Graphene van der Waals Heterostructures for High-Performance

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Printable transfer-free and wafer-size MoS2/graphene van der Waals heterostructures for high-performance photodetection Qingfeng Liu, Brent Cook, Maogang Gong, Youpin Gong, Ewing Dan, Matthew Casper, Alex Stramel, and Judy Z. Wu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b00912 • Publication Date (Web): 21 Mar 2017 Downloaded from http://pubs.acs.org on March 28, 2017

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ACS Applied Materials & Interfaces

Printable transfer-free and wafer-size MoS2/graphene van der Waals heterostructures for high-performance photodetection Qingfeng Liu,*† Brent Cook,† Maogang Gong,*† Youpin Gong,† Dan Ewing,‡ Matthew Casper,‡ Alex Stramel,‡ and Judy Wu*† †

Department of Physics and Astronomy, University of Kansas, Lawrence, KS, 66045, USA

‡

Department of Energy's National Security Campus, Kansas City, MO, 64147, USA

ABSTRACT: Two-dimensional (2D) MoS2/graphene van der Waals heterostructures integrate the superior light-solid interaction in MoS2 and charge mobility in graphene for highperformance optoelectronic devices. Key to the device performance lies in a clean MoS2/graphene interface to facilitate efficient transfer of photo-generated charges. Here we report a printable and transfer-free process for fabrication of wafer-size MoS2/graphene van der Waals heterostructures obtained using a metal-free-grown graphene, followed by lowtemperature growth of MoS2 from the printed thin film of ammonium thiomolybdate on graphene. The photodetectors based on the transfer-free MoS2/graphene heterostructures exhibit extraordinary short photoresponse rise/decay time of 20/30 ms, which is significantly faster than the previously reported MoS2/transferred-graphene photodetectors (0.28-1.5 s). In addition, high photoresponsivity up to 835 mA/W was observed in visible spectrum on such transfer-free MoS2/graphene heterostructures, which is much higher than the reported photodetectors based on the exfoliated layered MoS2 (0.42 mA/W), the graphene (6.1 mA/W) and transfer-free MoS2/graphene/SiC heterostructures (~40 mA/W). The enhanced performance is attributed to the clean interface on the transfer-free MoS2/graphene heterostructures. This printable and transferfree process paves the way for large-scale commercial applications of the emerging 2D heterostructures in optoelectronics and sensors. KEYWORDS: graphene, MoS2, transfer-free, heterostructures, printable, photodetectors.

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■ INTRODUCTION The significance of atomically thin two-dimensional (2D) materials in modern electronics and optoelectronic device has been verified by the fascinating electronic properties of graphene.1-4 However, the zero-band gap nature and poor optical responsivity of graphene become a fundamental obstacle to developing graphene-based electronics and optoelectronics.5 Vertically layer-by-layer stacking other 2D atomic layers on graphene can create van der Waals heterostructures, which can provide novel electronics and optoelectronic properties, thus open up unprecedented opportunities for fundamental research and practical applications in recent years.68

Especially, being a layered semiconductor with exceptional photoresponsivity and a tunable

band gap, the graphene-like hexagonal molybdenum disulfide (MoS2) is a natural photosensitizer in MoS2/graphene heterostructures with graphene being a broad-band transparent electrode.9-11 The density functional theory calculations suggested that graphene could bond weakly to MoS2 with an interlayer spacing of 0.332 nm and a binding energy of -23 meV per C atom irrespective of the adsorption arrangement.12 Such weak van der Waals bonding between MoS2 and graphene not only preserves the high carrier mobility in graphene, but also facilitates effective charge carrier transfer across the MoS2/graphene interface.13-17 For example, with the ingenious designing, the responsivity of the phototransistor can be up to 835 mA/W under by the applied back-gate voltage of -23 V,18 which is much larger than that of individual the exfoliated layered MoS2 (0.42 mA/W)

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and the graphene (6.1 mA/W).20 While an intensive effort has been

motivated in exploring direct CVD growth of MoS2 on graphene in the recent years,21-22 a scalable and facile fabrication of such heterostructures still remains a significant challenge. The use of graphene grown on Cu requires wet etching of Cu for transfer of graphene to the desired substrates such as SiO2/Si. This cumbersome transfer process not only inevitably degrades the graphene due to formation of wrinkles, cracks and contaminations,23 but also is not suitable for large-scale fabrication of MoS2/graphene heterostructures in a low cost.

