In situ TEM Characterization of Shear Stress-Induced Interlayer. Sliding in the Cross Section View of Molybdenum Disulfide

Transcription

1 In situ TEM Characterization of Shear Stress-Induced Interlayer Sliding in the Cross Section View of Molybdenum Disulfide Juan Pablo Oviedo, Santosh KC, Ning Lu, Jinguo Wang, Kyeongjae Cho, Robert M. Wallace, Moon J. Kim* Supplementary Figures Figure S1. HRTEM image of a free standing MoS 2 sample obtained by the Scotch tape clamping transfer method. 1

2 Figure S2. Manipulation of a single MoS 2 flake torn from the bulk. Left: Oxidized W tip at +10 V bias. Middle and right: MoS 2. Figure S3. Alignment of the MoS 2 flake in cross-section view. Left: Oxidized W tip at +10 V bias. Middle and right: MoS 2. 2

3 Figure S4. Bilayer 2H-MoS 2 under a 5 V/nm vertical electric field. (a) Charge difference between an electric field applied and zero field applied normal to bilayer 2H- MoS 2 (vertically periodic). The yellow (negative) regions show electron accumulation, while the turquoise (positive) regions represent electron depletion. (b) Charge difference plot along the Z direction showing the charge accumulation and depletion regions relative to the zero field case (horizontal line). Positive values represent electron accumulation and negative values electron depletion. The vertical line represents the middle of the bilayer gap. 3

4 Figure S5. Line profile of the 9 layer MoS 2 nanoflake cross section before the MoS 2 monolayer in situ shear exfoliation process. The separation between the arrows is 0.63nm, corresponding to the thickness of one layer of MoS 2. 4

5 Figure S6. Electron beam induced defects in MoS 2 at 200 kv. (a) MoS 2 flake oriented in cross section view. (b) The layers are bonded within the same basal plane. (c) After prolonged observation, the layers bond out of their same basal plane. (d) A defect in the MoS 2 lattice has been created (2 edges not bonded) 5

6 Figure S7. Interlayer energy as a function of separation distance for different values of electric field perpendicular to the basal plane of 2H-MoS 2. (a) (b) Figure S8. Interlayer sliding paths. (a) Side view of bilayer 2H-MoS 2 showing A-B stacking and sliding direction. (b) Top view of 2H-MoS 2 with various periodic sliding pathways shown by arrows. 6

7 Figure S9. In-plane sliding barriers for various paths, as shown in figure S4. Figure S10. SEM image of a MoS 2 flake on a 90 nm SiO 2 substrate prior to FIB patterning. The e-beam deposited Pt bar indicates the sample location used for the in situ micro-force shearing test. 7

8 Figure S11. 3D schematic of the in situ microforce shearing test sample. 8

9 Figure S12. Von Mises stress in the cross section sample before its failure during the in situ mechanical shearing test. Finite element simulation of the Von Mises stress across the MoS 2 flake cross section sample at the point of shearing. 9

10 Figure S13. Vertical stress distribution across a basal plane of the MoS 2 flake before shearing during the in situ mechanical test. Finite element simulation of the vertical stress across the MoS 2 flake at the point of shearing. Left side of the sample: Tensile stress. Right side of the sample: Compressive stress. Center: No stress. Figure S14. Shear stress distribution across a basal plane of the MoS 2 flake before shearing during the in situ mechanical test. Finite element simulation of the shear stress across the MoS 2 flake. The shear stress is evenly distributed in the middle area and peaks at the opposite horizontal edges, being highest at the edge closest to the indenter tip. This effect can be minimized by asymmetrically reinforcing the sample with FIB Pt. 10

11 Figure S15. Amorphized edge of a MoS 2 cross-section FIB prepared sample. Figure S16. Line profile of the MoS 2 cross section, as shown in figure S9. The distance between the peaks is 0.63 nm, corresponding to the thickness of a single layer of MoS 2. 11

12 Supplementary Methods STM tip etching Polycrystalline tungsten wire (Sigma Aldrich) of 0.25 mm thickness was straightened, cleaned with isopropyl alcohol, and cut into 12 cm long sections. It was then annealed in vacuum (10-6 torr) in a thermal evaporator tool at 6 A, 10 V DC for 30 min to undergo recrystallization. The wire was then cut into smaller sections which were etched using a tungsten loop immersed in 3 M KOH aqueous solution connected to the cathode terminal of a Keithley source meter 2400, while the wire was connected to the anode, using a 3V DC bias. A Labview program was used to control the etching, by setting a cutoff current of 0.1 A. Two tips were recovered per etching session (1 min.). Then, the tips were further sharpened in a FEI Nova nanolab 200 dualbeam, by simply imaging them for 10 s at a normal orientation using a 30 kv, 3 na Ga ion beam. Electrostatic probe manipulation test We verified that the electric field was responsible for the attachment of MoS 2 to the oxidized STM probe by making contact at different probe biases and observing the mechanical response of MoS 2 as the tip was retracted, noticing that no attachment occurred at a 0 V bias. As the bias was raised, stronger attraction between MoS 2 and the tip would occur as it was retracted. Custom lift-out grid preparation for side indentation A Cu Omniprobe lift-out grid was cut diagonally at a sharp angle, leaving only the first post of a row exposed. This grid was then glued using a two-part silver epoxy to a piece of

13 mm thick tungsten wire in a sideways orientation. It was then fixed to an SEM holder. Using a 30 kv, 20 na Ga ion beam in a dualbeam, the side of Cu post facing the cut side of the grid was etched away, in order to leave enough space for the indenter to access the sample. Then, the TEM sample lift-out procedure was used to transfer a sample to this post. The sample would then be thinned on the side opposite to the one meant for access to the nanoindenter tip. Finite element simulations of structural mechanics of MoS 2 during the in situ mechanical shearing test Finite element simulations were performed using commercial software (Comsol 5.0) where a 3D replica of the MoS 2 cross section sample was created. The sample was modeled as a linear elastic solid. Default material properties were introduced for Pt, SiO 2, and Si. MoS 2 was modeled as an isotropic material with the following properties: Poisson s ratio υ = 0.27; Young s modulus E = 270 GPa.* The bottom of the sample was fixed while a force, F = µn, was applied in the x direction on a rectangular surface of dimensions 0.2 x 0.3 µm at the position of the indenter tip pressed against the Pt cap, corresponding to the force at the point of failure of the sample. A stationary study was performed where the displacement in x and z was solved for the top and bottom faces of the MoS 2 layer, as well as the Von Mises stress distribution in the sample. A join operation was used to obtain the difference between the displacements of the MoS 2 faces in the x direction. This difference was then divided by the thickness of the MoS 2 flake, t = 50 nm, to obtain its shear strain distribution. A second join operation was used to obtain the difference between the displacements of the faces of the flake in the z direction. This was also divided by the thickness of the MoS 2 flake, t = 50 nm, to obtain the vertical strain distribution in the MoS 2 13

14 layer. The shear stress distribution could then be obtained by multiplying the shear strain in the MoS 2 layer with its shear modulus G. A shear modulus of 31.3 GPa was used in order to normalize the shear stress relative to the proposed value for the shear strength of MoS 2 in the [120] direction, τ f = 25.3 MPa. To obtain the vertical stress distribution, the shear strain value was multiplied by the Young s modulus of MoS 2. *Parameters obtained from: Bertolazzi, S.; Brivio, J.; Kis, A. Stretching and Breaking of Ultrathin MoS 2. ACS Nano 2011, 5,