FT-Raman and high-pressure infrared spectroscopic studies of dicalcium phosphate dihydrate (CaHPO4·2H2O) and anhydrous dicalcium phosphate (CaHPO4)

The FT-Raman spectra and the pressure dependence of the infrared spectra of the hydrated and anhydrous forms of dicalcium phosphate, CaHPO₄ · 2H₂O and CaHPO₄, have been studied. The hydrated salt exhibits a phase transition at 21 kbar (1.0 kbar=0.1 Gpa) but no high pressure transition was observed for anhydrous dicalcium phosphate. The O–H stretching frequencies of the water molecules in CaHPO₄·2H₂O all showed negative pressure dependences and correlate with the OO distances. The PO–H stretch increased with increasing pressure, indicating a strong hydrogen bond. The frequencies associated with the phosphate ion showed a normal pressure dependence.     

Low‐Temperature Cationic Rearrangement in a Bulk Metal Oxide

Cationic rearrangement is a compelling strategy for producing desirable physical properties by atomic‐scale manipulation. However, activating ionic diffusion typically requires high temperature, and in some cases also high pressure in bulk oxide materials. Herein, we present the cationic rearrangement in bulk Mn₂FeMoO₆ at unparalleled low temperatures of 150–300 ^oC. The irreversible ionic motion at ambient pressure, as evidenced by real‐time powder synchrotron X‐ray and neutron diffraction, and second harmonic generation, leads to a transition from a Ni₃TeO₆–type to an ordered‐ilmenite structure, and dramatic changes of the electrical and magnetic properties. This work demonstrates a remarkable cationic rearrangement, with corresponding large changes in the physical properties in a bulk oxide at unprecedented low temperatures.     

Oxygen self-doping in hollandite-type vanadium oxyhydroxide nanorods

A nonaqueous liquid-phase route involving the reaction of vanadium oxychloride with benzyl alcohol leads to the formation of single-crystalline and semiconducting VO 1.52(OH) 0.77 nanorods with an ellipsoidal morphology, up to 500 nm in length and typically about 100 nm in diameter. Composition, structure, and morphology were thoroughly analyzed by neutron and synchrotron powder X-ray diffraction as well as by different electron microscopy techniques (SEM, (HR)TEM, EDX, and SAED). The data obtained point to a hollandite-type structure which, unlike other vanadates, contains oxide ions in the channels along the c-axis, with hydrogen atoms attached to the edge-sharing oxygen atoms, forming OH groups. According to structural probes and magnetic measurements (1.94 mu B/V), the formal valence of vanadium is +3.81 (V (4+)/V (3+) atomic ratio approximately 4). The experimentally determined density of 3.53(5) g/cm (3) is in good agreement with the proposed structure and nonstoichiometry. The temperature-dependent DC electrical conductivity exhibits Arrhenius-type behavior with a band gap of 0.64 eV. The semiconducting behavior is interpreted in terms of electron hopping between vanadium cations of different valence states (small polaron model). Ab initio density-functional calculations with a local spin density approximation including orbital potential (LSDA + U with an effective U value of 4 eV) have been employed to extract the electronic structure. These calculations propose, on the one hand, that the electronic conductivity is based on electron hopping between neighboring V (3+) and V (4+) sites, and, on the other hand, that the oxide ions in the channels act as electron donors, increasing the fraction of V (3+) cations, and thus leading to self-doping. Experimental and simulated electron energy-loss spectroscopy data confirm both the presence of V (4+) and the validity of the density-of-states calculation. Temperature-dependent magnetic susceptibility measurements indicate strongly frustrated antiferromagnetic interactions between the vanadium ions. A model involving the charge order of the V (3+) sites is proposed to account for the observed formation of the magnetic moment below 25 K.     

Conversion of low density polyethylene foams into auxetic metamaterials

Cellular polymers, such as polyethylene foams, are commonly used in the packaging industry. These materials have short service life and generate a high volume of waste after use. In order to valorize this waste and produce added-value applications, it is proposed to convert these materials into highly efficient energy absorption structures. This was done by modifying the original cellular morphology of the foams (spheroidal or polygonal) into a re-entrant structure to produce auxetic materials. This work presents an optimized process combining mechanical compression and solvent vapor evaporation-condensation leading to low density foams (77–200 kg/m³) having negative Poisson's ratios (NPR). Three series of recycled low density polyethylene (LDPE) foams with an initial density of 16, 21, and 36 kg/m³ were used to optimize the processing conditions in terms of treatment temperature, time, and pressure. From all the samples prepared, a minimum Poisson's ratio of −3.5 was obtained. To further characterize the samples, the final foam structure was analyzed to relate with mechanical properties and compare with conventional foams having positive Poisson's ratios. The results are discussed using tensile properties and energy dissipation which were shown to be highly improved for auxetic foams. Overall, the resulting foams can be used in several applications such as sport and military protection equipment.     

Transformable Plasmonic Helix with Swinging Gold Nanoparticles

Control over multiple optical elements that can be dynamically rearranged to yield substantial three-dimensional structural transformations is of great importance to realize reconfigurable plasmonic nanoarchitectures with sensitive and distinct optical feedback. In this work, we demonstrate a transformable plasmonic helix system, in which multiple gold nanoparticles (AuNPs) can be directly transported by DNA swingarms to target positions without undergoing consecutive stepwise movements. The swingarms allow for programmable AuNP translocations in large leaps within plasmonic nanoarchitectures, giving rise to tailored circular dichroism spectra. Our work provides an instructive bottom-up solution to building complex dynamic plasmonic systems, which can exhibit prominent optical responses through cooperative rearrangements of the constituent optical elements with high fidelity and programmability.     

Graphic requirements for multistability and attractive cycles in a Boolean dynamical framework

To each Boolean function and each x{0,1}ⁿ, we associate a signed directed graph G(x), and we show that the existence of a positive circuit in G(x) for some x is a necessary condition for the existence of several fixed points in the dynamics (the sign of a circuit being defined as the product of the signs of its edges), and that the existence of a negative circuit is a necessary condition for the existence of an attractive cycle. These two results are inspired by rules for discrete models of genetic regulatory networks proposed by the biologist R. Thomas. The proof of the first result is modelled after a recent proof of the discrete Jacobian conjecture.     

Robust ligand shells for biological applications of gold nanoparticles

An important point regarding the development of stable biofunctional nanoparticles for biomedical applications is their potential for aspecific interactions with the molecules of the biological environment. Here we report a new self-assembled ligand monolayer system for gold nanoparticles called Mix-matrices, formed by a mixture of HS-PEG and alcohol peptides (peptidols) molecules. Stability of the Mix-capped nanoparticles prepared in various conditions was assessed using tests of increasing stringency. The results highlight the importance of identifying a concentration of ligands sufficiently high to obtain a compact matrix when preparing nanoparticles and that the stability of capped nanoparticles in biological environments cannot be predicted solely on their resistance to electrolyte-induced aggregation. The Mix-capped nanoparticles are resistant to aggregation induced by electrolytes and to aspecific interactions with proteins and ligand exchange. In addition, Mix-matrices allow the easy introduction of a single recognition function per nanoparticle, allowing the specific and stoichiometric labeling of proteins with gold nanoparticles. Therefore, the Mix-matrices provide a useful tool for the development of nanoparticle-based quantitative bioanalytical and imaging techniques, as well as for therapeutic purposes, such as the specific targeting of cancerous cells for photothermal destruction.