Distorted Monolayer ReS2 with Low-Magnetic-Field Controlled

Subscriber access provided by McMaster University Library

Article

Distorted Monolayer ReS2 with LowMagnetic-Field Controlled Magnetoelectricity Jinlei Zhang, Shuyi Wu, Yun Shan, JunHong Guo, Shuo Yan, Shuyu Xiao, Chunbing Yang, Jiancang Shen, Jian Chen, Lizhe Liu, and Xinglong Wu ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.8b09058 • Publication Date (Web): 08 Feb 2019 Downloaded from http://pubs.acs.org on February 8, 2019

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 25 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Nano

Distorted Monolayer ReS2 with Low-Magnetic-Field Controlled Magnetoelectricity Jinlei Zhang†, Shuyi Wu†, Yun Shan†,‡, JunHong Guo†,§, Shuo Yan, Shuyu Xiao†, Chunbing Yang†, Jiancang Shen†, Jian Chen‖, Lizhe Liu*,† and Xinglong Wu*,† † National

Laboratory of Solid State Microstructures and Department of Physics, Nanjing

University, Nanjing 210093, P. R. China ‡

China Key Laboratory of Advanced Functional Materials of Nanjing, Nanjing Xiaozhuang

University, Nanjing, 210093, P. R. China § School

of Optoelectronic Engineering and Grȕenberg Research Centre, Nanjing University of

Posts and Telecommunications, Nanjing, 210093, P. R. China ‖ Research

Institute of Superconductor Electronics, Nanjing University, Nanjing 210093, P. R.

China ABSTRACT. Two dimensional (2D) materials possessing ferroelectric/ferromagnetic orders and especially low-magnetic-filed controlled magnetoelectricity have great promise in spintronics and multistate data storage. However, ferroelectric and magnetoelectric (ME) dipoles in the atomthick 2D materials are difficult to be realized due to structural inversion symmetry, thermal actuation and depolarized field. To overcome these difficulties, the monolayer structure must possess an in-plane inversion asymmetry in order to provide out-of-plane ferroelectric polarization. Herein, Crystal chemistry is adopted to engineer specific atomic displacement in monolayer ReS2 to change the crystal symmetry to induce out-of-plane ferroelectric polarization at room

ACS Paragon Plus Environment

1

ACS Nano 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 25

temperature. The cationic Re vacancy in the atom-displaced ReS2 monolayer causes spin polarization of two immediate neighbor sulfur atoms to generate magnetic ordering and the ferroelectric distortion near the Re vacancy locally tunes the ferromagnetic order thereby triggering low-magnetic-field controlled ME polarization at about 28 K. As a result, 2D ME coupling multiferroic is achieved. Our results not only reveal a design methodology to attain coexistence of ferroelectric and ferromagnetic orders in 2D materials, but also provide insights into magnetoelectricity in 2D materials.

KEYWORDS: distorted monolayer ReS2, ferroelectric and ferromagnetic, magnetoelectric effect


2

Page 3 of 25 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Nano

Two dimensional (2D) materials possessing ferroelectric/ferromagnetic orders and especially low-magnetic-filed controlled magnetoelectricity have great promise in spintronics and multistate data storage.1-6 In ultrathin film such as perovskite ferroics, spontaneous out-of-planeelectric and magnetic orders begin to vanish at the thicknesses of few unit cells7-9 and 2D atomicthick materials have been explored rarely for out-of-plane ferroelectric polarization.

The

magnetoelectric (ME) dipoles in an atomic-thick 2D material are difficult to be realized due to structural inversion symmetry, thermal actuation and depolarized field. To overcome these difficulties, the monolayer structure must possess an in-plane inversion asymmetry in order to provide out-of-plane ferroelectric polarization. Very recently, it was predicted that breaking of the in-plane symmetry in van der Waals monolayer with a special structure can produce an asymmetrical out-of-plane structural configuration and 2D ferroelectric polarization can be achieved from atomic monolayer.10-12 If magnetic order is further introduced into such a 2D ferroelectric, a 2D multiferroic can be achieved and possibly shows applications in multifunctional nanodevices. In this work, the monolayer ReS2 with a non-centrosymmetric Td structure (Figure S1) is adopted as a 2D model to investigate the multiferroic and magnetoelectricity. In the Td structure, migrations of out-of-plane electric dipoles are severely restricted by the large barrier energy of about 4.57 eV (Figure S2a,b) which prevents the emergence of ferroelectricity.3 To overcome the hurdle, we change the structural environment of the Td phase on the sub-Angström scale to construct a transitional Tt phase.13,14 The Tt structure is formed by introducing the centrosymmetric metallic Tc phase into the pristine Td phase and the barrier energy in the Tt phase can be decreased to provide out-of-plane ferroelectricity. The structural transition of the Td phase is frequently accompanied with formation of cationic Re vacancies that can produce magnetic


3

ACS Nano 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 25

orders15-17 and consequently, multiferroic characteristics can be expected from the monolayer ReS2. More importantly, the magnetic moments are distributed nearby the ferroelectric dipoles, which can produce coupling to induce significant ME polarization. The ME coupling effect is generally induced below 1 Tesla which is appropriate for practical devices.4,18

