Metal oxide targets produced by the polymer-assisted deposition method
Introduction
Targets are an essential component in experimental nuclear science. They are the source of stationary nuclei in nuclear reactions with heavy-ion beams. Typically, targets should be chemically pure, uniform, homogeneous, and crack free over the irradiated area, while also being structurally rigid. Conventional methods of preparing targets include cold rolling [1], vacuum evaporation [2], molecular plating [3], painting/sedimentation [4], electrodeposition [5], and die compaction [6]. Each method has its own advantages and disadvantages with regard to target thickness, homogeneity, chemical composition, reproducibility, and deposition yields. Under high beam intensities commonly found in modern heavy-ion facilities, d- and f-transition metal oxide targets are preferred over elemental targets due to their higher melting points. Thus, the preparation of targets composed of homogeneous metal oxides (50–1000 μg/cm2) is of interest. The current methods for preparing transition metal oxides, molecular plating and electrodeposition, often suffer from poor adhesion to the backing material and lack homogeneity at target thicknesses less than about 300 nm [6]. Jia et al. [7] reported a new method of producing metal oxides, the polymer-assisted deposition (PAD). PAD is the process of spin coating an aqueous metal–organic polymer solution and annealing the film to produce a metal oxide layer. The method was shown to produce crack-free homogenous metal oxide films with uniform thicknesses between ∼20 and 400 μg/cm2 [8], [9]. The PAD method is the binding of a multidentate polymer to metal precursors in aqueous solution, and the subsequent spin coating of the solution on a substrate. The metal–organic films are then annealed to yield a high-quality metal oxide film.
Section snippets
Solution preparation
Varying weight percentages of metal chlorides (2–10%) and 15% polyethylenimine (PEI; 10 kDa, Aldrich) by weight (b.w.) were dissolved in aqueous solutions and the pH was adjusted between 6 and 6.5 using 37% HCl. A pH meter with attached electrode (Model 231, Orion Research) was used to monitor the adjustment. Solutions were mixed using a vortex mixer and stirred on a magnetic stir plate. The metal chloride hexahydrate or metal chloride of interest was dissolved in the polymer solution. All
Single-layer films
The thickness of a metal oxide layer produced by the PAD method is a function of several variables: angular acceleration of the spin coater, viscosity of the solution, metal ion concentration, maximum velocity of the spin coater, total time spun, and the annealing temperature profile. A systematic study was performed to determine film thickness as a function of metal ion concentration using Eu(III), Tm(III), and Hf(IV) (Fig. 1).
Spin coating yields film heights having an ηγ dependence [10],
Conclusion
Europium, thulium, and hafnium oxide thin films were created using the polymer-assisted deposition method. The films were characterized by SEM, profilometry, and XRD. The characterizations showed that the films were crack free, uniform, and homogeneous. The PAD method produces high-quality metal oxide layers below 300 nm in thickness, and reapplication of the method creates thicker metal oxide films of equal homogeneity.
Acknowledgements
SEM images were taken with the expert assistance of Virginia Altoe. The use of the profilometer was kindly provided by the Jean M.J. Fréchet group. Helpful advice and editing of this manuscript was provided by Jonathan Glidden. This work was supported by the National Nuclear Security Administration under the Stewardship Sciences Academic Alliance Program, Project NS00075, Award number DE-FG52-06NA27480. Work at the Molecular Foundry was supported by the Director, Office of Science, Office of
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