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Öğe Can Stable MoS2 Monolayers and Multilayers Be Constituted in the Biphenylene Network?(Amer Chemical Soc, 2023) Gorkan, Taylan; Demirci, Salih; V. Barth, Johannes; Aktürk, Ethem; Çıracı, SalimBased on the first-principles calculations, we predict that the well-known 2H-MoS2 monolayer of the trigonal prismatic phase appearing in a hexagonal network can also constitute another stable phase in the biphenylene network (B-MoS2). It consists of the connected octagon, hexagon and square rings and hence maintains the same numbers of neighbors of the constituent atoms, but its bonds between transition metal and chalcogen atoms are deformed to construct a direct but narrow band gap semiconductor with directional electronic conduction and optical properties with strong absorption in the near-infrared region. It has softer mechanical properties and site specific chemical activities of the same kind of constituent atoms. In the same way, vacancies of different chalcogen atoms in the cell attain different defect states in the band gap. This phase can remain stable above the room temperature and has a cohesive energy comparable to all the other 2D phases of the same compound. In fact, transitions from the 2H-phase to the B-phase can be possible. The B-phase can form multilayers and also a metallic 3D layered, van der Waals crystal with weak interlayer coupling. The narrow band gap of the monolayer is reduced in the bilayer but diminishes in multilayers and 3D layered crystals to change the semiconductor to a metal. Even more interesting is that B-MoS2 is versatile for the modulation of the band gap, even for the metal-insulator transition under applied strains.Öğe Düşük boyutlu selenyum, tellür ve ortorombik dibor dinitrür (o-B2N2) malzemelerinin yoğunluk fonksiyonel teorisi ile incelenmesi(Kırıkkale Üniversitesi, 2020) Demirci, Salih; Nezir, Saffet; Jahangırov, SeymurTermal uyarımlar, basma zorlanması ve pozitif ve negatif polaritenin aşırı yükü, selenyum ve tellür yarı iletkenlerini farklı metalik kristal yapılara dönüştürerek metal-yalıtkan geçişlerine aracılık etmektedir. Yüksek sıcaklığa ısıtıldıklarında veya sıkıştırıldıklarında veya pozitif olarak yüklendiklerinde, elementel kristaller arasında oldukça nadir görülen metalik bantlara sahip basit kübik yapıya dönüşürler. Negatif olarak yüklendiklerinde önce cisim merkezli dörtgen yapıya ve ardından negatif yükün artması ile cisim merkezli ortorombik yapıya dönüşürler. Fazla elektronlarla kararlı hale getirilen bu iki yeni yapı, aynı zamanda üst üste binen metalik bantlara ve daha düşük boyutsallığa sahip sanki iki-boyutlu ve bir-boyutlu alt yapılardan oluşmaktadır. Yük yükleme ile bu elementlerin iki-boyutlu nano tabakaları ve tek tabakalarında da benzer yapısal dönüşümler görülmektedir. Bu faz değişimleri dinamik, tersine çevrilebilir ve ayarlanabilirdir. Sonuç olarak elde edilen metal-yalıtkan geçişleri çok kısa zaman aralığında gerçekleşebilir ve önemli cihaz uygulamaları sunabilir. Ortorombik bir yapıda bor nitrürün iki-boyutlu tek tabakalı polimorfu(o-B2N2)tahmin edilmiştir. Yapısal optimizasyon, fonon dağılımı ve moleküler dinamik hesaplamalar, o-B2N2'nin termal ve dinamik olarak kararlı olduğunu göstermektedir. o-B2N2, 1.70 eV değerinde direkt bant genişliğine sahip bir yarı iletkendir ve y yönünde görünür aralıkta yüksek optik soğurmaya sahipken x yönünde düşük soğurmaya sahiptir. Bu muhteşem özellikler, o-B2N2'yi yarı iletken ve fotovoltaik uygulamalar için uygun bir malzeme yapmaktadır. o-B2N2, zorlanma ile görünür aralıkta optimum soğurma oranını korurken 2.92 eV değerine kadar arzu edilen herhangi bir bant genişliğini elde etmek için ideal bir seçenek olabilir. Zorlanma altında, iş fonksiyonu 2.98 eV değerine kadar düşürülebilmektedir. Bu değer o-B2N2'yi elektron yayılımına dayalı olarak çalışan ışık algılama cihazları için uygun bir malzeme yapmaktadır. Tüm çalışmalar, Yoğunluk Fonksiyonel Teorisi'ne (YFT) dayanan ilk ilke hesaplamaları kullanılarak gerçekleştirilmiştir.