Underpotential deposition (UPD) involves formation of two-dimensional monolayer of one metal onto foreign substrate preceding its bulk deposition. As such, it is used in the design of diverse electrodeposition protocols where the UPD monolayer acts as surfactant or flux mediator; a role that involves improving the growth kinetics and deposition morphology. More recent applications include monolayer restricted deposition methods where UPD phenomenon serves as enabling process. Recent literature has highlighted a very exciting development where the first reports of an electroless (e-less) lead and zinc monolayer deposition phenomenon. In general, the UPD can be phenomenologically described as potential dependent adsorption. The characteristic cyclic voltammetry involves one or more deposition/stripping peaks in the underpotential region of particular metal. The complexity of voltammetry arises from existence of one or more UPD monolayer superstructures and/or one or more UPD monolayers formed.
In a recent publication, University of Houston scientists: D. Wu, Dr. K. Ahmadi and Professor Stanko Brankovic in collaboration with Dr. Nikhil Dole, Dr Aniruddha Joi and Dr. Yezdi Dordi at Lam Research Corporation provided detailed and comparative information regarding lead UPD and lead electroless monolayer deposition on Ru(0001) substrate. Their approach involved the adoption and evaluation of two different ways of lead monolayer deposition on Ru(0001). Their work is currently published in Journal of The Electrochemical Society.
In brief, the team adopted a Ru(0001) disk of diameter 10 mm and 2 mm thickness which was then prepared using mechanical polishing and induction annealing at specific conditions – a process that yielded a highly reflective mirror-like surface. Next, an experimental routine for the e-less lead monolayer deposition was performed. Lastly, STM images of the lead UPD process on Ru(0001) were analyzed using custom made digital image processing (DIP) algorithm.
It was reported that based on the obtained electrochemical data, the lead monolayer deposition process was affected by the kinetics and thermodynamics of RuOH reduction. This was supported by the fact that it reflected on the decoration sequence of the Ru surface by lead monolayer where the nucleation and growth were first observed on terraces rather than on surface steps and defects. Additionally, the authors established that the very same phenomenon was responsible for atypical shape of the lead monolayer adsorption isotherm and on corresponding parameters of the S&B model.
In summary, the study by Professor Stanko Brankovic and the research team demonstrated two different ways for Pb monolayer deposition on Ru(0001) using UPD and e-less deposition phenomenon. Interestingly, for each approach, a 2D deposit was obtained with very similar morphology. Generally, the team demonstrated that the difference between e-less lead monolayer and lead UPD monolayer was the mass/charge associated with their formation and stripping. Overall, considerable applications of Ru metal in microelectronics and catalysis rise significance of these results presented for variety of future developments in respective areas.
Wu, K. Ahmadi, N. Dole, A. Joi, Y. Dordi, S. R. Brankovic. Underpotential and Electroless Pb Monolayer Deposition on Ru(0001). Journal of The Electrochemical Society, volume 166 (10) page D359-D365 (2019).Go To Journal of The Electrochemical Society