Scalable van der Waals Heterojunctions for High-Performance

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Scalable van der Waals Heterojunctions for High-Performance Photodetectors Chao-Hui Yeh, Zheng-Yong Liang, Yung-Chang Lin, Tien-Lin Wu, Ta Fan, Yu-Cheng Chu, Chun-Hao Ma, Yu-Chen Liu, Ying-Hao Chu, Kazutomo Suenaga, and Po-Wen Chiu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b10892 • Publication Date (Web): 25 Sep 2017 Downloaded from http://pubs.acs.org on September 26, 2017

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ACS Applied Materials & Interfaces

Scalable van der Waals Heterojunctions for High-Performance Photodetectors

Chao-Hui Yeh,1 Zheng-Yong Liang,1 Yung-Chang Lin,2 Tien-Lin Wu,1 Ta Fan,1 Yu-Cheng Chu,1 Chun-Hao Ma,1,3 Yu-Chen Liu,1 Ying-Hao Chu,3 Kazutomo Suenaga,2 Po-Wen Chiu1,4*

1

Department of Electrical Engineering, National Tsing Hua University, Hsinchu 30013, Taiwan

2

National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba 305-8565, Japan

3

Department of Materials Science and Engineering, National Chiao Tung University, Hsinchu 30010, Taiwan

4

Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei, 10617, Taiwan

*

Correspondence to: [email protected]

Abstract Atomically thin two-dimensional (2D) materials have attracted increasing attention for optoelectronic applications in view of their compact, ultrathin, flexible, and superior photosensing characteristics. Yet scalable growth of 2D heterostructures and fabrication of integrable optoelectronic devices remain unaddressed. Here, we show a scalable formation of 2D stacks and fabrication of phototransistor arrays with each photosensing element made of graphene-WS2 vertical heterojunction and individually addressable by a local top gate. The constituent layers in the heterojunction are grown using chemical vapor deposition in combination with sulfurization, providing a clean junction interface and processing scalability. The aluminum top gate possesses a self-limiting oxide around the gate structure, allowing for a self-aligned deposition of drain/source contacts to reduce the access (ungated) channel regions and to boost device performance. The generated photocurrent, inherently restricted by the limited optical absorption cross section of 2D materials, can be enhanced by two orders of magnitude by top gating. The resulting photoresponsivity can reach 4.0 A/W under illumination power density of 0.5 mW/cm2 and the dark current can be minimized to few picoamp, yielding a low noiseequivalent power of 2.5 × 10 W ∙ Hz /. Tailoring 2D heterostacks as well as device architecture moves the applications of 2D-based optoelectronic devices one big step forward.

Keywords: WS2, graphene, two dimensional materials, heterostructure, photoresponsivity

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INTRODUCTION Two-dimensional (2D) layered materials have been a subject of intensive research for more than a decade due to their wide range of fascinating properties. Of particular interest are their applications in nanoelectronics1-4 and optoelectronics5-7 considering their unique features that are absent in conventional semiconductor analogues such as the excellent mechanical flexibility and optical transparency. Versatile van der Waals stacks of 2D materials have delivered enormous opportunities to allow electronic functionalities by design.8 Examples are recently reported vertical field-effect transistors (FETs),9 barristors,10 and photovoltaic cells.11,

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The prototypical vertical FETs made of

transferred graphene-MoS2 heterojunction showed a remarkable on-current density, 2−5 orders of magnitude larger than those of conventional.9 A gate-controllable photocurrent with a responsivity of 0.22 AW-1 and an external quantum efficiency of 55% have also been realized using the same structure, rendering the layered 2D heterojunctions an exciting material system for electronics and optoelectronics with novel functionalities or better performance. To date, vertical 2D heterostructures are constructed via a sequential exfoliation and transfer. This technique gains an advantage over its ease of handling and provides 2D flakes with high quality, but is limited in its scalability and control over thickness. Routes which allow up-scaling and produce 2D heterostacks with desired functionalities are much needed. Of the various routes available for producing 2D materials, chemical vapor deposition (CVD) stands out as most useful for electronics applications due to its scalability, reliability, and integrability with silicon technology.2,13 Post sulfurization or selenization of transition metal oxides, on the other hand, takes the advantage of sitecontrollable

manufacturing.14,15

Recently,

in-plane

and

vertical

MoS2-WS2

heterojunctions have been grown using CVD and post sulfurization, respectively. However,

non-transfer

stacking

of

graphene

with

different

transition

metal

dichalcogenides (TMDs) remain unaddressed. Here, we bridge this gap and show a scalable fabrication of Graphene-WS2 (Gr-WS2) heterojunction arrays for highperformance photodetectors. The graphene layers are grown directly on transparent substrates by plasma-enhanced CVD, followed by patterned stacking of WS2 atop using

