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  • 1.
    Alfieri, Luisa
    et al.
    Department of Electrical Engineering and Information Technology, University of Naples Federico II, Italy.
    Bracale, Antonio
    Department of Engineering, University of Naples Parthenope, Centro Direzionale of Naples.
    Larsson, Anders
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Energy Science.
    New Power Quality Indices for the Assessment of Waveform Distortions from 0 to 150 kHz in Power Systems with Renewable Generation and Modern Non-Linear Loads2017In: Energies, ISSN 1996-1073, E-ISSN 1996-1073, Vol. 10, no 10, article id 1633Article in journal (Refereed)
    Abstract [en]

    The widespread use of power electronics converters, e.g., to interface renewable generation systems with the grid or to supply some high-efficiency loads, has caused increased levels of waveform distortions in the modern distribution system. Voltage and current waveforms include spectral components from 0 kHz to 150 kHz, characterized by a non-uniform time-frequency behavior. This wide interval of frequencies is currently divided into "low-frequency" (from 0 kHz to 2 kHz) and "high-frequency" (from 2 kHz to 150 kHz). While the low-frequencies have been exhaustively investigated in the relevant literature and are covered by adequate standardization, studies for the high-frequencies have been addressed only in the last decade to fill current regulatory gaps. In this paper, new power quality (PQ) indices for the assessment of waveform distortions from 0 kHz to 150 kHz are proposed. Specifically, some currently available indices have been properly modified in order to extend their application also to wide-spectrum waveforms. In the particular case of waveform distortions due to renewable generation, numerical applications prove that the proposed indices are useful tools for the characterization of problems (e.g., overheating, equipment malfunctioning, losses due to skin effects, hysteresis losses or eddy current losses) in cases of both low-frequency and high-frequency distortions

  • 2.
    Baran, Jakub D.
    et al.
    Tyndall National Institute, University College Cork.
    Larsson, Andreas
    Moriarty, Philip J.
    School of Physics and Astronomy, University of Nottingham.
    Theoretical and Experimental Study of Metal-phthalocyanines on Ag(111).2009Conference paper (Refereed)
    Abstract [en]

    Metal-phthalocyanines deposited and/or self-assembled on metal surface are considered as candidates for novel molecular sensors; molecular memories and photovoltaic cells. To study the adsorption of three metal-phthalocyanines (MPc (M=Co; Sn; Pb) on Ag(111) we performed density functional theory DFT calculations using the generalized gradient approximation (GGA) parameterization by Pedrew-Burke-Ernzerhof (PBE) for the exchange-correlation energy [1;2]. Three initial adsorption site were considered (hcp-hollow; fcc-hollow and on-top). Our results show that the most favourite adsorption site is hcp-hollow for SnPc and PbPc and on-top for CoPc. All calculated structures are compared with experimental data obtained by normal incidence X-ray standing wave spectroscopy (NIXSW) [3;4] Good agreement in binding geometries with experiment was found. To understand the hybridization of MPc s molecular orbitals with silver orbitals we have compared selected partial density of states PDOS for a free and adsorbed MPcs. SnPc and PbPc hybridize mostly with surface by central metal atom; however the effect from the aromatic rings is not negligible. Adsorption of CoPc on the silver surface results in a transfer of electron density from the surface to the central Co atom. After adsorption; the magnetic moment of CoPc is completely quenched which is in agreement with similar studies on adsorption on Au(111) [5]. Binding energies for all of systems are reported showing chemisorptive nature of the molecule-metal surface interaction

  • 3.
    Baran, Jakub D.
    et al.
    Tyndall National Institute, University College Cork.
    Cong, Yan
    School of Physics and Astronomy, University of Nottingham.
    Larsson, Andreas
    Density Functional Study of Metal-Phthalocyanines Interactions With a Silver Surface Ag(111).2007Conference paper (Refereed)
    Abstract [en]

    Metal phthalocyanines (MPc) are generally planar organic molecules comprising of a central metal atom surrounded by aromatic rings. Phthalocyanines are structurally similar to important biomolecules such as haemoglobin and chlorophyll and are commonly used in industry for pigmentation. They have been the focus of intense interest due to their electrical and (non-linear) optical properties. Deposited and/or slef-assembled on metal electrodes, phthalocyanines are attractive candidates for novel molecular sensors, memory, and light-harvesting components. Fundamental understanding of molecule-molecule and molecule-surface interaction is important when attempting to determine the charge transport characteristics of metal phthalocyanines. The most important are the interactions between first layer adsorbate molecules and the substrates, since these interactions determine the structural ordering of the organic films and therefore also have a considerable impact on the efficiency of optical, electronic, and magnetic properties of the system. To study the adsorption of metal-phthalocyanines (MPc (M=Co, Sn, Pb) on the Ag(111) surface we have performed electronic structure calculations using a cluster representation of the surface within the framework of density functional theory (DFT) [1]. Our calculations use the generalized gradient approximation (GGA) parameterization by Pedrew-Burke-Ernzerhof (PBE) for the exchange-correlation energy [2]. We have investigated bonding on three surface adsorption sites (hcp-hollow, fcc-hollow and on-top). SnPc was found to adsorb weakly to the surface, and to prefer hollow bonding rather than on-top bonding. The distance between the Sn atom and the top layer Ag-surface atoms (hcp-hollow and fcc-hollow) is consistent with experimental data obtained by normal incidence X-ray standing wave spectroscopy (NIXSW) [3,4]. Adsorption of CoPc was found very site specific and to prefer the on-top binding site. The distance between the Co atom and the top layer Ag atoms is ~3 . For PbPc, successful adsorption was only obtained on the hcp-hollow site. For SnPc and PbPc binding energy is small fraction of eV, however CoPc bind much strongly to Ag(111) .For each of these systems we have found good agreement in binding geometries with experimental data

  • 4.
    Baran, Jakub D.
    et al.
    School of Chemistry, University of Bath.
    Jarvis, Samuel P.
    School of Physics and Astronomy, University of Nottingham.
    Taylor, Simon
    School of Physics and Astronomy, University of Nottingham.
    Thompson, Damien
    Department of Physics and Energy, Materials and Surface Science Institute, University of Limerick.
    Saywell, Alex
    Fritz Haber Institute of the Max-Planck Society.
    Mangham, Berry
    School of Chemistry, the University of Nottingham.
    Champness, Neil R.
    School of Chemistry, the University of Nottingham.
    Moriarty, Philip
    School of Physics and Astronomy, University of Nottingham.
    Larsson, Andreas
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    A Case of Unusually Large Density of States Changes For Physisorption - TetraPhenyl-Porphyrins on Cu(111)2015Conference paper (Other academic)
    Abstract [en]

    Conformational changes caused by surface adsorption can dramatically affect a molecule’s properties. The conformational flexibility of the porphyrin family of molecules has been exploited particularly well in a number of contexts, including prototypical molecular switches. Despite this level of study, however, the exact mechanisms underpinning conformational switching are often unclear. We show that the conformation of the tetra(4-bromophenyl) porphyrin (Br4TPP) on Cu(111) depends critically on the precise adsorption site of the molecule, and that, remarkably, large conformational changes are driven entirely by van der Waals (vdW) interactions between the molecule and the substrate surface. A combination of scanning probe microscopy, low temperature single molecule manipulation, dispersion-corrected density functional theory (DFT) and molecular dynamics (MD) simulations shows that van der Waals forces dominate the adsorption of TPP molecules, causing significant distortions of the molecular architecture so that the porphyrin can adopt one of two low energy conformations. In addition, scanning probe manipulation has been used to translate and switch the Br4TPP molecule between conformations via an intermediary, ‘hybrid’ structure. We have used the generalized gradient approximation (GGA) parameterization by Perdew—Burke—Ernzerhof (PBE), and the sparse-matter optBP86b-vdW20 (vdW-DFT) exchange and correlation functional to account for the missing dispersion forces. In order to check for the presence of chemical bonding we have analyzed the molecule-surface complexes using electron localization function (ELF) and Bader charges. We find that vdW-forces alone are capable of causing large shifts in the molecular density of states, despite the complete absence of chemical interactions.

