Aligned Carbon Nanotube Arrays Research Articles

Electron Beam Interaction with Carbon Nanotubes in Scanning Electron Microscope: Mechanisms and Applications

AbstractA comprehensive understanding of the interaction between electron beams and carbon nanotubes (CNTs) in scanning electron microscope (SEM) can be challenging due to the complex surface charging behavior and various nanostructures formed by CNTs. Herein, an overview of the challenges and potential solutions are provided for achieving high‐resolution imaging of nanomaterials, such as CNTs, in SEM systems. First, imaging mechanisms of CNTs on conducting, insulating, and hybrid substrates are summarized, with morphology, material type, and charging effect covered. Second, measuring the conductivity, bandgap, and diameter of a CNT by SEM method is introduced. Finally, the utilization of super‐aligned CNT (SACNT) films for observing insulators and vertically aligned CNT arrays (VACNTAs) as electron blackbodies are discussed separately. New techniques for imaging and characterizing nanostructures will ultimately lead to the advancement of CNT‐based technologies.

Removing Conjugated Polymers from Aligned Carbon Nanotube Arrays.

Aligned carbon nanotube (A-CNT) with high semiconducting purity and high-density have been considered as one of the most promising active channels for field-effect transistors (FETs), but conjugated polymer dispersant residues on the surface of A-CNT have become the main obstacle for its further development in electronics applications. In this work, a series of removable conjugated polymers (CPs) are designed and synthesized to achieve favorable purification and alignment for CNT arrays with a high density of ≈360 CNTs/µm. Furthermore, a removal process of CPs on the CNT array film is developed. Raman spectra show that the CNTs in array film are almost not damaged after the removal process, and the G/D ratio is as high as 35. The field-effect transistors (FETs) are fabricated with a saturation current density up to 600µAµm-1 and a current on-off ratio of ≈105, even with a relatively long channel length of ≈3µm.

Removal of conjugated polymer on carbon nanotube array by dry process

With the progress in the purity and alignment of semiconductor carbon nanotubes (CNTs), how to efficiently remove the conjugated polymers twining round CNTs has become an important and urgent matter for the further development of CNT electronics. The removal of conjugated polymers (CPs) by wet cleaning will inevitably lead to the destruction of the morphology of aligned CNT array films, which will cause the monolayered CNTs to form agglomeration or stacking. Therefore, it is necessary to develop a dry method to remove CPs on the aligned CNT films. In this work, we studied the thermal decomposition characteristics of CNTs and CPs in detail, and found that the ratio of the oxidative decomposition rate of CNTs and CPs could reach up to 35.7 in the oxygen environment at 450 °C. The transmission electron microscopes (TEMs) show that such a dry treatment process can effectively remove the polymer on CNTs without affecting the morphology of CNT arrays. The Raman spectrum show that ratio of G/D of the annealed sample is as high as 41.8, which indicates that the CNTs are not detectable damaged during the annealed process. The fabricated field-effect transistors (FETs) with a length of 2 μm exhibit a current on/off ratio higher than 104 and a current density higher than 660 μA/μm, which further justifies that the CNT arrays treated by the dry method exhibit high quality.

Electromagnetic crosstalk isolation with transferred vertically aligned carbon nanotube arrays through thermocompression bonding

As electronic devices become smaller, more powerful, and operate at higher frequencies, electromagnetic compatibility (EMC) is becoming an ever-greater challenge due to the board real estate required for their proper operation. Thus, this paper presents a solution to augment existing electromagnetic (EM) isolation solutions to increase their effectiveness and reduce their required board real estate. Carbon nanotubes (CNTs) have long been touted as a promising, electrically conductive, high aspect ratio material for use in electronics. However, their widespread use has been limited due to the harsh environments needed for their growth. This paper describes the application of an improved vertically aligned carbon nanotube (VACNT) transfer technique that circumvents this limitation. The process of transferring a carbon nanotube fence wall (CNTFW) on existing grounded via-fence structures via thermocompression bonding augments the EM isolation capability of the CNTFW using the metal coating used for the thermocompression bond, along with a post-process metal epoxy coating. The combination of augmentations possible on a transferred CNTFW structure enables an electromagnetic crosstalk isolation improvement of up to 16 dB in the frequency range from 0 to 50 GHz. The integration of such a technology allows for further downscaling of electromagnetic isolation solutions in electronic devices.

