High-density Single-walled Carbon Nanotube Research Articles

Single-walled carbon nanotube synthesis with RuRhPdIrPt high entropy alloy catalysts

Using high-entropy alloy nanoparticles (HEA NPs) composed of five platinum-group metals (5PGM), Ru, Rh, Pd, Ir, and Pt, as catalysts, we succeeded in growing single-walled carbon nanotubes (SWCNTs) via chemical vapor deposition (CVD). After CVD growth with C2H2 feedstock at 750 ℃ for 10 min, high-density SWCNTs were grown from 5PGM HEA NPs. The diameters of most SWCNTs were in the range of 0.83–1.1 nm, exhibiting the growth of small-diameter SWCNTs. Compared with monometal PGM catalysts, the SWCNT yield with the 5PGM HEA NP catalysts was much higher and was compatible even with those with Fe and Co.

Growth of high-density horizontal SWNT arrays by pre-cracking of carbon source

Obtaining high-density horizontal single-walled carbon nanotube (SWNT) arrays is the key point of developing SWNT-based integrated circuits. However, some catalysts showing good control over SWNT structures possess relatively low activity to grow SWNT, which restricts the density of the grown arrays. Therefore, we developed a rational method to grow high-density SWNT arrays by iron (Fe)-assisted pre-cracking of carbon source in gas phase. A tiny amount of Fe, decomposed from ferrocene, was introduced into the chemical vapor deposition system with the gas flow without forming Fe nanoparticles on the substrates. The density of the SWNT arrays grown from Mo2C, WC and Cu catalysts can be significantly increased by 3–5 times with the help of Fe, reaching 40, 35 and 24 tubes μm−1, respectively. Through gas chromatography and Raman spectrum characterization, it is proved that carbon source decomposed more completely with Fe in the gas phase, which resulted in higher utilization efficiency of carbon source and better quality of the grown SWNTs. This pre-cracking approach is an effective way to improve the density of SWNT arrays with controlled structures.

Confined Fe Catalysts for High-Density SWNT Arrays Growth: a New Territory for Catalyst-Substrate Interaction Engineering.

Great efforts have been devoted to searching for efficient catalytic systems to produce ultra-high density single-walled carbon nanotube (SWNT) arrays, which lay the foundation for future electronic devices. However, one major obstacle for realizing high-density surface-aligned SWNT arrays is the poor stability of metal nanoparticles in chemical vapor deposition catalytic processes. Recently, Trojan catalyst has been reported to yield unprecedented high-density SWNT arrays with 130 SWNTs per µm on the a-plane (11-20) of the sapphire substrate. Herein, a concept of catalyst confinement effect is put forward to revealing the secret of remarkable growth efficiency of SWNT arrays by Trojan catalyst. Combined experimental and theoretical studies indicate that confinement of catalyst nanoparticles on discrete a-plane strips plays a key role in stabilizing the small nanoparticles. The highly dispersive and active states of catalysts are maintained, which promote the growth of super-dense SWNT arrays. By rationally designing the substrate reconstruction process, large areas of dense SWNT arrays (130 SWNTs per µm) covering the entire substrate are obtained. This approach may provide novel ideas for the synthesis of various high-density 1D nanomaterials.

Growth of Homogeneous High-Density Horizontal SWNT Arrays on Sapphire through a Magnesium-Assisted Catalyst Anchoring Strategy.

In-situ growth of high-density single-walled carbon nanotube (SWNT) arrays with homogeneity is highly desirable for integrated circuits. However, disastrous migration and aggregation of catalyst nanoparticles on substrate has greatly limited the area of as-grown SWNT arrays. Herein, we develop a magnesium-assisted catalyst anchoring strategy to restrain catalyst nanoparticles sintering on substrate. Magnesium modification ameliorates sapphire surface by high temperature solid reaction and thus provides a stronger metal-support interaction (SMSI). Hereby, we realize the direct growth of high-density SWNT arrays that fully cover an entire 10×10 mm2 substrate with the local highest density of ≈110 tubes μm-1 using iron as catalyst. This strategy was also proven universal when employing solid carbide catalysts.

