Theses supervised by Prof. Dr. Oğuz Gülseren
20 theses · İhsan Doğramacı Bilkent University
Ws2'nin 1h, 1t ve 1t' fazları için optimize edilmiş stıllınger-weber potansiyelleri: termal taşıma örneği
The advent of graphene has poured numerous amount of research effort into the study 2D materials and utilizing it for device fabrication. Monolayer Transi- tion Metal Dichalcogenides are one such class of polymorphic material with high prospect in versatile device applications due to its unique properties exhibited across the various phases. Classical Molecular Dynamics is a powerful tool that can be utilized to study the thermal and mechanical properties of these phases. Considering this, we optimise Stillinger-Weber type Potential for the seperate 1H, 1T and 1T′ phases of WS2 using Particle Swarm Optimization. These potentials are validated by comparison of phonon dispersion curves, Density Functional The- ory (DFT) based target characteristic data and through an accuracy assessment conducted using Non-Equilibrium Molecular Dynamic (NEMD) simulations to evaluate thermal conductivity of the polymorphic structures. Thermal conduc- tivity results obtained for 1H and 1T′ are in good agreement with first principle predictions calculated using Boltzmann Transport Equation. NEMD simulation of 1T phase prove to be challenging due to its dynamic instability with incoherent buckle structure formation along the symmetric directions.
Elektrik alan altında örnek iki boyutlu sistemlerin elektronik ve optik özellikleri
From optics to nanoelectronics, two-dimensional (2D) materials have drawn attention due to their extraordinary properties such as high carrier mobility, good thermal and electrical conductivity, and mechanical strength. Electronic and optical properties of example 2D systems containing single-layer graphene, 2D molybdenum carbide (Mo2C), and 2D tungsten diselenide (WSe2) under the vertical (perpendicular) static electric field (E-field) varying between 0.1 V/angstrom and 2.5 V/angstrom are investigated by the first principles calculations based on the density functional theory. Contributions of van der Waals interactions are included by selecting a suitable exchange-correlation functional. Electronic band structure and density of states information confirmed that monolayer graphene and single-layer Mo2C exhibit metallic properties whereas 2D WSe2 is a semiconductor with a direct band gap. For all systems up to some magnitude of the E-field, the bands in the valance band were found to be degenerate whereas shifts took place in the conduction band as the E-field was introduced to the system. By increasing E-field amplitudes, the Dirac point shifted upwards in graphene, and sigma∗ band shifted below the Fermi level at 0.5 V/angstrom. In addition to four well-known interband transitions (pi → pi∗ , sigma → sigma∗ , sigma → pi∗, pi → sigma∗), sigma∗ → pi∗ transition is observed. After an electric field amplitude (Ez) of 0.8 V/angstrom, bands due to the s-orbitals of Mo atoms in monolayer Mo2C shifted below the Fermi level. Additionally, pi plasmon peaks redshifted up to 0.4 V/angstrom and blueshifted between 0.6 V/angstrom and 2.5 V/angstrom. For the monolayer WSe2 system, the band gap becomes zero when Ez is greater than and equal to 1.0 V/angstrom which indicates a semiconductor-to-metal transition under the E-field. Shifts below the Fermi level enabled us to n-dope those systems.
Bükülmüş iki katmanlı grafenin geometrisinin, enerjitik ve elektronik yapısının incelenmesi
Twisted bilayer graphene (TBG) manifests unique electronic properties that hold substantial potential for advancements in nanotechnology, material science, and quantum computing. In this thesis, critical insights into the fundamental charac- teristics of TBG are uncovered through an in-depth exploration of its geometric configurations, interlayer interactions, and electronic properties. We begin our investigation with a thorough analysis of the geometrical prop- erties of the twisted bilayer graphene. By plotting the unit cell size against twist angles, we uncover distinct patterns and symmetries that emerge at different an- gles, offering insights into the fundamental structural properties that influence the material's behavior. Following this, we examine the interlayer energies using both Lennard-Jones (LJ) and Kolmogorov-Crespi (KC) potentials. Our analysis of local stacking configurations reveals that the interlayer energy remains invariant due to the averaging contributions from AA and AB regions. We then analyze the band structures across various twist angles using tight- binding calculations, computing parameters such as Fermi velocity and effective mass of the electrons. We observe the emergence of flat bands at "magic angles" and other unique band structures at specific twist angles, highlighting the complex electronic behavior of TBG.
