The film of overlapping graphene sheets was then formed by placing a glass slide in a vortexed mixture of graphite, water, and oil. The exfoliated graphene climbed the glass slide, separating the heptane vapor from the adsorbed water on the glass slide, creating an overlapping film of graphene sheets that was transparent and electrically conductive. The success of this approach is illustrated in Figure by the Raman spectrum of a graphene film created by graphene climbing a glass slide after vortex mixing. Because the film also contained occasional unexfoliated graphite flakes, the Raman spectra showed a combination of graphene and graphite. The broadening at lower wavenumbers of the film peak relative to the graphite flake peak indicated that a significant portion of the sample was exfoliated to some degree.