Modulating Electronic Properties of Monolayer MoS2 via Electron

Oct 26, 2016 - GO sheets are well-known as hole injection layers since they contain electron-withdrawing groups such as carboxyl, hydroxyl, and epoxy...
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Modulating Electronic Properties of Monolayer MoS2 via Electron-Withdrawing Functional Groups of Graphene Oxide Hye Min Oh,†,‡,$ Hyun Jeong,†,$ Gang Hee Han,† Hyun Kim,†,‡ Jung Ho Kim,†,‡ Si Young Lee,† Seung Yol Jeong,⊥,∥ Sooyeon Jeong,⊥ Doo Jae Park,⊗ Ki Kang Kim,# Young Hee Lee,*,†,‡,§ and Mun Seok Jeong*,†,‡ †

Center for Integrated Nanostructure Physics, Institute for Basic Science (IBS), Suwon 16419, Republic of Korea Department of Energy Science and §Department of Physics, Sungkyunkwan University, Suwon 16419, Republic of Korea. ⊥ Nano Hybrid Technology Research Center, Korea Electrotechnology Research Institute (KERI), Changwon 51543, Republic of Korea ∥ Department of Electrical Functionality Material Engineering, University of Science and Technology (UST), Daejeon 34113, Republic of Korea ⊗ Department of Physics, Hallym University, Hallymdaehaggil 1, Chuncheon 24252, Republic of Korea # Department of Energy and Materials Engineering, Dongguk University-Seoul, Seoul 04620, Republic of Korea ‡

S Supporting Information *

ABSTRACT: Modulation of the carrier concentration and electronic type of monolayer (1L) MoS2 is highly important for applications in logic circuits, solar cells, and light-emitting diodes. Here, we demonstrate the tuning of the electronic properties of large-area 1L-MoS2 using graphene oxide (GO). GO sheets are wellknown as hole injection layers since they contain electron-withdrawing groups such as carboxyl, hydroxyl, and epoxy. The optical and electronic properties of GO-treated 1LMoS2 are dramatically changed. The photoluminescence intensity of GO-treated 1LMoS2 is increases by more than 470% compared to the pristine sample because of the increase in neutral exciton contribution. In addition, the A1g peak in Raman spectra shifts considerably, revealing that GO treatment led to the formation of p-type doped 1L-MoS2. Moreover, the current vs voltage (I−V) curves of GO-coated 1L-MoS2 field effect transistors show that the electron concentration of 1L-MoS2 is significantly lower in comparison with pristine 1LMoS2. Current rectification is also observed from the I−V curve of the lateral diode structure with 1L-MoS2 and 1L-MoS2/ GO, indicating that the electronic structure of MoS2 is significantly modulated by the electron-withdrawing functional group of GO. KEYWORDS: monolayer MoS2, graphene oxide, electron-withdrawing effect, photoluminescence, Raman spectroscopy

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including control of the carrier concentration using a back gate or liquid gate,11,12 substitutional doping during growth,13 or gas molecule adsorption.14 However, these processes have not successfully achieved p-type doping of 1L-MoS2; hence, alternative doping methods such as chemical doping have also been explored.15,16 Chemical doping is known as an effective method to easily modify the carrier density of materials.17,18 However, reported chemical methods have been utilized for carrier doping of few or multilayer MoS2,16 which have an indirect band gap and lower luminescence

onolayer (1L) transition-metal dichalcogenides (TMDCs) such as molybdenum disulfide (MoS2) have attracted considerable attention for use in nextgeneration electronic and optoelectronic devices1−3 such as photodetectors,4,5 light-emitting devices,6 and solar cells7,8 because of their direct band gap.9,10 A practical difficulty that stands in the way of these applications is the significant suppression of the radiative recombination of excitons, which originates from a dominant formation of negative trions resulting from the strong n-type characteristic of natural 1LMoS2. To prohibit the formation of negative trions by modulating the highly n-type property of 1L-MoS2 and thereby enhance the radiative recombination, p-type doping has been attempted with several methods. Thus far, several techniques have been proposed for modulating the carrier type of MoS2, © 2016 American Chemical Society

