Fabrication of flexible optoelectronic devices based ...

Carbon 116 (2017) 167e173

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Fabrication of flexible optoelectronic devices based on MoS2/graphene hybrid patterns by a soft lithographic patterning method Min-A. Kang a, b, 1, Seong Jun Kim a, 1, Wooseok Song a, Sung-jin Chang c, Chong-Yun Park d, Sung Myung a, *, Jongsun Lim a, Sun Sook Lee a, Ki-Seok An a, ** a

Thin Film Materials Research Center, Korea Research Institute of Chemical Technology, Yuseong Post Office Box 107, Daejeon, 305-600, Republic of Korea Department of Energy Science, Sungkyunkwan University, Suwon, Gyeonggi-do, 440-746, Republic of Korea Department of Chemistry, Chung-Ang University, Seoul, 156-756, Republic of Korea d Department of Physics, Sungkyunkwan University, Suwon, Gyeonggi-do, 440-746, Republic of Korea b c

a r t i c l e i n f o

a b s t r a c t

Article history: Received 5 September 2016 Received in revised form 23 January 2017 Accepted 1 February 2017 Available online 2 February 2017

A cross-stacking MoS2/graphene hybrid patterns for the application to advanced flexible opto-electronic devices have been demonstrated by soft-lithographic patterning method. Well-defined MoS2/graphene hybrid pattern was fabricated simply by a soft lithographic patterning technique. In-depth exploration for the optical properties of diverse cross-stacking photodetectors based on MoS2/graphene patterns was carried out. In addition, cross-stacking MoS2/graphene was demonstrated onto a flexible polyethylene terephthalate (PET) substrate for the analysis of physical properties of devices. Substantially, this method should pave the way for realistic applications of transparent and flexible nano-electronic devices based on 2D materials. © 2017 Elsevier Ltd. All rights reserved.

Keywords: MoS2 Graphene MoS2-graphene hybrid film Soft-lithography Photodetector

1. Introduction Recently, two-dimensional (2D) materials, including graphene, hexagonal boron nitride (hBN), and transition metal dichalcogenides (TMDs) have attracted increasing attention due to their extraordinary features such as ultra-thin structures, superior bendability, and high optical transparency. Among these 2D materials, graphene is a promising building block for optoelectronic applications due to its remarkable electrical conductivity, broad absorption spectrum, and extreme bendability. However, the gapless and semi-metallic behavior of graphene have been an obstacle for its application to functional devices in spite of its high carrier mobility [1]. Further, graphene-based photodetectors also exhibit low photoresponsivity in a wide range of the visible light region because of their low optical absorption [2,3]. Among TMDs, semiconducting molybdenum disulfide (MoS2) exhibits great potential because its band gap can be tuned from indirect to direct by

* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (S. Myung), [email protected] (K.-S. An). 1 These authors contributed equally to this work. http://dx.doi.org/10.1016/j.carbon.2017.02.001 0008-6223/© 2017 Elsevier Ltd. All rights reserved.

adjusting the layer thickness [4]. In previous reports [5,6], the optoelectronic devices based on monolayer MoS2 nanosheets with direct bandgap of 1.8 eV showed high photoresponsivity. The hybridization of the MoS2/graphene heterostructure is particularly desired so as to satisfy the demand for high-performance optoelectronic devices based on 2D materials. MoS2/graphene hybrid structure by non-invasive physical attachment can preserve the high carrier mobility in graphene, and photogenerated carriers in MoS2 with high optical absorption capacity can be easily separated at the graphene/MoS2 interface. Due to this reason, MoS2/graphene-based hybrid photodetectors exhibited the excellent photoresponsivity. In addition, the remarkable flexibility of 2D materials such as graphene and MoS2 nanosheet allows us to reduce the degradation of performance by the external mechanical strain [7e12]. In this study, we demonstrated optical devices that can be fabricated based on cross-stacked MoS2-graphene hybrid patterns by using a soft-lithographic patterning technique, which can overcome the problems arising from mass fabrication. In the current study, surface energy modification of 2D materials such as graphene and MoS2 layer was employed for the fabrication of optoelectronic devices on arbitrary substrates [13e15]. The crossstacked MoS2/graphene hybrid film based on the woven structure

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was transferred on a polyethylene terephthalate (PET) substrate without deterioration of the properties of pristine 2D materials. The method provides a facile and quick route for the formation of a high-resolution cross-stacking patterned MoS2/graphene hybrid film without any defects and without an etching process. It hence paves the way for practical applications of transparent and flexible nano-electronic devices based on 2D materials [16e20].

water ¼ 1:2) was vaporized and the PDMS surface was coated with it at 270 C in order to increase the adhesion energy between the PDMS stamp and the MoS2 layer on a SiO2 substrate. The DMSOcoated PDMS stamp was brought in contact with a MoS2 layer for 90 s in order to attach the MoS2 layer to the PDMS stamp. The PDMS stamp with MoS2 patterns was then brought in contact with the surface of the target substrate at 70 C. The surface energy was restored, and MoS2 pattern was transferred to the target substrate.

