Fractal-like aggregates are usually modeled as monodisperse particles positioned in point contact. However, in reality, much more advanced connections between primary particles exist. In this work, new parameters for measuring both the intersection level and the neck level were introduced. Then, the impact of the connection type on the spectral behavior of fractal-like WO3 aggregates was studied. For light scattering simulations, in the visible spectrum, the ADDA algorithm was used. The results prove that necks have strong impact on the spectral behavior of WO3 aggregates and connections, which exist between primary particles, should not be excluded from models.
In the atmosphere, black carbon aggregates interact with both organic and inorganic matter. In many studies they are modeled using different, less complex, geometries. However, some common simplification might lead to many inaccuracies in the following light scattering simulations. The goal of this study was to compare the spectral behavior of different, commonly used soot particle models. For light scattering simulations, in the visible spectrum, the ADDA algorithm was used. The results prove that the relative extinction error δCext, in some cases, can be unexpectedly large. Therefore, before starting excessive simulations, it is important to know what error might occur.
Small particles tend to connect to each other and create large geometries, namely aggregates. To simplify the light scattering simulation process, they are usually modelled as assemblies of spheres positioned in point contact. This is a rough approximation because connections between them always exist. In this work we present answers to the three following questions: which optical properties of fractal-like aggregates are strongly dependent on the particle shape, what is the magnitude of the relative extinction error σCext when non-spherical particles are modelled as spheres and whether the relative extinction error σCext is dependent on the aggregate size Np. The paper was aimed at tropospheric black carbon particles and their complex refractive index m was based on the work by Chang and Charalampopoulos. The incident wavelength λ varied from λ = 300nm to λ = 900nm. For the light scattering simulations the ADDA algorithm was used. The polarizability expression was IGT_SO (approximate Integration of Greens Tensor over the dipole) and each particle, regardless of its shape, was composed of ca. Nd ≈ 1000 volume elements (dipoles). In the study, fractal-like aggregates consisted of up to Np = 300 primary particles with the volume equivalent to the volume of a sphere with the radius rp = 15nm. The fractal dimension was Df = 1:8 and the fractal prefactor was kf = 1:3. Geometries were generated with the tunable CC (Cluster-Cluster) algorithm proposed by Filippov et al. The results show that when the extinction cross section σCext is considered, the changes caused by the particle shape, which are especially visible for longer wavelengths λ cannot be neglected. The most significant difference can be observed for the regular tetrahedron. The relative extinction error σCext diminishes slightly along with the number of primary particles Np. However, even when large fractal-like aggregates are studied, it should not be considered as non-existent. On the contrary, when light scattering diagrams or the asymmetry parameter g are needed, spherical models can be used, even with relatively small fractal-like aggregates.
Small particles tend to aggregate and create large fractal-like structures which can be analysed using microscopy techniques. In this work we present an algorithm capable of measuring the basic morphological parameters of two-dimensional polystyrene layers. Our study was divided into two separate parts. The goal of the first one was to create high quality particle monolayers. Their purpose was to allow for monitoring of the two-dimensional aggregation process by means of optical microscopy. In the next step microscopy images were analysed in more detail. The size distribution function and the total number of particles were calculated. When an aggregate was larger than a specified size its fractal dimension was approximated using the box-counting technique. After retrieving the morphological parameters fractal-like aggregate models were created using the most common tunable algorithms. Our study proved that real structures resemble to geometries generated with CC (Cluster-Cluster) aggregation techniques. Initial clusters, i.e. those generated during early stages of the aggregation process, are characterized by slightly larger fractal dimension. However, its value decreases along with the aggregation time. The next step is to improve our algorithm even further and use it in a fully automatic on-line monitoring process.
Black carbon particles soon after emission interact with organic and inorganic matter. The primary goal of this work was to approximate the accuracy of the DDA method in determining the optical properties of such composites. For the light scattering simulations the ADDA code was selected and the superposition T-Matrix code by Mackowski was used as the reference algorithm. The first part of the study was to compare alternative models of a single primary particle. When only one material is considered the largest averaged relative extinction error is associated with black carbon (δCext ≈ 2.8%). However, for inorganic and organic matter it is lowered to δCext ≈ 0.75%. There is no significant difference between spheres and ellipsoids with the same volume, and therefore, both of them can be used interchangeably. The next step was to investigate aggregates composed of Np = 50 primary particles. When the coating is omitted, the averaged relative extinction error is δCext ≈ 2.6%. Otherwise, it can be lower than δCext < 0.2%.
Black carbon (BC) particles are a product of incomplete combustion of carbon-based fuels. One of the possibilities of studying the optical properties of BC structures is to use the DDA (Discrete Dipole Approximation) method. The main goal of this work was to investigate its accuracy and to approximate the most reliable simulation parameters. For the light scattering simulations the ADDA code was used and for the reference program the superposition T-Matrix code by Mackowski was selected. The study was divided into three parts. First, DDA simulations for a single particle (sphere) were performed. The results proved that the meshing algorithm can significantly affect the particle shape, and therefore, the extinction diagrams. The volume correction procedure is recommended for sparse or asymmetrical meshes. In the next step large fractal-like aggregates were investigated. When sparse meshes are used, the impact of the volume correction procedure cannot be easily predicted. In some cases it can even lead to more erroneous results. Finally, the optical properties of fractal-like aggregates composed of spheres in point contact were compared to much more realistic structures made up of connected, non-spherical primary particles.
The goal of our paper was to investigate the impact of the necking phenomenon on the optical properties of two
gold nanoparticles characterized by a large value of the imaginary part of the refractive index m. Our results
show that the difference between various neck types is small, therefore, for the modeling process, we recommend
the most simple one, i.e. the cylindrical connector. When a connection between two primary particles is caused
by the overlap factor Cov, and not by the neck size parameter Ycon, the results can be much more erroneous
and the possible error should be at least taken into consideration. The position of the extinction peak in a
function of the connection size Cov is dependent on the value of the complex refractive index m. Additionally,
the applicability of the DDA (Discrete Dipole Approximation) method for measuring the extinction cross section
Cext of gold structures was studied.
The main goal of our research was to investigate the optical properties of nanocomposites, created by combining
titanium dioxide with noble metals. Our research was focused on silver Ag, which reveals strong plasmonic
effects in the visible spectrum, and rutile TiO2 - a commonly used semiconductor material that does not present
any significant optical properties in the mentioned range of electromagnetic waves. Our results show that when
coating is considered (i.e. the TiO2 particle is covered by an additional Ag layer) the position of the extinction
peak can be manipulated and shifted towards lower wavelengths. However, when the main TiO2 particle is
surrounded by small Ag spheres, such extinction peak occurs at a fixed wavelength only and cannot be adjusted.
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