Size-selecting effect of water on fluorescent silicon clusters

Abstract

Silicon clusters were produced by gas aggregation in vacuum and co-deposited with water vapour onto a cold target where the water vapour froze. Melting of the ice yielded fluorescent silicon nanoparticles suspended in water which were investigated by photoluminescence spectroscopy (PL) and atomic force microscopy (AFM). The PL spectrum showed a prominent band at 420 nm and other, less intense bands at shorter wavelengths. No fluorescence was observed below 275 nm. The shortest wavelength observed was 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 by AFM. The images showed single and agglomerated clusters with heights of typically 0.6 up to 2 nm. The sizes displayed by our measurements are not correlated to the average sizes that result from gas aggregation, indicating a size-selecting effect of the water suspension. The cluster-cluster interaction in water is governed by repulsion due to thermal energy and attraction due to van der Waals forces. For very small clusters repulsion dominates; at 3 nm diameter the two forces are balanced. We identify this stable phase of small clusters as the origin of exceptionally stable fluorescence.

Nanoscale stripe arrays templated on Moiré patterns in graphite

“…Superlattices are regularly observed in HOPG and form due to interference 25 (Moiré) patterns formed when the top sheet of graphene is rotated with respect to the underlying crystal [13,14]. The same Moiré patterns are observed in epitactic graphene on Ruthenium and Iridium substrates.…”

“…The resulting solution was found to contain a stable phase of 45 silicon nanoparticles with a narrow size distribution centred at 1.3 nm. Analysis of this size distribution revealed that the nanoparticles had been produced entirely from the silicon atoms accompanying the cluster beam [25]. The produced particles showed stable luminescence over a period of 3 months [23], further measurements showing that fluorescence remains stable over years, in-50 dicating that the particles are chemically stable and have weak inter-particle reactions.…”

Fluorescent silicon clusters and nanoparticles

“…Atomic force microscopy in noncontact and constant force mode of cluster films produced by drop-casting colloidal solution onto freshly cleaved HOPG showed uncovered regions of graphite, and agglomerated monolayers, as well as double layers, of clusters (Torricelli et al, 2011). The height of the monolayers reflected the difference of the tip-HOPG and tip-cluster forces, and hence cannot be taken as a measure of cluster height.…”

“…When the photoexcitation wavelength was decreased from 310 to 240 nm, the wavelength of the fluorescent band remained at 420 nm; however, additional fluorescence bands appeared in the UV region (von Haeften et al, 2010a,b;Torricelli et al, 2011). When the clusters were embedded in liquid ethanol and isopropanol, the fluorescence wavelength shifted to 365 and 380 nm, respectively (Galinis et al, 2012b).…”

Towards biosensing via fluorescent surface sites of nanoparticles

“…After deposition, the ice was melted and a silicon nanoparticle-containing suspension was collected in a dish ). Silicon nanoparticles produced with this technique show a number of extraordinary properties: the average particle size is 1 nm in diameter (Torricelli et al 2011), they emit intense fluorescence in the deep blue spectral range when exposed to UV light, and the fluorescence intensity does not degrade over time. The interaction with water was found to be critical for the development of fluorescent activity; silicon clusters that were produced in our apparatus and directly deposited on a substrate without the water vapour beam on did not show fluorescence.…”