Browsing by Subject "Structural properties"

Now showing 1 - 13 of 13

Open Access
The effect of cucurbit[n]uril on the solubility, morphology, and the photophysical properties of nonionic conjugated polymers in an aqueous medium
(2010) Tuncel, D.; Artar, M.; Hanay, S. B.
The effects of cucurbit[n]uril on the dissolution and the photophysical properties of nonionic conjugated polymers in water are described. For this purpose, a fluorine-based polymer, namely, poly[9,9-bis{6(N,N-dimethylamino) hexyl}fluorene-co-2,5-thienylene (PFT) was synthesized and characterized by spectroscopic techniques including 1D and 2D NMR, UV-vis, fluorescent spectroscopy, and matrix-assisted laser desorption mass spectrometry (MALDI-MS). For the first time, it was demonstrated that a nonionic conjugated polymer can be made soluble in water through an inclusion complex formation with CB8. The structure of the complex was elucidated by NMR experiments including 1H and selective 1D-NOESY. This complex emits green and is highly fluorescent with fluorescent quantum yield of 35%. In contrast, CB6 or water-soluble CB7 although they are chemically identical to CB8 do not have any effect on the dissolution and photophysical properties of PFT. By preparing a protonated version of PFT, the optical properties of PFT in methanol, protonated PFT and PFT@CB8 in water have been studied and compared. It was also observed that the morphology of the polymer PFT was affected by the presence of CB8. Thus CB8-assisted self-assembly of polymer chains leads to vesicles formation; these structures were characterized by DLS, AFM, SEM, and TEM fluorescent optical microscopy.
Open Access
Electrical conduction and dielectric relaxation properties of AlN thin films grown by hollow-cathode plasma-assisted atomic layer deposition
(Institute of Physics Publishing, 2016) Altuntas, H.; Bayrak, T.; Kizir, S.; Haider, A.; Bıyıklı, Necmi
In this study, aluminum nitride (AlN) thin films were deposited at 200 ï¿½C, on p-type silicon substrates utilizing a capacitively coupled hollow-cathode plasma source integrated atomic layer deposition (ALD) reactor. The structural properties of AlN were characterized by grazing incidence x-ray diffraction, by which we confirmed the hexagonal wurtzite single-phase crystalline structure. The films exhibited an optical band edge around ∼5.7 eV. The refractive index and extinction coefficient of the AlN films were measured via a spectroscopic ellipsometer. In addition, to investigate the electrical conduction mechanisms and dielectric properties, Al/AlN/p-Si metal-insulator-semiconductor capacitor structures were fabricated, and current density-voltage and frequency dependent (7 kHz-5 MHz) dielectric constant measurements (within the strong accumulation region) were performed. A peak of dielectric loss was observed at a frequency of 3 MHz and the Cole-Davidson empirical formula was used to determine the relaxation time. It was concluded that the native point defects such as nitrogen vacancies and DX centers formed with the involvement of Si atoms into the AlN layers might have influenced the electrical conduction and dielectric relaxation properties of the plasma-assisted ALD grown AlN films.
Open Access
Electronic and elastic properties of the multiferroic crystals with the Kagome type lattices -Mn3V2O8 and Ni3V2O8: First principle calculations
(Taylor & Francis, 2019-08-16) Koç, H.; Palaz, S.; Mamedov, Amirullah M.; Özbay, Ekmel
The electronic, mechanical, and optical properties of the Kagome staircase compounds, Mn3V2O8 and Ni3V2O8, have been investigated using the VASP (Vienna ab-initio Simulation Program) that was developed within the density functional theory (DFT). The spin polarized generalized gradient approximation has been used for modeling exchange-correlation effects. The electronic band structures for both compounds and total and partial density of states corresponding to these band structures have been calculated. Spin up (spin down) Eg values for Mn3V2O8 and Ni3V2O8 compounds are 0.77 eV indirect (3.18 direct) and 1.58 eV indirect (0.62 eV) direct, respectively. The band gaps of both compound is in the d-d character. Bulk modulus, shear modulus, Young's modulus, Poisson's ratio, anisotropic factors, sound velocity, and Debye temperature were calculated and interpreted.
