7.1 Introduction
147
point depression, C 1s core-level shift, band gap expansion, edge and defect DiracFermi polarons generation and the associated magnetism consistently confirmed that
the shorter and stronger bonds between undercoordinated carbon atoms modulate
locally the atomic cohesive energy and the Hamiltonian which alter the detectable
bulk properties. The polarization of the unpaired sp
2 electrons by the densely, deeply,
and locally entrapped core and bonding electrons generates the massless, magnetic
and mobile Dirac-Fermi polarons at sites surrounding defects and ZGNR edges. The
pseudo-π-bond formation at edges discriminates the AGNR and the rec-ZGNR from
the AGNR in the electronic and magnetic anomalies.
7.2 Experimental Observations
7.2.1 STM/S-DFT: GNR Edge and Defect Polarization
Bond-order variation and the versatility of the sp-orbital hybridization enabled carbon
allotropes a group of amazing materials varying from diamond, graphite, fullerene
C 60 , nanotube (CNT), nanobud (CNB), graphene, and graphene nanoribbons (GNRs)
with different topological edges. Graphite is an electronic conductor and opaque but
diamond is an insulator yet transparent to light of almost all wavelengths; the former
shares nonbonding unpaired (or π-bond) electrons due to sp
2 –orbital hybridization
compared with the latter of an ideal sp
3 -hybridization. GNR performs quite differently from CNT or from an infinitely large sheet of graphene because of the
involvement of the two-coordinated edge atoms [20, 37, 49, 92–97].
Graphite point defects and GNRs with different types of edges demonstrate many
fascinating properties that neither the large graphene sheet nor the bulk graphite displays. One of such properties is the edge-selective generation of the Dirac Fermions
(DFs) with unexpectedly low effective mass, extremely high mobility [38], non-zero
spin [98–101], demonstrating the spin quantum Hall effect [48, 102]. The DFs perform abnormally in many aspects, which is beyond the description of Schrödinger
equation, but they follow Dirac equation of motion with a nearly linear dispersion
crossing E F [35, 55]. Because of the polarization and localization [93], it would be
comprehensive to name DFs as Dirac-Fermi polaritons (DFPs) that are associated
with atoms at the zigzag GNR edges or surrounding point defects with
√
3d lattice
spacing [92]. Performing differently from those unpaired nonbonding electrons in
the GNR interior, the DFPs determine the catalytic, electric, magnetic, optic, and
transport properties of the edged graphenes [48, 103, 104]. The zigzag-edged GNR
performs metallic like while the armchair-edged GNR semiconductor like.
STM/S probed the graphitic DFs as bright protrusions with resonant peak at E F
from sites surrounding atomic vacancies, shown in Fig. 7.1a [43–45, 87], from edges
of monolayer graphite terraces, and from graphene nanoribbons, see Fig. 7.1b [47–
49]. Resonant current flows between the STM tip and the GNR edge under zero bias.
147
point depression, C 1s core-level shift, band gap expansion, edge and defect DiracFermi polarons generation and the associated magnetism consistently confirmed that
the shorter and stronger bonds between undercoordinated carbon atoms modulate
locally the atomic cohesive energy and the Hamiltonian which alter the detectable
bulk properties. The polarization of the unpaired sp
2 electrons by the densely, deeply,
and locally entrapped core and bonding electrons generates the massless, magnetic
and mobile Dirac-Fermi polarons at sites surrounding defects and ZGNR edges. The
pseudo-π-bond formation at edges discriminates the AGNR and the rec-ZGNR from
the AGNR in the electronic and magnetic anomalies.
7.2 Experimental Observations
7.2.1 STM/S-DFT: GNR Edge and Defect Polarization
Bond-order variation and the versatility of the sp-orbital hybridization enabled carbon
allotropes a group of amazing materials varying from diamond, graphite, fullerene
C 60 , nanotube (CNT), nanobud (CNB), graphene, and graphene nanoribbons (GNRs)
with different topological edges. Graphite is an electronic conductor and opaque but
diamond is an insulator yet transparent to light of almost all wavelengths; the former
shares nonbonding unpaired (or π-bond) electrons due to sp
2 –orbital hybridization
compared with the latter of an ideal sp
3 -hybridization. GNR performs quite differently from CNT or from an infinitely large sheet of graphene because of the
involvement of the two-coordinated edge atoms [20, 37, 49, 92–97].
Graphite point defects and GNRs with different types of edges demonstrate many
fascinating properties that neither the large graphene sheet nor the bulk graphite displays. One of such properties is the edge-selective generation of the Dirac Fermions
(DFs) with unexpectedly low effective mass, extremely high mobility [38], non-zero
spin [98–101], demonstrating the spin quantum Hall effect [48, 102]. The DFs perform abnormally in many aspects, which is beyond the description of Schrödinger
equation, but they follow Dirac equation of motion with a nearly linear dispersion
crossing E F [35, 55]. Because of the polarization and localization [93], it would be
comprehensive to name DFs as Dirac-Fermi polaritons (DFPs) that are associated
with atoms at the zigzag GNR edges or surrounding point defects with
√
3d lattice
spacing [92]. Performing differently from those unpaired nonbonding electrons in
the GNR interior, the DFPs determine the catalytic, electric, magnetic, optic, and
transport properties of the edged graphenes [48, 103, 104]. The zigzag-edged GNR
performs metallic like while the armchair-edged GNR semiconductor like.
STM/S probed the graphitic DFs as bright protrusions with resonant peak at E F
from sites surrounding atomic vacancies, shown in Fig. 7.1a [43–45, 87], from edges
of monolayer graphite terraces, and from graphene nanoribbons, see Fig. 7.1b [47–
49]. Resonant current flows between the STM tip and the GNR edge under zero bias.