Physics / Fizik
Permanent URI for this collectionhttps://hdl.handle.net/11147/6
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Article Citation - WoS: 10Citation - Scopus: 10The Comparison of Transient Photocurrent Spectroscopy Measurements of Pulsed Electron Deposited Zno Thin Film for Air and Vacuum Ambient Conditions(Elsevier, 2019) Özdoğan, Mehmet; Yiğen, Serap; Çelebi, Cem; Utlu, GökhanPhotoconduction mechanism of ZnO thin films that produced by Pulsed Electron Deposition method is systematically investigated by taking Transient Photocurrent Spectroscopy measurements for different atmospheres including high vacuum and air environments. Response and recovery rates of photocurrent in the air are faster than the rates in high vacuum condition. The results in the presented work clearly indicate that the photoconduction of ZnO thin films with high surface-area-to-volume ratio are surface-related and mostly governed by adsorption/desorption of oxygen and water molecules in the atmosphere. Therefore, the high surface interaction tendency of ZnO surface with the atmosphere inevitably leads to charge transfer from surface to adsorbates and/or vice versa.Article Citation - WoS: 5Citation - Scopus: 5Enhanced Indirect Exchange Interactions in the Presence of Circular Potentials in Graphene(American Physical Society, 2019) Canbolat, Ahmet Utku; Çakır, ÖzgürWe calculate indirect exchange interaction between two magnetic impurities in pristine graphene in the presence of a circular potential. In bulk graphene structures indirect exchange interaction, also known as RKKY (Ruderman-Kittel-Kasuya-Yosida) interaction, shows a power-law decay with distance for both doped and undoped cases. Here we show that under a circular electric potential quasibound states lead to enhanced RKKY interactions between magnetic moments located in the vicinity of the potential well. It is shown that the strength of the potential well and Fermi energy can be tuned to create enhanced, nondecaying, long ranged RKKY interactions. We show that when the Fermi level lies at the quasibound state energy, the scattering processes between the states of the same chirality dominate over the other scattering channels and this leads to a predominantly ferromagnetic, nondecaying interaction between the impurities at long distances. The predicted effect can enable electrical control of RKKY interactions in graphene or other two-dimensional materials.Article Citation - WoS: 85Citation - Scopus: 91Ballistic Thermoelectric Properties of Monolayer Semiconducting Transition Metal Dichalcogenides and Oxides(American Physical Society, 2019) Özbal, Gözde; Senger, Ramazan Tuğrul; Sevik, Cem; Sevinçli, HaldunCombining first-principles calculations with Landauer-Mittiker formalism, ballistic thermoelectric transport properties of semiconducting two-dimensional transition metal dichalcogenides (TMDs) and oxides (TMOs) (namely MX2 with M = Cr, Mo, W, Ti, Zr, Hf; X = O, S, Se, Te) are investigated in their 2H and 1T phases. Having computed structural, as well as ballistic electronic and phononic transport properties for all structures, we report the thermoelectric properties of the semiconducting ones. We find that 2H phases of four of the studied structures have very promising thermoelectric properties, unlike their 1T phases. The maximum room temperature p-type thermoelectric figure of merit (ZT) of 1.57 is obtained for 2H-HfSe2, which can be as high as 3.30 at T = 800 K. Additionally, 2H-ZrSe2, 2H-ZrTe2, and 2H-HfS2 have considerable ZT values (both nand p-type), that are above 1 at room temperature. The 1T phases of Zr and Hf-based oxides possess relatively high power factors, however their high lattice thermal conductance values limit their ZT values to below 1 at room temperature.Article Citation - WoS: 7Citation - Scopus: 8Investigation of the Structural and Optical Properties of Copper-Titanium Oxide Thin Films Produced by Changing the Amount of Copper(Elsevier Ltd., 2019) Horzum, Şeyda; Gürakar, Sibel; Serin, TülayWe examine how the structural, morphological and optical properties of TiO2 thin films are changed with heavily copper (Cu) content. Variations in characteristic properties of the films with 0, 12.5, 25 and 50 wt% Cu contents, grown by sol-gel dip coating method, are observed by using X-ray diffraction (XRD), Raman scattering, atomic force microscopy, energy dispersive X-ray analysis and optical spectroscopy measurements. The XRD and Raman spectra indicate that pure TiO2 film forms in the anatase structure. At high Cu concentrations, XRD results also reveal the substitution of Ti with Cu and formation of extra compound Copper-Titanium oxide. Raman measurements also show that Cu is incorporated homogeneously into TiO2 matrix up to 12.5 wt% concentration and this uniformity is distorted at higher Cu contents. In addition, optical spectroscopy measurements show that the optical band gap energy decreases from 3.26 eV to 2.05 eV with increasing Cu concentration. Furthermore, it is observed that the refractive index values obtained by means of transmittance spectra at 550 nm wavelength; increases from 2.47 