AccChemRes - Effect of Covalent Chemistry on the Electronic Structure and Properties of Carbon Nanotubes and Graphene (original) (raw)
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Accounts of Chemical Research, 2013
I n 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) DielsÀAlder 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 DielsÀAlder 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 threedimensional (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.
We consider the covalent chemistry of the carbon allotropes with an emphasis on the newest member— graphene. We focus on the effect of such chemistry on the geometric and electronic structure of the functionalized materials and the way in which the conjugation is modified by such processes. We conclude that there are two limiting cases: (a) Conventional addition chemistry leading to the formation of s-bonds to the graphitic surface in which there is full rehybridization of the derivatized carbon atoms from sp2 to sp3; thus these carbon atoms are effectively removed from conjugation and from the electronic band structure (referred to as destructive rehybridization). (b) Covalent chemisorption with formation of an organometallic hexahapto-metal bond that largely preserves the graphitic band structure (constructive rehybridization) and accompanies the formation of bis-hexahapto-metal bonds such as those in (h6-SWNT)Cr(h6-SWNT) which serve to interconnect adjacent graphitic surfaces and significantly reduces the internanotube junction resistance in SWNT networks. The formation of h2 dihapto bonds represent an intermediate case of covalent chemistry and is known to be important in carbon nanotubes and particularly the fullerenes but this situation has been treated in detail in previous publications.
Chemical Science, 2011
We explore the organometallic chemistry of graphitic materials and demonstrate the h 6 -complexation reactions of chromium with graphene, graphite and carbon nanotubes. All of these extended periodic p-electron systems exhibit some degree of reactivity toward the reagents Cr(CO) 6 and (h 6 -benzene)Cr (CO) 3 , and we are able to demonstrate the formation of (h 6 -arene)Cr(CO) 3 or (h 6 -arene) 2 Cr, where arene ¼ single-walled carbon nanotubes (SWNT), exfoliated graphene (XG), epitaxial graphene (EG) and highly-oriented pyrolytic graphite (HOPG). We report the ATR-IR, Raman spectroscopy, XPS and chemistry of these new organometallic species and we observe clearly understandable trends in the chemistry and stability of the complexes based on curvature and surface presentation. For example, the SWNTs are the least reactive presumably as a result of the effect of curvature on the formation of the hexahapto bond; in the case of HOPG, (h 6 -HOPG)Cr(CO) 3 was isolated while the exfoliated graphene samples were found to give both (h 6 -graphene) 2 Cr, and (h 6 -graphene)Cr(CO) 3 structures. We report simple and efficient routes for the mild decomplexation of the graphene-chromium complexes which appears to restore the original pristine graphene state; exposure of the samples to white light in a solution of acetonitrile or the use of selected ligand competition reactions bring about a clean reversal of the metal complexation reactions and provide an independent proof of structure for the reaction products. This study represents the first example of the use of graphene as a ligand and is expected to expand the scope of graphene chemistry in connection with the application of this material in organometallic catalysis, where graphene can act as an electronically conjugated catalyst support.
Covalent chemistry in graphene electronics
Materials Today, 2012
The development of selective high precision chemical functionalization strategies for device fabrication, in conjunction with associated techniques for patterning graphene wafers with atomic accuracy would provide the necessary basis for a post-CMOS manufacturing technology. This requires a thorough understanding of the principles governing the reactivity and patterning of graphene at the subnanometer length scale. This article reviews our quest to delineate the principles of graphene chemistrythat is, the chemistry at the Dirac point and beyond, and the effect of covalent chemistry on the electronic structure, electrical transport and magnetic properties of this lowdimensional material in order to enable the scalable production of graphene-based devices for low-and high-end technology applications. The new carbon age, 1 which is the third wave in the carbon revolution, has witnessed overwhelming interest in low-dimensional carbon materials, with particular attention to graphene, the newest member of the series of carbon allotropes. This two-dimensional form of pure sp 2 hybridized carbonthe giant molecule 2 of atomic thickness has garnered tremendous attention among both physicists and chemists and has provided a test-bed for fundamental and device physics, 3,4 and a unique chemical substrate. 5-9 In line with theoretical predictions, charge carriers in graphene behave like massless Dirac fermions, which is a direct consequence of the linear energy dispersion relation. 10 Such features serve to recommend graphene
Advanced Materials
