A facile low-temperature growth of large-scale uniform two-end-open Ge nanotubes with hierarchical branches

We present a facile approach for the controlled fabrication of well-aligned arrays of Ge nanotubes (GeNTs) with tunable sizes and hierarchical branches inside the pre-designed nanochannels of porous anodic aluminum oxide (AAO) templates. Metal salts, such as nickel nitrate, silver nitrate, cobalt nitrate and copper sulphate are pre-decorated on the inner wall of the AAO nanochannels as catalyst precursors, where they are reduced into nickel, silver, cobalt, and copper clusters, and provide nucleation sites for subsequent Ge growth. GeNTs are formed by confining the Ge growth on the inner walls of the porous AAO template in a low temperature (300-380 degrees C) chemical vapor deposition process. The as-grown GeNTs have open ends with tailored wall thickness (between 10 and 26 nm), diameter (between 80 and 248 nm), and geometrical configuration (e. g., linear, Y-branching, multi-branching, and various multiple-generation branching). The GeNT formation process is sensitive to the choice of the catalyst precursor. Nickel salts lead to a uniform wall thickness of GeNTs compared with copper, silver, and cobalt salts. And GeNTs grown with copper salts as catalysts are polycrystalline, while nickel, silver and cobalt salts assisted GeNTs are amorphous though they can crystallize via post-annealing at 400 degrees C in Ar/H-2 atmosphere. These open-end hollow nanotubes with tunable sizes and hierarchical branches can serve as nanoscale containers or pipes to deliver fluids and molecular species, and are excellent building blocks for the construction of large-scale nanofluidic systems.