Supporting Information

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1 Supporting Information Defects and Surface Structural Stability of MoTe 2 Under Vacuum Annealing Hui Zhu, Qingxiao Wang, Lanxia Cheng, Rafik Addou, Jiyoung Kim, Moon J. Kim*, Robert M. Wallace* Department of Materials Science and Engineering, The University of Texas at Dallas, 800 West Campbell Road, Richardson, Texas 75080, United States * rmwallace@utdallas.edu and moonkim@utdallas.edu ; H.Z. and Q.W. contributed equally to this work 1

2 Figure S1. XPS analysis of 2H-MoTe 2 from HQ Graphene Figure S1. XPS spectra of (a) survey region, (b) C 1s and O 1s core levels from fresh exfoliated MoTe 2 samples purchased from the HQ Graphene vendor. As shown in the survey region, C and O are the only detectable impurities within the XPS detection limits. The C and O, associated with hydrocarbon C-C (284.4 ev) and C-O (285.6 ev) chemical states, are unavoidable for the ex situ preparation process due to surface re-adsorption from the atmosphere exposure or exfoliation tape residuals. Occasionally, a small concentration (less than 3 atomic %) of surface oxides, Te-O bonds (~576.1 ev) or Mo-O bonds (~232.2 ev), are also detectable and with a corresponding O 1s state located at ev. 2

3 Figure S2. Elemental analysis using inductively coupled plasma mass spectrometry Figure S2. Elemental analysis of MoTe 2 flake. The inductively coupled plasma mass spectrometry (ICPMS) technique with a detection limit of 0.1 parts per billion weight (ppbw) was used to search for 40 elements in a digested MoTe2 flake. 1 As highlighted in yellow and orange, only 10 elements (Na, Al, K, Ca, Nb, Cd, In, Ba, W, and Tb) were detected with concentrations larger than 0.11 ppbw. Elements highlighted in gray (blue) are not detected (measured). The equivalent concentration is normalized for a Si host matrix. The estimation of the equivalent impurity concentration levels normalized for a Si host matrix shows that this exceeds /cm 2 for only Cd and W, which is well below the detection limit of XPS or the defect concentration observed in STM images. 3

4 Figure S3. Surface variation of MoTe 2 from different sources Figure S3. Surface variation of MoTe 2 from different sources. (a-b) STM images ( nm 2 ) of exfoliated MoTe2 (0001) surfaces from (a) HQ Graphene ( HQ ) and (b) 2D Semiconductor Inc ( 2D ) with a protrusion areal density of 18.8% and 23.6%, respectively. The STM images are acquired at (a) Vb = 0.7 V, It = 1 na and (b) Vb = 1.5V, It = 0.5 na. (c) XPS spectra of Te 3d5/2 and Mo 3d core level regions from these two samples. All spectra are normalized with respect to the Mo 3d spectra to elucidate the stoichiometry variation. The corresponding Te/Mo atomic ratios range from and for the HQ and 2D samples, respectively, follows the trend of defect densities. 4

5 Figure S4. HAADF-STEM image of monolayer and bilayer MoTe 2 flake Figure S4. HAADF images of monolayer and bilayer MoTe 2 flakes. (a) Experimental raw image of a monolayer MoTe2 flake (b) Deconvoluted image of the monolayer MoTe2 flake (c) Experimental raw image of a bilayer MoTe2 flake (d) Deconvoluted image of the bilayer MoTe2 flake. 5

6 Figure S5. STEM simulation of adatoms on MoTe 2 thin films Figure S5. STEM image simulation of adatoms on MoTe 2 flakes with different thickness. (a-j) HAADF image simulations of Te, Mo, and C adatoms on MoTe2 thin films from monolayer up to 4 layers, respectively. (k) The top-view and side-view of the atomic model of adatoms on the surface of a monolayer MoTe2 flake. The adatoms may locate at the upper-left A site (above Te atom), middle-bottom B site (above Mo atom) and upper-right C site (hollow). The intensity profiles across the A site (red line), B site (blue line), and C site (cyan line) have been plotted with red, blue and cyan curves, respectively. 6

7 Te and Mo adatoms have similar image contrast, and they can be easily identified on the monolayer MoTe2 flake. The Te/Mo adatoms at the A and B sites on bilayer MoTe2 are still observable but not obvious at the C site. It is very difficult to locate Te/Mo adatoms on MoTe2 flakes more than 3 layers. The carbon adatoms contrast is invisible even for monolayer MoTe2 flake. This means that the light elements from polymer residues will not affect the HAADF images. Figure S6. Bias dependent protrusion features on MoTe 2 Figure S6. Bias dependent protrusions in MoTe 2. (a-b) nm 2 STM imaging on the same surface region at different sample biases. The sample bias and tunneling current are held constant for (a) Vb = V, It = 1.5 na and (b) Vb = -0.3 V, It = 1.5 na, respectively. (c) Z profiles across lines drawn in (a) and (b), respectively. The protrusions are higher under negative sample bias compared to the positive sample bias. Interestingly, some protrusions (indicated as blue dots) barely seen at positive sample biases can be pronounced at negative sample biases. Such observations confirm that the protrusions are originating from an electronic effect rather than a topographic effect. We attribute the variance of the protrusion heights in the STM image is related with different depths of the impurities beneath the sample surface detected by the associated electron density. 2 7

8 Figure S7. STM/STS characterization of Mo atomic vacancies Figure S7. STM/STS analysis of Mo atomic vacancies on one MoTe 2 sample from HQ Graphene. The STM images (a and b) are taken under a sample bias of 1V and a tunneling current of 0.3 na. It was noted that such a surface condition with some dark depression defects (lateral dimension of ~1 nm) and the low concentration of bright protrusion defects is only observed once on an exfoliated sample surface from HQ Graphene. The dark depression suggests electron depletion near those defects, consistent with the p-type behavior that obtained on those regions (see STS spectra in panel c). A similar phenomenon has been observed on other TMDs and usually is correlated to the atomic vacancy of a transition metal (VMo). 3,4 8

