• Corpus ID: 230524099

Highly Polarized Light Emission from 6T@BNNT Nanohybrids

@inproceedings{Badon2021HighlyPL,
  title={Highly Polarized Light Emission from 6T@BNNT Nanohybrids},
  author={Amaury Badon and Charlotte Allard and Fr{\'e}d{\'e}ric Fossard and Annick Loiseau and Laurent Cognet and Emmanuel Flahaut and Nicolas Izard and Richard Martel and Etienne Gaufr{\`e}s},
  year={2021}
}

• A. Badon, C. Allard, +6 authors E. Gaufrès
• Published 5 January 2021
• Physics

The polarized fluorescence emission of organic fluorophores has been extensively studied in photonics and is increasingly exploited in single molecule scale bio-imaging. Expanding the polarization properties of compact molecular assemblies is, however, extremely challenging due to depolarization and quenching effects associated with the self-aggregation of molecules into the sub-nanometer scale. Here we demonstrate that Boron Nitride Nanotubes (BNNTs) can act as a 1D host-template for the alignment of encapsulated a-sexithiophene (6T) inside BNNTs, leading to an optically active 6T@BNNT nanohybrid. We show that the fluorescence from the nanohybrid is strongly polarized with extinction ratios as high as 700 at room temperature. A statistical analysis of the 6T orientation inside BNNTs with inner diameter up to 1.5 nm shows that at least 80% of the encapsulated 6Ts exhibit a maximum deviation angle of less than 10° with respect to the BNNT axis. Despite a competition between molecule-molecule and molecule-BNNT adsorption in larger BNNTs, our results also show that more than 80% of the molecules display a preferential orientation along the BNNT axis with a deviation angle below 45°. Main: Polarized light-matter interaction in molecules offers additional functionalities of high interest in various applications, such as bio-imaging, solar energy harvesting and quantum photonics, to name just a few. In particular, common organic dyes, such as rod-like polythiophenes, combine huge photon absorption/emission transitions between well-oriented transition dipoles with bright polarized emission at the single molecule level. Yet, the unique polarization properties and bright emission of a single molecule is hard to translate to larger assemblies of that molecule because the self-assembly process generally prevents extended alignment of the molecular dipoles and hence lowers its emissivity. The formation of highly polarized dye aggregates requires: (1) Specific molecular ordering and alignment in the assembly and (2) a fine control of the interaction between molecules (e.g. van der Waals, hydrogen bonding, etc.). One emblematic molecular architecture for polarized emission is the head-to-tail stacking of dyes. This so-called J-aggregate favours intermolecular coupling that leads to bright lower emission states, giving higher fluorescence quantum yields, narrower red-shifted emission bands and a strong polarization dependency. Targeting this specific molecular arrangement is extremely challenging because the self-organization (or stacking) of common molecules rarely results in the desirable head-to-tail ordering. Many self-aggregation schemes have been investigated using supramolecular assemblies with or without selective bonding and many examples have been found thanks to specific adsorption mechanisms at surfaces or in solutions. Other approaches based on a host/guest concept and using nanoporous crystals, such as zeolites and more recently metal-organic frameworks (MOF), have also attracted significant interest for templating molecular dipoles. While these approaches have greatly expanded our understanding of the polarization effects related to the supramolecular structure of the emitters, the synthesis and manipulation of large assemblies of molecules for highly polarized emission remains a limiting step for wide-spread applications. Since their discovery, single-walled carbon nanotubes (SWCNTs) have been used as a host of molecules and as template for their assembly [18,19] and there are many examples related to the encapsulation of organic dyes (Dyes@CNT). Thanks to the 1D crystalline architecture, the nanotube offers a hollow centre that is readily available for molecular assembly and alignment. Their cavity is characterized by a diameter and chiral angle distributions with an impressive structural regularity along the whole nanotube axis. Although the SWCNT is structurally an ideal template for J-aggregates, this research has produced conflictual results on the emission properties of the dyes due to the presence of an efficient energy transfer processes between the photo-excited dye and the SWCNT host (Eg < 1 eV). It was recently shown that the fluorescence from most of the dyes are efficiently quenches by the SWCNT. However, our group has recently addressed the quenching issue by replacing SWCNTs with boron nitride nanotubes (BNNTs). Similar to a SWCNT, the BNNT has hollow cavity in the centre and presents a 1D crystalline structure having an inner diameter distribution ranging between 1-2 nm. However, they differ significantly by a strong dielectric behaviour up to the bandgap energy close to 6 eV. Our work showed that the BNNT sheet protects the luminescence properties organic