Deformation of colloidal silica particles using MeV Si ion irradiation Juan Carlos Cheang-Wong * , Ulises Morales, Eder Rese ´ndiz, Alicia Oliver, Luis Rodrı ´guez-Ferna ´ndez Instituto de Fı ´sica, Universidad Nacional Auto ´noma de Me ´xico, A.P. 20-364, Me ´xico, D.F. 01000, Mexico Available online 6 April 2007 Abstract Spherical submicrometer-sized silica particles were prepared by the Sto ¨ ber process, from a reaction mixture containing tetraethoxysi- lane, ammonia and ethanol, and deposited onto silicon wafers. While the properties of these SiO 2 particles depend on their size, size distribution and shape, monodisperse spherical particles were obtained with a narrow size distribution. The samples were then irradiated at room temperature with Si ions at two energies (4 MeV and 6 MeV) and fluences up to 5 · 10 15 Si/cm 2 , at an angle of 45° with respect to the sample surface. The size, size distribution and shape of the silica particles were determined using scanning electron and atomic force microscopes. By means of the Si ion irradiation, the as-prepared spherical silica particles can be turned into ellipsoidal particles, as a result of an increase of the particle dimensions perpendicular to the ion beam and a decrease in the direction parallel to the ion beam. This effect increases with the ion energy and fluence, and depends on the electronic energy loss processes to which the impinging ions are subjected. Our results also suggest that the particle deformation is more important in the case of larger as-prepared silica colloids. Ó 2007 Elsevier B.V. All rights reserved. PACS: 61.80.x; 61.82.Ms; 82.70.Dd Keywords: SEM; Atomic force and scanning tunneling microscopy; Colloids; Silica; Radiation 1. Introduction Colloidal silica particles are being intensively studied due to their potential applications in catalysis, intelligent materials, optoelectronic devices and coating technology [1,2]. The properties of these SiO 2 particles depend on their size, size distribution and shape, which in turn determine the different roles they can play as electronic substrates, thin film substrates, electrical and thermal insulators, etc., in nanodevices. On the other hand, shrinking device dimen- sions make it increasingly more difficult and costly to fab- ricate high-quality nanostructures, so several alternative approaches must be explored. It is well known that ion irradiation induces damage and structural changes in solids due to energy losses of multi-MeV heavy ions via ioniza- tion events and atomic collisions occurring in the near-sur- face region of the irradiated sample. Indeed, it has been observed that amorphous glassy materials like silicon diox- ide [3,4] can undergo extreme deformations under exposure to high-energy beams. This ion-beam induced anisotropic deformation of amorphous materials such as silica has been observed in the case of SiO 2 films on Si substrates [3] as well as in colloidal silica particles [4,5]. In both cases the resulting effect is an increase of the sample dimensions perpendicular to the ion beam and a decrease in the direc- tion parallel to the ion beam as a function of the fluence [5,6]. This effect has been observed in several classes of amorphous materials, but never in crystalline samples. The ion-induced anisotropic deformation has been described in terms of ion hammering: an amorphous mate- rial undergoes a dramatic shape change when bombarded with fast heavy ions at low temperature [7]. Independent of the detailed mechanisms responsible for this effect, it is clear that it is induced by the electronic energy loss pro- cesses to which the impinging MeV ions are subjected in 0022-3093/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2007.01.056 * Corresponding author. Tel.: +52 55 56225164; fax: +52 55 56225009. E-mail address: cheang@fisica.unam.mx (J.C. Cheang-Wong). www.elsevier.com/locate/jnoncrysol Journal of Non-Crystalline Solids 353 (2007) 1925–1929