Surface Defects on Natural MoS 2

Transcription

1 Supporting Information: Surface Defects on Natural MoS 2 Rafik Addou 1*, Luigi Colombo 2, and Robert M. Wallace 1* 1 Department of Materials Science and Engineering, The University of Texas at Dallas, 800 West Campbell Road, Richardson, Texas 75080, United States 2 Texas Instruments Incorporated, TI Boulevard, MS-365, Dallas, Texas 75243, United States *Address correspondence to: addou@utdallas.edu; rmwallace@utdallas.edu Highlights: 1- Large structural defect 2- FFT comparison between 2H-MoS 2 and the new phase 3- Bias dependence 4- Dark defects: depth profiles 5- Photoemission spectroscopy 6- Bandgap defect 7- Interface and surface science instrument 8- References S1

2 1- Large structural defect. Figure S1a shows large defect looks like a pit underneath the top most surface layer. The depth measured is less than 0.3 nm. The atomic structure remains the same around the defect as presented in Figure S1b. The hexagonal structure is still detectable inside the defect. The lattice constant measured in or far from defect is in agreement with the reported values in the main text. Figure S1. (a) STM image (14 nm 14 nm) shows missing portion of layer underneath the surface. (b) STM image (7 nm 7 nm) showing the atomic structure from an area near the defect in (a). S2

3 2- FFT comparison between 2H-MoS 2 and the new phase. Figure S2 shows a comparison between a FFT measured from the hexagonal structure describing the MoS 2 (0001) surface (Figure S2a) and the superstructure describing the new phase resembling to the monoclinic pyrrhotite 4C-Fe 7 S 8 (001) 1 surface (Figure S2c). The latter shows extra spots in the center as indicated by the white hexagon. Figure S2. (a) FFT measured from the STM image in (b). (c) FFT measured from the new phase shown in the STM image in (d). (b) STM image (10 nm 10 nm) shows the high resolution of the 2H-MoS 2. The bright defect corresponds to the Mo-like defect. (d) STM image (10 nm 10 nm) reveals the atomic structure of the new phase. S3

4 3- Bias dependence test Figure S3 describes the bias dependence test performed on c-mos 2 ; more details are explained in the main text. The dark defect at the positive bias shows two different characters at negative bias: dark defect in positive bias stays dark or changes to bright in negative bias (white circle, Figure S3(a-b)). We recall that at negative (positive) bias, the filled (empty) states of the surface are probed. The depth and the height of the defect are measured at ~0.6 nm corresponding to half unit cell of the MoS 2 crystal bulk along the c-axis. 2 The depression around the defect is caused by the Coulomb repulsion. 3 Figure S3. (a) STM image (V bias = +300 mv, I t = 0.1 na). (b) Same region as (a) recorded at V bias = -300 mv and I t = 0.1 na. Some thermal drift is evident between the two scans. (c) Height profile measured at positive bias on the dark defect outlined in (a). (d) Height profile measured at negative bias on the bright defect in (b). S4

5 4. Dark defects: depth profiles This section is related to Figure 3c in the main text. Different depth dimensions are measured from the same area (100 nm x 100 nm). Dark defects are observed with three depths measured to be 0.6±0.1 nm (Figure S4b), 1.3±0.3 nm (Figure S4c) and 2.1±0.2 nm (Figure S4d), which correspond in the bulk structure to 1/2 c, 1 c, and 3/2 c. Figure S4. (a) STM image recorded on highly defective surface. The line profiles measured across the dark defects show three different depths. (b, c, and d) Line profile across the dark defect outlined in (a) by black, white, and gray circle, respectively. The depths in (b, c, and d) correspond to one-layer MoS 2 (1/2 c), Two-layers (1 c), and three-layers (3/2 c), respectively. S5

6 5-Photoemission spectroscopy. Figure S5 shows the X-ray photoelectron spectroscopy survey of MoS 2 recorded immediately after exfoliation. Only two impurities were detected: Carbon and Oxygen. All other impurities reported in literature are below the detection limit of XPS (0.5 %). Desorbing C and O is possible by annealing the sample in UHV to temperature higher than 400 C for 15 min. Our core levels labeled in Figure S5 are in agreement with the recent XPS studies on MoS Figure S5. XPS survey of MoS 2 measured after exfoliation. Only two contaminations were detected: O and C. S6

