Browsing by Subject "Nanostructured materials."

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Open Access
Biomimetic self-assembled peptide nanofibers for bone regeneration
(2012) Kocabey, Samet
Self-assembled peptide nanofibers are exploited in regenerative medicine applications due to their versatile, biofunctional and extracellular-matrixresembling structures. These properties provide peptide nanofibers with osteoinductive and osteoconductive behaviors for bone regeneration applications through several approaches. In this thesis, two different approaches were discussed, which were developed to induce bone regeneration and mineralization including extracellular matrix mimicking peptide nanofibers based 2-D gel formation and surface functionalization of titanium implants. For this purpose, we designed glycosaminoglycan-mimetic peptide nanofibers inspired by chemical structure of glycosaminoglycans present in the bone extracellular matrix. We demonstrated that glycosaminoglycan-mimetic peptide nanofibers interact with BMP-2, a critical growth factor for osteogenic activity. Glycosaminoglycan-mimicking ability of the peptide nanofibers and their interaction with BMP-2 promoted osteogenic activity of and mineralization by osteoblastic cells. ALP activity, Alizarin Red Staining and EDAX spectroscopy indicated efficacy of the peptide nanofibers for inducing mineralization. We also developed a hybrid osteoconductive system for titanium biomedical implants inspired by mussel adhesion mechanism in order to overcome bone tissue integration problems. For this purpose, Dopa conjugated peptide nanofiber coating was used along with bioactive peptide sequences for osteogenic activity to enhance osseointegration of titanium surface. Dopamediated immobilization of osteogenic peptide nanofibers on titanium surfaces created an osteoconductive interface between osteoblast-like cells and inhibited adhesion and viability of soft tissue forming fibroblasts compared to the uncoated titanium substrate. In summary, osteoinductive and osteoconductive self-assembled peptide nanofibers were developed to promote osteogenic activity and mineralization of osteogenic cells. These bioactive nanofibers provide a potent platform in clinical applications of bone tissue engineering.
Open Access
Boron nitride and graphene 2D nanostructures from first-principles
(2010) Ovalı, Rasim Volga
In this thesis, the structures as well as mechanical and electronic properties of various boron nitride (BN) and graphene based two dimensional (2D) nano-structures are investigated in detail from rst-principle calculations using planewave pseudopotential method based on density functional theory. At the beginning of the thesis, essentials of the density functional theory (DFT) and a guidance for performing ab-initio calculations in the framework of DFT is presented. In addition, fundamentals about the exchange-correlation potential as well as approaches approximating it like local density approximation (LDA) and generalized gradient approximation (GGA) are discussed. Along with this thesis, rst of all, in order to understand the relation between the hexagonal boron nitride (h-BN) and cubic boron nitride (c-BN) and the growth of three dimensional (3D) BN structures, various defect structures introduce to BN monolayer, including point defects and especially highly curved defects such as n-fold rings, are investigated in detail. The calculated formation energies and structural analysis showed that 4-fold BN rings are the transient phase between h-BN to c-BN during c-BN nucleation. The charge density plots and density of states analysis further provide information about the electronic structure of these defect formations. Second of all, we have studied the formation of boron-nitride-carbon (BNC) ternary thin lms, so we observed the carbon nucleation in BN monolayer. These DFT based calculations show that carbon prefers the nitrogen site at rst step and the calculated defect energy indicates that carbon atoms tends to aggregate in BN hexagonal network, and hence increases the number of C-C bonds. BNC structures have magnetization of =1.0 B for odd number of carbon adsorption. Further substitution of carbon atoms into BN layer showed that carbon atomsform hexagonal rings instead of armchair or zigzag formations. Moreover, we calculated the vibrational modes of BN monolayer and BNC structures, and phonon density of states graphs are presented. The phonon frequencies intrinsic to C-C bond oscillations are observed, which is in good agreement with the experiment. Finally, point defects and ring formations on graphene are investigated in order to understand the Y-junction and kink formation in carbon nanotubes (CNTs). Pentagonal rings are the good candidates to initiate such 2D networks in CNTs. The curvature increases with increasing number of pentagonal rings. Moreover, interaction of sulphur atoms with graphene defects is studied. Final geometries and binding energies suggest that sulphur prefers to adsorb on defected regions, but it is not responsible for the formation of these structures or defects.
