Growth and Transfer of Seamless 3D Graphene-Nanotube Hybrids.

Seamlessly connected graphene and carbon nanotube hybrids (GCNTs) have great potential as carbon platform structures in electronics due to their high conductivity and high surface area. Here, we introduce a facile method for making patterned GCNTs and their intact transfer onto other substrates. The mechanism for selective growth of vertically aligned CNTs (VA-CNTs) on the patterned graphene is discussed. The complete transfer of the GCNT pattern onto other substrates is possible because of the mechanical strength of the GCNT hybrids. Electrical conductivity measurements of the transferred GCNT structures show Ohmic contact through the VA-CNTs to graphene--evidence of its integrity after the transfer process.

Generation of terahertz radiation by optical excitation of aligned carbon nanotubes.

We have generated coherent pulses of terahertz radiation from macroscopic arrays of aligned single-wall carbon nanotubes (SWCNTs) excited by femtosecond optical pulses without externally applied bias. The generated terahertz radiation is polarized along the SWCNT alignment direction. We propose that top-bottom asymmetry in the SWCNT arrays produces a built-in electric field in semiconducting SWCNTs, which enables generation of polarized terahertz radiation by a transient photocurrent surge directed along the nanotube axis.

Carbon nanotube terahertz detector.

Terahertz (THz) technologies are promising for diverse areas such as medicine, bioengineering, astronomy, environmental monitoring, and communications. However, despite decades of worldwide efforts, the THz region of the electromagnetic spectrum still continues to be elusive for solid state technology. Here, we report on the development of a powerless, compact, broadband, flexible, large-area, and polarization-sensitive carbon nanotube THz detector that works at room temperature. The detector is sensitive throughout the entire range of the THz technology gap, with responsivities as high as ∼2.5 V/W and polarization ratios as high as ∼5:1. Complete thermoelectric and opto-thermal characterization together unambiguously reveal the photothermoelectric origin of the THz photosignal, triggered by plasmonic absorption and collective antenna effects, and suggest that judicious design of thermal management and quantum engineering of Seebeck coefficients will lead to further enhancement of device performance.

3-Dimensional graphene carbon nanotube carpet-based microsupercapacitors with high electrochemical performance.

In this research, 3-dimensional (3D) graphene/carbon nanotube carpets (G/CNTCs)-based microsupercapacitors (G/CNTCs-MCs) were fabricated in situ on nickel electrodes. The G/CNTCs-MCs show impedance phase angle of -81.5° at a frequency of 120 Hz, comparable to commercial aluminum electrolytic capacitors (AECs) for alternating current (ac) line filtering applications. In addition, G/CNTCs-MCs deliver a high volumetric energy density of 2.42 mWh/cm(3) in the ionic liquid, more than 2 orders of magnitude higher than that of AECs. The ultrahigh rate capability of 400 V/s enables the microdevices to demonstrate a maximum power density of 115 W/cm(3) in aqueous electrolyte. The high-performance electrochemical properties of G/CNTCs-MCs can provide more compact ac filtering units and discrete power sources in future electronic devices. These elevated electrical features are likely enabled by the seamless nanotube/graphene junctions at the interface of the differing carbon allotropic forms.

Physical removal of metallic carbon nanotubes from nanotube network devices using a thermal and fluidic process.

Electronic and optoelectronic devices based on thin films of carbon nanotubes are currently limited by the presence of metallic nanotubes. Here we present a novel approach based on nanotube alkyl functionalization to physically remove the metallic nanotubes from such network devices. The process relies on preferential thermal desorption of the alkyls from the semiconducting nanotubes and the subsequent dissolution and selective removal of the metallic nanotubes in chloroform. The approach is versatile and is applied to devices post-fabrication.

Broadband terahertz polarizers with ideal performance based on aligned carbon nanotube stacks.

We demonstrate a terahertz polarizer built with stacks of aligned single-walled carbon nanotubes (SWCNTs) exhibiting ideal broadband terahertz properties: 99.9% degree of polarization and extinction ratios of 10(-3) (or 30 dB) from ~0.4 to 2.2 THz. Compared to structurally tuned and fragile wire-grid systems, the performance in these polarizers is driven by the inherent anistropic absorption of SWCNTs that enables a physically robust structure. Supported by a scalable dry contact-transfer approach, these SWCNT-based polarizers are ideal for emerging terahertz applications.

