Physics / Fizik
Permanent URI for this collectionhttps://hdl.handle.net/11147/6
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Research Project Grafen İçeren İşlevsel Nanoyapıların Tasarımı ve Elektronik Özellikleri(2015) Senger, Ramazan TuğrulGrafen ve türevi yapıların elektronik, manyetik özellikleri, işlevsel nanoyapıların tasarımına elverecek biçimde çeşitlilik göstermektedir. Tek tabaka grafen Fermi seviyesindeki Dirac noktaları ve civarındaki doğrusal enerji bantlarıyla kütlesiz fermiyonlara sahip bir yarımetal karakterindedir. Grafenin nano-ölçekli şeritleri (GNŞ) ise, şerit kalınlığına ve topolojisine bağlı olarak yasak bant aralığı değişen yarıiletkendirler. Çift tabaka grafenin, uygulanan bir dış elektrik alan yoluyla yasak enerji bant aralığı değiştirilebilen bir yarıiletken olduğu gösterilmiştir. Yine zigzag kenarlı GNŞ’lerde ve diğer grafen parçalarında, zigzag kenarlar boyunca yerelleşmiş ferromanyetik spin durumlarının olduğu bilinmektedir. Grafenin bu manyetik özelliği çeşitli spintronik uygulamalarında kullanılabilir. Grafen yapıların üzerine ve kenarlarına çok çeşitli element veya moleküller kullanarak kimyasal veya fiziksel işlevlendirme yapıp, yapıya farklı özellikler kazandırmak mümkündür.Research Project Ultra İnce Geçiş Metali Dikalkojenitleri Iıı-v Grubu Bileşikleri ve Bunların Heteroyapıları(2017) Senger, Ramazan Tuğrul;Projenin temel amacı geçiş-metali dikalkojenitlerinin tek tabakalı kristal yapılarının ve bunların kendi aralarında veya farklı metaryellerle oluşturduğu heteroyapıların yapısal, elektronik, manyetik, titreşimsel ve transport özelliklerinin kuramsal ve hesaplamalı yöntemlerle incelenmesidir. Tek tabakalı kristal malzemelerin üzerindeki yoğun akademik ilgi, grafenin, yani tek atom kalınlığındaki grafit tabakasının sentezlenmesinin ardından başlamıştır. Son yıllarda yapılan grafen araştırmaları, gerek deneysel gerekse teorik olarak bu yeni malzeme sınıfı konusunda hızlı bir bilgi birikimi sağlayarak farklı birçok grafen benzeri malzemelerin de önünü açmıştır. Bunlar arasında florografen, grafan, klorografen, silisen, germanen, III-V bileşiklerinin hegzagonal yapıları (h-BN, h-AlN) ve geçiş metali dikalkojenitleri (GMD, örneğin MoS2) sayılabilir. Bu yeni ve zengin malzeme sınıfının ortaya çıkışıyla ?iki boyutlu elektronik? adı altında bir araştırma alanı hızla gelişmektedir. Proje kapsamında yapılan teorik çalışmalar çoğunlukla Yoğunluk Fonksiyoneli Kuramı?nı temel alan hesap teknikleri ile yürütülmüştür. Teorik olarak umut vaat eden bu malzemelerin kapsamlı analizlerinin yapılması, dünyada pek çok grubun aktif çalışma konusu olan deneysel sentezlenme çalışmalarıyla sinerji yaratabilecek bir potansiyel taşımaktadır. Proje kapsamında öncelikle, ele alınan GMD?lerin iki boyutlu yapıların elektronik, manyetik, mekanik ve transport özellikleri hesaplandı. Ardından, bu yapılarda eksik atom durumlarına, diğer kusur durumlarına ve bu kusurlarla beraber elektronik, manyetik ve mekanik özelliklerindeki değişimler ele alındı. Sonrasında, yabancı atomlarla bu malzemelerin etkileşimleri ve buna bağlı olarak özelliklerindeki değişimler incelendi. Ayrıca, bu malzemelerin heteroyapılarının kararlılıkları ve kararlı olanların diğer özellikleriyle beraber elektronik ve transport özellikleri incelendi. Konusu, güncel ve hızla gelişen bir alana giren bu projede özgün çıktılar elde edilmiş ve bu çıktılar nitelikli uluslararası dergilerde yayınlanmıştır.Article Citation - WoS: 14Citation - Scopus: 17Enhancement of Thermoelectric Efficiency of T-Hfse2 Via Nanostructuring(American Physical Society, 2021) Ünsal, Elif; Senger, Ramazan Tuğrul; Sevinçli, HaldunIn this work, ab initio calculations based on density functional theory and the Landauer formalism are carried out to investigate ballistic thermoelectric properties of T-HfSe2 nanoribbons (NRs). The zigzag-edged NRs are metallic, and they are not included in this