Large-Area and Strain-Reduced Two-Dimensional Molybdenum

Research Article Cite This: ACS Appl. Mater. Interfaces 2019, 11, 26243−26249

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Large-Area and Strain-Reduced Two-Dimensional Molybdenum Disulfide Monolayer Emitters on a Three-Dimensional Substrate Chiao-Yun Chang,† Hsiang-Ting Lin,† Ming-Sheng Lai,†,‡ Cheng-Li Yu,† Chong-Rong Wu,†,§ He-Chun Chou,† Shih-Yen Lin,†,§ Chi Chen,† and Min-Hsiung Shih*,†,‡,∥ †

Research Center for Applied Sciences, Academia Sinica, Taipei 11529, Taiwan Department of Photonics and Institute of Electro-Optical Engineering, National Chiao-Tung University, Hsinchu 30010, Taiwan § Graduate Institute of Electronics Engineering, National Taiwan University, Taipei 10617, Taiwan ∥ Department of Photonics, National Sun Yat-sen University, Kaohsiung 80424, Taiwan

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‡

S Supporting Information *

ABSTRACT: Atomically thin membranes of two-dimensional (2-D) transition-metal dichalcogenides (TMDCs) have distinct emission properties, which can be utilized for realizing ultrathin optoelectronic integrated systems in the future. Growing a large-area and strain-reduced monolayer 2-D material on a three-dimensional (3-D) substrate with microstructures or nanostructures is a crucial technique because the electronic band structure of TMDC atomic layers is strongly affected by the number of stacked layers and strain. In this study, a large-area and strain-reduced MoS2 monolayer was fabricated on a 3D substrate through a two-step growth procedure. The material characteristics and optical properties of monolayer TMDCs fabricated on the nonplanar substrate were examined. The growth of monolayer MoS2 on a cone-shaped sapphire substrate effectively reduced the tensile strain induced by the substrate by decreasing the thermal expansion mismatch between the 2-D material and the substrate. Monolayer MoS2 grown on the nonplanar substrate exhibited uniform strain reduction and luminescence intensity. The fabrication of monolayer MoS2 on a nonplanar substrate increased the light extraction efficiency. In the future, large-area and strain-reduced 2-D TMDC materials grown on a nonplanar substrate can be employed as novel light-emitting devices for applications in lighting, communication, and displays for the development of ultrathin optoelectronic integrated systems. KEYWORDS: two-dimensional (2-D) material, transition metal dichalcogenides (TMDCs), molybdenum disulfide (MoS2), light Emitter, three-dimensional (3-D) substrate

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INTRODUCTION Monolayer two-dimensional (2-D) transition-metal dichalcogenides (TMDCs) are favorable for realizing ultrathin photon emitters because of their direct electronic band gaps, subnanometer thickness, and distinct optical properties.1,2 Because atomically thin membranes of TMDCs can be optimally shaped on a nonplanar substrate with nanoparticles or microcavities,3−6 a large-area monolayer 2-D material on a three-dimensional (3-D) substrate with microstructures or nanostructures can be favorably used for the development of ultrathin optoelectronic integrated systems in the future.7−11 However, when monolayer TMDCs are transferred on a nonplanar substrate with nanostructures, a large local strain is generated, and the occurrence of wrinkles and deformation in the 2-D material cannot be avoided.12−15 In-plane strain affects the electronic energy band of monolayer TMDCs.16−19 Monolayer TMDCs are subjected to in-plane tensile stress, which changes the lattice constant.20 Thus, the original direct energy gap of monolayer TMDCs is transformed to an indirect energy gap, and the luminescence characteristics are affected.17,18 For example, the photoluminescence (PL) intensity of monolayer molybdenum disulfide (MoS2) was reduced by 50% after applying 1% strain, and the red shift in © 2019 American Chemical Society