Received: September 19, 2016 Accepted: October 26, 2016 Published: October 26, 2016 10446

DOI: 10.1021/acsnano.6b06319 ACS Nano 2016, 10, 10446−10453

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Figure 1. Sample preparation. (a) Quartz plate used as a substrate, GO coating on half the area of the quartz plate, and transfer of 1L-MoS2 grown by CVD onto the GO-coated quartz plate. The prepared sample has 1L-MoS2 overlapped with GO sheets and 1L-MoS2 simultaneously. (b) PL spectrum of the 1L-MoS2 grown by CVD. (c) Raman spectrum of the 1L-MoS2 grown by CVD. The insets present schematics of the atomic displacements for each Raman mode. The interval between the two Raman modes is 18 cm−1, which indicates 1L-MoS2. (d) FTIR and (e) Raman spectra of the GO sheets used in this study; the GO sheets include abundant functional groups.

Figure 2. Tuning the optical properties of 1L-MoS2 by GO coatings. (a) PL spectra measured from 1L-MoS2 and 1L-MoS2/GO. PL intensity and peak positions were considerably modified by contact with GO sheets. (b) Fitted PL spectra of 1L-MoS2/GO and 1L-MoS2. A peaks in the PL spectra were reproduced by assuming three peaks with Lorentzian functions for biexciton (XX), trion (X*), and neutral exciton (X) peaks. (c) 1L-MoS2 and (d) 1L-MoS2/GO with increasing excitation power. (e) Raman spectra of 1L-MoS2 and 1L-MoS2/GO. The A1g mode of 1LMoS2/GO is blue-shifted by 1.9 cm−1 compared with that of 1L-MoS2. (f) Schematic representation of the electron-withdrawing model for tuning the electron density of 1L-MoS2 by coating with GO.

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ACS Nano efficiency than 1L-MoS2.19,20 Therefore, it is a challenge to find an effective approach to obtain p-type-doped 1L-MoS2. Recently, the use of graphene oxide (GO) was theoretically suggested for modulating the electronic structures of MoS2 by acting as a hole-injection layer; however, this idea has not yet been experimentally demonstrated.21 In this study, we report the p-type doping of 1L-MoS2 via the spin-casting of GO. It is observed that GO, with its apparent electron-withdrawing characteristics, effectively extracts electrons from a 1L-MoS2 sheet with which it is in contact. Enhancement of the neutral exciton peak intensity accompanied by relative reduction of the negative trion peak in the photoluminescence (PL) spectrum and a peak shift of the Raman spectrum suggest that the optical properties of 1L-MoS2 were tuned because of hole injection by the GO attachment. Moreover, the current versus voltage (I−V) curves of the 1LMoS2 with GO indicate the modulation of the electrical properties of 1L-MoS2. We also observed current rectification in the I−V curves of lateral 1L-MoS2 and 1L-MoS2/GO structures.

