Self-assembly of nanoscale lateral segregation profiles

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Self-assembly of nanoscale lateral segregation profiles

R. Stania, W. Heckel, I. Kalichava, C. Bernard, T. C. Kerscher, H. Y. Cun, P. R. Willmott, B. Schönfeld, J. Osterwalder, S. Müller, and T. Greber

Phys. Rev. B 93, 161402(R) – Published 7 April 2016

Abstract

The surface segregation profile of an intermetallic compound becomes vertically and laterally modulated upon epitaxial growth of a single-layer hexagonal boron nitride $(h$ -BN) nanomesh. $h$ -BN on PtRh(111) forms an 11-on-10 superhoneycomb, such as that on Rh(111) [Corso et al., Science 303, 217 (2004)], though with a smaller lattice constant of 2.73 nm. X-ray photoelectron diffraction shows that the $h$ -BN layer reduces the Pt enrichment of the first layer by promoting site swapping of about 10 Pt-Rh pairs within the $10 \times 10$ unit cell between the first and second layers. This segregation profile is confirmed by density-functional-theory-based cluster-expansion calculations. Generally, a strong modulation of the $h$ -BN bonding strength and a higher affinity to one of the constituents leads to self-assembly of top layer patches underneath the nanomesh pores.

Received 27 November 2015
Revised 26 January 2016

DOI:https://doi.org/10.1103/PhysRevB.93.161402

Physics Subject Headings (PhySH)

Granular flows

2-dimensional systems Alloys Metals

Density functional theory X-ray photoelectron diffraction

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

R. Stania^1,2,*, W. Heckel³, I. Kalichava², C. Bernard¹, T. C. Kerscher³, H. Y. Cun¹, P. R. Willmott², B. Schönfeld⁴, J. Osterwalder¹, S. Müller³, and T. Greber¹

¹Physik-Institut, Universität Zürich, Winterthurerstrasse 190, 8057 Zürich, Switzerland
²Paul Scherrer Institut, 5232 Villigen PSI, Switzerland
³Hamburg University of Technology, Denickestraße 15, 21073 Hamburg, Germany
⁴Laboratory of Metal Physics and Technology, Department of Materials, ETH Zürich, 8093 Zürich, Switzerland

^*Corresponding author: rostania@physik.uzh.ch

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Vol. 93, Iss. 16 — 15 April 2016

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Images

Figure 1
Mg $K α$ excited Rh $3 d$ and Pt $4 d_{5 / 2}$ x-ray photoelectron spectra of bare PtRh at (a) normal, (b) grazing and $h$ -BN/PtRh at (c) normal and (d) grazing emission angles. The spectra are normalized to the Rh $3 d_{5 / 2}$ intensity. The relative increase of the Pt $4 d_{5 / 2}$ signal at 315 eV in going from normal to grazing emission indicates Pt surface segregation. The decrease of the grazing Pt signal for $h$ -BN/PtRh indicates $h$ -BN-induced Rh segregation to the top layer. The pie diagrams represent the Pt (blue) and the Rh (red) atomic concentrations for the respective probing depths.
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Figure 2
Identification of the nanomesh superstructure of $h$ -BN/PtRh(111). (a) Low-energy electron diffraction (LEED) pattern $(E = 70$ eV). The $1 \times 1$ PtRh(111) principal spots are surrounded by $h$ -BN-induced superstructure spots. (b) Surface x-ray diffraction (SXRD) along the $h$ direction showing the crystal truncation rod and $h$ -BN-derived 11/10 and 9/10 superstructure rods. (c) Normal emission angular resolved He $I α$ spectra for bare PtRh (black) and $h$ -BN/PtRh(111) (red). The spectra are normalized to the intensity of the valence band peak at $\sim 1$ eV. The dashed lines indicate the positions of the $σ$ bands of the $h$ -BN layer corresponding to the pores $(α)$ or wires $(β)$ as shown in the inset. (d) Scanning tunneling microscopy images taken in constant current mode $(150 \times 150$ ${nm}^{2}, I_{t} = 1$ nA, $U_{t} = 5$ mV, drift corrected). The inset $(3.7 \times 3.7 {nm}^{2}, I_{t} = 1.5$ nA, $U_{t} = 0.5$ mV) reveals, after subtraction of the corrugation, the superhoneycomb unit cell with atomic resolution.
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Figure 3
Mg $K α$ excited x-ray photoelectron diffraction (XPD) patterns for Rh $3 d_{5 / 2}$ $(E_{kin} = 939.8$ eV) and Pt $4 f_{7 / 2}$ $(E_{k i n} = 1175.5$ eV) emission. (a) Rh bare, (b) Pt bare, (c) Rh $h$ -BN, and (d) Pt $h$ -BN. (e) Cross-ratio $X$ [Eq. (1)]. (f) Polar dependence of $X$ on the azimuths containing $〈 110 〉$ and $〈 001 〉$ (solid line) and the azimuthal average (open circles). The red line is $X$ as obtained from the Pt profiles in (g). (g) Pt concentrations as a function of the layer number. Note the swap of about 10% between the first and second layers.
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Figure 4
Cluster-expansion energies $(+)$ of approximately 13 000 different substrate structures each for the bare PtRh(111) surface (top left), for one of the $h$ -BN wire regions (bottom left), and for the $h$ -BN pore region (bottom right) as a function of Pt concentration. Information about the segregation profile in the two topmost substrate layers is color-coded by the number of Pt atoms per layer. The filled areas code for the topmost layer, and the colored $+$ for the second layer (blue denoting an all-Pt, red an all-Rh layer). The energy scale in all plots is given with respect to the bare case. The top right panel summarizes the lateral segregation scenario where the first two PtRh layers in the unit cell are represented by chains of 10 atoms.
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