Purification of graphene flakes by using radio-frequency thermal ar plasma

In this study, graphene flakes (GFs) were treated with a radio-frequency thermal Ar plasmas, and the effect of the plasma treatment on the change of GF properties such as impuries, structural defects and morphology, and electrical properties was investigated. As the plasma treatment, two different rf powers of 27 kW and 17 kW were used and, as the GF feeding position, the up-state and the down-state relative to the plasma area were used. When the GFs were treated with the higher rf power and up-state, the impurities such as oxygen and sulfur in GFs were completely removed after one plasma treatment time while, when the GFs were treated with the lower rf power and down-state, the longer plasma treatment time of about three times was required to remove all the impurities. However, the higher rf power of 27 kW and the use of up-state increased the defects on the graphene surface possibly due to the more damage on the graphene surface by high temperature Ar plasma. By using the lower rf power of 17 kW and down-state, the removal of the impurities in GFs while decreasing (or, at least, without increasing) the structural defects on the graphene surface could be obtained. When the sheet resistance of the GFs treated with down-state 17 kW was measured, the decrease of sheet resistance of about 50% compared with that of the raw GFs could be observed while the GFs treated with other conditions of the higher rf power and up-state showed higher sheet resistances.