Optimization of single-walled carbon nanotube growth and study of the hysteresis of random network carbon nanotube thin film transistors

Random network single-walled carbon nanotube (SWNT)-based thin film transistors show excellent properties in sensors, electronic circuits, and flexible devices. However, they exhibit a significant amount of hysteresis behavior, which should be solved prior to use in industrial applications. This paper provides optimum conditions for the growth of random network SWNTs and reveals that the observed hysteresis behavior originates from the charge exchange between the SWNTs and the dielectric layer rather than from changes in the intrinsic properties of the SWNTs. This was proven by studying the conditions of stepwise gate sweep experiments and time measurements. This paper also shows that top gate SWNT thin film transistors (TFTs) with an SU-8 dielectric layer could provide a practical solution to the hysteresis problem for SWNT TFTs in electronic circuit applications.     

Connection of macro-sized double-walled carbon nanotube strands by bandaging with double-walled carbon nanotube films

Based on their special shrinkage characteristics, double-walled carbon nanotube (DWCNT) films were used as bandages to bind the overlapped ends of macro-sized (centimeters long) DWCNT strands for connection to get structures of random length. Tensile tests indicated that the joints made in this way had relatively high tensile strength with a maximum value of 311 MPa corresponding to that of the original strands. The equivalent contact resistance of the joints was very small. And the connected strands showed better electronic properties in our investigation on the temperature dependence of resistivity and the same remarkable current capacity, in contrast to the original ones. This technique may offer a promising potential for the future extensive use of macro-sized CNTs in many fields, such as electrical cables and wires.     

Characterisation of carbon nanotube films deposited by electrophoretic deposition

Electrophoretic deposition (EPD) was shown to be a convenient method to fabricate uniform coatings of carbon nanotubes (CNTs) with desired thickness and excellent macroscopic homogeneity. The CNT deposition kinetics are controlled by the applied electric field and deposition time which, in turn, prove to be linearly correlated with the deposition yield and thickness. The CNT films were characterised by using a range of techniques including high resolution scanning electron microscopy, nanoindentation and atomic force microscopy. Nanoindentation results revealed differences in the local microstructure of CNT deposits leading to variations of Young’s modulus and hardness, which were ascribed to differences in the packing density of CNTs, as observed also by AFM. A mathematical model for the kinetic of EPD of CNTs based on Hamaker’s law was proposed and the predictions of the model were shown to be in good agreement with experimental results.     

Electrothermal halogenation of carbon nanotube films

We report an innovative approach to halogenation of carbon nanotube (CNT) films. Application of bias voltage across horizontally aligned CNT films resulted in the electrothermal effect, which enabled us to chemically modify CNT films at the entirely controlled range of temperatures up to 300 °C. Such heated CNT membranes were exposed to gaseous halogens (Cl₂, Br₂, I₂) whilst their electrical properties were monitored. The experiments were carried out at room temperature and 100–300 °C (in 50 °C steps) until no further change in electrical resistance could be seen. The procedure lasted less than one minute, during which we were able to successfully introduce up to 6.7%, 6.0% and 1.5% at. of Cl, Br and I into the CNT framework. In parallel, we observed a permanent increase in conductivity of the films and mild purification of the starting material due to the removal of various carbon–oxygen functional groups.     

Electronic properties of double-walled carbon nanotube films

The electronic properties of as-prepared and purified DWNT films were tested using the four-probe method. The resistivity of the purified DWNT films is about 1 mΩ cm at room temperature and has a positive dρ/dT above 55 K, which shows a good metallic property. The electrical resistivities of the purified DWNT films are quite similar to those of the SWNT bundles and acid-treated SWNTs. Our results are correspondent with the early theoretical calculation on the band structure of DWNTs.     

