Role of solvent in direct growth of gold nanostructures at the interface between focused ion beam-amorphized silicon and Au-ion-containing solution

The direct growth of gold nanostructures at the interface between a focused ion beam (FIB)-amorphized silicon surface and a HAuCl4-dissolved solution is demonstrated, and its mechanism is discussed in terms of the solvent for HAuCl4. Gold nanostructures successfully grew when a protic solvent such as water or alcohol, which has a hydroxyl group, was used as the solvent for HAuCl4. In contrast, no gold nanostructures were observed in the case of dimethyl sulfoxide (DMSO), an aprotic solvent having no hydroxyl group. Each protic solvent deactivates the FIB-amorphized silicon surface in the absence of Au ions. The growth of gold was inhibited when the FIB-amorphized silicon was exposed to the solvent before exposure to HAuCl4(aq). Silicon dangling bonds, which commonly exist in amorphized silicon, seem to reduce Au ions with the participation of protic solvent molecules in the redox reaction, initiating the growth of gold. The results of time-of-flight secondary ion mass spectrometry (TOF-SIMS) also support our suggestion.