Effect of Additive on the Formation of Cusn Alloy Nano-Trees Formed with DC Electroplating

We have found nano-tree of CuSn alloy by DC electroplating with a constant potential mode. The plating bath contained copper-sulphate, tin-nitrate, citric acid, and additive (PEG). Substrate was Pt thin film sputtered on a Si substrate.  Nano-trees have many branches which separate from trunk perpendicularly, and sub-branches grow perpendicular from them also.   The electroplating condition to grow nano-tree structure is limited in the very narrow window of cathode potential. From TEM-EDS analysis, it was clarified that each branch of nano-tree is single crystal having orthorhombic structure.  Diameter of branch distributes between a few tens of nm to a few hundreds of nm. 

   There is a significant effect of additive (PEG) for the formation of nano tree structure as shown in Fig.1. If there is no addition of PEG,  deposited Cu-Sn alloy has a round rock shape only (Fig.1-a). However, when adequate amount of PEG is added to the plating bath, a very interesting nano-tree structure grows almost perpendicularly to the substrate (Fig.1-b). The top view of the nano-tree clearly shows branches are formed with 4-fold symmetry.   

   Nano-tree has a huge ratio of surface area to its volume, and is regarded as a typical three-dimensional nano-porous structure.  The nano-tree may have a high potential to be used for electrode of Li-ion batteries, as well as chemical sensors when it is oxidized.

Further studies are in progress to clarify the effect of the additive for the growth of nano-tree structure.