RESULTS AND DISCUSSION Figure 1a shows the atomic force microscopy (AFM) image and height profile revealing monolayer ReS2 with a fairly flat and 0.91 nm thick terrace. Using spherical aberration corrected scanning transmission electron microscopy (Cs-corrected STEM), a series of Tt structures between the Td (turquoise dots) and Tc (orange dots) phases are identified (Figure 1b). The typical transition region from Td to Tt and finally to Tc with modified lattice interspacing is depicted at the bottom of Figure 1b. Figure 1c displays the corresponding lattice distortion and atomic displacement from the Td (cyan balls) to Tt/Tc (grey balls) sites by STEM. Statistical analysis based on the STEM images shows that the lattice plane distance of the zigzag chain decreases slowly from 0.36 (Td) to 0.27 nm (Tc) and the most probable zigzag chain distance is 0.32 nm (Figures S3 and S4), implying that the most possible Tt structure has an atomic displacement of about 30 % at room temperature. Some Re vacancies observed from the Tt structure are circled by magenta dots and the concentration of Re vacancies is estimated to be 5.5% according to energy-dispersive X-ray spectrometry (EDS) (Figure S5) and electron paramagnetic resonance (EPR) (Figure S6). The shift and broadening of the corresponding X-ray diffraction (XRD) peaks confirm that the synthesized ReS2 has good crystallinity and is produced due to the structural transition from the Td to Tc phases (Figure S7).19


4

Page 5 of 25 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Nano

The lattice structure of the transition region acquired from the STEM image is calculated by DFT (calculation details in Supplementary Information) to disclose the charge density distribution difference between the top and bottom surfaces (Figure 1d,e). Different from the Tc phase with high symmetry, in-plane symmetry breaking occurs in the Td and Tt structures. Hence, the electronic wave functions at the top and bottom surfaces become asymmetrical leading to out-ofplane electric dipoles. Moreover, the transitional potential barrier from the Tt to Tc phases diminishes dramatically compared to that from the Td to Tc phases (Figure S2a,b). The most probable kinetic pathway occurs in the Tt phases with above 30% atomic displacement (Figure S2b and Figure S4) and transitional barrier smaller than 0.78 eV (Figure 1f), which are comparable to those in Sn atom reversal of SnPc updown,20 hydrogen tautomerization of naphthalocyanine molecule,21 and I-vacancy-induced electric polarization in the CrI3 monolayer.3 This indicates that electric polarization reversal can be achievable by lattice distortion. To confirm the phase transition, in situ X-ray photoelectron spectroscopy (XPS) is performed on the synthesized ReS2 at different temperature as shown in Figure 1g. Both the Re 4f5/2 and 4f7/2 XPS spectra can be deconvoluted into three doublets. The Td doublet sub-peaks at 40.46 (4f5/2) and 42.86 eV (4f7/2) and Tc ones at 40.68 (4f5/2) and 43.08 eV (4f7/2) suggest coexistence of the Td and Tc structures consistent with XRD results (Figure S7). The third doublet sub-peaks correlate well with the Tt structure and show a blue-shift of about 0.12 eV as the temperature is increased from 270 to 300 K due to lattice distortion. Quantification analysis reveals that about 16.7% of ReS2 is transformed from the Tt to Tc structures from 270 to 300 K. Furthermore, the temperature dependence of the specific heat c confirms the phase transition from 285 to 305 K (Figure S8). To experimentally characterize the ferroelectricity, piezo-response force microscopy (PFM) is conducted to measure the local piezoelectric response and ferroelectric domains. As shown in


5

ACS Nano 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 25

Figure 2a,b, the room-temperature phase and amplitude mapping images of vertical PFM are partially overlaid on a 3-dimensional (3D) topography (area by green line), revealing that the ferroelectric domains have a belt-like form and no crosstalk with topography occurs. Since the domain pattern is closely associated with the distribution of the Tt phase, the obviously reduced domain content at room temperature should be due to the large drop in the Tt phase content in the monolayer ReS2 as shown by XPS (Figure 1g). The piezo-response is about 20 pm at only 0.5 V AC in Figure S9, and it is comparable to those of typical 3D ferroelectrics.22,23 Thus, the distorted monolayer ReS2 has excellent piezoelectricity. Switching PFM measurement is carried out with a DC voltage applied to the top surface with an AC voltage to switch polarization. The characteristic hysteresis and butterfly loops of vertical PFM further confirm that the spontaneous polarization is switchable under external electric fields (Figure 2c,d). The PFM reversal behavior of the ferroelectric domains is evaluated after electrical poling using a DC bias applied to the proximal tip and the ‘up’ and ‘down’ c-oriented domains appear in orange and purple (Figure 2e), respectively. This corroborates migration of the ferroelectric dipoles under an external electric field. The typical ferroelectric hysteresis loops at 50 Hz at different temperature clearly demonstrate ferroelectricity in the synthesized ReS2 (Figure 2f). The P  E hysteresis loops as direct evidence for ferroelectricity shrink as the temperature is increased from 260 to 300 K. Spontaneous polarization P decreases from 2.0 to 0.85 μC/cm2 and is almost invisible above 300 K due to the end of the ferroelectric phase transition. The coercive field of 0.65 kV cm-1 is smaller than those in 3D ferroelectrics such as BTO (10 kV cm–1), PZT (20 to 80 kV cm–1), DIPAC (9 kV cm–1), and DIPAB (5 kV cm-1).23,24 Owing to the relatively soft covalent bonding in the distorted ReS2 structure, Re vacancies can easily form as shown in Figure 1b. The ground state is calculated to be magnetic in the case of Re


