, University of Southampton

SCFED: SuperCritical Fluid Electrodeposition

Science Made Simple

 

You can read more about the specific properties and advantages of SuperCritical Fluids (SCFs)and Electrodeposition on the other Science Made Simple pages. SCFED or SuperCritical Fluid ElectroDeposition integrates all of these advantages to give an extremely powerful materials deposition and manufacturing technique.

 

Because SCFs do not exhibit the property of surface tension (the force which causes liquids to form drops) they can fill the smallest of spaces which scientists call 'nanopores'. These cylindrical holes can be just a few nanometres in diameter.

 

If an electrical contact (a sheet of metal through which electricity can flow) is put across the bottom of the nanopore, when an electrical charge is applied to this it draws useful molecules that have been dissolved in the SCF down to the bottom. When they reach the contact the molecules stop being dissolved and deposit at the bottom of the nanopore. If the molecules that are deposited act as conductors (metals) then they become part of the contact and the bottom of the contact moves further up the nanopore. This process can be continued until a conducting cylinder or 'nanowire'  of a suitable length has been grown inside the nanopore.

 

The animation below shows the SCFED process for manufacturing an electrical device in five basic steps:

 

 1.               

There are molecules of silver dissolved in a SCF around a template that has nanopores. This template is mounted on a gold electrical contact. No electrical current is flowing through the contact.

2.     

The electrical current is turned on and the molecules begin to be pulled into the nanopores.

3.     

 The dissolved silver molecules deposit at the bottom of the nanopore and a nanowire begins to grow.

4. Steps 1-3 are repeated with gold molecules and then with silver molecules again.
5. The layered structure in the nanowire created this way is called a 'junction'. Junctions are how electronic components work.

 

 

For Scientists... The use of supercritical fluids as electrolytes offers a far wider potential window than for liquids. This enables the deposition of a broad range of highly reactive but commercially interesting materials such as germanium and silicon for semiconductor devices both inside mesoporous templates and as ultra-high quality thin films on flat substrates. It is both the lack of surface tension and the suppression of bubble formation that allows SCF electrolytes to fully penetrate high aspect ratio, small diameter (currently tested at 3nm) nanopores. The low viscosity and high mass transport rates enhance control over the electrodeposition rate, an important feature for achieving well specified parameters as would be required for commercial applications. As electrodeposition can be performed at high temperatures from a SCF (>200 degC), increased surface mobility of atoms may be used to improve the crystalline quality and epitaxial growth. The directionality of electrodeposition negates pore blocking during  growth leading to the formation of a regular array of axially heterostructured nanowires. For applications these can either be left inside the mesoporous template or removed and isolated for individual nanowire contacting.