Ferroelectricity in Covalently functionalized Two-dimensional

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Ferroelectricity in Covalently functionalized Two-dimensional Materials: Integration of High-mobility Semiconductors and Non-volatile Memory Menghao Wu, Shuai Dong, Kailun Yao, Jun-Ming Liu, and Xiao Cheng Zeng Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.6b04309 • Publication Date (Web): 14 Oct 2016 Downloaded from http://pubs.acs.org on October 15, 2016

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Ferroelectricity in Covalently functionalized Two-dimensional Materials: Integration of High-mobility Semiconductors and Non-volatile Memory Menghao Wu1*, Shuai Dong2, Kailun Yao1, Junming Liu3*, Xiao Cheng Zeng4,5* School of Physics and Wuhan National High Magnetic Field Center, Huazhong University of Science and

1

Technology, Wuhan 430074, China Department of Physics, Southeast University, Nanjing 211189, China

2

Laboratory of Solid State Microstructures, Nanjing University, Nanjing 210093, China

3

Department of Chemistry, University of Nebraska-Lincoln, Lincoln, NE 68588, USA

4

Collaborative Innovation Center of Chemistry for Energy Materials, University of Science and Technology of China, Hefei, Anhui 230026, China

5

Abstract Realization of ferroelectric semiconductors by conjoining ferroelectricity with semiconductors remain a challenging task because most present-day ferroelectric materials are unsuitable for such a combination due to their wide bandgaps. Herein we show first-principles evidence towards the realization of a new class of 2D ferroelectric semiconductors through covalent functionalization of many prevailing 2D materials. Members in this new class of 2D ferroelectric semiconductors include covalently functionalized germanene, and stanene (Nat. Commun. 2014, 5, 3389.), as well as MoS2 monolayer (Nat. Chem. 2015, 7, 45); covalent functionalization of the surface of bulk semiconductors such as silicon (111), and the substrates of oxides such as silica with self-assembly monolayers (Nano Lett. 2014, 14, 1354). The newly predicted 2D ferroelectric semiconductors possess high mobility, modest bandgaps, and distinct ferroelectricity that can be exploited for developing various heterostructural devices with desired functionalities. For example, we propose applications of the 2D materials as 2D ferroelectric field-effect transistors with ultrahigh on/off ratio, topological transistors with Dirac fermions switchable between holes and electrons, ferroelectric junctions with ultrahigh electro-resistance, and multiferroic junctions for controlling spin by electric fields. All these heterostructural devices take advantage of the combination of high-mobility semiconductors with fast writing and non-destructive reading capability of non-volatile memory, thereby holding great potential for the development of future multifunctional devices.

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Introduction Since early this century, dilute magnetic semiconductors (DMS) have received intensive interests along with the arising of spintronics. When combined together, the semiconducting part can be used for data operations (e.g., signal amplification with the requirement of maintaining a gate voltage) while the ferromagnetic part can be used for non-volatile magnetic storage of information. So the conventional semiconductor materials (Si, GaAs, ZnO, etc.) doped with magnetic ions possess both advantages when directly integrated in current semiconductor-based circuits. However, their practical applications are still hindered by the weak saturation magnetic moments and low Curie temperature1, while doping stronger ferromagnetism may turn semiconductors into metals. Over the past ten years or so, two-dimensional (2D) high-mobility materials, such as graphene2, silicene3, germanene4, stanene5, transition-metal dichalcogenide (TMDC)6, phosphorene7, 8, have also received considerable research interests. A reason behind such high interests is that the performance of traditional transistors, when reduced to nanoscale, would be seriously influenced by the quantum effect, whereas the 2D materials, because of their atomic thickness and high mobility, are promising candidates to replace the current semiconductor materials in microelectronics and to sustain the Moore's Law for longer time. Nevertheless, it is even more challenging to achieve 2D ferromagnetic (FM) semiconductor compared with DMS because doping magnetic ions into 2D materials like graphene or phosphorene will be much more difficult than replacing Ga in GaAs or Zn in ZnO by 3d magnetic ions like Cr or Mn. Meanwhile, the saturation magnetization, along with the magnetic anisotropy energy, would be much lower than 3d magnetism in 3D DMS. On the other hand, most ferromagnets are not semiconductors but metals. It is known that ferroelectric (FE) materials can be used as non-volatile random access memory (RAM) as well. Destructive electrical reading is usually involved in FE RAM while high writing energy is required in FM RAM. So multiferroic materials combining fast electrical writing with magnetic reading are highly desirable9. Contrary to ferromagnets that are mostly metallic, FE materials must be non-metallic and does not conflict with semiconductivity. If semiconducting FE materials can be made with a moderate bandgap, they would entail both functions of non-volatile memory and manipulation of signals. Unlike 3D DMS that can be produced by directly combining semiconductors and ferromagnetism, ferroelectric semiconductors by combining semiconductors and ferroelectricity are scarce10 because most ferroelectric 2 ACS Paragon Plus Environment

