PDF Robust ferromagnetism in zigzag-edge rich MoS2 pyramids - NJU

(1)

PAPER

Cite this:Nanoscale, 2018,10, 11578

Received 14th April 2018, Accepted 18th May 2018 DOI: 10.1039/c8nr03038c rsc.li/nanoscale

Robust ferromagnetism in zigzag-edge rich MoS

₂

pyramids †

Qingwei Zhou, ^a,bShaoqiang Su,^bPengfei Cheng,^bXianbao Hu,^bMin Zeng,^b Xingsen Gao, ^bZhang Zhang *^band Jun-Ming Liu ^a,b

The intrinsic magnetism of MoS2has been extensively investigatedviasimulations, but few reliable experimental results have been explored. Herein, we develop zigzag-edge rich layered structural MoS2pyramids via chemical vapor deposition, triggering exceptional ferromagnetism. The magnetic measurements revealed the robust ferromagnetism of MoS2pyramids compared with MoS2flakes. The existence of ferromagnetism was mostly attributed to the presence of abundant zigzag-edges in the layered pyramids, confirmed by transmission electron microscopy, vibrating sample magnetometry, and magnetic force microscopy. Moreover, a clearly identified remnant and switchable magnetic moment was revealed for thefirst time in the MoS2pyramid. This study provides sound evidence with the zigzag-edge induced ferromagnetism of the MoS2materials, promising potential magnetic and spintronic applications.

Introduction

Owing to their unusual electronic structures and exceptional physical properties, two dimensional (2D) materials have attracted tremendous attention in recent years.^1–6As a kind of inorganic 2D semiconductor with a direct bandgap, 2D tran- sition metal dichalcogenides (TMDs) have attracted particular interest.^7–10 Among them, MoS2 is one of the most stable layered TMDs and naturally consists of covalently bonded three hexagonal atomic layers (S–Mo–S), enabling the for- mation of 2D layers.¹¹In particular, MoS2in its bulk form is a semiconductor with an indirect bandgap of about 1.0 eV while the monolayer material exhibits a direct bandgap of about 1.8 eV,¹² which makes the 2D MoS2 materials promising candi- dates for both fundamental research and nanodevice applications like field-eﬀect transistors (FETs), photodetectors, and gas sensors.^13–18However, in the extensive experimental and theoretical studies on the photoelectric properties of MoS2, only a few findings on the magnetic response of MoS2 have been reported.

It is well known that MoS2 in its bulk form exhibits no ferromagnetic or paramagnetic behavior.¹⁹Thus, to find spintronic applications, much eﬀort has been expended to activate

the ferromagnetism of MoS2. For example, proton and electron irradiation can introduce defects into pristine MoS2, leading the sample to be ferromagnetic.^20–22Co, Cu, and Mn doping was shown to produce ferromagnetism in doped MoS2too.^23–26 MoS2nanosheets with non-metal element adsorption also pre- sents ferromagnetic properties at room temperature.²⁷However, most of the theoretically and experimentally explored ferromagnetic properties in MoS2are extrinsic, because they are all induced by defects, absorbed or doped atoms, which are hard to precisely control and apply in devices, noting that the chemical doping would significantly change the electronic structure.^28,29Therefore, it is extremely urgent and important that we begin to realize the intrinsic magnetism in TMDs.

For the intrinsic magnetism in MoS2, Liet al.have theoretically presented that a zigzag-edge in MoS2nanoribbons exhibi- ted ferromagnetic behavior irrespective of the ribbon width and thickness.³⁰Zhanget al.have detected dilute magnetism from MoS2nanofilms and demonstrated that the magnetism mainly arises from the unsaturated atoms at the prismatic edge sites based on MoS2nanocluster models.³¹By first-prin- ciples calculations, dislocations and grain boundaries existing in 2D MoS2are also found to possess substantial magnetism, resulting from the partial occupancy of spin-resolved localized electronic states and significant spin–spin interactions.^32,33 However, recent studies on the intrinsic magnetism of MoS2

are still limited to theoretical calculations. And few reliable experimental results have been explored about the intrinsic magnetism of zigzag-edges, dislocations and grain boundaries in MoS2materials.