the peak energy of monolayer MoS2 was approximately 50 meV per percent of the tensile strain.17,21 The direct growth of TMDCs completely covers the 3-D substrate; however, controlling a single layer is difficult. The energy bands of MoS2 can be converted from a direct energy gap to an indirect energy gap by increasing the number of layers because the d orbital of an Mo atom has a strong coupling effect with the Pz orbital of an S atom, and an indirect energy gap is formed at the Γ point.22,23 By using one layer of MoS2 instead of multiple layers, the mixed orbital coupling effect is weakened, which transforms an indirect energy gap into a direct energy gap at the K point and enhances the emission efficiency.24 2-D TMDC atomic layers fabricated on a 3-D substrate with microstructures or nanostructures could be crucial for fabricating large-area monolayer TMDC materials and preventing the formation of large local strains. The designed microstructure or nanostructure substrates may enhance the light extraction efficiency of the emission from TMDC atomic layers. Because the electronic band structure of a TMDC Received: March 21, 2019 Accepted: June 25, 2019 Published: June 25, 2019 26243

DOI: 10.1021/acsami.9b05082 ACS Appl. Mater. Interfaces 2019, 11, 26243−26249

Research Article

ACS Applied Materials & Interfaces

procedure.36,37 In the future, a large-area and strain-reduced monolayer 2D material can be directly grown on a nonplanar substrate with a 3-D structure to design novel high-performance light-emitting devices for applications in lighting, light sources, communication, and displays.

atomic layer is strongly affected by the number of stacked layers and by strain,20,22,23 the growth of unstrained semiconductor materials is crucial. The residual thermal strain of III−V nitride-based bulk materials can be attributed to the mismatch between the thermal expansion coefficients of GaN and the substrate.25 The use of a patterned sapphire substrate (PSS) or surface patterning can release the thermal strain of GaN and increase the light extraction efficiency.26−28 Similarly, the growth of monolayer TMDC materials generates substrateinduced strain.29,30 The primary reason for the generation of substrate-induced strain is the mismatch between the thermal expansion or reduction of the monolayer MoS2 crystal lattice and the substrate during the temperature cooling procedure.31,32 The strain distribution of monolayer TMDCs, which are directly placed on a 3-D substrate, is not completely understood. Figure 1a,b illustrates the schematic of the

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RESULTS AND DISCUSSION Figure 1c,d displays the SEM images and cross-sectional views of the TEM images of the PSS. The height (H), diameter (D), and period (P) of the hexagonal lattice of the microcone of the PSS were approximately 1.56, 2.6, and 3 μm, respectively. The slant angle of the PSS cone was approximately 60°. Figure 2

Figure 2. Growth characteristics of the monolayer MoS2 on the PSS: (a) schematic of the thickness distribution and a 30°-tilted SEM image of the Mo deposited on the PSS, (b) TEM image of the Mo grown on the PSS cone, (c) TEM image of the Mo grown on the flat surface of the PSS, (d) schematic of the flow of MoO3 during the heating procedure at temperatures higher than the melting point, (e) TEM image of the monolayer MoS2 grown on the PSS cone after sulfurization, and (f) TEM image of the bilayer MoS2 grown on the flat surface of the PSS after sulfurization.

Figure 1. Thermal strain and light extraction of the monolayer MoS2 on the planar and nonplanar substrates. The increased light extraction and decreased tensile strain of the MoS2 fabricated on the (a) FSS and (b) PSS. (c) The top-view scanning electron microscopy (SEM) image of the PSS. (d) The cross-section of the transmission electron microscopy (TEM) images of the PSS.

