1,721,066 research outputs found
Modeling of Nanostructures
No full textMaterials properties show a dependence on the dimensionality of the systems studied. Due to the increased importance of surfaces and edges, lower-dimensional systems display behavior that may be widely different from their bulk counterparts. As a means to complement the newly developed experimental methods to study these reduced dimensional systems, a large fraction of the theoretical effort in the field continues to be channeled towards computer simulations. This chapter reviews briefly the computational methods used for the low dimensional materials and presents how the materials properties change with dimensionality. Low dimensional systems investigated are classified into a few broad classes: 0D nanoparticles, 1D nanotubes, nanowires, nanorods, and 2D graphene and derivatives. A comprehensive literature will guide the readers’ interest in computational materials sciences
Density functional theory investigation of electronic and geometric properties of the lattice mismatched III-V semiconducting nanowires
No full textDoktora TeziBu tezde, örgü sabitleri uyumayan III-V yarıiletkenlerinden InAs ve GaAs ile oluşturulan InAs/GaAs nanotel heteroyapılarının elektronik ve geometrik özellikleri incelenmiştir. (001) yönünde büyütülen InAs/GaAs bulk hetero yapıları ve InAs/GaAs nanotel heteroyapıları çalışılmıştır. Bulk heteroyapılar ve nanotel heteroyapıların her ikisi içinde farklı katmanlar çalışılmıştır.İncelenen sistemlerin denge durumu örgü sabitleri, toplam enerjileri, elektrostatik potansiyel eğrileri ve elektronik bant yapıları hesaplanmıştır. Ayrıca bu fiziksel niceliklerin tel yarıçapı ve katman sayısıyla olan ilişkileri incelenmiştir. Nanotel heteroyapılarda yarıçap değerinin artmasıyla denge durumu örgü sabitlerinin artığı gözlenmiştir. Nanotel heteroyapıların 0.9 nanometrenin altındaki yarıçapa sahip olduğu durumlarda örgü yapısındaki bozulmalar dikkati çekmiştir. Buna bağlı olarak elektrostatik potansiyel eğrilerde, InAs ve GaAs farkı gözlenmemiştir. Ancak 0.9 nm ve üzeri yarıçap değerlerine sahip nanotel heteroyapılarda hem örgü yapısı daha düzenli hem deInAs ve GaAs arasındaki potansiyel farkı bulk yapı özelliklerine benzer bir özellik göstermesine karşın bulk yapıdan elde edilen potansiyel farkından daha düşük olduğu anlaşılmıştır.Hesaplamalar, temeli yoğunluk fonsiyonel teorisine dayanan düzlem dalga özuyum alan programı ile gerçekleştirilmiştir.AbstractIn this thesis, we have investigated electronic and geometric properties of InAs/GaAs nanowire heterostructure which made up In As/GaAs from lattice mismatched III-V semiconductors. InAs/GaAs bulk het erostructures and InAs/GaAs nanowire heterostructures which are growth in the ( 001) direction have been investigated. We studied the different layers both of bulk heterostructures and nanowire heterostructures. Equilibrium lattice constants, to tal energies, electrostatic potential curves and electronic band structures of investigat ed systems have been calculated. Also, the relation between wire radius and layer nu mber of this physical quantities have been carried out. We observed that with increasing of the radius value equilibrium state lattice constant increases. We have noticed that di stortions of the lattice structure below the radius of 0.9 nanometers. We have not observed an obvious electrostatic potential difference between InAs and GaAs. Nanowire heterost ructures which have 0.9 nm and high value of radius show similar feature both lat tice structure which have more regular and between InAs and GaAs potential deference bulk structure properties. However the difference of potential from obtain bulk structure lower. Our calculations have been carried out using plane wave self consistent field program based on density functional theory
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Grafen nanoyapıların manyetik alan altında sıkı-bağlanım incelemesi.
No full textElectrons moving under the effects of a two dimensional periodic potential and a magnetic field perpendicular to this two dimensional plane has been the focus of many different studies for a long time. The interplay between the two length scales in this problem, lattice constant and the characteristic magnetic length, results in interesting phenomena such as the Hofstadter's butterfly. The bulk of the studies done so far has focused on uniform magnetic fields. The only requirement for the vector potential is that its closed loop integral resulting in the correct flux piercing through the loop. This allows us to use a rather unconventional gauge where we set certain values for the line integrals instead of solving the line integrals with a known vector potential. Using this gauge, we can study the effects of inhomogeneous fields in a very efficient way. The electronic structures of hexagonal flakes, Y-shaped junctions and cross-shaped junctions of different sizes have been studied using tight-binding method and the optimal gauge. The results show emergence of new states around the Fermi level localized on the lattice sites where the magnetic fields are applied.M.S. - Master of Scienc
Grafen bazlı malzemelerin yoğunluk fonksiyoneli teorisiyle incelenmesi.
