engleski

Quasiparticle spectra and excitons in organic molecules deposited on graphene or metal surface: G0 W0 -BSE approach

Organic molecules have become increasingly studied for many applications such as organic cir- cuits, field effect 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 first 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 first applied to the calculation of the QP HOMO-LUMO gap and excitons (triplet, singlet, optically active and inactive) in benzene C6 H6 . Good agreement with experimental results and other calculations is obtained [5]. The presence of the substrate requires simple modification of the present formulation ; the bare Coulomb interaction V should be replaced (everywhere through G0 W0 -BSE scheme) by the substrate induced dynamically screened Coulomb interaction W (ω) = V + ∆W (ω). 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 (C6 H6 ), teryllene, (C30 H16 ) 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 G0 W0 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).

molecular excitons ; quasi particles

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