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International audience; To enhance the performance of micro-patterned gaseous detectors (MPGDs) to meet thechallenging requirements of future high energy physics (HEP) experiments, two-dimensional (2D)materials are attractive candidates to address the back flow of positive ions, which affectsdetector performance by distorting electric field lines. In this context, graphene is promisingto work as selective filter for ion back flow suppression, being transparent to electrons while atthe same time blocking ions. Also, graphene membranes can physically separate drift andamplification regions of the detectors, offering additional flexibility in the choice of gasmixtures and allowing independent optimizations of detector sensitivity and electronmultiplication processes. Here we present an approach to integrate graphene grown via chemicalvapor deposition (CVD) on gaseous electron multiplier (GEM) prototypes via a wet transferprocedure in order to suspend graphene over thousands of holes with 60 μm diameter and overcomethe challenges encountered due to process steps involving liquids, mostly related with thecapillary effects during drying and evaporation of them. In order to overcome the risk of damagingthe membrane and decreasing the yield of suspended 2D material membranes, critical point dryer(CPD) and inverted floating method (IFM) procedures are investigated. In addition to thenecessity to cover the full holes in the active area, polymeric residuals have to be minimized inorder to evaluate the graphene transparency at the electron energies (i.e., < 15 eV) typicallyobtained in the operating conditions, measurements in these energy ranges are still not deeplyinvestigated.

The surface structure of Few-Layer Graphene (FLG) epitaxially grown on the C-face of SiC has been investigated by TM-AFM in ambient air and upon interaction with dilute aqueous solutions of bioorganic molecules (L-methionine and dimethyl sulfoxide, DMSO). Before interaction with molecular solutions, we observe nicely ordered, three-fold oriented rippled domains, with a 4.7 +/- 0.2 nm periodicity (small periodicity, SP) and a peak-to-valley distance in the range 0.1-0.2 nm. Upon mild interaction with the molecular solution, the ripple periodicity `` relaxes'' to 6.2 +/- 0.2 nm (large periodicity, LP), while the peak-to-valley height increases to 0.2-0.3 nm. When additional energy is transferred to the system through sonication in solution, graphene planes are peeled off, as shown by quantitative analysis of Raman spectroscopy and X-ray photoelectron spectroscopy which indicate a neat reduction of thickness. Upon exfoliation rippled domains are no longer observed. In comparative experiments on cleaved HOPG, we could not observe ripples on pristine samples in ambient air, while LP ripples develop upon interaction with the molecular solutions. Recent literature on similar systems is not univocal regarding the interpretation of rippling. The ensemble of our comparative observations on FLG and HOPG can be hardly rationalized solely on the basis of the surface assembly of molecules, either organic molecules coming from the solution or adventitious species. We propose to consider rippling as the manifestation of the free-energy minimization of quasi-2D layers, eventually affected by factors such as interplanar stacking, and interactions with molecules and/or with the AFM tip.

Analytical formulas are derived describing the coherent absorption of light from a realistic multilayer structure composed by an optically conducting surface on a sup- porting substrate. The model predicts two fundamental results. First, the absorption regime named coherent perfect transparency theoretically can always be reached. Second, the optical conductance of the surface can be extrapolated from absorption experimental data even when the substrate thickness is unknown. The theoretical predictions are experimentally verified by analyzing a multilayer graphene structure grown on a silicon carbide substrate. The graphene thickness estimated through the coherent absorption technique resulted in good agreement with the values obtained by two other spectroscopic techniques. Thanks to the high spatial resolution that can be reached and high sensitivity to the probed structure thickness, coherent absorp- tion spectroscopy represents an accurate and non-destructive diagnostic method for the spatial mapping of the optical properties of two-dimensional materials and of metasurfaces on a wafer scale.