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Journal of Materials Chemistry CMaterials for optical, magnetic and electronic deviceswww.rsc.org/MaterialsC

ISSN 2050-7526

PAPERNguyên T. K. Thanh, Xiaodi Su et al.Fine-tuning of gold nanorod dimensions and plasmonic properties using the Hofmeister eff ects

~

Volume 4 Number 1 7 January 2016 Pages 1–224

Journal of Materials Chemistry CMaterials for optical, magnetic and electronic devices

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J. Liu, B. Liang, G. Fu, W. Yu, K. Ren, S. Wang and C. Pan, J. Mater. Chem. C, 2018, DOI:

10.1039/C7TC05896A.

Journal of Materials Chemistry C

ARTICLE

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a. Hebei Key Laboratory of Optic-Electronic Information and Materials, College of Physics Science and Technology, Hebei University, Baoding 071002, P. R. China.

b. Beijing Institute of Nanoenergy and Nanosystems, Chinese Academy of Sciences, Beijing, 100083, China;

c. School of Nanoscience and Technology, University of Chinese Academy of Sciences, Beijing 100049, P. R. China.

† Corresponding authors: yuwei@hbu.edu.cn, renkailiang@binn.cas.cn, and cfpan@binn.cas.cn. Electronic Supplementary Information (ESI) available. See DOI: 10.1039/x0xx00000x

Received 00th January 20xx,

Accepted 00th January 20xx

DOI: 10.1039/x0xx00000x

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Vertical layered MoS2/Si heterojunction for ultrahigh and ultrafast photoresponse photodetector

Shuang Qiao,a,b,c Ridong Cong,a Jihong Liu,a Baolai Liang,a Guangsheng Fu,a Wei Yu,a,† Kailiang Ren,b,c† Shufang Wang,a and Caofeng Panb,c†

Recently, chemical vapor deposition (CVD) prepared vertically oriented few-layer 2D materials have attracted much

attention due to their attractive properties. However, the integration of these materials as heterojunctions in optoelectronic sensors were rarely reported. In this paper, large-area, high crystalline quality, and vertically few-layered MoS2 (V-MoS2) nanosheets were synthesized and transferred successfully onto a silicon substrate to form a V-MoS2/Si

heterojunction photodetector. The photodetector exhibits high photoelectric performances in a wide broadband ranging from visible to near-infrared with photoresponsivity up to 908.2 mA/W, detectivity up to 1.8891013 Jones. More importantly, an unprecedented response speed of (rise time ~ 56 ns, fall time ~ 825 ns) was found in the photodetector by

time response measurements, which is the best result achieved so far in any other 2D-based photodetectors or phototransistors. These excellent performances can be ascribed to the strong light absorption and the quick longitudinal intralayer carrier transport speed of the V-MoS2 nanosheets as well as the good heterojunction formed between V-MoS2

and Si. This intriguing vertical oriented structure in combination with both ultrafast time response and ultrahigh detectivity in V-MoS2/Si heterojunction makes it great potential application in optoelectronic devices.

1. Introduction

Molybdenum disulfide (MoS2), as an emerging layered two-dimensional (2D) material, has attracted extensive interest because of its excellent electronic, and optical properties. 1-3 It has been reported that the monolayer or few-layer MoS2-based field-effect transistors (FETs) exhibited a ultrahigh current on/off ratio (~108) and a very large mobility (>200 cm2/Vs) at room temperature, 4 and even a monolayer MoS2 layer can absorb ~10% of incident light and its spectral response can be modulated ranging from visible to mid-infrared depending on its layer thicknesses and introducing imperfections to the atomic lattices. 5-7 On account of these intriguing properties, researchers have made great efforts to develop the potential applications of MoS2 in a variety of fields, such as solar cells, transistors, energy storage, catalysis, and sensing devices. 1, 8-10 As one key component for the optoelectronic system, MoS2-based heterostructure photodetectors have been increasingly needed due to their unique photoresponse properties and easy integration in modern complex electronic and optoelectronic devices. For photodetection, photoresponsivity, detectivity, and response speed are three key figures of merit. However, from the previous reports, the parameters of the MoS2-based photodetectors are still far from satisfying practical applications, which may be attributed to the three related factors: poor optical

