Chemically Functionalized Phosphorene: Two-Dimensional

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Chemically-Functionalized Phosphorene: Two-dimensional Multiferroics with Vertical Polarization and Mobile Magnetism Qing Yang, Wei Xiong, Lin Zhu, Guoying Gao, and Menghao Wu J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.7b04422 • Publication Date (Web): 26 Jul 2017 Downloaded from http://pubs.acs.org on July 27, 2017

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Journal of the American Chemical Society

Chemically-Functionalized Phosphorene: Two-dimensional Multiferroics with Vertical Polarization and Mobile Magnetism Qing Yang1, Wei Xiong2, Lin Zhu1, Guoying Gao1, Menghao Wu1* 1

School of Physics and Wuhan National High Magnetic Field Center, Huazhong University of Science and Technology, Wuhan, Hubei, China 430074 2

Wuhan National Laboratory for Optoelectronics, Huazhong University of Science and Technology, Wuhan, Hubei, China 430074

Abstract In the future nanocircuits based on two-dimensional (2D) materials, the ideal nonvolatile memory would be based on 2D multiferroic materials that can combine both efficient ferroelectric writing and ferromagnetic reading, which remains hitherto unreported. Here we show first-principles evidences that halogen-intercalated phosphorene bilayer can be multiferroic with most long-sought advantages: their “mobile” magnetism can be controlled by ferroelectric switching upon external electric field, exhibiting either “on” state with spin-selective and highly p-doped channels, or “off” state insulating for both spin and electron transport, which renders efficient electrical writing and magnetic reading; vertical polarization can be maintained against depolarizing field, rendering high-density data storage possible; moreover, all those functions in the halogenated regions can be directly integrated into a 2D phosphorene wafer, like n/p channels by doping in a silicon wafer. Such formation of multiferroics with vertical polarization robust against depolarizing field can be attributed to the unique properties of covalent-bonded ferroelectrics distinct from ionic-bonded ferroelectrics, which may be extended to other van der Waals bilayer for design of non-volatile memory in future 2D wafers. Every intercalated adatom can be used to store one bit of data: “0” when binding to the down layer and “1” upon when binding to the up layer, giving rise to a possible approach of realizing single atom memory for highdensity data storage.

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1. INTRODUCTION Two-dimensional (2D) materials with atomic thickness and high mobility like graphene1 and transitionmetal dichalcogenide (TMDC)2 can be promising candidates for replacing silicon as future basic materials for nanoelectronics. The problems like quantum tunneling and gate leakage for traditional transistor at nanoscale, which has already lead to the end of Moore’s law, are likely to be solved in 2D materials. Phosphorene, monolayer of black phosphorous, has become a new family member of two-dimensional materials since 20143, 4. It is known that graphene possess ultra-high mobility but with a zero bandgap, while MoS2 has a suitable bandgap for semiconductors but with only a moderate mobility. In comparison, phosphorene possesses a moderate direct bandgap around 0.6~1.5eV depending on the number of layers and can be tuned via strain5. Moreover, it can also exhibit a high mobility up to 1000 cm2 V-1 S-1 and high on/off ratio up to 104 when applied as a field-effect transistor, which is highly desirable in nanoelectronic devices3. Interesting properties in strain-engineering have been predicted theoretically: superior mechanical flexibility6, strain-tunable bandgap5, 7, 8, phase transition upon strain9, 10

, ferroelasticity11, etc. Ionic (Li, Na) transport and covalently functionalizations in phosphorene have

also been studied both theoretically and experimentally12-17. In this paper, it will be revealed that halogen anions may transport on the surface of phosphorene at ambient condition, which may be used as magneto-electric devices since those covalently bonded anions carry both magnetic moments and charges. It is known that ferromagnetic (FM) materials exhibiting switchable magnetizations and ferroelectric (FE) materials exhibiting switchable electrical polarizations both possess potential applications as nonvolatile memories (NVMs). For their commercially random access memories (RAMs), data writing in FM RAMs is energy-consuming while reading operation in FE RAMs is destructive, so multiferroic materials18 which are both FM and FE19 and may combine efficient writing by electrical field and energy-saving


