Over 70 publications using NanoIntegris materials.
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Virus-templated Self-assembled Single-walled Carbon Nanotubes for Highly Efficient Electron Collection in Photovoltaic Devices
Application: Optoelectronic Devices
Summary: The performance of photovoltaic devices could be improved by using rationally designed nanocomposites with high electron mobility to efficiently collect photo-generated electrons. Single-walled carbon nanotubes exhibit very high electron mobility, but the incorporation of such nanotubes into nanocomposites to create efficient photovoltaic devices is challenging. Here, we report the synthesis of single-walled carbon nanotube–TiO2 nanocrystal core–shell nanocomposites using a genetically engineered M13 virus as a template. By using the nanocomposites as photoanodes in dye-sensitized solar cells, we demonstrate that even small fractions of nanotubes improve the power conversion efficiency by increasing the electron collection efficiency. We also show that both the electronic type and degree of bundling of the nanotubes in the nanotube/TiO2 complex are critical factors in determining device performance. With our approach, we achieve a power conversion efficiency in the dye-sensitized solar cells of 10.6%.
Harvesting Infrared Solar Energy by Semiconducting Single-Walled Carbon Nanotubes
Application: Optoelectronic Devices
Summary: Semiconducting single-walled carbon nanotubes (S-SWNTs) are used to fabricate p–n heterojuctions with n-type Si in order to harvest near-infrared solar energy. Our results demonstrate that S-SWNTs can be used to convert near-infrared light into electrical energy. The performance of solar cells based on C60-fullerene-encapsualted S-SWNTs is much better than that observed in solar cells fabricated by C60-encapsulated SWNTs containing both metallic and semiconducting SWNTs. It is found that when the light photon energy exceeds two times the band-gap energy of S-SWNTs, the efficiency suddenly increases, suggesting the occurrence of multiple exciton generation.
Semiconducting Enriched Carbon Nanotube Aligned Arrays of Tunable Density and Their Electrical Transport Properties
Application: Transistors
Citation: Biddut K. Sarker, Shashank Shekhar and Saiful I. Khondaker, ACS Nano (2011), 5, 8, 6297–6305.
Summary: We demonstrate assembly of solution-processed semiconducting enriched (99%) single-walled carbon nanotubes (s-SWNTs) in an array with varying linear density via ac dielectrophoresis (DEP) and investigate detailed 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 of the s-SWNTs in the array from 1/μm to 25/μm. The DEP assembled s-SWNT devices provide the opportunity to investigate the transport property of the arrays in the direct transport regime. Room temperature electron transport measurements of the fabricated devices show 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 μA/μm, on-conductance was 390 μS, and sheet resistance was 30 kΩ/. These values are the best reported so far for any semiconducting nanotube array.
Wettability Conversion from Superoleophobic to Superhydrophilic on Titania/Single-Walled Carbon Nanotube Composite Coatings
Application: Other Research
Citation: Min Zhang, Tao Zhang, and Tianhong Cui, Langmuir (2011), 27, 15, 9295–9301.
Summary: Superoleophobic surfaces were demonstrated on perfluorosilane-rendered titania (TiO2)/single-walled carbon nanotube (SWNT) composite coatings. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) observations revealed that SWNTs play a key role in the formation of overhanging structures and the nanoscale roughness on the coating surface, which compose the two critical morphologic factors for a superoleophobic surface. The wettability conversion from superoleophobic to superhydrophilic of the composite coatings was realized by the gradual decomposition of 1H,1H,2H,2H-perfluorodecyltrichlorosilane (FDTS) on the coating surface using UV irradiation. Contact angle measurement on both smooth TiO2 surface and rough composite coating surface under different UV irradiation time revealed that the wetting behavior of the liquids on the composite coating surface passes from the Cassie to the Wenzel and finally to the inversed-Cassie regime. Different liquids show different irradiation time for the wetting state change. By controlling the UV irradiation dose, liquids with surface tension difference smaller than 5 mN/m can exist in completely converse wetting states on the same coating surface, that is, superphobic for one liquid while superphilic for another with lower surface tension. Mixed organic liquids with different surface tension can be completely separated through a coated grid using this wettability tuning technique.
Radio Frequency and Linearity Performance of Transistors Using High-Purity Semiconducting Carbon Nanotubes
Application: Transistors
Summary: This paper reports the radio frequency (RF) and linearity performance of transistors using high-purity semiconducting carbon nanotubes. High-density, uniform semiconducting nanotube networks are deposited at wafer scale using our APTES-assisted nanotube deposition technique, and RF transistors with channel lengths down to 500 nm are fabricated. We report on transistors exhibiting a cutoff frequency (ft) of 5 GHz and with maximum oscillation frequency (fmax) of 1.5 GHz. Besides the cutoff frequency, the other important figure of merit for the RF transistors is the device linearity. For the first time, we report carbon nanotube RF transistor linearity metrics up to 1 GHz. Without the use of active probes to provide the high impedance termination, the measurement bandwidth is therefore not limited, and the linearity measurements can be conducted at the frequencies where the transistors are intended to be operating. We conclude that semiconducting nanotube-based transistors are potentially promising building blocks for highly linear RF electronics and circuit applications.