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post sulfurization, with thickness controlled by the pre-deposited metal oxide WOx. Through the modulation by an aluminum top gate, the generated photocurrent can be enhanced by more than two orders of magnitude while shining monochromatic light. The gated Gr-WS2 junctions can deliver a photoresponsibility as high as 4.0 AW-1, 2−4 orders of magnitude larger than that of devices solely comprising of graphene or WS2.16 More importantly, the dark current of the Gr-WS2 junction, which determines the signalto-noise ratio and is hard to be eliminated by signal processing through an external circuit, can be suppressed to a few picoamp, giving a low noise-equivalent power of 2.5 × 10 W ∙ Hz / . Moreover, we also transferred the Gr-WS2 junction arrays to a flexible substrate and fabricated top-gated Gr-WS2 phototransistors. Photodetection under bending was explored and discussed.

RESULTS AND DISCUSSION A horizontal 3-inch tube furnace equipped with a home-designed remote plasma system and a multi-coiled radio-frequency generator was used to grow graphene on sapphire and to chalcogenize transition metal oxides deposited by thermal evaporation. The chalcogenization takes place with the following two sequential reactions: MO + 2H ∗ → MO + H O MO +

(7 − ) ∗ (3 − ) X → MX + XO 2 2

where H ∗ and X ∗ are hydrogen and chalcogen radicals, respectively. Here, we show the reactions with M = Mo, W and X = S, Se. It has been known that pure metal M and stoichiometric oxide MO are hard to react directly with chalcogens.17 Here, hydrogen radicals serve as a reducing agent and are used to facilitate the conversion of MO to an intermediate MO that can react much more effectively with chalcogens. It should be noted that a higher level of hydrogen content will cause an excessive reduction of MO to M , making the chalcogenization reaction incomplete. As a result, the amount of

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hydrogen used in the reaction determines the layer quality, the number of nucleation sites and the grain size accordingly. We characterize the resulting graphene and TMD films using Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), and scanning transmission electron microscopy (STEM). Figure 1a-1d show the Raman properties of the WS2, MoS2, WSe2, and MoSe2 under a laser excitation wavelength of 532 nm. For WS2 and MoS2, the most prominent Raman peaks, corresponding to an in-plane ( ) and an out-of-plane (! ) optical modes, can be explicitly identified. Note that the frequency difference ∆ω between the and ! modes depends sensitively upon the number of layers due to the stronger dielectric screening of long-range Coulomb interactions between the effective charges in thicker samples.18, 19 The measured ∆ω are 63.5 cm-1 and 23.7 cm-1 for WS2 and MoS2, respectively. These frequency differences correspond to a thickness of 2−3 layers, in good agreement with the number of layers measured using AFM and STEM (Figure S1 and S2, Supporting Information). For WSe2 and MoSe2, on the other hand, they are featured in a single prominent vibrational mode, appearing at 250.5 cm-1 and 241.7 cm-1 for WSe2 and MoSe2 ! , respectively. The $2 mode, which is found at 307 cm-1 for WSe2, indicates a multilayer structure.20 What are also shown are the spatially resolved Raman maps of the most intense mode for WS2, MoS2, WSe2, and MoSe2. The areas, where the Raman responses, match well with the defined structures and are uniform in intensity at a fixed peak position. Figure 1e-1h show the atomic structure of the TMD films. The contrast of the annular dark field images varies as a power of atomic number ~ Z1.7, sensitively allowing us to differentiate the transition metal with the chalcogen atom and to assess the crystalline quality according to the defect density of chalcogens (Figure S2, Supporting Information). The selected area electron diffraction patterns (insets to Figure 1e-1h), which were taken with a 1-µm aperture in diameter, show diffraction spots arranged in multiple hexagons, indicating the polycrystalline structure with grain size smaller than 1 µm. Other characterizations of the TMD films including the chemical composition using XPS, topology using AFM, and layer-dependent photoluminescence can be found in Figure S2, S3 and S4 (Supporting Information). 4


