We have determined the dependence of the transition voltage (minimum in a ln(I/V-2) vs. I/V plot) on the vacuum gap width in ultra-high vacuum scanning tunneling microscopy junctions. We have performed dual bias room temperature experiments with a W tip and Au(111) as well as polycrystalline Pt surfaces. For both type of surfaces the transition voltage decreases linearly with increasing inverse gap width. This is in marked contrast with the standard models for quantum mechanical tunneling, which predict a linear increase of the transition voltage with increasing inverse gap width. This remarkable discrepancy can only be partly explained by the incorporation of an image charge effect and therefore there is a clear need for a revision of the standard models for quantum mechanical tunneling in vacuum scanning tunneling microscopy junctions.

Seed mediated growth assisted by surfactant is used to grow nanoparticles. The choice of seed particle, i.e. single crystal or multi-twinned, enables growth of nanorods and multi-branched nanoparticles. The latter are generally referred to as nanostars. The shape and size of the nanoparticles can be tuned by varying the solution composition and relative reactant concentrations. The optical properties of nanorods and nanostars are described and discussed in relation to their shape. For nanorods, the aspect ratio is a key parameter determining the plasmon resonance, while the absolute size has a much less pronounced effect. With the nanostars, the length of the branches turns out to be relevant. The optical properties of the multi-branched nanoparticles are discussed in terms of plasmon hybridization.