Atomically Precise Chemical Modification of Graphene

Graphene, a one-atom-thick planar sheet of carbon atoms in a honeycomb lattice, has attracted substantial attention for its superlative electronic, thermal and mechanical properties.  A wide range of transformative applications, including electronic devices, sensors, and composite materials, have been anticipated for graphene.  However, as has been shown in other materials systems, the emerging applications for graphene can be dramatically enhanced by creating chemically modified variants of the parent material.

Under the support of the W. M. Keck Foundation, researchers at Northwestern University led by principal investigator, Professor Mark Hersam, have explored chemical modification of graphene with the goal of establishing new classes of two-dimensional nanomaterials with tailored chemical, electrical and optical properties.  Rather than optimizing properties empirically, Hersam employs ultrahigh vacuum (UHV) scanning tunneling microscopy (STM) to characterize and understand chemical modification of graphene at the atomic scale.  These detailed studies have developed fundamental principles that are guiding future efforts to exploit chemically modified graphene in numerous societally pervasive applications, such as information technology, biotechnology, and renewable energy.


Figure 1. (left) UHV STM image of individual oxygen atoms covalently bonded to graphene. (right) UHV STM image of 10,12-pentacosadiynoic acid molecular chains noncovalently assembled on graphene.

Figure 1 illustrates two examples of atomically precise chemical modification of graphene.  In the first case, inorganic covalent functionalization of graphene was achieved with atomic oxygen in UHV.  Specifically, atomic oxygen deposition in UHV results in chemically homogeneous and fully reversible oxidization of graphene.  The structure of the UHV-oxidized graphene was studied at the atomic scale with STM and density functional theory, confirming uniform epoxide functionalization.  The resulting graphene epoxide modulates the electronic and chemical properties of graphene, which enables the templated growth of other materials such as metal oxide nanoparticles.

In the second case, highly ordered, one-dimensionally anisotropic monolayers were noncovalently assembled on graphene using 10,12 pentacosadiynoic acid (PCDA).  Molecular resolution UHV STM images confirm the one-dimensional ordering of the as-deposited PCDA monolayer.  By spatially tailoring chemical reactivity, these PCDA monolayers allow for the templated growth of metal oxide nanowires via atomic layer deposition.  The resulting nanowires possess desirable electronic and optical properties in addition to serving as a chemically resistant mask for the fabrication of graphene nanoribbons.

This highly interdisciplinary work has garnered several honors.  In particular, while being supported by the W. M. Keck Foundation, Hersam was elected to the rank of Fellow in six different professional societies: International Society for Optics and Photonics (SPIE), Materials Research Society (MRS), American Vacuum Society (AVS), American Physical Society (APS), American Association for the Advancement of Science (AAAS), and Institute of Electrical and Electronics Engineers (IEEE).  Hersam was also recognized with the ACS Arthur K. Doolittle Award in Polymeric Materials Science and Engineering and a MacArthur Fellowship for his research on chemically modified graphene and related nanomaterials.

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