Session Y6: Carbon Nanotubes: Devices, Capacitors and Other Applications

The Ultimate Electron Sources Using Millimeter Long Carbon Nanotubes

We are reporting the fabrication of a very efficient electron source using long and crystalline carbon nanotubes. These devices start to emit electrons at fields as low as 0.10 V/$\mu $m and reach threshold emission at 0.164 V/$\mu $m. In addition, these electron sources are very stable for long operation periods up to 200 hrs and can achieve peak current density of 2 Acm$^{-2}$ at only 0.28 V/$\mu $m. To demonstrate intense electron beam generation, these devices were used to produce white light by cathodoluminescence. Finally, to rational the measured properties in open carbon nanotubes of different lengths we used density functional theory. The modeling establishes a clear correlation between length and field enhancement factor.   

One-dimensional nature in transport property of SWNT thin film electrochemical transistor

Recent success in isolating single-walled carbon nanotubes (SWNTs) of narrow chirality distribution enabled making pure metallic (m-) and semiconducting (s-) SWNT films. Such films are expected to reflect the nature of individual SWNTs, that is their one dimensional subband structure. Therefore, it is interesting to investigate electronic transport in m- and s-SWNT films by controlling their Fermi level ($E_{F})$. Chemical doping or FET is unsuitable for the purpose because of the lack of precise and reversible $E_{F}$ controllability, and the narrow controllable $E_{F}$ range, respectively. The problems are solved by our electric double layer transistor technique,$^{1}$ where the gate voltage ($V_{G})$ is applied through an electrolyte. The conductance and optical absorption spectra of the resistance of s- and m-SWNT films were measured at various $V_{G}$. The conductance of the s-SWNT film showed stepwise change against $V_{G}$. The absorbance spectra indicate the steps correspond to reaching of the $E_{F}$ to a vHs. Furthermore, even m-SWNT films showed steep increases of conductance, demonstrating that the conductance strongly depend on the subband filling. $^{1}$ H. Shimotani et al., Appl. Phys. Lett. 88, 073104 (2006).   

A general approach for high yield fabrication of CMOS compatible all semiconducting carbon nanotube field effect transistor

We report strategies of achieving both high assembly yield of carbon nanotubes at selected position of the circuit via dielectrophoresis (DEP) and field effect transistor (FET) yield using semiconducting enriched single walled carbon nanotube (s-SWNT) aqueous solution. When the DEP parameters were optimized for the assembly of individual s-SWNT, 97{\%} of the devices show FET behavior with a maximum mobility of 210 cm$^{2}$/Vs, on-off current ratio $\sim $ 10$^{6}$ and on conductance up to 3 $\mu $S, however with an assembly yield of only 33{\%}. As the DEP parameters were optimized so that 1-5 s-SWNTs are connected per electrode pair, the assembly yield was almost 90{\%} with $\sim $ 90{\%} of these assembled devices demonstrating FET behavior. Further optimization gives an assembly yield of 100{\%} with up to 10 SWNT/site, however with a reduced FET yield of 59{\%}. Improved FET performance including higher current on--off ratio and high switching speed were obtained by integrating a local Al$_{2}$O$_{3}$ gate to the device. Our 90{\%} FET with 90{\%} assembly yield is the highest reported so far for carbon nanotube devices. Our study provides a pathway which could become a general approach for the high yield fabrication of CMOS compatible carbon nanotube FETs.   

Scanning Photocurrent Characterization of Absorption Resonances and Photocarrier Generation in Single-Walled Carbon Nanotubes

We use a scanning photocurrent microscope (SPCM) in conjunction with a supercontinuum laser source to study resonant absorption in individual single-walled carbon nanotubes (SWNTs). Characterization by spectrally-resolved SPCM is much faster than using resonant Raman scattering. The technique also complements existing Rayleigh scattering techniques because measurements can be performed on SWNTs in a standard field-effect transistor geometry. The broad band light source (0.67 eV to 2.76 eV) is monochromated and then focused onto the SWNT. Resonant absorption is manifested as peaks in photo-induced conductivity as a function of excitation energy. The wide range of photon energies gives us access to the excitonic transitions E11 to E44 in typical semiconducting SWNTs. This allows us to access information about the diameter/chirality of the nanotube, as well as probing phenomena associated with photogenerated carriers in SWNTs at room temperature.   