illustrates the structural characterization of the as-grown MoS2 on the PSS cone through TEM measurement. Supporting Information S1 provides details of the structural characterization. Figure 2a depicts the schematic of the thickness distribution of the Mo film on the PSS and the SEM image of the metal deposited on the PSS. The six-pointed star of the hexagonal lattice arrays of the Mo film can be observed on the flat bottom of the substrate. The thickness of the Mo film on the PSS was measured through TEM (Figure 2b,c). The average thicknesses of the MoS2 fabricated on the side of the cone, bottom edge of the cone, and flat bottom of the PSS were 2, 0.2, and 1 nm, respectively. The thickness of the Mo film deposited on the side of the cone was larger than that of the Mo film on the flat bottom of the PSS. After depositing the Mo films, Mo came into contact with air to form naturally oxidized MoO3 on the PSS. Figure 2d displays the schematic of the flow of MoO3 during heating, and Figure 2e,f shows the number of MoS2 layers on the PSS. During sulfurization, the competition between the mechanisms of Mo oxide segregation, which resulted in the formation of small clusters, and the sulfurization reaction, which resulted in the formation of a planar MoS2 film, was analyzed using the amount of background sulfur.36 When the temperature is higher than 600 °C,38,39 MoO3 segregation toward the flat bottom of the PSS, which is induced by gravity, may occur on the inclined surface.36 Therefore, after sulfurization, the MoO3 on the PSS was sulfurized to form monolayer MoS2 on the sapphire cone and bilayer MoS2 on the flat bottom of the PSS (Figure 2e,f). The number of MoS2 layers on the flat substrate was proportional to the thickness of the deposited Mo.36,40 The

monolayer MoS2 fabricated on a flat sapphire substrate (FSS) and a PSS under substrate-induced strain. The monolayer MoS2 fabricated on the c-plane of the FSS had an approximate strain of 0.2%.33 The tensile strain of MoS2 changed the electronic band structure, which caused a red shift of the PL peak. Furthermore, the thermal expansion coefficients along the a-axis and c-axis of the sapphire substrate were 7.28 × 10−6 and 8.11 × 10−6 °C−1 (at 25−800 °C), respectively.34 Because the thermal expansion coefficient along the c-axis was higher than that along the a-axis, the growth of monolayer MoS2 on the inclined plane of the c-plane sapphire substrate could reduce the residual tensile strain. Moreover, monolayer MoS2 had a high refractive index (n = 4.49),35 and the refractive index of the sapphire substrate (n = 1.7) was higher than that of air (n = 1). The refractive indices of the monolayer MoS2 and the substrate were higher than the refractive index of the surrounding environment. Partial emission light was easily trapped in the substrate. The monolayer MoS2 fabricated on the PSS with microstructures increased light scattering to enhance the light extraction efficiency. In this study, the optical and physical properties of monolayer MoS2 grown on a 3-D substrate were examined to develop a large-area and strain-reduced monolayer MoS2 emitter with a 3-D structure. To understand the substrateinduced strain in MoS2 atomic layers on the planar and nonplanar substrates, commercial FSS and PSS were employed to fabricate the monolayer MoS2 through a two-step 26244


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Figure 3. Optical properties of the MoS2 on the FSS and PSS: (a) Raman spectra of the monolayer MoS2 grown on the FSS and PSS, (b) PL spectra of the monolayer MoS2 grown on the c-plane of the FSS and PSS, (c) Raman peaks of the E12g and A1g modes of the MoS2 grown on the cplanes of the FSS and PSS, (d) strain and PL peak energy of the monolayer MoS2 grown on the FSS and PSS, (e) simulated light emission and light extraction distribution for the monolayer MoS2 grown on the FSS, and (f) simulated light emission and light extraction distribution for the monolayer MoS2 grown on the PSS.

