Microscopic structure of metal whiskers

We present TEM images of the interior of metal whiskers (MW) grown on electroplated Sn films. Along with earlier published information, our observations focus on a number of questions, such as why MWs' diameters are in the micron range (significantly exceeding the typical nano-sizes of nuclei in solids), why the diameters remain practically unchanged in the course of MW growth, what is the nature of MW diameter stochasticity, and what is the origin of the well-known striation structure of MW side surfaces. In an attempt to address such questions we performed an in-depth study of MW structure at the nanoscale by detaching a MW from its original film, reducing its size to a thin slice by cutting its sides by a focused ion beam, and performing TEM on that structure. Our observations revealed a rich nontrivial morphology suggesting that MW may consist of many side by side grown filaments. This structure appears to extend to the outside whisker surface and be the reason for the striation. In addition, we put forward a theory where nucleation of multiple thin metal needles results into micron-scale and larger MW diameters. This theory is developed in the average field approximation similar to the roughening transitions of metal surfaces. The theory also predicts MW nucleation barriers and other observed features.