Functionalization and Dissolution of Nitric Acid Treated Single-Walled Carbon Nanotubes

We report an investigation of the nature and chemical functionalization of nitric acid treated single-walled carbon nanotubes (SWNTs). SWNTs washed with diluted sodium hydroxide solutions were characterized by near-IR, mid-IR, and Raman spectroscopy as well as TEM, and the remaining carboxylic acid content was determined to assess the effect of base washing on the removal of carboxylated carbon fractions, which are generated by the nitric acid treatment. It was found that even after exhaustive washing with aqueous base the purified SWNTs contain carboxylic acid groups in sufficient quantity to prepare high quality soluble SWNT materials by covalent functionalization with octadecylamine

Diels−Alder Chemistry of Graphite and Graphene: Graphene as Diene and Dienophile

The zero-band-gap electronic structure of graphene enables it to function as either the diene or the dienophile in the Diels−Alder reaction, and this versatile synthetic method offers a powerful strategy for the reversible modification of the electronic properties of graphene under very mild conditions

Protection of Molecular Microcrystals by Encapsulation under Single-Layer Graphene

Microcrystals composed of the conjugated organic molecule perylene can be encapsulated beneath single-layer graphene using mild conditions. Scanning electron and atomic force microscopy images show that the graphene exists as a conformal coating on top of the crystal. Raman spectroscopy indicates that the graphene is only slightly perturbed by the underlying crystal, probably due to strain. The graphene layer provides complete protection from a variety of solvents and prevents sublimation of the crystal at elevated temperatures. Time-resolved photoluminescence measurements do not detect any quenching of the perylene emission by the graphene layer, although nonradiative energy transfer within a few nanometers of the crystal–graphene interface cannot be ruled out. The ability to encapsulate samples on a substrate under a graphene monolayer may provide a new way to access and interact with the organic crystal under ambient conditions

MoS₂‑Based Optoelectronic Gas Sensor with Sub-parts-per-billion Limit of NO₂ Gas Detection

Red light illumination with photon energy matching the direct band gap of chemical vapor deposition grown single-layer MoS2 with Au metal electrodes was used to induce a photocurrent which was employed instead of dark current for NO2 gas sensing. The resulting Au/MoS2/Au optoelectronic gas sensor showed a significant enhancement of the device sensitivity S toward ppb level of NO2 gas exposure reaching S = 4.9%/ppb (4900%/ppm), where S is a slope of dependence of relative change of the sensor resistance on NO2 concentration. Further optimization of the MoS2-based optoelectronic gas sensor by using graphene (Gr) with a work function lower than that of Au for the electrical contacts to the MoS2 channel allowed an increase of photocurrent. The limit of detection of NO2 gas at the level of 0.1 ppb was obtained for the MoS2 channel with graphene electrodes coated by Au. This value was calculated using experimentally obtained sensitivity and noise values and exceeds the U.S. Environment Protection Agency requirement for NO2 gas detection at ppb level

Chemically Functionalized Water-Soluble Single-Walled Carbon Nanotubes Modulate Morpho-Functional Characteristics of Astrocytes

We report the use of chemically functionalized water-soluble single-walled carbon nanotubes (ws-SWCNTs) for the modulation of morpho-functional characteristics of astrocytes. When added to the culturing medium, ws-SWCNTs were able to make astrocytes larger and stellate/mature, changes associated with the increase in glial fibrillary acidic protein immunoreactivity. Thus, ws-SWCNTs could have more beneficial effects at the injury site than previously thought; by affecting astrocytes, they could provide for a more comprehensive re-establishment of the brain computational power

Networks of Semiconducting SWNTs: Contribution of Midgap Electronic States to the Electrical Transport

ConspectusSingle-walled carbon nanotube (SWNT) thin films provide a unique platform for the development of electronic and photonic devices because they combine the advantages of the outstanding physical properties of individual SWNTs with the capabilities of large area thin film manufacturing and patterning technologies. Flexible SWNT thin film based field-effect transistors, sensors, detectors, photovoltaic cells, and light emitting diodes have been already demonstrated, and SWNT thin film transparent, conductive coatings for large area displays and smart windows are under development. While chirally pure SWNTs are not yet commercially available, the marketing of semiconducting (SC) and metallic (MT) SWNTs has facilitated progress toward applications by making available materials of consistent electronic structure. Nevertheless the electrical transport properties of networks of separated SWNTs are inferior to those of individual SWNTs. In particular, for semiconducting SWNTs, which are the subject of this Account, the electrical transport drastically differs from the behavior of traditional semiconductors: for example, the bandgap of germanium (<i>E</i> = 0.66 eV) roughly matches that of individual SC-SWNTs of diameter 1.5 nm, but in the range 300–100 K, the intrinsic carrier concentration in Ge decreases by more than 10 orders of magnitude while the conductivity of a typical SC-SWNT network decreases by less than a factor of 4. Clearly this weak modulation of the conductivity hinders the application of SC-SWNT films as field effect transistors and photodetectors, and it is the purpose of this Account to analyze the mechanism of the electrical transport leading to the unusually weak temperature dependence of the electrical conductivity of such networks. Extrinsic factors such as the contribution of residual amounts of MT-SWNTs arising from incomplete separation and doping of SWNTs are evaluated. However, the observed temperature dependence of the conductivity indicates the presence of midgap electronic states in the semiconducting SWNTs, which provide a source of low-energy excitations, which can contribute to hopping conductance along the nanotubes following fluctuation induced tunneling across the internanotube junctions, which together dominate the low temperature transport and limit the resistivity of the films. At high temperatures, the intrinsic carriers thermally activated across the bandgap as in a traditional semiconductor became available for band transport. The midgap states pin the Fermi level to the middle of the bandgap, and their origin is ascribed to defects in the SWNT walls. The presence of such midgap states has been reported in connection with scanning tunneling spectroscopy experiments, Coulomb blockade observations in low temperature electrical measurements, selective electrochemical deposition imaging, tip-enhanced Raman spectroscopy, high resolution photocurrent spectroscopy, and the modeling of the electronic density of states associated with various defects.Midgap states are present in conventional semiconductors, but what is unusual in the present context is the extent of their contribution to the electrical transport in networks of semiconducting SWNTs. In this Account, we sharpen the focus on the midgap states in SC-SWNTs, their effect on the electronic properties of SC-SWNT networks, and the importance of these effects on efforts to develop electronic and photonic applications of SC-SWNTs

