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  • 1.
    Löfgren, Robin
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Materialvetenskap.
    Pawar, Ravinder
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik.
    Öberg, Sven
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Materialvetenskap.
    Larsson, Andreas
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Materialvetenskap.
    The bulk conversion depth of the NV-center in diamond: computing a charged defect in a neutral slab2019Inngår i: New Journal of Physics, ISSN 1367-2630, E-ISSN 1367-2630, Vol. 21, artikkel-id 053037Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The negatively charged nitrogen vacancy (NV-) center in diamond has properties that make it a promising candidate for applications such as a qubit in room temperature quantum computing, single-molecule photoluminescence and NMR sensor, and as a single photon source for quantum cryptography. For many of its uses it is desirable to have the NV-center close to the diamond surface. In this work, we use density functional theory simulations to investigate how the distance of the NV- center to a surface, and its orientation, affect its properties, including the zero-phonon-line. We study the three technologically important surfaces terminated with fluorine, oxygen/hydroxyl and nitrogen. Since the NV-center is charged it requires special measures to simulate within a slab-model. We use the recently proposed charging with a substitutional donor in the diamond lattice resulting in a neutral super-cell, which provides very satisfactory results. We have found that the NV-centers properties converge to bulk values already at 5 angstrom depth.

  • 2.
    Löfgren, Robin
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Materialvetenskap.
    Pawar, Ravinder
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Materialvetenskap.
    Öberg, Sven
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Materialvetenskap.
    Larsson, Andreas
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Materialvetenskap.
    Charged dopants in neutral supercells through substitutional donor (acceptor): nitrogen donor charging of the nitrogen-vacancy center in diamond2018Inngår i: New Journal of Physics, ISSN 1367-2630, E-ISSN 1367-2630, Vol. 20, artikkel-id 023002Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Charged defects are traditionally computed by adding (subtracting) electrons for negative (positive) impurities. When using periodic boundary conditions this results in artificially charged supercells that also require a compensating background charge of the opposite sign, which makes slab supercells problematic because of an arbitrary dependence on the vacuum thickness. In this work, we test the method of using neutral supercells through the use of a substitutional electron donor (acceptor) to describe charged systems. We use density functional theory (DFT) to compare the effects of charging the well-studied NV-center in diamond by a substitutional donor nitrogen. We investigate the influence of the donor-N on the NV-center properties as a function of the distance between them, and find that they converge toward those obtained when adding an electron. We analyze the spin density and conclude that the donor-N has a zero magnetic moment, and thus, will not be seen in electron spin resonance. We validate our DFT energies through comparison to GW simulations. Charging the NV-center with a substitutional donor-N enables accurate calculations of slabs, without the ambiguity of using charged supercells. Implantation of donor-N atoms opens up the possibility to engineer NV-centers with the desired charge state for future ICT and sensor applications.

  • 3.
    Hedman, Daniel
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Materialvetenskap.
    Larsson, Andreas
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Materialvetenskap.
    Length dependent stability of single-walled carbon nanotubes and how it affects their growth2017Inngår i: Carbon, ISSN 0008-6223, E-ISSN 1873-3891, Vol. 116, s. 443-447Artikkel i tidsskrift (Fagfellevurdert)
    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.

  • 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 tekniska universitet, Institutionen för teknikvetenskap och matematik, Materialvetenskap.
    A Case of Unusually Large Density of States Changes For Physisorption - TetraPhenyl-Porphyrins on Cu(111)2015Konferansepaper (Annet vitenskapelig)
    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.
    Löfgren, Robin
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Materialvetenskap.
    Pawar, Ravinder
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik.
    Öberg, Sven
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Materialvetenskap.
    Larsson, Andreas
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Materialvetenskap.
    Density functional theory calculations on the diamond nitrogen-vacancy center2015Konferansepaper (Annet vitenskapelig)
  • 6.
    Silvearv, Fredrik
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Materialvetenskap.
    Larsson, Peter O.
    Department of Physics, University of Uppsala, Department of Physics, Condensed Matter Theory Group, Uppsala University, Division of Materials Theory, Department of Physics and Astronomy, Uppsala University.
    Jones, Sarah Louise T.
    Electronics Theory Group, Tyndall National Institute, Cork, Tyndall National Institute, University College Cork.
    Ahuja, Rajeev B.
    Division of Materials Theory, Department of Physics and Astronomy, Uppsala University, Department of Physics, Condensed Matter Theory Group, Uppsala University, Department of Physics, University of Uppsala, Applied Material Physics, Department of Materials and Engineering, Royal Institute of Technology (KTH).
    Larsson, Andreas
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Materialvetenskap.
    Establishing the most favorable metal-carbon bond strength for carbon nanotube catalysts2015Inngår i: Journal of Materials Chemistry C, ISSN 2050-7526, E-ISSN 2050-7534, Vol. 3, nr 14, s. 3422-3427Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    We have studied a wide range of transition metals to find potential carbon nanotube (CNT) catalysts for chemical vapor deposition (CVD) production. The adhesion strengths between a CNT and a metal cluster were calculated using first principle density functional theory (DFT) for all 1st, 2nd and 3rd row transition metals. We have developed the criterion that the metal-carbon adhesion strength per bond must fulfill a Goldilocks principle for catalyzing CNT growth and used it to identify, besides the well known catalysts Fe, Co and Ni, a number of other potential catalysts, namely Y, Zr, Rh, Pd, La, Ce and Pt. Our results are consistent with previous experiments performed either in a carbon arc discharge environment or by a CVD-process with regard to CNT catalyst activity

  • 7.
    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 tekniska universitet, Institutionen för teknikvetenskap och matematik, Materialvetenskap.
    Moriarty, Philip
    School of Physics and Astronomy, University of Nottingham.
    Measuring the mechanical properties of molecular conformers2015Inngår i: Nature Communications, ISSN 2041-1723, E-ISSN 2041-1723, Vol. 6, artikkel-id 8338Artikkel i tidsskrift (Fagfellevurdert)
    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

  • 8.
    Hedman, Daniel
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Materialvetenskap.
    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 tekniska universitet, Institutionen för teknikvetenskap och matematik, Materialvetenskap.
    On the Stability and Abundance of Single Walled Carbon Nanotubes2015Inngår i: Scientific Reports, ISSN 2045-2322, E-ISSN 2045-2322, Vol. 5, artikkel-id 16850Artikkel i tidsskrift (Fagfellevurdert)
    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.

