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Is graphene's future in the hands of the chemist?
Volume 14, Issue 10, Page 454 | James M. Tour
Courtesy of D. Kosynkin, Rice University.
The disclosure of graphene's superb properties has been credited to physicists A. Geim and K. Novoselov in their being awarded the 2010 Nobel Prize in Physics, and indeed those two researchers' images are indelibly part of the seminal graphene research. Geim and Novoselov's initial influential work [1] ignited a firestorm of activity among physicists and to a lesser extent materials scientists, although the latter group, notably through the work of R. Ruoff [2], made monolayer graphene far more accessible to the masses. Physicists were primarily the ones to record the amazing electronic, optical, and mechanical properties of the newly manipulated pseudo-2D material while predicting and explaining its behavior to the world, and they provided the underpinning through a series of cleverly designed structures and devices that launched graphene research on a trajectory that has few parallels in modern science. The scientific community owes the worldwide team of physicists gratitude for their passion that rapidly accelerated the flames of graphene research.
During a visit to Columbia University, a graphene research powerhouse, in the spring of 2011, Philip Kim, one of the founding-father-physicists of graphene research, said to me, "The future of graphene belongs to the chemist." As a synthetic chemist, I was flattered but also puzzled to hear that comment. I asked Philip to explain and he said that much of the physics has been done and understood, so chemists will be taking it from here. I had long admired the work of Philip and other physicists who had painstakingly fabricated graphene devices, suspending graphene in space, for example, studying its surface-unperturbed carrier mobility or its impervious balloon-like structure, and being able to constrain even the smallest gas atoms. Upon my return to Rice, I asked my physicist colleague, Doug Natelson, what he thought of Philip's comment. Interestingly, Doug agreed, citing much the same argument as did Philip: the basic physics is, for the most part, understood.
Feeling rather good about my own research field at that point, I began to reflect upon the more recent advances of chemists into the world of graphene. Indeed, chemists, by training and track record, are the pioneers in molecular structure synthesis, manipulation, and conversion. The ability to change one molecule type into another type, en masse (i.e., 1023 structures at a time), is a specialty predominantly reserved for the synthetic chemist.
Viewing graphene as a single molecule, though an exceedingly large molecule, could indeed lend itself to chemical manipulations that seemed obvious: an extended structure of fused benzenoid rings with their rich extended π-conjugation available to straightforward electrophilic and nucleophilic reaction chemistries that exploit the higher reactivity of the long-range π-structure. Accordingly, chemists began to burst on the scene, albeit late in comparison to the army of physicists, to synthesize, manipulate, modify, and apply graphene in ways that physicists and many materials scientists would never imagine.
For example, in came the chemists with methods to build graphene from the bottom up, not using a CVD process at 1000 °C, but through sequential ring-formation processes toward extended benzenoid sheet structures, or the remarkable assembly of brominated aromatics on a surface to build edge-precise nanoribbons [3]. There were other syntheses, not one molecule at a time, but in bulk, moles at a time, of graphene nanoribbons using the newly termed reaction of "unzipping" carbon nanotubes exploiting precise reaction chemistries that specifically cleave the strained carbon-carbon bonds (see Fig.), or metal-induced splitting of carbon nanotubes to nanoribbons through reductive insertion reactions. Furthermore, the functionalization of graphene using, for example, diazonium-based aryl radical additions became commonplace and it was further shown that the functionalization could be relegated predominantly to the near-edge domains of the graphene sheet.
Lately, chemists have been forging ahead using chemistry to open bandgaps in the zero-bandgap structure of graphene; to generate graphene devices that are superior in terms of conductivity and transparency to indium-tin-oxide, yet suitable for flexible substrates as well as glass; for spray-on applications providing access to large-area transparent thin-film electrodes; for precise layer-by-layer stripping of multilayer graphene using chemical techniques that amount to single-atom-resolution lithography in the vertical dimension, and so on.
The foundational advances of graphene undoubtedly came from the physics and materials communities, and though some might suggest that the future of graphene belongs to the chemist, I think that graphene is too big a molecule for such a constraining view. The future for graphene is indeed rich, but then again maybe the greatest wealth will be attained by chemists.
[1] K. Novoselov et al. Science, 306 (2004), p. 666
[2] X. Lie et al. Science, 324 (2009), p.1312
[3] J. Cai et al. Nature, 466 (2010), p. 4706
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