İki boyutlu (2B) malzemelerde anizotropik Meksika-şapkası bant yapısının termoelektrik özellikleri üzerindeki etkileri: Analitik ve hesaplamalı bir inceleme
The electronic band structures of two-dimensional (2D) materials often exhibit non-parabolic, highly anisotropic features near the valence band edge. A particularly intriguing case is the Mexican-hat-like dispersion, characterized by a ring-shaped energy extremum in momentum space. This topology gives rise to a van Hove singularity in the 2D density of states (DOS) and a sharp onset in the electronic transmission spectrum, both of which can significantly enhance thermoelectric performance. However, realistic materials frequently exhibit angular anisotropy in these dispersions, shifting the DOS singularity and degrading transport efficiency. In this thesis, we investigate the interplay between anisotropy in Mexican-hat-like band structures and thermoelectric properties across a family of 2D materials. Using first-principles density functional theory (DFT) and density functional perturbation theory (DFPT), we systematically study seven monolayer compounds, including PbBrF, PbClF, PbIF, BaHI, BiOCl, CaHBr, and SrHI. For each material, we perform structural optimization, electronic band structure analysis, and phonon stability checks. Thermoelectric transport coefficients are computed using the semi-classical Boltzmann transport equation via BoltzTraP, incorporating spin-orbit coupling (SOC) for the PbXF compounds. Phonon-limited lattice thermal conductivity is evaluated through third-order force constants obtained from thirdorder.py and ShengBTE. To analytically interpret the Mexican hat features, we develop a tight-binding model parameterized by the ratio of next-nearest to nearest-neighbor hopping amplitudes (ξ = t2/t1) and an angular anisotropy term β. We demonstrate how ξ and β jointly control the curvature and shape of the band edge, thus tuning the thermoelectric response. Across the PbXF series, SOC suppresses the valence-edge singularity and tends to lower the p-type performance while modestly enhancing the n-type response by sharpening conduction edges. Phonon calculations (second- and third-order force constants; ShengBTE) reveal intrinsically low lattice thermal conductivities due to soft optical modes and strong three-phonon scattering. As a case study in band-structure engineering, we apply biaxial tensile strain (0–6%) to SrHI and show that strain reduces the Mexican-hat anisotropy, slightly decreases the hat height and ring radius, and sharpens the DOS onset. Consistent with the Mott relation, p-type S and the power factor improve (with a mild trade off in conductivity). In contrast, the lattice thermal conductivity decreases as strain softens modes and enlarges anharmonic phase space. Taken together, the results establish clear descriptors and knobs—ξ, β, hat height, and strain—for designing high-performance 2D thermoelectrics.