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materials (e.g., perovskites and PVDFs) are large-gap insulators rather than semiconductors. Although bulk semiconductor-based ferroelectric cannot be achieved by directly doping like doping 3d ions in DMS, surface functionalization can make non-ferroelectric 2D materials ferroelectric (hereafter, we use FF2D to denote ferroelectric functionalized 2D materials). In our previous work, hydroxylized graphene, denoted as graphanol, was predicted as the first 2D van der Waals FE material with high polarizations11. Our subsequent calculations show that the Curie temperature of the graphanol can be higher than 700 K12. Note that the FE Curie temperature of 2D materials can be retained much higher than the room temperature as long as the barrier for switching is within a suitable range. In this work, we propose a novel approach to achieve 2D FE materials that can dodge various issues illustrated above. This approach can only apply to low-dimensional structures where most atoms are exposed. This approach is also practically feasible in view of many successes in synthesizing covalently functionalized 2D materials in recent years. For example, functionalized by hydroxyl13,

it has been reported that silicene, germanene and stanene can be , methyl15 or ligands like -CH2OCH3; boron nitride can be partially

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hydroxylized16; and MoS2 and phosphorene can be functionalized by amides17, carboxyl18 or aryl diazonium19. These functionalized 2D materials are all semiconductors with modest bandgaps. For the group-IV elements of Si, Ge and Sn, the sp3 state is more favorable, compared to sp2 state. Thus, 2D silicene, germanene and stanene are more easily functionalized than graphene. Moreover, silicene, germanene and stanene have larger lattice constants than graphene so that longer ligands could be chosen for the functionalization. For graphene, the hydroxyl group appears to be only choice since longer ligands would lead to stronger repulsion among adjacent functional groups. In this letter we show that many surface functionalized 2D materials are ferroelectric, and with proper substitution, the FE polarization can be tuned. In addition, similar functionalization of the surface of conventional semiconductors like Si and III-V compound can also give rise to ferroelectricity. The functionalization is through self-assembled monolayers (SAMs). Given the recent experimental evidence that a MoS2 monolayer on silica functionalized with various SAMs (-OH, -SH, -CH3, -CF3, -NH2, etc.) can exhibit distinct electronic and optical properties20, we predict that substrates like silica terminated with those SAMs can exhibit ferroelectricity and can be used to modify physical properties of supported 2D materials. With the ferroelectric 2D materials, various devices with distinct functions are readily designed. Computational Methods Our density-functional-theory (DFT) computations and scanning tunneling microscope (STM) simulation are performed with the generalized gradient approximation (GGA) in the Perdew-Burke-Ernzerhof (PBE)21 3 ACS Paragon Plus Environment

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form implemented in the Vienna Ab initio Simulation Package (VASP 5.3)22,

