Extreme structural stability of Ti0.5Sn0.5O2 nanoparticles: synergistic effect in the cationic sublattice.

Ti0.5Sn0.5O2 nanoparticles (∼5 nm and ∼10 nm) have been studied under high pressure by Raman spectroscopy. For particles with diameter ∼10 nm, a transformation has been observed at 20-25 GPa while for particles with ∼5 nm diameter no phase transition has been observed up to ∼30 GPa. The Ti0.5Sn0.5O2 solid solution shows an extended stability at the nanoscale, both of its cationic and anionic sublattices. This ultrastability originates from the contribution of Ti and Sn mixing: Sn stabilizes the cationic network at high pressure and Ti ensures a coupling between the cationic and anionic sublattices. This result questions a "traditional" crystallographic description based on polyhedra packing and this synergistic effect reported in this work is similar to the case of metamaterials but at the nanoscale.

Vitreous Carbon, Geometry and Topology: A Hollistic Approach.

Glass-like carbon (GLC) is a complex structure with astonishing properties: isotropic sp2 structure, low density and chemical robustness. Despite the expanded efforts to understand the structure, it remains little known. We review the different models and a physical route (pulsed laser deposition) based on a well controlled annealing of the native 2D/3D amorphous films. The many models all have compromises: neither all bad nor entirely satisfactory. Properties are understood in a single framework given by topological and geometrical properties. To do this, we present the basic tools of topology and geometry at a ground level for 2D surface, graphene being the best candidate to do this. With this in mind, special attention is paid to the hyperbolic geometry giving birth to triply periodic minimal surfaces. Such surfaces are the basic tools to understand the GLC network architecture. Using two theorems (the classification and the uniformisation), most of the GLC properties can be tackled at least at a heuristic level. All the properties presented can be extended to 2D materials. It is hoped that some researchers may find it useful for their experiments.

Microcanonical Nucleation Theory for Anisotropic Materials Validated on Alumina Clusters.

Nucleation kinetics in gas phase remains an open issue with no general model. The derivation of the reaction constants assuming a canonical ensemble fails to describe anisotropic materials such as oxides. We have developed a general and versatile model using activated complex kinetics with a microcanonical approach. This approach handles the kinetics issue in cluster growth when the transient nature of the processes hinders the use of the canonical ensemble. The model efficiently reproduces experimental size distributions of alumina clusters formed by laser ablation with different buffer gas densities, including magic numbers. We show that the thermodynamic equilibrium is not reached during the growth. The bounding energy measured is 10 times lower than the one deduced from DFT calculation, but also the one expected from the bulk cohesive energy.

Pressure-Induced Sublattice Disordering in SnO_{2}: Invasive Selective Percolation.

SnO_{2} powders and single crystal have been studied under high pressure using Raman spectroscopy and ab initio simulations. The pressure-induced changes are shown to drastically depend on the form of the samples. The single crystal exhibits phase transitions as reported in the literature, whereas powder samples show a disordering of the oxygen sublattice in the first steps of compression. This behavior is proposed to be related to the defect density, an interpretation supported by ab initio simulations. The link between the defect density and an amorphouslike Raman signal is discussed in terms of the invasive percolation of the anionic sublattice. The resistance of the cationic sublattice to the disorder propagation is discussed in terms of cation close packing. This result on SnO_{2} may be extended to other systems and questions a "traditional" crystallographic description based on polyhedra packing, as a decoupling between both sublattices is observed.

Nonisotropic Self-Assembly of Nanoparticles: From Compact Packing to Functional Aggregates.

Quantum strongly correlated systems that exhibit interesting features in condensed matter physics often need an unachievable temperature or pressure range in classical materials. One solution is to introduce a scaling factor, namely, the lattice parameter. Synthetic heterostructures named superlattices or supracrystals are synthesized by the assembling of colloidal atoms. These include semiconductors, metals, and insulators for the exploitation of their unique properties. Most of them are currently limited to dense packing. However, some of desired properties need to adjust the colloidal atoms neighboring number. Here, the current state of research in nondense packing is summarized, discussing the benefits, outlining possible scenarios and methodologies, describing examples reported in the literature, briefly discussing the challenges, and offering preliminary conclusions. Penetrating such new and intriguing research fields demands a multidisciplinary approach accounting for the coupling of statistic physics, solid state and quantum physics, chemistry, computational science, and mathematics. Standard interactions between colloidal atoms and emerging fields, such as the use of Casimir forces, are reported. In particular, the focus is on the novelty of patchy colloidal atoms to meet this challenge.