Open Access
Cascading logic gates using ion signals generated by photolabile caged compounds
(2013) Atılgan, Ahmet
Caged compounds have attracted great attention due to their use in the elucidation of numerous biochemical processes. Photolabile caged compounds release covalently bound moieties upon exposure of ultraviolet light. Releasing the active species in such a controlled manner enables concentration of the molecules to be manipulated in spatiotemporal way. Getting inspired from the knowledge of cellular information transfer through second messenger systems which Ca and Zn ions play important role, we synthesized a photolabile caged Zn(II) compound so that we benefit from its controlled ion release feature, so as to integrate two molecular logic gates physically. For that reason, a molecular logic operation was designed and the released ion was used as information carrier from one logic gate to other. After proving its practicality, we tested same principle for higher molecular logic systems. To do that, photolabile caged Zn(II) compound and previously proved supermolecule with coupled AND logic gates were physically integrated. Results proved that photolabile caged Zn(II) compounds is a useful way to combine two separate logic gates by means of free zinc ions. From this point of view, the approach physical integration of molecular logic gates through a metal ion or compound might be a solution for building more complex molecular logic systems.
Open Access
Characterization and corneal tissue engineering application of peptide amphiphiles
(2012) Dağdaş, Yavuz Selim
Molecular self-assembly is a powerful technique for developing novel nanostructures by using non-covalent interactions such as hydrogen bonding, hydrophobic, electrostatic, metal-ligand, π-π and van der Waals interactions. Hydrogen bonding, hydrophobic and electrostatic interactions promote self-assembly of peptide amphiphile molecules into nanofibers. Bundles of nanofibers form a three-dimensional network resulting in gel formation. Concentration and temperature dependent measurements of gel stiffness suggest that the mechanical properties of the gels are determined by a number of factors including the interfiber interactions and mechanical properties of individual nanofibers. Peptide amphiphile molecules provide a convenient model as extracellular matrix mimetic systems for regenerative medicine studies. Since the substrate stiffness is crucial for cellular behaviours such as proliferation, adhesion and differentiation, understanding the mechanisms behind the viscoelastic properties of the gels formed by self-assembling molecules can lead to development of new materials with controlled stiffness. In this study, regeneration of the corneal stroma was used as a model system for utilization of peptide amphiphile molecules in regenerative medicine studies. Corneal stroma is constituted by collagen fiber arrays that are closely packed forming a stiff environment for corneal fibroblasts. The tunability of mechanical properties of self-assembled peptide amphiphile nanostructures was aimed to be utilized in corneal stroma regeneration. Thinning of the corneal stroma is a debilitating problem that can be caused by diseases like keratoconus, infections or accidents. Since corneal stroma has a restricted regenerative capacity, thinning of stroma is usually treated with cornea transplantation, which is limited by the number of donors. In this thesis, I studied mechanical properties of self-assembled peptide amphiphile nanostructures in nanometer and micrometer scale. I found that the divergence in gel stiffness may arise from the difference of strength of interfiber bonds. An injectable, biocompatible, biodegradable and bioactive system that can be used for thickening the corneal stroma was developed. This system that is composed of nanofibers was observed to enhance viability and proliferation of keratocytes in vitro.
Open Access
Characterization of peptide amphiphile nanofibers their interactions with chondroprogenitor cells and morphological analysis of tissues from transgenic animals
(2012) Tombuloğlu, Ayşegül
Peptide amphiphiles, molecules able to self assemble into three dimensional networks resembling to extracellular matrix which is excessive in cartilage tissue, are suitable candidates for overcoming cartilage tissue defects and diseases which constitute central health problems throughout ages. Understanding developmental processes that underlie cartilage formation is also key for regenerating cartilage. In this study, peptide amphiphiles were synthesized, their potential for cartilage regeneration was investigated and a model for cellular aggregation, which is a central process in embryonic cartilage development, was established with chondroprogenitor cells and peptide amphiphile scaffolds. On scaffolds, chondroprogenitor cells aggregated without requiring any additional bioactive factors as opposed to cells grown without scaffolds. Addition of insulin to the medium enhanced the size of the aggregates suggesting scaffolds may be interacting with insulin. Similar to native cartilage tissue, collagen II was massively produced in