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

Abstract

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.

Conclusion and Perspective

“…Due to the specific electronic and transport properties of carbon-based nanostructures including carbon nanotube and graphene [37,38], their applications in sensing and nanoelectronics have been explored widely in recent years [39][40][41]. Of course, such investigations have also been proceeding in the field of spintronic devices with molecular nanomagnets and yielded some fascinating results especially the successful design of spin transistor and spin valves, which will be introduced in the next [43], but AFM images and molecular dynamics simulations demonstrated that those [TbPc 2 ] molecules on the surface form highly regular rectangular 2D nanocrystals, as shown in Fig.…”

Prospects of Spin Catalysis on Spin-Polarized Graphene Heterostructures

“…It was shown [22] that the formation of hexahapto-coordinated metal ions provides new possibilities to develop new-era lowdimensional graphene and carbon nanotube materials for organometallic catalysis and spintronics by preserving the graphitic band structure during formation of bis-hexahapto-metal bonds. It was shown that a network [23] of single Cr atoms with arene ligands trapped in a periodic potential well of a graphene layer opens new possibilities to quantum routeing and state manipulation through the conductive graphene substrate.…”

Organometallic Chemistry of Carbon Nanotubes and Graphene

“…While conventional addition chemistry in which the sp 2 -conjugated carbon atoms are rehybridized to sp 3 has been widely explored in the new carbon allotropes [4], the participation of graphene and CNTs in organometallic chemical reactions has received limited attention. The metal complexes of these carbon materials are potential candidates as reusable solid catalysts for organic synthetic applications [5], in organometallic catalysis as electronically conjugated catalyst supports [6], molecular wires [7] and new electronic materials [8,9]. Fullerenes [10][11][12][13] and CNTs [14] are curved carbon materials with demonstrated ability to serve as primary ligands, whereas graphene represents a new class of periodic, planar two-dimensional π-ligands which has been recently reported to have a rich organometallic chemistry [6], in analogy with the coordination chemistry of polyaromatic hydrocarbons (PAHs) [15].…”

“…Fullerenes [10][11][12][13] and CNTs [14] are curved carbon materials with demonstrated ability to serve as primary ligands, whereas graphene represents a new class of periodic, planar two-dimensional π-ligands which has been recently reported to have a rich organometallic chemistry [6], in analogy with the coordination chemistry of polyaromatic hydrocarbons (PAHs) [15]. The flat two-dimensional π-surface of graphene exhibits a unique chemical reactivity as a result of the electronic structure at the Dirac point and this provides the opportunity to perform a wide range of chemical reactions [6,9,16,17]. Functionalization of CNTs with transition metal complexes alters their electronic and chemical properties and single-walled carbon nanotubes (SWCNTs) have been already used as transition metal supports in heterogeneous [18][19][20] and homogeneous catalysis [5].…”