Compared with dislocations and grain boundaries, zigzag- edges are more generally present in 2D MoS2nanosheets with

†Electronic supplementary information (ESI) available. See DOI: 10.1039/

c8nr03038c

aLaboratory of Solid State Microstructures and Innovative Center of Advanced Microstructures, Nanjing University, Nanjing, 210093, China

bInstitute for Advanced Materials and Laboratory of Quantum Engineering and Quantum Materials, South China Academy of Advanced Optoelectronic, Guangzhou 510006, China. E-mail: zzhang@scnu.edu.cn

Published on 21 May 2018. Downloaded by NANJING UNIVERSITY on 12/19/2018 7:09:58 AM.

View Article Online

View Journal | View Issue

(2)

triangular and hexagonal shapes.^34,35One would surmise that ferromagnetism is prevalent in most few-layered MoS2

materials. However, the ferromagnetism of 2D MoS2 is too weak to detect experimentally, since the ratio of edge atomsvs.

total atoms is extremely small and decreases dramatically as the size of 2D MoS2 increases.³¹ Therefore, the edge-based intrinsic magnetism in MoS2 nanostructures is worth explor- ing where a reliable experimental verification can be done.

Here, the multi-layer MoS2pyramids with a high density of zigzag-edges were prepared through a chemical vapor deposition (CVD) method. The nanostructure and composition were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), Raman spectra and X-ray photoelectron spectroscopy (XPS). The magnetic properties of the MoS2 pyramids were determined by vibrating sample magnetometry (VSM) and magnetic force microscopy (MFM). Due to the high density of the ferromagnetic zigzag-edges, the MoS2 pyramids demonstrate a much larger saturated magnetic moment compared with other nanostructures of MoS2 materials.11,19,31,36–41 Moreover, a remnant and switchable magnetic moment was observed in MoS2for the first time. Additionally, it can be scaled up in the CVD process with a high yield, expecting to explore more potential applications in magnetic and spintronic devices based on these edge-rich MoS2pyramids.

Experimental

Fabrication of a MoS2pyramid

The MoS2 pyramid was synthesized using CVD on silica sub- strates, using MoO3 and S powder as precursors (see ESI, Fig. S1†). The silica substrate was cleaned with acetone, ethanol and DI-water in sequence and dried with nitrogen.

Then, the substrate was placed face-down above a crucible containing 3 mg of MoO3 (99.95%, Aladdin) and loaded into a 4 cm-diameter quartz tubular three-zone CVD furnace (OTF-1200X). The whole process was performed at atmospheric pressure, using high-purity Ar (99.999%) as a carrier gas.

Another crucible containing 500 mg of sulfur (99.99%, Aladdin) was located upstream. The distance between the S crucible and the growth substrate was varied to control the morphology of MoS2 pyramids grown on the substrate. The furnace temperature was ramped up to 300 °C with a rate of 20 °C min⁻¹and kept constant for 10 min. Then, the temperature was ramped up to 730 °C with a rate of 50 °C min⁻¹. After 10 min at 730 °C, the furnace was cooled down with the heater removed. The Ar flow rate was 50 sccm (standard cubic centi- meters per minute) when the temperature stayed at 730 °C and 200 sccm during temperature ramp up and cooling.

Sample characterization

The morphology and nanostructure of the as-prepared samples were characterized by scanning electron microscopy (SEM, ZEISS-Ultra55) and transmission electron microscopy (TEM, JEOL JEM-2100). The Raman spectra were recorded using an InVia Raman system (a42K864 Renishaw, InVia

system) with an excitation laser wavelength of 532 nm, and the laser power was set to 10%. The chemical bonding state and compositions of the samples were determined by X-ray photoelectron spectroscopy (XPS, ESCALAB 250 Xi).

The macroscopic magnetization measurements were carried out on a vibrating sample magnetometer (VSM) of physical property measurement system (PPMS, Quantum Design). Atomic/

magnetic force microscopy (AFM/MFM, Cypher™, Asylum Research) was used to characterize the surface morphologies and the magnetic properties of MoS2pyramids. To obtain clear MFM signals, the samples were magnetized before the measurements by a field (H = 5000 Oe) along the perpendicular direction, or by a reverse field (−H) along the opposite direction.

