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First-Principle Study of Electronic Properties of Ultrathin Layers
Nezval, David ; Vázquéz, Hector (referee) ; Friák, Martin (referee) ; Bartošík, Miroslav (advisor)
Tato práce charakterizuje strukturní vlastnosti adsorbovaných atomů galia a molekul vody na grafenu. Zkoumá také změny elektronových vlastností grafenu způsobené adsorpcí jednotlivých adsorbentů. Výpočty pomocí teorie funkcionálu hustoty (DFT) jsou ideálním nástrojem pro zkoumání a vysvětlení fyzikálních a chemických procesů, které probíhají během adsorpce. Elektronové vlastnosti jsou studovány pomocí výpočtů pásové struktury a Baderovy analýzy náboje. Nedávná experimentální zjištění odhalila, že při nízké koncentraci atomy Ga dopují grafen elektrony (n-dopování). Tento dopovací efekt se snižuje při vyšších koncentracích Ga atomů, kdy dochází ke tvorbě klastrů. Tato práce představuje adsorpci jednotlivých atomů Ga a klastrů tvořených více atomy Ga. Zatímco jednotlivé atomy n-dopují grafen 0,64 elektrony, atomy vázané v klastrech na sebe vzájemně působí, a tím oslabují dopování grafenu. Tvorba klastrů je zásadně ovlivněna difuzí atomů Ga po povrchu grafenu. Proto je část práce věnována výpočtům energie difuzní bariéry a tomu, jak může být tato bariéra ovlivněna nabitím grafenu. Experimentální pozorování ukazují, že u grafenu jež je vystaven působení molekul vody, dochází k dopování kladnými nosiči náboje (p-dopování). Tato pozorování však nebyla podpořena výpočty DFT. Tato práce zkoumá vliv vícevrstvé vody na elektronové vlastnosti grafenu. Pozornost byla věnována ovlivňování dopování grafenu v závislosti na orientaci molekul vody v první vrstvě nejblíže grafenu. Prezentované výsledky ukazují p-dopování grafenu, když je 6 nebo více vrstev vody orientováno kyslíkem ke grafenu.
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Anion-exchange enabled tuning of caesium lead mixed-halide perovskites for high-energy radiation detection
Matula, Radovan ; Friák, Martin (referee) ; Dvořák, Petr (advisor)
Olovnato-halogenidové perovskity (OHP) se svými bezprecedentními funkčními vlastnostmi, které umocňuje jednoduché ladění zakázaného pásma, vzaly svět polovodičů útokem. Proces aniontové výměny, možný i po syntéze, umožňuje ladění zakázaného pásu OHP, což vede k olovnato-mix-halogenidovým perovskitům (OMHP), čímž se rozšiřuje jejich potenciál pro aplikace, zejména v oblasti laditelných detektorů. Širokému přijetí LMHP však brání jejich chemická nestabilita, která vede k segregaci halogenidů v materiálu, což zásadně omezuje spolehlivý provoz zařízení na bázi OMHP. Pochopení kinetiky segregace halogenidu přes delší časové intervaly zůstává výzvou, nabízi se tak použití teoretických simulací, jako jsou metody Monte Carlo (MC). Avšak MC simulace spoléhají na dobře definované povrchy potenciální energie (PPE), typicky odvozené z výpočetně náročných výpočtů teorie funkcionálu hustoty (DFT). V této práci navrhujeme nový přístup pro konstrukci dobře definovaných PPE z vysoce přesných DFT dat se zlomkem výpočetního zatížení. Využitím activation-relaxation technique noveau (ARTn) pro motivaci hledání přechodových bodů v PPE v kombinaci s nejmodernějšími přístupy strojového učení se snažíme výrazně snížit výpočetní náklady. Navíc s využitím klasické teorie hodnotíme detekční schopnosti vybraných OMHP.
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Nanoparticels from brake pads wear
Švábenská, Eva ; Roupcová, Pavla ; Friák, Martin ; Schneeweiss, Oldřich
Exposure to particulate air pollution has been associated with a variety of health problems. One of the main\nsources of metal-rich airborne particulate pollution in roadside environments are brake-wear emissions. The \nscanning electron microscopy (SEM) and transmission electron microscopy (TEM), Mössbauer spectroscopy \n(MS), X-ray powder diffraction (XRD) was used to a make quantitative evaluation of wear particles in two \nsamples. Sizes, morphologies, oxidation state, and trace element compositions of wear particles were\ninvestigated using combination of several methods revealed high concentrations of Fe species and phases, \noften associated with carbonaceous material. SEM and TEM observations show that brake wear particles with \na size of tens to hundreds of nm mainly form particle agglomerates. However, ultrafine (<100 nm) particles are \nlikely to pose a health threat after inhalation and/or ingestion.
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First-principles studies of the electronic and structural properties of Lead Zirconate Titanate (PZT)
Planer, Jakub ; Friák, Martin (referee) ; Bartošík, Miroslav (advisor)
This work is focused on Density Functional Theory (DFT) calculations of oxygen vacancy diffusion barriers in mixed perovskite lead zirconate titanate and its pure counterparts. We found out that barrier heights are different in lead titanate and lead zirconate caused by the different localization of the excess electrons due to the oxygen vacancy formation. Diffusion barriers were also determined for titanium-rich mixed phases and compared to experimental values. This work contributes to clarify unusually low experimentally measured diffusion coefficients in PZT. We found out that the induced vacancy states are forming localized bonds to the lead atoms which causes the oxygen vacancies to become immobile due to the increase of the activation energy of the diffusion process.
