Nanocrystal-mediated crystallization of silicon and germanium nanowires in organic solvents: the role of catalysis and solid-phase seeding.

The size-dependent properties, large surface-area-to-volume ratios, dispersibility in solvents, and mechanical flexibility of semiconductor nanowires make them an exciting class of materials. Silicon nanowires in particular can be both n and p-doped and interface well with an insulating oxide and conducting metal silicide and these nanowires might be applied in silicon CMOS (complementary metal oxide semiconductor) circuits or with organic compounds on flexible plastic substrates. 5] Germanium is similar to silicon in many respects, but with a higher carrier mobility. Gold-seeded vapor–liquid–solid (VLS) growth is a common route to silicon and germanium nanowire synthesis at low growth temperatures ( 360 8C). 6–8] Unfortunately, gold traps electrons and holes in both silicon and germanium and poses a serious contamination problem for nanowire integration with silicon CMOSs. Surprisingly few gold alternatives have been investigated for nanowire seeding. Si nanowires have been seeded by chemical vapor deposition (CVD) with Ti particles and Ga droplets, and in organic solvents, Si and Ge nanowires were synthesized using Ni nanocrystals. 12] Gold alternatives have been explored more extensively for other semiconductors: Sn for ZnO wires, various metal films for vapor-grown tin oxide nanowires, and low-melting metals such as In and Bi for solution synthesis of Group II–VI, III-V and Ge nanowires. Herein, many different nanocrystals—Co, Ni, CuS, Mn, Ir, MnPt3, Fe2O3, and FePt—are explored as seeds for Si and Ge nanowire synthesis to develop a more general understanding of the role of the seed particles in the nanowire growth process. Si and Ge nanowires were grown by decomposing silanes or germanes in high temperature (450–500 8C), high pressure (10.3 MPa) supercritical toluene. Under these reaction conditions, Au nanocrystals seed nanowires by the “supercritical fluid–liquid–solid” (SFLS) mechanism in which nanowires evolve from a liquid Au:Si (or Au:Ge) eutectic. In combination with Si and Ge, many of the seed materials studied herein do not form liquid eutectics until reaching temperatures well above 500 8C, and are not expected to work for “VLS”-like growth. However, they form solid alloys below 500 8C, perhaps making solid-phase nanowire seeding possible. Figure 1 shows TEM images of the various colloidal nanocrystals used to seed Si and Ge nanowires; their size distributions had standard deviations less than 20% about mean diameters ranging between 4.2 and 10.2 nm. Figure 2 shows the reaction products. All of the nanocrystals seeded Si and Ge nanowires from monophenylsilane (MPS) and diphenylgermane (DPG), but with varying success (summar-