  • 9.
    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 tekniska universitet, Institutionen för teknikvetenskap och matematik, Materialvetenskap.
    Moriarty, Philip
    School of Physics and Astronomy, University of Nottingham.
    Physisorption Controls the Conformation and Density of States of an Adsorbed Porphyrin2015Inngår i: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 119, nr 50, s. 27982-27994Artikkel i tidsskrift (Fagfellevurdert)
    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.

  • 10.
    Hedman, Daniel
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Materialvetenskap.
    Barzegar, Hamid Reza
    Department of Physics, Umeå University.
    Rosén, Arne
    Physics Department, Göteborg University.
    Wågberg, Thomas
    Department of Physics, Umeå University.
    Larsson, Andreas
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Materialvetenskap.
    The relation between stability and abundance of single walled carbon nanotubes2015Konferansepaper (Annet vitenskapelig)
    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.

  • 11.
    Li, Yunguo
    et al.
    Applied Material Physics, Department of Materials and Engineering, Royal Institute of Technology (KTH).
    Ahuja, Rajeev B.
    Applied Material Physics, Department of Materials and Engineering, Royal Institute of Technology (KTH).
    Larsson, Andreas
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Materialvetenskap.
    Communication: Origin of the difference between carbon nanotube armchair and zigzag ends2014Inngår i: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 140, nr 9, artikkel-id 91102Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    In this work, we have found that the difference between armchair and zigzag ends of carbon nanotubes (CNTs) does not pertain at close study for individual bonds and thus alternative strategies need to be developed to reach the ultimate goals in selective growth. Based on first-principles simulations, the difference between binding strengths for CNTs of different chirality was investigated using hydrogen dissociation energies at their passivated ends. When all H atoms are removed collectively we find the well-known difference: that armchair bonds are much weaker than zigzag ones, which is typically seen for both CNT ends and graphene edges. However, when individual H atoms are removed we find almost no difference in hydrogen dissociation energies, small difference in bond lengths, which by association means small difference in C-C and M-C binding energies. We show convincingly that the difference in binding energy between armchair and zigzag ends is due to a fragment stabilization effect that is only manifested when all (or several neighbouring) bonds are broken. This is because at armchair ends/edges neighbouring dangling bonds can pair-up to form C≡C triple bonds that constitute a considerable stabilization effect compared to the isolated dangling bonds at zigzag ends/edges. Consequently, in many processes, e.g., catalytic growth where bonds are normally created/broken sequentially, not collectively, the difference between armchair and zigzag ends/edges cannot be used to discriminate growth of one type over the other to achieve chiral selective growth. Strategies are discussed to realize chirality selective growth in the light of the results presented, including addition of C2-fragments to favor armchair tubes

  • 12.
    Muthukumar, Kaliappan
    et al.
    Tyndall National Institute, University College Cork.
    Stróżecka, Anna
    Peter Grünberg Institut (PGI-3), and JARA-Fundamentals of Future Information Technology, Forschungszentrum Jülich.
    Mysliveček, Josef
    Charles University, Faculty of Mathematics and Physic.
    Dybek, Aneta
    Department of Physics, Queen Mary University of London.
    Dennis, John S.
    Department of Physics, Queen Mary University of London.
    Voigtländer, Bert
    Peter Grünberg Institut (PGI-3), and JARA-Fundamentals of Future Information Technology, Forschungszentrum Jülich.
    Larsson, Andreas
    Endohedral Fullerene Ce@C82 on Cu(111: Orientation, Electronic Structure, and Electron-Vibration Coupling2013Inngår i: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 117, nr 4, s. 1656-1662Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Structural, electronic, and vibrational properties of the endohedral fullerene Ce@C82 on Cu(111) have been studied by scanning tunneling microscopy (STM) and density functional theory (DFT). Ce@C82 forms islands on the substrate. Our STM measurements show relatively large differences in morphology and electron spectra of molecules within these islands indicating multiple molecular orientations on the substrate, while the vibrational spectra are more uniform. We have determined molecular orientations by comparing STM and DFT molecular morphology, and we have calculated Ce@C82 bound to Cu(111) and found that it is chemisorbed. We show that Ce@C82 adopts orientations on the surface that enables Ce to remain at its most favorable binding site inside C82. The effect of chemisorption on the structural and electronic properties of Ce@C82 is thus small, and the orientations are limited to configurations with Ce in the upper hemisphere of the molecular configurational space. We show that the variations in the dI/dV spectra between molecules of different orientations is due to Ce-cage orbitals that are localized in space and their involvement in tunneling depends on the molecular orientation on the substrate. The observed electron-vibration coupling modes in the STM-IETS (in-elastic tunneling spectroscopy) of Ce@C82 arise from cage modes only, and therefore, electron transport properties are expected to be different compared to Ce2@C80, which has active Ce-cage vibrations.