2. Experimental 2.3. Preparation and transferring of TCVD-grown graphene 2.1. CVD growth of MoS2 nanosheet on p-THPP promoter layer A 5,10,15,20-tetrakis(4-hydroxyphenyl)-21H,23H-porphyrin (pTHPP) thin film as a seeding promoter was deposited on a SiO2 substrate via thermal evaporation [21]. After the deposition was completed, the conventional TCVD was carried out for the synthesis of the MoS2 layer on the p-THPP promoter layer. We manipulated the thickness of the MoS2 layer by adjusting the distance between the Mo source and the p-THPP promoter layer. The Mo solution was prepared by dissolving 0.1 M ammonium heptamolybdate (Fluka, 99%) in 10 mL distilled water, which it was subsequently coated onto the UV-treated SiO2 (300 nm)/p-Si(100) substrates by spincoating at 2000 rpm for 30 s. 0.1 g sulfur powder (SAMCHUN, 98.0%) as the sulfur source was located upstream in the reactor. The distance between sulfur and Mo sources was 19 cm MoS2 layers were synthesized at 900 C and 1.5 Torr in Ar (flow-rate: 500 sccm) for 5 min. Poly(methylmethacrylate) (PMMA) was spin-coated onto the surface of the MoS2 layer at 2000 rpm for 30 s. The PMMAcoated MoS2 film was then soaked in a 4 M NaOH solution that contained 20 mL deionized (DI) water. After the SiO2 layer was completely etched away, the PMMA-coated MoS2 layer was transferred onto the new SiO2/Si substrates in order to carry out the softlithography patterning process. Finally, the PMMA layer was removed by using acetone, and the film was rinsed with DI water. 2.2. Transferring and patterning of MoS2 nanosheets Dimethyl sulfoxide (DMSO) diluted in DI water (DMSO/DI

A graphene monolayer was synthesized at 1050 C for 20 min on a 25 mm Cu foil by a conventional TCVD method. CH4 (2 sccm) and H2 (200 sccm) gases were introduced into a TCVD reactor during the graphene synthesis. The synthesized graphene was transferred onto a SiO2 (300 nm) substrate by a PMMA-assisted wet transfer method. The graphene layer was transferred onto the top of MoS2 patterns by using the DMSO-coated PDMS stamp via the same process used for transferring and patterning of the MoS2 film. 3. Results and discussion The soft lithographic patterning method was used for the fabrication of cross-stacking patterned MoS2/graphene hybrid structures as shown in Fig. 1. First, a large-scale MoS2 layer was synthesized on the promoter layer via the conventional thermal chemical vapor deposition (CVD) method, and MoS2 layers were transferred on a solid substrate using the process used in previous works [22] (Fig. 1(a)). Dimethyl sulfoxide (DMSO) diluted in water was then vaporized and coated on the polydimethylsiloxane (PDMS) surface to increase the adhesion energy between the PDMS stamp and the MoS2 layer. The DMSO-coated PDMS stamp was brought in contact with the MoS2 nanosheet, and MoS2 was transferred to the PDMS surface (Fig. 1(a)). The hydrophilic DMSO molecules increased the surface energy of PDMS, so the adhesive force between the MoS2 film and PDMS stamp was stronger than that between the MoS2 and SiO2 substrate. When PDMS with the MoS2 layer came in contact with the surface of the target substrate

Fig. 1. Schematic diagram of fabrication process for cross-stacking patterned MoS2/graphene hybrid structures. (a) Preparation of CVD-grown MoS2 sheets on a SiO2 substrate (left) and pattern fabrication of the MoS2 layer by removing a specific area of the MoS2 layer (right). (b) Transferring MoS2 patterns on the PDMS stamp toward a target substrate. (c) Preparation of CVD-grown graphene layer and patterning process. (d) Transferring graphene patterns on the PDMS stamp to the top of MoS2 patterns on the target substrate. (A colour version of this figure can be viewed online.)


