Transmission electron microscopy without aberrations: Applications to materials science Angus Kirkland * , Lan-Yun Chang, Sarah Haigh, Crispin Hetherington Oxford University, Department of Materials, Parks Road, Oxford OX13PH, United Kingdom Available online 1 November 2007 Abstract Aberration correction leads to a substantial improvement in the directly interpretable resolution of transmission electron microscopes. Direct electron optical correction based on a hexapole corrector and indirect computational analysis of a focal or tilt series of images offer complementary approaches and a combination of the two provides additional advantages. This paper describes aberration cor- rected instrumentation installed in Oxford which is equipped with correctors for both the image-forming and probe-forming lenses. Examples of the use of these instruments in the characterisation of nanocrystalline catalysts are given together with initial results com- bining direct and indirect methods. Ó 2007 Elsevier B.V. All rights reserved. PACS: 68.37.Àd; 41.85.Àp; 61.46.Df Keywords: Aberration correction; Nanocrystalline catalysis; Exit wave reconstruction 1. Introduction The atomic structure of materials and defects are now routinely characterized at atomic resolution by high resolu- tion transmission electron microscopy (HRTEM) which provides direct structural imaging at resolutions at or below 0.1 nm. Further improvements can be achieved using aberration correction by either direct or indirect methods. The former involves the insertion of multipole elements in the electron optical column that correct the inherent positive spherical aberration [1]. Indirect methods require a dataset of multi- ple images, from which the aberrations may be measured and computationally compensated a posteriori to recover the specimen exit wavefunction (see [2] for a review). In this article, we first outline the above two approaches to aberration correction and subsequently discuss the mer- its of applying them in combination. 2. Direct aberration correction A JEOL 200 kV FEG(S)TEM with both probe and imaging aberration correctors and an in column energy fil- ter was installed in Oxford in 2003 [3,4]. The correctors are based on a design due to Haider et al. [5] in which the pri- mary elements consist of a pair of strong hexapoles and two round-lens doublets. In practice adjustment of the imaging corrector is achieved using a Zemlin tableau of dif- fractograms calculated from images of a thin amorphous foil and recorded at several tilt azimuths with constant tilt magnitude [6,7]. These datasets provide measurements of the tilt-induced defocus and two fold astigmatism yielding linear estimates for the coefficients of the wave aberration function. For practical measurement a computationally efficient algorithm in which each experimental diffracto- gram is compared to a library of pre-calculated diffracto- grams is used [8]. The spherical aberration and other aberration coefficients may be set to zero for pure ampli- tude contrast, or the spherical aberration coefficient may be set to a small negative value for optimum phase contrast [9,10]. 1567-1739/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.cap.2007.10.065 * Corresponding author. E-mail address: angus.kirkland@materials.oxford.ac.uk (A. Kirkland). www.elsevier.com/locate/cap www.kps.or.kr Available online at www.sciencedirect.com Current Applied Physics 8 (2008) 425–428