Air-Stable Conversion of Separated Carbon Nanotube Thin-Film Transistors from p-Type to n-Type Using Atomic Layer Deposition of High-κ Oxide and Its Application in CMOS Logic Circuits
Application: Transistors
Citation: Jialu Zhang, Chuan Wang, Yue Fu, Yuchi Che, and Chongwu Zhou, ACS Nano (2011), 5, 4, 3284–3292.
Summary: Due to extraordinary electrical properties, preseparated, high purity semiconducting carbon nanotubes hold great potential for thin-film transistors (TFTs) and integrated circuit applications. One of the main challenges it still faces is the fabrication of air-stable n-type nanotube TFTs with industry-compatible techniques. Here in this paper, we report a novel and highly reliable method of converting the as-made p-type TFTs using preseparated semiconducting nanotubes into air-stable n-type transistors by adding a high-κ oxide passivation layer using atomic layer deposition (ALD). The n-type devices exhibit symmetric electrical performance compared with the p-type devices in terms of on-current, on/off ratio, and device mobility. Various factors affecting the conversion process, including ALD temperature, metal contact material, and channel length, have also been systematically studied by a series of designed experiments. A complementary metal−oxide−semiconductor (CMOS) inverter with rail-to-rail output, symmetric input/output behavior, and large noise margin has been further demonstrated. The excellent performance gives us the feasibility of cascading multiple stages of logic blocks and larger scale integration. Our approach can serve as the critical foundation for future nanotube-based thin-film macroelectronics.
Analyzing Absorption Backgrounds in Single-Walled Carbon Nanotube Spectra
Application: Other Research
Summary: The sources of broad backgrounds in visible−near-IR absorption spectra of single-walled carbon nanotube (SWCNT) dispersions are studied through a series of controlled experiments. Chemical functionalization of nanotube sidewalls generates background absorption while broadening and red-shifting the resonant transitions. Extensive ultrasonic agitation induces a similar background component that may reflect unintended chemical changes to the SWCNTs. No major differences are found between spectral backgrounds in sample fractions with average lengths between 120 and 650 nm. Broad background absorption from amorphous carbon is observed and quantified. Overlapping resonant absorption bands lead to elevated backgrounds from spectral congestion in samples containing many SWCNT structural species. A spectral modeling method is described for separating the background contributions from spectral congestion and other sources. Nanotube aggregation increases congestion backgrounds by broadening the resonant peaks. Essentially no background is seen in sorted pristine samples enriched in a single semiconducting (n,m) species. By contrast, samples enriched in mixed metallic SWCNTs show broad intrinsic absorption backgrounds far from the resonant transitions. The shape of this metallic background component and its absorptivity coefficient are quantitatively assessed. The results obtained here suggest procedures for preparing SWCNT dispersions with minimal extrinsic background absorptions and for quantifying the remaining intrinsic components. These findings should allow improved characterization of SWCNT samples by absorption spectroscopy.
A Review on Technological Aspects Influencing Commercialization of Carbon Nanotube Sensors
Application: Review Articles
Summary: Carbon nanotubes (CNTs) are one of the advanced functional materials of today and has been researched extensively since its discovery. Although much is still not known about the physical and chemical properties of CNTs, it has already found potential applications in many industries, from defense to electronics and even in environmental remediation. CNTs possess many desirable mechanical and chemical properties, which supercedes many of the advanced materials of today. It was also found that CNTs have excellent electronic properties like unprecedented mobilities of up to 100,000 cm2/V s, which can potentially result in a quantum leap in the electronics industry. Over the recent years, CNT and their derivatives (decorated/functionalized) were also intensively studied, especially in the field of bio and chemical sensing owing to the size similarity of nanotubes with the analytes such as biospecies that enable strong interactions between them. However, despite intensive research, commercialization of these potential applications still remains elusive mainly due to the lack of control in synthesis of specific chirality, diameter and length of CNTs, which influences the device performance. This short review focuses on addressing recent advances in CNT research especially on aspects such as controlled synthesis, decoration/functionalization for specific recognition, sensor device fabrication and commercialization strategies.
Graphene Electrochemistry: Surfactants Inherent to Graphene Can Dramatically Effect Electrochemical Processes
Application: Electrochemical
Summary: Surfactants are routinely used in the production of graphene and additionally in their solubilisation with the aim of reducing the likelihood of coalescing. We demonstrate that surfactants, which are an inherent property of graphene, are a major contribution to the electrochemical performance. Using well characterised commercially available graphene we demonstrate that the surfactant may be detrimental in electrochemical processes, for example in the electrochemical oxidation of NADH, used prolifically as the basis of over 300 biosensors, and in the electrochemical oxidation of acetaminophen, an analgesic and antipyretic drug which requires routine monitoring in a plethora of areas. The use of control experiments in the form of surfactant modified carbon electrodes is particularly encouraged in de-convoluting the origin of the electrochemical response of graphene modified electrodes.
