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Bismuth Oxychalocogenides: A New Class of Ferroelectric/ Ferroelastic Materials with Ultra High Mobility Menghao Wu, and Xiao Cheng Zeng Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.7b03020 • Publication Date (Web): 20 Sep 2017 Downloaded from http://pubs.acs.org on September 21, 2017

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Bismuth Oxychalocogenides: A New Class of Ferroelectric/Ferroelastic Materials with Ultra High Mobility Menghao Wu1* and Xiao Cheng Zeng2,3* 1

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

2

Department of Chemistry and Nebraska Center for Materials and Nanoscience,, University of NebraskaLincoln, Lincoln, NE 68588, USA 3

Collaborative Innovation Center of Chemistry for Energy Materials, University of Science and Technology of China, Hefei, Anhui 230026, China *Email: [email protected] (MW), [email protected] (XCZ). Tel: (402) 472-9894(XCZ)

Abstract Atomically thin Bi2O2Se has been recently synthesized and it possesses ultra-high mobility (Nat Nanotech. 2017, 12, 530; Nano Lett. 2017, 17, 3021). Herein, we show first-principles evidence that Bi2O2Se and a related class of bismuth oxychalcogenides, such as Bi2O2S and Bi2O2Te, not only are novel semiconductors

with

ultra-high

ferroelectricity/ferroelasticity.

Such

mobility a

but

unique

also

possesses

combination

of

previously

unreported

semiconducting

with

ferroelectric/ferroelastic properties enables bismuth oxychalcogenides to potentially meet a great challenge, that is, integration of room-temperature functional non-volatile memories into future nanocircuits. Specifically, we predict that bulk Bi2O2S is both ferroelastic and antiferroelectric, and that a thin film with odd number of layers can even be multiferroic with nonzero in-plane polarization, and this polarization can be switchable via ferroelasticity. Moreover, Bi2O2Te possesses intrinsic out-of-plane ferroelectricity while Bi2O2Se possesses piezoelectricity and ferroelectricity upon an in-plane strain. The in-plane strain on Bi2O2Se can induce giant polarizations (56.1 µC/cm2 upon 4.1% strain) with the piezoelectric coefficient being about 35 times higher than that of MoS2 monolayer. The in-plane strain can also enhance the bandgap or even convert indirect to direct bandgap beyond a critical value. The good match among the lattice constants of bismuth oxychalcogenides is also desirable, rendering the epitaxial growth of heterostructure devices free of fabrication issues related to lattice mismatch, thereby allowing high-quality bismuth oxychalcogenide heterostructures tailored by design for a variety of applications. Keywords: ferroelectrics, ferroelastics, piezoelectrics, high-mobility bismuth oxychalcogenides, vertical polarization

