Growth Of Vertical Carbon Nanotubes Research Articles

Application of multi-scale pore regulation for high thermal conductivity foam reinforcements in energy storage

A continuous diamond film layer was initially deposited on the surface of the foam skeleton by chemical vapor deposition (CVD), followed by the vertical growth of carbon nanotubes (CNTs) on diamond film using nickel particles as a catalyst. The CNTs extended into the interior of the pores inside the foam skeleton as a secondary heat transfer pathway. The effect of different pore densities on the thermal conductivity and latent heat of phase change composites (PCMs) was explored. The obtained diamond foam/carbon nanotube/paraffin composites (DF-CNT/PWs) was demonstrated to achieve an ultra-high conductivity of 5.3 W/m·K, which is 19.6 times that of the paraffin matrix. The latent heat of the DF-CNT/PWs is 83.37 J·g−1. In addition, finite element simulation confirmed that the thermal conductivity enhancer system could significantly improve the uniformity of heat transfer inside the foam skeleton.

Self-assembled nanostructures of 3D hierarchical faceted-iron oxide containing vertical carbon nanotubes on reduced graphene oxide hybrids for enhanced electromagnetic interface shielding

The self-assembled three dimensional (3D) hybrids nanostructure containing uniform growth of vertical carbon nanotubes (VCNTs) with faceted iron oxide nanoparticles (f-Fe3O4 NPs) on the surfaces of reduced graphene oxide nanosheets (rGO NSs) is achieved using microwave assisted approach. The formation of hierarchical 3D f-Fe3O4-VCNTs@rGO hybrids, using microwave method is a rapid, simple, and inexpensive synthetic route. First, the VCNTs grow with help of Fe NPs, and after oxidizing of Fe NPs in form of f-Fe3O4 NPs, the growth has terminated resulting in formation of small size (<500 nm) VCNTs containing f-Fe3O4 NPs on its tip. The defect- and oxygen-rich sites of rGO NSs favor the heterogeneous nucleation and growth of f-Fe3O4 NPs on the tip of VCNTs. The synthesized 3D f-Fe3O4-VCNTs@rGO hybrid shows the improved electromagnetic interference (EMI) for microwave shielding effectiveness (SE) as compared to both rGO NSs and Fe3O4 NPs@rGO NSs materials. This 3D f-Fe3O4-VCNTs@rGO hybrid demonstrates the shielding effectiveness value more than ∼25 dB as compared to Fe3O4 NPs@rGO NSs for 1.0 mm thin film of 3D f-Fe3O4-VCNTs@rGO hybrids in microwave X-band (8.2–12.4 GHz). This applied microwave synthesis approach for 3D f-Fe3O4-VCNTs@rGO hybrids is simple, fast, reproducible and scalable for advanced EMI shielding materials. It can be concluded that the faceted Fe3O4 NPs on the tip of VCNTs which are grown in-situ on rGO NSs shows synergetic performance for EMI shielding elements in advanced application areas like spacecraft and aircraft.

Monodisperse Pattern Nanoalloying for Synergistic Intermetallic Catalysis

Nanoscale alloys attract enormous research attentions in catalysis, magnetics, plasmonics and so on. Along with multicomponent synergy, quantum confinement and extreme large surface area of nanoalloys offer novel material properties, precisely and broadly tunable with chemical composition and nanoscale dimension. Despite substantial progress of nanoalloy synthesis, the randomized positional arrangement and dimensional/compositional inhomogeneity of nanoalloys remain significant technological challenges for advanced applications. Here we present a generalized route to synthesize single-crystalline intermetallic nanoalloy arrays with dimensional and compositional uniformity via self-assembly. Specific electrostatic association of multiple ionic metal complexes within self-assembled nanodomains of block copolymers generated patterned monodisperse bimetallic/trimetallic nanoalloy arrays consisting of various elements, including Au, Co, Fe, Pd, and Pt. The precise controllability of size, composition, and intermetallic crystalline structure of nanoalloys facilitated tailored synergistic properties, such as accelerated catalytic growth of vertical carbon nanotubes from Fe-Co nanoalloy arrays.