significant red shift compared with the Raman peaks of the unstrained MoS2. The variation in the peak shift of the E12g mode was at least four times higher than that of the A1g mode. The Raman signal of the monolayer MoS2 shifted with respect to the doping concentration, degree of strain, and number of layers. Because the MoS2 monolayers were grown on the two substrates under the same growth conditions, doping effects caused by the Raman peak shift were neglected. The red shift in the E12g mode was more sensitive to the in-plane strain than that in the A1g mode. Thus, the Raman red shift of the monolayer MoS2 could be attributed to the substrate-induced tensile strain. Figure 3d indicates that the tensile strains of the monolayer MoS 2 grown on the FSS and PSS were approximately 0.18 and 0.03%, respectively. The PL intensities of the monolayer MoS2 grown on the planar and nonplanar substrates were measured through continuous-wave 532 nm laser excitation (Figure 3b). The PL peaks of the monolayer MoS2 grown on the PSS exhibited a significant blue shift with a decrease in the tensile strain. The crystal lattice of the monolayer MoS2 was affected by the strain effect, which changed the electronic band structure. The energy gap of the monolayer MoS2 increased with decreasing tensile strain because of the transformation of the indirect energy gap into a direct energy gap at the K point. The exciton A emission of the unstrained monolayer MoS2 was approximately in a range of 1.88−1.89 eV,16,17,24,43 and the emission energy of exciton A of the monolayer MoS2 was red-shifted by approximately 50 meV per percent of the tensile strain.12,17 The emission energy of exciton A of the monolayer MoS2 grown on the nonplanar PSS was blue-shifted by approximately 18 meV compared with the emission energy of the monolayer MoS2 grown on the FSS. The growth of monolayer MoS2 on the nonplanar substrate effectively decreased the substrate-induced tensile strain. The PL intensity of the monolayer MoS2 grown on the PSS increased by a factor of 2.98. According to previous reports, the PL intensity of monolayer MoS2 decreases by 50% under a strain of 1%.17 Thus, the PL intensity of the monolayer MoS2 with a strain of 0.18% changed by approximately 9%. The variation in the PL intensity for the monolayer MoS2 grown on different substrates was not primarily influenced by the

thickness of MoO3 on the inclined surface of the sapphire cone was decreased to grow the monolayer MoS2 film on the PSS. Because MoO3 on the PSS could experience a flow situation during the growth process, the MoS2 monolayers were simultaneously grown on the c-planes of the FSS and PSS. The areas of MoS2 growth on the FSS and PSS were larger than 1.5 × 1.5 cm2. Supporting Information S2 presents the photographs of the large-area monolayer MoS2 on the two substrates. The strain effects of the monolayer MoS2 developed on different substrates were characterized using Raman spectrum analysis. Figure 3a displays the Raman spectrum of the monolayer MoS2 grown on the FSS and PSS. The Raman spectrum of the MoS2 on the sapphire substrate exhibited three 1 distinct peaks, including peaks at the in-plane of the E2g vibrational mode and out-plane of the A1g vibrational mode of the monolayer MoS2. The Raman peak at 417 cm−1 was attributed to the Raman signal of sapphire.41 A previous study specified that the E12g and A1g Raman shift peaks of unstrained MoS2 were observed at approximately 385.6 and 405.3 cm−1, respectively.33 The E12g and A1g Raman shift peaks of the MoS2 developed on the two substrates were obtained using the Lorentzian function to fit the Raman spectrum. Figure 3c illustrates the E12g and A1g modes of the Raman shift peak signal of the monolayer MoS2 grown on different substrates. The strain values of the monolayer MoS2 grown on different substrates were estimated by combining the Raman shift values of the E12g and A1g modes (Figure 3d).33,42 Figure 3b displays the PL spectrum of the monolayer MoS2 grown on the FSS and PSS. Moreover, the strained MoS2 affected the changes in the energy gap. The energy gap changes in the monolayer MoS2 grown on the two substrates with an increasing tensile strain were analyzed using the PL spectrum of the monolayer MoS2. The PL spectrum had two primary PL peaks. The dominant emission of MoS2 from neutral exciton A occurred at approximately 660 nm (1.88 eV). The PL peak of the monolayer MoS2 grown on different substrates was fitted using the Lorentzian function (Figure 3d). Figure 3c indicates that the Raman peaks of the E12g and A1g modes of the monolayer MoS2 grown on the FSS underwent 26245