Chemically Engineered Graphene-Based 2D Organic Molecular Magnet

Carbon-based magnetic materials and structures of mesoscopic dimensions may offer unique opportunities for future nanomagnetoelectronic/spintronic devices. To achieve their potential, carbon nanosystems must have controllable magnetic properties. We demonstrate that nitrophenyl functionalized graphene can act as a room-temperature 2D magnet. We report a comprehensive study of low-temperature magnetotransport, vibrating sample magnetometry (VSM), and superconducting quantum interference (SQUID) measurements before and after radical functionalization. Following nitrophenyl (NP) functionalization, epitaxially grown graphene systems can become organic molecular magnets with ferromagnetic and antiferromagnetic ordering that persists at temperatures above 400 K. The field-dependent, surface magnetoelectric properties were studied using scanning probe microscopy (SPM) techniques. The results indicate that the NP-functionalization orientation and degree of coverage directly affect the magnetic properties of the graphene surface. In addition, graphene-based organic magnetic nanostructures were found to demonstrate a pronounced magneto-optical Kerr effect (MOKE). The results were consistent across different characterization techniques and indicate room-temperature magnetic ordering along preferred graphene orientations in the NP-functionalized samples. Chemically isolated graphene nanoribbons (CINs) were observed along the preferred functionality directions. These results pave the way for future magnetoelectronic/spintronic applications based on promising concepts such as current-induced magnetization switching, magnetoelectricity, half-metallicity, and quantum tunneling of magnetization

Changes in the Morphology and Proliferation of Astrocytes Induced by Two Modalities of Chemically Functionalized Single-Walled Carbon Nanotubes are Differentially Mediated by Glial Fibrillary Acidic Protein

Alterations in glial fibrillary acidic protein (GFAP) levels accompany the changes in the morphology and proliferation of astrocytes induced by colloidal solutes and films of carbon nanotubes (CNTs). To determine if GFAP is required for the effects of CNTs on astrocytes, we used astrocytes isolated from GFAP null mice. We find that selected astrocytic changes induced by CNTs are mediated by GFAP, i.e., perimeter, shape, and cell death for solutes, and proliferation for films

Synthesis, Dispersion, and Viscosity of Poly(ethylene glycol)-Functionalized Water-Soluble Single-Walled Carbon Nanotubes

The carbon nanotube−polyethylene glycol (PEG) graft copolymer was synthesized by covalent functionalization of electric arc single-walled carbon nanotubes (SWNTs) with the monofunctional, tetrahydrofurfuryl-terminated polyethylene glycol PEG-THFF (MW∼200), to give a material composed of 80 wt % SWNTs. We show that the sequential processing of the resulting material by ultrasonication and high-shear mixing provides a means to disperse the SWNT-PEG-THFF macromolecules on two different length scales and leads to highly viscous solutions; at a concentration of 10 mg/mL the kinematic viscosity (ν) of an aqueous SWNT-PEG-THFF dispersion reaches a value of ν > 1000 cSt (for water ν ∼ 1 cSt). Analysis of this procedure by means of viscosity measurements and atomic force microscopy (AFM), shows that ultrasonication is effective in disrupting the SWNT bundles, while the high shear mixing disperses the individual SWNTs. The kinematic viscosity of aqueous dispersions of SWNT-PEG-THFF was measured as a function of nanotube concentration and compared to that of SWNT-PEG dispersions. The viscosity and AFM measurements show that the SWNT-PEG-THFF and SWNT-PEG graft copolymers form aqueous dispersions with distinct viscous characteristics; the use of monofunctional PEG-THFF for covalent functionalization of the SWNTs prevents cross-linking of the SWNTs in the final product, and this allows the production of more completely dispersed SWNTs than in the case of the SWNT-PEG graft copolymer, which is synthesized from a bifunctional glycol