Growth Of Vertically Aligned Carbon Nanotubes Research Articles

Integration of vertically-aligned carbon nanotubes with superconducting nanowire single photon detectors

We report on initial fabrication efforts in the integration of superconducting nanowire single-photon detectors (SNSPDs) with vertically aligned carbon nanotubes (VACNTs) with the goal of creating a wideband single-photon detector. SNSPDs provide high detection efficiencies and low dark count rates, while the VACNTs are excellent broadband optical absorbers. Combining these technologies could potentially enable the development of highly sensitive and versatile optical sensors for a variety of applications, such as spectroscopy, optical communication, and imaging in light starved environments. We developed two fabrication processes for the integration of VACNTs on SNSPDs. The first involves capping the SNSPDs with a protective layer and growing the VACNTs directly above nanowires. Thermal and electrical characterizations of the devices demonstrated a degradation of the superconducting qualities of the SNSPDs. The second process involved suspending the SNSPDs on a thin membrane via a backside etch, where VACNTs were then grown on the backside of the membranes below the nanowires. The membrane style devices showed no degradation in the superconducting properties of the nanowires. Measurements of the membrane style devices before and after the VACNT growth display similar superconducting properties and photon count rates.

Air and O2-Assisted Catalytic VACNT Growth Optimization for Uniformity and Throughput

The development of an optimized air or O2-assisted multi-wall vertically aligned carbon nanotubes (VACNT) process that adjusts the vertical height profile of a standard H2O vapor-assisted VACNT process is reported. The effect of the air or O2 chemical vapor deposition (CVD) precursor flow rate, the catalytic Fe layer thickness, the process growth temperature, and the H2/C2H4 ratio on VACNT length was first investigated to find the optimum growth conditions. Spatial distribution height mapping of VACNT structures on six patterned 4′′ catalyst Si wafers prepared with a 70–90 min long O2-assisted growth step shows an average growth height of 1.8–2.2 mm, with a standard deviation of less than 10%. Characterization techniques included Raman spectroscopy, scanning electron microscopy (SEM), and spatial height mapping analysis for a range of Fluid channel Array Brick (FAB) components with a length of 30 mm, a width range of 2.5–15 mm, a fluid channel diameter range of d = 5–100 mm, and a fluid channel closest gap range of g = 5–50 mm. A significant finding is that the O2-assisted VACNT growth process optimization efforts enable 2 mm parts processing with square edges, flat top surfaces, uniform height tolerances, and maximum catalyst wafer utilization for application in engineering devices.

A comparative study of high density Vertically Aligned Carbon Nanotubes grown onto different grades of aluminum – Application to supercapacitors

Vertically aligned carbon nanotubes (VACNT) have received extensive attention as electrode materials for supercapacitors owing to their superior features: no formatting or binder needed, anisotropy. They can be obtained by aerosol-assisted Catalytic Chemical Vapor Deposition (CCVD), a cost-effective and roll-to-roll compatible process, directly onto aluminum current collectors by lowering their growth temperature. In this paper, a comparative study of high density VACNT grown onto different grades of aluminum collectors is performed. Influence on electrochemical performances of carpet thickness up to 170 μm and VACNT volumetric density ranging from 70 to 350 mg/cm3 is evaluated. VACNT electrodes exhibit high areal capacitance up to 360 mF/cm2 and 25 F/cm3 and average gravimetric capacitance is 45 F/g. At high scan rate volumetric density has low impact while increasing height of VACNT limit performances. Symmetrical supercapacitor based on VACNT/Al electrodes assembled in coin-cell exhibits high energy and power density with high potential difference of 2.7 V and great energy retention capacity (90% at 13 A/g). Such results outline the benefits of the VACNT growth process directly applicable to aluminum current collector meeting industrial requirements (high purity and thin thickness) and exhibit the huge potentialities of such electrode to realize in supercapacitor applications.

Corrugated, concaved vertically aligned carbon nanotube structures in impeded CVD growth conditions

Vertically Aligned Carbon Nanotubes (VACNT) are promising soft materials in advanced devices, including wearable strain sensors. VACNT uniform growth depends on nucleation, growth, and deactivation of metal seed catalysts by chemical vapor deposition (CVD) at specific process conditions. CVD is performed at controlled process conditions to maximize VACNT growth with iron seed catalysts of various sizes, optimize water vapor concentration for enhanced VACNT growth, and probe their morphology by creating competition of the hydrocarbon feedstock to Fe seed catalyst of selected layer thicknesses. The analytical studies include ellipsometry to measure thermal oxide, barrier layer, and catalyst layer thicknesses; Raman spectroscopy to assess the characteristic peaks of carbon nanotubes; as well as scanning electron microscopy to evaluate VACNT morphology. The findings show a maximum VACNT growth of 1284 µm at 1 nm Fe catalyst, optimum water vapor content at 12.5–20 SCCM, and predominantly concaved and corrugated, a tube chamber position-dependent VACNT structures with low Ar/H2 ratio and C2H4 gas rates.