to 3.39 when the Cu concentration increases from 0 to 50 wt %.Article Citation - WoS: 24Citation - Scopus: 24Defect Tolerant and Dimension Dependent Ferromagnetism in Mnse2(Royal Society of Chemistry, 2019) Eren, İsmail; İyikanat, Fadıl; Şahin, HasanBy performing density functional theory-based calculations, we investigate the structural, vibrational, electronic and magnetic properties of 2D monolayers, nanoribbons and quantum dots of MnSe2. Vibrational spectrum analysis reveals the dynamical stability of not only ferromagnetic but also antiferromagnetic phases of single layer MnSe2 crystal structures. Electronically, calculations show that 1T-MnSe2 is a ferromagnetic structure displaying metallic behavior. It is also found that the structure preserves its dynamical stability and metallic behavior even under the presence of high density Se vacancies. Moreover, it was predicted that, differing from the 2D MnSe2, metal-metal interaction driven reconstructions result in ferromagnetic-to-antiferromagnetic crossover in the ground state of nanoribbons and quantum dots. With its robust ferromagnetic metallic character in the 2D ultra-thin limit and dimension-dependent magnetic properties, MnSe2 is an important candidate for spintronic device applications.Article Citation - WoS: 25Citation - Scopus: 27Symmergent Gravity, Seesawic New Physics, and Their Experimental Signatures(Hindawi Publishing Corporation, 2019) Demir, Durmuş AliThe standard model of elementary particles (SM) suffers from various problems, such as power-law ultraviolet (UV) sensitivity, exclusion of general relativity (GR), and absence of a dark matter candidate. The LHC experiments, according to which the TeV domain appears to be empty of new particles, started sidelining TeV-scale SUSY and other known cures of the UV sensitivity. In search for a remedy, in this work, it is revealed that affine curvature can emerge in a way restoring gauge symmetries explicitly broken by the UV cutoff. This emergent curvature cures the UV sensitivity and incorporates GR as symmetry-restoring emergent gravity (symmergent gravity, in brief) if a new physics sector (NP) exists to generate the Planck scale and if SM+NP is Fermi-Bose balanced. This setup, carrying fingerprints of trans-Planckian SUSY, predicts that gravity is Einstein (no higher-curvature terms), cosmic/gamma rays can originate from heavy NP scalars, and the UV cutoff might take right value to suppress the cosmological constant (alleviating fine-tuning with SUSY). The NP does not have to couple to the SM. In fact, NP-SM coupling can take any value from zero to Lambda SM2/Lambda NP2 if the SM is not to jump from Lambda SM approximate to 500GeV to the NP scale Lambda NP. The zero coupling, certifying an undetectable NP, agrees with all the collider and dark matter bounds at present. The seesawic bound Lambda SM2/Lambda NP2, directly verifiable at colliders, implies that (i) dark matter must have a mass less than or similar to Lambda SM, (ii) Higgs-curvature coupling must be approximate to 1.3%, (iii) the SM RGEs must remain nearly as in the SM, and (iv) right-handed neutrinos must have a mass less than or similar to 1000TeV. These signatures serve as a concise testbed for symmergence.Article Citation - WoS: 2Citation - Scopus: 2Impact of Encapsulation Method on the Adsorbate Induced Electrical Instability of Monolayer Graphene(AVS Science and Technology Society, 2019) Kalkan, Sırrı Batuhan; Yanılmaz, Alper; Çelebi, CemMonolayer graphene transferred onto a set of silicon carbide (SiC) substrates was encapsulated with a thin SiO2 film in order to prevent its interaction with atmospheric adsorbates. The encapsulation of graphene samples was realized by using two different thin film growth methods such as thermal evaporation (TE) and state-of-the-art pulsed electron deposition (PED). The encapsulation efficiency of these two techniques on the structural and electrical characteristics of graphene was compared with each other. Scanning electron microscopy (SEM) analysis showed that unlike the SiO2 thin film grown with PED, structural defects like cracks were readily formed on TE grown films due to the lack of surface wettability. The electronic transport measurements revealed that the electrical resistivity of graphene has been increased by two orders of magnitude, and the carrier mobility has been subsequently decreased upon the encapsulation process with the PED method. However, in-vacuum transient photocurrent spectroscopy (TPS) measurements conducted for short periods and a few cycles showed that the graphene layer encapsulated with the PED grown SiO2 film is electrically far more stable than the one encapsulated with TE grown SiO2 film. The results of TPS measurements were related to the SEM images to unravel the mechanism behind the improved electrical stability of graphene samples encapsulated with the PED grown SiO2 film.Article Citation - WoS: 30Citation - Scopus: 29Stacking-Dependent Excitonic Properties of Bilayer Blue Phosphorene(American Physical Society, 2019) İyikanat, Fadıl; Torun, Engin; Senger, Ramazan Tuğrul; Şahin, HasanAb initio calculations