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As learned from bulk semiconductors, the performance of electronic devices is intimately related to functional interface or heterostructures within the devices. Exquisite control over the composition and perfection of interfaces is required for the successful fabrication of high-performance planar devices. As such, we use the non-transfer method to make 2D heterojunctions, with focus on Gr-WS2 stacks in the current study. Figure 2 dipects the fabrication process flow of individually addressable phototransistor arrays based on Gr-WS2 junction. In brief, a few-layer graphene film was first grown on a transparent sapphire wafer using a remote-catalyzed CVD technique,21 followed by patterning into strips with 250 µm in width and 1000 µm in length using reactive oxygen plasma etching (Figure 2a). The graphene surface was cleaned prior to the deposition of metal oxide using the method described in our previous study.22 Tungsten oxide strips deposited perpendicular to the graphene strips was then defined by a hard mask (Figure S5, Supporting Information). Sulfurization was carried out in a plasma-enhanced CVD system at 700 ℃ for 20 min (Figure 2b and 2c). The Gr-WS2 junction is formed at the cross of the two strips, each side of which is contacted with Cr(1nm)/Au(50nm) as an electrical contact. In the last step, an Al-AlOx top gate was made atop the junction, followed by a self-aligned deposition of drain/source contacts (Figure 2d and Figure S6, Supporting Information).23 Figure 3a and 3b show the typical high-resolution TEM images of Gr-WS2 junction in the cross section. To ensure the continuity of the film, the averaged thickness of graphene and WS2 was grown to be 2−3 layers. A proper increase of the thickness gains the advantage of high light absorption, generating a higher photocurrent. However, growing multilayer graphene using plasma-enhanced CVD on dielectric substrates is not straightforward. It usually leads to a vertical outgrowth of graphene nanowalls or nanopedals due to the defective growth mechanism and fast deposition rate.24 Likewise, sulfurization of a metal oxide often occurs on the top surface and makes the reaction in the bottom incomplete. Figure 3a and 3b shows a thin and thick Gr-WS2 junctions, respectively. It can be seen that each constituent layer is well spaced: ~3.4 Å for graphene layers and ~ 6.3 Å for WS2. More importantly, the interface between graphene and WS2 is shape and clean. This characteristic is fundamentally critical in the performance of optoelectronic devices thus constructed. 5



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Figure 3c-3d shows the Raman spectra and spatial mapping of the Gr-WS2 junction. The characteristic Raman peaks of graphene and WS2 in the non-cross region are also taken for comparison. The G peak of graphene appears at 1598 cm-1 in the non-cross region, with a narrow full width at half maximum of 28 cm-1, indicative of decent crystallinity. For WS2, the characteristic and ! modes peak at 353.5 and 417 cm-1 in the non-cross region, respectively. No remarkable peak shift is found for both graphene and WS2 in the crossed junction region, in accord with other reported vertical TMD-TMD or Gr-TMD heterojunctions.25,

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Only a slight ! upshift of ~1 cm-1 is

noticed, presumably due to a weak charge transfer or strain induced by the underlying graphene layers.27-29 Similar ! upshift is also found in Gr-MoS2 junctions (Figure S7, Supporting Information). The spatially resolved Raman maps depict the uniformity of the graphene and WS2 strips as well as the Gr-WS2 junction, as shown in Figure 3d. WS2 has received considerable attention for optoelectronic applications due to its semiconducting properties with a direct bandgap in the visible spectrum,30 large exciton binding energy, large absorption coefficient,31 and high external quantum efficiency.32 Figure 4a shows a perspective view schematic and an optical microscopy top-view image of our photodetector comprising of perpendicular graphene and WS2 strips. The crossed junction area is 250 μm (Gr) × 80 μ m (WS2) in dimension, with WS2 lying above graphene and in intimate contact with an Al/AlOx top gate. The thickness of the graphene and WS2 strips was determined by TEM to be ~ 3 nm and ~ 4 nm, respectively. To measure the photoconductivity, the device was illuminated with a focused laser beam from the backside of the transparent substrate. Figure 4b shows the &'( − )'( curves for the WS2 strip under dark conditions at different gate voltages. The current increases with increasing the positive )*( , indicating the n-type nature of WS2. The low current density is attributed partly to an appreciable Schottky barrier formed at the metal-WS2 contacts and partly to a parasitic resistance in the access (ungated) regions. For charge transport going through the Gr-WS2 junction, the &'( − )'( curves become asymmetric (Figure 4c). The rectification stems from the two asymmetric Schottky barriers at the Au-Gr-WS2 and Au-WS2 junctions and allows current to pass through the Gr-WS2 junction only when

