(Invited) Controlling the Inner Dielectric Environment of Carbon Nanotubes to Tune Their Optical Properties

The optical properties of single-wall carbon nanotubes (SWCNTs) are extremely sensitive to their external and internal environment. For example, filling the endohedral cavity with water molecules results in characteristic shifts and broadening of both the SWCNTs’ radial breathing mode vibrational frequency and their optical (electronic) transitions, as well as a quenching of their emission.[1] Although mostly ignored in literature, solvent ingestion effects are substantial, and can also be used to passivate the nanotube interior, by controlled manipulation of the endohedral environment prior to dispersion.[2] In this work, specific and tunable modification of the optical properties of SWCNTs is demonstrated through the direct encapsulation of guest molecules with widely varying dielectric constants. Over 30 different compounds with varying static dielectric constant have been encapsulated inside the SWCNTs and their spectroscopic analysis, in comparison to unfilled (empty) SWCNTs, demonstrates experimentally that the general effect of filler static dielectric constant on the SWCNTs’ optical properties corresponds to a monotonic energy reduction (red-shifting) of the optical transitions with increased magnitude for higher dielectric constants. In addition to these spectral shifts, the filling also reveals a general increase of fluorescence intensity with lower dielectric constants. Our investigation hence demonstrates a new degree of modulation of the SWCNTs’ optical properties by simple endohedral ingestion of various guest molecules. [1] S. Cambré et al. Phys. Rev. Lett. 104, 207401 (2010); W. Wenseleers et al. Adv. Mater. 19, 2274 (2007); S. Cambré et al. ACS Nano 6, 2649 (2012) [2] J. Campo et al Nanoscale Horizons 1, 317 (2016)