First-principles calculations are used to investigate the electronic
properties of negatively cha... more First-principles calculations are used to investigate the electronic properties of negatively charged n-pyrrole oligomers with n ¼ 2–18. Chains of neutral oligomers are bent, whereas the negatively charged oligomers become almost planar due to accumulation of negative charge at the end monomers. Isomers of short oligomers (n < 6) display negative electron affinity although the corresponding anions are energetically stable. For longer oligomers with n 6, the electron affinity is small and positive, slowly increasing with oligopyrrole length. Doping of 12-pyrrole with lithium atoms shows that negative oxidation states are possible due to electron transfer from dopant to oligomer at locations close to dopant. These 12-pyrrole regions support extra negative charge and exhibit a local structural change from benzenoid to quinoid structure in the CAC backbone conjugation. Comparison between neutral and doped polypyrrole (PPy) indicates that doped polymers displays a substantial depletion of the band gap energy and the appearance of dopant-based bands in the gap for a 50% per monomer doping level. It is predicted that Li-doped PPy is not metallic.
The effect of oxidation on the energetics and structure of thiophene (Th) oligomers is studied wi... more The effect of oxidation on the energetics and structure of thiophene (Th) oligomers is studied with density functional theory at the B3PW91/6-311++G(d,p) level. Neutral n-Th oligomers (2 < n < 13) are gently curved planar chains. Ionization potential and electron affinity results show that n-Th oligomers are easier to be oxidized as their chain length increases. Oxidation states +2, +4, +6, and +8 are energetically stable in 12-Th. Upon oxidation the conjugated backbone of 12-Th switches from extended benzenoid phase to quinoid phase localized on groups of monomers regularly spaced along the chain. Oxidized states +2, +4, +6, and +8 of 12-Th display two +1e localized at the ends of their chains only because of the finite size of the chains. In 12-Th this end-effect extends over the two terminal monomers forming a positive-negative charge duet. This peculiar charge localization makes n-Th oligomers different from other conducting polymers with similar structure, such as polypyrrole. The spectrum of single-electron molecular states of oxidized 12-Th displays two localized single-electron states in the HOMO-LUMO energy gap per +2 oxidation state. Oligothiophene 12-Th doped with F atoms at 1:2 concentration presents a charge transfer of 3.4 e from oligomer to dopants that increases to 4.8 e in the presence of solvent. The charge distribution in these F-doped oligomers is similar to the +4 oxidation state of 12-Th. It is predicted that dopants produce an enhanced charge transfer localized in the proximity of their locations enhancing the formation of bipolarons in the central part of the oligomer chain.
To understand the reversible oxidation of graphene in a recent experiment, a density-functional t... more To understand the reversible oxidation of graphene in a recent experiment, a density-functional theory study is performed. The adsorption energy of isolated oxygen atom on graphene is 2.3 eV, indicating a strong interaction between them. However, the migration barrier of oxygen atoms on graphene is only 0.8 eV and oxygen diffusion is still possible. Provided with this possibility, we find that, although a single oxygen atom is very difficult to desorb, cooperative desorption of two oxygen atoms is feasible.
First-principles calculations are used to investigate the electronic
properties of negatively cha... more First-principles calculations are used to investigate the electronic properties of negatively charged n-pyrrole oligomers with n ¼ 2–18. Chains of neutral oligomers are bent, whereas the negatively charged oligomers become almost planar due to accumulation of negative charge at the end monomers. Isomers of short oligomers (n < 6) display negative electron affinity although the corresponding anions are energetically stable. For longer oligomers with n 6, the electron affinity is small and positive, slowly increasing with oligopyrrole length. Doping of 12-pyrrole with lithium atoms shows that negative oxidation states are possible due to electron transfer from dopant to oligomer at locations close to dopant. These 12-pyrrole regions support extra negative charge and exhibit a local structural change from benzenoid to quinoid structure in the CAC backbone conjugation. Comparison between neutral and doped polypyrrole (PPy) indicates that doped polymers displays a substantial depletion of the band gap energy and the appearance of dopant-based bands in the gap for a 50% per monomer doping level. It is predicted that Li-doped PPy is not metallic.
The effect of oxidation on the energetics and structure of thiophene (Th) oligomers is studied wi... more The effect of oxidation on the energetics and structure of thiophene (Th) oligomers is studied with density functional theory at the B3PW91/6-311++G(d,p) level. Neutral n-Th oligomers (2 < n < 13) are gently curved planar chains. Ionization potential and electron affinity results show that n-Th oligomers are easier to be oxidized as their chain length increases. Oxidation states +2, +4, +6, and +8 are energetically stable in 12-Th. Upon oxidation the conjugated backbone of 12-Th switches from extended benzenoid phase to quinoid phase localized on groups of monomers regularly spaced along the chain. Oxidized states +2, +4, +6, and +8 of 12-Th display two +1e localized at the ends of their chains only because of the finite size of the chains. In 12-Th this end-effect extends over the two terminal monomers forming a positive-negative charge duet. This peculiar charge localization makes n-Th oligomers different from other conducting polymers with similar structure, such as polypyrrole. The spectrum of single-electron molecular states of oxidized 12-Th displays two localized single-electron states in the HOMO-LUMO energy gap per +2 oxidation state. Oligothiophene 12-Th doped with F atoms at 1:2 concentration presents a charge transfer of 3.4 e from oligomer to dopants that increases to 4.8 e in the presence of solvent. The charge distribution in these F-doped oligomers is similar to the +4 oxidation state of 12-Th. It is predicted that dopants produce an enhanced charge transfer localized in the proximity of their locations enhancing the formation of bipolarons in the central part of the oligomer chain.
