Morphology of Quench Condensed Pb Films near the Insulator to Metal Transition

We present in situ scanning tunneling microscopy topographs of Pb films, formed by vapor deposition onto cold (T<20K), inert substrates, near their insulator to metal transition. At the critical mass deposited thickness for conduction, dG5.2nm, the films consist of approximately two layers of nanoclusters with diameter, 2r≈20nm and height, 3.5≤h≤5.5nm. We discuss how the nanocluster size and formation mechanism dictate the need for two layers to form in order for conduction to commence.