Supporting Information

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1 Supporting Information Tribological Properties of Ultrananocrystalline Diamond Films in Inert and Reactive Tribo-Atmospheres: XPS Depth-Resolved Chemical Analysis Revati Rani a, Kalpataru Panda b, Niranjan Kumar a *, Kamatchi Jothiramalingam Sankaran c,d, K. Ganesan a, I-Nan Lin e a Materials Science Group, Indira Gandhi Centre for Atomic Research, HBNI, Kalpakkam , Tamil Nadu, India b Center for Nanomaterials and Chemical Reactions, Institute for Basic Science (IBS), Daejeon 34141, Korea c Institute for Materials Research (IMO), Hasselt University, 3590 Diepenbeek, Belgium d IMOMEC, IMEC vzw, 3590 Diepenbeek, Belgium e Department of Physics, Tamkang University, New-Taipei 251, Taiwan, Republic of China *Corresponding author niranjan@igcar.gov.in, phystribology@gmail.com Tel.: (ext ) Fax: S1

2 Supplementary content: S1. Results and discussion: S3 S1.1. Microstructure of films S1.2. Surface topography and roughness of films S1.3. Raman spectroscopy of films Figures: Figure S1. (a) Bright field (BF) TEM micrograph of the UNCD films with selective area electron diffraction (SAED) pattern in the inset (b) TEM structure image of UNCD films corresponding to region A in (a). Fourier transformed diffractogram of the whole structure image (FT 0 ) and SAED pattern of region 1 and 2 (ft 1 and ft 2 ). Figure S2. 2D and 3D topography of films: (a) as-deposited (b) F Ar _15 and (c) F Ar _30; 2D topography: (a 1 ), (b 1 ) and (c 1 ) and 3D topography: (a 2 ), (b 2 ), and (c 2 ); (d) average roughness (R a ) and RMS roughness (R q ) of d 1 : as-deposited, d 2 : F Ar _15 and d 3 : F Ar _30 films, respectively (e) sputtering depth versus sputtering time 0, 5, 10, 15, 20, 25, 30 minutes for (a), (b), (c), (d), (e), (f) and (g), respectively. Figure S3. (a) Raman spectra of films and (b) peak position of v 1, v 2, v 3, D and G bands and I(D)/I(G) ratio: (a 1 ) as-deposited (a 2 ) F Ar _15 and (a 3 ) F Ar _30 films, respectively. Figure S4. Friction coefficient of as-deposited, F Ar _15 and F Ar _30 films in ambient humidatmosphere, N 2, Ar and CH 4 tribo-mediums. Figure S5. EDX mapping of deformed alumina ball scar region developed in ambient humid tribo-atmospheric condition: (a) SEM image of EDX mapping region (b) C K-edge (c) O K-edge (d) Al K-edge and (e) superimposed image and at.% of all the elements. Figure S6. EDX mapping of deformed alumina ball scar region developed in N 2 triboatmospheric condition: (a) SEM image of EDX mapping region (b) C K-edge (c) N K-edge (d) O K-edge (e) Al K-edge and (f) superimposed image and at.% of all the elements. Figure S7. EDX mapping of deformed alumina ball scar region developed in Ar triboatmospheric condition: (a) SEM image of EDX mapping region (b) C K-edge (c) O K-edge (d) Al K-edge and (e) superimposed image and at.% of all the elements. Figure S8. EDX mapping of deformed alumina ball scar region developed in CH 4 triboatmospheric condition: (a) SEM image of EDX mapping region (b) C K-edge (c) O K-edge (d) Al K-edge and (e) superimposed image and at.% of all the elements. References S3 S4 S6 S4 S5 S7 S8 S9 S10 S11 S12 S13 S2

3 S1. Results and discussion S1.1. Microstructure of films Figure S1a shows the bright field (BF) TEM micrograph of the UNCD films with an inset showing the selective area electron diffraction (SAED) pattern. The SAED contains sharp diffraction rings corresponding to (111), (220) and (311) lattices of diamond structure, confirming that the film content is diamond. The diffraction rings in the SAED pattern are smooth and continuous, indicating that the diamond grains are randomly oriented. A diffused ring in the center of SAED indicates the existence of amorphous carbon (a-c), which is probably the existence of trans-polyacetylene phases in the grain boundaries of UNCD films. This was further confirmed by Raman spectroscopy also and will be discussed later. Figure S1b shows a TEM structure image of UNCD films corresponding to region A in Figure S1a. Fourier transformed (FT) diffractogram of the whole structure image (FT 0 in Figure S1b) shows a spotted diffraction pattern arranged in ring, suggesting of nano-sized nature of the diamond. A large diffused ring of full-moon geometry located at the middle of FT image corresponds to a-c phases. The existence of diamond and a-c phases is highlighted by regions 1 and 2, respectively, which are identified by the FT images ft 1 and ft 2, respectively. S3