■ EXPERIMENTAL SECTION Metal-catalyst-free CVD synthesis of graphene. The CVD synthesis was performed in a quartz tube reactor (25 mm in diameter) inside a horizontal CVD furnace, as described in our


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previous report.24 In a typical experiment, the clean substrates were placed at the center of the quartz tube reactor. H2 (120 sccm) was introduced in the reactor while it was heated up to growth temperatures of 1065 oC. Then, CH4 (30 sccm) was fed into the reactor to initiate the graphene growth on the substrates. The growth time was maintained for 3 h. After growth, the furnace was quickly cooled to room temperature under the protection of H2. Printing fabrication of the MoS2/graphene heterostructures. The MoS2 precursor (NH4)2MoS4 was dissolved in (N,N-dimethylformamide, DMF) with a concentration of 0.1 wt %. The (NH4)2MoS4 precursor solution is used as the ink without any modification for printing of a uniform MoS2 precursor thin layer by a microplotter (SonoPlot, Inc.) through sonication of glass capillary tips. The plotter has a built-in calibration for the dispenser tips to measure the resonant frequency of glass capillary tips. The ultrasonic ink jet printing relies on vibration of the capillary through a piezoelectric. The resonance frequency of the capillary and fluid depends on the amount of fluid present in the capillary, so that always changes from calibration of the capillary piezoelectric unit after many printing cycles. The sonication frequency can be adjusted from 200-600 kHz for a given dispensing ink for optimal effects. The voltage amplitude to the piezoelectric can modulate the strength of this dispensing and is around 1-2 volts for very fluid solutions, but can go much higher for very viscous fluids. Printing schematics can be drawn through a software provided for lines, dots, squares, circles, and arch designs. The printing was carried out on the surface of the metal-free grown or transferred graphene on SiO2/Si substrates. For fabrication of the photodetectors, the Au/Ti (40 nm/10 nm) electrodes with spacing of 100 µm were fabricated on the graphene/SiO2/Si substrates using electron-beam evaporation through a metal shadow mask. Afterwards, the precursor layer was printed on the graphene between the electrodes. After the precursor layer was printed on the graphene between the electrodes, the substrates were placed in the CVD furnace and 10 sccm H2 and 50 sccm Ar were introduced into the quartz tube. The CVD system pressure was maintained at 50 mTorr. The temperature was raised to 450 oC. After the desired temperature is reached, the sample was subsequently annealed for 1 hour. Finally, the CVD furnace was cooled down to room temperature. Characterization. Raman spectra and mappings were taken on a confocal Raman system (WiTec alpha300) with laser excitation of 488 nm. Surface morphology of the samples was


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examined with Agilent 5500 atomic force microscope (Agilent Technologies, Tempe, AZ). TEM images were taken with Tecnai F20 equipped with an energy dispersive spectrometer. SEM image was obtained using FEI Versa 3D Dual Beam Field Emission Scanning Electron Microscope. The XPS spectra were taken using an AXISULTRA (KRATOS) system. The transmittance spectra were obtained with Varian 50 Bio UV-Visible Spectrophotometer. The current-voltage (I-V) characteristics of the MoS2/graphene film photodetectors were measured using a CHI660D electrochemical workstation. A spectral response for different wavelengths was recorded by using an Oriel Apex Monochromator illuminator.

■ RESULTS AND DISCUSSION

Figure 1. Schematic of the two-step transfer-free fabrication process for MoS2/graphene van der Waals heterostructures: (1) metal-free CVD growth of graphene directly on SiO2/Si or fused silica substrate; and (2) printing the (NH4)2MoS4 precursor on graphene followed by annealing to form 2D MoS2/graphene heterostructures. Inset is a typical SEM image of the printed sample.

The two-step fabrication process for the MoS2/graphene van der Waals heterostructures is schematically shown in Figure 1. The metal-free CVD growth is advantageous since it allows


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eliminating the cumbersome graphene transfer, and thus leads to a clean surface of the assynthesized graphene. Such clean graphene surface is critical to the formation of clean van der Waals interface between the MoS2 and graphene. In the second step, the low-temperature annealing of the printed (NH4)2MoS4 layer on graphene is beneficial to keep graphene intact and the graphene-MoS2 interface clean by avoiding formation of undesirable impurities.