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Fig. 2. (a) SEM images of (i) MoS2, (ii) graphene patterns, (iii) cross-stacking MoS2/graphene patterns. (b) The optical image of patterned MoS2/graphene hybrid film (c) Raman spectroscopy for each area of patterned MoS2/graphene hybrid film in the range of (i) MoS2 and (ii) graphene, respectively. (d) Raman mapping images of (i) MoS2 line pattern (red), (ii) graphene pattern (green), and (iii) cross-stacked MoS2/graphene array (blue) as the intensity of A1g mode of MoS2 and G-band of graphene. (e) XPS spectra of Mo 3d, S 2p, C 1s core levels for cross-stacking patterned MoS2/graphene hybrid film. (A colour version of this figure can be viewed online.)

at 70 C, the surface energy of PDMS was restored. The MoS2 layers then detached from the PDMS surface and transferred to the target substrate (Fig. 1(b)). In the case of graphene, we also utilized thermal CVD for large-scale synthesis of the monolayer graphene, which was transferred onto a SiO2 substrate via a PMMA-assisted wet transfer method [15,23]. Like the MoS2 layer, the graphene was patterned and transferred to the patterned MoS2 sheet on the target substrate by using the DMSO-coated PDMS stamp (Fig. 1(d)). Finally, the cross-stacking MoS2-graphene hybrid patterns were fabricated by transferring graphene lines onto the MoS2 patterns in perpendicular direction to the MoS2 lines. Scanning electron microscopy (SEM) was utilized to characterize the microstructure of MoS2, the graphene patterns, and the cross-stacking patterned MoS2/graphene hybrid structure

(Fig. 2(a)). SEM images confirmed the presence of uniform MoS2, graphene patterns, and cross-stacking MoS2/graphene linepatterns with 7 mm width. The uniform and transparent MoS2 patterns and the graphene patterns with the 7 mm width were transferred onto the target SiO2 substrate via surface energy modification using the PDMS stamp, as shown in Fig. 2(a). We also fabricated cross-stacked MoS2-graphene patterns electrodes by transferring the second graphene patterns on top of the patterned MoS2 lines (Fig. 2(a-iii)). Note that the transferred MoS2 patterns were stable after the transfer of the graphene patterns. This result shows the potential of our method for mass fabrication of flexible and transparent advanced devices based on 2D materials for thin film transistors or memory devices [24]. The quality of the cross-stacking patterned MoS2/graphene

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Fig. 3. (a) The real-time photocurrent and (b) the photo-responsivity versus bias voltage for the cross-stacking patterned MoS2/graphene hybrid photodetector based on MoS2 channel with mono-, bi-, tri-layer under UV light exposure. (c) Time-dependent photocurrent and (d) voltage-dependent photo-responsivity for cross patterned MoS2/graphene hybrid photodetector based on graphene channel as a function of MoS2 layer under UV light illumination. (A colour version of this figure can be viewed online.)

patterns was evaluated by Raman spectroscopy (Fig. 2(b) and (c)). Raman spectra showed no peaks corresponding to the bare SiO2 surface ((1) region in Fig. 2(b)) and two prominent peaks at 405.82 cm1 and 385.5 cm1 corresponding to the MoS2 areas ((3) and (4) regions in Fig. 2(b)). These peaks originated from the outof-plane vibration mode (A1g mode) of the sulfur atoms and the in-plane vibration mode (E2g mode) of molybdenum and sulfur atoms, respectively. Furthermore, the energy difference between the A1g and E2g modes of MoS2 areas was about 20 cm1, which indicated that the MoS2 patterns formed a monolayer [25]. In the case of graphene patterns, G-, D- and 2D-bands from the graphene and MoS2-graphene regions were observed [26]. It was observed that the G-band of only graphene patterns was slightly red-shifted in comparison to that of the MoS2/graphene region, and the intensity ratio of I2D/IG decreased by about 0.6, resulted in n-doping. The uniformity of the patterned MoS2, graphene, and crossstacking patterned MoS2/graphene lines was also confirmed by Raman mapping. Fig. 2(dei) and (d-ii) show Raman spectral imaging of the G-band intensity generated from a two-phonon double resonance process for the graphene layer and the intensity of the A1g mode originating from the vertical vibration of sulfur atoms in the MoS2 nanosheet, respectively. These results indicate that both the graphene and the MoS2 patterns were stable with high coverage after the soft lithographic patterning process. X-ray photoelectron spectroscopy (XPS) was also used to analyze the chemical components of the cross-stacked MoS2/graphene patterns

