Novel ${\mathrm{Pd}}_{2}{\mathrm{Se}}_{3}$ Two-Dimensional Phase Driven by Interlayer Fusion in Layered ${\mathrm{PdSe}}_{2}$

Novel ${Pd}_{2} {Se}_{3}$ Two-Dimensional Phase Driven by Interlayer Fusion in Layered ${PdSe}_{2}$

Junhao Lin, Sebastian Zuluaga, Peng Yu, Zheng Liu, Sokrates T. Pantelides, and Kazu Suenaga

Phys. Rev. Lett. 119, 016101 – Published 6 July 2017

Abstract

Two-dimensional (2D) materials are easily fabricated when their bulk form has a layered structure. The monolayer form in layered transition-metal dichalcogenides is typically the same as a single layer of the bulk material. However, ${PdSe}_{2}$ presents a puzzle. Its monolayer form has been theoretically shown to be stable, but there have been no reports that monolayer ${PdSe}_{2}$ has been fabricated. Here, combining atomic-scale imaging in a scanning transmission electron microscope and density functional theory, we demonstrate that the preferred monolayer form of this material amounts to a melding of two bulk monolayers accompanied by the emission of Se atoms so that the resulting stoichiometry is ${Pd}_{2} {Se}_{3}$ . We further verify the interlayer melding mechanism by creating Se vacancies in situ in the layered ${PdSe}_{2}$ matrix using electron irradiation. The discovery that strong interlayer interactions can be induced by defects and lead to the formation of new 2D materials opens a new venue for the exploration of defect engineering and novel 2D structures.

Received 10 February 2017

DOI:https://doi.org/10.1103/PhysRevLett.119.016101

Physics Subject Headings (PhySH)

Surface reconstruction

2-dimensional systems Monolayer films

Density functional theory Scanning transmission electron microscopy

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

Junhao Lin^1,*, Sebastian Zuluaga², Peng Yu³, Zheng Liu^3,4, Sokrates T. Pantelides^2,5, and Kazu Suenaga^1,6,†

¹National Institute of Advanced Industrial Science and Technology (AIST), AIST Central 5, Tsukuba 305-8565, Japan
²Department of Physics and Astronomy, Vanderbilt University, Nashville, Tennessee 37235, USA
³Centre for Programmed Materials, School of Materials Science and Engineering, Nanyang Technological University, Singapore 639798, Singapore
⁴NOVITAS, Nanoelectronics Centre of Excellence, School of Electrical and Electronic Engineering, Nanyang Technological University, Singapore 639798, Singapore
⁵Department of Electrical Engineering and Computer Science, Vanderbilt University, Nashville, Tennessee 37235, USA
⁶Department of Mechanical Engineering, The University of Tokyo, Tokyo 113-8656, Japan

^*To whom all correspondence should be addressed. lin.junhao@aist.go.jp
^†suenaga-kazu@aist.go.jp

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Vol. 119, Iss. 1 — 7 July 2017

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Figure 1
Exfoliation of layered ${PdSe}_{2}$ sample. (a) Schematic of the expected exfoliation of monolayer ${PdSe}_{2}$ from its bulk form. The atomic structural model is shown with the Se-Se dumbbell highlighted as the inset. (b) High resolution ADF STEM images showing a thin few-layer ${PdSe}_{2}$ region. The number of the layers can be identified by the difference of the image contrast. The inset is the FFT pattern of the whole image.
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Figure 2
Atomic resolution STEM images of the reconstructed monolayer ${Pd}_{2} {Se}_{3}$ and few-layer ${PdSe}_{2}$ . [(a)–(d)] High resolution experimental (grey) and simulated (yellow) ADF STEM image of reconstructed ${Pd}_{2} {Se}_{3}$ monolayer (a), the hypothetical ${PdSe}_{2}$ monolayer (b, only simulated, not observed in experiment), ${PdSe}_{2}$ bilayer (c), and ${PdSe}_{2}$ trilayer (d). The FFT of the ${Pd}_{2} {Se}_{3}$ monolayer and ${PdSe}_{2}$ bilayer region are displayed in the left-hand side of panels (a) and (c), respectively. The FFT indicates that even though the two systems have different lattice symmetry, they have similar lattice parameters. The principle diffraction planes of interest are highlighted by yellow circles. The unit cells of each structure are highlighted with red dashed squares.
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Figure 3
Interlayer fusion mechanism proposed by DFT calculations. (a) Schematic of the reconstruction mechanism from bilayer ${PdSe}_{2}$ to monolayer ${Pd}_{2} {Se}_{3}$ . The red and blue dashed squares represent the square Pd networks from the two layers before merging, and the green one displays the newly fused Pd squared network in the same layer after merging. The Se atoms are not displayed. (b) Interlayer distance as a function of chalcogen vacancy concentration in bilayer ${PdSe}_{2}$ (red) and ${MoS}_{2}$ (green). In ${PdSe}_{2}$ the interlayer distance shows a clear decreasing trend as the vacancy concentration increased, while the change is negligible in ${MoS}_{2}$ . The inset shows the energy landscape of each step of the interlayer fusion process.
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Figure 4
In situ observation of the reconstruction process from ${PdSe}_{2}$ bilayer to ${Pd}_{2} {Se}_{3}$ monolayer. [(a) and (b)] Sequential ADF STEM images showing the examples of the growth of the monolayer ${Pd}_{2} {Se}_{3}$ extending to the few-layer ${PdSe}_{2}$ matrix as assisted by electron irradiation, with the same crystal orientation as the parent ${PdSe}_{2}$ (direct lattice fusion, a) and with a misorientation angle of $\sim 37 °$ (fusion and realign, b). The time scale is indicated in both sets of the images. The blue crossings indicate the orientation of the squared Pd network.
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Physical Review Letters

Novel Pd2Se3 Two-Dimensional Phase Driven by Interlayer Fusion in Layered PdSe2

Junhao Lin, Sebastian Zuluaga, Peng Yu, Zheng Liu, Sokrates T. Pantelides, and Kazu Suenaga

Phys. Rev. Lett. 119, 016101 – Published 6 July 2017

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Novel ${Pd}_{2} {Se}_{3}$ Two-Dimensional Phase Driven by Interlayer Fusion in Layered ${PdSe}_{2}$