Figure 2. (a) Optical image of a typical MoS2/graphene heterostructure film. (b-c) AFM images and (d) Raman spectra taken starred regions of graphene (bottom) and the MoS2/graphene heterostructures (top) in (a). (e-f) Raman mapping images of (e) the 2D peak and (f) the E peak

taken at the framed region in (a).

Figure 2a shows a representative optical image of the transfer-free MoS2/graphene heterostructure. A uniform MoS2/graphene film is visible on the top side of the image through the color contrast between the MoS2 covered graphene (top) and uncovered graphene (bottom). Figure 2b-c compares the AFM images taken respectively on SiO2/Si, and the spots on graphene


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(bottom) and MoS2/graphene (top) indicated by the stars in Figure 2a. Before graphene growth, the SiO2 substrate shows a very smooth surface as expected (Figure S1, Supporting information). After graphene growth, the graphene sheet fully covered the substrate surface (Figure 2b). After annealing the (NH4)2MoS4 precursor printed on the graphene, an additional thin MoS2 layer was formed and covered the graphene, yielding the 2D MoS2/graphene van der Waals heterostructures. A low-magnification AFM image demonstrates that MoS2 layer coverage is over 80% (Figure S2, Supporting information) with a thickness in the range of 0.7 (1 layer as shown in Figure 2c) to 1.5 nm (3 layers). In our experiments, the thickness of MoS2 film on graphene was finely controlled by using low concentration (0.1 wt %) of (NH4)2MoS4 precursor solution ink at the selected printing conditions. If the (NH4)2MoS4 precursor concentration is higher than 0.15 wt %, the resulting MoS2 films of graphene would be thicker, while at a lower concentration than 0.5 wt %, the resulting MoS2 film is thin but discontinuous. Figure 2d compares the Raman spectra of the same sample taken on graphene (bottom, black star) and the MoS2/graphene (top, red star) in Figure 2a. Both Raman spectra exhibit three similar characteristic peaks in the high frequency region, i.e., the tangential G band at ~1590 cm1

derived from the in-plane vibration of the sp2 carbon atoms, the small disorder-induced D band

at ~1355 cm-1, and its second-order harmonic 2D band at ~2715 cm-1, verifying graphene grown on SiO2/Si (Figure S3, Supporting information). The intensity ratio of 2D to G band (I2D/IG) of ~2.4, the negligible D band and the small intensity ratio of D to G band (ID/IG = ~0.1) indicate the graphene is high-quality and monolayer. On the MoS2/graphene heterostructures, two additional prominent peaks, i.e, ~386 cm-1 and ~406 cm-1 appear in the low frequency region (Red), which correspond to the in-plane vibration of Mo and S atoms (E mode) and out-of-

plane vibration of the S atoms (A1g mode), respectively. The spacing between the two peaks is ~20 cm-1 and the full width at half-maximum of E is ~5 cm-1, which clearly demonstrate the

heterostructure of ultrathin MoS2 layer (1-3 layers) on graphene.25-26 Figure 2e-f illustrates the Raman maps of the 2D peak from graphene and the E peak from MoS2, respectively, taken on

the area marked with the rectangle in Figure 2a, exhibiting the high uniformity of each layer in the MoS2/graphene heterostructures. Figure 3a-c show the representative XPS spectra of the graphene and the transfer-free MoS2/graphene heterostructures. As expected, after the graphene growth on SiO2/Si, the XPS


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spectrum exhibits a dominant C1s peak at ~284.5 eV, verifying the formation of sp2 hybridized carbon with most carbon atoms remain embedded within the honeycomb lattice. After the lowtemperature annealing, the existence of Mo and S with ratio of ~0.5 confirms the formation of MoS2, in addition to the peak at ~284.5 eV for the underneath graphene. The XPS peaks at 229.3 and 232.5 eV correspond to the doublet of Mo3d5/2 and Mo3d3/2 orbital, respectively. Furthermore, the S2p orbit exhibits two peaks at 163.4 eV and 162.1 eV, which were attributed to the S2p1/2 and S2p3/2, respectively. These measured binding energies of Mo and S are consistent with MoS2 crystals with hexagonal symmetry 27.

Figure 3. (a-c) XPS spectra of (a) C1s, (b) S3p and (c) Mo3d of the graphene and MoS2/transferfree-graphene heterostructures.

