Integrated simulation of active carbon nanotube forest growth and mechanical compression

Carbon nanotube (CNT) forests are CNT populations that self-assemble into vertically oriented cellular arrays during growth. The anisotropic and inhomogeneous morphology of forests arises from complex mechanical interactions between CNTs during their collective growth and influences many forest properties. A time-resolved simulation is developed to model actively growing CNT populations having distributed properties and growth characteristics. The model considers van der Waals (vdW) attraction between neighboring CNTs and allows the growing and deforming CNTs to interact and react based on a balance of forces. Parametric variations of growth rate distribution and CNT occupation density generate variable CNT forest morphology in manners consistent with experimental observations. The forces opposing vdW bonding between contacting CNTs during forest growth are found to diminish with distance from the growth substrate and are proportional to CNT bending stiffness. Axial and transverse compression of simulated forests capture experimentally observed phenomena of coordinated axial buckling, transverse densification, and the foam-like force–displacement response that is typical of CNT forests. This new paradigm in CNT forest modeling may be used as an analytical tool to examine CNT forest growth kinetics, multi-physics CNT forest performance, and the post-synthesis processing and forming of CNT forest microstructures.     

Reversible tailoring of mechanical properties of carbon nanotube forests by immersing in solvents

The mechanical behavior of carbon nanotube (CNT) forests soaked in three solvents – toluene, acetonitrile, and isopropanol – is examined. Effective stiffness of the structure is evaluated in the dry and wet condition by micro-indentation using a 100 μm flat punch. With soaking of CNT forests in solvents, the stiffness decreases and deformation mechanism changes from buckling concentrated close to the bottom of the CNT forest to a distribution of local buckles along the height and global buckling of the entire length of CNTs. We use molecular dynamics simulations to relate the experimental observations to the reduced mechanical support from neighbor CNTs due to a decreased magnitude of van der Waals (vdW) interactions in the presence of solvents. Toluene, which produces the lowest average measured stiffness between the three solvents, produces the lowest vdW forces between individual CNTs. Furthermore, wet–dry cycling of CNT forests shows the reversibility and repeatability of change of stiffness by immersing in solvents. The results show that soaking CNT forests in solvents could be useful for applications such as interface materials where lower stiffness of CNT forests are needed and applications such as energy absorbing materials in which re-setting of stiffness is required.     

The experimental measurement of effective compressive modulus of carbon nanotube forests and the nature of deformation

Measurement of the effective compressive modulus of vertically aligned carbon nanotube (CNT) forests/turfs is evaluated with two different experimental methods. The first experimental method uses a high force nanoindentation system to uniformly compress CNT forests grown on rigid silicon substrates with a second silicon substrate on the top surface of the CNTs. This is performed for CNTs with heights of 61, 315, and 683 μm. Using this nanoindentation-based method, the measured effective compressive modulus values ranged from 0.12 to 1.2 MPa. Additionally, the effect of end constraints is investigated by testing the CNT forests with and without attaching the second rigid substrate to the CNT forest tips with an adhesive. It was found that attaching the second substrate to the CNT tips with an adhesive increases the measured effective compressive modulus by 10–30%. The second experimental method in this study is semi in situ and uses a scanning electron microscope and a compressive fixture with load cell. This method shows that under uniform compressive loading, the CNT forests demonstrate a local folding form of deformation with initial folding occurring near the growth substrate. The effective compressive modulus measured using this method was 0.11 MPa for 133 μm tall CNT forests.     

Diameter-dependent kinetics of activation and deactivation in carbon nanotube population growth

We reveal that the collective growth of vertically aligned carbon nanotube (CNT) forests by chemical vapor deposition (CVD) is governed by the size-dependent catalytic behavior of metal nanoparticles, which can be quantitatively related to the activation and deactivation kinetics of subpopulations of CNTs within the forest. We establish this understanding by uniquely combining real-time forest height kinetics with ex situ synchrotron X-ray scattering and mass-attenuation measurements. The growing CNT population is divided into subpopulations, each having a narrow diameter range, enabling the quantification of the diameter-dependent population dynamics. We find that the mass kinetics of different subpopulations are self-similar and are represented by the S-shaped Gompertz model of population growth, which reveals that smaller diameter CNTs activate more slowly but have longer catalytic lifetimes. While competition between growth activation and deactivation kinetics is diameter-dependent, CNTs are held in contact by van der Waals forces, thus preventing relative slip and resulting in a single collective growth rate of the forest. Therefore, we hypothesize that mechanical coupling gives rise to the inherent tortuosity of CNTs within forests and possibly causes structural defects which limit the properties of current CNT forests in comparison to pristine individual CNTs.     

