V. A. Karachevtsev

Raman spectroscopy of HiPCO single-walled carbon nanotubes at 300 and 5 K

Single-walled carbon nanotubes (SWNTs) produced by the high pressure CO disproportionation (HiPCO method) and purified by controlled thermal oxidation in air have been studied by Raman spectroscopy at 300 and 5 K. Raman spectra have been observed at λexc=632.8 and 441.6 nm laser excitation in the range of 160–1800 cm−1. In the low-frequency part of the spectra (the radial breathing mode range) eleven narrow lines can be detected at low temperatures, enabling an estimation of nanotube diameters (0.8–1.3 nm) and chirality. The width at half-maximum intensity of these spectral lines is about 3–4 cm−1 at 5 K. The Stokes and anti-Stokes spectra are measured at λexc=632.8 nm at room temperature. The most intense lines in these spectra are caused with the resonant Raman-scattering process. With increasing temperature from 5 to 300 K the shift (3–4 cm−1) of the most intense high-frequency component of the tangential mode (G mode) to lower frequency is observed. Based on the analysis of the Stokes/anti-Stokes spectra and the G band shape, the corresponding lines were identified with metallic or semiconducting type of nanotubes.

Combined Raman scattering and ab initio investigation of the interaction between pyrene and carbon SWNT

Raman spectra of HiPco SWNT and SWNT-pyrene films were measured in the 160–1800 cm−1 range. Due to the non-covalent interaction between SWNT and pyrene the most intensive component of the SWNT G mode (1590 cm−1) is downshifted by 2 cm−1 and becomes narrower. Also the intensity of the low-frequency component of the G mode (1550 cm−1) decreases by about 30%. Structures and interaction energies in the complexes of pyrene and zigzag (n, 0) SWNTs [6 ≤ n ≤ 20] were determined at the MP2 level of theory. The BSSE-free geometry optimization of the pyrene-zigzag (12,0) SWNT complex converged to a structure with a 1/2staggered conformation and with an intermolecular distance of 3.5 Å. The BSSE-free interaction energy in the complex is −30.8 kj mol−1. Increasing of the nanotube diameter leads to a higher interaction energy. This energy becomes equal to −37.2 kJ mol−1 in the case of a planar carbon surface.

RNA-Wrapped Carbon Nanotubes Aggregation Induced by Polymer Hybridization

The aggregation of poly(rA)-wrapped single-walled carbon nanotubes (SWNTs) induced by hybridization of the adsorbed polymer with free poly(rU) has been studied by differential UV absorption spectroscopy, NIR luminescence, and AFM. After the addition of poly(rU) into a poly(rA):SWNTs suspension, the double-stranded poly(rA)·poly(rU) was formed, which is evident from the characteristic S-like form of the melting curve for the polymer created. Hybridization of free poly(rU) with two complementary poly(rA), one of which was adsorbed to different individual nanotubes, links them together and causes the appearance of aggregates. The aggregation of nanotubes is accompanied with light scattering and can be monitored in an AFM image after the deposition of the suspension on a mica substrate. Molecular dynamic simulation demonstrates a possible structure of the SWNTs aggregate.

Fermi resonance in Ne, Ar and Kr-matrix infrared spectra of 5-bromouracil

Low-temperature matrix isolation Fourier-transform infrared spectroscopy and quantum-chemical calculations with DFT/B3LYP and MP2 methods were used for investigation of isolated 5-bromouracil (BrU) molecules. Only one tautomeric form of BrU was dominated in the low-temperature Ne, Ar, and Kr matrices. It was revealed that population of minor hydroxy-tautomers did not exceed 0.2%. Appearance of additional absorption bands in the region of stretching vibrations νCO (about 1710 cm−1) as well as of deformation ones (1297, 1093, 901 cm−1) was explained by Fermi resonance. In Ne matrices the peak intensities of absorption bands assigned to the out-of-plane vibrations of the ring and exocyclic atoms were decreased sharply. For the first time, least square method with the using of polynomial was proposed for the corrective scaling of calculated frequencies of vibrations. It is shown that the correction of calculated frequencies with the polynomial of degree two permits to decrease the root-mean-square discrepancy...

Photophysical Properties of SWNT Interfaced with DNA

This chapter is dedicated to photophysical properties of SWNTs on the surface of which such very important biological molecule as a DNA was adsorbed. This unique polymer can solubilize carbon nanotubes, occur separation of certain nanotube species, provide carbon nanotube biocompatibility and facilitates different applications of this nanomaterial. The photophysical characterization of these DNA-functionalized carbon nanotubes contributes to understanding of the fundamental mechanisms of carbon nanotube photophysics as well as gives some insight into the influence of biorganic environment on their photophysical properties. The important feature of this polymer is its different structural flexibility/stiffness, depending on nucleotides and/or their sequence, its ability to form double, triple and more complicated structures. All of these features influence substantially the polymer conformation at its adsorption on the nanotube surface: for example, some polymers can wrap round a nanotube, others adsorb along a tube in a stretching conformation, third ones form globules near the nanotube. It was demonstrated that photophysical properties of nanotubes with the polymer adsorbed, being in a water environment, strongly depend on the polymer conformation, which it acquires at adsorption. Therefore, in the first part of this chapter attention is focused on the analysis of different structures of the polymer, formed upon its adsorption on the nanotube, on the determination of the interaction energy between them, as all these have direct influence on spectral properties of nanotubes. In subsequent two parts of the chapter, devoted to absorption spectroscopy and photoluminescence of nanotubes with the polymer adsorbed (mainly with DNA), the current insight into these physical phenomena observed in carbon nanotubes is made. Thus, the influence of the ππ stacking interaction between nitrogen bases and the nanotube surface on absorption spectra of nanotubes and DNA was demonstrated as well the change of the electron structure of the nanotube in the exciting state, due to the helical negative sugar-phosphatic backbone of the polymer wrapped around nanotubes, was analyzed. The analysis of novel studying, directed to clarify the influence of interfaced organic and biological molecules as well as pH of water suspension on the quantum yield of photoluminescence of semiconducting nanotubes was executed too.