aggregates. GAG-PA which is designed to mimic glycosaminoglycans and Glu-PA which only presents glutamic acid were used to construct scaffolds with oppositely charged Lys-PA presenting lysine. Formation of aggregates was observed regardless of the PAs used. Use of both GAGPA and Glu-PA induced larger number of aggregates than only Glu-PA. Differential effect of GAG-PA couldn’t be inferred completely and might be investigated in more detail. In a second part of the study, tissue morphologies of lynx3 null mutant mice were studied. Lynx3 is a recently discovered protein belonging to Ly6-superfamily. It is expressed mainly within epithelial lining of respiratory, digestive and genital tracts and is involved in nicotinic acetylcholine receptor desensitization. In this study, morphologies of lynx3 null mice with that of wild type mice were compared to see whether lynx3 has a gross effect on the tissues in which it is expressed. Any significant difference in the morphologies of lung, trachea and thymus cannot be observed. Little variations in esophagus, stomach and female reproductive organ were seen, however, it was not clear whether these variations are related to individual differences or not and the relevance of the variations with lynx3 expression could not be seen clearly. More detailed analysis of tissues may provide additional insight to understand function of lynx3 and the cholinergic mechanisms within various tissues. Short peptides able to pass cell membrane and deliver genes into cells are outstanding alternatives to virus based transfection systems. In the third part of the study, peptide amphiphiles designed to mimic the natural polycationic proteins through forming nanofibers which exhibit positively charged residues at high density, were synthesized. Peptide amphiphiles could form stable complexes with DNA, through neutralization of charges and formation of hydrogen bonds. However, efficient transfection of the gene couldn’t be provided by any complexes in vitro. The study presents primary results upon which more detailed investigation can be built.
Open Access
Chemical vapor deposition of boron nitride nanotubes
(2013) Çiftçi, Niyazi Okan
Since ancient times materials that are available and used by people constitute indispensable constituent parts of world history. The historical ages are named after the materials which paved an irrevocable way to this stream and revolutionize the progress of history by changing the rhythm of anthropological breakthrough irreversibly. People have used these materials either directly borrowing from nature or by transforming those that are given. As we approach to modern society two methods namely a top-down method using transmission of experiences of before generations conventionally and bottom-up method by application of modern science and technology to understand subtleties of materials and engineering them for the specific goals. Boron nitride nanotubes are one class of these materials having superior properties to the conventional ones. High strength, electrically insulator property, controllable wide band gap and high oxidation resistance as compared to carbon nanotubes put them to the first ranks for the design of the future devices for our modern society. Hence a simple and inexpensive way of production of this kind of materials is utmost importance. For this in this thesis an inexpensive and a very practical way of BNNTs production is elucidated. The reactants boron (B), iron (III) oxide (Fe2O3) and magnesium oxide (MgO) are used as solid precursors. A CVD furnace reaching up till 1200 oC with a special design of added concentric quartz tubes equipped with NH3 and Argon gas served as reaction chamber.
Open Access
Chemically specific dynamic characterization of photovoltaic and photoconductivity effects of nanostructures by X-ray photoelectron spectroscopy
(2010) Ekiz, Okan Öner
X-Ray Photoelectron Spectroscopy is a widely used characterization method for chemical analysis of surfaces. In this thesis, We report characterization of photovoltaic and photoconductivity effects on nanostructured surfaces through light induced changes in the X-ray photoelectron spectra (XPS). The technique combines the chemical specificity of XPS and the power of surface photovoltage spectroscopy (SPV), with the addition of the ability to characterize photoconductivity under both static and dynamic optical excitation. A theoretical model that quantitatively describes the features of the observed spectra is presented. We demonstrate the applicability of the model on a multitude of sample systems, including homo- and heterojunction solar cells, CdS nanoparticles on metallic or semiconducting substrates, and carbon nanotube films on silicon substrates. A-Si/c-Si heterojunction solar cell is fabricated to characterize photovoltage generation in a nanostructured solar cell. Cadmium Sulfide (CdS) nanoparticles were synthesized by a solvothermal route. Both Multi Wall Carbon Nanotube (MWCNT) and Single Wall Carbon Nanotube (SWCNT) films were characterized with different substrates by XPS. Raman Microscopy, EDS, SEM, XRD, SAXS are used to characterize the samples and solar cells.
