Lattice strain effects on the optical properties of MoS 2 nanosheets

Transcription

1 Lattice strain effects on the optical properties of MoS 2 nanosheets Lei Yang, a Xudong Cui, b Jingyu Zhang, c Kan Wang, b Meng Shen, a Shuangshuang Zeng, a Shadi A. Dayeh, d Liang Feng, e Bin Xiang* a a Department of Materials Science & Engineering, CAS key Lab of Materials for Energy Conversion, University of Science and Technology of China, Hefei, Anhui, , China. * binxiang@ustc.edu.cn b Science and Technology on Plasma Physics Laboratory, Research Center of Laser Fusion, CAEP, Sichuan, , China c Molecular Foundry, Lawrence Berkeley National Laboratory, 1 Cyclotron Rd, Berkeley, CA 94720, USA d Department of Electrical and Computer Engineering, University of California San Diego, La Jolla, CA, 92093, USA e Department of Electrical Engineering, The State University of New York at Buffalo, Buffalo, NY, 14228, USA

2 Experiments All chemicals reagents used in this work are analytic purity and without further purification. Preparation of multilayered MoS 2 nanosheets (grown from ph<7 solution) The MoS 2 nanosheets were successfully synthesized via hydrothermal method S1-S3. A schematic illustration for the synthesis of MoS 2 nanosheets is shown in Figure S1. 3 mmol MoO 3 (The Company of Colloid Chemical, China) and 9 mmol of KSCN (Sinopharm Chemical reagent Co. Ltd, China) were dissolved in 80 ml distilled water. Subsequently, 0.28 ml HCl (Sinopharm Chemical reagent Co. Ltd, China) at a concentration of 12.5 mol/l were added into the reaction solution under violent stirring for 12 h. The prepared solution was transferred into a 100 ml Teflon-lined stainless autoclave and sealed tightly, and were then kept at 240 for 24 h in an oven. Afterwards, the autoclave was cooled down naturally to room temperature. The reaction product was filtered off, washed with distilled water, and absolute ethanol (Sinopharm Chemical reagent Co. Ltd, China) for 3 times respectively, and was then dried in vacuum at 60 for 6 h. Preparation of multilayered MoS 2 nanosheets (grown from ph=7 solution) The MoS 2 nanosheets were successfully synthesized via a hydrothermal method S1-S3. 3 mmol MoO 3 (The Company of Colloid Chemical, China) and 9 mmol of KSCN (Sinopharm Chemical reagent Co. Ltd, China) were dissolved in 80 ml distilled water under violent stirring for 12 h. The prepared solution was transferred into a 100 ml Teflon-lined stainless autoclave and sealed tightly, then kept at 240 for 24 h in an oven. Afterwards, the autoclave was cooled down naturally to room temperature. The reaction product was filtered off, washed with distilled water,

3 absolute ethanol (Sinopharm Chemical reagent Co. Ltd, China) for 3 times respectively, then is dried in vacuum at 60 for 6 h. CVD growth of monolayer MoS 2 on SiO 2 /Si substrate Monolayer MoS 2 was grown on SiO 2 (300 nm)/si substrate in a quartz tube furnace at atmospheric pressure by a chemical vapor deposition (CVD) method S4,S5. Prior to growth, SiO 2 /Si substrates were cleaned by sonication in acetone, absolute ethanol and distilled water for 20 minutes, respectively. The substrates were then placed face-down above the ceramic boat that was filled with 30 mg of MoO 3 and which was palced at the center of the quartz tube. Another ceramic boat filled with 10 mg of sulphur was located in the upstream of the tube with a 12 cm distance from the center. After purging the system with ultrahigh-purity argon for 20 min, the furnace was kept at 650 for 5 min at a heating rate of 15 /min with 10 sccm. After it was cooled to room temperature naturally, a very thin layer of coating on the substrate was observed by the naked eye. Preparation of strain-partially-released multilayered MoS 2 nanosheets We sonicated our as-synthesized multilayered MoS 2 nanosheets (grown from ph<7 solution) for 3 hours in absolute ethanol (Sinopharm Chemical reagent Co. Ltd, China). Several drops of the solution were applied onto a SiO 2 /Si substrate. Under the scanning electron microscopy (SEM), we observed micro-sized sheet-shape MoS 2 on the SiO 2 /Si substrate, which we refer to as strain-partially released multilayered MoS 2. The 3-hour sonication process provides enough force to break down part of the as-synthesized MoS 2 3D network into smaller pieces. Since the strain in the as-synthesized MoS 2 is mostly from the curvature morphology, the broken pieces with flat

