Materials Science and Engineering / Malzeme Bilimi ve Mühendisliği
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Article Citation - WoS: 4Citation - Scopus: 4Indirect Exchange Interaction in Two-Dimensional Materials With Quartic Dispersion(American Physical Society, 2022) Canbolat, Ahmet Utku; Sevinçli, Haldun; Çakır, ÖzgürWe investigate the indirect magnetic exchange interaction between two magnetic moments in a two-dimensional semiconductor with quartic dispersion, featuring a singularity at the band edge. We obtain the Green's functions analytically to calculate the magnetic exchange interaction at zero temperature. We show that the singularity in the density of states (DOS) for quartic dispersion gives rise to an enhancement in the amplitude of the Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction as the Fermi energy is swept toward the band edge. Furthermore, a region of finite exchange interaction arises, with a range increasing as the Fermi energy approaches the band edge. The results lay the possibility of an electrical/chemical control over the exchange interactions.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: 32Citation - Scopus: 33Ballistic Thermoelectric Transport Properties of Two-Dimensional Group Iii-Vi Monolayers(American Physical Society, 2021) Çınar, Mustafa Neşet; Özbal Sargın, Gözde; Sevim, Koray; Özdamar, Burak; Kurt, Gizem; Sevinçli, HaldunBallistic transport and thermoelectric properties of group III-VI compounds (XY: X = B, Al, Ga, In, Tl; Y = O, S, Se, Te, Po) are investigated based on first-principles calculations and Landauer formalism. This large family is composed of 25 compounds which stands out with their unique electronic band structures. Mexican hat shaped valence band, which exhibits quartic energy-momentum relation gives rise to a sharp peak in the density of states as well as a steplike electronic transmission spectrum near the valence band edge. The intriguing electronic band structure and transport properties motivate us to explore thermoelectric properties of group III-VI monolayers. We find that, in addition to the stepwise transmission at the band edge, flat bands, valley degeneracy, and band degeneracy are the factors that enhance thermoelectric efficiencies. For heavier compounds, better thermoelectric efficiencies are possible for both n-type and p-type carriers.Article Citation - WoS: 1Citation - Scopus: 1First-Principles Investigation of Photoisomeric Switching of Vibrational Heat Current Across Molecular Junctions(American Physical Society, 2020) Kurt, Gizem; Sevinçli, HaldunPhotoisomeric molecules rearrange their structure when exposed to light, which alters their chemical, electronic, mechanical, as well as vibrational properties. The present study explores the possibilities to tune the thermal transport across molecular junctions by using photoisomeric molecules. The effect of isomeric switching on phonon transport through single-molecule junctions linking two macroscopic reservoirs is investigated using density-functional-theory-based tight-binding calculations and Green-function formalism. The junctions are built using azobenzene and its derivatives (azobiphenyl and azotriphenyl) that display photoisomeric behavior. Effects of system setup on the heat current and the switching coefficient are studied systematically. Dependence on the molecular species, the choice of reservoir, as well as the type of linkers that bind the molecules to the reservoir are investigated with calculating the phonon-transmission spectra and temperature-dependent thermal conductance values. The results show that thermal conductance can be altered significantly by switching the molecule from trans- to cis-configuration since all molecules yield higher conductances in trans-configurations than their cis-configurations at temperatures higher than 50 K. In the low-temperature range, results reveal considerable switching coefficients exceeding 50%. At room temperature, the switching coefficient can be as high as 20%. It is shown that the effect is robust under the variation of both the molecular species and the linkers. © 2020 American Physical Society.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: 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.