Photoconversion of copper flakes to nanowires with ultrashort pulse laser irradiation

The size and shape of nanoscale materials provide important control over many of the physics and chemical properties, including electric and thermal conductivity, luminescence, and catalytic activity.1 Colloidal chemists have obtained excellent controlled nanosized particles for several spherical metal and semiconductor compositions, which has led to the discovery of the quantum size effect in colloidal nanocrystals.2 However, various bottom-up approaches for making anisotropic-shaped colloidal nanoparticles have been found, with most of these solution methods being based on a thermal process. On the other hand, top-down approaches have been developed for producing metal and semiconductor nanowires, nanobelts, and nanoprisms.3-5 In particular, the laser-induced ablation method has become an increasingly popular approach for making nanoparticles from the viewpoint of the concise procedure and application of a variety of materials.6-11 The ripple structures have been observed on the surface of the metal and semiconductor caused by the interferences between the scattering incident laser field and the surface plasmon-polariton waves (SPWs).11,12 However, until now, there has been no observation of nanowire formation in a colloidal suspension by pulsed laser irradiation and the mechanism of its appearance has not been fully understood. We recently reported the first experimental evidence of the periodic nanostructures embedded in silica glass after irradiation by a single focused beam of a femtosecond Ti:sapphire laser.13 The phenomenon is interpreted in terms of interference between the incident laser light field and the generated bulk electron plasma waves, resulting in the periodic modulation of electron plasma concentration and the structural changes in transparent material. Here, we report the first observation of the photoconversion from copper flakes to nanowires and nanospheres formation via ultrafast pulse laser irradiation. This phenomenon has provided two distinct surface plasmon resonances based on the characteristic shape. The observed copper nanowires of 50 nm diameter are fragmented from the initial flakes as a result of the interference between the light field and the SPWs. Beyond the fundamental issues of basic science, applications are possible in the areas of an optical polarization control, an electroconductive nanomaterial, and a catalytic metal