Carbon Nanotube Thin Films Research Articles

Growth and characterization of diamond particles, diamond films, and CNT-diamond composite films deposited simultaneously by hot filament CVD

It is important to understand the growth of CNT-diamond composite films in order to improve the inter-link between two carbon allotropes, and, in turn, their physical properties for field emission and other applications. Isolated diamond particles, continuous diamond thin films, and thin films of carbon nanotubes (CNTs) having non-uniformly distributed diamond particles (CNT-diamond composite films) were simultaneously grown on unseeded, seeded, and catalyst pre-treated substrates, respectively, using a large-area multi-wafer-scale hot filament chemical vapor deposition. Films were deposited for four different growth durations at a given deposition condition. The changes in surface morphology and growth behavior of diamond particles with growth duration were investigated ex situ using field emission scanning electron microscopy and 2D confocal Raman depth spectral imaging, respectively. A surface morphological transition from faceted microcrystalline nature to nanocrystalline nature was observed as a function of growth duration in the case of isolated diamond particles grown on both unseeded and catalyst pre-treated substrates. However, such a morphological transition was not observed on the simultaneously grown continuous diamond thin films on seeded substrates. 2D confocal Raman depth spectral imaging of diamond particles showed that the local growth of CNTs did not affect the growth behavior of neighboring diamond particles on catalyst pre-treated substrates. These observations emphasize the importance of surface chemical reactions at the growth site in deciding sp2 or sp3 carbon growth and the final grain size of the diamond films.

Conductivity enhancement of multiwalled carbon nanotube thin film via thermal compression method.

For the first time, the thermal compression method is applied to effectively enhance the electrical conductivity of carbon nanotube thin films (CNTFs). With the assistance of heat and pressure on the CNTFs, the neighbor multiwalled carbon nanotubes (CNTs) start to link with each other, and then these separated CNTs are twined into a continuous film while the compression force, duration, and temperature are quite enough for the reaction. Under the compression temperature of 400°C and the compression force of 100 N for 50 min, the sheet resistance can be reduced from 17 to 0.9 k Ω/sq for the CNTFs with a thickness of 230 nm. Moreover, the effects of compression temperature and the duration of thermal compression on the conductivity of CNTF are also discussed in this work.

Transparent and flexible supercapacitors with single walled carbon nanotube thin film electrodes.

We describe a simple process for the fabrication of transparent and flexible, solid-state supercapacitors. Symmetric electrodes made up of binder-free single walled carbon nanotube (SWCNT) thin films were deposited onto polydimethylsiloxane substrates by vacuum filtration followed by a stamping method, and solid-state supercapacitor devices were assembled using a gel electrolyte. An optical transmittance of 82% was found for 0.02 mg of SWCNTs, and a specific capacitance of 22.2 F/g was obtained. The power density can reach to 41.5 kW · kg(-1) and shows good capacity retention (94%) upon cycling over 500 times. Fabricated supercapacitors will be relevant for the realization of transparent and flexible devices with energy storage capabilities, displays and touch screens in particular.

Fully solution-processed transparent conducting oxide-free counter electrodes for dye-sensitized solar cells: spray-coated single-wall carbon nanotube thin films loaded with chemically-reduced platinum nanoparticles.

We report fully solution-processed fabrication of transparent conducting oxide-free counter electrodes (CEs) for dye-sensitized solar cells (DSSCs) by combining spray-coating of single-wall carbon nanotubes (SWCNTs) and chemical reduction of chloroplatinic acid precursor to platinum nanoparticles (Pt NPs) with formic acid. The power conversion efficiency of a semitransparent DSSC with such SWCNT-based CE loaded with Pt NPs is comparable to that of a control device with a conventional CE. Quantification of Pt loading shows that network morphology of entangled SWCNTs is efficient in forming and retaining chemically reduced Pt NPs. Moreover, electron microscopy and electrochemical impedance spectroscopy results show that mainly Pt NPs, which are tens of nanometers in diameter and reside at the surface of SWCNT CEs, contribute to electrocatalytic activity for triiodide reduction, to which we attribute strong correlation between power conversion efficiency of DSSCs and time constant deduced from equivalent-circuit analysis of impedance spectra.

Dielectrical properties of PET-MWCNT/P3HT:PC70BM/Al device: Impedance spectroscopy analysis

The dielectrical study in dark and under illumination of flexible organic photovoltaic cell (OPV) based on P3HT:PC70BM nanocomposite, which use transparent and conducting multi walled carbon nanotubes (MWCNT) thin films as anode on plastic substrate, is reported. The high frequency response appearing as a semi-circle indicates the homogeneous nature of the electrode–organic interface. On the other hand, inclined straight line localized at low frequency is attributed to recombination and bulk capacitance in the P3HT:PC70BM blend.