Catalyst-assisted vertical growth of carbon nanotubes on Inconel coated commercial copper foil substrates versus sputtered copper films

We have compared the growth of multi-walled carbon nanotubes using thermal chemical vapor deposition (CVD) on two types of substrates, copper foils and sputtered copper films. In both cases an initial 12nm thin film of Inconel is first deposited on the Cu before growing the nanotubes. The Inconel thin film can act as both a catalyst for nanotube growth as well as a support for the additional Fe catalyst that is supplied in the form of ferrocene during CVD growth. The surfaces of the underlying copper substrates are very different and play a role in the resulting carbon nanotube density. A quantitative analysis of the density and alignment of the resulting carbon nanotubes using scanning electron microscopy shows that the smooth surface of the sputtered copper substrate leads to improvement in vertical growth and density of nanotubes as opposed to the much rougher electropolished Cu foil. We show that this is related to the differences in catalyst islands distributions and graphitic crystallinity seen on the surfaces of the two types of substrates after heating the samples in the CVD chamber. This demonstrates that the surface of the starting substrate plays an important role in the subsequent catalyst surface distribution and therefore the resulting nanotube density.

Growth of vertical carbon nanotubes according to the Al2O3 buffer layer preparation

Carbon nanotubes (CNTs) have been extensively studied for several decades due to its promising properties. In this study, effects of the Al2O3 buffer layer on the growth of the wellorganized CNTs forest were studied. The buffer layers were prepared by the two methods, direct sputtering using Al with oxygen feeding and annealing the sputtered pure Al films to obtain Al2O3 films. FE-SEM and AFM results showed that the surface of the buffer layer by the first method was more clean and smooth. The CNTs forest using the first buffer layer showed better results both in the growth length and CNTs quality.

Nanostructuring Ultra-thin Co Films to Active Catalyst Particles for Vertically Aligned Single-walled CNT Growth

In the field of material synthesis using chemically-derived technique, nanostructuring metal catalyst particles towards high quality production of carbon nanotubes (CNTs) has been very attractive. In this work, cobalt (Co) which used as catalyst for vertical growth of CNTs were studied by means of transmission electron microscopy (TEM), scanning electron microscopy (SEM), and X-ray photoelectron spectroscopy (XPS). Aluminum (Al) films (20 nm) were thermally-oxidized to form aluminum oxide (Al-O) to support 0.5 nm Co catalyst during CNT growth. In growing CNTs by using chemical vapor deposition (CVD) technique, the role and characters of all involving materials are crucial to the growth result. From the Co/Al-O substrate and at 650°C of CVD temperature, 33-μm thick of vertically aligned single-walled CNT (VA-SWCNT) forest has been grown. TEM particle analysis revealed that the Co particles have an average of 3.50 nm experimentally and in principle favored the growth of highly demanded VA-SWCNTs. The as-prepared Co particles are suggested chemically active for the CNT growth. In addition, XPS analysis confirmed the surface chemical state of Co particles prior to the VA-SWCNT growth using ethanol based CVD system.

X-Ray and Morphological Characterization of Al-O Thin Films Used for Vertically Aligned Single-Walled Carbon Nanotube Growth

Al oxide (Al-O) films used as catalyst-support layer for vertical growth of carbon nanotubes (CNTs) were characterized by means of X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), transmission, and scanning electron microscopies (TEM and SEM). EB-deposited Al films (20 nm) were thermally-oxidized at 400 o C (10 min, static air) to produce the surface structure of Al-O. The Al-O was found amorphous and after the incorporation with Co catalyst, the grown CNTs were twisted together before vertically grown. The prepared Al-O surface is an electron-acceptor-dominated (oxygen-rich) surface where the formation of active catalyst could be enhanced to promote the vertically aligned CNT growth.