Pristine single layer graphene (SLG) has exceedingly high mobility, which is ∼ 4,000-20,000 cm 2 /Vs for typical devices supported on Si/SiO 2 substrates, and may reach as high as 250,000 cm 2 /Vs in suspended devices at room temperature. Such high mobilities make graphene an extremely attractive candidate for the next generation electronic materials. However, the absence of a band gap, which is necessary for digital electronics, presents a technological challenge. One effective approach to band gap engineering is the (partial) saturation of the valences of some of the conjugated carbon atoms. Nitrophenyl functionalization, in which a fully rehybridized sp 3 carbon atom is created in the lattice, dramatically modifi es the electronic and magnetic structure of graphene, with signifi cantly reduced fi eld effect mobility. Since this type of functionalization scheme introduces resonant scatters into the graphene lattice, we refer to this as destructive rehybridization. Most approaches for chemical modifi cation of graphene involve the creation of sp 3 carbon centers at the cost of conjugated sp 2 carbon atoms in the graphene lattice. We have recently investigated the application of organometallic chemistry by studying the covalent hexahapto modifi cation of graphitic surfaces with zero-valent transition metals such as chromium. [ 12 , 25 ] The formation of the hexahapto ( η 6 )-arene − metal bond leads to very little structural reorganization of the π -system. In the reaction of the zero-valent chromium metal with graphene, the vacant d π orbital of the metal (chromium) constructively overlaps with the occupied π -orbitals of graphene, without removing any of the sp 2 carbon atoms from conjugation. [ 12 , 25 ] Previously we have shown that the formation of such bishexahapto transition metal bonds between the conjugated surfaces of the benzenoid ring systems present in the surfaces of graphene and carbon nanotubes can dramatically change their electrical properties. [ 12 , 24-27 ] These prior works focus on using the bis-hexahapto-metal bond as an interconnect for electrical transport between the conjugated surfaces, thereby increasing the dimensionality of the carbon nanotube and graphene materials and thus we were concerned with the use of the bishexahapto-metal bond as a conduit for electron transport between surfaces. In contrast, the goal of the present study is to investigate the effect of the hexahapto-bonded chromium atoms on the electronic properties of graphene itself (within the plane of a single layer), by using mono-hexahapto-metal bonds to the graphene surface.
Materials Today Magazine - Sarkar - Covalent Chemistry in Graphene Electronics
Materials Today
The new carbon age 1 , which is the third wave in the carbon revolution, has witnessed overwhelming interest in low-dimensional carbon materials, with particular attention to graphene, the newest member of the series of carbon allotropes. This two-dimensional form of pure sp 2 hybridized carbon -a giant molecule 2 of atomic thickness -has garnered tremendous attention among both physicists and chemists and has provided a test-bed for fundamental and device physics 3,4 , and a unique chemical substrate 5-9 . In line with theoretical predictions, charge carriers in graphene behave like massless Dirac fermions, which is a direct consequence of the linear energy dispersion relation 10 . Such features serve to support the use of graphene for mechanical, thermal, electronic, magnetic, and optical applications, but the absence of a band-gap in graphene makes it unsuitable for conventional field effect transistors (FETs) 11,12 , and its lack of solution processability remains to be resolved 13 . These issues are potentially amenable to solution by chemical techniques, but the effect of chemistry on the mobility of functionalized graphene devices is an imposing challenge 14 . Dimensionality defines the physical and chemical behavior of a material and distinguishes one material from the other even among those of the same chemical composition 15 ; while the chemical concepts of structure and hybridization lead to the same conclusion 16-18 . From the standpoint of both physics and chemistry, the two-dimensional (2D) graphene materials with atomically flat surface are remarkably different from that of the quasi-zero-dimensional (0D) fullerenes, and the onedimensional (1D) carbon nanotube materials. The experimental isolation
In this Perspective, we present an overview of recent fundamental studies on the nature of the interaction between individual metal atoms and metal clusters and the conjugated surfaces of graphene and carbon nanotube with a particular focus on the electronic structure and chemical bonding at the metal−graphene interface. We discuss the relevance of organometallic complexes of graphitic materials to the development of a fundamental understanding of these interactions and their application in atomtronics as atomic interconnects, high mobility organometallic transistor devices, high-frequency electronic devices, organometallic catalysis (hydrogen fuel generation by photocatalytic water splitting, fuel cells, hydrogenation), spintronics, memory devices, and the next generation energy devices. We touch on chemical vapor deposition (CVD) graphene grown on metals, the reactivity of its surface, and its use as a template for asymmetric graphene functionalization chemistry (ultrathin Janus discs). We highlight some of the latest advances in understanding the nature of interactions between metals and graphene surfaces from the standpoint of metal overlayers deposited on graphene and SWNT thin films. Finally, we comment on the major challenges facing the field and the opportunities for technological applications.