Graphene as a functional layer for semiconducting carbon nanotube transistor sensors

A tight-binding study of the electron transport through single-walled carbon nanotube-graphene hybrid nanostructures

Hybrid carbon nanostructures based on the sp² hybridized allotropes of carbon, such as graphene and single-walled carbon nanotubes (SWCNTs), hold vast potential for applications in electronics of various forms. Electronic properties of such hybrid structures are modified due to the interaction between atoms of the components, which can be utilized to tailor the properties of the hybrid structures to suite the application. In this study, we have explored charge (electron) transport through the hybrid structures of single-layer graphene (SLG) and SWCNTs (both metallic and semiconducting) using the nonequilibrium Green's function formalism within the framework of tight-binding density functional theory. Our calculations show that the electronic transport in hybrid nanostructures is affected by the interactions between SWCNT and SLG in comparison to the individual components. The changes in the electronic structure and the transport properties with increasing interaction in hybrids (captured by decreasing the separation between SWCNT and SLG) are discussed, and it is demonstrated from this analysis that the hybrids with semiconducting SWCNTs and metallic SWCNTs show different behavior in the low bias regime while they show similar behavior at higher biases. The difference in the transport properties of hybrids with semiconducting and metallic SWCNTs is explained in terms of changes in the electronic structure, the local density of states, and the energy dispersion for electrons due to the interaction between atoms of the two components.

Tight-binding investigation of the structural and vibrational properties of graphene-single wall carbon nanotube junctions

Hybrid carbon nanostructures based on single walled carbon nanotubes (SWNTs) and single layer graphene (SLG) have drawn much attention lately for their applications in a range of efficient hybrid devices. A few recent studies, addressing the interaction behavior at the heterojunction, considered charge transfer between the constituents (SWNTs and SLG) to be responsible for changes in the electronic and vibrational properties of their hybrid system. We report the effect of various factors, arising due to the interactions between the atoms of SWNTs and SLG, on the structural and vibrational properties of hybrid nanostructures investigated computationally within the framework of tight-binding DFT. These factors, such as the van der Waals (vdW) forces, structural deformation and charge transfer, are seen to affect the Raman active phonon frequencies of SWNTs and SLG in the hybrid nanostructure. These factors are already known to affect the vibrational properties of SWNTs and SLG separately and in this work, we have explored their role and interplay between these factors in hybrid systems. The contribution of different factors to the total shift observed in phonon frequencies is estimated and it is perceived from our findings that not only the charge transfer but the structural deformations and the vdW forces also affect the vibrational properties of components within the hybrid, with structural deformation being the leading factor. With decreasing separation between SWNTs and SLG, the charge transfer and the vdW forces both increase. However, the increase in vdW forces is relatively much higher and likely to be the main cause for larger Raman shifts observed at smaller separations.

Molecular embroidering of graphene

Structured covalent two-dimensional patterning of graphene with different chemical functionalities constitutes a major challenge in nanotechnology. At the same time, it opens enormous opportunities towards tailoring of physical and chemical properties with limitless combinations of spatially defined surface functionalities. However, such highly integrated carbon-based architectures (graphene embroidery) are so far elusive. Here, we report a practical realization of molecular graphene embroidery by generating regular multiply functionalized patterns consisting of concentric regions of covalent addend binding. These spatially resolved hetero-architectures are generated by repetitive electron-beam lithography/reduction/covalent-binding sequences starting with polymethyl methacrylate covered graphene deposited on a Si/SiO₂ substrate. The corresponding functionalization zones carry bromobenzene-, deutero-, and chloro-addends. We employ statistical Raman spectroscopy together with scanning electron microscopy/energy dispersive X-ray spectroscopy for an unambiguous characterization. The exquisitely ordered nanoarchitectures of these covalently multi-patterned graphene sheets are clearly visualized.

Covalently 2D-patterning graphene with different chemical functionalities is an attractive way to tailor its physical and chemical properties. Here, the authors realize spatially defined 2D-hetereoarchitectures of graphene via a strategy of molecular embroidering.