Spectroscopic evidence and molecular simulation investigation of the pi-pi interaction between pyrene molecules and carbon nanotubes

The pi-pi interaction between pyrene molecules and single-walled carbon nanotubes (SWNTs) or multi-walled carbon nanotubes (MWNTs) was studied by fluorescence, FTIR, Raman spectroscopy and molecular simulation. The carbon nanotubes were incubated in pyrene solution and dried for characterization. A broadband fluorescence emission at 463 nm of the incubated samples was observed, which is similar to that of pyrene excimers but shifts to shorter wavelength. The typical FTIR bands of pyrene shift to lower wavenumbers in the incubated samples. D- and G-bands in Raman spectra of SWNTs also shift to low frequencies. All these spectroscopic evidences reveal the stronger pi-pi stacking interaction between the nanotubes and pyrene molecules over the pyrene dimers, which leads to the formation of pyrene-carbon nanotube complexes. The systems of SWNTs and pyrene molecules were also studied with molecular simulation. It was found from the binding energy calculation that a stronger interaction presents between the pyrene molecule and the nanotube. In addition, the simulation gives some structural information about the pyrene-nanotube complex, such as the staggered conformation of pyrene on nanotube. The effect of defects in carbon nanotube sidewall was also discussed.     

Detection of DNA hybridization using the near-infrared band-gap fluorescence of single-walled carbon nanotubes

We demonstrate the optical detection of DNA hybridization on the surface of solution suspended single-walled carbon nanotubes (SWNTs) through a SWNT band gap fluorescence modulation. Hybridization of a 24-mer oligonucleotide sequence with its complement produces a hypsochromic shift of 2 meV, with a detection sensitivity of 6 nM. The energy shift is modeled by correlating the surface coverage of DNA on SWNT to the exciton binding energy, yielding an estimated initial fractional coverage of 0.25 and a final coverage of 0.5. Hybridization on the nanotube surface is confirmed using Forster resonance energy transfer of fluorophore-labeled DNA oligonucleotides. This detection is enabled through a new technique to suspend SWNTs using adsorption of single-stranded DNA and subsequent removal of free DNA from solution. While the kinetics of free DNA hybridization are relatively fast (<10 min), the kinetics of the process on SWNTs are slower under comparable conditions, reaching steady state after 13 h at 25 degrees C. A second-order kinetic model yields a rate constant of k = 4.33 x 10(5) (M h)(-1). This optical, selective detection of specific DNA sequences may have applications in the life sciences and medicine as in vitro or in vivo detectors of oligonucleotides.     

Chemisorption and diffusion of hydrogen atoms on single-walled carbon nanotubes

Density functional theory has been performed to investigate the chemisorption and diffusion of H atoms on the surface of single-walled carbon nanotubes (SWNTs). The results show that the binding energy of a single hydrogen atom on the SWNTs surface decreases as the diameter of the tube increases and is not affected by the chirality of the tube much. Two hydrogen atoms favor binding at adjacent and opposite positions rather than at alternate carbon site. As for the diffusion of H atoms on the tube, it is found that an isolated H atom can diffuse rather than desorb on the small SWNT upon heating. As the tube diameter increases, the diffusion barrier for H atom on the surface decreases. Further study shows that when the H atom diffuses around another H atom, the diffusion barriers vary with the relative sites of the two H atoms.     

Excited excitonic states in single-walled carbon nanotubes

Polarized photoluminescence excitation spectroscopy on individual SWNTs reveals not only the longitudinal and transverse E 11, E 22, and E 12 ground-state excitons but also excited excitonic states including the continuum. When heated, SWNTs are known to undergo a bandgap shift transition (BST), which effectively changes the nanotube dielectric environment. Here, we show that the entire spectrum of excitonic resonances blue shifts under this transition, with excited states showing larger shifts, approaching 100 meV for a 1 nm diameter nanotube. The excitonic binding energy, Coulomb self-energy correction, and dielectric shift under the BST are estimated. Analysis of this blue shift reveals the dominant effect of dielectric screening on SWNT excitonic states.     

