Kazuyuki Hirao

Nanowire formation under femtosecond laser radiation in liquid

The size and shape of nanoscale materials provide excellent control over many of the physical and chemical properties, including electrical and thermal conductivity, magnetic properties, luminescence, and catalytic activity (Lieber, 1998). In particular, the synthesis and morphological control of nanosized particles, which exhibit surprising and novel phenomena based on the unique property called the quantum size effect, are attractive to chemists and physicists (Alivisatos, 1996). In recent years, nanoparticles are widely used in many applications ranging from biosensing (Anker et al., 2008; Elghanian et al., 1997; Lin et al., 2006), plasmonic devices (Ferry et al., 2008; Maier et al., 2003), and multifunctional catalysts (Hu et al., 1999; Lu et al., 2004). There are a wide variety of techniques that are capable of creating nanoparticles with various morphology and production yield. These nanoparticle formation approaches are typically grouped into two categories: ‘top-down’ and ‘bottom-up’. The first involves the breaking down of large pieces of bulk material into the required nanostructures. The second involves the building of nanostructures, atom-byatom or molecule-by-molecule in a gas phase or solution. These two approaches have evolved separately and reached the limits in terms of feature size and quality, in recent years, leading to exploring novel hybrid approaches in combining the top-down and bottom-up methods. Colloidal chemists have gained excellent controlled nanosized particles for several spherical metal and semiconductor compositions, which has led to the discovery of quantum size effect in colloidal nanocrystals (Alivisatos, 1996). However, various bottomup approaches for making morphologically controlled nanoparticles have been found; most of these solution methods are based on thermal process. On the other hand, top-down approaches have been developed for producing metal and semiconductor nanowires, nanobelts, and nanoprisms (Hu et al., 1999; Pan et al., 2001; Jin et al., 2001). In particular, the laser-induced ablation method has become an increasingly popular approach for making nanoparticles due to the applicability to various target materials in an ambient atmosphere (Jia et al., 2006a; Kawasaki & Masuda, 2005; Link et al., 1999; Sylvestre et al., 2004; Tamaki et al., 2002; Tull et al., 2006). Recently, various shape-controlled nanoparticles, such as nanowires (Morales & Lieber, 2008), nanotubes (Rao et al., 1997), and composite nanostructures (Zhang et al., 1998), have been fabricated by this technique. More recently, pulsed laser ablation in liquid has become profoundly intrigued for preparing nanoparticles from the viewpoint of the concise procedure and the ease of handling (Kawasaki & Masuda, 2005; Tamaki et al., 2002; Shimotsuma et al., 2007).

Morphology change from nanocrack into periodic pore array formed by femtosecond laser pulses

Defects inside single crystals are an important concern because they directly affect the physical or chemical properties of the material, especially in sapphire used as substrates for semiconductors. We have investigated the thermally activated transformations of nanometer-scale cracks and phase transitions inside sapphire by femtosecond laser irradiation and successive heat treatments. The nanocracks transformed into periodic arrays of pores and dislocations that aligned along the {11¯02} planes after heat treatments above 1300 °C. The amorphous phase at the focal point recovered into the initial single crystalline phase after the heat treatments. Our study provides useful information on the recovery behavior of nanometer-scale defects in a single crystal.