  • 13.
    Baran, Jakub D.
    et al.
    Tyndall National Institute, University College Cork.
    Larsson, Andreas
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Materialvetenskap.
    Theoretical insights into adsorption of cobalt phthalocyanine on Ag(111): A combination of chemical and van der Waals bonding2013Inngår i: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 117, nr 45, s. 23887-23898Artikkel i tidsskrift (Fagfellevurdert)
    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.

  • 14. Rosén, Arne
    et al.
    Li, Y.
    Larsson, Andreas
    Barzegar, H.R.
    Wågberg, T.
    Experimental and theoretical analysis of carbon nanotubes grown from catalytic nanoparticles of defined size distribution2012Konferansepaper (Fagfellevurdert)
    Abstract [en]

    We report use of a dip-coating method to prepare catalyst particles (mixture of iron and cobalt) with a controlled diameter distribution on silicon wafer substrates by changing the solution concentration and withdrawal velocity. The size and distribution of the prepared catalyst particles were analyzed by atomic force microscopy. Carbon nanotubes were grown by chemical vapor deposition, CVD, on the substrates with the prepared catalyst particles. By decreasing the particle size, the growth of carbon nanotubes can be tuned from few walled carbon nanotubes, with homogeneous diameters, to highly pure single walled carbon nanotubes, SWNT. Analysis of the Raman radial breathing mode, showed a relatively broad diameter distribution (0.8-1.4 nm) of single walled carbon nanotubes, SWNT, with different chiralities. By changing the size and composition of the catalyst particles but maintain the growth parameters, the chiralities of SWNT were reduced to mainly four different types: (12,1), (12.0), (8,5) and (7,5) of which quantity is 70 % of all the nanotubes.

  • 15.
    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 strengths2012Inngår i: Theoretical Chemistry accounts, ISSN 1432-881X, E-ISSN 1432-2234, Vol. 131, nr 9, s. 1-8Artikkel i tidsskrift (Fagfellevurdert)
    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

  • 16.
    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 correction2012Inngår i: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, s. 9487-9497Artikkel i tidsskrift (Fagfellevurdert)
    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

  • 17.
    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) surface2011Konferansepaper (Fagfellevurdert)
  • 18.
    Stróecka, Anna
    et al.
    Peter Grünberg Institut (PGI-3), JARA, Forschungszentrum Jülich.
    Muthukumar, Kaliappan
    Tyndall National Institute, University College Cork.
    Larsson, Andreas
    Dybek, Aneta
    Department of Physics, Queen Mary University of London.
    Dennis, T. John S.
    Department of Physics, Queen Mary University of London.
    Mysliveček, Josef
    Charles University, Faculty of Mathematics and Physic.
    Voigtländer, Bert
    Peter Grünberg Institut (PGI-3), JARA, Forschungszentrum Jülich.
    Electron-induced excitation of vibrations of Ce atoms inside a C 80 cage2011Inngår i: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 83, nr 16Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Inelastic electron tunneling spectroscopy of Ce 2@C 80 dimetallofullerenes reveals a low-energy inelastic excitation that is interpreted using ab initio calculations and associated with the movements of encapsulated Ce atoms inside the C 80 cage. The electron-vibration interaction in Ce 2@C 80 is unusually high, inducing a pronounced zero-bias anomaly in differential conductance of Ce 2@C 80. Our observations show that the atoms encapsulated in fullerene cages can actively participate in determining the properties of molecular junctions

  • 19.
    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 nanostructures2011Inngår i: Journal of Physical Chemistry B, ISSN 1520-6106, E-ISSN 1520-5207, Vol. 115, nr 49, s. 14518-14525Artikkel i tidsskrift (Fagfellevurdert)
    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

  • 20.
    O'Byrne, Justin P.
    et al.
    Department of Chemistry, Tyndall National Institute, University College Cork.
    Li, Zhong
    Department of Chemistry, Tyndall National Institute, University College Cork.
    Jones, Sarah Louise T.
    Electronics Theory Group, Tyndall National Institute, Cork.
    Fleming, Peter G.
    Department of Chemistry, Tyndall National Institute, University College Cork.
    Larsson, Andreas
    Morris, Michael A.
    Department of Chemistry, Tyndall National Institute, University College Cork.
    Holmes, Justin D.
    Department of Chemistry, Tyndall National Institute, University College Cork.
    Nitrogen-doped carbon nanotubes: Growth, mechanism and structure2011Inngår i: ChemPhysChem, ISSN 1439-4235, E-ISSN 1439-7641, Vol. 12, nr 16, s. 2995-3001Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Nitrogen-doped bamboo-structured carbon nanotubes have been successfully grown using a series of cobalt/molybdenum catalysts. The morphology and structure of the nanotubes were analysed by transmission electron microscopy and Raman spectroscopy. The level of nitrogen doping, as determined by X-ray photoelectron spectroscopy, was found to range between 0.5 to 2.5 at. %. The growth of bamboo-structured nanotubes in the presence of nitrogen, in preference to single-walled and multi-walled nanotubes, was due to the greater binding energy of nitrogen for cobalt in the catalyst compared to the binding strength of carbon to cobalt, as determined by density functional theory

  • 21.
    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 Centre2011Konferansepaper (Fagfellevurdert)
  • 22. Larsson, Andreas
    DFT Modelling of Single Molecule Spectoscopy of Fullerenes and Metal Phthalocyanines2010Konferansepaper (Fagfellevurdert)
  • 23. Muthukumar, Kaliappan
    et al.
    Larsson, Andreas
    Endohedral Metallofullerenes: Novel Materials for Nanoelectronics: Insights from Density Functional Theory Modeling2010Bok (Fagfellevurdert)
    Abstract [en]