absorption of traditional monolayer or few-layer 2D materials, 8 low longitudinal interlayer transport speed, 11 and lack of high quality heterojunctions 12. Moreover, although multilayer MoS2-based photodetectors can, to some extent, enhance optical absorption, its high intralayer mobility can still not be well developed, thus the photoresponse improves at the expense of response speed. 8, 13 Therefore, how to promote these performances together becomes a crucial problem. Fortunately, recently vertically oriented few- and multi- layered MoS2 (V-MoS2) has been proposed and successfully prepared, which has the advantages of both strong optical absorption and quick longitudinal intralayer transport, making enhancement of photoresponsivity and response speed workable. 14 More importantly, some researchers even have combined it with the traditional silicon semiconductor material to build high quality MoS2/Si hybrid heterojunctions in order to exploit the advantages of both materials and accelerate the practical application. 12, 13, 15, 16 In the designed MoS2/Si heterojunction form, MoS2 layers act as a light absorbing material, whereas the Si substrate provides an abrupt junction with MoS2 to promote the separation of photo-excited carriers near their interfaces. Previously, vertically standing layered MoS2/Si heterojunction photodetector has been prepared by magnetron sputtering and was reported to exhibit a high detectivity and fast response speed 12. However, compared with chemical vapor deposition (CVD) technique, the quality of magnetron sputtering-prepared V-MoS2 should be much worse with not intact layered structure, weak crystal quality, and lots of defects. Besides, although several high-quality and vertically oriented few-layered 2D nanosheets have been successfully prepared by CVD, 17-20 their heterojunctions as photodetectors have not been well fabricated and studied, and the photoelectric properties and time response of V-MoS2/Si heterjunction photodetector still remains unknown. In this paper, the large-area, high-quality, and vertically layered MoS2 nanosheets (V-MoS2) were synthesized by CVD. Raman,

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photoluminescence (PL), scanning electron microscopy (SEM) and X-ray diffraction (XRD) measurements suggest that the MoS2 nanosheets exhibit very high crystal quality with height of 2 μm and an averaged layer thickness of approximately 3 nm. Moreover, the MoS2 nanosheets were successfully transferred onto the p-type Si substrate to form the V-MoS2/Si heterojunction. Optoelectronic effect measurements demonstrate that the device showed a very wide photoresponse ranging from 350 to 1100 nm with photoresponsivity and detectivity up to 908.2 mA/W and

1.8891013 Jones, respectively. More importantly, the photodetector is able to work under very large power density range and wide pulsed frequency range, and demonstrates an unprecedented ultrafast time response of 56 ns (/825 ns), which is the best results for all 2D material-based photodetectors reported so far. Additionally, the V-MoS2/Si photodetector shows excellent durability in air without any protection for at least three months. These results indicate that the CVD-prepared V-MoS2/Si heterojunction may have great promising application in high-performance and ultrafast optoelectronic sensors.

2. Experimental methods

V-MoS2 nanosheets were synthesized on the Si/SiO2 (300 nm) substrate by the CVD technique. MoO3 powders and sulfur (S) pellets were used as Mo and S sources, respectively. First, the MoO3 and the S heating zones were heated to 65 °C and 160 °C, respectively, and the temperatures were maintained for ~40 min and ~30 min, respectively. Then, the heating zones were heated again to 150 °C and 760 °C for MoS2 preparation. During the preparation, Ar carrier gas was added into the quartz tube with flow rate of 35 sccm under atmospheric pressure. At last, after about 20 min, the V-MoS2 nanosheets were synthesized with height of 2 μm and an averaged layer thickness of 3 nm. The V-MoS2 nanosheets on the Si/SiO2 substrate were first spin-coated with polymethyl methacrylate (PMMA); then a hot plate was used to heat the substrate at ~80 °C for ~30 min, and a NaOH solution (2 mol/L) was used to remove the PMMA layer along with V-MoS2 nanosheets from the substrate at 60 °C; the layer was transferred to deionized water to wash away the residual NaOH and