Carbon Nanotube Flexible Electronics

Being mechanically flexible with superior electrical properties, thin-films of single-wall carbon nanotubes are proven to be one of the most promising material platforms for high performance flexible electronics. In this talk, I will discuss our solution-based platform for wafer-scale assembly of high-purity semiconducting carbon nanotube networks, which leads to low-cost fabrication of large quantity of thin-film transistors (TFTs) with excellent yield, uniformity, and respectable performance on flexible substrates. Based on the semiconducting carbon nanotube TFTs, a wide range of macro-scale system-level electronics have been demonstrated including flexible integrated circuits, flexible active-matrix organic light-emitting diode displays, and a user-interactive electronic skin that can simultaneously map and respond to various forms of stimuli. With emphasis on large-area systems where nm-scale accuracy in the assembly of nanotubes is not required, our demonstrations present a unique niche for nanotube electronics by taking full advantage of their superior electrical and physical properties. Our work shows carbon nanotubes’ immense promise as a low-cost and scalable TFT technology for nonconventional electronic systems with excellent performance.

Metal Decorated Carbon-Based Materials for Transparent Conductive Thin Films

A facile approach to decorate carbon-based materials, such as graphene oxides (GOx) and carbon nanotubes (CNTs), with metal nanoparticles was developed to enhance the electrical conductivity of coated thin films. CNTs were first surface-functionalized with hydroxyl, carbonyl, and carboxyl functional groups using a simple and green method without strong acid. The as-prepared CNTs were then dispersed in solvents and coated on PET or glass substrates to form conductive thin films. Copper or silver nanoparticles were then decorated on the functionalized CNT films using formaldehyde as a reduction agent. The microstructures of metallized thin films were characterized by FESEM, XRD, and UV measurements. The electrical and mechanical properties of the composite thin films were measured and compared with regular CNT films. The results showed that electrical conductivity of CNT thin films were greatly enhanced after the metallization process. This simple method can be also applied to two-dimensional nanostructured GOx materials to provide thin films with both good transparency and conductivity.

Manipulation of cell membrane using carbon nanotube scaffold as a photoresponsive stimuli generator

We describe, for the first time, the perforation of the cell membrane in the targeted single cell based on the nanosecond pulsed near-infrared (NIR) laser irradiation of a thin film of carbon nanotubes that act as an effective photon absorber as well as stimuli generator. When the power of NIR laser is over 17.5 μJ/pulse, the cell membrane after irradiation is irreversibly disrupted and results in cell death. In sharp contrast, the perforation of the cell membrane occurs at suitable laser power (∼15 μJ/pulse) without involving cell termination.

Electrical properties of PET-MWCNT/MEH-PPV/Al organic device

The study of electrical properties of flexible organic diode (OLEDs) based on MEH-PPV conjugated polymer, which utilize transparent and conducting multi walled carbon nanotube (MWCNT) thin films as anode on plastic substrate, is reported. Diode parameters, such as the series resistance Rs and the ideality factor n were calculated. Overall, our obtained results demonstrate the possibility of integrating the elaborated flexible electrodes in organic diodes having the structure PET-MWCNT/MEH-PPV/Al.

Multiwalled carbon nanotube thin films prepared by aerosol deposition process for use as highly efficient Pt-free counter electrodes of dye-sensitized solar cells

In this study, transparent and catalytic multiwalled carbon nanotube (MWCNT) thin films with controlled thickness were fabricated using a simple and conventional multiple nozzle-based aerosol deposition process (ADP). MWCNTs were homogeneously dispersed in deionized water via surfactant treatment, and the MWCNT-dispersed aqueous solution was then aerosolized and deposited on fluorine-doped tin oxide glass substrates to form MWCNT thin films. The characteristics of MWCNTs, degree of MWCNT dispersion stability in the aqueous solution, and morphology and light transmittance of the prepared MWCNT thin films were systematically examined. Finally, the prepared MWCNT thin films were used as the counter electrodes (CEs) of dye-sensitized solar cells (DSSCs). For comparison purposes, the photovoltaic performance of DSSCs composed of MWCNT thin-film CEs was compared to that of reference DSSCs composed of conventional Pt thin-film CEs. The results showed that the power conversion efficiency of DSSCs composed of critical amount of MWCNTs coated on CEs was almost equal or slightly higher than that of conventional Pt-based DSSCs. Thus, it can be concluded that the ADP-assisted precisely controlled accumulation of transparent and catalytic MWCNT thin films on the CEs of DSSCs is a very promising approach for replacing the expensive Pt metal that is currently used in DSSC industries.

