H. Häkkinen

University of Jyväskylä, Jyväskylä, Province of Western Finland, Finland

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Publications (64)184.65 Total impact

  • L. Nykänen, H. Häkkinen, K. Honkala
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    ABSTRACT: Density-functional-theory calculations were carried out for hydrogen capped linear carbon chains, polyynes and cumulenes, adsorbed dissociatively on the (1 1 1) and (2 1 1) surfaces of gold and silver. In the studied adsorption reactions, carbon–hydrogen bonds are broken and covalent carbon–metal bonds are created. The adsorption of cumulenes is highly endothermic, whereas the adsorption of polyynes is near thermoneutral. Also, the hydrogenation of adsorbed polyynyl radicals (·CnH) into adsorbed cumulene carbenes (:CnH2) was investigated, which was found to be exothermic on both metals. Vibrational calculations were conducted on the adsorption systems, and the results were compared with experimental surface enhanced Raman scattering spectra. An interpretation is proposed for the spectra of polyyne–silver solutions, and features of polyyne–gold spectra are predicted.
    Carbon. 07/2012; 50(8):2752–2763.
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    ABSTRACT: Charged monolayer-protected gold nanoparticles (AuNPs) have been studied in aqueous solution by performing atomistic molecular dynamics simulations at physiological temperature (310 K). Particular attention has been paid to electrostatic properties that modulate the formation of a complex comprised of the nanoparticle together with surrounding ions and water. We focus on Au144 nanoparticles that comprise a nearly spherical Au core (diameter 2 nm), a passivating Au–S interface, and functionalized alkanethiol chains. Cationic and anionic AuNPs have been modeled with amine and carboxyl terminal groups and Cl–/Na+ counterions, respectively. The radial distribution functions show that the side chains and terminal groups show significant flexibility. The orientation of water is distinct in the first solvation shell, and AuNPs cause a long-range effect in the solvent structure. The radial electrostatic potential displays a minimum for AuNP– at 1.9 nm from the center of the nanoparticle, marking a preferable location for Na+, while the AuNP+ potential (affecting the distribution of Cl–) rises almost monotonically with a local maximum. Comparison to Debye–Hückel theory shows very good agreement for radial ion distribution, as expected, with a Debye screening length of about 0.2–0.3 nm. Considerations of zeta potential predict that both anionic and cationic AuNPs avoid coagulation. The results highlight the importance of long-range electrostatic interactions in determining nanoparticle properties in aqueous solutions. They suggest that electrostatics is one of the central factors in complexation of AuNPs with other nanomaterials and biological systems, and that effects of electrostatics as water-mediated interactions are relatively long-ranged, which likely plays a role in, e.g., the interplay between nanoparticles and lipid membranes that surround cells.
    The Journal of Physical Chemistry C 04/2012; 116(17):9805–9815. · 4.84 Impact Factor
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    ABSTRACT: We show that linear shape isomers of small even-even nuclei exist with nearly any internucleon interactions. The shapes of the linear isomers look like chains of alpha-particles, but single-particle spectrum reveals that alpha-particle interpretation is not needed. Indeed, the same shapes are obtained even with noninteracting particles in a rectangular cavity. Linear shape isomers are shown to exist also in metal clusters.
    International Journal of Modern Physics E 01/2012; 06(03). · 0.63 Impact Factor
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    ABSTRACT: Using ab-initio computational techniques on crystal determined clusters, we report on the similarities and differences of Al$_{50}$(C$_5$(CH$_3)_5)_{12}$, Ga$_{23}$(N(Si(CH$_3)_{3}$)$_{2}$)$_{11}$, and Au$_{102}$(SC$_7$O$_2$H$_5$)$_{44}$ ligand-protected clusters. Each of the ligand-protected clusters in this study show the similar stable character which can be described via a electronic shell model. We show here that the same type of analysis leads consistently to derive a superatomic electronic counting rule, independently of the metal and ligand compositions. One can define the cluster core as the set of atoms where delocalized single-angular-momentum-character orbitals have hight weight using a combination of Bader analysis and the evaluation of Khon-Sham orbitals. Subsequently one can derive the nature of the ligand-core interaction. These results yield further insight into the superatom analogy for the class of ligand-protected metal clusters.
    Physical review. B, Condensed matter 04/2011; 84. · 3.77 Impact Factor