fished out with a target p-type Si substrate, and heated at ~80 °C for ~2 hours to dry the water and remove the possible wrinkles; the PMMA was dissolved by acetone. Afterward, suitable annealing (~200 °C) was carried out in vacuum chamber to strengthen the adhesion of V-MoS2 to the Si substrate. Finally, the Ag (50 nm) top contact layer and Ag (100 nm) bottom contact layer were deposited on the surface of MoS2 and the back of Si substrate by thermal evaporation, respectively. The active area of the device (diameter = 1 mm) is defined by the opened window locating at the center of the Ag contact area formed by a mask. The photoresponse properties of the V-MoS2/Si heterojunction photodetector were studied by using a SourceMeter (Keithley 4200) system. The illumination source was a continuous-wave laser of different wavelengths (with 405, 532, 671, 808, and 980 nm). An optical power meter (Coherent FieldMax II) was used to identify the laser power density. The time response measurement was performed by combining a light chopper (or fs-pulse-width pulsed lasers with frequency modulated by a Pulse Select) and a digital oscilloscope (Agilent DSO X 4022A).

3. Results and discussion

The morphology, crystalline structure, elemental and bond composition of the V-MoS2 nanosheets were characterized by SEM, Raman spectroscopy, XRD, and x-ray photoelectron spectroscopy (XPS), respectively. As demonstrated clearly in Figs. 1a and 1b, the surface structure of the as-synthesized MoS2 is composed of vertically layered nanosheets with averaged thickness of about 3 nm (corresponding to 4 layers of MoS2), height of about 2 μm. It is worth noting that the thickness of MoS2 nanosheet is non-uniform from down to top even with monolayer structure in the top area, as shown in Fig. S1, indicating a wedged morphology of the MoS2

nanosheets, which is in accordance with the morphology of other 2D material nanosheets caused by growth terraces or steps 18. The Raman spectrum is shown in Fig. 1c. The nanosheets exhibit two very strong characteristic MoS2 Raman peaks at 384.1 cm-1 (E2𝑔

1

mode) and 409.4 cm-1 (𝐴1𝑔 mode) respectively with a difference of

25.3 cm-1, which agrees well with the previous experimental findings for few-layered MoS2

21.

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Fig. 1 (a) Top-down, and (b) Cross-sectional SEM images of V-MoS2, (c) Raman spectra, (d) XRD result of V-MoS2nanosheets, e) XPS spectra of Mo 3d and S 2p.

XRD was used to characterize the crystalline structure of V-MoS2/Si. As shown in Fig. 1d, excepting diffraction peak at 2θ = 69.2°, which can be ascribed to the substrate of Si (400), only (00n) diffraction peaks of MoS2 at 2θ = 14.6°, 29.6°, 44.4° and 59.5° were observed, corresponding to (002), (004), (006) and (008) of MoS2 (JCPDS card No. 37-1492), respectively, indicating that individual MoS2

nanosheets are highly [001] oriented, as similar as other 2D material nanosheets with vertically layered structure 20. Finally, we also utilized the XPS to study the elemental and bond composition of V-MoS2, as shown in Fig. 1e. Peaks are observed at 233.33 eV, 230.14 eV, 164.19 eV, and 162.93 eV, which is expected for hexagonal molybdenite structure MoS2 corresponding to Mo 3d3/2 , Mo 3d5/2, S 2p1/2, and S 2p3/2, respectively 22. The structure of V-MoS2/Si heterojunction photodetector was illustrated in Fig. 2a. Ag conductive layer (50 nm) with a 1 mm-diameter opened window (determining the effective photoresponse area), was deposited on the MoS2 nanosheets as the top electrode. Fig. 2b gives the current-voltage (I-V) curves of the V-MoS2/Si heterojunction in dark and under laser illumination (808 nm, 80 mW/cm2), respectively. Obviously, the device exhibits a photodiode-like behavior with a significant response to 808 nm laser, indicating the high sensitivity to near-infrared light for this heterojunction. Moreover, the forward current keeps nearly constant, but the current increases with increasing reverse voltage under illumination as compared with that in dark, meaning that this