  • 5.
    Baran, Jakub D.
    et al.
    Tyndall National Institute, University College Cork.
    Kołodziejczyk, Wojciech
    Tyndall National Institute, University College Cork.
    Larsson, Peter O.
    Division of Materials Theory, Department of Physics and Astronomy, Uppsala University.
    Ahuja, Rajeev B.
    Division of Materials Theory, Department of Physics and Astronomy, Uppsala University.
    Larsson, Andreas
    On the stability of single-walled carbon nanotubes and their binding strengths2012In: Theoretical Chemistry accounts, ISSN 1432-881X, E-ISSN 1432-2234, Vol. 131, no 9, p. 1-8Article in journal (Refereed)
    Abstract [en]

    We have studied the relative stability of hydrogen-terminated single-walled carbon nanotubes (SWNTs) segments, and open-ended SWNT fragments of varying diameter and chirality that are present at the interface of the catalytic metal particles during growth. We have found that hydrogen-terminated SWNTs differ by <1 eV in stability among different chiralities, which presents a challenge for selective and property-controlled growth. In addition, both zigzag and armchair tubes can be the most stable chirality of hydrogen-terminated SWNTs, which is a fundamental obstacle for property-controlled growth utilizing thermodynamic stability. In contrast, the most armchair-like open-ended SWNTs segments are always the most stable ones, followed in sequence by chiral index up to the least stable zigzag segments. We explain the ordering by triple bond stabilization of the carbon dangling bonds at the open ends, which is a fragment stabilization effect that is only manifested when all bonds between two layers are broken. We show convincingly that the bond strength difference between zigzag and armchair tubes is not present when individual bonds are broken or formed

  • 6.
    Baran, Jakub D.
    et al.
    Tyndall National Institute, University College Cork.
    Larsson, Andreas
    A DFT study employing dispersion correction of adsorption of SnPc and CoPc on the Ag(111) surface2011Conference paper (Refereed)
  • 7. Baran, Jakub D.
    et al.
    Larsson, Andreas
    Adsorption Of Metal-Phthalocyanines on Ag(111): A First-Principles Study2008Conference paper (Refereed)
  • 8.
    Baran, Jakub D.
    et al.
    Tyndall National Institute, University College Cork.
    Larsson, Andreas
    DFT Computations of Metal Phthalocyanies on Ag(111)2008Conference paper (Refereed)
  • 9.
    Baran, Jakub D.
    et al.
    Tyndall National Institute, University College Cork.
    Larsson, Andreas
    Inversion of the shuttlecock shaped metal phthalocyanines MPc (M = Ge, Sn, Pb): A density functional study2010In: Physical Chemistry, Chemical Physics - PCCP, ISSN 1463-9076, E-ISSN 1463-9084, Vol. 12, no 23, p. 6179-6186Article in journal (Refereed)
    Abstract [en]

    Shuttlecock shaped metal-phthalocyanine (MPc) can adsorb on a substrate surface having the central metal atom either down or up and the possibility of reversible switching between these two adsorption configurations shows great promise for use in nanomechanical devices. Using density functional theory we investigate the mechanism of the internal conformational inversion of germanium, tin and lead phthalocyanine in terms of the geometry, energy barrier of the reaction, and redox properties of the central metal atom. We have found the same mechanism of inversion for GePc and SnPc but a different one for PbPc. Inversion proceeds through two transition states, separated by a planar local minimum, for GePc and SnPc, but through one transition state distorting the phthalocyanine macrocycle for PbPc. The energy barrier of inversion is 4.27 eV for PbPc and 2.12 and 3.16 eV for GePc and SnPc, respectively. Such high barriers are unlikely to be overcome at normal experimental conditions, and in many cases alternative explanations for switching between "up" and "down" conformation need to be sought, such as ionization assisted inversion or even flipping over of the molecules. Our calculations show that the inversion of GePc and SnPc is accompanied by reversible two electron oxidation (M II ↔ M IV) of the metal atom, through intersystem crossing. The difference in mechanism of inversion for GePc (SnPc) and PbPc is assigned to the different nature of the central metal atom

  • 10.
    Baran, Jakub D.
    et al.
    Tyndall National Institute, University College Cork.
    Larsson, Andreas
    Structure and energetics of shuttlecock-shaped tin-phthalocyanine on Ag(111: A density functional study employing dispersion correction2012In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, p. 9487-9497Article in journal (Refereed)
    Abstract [en]

    The reversible switch between two electronically and/or geometrically distinct states of a single molecule adsorbed on a well-characterized substrate is of high technological interest due to its possible use as single molecule devices and novel molecular memories. We have studied shuttlecock-shaped metal phthalocyanines, which can adsorb on surfaces in two distinct adsorption configurations, depending on if the central metal atom points toward or away from the surface, and we report on the adsorption of tin-phthalocyanine (SnPc) on an Ag(111) surface using density functional theory (DFT) including a semiempirical dispersion correction (DFT-D).We discuss the binding mechanism in detail and show that the adsorption of SnPc in these two orientations is driven by very different interactions. While "Sn-down" adsorption involves chemical bonding between Sn and the surface (chemisorption), the "Sn-up" configuration is bound only by weak van der Waals forces (physisorption). By comparing our theoretical results with a broad range of experimental data, we assess the effect of dispersion forces for the SnPc/Ag(111) system and how these impact adsorption energies, geometries, and the electronic structure. We show that an inclusion of dispersion forces improves the adsorption geometry with respect to experiment and is essential in order to capture the subtle electronic effects at molecule-metal interfaces. By analyzing the geometric and electronic structure of the adsorbed molecules we, in addition, shed light on the surprising 2-fold symmetry reduction of metal phthalocyanine molecules that has been observed upon adsorption on surfaces

  • 11.
    Baran, Jakub D.
    et al.
    Tyndall National Institute, University College Cork.
    Larsson, Andreas
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Theoretical insights into adsorption of cobalt phthalocyanine on Ag(111): A combination of chemical and van der Waals bonding2013In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 117, no 45, p. 23887-23898Article in journal (Refereed)
    Abstract [en]

    In this article we study in detail the interaction of cobalt phthalocyanine (CoPc) with the Ag(111) surface by means of density functional theory calculations (DFT). We discuss the electronic and geometric differences of the adsorbed CoPc as it interacts with the different binding sites of the surface, yielding deeper insight into the adsorption mechanism of organometallic molecules with noble metal surfaces. We interpret the experimentally observed 4-fold to 2-fold symmetry reduction upon interaction of phthalocyanine molecules with metal surfaces as caused by electronic effects originating from nonsymmetric interactions between the molecule and the surface. To asses the role of dispersion forces in bonding of CoPc to the surface we employ a semiempirical dispersion correction to standard DFT and compare the obtained molecule-surface separation with experimental measurements. We show that, in the case of CoPc, the molecule bonds to the surface mostly due to covalent bonding between Co and Ag, but with a considerable contribution from van der Waals bonding between the Pc ligand and the surface. We show in this case where the molecule-surface separation is mostly governed by covalent bonding between the central metal atom and the surface atoms that standard DFT performs reasonably well, as compared to the available experimental data.