No full textEmployability of graphene-based materials in various technological applications in order to exploit its exceptional properties is achieved/improved via their interactions with various atoms, molecules, nanostructures and surfaces. In this thesis, the interaction of cyclohexane and derived molecules, carbon nanotubes and metallic surfaces with both pristine and defected graphene structures are investigated using density functional theory.Ph.D. - Doctoral Progra
Altıgensel boron nitrür ve altın yüzeylerininin nanotribolojik özelliklerinin teorik olarak incelenmesi
No full textFrictional properties of two-dimensional structures on the nanoscale have gained importance, especially in the last decades, and a considerable amount of research is carried on to understand the interaction between their interfaces with well known lubricants. In industrial developments, these materials’ interfacial properties on metals have great importance. Therefore, in this thesis, we theoretically investigated the hexagonal BN and Au(111) interface with using molecular dynamics simulation. Calculation results have shown that the friction coefficient decreases with increasing load and a noticeable dependence on temperature is observed. We observed that the nanotribology properties of these two samples highly depend on sliding velocity, orientation and direction and also depend on the sizes, edges, and shapes of h-BN flake.Thesis (M.S.) -- Graduate School of Natural and Applied Sciences. Physics
Grafen ve au arayüzleri arasındaki sürtünme kuvvetinin katman sayısı ve yüke bağlı değişiminin yük yoğunluğu fonksiyoneli teorisi ile incelenmesi.
No full textAs the investigation of materials at nano scale become possible with today's technology it is observed that some physical phenomenons have different characteristics at atomistic scale than macroscopic one because of the quantum mechanical effects. One of these physical processes that differs at nano scale is the friction force. While it is expected that the friction force to be independent of contact area and velocity according to Amontos-Coulomb laws, it was observed that it changes by the effect of some parameters such as contact area, velocity, load and temperature at smaller scales. This study will include nanotribological calculations which analyzes the friction force between objects at atomic level. The lateral friction force that arises from the relative sliding motion of materials can be investigated in experimental manner using the Friction Force Microscopy (FFM) which is modified version of the Atomic Force Microscopy (AFM). It is also possible to obtain a detailed understanding of friction in atomic scale by modelling the mechanism of FFM using computational methods. The ab initio Density Functional Theory (DFT) is one of these methods that one can perform accurate calculations for the relative sliding motion of FFM probe tip and the surface. The importance of these kind of numerical methods is the convenience of investigating the wide range of material interactions that are not achievable with experimental methods. As it was observed that the two dimensional materials such as graphene, hexagonal boron nitride(h-BN) , provides good performance as dry lubricants, the scope of the nanotribology studies shifted into the frictional behaviors of these systems and their effects on other type of materials. In this thesis, we focus on the lateral friction force between two dimensional graphene sheets and three high-symmetry surfaces of gold. Our aim is to understand the results of FFM experiments by modelling the friction mechanism between Au coated probe-tip and graphene surface by implementing static calculations based on Density Functional Theory. As the Au coated probe slides over graphene, Au surfaces with different orientations would interact with graphene surface and three of them namely Au(100), Au(110) and Au(111) are reviewed as the subjects of this study. The effects of physical variables such as an external load applied to structures and increasing the thickness of the surfaces were interpreted. Results of this thesis can provide useful informations about minimizing the friction between objects with the help of different parameters which would be beneficial in industrial manner about reducing the loss of energy arises due to friction.M.S. - Master of Scienc
Güçlü korelasyonlu CeO2 malzemesinin katalitik özellikleri ve elektronik yapısının ilk ilkeler bilgisayar yöntemleri ile incelenmesi.
No full textDensity functional theory employed with Hubbard model addressing strong electronic local correlation source of which considered to be the occupation of d or f type atomic orbitals, since these are localized in space, is used to study ceria (CeO2 ) material. Since ceria exhibits occupied atomic f orbitals on Ce sites and these sites are not equivalent in correlation aspect when surface and defective structures considered, there has been no account of single Hubbard effective U value that captures or improves all the properties. Therefore, locally resolved Hubbard U, by this we mean diff erent U values for every distinct Ce site, is the first approach which should be taken in order to find a method that improves the theoretical accounts of this material and this approach is in the focus of this thesis. Ceria (111) and (110) surfaces, CO and Au adsorptions are investigated with this approach.M.S. - Master of Scienc
Alkollerin altın katalızörler üzerinde seçici oksıdasyon tepkime mekanizmalarının yük yoğunlugu fonksiyoneli ile incelenmesi.
No full textMost alcohols, when exposed to oxygen, are converted into CO2 and H2O as a result of a combustion reaction. Such reactions consist of several intermediate steps, which produce industrially valuable chemical species such as esters, ketones, carboxylic acids and aldehydes. These chemicals find uses in a wide range of applications in food, pharmaceutical, polymer and plating industries along with many others. As a result, there are considerable economical interests in the design of efficient, low-cost and environmentally low-impact synthetic routes of these products. Selective oxidation provides a way to interrupt the combustion reaction at the desired stage and extract the by-product of interest. The variety of these intermediate products and the particular mechanism is dictated by the environment and the catalyst used. Experimental studies have already indicated that the Au nanoparticles and Au surfaces pre-treated with atomic oxygen successfully catalyze selective oxidation. Current oxygen pretreatment techniques are, however, difficult and costly. The aim of this study is to design Au surfaces by means of doping the surface with Ni and Rh atoms in low concentrations to increase the performance of the surface for selective oxidation of ethanol. We utilize arguably the most successful theoretical method to ever have been used in surface science, namely Density Functional Theory (DFT), to understand the selective oxidation of ethanol. We present results concerning the dissociation of ethanol into subcomponents on these doped surfaces. M.S. - Master of Scienc
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