Open Access
Chiral Bodipy dyes & photosensitizers for photodynamic therapy and dye-sensitized solar cells
(2013) Çakmak, Yusuf
Bodipy is a molecule with many superior properties. After its discovery in 1968, most of the features were not recognized until mid 1990s. Thereafter, many research papers and patents have been produced and the number of publications and citations is still on the rise today. An important fraction of the research done with this fluorophore is in chemosensing field to probe various analytes including anions, cations and even biomolecules. However, in this research we have focused on different areas of subjects and tried to find novel applications for these dyes. First, we designed orthogonal bodipy dimers for efficient triplet photosensitization without heavy atoms in contrast to most other sensitizers and efficient singlet oxygen generation was achieved (Φ∆=0.51). In the second project, calix[4]arene molecules were designed and synthesized as carriers for photodynamic therapy, potentially behaving as a molecular basket carrying the agents to the tumor tissues. Later, we focused on obtaining axial chiral molecules by using solely bodipy dyes, and we were able to obtain enantiopure fragments were separated by using chiral HPLC. These rare molecules are desirable for modern biological labeling and advanced optoelectronic devices. Finally, we designed bodipy dyes for dye sensitized solar cells by adapting relevant functional groups, and following synthesis work, we constructed cells to assess the design parameters via measuring the electrical output results.
Open Access
Covalently functionalized MSNs as potential photosensitizing agents for PDT
(2011) Türkşanlı Kaplan, Merve
Photodynamic therapy (PDT) is a novel approach for the treatment of some cancers and other non-malignant diseases. PDT aims to kill cancer tissue by the generation of singlet oxygen as a result of excitation of the photosensitizer (PS) by illuminating with a light source at a certain wavelength. Mesoporous silica nanoparticles are promising in PDT issue due to their chemical inertness, biocompatibility, lowtoxicity, hydrophility and ease of surface modification. We have synthesized and characterized novel boradiazaindacene (BODIPY)-based PS that is covalently attached to the pore of mesoporous silica nanoparticles (MSNs). We have observed that near infrared absorbing photosensitizer attached MSNs successfully generate cytotoxic singlet oxygen.
Open Access
Design and fabrication of resonant nanoantennas on chalcogenide glasses for nonlinear photonic applications
(2013) Duman, Hüseyin
Optical nanoantennas are the metallic nanostructures which confine electromagnetic waves into sub-wavelength volumes at resonant conditions. They are used for various applications including biological and chemical sensing, single molecule spectroscopy, manipulation and generation of light. Combining extremely large electromagnetic field enhancement in plasmonic resonant nanoantenna with high optical nonlinearity of chalcogenide glass leads to a low-threshold broadband light generation scheme in sub-wavelength chip-scale structures. New frequency generation with ultra-low pumping power in plasmonic nanostructures allows compact on-chip light sources which can find applications in single molecule spectroscopy, optical signal processing and broadband lasers. We propose plasmonic nanoantenna chalcogenide glass systems for initiating nonlinear phenomena at low threshold. Size and shape of antennas are optimized according to linear refractive index of substrate and surrounding media for this purpose by finite difference time domain (FDTD) simulations. Resonant behaviour of antennas at their near-field and nonlinear response of optically highly nonlinear chalcogenide glasses are investigated. On resonance, strong field accumulation at the interface of the gold stripe and highly nonlinear As2Se3 glass triggers a start of the spectral broadening of incident beam accompanied by third harmonic generation at an ultra-low threshold power level of 3 W/µm2 . Moreover, we fabricate the designed structures by electron beam lithography, wet chemical techniques and optimize each fabrication step of processes by several experiments. Fabrication steps are explained and SEM images of related steps are presented.
Open Access
Design of compact optical devices based on periodic meta-structures
(2013) Akosman, Ahmet Emin
Manipulation of the flow of light is demanded for several applications such as communication, data storage, sensor, photovoltaic cells, microscopy, lasers and light emitting diodes for the purpose of designing compact, high-throughput and high efficiency optical devices. Nevertheless, the control of the propagation of the light becomes much harder in devices with smaller geometries mostly because of diffractions, loss mechanisms and fabrication difficulties. Furthermore, materials that are already available in the nature do not provide unprecedented optical properties for nanoscale optical applications. Due to this fact that fabrication of artificial materials is needed for utilizing novel and intriguing optical devices. For this purpose, some relatively new research fields have emerged like photonic crystals, metamaterials and high contrast gratings. We propose several designs based on aforementioned meta-structures to achieve compact and practically realizable optical devices. We presented compact optical demultiplexer, diode-like device and electro-optic modulator designs that are based on photonic crystals. We also proposed two circular polarizer designs based on metamaterials and high contrast gratings. Further, we investigated unidirectional transmission and polarization manipulation properties in chiral metamaterials. For most of the proposed designs, we also experimentally verified the numerical and theoretical findings. In conclusion, we can claim that the utilization of artifically structured materials give opportunity to realize the control of light much more easily in nanoscale designs.