    Fullerenes, hollow carbon cages on the order of 1 nm in diameter, are future building blocks in many areas of applied and fundamental nanoscience. Fullerenes show potential for applications in nanotechnology due to the possibility of tuning their properties by doping and/or functionlization. In particular, the endohedral doping of the hollow carbon cage with metal atoms and clusters allows changing of the electronic and magnetic properties of the molecule without significant distortion of the geometry of the outer shell. Despite their outstanding applications in technology and in bio-medical sciences the limited availability of endohedral metallofullerenes has hindered studies that describe their electronic and structural properties, the knowledge of which are essential prerequisites for many nanotechnological applications. In this book, we present our results on the characterization (structural, electronic and vibrational) of cerium endohedral fullerenes and provide precious clues about the formation mechanism of these species at the nanoscale.

  • 24. Kraka, Elif
    et al.
    Larsson, Andreas
    Cremer, Dieter
    Generalization of the badger rule based on the use of adiabatic vibrational modes2010Inngår i: Computational spectroscopy: methods, experiments and applications, Weinheim: Wiley-VCH Verlagsgesellschaft, 2010, s. 105-149Kapittel i bok, del av antologi (Fagfellevurdert)
  • 25.
    O'Byrne, Justin P.
    et al.
    Department of Chemistry, Tyndall National Institute, University College Cork.
    Li, Zhong
    Department of Chemistry, Tyndall National Institute, University College Cork.
    Tobin, Joseph M.
    Department of Chemistry, Tyndall National Institute, University College Cork.
    Larsson, Andreas
    Larsson, Peter O.
    Department of Physics, Condensed Matter Theory Group, Uppsala University.
    Ahuja, Rajeev B.
    Department of Physics, Condensed Matter Theory Group, Uppsala University.
    Holmes, Justin D.
    Department of Chemistry, Tyndall National Institute, University College Cork.
    Growth of carbon nanotubes from heterometallic palladium and copper catalysts2010Inngår i: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 114, nr 18, s. 8115-8119Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Bamboo-structured carbon nanotubes (BCNTs) were synthesized with MgO-supported Pd and Cu catalysts, doped with either Mo or W, by the catalytic chemical vapor decomposition of methane. No nanotubes were observed to grow from the catalysts in the absence of the dopant metals. Additionally, the level of dopant in the catalysts was found to strongly affect the morphology of carbon produced. Amorphous carbon was generated on a 10 wt % Cu/5 wt % W (2:1) catalyst, while BCNTs were produced on 20 wt % Cu/5 wt % W (4:1) and a 30 wt % Cu/5 wt % W (6:1) catalysts. A pure Pd catalyst produced carbon nanofibres (CNFs), while BCNTs were able to grow from Pd/Mo catalysts. Density functional theory simulations show that the composite Cu/W and Pd/Mo bimetallic particles which generated BCNTs have similar binding energies to carbon, and comparable to metals such as Fe, Co, and Ni which are traditionally used to grow CNTs by chemical vapor deposition

  • 26.
    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 study2010Inngår i: Physical Chemistry, Chemical Physics - PCCP, ISSN 1463-9076, E-ISSN 1463-9084, Vol. 12, nr 23, s. 6179-6186Artikkel i tidsskrift (Fagfellevurdert)
    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

  • 27.
    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 formation2010Inngår i: ACS Nano, ISSN 1936-0851, E-ISSN 1936-086X, Vol. 4, nr 2, s. 921-932Artikkel i tidsskrift (Fagfellevurdert)
    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

  • 28.
    Delaney, Paul
    et al.
    Queen's University Belfast.
    Larsson, Andreas
    Small cluster model of the NV centre in diamond2010Inngår i: Physics Procedia, ISSN 1875-3892, E-ISSN 1875-3892, Vol. 3, nr 4, s. 1533-1537Artikkel i tidsskrift (Fagfellevurdert)
    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.

  • 29.
    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 diamond2010Inngår i: Nano letters (Print), ISSN 1530-6984, E-ISSN 1530-6992, Vol. 10, nr 2, s. 610-614Artikkel i tidsskrift (Fagfellevurdert)
    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

  • 30.
    Muthukumar, Kaliappan
    et al.
    Tyndall National Institute, University College Cork.
    Larsson, Andreas
    The role of ellipticity on the preferential binding site of Ce and la in C78-D3h: A density functional theory study2010Inngår i: Nanoscale, ISSN 2040-3364, E-ISSN 2040-3372, Vol. 2, nr 7, s. 1250-1255Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Endohedral metallofullerenes that encapsulate one or several atoms, or a cluster of atoms have molecular properties making them useful both in technology and in bio-medical applications. Some fullerenes are found to have two metal atoms incarcerated and it has been recently found that two Ce atoms are incorporated into the C78-D3h (78:5) cage. In this study, we report calculations on the structural and electronic properties of Ce 2@C78 using density functional theory (DFT). While Ce 2@C80-Ih (D3d) and La 2@C80-Ih (D2h) have different ground state structures, we have found that Ce2@C78 has a D 3h ground state structure just as La2@C78. The encapsulated Ce atoms bind strongly to the C78-D3h cage with a binding energy (BE) of 5.925 eV but not as strong as in Ce@C 82-C2v nor in Ce2@C80-Ih. The elliptical nature of the cage plays a crucial role and accommodates the two Ce atoms at opposite ends of the C3 axis with a maximized inter atomic distance (4.078 Å). This means that the effect of the additional f-electron repulsion in M2@C78 with M = Ce compared to M = La, is less pronounced than in Ce2@C80 compared to La2@C80. We compare the results to the elliptical M 2@C72 (#10611) (M = La, Ce), and with a range of additional Ce and La endohedral fullerenes and explain the role ellipticity has in the preferential binding site of Ce and shed light on the formation mechanism of these nanostructures

