Negative differential resistance in carbon-silica nanocomposites

In this work, carbon–silica nanocomposites have been elaborated by sol–gel technique. SiO2 nanoparticles were mixed with resorcinol–formaldehyde (RF) carbon precursor solution with RF/SiO2 mass ratio being 1. The samples were dried by increasing temperature from ambient to 150°C by step 10°C/day and then pyrolysis at several temperatures 675°C (RF–SiO2–675), 700°C (RF–SiO2–700) and 725°C (RF–SiO2–725) under controlled argon atmosphere. The XRD investigations carried out on these samples outline that material has amorphous phase. TEM micrograph shows that RF–SiO2–675 sample was made by homogenous nanoparticles with about 14–20 nm in diameter. Current–voltage characteristics exhibit non–linear and symmetric behaviour for all measurement temperatures between 80 and 300 K. Therefore, a Negative Differential Resistance (NDR) phase has been detected at room temperature for RF–SiO2–675 sample. However, this non–linear behaviour appears for lower measurement temperatures when the pyrolysis temperature increases. The current threshold after each the NDR phase appears is found to increase with increasing the sample temperature while the threshold voltage decreases. (Vth)1/2 decreases linearly with measurement temperature and the threshold power (Vth*Ith) follows a Gaussian behaviour. The found results reveal that the NDR in the studied nanocomposites is a consequence of Joule heating effect.