Düzlemsel ve burulmalı iki boyutlu malzemelerin sürtünme özelliklerinin modellenmesi
The law of friction has been known since the 18th century but yet, the development on the tribology field was established in the last decades mainly by the invention of frictional force microscope (FFM), which enabled scientist to study friction on atomic levels. To describe the friction phenomena at nanoscale, molecular dynamics (MD) and density functional theory (DFT) models are commonly used, popular models and detailed information about friction can be obtained via those models. On the other hand, reduced-order simplified models such as Prandtl-Tomlinson (PT) model can also provide essential information about friction phenomena and understanding a phenomenon via a simplified model is always motivate. In this thesis, Prandtl-Tomlinson model is generalized into three dimensions and the model is illustrated in both two and three dimensions on various quasi two dimensional crystal structures such as graphene, silicene, germanene and hexagonal boron nitride. By solving the equation of motion of the PT model numerically, friction curves and some parametric dependences of the friction such as anisotropy and friction dependence on external loading force is analyzed. We concluded that the PT model in three dimensions provides good results and can be used to analyze friction phenomena to save from computational cost in MD and DFT models
2 boyutlu β-silion monotellürlerin termoelektrik özelliklerinin incelenmesi
Thermoelectric properties of novel 2D silicon monotelluride (SiTe) are studied using first principles calculations. The plane wave method based on den- sity functional theory as implemented in Vienna ab initio simulation package (VASP) is used to calculate the electronic structure. For the exchange correlation functionals, the generalized gradient approximation developed by Perdew-Burke-Ernzerhof (PBE-GGA) is taken into account. The calculated band gap for β-SiTe is 1.83 eV which is in consistence with the previous theoretical data. The electronic and lattice transport properties are investigated using the Boltzmann transport equation. For the electronic transport properties, BoltzTraP code is used which relies on the Fourier interpolation of electronic band structure and thus requires a large k-sampling to optimize the interpolation and produce better results. The Seebeck coefficient obtained at room temperature is 290 μV /K and the figure of merit with κ ` = 0 is 0.98. The density functional perturbation theory (DFPT) is used to calculate the 2nd order harmonic and 3 rd order anharmonic force constants. The phonon dispersion and density of states are computed from the 2 nd order harmonic force constants using Phonopy code. The lattice thermal conductivity and other lattice dependent transport properties are calculated using both the harmonic and anharmonic force constants via ShengBTE program. The specific heat and lattice thermal conductivity at room temperature is 305.5 J/mol K and 1.35 × 10 −3 W/m K, respectively. The figure of merit ZT for β-SiTe at room temperature using the κ' obtained from ShengBTE is 0.78 at 800k.
Haldane fazında bağ değiştiren spin-1/2 XXZ zinciri: DMRG ve fermiyonlaştırma çalışması
In this thesis, the subject of study is the Haldane phase in bond - alternating XXZ chain with S = 12 . We firstly mapped our model to the fermionic chain by the use of standard Jordan-Wigner Transformation, which leads to the famous interacting spinless Su-Schrieffer-Heeger (SSH) fermionic model with modifications. Firstly, we studied trivial quantum phase transition (QPT) under the magnetic field in exactly soluble non-interacting limit, which corresponds to the bond-alternating XX chain . Excitation spectrum, magnetization, magnetic susceptibility are used to characterize QPT and compared with the numerical results. Correlation func- tions for all components of spins are calculated exactly. Secondly, we studied symmetry - protected topological phase transition in the given non-interacting SSH model and characterized it by calculating topological winding number. Fermionized string order parameter as a function of spin coup- lings is obtained and the Haldane phase diagram in the XX limit is confirmed. The correspondence of the Haldane phase in the spin chain in XX limit and topological insulating phase of fermionic non-interacting SSH model is shown. Finally, by the use of entanglement spectrum as an order parameter, we numerically obtained Haldane phase diagram for XXZ model with bond-alternation. For numerical investigation, we used density matrix renormalization group theory in matrix product states formulation (MPS-DMRG) for a system with open boundary conditions (OBC) .
