Mechanical properties of nanoporous GaN and its application for separation and transfer of GaN thin films.

Nanoporous (NP) gallium nitride (GaN) as a new class of GaN material has many interesting properties that the conventional GaN material does not have. In this paper, we focus on the mechanical properties of NP GaN, and the detailed physical mechanism of porous GaN in the application of liftoff. A decrease in elastic modulus and hardness was identified in NP GaN compared to the conventional GaN film. The promising application of NP GaN as release layers in the mechanical liftoff of GaN thin films and devices was systematically studied. A phase diagram was generated to correlate the initial NP GaN profiles with the as-overgrown morphologies of the NP structures. The fracture toughness of the NP GaN release layer was studied in terms of the voided-space-ratio. It is shown that the transformed morphologies and fracture toughness of the NP GaN layer after overgrowth strongly depends on the initial porosity of NP GaN templates. The mechanical separation and transfer of a GaN film over a 2 in. wafer was demonstrated, which proves that this technique is useful in practical applications.

Constant current etching of gold tips suitable for tip-enhanced Raman spectroscopy.

We introduce a setup and method to produce gold tips that are suitable for tip-enhanced Raman spectroscopy by using a single step constant current electrochemical etch. The etching process is fully automated with only three preset parameters: the etching current, the reference voltage and the immersed length of gold wires. By optimizing these parameters, reproducible high quality tips with smooth surface and a radius curvature of about 20 nm can be formed. Tips prepared with this method were examined by tip-enhanced Raman spectroscopy experiments on the samples of single-wall carbon nanotube, p-aminothiophenol, and graphene. In the Raman mapping of single-wall carbon nanotubes, the spatial resolution is about 15 nm.

Hierarchical CdSe-gold hybrid nanocrystals: synthesis and optical properties.

The synthesis of hybrid nanostructures with controlled size, shape, composition and morphology has attracted increasing attention due to the fundamental and applicable interest. Here, we demonstrate the synthesis and optical properties of hierarchical CdSe-Au hybrid nanostructures with zinc blende (ZB) CdSe nanocrystals (NCs). For 3.5 nm ZB CdSe NCs, one Au cluster was deposited on each CdSe NC. Nevertheless, several Au clusters were selectively deposited on the apexes of 5 nm and 8 nm ZB CdSe NCs, resulting from the different reactivity of crystal facets. Furthermore, hierarchical CdSe-Au nanostructures with complex morphology were organized with the isolated CdSe-Au hybrid NCs by the coalescence of Au domains on the CdSe-Au hybrid NCs. UV-Vis spectra revealed a red tail upon the deposition of Au clusters. The chemical joint of Au on CdSe NCs was further confirmed by fluorescence quenching. The optical limiting performance of CdSe-Au hybrid NCs dispersed in toluene was investigated at 532 nm using a Nd:YAG laser with the pulse width of 8 ns.