Graft-induced midgap states in functionalized carbon nanotubes.

Covalent addition of functional groups onto carbon nanotubes is known to generate lattice point defects that disrupt the electronic wave function, resulting namely in a reduction of their optical response and electrical conductance. Here, conductance measurements combined with numerical simulations are used to unambiguously identify the presence of graft-induced midgap states in the electronic structure of covalently functionalized semiconducting carbon nanotubes. The main experimental evidence is an increase of the conductance in the OFF-state after covalent addition of 4-bromophenyl grafts on many single- and double-walled individual nanotubes, the effect of which is fully suppressed after thermodesorption of the adducts. The graft-induced current leakage is thermally activated and can reach several orders of magnitude above its highly insulating pristine-state level. Ab initio simulations of various configurations of functionalized nanotubes corroborate the presence of these midgap states and show their localization around the addends. Moreover, the electronic density of these localized states exhibits an extended hydrogenoid profile along the nanotube axis, providing access for long-range coupling between the grafts. We argue that covalent nanotube chemistry is a powerful tool to prepare and control midgap electronic states on nanotubes for enabling further studies of the intriguing properties of interacting 1D localized states.

Wall-selective probing of double-walled carbon nanotubes using covalent functionalization.

Double-walled carbon nanotubes (DWNTs) present an original coaxial geometry in which the inner wall is naturally protected from the environment by the outer wall. Covalent functionalization is introduced here as an effective approach to investigate DWNT devices. Performed using an aryldiazonium salt, the functionalization is reversible upon thermal annealing and occurs strictly at the surface of the outer wall, leaving the inner wall essentially unaltered by the chemical bonding. Measurements on functionalized DWNT transistors show that the electrical current is carried by the inner wall and provide unambiguous identification of the metallic or semiconducting character of both walls. New insights about current saturation at high bias in DWNTs are also presented as an illustration of new experiments unlocked by the method. The wall-selectivity of the functionalization not only enables selective optical and electrical probing of the DWNTs, but it also paves the way to designing novel electronic devices in which the inner wall is used for electrical transport while the outer wall chemically interacts with the environment.

Thermal chemistry of methylene- and phenyl-functionalized carbon nanotubes.

Thermal desorption of covalently functionalized SWNT was followed using Raman, X-ray photoemission (XPS), and thermodesorption (TDS) spectroscopies. By functionalizing different sources of SWNT, we assess the thermal stability of phenyl- and methylene-SWNT derivatives in relation to the source diameter and helicity distribution. For all samples, broad desorption features were observed at approximately 600 K for the phenyl-SWNT and at approximately 500 K for the methylene-SWNT derivatives. In both cases, no influence on helicity and on diameter was observed for the range studied. The study shows that the stability of methylene addends on SWNT is inferior to that of the phenyl and proves that the main desorption pathway of phenyl addends is a phenyl-phenyl coupling reaction.

Directed assembly of SWNTs by electrostatic interactions and its application for making network transistors.

Single-wall carbon nanotubes (SWNTs) are promising building blocks for the fabrication of nanoelectronic devices. However, achieving control over their assembly on substrates has been challenging and is still a bottleneck to their utilization. Herein, we present a general method for directing the chemical assembly of SWNTs on substrates through electrostatic interactions. By covalently functionalizing both the nanotube sidewalls and the SiO(2) substrate with charged groups, dense networks of SWNTs were produced. The method is selective and highly efficient to process network field-effect transistors.