Area-selective atomic layer deposition (ALD) is a technique that can be used for fabricating 3D structures with dimensions down to the nanoscale. A patterned resist, typically a self-assembled monolayer (SAM), directs film deposition through area-selective ALD, leading to lateral patterning. This article will describe the overall approach to area-selective ALD, introduce the process of atomic layer deposition, and discuss the development of monolayer ALD resists. We will describe the results of studies which show that monolayers with a high degree of packing and hydrophobicity perform best in blocking ALD, and that resistance against ALD can be used as a sensitive probe of SAM quality. We further describe patterning of the SAM through soft lithography, in particular microcontact printing (µCP), for the area-selective ALD process, and compare the area-selective ALD processes for HfO 2 and Pt ALD. The powerful patterning capability of µCP and the flexibility of area-selective ALD for various materials can impact many potential applications, and studies applying this process to fuel cell devices and integrated circuits will be described.

Using ͑methylcyclopentadienyl͒trimethylplatinum ͑MeCpPtMe 3 ͒ and oxygen as precursors, Pt has been deposited by atomic layer deposition ͑ALD͒ on the surfaces of yttria-stabilized zirconia ͑YSZ͒, a solid oxide electrolyte, as well as on oxide-covered silicon. Ex situ analyses have been carried out to examine the properties of both as-deposited and postannealed Pt films. X-ray photoelectron spectroscopy measurements demonstrate that there are no detectable impurities in the as-deposited Pt films, and four-point probe measurements show that the resistivity for a 30.2 nm film is as low as 18.3 ⍀ cm. The use of area-selective ALD to deposit patterned Pt has also been investigated. By coating these same substrates with octadecyltrichlorosilane ͑ODTS͒ selfassembled monolayers ͑SAMs͒, Pt ALD can be successfully blocked. Furthermore, it is shown that by transferring the ODTS SAMs to the substrates by microcontact printing ͑CP͒ using patterned stamps, platinum thin films are grown selectively on the SAM-free surface regions. Features with sizes as small as 2 m have been deposited by this combined ALD-CP method; the resolution is limited by the printed pattern and, likely, can be achieved at dimensions significantly smaller than a micrometer.

In this study, atomic layer deposition (ALD) was used to deposit Pt thin films as an electrode/catalyst layer for solid oxide fuel cells. I−V measurements were performed to determine the dependence of the fuel cell performance on the Pt film thickness at different operating temperatures. The measured fuel cell performance revealed that comparable peak power densities were achieved for ALD-deposited Pt anodes with only one-fifth of the platinum loading relative to dc-sputtered Pt anodes. The Pt films fabricated by dc sputtering and ALD had different microstructure, which accounted for the difference in their performance as a fuel cell anode. In addition to the continuous electrocatalyst layer, a micropatterned Pt structure was fabricated via area-selective ALD and used as a current collector grid/patterned catalyst for the fuel cells. An improvement of the fuel cell performance by a factor of 10 was observed using the Pt current collector grid/patterned catalyst integrated onto cathodic La0.6Sr0.4Co0.2Fe0.8O3-δ. The study suggests the potential to achieve improved performance and/or lower loadings using ALD for catalysts in fuel cells.

We demonstrate a method for growing metal nanoparticles (NPs) by atomic layer deposition (ALD) with the ability to vary aerial density and NP size using nucleation control. Self-assembled monolayers (SAMs) preadsorbed on the substrate serve as a template for subsequent growth of the NPs by ALD. Defects in the SAM resulting from incomplete formation time in solution are shown to act as nucleation sites for Pt. The strategy is demonstrated experimentally using ALD of Pt from a metal organic Pt precursor and O 2 counter reactant on silicon dioxide surfaces pretreated with octadecyltrichlorosilane SAMs. The aerial density and mean diameter of the Pt NPs are controlled by changing the SAM dip time and the number of ALD cycles. An isothermal nucleation model was developed in which several nucleation behaviors were considered in comparison with experimental data. A model incorporating nucleation incubation provided the best fit to the data.

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In this study, we demonstrate a novel approach;atomic layer deposition (ALD);for the synthesis and investigation of Pt-Ru catalyst structures toward the oxidation of stoichiometric (1:1) methanol solutions in advanced direct methanol fuel cells. Two types of thin-film materials are investigated as catalysts for methanol oxidation: Pt-Ru films of varying ruthenium content that are co-deposited by ALD, and Pt skin catalysts made by depositing porous platinum layers of different thickness by ALD on sputtered ruthenium films. MeCpPtMe 3 and Ru(Cp) 2 are used as precursors for Pt and Ru ALD, respectively, together with pure O 2 as the counter reactant. The electrochemical behavior of the co-deposited Pt-Ru catalysts and the Pt skin catalysts for methanol oxidation is characterized using chronoamperometry and cyclic voltammetry in a 0.5 M H 2 SO 4 /16.6 M CH 3 OH electrolyte at room temperature. The results illustrate that the optimal stoichiometric Pt:Ru ratio for the co-deposited catalysts is ∼1:1, which is consistent with our previous study on sputtered Pt-Ru catalysts using the same CH 3 OH concentration. Moreover, we report that the catalytic activity of sputtered ruthenium catalysts toward methanol oxidation is strongly enhanced by the ALD Pt overlayer, with such skin catalysts displaying superior catalytic activity over pure platinum. The mechanistic aspects of our observations are discussed.

