Changes in Optical Properties of Molecular Nanostructures (original) (raw)
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Optical Properties of Nanostructures
Journal of Computational and Theoretical Nanoscience, 2011
We analyzed linear molecular chain with exciton excitations for the case when number of excitons is not conserved. The results obtained are in some sense amazing. The dispersion law of finite chain is surface depending on two independent angles. The same conclusion is valid for concentrations of exitons and exciton pairs. As it was expectable physical characteristics of the finite chain depend on spatial coordinates. All results are compared to corresponding results of infinite chain.
Physica Scripta, 2009
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 method's application, we shall consider a molecular crystal of simple cubic structure: spatially unbounded (bulk) and strongly bounded alongside one direction (ultrathin film).
Adequate determination of micro and macro properties of optical nano-crystals
Opto-Electronics Review, 2017
The main problem in theoretical analysis of structures with strong confinement is the fact that standard mathematical tools: differential equations and Fourier's transformations are no longer applicable. In this paper we have demonstrated that the method of Green's functions can be successfully used on lowdimension crystal samples, as a consequence of quantum size effects. We can illustrate a modified model through the prime cubic structure molecular crystal: bulk and ultrathin film. Our analysis starts with standard exciton Hamiltonian with definition of commutative Green's function and equation of motion. We have presented a detailed procedure of calculations of Green's functions, and further dispersion law, distribution of states and relative permittivity for bulk samples. After this, we have followed the same procedures for obtaining the properties of excitons in ultra-thin films. The results have been presented graphically. Besides the modified method of Green's functions we have shown that the exciton energy spectrum is discrete in film structures (with a number of energy levels equal to the number of atomic planes of the film). Compared to the bulk structures, with a continual absorption zone, in film structures exist resonant absorption peaks. With increased film thickness differences between bulk and film vanish.
Specificities in Optical Absorption of Ultrathin Molecular Film-Structures
Contemporary Materials, 2010
This paper represents a theoretical study of changes of fundamental optical properties in symmetric perturbed quasi-two-dimensional structures-nanofilm molecular crystal patterns. The energy spectrum and the state of excitons, as well as their spatial distribution along the border layer (by layers) was found on the basis of adjusted Green's functions and analytic-numerical calculations. Relative permittivity was determined and the influence of limit parameters on the occurrence of resonant absorption was studied. Dependence of the absorption index on the frequency of external electromagnetic field was found and selective absorption defined with nanofilms with symmetric border surfaces.
Photonics and Nanostructures: Fundamentals and Applications, 2018
The basis of modern optical engineering involves makes finding ways to design predefined optical (and in generalphysical) properties of nanoscopic patterns. The boundary parameters of nano-patterns depend on (small) dimensions and the type of nanostructure substance, the external environment, as well as the form and the technical-technological of aspect of production. Fundamental properties of nano-structures can be drastically changed by changing these parameters. We have investigated the optical specificities of molecular dielectric crystal nanofilm under the influence of different confinement conditions. This paper presents a model of crystalline ultrathin molecular film and an analysis of dielectric, i.e. optical properties of these spatially much bounded structuresin their entirety. Using the two-time dependent Green's functions, the energy spectrum, the possible exciton states and their space distribution were determined and the dynamic permittivity was calculated. It was shown that the appearance of localized states in the boundary layers of the film depends on the thickness and the changing values of the system parameters in the boundary areas of the film. These localized states define the schedule and determine the number of resonant absorption peaks in the infrared region of the external electromagnetic radiation (EMR). Analytical analysis of the impact of the boundary parameters on the changes of the dielectric and optical properties of the nanofilm, as compared to the same properties of bulk samples with identical crystalline and chemical structure, is impossible. Thus, a software package (dubbed JOIG_S) has been developed and applied to perform the numerical analysis and plot graphic displays of the relation between microscopic (exciton) and macroscopic (dielectric and optical) properties as a function of the frequency of an external electromagnetic field (EMF), for a specified set of values of the boundary parameters. Optical, i.e. absorption and refraction properties of observed nanostructures demonstrate very narrow and strictly discrete characteristics. Characteristic resonant peaks appear in the dependence of the absorption index on the frequency of external EMF. All peaks fall into infrared region and indicate absorption of corresponding external EM frequencies. The number and distribution of these peaks depend on the number of layers in the film and the perturbation parameters, as a consequence of the quantum size and confinement effects. This proved that the outer environment of the film affects the given fundamental properties of a nano film, i.e. their choice/ change directly controls the optical properties of the film. Such an approach could be considered as a kind of optical engineering.
