High-Performance WS2 Monolayer Light-Emitting Tunneling Devices

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High-Performance WS2 Monolayer LightEmitting Tunneling Devices Using 2D Materials Grown by Chemical Vapor Deposition Downloaded via UNIV OF NEW ENGLAND on March 22, 2019 at 09:53:04 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

Yuewen Sheng,* Tongxin Chen, Yang Lu, Ren-Jie Chang, Sapna Sinha, and Jamie H. Warner* Department of Materials, University of Oxford, Parks Road, Oxford OX1 3PH, United Kingdom S Supporting Information *

ABSTRACT: The solid progress in the study of a single two-dimensional (2D) material underpins the development for creating 2D material assemblies with various electronic and optoelectronic properties. We introduce an asymmetric structure by stacking monolayer semiconducting tungsten disulfide, metallic graphene, and insulating boron nitride to fabricate numerous red channel lightemitting devices (LEDs). All the 2D crystals were grown by chemical vapor deposition (CVD), which has great potential for future industrial scale-up. Our LEDs exhibit visibly observable electroluminescence (EL) at both 5.5 V forward and 7.0 V backward biasing, which correlates well with our asymmetric design. The red emission can last for at least several minutes, and the success rate of the working device that can emit detectable EL is up to 80%. In addition, we show that sample degradation is prone to happen when a continuing bias, much higher than the threshold voltage, is applied. Our success of using high-quality CVD-grown 2D materials for red light emitters is expected to provide the basis for flexible and transparent displays. KEYWORDS: WS2, 2D materials, chemical vapor deposition, heterostructures, light-emitting devices

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Another advantage for using TMD material is its similar layered structures, which allows the van der Waals heterostructures to be built without having traditional lattice mismatch problems that cause interface strain.19−21 The primary element for conventional LED fabrication requires the formation of a p−n junction. Previous work on making TMD LEDs has been realized with electrostatic gating, where two individual transistors, one p-type and one n-type, are assembled on a single TMD material.22−24 However, it has the disadvantage that most TMDs cannot obtain ambipolar transport in conventional SiO2-back-gated transistors.25 In this regard, some other researchers introduced a new concept using ionic liquid, or other electrolytes, as gate dielectric materials,26,27 which has been reported successful and compatible with several TMD materials, thanks to its high capacitance.28−33 Instead of forming a p−n junction, another research group reported a simple structure and was able to

ne of the most important components in today’s optoelectronic devices is the light-emitting device (LED) that can directly convert current (electricity) to photons (light). In recent years, fundamental research in two-dimensional (2D) materials has developed rapidly since the first isolation of graphene in 2004.1 Other 2D materials, such as monolayered transition metal dichalcogenides (TMDs),2,3 boron nitride (BN),4,5 and black phosphorus,6,7 with properties ranging from superconducting, metallic, semimetallic, semiconducting, to insulating, have drawn increased attention as the best candidates for exploring nextgeneration electronic and optoelectronic applications, such as photodetectors, solar cells, LEDs, molecular sensors, and optical imaging sensors.8−13 In particular, monolayer TMDs, including tungsten disulfide (WS2) and molybdenum disulfide (MoS2), are semiconductors with direct band gaps and tunable optoelectronic properties.14−16 One merit of monolayer TMDs compared with conventional quantum well structures17,18 is their naturally passivated surfaces, which produce light emission dominated by excitons due to enhanced Coulomb interaction.15,16 © XXXX American Chemical Society

Received: January 9, 2019 Accepted: March 18, 2019

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DOI: 10.1021/acsnano.9b00211 ACS Nano XXXX, XXX, XXX−XXX