Lojistik otomatik hücreler
Cellular Automata (CA), initially formalized to investigate self-reproducing constructions, are among the most frequently used tools to model and understand complex systems. These computational frameworks are defined in discrete space-time- state domains, where time evolution occurs through local interactions. Despite the simple properties and the succinct absence of long range connections, these implementations have been proven proper for studying large scale collective behavior and self-organizing mechanisms which often emerge in dynamical systems. Following the spirit of the well-known Logistic Map, we introduce a single parameter that tunes the dynamics of totalistic CA by mapping their discrete state space into a Cantor set. By introducing this simple approach on two archetypal models, this study addresses further investigation of several complex phenomena: critical deterministic phase transitions, pattern formation and tunable emanation of self-organized morphologies in these discrete domains. We first apply this approach to Conway's Game of Life and observe sudden changes in asymptotic dynamics of the system accompanied by emergence of complex propagators. Incorporation of the new state space with system features is used to explain the critical points and formulate the tuning parameter range where the propagators adaptively survive, by investigating their autocatalytic sets of local interactions. Similar behavior is present when the same recipe is applied to Rule 90, a totalistic elementary one-dimensional CA. In addition, the latter case shows that transitions between Wolfram's universality classes of CA can be achieved by tuning a single parameter continuously. Finally, we implement the same idea in different models and qualitatively report the expanding complexity that these frameworks support.
Kaydırma ve yükleme altında çift katmanlı grafenin dinamikleri
Sliding and loading bilayer graphene is investigated using the Tight Binding method. We develop interaction-distance dependent tight binding parameter functions to allow for the calculation of band structure for different interacting distances. We investigate the band structures of sliding graphene and loading bilayer graphene, in which case the latter consists of varying interlayer distances between monolayers. We show that based on developed parameter models, band splittings can be seen to emerge in the band structures, which follow different patterns for different sliding directions. As expected we confirm that for varying vertical interlayer distances, monolayer graphene band structure is the limit for both AA and AB stacking configurations. By applying a quadratic energy model to the curvature of the band structures in the vicinity of the K-point for AB stacking configuration we predict the effective mass of electrons and holes in bilayer graphene, and electrons in monolayer graphene. We also show the pattern of change of effective mass with respect to changing interlayer distance, and try to investigate where our prescribed quadractic energy model breaks down, as we approach the limit of the monolayer band structure.
İki boyutlu seçilmiş grup II-VI kalkojenitlerin optik özelliklerinin çoklu-cisim teorilerine dayanan incelenmesi
Two-dimensional (2D) metal oxides (MOs) and metal chalcogenides (MChs) are emerging classes of 2D materials. Depending on the constituent elements, these materials can display various electronic and optical properties making them promising candidates in many device applications, such as solar cells and transparent circuits. Binary graphene-like structures of II-VI are the most straightforward structures of 2D MOs and MChs. We systematically examined the electronic and optical properties of selected 2D structures from this category: BeO, BaTe, CdO, CaO, CaS, MgO, SrS, SrSe and ZnO. The dynamical stability of these materials has been reported in previous studies. In 2D semiconductors, excitonic effects dominate the optical properties. Theoretical investigation of such phenomena requires employing many-body approaches beyond standard density functional theory. We utilized a single shot of GW approximation to predict the electronic band structure and solved the Bethe-Salpeter equation in the Tamm-Dancoff approximation to consider excitonic effects. Our results show that all structures possess indirect band gaps except ZnO and CdO. Furthermore, the considered structures have large exciton binding energies ranging from 0.72 eV in CdO to 2.84 eV in BeO. CdO has the smallest calculated optical band gap with a value of 1.43 eV. Analyzing the optical absorption spectra reveals that the CdO can absorb 7.9 % of the incident light in its optical band gap. The maximum amount of absorption appears in BeO, which can absorb 28% of incident light in the ultraviolet region. Among the structure mentioned above, there is a close matching between the lattice constants of ZnO and MgO, promising for creating lateral and vertical heterostructures. Due to the enhanced performance resulting from mixing distinct properties of individual monolayers, van der Waals heterostructures (vdWHs) are regarded as a revolutionary class among a plethora of presently fabricated or predicted 2D materials. Alongside vdWHs, recent studies have also reported 2D heterostructures with interlayer bonding. Motivated by the flourishing properties of vertical heterostructures, we comprehensively examined the mechanical, electronic and optical properties of ZnO/MgO structures in four different stackings. Structural relaxation has indicated two vdWHs and two structures with interlayer binding. All considered structures are mechanically stable. In addition, phonon dispersion curves show that the AB stacking formed by placing the Mg atom on top of the O atom of the ZnO layer is also dynamically stable at zero temperature. The s orbital of Zn atom dominates the minimum of the first conduction band of these structures. The optical absorbance spectra show that strong excitonic effects reduce the optical band gap to the visible light spectrum range, and all structures can absorb around 8% of incident light.
