Clusters

This work is carried out in conjunction with Prof. Brian Johnson and Dr. Scott McIndoe of Cambridge University and Dr. Paul Dyson of York University.
Projects in this field deal primarily with the creation and analysis of transition metal (TM) clusters. Clusters have increasing importance towards catalysis and nanoscale material manufacture, due to their intermediate size (and suspected properties) between molecular and bulk materials.

We principally investigate novel ways of producing metal clusters using lasers to induce the clustering process. Current work involves the use of transition metal carbonyl complexes to act as basic building blocks (precursors) to create larger metal cores. Performing laser desorption and ionisation (LDI) upon the precursors causes dissociation of the carbonyl ligands, leaving bonding sites available for other TM complexes, and hence increase the nuclearity of the metal core. This has already been proven to be a successful technique, with past work generating and analysing clusters in situ using a MALDI-TOF instrument - a standard mass analysis machine which can perform LDI and then analyse the formed clusters using time of flight (TOF) mass spectrometry.

An example spectra is shown below

The differently coloured peaks correspond to clusters of a set core size. Starting with a precursor of Rh₆(CO)₁₆ we see clusters with the following core sizes:
Red : 6 atom core, Blue: 12 atom core, Orange: 18 atom core, Green: 24 atom core. Light blue: 30 atom core, Grey: 36 atom core, and finally Purple:42 atom core.

An example of much large clusters made from Ru₆C(CO)₁₇can be seen below. In this case the clusters grow to approximately 200 metal atoms

The work described above has lead to the design and construction of many reaction vessels to produce clusters by LDI. Two different techniques are currently under investigation. One is to work in the solid state, similar to the experiments described above. The latest design has been designated the "Laser Anvil" or the "Photonic Hammer". This has produced some exciting results and is going through further testing.

In both cases the analysis of the clusters is carried out with electrospray mass spectrometry (ESI MS). Work is also planned to study the clusters using scanning electron microscopy and X-ray diffraction.

We are currently looking at the creation of clusters of a set size through selective fragmentation of the metal core. This is done through the exploitation of electrospray mass spectrometry. ESI MS requires a continuous flow of a sample dissolved in a solvent. The liquid is expelled from a narrow capillary as charged droplets in the presence of a high electric field. The applied potential causes the droplet's surface charge density to exceed the liquids surface tension causing a "Coulomb explosion". The droplets get smaller, with the solvent lost through evaporation until eventually all we have left is the original sample which is analysed.
This applied potential can also act as a soft fragmentation device, stripping our TM precursors of carbonyl ligands. Although in its preliminary stage this work has already produced results that are in the process of being published.

This work has also lead to the investigation of using the metal clusters as catalysts for performing important organic synthesis reactions. As yet this work is in its infancy, but current work looks extremely promising.