Hyperspectral Detection of the Fluorescence Shift between Chirality-Sorted Empty and Water-Filled Single-Wall Carbon Nanotube Enantiomers

This work was financially supported by the European Research Council (ERC Starting Grant, ORDERin1D, 679841) and the Research Foundation of Flanders (FWO) through a PhD fellowship for M.E. (11C9220N) and projects with grant numbers G036618N and G021112N. M.A.L.C. acknowledges support from the Research Fund from the University of Antwerp through a BOF-DOCPRO4 project. The authors also acknowledge the support of the Center of Integrated Nanotechnologies (CINT, Los Alamos National Laboratory, USA) for sharing the HiPco SWCNTs

Interfacial Metal Chlorides as a Tool to Enhance Charge Carrier Dynamics, Electroluminescence, and Overall Efficiency of Organic Optoelectronic Devices

The authors would like to thank the fund for scientific research-Flanders (FWO) for providing S.A. with a SB PhD grant with grant numbers 1SA4523N and 1SA4525N and BOF funding from UHasselt under grant number BOF22IN-CENT09 for the present work. S.C. thanks the FWO for providing infrastructure funding for the EPR instrumentation (I004920N). The XPS used in this work has been funded by the Research Foundation-Flanders under grant number I00620N

Advanced 1D heterostructures based on nanotube templates and molecules

Recent advancements in materials science have shed light on the potential of exploring hierarchical assemblies of molecules on surfaces, driven by both fundamental and applicative challenges. This field encompasses diverse areas including molecular storage, drug delivery, catalysis, and nanoscale chemical reactions. In this context, the utilization of nanotube templates (NTs) has emerged as promising platforms for achieving advanced one-dimensional (1D) molecular assemblies. NTs offer cylindrical, crystalline structures with high aspect ratios, capable of hosting molecules both externally and internally (Mol@NT). Furthermore, NTs possess a wide array of available diameters, providing tunability for tailored assembly. This review underscores recent breakthroughs in the field of Mol@NT. The first part focuses on the diverse panorama of structural properties in Mol@NT synthesized in the last decade. The advances in understanding encapsulation, adsorption, and ordering mechanisms are detailed. In a second part, the review highlights the physical interactions and photophysics properties of Mol@NT obtained by the confinement of molecules and nanotubes in the van der Waals distance regime. The last part of the review describes potential applicative fields of these 1D heterostructures, providing specific examples in photovoltaics, luminescent materials, and bio-imaging. A conclusion gathers current challenges and perspectives of the field to foster discussion in related communities

(Invited) Chirality-Dependent Surfactant Interactions Unravelled By Systematic Ultracentrifugal and Aqueous Two-Phase Separations

The discovery that natural bile salt surfactants are extraordinarily efficient at solubilizing individual, intact single-wall carbon nanotubes (SWCNTs) in water[1], soon enabled the separation of different SWCNT structures for the first time by density gradient ultracentrifugation (DGU),[2] and more recently also by aqueous two-phase extraction (ATP).[3,4] Despite very nice results for a limited number of small diameter SWCNT chiralities that have been isolated as individual species by DGU and ATP, the underlying mechanisms are not fully understood and the number of variables to be explored and optimized is enormous, preventing a systematic extension to other chiralities. Based on the unique capabilities of optical spectroscopy for characterizing SWCNTs, we present a systematic study of the structure sorting of DGU and ATP. We combine a direct in situ optical characterization of the centrifuge tubes after DGU[5] with systematic concentration-dependent ATP separations for different (mixed) surfactants and as such obtain a more detailed understanding of and control over both separation mechanisms. These systematic studies reveal that chirality-dependent surfactant interactions determine the separation order. [1] W. Wenseleers et al., Adv. Fucnt. Mater. 2004, 14, 1105 [2]. M.S. Arnold et al., Nature Nanotechnol. 2006, 1, 60 [3]. C.Y. Khripin et al., J. Am. Chem. Soc. 2013, 135, 6822 [4]. N.K. Subbaiyan et al., ACS Nano 2014, 8, 1619 [5]. S. Cambré et al., Nanoscale 2015 , 7, 20015</jats:p

(Invited) Systematic Aqueous Two-Phase Separations of Carbon Nanotubes to Investigate the Separation Mechanism

