Abstract
Low-temperature scanning tunneling spectroscopy (STS) allows us to probe electronic properties of clusters at surfaces with unprecedented accuracy. Recent experimental determination of the differential conductance of supported clusters yield considerable deviations with respect to the expected density of states and suggest that many cluster states are invisible to STS measurements. By means of fully self-consistent quantum transport calculations, using realistic tunneling tips, we show that, depending on the tip shape, only a small fraction of the electronic states contribute to the STS spectra, thus explaining the experimental findings. We demonstrate that the unambiguous characterization of the states on the supported clusters can be achieved with energy-resolved images, obtained from a theoretical analysis which mimics the experimental imaging procedure.