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Functional Nanostructured Materials (including low-D carbon)

Centimeter-Scale Periodically Corrugated Few Layer 2D MoS with Tensile Stretch-Driven Tunable Multifunctionalities 2

Emmanuel Okogbue, Jung Han Kim, Tae-Jun Ko, Hee-Suk Chung, Adithi Krishnaprasad, Jean Calderon Flores, Shraddha Nehate, Md Golam Kaium, Jong Bae Park, SeiJin Lee, Kalpathy B. Sundaram, Lei Zhai, Tania Roy, and Yeonwoong Jung ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b08178 • Publication Date (Web): 30 Jul 2018 Downloaded from http://pubs.acs.org on August 3, 2018

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ACS Applied Materials & Interfaces

Centimeter-Scale Periodically Corrugated Few Layer 2D MoS2 with Tensile Stretch-Driven Tunable Multifunctionalities

Emmanuel Okogbue, †,§ Jung Han Kim, † Tae-Jun Ko, † Hee-Suk Chung, ¶ Adithi Krishnaprasad, † Jean Calderon Flores, ‡ Shraddha Nehate, § Md Golam Kaium, † Jong Bae Park, ¶ Sei-Jin Lee, ¶ Kalpathy B. Sundaram, § Lei Zhai, †,‡,¢ Tania Roy, †,§,¢

†

and Yeonwoong Jung*,†,§,¢

NanoScience Technology Center, University of Central Florida, Orlando, Florida 32826,

USA §

Department of Electrical and Computer Engineering, University of Central Florida, Orlando,

Florida 32826, USA ¶

Analytical Research Division, Korea Basic Science Institute, Jeonju 54907, South Korea

‡

Department of Chemistry, University of Central Florida, Orlando, Florida 32826, USA

¢

Department of Materials Science and Engineering, University of Central Florida, Orlando,

Florida 32826, USA

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ABSTRACT: Two-dimensional (2D) transition metal dichalcogenide (TMD) layers exhibit superior optical, electrical, and structural properties unattainable in any traditional materials. Many of these properties are known to be controllable via external mechanical inputs, benefiting from their extremely small thickness coupled with large in-plane strain limits. However, realization of such mechanically-driven tunability often demands highly complicated engineering of 2D TMD layer structures, which is difficult to achieve on a large wafer-scale in a controlled manner.

Herein, we explore centimeter-scale periodically

corrugated 2D TMDs particularly, 2D molybdenum disulfide (MoS2) and report their mechanically tunable multifunctionalities. We developed a water-assisted process to homogeneously integrate few-layers of 2D MoS2 on three-dimensionally corrugated elastomeric substrates on a large-area (> 2cm2). The evolution of electrical, optical, and structural properties in these three-dimensionally corrugated 2D MoS2 layers were systematically studied under controlled tensile stretch. We identified that they present excellent electrical conductivity and photoresponsiveness as well as systematically tunable surface wettability and optical absorbance even under significant mechanical deformation. These novel three-dimensionally structured 2D materials are believed to offer exciting opportunities for large-scale, mechanically deformable devices of various form factors and unprecedented multifunctionlaities.

KEYWORDS: 2D materials, 2D MoS2, 2D layer transfer, strechable devices, multifunctionalities


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INTRODUCTION With increasing need for wearable/stretchable devices in a variety of applications,1 there is strong demand for developing new materials that can preserve their intrinsic material properties under harsh environment involving severe mechanical deformation. The limited mechanical flexibility inherent to covalently bonded inorganic thin film materials has sparkled great interest in two-dimensional (2D) materials of extremely small thickness and large inplane strain limits.2-4 Particularly, transition metal dichalcogenide (TMD) materials such as 2D molybdenum disulfide (2D MoS2) present enormous potential owing to their tunable bandgap energy and semiconducting transport, overcoming the intrinsic shortcomings of graphene.5-8 Despite the projected merits, it has remained technically challenging to fabricate large-scale 2D MoS2 layers in a tailored structure which allows for such excellent property advantages under severe mechanical deformation. Although it has been theoretically predicated that 2D MoS2 layers can present the high in-strain limit of ~30 %,9-10 chemicallygrown 2D MoS2 layers generally exhibit much limited stretchability; for example, significant electrical failure was observed in CVD grown-2D MoS2 layers at a tensile strain of only ~5 %,11 severely limiting their applicability requiring larger mechanical tolerance. In addition to the preservation of such intrinsic material properties, it is highly desirable to modulate them in a controlled manner via mechanical deformation in a way to further broaden the versatility of the material. Ideally, the structure of 2D MoS2 layers should be precisely engineered to accomodate the aforementioned property requirements on a large wafer-scale, which would only

