Permittivity in Molecular Nanofilms
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1017-DD08-29
Permittivity in Molecular Nanofilms Sinisa Vucenovic1, Dusan Ilic2, Jovan Setrajcic3, Vjekoslav Sajfert4, and Dragoljub Mirjanic1 1 Medical faculty, University of Banja Luka, Banja Luka, 78000, Bosnia and Herzegovina 2 Faculty of technical sciences, University of Novi Sad, Novi Sad, 21000, Yugoslavia 3 Department of Physics, Faculty of natural sciences, University of Novi Sad, Novi Sad, Yugoslavia 4 Technical faculty "Mihajlo Pupin", University of Novi Sad, Zrenjanin, 23000, Yugoslavia ABSTRACT A microscopic theory of dielectrical properties of thin molecular films, i.e. quasi 2D systems bounded by two surfaces parallel to XY planes was formulated. Harmonic exciton states were calculated using the method of two-time, retarded, temperature dependent Green's functions. It has been shown that two types of excitations can occur: bulk and surface exciton states. Analysis of the optical properties of these crystalline systems for low exciton concentration shows that the permittivity strongly depends on boundary parameters and the thickness of the film. Conditions for the appearance of localized exciton states have been especially analyzed. INTRODUCTION Application of dielectrics in electronic packaging is strongly connected with use of electronic components in extreme physical conditions (high pressures and temperatures). On the other hand, molecular crystals are used as materials for light energy conversion. We study the basic physical characteristics of ultrathin dielectrics - ultrathin molecular crystalline films, which could be used as surface layers for electronic component protection. The first aim of this paper is the analysis of the influence of finite dimensions of crystalline ultrathin films and boundary conditions onto the energy spectrum of the excitons. We base our analysis on the standard exciton Hamiltonian [1,2]: H = ∑ ∆ nr Pnr+ Pnr − ∑ Vnrmr Pnr+ Pmr − ∑ Wnrmr Pnr+ Pnr Pmr+ Pmr , (1) r
n
rr
nm
rr
nm
where ∆ nr denotes the excitation energy of an isolated molecule localized at the site r n ≡ (nx , ny , nz ) while Vnrmr and Wnrmr represent the matrix elements of dipole-dipole interaction. As orbitals overlap only for neighbor molecules, nearest-neighbor approximation is used. For low exciton concentrations, in harmonic (Bose) approximation [3], we get: H = ∑ ∆ nr Bnr+ Bnr − ∑ Vλr Bnr+ Bnr + λr (2) r
n
r r
n,λ
The fact that the film is of finite dimension only along z-directions (orthogonal to the film boundary surfaces) is expressed in terms of the conditions: nz = 0,1,2,..., N z , N z ∝ 10 ,
nα ∈ [− Nα / 2,+ Nα / 2] , Nα ∝ 108 , α = ( x, y ) . Exciton energies must be redefined because of the presence of film boundaries. The parameters ε 0 / N z define the change of the exciton energy at the
surface layers of the film, v0 / N z define the change of the transfer energy within the surface layers
and the layers nearest to them (along z direction), while parameters v0x// Ny z define the change of exciton transfer in boundary layers (along x and y directions). ∆ nr ≡ ∆ 1 + ε 0δ n z ,
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