Browsing by Subject "Boron nitride."

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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
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
Growth and characterization of boron nitride thin films and nanostructures using atomic layer deposition = Bor nitrür ince filmlerin ve nanoyapıların atomik katman biriktirme yöntemi ile büyütülmesi ve karakterizasyonu
(2014) Haider, Ali
Being a member of III-nitride family, boron nitride (BN) and its nanostructures have recently attracted a lot of attention, mainly due to their distinctive and superior material properties, including wide band gap, high-temperature stability, high oxidation and corrosion resistance, as well as high thermal conductivity. This versatile material has found applications in UV emission, lubrication, composite reinforcement, gas adsorption, cosmetics, and thermal management. For modern electronic applications, it is imperative to obtain high quality BN films on large area substrates with a controlled thickness in order to fulfill the entire spectrum of hBN applications. Also, a facile method such as atomic layer deposition (ALD) using non halide precursors is necessary to obtain BN films at low temperatures compliant with the standards in terms of having nontoxic byproducts. ALD is a special case of chemical vapor deposition (CVD), in which two or more precursors are sequentially exposed to substrate surface separated by purging periods. In comparison with other thin film growth methods, hall mark of ALD is self limiting growth mechanism which enables deposition of highly uniform and conformal thin films with sub-angstrom thickness control. The precise and conformal layer by layer growth of ALD can be exploited to achieve growth of BN hollow nanofibers (HNFs) on high aspect ratio electrospun polymer nanofibrous templates. BN HNFs fabricated by combination of ALD and electrospinning can be utilized to address and solve important constraints associated with previous methods of fabrication such as severe preparation conditions, limited control over morphology, and low purity of the resulting BN HNFs. In this thesis, we report on the controlled deposition of BN films and its nanostructures with the use of a hollow-cathode plasma source integrated (HCPA-ALD) reactor and present detailed materials characterization results of deposited thin films and fabricated nanostructures. Depositions are carried out at low substrate temperatures (less than 450 °C) using sequential injection of nonhalide triethylboron (TEB) and N2/H2 plasma as boron and nitrogen precursors, respectively. The deposition process parameters such as pulse length of TEB and substrate temperature, as well as the influence of post-deposition annealing are studied. Moreover, another nonhalide alternative precursor named tris(dimethyl)amidoboron (TDMAB) was studied for deposition of BN films. Initial experiments were performed using TDMAB and N2/H2 plasma as boron and nitrogen precursor. In addition to BN thin film growth studies, we report on electrospun polymeric nanofibrous template-based fabrication and characterization of AlN/BN bishell HNFs. Synthesized AlN/BN bishell HNFs were found to be polycrystalline with a hexagonal structure along with lowimpurity content.