GaAs tabanlı iki boyutlu (2b) yapıların kararlılıkları
Graphene is a two dimensional material isolated for the first time in 2004. After this, two dimensional materials has become an appealing research area for the scientists because of their exotic properties. In search for two dimensional materials, both experimental and theoretical investigations have been carried out. First-principles approaches have been used to predict silicene and germane theoretically. A technologically important semiconductor material is GaAs, however there is no report of two dimensional materials which is based on GaAs. We attempted to find a new stable 2D structure which is formed from either Ga and As atoms based on GaAs or its functionalized form with O atoms. In search for such a system, we performed density functional theory based calculations by using a plane-wave pseudopotential method. We used local density approximation for the exchange correlation potential. First, we performed geometrical optimization calculation in order to identify possible stable structures. We obtained electron band diagrams and phonon dispersion relations to check electronic properties and stability of these materials. We found three structures which are based on GaAs (100), (110) and (110) surfaces. We found that these two dimensional materials are geometrically stable but after performing phonon calculations we observe that there are some negative energy modes. In addition we identified one system which is based on Ga, As, and O atoms. Even though this structure is stable after geometry optimization, it has negative phonon modes in its phonon band diagrams.
Prandtl-Tomlinson modelini kullanarak iki boyutumsu sistemlerin sürtünme özelliklerinin incelenmesi
Tribology, the study of friction, is both an old theoretical problem in physics and an area of great practical importance. The invention of experimental instru- ments such as Atomic Force Microscope (AFM) has lead to the emergence of the eld of nanotribology, the exploration of friction phenomenon at the nanoscale. While more complete descriptions of friction make use of density functional the- ory (DFT) and molecular dynamics (MD) simulations, many essential features of frictional phenomena are accurately modeled by so called "reduced order models" such as the Prandtl-Tomlinson (PT) Model. We illustrate the PT model in both one-dimensional and two-dimensional forms via application to various crystal lattice surfaces (cubic, planar hexagonal) and reproduce important results from the literature by solving the resulting Langevin equation within the PT model. We also discuss the parameter dependence in this model via relevant simulations. We then generalize the PT model to a three-dimensional case and analyse quasi-two-dimensional systems. These systems thus exhibit a small amount of "buckling" - i.e. with out-of-plane basis atoms. The equations of motion of the Prandtl-Tomlinson model are solved numerically and the resulting friction force curves, tip path and lattice are analysed comparatively. The results agree with underlying theory and make testable predictions. We conclude that our gen- eralized, three-dimensional PT model is a good approximation to the frictional dynamics at this scale for these systems and has the advantage of being compu- tationally less intensive than full scale MD or DFT calculations.
