The high-pressure behavior of LuFe2O4 is characterized based on synchrotron X-ray diffraction and neutron diffraction, resistivity measurements, X-ray absorption spectroscopy and infrared spectroscopy studies. The results obtained enabled us to propose a P-T phase diagram. In this study, the low pressure charge-ordering melting could be detected by synchrotron XRD in the P-T space. In addition to the ambient pressure monoclinic C2/m and rhombohedral R(3) over bar m phases, the possible P (1) over bar triclinic phase, the monoclinic high pressure form Pm and metastable modulated monoclinic phases were observed; the latter modulated monoclinic phases were not observed in the present neutron diffraction data. Furthermore, the transition to the Pm phase which was already characterized by strong kinetics is found to be favored at high temperature (373 K). Based on X-ray absorption spectroscopy data the Pm phase, which could be recovered at atmospheric pressure, can be explained by a change in the Fe-local environment from a five-fold coordination to a distorted 5 + 1 one.

Double-walled carbon nanotubes (DWNTs), consisting of two coaxial and weakly van der Waals coupled single-walled carbon nanotubes (SWNTs), are one of the most ideal and fundamental systems to study the effects of inter-tube coupling on the physical properties in one-dimensional structures.On this poster, we report on the investigations of individual, spatially isolated and suspended double-walled carbon nanotubes (DWNTs) by combination of high-resolution electron microscopy, electron diffraction, Rayleigh spectroscopy and Raman spectroscopy (Figure 1) [1,2]. We first present an overview of the effect of the diameters, inter-tube distance and nature (metallic or semiconducting) on the Raman signature (Radial Breathing Like [1,3,4] and Tangential Modes [4-6]) of DWNTs. We then examine the features of tangential modes and find a clear frequency shift of the inner-layer G modes as a function of the inter-tube distance with respect to the corresponding G modes in equivalent single-walled carbon nanotubes (SWNT).[4-6] These results are understood by considering the effects of the relaxation of the layers and the interaction between the relaxed structures.[7] Finally, we study the experimental excitation dependence of the G modes intensity of the constituent inner and outer SWNTs. In particular, we discuss the effects of the quantum interference between different electronic transitions on the observed behaviors.[8]References[1] D.I. Levshov et al., Carbon, 114, 141 (2017). [2] D.I. Levshov et al., Phys Rev. B 96 (2017), 195410.[3] D.I. Levshov et al., Nano Lett. 11(11) (2011), 4800. [4] H.-N. Tran et al., Adv. Nat. Sci. Nanosci. Nanotechnol. (2017), 8, 15018.[5] D. Levshov et al., J. Phys. Chem. C 119(40) (2015), 23196.[6] D.I. Levshov et al., Phys. Status Solidi B 254(11) (2017), 1700251.[7] V.N. Popov et al., Phys. Rev. B 97 (2018), 165417.[8] H.-N. Tran et al., Phys. Rev. B 95 (2017), 95 (20), 205411

We report the direct and unambiguous evidence of the existence of inner semiconducting tube (iSCT) photoluminescence (PL) from measurements performed on four individual free-standing index-identified double-walled carbon nanotubes (DWNTs). Based on thorough Rayleigh scattering, Raman scattering and PL experiments, we are able to demonstrate that the iSCT PL is observed with a quantum yield estimated as a few 10-6 independently of the semiconducting or metallic nature of the outer tube. This result is mainly attributed to ultrafast exciton transfer from inner to outer tube. Furthermore, by carrying out PL excitation experiments on the (14,1)@(15,12) DWNT, we show that the iSCT PL can be detected through the optical excitation of the outer tube (Figure 1) indicating that the exciton transfer can also occur in the opposite way.References[1] D.I. Levshov et al., Phys Rev. B 96 (2017), pp. 195410