Local and broadband photovoltaic response of aligned carbon nanotube films

Although individual semiconducting single-walled carbon nanotubes (SWCNT) have exhibited clear photovoltaic responses, it remains unclear whether macroscopic films of carbon nanotubes can also behave this way. While some groups have explained finite photovoltages as Schottky barrier effects, other groups have proposed photo-thermoelectric effects in suspended films. Here, we have studied highly aligned SWCNT films that work well as photovoltaics. SWCNTs grown by CVD were transferred onto a SiO2 substrate. There was a broad diameter distribution in the films to obtain a large wavelength range of interband absorption. The films were top-contacted with various metals. We made a systematic scanning photocurrent study of such samples at 660 and 1350 nm. A strong local photovoltage appeared at electrode-SWCNT interfaces. Detailed comparison with theoretical calculations of the dependence of photo-response on the nanotube orientation, metal electrode type, and temperature unambiguously revealed the photovoltaic nature of the observed photovoltage. We assign these effects to the doping of both metallic and semiconducting SWCNTs under the electrodes, in a similar fashion to graphene, its lineshape being determined by the diffusion of photoexcited carriers. Finally, to obtain a finite net signal under global illumination, we utilized different electrode combinations and studied their photoresponses from the visible up to mid-infrared and terahertz.   

Separated Carbon Nanotube Macroelectronics for Active Matrix Organic Light-Emitting Diode Displays

Active matrix organic light-emitting diode (AMOLED) display holds great potential for the next generation visual technologies due to its high light efficiency, flexibility, lightweight, and low-temperature processing. However, suitable thin-film transistors (TFTs) are required to realize the advantages of AMOLED. Pre-separated, semiconducting enriched carbon nanotubes are excellent candidates for this purpose because of their excellent mobility, high percentage of semiconducting nanotubes, and room-temperature processing compatibility. Here we report, for the first time, the demonstration of AMOLED displays driven by separated nanotube thin-film transistors (SN-TFTs) including key technology components such as large-scale high-yield fabrication of devices with superior performance, carbon nanotube film density optimization, bilayer gate dielectric for improved substrate adhesion to the deposited nanotube film, and the demonstration of monolithically integrated AMOLED display elements with 500 pixels driven by 1000 SN-TFTs. Our approach can serve as the critical foundation for future nanotube-based thin-film display electronics.   

Semiconducting Enriched Carbon Nanotube Aligned Arrays of Tunable Density and Their Electrical Transport Properties

Many proposed applications of semiconducting single walled carbon nanotubes (s-SWNT) require massively parallel array as they can average out inhomogeneity of individual tubes, provide larger on- currents and better transistor properties. Here, we report assembly of solution processed semiconducting enriched (99{\%}) SWNT in an array with varying linear density via ac-dielectrophoresis and investigate electronic transport properties of the fabricated devices. We show that (i) the quality of the alignment varies with frequency of the applied voltage and that (ii) by varying the frequency and concentration of the solution, we can control the linear density in the array from 1 to 25 s-SWNT/$\mu $m. We found that with increasing nanotube density the device mobility increases while the current on-off ratio decreases dramatically. For the dense array, the device current density was 16 $\mu $A/$\mu $m, on-conductance was 390 $\mu $S, and sheet resistance was 30 k$\Omega $/. These values are the best reported so far for any semiconducting nanotube array. Our study will have important implications in fabricating high quality devices for digital and analog electronics.   

Fully Transparent Separated Carbon Nanotube Based Thin-film Transistors and their Application in Display Electronics

Transparent electronics have attracted numerous research efforts in recent years due to its great potential to make significant commercial impact in a wide variety of areas such as transparent displays. High optical transparency as well as good electrical performance is required for this kind of applications. Pre-separated, semiconducting enriched carbon nanotubes are excellent candidates for this purpose due to their excellent mobility, high percentage of semiconducting nanotubes, and room-temperature processing compatibility. Here in this paper, we report fully transparent high-yield transistors based on separated carbon nanotube random network. High electrical performance is achieved by using large work function thin metal layer and indium-tin oxide (ITO) as contacts and all devices show excellent transparency ($\sim $82{\%}). Furthermore, OLED control circuit has been demonstrated with transparent separated nanotube thin-film transistors and large range output light intensity modulation has been observed. Our results suggest the promising future of separated carbon nanotube based transparent electronics and can serve as the critical foundation for the next generation transparent display applications.   

Fabrication of Carbon nanotube TFTs for pressure-sensing device by printing method

Printing technology is very promising from many advantages, for example, low cost, flexible etc. We previously have developed a high-performance printed thin-film transistor(TFT) using single-walled carbon nanotube(CNT). A simple ink-jet printing system was used for drawing the device patterns. The maximum temperature was 200 degrees during nano-silver electrode fabrication. The temperature of CNT-channel patterning is under 50 degrees. The widths of the source and drain electrodes were about 1 mm and the channel length were about 150 $\mu$m. The thickness of the gate insulator was about 650 nm. The estimated mobilities using over 95{\%} purified semiconductive-CNT were $\sim $5.1 cm2/Vs for the TFTs whose on/off ratio were more than 5,000. This time we fabricated CNT-TFT arrays for pressure-sensing sheet devices, using the printed-process on a plastic film. CNT-TFT sheet has a dimension of 16 x 16 TFTs. The average mobility of the TFTs is 4.6 cm2/Vs. The pressure-sensing cell was prepared to combine Printed-TFT, a conductive rubber sheet and a film with a copper foil. Drain current changes in response to pressure applied to the current changes were observed up to 100nA from 10pA. These results were very promising for CNT-TFT applications to printable electronics.   