Quantitative trace level voltammetry in the presence of electrode fouling agents: Comparison of single-walled carbon nanotube network electrodes and screen-printed carbon electrodes

Single walled carbon nanotube (SWNT) network electrodes, in which a planar arrangement of SWNTs on an inert surface serve as a working electrode for voltammetry, offer considerable attributes for electroanalysis. Here, the effect of SWNT network density on the trace voltammetric analysis of a water-soluble ferrocene derivative (FcCOOH) is investigated in the presence of polyethylene glycol (PEG) or albumin, species that can foul (block) an electrode via adsorption. Fc-based analytes typically find use in redox labelling or redox shuttling, point-of-care electrochemical detection devices. Comparison is made between SWNT electrodes, grown by catalyzed chemical vapour deposition at three different surface coverages, 5, ~20 and ~30 μmSWNT μm-2, and commercial screen-printed carbon electrodes (SPCEs). In the presence of PEG (8% 2 K), for cyclic voltammetry, the lowest detectable concentration decreases as SWNT network density decreases. However, when employing differential pulse voltammetry, all three networks show a 1 nM FcCOOH limit of detection, three orders of magnitude smaller than achievable with SPCEs. This is attributed to the low capacitance of the SWNTs and absence of amorphous carbon structures which can contribute a pseudo-capacitive response. For both polyethylene glycol (PEG) and albumin (4%), repeat cycling shows the higher density SWNT network electrodes (≥20 μmSWNT μm-2) are far less susceptible to electrode fouling. Toward practical devices, a three-electrode chip, similar in design to that used in SPCEs, but using high density SWNT network electrodes, is also demonstrated to have impressive detection sensitivity for FcCOOH (nM level) in PEG solutions. The simplicity and practicality of the design widens the potential applications of these ultra-sensitive diagnostic tools based on planar network SWNTs.

Synthesis Procedure of Highly Densely Packed Carbon Nanotube Forests on TiN.

The goal of this research was to obtain high-density single-walled carbon nanotube forests (SWNTs) on conductive substrates for different applications, including field emission. For this, dip-coating was chosen as the catalyst deposition method, to subsequently grow SWNTs by Alcohol Catalytic Chemical Vapor Deposition (AC-CVD). Si (100) was chosen as the substrate, which was then coated with a TiN thin film. By sputtering with Ar, it was possible to generate alternating TiN and Si lanes, with a different wettability and, therefore, a different affinity for the catalysts. As a result, the Mo-Co catalyst was mainly deposited on TiN and not on sputtered-Si, which allowed the selective growth of SWNT forests on the TiN conductive surfaces. These as-synthesized SWNTs were used for field emission measurements in a high vacuum chamber.

Increasing the Density of Single-Walled Carbon Nanotube Arrays by Multiple Catalysts Reactivation

High-density single-walled carbon nanotube (SWNT) array is a basic premise for its application in nanoelectronic devices in the future. Generally, it is considered that the improvement in catalyst efficiency is a possible route to increase the density. Here, we designed a multiple-catalysts reactivation method to increase the catalyst efficiency. By multiple H2 treatment, we could successfully improve the density of the SWNT arrays by an average of 65%. Moreover, a new model to describe the multiple-catalysts reactivation process was established to deeply understand the mechanism of catalyst poisoning, and two possible types of catalyst poisoning: physical poisoning (PP) and structural poisoning (SP) were proposed. Only poisoned catalysts with PP type could be reactivated to produce new carbon nanotubes. By comparing different metal catalysts, we found that the ratio of PP to SP catalysts was highly related to the growth model, including vapor–solid (VS) and vapor–liquid–solid (VLS) model. Specifically, catalysts with VS mechanism (like W or Cu) showed higher PP ratio than those with VLS mechanism (like Co). The relationship between poisoning type and growth model mode may also contribute to the further catalyst design for high-density SWNT arrays.

Versatile Bottom-Up Synthesis of Tethered Bilayer Lipid Membranes on Nanoelectronic Biosensor Devices.

