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Chemical Vapour Deposition

Direct Liquid Injection CVD (DLI-CVD) and Aerosol-Assisted CVD (AACVD) for the Growth of Advanced Layered Materials: Multiferroics and Electrochromics

Chemical vapour deposition or CVD is a method by which thin layers of materials may be deposited (grown) onto a variety of materials so as to perform some useful function or provide the materials combination needed in the fabrication of a device of some sort. The Advanced Materials and Surfaces Group are well-known for their expertise in this area as well as in other methods of thin film deposition such as chemical solution deposition (CSD) and atomic layer deposition (ALD).

In a typical CVD experiment precursor chemicals containing the atoms that are necessary to form the required coating are made to react on, or near to the surface to be coated, byproducts are removed and the coating material deposited. It is usually desirable that the precursor chemicals are volatile such that they can be easily transported to the reaction zone, although this cannot always be achieved. Sometimes the precursor chemicals are not sufficiently volatile to permit this and so under these circumstances the chemicals are made into a solution using a volatile solvent. Droplets of the solution are then vapourised and transported to the reaction zone, where the solvent evaporates leaving the reactive precursor molecules at the surface to be coated. They then break down to form the film and the byproducts are sent to the reactor exhaust. Under these conditions the technique in question is known as aerosol-assisted CVD (AACVD) or more commonly, direct liquid injection CVD (DLI-CVD).

Chemical Vapour Deposition
Left: a transmission electron microscopy image of a natural ‘superlattice’ material, the room temperature multi-ferroic material bismuth iron manganese titanate Bi6Ti2.8Fe1.52Mn0.68O18 grown by direct liquid injection CVD. We have shown that these so-called Aurivillius-phase thin films are both ferroelectric and ferromagnetic and demonstrate magnetic field-induced switching of ferroelectric polarization in individual Aurivillius phase grains at room temperature. Such materials may find application in areas such as switches and actuators as well as high density data storage media. Right: showing how ferroelectric domains in this material can be ‘poled’ or patterned. Taken from Faraz et al., AIP Advances, 5, 087123 (2015).

 

This work, which is led by team member Dr Lynette Keeney, (pictured below) has received funding directly from industry and most recently the Royal Society/Science Foundation Ireland.  Dr Keeney is the recipient of a prestigious Royal Society University Research Fellowship Award which will allow her to continue and extend her studies of these highly novel materials.

Dr Lynette Keeney pictured with Professor Mark Ferguson
Team member Dr Lynette Keeney pictured with Professor Mark Ferguson, Director General of Science Foundation Ireland and Chief Scientific Adviser to the Government of Ireland, at the formal announcement of her Royal Society Award.

 

AIXTRON atomic vapour deposition system at Tyndall
A photograph of the AIXTRON atomic vapour deposition system at Tyndall which is used to prepare complex oxides via the process known as direct liquid injection CVD. The photograph shows the injection unit at the top left of the picture and the quartz reactor assembly in the main body of the picture.

 

Cyclic voltammograms of the silver-doped V2O5 films grown using aerosol assisted CVD at Tyndall
Cyclic voltammograms of the silver-doped V2O5 films grown using aerosol assisted CVD at Tyndall.

 

The voltammograms are used to simulate repeated Li+ ion insertion and de-insertion such as may occur in a Li+ ion battery or in an electrochromic (colour-change) device. Voltammograms were recorded at a scan rate of10 mVs-1. The first scan of the 5% and 15% samples are given with the 500th scan of the 15% sample highlighting the cycle stability. The 0% silver sample is shown as an insert. This figure is taken from Vernardou et al., Electrochimica, Acta, 196, (2016), 294–299, work that has taken place via a long-term collaboration with colleagues at the Technological Educational Institute of Crete, 71004, Heraklion, and the Institute of Electronic Structure and Laser, Foundation for Research & Technology-Hellas, P.O. Box 1527, Vassilika Vouton, 71110 Heraklion Crete, Greece.

 

At Tyndall the DLI-CVD method is currently being used to grow thin films of complex oxides such as the multiferroic and electrochromic materials depicted in the figures above.

 

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