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4b). After statistical analysis, the Raman shift peak of the E12g vibrational mode of the monolayer MoS2 on the PSS cone was approximately at 385.4 ± 0.15 cm−1, and the substrate-induced strain was 0.04 ± 0.019%. The Raman distribution of the E12g mode of the monolayer MoS2 grown on the PSS cone was reasonably uniform, whereas the value of E12g in the monolayer MoS2 grown on the bottom plane of the PSS significantly decreased. The number of MoS2 layers was determined using the Raman signal difference (Δω = A1g − E12g). Figure 4c displays the Δω mapping of the monolayer MoS2 grown on the PSS. The Δω mapping values of the MoS2 grown on the conical surface and flat bottom of the PSS were approximately 19.9 and 21.3 cm−1, respectively. These values indicated that monolayer MoS2 was deposited on the PSS cone, and bilayer MoS2 was deposited on the flat bottom of the PSS.43 The sixpointed star of hexagonal lattice arrays of the bilayer MoS2 was observed on the flat bottom of the substrate. The Δω mapping analysis of the number of layers was consistent with the TEM results. The full width at half-maximum (fwhm) of the Raman and PL spectra for the monolayer MoS2 on the PSS was narrower than that for the monolayer MoS2 on the FSS, which exhibited improved crystal quality. Supporting Information S3 provides the details of the fwhm of the Raman and PL spectra for the monolayer MoS2 on different substrates. Figure 4d depicts the schematic of the change in the lattice constant of the monolayer MoS2 grown on the inclined surface of the PSS cone when decreasing the temperature from 750 to 25 °C. The monolayer MoS2 grown on the PSS cone had a low tensile strain, due to which the substrate-induced strain of the monolayer MoS2 decreased after cooling to room temperature. The terms XAl2O3750 and XAl2O325 represent the radii of the sapphire cone at 750 and 25 °C, respectively; YAl2O3750 and YAl2O325 represent the heights of the sapphire cone at 750 and 25 °C, respectively; and θ indicates the angle of inclination of the cone at 25 °C. The thermal strains of the monolayer 2-D material grown on the FSS and the conical surface of the PSS were estimated. The simulated strain of the monolayer MoS2 grown on the PSS was smaller than that of the monolayer MoS2 grown on the FSS. This observation indicates that compared with the MoS2 grown on the FSS, the monolayer MoS2 on the PSS had a lower thermal expansion coefficient mismatch between the 2D material and the inclined surface of the cone. The thermal expansion efficiencies of the sapphire substrate along the a-axis and c-axis were different. The thermal strain of monolayer MoS2 with varying angles of inclination of the PSS cone (θ) could be calculated using a simple physical model. A θ value of 0 is parallel to the MoS2 grown on the c-plane FSS. The strain of monolayer MoS2 decreased due to the reduction of the thermal expansion mismatch. Supporting Information S4 presents the details of the simple physical model for calculating the thermal strain variation. To validate the uniformity of the PL peak intensity of the monolayer MoS2 grown on the PSS, depth-dependent PL mapping of MoS2 was performed by varying the focal plane. Figure 5a illustrates the TEM image of the PSS. The PL peak intensity mapping image of the monolayer MoS2 was obtained by varying the focal plane from z = 1.5 to 0 μm (Figure 5b−e). At the bottom of the sapphire cone, z = 0. When the focal plane was at z = 1.5 μm, the PL intensity distribution was concentrated at the top of the cone. The highest PL intensity gradually shifted downward when the height of the focal plane

substrate-induced strain. To understand the enhancement factor of the PL intensity of the monolayer MoS2 grown on the PSS, the light extraction efficiencies of the monolayer MoS2 grown on different substrates were simulated using the finiteelement method. The integrated optical powers of the far fields above different substrates were calculated using the same initial value of luminescence intensity on the monolayer MoS2. Figure 3e,f displays the simulated distribution of the electric field on the FSS and PSS. Most of the light emitted from the FSS device was trapped in the substrate. The light emitted from the PSS device was scattered and refracted because of the structural relationship, which increased the light extraction efficiency. The integrated emission intensities of the monolayer MoS2 grown on the PSS in the far field were 3.3 times higher those of the monolayer MoS2 grown on the FSS. The simulated results were consistent with the experimental results, which indicates that growing monolayer MoS2 on a PSS increases the light extraction efficiency. The physical and optical properties of the monolayer MoS2 grown on the nonplanar substrate were examined through Raman and PL mapping. Figure 4a displays the Raman

