Materials Science and Engineering / Malzeme Bilimi ve Mühendisliği
Permanent URI for this collectionhttps://hdl.handle.net/11147/4719
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Article Citation - WoS: 4Citation - Scopus: 4The Peculiar Potential of Transition Metal Dichalcogenides for Thermoelectric Applications: a Perspective on Future Computational Research(American Institute of Physics, 2023) Özbal, Gözde; Sarıkurt, Sevil; Sevinçli, Haldun; Sevik, CemThe peculiar potential transition metal dichalcogenides in regard to sensor and device applications have been exhibited by both experimental and theoretical studies. The use of these materials, thermodynamically stable even at elevated temperatures, particularly in nano- and optoelectronic technology, is about to come true. On the other hand, the distinct electronic and thermal transport properties possessing unique coherency, which may result in higher thermoelectric efficiency, have also been reported. However, exploiting this potential in terms of power generation and cooling applications requires a deeper understanding of these materials in this regard. This perspective study, concentrated with this intention, summarizes thermoelectric research based on transition metal dichalcogenides from a broad perspective and also provides a general evaluation of future theoretical investigations inevitable to shed more light on the physics of electronic and thermal transport in these materials and to lead future experimental research. © 2023 Author(s).Article Citation - WoS: 3Citation - Scopus: 2Effects of Interphase Boundaries in Ginzburg-Landau One-Dimensional Model of Two-Phase States in Clamped Systems(American Institute of Physics, 2021) Levanyuk, Arkady P.; Minyukov, Sergey A.; Mısırlıoğlu, İbrahim Burç; Okatan, Mahmut BarışPrevious Landau-type models of two-phase state formation in clamped systems whose material exhibits first-order phase transitions in free state neglects the existence of interphase boundaries. Here, we take them into account in the framework of a Ginzburg-Landau one-dimensional model to study the dependence of characteristics of the two-phase state on system size. Unlike earlier works, we find that the transition to the two-phase state from both the symmetrical and nonsymmetrical phases is not continuous but abrupt. For a one-dimensional system with length L studied in this work, we show that the formation of two-phase state begins with a region whose size is proportional to root L. The latent heat of the transition is also proportional to root L -> infinity, recovering the earlier result for infinite systems. The temperature width of the two-phase region decreases with decreasing of L, but we are unable to answer the question about the critical length for two-phase state formation because the approximation used in analytical calculations is valid for sufficiently large L. A region of small values of L was studied partially to reveal the limits of validity of the analytical calculations. The main physical results are also obtainable within a simple approximation that considers the energy of interphase boundary as a fixed value, neglecting its temperature dependence and the thickness of the boundary. A more involved but consistent treatment provides the same results within the accepted approximation and sheds light on the reason of validity of the simplified approach.Article Citation - WoS: 14Citation - Scopus: 13Effect of Texture on the Electrical and Electrocaloric Properties of 0.90pb(mg1/3nb2 Relaxor Ceramics(American Institute of Physics, 2020) Mensur Alkoy, Ebru; Okatan, M. Barış; Aydın, Ecem; Kılıç, Yusuf; Mısırlıoğlu, I. Burç; Alkoy, SedatFunctional properties of ferroelectric compositions depend strongly on the type of stable crystalline phase at a given temperature and texturing. In addition to defining a governing crystallographic axis among the grains with respect to a global reference axis, texturing in these systems is also often meant to imply the relationship between an external applied field and the polar axis. Here, we synthesize randomly oriented and 001(pc) textured 0.90Pb(Mg1/3Nb2/3)O-3-0.10PbTiO(3) solid solutions that fall into the relaxor category. A >95% degree of 001(pc) texturing was achieved by the use of single crystal BaTiO3 template crystallites whose volume fraction does not exceed 5% of the entire sample volume. Electrical measurements made on random and textured samples reveal the impact of texture on the hysteresis and the dielectric response. A Curie-Weiss analysis of the temperature dependent dielectric data shows the degree of relaxor behavior in random and textured samples. As similar compositions have recently been getting interest for electrothermal management applications, we compute the electrocaloric (EC) response of the random and textured samples using indirect methods followed by an estimation of the EC response with a subsequent thermodynamic analysis to shed light on the effect of texture on the observed differences between the textured and the random oriented samples. We finally compare and contrast on the desirability of texturing in these systems for use as EC components for thermal management applications.Article Citation - WoS: 3Citation - Scopus: 4Experimental and First-Principles Investigation of Cr-Driven Color Change in Cesium Lead Halide Perovskites(American Institute of Physics, 2019) Özen, Sercan; Güner, Tuğrul; Topçu, Gökhan; Özcan, Mehmet; Demir, Mustafa Muammer; Şahin, HasanHerein, we report room temperature Cr-doping for all-inorganic perovskites that have attracted great attention in recent years due to their extraordinary optical properties, low cost, and ease of synthesis. Incorporation of Cr 3 + ions into the perovskite crystal lattices is achieved by following a facile route involving an antisolvent recrystallization method at room temperature. It is shown that both Cr-doping and formation of crystals in the CsPbBr x Cl 3 - x phase are provided by increasing the concentration of the CrCl 3 solution. It is also observed that the doping procedure leads to the emergence of three types of distinctive peaks in the PL spectrum originating from CsPbBr x Cl 3 - x domains (476-427nm), Cr-strained host lattices (515nm), and midgap states formed by Cr dopants (675-775nm). It is also found that the Cr-doped perovskites emitting a dark violaceous color change their color to white with a high color rendering index (88) in 30-day time intervals. Easy-tunable optical properties of all-inorganic Cs perovskites indicate their great potential for future optoelectronic