Figure 2a presents the PL spectra measured for 1L-MoS2/ GO (red arrow) and 1L-MoS2 (black arrow). The samples for the measurement are schematically illustrated in the inset of Figure 2a. PL peaks associated with A and B excitons were observed in both samples; however, the PL intensity of 1LMoS2/GO (red curve) was greatly enhanced (by approximately 470%) compared with that of 1L-MoS2 (black curve). Moreover, the peak position of the PL spectrum of 1L-MoS2 was blue-shifted in the 1L-MoS2/GO. The PL peak of the A excitons in 1L-MoS2 appeared at 1.85 eV. However, in the PL spectrum of 1L-MoS2/GO, the peak positions for the A excitons appeared at higher energy compared with those of 1LMoS2; i.e., the A peak is observed at ∼1.92 eV. According to previous reports, these blue-shifted peak positions for A and B excitons could be explained by p-type doping or compressive strain of MoS2 owing to lattice mismatch between the GO and MoS2 layer.15,26−29 The two peaks at approximately 2.15 eV represent Raman modes of the GO sheets, which correspond to the D and G bands.30,31 To analyze the peak shift of the PL spectra, Lorentzian fitting and intensity normalization were applied as shown in Figure 2b. The top and bottom panels present the fitted PL spectra of 1LMoS2/GO and 1L-MoS2, respectively. The PL peak for the A exciton is composed of the biexciton peak (XX) at 1.85 eV (red line), trion peak (X*) at 1.88 eV (green line), and neutral exciton peak (X) at 1.91 eV (blue line). According to a previous report, neutral excitons are converted into negative or positive trions depending on the excess carrier type.32 Here, trion peak (X*) in the pristine MoS2 is a negative trion, as reported by several studies;14,15,33 meanwhile, the trion peak (X*) from MoS2/GO in Figure 2b can be estimated as a positive trion or negative/positive trion from the p-type doped feature of the blue-shifted PL peak.32 The biexciton (XX) is predominant in the A exciton peak of 1L-MoS2 at high excitation power.32 The negative trion emission emergence in the as-prepared 1L-MoS2 is attributed to the heavy electron (n-type) doping.33 However, neutral exciton recombination governs the A exciton peak of 1L-MoS2/ GO, as observed in Figure 2b, and can be attributed to the decrease in the number of excess electrons resulting from the loss of electrons to the GO.34 For further investigation of the optical properties of 1L-MoS2 and 1L-MoS2/GO, we examined the excitation-laser-power-dependent PL emission. Parts c and d of Figure 2 show the power-dependent PL intensities of 1LMoS2 and 1L-MoS2/GO, respectively. Figure 2c presents the integrated PL intensity of the biexciton (XX), trion (X*), and neutral exciton (X) of 1L-MoS2 with excitation powers ranging from ∼0.04 to 3.2 mW (see Figure S4). In 1L-MoS2, the PL intensity of the biexciton peak increased upon increasing the incident laser power, as reported for MoS232 and WS2.35 The biexciton (XX) intensity increased with an increase in the excitation power, which could be interpreted as an increased biexciton population induced by abundant exciton density.36,37 However, the trion (X*) intensity decreased at high excitation powers and exhibited an opposite trend as the biexciton (XX) intensity, which is thought to have contributed to the formation of biexcitons (XX) with increasing excitation power.36,37 The integrated PL intensity of the neutral exciton (X) peak exhibited almost negligible changes as a function of the excitation power compared with those of the biexciton (XX) and trion (X*) peaks. The saturated PL intensity of the neutral exciton (X) peak is attributed to Coulomb screening and Pauli blocking effects induced by the presence of excess free

RESULTS The flakes of 1L-MoS2 were synthesized on a SiO2 substrate using chemical vapor deposition (CVD).22 The shape of the 1L-MoS2 flakes was triangular, and the average size of the flakes was 30 μm (see the Figure S1). The detailed sample preparation procedure is depicted in Figure 1a. Hydrophilic treatment was applied to the entire surface of the quartz substrate using UV-lamp irradiation for 20 min to achieve better adhesion of the GO sheets. The GO sheets were then coated onto half of the substrate using the spin-casting method, as illustrated in Figure 1a. The average thickness of the coated GO sheets on the substrate was approximately 10 ± 4 nm (∼9 ± 3 layers) (see Figure S2). Across the entire surface of the prepared template, large flakes of 1L-MoS2 were transferred onto the coated GO on the SiO2/Si substrate (1L-MoS2/GO) using the wet-transfer method (see the Methods for details). To examine the optical properties of the 1L-MoS2, PL and Raman spectroscopy measurements were performed at room temperature. A solid-state laser with a wavelength of 532 nm was used as the excitation source, and a spectrometer equipped with three gratings (150 grooves/500 nm blaze, 600 grooves/ 600 nm blaze, and 1800 grooves/500 nm blaze) was utilized for the measurements. Two peaks, A (1.88 eV) and B (2.06 eV), appeared in the PL spectrum, as observed in Figure 1b, which represent the exciton transition corresponding to direct optical transitions from the near band edge.9,23 In the Raman spectrum, peaks appeared at 385 (E12g) and 403 cm−1 (A1g), as shown in Figure 1c. The distance between the A1g and E12g peaks (Δ = A1g − E12g) was approximately 18 cm−1, which is characteristic of 1L-MoS2.24 To verify the presence of the functional groups of GO, Fourier-transform infrared (FTIR) spectroscopy measurements were performed at room temperature as depicted in Figure 1d. The observed functional groups are the well-known electron-withdrawing groups such as phenol, carboxyl, hydroxyl, and epoxy groups.25 The density of negative charge in 1L-MoS2 was much higher than that in GO because of the strong n-type nature of MoS2, as confirmed by the zeta potential measurements (see Figure S3). This finding suggests that the electrons in 1L-MoS2 can be attracted to the GO sheets. The presence of GO was verified by Raman modes of the D band (1347 cm−1) and G band (1600 cm−1) in the Raman spectrum, as observed in Figure 1e. 10448