Flexible Single-wall Carbon Nanotube Membrane Symmetric Aqueous Double Layer Electrochemical Capacitor

We present preliminary results on an aqueous symmetric double layer electrochemical capacitor (EDLC) constructed with flexible binder-free single wall carbon (SWCNTs) membrane as electrodes. The capacitors were cycled from 0 to 1V @ 10 A/g for 10,000 cycles with 99.9{\%} coulombic efficiency and 94{\%} energy efficiency, and 100{\%} depth of discharge. The power performance of the aqueous symmetric SWCNTs membrane capacitor is almost 100 --1000 times better than commercial non-aqueous EDLC capacitors.   

Optimizing Efficiency in Conducting Polymer/Single-walled Carbon Nanotube Hybrids for Organic Photovoltaics

Several unique properties of single-walled carbon nanotubes (SWCNTs) have motivated their investigation as potential replacements for fullerene derivatives as the acceptor phase of organic photovoltaic (OPV) devices. Although replacement of the ubiquitous fullerene acceptors by SWCNTs in OPV devices has shown limited success thus far, better understanding of charge transfer between SWCNTs and conjugated polymers has promoted its viability. We provide experimental evidence that m-SWNTs limit the generation efficiency and lifetime of the charge-separated state in these composites. We also probe the photo-carrier generation and decay dynamics in poly(3-hexylthiophene) (P3HT) paired with a broad diameter range of SWCNTs. We witness electron transfer from the polymer to SWCNT and \textit{selective} hole transfer from the SWCNT to polymer by varying the nanotube HOMO via its diameter. We finally extend our investigation to additional semi-conducting polymers that have contributed to high OPV efficiencies, pBTTT and PCDTBT.   

Ultra-high density aligned Carbon-nanotube with controled nano-morphology for supercapacitors

Recent advances in fabricating controlled-morphology vertically aligned carbon nanotubes (VA-CNTs) with ultrahigh volume fractioncreate unique opportunities for developing unconventional supercapacitors with ultra-high energy density, power density, and long charge/discharge cycle life.Continuous paths through inter-VA-CNT channels allow fast ion transport, and high electrical conduction of the aligned CNTs in the composite electrodes lead to fast discharge speed. We investigate the charge-discharge characteristics of VA-CNTs with $>$20 vol{\%} of CNT and ionic liquids as electrolytes. By employing both the electric and electromechanical spectroscopes, as well as nanostructured materials characterization, the ion transport and storage behaviors in porous electrodes are studied. The results suggest pathways for optimizing the electrode morphology in supercapacitorsusing ultra-high volume fraction VA-CNTs to further enhance performance.   

Capacitance of highly ordered nanocapacitors arrays: Model and microscopy

It is described briefly the process used to build an ordered porous array in an anodic aluminum oxide (AAO) template, filled with multiwall carbon nanotubes (MWCNTs). The MWCNTs were grown directly inside the template through chemical vapor deposition (CVD). The role of the CNTs is to provide narrow metal electrodes wich contact with a dielectric surface barrier, hence, forming a capacitor. This procedure allows the construction of an array of 10$^{10}$ parallel nano-spherical capacitors/cm$^{2}$. A central part of this contribution is the use of physical parameters obtained from processing high-resolution transmission electron microscopy (HRTEM) images, to predict the specific capacitance of the AAO arrays. Electrical parameters were obtained by solving Laplace's equation through finite element methods   

Gecko inspired carbon nanotube based thermal gap pads

Thermal management has become a critical factor in designing the next generation of microprocessors. The bottleneck in design of material for efficient heat transfer from electronic units to heat sinks is to enhance heat flow across interface between two dissimilar, rough surfaces. Carbon nanotubes (CNT) have been shown to be promising candidates for thermal transport. However, the heat transport across the interface continues to be a challenging hurdle. In the current work we designed free standing thermal pads based on gecko-inspired carbon nanotube adhesives. The pads were made of metallic carbon nanotubes and the structure was designed such that it would allow large area of intimate contact. We showed that these adhesive pads can be used as electrical and thermal interconnects.   

Stiff and Multifunctional Carbon Nanotube Composites

It has been a challenge for two decades to assemble the extremely strong carbon nanotubes (CNTs) into macroscopic CNT composites that break the strength ceiling of carbon fiber composites. Here we report the fast incorporation of long CNTs into polymer matrix using a novel approach, stretch-winding, to produce composites that are much stronger than any current engineering composite. The CNT composites reach a strength of 3.8 GPa, an excellent electrical conductivity and a high thermal conductivity. These superior properties are primarily derived from the long length, high volume fraction, good alignment and reduced waviness of the CNTs that are produced. The combination of high strength and excellent electrical and thermal conductivities makes CNT composites a promising enabler of new aerospace technologies and adventures.