Critical Role of the Acetylene Content and Fe/C Ratio on the Thickness and Density of Vertically Aligned Carbon Nanotubes Grown at Low Temperature by a One-Step Catalytic Chemical Vapor Deposition Process.

The present work explores the role of the carbon source content and the Fe/C ratio on the synthesis of vertically aligned carbon nanotubes (VACNTs) by one-step aerosol-assisted CCVD operated at a medium temperature (615 °C) on aluminum substrates. The main objective was to overcome the limitations of VACNT growth, constituting a drawback for applications requiring thick VACNTs. By using acetylene as carbon feedstock and ferrocene as a catalyst precursor, we demonstrate that when acetylene content is reduced to 1.5 vol%, it is possible to grow VACNT carpets up to 700 µm thick while maintaining constant VACNT growth for a long duration (up to 160 min). The carbon conversion yield is significantly improved when the acetylene content reaches 1.5 vol%. The Al surface roughness also influences VACNT growth. An optimum Fe/C ratio of 0.8 wt.% coupled with a low acetylene content gives the highest growth rate (5.4 µm/min) ever reported for a thermal aerosol-assisted CCVD process operated at such a low temperature. The CNT number density can be controlled by varying the Fe/C ratio, enabling high density growth (e.g., 1.3 × 1011 CNT/cm2).

Conducting Interface for Efficient Growth of Vertically Aligned Carbon Nanotubes: Towards Nano-Engineered Carbon Composite.

Vertically aligned carbon nanotubes (VACNT) are manufactured nanomaterials with excellent properties and great potential for numerous applications. Recently, research has intensified toward achieving VACNT synthesis on different planar and non-planar substrates of various natures, mainly dependent on the user-defined application. Indeed, VACNT growth has to be adjusted and optimized according to the substrate nature and shape to reach the requirements for the application envisaged. To date, different substrates have been decorated with VACNT, involving the use of diffusion barrier layers (DBLs) that are often insulating, such as SiO2 or Al2O3. These commonly used DBLs limit the conducting and other vital physico-chemical properties of the final nanomaterial composite. One interesting route to improve the contact resistance of VACNT on a substrate surface and the deficient composite properties is the development of semi-/conducting interlayers. The present review summarizes different methods and techniques for the deposition of suitable conducting interfaces and controlled growth of VACNT on diverse flat and 3-D fibrous substrates. Apart from exhibiting a catalytic efficiency, the DBL can generate a conducting and adhesive interface involving performance enhancements in VACNT composites. The abilities of different conducting interlayers are compared for VACNT growth and subsequent composite properties. A conducting interface is also emphasized for the synthesis of VACNT on carbonaceous substrates in order to produce cost-effective and high-performance nano-engineered carbon composites.

Analysis of the Continuous Feeding of Catalyst Particles during the Growth of Vertically Aligned Carbon Nanotubes by Aerosol-Assisted CCVD.

Aerosol-assisted catalytic chemical vapor deposition (AACCVD) is a powerful one-step process to produce vertically aligned carbon nanotubes (VACNTs), characterized by the continuous supply of the catalyst precursor (metallocene). The behavior of catalyst species all along the synthesis is essential for the continuous growth of VACNTs. It is there investigated through detailed observations and elemental analyses at scales of VACNT carpets and of individual CNTs. Our approach is based on two complementary experiments: quenching of the sample cooling, and sequential injection of two distinct metallocenes. Metal-based nanoparticles nucleated in the gas-phase during the whole synthesis duration are shown to diffuse in between the growing VACNTs from the top of the CNT carpet towards the substrate. They are much smaller than the catalyst particles formed on the substrate in the initial steps of the process and evidences are given that they continuously feed these catalyst particles at the VACNT roots. Particularly, the electron energy-loss spectroscopy (EELS) analyses of metal-based segments found into a single CNT show that the second injected metal is very gradually incorporated in the particle initially formed from the metal firstly injected. The feeding of the catalyst particles by the nanoparticles continuously nucleated in the gas-phase is therefore an essential feature of the base-growth of CNTs by AACCVD.