6

Page 7 of 25 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Nano

vacancies in the Td structure except the Tc structure (Figures S10 and S11).15 The magnetic moments of two immediate neighbor sulfur atoms S1 and S2 near the Re vacancy in the Tt structures with different atomic displacements from the Td to Tc structure are presented in Figure 3a (calculation details in Supplementary Information). Orbital analysis discloses that the main contribution to the magnetization density (i.e., Δρ↑,↓ = ρ↑ − ρ↓) for the Re vacancy) arises from the electrons in p orbitals of the two S atoms (the inset of Figure 3a and Figure S12). For the higher atomic displacements toward the Tc structure ( 40%), both atoms S1 and S2 have negligible magnetic moments. As for the lower atomic displacements ( 40%) of the Tt structure, the magnetic moments increase dramatically to 0.64 and 0.76 μB in the Td structure, respectively. For less than 30% atomic displacements, the energy difference between the spin polarized and spin unpolarized states is more than 42 meV (Figure 3a, right side), indicating stability of the magnetic conFigureuration at a fairly high temperature. Based on the theoretical calculations, we determine the magnetic characteristics of the synthesized ReS2 using a superconducting quantum interference device (SQUID) and vibrating sample magnetometer (VSM). Figure 3b shows the room-temperature out-of-plane (along the c axis) and in-plane (perpendicular to the c axis) magnetization loops of the sample with 5.5% Re vacancy content. The nonlinear hysteresis loop with nonzero residual magnetization and nonzero coercivity indicates the room-temperature ferromagnetic order. The out-of-plane magnetization is saturated at a value 3.4 times larger than the in-plane one and the corresponding coercive field is also 2 times bigger.25,26 The MH curves at different temperature are shown in Figure S13 and the ferromagnetic order occurs at above 350 K. The magnetic susceptibilities M(T) in the temperature range of 5400 K under zero-field-cooling (ZFC) and field-cooling (FC) with a steady external field of 200 Oe are shown in Figure 3c. The magnetic ordering temperature is  300 K and in the


7

ACS Nano 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 25

temperature range of the phase transition from the Tt to Tc structure (Figure S8) the magnetic moments in atoms S1 and S2 begin to disappear. The Curie temperature is estimated to be above 704 K based on mean field approximation calculation by extrapolating the M(T) curve acquired from VSM (the inset of Figure 3c). Figure 3c shows a magnetic anomaly at 60 K. For confirmation, we obtain the in-plane and out-of-plane M(T) curves at a 200 Oe external magnetic field as shown in Figure 3d. The magnetic anomaly associated with the magnetic transition mainly occurs in the in-plane at 60 K, which clues a possible ME coupling. Here we would like to mention that our EDS and XPS reveal the absence of magnetic metal impurities/contaminants (Figure S14).26 We also conduct more AFM observations and find that the number of few nanosheets (such as monolayer and bilayer) obviously increases with Re vacancy concentration in our ReS2 sample (Figure S15). The saturation magnetization tracks well the change in the Re vacancy concentration and the sample without Re vacancies has no magnetism (Figure S16). Hence, the observed ferromagnetism is intimately related to the Re vacancies. Using a classical illustration of spin-orbital coupling based on the above DFT calculation, the magnetic moments (Si and Sj) on atoms S1 and S2 are canted from each other (Figure 4a) and the horizontal mirror-plane symmetry is broken due to different electric transfer to the two S atoms in the various distorted structures (Figure S17) thereby generating individual polarizations (Pi and Pj) on the two S atoms. This can be defined as a ME monopole that is given by the space integration of the magnetization density timing position operator.2 Therefore, the magnetically induced polarization as a function of applied magnetic field can be calculated as shown in Figure S18. By measuring the pyroelectric current, the total polarization P versus temperature is obtained as shown in Figure 4b, where P = ∑(Pi +Pj) is calculated by integrating the measured pyroelectric current. P increases with decreasing temperature from 65 K confirming onset of the ferroelectric phase