  • 31.
    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)2010Inngår i: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 81, nr 7Artikkel i tidsskrift (Fagfellevurdert)
    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

  • 32.
    Larsson, Andreas
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Materialvetenskap.
    Activity: Carbon Nanotubes and Their Interactions With Metals2009Konferansepaper (Annet (populærvitenskap, debatt, mm))
  • 33.
    Muthukumar, Kaliappan
    et al.
    Tyndall National Institute, University College Cork.
    Larsson, Andreas
    Endohedral fullerenes and their applications in technology2009Inngår i: Advances in nanotechnology and applications, Louisville, KY: Center for Nanotechnology, Research, & Applications (CENTERA), Sullivan University College of Pharmacy , 2009, s. 127-135Kapittel i bok, del av antologi (Fagfellevurdert)
  • 34.
    Muthukumar, Kaliappan
    et al.
    Tyndall National Institute, University College Cork.
    Stróecka, Anna
    Institut für Bio- und Nanosysteme, Forschungszentrum Jülich.
    Voigtländer, Bert
    Institut für Bio- und Nanosysteme, Forschungszentrum Jülich.
    Larsson, Andreas
    Endohedral fullerenes and their implications in technology2009Konferansepaper (Fagfellevurdert)
    Abstract [en]

    The interest in nanoscale materials is due to their positive impact in various scientific areas such as energy; environmental and in biomedical sciences. Carbon has a wide range of interesting properties; which makes it a versatile element and special attention has been paid to carbon based nanostructures such as fullerenes and nanotubes among several other investigated materials. Fullerenes; with their distinctive carbon cage structure; have been the object of intense research since their discovery and they have become a building block of nanoscience and a base for novel molecular materials. Fullerenes have also played a central role in the development of a variety of single molecule techniques and spectroscopies; each of which is currently an intensely active sub-field of nanoscience and nanotechnology. These include: single molecule manipulation; single molecule electronic and optical microscopy; and monomolecular electronic devices

  • 35.
    Stróecka, Anna
    et al.
    Institut für Bio- und Nanosysteme, Forschungszentrum Jülich.
    Muthukumar, Kaliappan
    Tyndall National Institute, University College Cork.
    Dybek, Aneta
    Department of Physics, Queen Mary University of London.
    Dennis, T. Jiohn S.
    Department of Physics, Queen Mary University of London.
    Larsson, Andreas
    Mysliveček, Josef
    Institut für Bio- und Nanosysteme, Forschungszentrum Jülich.
    Voigtländer, Bert
    Institut für Bio- und Nanosysteme, Forschungszentrum Jülich.
    Modification of the conductance of single fullerene molecules by endohedral doping2009Inngår i: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 95, nr 13Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    We use scanning tunneling microscopy to establish controlled contacts to single molecules of endohedrally doped Ce 2@C 80 fullerenes with C 60 as a reference. The stability of the experimental setup allows for the determination of the conductance of Ce 2@C 80 relative to the conductance of C 60. The endohedral doping reduces the conductance of Ce 2@C 80 by a factor of about five with respect to C 60. Ab initio calculations show that the reason for this reduced conductance is the absence of electron orbitals delocalized over the cage of Ce 2@C 80 in the energy window of the conductance measurement

  • 36.
    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 molecular2009Inngår i: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 113, nr 17, s. 7298-7304Artikkel i tidsskrift (Fagfellevurdert)
    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

  • 37.
    O'Byrne, Justin P.
    et al.
    Department of Chemistry, Materials Section and Supercritical Fluid Centre, University College Cork.
    Tobin, Joseph M.
    Department of Chemistry, Materials Section and Supercritical Fluid Centre, University College Cork.
    Li, Zhong
    Department of Chemistry, Materials Section and Supercritical Fluid Centre, University College Cork.
    Larsson, Andreas
    Holmes, Justin D.
    Department of Chemistry, Materials Section and Supercritical Fluid Centre, University College Cork.
    Novel catalysts for carbon nanotube and nanofibre synthesis2009Konferansepaper (Fagfellevurdert)
    Abstract [en]

    Carbon nanotubes are of huge interest to the scientific community for their physical and electronic properties[1]. We have synthesized carbon nanotubes (CNTs) on novel non-traditonal catalysts. Bamboo-shaped carbon nanotubes (BCNTs) were synthesized on Pd and Cu catalysts doped with either Mo or W. BCNTs were sythesised using the catalytic chemical vapour decomposition (CVD) of methane. Cu is catalytically inactive for the synthesis of CNTs; however this is not the case when Cu is doped with Mo[2] and W. When Cu is doped with these metals bamboo structured CNTs can be produced via a CVD synthesis method. The addition of a dopant alters the electronic structure of catalyst nanoparticle causing the binding strength between the nanoparticle and carbon to approach that of traditional catalysts for CNT growth i.e. Fe; Co; Ni and their alloys which has been shown by computer modeling. We have performed full geometry optimization DFT calculations of Cu/W and Pd/Mo particles of different compositions to determine their ability to stabilize and support the growing end of a CNT. We have computed a (5;0) SWNTs binding energy to a M13 metal cluster (M=Cu; W or Pd; Mo); which in a model system that has been proven to correctly predict the trends in bonding[2; 3]. The resulting binding energies are compared to Fe; Co; Ni and Cu/Mo clusters of these previous reports. All calculations were made using the generalized gradient approximation (GGA) exchange and correlation potential by Perdew-Burke-Ernzenhof [PBE]; utilizing the polarized valence triple-ζ (TZVP) basis set[4] and relativistic effective core potentials (ECPs) for Cu; Pd; W and Mo[5; 6]; as implemented in the Turbomole suite of programs[7-10].

