Browsing by Subject "Nanotubes, Carbon."

Now showing 1 - 6 of 6

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
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
Non-covalently functionalized SWNTs as potential delivery agents for novel BODIPY*based PDT sensitizers & the design and synthesis of dendritic light harvesters
(2009) Erbaş, Sündüs
Photodynamic therapy (PDT) is a non-invasive method used for the treatment of a number of diseases including certain cancers and some cardiovascular diseases. Photodynamic action depends on absorbance of photosensitizer (PS) in near IR region of the spectrum and subsequent generation of cytotoxic singlet oxygen in the vicinity of the PS. Carbon nanotubes are widely used for biomedical applications due to their inertness, biocompatibility, cellular internalization, facile and multi-modification. We have synthesized and characterized novel water soluble boradiazaindacene (BODIPY)-based PS, noncovalently attached on to the single wall carbon nanotube (SWNT). We have observed that near infrared absorbing photosensitizer preserve its singlet oxygen generation capability upon adsorption on SWNT. We have demonstrated that SWNTs can be used as the delivery vehicle of PS for the use in PDT. In the second part of this research, multichromatic cascade-type light harvesting BODIPY dendrimers were synthesized and highly efficient energy transfer was observed. Successful channeling of energy in a predefined region of the dendrimer was revealed.
Open Access
Probing interfacial processes on carbon nanotubes and graphene surfaces
(2012) Kakenov, Nurbek
The surface of low-dimensional carbon (carbon nanotubes and graphene) has unique electronic properties due to the delocalized p-orbitals. Very high carrier mobility with nanoscale dimension make carbon nanotubes and graphene promising candidates for high performance electronics. Besides electronic properties, the delocalized orbitals have a strong tendency to adsorb aromatic molecules via p-electronic interactions. The strong non-covalent interactions between the graphitic surface and organic molecules provide a unique template for supramolecular chemistry and sensing applications. A comprehensive understanding of these forces at atomic and molecular level still remains a challenge. In this thesis, we have used carbon nanotube networks and graphene as model systems to understand molecular interactions on carbon surface. We have developed processes to integrate these model materials with sensitive and surface specific sensors, such as surface plasmon sensor and quartz crystal microbalance. In the first part of the thesis, we integrated surface plasmon resonance (SPR) sensors with networks of single-walled carbon nanotubes to study interactions between SWNT and organic molecules. In the second part, we probe interfacial processes on graphene surface by mass detection. We anticipate that the developed methods could provide a sensitive means of detecting fundamental interaction on carbon surfaces.
Open Access
Synthesis of vertically aligned CNT arrays using liquid based precursors and their functionalization by conjugated polymers
(2011) Baykal, Beril
In the first part of this work, a new solution based catalyst precursor application method is developed for growing high quality vertically aligned carbon nanotubes arrays (VANTA) through alcohol catalyzed chemical vapor deposition (AC-CVD). For this purpose, various solution based precursor preparation routes are investigated starting from γ-Al2O3 / Fe(NO3)3.9H2O mixtures and ranging to catalyst precursors prepared by mixing aqueous aluminium and iron nitrate solutions. Application of these solutions separately layer by layer on Si(100) substrate resulted in high quality dense vertically aligned CNT arrays. By varying the metal nitrate concentration in the precursor solutions, the dependence of the density and quality of CNT arrays on the catalyst layers are investigated. The CNT array quality and density are characterized by dynamic contact angle measurements using water droplets. Higher density CNT arrays resulted in higher contact angle measurements. The chemical and structural characterizations of CNTs are also done by using TEM, SEM, EDX and Raman spectroscopy. Some of the samples are found to be super hydrophobic even after 30 minutes of exposure to water. In this effort, application of subsequent layers of aqueous aluminium nitrate and iron nitrate on oxidized Si(100) surfaces are found to be most efficient catalyst layer preparation technique resulting in the highest density of CNT arrays. In the second part of this work, functionalization of the synthesized CNT arrays is done for the purpose of achieving good dispersibility of CNTs in aqueous media. To this end, a new approach is used to ensure stability of the CNT-water solution. In this approach, conjugated polymer nanoparticles (CPNs) are successfully used to disperse CNTs through non-covalent functionalization of the sidewalls of CNTs. The attachment of CPNs to CNTs is characterized by SEM, EDX and TEM. Moreover, interactions are investigated by UV-VIS, and Raman spectroscopy. The interaction mechanism of polymer chains with side-walls of CNTs are further scrutinized by follow-up experiments where two different conjugated polymers with brominated-alkyl and bare alkyl groups in THF media are mixed with SWCNTs (commercial), MWCNTs and an-MWCNTs (synthesized in the first part of this study). The results of this investigation suggested a limited number of docking configurations of the polymers with the CNT side-walls. Also, it is found that the defect density of the CNT side-walls play an important role in the nature of the interaction. Overall, in this work a cheap and effective route for application of catalyst is developed for the synthesis of dense, super-hydrophobic CNT arrays using ACCVD. Then, well-dispersion of these CNTs is successfully achieved using CPNs. Finally, the nature of the interaction between conjugate polymers and CNTs sidewalls are investigated using experimental techniques.