V-MoS2/Si heterojunction works in the typical photovoltaic-like mode due to p-n junction forming between V-MoS2 and Si, where part of photo-excited carriers in V-MoS2 or Si diffuse towards the built-in field of the interface, then the electrons are separated into the MoS2 side and the holes are separated into the Si side. It is also proved that this rectifying property is derived from the junction between Si and V-MoS2, but not from Ag/MoS2 contact or Si/Ag contact because of the linear I-V characteristic of the Ag/V-MoS2/Ag and Ag/Si/Ag stacking structures between -2 V and 2 V (Fig. S2). The laser wavelength-dependent photoresponse of the photodetector is shown in Fig. 2c. It is clear that the V-MoS2/Si heterojunction exhibits a broadband spectral response ranging from 350 to 1100 nm with an optimal photoresponse wavelength of about 810 nm, indicating that the device is very suitable for applying in visible-near-infrared optoelectric sensors. To further understand the photoresponse characteristics, photocurrent as a function of time was measured under periodically turning on and off laser (808 nm, 80 mW/cm2) at -1 V and -2 V, respectively, as shown in Fig. 2d. The photocurrent of the device increases and decreases greatly with the laser illumination on and off, generating a repeatable and stable

photocurrent /ON OFFI I ratio of 8.72104 and 2.96105 at -1 V and -

2 V, respectively. From the above results, it is suggested that the V-MoS2/Si heterojunction has great potential application in optoelectronic sensors.

Fig. 2 (a) Schematic illustration of photoresponse of the V-MoS2/Si heterojunction device. (b) The typical I-V curves of V-MoS2/Si heterojunction in dark and under light illuminations, respectively. (c) Spectral response as a function of wavelength. (d) Photocurrent as a function of time under -1 V and -2 V bias voltages under illumination of 240 mW/cm2 for 808 nm.

Then, to evaluate the photodetection performance of the V-MoS2/Si heterojunction device, I-V curves under illumination of different light power densities and wavelengths. Fig. 3a presents the I-V properties of the heterojunction at different laser power densities of the 808 nm laser. Upon the monochromatic laser

illumination (808 nm), the reverse photocurrent increases greatly with increasing the laser power density due to the increased photo-excited carriers, and approaches to a huge value of 0.626 mA at -2 V and 240 mW/cm2, suggesting a strong photoresponse of the device. It is noteworthy that even under illumination of so large power

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density range, the V-MoS2/Si heterojunction can still work very well with excellent repeatability of photoresponse. Moreover, the I-V curves of the heterojunction under illumination of different lasers (405, 532, 671, 808 and 980 nm, respectively) at laser power density of 240 mW/cm2 were also investigated, as shown in Fig. 3b. The reverse photocurrent, which gets considerable improved with wavelength changing from 405 nm to 808 nm, then starts to decrease for 980 nm laser, exhibits a non-monotonic and complex wavelength-dependency behavior, the tendency of which is in accordance with the spectra response in Fig. 2c, indicating the vital effect of photo-generated efficiency on the photocurrent. The laser power density-dependent photocurrents at different external voltages are shown in Fig. S3a. The photocurrent shows a strong dependence on the laser power densities, especially for large reverse bias voltages. The relationship between photocurrent and power density can be effectively fitted with a power law

phI CP ,

where phI is the photocurrent, C is a proportional coefficient,

is a photoresponse-related exponent parameter, and P represents the incident laser power density. 12, 23 By fitting the measured data, the values of 0.59,0.61,0.64,0.71,0.85,0.96 were extracted

for reverse bias voltage of 0, -0.5, -0.8, -1.0, -1.5, and -2.0 V, respectively. It is clear that the exponent parameter approaches to the ideal factor ( =1.0 ) of low trap state junctions with increasing

reverse bias voltage gradually, indicating that the photocurrent loss, which is attributed to the complicated process of carriers generation, recombination, defects, or trapping within the V-MoS2/Si heterojunction, 24-26 can be alleviated by the bias voltage in the device 27. Moreover, the laser power density-dependent photocurrent curves of different lasers at bias voltage of -2 V were also fitted by the power law equation with the best fitting

parameters of 0.89,0.96,0.94,0.97,0.96 for 405, 532, 671,

808, and 980 nm, respectively, as shown in Fig. S3b. It is obtained that the fitting factor is nearly independent of laser wavelength, which implies that the trap in this heterojunction should be the shallow donor states located between fermi level and conduction band. 28

Photoresponsivity (R) and detectivity (D*) are the two crucial Figure of merit factors for a photodetector device. R represents the photocurrent that generated per unit incident laser power on the effective photoresponse area of a photodetector, and can be written as follows: 1, 8, 12

=phI

RPS

(1)

Where phI is the photocurrent, and S is the effective

photoresponse area that illuminated by the laser. Here, the effective area of the V-MoS2/Si photodetector is determined by the area of the opened window (~0.785 mm2). Apparently, high R means a large photocurrent generating under a relatively low laser power. While D* indicates the performance of a device for detecting weak signal, and can be deduced by using the following equation: 8, 12

2 2d d

A RD R

qI qI

(2)

Where dI and

dJ are the current and the current density in dark,

respectively. From Equation (2), it is suggested that high detectivity is determined by both a large responsivity and a low dark current density.

