PHYSICAL REVIEW B 85, 214113 (2012) Anomalous grain growth in the surface region of a nanocrystalline CeO 2 film under low-temperature heavy ion irradiation P. D. Edmondson, 1,2,* Y. Zhang, 2,3 S. Moll, 4 T. Varga, 5 F. Namavar, 6 and W. J. Weber 2,3 1 Department of Materials, University of Oxford, Parks Road, Oxford, OX1 3PH, United Kingdom 2 Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA 3 Department of Materials Science and Engineering, University of Tennessee, Knoxville, Tennessee 37996, USA 4 CEA-DEN, Service de Recherches de M´ etallurgie Physique, Centre d’ ´ Etudes de Saclay, 91191 Gif-sur-Yvette Cedex, France 5 Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory, P.O. Box 999, Richland, Washington 99352, USA 6 University of Nebraska Medical Center, Omaha, Nebraska 68198, USA (Received 3 February 2012; revised manuscript received 17 April 2012; published 15 June 2012) Grain growth and phase stability of nanocrystalline ceria are investigated under ion irradiation at different temperatures. Irradiations at temperatures of 300 and 400 K result in uniform grain growth throughout the film. Anomalous grain growth is observed in thin films of nanocrystalline ceria under 3-MeV Au + irradiation at 160 K. At this low temperature, significant grain growth is observed within 100 nm from the surface, and no obvious growth is detected in the rest of the films. While the grain growth is attributed to a defect-stimulated mechanism at room temperature and above, a defect diffusion-limited mechanism is significant at low temperatures with the primary defect responsible being the oxygen vacancy. The nanocrystalline grains remain in the cubic phase regardless of defect kinetics. DOI: 10.1103/PhysRevB.85.214113 PACS number(s): 61.80.−x, 61.82.Rx, 61.72.−y I. INTRODUCTION Thin films of binary oxide ceramics, such as zirconia (ZrO 2 ) and ceria (CeO 2 ) are technologically important materials. Due to their exceptional ionic conductivity, they are particularly attractive for use in solid oxide fuel cells. 1,2 However, it has been suggested that nanocrystalline films may be used due to the enhancement of the material properties and the ability to tailor those properties with grain size. 3,4 A 4-orders-of- magnitude increase in the electrical conductivity of CeO 2 has been observed when the grain size is reduced from the micro- to the nanocrystalline scale. 5 Recently, the possibility to achieve higher ionic conductivity in nanocrystalline zirconia has been discussed. 6 It is well known that the ionic conductivity of zirconia is via an oxygen vacancy mechanism, and as such, enhanced conductivity may be achieved by controlling the oxygen vacancy concentration. In a later paper, it was demon- strated that oxygen concentration or a saturation of oxygen vacancies may be controlled in zirconia film by ion beams while maintaining nanosized grains. 7 This demonstrated the novel use of ion beams to effectively defect engineer materials to enhance their properties. In this paper, thin films of nanocrystalline ceria have been irradiated with 3-MeV Au + ions at 160, 300, and 400 K in order to evaluate the response of the material to energy deposition and to examine if ion-beam manipulation of the thin films may be used to enhance the properties of the films. II. EXPERIMENTAL METHODS Thin films, approximately 330-nm thick nanocrystalline CeO 2 , have been deposited onto a (001) silicon substrate using an ion-beam-assisted-deposition (IBAD) technique chronicled elsewhere. 8 The average initial grain diameter is ∼6 nm. These films have previously been shown to be in the cubic form. 9 The films were then irradiated with 3-MeV Au + ions at temperatures ranging between 160 and 400 K and up to doses of ∼35 displacements per atom (dpa) using the 3-MV tandem accelerator facilities located at the Environmental Molecular Sciences Laboratory (EMSL) at Pacific Northwest National Laboratory. Particular care was taken during the 160-K experiment so as to ensure that thermal steady state was achieved prior to ion bombardment. The ion energy was chosen such that the energy deposition into the CeO 2 film was maximized while minimizing the number of ions implanted into the film. Following irradiation, the thin films were examined using a combination of glancing incidence x-ray diffraction (GIXRD), Rutherford backscattering spectroscopy (RBS), and cross-sectional transmission electron microscopy (XTEM). The GIXRD was performed to determine nominal grain size using a Philips X’Pert diffractometer with Cu K α 1 x rays. The average grain size of the films was determined using pseudo-Voigt profiles of the main diffraction peaks. The results from the RBS measurements of the as-deposited and irradiated samples are used to determine the film stoichiometry and thickness (in atom cm −2 ). Specimens to be examined in XTEM were prepared using a tripod polishing technique in which the samples are mechanically thinned to a thickness of 15–20 μm before ion milling to perforation using a Gatan precision ion polishing system with beam energy reduced from 4.5 to 3 keV. A JEOL 2010 transmission electron microscope (TEM) operating at 200 keV was used to image the specimens. III. RESULTS A. As-deposited film TEM micrographs of the as-deposited material are shown in Fig. 1. The diffraction contrast image in Fig. 1(a) shows the nanostructured ceria (NSC) film on the top of a silicon substrate with an approximately 5-nm buffer layer of SiO 2 . The structure of the NSC film appears to be uniform throughout with no abrupt changes in contrast. The selected area electron diffraction (SAED) pattern shown in Fig. 1(b) indicates that 214113-1 1098-0121/2012/85(21)/214113(5) ©2012 American Physical Society