Resolution High-angle Dark Field Research Articles

Metal−Graphene Interaction Studied via Atomic Resolution Scanning Transmission Electron Microscopy

Distributions and atomic sites of transition metals and gold on suspended graphene were investigated via high-resolution scanning transmission electron microscopy, especially using atomic resolution high angle dark field imaging. All metals, albeit as singular atoms or atom aggregates, reside in the omni-present hydrocarbon surface contamination; they do not form continuous films, but clusters or nanocrystals. No interaction was found between Au atoms and clean single-layer graphene surfaces, i.e., no Au atoms are retained on such surfaces. Au and also Fe atoms do, however, bond to clean few-layer graphene surfaces, where they assume T and B sites, respectively. Cr atoms were found to interact more strongly with clean monolayer graphene, they are possibly incorporated at graphene lattice imperfections and have been observed to catalyze dissociation of C-C bonds. This behavior might explain the observed high frequency of Cr-cluster nucleation, and the usefulness as wetting layer, for depositing electrical contacts on graphene.

Nanotopography of graphene

AbstractMicroscopically 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.