Fig. 3 (a) Photoresponse characteristics of the V-MoS2/Si heterojunction under illumination of different light power densities for 808 nm laser. (b) Photoresponse characteristics under light illumination of 240 mW/cm2 with different wavelengths. (c) Responsivity and detectivity as a function of reverse bias voltage under illumination of 1.6 mW/cm2 for 808 nm laser. (d) Responsivity and detectivity as a function of laser wavelength under -2 V bias voltage with 1.6 mW/cm2 illumination.

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Fig. 3c gives the reverse bias voltage-dependent photoresponsivity and detectivity under illumination of 1.6 mW/cm2 with 808 nm laser. Both the photoresponsivity and the detectivity increase considerably with increasing bias voltage, and reach to 908.2 mA/W

and 1.8891013 Jones (Jones=cmHz1/2/W) at -2 V, respectively. The photoresponsivity, as well as the detectivity, is much better than that obtained in parallel MoS2-based photodetectors and simple vertical layered 2D nanosheets photodetectors 8, 10, 17, 19. Moreover, we also got the photoresponsivities and the detectivities under illumination of other different power densities, as shown in Fig. S4a and S4c. It is obtained that both the photoresponsivity and the detectivity increases with decreasing power density, which is suggested to be directly related to the enhanced recombination or scattering rate of the generated carriers. 29, 30 Fig. 3d shows the laser wavelength-dependent photoresponsivity and detectivity at bias voltage of -2 V and illumination power density of 1.6 mW/cm2. The photoresponsivity (/detectivity) improves from 197.3 mA/W

(/0.7091013 Jones) to a maximum of 908.2 mA/W (/1.8891013

Jones) with laser wavelength changing from 405 nm to 808 nm and

then reduces to 286.4 mA/W (/1.0581013 Jones) for 980 nm, the tendency of which is also consistent with the spectra response in Fig. 2c. Notably, the large photoresponsivity and detectivity nearly outshines most of previous reported 2D material-based photodetectors, 8, 12, 19, 31-36 especially in near-infrared range, suggesting that the V-MoS2/Si heterojunction shows great potential application in infrared photodetectors. In addition, the non-monotonic dependence of photoresponsitivity and detectivity on laser wavelength is nearly independent of both bias voltage and laser power density (Fig. S4b and S4d), which indicates that it is an intrinsic property determined by spectra response, but not a random phenomenon. The high photoresponse properties can be ascribed to the very strong light absorption of the V-MoS2

nanosheets because of the large thickness of the film, and also the high quality of this V-MoS2/Si heterojunction with strong interface built-in field and fewer bulk defects.

Fig. 4 Time-dependent photovoltages at frequencies of (a) 100 Hz (chopper-induced pulse laser), and (b) 0.1 MHz (100 fs-pulse-width pulsed laser). The magnified plots of one response cycle for 0.1 MHz at laser (c) being turned on, and (d) being turned off.

The response time is another key parameter of a photodetector, which represents its capability to follow high-frequency pulsed optical signals. In order to further investigate the time response of this V-MoS2/Si device, a pulsed photovoltage signal was measured first by combining a digital oscilloscope and a laser chopper with modulating frequency ranging from 0 Hz to 4000 Hz. Fig. 4a shows the time-dependent photovoltages at typical frequency of 100 Hz under illumination of 80 mW/cm2 with 808 nm laser, the

photovoltage gets the maximum (maxV ) in the period of laser on,

and gets the minimum (minV ) in the period of laser off. It was found

that when the chopper frequency was modulated from 100 Hz to

4000 Hz, both the maxV and the

minV nearly did not decrease or

increase and the relative balance max min max( ) /V V V kept nearly

100% over the whole frequency range, as shown in Fig. S5,

suggesting that the V-MoS2/Si heterojunction photodetector has a very wide frequency response range. However, the rise (/fall) time from the magnified time-dependent photovoltage curve even at 4000 Hz cannot represent its true response speed, as the resolution of the response time is related to both the laser-pulse frequency and width under this condition 37, and more importantly, the