Aqueous two-phase (ATP) separation has been demonstrated as a fast, highly-scalable separation technique to sort single-wall carbon nanotubes (SWCNTs) by diameter, chirality, metallicity, etc.[1-2] In ATP, two water-soluble, yet immiscible polymers are mixed together at sufficiently high concentration after which they spontaneously form two phases with different hydrophilicity. In a typical separation protocol, SWCNTs are added to this two-polymer mixture with a combination of different surfactants, resulting in separation of different chiral species among both phases. While very promising separations have been achieved, the separation mechanism is not well understood In this paper, we present a series of ATP separations, in which surfactant combinations and concentrations are systematically varied. Absorption spectroscopy, multi-wavelength resonant Raman scattering and wavelength-dependent photoluminescence-excitation spectroscopy, combined with detailed two-dimensional spectral analysis, is applied to characterize the full composition of the resulting separated fractions and as such study the underlying separation mechanism. These systematic studies reveal that chirality-dependent surfactant interactions determine the separation order. [1]. C.Y. Khripin et al., J. Am. Chem. Soc. 2013, 135, 6822 [2]. N.K. Subbaiyan et al., ACS Nano 2014, 8, 1619 </jats:p

(Invited) Quasi Phase Transition in a Single File of Water Molecules Encapsulated inside (6,5) Carbon Nanotubes

The confinement of water molecules in one-dimensional channels leads to fascinating new properties which are of critical importance for many processes in biology, geology, catalysis and nanofluidics. Single-walled carbon nanotubes (SWCNTs) are one of the most promising model systems to investigate molecules under confinement. They combined atomically-precise diameters that deliver the necessary level of confinement with a weak interaction of the water molecules with their smooth inner walls. Despite significant efforts, theoretical predictions of the behavior and peculiar properties of a single-file chain of water water molecules (e.g. interesting for ultrafast proton transport) have proven to be very hard to validate experimentally. In this work, we present temperature-dependent (4.2K up to room temperature) photoluminescence (PL) spectra of water-filled and empty chirality-sorted (6,5) SWCNTs. Superimposed on a linear temperature-depenent PL spectral shift of the empty tubes, an additional stepwise PL shift of the water-filled SWCNTs is observed at around 150K. With the empty SWCNTs serving as the ideal reference system, we can assign this observation to temperature-induced changes occuring within the single file molecular chain. Supported by new Molecular Dynamics simulations, we obtain the first experimental evidence of a quasi phase transition of the orientation of the molecular dipoles within such a one-dimensional single file chain of water molecules. </jats:p

(Invited) Aligning Organic Dipolar Molecules in Carbon Nanotubes for Nonlinear Optics

The 1D character of single-walled carbon nanotubes (SWCNT) combined with their mechanical robustness, chemical inertness and wide range of diameters makes them ideal nanocontainers for various molecules. Here we show that by encapsulating elongated dipolar molecules in the 1D internal channel of SWCNTs, Coulomb interactions naturally favour a polar head-to-tail alignment of the molecular dipoles, resulting in a cooperative enhancement of directional properties, such as the dipole moment and the second-order nonlinear optical (NLO) response.[1]  This principle is demonstrated for a prototypical dipolar molecule encapsulated in various SWCNT samples. The encapsulation is evidenced by extensive wavelength-dependent fluorescence-excitation and resonant Raman experiments on bile-salt solubilised dye-filled SWCNTs[2], revealing the effect of encapsulation on the vibrational and electronic properties of the SWCNTs and the encapsulated molecules.[3-6] The polar alignment is demonstrated by wavelength-dependent hyper Rayleigh scattering experiments (HRS)[7-9] (i.e. second harmonic light scattering). The newly synthesized organic nanohybrids possess a giant total dipole moment and NLO response, corresponding to ~70 identically aligned molecules (HRS intensity scaling quadratically with this number). These nanohybrids form solution processible building blocks highly interesting for the development of electro-optic modulators and switches. References [1] S. Cambré et al. Nature Nanotechnol. 10, 248 (2015) [2] W. Wenseleers et al. Adv. Funct. Mater. 14, 1105 (2004) [3] W. Wenseleers et al. Adv. Mater. 19, 2274 (2007) [4] S. Cambré et al. Phys. Rev. Lett. 104, 207401 (2010) [5] S. Cambré et al, ACS nano 6, 2649 (2012) [6] S. Cambré et al. Angew. Chem. Int. Ed. 50, 2764 (2011) [7] E. Goovaerts et al. In Handbook of advanced electronic and photonic Materials and Devices, Vol. 7: Nonlinear optical materials, Academic Press, San Diego, 127-191 (2001) [8] J. Campo et al. Optics Express 17, 4587 (2009) [9] J. Campo et al. J. Phys. Chem. Lett. .3, 2248 (2012)</jats:p