Exciton States Spectroscopy in Quasi - Zero - Dimensional Nanostruсtures: Theory
Optics, 2014
The theory of exciton states in a quantum dot under conditions of dominating polarization interaction of an electron and a hole with a spherical (quantum dot-dielectric matrix) interface is developed. An shown, that the energy spectrum of heavy hole in the valence band quantum dot is equivalent to the spectrum of hole carrying out oscillator vibrations in the adiabatic electron potential. In the framework of the dipole approximation studied interband absorption of light in a quasi-zero-dimensional nanosystems. We show that the absorption and emission edge of quantum dots is formed by two transitions of comparable intensity from different hole size-quantization levels and into a lower electron sizequantization level. Propose a theoretical prospect of using hole transitions between equidistant series of quantum levels observed in nanocrystals for desining a nanolaser.
Nature Materials, 2006
Nanoscale systems are forecast to be a means of integrating desirable attributes of molecular and bulk regimes into easily processed materials. Notable examples include plastic light-emitting devices and organic solar cells, the operation of which hinge on the formation of electronic excited states, excitons, in complex nanostructured materials. The spectroscopy of nanoscale materials reveals details of their collective excited states, characterized by atoms or molecules working together to capture and redistribute excitation. What is special about excitons in nanometre-sized materials? Here we present a cross-disciplinary review of the essential characteristics of excitons in nanoscience. Topics covered include confinement effects, localization versus delocalization, exciton binding energy, exchange interactions and exciton fine structure, exciton-vibration coupling and dynamics of excitons. Important examples are presented in a commentary that overviews the present understanding of excitons in quantum dots, conjugated polymers, carbon nanotubes and photosynthetic light-harvesting antenna complexes.
Boundary Influence on Permittivity in Molecular Films
Acta Physica Polonica A, 2007
A microscopic theory of optical 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 was shown that two types of excitations can occur: bulk and surface exciton states. Analysis of the optical properties (i.e. dielectrical permittivity) of these crystalline systems for low exciton concentration shows that the permittivity strongly depends on boundary parameters and the thickness of the film. Influences of boundary conditions on optical characteristics of these nanostructures were especially analyzed.
Different regions of exciton localized states in ultrathin dielectric films
Journal of Physics and Chemistry of Solids, 2017
This paper presents the results of research conducted in the field of confinement effects in ultrathin dielectric films. Method of choice is the method of Green's functions which proved to be a very powerful tool for theoretical research in solid state physics. Among many other properties of materials, such as optical, conductive, etc. with this method it is possible to obtain critical values of boundary parameters corresponding to all transitions from bulk to localized exciton states. This research is valid for four layered crystalline film, since that type of ultrathin structure was the object of research. Conditions for occurrence of one, two, three, or even four localized states have been found and analyzed and regions with equal number of localized states have been defined. This corresponds to the process of reconstruction of the energy spectra of excitons in quasi 2D nanostructures with respect to those in corresponding bulk-structures.
Organic thin film based excitonic nanostructures are of a great interest in modern resonant nanophotonics as a promising alternative for plasmonic systems. Such nanostructures sustain propagating and localized surface exciton modes which can be exploited in refractive index sensing and near-field enhanced spectroscopy. To realize these surface excitonic modes and to enhance their optical performance, the concentration of the excitonic molecules present in the organic thin film has to be quite high so that a large oscillator strength can be achieved. Unfortunately, this often results in a broadening of the material response which might prevent achieving the very goal. Therefore, systematic and in-depth studies are needed on the molecular concentration dependence of the surface excitonic modes to acquire optimal performance from them. Here, we study the effect of molecular concentration in terms of oscillator strength and Lorentzian broadening on various surface excitonic modes when e...