Size-dependent pressure-induced amorphization: a thermodynamic panorama.

Below a critical particle size, some pressurized compounds (e.g. TiO2, Y2O3, PbTe) undergo a crystal-to-amorphous transformation instead of a polymorphic transition. This effect reflects the greater propensity of nanomaterials for amorphization. In this work, a panorama of thermodynamic interpretations is given: first, a descriptive analysis based on the energy landscape concept gives a general comprehension of the balance between thermodynamics and kinetics to obtain an amorphous state. Then, a formal approach based on Gibbs energy to describe the thermodynamics and phase transitions in nanoparticles gives a basic explanation of size-dependent pressure-induced amorphization. The features of this transformation (amorphization occurs at pressures lower than the polymorphic transition pressure!) and the nanostructuration can be explained in an elaborated model based on the Ginzburg-Landau theory of phase transition and on percolation theory. It is shown that the crossover between polymorphic transition and amorphization is highly dependent on the defect density and interfacial energy, i.e., on the synthesis process. Their behavior at high pressure is a quality control test for the nanoparticles.

Thermodynamics of nanoparticles: experimental protocol based on a comprehensive Ginzburg-Landau interpretation.

The effects of surface and interface on the thermodynamics of small particles require a deeper understanding. This step is crucial for the development of models that can be used for decision-making support to design nanomaterials with original properties. On the basis of experimental results for phase transitions in compressed ZnO nanoparticles, we show the limitations of classical thermodynamics approaches (Gibbs and Landau). We develop a new model based on the Ginzburg-Landau theory that requires the consideration of several terms, such as the interaction between nanoparticles, pressure gradients, defect density, and so on. This phenomenological approach sheds light on the discrepancies in the literature as it identifies several possible parameters that should be taken into account to properly describe the transformations. For the sake of clarity and standardization, we propose an experimental protocol that must be followed during high-pressure investigations of nanoparticles in order to obtain coherent, reliable data that can be used by the scientific community.

Tunable synthesis and in situ growth of silicon-carbon mesostructures using impermeable plasma.

In recent years, plasma-assisted synthesis has been extensively used in large scale production of functional nano- and micro-scale materials for numerous applications in optoelectronics, photonics, plasmonics, magnetism and drug delivery, however systematic formation of these minuscule structures has remained a challenge. Here we demonstrate a new method to closely manipulate mesostructures in terms of size, composition and morphology by controlling permeability at the boundaries of an impermeable plasma surrounded by a blanket of neutrals. In situ and rapid growth of thin films in the core region due to ion screening is among other benefits of our method. Similarly we can take advantage of exceptional properties of plasma to control the morphology of the as deposited nanostructures. Probing the plasma at boundaries by means of observing the nanostructures, further provides interesting insights into the behaviour of gas-insulated plasmas with possible implications on efficacy of viscous heating and non-magnetic confinement.

Mutual orientation of two C60 molecules: an ab initio study.

The orientational dependence of the interaction between two C(60) molecules is investigated using ab initio calculations. The binding energy, computed within density functional theory in the local density approximation, is substantially smaller than the one derived from the experimental heat of sublimation of fullerite, which calls into question the nature of inter-C(60) bonding. According to our calculations, the experimentally observed orientation with a C(60) presenting a hexagon-hexagon bond to a pentagonal face of the other C(60) is not really favored. Some other configurations are very close in energy and in fact a pentagon facing a pentagon and a hexagon facing a hexagon-hexagon bond are found to be slightly more favorable situations. Our results are compared to previous ones obtained either with previous empirical intermolecular potentials or to existing ab initio studies of crystalline C(60). In addition, the stacking of C(60) in a crystal and in a decahedral (C(60))(7) cluster is discussed.