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code. The projected

augmented wave (PAW)24 method with a plane-wave basis set was used. The energy cutoff and convergence for the force are set to be 400 eV and 0.01eV/ Å. A vacuum space of 15 Å is adopted to minimize the artificial interaction between 2D material layer and its images. The PBE-D2 functional of Grimme25 was utilized to account for the dispersive forces. The Berry-phase method26 was employed to evaluate crystalline polarization. Transmission spectra are computed by using the nonequilibrium Green’s function (NEGF) and Landauer-Buttiker formula,27 implemented in the QuantumWise ATK code28, with which the 30 × 1 × 100 k-point mesh is employed in the Brillouin zone. For the ab initio Born-Oppenheim molecular dynamics simulation (BOMD) (see below), we adopt the PBE-D2 functional and the same vacuum spacing. The simulation is performed in the constant temperature and volume ensemble with the temperature controlled at 350 K. Functionalized 2D Materials and Surfaces of Bulk Materials We first investigate covalent functionalized germanene and stanene. Importantly, methyl-terminate d germanene and stanene [Fig. 1(a)] have been fabricated by Goldberger and coworkers recently, which are air stable and free-standing with moderate bandgaps, and have high mobility comparable to phosphorene14, 15, 29. Goldberger and coworkers also reported successful synthesis of a 2D analogue Sn(P, As, Sb)-CH2OCH3. We speculate that this ligand with a dipole moment may induce ferroelectricity as long as it is switchable upon an external electric field, as shown in Fig. 1(b). By using nudged elastic band (NEB) method, we estimated the average rotation barrier (or the barrier of switching) to be about 0.09 eV per ligand [Figure S1(a)], which is much higher than the ferroelectric switching barrier reported previously for 3D ferroelectric BaTiO330 and 2D ferroelectric SnSe10. This rotational barrier, defined as Ek, is the collective rotational barrier [supposing all spins/dipoles rotating spontaneously towards one direction in ferromagnetic/ferroelectric materials]. Note that in the Ek computation, the nearest-neighbor interactions that play the key role in Curie temperature estimation are not taken into consideration. So Ek cannot be used to determine the Curie temperature. However, Ek can be used to determine whether a ferroic material is “hard” or “soft”. For a material with a higher Ek, a higher external magnetic/electric field is required for the polarization switching. Note also that thermal activation can increase structural disorder in the system and make spins/dipoles rotation more randomly, but it cannot make all (infinite number of) spins/dipoles switch uniformly towards one direction. What determines the Curie temperature should be Ej, defined as the switching barrier for one spin/dipole while all other surrounding spins/dipoles are fixed. In the ground state, the dipole moments of ligands are along the zigzag direction of the honeycomb lattice. 4 ACS Paragon Plus Environment

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Here, the Ej value will be very high because the dipole of adjacent ligands cannot be opposite (to avoid collision) since -CH2OCH3 is a long stick-like ligand anchored at one side. As such, all the ligands are aligned towards the same direction, thereby making the intrinsic ferroelectricity ultra-robust. The computed 2D switchable polarization is 0.31×10-10C/m for SnSb-CH2OCH3, which is around 4.5 µC/cm2 in 3D unit when the thickness of monolayer is taken as 7 Å. This polarization may be further enhanced by substitution of ligands with larger dipole moments. For methyl-terminated germanene or stanene, substitution of a hydrogen atom in each methyl group by a halogen atom would make the system ferroelectric. All ligands can be also substituted by other ligands such as -CHO or –COOH. As shown in Fig. 1(c), for germanene or stanene terminated by –CH2F or -COOH, the polarization is aligned along the zigzag direction, while for those terminated by -COH, the ligands form zigzag hydrogen-bonded chains just like hydroxyls in graphanol, and the polarization is along the armchair direction. For all three liganded monolayers, their polarizations can be switched upon rotation of ligands. With -COOH, the ferroelectricity can be also switched by proton transfer along the hydrogen-bonded chains. Taking functionalized germanene as an example, the computed polarizations (see Table 1) are much higher than those of SnSb-CH2OCH3, while their bandgaps seem suitable for nanoelectronic applications. Various system configurations including antiferroelectric configurations are examined to confirm that the ferroelectric states shown in Fig. 1 are indeed the ground state (see Figure S1). We also performed ab initio BOMD simulation with the temperature controlled at 350 K to confirm that the ferroelectricity can still be retained above the ambient temperature (Figure S2). Meanwhile, we also simulated the STM images for comparison with future STM experiments (Figure S2). Moreover, we computed the strain energy following previous studies31, 32. It turns out that the strain energy is negative for -CH2OCH3, -CH3F, -CHO and –COOH functionalization. When the coverage of ligands decreases from 100% to 50% (half passivated by ligands and another half by hydrogen), the (negative) strain energy becomes less negative, with an energy change of 0.056, 0.075, 0.067, 0.042 eV per ligand, respectively. This is understandable as ligands are separated by more hydrogen at the lower coverage while hydrogen bonds are interrupted. The combined situation may not be favorable in energy change.