be

possible

by

achieving;

(1)

Large-scale

2D

MoS2

layers

should

be

transferred/integrated on mechanically deformable platforms from their growth substrates without compromising their intrinsic material properties. (2) The integrated large-scale 2D MoS2 layers should maintain a three-dimensional geometry with well-defined structures to ensure improved mechanical flexibility/tunability. The separation/transfer of 2D MoS2 layers from their rigid growth substrates has generally employed the use of strong acid/base agents 3 ACS Paragon Plus Environment


such as buffered oxide etchant (BOE) or potassium hydroxide (KOH) which are known to be detrimental to their material properties.12-14 These methods require protective polymeric layers such as polymethylmethacrylate (PMMA) , which often introduces the unwanted dissolution of 2D layers during their removal. Moreover, the intrinsically limited mechanical flexibility/stretchability of 2D MoS2 layers can be amended by engineering their structures into tailored three-dimensional (3D) forms.11,

15

Recently, substantive efforts on the 3D

structuring of 2D TMD layers been devoted with a variety of motivations,16 including “crumpled” 2D MoS2 layers integrated on mechanically/thermally responsive substrates.17-18 Three-dimensionally oriented 2D layers have also been achieved during their natural growth stages dictated by material growth parameters.19-20 The integration of pre-grown 2D MoS2 layers on flexible substrates has previously been attempted to achieve three-dimensionally structured 2D layers via substrate engineering,21-22 wherein the integration typically remains on small areas (< cm2). Moreover, in such cases, the integrated 3D MoS2 layers suffer from a large degree of random layer orientation with uncontrolled morphologies. In this work, we report centimeter-scale periodically corrugated 2D MoS2 layers and study their mechanically tunable multifunctionalities. We developed a water-assisted transfer method to homogeneously integrate 2D MoS2 layers on three-dimensionally patterned elastomeric substrates on a large wafer-scale (> 2cm2). The integrated 2D MoS2 layers resemble the pre-defined patterns provided by the donor substrates, exhibiting periodically corrugated layer structures with well-defined dimensions. We systematically investigated the electrical, optical, and structural properties of these novel 3D-structured 2D MoS2 layers and identified their simultaneous and excellent tunability under controlled tensile stretch.

RESULTS AND DISCUSSION Figure 1a illustrates the fabrication process of centimeter-scale periodically corrugated 2D MoS2 layers. The process starts with the preparation of a corrugated Polydimethylsiloxane 4 ACS Paragon Plus Environment

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(PDMS) substrate using a sapphire with periodically (1 µm pitch) grooved sinusoid structures as the patterning mold. The PDMS is poured and cured on the mold surface at room temperature for 48 hours to ensure its desired surface adhesiveness.23 Once peeled off the mold, the prepared PDMS substrate is then bi-axially stretched to a certain extent (e.g., 50% of its original length) using a home-made automated tensile stretcher. A separately prepared as-grown 2D MoS2 layers on a SiO2/Si substrate are subsequently integrated on the surface of the pre-stretched PDMS. Details for the growth of 2D MoS2 layers are presented in the experimental section. The 2D MoS2 layers-grown substrate is mildly pressed against the PDMS to ensure a conformal contact with its surface under stretching. Subsequently, water droplet is deposited on the growth substrate-integrated PDMS which is then allowed for a few minutes. The water penetrates into the van der Waals gaps in between the as-grown 2D MoS2 layers and the underlying SiO2, taking advantages of the imbalance in the surface energies of 2D MoS2 and SiO2.12, 24 Once the remaining water droplet is completely removed by a paper towel, the integrated SiO2/Si substrate is gently removed. As a result, only the 2D MoS2 layers become delaminated preserving their pristine material quality since they were in intimate contact with the PDMS. Lastly, the pre-applied tensile stretch is released in the PDMS, which yields periodically corrugated 2D MoS2 layers/PDMS. Previous approaches based on the water-aided transfer of 2D layers12,

