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    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, Salim
    Based 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.
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    Functional Carbon and Silicon Monolayers in Biphenylene Network
    (Amer Chemical Soc, 2022) Gorkan, Taylan; Callioglu, Safak; Demirci, Salih; Aktuerk, Ethem; Ciraci, Salim
    We 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.
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    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, Salim
    A 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.
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    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, Salim
    Graphene 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.
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    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, Salim
    When 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.
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    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, Salim
    We 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.