Anisotropic Microwave Properties of Vertically Aligned Carbon Nanotube Arrays

Anisotropic Microwave Properties of Vertically Aligned Carbon Nanotube Arrays

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Small Molecule Additives to Suppress Bundling in Dimensional‐Limited Self‐Alignment Method for High‐Density Aligned Carbon Nanotube Array

AbstractSemiconducting single‐walled carbon nanotube (CNT) is a promising candidate as a channel material for advanced logic transistors, attributed to the ultra‐thin 1‐nm cylindrical geometry, high mobility, and high carrier injection velocity. However, the presence of undesired CNT bundles in the CNT arrays for wafer‐scale device fabrication, even when utilizing the state‐of‐the‐art dimension‐limited self‐alignment (DLSA) method, poses challenges. These CNT bundles degrade the transistor gate's efficiency in controlling the flow of charge carriers in the CNT channel, leading to pronounced device‐to‐device variability. Here, a novel method is introduced to alleviate bundling in CNT arrays assembled via DLSA, by involving small molecule additive to screen the attractive van der Waals force between neighboring CNTs during the DLSA process, resulting in over 50% reduction in CNT bundling. Furthermore, a pioneering methodology for quantifying CNT bundles is presented and employed experimentally to assess bundles in dense CNT arrays assembled by DLSA using transmission electron microscopy. Both experimental data and molecular dynamics simulation reveal that CNT bundling originates from van der Waals attraction between CNTs, and the disturbed liquid‐liquid interface by accumulating excess polar molecules. These findings illuminate new pathways for realizing dense, bundle‐free CNT arrays.

Construction of Medusa-Like Adhesive Carbon Nanotube Array Induced by Deformation of Alumina Sheets.

To change the binary structure of nanotube and nanotube array in vertically aligned carbon nanotube arrays, this work deposits regularly arranged amorphous alumina sheets on the classical array growth catalyst (10nm-thick alumina and 2nm-thick iron) and obtains an array similar to the Medusa head. Subsequent experiments revealed that these alumina sheets show both unstable and stable qualities during growth: unstable in that they thermally deform and change their newly discovered characteristics of blocking carbon source diffusion, which regulates the nanotube growth order in specific areas; stable in that they withstand the deformation caused by heat and sequential growth of nanotubes, serving as a substrate and buffer layer for Medusa's hair, i.e., nanotube bundles on the array surface. Their combination splits this binary structure into a tertiary architecture consisting of nanotubes, nanotube bundles, and the array spanning nano-, micro-, and milli-meter. Benefiting from this structure, this array exhibits a unique near-isotropic adhesion characteristic compared to existing reports and outperforms classical and patterned arrays with the same classical catalyst and growth conditions.

Facile synthesis of high-density and vertically aligned carbon nanotube arrays on an acid treated stainless steel mesh

Directly growing vertically aligned carbon nanotube arrays (VACNTs) on conductive substrates has shown great potential in a wide variety of applications. Among the various substrates, stainless steel (SS) possesses the advantageous features of wide availability, diverse compositions, and Fe containing, all of which are favorable for direct use as a low-cost and efficient catalytic substrate for VACNT growth. However, it is still unachieved to grow high-density VACNTs on SS in a facile way due to the lack of effective engineering strategies on the SS surface. Herein, we demonstrate a facile but efficient method to grow dense and highly VACNTs on an SS mesh by simply etching the SS mesh with an optimized HCl solution. Experimental results show that the etching can remove the chromium-rich passive layer and create highly active sites for chemical vapor deposition. In addition, it is revealed that the presence of the high-melting element of Mo in SS 316 is essential for prolonging the lifetime of active sites. Compared to the case on the SS 304 without Mo element, the VACNTs grown on SS 316 exhibit a higher purity, better alignment, higher degree of graphitization, and more importantly, large height of 80 μm.

Quartz Nonadherent and Clean Exfoliation of the Heteroatom-Doped Bulk Carbon Nanotubes Array.

Vertically aligned carbon nanotubes array offers unique properties for various applications. Detaching them from the growth substrate, while preserving their vertical structure, is essential. Quartz, a cost-effective alternative to silicon wafers and metal-based substrates, can serve as both a reaction chamber and a growth substrate. However, the strong adhesive interaction with the quartz substrate remains an obstacle for further applications. Herein, we presented a simple and well-controlled exfoliation strategy assisted by the introduction of heteroatoms at root ends of a carbon nanotubes array. This strategy forms lower surface polarity of the carbon fragment to significantly reduce adhesion to the quartz substrate, which contributes to the effortless exfoliation. Furthermore, this scalable approach enables potential mass production on recyclable quartz substrates, enhancing the cost-effectiveness and efficiency. This work can establish a solid foundation for cost-competitive carbon nanotube-based technologies, offering a promising avenue for their widespread applications.