7 6- Band-gap defect One of the frequent features appearing in the STS data is presented in Figure S5. This defect feature appears in the band-gap at the voltage of V near the VBM. A similar feature was observed in the calculated DOS induced by S vacancy. 6 Another STM/STS study showed the same feature located at and -0.7 V. 7 Figure S6. Defect feature appears at V in the band-gap of natural MoS 2. S7

8 7- Interface and surface science instrument Figure S7 shows a schematic of the Omicron 8 ultra high vacuum (UHV) system with a base pressure lower than mbar utilized to characterize the MoS 2 samples. The UHV system is a customized Omicron multiprobe-xps/spm 9. The analysis chamber consists of three photon sources, monochromated Al Kα, unmonochromated dual source Mg Kα and Al Kα, and a high intensity helium ultraviolet source (red box, Figure S6). The generated photoelectrons are detected by a multichannel plate MCD 128 Argus analyzer with dynamic-xps 10 and imaging capability (green box, Figure S6). The chamber contains also an Ar sputter and electron flood guns. The manipulator in the analysis chamber is connected to LN 2 cooling (<100 K), radiative heating (1170 K) and direct current heating. A coupled chamber houses a scanning probe microscope (SPM) with spring suspensions and magnetic damping to reduce vibration-induced noise (blue box, Figure S6). The SPM is a variable temperature design and allows analysis using traditional scanning tunneling microscopy/spectroscopy (STM/STS). An interconnected preparation chamber (yellow box, Figure S6) consists of manipulator with LN 2 cooling, radiative heating and direct current, pyrometer, precise gas dosing, four-pocket e-beam evaporator and thermal gas cracker for deposition and surface treatments. For analysis, the preparation chamber possesses a 300 AMU Quadrupole mass spectrometer and low energy electron diffraction. S8

9 Figure S7. In-situ surface science system dedicated to the nanoscale characterization (STM/AFM/LEED), Surface/interface chemistry (XPS) and electronic properties (UPS/STS). 8- References. (1) Becker, U.; Munz, A. W.; Lennie, A. R.; Thornton, G.; Vaughan, D. J. The Atomic and Electronic Structure of the (001) Surface of Monoclinic Pyrrhotite (Fe 7 S 8 ) as Studied Using STM, LEED and Quantum Mechanical Calculations. Surf. Sci. 1997, 389, (2) Dickinson, R. G.; Pauling, L. The Crystal Structure of Molybdenite. J. Am. Chem. Soc. 1923, 45, (3) Inoue, A.; Komori, T.; Shudo, K. I. Atomic-Scale Structures and Electronic States of Defects on Ar + -Ion Irradiated MoS 2. J. Electron Spectrosc. Relat. Phenom. 2013, 189, (4) Ganta, D.; Sinha, S.; Haasch, R. T. 2-D Material Molybdenum Disulphide Analyzed by XPS. Surf. Sci. Spec. 2014, 21, 19. (5) McDonnell, S.; Addou, R.; Buie, C.; Wallace, R. M.; Hinkle, C. L. Defect- Dominated Doping and Contact Resistance in MoS 2. ACS Nano 2014, S9

10 (6) KC, S.; Longo, R. C.; Addou, R.; Wallace, R. M.; Cho, K. Impact of Intrinsic Atomic Defects on the Electronic Structure of MoS 2 Monolayers. Nanotechnology 2014, 25, (7) Lu, C.-P.; Li, G.; Mao, J.; Wang, L.-M.; Andrei, E. Y. Bandgap, Mid-Gap States, and Gating Effects in MoS 2. Nano Lett. 2014, 14, (8) (9) Wallace, R. M. In-Situ Studies on 2D Materials. ECS Trans. 2014, 64, (10) McDonnell, S.; Brennan, B.; Bursa, E.; Wallace, R. M.; Winkler, K.; Baumann, P. GaSb Oxide Thermal Stability Studied by Dynamic-XPS. J. Vac. Sci. Technol. B, 2014, 32, S10