STRUCTURAL AND EXCITONIC PROPERTIES OF QUANTUM DEFECT-TAILORED ULTRASHORT CARBON NANOTUBES FROM DENSITY FUNCTIONAL THEORY

Abstract

Fluorescent ultrashort nanotubes (FUNs) have recently received renewed interest as a class of luminescent nanomaterials, owing to developments in sp^3 quantum defect chemistry in single-wall carbon nanotubes (SWCNTs). By tailoring ultrashort SWCNTs with quantum defects, their typically quenched photoluminescence is re-activated and brightened. Since the potential uses of FUNs for applied and fundamental science have only begun to be explored, there exists a need for a theoretical understanding of their basic optical and electronic properties. To that end, we have performed quantum chemical calculations within a density functional theory framework to explore these properties. By performing ground and excited electronic state calculations on molecular models of ultrashort SWCNTs, including those with sp^3 quantum defects, we predict the following properties: 1) there exist natural molecular models of ultrashort SWCNTs, in the sense that they have the lowest ground state energy amongst their isomers, and exhibit the correct absorption spectra, 2) the lowest bandgap excitonic absorption in SWCNTs has a length-dependent energy shift ∆E that follows a scaling law ∆E ∼ L^{−1/2}, for a SWCNT model of length L, departing from a standard particle-in-a-box view of quantum confinement in SWCNTs, and 3) ultrashort metallic SWCNTs can host organic color centers made by sp^3 defects due to the quantum confinement effect, contrasting with the usual view of metals as fluorescent quenchers. These predictions have implications for the edge-structure of SWCNTs and the length-dependence of their optical properties, providing a theoretical picture of FUNs that points to a gap in understanding of these materials and suggests a need for greater experimental focus on the properties of ultrashort nanotubes.