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Highly Effi cient Photocurrent Generation from Nano-crystalline Graphene–Molybdenum Disulfi de Lateral Interfaces

Kang Hyuck Lee , Tae-Ho Kim , Hyun-Jin Shin , and Sang-Woo Kim*

Dr. K. H. Lee, T.-H. Kim, Prof. S.-W. Kim School of Advanced Materials Science and Engineering Sungkyunkwan University (SKKU) Suwon 440-746 , Republic of Korea E-mail: kimsw1@skku.edu Dr. H.-J. Shin Samsung Advanced Institute of Technology (SAIT) Yongin 446-712 , Republic of Korea Prof. S.-W. Kim SKKU Advanced Institute of Nanotechnology (SAINT) Center for Human Interface Nanotechnology (HINT) SKKU-SAMSUNG Graphene Center, Sungkyunkwan University (SKKU) Suwon 440-746 , Republic of Korea

DOI: 10.1002/adma.201504865

500 times larger than that from a graphene–MoS 2 vertical inter-face heterostructure.

The G nc –MoS 2 lateral-interface heterostructure thin fi lm was synthesized through the sulfurization of a MoO x thin fi lm on a G nc layer, which has many steps and edges. In a previous study, we found that the G nc layer was formed by the 4–7 nm size of the nanocrystalline graphene fl akes. [ 36 ] Figure 1 a shows that the G nc has a rough surface with sub-micrometer islands. Even with the 15 nm thick MoO x thin-fi lm deposition, the mor-phology still shows a rough surface morphology, as shown in Figure 1 b. However, the G nc /MoO x islands-structured rough surface transformed to a fl at surface during sulfurization (Figure 1 c) because the lateral volume of the MoO x was expanded and the gap spaces were fi lled when the MoO x was transformed to MoS 2 . [ 37 ] A Raman spectrum shows the E 2g (382 cm −1 ) mode and the A 1g (406 cm −1 ) mode of the MoS 2 and the G band (1583 cm −1 ), D band (1344 cm −1 ), and 2D band (2686 cm −1 ) of G nc (Figure 1 d). To investigate the structural properties of the G nc –MoS 2 lateral-interface thin fi lm, cross-sectional high-res-olution transmission electron microscopy (HRTEM) measure-ments were carried out. From the measurements, it was found that the thin fi lm is composed of long-range-ordered crystalline (more than 10 nm size) MoS 2 layers and short-range-ordered G nc fl akes (2–4 nm size) with a very rough surface morphology (Figure 1 e). The element maps obtained by energy-fi ltered TEM (EFTEM) indicate the existence of a distinct MoS 2 (Mo and S atoms) region and a distinct carbon (C) atoms region (see Figure S1, Supporting Information). The element maps further confi rm that crystalline MoS 2 was grown on the short-range-ordered carbon-stacked structure with a large number of lateral interconnections. The X-ray photoelectron spectroscopy spectra also show components of the G nc –MoS 2 lateral-interface heterostructure thin fi lm. These binding energies are all con-sistent with the reported values for a MoS 2 crystal and graphitic carbon (Figure S2, Supporting Information).

To investigate the current–voltage ( I–V ) properties of the G nc –MoS 2 lateral interfaces, we fabricated a vertically stacked monolayer graphene–MoS 2 device (inset of Figure 2 a) and a G nc –MoS 2 lateral-interface-structured device (inset of Figure 2 b). The I–V curve of the vertically stacked graphene–MoS 2 device is almost linear, while the I–V curve of the G nc –MoS 2 lateral-inter-face heterostructure thin-fi lm device clearly shows current-rec-tifi cation behavior, as expected for a graphene–metal interface. The series resistance ( R s ) of the G nc –MoS 2 device is 2.9 × 10 5 Ω. An increase in current at high reverse bias indicates a 3.2 × 10 7 Ω shunt resistance ( R h ) across the junction. This large R h indicates the increased depletion of interfaces. Actually, our CVD MoS 2 thin fi lm shows n-type semiconductor behavior (Figure S3, Supporting Information). Therefore, to make a Schottky junction in the G nc –MoS 2 interface, the work function of the

In 2D materials, it is found that heterointerface interactions infl uence their electrical and optical properties more strongly than a covalently bonded bulk heterostructure. [ 1–10 ] Recently, artifi cial van der Waals (VdW) heterostructures, which are vertical stacks of different 2D materials such as graphene–hexagonal boron nitride, [ 11–14 ] graphene–transition-metal dichalcogenides (TMDs), [ 15,16 ] and TMD–TMD, [ 17,18 ] held together by weak intermolecular VdW forces, have attracted strong attention. [ 19 ] Although artifi cial VdW heterostructures have already been realized using the mechanical exfoliation method, the low throughput of mechanical exfoliation and the lack of scalability make this technique nonideal, in particular for industrial adaptation. Thus, direct synthesis of large-scale vertical 2D heterostructures using chemical vapor deposition (CVD) is regarded as a promising approach. [ 20–25 ]

