Elsevier

Thin Solid Films

Volume 636, 31 August 2017, Pages 552-557
Thin Solid Films

Shape-controlled fabrication of nanopatterned samarium-doped cerium oxide thin films using ultraviolet nanoimprint lithography

https://doi.org/10.1016/j.tsf.2017.06.014Get rights and content

Highlights

  • A shape-controlled method for top-down nanopatterning was demonstrated.

  • A UV-curable resin containing precursors was developed.

  • Ultraviolet nanoimprint lithography was employed for the nanopatterning.

  • A minimum feature width of 45 nm was realized by patterning and annealing.

  • Undoped and samarium-doped CeO2 thin films were confirmed by XRD and EDX.

Abstract

A shape-controlled method for the nanoscale patterning of cerium oxide thin films was demonstrated in this work. An ultraviolet-curable precursor-containing resin was newly developed for samarium-doped and undoped cerium oxide films, and ultraviolet nanoimprint lithography was employed for the nanopatterning. Various nanostructure shapes such as lines, circular pillars, and circular holes were fabricated, and a minimum feature width of 45 nm was realized by patterning and subsequently shrinking the structures. The fabricated film was confirmed to be cerium dioxide, both undoped and doped with Sm. The effects of the annealing temperature on the crystallinity of the nanostructure and the controllability of the doping ratio were investigated. We believe that this work will provide a new path for preparing nanostructured cerium oxide for energy applications and a basic platform for analyzing the relationship between the structure and performance of size-controlled nanostructures.

Introduction

Cerium dioxide has garnered considerable attention as one of the most promising materials for energy and environmental applications, such as fuel cells [1], [2], photocatalytic systems [3], [4], hydrogen production and purification [5], [6], etc. Recently, many researchers have focused on decreasing the size of cerium dioxide structures to the nanoscale level to achieve higher performance for these applications [7], [8], [9], [10], [11], [12]. Several methods to synthesize cerium dioxide nanostructures such as nanoparticles [8], [9], [10], nanorods [11], [12] nanowires [13], [14], and nanotubes [15] have been introduced, and these nanostructures have been utilized as building blocks for hierarchical systems. However, there is still necessity for the consistent and economical large-scale fabrication, and comprehensive study of the structure-performance relationship using designed and size-controlled nanostructure [16], [17]. Nano-patterning based on a cerium oxide thin film can be a solution to this problem. In order to fabricate a cerium oxide thin film, sputtering [18], [19], metal-organic chemical vapor deposition (MOCVD) [20], pulsed laser deposition (PLD) [21], sol-gel process [22], [23], etc. have been used. Sol-gel process has gained popularity due to its solution-based facile process without requiring high vacuum and expensive equipment.

Recently, the sol-gel based direct nanopatterning of metal oxides by nanoimprinting a precursor-containing resin was used to successfully fabricate metal oxide nanostructures such as TiO2, ZnO, and ZrO2 [24], [25], [26]. This technique offers advantages for the consistent, size-controllable, large-area fabrication of nanostructures at a low cost with high productivity on flexible or non-uniform substrates.

In this work, we demonstrate a facile, shape-controlled nanoscale patterning method for cerium oxide thin films. An ultraviolet-curable precursor-containing resin was prepared for the patterning, which was performed by ultraviolet nanoimprint lithography (UVNIL). After the lithography, the patterned structure was calcined at various temperatures. The fabricated structure was confirmed to be cerium dioxide. To the best of our knowledge, this is the first report of nanopatterning cerium oxide thin films by UVNIL. We believe that this work will open a new avenue for preparing nanostructured cerium oxide for energy applications and a good platform for analyzing the relationship between structure and performance using size-controlled nanostructures.

Section snippets

Preparation of the precursor resin

The CeO2 precursor resin was prepared by dissolving 0.4 M cerium acetate hydrate (Ce(CH3COO)3  xH2O, Aldrich, 99.9%), 3.6 M monoethanolamine (MEA, (NH2CH2CH2OH, Aldrich, 99.5%), and 0.5 M 2-nitrobenzaldehyde (Aldrich UV-linker) in 2-methoxyethanol (CH3OCH2CH2OH, Aldrich, 99.5%). Notably, the molar ratio of MEA to metal oxide precursor was 9:1, in contrast with the 1:1 molar ratio used in the previous study [26]. The higher MEA content increased the solubility of the cerium acetate hydrate in the

Results and discussions

Fig. 2 shows the morphologies of CeO2 thin films with various nanoscale patterns. Nanoscale patterned arrays of lines, circular pillars, and circular holes were formed on the CeO2 thin film. A minimum feature width of 45 nm and maximum cross-sectional aspect ratio of 1.7 were achieved for the pillars and lines, respectively. A minimum feature size of 45 nm is difficult to realize without e-beam lithography in top-down fabrication methods, but such a small feature size was achieved in this work

Conclusion

A facile, shape-controlled nanoscale patterning method for cerium oxide thin films was demonstrated in this work. An ultraviolet-curable precursor-containing resin was newly developed for undoped and samarium-doped cerium oxide film, and UVNIL was employed for nanopatterning. The fabricated films were confirmed to be cerium dioxide with and without samarium. We believe that this work will offer both a new avenue for preparing nanostructured cerium oxide for energy applications and a good

Acknowledgement

This work was supported by the Main Research Program (NK196B, NK196E) of the Korea Institute of Machinery and Materials and by the Center for Advanced Meta-Materials (CAMM) funded by the Ministry of Science, ICT and Future Planning as Global Frontier Project (CAMM-2014M3A6B3963707). The authors would like to thank Mr. Yun-Chang Park at the National Nanofab Center (NNFC) for his valuable discussion.

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