Chalcogenide glasses exhibit remarkable properties, which could be used in the development of electrical memory susceptible to replace the current flash memories. During the last decades, a strong interest has been focused on superionic Ag-Ge-Se glasses for their high ionic conductivity, a property that could be used to develop ionic Conductive-Bridging Random Access Memories (CB-RAM). Bulk Ag-Ge-Se glasses have already been widely studied for their electrical, structural and thermal properties. In particular it was shown that the conductivity of phase separated Agx(Ge0.25Se0.75)100-x glasses increases suddenly by seven to eight orders of magnitude at 7-10 at.%Ag. The percolation of the Ag-rich phase was at the origin of this sudden jump in conductivity. Despite a convergence on the inhomogeneous nature of these glasses, some questions remain open such as the composition of the Ag-rich and Ag-poor phases, size and shape of the different phases. These questions has hardly been treated in literature to date. In this talk, investigations on bulk Agx(Ge0.25Se0.75)100-x glasses will be performed focusing on the inhomogeneous nature of the glasses. Macroscopic and local studies of vibrational and mechanical properties will conduct using Raman (mapping) spectroscopy, Vickers microhardness and Contact Resonance Atomic Force Microscopy (CR-AFM). For the glass containing 10 at. % in Ag, Raman mapping will give evidence of a phase separation for through continuous interpenetrating phases in the spinodal decomposition process. Combined mechanical characterizations will indicate that the microhardness and rigidity modulus decrease with the silver content in the glass. At nanoscale level, CR-AFM measurements will highlight a modulation of the rigidity with Ag content. The structural origin of these changes will be confirmed using Raman mapping evidencing modifications in the tetrahedral network between two phases. The results could suggest a different Ge/Se ratio in Ag-poor and Ag-rich phases.

Encapsulating molecules inside nanotubes (called hybrid nanotubes in the following) is a common strategy to add new functionalities to the nanocontainer. The one-dimensional nature of single-walled carbon nanotubes (SWCNT) internal channels is exploited here to induce a molecular order and specific interactions which are dependent of the size of the nano container (figure). I present in this seminar the study of the relation between structural, vibrational and electronic properties of hybrid nanotubes encapsulating dyes. Dimethyl-quarterthiophene (4TCH3) encapsulated inside single-walled carbon nanotubes is studied by inelastic neutron scattering. The results reveal specific dynamics of the 4TCH3 modes characterized in particular by a large sensitivity in the low frequency domain of the methyl libration to the confinement. DFT based calculations consistently describe the observed phenomena and give insights on the structure of the hybrid nanotubes at low temperature where molecules preferentially interact with the nanotube wall. We evidence also by means of Raman spectroscopy and transmission electron microscopy that the supramolecular organizations of the confined oligothiophenes depend on the nanocontainer size. Charge transfer between the dyes and the nanocontainer is shown. A strong dependence on the infrared response of the hybrid nanotube due to confinement effect is finally investigated. The results are compared to experiments performed on nanotubes where dyes are π-stacked at the outer surface. Surprisingly, the confinement properties lead to an exaltation of the infrared absorption response in the carbon nanotubes from dye molecule interactions. Thanks to the comparison between the experimental investigations and DFT calculations, we elucidate the origin of the large enhancement of this infrared absorption.

Middle infrared response on a carbon nanotube is very weak because this homonuclear allotrope of carbon does not bear permanent dipoles. Nevertheless, a significant number of infrared phonons are predicted by the group theory. Encapsulating molecules inside nanotubes (called hybrid nanotubes in the following) is a common strategy to add new functionalities. The one-dimensional nature of single-walled carbon nanotubes (SWCNT) internal channels has been exploited here to induce a molecular order and specific interactions which are dependent of the size of the nano container.Here we report the discovery of a strong dependence on the infrared response of the hybrid nanotube due to confinement effect when dimethylquaterthiophenes are encapsulated inside SWCNTs. Results are compared to experiments performed on nanotubes where dyes are π-stacked at the outer surface. Surprisingly, the confinement properties lead to an exaltation of the infrared absorption response in the carbon nanotubes from dye molecule interactions. Thanks to the comparison between the experimental investigations and DFT calculations, we elucidate the origin of the large enhancement of this infrared absorption and demonstrate that interactions between conjugated oligomers and nanotubes can be probed.