Ikı boyutlu ve ıkı boyutumsu malzemelerın sıkı bağ metoduyla modellenmesı
Since the advent of graphene, two-dimensional (2D) materials have consistently been studied owing to their exceptional electronic and optical properties. While graphene is completely two-dimensional in nature, its other analogues from the group IV A elements in the periodic table have been proven to have a low-buckled structure which adds up the exotic properties exhibited by them. The semiconductor industry is striving for such materials exhibiting exotic electronic, optical and mechanical properties. In this thesis work we are primarily working towards a generalized tight-binding (TB) model for the 2D family of group IV A elements. Graphene has been studied extensively and we have successfully reproduced its energy band-structure accounting up to the third nearest neighbor contributions. The results have been checked extensively by performing simulations over a large set of avail- able parameters and are found to be accurate. The other graphene analogues (viz; silicene, germanene and stanene) exhibiting a hexagonal 2D structure have been reported to have a buckling associated to them. We have analytically built up a TB model by considering the orbital projections along the bond length which accounts for the buckling in these 2D structures. Electronic band-structures have been reproduced and compared by taking into account the nearest neighbor and next-nearest neighbor contributions. Since these Structures exhibit a Dirac like cone at the Dirac point and showing linear dispersion, study of electronic band-structures in detail becomes indispensable. After the famous Kane and Mele paper on Quantum Spin Hall Effect in Graphene, condensed matter physicists have been looking for similar phenomena in other 2D materials. We have successfully included the spin-orbit coupling (SOC) contribution to our unperturbed Hamiltonian and were able to produce splitting around the Dirac points. Since, Silicene and its other analogues exhibit same structure with different amount of buckling, we were able to track down the whole energy band-structure. Alongside this thesis also focuses on calculating optical properties of these materials. In essence, this thesis work is an insight to the electronic and optical properties of the hexagonal 2D structures from the carbon family group. Derived structures from these 2D materials (viz; quantum dot, nano ribbon) could easily be studied utilizing the tight-binding formulation presented here. The proposed future work is the inclusion of nitrides and transition metal dichalcogenides (TMDCs) in the TB model.
Grafenin elektronik yapısının dış alanların etkisi altında incelenmesi
In this thesis, the electronic structure of graphene under the influence of external fields such as strain or magnetic fields is investigated by using tight-binding method. Firstly, we study graphene for a band gap opening due to uniaxial strain. In contrast to the literature, we find that by considering all the bands (both ? and ? bands) in graphene and including the second nearest neighbor interactions, there is no systematic band gap opening as a function of applied strain. Our results correct the previous works on the subject. Secondly, we examine the bandstructure and Hall conductance of graphene under the influence of perpendicularmagnetic field. For graphene, we demonstrate the energy spectrum in the presence of magnetic field (Hofstadter Butterfly) where all orbitals are included. We recover both the usual and the anomalous integer quantum Hall effects depending on the proximity of the Dirac points for pure graphene and the usualinteger quantum Hall effect for pure square lattice. Then, we explore the evolution of electronic properties when imperfections are introduced systematically to the system. We also demonstrate the results for a square lattice which has adistinct position in cold atom experiments. For the energy spectrum of imperfect graphene and square lattice under magnetic field (Hofstadter Butterflies), we find that impurity atoms with smaller hopping constants result in highly localized states which are decoupled from the rest of the system. The bands associated with these states form close to E = 0 eV line. On the other hand, impurity atoms with higher hopping constants are strongly coupled with the neighboring atoms. These states modify the Hofstadter Butterfly around the minimum and maximum values of the energy and for the case of graphene they form two self-similar bands decoupled from the original butterfly. We also show that the bands and gaps due to the impurity states are robust with respect to the second order hopping. For the Hall conductance, in accordance with energy spectra, the localized states associated to the smaller hopping constant impurities or vacancies donot contribute to Hall conduction. However the higher hopping constant impurities are responsible for new extended states which contribute to Hall conduction. Our results for Hall conduction are also robust with respect to the second order interactions.