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WS2 is negatively biased. Here, graphene plays a critical role in Fermi level unpinning which yields a low barrier at the Au-Gr-WS2 junction.33 As the device is illuminated with a focused laser beam (wavelength: 514 nm and power: 50 μW), the current density going across the Gr-WS2 junction is enhanced by a factor of 150 in comparison to that going along the WS2 strip at )*( = 1 V. This remarkable enhancement is also seen in the dependence of photocurrent generation upon the application of a top gate voltage, shown in Figure 4d. In the dark state, the Gr-WS2 junction functions as an n-type vertical FET, i.e., the channel only conducts at )*( > 0. When we illuminated the device biased at )'( = −1 V, the channel current remains blocked at )*( < 0 but remarkably increased at )*( > 0. The room-temperature ON/OFF ratio of the Gr-WS2 vertical FET reaches 104, higher than the similar device made of GrMoS2.30 This result indicates that massive photo-excided carriers can be generated in the atomic thin WS2 layers and effectively modulated by an external electric field. Figure 4d inset shows the time-resolved photoresponse of the device. Under repeated ON-OFF cycles of light illumination, the time-resolved photoresponse of the Gr-WS2 junction at a fixed bias of )'( = −1 V and )*( = 1 V yields an alternating high and low impedance states, with an ON/OFF ratio (defined as &,- ⁄&./01 ) of ~ 50, rendering the device a high quality photosensitive switch. To determine the sensitivity of our phototransistor, we measure the noise in the dark current. Given that shot noise is the dominant source of the total noise in our device (higher than the thermal noise and 1/f noise), the dark current shot noise is written as 324&./01 ~1 × 105 A ∙ Hz / for Idark = 3 pA, where e stands for electronic charge. The noise-equivalent power, which is a measure of sensitivity of a photodetector and equal to the shot noise divided by the responsivity, is obtained as ~2.5 × 10 W ∙ Hz / , two orders of magnitude smaller than that of commercial silicon avalanche photodiode based on p-n junctions.34 For our Gr-WS2 junction, it increases at )*( > 0. The observed gate modulation of photocurrent in our devices can be understood from the junction band diagram (Figure 4e). Because the vertical channel length of WS2 (3−5 nm) is much shorter than the

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depletion length of the Gr-WS2 junction, the band bending at the Gr-WS2 interface dominates the band slope of the entire WS2 layers. As such, application of an external electric field provides an effective means to modulate this monotonic band slope, capable of controlling over the separation and transport of photocarriers in vertically stacked layer heterojunctions. At )*( > 0, the band bends downwards at the WS2-gate interface, driving the photo-excited holes to the graphene side and the electrons to WS2. Under a forward bias, the current increases exponentially due to the lowering of both contact and Gr-WS2 interjunction barriers. On the other hand, a negative )*( causes an upward bending of the WS2 band, depleting both graphene and WS2 as photocarriers are generated. In this case, the photocurrent is suppressed. Photoresponsivity, external quantum efficiency, and photogain are important figures of merit in evaluating the performance of photodetection. Photoresponsivity is a measure of a device’s electrical response to an incident light and defined as 8 =

&,: , where &,9

and P stand for measured photocurrent and illumination power of laser, respectively. Figure 4f depicts the photocurrent and photoresponsivity as a function of incident power at a bias of )'( = −1 V and )*( = 1 V. Under this condition, the transistor is tuned to the ON state and the photocurrent monotonically increases with the increase of incident power. On the contrary, the photoresponsivity monotonically decreases with the increase of incident power from 4 A/W at 9 = 0.15 mW/cm to 0.3 A/W at 9 = 60 mW/cm . This behavior is consistent with other photodetectors made of solely TMDs and attributed to the shortening of exciton recommendation time caused by the increasing number of electron-hole pairs and the influence of trap states between the gate oxide and semiconductor.31 The external quantum efficiency and photogain of our Gr-WS2 photodetectors are given in Figure S8 (Supporting Information). In addition to the operation on rigid substrates, we also transferred the Gr-WS2 junctions onto polyethylene terephthalate (PET). The same top gate structure was fabricated after the transfer, forming bendable Gr-WS2 phototransistor arrays. Figure 5a5c show the optical images of the Gr-WS2 junctions on PET, with an individually addressable top gate for each junction. In general, the transistor behaviors of the devices