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Figure 3. Variations in the optical properties of 1L-MoS2 with different GO thicknesses: (a) PL spectra, (b) integrated PL intensity (left) and A exciton peak energy (right), (c) fwhm of the PL emission peak, (d) Raman spectra, (e) E12g mode, and (f) A1g mode.

electrons.38−40 Figure 2d presents the integrated intensity of the biexciton (XX) and neutral exciton (X) peaks of 1L-MoS2/ GO. The intensities of the neutral exciton (X), biexciton (XX), and trion (X*) peaks increased with increasing excitation power (see Figure S5). Because the exciton−exciton annihilation and/or charge screening effect are suppressed by the reduction of excess free electrons,15,33,41 the intensity of the neutral exciton (X) is increased with increasing excitation power. For 1L-MoS2/GO, the PL intensity of the neutral exciton is dominant in the entire range of incident laser powers. Furthermore, similarly increased PL intensities of the biexciton (XX) and trion (X*) are attributed to reduced excess free electrons, which result in contact with the electron-withdrawing group of GO.12,21 In addition, we also performed confocal Raman spectroscopy on the 1L-MoS2 and 1L-MoS2/GO samples. The measured Raman spectra of 1L-MoS2 and 1L-MoS2/GO are represented as scatter plots with blue and red circles, respectively, in Figure 2e. The solid and dotted lines represent the fitted spectra obtained using the Lorentzian function. The A1g peak in the Raman spectrum of 1L-MoS2/GO was blue-shifted by ∼1.9 cm−1 compared with that of 1L-MoS2. The full-width at halfmaximum (fwhm) values for the A1g modes of 1L-MoS2 and 1L-MoS2/GO were 5.5 and 3.4 cm−1, respectively, and the Raman peak for the E12g mode of 1L-MoS2/GO was almost unchanged within spectral resolution compared with that of 1LMoS2. A previous report indicated that the A1g phonon mode is remarkably sensitive to coupling with electrons and the bonding strength.42 Therefore, loss of the excess electrons in the GO-coated 1L-MoS2 yields reduced electron−phonon coupling, resulting in a narrower fwhm and stronger bonds and, thus, the blueshift. Hence, the shifted electronic properties of the 1L-MoS2 owing to the contact with the GO sheets are attributed to electron donation from the 1L-MoS2 to the GO sheets induced by the electron-withdrawing groups of the GO. The process of electron withdrawal between 1L-MoS2 and GO is schematically illustrated in Figure 2f. In other words, the considerable change in the PL/Raman spectra can be attributed to the withdrawal of electrons from the 1L-MoS2 to the GO.

