(Invited) Modulation of the Fermi Level in Single-Walled Carbon Nanotubes By Chromophore Encapsulation

Abstract

The one-dimensional structure of single-walled carbon nanotubes renders these materials ideal for optoelectronic applications as they can transport electrons or holes with quasi-ballistic features and as they display optical absorption and near-infrared emission (thanks to van Hove singularities). Nevertheless, functionalities can to be added to nanotube to both improve and control the physical properties. To this aim, chromophore encapsulation into host single-walled carbon nanotubes allows to create hybrid nano-systems with tunable opto-electronic properties. Up to now, we have been confining different kinds of molecules,1-4 either absorbing in the blue/green range, being electron donor (quaterthiophene derivatives (4T) and tetramethyl-paraphenylenediamine (TMPD)) or electron acceptor (tetracyanoquinodimethane (TCNQ)), or absorbing in the red visible range (phthalocyanine (MPc)).In this study, we investigate, at both the macroscopic and the individual scales, the electronic and the optical properties of our hybrid systems by means of Raman and photoluminescence spectroscopies.From Raman measurements, a significant charge transfer from the confined dye to the nanotube is evidenced. Experiments also suggest a photo-activated electron transfer for small diameter (~9 Å) semiconducting and metallic tubes. The main relevant parameters that govern the charge transfer are the nanotube diameter and the electronic properties of both the nanotube (metallic or semiconducting) and the chromophores (electron donor or acceptor).Photoluminescence experiments clearly demonstrate changes on the emission properties after encapsulation. The intensities can be increased or reduced depending on the nature of the confined chromophores (electron donor or acceptor). These behaviors are consistent with a charge transfer.Therefore, Raman and photoluminescence experiments strongly suggest charge transfer between the confined molecules and the nanotubes, leading to a Fermi level shift which governs the radiative de-excitation efficiency and the electron-phonon coupling.