I have recently performed Impedance spectroscopy of ferroelectric Bi4Ti3O12 thin films on platinum, the cole-cole plot of which can be seen in the attached figure. I was expecting a single semi-circle, however the irregularly semi circle faces in the negative z' region rather than the positive.
I am currently looking for a reason to explain this phenomena. I have tested the apparatus using standard (RC) circuits and found only regular semicircle cole-cole plots. If anyone could help me explain this irregularity, or correct it, I would be very grateful.
I have experience with Impedance spectroscopy of (RC) circuits but not ferroelectric ones. However from my literature reviews, they appear to behave as normal capacitors, which I expect is due to the measuring signal being significantly smaller than the coercive field. If anyone could correct or further my knowledge on this as well, I would again be grateful.
Dear David,
I also had obtained the effect of the negative values of Z' in impedance spectroscopy for the supercapacitors (SC) or electrochemical capacitors (EC).
This effect occurs after fast supercapacitor charge by a constant current source and than there was no relaxation time between charging mode and impedance measurement where ferformed immadiately. This effect was explained by me and my friend Prof. Manuel Ortigueira in our new paper "Negative Active Resistance Effect of Supercapacitors", which is non-published yet. We call this phenomenon “ The EC relaxation phenomenon”.
This effect is explained with the EC nonstationary measurement mode and the fractional order EC model which was described in our paper "V. V. Martynyuk and M. D. Ortigueira, “Fractional Model of an Electrochemical Capacitor,” Signal Processing. – DOI: 10.1016/j.sigpro.2014.02.021".
If you are interested in the mutual scientific cooperation with me and Prof. Manuel Ortigueira we will extended our non-published paper for case of your ferroelectric thin films and you will be our co-outhor. Please let me know your opinion and send me your letter to my e-mail [email protected]
If you are agreed with me I will send you our new non-published paper "Negative Active Resistance Effect of Supercapacitors" to your e-mail and we will extend it for case of your ferroelectric thin films. After that will publish paper together with you.
Best regards,
Valeriy
Dear David Coathup,
in order to help (on your peculiar Nyquist plot) give us, please, some more infos on the :
1. thickness tF of your film ?
2. electrical contacts. Apart Pt el.-contact, what is the other ?
3. measuring system (company & model of your set up) ?
Dear Valeriy,
Thank you for your offer. I need time to process what you have said, and I promise to get back to you soon.
To answer your questions Ioannis, the thickness of the film is 400nm. The contacts are on the platinum substrate, and on a 430um diamater gold contact evaporated onto the thin film.
The system itself is Autolab PGSTAT302N.
Again, many thanks to you both for you assistance.
-David
Dear David Coathup,
a possible error might be a leakage (parallel) current from the sample holder SH. Try another SH and remeasure it.
Dear Ioannis, Could you define what you mean by SH? I have measured the Leakage current desperately, and it's only 6 micro amps at 5V applied.
Best wishes
David
SH : Sample Holder.
If I have to be more specific then send 1 or 2 photos of your sample (you measure) inside your Sample Holder.
Note (similar terminology for Sample Holder) : 1.Test Fixture, 2.probe, etc.
Dear Ioannis,
To clarify, I am using thin films and do not require a sample holder. I use a standard probing station to perform the measurements.
The substrates themselves have a 500um thickness, so I guess some parallel capacitance with the metalic probing station stage is possible. I will attempt to repeat these experiments sitting the sample on a thick insulative layer to see how it behaves.
Best wishes
David.
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