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on PET are qualitatively similar with those on rigid substrates, except that the photocurrent on PET is lowered by a factor of ~7. The degraded performance in photoresponse might be attributed to the rough surface and the trapped water layer between the substrate and graphene. Figure 5d shows the time-resolved photocurrent for Gr-WS2 junctions on PET under alternating ON/OFF light illuminations at )'( = −1V in an ambient environment. A rise and decay response time of 800 ms and 3 s are measured, respectively. No clear persistent photocurrent is seen in the repeated ON/OFF switch of light illumination. Nevertheless, the Gr-WS2 photodetectors on PET takes longer time to saturate and decay in comparison to the same structure lying on sapphire. Figure 5e shows the ON-state photocurrent as a function of bending cycles. A bending cycle is referred to as the completion of an ON/OFF light illumination under a bending radius of 10 mm and the bias conditions of )'( = −1 V and )*( = 1 V. The Gr-WS2 junction maintains its structural integrity and exhibits stable photocurrent within the tested cycles, indicative of its decent flexibility and reliable durability for the applications in wearable optoelectronic devices.

CONCLUSION In summary, we show all-CVD growth of vertical Gr-WS2 heterojunctions. An individually addressable Al/AlOx top gate was fabricated on top of the Gr-WS2 junction, forming phototransistors on either sapphire or PET substrates. The Al/AlOx gate architecture allows for a self-aligned deposition of drain/source contacts, reducing the access regions and lowering the parasitic resistance accordingly. The resulting Gr-WS2 photodetectors exhibit gate-tunable high photocurrent. The photoresponsivity reaches 4.0 A/W. The low dark current yields an equivalent noise power as low as 2.5 × 10 W ∙ Hz / . Further, the self-passivated interface oxidation layer has shown its potential to act as a robust gate insulator on our Gr-WS2 photodetectors and is also anticipated to extend its unique features to other 2D materials for widespread applications35,36. An ongoing research is to optimize the number of constituent layers in the Gr-TMD junctions and to explore other TMD-TMD junctions for superior performance of photodetection.

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The better control over the thickness, clean interjunction interface, scalability, reliability, and integrability with silicon technology make the 2D heterojunctions an ideal candidate for future optoelectronic applications. EXPERIMENTAL SECTION Growth of Vertical Heterojunctions Few-layer graphene was grown using our previously reported method involving direct formation of graphene on oxide with remote catalyzation. The graphene film was then patterned as 250×1000 µm2 stripes. The Raman characterization and the van der Pauw measurements of our as-grown graphene film are given in Figure S9 (Supporting Information). For TMDs, a patterned transition metal oxide seed layer was deposited by thermal evaporation using tungsten(VI) oxide with 99.995% in purity or molybdenum (VI) oxide with 99.97% in purity. Subsequently, sulfurization or selenization was carried out in a 3-inch quartz tube placed in a dual-walled tube furnace equipped with RF plasma. The samples were placed at the center of the tube furnace, and the chalcogen precursor (sulfur or selenium powders with 99.998% in purity) were located at the upstream of the gas flow, set up with an independent heating source. The samples and the chalcogen powders were heated up to 700–800 ℃ and 140 ℃, respectively, depending on the kind of transition metal. Vaporized chalcogen precursor was carried by argon (200 sccm) and hydrogen (5–40 sccm) through a plasma region. The chalcogenization reaction takes ~20 min. The samples were rapidly cooled down to room temperature as the reaction was completed. Device Fabrication Drain/source metal contacts, Cr(1nm)/Au(50nm), were first deposited on each side of the graphene and WS2 strips. Then, standard photolithography was applied to define a topgate window (500×250 µm2) covering the crossed Gr-WS2 junction area. A thin Al film with thickness of 7 nm was deposited, followed by an oxidation process in a sealed chamber filled with high-purity oxygen (>99.999%) at a pressure of 3 kgf/cm2 overnight. Then, the Al top gate (100 nm) was deposited and staked on the surface oxidation layer (AlOx). Afterward, a self-aligned deposition of an 8–nm–thick gold film was applied to reduce access resistance of the device. Optoelectronic Measurements The Gr-WS2 junction photodetectors were made on sapphire which is glued on a glass substrate with a hole of 5 mm in diameter for laser illumination. A solid-state laser was used to provide a light source with wavelength of 532 nm. The output power is controllable in a range from 100 µW to 80 mW and coupled with a chopper to define the

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switching ON/OFF intervals. The output power was calibrated with a hand-held continuous wave (CW) laser power meter. For & − ) measurements, the drain and gate biases were provided through Keithley 2400 source meters, and the generated photocurrent was detected via a Keithley 2000 multimeter connected to a current preamplifier.