Controlled growth of single-walled carbon nanotubes for unique nanodevices

The controlled growth of bent and horizontally aligned single-walled carbon nanotubes (SWNTs) is demonstrated in this study. The bent SWNTs growth is attributed to the interaction between van der Waals force with substrate and aerodynamic force from gas flow. The curvature of bent SWNTs can be tailored by adjusting the angle between gas flow and step-edge direction. Electrical characterization shows that the one-dimensional resistivity of bent SWNTs is correlated with the curvature, which is due to strain induced energy bandgap variation. Additionally, a downshift of 10 cm(-1) in G-band is found at curved part by Raman analysis, which may be resulted from the bending induced carbon-carbon bond variation. In addition, horizontally aligned SWNTs and crossbar SWNTs were demonstrated. To prove the possibility of integrating the SWNTs having controllable morphology in carbon nanotube based electronics, an inverter with a gain of 2 was built on an individual horizontally aligned carbon nanotube.     

Transparent boron-doped carbon nanotube films

We report results of studies on the sheet resistance and optical transmission of thin films of boron-doped single-walled carbon nanotubes (SWNTs). Boron doping was carried out by exposure of SWNTs to B 2O 3 and NH 3 at 900 degrees C and 1-3 atom % boron was found in the SWNT bundles via electron energy loss spectroscopy (EELS). Boron doping was found to downshift the positions of the optical absorption bands associated with the van Hove singularities (E 11 (s) E 22 (s) and E 11 (m)) by approximately 30 meV relative to their positions in acid-treated and annealed SWNTs. Raman spectroscopy, EELS, and optical data are consistent with the picture that a few atom % boron has been substituted for carbon in the sp (2) framework of SWNTs. Finally, our results show that boron doping does not significantly affect the optical transmittance in the visible region. However, boron doping lowers the sheet resistance by approximately 30% relative to pristine SWNT films from the same batch. Boron-doped SWNT may provide a better approach to touch-screen technology.     

Optical properties of single-walled carbon nanotubes functionalized with copolymer poly(3,4-ethylenedioxythiophene-co-pyrene)

Optical properties are reported for composites based on single-walled carbon nanotubes (SWNTs) and copolymer poly(3,4-ethylenedioxythiophene-co-pyrene) (PEDOT-Py) prepared by chemical polymerization of two monomers in the presence of carbon nanotubes. A charge transfer between SWNTs and the PEDOT-Py copolymer was demonstrated by Raman scattering. The increase in the relative intensity of the Raman lines peaked at 440–577 cm⁻¹, which were assigned to the ethylenedioxy ring vibrational modes, indicated a significant hindrance steric in the case of the composites based on the PEDOT-Py copolymer and metallic SWNTs. The increase in the absorbance of IR band peaked at 984 cm⁻¹ occurred simultaneously with the disappearance of the IR band at 1639 cm⁻¹. This finding was a consequence of the formation of new covalent bonds between SWNTs and the thiophene and benzene rings of the repeating units of the PEDOT-Py copolymer. The photoluminescence (PL) quenching process of the PEDOT-Py copolymer was induced by semiconducting SWNTs. The PL quenching of PEDOT-Py copolymer in the presence of SWNTs was demonstrated based on the energy level diagrams of the two constituents of the PEDOT-Py/SWNTs composite material.     

Increasing the efficiency of single walled carbon nanotube amplification by Fe-Co catalysts through the optimization of CH4/H2 partial pressures

Single walled carbon nanotubes (SWNTs) seeds are grown using Fe-Co nanoparticles on spin-on-glass. The relative efficiency of nucleation and amplification (versus etching) was investigated as a function of the CH(4)/H(2) feedstock ratio and growth temperature. At 900 °C, maximum amplification is obtained with CH(4)/H(2) ratio of 80:20 but 60:40 for nucleation. Amplification is further enhanced at 800 °C, compared with etching dominating at 1000 °C. Amplification of SWNTs is in equilibrium with etching; higher carbon feedstock pressure and decreased temperature increase the rate of amplification; the converse increases etching.     

