Physics / Fizik
Permanent URI for this collectionhttps://hdl.handle.net/11147/6
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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: 110Citation - Scopus: 109Structural, Electronic and Phononic Properties of Ptse2: From Monolayer To Bulk(IOP Publishing Ltd., 2018) Kandemir, Ali; Akbalı, Barış; Kahraman, Z.; Badalov, S. V.; Özcan, Mehmet; İyikanat, Fadıl; Şahin, HasanThe layer dependent structural, electronic and vibrational properties of the 1T phase of two dimensional (2D) platinum diselenide are investigated by means of state-of-the-art first-principles calculations. The main findings of the study are: (i) monolayer platinum diselenide has a dynamically stable 2D octahedral structure with 1.66 eV indirect band gap, (ii) the semiconducting nature of 1T-PtSe2 monolayers remains unaffected even at high biaxial strains, (iii) top-to-top (AA) arrangement is found to be energetically the most favorable stacking of 1T-PtSe2 layers, (iv) the lattice constant (layer-layer distance) increases (decreases) with increasing number of layers, (v) while monolayer and bilayer 1T-PtSe2 are indirect semiconductors, bulk and few-layered 1T-PtSe2 are metals, (vi) Raman intensity and peak positions of the A1g and Eg modes are found to be highly dependent on the layer thickness of the material, hence; the number of layers of the material can be determined via Raman measurements.Article Citation - WoS: 7Citation - Scopus: 8Hydrogenation-driven phase transition in single-layer TiSe2(IOP Publishing Ltd., 2017) İyikanat, Fadıl; Kandemir, Ali; Özaydın, H. Duygu; Senger, Ramazan Tuğrul; Şahin, HasanFirst-principles calculations based on density-functional theory are used to investigate the effects of hydrogenation on the structural, vibrational, thermal and electronic properties of the charge density wave (CDW) phase of single-layer TiSe2. It is found that hydrogenation of single-layer TiSe2 is possible through adsorption of a H atom on each Se site. Our total energy and phonon calculations reveal that a structural phase transition occurs from the CDW phase to the T d phase upon full hydrogenation. Fully hydrogenated TiSe2 presents a direct gap semiconducting behavior with a band gap of 119 meV. Full hydrogenation also leads to a significant decrease in the heat capacity of single-layer TiSe2.Article Citation - WoS: 25Citation - Scopus: 30Thinning Cspb2br5 Perovskite Down To Monolayers: Cs-Dependent Stability(American Physical Society, 2017) İyikanat, Fadıl; Sarı, Emre; Şahin, HasanUsing first-principles density functional theory calculations, we systematically investigate the structural, electronic, and vibrational properties of bulk and potential single-layer structures of perovskitelike CsPb2Br5 crystal. It is found that while Cs atoms have no effect on the electronic structure, their presence is essential for the formation of stable CsPb2Br5 crystals. The calculated vibrational spectra of the crystal reveal that not only the bulk form but also the single-layer forms of CsPb2Br5 are dynamically stable. Predicted single-layer forms can exhibit either semiconducting or metallic character. Moreover, the modification of the structural, electronic, and magnetic properties of single-layer CsPb2Br5 upon formation of vacancy defects is investigated. It is found that the formation of Br vacancy (i) has the lowest formation energy, (ii) significantly changes the electronic structure, and (iii) leads to ferromagnetic ground state in the single-layer CsPb2Br5. However, the formation of Pb and Cs vacancies leads to p-type doping of the single-layer structure. Results reported herein reveal that the single-layer CsPb2Br5 crystal is a novel stable perovskite with enhanced functionality and a promising candidate for nanodevice applications.Article Citation - WoS: 8Citation - Scopus: 10Stability, Electronic and Phononic Properties of Ss and 1t Structures of Sitex (x = 1, 2) and Their Vertical Heterostructures(IOP Publishing Ltd., 2017) Kandemir, Ali; İyikanat, Fadıl; Şahin, HasanBy performing first-principles calculations, we predict a novel, stable single layer phase of silicon ditelluride, 1T-SiTe2, and its possible vertical heterostructures with single layer β-SiTe. Structural optimization and phonon calculations reveal that 1T-SiTe2 structure has a dynamically stable ground state. Further analysis of the vibrational spectrum at the - point shows that single layer 1T-SiTe2 has characteristic phonon modes at 80, 149, 191 and 294 cm-1. Electronic-band structure demonstrates that 1T-SiTe2 phase exhibits a nonmagnetic metallic ground state with a negligible intrinsic spinorbit splitting. Moreover, it is shown that similar structural parameters of 1T-SiTe2 and existing β-SiTe phases allows construction of 1T-β heterostructures with a negligible lattice mismatch. In this regard, it is found that two energetically favorable stacking orders, namely AA and ATB, have distinctive shear and layer breathing phonon modes. It is important to note