2D Materials for the Investigation of Plasma-Surface Interaction

Low-temperature plasmas are well-known for being capable of accelerating the kinetics of surface reactions, with important consequences in applications such as reactive ion-etching, atomic layer deposition, and plasma-enhanced heterogeneous catalysis. On the other hand, these systems are notoriously complex and challenging to probe experimentally. In particular, plasma-induced heating is a poorly understood phenomenon. This term is broadly utilized to describe the energy released at the surface of plasma-exposed materials, resulting from the recombination of plasma-produced charge-carriers and radicals. In this work, we address this issue by performing Raman thermometry of surfaces exposed to a low-temperature plasma jet. We choose to use graphene as a surface-specific temperature probe, enabling the measurements of the localized temperature at the plasma-exposed surface. Measurements are performed by simultaneously recording the Stokes and anti-Stokes G bands of the Raman signal of graphene, while the plasma is impinging onto the surface. The system response is carefully calibrated by using a heater in a vacuum chamber, by bringing the graphene to temperatures as high as 900 °C. When exposed to a radiofrequency (RF) argon plasma, and without any additional heating, the graphene temperature raises to approximately 350 °C even at modest plasma input power. We further analyze our experimental results using a heat transfer model, which rules out the possibility of a temperature increase due to gas heating by the plasma. In addition, we find that heating strongly depends on the plasma composition. We stress that the measurements were performed using a few layers-thick graphene sample, as initial experiments demonstrated that single-layer graphene is rapidly degraded under plasma exposure impeding accurate measurements. We find strong evidence of anisotropic etching of graphene by hydrogenation, which increases disorder in the material structure and reduces the size of graphene domains. This study highlights the potential of two-dimensional materials, particularly graphene, to quantitatively characterize plasma related phenomena which to this day remain poorly understood.[1]<br/> <br/>1. Berrospe-Rodriguez, C., et al., <i>Interaction Between a Low-Temperature Plasma and Graphene: An in situ Raman Thermometry Study.</i> Physical Review Applied, 2021. <b>15</b>(2): p. 024018.