Raman and QCM Studies of PPG and PEG Adsorption on Cu Electrode Surfaces

Abstract

We investigate the behavior of Cu plating bath suppressor additives poly(ethylene glycol) (PEG) and poly(propylene glycol) (PPG) using normal Raman spectroscopy, surface-enhanced Raman spectroscopy (SERS), and electrochemical quartz crystal microbalance (QCM) measurements. Raman and SERS show a clear spectroscopic trend of increased intensity in higher wavenumber modes in the CH stretching region as the environment is changed from pure material to solution to surface for both PEG and PPG. The spectral changes associated with PEG are larger than those associated with PPG, suggesting that the relatively more hydrophilic PEG undergoes more conformational changes upon surface association relative to the more hydrophobic PPG. Calculations show that the observed spectroscopic trend is associated with increased gauche character in the polymer backbone. QCM measurements show PEG adsorbs to the surface only in the presence of Cl−, while PPG adsorbs to the surface both with and without Cl− present. In the presence of Cl−, PPG forms a denser surface layer (0.598 μg/cm2) compared to PEG (0.336 μg/cm2) on a Cu underpotential deposition (UPD) layer on Au. These differences are consistent with the increased hydrophobicity of PPG relative to PEG.