S.M. Vučenović

Selective IR absorption in molecular nanofilms

We study the dynamics of a single Frenkel exciton in a disordered molecular chain. The coherent-potential approximation (CPA) is applied to the situation when the single-molecule excitation energies as well as the transition dipole moments, both their absolute values and orientations, are random. Such model is believed to be relevant for the description of the linear optical properties of one-dimensional

Phonon Thermodynamics in Crystalline Nanofilms

J

Exciton Dispersion Law and States of Bimolecular Thin Films

aggregates. We calculate the exciton density of states, the linear absorption spectra and the exciton coherence length which reveals itself in the linear optics. A detailed analysis of the low-disorder limit of the theory is presented. In particular, we derive asymptotic formulas relating the absorption linewidth and the exciton coherence length to the strength of disorder. Such expressions account simultaneously for all the above types of disorder and reduce to well-established form when no disorder in the transition dipoles is present. The theory is applied to the case of purely orientational disorder and is shown to agree well with exact numerical diagonalization.Abstract We have formulated a microscopic theory of optical properties of ultrathin molecular films (nanofilms), i.e. quasi 2D systems parallel to XY planes bounded by two surfaces. Exposure of nanofilms to the external electromagnetic fields has result in creation of excitons – but different than bulk ones. 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 optical excitations can occur: bulk and surface exciton states. Exciton energy dispersion law shows discrete behavior with non-zero values. Analysis of the dielectric properties of these crystalline systems for low exciton concentration shows that the permittivity strongly depends on boundary parameters and the thickness of the film. In addition, permittivity shows very narrow and discrete dependence of external electromagnetic field frequency, which is a consequence of both resonance and quantum size effects. Influences of boundary conditions on optical characteristics (through analyses of dynamical absorption coefficient) of these nanostructures were specially and in details explored.

PERMITTIVITY IN PERTURBED MOLECULAR NANOFILMS

The dispersion law and density of states of phonons in ultrathin films was analyzed in this paper. It turned out that phonons in a thin film require activation energy for exciting. This leads to extremely low specific heat and specific conductivity at low temperatures. Consequences of quoted facts were discussed in detail and their influence on kinetic and thermodynamic properties of thin films is estimated.

Absorption features of symmetric molecular nanofilms

Dispersion laws and states (i.e. probability of finding) of Frankel excitons in ultra-thin molecular films are found using a well-known method of Green’s functions. Space boundaries and changes of energetic parameters on boundaries are considered as perturbations. The cubic crystalline system with complex cell consisted of two molecules (a and b), i.e. bimolecular film, was analyzed in harmonic approximation, and then compared with the results obtained for simple cubic cell systems (i.e. monomolecular film). In both cases the energy spectra show sharp discrete levels, although the energy spectra of bimolecular films split into two zones with discrete levels. Probability of finding exciton in the mono- or bimolecular ultra-thin films is significantly influenced by the perturbation and the values of on-site energies of molecules a and b. Obtained conditions of the existence of localized exciton states at boundaries are of special interest.

ELECTRON THERMODYNAMICS OF NANOFILM-STRUCTURES

A microscopic theory of dielectric 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 Greens 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 or unoccupied exciton states have been especially analyzed.

REFRACTIVE PROPERTIES OF MOLECULAR CRYSTALLINE SUPERLATTICES

A microscopic theory of dielectric properties of symmetrical ultrathin molecular films, was formulated in bosonic and nearest-neighbor approximation. The dispersion law of harmonic exciton states were calculated using the method of two-time, temperature dependent Greens functions. It has been shown that two types of excitations can be occurred: bulk and surface exciton states. Exciton spectral weights and space distribution along the direction of broken symmetry were analyzed as well. Calculating of dynamical permittivity by the single-pole Greens functions have shown that the threshold of light absorption can be moved along frequencies, changing the film thickness and the intensity of boundary perturbations. This can give a great contribution to practical ultrathin film engineering.

Adaption and application of the Green function method to research on molecular ultrathin film optical properties

Recent research of physical properties of materials have shown that low-dimensional systems have more emphasized characteristics than bulk ones, due to quantum size effects. Superconductive films with higher critical parameters than the bulk ones are especially interesting. The basic characteristic of dispersion law of electrons in superconductive films is the presence of energy gaps. The gap size depends strongly on film thickness. Thermodynamic behavior of this system is strongly influenced by gap presence. Electron contribution in specific heat and entropy of superconductive thin film were analyzed on the basis of electron dispersion law in long-wave approximation, as well as their behavior in low temperature (T<TC). It has been shown that both of them linearly depend on temperature, similarly as in bulk structures, but with different slope coefficient (film heat capacity is lower and entropy is higher than in the bulk at the same temperature). Due to poor heat (as well as electric) conducting properties, films are better superconductors (which is experimentally proved). Films are also less ordered systems and closer to the equilibrium state. The explanation of high temperature superconductivity can be found by studying spatially-bounded systems.

Mechanical Oscillations and Charge Carriers in Nanostructures

In this paper, we continue the theoretical study of changes in optical characteristics of superlattices as a consequence of some specific behavior of excitons. Based on the particular dispersion law and the spectral distribution of excitons, we have formulated and calculated the refractive indices of these structures and defined how refractive indices change with external electromagnetic field. It was determined that among wide variety of parameters governing calculus, only two parameters have a significant effect on exciton energy spectra and calculated optical quantities: (a) the number of film layers which constitute superlattice; (b) ratio of the exciton energy transfer between the layers of one film and on the boundary between two different films.

Dielectric properties of molecular crystalline films

Interest in the study of the exciton subsystem in crystalline structures (in this case nanostructures, i.e. thin films) occurred because dielectric, optical, photoelectric and other properties of materials can be explained by means of it. The basic question to be solved concerning theoretical research into the spatially strongly bounded structures is the inability to apply the standard mathematical tools: differential equations and Fourier analysis. In this paper, it is shown how the Green function method can also be efficiently applied to crystalline samples so constrained that quantum size effects play a significant role on them. For the purpose of exemplification of this methods application, we shall consider a molecular crystal of simple cubic structure: spatially unbounded (bulk) and strongly bounded alongside one direction (ultrathin film).