We tested the atomic layer deposition of platinum layers onto various sputtered porous metals including silver, palladium, ruthenium, and gold as catalysts for solid oxide fuel cells. We investigated thermal and chemical stability against oxidation using X-ray photoelectron spectroscopy. Fuel cell tests were conducted using the metal-atomic layer deposition platinum cathodes with 200 m thick, 8% yttria-stabilized zirconia electrolytes and sputtered platinum anodes. Performance of the fuel cells was measured by comparing the current-voltage curves and maximum power densities with varying temperature.Ceramic fuel cells have attracted recent attention as a future energy conversion system because of their potentials for high efficiency and fuel flexibility. However, the high operating temperature of ceramic fuel cells of 700-1000°C limits their practical usage. There have been several efforts to reduce the operating temperature to below 400°C by employing thin electrolyte membranes, by replacing conventional zirconia-based electrolytes with other doped oxides, which exhibit higher ionic conductivity, or by replacing conventional cathodes based on perovskite oxides with novel metal oxide cermets. 1-9 At intermediate temperatures, platinum or platinum alloys are known as the best catalysts. However, the high price of platinum has become a main bottleneck against the commercialization of fuel cells equipped with Pt-based catalysts. Recently, it was reported that several Pt metal alloys exhibited higher catalytic activities compared to pure Pt with a stable formation of a Pt skin segregated from the bulk of the cluster. 10 Atomic layer deposition ͑ALD͒ is a modified version of metallorganic chemical vapor deposition relying on the self-limiting chemistry of precursors and the interaction between substrates and precursor molecules. The ALD process enables conformal deposition along complex three-dimensional contours because the surface reaction takes place regardless of reactant flow direction. ALD films can be deposited along trenches with a high aspect ratio. 11,12 Therefore, ALD is expected to achieve good surface alloying over a nanoscale metal mesh used for current collection. Pure ALD Pt was investigated as a fuel cell catalyst in our previous work. 13 In this study, we tested metal catalysts surface-coated with Pt layers by ALD as cathodes of fuel cells to minimize the high cost of platinum and to enhance surface kinetics on the Pt metal surface alloys. ExperimentalVarious nanoporous metal meshes, including Ag, Pd, Ru, and Au, were used as template metals for cathodes. There was a prior investigation of Ag, Pd, and Au as solid oxide fuel cell ͑SOFC͒ catalysts. 14 There are also several publications reporting reasonably high or better performance of bimetallic alloys, including Ag-Pt and Pd-Pt, as cathodes for fuel cells compared to pure Pt. 15,16 The metal meshes used in our tests were produced by dc sputtering with 10 Pa of Ar background gas for 80 s of sputtering time to grow ϳ80 nm thick porous layers. On the me...

For possible catalytic anodes in direct methanol fuel cells ͑DMFCs͒ employing a 1:1 stoichiometric methanol-water reforming mixture, we have studied sputtered Pt-Ru catalysts over a wide composition range. The surface morphology of the catalyst films, determined from scanning electron microscopy studies, is rough and nanoporous and is dependent on the composition. The structure of the films has been verified as polycrystalline by X-ray diffraction analysis, which further shows that the cosputtered films are highly alloyed. The electrochemical behavior of the sputtered films has been evaluated for methanol oxidation using cyclic voltammetry and chronoamperometry in the H 2 SO 4 /CH 3 OH electrolyte at room temperature. The results indicate that Pt 0.53 Ru 0.47 is the optimal alloy composition for highly concentrated 16.6 M CH 3 OH, which corresponds to the stoichiometric fuel that will be used in next-generation DMFCs designed to mitigate methanol crossover. Long-time chronoamperometry measurements show that sputtered Pt-Ru catalysts maintain a stable performance after the initial decay.Much attention has been paid to direct methanol fuel cells ͑DMFCs͒ because they offer a highly efficient and environmentalfriendly technology for energy conversion. 1,2 Methanol has a relatively low theoretical oxidation potential ͑E o = 0.03 V͒ comparable to that of hydrogen ͑E o = 0.00 V͒ and a higher volumetric energy density ͑16 MJ L −1 ͒ even when compared to liquid hydrogen ͑9 MJ L −1 ͒. Hence, it is an attractive fuel for portable power applications. Moreover, methanol offers improved logistics over hydrogen because it is naturally a liquid at room temperature and at atmospheric pressure, and its use in DMFCs does not require preprocessing modules such as external reformers.The study presented in this paper was undertaken in the context of exploring the possibility of replacing Nafion-based hydrated polymer membranes typically employed in DMFCs with a dense ceramic-based proton-conducting membrane impervious to methanol ͑or water͒ to eliminate the persistent problem of methanol crossover. Due to the large deficiency in the proton conductivity of ceramic membranes compared to hydrated polymers, the ceramic membranes need to be fabricated very thinly and have to be pinholefree to achieve comparable transport rates while physically blocking methanol crossover. Indeed, using the atomic layer deposition method, ongoing work in our laboratory has successfully demonstrated the fabrication of high quality yttria-doped barium zirconate ͑BYZ͒ proton-conducting films that are nominally 100 nm thick. [3][4][5] At this thickness and at the nominal operating temperature of 60°C for Nafion-based DMFCs, the ohmic contribution to cell impedance from proton conduction in the ceramic membrane is expected to be comparable, albeit lower by factors of 5-10, than the 100-150 mm thick Nafion-based membranes commonly employed in DMFCs. Although such ceramic-based DMFCs are expected to operate at temperatures higher than 100°C, a similar comparison w...