relative balance of photovoltage max min max( ) /V V V does not

decrease in the measurement frequency range. Then, the response time of the V-MoS2/Si heterojunction was studied by using an oscilloscope to measure the photovoltage variation under illumination of a ultrashort pulse width laser with the same power density. The pulsed light (800 nm) was produced from a 100 fs-pulse-width Ti: sapphire laser (Spectra Physics, Mai Tai HP) using a pulse select to modulate the laser repetition frequency ranging from 16 KHz to 4 MHz. When the frequency of laser input increases

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from 16 KHz to even 0.1 MHz, the V-MoS2/Si device can still maintain excellent switching performance, as shown in Fig. 4b. The detailed information of the rising and falling process in a single magnified switching cycle is provided in Fig. 4c and 4d. By fitting time-dependent photovoltage curve at rising and falling edges using

exponential equations (1 exp( / ))ph offV V A t and

(1 exp( / ))ph onV V B t , respectively, where onV ,

offV , A ,

and B are fitting parameters, an ultrafast time response with

rising time rise of 56 ns and falling time fall of 825 ns can be

extracted (the response time is a little different from that reported in Ref. 37 due to different samples). Notably, this ultrafast response speed is much quicker than that reported in any other 2D-based photodetectors or Si-based photodetectors 8-10, 12, 14, 31, 38, 39 and may also represent the best result in 2D materials till now, suggesting that the CVD-prepared V-MoS2 can indeed greatly facilitate the transport of photo-excited carriers along the longitudinal intralayer direction to reach the top Ag electrode. Moreover, we also investigated the air stability of this V-MoS2/Si heterojunction by placing it in air without any protection. The photoresponse stays unchanged in both chopper modulated laser and 100 fs-width pulsed laser for at least three months, (as shown in Fig. S6a and S6b), demonstrating that the device is stable for practical application in high performance photodetector.

4. Conclusions

In conclusion, we have prepared large-area, high-quality, and vertically few-layered MoS2 nanosheets by CVD and successfully transferred them onto the Si substrate to form the V-MoS2/Si heterojunction. Photoelectric measurements suggest that the V-MoS2/Si heterojunction photodetector exhibits excellent characterizes with a wide broadband response ranging from 350 nm to 1100 nm, large responsivity of 908.2 mA/W, and extremely high

detectivity of 1.8891013 Jones. More importantly, an unprecedented ultrafast response speed of 56 ns (/825 ns) was observed in this device by time-response measurements, which represents the best results reported so far in all 2D materials. The high photoresponse performances and ultrafast time response can be ascribed to the good quality of the CVD-prepared V-MoS2/Si heterojunction with strong light absorption and quick carrier transport speed in the unique vertically oriented few-layer MoS2

nanosheets as well as the good heterojunction formed between V-MoS2 and Si. Our results demonstrate that the CVD-prepared large-area, high-quality, and vertically oriented few-layer 2D material heterojunction shows great potential applications in ultrafast, high responsivity and broadband photodetecting sensors.

Conflicts of interest

They authors declare no competing financial interests.

Acknowledgements

This work is supported by the National Nature Science Foundation of China (Grant Nos. 51372064, 61504036, 11704094, 11504076, 51622205, 61675027, 51432005 and 61505010), the Nature Science Foundation of Hebei Province (Grant Nos. A2016201087, E2017201227, F2017201141 and F2018201198), the Science and technology research project of higher education institution of Hebei Province (Grant Nos. ZD2016036, and Z2015121), Nature Science Foundation for Distinguished Young Scholars of Hebei University

(Grant No. 2015JQ03), the Second Batch of Young Talents of Hebei Province, the support of national key R & D project from Minister of Science and Technology, China (2016YFA0202703), and Beijing Natural Science Foundation (4181004 and 4182080).

