The ORDERin1D ERC starting grant research team has researched several aspects of one-dimensional (1D) carbon nanotube hybrids, created by filling the chirality-sorted CNTs with various molecules, resulting in several breakthroughs.
First, the ERC team was able to observe a peculiar quasi phase transition that occurred when confining water inside very tiny CNTs, that can fit only a single row of water molecules. It was found that at low temperature (below 150 Kelvin), the water dipoles all align head to tail in a ferroelectric manner with all dipoles pointing in the same sense. When increasing the temperature, other ordered structures can be formed until a random orientation of the water dipoles is obtained at high temperature. Corroborated with molecular dynamics simulations, this provided the first demonstration that water molecules behave entirely different under confinement. These results were afterwards also extended to other chiralities, showing a clear dependence on the phase transition temperature with the surrounding SWCNT diameter, each diameter resulting in a different configuration of the encapsulated water molecules.
Secondly, the ERC team filled the CNTs with more than 30 different solvents, each with a different static dielectric constant, which enabled us to unravel the origin of electronic shifts of the optical transitions of CNTs induced by the dielectric constant of the fillers. Moreover, for each of the fillers a minimal encapsulation diameter was experimentally determined and compared to theoretical models, showing that as long as the molecule fits inside the CNTs it can be encapsulated. Surprisingly, even a very short, 30 seconds, exposure of the CNTs to the solvent results in a complete filling of the CNTs, which can have drastic implications on the resulting CNT properties. For example, it was found that more polar fillers quench the emission efficiency of the CNTs significantly, while more apolar fillers approach the optical properties of empty CNTs. These results proved important for the CNT research community, where researchers often unwittingly worked with filled CNTs.
Aside from these solvents, also organic dye molecules were encapsulated inside CNTs with different diameters. After optical excitation of the dye molecules, it was found that the dye molecules efficiently transfer their excitation energy to the CNTs (close to 100%), thereby photosensitizing the CNTs for other wavelengths of light. Interestingly, we found that depending on the CNT diameter, the dyes adopt specific molecular arrangements (e.g. single file to double file arrangements in smaller and larger diameter CNTs) which strongly influences the excitation energy needed to excite the dye molecules. As such, one is able to tune the excitation energy of the dye, by selecting a specific CNT diameter.
Within this project also a new setup was designed to perform hyperspectral imaging of individual SWCNTs along their length, where each pixel in the image contains a full emission spectrum of the SWCNT under investigation. Using this technique, we were able to unravel the peculiar interaction of chiral bile salt surfactants with the SWCNT surface, stacking differently around left- and right-handed SWCNT enantiomers. This stacking also lies at the basis of the separation of such enantiomers from each other.
The inner diameter of SWCNTs can furthermore also be used to synthesize new carbon structures in one dimension, which can then be identified and characterized by subsequent wavelength-dependent resonant Raman spectroscopy. As such we investigated the electronic and optical properties of 6- and 7-armchair graphene nanoribbons and ultralong linear carbon chains up to thousands of carbon atoms, synthesized within the hollow core of the SWCNTs.
Finally, we unravelled the mechanism behind the sorting of SWCNTs by diameter and chiral structure, thereby enabling new sorting protocols to be developed for large diameter SWCNTs filled with dye molecules, thus resulting in nanohybrids of SWCNTs with a well-defined chiral structure of the SWCNT host.
These research findings resulted in several high impact publications and have been the topic of various invited and contributed presentations at the most important scientific meetings in the field.
