2.4 Valence Band and Nonbonding States
39
orbits, each capable of being occupied by two electrons, forming a quasi-tetrahedron.
These four directional orbits can hence accommodate eight electrons. The central
O atom, for example, has six 2s
2 2p
4 electrons and needs another two to complete
its valence shell. Therefore, the O forms two bonds with the nearest neighbors by
sharing one electron with each of its two neighbors, while its lone electron pairs
occupy its remaining two orbits. Likewise, a nitrogen atom needs three electrons for
sharing and generates one lone pair; and an F atom forms a tetrahedron with three
lone pairs. The manner of electronic distribution, bond type, bond length and energy
surrounding the central F, O, and N atom in the tetrahedron are anisotropic at the
atomic scale [2].
Counting from lower to higher energy, the DOS features are the bonding states,
electronic holes, electron lone pairs of F
− , O
2− , or N
3− , and the anti-bonding dipoles
of the host. In a semiconductor compound, holes create at the top of the valence
band, which further expands the semiconductor’s band gap, turning a semiconductor
into an insulator, such as Si 3 N 4 and SiO 2 .
In metallic compounds, holes generate near the Fermi surface, which open a band
gap and turns the conductor into an insulator or a semiconductor, such as Al 2 O 3 , TiO 2 ,
ZnO, and AlN. Nonbonding states located in the band gap form impurity states, while
dipoles form antibonding states above the E F . The production of dipoles shifts the
surface potential barrier outwardly with high saturation [36], opposing to the effect of
the positively charged ions. These dipoles also screen and split the local potentials,
adding excessive features to the core bands. STM probes dipoles as protrusions,
while the positive ions are probed as depressions.
2.4.3 Impact of Nonbonding States
The “nonbonding” states also refer to the energetic electrons involved in the antibonding dipoles, as well as H-bond like and C–H-bond like. An ionic impurity in a
metal also polarizes neighbors to create polarized states [3, 37]. The van der Waals
bond having a maximal energy of several tenths of an eV is in this category as it
stands for dipole-dipole interaction instead of charge sharing exchange interactions.
Contributing insignificantly to the Hamiltonian or to the atomic cohesive energy,
the nonbonding electrons add, however, impurity states near E F , which neither follow
the regular dispersion relations nor occupy the allowed states of the valence band
and below. They are located at energies within the energy window of an STM/S.
In additional to the weak interactions with energies of ~50 meV, as detected using
Raman and electron energy loss spectroscopy (EELS) [3], these lone pairs, however,
polarize the neighboring atoms, instead, causing their neighbors changing to dipoles.
The impact of the nonbonding lone pairs and the antibonding dipoles is ubiquitously abundant and profound. For example, the presence of the antibonding dipoles
lowers drastically the work function by more than 1 eV [38], which aids greatly the
electron emission for imaging and display [39, 40]. Nitrogenating of diamond and
carbon nanotubes, oxidation and fluorination of metals, etc., have the same effect
39
orbits, each capable of being occupied by two electrons, forming a quasi-tetrahedron.
These four directional orbits can hence accommodate eight electrons. The central
O atom, for example, has six 2s
2 2p
4 electrons and needs another two to complete
its valence shell. Therefore, the O forms two bonds with the nearest neighbors by
sharing one electron with each of its two neighbors, while its lone electron pairs
occupy its remaining two orbits. Likewise, a nitrogen atom needs three electrons for
sharing and generates one lone pair; and an F atom forms a tetrahedron with three
lone pairs. The manner of electronic distribution, bond type, bond length and energy
surrounding the central F, O, and N atom in the tetrahedron are anisotropic at the
atomic scale [2].
Counting from lower to higher energy, the DOS features are the bonding states,
electronic holes, electron lone pairs of F
− , O
2− , or N
3− , and the anti-bonding dipoles
of the host. In a semiconductor compound, holes create at the top of the valence
band, which further expands the semiconductor’s band gap, turning a semiconductor
into an insulator, such as Si 3 N 4 and SiO 2 .
In metallic compounds, holes generate near the Fermi surface, which open a band
gap and turns the conductor into an insulator or a semiconductor, such as Al 2 O 3 , TiO 2 ,
ZnO, and AlN. Nonbonding states located in the band gap form impurity states, while
dipoles form antibonding states above the E F . The production of dipoles shifts the
surface potential barrier outwardly with high saturation [36], opposing to the effect of
the positively charged ions. These dipoles also screen and split the local potentials,
adding excessive features to the core bands. STM probes dipoles as protrusions,
while the positive ions are probed as depressions.
2.4.3 Impact of Nonbonding States
The “nonbonding” states also refer to the energetic electrons involved in the antibonding dipoles, as well as H-bond like and C–H-bond like. An ionic impurity in a
metal also polarizes neighbors to create polarized states [3, 37]. The van der Waals
bond having a maximal energy of several tenths of an eV is in this category as it
stands for dipole-dipole interaction instead of charge sharing exchange interactions.
Contributing insignificantly to the Hamiltonian or to the atomic cohesive energy,
the nonbonding electrons add, however, impurity states near E F , which neither follow
the regular dispersion relations nor occupy the allowed states of the valence band
and below. They are located at energies within the energy window of an STM/S.
In additional to the weak interactions with energies of ~50 meV, as detected using
Raman and electron energy loss spectroscopy (EELS) [3], these lone pairs, however,
polarize the neighboring atoms, instead, causing their neighbors changing to dipoles.
The impact of the nonbonding lone pairs and the antibonding dipoles is ubiquitously abundant and profound. For example, the presence of the antibonding dipoles
lowers drastically the work function by more than 1 eV [38], which aids greatly the
electron emission for imaging and display [39, 40]. Nitrogenating of diamond and
carbon nanotubes, oxidation and fluorination of metals, etc., have the same effect