8

Page 9 of 25 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Nano

transition. P begins to change gently at 18 μC/m2 below 45 K indicating disappearance of the ferroelectric effect. The dielectric anomaly as an important signature that the ferroelectric phase transition exist also takes place at the same temperature in the dielectric constant mapping image (Figure S19). The temperature dependence of the magnetic susceptibility (χ) and specific heat c also exhibit signals consistent with the dielectric anomaly (Figure 4c). Therefore, nontrivial spin– charge-orbital coupling may exist during the magnetic phase transition below the temperature of the dielectric anomaly. Since magnetoelectricity is a kind of spin-charge-orbit driven phase transition in the case without exchange-striction effects,2,27,28 the magnetic-field-dependent ferroelectricity and magnetically induced polarization are measured from the synthesized sample with 5.5% Re vacancy content to further determine the intrinsic ME properties.29-31 The magnetic dependence of the ferroelectric polarization P is obtained by integrating the pyroelectric currents at different applied magnetic fields as shown in Figure 4d. Even 0.5 T imposes large effects on P. The magnetically induced polarization PH obtained by integrating the ME current as the direct evidence of a ME coupling multiferroic is shown in Figure 4e. PH flops with a nonlinear decrease of 3.7 μC/m2 as the low-magnetic-field H increases from 0 to less than 1 T at 28 K and then exhibits negligible change with higher H. This is in line with the behavior of M(H) which is saturated for H along the in-plane above 0.9 T (Figure 3b). This result clearly shows the existence of the Hinduced ME coupling phase transition in the synthesized ReS2. When the temperature is above 32 K, ME polarization is reduced largely, meaning the end of the H-induced phase transition. We have also further calculated the magnetoelectric (ME) effect by obtaining the change of P per magnetic field strength, P = P0 – PT, here P0 and PT are the ferroelectric polarizations under without and with applied magnetic field (Figure S20). In Figure S20a, the ME effect defined as


9

ACS Nano 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 25

P/P0 × 100% is as high as 28% when H = 2 T, whereas the ME effect is ~27 % at H = 9 T in the perovskite DyMnO332 and ~21 % at H = 9 T in the double perovskite Y2NiMnO6.33 We have also estimated the ME effect by defining the ME coefficient α as α = μodPH/dH (here μo is the magnetic permeability of vacuum) as shown in Figure S20b. At H = 0.4 T, α reaches a maximum (αmax  8.8 ps/m) at 30 K. It is 2 times larger than the coefficient of a linear ME effect in Cr2O3.2,30 When the magnetic field is decreased to ~ 0.3 T, α still exceeds 4 ps/m. These results clearly demonstrate that the current ME effect can occur at a low magnetic field. This ME coupling effect in 2D layered materials induced by lattice distortion and cationic vacancy can be generated and controlled by external low-magnetic-field and expected to have important applications.30,31,34,35

CONCLUSION 2D distorted monolayer ReS2 with simultaneously ferroelectric and ferromagnetic orders is synthesized at above room temperature. The ferroelectric distortion of the two immediate neighbor S atoms near the Re vacancy can be utilized to locally tune the ferromagnetism forming spincharge-orbital coupling. Under an applied magnetic field, spontaneous ferroelectric polarization is obviously suppressed due to decreased spin-charge-transfer coupling and the ferromagnetic ferroelectric polarization is realized below 32 K at a low magnetic field. A good understanding of the coupling mechanism in 2D materials provides insights into the design of ferromagnetic ferroelectrics and boasting reduced size, weight, and energy consumption, two-dimensional multiferroic materials have large potentials in ultrasensitive sensors, nanoscale electromechnical systems, as well as next-generation high-speed and low-power nanoelectronics and nanospintronics.


10

Page 11 of 25 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Nano

METHODS All The single-crystal Td-ReS2 was produced by the Br2-assisted chemical vapor transport method described in ref. 17. A standard mechanical exfoliation method was adopted to isolate the monolayer and few-layer ReS2 films. The T@Td-ReS2 nanosheets were prepared by ultrasonic chemical exfoliation as shown in the following. Ten mg of the single crystals of Td-ReS2 were mixed with 5 mL of 9 M nitric acid in a centrifugal tube and sonicated for 1 h (100W) at room temperature. The temperature was controlled to be less than 35 oC using an ice bag. The resulting mixture was centrifuged and the supernatant was decanted. The single-crystal Td-ReS2 were processed 1, 2, 3, and 4 times, respectively, to obtain T@Td-ReS2 nanosheets with different TReS2 concentrations. Afterwards, the samples were thoroughly washed with absolute ethanol and water sequentially until neutral and to remove impurities adsorbed on the T@Td-ReS2 nanosheets. They were then dried in air at 60 °C for 5 h. The number of layers can be identified by the color interface of a 285-nm thick SiO2 wafer and confirmed by measuring the thickness of the flakes using a Bruker Multimode 8 atomic force microscope. The XPS spectra were collected in situ on a PHI 5000 VersaProbe at different temperature using samples deposited on silicon wafers. The spectra were acquired at 300 K and then 270 K by cooling. The Cs-corrected STEM images were obtained on the JEM-200CX and FEI Titan3 G2 Cube 60-300 kV with double Cs correctors in the TEM and STEM modes. AFM and PFM were conducted on a Bruker NanoScope IV NS4-1 instrument at the same time. Due to the conductive demand in the PFM measurements, the layered ReS2 samples are deposited consistently on Ptcoated Si wafers for magnetic measurements. Probe-type Premier II was used to acquire the


