Physics / Fizik
Permanent URI for this collectionhttps://hdl.handle.net/11147/6
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Article Citation - WoS: 11Citation - Scopus: 13Effect of Cnt Incorporation on Pan/Ppy Nanofibers Synthesized by Electrospinning Method(TÜBİTAK, 2020) İnce Yardımcı, Atike; Tanoğlu, Metin; Yılmaz, Selahattin; Selamet, YusufIn this study, carbon nanotubes (CNTs) added polyacrylonitrile/polypyrrole (PAN/PPy) electrospun nanofibers were produced. Average diameters of the nanofibers were measured as 268 and 153 nm for 10 and 25 wt% of PPy contents, respectively. A relatively higher strain to failure values (23.3%) were observed for the low PPy content. When as-grown CNTs (1 and 4 wt%) were added into the PAN/PPy blends, disordered nanofibers were observed to form within the microstructure. To improve the interfacial properties of CNTs/PAN/PPy composites, CNTs were functionalized with H2SO4/HNO3/HCl solution. The functionalized CNTs were well dispersed within the nanofibers and aligned along the direction of nanofibers. Therefore, beads formation on nanofibers decreased. The impedance of the nanofibers was found to decrease with the PPy content and CNT addition. These nanofibers had a great potential to be used as an electrochemical actuator or a tissue engineering scaffold.Article Citation - WoS: 4Citation - Scopus: 5Identifying Threading Dislocations in Cdte Films by Reciprocal Space Mapping and Defect Decoration Etching(American Institute of Physics, 2018) Polat, Mustafa; Bilgilisoy, Elif; Arı, Ozan; Öztürk, Orhan; Selamet, YusufWe study threading dislocation (TD) density of high-quality cadmium telluride (CdTe) layers grown on a (211) oriented GaAs substrate by molecular beam epitaxy. High-resolution X-ray diffraction was performed to calculate the density of screw-type TDs by measuring the broadening of the asymmetrical (511) Bragg reflections of CdTe epilayers. In addition, total TD densities were determined by the Everson-etching method and were compared with screw TDs. Our results show that the total TD densities in CdTe films were dominated by those with screw character. The screw component TDs are estimated to account for more than 90% of the total TD density. CdTe layers grown at a thickness of less than 3.0 μm typically exhibit the screw TD densities in the 106 cm-2 and 107 cm-2 range. It can be noted that as the nucleation temperature increases, i.e., ≥222 °C, both the area density of TDs with the screw component of the CdTe films and the total TD density are roughly four times larger than those of the epilayer grown at the nucleation temperature of 215 °C. Furthermore, we discuss the influence of the II/VI flux ratio on the density of threading dislocations. The contribution of screw TDs to the total TD density showed a significant decrease in roughly 30% in the case of a high II/VI flux ratio. We further examine the reciprocal space maps in the vicinity of the (422) reflections.Article Citation - WoS: 7Citation - Scopus: 7Stable Ultra-Thin Cdte Crystal: a Robust Direct Gap Semiconductor(IOP Publishing Ltd., 2017) İyikanat, Fadıl; Akbalı, Barış; Kang, J.; Senger, Ramazan Tuğrul; Selamet, Yusuf; Şahin, HasanEmploying density functional theory based calculations, we investigate structural, vibrational and strain-dependent electronic properties of an ultra-thin CdTe crystal structure that can be derived from its bulk counterpart. It is found that this ultra-thin crystal has an 8-atom primitive unit cell with considerable surface reconstructions. Dynamic stability of the structure is predicted based on its calculated vibrational spectrum. Electronic band structure calculations reveal that both electrons and holes in single layer CdTe possess anisotropic in-plane masses and mobilities. Moreover, we show that the ultra-thin CdTe has some interesting electromechanical features, such as strain-dependent anisotropic variation of the band gap value, and its rapid increase under perpendicular compression. The direct band gap semiconducting nature of the ultra-thin CdTe crystal remains unchanged under all types of applied strain. With a robust and moderate direct band gap, single-layer CdTe is a promising material for nanoscale strain dependent device