Browsing by Subject "Nanostructured materials--Magnetic properties."

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Open Access
Biofunctionalization of superparamagnetic iron oxide nanoparticles
(2011) Sülek, Selim
Magnetic resonance imaging (MRI) has attracted intensive interest due to its non-invasive monitoring capacity. Gadolinium based contrast agents, most widely used CA, suffer from high level of toxicity and high threshold of detection. Superparamagnetic iron oxide nanoparticles (SPION) based contrast agents (CA) are good alternatives for gadolinium based CAs, since they have extraordinary magnetic properties within nanometer size and relatively low toxicity. Surface active group of SPIONs are mostly responsible for these advantages. In this thesis, we studied biofunctionalization of iron oxide magnetic nanoparticles with variety of peptide molecules for the solubilization and biofunctionalization of SPIONs. Particle synthesis was carried out via two methods: co-precipitation and thermal decomposition and they were compared by means of size and stability. Several characterization methods, such as Fourier Transform Infrared Spectroscopy (FT-IR), Circular Dichroism (CD), Rheology, X-ray diffraction (XRD) X-ray photon spectroscopy (XPS), vibrating sample magnetometer (VSM), Magnetic resonance imaging (MRI), Atomic Force Microscopy (AFM), Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM) were used in order to fully characterized the SPIONs prepared.methods were used in order to fully characterize the SPIONs. Thermal decomposition is the best method to control the particle size and avoid aggregation problems. Peptide amphiphile molecules are used to non-covalently functionalize SPIONs synthesized by thermal decomposition method to provide water solubility and biocompatibility. Particles are found to be around 35 nm with r2 values of 100.4 and 93.7 s-1mM-1 which are comparable with commercially available SPIONs. In vitro cell culture experiments revealed that peptide-SPION complexes are biocompatible and are localized around the cells due to their peptide coating. Finally, SPIONs were evaluated in terms of their potential use as MRI contrast agent.
Open Access
Electro-magnetic properties and phononic energy dissipation in graphene based structures
(2008) Sevinçli, Haldun
With the synthesis of a single atomic plane of graphite, namely graphene honeycomb structure, active research has been focused on the massless Dirac fermion behavior and related artifacts of the electronic bands crossing the linearly at the Fermi level. This thesis presents a theoretical study on the electronic and magnetic properties of graphene based structures, and phononic energy dissipation. First, functionalization of these structures by 3d-transition metal (TM) atoms is investigated. The binding energies, electronic and magnetic properties have been investigated for the cases where TM-atoms adsorbed to a single side and double sides of graphene. It is found that 3d-TM atoms can be adsorbed on graphene with binding energies ranging between 0.10 to 1.95 eV depending on their species and coverage density. Upon TM-atom adsorption graphene becomes a magnetic metal. TM-atoms can also be adsorbed to graphene nanoribbons with armchair edge shapes (AGNRs). Binding of TM-atoms to the edge hexagons of AGNR yield the minimum energy state for all TM-atom species examined in this work and in all ribbon widths under consideration. Dependingon the ribbon width and adsorbed TM-atom species, AGNR, a non-magnetic semiconductor, can either be a metal or a semiconductor with ferromagnetic or anti-ferromagnetic spin alignment. Interestingly, Fe or Ti adsorption makes certain AGNRs half-metallic with a 100% spin polarization at the Fermi level. These results indicate that the properties of graphene and graphene nanoribbons can be strongly modified through the adsorption of 3d TM atoms. Second, repeated heterostructures of zigzag graphene nanoribbons of different widths are shown to form multiple quantum well structures. Edge states of specific spin directions can be confined in these wells. The electronic and magnetic state of the ribbon can be modulated in real space. In specific geometries, the absence of reflection symmetry causes the magnetic ground state of whole heterostructure to change from antiferromagnetic to ferrimagnetic. These quantum structures of different geometries provide novel features for spintronic applications. Third, as a possible device application, a resonant tunnelling double barrier structure formed from a finite segment of armchair graphene nanoribbon with varying widths has been proposed based on first-principles transport calculations. Highest occupied and lowest unoccupied states are confined in the wider region, whereas the narrow regions act as tunnelling barriers. These confined states are identified through the energy level diagram and isosurface charge density plots which give rise to sharp peaks originating from resonant tunnelling effect. Finally, we studied dynamics of dissipation of local vibrations to the surrounding substrate. A model system consisting of an excited nano-particle which is weakly coupled with a substrate is considered. Using three different methods, the dynamics of energy dissipation for different types of coupling between the nano-particle and the substrate is studied, where different types of dimensionality and phonon densities of states were also considered for the substrate. Results of this theoretical analysis are verified by a realistic study. To this end the phonon modes and interaction parameters involved in the energy dissipation from an excited benzene molecule to the graphene are calculated performing first-principles calculations.