Interestingly, it was found that the CVD synthesis allows the growth of in-plane monolayer heterostructures, making the ultrathin interface between 2D materials. [ 20–22 ] Such an ultrathin interface formed by direct synthesis is much more sensitive to interface interaction than the artifi cial VdW heterointerface formed by exfoliation. A band structure of 2D materials can be changed by vertically stack ordering, [ 26,27 ] and isotropic exciton dynamics [ 28 ] and different band struc-tures are observed between in-plane and out-of-plane. [ 29–35 ] However, in spite of the importance of interface control, the study of interface interactions is still lacking for large-scale directly grown 2D heterointerfaces for practical device applica-tions. Here, we demonstrate that a nanocrystalline graphene (G nc )–molybdenum disulfi de (MoS 2 ) lateral-interface hetero-structure, realized using CVD, can reveal distinct current-recti-fi ed characteristics similar to a p–n diode. The lateral interfaces can create an internal electric fi eld between graphene and MoS 2 because metallic MoS 2 edges induce charge reordering and a potential shift in graphene. We found that the photocurrent generated from a G nc –MoS 2 lateral-interface heterostructure is

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G nc should be much higher than the work function of the MoS 2 thin fi lm. However, the experimentally measured (Figure S4 and Table S1, Supporting Information) work function of G nc was lower than that of the MoS 2 thin fi lm; nevertheless, current rectifi cation occurred at the G nc –MoS 2 interface. This means that the unexpected phenomenon affects the electronic struc-ture of the G nc –MoS 2 lateral interface.

Figure 2 c shows a side-view schematic illustration of the G nc –MoS 2 system. The MoS 2 edges generate a potential gradient in the graphene due to the metallic edge effect of the MoS 2 . [ 29–35 ] Similar to the case of graphene with metal con-tacts (in which the metal contacts generally show a difference in the work function at the interface between the graphene and metal/graphene regions), [ 9,10,30 ] an electric fi eld develops, resulting in charge separation, i.e., a p–n junction. Due to this edge effect of MoS 2 , the charge separation is generated at every lateral interconnection of interfaces between G nc and MoS 2 as shown in Figure 2 c. Namely, the local charge-separated regions act as depletion regions and fi nally the G nc –MoS 2 device shows rectifi cation behavior.

To clarify the interface effect between the graphene and the MoS 2 edges, we fabricated MoS 2 -line-patterned monolayer gra-phene devices as shown in Figure 3 a, and measured the I–V curves. The line-pattered monolayer graphene has line widths of 100, 50, and 20 µm, and the width of the gap between each

line is the same as the line width. We found that the I–V curves of the MoS 2 -line-patterned monolayer graphene devices clearly show an unsymmetrical shape according to the decrease of the pattern width as shown in Figure 3 b. This means that the increase of the graphene edge density leads to a distinct expan-sion of charge depletion region due to the potential gradient generated at the MoS 2 edge. However, the edge effect between the MoS 2 and the patterned graphene is not signifi cant because the minimum width of the line-patterned graphene is on the sub-micrometer scale. Moreover, the range of potential gra-dients caused by the MoS 2 edge effect is a few nanometers in scale. [ 29,30 ] This means that the depletion region between the patterned graphene and the MoS 2 cannot be fully cov-ered by the entire interface. On the other hand, G nc consists of graphene fl akes of 3–4 nm in size, as shown in Figure 1 e, so it can be covered by the entire interface. Figure 3 c shows the R h of line-patterned graphene–MoS 2 devices and the G nc –MoS 2 device. The R h of the G nc –MoS 2 device was dramatically increased compared with the graphene–MoS 2 devices.

To further examine the optoelectronic properties of the G nc –MoS 2 lateral-interface heterostructure, we measured the response of phototransistors. Figure 4 a,b presents the photocur-rent as a function of time under solar-simulated air-mass (AM) 1.5 global (G) illuminations (light intensity is 100 mW cm −2 ) under 1 V bias condition. Under illumination, the current rises

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Figure 1. a) AFM images of G nc on SiO 2 /Si (a), MoO x /G nc on SiO 2 /Si (b), and MoS 2 /G nc on SiO 2 /Si samples (c). The insets of (a–c) show schematics of the heterointerfaces. d) Raman spectrum of MoS 2 /G nc . The inset is a magnifi ed 2D peak band of the G nc . e) Cross-sectional HRTEM image of the MoS 2 /G nc interface.

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to a high value (on state) and then returns to a low value when the light is off (off state). The generated photocurrent of the lateral-interface G nc –MoS 2 device at on state is about 1.1 µA which is 500 times higher than the vertical stacked graphene–MoS 2 device (generated photocurrent is ≈2.1 nA). The photo-response is characterized by typical decay times of τ decay = 2.8 s and τ decay = 4.5 s for the G nc –MoS 2 device and the vertically stacked graphene–MoS 2 device, respectively (Figure S7, Sup-porting Information). This result further confi rms that the dramatic decrease of the photoresponse decay time was caused by internal electric fi eld, which is very similar behavior to the previous report on the performance enhancement of MoS 2 photodetectors by applying an external gate voltage. [ 38 ]