Interfacing nanoelectronic devices with cell membranes can enable multiplexed detection of fundamental biological processes (such as signal transduction, electrophysiology, and import/export control) even down to the single ion channel level, which can lead to a variety of applications in pharmacology and clinical diagnosis. Therefore, it is necessary to understand and control the chemical and electrical interface between the device and the lipid bilayer membrane. Here, we develop a simple bottom-up approach to assemble tethered bilayer lipid membranes (tBLMs) on silicon wafers and glass slides, using a covalent tether attachment chemistry based on silane functionalization, followed by step-by-step stacking of two other functional molecular building blocks (oligo-poly(ethylene glycol) (PEG) and lipid). A standard vesicle fusion process was used to complete the bilayer formation. The monolayer synthetic scheme includes three well-established chemical reactions: self-assembly, epoxy-amine reaction, and EDC/NHS cross-linking reaction. All three reactions are facile and simple and can be easily implemented in many research labs, on the basis of common, commercially available precursors using mild reaction conditions. The oligo-PEG acts as the hydrophilic spacer, a key role in the formation of a homogeneous bilayer membrane. To explore the broad applicability of this approach, we have further demonstrated the formation of tBLMs on three common classes of (nano)electronic biosensor devices: indium-tin oxide-coated glass, silicon nanoribbon devices, and high-density single-walled carbon nanotubes (SWNT) networks on glass. More importantly, we incorporated alemethicin into tBLMs and realized the real-time recording of single ion channel activity with high sensitivity and high temporal resolution using the tBLMs/SWNT network transistor hybrid platform. This approach can provide a covalently bonded lipid coating on the oxide layer of nanoelectronic devices, which will enable a variety of applications in the emerging field of nanoelectronic interfaces to electrophysiology.

Selective synthesis of large diameter, highly conductive and high density single-walled carbon nanotubes by a thiophene-assisted chemical vapor deposition method on transparent substrates

Selective synthesis of single-walled carbon nanotubes (SWNTs) with controlled properties is an important research topic for SWNT studies. Here we report a thiophene-assisted chemical vapor deposition (CVD) method to directly grow highly conductive SWNT thin films on substrates, including transparent ones. By adding low concentration thiophene into the carbon feedstock (ethanol), the as-prepared carbon nanotubes demonstrate an obvious up-shift in the diameter distribution while the single-walled structure is still retained. In the proposed mechanism, the change in the diameter is sourced from the increase in the carbon yield induced by the sulfur-containing compound. Such SWNTs are found to possess high conductivity with 95% SWNTs demonstrating on/off ratios lower than 100 in transistors. More importantly, it is further demonstrated that this method can be used to directly synthesize dense SWNT networks on transparent substrates which can be utilized as transparent conductive films (TCFs) with very high transparency. Such TCFs can be applied to fabricate a light modulating window as a proof-of-concept. The present work provides important insights into the growth mechanism of SWNTs and great potential for the preparation of TCFs with high scalability, easy operation and low cost.

Chemical Vapor Deposition Using Ethylene Gas toward Low Temperature Growth of Single-Walled Carbon Nanotubes

We demonstrate the growth of single-walled carbon nanotubes (SWNTs) using ethylene-based chemical vapor deposition (CVD) and ferritin-induced catalytic particles toward growth temperature reduction. We first optimized the gas composition of H₂ and C₂H₄ at 500 and 30 sccm, respectively. On a planar SiO₂ substrate, high density SWNTs were grown at a minimum temperature of 760℃. In the case of growth using nanoporous templates, many suspended SWNTs were also observed from the samples grown at 760℃; low values of I D /I G in the Raman spectra were also obtained. This means that the temperature of 760℃ is sufficient for SWNT growth in ethylene-based CVD and that ethylene is more effective that methane for low temperature growth. Our results provide a recipe for low temperature growth of SWNT; such growth is crucial for SWNT-based applications.