Results and discussion

The morphology of the atomically multi-layered MoS2 pyramids was characterized by SEM, TEM and AFM. Fig. 1a is a schematic illustration of the 3D pyramid structure, which consists of atomically stacked monolayers of MoS2 in decreasing sizes. Fig. 1b and c are the top-view SEM images of MoS2pyra- mids of diﬀerent magnifications. The pyramid size is around 10μm, which is defined by the edge length of the largest basal plane. Fig. 1d is the enlarged SEM image of the white dashed rectangle marked in Fig. 1c. The 3D pyramid has three step- shaped slopes and a flat top. The pyramid top consists of a complicated stack of small triangular MoS2monolayers, which was further investigated using the AFM images ( phase signal) of a single MoS2pyramid (see Fig. S2†). Overall, the atomically multi-layered structure looks like a pyramid. Each basal plane has a triangular shape and shrinks layer-by-layer to the summit from the base triangle. An AFM height image of a MoS2pyramid’s summit is shown in Fig. 1e with the relevant steps. The height profile (Fig. 1f ) illustrates four obvious ter- races with a similar step height of 0.62 nm, which matches the thickness of a single MoS2atomic layer.⁴²

The atomically multi-layered structure of the MoS2pyramid could also be confirmed by Raman characterization. MoS2has two typical Raman peaks corresponding to the E¹2g and A1g

modes, respectively. The frequency diﬀerence between the A1g

and E¹2gmodes depends on the number of layers of MoS2. For monolayer MoS2, the frequency difference is usually about 19 cm⁻¹. The Raman mapping of the frequency difference for the monolayer MoS2 flakes (see Fig. S3a†) displays a uniform distribution, as revealed in our previous studies.^43,44 Fig. 1g shows a typical Raman mapping of the frequency difference between the A1g and E¹2g modes for a single MoS2 pyramid.

The color change was observed along the triangular edges of the MoS2pyramid from the periphery to the center, indicating increasing layer numbers from the base to the top. A large scale Raman mapping (see Fig. S3b†) confirms the large-scale uniform size distribution of the MoS2 pyramids. The Raman spectra of the MoS2 pyramid from the base to the top (as marked by 1, 2, 3 and 4 in Fig. 1g) are shown in Fig. 1h. The frequency diﬀerence at the base of the MoS2pyramid (marked

Nanoscale Paper

(3)

by 1 in Fig. 1g) is 19 cm⁻¹, indicating a monolayer structure.

Clearly, a red shift for the E¹2g mode and a blue shift for the A1g mode are both recognized from the base of the MoS2

pyramid to the top, indicating the multi-layered pyramid structure as well.

To further elucidate the crystalline structure, the MoS2pyra- mids are transferred to the TEM copper grid by polymethyl methacrylate (PMMA). Fig. 2a is the TEM image of a multi- layered MoS2pyramid with a size of hundreds of nanometers.

We observed the triangle edges with a contrast diﬀerence by layer-by-layer stacking, and the size of the edge shrinks gradu-

ally to the center summit. Schematically, a MoS2 monolayer consists of covalently bonded three hexagonal atomic layers (S–Mo–S), and the three zigzag-edges are the dominant morphologies of the MoS2triangle as shown in Fig. 2b. The zigzag- edge of MoS2is the most energetically stable edge orientations ({100} plane),^34,35,45 and has been proved to be magnetic in theoretical research studies.^30,46

As shown in Fig. 2c for an enlarged TEM image of the dashed rectangle area in Fig. 2a, the MoS2pyramid has a high density of zigzag-edges of about 3.43 × 10¹³ nm cm⁻². The corresponding high-resolution TEM (HRTEM) image (Fig. 2d)

Fig. 2 Crystalline structure of atomically multi-layered MoS₂pyramids. (a) TEM image of a single MoS₂pyramid. (b) Top and side view of the schematic lattice structure of a triangle 2D MoS₂monolayer with Mo-zigzag edges. The yellow and purple spheres represent S and Mo atoms, respectively. (c) Enlarged TEM image of the dashed rectangle indicated in (a). (d) HRTEM image of the edges in the MoS₂pyramid. (f ) The corresponding SAED pattern.

