New Nitride Nanoceramics from Synthesis-Mixed Nanopowders in the Composite System Gallium Nitride GaN-Titanium Nitride TiN

Presented is a study on the preparation, via original precursor solution chemistry, of intimately mixed composite nanocrystalline powders in the system gallium nitride GaN–titanium nitride TiN, atomic ratio Ga/Ti = 1/1, which were subjected to high-pressure (7.7 GPa) and high-temperature (650, 1000, and 1200 °C) sintering with no additives. Potential equilibration toward bimetallic compounds upon mixing of the solutions of the metal dimethylamide precursors, dimeric {Ga[N(CH₃)₂]₃}₂ and monomeric Ti[N(CH₃)₂]₄, was studied with ¹H- and ¹³C{H}-NMR spectroscopy in C₆D₆ solution. The different nitridation temperatures of 800 and 950 °C afforded a pool of in situ synthesis-mixed composite nanopowders of hexagonal h-GaN and cubic c-TiN with varying average crystallite sizes. The applied sintering temperatures were either to prevent temperature-induced recrystallization (650 °C) or promote crystal growth (1000 and 1200 °C) of the initial powders with the high sintering pressure of 7.7 GPa having a detrimental effect on crystal growth. The powders and nanoceramics, both of the composites and of the individual nitrides, were characterized if applicable by powder XRD, SEM/EDX, Raman spectroscopy, Vicker’s hardness, and helium density. No evidence was found for metastable alloying of the two crystallographically different nitrides under the applied synthesis and sintering conditions, while the nitride domain segregation on the micrometer scale was observed on sintering. The Vicker’s hardness tests for many of the composite and individual nanoceramics provided values with high hardness comparable with those of the individual h-GaN and c-TiN ceramics.

Demonstration of Self-Starting Nonlinear Mode Locking in Random Lasers

In ultrafast multimode lasers, mode locking is implemented by means of saturable absorbers or modulators, allowing for very short pulses. This occurs because of nonlinear interactions of modes with well equispaced frequencies. Though theory predicts that, in the absence of any device, mode locking would occur in random lasers, this has never been demonstrated so far. Through the analysis of multimode correlations we provide clear evidence for nonlinear mode coupling in random lasers. The behavior of multiresonance intensity correlations is tested against the nonlinear frequency matching condition equivalent to the one underlying phase locking in ordered ultrafast lasers. Nontrivially large correlations are clearly observed for spatially overlapping resonances that sensitively depend on the frequency matching condition to be satisfied, eventually demonstrating the occurrence of nonlinear mode-locked mode coupling. This is the first example, to our knowledge, of an experimental realization of self-starting mode locking in random lasers, allowing for many new developments in the design and use of nanostructured devices.

Free-Standing Self-Assemblies of Gallium Nitride Nanoparticles: A Review

Gallium nitride (GaN) is an III-V semiconductor with a direct band-gap of 3 . 4 e V . GaN has important potentials in white light-emitting diodes, blue lasers, and field effect transistors because of its super thermal stability and excellent optical properties, playing main roles in future lighting to reduce energy cost and sensors to resist radiations. GaN nanomaterials inherit bulk properties of the compound while possess novel photoelectric properties of nanomaterials. The review focuses on self-assemblies of GaN nanoparticles without templates, growth mechanisms of self-assemblies, and potential applications of the assembled nanostructures on renewable energy.