Open Access
Electronic structure of graphene nano-ribbons
(2008) Şen, Hüseyin Şener
Graphite is a known material to human kind for centuries as the lead of a pencil. Graphene as a two dimensional material, is the single layer of graphite. Many theoretical works have been done about it so far, however, it newer took attention as it takes nowadays. In 2004, Novoselov et al. was able to produce graphene in 2D. Now that, making experiments on graphene is possible scientists have to renew their theoretical knowledge about systems in two dimension because graphene, due to its electronic structure, is able to prove the ideas in quantum relativistic phenomena. Indeed, recent theoretical studies were able to show that, electrons and holes behave as if they are massless fermions moving at a speed about 106m/s (c/300, c being speed of light) due to the linear electronic band dispersion near K points in the brillouin zone which was observed experimentally as well. Having zero band gap, graphene cannot be used directly in applications as a semiconductor. Graphene Nano-Ribbons (GNRs) are finite sized graphenes. They can have band gaps differing from graphene, so they are one of the new candidates for band gap engineering applications such as field effect transistors. This work presents theoretical calculation of the band structures of Graphene NanoRibbons in both one (infinite in one dimension) and zero dimensions (finite in both dimensions) with the help of tight binding method. The calculations were made for Zigzag, Armchair and Chiral Graphene Nano-Ribbons (ZGNR,AGNR,CGNR) in both 1D and 0D. Graphene nano-ribbons with zero band gap (ZGNR and AGNR) are observed in the calculations as well as the ribbons with finite band gaps (AGNR and CGNR) which increase with the decrease in the size of the ribbon making them much more suitable and strong candidate to replace silicon as a semiconductor.
Open Access
First-principles investigation of functionalization of graphene
(2013) Korkmaz, Yaprak
The graphene sheet is a single-atom thick novel material and attracts great interest due to its unique features. However, it is a metallic material with no bandgap, which makes it difficult to integrate in electronic applications. Adatom adsorption is one of the promising ways to make this structure functional. To this end, electronic and structural properties of graphene have been investigated by using density functional theory formalism in order to understand atomic level interaction between halogen adatoms and graphene layer. The most common adatom, hydrogen, has also been studied. In this study, plane-wave, pseudopotential density functional theory calculations were carried out using generalized gradient approximations for the exchange correlation potential with the Quantum Espresso package. In order to obtain fully relaxed structures, geometry optimization has been performed in all of the calculations. The adatom-graphene system is modelled with a 4 × 4 graphene supercell. Adsorption energies of halogen adatoms and dimers adsorbed on highly symmetric positions on graphene layer are calculated. Different configurations of adatoms have been tested. Specific properties such as band structure and density of states of these system have been investigated. The results show that a fully covered graphene layer is stable and optimized structures exhibit a band gap of a few eV. The most stable structure among halogen adatoms is the fluorine adsorbed on graphene. It has the highest electronegativity, which is the reason for high electron transfer from the graphene layer. This is the reason of the formation of covalent bonds. Furthermore, the most stable configuration is found to be chair configuration with the halogen atoms alternating in both sides of the layer.
Open Access
First-principles investigation of graphitic nanostructures
(2013) Şen, Hüseyin Şener
In this thesis, first-principles investigations of several graphene related nanosystems 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.