  • 38.
    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 monolayers2009Inngår i: Langmuir, ISSN 0743-7463, E-ISSN 1520-5827, Vol. 25, nr 1, s. 242-247Artikkel i tidsskrift (Fagfellevurdert)
    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

  • 39.
    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).2009Konferansepaper (Fagfellevurdert)
    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

  • 40.
    Muthukumar, Kaliappan
    et al.
    Tyndall National Institute, Computational Modelling Group, University College Cork.
    Larsson, Andreas
    Strózecka, Anna
    Institut für Bio- and Nanosysteme (IBN 3) and CNI – Forschungszentrum Jülich.
    Voigtländer, Bert
    Institut für Bio- and Nanosysteme (IBN 3) and CNI – Forschungszentrum Jülich.
    Theoretical explanation of the nature of free and surface bound cerium endohedral fullerenes2009Konferansepaper (Fagfellevurdert)
    Abstract [en]

    Cerium is one of the most reactive elements of the rare earth group, and its complex physics accompanying its single occupied 4f band, close to the Fermi level, has baffled scientists for many decades. Cerium can be encapsulated into fullerene (C72, C78, C80 and C82) cages, which works as an n-dopant. Metallofullerenes can be used as quantum bits in quantum computers, as molecular devices and as MRI contrasting agents. Here we address the challenges in modelling cerium endohedral fullerenes using density functional theory (DFT) and report our theoretical inspection that gives a detailed picture of metal-cage interaction and a closer look on the electronic structure of these cerium doped fullerenes. Experimental and theoretical results for these endohedral species bound to Cu(111) will also be presented with a fundamental and applied viewpoint to expand our knowledge about endohedral fullerene-surface interaction. We have found that the conductance of empty C60 to be much higher than the endohedrally doped cerium fullerene Ce2@C80 when bound to the same Cu(111) surface using low-temperature scanning tunnelling microscopy (LT-STM), which we explain through DFT calculations.

  • 41.
    Muthukumar, Kaliappan
    et al.
    Tyndall National Institute, University College Cork.
    Larsson, Andreas
    A density functional study of Ce@C82: Explanation of the Ce preferential bonding site2008Inngår i: Journal of Physical Chemistry A, ISSN 1089-5639, E-ISSN 1520-5215, Vol. 112, nr 5, s. 1071-1075Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Ce has been found experimentally to be preferentially incorporated into the C82 isomer of C2v symmetry as have other lanthanoids in M@C82 (M = La, Pr, Nd, etc.). We have investigated the underlying reason for this preference by calculating structural and electronic properties of Ce@C82 using density functional theory. The ground-state structure of Ce@C82 is found to have the cerium atom attached to the six-membered ring on the C2 axis of the C82-C2v cage, and the encapsulated atom is found to perturb the carbon cage due to chemical bonding. We have found Ce to favor this C2v chemisorption site in Cv2 by 0.62 eV compared to other positions on the inside wall of the cage. The specific preference of the metal atom to this six-membered ring is explained through electronic structure analysis, which reveals strong hybridization between the d orbitals of cerium and the π orbitals of the cage that is particularly favorable for this chemisorption site. We propose that this symmetry dictated interaction between the cage and the lanthanide d orbital plays a crucial role when C82 forms in the presence of Ce to produce Ce@C82 and is also more generally applicable for the formation of other lanthanoid M@C82 molecules. Our theoretical computations are the first to explain this well-established fact. Last, the vibrational spectrum of Ce@C82 has been simulated and analyzed to gain insight into the metal-cage vibrations

  • 42.
    Larsson, Peter O.
    et al.
    Department of Physics, Condensed Matter Theory Group, Uppsala University.
    Araújo, Carlos Moysés
    Department of Physics, Condensed Matter Theory Group, Uppsala University.
    Larsson, Andreas
    Jena, Puru
    Department of Physics, Virginia Commonwealth University, Richmond.
    Ahuja, Rajeev B.
    Department of Physics, Condensed Matter Theory Group, Uppsala University.
    A detailed look at the catalytic action of transition metal atom dopants in MgH2 nanoclusters2008Konferansepaper (Fagfellevurdert)
    Abstract [en]

    Magnesium hydride (MgH2 ) is a candidate for solid-state hydrogen storage applications. In this work; hydrogen desorption from transition metal doped MgH2 clusters of approximately 1 nm was investigated by density functional theory (PBE functional and Gaussian basis sets). It was found that transition metals were more stable at the surface than in the center of the cluster and that desorption energies of hydrogen atoms bound to such surface doped transition metals were significantly lowered. It was furthermore observed how transition metals attract hydrogens to keep at least four hydrogen atoms coordinated even when the total hydrogen content of the cluster decreases. This effect is associated with migration of the transition metal atoms from the surface sites to the interior sites during the dehydrogenation process; releasing more hydrogen as they diffuse. This diffusion mechanism may account for the fact that a small amount of catalysts is sufficient to improve the kinetics of MgH2 .