Open Access
Design, synthesis and characterization of activatable photosensitizers for photodynamic therapy
(2012) Çakmak, Fatma Pir
Search for new noninvasive methods for diseases has been significant question for years. Therapeutic properties of light are combined with proper chromophore in order to create fundamentals of photodynamic therapy which is a new treatment modality for cancer and other various non-oncological diseases. The method relies on the activation of photosensitizer by using light of certain wavelength and generation of cytotoxic singlet oxygen species in response. Reactive oxygen species kill the targeted tissue within smaller effective diameter through apoptosis/ necrosis mechanism. Through this method, new PDT agents can be proposed and their properties can be tuned by manipulation of other photophysical processes. In this thesis, synthesis, characterization novel water soluble, near IR absorbing Bodipy photosensitizer will be discussed. As opposed to other photosensitizers in literature, this photosensitizer is rationally designed to have singlet oxygen generation capability only in cancer tissue as a result of glutathione triggered activation.
Open Access
Design, synthesis and characterization of bioinspired nanomaterials for engineering and biomedicine
(2014) Ceylan, Hakan
Nature is an inspirational school for materials scientists. Natural selection process puts a massive pressure on biological organisms giving rise to effective strategies for fabricating materials, which generally outperform their man-made counterparts. Mimicking physical and chemical features of biological materials can greatly aid in overcoming existing design constraints of engineering and medicine. In this dissertation, a reductionist, bottom-up approach is demonstrated to recapitulate biological functionalities in fully-synthetic hybrid constructs. For material design, the potential of short, rationally-designed peptides for programmed organization into nanostructured materials is explored. The resulting nano-ordered materials exhibit multifunctional and adaptive properties, which can be tailored by the information within monomeric peptide sequences as well as the emerging properties upon their self-assembly. In light of these, design, synthesis and characterization of the prototypes of nanostructured functional materials are described in the context of regenerative medicine and biomineralization.
Open Access
Effects of charging on two-dimensional honeycomb nanostructures
(2012) Topsakal, Mehmet
In this thesis we employ state-of-the-art first-principles calculations based on density functional theory (DFT) to investigate the effects of static charging on two-dimensional (2D) honeycomb nanostructures. Free standing, single-layer graphene and other similar single-layer honeycomb structures such as boron nitride (BN), molybenum disulfide (MoS2), graphane (CH) and fluorographene (CF) have been recently synthesized with their unusual structural, electronic and magnetic properties. Through understanding of the effects of charging on these nanostructures is essential from our points of view in order to better understand their fundamental physics and developing useful applications. We show that the bond lengths and hence 2D lattice constants increase as a result of electron removal from the single layer. Consequently, phonons soften and the frequencies of Raman active modes are lowered. Three-layer, wide band gap BN and MoS2 sheets are metalized while these slabs are wide band semiconductors, and excess positive charge is accumulated mainly at the outermost atomic layers. Excess charges accumulated on the surfaces of slabs induce repulsive force between outermost layers. With increasing positive charging the spacing between these layers increases, which eventually ends up with exfoliation when exceeded the weak van der Waals (vdW) attractions between layers. This result may be exploited to develop a method for intact exfoliation of graphene, BN and MoS2 multilayers. In addition we also show that the binding energy and local magnetic moments of specific adatoms can be tuned by charging. We have addressed the deficiencies that can occur as an artifact of using plane-wave basis sets in periodic boundary conditions and proposed advantages of using atomicorbital based methods to overcome these deficiencies. Using the methods and computation elucidated in this thesis, the effects of charging on periodic as well as finite systems and the related properties can now be treated with reasonable accuracy. The adsorption of oxygen atoms on graphene have been investigated extensively before dealing with the charging of graphene oxide (GOX). The energetics and the patterns of oxygen coverage trends are shown to be directly related with the amount of bond charge at the bridge sites of graphene structure. We finally showed that the diffusion barriers for an oxygen atom to migrate on graphene surface is significantly modified with charging. While the present results comply with the trends observed in the experimental studies under charging, we believe that there are other factors affecting the reversible oxidation-reduction processes.
Open Access
Electronic structure of graphene under the influence of external fields
(2012) İslamoğlu, Selcen
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 σ and π 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 band structure and Hall conductance of graphene under the influence of perpendicular magnetic 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 usual integer 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 a distinct 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 do not 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.