Grafenin işlevleştirlmesinin ilk prensip teknikleriyle incelenmesi
The graphene sheet is a single-atom thick novel material and attracts greatinterest due to its unique features. However, it is a metallic material with nobandgap, which makes it di cult to integrate in electronic applications. Adatomadsorption is one of the promising ways to make this structure functional. Tothis end, electronic and structural properties of graphene have been investigatedby using density functional theory formalism in order to understand atomic levelinteraction between halogen adatoms and graphene layer. The most commonadatom, hydrogen, has also been studied. In this study, plane-wave, pseudo-potential density functional theory calculations were carried out using generalizedgradient approximations for the exchange correlation potential with the QuantumEspresso package. In order to obtain fully relaxed structures, geometry optimiza-tion has been performed in all of the calculations. The adatom-graphene systemis modelled with a 4 4 graphene supercell. Adsorption energies of halogenadatoms and dimers adsorbed on highly symmetric positions on graphene layerare calculated. Dierent con gurations of adatoms have been tested. Speci cproperties such as band structure and density of states of these system have beeninvestigated. The results show that a fully covered graphene layer is stable andoptimized structures exhibit a band gap of a few eV. The most stable structureamong halogen adatoms is the uorine adsorbed on graphene. It has the highestelectronegativity, which is the reason for high electron transfer from the graphenelayer. This is the reason of the formation of covalent bonds. Furthermore, themost stable con guration is found to be chair con guration with the halogenatoms alternating in both sides of the layer.Keywords: graphene, adsorption on graphene, graphane, halogen atoms, DFT,plane-wave pseudo-potential calculations
Güneş pilleri için metal nanoparçacık plazmonik uygulamaları
In today's economy, need for development in energy is essential. Solar energyis safe, and at the same time is one of the cleanest, cheapest choices of energyalternative to fossil fuels. In this perspective, using the sun light eectively is infundamental importance. One of the problems, because of the indirect band gapof the material Si, is small energy conversion ratios of various solar cell structuresand limited absorption of red light. Because of the material properties, Si cellscannot absorb red light, which contributes great amount of the sun light. Oneof the recent developed techniques to use red light is using metal nanoparticles(MNP) embedded in a semiconductor medium as sub-wavelength antennas orMNP scatterers, hence increasing the eective path length of light in the cell.Absorption and scattering are mostly in plasmon resonances. Shifting theplasmon resonance peaks is possible by changing various parameters of the system like the size of the MNPs. In this work, Finite-Dierence Time-Domain(FDTD) method is used to analyze various systems worked. Mainly the MEEPpackage, developed at MIT, is used to simulate systems and other codes, relatedto analytical work, have also used to compare results. The plasmon resonances ofvarious sizes of Ag MNPs embedded in dierent mediums at dierent positionsare analyzed. Critical parameters like particle size, shape, dielectric medium, lmthickness are discussed for improved solar cell applications.Keywords: Solar Cells, Plasmonics, metal nanoparticles, Ag nanoparticles,FDTD, MEEP, Drude, Lorentz..
Silisyum (001) yüzeylerinde iridyum atomlarının ilk prensip hesaplardan incelenmesi
Self-assembled nanowire growth on semiconductor surface is based on deposing sub-monolayer material over the surface. Even though high resolution STM image gives plausible surface analysis, determination of the nanowire structure is the most diffucult part of these experiments. Due to the this reason, first-principles investigation is essential to understand the one dimensional nanowire structure grown over the surface as well as the STM images of these structures. Recently, iridium silicide nanowire on Si (001) surface is observed. In this thesis, we study formation of the nanowire after deposition of Ir on Si(001). Ab-initio plane wave pseudopotential calculations are performed for number of iridium silicide nanowires generated by increasing iridium coverage on Si(001) surface. For the iridium coverage as 0.125 ML, the possible nanowire formation is analyzed and its calculated STM images are compared with experimental STM image. As a result of our detailed analysis, we suggest that the STM image observed at experiment doesn?t consist of Ir atoms since Ir atom tends to be buried into the Si bulk. We model the possible nanowire formation which is consistent with pseudo-STM calculation. According to our model, iridium is placed at the troughs between the dimer rows on the surface and it creates a structure by breaking the Si dimer bonds. The coverage implied by the model, is consistent with experimental numbers.