Functionalized Carbon Nanotube Thin Films as the pH Sensing Membranes of Extended-Gate Field-Effect Transistors on the Flexible Substrates

The oxygen-plasma-functionalized carbon nanotube thin films on the flexible substrates as the pH sensing membranes of extended-gate field-effect transistors are proposed for the first time. The carbon nanotubes are ultrasonically sprayed onto the polyimide substrates followed by an oxygen-plasma functionalization. Such oxygen-plasma-treated carbon nanotube thin films (CNTFs) exhibit superior pH sensing characteristics with the sensitivity of 55.7 mV/pH and voltage linearity of 0.9996 in a wide sensing range of pH 1-13. Moreover, the excellent flexibility of carbon nanotube is also demonstrated and the oxygen-plasma-treated CNTFs still maintain the sensitivity of 53.6 mV/pH and voltage linearity of 0.9943 even after five-cycle bending test. These results reveal that the oxygen-plasma-treated CNTFs have great potentials in the practically disposal and wearable biosensor applications.

Generation of mirage effect by heated carbon nanotube thin film

Mirage effect, a common phenomenon in nature, is a naturally occurring optical phenomenon in which lights are bent due to the gradient variation of refraction in the temperature gradient medium. The theoretical analysis of mirage effect generated by heated carbon nanotube thin film is presented both for gas and liquid. Excellent agreement is demonstrated through comparing the theoretical prediction with published experimental results. It is concluded from the theoretical prediction and experimental observation that the mirage effect is more likely to happen in liquid. The phase of deflected optical beam is also discussed and the method for measurement of thermal diffusivity of medium is theoretically verified. Furthermore, a method for measuring the refractive index of gas by detecting optical beam deflection is also presented in this paper.

Carbon Nanotube Thin Film-Supported Fibroblast and Pluripotent Stem Cell Growth

In this study, we functionalized superaligned carbon nanotube (CNT) thin film and performed detailed analysis to characterize the surface modifications. Our results demonstrated that chemical and biochemical modification of CNT thin films significantly altered the level of hydrophilicity. We also showed that the growth of mammalian fibroblasts and pluripotent stem cells on functionalized CNT thin films were better than that on untreated CNT films. Moreover, extracellular matrix protein-conjugated CNT thin film can support pluripotent stem cell growth without feeder cells. These results suggest that once properly functionalized, CNT thin film can be a suitable scaffolding material for a broad range of mammalian cell culture. Our work provided basis for future cell/tissue engineering with this material.

Meniscus-dragging deposition of single-walled carbon nanotubes for highly uniform, large-area, transparent conductors

We report the simple production of highly uniform single-walled carbon nanotube (SWCNT) thin films with significant reduction of the conducting material consumption by applying an effective microliter-scale solution process. The transparent thin film conductors were formed directly on a rigid or flexible substrate within a short process time by dragging a meniscus of an aqueous SWCNT solution with a volume of tens of microliters. The easily controllable coating parameters enabled the fine-tuning of the film thickness on a nanometer scale. This coating process is well suited for large-area, bendable SWCNT thin films whose randomly interconnected networks exhibit two-dimensional percolation behavior.

Effect of atomic interconnects on percolation in single-walled carbon nanotube thin film networks.

The formation of covalent bonds to single-walled carbon nanotube (SWNT) or graphene surfaces usually leads to a decrease in the electrical conductivity and mobility as a result of the structural rehybridization of the functionalized carbon atoms from sp(2) to sp(3). In the present study, we explore the effect of metal deposition on semiconducting (SC-) and metallic (MT-) SWNT thin films in the vicinity of the percolation threshold and we are able to clearly delineate the effects of weak physisorption, ionic chemisorption with charge transfer, and covalent hexahapto (η(6)) chemisorption on these percolating networks. The results support the idea that for those metals capable of forming bis-hexahapto-bonds, the generation of covalent (η(6)-SWNT)M(η(6)-SWNT) interconnects provides a conducting pathway in the SWNT films and establishes the transition metal bis-hexahapto organometallic bond as an electronically conjugating linkage between graphene surfaces.