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Figure 1. Side and top views of (a) Methyl-terminated germanene/stanene and (b) Sn(P, As, Sb)-CH2OCH3. Side and top views of (c) Germanene/stanene functionalized by –CH2F, -CHO, and –COOH, respectively. (d) and (e) Side view of MoS2 monolayer functionalized by –COOH and –CONH2, respectively. The report of fabrications for (a), (b), (d) and (e) is referred to Refs. 14 and 13, 17, and 16.


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It is worthy of mentioning that the partially hydroxylized counterparts, like Si6H3(OH)3 (siloxene), Ge1xSnxH1-x(OH)x

have also been synthesized14. Here, the hydroxylized part also possesses ferroelectricity like

graphanol. For example, the polarization of a fully hydroxylized silicene (silicanol) is actually greater than that of SnSb-CH2OCH3. Moreover, TMDCs like MoS2 were also functionalized with different functional groups such as amide17 and carboxyl18 in experiments. In particular, the 1T phase of MoS2 was shown to become stable semiconductors when functionalized17, 33, and the associated polar groups may induce ferroelectricity as well. As shown in Fig. 1(d, e), the ligands (-COOH, -CONH2) are aligned in one direction, which should be switchable upon an external electric field. The computed polarizations (Table 1) are comparable to the functionalized germanene. The (111) surface configuration of cubic Si, Ge and Sn is similar to silicene, germanene and stanene, and thus, similar functionalization may also induce ferroelectricity on these surfaces. Silicon (111) surface, for example, can become ferroelectric when functionalized by SAM of ligands like –SH (Fig. 2(a)). The ligands can also serve as n or p dopants: -OH is an electron withdrawing group while ligands like – CHO, -COOH and –CONH2 are electron-donating groups. Similarly, binary semiconductors like III-V or II-VI compounds can become ferroelectric when functionalized with ferroelectric SAMs (see Fig. 2(b)). Compared to recent progresses on functionalization of 2D materials, functionalization of surfaces of conventional bulk semiconductors is expected to be much more practical at present, especially noting that the covalent functionalization of silicon surfaces has already been reported for decades31, 32, 34. It may be also possible that with current techniques the surface functionalization of particular region of a wafer can make that local region ferroelectric, thereby the latter region can be directly integrated with current silicon-based circuits. SAMs can be also used to functionalize 2D insulating materials. A previous experiment20 demonstrated that MoS2 monolayer on silica functionalized with various SAMs (terminated by -OH, -SH, -CH3, -CF3, -NH2, etc.) can exhibit distinct electronic and optical properties. Here, we suggest that the MoS2 monolayer on silica functionalized with SAMs can be ferroelectric also, e.g., with –OH functionalization (Fig. 2c). The surface-hydroxylized silica exhibits polarization (Table 1), thereby resulting a horizontal electric field along the MoS2 monolayer. When used in photovoltaics, the electric field can facilitate the departure of electrons and holes so that the lifetime of excitons and the photovoltaics efficiency may be enhanced.


