Xirong Jiang

Area-Selective Atomic Layer Deposition of Platinum on YSZ Substrates Using Microcontact Printed SAMs

Using (methylcyclopentadienyl)trimethylplatinum (MeCpPtMe 3 ) and oxygen as precursors, Pt has been deposited by atomic layer deposition (ALD) on the surfaces of yttria-stabilized zirconia (YSZ), 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 μΩ cm. The use of area-selective ALD to deposit patterned Pt has also been investigated. By coating these same substrates with octadecyltrichlorosilane (ODTS) self-assembled monolayers (SAMs), Pt ALD can be successfully blocked. Furthermore, it is shown that by transferring the ODTS SAMs to the substrates by microcontact printing (μCP) using patterned stamps, platinum thin films are grown selectively on the SAM-free surface regions. Features with sizes as small as 2 μm have been deposited by this combined ALD-μCP method; the resolution is limited by the printed pattern and, likely, can be achieved at dimensions significantly smaller than a micrometer.

Catalysts with Pt Surface Coating by Atomic Layer Deposition for Solid Oxide Fuel Cells

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 μm 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.

Sputtered Pt–Ru Alloys as Catalysts for Highly Concentrated Methanol Oxidation

For possible catalytic anodes in direct methanol fuel cells (DMFCs) 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.

Oxidative removal of self-assembled monolayers for selective atomic layer deposition

Atomic layer deposition (ALD) is a thin film growth technique that employs a sequence of selflimiting surface reaction steps to afford subnanometer control of the growth process [1-2]. The self-limiting adsorption reactions ensure the precise control of film thickness and uniformity over large areas. While ALD provides perhaps the best available control of material thickness in the zdirection, fabricating precise patterns within the film can be challenging. Several groups have used chemical resists for area selective ALD [3-5], mostly employing selfassembled monolayers (SAMs). SAMs are thin organic films which form spontaneously on solid surfaces. They are well known for modifying the physical, chemical, and electrical properties of surfaces. Moreover, it is known that SAMs can inhibit surface reaction of ALD precursors. A variety of SAMs are stable at temperatures up a few hundred degrees centigrade, unlike the resist layers for photolithography and electron beam lithography. In this presentation, we will demonstrate a new nano-scale fabrication process combining atomic force microscopy (AFM) lithography of SAM for ALD patterning: AFM lithography as a nano-scale patterning method, SAM as a chemical resist for following hydrofluoric acid (HF) etching and ALD process. Due to the unique capability of the precise positioning and imaging, AFM lithography is able to create site-specific and localized patterning. In addition, the topological and electrochemical properties of patterns can be immediately characterized with AFM, and thus in situ pattern fabrication and characterization is possible with AFM lithography. Figure 1 shows the sequence process steps. When an electric field is applied through the AFM tip, an anodic bias can induce local oxidation of SAM-modified silicon substrate. Moving the AFM tip in a predefined fashion enables the creation of oxide patterns on the silicon surface. Subsequent HF etching locally removes oxide together with SAM, exposing silicon layer underneath. This patterned substrate can now be used as template for further ALD processing. To verify the complete removal of SAM at AFM oxidation step, Auger electron spectroscopy (AES) together with scanning electron microscopy (SEM) gas has been used for elemental mapping. (Figure 2) Zirconia ALD patterns with ~100nm line width and ~5nm height were fabricated, as shown in Figure 3. AFM scanning at each step is performed for imaging height/depth profiles. Considering that the negative pattern has a depth of ~3nm, and that the height of the ALD patterns is at least as thick as the SAM mold, a growth rate of ~0.8A per cycle is estimated, which is comparable to the typical ALD growth rates of zirconia.

Synthesis of Microscale Lead Sulfide Disks by Patterned Self-Assembled Monolayer

This work explores an approach to utilize nanosphere lithography (NSL) and atomic layer deposition (ALD) to fabricate an array of microscale disks of lead sulfide (PbS). In this approach, a mold to fabricate polymer stamps was produced by NSL. Using these stamps a patterned self-assembled monolayer (SAM) of octadecyltrichlorosilane (ODTS) was deposited using microcontact printing. The ODTS SAM functioned as a resist to block the growth of ALD PbS. The resulting PbS disks were characterized by scanning electron microscopy (SEM) and Auger electron spectroscopy (AES) to confirm the morphology and stoichiometry. Regular arrays of microscale PbS disks were successfully fabricated. This is a potentially attractive methodology for fabrication of multi-layer devices.