To verify the effect of the GO thickness in 1L-MoS2/GO, PL and Raman spectra were obtained as a function of the GO thickness (Figure 3a). The thickness of the GO sheets was controlled by modifying the number of coating sequences performed to form the GO sheets on the substrate (see Figure S2). Since thickness fluctuations of GO sheets are considerably higher due to a simple coating process, three representative thicknesses of 10 ± 4, 15 ± 5, and 50 ± 10 nm have been applied in this study. Notably, we have used the average data points of PL and Raman spectra for each sample because of the slightly inhomogeneous distribution of optical properties of 1LMoS2/GO. As revealed in Figure 3a, the PL peak position and spectral shape of the A exciton changed markedly as the GO thickness was varied. Figure 3b presents plots of the integrated PL intensities (left) and peak positions (right) of the A exciton measured for 1L-MoS2/GO with various GO thicknesses. As the thickness of the GO was increased, the PL intensity of 1LMoS2/GO gradually decreased, but, still higher than that of the pristine 1L-MoS2 up to 50 nm of GO. This finding contradicts that from a previous report,15 which stated that PL intensity increased upon increasing the doping step. According to another previous report, as the tensile strain in 1L-MoS2 increased, the PL intensity decreased and the PL peak position was red-shifted.43 Using atomic force microscopy (AFM) (see Figure S6), we observed that the surface roughness and topographical fluctuation of the GO sheets increased upon increasing the thickness of the GO sheets. Therefore, the decrease in the PL intensity could be explained by higher strain of 1L-MoS2 arising from the rough surface of the coated GO. The fwhm values of PL emission in 1L-MoS2 with different GO thicknesses are presented in Figure 3c. As observed in Figure 2b, the spectral shape of PL emission in 1L-MoS2 becomes sharper after GO doping because of the affected neutral exciton peak and suppressed biexciton peak. However, the fwhm of the PL emission in 1L-MoS2/GO slightly increased when the thickness of GO was increased, which could reflect a decrease in the doping effect (see Figure S8). Such a reduction in the electron-withdrawing phenomenon with increasing coating thickness is supported by the Raman 10449

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ACS Nano spectra presented in Figure 3d. Here, the peak position and fwhm of the E12g and A1g modes in the Raman spectra were changed by varying the thickness of the coated GO sheets. The E12g mode of 1L-MoS2 was red-shifted with increased fwhm by increasing the thickness of the coated GO sheets (Figure 3e). In contrast, the A1g mode of 1L-MoS2 was blue-shifted with decreased fwhm through contact with the GO sheets; the amount of blueshift was almost the same with increasing thickness of the GO sheet (Figure 3f). Note that the increased fwhm of both E12g and A1g modes indicates that tensile strain of 1L-MoS2 is increased.44 These findings indicate that the thickness of the coated GO sheets affects the strain of 1LMoS2 because the E12g mode is highly sensitive to the strain level.42,43 Consequently, changes in the PL and Raman spectra as a function of the GO thickness could be induced by a change in the strain effect between the 1L-MoS2 and GO sheets with different thicknesses. In addition, PL and Raman data of 1LMoS2 on 1L-GO show a similar tendency as that of 1L-MoS2 on multilayer GO, even though PL enhancement of 1L-MoS2 on 1L-GO is lower than that of 1L-MoS2 on 10 nm-thick GO, which is attributed to the lower number of functional groups of GO (see Figures S7 and S11). To clarify the dominant reason for the dramatic changes in the optical properties of 1L-MoS2 resulting from coating with GO, we performed confocal PL and Raman mapping of the samples. Figure 4a presents a confocal PL intensity image of 1L-MoS2/GO that is in accordance with Figure 2a. The inset in Figure 4a presents an optical microscopy image of the sample; the white dotted line indicates the edge of the GO region. As observed in Figure 4a, the 1L-MoS2/GO region has a greater PL intensity with higher intensity fluctuations compared with the1L-MoS2 region without GO. The higher PL intensity appearing in a broad area near the GO may be attributed to the locally strain-relaxed areas of 1L-MoS2 induced by partial floating because of the slight height difference between the GO and substrate.45 Figure 4b presents PL spectra extracted from the points marked I and II in Figure 4a, which are representative positions for 1L-MoS2 and 1L-MoS2/GO, respectively. Figure 4c presents a Raman intensity image for the wavenumber range of 370−420 cm−1. The Raman scattering intensity of 1L-MoS2/GO is slightly higher than that of 1L-MoS2 without GO. Figure 4d presents local Raman spectra obtained from points I and II in Figure 4c. The black and red curves represent 1L-MoS2 and 1L-MoS2/GO, respectively. The E12g peak in the Raman spectrum of 1LMoS2/GO was red-shifted by ∼1.1 cm−1 compared with that of 1L-MoS2. The A1g peak of 1L-MoS2/GO was blue-shifted by ∼1.5 cm−1 compared with that of 1L-MoS2. The fwhm for the A1g modes of 1L-MoS2 and 1L-MoS2/GO were 11.2 and 5.3 cm−1, respectively. The red-shift of the E12g mode reflects the effect of interfacial strain resulting in increased tensile strain of 1L-MoS2.44 The blue-shift and decreased fwhm of the A1g mode indicate p-doping of 1L-MoS2 and/or an increase in van der Waals interactions at the interface between GO and 1L-MoS246 (see Figure S9). To verify the presence of GO sheets under the 1L-MoS2, confocal Raman measurements were conducted in the wavenumber range from 1000 to 1900 cm−1, which includes the D and G Raman modes of GO. The confocal Raman intensity image for the D and G modes of GO is presented in Figure 4e. The white dotted line represents an edge of the underlying GO layer. For comparison of the strain effect in 1L-MoS2, GO sheets were coated on 1L-MoS2 (see Figure S10). The PL and Raman