4 Figure S1. (a) bright field (BF) TEM micrograph of the UNCD films with selective area electron diffraction (SAED) pattern in the inset (b) TEM structure image of UNCD films corresponding to region A in (a). Fourier transformed diffractogram of the whole structure image (FT 0 ) and SAED pattern of region 1 and 2 (ft 1 and ft 2 ). S1.2. Surface topography and roughness of films The 2D and 3D topographical feature of the Ar + -ion sputtered films are quite similar to asdeposited films (Figure S2a-c). However, variation in the roughness value was observed after the Ar + -ion sputtering process (Figure S2d). This was associated to Ar + -ion sputtering induced surface etching and decontamination of the film surface. It is worth mentioning that after the Ar + -ion sputtering, the F Ar _15 and F Ar _30 films showed sputtered depth of roughly 18±3 nm and S4

5 36±5 nm, respectively. The sputtered depth for all the sputtering times is presented in Figure S2e. Figure S2. 2D and 3D topography of films: (a) as-deposited (b) F Ar _15 and (c) F Ar _30; 2D topography: (a 1 ), (b 1 ) and (c 1 ) and 3D topography: (a 2 ), (b 2 ), and (c 2 ); (d) average roughness S5

6 (R a ) and RMS roughness (R q ) of d 1 : as-deposited, d 2 : F Ar _15 and d 3 : F Ar _30 films, respectively (e) sputtering depth versus sputtering time 0, 5, 10, 15, 20, 25, 30 minutes for (a), (b), (c), (d), (e), (f) and (g), respectively. S1.3. Raman spectroscopy of films Bulk chemical properties of as-deposited, F Ar _15 and F Ar _30 films were analyzed by Micro- Raman spectroscopy (Figure S3). All Raman spectra are deconvoluted into five individual peaks designated as v 1, v 2, v 3, D and G bands (Figure S3a). All these peaks are associated to phonon behavior of grain boundary phases of UNCD films. 1 3 First three peaks belong to transpolyacetylene (TPA) phases and arise due to C-H in-plane bending, C-H stretching and C=C symmetric stretching vibrations, respectively. 3 5 Moreover, D and G bands are signature of a-c and graphite-sp 2 phases, respectively. 6 Raman band appearing at 1332 cm -1 (zone-center optical phonon mode) 7 in microcrystalline and single crystals diamond becomes broader in ultrananodiamond crystallites due to phonon confinement effect. 8 Moreover, stronger D band of a-c and graphite-sp 2 phase may possibly quench the weaker diamond band if it appears and thus, remains unresolved. S6

7 Figure S3. (a) Raman spectra of films and (b) peak position of v 1, v 2, v 3, D and G bands and I(D)/I(G) ratio: (a 1 ) as-deposited (a 2 ) F Ar _15 and (a 3 ) F Ar _30 films, respectively. The systematic peak intensity reduction and blue shift of v 1, v 2 and v 3 bands in TPA phase was observed in F Ar _15 and F Ar _30 films compared to as-deposited films (Figure S3a-b). These two facts directly indicate the change of longer TPA chains in as-deposited films into shorter one after Ar + -ion sputtering. 9 Furthermore, Blue shift of v 2 and v 3 bands is more significant compared to v 1 which is related to C-H bond rigidity and strength. Moreover, blue shift of G band and increase in I(D)/I(G) ratio in Ar + -ion sputtered films is fingerprint of smaller in-plane size of sp 2 phase (Figure S3a). 10 The repeated Raman spectra at several locations also validate our observation for TPA, D and G band shifts, I(D)/I(G) ratio in as-deposited, F Ar _15 and F Ar _30 films (Figure S3b). Finally, Raman analysis confirmed the decrease in graphite phase with the increase in D band intensity in Ar + -ion sputtered films which is directly related to increase in a-c S7

