The role of big metal clusters in nanoscience

The miniaturisation of a metal particle to the nanosize regime produces quantum size behavior, i.e. the quasi-delocalized metallic electrons begin to form discrete energy levels, This transition from bulk to molecule is frequently observed for ligand-protected clusters in the size range 1-4 nm, Tunneling spectroscopic (STS) experiments on single clusters not only prove the size dependence of the Coulomb gap, but also the temperature dependence. As the electrostatic energy e(2)/2C must be large compared with the thermal energy k(g)T of the electron to induce Coulomb steps, very low temperatures are necessary to observe this event on larger particles. A 17 nm Pd cluster behaves like a piece of metal at room temperature, but shows a Coulomb barrier at 4.2 K. On the contrary, a ligand-stabilized 1.4 nm Au cluster shows a Coulomb step at room temperature (small C), but a series of Coulomb steps (SET processes) at 90 K. The practical use of clusters in nanoelectronics is coupled to the ability to organize them three-, two- or one-dimensionally. A quasi 3-D organization of clusters can be reached by linking them with spacer molecules, chemically fixed to the cluster surfaces. A direct relation between spacer length and activation energy for electronic inter-cluster tunneling processes is observed. Two-dimensional arrangements of different kinds of clusters have been reached by self-assembly processes on chemically modified surfaces or by using Langmuir-Blodgett (LB) films transferred onto a substrate, A deficiency of both methods is the lack of long-range order. First attempts to generate 1-D chains show good promise if nanopores in alumina membranes are used as tubes to be filled by the clusters. Electrophoretic filling has turned out to be the better method to introduce clusters compared with vacuum filling.