24

rely on the capillary force for their

delamination in water, which generally leads to the formation of unwanted layer wrinkles and overlaps. The present approach avoids such drawbacks as the adhesiveness of the PDMS enables the conformal contact of the entire 2D layer surface as well as achieving a transfer of precisely-defined patterns. Figure 1b-e demonstrate the structural characterization of 2D MoS2 layers grown on SiO2/Si before their integration to PDMS. The growth was carried out by the CVD sulfurization of pre-deposited Mo (typically, ~2 nm thickness) on SiO2/Si, following the method previously developed in our lab.25 The thickness of Mo is carefully selected to ensure the growth of 2D MoS2 layers in a horizontal geometry avoiding unwanted 5 ACS Paragon Plus Environment


vertical 2D layer orientation.26-28 Figure 1b shows the Raman spectrum of as-grown 2D MoS2 layers on a SiO2/Si substrate (area ~ 2 cm2), pictured in the inset. The Raman profile shows two distinct peaks corresponding to the in-plane (E12g) and out-of-plane (A1g) vibration modes of 2D MoS2 layers, respectively.26, 29-30 The cross-sectional transmission electron microscopy (TEM) image of the corresponding sample reveals the horizontally oriented 2D MoS2 layers of well-resolved 2D layer number (Figure 1c).

The plane-view high-resolution TEM

(HRTEM) image in Figure 1d reveals the Moiré pattern of hexagonal MoS2 basal planes,25 which is indicaticative of the lattice mis-orientation of stacked layers and is consistent with the cross-sectional TEM. The energy dispersive X-ray spectroscopy (EDS) mapping images of the same sample obtained in a scanning TEM (STEM) mode show the spatial homogeneity of Mo and S (Figure 1e), further confirming the complete conversion of Mo and S into 2D MoS2 layers.31 Figure 1f evidences the viability of the water-assisted integration of 2D MoS2 layers onto a pre-patterned/stretched PDMS, as illustrated in Figure 1a. The top image demonstrates the application of water droplet on 2D MoS2 layers-grown SiO2/Si attached on a pre-stretched PDMS and the bottom image reveals the successful transfer of 2D MoS2 layersonly. It is worth mentioning that our integration approach enables a complete transfer of asgrown, large-area 2D MoS2 layers in their original shape while precisely replicating the 3D pattern of the secondary substrate. Figure 2 describes the structural characterization of 2D MoS2 layers integrated on a pre-patterned PDMS. Figure 2a shows a large-area (~2 cm2) homogenously continuous film integrated on a PDMS, revealing distinguishable color compared to the surrounding PDMS which is optically transparent. The film is indeed confirmed to be 2D MoS2 layers, characterized by Raman spectroscopy (Supporting Information, Figure S1) as well as optical absorption measurements (to be discussed below). The enlarged optical image of the red square box reveals the periodically spaced patterns of continuous 2D MoS2 layers which cover the entire area of targeted integration. Figure 2b demonstrates the mechanical 6 ACS Paragon Plus Environment

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flexibility/bendability of the 2D MoS2 layers integrated on PDMS – the same sample as described in Figure 2a. The vivid optical contrast obtained only from the region of 2D MoS2 layers reflects that they are in the corrugated structure,32 which is in sharp contrast to the optically transparent PDMS.

