A team from the Technical University of Munich (TUM) has formulated a simple method to synthesize silicon clusters. Small silicon clusters act as a source of accordingly modified silicon. This is a significant achievement because, until now, silicon clusters have not been made in soluble form, which is used for flexible processing. Prime silicon cells have an efficiency of nearly 24 percent, with the theoretical limit being around 29 percent, as silicon typically crystallizes in a diamond structure that gives only an indirect band gap, explains Thomas Fässler, Professor, Inorganic Chemistry with Focus on New Materials, TUM.
Researchers believe the small silicon clusters are ideal for creating materials wherein the silicon atoms are arranged in a way that makes a direct band gap that they can use to generate solar energy, as the atoms in these clusters can be arranged in another way that in crystalline silicon. Professor Fässler says these compounds are also useful for several other chemical experiments. With merely a few synthesis steps, four and nine silicon atoms can be joined to create tetrahedrons or near-spherical structures, although, this synthesis and isolation of atomic clusters was a tedious process. The new method is a major step forward, adds Professor Fässler. Fusing silicon and potassium creates a gray powder-like compound with 17 silicon and 12 potassium atoms. The first author of the study, Lorenz Schiegerl, was able to stabilize the soluble clusters consisting of nine atoms in liquid ammonia by adding an organic molecule to the ammonia, which encircles the potassium atoms.
Professor Fässler says that this simple process, right from elemental silicon, paves the way to myriad chemical experiments with the clusters. Another reaction path where these silicon clusters would be valuable is reactions for creating compounds made from silicon clusters, wherein three of the nine silicon atoms fuse with molecules containing silicon, or for instance, tin or carbon. Clusters that have the highest concentration of silicon currently recorded are present in these solutions, indicating that there are many possibilities for depositing silicon with modified structures out of solution.