Gilded Graphene

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graphene, fluorescence microscopy

Lianfeng Sun/NCNT

Color CodedIn this SEM image, a thin gold coat helps distinguish a four-layer-thick region of graphene (left) from bare silica (middle) and a bilayer region (right).

Graphene films between one and four atomic layers thick can be distinguished by coating them with a layer of gold. According to a new study, the gold layer adopts a unique appearance based on the number of graphene layers (J. Am. Chem. Soc., DOI: 10.1021/ja909228n).

Graphene’s exceptional electronic, optical, and mechanical properties have recently focused attention on the sheet of carbon as little as one atom thick. Yet advances in this area have been hampered by the small number of microscopy and spectroscopy techniques capable of “seeing” graphene and distinguishing between samples of various thicknesses.

The study, by Lianfeng Sun of the National Center for Nanoscience & Technology, in Beijing, and coworkers, adds to a short list of recently developed techniques for enhancing contrast between graphene films of various thicknesses and between the carbon films and the solid surfaces that typically support them.

Sun and coworkers used Raman microspectroscopy to benchmark the number of atomic layers in graphene samples and mapped out regions of differing thickness within a single sample. Then they evaporated gold onto the samples. They found that they could use scanning electron microscopy to recognize differences in the morphology, grain size, and general appearance of the gold films and that those differences depend directly on the number of underlying graphene layers. The SEM analysis can be done faster and with higher spatial resolution than the Raman analysis, they say.

“This work reveals an intriguing layer-dependent surface property of graphene,” says Jiaxing Huang, a materials science professor at Northwestern University. That property may play a diagnostic role in future hybrid materials and electronic devices built from metal-graphene composites, he adds.

Huang’s group just developed an alternative method for graphene imaging that exploits graphene’s knack for quenching fluorescence in nearby dye molecules. Treating a solid that supports numerous graphene samples with fluorescein (which can be removed after analysis) causes the carbon films to appear dark, in strong contrast to bare regions of the support, which fluoresce brightly. The technique can distinguish between films of various thicknesses and works on films suspended in solution, they report (J. Am. Chem. Soc. 2010, 132, 260).

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