Open Access
Electronic and optical properties of stanane and armchair stanane nanoribbons
(Springer, 2020-04) Fadaie, M.; Dideban, D.; Gülseren, Og̈uz
In this study, we performed a density functional theory based investigation of the structural, electronic, and optical properties of a stanane, fully hydrogenated stanene SnH, and armchair stanane nanoribbons ASnHNRs. Our full geometry optimization calculations show stanane has 0.84 Å buckled height and the buckled structure is preserved in ASnHNRs. The optimized lattice parameter of stanane, Sn–Sn, and Sn–H bond length are 4.58 Å, 2.75Å, and 1.73 Å, respectively. Electronic structure calculations show that stanane is a moderate-band-gap semiconductor with a direct band gap of 1.2 eV and ASnHNRs are wide-band-gap semiconductors. The band gap of ASnHNRs decreases as the ribbons width increases. We investigated the optical properties for two directions of polarization. For perpendicular-polarized light, the imaginary part of dielectric function ε2(ω)ε2(ω) of stanane peaks between 5 and 10 eV; while for the parallel-polarized light, the peaks are seen in a wide range of energy. According to the results, stanane is a good absorptive matter, especially for visible regions of the electromagnetic spectrum. The presence of anisotropy with respect to the type of light polarization is observed in ASnHNRs also. In these structures, the main peak of ε2(ω)ε2(ω) is located at 3.4 eV for parallel- and in 6–8 eV for perpendicular-polarized light.
Open Access
Electronic structure of conventional slater type antiferromagnetic insulators: AIrO3 (A=Sr, Ba) perovskites
(Institute of Physics, 2022) Koc, Husnu; Mamedov, Amirullah M.; Özbay, Ekmel
The structural, mechanical, and electronic properties of Perovskite BaIrO3 and SrIrO3 compounds based on the density functional theory (DFT) have been examined in four different structures (C2/c, R-3m, P6_3/mmc and Pm-3m) and Pnma structure, respectively. The spin polarized generalized gradient approximation has been used for modeling exchange-correlation effects. As a result of spin polarized calculations, it has been observed that BaIrO3 compound showed magnetic properties in C2/c and R-3m structures, but not in Pm-3m and P6_3/mmc structures. SrIrO3 compound also shows magnetic properties in Pnma structure. The elastic constants have been calculated using the strain-stress method and the other related quantities (the bulk modulus, shear modulus, Young's modulus, Poisson's ratio, anisotropy factor, sound velocities, and Debye temperature) have also been estimated. In electronic band structure calculations, while Pm-3m and P6_3/mmc structures of NaIrO3 compound are metallic and semiconductor (Eg = 1.190 eV indirect), respectively, while C2/c and R-3m structures showing magnetic properties are metallic in spin down state and semiconductor (Eg=0.992 eV indirect and Eg=0.665 eV direct, respectively) in the spin up state. The Pmna structure in the SrIrO3 compound is a semiconductor in both spin states (Eg=0.701 eV “0.632 eV” indirect in the spin up “spin down”). © 2022 Institute of Physics Publishing. All rights reserved.
Open Access
Fe promoted NOx storage materials: structural properties and NOx uptake
(American Chemical Society, 2010) Kayhan, E.; Andonova, S. M.; Şentürk, G. S.; Chusuei, C. C.; Ozensoy, E.
Fe promoted NOx storage materials were synthesized in the form of FeOx/BaO/Al2O3 ternary oxides with varying BaO (8 and 20 wt %) and Fe (5 and 10 wt %) contents. Synthesized NOx storage materials were investigated via TEM, EELS, BET, FTIR, TPD, XRD, XPS, and Raman spectroscopy, and the results were compared with the conventional BaO/Al2O3 NOx storage system. Our results suggest that the introduction of Fe in the BaO/Al2O3 system leads to the formation of additional NOx storage sites which store NOx mostly in the form of bidentate nitrates. NO2 adsorption experiments at 323 K via FTIR indicate that, particularly in the early stages of the NOx uptake, the NOx storage mechanism is significantly altered in the presence of Fe sites where a set of new surface nitrosyl and nitrite groups were detected on the Fe sites and the surface oxidation of nitrites to nitrates is significantly hindered with respect to the BaO/Al2O3 system. Evidence for the existence of both Fe3+ as well as reduced Fe2+/(3-x)+ sites on the freshly pretreated materials was detected via EELS, FTIR, Raman, and XRD experiments. The influence of the Fe sites on the structural properties of the synthesized materials was also studied by performing ex situ annealing protocols within 323-1273 K followed by XRD and Raman experiments where the temperature dependent changes in the morphology and the composition of the surface domains were analyzed in detail. On the basis of the TPD data, it was found that the relative stability of the stored NOx species is influenced by the morphology of the Ba and Fe containing NOx-storage domains. The relative stabilities of the investigated NOx species were found to increase in the following order: N2O3/NO+ < nitrates on γ-Al2O3 < surface nitrates on BaO < bidentate nitrates on FeOx sites < bulk nitrates on BaO.