Notes and references

1 O. L. Sanchez, D. Lembke, M. Kayci, A. Radenovic, A. Kis, Nat. Nanotechnol., 2013, 8, 497.

2 Y. Yoon, K. Ganapathi, S. Salahuddin, Nano Lett., 2011, 11, 3768. 3 Z. Yin, H. Li, H. Li, L. Jiang, Y. Shi, Y. Sun, G. Lu, Q. Zhang, X. Chen,

H. Zhang, ACS Nano, 2012, 6, 74. 4 B. Radisavljevic, A. Radenovic, J. Brivio, V. Giacometti, A. Kis, Nat.

Nanotechnol., 2011, 6, 147. 5 K. F. Mak, C. Lee, J. Hone, J. Shan, T. F. Heinz, Phys. Rev. Lett.,

2010, 105, 136805. 6 Y. Xie, B. Zhang, S. X. Wang, D. Wang, A. Z. Wang, Z. Y. Wang, H. H.

Yu, H. J. Zhang, Y. X. Chen, M. W. Zhao, B. B. Huang, L. M. Mei, J. Y. Wang, Adv. Mater., 2017, 29, 1605972.

7 M. Bernardi, M. Palummo, J. C. Grossman, Nano Lett., 2013, 13, 3664.

8 W. Choi, M. Y. Cho, A. Konar, J. H. Lee, G. B. Cha, S. C. Hong, S. Kim, J. Kim, D. Jena, J. Joo, S. Kim, Adv. Mater., 2012, 24, 5832.

9 W. J. Zhang, C. P. Chuu, J. K. Huang, C. H. Chen, M. L. Tsai, Y. H. Chang, C. T. Liang, Y. Z. Chen, Y. L. Chueh, J. H. He, M. Y. Chou, L. J. Li, Sci. Rep., 2014, 4, 3826.

10 X. D. Wang, P. Wang. J. L. Wang, W. D. Hu, X. H. Zhou, N. Guo, H. Huang, S. Sun, H. Shen, T. Lin, M. H. Tang, L. Liao, A. Q. Jiang, J. L. Sun, X. J. Meng, X. S. Chen, W. Lu, J. H. Chu, Adv. Mater., 2015, 27, 6575.

11 S. Das, H. Y. Chen, A. V. Penumatcha, J. Appenzeller, Nano Lett., 2013, 13, 100.

12 L. Wang, J. S. Jie, Z. B. Shao, Q. Zhang, X. H. Zhang, Y. M. Wang, Z. Sun, S. T. Lee, Adv. Funct. Mater., 2015, 25, 2910.

13 Y. Zhang, Y. Q. Yu, L. F. Mi, H. Wang, Z. F. Zhu, Q. Y. Wu, Y. G. Zhang, Y. Jiang, Small, 2016, 12, 1062.

14 D. S. Kong, H. T. Wang, J. J. Cha, M. Pasta, K. J. Koski, J. Yao, Y. Cui, Nano Lett., 2013, 13, 1341.

15 Y. Li, C. Y. Xu, J. Y. Wang, L. Zhen, Sci. Rep., 2014, 4, 7186. 16 S. Qiao, B. Zhang, K. Y. Feng, R. D. Cong, W. Yu, G. S. Fu, S. F.

Wang, ACS Appl. Mater. Interfaces, 2017, 9, 18377. 17 B. J. Zheng, Y. F. Chen, Z. G. Wang, F. Qi, Z. S. Huang, X. Hao, P. J.

Li, W. L. Zhang, Y. R. Li, V 2D Mater., 2016, 3, 035024. 18 H. Li, H. Q. Wu, S. G. Yuan, H. Qian, Sci. Rep., 2016, 6, 21171. 19 G. B. Liu, Z. H. Li, X. S. Chen, W. Zheng, W. Feng, M. J. Dai, D. C.

Jia, Y. Zhou, P. A. Hu, Nanoscale, 2017, 9, 9167. 20 J. Gao, L. Li, J. W. Tan, H. Sun, B. C. Li, J. C. Idrobo, C. V. Singh, T.

