H. Häkkinen

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

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Publications (66)168.75 Total impact

  • J. Nevalaita · H. Häkkinen · K. Honkala
    [Show abstract] [Hide abstract] ABSTRACT: The molecular and dissociative adsorption of water on a Ag-supported 1ML, 2ML and 3ML-thick MgO films with a single Au adatom is investigated using density functional theory calculations. The obtained results are compared to a bulk MgO(001) surface with an Au atom. On thin films the negatively charged Au strengthens the binding of the polar water molecule due to the attractive Au-H interaction. The adsorption energy trends of OH and H with respect to the film thickness depends on an adsorption site. In the case OH or H binds atop Au on MgO/Ag(001), the adsorption becomes more exothermic with the increasing film thickness, while the reverse trend is seen when the adsorption takes place on bare MgO/Ag(001). This behaviour can be explained by different bonding mechanisms identified with the Bader analysis. Interestingly, we find that the rumpling of the MgO film and the MgO-Ag interface distance correlate with the charge transfer over the thin film and the interface charge, respectively. Moreover, we employ a modified Born-Haber-cycle to analyze the effect of film thickness to the adsorption energy of isolated Au and OH species on MgO/Ag(001). The analysis shows that the attractive Coulomb interaction between the negatively charged adsorbate and the positive MgO-Ag-interface does not completely account for the weaker binding with increasing film thickness. The redox energy associated with the charge transfer from the interface to the adsorbate is more exothermic with the increasing film thickness and partly compensates the decrease in the attractive Coulomb interaction.
    No preview · Article · Apr 2015 · Surface Science
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    V. Mäkinen · H. Häkkinen
    [Show abstract] [Hide abstract] ABSTRACT: The experimental structure of the Au25(SR) 18− cluster has been previously accurately determined. However, it’s thermodynamical behaviour is not well studied. We performed molecular dynamics simulations of 10 ps duration on the model cluster Au25(SH) 18− to gain information about the thermodynamical behaviour and stability of the cluster at temperatures between 300 K and 600 K. Our results suggest that the gold-sulfur bonds at the core-thiolate interface are the weakest ones in the system. However, the interface remains well defined during the simulations. The most significant structural changes take place in the gold core, where the ground state gold-gold bond length profile is completely changed by the thermal vibrations. The thermal movement does not affect the electronic structure notably. The HOMO and LUMO states broaden and the HOMO-LUMO gap narrows as a function of temperature, but the superatom electronic structure can be seen at all the temperatures clearly.
    Preview · Article · Nov 2012 · The European Physical Journal D
  • L. Nykänen · H. Häkkinen · K. Honkala
    [Show abstract] [Hide abstract] 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.
    No preview · Article · Jul 2012 · Carbon
  • J.kolehmainen · H.häkkinen · M.manninen · M.koskinen
    [Show abstract] [Hide abstract] 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.
    No preview · Article · Jan 2012 · International Journal of Modern Physics E
  • O. Lopez-Acevedo · H. Häkkinen
    [Show abstract] [Hide abstract] 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.
    No preview · Article · Jul 2011 · The European Physical Journal D
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    O. Lopez-Acevedo · P. A. Clayborne · H. Häkkinen
    [Show abstract] [Hide abstract] 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.
    Full-text · Article · Apr 2011 · Physical review. B, Condensed matter
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    M Strange · C Rostgaard · H Häkkinen · K. S. Thygesen
    [Show abstract] [Hide abstract] 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.
    Full-text · Article · Mar 2011 · Physical review. B, Condensed matter
  • P. Heino · H. Häkkinen · L. Perondi · K. Kaski
    [Show abstract] [Hide abstract] ABSTRACT: Mechanical properties of copper and aluminum have been studied using finite temperature molecular dynamics simulations. Atomic interactions have been described by a many-atom effective medium potential, which takes into account interactions up to third neighbors. The computed elastic constants showed good agreement with experimental data. Encouraged by these results the model was applied to study fracture in copper. Systems with a grain boundary and an initial cut serving as a crack seed have been studied. In the first case, crack nucleation and propagation took place exclusively at the grain boundary. In the second case, dislocation propagation was observed in one of the 〈110〉 directions, with a speed of about 60% of the longitudinal speed of sound. For thin systems crack propagation occurred through micro-void coalescence with a speed of about 30% of the Rayleigh wave speed in copper.
    No preview · Article · Jan 2011 · MRS Online Proceeding Library
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    Full-text · Article · Jul 2010 · Microscopy and Microanalysis
  • [Show abstract] [Hide abstract] 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.
    No preview · Article · Jun 2010 · Journal of Physics Condensed Matter
  • [Show abstract] [Hide abstract] 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.
    No preview · Article · Feb 2010 · ChemInform
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    X Lin · N Nilius · H-J Freund · M Walter · P Frondelius · K Honkala · H Häkkinen
    [Show abstract] [Hide abstract] ABSTRACT: The electronic structure of ultra-small Au clusters on thin MgO/Ag(001) films has been analyzed by scanning tunneling spectroscopy and density functional theory. The clusters exhibit two-dimensional (2D) quantum well states, whose shapes resemble the eigen-states of a 2D electron gas confined in a parabolic potential. From the symmetries of the HOMO and LUMO of a particular cluster, its electron filling and charge state is determined. In accordance to a DFT Bader-charge analysis, aggregates containing up to twenty 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.
    Full-text · Article · Jun 2009 · Physical Review Letters
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    S. Malola · H. Häkkinen · P. Koskinen
    [Show abstract] [Hide abstract] 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
    Full-text · Article · Sep 2008 · The European Physical Journal D
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    D. Koudela · A. -M. Uimonen · H. Häkkinen
    [Show abstract] [Hide abstract] 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
    Preview · Article · Aug 2008 · Physics of Condensed Matter
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    O. Lopez-Acevedo · D. Koudela · H. Häkkinen
    [Show abstract] [Hide abstract] 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.
    Full-text · Article · Jun 2008 · Physics of Condensed Matter
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    P Frondelius · H Häkkinen · K Honkala
    [Show abstract] [Hide abstract] 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.
    Full-text · Article · Sep 2007 · New Journal of Physics
  • H. Häkkinen · R. N Barnett · U. Landman
    [Show abstract] [Hide abstract] 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.
    No preview · Article · Jul 2007 · EPL (Europhysics Letters)
  • H. Häkkinen · S. Mäkinen · M. Manninen
    [Show abstract] [Hide abstract] 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.
    No preview · Article · Jul 2007 · EPL (Europhysics Letters)
  • H Häkkinen · M Manninen
    [Show abstract] [Hide abstract] 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.
    No preview · Article · Jan 2007 · Physica Scripta
  • S Mäkinen · H Häkkinen · M Manninen
    [Show abstract] [Hide abstract] 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.
    No preview · Article · Jan 2007 · Physica Scripta

Publication Stats

2k Citations
168.75 Total Impact Points


  • 1990-2012
    • University of Jyväskylä
      • • Nanoscience Center
      • • Department of Physics
      Jyväskylä, Province of Western Finland, Finland
  • 1995-2003
    • Georgia Institute of Technology
      • School of Physics
      Atlanta, GA, United States
  • 2001
    • University of Lausanne
      Lausanne, Vaud, Switzerland
  • 1994
    • University of Helsinki
      Helsinki, Uusimaa, Finland