Letter pubs.acs.org/NanoLett
Tuning Optical Signatures of Single- and Few-Layer MoS2 by BlownBubble Bulge Straining up to Fracture Rui Yang,† Jaesung Lee,† Souvik Ghosh,‡ Hao Tang,† R. Mohan Sankaran,‡ Christian A. Zorman,† and Philip X.-L. Feng*,† †
Electrical Engineering, ‡Chemical and Biomolecular Engineering, Case School of Engineering, Case Western Reserve University, 10900 Euclid Avenue, Cleveland, Ohio 44106, United States S Supporting Information *
ABSTRACT: Emerging atomic layer semiconducting crystals such as molybdenum disulfide (MoS2) are promising candidates for flexible electronics and strain-tunable devices due to their ultrahigh strain limits (up to ∼20−30%) and strain-tunable bandgaps. However, high strain levels, controllable isotropic and anisotropic biaxial strains in single- and fewlayer MoS2 on device-oriented flexible substrates permitting convenient and fast strain tuning, remain unexplored. Here, we demonstrate a “blown-bubble” bulge technique for efficiently applying large strains to atomic layer MoS2 devices on a flexible substrate. As the strain increases via bulging, we achieve continuous tuning of Raman and photoluminescence (PL) signatures in single- and few-layer MoS2, including splitting of Raman peaks. With proper clamping of the MoS2 crystals, we apply up to ∼9.4% strain in the flexible substrate, which causes a doubly clamped single-layer MoS2 to fracture at 2.2−2.6% strain measured by PL and 2.9−3.5% strain measured by Raman spectroscopy. This study opens new pathways for exploiting 2D semiconductors on stretchable substrates for flexible electronics, mechanical transducers, tunable optoelectronics, and biomedical transducers on curved and bulging surfaces. KEYWORDS: 2D semiconductors, atomic layer MoS2, bulge test, strain tuning, Raman spectroscopy, photoluminescence (PL)
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This biaxial strain, however, was predefined by the substrate and growth, thus may not be readily tuned or controlled once the CVD growth is finished.24 In situ continuous monitoring of Raman and PL characteristics of atomic layer MoS2 crystals under controllable biaxial strain levels on flexible substrates and in device-oriented platforms have not yet been demonstrated, and detailed device characteristics of MoS2 at a large strain until fracture remain to be explored. Bulge measurement is an effective method established for characterizing mechanical properties of thin film materials until their fracture, with a wide range of applications in research and industry. A typical configuration includes creating a differential pressure on two sides of the thin film to bulge it, while monitoring the film’s load-deflection behavior, and then extracting the stress−strain relationship. Starting 1959, bulge measurement has been performed on gold and silver films25 and then on polyimide films,26 where the Young’s modulus, residual stress, and fracturing strain are measured. Later this method has been further developed and extended to other technologically important materials such as thin copper (Cu) films that are key to metallic interconnects in integrated circuits
wo-dimensional (2D) semiconductors such as atomic layers of transition metal dichalcogenides (TMDCs) have excited great interest toward flexible and transparent electronics because of their high mobilities maintained at atomic-scale thickness, exceptional flexibility and stretchability, and sizable bandgaps.1−4 As a forerunner and representative TMDC, molybdenum disulfide (MoS2) is a layered semiconductor with strain-tunable band structure and electron mobility,5−10 making it highly suitable for foldable and stretchable electronics, mechanical transducers, and strain-tunable optoelectronics.11−14 To use MoS2 in flexible devices, a quantitative understanding of its material properties under varying strain until its fracture is necessary. Different methods have already been attempted to strain 2D semiconductors; and among them, uniaxial straining techniques are mainly based on bending of the polymeric substrate as a beam;5−7,12−18 and biaxial straining is either based on piezoelectric effect in the substrate19 or thermal expansion of the substrate.20 The shifts of photoluminescence (PL) peaks,5,6 Raman modes,21,22 and changes in transistor performance,13 as well as nanomechanical resonances23 with strain in MoS2 structures have been characterized. There have also been measurements of Raman and PL peak shifts that reveal biaxial tensile strain caused by the mismatch in thermal expansion coefficients between MoS2 and SiO2/Si substrate during chemical vapor deposition (CVD) growth. © 2017 American Chemical Society
Received: February 19, 2017 Revised: May 15, 2017 Published: June 19, 2017 4568
DOI: 10.1021/acs.nanolett.7b00730 Nano Lett. 2017, 17, 4568−4575
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Figure 1. Experimental setup and representative MoS2 device for blown-bubble bulge measurement. (a) 3D illustration of the experimental setup with gas inlet and outlet (covered by the PDMS) on the pressure chamber and the acrylic holder for fixing the PDMS. (b) Illustration of the cross section of the MoS2 device, showing that the MoS2 is clamped with silicone adhesive and strained on top of the bulging PDMS. (c) Photograph of the bulging PDMS observed during experiment. (d) FEM simulation of the PDMS deformation under a differential gas pressure of 2.4 × 104 Pa (3.5 psi). (e) Optical image of a clamped 2L MoS2 device, with the yellow dashed lines showing the boundary of the clamp and the white dashed lines showing the boundary of the 2L MoS2.
