Rakesh Voggu

Effects of charge transfer interaction of graphene with electron donor and acceptor molecules examined using Raman spectroscopy and cognate techniques

The effects of the interaction of few-layer graphene with electron donor and acceptor molecules have been investigated by employing Raman spectroscopy, and the results compared with those from electrochemical doping. The G-band softens progressively with increasing concentration of tetrathiafulvalene (TTF) which is an electron donor, while the band stiffens with increasing concentration of tetracyanoethylene (TCNE) which is an electron acceptor. Interaction with both TTF and TCNE broadens the G-band. Hole and electron doping by electrochemical means, however, stiffen and sharpen the G-band. The 2D-band position is also affected by interaction with TTF and TCNE. More importantly, the intensity of the 2D-band decreases markedly with the concentration of either. The ratio of intensities of the 2D-band and G-band decreases with an increase in TTF or TCNE concentration, and provides a means for carrier titration in the charge transfer system. Unlike the intensity of the 2D-band, that of the D-band increases on interaction with TTF or TCNE. All of these effects occur due to molecular charge transfer, also evidenced by the occurrence of charge transfer bands in the electronic absorption spectra. The electrical resistivity of graphene varies in opposite directions on interaction with TTF and TCNE, the resistivity depending on the concentration of either compound.

Charge-transfer with graphene and nanotubes

Charge-transfer between electron–donor and –acceptor molecules is a widely studied subject of great chemical interest. Some of the charge-transfer compounds in solid state exhibit novel electronic properties. In the last two to three years, occurrence of molecular charge-transfer involving single-walled carbon nanotubes (SWNTs) and graphene has been demonstrated. This interaction gives rise to significant changes in the electronic properties of these nanocarbons. We examine charge-transfer phenomenon in graphene and SWNTs in this article in view of its potential utility in device applications.

A Simple Method of Separating Metallic and Semiconducting Single-Walled Carbon Nanotubes Based on Molecular Charge Transfer

Interaction of as-prepared single-walled carbon nanotubes (SWNTs), containing a mixture of metallic and semiconducting species with the potassium salt of coronene tetracarboxylic acid, I, in an aqueous medium provides a simple method of separating semiconducting and metallic species. The metallic nanotubes precipitate out on interaction with I while the semiconducting nanotubes remain in solution. The method avoids centrifugation and is amenable for large-scale separation and can be used as a routine laboratory procedure. Interestingly, interaction with strong electron acceptor molecules brings about metal-semiconductor transitions in SWNTs.

Selective generation of single-walled carbon nanotubes with metallic, semiconducting and other unique electronic properties

As-synthesized single-walled carbon nanotubes (SWNTs) are mixtures of semiconducting and metallic species and separation of the two is of crucial importance for many applications. In this article, the methods employed for the enrichment of semiconducting and metallic SWNTs are presented, along with possible procedures to prepare either of the species selectively. Equally important are the methods for chirality selection. The discovery of metal-semiconductor transitions in SWNTs induced by interaction with electron donor and acceptor molecules is not only of academic interest, but may also find applications. Synthesis of Y-junction SWNTs with unique electronic properties at the junction is yet to be fully accomplished.

Semiconductor to metal transition in SWNTs caused by interaction with gold?and platinum nanoparticles

Single-walled carbon nanotubes (SWNTs) have been coated with gold and platinum nanoparticles either by microwave treatment or by the click reaction. The Raman spectra of these SWNT–metal nanoparticle composites have been investigated. Analysis of the G bands in the Raman spectra shows an increase in the proportion of metallic SWNTs attached to metal nanoparticles. This conclusion is also supported by the changes observed in the radial breathing mode (RBM) bands. Ab initio calculations reveal that semiconductor–metal transition occurs in SWNTs due to Coulombic charge transfer between the metal nanoparticles and the semiconducting SWNTs.

Self-assembly of C60, SWNTs and few-layer graphene and their binary composites at the organic-aqueous interface.

Self-assembly of C(60), single-walled carbon nanotubes (SWNTs) and few-layer graphene at the toluene-water interface has been investigated, starting with different concentrations of the nanocarbons in the organic phase and carrying out the assembly to different extents. Morphologies and structures of the films formed at the interface have been investigated by electron microscopy and other techniques. In the case of C(60), the films exhibit hcp and fcc structures depending on the starting concentration in the organic phase, the films being single crystalline under certain conditions. Self-assembly of the composites formed by pairs of nanocarbons (C(60)-SWNT, C(60)-few-layer graphene and SWNT-few-layer graphene) at the interface has been studied by electron microscopy. Raman spectroscopy and electronic absorption spectroscopy of the films formed at the interface have revealed the occurrence of charge-transfer interaction between SWNTs and C(60) as well as between few-layer graphene and C(60).