1 ACS Paragon Plus Environment

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With the continuation of reducing integrated circuit size toward sub-10nm scale, two major issues encountered for the current prevailing random access memories (RAMs) based on silicon materials are: 1) Quantum tunneling and memory wear tend to be aggravated since their “0” and “1” states are not degenerate in energy, and 2) the data storage is lost upon power outage, while continuous supply of power is largely offset by the power dissipation. One possible solution to resolve both issues is to utilize either ferroelectric RAMs (FeRAMs) or magnetic RAMs (MRAMs), both being non-volatile with equivalent “0” and “1” states. However, their integration into future semiconductor nano-circuits is a great challenge. Although intensive research has been devoted to diluted magnetic semiconductors to incorporating ferromagnetism (FM) into semiconductors,1 none of the materials fabricated thus far has achieved the Curie temperature near room temperature. Moreover, the associated superparamagnetism often leads to memory wear at the nanoscale. On synthetic approach in making ferroelectric RAMs (FeRAMs), the similar strategy on doping-induced FE in semiconductors has not been as effective. Meanwhile the traditional ferroelectrics like perovskites usually possess large bandgaps, and thus are insulators rather than semiconductors. To date, neither FeRAMs nor MRAMs have been ready to substitute silicon-based RAMs, as the latter ones can be integrated into silicon wafer with mature silicon process. Apart from silicon-based memories, the performance of silicon-based transistors, when reduced to sub10nm scale, can be also seriously influenced by the quantum effect. Nevertheless, successful synthesis of a large family of two-dimensional (2D) electronic materials, such as, among others, graphene2, transitionmetal dichalcogenide3 and phosphorene4, 5, with atomic thickness and high mobility, offers more possibilities to replace the current semiconductor materials in microelectronics, and to sustain the Moore's Law for longer time. Still, most star 2D materials still have their own shortcomings. For example, graphene lacks a bandgap suitable for nanoelectronics, MoS2 monolayer possesses only a moderate electron mobility, and phosphorene is chemically unstable when exposed to the air. Recently, large single crystals of 2D Bi2O2Se have been synthesized experimentally, displaying size-tunable bandgap and ultrahigh Hall mobility of > 20000 cm2 V-1 S-1 at 2K.6, 7 This new member in 2D materials family is also airstable, hence free of the major issues mentioned above for many current 2D materials. In this letter, we show first-principles evidence that Bi2O2Se and a related class of bismuth oxychalcogenides like Bi2O2S and Bi2O2Te are not only remarkable semiconducting materials with ultrahigh mobility, but also entail ferroelectric/ferroelastic properties. Such a unique combination of high-mobility semiconductors and non-volatile memories (NVMs) offers a possible solution to address a great challenge, that is, integration of room-temperature functional non-volatile memories into future nanocircuits.


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Since the prediction of ferroelectric graphanol in 20138, a series of 2D ferroelectrics have been predicted9-17 or some of them even synthesized18, 19. However, none of the fabricated 2D ferroelectrics in the laboratory are high-mobility semiconductors, required as the materials for integration into future wafers. Although some 2D ferroelectrics are predicted to possess high mobility, e.g., SnSe9 and functionalized germanene10, their monolayer structures are yet to be fabricated. Hence, once the predicted ferroelectricity and ferroelasticity in already-synthesized bismuth oxychalocogenides are confirmed in the laboratory, their possibility for direct integration of NVMs into nanocircuits are ready to be tested. Furthermore, we find that application an in-plane strain on Bi2O2Se not only can induce giant polarizations, but also enhance the bandgap or even tune bandgap from indirect to direct when the strain is beyond a critical value. The good match in lattice constants of bismuth oxychalocogenides is highly desirable as well, as it allows fabrication of heterostructure devices with tailored properties and yet free of lattice-mismatch induced problems. First-principles calculations are performed within the framework of density-functional-theory (DFT) calculations implemented in the Vienna ab initio Simulation Package (VASP 5.3)20, 21. The projector augmented wave (PAW)22 potentials for the core and the generalized gradient approximation (GGA) in the Perdew-Burke-Ernzerhof (PBE)23 form for the exchange-correlation functional are used. The Monkhorst-Pack k-meshes are set to 7 × 7 × 3 in the Brillouin zone and all atoms are relaxed in each optimization cycle until atomic forces on each atom are smaller than 0.01 eV Å−1 and the energy variation between subsequent iterations fall below 10−6 eV. All the band structures are computed by using the screened exchange hybrid Heyd-Scuseria-Ernzerhof (HSE06)24 functional with taking spin-orbit coupling (SOC) into account.


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Figure 1 Geometric and band structures of (a) Bi2O2S, (b) Bi2O2Se, (c) Bi2O2Te, and (d) BiCuOSe, where red, purple, yellow, orange, green and pink spheres denote O, Bi, S, Se, Te, Cu atoms, respectively. X(0.5, 0, 0), (0, 0, 0), U(0.5, 0.5, 0), R(0.5, 0.5, 0.5) denote symmetry points in the band structures.

