Synthesis Of Novel Phases Research Articles

Synthesis of Novel Phases in Si Nanowires Using Diamond Anvil Cells at High Pressures and Temperatures

Silicon has several technologically promising allotropes that are formed via high-pressure synthesis. One of these phases (hd) has been predicted to have a direct band gap under tensile strain, whereas other (r8 and bc8) phases are predicted to have narrow band gaps and good absorption across the solar spectrum. Pure volumes of these phases cannot be made using conventional nanowire growth techniques. In this work, Si nanowires were compressed up to ∼20 GPa and then decompressed using a diamond anvil cell in the temperature range of 25–165 °C. It was found that at intermediate temperatures, near-phase-pure bc8-Si nanowires were produced, whereas amorphous Si (a-Si) dominated at lower temperatures, and a direct transformation to the diamond cubic phase (dc-Si) occurred at higher temperatures under compression. Thus this study has opened up a new pressure–temperature pathway for the synthesis of novel Si nanowires consisting of designed phase components with transformative properties.

Novel binary and ternary phases in the Si-C-N system

The present article reviews recent advances in synthesis of novel phases in the ternary Si-C-N system. A dense carbon nitride phase, C2N2(NH), was synthesized for the first time at high pressures and high temperatures in a laser heated diamond anvil cell (LH-DAC). Based on results of electron diffraction, EELS- and SIMS-measurements combined with theoretical calculations the structure of this new C-N-H compound was analysed to be a defect wurtzite structure of the sinoite (Si2N2O)-type. Farther, a variety of amorphous SiCN phases and the first ternary crystalline phases, namely Si(NCN)2 and Si2N2(NCN), were synthesized at ambient pressure. In general, the high-pressure polymorphs of Si-C-N materials are predicted to exhibit a unique combination of high hardness, thermal stability and oxidation resistance with interesting optoelectronic properties.

Sputtering on He cooled substrate: a new method for producing nanometre-size materials

A multi-opposed targets sputtering system with a substrate cooled by liquid helium is proposed as a novel method for preparing small particles and nanocrystalline composites. Three pairs of targets are supplied with both dc and rf power supplies. All of the materials including ceramics and polymers, can be sputtered simultaneously. Sputtering of the various target materials is accompanied by a properly controlled process in which the media gas (Ar, for example) is solidified. The sputtered particles are embedded into the solid inert gas matrix on the substrate. After the sputtering is terminated, the substrate temperature and the pressure are increased; the solid argon is liquified and, together with the particles, collected in the container. However, such particles are not stable. They aggregate rapidly in liquid argon and coalesce spontaneously after the argon is evaporated. The application of this inert-gas matrix isolation technique allows us to produce free-standing particles of low contamination metal-metal, metal-ceramic, and metal-polymer composites at low temperatures (below 84 K). Addition of other gases makes it possible to react or stabilize the particles. In this way a new route for the synthesis of novel phases of nano-sized materials is achieved.