Elena Bekyarova

Chemical Modification of Epitaxial Graphene: Spontaneous Grafting of Aryl Groups

The addition of nitrophenyl groups to the surface of few-layer epitaxial graphene (EG) by the formation of covalent carbon-carbon bonds changed the electronic structure and transport properties of the EG from near-metallic to semiconducting.

Spectroscopy of Covalently Functionalized Graphene

In order to engineer a band gap into graphene, covalent bond-forming reactions can be used to change the hybridization of the graphitic atoms from sp(2) to sp(3), thereby modifying the conjugation length of the delocalized carbon lattice; similar side-wall chemistry has been shown to introduce a band gap into metallic single-walled carbon nanotubes. Here we demonstrate that the application of such covalent bond-forming chemistry modifies the periodicity of the graphene network thereby introducing a band gap (∼0.4 eV), which is observable in the angle-resolved photoelectron spectroscopy of aryl-functionalized graphene. We further show that the chemically-induced changes can be detected by Raman spectroscopy; the in-plane vibrations of the conjugated π-bonds exhibit characteristic Raman spectra and we find that the changes in D, G, and 2D-bands as a result of chemical functionalization of the graphene basal plane are quite distinct from that due to localized, physical defects in sp(2)-conjugated carbon.

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.

Conductive single-walled carbon nanotube substrates modulate neuronal growth.

We used conductive nanotube films as substrates with which we could systematically vary the conductance to see how this property affects neuronal growth. Here we show that nanotube substrates in a narrow range of conductivity promote the outgrowth of neurites with a decrease in the number of growth cones as well as an increase in cell body area, while at higher conductance these effects disappear.

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.

Effect of single-walled carbon nanotube purity on the thermal conductivity of carbon nanotube-based composites

Raw [as-prepared (AP)] and purified single-walled carbon nanotubes (SWNTs) were utilized for the preparation of SWNT-epoxy composites. Purified functionalized SWNTs provide a significantly greater enhancement of the thermal conductivity, whereas AP-SWNTs allow the best electrical properties because of their ability to form efficient percolating network. A series of SWNT samples of varying purity but identical chemical functionality was prepared to delineate the effect of SWNT purity on the thermal conductivity of SWNT-epoxy composites. The authors found that purified SWNTs provide approximately five times greater enhancement of the thermal conductivity than the impure SWNT fraction demonstrating the significance of SWNT quality for thermal management.

Chemistry at the Dirac point: Diels-Alder reactivity of graphene.

Most of the interesting physics of graphene results from the singular electronic band structure at the so-called Dirac point, where the conduction and valence bands cross in momentum space. Although graphene is very stable thermodynamically, the electronic structure at the Dirac point facilitates basal plane chemistry including pericyclic reactions such as the Diels-Alder reaction. We have discovered a series of facile Diels-Alder reactions in which graphene can function either as a diene when paired with tetracyanoethylene and maleic anhydride or as a dienophile when paired with 2,3-dimethoxybutadiene and 9-methylanthracene. In this Account, we seek to rationalize these findings using simple arguments based on considerations of orbital symmetry and the frontier molecular orbital theory. The graphene conduction and valence bands (HOMO and LUMO) cross at the Dirac point, which defines the work function (W = 4.6 eV). Thus, the HOMO and LUMO form a degenerate pair of orbitals at this point in momentum space with the same ionization potential (IP) and electron affinity (EA). Based on the importance of the energies of the HOMO (-IP) and LUMO (-EA) in frontier molecular orbital (FMO) theory, graphene should be a reactive partner in Diels-Alder reactions due to the very high-lying HOMO and low-lying LUMO (energies of -4.6 eV). Inspection of the orbital symmetries of the degenerate pair of half-occupied band orbitals at the Dirac point confirms that with the appropriate orbital occupancies, both diene and dienophile reaction partners should undergo concerted Diels-Alder reactions with graphene that are allowed based on the Woodward-Hoffmann principles of orbital symmetry.

Aryl functionalization as a route to band gap engineering in single layer graphene devices.

Chemical functionalization is a promising route to band gap engineering of graphene. We chemically grafted nitrophenyl groups onto exfoliated single-layer graphene sheets in the form of substrate-supported or free-standing films. Our transport measurements demonstrate that nonsuspended functionalized graphene behaves as a granular metal, with variable range hopping transport and a mobility gap ∼0.1 eV at low temperature. For suspended graphene that allows functionalization on both surfaces, we demonstrate tuning of its electronic properties from a granular metal to a semiconductor in which transport occurs via thermal activation over a transport gap ∼80 meV from 4 to 300 K. This noninvasive and scalable functionalization technique paves the way for CMOS-compatible band gap engineering of graphene electronic devices.

Poly(m-aminobenzene sulfonic acid) functionalized single-walled carbon nanotubes based gas sensor

We have demonstrated a NH3, NO2 and water vapour sensor based on poly(m-aminobenzene sulfonic acid) functionalized single-walled carbon nanotube (SWNT–PABS) networks. The SWNT–PABS based sensors were fabricated by simple dispersion of SWNT–PABS on top of pre-fabricated gold electrodes. SWNT–PABS sensors showed excellent sensitivity with ppbv level detection limits (i.e., 100 ppbv for NH3 and 20 ppbv for NO2) at room temperature. The response time was short and the response was totally reversible. The sensitivity could be tuned by adjusting the sensor initial resistance. The sensors were also suitable for monitoring relative humidity in air.