Advances in Carbon Nanotube Based Electrochemical Sensors for Bioanalytical Applications
Application: Review Articles
Summary: Electrochemical (EC) sensing approaches have exploited the use of carbon nanotubes (CNTs) as electrode materials owing to their unique structures and properties to provide strong electrocatalytic activity with minimal surface fouling. Nanofabrication and device integration technologies have emerged along with significant advances in the synthesis, purification, conjugation and biofunctionalization of CNTs. Such combined efforts have contributed towards the rapid development of CNT-based sensors for a plethora of important analytes with improved detection sensitivity and selectivity. The use of CNTs opens an opportunity for the direct electron transfer between the enzyme and the active electrode area. Of particular interest are also excellent electrocatalytic activities of CNTs on the redox reaction of hydrogen peroxide and nicotinamide adenine dinucleotide, two major by-products of enzymatic reactions. This excellent electrocatalysis holds a promising future for the simple design and implementation of on-site biosensors for oxidases and dehydrogenases with enhanced selectivity. To date, the use of an anti-interference layer or an artificial electron mediator is critically needed to circumvent unwanted endogenous electroactive species. Such interfering species are effectively suppressed by using CNT based electrodes since the oxidation of NADH, thiols, hydrogen peroxide, etc. by CNTs can be performed at low potentials. Nevertheless, the major future challenges for the development of CNT-EC sensors include miniaturization, optimization and simplification of the procedure for fabricating CNT based electrodes with minimal non-specific binding, high sensitivity and rapid response followed by their extensive validation using “real world” samples. A high resistance to electrode fouling and selectivity are the two key pending issues for the application of CNT-based biosensors in clinical chemistry, food quality and control, waste water treatment and bioprocessing.
Biosensors Based on One-dimensional Nanostructures
Application: Review Articles
Summary: Over the past decade, one-dimensional nanostructures (1D-NS) have been studied for the detection of biological molecules. These nanometre-scale materials, with diameters comparable to the size of individual biomolecules, offer the advantage of high sensitivity. In this feature article we discuss different techniques of biosensing using 1D-NS, namely electrical, electrochemical, optical, and mechanical methods, with a focus on the advancement of these techniques. Advantages and disadvantages of various synthesis and functionalization methods of 1D-NS, as well as biosensor device fabrication procedures are discussed. The main focus of this review is to demonstrate the progress of protein and DNA sensors based on 1D-NS over the past decade, and in addition we present an outlook for the future of this technology.
High-performance Local Back Gate Thin-Film Field-Effect Transistors Using Sorted Carbon Nanotubes on an Amino-Silane Treated Hafnium Oxide Surface
Application: Transistors
Citation: K C Narasimhamurthy and Roy Paily, Semiconductor Science and Technology (2011), 26, 075002.
Summary: 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.
Fabricating Devices with Dielectrophoretically Assembled, Suspended Single Walled Carbon Nanotubes for Improved Nanoelectronic Device Characterization
Application: Transistors
Integrating carbon nanotubes (CNTs) and nanowires into devices for sensing, actuation and other nanoelectronic applications has the potential to increase device efficiency and lower power consumption. Examples include ultra-high frequency CNT filters and resonators for high sensitivity gas/mass detection. Reliable operation requires careful mechanical and electrical characterization of the integrated CNTs and their contact with electrodes. In this work, we demonstrate a fabrication strategy and integration of suspended single walled CNTs (SWCNTs) on a chip for investigation of the metal nanotube interfacial adhesion strength. A multi-step etching process is used to prepare SWCNTs integrated on TEM compatible chips. Alternating current (AC) dielectrophoresis (DEP) is used for selective SWCNT integration simultaneously overcoming localization issues. The suspended tubes are conducive to mechanical manipulation or electrostatic actuation. In addition, our approach provides fully suspended electrodes for TEM analysis with reduced charging issues that are typically caused by supporting insulating layers. This enables the visualization of failure modes of the tube/electrode contact that have not been previously observed.
Graphene Electrochemistry: Fabricating Amperometric Biosensors
Application: Chemical and Biological Sensors
Citation: Dale A. C. Brownson and Craig E. Banks, Analyst (2011), 136, 10, 2084-2089.
Summary: The electrochemical sensing of hydrogen peroxide is of substantial interest to the operation of oxidase-based amperometric biosensors. We explore the fabrication of a novel and highly sensitive electro-analytical biosensor using well characterised commercially available graphene and compare and contrast responses using Nafion -graphene and -graphite modified electrodes. Interestingly we observe that graphite exhibits a superior electrochemical response due to its enhanced percentage of edge plane sites when compared to graphene. However, when Nafion, routinely used in amperometric biosensors, is introduced onto graphene and graphite modified electrodes, re-orientation occurs in both cases which is beneficial in the former and detrimental in the latter; insights into this contrasting behaviour are consequently presented providing acuity into sensor design and development where graphene is utilised in biosensors.
Graphene Electrochemistry: Surfactants Inherent to Graphene Inhibit Metal Analysis
Application: Other Research
Citation: Dale A.C. Brownson and Craig E. Banks, Electrochemistry Communications (2011), 13, 2, 111-113.
Summary: We demonstrate for well characterised and commercially available graphene that the surfactant used in its fabrication inhibits the electro-analytical sensing of cadmium(II) via anodic stripping voltammetry. Inspection of the deposition and stripping steps reveals that the surfactant inhibits the latter corresponding to the transition of cadmium metal to cadmium ions; this observation is in distinct contrast to the current electrochemical literature of graphene for metal analysis.
