Growing nanoparticles in helium droplets: a route to new nanoscience and nanotechnology
This is one of our newest areas of research and offers many exciting prospects. Here large liquid helium droplets are used as a medium for growing nanoparticles. A strength of the technique is the ability of the droplets to capture any material that can be produced at low pressure in gaseous form, whether a gas or a vapour from a liquid or heated solid. The gaseous atoms or molecules collide with the droplets and then enter.
This process is repeated and allows the formation of particles inside the helium droplets with controllable dimensions ranging from sub-nm diameters to tens of nm. Furthermore the very low temperature of the helium droplets (0.4 K) allows unusual combinations of materials to form in layers, i.e. as core-shell and core-multiple shell structures.
The new particles can be investigated while inside the droplets, e.g. by laser spectroscopy, or can be removed from the helium droplets by collision with a solid substrate. High resolution microscopy shows that the helium acts as a ‘cushion’ and the particle lands on the surface intact.
As well as spherical nanoparticles, we have also recently observed metallic nanowires from helium droplets, which are believed to be formed by condensation of metal atoms on so-called quantized vortices in superfluid helium.
In our laboratory we are exploring ways to control particle and wire formation using helium droplets. We are also attempting to make a whole new range of nanostructures with many possible applications, including in sensors, magnetic storage devices, and heterogeneous catalysis.
Further images can be seen below (click to enlarge):
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