The projected scanning electron microscopy (SEM) image

(Figure 2c) confirms the periodically (1 µm pitch) corrugated 2D MoS2 layers conformally attached to the underlying PDMS. Figure 2d-g present the atomic force microscopy (AFM) characterization of the 2D MoS2 layers on PDMS.

Figure 2d, e show the plane- and

projected-view AFM topography profiles of the same material in Figure 2a, b, respectively. The bright and dark AFM image contrasts present the periodic appearance of “hill” and “valley” regions in the corrugated 2D MoS2 layers respectively, consistent with the optical and SEM observations. Figure 2f shows a corresponding AFM adhesion map confirming the conformal coating of 2D MoS2 layers on PDMS, and its quantitative analysis is presented in Figure 2g. The AFM height profile (red) and Young’s modulus distribution (blue) in Figure 2g across the periodically corrugated 2D MoS2 layers infer that the hill regions are in the highest strain state. This observation of strain distribution is consistent with previous studies on the 2D MoS2 layers of wrinkled/crumpled structures.33-34 Moreover, the modulus distribution is observed to be highly periodic and uniform across the repeatedly appearing hills/valleys, further suggesting the structural homogeneity of the material. In the AFM analysis, the modulus is calculated by using the following equation:

= ∗ √ +

(1)

where E* is the reduced Young’s Modulus, Ftip is the force on the tip, Fadh is the adhesion force, R is the tip radius, and d is the tip-sample separation.35 Having confirmed the achievements of large-area, periodically corrugated, and continuous 2D MoS2 layers on PDMS, we then assess their multi-functional material properties under controlled mechanical deformation. We systematically characterize their 7 ACS Paragon Plus Environment


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electrical, optical, and structural properties under unidirectional tensile stretch and evaluate their strain-driven evolution.

Figure 3a,b shows a representative image of periodically

corrugated 2D MoS2 layers on PDMS clamped with an automated mechanical stretcher which applies tensile stretch at a rate of 0.5 mm/sec. In Figure 3a, the as-prepared sample is initially in a pristine state (AFM and SEM images in Figure 2c, 2d) with an original length along the orientation of the tensile stretch. The sample is then subjected to a stretch of 50% with a new length = + 0.5 (Figure 3b). From the images, it is confirmed that there is no slippage of the 2D MoS2 layers on the underlying PDMS owing to its high conformity, which is crucial for the proper measurements of material properties. Moreover, we quantified the surface roughness of corrugated 2D MoS2 layers by characterizing their topography with AFM and confirmed its gradual reduction with increasing stretching (Supporting Information, Figure S2). Having confrimed the excellent mechanical stretchability of corrugated 2D MoS2 layers, we first characterized their electrical properties under systematically controlled tensile stretch. Figure 3c shows the two-terminal current (I)-voltage (V) transport characteristics with varying tensile stretch. The results indicate a very slight decrease of current with increasing stretching, implying that the corrugated 2D MoS2 layers well preserve their structural/electrical integrity upon being flattened out. In contrast, the I-V characteristics from plain 2D MoS2 layers on PDMS without corrugation show a drastic current decrease ∆

with increasing stretching (Figure 3d). We calculate the relative change of resistance, i.e.,

,

where ∆ is an increase in resistance over initial resistance and plot it in Figure 3e red dots. The corrugated 2D MoS2 layers present ~60 % increase of

∆

upon a tensile stretch up

to 30 %, confirming their excellent electrical tunability under severe mechanical deformation. It is worth mentioning that significant electrical failure (i.e. > 6000 % increase of

∆

) was

previously reported in CVD-grown un-structured 2D MoS2 layers even at a tensile stretch of < 8 ACS Paragon Plus Environment

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~5 %.11 For comparison, a plot of resistance change vs. stretching level for plain 2D MoS2 layers/PDMS wihtout corrugation is also prsented (Figure 3e black dots), which shows a much steeper (> 10 times) change in

∆

under the identical stretch level consistent with

previous studies.11 We note that there exists a few studies on three-dimensionally crumpled graphene and other 2D materials and their stretch-driven electrical responses while these materials are limited to 2D layers of random orientation on small areas.36-37 We emphasize that the present study explores 2D MoS2 layers with precisely defined structures on a waferscale (> 2 cm2) and their multi-functionalities, significantly advancing over the previous studies in terms of dimensional scalability, structural controllability, and the versatility of tunable properties (to be further discussed).