Table 1 Lattice constants and bandgaps for Bi2O2S, Bi2O2Se, Bi2O2Te, and BiCuOSe. Space Group

|a|( Å)

|b|( Å)

|c|( Å)

Eg(eV)

Bi2O2S

Pnnm

3.85

3.89

11.97

1.25

Bi2O2Se

I4/mmm

3.90

3.90

12.39

0.89

Bi2O2Te

I4MM

4.01

4.01

12.90

0.16

BiCuOSe

I4/mmm

3.93

3.93

9.01

1.25

Figure 1 displays the optimized geometric structures and computed band structures for Bi2O2S, Bi2O2Se, Bi2O2Te,25 and BiCuOSe. HSE06/SOC computations indicate that they are all semiconductors with indirect bandgaps of 1.25, 0.89, 0.16 and 1.25 eV, respectively. Here the structures in Fig. 1(a-c) are Bi2O2 layers


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intercalated by chalcogen atoms, while BiCuOSe is composed of Bi2O2 layers intercalated by CuSe layers which withdraw one electron from every BiO format unit like chalcogen atoms. As a result, the band structures in Fig. 1 are similar, while that in Fig. 1(d) is slightly more distinct from others. Their lattice parameters in –x and –y direction can match each other quite well, as summarized in Table 1. Here, the structure of Bi2O2Se in Fig. 1(b) is highly symmetrical (S.G. I4/mmm) where each Se atom is exactly located in the center of a cubic composed of 8 Bi atoms. In Fig. 1(a) and (c), however, the geometric structure of Bi2O2S and Bi2O2Te are both slightly distorted, compared with the highly symmetrical lattice of Bi2O2Se, as shown by the interlayer displacement of adjacent Bi2O2 layers in Bi2O2S, and the slightly distortion of Te atoms away from the center between adjacent Bi2O2 layers in Bi2O2Te. We also notice the Rashba splitting in the band structure of Bi2O2Te, indicating the breaking of inversion symmetry. Ferroelectric-like distortions induced by lone-pair electrons have already been studied in numerous materials like SnSe9 and BiFeO3,26 and here, similar mechanism may emerge in the bismuth oxychalocogenides with similar lonepair electrons.


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Figure 2 (a) Ferroelasticity of Bi2O2S; (b) antiferroelectric-coupling Bi2O2 layers in bulk Bi2O2S and in thin film. Red, purple, yellow, and white spheres denote O, Bi, S, H atoms, respectively. Δd1 represents the interlayer displacement of adjacent Bi2O2 layers (the coordinate difference between layers is along the – y axis). Δd2 represents the relative intra-layer displacement of Bi and O atoms along –y axis with respect to the symmetrical Bi2O2 layer. The directions of polarization for Bi2O2 layers (same to the displacement of Bi while opposite to the displacement of O) are marked by gray arrows.

We first investigate the geometric structures of Bi2O2S, with its top view of xy plane displayed in Fig. 2(a). Although the centrosymmetry of Bi2O2S structure is still preserved (S.G. Pnnm), the interlayer displacement of adjacent Bi2O2 layers Δd1= 0.56 Å is aligned in –y direction, giving rise to an anisotropy between –x and –y direction. The lattice parameter in –y direction |b| will be slightly larger (around 1 %) than |a| in –x direction, and the system can be 2D ferroelastics27, 28: Upon a strain in –x direction, the structure can be transformed to an equivalent configuration where |a|’=|b| and |b|’=|a|, and the interlayer displacement is switched to the –x direction, same as the initial state with 90 degree rotation. The intermediate state (S.G. I4/mmm) with a square lattice |a|’’=|b|’’ in xy plane, is 10.0 meV higher in energy than the ground state. This difference can be viewed as the collective switching barrier upon strain, which is higher than some 2D ferroelastics known to date (e.g., about 4 and 1 meV for SnS and SnSe, respectively), although the ferroelastic strain around 1% is lower compared with that applied to SnS and SnSe (4.9% and 2.1%, respectively).9 Distinct from the symmetrical Bi2O2 layer in Bi2O2Se, here in Bi2O2S, every Bi2O2 layer is actually ferroelectric, as Bi and O atoms will move towards opposite directions along –y axis with by Δd2= 0.036 Å, as shown in Fig. 2(b). However, the polarization directions of adjacent layers are opposite so that they are anti-ferroelectrically coupled. Therefore, for a Bi2O2S thin film with odd number of Bi2O2 layers, it can exhibit multiferroicity with a net polarization. For example, the polarization of 2D Bi2O2S monolayer and tri-layer (passivated by hydrogen at both sides, following the model in Ref. 6) are, respectively, 0.12 and 0.53 × 10-10 C/m. Similar to phosphorene analogues like SnS monolayer, here, the ferroelasticity and ferrielectricity are coupled: The net polarization will switch by 90 degree concomitantly with ferroelastic switching, while the ferroelastic switching can be performed by applying an in-plane electric field perpendicular to the polarization direction. This coupling may enable non-volatile memory with efficient data reading/writing that can be directly integrated into nanocircuits because of the combination with high-mobility semiconducting property.


