Over the past two decades there have been significant advances in the development of one-dimensional (1D) nanostructures.  The wire-like geometry of these structures not only introduces remarkably enlarged and well-defined crystal surfaces relative to planar structures, but also provides 1D confined channels which have the potential to fundamentally tailor the transportation of electrons, phonons and photons.  Fully capturing the promising surface and transport properties of these structures in practical devices or systems, however, relies on the capability of effectively translating their extraordinary 1D characteristics into the three-dimensional (3D) space. 
Current techniques for growing single-crystalline nanowires (NWs) from bottom up rely on precipitation of precursors.  Next generation devices will likely require precise material chemistry in engineered three dimensional (3D) architectures.  To date, however, to synthesize such a 3D nanowire array inside a highly confined space is very challenging, particularly when this space is comparable to the size of the NW itself.  The challenge faced by current techniques arises from the coupling between the crystal growth rate and the precursor concentration.  Due to the non-uniform distribution of precursors within a confined diffusion channels, uniform growth of NWs inside such channels is difficult. 
Among all bottom-up nanostructure synthesis techniques, atomic layer deposition (ALD) is a state-of-the art approach that has a growth rate independent of the precursor concentration owing to its self-limiting surface reaction.  ALD has been widely applied to grow conformal thin film coatings with precisely-controlled thicknesses down to the subnanometer level.  A recent discovery has also showed that introducing metal catalysts into the ALD process can lead to the transition from conformal coatings to the vapor-liquid-solid growth of NW morphologies. ( See Yang, R. B.; Zakharov, N.; Moutanabbir, O.; Scheerschmidt, K.; Wu, L. M.; Gosele, U.; Bachmann, J.; Nielsch, K. Journal of the American Chemical Society 2010, 132, (22), 7592-7594.)