Abstract
Silicon clusters were produced by gas aggregation in vacuum and co-deposited with watervapour onto a cold target where the water vapour froze. Melting of the ice yieldedfluorescent silicon nanoparticles suspended in water which were investigated byphotoluminescence spectroscopy (PL) and atomic force microscopy (AFM). The PLspectrum showed a prominent band at 420 nm and other, less intense bands at shorterwavelengths. No fluorescence was observed below 275 nm. The shortest wavelength observedwas related to a silicon cluster diameter of 0.9 nm using a simple particle-in-a-box model.Drops of the suspension were also deposited on freshly cleaved HOPG and investigated byAFM. The images showed single and agglomerated clusters with heights of typically 0.6 upto 2 nm. The sizes displayed by our measurements are not correlated to the average sizesthat result from gas aggregation, indicating a size-selecting effect of the watersuspension. The cluster–cluster interaction in water is governed by repulsion due tothermal energy and attraction due to van der Waals forces. For very small clustersrepulsion dominates; at 3 nm diameter the two forces are balanced. We identify thisstable phase of small clusters as the origin of exceptionally stable fluorescence.
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