M. Lu, N. Koratkar, Nano Lett., 2016, 16, 3780. 21 C. Lee, H. Yan, L. E. Brus, T. F. Heinz, J. Home, S. Ryu, ACS Nano,

2010, 4, 2695. 22 G. Eda, H. Yamaguchi, D. Voiry, T. Fujita, M. Chen, M. Chhowalla,

Nano Lett., 2011, 11, 5111. 23 M. Hafeez, L. Gan, H. Li, Y. Ma, T. Zhai, Adv. Mater., 2016, 28,

8296. 24 L. Li, E. Auer, M. Liao, X. Fang, T. Zhai, U. K. Gautam, A. Lugstein,

Y. Koide, Y. Bando, D. Golberg, Nanoscale, 2011, 3, 1120. 25 L. H. Zeng, M. Z. Wang, H. Hu, B. Nie, Y. Q. Yu, C. Y. Wu, L. Wang,

J. G. Hu, C. Xie, F. X. Liang, L. B. Luo, ACS Appl. Mater. Interfaces, 2013, 5, 9362.

26 Y. Huang, H. X. Deng, K. Xu, Z. X. Wang, Q. S. Wang, F. M. Wang, F. Wang, X. Y. Zhan, S. S. Li, J. W. Luo, J. He, Nanoscale, 2015, 7, 14093.

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d by

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jing

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27 S. S. Liu, X. Yuan, P. Wang, Z. G. Chen, L. Tang, E. Z. Zhang, C. Zhang, Y. W. Liu, W. Y. Wang, C. Liu, C. Chen, J. Zou, W. D. Hu, F. X. Xiu, ACS Nano, 2015, 9, 8592.

28 S. C. Kung, W. E. van der Veer, F. Yang, K. C. Donavan, R. M. Penner, Nano Lett., 2010, 10, 1481.

29 F. Liu, H. Shimotani, H. Shang, T. Kanagasekaran, V. Zolyomi, N. Drummond, V. I. Falko, K. Tanigaki, ACS Nano, 2014, 8, 752.

30 G. Su, V. G. Hadjiev, P. E. Loya, J. Zhang, S. Lei, S. Maharjan, P. Dong, P. M. Ajayan, J. Lou, H. Peng,. Nano Lett., 2015, 15, 506.

31 M. R. Esmaeili-Rad, S. Salahuddin, Sci. Rep., 2013, 3, 2345. 32 M. Massicotte, P. Schmidt, F. Vialla, K. G. Schädler, A. Reserbat-

Plantey, K. Watanabe, T. Taniguchi, K. J. Tielrooij, F. H. Koppens, Nat. Nanotechnol., 2016, 11, 42.

33 H. Yuan, X. Liu, F. Afshinmanesh, W. Li, G. Xu, J. Sun, B. Lian, A. G. Curto, G. Ye, Y. Hikita, Z. Shen, S. C. Zhang, X. Chen, M. Brongersma, H. Y. Hwang, Y. Cui, Nat. Nanotechnol., 2015, 10, 707.

34 C. Ma, Y. Shi, W. Hu, M. H. Chiu, Z. Liu, A. Bera, F. Li, H. Wang, L. J. Li, T. Wu, Adv. Mater., 2016, 28, 3683.

35 B. W. H. Baugher, H. O. H. Churchill, Y. F. Yang, P. J. Herrero, Nat. Nanotechnol., 2014, 9, 262.

36 Y. H. Chang, W. Zhang, Y. Zhu, Y. Han, J. Pu, J. K. Chang, W. T. Hsu, J. K. Huang, C. L. Hsu, M. H. Chiu, T. Takenobu, H. Li, C. Wu, W. H. Chang, A. T. S. Wee, L. J. Li, ACS Nano, 2014, 8, 8582.

37 R. D. Cong, S. Qiao, J. H. Liu, J. S. Mi, W. Yu, B. L. Liang, G. S. Fu, C. F. Pan, S. F. Wang, Adv. Sci., 2017, DOI: 10.1002/advs.201700502.

38 X. Yuan, L. Tang, S. S. Liu, P. Wang, Z. G. Chen, C. Zhang, Y. W. Liu, W. Y. Wang, Y. C. Zou, C. Liu, N. Guo, J. Zou, P. Zhou, W. D. Hu, F. X. Xiu, Nano Lett., 2015, 15, 3571.

39 P. Wang, S. S. Liu, W. J. Luo, H. H. Fang, F. Gong, N. Guo, Z. G. Chen, J. Zou, Y. Huang, X. H. Zhou, J. L. Wang, X. S. Chen, W. Lu, F. X. Xiu, W. D. Hu, Adv. Mater., 2017, 29, 1604439.

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