Scalable Complementary Logic Gates with Chemically Doped Semiconducting Carbon Nanotube Transistors
Application: Transistors
Summary: Use of random network carbon nanotube (CNT) transistors and their applications to complementary logic gates have been limited by several factors such as control of CNT density, existence of metallic CNTs producing a poor yield of devices, absence of stable n-dopant and control of precise position of the dopant, and absence of a scalable and cost-effective fabrication process. Here, we report a scalable and cost-effective fabrication of complementary logic gates by precisely positioning an air-stable n-type dopant, viologen, by inkjet printing on a separated semiconducting CNTs network. The obtained CNT transistors showed a high yield of nearly 100% with an on/off ratio of greater than 103 in an optimized channel length (9 μm). The n-doped semiconducting carbon nanotube transistors showed a nearly symmetric behavior in the on/off current and threshold voltage with p-type transistors. CMOS inverter, NAND, and NOR logic gates were integrated on a HfO2/Si substrate using the n/p transistor arrays. The gain of inverter is extraordinarily high, which is around 45, and NAND and NOR logic gates revealed excellent output on and off voltages. These series of whole processes were conducted under ambient conditions, which can be used for large-area and flexible thin film technology.
An Ultrasensitive and Low-Cost Graphene Sensor Based on Layer-By-Layer Nano Self-Assembly
Application: Chemical and Biological Sensors, Biomedical
Citation: Bo Zhang and Tianhong Cui, Applied Physics Letters (2011), 98, 073116.
Summary: The flexible cancer sensor based on layer-by-layer self-assembled graphene reported in this letter demonstrates features including ultrahigh sensitivity and low cost due to graphene material properties in nature, self-assembly technique, and polyethylene terephthalate substrate. According to the conductance change of self-assembled graphene, the label free and labeled graphene sensors are capable of detecting very low concentrations of prostate specific antigen down to 4 fg/ml (0.11 fM) and 0.4 pg/ml (11 fM), respectively, which are three orders of magnitude lower than carbon nanotube sensors under the same conditions of design, manufacture, and measurement.
Fundamental Limits on the Mobility of Nanotube-Based Semiconducting Inks
Application: Transistors
Citation: Nima Rouhi, Dheeraj Jain, Katayoun Zand, Peter John Burke, Advanced Materials (2011), 23, 1, 94-99.
Summary: High mobility and high on/off ratio thin-film transistors are fabricated using solution-based deposition of purified semiconducting carbon nanotubes. A comprehensive spectrum of the density starting from less than 10 tubes μm-2 to the high end of around 100 tubes μm-2 is investigated. This study provides the first important roadmap for the tradeoffs between mobility and on/off ratio in nanotube-based semiconducting inks.
Aligned Dense Single-Walled Carbon Nanotube Beams and Cantilevers for Nanoelectromechanical Systems Applications
Application: Electromechanical, Other Research
Summary: A processable approach to fabricate suspended and aligned single-walled carbon nanotube (SWNT) beams and cantilevers is presented in this article. Suspended dense SWNT membranes were aligned and deposited by a controlled dielectrophoresis process. A gallium focused ion beam at 30 keV and 50 pA with an optimized dose bombarded the SWNT membranes to prepare them for suspended nanoscale beams and cantilevers. To demonstrate the application of this process to nanoelectromechanical systems (NEMS), an SWNT switch was realized with a pull-in voltage of ∼ 7.8 V. Accordingly, the fabrication process of SWNT beams and cantilevers is believed to be very promising for prototyping of many NEMS devices such as switches, resonators, and biosensors.
Challenges in the Use of 1D Nanostructures for On-chip Biosensing and Diagnostics: A Review
Application: Review Articles
Citation: Kannan Balasubramanian, Biosensors and Bioelectronics (2010), 26, 4, 1195-1204.
Summary: This review outlines the use of one-dimensional nanostructures (1D-NS) for the detection of biomolecules on a chip. The materials discussed here include carbon nanotubes, metallic and semiconducting nanowires and nanochannels. While nanotubes and naowires have predominantly been used as electrical detectors, nanochannels are promising frameworks for optical detection in applications such as separation, preconcentration and DNA mapping. The primary expectation for all the three types of 1D-NS lies in the promise for ultimate single molecule detection. Furthermore, the electrical double layer governs the physics behind biosensing in all the three systems. The review starts by shedding light on the advantages arising due to the use of 1D nanostructures, followed by a discussion of fundamental aspects such as double layer effects and sensing methodologies. After this, the three kinds of 1D-NS are introduced. The main focus of the review is an in-depth analysis of the current achievements in the field and the major challenges that are to be overcome for the widespread use of such nanostructures in applications such as lab-on-a-chip devices and point-of-care diagnostics.
Evaluating Defects in Solution-Processed Carbon Nanotube Devices via Low-Temperature Transport Spectroscopy
Application: Review Articles
Citation: Paul Stokes and Saiful I. Khondaker, ACS Nano (2010), 4, 5, 2659–2666.
Summary: We performed low-temperature electron transport spectroscopy to evaluate defects in individual single-walled carbon nanotube (SWNT) devices assembled via dielectrophoresis from a surfactant-free solution. At 4.2 K, the majority of the devices show periodic and well-defined Coulomb diamonds near zero gate voltage corresponding to transport through a single quantum dot, while at higher gate voltages, beating behavior is observed due to small potential fluctuations induced by the substrate. The Coulomb diamonds were further modeled using a single electron transistor simulator. Our study suggests that SWNTs derived from stable solutions in this work are free from hard defects and are relatively clean. Our observations have strong implications on the use of solution-processed SWNTs for future nanoelectronic device applications.
Exploring the Physicoelectrochemical Properties of Graphene
Application: Other Research
Citation: Dimitrios K. Kampouris and Craig E. Banks, Chemistry Communications (2010), 46, 47, 8865-9068.