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    ABSTRACT: The electronic conductance of a benzene molecule connected to gold electrodes via thiol, thiolate, or amino anchoring groups is calculated using nonequilibrium Green functions in combination with the fully self-consistent GW approximation for exchange and correlation. The calculated conductance of benzenedithiol and benzenediamine is one-fifth that predicted by standard density functional theory (DFT), in very good agreement with experiments. In contrast, the widely studied benzenedithiolate structure is found to have a significantly higher conductance due to the unsaturated sulfur bonds. These findings suggest that more complex gold-thiolate structures where the thiolate anchors are chemically passivated by Au adatoms are responsible for the measured conductance. Analysis of the energy level alignment obtained with DFT, Hartree-Fock, and GW reveals the importance of self-interaction corrections (exchange) on the molecule and dynamical screening at the metal-molecule interface. The main effect of the GW self-energy is to renormalize the level positions; however, its influence on the shape of molecular resonances also affects the conductance. Non-self-consistent G 0 W 0 calculations, starting from either DFT or Hartree-Fock, yield conductance values within 50% of the self-consistent GW results.
    Physical review. B, Condensed matter 01/2011; 83. · 3.77 Impact Factor
  • O. Lopez-Acevedo, H. Häkkinen
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    ABSTRACT: . Loss of small fragments (like AuL, Au2L3, Au4L4) have been found systematically in several MALDI and FAB experiments on thiolate-protected gold clusters of different sizes. When using the cluster Au25L18 –1 as parent cluster, the fragmented cluster Au21L14 –1 has been reported to be obtained in high proportion (L = SCH2CH2Ph). Here we analyse a few possible fragmentation patterns of the well-known parent cluster Au25L18 –1 (L = SCH3). Using DFT calculations we study the different atomic configurations obtained after a AuL fragment is lost from Au25L18 –1. We found energetically favourable configurations that can be written as Au13 [ Au2L3]6−z [AuL2] z –1, where the modification can be described as a replacement of the long protecting unit by a short one (Au2L3 → AuL2). A full replacement (z = 6) gives rise to a protected Au19L12 –1 cluster. This mechanism does not modify the super-atomic electronic structure of the gold core, i.e., all these fragments remain an 8 electron super-atom clusters exactly like the parent Au25L18 –1. We suggest that the Au19L12 –1 cluster could be realized by using a bulky thiolate, such as the tert-butyl thiolate SC(CH3)3.
    The European Physical Journal D 01/2011; 63:1-4. · 1.51 Impact Factor
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    ABSTRACT: Electronic structure calculations have become an indispensable tool in many areas of materials science and quantum chemistry. Even though the Kohn-Sham formulation of the density-functional theory (DFT) simplifies the many-body problem significantly, one is still confronted with several numerical challenges. In this article we present the projector augmented-wave (PAW) method as implemented in the GPAW program package (https://wiki.fysik.dtu.dk/gpaw) using a uniform real-space grid representation of the electronic wavefunctions. Compared to more traditional plane wave or localized basis set approaches, real-space grids offer several advantages, most notably good computational scalability and systematic convergence properties. However, as a unique feature GPAW also facilitates a localized atomic-orbital basis set in addition to the grid. The efficient atomic basis set is complementary to the more accurate grid, and the possibility to seamlessly switch between the two representations provides great flexibility. While DFT allows one to study ground state properties, time-dependent density-functional theory (TDDFT) provides access to the excited states. We have implemented the two common formulations of TDDFT, namely the linear-response and the time propagation schemes. Electron transport calculations under finite-bias conditions can be performed with GPAW using non-equilibrium Green functions and the localized basis set. In addition to the basic features of the real-space PAW method, we also describe the implementation of selected exchange-correlation functionals, parallelization schemes, ΔSCF-method, x-ray absorption spectra, and maximally localized Wannier orbitals.
    Journal of Physics Condensed Matter 06/2010; 22(25):253202. · 2.22 Impact Factor
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    ABSTRACT: ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.
    ChemInform 01/2010; 31(8).
  • Microscopy and Microanalysis 01/2010; 16:1652-1653. · 2.50 Impact Factor
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    ABSTRACT: The electronic structure of ultrasmall Au clusters on thin MgO/Ag(001) films has been analyzed by scanning tunneling spectroscopy and density functional theory. The clusters exhibit two-dimensional quantum well states, whose shapes resemble the eigenstates of a 2D electron gas confined in a parabolic potential. From the symmetry of the highest occupied (HOMO) and lowest unoccupied molecular orbital (LUMO) of a particular cluster, its electron filling and charge state is determined. In accordance with a Bader charge analysis, aggregates containing up to 20 atoms accumulate one to four extra electrons due to a charge transfer from the MgO/Ag interface. The HOMO-LUMO gap is found to close for clusters containing between 70 and 100 atoms.