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Figure 1. Asymmetric heterostructure LEDs based on CVD-grown 2D materials. Schematic (a) side and (b) top views of the Cr/Au/GrB/ BNB/1L WS2/BNT/AuT asymmetric semivertical light-emitting device. The red dashed rectangle indicates the active region. (c) Zoomed-in cross-sectional view of the active area, featuring monolayer WS2 as the active semiconducting material, graphene as the bottom contact, Au as the top contact, and multilayer BN as the tunnel barriers. (d) SEM image showing a typical area of the WS2-based devices with customized top Au bond pads. Three different types of designs are presented. Scale bar: 100 μm. (e) Enlarged SEM image of the green dashed rectangle in (d), illustrating a representative device that has a channel length of 5 μm; same for all devices. Scale bar: 10 μm. (f) Schematic band diagram of the asymmetric structure in (a) when Vds is not applied (equilibrium state). (g,h) Band diagrams for the cases of (g) positive and (h) negative high bias where EL signals are observed.

obtain electroluminescence (EL) by applying an AC voltage.34 However, all the configurations mentioned above require local gate control, which seriously hampers its applicability and scalability in transparent optoelectronics. In addition, to make devices using only one single 2D material, another important method to realize light emission is stacking different 2D materials together. By utilizing graphene as a conductive electrode, BN as an insulator, and TMDs as the active semiconducting material, it would be feasible to produce vertically stacked 2D heterostructures.35−39 However, the most popular technique so far for assembling van der Waals heterostructures remains mechanical exfoliation, which is not suitable for future industrial scale-up. Compared with vertically stacked heterostructures,40−42 lateral heterojunctions can be easily realized via either direct growth or manual transfer.43−45 However, their LED application has not yet been realized. In this study, we introduce an asymmetric lateral structure for the fabrication of red channel light-emitting devices based

on single-layer WS2 (1L WS2) and measure their EL properties. All the 2D materials are grown by ambient-pressure chemical vapor deposition (APCVD) as previously reported.46−49 This allows exciting opportunities for the production of large-area device arrays, which cannot be achieved by mechanically exfoliated approaches. By stacking 2D materials together via layer by layer transfer, 2D heterostructures can be constructed with different stacking sequences. Our asymmetric structure enables us to observe the red electroluminescent signals in 1L WS2 at different forward and backward biasing onsets. The devices were characterized by optical microscopy (OM), scanning electron microscopy (SEM), transmission electron microscopy (TEM), as well as photoluminescence (PL) and Raman spectroscopies. The performance and durability of the red channel emitters were also examined under different biasing and time scale. B


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Figure 2. Optical characterization of the asymmetric heterostructures. (a) Optical image of a typical WS2 light-emitting device. Scale bar: 10 μm. (b) Raman spectrum of the graphene electrode, obtained from the blue cross in (a). The 2D/G ratio is 2.5, indicating monolayer graphene. (c) Raman spectrum of the BN films, the black cross in (a), with a peak position of ∼1367.7 cm−1. (d) PL integrated intensity mapping obtained from the red dashed square in (a). (e) PL spectrum taken from the red cross in (a), with two fitted Lorentzian peaks representing exciton (A) emission (red) and negative trion (A−) emission (blue). (c) Raman spectrum of the same spot, fitted with six Lorentzian peaks, two of which are highlighted showing E′ (red) and A′1 (blue) modes.

RESULTS AND DISCUSSION Figure 1a,b schematically presents the side and top views of our asymmetric architecture featuring a semiconducting 1L WS2 monolayer sandwiched between two layers of multilayer BN films (3−5 layers), with monolayer graphene as a bottom electrode and gold (Au) as a top electrode. The red dashed rectangles in Figure 1a,b indicate the active region, and an enlarged cross-sectional schematic view is shown in Figure 1c. The fabrication process for such an asymmetric light-emitting device can be divided into six steps (see Supporting Information Figure S1). The Cr/Au electrodes were first deposited onto a silicon chip with a 300 nm oxide layer, followed by making bottom graphene electrodes utilizing electron-beam lithography (EBL) and reaction ion etching (RIE). Then, three layers of 2D materials (i.e., bottom BN, 1L WS2, and top BN) were transferred accordingly on top of the as-prepared substrate using a poly(methyl methacrylate) (PMMA) support.50 Finally, the top Au bond pads were designed depending on the relative positions of the 1L WS2 domains and then deposited by thermal evaporation. Our study includes three types of top Au contact designs in order to maximize the number of devices (see Supporting Information Figure S2). We were able to fabricate over 110 devices on a single 1 cm × 1 cm chip (see Supporting Information Figure S3). The channel length for each device is the same, featuring a 5 μm gap between bottom graphene and top Au electrodes. Figure 1d is the SEM image of a typical area after all the

