Nanowires laser ablation

Morales, A. M. Lieber, C. M. 1998. A laser ablation method for the synthesis of crystalline semiconductor nanowires. Science 279 208-211. 

Recently, a laser ablation-condensation technique was used to produce nanometer-sized catalyst clusters to grow nanowires by the VLS method. A schematic of the laser ablation apparatus used by Morales and Lieber (1998) to produce silicon nanowires is shown in Fig. 11. The target consists of silicon and the catalyst material (e.g., Sii AFeA), and a pulsed laser is used to produce nanometer-sized catalyst clusters within a reaction chamber at 1200°C. The ablated materials are carried by an argon gas flow, and the... 

Fig. 11. Schematicof the laser ablation apparatus for the production of nanowires (Morales and Lieber, 1998).

Although laser ablation methods have been used to fabricate large quantities of single-crystalline semiconducting nanowires with high aspect ratios, the wires obtained are randomly oriented with a variety of morphologies... 

Recently, the VLS growth method has been extended beyond the gas-phase reaction to synthesis of Si nanowires in Si-containing solvent (Holmes et al, 2000). In this case 2.5-nm Au nanocrystals were dispersed in supercritical hexane with a silicon precursor (e.g., diphenylsilane) under a pressure of 200-270 bar at 500°C, at which temperature the diphenylsilane decomposes to Si atoms. The Au nanocrystals serve as seeds for the Si nanowire growth, because they form an alloy with Si, which is in equilibrium with pure Si. It is suggested that the Si atoms would dissolve in the Au crystals until the saturation point is reached then they are expelled from the particle to form a nanowire with a diameter similar to the catalyst particle. This method has an advantage over the laser-ablated Si nanowire in that the nanowire diameter can be well controlled by the Au particle size, whereas liquid metal droplets produced by the laser ablation process tend to exhibit a much broader size distribution. With this approach, highly crystalline Si nanowires with diameters ranging from 4 nm to 5 nm have been produced by Holmes et al. (2000). The crystal orientation of these Si nanowires can be controlled by the reaction pressure. 

Si nanowires were first produced using the classical metal catalyst VLS approach [21, 22, 46]. Laser ablation of a metal-containing Si target produces metal/metal silicide nanoparticles that act as the critical catalyst needed for the nucleation of SiNWs. The wires grow further by dissolution of silicon in the metallic nano-cap and concurrent Si segregation from the cap. In a typical experiment, an excimer laser is used to ablate the target placed in an evacuated quartz tube filled with an inert gas, e.g. argon [22]. 

The OAG method has a general nature and can be applied to a variety of materials other than Si. Based on the OAG method, we have synthesized nanowires of a wide range of semiconducting materials including Ge [35], GaN [36, 37], GaAs [38, 39], GaP [41], SiG [40], and ZnO (whiskers) [42]. The actual OAG process was activated by laser ablation, hot-filament chemical vapor deposition (HFCVD) or thermal evaporation. 

Gallium arsenide nanowires with a zinc-blende structure were fabricated by laser ablation of GaAs powders mixed with GazOs (no metal catalyst used). SEM obser-... 

Dozens of methods to synthesize nanotubes, nanowires, and nanorods have been reported that can be found in the references included in Table 1. In addition to the most well known ones, such as hot plasmas, laser ablation, chemical vapor deposition, high temperature solid state and hydrothermal synthesis, fill-ing/coating of carbon nanotubes and similar types of materials, three methods have been developed that enable the synthesis of a wealth of new anisotropic nanoparticles. 

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