Figure 4. Spatially resolved optical properties of 1L-MoS2/GO. (a) Confocal PL intensity image of 1L-MoS2/GO. The inset is an optical image of 1L-MoS2/GO used for the measurements. (b) Local PL spectra obtained from points I and II in (a). (c) Confocal Raman intensity images of 1L-MoS2/GO for 370−420 cm−1 corresponding to the MoS2 and (d) the local Raman spectra acquired from points I and II in (c). (e) Confocal Raman intensity images of 1L-MoS2/GO for 1000−1900 cm−1 corresponding to the GO Raman modes and (f) local Raman spectra acquired from points I and II in the confocal Raman intensity image.

enhancement with peak shift of 1L-MoS2 under GO exhibit similar tendencies as those of 1L-MoS2 on GO (see Figure S10). Note that 1L-MoS2 under the GO exhibited higher strain than 1L-MoS2 on GO because 1L-MoS2 was sandwiched by GO and SiO2. Figure 4f presents the local Raman spectra at points I and II in Figure 4e. The Raman peaks at 1311 and 1570 cm−1 correspond to the D and G modes of GO, respectively. The confocal Raman spectroscopy results for GO confirm that the 1L-MoS2 is placed across the edge of GO. Consequently, we can conclude that the p-doping effect dominates the enhancement of the PL intensity of 1L-MoS2 with GO even though the 1L-MoS2 is under tensile strain. Furthermore, to clarify that the electron withdrawing characteristic of GO affected to PL enhancement of 1L-MoS2, we functionalized GO sheets with diverse oxygen functional groups. The PL measurements revealed higher PL enhancement of 1L-MoS2 upon increasing the density of functional groups in GO (see Figure S11). This result indicates that the pdoping effect of 1L-MoS2/GO mainly arose by electron withdrawal from 1L-MoS2 to GO. To confirm that a reduced electron concentration in 1LMoS2 results from coating with GO, the electrical transport 10450