(IC),27,28 as well as silicon nitride (SiNx),29,30 polysilicon (polySi),31 and silicon carbide (SiC) thin films32 that are the major structural materials in both IC and microelectromechanical systems (MEMS). Bulging of polyimide and Cu films have led to fracturing of the materials, both at approximately 4% strain,26,27 while bulge and tensile measurements of poly-Si, SiNx, and SiC have demonstrated fracturing strain levels at 0.7− 1.7%, 0.3−2.3%, and 0.1−0.4%, respectively.30,33−37 Although recently it has been reported that quite high strain can be achieved in fully clamped suspended MoS2 by pressurizing the cavity formed between the MoS2 membrane and the etched substrate,38 this method is stated to rely on very slow gas diffusion through the SiO2 layer to pressurize the cavity, which usually requires days to build up the necessary pressure difference for straining. Therefore, facile, fast, and efficient straining techniques that are device-oriented and compatible with flexible substrates are highly desirable. Today, it is highly intriguing to explore the bulging of 2D semiconductors on various substrates to attain high strain levels simply and quickly in device platforms compatible with mainstream MEMS bulging and industrial protocols. Moreover, when combined with flexible substrates such as polydimethylsiloxane (PDMS), we envision and demonstrate here that the conventional bulge testing becomes a “blown-bubble” bulge technique, featuring much boosted deformations. In addition to this blown-bubble bulge from planar to hemisphere, we note that another relevant method of blowing flexible polymer “balloons” has been exploited to strain, stretch, and align nanostructures adhered on the balloon surface, so as to fabricate large arrays of aligned silicon nanowires and nanotubes.39−43 By employing the blown-bubble method to bulge the polymer substrate to efficiently strain the MoS2 crystals on top, here we attain continuous and broad tuning of the optical signatures of 2D TMDCs on flexible substrates. The amount of strain can be continuously tuned by changing the gas pressure underneath
the PDMS, and the gas pressure and thus the strain can be changed conveniently and quickly (in seconds). In this study, we first strain single- and few-layer MoS2 structures until they fracture using the novel blown-bubble bulge technique, on flexible PDMS substrate. The PDMS is mounted onto a specially designed holder, and the differential gas pressure or amount of strain on PDMS can be applied fast (in seconds) with precise control. The MoS2 flakes are clamped with silicone adhesive to prevent slippage during straining, while unclamped devices are also measured to evaluate the effectiveness of this anchoring approach. We continuously record the optical images and the signatory Raman and PL spectra with increasing strain until the MoS2 structures slide and finally fracture. For single-layer MoS2, PL peak shifts linearly by −41 ± 3 meV/% strain, and Raman peak E12g shifts by −2.1 ± 0.2 cm−1/% strain; the MoS2 structure fractures at 2.2−2.6% strain measured with PL and 2.9−3.5% strain measured with Raman. Though the MoS2 is only clamped at two sides, the strain is isotropic biaxial at levels below 1.2%, confirmed by the linear shift of optical signatures. Above this level, the strain becomes anisotropic due to sliding at larger strain (1.2% to 7% strain in PDMS). At large strain, splitting of the Raman peak E12g is observed, because strain breaks the symmetry of the lattice. At 7% to 9.4% strain in PDMS, the MoS2 structure fractures. While other methods of straining MoS2 have been attempted, we demonstrate here the blownbubble bulge technique with flexible substrates, which is not only convenient and fast to achieve large strain until MoS2 fractures but also is carried out in device-oriented configurations, which is realistic and pertinent to many device applications such as flexible electronics, optoelectronics, and biomedical transducers on curved substrates. The blown-bubble bulge technique is also readily applicable to other 2D semiconductor devices for studying their strain responses. Blown-Bubble Bulge Characterization Apparatus. Figure 1a illustrates the experimental configuration for applying 4569
