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nanotimes News in Brief

11-08 :: August 2011

Nanoparticles // Water keeps Nanoparticle Size Under 3nm Diameter

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cientists at the University of Leicester, UK, have discovered a size-selecting effect of water on

nanoparticles that are dissolved in it. First, the team injected silicon nanoparticles with different size distributions into liquid water. Next, the researchers placed a drop of the suspension on a graphite sur- face, dried it in vacuum and measured the heights of nanoparticles using atomic force microscopy. All samples showed small heights of about 1nm on average. The heights never exceeded 3nm regardless of the size distribution of the nanoparticles that were injected in the water.

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 3nm diameter the forces are balanced, which means that bigger nanoparticles agglomerate to macroscopic grains. Silicon nanoparticles that are smaller than 3nm in diameter can never agglomerate and form a stable phase in water.

The team believes that this stable phase is responsi- ble 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 micro- scopy analysis.

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