STM and STS investigations of transition metals’ clusters (Cr, CoCr, Ni) produced by the plasma gas condensation source

Transition metals’ clusters were created in plasma gas condensation (PGC) source and deposited on solid state surfaces as Si(1 1 1), Au on Si(1 1 1), and graphite (highly oriented pyrolytic graphite, HOPG). The clusters’ size distribution was investigated by means of the transmission electron microscope and time of flight mass spectrometer. The diameters of these clusters, depending on the PGC source parameters, were between 2.5 and 13 nm. However, the clusters’ diameters, as seen by the scanning tunneling microscopy (STM), vary from few to several nanometers as a function of the tip radius. There is a much better correlation between the TOF results and the apparent height of clusters measured by the STM. The scanning tunneling spectroscopy measurements (I–V characteristics and dI/dV vs. the bias voltage) were performed down to 25 K and at the room temperature. The local electron density curves were calculated by numerical calculation of d(lnI)/d(lnV) or other (Bando) methods. The spectra showed a difference between the curves measured above the substrate and clusters, and also between spectra measured by different e.g. tungsten or iron tips. Images taken at higher temperatures have not revealed enhanced surface diffusion, surprisingly. The creation of fractal type structures as a result of an annealing process was observed only in the case of highly oriented pyrolytic graphite (HOPG) substrate. We have also observed the Si atoms deficiency in the close vicinity of clusters deposited on Si substrate during an annealing process starting from 500 K. This can be interpreted as a result of cluster decoration by the Si atoms. Finally, we showed some possibility of clusters’ removal from the surface by applying an appropriate voltage pulse between the tip and the sample.