Moreover, we performed fatigue tests on

corrugated 2D MoS2 layers by characterizing their electrical response upon repeated stretch for a large number of cycles and confirmed excellent mechanical tolerance (Supporting Information, Figure S3). Representative photo image of one of the elctrically tested devices are presented in Supporting Information, Figure S4, where typical dimensions for channel length and areas are 400 µm and 2 cm2, respectively. Having proved the tunable electrical properties, we also assessed the optical properties of periodically corrugated 2D MoS2 layers and investigated the influence of mechanical deformation. We performed the ultraviolet-visible (UV-Vis) spectroscopy characterization of another sample and identified its optical absorbance under varying tensile stretch levels in a wide range of spectral wavelength. Figure 4a indicates that the absorbance monotonically decreases with increasing stretching, which reflects the reduction in the surface-volume ratio in the given area of the sample.36-37 Two absorbance peaks are visible in the spectral range of ~600 - 700 nm, which correspond to A and B excitonic peaks from the K point of the Broullion zone in 2D MoS2 layers.33, 38-41 The observed absorbance characteristics are quite consistent with previous studies revealing distinguishable features of 2D MoS2 layers,33, 38-41 9 ACS Paragon Plus Environment


which further confirms their successful large-area transfer/integration on PDMS. Interestingly, we observe that these excitonic peaks exhibit systematic blue shift with increasing stretch levels, manifested by the reduction in the A excitonic peak energy as shown in Figure 4b. We believe that this stretch-driven blue shift is attributed to the “funnel effect” in strained 2D materials;33 theoretical/experimental studies on intrinsically and/or extrinsically strained 2D materials indicate that excitons generated by light illumination drift to the most strained regions with their layers.42 Our 2D MoS2 layers are highly strained in their initially corrugated state, possessing the maximally-strained hill regions (AFM modulus in Figure 2g). Increasing tensile stretch across the hills introduces the “relaxation” of this built-in strain as the initially corrugated 2D MoS2 layers in their highest strain state become systematically flatten out (Figure 3a,b). Consequently, 2D MoS2 layers in a more strained state corresponding to the smaller stretch level in Figure 4a result in significant recombination of drifted excitons, hence a lower exciton energy.33,38 Previous studies with randomly crumpled graphene have reported similar absorbance tunability driven by external strain,36-37 while our materials exhibit a highly systematic trend retained on a centimeter- scale area owing to their controlled geometry. The stretch-driven tunability of the opto-electrical properties in corrugated 2D MoS2 layers was further verified by characterizing their photoresponsiveness. We confirmed dynamic and reversible photocurrent generation from a sample in a pristine (corrugated) state under repeated optical illumination (Figure 4c) – measured at 10 V with 17 second intervals. Moreover, we systematically measured alternating ON/OFF photocurrents under increasing stretching levels with/without optical illumination as showin in Figure 4d – obtained from a sample different from that for Figure 4c. The result confirms that the ON/OFF photocurrent responsiveness is well retained even up to the stretch of ~30 % under the illumination of an identical intensity. The gradual decrease of the photocurrent with increasing stretch is attributed to the decrease of the optical absorbance (Figure 4a), i.e., decrease in the number of photo-excited carriers under a given illumination area. 10 ACS Paragon Plus Environment