Fig. 1 Morphology of atomically multi-layered MoS₂pyramids. (a) Schematic illustration of the MoS₂pyramid. (b) Top-view SEM images of MoS₂ pyramids. (c) Magnified view of a single MoS₂pyramid. (d) Enlarged SEM image of the dashed rectangle indicated in (c). (e) AFM height image of a MoS₂pyramid’s summit and (f ) height profile across the red line in (e), showing obvious single layer steps (marked byα,β,γandδ). (g) Raman mapping of the frequency difference between A_1gand E¹_2gfor a single MoS₂pyramid. (h) Raman spectrum of the points 1, 2, 3 and 4 marked in (g).

(4)

clearly demonstrates three MoS2layers with two parallel edges (indicated with white dashed lines). The HRTEM image reveals the periodic atom arrangement of each MoS2layer. The corresponding inter-planar spacing was found to be 0.27 nm based on the periodic pattern in the lattice fringe image, which matches with the (100) facet of MoS2. The two zigzag edges marked with white dashed lines are all parallel to the (100) facet of MoS2. The selected area electron diffraction (SAED) obtained from the same region shows a single set of spot pat- terns characteristic of trigonal structures (Fig. 2e), indicating that all layers are of the same crystalline orientation. The intensity of the (110) diffraction points is much stronger than that of the (100) diffraction points, which is in agreement with a previous structural study of the MoS2pyramids.⁴⁷

In order to eliminate the eﬀect of the magnetic impurities causing ferromagnetism, the chemical composition was investigated. The XPS spectra of the MoS2 pyramids are shown in Fig. 3. Obviously, due to the spin–orbit splitting of the Mo 3d, the Mo 3d doublet with an energy separation of 3.2 eV was observed (Fig. 3a). The peaks at 232.3 eV and 229.1 eV are identified as Mo 3d3/2and Mo 3d5/2, respectively, representing the Mo⁴⁺state. The small shoulder that appeared at 226.3 eV corresponds to the S 2s peak.²⁰The spectrum of S consists of S 2p3/2at 161.8 eV and S 2p1/2at 163.0 eV (Fig. 3b). The full XPS spectrum scanned from 0 to 1400 eV is shown in Fig. 3c, con- firming the absence of any significant magnetic impurity (green dashed rectangle marked in the spectrum). The observed O and C peaks result from the interactions with air and carbon contamination. The XPS results suggest that the detected ferromagnetism simply comes from the prepared MoS2pyramids.

The morphology and crystalline structure characterization studies of the MoS2 pyramid indicate its atomically multi-

layered structure with a high density of zigzag edges as schematically illustrated in Fig. 4a and b, which motivated us to carry out a comparative study on the magnetic properties of the single layer MoS2flakes and the MoS2pyramids. The size of the single layer MoS2flakes is tens of microns (see Fig. S4†).

The macroscopic magnetization measurements were carried out on a vibrating sample magnetometer (VSM) of the physical property measurement system (PPMS, Quantum Design).

Fig. 4c shows the magnetizationversus magnetic field (M–H) curves in the field range of−4.0 kOe <H < 4.0 kOe at room temperature (300 K) for the MoS2 flakes and the MoS2 pyramids on the Si/SiO2substrate, and the substrate only. The Si/

SiO2substrate simply gives diamagnetic signals (black lines in Fig. 4c), and the MoS2flakes on the Si/SiO2substrate display similar diamagnetic signals to the substrate (blue line in Fig. 4c). For the MoS2 pyramids on the Si/SiO2 substrate, although the magnetic response is dominated by diamagnetism (red line in Fig. 4c), we find that the diamagnetic background is superimposed onto the ferromagnetic loop (Fig. 4d).

TheM–Hcurve implies that the total magnetic susceptibility comprises both diamagnetic and ferromagnetic parts. Hence, the diamagnetic and ferromagnetic signals can be attributed to the Si/SiO2substrate and the MoS2 pyramids, respectively.

And the Si/SiO2substrate can only give diamagnetic signals at various temperatures (see Fig. S5†). Fig. 4e illustrates theM–H curves for the MoS2 pyramids at three temperatures (300 K, 100 K and 2 K) with the diamagnetic signals subtracted.