Open Access
Size modulation and defects in graphene based ribbons : magnetism and charge confinement
(2008) Topsakal, Mehmet
In this thesis, we investigated the effects of vacancy and heterojunction formation on electronic and magnetic properties of graphene nanoribbons (GNRs) by using first principles pseudopotential plane wave method within Density Functional Theory. Graphene based materials are expected to be very important in future technology. Through understanding of all the factors which influence their physical properties is essential. We have shown that electronic and magnetic properties of graphene nanoribbons can be affected by defect-induced itinerant states. The band gaps of armchair nanoribbons can be modified by hydrogen saturated holes. Defects due to periodically repeating vacancies or divacancies induce metallization, as well as magnetization in non-magnetic semiconducting nanoribbons due to the spin-polarization of local defect states. Antiferromagnetic ground state of semiconducting zigzag ribbons can change to ferrimagnetic state upon creation of vacancy defects, which reconstruct and interact with edge states. Even more remarkable is that all these effects of vacancy defects are found to depend on their geometry and position relative to edges. We also predicted that periodically repeated junctions of graphene ribbons of different widths form multiple quantum well structures having confined states. These quantum structures are unique, since both constituents of heterostructures are of the same material. The width as well as the band gap, for specific superlattices are modulated in direct space. Orientation of constituent nanoribbons, their widths, lengths and the symmetry of the junction are some of the crucial structural parameters to engineer electronic properties of these systems. Our further studies on nanoribbons and nanoribbon superlattices showed the strong dependence of band gaps and magnetic moments on applied uniaxial stress. This thesis presents an extensive study of size modulation and defect formation on graphene nanoribbons.
Open Access
Titanium dioxide nanostructures for photocatalytic and photovoltaic applications
(2008) Çakır, Deniz
In this thesis, TiO2 nanostructures and their photocatalytic and photovoltaic ap- plications have been investigated by using the ¯rst-principles calculations based on density functional theory. We have concentrated on three di®erent systems, namely TiO2 clusters, nanowires and surfaces. TiO2 is widely used in various applications, since it is chemically stable in di®erent conditions, ¯rm under il- lumination, non toxic, and relatively easy and cheap to produce. Most of the technological applications such as photovoltaic and photocatalytic of TiO2 are mainly related to its optical properties. First of all, structural, electronic, and magnetic properties of small (TiO2)n (n=1{10) clusters have been studied. Various initial geometries for each n have been searched to ¯nd out the ground state geometries. In general, it has been found that the ground state structures (for n=1{9) have at least one dangling or pendant O atom. Only the lowest lying structure of n=10 cluster does not have any pendant O atom. In the ground state structures, Ti atoms are at least 4{fold coordinated for n ¸ 4. Clusters prefer to form three{dimensional and compact structures. All clusters have singlet ground state. The formation energy and HOMO{LUMO gap have also been calculated as a function of the number of TiO2 unit to study the stability and electronic properties. The formation energy increases with increasing size of the cluster. This means that clusters become stronger as their size grows. The interaction of the ground state structure of each (TiO2)n cluster with H2O has been investigated. The binding energy Eb of H2O molecule decreases and oscillates as the cluster size increases. The interaction of the ground state structure of n=3, 4, 10 clusters with more than one H2O molecule has also been studied. We have calculated Eb per adsorbed molecule and we have shown that it decreases with increasing number of adsorbed H2Omolecule (N). When N ¸ 2 for n=3 and N ¸3 for n=4 clusters, H2O molecules bind more strongly to n=10 cluster. The adsorption of transition metal (TM) atoms such as V, Co, and Pt on n=10 cluster has been studied as well. All these elements interact with the cluster forming strong chemisorption bonds, and the permanent magnetic moment is induced upon the adsorption of Co or V atoms. Second of all, structural, electronic and magnetic properties of very thin TiOx (x=1,2) nanowires have been presented. All stoichiometric TiO2 nanowires ex- hibit semiconducting behavior and have non{magnetic ground state. There is a correlation between binding energy (Eb) and the energy band gap (Eg) of these TiO2 nanowires. In general, Eb increases with Eg. In non-stoichiometric TiO nanowires, we have both metallic and semiconducting nanowires. In addition to non{magnetic TiO