Nanowires

Now that my new semester started I thought I'd share a short excerpt about what "we" do here in Lund - what is special for Lund research. It is everywhere here. If you study here you might even forget that there is something else going on (in physics). It is nanowires. In principle they are exactly what the name implies. Wires. Small. But first of course the always most important question first: Why would you investigate them?

They show some very interesting properties that conventional wires do not show. These properties lead to promising application possibilities such as (of course) wires in ever modern devices which are getting smaller and smaller. But here besides being small they already show further promising advantages. By combining materials in a way described further below they might allow transmission of two signals in opposing directions at the same time or introduce barriers or quantum wells to quantise conductance. Furthermore, junctions of nanowires might allow to study Majorana fermions which is an interesting scientific field. (Should I roughly understand what Majorana fermions are I might write an article about them but so far I only understand that they seem to very interesting ...) There are some possibilities for medical sciences as well but I do not know much about this. (E.g. something with detecting cancer I think.) Not to forget applications in power generation as I described in the article "The future of solar cells?".

Figure 1: Examples of nanowire
structures consisting of different
materials. The right image is a
cross-section through a wire.
A structure of this kind allows to
transport two signals at the same
time.

Now how are they fabricated and why do they have further interesting properties than just being small? And what actually is small? Nanowires again already implies the size: The diameter of nanowires usually ranges from a few to a few ten nanometers. Their length is usually at least 20 times as long. Otherwise it would not be a wire ... The small diameter now gives the wires a large surface to volume ratio. And this is the interesting part!

If you grow different elements on top of each other in crystalline structures often the top layer will break after a few nanometers. This is because different elements have different distances between the atoms in the crystals which causes strain in the growing layer. After a few nanometers this strain will be too large to allow a monocrystalline structure and the crystal breaks. You cannot use it for devices anymore. With the large surface to volume ratio the nanowires can balance the strain as there is a lot of space for the atoms to gradually change from one crystal structure to another and the different crystals do not break. This can be used to grow heterostructures in ways shown in figure 1.

The principle of fabricating nanowires is as easy as elegant. (Note: the principle.) On a substrate seed particles are distributed by nanoimprint (basically using a nanostamp) or from a gas. Afterwards the nanowire material is applied as gas as well. It will settle on the seed particles, diffuse through them and push the seed up. So underneath the seed there grow the wires. Afterwards branches can be grown and stuff like that. Which is basically what happens in Lund all day. ;)

Figures 2a) (top) Seed particles on a substrate. 2b) (bottom) Different stages of growing nanowires.