study. The armchair NRs possess two types of edge symmetries depending on the number of atoms present in a row; odd-numbered NRs have mirror symmetry, whereas the even-numbered NRs have glide reflection symmetry. The armchair-edged NRs are dynamically stable and show semiconducting properties with varying band gap values in the infrared and visible regions. Detailed transport analyses show that the n-type Seebeck coefficient and the power factor differ because of the structural symmetry, whereas the p-type thermoelectric coefficients are not significantly influenced. It is shown that the phonon thermal conductance is reduced to a third of its two-dimensional value via nanostructuring. The p-type Seebeck coefficient and the power factor for T-phase HfSe(2 )are enhanced in NRs. We report that the p-type ZT value of HfSe2 NRs at 300 and 800 K are enhanced by factors of 4 and 3, respectively.Article Citation - WoS: 15Citation - Scopus: 15Ag and Au Atoms Intercalated in Bilayer Heterostructures of Transition Metal Dichalcogenides and Graphene(American Institute of Physics, 2014) İyikanat, Fadıl; Şahin, Hasan; Senger, Ramazan Tuğrul; Peeters, François M.The diffusive motion of metal nanoparticles Au and Ag on monolayer and between bilayer heterostructures of transition metal dichalcogenides and graphene are investigated in the framework of density functional theory. We found that the minimum energy barriers for diffusion and the possibility of cluster formation depend strongly on both the type of nanoparticle and the type of monolayers and bilayers. Moreover, the tendency to form clusters of Ag and Au can be tuned by creating various bilayers. Tunability of the diffusion characteristics of adatoms in van der Waals heterostructures holds promise for controllable growth of nanostructures. (C) 2014 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution 3.0 Unported License.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: 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: 3Citation - Scopus: 3Hydrogenated Derivatives of Hexacoordinated Metallic Cu2si Monolayer(Royal Society of Chemistry, 2018) Ünsal, Elif; İyikanat, Fadıl; Şahin, Hasan; Senger, Ramazan TuğrulHerein, we carried out first-principles calculations based on density functional theory to investigate the effects of surface functionalization with hydrogen atoms on structural, dynamical and electronic properties of Cu2Si monolayer. Pristine Cu2Si, a metallic monolayer, has a planar hexacoordinate structure. Calculations revealed that the most favorable position of a single H atom on the Cu2Si monolayer is at the top of a Si site. Derivatives of Cu2Si monolayer with various H concentrations were investigated, and by performing phonon calculations, it was found that there are three stable hydrogenated structures. Specific heat of these monolayers was found to increase with the hydrogen concentration at temperatures higher than 100 K. Electronically, the hydrogenated derivatives of Cu2Si monolayer preserve the metallic character.Article Citation - WoS: 1Citation - Scopus: 1Directed Growth of Hydrogen Lines on Graphene: High-Throughput Simulations Powered by Evolutionary Algorithm(American Physical Society, 2018) Özbal, Gözde; Falkenberg, J. T.; Brandbyge, M.; Senger, Ramazan Tuğrul; Sevinçli, HaldunWe set up an evolutionary algorithm combined with density functional tight-binding calculations to investigate hydrogen adsorption on flat graphene and graphene monolayers curved over substrate steps. During the evolution, candidates for the new generations are created by adsorption of an additional hydrogen atom to the stable configurations of the previous generation, where a mutation mechanism is also incorporated. Afterwards a two-stage selection procedure is employed. Selected candidates act as the parents of the next generation. The evolutionary algorithm predicts formation of lines of