fabrication processes with different designs of top Au contacts. Figure 1e shows a zoomed-in SEM image of the green dashed rectangle in Figure 1d, with a 1L WS2 triangular domain highlighted in red bridging across the gap between the bottom graphene (blue) and top Au electrodes (yellow) but sandwiched by two layers of BN films. Figure 1f is the band diagram of our asymmetric 1L-WS2-based light-emitting device with no bias applied. When applying a positive bias into the system, at low Vds, minimal carrier injection from graphene occurs and the current flowing through the device is relatively low, with electrons as majority charge carriers. When it reaches a certain high voltage (Figure 1g), minority charge carriers, holes, from the top Au electrode could also be injected along with the electrons from the bottom graphene electrode. The recombination of these injected holes and electrons could generate EL. Alternatively, a similar mechanism for EL production could be realized when applying a higher negative bias to the structure, with electrons (majority charge carriers) injected from top Au side and holes (minority charge carriers) from bottom graphene side (Figure 1h). Before bias was applied to the system, the 1L WS2 lightemitting devices were first characterized by optical microscopy, as well as PL and Raman spectroscopies. Figure 2a shows the optical image of a typical device after all of the fabrication processes. The graphene electrode is mostly monolayer, and the Raman spectrum (Figure 2b), with a 532 nm excitation, was taken from the blue marked spot in Figure 2a to determine the layer number. Two important features of graphene are C


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Figure 3. Observation of visible EL from the WS2-based LEDs. (a) Optical image showing a typical red channel light emitter based on monolayer WS2, graphene, and BN. Scale bar: 50 μm. (b) Image showing observable EL in the active region at high bias. Scale bar: 50 μm. (c) Comparison of the PL and EL spectra (taken at Vds = 7 V), indicating very similar profile. The PL was acquired at the same spot before source−drain bias was applied to generate EL.

Figure 4. Bias-dependent EL characteristics of the 1L WS2 LEDs. (a) Full ramping profile of the Ids−Vds curve for a typical WS2 light-emitting device. (b) 2D mapping of the EL spectra as a function of Vds for the WS2 light-emitting device when Vds < 0. (c) Spatial mapping of the EL spectra for the same device when Vds > 0.

highlighted: a G peak at ∼1586 cm−1 and a 2D peak at ∼2693 cm−1. The 2D/G intensity ratio is 2.5, indicating single-layer graphene.51−53 Figure 2c presents the Raman vibration mode of the BN films used in this work to sandwich 1L WS2 domains. The spectrum was obtained from the black marked spot in Figure 2a and has a peak positioning at around 1367.7 cm−1. The low-magnification TEM image indicates that our CVD-grown BN film is approximately five layers thick (see Supporting Information Figure S4), similar to our previous report.49 Figure 2d is the PL integrated intensity mapping of the red dashed square in Figure 2a, indicating high areal uniformity of the 1L WS2 domain. No PL quenching is found in the region with graphene lying underneath, confirming the existence of insulating BN films that effectively separate the 2D materials on either side and prevent the charge transfer from 1L WS2 to graphene.54 The quality of the 1L WS2 triangular domain was further investigated by taking PL and Raman spectra from the red marked spot in Figure 2a. Figure 2e shows a strong PL peak between the energy range of 1.8 and 2.2 eV, typical for a transferred CVD-grown 1L WS2 sample. The PL spectrum was then fitted by two Lorentzian curves, corresponding to the exciton (A) and negative trion (A−) emissions. The peak positions for A and A− are 2.01 and 1.98 eV, respectively, leading to a binding energy of ∼30 meV for