    Physical Review Letters 06/2009; 102(20):206801. · 7.73 Impact Factor
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    S. Malola, H. Häkkinen, P. Koskinen
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    ABSTRACT: We study the bonding and diffusion of Au in graphene vacancies using density-functional theory. Energetics show that Au adsorbs preferably to double vacancies, steadily in-plane with graphene. All diffusion barriers for the complex of Au in double vacancy are above 4 eV, whereas the barriers for larger vacancies are below 2 eV. Our results support the main results of a recent experiment [Gan et al., Small 4, 587 (2008)], but suggest that the observed diffusion mechanism is not thermally activated, but radiation-enhanced.
    Applied Physics Letters 01/2009; · 3.79 Impact Factor
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    S. Malola, H. Häkkinen, P. Koskinen
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    ABSTRACT: Vibrational properties of graphene nanoribbons are examined with density functional based tight-binding method and non-resonant bond polarization theory. We show that the recently discovered reconstructed zigzag edge can be identified from the emergence of high-energy vibrational mode due to strong triple bonds at the edges. This mode is visible also in the Raman spectrum. Total vibrational density of states of the reconstructed zigzag edge is observed to resemble the vibrational density of states of armchair, rather than zigzag, graphene nanoribbon. Edge-related vibrational states increase in energy which corroborates increased ridigity of the reconstructed zigzag edge. Comment: 4 pages, 4 figures
    The European Physical Journal D 09/2008; · 1.51 Impact Factor
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    D. Koudela, A. -M. Uimonen, H. Häkkinen
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    ABSTRACT: We study the dependence of the intrinsic conductance of a nanocontact on its shape by using the recursion-transfer-matrix method. Hour-glass, torus, and spherical shapes are defined through analytic potentials, the latter two serving as rough models for ring-like and spherical molecules, respectively. The sensitivity of the conductance to geometric details is analyzed and discussed. Strong resonance effects are found for a spherical contact weakly coupled to electron reservoirs. Comment: 7 pages, 12 figures
    Physics of Condensed Matter 08/2008; · 1.28 Impact Factor
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    O. Lopez-Acevedo, D. Koudela, H. Häkkinen
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    ABSTRACT: Electrical conductance through various nanocontacts between gold electrodes is studied by using the density functional theory, scalar-relativistic pseudopotentials, generalized gradient approximation for the exchange-correlation energy and the recursion-transfer-matrix method along with channel decomposition. The nanocontact is modeled with pyramidal fcc(100) tips and 1 to 5 gold atoms between the tips. Upon elongation of the contact by adding gold atoms between the tips, the conductance at Fermi energy E_F evolves from G ~ 3 G_0 to G ~ 1 G_0 (G_0 = 2e/h^2). Formation of a true one-atom point contact, with G ~ 1 G_0 and only one open channel, requires at least one atom with coordination number 2 in the wire. Tips that share a common vertex atom or tips with touching vertex atoms have three partially open conductance channels at E_F, and the symmetries of the channels are governed by the wave functions of the tips. The long 5-atom contact develops conductance oscillations and conductance gaps in the studied energy range -3 < E-E_F < 5 eV, which reflects oscillations in the local density of electron states in the 5-atom linear "gold molecule" between the electrodes, and a weak coupling of this "molecule" to the tips.
    Physics of Condensed Matter 06/2008; · 1.28 Impact Factor
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    P Frondelius, H Häkkinen, K Honkala
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    ABSTRACT: We report a systematic density functional theory investigation of adsorption of small Aun (n =1–6) clusters on ideal and defected MgO(100) single crystal surfaces and Mo(100) supported thin MgO(100) films. As a model defect, we consider a neutral surface oxygen vacancy (Fs). Optimal adsorption geometries and energies, cluster formation energies and cluster charges are discussed and compared in detail over four different substrates. For a given cluster size, the adsorption energy among these substrates increases in the order MgO, Fs/MgO, MgO/Mo and Fs/MgO/Mo. While cluster growth by association of atoms from gas phase is exothermic on all the substrates, cluster growth by diffusion and aggregation of pre-adsorbed Au atoms is an endothermic process for Au1→Au2, Au3→Au4 and Au5→Au6 on MgO/Mo and Au2→Au3 and Au5→Au6 on Fs/MgO/Mo. The adsorbed clusters are close to neutral on MgO, but adopt a significant anionic charge on other supports with the increasing order: MgO/Mo, Fs/MgO and Fs/MgO/Mo. The adsorption strength thus correlates with the amount of negative charge transferred from the substrate to gold.