(Fig. 2(e)). In the Mo 3d, S 2p, and C 1s core level spectra for the cross-stacked MoS2/graphene, the Mo 3d3/2 and 3d5/2 peaks were located at the binding energy (EB) of 232.5 eV and 229.3 eV. The S 2p1/2 and 2p3/2 peaks at EB of 163.3 eV and 162 eV were observed in the MoS2 regions, which implied the existence of MoS2 [25,27]. The C 1s peak at 284.6 eV, corresponding to the CeC bond of graphene was also observed for the graphene pattern regions [26]. The number of layers of the CVD-grown MoS2 film could be controlled by manipulating the distance between the MoO3 source and a target substrate during the CVD growth process. Here, the distance between the MoO3 source and a target substrate with the promoter is 1, 2, and 3 cm and the thickness of grown MoS2 layers is about 0.8, 1.4, and 2.0 nm, respectively. This thickness range is consistent with that of mono-, bi-, and tri-layer MoS2 layers (Fig. S2 in Supporting Information) [25,28e31]. Atomic force microscopy (AFM) was used for the analysis of the thickness of the layer-controlled MoS2 films (Fig. S1 in Supporting Information). Raman spectroscopy and ultravioletevisible (UVevis) spectroscopy were also carried out to investigate the thickness and transmittance of MoS2 layers, the results of which were also consistent with AFM analysis results (Fig. S1(b) in Supporting Information) [29,32,33]. The hybrid photodetector based on cross-stacking MoS2/graphene patterns was fabricated via a soft-lithographic transferring and patterning process. The resulting MoS2/graphene photodetectors were evaluated by measuring the time- and voltage-dependent photocurrent under UV-lamp illumination with a wavelength of


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Fig. 4. (a) Optical microscope images of cross-stacked patterns based on MoS2 with the width of (i) 1 mm, (ii) 500 mm, and (iii) 100 mm, and fixed graphene patterns with 500 mm width. (b) Photograph of cross-linking MoS2/graphene hybrid patterns onto the flexible substrate (PET) by a soft-lithographic patterning technique. (c) Photograph of MoS2/graphene patterns on PET (i) before and (ii) after device bending. (d) Time-dependent photocurrent for flexible hybrid photodetector with 10 mm-MoS2 and graphene as a function of bending cycles. Inset of (d) show the optical image of cross-stacking patterned MoS2 and graphene with 10 mm and 500 mm width, respectively. (e) Photo-responsivity according to the number of bending cycles (1e10,000) in photodetectors with different pattern sizes of MoS2. Here, the bending radius is 9 mm, and bias voltage is 1 V. (A colour version of this figure can be viewed online.)

254 nm at gate voltage (VG) ¼ 0 V, as shown in Fig. 3. Here, the width of the hybrid photodetector with the cross-stacking MoS2/ graphene patterns was 500 mm. We designed two different types of photodetectors based on cross-stacking MoS2/graphene patterns. First, the MoS2 channel was brought in contact with the graphene electrode patterns, and the conductance difference through the MoS2 channel was monitored at 30 V bias voltages (Fig. 3(a) and (b)). Fig. 3(a) shows that the MoS2-based device with the tri-layer MoS2 film has a photocurrent of 14.3 nA, which indicates that this device has a higher photocurrent value than those with monoand bi-layer MoS2 films [34,35]. The photoresponsivities for MoS2/ graphene hybrid photodetectors with the mono-, bi-, and tri-layer MoS2 films were also estimated under various bias voltages. As shown in Fig. 3(b), the photoresponsivity of the MoS2 device increased linearly with the bias voltage from 10 to 50 V. Additionally, this result shows that the tri-layer MoS2 film shows the highest photoresponsivity. This phenomenon can be understood by the number of charge carriers contributing to the electron transfer kinetics. In case of multilayer MoS2, the incident light is converted into a substantial amount of charge carriers, and lead to a large amount of absorption in comparison to monolayer MoS2. The layerdependent absorption spectra under UV light irradiation of Fig. S1(c) in Supporting Information show that tri-layer MoS2 exhibited the largest absorption spectra, which is mostly consistent

