What? Why? Who? How?

Let’s take it back to the start.

 What is Beyond the Big Bang? This blog started as a part of my dissertation as my public dissemination piece which is a form of information that explained my dissertation question to members of the public which are not an expert in the field I was exploring. As I was forming the blog I got friends to overview it and make sure there were no mistakes and as I got feedback I realised that most people who are no longer studying science are still interested in learning about it.

Why make a science blog? From the age of 16 you are made to slim down your education to focus more on a select subject. For most people this meant the end of science education and although at the time most people are excited by the prospect of no more chemical reactions or biological field studies, as they have gotten older they miss learning more about how the world works. So I have decided to carry on this blog to explain in more depth some of the everyday science around us.

How are you going to explain everything? In my further education I have studied in depth chemistry and areas of biology, so don’t expect anything detailed about physics or engineering as those things are still a mystery to me! But I do plan to try and explain more of the everyday scientific terms that are thrown at us in advertising, literature, movies etc.  I will be doing my research alongside to make sure everything that is presented is correct and that there are no fibs being told.

Who are you? Let’s be honest, at this stage no one really cares who I am but expel the basics. I am a 22-year-old student living in London who has just graduated from their undergraduate degree in pharmaceutical chemistry and is going on to a masters in biopharmaceuticals. So I ain’t know David Attenborough with years of knowledge behind them but I am a young adult who has scientific jargon thrown at them daily and is curious to what it all actually means! For explain what actually is gluten? What makes smart water smart? Why are there antioxidants in my foundation? How does purple shampoo make blonde hair more blonde?

So if science doesn’t interest you at all then thank you for reading this far but if you do have a curious mind to how this wacky world works then stay tuned. The overall goal for this blog is to make science understandable for everyone from every walk of life.

Now let’s get this nerd party started…

I got 99 problems...

With all of these developments and continuing research going into finding better ways and better precursors to make better thin films there are definitely going to be problems that are going to arise. For this post we are going to focus on thin films made from the group IV and V transition metals with nitrogen to made nitride thin films. There is a demand in industry for nitride thin films due the desirable qualities they possess, including how hard they are, long wear residence, high melting point and good chemical resistance. These qualities of nitride thin films makes them the major component in many applications, including  coatings for cutting and grinding tools, wear surfaces and semiconductors.

Group IV and V transition metals, Rf and Db are not looked at though as they are both highly radioactive and do not exist in nature.
One of the biggest problems when depositing thin films is contamination from other elements. When a nitride thin film has contamination from carbon and/or oxygen it reduces the properties of the thin film. Contaminations cause an increase in electrical resistance (which is a problem when depositing TiN films which need to be conductive) and also decrease the hardness of the film which will make it lose its ability to coat things. To overcome this issue, more research has gone into finding single source precursors. A single source precursor is a complex that will be deposited to form a thin film without the addition of any other complexes i.e the addition of a reducing agent or a nitrogen source (ammonia).

TDEAT, an example of a single source precursor used to deposit TiN thin films.
Another focus for future precursors is to be able to synthesis complexes in lower oxidation states (as discussed in Where did that plane get its nice coat?). A key example of this is TaN which was mentioned above. When in the 3+ state it forms the stoichiometric form which is conductive whereas if the +5 state is formed it dimerizes and forms the Ta₃N₅ which is insulating. It’s clear what issues can arise if the wrong oxidation state is formed. The same issue arises for other group IV and V transition metals when using lower oxidation state complexes. When in a lower oxidation state, the complexes have a tendency to dimerize like in the case of zirconium coordination complexes in a 3+ state. An idea to overcome this is to use bulky ligands. The bulkiness of the ligands will cause strict steric conditions which will prevent the complexes from dimerises and keeping the final thin film in the desired low oxidation state. 

