Physics / Fizik
Permanent URI for this collectionhttps://hdl.handle.net/11147/6
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Article Citation - Scopus: 1Production and Dispersion of Plasma Functionalized Carbon Nanotubes in Pan Fiber Spinning Solution With Different Surfactants(MMOB Tekstil Mühendisleri Odası, 2022) Kutlu, Bengi; Akşit, Aysun; Özyüzer, Lütfi; Yağmurcukardeş, Nesli; Selamet, YusufIn this study, the distribution of plasma-functionalized multi-walled carbon nanotubes in polyacrylonitrile nanocomposite fibers produced by wet spinning method using different surfactants (Triton-X and sodium dodecyl sulfate) was investigated. Firstly, we produced CNTs by chemical vapor deposition (CVD) technique. Secondly, low-pressure plasma functionalization of CNTs was realized. Finally, nanocomposite polyacrylonitrile fibers doped by CNTs were obtained using wet spinning technique. Properties of produced carbon nanotubes, functionalized carbon nanotubes and nanocomposite polyacrylonitrile fibers were examined by the analyses of chemical composition, surface structure, structural and mechanical properties.Article Echoes From the Event Horizon of a Superfluid Vortex(IOP Publishing, 2022) Güven, Kaan; Demirkaya, BetülA vortex formed in the superfluid state of a Bose-Einstein condensate may exhibit superradiance a la blackhole for radially propagating acoustic fluctuations. The analogy is usually based on the so-called draining bathtub model of the vortex, in which an event horizon and ergosphere emerges when the radial velocity of the superfluid exceeds the propagation speed of sound in the condensate. The acoustic fluctuations mimic a massless scalar field in the curved Lorentzian space-time of the vortex and are governed by the Klein-Gordon wave equation. One common main approximation is the constant background density of the superfluid even in the presence of the vortex. This sets a constant relativistic sound speed. However, the vortex state solution of the Gross-Pitaevskii equation clearly shows that both the density and the speed of sound vary radially near the vortex core, where the event horizon and thus the superradiance will take place. What changes would this complex interdependence bring to the formulation and to the outcomes of the superradiance based on constant density approximation? Here, we recount this question posed under the guidance of Prof. Tekin Dereli and present recent results. We show that the self-consistent density modifies the amplification dynamics near the event horizon significantly, thereby altering the temporal and spectral fingerprint of the superradiance of the vortex.Article Study of the Effect of Various Chemical Polishing Treatments on Mbe-Grown Cdte/Gaas (211)b Heterostructures(Sakarya University, 2020) Bilgilisoy, E.; Özçeri, E.; Tarhan, E.A three-inch-diameter high quality CdTe thin film was grown on a GaAs (211)B substrate by molecular beam epitaxy (MBE) in ultra-high vacuum conditions. The CdTe/GaAs (211)B heterostructure was then cut into several sample pieces. A few as-grown sample pieces were subjected to chemical etching solutions which created etch pits on the surface. The scanning electron microscopy images of such samples were used to calculate the etch pit densities on the surface. In addition, several as-grown samples were subjected to chemical polishing treatments under different conditions to quantify the removal of O and Te-O structures from the surface. Atomic force microscopy was used to determine as-grown and polished surface morphology and the polish rate of chemical solutions. A study of the surface stoichiometry and the chemical composition of the as-grown and polished CdTe (211)B surfaces were carried out by using X-ray photoelectron spectroscopy. Bulk structural qualities of the as-grown and polished samples were studied in terms of the vibrational and phonon modes via confocal Raman spectroscopy. From a comparative analyses of the results, the best chemical polishing conditions for the MBE-grown CdTe (211)B heterostructure were determined. © 2020, Sakarya University. All rights reserved.Article Single Layer Res2h2: Stability, Raman Activity and Electronic Properties(Eskişehir Teknik Üniversitesi, 2018) Ünsal, Elif; Şahin, HasanIn this study, the structural, vibrational and electronic properties of the hydrogenated single layer of ReS2 are investigated byperforming the first principle calculations based on density functional theory. We found that the characteristic properties ofthe monolayer ReS2 can be manipulated upon the hydrogen functionalization. As the monolayer ReS2, the ReS2H2 hasdistorted 1T phase; however, the bonding in Re slab significantly varies with the hydrogenation. Our results demonstrate thatthe full-surface hydrogenation leads to an expansion in lattice and the Re4 tetramer-chains in the monolayer ReS2 areseparated into two dimers in the hydrogenated monolayer. It is calculated that the dynamically stable monolayer of ReS2H2has 26 Raman-active vibrational modes. Constant volume specific heat calculations are also performed and the resultsindicate that at high temperature, the monolayer ReS2 approaches to limit of 3R before the monolayer ReS2H2. By performingthe electronic band structure calculations, it is shown that when the ReS2 surface is fully hydrogenated, there occurs a directto indirect band gap transition and the semiconducting hydrogen-induced monolayer has a band gap of 0.74 eV.Article Citation - Scopus: 1Oxidizer Gases Effects on the Diameter-Controlled Synthesis of Carbon Nanotubes(MIM Research Group, 2021) İnce Yardımcı, Atike; Öğütlü, Ahmet Sabri; Öğütlü, DenizIn this study, the influence of the oxidizers on the synthesis of carbon nanotubes by C2H4 decomposition over Fe catalyst has been investigated. CO2, O2, and H2O have been used as oxidizers, and to control catalyst particle formation and their sizes in the pretreatment stage. The same oxidizers have also been used in the growth stage to maintain the catalyst particle size, remove amorphous carbon formation to keep catalyst particle active. The results of scanning electron microscopy indicated that the average diameters of nanotubes decreased