Lyudmyla Adamska

University of South Florida, Tampa, FL, United States

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Publications (10)50.18 Total impact

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    ABSTRACT: First-principles density functional theory calculations were performed to investigate the effect of Sn surface alloying on the strength of interface interactions between graphene and Ni(111) or Cu(111) substrates. A substantial reduction of graphene-metal interactions was observed for the graphene/Sn-Ni(111) interface: binding energy was reduced from 0.055 eV/(C atom) to 0.015 eV/(C atom); interface distance was increased from 2.12 Å to 3.52 Å. The initially weak graphene/Cu(111) interface was hardly affected by Sn surface alloying. Electronic structure calculations, including local density of states and simulated scanning tunneling microscopy images, provide further details on the changing character of graphene-Ni(111) interactions upon Sn alloying.
    Applied Physics Letters 07/2012; 101(5). · 3.79 Impact Factor
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    ABSTRACT: Structural, electronic, and magnetic properties of simple interfaces (graphene on top of a metallic substrate) and complex interfaces (a single metallic adlayer on a simple graphene/metal system, either on top or between the graphene and metallic substrate) have been studied using density functional theory. Two types of simple interface with strong (Ni/graphene) and weak (Cu/graphene) bonding were considered. In addition to binding energies and interface distances, which are used to quantify the strength of graphene-substrate interactions, the bonding in simple and complex interfaces was analyzed using charge density distributions and bond orders. Substantial enhancement of the metallic substrate/graphene binding was observed in complex interfaces, consisting of a Ni monolayer on top of a simple {Ni or Cu}/graphene interface. The increase of substrate-graphene bonding in such complex interfaces is accompanied by weakening of in-plane C-C bonds in graphene, as quantified by the bond orders. A weak ferrimagnetism in graphene, i.e., unequal magnetic moments −0.04μB and +0.06μB on the C atoms, is induced by a ferromagnetic Ni substrate. The strength of graphene-substrate interactions is also reflected in simulated scanning tunneling microscopy images.
    Physical review. B, Condensed matter 05/2012; 85(19). · 3.77 Impact Factor
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    ABSTRACT: The epitaxial growth of graphene on metal substrates is one of the major methods of graphene production for electronic applications. Therefore, the metal/graphene interface interactions as well as the graphene defects appeared during the growth affect in a substantial way the electronic properties of both graphene and graphene/metal contacts, which are both important for device applications. Structural and electronic properties of simple and complex graphene/metal as well as graphene/metal-alloy interfaces were investigated using first principles density functional theory. The point defect structures in graphene on metal substrate were studied and compared with those in free standing graphene.
    02/2012;
  • Andrew Ross, Lyudmyla Adamska, You Lin, Ivan Oleynik
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    ABSTRACT: Epitaxial growth of graphene on Ni(111) substrates is one promising method of large-scale, high-quality graphene wafer production, due to the small lattice mismatch between these two materials. We present results of first-principles density functional theory (DFT) investigation of thestructural, electronic, and magnetic properties of graphene/Ni(111) interfaces relevant to experimental studies of graphene growth on nickel substrates. DFT calculations were performed to identify the favored interface geometries and binding sites for different interface configurations. Additional adlayers of Ni and Cu were either adsorbed on top of the graphene/metal interface, or placed between the graphene and substrate to model processes of metal intercalation. It was also found that the interaction between graphene/Ni(111) and the top Cu adlayer is much weaker compared to that for Ni adlayer. The atomic, electronic, and magnetic properties of these interfaces, including induced magnetic moments in graphene/Ni(111) and Cu,Ni/graphene/Ni(111) systems are also discussed.
    03/2011;
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    ABSTRACT: The formation of single-layer graphene by exposure of a Ni(111) surface to ethylene at low pressure has been investigated. Two different growth regimes were found. At temperatures between 480 and 650 °C, graphene grows on a pure Ni(111) surface in the absence of a carbide. Below 480 °C, graphene growth competes with the formation of a surface Ni2C carbide. This Ni2C phase suppresses the nucleation of graphene. Destabilization of the surface carbide by the addition of Cu to the surface layer facilitates the nucleation and growth of graphene at temperatures below 480 °C. In addition to the growth of graphene on Ni substrates, the interaction between graphene and Ni was also studied. This was done both experimentally by Ni deposition on Ni-supported graphene and by density functional theory calculation of the work of adhesion between graphene and Ni. For graphene sandwiched between two Ni-layers, the work of adhesion between graphene and the Ni substrate was found to be four times as large as that for the Ni-supported graphene without a top layer. This stronger interaction may cause the destruction of graphene that is shown experimentally to occur at ~200 °C when Ni is deposited on top of Ni-supported graphene. The destruction of graphene allows the Ni deposits to merge with the substrate Ni. After the completion of this process, the graphene sheet is re-formed on top of the Ni substrate, leaving no Ni at the surface.
