Over last five years, we have developed a focused materials synthesis effort toward conceptualizing, rationalizing and developing a series of process concepts for bulk synthesis of inorganic nanowires, directed synthesis of nanostructures and nanostructure networks. These efforts have led to a series of discoveries both in terms of process concepts and in terms of new structural discoveries for several materials. Examples of new structures include carbon nanopipettes with varying external diameter and carbon microtubes with large ‘tunable’ internal diameters and ‘tunable’ morphologies.

The concepts we developed for controlled synthesis of nanowires can be broadly divided into two different techniques

a. Multiple nucleation and growth of nanowires from bulk pool of low melting metals

We discovered a new phenomenon in which multiple nanowires nucleate from low melting metal melt pools. This concept has been successfully demonstrated with silicon, silicon oxide, silicon nitride, gallium nitride, gallium oxide nanowires/nanotubes and nanowebs. The phenomena observed is quite different than the original VLS concept by Wagner and Ellis (1964) in which a catalyst cluster confines crystal growth in one-dimension.

In our approach, multiple nuclei grow vertically over low-melting metal melt surface directly with basal attachment. Our concept is clearly illustrated in the figure below, where we show the growth of gallium oxide nanowires/whiskers from bulk pool of gallium. The unique aspect of this technique is that one can control the diameter of nanowires and orientation (growth direction) by adjusting the supersaturation of solute in bulk pool of low melting metals by simple variations in the gas phase chemistries and substrate temperatures. This method can be used to synthesize nanowires from various low melting metals: Aluminum, gallium, indium etc.

We believe that our approach could be developed further for bulk manufacturing of nanowires and for direct synthesis of nanoelectronic device components such as nanowires with built-in atomically sharp hetero-interfaces.

 

 

 

 

 

 

 

 

b. Novel vapor phase method for the synthesis of metal and metal oxide nanowires

We have developed a novel vapor phase concept for the synthesis of metal and metal oxide nanowires of transition metals, without the use of either templates or catalysts. Chemical vapor transport of metal oxides onto substrates maintained above their decomposition temperature leads to the formation of the respective metal nanowires. Similarly, vapor phase transport of oxides onto substrates maintained below their respective decomposition temperatures leads to the formation of metal oxide nanowires. We specifically demonstrated this concept with the synthesis of tungsten and tungsten oxide nanowires. This concept could potentially be extended for synthesizing nanowires of other transition metals and metal oxides.