Influence of polarity on filling polymer molecules into carbon nanotubes

Using molecular dynamics (MD) simulation, we studied the influence of polarity of polymer chains and modified single-walled nanotubes (SWNTs) on filling polymer chains into SWNTs. The center of mass (COM) distance and the interaction energy between the polymer molecules and SWNTs, as well as non-bond energy (including van der Waals energy and electrostatics energy) differences (ΔE) between initial structure and final structure in the SWNTs–polymers systems were calculated. The simulations indicate that both the polarity of polymer molecules and the polarity of modified groups attached to SWNTs can obstruct filling polymer chains into SWNTs. The general conclusions may be of importance in the production of high-performance SWNTs–polymers nanocomposites and have important theoretical significance on the application of carbon nanotubes as drug carrier and transportation channels.     

Enrichment of (8,4) Single‐Walled Carbon Nanotubes Through Coextraction with Heparin

Heparin sodium salt is investigated as a dispersant for dispersing single‐walled carbon nanotubes (SWNTs). Photoluminescence excitation (PLE) spectroscopy is used for identification and abundance estimation of the chiral species. It is found that heparin sodium salt preferentially disperses larger‐diameter Hipco SWNTs. When used to disperse CoMoCAT nanotube samples, heparin has a strong preference for (8,4) tubes, which have larger diameter than the predominant (6,5) in pristine CoMoCAT samples. PLE intensity due to (8,4) tubes increases from 7% to 60% of the total after threefold extractions. Computer modeling verifies that the complex of (8,4) SWNTs and heparin has the lowest binding energy amongst the four semiconducting species present in CoMoCAT. Network field‐effect transistors are successfully made with CoMoCAT/heparin and CoMoCAT/sodium dodecylbenzene sulfonate (SDBS)–heparin (x3), confirming the easy removability of heparin.     

The importance of strong carbon-metal adhesion for catalytic nucleation of single-walled carbon nanotubes

Density functional theory is used to show that the adhesion between single-walled carbon nanotubes (SWNTs) and the catalyst particles from which they grow needs to be strong to support nanotube growth. It is found that Fe, Co, and Ni, commonly used to catalyze SWNT growth, have larger adhesion strengths to SWNTs than Cu, Pd, and Au and are therefore likely to be more efficient for supporting growth. The calculations also show that to maintain an open end of the SWNT it is necessary that the SWNT adhesion strength to the metal particle is comparable to the cap formation energy of the SWNT end. This implies that the difference between continued and discontinued SWNT growth to a large extent depends on the carbon-metal binding strength, which we demonstrate by molecular dynamics (MD) simulations. The results highlight that first principles computations are vital for the understanding of the binding strength's role in the SWNT growth mechanism and are needed to get accurate force field parameters for MD.     

Noncovalent assembly of carbon nanotubes and single-stranded DNA: an effective sensing platform for probing biomolecular interactions

In this paper, we report the assembly of single-walled carbon nanotubes (SWNTs) and single-stranded DNA to develop a new class of fluorescent biosensors which are able to probe and recognize biomolecular interactions in a homogeneous format. This novel sensing platform consists of a structure formed by the interaction of SWNTs and dye-labeled DNA oligonucleotides such that the proximity of the nanotube to the dye effectively quenches the fluorescence in the absence of a target. Conversely, and very importantly, the competitive binding of a target DNA or protein with SWNTs for the oligonucleotide results in the restoration of fluorescence signal in increments relative to the fluorescence without a target. This signaling mechanism makes it possible to detect the target by fluorescence spectroscopy. In the present study, the schemes for such fluorescence changes were examined by fluorescence anisotropy and fluorescence intensity measurements for DNA hybridization and aptamer-protein interaction studies.     