Accordingly, the saturation magnetization (Ms) of the MoS2

pyramids is about 2.5 emu g⁻¹. TheMsis much larger than the previous experimental studies on the MoS2 materials,^31,40,41 being probably related to the greatly enhanced density of zigzag-edges in the MoS2 pyramids. The enlarged view of the ferromagnetic loops in the low field range is shown in Fig. 4f.

Obviously, the ferromagnetic behavior of the MoS2pyramids is robust at this temperature range (2 K–300 K), and the remnant magnetic moment (Mr) and coercivity (Hc) both decrease with increasing temperature.

The direct evidence for magnetic regions in the MoS2

pyramid was obtained through MFM measurements. MFM is a quick and reliable method to detect and localize magnetic regions in nanomaterials, and has been adopted to study the magnetic domains and regions in TMDs.^28,39,48The MFM was done via a two-step measurement. Each scan line was measured twice using a magnetic tip under the lift mode.

First, the height profile of the sample surface under the tapping mode was obtained. Then, the magnetic tip was lifted at a fixed height of 50 nm over the sample based on the height profile. In this way, the influence of height and short-term interactions, such as the static electrical force between the tips and the sample can be eliminated. In order to avoid the response variation induced by diﬀerent tips, the images were always obtained using the same tip. Fig. 5a is an AFM topography image of a single MoS2pyramid, and Fig. 5b is the corresponding MFM phase image during the remnant state after magnetization by a field (H) along the surface normal. As a comparison, the AFM image and the corresponding MFM Fig. 3 XPS spectra of (a) Mo 3d, (b) S 2p and (c) the full region of the

MoS₂pyramids. The green dashed rectangle represents the position of magnetic impurity.

Nanoscale Paper

(5)

image of another MoS2 pyramid after magnetization by a reverse field (−H) are shown in Fig. 5c and d. The green circles marked in the AFM images (Fig. 5a and c) indicate impurities with a relatively higher altitude, while no phase shift was found in the corresponding MFM images (green circles marked in Fig. 5b and c), which suggests that the influence of height could be eliminated during the MFM measurements.

Specifically, the heights of these two MoS2 pyramids are similar (∼18 nm, Fig. 5e and g), which means that the number of MoS2layers with zigzag-edges is almost the same.

In the top-view AFM images (Fig. 5a and c), the density of the zigzag edges in the pyramid increases from the base to the top, which is consistent with the microscopy data. The MFM phase shift profiles of Fig. 5f and h are across the two pyramids from one side to another (red dashed lines in Fig. 5b and d), all demonstrating a larger contrast difference in the central area. Accordingly, the highest zigzag-edge density should exist in the center of the MoS2pyramid. Moreover, with the reverse contrast difference, both MFM images display a remanent switchable magnetic moment along the surface normal after Fig. 4 (a) Top and (b) side view of the schematic atomic structure of the MoS2pyramid: the yellow and purple spheres represent S and Mo atoms, respectively. (c) TheM–H curves at 300 K of MoS2pyramids (red), MoS2flakes (blue) and the Si/SiO2substrate (black). (d) The corresponding enlarged view of (c). (e) TheM–Hcurves at various temperatures of 300 K (black), 100 K (blue) and 2 K (red) for the MoS2pyramids after subtracting the diamagnetic background. (f ) The corresponding enlarged view of (d).

Fig. 5 (a, c) AFM topography images of a single MoS₂pyramid and (b, d) the corresponding MFM phase images in the remanence state that has been premagnetized by afield (H) along the perpendicular direction (b) and by a reversefield (−H) along the opposite direction (d). (e, g) AFM height profiles and (f, h) MFM phase shift profiles of the red dashed lines in (a, c) and (b, d), respectively.

(6)

magnetization in a largeHeither in the up or down direction.

In the corresponding MFM phase shift profiles, the reverse contrast of the MFM phase in these two pyramids is also confirmed, with one demonstrating 100 milli-degree (m°) by a field (H) and−100 m° for another one by a reverse field (−H).

