Leah B. Sheridan

Formation of palladium nanofilms using electrochemical atomic layer deposition (E-ALD) with chloride complexation.

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 Cu(UPD) atom and a Pd(2+) ion that would take its position. Instead, it was proposed that electrons are transferred through the metallic surface to reduce Pd(2+) 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 Cu(UPD) 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 Pd(2+) ions, forming PdCl(4)(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.

Electrochemical Atomic Layer Deposition (E-ALD) of Palladium Nanofilms by Surface Limited Redox Replacement (SLRR), with EDTA Complexation

Atomic-scale control in the formation of Pd thin films is being developed using electrochemical atomic layer deposition (E-ALD) via surface limited redox replacement (SLRR). Pd has unique hydrogen storage properties. To study hydrogen storage capacity, hydrogen charging and discharging kinetics and its catalytic properties at the nanoscale will require films with well-defined thickness and structure. SLRR is the use of underpotential deposition (UPD) to form a sacrificial atomic layer of a less noble metal, such as Cu or Pb, and to exchange it at open circuit potential (OCP) for a more noble metal (Pd) via galvanic displacement. The deposits were grown using an automated electrochemical flow cell system which allowed sequential variation of solutions and potentials. Electron probe microanalysis (EPMA) revealed excess growth at the flow cell ingress, suggesting that the SLRR mechanism involved electron transfer from substrate to Pd2+ ions, rather than direct electron exchange from sacrificial metal atom(s) to Pd2+ ions. Ethylenediaminetetraacetic acid (EDTA) was used to slow the galvanic displacement by complexing the Pd2+, in an attempt to form more uniform Pd deposits. The resulting films were more homogeneous and displayed the expected Pd voltammetry in H2SO4. The charge for UPD remained constant from cycle to cycle, indicating no roughening of the surface. Ways of optimizing complexing agent properties, as well as the flow cell design and deposition parameters are discussed.

Electrodeposition of CuInSe2 (CIS) via electrochemical atomic layer deposition (E-ALD).

The growth of stoichiometric CuInSe(2) (CIS) on Au substrates using electrochemical atomic layer deposition (E-ALD) is reported here. Parameters for a ternary E-ALD cycle were investigated and included potentials, step sequence, solution compositions and timing. CIS was also grown by combining cycles for two binary compounds, InSe and Cu(2)Se, using a superlattice sequence. The formation, composition, and crystal structure of each are discussed. Stoichiometric CIS samples were formed using the superlattice sequence by performing 25 periods, each consisting of 3 cycles of InSe and 1 cycle of Cu(2)Se. The deposits were grown using 0.14, -0.7, and -0.65 V for Cu, In, and Se precursor solutions, respectively. XRD patterns displayed peaks consistent with the chalcopyrite phase of CIS, for the as-deposited samples, with the (112) reflection as the most prominent. AFM images of deposits suggested conformal deposition, when compared with corresponding image of the Au on glass substrate.