Nanohybrid Structures Formed by Carbon Nanotubes with Long Polynucleotide

Adsorption of poly(rA) on the single-walled carbon nanotubes (SWNTs) surface have been studied by AFM and computer modeling. It follows from AFM images that relatively long fragments of polymers attached to two individual nanotubes in aqueous suspension form branched structures. The size of these structures increases after the addition of a complementary poly(rU) to the aqueous suspension of nanotubes with adsorbed poly(rA), which is induced by hybridization of the adsorbed and free polymer. Molecular dynamic simulation demonstrates different possible structures of these nanohybrids.

Luminescence and Raman scattering of nonpolymerized and photopolymerized fullerene films at 297 and 5K

Luminescence and Raman scattering spectroscopy are used to study nonpolymerized and photopolymerized (with 45% and 85% polymerization) fullerene films (0.5μm thick on a Si substrate) at 5 and 297K. The films were polymerized while they were being deposited and irradiated with UV light. The wide-band emission observed at room temperature from a nonpolymerized fullerene film becomes structured at 5K. A short-wavelength band peaking at 695nm appears in the emission. The intensity of this band decreases with polymerization. Analysis of the low-temperature luminescence spectra of fullerene shows that polymerization is accompanied by a shift of the luminescence bands into the red region. Low-temperature investigations revealed lines in the Raman scattering spectrum of an 85% polymerized film which peak at the frequencies 1466 and 1461cm−1. These lines are due to the vibrations of fullerene dimers and a polymerized chain, respectively. Dimers predominate in a film with 45% polymerization in the polymerized phase...

Raman spectroscopy and SEM study of SWNTs in aqueous solution and films with surfactant or polymer surroundings

Abstract Raman spectra of single‐walled carbon nanotubes (SWNTs) in aqueous solutions with sodium dodecylsulfate (SDS) or fragmented single‐stranded DNA (ss‐DNA) and films obtained from these solutions have been studied. Scanning electron microscope (SEM) film study shows that micelles formed by SDS molecules around SWNT in solution do not keep individual nanotubes from sticking together in bundles during drying out the film. DNA wrapped around SWNT precludes the full nanotubes sticking in the film that facilitates the following splitting of these bundles.

Interaction of a tricationic meso-substituted porphyrin with guanine-containing polyribonucleotides of various structures

The interaction of a tricationic water-soluble meso-(N-methylpyridinium)-substituted porphyrin, TMPyP3+, derived from classic TMPyP4, with double-stranded poly(G)  ⋅  poly(C) and four-stranded poly(G) polyribonucleotides has been studied in aqueous buffered solutions, pH 6.9, of low and near-physiological ionic strengths in a wide range of molar phosphate-to-dye ratios (P/D). To clarify the binding modes of TMPyP3+ to biopolymers various spectroscopic techniques, including absorption and polarized fluorescence spectroscopy, Raman spectroscopy, and resonance light scattering, were used. As a result, two competitive binding modes were revealed. In solution of low ionic strength outside binding of the porphyrin to the polynucleotide backbone with self-stacking prevailed at low P/D ratios (P/D  <  3.5). It manifested itself by the substantial quenching of porphyrin fluorescence. Also the formation of large-scale porphyrin aggregates was observed near the stoichiometric binding ratio. The spectral changes observed at P/D  >  30 including emission enhancement were supposed to be caused by the embedding of partially stacked porphyrin J-dimers into the polymer groove. TMPyP3+ binding to poly(G) induced a fluorescence increase 2.5 times as large as that observed for poly(G)  ⋅  poly(C). In solution of near-physiological ionic strength the efficiency of external porphyrin binding was reduced substantially due to the competitive binding of Na+ ions with the polymer backbone. The spectroscopic characteristics of porphyrin bound to polynucleotides at different conditions were compared with those for free porphyrin.

Noncovalent Interaction of Graphene with Heterocyclic Compounds: Benzene, Imidazole, Tetracene, and Imidazophenazines

Noncovalent functionalization of graphene with organic molecules offers a direct route to multifunctional modification of this nanomaterial, leading to its various possible practical applications. In this work, the structures of hybrids formed by linear heterocyclic compounds such as imidazophenazine (F1) and its derivatives (F2-F4) with graphene and the corresponding interaction energies are studied by using the DFT method. Special attention is paid to the hybrids where the attached molecule is located along the graphene zigzag (GZZ ) and armchair (GAC ) directions. The interaction energies corresponding to the graphene hybrids of the F1-F4 compounds for the two directions are found to be distinct, while tetracene (being a symmetrical molecule) shows a small difference between these binding energies. It is found that the back-side CH3 and CF3 groups have an important influence on the arrangements of F1 derivatives on graphene and on their binding energies. The contribution of the CF3 group to the total binding energy of the F3 molecule with graphene is the largest (3.4 kcal mol(-1) ) (the GZZ direction) while the CH3 group increases this energy of F2 only by 2.0 kcal mol(-1) (the GAC direction). It is shown that replacing the carbons with other atoms or adding a back-side group enables one to vary the polarizability of graphene.