  • 12.
    Baran, Jakub D.
    et al.
    Tyndall National Institute, University College Cork.
    Larsson, Andreas
    Theoretical Study Of Metal-Phthalocyanies MPc (M=Co; Sn; Pb) With Silver Surface Ag(111) And Reversible Conformational Inversion2008Conference paper (Refereed)
    Abstract [en]

    Metal-phthalocyanie adsorbed on metal surfaces are class of materials particularly promising as a building blocks for molecular electronic devices. Their application rely on the electrochemically induced switching of their electronic and magnetic state has been demonstrated [1]. The knowledge of their molecular geometry and electronic structure as a single entities and when adsorbed on surface are crucial points in order to understand their interaction with surfaces. By means of density functional theory (DFT) we have investigated conformational interconversion ( see Fig. 1) of single MPc molecules as their interaction with Ag(111) silver surfaces. Structural analysis using B3-LYP functional; and DZVP2 and TZVPP2 [2] basis set has been performed to evaluate the transition state (TS) and energy barrier of this conversion. We have found two different mechanism of inversion for SnPc and PbPc. To study the adsorption of metal-phthalocyanies MPc (M=Co; Sn; Pb) bonded parallel to the Ag(111) surface we used cluster representation of surface (55 and 169 silver atoms). To perform this calculations the generalized gradient approximation (GGA) parameterisation by Pedrew-Burke-Ernzerhof (PBE) for exchange-correlation energy [3] and multipole resolution of identity [4] method were used. We have investigated bonding on three surface adsorption sites (hcp-hollow; fcc-hollow and on-top). For each of these systems we have found good agreement in binding geometries with experimental data obtained by normal incidence X-ray standing wave spectroscopy (NIXSW) [4;5]. Binding energies and geometries for all systems are given. We have used electronic structure calculations to better understand these molecules as a separate entities and as a devices

  • 13.
    Baran, Jakub D.
    et al.
    Tyndall National Institute, University College Cork.
    Larsson, Andreas
    Cong, Yan
    School of Physics and Astronomy, University of Nottingham.
    Interactions of Metal-phthalocyanines MPc (M=Co; Sn; Pb) with Silver Surface Ag(111)2007Conference paper (Refereed)
  • 14.
    Baran, Jakub D.
    et al.
    Tyndall National Institute, University College Cork.
    Larsson, Andreas
    Cong, Yan
    School of Physics and Astronomy, University of Nottingham.
    Moriarty, Philip J.
    School of Physics and Astronomy, University of Nottingham.
    DFT Computation Of Metal-Phthalocyanines Bonded To Ag(111)2007Conference paper (Refereed)
    Abstract [en]

    To study the adsorption of metal-phthalocyanines (MPc (M=Co; Sn; Pb) on the Ag(111) surface we have performed electronic structure calculations using a cluster representation of the surface within the framework of density functional theory (DFT) [1]. Our calculations use the generalized gradient approximation (GGA) parameterization by Pedrew-Burke-Ernzerhof (PBE) for the exchange-correlation energy [2]. We have investigated bonding on three surface adsorption sites (hcp-hollow; fcc-hollow and on-top). SnPc was found to adsorb weakly to the surface (0.15 to 0.25 eV); and to prefer hollow bonding rather than on-top bonding. The distance between the Sn atom and the top layer Ag-surface atoms (hcp-hollow and fcc-hollow) is consistent with experimental data obtained by normal incidence X-ray standing wave spectroscopy (NIXSW) [3;4]. CoPc is much more strongly bound to the Ag(111) surface and was found to prefer the on-top site. The calculated binding energy is 1.2 eV and the distance between the Co atom and the top layer Ag atoms is 3 (which also matched the experimental data well). For PbPc; successful adsorption was only obtained on the hcp-hollow site with a binding energy of 0.5 eV. For each of these systems we have found good agreement in binding geometries with experimental data.

  • 15.
    Baran, Jakub D.
    et al.
    Tyndall National Institute, University College Cork.
    Larsson, Andreas
    Cong, Yan
    School of Physics and Astronomy, University of Nottingham.
    Moriarty, Philip J.
    School of Physics and Astronomy, University of Nottingham.
    Interactions of Metal-phthalocyanines MPc (M=Co; Sn; Pb) with Silver Surface Ag(111): A Density Functional Study2008Conference paper (Refereed)
    Abstract [en]

    Deposited and/or self-assembled on metal electrodes; metal-phthalocyanine are attractive candidates for novel molecular sensors; memory; and light-harvesting components. The knowledge of the their molecular geometry and electronic structure are crucial points in order to understand their interactions with surfaces. To study the adsorption of metal-phthalocyanines (MPc (M=Co; Sn; Pb) bonded parallel on the Ag(111) surface we have performed electronic structure calculations using a cluster representation (55 and 169 silver atoms) of the surface within the framework of density functional theory (DFT) [1]. Our calculations use the generalized gradient approximation (GGA) parameterization by Pedrew-Burke-Ernzerhof (PBE) for the exchange-correlation energy [2] and multipole accelerated resolution of identity method [3]. We have investigated bonding on three surface adsorption sites (hcp-hollow; fcc-hollow and on-top). For each of these systems we have found good agreement in binding geometries with experimental data obtained by normal incidence X-ray standing wave spectroscopy (NIXSW) [4;5]. Binding energies and geometries for all systems are given. We propose flat chemisorption of respective MPcs on Ag(111).

  • 16.
    Baran, Jakub D.
    et al.
    Tyndall National Institute, University College Cork.
    Larsson, Andreas
    cong, Yan
    School of Physics and Astronomy, University of Nottingham.
    Moriarty, Philiph J.
    School of Physics and Astronomy, University of Nottingham.
    Interactions of Metal-phthalocyanines with Silver Surface Ag(111: A Density Functional Theory Study2008Conference paper (Refereed)
  • 17.
    Baran, Jakub D.
    et al.
    Tyndall National Institute, University College Cork.
    Larsson, Andreas
    Woolley, Richard A.J-
    School of Physics and Astronomy, University of Nottingham.
    Cong, Yan
    School of Physics and Astronomy, University of Nottingham.
    Moriarty, Philip J.
    School of Physics and Astronomy, University of Nottingham.
    Cafolla, Attilio Anthony
    School of Physics and Astronomy, University of Nottingham.
    Schulte, Karina H.G.
    MAX-lab, Lund University.
    Dhanak, Vinod R.
    Department of Physics, University of Liverpool.
    Theoretical and experimental comparison of SnPc, PbPc, and CoPc adsorption on Ag(111)2010In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 81, no 7Article in journal (Refereed)
    Abstract [en]

    A combination of normal-incidence x-ray standing-wave (NIXSW) spectroscopy, x-ray photoelectron spectroscopy (XPS), scanning tunneling microscopy (STM), and density-functional theory (DFT) has been used to investigate the interaction of a number of phthalocyanine molecules (specifically, SnPc, PbPc, and CoPc) with the Ag(111) surface. The metal-surface distances predicted by the DFT calculations for SnPc/Ag(111) (2.48Å) and CoPc/Ag(111) (2.88Å) are in good agreement with our NIXSW experimental results for these systems (2.31±0.09 and 2.90±0.05Å, respectively). Good agreement is also found between calculated partial density-of-states plots and STM images of CoPc on Ag(111). Although the DFT and Pb4f NIXSW results for the Pb-Ag(111) distance are similarly in apparently good agreement, the Pb4f core-level data suggest that a chemical reaction between PbPc and Ag(111) occurs due to the annealing procedure used in our experiments and that the similarity of the DFT and Pb4f NIXSW values for the Pb-Ag(111) distance is likely to be fortuitous. We interpret the Pb4f XPS data as indicating that the Pb atom can detach from the PbPc molecule when it is adsorbed in the "Pb-down" position, leading to the formation of a Pb-Ag alloy and the concomitant reduction in Pb from a Pb2 + state (in bulklike films of PbPc) to Pb0. In contrast to SnPc, neither PbPc nor CoPc forms a well-ordered monolayer on Ag(111) via the deposition and annealing procedures we have used. Our DFT calculations show that each of the phthalocyanine molecules donate charge to the silver surface, and that back donation from Ag to the metal atom (Co, Sn, or Pb) is only significant for CoPc