Öğe Functional Carbon and Silicon Monolayers in Biphenylene Network(Amer Chemical Soc, 2022) Gorkan, Taylan; Callioglu, Safak; Demirci, Salih; Aktuerk, Ethem; Ciraci, SalimWe investigated the effects of vacancy, void, substitutional impurity, isolated adsorption of selected adatoms, and their patterned coverage on the physical and chemical properties of metallic carbon and silicon monolayers in a biphenylene network. These monolayers can acquire diverse electronic and magnetic properties to become more functional depending on the repeating symmetry, size of the point defects, and on the type of adsorbed adatoms. While a carbon monovacancy attains a local magnetic moment, its void can display closed edge states with interesting physical effects. Adsorbed light-transition or rare-earth metal atoms attribute magnetism to these monolayers. The opening of a gap in the metallic density of states, which depends on the pattern and density of adsorbed hydrogen, oxygen, and carbon adatoms, can be used as the band gap engineering of these two-dimensional materials. The energy barriers against the passage of oxygen atoms through the centers of hexagon and octagon rings are investigated, and the coating of the active surfaces with carbon monolayers is exploited as a means of protection against oxidation. We showed that the repulsive forces exerting even at distant separations between two parallel, hydrogenated carbon monolayers in a biphenylene network can lead to the superlow friction features in their sliding motion. All these results obtained from the calculations using the density functional theory herald critical applications.Öğe Hydrogenated Carbon Monolayer in Biphenylene Network Offers a Potential Paradigm for Nanoelectronic Devices(Amer Chemical Soc, 2022) Demirci, Salih; Gorkan, Taylan; Callioglu, Safak; Ozcelik, V. Ongun; Barth, Johannes, V; Aktuerk, Ethem; Ciraci, SalimA metallic carbon monolayer in the biphenylene network (specified as C ohs) becomes an insulator upon hydrogenation (specified as CH ohs). Patterned dehydrogenation of this CH ohs can offer a variety of intriguing functionalities. Composite structures constituted by alternating stripes of C and CH ohs with different repeat periodicity and chirality display topological properties and can form heterostructures with a tunable band-lineup or Schottky barrier height. Alternating arrangements of these stripes of finite size enable one to also construct double barrier resonant tunneling structures and 2D, lateral nanocapacitors with high gravimetric capacitance for an efficient energy storage device. By controlled removal of H atom from a specific site or dehydrogenation of an extended zone, one can achieve antidoping or construct OD quantum structures like antidots, antirings/loops, and supercrystals, the energy level spacing of which can be controlled with their geometry and size for optoelectronic applications. Conversely, all these device functions can be acquired also by controlled hydrogenation of a bare C ohs monolayer. Since all these processes are applied to a monolayer, the commensurability of electronically different materials is assured. These features pertain not only to CH ohs but also to fully hydrogenated Si ohs.Öğe Lateral Composite Structures of Graphene/Graphane/Graphone: Electronic Confinement, Heterostructures with Tunable Band Alignment, and Magnetic State(Amer Chemical Soc, 2023) Demirci, Salih; Gorkan, Taylan; Akturk, Ethem; Ciraci, SalimGraphene can be hydrogenated fully on both sides andalso semihydrogenatedon one side to constitute graphane and graphone, respectively. Whileboth are wide band gap semiconductors, graphone also acquires a magneticground state originating from unpaired & pi;-bonds. We predict thatlateral composite structures/heterostructures can be constructed bythe patterned dehydrogenation of graphane or graphone with commensurateinterfaces, which display diverse physical properties depending ontheir constituents, interface geometry, and size. When constructedby consecutive graphane and graphene strips of very narrow width,they can attain exclusive electronic and magnetic properties in 2D,which are different from those of both parent materials. However,periodic and commensurate semiconductor-semiconductor heterostructureswith straddling band alignment and tunable band gaps can form, ifthe widths of strips with the armchair interface are wide enough toentail confinements of electronic states and hence to change the