A simple method to grow millimeters long vertically aligned carbon nanotube forests

Growing long vertically aligned carbon nanotube (VACNT) forests have been achieved so far. However, it requires some additional steps or use of etchants to keep catalyst nanoparticles (NPs) active during VACNT growth in chemical vapor deposition (CVD). Here, we were able to grow a 2.6 mm long VACNT forest with a very simple and cost effective method without using an etchant or other complicated systems. Reducing the catalyst at lower temperature than the growth temperature before VACNT growth provided dense and uniform NPs formation. We observed that NPs, formed after low temperature reduction, did not coarsen at growth temperature at least for 20 min which extends catalyst NP lifetime. Moreover, we find that time varying forest height of VACNT is accurately represented by the Gompertz model of height evolution as a result of low temperature catalyst reduction otherwise it is represented by the monomolecular model. Additionally, use of very low carbon feedstock (10 sccm C2H4) enabled us to prevent carbon coating of active catalyst NPs and ensured a longer NP lifetime. Low carbon use also provided a cost effective and an eco-friendly synthesis of VACNT.

Investigation and Optimization of Temperature Dependent Parameters for Growing Millimeter-Long Vertically Aligned Carbon Nanotubes

This study aimed to investigate effects of temperature dependent parameters on vertically aligned carbon nanotube (VACNT) forest height such as catalyst treatment temperature and growth temperature and optimize these parameters to grow long VACNT forests. These growth parameters were examined and optimized on samples including three different thicknesses of Fe catalyst layers which catalyze VACNT growth. Heights of VACNT forests grown on different Fe catalyst layers linearly decreased at various rates with the increment of the catalyst treatment temperature from 500 ℃ to 800 ℃. Moreover, optimum growth temperature to grow long VACNTs was found to be independent from Fe catalyst layer thickness while their height distributions showed variations. As a result of optimization of these parameters, we have found that it is possible to grow millimeter long VACNT forests from all three Fe catalyst layers with substantially low gas flows in 30 min growth. Raman spectroscopy approved that disorder of grown VACNTs is very low. Use of low gas feedstock, achieved in this study, ensures green and economic production of VACNTs.

Hot-filament CVD Growth of Vertically-aligned Carbon Nanotubes on Support Materials for Field Electron Emitters

Carbon nanotubes are used in applications such as energy storages, electronic devices. For these applications, direct assembly of the well aligned and densely packed CNTs (VACNTs) on electrically conducting substrates is of interest. In this paper, we report a direct growth of the VACNTs on several substrates by using hot-filament chemical vapor deposition (hot-filament CVD) and a catalytic buffer-layer structure. The buffer layer was the Aluminum (Al) thin film used together with iron (Fe) as the catalyst. It was found that the underneath layer (Al) was critical for obtaining the VACNT forests with high purity and good contact. The as-fabricated VACNT/support-material structures are object to test field electron emission properties. Testing results of the VACNT samples demonstrated promising field emission characteristics in term of high emitting currents.

Atmospheric pressure atomic layer deposition of iron oxide nanolayer on the Al2O3/SiO2/Si substrate for mm-tall vertically aligned CNTs growth

Atmospheric pressure atomic layer deposition (AP-ALD) of iron oxide, as a catalyst for water-assisted chemical vapor deposition (WA-CVD) growth of vertically aligned carbon nanotubes (VA-CNTs), was studied. Fe2O3 film was deposited on the porous Al2O3/SiO2/Si substrate using ferrocene and O2 as precursors. The self-liming film growth was obtained at the temperature window of about 180–250 °C with the optimum dosing and purging duration of about 20 min. The rather longer dosing time here compared to the conventional ALD is attributed to the high pressure as well as high porosity of the substrate. Considering the lower deposition rate at the beginning of the growth, it is suggested that the substrate-inhibited growth mode takes place in this work. The grown films have been characterized by SEM, AFM, Raman, and XRR. Raman spectroscopy peaks were consistent with the formation of Fe2O3. SEM images showed that the uniform coating of Fe2O3 follows the Volmer–Weber growth mode. The catalyst thickness-dependent growth of VACNTs revealed that the deposition through 12 cycles at 250 °C provides sufficient thickness, which resulted in the growth of mm-tall VA-CNTs within a relatively short time of WA-CVD. The changes in the catalyst, i.e. surface roughness and particle size, as a function of WA-CVD time were investigated. The time-dependent evolutions were mainly due to the subsurface diffusion and Ostwald ripening that ultimately led to the growth termination of CNTs.