Figure 4. Raman and thermal strain analysis of the MoS2 on the PSS: (a) Raman spectra of the MoS2 grown on the top of the cone, side of the cone, and flat bottom of the PSS; (b) Raman mapping of the E12g peak for the monolayer MoS2 grown on the PSS; (c) Raman mapping of Δω for the monolayer MoS2 grown on the PSS; (d) schematic of lattice compression for the monolayer MoS2 grown inclined to the cplane of sapphire when the temperature decreased from 750 °C (growth temperature) to 25 °C (room temperature).

spectrum of the monolayer MoS2 grown on the PSS. The characteristics of the Raman spectrum of the monolayer MoS2 were the same at the top and side of the PSS cone, whereas the Raman peak shifts of the E12g and A1g modes of the monolayer MoS2 at the flat bottom of the PSS were reversed. The displacement of the A1g mode was significantly higher than that of the E12g mode, which is different from previous trends observed in the growth of monolayer MoS2 on different substrates. The A1g mode was more sensitive to changes in the number of layers than the E12g mode was. The Raman shift of the monolayer MoS2 at the flat bottom of the PSS could be attributed to the change in the number of MoS2 layers. The tensile strain distribution of the monolayer MoS2 grown on the PSS was validated through the Raman mapping of E12g (Figure 26246


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Figure 5. Depth-dependent PL measurement of the monolayer MoS2 grown on the PSS. (a) Cross-section of the TEM image of the monolayer MoS2 grown on the PSS. The measured height (z) is relative to the bottom of the cone of the PSS. Points A−F are the labeled positions for PL measurements. PL peak intensity mapping of the monolayer MoS2 grown on the PSS with the focal plane at (b) z = 1.5 μm, (c) z = 1.0 μm, (d) z = 0.5 μm, and (e) z = 0.0 μm. (f) Peak wavelength mapping of the monolayer MoS2 grown on the PSS. (g) PL spectrum of the monolayer MoS2 grown on the PSS with various measured focal planes and positions as marked in Figure 1a. The PL spectrum of A−F exhibits MoS2 emission at various positions on the PSS. The inset figure is the SEM image of the monolayer MoS2 grown on the PSS.

was reduced from z = 1.5 to 0.5 μm. The PL intensity of the monolayer MoS2 at z = 0 μm exhibited a six-pointed star distribution of hexagonal lattice arrays at the flat bottom of the substrate. Gaussian fitting analysis was applied to the PL spectrum at z = 0 μm to obtain the PL peak wavelength mapping image (Figure 5f). The PL peak of the triangular region at the flat bottom of the substrate exhibited a red shift, which was consistent with the Raman results. Figure 5g depicts the PL spectrum of the monolayer MoS2 for different positions on the surface of the PSS (from point A to F in Figure 5a). The inset in Figure 5g displays the SEM image of the monolayer MoS2 grown on the PSS. The PL intensity of MoS2 decreased with the decreasing height of the focal plane from point A to C. The change in the PL intensity for different heights of the cone was limited by the effective light reception range and was uniform. Supporting Information S5 presents the details of the effective light reception. The PL and Raman results indicated an improved crystalline quality for the monolayer MoS2 grown on the PSS. In the future, the growth of monolayer MoS2 on the PSS can be examined with a series of gradual changes in the cone geometry and the crystalline quality can be characterized. Because the migration of MoO3 segregation on the inclined surface of the PSS cone reduced defects and improved the crystalline quality, the growth mechanism of MoS2 on the nonplanar substrate increased the step coverage. Thus, MoS2 can be employed in nonplanar photoelectric devices.

on the FSS. In the future, large-area and strain-reduced 2D materials can be grown on nonplanar sapphire substrates with patterns, such as bevels, pyramids, and grooves, to design novel light-emitting devices for lighting, communication, and display applications.