in the framework of many-body perturbation theory (MBPT) are performed to calculate the electronic and optical properties of monolayer and bilayer blue phosphorene with different stacking configurations. It is found that the stacking configuration of bilayer blue phosphorene strongly affects the electronic band gap of the material. By solving the Bethe-Salpeter equation (BSE) on top of the G(0)W(0) calculation, the binding energies, spectral positions, and band decomposition of excitons of monolayer and bilayer configurations are investigated. The most prominent two excitonic peaks of bilayers are examined in detail. Our calculations show that different stacking configurations lead to distinct interlayer interaction characteristics which lead to substantial change in the optical spectrum of bilayer blue phosphorene. Mostly intralayer and mixed interlayer excitons with quite high binding energies are obtained in bilayer blue phosphorene. Our results show that excitonic properties of ultrathin materials play an important role in tuning and improving the optoelectronic performance of two-dimensional materials.Article Citation - WoS: 8Citation - Scopus: 8Oxide Shell Layer Influences on Size-Dependent Tensile and Compressive Mechanical Properties of Iron Nanowires: a Reaxff Molecular Dynamics Study(American Institute of Physics, 2019) Aral, GürcanThe systematic understanding of an overall deformation mechanism of metallic iron (Fe) nanowires (NWs) with the pre-existing oxide shell layer (Fe/FexOy) under various mechanical loading conditions is of critical importance for their various applications. Herein, we perform molecular dynamics simulations using ReaxFF reactive interatomic potential to systematically investigate the effect of the pre-existing oxide shell layer on the underlying intrinsic mechanical deformation mechanism and related mechanical properties of metallic [001]-oriented Fe NWs under both uniaxial tension and compressive loading. Three different diameters of the NWs are investigated to elucidate the size effect. The Fe NWs with the preoxide shell layer possess unique and intriguing mechanical properties and deformation mechanisms. In particular, the oxide shell layer with the combined effect of the diameter and the applied uniaxial loading mode dictates the strength and the overall stress-strain behaviors of the NWs. Interestingly, the oxide-coated NWs clearly exhibit the diameter-dependent elastic deformation intrinsic mechanism and related properties as compared to the pristine counterparts. Specifically, the pre-existing oxide shell layer expedites the onset of tensile plasticity by drastically reducing the tensile yield stress and significantly decreasing the tensile elastic limit. Contrary to the tensile loading, the presence of the oxide shell layer reduces or increases the compressive yield stress of the pristine Fe NW with respect to its diameter. However, the pre-existing oxide shell layer leads to a significantly delayed onset of compressive plasticity, that is, a significant increase in the compressive elastic limit. Published under license by AIP Publishing.Article Citation - WoS: 89Citation - Scopus: 85Cspbbr3 Perovskites: Theoretical and Experimental Investigation on Water-Assisted Transition From Nanowire Formation To Degradation(American Physical Society, 2018) Akbalı, Barış; Topçu, Gökhan; Güner, Tuğrul; Özcan, Mehmet; Demir, Mustafa Muammer; Şahin, HasanRecent advances in colloidal synthesis methods have led to an increased research focus on halide perovskites. Due to the highly ionic crystal structure of perovskite materials, a stability issue pops up, especially against polar solvents such as water. In this study, we investigate water-driven structural evolution of CsPbBr3 by performing experiments and state-of-the-art first-principles calculations. It is seen that while an optical image shows the gradual degradation of the yellowish CsPbBr3 structure under daylight, UV illumination reveals that the degradation of crystals takes place in two steps: transition from a blue-emitting to green-emitting structure and and then a transition from a green-emitting phase to complete degradation. We found that as-synthesized CsPbBr3 nanowires (NWs) emit blue light under a 254 nm UV source. Before the degradation, first, CsPbBr3 NWs undergo a water-driven structural transition to form large bundles. It is also seen that formation of such bundles provides longer-term environmental stability. In addition theoretical calculations revealed the strength of the interaction of water molecules with ligands and surfaces of CsPbBr3 and provide an atomistic-level explanation to a transition from ligand-covered NWs to bundle formation. Further interaction of green-light-emitting bundles with water causes complete degradation of CsPbBr3 and the photoluminescence signal is entirely quenched. Moreover, Raman and x-ray-diffraction measurements revealed that completely degraded regions are decomposed to PbBr2 and CsBr precursors. We believe that the findings of this study may provide further insight into the degradation mechanism of CsPbBr3 perovskite by water.