Integration of High-mobility Semiconductors and Nonvola

Ferroelectricity in Covalently functionalized Two-dimensional Materials: Integration of High-mobility Semiconductors and Nonvolatile Memory

Menghao Wu,*

^,†

Shuai Dong,

^‡

Kailun Yao,

^†

Junming Liu,*

^,§

and Xiao Cheng Zeng*

^,⊥,¶

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

‡Department of Physics, Southeast University, Nanjing 211189, China

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

⊥Department of Chemistry, University of Nebraska-Lincoln, Lincoln, Nebraska 68588, United States

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

*^S Supporting Information

ABSTRACT: Realization of ferroelectric semiconductors by conjoining ferroelectricity with semiconductors remains 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 toward the realization of a new class of two-dimensional (2D) ferroelectric semiconductors through covalent functionaliza- tion 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 MoS₂monolayer (Nat. Chem.2015,7, 45), covalent functionalization of the surface of bulk semi-

conductors such as silicon (111) (J. Phys. Chem. B 2006,110 , 23898), 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 ferroelectricfield-effect transistors with ultrahigh on/offratio, topological transistors with Dirac Fermions switchable between holes and electrons, ferroelectric junctions with ultrahigh electro-resistance, and multiferroic junctions for controlling spin by electricfields. All these heterostructural devices take advantage of the combination of high-mobility semiconductors with fast writing and nondestructive reading capability of nonvolatile memory, thereby holding great potential for the development of future multifunctional devices.

KEYWORDS: Ferroelectric semiconductors, multifunctional devices, covalent functionalization

S

ince early this century, dilute magnetic semiconductors (DMS) have received intensive interest along with the arising of spintronics. When combined together, the semi- conducting 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 nonvolatile magnetic storage of information. Therefore, 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 temper- ature,¹ while doping stronger ferromagnetism may turn semiconductors into metals.

Over the past ten years or so, two-dimensional (2D) high- mobility materials, such as graphene,² silicene,³ germanene,⁴

stanene,⁵ transition-metal dichalcogenide (TMDC),⁶ and phosphorene,^7,8 have also received considerable research interests. A reason behind such high interest 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 a 2D ferromagnetic (FM) semiconductor compared with DMS because doping magnetic ions into 2D materials like graphene or phosphorene will be

Received: October 14, 2016 Published: October 14, 2016

pubs.acs.org/NanoLett

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 nonvolatile random access memory (RAM) as well. Destructive electrical reading is usually involved in FE RAM, while high writing energy is required in FM RAM. Therefore, multiferroic materials combining fast electrical writing with magnetic reading are highly desirable.⁹ Contrary to ferromagnets that are mostly metallic, FE materials must be nonmetallic and do not conflict with semiconductivity. If semiconducting FE materials can be made with a moderate bandgap, they would entail both functions of nonvolatile memory and manipulation of signals. Unlike 3D DMS that can be produced by directly combining semiconductors and ferromagnetism, ferroelectric semiconductors by combining semiconductors and ferroelec-

tricity are scarce¹⁰ because most ferroelectric materials (e.g., perovskites and polyvinylidenefluoride materials) are large-gap insulators rather than semiconductors. Although bulk semi- conductor-based ferroelectric cannot be achieved by directly doping like doping 3D ions in DMS, surface functionalization can make nonferroelectric 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 polarizations.¹¹ Subsequent calculations show that the Curie temperature of the graphanol can be higher than 700 K.¹² 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.¹⁰In 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 Figure 1.Side and top views of (a) methyl-terminated germanene/stanene and (b) Sn(P, As, Sb)−CH2OCH₃, where the blue arrow denotes that the polarization is switchable. Side and top views of (c) germanene/stanene functionalized by−CH2F,−CHO, and−COOH, respectively. Side view of MoS₂monolayer functionalized by (d)−COOH and (e)−CONH2, respectively. The reports of fabrications for panels a, b, d, and e are in refs15, 14,18, and17.

