Synthesis of carbon nanotube composites and their optoelectronic properties characterized by scanning probe microscopic techniques

Abstract

Carbon nanotubes (CNTs) are one-dimensional nanomaterials with interesting electronic properties and have potential applications in miniaturization of optoelectronic devices. Although various dispersants have been designed to functionalize CNTs, the optoelectronic properties of individual functionalized CNTs are yet to be fully explored. In this work, electrostatic force microscopy (EFM) and photoconductive atomic force microscopy (PCAFM) were utilized to characterize individual functionalized CNTs and rigorous statistical methodology for data analysis was applied to draw tenable conclusions. A poly(alkylthiophene)-based conjugated polyelectrolyte was used as a photosensitizer and a dispersant of CNTs to form a composite. EFM was performed for individual pristine CNTs and individual functionalized CNTs for comparison. It was the first time that charges of a polyelectrolyte on individual CNTs were quantitatively detected by EFM. PCAFM has revealed that the dominant charge transport mechanism of individual functionalized CNTs is Simmons tunneling. The large statistical dispersion of the PCAFM data necessitated rigorous statistical analysis to distinguish real effects of illumination from artifacts. The nonparametric bias-corrected and accelerated bootstrap method quantified the photoresponses conveniently without any presumption of the data distributions. The equilibrium conductance of an individual functionalized CNT exhibited a one-fold increase upon white light illumination. A series of ruthenium-containing diblock copolymers were synthesized by reversible addition-fragmentation chain transfer polymerization. The pyrene-containing first block acted as an anchoring group to interact with CNTs and the ruthenium(II) complexes in the second block acted as photosensitizers. Moieties with different electronic properties were conjugated to pyrene to tune the energy levels. The photoresponses of composites formed by these polymers were compared by PCAFM. The multiway factorial analysis of variance was employed to analyze the PCAFM data to investigate the effects of the dispersants, illumination condition and applied bias on the charge transport of the composites. The difference in photoresponse among different composites was related to the conformations of the composites, which were elucidated by molecular dynamics simulations, and the electronic properties of polymers, which were deduced by cyclic voltammetry and density functional theory calculations. The analysis suggests that the photogenerated charges from the metal complexes in the second block pass through the first block before reaching the CNT. The photoresponses of the CNT composites have potential applications in optoelectronic devices. With EFM to identify charged coatings and PCAFM to probe the photoresponses, individual functionalized CNTs can be characterized in detail. The rigorous statistical methodology can unravel the charge transport mechanism, which may provide guidelines for designing molecular devices.