The control of structuring and morphology in hybrid solids is a key parameter for the design of functional materials. In particular, it has been shown that structure and morphology could be tuned through sol gel synthesis conditions in bridged silsesquioxanes (BS). As a matter of fact, during sol-gel process, the competition between self-assembing through non covalent interactions between the organic sub-structures and condensation of the inorganic siliceous sub-structure can lead to the formation of crystalline compounds or amorphous materials.In order to go further with the understanding of the control of the nanostructuring in bridged silsequioxane, we designed the organic moieties of the precursors with either 2 symetrical through hydrogen bond and the strength of these hydrogen bonds is studied through vibrational spectroscopies in-situ at low temperature or at high pressure. The results are analyzed and discussed in terms of H bond strength and geometric factors.In the second part of this talk, we will focus on the influence of the central subunit lying between the two urea/thiourea groups_either a long flexible alkyl chain or rigid phenyl group_ on the self-assembling properties through H bonds.

Bridged silsesquioxane nanomaterials exhibit original mechanical properties thanks to the association of non-covalent and covalent interactions. Thanks to in situ high pressure spectroscopic studies, achieved in diamond anvil cells, the mechanical behavior of these materials was followed as a function of pressure. Vibrational studies coupled to ab-initio simulations show that mechanical constrains are absorbed by the modulation of H bond interactions. We thus show that the rigidity yielded by the inorganic polymerization is counterbalanced by the presence of the intermolecular H bond network. In a large range of pressures, these hybrid materials have a reversible behavior, and thus behave as molecular springs. In a second time, we demonstrate that the pressure behavior of these molecular spring is sensitive to the conformation of H bonds (cyclic versus linear) and to the constraints imposed the covalent inorganic network.

Energy decay plays a central role in a wide range of phenomena1,2,3, such as optical emission, nuclear fission, and dissipation in quantum systems. Energy decay is usually described as a system leaking energy irreversibly into an environmental bath. Here, we report on energy decay measurements in nanomechanical systems based on multilayer graphene that cannot be explained by the paradigm of a system directly coupled to a bath. As the energy of a vibrational mode freely decays, the rate of energy decay changes abruptly to a lower value. This finding can be explained by a model where the measured mode hybridizes with other modes of the resonator at high energy. Below a threshold energy, modes are decoupled, resulting in comparatively low decay rates and giant quality factors exceeding 1 million. Our work opens up new possibilities to manipulate vibrational states4,5,6,7, engineer hybrid states with mechanical modes at completely different frequencies, and to study the collective motion of this highly tunable system.

Ultralight mechanical resonators based on low dimensional materials are well suited as exceptional transducers of minuscule forces or mass changes. However, the low dimensionality also provides a challenge to minimize resistive losses and heating. Here, we report on a novel approach that aims to combine different two-dimensional (2D) materials to tackle this challenge. We fabricated a heterostructure mechanical resonator consisting of few layers of niobium diselenide (NbSe2) encapsulated by two graphene sheets. The hybrid membrane shows high quality factors up to 245,000 at low temperatures, comparable to the best few-layer graphene mechanical resonators. In contrast to few-layer graphene resonators, the device shows reduced electrical losses attributed to the lower resistivity of the NbSe2 layer. The peculiar low-temperature dependence of the intrinsic quality factor points to dissipation over two-level systems which in turn relax over the electronic system. Our high sensitivity readout is enabled by coupling the membrane to a superconducting cavity which allows for the integration of the hybrid mechanical resonator as a sensitive and low loss transducer in future quantum circuits.

An organogelator based on a N,N′-substituted H-bonding perylenebisimide (PBI-C10) self-assembles to form either a green J-type (form I) or a red H-type (form II) aggregate structure. The molecular packing of both polymorphs was determined from a combination of Transmission Electron Microscopy (TEM) (low dose electron diffraction and high resolution), Grazing incidence X-ray diffraction and polarized infrared spectroscopy. To that aim, highly oriented films have been prepared by mechanical rubbing at controlled film temperature and DFT calculations were performed to identify representative vibrational IR bands and their associated polarizations. H-Bonding between amides generates either a rectangular columnar phase (form I) in the dried gel or a hexagonal packing of supramolecular 21/1 helices with a long period of 97 Å (form II) in annealed thin films. In aligned films of form I, polarized FTIR spectroscopy helps determine the orientation of both intermolecular H-bonds and the PBI core with respect to the substrate. In form II, PBI-C10 molecules assemble into pairs to form off-centered 21/1 helices whose helical axis is made of strongly H-bonded amides. TEM investigations show that three 21/1 helices are packed in a frustrated trigonal structure formed by H-bonding. The Form I → Form II transformation implies a redistribution of a single population of strong intra-columnar H-bonds between amides in form I to a mixture of strong and weak H-bonds in the supramolecular helices, the strong H-bonds forming the spine of the helices.

alpha-Quartz-type gallium phosphate and representative compositions in the AlPO4-GaPO4 solid solution were studied by x-ray powder diffraction and absorption spectroscopy, Raman scattering, and by first-principles calculations up to pressures of close to 30 GPa. A phase transition to a metastable orthorhombic high-pressure phase along with some of the stable orthorhombic Cmcm CrVO4-type material is found to occur beginning at 9 GPa at 320 degrees C in GaPO4. In the case of the AlPO4-GaPO4 solid solution at room temperature, only the metastable orthorhombic phase was obtained above 10 GPa. The possible crystal structures of the high-pressure forms of GaPO4 were predicted from first-principles calculations and the evolutionary algorithm USPEX. A predicted orthorhombic structure with a Pmn2(1) space group with the gallium in sixfold and phosphorus in fourfold coordination was found to be in the best agreement with the combined experimental data from x-ray diffraction and absorption and Raman spectroscopy. This method is found to very powerful to better understand competition between different phase transition pathways at high pressure.