our 1L WS2 crystal, which falls in the range of reported values ranging from 20 to 40 meV.55,56 In addition, a Raman spectrum obtained from the same spot was fitted by six Lorentzian peaks (Figure 2f). The peak positions are 355.8 and 417.6 cm−1 for in-plane (E′) and out-of-plane (A′1) vibration modes, respectively. The frequency difference between E′ and A′1 modes is measured to be 61.8 cm−1, in accordance with those previously reported in the literature.3,57,58 All the EL measurements were carried out at room temperature and in ambient conditions. Figure 3a shows an example of our light-emitting devices with no source−drain bias applied. When Vds ramps up to a certain value, current starts to flow through the active region and a red EL dot can be observed across the bottom graphene and top Au electrodes. In most cases, the EL was sufficiently bright enough that it could be detected using a cheap CMOS camera. Figure 3b is a single frame of a video, confirming that the position of EL locates between the two contacts. The bright EL also enabled us to measure the PL at the same spot that generated EL. Figure 3c plots the PL and EL spectra together, measured at the same position, showing very similar profiles. In our study, it is worth mentioning that no biasing is applied during the PL measurement, whereas there is no laser excitation when the EL measurement is in process. The high similarity of the PL D


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Figure 5. Variable behavior and time-dependent EL characteristics of the 1L WS2 LEDs. (a) Variable behavior of the WS2-based LEDs, with a success rate of 80%. (b) Transient EL intensity characteristics of the asymmetric structure under negative switch-on bias (Vds = −7 V) and zero applied bias. (c) Color map of the same transient EL signals in (b), showing very similar EL peak positions. (d) Five corresponding EL spectra of the time-dependent EL measurements. The spectra were fitted with two Lorentzian peaks, indicating similar profiles. (e) Color map of the EL spectra as a function of time under −10 V, a higher and continuing bias. The EL intensity started to decrease after 4 min biasing, resulting from sample degradation.

graphene or metal contact, thus, no efficient EL emission; (ii) the actual EL spot is hard to determine before biasing, and it might be out of our detectable radius defined by the 5 μm spot of the microscope objective and spectrometer system. In addition, we examine the stability and durability of our 1L WS2 LEDs. Figure 5b shows EL intensity characteristics under a negative bias of 7 V (the threshold voltage at backward biasing), switched on and off over several cycles. Five EL spectra were taken, marked as (i−v), and the time interval between every two measurements is 1 min. The black dashed line in Figure 5b indicates the EL average intensity value, and the magenta dashed line marks the zero EL emission background when no bias is applied. The standard deviation of the EL intensity is measured to be about 12% of the mean. Despite the slight difference in peak intensity, the peak positions are almost the same, with an average value of 2.016 eV and a negligible standard deviation, according to Figure 5c, a color map plotted with the same set of data. In Figure 5d, the five EL profiles were plotted together and no significant difference can be observed. Each spectrum was then deconvoluted into two separate peaks by Lorentzian fitting, attributed to A and A− emissions. The average photon energies for A and A− are 2.022 and 1.996 eV with standard deviations of 0.001 and 0.002 eV, respectively, which indicates high stability and durability of the structure and materials involved when source−drain bias reaches the switch-on point. The weight of neutral exciton emission is slightly over 50% in our

and EL peak shapes indicates that the EL comes from the recombination of the injected electrons and holes across the direct band gap. To further investigate the mechanism of red light production by EL, we measured the electrical properties of a typical device within a ±10 V source−drain bias. As can be seen from Figure 4a, an asymmetric Ids−Vds curve is expected due to our asymmetric design. Current starts to ramp up quickly at a positive voltage of 5 V, whereas it does not begin to soar until ∼7 V at backward biasing. Figure 4b,c shows the color maps of the EL spectra as a function of Vds. The results show that the switch points for triggering EL are 5.5 V at forward biasing and 7.0 V at backward biasing, which confirms that it is easier to inject electrons from the graphene end rather than the Au end because the barrier (energy difference) between the Fermi level of graphene and the conduction band of 1L WS2 is smaller than that between 1L WS2 and the work function of Au. We measured more than two dozen of the asymmetric structures. The success rate of the fabrication process for electrical transport was nearly 100%, with every device showing asymmetric transport characteristics, and then 80% of these showed detectable EL signals. Most working devices exhibited EL signals with similar peak positions and intensities (Figure 5a). We identified two ways in which the collection of EL signals can fail: (i) the transferred BN films are broken in some region, which causes charge transfer from 1L WS2 to either E


