Many types of sensor now use graphene as the response material and has become a hot area of graphene research. Despite not being the most obvious use of graphene to outsiders, it is an area in which a lot of research is being undertaken and commercialized.
A new addition to the graphene sensor family has now made its presence known in the academic world because a team of Researchers from South Korea have created a graphene nanoribbon sensor which can measure high vacuum pressures.
There are many sensors nowadays coming out of academia which use graphene, at least in part, to detect an environmental or invoked response. Many sensors have gained interest from industry, and as a result, new graphene-based sensors are appearing all of the time. To date, the most common sensors using graphene are humidity, temperature, gas and pressure sensors.
The area of graphene pressure sensors is a growing area within itself and already consists of MEMS piezoresistive pressure sensors, field emission pressure sensors and squeeze-film pressure sensors, to name a few.
The area of graphene pressure sensors has exploded in recent years and has been brought on by the high flexibility exhibited by graphene and the easily measurable response from the sheet to the detector.
A new type of graphene-based pressure sensor has been realized by the Korean Researchers and uses graphene nanoribbons to detect changes in high vacuum pressures.
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 solubilsed 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 Researchers also characterized the graphene nanoribbon materials using a combination of X-ray photoelectron spectroscopy (XPS, VG Scientifc) transmission electron microscopy (TEM, JEOL, JEM-2200FS), Fourier transform infrared (FTIR) spectroscopy (Shimadzu, IRTracer-100), Raman spectroscopy (Horiba, LabRam HR) and X-ray diffraction (XRD, Rigaku, D/MAX 2500).
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.
The response of the graphene sensor to changes in pressure within the high vacuum was rapid and possessed a response of the order of a few seconds. The graphene nanoribbons reduced the response time (compared to the reference) by providing a shorter diffusion path for the detectable gas molecules.
The sensitivity of the sensor was also found to be three times greater than the device containing reduced graphene oxide, as well as other common pressure sensors. The sensor could also detect changes in vacuum pressure up to 8x10-7 Torr.
The sensor is also expected to be able to read high vacuum pressures and possess a high durability to pressure shocks and now offers a new way of measuring changes in pressure and joins the family of graphene pressure sensors, whilst opening a new area of potential sensor research.
“Ultra-sensitive graphene sensor for measuring high vacuum pressure”- Ahn S. I., et al, Scientific Reports, 2017, DOI:10.1038/s41598-017-13038-3
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