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 $(T<\mathrm{}20\mathrm{}\mathrm{K})$, inert substrates, near their insulator to metal transition. At the critical mass deposited thickness for conduction, $d_G\cong 5.2\mathrm{nm}$, the films consist of approximately two layers of nanoclusters with diameter, $2r\approx 20\mathrm{nm}$ and height, $3.5\le h\le 5.5\mathrm{nm}$. We discuss how the nanocluster size and formation mechanism dictate the need for two layers to form in order for conduction to commence.