Summary: Convincing evidence is presented demonstrating that the electro-catalytic nature of graphene resides in electron transfer from the edge of graphene which structurally resembles the behaviour of edge plane (rather than basal plane) of highly ordered pyrolytic graphite. The impact of surfactants intrinsic to graphene on the electrochemical response is highlighted.
The Electrochemical Response of Graphene Sheets is Independent of the Number of Layers from a Single Graphene Sheet to Multilayer Stacked Graphene Platelets
Application: Electrochemical
Citation: Madeline Shuhua Goh, Martin Pumera, Chemistry - An Asian Journal (2010), 5, 11, 2355–2357.
Summary: We compare electrochemical response of single-, few-, and multilayer graphene sheets and conclude that there is no significant difference between them. Therefore, there is no need for single-layer graphene sheets for electrochemical applications because multilayer graphene provides equal voltammetric performance.
Radiation Effects in Single-Walled Carbon Nanotube Thin-Film-Transistors
Application: Transistors
Summary: The fabrication, characterization, and radiation response of single-walled carbon nanotube (SWCNT) thin-film field effect transistors (SWCNT-TFTs) has been performed. SWCNT-TFTs were fabricated on SiO2-Si substrates from 98% pure semiconducting SWCNTs separated by density gradient ultracentrifugation. Optical and Raman characterization, in concert with measured drain current Ion/Ioff ratios, up to 104, confirmed the high enrichment of semiconducting-SWCNTs. Total ionizing dose (TID) effects, up to 10 MRads, were measured in situ for a SWCNT-TFT under static vacuum. The results revealed a lateral translation of the SWCNT-TFT transfer characteristics to negative gate bias resulting from hole trapping within the SiO2 and SiO2-SWCNT interface. Additional TID exposure conducted in air on the same device had the opposite effect, shifting the transfer characteristics to higher gate voltage, and increasing the channel conductance. No significant change was observed in the device mobility or the SWCNT Raman spectra following a TID exposure of 10 Mrad(Si), indicating extrinsic factors dominate the transfer characteristics in the SWCNT-TFT devices during irradiation. The extrinsic effects of charge trapping and the role that gas adsorption plays in the radiation response are discussed.
A Generalized Method for Evaluating the Metallic-to-Semiconducting Ratio of Separated Single-Walled Carbon Nanotubes by UV−vis−NIR Characterization
Application: Other Research
Summary:A general and useful method has been developed to evaluate the metallic-to-semiconducting (M/S) ratio for separated single-walled carbon nanotubes (SWNTs). By virtue of measuring UV−vis−NIR spectra of a variety of solutions with different ratios of metallic-rich to semiconducting-rich SWNTs, the commercial IsoNanotubes samples as well as metallic-rich HiPCO SWNTs (HiPCO-M) separated by an Agarose gel method have been evaluated. Values of 99.5% metallic contents for IsoNanotubes-M, 98.9% semiconducting contents for IsoNanotubes-S, and 1.24 for the absorption coefficient of IsoNanotubes, whereas 80.4% metallic contents for HiPCO-M and 1.05 for the absorption coefficient of HiPCO SWNTs were obtained. This method does not need pure metallic (M-) or semiconducting (S-) SWNTs as references. Furthermore, we found that this method can also be applied to evaluate the M/S ratio for any SWNT samples.
Tip-Enhanced Raman Spectroscopic Imaging of Localized Defects in Carbon Nanotubes
Application: Other Research
Citation: Carsten Georgi, Achim Hartschuh, Applied Physics Letters (2010), 97, 143117.
Summary: We used tip-enhanced Raman spectroscopy to study defect induced D-band Raman scattering in metallic single-walled carbon nanotubes with a spatial resolution of 15 nm. The spatial extent of the D-band signal in the vicinity of localized defects is visualized and found to be about 2 nm only. Using the strong optical fields underneath the tip, we photogenerate localized defects and derive a relation between defect density and resulting D-band intensity.
Effect of Source, Surfactant, and Deposition Process on Electronic Properties of Nanotube Arrays
Application: Transistors
Summary: The electronic properties of arrays of carbon nanotubes from several different sources differing in the manufacturing process used with a variety of average properties such as length, diameter, and chirality are studied. We used several common surfactants to disperse each of these nanotubes and then deposited them on Si wafers from their aqueous solutions using dielectrophoresis. Transport measurements were performed to compare and determine the effect of different surfactants, deposition processes, and synthesis processes on nanotubes synthesized using CVD, CoMoCAT, laser ablation, and HiPCO.
Micro Tactile Sensors with a Suspended and Oriented Single Walled Carbon Nanotube Beam Embedded in Polydimethylsiloxane Elastomer
Application: Other Research
Citation: Miao Lu, Dongjin Lee, Taiho Yeom, Tianhong Cui, Sensor Letters (2010), 8, 4, 639-644.
Summary:A tactile sensor utilizing a patterned, aligned, and suspended SWNT film as a sensing element is reported in this paper. The sensor was prepared on both silicon and polymer substrates to expand its potential applications to different working conditions. First, a trench 10 μm deep with Cr/Au electrodes on both sides of the trench was realized. Next, dense and oriented SWNT films were self-assembled using dielectrophoresis through in-situ control of the dc resistance of the film. Follow that, the SWNT film was patterned by lithography and oxygen plasma etching to prepare a suspended SWNT beam. Finally, PDMS primer was spin-coated on the structure and cured to protect the SWNT beam and realize a robust tactile sensor. In nanoindentation test, a piezoresistive sensitivity of 5%/mN and a detection limitation of 2 μN were demonstrated. This simple and low temperature fabrication technology is believed to be very promising for flexible tactile sensor and sensor array in applications to smart robots, implantable clinic tools, or embedded pressure sensors in micro fluidic systems.
