Surface buckling of black phosphorus: Determination, origin, and influence on electronic structure

Zhongwei Dai, Wencan Jin, Jie-Xiang Yu, Maxwell Grady, Jerzy T. Sadowski, Young Duck Kim, James Hone, Jerry I. Dadap, Jiadong Zang, Richard M. Osgood, Jr., and Karsten Pohl

Phys. Rev. Materials 1, 074003 – Published 29 December 2017

Abstract

The surface structure of black phosphorus materials is determined using surface-sensitive dynamical microspot low energy electron diffraction ( $μ LEED$ ) analysis using a high spatial resolution low energy electron microscopy (LEEM) system. Samples of (i) crystalline cleaved black phosphorus (BP) at 300 K and (ii) exfoliated few-layer phosphorene (FLP) of about 10 nm thickness which were annealed at 573 K in vacuum were studied. In both samples, a significant surface buckling of 0.22 Å and 0.30 Å, respectively, is measured, which is one order of magnitude larger than previously reported. As direct evidence for large buckling, we observe a set of (for the flat surface forbidden) diffraction spots. Using first-principles calculations, we find that the presence of surface vacancies is responsible for the surface buckling in both BP and FLP, and is related to the intrinsic hole doping of phosphoresce materials previously reported.

Received 21 September 2017

DOI:https://doi.org/10.1103/PhysRevMaterials.1.074003

Physics Subject Headings (PhySH)

Band gap Crystal defects Crystal structure Crystal symmetry Defects Density of states Edge states Electronic structure Structural properties Surface & interfacial phenomena Surface instabilities Surface reconstruction

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

Zhongwei Dai^1,*, Wencan Jin^2,†, Jie-Xiang Yu¹, Maxwell Grady¹, Jerzy T. Sadowski³, Young Duck Kim^2,‡, James Hone², Jerry I. Dadap², Jiadong Zang¹, Richard M. Osgood, Jr.², and Karsten Pohl¹

¹Department of Physics and Materials Science Program, University of New Hampshire, Durham, New Hampshire 03824, USA
²Columbia University, New York, New York 10027, USA
³Center for Functional Nanomaterials, Brookhaven National Laboratory, Upton, New York 11973, USA

^*zbr5@wildcats.unh.edu
^†Current address: Department of Physics, University of Michigan, Ann Arbor, Michigan 48109, USA.
^‡Current address: Department of Physics, Kyung Hee University, Seoul 02447, Republic of Korea.

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Issue

Vol. 1, Iss. 7 — December 2017

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Images

Figure 1
(a)–(c) BP bulk crystal structure: 3D rendering, top view, and side view along the dashed line in (b), respectively. (d) Side view of BP and FLP relaxed surface structure, along dashed line in (b). Dotted square in (b) indicates the unit cell of BP, containing 8 P atoms.
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Figure 2
(a) LEEM image and (b) $μ LEED$ diffraction pattern of red-circled area in (a) taken at 30 eV electron energy of freshly cleaved BP crystal surface. (c) Optical, (d) PEEM, (e) LEEM, and (f) $μ LEED$ image of red-circled area in (e) taken at 30 eV electron energy of mechanically exfoliated flake of FLP, of about 10 nm thickness. (g) LEEM image and (h) $μ LEED$ diffraction pattern of an exfoliated flake after annealing at $370^{\circ} C$ , taken at 24 eV electron energy. Sharp diffraction pattern indicates that the surface is pristine and well ordered. An extra set of “forbidden spots,” the (10) beams denoted as C in (g), is clearly visible and unequivocal evidence of surface buckling on BP.
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Figure 3
(a)–(d) (00) and (01) low-electron energy diffraction beam $IV$ curves for cleaved BP crystal and exfoliated FLP flake, respectively. Red dotted curves are experimental and green solid curves are calculated using optimized surface structural parameters. (e), (f) Reliability $R_{2}$ factor plotted vs $b_{1}$ and $b_{2}$ for cleaved BP crystal and exfoliated FLP flake, respectively.
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Figure 4
Phosphorene atomic structure with defect introduced. Upper panels: Side and top view of an $n \times 4$ ( $n = 2$ , 4, 6, 8) supercell of the monolayer phosphorene with a point defect introduced at row 3. Blue and gray colors of balls distinguish the top and second P atomic layers. Lower panel: Average magnitude of buckling in each row for various $n \times 4$ supercells.
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Figure 5
Buckling and hole doping induced by defects. The DOS for (a) the ideal monolayer and (b) the $4 \times 4$ defect-included supercell BP. The Fermi level is set to zero. (c) Energy difference (blue solid squares) between the buckled and nonbuckled configurations and the magnitude of buckling (red open circles) with increasing hole-doping number. (d) Dependence of band gap on the buckling magnitude in monolayer (blue solid line) and bilayer (green dashed line) phosphorenes. The magnitude of the buckling is adjusted in single-layer phosphorene (1 ML) and the top bilayer of two-layer phosphorene (2 ML) from the range of 0 Å to 0.4 Å. The band gap of each structure is calculated accordingly and shown in (d).
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