The effects of additives on the nucleation and growth kinetics of electrodeposited copper nanostructures and thin films

The effects of additives on copper electrodeposition processes have been investigated with electrochemical methods, atomic scale imaging techniques, surface nonlinear optical spectroscopy, and by molecular dynamics simulations.Linear potential sweep voltammograms (LPSV) and cyclic voltammograms (CV) of additive-free CuSO4 solutions, and with addition of 1-propanol or malachite green (MG) to the electrolyte were measured on Au electrodes. The morphologies of electrodeposited Cu films on Au thin film electrodes (TFE) were investigated by means of atomic force microscopy (AFM) and scanning electron microscopy (SEM). The copper grain takes the shape of trigonal pyramids formed in solutions containing 1-propanol, and square pyramids from solutions containing MG. X-ray diffraction (XRD) analysis indicated polycrystallites are formed. In situ electrochemical scanning tunneling microscopy (EC-STM) was employed to monitor copper nucleation and initial growth on Au (111) and on highly oriented pyrolytic graphite (HOPG) electrodes from CuSO4 solutions with and without additives. The in situ EC-STM images provided visual information of nucleation and initial growth of Cu electrodeposition. It was revealed that additives (particularly MG) significantly altered the nucleation and growth characteristics of Cu, resulting in nucleation and growth along terrace edges. The initial growth was changed from 3-dimensional in additive-free electrolyte to 2-dimensional in electrolytes containing MG. The effects of MG on copper electrodeposition have also been investigated by the surface sensitive nonlinear optical spectroscopy, second harmonic generation (SHG). The rotational anisotropy of Cu(111) and Cu(100) was detected; SHG response to incident light polarization on interfaces of Cu(111)/MG(aq) and Cu(100)/MG(aq) were also obtained and indicated sensitivity to the organic additive. Fitting the polarization-resolved SHG data provided information regarding the averaged orientation of the additive MG on the Cu surfaces. Further understanding of the effects of MG on copper electrodeposition was provided by molecular dynamics simulations. The adsorption of MG on crystal facets were simulated. The results indicate that both neutral MG molecules and their positive ions prefer the (111) plane to (100) facets of Cu, resulting in faster Cu growth kinetics on the normal of Cu(100) facet.