  • 18. Beton, P.
    et al.
    Moriarty, P.
    Keeling, D.
    Dunsch, L.
    Georgi, P.
    Wang, Chun-Ru
    Greer, J.C.
    Larsson, Andreas
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Smith, R.
    Godwin, P.
    Suter, D.
    Udovicic, L.
    Weidinger, A.
    Harneit, W.
    Waiblinger, M.
    Welland, M.
    Durkan, C.
    A study for the construction of a quantum information precessing device using doped fullerenes2000Conference paper (Refereed)
  • 19.
    Bolton, Kim
    et al.
    Physics Department, Göteborg University.
    Larsson, Peter
    Department of Physics, Condensed Matter Theory Group, Uppsala University.
    Larsson, Andreas
    Aruja, Rajeev B.
    Department of Physics, Condensed Matter Theory Group, Uppsala University.
    Ding, Feng
    Physics Department, Göteborg University.
    Duan, HaiMing
    Physics Department, Göteborg University.
    Rosén, Arne E.
    Physics Department, Göteborg University.
    Börjesson, A.
    Harutyunyan, A.R.
    Mora, E.
    Tokune, T.
    Jiang, A.
    Awasthi, N.
    Setyawan, W.
    Curtarolo, S.
    Computational Modeling of SWNTs and Their Growth2007Conference paper (Refereed)
  • 20.
    Bolton, Kim
    et al.
    Physics Department, Göteborg University.
    Larsson, Peter O.
    Department of Physics, Condensed Matter Theory Group, Uppsala University.
    Larsson, Andreas
    Ahuja, Rajeev B.
    Department of Physics, Condensed Matter Theory Group, Uppsala University.
    Ding, Feng
    Physics Department, Göteborg University.
    Duan, HaiWing
    Physics Department, Göteborg University.
    Zhu, W.
    Börjesson, A.
    Harutyunyan, H.R.
    Tokune, T.
    Curtarolo, S
    Carbon Nanotube Growth Mechanisms2007In: Proceedings of Diamond 2007, the 18th European Conference on Diamond, Diamond-Like Materials, Carbon Nanotubes, Nitrides and Silicon Carbide: [... (Diamond 2007), Berlin, Germany, 10-14 September 2007], 2007Conference paper (Refereed)
    Abstract [en]

    We have used a variety of computational methods to study key aspects of single-walled carbon nanotube (SWNT) growth. Molecular dynamics (MD) studies based on an empirical force field showed; for example; why SWNT growth occurs in a temperature window and why; for 1-2 nm catalyst particles; the SWNT diameter varies linearly with the size of the particle. In addition; the liquid or solid phase of the catalyst particle is strongly dependent on particle size; and smaller particles (< 1.5 nm) are liquid at typical chemical vapor deposition temperatures whereas larger particles (> 5 nm) are solid. The phase of particles of intermediate sizes depends on the exact temperature and on their carbon content. The effect of substrates on metal-carbide properties and SWNT growth has been studied by combing density functional (DFT) and MD methods. A major effect of flat; inert substrates is to flatten the catalyst particles thereby increasing their melting points. DFT has also been used to study the catalyst-SWNT interaction which is critical for the growth of long SWNTs; and is also being used to study the importance of the SWNT cap structure on its chirality. This knowledge is important; for example; when using SWNTs as seeds for the growth of longer nanotubes.

  • 21.
    Christen, Dines
    et al.
    Institut für Physikalische und Theoretische Chemie, Universität Tübingen.
    Coudert, Laurent H.
    Institut für Physikalische und Theoretische Chemie, Universität Tübingen.
    Larsson, Andreas
    Cremer, Dieter
    Institut für Physikalische und Theoretische Chemie, Universität Tübingen.
    The rotational-torsional spectrum of the g′Gg conformer of ethylene glycol: Elucidation of an unusual tunneling path2001In: Journal of Molecular Spectroscopy, ISSN 0022-2852, E-ISSN 1096-083X, Vol. 205, no 2, p. 185-196Article in journal (Refereed)
    Abstract [en]

    The microwave spectrum of the energetically unfavored g′Gg conformer of ethylene glycol (CH 2OH - CH 2OH) is reported. This spectrum is dominated by an interconversion geared-type large-amplitude motion during which each OH group in turn forms the intramolecular hydrogen bond. The microwave spectrum has been analyzed with the help of a Watson-type Hamiltonian plus a 1.4-GHz tunneling splitting. The rotational dependence of this tunneling splitting has been examined using an IAM approach and this yielded qualitative information on the tunneling path the molecule uses to interconvert between its two most stable conformers. Unexpectedly, but in agreement with ab initio calculations, when tunneling occurs between the energetically equivalent g′Gg and gGg′ conformers, the OH groups are rotated stepwise through 240° in the sense of a flip-flop rather than a concerted rotation and the molecule goes through the more stable g′Ga and aGg′ forms. The electronic reasons for preferring a long rather than a short rotational path via a gGg form are discussed using calculated adiabatic vibrational modes

  • 22. Cremer, Dieter
    et al.
    Larsson, Andreas
    Kraka, Elif
    Department of Theoretical Chemistry, Göteborg University.
    New developments in the analysis of vibrational spectra On the use of adiabatic internal vibrational modes1998In: Theoretical and Computational Chemistry, ISSN 1380-7323, Vol. 5, no C, p. 259-327Article in journal (Refereed)
  • 23.
    Cremer, Dieter
    et al.
    Department of Theoretical Chemistry, Göteborg University.
    Wu, A.N.
    Department of Theoretical Chemistry, Göteborg University.
    Larsson, Andreas
    Kraka, Elif
    Department of Theoretical Chemistry, Göteborg University.
    Some thoughts about bond energies, bond lengths, and force constants2000In: Journal of Molecular Modeling, ISSN 1610-2940, E-ISSN 0948-5023, Vol. 6, no 4, p. 396-412Article in journal (Refereed)
    Abstract [en]

    The bond energy (BE) of a polyatomic molecule cannot be measured and, therefore, determination of BEs can only be done within a model using a set of assumptions. The bond strength is reflected by the intrinsic BE (IBE), which is related to the intrinsic atomization energy (IAE) and which represents the energy of dissociation under the provision that the degree of hybridization is maintained for all atoms of the molecule. IBE and BE differ in the case of CC and CH bonds by the promotion, the hybridization, and the charge reorganization energy of carbon. Since the latter terms differ from molecule to molecule, IBE and BE are not necessarily parallel and the use of BEs from thermochemical models can be misleading. The stretching force constant is a dynamical quantity and, therefore, it is related to the bond dissociation energy (BDE). Calculation and interpretation of stretching force constants for local internal coordinate modes are discussed and it is demonstrated that the best relationship between BDEs and stretching force constants is obtained within the model of adiabatic internal modes. The valence stretching force constants are less suitable since they are related to an artificial bond dissociation process with geometrical relaxation effects suppressed, which leads to an intrinsic BDE (IBDE). In the case of AX(n) molecules, symmetric coordinates can be used to get an appropriate stretching force constant that is related to the BE. However, in general stretching force constants determined for symmetry coordinates do not reflect the strength of a particular bond since the related dissociation processes are strongly influenced by the stability of the products formed.