Unique origin of colors of armchair carbon nanotubes.

The colors of suspended metallic colloidal particles are determined by their size-dependent plasma resonance, while those of semiconducting colloidal particles are determined by their size-dependent band gap. Here, we present a novel case for armchair carbon nanotubes, suspended in aqueous medium, for which the color depends on their size-dependent excitonic resonance, even though the individual particles are metallic. We observe distinct colors of a series of armchair-enriched nanotube suspensions, highlighting the unique coloration mechanism of these one-dimensional metals.

Carbon nanotube terahertz polarizer.

We describe a film of highly aligned single-walled carbon nanotubes that acts as an excellent terahertz linear polarizer. There is virtually no attenuation (strong absorption) when the terahertz polarization is perpendicular (parallel) to the nanotube axis. From the data, the reduced linear dichrosim was calculated to be 3, corresponding to a nematic order parameter of 1, which demonstrates nearly perfect alignment as well as intrinsically anisotropic terahertz response of single-walled carbon nanotubes in the film.

Abrasion as a catalyst deposition technique for carbon nanotube growth.

Mechanical abrasion of stainless steel (SS) surfaces is demonstrated as an effective technique for the deposition of catalyst to support growth of high density layers of carbon nanotubes (CNTs) in water-assisted catalytic chemical vapor deposition. In all cases of Fe-containing materials abraded on Al(2)O(3) substrates, CNT growth is observed; the 400 series of SS appears to deposit catalyst most efficiently. We demonstrate that this simple abrasion technique enables both micro- and nanoscale accuracy in catalyst patterning as well as large area catalyst deposition for uniform, dense CNT growth. Raman spectroscopy characterization indicates high quality CNTs grown by this approach. This technique provides an inexpensive and simple route for addition of catalyst for Fe-based surface growth of CNTs.

Templated growth of graphenic materials.

A novel strategy is proposed for the topologically controlled synthesis of extended graphenic sheets by additively reacting carbon into a pre-existing graphene sheet which is on top of a templating substrate. This concept is implemented and demonstrated using chemical vapor deposition (CVD). Novel morphological features observed in this study suggest unusual aspects of the CVD growth process. CVD results demonstrate the basic soundness of the synthesis strategy but highlight the sensitivity of the process to certain types of disruption and the need for alternative forms of embodiment.

Selective photochemical functionalization of surfactant-dispersed single wall carbon nanotubes in water.

Ultraviolet (UV) irradiation of single wall carbon nanotubes (SWCNTs) individually dispersed in surfactants leads to diameter and type-selective photohydroxylation of the nanotubes. Photohydroxylation of first semiconductor and then small diameter metallic SWCNTs was confirmed after 254 nm UV irradiation in acidic, neutral, and basic aqueous solutions at ambient and elevated temperatures. The increased oxygen content of the SWCNTs after UV irradiation, as detected by X-ray photoelectron spectroscopy, suggests that SWCNTs were hydroxylated by reaction with water. Attenuated total reflectance Fourier transform infrared analysis provides evidence of hydroxyl functional groups on their surface. This photochemical reaction is impeded by molecular oxygen and appears to involve a reactive intermediate generated in the vicinity of semiconducting SWCNTs. This represents a noncontaminating selective reaction in the liquid phase that uses an intrinsic property of the tubes.

A highly selective, one-pot purification method for single-walled carbon nanotubes.

We report on a one-pot, highly selective chemistry to remove residual catalysts from single-walled carbon nanotubes (SWNTs). The impurities, initially present at approximately 35 wt % and mostly as carbon-coated iron nanoparticles, can be driven below 5 wt % with nearly no loss of SWNTs. The carbon-coated iron impurities are dissolved simply by reacting with an aqueous mixture of H2O2 and HCl at 40-70 degrees C for 4-8 h. This purification combines two known reactions involving H2O2 and HCl, respectively; however, by combining these two typically inefficient reactions into a one-pot reaction, the new process is surprisingly selective toward the removal of the metal impurities. This high selectivity derives from the proximity effect of the iron-catalyzed Fenton chemistry. At pH approximately 1-3, iron is dissolved upon exposure, avoiding the otherwise aggressive iron-catalyzed digestion of SWNTs by H2O2. This extremely simple and selective chemistry offers a "green" and scalable process to purify carbon nanotube materials.

