Sometimes ago I decided to bring one of my semiconductor device to life. A plan was easy (at least in theory) take one of my device, add some personality to it and move it to a virtual world. And that was the start of Mr. Gansy. Probably the first living core-shell nano light emitting diode (LED).
So what is the core-shell nano-LED? Firstly, I will describe a classical LED. Let’s named it a planar type LED. So in order to make diode we need two materials with a slightly different electrical properties, namely material type ‘p’ (positive) and material type ‘n’ negative. By connecting these two materials together we create a junction, which is called very often ‘p-n’ junction. This junction is essential for most of diodes, not necessarily light emitting diodes (LED).
As we now know we need the junction to make a diode. So what is the difference between a planar LED and a core-shell LED? The top part of the image (A) shows the diagram of a planar LED, we see that diode structure is organized like a sandwich where one type of material is laying on the other one and the junction area is between them. This also can be seen as 2D approach as the junction is a plane in 2D space.
What is a core-shell LED? Check out the next image (B). The bottom material (i.e. n-type) is in form of a pillar instead of a layer. This pillar can be small as 1-5 micro meter height and less than 1 micro meter diameter. This is truly nano-pillar, sometimes it is also named nano-rod. Let’s called this nano-pillar the CORE. To finish our diode we have to create a junction with other type of material. We can do it by depositing the p-type material around the CORE and as you guess this is formation of the SHELL. The ‘p-n’ junction is between CORE and SHELL and it is not anymore a plane in 2D space, but is an area in 3D space.
Let’s say (this is rough approximation) that an efficacy of our device is related to the surface area of ‘p-n’ junction. To design better device we need a larger surface of ‘p-n’ junction. This can be achieved by organizing the nanoscale core-shell LEDs into a grid (array). Due to nano scale of a single core-shell LED we can put a massive number of them into one LED chip. And then if we had two LED chips: a) one fabricated as a planar, b) second as a core-shell LED. Both with the same size, in terms of a final size of a chip. The one with larger surface area of ‘p-n’ junction would be generating more light, and that would be a core-shell LED. More surface area of ‘p-n’ junction means higher light output and brighter device at the end.
For more technical read, please check this link. There are quite impressive electron microscope images as well.