Shortwave Infrared Polarization Imaging and Monolithic Integrated Polarization Amplification Systems Based on Aligned Carbon Nanotube Arrays

AbstractNext‐generation shortwave infrared (SWIR) imaging systems generally require a multidimensional information sensing capability (including intensity, wavelength, polarization, phase, etc.), a highly integrated photodetector unit, and information processing, allowing for miniaturization and low‐cost production. However, traditional polarized SWIR imaging systems with integrated polarizer arrays as supplementary filters and silicon‐based amplifying circuits are complicated and very expensive. Here, a SWIR polarization‐sensitive photodetector and a monolithic integrated polarization amplification system (MIPAS) based on well‐aligned carbon nanotube (CNT) arrays are demonstrated. The polarization‐sensitive CNT photodetector exhibits anisotropic ratios of ≈5.18 and ≈7.56 at 1800 and 2000 nm wavelengths, respectively, and high‐resolution characteristics that can be utilized to image SWIR laser spots with a radius of less than 10 µm. Furthermore, MIPAS including a CNT field‐effect transistor, a CNT loading resistor, and a polarization‐sensitive CNT photodetector is used to increase the anisotropic ratio of the CNT photodetector. The amplified anisotropic ratio is improved up to 173 and 243 at 1800 and 2000 nm wavelengths, respectively, which is the maximum reported in the SWIR band. Our work demonstrates that the CNT polarization‐sensitive photodetector has the potential for SWIR polarization imaging with a monolithic integrated polarization amplification system.

Synthesis of Vertically Aligned Carbon Nanotube Arrays from Premade Bimetallic Aluminum-Iron Oxide Nanoparticles Monolayer

: Carbon Nanotubes (CNTs) are excellent materials because of their unique physical and chemical properties. Though catalyst nanoparticles preparation from thin film catalysts are common but premade catalysts nanoparticles in liquid phase has control over the size and composition which control the diameter and length of CNTs. Here, we report the synthesis of vertically aligned (VA) CNTs on metal oxide substrates including silicon oxide and stainless-steel using aluminum-iron oxides Al2O3-Fe2O3 nanoparticles monolayer. Oleic acid functionalized Al2O3-Fe2O3 nanoparticles were assembled onto metal oxide substrate as a monolayer using 11-phousnoundecanoic acid (PNDA) linker molecule where phosphonate group bind to the substrate and terminal carboxylic group of PNDA bind to the surface of bimetallic oxide catalysts. Homogenous catalyst nanoparticles distribution from monolayer assembly can reduce the aggregation of catalyst particles which can increases the catalyst lifetime during CNTs growth. Synthesis of VA-CNTs require Al2O3 catalyst support on substrate surface, but our bimetallic Al2O3-Fe2O3 nanoparticles contain both catalyst support (Al2O3) and catalyst (Fe) in the same particle. The successful VA-CNTs synthesis were achieved by chemical vapor deposition (CVD) process using acetylene as carbon precursor in atmospheric pressure. We envision that our catalyst preparation for VA-CNTs growth can be a cost-efficient method for large scale synthesis of CNTs. Further, the VA-CNTs growth on conductive stainless-steel substrate can facilitate the electrochemical application of this hybrid materials for biomolecule sensing and heavy metals detection.

Performance evaluation of VACNT arrays under simulated LEO plasma environment

The space applications of vertical aligned carbon nanotube (VACNT) arrays have attracted many researchers' interest in recent years. However, the influences of various space environmental factors on the VACNT arrays are mostly unknown. In this paper, the ground-based experiments of simulated plasma environment in low earth orbit (LEO) were presented. VACNT arrays synthesized by chemical vapor deposition are exposed in the vacuum chamber under different plasma parameters (time, plasma temperature, and plasma density). The morphological, structural as well as chemical surface modifications were investigated by scanning electron microscopy, Raman spectroscopy, transmission electron microscopy and x-ray photoelectron spectroscopy. Correlations of the plasma parameters to the adhesion of VACNT arrays were analyzed at micro-scale and nano-scale by nanoindentation and atomic force microscopy. The obtained data confirm that exposure to simulated LEO plasma environment will lead to the increase of defect degree, the damage of tube walls between CNTs, which will weaken the adhesion of VACNT arrays. However, the plasma will also react with the carbon tubes on the surface and introduce many polar groups, thus consolidating the adhesion of VACNT arrays. Our results provide experimental predictions needed to guide the design of VACNT arrays for space applications.