For comparison, we also fabricated the MoS2/graphene heterostructures on the transferred graphene. Graphene grown on Cu foil was transferred onto SiO2/Si substrates by using our previously reported method.23 Figure S4 (Supporting Information) shows TEM images of the MoS2/transferred-graphene heterostructures. The graphene and MoS2 layers can be identified clearly in high-resolution TEM by difference in image contrast (MoS2 typically showing higher contrast than graphene) in Figure S4b. The lattice constants can be measured to be a = b = ~0.32, which are consistent with that of bulk MoS2.25 The distributions of C, Mo and S within the MoS2/graphene heterostructure were analyzed using energy dispersive spectroscopy elemental mappings of C, Mo and S (Figure S4c-d, Supporting Information). The color


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distribution within the mappings shows two distinct phases with C (green) standing out as a separate phase, and Mo (yellow) mixing with S (red) forming the MoS2 as the other phase. It should be noted that the non-uniform and partial covering of MoS2 layers on the transferred graphene is attributed to the degradation of the surface properties of the transferred graphene due to the introduction of defects (wrinkles, cracks, etc.) and contaminations such as the residual metal impurities and PMMA contaminants during the transfer process. In stark contrast, the metal-free grown CVD graphene has a clean graphene surface, which is critical to the formation of continuous and uniform covering of MoS2 layer on the graphene surface with clean van der Waals interface.

Figure 4. (a) Photograph of an array of printed MoS2/graphene heterostructure photodetectors on fused silica. Inset (lower left) is a schematic diagram of the photodetector. (b) Schematic diagram of the MoS2/graphene interface band. (c) I-V characteristics measured in dark and upon 540-nm illumination with 0.1 mW/cm2. (d) Dynamic photocurrent measured in response to 540nm light on and off. Inset is enlarged portions of (d).


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Figure 4a shows a typical photograph of an array of the printed MoS2/graphene devices with interdigitated Au/Ti (40 nm/10 nm) electrodes (channel length 250 µm). It can be seen that this method is therefore suitable for large-scale fabrication of the MoS2/graphene van der Waals heterostructure photodetectors. The photodetector configuration is schematically illustrated in Inset of Figure 4a (lower left). For such photodetectors, MoS2 layer acts as photosensitizer to absorb light, while graphene is used an expressway for carrier transport. The MoS2/graphene interface band diagram of our devices is schematically illustrated in Figure 4b. Under light irradiation, the electron-hole pairs (excitons) are generated in the semiconducting MoS2 layer and charge transfer is induced by an internal electric field, arising at the MoS2graphene interface due to the workfunction mismatch between them.18 The photo-excited holes transfer into the graphene and drift to the drain under an applied bias voltage (Vds) with a typical timescale of τtransit, which can be expressed as τtransit =L2/(µVds), where L is the channel length and is the carrier mobility in photoconductive channel. The photo-excited electrons are trapped in the MoS2 layer with a typical timescale of τlife, which directly determines the transient dynamic response time of the observed photoresponse. Charge conservation in the graphene channel results in hole replenishment from the source as soon as a hole reaches the drain.28 Accordingly, benefiting from the ultrahigh mobility of graphene, the transferred photo-excited holes into graphene can circulate many times through an external circuit under the driving of the applied bias voltage until they recombine with the photo-excited electrons trapped MoS2 layers, which results in a high responsivity (or gain) photodetector. This photoconductor mechanism in the MoS2/graphene devices can be given by G = Rhv/e = τlife/τtransit = τlifeµVds/L2, where G is the photoresponse gain, R is the responsivity, h is Planck’s constant, e is the charge of an electron, and v is the frequency of incident light.29 Therefore, high responsivity of the MoS2/graphene photodetector can be expected as a result of the long life time of excess carriers in MoS2 and high carrier mobility of graphene. Figure 4c shows the typical I-V curves of the MoS2/graphene photodetectors measured in dark (IDark) and upon 540-nm illumination of 0.1 mW/cm2 (ILight). It can be seen that the photocurrent (IPh=ILight-IDark) increases with increasing applied bias. At the bias of 5 V, the device has a large photocurrent of ~8.34 µA, indicative of a strong photoresponse. Figure 4d shows the time-resolved photocurrent of the device in response to “on” and “off” of the light illumination. The rise and decay time was 20 ms and 30 ms, respectively. Such response speed is over 10


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times faster than that (0.28-1.5 s) of the previously reported MoS2/transferred-graphene heterostructures.17-18 This much improved response speed of our devices is attributed to the clean MoS2-graphene interface, which is beneficial to reducing charge traps for fast photoresponse.