11

ACS Nano 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 25

electric hysteresis loop of the layered ReS2 at different temperatures. The magnetization of the synthesized layered ReS2 sample and applied magnetic field (M  H) was determined at different temperature on a superconducting quantum interference device (MPMS SQUID VSM of Quantum Design). The temperature dependence (MT) of the magnetic susceptibility was assessed in the temperature ranges of 5-400 and 400-650 K by SQUID and VSM, respectively. The layered ReS2 is deposited on the surface of a Pt-coated silicon for the measurement of magnetic anisotropy. The EPR spectra were recorded on an EMX-10/12 instrument with f = 9.7662 GHz. The synthesized and pure Td structured ReS2 samples were placed in the EPR pressure tube and EPR spectra were acquired from the pure Td and ReS2 samples for reaction time durations of 1, 2, 3, and 4 h. The gold electrode was evaporated on the widest side of the pressing plate. The dependence of the electric polarization on the magnetic field was determined by measuring the magnetoelectric current in customized PPMS-9 of Quantum Design. Before each measurement, the sample was poled by the following procedures. A magnetic field (B) was applied to bring the system to the paraelectric collinear-ferromagnetic phase. An electric field E (Epole = 4 kV/cm) was applied perpendicular to B to polarize the magnetic-field-induced ferroelectric phase. After poling, E was removed and the magnetoelectric current was measured with an electrometer by scanning B at a rate of 80 Oe/s. Electric polarization was determined by integrating the magnetoelectric current using a time electrometer. During the measurement of the temperature-dependent pyroelectric current, the sample was cooled from 310 to 3 K in a static electric field, Epole = 4 kV/cm, and zero magnetic field (ZFC). The pyroelectric current was measured by heating the sample at a rate of 2 K/min in a constant magnetic field such as 0, 0.5, 1.5, 2.0 T.


12

Page 13 of 25 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Nano

Figure 1. (a) AFM topographical image and height profile of a synthesized monolayer ReS2 on a silicon wafer. The height profile data are obtained along the red line in the AFM image. (b) Cscorrected STEM image of a distorted monolayer in which the Td (cyan balls), Tc (orange balls), Tt structures as well as Re vacancies (surrounded by pink balls) are observed clearly. The lattice interspacing of the zigzag chain in the Tt phase shows a noticeable decrease from 0.36 nm of Td phase to 0.27 nm of Tc phase (the bottom with shifty colors). Scale bar: 0.5 nm. (c) Corresponding lattice distortion and atomic displacement from Td (cyan balls) to Tt/Tc (grey balls) sites acquired from the STEM image. The grey balls represent Re atoms in Tt and Tc structures displaying the displacement away from their initial positions (cyan balls in Td structures). (d,e) Charge density distributions of the monolayer structures at the top (d) and bottom (e) surfaces. (f) Kinetic pathways of the polarization reversal processes of one typical Tt structure. IS, TS, and FS


13

ACS Nano 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 25

represent initial, transitional and final states, respectively. (g) XPS spectra of the Re atoms acquired from the synthesized ReS2 film at 300 K (upper) and 270 K (down), respectively.


14

Page 15 of 25 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Nano

Figure. 2 (a,b) Ferroelectric domain phase (a) and amplitude (b) mapping images of the lateral PFM overlaid on the 3D topography map of a synthesized monolayer ReS2 (5×5 μm2). The area of the monolayer is marked by the green dashed line. (c,d) Phase-voltage hysteresis loop (c) and amplitude-voltage butterfly loop (d) of the ReS2 monolayer acquired by PFM. (e) 0.15×0.15 μm2 domain reversal (PFM) image of the ReS2 monolayer after electrical poling with a DC bias applied to the proximal tip (0.3×0.3 μm2). (f) Ferroelectric hysteresis loops of a synthesized ReS2 sample at different temperature.


15

ACS Nano 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 25

Figure 3. (a) Calculated magnetic moment (magenta ones) of atoms S1 and S2 and energy difference between the spin polarized and spin unpolarized states (olive one) versus atomic displacement from the Td to Tc structures (the solid line as a guide to the eye). The inset shows the spin resolved charge density of the two immediate neighbor atoms S1 and S2 near the Re vacancy (dashed circle). (b) Orientation dependence of magnetization at 300 K. The magnetic fields are parallel (out-of-plane) and vertical (in-plane) to the c axis. (c) Temperature dependence of the magnetic susceptibility measured at 200 Oe external magnetic field cooling (FC) and zero magnetic field cooling (ZFC).

The extrapolated line at higher temperature intersects the

temperature axis at 704 K indicating the Curie temperature (inset). (d) Temperature dependence of the magnetic susceptibility measured at the 200 Oe external magnetic field in the out-of-plane and in-plane directions.


