Engineering Polarons at a Metal Oxide Surface

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Engineering Polarons at a Metal Oxide Surface

C. M. Yim, M. B. Watkins, M. J. Wolf, C. L. Pang, K. Hermansson, and G. Thornton

Phys. Rev. Lett. 117, 116402 – Published 9 September 2016

Abstract

Polarons in metal oxides are important in processes such as catalysis, high temperature superconductivity, and dielectric breakdown in nanoscale electronics. Here, we study the behavior of electron small polarons associated with oxygen vacancies at rutile ${TiO}_{2} (110)$ , using a combination of low temperature scanning tunneling microscopy (STM), density functional theory, and classical molecular dynamics calculations. We find that the electrons are symmetrically distributed around isolated vacancies at 78 K, but as the temperature is reduced, their distributions become increasingly asymmetric, confirming their polaronic nature. By manipulating isolated vacancies with the STM tip, we show that particular configurations of polarons are preferred for given locations of the vacancies, which we ascribe to small residual electric fields in the surface. We also form a series of vacancy complexes and manipulate the Ti ions surrounding them, both of which change the associated electronic distributions. Thus, we demonstrate that the configurations of polarons can be engineered, paving the way for the construction of conductive pathways relevant to resistive switching devices.

Received 4 February 2016

DOI:https://doi.org/10.1103/PhysRevLett.117.116402

This article is available 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 Society

Physics Subject Headings (PhySH)

Local density of states Point defects Polarons

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

C. M. Yim¹, M. B. Watkins², M. J. Wolf^3,4, C. L. Pang¹, K. Hermansson⁴, and G. Thornton^1,*

¹Department of Chemistry and London Centre for Nanotechnology, University College London, 20 Gordon Street, London WC1H 0AJ, United Kingdom
²School of Mathematics and Physics, University of Lincoln, Brayford Pool, Lincoln LN6 7TS, United Kingdom
³Department of Physics & Astronomy and London Centre for Nanotechnology, University College London, Gower Street, London WC1E 6BT, United Kingdom
⁴Department of Chemistry, Ångström Laboratory, Uppsala University, Box 538, S-751 21 Uppsala, Sweden

^*Corresponding author. g.thornton@ucl.ac.uk

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Issue

Vol. 117, Iss. 11 — 9 September 2016

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Images

Figure 1
$3.2 \times 3.2 {nm}^{2}$ representative STM images of isolated $O_{b}$ -vacs on ${TiO}_{2} (110)$ recorded using (a) positive (empty states) and (b)–(f) negative (filled states) sample bias at different temperatures: (a),(b) 78 K, (c) 16 K, and (d)–(f) 7 K. Filled-state images in (b)–(f) were recorded in the vicinity of different isolated $O_{b}$ -vacs. Intersections of the white grids mark the positions of the ${O_{b}}^{2 -}$ . Rectangles are drawn around the $O_{b}$ -vacs and mark the same areas between the empty- and filled-state images in (a),(b). Scan parameters: (a) $(V_{S}, I_{T}) = + 0.9 V$ , 20 pA; (b) $- 1.1 V$ , 5 pA; (c) $- 2 V$ , 1.6 pA; and (d)–(f) $- 2 V$ , 0.3 pA.
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Figure 2
STM images of an individual $O_{b}$ -vac of ${TiO}_{2} (110)$ taken (a),(d) before manipulation, (b),(e) after the $O_{b}$ -vac was displaced along the $O_{b}$ row by a series of $+ 4 V$ , 6 s tip pulses, and (c),(f) after the $O_{b}$ -vac was returned to its initial position by tip pulses. Black and pink open circles mark the initial and current positions of the $O_{b}$ -vac. Intersections of the white grids mark the positions of ${O_{b}}^{2 -}$ . All images were taken at 7 K. Scan parameters: (a)–(c) $(V_{S}, I_{T}) = + 1.0 V$ , 10 pA and (d)–(f) $- 2.0 V$ , 0.3 pA.
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Figure 3
Calculation of individual $O_{b}$ -vacs on ${TiO}_{2} (110)$ . (a) Calculated structure of a single $O_{b}$ -vac and one of its ground-state excess electron configurations on rutile ${TiO}_{2} (110)$ . Turquoise spheres are sites of the removed bridging oxygen ions ( ${O_{b}}^{2 -}$ ). Small pink and red spheres are the Ti and O ions, respectively. Highlighted in blue is an isosurface of the excess electron density at $10^{- 3} electrons / Å^{3}$ . (b)–(f) Simulated STM images of individual $O_{b}$ -vacs of ${TiO}_{2} (110)$ with four nonequivalent polaron configurations. The excess electrons, visualized as red dumbbells in the filled-state images in (c)–(f), are located at different Ti sites beneath the $O_{b}$ -vacs. Relative energies after ionic relaxation are indicated. Open circles mark the positions of $O_{b}$ -vacs. Intersections of the white grids mark the positions of the ${O_{b}}^{2 -}$ .
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Figure 4
Experimental STM images of $O_{b}$ -vac multimers obtained simultaneously at positive and negative sample biases. The $O_{b}$ -vac dimer in (a) and (b), cross-dumbbell trimer in (c) and (d), and tetramer in (i) and (j) were formed using a series of $+ 4 V$ , 6 s tip pulses at 7 K. The mushroom trimers in (e)–(h) were formed by applying $+ 3.75 V$ , 0.2 s tip pulses over a cross-dumbbell trimer at 78 K. The intersections of the white grids mark the positions of ${O_{b}}^{2 -}$ . Circles mark the removed ${O_{b}}^{2 -}$ . Scan parameters: (a),(c),(i) $(V_{S}, I_{T}) = + 1.0 V$ , 10 pA, (b),(d),(j) $- 2.0 V$ , 0.3 pA, (e),(g) $+ 0.7 V$ , 10 pA, and (f),(h) $- 1.0 V$ , 1.6 pA. STM images in (a)–(d) and (i) and (j) were taken at 7 K, those in (e)–(h) taken at 78 K.
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