from 13.4±1.2 nm to 6.2±0.5 nm and extremely dense nanotubes were obtained when we added a small amount of CO2. Adding O2 extremely decreased the areal carbon nanotube density while widens the diameter distribution. H2O addition resulted in larger average diameters and made the growth strongly pretreatment dependent. Within the parameters tried for catalyst pretreatment and CNT growth processes, CO2 seemed the best choice for a weak oxidizing assistant. The strong dependency of the average diameter on pretreatment conditions indicated that pretreatment is a very important step in deciding the final diameters and their distribution.Conference Object Investigation of In-Gap Field Enhancement at Terahertz Frequencies for a Metasurface Enhanced Sensor(Institute of Electrical and Electronics Engineers, 2021) Tugay, Halime; Altan, Hakan; Demirhan, Yasemin; Özyüzer, Lütfi; Sabah, CumaliThe arrangements of subwavelength inclusions in a metasurface can serve as an effective absorber for the terahertz region. When such an absorber is combined with a unique material, the absorption can induce effects that can lead to a change in the materials electrical properties. Vanadium dioxide shows a passive and reversible change from monoclinic insulator phase to metallic tetragonal rutile structure by using external stimuli such as temperature (340K), photo excitation, electric field, mechanical strain or magnetic field [1,2]. Upon absorption of the THz radiation, the high electric fields that are generated inside the gaps of the metasurface can serve as trigger points, as was shown previously using kV strength THz E-fields.Article Investigation of Interactions of Acetylene Molecules With an Iron Nanowire and Its Effects on Mechanical Tensile Properties(Sakarya University, 2021) Aral, G.Understanding complex atomistic-scales interactions mechanisms of reactive acetylene (C2H2) molecules with reactive pure iron nanowires (Fe NWs) including its effects on the tensile mechanical properties of NWs is a crucial task in nanotechnology, especially having practical significance in the mechanical reliability, durability and stability. Therefore, we performed molecular dynamics (MD) simulations based on ReaxFF reactive force field interatomic potential model to investigate the interactions of C2H2 molecules with surface of cylindrical pure Fe NW and its fundamental effects on the tensile mechanical deformations properties of NWs at three different strain rates. Our results reveal that the chemical energetic reactions on the free surface of cylindrical Fe NW with C2H2 molecules in the gas phase form FexCyHz shell layer at temperature T=300 K. The presence of FexCyHz shell layer on the free surface of NW has a significant effect on the mechanical tensile deformation mechanism of the NWs. © 2021, Sakarya University. All rights reserved.Book Part Citation - Scopus: 1Thz Sources and Detectors Fabricated From High Temperature Superconductors(Springer, 2021) Demirhan, Yasemin; Türkoğlu, Fulya; Altan, Hakan; Sabah, Cumali; Özyüzer, LütfiHigh temperature superconductors have unique properties that can be useful in the THz region, single crystal constituted from superconducting CuO2-layers could sustain high voltages across the junctions and they are coupled through the intrinsic Josephson Effect this maintains the potential for very intense, coherent radiation which spreads over the THz gap. We investigated various experimental techniques to fabricate THz sources, bolometers and filters for efficient THz emission and detection. Rectangular mesa structures were fabricated on Bi2Sr2CaCu2O8-x (Bi2212) single crystal superconductors using standard e-beam lithography and Ar ion beam etching systems and an emitted power as high as 60 ?W at frequencies up to 0.85 THz was detected from micron sized continuous wave terahertz sources. We also fabricated bolometric microchips for THz detection purpose from Bi2212 single crystals. Bi2212 microchips detected the signals and response time were calculated, our results have clearly shown that Bi2212 single crystals are potential candidates for THz detection. The detection properties and sensitivity of bolometer chips can be further improved by integrating an antenna and filter structures. In order to investigate this we have fabricated metamaterial THz filters based on metals and YBa2Cu3Ox superconducting thin films with metal-mesh shape and a unique fourcross shape pattern. Both a THz time domain spectrometer and a Fourier transform infrared spectrometer (FTIR) were used to investigate the performance of these filters and the results were compared with simulations done with a commercially available electromagnetic simulation software. © Springer Nature B.V. 2021.Conference Object Native and Light Induced Defect States in Wide Band Gap Hydrogenated Amorphous Silicon-Carbon (a-Si1 : H) Alloy Thin Films(Springer, 1997) Güneş, MehmetIn this study, wide band gap a-Si1-x C-x:H alloy thin films prepared with and without hydrogen diluation of (SiH4, CH4) were characterized using optical absorption, dark conductivity, steady-state photoconductivity, sub-bandgap absorption obtained with both photothermal deflection spectroscopy (PDS) and dual beam photoconductivity (DBP), and electron spin resonance (ESR) techniques. Experimental results of steady-state photoconductivity and sub-bandgap absorption for different generation rates were analyzed using a detailed numerical model based on Simmons-Taylor statistics. The densities, energy location and nature of the native and light induced defect states in diluted and undiluted a-Si1-xCx:H alloy thin films were derived from the best fits to the experimental data. The extracted parameters for defect states were compared with those of a-Si:H films both in the annealed and light degraded states.Conference Object Can Cpt Be Violated Through Extended Time Reversal?(World Scientific Publishing, 2001) Erdem, Recai; Ufuktepe, ÜnalWe consider the implications of the extension of time reversal through Wigner types and group extensions. We clarify its physical content and apply the results in a toy model. Finally we point out the possibility of violation of CPT in this framework.