dyes with clear signatures of bright J-like aggregation states and enhanced photostability. Here we explore the fluorescence polarization properties of a-sexithiophene (6T) molecules encapsulated inside BNNTs (6T@BNNTs). By using aberration corrected High Resolution Transmission Electronic Microscopy (HRTEM) and fluorescence imaging of individual 6T@BNNTs, we observe that the molecules are remarkably well aligned along the BNNT axis for inner diameter below 3 nm. We report that this 1D ordering is independent from the length of the BNNT and of the filling level. We finally show that a complex adsorption and stacking competition induces molecular disorder in the larger inner diameter BNNTs, which complicates the polarization dependency when the inner diameter is above ~3 nm. Results and discussion. The sample preparation begins by opening raw BNNTs (BNNT LLC supplier) using mechanical grinding and ultrasound treatments, followed by a thermal purification at 800°C in air to remove the content of elemental boron and other h-BN sub-products. Purified BNNTs are then dispersed in DMF by ultrasounds and a small fraction of the solution is either spincoated on a Si/SiO2 substrate (previously patterned with localisation marks) or drop casted onto a Molybdenum TEM grid coated with a holey SiO2 membrane. The BNNTs are then liquid-filled with 6Ts by placing the sample in a solution of 6T dissolved in toluene at 115°C and at a concentration of 5x10 M. The last step of this liquid phase encapsulation consists in removing the non-encapsulated 6T molecules using toluene rinsing cycles at room temperature. Residues of 6Ts that are not encapsulated are finally removed using an oxidative piranha solution (or a mild oxygen plasma in some experiments). More details about the sample preparation can be found in the supplementary information file. Figure 1: a Schematic illustration of a-sexithiophene (6T) molecules aligned inside a boron nitride nanotube (BNNT). b Diameter distribution of BNNTs probed on the grid using Transmission Electron Microscopy. Note that BNNTs generally have 2 to 4 walls; wall numbers superior to 5 and diameters larger than 10 nm are seldom found in our samples. Figure 1a presents a schematic representation of 6T molecules aligned inside a BNNT. In practice, the measurements of the inner diameter, dinner, of our BNNT samples is obtained statistically using aberration corrected High Resolution Transmission Electron Microscopy (ac-HRTEM) experiments and the results indicate a relatively broad distribution, ranging from 0.7 nm to 7 nm with an occurrence maximum centred at about 2 nm (see Fig 1b). Considering the 1 nm length of an elongated 6T molecule, this large distribution of the size of the BNNT cavity should induce various stacking configurations that can compete with the head-to-tail alignment required for highly polarized emission. To investigate this configuration distribution, we performed polarized fluorescence imaging at the single nanotube level of 6T@BNNTs lying on Si/SiO2 substrate (Fig. S1). Briefly, a polarized 532 nm laser (1:1000 polarization ratio) was collimated onto the sample using the 60x objective to homogeneously photo-excite individual 6T@BNNTs within a ~ 45x45 μm field-of-view. A combination of half and quarter wave plates allowed to excite the sample with a well-defined polarization state (linear or circular with controlled orientations). The emitted fluorescence was collected and analysed through a polarization beam splitter prism cube acting as the “analyser” and a 660±5 nm band-pass filter. Details on the optical set up are available in the supplementary information. Figure 2a shows a representative series of polarized fluorescence images obtained from individual 6T@BNNTs recorded using different excitation polarization conditions (blue arrows). The top series displays the fluorescence of randomly oriented 6T@BNNTs having lengths comparable to the optical resolution of our apparatus (~1 μm) and excited with circularly polarized light at a laser wavelength of 532 nm. Note that the circular polarization condition ensures that all orientations are simultaneously detected with our setup. The middle and bottom series present the response of two orthogonal 6T@BNNTs having relatively longer lengths of ~4 μm. The first one presents a homogeneous fluorescence intensity along the BNNT axis while the second displays spotted emission due to a partial filling of 6T dyes. The two 6T@BNNT are excited with a linearly polarized light oriented approximatively along the 6T@BNNT axis. Figure 2: a Polarized fluorescence images of short (< 1 μm) and long (~ 4 μm) individual 6T@BNNT candidates deposited on Si/SiO2 substrate. The polarization angle of the signal ranges from 0° to 270° with an incremental step of 30° between each image. The blue arrows indicate the direction of the excitation polarization for the series: circular, horizontal and vertical polarization from top, middle to bottom, respectively. b-c Polar plots of the fluorescence intensity for two 6T@BNNT as a function of the analyser position and for two excitation polarizations, i.e. along and orthogonal to the nanotube axis. The excitation wavelength is 532 nm. The fitt