We analyze the stability of the atomic configurations of tips in the adhesion process in noncontact atomic force microscopy (AFM) using a potential energy surface (PES). We calculate the PES for two types of atomic configurations of the tip as typical cases of reconstruction and irreversible change during the adhesion process. The stability of the tips after atomic contact with the surface is explained on the basis of the calculated PESs, which are affected by the strength of the atomic bond between the tip and the surface. It is shown from the computational model for the AFM that an unstable tip leads to a larger energy dissipation compared to that for a stable tip.

Received 25 April 2018Revised 18 June 2018DOI:https://doi.org/10.1103/PhysRevX.8.039902Published by the American Physical Society under the terms of the Creative Commons Attribution 3.0 License. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.Published by the American Physical SocietyPhysics Subject Headings (PhySH)Research AreasDefectsPhysical Systems2-dimensional systemsCondensed Matter, Materials & Applied Physics

We investigated the stability of ionic configurations of the tip of the cantilever in non-contact AFM.; For this, we used a computational model that couples the ionic motion of the MgO surface and the oscillating cantilever. The motion of ions was connected to the oscillating cantilever using a coupling method that had been recently developed. The adhesive process on the ionic MgO surface leads to energy dissipation of the cantilever. It is shown that limited types of ionic configurations of the tip are stable during the adhesive process. Based on the present computational model, we discuss the adhesive mechanism leading to energy dissipation.

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Electronic excitations can be efficiently analyzed in terms of the underlying Kohn-Sham (KS) electron-hole transitions. While such a decomposition is readily available in the linear-response time-dependent density-functional theory (TDDFT) approaches based on the Casida equations, a comparable analysis is less commonly conducted within the real-time-propagation TDDFT (RT-TDDFT). To improve this situation, we present here an implementation of a KS decomposition tool within the local-basis-set RT-TDDFT code in the free GPAW package. Our implementation is based on postprocessing of data that is readily available during time propagation, which is important for retaining the efficiency of the underlying RT-TDDFT to large systems. After benchmarking our implementation on small benzene derivatives by explicitly reconstructing the Casida eigenvectors from RT-TDDFT, we demonstrate the performance of the method by analyzing the plasmon resonances of icosahedral silver nanoparticles up to Ag561. The method provides a clear description of the splitting of the plasmon in small nanoparticles due to individual single-electron transitions as well as the formation of a distinct d-electron-screened plasmon resonance in larger nanoparticles.

Powered by TCPDF (www.tcpdf.org) This material is protected by copyright and other intellectual property rights, and duplication or sale of all or part of any of the repository collections is not permitted, except that material may be duplicated by you for your research use or educational purposes in electronic or print form. You must obtain permission for any other use. Electronic or print copies may not be offered, whether for sale or otherwise to anyone who is not an authorised user. Hakala, M. H.; Foster, A. S.; Gavartin, J.L.; Havu, P.; Puska, M. J.; Nieminen, R. M.

The influence of magnetic ordering on the stability of Ni–Mn–Ga(–Co–Cu) Heusler alloys is investigated using the first-principles exact muffin-tin orbital method in combination with the coherent-potential approximation. The paramagnetic (PM) state is described by disordered local moment approach. In stoichiometric Ni 2 MnGa alloy, the total energy profile along the tetragonal deformation path differs between ferromagnetic (FM) ground state and PM state with high energy, where cubic structure of austenite exhibits lower total energy than tetragonally distorted structure of martensite. Martensitic structure is stabilized in ground state by FM interaction. In PM state it can be stabilized by partial substitution of Ni by Co or by partial substitution of Mn/Ga by Cu. Energy difference between PM and FM state $\Delta E_{\mathrm {PM{-}FM}}$ can be used for qualitative estimation of Curie temperature $T_{C}$ . Since Co doping to Ni sublattice slightly increases $\Delta E_{\mathrm {PM{-}FM}}$ , the $T_{C}$ should also increase, which corresponds to experimental findings. Analogically, Cu doping to Mn sublattice strongly decreases $\Delta E_{\mathrm {PM{-}FM}}$ , which corresponds to strong decrease of $T_{C}$ , also confirmed experimentally. For Cu doping in Ga sublattice the decrease in $T_{C}$ is weaker.

Finite MoS2 nanoparticles are known to support metallic edge states that are responsible for their catalytic activity. In this work we employ time-dependent density-functional theory (TDDFT) to study the influence of such edge states on the optical properties of triangular MoS2 monolayer flakes. We find that the edge states support collective plasmon-like excitations that couple strongly to the optical field leading to pronounced absorption peaks below the onset of interband transitions on the basal plane. Additionally, structural relaxation of the flakes can significantly distort the edge states. Thus, we observe that while an evenly-spaced edge configuration supports one-dimensional (1D) plasmon modes similar to those of an ideal 1D electron gas, the relaxed structures show mixed plasmon and single-electron excitations in the low-energy response. Our findings illustrate the sensitivity of the optical response of MoS2 nanostructures to the details of the edge configuration.