4 shape possess much less strain in contrast to that in 3D network. Preparation of MoS 2 nanosheets on SiO 2 /Si substrate using the micromechanical exfoliation method MoS 2 nanosheets were exfoliated from bulk MoS 2 (SPI Supplies Brand Moly Disulfide) using the Scotch-tape micromechanical cleavage method similar to the production of graphene S6. First, Scotch-tape was pressed onto the MoS 2 bulk crystal lightly and removed carefully with a small angle. Many thick MoS 2 sheets were attached on the Scotch tape during this step. The MoS 2 nanosheets were repeatedly peeled off on Scotch tape several times and transferred onto the pre-cleaned SiO 2 /Si substrate by pressing the Scotch-tape with MoS 2 nanosheets onto the SiO 2 /Si substrate by applying a slight pressure with tweezers. Finally, the Scotch-tape was removed from the substrate slowly and several MoS 2 nanosheets were left on the SiO 2 /Si substrate. Preparation of exfoliated MoS 2 nanosheets on flexible substrate The PDMS flexible substrate was compound from DOW CORNING SYLGARD 184 silicone rubber (including the basic components and curing agent). First, the curing agent was added into the basic components under violent stirring for a few minutes with a weight ratio of 1:10. Then the mixture liquid was coated on the SiO 2 /Si substrate containing MoS 2 nanosheets which was prepared using the micromechanical exfoliation method discussed above. In order to remove the bubbles in the liquid mixture, the substrate was left in vacuum drier for 10 min. In order for the bubbles to be removed from the mixture liquid during solidification, the substrate was put in the oven at 100 for 1h. Finally, the solidified PDMS was peeled off from the SiO 2 /Si

5 handle substrate with the aid of tweezers atop which several pieces of MoS 2 nanosheets were retained. A schematic illustration of this process is shown in Figure S7. The TEM sample preparation process for cross-sectional images The MoS 2 nanosheets were embedded in EPON812. After a curing process in an oven at 60 o C for 24 hours, the embedding medium was sliced into a 40-nm-thick electron transparent slices. Computation methods The calculations are performed using the Vienna ab initio simulation package (VASP) in the framework of density function theory (DFT). The projector augmented-wave (PAW) pseudopotential is used with an energy cutoff of 450 ev for the plane-wave basis functions. The Monkhorst-Pack k-point mesh of is employed for the structural optimization of the nanosheets, and for the bulk. For the nanosheets, a vacuum space along z direction larger than 15 Å is employed. The atomic positions are optimized according to the guidance of total energy, with a criterion smaller than 1 10 ev. In the whole calculations, the generalized gradient approximation (GGA) of Perdew-Burke-Ernzerhof (PBE) is used as the exchange-correlation function. References S1. Ye, L., Wu, C. Z., Guo, W. & Xie, Y. MoS 2 hierarchical hollow cubic cages assembled by bilayers: one-step synthesis and their electrochemical hydrogen storage properties. Chem. Commun. 45, (2006).

6 S2. Li, X.-L. & Li Y.-D. MoS 2 nanostructures: synthesis and electrochemical Mg 2+ intercalation. J. Phys. Chem. B 108, (2004). S3. Chirayil, T., Zavalij, P. Y. & Whittingham, M. S. Hydrothermal synthesis of vanadium oxides. Chem. Mater. 10, (1998). S4. Lee, Y.-H. et al. Synthesis and transfer of single-layer transition metal disulfides on diverse surfaces. Nano Lett. 13, (2013). S5. Lee, Y.-H. et al. Synthesis of large-area MoS 2 atomic layers with chemical vapor deposition. Adv.Mater. 24, (2012). S6. Novoselov, K. S. et al. Electric field effect in atomically thin carbon films. Science 306, (2004).

7 Figure S1. Schematic illustration for the synthesis of multilayered MoS 2 nanosheets.

8 Figure S2. XRD of MoS 2 nanosheets grown from PH=7 solution. The calculated (002) spacing is Å. And the calculated (100) spacing is Å. ~ 2% out of plane uniaxial tensile strain was confirmed from this XRD data.

9 Figure S3. SEM image of our as-synthesized MoS 2 nanosheets grown from PH<7 solution.

10 (a) (b) (c) (d) Figure S4. Cross-sectional HRTEM images of the MoS 2 nanosheets grown from PH<7 solution. Each layer is denoted by the yellow spheres representing the Mo atoms. The direction of the cross-section HRTEM is along [001]. The synthesized MoS 2 nanosheets consist of (b) 5, (c) 6, (d) 9 and (e) 10 layers, respectively.

11 (a) (b) (c) (d) Figure S5. The measured interlayer spacing from HRTEM images of Figure S4 for different MoS 2 nanosheets consisting of (a) 5, (b) 6, (c) 9 and (d) 10 layers, respectively. The dotted lines represent the bulk interlayer spacing.

12 Figure S6. Schematic illustration for the preparation of the MoS 2 cross-sectional cuts for HRTEM.

13 Figure S7. Schematic illustration for the preparation of exfoliated MoS 2 nanosheets on a flexible substrate.

14 (a) (b) (c) (d) Figure S8. The sample morphologies of MoS 2 nanosheets including grown from ph=7 (a) and ph<7 (b) solution, strain-partially-released MoS 2 nanosheets (c), and monolayer MoS 2 (CVD growth on SiO 2 /Si substrate) (d). Scale bar is 300 nm.

15 (a) (b) Figure S9. (a) Raman spectra of MoS 2 nanosheets with strain completely released by ultrasonication process, and bulk MoS 2. (b) PL spectra of MoS 2 nanosheets with strain completely released by ultrasonication process, and bulk MoS 2.

16 Figure S10. Calculated band-gap shift as a function of in-plane tensile strain.