hydrogen atoms on flat graphene. In curved graphene, the evolution follows a similar path except for a new mechanism, which aligns hydrogen atoms on the line of minimum curvature. The mechanism is due to the increased chemical reactivity of graphene along the minimum radius of curvature line (MRCL) and to sp(3) bond angles being commensurate with the kinked geometry of hydrogenated graphene at the substrate edge. As a result, the reaction barrier is reduced considerably along the MRCL and hydrogenation continues like a mechanical chain reaction. This growth mechanism enables lines of hydrogen atoms along the MRCL, which has the potential to overcome substrate or rippling effects and could make it possible to define edges or nanoribbons without actually cutting the material.Article Citation - WoS: 50Citation - Scopus: 52Monitoring the Characteristic Properties of Ga-Doped Zno by Raman Spectroscopy and Atomic Scale Calculations(Elsevier, 2019) Horzum, Şeyda; İyikanat, Fadıl; Senger, Ramazan Tuğrul; Çelebi, Cem; Sbeta, Mohamed; Yıldız, Abdullah; Serin, TülayWe experimentally and theoretically study how the structural and vibrational properties of zinc oxide (ZnO) are modified upon Gallium (Ga) doping. The characteristics of Ga-doped ZnO thin films which are synthesized by sol-gel spin coating method on glass substrates are monitored by using X-ray diffraction (XRD) and Raman scattering measurements. For atomic-level understanding of the experimental findings state-of-the-art density functional theory (DFT) based calculations are also performed. DFT calculations reveal that both the substitution and adsorption of Ga atoms in ZnO are energetically possible and substitutional doping in ZnO is the most favourable scenario. XRD measurements show that all the films are in wurtzite structure and the crystallite size of the films decreases with increasing Ga doping. In addition, Raman analysis show that strong vibrational modes at about 100 and 441 cm(-1) are associated with E-2(low) and E-2(high) phonon branches of ZnO, respectively. While the frequency of the E-2(low) mode downshifts with increasing Ga concentration, the E-2(high) phonon mode is not affected by the Ga doping. Furthermore, E-Ga phonon branch, stemming from the substituted Ga atoms, emerges at low frequencies. It is also seen that the Raman intensity of the E-G(a) peak linearly increases with increasing Ga concentration. Experimental results on the vibrational properties are in good agreement with the ab initio phonon calculations. (C) 2018 Elsevier B.V. All rights reserved.Article Citation - WoS: 16Citation - Scopus: 16Temperature and Gate Dependence of Carrier Diffusion in Single Crystal Methylammonium Lead Iodide Perovskite Microstructures(American Chemical Society, 2020) McClintock, Luke; Xiao, Rui; Hou, Yasen; Gibson, Clinton; Travaglini, Henry Clark; Abramovitch, David; Tan, Liang Z.; Senger, Ramazan Tuğrul; Fu, Yongping; Jin, SongWe investigate temperature-dependent photogenerated carrier diffusion in single-crystal methylammonium lead iodide microstuctures via scanning photocurrent microscopy, Carrier diffusion lengths increased abruptly across the tetragonal to orthorhombic phase transition and reached 200 +/- 50 mu m at 80 K. In combination with the microsecond carrier lifetime measured by a transient photocurrent method, an enormous carrier mobility value of 3 x 10(4) cm(2)/V s was extracted at 80 K. The observed highly nonlocal photocurrent and the rapid increase of the carrier diffusion length at low temperatures can be understood by the formation and efficient transport of free excitons in the orthorhombic phase as a result of reduced optical phonon scattering due to the dipolar nature of the excitons. Carrier diffusion lengths were tuned by a factor of 8 by gate voltage and increased with increasing majority carrier (electron) concentration, consistent with the exciton model.