dimensionalityof the system from 2D to 1D. Depending on the type of zigzag interface,periodic heterostructures attain spin polarized straddling band alignments.Composite structures patterned on graphone can form magnetic semiconductor-semiconductorheterostructures, which have different staggered band alignments fordifferent spin polarization. Specifically, under the in-plane electricfield, a single heterostructure constructed on zigzag nanoribbonscan change its magnetic state and start to operate as a magnetic diodefor one spin direction. All of these composite structures, which allowelectronic confinement followed by a change of dimensionality, offervarious quantum structures and functionalities with potential applicationsin spintronics.Öğe Magnetization of silicene via coverage with gadolinium: Effects of thickness, symmetry, strain, and coverage(Amer Physical Soc, 2021) Demirci, Salih; Gorkan, Taylan; Callioglu, Safak; Yuksel, Yusuf; Akinci, Umit; Akturk, Ethem; Ciraci, SalimWhen covered by gadolinium (Gd) atoms, silicene, a freestanding monolayer of Si atoms in a honeycomb network, remains stable above the room temperature and becomes a two-dimensional (2D) ferromagnetic semiconductor, despite the antiferromagnetic ground state of three-dimensional bulk GdSi2 crystal. In thin GdSi2 multilayers, even if magnetic moments are ordered parallel in the same Gd atomic planes, they are antiparallel between nearest Gd planes; hence they exhibit a ferrimagnetic behavior. In contrast, a freestanding Gd2Si2 monolayer constructed by covering silicene from both sides by Gd atoms is a stable antiferromagnetic metal due to the mirror symmetry. While multilayers covered by Gd from both sides having an odd number of Gd planes have a ferrimagneticlike ground state, even-numbered ones have antiferromagnetic ground state, but none of them is ferromagnetic. Silicon atoms intervening between Gd planes are responsible for these intriguing magnetic orders conforming with the recent experiments performed on Si(111) surface. Additionally, the magnetic states of these 2D gadolinium disilicide monolayers can be monitored by applied tensile strain and by the coverage/decoration of Gd. These predictions obtained by using first-principles, spin-polarized, density functional theory calculations combined with Monte Carlo simulations herald that C, B, Si, Ge, Sn, and their compounds functionalized by rare-earth atoms can lead to novel nanostructures in 2D spintronics.Öğe Modification of electronic structure, magnetic structure, and topological phase of bismuthene by point defects(Amer Physical Soc, 2017) Kadioglu, Yelda; Kilic, Sevket Berkay; Demirci, Salih; Akturk, O. Uzengi; Akturk, Ethem; Ciraci, SalimThis paper reveals how the electronic structure, magnetic structure, and topological phase of two-dimensional (2D), single-layer structures of bismuth are modified by point defects. We first showed that a free-standing, single-layer, hexagonal structure of bismuth, named h-bismuthene, exhibits nontrivial band topology. We then investigated interactions between single foreign adatoms and bismuthene structures, which comprise stability, bonding, electronic structure, and magnetic structures. Localized states in diverse locations of the band gap and resonant states in band continua of bismuthene are induced upon the adsorption of different adatoms, which modify electronic and magnetic properties. Specific adatoms result in reconstruction around the adsorption site. Single vacancies and divacancies can form readily in bismuthene structures and remain stable at high temperatures. Through rebondings, Stone-Whales-type defects are constructed by divacancies, which transform into a large hole at high temperature. Like adsorbed adatoms, vacancies induce also localized gap states, which can be eliminated through rebondings in divacancies. We also showed that not only the optical and magnetic properties, but also the topological features of pristine h-bismuthene can be modified by point defects. The modification of the topological features depends on the energies of localized states and also on the strength of coupling between point defects.