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: 5Citation - Scopus: 5Collapse of the Vacuum in Hexagonal Graphene Quantum Dots: a Comparative Study Between Tight-Binding and Mean-Field Hubbard Models(American Physical Society, 2020) Polat, Mustafa; Sevinçli, Haldun; Güçlü, Alev DevrimIn this paper, we perform a systematic study on the electronic, magnetic, and transport properties of the hexagonal graphene quantum dots (GQDs) with armchair edges in the presence of a charged impurity using two different configurations: (1) a central Coulomb potential and (2) a positively charged carbon vacancy. The tight-binding and the half-filled extended Hubbard models are numerically solved and compared with each other in order to reveal the effect of electron interactions and system sizes. Numerical results point out that off-site Coulomb repulsion leads to an increase in the critical coupling constant to beta(c) = 0.6 for a central Coulomb potential. This critical value of beta is found to be independent of the GQD size, reflecting its universality even in the presence of electron-electron interactions. In addition, a sudden downshift in the transmission peaks shows a clear signature of the transition from subcritical beta < beta(c) to the supercritical beta > beta(c) regime. On the other hand, for a positively charged vacancy, collapse of the lowest bound state occurs at beta(c) = 0.7 for the interacting case. Interestingly, the local magnetic moment, induced by a bare carbon vacancy, is totally quenched when the vacancy is subcritically charged, whereas the valley splittings in electron and hole channels continue to exist in both regimes.Article Citation - WoS: 134Citation - Scopus: 137Structural, Vibrational, and Electronic Properties of Single-Layer Hexagonal Crystals of Group Iv and V Elements(American Physical Society, 2018) Özdamar, Burak; Özbal, Gözde; Çınar, Mustafa Neşet; Sevim, Koray; Kurt, Gizem; Kaya, Birnur; Sevinçli, HaldunUsing first-principles density functional theory calculations, we investigate a family of stable two-dimensional crystals with chemical formula A2B2, where A and B belong to groups IV and V, respectively (A=C, Si, Ge, Sn, Pb; B=N, P, As, Sb, Bi). Two structural symmetries of hexagonal lattices P6m2 and P3m1 are shown to be dynamically stable, named as α- and β -phases correspondingly. Both phases have similar cohesive energies, and the α phase is found to be energetically favorable for structures except CP, CAs, CSb, and CBi, for which the β phase is favored. The effects of spin-orbit coupling and Hartree-Fock corrections to exchange correlation are included to elucidate the electronic structures. All structures are semiconductors except CBi and PbN, which have metallic character. SiBi, GeBi, and SnBi have direct band gaps, whereas the remaining semiconductor structures have indirect band gaps. All structures have quartic dispersion in their valence bands, some of which make the valence band maximum and resemble a mexican-hat shape. SnAs and PbAs have purely quartic valence band edges, i.e., E-αk4, a property reported for the first time. The predicted materials are candidates for a variety of applications. Owing to their wide band gaps, CP, SiN, SiP, SiAs, GeN, GeP can find their applications in optoelectronics. The relative band positions qualify a number of the structures as suitable for water splitting, where CN and SiAs are favorable at all pH values. Structures with quartic band edges are expected to be efficient for thermoelectric applications.Article Citation - WoS: 171Citation - Scopus: 169Janus Single Layers of In2sse: a First-Principles Study(American Physical Society, 2018) Kandemir, Ali; Şahin, HasanBy performing first-principles calculations, we propose a stable direct band gap semiconductor Janus single-layer structure, In2SSe. The binary analogs of the Janus structure, InS and InSe single layers are reviewed to evince the structural and electronic relation with In2SSe. The structural optimization calculations reveal that a Janus In2SSe single layer has hexagonal geometry like the InS and InSe single layers, which are also its structural analogs. The Janus single layer is dynamically stable, as indicated by the phonon spectrum. The electronic band diagram of the Janus structure shows that an In2SSe single layer is a direct band gap semiconductor, in contrast to its analogs, InS and InSe single layers, which are indirect band gap semiconductors. Nevertheless, it is found that the strain effect on electronic properties of the InS and InSe single layers designates the electronic structure of the Janus single layer. A rough model for the construction of the electronic band diagram of the Janus structures is discussed, and it is indicated that the difference in work functions of chalcogenide sides in the Janus structure determines the construction of the electronic structure. It is found that the Janus structure is a robust direct gap semiconductor under tolerable strain; for that reason, the Janus In2SSe single layer is a candidate for optoelectronic nanodevice applications.