Electric-field-controlled reversible order-disorder switching of a metal tip surface

Ludvig de Knoop, Mikael Juhani Kuisma, Joakim Löfgren, Kristof Lodewijks, Mattias Thuvander, Paul Erhart, Alexandre Dmitriev, and Eva Olsson

Phys. Rev. Materials 2, 085006 – Published 22 August 2018

Abstract

While it is well established that elevated temperatures can induce surface roughening of metal surfaces, the effect of a high electric field on the atomic structure at ambient temperature has not been investigated in detail. Here we show with atomic resolution using in situ transmission electron microscopy how intense electric fields induce reversible switching between perfect crystalline and disordered phases of gold surfaces at room temperature. Ab initio molecular dynamics simulations reveal that the mechanism behind the structural change can be attributed to a vanishing energy cost in forming surface defects in high electric fields. Our results demonstrate how surface processes can be directly controlled at the atomic scale by an externally applied electric field, which promotes an effective decoupling of the topmost surface layers from the underlying bulk. This opens up opportunities for development of active nanodevices in, e.g., nanophotonics and field-effect transistor technology as well as fundamental research in materials characterization and of yet unexplored dynamically controlled low-dimensional phases of matter.

Received 26 October 2017
Revised 11 June 2018

DOI:https://doi.org/10.1103/PhysRevMaterials.2.085006

Physics Subject Headings (PhySH)

Crystal structure Metals Phase transitions Roughness Surface & interfacial phenomena

Nanoparticles Nanowires Single crystal materials

Density functional theory Electron diffraction High-resolution transmission electron microscopy Molecular dynamics Scanning probe microscopy

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

Ludvig de Knoop^1,*, Mikael Juhani Kuisma^1,2, Joakim Löfgren¹, Kristof Lodewijks¹, Mattias Thuvander¹, Paul Erhart^1,†, Alexandre Dmitriev^1,3,4,‡, and Eva Olsson^1,§

¹Department of Physics, Chalmers University of Technology, 412 96 Gothenburg, Sweden
²Department of Chemistry, University of Jyväskylä, 40014 Jyväskylä, Finland
³Department of Physics, University of Gothenburg, 412 96 Gothenburg, Sweden
⁴Geballe Laboratory for Advanced Materials, Stanford University, Stanford, California 94305-4045, USA

^*ludvig.deknoop@chalmers.se
^†erhart@chalmers.se
^‡alexd@physics.gu.se
^§eva.olsson@chalmers.se

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Vol. 2, Iss. 8 — August 2018

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Images

Figure 1
Experimental setup and results of electric-field-induced order-disorder switching of atomic layers. (a) Schematic of parts of the in situ TEM sample holder with the nanomanipulator and the corresponding electrical circuit. The large inset shows a TEM micrograph of the investigated Au nanocones on a carbon (C) film. (b), (c) 10 nm of the apex of one of the nanocones as frames extracted from In Situ TEM Movie 2 in Supplemental Material [27]. The disordered phase was switched off and on, respectively, by applying a voltage $V$ of (b) 38 V resulting in an electric field $F$ of around 12 V/nm and (c) $V = 80 V$ and $F \approx 25 V / nm$ .
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Figure 2
Evolution of a Au nanocone apex under an electron beam and an intense electric field, from ordered to disordered to field-evaporated phase. (a) Number of atomic layers removed as a function of time $t$ . (b)–(j) Extracted frames from In Situ TEM Movies 1 and 2 in Supplemental Material [27], corresponding to the points labeled b–j in (a). In the first and crystalline regime [orange, (b)–(d)], no electrical bias was applied and the electron beam was first turned on, then off, and then turned on again (note the truncated $x$ axis); in the second and disordered regime [red, (e)–(g)], the electron beam was on and a bias from 68 to 80 V was applied; in the third regime [blue, (h)–(j)], the electron beam was on and the external bias was increased until field evaporation occurred (102–140 V). The error bars in (a), with an average height of 0.9 atomic layers, represent the uncertainty in measuring the number of atomic layers. $V$ , $F$ , and $r$ denote the applied voltage, the corresponding electric field, and the tip radius, respectively. The red dotted lines in (b)–(j) mark points of reference. The scale bar in (b) also applies to (c)–(j).
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Figure 3
Reversibility, from disordered to ordered during voltage decrease. The apex of a Au nanocone (different from the nanocone in Figs. 1 and 2) with tip radius of 2.5 nm and distance to cathode of 58 nm at different voltages. (a) The outmost layers are in a disordered phase, at an applied bias $V$ of 78 V and an electric field $F$ of around 28 V/nm. (b) 27 s after (a), with $V = 49 V$ and $F \approx 17 V / nm$ , some of the outmost layers remain in a disordered phase and in (c), 7 s later, the nanocone has reverted to a crystalline phase, at a bias of 40 V and an electric field of around 14 V/nm.
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Figure 4
Ab initio simulations of the crystalline phase, the disordered phase, and of field evaporation. (a) Effective surface area of a gold nanoparticle at 360 K as a function of the electric field. (b) Excess charge distribution (top) and shape (bottom) for three gold nanoparticles representative of the three stages indicated in (a). The color scale indicates the accumulation (light blue, maximum of $0.05 electrons / Å^{3}$ ) and depletion (cerise, minimum of $- 0.05 electrons / Å^{3}$ ) of electronic charge. (c) Approximate phase diagram showing the temperature dependence of the electric fields at which disordering (II) and field evaporation (III) occur.
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