Öğe Monolayer diboron dinitride: Direct band-gap semiconductor with high absorption in the visible range(AMER PHYSICAL SOC, 2020) Demirci, Salih; Rad, Soheil Ershad; Kazak, Sahmurat; Nezir, Saffet; Jahangirov, SeymurWe predict a two-dimensional monolayer polymorph of boron nitride in an orthorhombic structure (o-B2N2) using first-principles calculations. Structural optimization, phonon dispersion, and molecular dynamics calculations show that o-B2N2 is thermally and dynamically stable. o-B2N2 is a semiconductor with a direct band gap of 1.70 eV according to calculations based on hybrid functionals. The structure has high optical absorption in the visible range in the armchair direction while low absorption in the zigzag direction. This anisotropy is also present in electronic and mechanical properties. The in-plane stiffness of o-B2N2 is very close to that of hexagonal boron nitride. The diatomic building blocks of this structure hint at its possible synthesis from precursors having B-B and N-N bonds.Öğe Nanostructures of Molybdenum, Chromium, and Tungsten Constructed by a Basic Structural Unit(Amer Chemical Soc, 2024) Demirci, Salih; Jahangirov, Seymur; Ciraci, SalimWe predicted a stable, zigzag chain structure of Molybdenum Mo, which, being a basic structural unit, can construct stable 0D or quantum dot (segments of atomic chains and rings), 1D (various infinite chains), quasi-1D (nanoribbons and nanotubes), and 2D (bilayers) structures entailing critical magnetic, elastic, and electronic properties. This zigzag chain, constituted by strong Mo-Mo covalent bonds, is a nonmagnetic semiconductor but undergoes an insulator-metal transition under compression acquiring a ferromagnetic state. Although Mo cannot form stable, suspended, 2D monolayers of a single atomic plane, parallel zigzag chains bound by metallic interchain interaction can construct stable bilayers of different symmetry, and also their stacks forming slabs. These bilayers, being elemental Dirac materials and displaying directional properties, clarify the ambiguous situation in recent studies synthesizing and/or predicting different 2D molybdenene structures. Notably, other Group VIB elements, chromium, and tungsten, also construct similar atomic chains and 2D bilayers with diverse physical properties. Intriguingly, an atomic chain structure serves as a building block for many low-dimensional materials with diverse physical properties, making it unique in nanoscience.Öğe Stability and electronic properties of monolayer and multilayer structures of group-IV elements and compounds of complementary groups in biphenylene network(Amer Physical Soc, 2022) Demirci, Salih; Callioglu, Safak; Gorkan, Taylan; Akturk, Ethem; Ciraci, SalimWe predict that specific group-IV elements and IV-IV, III-V, and II-VI compounds can form stable, free-standing two-dimensional (2D) monolayers consisting of octagon, hexagon, and square rings (ohs), in which the threefold coordination of atoms is preserved to allow sp(2)-type hybridization. These monolayers can also construct bilayers, multilayers, three-dimensional (3D) layered van der Waals solids, and 3D crystals with strong vertical bonds between layers as well as quasi-one-dimensional nanotubes and nanoribbons with diverse edge geometries. All these ohs structures can constitute a large class of 2D materials ranging from good metals to wide bandgap semiconductors and display physical and chemical properties rather different from those of their counterparts in the hexagonal (honeycomb) network. The metallic state of freestanding 2D C, Si, and Ge ohs monolayers and 3D C ohs bulk contrast, respectively, with graphene, silicene, germanene, and graphite.Öğe Strain engineering of electronic and optical properties of monolayer diboron dinitride(Amer Physical Soc, 2021) Demirci, Salih; Rad, Soheil Ershad; Jahangirov, SeymurWe studied the effect of strain engineering on the electronic, structural, mechanical, and optical properties of orthorhombic diboron dinitride (o-B2N2) through first-principles calculations. The 1.7-eV direct band gap observed in the unstrained o-B2N2 can be tuned up to 3 eV or down to 1 eV by applying 12% tensile strain in armchair and zigzag directions, respectively. Ultimate strain values of o-B2N2 were found to be comparable with that of graphene. Our calculations revealed that the partial