Waviness reduces effective modulus of carbon nanotube forests by several orders of magnitude

Waviness is invariably present in vertically-aligned Carbon Nanotubes (CNTs) regardless of how controlled the fabrication process is. This study, using experiments and models, shows that such inherent waviness is the main mechanism by which the effective modulus of CNTs is reduced by several orders of magnitude. At this time, most studies have shown that the compliant mechanical response of the CNT forests under compressive loading is due to bending and buckling of CNTs as well as the variation of CNT density throughout the forest height. Subjecting CNT forests to tensile loads as well as to compressive loads, it is shown here that the high compliance of CNT forests is due to the inherent waviness of individual CNTs, and not necessarily due to bending and buckling of CNTs. The experimental findings are also supported through analytical models and numerical models that show that the CNT wavy geometry causes the CNTs to have 4–5 orders of magnitude greater compliance than a straight CNT.     

Length dependent foam-like mechanical response of axially indented vertically oriented carbon nanotube arrays

The axial compressive mechanical response of substrate-supported carbon nanotube (CNT) arrays with heights from 35 to 1200 μm is evaluated using flat punch nanoindentation with indentation depths to 200 μm. The compressive behavior is consistent with that of an open-cell foam material with array height playing a role similar to that of occupation density for traditional foam. Mechanical yielding of all arrays is initiated between 0.03 and 0.12 strain and arises from localized coordinated plastic buckling. For intermediate CNT array heights between 190 and 650 μm, buckle formation is highly periodic, with characteristic wavelengths between 3 and 6 μm. Buckle formation produced substantial force oscillations in both the compressive and lateral directions. The compressive elastic modulus of the arrays is obtained as a continuous function of penetration depth and attains a value between 10 and 20 MPa for all arrays during mechanical yield. A qualitative model based upon concepts of cellular foam geometry is advanced to explain the observed CNT buckling behavior.     

Controlling the growth morphology of carbon nanotubes: from suspended bridges to upright forests

We have developed two strategies to produce carbon nanotubes (CNTs) from low-density surface growth to high-density forest growth. We have demonstrated that by introducing a C(2)H(2) pulse at the beginning of the growth, where methane is still used as the main carbon feeding gas, the growth tendency of CNTs can be changed and the resulting growth morphology will vary from surface growth to forest growth. Similarly, the growth morphology can be changed when the growth temperature is raised. The further characterization via Raman spectroscopy indicates that an increasing C(2)H(2) pulse time will lead to a rise of the D peak for as-grown CNTs, due to the formation of more multi-walled CNTs and the amorphous carbon contamination introduced by extra C(2)H(2), while a high growth temperature tends to produce high-quality CNTs and to reduce the amorphous carbon contamination. Furthermore, by appropriately adjusting the growth temperature and controlling the C(2)H(2) pulse time, we have managed to produce both suspended CNT bridges and upright forests within a single growth procedure and to form suspended pristine CNT transistors with a relatively high yield. In addition, the electrical properties of these CNT nanostructures have been investigated by electrical transport and scanning photocurrent measurements.     

Multiple “optimum” conditions for Co-Mo catalyzed growth of vertically aligned single-walled carbon nanotube forests

Carbon nanotubes (CNTs) were grown directly on substrates by alcohol catalytic chemical vapor deposition using a Co-Mo binary catalyst. Optimum catalytic and reaction conditions were investigated using a combinatorial catalyst library. High catalytic activity areas on the substrate were identified by mapping the CNT yield against the orthogonal gradient thickness profiles of Co and Mo. The location of these areas shifted with changes in reaction temperature, ethanol pressure and ethanol flow rate. Vertically aligned single-walled CNT (SWCNT) forests grew in several areas to a maximum height of ca. 30 μm in 10 min. A pure Co catalyst yielded a vertically aligned SWCNT forest with a bimodal diameter distribution. The effects of Mo on the formation of catalyst nanoparticles and on the diameter distribution of SWCNTs are discussed and Mo as thin as a monolayer or thinner was found to suppress the broadening of SWCNT diameter distributions.     

垂直碳纳米管阵列的生长控制研究进展

垂直碳纳米管（VACNT）阵列由于具有良好的排列、优异的导电导热能力、高比表面积、高纯度等优点而得到广泛应用。本文概述了用于碳纳米管阵列生长的热化学气相沉积（CVD）制备方法的最新进展,重点阐述了CVD法生长碳纳米管阵列的动力学与生长终止机理,指出CVD过程中的催化剂形貌演化是引发碳纳米管阵列生长停止的重要原因。介绍了人们通过生长条件控制与催化剂设计等方法调控碳纳米管阵列结构（包括管壁数、管径和密度）方面取得的进展,指出碳纳米管阵列的大批量制备及结构参数的精确调控是未来发展的 重点。     

Strain engineering for thermal conductivity of single-walled carbon nanotube forests

