I am using a two probe method to measure the resistance of my graphene nanocomposites. I have observed that at concentrations around 1wt% the resistance does not depend on the area of the sample! Actually it seems that the current finds the least resistant path in the sample (like a short circuit).
For example for a sample that shows a resistance of almost 100 kilo ohms, if I cut the sample in half and measure the resistance again, one piece shows even lower resistance (80 kilo ohms) but resistance of the other one is around 6 orders of magnitude higher!
Also some samples with a very poor dispersion of graphene may show a very low resistance!
In these cases, how should I report the conductivity as it does not depend on the area of the sample and the presence of such low resistance paths determines the overall resistance of the sample?
Dear Catherine, two probe method (especially if you use only point or needle electrode) can be used only to assess the order of magnitude in conductivity. If you want to have reportable quantities you should use three electrode system (with guard electrode) for higher resistivity samples of the four electrode arrangement with needle electodes at ower resitivities. You have to evaporate or paint electrodes in the first case. In each case it is advisable to use a range of voltages for the measurements, as the resistivity is usually nonlinear.
Dear Catherine, two probe method (especially if you use only point or needle electrode) can be used only to assess the order of magnitude in conductivity. If you want to have reportable quantities you should use three electrode system (with guard electrode) for higher resistivity samples of the four electrode arrangement with needle electodes at ower resitivities. You have to evaporate or paint electrodes in the first case. In each case it is advisable to use a range of voltages for the measurements, as the resistivity is usually nonlinear.
Two-probe resistance measurements can give you very inconsistent answers due to variable contact resistance of the contacts/probes. You should utilize a standard 4-probe resistance measurement topology in order to compare and contrast results across different samples. What you measure in a 4-probe geometry will be the raw resistance, which can be converted into a sheet resistance that is independent of sample geometry.
Dear Dr. Banhegy and Dr. Trivedi,
Thank you so much for your answers. 4-probe method measures the sheet resistance and then calculates volume resistivity based on sheet resistance. In this method we assume that the sample is completely homogeneous so the problem of heterogeneity of graphene nanocomposites still exists in this method! I measured sheet resistance of my samples but the results were very different from those of 2-probe method.
For example, some samples showed a sheet resistance of around 10^5 ohm/square but almost non-conductive in 2-probe method!
Do you think the problem of low resistance paths (something like short circuits) is resolved in 3-probe method?
In order to investigate the conductivity of the heterogeneous polymer/nanorod composites, we discretize Laplace’s equation for a static current distribution onto a square lattice
Dear Catherine, could you provide the dimensions (sample thickness, diameter and distance of the electrodes, penetration of the electrodes) of your 2 and 4 probe tests? How did you establish possibly perfect contact between your electrodes and the test material? Is it not possible to perform classical three-electrode tests (as in dielectric test) on larger sheets with evaporated electode? In the three electrode arrangement it is also possible to measure the surface resistance between the central electrode and the guard ring.
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