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DFT calculations of graphene regarding to biosenzoric applications
Špaček, Ondřej ; Friák, Martin (referee) ; Bartošík, Miroslav (advisor)
This diploma thesis is focused on calculation of both structure and electronic properties of the graphene after the adsorption of atomic and molecular oxygen and urea using the Density Functional Theory (DFT). The influence of van der Waals interactions on the structure and adsorption energy is studied, as well as influence of the thermal corrections, the charge density spatial distribution and the electronic doping of graphene after the adsorption of the adsorbant on the graphene.
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Computational prediction of solubility limits in solid solutions
Fikar, Ondřej ; Friák, Martin (referee) ; Zelený, Martin (advisor)
This diploma thesis is focused on a theoretical study of the phase stability of solid solutions in selected aluminium and silver alloys. The ab initio calculations were performed using projected augmented waves method and the thermal dependencies of thermodynamic quantities were acquired using phonon calculations. The main focus of this work is the of aluminium-germanium alloy, while the other examined alloys (Al-Pb, Ag-Ge and Ag-Pb) serve the purpose of comparison of the solid solubility prediction and its reliability in systems with different composition. The temperatures, at which the solid solutions become stable, were evaluated using the energy difference between possible states and this evaluation was carried out for different contributions to the total energy. Also the electronic and phonon densities of states were calculated for all pure elements and solid solutions. The temperatures of solubility were compared to the experimental ones provided by the CALPHAD method and the individual contributions to the total energy were determined and depicted. The results obtained in this work tend to underestimate temperatures of solubility of individual solid solutions by hundreds of Kelvin.
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AB INITIO STUDY OF SILVER NANOPARTICLES, GRAIN BOUNDARIES AND THEIR \nQUADRUPLE JUNCTIONS
Polsterová, Svatava ; Všianská, Monika ; Friák, Martin ; Pizúrová, Naděžda ; Sokovnin, S. ; Šob, Mojmír
Motivated by our experimental research related to silver nanoparticles with various morphologies, we have employed quantum-mechanical calculations to provide our experiments with theoretical insight. We have computed properties of a 181-atom decahedral silver nanoparticle and two types of internal extended defects, -5(210) grain boundaries (GBs) and quadruple junctions (QJs) of these GBs. We have employed a supercell approach with periodic boundary conditions. Regarding the thermodynamic stability of the decahedral nanoparticle, its energy is higher than that of a defect-free face-centered cubic (fcc) Ag by 0.34 eV/atom. As far as the -5(210) GB is concerned, its energy amounts to 0.7 J/m2 and we predict that the studied GBs would locally expand the volume of the lattice. Importantly, the system with GBs is found rather close to the limit of mechanical stability. In particular, the computed value of the shear-related elastic constant C66 is as low as 9.4 GPa with the zero/negative value representing a mechanically unstable system. We thus predict that the -5(210) GBs may be prone to failure due to specific shearing deformation modes. The studied GBs have also the value of Poisson’s ratio for some loading directions close to zero. Next, we compare our results related solely to -5(210) GBs with those of a system where multiple intersecting -5(210) GBs form a network of quadruple junctions. The value of the critical elastic constant C66 is higher in this case, 13 GPa, and the mechanical stability is, therefore, better in the system with QJs.
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DFT calculations of graphene regarding to biosenzoric applications
Špaček, Ondřej ; Friák, Martin (referee) ; Bartošík, Miroslav (advisor)
This diploma thesis is focused on calculation of both structure and electronic properties of the graphene after the adsorption of atomic and molecular oxygen and urea using the Density Functional Theory (DFT). The influence of van der Waals interactions on the structure and adsorption energy is studied, as well as influence of the thermal corrections, the charge density spatial distribution and the electronic doping of graphene after the adsorption of the adsorbant on the graphene.
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Computational prediction of solubility limits in solid solutions
Fikar, Ondřej ; Friák, Martin (referee) ; Zelený, Martin (advisor)
This diploma thesis is focused on a theoretical study of the phase stability of solid solutions in selected aluminium and silver alloys. The ab initio calculations were performed using projected augmented waves method and the thermal dependencies of thermodynamic quantities were acquired using phonon calculations. The main focus of this work is the of aluminium-germanium alloy, while the other examined alloys (Al-Pb, Ag-Ge and Ag-Pb) serve the purpose of comparison of the solid solubility prediction and its reliability in systems with different composition. The temperatures, at which the solid solutions become stable, were evaluated using the energy difference between possible states and this evaluation was carried out for different contributions to the total energy. Also the electronic and phonon densities of states were calculated for all pure elements and solid solutions. The temperatures of solubility were compared to the experimental ones provided by the CALPHAD method and the individual contributions to the total energy were determined and depicted. The results obtained in this work tend to underestimate temperatures of solubility of individual solid solutions by hundreds of Kelvin.
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