Morphology and growth of metal clusters in the gas phase:: A transition from spherical to ramified structures

Metal cluster cations are produced in a laser vaporization source, size selected in the gas phase by an electrostatic quadrupole deflector, and deposited without coalescence on carbon coated electron microscopy grids for further observation. For platinum clusters, a transition from spherical to strongly ramified shapes is observed when the cluster size increases beyond a critical diameter d(c) of about 2.5 nm. In ramified structures, the branch width is almost independent of the size and of the order of d(c). This study shows evidence of the role of coagulation in the kinetics of growth and of the quenching of particle coalescence above the critical size. The results are discussed in the framework of previous models developed in aerosol and thin film growth experiments. A comparison with indium and gold clusters confirms that the border size at the transition from coalescence to ramification depends on the thermodynamic properties of the element.