chemical vapor deposition is studied. In this contribution, we report the influence of the carbon source (i.e. acetylene, ethylene and propylene), the reaction/activation temperature, the rate of heating, the reaction time, the metal loading, and the metallic nanoparticle size and distribution on the growth and alignment of carbon nanotubes. Fe/Al thin films deposited onto silicon samples by electron-beam evaporation are used as catalyst. A phenomenological growth mechanism is proposed to explain the interaction between these multiple factors. Three different outcomes of the synthesis process are found: i) formation of forests of non-aligned, randomly oriented multi-walled carbon nanotubes, ii) growth of vertically aligned tubes with a thin and homogeneous carbonaceous layer on the top, and iii) formation of vertically aligned carbon nanotubes. This carbonaceous layer (ii) has not been reported before. The main requirements to promote vertically aligned carbon nanotube growth are determined.

Pulsed Plasma Assisted Growth of Vertically Aligned Carbon Nanotubes at Low Temperature on Mo Substrate

Aligned carbon nanotubes (CNTs)/carbon nanofibers (CNFs) are synthesized at temperatures of 350, 400 and 450 °C using pulsed plasma enhanced chemical vapour deposition (Pulsed-PECVD) process. Ni catalyst film of thickness 10 nm is directly deposited on molybdenum (Mo) substrate and growth of CNT is performed at various temperatures. Pulsed-PECVD facilitates aligned CNT growth on Mo substrates at temperatures 400 and 450 °C, whereas at 350 °C the structure as obtained, are CNFs. A significant change is observed on the as grown CNTs/CNFs morphology with the catalyst film pre-treatment time and the growth temperature. Prolonged catalyst pre-treatment at 400–450 °C leads to growth of deformed structure CNTs which is due to the accumulation of amorphous carbon. At low temperature of 350 °C, longer CNFs are obtained for longer treatment time. The growth mechanism can be explained based on the wettability of Ni film on Mo substrate and solubility of carbon atoms during growth process. Using field emission scanning electron microscopy and high resolution transmission electron microscopy, dimension and structure as well as crystallinity of the samples are analyzed. At synthesis temperatures of 450 and 400 °C, multiwall carbon nanotubes are grown which shows some degree of crystallinity, whereas at 350 °C CNFs are obtained which is confirmed by HRTEM analysis and Raman spectroscopy characterization.