Synthesis of Cu3N and Cu3N-Cu2O multicomponent mesocrystals: non-classical crystallization and nanoscale Kirkendall effect.

Mesocrystals are superstructures of crystallographically aligned nanoparticles and are a rapidly emerging class of crystalline materials displaying sophisticated morphologies and properties, beyond those originating from size and shape of nanoparticles alone. This study reports the first synthesis of Cu3N mesocrystals employing structure-directing agents with a subtle tuning of the reaction parameters. Detailed structural characterizations carried out with a combination of transmission electron microscopy techniques (HRTEM, HAADF-STEM-EXDS) reveal that Cu3N mesocrystals form by non-classical crystallization, and variations in their sizes and morphologies are traced back to distinct attachment scenarios of corresponding mesocrystal subunits. In the presence of oleylamine, the mesocrystal subunits in the early reaction stages prealign in a crystallographic fashion and afterwards grow into the final mesocrystals, while in the presence of hexadecylamine the subunits come into contact through misaligned attachment, and subsequently, to some degree, realign in crystallographic register. Upon prolonged heating both types of mesocrystals undergo chemical conversion processes resulting in structural and morphological changes. A two-step mechanism of chemical conversion is proposed, involving Cu3N decomposition and anion exchange driven by the nanoscale Kirkendall effect, resulting first in multicomponent/heterostructured Cu3N-Cu2O mesocrystals, which subsequently convert into Cu2O nanocages. It is anticipated that combining nanostructured Cu3N and Cu2O in a mesocrystalline and hollow morphology will provide a platform to expand their application potential.

Single Nanoparticle Magnetic Spin Memristor.

There is an increasing demand for the development of a simple Si-based universal memory device at the nanoscale that operates at high frequencies. Spin-electronics (spintronics) can, in principle, increase the efficiency of devices and allow them to operate at high frequencies. A primary challenge for reducing the dimensions of spintronic devices is the requirement for high spin currents. To overcome this problem, a new approach is presented that uses helical chiral molecules exhibiting spin-selective electron transport, which is called the chiral-induced spin selectivity (CISS) effect. Using the CISS effect, the active memory device is miniaturized for the first time from the micrometer scale to 30 nm in size, and this device presents memristor-like nonlinear logic operation at low voltages under ambient conditions and room temperature. A single nanoparticle, along with Au contacts and chiral molecules, is sufficient to function as a memory device. A single ferromagnetic nanoplatelet is used as a fixed hard magnet combined with Au contacts in which the gold contacts act as soft magnets due to the adsorbed chiral molecules.

Magnetic Nanoplatelet-Based Spin Memory Device Operating at Ambient Temperatures.

There is an increasing demand for realizing a simple Si based universal memory device working at ambient temperatures. In principle, nonvolatile magnetic memory can operate at low power consumption and high frequencies. However, in order to compete with existing memory technology, size reduction and simplification of the used material systems are essential. In this work, the chiral-induced spin selectivity effect is used along with 30-50 nm ferromagnetic nanoplatelets in order to realize a simple magnetic memory device. The vertical memory is Si compatible, easy to fabricate, and in principle can be scaled down to a single nanoparticle size. Results show clear dual magnetization behavior with threefold enhancement between the one and zero states. The magnetization of the device is accompanied with large avalanche like noise that is ascribed to the redistribution of current densities due to spin accumulation inducing coupling effects between the different nanoplatelets.

Direct solvothermal preparation of nanostructured fluoride aerogels based on AlF3.

AlF3-based aerogels, a new class of inorganic aerogels, are prepared in a novel direct process that combines fluoride sol-gel synthesis with high temperature supercritical drying. The bulk structure of the solid products depends decisively on the applied solvent(s); very voluminous bulk aerogels are obtained only with MeOH that is used either alone or in combination with some other polar solvents. MeOH acts as a methoxylation agent; and formed methoxy (MeO) species are remarkably stable and deactivate the surface acidic sites. Removal of MeO species under moderate conditions results in catalytically active fluorides with a preserved nanostructure. In preparations with MeOH, preferential growth of anisotropic nanoparticles (nanorods) is the key step that leads to the formation of very open aerogel structures. Another process, dehydration of alcohols, results in some hydroxylation and hydration that lead to the formation of distinctive surface and bulk OH/H2O species. The structure of AlF3-based aerogels is consistent with the hexagonal tungsten bronze (HTB) ß-AlF3 although their composition corresponds to a formula AlF3-x(OH, OMe)x·yH2O (x = 0.1 ± 0.05). Some other characteristics of the fluoride nanoparticles, like crystallinity, particle size, and uniformity, can be effectively controlled by the temperature of the solvothermal process. The described methodology allows a controllable preparation of catalytically active fluorides in the form of regularly shaped and uniformly sized nanoparticles.

Multiscale anode materials in lithium ion batteries by combining micro- with nanoparticles: design of mesoporous TiO2 microfibers@nitrogen doped carbon composites.

TiO2 has been considered as a promising anode material for lithium ion batteries. However, its poor rate capability originating from the intrinsically low lithium ion diffusivity and its poor electronic conductivity hampers putting such an application into practice. Both issues can be addressed by nanostructure engineering and conductive surface coating. Herein, we report a template-assisted synthesis of micron sized TiO2 fibers consisting of a mesoporous network of anatase nanoparticles of about 7.5 nm and coated by N doped carbon. In a first step, an amorphous layer of TiO2 was deposited on cobalt silicate nanobelts and subsequently transformed into crystalline anatase nanoparticles by hydrothermal treatment. The N doped carbon coating was realized by in situ polymerization of dopamine on the crystalline TiO2 followed by annealing under N2. After removal of the template, we obtained the final mesoporous TiO2 fibers@N doped carbon composite. Electrochemical tests revealed that the composite electrode exhibited excellent electrochemical properties in terms of specific capacity, rate performance and long term stability.

Large-area alignment of tungsten oxide nanowires over flat and patterned substrates for room-temperature gas sensing.

Alignment of nanowires over a large area of flat and patterned substrates is a prerequisite to use their collective properties in devices such as gas sensors. In this work, uniform single-crystalline ultrathin W18 O49 nanowires with diameters less than 2 nm and aspect ratios larger than 100 have been synthesized, and, despite their flexibility, assembled into thin films with high orientational order over a macroscopic area by the Langmuir-Blodgett technique. Alignment of the tungsten oxide nanowires was also possible on top of sensor substrates equipped with electrodes. Such sensor devices were found to exhibit outstanding sensitivity to H2 at room temperature.