Figure 5. (a) Dynamic photoresponse of the MoS2/graphene heterostructure photodetector measured in response to 540-nm light on and off with different incident light powers. (b) Photocurrent and (c) photoresponsivity as function of the incident light intensity. (d) Spectral photoresponsivity of the MoS2/graphene heterostructure photodetector.

Figure 5a shows the incident light power dependence of photocurrent for the MoS2/graphene heterostructure photodetector. It can be seen that the MoS2/graphene heterostructure photodetectors keeps similar fast photoresponse speed upon different incident light power. The photocurrent increases with increasing the incident light power (Figure 5b). The responsivity R is a key parameter to evaluating the performance of a photodetector, which can be calculated by the equation: R= (ILight-IDark)/ Pph, where Pph is the incident illumination power on the effective


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detection area. The decreasing trend of responsivity (Figure 5c) with increasing PPh is anticipated and consistent with the previously-reported MoS2/ graphene photodetectors based on the transferred graphene.17-18 Overall, a maximum R value of 835 mA/W was obtained in the minimum PPh of 0.05 mW/cm.2 This performance represents an improvement of two orders of magnitude over that of the high-crystallinity graphene (~6.1 mA/W),20 three orders of magnitude higher than that of the exfoliated layered MoS2 (~0.42 mA/W),

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and one order of magnitude

higher than that of the transfer-free MoS2/graphene/SiC heterostructures (~40 mA/W).30 Figure 5d shows the spectral photoresponsivity of the MoS2/graphene heterostructure photodetector measured at 5V bias voltage. A clear band edge can be observed at ~680 nm wavelength, which corresponds well to the optical energy gap of MoS2 (~1.8 eV) and confirms MoS2 dominates the optical absorption in the MoS2/graphene heterostructures (Figure S5, Supporting Information). This illustrates the advantage of the wavelength selectivity in the MoS2/graphene heterostructures, when compared the pure graphene photodetectors. For a direct comparison, we also fabricated the photodetectors based on the MoS2/transferredgraphene heterostructures and the pure MoS2 (without graphene) on SiO2/Si substrates. Upon 540-nm illumination at 0.1 mW/cm2, the photocurrent of the MoS2/transferred-graphene heterostructures decrease by ~22 times (Figure S6, Supporting Information) when compared to the MoS2/transfer-free-graphene counterpart measured under the same conditions. Accordingly, the photoresponsivity of the MoS2/transferred-graphene decreased to ~26 mA/W. Since both the device fabrication and measurement conditions are comparable, the photoresponse decrease of the MoS2/transferred-graphene photodetector is reasonably caused by interface layer blocking charge transfer to graphene, in addition to defects and impurities formed on graphene during transfer process.31-32

■ CONCLUSIONS In summary, we have developed a facile, printable and transfer-free method for wafer-size fabrication of MoS2/graphene van der Waals heterostructure photodetectors with high responsivity and photoresponse speed. By eliminating the cumbersome graphene transfer


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process, the resulting clean MoS2/graphene interface leads to the high performance of the optoelectronic devices. Moreover, this process can be extended for wafer-size fabrication of van der Waals heterostructures of other 2D materials such as MoSe2, WS2 and WSe2 on metal-free CVD grown graphene for commercial optoelectronics.

■ ASSOCIATED CONTENT Supporting Information AFM and Raman of SiO2/Si substrate, Low-magnification AFM and optical transmittance of the MoS2/graphene film of transfer-free MoS2/graphene, TEM with elemental mappings (e.g., C, Mo and S) and time-dependent photocurrent of the MoS2/transferred-graphene heterostructures. This material is available free of charge via the Internet at http://pubs.acs.org.

■ AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected]; [email protected]; [email protected] ■ ACKNOWLEDGMENTS The authors acknowledge support in part by ARO contract No. ARO-W911NF-16-1-0029, and NSF contracts Nos. NSF-DMR-1337737, and NSF-DMR-1508494. Q Liu would like to thank Dr. Tsuyohiko Fujigaya and Mr. Yuki Hamasaki of Kyushu University for the help and discussion of the XPS part. This research was supported by Plant Directed Research and Development funds from the Department of Energy’s National Security Campus, operated and managed by Honeywell Federal Manufacturing and Technologies, LLC under contract No. DENA0002839.

■ REFERENCES (1) Geim, A. K. Graphene: Status and Prospects. Science 2009, 324, 1530-1534.


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17

Graphene van der Waals Heterostructures for High-Performance

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