  • 24.
    Delaney, Paul
    et al.
    Queen's University Belfast.
    Larsson, Andreas
    Greer, James C.
    Tyndall National Institute, University College Cork.
    Singlet and Triplet Levels of the NV Centre2011Conference paper (Refereed)
  • 25.
    Delaney, Paul
    et al.
    Queen's University Belfast.
    Greer, James C.
    Tyndall National Institute, University College Cork.
    Larsson, Andreas
    Spin-polarization mechanisms of the nitrogen-vacancy center in diamond2010In: Nano letters (Print), ISSN 1530-6984, E-ISSN 1530-6992, Vol. 10, no 2, p. 610-614Article in journal (Refereed)
    Abstract [en]

    The nitrogen-vacancy (NV) center in diamond has shown great promise for quantum information due to the ease of initializing the qubit and of reading out its state. Here we show the leading mechanism for these effects gives results opposite from experiment; instead both must rely on new physics. Furthermore, NV centers fabricated in nanometer-sized diamond clusters are stable, motivating a bottom-up qubit approach, with the possibility of quite different optical properties to bulk

  • 26.
    Delaney, Paul
    et al.
    Queen's University Belfast.
    Larsson, Andreas
    Small cluster model of the NV centre in diamond2010In: Physics Procedia, ISSN 1875-3892, E-ISSN 1875-3892, Vol. 3, no 4, p. 1533-1537Article in journal (Refereed)
    Abstract [en]

    The singlet 1E and 1A 1 energy levels of the Nitrogen-Vacancy centre's ground state configuration each need two Slater determinants in theoretical models, posing difficulties for Density-Functional Theory (DFT) and Hartree-Fock approaches. Configuration Interaction (CI) can handle such states, but not the C 284H 144N - and C 163H 100N - clusters of our recent DFT study as CI computer time and memory scale worse than DFT with system size. Using smaller clusters to model bulk diamond introduces size errors. We examine the smaller diamond cluster C 42H 42N - using DFT to quantify the size error: if not too large it opens the way to CI calculations of these states.

  • 27.
    Ding, Feng
    et al.
    Physics Department, Göteborg University.
    Larsson, Oeter
    Department of Physics, University of Uppsala.
    Larsson, Andreas
    Ahuja, Rajeev
    Department of Physics, University of Uppsala.
    Duan, Haiming
    Physics Department, Göteborg University.
    Rosén, Arne
    Physics Department, Göteborg University.
    Bolton, Kim
    Physics Department, Göteborg University.
    Strong SWNT-catalyst adhesion strength as a necessary condition for SWNT growth2007Conference paper (Refereed)
  • 28.
    Ding, Feng
    et al.
    Physics Department, Göteborg University.
    Larsson, Peter O.
    Department of Physics, Condensed Matter Theory Group, Uppsala University.
    Ahuja, Rajeev B.
    Department of Physics, Condensed Matter Theory Group, Uppsala University.
    Larsson, Andreas
    Duan, HaiMing
    Physics Department, Göteborg University.
    Rosén, Arne E.
    Physics Department, Göteborg University.
    Bolton, Kim
    Physics Department, Göteborg University.
    The Importance of Metal particle: Nanotube Binding for Single Walled Nanotube Growth2006Conference paper (Refereed)
  • 29.
    Ding, Feng
    et al.
    Physics Department, Göteborg University.
    Larsson, Peter O.
    Department of Physics, Condensed Matter Theory Group, Uppsala University.
    Larsson, Andreas
    Ahuja, Rajeev B.
    Department of Physics, Condensed Matter Theory Group, Uppsala University.
    Duan, HaiMing
    Physics Department, Göteborg University.
    Rosen, Arne E.
    Physics Department, Göteborg University.
    Bolton, Kim
    Physics Department, Göteborg University.
    The importance of strong carbon-metal adhesion for catalytic nucleation of single-walled carbon nanotubes2008In: Nano letters (Print), ISSN 1530-6984, E-ISSN 1530-6992, Vol. 8, no 2, p. 463-468Article in journal (Refereed)
    Abstract [en]

    Density functional theory is used to show that the adhesion between single-walled carbon nanotubes (SWNTs) and the catalyst particles from which they grow needs to be strong to support nanotube growth. It is found that Fe, Co, and Ni, commonly used to catalyze SWNT growth, have larger adhesion strengths to SWNTs than Cu, Pd, and Au and are therefore likely to be more efficient for supporting growth. The calculations also show that to maintain an open end of the SWNT it is necessary that the SWNT adhesion strength to the metal particle is comparable to the cap formation energy of the SWNT end. This implies that the difference between continued and discontinued SWNT growth to a large extent depends on the carbon-metal binding strength, which we demonstrate by molecular dynamics (MD) simulations. The results highlight that first principles computations are vital for the understanding of the binding strength's role in the SWNT growth mechanism and are needed to get accurate force field parameters for MD

  • 30.
    Fransson, Jonas
    et al.
    Department of Materials Science and Engineering, Royal Institute of Technology.
    Bengone, Oliver M.
    Department of Physical Electronic/Photonic, Mitthögskolan.
    Larsson, Andreas
    Greer, James C.
    Tyndall National Institute, University College Cork.
    A physical compact model for electron transport across single molecules2006In: IEEE transactions on nanotechnology, ISSN 1536-125X, E-ISSN 1941-0085, Vol. 5, no 6, p. 745-749Article in journal (Refereed)
    Abstract [en]

    Prediction of current flow across single molecules requires ab initio electronic structure calculations along with their associated high computational demand, and a means for incorporating open system boundary conditions to describe the voltage sources driving the current. To date, first principle predictions of electron transport across single molecules have not fully achieved a predictive capability. The situation for molecular electronics may be compared to conventional technology computer-aided design (TCAD), whereby various approximations to the Boltzmann transport equation are solved to predict electronic device behavior, but in practice are too time consuming for most circuit design applications. To simplify device models for circuit design, analytical but physically motivated models are introduced to capture the behavior of active and passive devices; however, similar models do not yet exist for molecular electronics. We follow a similar approach by evaluating an analytical model achieved by combining a mesoscopic transport model with parameterizations taken from quantum chemical calculations of the electronic structure of single molecule bonded between two metal contacts. Using the model to describe electron transport across benzene-1,4-dithiol and by comparing to experiment, we are able to extract the coupling strength of the molecule attached to two infinite metal electrodes. The resulting procedure allows for accurate and computationally efficient modeling of the static (dc) characteristics of a single molecule, with the added capability of being able to study the physical model parameter variations across a range of experiments. Such simple physical models are also an important step towards developing a design methodology for molecular electronics

  • 31.
    Gannon, Greg
    et al.
    Tyndall National Institute, University College Cork.
    Greer, James C.
    Tyndall National Institute, University College Cork.
    Larsson, Andreas
    Thompson, Damien J.
    Tyndall National Institute, University College Cork.
    Molecular dynamics study of naturally occurring defects in self-assembled monolayer formation2010In: ACS Nano, ISSN 1936-0851, E-ISSN 1936-086X, Vol. 4, no 2, p. 921-932Article in journal (Refereed)
    Abstract [en]

    One of the major challenges for nanofabrication, in particular microcontact printing (μ-CP), is the control of molecular diffusion, or "ink spreading", for the creation of nanopatterns with minimized "smudging"at pattern boundaries. In this study, fully atomistic computer simulations were used to measure the impact of naturally occurring domain boundaries on the diffusion of excess alkanethiol ink molecules on printed alkanethiol self-assembled monolayers (SAM). A periodic unit cell containing approximately one million atoms and with a surface area of 56 nm×55 nm was used to model a hexadecanethiol SAM on Au(111), featuring SAM domain boundaries and a range of concentrations of excess hexadecanethiol ink molecules diffusing on top. This model was simulated for a total of approximately 80 ns of molecular dynamics. The simulations reveal that domain boundaries impede the diffusion of excess ink molecules and can, in some cases, permanently trap excess inks. There is competition between ink spreading and ink trapping, with the ink/SAM interaction strongly dependent on both the ink concentration and the SAM orientation at domain boundaries. SAM defects thus provide potential diffusion barriers for the control of excess ink spreading, and simulations also illustrate atom-scale mechanisms for the repair of damaged areas of the SAM via self-healing. The ability of domain boundaries to trap excess ink molecules is accounted for using an accessible volume argument, and trapping is discussed in relation to experimental efforts to reduce molecular spreading on SAMs for the creation of ultrahigh resolution nanopatterns