Ti(III) complexes can be generated by using the stable nitroxyl radical TEMPO (2,2,6,6-tetramethylypiperidine-N-oxyl), an example of using cyclopentadienyl as a ligand
It is clear from the published literature available, that currently low oxidation state precursors for group IV and V nitride thin films are not widely or commercially available. There is a lot of attention going into research to find low oxidation state precursors, especially for TiN and TaN thin films for application in semiconductors. Although there is not been found yet on making precursors themselves, a lot as been done to find low oxidation state coordination compounds that could be carried forward in the future and applied as precursors. Key ligands that have emerged as potential precursors are hydrazine’s, amido containing ligands, cyclopentadienyl and guanidinates. The characteristic which is prominent in all of these ligands is their ability to stabilise the low valence nature of the transition metals in question as they are all electron rich. An example of the successful application of precursors which incorporate these ligands include the deposition of TiN films from 1,1-dimethylhydrazine which was also able to promote the reduction of Ti(+4) to Ti(+3). 

An example of the use of guanidinato ligands, [Ta(NEtMe)(N‑^tBu){C(N-ⁱPr)₂-(NEtMe)}₂] (RHS compound) was successfully used to deposit (+4) TaN thin films
Almost all of group IV and V have shown potential to be deposited as nitride thin films from low oxidation state precursors. The next step in the research to find this low oxidation state precursors will be to test if they have the right qualities to be used in CVD/ALD deposition to make the thin films. 

Where did that plane get its nice coat?

Ever wonder how they get objects to look gold without using real gold? Or how aeroplanes manage to go through intense weather conditions and corrosion and yet still look so shiny and new? The answer lies in the application and science of thin films. As mentioned in The ever shrinking world of microelectronics, thin films are a growing area of applied science that is used all around us! A major part of thin films and an area of research that is continuing to grow in interest is the deposition process used to make them.

Thin films have been in research and development for hundreds of years. They are in application all around us including mirrors, solar panels, coatings for tools, aerospace engineering and much more. They physical and chemical properties of the thin films is decided by the substrate (the base) the thin film is coated on and the precursors used. The precursor is the complex which is deposited onto the substrate, often the precursor is not the compound that is wanted to be deposited (the compound that the thin film is made up of) so reactions are often carried out during the deposition process. 

There are two main types of deposition that are used to make thin films, physical vapour deposition (PVD) and chemical vapour deposition (CVD). PVD was the old favourite to be used to deposit most thin films however as technology develops, CVD and its subsets have become the deposition of choice. This is because CVD offers thin films with more precise conformity and better control over the thickness of the films. A subset of CVD that is growing in interest is atomic layer deposition (ALD) which has even more control over the thin film as they deposited therefore again synthesising thin films of higher quality.

Why the demand for better deposition methods? 

As the demand for better, smaller electronics grows the pressure is on to find new, more efficient materials and more precise deposition methods is also demanded! As substrates (microelectronics and other electronics) get more delicate and sensitive, the way in which the thin films are deposited has to also change so the substrates don’t get damaged. A major component of this is the temperature at which the films are deposited. Older methods of deposition use rather high temperatures (500 ^oC +) and these new sensitive substrates would be damaged if exposed to these high temperatures. This is a big problem in that to be able to deposit thin films at lower temperatures not only does the deposition process have to change but so do the precursors used. Temperature is such a major control of deposition because for the precursors to react they all need to be a certain temperature. If this temperature is not reached, then the reaction won’t happen and the correct thin films won’t be made.

The burning of lithium
 How reactive a precursor is can be referred to as its volatility. So the more volatile a precursor, the more reactive is. A key part of a precursors that contributes to its volatility is its oxidation state. Much research has been done and is being done to look into if by lowering the oxidation state of a precursor means that it can be deposited at lower temperatures. To read more about this have a look at 99 problems where some of this research and its results are discussed. 

With this fast developing world of technology it is inevitable that problems will arise. It is down to material chemists and physicists to find and develop new chemicals and technology that will be able to keep up! 
The future of new technology is all down to the research done by these scientists and who knows what the next big breakthrough will be?