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flow (25% H2 and 75% Ar) when the chamber containing ammonia borane powder reached 120 °C. After 20 min growth, the furnace was powered off and slid away from the sample, allowing a fast cooling process. Transfer of CVD-Grown 2D Materials. The as-grown 2D materials were first spin-coated with a PMMA supporting layer (495 K, A8) at 4500 rpm for 1 min and cured at 180 °C for 1.5 min. The Cu substrates for the growth of graphene and BN were chemically etched away by ammonium persulfate solution (APS, 0.1 M), while the SiO2/Si substrates for WS2 growth were chemically etched by potassium hydroxide solution (KOH, 1 M) at room temperature. The detached PMMA/graphene, PMMA/BN, and PMMA/WS2 films were carefully cleaned several times by deionized water. Subsequently, the films were transferred onto other substrates and baked at 180 °C for 15 min. Finally, PMMA was removed by acetone. Device Fabrication. An electron-beam lithography (JEOL 5500 FS) system was used to pattern metal contacts with a bilayer positive photoresist. Ten nanometer Cr and 70 nm Au were deposited by a thermal evaporator, followed by a lift-off process in acetone. The graphene film was then transferred onto it and patterned using EBL with a negative photoresist and RIE etching. After that, three layers of 2D materials were transferred onto the sample accordingly, namely, bottom BN, WS2, and top BN. After each transfer, PMMA was removed by acetone. Finally, the top layer of Au bond pads was patterned and deposited using EBL and thermal evaporation, respectively. Characterization of WS2 Light-Emitting Devices. SEM (Hitachi S-4300) was used to examine the morphology of the heterostructures with a 3.0 kV accelerating voltage. The PL and Raman results in Figure 2 were conducted using a confocal Raman microscope (JY Horiba LabRAM Aramis) with a 532 nm laser excitation. The estimated laser spot size is ∼1 μm. The laser power used was 12.5 mW and 12.5 μW for Raman and PL measurements, respectively. Electrical Measurement and Electroluminescence Collection. The bias was applied through a pair of tungsten tips (5 μm diameter, Signatone SE-T) and controlled by a source meter (Keithley 2400-LV). When a continuous high bias was applied, the EL video was recorded by a CMOS camera (Thorlabs DCC1645C CMOS Camera) under a 10× magnification objective lens. The EL spectra were collected using a home-built spectrometer system (Princeton Instruments Acton SP-2300 spectrometer and a PIXIS 100 CCD). The corresponding PL (Figure 3) was measured at the same spot that generated EL. The estimated laser spot size for PL measurements is ∼5 μm, and the laser power was 0.1 mW.

EL, probably due to lattice strain and impurity doping introduced by our APCVD growth. To further examine its robustness, we increased the source−drain voltage from 7 to 10 V (backward biasing). As expected, a higher intensity of EL can be detected. However, when we continue applying the high bias, the EL intensity started to collapse after 4 min biasing (Figure 5e), which is due to WS2 degradation (see Supporting Information Figure S5).