  • 32.
    Gannon, Greg
    et al.
    Tyndall National Institute, University College Cork.
    Larsson, Andreas
    Greer, James C.
    Tyndall National Institute, University College Cork.
    Thompson, Damien J.
    Tyndall National Institute, University College Cork.
    Guanidinium chloride molecular diffusion in aqueous and mixed water-ethanol solutions2008In: Journal of Physical Chemistry B, ISSN 1520-6106, E-ISSN 1520-5207, Vol. 112, no 30, p. 8906-8911Article in journal (Refereed)
    Abstract [en]

    Solutions containing guanidinium chloride (GdmCl), or equivalently guanidine hydrochloride (GdnHCl), are commonly used to denature macromolecules such as proteins and DNA in, for example, microfluidics studies of protein unfolding. To design and study such applications, it is necessary to know the diffusion coefficients for GdmCl in the solution. To this end, we use molecular dynamics simulations to calculate the diffusion coefficients of GdmCl in water and in water-ethanol solutions, for which no direct experimental measurements exist. The fully atomistic simulations show that the guandinium cation Gdm + diffusion decreases as the concentration of both Gdm + and ethanol in the solution increases. The simulations are validated against available literature data, both transformed measured viscosity values and computed diffusion coefficients, and we show that a prudent choice of water model, namely TIP4P-Ew, gives calculated diffusion coefficients in good agreement with the transformed measured viscosity values. The calculated Gdm + diffusion behavior is explained as a dynamic mixture of free cation, stacked cation, and ion-paired species in solution, with weighted contributions to Gdm + diffusion from the stacked and paired states helping explain measured viscosity data in terms of atom-scale dynamics

  • 33.
    Gannon, Greg
    et al.
    Tyndall National Institute, Lee Makings, Prospect Row, Cork.
    Larsson, Andreas
    Greer, James C.
    Tyndall National Institute, Lee Makings, Prospect Row, Cork.
    Thompson, Damien J.
    Tyndall National Institute, Lee Makings, Prospect Row, Cork.
    Modelling ink spreading on self-assembled monolayers for nanopatterning applications2008Conference paper (Refereed)
    Abstract [en]

    Alkanethiol self-assembled monolayers (SAMs) on gold; specifically the Au(111) surface; have been widely studied since their discovery in the early 1980 s. The interest in SAMs is due in part to their ease of production but also due to their present and potential application in technologies as diverse as biosensors; corrosion protection and nanolithography. Spreading of ink outside the desired printed area is one of the major limitations of microcontact printing (m-CP) with alkanethiol self-assembled monolayers (SAMs) on gold.1;2 We use molecular dynamics (MD) computer simulations to quantify the temperature and concentration dependence of hexadecanethiol (HDT) ink spreading on HDT SAMs; modelling 18 distinct printing conditions using periodic simulation cells of ~7 nm edge length and printing conditions ranging from 7 ink molecules per cell at 270 K to 42 ink molecules per cell at 371K.3 The computed alkanethiol ink diffusion rates on the SAM are of the same order of magnitude as bulk liquid alkanethiol diffusion rates at all but the lowest ink concentrations and highest temperatures; with up to 20-30 times increases in diffusion rates at the lowest concentration-highest temperature conditions. We show that although alkanethiol surfaces are autophobic; autophobicity is not enough to pin the ink solutions on the SAM and so any over-inking of the SAM will lead to spreading of the printed pattern. Comparison of experimental and calculated diffusion data supports an interpretation of pattern broadening as a mixture of spreading on fully- and partially-formed SAMs; and the calculated spreading rates establish some of the fundamental limitations of m-CP in terms of stamp contact time and desired pattern width

  • 34.
    Gannon, Greg
    et al.
    Tyndall National Institute, Lee Makings, Prospect Row, Cork.
    Larsson, Andreas
    Greer, James C.
    Tyndall National Institute, Lee Makings, Prospect Row, Cork.
    Thompson, Damien J.
    Tyndall National Institute, Lee Makings, Prospect Row, Cork.
    Quantification of ink diffusion in microcontact printing with self-assembled monolayers2009In: Langmuir, ISSN 0743-7463, E-ISSN 1520-5827, Vol. 25, no 1, p. 242-247Article in journal (Refereed)
    Abstract [en]

    Spreading of ink outside the desired printed area is one of the major limitations of microcontact printing (μ-CP) with alkanethiol self-assembled monolayers (SAMs) on gold. We use molecular dynamics (MD) computer simulations to quantify the temperature and concentration dependence of hexadecanethiol (HDT) ink spreading on HDT SAMs, modeling 18 distinct printing conditions using periodic simulation cells of ∼7 nm edge length and printing conditions ranging from 7 ink molecules per cell at 270 K to 42 ink molecules per cell at 371K. The computed alkanethiol ink diffusion rates on the SAM are of the same order of magnitude as bulk liquid alkanethiol diffusion rates at all but the lowest ink concentrations and highest temperatures, with up to 20-30 times increases in diffusion rates at the lowest concentration-highest temperature conditions. We show that although alkanethiol surfaces are autophobic, autophobicity is not enough to pin the ink solutions on the SAM, and so any overinking of the SAM will lead to spreading of the printed pattern. Comparison of experimental and calculated diffusion data supports an interpretation of pattern broadening as a mixture of spreading on fully and partially formed SAMs, and the calculated spreading rates establish some of the fundamental limitations of μ-CP in terms of stamp contact time and desired pattern width

  • 35.
    Gannon, Greg
    et al.
    Tyndall National Institute, University College Cork.
    Larsson, Andreas
    Thompson, Damien J.
    Tyndall National Institute, University College Cork.
    Monolayer packing, dehydration, and ink-binding dynamics at the molecular2009In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 113, no 17, p. 7298-7304Article in journal (Refereed)
    Abstract [en]

    Gold-bound self-assembled monolayers (SAMs) terminating in β-cyclodextrin (β-CD) cavities provide a highly ordered surface array of hydrophobic binding pockets and so are used as "molecular printboards" for nanopatterning applications. The present work complements ongoing nanoscale experiments by providing the atom-scale structure, dynamics, and energetics of the printboard, which may aid the design of functional platforms for nanotechnology. We use fully atomistic molecular dynamics (MD) computer simulations to probe the printboard lattice constant, height, steric packing, hydrophobicity, and ink-binding properties as a function of gold-β-CD "linker" molecule and degree of binding to gold. The simulations reveal the stabilization associated with the experimentally observed surface lattice constant of ∼2 nm, alkanethioether linkers, and partial unbinding from gold. Additional ink-binding simulations indicate that multivalent ink molecules can offset disordering in the more loosely packed alkanethiol-linked printboard, with the attendant steric penalty similar in magnitude to the favorable multivalent ink:β-CD complexation

  • 36.
    Gannon, Greg
    et al.
    Theory Modelling and Design Centre, Tyndall National Institute, University College Cork.
    O'Dwyer, Colm
    Department of Physics and Energy, Materials and Surface Science Institute, University of Limerick.
    Larsson, Andreas
    Thompson, Damien J.
    Theory Modelling and Design Centre, Tyndall National Institute, University College Cork.
    Interdigitating organic bilayers direct the short interlayer spacing in hybrid organic-inorganic layered vanadium oxide nanostructures2011In: Journal of Physical Chemistry B, ISSN 1520-6106, E-ISSN 1520-5207, Vol. 115, no 49, p. 14518-14525Article in journal (Refereed)
    Abstract [en]

