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S. Yanagisawa and I. Hamada
Understanding of unoccupied states including image potential state (IPS) at
organic-metal interfaces is of importance, and experimental or theoretical insight
into prototype systems such as physisorption of a typical aromatic hydrocarbon on
a graphite surface is required. Theoretically, GW approximation is one of the most
promising electronic structure methods [169], because of its rigorous description
of the long-ranged tail of the (screened) Coulomb potential. However, as far as
the authors know, there is no report at present successfully investigating the IPS
at organic-metal interfaces based on the same methodology, possibly because of
the numerical or technical difficulty, as mentioned above. The approach based on
a variant of vdW-DF may be a method of choice at present to gain insights into
the IPS at organic-metal interfaces, with its long-range nature of the potential and
accuracy in prediction of the interface geometry, along with its reasonable cost
almost comparable to DFT-LDA or DFT-GGA [19, 134].
Thirdly, we have outlined recent theoretical approaches for determination of
energy levels for charge injection, i.e., ionization energy (IE) and electron affinity
(EA), at an organic semiconductor surface, which essentially determines barrier for
charge injection. As with the previous topics, GW approximation is promising also
for this problem. However, to avoid the numerical or technical difficulties involved
in treatment of surfaces and interfaces with the periodic slab, there are some
treatments proposed [21, 22], in which the vacuum level is determined based on
the calculation of the slab model within DFT-GGA. The approach proposed in Ref.
[22] successfully elucidated the dependence of the ionization energy and electron
affinity on the surface morphology or the molecular orientation at the surface as
measured by the LEIPS technique [170–172]. Nevertheless, in these approaches,
the induced polarization effect in the bulk was treated. To be more quantitative
in comparison to experimental measurements, the polarization effect on a surface
should be taken into account, which is weaker than that in the bulk. A recently
proposed approach based on QM/MM, in which IE and EA of a single molecule
are treated at the GW level of theory, while the surrounding molecules are treated
as continuum of dielectric medium, and thus the macroscopic shape of the surface
can be treated within MM [23]. The result demonstrates quantitative treatment of IE
and EA taking into account the presence of the organic crystal surface [23]. As far
as the theoretical treatment of IE and EA of organic single crystals or thin films is
concerned, this treatment may be a method of choice at present.
The first-principles theoretical methods, as outlined in this article, have allowed
precise determination or prediction of the intrinsic basic electronic properties
of organic semiconductor materials measured or observed in recent well-defined
experimental measurements. Compared to the vast amount of database available and
the understanding on the electronic properties of inorganic semiconductor materials,
there is much room for understanding the basic electronic origin of the materials
properties, and there is much need for constructing database for organic semiconductor materials. The roles of the first-principles electronic structure methods as
described here, aided by future increase in computational resources and technical
development in program codes, will become more and more important for more
S. Yanagisawa and I. Hamada
Understanding of unoccupied states including image potential state (IPS) at
organic-metal interfaces is of importance, and experimental or theoretical insight
into prototype systems such as physisorption of a typical aromatic hydrocarbon on
a graphite surface is required. Theoretically, GW approximation is one of the most
promising electronic structure methods [169], because of its rigorous description
of the long-ranged tail of the (screened) Coulomb potential. However, as far as
the authors know, there is no report at present successfully investigating the IPS
at organic-metal interfaces based on the same methodology, possibly because of
the numerical or technical difficulty, as mentioned above. The approach based on
a variant of vdW-DF may be a method of choice at present to gain insights into
the IPS at organic-metal interfaces, with its long-range nature of the potential and
accuracy in prediction of the interface geometry, along with its reasonable cost
almost comparable to DFT-LDA or DFT-GGA [19, 134].
Thirdly, we have outlined recent theoretical approaches for determination of
energy levels for charge injection, i.e., ionization energy (IE) and electron affinity
(EA), at an organic semiconductor surface, which essentially determines barrier for
charge injection. As with the previous topics, GW approximation is promising also
for this problem. However, to avoid the numerical or technical difficulties involved
in treatment of surfaces and interfaces with the periodic slab, there are some
treatments proposed [21, 22], in which the vacuum level is determined based on
the calculation of the slab model within DFT-GGA. The approach proposed in Ref.
[22] successfully elucidated the dependence of the ionization energy and electron
affinity on the surface morphology or the molecular orientation at the surface as
measured by the LEIPS technique [170–172]. Nevertheless, in these approaches,
the induced polarization effect in the bulk was treated. To be more quantitative
in comparison to experimental measurements, the polarization effect on a surface
should be taken into account, which is weaker than that in the bulk. A recently
proposed approach based on QM/MM, in which IE and EA of a single molecule
are treated at the GW level of theory, while the surrounding molecules are treated
as continuum of dielectric medium, and thus the macroscopic shape of the surface
can be treated within MM [23]. The result demonstrates quantitative treatment of IE
and EA taking into account the presence of the organic crystal surface [23]. As far
as the theoretical treatment of IE and EA of organic single crystals or thin films is
concerned, this treatment may be a method of choice at present.
The first-principles theoretical methods, as outlined in this article, have allowed
precise determination or prediction of the intrinsic basic electronic properties
of organic semiconductor materials measured or observed in recent well-defined
experimental measurements. Compared to the vast amount of database available and
the understanding on the electronic properties of inorganic semiconductor materials,
there is much room for understanding the basic electronic origin of the materials
properties, and there is much need for constructing database for organic semiconductor materials. The roles of the first-principles electronic structure methods as
described here, aided by future increase in computational resources and technical
development in program codes, will become more and more important for more