that the combination of semiconducting β-SiTe and metallic 1T-SiTe2 building blocks forms ultra-thin Schottky barriers that can be used in nanoscale optoelectronic device technologies.Article Citation - WoS: 2Citation - Scopus: 2Structural Changes in a Schiff Base Molecular Assembly Initiated by Scanning Tunneling Microscopy Tip(IOP Publishing Ltd., 2016) Tomak, Aysel; Bacaksız, Cihan; Mendirek, Gizem; Şahin, Hasan; Hür, Deniz; Görgün, Kamuran; Senger, Ramazan Tuğrul; Birer, Özgür; Peeters, François M.; Zareie, Hadi M.We report the controlled self-organization and switching of newly designed Schiff base (E)-4-((4-(phenylethynyl) benzylidene) amino) benzenethiol (EPBB) molecules on a Au (111) surface at room temperature. Scanning tunneling microscopy and spectroscopy (STM/STS) were used to image and analyze the conformational changes of the EPBB molecules. The conformational change of the molecules was induced by using the STM tip while increasing the tunneling current. The switching of a domain or island of molecules was shown to be induced by the STM tip during scanning. Unambiguous fingerprints of the switching mechanism were observed via STM/STS measurements. Surface-enhanced Raman scattering was employed, to control and identify quantitatively the switching mechanism of molecules in a monolayer. Density functional theory calculations were also performed in order to understand the microscopic details of the switching mechanism. These calculations revealed that the molecular switching behavior stemmed from the strong interaction of the EPBB molecules with the STM tip. Our approach to controlling intermolecular mechanics provides a path towards the bottom-up assembly of more sophisticated molecular machines.Article Citation - WoS: 166Citation - Scopus: 170Mechanical Properties of Monolayer Gas and Gase Crystals(American Physical Society, 2016) Yağmurcukardeş, Mehmet; Senger, Ramazan Tuğrul; Peeters, François M.; Şahin, HasanThe mechanical properties of monolayer GaS and GaSe crystals are investigated in terms of their elastic constants: in-plane stiffness (C), Poisson ratio (ν), and ultimate strength (σU) by means of first-principles calculations. The calculated elastic constants are compared with those of graphene and monolayer MoS2. Our results indicate that monolayer GaS is a stiffer material than monolayer GaSe crystals due to the more ionic character of the Ga-S bonds than the Ga-Se bonds. Although their Poisson ratio values are very close to each other, 0.26 and 0.25 for GaS and GaSe, respectively, monolayer GaS is a stronger material than monolayer GaSe due to its slightly higher σU value. However, GaS and GaSe crystals are found to be more ductile and flexible materials than graphene and MoS2. We have also analyzed the band-gap response of GaS and GaSe monolayers to biaxial tensile strain and predicted a semiconductor-metal crossover after 17% and 14% applied strain, respectively, for monolayer GaS and GaSe. In addition, we investigated how the mechanical properties are affected by charging. We found that the flexibility of single layer GaS and GaSe displays a sharp increase under 0.1e/cell charging due to the repulsive interactions between extra charges located on chalcogen atoms. These charging-controllable mechanical properties of single layers of GaS and GaSe can be of potential use for electromechanical applications. © 2016 American Physical Society.Article Citation - WoS: 14Citation - Scopus: 14Formation and Diffusion Characteristics of Pt Clusters on Graphene, 1h-Mos2 and 1t-Tas2(John Wiley and Sons Inc., 2014) Özaydın, H. Duygu; Şahin, Hasan; Şenger, Ramazan Tuğrul; Peeters, François M.Many experiments have revealed that the surfaces of graphene and graphene-like structures can play an active role as a host surface for clusterization of transition metal atoms. Motivated by these observations, we investigate theoretically the adsorption, diffusion and magnetic properties of Pt clusters on three different two-dimensional atomic crystals using first principles density functional theory. We found that monolayers of graphene, molybdenum disulfide (1H-MoS2) and tantalum disulfide (1T-TaS2) provide different nucleation characteristics for Pt cluster formation. At low temperatures, while the bridge site is the most favorable site where the growth of a Pt cluster starts on graphene, top-Mo and top-Ta sites are preferred on 1H-MoS2 and 1T-TaS2, respectively. Ground state structures and magnetic properties of Ptn clusters (n = 2,3,4) on three different monolayer crystal structures are obtained. We found that the formation of Pt2 dimer and a triangle-shaped Pt3 cluster perpendicular to the surface are favored over the three different surfaces. While bent rhombus shaped Pt4 is formed on graphene, the formation of tetrahedral shaped clusters are more favorable on 1H-MoS2 and 1T-TaS2. Our study of the formation of Ptn clusters on three different monolayers provides a gateway for further exploration of nanocluster formations on various surfaces.