We perform classical molecular dynamics simulations to investigate the mechanical compression effect on the thermal conductivity of the single-walled carbon nanotube (SWCNT) forest, in which SWCNTs are closely aligned and parallel with each other. We find that the thermal conductivity can be linearly enhanced by increasing compression before the buckling of SWCNT forests, but the thermal conductivity decreases quickly with further increasing compression after the forest is buckled. Our phonon mode analysis reveals that, before buckling, the smoothness of the inter-tube interface is maintained during compression, and the inter-tube van der Waals interaction is strengthened by the compression. Consequently, the twisting-like mode (good heat carrier) is well preserved and its group velocity is increased by increasing compression, resulting in the enhancement of the thermal conductivity. The buckling phenomenon changes the circular cross section of the SWCNT into ellipse, which causes effective roughness at the inter-tube interface for the twisting motion. As a result, in ellipse SWCNTs, the radial breathing mode (poor heat carrier) becomes the most favorable motion instead of the twisting-like mode and the group velocity of the twisting-like mode drops considerably, both of which lead to the quick decrease of the thermal conductivity with further increasing compression after buckling.     

Carbon nanotube bumps for the flip chip packaging system

ABSTRACT: Carbon nanotube [CNT] interconnection bump joining methodology has been successfully demonstrated using flip chip test structures with bump pitches smaller than 150 μm. In this study, plasma-enhanced chemical vapor deposition approach is used to grow the CNT bumps onto the Au metallization lines. The CNT bumps on the die substrate are then 'inserted' into the CNT bumps on the carrier substrate to form the electrical connections (interconnection bumps) between each other. The mechanical strength and the concept of reworkable capabilities of the CNT interconnection bumps are investigated. Preliminary electrical characteristics show a linear relationship between current and voltage, suggesting that ohmic contacts are attained.     

Influence of lengths of millimeter-scale single-walled carbon nanotube on electrical and mechanical properties of buckypaper

The electrical conductivity and mechanical strength of carbon nanotube (CNT) buckypaper comprised of millimeter-scale long single-walled CNT (SWCNT) was markedly improved by the use of longer SWCNTs. A series of buckypapers, fabricated from SWCNT forests of varying heights (350, 700, 1,500 μm), showed that both the electrical conductivity (19 to 45 S/cm) and tensile strength (27 to 52 MPa) doubled. These improvements were due to improved transfer of electron and load through a reduced number of junctions for longer SWCNTs. Interestingly, no effects of forest height on the thermal diffusivity of SWCNT buckypapers were observed. Further, these findings provide evidence that the actual SWCNT length in forests is similar to the height.     

Buckling initiation and displacement dependence in compression of vertically aligned carbon nanotube arrays

Carbon nanotube (CNT) arrays have shown the remarkable ability to react as foam-like structures and exhibit localized buckling coordinated within specific regions. Here, we report on the low-cycle compression of bulk vertically aligned CNT arrays to observe initiation and growth of the buckling as a function of compressive strain. A critical strain is found above which the buckling region length increased and below which it remained at or below the applied strain. As previously observed, the buckling region of the CNT array propagates from the surface where growth occurred, which, in the test specimen, is a free surface and later receives compressive contact by a polished silicon substrate. The results are corroborated with nanoindentation on the surfaces, which indicate a stiffening of the near surface with increasing applied strain. Observation and results of the buckling region nature are important for applications of nanotube arrays as energy absorbing cushions, tunable dampers, thermal contacts, or in sliding contact.     

Thickness-, alignment- and defect-tunable growth of carbon nanotube arrays using designed mechanical loads

We demonstrate the thickness-, morphology-, and defect-tunable growth and simultaneous integration of aligned carbon nanotube (CNT) arrays using a novel microscale platform. This platform consists of a micromechanical spring of desired stiffness, which applies a precise vertical load to a vertically aligned CNT array during its growth by chemical vapor deposition (CVD). The micromechanical spring is strained by the extrusive growth force output from the aligned CNT array during its growth and, at the same time, exerts a mechanical restoring force against the buckling resistance of the CNTs. This application of a designed vertical load on the CNTs allows modulation of the thickness and degree of alignment of the CNT array, as well as the structural quality of the individual CNTs. Consequently, the electrical resistance between two opposing CNT arrays can be tuned by adjusting the vertical load. In addition, their sensing responsiveness toward chemical species can also be enhanced by applying larger vertical load on the CNTs. In contrast to conventional growth methods for producing aligned CNT arrays, our approach offers an efficient way for the growth engineering and on-chip integration of aligned CNT arrays in a single step of the CVD.     

Liquefied petroleum gas containing sulfur as the carbon source for carbon nanotube forests

Carbon nanotube (CNT) forests were obtained from liquefied petroleum gas (LPG) as the carbon source in the floating catalyst process. The CNTs obtained in the forest had a thinner diameter and lower growth rate than those obtained with other carbon sources, which was attributed to the existence of sulfur in the LPG. The use of unpurified LPG provides a controllable way to synthesize a CNT forest at low cost.     

