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To verify the chemical composition of the samples, in particular to investigate a possible presence of oxygen in the adhesion layer, STEM-EELS analysis was used. A line scan across the layer interfaces of the 2nm Ti/2nm Au sample shows the presence of a Ti core loss L3 edge at 460 eV and an L2 edge at 465 eV. A SiO2 O-K edge is visible at 538 eV, while the O-K edge of O bounded to Ti is found at 532 eV (Fig. 9a). The same investigation was performed on the 2nm Cr/2nm Au sample (Fig. 9b). The analysis showed a Cr L3 edge at 585 eV and an L2 edge at 594 eV. The SiO2 O-K edge is visible at 545 eV, while at 540 eV a weak OCr-K edge of O bounded to Cr is visible for a limited thickness below Au. Furthermore, the Cr edge presents a compositional tail along the scan direction, which confirms diffusion into the Au layer. For the length of the tail there is no presence of OCr-K edge, indicating that Cr inside Au is in metallic form. This is in good agreement with the observed diffusion, which involves only metallic Cr.
To verify the chemical composition of the samples, in particular to investigate a possible presence of oxygen in the adhesion layer, STEM-EELS analysis was used. A line scan across the layer interfaces of the 2nm Ti/2nm Au sample shows the presence of a Ti core loss L3 edge at 460 eV and an L2 edge at 465 eV. A SiO2 O-K edge is visible at 538 eV, while the O-K edge of O bounded to Ti is found at 532 eV (Fig. 9a). The same investigation was performed on the 2nm Cr/2nm Au sample (Fig. 9b). The analysis showed a Cr L3 edge at 585 eV and an L2 edge at 594 eV. The SiO2 O-K edge is visible at 545 eV, while at 540 eV a weak OCr-K edge of O bounded to Cr is visible for a limited thickness below Au. Furthermore, the Cr edge presents a compositional tail along the scan direction, which confirms diffusion into the Au layer. For the length of the tail there is no presence of OCr-K edge, indicating that Cr inside Au is in metallic form. This is in good agreement with the observed diffusion, which involves only metallic Cr.


<gallery widths="450px" heights="450px" perrow="2" halign="center"> image:Picture29.png|Fig. 8: STEM-EDX maps of the 2nm Ti/2nm Au sample (a), 2nm Cr/2nm Au sample (b) and 2nm Cr/20nm Au sample (c). The Au L-alpha signal is acquired at 9713 eV, the Ti K-alpha signal at 4510.9 eV and the Cr K-alpha signal at 5414.7 eV.  
<gallery widths="400px" heights="400px" perrow="2" halign="center"> image:Picture29.png|Fig. 8: STEM-EDX maps of the 2nm Ti/2nm Au sample (a), 2nm Cr/2nm Au sample (b) and 2nm Cr/20nm Au sample (c). The Au L-alpha signal is acquired at 9713 eV, the Ti K-alpha signal at 4510.9 eV and the Cr K-alpha signal at 5414.7 eV.  
image:Picture12.png|Fig. 9: (a) STEM-EELS linear scan of the 2nm Ti/2nm Au sample, showing the presence of oxygen in the Ti layer. (b) STEM-EELS linear scan of the 2nm Cr/2nm Au sample, which shows the presence of oxygen that is bounded to Cr and Cr diffusion into the Au layer. </gallery>
image:Picture12.png|Fig. 9: (a) STEM-EELS linear scan of the 2nm Ti/2nm Au sample, showing the presence of oxygen in the Ti layer. (b) STEM-EELS linear scan of the 2nm Cr/2nm Au sample, which shows the presence of oxygen that is bounded to Cr and Cr diffusion into the Au layer. </gallery>


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The first analysis was performed to clarify if oxygen was originating from source i). To rule out possible oxygen migration from the oxygen-rich SiO2 substrate, the metal thin-films were deposited on amorphous Si3N4. For both samples, after Ar ion milling of 20 nm of Au, the Au 4f signal intensity decreased and Cr 2p and Ti 2p signals started to appear together with the O 1s signal (Fig. 10a for Ti and 10b for Cr, respectively).
The first analysis was performed to clarify if oxygen was originating from source i). To rule out possible oxygen migration from the oxygen-rich SiO2 substrate, the metal thin-films were deposited on amorphous Si3N4. For both samples, after Ar ion milling of 20 nm of Au, the Au 4f signal intensity decreased and Cr 2p and Ti 2p signals started to appear together with the O 1s signal (Fig. 10a for Ti and 10b for Cr, respectively).


[[File:Picture13.png|450px|center|thumb|Fig. 10: XPS depth profiling of 2nm Ti/20nm Au (a) and 2nm Cr/20nm Au (b) samples deposited on Si3N4 substrate. Both samples show the presence of a Au 4f signal from the surface to a depth of 20 nm. For a depth deeper than 20 nm, both Ti 2p and Cr 2p signals appear together with the O 1s signal.]]
[[File:Picture13.png|500px|center|thumb|Fig. 10: XPS depth profiling of 2nm Ti/20nm Au (a) and 2nm Cr/20nm Au (b) samples deposited on Si3N4 substrate. Both samples show the presence of a Au 4f signal from the surface to a depth of 20 nm. For a depth deeper than 20 nm, both Ti 2p and Cr 2p signals appear together with the O 1s signal.]]


Peak fits for the Ti and Cr signals were performed, and are reported in Fig. 11. The Ti 2p peak is a convolution of three components: a TiN doublet at 455.0 and 461.1 eV, a TiO2 doublet at 457.7 and 464.0 eV and a TiOx signal, which forms the descending background tail. The fit gives the following information: 1) Ti has formed Ti-N bonds with the Si3N4 substrate; 2) the adhesion layer was partially oxidized during deposition and not from source i). Metallic Ti could not be detected, but its presence cannot be excluded: metallic Ti is highly reactive with respect to oxygen and nitrogen, and the destructive sputtering process used for the depth profiling could have enhanced the mixing between Ti, O and N, catalyzing the reaction of the metallic Ti bound to Au to form an oxide or a nitride. The Cr 2p peak fit is formed by three components: a metallic Cr doublet at 574.4 and 583.6 eV, a Cr2O3 doublet at 576.3 and 585.6 eV and a CrO3 doublet at 580 and 589.2 eV. The result of the fit indicates a partial oxidation of Cr, as in the Ti case.
Peak fits for the Ti and Cr signals were performed, and are reported in Fig. 11. The Ti 2p peak is a convolution of three components: a TiN doublet at 455.0 and 461.1 eV, a TiO2 doublet at 457.7 and 464.0 eV and a TiOx signal, which forms the descending background tail. The fit gives the following information: 1) Ti has formed Ti-N bonds with the Si3N4 substrate; 2) the adhesion layer was partially oxidized during deposition and not from source i). Metallic Ti could not be detected, but its presence cannot be excluded: metallic Ti is highly reactive with respect to oxygen and nitrogen, and the destructive sputtering process used for the depth profiling could have enhanced the mixing between Ti, O and N, catalyzing the reaction of the metallic Ti bound to Au to form an oxide or a nitride. The Cr 2p peak fit is formed by three components: a metallic Cr doublet at 574.4 and 583.6 eV, a Cr2O3 doublet at 576.3 and 585.6 eV and a CrO3 doublet at 580 and 589.2 eV. The result of the fit indicates a partial oxidation of Cr, as in the Ti case.
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