Effects of Surfactants on Spinning Carbon Nanotube Fibers by an Electrophoretic Method
Application: Other Research
Summary: Thin fibers were spun from a colloidal solution of single-walled carbon nanotubes (SWNTs) using an electrophoretic method. Sodium dodecylbenzenesulfonate (NaDDBS) was chosen as a surfactant and showed good performance owing to its special chemical structure. The highest spinning velocity reached 0.5 mm s−1. The resulting SWNT fibers had a tensile strength of 400 MPa and a conductivity of 355 S cm−1. Their mechanical and electrical properties were markedly improved after adding NaDDBS as the dispersant in water.
The Polarized Carbon Nanotube Thin Film LED
Application: Optoelectronic Devices, Transistors
Summary: We demonstrate a light emitting p-i-n diode made of a highly aligned film of separated (99%) semiconducting carbon nanotubes, self- assembled from solution. By using a split gate technique, we create p- and n-doped regions in the nanotube film that are separated by a micron-wide gap. We inject p- and n-type charge carriers into the device channel from opposite contacts and investigate the radiative recombination using optical micro-spectroscopy. We find that the threshold-less light generation efficiency in the intrinsic carbon nanotube film segment can be enhanced by increasing the potential drop across the junction, demonstrating the LED- principle in a carbon nanotube film for the first time. The device emits infrared light that is polarized along the long axes of the carbon nanotubes that form the aligned film.
Flexible, Transparent Single-Walled Carbon Nanotube Transistors with Graphene Electrodes
Application: Transistors
Summary: This paper reports a mechanically flexible, transparent thin film transistor that uses graphene as a conducting electrode and single-walled carbon nanotubes (SWNTs) as a semiconducting channel. These SWNTs and graphene films were printed on flexible plastic substrates using a printing method. The resulting devices exhibited a mobility of ~ 2 cm2 V -1 s -1, On/Off ratio of ~ 102, transmittance of ~ 81% and excellent mechanical bendability.
Quantifying the Electron Transfer Sites of Graphene
Application: Other Research
Summary: We demonstrate that graphene modified electrodes do not suffer from thin layer effects which is commonly observed in carbon nanotube modified electrodes which precludes mechanistic information to be deduced and false claims of electro-catalysis to be inferred. A simple methodology is presented allowing the electron transfer sites of graphene, viz edge plane sites to be readily determined, allowing researchers to make comparisons in the graphene field such as electrochemical generation and storage devices where graphene has been beneficially applied. Interestingly we find that in comparison of graphene orientated on a surface with that of multi-walled carbon nanotubes, the latter has an identical % of electron transfer sites (edge plane content) with that of the former.
Ultrabroadband Photodetection Based on Graphene Ink
Application: Optoelectronic Devices
Citation: A. Radoi, A. Iordanescu, A. Cismaru, M. Dragoman, D. Dragoman, Nanotechnology (2010), 21 455202.
Summary: We report photodetection in a very large spectral bandwidth, which encompasses ultraviolet, visible and near infrared, using graphene inks or graphene inks functionalized with either gold or silver nanoparticles, or gold nanoparticles further encapsulated with bovine serum albumin deposited on interdigitated electrodes fabricated on a silicon dioxide/silicon substrate. In contrast to gold-functionalized graphene inks, which have responsivities better than 1 mA W-1 at a 0.1 V bias over the huge bandwidth extending from 215 to 2500 nm, Ag-functionalized inks show at least a four-fold increased responsivity, with a record value of 13.7 mA W-1 in near infrared.
Solvation Dynamics of Coumarin 153 in SDS Dispersed Single Walled Carbon Nanotubes (SWNTs)
Application: Other Research
Citation: Abhigyan Sengupta, Partha Hazra, Chemical Physics Letters (2010), 10.1016/j.cplett.2010.10.049.
Summary: We have studied the solvation dynamics and rotational relaxation of Coumarin 153 (C-153) in SDS dispersed two different types of single walled carbon nanotubes (SWNTs), namely metallic and semiconducting, using picosecond fluorescence spectroscopy. It has been observed that solvation dynamics of C-153 in SWNTs is severely retarded compared to pure water and SDS micelle. Time resolved fluorescence anisotropy study suggests that C-153 molecules are located on the surface of SWNT, where the rotational motion of the probe is severely hindered compared to SDS micelle due to the restriction imposed by SWNT surface as well as surrounding SDS monomers or SDS half-cylindrical micelles adsorbed on SWNT surface.
Macroelectronic Integrated Circuits Using High-Performance Separated Carbon Nanotube Thin-Film Transistors
Application: Transistors
Citation: Chuan Wang, Jialu Zhang, Chongwu Zhou, ACS Nano (2010), 4, 12, 7123–7132.