8 and sp 3 phase. 10 This change is simply related to the depth dependent variation in the chemical nature of the films and elimination of sp 2 rich phase with longer TPA chain from the film surface during sputtering process. Bouchet et al. noticed the sp 2 phase rich layer formation at the film surface region during the final steps of the deposition process. 11 Here, it is important to mention that there must not be any Ar + -ion sputtering induced conversion of sp 2 into sp 3 phase and vice versa because such conversion needs very high activation energy 12 and therefore, we observe merely depth dependent variation in the chemical nature of films. Figure S4. Friction coefficient of as-deposited, F Ar _15 and F Ar _30 films in ambient humidatmosphere, N 2, Ar and CH 4 tribo-mediums. S8

9 Figure S5. EDX mapping of deformed alumina ball scar region developed in ambient humid tribo-atmospheric condition: (a) SEM image of EDX mapping region (b) C K-edge (c) O K-edge (d) Al K-edge and (e) superimposed image and at.% of all the elements. S9

10 Figure S6. EDX mapping of deformed alumina ball scar region developed in N 2 triboatmospheric condition: (a) SEM image of EDX mapping region (b) C K-edge (c) N K-edge (d) O K-edge (e) Al K-edge and (f) superimposed image and at.% of all the elements. S10

11 Figure S7. EDX mapping of deformed alumina ball scar region developed in Ar triboatmospheric condition: (a) SEM image of EDX mapping region (b) C K-edge (c) O K-edge (d) Al K-edge and (e) superimposed image and at.% of all the elements. S11

12 Figure S8. EDX mapping of deformed alumina ball scar region developed in CH 4 triboatmospheric condition: (a) SEM image of EDX mapping region (b) C K-edge (c) O K-edge (d) Al K-edge and (e) superimposed image and at.% of all the elements. S12

13 References (1) Rani, R.; Kumar, N.; Kozakov, A. T.; Googlev, K. A.; Sankaran, K. J.; Das, P. K.; Dash, S.; Tyagi, A. K.; Lin, I. N. Superlubrication Properties of Ultra-nanocrystalline Diamond Film Sliding against a Zirconia Ball. RSC Adv. 2015, 5, (2) Birrell, J.; Gerbi, J. E.; Auciello, O.; Gibson, J. M.; Johnson, J.; Carlisle, J. A. Interpretation of the Raman Spectra of Ultrananocrystalline Diamond. Diam. Relat. Mater. 2005, 14, (3) Mishra, K. K.; Rani, R.; Kumar, N.; Ravindran, T. R.; Sankaran, K. J.; Lin, I. N. High Pressure Raman Spectroscopic Studies on Ultrananocrystalline Diamond Thin Films: Anharmonicity and Thermal Properties of the Grain Boundary. Diam. Relat. Mater. 2017, 80, (4) López-Ríos, T.; Sandré, É.; Leclercq, S.; Sauvain, É. Polyacetylene in Diamond Films Evidenced by Surface Enhanced Raman Scattering. Phys. Rev. Lett. 1996, 76, (5) Ferrari, A. C.; Robertson, J. Origin of the 1150 cm -1 Raman Mode in Nanocrystalline Diamond. Phys. Rev. B 2001, 63, (6) Ferrari, A. C.; Robertson, J. Resonant Raman Spectroscopy of Disordered, Amorphous, and Diamondlike Carbon. Phys. Rev. B 2001, 64, (7) Solin, S. A.; Ramdas, A. K. Raman Spectrum of Diamond. Phys. Rev. B 1970, 1(4), (8) Sun, K. W.; Wang, J. Y.; Ko, T. Y. Raman Spectroscopy of Single Nanodiamond: Phonon- Confinement Effects. Appl. Phys. Lett. 2008, 92, (9) Rybachuk, M.; Bell, J. M. Electronic States of Trans-polyacetylene, Poly(p-phenylene vinylene) and sp-hybridised Carbon Species in Amorphous Hydrogenated Carbon Probed by Resonant Raman Scattering. Carbon, 2009, 47, (10) Ferrari, A. C.; Robertson, J. Interpretation of Raman Spectra of Disordered and Amorphous Carbon. Phys. Rev. B. 2000, 61, (11) De Barros Bouchet, M. I.; Matta, C.; Vacher, B.; Le-Mogne, T.; Martin, J. M.; von Lautz, J.; Ma, T.; Pastewka, L.; Otschik, J.; Gumbsch, P. et al. Energy Filtering Transmission Electron Microscopy and Atomistic Simulations of Tribo-induced Hybridization Change of Nanocrystalline Diamond Coating. Carbon 2015, 87, (12) Narayan, J.; Bhaumik, A. Direct Conversion of Amorphous Carbon into Diamond at Ambient Pressures and Temperatures in Air. APL Mater. 2015, 3, S13