Open Access
Fundamentals, progress, and future directions of nitride-based semiconductors and their composites in two-dimensional limit: a first-principles perspective to recent synthesis
(American Institute of Physics Inc., 2018) Kecik D.; Onen, A.; Konuk, M.; Gürbüz, E.; Ersan, F.; Cahangirov, S.; Aktürk, E.; Durgun, Engin; Çıracı, Salim
Potential applications of bulk GaN and AlN crystals have made possible single and multilayer allotropes of these III-V compounds to be a focus of interest recently. As of 2005, the theoretical studies have predicted that GaN and AlN can form two-dimensional (2D) stable, single-layer (SL) structures being wide band gap semiconductors and showing electronic and optical properties different from those of their bulk parents. Research on these 2D structures have gained importance with recent experimental studies achieving the growth of ultrathin 2D GaN and AlN on substrates. It is expected that these two materials will open an active field of research like graphene, silicene, and transition metal dichalcogenides. This topical review aims at the evaluation of previous experimental and theoretical works until 2018 in order to provide input for further research attempts in this field. To this end, starting from three-dimensional (3D) GaN and AlN crystals, we review 2D SL and multilayer (ML) structures, which were predicted to be stable in free-standing states. These are planar hexagonal (or honeycomb), tetragonal, and square-octagon structures. First, we discuss earlier results on dynamical and thermal stability of these SL structures, as well as the predicted mechanical properties. Next, their electronic and optical properties with and without the effect of strain are reviewed and compared with those of the 3D parent crystals. The formation of multilayers, hence prediction of new periodic layered structures and also tuning their physical properties with the number of layers are other critical subjects that have been actively studied and discussed here. In particular, an extensive analysis pertaining to the nature of perpendicular interlayer bonds causing planar GaN and AlN to buckle is presented. In view of the fact that SL GaN and AlN can be fabricated only on a substrate, the question of how the properties of free-standing, SL structures are affected if they are grown on a substrate is addressed. We also examine recent works treating the composite structures of GaN and AlN joined commensurately along their zigzag and armchair edges and forming heterostructures, δ-doping, single, and multiple quantum wells, as well as core/shell structures. Finally, outlooks and possible new research directions are briefly discussed. © 2018 Author(s).
Open Access
Growth of high crystalline quality semi-insulating GaN layers for high electron mobility transistor applications
(2006) Yu H.; Caliskan, D.; Özbay, Ekmel
Semi-insulating character (sheet resistivity of 3.26 × 10 11 Ω/sq) of thick GaN layers was developed for AlGaN/GaN high electron mobility transistor (HEMT) applications on an AlN buffer layer. Electrical and structural properties were characterized by a dark current-voltage transmission line model, x-ray diffraction, and atomic force microscope measurements. The experimental results showed that compared to semi-insulating GaN grown on low temperature GaN nucleation, the crystal quality as well as surface morphology were remarkably improved. It was ascribed to the utilization of a high quality insulating AlN buffer layer and the GaN initial coalescence growth mode. Moreover, the significant increase of electron mobility in a HEMT structure suggests that this is a very promising method to obtain high performance AlGaN/GaN HEMT structures on sapphire substrates. © 2006 American Institute of Physics.
Open Access
Investigation of new two-dimensional materials derived from stanene
(Elsevier, 2017-09) Fadaie, M.; Shahtahmassebi, N.; Roknabad, M. R.; Gulseren, O.
In this study, we have explored new structures which are derived from stanene. In these new proposed structures, half of the Sn atoms, every other Sn atom in two-dimensional (2D) buckled hexagonal stanene structure, are replaced with a group- IV atom, namely C, Si or Ge. So, we investigate the structural, electronic and optical properties of SnC, SnGe and SnSi by means of density functional theory based first-principles calculations. Based on our structure optimization calculations, we conclude that while SnC assumes almost flat structure, the other ones have buckled geometry like stanene. In terms of the cohesive energy, SnC is the most stable structure among them. The electronic properties of these structures strongly depend on the substituted atom. We found that SnC is a large indirect band gap semiconductor, but SnSi and SnGe are direct band gap ones. Optical properties are investigated for two different polarization of light. In all structures considered in this study, the optical properties are anisotropic with respect to the polarization of light. While optical properties exhibit features at low energies for parallel polarization, there is sort of broad band at higher energies after 5 eV for perpendicular polarization of the light. This anisotropy is due to the 2D nature of the structures. © 2017 Elsevier B.V.
Open Access
Lattice dynamics and elastic properties of lanthanum monopnictides
(2008) Gökoǧlu G.; Erkişi, A.
In this study, first principles calculation results of the second order elastic constants and lattice dynamics of two lanthanum monopnictides, LaN and LaBi, which crystallize in rock-salt structure (B1 phase), are presented. Calculations were based on plane wave basis sets and pseudopotential methods in the framework of Density Functional Theory (DFT) with generalized gradient approximation. Elastic constants are calculated by tetragonal and orthorhombic distortions on cubic structure. Phonon dispersion spectra was constructed in the linear response approach of the Density Functional Perturbation Theory (DFPT). The complete phonon softening with negative frequencies and large elastic anisotropy were observed for LaN single crystal as a sign of the structural instability. The phonon dispersion curve for LaBi is typical for lanthanum monopnictides and does not show any anomalous physical property. The calculated structural quantities for both LaN and LaBi systems agree well with the available experimental and theoretical data. © 2008 Elsevier Ltd. All rights reserved.