The Mo atom in the bulk MoS2 crystal adopts a trigonal- prismatic structure with respect to the six S atoms around it.

The 4d orbitals of Mo are spin-paired because of ligand-field splitting, resulting in diamagnetism for the bulk MoS2

crystal.⁴⁹ However, the edges of MoS2 pyramids are zigzag- edges with unsaturated edge atoms, which are responsible for the ferromagnetic behavior of MoS2 zigzag-edges since the coordination of these atoms is diﬀerent from that of the inner atoms. Theoretical results have shown spin-up and spin-down peaks of Mo-4d of the zigzag-edge Mo atoms splitting asymme- trically about the Fermi level, which means that the 4d orbitals of zigzag-edge Mo atoms are spin-polarized.³¹The magnetism of the MoS2 zigzag-edges is ferromagnetically coupled, which can form long-range ferromagnetism.³⁰Further theoretical calculations show that the Mo atoms in zigzag-edges have a robust intrinsic ferromagnetic response of 2μB per Mo.³¹The zigzag-edges of MoS2consist of about 3.25 Mo atoms per nano- meter.³⁰Therefore, based on the size of each layer in the MoS2

pyramids shown in the TEM characterization studies (Fig. S6†), we counted the total length of the zigzag-edges in this MoS2 pyramid. A rough estimation allows us to obtain a magnetization of 0.92 emu g⁻¹ (see calculation details in the ESI†), which is close to the experimentally measured magnetization of∼2 emu g⁻¹in this work.

Conclusions

In summary, we report a detailed magnetism study of CVD- grown MoS2pyramids and observe their intrinsic ferromagnetism from 2 to 300 K. The XPS results confirm the absence of any significant magnetic impurity, which suggests that the detected ferromagnetism simply comes from the prepared MoS2 pyramids. The nanostructure (TEM) and magnetic characterization studies (VSM and MFM) demonstrate clear evidence of the ferromagnetism in MoS2 pyramids, which is mainly related to the high density of zigzag edges in the atomically multi-layered pyramid structure. The MoS2 pyramids demonstrate a much larger saturated magnetic moment compared with the other nanostructures of MoS2 materials.

Moreover, for the first time, a remanent switchable magnetic moment was observed in the MoS2 pyramid. This work provides new insight into the magnetism of 2D MoS2 materials and sheds light on their potential magnetic applications.

Con ﬂ icts of interest

There are no conflicts to declare.

Acknowledgements

We acknowledge financial support from the National Key Research Projects of China (2016YFA0300101, 2015CB654602), the National Natural Science Foundation of China (51431006, 51721001), the National Natural Science Foundation of Guangdong, China (2014A030313434 and 2016A030308019), the Pearl River Nova Program of Guangzhou (201506010019), and the Key Projects of Applied Special Funds of Guangdong Science and Technology Project (2015B090927006).

Notes and references

1 A. J. Mannix, B. Kiraly, M. C. Hersam and N. P. Guisinger, Nat. Rev. Chem., 2017,1, 0014.

2 C. Tan, X. Cao, X.-J. Wu, Q. He, J. Yang, X. Zhang, J. Chen, W. Zhao, S. Han, G.-H. Nam, M. Sindoro and H. Zhang, Chem. Rev., 2017,117, 6225–6331.

3 F. Liu, Y. Zhou, Y. Wang, X. Liu, J. Wang and H. Guo,NPJ Quantum Mater., 2016,1, 16004.

4 N. Mounet, M. Gibertini, P. Schwaller, D. Campi, A. Merkys, A. Marrazzo, T. Sohier, I. E. Castelli, A. Cepellotti, G. Pizzi and N. Marzari, Nat. Nanotechnol., 2018,13, 246–252.

5 S. Zhou, C.-C. Liu, J. Zhao and Y. Yao,NPJ Quantum Mater., 2018,3, 16.

6 Z. Zheng, J. Chen, Y. Wang, X. Wang, X. Chen, P. Liu, J. Xu, W. Xie, H. Chen, S. Deng and N. Xu,Adv. Mater., 2018,30, 1705318.

