The Researchers synthesized a mixture of graphene nanoribbons (of varying size and chemical composition) from a combination of multi-walled carbon nanotubes, sulphuric acid and phosphoric acid in a chemical exfoliation approach. The result was a mixture of several graphene nanoribbons which were separated and purified ready for device implementation and testing. The Researchers also synthesized graphene oxide through a modified Hummers’ method for use as a reference material.
The different devices were fabricated using the different graphene nanoribbon sheets and one was fabricated with graphene oxide for comparison. The graphene nanoribbons were solubilzed using hydrazine and inserted into a hole placed within an indium tin oxide (ITO) electrode. The ITO-graphene sensor electrode was then coupled to a four-point probe in a vacuum chamber. The vacuum pressure was regulated through a calibrated capacitor gauge, and the sheet resistance and activities of the graphene nanoribbons within the sensor devices were measured throughout pressure leakages and elevations.
The sheet resistance of the sensor devices decreased with a decreasing pressure, until the pressure reached between 1 and 10 Torr. A reduction in the pressure beyond this resulted in a change in the electrical behavior of the graphene sheet(s), and the specific changes were largely dependent upon the temperature of the surrounding environment. At temperatures around 30 °C, the sheet resistance was also found to decrease, whereas at temperatures around 100 °C, the sheet resistance was found to increase.
The changes in the sheet resistance with respect to atmospheric temperature were also explained in the paper by a hypothesis based on van der Waals attraction chemistry. The hypothesis states that the local effect of van der Waals forces created a shorter distance between the graphene sheets due to the attractive force of the carbon clusters in the sheets possessing a smaller value than the sum of their vibrational and elastic forces.
The shorter sheet distance is thought to be directly responsible for the decrease in the sheet resistance, and was verified experimentally in their XRD analyzes. On the other hand, an increase in the sheet resistance was also found to be a direct result of the shortening of the local distance of carbon clusters.