DOI: 10.1021/acs.nanolett.7b00730 Nano Lett. 2017, 17, 4568−4575
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Figure 2. Blown-bubble bulge measurements of 1L and 3L MoS2 on PDMS. (a, b) Optical images showing the MoS2 flake (a) after exfoliation onto PDMS and (b) after being clamped with silicone adhesive, with the boundary of the clamp and the MoS2 flake outlined by the yellow and white dashed lines, respectively. (c, d) Characterization of the 1L region as indicated in panel a under different bulging and straining conditions, using (c) PL and (d) Raman spectroscopy. The differential pressure on PDMS is indicated on top of the corresponding spectrum. (e, f) Characterization of the 3L region as indicated in panel a under different bulge levels, using (e) PL and (f) Raman spectroscopy. The indirect transition (I) peak data in panel e are magnified for clarity. The spectra are vertically shifted, and the red dashed lines are guides to show the trend of the peak shift.
signatures. To prevent sliding of the MoS2 flake during the bulging and straining measurements, the MoS2 is clamped on the PDMS with silicone adhesive, as shown in Figure 1b. Upon the application of pressure, the PDMS bulges upward, and the MoS2 attached to the center of the top PDMS surface is subjected to a tensile strain, which causes changes in the MoS2 band structure and phonon modes, represented by shifts in PL and Raman peaks, respectively. The excitation laser wavelength for Raman and PL measurement is 532 nm, with the laser power incident on the MoS2 structure typically within 100−200 μW to minimize laser heating effects while providing enough signal-to-noise ratio to resolve the peak shifts. The Raman and PL signals are collected with a spectrometer and then a CCD detector. The setup is versatile and can be extended to study
strain to MoS2. We change the pressure applied at the bottom surface of the PDMS, while the top surface where the MoS2 structure is adhered to is kept at atmosphere pressure, thereby bulging up the PDMS diaphragm. The sample is mounted on a custom-built chamber that enables N2 gas pressure to be applied to the back side of PDMS via a gas manifold and a pressure controller, thus causing it to bulge as shown in Figure 1b and c, and the pressure stabilizes in seconds. The PDMS sheet is clamped firmly to the top opening of the chamber by a specially designed circular acrylic holder with a circular opening in the middle that serves to define the bulging region. Prior to mounting, MoS2 flakes are mechanically exfoliated onto PDMS; then thin flakes are identified under an optical microscope, and their thicknesses are confirmed by their Raman44 and PL45,46 4570
DOI: 10.1021/acs.nanolett.7b00730 Nano Lett. 2017, 17, 4568−4575
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Figure 3. Strain tuning of the MoS2 device shown in Figure 2, for the 1L region. (a) Measurement and modeling of the corresponding strain in PDMS versus varying differential gas pressure. (b) Optical image showing that the MoS2 finally fractures under applied strain, with the crack position indicated by the red dashed line and the red arrow. (c, d) Extracted (c) PL peak A and (d) Raman peak E12g position shift with applied strain in PDMS, with the corresponding strain in MoS2 indicated on the top axis (εMoS2,Raman- indicates the strain in MoS2 calculated from the strain response of E1−2g Raman peak). The shaded green area indicates that the device is still in the linear regime without sliding, while the pink region suggests that the MoS2 could have some sliding with substrate due to observed change in the slope. The MoS2 device finally fractures at strain of ∼7−9.4% in PDMS, as shown by the red star.
