offering the possibility of relatively inexpensive, large scale production is a
re-activated old method of graphite exfoliation [9, 10], which involves oxidation
of graphite by harsh methods and the thermal or chemical reduction of the resulting
graphene oxide sheets after dispersion and processing. We shall return to such
“top-down” methods in Sect. 8, but we can anticipate that they will often lack
structural perfection, reproducibility, and efficiency. Furthermore, they do not
allow precise control over the size or the periphery of the graphene sheets. When
considered from the point of view of polymer chemistry, graphene can be regarded
as a one-atom thick 2D polymer, and such a structure cries out for chemical
“bottom-up” approaches.
Before we introduce the polymer chemistry of graphene materials, however, one
additional electronic feature must be introduced: graphene as a 2D π-electron system
is a semimetal and thus characterized to have a vanishing band gap. This would not
allow on/off behavior in, for example, a field-effect transistor using graphene as the
semiconductor [11–15]. Thus, while graphene itself appears of limited use for digital
electronics, it has been predicted that a band gap could be opened by geometric
confinement such as occurs in graphene nanoribbons (GNRs) [16]. Therefore, a
number of further top-down methods have been applied such as “unzipping” of
carbon nanotubes [17] or lithography of graphene [18]. Again, from the point of
view of polymer synthesis, the resulting nanoribbons still lack structural precision,
particularly when looking at the edges of the GNRs. This lack of control is especially
important in regards to the widths of GNRs, as it is essential to reach nanometer
length scales in order to introduce a band gap.
The present text will focus on concepts towards establishing a reliable polymer
chemistry approach for graphenes and graphene nanoribbons, with precise control
over their size, edges, and chemical functionalities. Size is critical because electronic
transport measurements, for example, require the sheets to form efficient contacts
between electrodes. Exfoliated graphene flakes [19] extend well into the micrometer
length scale, while graphene monolayers made by CVD cover areas of more than
30 Â 30 inches (76 Â 76 cm) [7] and, after etching of the substrate, can be processed
by roll-to-roll printing [20, 21].
2 Expanding Synthetic Chemistry to Complex
Macromolecules
At this point, the reader may ask, is the title just an attempt to follow current
fashion, or is graphene a true target for polymer synthesis? This then may provoke
the more general question, how does polymer synthesis position itself between
classical organic synthesis and other physical methods of material fabrication such
as, for example, exfoliation of graphite? Classical organic chemistry takes pride in
the complexity of the structures, often natural products, produced. It employs
parameters such as reducing the number of synthetic steps (e.g., by using elegant
Graphene as a Target for Polymer Synthesis
63
re-activated old method of graphite exfoliation [9, 10], which involves oxidation
of graphite by harsh methods and the thermal or chemical reduction of the resulting
graphene oxide sheets after dispersion and processing. We shall return to such
“top-down” methods in Sect. 8, but we can anticipate that they will often lack
structural perfection, reproducibility, and efficiency. Furthermore, they do not
allow precise control over the size or the periphery of the graphene sheets. When
considered from the point of view of polymer chemistry, graphene can be regarded
as a one-atom thick 2D polymer, and such a structure cries out for chemical
“bottom-up” approaches.
Before we introduce the polymer chemistry of graphene materials, however, one
additional electronic feature must be introduced: graphene as a 2D π-electron system
is a semimetal and thus characterized to have a vanishing band gap. This would not
allow on/off behavior in, for example, a field-effect transistor using graphene as the
semiconductor [11–15]. Thus, while graphene itself appears of limited use for digital
electronics, it has been predicted that a band gap could be opened by geometric
confinement such as occurs in graphene nanoribbons (GNRs) [16]. Therefore, a
number of further top-down methods have been applied such as “unzipping” of
carbon nanotubes [17] or lithography of graphene [18]. Again, from the point of
view of polymer synthesis, the resulting nanoribbons still lack structural precision,
particularly when looking at the edges of the GNRs. This lack of control is especially
important in regards to the widths of GNRs, as it is essential to reach nanometer
length scales in order to introduce a band gap.
The present text will focus on concepts towards establishing a reliable polymer
chemistry approach for graphenes and graphene nanoribbons, with precise control
over their size, edges, and chemical functionalities. Size is critical because electronic
transport measurements, for example, require the sheets to form efficient contacts
between electrodes. Exfoliated graphene flakes [19] extend well into the micrometer
length scale, while graphene monolayers made by CVD cover areas of more than
30 Â 30 inches (76 Â 76 cm) [7] and, after etching of the substrate, can be processed
by roll-to-roll printing [20, 21].
2 Expanding Synthetic Chemistry to Complex
Macromolecules
At this point, the reader may ask, is the title just an attempt to follow current
fashion, or is graphene a true target for polymer synthesis? This then may provoke
the more general question, how does polymer synthesis position itself between
classical organic synthesis and other physical methods of material fabrication such
as, for example, exfoliation of graphite? Classical organic chemistry takes pride in
the complexity of the structures, often natural products, produced. It employs
parameters such as reducing the number of synthetic steps (e.g., by using elegant
Graphene as a Target for Polymer Synthesis
63