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For Bi2O2Te in Fig. 3(a), the interlayer displacement of adjacent Bi2O2 layers is vanished. However, compared with Se atoms in Bi2O2Se, here, the Te atoms tend to move away from the center of Bi cubic, giving rise to FE with switchable polarization that can be either in-plane or out-of-plane. When such a displacement Δd is aligned in xy plane (state I), the favorable direction will be the diagonal line of the square, with Δd1=0.071 Å, giving rise to a polarization of 7.1 µC/cm2. When they are switched to the outof-plane direction (state II), Δd2 = 0.055 Å and the polarization decreases to a lower value of 5.3 µC/cm2. However, for every unit cell, state II is 10.2 meV lower in energy compared with state I so that the polarization would favor being aligned vertically. Due to the distortion, the length of Te-Bi bonds in Bi2O2Te ranges from 3.36 to 3.50 Å, while the Te-Bi bonds in Bi2Te3 should be much stronger due to shorter bond length (3.10 and 3.29 Å). In Fig. 3(b) we also examine two anti-ferroelectric configurations, where the intra-layer and interlayer configurations are around 23 and 29 meV higher in energy, respectively, compared with the ferroelectric state. This also confirms the ferroelectric state as the ground state.

Figure 3 (a) Ferroelectricity in Bi2O2Te upon in-plane (I) or out-of-plane (II) displacement of Te atoms, and (b) two anti-ferroelectric configurations. The displacement directions for Te atoms are marked by black arrows. 7 ACS Paragon Plus Environment

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The ground state of Bi2O2Se is highly symmetrical, which should not exhibit bulk polarization. However, upon an in-plane strain, it exhibits not only piezoelectricity but also ferroelectricity. As displayed in Fig. 4(a), upon an in-plane biaxial strain, the square lattice of Bi2O2 layer (from the top view) may be distorted: Se/O atoms carrying negative charges and Bi atoms carrying positive charges will deviate from original position along the diagonal of the square lattice but in opposite directions, giving rise to a net polarization along the diagonal direction. If the strain is uniaxial, the displacement and polarization will be both aligned along the direction of strain. Distinct from the antiferroelectric interlayer coupling configuration of Bi2O2S, here, the polarizations of the layers in Bi2O2Se upon strain are all in the same direction so that the interlayer coupling is ferroelectric. The distorted state will be more favorable in energy, compared with symmetrical state, only when the biaxial strain is larger than 1.7% (see Fig. S2), so there will be a critical point where the system becomes FE. As shown in Fig. 4(b), the out-of-plane polarization becomes nonzero when the strain is larger than 1.7%, and can increase to 56.1 µC/cm2 upon a 4.1% biaxial strain. The strain can be achieved by epitaxial growth on substrates like AlAs, CaS, BaTiO3 or even Bi2O2Te, with in-plane lattice parameters a few percent larger than that of Bi2O2Se. Compared with monolayer MoS2 which has a high piezoelectric coefficient of 2.9× 10-10 C/m as measured in previous experiment29 (which is around 0.4 C/m2 in 3D if the thickness of monolayer is taken as 7.1 Å), Bi2O2Se possesses a piezoelectric coefficient that is at least 35 times higher, assuming a linear relationship between the polarization and strain below 4.1%. The Rashba splitting in its band structure upon a biaxial strain of 5% is another evidence of FE (Fig. 4(c)). Additionally, the bandgap is also much enhanced. The data in Fig. S1 show that the bandgap is tunable by strain, and can even increase to 1.55 eV from indirect to direct upon a strain above a critical strain of 7.7%.


