Formation of Palladium Nanofilms Using Electrochemical Atomic Layer Deposition (E-ALD) with Chloride Complexation

• Published in: Langmuir Vol. 29; no. 5; pp. 1592 - 1600
• Main Authors: Sheridan, Leah B, Gebregziabiher, Daniel K, Stickney, John L, Robinson, David B
• Format: Journal Article
• Language: English
• Published: Washington, DC American Chemical Society 05.02.2013
• Subjects: Chemistry; Chlorides - chemistry; Copper - chemistry; Cross-disciplinary physics: materials science; rheology; Electrochemical Techniques; Electrochemistry; Exact sciences and technology; General and physical chemistry; Gold - chemistry; Materials science; Metal Nanoparticles - chemistry; Methods of deposition of films and coatings; film growth and epitaxy; Palladium - chemistry; Physics; Complexation; Gold; Electrochemical method; Chlorides; Transition metal; Palladium; Platinoid
• ISSN: 0743-7463; 1520-5827
• DOI: 10.1021/la303816z

Pd thin films were formed by electrochemical atomic layer deposition (E-ALD) using surface-limited redox replacement (SLRR) of Cu underpotential deposits (UPD) on polycrystalline Au substrates. An automated electrochemical flow deposition system was used to deposit Pd atomic layers using a sequence of steps referred to as a cycle. The initial step was Cu UPD, followed by its exchange for Pd ions at open circuit, and finishing with a blank rinse to complete the cycle. Deposits were formed with up to 75 cycles and displayed proportional deposit thicknesses. Previous reports by this group indicated excess Pd deposition at the flow cell ingress, from electron probe microanalysis (EPMA). Those results suggested that the SLRR mechanism did not involve direct transfer between a CuUPD atom and a Pd2+ ion that would take its position. Instead, it was proposed that electrons are transferred through the metallic surface to reduce Pd2+ ions near the surface where their activity is highest. It was proposed that if the cell was filled completely before a significant fraction of the CuUPD atoms had been oxidized then the deposit would be homogeneous. Previous work with EDTA indicated that the hypothesis had merit, but it proved to be very sensitive to the EDTA concentration. In the present study, chloride was used to complex Pd2+ ions, forming PdCl4 2–, to slow the exchange rate. Both complexing agents led to a decrease in the rate of replacement, producing more homogeneous films. Although the use of EDTA improved the homogeneity, it also decreased the deposit thickness by a factor of 3 compared to the thickness obtained via the use of chloride.