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reading by magnetism are highly desirable. Their rare existence in nature may be due to the confliction that most FM are metallic while FE must be insulating: almost all the multiferroic materials known to date are antiferromagnetic or ferrimagnetic, except EuTiO3 that will become FM upon large strain20. Additionally, they lack vertical polarizations in thin-films for high-density data storage, due to the effect of surface depolarization field. Currently neither FM nor FE RAMs have commercially substituted flash memory as the most prevailing NVMs, even though flash memory cannot offer arbitrary random-access rewrite or erase operation, in addition to memory wear and read disturb21. The major reason is siliconbased flash memory can be directly integrated in silicon wafer utilizing mature silicon process. In comparison, the combination of FE or FM RAMs into silicon is technically challenged by many problems. Similar to n/p doping channels in silicon wafer, ferroics induced by dopants will be desirable for formation of NVM regions. However, despite intensive research devoted to diluted magnetic semiconductors trying to incorporate FM into semiconductors, their Curie temperature can scarcely reach room-temperature22; efforts on combining FE with semiconductors have been scarcely reported, as FE cannot be induced simply by doping 3d metal ions. With the downscaling of integrated circuits size to nanoscale, however, flash memories as well as FM and FE RAMs need to be redesigned to be incorporated into circuits based on 2D materials with atomic thickness. The issue of quantum tunneling in flash memories due to non-degenerate “0” and “1” state will be difficult to solve in nanoscale, which may be a chance for FM or FE RAMs with two equivalent states. However, doping 3d magnetic ions in 2D materials like graphene will be much more difficult than in traditional semiconductors. Meanwhile the number of reports on 2D FE materials is increasing but still limited

11, 23-28

since the first related prediction in 201329, especially for those with vertical polarization

desirable for high density data storage30. Here we show the first-principles evidence that halogendecorated phosphorene systems can meet all those challenges, which are 2D multiferroic metal-free materials combining most advantages simultaneously: efficient FE writing + FM reading; vertical



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polarization for high density data storage; directly integration in 2D wafer. It is also revealed that vertical polarization in 2D will be more likely to survive in covalent-bonded ferroelectrics compared with ionicbonded ferroelectrics, which may be extended to other 2D materials as NVMs.

2. COMPUTATIONAL DETAILS The calculations were performed by using the spin-polarized density-functional-theory (DFT) in the generalized gradient approximation (GGA) implemented by the VASP code31, 32, in which the Perdew– Burke–Ernzerhof (PBE)33 exchange-correlation functional and the projector augmented wave (PAW)34 formalism are employed. The layered structures are placed in the xy plane, and a large vacuum region with a thickness of 15 Å is added in z direction. The plane-wave cutoff is set as 400 eV and the Brillouinzone integration of the supercell is sampled with 7× 7 × 1 Monkhorst–Pack grid. A semi-empirical correction using Grimme method35 is applied to take van der Waals interaction into account. The nudged elastic band (NEB) method36 was used for determining the migration paths and diffusion energy barrier.

3. RESULTS AND DISCUSSION 3.1 Intralayer Magnetoelectrics


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Figure 1 (a)Geometric structure and spin density distribution (in blue color) for phosphorene (P atoms denoted by pink spheres) bonded with a Cl adatom (denoted by green spheres). (b) Illustrations of migration directions for halogen adatoms. (c)The computed diffusion pathway for Cl adatom along the armchair and zigzag direction. (d)Comparison of diffusion pathway for –F, -Cl and -Br adatom along the zigzag direction.



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Eb (eV/atom)

Charge (e)

Monolayer

diffusion

barrier Bilayer

(eV)

(eV)

F

-2.0

-0.221

0.72

0.59

Cl

-0.20

-0.222

0.39

0.19

Br

-0.022

-0.224

0.29

0.074

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diffusion

barrier

Table 1 Binding energies compared with X2 (defined as Eb=E(phosphorene-X)-E(phosphorene)-E(X2)/2, and negative values indicate to be favorable in energy) , Hirshfeld charge analysis and diffusion barrier of F, Cl and Br adatom in phosphorene.

We first investigate an isolated halogen (X=F, Cl, Br) adatom covalently bonded to phosphorene monolayer per 2×3 supercell, as shown in Fig. 1(a), where a magnetic moment of 1.0 µB is induced by every halogen and mainly distributed around its adjacent P atoms. The binding energies compared with X2 (defined as Eb=E(phosphorene-X)-E(phosphorene)-E(X2)/2) and the charge on the halogen adatom X are listed in Table 1. Those charges on halogen adatoms reveal that they will be able to diffuse when driven by electrical field if the diffusion barriers are within a moderate range. Take X=Cl as an example, in Fig 1(c) we calculated the diffusion barrier for Cl adatom along the zigzag direction (1 to 2) and armchair direction (1 to 3) using NEB method. It turns out that the diffusion is more efficient along zigzag direction with a relatively low barrier of 0.39eV, but much more blocked along the armchair direction with a high barrier of 0.72eV. At ambient condition, the diffusion will be feasible along zigzag direction but almost forbidden along armchair direction. The diffusion path for all X=F, Cl, Br along the zigzag direction is also displayed in Fig. 1(d). It is shown that F has the largest diffusion barrier (>0.7eV) which is obstructive for its diffusion, while Br has a much lower diffusion barrier (

Chemically Functionalized Phosphorene: Two-Dimensional

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