CAREER: Linking the Forward and Reverse Vapor-Liquid-Solid Mechanisms to Synthesize Ordered Integrated Metal-Oxide Nanostructures

Though the vapor-liquid-solid (VLS) mechanism has been extensively studied in its role in metal-catalyzed nanowire (NW) growth, the reverse of this process – the solid-liquid-vapor (SLV) mechanism – is not well understood, yet it could well provide the key to understanding and utilizing VLS-grown wires. VLS-grown wires are highly attractive components for functional nanomaterials since they grow along unique crystallographic axes to form defect-free single crystals with well-controlled dimensions. To date, however, these free-standing wires have been put to no technological use, since their ordered arrangement or placement is highly challenging. Furthermore, several aspects of the VLS mechanism critical to controlling metal-catalyzed growth of NWs are not well understood, such as the role of metal droplet stability, or the conditions required to achieve a steady-state balancing of the forward and reverse mechanisms – VLS with SLV – to produce NW growth. SLV has been observed both in free-standing wires, resulting in complete dissolution of the wire, and in large single crystals, resulting in welloriented etched pores; the first observation of this phenomenon was reported in the 1970’s, shortly after the discussion of VLS was initiated. Yet, the parameters required to induce or control the SLV mechanism, and the interplay between VLS and SLV, have gone virtually uninvestigated. The SLV mechanism conveys the exact same advantages to nanostructure growth that enabled the VLS mechanism to revolutionize the field of NW synthesis, namely that defect-free structures (negative nanowires) may be produced with great control over growth direction and dimensions. Moreover, SLV could be used to synthesize highly-crystalline porous structures from materials as versatile as those used for VLS NW growth, direct the growth of VLS NWs along specific crystallographic directions within a porous material, and produce highly ordered integrated nanostructures containing interfaces with well-defined chemistry, crystallography, and spacing. Thus, there is a critical need to determine the key factors governing the SLV mechanism both in freestanding NWs and bulk crystals, and its fundamental relationship to VLS growth. In the absence of such knowledge, it will likely remain highly challenging to utilize VLS-grown NWs for any technological application, or to exploit the solid state chemistry of a templating material to direct the functionality of a nanocomposite.