How small the world can be?
and the things we've learned to do with it

Imagine how big the solar system is…. but in reverse….

Well, this is how small the combination of a few atoms is.

There are some striking similarities between the entire solar system and a single atom, yet the size difference is enormous. The Solar System on the left side is 30e10 m in diameter, whereas the diameter of an atom is 3e-10m. Now you can imagine how difficult it is to manipulate those atoms. For those little guys in their quantum universe, the laws of physics work differently. What the human eye can see as a solid material would possibly be hollow on nanoscales, what we imagine as a ball can actually be just a wave and won’t even exist in the limited materialistic world. Weird, right? But don’t be afraid to be confused, as well-known scientist Richard Feynman put it:

If you think you understand quantum mechanics, you don’t understand quantum mechanics.

Richard Phillips Feynman

Operating on nanoscales can change perspectives of modern technologies. Putting NANO at a higher level, we will have the ability to improve energy efficiency, assist in environmental issues, and solve some big health problems. Moreover, we can greatly boost industrial output while lowering expenses.

Just to generalize – if you put atoms in a specified position they can do beautiful things.

Please, meet nanowires!

I didn’t know much about them before applying for PhD., and couldn’t even imagine myself working with nanowires. The most I’ve done before regarding research is thin-films (2D structures), but not 1D 😱. But here I am, and I am awaiting to do great interesting work along next years.

So what actually nanowire is? Let’s ask Wikipedia: “nanowire is a nanostructure, with the diameter of the order of a nanometre (10−9 metres). It can also be defined as the ratio of the length to width being greater than 1000.”

a microscopy photograph showing nanowires which look like gray rods with roundish tips
Here my colleague David Alcer shared a picture of GaInP nanowires 1,4-1,6 micrometers long standing on their substrate taken by scanning electron microscope. The picture’s source – Comparison of Triethylgallium and Trimethylgallium
Precursors for GaInP Nanowire Growth

In reality it is not that perfect, but generally, nanowires are ultra-thin and ultra-long configurations. And here’s the rub: even spotting such structures is difficult, because having the finest microscope’s resolution of the nanowire’s length makes it hard to see details in the tiny diameter. To accomplish this task scanning electron microscopy or other advanced techniques are used.

How it’s made

I would like to describe only one of the common techniques to grow semiconductor nanowires – MOVPE (which stands for metal-organic vapor-phase epitaxy). And, because different types of nanowires may and should be manufactured in different methods, I’ll focus on typical III-V group materials.

Deposistion of “seeds”

The first step in III-V nanowires growth is arranging the gold particles on a substrate. Those particles will have a role of a “seed”, from which structured nanowire is appearing. To begin, as usual for semiconductor devices, we take substrate, which is normally composed of the relevant semiconductor, and clean it of contaminants using Acetone and Isopropyl, heating alongside, to get rid of everything that can interfere with the next processes (organic and inorganic dirt, resist, chemicals, etc. ). The substrate is then dried using nitrogen flow.

With help of a process called spin-coating, we deposit a thin layer of resist (it can be some oxide). Usually, a small amount of resistive material is applied to the center of the substrate and placed on the plate which rotates fast for some time spreading liquid resist all over the substrate until the desired thickness is achieved. Then our pre-sample is baked at around 150-180oC. Those 2 stages take around 10 minutes and require no heavy equipment.

As shown in the picture below – in the resistive layer we make some holes using the technique Electron Beam Lithography. During this process, a powerful microscopic system and laser produce a thin beam containing electrons, that shoots a sensitive resistive layer destroying it. In the end, a desirable pattern (in our case in form of tiny holes) emerges. Almost every process is, of course, followed by cleaning, because even the cleanest laboratory will have microscopic impurities that can ruin all the work.

Thermal evaporation is then conducted for the deposition of a thin layer of gold. With help of high current gold is heated to a super high temperature until starts evaporating. The process happens in a high vacuum so evaporated particles don’t collide and go straight to our sample. The sacrificial resist layer together with some gold on top of it then gets removed using chemicals and heating, and at the end, only golden parts which stuck to a substrate itself are left. This step is called lift-off.

The scheme above shows the first stage of nanowires manufacturing called E-beam.
Result of this stage – golden particles deposited on a substrate.

Nanowires growth

The part where all the magic happens is called VLS (vapor-liquid-solid method).

In the chamber with some low pressure, carrier gases, and increasing temperature to around 420oC, desired III-V material is injected. With high temperature gold starts melting and gets saturated by III-V material. Those molecules are able to diffuse to their grow front and build up nanowire layer by layer, and some of them, which are almost insoluble in gold – on a created nanowire’s surface or substrate. Changing the temperature, pressure, and gas concentration we can control the growth rate and some of the other properties, like crystallographic structure for instance. By modifying an angle in which atoms got absorbed – we can make nanowires look or lean in different directions, sometimes even touch each other’s golden tips.

Here the representation of the second stage of growing nanowires is depicted
schematically – VLS (vapor–liquid–solid method)
.

Performing all the processes from the very beginning can take up to a month for an average Ph.D. in the university, including waiting for the equipment and preparation of a few references samples, checking every step on the microscope, and so on.

So why do we need nanowires?

Regarding small size, nanowires can be good conductors and successfully pass electrons through. First, it is a big improvement in computation, which means that millions of more transistors can fit on the same microprocessor. It results in a rapid increase in computer speed.

Another reported feature is next-generation solar cells – nanowires on the surface will increase absorption of the sunlight by hundred times, so we have not the only flat panel, but many many tiny wires, that can trap light better, reduce reflection, and lower the quality and amount of needed materials.

At the end of the day, those structures can modify all the properties of other nanoassembly.

Nelia Zaiats

Aimed to fly high successful university graduate in engineering with considerable international experience.

Leave a Reply

Your email address will not be published. Required fields are marked *

This website is using cookies to improve the user-friendliness. You agree by using the website further. Privacy policy