Graphene-coated microballs for a hyper-sensitive vacuum sensor

Reduced graphene oxide (RGO)-coated microballs of poly (methyl methacrylate) (PMMA) used for fabricating three-dimensional sensor (3D sensor), which are expected to exhibit high sensitivity compared with conventional two-dimensional (2D) sensors, were prepared using a reaction-based assembly process. The sheet resistance and transmittance of the RGO-coated balls decreased with increasing number of coatings, implying that the RGO was well adhered to the ball by the assembly method. Two types of vacuum pressure sensors using multiple balls and a single ball were fabricated using lift-off and air-blowing methods, respectively. At pressures <1 torr, the sensors showed an increased resistance value due to the bending of graphene sheets by the Van der Waals attractive force. Further, the pressure versus resistance values at the logarithmic scale showed a linear relation, with a pressure reading error <6%. Compared with the 2D sensor fabricated using RGO, the multiball sensor exhibited almost 4–5 times higher RRC value. The single-ball sensor showed reasonable reproducibility at various temperatures. Given the size and pressure reading range of the sensor, the sensitivity of the single-ball sensor at 100 °C was approximately 6,000 times greater than that of the sensor with the highest sensitivity reported in the literature. The increase in surface area and the geometric effect of the sensing part of the single-ball sensor appeared to be responsible for its abnormally high sensitivity.