synthesizing covalently functionalized 2D materials in recent years. For example, it has been reported that silicene, germanene, and stanene can be functionalized by hydroxyl,^13,14 methyl,¹⁵ or ligands like−CH₂OCH₃; boron nitride can be partially hydroxylized;¹⁶ and MoS₂ and phosphorene can be functionalized by amides,¹⁷ carboxyl,¹⁸ or aryl diazonium.¹⁹ These functionalized 2D materials are all semiconductors with modest bandgaps. For the group-IV elements of Si, Ge, and Sn, the sp³state is more favorable compared to the sp²state. Thus, 2D silicene, germanene, and stanene are more easily function- alized 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 properties,²⁰ 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)²¹form implemented in the Vienna Ab initio Simulation Package (VASP 5.3)^22,23code. The projected augmented wave (PAW)²⁴method with a plane-wave basis set was used. The energy cutoffand convergence for the force were set to be 400 eV and 0.01 eV/Å, respectively. A vacuum space of 15 Å was adopted to minimize the artificial interaction between 2D material layer and its images. The PBE-D2 functional of Grimme²⁵was utilized to account for the dispersive forces. The Berry-phase method²⁶ was employed to evaluate crystalline polarization. Transmission spectra were computed by using the nonequilibrium Green’s function (NEGF) and Landauer- Buttiker formula,²⁷ implemented in the QuantumWise ATK code,²⁸ with which the 30 × 1 × 100 k-point mesh was employed in the Brillouin zone. For the ab initio Born- Oppenheim molecular dynamics simulation (BOMD) (see below), we adopted the PBE-D2 functional and the same vacuum spacing. The simulation was 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-terminated germanene and stanene (Figure 1a) have been fabricated by

Goldberger and co-workers recently, which are air stable and free-standing with moderate bandgaps and have high mobility comparable to phosphorene.^14,15,29Goldberger and co-workers 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 electricfield, as shown inFigure 1, panel 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 S1a), which is much higher than the ferroelectric switching barrier reported previously for 3D ferroelectric BaTiO₃³⁰ and 2D ferroelectric SnSe.¹⁰ This rotational barrier, defined as Ek, is the collective rotational barrier (supposing all spins/dipoles rotating spontaneously toward 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. Therefore, Ek

cannot be used to determine the Curie temperature. However, Ekcan 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 switch- ing. 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 toward 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. Here, theEjvalue will be very high because the dipole of adjacent ligands cannot be opposite (to avoid collision) since−CH₂OCH₃is a long stick-like ligand anchored at one side. As such, all the ligands are aligned toward 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 4.5μC/cm²in 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 Figure 1, panel 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. By taking function- alized germanene as an example, the computed polarizations (seeTable 1) are much higher than those of SnSb-CH₂OCH₃, while their bandgaps seem suitable for nanoelectronic applications. Various system configurations including antiferro- Table 1. Computed Polarizations and Bandgaps of Various Functionalized Monolayers

SnSb-CH₂OCH₃ Ge-CH₂F Ge-CHO Ge-COOH Si−OH MoS₂−COOH MoS₂−CONH₂ MoS₂+ Silica−OH

polarization (10⁻¹⁰C/m) 0.31 1.17 0.81 0.68 0.76 0.55 0.50 0.37

bandgap (eV) 1.1 1.0 1.66 1.36 0.51 0.20 0.11 1.80

Nano Letters

electric configurations are examined to confirm that the ferroelectric states shown in Figure 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 studies.^31,32 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, and 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.

It is worthy of mentioning that the partially hydroxylized c o u n t e r p a r t s , l i k e S i₆H₃( O H )₃ ( s i l o x e n e ) a n d Ge_1−xSn_xH_1−x(OH)_x, have also been synthesized.¹⁴Here, 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 MoS₂was shown to become a stable semiconductor when functionalized,^17,33 and the associated polar groups may induce ferroelectricity as well.

As shown inFigure 1, panels d and 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 that of 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 (Figure 2a). 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 (seeFigure 2b). Compared to recent progress 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 decades.^31,32,34It 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 experiment²⁰demonstrated 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 MoS₂monolayer on silica functionalized with SAMs can be ferroelectric also, for example, with −OH functionalization (Figure 2c). The surface-hydroxylized silica exhibits polarization (Table 1), which thereby results in a horizontal electricfield along the MoS₂monolayer. When used

in photovoltaics, the electricfield can facilitate the departure of electrons and holes so that the lifetime of excitons and the photovoltaics efficiency may be enhanced.