CONCLUSION In summary, we demonstrate an asymmetric structure to fabricate red channel light-emitting devices based on 1L WS2, graphene, and BN. All the 2D materials were grown by APCVD, which is suitable for mass production in the future. The thresholds for EL generation are 5.5 V at forward biasing and 7.0 V at backward biasing, which correspond well to our asymmetric structure. The PL and EL profiles show almost no difference in spectral profile, indicating that the EL arises from the electron−hole recombination across the direct band gap of 1L WS2. Under EL switch-on voltages, our WS2 LEDs show excellent performance, high success rate, and long durability. Furthermore, the robustness testing proves that it may cause sample degradation if applying a continuous bias that is much higher than switch-on voltage, which could lead to device breakdown. This study provides significant insight for exploring LED applications based on CVD-grown 2D materials in a lateral heterojunction, which might be a breakthrough point for achieving large-scale display technology. EXPERIMENTAL METHODS CVD Synthesis of Monolayer WS2. The 1L WS2 was grown on Si chips with a 300 nm oxide layer (University Wafer) based on our previously reported CVD method with S and WO3 being precursors.47 Sulfur powder (200 mg, >99.5%, Sigma-Aldrich) and WO3 powder (100 mg, >99.5%, Sigma-Aldrich) were placed at the center of the low-temperature zone (furnace 1) and the high-temperature zone (furnace 2), respectively. The cleaned substrate was then placed downstream, about 9 cm away from WO3 powder in furnace 2. The whole system was first purged using argon gas (Ar) with a flow rate of 500 sccm for 30 min. Then furnace 1 temperature was set to 180 °C to create a S-rich environment, while furnace 2 temperature was set to 1145 °C with a rate of 40 °C/min. The reaction was carried out when both furnaces reached the target temperatures under a mixed gas flow of 95% Ar and 5% hydrogen (H2). After 4 min growth, furnace 2 temperature was set to 0 and the gas flow was reduced to 10 sccm Ar. When furnace 2 reached 1000 °C, the temperature of furnace 1 was increased to 380 °C. The gas flow rate was set back to 500 sccm Ar once furnace 1 reached 380 °C in order to blow the remaining sulfur away from the reaction zone. Finally, when furnace 2 temperature reached 700 °C, both furnaces were fast cooled to room temperature. CVD Synthesis of Monolayer Graphene. The CVD reaction was conducted in ambient pressure based on our previous report.59 The system was first flushed with Ar, H2, and methane (CH4) for 20 min, followed by another 10 min flushing with Ar and H2 only. The furnace was then ramped up to 1070 °C at a rate of 60 °C/min with 500 sccm Ar and 100 sccm H2. The copper sheet (25 μm thickness, Alfa Aesar) was annealed at 1070 °C under the same flow rate for another 1 h. After that, the reaction was carried out by introducing 5 sccm CH4. After 1 h growth, the furnace was switched off and the sample was quickly cooled to room temperature. CVD Synthesis of Multilayer BN. The multilayer h-BN films were synthesized on copper foil (25 μm thickness, Alfa Aesar) with ammonia borane (≥97%, Sigma-Aldrich) as precursor by APCVD.49 The system was first purged with Ar and H2 for 30 min, followed by a heating up process and a copper annealing process. The reaction was carried out under the condition of 1070 °C and 120 sccm mixed gas

ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.9b00211. Device fabrication process, designs of customized top bond pad, overview of the final product, characterization of multilayer BN, robustness testing with a higher and continuing bias, quantum efficiency estimation (PDF) Video 1 (AVI)

AUTHOR INFORMATION Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Yuewen Sheng: 0000-0003-3067-9520 Tongxin Chen: 0000-0001-6333-7856 Ren-Jie Chang: 0000-0001-8215-9469 Jamie H. Warner: 0000-0002-1271-2019 F