peaks as E1+2g and E1−2g and fit the curves to Lorentzian functions to extract their peak positions. The A1g Raman mode is the out-of-plane phonon mode and has a relatively small peak position shift with strain compared with E12g mode, which agrees with previous reports.21 In the 3L region, we observe redshift of both PL peak A (direct transition) and I (indirect transition), as well as Raman peak E12g, and the amount of the shift is smaller than that of the 1L region, as shown in Figure 2e and f. This is in accordance with previous reports that the responsivity of peak position shift for 3L is smaller than that of the 1L region.47 To carefully examine how the strain in MoS2 correlates with the applied bulging of the PDMS, we first calibrate the displacement near the center of the PDMS (h) under different pressures, as shown in Figure 3a. This is achieved by measuring the change in focus at different pressures using an optical microscope. The displacement is also simulated with FEM, which shows a trend similar to that of the measured displacement, while the minor discrepancy could be attributed to the nonidealities in the clamping conditions of the PDMS. The displacement is further converted to strain in the PDMS using Hencky’s model for a large deflection of circular membranes,48 using the radius of the acrylic clamp opening a = 3.2 mm, thickness of PDMS t = 432 μm, PDMS Young’s modulus EY = 1 MPa, PDMS Poisson’s ratio ν = 0.5, and assuming there is no initial tension in the PDMS film. Note that here we calculate the strain in the PDMS without considering the local effect of the MoS2. Since MoS2 has a much higher Young’s modulus (270 GPa) than that of the PDMS, although MoS2 thickness (0.7 nm for 1L MoS2) is much smaller than the PDMS thickness, it could still cause a deviation in the local
the strain tuning of other 2D semiconductors with different geometries. At high differential pressures, the PDMS bulges as a blown bubble, as shown in the optical image in Figure 1c. Finite element modeling (FEM) in Figure 1d as well as analytical solution is used to discern the PDMS deflection and strain at a given pressure. A photograph of a representative clamped bilayer (2L) MoS2 structure on PDMS is shown in Figure 1e. Device Responses to Bulge at Varying Pressures. Representative optical images of a MoS2 structure on PDMS are shown in Figure 2a (before clamping) and Figure 2b (after clamping), which contains both single-layer (1L) and trilayer (3L) regions. As the differential pressure is gradually ramped up, we simultaneously record the PL and Raman spectra. In the 1L region, as we increase the pressure, we observe a continuous redshift of the direct transition PL peak A and Raman peak E12g as shown in Figure 2c and d, confirming that strain is applied to MoS2 by bulging the PDMS. The tendency of redshift and the decrease of PL intensity with tensile strain in MoS2 is in accordance with previous reports.5,6 The largest PL peak shift observed is ∼90 meV, and the greatest Raman peak shift is ∼6 cm−1 for the E12g mode. We observe in Figure 2d that the E12g peak clearly splits into two peaks when the strain is large because the strain modifies the lattice structure and breaks the symmetry, which is in accordance with a previous report.6 The Raman peak splitting indicates that the strain is anisotropic at large strain, due to the sliding between MoS2 and the flexible substrate, especially in the unclamped direction. The new observations of both the isotropic biaxial strain and the anisotropic strain at different bulging stages, plus the transition phenomena, are interesting and have implications to structures and devices with asymmetric geometries. We note the two split 4571
DOI: 10.1021/acs.nanolett.7b00730 Nano Lett. 2017, 17, 4568−4575
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Figure 4. Strain tuning of the MoS2 device shown in Figure 2, for the 3L region. (a, b) PL peak shift of (a) direct transition peak A and (b) indirect transition peak I, with applied strain in PDMS. (c) Raman peak E12g position shift with applied strain in PDMS. The extracted strain in MoS2 is indicated on the top axis, and the red star represents fracturing of the MoS2 structure.