    New Journal of Physics 09/2007; 9(9):339. · 4.06 Impact Factor
  • H. Häkkinen, R. N. Barnett, U. Landman
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    ABSTRACT: The structure and energetics of multiple-excess-electron alkali halide clusters Na14Cl14-n, (1 ≤ n ≤ 6) is studied by ab initio calculations using norm-conserving pseudopotentials and local-spin-density-functional theory. Analysis of various electronic properties (Kohn-Sham one-electron eigenvalue spectra, ionization potentials, participation ratios of Kohn-Sham orbitals), as well as multiple F-center formation energies, suggests that these clusters can be characterized as Nan(NaCl)14-n having a "phase-separated" metallic part NaI. The Na14Cl9 (or Na14Cl9+) cluster exhibits a face (surface) segregated metallic Na5 (Na5+) overlayer, the stability of which is demonstrated by a molecular-dynamics simulation at 660 K.
    EPL (Europhysics Letters) 07/2007; 28(4):263. · 2.26 Impact Factor
  • H. Häkkinen, S. Mäkinen, M. Manninen
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    ABSTRACT: Structures of edge dislocations in aluminum are calculated using molecular dynamics and an effective medium theory with many-atom interactions. The local density approximation is used to calculate the positron states at different trapping sites. The partial [211]-dislocation is a very shallow trap with a positron binding energy of less than 80 meV and a positron lifetime similar to the bulk lifetime. Vacancies and single jogs at the dislocation line result in a lifetime of about 224 ps which is in a good agreement with the experimentally observed lifetimes in aluminum containing dislocations.
    EPL (Europhysics Letters) 07/2007; 9(8):809. · 2.26 Impact Factor
  • H Häkkinen, M Manninen
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    ABSTRACT: The thermal properties of simple metals, Al and Cu, have been studied using the molecular dynamics simulation method and the effective medium theory (EMT). The results obtained from EMT have a strongly predictive nature, because the model includes many-atom interactions and is based on ab initio-calculations within the framework of the density-functional theory. We have calculated the thermal expansion coefficient of solid and melt phase, melting point and latent heat of fusion for both metals. All the results are in good agreement with the experimental data. The long-range interactions (beyond the nearest neighbours) have been found to be important in the simulations of high-temperature processes (melting, disordered structure) and also in the case of extended crystal defects. The theory gives a good description of the vacancy formation energy and the stacking fault energy. The presence of vacancies lowers the simulated melting point of Al indicating the importance of crystal defects in the melting mechanism. No significant temperature dependence of the vacancy formation energy in Al was observed.
    Physica Scripta 01/2007; 1990(T33):210. · 1.03 Impact Factor
  • S Mäkinen, H Häkkinen, M Manninen
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    ABSTRACT: Atomic arrangement around a vacancy-type dislocation loop on a (111)-plane in Al is calculated by using the molecular dynamics simulation method. Interatomic potential used in the calculations is derived from the effective-medium theory. The potential includes many-atom interactions and is extended beyond the nearest neighbours. Annihilation characteristics and trapping of positrons at the dislocation loop are studied by calculating positron states for the relaxed atomic structure. The results show that the pure vacancy loop is a relatively shallow trap with a binding energy of 0.43 eV and a positron lifetime of 191 ps.
    Physica Scripta 01/2007; 1990(T33):206. · 1.03 Impact Factor
  • H. Häkkinen, M. Manninen
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    ABSTRACT: The effect of atomic and electronic structure on mechanical properties of a piece of sodium nanowire is studied with a total-energy method based on the density functional theory. We find that adiabatic elongation or compression of the wire results in discrete jumps in total energy due to a correlated rearrangement of electron states and atomic structure, and the resulting force shows oscillations in the nanonewton range. Furthermore, our results indicate that under certain conditions the break contact could support well-localized electron states. We discuss the results in connection with recent experiments and calculations on extended nanowires.
    EPL (Europhysics Letters) 01/2007; 44(1):80. · 2.26 Impact Factor

Publication Stats

1k Citations
184.65 Total Impact Points

Institutions

  • 1990–2012
    • University of Jyväskylä
      • • Nanoscience Center
      • • Department of Physics
      Jyväskylä, Province of Western Finland, Finland
  • 2009
    • MPG Ranch
      Lolo, Montana, United States
  • 1995–2003
    • Georgia Institute of Technology
      • School of Physics
      Atlanta, GA, United States
  • 2001–2002
    • University of Freiburg
      Freiburg, Baden-Württemberg, Germany
    • University of Lausanne
      Lausanne, Vaud, Switzerland