Multi-Walled Carbon Nanotube Array Modified Electrode with 3D Sensing Interface as Electrochemical DNA Biosensor for Multidrug-Resistant Gene Detection.

Drug resistance in cancer is associated with overexpression of the multidrug resistance (MDR1) gene, leading to the failure of cancer chemotherapy treatment. Therefore, the establishment of an effective method for the detection of the MDR1 gene is extremely crucial in cancer clinical therapy. Here, we report a novel DNA biosensor based on an aligned multi-walled carbon nanotube (MWCNT) array modified electrode with 3D nanostructure for the determination of the MDR1 gene. The microstructure of the modified electrode was observed by an atomic force microscope (AFM), which demonstrated that the electrode interface was arranged in orderly needle-shaped protrusion arrays. The electrochemical properties of the biosensor were characterized by cyclic voltammetry (CV), differential pulse voltammetry (DPV), and electrochemical impedance spectroscopy (EIS). Chronocoulometry (CC) was used for the quantitative detection of the MDR1 gene. Taking advantage of the good conductivity and large electrode area of the MWCNT arrays, this electrochemical DNA sensor achieved a dynamic range from 1.0 × 10-12 M to 1.0 × 10-8 M with a minimal detection limit of 6.4 × 10-13 M. In addition, this proposed DNA biosensor exhibited high sensitivity, selectivity, and stability, which may be useful for the trace analysis of the MDR1 gene in complex samples.

High-Temperature-Operable Electromechanical Computing Units Enabled by Aligned Carbon Nanotube Arrays.

Nano/micro-electromechanical (NEM/MEM) contact switches have great potential as energy-efficient and high-temperature-operable computing units to surmount those limitations of transistors. However, despite recent advances, the high-temperature operation of the mechanical switch is not fully stable nor repetitive due to the melting and softening of the contact material in the mechanical switch. Herein, MEM switches with carbon nanotube (CNT) arrays capable of operating at high temperatures are presented. In addition to the excellent thermal stability of CNT arrays, the absence of a melting point of CNTs allows the proposed switches to operate successfully at up to 550 °C, surpassing the maximum operating temperatures of state-of-the-art mechanical switches. The switches with CNTs also show a highly reliable contact lifetime of over 1 million cycles, even at a high temperature of 550 °C. Moreover, symmetrical pairs of normally open and normally closed MEM switches, whose interfaces are initially in contact and separated, respectively, are introduced. Consequently, the complementary inverters and logic gates operating at high temperatures can be easily configured such as NOT, NOR, and NAND gates. These switches and logic gates reveal the possibility for developing low-power, high-performance integrated circuits for high-temperature operations.

Stochastic models of free-molecular nanopore flows.

In gas transport systems of the nanoscale, fluid-surface interactions become the main forces governing the evolution of the flow state. In ideal nanoscale systems, such as atomically smooth carbon nanotubes, the characteristic lengths reduce to such an extent that the non-equilibrium entrance region comprises a large proportion of the domain. In this regime, the added effective resistance induced by the non-equilibrium entrance region becomes large enough that classical effusion models break down. The mechanisms behind the resistance in this regime are still poorly understood. A stochastic model of interfacial resistance is developed here, which allows for the determination of the effective diffusion coefficient via a novel finite-difference solution. We use this method to model free-molecular gas flow through long nanotubes, showing that such non-equilibrium effects may be present in systems of length scales currently within manufacturing capabilities. Finally, this model is used to discuss gas separation through aligned carbon nanotube arrays, with a focus on the effect of membrane length on the separation of a H2-CH4 mixture.

Terahertz Schottky Barrier Diodes Based on Aligned Carbon Nanotube Arrays

Carbon nanotubes (CNTs) with superior electrical characteristics are among the most promising candidate materials for constructing millimeter wave and terahertz wave electronic devices. However, the reported Schottky barrier diodes (SBDs) based on CNTs usually exhibited high series resistance and severe mismatch to lower impedance external circuits, which limits the devices for high frequency applications. In this work, CNT SBDs of lateral structure with a rectify current density of -0.78 mA/μm and a short circuit current responsivity of 6.8 A/W are demonstrated based on aligned carbon nanotube arrays (A-CNTs) with high-density and high-semiconducting purity on a quartz substrate. The A-CNT SBD with a width of 10 μm and an L <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ch</sub> of 50 nm under zero bias exhibits an ultra-low intrinsic capacitance of 2.5 fF and an intrinsic cutoff frequency <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">f</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">C</sub> of up to 840 GHz, which suggests that CNT-based SBDs are promising for millimeter wave and terahertz wave applications.