Substrate geometry-dependent nonlinear absorption of carbon nanotubes deposited onto D-shaped optical fibers

The effect of single-wall carbon nanotubes (SWNTs) on nonlinear optical absorption of D-shaped fibers with versatile the remaining length of the cladding region and the interaction length are investigated. The optical absorption based on SWNTs is induced by the energy bandgap in SWNTs. The bandgap energy depends on the tube diameter of SWNTs. After fabricating versatile D-shaped fiber, SWNTs are deposited on the polished surface of D-shaped fibers. The cladding region of single mode fibers is removed by a side-polishing technique and the D-shaped fiber is obtained. In the D-shaped fiber, the cladding region is thin enough to induce the evanescent field coupling of core mode to the other modes of the SWNT-overlay. The nonlinear absorption based on the SWNTs-overlay is changed by the remaining length of cladding region and the interaction length because the coupling strength of evanescent field strongly depends on the different remaining lengths of the cladding region and the interaction lengths as well.     

Effect of constructive rehybridization on transverse conductivity of aligned single-walled carbon nanotube films

Alignment of densely packed single-walled carbon nanotubes (SWNTs) largely preserves the extraordinary electronic properties of individual SWNTs in the alignment direction, while in transverse direction the films are very resistive due to large energy barriers for tunneling between adjacent SWNTs. We demonstrate that chromium atoms inserted between the sidewalls of parallel SWNTs effectively coordinate to the benzene rings of the nanotubes via hexahapto bonds that preserve the nanotube-conjugated electronic structure and serve as a conduit for electron transfer. The atomically interconnected aligned SWNTs exhibit enhanced transverse conductivity, which increases by ～2100% as a result of the photoactivated organometallic functionalization with Cr. The hexahapto mode of bonding the graphitic surfaces of carbon nanotubes with transition metal atoms offers an attractive route to the reversible chemical engineering of the transport properties of aligned carbon nanotube thin films. We demonstrate that a device fabricated with aligned SWNTs can be reversibly switched between a state of high electrical conductivity (ON) by light and low electrical conductivity (OFF) by applied potential. This study provides a route to the design of novel nanomaterials for applications in electrical atomic switches, optoelectronic and spintronic devices.     

碳纳米管填充硅橡胶膜回收大豆油/正己烷混合油的溶剂

采用膜分离技术取代当前的蒸发工艺回收植物油浸取工艺中的浸取溶剂,这一新技术不涉及相变,节能效果明显.为此,研制了单壁碳纳米管填充聚二甲基硅氧烷/聚偏氟乙烯(PDMS/PVDF)复合膜,通过接触角和SEM等方法对复合膜的表面进行表征.将制备的复合膜应用于从大豆油/正己烷(质量比1∶3)混合油中回收溶剂正己烷,考察了碳纳米管填充量和高温纯化处理对复合膜分离性能的影响.结果表明,碳纳米管经高温纯化预处理后,所制成填充复合膜的分离性能得到了较大的提高.     

Electron transfer properties of iodine-doped single-walled carbon nanotubes using field effect transistor

Single-walled carbon nanotubes (SWNTs) are known to have a p-type charge transfer character in the atmosphere. The energy state of SWNTs can be modulated by doping with either an electron donor or an acceptor. In this study, iodine molecules are chosen for intercalation to SWNTs to predict the charge transfer tendency between them. Field-effect transistors (FETs) using iodine intercalated SWNTs (I-SWNTs) are fabricated and their electronic properties are investigated to better understand the charge transfer between iodine and SWNTs by changing gate voltages. Under vacuum, I-SWNT FETs exhibit weak n-type character, indicating that electrons are transferred slightly from the iodine to the SWNTs. After exposure to O2 gas, n-type characters are reduced; however, they still retain their original type.     