Fabrication of stable pn junction single-walled carbon nanotube thin films by position selective Cs plasma irradiation method

Stable n-type thin film transistors (TFTs) are fabricated with Cs encapsulated single-walled carbon nanotubes (Cs@SWNTs). The transport property of SWNTs-TFTs clearly changes from p- to n-type characteristic after the Cs plasma irradiation. Based on the systematic investigations, it is revealed that there is an optimum ion energy for the effective Cs encapsulation, which is around 50 eV. Furthermore, it is also found that the n-type feature is stable even in water and high temperature (< 400 °C) conditions. The pn junction structure is also realized by position selective doping of Cs. This very stable pn junction TFT is important for the practical application of SWNTs-based thin film electronics.

Layer-by-layer approach for deposition of pure carbon nanotubes and composite films for use as electrodes in electrochemical devices

A series of thin films of carbon nanotubes (CNT) was made using the layer-by-layer technique. Experiments were conducted to investigate the need for polymer incorporation since pure CNT films are considered too brittle for application. However, the transition to a CNT–polymer composite material limits its suitability for many applications such as electrode material in electrochemical devices. This is due to the high electrical resistance of more common polymers. This study will explore the mechanisms of film growth and analyze the resulting material in terms of porosity and electrical conductivity for a pure CNT and a composite approach.

Analyte-Dependent Sensing Mechanisms: The Fabrication and Characterization of a 32-Channel Array of SWCNT-TF Chemiresistive Sensors

In this article, we discuss the fabrication and characterization of a 32-channel array of single-walled carbon nanotube thin film (SWCNT-TF) chemiresistor sensors. By simply varying the SWCNT-TF device geometry from a length-to-width (L/W) ratio of 1.5:1?15:1, the device switching characteristics were tuned, which provided a unique platform for the investigation of analyte-dependent sensing mechanisms. Our results indicate that the dominant mechanisms for pH sensing and M13 bacteriophage sensing are distinctively different. The percentage resistance change (?R/R0) of the SWCNT-TF devices in response to pH change can be well explained by chemical doping due to OH?/H+ adsorption on nanotubes. In contrast, electrostatic gating was found dominant in the sensing of M13 bacteriophage, where a decrease in the ?double-layer capacitance by M13 captured on the sensor?s surface significantly reduced the overall SWCNT-TF conductance. The of all devices was found to exhibit a logarithmic dependence on M13 concentration. An experimental detection limit of 0.5 pM M13 was demonstrated on all devices, with a theoretical detection limit down to the fM range on high-aspect-ratio (15:1) devices. Using M13 bacteriophage as a model virus, this study demonstrated the potential of SWCNT-TF chemiresistor sensors for real-time electrical virus detection, provided better understanding of sensing mechanisms, and outlined the importance of film/device geometry in the optimization of sensitivity and reproducibility.

Optical properties of carbon nanotube thin films in subterahertz frequency regime

Multiwalled carbon nanotube (MWCNT) thin films deposited on fused quartz substrates are characterized by continuous wave subterahertz free space technique using a vector network analyzer (VNA). The scattering (S) parameters are measured by a VNA in the frequency range of 220–325 GHz (G-band) and 325–500 GHz (Y-band). The complex refractive index is extracted with the help of Nicholson–Ross–Weir method. The transmission and absorption of the thin films are also studied. VNA offers a good spectral resolution and a dynamic range of 60–70 dB around 500 GHz. The experimental technique and material parameter extraction procedures are described. The refractive index, transmission and absorption coefficient obtained for MWCNT thin films are plotted and the results are discussed. © 2014 Wiley Periodicals, Inc. Microwave Opt Technol Lett 56:1895–1898, 2014

Controlled doping of carbon nanotubes with metallocenes for application in hybrid carbon nanotube/Si solar cells.

There is considerable interest in the controlled p-type and n-type doping of carbon nanotubes (CNT) for use in a range of important electronics applications, including the development of hybrid CNT/silicon (Si) photovoltaic devices. Here, we demonstrate that easy to handle metallocenes and related complexes can be used to both p-type and n-type dope single-walled carbon nanotube (SWNT) thin films, using a simple spin coating process. We report n-SWNT/p-Si photovoltaic devices that are >450 times more efficient than the best solar cells of this type currently reported and show that the performance of both our n-SWNT/p-Si and p-SWNT/n-Si devices is related to the doping level of the SWNT. Furthermore, we establish that the electronic structure of the metallocene or related molecule can be correlated to the doping level of the SWNT, which may provide the foundation for controlled doping of SWNT thin films in the future.