High-Speed in Situ X-ray Scattering of Carbon Nanotube Film Nucleation and Self-Organization

The production of high-performance carbon nanotube (CNT) materials demands understanding of the growth behavior of individual CNTs as well as collective effects among CNTs. We demonstrate the first use of grazing incidence small-angle X-ray scattering to monitor in real time the synthesis of CNT films by chemical vapor deposition. We use a custom-built cold-wall reactor along with a high-speed pixel array detector resulting in a time resolution of 10 msec. Quantitative models applied to time-resolved X-ray scattering patterns reveal that the Fe catalyst film first rapidly dewets into well-defined hemispherical particles during heating in a reducing atmosphere, and then the particles coarsen slowly upon continued annealing. After introduction of the carbon source, the initial CNT diameter distribution closely matches that of the catalyst particles. However, significant changes in CNT diameter can occur quickly during the subsequent CNT self-organization process. Correlation of time-resolved orientation data to X-ray scattering intensity and height kinetics suggests that the rate of self-organization is driven by both the CNT growth rate and density, and vertical CNT growth begins abruptly when CNT alignment reaches a critical threshold. The dynamics of CNT size evolution and self-organization vary according to the catalyst annealing conditions and substrate temperature. Knowledge of these intrinsically rapid processes is vital to improve control of CNT structure and to enable efficient manufacturing of high-density arrays of long, straight CNTs.

A Self-Defined Hollow Needle Formation on Silicon Membranes with the Aids of Carbon Nanotubes

A self-defined method for the formation of hollow needles and micro-cylindrical structures on silicon substrates is reported which uses the combination of high aspect ratio vertical etching of silicon with a small angle vacuum deposition technique. These structures can be used as the media to transfer gas and liquid through their tiny holes. The fabrication is further continued with the growth of carbon nanotubes on the already created silicon-based features to form a cage-like cylindrical structure. In addition, the growth of vertical carbon nanotubes just inside the cup-like structures has been demonstrated without a need for an extra lithography step.

Nickel Catalyst-Assisted Vertical Growth of Dense Carbon Nanotube Forests on Bulk Copper

Vertical growth of carbon nanotubes using thermal chemical vapor deposition (CVD) is demonstrated on bulk copper substrates by first sputtering a thin Ni film on the surface of copper. Vertical growth of carbon nanotubes occurred when the nickel film thickness was 20 nm and the carbon nanotubes were grown using a xylene source and additional ferrocene catalyst during CVD. These results show the effectiveness of this method in directly integrating carbon nanotubes with highly conductive substrates for applications where a conductive carbon nanotube network is desirable.

Effects of a Sandwich-Like Catalyst on the Vertical Growth of Carbon Nanotubes Synthesized by Using Chemical Vapor Deposition

Effects of a Sandwich-Like Catalyst on the Vertical Growth of Carbon Nanotubes Synthesized by Using Chemical Vapor Deposition

A Computational Study of Vertical Partial-Gate Carbon-Nanotube FETs

A vertical partial gate carbon nanotube (CNT) field-effect transistor (FET), which is amenable to the vertical CNT growth process and offers the potential for a parallel CNT array channel, is simulated using a self-consistent atomistic approach. We show that the underlap between the gate and the bottom electrode (required for isolation between electrodes) is advantageous for transistor operation because it suppresses ambipolar conduction. A vertical CNTFET with a gate length that covers only 1/6 of the channel length has a much smaller minimum leakage current than one without underlap, while maintaining comparable on current. Both n-type and p-type transistor operations with balanced performance metrics can be achieved on a single partial gate FET by using proper bias schemes. Even with a gate underlap, it is demonstrated that increasing the CNT diameter still leads to a simultaneous increase of on current and minimum leakage current. Along with a partial gate, the simulated transistor features a significant amount of air between the surface of the channel CNT and the gate insulator, as is caused by the vertical CNT growth process. Filing this pore with a high-k insulator is shown to have the potential to decrease the on current, due to electrostatic phenomena at the source-channel contact.