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Figure 2. (a) Silicon (111) passivated by –SH, and (b) cubic boron nitride (111) passivated by –OH. (c) MoS2 monolayer on a silica substrate passivated by hydroxyl (fabrication was reported in Ref. 19).

Table 1. Computed polarizations and bandgaps of various functionalized monolayers. SnSb-

Ge-

Ge-

Ge-

CH2OCH3

CH2F

CHO

COOH

Si-OH

MoS2-

MoS2-

MoS2+

COOH

CONH2

SilicaOH

Polarization

0.31

1.17

0.81

0.68

0.76

0.55

0.50

0.37

1.1

1.0

1.66

1.36

0.51

0.20

0.11

1.80

(10-10C/m) Bandgap (eV)


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Heterostructure Devices It is known that polar discontinuity at the interface can induce polarization charges and electric fields that drive metal-insulator transition35. Here we build a heterojunction by functionalizing a single 2D material or a semiconductor surface by two different ligands [e.g., Si (111) by –SH and Cl] so that one region is ferroelectric and the other non-ferroelectric, and within the associated boundary free charge accumulates along the 1D interface. The density of free carrier λF will balance the polarization charge density λP: λF=(PF-PNF)·n =-λP

,

Here PF, PNF are the polarization of ferroelectric and non-ferroelectric regions, and n is a unit vector pointing from the ferroelectric region to non-ferroelectric region, normal to the interface. By switching the direction of polarization, the free carriers at the interface can be switched between electrons and holes, as shown in Fig. 3(a). To simulate the metal-insulator transition upon polar discontinuity, we consider a graphene sheet functionalized by arrays of –OH and –F nanostripes, where the hydroxylized regions are ferroelectric and the polarization of zigzag O-H…O-H chain is aligned in the armchair direction of graphene lattice11, generating a difference in potential between two sides: as shown in Fig. 3(b). When the polarization is aligned in the direction of 60 degree away from –X, the system is semiconducting with a bandgap ~0.15 eV. When it is directly along –X, the system is metallic along the –Y direction with a 1D Dirac cone located at the Fermi level. In this case, the 1D free electron gas and hole gas are formed, respectively, at different sides of hydroxylized nanostripes. In contrast, the hydroxylized graphene and fluorinated graphene are insulating with bandgaps larger than 2.7 eV. Thus, the interface of functionalized ferroelectric and non-ferroelectric regions can be used as ferroelectric FET with ultrahigh on/off ratio (on/off states are respectively metallic/semiconducting). If the non-ferroelectric regions are pristine graphene nanostripe, where the two ferromagnetic zigzag edges of the graphene nanostripe are antiferromagnetically-coupled in the ground state, the difference of potential between two spin-polarized edges may push the edge states towards the Fermi level in one spin-channel, making the system halfmetallic. As shown in Fig. 3(c), the system is metallic in the spin-up channel but insulating in the spin-down channel when the polarization of hydroxylized nanostripes is aligned to the right, but vice versa when it is switched towards the left. Therefore, it is feasible to using electric field to control spin, rendering the combined electrical writing and magnetic reading possible in data storage.


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Figure 3. Top-view and side-view for (a) silicon (111) passivated by –SH and –Cl; (b) graphene nanostripes functionalized by –OH and –F; (c) partially hydroxylized graphene nanostripes, where spin density distributions are marked by yellow (spin-up) and blue (spin down), and in the band structures, red and black lines denote different spin channels. The directions of polarization (marked by arrows) in ferroelectric regions are pointing from the center of negative ions (like S, F, O) to the center of positive ions (like H), the same as those of the overall dipole of the ligands,

Ferroelectric tunnel junction (FTJ)36 is an alternative way for non-destructive reading, where switching the polarization of a sandwiched ferroelectric layer between two different metals can produce a change in tunneling resistance, known as tunneling electroresistance (TER). Previous studies on TER were focused 10 ACS Paragon Plus Environment

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on metal oxide bulk systems. Here, we demonstrate a high TER from the design of a 2D ferroelectric/nonferroelectric junction based functionalized 2D materials or semiconductor surfaces. For example, as shown in Figure 4(a), our first design is a p-doped (0.001e/atom) junction of germanene passivated by – CH3 and germanene passivated by –CH2F, where the latter region is ferroelectric. With the switching of polarization in the region passivated by –CH2F from right to left, more holes are accumulated at the boundary, and the transmission would be greatly enhanced from 1.4×10-4 to 0.042, as shown in the clear transmission spectrum in Figure 4(a) upon different polarization directions. In this case, the TER is over 300, much larger than current TER measured in experiments.