Growth of high quality, high density single-walled carbon nanotube forests on copper foils

We demonstrate the growth of high quality single-walled carbon nanotube (SWCNT) forests on commercial Cu foils by cold-wall chemical vapor deposition. Time-of-flight secondary ion mass spectrometry was employed to study the effect of annealing on the catalyst evolution with or without an AlOx barrier layer. X-ray photoelectron spectroscopy was used to investigate the chemical states of the catalyst and the barrier layer. SWCNT forests can be reproducibly grown on Cu foils sputter-coated with Al and Fe layers as thin as 6 nm and 0.4 nm, respectively. Al transforms into AlOx on exposure to air and during annealing. Most importantly, such a thin AlOx barrier layer ensures not only the growth of SWCNTs but also an Ohmic contact between the as grown SWCNTs and the Cu base as measured by a two-point probe station. The as-grown SWCNTs exhibit a bimodal distribution of diameters ranging from 0.6 to 4.5 nm, with two peaks centered at 0.8 nn and 2.6 nm, respectively.

Growth of high-density horizontally aligned SWNT arrays using Trojan catalysts.

Single-walled carbon nanotube (SWNT)-based electronics have been regarded as one of the most promising candidate technologies to replace or supplement silicon-based electronics in the future. These applications require high-density horizontally aligned SWNT arrays. During the past decade, significant efforts have been directed towards growth of high-density SWNT arrays. However, obtaining SWNT arrays with suitable density and quality still remains a big challenge. Herein, we develop a rational approach to grow SWNT arrays with ultra-high density using Trojan catalysts. The density can be as high as 130 SWNTs μm(-1). Field-effect transistors fabricated with our SWNT arrays exhibit a record drive current density of -467.09 μA μm(-1) and an on-conductance of 233.55 μS μm(-1). Radio frequency transistors fabricated on these samples exhibit high intrinsic fT and fMAX of 6.94 and 14.01 GHz, respectively. These results confirm our high-density SWNT arrays are strong candidates for applications in electronics.

Highly uniform carbon nanotube nanomesh network transistors

A new type of single-walled carbon nanotube (SWNT) thin-film transistor (TFT) structure with a nanomesh network channel has been fabricated from a preseparated semiconducting nanotube solution and simultaneously achieved both high uniformity and a high on/off ratio for application in large-scale integrated circuits. The nanomesh structure is prepared on a high-density SWNT network channel and enables a high on/off ratio while maintaining the excellent uniformity of the electrical properties of the SWNT TFTs. These effects are attributed to the effective elimination of metallic paths across the source/drain electrodes by forming the nanomesh structure in the high-density SWNT network channel. Therefore, our approach can serve as a critical foundation for future nanotube-based thinfilm display electronics.

Temperature-dependent nonlinear phonon behavior in high-density carbon nanotube thin films

We report the temperature-dependent Raman spectra for high-density single-walled carbon nanotube thin films. We show that the position of the main Raman mode (G) softens as the temperature increases and is nonlinear in the range of 70–270 K. This effect is explained by optical phonon decay. In the linear regime, the first-order temperature coefficient (χT) equals −0.02 cm−1/K, which is lower than for any other carbon nanotubes. Importantly, we found that local laser-induced temperature change shows a nonlinear trend as a function of global temperature with a minimum at 270 K. Our results contribute to understand the thermal properties of carbon nanotube thin films that could be applied, for example, in photovoltaic or thermoelectric devices.

Solution-Processed Logic Gates Based On Nanotube/Polymer Composite

Hysteresis-free logic gates capable of operation at 100 kHz are fabricated basing on local-gate thin-film transistors with their channel featuring solution-processed composite films of single-walled carbon nanotubes (SWCNTs) and poly(9,9-dioctylfluorene-co-bithiophene) (F8T2). Using dip-coating for deposition of composite films, high-density SWCNTs are found to be embedded in an F8T2 layer and thus being kept from the underlying AlO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> gate dielectric by a certain distance. The presence of the F8T2 interlayer effectively suppresses hysteresis although it also weakens the gate electrostatic control. The fabricated transistors are characterized by nil hysteresis, high carrier mobility, large ON/OFF current ratio, low operation voltage, small subthreshold swing, and remarkable scalability. These properties are crucial for the realization of the well-performing logic circuits.

