Quasiparticle spectra and excitons in organic molecules deposited on graphene and metal surfaces: G0W0-BSE approach
DIPC Seminars
- Speaker
-
Vito Despoja, Dep. of Physics, Univ. of Zagrev, Croatia
- When
-
2013/09/06
14:00 - Place
- Donostia International Physics Center (DIPC).Paseo Manuel de Lardizabal, 4 (nearby the Facultad de Quimica), Donostia
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**Quasiparticle spectra and excitons in organic molecules deposited on
graphene or
metal surface: G0W0-BSE approach
**
V. Despoja
Department of Physics, University of Zagreb, Bijeni_x0014_cka 32, HR-10000 Zagreb,
Croatia
Organic molecules have become increasingly studied for many applications such
as organic cir-
cuits, eld e ect transistors and suitable for photovoltaic applications in
solar cells and for biosensing
applications. This trend requires a more accurate theoretical and experimental
investigation of the
molecular electronic structure and influence of the substrate on the molecular
spectra. This work
is focused on investigating the quasi-particle (QP) spectra and electronic
excitations in organic
molecules. The QP properties of the molecules are investigated in the
framework of Hedins's GW
theory [1, 2], while the excitations (optical and energy loss spectra) inside
the molecule are investi-
gated by solving the Bethe-Salpeter equation (BSE) [3, 4].
The fi rst part of the presentation is focused on explaining an alternative
methodology for calculat-
ing the QP states and excitons in the molecule. To check the accuracy of the
method it is rst applied
to the calculation of the QP HOMO-LUMO gap and excitons (triplet, singlet,
optically active and
inactive) in benzene C6H6. Good agreement with experimental results and other
calculations is
obtained [5]. The presence of the substrate requires simple modi cation of the
present formulation;
the bare Coulomb interaction V should be replaced (everywhere through G0W0-BSE
scheme) by
the substrate induced dynamically screened Coulomb interaction W(f) = V +
_x0001_DW(f). We find
the substrate reduces the QP HOMO-LUMO gap but the exciton energy remains
unchanged. The
spectra and decay of the singlet excitons to the electronic excitations
(electron-hole and plasmons)
in the substrate will also be presented.
In the second part of the presentation we will demonstrated how to use BSE and
a very few states
in the HOMO-LUMO region as a basis set to obtain accurate energies for the low
lying molecular
excitons. The method is applied to methane (CH4), benzene (C6H6), teryllene,
(C30H16) and
fullerene (C60) and, as shown in Tab.I, good agreement with
experimental/theoretical results [6, 7] is obtained. Calculation of accurate
HOMO-LUMO gaps in the framework of G0W0 is computationally
very heavy because it requires many unoccupied states. In this presentation we
will demonstrate
that satisfactory results may be obtained if the screened Coulomb interaction
W0 is constructed just
from low lying electronic transitions (transitions between LDA-HOMO and LDA-
LUMO states).
CH4(Td) | C6H6 C30H16 C60
Triplet/Singlet T S | T S | T S | T
S
This work /(eV) 10.3 10.5 | 3.93 7.02 | 1.36 2.42 | 2.14, 2.37 2.33,
2.41, 2.5, 2.93
Exp./Theor. /(eV) 10.1 10.5| 3.95 6.94 | / 2.35 | 2.24
2.98
TABLE I: Energies of the low lying molecular excitons obtained by solving
reduced BSE.
[1] L. Hedin, Phys. Rev. 139, 796 (1965)
[2] Mark S. Hybertsen, Steven G. Louie, Phys. Rev. B 34,
5390 (1986)
[3] G. Strinati, Phys. Rev. B 29, 5718 (1984)
[4] M. Rohl ng, S. G. Louie, Phys. Rev. Lett. 81, 2312 (1998)
[5] J. B. Neaton, Mark S. Hybertsen, and Steven G. Louie,
Phys. Rev. Lett 97, 216405 (2006)
[6] J. D. Sau, J. B. Neaton, H. J. Choi, S. G. Louie, M. L.
Cohen, Phys. Rev. Lett. 101, 026804 (2008)
[7] G. Malloci, G. Cappellini, G. Mulas, A. Mattoni, Chem.
Phys. 384, 19 (2011)