Periodic Reporting for period 4 - ORDERin1D (Order in one dimension: Functional hybrids of chirality-sorted carbon nanotubes)
Période du rapport: 2020-11-01 au 2021-10-31
CNTs can be visualized as a one atom thick layer of carbon atoms (i.e. a sheet of graphene) rolled up into a hollow cylindrical structure with a diameter of only 0.5 to 2 nanometers. These nanoscale structures possess remarkable electronic and optical properties, strongly depending on the way the graphene sheet is rolled up into a cylinder and thus the exact chiral structure of the CNTs. Each CNT structure will absorb and emit light at different wavelengths, thereby allowing the identification of the different CNT structures present in a specific sample by optical spectroscopy.
The hollow cores of CNTs are just large enough to fit different molecules inside that, depending on the diameter, form single or multiple molecular files after encapsulation. In particular such a single row of molecules can behave very differently in comparison to the same molecules embedded in a macroscopic material. Not only the properties of the molecules themselves, but also the properties of the CNTs change drastically due to the close interaction between both components. In this project, filling of CNTs with various functional molecules is being investigated to promote new functionalities of both the individual building blocks (CNTs and encapsulated molecules) and the novel nanohybrid materials that are formed through the one-dimensional confinement of the molecules inside the CNTs.
There exist more than 160 different CNT structures within a diameter range of 0.5-2nm each having a slightly different chiral structure. Although this provides a very large platform where for each molecule an ideal CNT structure can be found that nicely fits around it, unfortunately, synthesis methods invariably produce a mixture of different chiral structures and diameters, each with different electronic and optical properties. Therefore, sorting of the CNTs based on their specific chiral structure is extremely important and one of the overall objectives of this project is to develop new sorting protocols for specific CNT chiral structures.
Such novel nanhybrid materials, with well-defined structure, can have a direct impact on the societal challenges of the future: including energy harvesting, water purification, ultrafast electro-optical modulators for optical data communication, ultrasensitive sensors, memory elements and nanoprobes for bio-imaging applications.
The hollow cores of CNTs are just large enough to fit different molecules inside that, depending on the diameter, form single or multiple molecular files after encapsulation. In particular such a single row of molecules can behave very differently in comparison to the same molecules embedded in a macroscopic material. Not only the properties of the molecules themselves, but also the properties of the CNTs change drastically due to the close interaction between both components. In this project, filling of CNTs with various functional molecules is being investigated to promote new functionalities of both the individual building blocks (CNTs and encapsulated molecules) and the novel nanohybrid materials that are formed through the one-dimensional confinement of the molecules inside the CNTs.
There exist more than 160 different CNT structures within a diameter range of 0.5-2nm each having a slightly different chiral structure. Although this provides a very large platform where for each molecule an ideal CNT structure can be found that nicely fits around it, unfortunately, synthesis methods invariably produce a mixture of different chiral structures and diameters, each with different electronic and optical properties. Therefore, sorting of the CNTs based on their specific chiral structure is extremely important and one of the overall objectives of this project is to develop new sorting protocols for specific CNT chiral structures.
Such novel nanhybrid materials, with well-defined structure, can have a direct impact on the societal challenges of the future: including energy harvesting, water purification, ultrafast electro-optical modulators for optical data communication, ultrasensitive sensors, memory elements and nanoprobes for bio-imaging applications.
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.
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.
In summary, this project has developed new methods for sorting carbon nanotubes and modifying their interior hollow space. We also addressed more fundamental scientific questions such as the observation of a quasi-phase transition in a one-dimensionally confined chain of water molecules, which was unexpected as theory did not predict such phase transitions to exist. Further, we also focused on developing new methodologies to characterize the filling of CNTs as well as to investigate the mechanism behind the sorting techniques of different CNT structures.
Further work beyond the current project, will explore the specific alignment of dipolar dyes inside CNTs, phase transitions in other CNT diameters as well as investigation of the transport of molecules inside the CNTs.
Further work beyond the current project, will explore the specific alignment of dipolar dyes inside CNTs, phase transitions in other CNT diameters as well as investigation of the transport of molecules inside the CNTs.