Open Access
A structural analysis of ultrathin barrier (In)AlN/GaN heterostructures for GaN-based high-frequency power electronics
(Wiley, 2022-04-08) Narin, P.; Kutlu-Narin, E.; Atmaca, G.; Sarikavak-Lisesivdin, B.; Lisesivdin, S. B.; Özbay, Ekmel
Metal–organic chemical vapor deposition (MOCVD) is one of the best growth methods for GaN-based materials as well-known. GaN-based materials with very quality are grown the MOCVD, so we used this growth technique to grow InAlN/ GaN and AlN/GaN heterostructures in this study. The structural and surface properties of ultrathin barrier AlN/GaN and InAlN/GaN heterostructures are studied by X-ray diffraction (XRD) and atomic force microscopy (AFM) measurements. Screw, edge, and total dislocation densities for the grown samples have been calculated by using XRD results. The lowest dislocation density is found to be 1.69 108 cm2 for Sample B with a lattice-matched In0.17Al0.83N barrier. The crystal quality of the studied samples is determined using (002) symmetric and (102) asymmetric diffractions of the GaN material. In terms of the surface roughness, although reference sample has a lower value as 0.27 nm of root mean square values (RMS), Sample A with 4-nm AlN barrier layer exhibits the highest rough surface as 1.52 nm of RMS. The structural quality of the studied samples is significantly affected by the barrier layer thickness. The obtained structural properties of the samples are very important for potential applications like high-electron mobility transistors (HEMTs).
Open Access
The sub-terahertz region absorption of sputter deposited nanoscale TiAlV thin films
(Elsevier B.V., 2022-12-05) Öksüzoğlu, Ramis Mustafa; Altan, Hakan; Abdüsselamoğlu, Mehmet Sait; Özkan, Özlem Başak; Bayram, Yasin; Chakar, Erkan Syuleyman
Absorption in the sub-terahertz region in TiAlV thin films with their potential usage in detectors and plasmonic applications is a crucial point for device performance. This work aims to investigate the thickness-dependent evolution of the sub-terahertz region absorption, electrical resistivity in TiAlV thin films deposited by DC magnetron sputtering and to study the structure-property correlation. For structural analyses, X-ray diffraction and transmission electron microscopy techniques have been used. In different deposition conditions, TiAlV thin films indicating the β-Ti phase with anisotropic growth and the hexagonal AlTi3 phase with equiaxial growth have been produced. High resistivity values have been measured in films with the AlTi3 phase. In all TiAlV thin films with different structures, thickness dependent resistivity change in the range 3–23 nm, whereby a strong increase in electrical resistivity with decreasing film thickness have been observed below a thickness of 6 nm. The sub-terahertz absorption increases with increasing film thickness. The highest terahertz region absorption has been found for the films with the β-Ti phase and anisotropic growth also indicating higher electrical conductivity, which favors absorption sensitive applications in the terahertz region. © 2022 Elsevier B.V.
Open Access
Tuning structural and electronic properties of two-dimensional aluminum monochalcogenides: prediction of Janus Al2 X X′ (X / X′ : O, S, Se, Te) monolayers
(American Physical Society, 2020) Demirtaş, Mehmet; Varjovi, M. Jahangirzadeh; Çiçek, Mert Miraç; Durgun, Engin
The realization of ternary, single-layer transition metal dichalcogenides has suggested a promising strategy to develop two-dimensional (2D) materials with alternative features. In this study, we design and investigate Janus aluminum monochalcogenide monolayers, Al2XX′ (X/X′=O,S,Se,Te) by using first-principles methods. Starting from binary constituents, the ternary structures are optimized without any constraint and ground-state configurations are obtained. The stability of these systems is tested by performing phonon spectra analysis and ab initio molecular dynamics simulations and all Al2XX′ monolayers other than AlTeO are confirmed to be dynamically stable. Mechanical properties are examined by calculating Young's modulus and Poisson's ratio and subsequently compared with binary counterparts. Monolayers of Al2XX′ have a brittle character but oxygenation makes them less stiff. The electronic structure is also analyzed and variation of the band gap with the type of chalcogen atoms is revealed. It is found that different from their binary counterparts, Al2XO monolayers are direct band-gap semiconductors. Additionally, modification of the electronic structure in the presence of biaxial compressive or tensile strain is investigated by taking into account possible indirect-direct band-gap transitions. Our results not only predict stable 2D ternary Al2XX′ structures but also point out them as promising materials for optoelectronic applications.