16

Page 17 of 25 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Nano

Figure 4. (a) Phase transition caused by the applied magnetic field which induces ME polarization PH (reversible).

b) Spontaneous polarization P and pyroelectric current as a function of

temperature. The polarization is calculated by integrating the measured pyroelectric current. c) Temperature derivative of the magnetic susceptibility (upper panel) and temperature dependence of the specific heat c (lower panel) confirming the phase transition related to the magnetic ordering. d) Temperature dependence of the total electric polarization at the four applied magnetic fields of 0, 0.5, 1.5 and 2 T. e) ME polarization PH versus applied magnetic field at 28, 30, and 32 K.


17

ACS Nano 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 25

ASSOCIATED CONTENT Supporting Information. Density functional theory calculations, spin-charge-orbital coupling, first-principles parameterized equivalent Heisenberg model, mechanism of magnetoelectric coupling, STEM images and symmetrical change of Td and Tc structure, evolution of the total energy of monolayer ReS2 and calculated activation barrier energy of different Tt phases, lattice distance distribution of the zigzag chain, Re vacancy concentrations, X-band EPR spectra, XRD patterns, temperature dependence of the specific heat c, ferrelectric piezoresponse, formation energy and total magnetic moment of the system with different point defects, energy difference between the antiferromagnetic and ferromagnetic states versus atomic displacement, magnetic hysteresis loops, XPS spectra, calculated electric transfer of the two immediate neighbor S atoms, AFM images and thickness distributions of the layered ReS2 with different mean Re vacancy concentrations, calculated total PH and ME effect as a function of the applied magnetic field, and dielectric constant versus temperature, and This material is available free of charge via the Internet at http://pubs.acs.org. The authors declare no competing financial interest.

AUTHOR INFORMATION Corresponding Author L. Z. Liu, e-mail: [email protected], X. L. Wu, e-mail: [email protected] Author Contributions J.L.Z, S.Y.W, Y.S, and J.H.G contribute equally to this work.


18

Page 19 of 25 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Nano

ACKNOWLEDGMENTS This work was supported by National Key R&D Program of China (No. 2017YFA0303200 and 2018YFA0306004), National Natural Science Foundations of China (Nos. 61521001, 11674163, and 14041621), and Natural Science Foundation of Jiangsu Province (BK20171332 and BK20161117). This work was also supported by the Fundamental Research Funds for the Central Universities (0204-14380066 and 0204-14380083) and high Performance Computing Centers of Nanjing University and Shenzhen. Partial support is also supported by program B for Outstanding Ph.D candidate of Nanjing University (201701B011).


19

ACS Nano 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 25

REFERENCES (1) Han, W.; Kawakami, R. K.; Gmitra, M.; Fabian, J. Graphene Spintronics. Nat. Nanotechnol. 2014, 9, 794-807. (2) Ricci, F.; Bousquet, E. Unveiling the Room-Temperature Magnetoelectricity of Troilite FeS. Phys. Rev. Lett. 2015, 116, 227601. (3) Zhao, Y. H.; Lin, L. F.; Zhou, Q. H.; Li, Y.H.; Yuan, S. J.; Chen, Q.; Dong, S.; Wang, J. L. Surface Vacancy-Induced Switchable Electric Polarization and Enhanced Ferromagnetism in Monolayer Metal Trihalides. Nano Lett. 2018, 18, 2943-2949. (4) Ramesh, R.; Spaldin, N. A. Multiferroics: Progress and Prospects in Thin Films. Nat. Mater. 2007, 6, 21-29. (5) Liu, W. Q.; Wong, P. K. J.; Xu Y. B. Hybrid Spintronic Materials: Growth, Structure and Properties. Prog. Mater. Sci. 2019, 99, 27-105. (6) Liu, W. Q.; West, D.; He, L.; Xu, Y. B.; Liu, J.; Wang, K. J.; Wang, Y.; van der Laan, G.; Zhang, R.; Zhang, S. B. ; Wang, K. L. Atomic-Scale Magnetism of Cr-Doped Bi2Se3 Thin Film Topological Insulators. ACS Nano 2015, 10, 10237-10243. (7) Junquera, J.; Ghosez, P. Critical Thickness for Ferroelectricity in Perovskite Ultrathin Films. Nature 2003, 422, 506-509. (8) Fong, D. D.; Stephenson, G. B.; Streiffer, S. K.; Eastman, J. A.; Auciello,O.; Fuoss, P. H.; Thompson, C. Ferroelectricity in Ultrathin Perovskite Films. Science 2004, 304, 1605-1653. (9) Chang, K.; Liu, J. W.; Lin, H.C.; Wang, N.; Zhao, K.; Zhang, A. M.; Jin, F.; Zhong, Y.; Hu, X. P.; Duan, W. H.; Zhang, Q. M.; Fu, L.; Xue, Q. K.; Chen, X.; Ji, S. H. Discovery of Robust in-Plane Ferroelectricity in Atomic-Thick SnTe. Science 2016, 353, 274-278.