We fabricated a transparent fl exible photovoltaic (PV) device using the G nc –MoS 2 heterointerfaces. The G nc –MoS 2 was transferred on a poly(ethylene naphthalate) (PEN) sub-strate as shown in Figure 4 c. The photovoltaic performance of the G nc –MoS 2 heterostructure device was measured under

solar-simulated AM 1.5G illuminations. Figure 4 d presents a zoomed-in view of the I – V curve, focusing on the quadrant of the PV power generation. The short-circuit current I sc was ≈2 µA at an excitation optical power ( P opt ) of ≈90 µW (corresponding to 100 mW cm −2 ). The open-circuit voltage ( V oc ) was ≈0.2 V. This value is comparable to that of V oc of a single-gated gra-phene/MoS 2 /graphene PV device [ 13 ] (at a gate voltage = −30 V). This relatively high V oc means that the lateral interfaces of G nc –MoS 2 heterostructure have an important role to increase barrier potential, which is similar to the negative gating effect in single-gated graphene/MoS 2 /graphene PV devices. We further esti-mated the power conversion effi ciency (PCE) of 0.12% with a fi ll factor of 0.31. Our PV device has a lateral transport channel with a quite long length (≈500 µm), resulting in the relatively low PCE. [ 17 ]

In conclusion, we have demonstrated a G nc –MoS 2 lateral-inter-face heterostructure thin fi lm. The G nc –MoS 2 lateral-interface het-erostructure can show distinct current-rectifi ed characteristics that

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Figure 2. a,b) I–V properties of the monolayer graphene–MoS 2 vertical stacked junction (a), and G nc –MoS 2 lateral-interface heterostucture devices (b). c) Schematic image of Schottky barrier increase at lateral interfaces between MoS 2 edge and G nc edges.

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are rarely observed in the monolayer graphene–MoS 2 vertically stacked heterostructure. The metallic MoS 2 edges give rise to a potential gradient in graphene. Similar to the case of graphene

with metal contacts, which generally show a work-function differ-ence at the interface between the graphene and metal/graphene regions, an electric fi eld develops, resulting in charge separation.

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Figure 3. a) Schematic illustration of line-patterned graphene–MoS 2 device fabrication. b) I–V properties of the line-patterned graphene–MoS 2 lateral-interface devices with different line widths of graphene. c) The log plot of the R h values of the devices with different line widths of graphene.

Figure 4. Photoresponse behaviors of a) graphene–MoS 2 vertical stacked device and b) G nc –MoS 2 lateral-interface device. c) Real photo image of a transparent fl exible photovoltaic device based on G nc –MoS 2 heterostructure and d) its PV characteristic under AM 1.5G solar light illumination.

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This local charge-separated region acts as a depletion region; thus, the G nc –MoS 2 device shows current-rectifi cation behavior similar to a p–n diode. The photocurrent generated from the G nc –MoS 2 lateral-interface heterostructure was 500 times higher than that from a graphene–MoS 2 vertical junction. Moreover, the G nc –MoS 2 lateral-interface heterostructure shows clear photovoltaic perfor-mance. Our approach introduces the possibility of providing a novel strategy for designing 2D material-based device platforms without the need of p–n junction formation.

Experimental Section Synthesis of G nc –MoS 2 Lateral-Interface Heterostructure Thin Film : The

growth of the G nc thin fi lm was conducted under atmospheric pressure for the CVD growth. Cu foil (99.9%, 125 µm thickness, Alfa Aesar, Ward Hill, MA, USA) was placed into a quartz tube. The Cu foil was annealed at 900 °C for 60 min using 500 sccm N 2 and 500 sccm H 2 , and was then cooled to 400 °C. After cooling, a G nc layer formed on the surface of the Cu foil. The G nc thin fi lms were then transferred onto the SiO 2 /Si substrates. Various thicknesses (5–60 nm) of MoO x fi lms were deposited on the G nc thin fi lm transferred SiO 2 /Si substrate using a thermal evaporator. The MoO x /G nc on SiO 2 /Si samples were placed into a quartz tube. The samples were annealed at 900 °C for 60 min using 100 sccm H 2 S and cooled to RT. The MoS 2 was formed by sulfurization of MoO x thin fi lm. [ 39 ]

Characterizations : Atomic force microscopy (AFM) (Park systems, XE-100), Raman spectroscopy (WItec, ALPHA300R), and HRTEM were used to ensure the quality, thickness, and elemental components of the G nc and MoS 2 . The electrical properties of the fabricated devices were measured using a semiconductor parameter analyzer in vacuum and at room temperature.

Supporting Information Supporting Information is available from the Wiley Online Library or from the author.

Acknowledgements This research was fi nancially supported by the Center for Advanced Soft-Electronics as the Global Frontier Project (2013M3A6A5073177), Basic Science Research Program (2009–0083540) through the National Research Foundation (NRF) of Korea Grant funded by the Ministry of Science, ICT & Future Planning and Human Resources Program in Energy Technology of the Korea Institute of Energy Technology Evaluation and Planning (KETEP), granted fi nancial resource from the Ministry of Trade, Industry & Energy, Korea (No. 20154030200870).

Received: October 2, 2015 Revised: November 9, 2015

Published online: December 28, 2015

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