Ultrafast electronic response of graphene to a strong and localized electric field

A challenging part of the experiment is the preparation of a macroscopic, freestanding single layer of high quality graphene. The standard approach is to transfer graphene, grown on a copper foil via chemical vapour deposition (CVD), onto a grid used for transmission electron microscopy (TEM) with the help of a thin layer of polymethylmethacrylat (PMMA) for mechanical stabilization 1. Later, the PMMA layer has to be removed and this always constitutes a source of severe contamination 2. The use of PMMA is therefore avoided entirely, and instead the sample preparation procedure from 3 is adapted. A 300-mesh gold TEM-Grid with a quantifoil (regular array of 1.2 µm holes) on top was used as the graphene carrier. Cross-check using highly charged ion transmission experiments with bare (i.e. without graphene coverage) 300-mesh gold quantifoil TEM grids confirmed that ions were fully neutralized in the Cu/Au grid bars as well as the tens of nm thick quantifoil, or were transmitted unhindered through the hole arrays. Thus any measured signal from the TEM grids with graphene spanning over the grid holes exclusively probes the intrinsic nature of the suspended graphene membrane. The grid was placed onto commercially available CVD graphene (Graphenea). Isopropyl alcohol was used to increase the adhesion between the quantifoil and the graphene layer. To remove the copper foil, the stack was placed onto the surface of an ammonium persulphate solution. After approximately 24 hours the etchant was substituted by deionized water and the TEM-Grid was carefully lifted out of the fluid. To remove the contaminations introduced during this procedure, we embedded the transferred graphene on TEM grid into active carbon within a ceramic dish and then heated it in an oven. The temperature of the samples was increased with a heating rate of ≈ 300 • C h −1 to a maximum temperature of 260 • C. This temperature was held for 30 min, and then the sample was let to cool down. Reaching ≈ 70 • C, we took the sample out of the active carbon. Finally, a smooth nitrogen stream was used to remove the remaining active carbon from the grid. The quality of the transferred graphene was checked by Raman spectroscopy. A typical Raman spectrum is shown in Supplementary Figure. 1a. The FWHM of the 2D mode peak provides the number of layers of the graphene sample. The inspected graphene samples show a FWHM of the 2D peak …

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