Preparation and properties of polyimide/carbon nanotube composite films with electromagnetic wave absorption performance

The polyimide (PI)/carbon nanotube (CNT) films including 3,3′,4,4′-biphenyl tetracarboxylic dianhydride (BPDA), p-phenylenediamine (p-PDA), and CNTs were prepared, which have prominent electromagnetic (EM) wave absorption performance. Experimental analyses of the mechanical properties, thermal stabilities, coefficient of thermal expansion (CTE), the glass transition temperature (T_g), and EM parameter revealed the beneficial effects of the CNTs on the resulting composite films. In particular, when the content of CNTs is 6 wt%, the film shows the highest EM wave absorption performance, which exhibits the effective absorption bandwidth of 2.72 GHz with the matching thickness of only 2.0 mm. These results indicate that PI-based films have a certain potential application in the area of EM wave-absorbing materials.     

Impact of ink formulation on carbon nanotube network organization within inkjet printed conductive films

The inkjet printing of an aqueous suspension of carboxylic acid-functionalized single walled carbon nanotubes (SWCNT-COOH) and of a conductive ink combining SWCNT-COOH with the conductive polymer poly(3,4-ethylenedioxythiophene) doped with poly(styrene sulfonic acid) (PEDOT-PSS) was studied. A dimensionless study predicted the behavior of these two fluids in a given printing system. Observations at different scales were performed on the printed samples to visualize the arrangement of the carbon nanotube (CNT) network within the printed layer. An innovative way to localize CNTs within the printed patterns was developed by using a mapping technique of surface sample, based on a scanning electron microscope coupled with an energy dispersive X-ray spectroscope. The SWCNT-COOH aqueous suspension is subject to the halo (or “coffee ring”) effect, which is a well-known phenomenon in inkjet printing, whereas the SWCNT-COOH/PEDOT-PSS ink offers a more homogeneous CNT network. The CNT orientation has also been under investigation. For the SWCNT-COOH suspension, specific orientations of the CNTs were recorded, whereas for the SWCNT-COOH/PEDOT-PSS ink, a more homogeneous CNT distribution with a random orientation was obtained. This study proved also that the droplet ejection velocity can have an impact on the CNT distribution and consequently on the electrical performances of the ink.     

The influence of the transparent layer thickness on the absorption capacity of epoxy/carbon nanotube buckypaper at X-band

Carbon nanotube films (BPs) as EMI shielding materials can be applied in electronic and communication devices due to their high electrical conductivity. Sandwich structures can offer excellent shielding effectiveness by introducing a wave-transmitting layer between conductive films. However, the optimization of the structure demands a deep investigation and plays a crucial role in the final shielding properties of the composites. In this work, BPs are incorporated into epoxy substrates with variable thicknesses (1–6 mm) to fabricate epoxy/BP sandwich structures. The morphology of the CNT films is analyzed by SEM, and the electrical conductivity of all prepared samples is measured by 4-point method. The electromagnetic tests are carried out in the X-band (8.2–12.4 GHz) through the scattering parameters. SEM images reveal a porous structure without visible agglomeration. The electrical conductivity of the BP reaches up to 996 S/m, whereas the values for epoxy/BP composites varies in the range of 8.51–3.13 S/m (1 to 3 mm). BP total shielding efficiency (SE_T) is approximately 14 dB along the X-band spectrum, with similar contributions of reflection and absorption losses. While, the composites show mainly absorbing behavior, especially in the thicker samples, with more significant SE_T values (23.4 dB–6 mm).     

Growth of novel carbon phases by methane infiltration of free-standing single-walled carbon nanotube films

High-temperature methane infiltration of thin, free-standing films of acid-treated single-walled carbon nanotubes (SWCNT) has been studied by means of scanning electron microscopy, transmission electron microscopy and Raman spectroscopy. In the early stages of infiltration, carbon nuclei form predominantly at SWCNT bundle intersections. Further growth proceeds via the formation of graphite nanosheets - without further influence of the nanotube support. Both sheet edges and their structural imperfections act as reaction centers for subsequent deposition, likely giving rise to autocatalytic deposition kinetics. In contrast, infiltration with a H₂:CH₄ (24:1) mixture leads to the reductive activation of residual Ni/Co impurities embedded in the precursor SWCNT-felt. This is associated with a different predominant carbon deposition mode in which multiwalled carbon nanotubes grow out from the substrate.     

