The Leicester group explains this observation by the balance of repulsive and attractive forces acting on the nanoparticles. The silicon nanoparticles are attracted by van der Waals forces: this attraction energy increases proportionally to the diameter of the nanoparticles and inversely proportional to their separation. The nanoparticles' thermal energy acts against the van der Waals attraction and prevents particle clusters from agglomeration. This thermal energy is greater than the attractive energy for small nanoparticles. For nanoparticles with 3 nm diameter the forces are balanced, which means that bigger nanoparticles agglomerate to macroscopic grains. Silicon nanoparticles that are smaller than 3 nm in diameter can never agglomerate and form a stable phase in water.

The team believes that this stable phase is responsible for the exceptional fluorescent properties shown by the silicon nanoparticles. In the study, the group presents evidence that the fluorescence stems from very small silicon clusters of around 1 nm in size, which matches up well with the atomic force microscopy analysis.

Deep blue fluorescence

The silicon nanoparticles exhibit fluorescence in the deep blue spectral range after UV light excitation and the fluorescence intensity remains stable over more than a year. These properties are the result of a novel production technique developed by the team – the group first produces silicon nanoparticle beams in the gas phase and then co-deposits the nanoparticles with water vapour onto a cold target. Melting of the ice yields a suspension of fluorescent nanoparticles.

The researchers believe that their technique can be used for the production of nanoparticles other than silicon and the next step is to investigate whether the size-selecting effect of water will be displayed in other solvents as well. The discovery of the size-selecting effect may be relevant to applications of nanostructures in optoelectronics, catalysis, and biomedical diagnostics and therapy.

More information can be found in the journal Nanotechnology.