Model nanotellerde termoelektrik verimlilik
Nowadays, the use of thermoelectric semiconductor devices are limited by their low efficiencies. Therefore, there is a huge amount of research effort to get high thermoelectric efficient materials with a fair production value. To this end, one important possibility for optimizing a material's thermoelectric properties is reshaping their geometry. The main purpose of this thesis is to present a detailed analysis of thermoelectric efficiency of 2 lead systems with various geometries in terms of linear response theory, as well as 3 lead nanowire system in terms of the linear response and nonlinear response theories. The thermoelectric efficiency both in the linear response and nonlinear response regime of a model nanowire was calculated based on Landauer-Büttiker formalism. In this thesis, first of all, the electron transmission probability of the system at the hand, i.e. 2 lead or 3 lead systems are investigated by using R-matrix theory. Next, we make use of these electron transmission probability of model systems to find thermoelectric transport coefficients in 2 lead and 3 lead nanowires. Consequently, the effect of inelastic scattering is incorporated with a fictitious third lead in the 3 lead system. The efficiency at maximum power is especially useful to define the optimum working conditions of nanowire as a heat engine. Contrary to general expectation, increasing the strength of inelastic scattering is shown to be a means of making improved thermoelectric materials. A controlled coupling of the nanowire to a phonon reservoir for instance could be a way to increase the efficiency of nanowires for better heat engines.
Grafen tabanlı nanoyapıların ilk-prensip hesaplar ile incelenmesi
In this thesis, first-principles investigations of several graphene related nano-systems based on density functional theory are presented. First, the electronic structure of several graphene nano-ribbons both in 1D and 0D (up to systems with more than 1000 atoms) including all types (armchair, zigzag and chiral) are discussed using tight binding calculations. We observed that the band gap of the ribbons depend both on the length of the ribbon and the angle of chirality. Second, the effect of phosphorus and sulfur during the growth of carbon nanotubes is investigated from ab-initio density functional theory based calculations. To this end, we present the binding chemistry of phosphorus and sulfur atoms on graphene with and without vacancies and kink like defect structures. Consequently, the difference between the bindings of these two atoms is discussed in order to understand the reason behind their effects on the growth mechanism. The details of the phosphorus or sulfur binding are important in order to understand the occurrence of Y-junctions and kinks in carbon nanotubes as well. Third, we focus on the interaction of bilayer graphite and multi-walled carbon nanotubes with the Li atom since these materials are prime candidates for the electrodes for battery applications. The need for rechargeable batteries with high capacity increased enormously by the invention of electronic devices like cell phones or MP3 players. Hence, there is a huge effort to develop and improve Li-ion batteries. Therefore, we have investigated interaction of Li with graphene and Li intercalation to bilayer graphene and multi-walled carbon nanotubes from planewave pseudo potential calculations. Finally, super-periodic graphitic structures observed through scanning tunnelling microscope are described and investigated from density functional calculations. The difference between the observed and actual periodicity and the occurrence of the so-called Moire patterns are explained in terms of geometrical calculations and the charge density of these systems.
Doğrusal karbon zincirlerinin yoğunluk fonksiyoneli teorisi ile incelenmesi
In this thesis the structural and electronic properties of linear carbon chains are investigated using density functional theory. Polyyne structure of alternating single and triple bonds was shown to be energetically favored structure compared to successive double-double bonded cumulene structure. Band calculations showed that polyyne is a semiconductor whereas cumulene is a metal. Phonon calculations showed that cumulene is unstable. When put in a hexagonal formation these chains are found to form three possibly stable structures, one tightly bound hexagonal tube, and two loosely bound structures one which can be described as a hexagonal assembly of polyyne chains and one which can be considered stacks of hexagonal carbon flakes. Electronic band structure calculations showed that all three structures are semiconductors. Charge density profile showed strong chemical bonds both in vertical and horizontal directions for the first structure, whereas second structure of polyyne chains had no strong bonds between chains and third structure of hexagon flakes showed no strong bond between hexagon flakes. It is also found that as hexagon size shrinks the favored structure of chains changes from polyyne to cumulene and a band structure calculation showed that a semiconductor to metal transition happens.