Measuring the thickness of ultra-thin diamond-like carbon films

This paper examines the challenge posed by the measurement of thickness of sub-50 nm diamond-like carbon (DLC) films deposited onto silicon substrates. We compared contact profilometry (CP), optical profilometry (OP), contact atomic force microscopy (CAFM), tapping atomic force microscopy (TAFM) and X-ray reflectometry (XRR). Generally, CP, CAFM, TAFM and XRR give similar thickness values except for the case of the more compliant samples measured by CP and CAFM. Moreover, the theoretically precise XRR technique gives significant standard deviation due to the layering of the DLC film. For those transparent samples, OP always gives an erroneous measurement. These metrological artefacts are compared to calculations of mechanical deformation (CP and CAFM), energy dissipation (TAFM) and thin film interferences (OP). The OP artefact is used to extract the film’s refractive index, in good agreement with literature values. Finally, the comparative data obtained in this study also shows that the density and refractive index of the 10 nm thick films are constituently lower than those of the 50 nm thick films. This scaling effect, which is consistent with known growth mechanisms for DLC, further complicates the measurement of thickness by optical techniques.     

Controlling the thickness of a fluoroplastic coating for polyimide films

The order of developing instruments for process control proposed in [3] was confirmed on a concrete example. The ITP-1 instrument allows controlling the thickness of a fluoroplastic coating on a polyimide film directly in process equipment by a contactless method of measurement. The technical solutions for controlling the thickness of film and fibre materials when the object measured exhibits transverse mechanical oscillations are reported. A method is proposed for checking the ITP-1 instrument using a calculation equation. A possible area of application of these process solutions is indicated. __________ Translated from Khimicheskie Volokna, No. 6, pp. 61–63, November–December, 2006.     

Risk-efficient sequential estimation of multivariate random coefficient autoregressive process

A vector-valued autoregressive time series model is considered. The autoregressive coefficients of the model are random with possible dependencies among them. Estimation of the large number of parameters in such models becomes costly with an increase in dimension. A sequential procedure is proposed that promises a significant gain in the sample size thus reduction in the cost of implementation. The procedure is also risk efficient in the sense that as the cost of sampling becomes negligible the asymptotic predictive risk of the proposed procedure reaches the oracle predictive risk corresponding to the best fixed sample size procedure that assumes the values of the nuisance parameters to be known. Extensive simulation results are presented to illustrate the properties of the proposed procedure in a finite sample.     

Frictional behaviour of vertically aligned carbon nanotube films

Vertically aligned CNT films were grown on polycrystalline β-SiC wafers by the surface decomposition method. Their frictional behaviours were investigated by AFM at the nanometer scales. Compared with DLC film and silicon wafers, they demonstrate an extremely low friction coefficient at the nanometer scale about 0.03-0.04. The effect of the surface topography on the friction coefficient is obvious for the aligned CNT film sliding at the nanometer scale. This implies that the excellent tribological properties of the vertically aligned CNT films, combined with their small dimensions and structural perfection, might lead to significant improvement of the performance of nano-devices.     

Work-function engineering of carbon nanotube transparent conductive films

We developed a method that allows control of the work function of carbon nanotube (CNT) transparent conductive films on a flexible sheet of plastic. The approach involves the deposition of small amount of metals, such as aluminum, on CNT transparent conductive films followed by a measure of the work function of the films using in situ ultraviolet photoelectron spectroscopy. Core-level spectra of the films were collected in order to investigate the chemical reaction when a small amount of aluminum was deposited on their surface in a stepwise manner. The measurements revealed that deposition of less than 0.5 nm of aluminum was enough to control the work function of CNT transparent conductive films.     