20

Page 21 of 25 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Nano

(10) Shirodkar, S. N.; Waghmare, U. V. Emergence of Ferroelectricity at a Metal-Semiconductor Transition in a 1T Monolayer of MoS2. Phys. Rev. Lett. 2014, 112, 157601. (11) Huang, C. X.; Du, Y. P.; Wu, H. P.; Xiang, H. J.; Deng, K. M.; Kan, E. J. Prediction of Intrinsic Ferromagnetic Ferroelectricity in a Transition-Metal Halide Monolayer. Phys. Rev. Lett. 2018, 120, 147601. (12) Zhu, H. Y.; Wang, Y.; Xiao, J.; Liu, M.; Xiong, S. M.; Wong, Z. J.; Ye, Z. L.; Ye, Y.; Yin, X. B.; Zhang, X. Observation of Piezoelectricity in Free-Standing Monolayer MoS2. Nat. Nanotechnol. 2015, 10, 151-155. (13) Tongay, S.; Sahin, H.; Ko, C.; Luce, A.; Fan, W.; Liu, K.; Zhou, J.; Huang, Y. S.; Ho, C. H.; Yan, J. Y.; Ogletree, D. F.; Aloni, S.; Ji, J.; Li, S. S.; Li, J. B.; Peeters, F. M.; Wu, J. Q. Monolayer Behaviour in Bulk ReS2 due to Electronic and Vibrational Decoupling. Nat. Commun. 2014, 10, 1038. (14) Pitcher, M. J.; Mandal, P.; Dyer, M. S.; Alaria, J.; Borisov, P.; Niu, H.; Claridge, J. B.; Rosseinsky, M. J. Tilt Engineering of Spontaneous Polarization and Magnetization above 300 K in a Bulk Layered Perovskite. Science 2015, 347, 420-424. (15) Horzum, S.; ćakır, D.; Suh, J.; Tongay, S.; Huang, Y. S.; Ho, C. H.; Wu, J.; Sahin, H.; Peeters, F. M. Formation and Stability of Point Defects in Monolayer Rhenium Disulfide. Phys. Rev. B 2014, 89, 162-169. (16) Xiao, J.; Zhu, H. Y.; Wang, Y.; Feng, W.; Hu, Y. X.; Dasgupta, A.; Han, Y. M.; Wang, Y.; Muller, D. A.; Martin, L. W.; Hu, P. A.; Zhang, X. Intrinsic Two-Dimensional Ferroelectricity with Dipole Locking. Phys. Rev. Lett. 2018, 120, 227601.


21

ACS Nano 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 25

(17) Li, L. Z.; Jokisaari, J. R.; Zhang, Y.; Cheng, X. X.; Yan, X. X.; Heikes, C.; Lin, Q. Y.; Gadre, C.; Schlom, D. G.; Chen, L. Q.; Pan, X. Q. Control of Domain Structures in Multiferroic Thin Films through Defect Engineering. Adv. Mater. 2018, 30, 1802737. (18) Hur, N.; Park, S.; Sharma, P. A.; Ahn, J. S.; Guha, S.; Cheong, S. W. Electric Polarization Reversal and Memory in a Multiferroic Material Induced by Magnetic Fields. Nature 2004, 429, 392-395. (19) Wu, S. Y.; Shan, Y.; Guo, J. H.; Liu, L. Z. X.; Liu, X.; Zhu, X. B.; Zhang, J. L.; Shen, J. C.; Xiong, S. J.; Wu, X. L. Phase-Engineering-Induced Generation and Control of Highly Anisotropic and Robust Excitons in Few-Layer ReS2. J. Phys. Chem. Lett. 2017, 8, 27192724. (20) Wang, Y. F.; Kröger, J.; Berndt, R.; Hofer, W. A. Pushing and Pulling a Sn Ion through an Adsorbed Phthalocyanine Molecule. J. Am. Chem. Soc. 2009, 131, 3639-3643. (21) Liljeroth, P.; Repp, J.; Meyer, G. Current-Induced Hydrogen Tautomerization and Conductance Switching of Naphthalocyanine Molecules. Science 2007, 317, 1203-1206. (22) Zhang, Q.; Xie, L.; Liu, G. Q.; Prokhorenko, S.; Nahas, Y.; Pan, X. Q.; Bellaiche, L.; Gruverman, A.; Valanoor, N. Nanoscale Bubble Domains and Topological Transitions in Ultrathin Ferroelectric Films. Adv. Mater. 2017, 29, 1702375. (23) Fu, D. W.; Cai, H. L.; Liu, Y. M.; Ye, Q.; Zhang, W.; Zhang, Y.; Chen, X. Y.; Giovannetti, G.; Capone, M.; Li, J. Y.; Xiong, R. G. Diisopropylammonium Bromide Is a HighTemperature Molecular Ferroelectric Crystal. Science 2013, 339, 425-428. (24) Zhang, M. H.; Wang, K.; Du, Y. J.; Dai, G.; Sun, W. Li, G.; Hu, D.; Thong, H. C.; Zhao, C. L.; Xi, X. Q.; Yue, Z. X.; Li, J. F. High and Temperature-Insensitive Piezoelectric Strain in Alkali Niobate Lead-free Perovskite. J. Am. Chem. Soc. 2017, 139, 3889-3895.