applications.Article Citation - WoS: 13Citation - Scopus: 16Reciprocal Space Mapping Study of Cdte Epilayer Grown by Molecular Beam Epitaxy on (2 1 1)b Gaas Substrate(IOP Publishing Ltd., 2017) Polat, Mustafa; Arı, Ozan; Öztürk, Orhan; Selamet, YusufWe examine high quality, single crystal CdTe epilayer grown by molecular beam epitaxy (MBE) on (2 1 1)B GaAs substrate using both positions and full width at half maximums (FWHMs) of reciprocal lattice points (RLPs). Our results demonstrate that reciprocal space mapping (RSM) is an effective way to study the structural characteristics of the high-index oriented epitaxial thin films having a large lattice mismatch with the substrate. The measurement method is defined first, and then the influence of shear strain ( xz) on the position of the (5 1 1) node of epilayer is clarified. It is concluded that the lattice tilting is likely to be related with the lattice mismatch. Nondestructive measurement of the dislocation density is achieved by applying the mosaic crystal model. The screw dislocation density, estimated to be 7.56×107 cm2, was calculated utilizing the broadened peakwidths of the asymmetric RLP of the epilayer lattice.Article Citation - WoS: 4Citation - Scopus: 4Effect of Aromatic Sams Molecules on Graphene/Silicon Schottky Diode Performance(Electrochemical Society, Inc., 2016) Yağmurcukardeş, Nesli; Aydın, Hasan; Can, Mustafa; Yanılmaz, Alper; Mermer, Ömer; Okur, Salih; Selamet, YusufAu/n-Si/Graphene/Au Schottky diodes were fabricated by transferring atmospheric pressure chemical vapor deposited (APCVD) graphene on silicon substrates. Graphene/n-Si interface properties were improved by using 5-[(3-methylphenyl)(phenyl) amino]isophthalic acid (MePIFA) and 5-(diphenyl)amino]isophthalic acid (DPIFA) aromatic self-assembled monolayer (SAM) molecules. The surface morphologies of modified and non-modified films were investigated by atomic force microscopy and scanning electron microscopy. The surface potential characteristics were obtained by Kelvin-probe force microscopy and found as 0.158 V, 0.188 V and 0,383 V as a result of SAMs modification. The ideality factors of n-Si/Graphene, n-Si/MePIFA/Graphene and n-Si/DPIFA/Graphene diodes were found as 1.07, 1.13 and 1.15, respectively. Due to the chain length of aromatic organic MePIFA and DPIFA molecules, also the barrier height φB values of the devices were decreased. While the barrier height of n-Si/Graphene diode was obtained as 0.931 eV, n-Si/MePIFA/Graphene and n-Si/DPIFA/Graphene diodes have barrier height of 0.820 and 0.720 eV, respectively.Article Citation - WoS: 89Citation - Scopus: 94Effects of Li Substitution in Bi-2223 Superconductors(Springer Verlag, 2008) Bilgili, Özlem; Selamet, Yusuf; Kocabaş, KemalThe effects of Li substitution on the properties of high temperature superconductor Bi 17Pb 0.3Sr 2C 2Cu 3-x Li x O y were investigated. The samples were prepared by substituting Li (x=0.00-0.20) with changing ratios by a solid state reaction method. The samples were characterized by X-ray diffraction, DC electrical resistivity, AC magnetic susceptibility, and scanning electronic microscopy (SEM). The X-ray diffraction studies were done at room temperature and the lattice constants of the material were determined by indexing all the peaks observed. This study shows that there are two coexisting phases; high-T c (2223) phase and low-T c (2212) phase. The lattice structure of the material belongs to the orthorhombic unit cell. The volume fraction was estimated from the intensities of Bi-(2223) and Bi-(2212) phases. The sample with 20 wt% of added Li showed the higher volume fraction of Bi-(2223) phase formed (81%) compared to the other samples. The DC electrical resistivity of all the samples decreased as the wt% of Li increased. Both the onset critical temperatures T c (onset) and zero electrical resistivity critical temperatures T c (R=0) of the samples were determined from the DC electrical resistivity measurements. The observed value of the onset critical T c (onset) temperature was 110 K agreeing well with the magnetic susceptibility measurements. We obtained T c onset at 112 K from AC magnetic susceptibility measurements. © 2008 Springer Science+Business Media, LLC.