Statistically accurate length measurements of single-walled carbon nanotubes.

The ability to accurately measure the length of nanotubes is important to understanding nanotube growth and cutting processes. To date, there have been few methods available to obtain a statistically significant length measurement of any nanotube sample due to difficulties in obtaining a complete suspension of individual nanotubes and the tedious nature of measuring 1000+ nanotubes. Here we describe a relatively simple method that functionalizes single-walled carbon nanotubes to achieve a high propensity of individual nanotubes in chloroform as high as 92%. This suspension can be dispersed on mica substrates for AFM analysis. Nanotube lengths and heights can be determined using the Nanotube Length Analysis module of SIMAGIS yielding an accurate measure of length and height distribution of a large population of the nanotube sample.

Dispersions of functionalized single-walled carbon nanotubes in strong acids: solubility and rheology.

The manipulation and processing of single-wall carbon nanotubes (SWNTs) is limited by their poor solubility in most common solvents. Covalent sidewall functionalization of SWNTs provides an excellent route to improve their solubility. Here we have studied the relationship between sidewall functionalization and phase behavior of solutions of functionalized SWNTs (f-SWNTs) in strong acids. We use centrifugation in conjunction with UV-Vis-nlR spectroscopy to quantify the solubility of f-SWNTs in strong acids. We image the dispersions of functionalized tubes by polarized light microscopy. We find that adding butyl groups increases marginally the solubility of SWNTs in 102% sulfuric acid in the isotropic phase; adding 9-nonadecyne groups roughly doubles the solubility of SWNTs. Viscosity measurements in dilute solutions are sensitive to de-bundling. We compare the viscosity-concentration dependence of dilute pristine and f-SWNTs to assess whether and how functionalization promotes de-bundling and stabilizes the tubes. The phase behavior and rheology of these f-SWNTs parallels with that of pristine SWNTs; 9-nonadecylated SWNTs have higher solubility and should be easier to process.

Atomic H-Induced Mo2C Hybrid as an Active and Stable Bifunctional Electrocatalyst.

Mo2C nanocrystals (NCs) anchored on vertically aligned graphene nanoribbons (VA-GNR) as hybrid nanoelectrocatalysts (Mo2C-GNR) are synthesized through the direct carbonization of metallic Mo with atomic H treatment. The growth mechanism of Mo2C NCs with atomic H treatment is discussed. The Mo2C-GNR hybrid exhibits highly active and durable electrocatalytic performance for the hydrogen-evolution reaction (HER) and oxygen-reduction reaction (ORR). For HER, in an acidic solution the Mo2C-GNR has an onset potential of 39 mV and a Tafel slope of 65 mV dec-1, and in a basic solution Mo2C-GNR has an onset potential of 53 mV, and Tafel slope of 54 mV dec-1. It is stable in both acidic and basic media. Mo2C-GNR is a high-activity ORR catalyst with a high peak current density of 2.01 mA cm-2, an onset potential of 0.93 V that is more positive vs reversible hydrogen electrode (RHE), a high electron transfer number n (∼3.90), and long-term stability.

Overcoming Catalyst Residue Inhibition of the Functionalization of Single-Walled Carbon Nanotubes via the Billups-Birch Reduction.

The Billups-Birch Reduction chemistry has been shown to functionalize single-walled carbon nanotubes (SWCNTs) without damaging the sidewalls, but has challenges in scalability. Currently published work uses a large mole ratio of Li to carbon atoms in the SWCNT (Li:C) to account for lithium amide formation, however this increases the cost and hazard of the reaction. We report here the systematic understanding of the effect of various parameters on the extent of functionalization using resonant Raman spectroscopy. Addition of 1-iodododecane yielded alkyl-functionalized SWCNTs, which were isolated by solvent extraction and evaporation, and purified by a hydrocarbon wash. The presence of SWCNT growth catalyst residue (Fe) was shown to have a strong adverse effect on SWCNT functionalization. Chlorination-based SWCNT purification reduced the amount of residual Fe, and achieve a maximum ID/IG ratio using a Li:C ratio of 6:1 in a reaction time of 30 min. This result is consistent with published literature requiring 20-fold mole equivalents of Li per mole SWCNT with a reaction time of over 12 h. This new understanding of the factors influencing the functionalization chemistry will help cut down material and process costs, and also increase the selectivity of the reaction toward the desired product.