    Layered metal oxides provide a single-step route to sheathed superlattices of atomic layers of a variety of inorganic materials, where the interlayer spacing and overall layered structure forms the most critical feature in the nanomaterials' growth and application in electronics, health, and energy storage. We use a combination of computer simulations and experiments to describe the atomic-scale structure, dynamics and energetics of alkanethiol-intercalated layered vanadium oxide-based nanostructures. Molecular dynamics (MD) simulations identify the unusual substrate-constrained packing of the alkanethiol surfactant chains along each V 2O 5 (010) face that combines with extensive interdigitation between chains on opposing faces to maximize three-dimensional packing in the interlayer regions. The findings are supported by high resolution electron microscopy analyses of synthesized alkanethiol-intercalated vanadium oxide nanostructures, and the preference for this new interdigitated model is clarified using a large set of MD simulations. This dependency stresses the importance of organic-inorganic interactions in layered material systems, the control of which is central to technological applications of flexible hybrid nanomaterials

  • 37.
    Gannon, Greg
    et al.
    Tyndall National Institute, University College Cork.
    Thompson, Damien J.
    Tyndall National Institute, University College Cork.
    Larsson, Andreas
    Greer, James C.
    Tyndall National Institute, University College Cork.
    A Molecular dynamics study of alkanoethiol diffusion on alkanethiol self-assembled monolayers2007Conference paper (Refereed)
  • 38.
    Gannon, Greg
    et al.
    Tyndall National Institute, University College Cork.
    Thompson, Damien J.
    Tyndall National Institute, University College Cork.
    Larsson, Andreas
    Greer, James C.
    Tyndall National Institute, University College Cork.
    Atomistic simulations of nanopatterning systems2006Conference paper (Refereed)
  • 39.
    Gannon, Greg
    et al.
    Tyndall National Institute, University College Cork.
    Thompson, Damien J.
    Tyndall National Institute, University College Cork.
    Larsson, Andreas
    Greer, James C.
    Tyndall National Institute, University College Cork.
    Computational study of self-assembled monolayers of thiolates and hepththioether functionalized Beta-cyclodextrins on a Au(III) surface2005Conference paper (Refereed)
  • 40.
    Gannon, Greg
    et al.
    Tyndall National Institute, University College Cork.
    Thompson, Damien J.
    Tyndall National Institute, University College Cork.
    Larsson, Andreas
    Greer, James C.
    Tyndall National Institute, University College Cork.
    Molecular dynamics simulations of self-assembled monolayers for nanopatterning applications2007Conference paper (Refereed)
    Abstract [en]

    Self-Assembled Monolayers (SAMs) are used in areas as diverse as corrosion protection; nanodevices and biotechnology. Despite a wealth of both experimental and theoretical studies of SAM structure and behaviour[1]; there still remain some unanswered questions. Alkanethiol molecular ink diffusion on alkanethiol SAMs is one such area; and one amenable to computational study. Ink diffusion is an important consideration when one performs microcontact printing - "... the quality of the printed pattern strongly depends on the mobility of the ink compound ..."[2]. An understanding of ink diffusion is therefore crucial to the production of stable; high-resolution nanopatterns. We first of all calculated alkanethiol self-diffusion coefficients in bulk liquid and obtained good agreement with measured values. The Einstein diffusion equation was used to calculate the diffusion coefficients from 1 nanosecond molecular dynamics (MD) simulations Simulations of alkanethiol SAMs were performed and the temperature dependence of the SAM tilt angle found to be in good agreement with literature values. Having validated our method both for calculating diffusion and the SAM structural models; we calculated the diffusion of both a single alkanethiol molecule and a 75-molecule drop ; on a range of perfect and defect SAMs; establishing a range for ink diffusion in different environments and; ultimately; allowing identification of optimum ink molecular weights for microcontact printing applications

  • 41.
    Hedman, Daniel
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Barzegar, Hamid Reza
    Department of Physics, Umeå University, Department of Physics, University of California.
    Rosén, Arne
    Physics Department, Göteborg University.
    Wågberg, Thomas
    Department of Physics, Umeå University.
    Larsson, Andreas
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    On the Stability and Abundance of Single Walled Carbon Nanotubes2015In: Scientific Reports, ISSN 2045-2322, E-ISSN 2045-2322, Vol. 5, article id 16850Article in journal (Refereed)
    Abstract [en]

    Many nanotechnological applications, using single-walled carbon nanotubes (SWNTs), are only possible with a uniform product. Thus, direct control over the product during chemical vapor deposition (CVD) growth of SWNT is desirable, and much effort has been made towards the ultimate goal of chirality-controlled growth of SWNTs. We have used density functional theory (DFT) to compute the stability of SWNT fragments of all chiralities in the series representing the targeted products for such applications, which we compare to the chiralities of the actual CVD products from all properly analyzed experiments. From this comparison we find that in 84% of the cases the experimental product represents chiralities among the most stable SWNT fragments (within 0.2 eV) from the computations. Our analysis shows that the diameter of the SWNT product is governed by the well-known relation to size of the catalytic nanoparticles, and the specific chirality is normally determined by the product’s relative stability, suggesting thermodynamic control at the early stage of product formation. Based on our findings, we discuss the effect of other experimental parameters on the chirality of the product. Furthermore, we highlight the possibility to produce any tube chirality in the context of recent published work on seeded-controlled growth.

  • 42.
    Hedman, Daniel
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Barzegar, Hamid Reza
    Department of Physics, Umeå University.
    Rosén, Arne
    Physics Department, Göteborg University.
    Wågberg, Thomas
    Department of Physics, Umeå University.
    Larsson, Andreas
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    The relation between stability and abundance of single walled carbon nanotubes2015Conference paper (Other academic)
    Abstract [en]

    The ability to utilize the remarkable electrical and mechanical properties of single walled carbon nanotubes (SWNTs) can be hugely beneficial for technological applications. The limiting factors for these technological applications is that many of them rely on specific electrical/mechanical properties of the SWNT. The mechanical and electrical properties of a SWNT depends on its chiral indecencies n and m, which means that in order to get a desired electrical/mechanical property one needs to synthesize SWNTs with specific chiral indecencies.Huge effort has been put on trying to synthesize SWNTs with specific chiral indecencies or to post-purify them after synthesis. Although post-purification methods can result in a relatively high yield of SWNTs with specific chiralities, such methods are expensive, time consuming and may damage the SWNTs. A more efficient method would be to selectively grow/synthesize SWNTs with the desired properties. Chemical vapor deposition (CVD) has become a favored technique for trying to achieve selective SWNT growth since the process involves several controllable growth parameters.In our work we have investigated the relation between the relative stability of different SWNTs and compared that to the experimentally observed statistical abundance of the same SWNTs. The relative energy of the SWNTs was calculated using density functional theory with the VASP-code. We have chosen to include all the SWNTs in the (n+m) = 8,9,10,11,12,13,14,15,16,17 and 18-series in our calculations, this equals 80 SWNTs in total. The SWNT models used in our calculations are six layered hydrogen terminated SWNT fragments where each layer contains 2(n+m) carbon atoms.Our calculations show a remarkable connection between the relative stability of the SWNTs and their statistical abundance in experiments. The most stable SWNT in each series correlates with the most abundant SWNT in that series, as found in the experimental results gathered from the literature.

  • 43.
    Hedman, Daniel
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Larsson, Andreas
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Length dependent stability of single-walled carbon nanotubes and how it affects their growth2017In: Carbon, ISSN 0008-6223, E-ISSN 1873-3891, Vol. 116, p. 443-447Article in journal (Refereed)
    Abstract [en]

    Using density-functional theory the stability of armchair and zigzag single-walled carbon nanotubes and graphene nanoribbons was investigated. We found that the stability of armchair and zigzag nanotubes has different linear dependence with regard to their length, with switches in the most stable chirality occurring at specific lengths for each nanotube series. We explain these dependencies by competing edge and curvature effects. We have found that within each series armchair nanotubes are the most stable at short lengths, while zigzag nanotubes are the most stable at long lengths. These results shed new insights into why (near) armchair nanotubes are the dominant product from catalytic chemical vapor deposition growth, if templating is not used. Paradoxically, the stability of armchair nanotubes at short lengths favors their growth although zigzag nanotubes are more stable at long lengths, resulting in the production of the least stable nanotubes.

