BACKGROUND
Graphene has emerged as a promising ultra-thin replacement for traditional carbon (graphite) foils currently used to characterize the plasma environment in space. Performance improvements from graphene can lead to savings in precious resources, such as instrument mass, and it can help reduce high voltages and thus reduce risk. A key challenge to producing the graphene foils is the transfer from the copper substrate onto which single layer graphene (SLG) is grown to the nickel grid that will support the graphene foil. The primary objective of this research was to develop the graphene transfer method in-house to reliably produce graphene foils for space plasma instruments.
APPROACH
The first step is to spin coat poly(methyl methacrylate) or PMMA polymer over the SLG which provides mechanical support throughout the stacking and transfer process. Then the copper substrate underneath the graphene is etched by placing the copper on the air/liquid interface of the etchant. After etching, the PMMA-SLG film is lifted out of the etchant using a clean glass slide. After cleaning, the PMMA-SLG film is deposited onto another copper-SLG sample. The process is repeated until there are 2 or 3 layers of graphene, and then transferred onto a nickel support grid. The last step is to dissolve/remove the PMMA layer from the graphene/grid structure. We evaluate the performance of the foils by quantifying the coverage and uniformity of the foils by measuring light transmission through the foil. We also evaluate their thickness by measuring the angular scattering of protons passing through them.
ACCOMPLISHMENTS
At the end of this project, we produced at least thirteen graphene foils, made of two or three stacked single layer graphene. We defined three criteria to evaluate the quality of the foils: 1) graphene coverage of greater than 95%, 2) graphene foil thickness lower or equal to thinnest practical carbon foils, and 3) PMMA residues coverage of less than 5% of the foil area. Ten out of thirteen met the coverage criterion and all of them met the PMMA residue criterion, and the initial thickness measurements showed that the graphene foils were less than half the thickness of the regular carbon foils. SwRI now has the capability to produce high quality graphene foils for space plasma instrument.