2. General chemical synthesis of metal silicide nanowires.

Nanomaterials extensively studied so far are usually made of elements or prototypical compound semiconductors with simple stoichiometries. In contrast, intermetallic compounds such as metal silicides have multiple and unpredictable stoichiometries and complex phase behavior, making them challenging to synthesize and rarely explored so far. However, novel nanowire (NW) materials of silicides have many new physical properties and significant applications in nanoelectronics, nanophotonics, nanospintronics (see below), and thermoelectrics. We are developing general synthetic approaches to silicide NWs to overcome such previously unaddressed complexity. Our first approach utilizes chemical vapor deposition (CVD) of single source organometallic precursors (SSPs) to reproducibly deliver both silicon and metals with stoichiometric control. For example, using SSPs Fe(SiCl3)2(CO)4 and Co(SiCl3)(CO)4, FeSi (Fig. B) and CoSi nanowires are produced respectively on silicon substrates covered with a thin (1-2 nm) layer of silicon oxide without any catalyst seeds (see Figure 2).

Figure 2. A) Our unique synthetic route to silicide NWs using SSP; B) SEM of FeSi NWs we synthesized; C) HRTEM of a FeSi nanowire.

We have also developed a complementary chemical vapor transport (CVT) method to synthesize NWs of silicides for which suitable organometallic precursors are not readily accessible, such as CrSi2 and Ni2Si. We discovered a new and general nanowire growth mechanism that is different from the typical vapor-liquid-solid (VLS) NW growth and critically depends on the oxide thickness. Taking advantage of such unique oxide dependence, we have developed a simple method to pattern the location of NW growth by modulating the surface oxide without the use of metal catalysts. We are systematically investigating families of metal-silicon organometallic complexes as SSPs and working on elucidating the detailed NW growth mechanism and the chemical rules governing the formation of the nanoscale intermetallic phases towards the goal of rational synthesis of nanomaterials of any pure or alloyed metal silicides.

Figure 3. CVT synthesis of silicide nanowires using Ni2Si nanowires as an example.