Summary: Macroelectronic integrated circuits are widely used in applications such as flat panel display and transparent electronics, as well as flexible and stretchable electronics. However, the challenge is to find the channel material that can simultaneously offer low temperature processing, high mobility, transparency, and flexibility. Here in this paper, we report the application of high-performance separated nanotube thin-film transistors for macroelectronic integrated circuits. We have systematically investigated the performance of thin- film transistors using separated nanotubes with 95% and 98% semiconducting nanotubes, and high mobility transistors have been achieved. In addition, we observed that while 95% semiconducting nanotubes are ideal for applications requiring high mobility (up to 67 cm2 V-1 s-1) such as analog and radio frequency applications, 98% semiconducting nanotubes are ideal for applications requiring high on/off ratios (>104 with channel length down to 4µm). Furthermore, integrated logic gates such as inverter, NAND, and NOR have been designed and demonstrated using 98% semiconducting nanotube devices with individual gating, and symmetric input/output behavior is achieved, which is crucial for the cascading of multiple stages of logic blocks and larger scale integration. Our approach can serve as the critical foundation for future nanotube-based thin-film macroelectronics.
Ultrasensitive Detection of DNA Molecules with High On/Off Single-Walled Carbon Nanotube Network
Application: Chemical and Biological Sensors
Summary: Semiconducting networks were found to be extremely sensitive to charges, which promises the electrical detection of ultralow concentrations of DNA (down to 0.1 fM, ∼100 DNA molecules).
Electronic-Structure-Dependent Bacterial Cytotoxicity of Single-Walled Carbon Nanotubes
Application: Biomedical
Summary: Single-walled carbon nanotubes (SWNTs) have been previously observed to be strong antimicrobial agents, and SWNT coatings can significantly reduce biofilm formation. However, the SWNT antimicrobial mechanism is not fully understood. Previous studies on SWNT cytotoxicity have concluded that membrane stress (i.e., direct SWN--bacteria contact resulting in membrane perturbation and the release of intracellular contents) was the primary cause of cell death. Gene expression studies have indicated oxidative stress may be active, as well. Here, it is demonstrated for the first time how SWNT electronic structure (i.e., metallic versus semiconducting) is a key factor regulating SWNT antimicrobial activity. Experiments were performed with well-characterized SWNTs of similar length and diameter but varying fraction of metallic nanotubes. Loss of Escherichia coli viability was observed to increase with an increasing fraction of metallic SWNTs. Time-dependent cytotoxicity measurements indicated that in all cases the majority of the SWNT antimicrobial action occurs shortly after (<15 min) bacteria--SWNT contact. The SWNT toxicity mechanism was investigated by in vitro SWNT-mediated oxidation of glutathione, a common intracellular thiol that serves as an antioxidant and redox state mediator. The extent of glutathione oxidation was observed to increase with increasing fraction of metallic SWNTs, indicating an elevated role of oxidative stress. Scanning electron microscopy images of E. coli in contact with the SWNTs demonstrated electronic structure-dependent morphological changes consistent with cytotoxicity and glutathione oxidation results. A three-step SWNT antimicrobial mechanism is proposed involving (i) initial SWN--bacteria contact, (ii) perturbation of the cell membrane, and (iii) electronic structure-dependent bacterial oxidation.
Characterization of Carbon Nanotube Nanoswitches with Gigahertz Resonance Frequency and Low Pull-In Voltages Using Electrostatic Force Microscopy
Application: Electromechanical
Summary: An electrostatic force microscope (EFM) was used to characterize single-walled carbon nanotube (SWNT)-based nanoswitches in this paper. A conductive atomic force microscopy (AFM) tip acted as a mechanical probe as well as a positioning electrode in the experiment. The resonance frequency of the SWNT beams was computed from the measured SWNTs' dimension and spring constant. The pull-in voltages and the corresponding gaps were extracted simultaneously from a set of force curves at different EFM probe bias voltages. The adhesive force between the AFM tip and the SWNT beam was measured through the analysis of retract force curves. The relationship between the pull-in voltage and the SWNT nanoswitch gap was in agreement with the electrostatic pull-in theory. Long-range forces such as meniscus force or electrostatic force from surface charges engaged the SWNT beam when the gap was below 6 nm in atmosphere. The SWNT beam with a resonance frequency of 1.1 GHz was actuated by a voltage of 2 V for a gap of 6.5 nm. The average adhesive force between an SWNT beam and a platinum/iridium (PtIr5)-coated tip was found to be about 50 nN. Considering the stiffness of the 1.1 GHz SWNT beam, the elastic restoring force at 6.5 nm exceeds 53 nN, which will overcome the adhesive force and release the 1.1 GHz SWNT beam. Finally, some possible approaches to further improve the behavior of SWNT nanoswitches are discussed.
Sorting Carbon Nanotubes by Electronic Structure Using Density Differentiation
Application: CNT Sorting Technologies
Summary: This foundational paper established density gradient ultracentrifugation as the technical and commercial solution to the "carbon nanotube polydispersity" problem. It is the most cited paper in the history of Nature Nanotechnology.
Wafer-Scale Fabrication of Separated Carbon Nanotube Thin-Film Transistors for Display Applications
Application: Transistors
Summary: This paper demonstrates a functioning OLED display device based on a waferscale assembly of carbon nanotube thinfilm transistors. Using IsoNanotubes S 95%, the University of California produced transistors with high yield (>98%), low sheet resistance (25kΩ/sq), high current density ( 10µA/µm), and superior mobility (52 cm 2 V-1s-1). Moreover, on/off rations of >10^4 were achieved in devices with channel length L>20µm. To the best of our knowledge, these are the best concurrent CNT transistor numbers reported in the literature to date.
