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Cluster Physics

Overview

Clusters are small aggregates of atoms with dimensions on the order of 1 - 100 nanometers. When bulk matter is scaled down to these dimensions its properties begin to change in a variety of interesting ways. Small particles of metals and semiconductors have electric, magnetic, and optical properties which are different from the bulk and depend on size. A familiar example is the coloration of stained glass which is due to small metal particles annealed into the glass from metal salts. In recent years, chemists have discovered reactions to grow metal and semiconductor nanoparticles in organic solutions which allow them to be segregated by size. Like the particles preciptated in stained glass, these nanoparticles have brilliant colors that differ from those of the bulk and depend on the particle size. There has also been much interest in the magnetic properties of small metal clusters - which have technological applications in magnetic data storage, and medical imaging.

The projects in our group study the properties of free clusters in a molecular beam. Because of their small size, a typical cluster will have a large fraction of its atoms on the surface. This makes it very difficult to distinguish between effects which are due to their small size and effects which are due to surface interactions with a solvent or matrix. Free clusters, which condense in the gas phase and can be collimated into a molecular beam, are an experimental platform where the properties of small clusters can be studied without the complication of surface interations.

Experiments

Our experiments focus on how the electric and magnetic properties of clusters vary with size, to try and understand the emergence of these properties in the bulk.

Click this link to see a diagram of our apparatus

Clusters of all sizes are produced in a molecular beam by ablating a sample rod with a pulsed laser. The beam is then analyzed by a Time of Flight Mass Spectrometer (TOFMS) which disperses the clusters into a spectrum by their mass. In the TOF spectrometer, the cluster beam (which contains clusters of all sizes) is ionized by a UV laser and accelerated toward a detector by an electric field. The time of flight depends on the mass of the cluster, with the lightest clusters arriving first and the heaviest clusters arriving last. An actual spectrum is shown below, taken from a beam of cobalt clusters.

Each peak in the above spectrum represents a cluster with a specific number of cobalt atoms.

In many of our experiments we direct the cluster beam through an electric or magnetic field and we want to measure the deflection of cluster beam by the field. The TOF spectrometer also gives detailed information about the deflection of the beam, because a cluster that has been deflected by an electric or magnetic field will have a slight lead or lag compared to the time of flight of an undeflected cluster. The deflection is manifested in the mass spectrum by a shifting of the peak of the deflected cluster. The magnitude of the deflection can be calculated from the magnitude of the peak's shift.

Recent Work

Our recent work has focused on corellated electron effects in clusters and alloys such as ferromagnetism and ferroelectricity. We are studying the details of how these propeties vary with cluster size and temperature. Another interesting series of experiments involves how the properties of clusters change when magnetic impurities are added to make alloys. See our publications page for details.