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44743-B10
Glow-Discharge Plasma as a Synthetic Medium for Nanocrystalline Non-Molecular Metal Oxides

Richard W. Schaeffer, Messiah College

Introduction

The plasma synthesis project has continued to build on the progress from 2006-2007, which improved the plasma chamber design, added a second plasma chamber, and added a mass-flow controller system to better regulate gas delivery to the plasma.  During 2007-2008, we were able to synthesize more than two dozen metal oxide films on glass and metal substrates.  These films were characterized for stoichiometry using atomic absorption spectroscopy, phase composition using powder x-ray diffraction, and morphology using optical and atomic force microscopy.   

The Plasma Reactor and Associated Systems

We continued modifying the shape and orientation of the reactant cathode and substrate this year.  The hollow cathode design implimented last year (using custom made glass sheaths to house the metal cathode) did not resolve all of the problems associated with the plasma synthesis.  Although, as reported last year, the hollow cathode design resulted in a more spatially directed plasma, a more concentrated deposition of product on the substrate, and improved product deposition rates, we are still struggling with poor reproduciblity.  We are successfully making metal and mixed metal oxides, but product yields, stoichiometry, and morphology vary significantly from sample to sample.  We attempted to exercise better control over the system by directing the plasma gas flow through the cathode to promote a more uniform product formation in the plasma and a more evenly and completely deposited product on the substrate, however, improvements were not consistently realized.  We are currently working on another cathode design to address these difficulties. 

The enhancements to the plasma system made at the end of the last report period have worked well.  The electronic mass flow control of the plasma gases, argon and oxygen, have vastly improved the control and stability of the plasma gas composition and flow.   The MKS type M100B mass-flow controllers and M10MB mass-flow meters allow excellent stability and reproducibility in both gas pressure, flow, and mixing.  We are confident that the problems in product formation (described above) are not derived from control over the plasma gases.  Moreover, we have been successful in stabilizing the second plasma chamber so that it now behaves very consistently like the first plasma system with respect to gas control and voltage-current realtionships.  Therefore our continuing work will focus on redesigning the plasma cathode and substrate to improve product yield, stoichiometry, and morphology.

Formation and Characterization of Products

As mentioned previously, we have made about two dozen different samples of metal oxides from the  plasma system over this reporting period.  As before, the products have been analyzed for stoichiometry via atomic absorption spectroscopy, phase composition via powder x-ray diffraction, and in several cases, morphology via optical and atomic force microscopy. As reported in 2006-2007, products from the reaction of a copper cathode in an oxygen plasma have shown a curiously wide ranging and unusual elemental composition.  This reporting period values ranged from Cu_(1.05)O to Cu_(1.22)O in an argon rich plasma (3:1 moles Ar to O₂) to Cu_(2.31) to Cu_(3.27)O in an oxygen rich plasma (3:1 moles O₂ to Ar).  Again, the reproducibility of the stoichiometry is not as good as desired, ranging between 10-30% rsd.  Like past work, part of the difficulty has been the small yield of product ranging from less than 1.00 to 2.14 mg for 24-48 hour run times.  This increases the relative uncertainty of stoichiometric determinations.  We are hopeful that continued work on the cathode design will lead to higher and more reproducible yields and more consistent and predictible product stoichiometry. 

We have analyzed many of the plasma products with powder x-ray diffraction, which typically reveal a crystalline phase of CuO (tenorite) with varying peak profiles and an amorphous phase as illustrated in Figure 1 and Figure 2.  The variation in peak profile probably represents differences in particle size and crystallinity. Finally, for many of the  samples, atomic force microscopy revealed an uneven “mountain range” product morphology on the surface of the substrate as illustrated in Figure 3.  The height and diameter of the individual “peaks” were about 50 and 400 nm, respectively. 

Figure  SEQ Figure \* ARABIC 1.  Powder XRD patterns from four product films from a plasma synthesis
with a copper hollow cathode design.  The peak 2θ positions are consistent with
CuO, but with apparently varying stoichiometry.

Figure 2.  Powder XRD patterns from four product films from a plasma synthesis
with a copper hollow cathode design.  The peak 2θ positions are consistent with
CuO, but with profiles that indicate smaller particle size or less crystallinity.

Figure  SEQ Figure \* ARABIC 3.  Three dimensional AFM image of CuO
plasma product.

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