Catalytic-free growth of VACNTs for energy harvesting

The present study introduces a process to grow micro-honeycomb (µ-HC) vertically aligned carbon nanotubes (VACNTs) using thermal chemical vapor deposition technique. Methane is used as a source of carbon and hydrogen gas as a reducing agent. Where, the fabricated µ-HC structure reported in literature involves complex synthesis process and requires a catalyst layer, the novelty of the process used here lies in the fact that no catalyst layer is used for the growth of CNT network, rather copper foil is used as a substrate. The in-situ cracking of CNTs due to water treatment leads to the formation of µ-HC CNT network, which is confirmed by Raman spectroscopy. Further scanning electron microscopy analysis shows that the length of developed µ-HC CNT is ∼5 µm. Hexagonal µ-HC network shows more than 94% absorption in UV-Vis-NIR wavelength region. The designed process provides high-yield with a low-cost synthesis of vertically aligned CNTs having 3 D microarchitecture. The fabricated CNT network can be used as an electrode for supercapacitor, as an active layer in a photovoltaic cell and most of the energy harvesting devices.

The Roles of an Aluminum Underlayer in the Biocompatibility and Mechanical Integrity of Vertically Aligned Carbon Nanotubes for Interfacing with Retinal Neurons.

Retinal implant devices are becoming an increasingly realizable way to improve the vision of patients blinded by photoreceptor degeneration. As an electrode material that can improve restored visual acuity, carbon nanotubes (CNTs) excel due to their nanoscale topography, flexibility, surface chemistry, and double-layer capacitance. If vertically aligned carbon nanotubes (VACNTs) are biocompatible with retinal neurons and mechanically robust, they can further improve visual acuity—most notably in subretinal implants—because they can be patterned into high-aspect-ratio, micrometer-size electrodes. We investigated the role of an aluminum (Al) underlayer beneath an iron (Fe) catalyst layer used in the growth of VACNTs by chemical vapor deposition (CVD). In particular, we cultured dissociated retinal cells for three days in vitro (DIV) on unfunctionalized and oxygen plasma functionalized VACNTs grown from a Fe catalyst (Fe and Fe+Pl preparations, where Pl signifies the plasma functionalization) and an Fe catalyst with an Al underlayer (Al/Fe and Al/Fe+Pl preparations). The addition of the Al layer increased the mechanical integrity of the VACNT interface and enhanced retinal neurite outgrowth over the Fe preparation. Unexpectedly, the extent of neurite outgrowth was significantly greater in the Al/Fe than in the Al/Fe+Pl preparation, suggesting plasma functionalization can negatively impact biocompatibility for some VACNT preparations. Additionally, we show our VACNT growth process for the Al/Fe preparation can support neurite outgrowth for up to 7 DIV. By demonstrating the retinal neuron biocompatibility, mechanical integrity, and pattern control of our VACNTs, this work offers VACNT electrodes as a solution for improving the restored visual acuity provided by modern retinal implants.

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.

In Situ Measurement of Carbon Nanotube Growth Kinetics in a Rapid Thermal Chemical Vapor Deposition Reactor With Multizone Infrared Heating

Abstract Understanding and controlling the growth of vertically aligned carbon nanotube (VACNT) forests by chemical vapor deposition (CVD) is essential for unlocking their potential as candidate materials for next generation energy and mass transport devices. These advances in CNT manufacturing require developing in situ characterization techniques capable of interrogating how CNTs grow, interact, and self-assemble. Here we present a technique for real-time monitoring of VACNT forest height kinetics applied to a unique custom designed rapid thermal processing (RTP) reactor for CVD of VACNTs. While the integration of multiple infrared heating lamps enables creating designed spatiotemporal temperature profiles inside the reactor, they pose challenges for in situ measurements. Hence, our approach relies on contrast-adjusted videography and image processing, combined with calibration using 3D optical microscopy with large depth-of-field. Our work enables reliably measuring VACNT growth rates and catalytic lifetimes, which are not possible to measure using ex situ methods.

Bipolar electrochemical capacitors using double-sided carbon nanotubes on graphite electrodes

The electrochemical capacitor (EC) is a key enabler for the miniaturized self-powered systems expected to become ubiquitous with the advent of the internet-of-things (IoT). Vertically aligned carbon nanotubes (VACNTs) on graphite holds promise as electrodes for compact and low-loss ECs. However, as with all ECs, the operating voltage is low, and miniaturization of higher voltage devices necessitates a bipolar design. In this paper, we demonstrate a bipolar EC using graphite/VACNTs electrodes fabricated using a joule heating chemical vapor deposition (CVD) setup. The constructed EC contains one layer of double-sided VACNTs on graphite as bipolar electrode. Compared to a series connection of two individual devices, the bipolar EC has 22% boost in volumetric energy density. More significant boost is envisaged for stacking more bipolar electrode layers. The energy enhancement is achieved without aggravating self-discharge (71.2% retention after 1 h), and at no sacrifice of cycling stability (96.7% over 50000 cycles) owing to uniform growth of VACNTs and thus eliminating cell imbalance problems.

