Au/Ag bilayered metal mesh with arrays of nanoholes were devised as a catalyst for metal-assisted chemical etching of silicon. The present metal catalyst allows us not only to overcome drawbacks involved in conventional Ag-based etching processes, but also to fabricate extended arrays of silicon nanowires (SiNWs) with controlled dimension and density. We demonstrate that SiNWs with different morphologies and axial orientations can be prepared from silicon wafers of a given orientation by controlling the etching conditions. We explored a phenomenological model that explains the evolution of the morphology and axial crystal orientation of SiNWs within the framework of the reaction kinetics.

Intensive studies have recently been performed on graphene-based photodetectors, but most of them are based on field effect transistor structures containing mechanically exfoliated graphene, not suitable for practical large-scale device applications. Here we report highefficient photodetector behaviours of chemical vapor deposition grown all-graphene p-n vertical-type tunnelling diodes. The observed photodetector characteristics well follow what are expected from its band structure and the tunnelling of current through the interlayer between the metallic p-and n-graphene layers. High detectivity (B10 12 cm Hz 1/2 W À 1 ) and responsivity (0.4B1.0 A W À 1 ) are achieved in the broad spectral range from ultraviolet to near-infrared and the photoresponse is almost consistent under 6-month operations. The high photodetector performance of the graphene p-n vertical diodes can be understood by the high photocurrent gain and the carrier multiplication arising from impact ionization in graphene.

A generic process for the preparation of curved silicon nanowires (SiNWs) with ribbon-like cross sections was developed. The present synthetic approach is based on chemical etching of (100)-oriented silicon wafers in mixture solutions of HF and H(2)O(2) by using patterned thin gold films as catalyst and provides a unique opportunity for the fabrication of extended arrays of zigzag SiNWs, ultrathin straight [111] SiNWs, and curved SiNWs with controlled turning angles. On the basis of our experiments performed under various etching conditions, the factors governing the axial crystal orientation and morphology of SiNWs were systematically analyzed. We proposed a model that explains the formation of the present novel silicon nanostructures during chemical etching of silicon.

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