Controllable fabrication and electromechanical characterization of electrophoresis assembled single-walled carbon nanotube-polymer film transducers

This paper presents a controllable electrophoresis allowing a directed deposition of negatively charged single-walled carbon nanotubes (SWNTs) on an electro-active polymer to fabricate thin-film transducers under an electric field excitation. The assembled high-density SWNT networks are verified with SEM micrograph and Raman spectroscopy, and the electric filed-induced mechanism for electrophoresis of SWNT is investigated using electrochemical analysis technique. The dynamic electromechanical properties are characterized by a combinative approach of piezoelectric excitation and laser vibrometer measurement. A remarkable performance enhancement and tunability for such thin-film transducers in both resonant frequency and quality factor is demonstrated, compared with pure polymer. This observed enhancement can not only be exploited to tailor the thin-film transducers for desired electromechanical properties, but also create versatile pathways for variety of applications including polymeric electronic technologies.

High-performance local back gate thin-film field-effect transistors using sorted carbon nanotubes on an amino-silane treated hafnium oxide surface

Wafer-scale fabrication and characterization of local back gate semiconducting nanotube thin-film transistors (SN-TFTs) are reported in this paper. The local back gate voltage of the corresponding SN-TFT controls the individual transistor switching. In order to achieve high performance, a high-k dielectric material is employed as a gate oxide and this helped to achieve low-voltage operations, much steeper sub-threshold voltage swings and higher transconductance values. A simple procedure to deposit a high-density single-walled carbon nanotube thin film on an amino-silane-treated hafnium oxide (HfOX) surface is suggested such that a good density of nanotubes is realized without degrading the device on–off current ratio and mobility values. The density of the nanotubes achieved on the silanized HfOX surface is about 40–45 nanotubes µm−2. SN-TFTs exhibit an excellent p-type output characteristic with distinct linear and saturation regions. Local back gate SN-TFTs exhibit an on–off current ratio exceeding 104 and a steep sub-threshold slope of 400 mV/decade. SN-TFTs achieve a maximum current density of 13 μA µm−1, an average threshold voltage of −0.5 V, a maximum normalized transconductance of 18.5 μS µm−1 and exhibit a maximum carrier mobility of 60.6 cm2(Vs)−1.

Facile and Scalable Fabrication of Well‐Aligned and Closely Packed Single‐Walled Carbon Nanotube Films on Various Substrates

The superior electrical and mechanical properties of singlewalled carbon nanotubes (SWNTs) have made them promising candidates for many applications in the fi eld of electronics, optoelectronics, and composites. [ 1 ] Since SWNTs exhibit most of their remarkable properties along the tube axis, a key prerequisite for their application is a cost-effective and facile fabrication of well-aligned and high-density SWNT structures on a large scale on substrates with controlled location and orientation. [ 2 ]

Single Walled Carbon Nanotubes (SWCNTs) Assembled Site-Selectively on Flexible Substrate

Site-selectively array of single walled carbon nanotubes (SWCNTs) is one of the key points for the fabrication and application of nanodevices. This research is focused on developing a technology to assemble site-selective high density SWCNTs on flexible substrate. Patterned SWCNTs were assembled on flexible poly (ethylene terephthalate) (PET) substrate via inducing directed charge (DC) field. In this process, the modification of hydrophilic or hydrophobic substrate surface is accomplished by selective ultraviolet (UV) irradiation, and which is very important to the self-assemble behavior and accurately site-selective location of SWCNTs. This method, a composition of “Top-Down” approach and “Bottom-Up” approach, is expected to give a way for the fabrication of nanodeivces and nanosystems.

Carbon nanotubes in nanopackaging applications

CNTs are recently discovered materials made by rolled-up sheets of graphene. They may be made either by a single shell [single-walled CNT (SWCNT) with radius ranging from 0.7 to 3-4 nm] or several nested shells [multiwalled CNT (MWCNT) with outer radius typically of the order of some tens of nanometers]. The length of a nanotube may reach the order of millimeters, and hence, this nanostructured material exhibits an excellent form factor, able to comply with the ultrafine pitches required in nanopackaging. CNT (carbon nanotubes) to be used for vertical vias or interconnects for packaging applications, very high-density SWCNT bundles must be demonstrated. When using MWCNT bundles instead, the achieved density comes from a compromise between the CNT radius and its shell number. The fabrication process must provide low contact resistance, good direction control, and compatibility with CMOS technology. The road for CNTs to replace copper in chip packaging is still long, but the gap between theoretical predictions and practical applications is reducing faster and faster.