Open Access
Electrostatic effects on the self-assembly mechanism of peptide amphiphiles
(2010) Toksöz, Sıla
Self-assembling peptide amphiphiles, synthesized through solid phase peptide synthesis – a bottom-up approach, have been used with various tissue engineering purposes. Peptide amphiphile molecules self-assemble into nanofibers, which form three dimensional networks mimicking the extracellular matrix. Electrostatic interactions affect the formation of nanofibers. The effect of charged groups on the peptides on nanofiber formation were studied in this work. Neutralization of the charged groups by pH change, electrolyte addition or addition of oppositely charged biomacromolecules triggered the aggregation of the peptides. To understand the controlled formation of the gels composed of peptide nanofibers better can help the researchers develop bioactive collagen mimetic nanofibers for tissue engineering studies and use them in angiogenesis. Results obtained by Fourier Transform Infrared Spectroscopy (FT-IR), Circular Dichroism (CD), Rheology, pH titration, Atomic Force Microscopy (AFM), Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM); as well as the potential of using the peptide amphiphile molecules to promote angiogenesis, are described.
Open Access
Environmental friendly InP/ZnS nanocrystals
(2012) Coşkun, Yasemin
Semiconductor nanocrystals are nanometer scale fluorescent crystallites with tunable optical properties, which can be controlled by the material composition and particle size. They can be prepared using various synthesis techniques and find applications in many different areas ranging from life sciences to electronics. In this thesis, indium phosphide based nanocrystals are studied for LED applications. The thesis research work focuses on the colloidal synthesis method and material characterization of these nanocrystals. Using one pot synthesis method, the indium phosphide/zinc sulfide (InP/ZnS) core/shell nanocrystal structures are synthesized. This synthesis technique allows for a reproducible and tunable preparation method. The material characterization techniques used in this thesis include UV-Vis spectroscopy, photoluminescence spectroscopy, transmission electron microscopy (TEM), X-ray diffraction, X-ray photoelectron spectroscopy (XPS), inductively coupled plasma-mass spectrometry (ICP-MS), and Zeta Sizer (or DLS). These properties make InP/ZnS nanocrystals comparable to their cadmium (Cd) containing counterparts with respect to their optical properties, and InP/ZnS nanocrystals offer the potential to replace them because of environmental concerns in LED applications.
Open Access
Fabrication of novel core-shell nanostructures for photonics applications
(2013) Khudiyev, Tural
Developments in nanoscale fabrication and characterization techniques have led to fundamental changes in the scientific understanding of many fields, and novel nanostructures have been utilized to investigate the conceptual underpinnings behind a diverse array of natural phenomena. However, nanofabrication methods are frequently hindered by issues such as misalignment, small batch sizes, high production costs and constraints in material choice or nanostructure diversity, which decrease their potential utility and prevent their widespread application in nanoscale optics and photonics. In this work, a new top-down nanofabrication method is described, which is called Iterative Size Reduction (ISR), where step-by-step reduction is utilized to decrease structure dimensions from macro- to nanosizes and produce indefinitely long one-dimensional core-shell nanostructures with properties highly suitable for use in optical applications. Plateau-Rayleigh instabilities are then utilized to thermally degrade ISR-produced nanowire arrays into complex core-shell schemes, which are produced successively in a hitherto-undescribed transitory region between core-shell nanowires and core-shell nanospheres. A diverse array of optical phenomena have been observed on fabricated novel core-shell nano-platforms, which are utilized in the design of novel nanostructures for emerging photonics applications. Briefly, (a) the resonant Mie scattering behavior is characterized on glass-polymer nanostructures and these nanostructures are designed for large-area structural coloration, (b) a novel non-resonant Mie scattering regime responsible for the scattering characteristics exhibited by all-polymer core-shell nanowires is described, (c) a nanoscale analogue to the thin film interference phenomenon is demonstrated that occurs on the core-shell boundary of ISR-produced micro- and nanostructures, (d) an unusual photonic crystal structure observed in the neck feathers of mallard drakes is investigated and imitated, (e) a series of all-polymer core-shell nanowires to function as novel light-trapping platforms and sensors are engineered and (f) the generation of supercontinuum light in well-ordered arrays of As2Se3 nanowires is reported.