Vertically Aligned CNTs Embedded in Cr/TiO2 Membranes For Realization of Ion Sources

AbstractPECVD-grown carbon nanotubes on (100) silicon membranes have been realized and exploited for electron and ion emission applications. The growth of CNT's is achieved by a mixture of hydrogen and acetylene gases in a CVD reactor and a 5-10nm thick nickel is used as the seed for the growth. The presence of a DC-plasma yields a vertical growth of carbon nanotubes. The as-grown nanotubes are encapsulated by means of an insulating TiO2 layer. The formation of a thin membrane is possible by means of a chemical anisotropic etching technique. The membrane is then removed from the back side to fully suspend the CNT-holding TiO2 layer. Upon exposure to a plasma ashing step the nanotubes are partially removed and a both-end opened hollow nanostructure is formed which can be used as a miniaturized ion source. The CNT-holding substrate can be exploited as a grid to extract the ions from an ionization chamber just underneath the membrane. Applying a proper accelerating electric field, positive ions made inside a DC discharge cavity can form a beam-shape emission of ions towards the opposite negative electrode. The beam is well suited for a source of ion lithography. In which, the emission has the ability of direct writing on a photo-resist coated substrates. Preliminary nano-scale dots have been created with sizes between 50 and 80nm. Scanning electron microscopy has been used to investigate the results.

Effect of supporting layer on growth of carbon nanotubes by thermal chemical vapor deposition

Selective growth of vertically aligned and highly dense carbon nanotubes was achieved by using thermal chemical vapor deposition via careful selection of a thin catalyst layer and an appropriate supporting layer. It was found that carbon nanotube growth was significantly enhanced when tantalum was used as the supporting layer on which a thin iron catalyst was deposited. Cross-sectional transmission electron microscopy revealed a Stranski-Krastanov mode of iron island growth on tantalum with relatively small contact angles controlled by the relative surface energies of the supporting layer, the catalyst, and their interface. The as-formed iron island morphology promoted vertical growth of carbon nanotubes.

Controlling the Morphology of Carbon Nanotube Films by Varying the Areal Density of Catalyst Nanoclusters Using Block‐Copolymer Micellar Thin Films