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voltage of approximately 11 V under forward bias. This rectification of current is attributed to the relatively lower electron concentration of 1L-MoS2/GO compared with that of 1L-MoS2. Electrons cannot flow through this structure when reverse bias is applied because of the increased built-in potential that is formed at the interface between 1L-MoS2 and 1L-MoS2/ GO. In contrast, electrons can easily flow through this structure under forward bias because of the decreased built-in potential. This clear current rectification phenomenon can be regarded as a typical p−n junction diode characteristic. Since the goldcoated tungsten tip was used for electrical measurement, a Schottky barrier (SB) is formed at the interface between the metal tip and 1L-MoS2. To confirm the influence of SB on current rectification, we measured the I−V curve of 1L-MoS2− 1L-MoS2 structure (see SI, Figure S12). In the I−V curve, the current rectification was not observed. This indicates that the current rectification is mostly induced by the lateral 1L-MoS2− 1L-MoS2/GO homojunction diode structure. Consequently, the electrical transport between 1L-MoS2/GO and 1L-MoS2 supports the idea that the adsorption of GO modulates the electronic property of 1L-MoS2.

CONCLUSIONS In this work, we have demonstrated p-type doping of 1L-MoS2 through contact with the GO sheets. Upon decreasing the thickness of the GO sheets, increased PL intensity and shifted A1g peak positions in the Raman spectra were clearly observed and served as evidence of the tuned electronic properties of the 1L-MoS2. The tuning of the electronic properties of 1L-MoS2 by coating with GO can be explained by the abundant electronwithdrawing characteristics from various oxygen functional groups presented on the GO sheets. Using confocal PL/Raman spectroscopy, we observed that the p-doping effect was dominant in 1L-MoS2 with GO through the enhancement of the PL intensity of neutral exciton and blue-shifted A1g Raman peak. Moreover, the electrical transport characteristics of a GO/ MoS2 FET demonstrated that a simple GO coating on 1LMoS2 significantly reduces the electron concentration and slightly increases the hole concentration of the 1L-MoS2. Our technique for tuning the optical and electronic properties of 1LMoS2 by coating with GO sheets offers remarkable advantages for optoelectronic applications; the process is simple and low cost, and it allows large-scale fabrication and modulation of the electronic properties.

Figure 5. Electrical transport characteristics of 1L-MoS2 and GO/ 1L-MoS2 at room temperature. (a) Optical micrograph of a 1LMoS2 device and schematic of the back-gated FET of 1L-MoS2. (b) ID−VGS curves under VDS = 1 − V of the as-prepared 1L-MoS2 and GO/1L-MoS2 sample. (c) I−V characteristics of the lateral 1LMoS2−1L-MoS2/GO structure. The inset shows I−V curve in the log scale with a 3D schematic.

schematic illustration of the 1L-MoS2 FET device and the Cr/ Au (10 nm/70 nm) used as the source and drain electrodes. Figure 5b presents the source−drain current (IDS) versus gate voltage transfer curves of 1L-MoS2 before and after GO coating. The filled black squares and red circles represent I−V curves of 1L-MoS2 and GO-coated 1L-MoS2, respectively. A substantially lower threshold voltage with remarkably lower current level was observed for the 1L-MoS2 FET with GO compared with the 1L-MoS2 FET without GO. This shows that electron concentration of 1L-MoS2 with GO is significantly decreased and hole concentration is meaningfully increased compared with that of 1L-MoS2 without GO, as shown in Figure 5b. Therefore, this supports the idea that the functional groups of GO induce the withdrawing the electrons from MoS2 resulting in p-type doping. Figure 5c shows an I−V curve of the lateral 1L-MoS2−1LMoS2/GO structure with voltage sweep range from −20 to +20 V. The inset is a log scale I−V curve and schematic of the lateral 1L-MoS2−1L-MoS2/GO structure designed for the I−V measurement. The distance between the anode and cathode was maintained at two metal probes maintained at 500 μm. As observed in the I−V curve, a considerably lower current close to zero was observed under reverse bias; however, a dramatically increased current was observed from the threshold