7 L. S. Hart, J. L. Webb, S. Murkin, D. Wolverson and D.-Y. Lin,NPJ 2D Mater. Appl., 2017,1, 41.

8 J. R. Dunklin, P. Lafargue, T. M. Higgins, G. T. Forcherio, M. Benamara, N. McEvoy, D. K. Roper, J. N. Coleman, Y. Vaynzof and C. Backes,NPJ 2D Mater. Appl., 2018,1, 43.

9 X.-J. Yan, Y.-Y. Lv, L. Li, X. Li, S.-H. Yao, Y.-B. Chen, X.-P. Liu, H. Lu, M.-H. Lu and Y.-F. Chen, NPJ Quantum Mater., 2017,2, 31.

10 J. Wen, H. Wang, W. Wang, Z. Deng, C. Zhuang, Y. Zhang, F. Liu, J. She, J. Chen, H. Chen, S. Deng and N. Xu,Nano Lett., 2017,17, 4689–4697.

11 S. Tongay, S. S. Varnoosfaderani, B. R. Appleton, J. Wu and A. F. Hebard,Appl. Phys. Lett., 2012,101, 123105.

12 R. Ganatra and Q. Zhang,ACS Nano, 2014,8, 4074–4099.

13 B. Radisavljevic, A. Radenovic, J. Brivio, V. Giacometti and A. Kis,Nat. Nanotechnol., 2011,6, 147–150.

14 S. Kim, A. Konar, W.-S. Hwang, J. H. Lee, J. Lee, J. Yang, C. Jung, H. Kim, J.-B. Yoo, J.-Y. Choi, Y. W. Jin, S. Y. Lee, D. Jena, W. Choi and K. Kim,Nat. Commun., 2012,3, 1011.

15 Z. Yin, H. Li, H. Li, L. Jiang, Y. Shi, Y. Sun, G. Lu, Q. Zhang, X. Chen and H. Zhang,ACS Nano, 2012,6, 74–80.

16 O. Lopez-Sanchez, D. Lembke, M. Kayci, A. Radenovic and A. Kis,Nat. Nanotechnol., 2013,8, 497–501.

17 G. Berghäuser and E. Malic, Phys. Rev. B: Condens. Matter Mater. Phys., 2014,89, 125309.

Nanoscale Paper

(7)

18 Y. Fu, E. Liu, H. Yuan, P. Tang, B. Lian, G. Xu, J. Zeng, Z. Chen, Y. Wang, W. Zhou, K. Xu, A. Gao, C. Pan, M. Wang, B. Wang, S.-C. Zhang, Y. Cui, H. Y. Hwang and F. Miao,NPJ Quantum Mater., 2017,2, 52.

19 J. Luxa, O. Jankovský, D. Sedmidubský, R. Medlín, M. Maryško, M. Pumera and Z. Sofer, Nanoscale, 2016,8, 1960–1967.

20 S. Mathew, K. Gopinadhan, T. K. Chan, X. J. Yu, D. Zhan, L. Cao, A. Rusydi, M. B. H. Breese, S. Dhar, Z. X. Shen, T. Venkatesan and J. T. L. Thong, Appl. Phys. Lett., 2012, 101, 102103.

21 S. W. Han, Y. H. Hwang, S.-H. Kim, W. S. Yun, J. D. Lee, M. G. Park, S. Ryu, J. S. Park, D.-H. Yoo, S.-P. Yoon, S. C. Hong, K. S. Kim and Y. S. Park,Phys. Rev. Lett., 2013, 110, 247201.

22 S. W. Han, Y. Park, Y. H. Hwang, W. G. Lee and S. C. Hong, Appl. Phys. Lett., 2016,109, 252403.

23 Z. Xiang, Z. Zhang, X. Xu, Q. Zhang, Q. Wang and C. Yuan, Phys. Chem. Chem. Phys., 2015,17, 15822–15828.

24 B. Xia, Q. Guo, D. Gao, S. Shi and K. Tao,J. Phys. D: Appl.

Phys., 2016,49, 165003.

25 J. Wang, F. Sun, S. Yang, Y. Li, C. Zhao, M. Xu, Y. Zhang and H. Zeng,Appl. Phys. Lett., 2016,109, 092401.

26 H. Tan, W. Hu, C. Wang, C. Ma, H. Duan, W. Yan, L. Cai, P. Guo, Z. Sun, Q. Liu, X. Zheng, F. Hu and S. Wei,Small, 2017,13, 1701389.