Figure 4. Ferroelectric junction of germanene passivated by –CH2F and –CH3. The direction of polarization in ferroelectric regions are marked by arrows.


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Figure 5. (a) In-plane heterostructure of germanene passivated by –CH2F and –I. (b) Ferroelectric PN junction based on hydroxylized Si (111) surface and MoS2 on hydroxylized silica surface.

Other Device Designs Previous studies have predicted that germanene, stanene and their functionalized counterpart are 2D topological insulators (TIs)37, 38, while half-passivated Ge-I or Sn-I exhibit quantum anomalous Hall effect39. Among them, the hydroxylized stanene is also ferroelectric. The ferroelectric TIs with switchable polar surfaces and spin-momentum locked Dirac cones can render electric-field control of topological surface states and the surface spin current possible. Figure 5(a) displays an in-plane heterostructure of TI and ferroelectric functionalized region (Ge-I and Ge- CH2F, for example). At the interface, the Dirac fermions at the edge of TI can switch between holes and electrons upon the polarization switching of Ge-CH2F. A topological transistor can be composed by two ferroelectric domains of functionalized germanene or stanene, and the on/off state is switchable and depends on the parallel/anti-parallel configurations of ferroelectric domains, where the edge in anti-parallel configuration is actually a 1D PN junction. It is known that PN junction typically has a low resistance state with narrower depletion region upon a forward bias, and a high resistance state with wider depletion region upon a reverse bias. The effect of ferroelectric polarization can be equivalent to an external field. So the ferroelectric PN junction, like a silicon PN junction with hydroxylized surface, or a MoS2 PN junction on a hydroxylized silica substrate (Fig. 5(b)), can 12 ACS Paragon Plus Environment

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switch between high/low resistance states upon the switching of polarization. Since the diffusion length can be up to micrometers, our designs are still qualitatively. Conclusion On basis of the first-principles calculations, we show the rise of ferroelectricity in a series of covalent functionalized silicene, germanene, stanene and MoS2 monolayer, as well as the surface of bulk semiconductors like silicon (111) or substrates like silica functionalized with SAMs. Most of these systems have already been synthesized in the laboratory. These FF2Ds mostly possess both high mobility and moderate bandgaps for nanoelectronic applications together with ferroelectricity for non-volatile memory. Based on these ferroelectric 2D materials, we design a number of heterostructure devices with various useful functions: 2D ferroelectric FTEs with ultrahigh on/off ratio, topological transistors with Dirac fermions switchable between holes and electrons, ferroelectric junctions with ultrahigh electroresistance, multiferroic junctions controlling spin by electric fields. These systems can combine highmobility semiconductors for nanoelectronics and fast writing and non-destructive reading for non-volatile memory, holding great promise as multifunctional devices. AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] (MHW), [email protected] (JML), [email protected] (XCZ) Acknowledgements MHW, KLY and JML are supported by the National Natural Science Foundation of China (Nos. 21573084, 11274130 and 51431006). 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 (2016YFA0200600 and 2016YFA0200604) to USTC. We also thank Shanghai Supercomputing Center for providing computational resources. The authors declare no competing financial interests. Supporting Information This material is available free of charge via the Internet at http://pubs.acs.org. 13 ACS Paragon Plus Environment

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Nano Letters

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Nano Letters

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TOC Graphic

Two-dimensional Functionalized Ferroelectric Tunnel Junction


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Ferroelectricity in Covalently functionalized Two-dimensional

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