9 Figure S8. XPS investigation of air stability of MoTe 2 Figure S8. XPS spectra of Te 3d 5/2, Mo 3d, O 1s and C 1s core level regions from a freshly exfoliated sample after being exposed to air for 5 min, 15 min, 30 min, and 2 days, respectively. The surface chemistry has been compared with one as-received MoTe2 sample with native oxides. The Te/Mo-O peak in the O 1s core level spectrum is near the XPS detection limit during the initial 30 min air exposure, suggesting there are no detectable surface oxides for a short time air exposure. After exposing to air for 2 days, analysis of the sample surface reveals a small concentration of surface oxides with Te-O bonds of 2.7 at.% and Mo-O bonds of 3.5 at.%. The oxidation rate is much slower than previous reports on MBE synthesized MoTe2 with substantial Te vacancies, which was immediately oxidized after exposing to air. 5 The results suggest that the interstitial Te defects within the MoTe2 do not substantially degrade the crystal s chemical stability. 9

10 Figure S9. STS spectra of un-dissociated molybdenum telluride clusters at 300 C Figure S9. Two examples of STS measurements along the line moving from 2H-MoTe 2 region toward cluster defects generated at 300 C. Both results are similar in that the bandgap, especially the valence band edge near the defect regions, is reduced. This behavior is attributed to the formation of gap states. 10

11 Figure S10. Different domain morphologies of MoTe 2 Figure S10. Different domain morphologies of MoTe 2. (a-b) Topographic STM images of triangular domains located at the boundary of the 2H phase and wagon wheel (WW) pattern. This 2H-WW boundary is less than 2% coverage of the bulk sample surface (c-e) STEM images of different domain morphologies identified on a bilayer MoTe2 thin film. The observations suggest that WW networks are stacked by dense inversion domain boundaries (IDBs) driven by thermodynamic equilibrium rather than from a moiré interference explanation. 6 Previous STEM study on MoSe2 IDBs suggested that it is the releasing of compressive strain, originated from chalcogen vacancies, initiates the formation of IDBs. 7 It is noted that both the panel (d) and (e) are measurable WW morphologies in a bilayer region given the free-standing property of the layers utilized for STEM imaging windows. A Te vacancy-induced Mo gliding-plane can take place independently in the topmost and bottom MoTe2 layer, thus changing the final morphology contrast. (f and g) Structural model (top) and corresponding image simulation (bottom) of an IDB on a pristine monolayer MoTe2 and on a monolayer MoTe2 with IDBs, respectively. The experimental results match with the simulated morphology in (g) only, suggesting that both the top and bottom Mo planes have glided. 11

12 Figure S11. HAADF-STEM image and simulation of inversion domains in a monolayer MoTe 2 flake Figure S11. STEM image and simulation of inversion domains in monolayer MoTe 2 flake. (a) Experimental HAADF raw image of a monolayer MoTe 2 flake (b) Deconvoluted image of the monolayer MoTe 2 flake (c) Inversion domains model according to the experimental data (d) HAADF-STEM image simulation based on the model. Te single vacancies and Te2 column vacancies (losing both the top and bottom Te atoms) are indicated with green and red circles, respectively. Two neighboring domains have been highlighted with lime and red color lines. 12

13 Figure S12. Tape residues of the exfoliated MoTe 2 flake Figure S12. Tape residues of the exfoliated MoTe 2 flake. The tape residues associated from the sample transfer process are difficult to remove by UHV annealing or acetone, isopropanol, and DI water cleaning. References. (1) Addou, R.; McDonnell, S.; Barrera, D.; Guo, Z.; Azcatl, A.; Wang, J.; Zhu, H.; Hinkle, C. L.; Quevedo-Lopez, M.; Alshareef, H. N.; Colombo, L.; Hsu, J. W. P.; Wallace, R. M. Impurities and Electronic Property Variations of Natural MoS2 Crystal Surfaces. ACS Nano 2015, 9, (2) Addou, R.; Wallace, R. M. Surface Analysis of WSe2 Crystals: Spatial and Electronic Variability. ACS Appl. Mater. Interfaces 2016, 8, (3) Caulfield, J. C.; Fisher, A. J. Electronic Structure and Scanning Tunnelling Microscope Images of Missing-Atom Defects on MoS2 and MoTe2 Surfaces. J. Phys. Condens. Matter 1997, 9, (4) Sommerhalter, C.; Matthes, T. W.; Boneberg, J.; Lux-Steiner, M. C.; Leiderer, P. Investigation of Acceptors in P-Type WS2 by Standard and Photo-Assisted Scanning Tunneling Microscopy/spectroscopy. Appl. Surf. Sci. 1999, 144, (5) Diaz, H. C.; Chaghi, R.; Ma, Y.; Batzill, M. Molecular Beam Epitaxy of the van Der Waals Heterostructure MoTe2 on MoS2: Phase, Thermal, and Chemical Stability. 2D Mater. 2015, 2, 1 5. (6) Murata, H.; Koma, A. Modulated STM Images of Ultrathin MoSe2 Films Grown on MoS2(0001) Studied by STM/STS. Phys. Rev. B 1999, 59, (7) Lin, J.; Pantelides, S. T.; Zhou, W. Vacancy-Induced Formation and Growth of Inversion Domains in Transition-Metal Fichalcogenide Monolayer. ACS Nano 2015, 9,