Open Access
Synthesis, characterization and functionalization of vertically aligned carbon nanotube arrays
(2012) Küçükayan Doğu, Gökçe
In the last decade, there has been an increased interest on carbon nanotubes (CNTs) for various applications due to their unique structural, electronic, mechanical and chemical properties. Synthesis of CNTs is no more a challenge with the enhancements and diversity in production techniques. The remaining challenges regarding CNTs for high-volume manufacturing and commercial applications are related to the followings; firstly, gaining control over orientation and density of CNTs during growth for building two and/or three dimensional functional structures and secondly, modulating properties of these structures through a facile route. With regards to these challenges, the growth dynamics of vertically aligned carbon nanotubes (VA-CNTs) were investigated in this thesis. The first part of this thesis explains synthesis of VA-CNTs achieved through the use of a newly designed alcohol catalyzed chemical vapor deposition system in detail. Various catalyst layers were used in the experiments for understanding growth mechanism and thereby the effect of synthesis parameters. The catalyst layers were deposited on SiO2 wafers through physical vapor deposition techniques. The configuration of these catalyst layers were engineered to tune the density and alignment of VA-CNTs by considering the competing mechanism between the subsurface diffusion and migration of catalyst particles. In addition, the annealing parameters were investigated for synthesizing taller and aligned CNTs. The characterization of catalyst layers and VA-CNTs were performed using analysis of Scanning Electron Microscopy, Raman Spectroscopy, Atomic Force Microscopy, X-ray Photoelectron Spectroscopy, Electron Energy Loss Spectroscopy, High Resolution Transmission Electron Microscopy and Raman Spectroscopy. In the second part, effect of synthesis parameters such as growth temperature and pressure, carbon source type and concentration were examined to better understand the growth dynamics of VA-CNTs. Physical and structural transitions of CNTs were observed induced by the decomposition reaction processes of various carbon sources at a related growth temperature. Growth behavior of VA-CNTs was investigated under different carbon source concentration and pressure to find an optimum growth range. The results indicated that while the synthesis method followed in this work is a catalytic based process where the reaction kinetics has a profound influence on the growth of VA-CNTs molecular diffusion mechanisms were found to be playing a key role in determining the growth, size, orientation and structural properties of VA-CNTs. Hence, an approach incorporating the kinetic and diffusion related processes were followed for building an empirical model for uncovering the dominant mechanisms responsible for the termination of growth of VA-CNTs. In the following sections of the thesis, the preliminary studies regarding Li intercalation to VA-CNTs and cell growth on CNTs were performed for possible future applications of two and three-dimensional structures based on CNTs. In situ Li intercalation was studied during the growth of VA-CNTs which does not require post processing for the intercalation mechanism as commonly performed in the existing literature. Li intercalation in the CNTs was confirmed by using X-ray Photoelectron Spectroscopy, Electron Energy Loss Spectroscopy and Raman Spectroscopy following the changes induced by the charge transfer from Li to the carbon lattice. In the second application case, used of VA-CNTs were examined as a scaffold for growing mesenchymal stem cells (MSCs). Surfaces covered with VA-CNTs were patterned by using elasto-capillary mechanism to create suitable ‘nests’ for MSCs to be anchored. The cell viability test was conducted on seeded MSCs on CNT nests and indicated no toxic effect of CNT nests when they were used as scaffold. Furthermore, an aging effect of cells on adhesion was investigated. As a conclusion, the work presented here demonstrated that control over structural and surface properties of VA-CNTs could be attained by taking advantage of a wide range of growth parameters such as temperature, pressure and carbon source type. Hence, the two case studies examined in this study demonstrated a path for aligned and denser CNTs synthesized with desired properties using the learnings attained in the first part of the thesis to be used as anode materials for Li ion batteries and as alternative scaffolds for tissue engineering applications.