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Moreover, we study the stretch-tunable structural properties of periodically corrugated 2D MoS2 layers by characterizing their surface wettability through water contact angle (WCA) measurements. Figure 5a presents the morphological evolution of water droplet on the surface of corrugated 2D MoS2 layers/PDMS under varying stretch levels as well as corresponding WCA values. The plots show the excellent hydrophobicity of the material and the WCA values are observed to systematically decrease with increasing stretching levels in the tested range of 0-50 %. This trend is attributed to the reduced average surface roughness of the sample, quantitatively agreeing with the prediction from the Wenzel equation.43-45 We also performed the WCA measurements with as-grown 2D MoS2 layers transferred onto the surface of flat PDMS with no corrugated patterns (Supporting Information, Figure S5) . We identified that the WCA value is ~90o which is similar to that of the corrugated sample but flattened under 50 % stretch in Figure 5a. These observation and analysis indicate the intrinsic hydrophobicity of our CVD-grown 2D MoS2 layers consistent with previous studies43,46 as well as confirming its stretch-driven tunability. By taking advantage of the intrinsically corrugated yet tunable 2D MoS2 layer structure, we further demonstrate a more dynamic surface-wetting tunability by incorporating surface functional polymers such as 1H,1H,2H,2H-perfluorooctyltriethoxysilane (PFOTES). The polymer was integrated on the surface of the material by evaporation in a vacuum chamber, which results in a uniform conformal coating. The PFOTES polymer is known to reduce the surface energy of materials, resulting in super-hydrophobic surface wettability.47 Figure 5b and 5c show the WCA measurements of the PFOTES-treated, periodically corrugated 2D MoS2 layers in their pristine state (WCA ~141o at 0%) and after a tensile stretch (WCA ~ 120o at 50%), respectively.

CONCLUSIONS



In summary, we developed a reliable process to transfer and integrate large-area 2D MoS2 layers on pre-structured elastomeric polymers. This transfer/integration approach overcomes the intrinsic limitations of previously developed crumpled 2D MoS2 layers in terms of achieving the scalable controllability of tailored dimension and targeted geometries. The comprehensive measurements of electrical, optical, opto-electrical, and structural properties in the periodically corrugated 2D MoS2 layers testify their high versatility for realizing a large set of dynamically tunable multi-functionalities.

EXPERIMENTAL SECTION 2D MoS2 layer growth: 2D MoS2 layers were grown by a chemical vapor deposition (CVD) method. Mo films (thickness ~2 nm) were deposited on SiO2/Si substrates (300nm SiO2 thickness) at a rate of 0.15 Å/s by electron beam evaporation. Mo deposited substrates were placed in the middle of a quartz tube CVD furnace along with an alumina boat containing sulfur positioned at the upstream side. The quartz tube was then evacuated to a pressure of ~1mTorr and purged with argon (Ar) to remove residual gases in the tube. Afterward, the furnace was heated up to ~750 - 800 ℃ in 50 minutes and was held there for another 50 minutes under the continuous supply of Ar gas. The furnace was allowed to naturally cool down to room temperature after the reaction. TEM and STEM characterization: The structural and chemical analysis of as-grown 2D MoS2 layers were performed using a TEM/STEM (JEOL ARM200F) at an accelerating voltage of 200 kV. The STEM-EDS analysis was utilized with EDAX detector (SDD type 80T) and software (AZtecTEM, Oxford) with a probe current of ~100 pA. All STEM operations were carried out at a convergence semi-angle of 20 mrad. AFM characterization: Atomic force microscopy (AFM), Multimode 8 AFM, Nanoscope V, Bruker was performed in a peak force quantitative nanomechanical (PF-QNM) mode in air at a scan rate of 0.517 Hz. ScanAsyst AFM probe (radius > 10 nm, Bruker) was used with a 12 ACS Paragon Plus Environment