    New Journal of Physics 02/2011; 13(2):025001. · 4.06 Impact Factor
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    ABSTRACT: A novel growth mechanism of graphene on Ni(111) has been discovered that occurs at temperatures below 460 °C. At these conditions, a surface-confined nickel-carbide phase coexists with single layer graphene. The graphene grows by in-plane transformation of the carbide along a one-dimensional phase-boundary, which is distinctively different from known growth processes on other transition metals and on Ni above 460 °C, where carbon atoms attach to "free" edges of graphene islands.
    Nano Letters 02/2011; 11(2):518-22. · 13.03 Impact Factor
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    ABSTRACT: In this paper, we show that electron-plasmon interactions play a critical role in resonant charge transfer in molecular tunnel junctions (MTJs). Only when such interactions are taken into account, does it become possible to obtain the correct values of the turn-on threshold voltage in the current-voltage characteristics observed in experiment. In addition, we predict the emission of photons from MTJs with a large quantum yield ∼10−2 as a result of the excitation of surface plasmons upon the application of a voltage across the junction. Finally, we discuss physical conditions, which are necessary for the experimental observation of this phenomenon.
    Physical review. B, Condensed matter 01/2010; 81(3). · 3.77 Impact Factor
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    ABSTRACT: In the molecular electronics field it is highly desirable to engineer the structure of molecules to achieve specific functions. In particular, diode (or rectification) behaviour in single molecules is an attractive device function. Here we study charge transport through symmetric tetraphenyl and non-symmetric diblock dipyrimidinyldiphenyl molecules covalently bound to two electrodes. The orientation of the diblock is controlled through a selective deprotection strategy, and a method in which the electrode–electrode distance is modulated unambiguously determines the current–voltage characteristics of the single-molecule device. The diblock molecule exhibits pronounced rectification behaviour compared with its homologous symmetric block, with current flowing from the dipyrimidinyl to the diphenyl moieties. This behaviour is interpreted in terms of localization of the wave function of the hole ground state at one end of the diblock under the applied field. At large forward current, the molecular diode becomes unstable and quantum point contacts between the electrodes form.
    Nature Chemistry 10/2009; 1(8):635-641. · 21.76 Impact Factor
  • Lyudmyla Adamska, Ivan Oleynik, Mortko Kozhushner
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    ABSTRACT: It has recently been realized that the image potential plays an important role in charge transport through single organic molecules. In most cases, the classical image potential -1/4z is used to calculate the modified energy spectrum of the charge carriers in the molecule. In this talk, we will present the theory of resonant tunneling transitions that include the quantum mechanical effects of dynamic image potential due to the polarization interaction of the tunneling charge carrier (electron or hole) with surface plasmons. The application of this theory to organic molecular junctions of experimental interest will be discussed.
    01/2009;
  • Lyudmyla Adamska, Ivan Oleynik, Mortko Kozhushner
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    ABSTRACT: The electrical conduction of relatively long (1-2 nm) single organic molecules occurs via resonant tunneling of charge carriers, electrons and/or holes, through the energy levels of negative molecular ion (electrons) and/or positive molecular ion (holes). The position of these resonant energy levels with respect to the Fermi levels of the anode and cathode determines the relative contributions of electron and hole conduction to the resonant current. These resonant levels depend on the applied bias, and are also influenced by several physical factors such as the polarization of the molecule, image potential and metal/molecule interfaces that are difficult to control under conditions of real experiment. In this presentation we suggest a method of unambiguous experimental determination of specific type of the conduction mechanism (electron or hole conduction) which is based on the idea of utilizing experimental techniques of nanocalorimetry.
    03/2008;