  • 43.
    Muthukumar, Kaliappan
    et al.
    Tyndall National Institute, University College Cork.
    Larsson, Andreas
    Stróżecka, Anna
    Institut für Bio- and Nanosysteme (IBN 3) and CNI – Forschungszentrum Jülich.
    Viogtländer, Bert
    Institut für Bio- and Nanosysteme (IBN 3) and CNI – Forschungszentrum Jülich.
    A Joint Theoretical and Experimental Characterization of the In-elastic Tunneling modes of Ce@C82 and Ce2@C802008Konferansepaper (Fagfellevurdert)
    Abstract [en]

    The drive for miniaturization of electronic devices involves nanotechnology; where single molecules are investigated for the use as electric components.1 Owing to their electronic and magnetic flexibility; endohedral fullerenes have been suggested as one of a possible molecular candidate in future electronics.2 Such molecular components differ from the conventional electronic materials by the degree to which mechanical degrees of freedom affect electrical conductivity.3;4 Understanding of these processes are still in its infancy and in this study in-elastic tunnelling spectroscopy (IETS) which probes the tunnelling current of a single molecule using scanning tunnelling microscopy (STM); with controlled excitation of their vibration modes has been applied to cerium endohedral fullerenes (Ce@C82 and Ce2@C80) to investigate the interplay between electrons and vibrations of these molecules on Cu(111).5 We report here our measured IETS spectrum of single Ce@C82 and Ce2@C80 on Cu (111) and explain the observed IETS peaks by analysing the molecular vibrational spectra simulated using density functional theory calculations.

  • 44. Baran, Jakub D.
    et al.
    Larsson, Andreas
    Adsorption Of Metal-Phthalocyanines on Ag(111): A First-Principles Study2008Konferansepaper (Fagfellevurdert)
  • 45.
    Muthukumar, Kaliappan
    et al.
    Tyndall National Institute, Computational Modelling Group, University College Cork.
    Larsson, Andreas
    Strózecka, Anna
    Institut für Bio- and Nanosysteme (IBN 3) and CNI – Forschungszentrum Jülich.
    Voigtländer, Bert
    Institut für Bio- and Nanosysteme (IBN 3) and CNI – Forschungszentrum Jülich.
    Cerium doped Fullerenes (Ce@C82 & Ce2@C80) adsorbed on Cu(111) surface: Theoretical Explanation for Surprising Experimental Findings2008Konferansepaper (Fagfellevurdert)
    Abstract [en]

    Cerium is one of the most reactive elements of the rare earth group; and the complex physics accompanying its single occupied 4f band; close to the Fermi level; has baffled scientists for many decades. Cerium can be encapsulated into fullerene (C82 & C80) cages;1-4 which works as an n-dopant and in the incarcerated form it can be used as quantum bits in quantum computing2. Theoretical studies aiming at the detailed picture of metal-cage interaction and a closer look on the electronic structure of these Ce doped fullerenes have been reported.3-4 But; only a few theoretical studies have been conducted with an aim to address the nature of interaction of these endohedral fullerene molecules with semiconductors and metal surfaces. Hence; in this study; we apply density functional theory (DFT) to expand the investigation of Ce@C82 and Ce2@C80 on Cu (111) surface to have a more detailed study from both a fundamental and applied viewpoint to what extent the fullerene-surface interaction influences the encapsulated atom. Our calculations reveal a surprising observation in the electronic structure of doped Ce fullerenes on Cu (111) surface. We discuss and elucidate our results with the measured STM/STS spectra

  • 46.
    Muthukumar, Kaliappan
    et al.
    Tyndall National Institute, University College Cork.
    Larsson, Andreas
    Schulte, Karina H.G.
    MAX-lab, Lund University.
    Moriarty, Philip J.
    School of Physics and Astronomy, University of Nottingham.
    Stróżecka, Anna
    Institut für Bio- and Nanosysteme (IBN 3) and CNI – Forschungszentrum Jülich.
    Voigtländer, Bert
    Institut für Bio- and Nanosysteme (IBN 3) and CNI – Forschungszentrum Jülich.
    Cerium Endohedral fullerenes (Ce@C82 and Ce2@C80) Theoretical Interpretations for Experimental observation2008Konferansepaper (Fagfellevurdert)
    Abstract [en]

    Many questions remain unanswered for the endohedral fullerenes. Owing to the improvement in separation of isomers and in theoretical studies (DFT and ab-initio) evolving as imperative tool for characterization; these can be addressed.1;2 Understanding of the position and binding configuration of the metal atom inside the cage is crucial as it controls the structural and electronic properties of the molecule.3 Theoretical calculations proved to be efficient in explaining many controversies in the field of lanthanoid endohedral fullerenes.2-5 Fig; Ce2@C80; Ce2@C78) DFT optimized structures of Ce2@C80 D3d and Ce2@C78 D3h Here in this study we use DFT to characterize Ce doped metallofullerenes and report some surprising theoretical findings on the binding of cerium inside various carbon cages; (C60; C78; C80; C82). We observe that the presence of an additional Ce atom puts restrictions on the binding in the C80 cage6; but this does not happen in the C78 cage. We explain the reason behind this by analyzing the electronic structure. Further various spectra (RESPES; IETS; STM/STS) have been simulated for Ce@C82 and Ce2@C80 which we compare and discuss with experiments

