Irina Kalinina

Effect of Covalent Chemistry on the Electronic Structure and Properties of Carbon Nanotubes and Graphene

In this Account, we discuss the chemistry of graphitic materials with particular reference to three reactions studied by our research group: (1) aryl radical addition, from diazonium precursors, (2) Diels-Alder pericyclic reactions, and (3) organometallic complexation with transition metals. We provide a unified treatment of these reactions in terms of the degenerate valence and conduction bands of graphene at the Dirac point and the relationship of their orbital coefficients to the HOMO and LUMO of benzene and to the Clar structures of graphene. In the case of the aryl radical addition and the Diels-Alder reactions, there is full rehybridization of the derivatized carbon atoms in graphene from sp(2) to sp(3), which removes these carbon atoms from conjugation and from the electronic band structure of graphene (referred to as destructive rehybridization). The radical addition process requires an electron transfer step followed by the formation of a σ-bond and the creation of a π-radical in the graphene lattice, and thus, there is the potential for unequal degrees of functionalization in the A and B sublattices and the possibility of ferromagnetism and superparamagnetism in the reaction products. With regard to metal functionalization, we distinguish four limiting cases: (a) weak physisorption, (b) ionic chemisorption, in which there is charge transfer to the graphitic structure and preservation of the conjugation and band structure, (c) covalent chemisorption, in which there is strong rehybridization of the graphitic band structure, and (d) covalent chemisorption with formation of an organometallic hexahapto-metal bond that largely preserves the graphitic band structure (constructive rehybridization). The constructive rehybridization that accompanies the formation of bis-hexahapto-metal bonds, such as those in (η(6)-SWNT)Cr(η(6)-SWNT), interconnects adjacent graphitic surfaces and significantly reduces the internanotube junction resistance in single-walled carbon nanotube (SWNT) networks. The conversion of sp(2) hybridized carbon atoms to sp(3) can introduce a band gap into graphene, influence the electronic scattering, and create dielectric regions in a graphene wafer. However, the organometallic hexahapto (η(6)) functionalization of the two-dimensional (2D) graphene π-surface with transition metals provides a new way to modify graphitic structures that does not saturate the functionalized carbon atoms and, by preserving their structural integrity, maintains the delocalization in these extended periodic π-electron systems and offers the possibility of three-dimensional (3D) interconnections between adjacent graphene sheets. These structures may find applications in interconnects, 3D-electronics, organometallic catalysis, atomic spintronics and in the fabrication of new electronic materials.

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.

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.

Effect of first row transition metals on the conductivity of semiconducting single-walled carbon nanotube networks

We demonstrate the ability of first row transition metals to form electrically conducting interconnects between semiconducting single-walled carbon nanotubes (SWNTs) by constructive rehybridization between sidewall benzene rings as a result of the formation of bis-hexahapto-metal-bonds [(η6-SWNT)M(η6-SWNT)], which bridge adjacent SWNTs. Metal deposition on SWNT films enhances the conductivity by three distinct mechanisms: physisorption of gold leads to the formation of a non-interacting gold film and a monotonic conductivity increase; ionic chemisorption of lithium strongly increases the conductivity due to charge transfer to the SWNTs; covalent chemisorption of first row transition metals leads to an abrupt change in conductivity due to formation of (η6-SWNT)M(η6-SWNT) interconnects.

Chemically Engineered Single‐Walled Carbon Nanotube Materials for the Electronic Detection of Hydrogen Chloride

p-SWNT purified SWNT DMF The detection of gas analytes relies on a variety of operating mechanisms which include ionization of gases, modulation of optical properties, gas chromatography, mass spectrometry, electrochemistry, conductance modulation in field effect transistors, and chemical resistors. The modulation of the electronic characteristics of a sensor device based on a chemical resistor is a very attractive approach because of its simplicity. In the search for novel sensor materials, which can broaden the range of detected molecules and the limits of detection, single-walled carbon nanotubes (SWNTs) have emerged as a promising candidate. Most of the research in the field has been focused on understanding and exploring the sensitivity of the electronic structure of SWNTs to charge transfer from adsorbed molecules. It has been shown that defects, non-covalently attached polymers, and metal or metal oxide nanoparticles can affect the sensitivity of SWNTs to gas molecules. The research on the ability of SWNTs to act as a sensor material has primarily used gas molecules with electron donating or withdrawing properties such as ammonia and nitrogen dioxide. The mechanism of modulation of the transport properties of SWNTs is usually attributed to charge transfer between the dopant and carbon nanotubes, which is assumed to lead to refilling or depletion of the valence band of the semiconducting SWNTs. Previously we reported that covalent functionalization can dramatically increase the resistance change of SWNT films during exposure to gas molecules. The advantages of covalently functionalized SWNTmaterials for sensor applications are obvious: i) facile dispersibility in solvents and enhanced processability, ii) reproducible and well-defined chemical composition, and iii) possibility of attaching organic moieties specifically engineered to interact with analyte molecules. In our previous work on the sensing mechanism of poly(m-aminobenzene sulfonic acid)-functionalized SWNTs (SWNT-PABS) toward ammonia, we argued that the electron affinity of the attached PABS functionality was modulated by an acid–base equilibrium and that this directly influenced the conductivity by controlling the concentration of holes in the valence band of the semiconducting SWNTs. In the present study we explore the generality of this approach by employing chemically functionalized SWNTmaterials with a range of basicities and electronic structures and studying the change of their electronic properties on exposure to hydrogen chloride. For this study we synthesized three types of functionalized SWNT materials with covalently attached functional groups of varying basicity (Table 1) and compared the change of their electronic properties to that of purified non-functionalized SWNTs on exposure to HCl. Thin films of the SWNT materials were deposited on a glass substrate with pre-patterned interdigitated gold contacts (Fig. 1a). The SWNTmaterials were dispersed in a solvent (Table 1) by ultrasonication and sprayed on the substrate held at 120 8C; the resulting films had resistances in the range of 30 kV. The thickness of the films (t) was estimated from the intensity of the first semiconducting interband transition (S11) and the extinction coefficient of the materials; [20]

Chemically functionalized single-walled carbon nanotube films modulate the morpho-functional and proliferative characteristics of astrocytes.

We used single-walled carbon nanotube (CNT) films to modulate the morpho-functional and proliferative characteristics of astrocytes. When plated on the CNT films of various thicknesses, astrocytes grow bigger and rounder in shape with a decrease in the immunoreactivity of glial fibrillary acidic protein along with an increase in their proliferation, changes associated with the dedifferentiation of astrocytes in culture. Thus, CNT films, as a coating material for electrodes used in brain machine interface, could reduce astrogliosis around the site of implantation.

Formation of Transition Metal Cluster Adducts on the Surface of Single-walled Carbon Nanotubes: HRTEM Studies

We report the formation of chromium clusters on the outer walls of single-walled carbon nanotubes (SWNTs). The clusters were obtained by reacting purified SWNTs with chromium hexacarbonyl in dibutyl ether at 100°C. The functionalized SWNTs were characterized by thermogravimetic analysis, XPS, and high-resolution TEM. The curvature of the SWNTs and the high mobility of the chromium moieties on graphitic surfaces allow the growth of the metal clusters and we propose a mechanism for their formation.