22

Page 23 of 25 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Nano

(25) Huang, B.; Clark, G.; Navarro-Moratalla, E.; Klein, D. R.; Cheng, R.; Seyler, K. L.; Zhong, D.; Schmidgall, E.; McGuire, M. A.; Cobden, D. H.; Yao, W.; Xiao, D.; Jarillo-Herrero, P.; Xu, X. D. Layer-Dependent Ferromagnetism in a van der Waals Crystal Down to the Monolayer Limit. Nature 2017, 546, 270-273. (26) Cervenka, J; Katsnelson, M.I.; Flipse, C. F. J. Room-temperature Ferromagnetism in Graphite Driven by Two-Dimensional Networks of Point Defects. Nat. Phys. 2009, 5, 840844. (27) Mundy, J. A.; Brooks, C. M.; Holtz, M. E.; Moyer, J. A.; Das, H.; Rebola, A. F.; Heron, J. T.; Clarkson, J. D.; Disseler, S. M.; Liu, Z. Q.; Farhan, A.; Held, R.; Hovden, R.; Padgett, E.; Mao, Q. Y.; Paik, H.; Misra, R.; Kourkoutis, L. F.; Arenholz, E.; Scholl, A.; Borchers, J. A.; Ratcliff, W. D.; Ramesh, R.; Fennie, C. J.; Schiffer, P.; Muller, D. A.; Schlom, D. G. Atomically Engineered Ferroic Layers Yield a Room-Temperature Magnetoelectric Multiferroic. Nature 2016, 537, 523-526. (28) Ding, W. J.; Zhu, J. B.; Wang, Z.; Gao, Y. Y.; Xiao, D.; Gu, Y.; Zhang, Z. Y.; Zhu, W. G. Prediction of Intrinsic Two-Dimensional Ferroelectrics in In2Se3 and Other III2-VI3 van der Waals Materials. Nat. Commun. 2017, 8, 14956. (29)Infante, I. C.; Juraszek, J.; Fusil, S.; Dupe, B.; Gemeiner, P.; Dieguez, O.; Pailloux, F.; Jouen, S.; Jacquet, E.; Geneste, G.; Pacaud, J.; Iniguez, J.; Bellaiche, L.; Barthelemy, A.; Dkhil, B.; Bibes, M. Multiferroic Phase Transition near Room Temperature in BiFeO3 Films. Phys. Rev. Lett. 2011, 107, 237601. (30) Kitaro, Y.; Hiraoka, Y.; Honda, T.; Ishikura, T.; Nakamura, H.; Kimura, T. Low-Field Magnetoelectric Effect at Room Temperature. Nat. Mater. 2010, 9, 797-800.


23

ACS Nano 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 25

(31) Zhou, L.; Dai, J. H.; Chai, Y. S.; Zhang, H. M.; Dong, S.; Cao, H. B.; Calder, S.; Yin, Y. Y.; Wang, X.; Shen, X. D.; Liu, Z. H.; Saito, T.; Shimakawa, Y.; Hojo, H.; Ikuhara, Y.; Azuma, M.; Hu, Z. W.; Sun, Y.; Jin, C. Q.; Long, Y. W. Realization of Large Electric Polarization and Strong Magnetoelectric Coupling in BiMn3Cr4O12. Adv.Mater. 2017, 29, 170343. (32) Zhang, N.; Guo, Y. Y.; Lin, L.; Dong, S.; Yan, Z. B.; Li, X. G.; Liu, J.-M. Ho Substitution Suppresses Collinear Dy Spin Order and Enhances Polarization in DyMnO3. Appl. Phys. Lett. 2011, 99, 102509. (33) Su, J.; Yang, Z. Z.; Lu, X. M.; Zhang, J. T.; Gu, L.; Lu, C. J.; Li, Q. C.; Liu, J.-M.;. Zhu, J. S. Magnetism-Driven Ferroelectricity in Double Perovskite Y2NiMnO6, ACS Appl. Mater. Interfaces 2015, 7, 13260-13265. (34) Ishiwata, S.; Taguchi, Y.; Murakawa, H.; Onose, Y.; Tokura, Y. Low-Magnetic-Field Control of Electric Polarization Vector in a Helimagnet. Science 2008, 319, 1643-164. (35) Wang, J.; Neaton, J. B.; Zheng, H.; Nagarajan, V.; Ogale, S. B.; Liu, B.; Viehland, D.; Vaithyanathan, V.; Schlom, D. G.; Waghmare, U. V.; Spaldin, N. A.; Rabe, K. M.; Wuttig, M.; Ramesh, R. Epitaxial BiFeO3 Multiferroic Thin Film Heterostructures. Science 2003, 299, 1719-1722.


24

Page 25 of 25 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Nano

Table of Content Artwork


25

Distorted Monolayer ReS2 with Low-Magnetic-Field Controlled

Recommend Documents