Localized synthesis of horizontally suspended carbon nanotubes

Horizontally suspended carbon nanotube (CNT) forests and bundle arrays were grown locally from selected trench sidewalls with the assistance of microheaters integrated on the silicon substrate. The patterned iron (Fe) catalyst layer was deposited onto these trench sidewalls by the tilted electron beam evaporation through a shadow mask placed on top of the trenches. Only local areas of the substrate were heated to high temperatures for the CNT growth by integrated microheaters, while most other areas remained at much lower temperatures, improving the process compatibility of the CNT growth with CMOS semiconductor technologies. The length and quality of the CNTs display a strong dependence on the local growth temperatures, and generally long and high-quality CNTs are grown in the high temperature regions.     

Evolution of directly-spinnable carbon nanotube catalyst structure by recycling analysis

Carbon nanotubes (CNTs), particularly of the substrate-grown directly spinnable variety, have a vast number of potential applications. We have devised a recycling methodology to enable the evolution of catalyst morphology and activity and its effect on CNT growth quality and failure to be studied incrementally. Direct spinnability is particularly sensitive to many variables and so provides verification that each cycle is essentially identical.Two processes, utilising acetylene with and without hydrogen, are studied. Acetylene alone gives four cycles of spinnable CNTs before failing abruptly at the fifth at which point catalyst particle (and CNT) areal density increase while CNT diameter and length fall sharply. In contrast, addition of hydrogen doubles the initial growth rate but spinnability declines on the third cycle and fails on the fourth as catalyst (and CNT) areal density decreases sharply. CNT length also falls although diameter increases.Our observations support a proposed ‘sinking plateau’ model of catalyst behaviour where growth rate is driven by the essentially flat accumulation area or ‘plateau plain’ surrounding a local high spot (active catalyst particle). The growth rate remains stable until the plateau plain drops below the substrate surface through diffusion at the base, at which point it falls sharply.     

Growth evolution of rapid grown aligned carbon nanotube forests without water vapor on Fe/Al2O3/SiO2/Si substrate

This paper presents the growth evolutions in terms of the structure, growth direction and density of rapid grown carbon nanotube (CNT) forests observed by scanning and transmission electron microcopies (SEM/TEM). A thermal CVD system at around 700 °C was used with a catalyst of Fe films deposited on thin alumina (Al₂O₃) supporting layers, a very fast raising time to the growth temperature below 25 °C/s, and a carbon source gas of acetylene diluted with hydrogen and nitrogen without water vapor. Activity of Fe catalyst nanoparticles was maintained for 5 min during CVD process, and it results in CNT forests with heights up to 0.6 mm. SEM images suggest that the disorder in CNT alignment at the initial stage of CNTs plays a critical role in the formation of continuous CNT growth. Also, the prolonged heating process leads to increased disorder in CNT alignment that may be due to the oxidation process occurring at the Fe nanoparticles. TEM images revealed that both double- and few-walled CNTs with diameters of 5-7 nm were obtained and the CNT density was controlled by thickness of Fe catalytic layer. The number of experiments at the same conditions showed a very good repeatability and reproducibility of rapid grown CNT forests.     

Carbon nanotubes/SiC whiskers composite prepared by CVD method

Selective growth of carbon nanotubes (CNTs) on silicon carbide (SiC) substrate will create some new applications in composites and electronic devices by combining their mechanical and physical properties. Multi-walled CNTs were successfully grown on SiC whiskers using a conventional xylene–ferrocene chemical vapor deposition process. A thin oxide layer was created on the surface of the SiC whiskers by high-temperature annealing in air before CNT growth. The effect of catalyst morphology and chemistry on the growth of CNTs was analyzed. Our technique may be further applied to the controlled growth of CNTs on any other SiC substrates.     

Prevention of Si-contaminated nanocone formation during plasma enhanced CVD growth of carbon nanotubes

We investigated the growth behavior and morphology of vertically aligned carbon nanotubes (CNTs) on silicon (Si) substrates by direct current (DC) plasma enhanced chemical vapor deposition (PECVD). We found that plasma etching and precipitation of the Si substrate material significantly modified the morphology and chemistry of the synthesized CNTs, often resulting in the formation of tapered-diameter nanocones containing Si. Either low bias voltage (∼500 V) or deposition of a protective layer (tungsten or titanium film with 10-200 nm thickness) on the Si surface suppressed the unwanted Si etching during growth and enabled us to obtain cylindrical CNTs with minimal Si-related defects. We also demonstrated that a gate electrode, surrounding a CNT in a traditional field emitter structure, could be utilized as a protection layer to allow growth of a CNT with desirable high aspect ratio by preventing the nanocone formation.