Dear Pouria

According to what you described, your CNT films are not conducting enough to be measured using the four-point set-up. Normally a four point probe can only pick up the surface resistance around a few K ohm/sq range. The more sensitive meaurement set-up for less conducting films or even semiconductors is a Hall-effect system.

p. s What size of your CNT film?

How did you measure IV? It must be two contacts then if I am right.

Hope it helps.

Xuhua

Thank you Dr. Xuhua Wang for your comment. The four probe consist of parallel wires due to the dimensional characteristics of the films which is 30mm in length by 5mm in width. The combination of current source and nanovoltmeter that is used here is capable of resistance measurements up to 100M.Ohm (According to manufacturer) and I am not sure how the limits of the setup may have caused this (since a limit of 10V is set in this particular measurement). I greatly appreciate if you would further explain your last question (How the IV was measured) in case I have not answered.

Thanks for your feedback Pouria.

Firstly, the set-up you have for sure is not sentive to the measurements of high resistive thin films. Actually we talked the same things regarding sensitivity. The supplier referred you as linear resistance. What I mentioned is the surface resistance. To be clear, a four point probe is to measure a surface resistance of a thin film by setting a current of the outsider two probes and measuring the voltage of the inner two probes. Then the surface resistance can be obtained from a formula of 4.5324×V/I. By in mind, the formula assuming the dimension of a thin film sample is much larger than the dimension of the four probes. Therefore your film is not big enough in one dimension. Even you can measure it, the errors can be large. You had better have some square or round dimension samples which are much bigger than the dimention of your four probes occupied.

Now i understand that your IV refered to the four probe measurements. From the slope of your IV curve, you can calculate the surface resistance of your films from the above formula I mentioned.

Hope it is clear.

Hello Pouria,

Looks like you’ve swept current instead of voltage and you have your voltage limit set to +-1.5V. Since it appears linear despite the limit, resistance is roughly 33 kOhm (1V/~30uA). Try providing a current of exactly 4.5324 mA and your voltage reading (in mV) will be exactly your sheet resistance (in Ohm/sq). As Xuhua mentioned, you may see increased resistance with a 2T terminal measurement from lead resistance, so try a 4T measurement.

It is interesting to note that voltage while saturating at +-1.5V becomes noisy.

Can it be some kind of breakdown? Although the voltage is very and pattern seems too symmetrical..

Still the first thing to suspect is clipping by the instrument although it should not be noisy then.

Sorry all that this question turned into a discussion :)

Thank you Dr. Xuhua Wang for you answer. Your answer was clear and made me look more into different techniques in more detail. Unfortunately, at this stage of the experiment, I don't have the option of changing the dimensions of the samples. Although, changes may be possible to the four probe measurement setup to make that happen which I'm currently investigating.

Thank you George C. Wilkes for your input. The setup is four probe and I should mention that the voltage limit is set to 10V. Hence, I'm not sure why it limits at 1.5V. I still have not figured out where this limit comes from.

Thank you Nikolay Pavlov for your comment. I'm quite certain there's no break down since this result is repeatable with different parameters on the same sample. I agree with you, clipping by the instrument should have taken place at 1.5V when it's set at 10V limit and it should not have been noisy.

Following..

Article Preparation of ZnO film on p-Si and I-V characteristics of p-Si/n-ZnO

Dear Pouria Khanbolouki ,

is it easy to say more about the CNT-deposition method, e.g. how did you deposited the CNT film on Si (or maybe: SiO2//Si) ?

Is there any idea about a remaining "binder" marerial or any other (possible, secondary, non 1-Dim) phase[1] inside the (nominally pure) CNT film ?

Are there any SEM (or other micro-) images[2], you could show?

Also, is it possible to measure EIS[3,4], in addition to your quasi-DC iv-sweep ?

1. Nanotubes and Their Alignment and Deposition as a Film in a dc Field https://pubs.acs.org/doi/pdf/10.1021/ja0479888?rand=c7bougf1

2. Recent advances in nanorobotic manipulation inside scanning electron microscopes https://www.nature.com/articles/micronano201624#ref50

3. Overview of EIS (Electrochemical Impedance Spectroscopy) https://www.gamry.com/application-notes/EIS/potentiostatic-eis-tutorial/

4. Four-Electrode Experiments, in Two, Three and Four Electrode Experiments https://www.gamry.com/application-notes/instrumentation/two-three-and-four-electrode-experiments/

Dear Pouria Khanbolouki

welcome,

You have a very good I-V curve. Normally one has to plot I versus V. But this is irrelevant. Your specimen shows breakdown at voltages at -1.5 to 1.5 V. Then it has an ohmic behavior. The resistance is deltaV/deltaI= 3V/ 100 uA= 30 kohm.

Best wishes. The most important condition to measure the measure the sheet resistance using the 4 point probe is that the substrate must be insulating for staight forward assessment of the sheet resistance as rohs= 4.44 V/I= about 130 kohm per square.

The resistivity of the layer will be rohs x the thickness of the layer= 130 kx 1 nano=.013 ohm.cm which means that it has a very low resistivity or high conductivity.

Best wishes

make sure the Si substrate conductivity type is different from that for your CNT film. i.e. p-type substrate for n-type CNT.

or that the substrate is insulating as Abdelhalim Zekry has mentioned earlier.

Dear Pouria Khanbolouki ,

if it is possible to (re-)measure some impedance spectra[1,2] by (a 4-Electrodes) EIS[3,4] technique, then

you might distinguish (in frequency domain, and/or by Z-modeling) the main impedance contribution of the deposited CNT thin film from the possible (side-effect) impedance of the (Si-) substrate[1,2], or even an extreme scenario for a (as a 3rd possible Z-contribution) conducting interfacial layer[5,6].

1. Impedance Spectroscopy and Electrophysiological Imaging of Cells With a High-Density CMOS Microelectrode Array System https://www.researchgate.net/publication/328892033_Impedance_Spectroscopy_and_Electrophysiological_Imaging_of_Cells_With_a_High-Density_CMOS_Microelectrode_Array_System

2. On-Chip Electrochemical Impedance Spectroscopy for Biosensor Arrays https://www.egr.msu.edu/~mason/pubs/sensors_conf06-Chao-EIScircuit.pdf

3. Overview of EIS (Electrochemical Impedance Spectroscopy) https://www.gamry.com/application-notes/EIS/potentiostatic-eis-tutorial/ 4. Four-Electrode Experiments, in Two, Three and Four Electrode Experiments https://www.gamry.com/application-notes/instrumentation/two-three-and-four-electrode-experiments/

5. Ionic Strength Gated Redox Current Rectification by Ferrocene Grafted in Silica Nanochannels https://www.researchgate.net/publication/336587140_Ionic_Strength_Gated_Redox_Current_Rectification_by_Ferrocene_Grafted_in_Silica_Nanochannels

6. Directed Growth and Electrical‐ Transport Properties of Carbon Nanotube Architectures on Indium Tin Oxide Films on Silicon‐Based Substrates https://www.researchgate.net/publication/229782013_Directed_Growth_and_Electrical-_Transport_Properties_of_Carbon_Nanotube_Architectures_on_Indium_Tin_Oxide_Films_on_Silicon-Based_Substrates