Supporting Data. The University of Texas at Dallas, 800 West Campbell Road, Richardson, Texas 75080, United

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1 Supporting Data MoS 2 Functionalization for Ultra-thin Atomic Layer Deposited Dielectrics Angelica Azcatl, 1 Stephen McDonnell, 1 Santosh KC, 1 Xing Peng, 1 Hong Dong, 1 Xiaoye Qin, 1 Rafik Addou, 1 Greg I Mordi, 2 Ning Lu, 1 Jiyoung Kim, 1,2 Moon J. Kim, 1 Kyeongae Cho, 1 and Robert M. Wallace 1 1 Department of Materials Science and Engineering, 2 Department of Electrical Engineering, The University of Texas at Dallas, 800 West Campbell Road, Richardson, Texas 75080, United States Experimental and Calculation Details The in situ study was performed in a multifunctional deposition/characterization ultrahigh vacuum (UHV) system, described elsewhere. 1 X-ray Photoelectron Spectroscopy was carried out using a monochromated Al Kα ( ev) x-ray source, equipped with a 7 channel analyzer, using pass energy of 15 ev and scanning with a take-off angle of 45 with respect to the sample normal. Deconvolution of XPS peaks was carried out using AAnalyzer software. 2 As noted in the text, UV-O 3 exposure was accomplished in-situ by performing the exposure in an attached chamber. This was done so as to avoid spurious atmospheric (mainly physisorbed water and organic) contamination of the treated MoS 2 surface after the UV-O 3 exposure. Given the parameters used to accomplish the exposure and subsequent ALD of Al 2 O 3, one may also expect similar functionalization for atmospheric bench top UV-O 3 generators and ex-situ ALD reactors as well, although this was not examined in this study. One may speculate that the physisorbed contamination may be removed by the ALD reactor heating under vacuum, while

2 preserving the S-O observed functionalization prior to reaching ~200 C and/or initiating the precursor pulses. Low energy electron diffraction (LEED) was performed in an Omicron reverse-view instrument at a beam energy of 127 ev under a UHV environment in an interconnected chamber. For the DFT calculations, a plane wave basis set and Projector Augmented Wave (PAW) pseudopotentials were implemented in the Vienna Ab-initio Simulation Package (VASP). The electronic wave functions are represented by plane wave basis with a cutoff energy of 500 ev. The exchange correlation interactions are incorporated as a functional of Generalized Gradient Approximation (GGA). A model of 5 5 supercell of MoS 2 with the periodic slab separation of 1.6 nm vacuum in order to avoid spurious interaction between the two surfaces of the slab was used. In these calculations, the atoms are allowed to relax while the cell size is kept fixed. AFM images were obtained ex-situ from an Atomic Probe Microscope Veeco, Model 3100 Dimension V, located at the UT-Dallas Cleanroom. Tapping mode was performed using an OTESPa tip. TEM cross-sectional samples were made by the lift-out approach 3 using a FEI Nova 200 NanoLab dual-beam FIB/SEM system. A Au layer (~70 nm) was e-beam deposited on the Al 2 O 3 surface to eliminate electron charging. Layers of SiO 2 and Pt were also deposited to protect the region of interest during focused ion beam milling. HRTEM imaging was performed using a JEOL 2100F operated at 200 kv. Intensity ratios and attenuation

3 The oxygen coverage on the MoS 2 surface was calculated by using the ratio of the integrated intensities of S-O to total Sulfur (I S-O /I S ). The integrated intensity of S 2p core level spectra from the S atoms bond to oxygen atoms corresponds to I S-O and the total integrated intensity of S 2p spectra from both the S-O and S-Mo corresponds to I S. Here, I S1 and I S2 represent the sulfur intensity corresponding to the summarized intensity from top (S 1 ) and bottom (S 2 ) layers in each MoS 2 layer, as shown in Figure S1. The total intensity of S (including S-O and S-Mo) equals to I S1 + I S2. Therefore, I S-O /I S follows the equation: I s O I S = I s O I S1 +I S2 This equation assumes that S-O layer is not attenuated by any overlayer (I s O = I monolayer ) and that the oxidation process did not change the structure of MoS 2. Figure S1. Molybdenum disulfide atomic layer model and characteristic distances. The signal intensities follow:

4 I S1 = I monolayer e 0 + e δ + e 2δ + = I monolayer 1 e δ I S2 = I monolayer e d 1 λsinθ e 0 + e δ + e 2δ e + = I monolayer d 1 λsinθ 1 e δ Where, δ = (d 1+d 2 ) λsinθ Thus, I s O I S = e d 1 λsinθ 1 e δ The distances, d 1= nm and d 2 = 0.30 nm according to previous reports. 4 The effective attenuation length (EAL) λ was calculated using NIST electron EAL Database, Version 1.3; for an electron traveling through MoS 2, λ MoS2 =2.542 nm. Effect of remotely generated O 3 without UV exposure A bulk MoS 2 crystal (SPI) sample, prepared as described in the methods section, was loaded into the UHV system. The MoS 2 sample was transferred to the same clustered chamber as the one used for UV-O 3 treatments. O 3 was remotely generated and leaked to the chamber up to a pressure of 900 mbar; same exposure time as UV-O 3 treatment (15 min) was used.