(Invited) Optically Detected Magnetic Resonance of Triplet Excitons in Sorted (6,5) and (7,5) SWCNTs

Single-wall carbon nanotubes (SWCNTs) possess unique electronic and optical properties, strongly depending on their exact chiral structure. Their quasi one-dimensional character results in the formation of strongly bound electron-hole pairs (excitons) that can even be observed at room temperature (i.e. binding energies of the order of several hundred meV).[1] The exciton fine structure of SWCNTs is quite complex, with multiple singlet and triplet excitonic states, of which only one is optically allowed, thereby resulting in extremely low fluorescence (PL) quantum yields. While the singlet excitons have already been investigated thoroughly, little is known about the longer-living triplet excitons. In this work, we report the characterization of triplet excitons in chirality-purified samples of (7,5) and (6,5) SWCNTs by means of optically detected magnetic resonance spectroscopy (ODMR), a technique that measures the influence on the emission of CNTs when making transitions between the spin levels of the triplet excitons. In both chiralities the nanotubes are shown to sustain two types of triplet exciton states, as shown by the symmetry and magnitude of their zero-field splitting parameters in the spin Hamiltonian, which are determined by careful fitting of magnetic-field orientation-dependent ODMR spectra of in-plane aligned SWCNTs. While the triplet excitons only weakly depend on the nanotube environment, an increase in zero-field splitting is found for (6,5) with respect to (7,5) nanotubes, in good agreement with the tighter confinement expected in the narrower-diameter nanotubes. [1] M.S. Dresselhaus et al, Ann. Rev. Phys. Chem. 2007, 58, 719 [2] D. Stich et al, Nature Photon. 2014 , 8,139; J. Palotás et al, ACS Nano 2020, 14, 11254 </jats:p

(Invited) Hyperspectral Detection of the Fluorescence Shift between Enantiomers of Empty and Water-Filled Single-Wall Carbon Nanotubes

The unusually chiral-structure-dependent properties of single-wall carbon nanotubes (SWCNTs) are also strongly influenced by their internal and external environment, which can therefore be investigated through optical spectroscopy.[1,2] In order to study this influence in detail, we developed a hyperspectral IR fluorescence imaging setup based on a microscope with a liquid crystal tunable filter. By resolving the spectra of individual SWCNTs, and even along the length of SWCNTs, the effect of inhomogeneous broadening is largely eliminated, and spectral details can be resolved which are inaccessible in bulk spectroscopy. An automated image processing scheme is used to obtain statistics on large numbers of individual SWCNTs. In particular, we show that not only the spectral shift in emission between empty and water-filled[1] chirality sorted SWCNTs can be resolved, but even separate emission peaks are observed for the left- and right-handed enantiomers, which interact slightly differently with the chiral surfactant with which they are coated. The approach is particularly promising for the quantification of enantio-selective separation results. [1] W. Wenseleers et al., Adv. Mater. 19, 2274 (2007); S. Cambré and W. Wenseleers, Angew. Chem. 50, 2764 (2011); S. Cambré et al., ACS Nano 6, 2649 (2012). [2] J. Campo et al., ACS Nano 2021, 15, 2301−2317. </jats:p

(Invited) Modulation of Nanotube Optical Properties By Controlling the Dielectric Environment inside of Single-Wall Carbon Nanotubes

Specific and tunable modification to the optical properties of single-wall carbon nanotubes is demonstrated through the direct encapsulation into the nanotube interior of guest molecules with measured static dielectric constants. Optical measurements on SWCNT populations containing over thirty distinct simple compounds of varying static dielectric from 1.8 to 109 in large diameter nanotubes, and 15 compounds in small diameter nanotubes demonstrate for the first time experimentally the general effect of filler static dielectric on the nanotube optical properties. Comparison to effective medium theory predictions is presented, and guest molecules are identified that lead to effects disobeying the general trend and theory. These results both demonstrate a new degree of exploitable modulation in the optical properties of SWCNTs, and provide a foundation for examining higher order effects in host-guest interactions in well controlled pore size materials. </jats:p