In spite of much research progress in the science and synthesis of carbon nanotubes (CNTs), control over the location and orientation of CNTs on substrates remains a major challenge. A key breakthrough was the synthesis of vertically aligned CNT (VA-CNT) arrays using thermal chemical vapor deposition (CVD) and plasma-enhanced CVD, where the CNTs self-orient perpendicular to the substrate surface due to initial crowding and continue to grow upward in this direction. These CNT arrays have wide-ranging applications, including membranes, heat dissipation, electrical interconnects, and nanoelectronics. The catalysts for synthesis of VACNTs are commonly prepared by sputtering or evaporating a thin metal film onto a substrate, which dewets to form catalyst nanoparticles at an elevated temperature prior to growth. While these catalysts are easily prepared and patterned by shadow masking or lithography, these approaches are not easily able to create nanocluster catalysts that have monodisperse diameters and quantifiable areal densities. In thin metal films, both the nanocluster size and areal density are coupled to the film thickness, and the annealing procedure affects the size, density, and the chemical state of the nanoclusters. Recently, Huh et al. demonstrated a route for controlling the density of CNT growth using varying densities of colloidal cobalt nanoparticles; however, due to nanoparticle coalescence their route does not enable precise quantification of nanocluster areal density and leads to a broad distribution of CNT diameters. Zhang et al. also recently demonstrated the control of CNT growth by varying the density of Co–Mo nanoparticles, although their route is unable to independently vary the diameter and the areal density of nanoparticles. We employ a methodology for synthesizing iron oxide nanoclusters that utilizes micelles formed by the amphiphilic block copolymer, polystyrene-b-poly(acrylic acid) (PS-bPAA). This catalyst system has significant value because it enables the creation of nanocluster arrays of a chosen metal species, with independent control of the nanocluster diameter and areal density. In previous work, nanocluster diameters were varied between 5 and 16 nm and the areal density was varied from 6.0 × 10 to 1 × 10 cm, although variation outside of these ranges is easily accessible. At higher areal densities the nanoclusters are hexagonally ordered. Further, as we have presented separately, the nanocluster arrays can be patterned on the micrometer length scale using microcontact printing. Here, we utilize this system to create arrays of uniform-diameter iron oxide nanoclusters, with quantifiable areal densities that can be varied over more than an order of magnitude. We achieve vertical CNT growth from our catalyst system through appropriate selection of the substrate, catalyst preparation procedure, and reaction conditions. To the best of our knowledge, our work is the first example of vertical growth of CNTs from a block-copolymer-based catalyst. Because this catalyst system allows for precise quantification of the nanocluster areal density, we can estimate the percentage of nanoclusters that nucleate the growth of a CNT. By uniformly varying the areal density of iron oxide nanoclusters on the substrate surface, we manipulate the morphology of the CNT film from a tangled and sparse arrangement of individual CNTs, through a transition region with locally bunched and self-aligned CNTs, to rapid growth of thick vertical CNT films. The procedure for synthesizing the iron oxide nanocluster arrays utilized spherical micelles formed by PS-b-PAA in toluene, which were loaded with FeCl3, and then spin-cast onto a substrate (see Experimental Section). The polymer thin film was then treated in oxygen plasma to remove the organic components, leaving an iron oxide nanocluster array, as shown in Figure 1A. Previous studies have demonstrated that the iron oxide nanocluster structure is Fe2O3. [22,25] The molecular weight and metal-loading ratios of Figure 1A led to iron oxide nanocluster arrays with diameters of 16 ± 1.6 nm that were arranged in a quasihexagonal array with an areal density of 6.0 × 10 cm. The root-mean-square roughness of the nanocluster arrays was 2.2 nm, as determined from using atomic force microscopy (AFM). C O M M U N IC A TI O N S

Transplanting Assembly of Carbon Nanotubes

The manufacture of nanotechnology products requires assembly of nano scale components to macro/micro-scale systems, which is a major challenge in attempting to industrialize nanotechnologies. A new approach to assemble nanostructures is developed with a control of individual nanostructure's location and alignment in a long-range order. A concept of transplanting assembly has been developed to embed carbon nanotubes into micro-scale polymer blocks, which then serve as carriers for the embedded carbon nanotubes, facilitating the handling and assembly of them. This technique includes vertical growth of carbon nanotubes, nanopellet casting, planarization, nanopellet separation, transplantation and bonding. This assembly technology can be broadly applied to manufacture many nano scale products including carbon nanotube tipped atomic force microscope probes.

PECVD‐Grown Carbon Nanotubes on Silicon Substrates Suitable for Realization of Field‐Emission Devices

We report vertical growth of carbon nanotubes using a plasma enhanced CVD technique suitable for realization of field‐emission electronic devices on silicon substrates. Vertically grown carbon nanotubes are formed on silicon substrates at temperatures ranging from 500 to 650°C and at a pressure of 3.5 torr using a mixture of C2H2 and H2 and in the presence of a DC plasma with an applied voltage between 300 and 500 V. Vertical tubes with a height up to 10 µm have been realized. The growth conditions and the emission of electrons from nanotubes have been investigated. By varying the growth temperature during the deposition process, conical structures are formed. The sharp points of such structures make them suitable for realization of field‐emission devices and transistors.

Transplanting carbon nanotubes

Current efforts of guided growth of carbon nanotubes alone cannot make large-scale and directed assembly of them. The nanopelleting concept overcomes this limitation by embedding carbon nanotubes in microscale pellets that can be transplanted readily. This technique includes vertical growth of carbon nanotubes, pellet casting, planarization, pellet separation, and transplantation. A specific manufacturing process is developed and tested with favorable results. This technology will enable directed assembly of carbon nanotubes in a long-range order.