5 Figure S2. Mo3d and S 2p spectra of exfoliated MoS 2 before and after O 3 exposure. O 3 in this case was remotely generated and no UV light was incident on the sample surface. Figure S2 shows the XPS spectra of the MoS 2 surface, as exfoliated and after O 3 exposure. In should be noted that the as exfoliated MoS 2 sample exhibit a 2nd feature for both Mo 3d and S 2p core levels, which does not represent an additional chemical state; instead, it reflects the intrinsic n- and p-type behavior of MoS 2 that is detected simultaneously by XPS and can vary significantly across a single sample, as described elsewhere. 5,6 After O 3 exposure, oxidation of sulfur was not achieved; thus, the O 3 exposure alone on MoS 2, under these conditions, does not result in S-O bond formation. This is in agreement with the DFT calculation, in which an energetic barrier must be exceeded in order to form S-O bonds, which, in the case of a UV-O 3 treatment, is further facilitated by UV light exposure. Lastly, the decrease in intensity of the 2 nd feature occurring upon O 3 exposure could be attributed to either small

6 shifts in sample analysis position or suggests that O 3 (or other decomposition products) does interact with the surface but not to the point to cause a chemical state change. As Exfoliated MoS 2 (a) (b) MoS 2 + UV-O 3 1 nm 0 nm Figure S3. AFM images of (a) bulk MoS 2 exfoliated as described in the methods section and (b) bulk MoS 2 after UV-O 3 treatment. Scale Bar: 60 nm. Surface morphology Previous reports showed that when natural MoS 2 is treated with RF-oxygen plasma, new features on the surface with height of a few nanometers and length of nm were formed. 7 Such features were attributed to the breakage of the surface basal plane via etching and oxidation of molybdenum and sulfur. As a comparative study, the effect of UV-O 3 treatment on the surface morphology of MoS 2 was performed. AFM images are presented in Figure S3 of the as-exfoliated and UV-O 3 treated MoS 2 surface. It should be noted that after the UV-O 3 treatment, no additional nanoscale features are observed, as reported for a RF-oxygen plasma treatment.7,8. This result is consistent with our observations of lack of disruption of the MoS 2 surface given by in situ XPS and LEED.

7 ALD Al 2 O 3 film uniformity Cross sectional high-resolution transmission electron microscopy HR-TEM was conducted on Al 2 O 3 films deposited on O-MoS 2 at different temperatures. Figure S4 shows uniform Al 2 O 3 films obtained at 200 C and 250 C, but for the case of deposition at 300 C, pin-hole formation in the film is detected. This suggests that the enhancement in ALD nucleation given by the chemisorbed oxygen at lower temperatures is not spatially confined; therefore, continuous and fully covered Al 2 O 3 thin films are constant over a large length scale (~1μm). (a) (b) (c) Figure S4. Low magnification HRTEM cross-section images of Al 2 O 3 deposited by 30 ALD cycles on the oxygen functionalized MoS 2 confirming the uniformity of the films deposited at (a) 200 C and (b) 250 C, and the island growth mode at (c) 300 C.

8 Effect of Annealing on Oxygen functionalized MoS 2 To determine the thermal stability of the chemisorbed oxygen on MoS 2, in situ anneals of 20 min in the ALD chamber vacuum, at the temperatures used for Al 2 O 3 deposition, were performed. The XPS spectra in Figure S5 show that the S-O intensity starts to decreases as temperature is increased, to the point of being under the detection limits at 300 C. Desorption of oxygen is probable to occur driven by the increase in thermal energy. Figure S5. XPS spectra of the S 2p showing the thermal stability of the chemisorbed oxygen on MoS 2. O-MoS 2 was annealed for 20 min under N 2 environment at ~10 mbar at the ALD chamber, without precursor exposure.

9 1 R. M. Wallace, ECS Trans. 16, 255 (2008). 2 A. Herrera-Gomez, aherrera@qro.cinvestav.mx CINVESTAV Queretaro, MX. 3 L.A. Giannuzzi and F.A. Stevie, Micron, 30, 197 (1999). 4 P. D. Fleischauer, J. R.Lince, P.A. Bertrand, and R. Bauer, Langmuir 5, 1009 (1989) 5 S. McDonnell, R. Addou, C. Buie, R. M. Wallace, C. L. Hinkle, Submitted (2013). 6 J.A. Wilson, A. D. Yoffe, Adv. Phys. 18, 193 (1969). 7 N. Cui, N. M. D. Brown, A. Mckinley, Appl. Surf. Sci. 151, 17 (1999). 8 J. Yang, S. Kim, W. Choi, S. H. Park, Y. Jung, M.-H. Cho, and H. Kim, ACS Appl. Mater. Interfaces 5, 4739 (2013).