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Figure 4 (a) Geometric structures of Bi2O2Se upon a strain, (b) dependence of the polarization on biaxial strain, and (c) band structure upon a biaxial strain of 5% (in black) compared with pristine Bi2O2Se (in red).

The computed bandgap of Bi2O2Te with intrinsic vertical ferroelectricity is much lower compared with other bismuth oxychalcogenides, noting that a bandgap around 1 eV is more desired for nanoelectronic applications. However, the bandgap for alloy Bi2O2SxTe1-x or Bi2O2SexTe1-x can be tunable via changing alloy composition x (see Fig. S3). As shown in Fig. 5(a), at x = 0.5, the bandgap of Bi2O2SxTe1-x can be greatly enhanced to > 0.95 eV. Meanwhile, the vertical ferroelectricity can be maintained or even enhanced compared with the pristine Bi2O2Te because not only Te atoms but also Se atoms will move away from the centers of Bi cubic, giving rise to a vertical polarization of 6.1 µC/cm2. Lastly, the good match in the lattice constants of different bismuth oxychalcogenides is also noteworthy, as it renders the epitaxial growth of heterostructure devices free of fabrication issues due to the lattice mismatch. As such, high-quality bismuth oxychalcogenide heterostructures can be tailored by design for a variety of applications. For example, Bi2O2Te can be grown on Bi2O2S/ Bi2O2Se, or even BiCuOSe, giving rise to a ferroelectrics/semiconductor heterostructure as a ferroelectric field-effect device10, 30, where 9 ACS Paragon Plus Environment

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nonvolatile switching between electron and hole carrier at the interface can be available due to polar discontinuity30, as shown in Fig. 5(b). Magnetoelectric device may also be designed, noting that the Rashba splitting in Fig. 1(c) and Fig. 4(c) also enables making of devices to control spin via external electric field.

Figure 5 (a) Geometric and band structure of Bi2O2S0.5Te0.5, (b) design of ferroelectric/semiconductor heterostructure as ferroelectric field-effect device.

In summary, based on first-principles calculations, we predict that Bi2O2Se and other bismuth oxychalcogenides like Bi2O2S and Bi2O2Te possess ferroelectricity/ferroelasticity. Such a unique combination of 2D high-mobility semiconductor with ferroelectricity/ferroelasticity renders the integration of room-temperature functional NVMs into future nanocircuits possible. The associated Rashba spitting may be exploited to control spin via ferroelectric switching. Finally, the good match in the lattice constants of bismuth oxychalcogenides renders heterostructure devices free of lattice-mismatch issue, thereby being tailored with flexibility for applications.


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Supporting Information The Supporting Information is available free of charge Dependence of bandgap on strain and alloy fraction, and dependence of energy difference between symmetrical state and distorted state on strain.

Author Information Corresponding Authors: [email protected] (MHW) and [email protected] (XCZ) The authors declare no competing financial interest Acknowledgements MHW is supported by the National Natural Science Foundation of China (Nos. 21573084). XCZ is supported by the US National Science Foundation through the Nebraska Materials Research Science and Engineering Center (MRSEC) (grant No. DMR-1420645), a Qian-ren B (One Thousand Talents Plan B) summer research fund from USTC, and by a State Key R&D Fund of China (2016YFA0200604) to USTC. We also thank Shanghai Supercomputing Center for providing computational resources.

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