Figure 5. Comparison of MoS2 devices with and without silicone clamp. (a, b) Optical images of the clamped device (a) before and (b) under straining. (c, d) The PL spectra under applied varying pressure for (c) 1L region and (d) 2L region of the clamped device. (e, f) Summary of PL peak shifts of the clamped MoS2, for (e) the A peak of the 1L region, and (f) the I peak for the 2L region. (g, h) Optical images of the unclamped device (g) before and (h) under straining. (i) PL spectra under applied pressure for the unclamped device, showing a small peak shift. (j) Summary of the PL peak shift for the MoS2 structure without a clamp.
with strain in the PDMS for the 1L region of the same MoS2 structure as shown in Figure 2. Figure 3c shows that, in the range of 0−1.2% strain (green area), the PL peak shifts linearly with the responsivity of −41 ± 3 meV/% strain, and Figure 3d shows that the Raman peak E1−2g shifts with the responsivity of −2.1 ± 0.2 cm−1/% strain. We use the E1−2g peak instead of E1+2g peak to calculate the strain responsivity and strain in MoS2 because it shows a clearer trend and larger shift, and calculation using E1+2g peak results in different strain responsivity but a similar strain level in MoS2. The A1g Raman peak shifts by −0.5 ± 0.1 cm−1/% strain. In the range
strain as compared to the rest of the PDMS substrate. Nevertheless, this calculation provides a reasonable estimate of the strain in PDMS (εPDMS) near the MoS2 structure, and the strain in MoS2 (εMoS2) will be determined separately, as we will discuss later. Details about the Hencky’s model and calculation of strain in the PDMS membrane are described in Supporting Information (SI). We obtain nearly linear relationship between the applied pressure and the strain in PDMS, which is ∼0.3% strain in PDMS per kilopascal of differential pressure applied. To determine the strain actually applied to the MoS2, we summarize in Figure 3 the Raman and PL peak position shifts 4572
DOI: 10.1021/acs.nanolett.7b00730 Nano Lett. 2017, 17, 4568−4575
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displays a redshift versus strain response, similar to that of the 1L region, with −0.5 ± 0.1 cm−1/% strain. We do not observe Raman peak splitting for the 3L region, and the fracture strain calculated from Raman measurement is 2.5−3.1% in the 3L MoS2. Effect of Clamping on the Blown-Bubble Bulge Measurement. We also determine the effectiveness of our clamping approach by comparing the strain responses of MoS2 structures with and without clamping, as shown in Figure 5. Figure 5a shows an optical image of a clamped sample containing a MoS2 structure with varying thicknesses. We perform blown-bubble bulge with in situ PL measurements on the 1L and 2L regions under varying pressure, and the results are shown in Figure 5c and d, respectively. A similar redshift occurs for the A and I peaks in the PL spectra over a differential pressure range of 0−4.14 kPa (0.8% strain in PDMS), and no sliding is observed at 2.07 kPa (0.25% strain in PDMS) with the silicone clamp (Figure 5b). The PL peak position shift with strain is summarized in Figure 5e (1L region) and 5f (2L region), showing that for the 1L region, the A peak shifts with a responsivity of −18.5 meV/% strain; for the 2L region, the A peak responsivity is −27.3 meV/% strain, and the I peak responsivity is −89.1 meV/% strain. The A peak responsivity for the 1L region is smaller than the value measured earlier, probably because this 1L region is connected to the thicker MoS2 region, and the clamping condition is not ideal, which again proves the importance of clamping for the correct measurement of the strain response. We assume that, for the amount of strain applied, the strain in PDMS is the same as that in MoS2 because the peak shift is still linear and the slope does not change much. For another 1L MoS2 structure on PDMS without clamping (Figure 5g), we observe that MoS2 already detaches from the PDMS substrate and forms wrinkles at a differential pressure of 2.07 kPa (Figure 5h). From the PL spectra in Figure 5i and summary in Figure 5j, a small peak shift of −2.9 meV/% strain is observed. The strain in MoS2 is hard to determine here because the MoS2 already delaminates. The results show that the robust clamping condition is essential to observe the correct shift in phonon modes and band structures until the fracture strain. In conclusion, we have demonstrated a blown-bubble bulge measurement technique and showed that it can efficiently attain large strains in single- and few-layer MoS2 on top of flexible PDMS substrates, until the MoS2 crystals fracture. PL and Raman spectroscopy show that the bandgap and phonon modes shift with the applied strain, with the signatory peaks continuously red-shifting upon increasing strain, up to the fracturing strain. The strain is found to be isotropic biaxial at
