Take a break: Building nanoscale structures with electrophoresis

Building a wall

Electrophoresis is turning out to be a very effective nanoscale builder, say Irish researchers who have used it to construct walls made of nanorod ‘bricks’ all aligned in the same direction. Like many normal-size builders, though, it doesn’t like to be rushed.

These nanorod ‘bricks’, which are around 7nm wide and 30nm long, are made from cadmium sulfide (CdS) or cadmium selenide (CdSe) and covered in organic ligands. When he placed these nanorods in a solution, Kevin Ryan at the University of Limerick found they spontaneously clumped together to form stable two-dimensional sheets that dropped to the bottom of the solution under the force of gravity.

This clumping occurs because the nanorods possess a dipole moment, with negative and positive charges separated at opposite ends of the nanorod. The negative and positive charges on different nanorods attract each other, causing the nanorods to agglomerate. Once the nanorods are brought close together, the ligands help to hold them in place, producing the stable sheets.

However, the nanorods in these sheets are fairly disorganised, like a big pile of bricks, limiting their usefulness. Ryan wondered whether applying an electric current across the solution would provide the nanorods with some guidance as they come together, helping them form regular arrays, like lines of bricks in a wall.

 

Lining up the bricks

Using an electric current to guide the assembly of nanoparticles into layers and structures is known as electrophoretic deposition and is being investigated by several research groups. But Ryan and his colleagues are one of the first to try using it to induce a high degree of order in these structures.

As a first step, they applied an electric current across a toluene solution of CdS nanorods covered in alkyl groups. The current caused the nanorods to migrate to the anode, where they formed a regular layer in which all the nanorods lined up at right angles to the surface of the anode, like soldiers stood to attention. Subsequent layers would then form on top of the original layer, with all the nanorods in all the layers aligned in the same direction.

Exactly how many layers formed depended on how long the electric current was applied for: applying the current for 30 seconds produced two layers, 60 seconds produced four layers, 120 seconds produced six layers and 180 seconds produced nine layers. The covering of ligands ensured that the layers stayed in place when the current was switched off, allowing them to be extracted from the solution.

As a second test, Ryan and his colleagues tried the same thing with CdSe nanorods covered in alkyl groups, in order to show that this construction technique works with more than one kind of nanoparticle. CdSe nanorods are more curved that CdS nanorods, giving them a rice-like appearance. As a consequence, although the electric current again caused all the nanorods to line up at right angles to the anode, they weren’t able to fit together as neatly and the so layers they formed weren’t quite as regular.

 

Carefully does it

Next, Ryan tested the influence of the ligands, by swapping the covering of alkyl groups on the CdSe nanorods with a covering of pyridine molecules. As well as locking the nanorods together, the ligands also tune their overall charge state, and pyridine confers a higher charge than the alkyl groups.

When they tried electrophoretic deposition with these pyridine-covered nanorods, Ryan and his colleagues found the nanorods formed disorganised sheets. The reason for this seems to be that the high charge causes the nanorods to move at a faster speed through the solution, meaning the nanorods arrive at the anode too quickly to be able to form an organised layer. This shows that the process of constructing an aligned, high-quality nanoscale layer simply can’t be rushed.

As long as the aligned layers are allowed to dry slowly, they can be extracted from the solution without cracking, and Ryan is now planning to investigate whether these regular arrays of nanorods might prove of use in electronic, photonic or photovoltaic devices.