27 D. Gao, S. Shi, K. Tao, B. Xia and D. Xue,Nanoscale, 2015, 7, 4211–4216.

28 L. Tao, F. Meng, S. Zhao, Y. Song, J. Yu, X. Wang, Z. Liu, Y. Wang, B. Li, Y. Wang and Y. Sui, Nanoscale, 2017, 9, 4898–4906.

29 Y. Zhou, Z. Wang, P. Yang, X. Zu, L. Yang, X. Sun and F. Gao,ACS Nano, 2012,6, 9727–9736.

30 Y. Li, Z. Zhou, S. Zhang and Z. Chen,J. Am. Chem. Soc., 2008,130, 16739–16744.

31 J. Zhang, J. M. Soon, K. P. Loh, J. Yin, J. Ding, M. B. Sullivian and P. Wu,Nano Lett., 2007,7, 2370–2376.

32 Z. Zhang, X. Zou, V. H. Crespi and B. I. Yakobson, ACS Nano, 2013,7, 10475–10481.

33 N. Gao, Y. Guo, S. Zhou, Y. Bai and J. Zhao,J. Phys. Chem.

C, 2017,121, 12261–12269.

34 J. V. Lauritsen, J. Kibsgaard, S. Helveg, H. Topsøe, B. S. Clausen, E. Lægsgaard and F. Besenbacher, Nat.

Nanotechnol., 2007,2, 53.

35 A. M. van der Zande, P. Y. Huang, D. A. Chenet, T. C. Berkelbach, Y. You, G.-H. Lee, T. F. Heinz, D. R. Reichman, D. A. Muller and J. C. Hone,Nat. Mater., 2013,12, 554.

36 D. Gao, M. Si, J. Li, J. Zhang, Z. Zhang, Z. Yang and D. Xue, Nanoscale Res. Lett., 2013,8, 129.

37 B. Sun, Q. L. Li and P. Chen,Micro Nano Lett., 2014,9, 468– 470.

38 R. Zhang, Y. Li, J. Qi and D. Gao,Nanoscale Res. Lett., 2014, 9, 586.

39 G. Gao, C. Chen, X. Xie, Y. Su, S. Kang, G. Zhu, D. Gao, A. Trampert and L. Cai, Mater. Res. Lett., 2017, 5, 267–275.

40 R. Qi, S. Wang, M. Wang, W. Liu, Z. Yan, X. Bi and Q. Huang,Chem. Commun., 2017,53, 9757–9760.

41 Z. Yang, D. Gao, J. Zhang, Q. Xu, S. Shi, K. Tao and D. Xue, Nanoscale, 2015,7, 650–658.

42 W. O. Winer,Wear, 1967,10, 422–452.

43 Q. Zhou, S. Su, D. Hu, L. Lin, Z. Yan, X. Gao, Z. Zhang and J.-M. Liu,Nanotechnology, 2018,29, 105402.

44 S. Su, Q. Zhou, Z. Zeng, D. Hu, X. Wang, M. Jin, X. Gao, R. Nötzel, G. Zhou, Z. Zhang and J. Liu, ACS Appl. Mater.

Interfaces, 2018,10, 8026–8035.

45 L. S. Byskov, J. K. Nørskov, B. S. Clausen and H. Topsøe, Catal. Lett., 2000,64, 95–99.

46 P. Cui, J.-H. Choi, W. Chen, J. Zeng, C.-K. Shih, Z. Li and Z. Zhang,Nano Lett., 2017,17, 1097–1101.

47 L. Zhang, K. Liu, A. B. Wong, J. Kim, X. Hong, C. Liu, T. Cao, S. G. Louie, F. Wang and P. Yang,Nano Lett., 2014, 14, 6418–6423.

48 H. Li, X. Qi, J. Wu, Z. Zeng, J. Wei and H. Zhang,ACS Nano, 2013,7, 2842–2849.

49 R. Huisman, R. de Jonge, C. Haas and F. Jellinek,J. Solid State Chem., 1971,3, 56–66.