Cutting of single-walled carbon nanotubes by ozonolysis.

Cutting of single-walled carbon nanotubes (SWNT) has been achieved by extensive ozonolysis at room temperature. Perfluoropolyether (PFPE) was selected as a medium for cutting SWNT due to its high solubility for ozone (O3). A mixture of 9 wt % of O3 in O2 was bubbled through a homogeneous suspension of pristine SWNT in PFPE, at room temperature. The intense disorder mode in the Raman spectra of ozonated SWNT indicates that extensive reaction with the sidewalls of SWNT occurs during ozonolysis. Atomic force microscopy (AFM) images of SWNT, before and after ozonolysis, provided a measure of the extent of the cutting effects. Monitoring of the evolved gases for both pristine and purified SWNT indicates CO2 was produced during the ozonolysis process with a dependence on both system pressure and temperature. During heating, FTIR analysis of gases released indicated that carbon oxygen groups on the sidewalls of SWNT are released as CO2. SWNT was found to be extensively cut after an ozone treatment with a yield of approximately 80% of the original carbon.

Surface area measurement of functionalized single-walled carbon nanotubes.

Single-walled carbon nanotubes have been functionalized and the specific surface areas of the functionalized nanotubes measured. Contrary to expectations, functionalization leads to a decrease in specific surface area compared to that of the unfunctionalized nanotubes. Treatment with a concentrated 1:1 nitric/sulfuric acid mixture followed by high-temperature baking at 1000 degrees C was found to increase the specific surface area of the nanotubes. For the unfunctionalized SWNTs, this treatment increases the specific surface area (SSA) by 20%. In the case of SWNTs functionalized by n-butyl groups the increase in the SSA was nearly 2-fold with the value increasing from 410 (drying at 110 degrees C) to 770 m2/gm (acid and bake treatment followed by drying at 110 degrees C). For the ozonized SWNTs, the SSA increases more than 3-fold from 381 (drying at 110 degrees C) to 1068 m2/gm (acid and bake treatment followed by drying at 110 degrees C). SEM images indicate that the nanotubes rebundle in the solid state with an average bundle size of 10-30 nm. AFM studies show that the ozonized tubes have been cut to short bundles after ozonolysis. Hydrogen uptake studies carried out on the baked ozonized tubes led to a 3 wt % hydrogen uptake at 77 K and 30 bar.

Ozonolysis of functionalized single-walled carbon nanotubes.

Room temperature ozonolysis of fluorinated SWNT and phenyl-sulfonated SWNT have been studied in perfluoropolyether (PFPE) solvents. Etching at the end caps (approximately 70 nm/hour for fluorinated SWNT/PFPE suspension with 1 g/l concentration) has been demonstrated to be the dominating effect during the ozonolysis of fluorinated SWNT. Base on characterization by AFM analysis, X-ray Photoelectron Spectroscopy (XPS) and Raman Spectroscopy, fluorination along the SWNT sidewalls protects F-SWNT from extensive functionalization by ozonolysis. An ozone reaction with fluorinated SWNT has been found to improve its solubility in 96% sulfuric acid. This allows oxidative cutting by ammonium peroxydisulfate without defluorination. In comparison to fluorinated SWNT, phenyl-sulfonated SWNT was found to be effectively and homogeneous cut by ozonolysis in a water suspension.

Growing Carbon Nanotubes from Both Sides of Graphene.

The design and synthesis of hybrid structures between graphene and carbon nanotubes is an intriguing topic in the field of carbon nanomaterials. Here the synthesis of vertically aligned CNT carpets underneath graphene and from both sides of graphene is described with continuous ordering over a large area. Scanning electron microscopy and Raman spectroscopic characterizations show that CNT carpets grow underneath graphene through a base-growth mechanism, and grow on top of graphene through a tip-growth mechanism. Good electrical contact is observed from the top CNT carpets, through the graphene layer, to the bottom CNT carpets. This sandwich-like CNT/graphene/CNT hybrid structure could provide an approach to design and fabricate multilayered graphene/CNTs materials, as well as potential applications in the fields of nanomanufacturing and energy storage.