Water Vapor Condensation from Atmospheric Air by Super-Hydrophobic VACNTs Growth on Stainless Steel Pipes

AbstractEnergy-efficient condensation of steam contained in atmospheric air has emerged as a solution to the water scarcity. Academic and industrial research works that seeks to develop water collection devices with high efficiency has great relevance for the scientific community. In this work, we aim to show that modified carbon nanotubes forest can remove the condensed drops easier than a hydrophobic and a super-hydrophilic surface. In addition, this result was reached at high super saturation level which is an innovative aspect of this work. The Vertically Aligned Carbon Nanotubes (VACNTs) were grown on steel pipes. We used a CO2 laser and an O2 plasma to perform the post treatments that changed the CNTs to super-hydrophobic and super-hydrophilic, respectively. In addition, the CO2 laser treatment added a second level of roughness in the surface by etching the nanotubes walls. A polyethylene coating attached the carbon nanotubes to the substrate. We experimentally demonstrated a 24% higher vapor condensation rate at high supersaturations levels.

Growth of vertically-aligned carbon nanotubes on graphite for electric double-layer capacitors

Recently, electric double-layer capacitors (EDLCs) have attracted great attention as an energy storage device with high input-output power density. In this study, we synthesized carbon nanotubes (CNTs)/graphite with high surface area as a novel electrode component for EDLC. CNTs were grown on a defective graphite surface by alcohol catalytic chemical vapor deposition for the first time. We found that oxidative plasma treatment time for graphite surface attributes to selective growth of laterally-aligned (LA) or vertically-aligned (VA) CNTs. Electrochemical analysis reveals VACNTs/graphite possesses the highest electrostatic capacity of 1.5 mF cm−2 among pristine graphite, LACNTs/graphite, and VACNTs/graphite. We suggest that increase of specific surface area with CNTs results in the increase of electrostatic capacity for the EDLC electrode. The results indicate that VACNTs/graphite electrodes are expected to improve the performance of EDLCs.

One-pot preparation of iron/alumina catalyst for the efficient growth of vertically-aligned carbon nanotube forests

The catalytic growth of vertically-aligned carbon nanotubes (VA-CNTs) forest usually requires thin catalyst films deposited by multi-step and costly physical vapor deposition techniques. Here, we demonstrate that an efficient catalyst and its supporting layer for VACNT growth can be prepared by using a simple solution of Fe(NO3)3 and Al(NO3)3 deposited on silica in a single step. This process being much simpler and cheaper than existing preparation methods, it can easily be transferred to industry for the low-cost, thin and large-area coating of catalyst for VA-CNT growth. Our study shows that aluminum hydroxides preferentially react with the SiO2 surface while iron hydroxides tend to form oxide or hydroxide nanoparticles, thus allowing preparation of an aluminum-based buffer layer with iron-based nanoparticles at its surface. Optimization of the Fe/Al ratio and salt concentrations yielded catalysts with performances similar to standard Fe/Al2O3 catalysts prepared by physical vapor deposition.

The improvement of mechanical properties of SiC/SiC composites by in situ introducing vertically aligned carbon nanotubes on the PyC interface

In order to improve the mechanical properties, vertically aligned carbon nanotubes (VACNTs) were in situ introduced on the pyrocarbon (PyC) interfaces of the multilayer preform via chemical vapor deposition (CVD) process under tailored parameters. Chemical vapor infiltration (CVI) process was then employed to densify the multilayer preform to acquire SiC/SiC composites. The results show that the growth of VACNTs on PyC interface is highly dependent to the deposition temperature, time and constituent of gas during CVD process. The preferred orientation and high graphitization of VACNTs were obtained when temperature is 800 ℃ and C2H4/H2 ratio is 1:3. The bending strength and fracture toughness of SiC/SiC composites with PyC and PyC-VACNTs interfaces were compared. Compared to the SiC/SiC composite with PyC interface, the bending strength and fracture toughness increase 1.298 and 1.359 times, respectively after the introduction of PyC-VACNTs interface to the SiC/SiC composites. It is also demonstrated that the modification of PyC interface with VACNTs enhances the mechanical properties of SiC/SiC composites due to the occurrence of more fiber pull-outs, interfacial debonding, crack branching and deflection