To understand the reversible oxidation of graphene in a recent experiment, a density-functional t... more To understand the reversible oxidation of graphene in a recent experiment, a density-functional theory study is performed. The adsorption energy of isolated oxygen atom on graphene is 2.3 eV, indicating a strong interaction between them. However, the migration barrier of oxygen atoms on graphene is only 0.8 eV and oxygen diffusion is still possible. Provided with this possibility, we find that, although a single oxygen atom is very difficult to desorb, cooperative desorption of two oxygen atoms is feasible.
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properties of negatively charged n-pyrrole oligomers with n ¼ 2–18. Chains of neutral
oligomers are bent, whereas the negatively charged oligomers become almost planar
due to accumulation of negative charge at the end monomers. Isomers of short
oligomers (n < 6) display negative electron affinity although the corresponding anions
are energetically stable. For longer oligomers with n 6, the electron affinity is small
and positive, slowly increasing with oligopyrrole length. Doping of 12-pyrrole with
lithium atoms shows that negative oxidation states are possible due to electron transfer
from dopant to oligomer at locations close to dopant. These 12-pyrrole regions support
extra negative charge and exhibit a local structural change from benzenoid to quinoid
structure in the CAC backbone conjugation. Comparison between neutral and doped
polypyrrole (PPy) indicates that doped polymers displays a substantial depletion of the
band gap energy and the appearance of dopant-based bands in the gap for a 50% per
monomer doping level. It is predicted that Li-doped PPy is not metallic.
density functional theory at the B3PW91/6-311++G(d,p) level. Neutral n-Th oligomers (2 < n
< 13) are gently curved planar chains. Ionization potential and electron affinity results show that
n-Th oligomers are easier to be oxidized as their chain length increases. Oxidation states +2, +4,
+6, and +8 are energetically stable in 12-Th. Upon oxidation the conjugated backbone of 12-Th
switches from extended benzenoid phase to quinoid phase localized on groups of monomers regularly
spaced along the chain. Oxidized states +2, +4, +6, and +8 of 12-Th display two +1e
localized at the ends of their chains only because of the finite size of the chains. In 12-Th this
end-effect extends over the two terminal monomers forming a positive-negative charge duet. This
peculiar charge localization makes n-Th oligomers different from other conducting polymers with
similar structure, such as polypyrrole. The spectrum of single-electron molecular states of oxidized
12-Th displays two localized single-electron states in the HOMO-LUMO energy gap per +2 oxidation
state. Oligothiophene 12-Th doped with F atoms at 1:2 concentration presents a charge transfer
of 3.4 e from oligomer to dopants that increases to 4.8 e in the presence of solvent. The charge distribution
in these F-doped oligomers is similar to the +4 oxidation state of 12-Th. It is predicted that
dopants produce an enhanced charge transfer localized in the proximity of their locations enhancing
the formation of bipolarons in the central part of the oligomer chain.
properties of negatively charged n-pyrrole oligomers with n ¼ 2–18. Chains of neutral
oligomers are bent, whereas the negatively charged oligomers become almost planar
due to accumulation of negative charge at the end monomers. Isomers of short
oligomers (n < 6) display negative electron affinity although the corresponding anions
are energetically stable. For longer oligomers with n 6, the electron affinity is small
and positive, slowly increasing with oligopyrrole length. Doping of 12-pyrrole with
lithium atoms shows that negative oxidation states are possible due to electron transfer
from dopant to oligomer at locations close to dopant. These 12-pyrrole regions support
extra negative charge and exhibit a local structural change from benzenoid to quinoid
structure in the CAC backbone conjugation. Comparison between neutral and doped
polypyrrole (PPy) indicates that doped polymers displays a substantial depletion of the
band gap energy and the appearance of dopant-based bands in the gap for a 50% per
monomer doping level. It is predicted that Li-doped PPy is not metallic.
density functional theory at the B3PW91/6-311++G(d,p) level. Neutral n-Th oligomers (2 < n
< 13) are gently curved planar chains. Ionization potential and electron affinity results show that
n-Th oligomers are easier to be oxidized as their chain length increases. Oxidation states +2, +4,
+6, and +8 are energetically stable in 12-Th. Upon oxidation the conjugated backbone of 12-Th
switches from extended benzenoid phase to quinoid phase localized on groups of monomers regularly
spaced along the chain. Oxidized states +2, +4, +6, and +8 of 12-Th display two +1e
localized at the ends of their chains only because of the finite size of the chains. In 12-Th this
end-effect extends over the two terminal monomers forming a positive-negative charge duet. This
peculiar charge localization makes n-Th oligomers different from other conducting polymers with
similar structure, such as polypyrrole. The spectrum of single-electron molecular states of oxidized
12-Th displays two localized single-electron states in the HOMO-LUMO energy gap per +2 oxidation
state. Oligothiophene 12-Th doped with F atoms at 1:2 concentration presents a charge transfer
of 3.4 e from oligomer to dopants that increases to 4.8 e in the presence of solvent. The charge distribution
in these F-doped oligomers is similar to the +4 oxidation state of 12-Th. It is predicted that
dopants produce an enhanced charge transfer localized in the proximity of their locations enhancing
the formation of bipolarons in the central part of the oligomer chain.