In Situ Surface Roughness Analysis of Electrodeposited Co Films in an Ionic Liquid Using Electrochemical Surface Plasmon Resonance: Effect of Leveling Additives

The effect of two leveling additives, thiourea (TU) and coumarin (CM), on cobalt electrodeposition process in an ionic liquid (IL), 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)amide (C4mimTFSA), at the gold surface has been analyzed in situ using electrochemical surface plasmon resonance (ESPR), in which the SPR resonance angle (Δθ) has been recorded simultaneously with cyclic voltammograms. The two additives show Δθ behaviors different from the additive-free case and from each other. In the case of TU, the positive Δθ shift due to Co cathodic deposition is larger than the additive-free case. Even after the subsequent negative Δθ shift due to Co anodic dissolution, Δθ remains positive not returning to the original value. This indicates that the dissolution of gold facilitated by TU roughens the surface of the gold electrode. In contrast, in the case of CM, the positive Δθ shift due to Co cathodic deposition is smaller than the additive-free case. A model analysis using Fresnel reflectivity has revealed that CM actually smooths the surface of the Co film even in the initial process of the electrodeposition. These findings illustrate that ESPR can sense in situ the surface roughness change during electrodeposition on the order of Å