We investigate how different interface geometries of an Al/Al2O3 junction, a common component of modern tunnel devices, affect electron transport through the tunnel barrier. We study six distinct Al/Al2O3 interfaces which differ in stacking sequences of the metal and the oxide surface atoms and the oxide termination. To construct model potential barrier profiles for each examined geometry, we rely on first-principles density-functional theory (DFT) calculations for the barrier heights and the shapes of the interface regions as well as on experimental data for the barrier widths. We show that even tiny variations in the atomic arrangement at the interface cause significant changes in the tunnel barrier parameters and, consequently, in electron transport properties. Especially, we find that variations in the crucial barrier heights and widths can be as large as 2 eV and 5 angstrom, respectively. Finally, to gain information about the average properties of the measured junction, we fit the conductance calculated within the Wentzel-Kramers-Brillouin approximation to the experimental data and interpret the fit parameters with the help of the DFT results.

Rights: © 2002 American Physical Society (APS). This is the accepted version of the following article: Lee, Young Joo & von Boehm, J. & Pesola, M. & Nieminen, Risto M. 2002. First-principles study of migration, restructuring, and dissociation energies of oxygen complexes in silicon. Physical Review B. Volume 65, Issue 8. 085205/1-12. ISSN 1550-235X (electronic). DOI: 10.1103/physrevb.65.085205, which has been published in final form at http://journals.aps.org/prb/abstract/10.1103/PhysRevB.65.085205.

Rights: © 2002 American Physical Society (APS). This is the accepted version of the following article: Lee, Y. J. & Pesola, M. & von Boehm, J. & Nieminen, Risto M. 2002. Local vibrations of thermal double donors in silicon. Physical Review B. Volume 66, Issue 7. 075219/1-4. ISSN 1550-235X (electronic). DOI: 10.1103/physrevb.66.075219, which has been published in final form at http://journals.aps.org/prb/abstract/10.1103/PhysRevB.66.075219.

Rights: © 1996 American Physical Society (APS). This is the accepted version of the following article: von Boehm, Juhani & Nieminen, Risto M. 1996. Molecular-dynamics study of partial edge dislocations in copper and gold: Interactions, structures, and self-diffusion. Physical Review B. Volume 53, Issue 14. 8956-8966. ISSN 1550-235X (electronic). DOI: 10.1103/physrevb.53.8956, which has been published in final form at http://journals.aps.org/prb/abstract/10.1103/PhysRevB.53.8956.

Rights: © 2013 American Physical Society (APS). This is the accepted version of the following article: Berseneva, Natalia & Gulans, Andris & Krasheninnikov, Arkady V. & Nieminen, Risto M. 2013. Electronic structure of boron nitride sheets doped with carbon from first-principles calculations. Physical Review B. Volume 87, Issue 3. 035404/1-9. ISSN 1550-235X (electronic). DOI: 10.1103/physrevb.87.035404, which has been published in final form at http://journals.aps.org/prb/abstract/10.1103/PhysRevB.87.035404.

Rights: © 2009 American Physical Society (APS). This is the accepted version of the following article: Krasheninnikov, A. V. & Lehtinen, P. O. & Foster, A. S. & Pyykkö, P. & Nieminen, Risto M. 2009. Embedding Transition-Metal Atoms in Graphene: Structure, Bonding, and Magnetism. Physical Review Letters. Volume 102, Issue 12. 126807/1-4. ISSN 0031-9007 (printed). DOI: 10.1103/physrevlett.102.126807, which has been published in final form at http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.102.126807.

Rights: © 2009 American Physical Society (APS). This is the accepted version of the following article: Gulans, Andris & Puska, Martti J. & Nieminen, Risto M. 2009. Linear-scaling self-consistent implementation of the van der Waals density functional. Physical Review B. Volume 79, Issue 20. 201105/1-4. ISSN 1550-235X (electronic). DOI: 10.1103/physrevb.79.201105, which has been published in final form at http://journals.aps.org/prb/abstract/10.1103/PhysRevB.79.201105.

Rights: © 2007 American Physical Society (APS). This is the accepted version of the following article: Foster, A. S. & Gosálvez, M. A. & Hynninen, T. & Nieminen, Risto M. & Sato, K. 2007. First-principles calculations of Cu adsorption on an H-terminated Si surface. Physical Review B. Volume 76, Issue 7. 075315/1-8. ISSN 1550-235X (electronic). DOI: 10.1103/physrevb.76.075315, which has been published in final form at http://journals.aps.org/prb/abstract/10.1103/PhysRevB.76.075315.