Electronic devices based on purified carbon nanotubes grown by high-pressure decomposition of carbon monoxide

The excellent properties of transistors, wires and sensors made from single-walled carbon nanotubes (SWNTs) make them promising candidates for use in advanced nanoelectronic systems. Gas-phase growth procedures such as the high-pressure decomposition of carbon monoxide (HiPCO) method yield large quantities of small-diameter semiconducting SWNTs, which are ideal for use in nanoelectronic circuits. As-grown HiPCO material, however, commonly contains a large fraction of carbonaceous impurities that degrade the properties of SWNT devices. Here we demonstrate a purification, deposition and fabrication process that yields devices consisting of metallic and semiconducting nanotubes with electronic characteristics vastly superior to those of circuits made from raw HiPCO. Source-drain current measurements on the circuits as a function of temperature and backgate voltage are used to quantify the energy gap of semiconducting nanotubes in a field-effect transistor geometry. This work demonstrates significant progress towards the goal of producing complex integrated circuits from bulk-grown SWNT material.     

Stable and controlled amphoteric doping by encapsulation of organic molecules inside carbon nanotubes

Single-walled carbon nanotubes (SWNTs) have strong potential for molecular electronics, owing to their unique structural and electronic properties. However, various outstanding issues still need to be resolved before SWNT-based devices can be made. In particular, large-scale, air-stable and controlled doping is highly desirable. Here we present a method for integrating organic molecules into SWNTs that promises to push the performance limit of these materials for molecular electronics. Reaction of SWNTs with molecules having large electron affinity and small ionization energy achieved p- and n-type doping, respectively. Optical characterization revealed that charge transfer between SWNTs and molecules starts at certain critical energies. X-ray diffraction experiments revealed that molecules are predominantly encapsulated inside SWNTs, resulting in an improved stability in air. The simplicity of the synthetic process offers a viable route for the large-scale production of SWNTs with controlled doping states.     

Attachment of functionalized single-walled carbon nanotubes (SWNTs) to silicon surfaces

Single-walled carbon nanotubes (SWNTs) were functionalized by direct fluorination and subsequent reaction with 6-aminohexanoic acid for water-soluble carboxylic acid functionalized SWNTs (AHA-SWNTs). Both of the compounds were used as precursors to attach SWNTs to APTES coated silicon surfaces. AHA-SWNTs in aqueous solution were reacted with APTES self-assembled monolayers (SAMs) with coupling reagents N-(3-dimethylaminopropyl)-N'-ethylcarbodiimide (EDC) and N-hydroxysuccinimide (NHS). The surface coverage is a function of concentration of AHA-SWNTs, solvent and coupling method. While for the fluorinated SWNTs (F-SWNTs), direct addition of F-SWNTs to preformed APTES SAMs at 90 degrees C shows essentially no reaction, in contrast to the one-pot reaction of F-SWNTs with APTES molecules in the presence of SWNTs on a silicon substrate. This reaction route provides a convenient method to attach SWNTs to silicon surfaces.     

A nanohybrid material of SWNTs covalently functionalized with porphyrin for light harvesting antenna: synthesis and photophysical properties

Novel covalently porphyrin functionalized single-walled carbon nanotubes (SWNTs) were synthesized using carboxylic group functionalized carbon nanotubes (o-SWNTs) with meso-aniline substituted porphyrin. The structure and morphology of this SWNT nanohybrid material were fully characterized with FTIR, Raman, UV-Vis-NIR spectra as well as TGA and TEM measurements. The energy transfer efficiency from porphyrin to SWNTs and porphyrin fluorescence quenching mechanism were studied by means of steady state fluorescence and time-resolved fluorescence measurement. The fast and efficient electron transfer occurring in this nanohybrid illustrates that they can be utilized as a good candidate for light harvesting materials in molecular photonic devices and solar energy utilization.