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METHODS

Growth Procedure of MoS2. Monolayer MoS2 was grown on the c-plane FSS and PSS by using a two-step method. First, the Mo metal layer was deposited on the FSS and PSS by using a radio frequency sputtering system. The operation power, operation pressure, and deposition time were 40 W, 5 × 10−3 Torr, and 30 s, respectively. After deposition, Mo came into contact with air to form naturally oxidized MoO3 on the substrates. The MoO3 on the substrates was then sulfurized using a tube furnace. After Mo deposition, the samples were placed at the center of a furnace tube under a pressure of 5 × 10−3 Torr, and the temperature was increased from 25 to 750 °C in 40 min. The MoO3 on the sapphire substrates was sulfurized at a growth temperature of 750 °C and under a furnace pressure of 0.7 Torr. The thermal evaporation of S powder at 120 °C was placed at the upstream of 130 sccm Ar gas flow. Optical Measurements and Structural Analyses. Raman spectra mapping was performed using a HORIBA Jobin Yvon LabRam HR800 Raman spectroscopy system. For a pumping system with a laser wavelength of 488 nm and a 100× objective (NA = 0.8), the spatial resolution was estimated to be approximately 372 nm by using the Rayleigh criterion. Therefore, the spatial resolution of the Raman spectra was sufficient for resolving the Raman signal distribution of MoS2 on the PSS cone. The PL mapping measurements were performed on a commercial system (WiTec alpha300) by using a laser excitation of 532 nm. The spot diameter of the laser beam was approximately 800 nm with a light-collected objective lens magnification of 100×, a focal length of 0.22 mm, and an NA of 0.9. This system is a high-resolution confocal μ-Raman system for mapping and depth-dependent PL measurements. The electron microscope images of the samples and devices were obtained using the FEI Inspect F SEM system. The TEM images of the MoS2 atomic layers on the sapphire substrates were obtained using a JEM-2800F transmission electron microscope.

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CONCLUSIONS The physical and optical properties of monolayer MoS2 grown on a nonplanar sapphire substrate were examined in this study. The substrate-induced tensile strain of the monolayer MoS2 grown on the PSS dramatically decreased because of the reduction in the thermal expansion coefficient mismatch between the material and the substrate. The monolayer MoS2 grown on a sapphire cone reduced the strain and exhibited uniform PL intensity. Moreover, growing monolayer MoS2 on the PSS increased the light extraction efficiency. The PL intensities of the monolayer MoS2 grown on the PSS were three times higher than those of the monolayer MoS2 grown 26247


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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/acsami.9b05082. Structural characterization of the as-grown MoS2 on the PSS cone; the large-area MoS2 monolayer on the substrates; discussion of the optical properties of the MoS2 monolayer on the FSS and PSS; calculated strain of the monolayer MoS2 on the nonplanar sapphire; and PL mapping measurement for the monolayer MoS2 on the PSS (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Chiao-Yun Chang: 0000-0002-6740-2528 Hsiang-Ting Lin: 0000-0002-4715-8943 Cheng-Li Yu: 0000-0002-7114-3346 Shih-Yen Lin: 0000-0001-7028-481X Min-Hsiung Shih: 0000-0002-5627-7098 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the Innovative Materials and Analytical Technology Exploration (i-MATE) program, Academia Sinica, Taiwan and the Ministry of Science and Technology (MOST), Taiwan, under Contract No. 105-2112M-001-011-MY3.

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REFERENCES

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