ASSOCIATED CONTENT Supporting Information Supporting information is available free of charge on the ACS Publication website at DOI: AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] ORCID Po-Wen Chiu: 0000-0003-4909-0310 Chao-Hui Yeh: 0000-0002-9437-055X Author Contributions C.H.Y. and Z.Y.L. contributed equally to the work, and both contributors are considered first authors. Notes The authors declare no competing financial interests.

ACKNOWLEDGMENTS We are thankful to the Best Champion Technology Co., Ltd. for technical support. C.H.Y. acknowledges Mr. Chiu-Chuan Liao for valuable discussion. P.-W.C appreciates the project support of Taiwan Ministry of Science and Technology: MOST 103-2628-M-007004-MY3; and MOST 103-2119-M-007-008-MY3.

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Supporting Information. Brief statement on additional AFM, XPS, Raman, PL, HRTEM, and STEM characterizations of as-grown TMD films, electrical properties of as-grown graphene, contact improvement via access resistance reduction, information of homemade hard mask kit as well as chalcogenization procedure, and the external quantum efficiency and photocurrent gain of Gr-WS2 phototransistors.

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Figure 1 Raman and STEM spectroscopy characterizations of synthesized TMDs. (a-d) Raman spectra and characteristic maps acquired for four different TMDs: WS2, MoS2, WSe2, and MoSe2. Scale bars: 50 µm. (e-h) STEM images taken from the four materials shown in (a-d). Scale bars: 2nm. Inset: corresponding selective-area electron diffraction pattern.

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Figure 2 Cartoon illustration of fabrication process of phototransistor based on the vertical Gr-TMD junction. (a) Direct CVD growth of graphene and strip patterning; (bc) Patterned deposition of transition metal oxide and chalcogenization. (d) Formation of Al/AlOx top-gate electrode and self-alignment metals.

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Figure 3 Raman and TEM spectroscopy characterizations of Gr-WS2 vertical junction. (a-b) High-resolution cross-sectional TEM images of Gr-WS2 junction. (c) Raman spectra of the multilayer WS2 (taken on the red spot of the optical image in the inset) and crossed junction area (taken on the green spot of the optical image in the inset). Scale bar: 100 µm. (d) Spatial Raman mapping of the G-band (1598 cm-1) and mode (351 cm-1) at the Gr-WS2 junction area. Scale bar: 40 µm.

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Figure 4 Device geometry, energy band diagram, and electrical properties of GrWS2 photodetectors on a rigid substrate. (a) Optical image of Gr-WS2 junction photodetectors on sapphire, with a close-up of the junction area at the lower panel and a schematic of light illumination through the device at the right panel. (b) and (c) are the output characteristics of the device. The inset shows the direction of current flow through the Gr-WS2 junction. (d) Gating response ( &'( − )*( ) of the Gr-WS2 junction photodetector in dark and illuminated states, acquired at a bias voltage of )'( = −1 V. Illumination power is 30 mW/cm2. Inset shows the time-resolved photoresponse of the device, recorded at a fixed bias of )'( = −1 V and )*( = 1 V. The modulation frequency of the chopper is 0.1 Hz. (e) Band diagram of the top-gated Gr-WS2 junction photodetector. An electrostatic gating changes the band slope of WS2, determining the electron density in the n-type WS2 as illuminated with light. (f) Photoresponsivity and photocurrent as a function of incident laser power.

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Figure 5 Optical image and photoresponse of Gr-WS2 photodetectors on a flexible PET substrate. (a-c) Optical images of Gr-WS2 photodetectors on PET. Scale bar: 50 µm. (d) Time-resolved photoresponse of the device recorded at a fixed bias of )'( = −1 V and )*( = 1 V. Photocurrent at an illumination power of 1 mW/cm2 and 30 mW/cm2 was acquired. (e) Photocurrent as a function of bending cycle at an illumination power of 30 mW/cm2.

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WS2 Graphene 5 nm

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Scalable van der Waals Heterojunctions for High-Performance

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