nanowires, we have also ferromagnetic nanowires. Three{ dimensional (3D) structures are more energetic than planar ones for both stoi- chiometries. The stability of TiOx nanowires is enhanced by the increase of the size and coordination number of Ti and O atoms which tend to possess at least four and two nearest neighbors, respectively. We have also studied the structural and electronic properties of rutile (110) nanowires obtained by cutting bulk ru- tile along the [110] direction with a certain cross section. The bulk nanowires are more energetic than the thin nanowires after a certain diameter. Like thin TiO2 nanowires, all bulk wires are semiconducting and Eg oscillates with the cross section of these (110) nanowires. Third of all, we have studied the interaction of perylenediimide (PDI){based dye compounds (BrPDI, BrGly, and BrAsp) with both the unreconstructed (UR) and reconstructed (RC) anatase TiO2 (001) surfaces. All dye molecules form strong chemical bonds with the surface in the most favorable adsorption struc- tures. The lowest binding energy is 2.60 eV which has been obtained in the adsorption of BrPDI dye on the UR surface. In UR{BrGly, RC{BrGly and RC{ BrAsp cases, we have observed that HOMO and LUMO levels of the adsorbed molecules appear within the band gap and conduction band regions, respectively. Moreover, we have obtained a gap narrowing upon adsorption of BrPDI on the RC surface. Because of the reduction in the e®ective band gap of the surface{dye system and possibly achieved the visible light activity, these results are valuable for photovoltaic and photocatalytic applications. We have also considered the e®ects of the hydration of surface on the binding of BrPDI. It has been found that the binding energy drops signi¯cantly for the completely hydrated surfaces.Fourth of all, we have considered the interaction of BrPDI, BrGly, and BrAsp dye molecules with defect free rutile TiO2 (110) surfaces. All dye molecules form moderate chemical bonds with surface in the most stable adsorption structures. The average binding energy of dye molecules is about 1 eV. Regardless of the type of dye molecules, HOMO and LUMO levels of the adsorbed dye appear within the gap and the conduction band region of defect free surface, respectively. The e®ect of the slab thickness on the interaction strength between the dye and the surface has also been examined. Unlike the four layers slab, BrGly and BrAsp molecules are dissociatively adsorbed on the three layers slab. The interaction between BrPDI and partially reduced rutile (110) as well as platinized surface has been also considered in order to ¯gure out the e®ects of O vacancy and preadsorbed small Ptn (n=1, 3 and 5 ) clusters on the binding, electronic, and structural properties of the dye{surface system. It has been found that BrPDI dye prefers to bind to the O vacancy site for the partially reduced surface case. Transition metal deposition on metal oxides plays a crucial role in various industrial areas such as catalysts and photovoltaic cells. Finally, an extensive study of the adsorption of small Ptn (n=1{8) and bimetallic Pt2Aum (m=1{5) clusters on partially reduced rutile TiO2 (110) has been presented. The e®ect of surface O vacancies on the adsorption and growth of Pt and bimetallic Pt{Au clusters over the defective site of the 4£2 rutile surface has been studied. Struc- tures, energetics and electronic properties of adsorbed Ptn and Pt2Aum clusters have been analyzed. The surface O vacancy site has been found to be the most active site for a single Pt monomer. Other Pt clusters, namely dimer, trimer and so on, tend to grow around this monomer. As a result, O vacancy site behaves as a nucleation center for the clustering of Pt atoms. Small Pt clusters interact strongly with the partially reduced surface. Eb per adsorbed Pt atom is 3.38 eV for Pt1 case and Eb increases as the cluster size grows due to the formation of strong Pt{Pt bonds. Pt clusters prefer to form planar structures for n = 1{6 cases. The calculated partial density of states of Ptn{TiO2 surface has revealed that the surface becomes metallic when n ¸ 3. In the case of bimetallic Pt-Au clusters, Aum clusters have been grown on the Pt2{TiO2 surface. Previously ad- sorbed Pt dimer at the vacancy site of the reduced surface acts as a clustering center for Au atoms. This Pt2 cluster also inhibits sintering of the Au clusters on the surface. The interaction between the adsorbed Au atoms and titania surface as well as previously adsorbed Pt dimer is weak compared to Pt{TiO2 surface interactions. Since charge state of the clusters adsorbed on the oxide surfaces iscrucial for catalysis applications of these clusters, total charge on each atom of the metal clusters has also been calculated. Charge transfer among the cluster atoms and underlying oxide surface is more pronounced for Ptn clusters. Furthermore, the absolute value of total charge on the clusters is greater for Ptn than that of bimetallic Pt{Au case.