device applications.Article Citation - WoS: 9Structural, Electronic, and Magnetic Properties of Point Defects in Polyaniline (c3n) and Graphene Monolayers: a Comparative Study(American Institute of Physics, 2020) Sevim, Koray; Sevinçli, HaldunThe newly synthesized two-dimensional polyaniline (C3N) is structurally similar to graphene and has interesting electronic, magnetic, optical, and thermal properties. Motivated by the fact that point defects in graphene give rise to interesting features, like magnetization in an all carbon material, we perform density functional theory calculations to investigate vacancy and Stone-Wales type point defects in monolayer C3N. We compare and contrast the structural, electronic, and magnetic properties of these defects with those in graphene. While monovacancies and Stone-Wales defects of C3N result in reconstructions similar to those in graphene, divacancies display dissimilar geometrical features. Different from graphene, all vacancies in C3N have metallic character because of altered stoichiometry; those that have low-coordinated atoms have finite magnetic moments. We further investigate the robustness of the reconstructed structures and the changes in the magnetic moments by applying tensile and compressive biaxial strain. We find that, with the advantage of finite bandgap, point defects in C3N are qualified as good candidates for future spintronics applications.Article Citation - WoS: 2Citation - Scopus: 2Surface Roughness Estimation of Mbe Grown Cdte/Gaas(211)b by Ex-Situ Spectroscopic Ellipsometry(American Institute of Physics, 2016) Karakaya, Merve; Bilgilisoy, Elif; Arı, Ozan; Selamet, YusufSpectroscopic ellipsometry (SE) ranging from 1.24 eV to 5.05 eV is used to obtain the film thickness and optical properties of high index (211) CdTe films. A three-layer optical model (oxide/CdTe/GaAs) was chosen for the ex-situ ellipsometric data analysis. Surface roughness cannot be determined by the optical model if oxide is included. We show that roughness can be accurately estimated, without any optical model, by utilizing the correlation between SE data (namely the imaginary part of the dielectric function, <ϵ2 > or phase angle, ψ) and atomic force microscopy (AFM) roughness. <ϵ2 > and ψ values at 3.31 eV, which corresponds to E1 critical transition energy of CdTe band structure, are chosen for the correlation since E1 gives higher resolution than the other critical transition energies. On the other hand, due to the anisotropic characteristic of (211) oriented CdTe surfaces, SE data (<ϵ2 > and ψ) shows varieties for different azimuthal angle measurements. For this reason, in order to estimate the surface roughness by considering these correlations, it is shown that SE measurements need to be taken at the same surface azimuthal angle. Estimating surface roughness in this manner is an accurate way to eliminate cumbersome surface roughness measurement by AFM.Article Citation - WoS: 22Citation - Scopus: 24Quantum Interference in Thermoelectric Molecular Junctions: a Toy Model Perspective(American Institute of Physics, 2014) Nozaki, Daijiro; Avdoshenko, Stanislav M.; Sevinçli, Haldun; Cuniberti, GianaurelioQuantum interference (QI) phenomena between electronic states in molecular circuits offer a new opportunity to design new types of molecular devices such as molecular sensors, interferometers, and thermoelectric devices. Controlling the QI effect is a key challenge for such applications. For the development of single molecular devices employing QI effects, a systematic study of the relationship between electronic structure and the quantum interference is needed. In order to uncover the essential topological requirements for the appearance of QI effects and the relationship between the QI-affected line shape of the transmission spectra and the electronic structures, we consider a homogeneous toy model where all on-site energies are identical and model four types of molecular junctions due to their topological connectivities. We systematically analyze their transmission spectra, density of states, and thermoelectric properties. Even without the degree of freedom for on-site energies an asymmetric Fano peak could be realized in the homogeneous systems with the cyclic configuration. We also calculate the thermoelectric properties of the model systems with and without fluctuation of on-site energies. Even under the fluctuation of the on-site energies, the finite thermoelectrics are preserved for the Fano resonance, thus cyclic configuration is promising for thermoelectric applications. This result also suggests the possibility to detect the cyclic configuration in the homogeneous systems and the presence of the QI features from thermoelectric measurements.Article Citation - WoS: 10Citation - Scopus: 11Phonon scattering in graphene over substrate steps(American Institute of Physics, 2014) Sevinçli, Haldun; Brandbyge, MadsWe calculate the effect on phonon transport of substrate-induced bends in graphene. We consider bending induced by an abrupt kink in the substrate, and provide results for different step-heights and substrate interaction strengths. We find that individual substrate steps reduce thermal conductance in the range between 5% and 47%. We also consider the transmission across linear kinks formed by adsorption of atomic hydrogen at the bends and find that individual kinks suppress thermal conduction substantially, especially at high temperatures. Our analysis show that substrate irregularities can be detrimental for thermal conduction even for small step heights.Article Citation - WoS: 72Citation - Scopus: 80Electronic, Phononic, and Thermoelectric Properties of Graphyne Sheets(American Institute of Physics, 2014) Sevinçli, Haldun; Sevik, CemElectron, phonon, and thermoelectric transport properties of α-, β-, γ-, and 6,6,12-graphyne sheets are compared and contrasted with those of graphene. α-, β-, and 6,6,12-graphynes, with direction dependent Dirac dispersions, have higher electronic transmittance than graphene. γ-graphyne also attains better electrical conduction than graphene except at its band gap. Vibrationally, graphene conducts heat much more efficiently than graphynes, a behavior beyond an atomic density differences explanation. Seebeck coefficients of the considered Dirac materials are similar but thermoelectric power factors decrease with increasing effective speeds of light. γ-graphyne yields the highest thermoelectric efficiency with a thermoelectric figure of merit as high as ZT-=-0.45, almost an order of magnitude higher than that of graphene.