  • 47.
    Li, Zhong
    et al.
    Department of Chemistry, Materials Section and Supercritical Fluid Centre, University College Cork.
    Larsson, Andreas
    Larsson, Peter O.
    Department of Physics, Condensed Matter Theory Group, Uppsala University.
    Ahuja, Rajeev B.
    Department of Physics, Condensed Matter Theory Group, Uppsala University.
    Tobin, Joseph M.
    Department of Chemistry, Materials Section and Supercritical Fluid Centre, University College Cork.
    O'Byrne, Justin P.
    Department of Chemistry, Materials Section and Supercritical Fluid Centre, University College Cork.
    Morris, Michael A.
    Department of Chemistry, Materials Section and Supercritical Fluid Centre, University College Cork.
    Attard, Gary Anthony
    School of Chemistry, Cardiff University.
    Holmes, Justin D.
    Department of Chemistry, Materials Section and Supercritical Fluid Centre, University College Cork.
    Copper/molybdenum nanocomposite particles as catalysts for the growth of bamboo-structured carbon nanotubes2008Inngår i: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 112, nr 32, s. 12201-12206Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Bamboo-structured carbon nanotubes (BCNTs), with mean diameters of 20 nm, have been synthesized on MgO-supported Cu and Mo catalysts by the catalytic chemical vapor deposition of methane. BCNTs could only be generated using a combination of Cu and Mo catalysts. No BCNTs were produced from either individual Cu or Mo catalysts. In combination, Mo was found to be essential for cracking the methane precursor, while Cu was required for BCNT formation. Energy dispersive X-ray analysis of the individual particles at the tips of the nanotubes suggest that Cu and Mo are present as a "composite" nanoparticle catalyst after growth. First-principles modeling has been used to describe the interaction of the Cu/Mo catalyst with the nanotubes, suggesting that the catalyst binds with the same energy as traditional catalysts such as Fe, Ni, and Co

  • 48.
    Muthukumar, Kaliappan
    et al.
    Tyndall National Institute, University College Cork.
    Larsson, Andreas
    Density Functional Characterization of Ce2@C80: Explanation of the Ce Preferential bonding site inside the Ih-C80 cage2008Konferansepaper (Annet vitenskapelig)
    Abstract [en]

    Metallofullerenes have many possible uses in technology and even in bio-medical applications.1 Some fullerenes have been found to have two incarcerated metal atoms such as M2@C80 (M = La; Ce; etc.).2 Owing to the difficulty in synthesis and purification; structural characterisation has not been reported for many fullerenes that has two metal atoms. Here in this study we report the structural and electronic properties of Ce2@C80; calculated using density functional theory. Ce in Ce2@C80 is found to have a novel binding site which minimizes Ce(f) Ce(f) overlap to favor Ce-C bonding leading to D3d configuration while La in La2@C80 has D2h symmetric ground state structure.3 (Picture) HOMO and LUMO of D3d Ce2@C80 The analysis of the binding energy of Ce shows that the two endohedral atoms have a weaker binding in Ih-C80 than it has in Ce@C80 and in Ce@C82.4 We discuss the reason for this preference of binding site where Ce has a weaker binding strength; using the frontier level orbitals of Ce2@C80 in comparison with the orbitals of La2@C80. Further; the nature of charge transfer and the oxidation state of the Ce atoms in Ce2@C80; correlating to the conventional Ce tri-halide compounds (CeF3;CeCl3) will be explained. The investigation on the simulated vibrational spectrum will also be presented

  • 49.
    Muthukumar, Kaliappan
    et al.
    Tyndall National Institute, University College Cork.
    Larsson, Andreas
    Density Functional Theory Description of the Preferential Binding Site of Cerium Atoms in Ce2@C802008Konferansepaper (Fagfellevurdert)
    Abstract [en]

    iCeC80; Ce2@C80; Cerium; DFT; Electronic Structure Theory; Turbomole Abstract: Fullerenes encapsulating one or more lanthanoides such as M@C82 or M2@C80 (M = La; Ce; etc.) have interesting properties for potential applications as devices in nanoelectronics and contrasting agents in biomedical sciences.1-3 The properties of these species; like electronic structure; stability and the symmetry of the molecule; depends upon the position of the metal atom inside the cage. In this study we characterize Ce2@C80 through density functional theory calculations and establish the competeting nature of binding between the two Ce atoms and Ce-C bonding; the nature of which causes cerium bond to a different binding site in Ih-C80 compared to fullerenes with only one incarcerated Ce atoms. We observe a strong hybridization between the d orbitals of cerium and the π electrons of the Ih-C80 cage. Here we report and discuss the ground state configurations of Ce2@C80 in connection with our recent study of Ce@C82.4;5We; in addition; show that Ce2@C80 has a different ground state geometry than La2@C80.6

  • 50.
    Muthukumar, Kaliappan
    et al.
    Tyndall National Institute, University College Cork.
    Larsson, Andreas
    Density Functional Theory Explanation of the Different Preferential Binding Sites for Ce and La In M2@C80 (M=Ce; La)2008Konferansepaper (Fagfellevurdert)
    Abstract [en]

    Fullerenes encapsulating metal atoms have many possible uses in technology and even in bio-medical applications.1 Some fullerenes have been found to have two incarcerated metal atoms such as M2@C80 (M = La; Ce; etc.).2 Here we report the structural and electronic properties of Ce2@C80 calculated using density functional theory. We have found that Ce2@C80 has a D3d symmetric ground state structure and that Ce binds to a different type of binding site compared to other cerium containing fullerenes; such as Ce@C823 and even in Ce@C80. Fig: This binding site also differs compared to La in La2@C80; which is D2h symmetric. Since each of the six-membered rings in Ih-C80 satisfies the proposed criteria for (M=La; Ce) binding3; we here analyze and discuss the reason of Ce binding to another site based on its frontier level orbitals in connection with the orbitals of La2@C80. We also discuss oxidation state of Ce in Ce2@C80; in comparison with the traditional Ce tri-halides compounds (CeCl3; CeF3) and the investigation on the simulated vibrational spectrum of the ground state isomer of Ce2@C80.

123 1 - 50 of 127
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