Figure 1: Scanning Probe Microscope image of Pt clusters deposited on a graphite surface by applying voltage pulses between a Pt tip and graphite surface. The pulse width was 200ns and the voltage was 8 V to the surface.
Donald Baer and Youg Liang
This research program is investigating the formation and properties of nanoscale functional structures with the ultimate objective of integration of active nanometer sized components into "smart" microtechnology devices. The current focus is toward the creation of ordered arrays of nano-sized clusters with a potential for catalytic studies. Two different approaches are being made to create and stabilize these arrays. The first approach involves deposition of thin layers on stepped oxide surfaces and heating these layers to allow formation of metal nano-strings at the step edges. The second approach involved use of a scanning probe microscope to deposit nano-dimensioned clusters. An atomic force microscope is being used to determine the size and structure of the strings formed.
This effort on specific issues related to nanoscale component development uses a range of new experimental tools recently made available in the William R. Wiley Environmental Molecular Science Laboratory (EMSL) that enhance our ability to work in the nanometer dimension. Two approaches have been used to create nano-dimensioned arrays with potential for use in catalytic studies. Both approaches use different methods in the attempt to stabilize or pin the nano-dimensional features thereby offering greater stability than might otherwise be anticipated. To date we have demonstrated the general ability to create the desired types of surfaces, and much of the future efforts will be directed toward testing the stability and functionality of these surfaces.
An overall objective in this task is to create ordered arrays of nano-sized clusters on oxide surfaces. The intent here has been to understand and control the deposition process to allow different sized and spaced arrays to be created to optimize catalytic reactions. The three barriers to this effort are I) the deposition of the clusters of appropriate material and size, ii) generation of an oxide substrate with appropriate smoothness to allow observation of the deposited array and iii) combining the two in a manner that stabilized the arrays on the surface. Our initial deposition effort was to show that clusters could be deposited on smooth graphite by a field emission process from a STM tip. This effort has shown that such clusters can be deposited on graphite by field deposition from a tip (Fig. 1).
Figure 1: Scanning Probe Microscope image of Pt clusters deposited on a graphite surface by applying voltage pulses between a Pt tip and graphite surface. The pulse width was 200ns and the voltage was 8 V to the surface. |
It is possible to deposit clusters over a wide size range by the method, but the approach is less applicable for oxide surfaces. Our second effort has been to combine electrochemical and mechanical deposition of metals on different smooth substrates using scanning probe technology. The overall objective is to place Rh clusters of controlled sizes on TiO2 surfaces although initial efforts involve Cu dots on Au and graphite. This mechanical-electrochemical deposition method involves both the mechanics of placing the metal on the surface and a flow of current during the deposition. We anticipate that this process will "fix" the metal clusters on the surface and minimize mobility.
This approach has previously been used to deposit Cu on Au and graphite and we are testing our ability by using this system on our. Our challenge is the areas involve using a new Scanning probe microscope (SPM) system and depositing on a smooth oxide surface. The properties of an electrochemical cell for the Digital Instruments (DI) SPM have been determined. This work is involving a new DI instrument in EMSL that has not previously been used with the electrochemical cell. Electrochemical tests now show a reproducible and expected cyclic deposition behavior of Cu on both a metal substrate and the SPM tip. This initial step verifies cell operation and serves as the basis for deposition for rhodium or other metals. After deposition of Cu is verified on a gold or graphite substrate, we will deposit the material on a rutile substrate and then move to Rh deposition.
The second method being explored for forming nano-dimensioned catalytic structures involves the use of stepped oxide surfaces and metal deposition. This approach takes advantage of the stronger interaction of atoms at steps in comparison to smooth surfaces. An atomic force microscope has been used to examine the structures formed upon evaporation and heating of the metal-coated oxide-surfaces. This experiment takes advantage of the new vacuum STM/AFM capability in EMSL and the film deposition system available as part of the EMSL specimen transfer capability. Because TiO2 (110) surfaces can be processed into relatively smooth planes with steps (Fig. 2a) this material makes a very appropriate substrate for this methods and serves as a useful support for catalytic systems.
 Figure 2a |
The ability to form Rhodium (Rh) clusters that accumulate at steps on a rutile (TiO2) surface has been demonstrated. This has been accomplished by depositing 1 monolayer (ML) and 4 ML of Rh on a characterized stepped surface of the rutile. The 4 ML coverage caused the formation of cluster covering the whole surface. However, at 1 ML coverage the clusters were associated with the surface steps (Fig. 2b). These initial tests were conducted with a deposition system connected to a surface analysis spectrometer with AFM measurements conducted in a different chamber. Current efforts are focused on lower metal coverages and using a different system for which the AFM work can be conducted in the same chamber. This will allow thermal stability to be examined by repeated heating of a single specimen.
 Figure 2b: a) AFM image of stepped rutile surface showing single atomic layer height steps, b) AFM image of stepped surface covered with 1 monolayer of Rh that accumulates at steps. |
Dr. John Daschbach completed the electrochemical aspects of this work. Mark Engelhard assisted the deposition experiments by working with and connecting the EMSL portable deposition system to a surface analysis spectrometer.
Interaction of Ultra-Thin Rh Films with the Vicinal TiO2(110) Surfaces, Y. Liang, D.R. Baer, M.H. Engelhard, and G.S. Herman. Presented at 20th Conference on Applied Surface Analysis, Richland, WA June 1998
Enhancement of catalytic properties using surface and interface engineering Y. Liang, J. Daschback, A. Joly, D. Baer Pacific Northwest National Laboratory* M. Na, H. Luo State University of New York, Buffalo, NY. To be presented at MRS Fall meeting, Boston November 1998
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