16 Topological Mechanochemistry of Graphene
291
tricotage sheet when either the sheet rupture has both commenced and completed by
the rupture of a single stitch row (armchair mode) or the rupture of one stitch is ‘tugging at thread’ the other stitches that are replaced by still elongated one-atom chain
of the carbon atoms (zigzag mode). In the former case, the deformation is one-stage
and is terminated on the 17th step of the deformation. In contrast, the deformational
mode zigzag is multi-stage and consists of 250 consequent steps with elongation of
0.1 Å at each step [29, 30]. The formation of the one-atom chain under zigzag-mode
tension of the naked graphene piece has been supported experimentally [39].
Quite unexpectedly, the character of the deformation has occurred to be strongly
dependent on the chemical situation at the molecule edges. As seen in Fig. 16.2b,
the addition of one hydrogen atom to each of the molecule edge atoms does not
change the general character of the deformation: it remains a tricotage-like one so
that there is still a large difference between the behavior of zigzag and armchair
modes. At the same time, the number of the deformation steps of the zigzag mode
reduces to 125.
Even more drastic changes for this mode are caused by the addition of the second
hydrogen atoms to the edge ones (Fig. 16.2c). Still, the armchair mode is quite
conservative while the zigzag one becomes practically identical to the former. The
tricotage-like character of the deformation is completely lost and the rupture occurs
at the 20th step.
Figure 16.3 presents a set of the ‘stress-strain’ relations that fairly well highlight the difference in the mechanical behavior of all the three molecules. Table 16.1
presents the Young modules that were defined in the region of the elastic deformation. As seen from the table, the Young modules depend on the character of the
edge atom chemical modification. As shown in [31], elastic properties of extended
molecules such as polymers [35, 40] and nanographenes [31] are determined by
dynamic characteristics of the objects, namely, by force constants of the related
vibrations. Since benzenoid units provide the determining resistance to any deformation of the graphene molecules, the dynamic parameters of the stretching C–C
vibrations of the units are mainly responsible in the case of the uniaxial tension.
Changing in Young’s modules means changing in the force constants (and, consequently, frequencies) of these vibrations. The latter are attributed to the G-band of
graphene that lays the foundation of a mandatory testing of any graphenium system by the Raman spectroscopy. In numerous cases, the relevant band is quite wide
which might indicate the chemical modification of the edge zone of the graphene
objects under investigation.
Since the deformation-induced molecule distortion mainly concerns the basal
atoms, so drastic changes in the deformation behavior points to a significant influence of the chemical state of the edge atoms on the electronic properties in the basal
plane. The observed phenomenon can be understood if suggest that (1) the deformation and rupture of the molecule are a collective event that involves the electron
system of the molecule as a whole; (2) the electron system of the graphene molecule
is highly delocalized due to extreme correlation of the odd electrons; and (3) the
electrons correlation is topologically sensitive due to which the chemical termination of the edge atoms so strongly influences the behavior of the entire molecule.
The latter has turned out to be the reality, indeed.
291
tricotage sheet when either the sheet rupture has both commenced and completed by
the rupture of a single stitch row (armchair mode) or the rupture of one stitch is ‘tugging at thread’ the other stitches that are replaced by still elongated one-atom chain
of the carbon atoms (zigzag mode). In the former case, the deformation is one-stage
and is terminated on the 17th step of the deformation. In contrast, the deformational
mode zigzag is multi-stage and consists of 250 consequent steps with elongation of
0.1 Å at each step [29, 30]. The formation of the one-atom chain under zigzag-mode
tension of the naked graphene piece has been supported experimentally [39].
Quite unexpectedly, the character of the deformation has occurred to be strongly
dependent on the chemical situation at the molecule edges. As seen in Fig. 16.2b,
the addition of one hydrogen atom to each of the molecule edge atoms does not
change the general character of the deformation: it remains a tricotage-like one so
that there is still a large difference between the behavior of zigzag and armchair
modes. At the same time, the number of the deformation steps of the zigzag mode
reduces to 125.
Even more drastic changes for this mode are caused by the addition of the second
hydrogen atoms to the edge ones (Fig. 16.2c). Still, the armchair mode is quite
conservative while the zigzag one becomes practically identical to the former. The
tricotage-like character of the deformation is completely lost and the rupture occurs
at the 20th step.
Figure 16.3 presents a set of the ‘stress-strain’ relations that fairly well highlight the difference in the mechanical behavior of all the three molecules. Table 16.1
presents the Young modules that were defined in the region of the elastic deformation. As seen from the table, the Young modules depend on the character of the
edge atom chemical modification. As shown in [31], elastic properties of extended
molecules such as polymers [35, 40] and nanographenes [31] are determined by
dynamic characteristics of the objects, namely, by force constants of the related
vibrations. Since benzenoid units provide the determining resistance to any deformation of the graphene molecules, the dynamic parameters of the stretching C–C
vibrations of the units are mainly responsible in the case of the uniaxial tension.
Changing in Young’s modules means changing in the force constants (and, consequently, frequencies) of these vibrations. The latter are attributed to the G-band of
graphene that lays the foundation of a mandatory testing of any graphenium system by the Raman spectroscopy. In numerous cases, the relevant band is quite wide
which might indicate the chemical modification of the edge zone of the graphene
objects under investigation.
Since the deformation-induced molecule distortion mainly concerns the basal
atoms, so drastic changes in the deformation behavior points to a significant influence of the chemical state of the edge atoms on the electronic properties in the basal
plane. The observed phenomenon can be understood if suggest that (1) the deformation and rupture of the molecule are a collective event that involves the electron
system of the molecule as a whole; (2) the electron system of the graphene molecule
is highly delocalized due to extreme correlation of the odd electrons; and (3) the
electrons correlation is topologically sensitive due to which the chemical termination of the edge atoms so strongly influences the behavior of the entire molecule.
The latter has turned out to be the reality, indeed.