We describe scanning tunneling microscopy and spectroscopy results on the ligand stabilized metal cluster Pt30~Phen~(,O30 at 4.2 K. In these measurements we observed very clear charging effects. In addition to the regular Coulomb staircase, sometimes an irregular staircase pattern has been observed. This irregular staircase pattern is consistent with a discrete density of states of the cluster. However, also the unknown influence of the ligands has to be considered.
The interest in small metallic particles is mainly due to the fact that these particles allow the study of the transition from atomic to solid state properties. Especially the discreteness of the electronic states (the quantum size effect (QSE)) in these particles is of interest. This QSE will becoine important for temperatures that are small compared to the average level splitting between the individual discrete electronic states (AE). The QSE has been addressed theoretically in several papers [1][2][3]. The result of these papers is that the level splitting has a statistical distribution, which depends on the symmetry of the Hamiltonian. In general the distance between two levels is roughly equal to the average level splitting. For extensive overviews of the electronic properties of small metallic particles we refer to papers by Halperin [4] and Perenboom et al. [5].
The main effect that is observed in tunneling spectroscopy on small metallic particles is a stepwise increase of the current at equal voltage intervals A V. This is called the Coulomb staircase (CSC) and the origin of it is the charging energy, (e2/2C) that is required to charge the metallic particle with an electron. At low temperature (kBY<<e2,/2C) this energy is not available from thermal fluctuations and therefore it has to be supplied by the external voltage source. At low bias voltages (eV<<e2/2C) the energy of the electrons is too small to overcome this Coulomb blockade and therefore no current will flow.