Heterostructure Devices. It is known that polar disconti- nuity at the interface can induce polarization charges and electric fields that drive metal−insulator transition.³⁵Here 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 nonferroelectric, and within the associated boundary free charge accumulates along the 1D interface. The density of free carrierλFwill balance the polarization charge densityλP:

λ_F=(P_F−P_NF)n= −λ_P

Here P_F and P_NF are the polarization of ferroelectric and nonferroelectric regions, respectively, and n is a unit vector pointing from the ferroelectric region to nonferroelectric 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 inFigure 3, panel a. To simulate the metal−insulator transition upon polar disconti- nuity, 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 lattice,¹¹which generates a difference in potential between two sides, as shown in Figure 3, panel b. When the polarization is aligned in the direction of 60 degrees 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 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 ref20).

hydroxylized graphene andfluorinated graphene are insulating with bandgaps larger than 2.7 eV. Thus, the interface of functionalized ferroelectric and nonferroelectric regions can be used as ferroelectricfield-effect transistor with ultrahigh on/off ratio (on/offstates are respectively metallic/semiconducting).

If the nonferroelectric 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 toward the Fermi level in one spin-channel, which makes the system half-metallic. As shown

in Figure 3, panel 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 left, but vice versa when it is switched toward the right. Therefore, it is feasible to use electric field to control spin, which would render the combined electrical writing and magnetic reading possible in data storage.

Ferroelectric tunnel junction (FTJ)³⁶is an alternative way for nondestructive reading, where switching the polarization of a sandwiched ferroelectric layer between two different metals can produce a change in tunneling resistance, known as tunneling 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 point 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,.

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

Nano Letters

electroresistance (TER). Previous studies on TER were focused 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 inFigure 4, ourfirst design is a p-doped (0.001e/atom) junction of germanene passivated by−CH₃and 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, upon different polarization directions. In this case, the TER is over 300, much larger than current TER measured in experiments.

Other Device Designs.Previous studies have predicted that germanene, stanene, and their functionalized counterpart are 2D topological insulators (TIs),^37,38while half-passivated Ge−I or Sn−I exhibit quantum anomalous Hall effect.³⁹ 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, panel 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 of two ferroelectric domains of functionalized germanene or stanene, and the on/offstate is switchable and depends on the parallel/antiparallel configurations of ferro- electric domains, where the edge in antiparallel 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 externalfield.

Therefore, the ferroelectric PN junction, like a silicon PN junction with hydroxylized surface or a MoS₂PN junction on a hydroxylized silica substrate (Figure 5b), can switch between high/low resistance states upon the switching of polarization.

Since the diffusion length can be up to micrometers, our designs are still qualitative.

Conclusion.On the basis of thefirst-principles calculations, we show the rise of ferroelectricity^40,41in 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 nonvolatile memory. On the basis of 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 electro-resist- ance, and multiferroic junctions controlling spin by electric fields. These systems can combine high-mobility semiconduc- tors for nanoelectronics and fast writing and nondestructive reading for nonvolatile memory, holding great promise as multifunctional devices.

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ASSOCIATED CONTENT

*^S Supporting Information

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.nano- lett.6b04309.

Configurations of SnSb-CH2OCH₃, and germanene functionalized by −CH₂F, −CHO, and −COOH;

simulated STM images uponVbias= 1.5 V and snapshots of equilibrium structures at 350 K at the end of BOMD simulation (PDF)

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AUTHOR INFORMATION Corresponding Authors

*E-mail: [email protected].

*E-mail: [email protected].

*E-mail: [email protected].

Notes

The authors declare no competingfinancial interest.

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ACKNOWLEDGMENTS

M.W., K.Y., and J.L. are supported by the National Natural Science Foundation of China (Nos. 21573084, 11274130, and 51431006). X.C.Z. is supported by the U.S. National Science Foundation through the Nebraska Materials Research Science and Engineering Center (MRSEC) (Grant No. DMR- Figure 5.(a) In-plane heterostructure of germanene passivated by−CH₂F and−I. (b) Ferroelectric PN junction based on hydroxylized Si (111) surface and MoS₂on hydroxylized silica surface.

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.

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NOTE ADDED AFTER ASAP PUBLICATION

This paper was published on the Web on October 18, 2016.

Additional text corrections were implemented, along with a revised Supporting Informationfile, and the paper was reposted on October 19, 2016.

Nano Letters