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Y.S. and T.C. contributed equally to this work. Y.S., T.C., and J.H.W. conceived the idea for the project and designed the experiments. Y.S. and T.C. fabricated devices and performed the transport and EL measurements. Y.S. analyzed the experimental data and wrote the manuscript. Y.S. synthesized monolayer graphene and multilayer BN films. Y.S. and Y.L. synthesized monolayer WS2. Y.S. performed Raman and PL measurements. Y.S., T.C., Y.L., R.C., and S.S. characterized the devices under OM, SEM, and TEM. J.H.W. supervised the project. All authors discussed the results and commented on the manuscript. Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS J.H.W. acknowledges the support from the Royal Society and the ERC Consolidator grant (725258 CoG 2016 LATO). We thank Xinya Bian, Junjie Liu, Yutian Wen, Qianyang Zhang, and Linlin Hou for the help with the device fabrication and the useful discussions. REFERENCES (1) Geim, A. K.; Novoselov, K. S. The Rise of Graphene. Nat. Mater. 2007, 6, 183−191. (2) Mak, K. F.; Lee, C.; Hone, J.; Shan, J.; Heinz, T. F. Atomically Thin MoS2A New Direct-Gap Semiconductor. Phys. Rev. Lett. 2010, 105, 136805. (3) Gutiérrez, H. R.; Perea-López, N.; Elías, A. L.; Berkdemir, A.; Wang, B.; Lv, R.; López-Urías, F.; Crespi, V. H.; Terrones, H.; Terrones, M. Extraordinary Room-Temperature Photoluminescence in Triangular WS2 Monolayers. Nano Lett. 2013, 13, 3447−3454. (4) Ci, L.; Song, L.; Jin, C.; Jariwala, D.; Wu, D.; Li, Y.; Srivastava, A.; Wang, Z. F.; Storr, K.; Balicas, L.; Liu, F.; Ajayan, P. M. Atomic Layers of Hybridized Boron Nitride and Graphene Domains. Nat. Mater. 2010, 9, 430−435. (5) Dean, C. R.; Young, A. F.; Meric, I.; Lee, C.; Wang, L.; Sorgenfrei, S.; Watanabe, K.; Taniguchi, T.; Kim, P.; Shepard, K. L.; Hone, J. Boron Nitride Substrates for High-Quality Graphene Electronics. Nat. Nanotechnol. 2010, 5, 722−726. (6) Yi, Y.; Yu, X. F.; Zhou, W.; Wang, J.; Chu, P. K. TwoDimensional Black Phosphorus: Synthesis, Modification, Properties, and Applications. Mater. Sci. Eng., R 2017, 120, 1−33. (7) Chen, P.; Li, N.; Chen, X.; Ong, W. J.; Zhao, X. The Rising Star of 2D Black Phosphorus beyond Graphene: Synthesis, Properties and Electronic Applications. 2D Mater. 2018, 5, 014002. (8) Loan, P. T. K.; Zhang, W.; Lin, C. Te; Wei, K. H.; Li, L. J.; Chen, C. H. Graphene/MoS2 Heterostructures for Ultrasensitive Detection of DNA Hybridisation. Adv. Mater. 2014, 26, 4838−4844. (9) Lopez-Sanchez, O.; Lembke, D.; Kayci, M.; Radenovic, A.; Kis, A. Ultrasensitive Photodetectors Based on Monolayer MoS2. Nat. Nanotechnol. 2013, 8, 497−501. (10) Lopez-Sanchez, O.; Alarcon Llado, E.; Koman, V.; Fontcuberta I Morral, A.; Radenovic, A.; Kis, A. Light Generation and Harvesting in a van der Waals Heterostructure. ACS Nano 2014, 8, 3042−3048. (11) Bernardi, M.; Palummo, M.; Grossman, J. C. Extraordinary Sunlight Absorption and One Nanometer Thick Photovoltaics Using Two-Dimensional Monolayer Materials. Nano Lett. 2013, 13, 3664− 3670. (12) Mao, D.; Wang, Y.; Ma, C.; Han, L.; Jiang, B.; Gan, X.; Hua, S.; Zhang, W.; Mei, T.; Zhao, J. WS2 mode-Locked Ultrafast Fiber Laser. Sci. Rep. 2015, 5, 7965. (13) Eda, G.; Maier, S. A. Two-Dimensional Crystals: Managing Light for Optoelectronics. ACS Nano 2013, 7, 5660−5665. (14) Wang, Q. H.; Kalantar-Zadeh, K.; Kis, A.; Coleman, J. N.; Strano, M. S. Electronics and Optoelectronics of Two-Dimensional G


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High-Performance WS2 Monolayer Light-Emitting Tunneling Devices

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