The cluster related research is one of the main research directions of the materials research in ELSP group. The research aims to characterize the electronic structure of  nanoparticles - information  which is vital for various applications of nanostructures in applied research and applications.  In an international collaboration project the group has developed a so called Exchange Metal Cluster Source (EXMEC). The first scientific results from this collaboration were published in years 2009-2010.

The EXMEC has so far been used to form nanoclusters from various metals, semimetals and ionic molecules. It has also been used as an atmospheric reaction chamber for airborne aerosols. With the aid of Oulu University strategic funding  an upgraded version of the EXMEC source is commissioned (2011-2012). The  new source provides a possibility for constant usage at the ELSP-lab in Oulu as well as in synchrotron radiation sources.  The EXMEC source is widely used in the interdisciplinary collaboration projects inside Oulu University as well as international projects.

EXMEC keynotes:

• Uncharged clusters
• Size range up to few nanometers max
• Conducted with raregas flow (see below for details)
• Layered clusters
• Metals, semimetals, salts
• Multipurpose

Working principle of the EXMEC

Metal clusters in EXMEC are produced in two steps which are depicted below:

Click the image to enlarge.

1. Rare gas atoms (blue) are guided through a shaped nozzle into a vacuum chamber. Pressure difference between the gas line feeding the nozzle and the chamber generates an adiabatic expansion of gas where the gas temperature decreases rapidly and the gas enters a supercooled state. This state makes it possible for the rare gas clusters to form. 

2. Rare gas cluster go through an small opening in a conical skimmer. On the other side is the experimental chamber where the rare gas clusters enter the oven. Inside the oven, metal atoms from the vapor hit the rare gas clusters and stick to them. If couple of metal atoms attach to same cluster, the metal atoms can form bonds with each other. Rare gas atoms start to evaporate from the cluster complex which transports the formation heat away from the newly formed metal clusters.

Size of the clusters can be changed by modifying the geometry or the temperature of the nozzle, and regulating the pressure of the rare gas stream. Some examples of our groups research, where EXMEC has been used:

• Size-varied photoelectron spectroscopy of metal cluster using the Exchange metal cluster source
• Photon Energy Dependent Valence Band Response of Metallic Nanoparticles