7.1 Introduction
145
1. GNRs and CNTs are mechanically stronger yet chemically and thermally less
stable. CNTs exhibit extremely high strength yet relatively lower chemical and
thermal stability compared to their bulk counterparts. Compared with the bulk
value of 1.05 TPa, the elastic modulus of the SWCNT was measured to vary from
0.5 to 5.5 TPa depending on the presumption of the wall thickness of the CNT
[62–69]. The Young’s modulus of the multi-walled CNTs (MWCNTs) drops
with the inverse wall thickness and it is less sensitive to the outermost radius of
the MWCNTs if the wall thickness remains unchanged [70, 71]. Atoms in the
open edge of a SWCNT coalesce at 1593 K [72] and a ~280% extensibility of the
CNT occurs at ~2000 K [73]. Under the flash of an ordinary camera, the SWCNT
burns under the ambient conditions [74]. Generally, for bulk materials, the elastic
modulus is always proportional to their melting points. The mechanism behind
the paradox of elastic enhancement and T m suppression of the CNTs is still a
puzzle.
2. The wall thickness, the C–C bond length and energy, and the role of atomic undercoordination remain challenge. The wall thickness and the Young’s modulus of
the C–C bond in the SWCNTs are correlated, which leads to the uncertainty in
both quantities. Although atoms that surround defects or are located at the tip
ends or at the surface are expected to play some unusual, yet unclear, roles in
dominating the mechanical and thermal properties of CNTs and GNRs. A consistent insight into the mechanism behind the fascinations from the perspective
of atomic under-coordination is necessary.
3. Mechanisms for the metallic and magnetic ZGNR and the semiconductive AGNR
remain unclear. Compared with the LGS or CNTs, the ZGNRs possess stronglylocalized edge states [75] with magnetic and metallic nature, whereas the AGNRs
have larger band gap (E G ) with semiconductive nature. The E G is roughly proportional to the inverse width of the GNRs [76–78]. However, discrepancy remains
between the theory and the experimental derived E G of GNR. Measurements
[77] and theoretical calculations [51, 78, 79] showed less consistent in the band
gap opening of GNR. Mechanisms remain yet unclear regarding the generation
of the localized edge states and the expansion trends of the E G in spite of the
possible mechanisms such as doping [80], defects forming [51, 81, 82], symmetry breaking [83], substrate interaction [84], edge distortion [76], strain [85],
quantum confinement [48], and the staggered sublattice potentials modulation
[86].
4. Mechanism for the edge selective generation and hydrogen annihilation of the
Dirac fermions remains opening. The Dirac-Fermi polaritons [44, 45] generate
at sites surrounding atomic vacancies [43, 87], the edges of monolayer graphite
terrace and the ZGNRs [46–49], other than edges of the AGNR or the rec-AGNR.
It remains unclear why the edge and site discriminate the generation of Dirac
fermions.
5. Origin of and correlation between the positive C 1s core level shift of the GNR
edge, GNR interior and the associated work function reduction with the number of GNR layers are ambiguous. Three XPS C 1s components have been
resolved from graphene flakes produce [88], corresponding, respectively, from
145
1. GNRs and CNTs are mechanically stronger yet chemically and thermally less
stable. CNTs exhibit extremely high strength yet relatively lower chemical and
thermal stability compared to their bulk counterparts. Compared with the bulk
value of 1.05 TPa, the elastic modulus of the SWCNT was measured to vary from
0.5 to 5.5 TPa depending on the presumption of the wall thickness of the CNT
[62–69]. The Young’s modulus of the multi-walled CNTs (MWCNTs) drops
with the inverse wall thickness and it is less sensitive to the outermost radius of
the MWCNTs if the wall thickness remains unchanged [70, 71]. Atoms in the
open edge of a SWCNT coalesce at 1593 K [72] and a ~280% extensibility of the
CNT occurs at ~2000 K [73]. Under the flash of an ordinary camera, the SWCNT
burns under the ambient conditions [74]. Generally, for bulk materials, the elastic
modulus is always proportional to their melting points. The mechanism behind
the paradox of elastic enhancement and T m suppression of the CNTs is still a
puzzle.
2. The wall thickness, the C–C bond length and energy, and the role of atomic undercoordination remain challenge. The wall thickness and the Young’s modulus of
the C–C bond in the SWCNTs are correlated, which leads to the uncertainty in
both quantities. Although atoms that surround defects or are located at the tip
ends or at the surface are expected to play some unusual, yet unclear, roles in
dominating the mechanical and thermal properties of CNTs and GNRs. A consistent insight into the mechanism behind the fascinations from the perspective
of atomic under-coordination is necessary.
3. Mechanisms for the metallic and magnetic ZGNR and the semiconductive AGNR
remain unclear. Compared with the LGS or CNTs, the ZGNRs possess stronglylocalized edge states [75] with magnetic and metallic nature, whereas the AGNRs
have larger band gap (E G ) with semiconductive nature. The E G is roughly proportional to the inverse width of the GNRs [76–78]. However, discrepancy remains
between the theory and the experimental derived E G of GNR. Measurements
[77] and theoretical calculations [51, 78, 79] showed less consistent in the band
gap opening of GNR. Mechanisms remain yet unclear regarding the generation
of the localized edge states and the expansion trends of the E G in spite of the
possible mechanisms such as doping [80], defects forming [51, 81, 82], symmetry breaking [83], substrate interaction [84], edge distortion [76], strain [85],
quantum confinement [48], and the staggered sublattice potentials modulation
[86].
4. Mechanism for the edge selective generation and hydrogen annihilation of the
Dirac fermions remains opening. The Dirac-Fermi polaritons [44, 45] generate
at sites surrounding atomic vacancies [43, 87], the edges of monolayer graphite
terrace and the ZGNRs [46–49], other than edges of the AGNR or the rec-AGNR.
It remains unclear why the edge and site discriminate the generation of Dirac
fermions.
5. Origin of and correlation between the positive C 1s core level shift of the GNR
edge, GNR interior and the associated work function reduction with the number of GNR layers are ambiguous. Three XPS C 1s components have been
resolved from graphene flakes produce [88], corresponding, respectively, from