Electrochemically Oxidized Aligned Carbon Nanotube Arrays Embedding Foam Substrates for Water Splitting

Designing highly active electrocatalysts for both hydrogen and oxygen evolution with good durability at large current densities is very significant for water splitting. In this study, a new type of catalytic electrode was developed for water splitting. This electrode was made by growing microflower-like NiFeP on an aligned carbon nanotube (CNT) array network skeleton that was embedded in a Ni foam (NF) substrate. This design is aimed at enhancing the electrocatalytic activity for both hydrogen and oxygen evolution. The results showed that the resulting electrode (ECO-NF-ACNT@NiFeP) had a porous structure and was hydrophilic and highly resistant to corrosion, leading to improved activity for both the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER). The overpotential for harvesting a 10 mA cm–2 OER and HER current density was 97 and 190 mV, respectively, and the overpotential for reaching 100 mA cm–2 was 345 and 348 mV, respectively. These results are significantly better than those of commercial RuO2 and Pt/C catalysts and provide a promising direction for developing water splitting catalysts.

Selectively Tuning the Substrate Adhesion Strength of Aligned Carbon Nanotube Arrays via Thermal Postgrowth Processing.

The excellent intrinsic properties of aligned nanofibers, such as carbon nanotubes (CNTs), and their ability to be easily formed into multifunctional 3D architectures motivate their use for a variety of commercial applications, such as batteries, chemical sensors for environmental monitoring, and energy harvesting devices. While controlling nanofiber adhesion to the growth substrate is essential for bulk-scale manufacturing and device performance, experimental approaches and models to date have not addressed tuning the CNT array-substrate adhesion strength with thermal processing conditions. In this work, facile "one-pot" thermal postgrowth processing (at temperatures Tp = 700-950 °C) is used to study CNT-substrate pull-off strength for millimeter-tall aligned CNT arrays. CNT array pull-off from the flat growth substrate (Fe/Al2O3/SiO2/Si wafers) via tensile testing shows that the array fails progressively, similar to the response of brittle microfiber bundles in tension. The pull-off strength evolves nonmonotonically with Tp in three regimes, first increasing by 10 times through Tp = 800 °C due to graphitization of disordered carbon at the CNT-catalyst interface, and then decreasing back to a weak interface through Tp = 950 °C due to diffusion of the Fe catalyst into the substrate, Al2O3 crystallization, and substrate cracking. Failure is observed to occur at the CNT-catalyst interface below 750 °C, and the CNTs themselves break during pull-off after higher Tp processing, leaving residual CNTs on the substrate. Morphological and chemical analyses indicate that the Fe catalyst remains on the substrate after pull-off in all regimes. This work provides new insights into the interfacial interactions responsible for nanofiber-substrate adhesion and allows tuning to increase or decrease array strength for applications such as advanced sensors, energy devices, and nanoelectromechanical systems (NEMS).

Aligned Carbon Nanotube Arrays with Germanium Protective Layers for Improving the Performance of Radio Frequency Transistors

Aligned carbon nanotube arrays with high semiconducting purity and high density have been regarded as an ideal channel material for constructing high-frequency field effect transistors and high-speed integrated circuits. However, contaminants (mainly resist residues) that leave the surface of carbon nanotubes during the fabrication process seriously deteriorate the interface contact quality, which will trigger large contact resistance and thus inhibit the ultimate performance of the carbon nanotube-based transistors. Here, we demonstrate a germanium protective layer technique that can keep a clean surface of carbon nanotube films from the contaminants during device fabrication and thus boost the quality of ohmic contact between carbon nanotubes (CNTs) and metal. The transfer length method (TLM) is used to evaluate the effect of the germanium protective layer technique on the ohmic contact resistance. The contact resistance from metal–CNTs extracted using the TLM was reduced by 93%, from 1.66 to 0.123 KΩ μm before and after the germanium process treatment. Owing to the great improvement of contact between metal and CNTs by introducing the germanium protective layer technique, the fabricated aligned CNT array-based radio-frequency field-effect transistors with a gate length of 110 nm on the quartz substrate present excellent DC performance, with an on-state current of 1.28 mA/μm and maximum transconductance of up to 1.05 mS/μm at a bias of −1.2 V. Meanwhile, the as-measured current gain cut-off frequency (fT,EXT) and power gain frequency (fMAX,EXT) increase from 12 to 110 GHz and 13 to 105 GHz, respectively. This approach provides a simple and reproducible means of improving the interface quality and reducing contact resistance in CNT-based devices to enhance performance.