Thin Film Nanotube Transistors Based on Self-Assembled, Aligned Semiconducting Carbon Nanotube Arrays
Application: Transistors
Summary: The IBM T.J. Watson Research Center with Northwestern University fabricated thin-film transistors (TFTs) from DGU produced semiconducting CNTs. To confirm the semiconducting purity of the CNTs, the team synthesized 83 single nanotube transistors from the same DGU produced source material. 82 of the 83 transistors were found to contain a semiconducting nanotube, empirically confirming the material's calculated level (99%) of semiconducting enrichment.
Colored Semitransparent Conductive Coatings Consisting of Monodiserse Metallic Single-Walled Carbon Nanotubes
Citation: Alexander A. Green, Mark C. Hersam, Nano Letters (2008) 8, 5, 1417-1422.
Summary: This paper discusses the performance of DGU-produced metallic nanotubes in transparent conductive films. In comparison to unsorted-CNT films, metallic films were found to be up to 5.6x more conductive in the visible spectrum, and 10x more conductive in the near infrared (NIR) at similar transparencies.
80 GHz Field-Effect Transistors Produced Using High Purity Semiconducting Single-Walled Carbon Nanotubes
Application: Transistors
Summary: In this study, solutions of 99% pure semiconducting nanotubes were used to fabricate SWNT field-effect transistors (FETs) with extrinsic and intrinsic current gain cutoff frequencies of ~15 and ~80 GHz, respectively. Importantly, this study also demonstrates that precise nanotube alignment is not required to achieve excellent performance in high-frequency devices.
Progress Towards Monodisperse Single-Walled Carbon Nanotubes
Application: Review Articles
Citation: Mark C. Hersam, Nature Nanotechnology (2008) 3, 387-394.
Summary: This paper discusses the advantages of DGU over other nanotube sorting strategies, such as dielectrophoresis, selective chemistry, controlled electrical breakdown, and chromatography. In brief, the principle advantages of DGU are its:
- Demonstrated scalability
- Compatibility with a wide range of starting materials
- Use of reversible functionalization chemistry
- Iterative repeatability
Electrochemical Analysis of Single-Walled Carbon Nanotubes Functionalized with Pyrene-Pendant Transition Metal Complexes
Application: Electrochemical, Other Research
Citation: Eden W. McQueen, Jonas I. Goldsmith, JACS (2009), 131, 48, 17554-17556.
Summary: The noncovalent functionalization of single-walled carbon nanotubes (SWNTs) is important in the development of advanced materials and nanoelectronic sensors and devices. A cobalt-terpyridine transition metal complex with pendant pyrene moieties has been shown to successfully functionalize SWNTs via noncovalent π−π stacking interactions. Cyclic voltammetry at SWNT coated platinum electrodes has been utilized to investigate the process of surface modification and provides conclusive evidence of robust surface functionalization. The electrochemical methodology for examining surface functionalization of SWNTs described herein is generalizable to any redox-active system and provides a simple and powerful means for in situ examination of processes occurring at the surface of nanostructured materials.
Evaluation of Transparent Carbon Nanotube Networks of Homogeneous Electronic Type
Application: Other Research
Summary: In this report, we present a description of the optical and electronic properties of as-deposited, annealed, and chemically treated single-walled carbon nanotube (SWNT) films showing metallic or semiconducting behavior. As-deposited and annealed semiconducting SWNT films were significantly less conductive than metallic SWNT films; however, chemical treatment of semiconducting SWNT films resulted in sheet resistance values as low as 60 Ω·sq−1 in comparison to 76 Ω·sq−1 for similarly processed metallic SWNT films. We conclude that the greater improvement of electrical conductivity observed in the semiconducting SWNT film results from the difference in the density of available electronic states between metallic and semiconducting SWNTs. A corroborative investigation of the change in surface work function and the chemical composition of SWNT films, as revealed by X-ray photoelectron spectroscopy, is provided to support these conclusions and to give new perspective to the formation of electronically homogeneous SWNT networks.
Enhanced Electromodulation of Infrared Transmittance in Semitransparent Films of Large Diameter Semiconducting Single-Walled Carbon Nanotubes
Application: Optoelectronic Devices
Citation: Feihu Wang, Mikhail E. Itkis, Robert C. Haddon, Nano Letters (2010), 10, 3, 937-942.
Summary: We report a comprehensive study of the gate-induced electromodulated transmittance of infrared light by single-walled carbon nanotube (SWNT) thin films. The observed electromodulation is significantly enhanced by utilizing large diameter SWNTs, increasing the ratio of semiconducting to metal SWNTs, and by decreasing the SWNT film thickness. The amplitude of the effect reported herein (∼7%) is more than an order of magnitude larger than in previous SWNT thin film solid state devices.
Memory Effect of a Single-Walled Carbon Nanotube on Nitride-Oxide Structure Under Various Bias Conditions
Application: Other Research
Summary: We report on the memory effect of single-walled carbon nanotubes (SWNTs (placed on a nitride-oxide layer structure designed as a charge storage medium. The conductance of the SWNT was modulated by the injected charge in the nitride-oxide interface and the polarities of injected charges were then detected. A large on/off-state current ratio ≶104(was obtained at a small program/erase voltage range ≶3 V(. We also studied the effect of a half-selected cell on the conductance of the SWNTs to identify the issues with cross-point memory architecture.
