Nanotopography of graphene

Microscopically clean graphene surfaces are scrutinized via atomic resolution high angle dark field imaging and electron energy loss spectroscopy for presence of adsorbed atoms. Carbon ad-atoms can be observed in dark field images, whereas hydrogen is revealed in electron loss spectra. Spectrum images show the spatial distribution of hydrogen in pristine and deliberately H-dosed graphene; hydrogen is found on all graphene surfaces but the coverage is higher and more uniform in dosed samples. On clean surfaces the energy loss is characteristic of the ground level excitation in atomic hydrogen, whereas a spread in the hydrogen energy levels is found in hydrocarbon deposits. A rippling effect in the graphene sheets is revealed when lattice images are processed using fast Fourier transform (FFT) techniques. This contrast effect originates from changes in the bond length projection, due to inclinations of the sheet. It is suggested that point defects - vacancies and ad-atomes - influence the ripple patterns.