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nominal spring constant of 0.4 N/m which was calibrated using a standard PDMS sample of 3.5 MPa (Bruker). The loading forces (peak force set point) on the surface of 2D MoS2/PDMS samples were 0.245 nN in PF-QNM process. The adhesion and Derjaguin-Müller-Toporov (DMT) modulus (Young's modulus) of materials were analyzed using Nanoscope analysis software 1.8, Bruker. Optical characterizations: The optical absorbance of corrugated 2D MoS2 layers under varying stretch levels was characterzed with UV-Vis spectroscopy (Cary Win UV spectrometer) in the wavelength range of 200-800 nm. The absorbance of a pre-patterned PDMS substrate was measured and subtracted from all the collected data to take into account of its contribution to peak intensities. The optical energy gap of the samples was determined by analyzing the obtained absorption A (λ) spectra. Electrical and photocurrent characterizations: For electrical characterization, two-terminal devices were configured on corrugated 2D MoS2/PDMS with metal contacts (gold). The transport properties of the material under stretch were characterized using a Keithley 2450 source meter coupled with a probe station. For photocurrent characterization, the electrical responses of the material were characterized under a solar lamp illumination (intensity of 1000 watt/m2) by a semiconductor paramater analyzer (Keyseight B1500). A home-built tensile stretcher was mounted on the probe station for both the characterization.

ASSOCIATED CONTENT Supporting Information Additional Raman and AFM data, water contact angle data, fatigue test data, and photos of tested devices. This material is available free of charge via the internet at ACS publications website

AUTHOR INFORMATION 13 ACS Paragon Plus Environment


Corresponding Authors *Email: [email protected] Notes The authors declare no competing financial interests

ACKNOWLEDGEMENTS This work was supported by the Korea Institute of Energy Technology Evaluation and Planning (KETEP) and the Ministry of Trade, Industry & Energy (MOTIE) of the Republic of Korea (No. 20173010013340) (Y. J.).

H-S.C. was supported by National Research

Foundation of Korea (NRF) grant funded by the Korea Government (MSIP) (No. 2015R1C1A1A01052727).


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Figure 1. (a) Illustration of the process to fabricate periodically corrugated 2D MoS2 layers integrated on PDMS. (b) Raman characterization of large-area 2D MoS2 layers grown on a SiO2/Si substrate along with the corresponding sample image (inset). (c) Cross-sectional TEM image of as-grown 2D MoS2 with horizontally-aligned layers. (d) Plane-view HRTEM image showing the Moiré fringes from as-grown 2D MoS2 layers. (e) STEM-EDS maps revealing the uniform spatial distribution of Mo and S. The scale bar is 100 nm (f) Demonstration of the transfer and integration of 2D MoS2 layers on a pre-stretched PDMS.



Figure 2. (a) Optical microscopy images of periodically corrugated 2D MoS2 layers on PDMS over an area of ~2cm2. (b) Image of the same sample in sunlight showing holographic reflection from the 2D MoS2 layers. (c) Side-view SEM image showing the periodically corrugated structure of 2D MoS2 layers on PDMS. (d) Top-view AFM topography image of the same sample. (e) 3D projected AFM topography image of the same sample revealing the alternating appearance of hills and valleys with a periodicity of 1 µm. (f) AFM adhesion map profile corresponding to (d) and (e). (g) AFM height profile (blue) and Young’s modulus mapping (red) of the same sample, revealing that the hill regions are highly strained.


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Figure 3. Periodically corrugated 2D MoS2 layers on PDMS in their (a) pristine and (b) stretched states along with the corresponding schematic illustrations. (c),(d) Two-terminal IV characteristics with increasing stretching for (c) periodically corrugated, and (d) plain 2D MoS2 layers without corrugation. (e) Comparison of

∆

for periodically corrugated vs. plain

2D MoS2 layers.



Figure 4. (a) Optical absorbance spectra of periodically corrugated 2D MoS2 layers on PDMS under varying stretch levels. A and B excitonic peaks are denoted. (b) Variation of A excitonic peak energy under varying stretch levels. (c) Dynamic and reverisble phhotocurrent generation from 2D MoS2 layers on PDMS in a corrugated state. (d) Variation of ON/OFF photocurrent under varying stretch levels.


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Figure 5. (a) Variation of WCA values for periodically corrugated 2D MoS2 layers under varying stretch levels. (b),(c) WCA measurements of PFOTES-treated periodically corrugated 2D MoS2 layers at (b) 0%, and (c) 50 % tensile stretch



TOC figure


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Centimeter-Scale Periodically Corrugated Few ... - ACS Publications

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