  • 44.
    Jarvis, Samuel P.
    et al.
    School of Physics and Astronomy, University of Nottingham.
    Taylor, Simon
    School of Physics and Astronomy, University of Nottingham.
    Baran, Jakub D.
    Tyndall National Institute, University College Cork, School of Chemistry, University of Bath.
    Champness, Neil R.
    School of Chemistry, the University of Nottingham.
    Larsson, Andreas
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Moriarty, Philip
    School of Physics and Astronomy, University of Nottingham.
    Measuring the mechanical properties of molecular conformers2015In: Nature Communications, ISSN 2041-1723, E-ISSN 2041-1723, Vol. 6, article id 8338Article in journal (Refereed)
    Abstract [en]

    Scanning probe-actuated single molecule manipulation has proven to be an exceptionally powerful tool for the systematic atomic-scale interrogation of molecular adsorbates. To date, however, the extent to which molecular conformation affects the force required to push or pull a single molecule has not been explored. Here we probe the mechanochemical response of two tetra(4-bromophenyl)porphyrin conformers using non-contact atomic force microscopy where we find a large difference between the lateral forces required for manipulation. Remarkably, despite sharing very similar adsorption characteristics, variations in the potential energy surface are capable of prohibiting probe-induced positioning of one conformer, while simultaneously permitting manipulation of the alternative conformational form. Our results are interpreted in the context of dispersion-corrected density functional theory calculations which reveal significant differences in the diffusion barriers for each conformer. These results demonstrate that conformational variation significantly modifies the mechanical response of even simple porpyhrins, potentially affecting many other flexible molecules

  • 45.
    Jarvis, Samuel P.
    et al.
    School of Physics and Astronomy, University of Nottingham.
    Taylor, Simon
    School of Physics and Astronomy, University of Nottingham.
    Baran, Jakub D.
    Tyndall National Institute, University College Cork, School of Chemistry, University of Bath, Department of Chemistry, University of Bath.
    Thompson, Damien
    Department of Physics and Energy, Materials and Surface Science Institute, University of Limerick.
    Saywell, Alex
    Fritz Haber Institute of the Max-Planck Society, School of Physics and Astronomy, University of Nottingham.
    Mangham, Berry
    School of Chemistry, the University of Nottingham.
    Champness, Neil R.
    School of Chemistry, the University of Nottingham.
    Larsson, Andreas
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Moriarty, Philip
    School of Physics and Astronomy, University of Nottingham.
    Physisorption Controls the Conformation and Density of States of an Adsorbed Porphyrin2015In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 119, no 50, p. 27982-27994Article in journal (Refereed)
    Abstract [en]

    Conformational changes caused by adsorption can dramatically affect a molecule's properties. Despite extensive study, however, the exact mechanisms underpinning conformational switching are often unclear. Here we show that the conformation of a prototypical flexible molecule, the free-base tetra(4-bromophenyl) porphyrin, adsorbed on Cu(111), depends critically on its precise adsorption site and that, remarkably, large conformational changes are dominated by van der Waals interactions between the molecule and the substrate surface. A combination of scanning probe microscopy, single-molecule manipulation, DFT with dispersion density functional theory, and molecular dynamics simulations show that van der Waals forces drive significant distortions of the molecular architecture so that the porphyrin can adopt one of two low-energy conformations. We find that adsorption driven by van der Waals forces alone is capable of causing large shifts in the molecular density of states, despite the apparent absence of chemical interactions. These findings highlight the essential role that van der Waals forces play in determining key molecular properties.

  • 46.
    Konkoli, Zoran
    et al.
    Department of Theoretical Chemistry, Göteborg University.
    Larsson, Andreas
    Cremer, Dieter
    Department of Theoretical Chemistry, Göteborg University.
    A new way of analyzing vibrational spectra: II. Comparison of internal mode frequencies1998In: International Journal of Quantum Chemistry, ISSN 0020-7608, E-ISSN 1097-461X, Vol. 67, no 1, p. 11-27Article in journal (Refereed)
    Abstract [en]

    Adiabatic internal frequencies are compared with c-vector frequencies and intrinsic frequencies. It is shown that c-vector modes are not suitable to characterize molecular fragments φ n since they are not localized in φ n and their definition leads to unreasonable frequency values. Intrinsic frequencies suffer from a strong dependence on the set of internal parameters chosen to describe the geometry of the molecule. Apart from this, they represent averaged frequencies, for which mass effects and electronic effects are not properly separated. Adiabatic frequencies are based on a dynamic principle, separate properly mass effects and electronic effects and do not depend in any way on the set of internal parameters. This is shown for HF/6-31G(d, p) vibrational frequencies of ethene, dichloroethene, benzene, the cyclooctatetraene dication, benzocyclobutadiene, and some of their isotopomers

  • 47.
    Konkoli, Zoran
    et al.
    Department of Theoretical Chemistry, Göteborg University.
    Larsson, Andreas
    Cremer, Dieter
    Department of Theoretical Chemistry, Göteborg University.
    A new way of analyzing vibrational spectra: IV. Application and testing of adiabatic modes within the concept of the characterization of normal modes1998In: International Journal of Quantum Chemistry, ISSN 0020-7608, E-ISSN 1097-461X, Vol. 67, no 1, p. 41-44Article in journal (Refereed)
    Abstract [en]

    The CNM (characterization of normal modes) method for extracting chemical information out of vibrational spectra is tested for vibrational spectra of molecules with relatively strong or relatively weak coupling between internal vibrational modes. Symmetry, parameter set stability, and frequency uncertainty tests are applied to check whether internal vibrational modes, internal mode frequencies, and amplitudes Script A sign nμ comply with symmetry, are independent of the set of internal parameters ζ n used to describe molecular geometry or fulfill a Lorentzian correlation between amplitudes Script A sign nμ and frequency differences Δω nμ = ω n - ω μ. In all cases considered, amplitudes Script A sign nμ based on adiabatic internal modes and mass or force constant matrices as metric O are superior to any other definition of amplitude. They represent the basic elements of the new CNM method that leads to chemically reasonable results and presents a new way of extracting chemical information out of vibrational spectra. A number of deficiencies of the potential energy distribution (PED) analysis is discussed

  • 48.
    Kołodziejczyk, Wojciech
    et al.
    Tyndall National Institute, University College Cork.
    Baran, Jakub D.
    Tyndall National Institute, University College Cork.
    Larsson, Peter O.
    Division of Materials Theory, Department of Physics and Astronomy, Uppsala University.
    Ahuja, Rajeev B.
    Division of Materials Theory, Department of Physics and Astronomy, Uppsala University.
    Larsson, Andreas
    Stability of Single-Walled Carbon Nanotubes and Their Cut Ends2008Conference paper (Refereed)
  • 49. Kraka, Elif
    et al.
    Larsson, Andreas
    Cremer, Dieter
    Generalization of the badger rule based on the use of adiabatic vibrational modes2010In: Computational spectroscopy: methods, experiments and applications, Weinheim: Wiley-VCH Verlagsgesellschaft, 2010, p. 105-149Chapter in book (Refereed)
  • 50.
    Larsson, Andreas
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Activity: Carbon Nanotubes and Their Interactions With Metals2009Conference paper (Other (popular science, discussion, etc.))
123 1 - 50 of 127
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