Open Access
Frictional and vibrational properties of nanostructures
(2012) Cahangirov, Seymur
Frictional and vibrational properties of low-dimensional nanostructures have been investigated using the state-of-the-art ab-initio calculations. Stringent test of stability based on calculation of phonon dispersions have been performed for various materials having important potential applications in nanoscience and nanotechnology. Silicene, a counterpart of graphene composed of silicon atoms, is one of such materials with its suitability to well established silicon technology together with eccentric electronic structure due to its honeycomb symmetry. Vibrational spectrum of silicene is found to be exempt from imaginary frequencies upon the puckering of atoms in adjacent sublattices while preserving the symmetry necessary for occurrence of massless Dirac Fermions. Analyses of vibrational properties of silicene nanoribbons and carbon atomic chains revealed new interesting physics like fourth acoustical mode and long-ranged interactions due to Friedel oscillations. Basic concepts of friction science like dissipation phenomena, adiabatic and sudden processes together with several simple models of friction have been summarized. A new method for calculation of corrugation potential between layered lubricants under constant loading pressure is introduced. Transition from stickslip to continuous sliding regime is quantified through definition of frictional figure of merit for layered lubricants. Using this measure tungsten oxide is proposed as an oxidation resistant material which can outperform molybdenum disulfide as a superlubricant. It was found that, the corrugation strengths of graphene layers sandwiched between Ni slabs decrease as the number of layers increase.
Open Access
Functionalization of graphene and stoichiometric graphene derivatives
(2011) Şahin, Hasan
Re ent developments in experimental te hniques have made the design and produ tion of materials at nanos ale possible. In parti ular, graphene has been the fo us of resear h in diverse elds owing to high mobility arrier transport and other ex eptional properties. Over the past four years experimental studies have demonstrated that hemi al onversion of graphene to its stoi hiometri derivatives is possible by hydrogenation, uorination and hlorination. The aim of this thesis is to predi t stable stoi hiometri graphene derivatives and explore their me hani al, ele troni and magneti properties. Moreover, the fun tionalization of graphene and its derivatives are a hieved, whereby their physi al properties are modi ed to derive novel materials. Our predi tions revealing stable 2D single layer onformers, whi h an be used as novel nano oeting materials, are obtained from state-of-the art rst-prin iples Density Fun tional al ulations of total energy, phonons, transition state analysis and ab-initio mole ular dynami s. An extensive theoreti al study on the stability of hydrogenated graphene (CnH), fully hydrogenated graphane i.e graphane (CH), and their quasi onedimensional nanoribbons is performed. The formation of meshes of dehydrogenated domains on graphane resulted in geometry spe i magneti stru tures showing interesting magneti intera tions. Creation of H and CH va an ies, as well as adsorption of transition metal atoms give rise to signi ant spinpolarization in graphane nanoribbons. It is shown that as a result of one-sided or two-sided uorination of graphene one an obtain nanostru tures with diverse ele troni and magneti properties. Fully uorinated graphene or uorographene CF is a stable, sti and non-magneti semi ondu tor. Additionally, this onformer of bu kled graphene is fun tionalized by alkali, non-metal, metalloid and transition metal atoms, and ea h group leads to diverse adsorption properties. Adsorption of hlorine to graphene is dramati ally di erent from those of hydrogen and uorine. While the binding energy of hlorine is signi ant, its migration on the surfa e of perfe t graphene takes pla e almost without barrier. This is ru ial for energy harvesting on graphene surfa e. Energy optimization and phonon al ulations indi ate that the hair on guration of fully hlorinated graphene ( hlorographene) is energeti ally most favorable and stable. It is a nonmagneti semi ondu tor with 1.2 eV dire t band gap, whi h an be tuned by applied uniform strain. Graphene by itself an be fun tionalized by reating meshes of va an ies or adatoms onserving spe i symmetries. Under these ir umstan es linearly rossing bands and hen e the massless Dira Fermion behavior an be maintained. Finally, it is demonstrated that multilayer, even single layer graphene onstitute an ex ellent nanos ale oating, whi h an prevent a rea tive metal surfa e from oxidation without hanging the size and other physi al properties. Graphene an sti k to at metal surfa es and hinders free oxygen atom and mole ule from penetrating to the metal surfa e. Single layer uorographene an be used also for the same purposes. Design of novel nanomaterials, in parti ular biologi al mole ules and omplexes using rst-prin iples methods derived from quantum theory indi ates a new dire tion in theory, whi h promises a produ tive hybridization with experimental studies.