Recent advances in hybrids of carbon nanotube network films and nanomaterials for their potential applications as transparent conducting films

The use of carbon nanotubes (CNTs) as transparent conducting films is one of the most promising aspects of CNT-based applications due to their high electrical conductivity, transparency, and flexibility. However, despite many efforts in this field, the conductivity of carbon nanotube network films at high transmittance is still not sufficient to replace the present electrodes, indium tin oxide (ITO), due to the contact resistances and semi-conducting nanotubes of the nanotube network films. Many studies have attempted to overcome such problems by the chemical doping and hybridization of conducting guest components by various methods, including acid treatment, deposition of metal nanoparticles, and the creation of a composite of conducting polymers. This review focuses on recent advances in surface-modified carbon nanotube networks for transparent conducting film applications. Fabrication methods will be described, and the stability of carbon nanotube network films prepared by various methods will be demonstrated.     

High-performance biosensors based on enzyme precipitate coating in gold nanoparticle-conjugated single-walled carbon nanotube network films

Single-walled carbon nanotube (SWCNT) network films with high network density were prepared by vacuum-filtering a suspension of SWCNTs, and used as a host of enzyme precipitate coating of glucose oxidase (EPC-GOx). EPC-GOx was fabricated into the SWCNT network films in a two-step process of enzyme precipitation and crosslinking. High GOx loading in a form of EPC expedited the generation of electrons while the good connectivity of conductive SWCNTs in the network structure increased the electron transfer rate. According to amperometric measurements, the sensitivities of GOx/SWCNT electrodes, governed by both generation and transfer of electrons, were significantly enhanced by filling up the open pores of SWCNT films with the EPC-GOx when compared to the approaches of covalent-attachment (CA) and enzyme coating (EC) with no step of enzyme precipitation. For example, the sensitivities of CA, EC and EPC-GOx were 0.039, 0.140, and 5.72 μA mM⁻¹, respectively. High sensitivity of EPC-GOx was maintained under iterative uses for 10 days. The deposition of gold nanoparticles into SWCNT films has resulted in high-performance glucose sensors with a remarkable sensitivity of 24.5 μA mM⁻¹, which can be explained by further expedited electron transfer due to deposited gold nanoparticles.     

Effect of sidewall modification in the determination of friction coefficient of vertically aligned carbon nanotube films using friction force microscopy

The significance of the sidewall surface of vertically aligned carbon nanotubes (VACNTs) and the effect of humidity in the determination of VACNT film friction coefficient have been investigated. VACNT films of 2 μm thick were sidewall-modified by means of CF₄ and O₂ plasma treatments, and verified for the functionalization of the sidewalls. They were then characterized for wettability properties, as well as friction coefficient using friction force microscopy at different humidity levels. It was found that humidity had insignificant effect on the friction coefficient, and sidewall friction formed a major component of the friction force experienced by the tip. Sidewall modifications resulted in friction coefficient changes of up to 50%.     

Densified aligned carbon nanotube films via vapor phase infiltration of carbon

We report a simple way to produce fully densified aligned carbon nanotube (ACNT) films. The simultaneous growth of nanotubes and densification of the ACNT films by carbon infiltration in the interstitial spaces between nanotubes are accomplished in a single step by the combination of the chemical vapor deposition and chemical vapor infiltration processes. Scanning electron microscope analysis and microbalance measurements showed that after infiltration, the diameters of nanotubes and bulk density of the ACNT films are increased by an order of magnitude (and hence the porosity of the ACNT films is decreased). Transmission electron microscope and Raman scattering analysis showed that after densification, the nanotubes are conformally coated by partially graphitized pyrolytic carbon. The compressive modulus of the densified ACNT films could be increased by three orders of magnitude compared to the pristine ACNT films. Electrical properties are also measured for the densified films showing marked differences with the ACNT films. The property enhanced densified ACNT films constitute a new form of carbon-carbon nanocomposites and could find applications as multifunctional nanocomposites.