Methods for VLS Synthesis of Nanowires

The VLS synthesis requires only the presence of a metaUic catalyst and nanowire material in the vapor. Consequently, a broad spectrum of VLS-based techniques has been developed for synthesizing nanowires. Here, only the three primary techniques will be described, namely, flow reaction, laser ablation and CVD, which indudes plasma-enhanced chemical vapor deposition (PECVD). However, the primary focus will be on CVD-based techniques utUized in the authors laboratory. 

The catalyst can be introduced either by seeding the substrate prior to insertion into the reactor, or by simultaneously introducing it into the vapor with the source materials for the nanowires. In the latter case, the catalyst is either placed in zone A with the source material, mixed in with the source material, or placed in an intermediate zone. The operating temperatures and gas flow rates are dependent on the geometry of the reactor, the type of nanowires to be synthesized, and the types of source material and catalyst used in the process. 

The advantages of these reactors are that they are relatively inexpensive to build, and they are versatile, simple to operate and easy to maintain. In addition, the use of solid, sublimeable precursors in the form of powders simplifles the identification of optimal operating conditions for the synthesis of nanowires. One shortcoming of these systems is that oxygen contamination can be problematic, although if judicious efforts are made to eliminate any oxygen then extremely pure nanowires can be realized. 

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