METHODS Synthesis of Graphene Oxide. GO sheets were prepared using a modified Hummers’ method. Natural graphite was obtained from Alfa Aesar (99.999% purity, 200 mesh). First, 5 g of graphite and 350 mL of 10 M H2SO4 were blended. Subsequently, KMnO4 (15 g) was slowly added over approximately 1 h and stirred continuously for 2 h in a cooled water bath. After the mixture had been strongly stirred for 3 days at room temperature, deionized (DI) water was added, and stirring proceeded for 10 min in a cooled water bath. The mixture was then stirred for an additional 2 h at room temperature after the addition of an aqueous solution of H2O2 (30 wt%). A diluted HCl (35 wt%) solution was then added and stirred for 30 min at room temperature. After the supernatant solution was decanted, DI water was slowly added and stirred for 30 min. Then, 1 g/L of the graphite oxide solution in water was sonicated for 1 h to exfoliate the GO sheets. To obtain dispersed GO, centrifugation at 10,000 rpm was performed for 1 h, and then the supernatant solution was decanted. Wet Transfer of 1L-MoS2. The wet-transfer process used in this study was conducted as follows: PMMA C4 (Micro Chem, 4 wt% in 10451

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ACS Nano chlorobenzene) was coated onto large flakes of 1L-MoS2 grown on a SiO2/Si substrate to serve as a supporting layer for the transfer process. After being coated with PMMA, the sample was floated on a 1 M potassium hydroxide (KOH) solution at 80 °C for 15 min to remove the SiO2 layer. Subsequently, only large flakes of 1L-MoS2 with PMMA remained on the KOH solution. The remaining PMMA/ 1L-MoS2 was washed with DI water to remove any residual KOH etchant. The washed PMMA/1L-MoS2 was transferred onto the prepared GO-coated template and SiO2/Si substrate described in the main text. After the drying process was complete, the PMMA was removed using acetone. Device Fabrication. The source and drain electrodes were patterned using electron-beam lithography followed by Cr/Au (10 nm/70 nm) deposition using e-beam/thermal evaporation. After the lift-off process, I−V measurements were performed using a Keithley 4200 probe station at room temperature. Characterization Methods. FTIR spectra were recorded using an FTIR spectrometer (4700, JASCO). UV−vis absorption spectra were measured using a commercial spectrophotometer (V-670, JASCO). Raman and PL spectra were obtained using a multifunctional optical microscopy system (NTEGRA SPECTRA, NT-MDT). In this system, a 532 nm (2.33 eV) laser was used as the excitation source. To measure the electric potentials of GO and 1L-MoS2 in solution, zeta potential analysis (ELS-8000, Otsuka Electronics) was performed. The topography of the sample surfaces was examined using a commercial atomic force microscope in the contact mode (Anasys Instruments). Field-emission scanning electron microscopy (SEM, JSM7000F, JEOL) was performed to examine the surface morphology of the samples.

18). K.K.K. acknowledges the support from Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science.

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ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.6b06319. Scanning electron microscopy images of 1L-MoS2 flakes, optical microscopy images of 1L-MoS2 alone and 1LMoS2 on GO, zeta potential spectra of GO and 1L-MoS2, PL spectra fitting 1L-MoS2 at various excitation laser powers, PL spectra fitting of 1L-MoS2/GO at various excitation laser powers, atomic force microscopy images of GO, optical properties of 1L-MoS2/1L-GO, PL spectra fitting, spatially resolved optical properties of 1L-MoS2/GO, spatially resolved optical properties of GO/1L-MoS2, and PL enhancement factor analysis, and I−V curve of 1L-MoS2−1L-MoS2 (PDF)

AUTHOR INFORMATION Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Author Contributions $

H.M.O. and H.J. contributed equally.

Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was supported by IBS-R011-D1 of Korea, the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (2016R1A2B2015581), the Human Resources Development program (No. 20124010203270) of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korea government Ministry of Trade, Industry and Energy, and KERI Primary Research Program of MSIP/NST (No. 16-12-N010110452

DOI: 10.1021/acsnano.6b06319 ACS Nano 2016, 10, 10446−10453

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ACS Nano

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DOI: 10.1021/acsnano.6b06319 ACS Nano 2016, 10, 10446−10453