alignment of the band edges with the redox potentials of water in pristine o-B2N2 can be tuned into a full alignment under the armchair and biaxial tensile strains. The anisotropic charge carrier mobility found in o-B2N2 prolongs the average lifetime of the carrier drift, creating a suitable condition for photoinduced catalytic reactions on its surface. Finally, we found that even in extreme straining regimes, the highly anisotropic optical absorption of o-B2N2 with strong absorption in the visible range is preserved. Having strong visible light absorption and prolonged carrier migration time, we propose that strain engineering is an effective route to tune the band gap energy and band alignment of o-B2N2 and turn this two-dimensional material into a promising photocatalyst for efficient hydrogen production from water splitting.Öğe Structural and electronic properties of monolayer group III monochalcogenides (conferenceObject)(Amer Chemical Soc, 2019) Demirci, Salih; Avazli, Nurlan; Durgun, Engin; Jahangirov, Seymur…Öğe Temperature, strain and charge mediated multiple and dynamical phase changes of selenium and tellurium(ROYAL SOC CHEMISTRY, 2020) Demirci, Salih; Gürel, Hikmet Hakan; Jahangirov, Seymur; Çıracı, SalimSemiconducting selenium and tellurium in their 3D bulk trigonal structures consist of parallel and weakly interacting helical chains of atoms and display a number of peculiarities. We predict that thermal excitations, 2D compressive strain and excess charge of positive and negative polarity mediate metal-insulator transitions by transforming these semiconductors into different metallic crystal structures. When heated to high temperature, or compressed, or charged positively, they change into a simple cubic structure with metallic bands, which is very rare among elemental crystals. When charged negatively, they transform first into body-centered tetragonal and subsequently into the body-centered orthorhombic structures with increasing negative charging. These two new structures stabilized by excess electrons also have overlapping metallic bands and quasi 2D and 1D substructures of lower dimensionality. Since the external charging of crystals can be achieved through their surfaces, the effects of charging on 2D structures of selenium and tellurium are also investigated. Similar structural transformations have been mediated also in 2D nanosheets and free-standing monolayers of these elements. These phase changes assisted by phonons are dynamical, reversible and tunable; the resulting metal-insulator transitions can occur within very short time intervals and may offer important device applications.Öğe Two dimensional ruthenium carbide: structural and electronic features(ROYAL SOC CHEMISTRY, 2020) Görkan, Taylan; Demirci, Salih; Jahangirov, S.; Gokoglu, Gökhan; Aktürk, EthemThe design and realization of novel 2D materials and their functionalities have been a focus of research inspired by the successful synthesis of graphene and many other 2D materials. In this study, in view of first principles calculations, we predict a novel 2D material ruthenium carbide (RuC) in graphene-like honeycomb hexagonal lattice with planar geometry. Phonon dispersion spectra display a dynamically stable structure. Comprehensive molecular dynamics calculations confirm the stability of the structure up to high temperatures as approximate to 1000 K. The system is a narrow gap semiconductor with a band gap of 53 meV (345 meV) due to GGA-PBE (HSE) calculations. Band gap exhibits significant changes by applied strain. Elastic and optical properties of the system are examined in monolayer form. RuC/RuC bilayer, RuC/graphene and RuC/h-BN heterostructures are also investigated. By calculating the phonon dispersion it is verified that RuC bilayer is the most stable in AA type-stacking configuration where Ru and C atoms of both layers have identical lateral coordinates. The effects of atomic substitutions on electronic band structures, acting as p-type and n-type doping, are revealed. A novel 3D RuCLi structure is also predicted to be stable and the isolation of its monolayer forms are discussed. Ruthenium carbide, as a 2D material which is dynamically and thermally stable, holds promise for applications in nanoelectronics.