with the previous report [34]. In the second photodetector, a single graphene layer was utilized as channels, and a MoS2 layer was placed on bottom of the single graphene layer, where the MoS2 layer under the graphene layer acts as a dopant. In contrast to the first device with only the MoS2 channel, the current level of photo-devices based on the MoS2/graphene heterostructure decreased under UV-light illumination. In addition, we applied mono-, bi-, and tri-layer MoS2 patterns under the graphene channel. The photocurrent of graphene devices with the tri-layer MoS2 film was significantly lower than the current of those with the mono- or bi-layer MoS2 films (Fig. 3(c)). This behavior can be explained by the charge transfer at the interface between MoS2 and graphene under UV-light exposure [36]. It is suggested that when the MoS2/graphene heterostructure was under UV-light illumination, photo-excited electron-hole pairs were generated in the MoS2 layer. These electron-hole pairs were separated by the electric field at the interface between MoS2 and graphene, and the electrons (holes) were transferred to the graphene (MoS2) layer. This led to a reduction in the hole concentration in the graphene and an upward shift of the Fermi level of graphene, which resulted in a decrease in the conductance of graphene devices. When the graphene channel with MoS2 was exposed to light, the Dirac voltage of graphene shifted toward negative values, as shown in Fig. S3(a) in the Supporting

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Information. The photoresponsivity also improved gradually with the increase in the bias voltage. The graphene device with the trilayer MoS2 film was found to exhibit a higher photoresponsivity of 5 A/W at 2 V than those with the mono- and bi-layer MoS2 films (Fig. 3(d)) [34,35]. Cross-stacked patterns of graphene and MoS2 layers with various widths (10 mme1 mm) were fabricated to analyze their optoelectrical properties and optimize bending durability (Fig. 4(a)). In the current work, the width of the graphene patterns was 500 mm. The cross-stacked MoS2/graphene hybrid patterns were successfully transferred on a flexible polyethylene terephthalate (PET) substrate via the soft-lithographic patterning process, as shown in Fig. 4(b). We also evaluated the photo-electric properties of the graphene devices with the MoS2 patterns having a width of 10 mm, 100 mm, 500 mm, and 1 mm, as a function of the number of bending cycles. In our work, a bending tester (JUNIL TECH Co., LTD - JIBT-610 (Radius Bending)) was used, and the bending radius was 9 mm (Fig. 4(c)). The time-dependent photocurrent of the graphene photodetector with 10 mm-width MoS2 patterns was obtained after 1 to 10,000 bending cycles. Fig. 4(d) showed that the photocurrent decreased from 4.3 to 3.7 A with increasing bending cycles. In addition, electrical transport measurement was carried out under cyclic UV irradiation after 1 to 10,000 bending cycles to investigate the photoresponsivity of the MoS2/graphene photodetectors with various widths of the MoS2 patterns. Interestingly, among these devices, the one with the 10mm-wide MoS2 patterns exhibited the lowest reduction in the photoresponsivity with increasing the number of bending cycles (Fig. 4(e)). This result can be explained by the facts that MoS2 patterns with a small width have relatively less compressive stress during the bending process and the physical damage caused by the bending process result in the reduction of the photocurrent. To confirm cracks of the patterned MoS2 film caused by mechanical bending stress, AFM measurement was performed after 10,000 bending cycles with a bending radius of 9 mm. Fig. S4 show AFM images of before and after 10,000 bending for a 100-mm, 500-mm and 1-mm patterned MoS2 films transferred on PET substrates, respectively. These AFM images showed that any remarkable crack could not be observed for all of patterned MoS2 films (100 mm, 500 mm and 1 mm) before and after bending cycles. However, the RMS roughness of MoS2 surface increased after bending. It is considered that the structural deformations caused by the mechanical bending damage are the factors that degrade the photoelectrical characteristics of the device. 4. Conclusion In summary, we demonstrated the fabrication of novel crossstacked MoS2/graphene hybrid patterns by using a softlithographic patterning method. In the current work, a simple soft lithographic patterning process was applied for patterning and transferring of large-scale CVD-grown graphene and MoS2 layers using polymer stamps coated with polar molecules. The structural and chemical characterizations of the cross-stacked MoS2/graphene hybrid patterns were performed via XPS and Raman analysis. In addition, we measured time-dependent photoresponsivity for hybrid photodetectors based on MoS2 and graphene channels under UV-light illumination. The graphene photodetector with MoS2 patterns showed a high photoresponsivity of 6.3 A/W. The bending test for the MoS2/graphene hybrid film transferred on flexible PET substrate was also performed as a function of width of MoS2 patterns. It was found that with decreasing width of MoS2 patterns, the degradation of photoresponsivity induced by the bending process was gradually alleviated. Most significantly, our cross-stacked patterns of 2D materials provide a new approach for

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